JP2003143077A - Optical transmission system and optical signal modulation device used therefor - Google Patents

Abstract

(57) [Summary] [Problem] A downlink CW of a constant strength transmitted from a station side device as an upstream signal transmitted from a user side device to a station side device.
In a bidirectional optical transmission system that modulates and returns light, the transmission distance is increased with low power consumption. SOLUTION: A downlink CW light having a constant intensity is transmitted from a first transmission device (an intra-station device) to a second transmission device (user-side ONU) via an optical fiber transmission line, and the second transmission device transmits the downlink CW light. An upstream optical signal obtained by phase-modulating the CW light is generated and the first optical signal is generated.
Return to the transmission device. In the first transmission device, the downlink CW
By multiplexing the light and the received upstream optical signal to cause interference, the phase fluctuation of the phase-modulated upstream optical signal is detected and demodulated as the intensity fluctuation, and the upstream signal is transmitted from the second transmission device.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system mainly used in an access system, in which a downlink signal of a constant intensity transmitted from a station side device is transmitted as an upstream optical signal transmitted from a user side device to a station side device. The present invention relates to an optical transmission system configured to modulate and return CW light and an optical signal modulator used for the optical transmission system.

[0002]

2. Description of the Related Art In conventional optical communication, a laser diode (LD) or a light emitting diode (LE) is generally used as a transmission light source.
D) is widely used. When the output light from these light sources is modulated by an electric signal, the direct modulation method in which the injection current to the LD or LED is changed to directly modulate the output light intensity, or the output light from the LD is made into CW light of constant intensity An external modulation method has been used in which intensity modulation or phase modulation is performed by an external modulator prepared separately.

In general, for LDs and LEDs, when inputting the output light to an optical fiber having a small core diameter such as a single mode optical fiber, precise alignment using a minute optical system is required. . Moreover, since the adjustment is difficult to automate, it is often done manually. As a result, the cost of the optical transmission module becomes high, and especially in a communication system such as an access system that requires a light source for each line.
It is a factor that pushes up communication costs.

Further, although the LD direct modulation method and the external modulation method are capable of high-speed modulation, the LD driving current is large, so that the power consumption is high and the modulation characteristics and the oscillation wavelength are apt to fluctuate depending on the ambient temperature. . Therefore, when a stable modulation operation is required, or particularly when wavelength stability is required, such as when used in a wavelength division multiplex transmission method (WDM), a temperature adjusting means such as a Peltier element is used to generate a light source or external modulation. It was necessary to keep the temperature of the vessel constant, which inevitably complicated the device and increased power consumption.

In the access system, a plurality of lines cannot be multiplexed on one optical fiber like a relay network between upper stations, and the optical fiber line is occupied by one user. Track costs make up a large proportion of communication costs. Therefore, when bidirectional optical communication is performed, bidirectional communication is multiplexed with one optical fiber rather than the two-core bidirectional communication method in which one optical fiber is used for upstream and one for optical communication. The core bidirectional communication method is more advantageous in terms of cost. In this one-core bidirectional communication method, conventionally, time division multiplexing method (ping-pong transmission method) or WDM is used.
Is used. However, the time division multiplexing method has a problem that the transmission capacity in one direction is substantially half the transmission line capacity. The WDM is an optical line terminal (ON) on the user side.
Since a filter for demultiplexing bidirectional wavelengths is required in U) and a light source for outputting a specific wavelength for upstream is also required, there is a problem that the ONU becomes complicated and expensive.

[0006]

In order to address the above problems, in the access system, the CW light for upstream signals is transmitted in the downstream direction from the CW light source installed on the station side, and the user side is turned ON.
A means has been proposed for realizing upstream optical signal transmission by modulating the downstream CW light with an upstream signal at the U and returning the modulated CW light. One of them is electro-absorption or semiconductor amplifier type semiconductor optical modulator, Mach-Zehnder type (MZ
Type) optical modulator is used to intensity-modulate the downstream CW light and generate an upstream optical signal. However, the semiconductor optical modulator is required to have the same precise alignment as that of the LD module, and particularly the semiconductor amplifier type has a large driving current, which causes a problem of an increase in cost and an increase in power consumption. Further, the MZ type optical modulator requires an additional device for compensating for a change in bias condition due to drift in the modulator, and has a limit to downsizing the modulation module structurally, which makes the device complicated and large. Was unavoidable.

On the other hand, since the CW light transmitted from the station side is modulated and returned, when the upstream signal is received on the station side, a loss of twice the loss of the optical fiber line in the access section is added. Become. Therefore, the optical signal intensity is considerably attenuated, and there is a problem that the limitation of the transmission distance to obtain the required SN ratio becomes severe.

The present invention is a bidirectional optical transmission system that modulates and returns the downlink CW light of a constant intensity transmitted from the station side device as an upstream signal transmitted from the user side device to the station side device, with low power consumption. It is an object of the present invention to provide an optical transmission system capable of increasing the transmission distance and an optical signal modulator used for the optical transmission system.

[0009]

An optical transmission system according to the present invention is a downlink CW having a constant intensity from a first transmission device (intra-station device) to a second transmission device (user-side ONU) via an optical fiber transmission line. The second transmission device transmits light and the downlink C
An upstream optical signal obtained by phase-modulating W light is generated and returned to the first transmission device. In the first transmission device, the downlink CW light and the received upstream optical signal are multiplexed and interfered with each other to detect and demodulate the phase variation of the phase-modulated upstream optical signal as intensity variation, and the second transmission device The upstream signal is transmitted.

Here, there is provided a variable optical attenuating means for attenuating the optical intensity of the combined downstream CW light or upstream optical signal. In addition, a polarization adjusting means for adjusting the polarization state of the downlink CW light or the upstream optical signal to be multiplexed is provided, and a phase adjusting means for adjusting the phase state of the downlink CW light or the upstream optical signal to be multiplexed is provided. Set so that light interference will occur most strongly.

Further, the optical multiplexing means of the first transmission device is 2
This is a configuration in which the interference light of the downstream CW light and the interference light of the upstream light signal are complementarily output to one output port, and the light receiving circuit photoelectrically converts the interference light that is complementarily output by two light receiving elements, and outputs the difference between the electric signals It may be configured to.

The optical folding means of the second transmission device reflects the modulated light output from the optical phase modulating means, phase-modulates the modulated light again by the optical phase modulating means, and returns it to the optical fiber transmission line. It may be configured.

Further, as an optical fiber transmission line, a downlink CW is used.
A first optical fiber transmission line for transmitting light and a second optical fiber transmission line for transmitting upstream optical signals are provided, and the optical folding means optically phase-modulates the downstream CW light from the first optical fiber transmission line. The modulated light input to the means and output from the optical phase modulation means may be sent to the second optical fiber transmission line.

The optical signal modulator of the present invention comprises an optical phase modulator for modulating the phase of the downlink CW light transmitted through the optical fiber transmission line with a transmission signal, and a modulated light output from the optical phase modulator. Reflecting means for reflecting and modulating the reflected modulated light again by the optical phase modulating means and returning it to the optical fiber transmission line as an upstream optical signal is provided.

Further, a branching means for branching the downstream CW light from the optical fiber transmission line into two, an optical waveguide for ring-connecting the branching ports of the branching means to transmit the downstream CW light clockwise and counterclockwise, and an optical waveguide. Optical phase modulation means for modulating the phase of the downstream CW light that is inserted into and transmitted by the transmission signal, and the clockwise and counterclockwise optical signals phase-modulated by the optical phase modulation means are combined by the branching means and then rise. The optical signal may be returned to the optical fiber transmission line as an optical signal. Further, polarization adjusting means for setting the upstream optical signal in a predetermined polarization state may be inserted in the optical waveguide.

A polarization rotation means for rotating the polarization of the passing light by 45 degrees is provided between the optical phase modulation means and the reflection means, and the polarization of the light traveling back and forth through the polarization rotation means is rotated by 90 degrees. It may be configured to allow it. Further, the optical phase modulation means has an optical axis of 90
A configuration using two optical phase modulation means arranged with a deviation may be used.

Further, the downstream CW light output from one end of the optical fiber transmission line is condensed between the optical fiber transmission line and the optical phase modulation means so as to focus on the reflection means, and the reflected light is collected. An optical lens coupled to the optical fiber transmission line may be provided.

The reflecting means reflects the modulated light output from the optical phase modulating means, phase-modulates the reflected modulated light again by the optical phase modulating means, and returns it to the optical fiber transmission line as an upstream optical signal. A wavelength-selective reflection unit that transmits a downstream optical signal having a wavelength different from that of the downstream CW light, and a light-receiving circuit that receives the downstream optical signal transmitted through the wavelength-selective reflection unit may be provided.

Further, the downstream CW light from the optical fiber transmission line is branched into two, one of the downstream CW light is input to the optical phase modulation means, and the upstream optical signal is branched into two, one of which is the optical fiber transmission line. The optical branching means for returning to the optical branching means, the other downstream CW light and the upstream optical signal branched by the optical branching means are combined, and the optical combining means for outputting the interference light and the interference light output from the optical combining means are combined. A modulated light monitor circuit that outputs a monitor signal that is photoelectrically converted may be provided. Here, when modulating the digital baseband signal by the optical phase modulating means, the monitor signal level becomes the minimum when the phase of the downstream CW light input to the optical multiplexing means and the phase of the upstream optical signal are the same, and A modulation condition adjusting circuit for controlling the modulation condition of the optical phase modulation means may be provided so that the monitor signal level becomes maximum when the phase of the CW light and the phase of the upstream optical signal deviate by π.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment of Optical Transmission System: Claims 1 and 6) FIG. 1 shows a first embodiment of the optical transmission system of the present invention. In the figure, the optical transmission system according to the present embodiment includes an intra-station device 10 that is normally placed in a building of a telecommunications carrier or the like for terminating an access line, concentrating lines, etc. The optical signal modulator 20 is a function in the ONU. The intra-station device 10 and the optical signal modulator 20 are connected via an optical fiber transmission line 30. Note that the optical transmission system of the present invention is also applicable to a configuration in which a plurality of optical signal modulators 20 are star-connected to the intra-station device 10 and a plurality of optical signal modulators 20 share the light source of the intra-station device 10. In the following description of the embodiments, a one-to-one configuration will be described.

The intra-station device 10 is an isolator 1 inserted in a light source 11 which outputs CW light of a constant intensity and an optical waveguide 12-1 which connects the light source 11 and the optical fiber transmission line 30.
3 and 2 × 2 optical couplers 14 and 2 × 1 which are connected to the 2 × 2 optical coupler 14 via optical waveguides 12-2 and 12-3.
The optical coupler 15 and the light receiving circuit 16 that receives the output light of the optical coupler 15 are included. As the light source 11, a light source with good coherence such as a laser diode (LD), a solid-state laser, or a fiber ring laser is used. The optical waveguide 12-1 includes a port A and a port C of the 2 × 2 optical coupler 14.
, The optical waveguide 12-2 is connected to the port B of the 2 × 2 optical coupler 14, and the optical waveguide 12-3 is connected to the port D of the 2 × 2 optical coupler 14.

The optical signal modulator 20 includes a phase modulator 21 for phase-modulating the CW light transmitted through the optical fiber transmission line 30 with a modulation signal, and the transmitted light of the phase modulator 21 to the optical fiber transmission line 30. The reflection plate 22 is fixed by adjusting the angle so that the light is incident again. The phase modulator 21 is an element that internally changes the refractive index by applying a voltage according to the modulation signal to the electrodes, and phase-modulates the transmitted light in accordance with this, for example, an electro-optical effect such as LiNbO _3. It is realized by using a crystalline material having
Since the phase modulator is not particularly limited in shape, it does not require precise alignment and can be easily miniaturized and integrated by a planar waveguide structure or the like. Also,
Generally, the current flowing through the phase modulator 21 itself is very small, so that the power consumption can be suppressed to a low level.

The operation of the optical transmission system having the above configuration will be described. The CW light output from the light source 11 is transmitted through the isolator 13 to the port A of the 2 × 2 optical coupler 14.
Is input to the port C and is divided into two, port C and port D.
The CW light output from the port C is transmitted through the optical fiber transmission line 3
0 to the optical signal modulator 20 in the user-side ONU. The CW light input to the optical signal modulator 20 is
The phase modulator 21 receives the phase modulation by the modulation signal and reflects it by the reflection plate 22, and as an upstream optical signal, the optical fiber transmission line 3
Wraps to 0.

The upstream optical signal transmitted via the optical fiber transmission line 30 is input to the port C of the 2 × 2 optical coupler 14 of the intra-station device 10 and is branched into two ports A and B. The upstream optical signal output from the port A is blocked by the isolator 13. The upstream optical signal output from the port B is input to one port of the 2 × 1 optical coupler 15 via the optical waveguide 12-2. Further, the CW light from the light source 11 branched to the port D is input to the other port of the 2 × 1 optical coupler 15 via the optical waveguide 12-3. 2x
In one optical coupler 15, the phase-modulated upstream optical signal and C
The W light is multiplexed and input to the light receiving circuit 16.

Here, since the upstream optical signal and the CW light originally have the same coherence from the same light source 11, they interfere with each other in the 2 × 1 optical coupler 15 and have an intensity corresponding to the phase difference between the two lights. Fluctuation occurs. The phase-modulated upstream optical signal can be demodulated as an electric signal by photoelectrically converting the output light after the interference by the light receiving circuit 16 and amplifying it.

(Second Embodiment of Optical Transmission System: Claims 1, 2, 3, 4, 6) FIG. 2 shows a second embodiment of the optical transmission system of the present invention. The feature of this embodiment is that the first
The optical signal modulator 20 in the user side ONU is the same as that in the first embodiment.

The intra-station device 10 includes a light source 11 that outputs CW light of a constant intensity, a 1 × 2 optical coupler 17 and an optical circulator that are inserted in an optical waveguide 12-1 that connects the light source 11 and the optical fiber transmission line 30. 18 and 1 × 2 optical coupler 1
7 and the optical circulator 18 to the optical waveguides 12-2, 1
A 2 × 1 optical coupler 15 connected via 2-3, a light receiving circuit 16 for receiving the output light of the optical coupler 15, a variable attenuator 41 inserted in the optical waveguide 12-2, and a polarization adjuster 42. And a phase adjuster 43.
The polarization adjuster 42 can be realized by, for example, a method of sequentially transmitting a target optical signal to a ½ wavelength plate and a ¼ wavelength plate that can rotate at an arbitrary angle. Phase adjuster 43
Can be realized by adjusting the applied voltage for phase adjustment with the same configuration as the phase modulator 21 used in the optical signal modulator 20.

The CW light output from the light source 11 is 1 × 2.
The optical coupler 17 splits the light into two ports. One of the CW lights is input from the port A of the optical circulator 18 and output to the port B, and is transmitted to the optical signal modulator 20 in the user-side ONU via the optical fiber transmission line 30. In the same way as in the first embodiment, the CW is performed by the optical signal modulator 20.
The upstream optical signal obtained by phase-modulating the light and returning it is input from the port B of the optical circulator 18 of the intra-station device 10 and output to the port C, and one of the 2 × 1 optical coupler 15 is passed through the optical waveguide 12-3. Input to the port. Also, 1 × 2
The other CW light branched by the optical coupler 17 is the optical waveguide 1
2-2 via the variable attenuator 41, the polarization adjuster 42 and the phase adjuster 43 inserted in 2-2.
Input to the other port of. In the 2 × 1 optical coupler 15, the phase-modulated upstream optical signal and CW light are multiplexed and input to the light receiving circuit 16. As in the first embodiment, the light receiving circuit 16 can demodulate the phase-modulated upstream optical signal as an electrical signal by optoelectrically converting and amplifying the output light after interference.

In the configuration of this embodiment, the use of the optical circulator 18 eliminates the need for an isolator for blocking the return light to the light source 11. Further, in the variable attenuator 41, the requirement of the dynamic range of the light receiving circuit 16 can be relaxed by attenuating the intensity of CW light to a predetermined value. By adjusting the polarization state of the CW light, the polarization adjuster 42 can interfere in the polarization state with the best interference efficiency. Phase adjuster 43
Since the extinction ratio can be set to the maximum state by adjusting the phase difference between the CW light and the upstream optical signal, the reception sensitivity of the upstream optical signal can be optimized.

The variable attenuator 41, the polarization adjuster 42, and the phase adjuster 43 are independent of each other, and it is not always necessary to provide the three at the same time. Moreover, you may insert them in the optical waveguide 12-3 side.

(Third Embodiment of Optical Transmission System: Claims 1, 2, 3, 4, 7) FIG. 3 shows a third embodiment of the optical transmission system of the present invention. The feature of this embodiment is that the second
The optical signal modulator 20 in the user-side ONU in the embodiment of the present invention is changed from the reflection type to the transmission type, the downstream optical fiber transmission line 30-1 and the upstream optical fiber transmission line 30-2 are separated, and the intra-station device 10 The optical circulator 18 in the inside is unnecessary.

The intra-station device 10 includes a light source 11 which outputs CW light of a constant intensity, a light source 11 and a downstream optical fiber transmission line 3.
1 × to be inserted into the optical waveguide 12-1 for connecting to 0-1
The two optical couplers 17, the 1 × 2 optical coupler 17, and the optical waveguides 12-2 and 12-3 in the upstream optical fiber transmission line 30-2.
2 × 1 optical coupler 15 and optical coupler 1 which are connected via
5, a variable attenuator 41, a polarization adjuster 42, and a phase adjuster 43, which are inserted into the optical waveguide 12-2.

The optical signal modulator 20 comprises a phase modulator 21 for phase-modulating the CW light transmitted through the downstream optical fiber transmission line 30-1 with a modulation signal.
In this configuration, the transmitted light of No. 1 is connected to the upstream optical fiber transmission line 30-2.

The CW light output from the light source 11 is 1 × 2.
The optical coupler 17 splits the light into two ports. One of the CW lights is transmitted to the optical signal modulator 20 in the user side ONU via the downstream optical fiber transmission line 30-1.
The CW light input to the optical signal modulation device 20 is phase-modulated by the modulation signal in the phase modulator 21 and is transmitted, and is transmitted to the upstream optical fiber transmission line 30-2 as an upstream optical signal. The upstream optical signal transmitted through the optical fiber transmission line 30-2 is input to one port of the 2 × 1 optical coupler 15 via the optical waveguide 12-3 of the intra-station device 10. Also, 1 × 2
The other CW light branched by the optical coupler 17 is the optical waveguide 1
2-2 via the variable attenuator 41, the polarization adjuster 42 and the phase adjuster 43 inserted in 2-2.
Input to the other port of. In the 2 × 1 optical coupler 15, the phase-modulated upstream optical signal and CW light are multiplexed and input to the light receiving circuit 16. As in the second embodiment, the light receiving circuit 16 can demodulate the phase-modulated upstream optical signal as an electrical signal by optoelectrically converting and amplifying the output light after interference.

In this embodiment, the optical circulator 18 in the second embodiment is not necessary, and the reflector 22 in the optical signal modulator 20 is not necessary, which simplifies the structure. However, although two optical fiber transmission lines in the access section are required, one for the upstream and one for the downstream, even if a part of the downstream CW light is reflected somewhere in the downstream optical fiber transmission line 30-1, the upstream optical signal is transmitted. Therefore, high quality transmission of the upstream signal is possible.

(Fourth Embodiment of Optical Transmission System: Claims 1 and 5) FIG. 4 shows a fourth embodiment of the optical transmission system of the present invention. The feature of this embodiment lies in the configuration of the intra-station device 10 in the first embodiment, and the optical signal modulator 20 in the user-side ONU is the same as in the first embodiment.

The intra-station device 10 includes a light source 11 which outputs CW light of a constant intensity, and a 2 × 2 optical coupler 14-1 which is inserted into an optical waveguide 12-1 which connects the light source 11 and the optical fiber transmission line 30. The optical circulator 18, the 2 × 2 optical coupler 14-1 and the optical circulator 18 are connected to the optical waveguide 12.
2 × 2 optical coupler 14 connected via -2 and 12-3
-2, a differential type light receiving circuit 44 that receives output light from the two ports of the 2 × 2 optical coupler 14-2, and the optical waveguide 1
2-2 to insert the phase adjuster 43 and the optical waveguide 12
-3 of the polarization adjuster 42.

Here, the 2 × 2 optical couplers 14-1, 14-
2, the optical waveguide 12-2, a part of the optical waveguide 12-1 and the optical waveguide 12-3, and the phase adjuster 43.
An interferometer type optical waveguide circuit 47 is configured. These can be integrated on the same optical waveguide substrate, quartz or L
It is realized by a planar waveguide circuit of iNbO ₃ .

The CW light output from the light source 11 is 2 × 2.
The optical coupler 14-1 splits the light into two ports. One of the CW lights is input from the port A of the optical circulator 18 through the optical waveguide 12-1 and is output to the port B, and is transmitted to the optical signal modulator 20 in the user-side ONU via the optical fiber transmission line 30. Is transmitted. Similarly to the first embodiment, the upstream optical signal obtained by phase-modulating and returning the CW light by the optical signal modulator 20 is input from the port B of the optical circulator 18 of the intra-station device 10 and output to the port C. It is input to one port of the 2 × 2 optical coupler 14-2 via the waveguide 12-3 and the polarization adjuster 42. Also 2x
The other CW light branched by the two-optical coupler 14-1 is input to the other port of the 2 × 2 optical coupler 14-2 via the optical waveguide 12-2 and the phase adjuster 43.

In the 2 × 2 optical coupler 14-2, the phase-modulated upstream optical signal and the CW light are multiplexed, and two lights with inverted interference light intensities are taken out from the two output ports, and differential signals are obtained. It is input to the mold light receiving circuit 44. In the differential type light receiving circuit 44, the complementary outputs extracted to the two output ports of the 2 × 2 optical coupler 14-2 are supplied to the two light receiving elements 45-1 and 45-4.
Opto-electric conversion is performed at 5-2, and the difference between the output electric signals is amplified by the differential amplifier circuit 46. This doubles the intensity fluctuation component generated by the demodulation of the phase modulation, and cancels the fluctuation of the light intensity and the noise component which are additionally generated in the process of optical transmission, and the signal-to-noise ratio (SNR) High-sensitivity light reception and demodulation are possible.

The polarization adjuster 42 and the phase adjuster 4
The function of 3 is the same as that shown in the second embodiment. Further, as in the third embodiment, the optical signal modulation device 20 in the user-side ONU is changed from the reflective type to the transmissive type, and the downstream optical fiber transmission line 30-1 and the upstream optical fiber transmission line 30-2. May be separated and the optical circulator 18 in the intra-station device 10 may be eliminated.

(First Embodiment of Optical Signal Modulator: Claim 8) The first embodiment of the optical signal modulator of the present invention is the embodiment of the optical transmission system shown in FIGS. This is the reflection type configuration described in.

(Second Embodiment of Optical Signal Modulator: Claims 9 and 10) FIG. 5 shows a second embodiment of the optical signal modulator of the present invention. The optical signal modulator of the present embodiment has a configuration in which the CW light from the optical fiber transmission line 30 is phase-modulated and returned as an upstream optical signal, and is classified into a reflection type similar to the embodiments shown in FIGS. It is what is done.

In the figure, an optical signal modulator 20 includes a 1 × 2 optical coupler (or 2 × 2 optical coupler) 23 for branching CW light transmitted through an optical fiber transmission line 30 into two output ports, The optical waveguide 24 connects the two output ports in a ring shape, and the phase modulator 21 and the polarization adjuster 42 inserted in the optical waveguide 24. The phase modulator 21 phase-modulates the CW light in each direction by the modulation signal and transmits it.

Here, the clockwise CW light and the counterclockwise CW light branched to the ring-shaped optical waveguide 24 are subjected to the same phase modulation by the phase modulator 21 and make one round to make a 1 × 2 optical coupler. 23. At this time, since the path lengths of the lights in the respective directions are equal and the phases are the same, they merge and are returned to the optical fiber transmission line 30. Even if a 2 × 2 optical coupler is used instead of the 1 × 2 optical coupler 23, the phase-modulated upstream optical signal can be output from the same input port into which the CW light is input.

The optical signal modulator 20 of the present embodiment does not require a reflector and can be constituted by a planar optical waveguide for all other components. Therefore, the entire modulator can be integrated on the planar optical waveguide and miniaturized. It becomes possible to do.

Further, although the polarization controller 42 is arranged as necessary, when it is arranged, the upstream optical signal can be folded back in an arbitrary polarization state. Thereby, for example, a polarization maintaining optical fiber may be used for the optical fiber transmission line 30, the downstream CW light may be linearly polarized, and the upstream optical signal may be controlled to be linearly polarized orthogonal to this and folded back. In this case, for example, even if a part of the downstream CW light is reflected by the reflecting surface of the optical connector, etc., since the polarizations of the reflected light and the upstream optical signal are orthogonal to each other, these interferences are suppressed, and the reflected light is reflected. An optical transmission system having high light resistance can be configured. However, it is necessary to use a polarization independent type as the phase modulator 21.

(Third Embodiment of Optical Signal Modulator: Claim 11) FIG. 6 shows an optical signal modulator according to a third embodiment of the present invention. The optical signal modulator of the present embodiment is shown in FIG.
2 is a configuration example for obtaining a phase modulation effect that does not depend on the polarization state of an incident optical signal in the reflection type configurations shown in 2 and 4.

In the figure, an optical signal modulator 20 includes a phase modulator 21 for phase-modulating the CW light transmitted through an optical fiber transmission line 30 with a modulation signal, and a polarization rotator 25.
And a reflection plate 22 that is fixed by adjusting the angle so that the transmitted light of the phase modulator 21 and the polarization rotator 25 is incident on the optical fiber transmission line 30 again.

The polarization rotator 25 is composed of a Faraday rotator and a magnet for applying a magnetic field, and rotates its polarization state by 45 degrees when transmitting the output light of the phase modulator 21 by the Faraday effect. When the light reflected by the reflection plate 22 is transmitted in the opposite direction, the light is rotated by 45 degrees in the same direction, and by reciprocating, the polarization state is set to rotate by 90 degrees. That is, the polarization state of the CW light is rotated 90 degrees and returned.
Thus, even if the phase modulator 21 has polarization dependency of incident light such as birefringence, the polarization dependency is canceled by the incident light and the reflected light, and the phase modulation does not depend on the polarization state of the CW light. It can be performed.

When a laser diode is used as a light source of incident CW light, its polarization state is usually close to linear polarization, and the upstream optical signal returned from the optical signal modulator 20 of this embodiment is orthogonal to this. The state is close to linear polarization. Therefore, as in the third embodiment, even if a part of the downstream CW light is reflected by the reflection surface of the optical connector or the like, the polarizations of the reflected light and the upstream optical signal are orthogonal to each other. Interference is suppressed and high quality transmission is possible.

In this embodiment, the phase modulator 21, the polarization rotator 25, and the reflector 22 are integrated into a module after the alignment of the optical system is adjusted, so that the optical signal is small and has low power consumption. A modulator can be constructed.

(Fourth Embodiment of Optical Signal Modulator: Claim 12) FIG. 7 shows a fourth embodiment of the optical signal modulator of the present invention. The optical signal modulator of the present embodiment is a configuration example for obtaining a phase modulation effect that does not depend on the polarization state of the incident optical signal, as in the third embodiment.

In the figure, an optical signal modulator 20 includes phase modulators 21-1 and 21-2 for phase-modulating the CW light transmitted through the optical fiber transmission line 30 with a modulation signal.
The reflection plate 22 is fixed by adjusting the angle so that the transmitted light of the phase modulators 21-1 and 21-2 is incident on the optical fiber transmission line 30 again.

Here, the phase modulator 21-1 and the phase modulator 21-2 have the same function, performance, size, etc., but the phase modulator 21-2 is different from the phase modulator 21-1. , It is set to a state rotated by 90 degrees in a plane perpendicular to the optical axis of the incident light. Thereby, the phase modulators 21-1 and 21-2
Even when there is incident light polarization dependency such as birefringence, the effect is canceled by passing through the phase modulators 21-1 and 21-2 whose optical axes are shifted by 90 degrees, and as a result, the CW light It is possible to perform phase modulation that does not depend on the polarization state.

(Fifth Embodiment of Optical Signal Modulator: Claim 13) FIG. 8 shows an optical signal modulator according to a fifth embodiment of the present invention. The optical signal modulator of the present embodiment has a configuration in which the CW light from the optical fiber transmission line 30 is phase-modulated and returned as an upstream optical signal, and is basically similar to the embodiments shown in FIGS. It is a thing. Further, it is similarly applicable to FIGS. 6 and 7 and the embodiments shown below.

The feature of this embodiment is that the optical fiber transmission line 30 and the phase modulator 2 in the embodiment shown in FIGS.
CW incident from the optical fiber transmission line 30 between the CW
The light is transmitted through the phase modulator 21 (in the embodiment of FIG. 6, the phase modulator 2
1 and the polarization rotator 25, in the embodiment of FIG. 7, an optical lens 26 is arranged so as to focus on one point on the reflection plate 22 after passing through the phase modulators 21-1, 21-2). . It should be noted that the broken line in the figure indicates the ray trajectory. Reflector 2
2 totally reflects this light, the reflected light is again phase modulator 2
1 and the optical lens 26, the optical fiber transmission line 30
The angle of the reflection plate 22 is adjusted and fixed so that the light enters the.

(Sixth Embodiment of Optical Signal Modulator: Claim 14) FIG. 9 shows a sixth embodiment of the optical signal modulator of the present invention. The optical signal modulator of the present embodiment wavelength-multiplexes and transmits the CW light and the downstream optical signal for the upstream optical signal from the intra-station device 10 to the optical signal modulator 20, and phase-modulates and returns the CW light and the downstream optical signal. The structural example with a light receiving circuit which receives a signal is shown.

The optical signal modulator 20 of this embodiment is basically the same as that of the embodiment shown in FIGS. 1, 2 and 4, except that a wavelength selective reflection plate 27 is used instead of the reflection plate 22. ,
A light receiving circuit 50 for the downstream optical signal is provided. Assuming that the wavelength of the CW light (upstream optical signal) is λ1 and the wavelength of the downstream optical signal is λ2, the wavelength selective reflection plate 27 totally reflects the optical signal of wavelength λ1 and transmits the downstream optical signal of wavelength λ2 to receive it. The circuit 50 receives light. Such a wavelength selective reflection plate 2
7 is realized, for example, by a dielectric multilayer film filter in which dielectric thin films having different refractive indexes are laminated in multiple layers.

As a result, the downstream optical signal of wavelength λ2 is photoelectrically converted by the light receiving circuit 51, amplified, and output as a downstream signal. The downlink optical signal is an intensity-modulated signal, so that demodulation is not hindered even if it passes through the phase modulator 21 and undergoes phase modulation.

In this embodiment, the phase modulator 21, the wavelength selective reflection plate 27, and the light receiving circuit 50 are integrated into a module after alignment adjustment of the optical system.
An optical signal modulation device for bidirectional transmission capable of receiving a downstream optical signal can be realized with a small configuration. An example of this module configuration is shown in Fig. 9 (2). In FIG. 9 (2), on the planar optical circuit 28, the phase modulator 21, the wavelength selective reflection plate 27, the light receiving element 51 of the light receiving circuit 50, and the optical waveguide 29 are provided.
Are arranged so as to be optically coupled to each other. Further, the electrodes of the phase modulator 21 are drawn out onto the planar optical circuit 28,
A modulation signal is applied.

(Seventh Embodiment of Optical Signal Modulator: Claims 15 and 16) FIG. 10 shows a seventh embodiment of the optical signal modulator of the present invention. Optical signal modulation circuit 10 of the present embodiment
Shows a configuration example with a monitor circuit that monitors the upstream optical signal that phase-modulates and returns the CW light and feedback-controls the modulation condition.

In the figure, in addition to the phase modulator 21 and the reflection plate 22 in the first embodiment, the optical signal modulator is provided in an optical waveguide 52-1 connecting the optical fiber transmission line 30 and the phase modulator 21. 2x2 optical coupler 53 to be inserted
And the optical waveguides 52-2 and 52-3 to the 2 × 2 optical coupler 53.
2 × 1 optical coupler 54 and optical coupler 5 connected via
The modulated light monitor circuit 55 that receives the output light of No. 4 and the modulation condition adjustment circuit 56 that controls the applied voltage of the phase modulator 21 according to the monitor output. Optical waveguide 52
−1 is connected to the ports A and C of the 2 × 2 optical coupler 53, the optical waveguide 52-2 is connected to the port B of the 2 × 2 optical coupler 53, and the optical waveguide 52-3 is a 2 × 2 optical coupler. 5
3 to port D.

The CW light input to the optical signal modulator 20 is input to the port A of the 2 × 2 optical coupler 53 and is split into two ports C and D. The CW light output from the port C is input to the phase modulator 21 and the phase modulator 2
At 1, the signal is phase-modulated by the modulation signal, reflected by the reflector 22, and returned as an upstream optical signal. This upstream optical signal is input to the port C of the 2 × 2 optical coupler 53 and is branched into two ports A and B. The upstream optical signal output from the port A is returned to the optical fiber transmission line 30 and transmitted to the intra-station device 10. The upstream optical signal output from the port B is input to one port of the 2 × 1 optical coupler 54 via the optical waveguide 52-2. The CW light branched to the port D is 2 × 1 via the optical waveguide 52-3.
It is input to the other port of the optical coupler 54. In the 2 × 1 optical coupler 54, the upstream optical signal that has been phase-modulated and the CW light are multiplexed, and an intensity variation according to the phase difference between the two lights occurs. The modulated light monitor circuit 55 outputs the modulated light monitor signal corresponding to the phase-modulated upstream optical signal by inputting this interference light, optoelectrically converting it, and amplifying it.

On the other hand, the phase modulator 21 is modulated by the modulation signal, but by controlling the driving condition thereof by the modulation condition adjusting circuit 56 according to the modulated light monitor signal, the phase modulator 21 is always in the optimum modulation state. Can be operated.

When the digital baseband signal is modulated, the output of the modulated light monitor circuit 55 is output when the phase of the CW light input to the 2 × 1 optical coupler 54 and the phase of the upstream optical signal are the same. The modulation condition adjusting circuit is such that the output of the modulated light monitor circuit 55 becomes maximum (1 level, mark) when the phase of the CW light and the phase of the upstream optical signal deviate by just π. By controlling with 56, the modulation state with the best extinction ratio is realized.

[0067]

As described above, in the optical transmission system of the present invention, the downlink CW light transmitted from the first transmission device to the second transmission device is phase-modulated and the first transmission is performed as an upstream optical signal. And transmitting the upstream signal from the second transmission device by returning to the device, multiplexing the downstream CW light and the received upstream optical signal by the first transmission device, and causing the interference to detect the intensity change. You can As a result, high quality and long-distance transmission of upstream signals can be achieved with a simple configuration that does not require a light source on the side of the second transmission device (ONU). Also, CW
Since the light source of the first transmission device that generates light can be easily shared by a plurality of second transmission devices (ONUs), 1
The cost of the light source per transmission device can be reduced, and the economical efficiency of the system can be improved.

Further, when the downstream CW light and the upstream optical signal are multiplexed on the side of the first transmission device, by taking the difference between the complementary outputs of the interference light, highly sensitive reception demodulation with good SNR is possible. Becomes Also, by changing the polarization of the downlink CW light and the polarization of the upstream optical signal on the second transmission device side, it is possible to avoid the influence of unnecessary interference waves on the upstream optical signal and perform high-quality transmission. .

Further, even when the optical phase modulation means of the second transmission device has an incident light polarization dependency, by using the polarization rotation means or the like, the phase modulation effect which does not depend on the polarization state of the incident optical signal is obtained. Can be obtained. Furthermore, the polarization of the downlink CW light and the polarization of the uplink optical signal can be made orthogonal to each other, and interference from the downlink CW light can be suppressed.

The optical signal modulator of the present invention is highly economical because it does not require precise alignment, can be easily miniaturized and integrated, and can achieve low power consumption.
Further, it is possible to easily cope with the uplink and downlink bidirectional wavelength division multiplex transmission and the monitoring of the modulation state of the uplink signal. By using this optical signal modulator in the optical transmission system of the present invention, an economical and high quality system can be constructed.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of an optical transmission system of the present invention.

FIG. 2 is a block diagram showing a second embodiment of the optical transmission system of the present invention.

FIG. 3 is a block diagram showing a third embodiment of the optical transmission system of the present invention.

FIG. 4 is a block diagram showing a fourth embodiment of the optical transmission system of the present invention.

FIG. 5 is a block diagram showing a second embodiment of the optical signal modulator of the present invention.

FIG. 6 is a block diagram showing a third embodiment of the optical signal modulation device of the present invention.

FIG. 7 is a block diagram showing a fourth embodiment of the optical signal modulator of the present invention.

FIG. 8 is a block diagram showing a fifth embodiment of the optical signal modulator of the present invention.

FIG. 9 is a block diagram showing a sixth embodiment of the optical signal modulator of the present invention.

FIG. 10 is a block diagram showing a seventh embodiment of the optical signal modulator of the present invention.

[Explanation of symbols]

10 Station equipment 11 light source 12 Optical waveguide 13 Isolator 14 2 × 2 optical coupler 15 2 × 1 optical coupler 16 Light receiving circuit 17 1 × 2 optical coupler 18 Optical circulator 20 Optical signal modulator 21 Phase modulator 22 Reflector 23 1 × 2 optical coupler 24 Optical Waveguide 25 polarization rotator 26 Optical lens 27 Wavelength selective reflector 28 Flat optical circuit 29 Optical Waveguide 30 optical fiber transmission line 41 Variable attenuator 42 Polarization adjuster 43 Phase adjuster 44 Differential type photo detector 45 Light receiving element 46 Differential amplifier circuit 47 MZ interferometer type optical waveguide circuit 50 Light receiving circuit 51 Light receiving element 52 optical waveguide 53 2 × 2 optical coupler 54 2 × 1 optical coupler 55 Modulated light monitor circuit 56 Modulation condition adjustment circuit

Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04B 10/152

Claims (16)

[Claims] 1. An upstream optical signal, wherein downlink CW light of constant intensity is transmitted from a first transmission device to a second transmission device via an optical fiber transmission line, and the downlink CW light is modulated by the second transmission device. In the optical transmission system configured to fold back to the first transmission device via the optical fiber transmission path, the first transmission device includes a laser light source that outputs light with high coherence as the downlink CW light, and the laser. The downstream CW light output from the light source is split into two,
An optical branching unit for sending one of the downlink CW lights to the optical fiber transmission line, another downlink CW light branched by the optical branching unit, and the upstream optical signal transmitted via the optical fiber transmission line. And an optical multiplexing means for multiplexing and outputting the interference light, and photoelectric conversion of the interference light output from the optical multiplexing means,
A light receiving circuit that outputs an electrical signal obtained by demodulating the upstream optical signal, wherein the second transmission device transmits the downlink CW transmitted via the optical fiber transmission path.
An optical phase modulating means for modulating the phase of light by a transmission signal; and an optical returning means for returning the optical signal phase-modulated by the optical phase modulating means to the optical fiber transmission line as an upstream optical signal, Optical transmission system. 2. The optical transmission system according to claim 1, wherein the first transmission device includes a variable optical attenuator that attenuates the optical intensity of the downstream CW light or the upstream optical signal input to the optical multiplexer. An optical transmission system characterized by that. 3. The optical transmission system according to claim 1, wherein the first transmission device includes polarization adjusting means for adjusting the polarization state of the downstream CW light or the upstream optical signal input to the optical multiplexing means. An optical transmission system characterized by being provided. 4. The optical transmission system according to claim 1, wherein the first transmission device includes a phase adjusting unit that adjusts a phase state of a downstream CW light or an upstream optical signal input to the optical multiplexing unit. An optical transmission system characterized in that 5. The optical transmission system according to claim 1, wherein the optical multiplexing means of the first transmission device complementarily outputs the downstream CW light and the interference light of the upstream optical signal to two output ports. The optical transmission system is characterized in that the light receiving circuit is configured to photoelectrically convert the complementary output interference light by two light receiving elements and output a difference between the electric signals. 6. The optical transmission system according to claim 1, wherein the optical folding means of the second transmission device reflects the modulated light output from the optical phase modulation means, and the modulated light is regenerated by the optical transmission means. An optical transmission system characterized in that it is phase-modulated by a phase modulation means and folded back to the optical fiber transmission line. 7. The optical transmission system according to claim 1, wherein the optical fiber transmission path is a first optical fiber transmission path for transmitting the downlink CW light and a second optical fiber for transmitting the upstream optical signal. A fiber transmission path, wherein the optical folding means inputs the downlink CW light from the first optical fiber transmission path to the optical phase modulation means, and outputs the modulated light output from the optical phase modulation means to the optical phase modulation means. 2. An optical transmission system characterized in that the optical transmission system is configured to send to the optical fiber transmission line 2. 8. A second optical transmission system according to claim 1.
In the transmission device, the downlink CW transmitted via the optical fiber transmission path.
Optical phase modulation means for modulating the phase of light by a transmission signal, and the modulated light output from the optical phase modulation means is reflected, and the reflected modulated light is phase-modulated again by the optical phase modulation means to obtain an upstream optical signal. And a reflection means that is turned back to the optical fiber transmission line. 9. A second optical transmission system according to claim 1.
In the transmission device, the branching unit for branching the downlink CW light from the optical fiber transmission path into two and the branching port of the branching unit are ring-connected to each other for the downlink C
An optical waveguide that transmits the W light clockwise and counterclockwise, and an optical phase modulator that modulates the phase of the downstream CW light that is inserted into the optical waveguide and that is transmitted by the transmission signal are provided. An optical signal modulation device, characterized in that the modulated clockwise and counterclockwise optical signals are combined by the branching means and returned to the optical fiber transmission line as an upstream optical signal. 10. The optical signal modulator according to claim 9, further comprising a polarization adjusting unit that is inserted into the optical waveguide and sets the upstream optical signal into a predetermined polarization state. Transmission system. 11. The optical signal modulator according to claim 8, further comprising a polarization rotation means for rotating the polarization of the passing light by 45 degrees between the optical phase modulation means and the reflection means. An optical signal modulation device having a configuration for rotating the polarization of light that reciprocates through a polarization rotation means by 90 degrees. 12. The optical signal modulator according to claim 8, wherein the optical phase modulator is arranged with its optical axis shifted by 90 degrees.
An optical signal modulation device having a configuration using two optical phase modulation means. 13. The optical signal modulator according to claim 8, wherein between the optical fiber transmission line and the optical phase modulator,
An optical lens is provided, which collects downlink CW light output from one end of the optical fiber transmission line so as to focus on the reflection means, and couples the reflected light to the optical fiber transmission line. Optical signal modulator. 14. The optical signal modulation device according to claim 8, wherein the reflection means reflects the modulated light output from the optical phase modulation means, and the reflected modulated light is again reflected by the optical phase modulation means. It is a wavelength selective reflection means for phase-modulating and returning to the optical fiber transmission line as an upstream optical signal, and transmitting a downstream optical signal having a wavelength different from that of the downstream CW light. The downstream light transmitted through the wavelength selective reflection means. An optical signal modulator comprising a light receiving circuit for receiving a signal. 15. The optical signal modulator according to claim 8, wherein the downstream CW light from the optical fiber transmission path is branched into two, and one of the downstream CW light is input to the optical phase modulation means, and Optical branching means for branching the upstream optical signal into two and returning one of them to the optical fiber transmission line, and the other downstream CW light and upstream optical signal branched by the optical branching means are combined, and the interference light is generated. An optical signal modulation device comprising: an optical multiplexing means for outputting; and a modulated light monitor circuit for outputting a monitor signal that photoelectrically converts the interference light output from the optical multiplexing means. 16. The optical signal modulator according to claim 15, wherein when the optical phase modulator modulates the digital baseband signal, the phase of the downstream CW light and the upstream optical signal input to the optical multiplexer. Of the optical phase modulation means such that the monitor signal level becomes minimum when the phases of the two are the same, and the monitor signal level becomes maximum when the phase of the downstream CW light and the phase of the upstream optical signal deviate by π. An optical signal modulation device comprising a modulation condition adjusting circuit for controlling conditions.