JPH0879218A - Tunable filter control system and optical communication system using the same - Google Patents

Abstract

(57) [Abstract] [Purpose] A wavelength tunable filter control system that can obtain a large error signal during both pull-in and stable conditions and can suppress periodic wavelength fluctuations during stable conditions. In a variable wavelength filter control system, a central wavelength of a variable wavelength filter 1-2-1, 1-2-2 is modulated to generate an error signal, which is tracked to a wavelength of transmitted light.
A plurality of wavelength tunable filters 1-2-1, 1-2-2 having different half-widths of transmission spectra, and devices 1-9-1, 1-for synchronizing and modulating the central wavelengths of these wavelength tunable filters.
9-2 and the like. An error signal is generated based on a plurality of signals obtained from a plurality of wavelength tunable filters. If the transmitted light from the wavelength tunable filter 1-2-1 having a wide half-value width is supplied to the receiving unit, the fluctuation of the light intensity in the stable state is small and the receiving characteristic is not deteriorated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control system and a control device for a wavelength tunable filter used in a receiver or the like of a wavelength division multiplexing communication system.

[0002]

2. Description of the Related Art In recent years, as information has become multimedia, support for high-speed and large-capacity communication has been studied regardless of subscriber system or LAN. In wavelength division multiplexing, it is possible to have a large number of independent lines for each optical fiber that is a transmission line. As a result, it is possible to provide a communication system that has flexibility for communication in addition to the high speed inherent in optical communication.

A wavelength filter that selects a signal of one wavelength from the multiplexed optical signals is one of the essential elements for this wavelength multiplexing communication. In a system in which the number of multiplexes is about several waves and the wavelength spacing is wide, a dielectric multilayer fixed filter can be used. However, in a system with a large number of multiplexes and a narrow wavelength interval, a wavelength filter having a variable wavelength and a narrow transmission wavelength width is required.

When such a wavelength tunable filter is used, the center wavelength of the transmission spectrum of the wavelength tunable filter (hereinafter, simply referred to as the center wavelength) is pulled into the wavelength of the optical signal to be received and stabilized, that is, tracking control is performed. Will be needed. Hereinafter, a conventional example of this wavelength tunable filter control system will be described. This example is from the publication Elec
Tonics Letters, Volume 26, 25
No. (December 1990) pages 2122 to 2123.

FIG. 7 shows a block diagram. The optical signal transmitted through the wavelength tunable filter 7-1 enters the light receiving element 7-2. This optical signal is converted into an electric signal by the light receiving element 7-2 and amplified by the amplifier 7-3. This amplified signal is further amplified by the narrowband amplifier 7-4 that amplifies only the frequency component near the frequency f, and only the modulated component is amplified.
Input to -5. The synchronous detection circuit 7-5 synchronously detects this signal with the modulation signal. The output of the synchronous detection circuit 7-5 is amplified by the integrator 7-6 only for low frequency components. In addition,
The time constant of the integrator 7-6 is set to be sufficiently longer than the period of the modulation signal. The feedback control circuit 7-7 is
A control signal is generated using this low frequency component as an error signal. The filter drive circuit 7-8 injects a current into the wavelength tunable filter 7-1 based on the three input signals of the offset signal, the modulation signal, and the control signal, and outputs the central wavelength λ of the filter.
Control c. The sine wave oscillator 7-9 has a frequency f for modulation.
The sine wave of is generated and output to the filter drive circuit 7-8 and the delay circuit 7-10.

FIG. 8 is a schematic diagram of the relationship between the transmission spectrum of the wavelength tunable filter and the wavelength of the optical signal to be received. FIG.
Where λc is the center wavelength of the transmission spectrum of the wavelength tunable filter, and λs is the wavelength of the received optical signal. λc is calculated from tracking from left to right when λc <λs by tracking.
When> λs, it is pulled from right to left and stabilized.
The range in which λc can be drawn is a range from the bottom of the transmission spectrum to a certain extent from the peak. This range is determined by the characteristics of the control system. Hereinafter, this range is referred to as a pull-in area.

The central wavelength λc of the transmission spectrum of the variable wavelength filter 7-1 is modulated by the sinusoidal modulation signal input to the filter control circuit 7-8. The modulation amplitude of this wavelength is converted into a change in the amount of light of the wavelength λs that passes through the wavelength tunable filter 7-1, and the light receiving element 7-2 further.
Is converted into an electric signal. The phase of the sine wave of the frequency f included in this electric signal is opposite between λc <λs and λc> λs.

The low-frequency component of the output of the synchronous detection circuit 7-5 (that is, the output of the integrator 7-6) becomes positive when the phases of the two inputs are the same and negative when the phases of the two inputs are opposite. When the aforementioned electric signal and the modulation signal from the sine wave oscillator 7-9 are used as these two inputs, the positive / negative of the output of the integrator 7-6 is
The case of λc <λs and the case of λc> λs are opposite. By performing feedback control using this signal as an error signal, λ
c can be pulled into λs and stabilized.

[0009]

In the above tracking method, after the center wavelength λc of the wavelength tunable filter 7-1 is pulled into the wavelength λs of the light source (LD) (hereinafter, referred to as stable time), the modulation signal is used. There is a periodic variation in wavelength λc. This wavelength fluctuation is converted into a fluctuation of the light intensity incident on the light receiving element by the transmission spectrum of the wavelength tunable filter 7-1. This fluctuation of the light intensity deteriorates the reception characteristics. If the modulation amplitude is reduced, fluctuations in light intensity can be suppressed. However, in such a case, the error signal becomes small, the response characteristic of the pull-in deteriorates at the time of pulling in, and the stability of the wavelength decreases at the time of stable.

That is, in the conventional tracking method, when the modulation amplitude is large, the pull-in response characteristic is good but the reception characteristic is deteriorated, and when the modulation amplitude is small, the receive characteristic is good but the pull-in response characteristic is deteriorated. There was a drawback.

Therefore, an object of the present invention is to provide a control system and a control device for a wavelength tunable filter used in a receiving device or the like of a wavelength division multiplexing communication system, which solves the above-mentioned drawbacks.

[0012]

A wavelength tunable filter control system or apparatus according to the present invention modulates the central wavelength of a wavelength tunable filter to generate an error signal and tracks the wavelength of transmitted light. In the apparatus, a plurality of wavelength tunable filters having different half-widths of transmission spectra, and means for synchronizing and modulating the central wavelength of the wavelength tunable filter, and an error signal based on a plurality of signals obtained from the wavelength tunable filter It is characterized by generating.

[0013]

Since the error signal is generated based on a plurality of signals obtained from a plurality of wavelength tunable filters having different half-widths of the transmission spectrum, a large error signal can be obtained both at the time of pulling in and at the time of stable. In addition, it is possible to suppress periodic wavelength fluctuations when stable. Furthermore, if the transmitted light from the wavelength tunable filter having a wide half-value width is supplied to the receiving unit, the fluctuation of the light intensity in the stable state is small and the receiving characteristic is not deteriorated.

[0014]

[First Embodiment] A first embodiment of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a block diagram of a first embodiment of a wavelength tunable filter control system to which the present invention is applied. As shown in the figure, the control system is an optical branching device 1-1-1,
1-1-2, wavelength tunable filter 1-2-1, 1-2
2, light receiving elements 1-3-1, 1-3-2, amplifier 1-4-
1, 1-4-2, adder 1-5, synchronous detection circuit 1-6,
Low-pass filter (hereinafter referred to as LPF) 1-7, feedback control circuit 1-8, filter drive circuits 1-9-1, 1-9
-2, the sine wave oscillator 1-10 is configured as a main part.

The optical splitter 1-1-1 splits the optical signal from the light source into two. The branched lights enter the wavelength tunable filters 1-2-1 and 1-2-2, respectively. These filters are distributed feedback (DFB) filters, and the center wavelength to be transmitted can be controlled by changing the injection current into the filters near the oscillation threshold. Since the transmission center wavelength is proportional to the injection current in a narrow range, by superimposing a modulation signal having a small amplitude on the bias component of the injection current, the center wavelength can be modulated according to the modulation signal. Regarding the DFB filter, publications: The Institute of Electronics, Information and Communication Engineers, CI, J73-C-
I, No. 5 (May 1990), pages 347-353.

The optical branching device 1-1-2 branches a part of the light transmitted through the wavelength tunable filter 1-2-1 for tracking and the rest for reception. The light receiving element 1-3-1 converts the optical signal split by the optical splitter 1-1-2 into an electrical signal. The amplifier 1-4-1 amplifies the electric signal. The other light receiving element 1-3-2 is a wavelength tunable filter 1-2-2.
The optical signal transmitted through is converted into an electric signal. Amplifier 1-4
-2 amplifies the electric signal. The adder 1-5 adds the electric signals from the amplifiers 1-4-1 and 1-4-2. The synchronous detection circuit 1-6 is a circuit for comparing the phases of the signal from the adder 1-5 and the modulation signal from the sine wave oscillator 1-10, and outputs a signal proportional to the phase difference between the two. The LPF 1-7 is a filter that extracts a low frequency component from the output signal of the synchronous detection circuit 1-6, and its cutoff frequency is sufficient for the modulation signal (frequency: f) that is the output of the sine wave oscillator 1-10. It is set to attenuate. The feedback control circuit 1-8 generates a control signal using the signal from the LFP 1-7 as an error signal. As a feedback control method, a well-known PID (Proportional In
Tegral Differential) control is preferably used.

Filter drive circuits 1-9-1, 1-9-2
The control signal, the modulation signal from the sine wave oscillator 1-10, the offset signal from the receiving unit, and the tracking ON / OFF signal are input to the. Filter drive circuit 1
The output of -9-1 is the injection current to the DFB filter 1-2-1, and the output of the filter drive circuit 1-9-2 is the injection current to the DFB filter 1-2-2. Each drive circuit 1-9-
1, 1-9-2 are ON / OFF of tracking from the receiving unit.
The method of generating the injection current is switched by the OFF control signal. When tracking is ON, the output current is determined from the control signal, modulation signal, and offset signal, and tracking O
In FF, the output current is determined only by the offset signal. The receiving unit mentioned here refers to an apparatus in which the control system of the present invention is incorporated.

The drive circuits 1-9-1 and 1-9-2 are set to generate output signals corresponding to the connected filters 1-2-1 and 1-2-2. That is, each drive circuit outputs, from the same control signal and offset signal, an output signal such that the center wavelengths of both filters 1-2-1 and 1-2-2 are always the same, for the filters connected to each. To generate.

The sine wave oscillator 1-10 outputs a modulation signal for modulating the center wavelength of the DFB filters 1-2-1 and 1-2-2 with a minute amplitude. The output is divided into three, one is input to the filter drive circuit 1-9-1, the other is input to the filter drive circuit 1-9-2, and the remaining one is input to the synchronous detection circuit 1-6. . Both filters 1-2-1, 1-2
-2 has the center wavelength of each drive circuit 1-9-1, 1-9
-2, they are modulated in synchronization with each other. Regarding the frequency f of the modulation signal from the sine wave oscillator 1-10, the upper limit is determined by the band of the transmission signal, and the lower limit is determined by the response speed of tracking. Usually, a frequency of several 100 Hz to several kHz is used.

When multi-electrode DFB filters are used as the wavelength tunable filters 1-2-1 and 1-2-2, the outputs of the filter drive circuits 1-9-1 and 1-9-2 are equal to the number of electrodes. Yes, the center wavelength can be changed by controlling the injection current of each electrode. Further, the modulation of the center wavelength for tracking is performed by superimposing a sinusoidal component on the injection current of one or more electrodes.

FIG. 2 is a schematic diagram for explaining the transmission spectra of the wavelength tunable filters 1-2-1, 1-2-2 used in this embodiment. In FIG. 2, 2-1 is a wavelength tunable filter 1-2-1, 2-2 is a wavelength tunable filter 1-
2-2 shows the transmission spectra of 2-2, respectively. Also,
λs represents the wavelength of the received optical signal, and λs ′ represents the wavelength of the optical signal of the channel adjacent to the received optical signal. The wavelength tunable filter 1-2-1 is assumed to have characteristics equivalent to those used in the conventional optical wavelength division multiplexing communication system. The full width at half maximum of the transmission spectrum is determined by the wavelength interval | λs-λs' | of the system and the crosstalk required by the system. On the other hand, the wavelength tunable filter 1-
The half width of 2-2 is narrower than that of the wavelength tunable filter 1-2-1.

FIG. 3 shows the wavelengths λs and D of the received optical signal.
Difference in center wavelength λc of FB filter | λs−λc |
FIG. 3 is a schematic diagram showing the relationship between the error signal and the error signal. The shape of the transmission spectrum of the tunable filter is as shown in FIG.
Usually, the inclination is loose at the apex and hem. Since the wavelength modulation component is generated based on this component, its magnitude becomes small when λs is at the apex and the skirt of the transmission spectrum of the wavelength tunable filter. Therefore, the erroneous calculation signal of the typical wavelength tunable filter 1-2-1 becomes like the curve shown by the dotted line 3-1 in FIG. Similarly, the error signal from the wavelength tunable filter 1-2-2 having a narrow transmission spectrum has a curve indicated by the one-dot chain line 3-2 in FIG. The peak wavelength of the intensity of the error signal becomes closer to the wavelength λs of the optical signal to be received, as the half width of the wavelength tunable filter becomes narrower.

In this embodiment, two types of wavelength tunable filters 1-2-1, 1-2 having different half-widths of transmission spectra are used.
2 is used for tracking. Each drive circuit 1-9-
1, 1-9-2, both filters 1-2-1, 1-
The transmission center wavelengths of 2-2 are the same. Further, the modulation of the central wavelength of both filters is synchronized. In this embodiment, an error signal is obtained by modulating the central wavelength of the filter and synchronously detecting the transmitted light intensity. That is, the fluctuation of the central wavelength is converted into the fluctuation of the transmitted light intensity, and the magnitude of the filter central wavelength λc and the wavelength λs of the received light is judged from the phase of the fluctuation. When a plurality of wavelength tunable filters are used and one detector is used as in the present embodiment, it is necessary to make the modulation phases equal.

The wavelength tunable filter 1-2-1 is a DFB filter and is used for reception. The half-value width of the transmission spectrum is as wide as the half-value width of the conventional wavelength tunable filter for the optical wavelength division multiplexing communication system. The filter 1-
The optical signal transmitted through 2-1 contributes to the error signal only by the signal represented by the curve 3-1 in FIG. Wavelength tunable filter 1
-2-2 is also a DFB filter, which is used for tracking when stable. The half-value width of the transmission spectrum is narrower than the half-value width of the wavelength tunable filter 1-2-1. The optical signal transmitted through the wavelength tunable filter 1-2-2 is 3-2 in FIG.
Only the signal represented by the curve of contributes to the error signal. The optical signals from both filters are received by the light receiving element 1-3 as described above.
1, 1-3-2 is converted into an electric signal, and the amplifier 1-4-
After being amplified by 1, 1-4-2, they are added by the adder 1-5. Therefore, the error signal in this embodiment has a curve represented by 3-3 in FIG.

Next, the operation of the control system of this embodiment will be described with reference to FIGS. In this embodiment, two wavelength tunable filters 1-2-1 and 1-2-2 are used, but the operation of modulating the center wavelength λc of the filter to generate an error signal and tracking to the wavelength λs of the optical signal is not performed. This is similar to the conventional example. Further, λc is λs before the tracking operation is started by the bias signal from the receiving unit.
Is set so as to enter the pull-in area of the wavelength tunable filter 1-2-1.

In this embodiment, the error signal is │λs-λc
Since it changes with respect to | as shown by 3-3 in FIG. 3, compared with the change of the error signal in the conventional example shown by the curve 3-1 in FIG. 3, | λs−λc | However, the rising edge of the error signal is sharp. Therefore, the error signal in the stable state is large and the wavelength stability is high. Further, since the transmitted light from the wavelength tunable filter 1-2-1 having a wide half-value width is supplied to the receiving unit, the fluctuation of the light intensity at the time of stability is small and the receiving characteristic is not deteriorated.

[0027]

[Second Embodiment] A second embodiment of the present invention will be described below with reference to the drawings. FIG. 4 is a block diagram of a second embodiment of the variable wavelength filter control system of the present invention. In this embodiment, three variable wavelength filters are used. The control system is the optical splitter 4
-1-1, 4-1-2, wavelength tunable filter 4-2-1,
4-2-2, 4-2-3, light receiving elements 4-3-1, 4-3
-2, 4-3-3, amplifier 4-4-1, 4-4-2, 4
-4-3, adder 4-5, synchronous detection circuit 4-6, LFP
4-7, feedback control circuit 4-8, filter drive circuit 4-9
-1, 4-9-2, 4-9-3, sine wave oscillator 4-10
Is configured as a main part.

In this embodiment, the wavelength tunable filter 4-2 is used.
1 has the widest half-value width of the transmission spectrum, which is almost equal to the magnitude of the half-value width of the wavelength tunable filter used in the conventional example. The FWHM of the wavelength tunable filter 4-2-3 is narrow, and the FWHM of the wavelength tunable filter 4-2-2 has an intermediate width. By the drive circuits 4-9-1, 4-9-2, and 4-9-3, the center wavelengths of the filters are the same, and the modulation is also synchronized. The output to the receiving unit is the optical branching device 4-1-2 that transmits the transmitted light of the wavelength tunable filter 4-2-1.
It branches off at and is taken out.

Similar to the first embodiment, the optical signal from each filter is converted into an electric signal by the light receiving elements 4-3-1, 4-3-2, 4-3-3, and then the amplifier 4-4-. 1,4-4-
It is amplified by 2, 4-4-3 and added by the adder 4-5. The output from the adder is input to the synchronous detection circuit 4-6,
The output is passed through the LPF 4-7 to obtain an error signal. Figure 5
5-4 shows the relationship between the difference | λs−λc | between the input light wavelength and the filter center wavelength and the error signal in this embodiment. The curve 5-1 in FIG.
The relationship between s-λc | and the error signal is shown. 5-2 is a contribution of the optical signal transmitted through the wavelength tunable filter 4-2-2 to the error signal, and 5-3 is a contribution of the optical signal transmitted through the wavelength tunable filter 4-2-3 to the error signal. It represents.

Change in Error Signal in this Embodiment 5-4
Comparing the change 5-1 of the error signal in the conventional example, the rise of the error signal is sharp and the intensity is large as a whole in the case of the present embodiment. Therefore, the stability when stable is high,
It takes less time to pull in. Further, compared to the first embodiment, since there is one wavelength tunable filter, the error signal in the middle part of the wavelength range that can be pulled in is large. Here, when the upper limit or the lower limit of the wavelength that the filter can pull in is set to λ _lim , the difference | λs−λc | between the input light wavelength and the filter center wavelength is | λs−λ _lim | /.
It is about 2. Further, since the transmitted light from the filter 4-2-1 having a wide half-width is supplied to the receiving unit, the fluctuation of the light intensity at the time of stability is small and the receiving characteristic is good.

[0031]

[Third Embodiment] FIG. 6 is a block diagram of a wavelength division multiplexing optical communication system according to a third embodiment of the present invention. This system is preferably used for video distribution. The number of channels is n, the number of receiving devices is m, and the wavelength of each channel is λ1, λ2 to λn.
Indicate. The transmitter 6-1 modulates each of the n light sources with a digital video signal and sends it to the optical fiber 6-2. This optical signal is sent by the optical branching device 6-3 to m optical fibers 6-4-
It is divided into 1 to m and sent to the receiving devices 6-5-1 to 6-m. Each of the receiving devices 6-5-1 to 6-m has a wavelength tunable filter to which the control method of the first or second embodiment is applied, whereby only the wavelength to be received is selected from the wavelength division multiplexed signal,
Receive the video signal.

FIG. 2 shows an example of the reception status in this embodiment. Receiver 6-5-k (1≤k≤m) having a wavelength tunable filter to which the control method of the first embodiment is applied.
However, when tracking is started to receive an optical signal of wavelength λs, the relationship between the received optical signal and the wavelength of the wavelength tunable filter and the signal strength is expressed as shown in FIG.

By using the wavelength tunable filter control system of the present invention, the center wavelength of the wavelength tunable filter can be tracked to the wavelength to be received, and the fluctuation due to the modulation signal at the time of tracking can be suppressed, so that a good reception state can be maintained. can do.

[0034]

Other Embodiments In the above embodiments, the signals from the respective wavelength tunable filters are added and then detected. On the contrary,
A configuration in which each signal is first detected and then added is also conceivable. Further, a plurality of oscillators may be prepared and the center wavelength of each filter may be modulated at different frequencies. In this case, at least as many synchronous detection circuits as the types of modulation frequencies used are required.

Although the DFB filter is used as the wavelength tunable filter in the above embodiment, another wavelength tunable filter capable of controlling the central wavelength of the transmission spectrum may be used.

Further, although the sine wave is used as the modulation signal for obtaining the error signal in the above embodiment, other signals having phase information may be used. When the phase shift in the control system of the two input signals of the phase comparison circuit is so large that it cannot be ignored with respect to the period of the modulation signal, a phase shifter such as a delay circuit is required between the oscillator and the synchronous detector. .

In the above embodiment, the center wavelengths of the filters are the same, but they need not be exactly the same. However, the further apart the wavelength of the received light and the central wavelength of the wavelength tunable filter that extracts the optical signal to the receiving unit,
The intensity of the optical signal extracted to the receiving unit becomes low.

In the above embodiment, the transmitted light of the wavelength tunable filter 1-2-1 for reception is similarly transmitted by the wavelength tunable filter 4-2-1 by using the optical branching device 1-1-2. The optical branching device 4-1-2 is used to branch the optical signal to the receiver and the optical signal for tracking. However, instead of using the optical branching device for extracting the received light, the optical frequency signal may be converted into an electric signal and then the component of the modulation frequency f may be separated by an electric filter to be used as a tracking signal.

[0039]

According to the structure of the present invention, since the error signal is large both at the time of pulling in and at the time of stabilization, and the wavelength fluctuation at the time of stability due to the modulation is small, the tracking characteristics are improved and good reception is performed. be able to.

[Brief description of drawings]

FIG. 1 is a block diagram of a wavelength tunable filter control system according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a transmission spectrum in the first example.

FIG. 3 is a schematic diagram showing a relationship between | λs−λc | and an error signal in the first embodiment.

FIG. 4 is a block diagram of a wavelength tunable filter control system according to a second embodiment of the present invention.

FIG. 5 is a schematic diagram showing a relationship between | λs−λc | and an error signal in the second embodiment.

FIG. 6 is a configuration diagram of an optical communication system which is a third embodiment of the present invention.

FIG. 7 is a block diagram of a wavelength tunable filter control system in a conventional example.

FIG. 8 is a schematic diagram showing a relationship between a transmission spectrum of a wavelength tunable filter and a wavelength of an optical signal in a conventional example.

[Explanation of symbols]

1-2-1, 1-2-2, 4-2-1, 4-2-2
4-2-3 Wavelength tunable filter 1-9-1, 1-9-2, 4-9-1, 4-9-2,
4-9-3 Filter drive circuit

Claims (8)

[Claims] 1. A wavelength tunable filter control system for modulating a center wavelength of a wavelength tunable filter to generate an error signal and tracking the center wavelength of the wavelength tunable filter to the wavelength of an optical signal based on the error signal. A plurality of wavelength tunable filters having different half-value widths and a means for synchronizing and modulating the central wavelengths of the plurality of wavelength tunable filters are provided, and an error signal is generated based on the plurality of signals obtained from the wavelength tunable filter. Characteristic tunable filter control method. 2. The wavelength tunable filter control method according to claim 1, wherein the number of the wavelength tunable filters is two. 3. The wavelength tunable filter control system according to claim 1, wherein the number of the wavelength tunable filters is three. 4. The tunable filter control system according to claim 1, wherein the tunable filter is a distributed feedback filter. 5. The wavelength tunable filter control method according to claim 1, wherein the center wavelength of the wavelength tunable filter is modulated by using a sinusoidal modulation signal. 6. The wavelength tunable filter control system according to claim 1, wherein the central wavelengths of the tunable filters are the same. 7. A wavelength tunable filter controller that modulates the center wavelength of a tunable filter to generate an error signal and tracks the center wavelength of the tunable filter to the wavelength of an optical signal based on the error signal. A plurality of wavelength tunable filters having different half-value widths and a means for synchronizing and modulating the central wavelengths of the plurality of wavelength tunable filters are provided, and an error signal is generated based on the plurality of signals obtained from the wavelength tunable filter. Characteristic wavelength tunable filter control device. 8. An optical communication system comprising a receiver using the variable wavelength filter control system according to claim 1. Description: