JP2000196523A - Optical communication system - Google Patents

Abstract

(57) [Problem] To solve the problem that stable modulation cannot be performed due to polarization fluctuation of an optical fiber when an optical modulator having polarization dependence is installed at a place away from a light source. SOLUTION: Light incident on an optical modulator 5 is polarization-scrambled by a polarization scrambler 2 in advance. The polarization scramble frequency is a frequency lower than the center frequency of the band signal to be transmitted. As a result, even if the optical fiber 6 receives unpredictable polarization fluctuation, stable modulation can always be performed. Further, since the polarization scramble frequency is low, a system can be constructed at low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical communication system using an external modulator.

[0002]

2. Description of the Related Art An optical communication system modulates light from a light source with a signal of information to be transmitted and transmits the modulated signal through an optical communication path such as an optical fiber, thereby enabling high-speed and wide-band communication. Conventionally, such an optical communication system is mainly used for a trunk system or the like. Therefore, the optical transceiver is often installed in a place where temperature conditions are stable, such as in a station of a communication company.

However, in recent years, demands for high-speed, wide-band communication have been ubiquitous, and accordingly, optical communication systems have been called optical subscriber systems, wireless base stations, etc. in addition to trunk systems. Has been widely used. As a result, there are more opportunities to install optical transceivers outdoors, especially in optical subscriber systems and optical accommodation of wireless base stations.

[0004] By the way, there are many places where an optical transmitter / receiver installed outdoors has a severe temperature condition such as on a telephone pole, and how to use a component which is vulnerable to high temperature and low temperature among components constituting the optical transmitter / receiver is to be used. Is a problem.

[0005] For example, a semiconductor laser device is used as a light source for optical communication. This semiconductor laser device is particularly vulnerable to a temperature change. Therefore, in the case where an optical transmitter is installed on a telephone pole as described above, as an example of a countermeasure, a semiconductor laser element is installed in a station where the environment is stable, and the output light is transmitted over the telephone pole using an optical fiber. Has been proposed, in which the light is transmitted to an optical transmitter, modulated by an optical modulator provided in the optical transmitter on the telephone pole, and sent back to the station.

On the other hand, there are mainly two types of optical modulators currently used in optical communication, which are of the electroabsorption type (EA type).
A modulator and a Mach-Zehnder (MZ) modulator.
When any one of these optical modulators is applied to a system for modulating and returning light transmitted from a station as described above, the polarization dependency of the optical modulator becomes a problem.

[0007] Since an ordinary optical fiber used as an optical communication path does not maintain polarization, what kind of polarization fluctuation occurs before a certain polarization output from the laser element reaches the optical modulator. I can not predict.

[0008] Of course, optical fibers that maintain polarization have also been put to practical use. However, the optical fiber that maintains this polarization is
It is mainly used for several meters, such as a patch cord, and its price is very high. Therefore, it is difficult to use it as a long-distance transmission fiber outdoors in terms of cost.

Therefore, since an ordinary optical fiber which does not maintain polarization is used as the optical communication path, the polarization dependence is not applied to the optical modulator used in the system for modulating and transmitting light as described above. If there is, the polarization input to the optical modulator fluctuates, so that stable modulation cannot be performed.

By the way, some EA type modulators have small polarization dependence, but MZ type modulators usually have large polarization dependence, and it has been considered difficult to apply them to the above-mentioned system. Was.

[0011] However, the MZ modulator has better characteristics such as distortion, loss, and chirping than the EA modulator. Therefore, if possible, we would like to construct a system that modulates and sends back light using an MZ modulator, but until now, it was not possible to solve the problem of polarization fluctuation at an appropriate price range. .

[0012] Of course, there is no example in which the MZ modulator is applied to the optical communication system as described above. For example, a document “O
MI95-8 "and the like.

In the technique disclosed in this document, two solid-state laser devices having slightly different wavelengths are prepared as light sources, and the output lights of these two solid-state laser devices are combined with their polarizations orthogonal to each other. Used. Light of orthogonal polarizations with slightly different wavelengths can be considered independent because of the different wavelengths.

When this is modulated by the optical modulator, a certain amount of the total power of the two lights is always modulated.
Since the system constructed in the document "OMI95-8" is a system with very severe noise conditions, there is a great merit of using a solid-state laser device with lower noise than a semiconductor laser device.

For this reason, the use of the solid-state laser device is acceptable in terms of cost. However, in a system in which the requirement for noise is not strict, such as accommodating a mobile radio base station, the solid-state laser device is used only for the reason of polarization dependence. Such expensive items cannot be used in terms of system cost. Then, it is difficult to use a method of combining two laser outputs having slightly different wavelengths in the same manner by using a semiconductor laser element instead of the solid-state laser device so that the polarizations are orthogonal to each other.

For example, a semiconductor laser device has extremely low wavelength stability as compared with a solid-state laser device, so how to stabilize the wavelength is a major issue.
When the wavelength is unstable, the following problem occurs.

That is, if the wavelengths of the two lights are too close to each other without being stabilized, beat noise occurs due to interference. Further, if the wavelengths of the two lights are completely equal, the independence of each of the lights cannot be maintained, and the orthogonality cannot be maintained. Can not be regarded as two lights in the polarized state. Therefore, it is absolutely necessary to stabilize the wavelength of the semiconductor laser device. However, the wavelength stabilization of the semiconductor laser device requires a complicated mechanism and is costly.

[0018]

As described above, an optical transmitter installed in a place where temperature conditions are severe, such as on a utility pole, does not include a delicate component such as a semiconductor laser device, and an optical modulator. There is a demand for a system in which only a single light source is arranged, and the unmodulated light transmitted from the laser light source in the station via an optical fiber is modulated and returned.

In this case, since the polarization randomly fluctuates depending on the optical fiber from the station to the optical modulator, an optical modulator having polarization dependence cannot be used. There has been proposed a method of combining light having slightly different wavelengths in a state where the polarizations are orthogonal to each other and sending the light to an optical modulator, but the cost is high.

Therefore, development of a method for solving this problem has been desired.

An object of the present invention is to solve such a problem and to change the polarization from the light source to the modulator.
Another object of the present invention is to provide an optical communication system that can operate stably in an appropriate price range even if the modulator has polarization dependence. Another object of the present invention is to provide an optical communication system capable of reducing the cost of the system even if the frequency band to be handled becomes high.

[0022]

To solve such a problem, the present invention is as follows.

[1] In the first aspect of the present invention, a light source for outputting output light having temporal coherence is connected to the light source so that the polarization of the output light is substantially non-polarized in time average. A polarization scrambler that scrambles in time, and a part or all of the output light that is polarization scrambled by the polarization scrambler is modulated by an electric signal corresponding to an input data signal, and output to an optical fiber. An optical modulator having a modulation characteristic that is dependent on the incident polarization.

Light sources generally used for high-speed optical communication have temporal coherence and always have a specific polarization. It suffices if the optical modulator can be input to the optical modulator without any polarization fluctuation from the light source.

Therefore, in the present invention, a polarization scrambler for temporally scrambling the polarization is inserted between the light source and the optical modulator. The polarization scrambler modulates the polarization of the input light and makes the time average of the polarization of the output light almost non-polarized. In order to explain the operation, a Poincare sphere for graphically illustrating the polarization state is introduced.

The Poincare sphere is a sphere as shown in FIG.
It maps polarization on a sphere. Fully coherent light has perfect polarization, which is mapped onto any of the surfaces of the sphere. The radius of the sphere indicates the degree of polarization. Light that is not partially coherent (partially polarized light) cannot completely define the polarization, and remains ambiguous and is mapped inside the sphere. The center of the sphere is completely incoherent light and cannot specify any polarization. The state mapped to the center of the sphere is called unpolarized light.

As described above, since the light source for high-speed optical communication is coherent light, it is mapped on the surface of a sphere.
It is almost impossible to make this completely incoherent light and non-polarized light. However, it is possible to modulate the polarization at high speed so that its time average is at the center of the sphere. For example, the time between points A and B in FIG.
If you switch fast, the mean will be at the center of the sphere.

Such a state can be said to be non-polarized on a time average.

As described above, the polarization scrambler is a component that realizes a substantially non-polarized state on a time average.

To perform modulation with an optical modulator having polarization dependence, incident light is separated corresponding to two orthogonal polarization axes of the optical modulator, and modulation is performed with different efficiencies and losses. After that, it is equivalent to multiplexing. There is a polarizer as an element that cuts out a specific linearly polarized component, and a polarizer is not necessarily included in the configuration of the present invention, but in order to explain a process of separating into two orthogonal polarizations, a polarizer is schematically illustrated. Use

The behavior of the polarizer on the Poincare sphere will be described. When a certain polarization enters the polarizer, only the component projected on the axis is output. When an arbitrary polarized wave is incident on the polarizer, the magnitude of the output can be calculated as follows using a Poincare sphere.

A point on the Poincare sphere corresponding to an arbitrary polarization is mapped. For example, suppose that it is point A in FIG.

The equator of the Poincare sphere corresponds to linear polarization, and the position on the equator differs depending on the inclination of the linear polarization.
A point P on the Poincare sphere corresponding to the linearly polarized light inclined in the same direction as the direction of the axis of the polarizer is defined. The diameter of the sphere including the point P always passes through the center of the sphere. Let this be a straight line L,
Let Q be the point where the straight line L intersects the surface on the opposite side of the sphere (not point P).

Thus, a plane including the point A and perpendicular to the straight line L is uniquely determined. The intersection of this plane and the straight line L is denoted by C. The ratio of the output light power of the polarizer to the total power of the light incident on the polarizer is equal to PQ / CQ. In addition, the arbitrary polarization is Q
, And P, the ratio becomes equal to PC: CQ.

When light passes through a medium that causes polarization fluctuation, for example, a normal optical fiber that does not maintain polarization, the position of the output polarization on the Poincare sphere changes. But,
If the polarization-causing medium has no polarization-dependent loss,
Only the polarization changes so as to slide up the surface of the sphere, but the relative positional relationship of the plurality of points does not change. Therefore, the time average is unpolarized, that is, light that is polarization-scrambled so as to be at the center of the sphere passes through an optical fiber that is not polarization-maintaining. The non-polarized state at is maintained.

Consider a case where such light is modulated by a polarization-dependent optical modulator. As described above, it is assumed that the polarization scrambled light is separated according to two orthogonal polarization axes of the optical modulator. Since the time average of the polarization-scrambled light is at the center of the sphere, the separated light is always time-averaged to 1: 1 power, regardless of the direction of the axis of the optical modulator. Is done. Even if the modulation efficiency and loss in each polarization axis of the optical modulator are different, the ratio of the separated power is constant,
The light that is combined after being modulated also undergoes a certain fixed rate of loss and modulation. Even if the light is changed by an optical fiber or the like before entering the optical modulator, it is always constant regardless of the degree of the change.

A polarization scrambler can generally be easily constructed using a phase modulator. The phase modulator does not require a complicated configuration such as bias control, and is a device that can be constructed at a relatively low cost.

In this way, even if the polarization modulator which has polarization dependence is used and the polarization fluctuation which cannot be predicted before the light enters the optical modulator, the modulation efficiency and the loss are always stable. And stable transmission quality can be ensured at relatively low cost.

[2] Next, the second invention of the present application provides the optical communication system of the first invention, wherein the optical modulator is a Mach-Zehnder type optical modulator.

A Mach-Zehnder (MZ) modulator generally has large polarization dependence. In many cases, a polarizer is inserted at the input of the modulator so that only the polarization having the highest modulation efficiency is modulated. For this reason, hitherto, it has only been used in systems where the incident polarization does not fluctuate. However, as described above, the MZ modulator has distortion characteristics, chirping characteristics, loss characteristics, and the like.
In many cases, the performance is superior to that of an electroabsorption (EA) modulator.

Therefore, by applying the first invention of the present application to a system in which an MZ modulator having a high performance is desired but cannot be used due to polarization fluctuation, transmission of higher quality can be achieved.

[3] Next, in the third invention of the present application, an optical fiber transmission line that does not maintain polarization is inserted between the polarization scrambler and the optical modulator. An optical communication system according to the first or second aspect of the present invention is provided.

The present invention is applied to a system in which an optical fiber transmission line is inserted between a light source and an optical modulator, for example, a system in which a light source is located inside a station and an optical modulator is arranged on a utility pole. Apply. As a result, the polarization fluctuation of the optical fiber transmission line is allowed while using the polarization-dependent optical modulator.
By doing so, the selection range of the device is expanded, and the device can be selected with emphasis on other performances regardless of the polarization dependence.

The position where the polarization scrambler is inserted is desirably immediately after the light source where the polarization scrambler can be configured more simply than immediately before the optical modulator. That is, the optical fiber transmission line is desirably provided between the polarization scrambler and the optical modulator. There are two types of polarization scramblers: one that can change an arbitrary polarization into an unpolarized state, and one that can change the polarization into an unpolarized state only when a specific polarization is input.
The latter is simpler in configuration and lower in cost.

Since the light source normally outputs a fixed polarization, the polarization scrambler is connected to the polarization scrambler by a patch cord of a polarization maintaining optical fiber or the like, so that only a specific polarization is output to the polarization scrambler. It is easy to make it incident.

By doing so, a system can be constructed at lower cost.

[4] Next, in the fourth invention of the present application, the data signal is a band signal, and the output light is modulated as a subcarrier signal by the optical modulator. An optical communication system according to a third invention is provided.

In the present invention, the polarization-scrambled light is subjected to subcarrier modulation with a band signal by a polarization-dependent modulator. In this way, when transmitting a radio signal or the like received by an antenna installed on a telephone pole to a station without demodulation, it is possible to use an optical modulator having polarization dependency but good distortion characteristics. It becomes possible. Generally, band signals such as radio signals are often frequency-multiplexed and have a large dynamic range, and thus are sensitive to distortion characteristics. Therefore, as in the past, it has been difficult for an EA modulator having no polarization dependence but poor distortion characteristics to transmit such a signal with good quality.

According to the present invention, such band signals can be transmitted with good quality.

[5] Further, in the fifth invention of the present application, the fundamental frequency of the scramble signal for scrambling the polarization applied to the polarization scrambler is smaller than the lower limit frequency of the band signal, and An optical communication system according to a fourth aspect of the present invention is provided, wherein the frequency is higher than a frequency corresponding to the bandwidth of the band signal.

The polarization scrambler which has been used in the optical communication system scrambles the light modulated by the baseband signal at a frequency higher than the bit rate (at a speed of twice or more the bit rate). To switch the waves). The reason is that when polarization scrambling is performed in a system in which light passes through a device having extremely large polarization dependence, information is lost unless polarization is switched at twice or more the bit rate. It has been done.

In the present invention, the polarization-scrambled light is subjected to subcarrier modulation by a band signal using a polarization-dependent optical modulator. Up to now, there has been no study on using subcarrier-modulated light with a band signal together with polarization scrambling, and analogy from the baseband system requires that polarization scrambling be performed at a frequency that is at least twice the subcarrier frequency. It seems there is.

Considering the current state and future prospects of the communication technology field, an optical subcarrier system using an external optical modulator as assumed in the present invention has a subcarrier frequency of several [GHz] to several tens [GHz]. ] Is likely to be high. In this case, polarization scrambling may be easily applied at a frequency equal to or more than twice the subcarrier frequency, but if the frequency becomes very high, a frequency response characteristic corresponding to the scrambling is required. It is undeniable that the cost may be significantly higher than that of a low frequency.

One of the objects of the present invention is to enable polarization scrambling at a frequency lower than the subcarrier frequency in order to avoid such a high cost.

Consider a case where the polarization scrambled light is subcarrier-modulated by a band signal by a modulator having a very high polarization dependency, such as an MZ modulator having a polarizer at the input. . The intensity of the light before being modulated at the stage of passing through the polarizer oscillates around a certain DC level in a waveform composed of a scramble frequency and its harmonics. FIG. 7 shows an electric spectrum in the case where light undergoes a photoelectric change at the stage when light passes through the polarizer. There is a constant power to the direct current, that is, a power in the case where half of the light power incident on the polarizer is photoelectrically converted. Furthermore, a linear spectrum of some magnitude exists in the optical modulator at the scramble frequency fsc and its harmonics, depending on the direction of the axis of the polarizer and the polarization fluctuation received before the light enters the polarizer.

To modulate this with an optical modulator means to create a convolution of the modulated signal and the signal of FIG. When the modulated signal is a band signal having the center frequency fs, a spectrum as shown in FIG. 10B is obtained by the following convolution operation. That is, each line spectrum of FIG. 7 and its image component (FIG. 10)
(A)) is shifted in the plus direction by the center frequency fs of the band signal (it shifts to the minus, but this is not considered because it eventually becomes an image of a component shifted to the plus). Each line is shaped into a band signal, and then folded around "0". Since the signal applied to the optical modulator is a signal centered on a DC bias point other than “0”, a part of the power of the signal in FIG. Thus, FIG. 10B is obtained.

The optical receiver extracts a component at the original signal frequency fs from such a signal obtained by photoelectric conversion. This component is a component generated by convolution of the DC component of FIG. 10A and the modulation signal of the center frequency fs. The components generated by the scramble frequency and its harmonics cannot be used because their magnitudes fluctuate due to the influence of the polarization fluctuations received before they enter the optical modulator, and sometimes they may not be present at all.

Therefore, regardless of the number of polarization scramble frequencies, the received signal can be obtained stably as long as the DC component is obtained stably.

As can be seen from the above, there is no need to scramble at a frequency twice or more the subcarrier frequency. Although it may be higher than the subcarrier frequency,
A small frequency, at which system construction is easier, is sufficient.

However, as shown in FIG. 10B, a component generated by passing the scrambled signal through the polarizer may eventually remain due to the DC component added to the band signal. When this overlaps with the signal to be received, the receiving sensitivity is degraded. Therefore, it is desirable that the scrambling frequency be a frequency lower than the lower limit frequency of the band signal, avoiding the frequency band of the band signal.

From the viewpoint of the device driving system, the lower the scramble frequency, the better. However, as can be seen from FIG. 10B, a component occurs due to the convolution of the scramble frequency and the band signal. In addition, if the received signals overlap, the receiving sensitivity deteriorates. Therefore, it is desirable that the scramble frequency is a frequency larger than the frequency of the bandwidth of the band signal.

As described above, the scramble frequency can be kept low, and the cost of the polarization scrambler can be reduced. By setting the lower limit of the scramble frequency, transmission quality can be ensured.

[6] The sixth invention of the present application provides the optical communication system of the fifth invention, wherein the fundamental frequency of the scrambled signal is smaller than 1/6 of the lower limit frequency of the band signal. I do.

As shown in FIG. 7, when the polarization scrambled signal at the frequency fsc passes through a polarizer or a component having a polarization dependent loss, even if the scrambled signal is a sine wave, the signal after photoelectric conversion is obtained. Electric signals include
A harmonic component of fsc is generated in addition to the component of frequency fsc. The magnitudes of these components vary depending on the magnitude of the polarization-dependent loss of the component and the polarization fluctuations received before entering the component, but tend to be smaller as the order of the harmonic is higher. Therefore, it is sufficient that these harmonics are small enough to not affect the received signal even if they overlap with the signal to be received.

Therefore, in the sixth invention of the present application, the scramble frequency is sufficiently reduced. As a result, even if a harmonic is generated and overlaps the received signal, the order of the overlapped harmonic is high and its magnitude is sufficiently small.

The order at which the harmonics become sufficiently small is expected to be about the seventh order. Therefore, in the present invention, the scrambling frequency is set to 1/6 or less of the lower limit frequency of the band signal so that the sixth or lower harmonic does not overlap within the signal band.

By doing so, it is possible to perform good transmission without being affected by the component generated when the polarization-scrambled light passes through the component having the polarization-dependent loss.

As another method for avoiding the influence of such harmonic components, in the seventh invention of the present application, the fundamental frequency of the scramble signal is such that a harmonic of an arbitrary order of the fundamental frequency is a signal band of the band signal. The optical communication system according to the fifth aspect of the present invention is characterized in that the optical communication system is determined so as not to overlap.

[7] The relative bandwidth of the band signal is relatively large,
If the frequency of the sixth invention of the present application is lower than the frequency corresponding to the bandwidth of the band signal, the sixth invention of the present application cannot be applied. If the scrambling frequency is relatively high and the frequency of the sixth invention of the present application is lower than the frequency corresponding to the bandwidth of the band signal, the sixth invention of the present application cannot be applied. If the scramble frequency cannot be lowered as shown in FIG. 10 (b), the frequency-shifted band signal folded around "0" may overlap with the signal to be received.

Therefore, in the seventh invention of the present application, a scramble frequency in which harmonics do not overlap in the signal band is selected and used. That is, assuming that the fundamental frequency of the scrambled signal is fsc, the center frequency of the band signal is fs, and the bandwidth (full width) of the band signal is △ F, the high frequency of any order n (≧ 2) is n * fsc <fs− △ The scrambling frequency fsc is selected such that F / 2 and n * fsc> fs + △ F / 2.
When the scramble frequency fsc is relatively small, this condition alone is sufficient.

However, if the scramble frequency fsc cannot be lowered for some reason and the loopback signal overlaps as shown in FIG. 10B, the loopback signal does not enter the signal band to be received. So, furthermore,
For a harmonic of any order p (≧ 2), the scrambling frequency fsc is set so that 2 (fs + ΔF / 2) / p <fsc and 2 (fs−ΔF / 2) / p> fsc Select

By doing so, good transmission can be achieved without the high frequency of the scrambled signal or the folded band signal overlapping the received signal.

[8] Further, in the eighth invention of the present application, the data signal is a radio signal received by an antenna as it is or a signal that has been converted to an intermediate frequency and has not been demodulated. An optical communication system according to a fourth to seventh aspects of the present invention is provided.

In the present invention, since the polarization-scrambled light is modulated by the polarization-dependent optical modulator, it cannot be ruled out that there is a loss in optical power in principle. But,
The amount of optical power loss caused by modulating the polarization scrambled light with a polarization-dependent optical modulator is 3 [dB] at the maximum. (Of course, there are also other losses such as excess loss of the polarization scrambler, excess loss of the optical modulator, and loss due to optical modulation. However, these are the amounts that can be reduced by the progress of the device.) As described in, the power of the polarization scrambled light is equally divided into two orthogonal polarization axes of the optical modulator,
This is because, when the polarization dependence of the optical modulator is maximum, that is, when a polarizer is inserted, light incident on one polarization axis is not transmitted at all.

In an optical communication system, reception sensitivity generally deteriorates due to a decrease in optical power. The degree of deterioration varies depending on the applied system. However, when the present invention is applied to a radio signal received by an antenna, the reception sensitivity hardly deteriorates.

It is explained as follows.

When transmitting a radio signal received by an antenna, particularly when the radio signal is a signal for mobile communication, the radio signal is received by an antenna and an LNA (Low Noise) (not shown) is used.
Amplifier), but a lot of noise is already mixed at the stage of amplification, and the signal-to-noise ratio (SNR) is often not so large.

On the other hand, the noise added to the optical communication system of the present invention, particularly in the optical transmission unit, is much smaller than the noise mixed in the signal received by the antenna. If there is no loss caused by polarization scrambling, the SNR degradation of the radio signal in the optical transmission unit is very small. The optical loss received by polarization scrambling is at most 3 [dB] as described above.

As a result, the relative increase in the noise amount of the optical link is at most 3 [dB]. When the additional noise in the originally small optical transmission unit increases by about 3 [dB], the SNR degradation of the radio signal is reduced. Contributes little.

Accordingly, in the present invention, although light is lost due to polarization scrambling, if the applied signal is a radio signal received by an antenna, the reception sensitivity is hardly degraded due to the loss. equal.

By applying the present invention to such a system, it is possible to enjoy only the advantages of the present invention, without any disadvantages present in the present invention.

[9] Further, in the ninth invention of the present application, the light source is installed indoors, and the optical modulator is installed in a device installed outdoors. An optical communication system according to an eighth aspect of the present invention is provided.

By applying the present invention, it is possible to install an optical modulator having good characteristics, but having polarization dependence, at a location away from the light source, and to connect the optical modulator with a normal optical fiber. In this way, it is possible to place the light source in the office building of the service provider and place the optical modulator in the device installed on the telephone pole, and on the telephone pole it is possible to return only the modulated light Becomes As a result, it becomes possible to operate the light source in a stable temperature condition, and it is possible to use an optical modulator excellent in other characteristics irrespective of the polarization dependence, and to achieve high quality. , Stable transmission becomes possible.

[10] Next, in the tenth invention of the present application, a light source for outputting output light having temporal coherence and a polarization signal for subcarrier-modulating the output light of the light source with an electric signal corresponding to a band signal are provided. Optical modulator with wave dependence,
Alternatively, a device having a polarization dependent loss and an optical modulator that performs subcarrier modulation on the output light of the light source, inserted between the light source and the optical modulator, the output light of the light source at a specific frequency A polarization scrambler for polarization scrambling,
An optical receiver that photoelectrically converts the light modulated by the optical modulator and receives a component generated by convolution of the scrambling frequency of the polarization scrambler or a harmonic thereof and the band signal. A communication system is provided.

As described above, when the polarization-scrambled light is passed through a polarization-dependent device, many harmonics are generated in addition to the scramble frequency. The amount of generated harmonics depends on the magnitude of polarization modulation during polarization scrambling,
It depends on the waveform of the scrambled signal, the polarization fluctuation received while the light arrives at the device having polarization dependence, and the magnitude of the polarization dependence of the device. In the present invention, the harmonic has been treated as an interfering wave so far, but by using this, a system with an unprecedented operation can be constructed.

The present invention has mainly been considered to be applied to an optical transmission system for a band signal in a millimeter wave or quasi-millimeter wave band, but components operating at such a high frequency are expensive. A method of generating such a high-frequency data signal is generally as follows. After modulating the low-frequency carrier with the baseband data, using a mixer or the like, multiplying the carrier with millimeter-wave or quasi-millimeter-wave band,
Upconvert to the desired frequency band. The carrier used at this time is usually generated by multiplying a low-frequency carrier having a small phase noise by a multiplier. This is because it is difficult to produce a millimeter-wave or quasi-millimeter-wave band oscillator having good noise characteristics with the current technology. Multipliers, mixers, and the like are expensive because they are components in the millimeter-wave and quasi-millimeter-wave bands, and optical modulators operating at such high frequencies, their driving systems, and optical receiving systems are also expensive.

On the other hand, in a system using a polarization scrambler as in the present invention, it is possible to easily generate a harmonic of a scramble frequency. Devices for generating harmonics are devices having high polarization dependence, and at most low cost passive devices such as polarizers. In addition, the level of the harmonic generated in this way is higher in principle than the harmonic generated by an ordinary nonlinear device (such as a diode). A typical nonlinear device is linear based,
This is because the process of cutting out the polarization scrambled light with a polarizer has little linearity, and in principle can only be non-linear, while using a small amount of deviation from linearity. However, as described above, since the magnitude of the harmonic depends on various parameters until the light is incident on the polarizer, it is necessary to adjust the parameters so that the harmonic is generated as efficiently as possible. In particular, it is necessary to configure the system by paying attention to the state of polarization that is passed from the polarization scrambler to the polarizer. In some cases, adjustment may be made by aggressive polarization conversion using a wave plate or its equivalent device.

As a result, a high-order high-order harmonic can be easily obtained. Optical modulator modulates light with harmonic components generated by passing through a polarizer, or an optical modulator with high polarization dependence on polarization scrambled light (or a polarizer mounted at the signal input end) When modulation is performed by the modulated optical modulator), a convolution component of the modulation signal, the scrambled signal frequency, and its harmonics is generated as shown in FIG. When harmonics are generated mainly by a device having a polarization dependent loss instead of an optical modulator, the order of the optical modulator and the device having a polarization dependent loss may be interchanged. That is, the operation of the present invention does not change even if the light is passed through a device having polarization dependence after being modulated by the optical modulator.

As described above, since the level of the harmonic is large, a component obtained by frequency-converting the band signal by the harmonic can be obtained with sufficiently large power. Since these components are replicas of the original band signal, they may be cut out by a filter or the like as necessary and used.

The principle of the present invention is to generate a convolution of a plurality of signals. Therefore, it can be realized with another configuration. For example, light whose fundamental frequency is equal to the scramble frequency and whose intensity is modulated by a waveform (rectangular wave, triangular wave, short pulse train, etc.) containing many harmonic components, or a local carrier input to a mixer when frequency conversion is performed electrically May be further modulated by a data signal by an external optical modulator.

However, generating a rectangular wave or a short pulse train containing many harmonic components requires a device having a frequency band extending to the region of the harmonic components, so that it is difficult to reduce the cost. . Also, it is costly to apply optical modulation at a high frequency such as a millimeter wave or a quasi-millimeter wave from the beginning.

However, according to the present invention, by using the polarization scrambling, it is possible to reduce unwieldy and expensive components such as a multiplier and a mixer in the millimeter-wave and quasi-millimeter-wave bands. At this time, the polarization scrambling frequency may be a low frequency. The mechanism for generating harmonics is a passive process called polarization dependence, but large harmonics can be easily obtained. As a result, high-quality frequency conversion can be performed at low cost.

[11] More specifically, the eleventh embodiment of the present invention
According to the invention, the scramble frequency is a frequency smaller than the lower limit frequency of the band signal, and the optical receiver is a component generated by convolution of the band signal with the scramble frequency or a harmonic thereof obtained by photoelectric conversion. Among them, the optical communication system according to the tenth aspect of the present invention is characterized in that a component having the lowest frequency is extracted.

As can be seen from FIG. 10, the configuration of the present invention also substantially performs downsampling. Therefore, according to this configuration, a band signal converted to a low frequency is generated. Among them, the signal generated at the lowest frequency is cut out by a low-pass filter and used. The signal extracted by a low-pass filter or the like may be processed by inputting it to a demodulator depending on the intended use. If the cut-out signal is to be demodulated, the scramble frequency may be determined so that the signal is converted to a frequency suitable for demodulation.

Although downsampling originally requires an A / D converter operating near the frequency of the original band signal, according to the present invention, a converter for a millimeter wave, a quasi-millimeter wave band or the like is used. Downsampling can be easily performed without using, and a signal having a frequency that is easy to handle can be obtained.

The quality of the obtained signal is better than that obtained by down-converting the frequency by using a frequency multiplier or a mixer operating in the millimeter wave or quasi-millimeter wave band.
Also, since the part that operates in the millimeter wave and quasi-millimeter wave band is only around the optical modulator, the system can be constructed at low cost.

With such a configuration, the optical receiver only needs to operate in the frequency band of a signal to be received, that is, a signal converted to a low frequency. Therefore, despite transmitting signals in the millimeter wave and quasi-millimeter wave bands by the optical modulator, the optical receiver may be of a low frequency operating at, for example, 1 [GHz] or less. Can be configured at very low cost. [12] Further, in the twelfth invention of the present application, the optical receiver uses the frequency of the band signal among the components generated by convolution of the scramble frequency or its harmonics and the band signal obtained by photoelectric conversion. An optical communication system according to the tenth aspect of the present invention is characterized in that a component having a higher frequency is extracted.

Contrary to the tenth aspect, it is also possible to up-convert a signal by using a mode as in the present invention. For example, in FIG. 10B, an up-converted band signal can be obtained by cutting out a band signal whose frequency has been shifted to a higher side than fs by a band-pass filter or the like.

The components in the millimeter and quasi-millimeter wave bands are much more expensive than those in the low frequency band. Millimeter-wave and quasi-millimeter-wave optical modulators and their driving systems are not low-cost. In the present invention, the polarization-scrambled light is data-modulated using a polarization-dependent optical modulator, and the band signal used at that time is a low-frequency band signal. Also,
The scramble frequency of the polarization scrambler is not high frequency such as millimeter wave and quasi-millimeter wave band (although it may be scrambled at such frequency if there is no problem of cost), but it is scrambled at lower frequency, Harmonics generated by passing through components having polarization dependence are used. As a result, high-quality frequency conversion can be performed at lower cost.

In the present invention, the optical receiver needs to operate at an up-converted frequency, that is, a millimeter wave or a quasi-millimeter wave band. However, all components of the optical transmission system can be of low frequency, and there is no need for expensive electrical devices such as mixers and multipliers for converting the low frequency to high frequency, so the system can be manufactured at low cost. Can be built. Also, since the harmonics generated by the present invention have a high level (high power), high-quality frequency conversion is possible, and the quality of the signal obtained by the optical receiver is high.

[0101]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. In the embodiment, only a portion related to the operation of the present invention is shown.

(First Embodiment) Here, it is assumed that a polarization-dependent device is used as an optical modulator, and that unpredictable polarization fluctuation is given before light enters the optical modulator. In addition, a description will be given of an embodiment of a system that can always perform modulation with stable modulation efficiency and loss and that can secure stable transmission quality at relatively low cost without requiring wavelength stability of a light source. .

FIG. 1 is a diagram showing an embodiment of the first invention of the present application. In the figure, 1 is a coherent light source, 2 is a polarization scrambler, 3 is a scramble signal source, 5 is an optical modulator, and 6 is an optical fiber. The coherent light source 1 is a light source that outputs light having temporal coherence, a scramble signal source 3 is for generating a scramble signal, and a polarization scrambler 2 is a scramble signal input from the scramble signal source 3. Is used to scramble the polarization of the input light so that the time average of the polarization becomes non-polarized.

The optical modulator 5 modulates the intensity of the polarization-scrambled light in accordance with the data signal input from the data input 7 and outputs the modulated light to the optical fiber 6. Reference numeral 5 has polarization dependency such that the modulation efficiency and the loss are different depending on the polarization of the incident light.

The present apparatus having such a configuration generates light having temporal coherence by the coherent light source 1,
The output light of the coherent light source 1 is input to the polarization scrambler 2. The polarization scrambler 2 uses the scramble signal input from the scramble signal source 3 to scramble the polarization of the input light so that the time average of the polarization becomes non-polarized.

The polarization-scrambled light is supplied to the optical modulator 5.
, Undergoes intensity modulation corresponding to the data signal input from the data input 7. The optical modulator 5 has polarization dependency such that the modulation efficiency and the loss are different depending on the polarization of the incident light. The light having undergone the intensity modulation is output to the optical fiber 6 and transmitted to the optical receiver.

With such a configuration, even if a polarization-dependent device is used as the optical modulator and an unpredictable polarization fluctuation is given before the light is incident on the optical modulator, the polarization modulator is always used. Modulation with stable modulation efficiency and loss becomes possible. In addition, stable transmission quality can be ensured at relatively low cost without requiring wavelength stability of the light source.

<Configuration Example of Polarization Scrambler> A configuration example of the polarization scrambler 2 used here will be described. Various types of polarization scramblers have been proposed so far. For example, a device that applies phase modulation to only one of two orthogonal polarizations of incident light using a waveguide having an electro-optic effect, or a phase difference between two orthogonal circular polarizations using a Faraday rotator And the like that modulates

Light sources generally used for high-speed optical communication have temporal coherence and always have a specific polarization. It suffices if the optical modulator can be input to the optical modulator without polarization fluctuation from the light source, but there are some systems that cannot.

Therefore, according to the present invention, a polarization scrambler 2 for scrambling the polarization temporally between the light source and the optical modulator is provided.
Insert The polarization scrambler 2 modulates the polarization of the input light and makes the time average of the polarization of the output light substantially non-polarized.

In order to explain the operation, a Poincare sphere for graphically illustrating the polarization state is introduced.

The Poincare sphere is a sphere as shown in FIG.
It maps polarization on a sphere. Fully coherent light has perfect polarization, which is mapped onto any of the surfaces of the sphere. The radius of the sphere indicates the degree of polarization. Partially non-coherent light (partially polarized light) cannot completely define the polarization, remains ambiguous, and is mapped inside the sphere. The center of the sphere is completely incoherent light and cannot specify any polarization. The state mapped to the center of the sphere is called unpolarized light.

As described above, since the light source for high-speed optical communication is coherent light, it is mapped on the surface of the sphere.
It is almost impossible to make this completely incoherent light and non-polarized light. However, it is possible to modulate the polarization at high speed so that its time average is at the center of the sphere. For example, the time between points A and B in FIG.
If you switch fast, the mean will be at the center of the sphere.

Such a state can be said to be non-polarized on a time average.

As described above, the polarization scrambler is a component that realizes a substantially non-polarized state on a time average.

To perform modulation with an optical modulator having polarization dependence, incident light is separated corresponding to two orthogonal polarization axes of the optical modulator, and modulation is performed with different efficiencies and losses. After that, it is equivalent to multiplexing. There is a polarizer as an element that cuts out a specific linearly polarized component, and a polarizer is not necessarily included in the configuration of the present invention, but in order to explain a process of separating into two orthogonal polarizations, a polarizer is schematically illustrated. Use

Here, the behavior of the polarizer on the Poincare sphere will be described.

When a certain polarization enters the polarizer, only the component projected on the axis is output. Therefore, when an arbitrary polarized wave is incident on the polarizer, the magnitude of the output can be calculated as follows using the Poincare sphere.

A point on the Poincare sphere corresponding to an arbitrary polarization is mapped. For example, suppose that it is point A in FIG.

The equator of the Poincare sphere corresponds to linear polarization, and the position on the equator differs depending on the inclination of the linear polarization.
A point P on the Poincare sphere corresponding to the linearly polarized light inclined in the same direction as the direction of the axis of the polarizer is defined. The diameter of the sphere including the point P always passes through the center of the sphere. Let this be a straight line L, and let Q be the point at which this straight line L intersects the opposite surface of the sphere (not point P).

In this way, a plane including the point A and perpendicular to the straight line L is uniquely determined. The intersection of this plane and the straight line L is denoted by C. The ratio of the output light power of the polarizer to the total power of the light incident on the polarizer is equal to PQ / CQ. In addition, the arbitrary polarization is Q
, And P, the ratio becomes equal to PC: CQ.

When light passes through a medium that causes polarization fluctuation, for example, an ordinary optical fiber that does not maintain polarization, the position of the output polarization on the Poincare sphere changes. But,
If the polarization-causing medium has no polarization-dependent loss,
Only the polarization changes so as to slide up the surface of the sphere, but the relative positional relationship of the plurality of points does not change. Therefore, the time average is unpolarized, that is, light that is polarization-scrambled so as to be at the center of the sphere passes through an optical fiber that is not polarization-maintaining. The non-polarized state at is maintained.

Consider a case where such light is modulated by a polarization-dependent optical modulator. As described above, it is assumed that the polarization scrambled light is separated according to two orthogonal polarization axes of the optical modulator. Since the time average of the polarization-scrambled light is at the center of the sphere, the separated light is always time-averaged to 1: 1 power, regardless of the direction of the axis of the optical modulator. Is done. Even if the modulation efficiency and loss in each polarization axis of the optical modulator are different, the ratio of the separated power is constant,
The light that is combined after being modulated also undergoes a certain fixed rate of loss and modulation. Even if the light is changed by an optical fiber or the like before entering the optical modulator, it is always constant regardless of the degree of the change.

In general, the polarization scrambler 2 can be easily constructed using a phase modulator. And
The phase modulator does not require a complicated configuration such as bias control, and is a device that can be constructed at a relatively low cost. <Example of use of MZ modulator in system where incident polarization fluctuates> If the optical modulator has no polarization dependency, there is no need to separately use polarization scrambler 2 to polarization scramble coherent light, which is simple. Configuration. However, optical modulators generally have a greater or lesser polarization dependence. Some electroabsorption (EA) modulators have relatively small polarization dependence, but the EA modulator generally has a Mach-Zehnder (MZ-Zehnder) distortion characteristic, loss characteristic, and chirping characteristic.
Type) worse than modulator.

On the other hand, the MZ modulator generally has a large polarization dependency, and stable modulation cannot be performed in a system in which the incident polarization fluctuates. However, by applying the present invention, the MZ modulator can be used even in a system where the incident polarization fluctuates. FIG. 2 is a block diagram showing the embodiment.

FIG. 2 is a block diagram showing an embodiment in which a Mach-Zehnder (MZ) modulator can be used even in a system in which the incident polarization fluctuates by applying the present invention. As shown in FIG. 2, the polarization obtained by scrambling the input light by the polarization scrambler 2 so that the time average of the polarization becomes non-polarized is input from the data input 7 by the MZ modulator 8. The intensity is modulated according to the data signal and output to the optical fiber 6.

In this system having such a configuration, the output light of the coherent light source 1 that outputs light having temporal coherence is input to the polarization scrambler 2. The polarization scrambler 2 uses the scramble signal input from the scramble signal source 3 to scramble the polarization of the input light so that the time average of the polarization becomes non-polarized.

The polarization-scrambled light is subjected to intensity modulation by a Mach-Zehnder (MZ) modulator 8 corresponding to the data signal input from the data input 7. The MZ modulator 8 has polarization dependence such that the modulation efficiency and the loss are different depending on the incident polarization. The light having undergone the intensity modulation is output to the optical fiber 6 and transmitted to the optical receiver.

The MZ modulator 8 is superior to the EA modulator in distortion characteristics, loss characteristics, chirping characteristics and the like.

Therefore, by using the polarization scrambler 2 as in the present invention, the polarization of the input light is scrambled so that the time average of the polarization becomes non-polarized, and this polarized light is converted into an MZ modulator. By using the MZ modulator, even in a system where the input polarization fluctuates, better transmission characteristics can be obtained using the MZ modulator.

As described above, according to the present invention, the polarization of the input light is scrambled by using the polarization scrambler 2 so that the time average of the polarization becomes non-polarized, and the polarized light is modulated. The optical signal is transmitted to an optical communication path after being modulated by a data signal, so that light that has passed through a block with polarization fluctuations, such as an optical fiber, can be input to an MZ-type modulator. An optical modulator that does not tolerate fluctuations in polarization becomes available.

This means that the coherent light source 1 is installed indoors with little influence on the environment, and the modulator can be applied to the configuration of an optical communication system installed outside without any problem.

Therefore, next, the indoor coherent light source 1
Is guided through a fiber optic transmission line 11, which is a system with polarization fluctuation, into equipment 10 installed on a telephone pole or the like, where the data signal is optically modulated by a MZ modulator to correspond to the data signal, and the data signal is converted to light. An example of a system configuration for transmission will be described below. <Example of application in a system in which incident polarization fluctuates> An optical fiber transmission line 1 having a polarization fluctuation from an indoor coherent light source 1
1 guides light into the equipment 10 installed on a utility pole, etc.
Here, FIG. 3 shows an example of a system configuration in which the MZ modulator optically modulates the data signal corresponding to the data signal and optically transmits the data signal.

FIG. 3 shows a case where the coherent light source 1 is installed indoors where the influence of the environment is small, and the coherent light source 1 is installed on an electric pole or the like via an optical fiber transmission line 11 having a polarization fluctuation from the coherent light source 1. 1 is a specific example of an optical communication system in which a data signal is modulated by an optical modulator 5 and transmitted to an optical communication path 6. In the figure, reference numeral 9 denotes a device having a light source, which is installed indoors. Reference numeral 10 denotes a device having an optical modulator, which is installed in a place where the environmental change is large, such as on a utility pole. Reference numeral 11 denotes an optical fiber transmission line, which is an optical transmission line connecting the device 9 and the device 10. 1 is a coherent light source provided in the device 9,
2 is a polarization scrambler provided in the device 9, 3 is a scramble signal source provided in the device 9, 5 is an optical modulator,
6 is an optical fiber.

The coherent light source 1 is a light source for outputting light having temporal coherence, and the scramble signal source 3 is for generating a scramble signal.
The polarization scrambler 2 scrambles the polarization of the input light using the scramble signal input from the scramble signal source 3 so that the time average of the polarization is non-polarized, and outputs it to the optical fiber transmission line 11. It is.

The optical modulator 5 is an MZ type optical modulator,
The polarization-scrambled light transmitted through the optical fiber transmission line 11 is intensity-modulated in accordance with the data signal input as the data input 7 and is output to the optical fiber 6. The modulator 5 has polarization dependency such that the modulation efficiency and the loss are different depending on the polarization of the incident light.

The present system includes a device 9 having a light source,
A light source 1 for outputting light having temporal coherence and a polarization scrambler 2 are provided. The light output from the light source 1 is input to the polarization scrambler 2, and is subjected to polarization scrambling by the signal input from the scramble signal source 3 so that the time average becomes non-polarized.

The light output from the polarization scrambler 2 propagates through the optical fiber transmission line 11. Optical fiber transmission line 11
Is made of a normal single mode fiber that does not maintain polarization. Even if the polarization at the time of incidence is constant, the output polarization fluctuates due to changes in temperature, tension, pressure, and the like.

Therefore, when the optical fiber transmission line 11 is in front of the optical modulator 5, when the polarization scrambler 2 is not inserted, the polarization incident on the optical modulator 5 is not stabilized, and Due to the polarization dependence of 5, the modulation efficiency and the output light power fluctuate. However, since the polarization scrambler 2 is inserted, a stable amount of modulation and stable output light power can always be obtained.

When the scramble characteristic of the polarization scrambler 2 depends on the incident polarization, it is necessary that the polarization does not fluctuate between the coherent light source 1 and the polarization scrambler 2. For example, they are connected by a polarization maintaining fiber, or are spatially coupled between the light source 1 and the polarization scrambler 2.

The insertion position of the polarization scrambler 2 may be not immediately after the light source but immediately before the optical modulator as long as the polarization scrambler 2 has no incident polarization dependency. That is, the configuration is such that the optical fiber transmission line is between the light source and the polarization scrambler. However, a polarization scrambler that can make the time average nonpolarized without depending on the incident polarization has a complicated configuration and is expensive. Therefore, the configuration in which the polarization scrambler is inserted immediately after the light source and there is an optical fiber transmission line between the polarization scrambler and the optical modulator can use a relatively low-cost polarization scrambler. desirable.

Such a form in which the light source and the optical modulator are separated is suitable for a form in which the signal source is an antenna of a radio base station on a telephone pole, which is a place where the temperature environment is poor. This is because it is difficult for a laser element serving as a light source for optical communication to operate stably with respect to a temperature change.

<Example of Application to Wireless Antenna Station> FIG. 4 shows an embodiment in which a wireless antenna station is connected using the present invention as a form in which the light source and the optical modulator are separated. In the figure, reference numeral 14 denotes a center station, 15 denotes an antenna station, and the center station 14 includes a coherent light source 1, a polarization scrambler 2, a scramble signal source 3, and an optical receiver 12. A device 5, a driver amplifier 4, and an antenna 16 are provided. The center station 14 and the antenna station 15 are connected by optical fiber transmission lines 11 and 13.

That is, the center station 14 is a station for modulating and demodulating a radio signal, connecting to a public network, and the like. The center station 14 is provided with the coherent light source 1 and the polarization scrambler 2. The center station 14 is a building, which has a mild internal temperature condition and is suitable for installing delicate components such as laser elements.

The light output from the light source 1 having temporal coherence is polarization-scrambled by the polarization scrambler 2 in accordance with the signal from the scramble signal source 3. The polarization scrambled light is transmitted through the optical fiber transmission line 11.
And transmitted to the antenna station 15. Antenna station 1
5 has an antenna 16 and an optical modulator 5.

The radio signal received by the antenna 16 is amplified by the LNA, which is an amplifier immediately after the antenna, and then amplified by the driver amplifier 4 and applied to the optical modulator 5. The optical modulator 5 is an optical modulator that has polarization dependency such as an MZ modulator, but is excellent in other points such as distortion, loss, and chirping.

In this system, the center station 14 has the polarization scrambler 2, and the light input from the center station 14 to the optical modulator 5 of the antenna station 15 via the optical fiber transmission line 11 has a time average. It is polarization scrambled so as to be non-polarized.

Therefore, stable modulation is possible irrespective of the polarization fluctuation received by the optical fiber transmission line 11. The light modulated by the wireless signal in the optical modulator 5 enters the optical fiber transmission line 13 and propagates toward the optical receiver 12.

Although the optical receiver 12 is installed in the center station 14 in the configuration shown in FIG. 4, it may be installed in another station not shown. After the photoelectric conversion, the optical receiver 12 cuts out the frequency near the wireless signal received by the antenna 16 by using a filter or the like, and receives it. The received wireless signal is demodulated or relayed in the form of the original wireless signal depending on the subsequent use.

By doing so, it is not necessary to dispose a delicate component such as a laser element in the antenna station 15 exposed to the external environment, and to ignore other factors such as polarization dependence and obtain other characteristics. It is possible to select an optical modulator for better or worse, and more stable transmission is possible.

A band signal such as a radio signal is transmitted as shown in FIG.
As shown in (1), frequency multiplexing is often performed, and the dynamic range of an uplink radio signal is often large. To ensure that low-power signals are not buried in high-power signals or their intermodulation,
The optical modulator 5 is required to have good distortion characteristics.

By applying the present invention to a band signal such as a radio signal, it becomes possible to select an optical modulator centering on distortion characteristics irrespective of polarization dependence, and to transmit a band signal with good quality. It becomes possible.

In the present invention, a system using an external modulator as an optical modulator and having a high subcarrier frequency is assumed. The subcarrier frequency is, for example, several [GH
z] to several tens [GHz], which is a high frequency that is a little cumbersome to handle. It is desirable to use such a high frequency as much as possible in the system. In the case of a band signal such as a radio signal, the band that limits the information amount of the signal is not the carrier frequency but the bandwidth.

Therefore, if polarization scrambling is performed at a speed higher than the bandwidth, scrambling can be performed without losing information, and as previously considered, scrambling is performed at twice the speed of the subcarrier frequency. You don't have to.

Therefore, taking advantage of this fact, the present invention sets the scramble frequency lower than the lower limit frequency of the band signal as shown in FIG.

<Embodiment in which Scrambling Frequency is Lowered> An embodiment in which the scrambling frequency is lower than the lower limit frequency of the band signal will be described. As shown in FIG. 6, the center frequency of the band signal is fs, and the bandwidth of the band signal is ΔF. ΔF is a width which is not affected by the demodulation even if there is any signal component outside ΔF when demodulating the band signal.

In the present invention, the polarization scrambling frequency f
Let sc be fsc <fs-ΔF / 2.

This is because if the scramble frequency fsc overlaps the band of the band signal, the scrambled signal component remains when the signal is received by the optical receiver, and the reception sensitivity may be degraded. The scramble frequency is set to be higher than the frequency corresponding to the bandwidth ΔF of the band signal. This is because if the scramble frequency is smaller than ΔF, the convolution component of the band signal and the scramble frequency generated at the center frequency fs + fsc partially overlaps with the band signal to be received.

By keeping the scramble frequency fsc low as described above, the polarization scrambler and its driving system can be constituted by low-cost devices, and the system cost can be reduced.

As shown in FIG. 7, when the signal scrambled at the frequency fsc passes through a polarizer or a component having polarization-dependent loss, even if the scrambled signal is a sine wave, the photoelectric conversion is performed. In the subsequent electric signal, a harmonic component of fsc is generated in addition to the component of frequency fsc. The magnitudes of these components vary depending on the magnitude of the polarization-dependent loss of the component and the polarization fluctuations received before entering the component, but tend to be smaller as the order of the harmonic is higher. Therefore, it is sufficient that these harmonics are small enough to not affect the received signal even if they overlap with the signal to be received. Therefore, the scramble frequency fsc is made sufficiently small.

That is, when the polarization-scrambled signal passes through the polarization-dependent component, the harmonics 2fsc, 3f of the scramble frequency fsc as shown in FIG.
fsc,... occur.

The magnitude of the harmonic depends on the waveform of the polarization scrambled signal (sine wave, rectangular wave, triangular wave,...), The polarization fluctuation received before the polarization-dependent component is incident, and the polarization. It depends on the magnitude of the wave dependence.

The harmonics generated in this manner are very large in principle, and do not easily attenuate to higher harmonics. From experience, it has been found that they are sufficiently attenuated at about the seventh order. ing.

Therefore, in the present invention, the scramble frequency fsc is set to be smaller than 1/6 of the lower limit frequency of the band signal as shown in FIG. 8 so that harmonics of the sixth order or less do not overlap within the band of the band signal. In addition, for the same reason as described above, the bandwidth is made larger than the bandwidth ΔF of the band signal.

As a result, even if the generated harmonic wave overlaps with the band signal to be received, the power of the overlapped harmonic wave is small, so that the reception sensitivity does not largely deteriorate. Further, since the scramble frequency is sufficiently small, the configuration around the polarization scrambler can be reduced in cost.

For some reason, the polarization scrambling frequency may not be sufficiently reduced, or the desired reception sensitivity may not be obtained if any small harmonics overlap the band signal. In such a case, the scrambling frequency fsc may be set so that harmonics do not overlap in the signal band. <Setting method of scramble frequency fsc> A method of setting the scramble frequency fsc will be described with reference to FIG.

The lower limit frequency of the band signal 17 is fs-Δ
F / 2, and the upper limit frequency is fs + ΔF / 2. The scramble frequencies at which the second harmonic does not come within this band are a frequency smaller than (fs−ΔF / 2) / 2 and a frequency larger than (fs + ΔF / 2) / 2. Therefore, in order to prevent the second harmonic from overlapping with the band signal, the scramble frequency fsc is changed from “(fs−ΔF / 2) / 2” to “(f
s + ΔF / 2) / 2 ”.

Next, the scramble frequency fsc at which the third harmonic does not overlap within the signal band is a frequency smaller than (fs−ΔF / 2) / 3 and a frequency larger than (fs + ΔF / 2) / 3. Therefore, in order to prevent the third harmonic from overlapping the band signal, the scramble frequency fsc is changed from “(fs−ΔF / 2) / 3” to “(f
s + ΔF / 2) / 3 ”.

Similarly, the fourth harmonic, the fifth harmonic,...
Can also define a frequency band in which the scramble frequency fsc must not be set.

The scramble frequency fsc is determined so as not to fall into any of the frequency bands defined as above. For example, it is like the scramble frequency fsc shown in FIG.

As a result, the harmonics of the scramble frequency fsc do not overlap within the band of the band signal to be received, and good transmission is possible.

If the scramble frequency fsc cannot be reduced sufficiently, there may be a problem with the component turned back around the frequency "0" as shown in FIG. 10B. These components are high-order harmonics of the scramble frequency and convolutional components of the band signal. The frequency overlaps with the band signal to be received, which is centered on fs, and if present without sufficient attenuation, the reception sensitivity deteriorates. Wake up. Therefore, it is necessary to determine the scramble frequency so that the folded signals do not overlap.

The spectrum shown in FIG. 10A is shifted to the high frequency side by fs, the bandwidth of the band signal is given, the signal is looped back at the frequency “0”, and the power of the spectrum shown in FIG. 10A is further reduced. Are added, the optical receiver 12
Will be the electrical spectrum received at
FIG. 11 shows the spectrum before folding.

Referring to FIG. There is a small replica of the band signal near −fs, and when this folds back, the folded signal occurs near the center frequency fs and may overlap.

This signal is a component generated by the fourth harmonic of the scramble frequency. To prevent the signal from overlapping with a desired band signal, any one of the following conditions should be satisfied. .

That is, when the replica comes on the higher frequency side than the center frequency fs, 2 * fs + ΔF <4 * fsc, and when the replica comes on the lower frequency side than the center frequency fs, 2 * fs−ΔF> 4 * fsc. To do.

When this is extended not only to the fourth order but to all harmonics, the order of the harmonic is expressed as n, and 2 (fs + ΔF / 2) / n <fsc and 2 (fs−ΔF / 2) / The scrambling frequency fsc may be determined so as to satisfy n> fsc.

FIG. 12 shows the range of the scramble frequency fsc determined by such conditions, more precisely, the range in which the scramble frequency fsc must not be set. The scramble frequency fsc may be set so as to avoid both such a range and the range shown in FIG.

By doing so, good transmission is possible even when the scramble frequency cannot be lowered sufficiently.

There is also a theoretical scrambled signal waveform that eliminates any harmonics generated by the polarization scrambled light passing through polarization dependent components. For example, the polarization may be modulated using an infinite phase modulator or the like so that the polarization continues to travel in one direction at a constant speed on the outer circumference of the cross-section circle including the center of the sphere on the Poincare sphere.

At present, the infinite phase modulator is not at the stage of practical use, but when an infinite phase modulator with an appropriate capacity at a reasonable price is put to practical use in the future, such an infinite phase modulator will be used to make use of the polarization. By performing scrambling, the problem caused by the generation of harmonics as described above can be solved.

As described above, the present invention is effective when applied to a system for transmitting a radio signal received by an antenna through an optical fiber without demodulation.
This is for the following reasons.

The present invention has the potential to lose optical power that has essentially useful signal components. In a system in which SNR (signal / noise ratio) is limited by noise generated in an optical transmission system, loss of optical power directly leads to deterioration of reception sensitivity. However, the SNR of a radio signal received by an antenna, particularly a radio signal of mobile communication, is determined by the gain of the antenna and the noise of the LNA which is an amplifier immediately after the antenna. The amount of noise included at that stage is much greater than the amount of noise added on the optical link.

[0184] Decreasing the optical power by introducing polarization scrambling is equivalent to relatively increasing the amount of noise added in the optical link, but the amount of noise contained in the original signal is reduced. Since it is much larger than the amount of noise added in the optical link, the SNR of the signal hardly changes even if the additional noise in the optical link slightly increases.

This will be described with reference to a numerical example using the system shown in FIG. 4 as an example.

For simplification of the description, the excess loss of each element is not considered. In the system shown in FIG. 4, the power of the signal received by the antenna 16 and amplified by the LNA is 0 [dBm], and the power of noise accompanying the signal is −
It is assumed that it is 10 [dBm]. At this time, the SNR is 10 [d
B].

On the other hand, the optical power transmitted through the optical fiber transmission line 11 is 5 [dBm], and this light is polarization-scrambled and 3 [dB] due to the polarization dependence of the optical modulator 5. Suppose you suffer a loss.

As a result, the light that is effectively modulated is 2
[DBm]. Assuming that the bias point of the optical modulator 5 is a point of power "1/2", the loss corresponding to that point is 3 [d
B] Yes.

In the optical modulator 5, when the driving signal power is P [dBm] with respect to the input light power “I”, the power of the driving signal included in the output light is “2I + P−20 [d
B] ″, the light emitted from the optical modulator 5 is −1 [d
Bm] light has a signal power of -22 [dBm] and -3 [dBm].
Noise power for 2 [dBm] is included.

The loss of the optical fiber transmission line 13 is 4
[DB], the optical receiver 12 has a sensitivity of 20 × log with respect to the increase or decrease of the optical power, so that the signal power output from the optical receiver is −30 [dBm], and
The noise power is -40 [dBm].

On the other hand, it is assumed that the additional noise of the optical link when the light transmitted through the optical fiber transmission line 13 is received by the optical receiver 12 is -60 [dBm] without depending on the received optical power. Even if noise of -60 [dBm] is added to -40 [dBm], the noise becomes -39.957 [dBm] in total, and the noise amount hardly changes. The SNR of the received signal is 9.957 [dB].

[0192] If the polarization-scrambled light is passed through the polarization-dependent optical modulator 5, 3 [dB] is obtained.
, The signal power after reception by the optical receiver 12 is −24 [dBm], and the noise power is −34 [d
Bm].

The additional noise due to the optical link remains unchanged, -60 [dBm], and the total noise is -3 [dBm].
3.989 [dBm], the SN of the received signal
R is 9.989 [dB].

From this, it can be seen that the SNR hardly changes even without the polarization scrambler 2 as compared with the case with the polarization scrambler 2.

In the optical link, even if the optical power slightly decreases, unless the optical power is reduced significantly, the optical link is amplified by the optical amplifier as shown in FIG. 13 to recover the optical SN close to the original optical SN. Can be done. In addition,
In the configuration of FIG.
However, when the loss received by the optical fiber transmission line 13 is small, a configuration in which the antenna station 15 is disposed immediately before the optical receiver 12 in the center station 14 can be adopted. You can make it.

As described above, by applying the present invention to a system in which the signal-to-noise ratio of a signal to be transmitted is not good, an optical loss becomes invisible and an optical modulator having polarization dependence can be used. Only the advantages of the present invention that good performance can be ensured can be enjoyed.

(Second Embodiment) Next, an embodiment of frequency conversion of a band signal using a polarization scrambler and a polarization-dependent optical modulator will be described with reference to the drawings.

FIG. 14 is a block diagram showing one of the embodiments. In FIG. 14, 1 is a coherent light source, 2 is a polarization scrambler, 3 is a scramble signal source,
21 is a polarizer, 22 is an optical modulator, 6 is an optical fiber, 25
Is an optical receiver.

The coherent light source 1 is a light source for outputting light having temporal coherence, and the scramble signal source 3 is for generating a scramble signal.
The polarization scrambler 2 scrambles the polarization of the input light using the scramble signal input from the scramble signal source 3. Here, the polarization scrambler 2 simply scrambles the polarization of the input light. It is not necessary to scramble the polarization so that the time average is unpolarized.

The polarizer 21 receives the polarization-scrambled light and generates a scramble frequency harmonic therefrom. The polarization axis of the polarizer 21 is such that a harmonic of a desired order is generated. It is oriented in such a way that it occurs at the desired size. Further, the optical modulator 22 includes the polarizer 21.
The light obtained through the above is light-modulated in response to data input and output to the optical fiber 6.

In such a configuration, the light output from the light source 1 that outputs light having temporal coherence is:
The polarization scrambler 2 scrambles the polarization.
However, at this time, the point different from the invention in the first embodiment is that "the time average of the polarization by the polarization scrambler 2 does not need to be non-polarized".

[0202] The polarization scrambled light passes through the polarizer 21 whose polarization axis is oriented in an appropriate direction, and a harmonic of a scramble frequency is generated. The polarization axis of the polarizer 21 is oriented such that a desired order of harmonics is generated with a desired magnitude. The light that has passed through the polarizer 21 undergoes intensity modulation by the light modulator 22.

The optical modulator 22 receives a band signal as the data input 7 and is driven by a voltage corresponding to the band signal. The output of the optical modulator 22 enters the optical fiber 6 and is transmitted to the optical receiver 25.

After performing the photoelectric conversion, the optical receiver 25 extracts a desired frequency component from a large number of components having band signal information using a filter having appropriate frequency characteristics. At this time, between the coherent light source 1 and the polarization scrambler 2 and between the polarization scrambler 2 and the polarizer 21, the polarization is not subject to unpredictable fluctuation.
They are connected by a polarization maintaining fiber or are spatially coupled.

In this case, the optical modulator 22 may or may not have polarization dependence. If there is no polarization dependence, all light that has passed through the polarizer 21 is to be modulated. If there is polarization dependence, light is lost due to a shift between the polarization axis of the polarizer 22 and the polarization axis of the optical modulator 22. However, upon passing through the polarizer 22, the polarization scrambled light returns to a single polarization, and the effect of the polarization scrambling is converted into a form of intensity modulation. for that reason,
When passing through the optical modulator, the generation of further harmonics due to polarization scrambling does not occur. However, polarizer 2
If there is a time variation in the polarization between the optical modulator 1 and the optical modulator 22,
Since the optical power and the modulation efficiency modulated by the optical modulator fluctuate, it is preferable that the polarization between the polarizer 21 and the optical modulator 22 is configured not to fluctuate.

In the configuration shown in FIG. 14, the polarizer 21 is disposed between the polarization scrambler 2 and the optical modulator 22. The same effect can be obtained by disposing the optical modulator 22 behind the optical modulator 22 so that the polarization between the optical modulator 22 and the polarizer 21 does not change. In that case, the direction of the axis of the polarizer 21 is
If 2 has polarization dependence, it is adjusted to the direction of the polarization axis of the optical modulator 22.

At this time, the polarization axis of the optical modulator 22 is set in such a direction that a component of a desired frequency appears best after passing through the polarizer 21. However, if the modulator 22 does not have polarization dependence, the polarization axis of the polarizer 21 is set in such a direction that a component of a desired frequency can be generated best.
Furthermore, if the optical modulator 22 has no polarization dependence, the polarizer 21 may be located immediately before the optical receiver 25. In that case, since the polarization changes in the optical fiber 6, the optical receiver 25
Therefore, a mechanism such as controlling the direction of the axis of the polarizer 21 is required so that a desired frequency component can be obtained with a desired size or more when received.

<Example of System Using Optical Modulator with Large Polarization Dependence> When the optical modulator to be used has a very large polarization dependence, and a polarizer is provided at the entrance of the optical modulator. Is implemented, the configuration shown in FIG. 15 may be used.

That is, in this case, when an optical modulator 23 having a large polarization dependence is used as shown in FIG. 15 instead of the optical modulator 22 in the configuration of FIG. 14, a polarizer is provided at the entrance of the optical modulator. When the mounted optical modulator 23 is used, the polarization axis of the optical modulator 23 is directed in a direction such that a desired frequency component is generated with a desired magnitude or more. The polarization maintaining fiber is connected between the coherent light source 1 and the polarization scrambler 2 and between the polarization scrambler 2 and the optical modulator 23 so that the polarization does not undergo unpredictable fluctuation. Or spatially coupled.

In the optical communication system having such a configuration, the light output from the light source 1 that outputs light having temporal coherence is guided to the polarization scrambler 2.
The two are connected by a polarization maintaining fiber or are spatially coupled so that the polarization does not undergo unpredictable fluctuation.

Therefore, the light having the temporal coherence is supplied to the polarization scrambler 2 without undergoing unpredictable fluctuations in the polarization, and the polarization scrambler 2 scrambles the polarization and scrambles the polarization. 23. At this time, it is not necessary for the polarization scrambler 2 to make the time average of the polarized light non-polarized.

The polarization scrambler 2 and the optical modulator 23 are connected by a polarization maintaining fiber or are spatially coupled so that the polarization does not undergo unpredictable fluctuation.

Therefore, the light that is polarization scrambled from the polarization scrambler 2 and sent to the optical modulator 23 is sent to the optical modulator 23 without undergoing unpredictable fluctuations in the polarization. The light transmitted to the optical modulator 23 in this manner is subjected to intensity modulation by the optical modulator 23.

The polarization axis of the optical modulator 23 is oriented in a direction such that a desired frequency component is generated with a desired magnitude or more. In addition, a band signal is input to the optical modulator 23 as the data input 7. Therefore, the optical modulator 23 is driven by the voltage corresponding to the band signal, and the input light is light intensity modulated. Then, the output of the optical modulator 23 whose optical intensity has been modulated enters the optical fiber 6 and is transmitted to the optical receiver 25.

The optical receiver 25, which has received the signal via the optical fiber 6, performs photoelectric conversion, and then uses a filter or the like having an appropriate frequency characteristic to output a desired signal from among a large number of components having band signal information. Is extracted.

As described above, when the used optical modulator has a very large polarization dependence, and when a polarizer is mounted at the entrance of the optical modulator, the coherent light source 1 and the polarizer are not used. Between the wave scrambler 2 and between the polarization scrambler 2 and the optical modulator 23, a polarization maintaining fiber is connected or spatially coupled so as to prevent the polarization from undergoing unpredictable fluctuation. By setting the polarization axis of the modulator 23 in such a direction that a desired frequency component is generated at a desired size or more, an optical modulator having polarization dependency can be used. The advantage of the present invention that performance can be ensured can be enjoyed.

In the configurations shown in FIGS. 14 and 15, the directions of the polarization axes of the polarizer and the optical modulator are adjusted so that a desired harmonic or a desired frequency component becomes maximum (or a desired size or more). It was explained.

However, a polarizer can basically only cut out linearly polarized waves. Depending on the configuration of the polarization scrambler and the imperfection of the coupling system on the way, the axis at which the desired harmonic becomes maximum may be circular polarization or elliptical polarization instead of linear polarization. Further, depending on the shape of the optical connector, it may not be possible to freely change the direction of the polarization axis to connect the components. In such a case, a polarization converter may be inserted in front of a polarization-dependent device, that is, a polarizer or a polarization-dependent optical modulator, to convert the polarization into an appropriate polarization.

Such an example will be described next as a third embodiment.

(Third Embodiment) FIG. 16 shows a configuration example in which a polarization converter 24 is inserted in the system shown in FIG. That is, as shown in FIG. 16, the polarization converter 24 is interposed between the polarization scrambler 2 and the polarization-dependent optical modulator 23.

Here, the polarization converter 24 is obtained by cutting out the polarization-scrambled light at the polarization axis of the polarization-dependent optical modulator 23 to obtain a desired harmonic or a desired frequency component. Is maximum (or larger than desired)
The polarization converter 24 converts the polarization incident on the optical modulator 23 having polarization dependence, and the polarization converter 24 is constituted by a wave plate or a device having the same function as the wave plate. I have.

In this system having such a configuration, the light generated by the coherent light source 1 is polarization-scrambled by the polarization scrambler 2. Then, the polarization-scrambled light is polarization-converted by the polarization converter 24 and input to the optical modulator 23 having polarization dependency. The polarization converter 24 converts the polarization scrambled light into the polarization-dependent optical modulator 2.
3, the polarization-dependent optical modulator 23 is controlled so that a desired harmonic or a desired frequency component becomes maximum (or a desired size or more).
To convert the polarization incident on.

With this configuration, it is possible to reduce the number of electrical components operating in the millimeter-wave or quasi-millimeter-wave band required for transmitting and receiving millimeter-wave and quasi-millimeter-wave signals, thereby reducing cost. Leads to. Further, since harmonics generated by the polarization scrambled light passing through the polarizer are large, high-quality frequency conversion can be performed.

The electric spectrum after being received by the optical receiver 25 is as shown in FIG. If the configuration of the present invention is used for down-converting a signal, for example, a component having a center frequency of 2 fsc-fs is cut out using a low-pass filter and used.

If the frequency is used for up-conversion, for example, the center frequency fs + 2f
The component of sc is extracted by a band pass filter.
At this time, it is assumed that the polarization scrambling frequency is determined in advance so that each component has a desired frequency.

A specific application example for down-converting the frequency of a band signal will be described below with reference to the drawings.

<Application Example of Down-Conversion of Band Signal Frequency> FIG. 17 is a functional block diagram of an optical communication system in which a radio signal is transmitted from the antenna station 15 to the center station 14. Based on the configuration of FIG. 15, it is applied to a wireless system. Therefore, the same components as those in FIG. 15 are denoted by the same reference numerals, and description thereof is omitted.

The antenna station 15 has the coherent light source 1,
It comprises a polarization scrambler 2, a scramble signal source 3, an optical modulator 23, a driver amplifier 4, and an antenna 16.

Numeral 14 denotes a center station, and an antenna station 15
The optical fiber transmission line 11 connects between the central station 14 and the center station 14.

The center station 14 includes a photoelectric converter 30, a low-pass filter 26, and a demodulator 27. The photoelectric converter 30 converts an optical signal into an electric signal. The photoelectric converter 30 photoelectrically converts light transmitted through the optical fiber transmission line 11 and outputs the light to the low-pass filter 26. The low-pass filter 26 is for cutting out a signal down-sampled to a low frequency from a signal that has been photoelectrically converted and converted into an electric signal.
The demodulator 27 demodulates this low frequency signal into a base band digital signal.

In this system having such a configuration, in the antenna station 15, the polarization scrambler 2 scrambles the light output from the light source 1 based on the signal from the scramble signal source 3.

On the other hand, the radio signal received by the antenna 16 and amplified by the LNA is amplified by the driver amplifier 4 and
The light is applied to the light modulator 23. The optical modulator 23 has polarization dependence.

The output light of the polarization scrambler 2 is input to the optical modulator 23 and is subjected to intensity modulation by a radio signal.
The output of the optical modulator 23 is transmitted to the center station 14 via the optical fiber transmission line 11.

In the center station 14, the optical fiber transmission line 1
1 is photoelectrically converted by the photoelectric converter 30. The low-pass filter 26 cuts out a low-frequency down-sampled signal from the photoelectrically converted signal. The cut-out low frequency signal is demodulated by the demodulator 27 into a baseband digital signal. Then, the demodulated signal is subjected to appropriate signal processing and then sent out to a network or the like.

The transition of the frequency of the system shown in FIG. 17 will be described with reference to a specific example.

A signal incident on the antenna 16 is, for example,
With a center frequency of 22.5 GHz, a bandwidth of 0.3 [GH
z]. The polarization scramble signal is, for example, a 5.5 [GHz] sine wave signal. 5.5 [G
[Hz] is intensity-modulated by a polarization-dependent optical modulator 23 with a band signal of 22.5 GHz.

As a result, the "fourth harmonic" of 5.5 [GHz] and the "convolution component of the band signal" are 0.5 [GHz].
Appear in the center. The bandwidth of the band signal is 0.3 [GHz]
Therefore, a band signal frequency-converted to a band of 0.5 ± 0.15 [GHz] is generated.

This is cut out by, for example, a low-pass filter 26 having a high cutoff frequency of 0.7 [GHz] and demodulated by a demodulator 27. If the demodulator 27 is 0.5 [GH
z] to demodulate the band signal centered on
The center frequency of the frequency-converted signal is 0.5 [GHz]
5.5 in advance so that
[GHz].

According to the present invention shown in the third embodiment, high efficiency and high quality can be achieved by utilizing a large harmonic generated by a combination of a polarization scrambler and a device having a polarization dependent loss. Good downsampling becomes possible. Further, for example, A operating in the 22 [GHz] band
A high frequency electric component such as a / D converter is not required. Further, the optical receiver including the photoelectric converter only needs to be able to receive a signal converted to a low frequency, so that a low-cost optical receiver that operates only at a low frequency may be used. Such an optical receiver that operates only at a low frequency generally has better reception sensitivity than that that operates at a high frequency, and enables high-quality reception.

Next, a specific application example for up-converting the frequency of a band signal will be described with reference to the drawings.

<Application Example of Up-Conversion of Band Signal Frequency> FIG. 18 is a functional block diagram of a system for transmitting a radio signal from the center station 14 to the antenna station 15. This example is based on the configuration of FIG. 15 and is applied to a wireless system.

In the figure, reference numeral 14 denotes a center station,
The center station 14 includes a coherent light source 1, a polarization scrambler 2, and an optical modulator 23. Also,
Reference numeral 15 denotes an antenna station. The antenna station 15 includes a photoelectric converter 30, a band-pass filter 28, and a power amplifier 29.
And an antenna 16. An optical fiber transmission line 11 is provided between the antenna station 15 and the center station 14.
Connected at

The photoelectric converter 30 of the antenna station 15 converts an optical signal into an electric signal, and photoelectrically converts light transmitted through the optical fiber transmission line 11 and outputs the light to the bandpass filter 28. It is. The band-pass filter 28 cuts out a band signal that is up-converted to a desired frequency from a signal that has been photoelectrically converted and converted into an electric signal. The power amplifier 29 amplifies the cut-out band signal to the power required to transmit it as radio waves, and the antenna 16 emits the power-amplified signal to space.

In the system having such a configuration, in the center station 14, the output light from the light source 1 is supplied to the polarization scrambler 2, and the polarization scrambler 2 uses the polarization scrambler 2 based on the signal from the scramble signal source 3. Wave scramble. Then, this is given to the optical modulator 23 having polarization dependency.

A low frequency band signal is input to the optical modulator 23 as the data input 7, and the output light of the polarization scrambler 2 is light intensity modulated according to the band signal. Output light from the optical modulator 23 is transmitted to the antenna station 15 via the optical fiber transmission line 11.

In the antenna station 15, the photoelectric conversion is performed by the photoelectric converter 30. From the obtained electric signal, a band signal up-converted to a desired frequency is cut out by a band-pass filter 28. The cut-out band signal is amplified to a required size by the power amplifier 29 and radiated from the antenna 16 into space.

The change in the frequency of the system shown in FIG. 18 will be described by way of an example. The frequency at which the band signal is radiated from the antenna 16 is, for example, 22.9 [GHz], the center frequency of the band signal provided as the data input 7 is 0.5 [GHz], and the bandwidth is 0. .3 [GH
z].

In this case, 22.9 [GHz] and 0.5
The difference of [GHz] is 22.4 [GHz], and the difference is up-converted by the mechanism of the present invention.

[0249] Of the harmonics generated when the polarization-scrambled light passes through a polarization-dependent device, the order of the harmonic to be used is determined by various factors. For example, as shown in FIG. 10 (b), when the return signal and the harmonic of the polarization scramble frequency overlap, "do not enter the desired signal band", and "the polarization scrambler is The scramble frequency should not be too high to facilitate the determination. "

Here, for example, it is assumed that the order of "4th order" is selected. As a result, the scramble frequency becomes 22.
4 ÷ 4 = 5.6 [GHz].

With the polarization scrambler 2, this 5.6 [GH
The polarization scrambled light using the scramble signal of [z] is subjected to light intensity modulation by the polarization-dependent optical modulator 23.

As a result, a harmonic component of the scrambled signal frequency and a convolution component of the band signal for driving the optical modulator are generated. Of the signals converted into electric signals by the photoelectric converter 30, a desired frequency, "center frequency 22.9"
[GHz] "is cut out by the band-pass filter 28. Then, the power is amplified and transmitted to the air.

According to the present invention shown in the third embodiment, high efficiency and high quality can be achieved by utilizing a large harmonic generated by a combination of a polarization scrambler and a device having a polarization dependent loss. A good Ab conversion is possible. Further, for example, a high-frequency electric component such as a mixer or a multiplier operating in the 22 [GHz] band is not required, and furthermore, a low-frequency electric component such as an optical modulator or a drive system of the optical modulator operates with a low-frequency signal. The cost is better.

Although various embodiments have been described above, the present invention is not limited to the above-described embodiments, and can be implemented with various modifications. Further, in all of the above embodiments, the polarization scramble signal is basically described as a sine wave, but if a driving system can be easily constructed with a triangular wave, a rectangular wave, a pulse wave, or the like, Such a waveform may be used.

[0255]

As described above, in the present invention, the coherent light incident on the polarization-dependent optical modulator is polarization-scrambled in advance so that the time average becomes non-polarization. As a result, even if an unpredictable polarization fluctuation occurs before light enters the optical modulator, stable modulation can always be performed. By doing so, it is possible to use an optical modulator that is excellent in other performances even if it has polarization dependence even in a system where the incident polarization fluctuates, and higher quality transmission is achieved. It becomes possible.

In the present invention, the polarization-scrambled light is intensity-modulated by a polarization-dependent optical modulator corresponding to a band signal. As a result, a convolution component of the generated polarization scrambling frequency and the convolution component of the band signal is cut out by a filter or the like, thereby obtaining a signal obtained by frequency-converting the band signal. The magnitude of harmonics generated by the polarization-scrambled light passing through a device having a polarization-dependent loss is large, and high-quality frequency conversion is possible. Further, since the frequency conversion is performed in the process of light, an electric device for performing the frequency conversion is not required, and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the present invention, and is a block diagram for explaining a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the present invention, and is a block diagram for explaining the first embodiment of the present invention.

FIG. 3 is a diagram for explaining the present invention, and is a block diagram for explaining the first embodiment of the present invention.

FIG. 4 is a diagram for explaining the present invention, and is a diagram for explaining the first embodiment of the present invention.

FIG. 5 is a diagram for explaining the present invention, and is a diagram for explaining the first embodiment of the present invention.

FIG. 6 is a diagram illustrating an embodiment of the present invention.

FIG. 7 is a diagram illustrating an embodiment of the present invention. It is a figure for explaining the present invention.

FIG. 8 is a diagram illustrating an embodiment of the present invention. It is a figure showing an embodiment of the invention.

FIG. 9 is a diagram for describing an embodiment of the present invention.

FIG. 10 is a diagram illustrating an embodiment of the present invention.

FIG. 11 is a diagram illustrating an embodiment of the present invention.

FIG. 12 is a diagram illustrating an embodiment of the present invention.

FIG. 13 is a diagram for explaining the present invention, and is a diagram for explaining the first embodiment of the present invention.

FIG. 14 is a diagram for explaining the present invention, and is a diagram for explaining a second embodiment of the present invention.

FIG. 15 is a diagram for explaining the present invention, and is a diagram for explaining a second embodiment of the present invention.

FIG. 16 is a diagram for explaining the present invention and is a diagram for explaining a third embodiment of the present invention.

FIG. 17 is a diagram for explaining the present invention, and is a diagram for explaining a third embodiment of the present invention.

FIG. 18 is a diagram for explaining the present invention, and is a diagram for explaining a third embodiment of the present invention.

FIG. 19 is a diagram for explaining the behavior of the polarizer on the Poincare sphere.

FIG. 20 is a diagram for explaining the behavior of the polarizer on the Poincare sphere.

FIG. 21 is a diagram for explaining the behavior of the polarizer on the Poincare sphere.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Coherent light source bipolarization scrambler 2 ... Polarization scrambler 3 ... Scramble signal source 4 ... Driver amplifier Optical modulator and optical fiber data input 8 ... Mach-Zehnder modulator 9 ... Device having a light source 10 ... Optical modulation Device having optical device 11 Optical fiber transmission line 12 Optical receiver 13 Optical fiber transmission line 14 Center station 15 Antenna station 16 Antenna 17 Band signal 18 Area where scramble frequency cannot be set 19 Scramble frequency is set Unavailable region 20 ... Optical amplifier 21 ... Polarizer 22 ... Optical modulator 23 ... Polarization-dependent optical modulator 24 ... Polarization converter 25 ... Optical receiver 26 ... Low-pass filter 27 ... Demodulator 28 ... Band bus filter 29: power amplifier 30: photoelectric converter

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) H04B 10/14 10/04 10/06 10/152 10/142

Claims (12)

[Claims] 1. A light source for outputting output light having temporal coherence, and a polarization scrambler connected to the light source for scrambling the polarization of the output light temporally so that a time average is substantially non-polarized. And an optical modulator having a modulation characteristic dependent on the incident polarization, wherein a part or all of the output light polarization-scrambled by the polarization scrambler corresponds to an input data signal. An optical communication system comprising: an optical modulator that modulates with an electrical signal and outputs the modulated signal to an optical fiber. 2. The optical communication system according to claim 1, wherein said optical modulator is a Mach-Zehnder type optical modulator. 3. The optical fiber transmission line according to claim 1, wherein an optical fiber transmission line that does not maintain polarization is inserted between the polarization scrambler and the optical modulator. Optical communication system. 4. The optical communication according to claim 1, wherein said data signal is a band signal, and is modulated as a subcarrier signal on said output light by said optical modulator. system. 5. A fundamental frequency of a scramble signal applied to the polarization scrambler for scrambling polarization is lower than a lower limit frequency of the band signal and higher than a frequency corresponding to a bandwidth of the band signal. 5. The optical communication system according to claim 4, wherein 6. The optical communication system according to claim 5, wherein a fundamental frequency of said scrambled signal is smaller than 1/6 of a lower limit frequency of said band signal. 7. The fundamental frequency of the scramble signal is determined such that harmonics of any order of the fundamental frequency do not overlap within the signal band of the band signal. Optical communication system. 8. The data signal as claimed in claim 4, wherein the data signal is a radio signal received by an antenna as it is or a signal which has not been demodulated after being converted to an intermediate frequency. Optical communication system. 9. The optical communication system according to claim 4, wherein said light source is installed indoors, and said optical modulator is installed in an equipment installed outdoors. 10. A light source for outputting output light having temporal coherence, and a polarization-dependent optical modulator, wherein sub-carrier modulation is performed on output light of the light source by an electric signal corresponding to a band signal. An optical modulator to be applied, or a device having a polarization dependent loss, an optical modulator for performing subcarrier modulation on output light of the light source, and an output light of the light source inserted between the light source and the optical modulator. A polarization scrambler that performs polarization scrambling at a specific frequency, photoelectrically converts the light modulated by the optical modulator, and is generated by convolution of the scrambling frequency of the polarization scrambler or a harmonic thereof and the band signal. An optical communication system comprising: an optical receiver that receives a component. 11. The scramble frequency is lower than a lower limit frequency of the band signal, and the optical receiver is generated by convolving the band signal with the scramble frequency or a harmonic thereof obtained by photoelectric conversion. The optical communication system according to claim 10, wherein a component having the lowest frequency is extracted from the components. 12. The optical receiver extracts a component having a frequency higher than the frequency of the band signal among components generated by convolution of the scramble frequency or a harmonic thereof and the band signal obtained by photoelectric conversion. The optical communication system according to claim 10, wherein: