JPH09261181A - Multichannel optical frequency stabilizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize not only the optical frequency interval but the absolute frequency of a light source by controlling the frequency intervals of plural light sources based on the changing cycle of transmittance of a cyclic optical filter and also stabilizing the transmission center frequency or toe filter. SOLUTION: The output of a cyclic optical filter 33 is demultiplexed by an optical demultiplexer 40 and received by the photodetectors 511 to 516. The outputs of these photodetectors are inputted to the lock-in amplifiers 521 to 526 to execute the lock-in detection based on the signals of an oscillator 34. The error signal output is acquired from the outputs of amplifiers 521 to 526 in response to a frequency shift amount δf=f1-fio (i=1, 2...5, r) set against zero output fi . Under such conditions, the output of the amplifier 526 corresponds to the difference between the reference optical frequency fr0 and the peak frequency fr of the filter 33. Then the frequency fr can be stabilized at the frequency fr0 when the output of the amplifier 526 is fed back to a filter tuning electrode 32 via an adder 35. Furthermore, the oscillation frequency of semiconductor lasers 121 to 125 can be stabilized at a prescribed absolute temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used as a light source for optical frequency multiplex (or wavelength multiplex) communication. In particular, it relates to stabilization of the oscillation frequency (oscillation wavelength).

[0002]

2. Description of the Related Art Optical Frequency Division Multiplexing (OFDM)
uency Division Multiplexing, wavelength multiplexing, WDM: Wa
(Synonymous with velength Frequency Division Multiplexing) communication method, multiplex the signals of light sources with different oscillation frequencies on the transmitting side and transmit, and separate the signal for each optical frequency by the optical demultiplexer or optical filter on the receiving side, It receives and demodulates each channel. At that time, if the oscillation frequency of the light source fluctuates, the receiving side may not be able to output a signal component to a predetermined output port of the optical demultiplexer, or crosstalk to other channels may occur. There is a problem that signals cannot be received. In order to solve such a problem, stabilizing the oscillation frequency of each light source to a predetermined value is important for performing optical frequency multiplex communication.

14 and 15 show the structure of a conventional optical frequency multiplex light source in which the oscillation frequencies of a plurality of light sources are stabilized, and the operating principle thereof. This conventional example is disclosed in JP-A-8-5.
No. 1411, a plurality of semiconductor lasers 121 to 125 are used as a light source, the output of each laser is combined by a star coupler 21, and the combined output is a Mach-Zehnder interferometer type optical filter (hereinafter referred to as ""MZfilter")
The signal is input to the output signal 33, the output is demultiplexed using the arrayed waveguide grating type optical demultiplexer 44, and then received by the photodetectors 511 to 515 for each light source output. At that time, FIG.
As shown in, the frequency intervals of the predetermined frequencies of the light sources are set to be evenly spaced, and the cycle of the peak frequency of the transmittance of the MZ filter 33 having the sine wave characteristic on the frequency axis is set to the frequency interval of the light sources. To match. Further, the oscillator 3 for oscillating the transmission characteristic of the MZ filter 33 at a constant frequency.
4 is used for modulation on the optical frequency axis. At this time, the output obtained from the photo detectors 511 to 515 is the lock-in amplifier 52 whose output is the reference of the oscillator 34.
Enter 1 to 525. As a result, the lock-in amplifier 5
The outputs of 21 to 525 are differential waveforms of the transmitted signal, and M
It becomes zero at the peak frequency of the transmittance of the Z filter 33. Therefore, an error signal output corresponding to the frequency difference from the center frequency can be obtained. By feeding back to the variable frequency power supplies 111 to 115 of the respective light sources according to the output of the error signal, the oscillation frequencies of the semiconductor lasers 121 to 125 are M.
It is possible to perform stabilization control to the peak frequency of the transmittance of the Z filter 33.

[0004]

However, in the configuration of the conventional example, the frequency (wavelength) interval of the light source can be stabilized, but the center frequency fluctuation of the MZ filter, which is the frequency reference, is not taken into consideration. There is a problem that the absolute frequency cannot be stabilized. Further, in the conventional example, it is premised that the frequency intervals of the respective light sources are constant, and the peak frequency interval of the transmittance of the MZ filter is set to match the set frequency interval of the light source. It is impossible to stabilize the optical frequency multiplex light source whose intervals are not constant.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and to provide a multi-channel optical frequency stabilizer which can stabilize the absolute frequency of a light source and can be used even when the light source frequency interval is not constant. And

[0006]

According to a first aspect of the present invention, the combined light of the output lights of a plurality of light sources whose output optical frequencies are set to different values is input, and each of the plurality of light sources is input. The optical frequency discriminating means whose transmittance changes with respect to the deviation from the optical frequency set in, the optical demultiplexing means for demultiplexing the output light of the light source from the output light of the optical frequency discriminating means, and Optical frequency error detecting means for detecting an optical frequency error of each output light of the light source from the demultiplexed light of the optical demultiplexing means, and a feedback loop for controlling each output optical frequency of the light source according to the output of the optical frequency error detecting means. (Hereinafter, referred to as “first feedback loop”), a multi-channel optical frequency stabilizer including a frequency reference light source whose output optical frequency is stabilized, and a frequency reference light source Discrimination error detection means for inputting the output light of the light source to the optical frequency discrimination means and detecting discrimination error from the discrimination output, and control means for controlling the transmission center frequency of the optical frequency discrimination means according to the output of the discrimination error detection means. (Hereinafter, referred to as “second feedback loop”) is provided.

With this configuration, not only the frequency interval of the light source can be stabilized by the first feedback loop, but also the fluctuation of the transmission center frequency of the optical frequency discriminating means can be stabilized by the second feedback loop.
The absolute frequency of the light source can be stabilized.

The optical frequency discriminating means includes means for modulating the transmission characteristics of the optical frequency discriminating means, and the optical frequency error detecting means and the optical discriminating error detecting means respectively receive the demultiplexed light of the optical demultiplexing means and the light receiving means. It is preferable to include means for performing phase-coherent detection of the output of the light receiving means by the modulation signal.

Separately from this, the optical frequency discriminating means includes an optical filter having a structure in which complementary outputs are obtained at the two output ports, and the optical demultiplexing means is provided at each of the two output ports of the optical filter. Of the optical frequency error detecting means and the optical discriminating error detecting means, and the same light source as the light receiving means provided corresponding to the two output ports of the optical filter, respectively. It is also possible to include means for outputting the difference or ratio component of the two light receiving means corresponding to the optical frequency component of. With this configuration, the light source frequency can be stabilized without modulating the optical frequency discriminating means. In this case, the optical demultiplexing means includes an array waveguide grating type optical demultiplexer provided with a plurality of input / output ports facing each other, and a pair of facing input / output ports of the array waveguide grating type optical demultiplexer. The two output ports of the optical filter may be connected to the optical filter, and the light receiving means may be connected to input / output ports other than the one set of input / output ports of the arrayed waveguide grating type optical demultiplexer.

The light demultiplexing means can be formed as the same element as the means for combining the output lights of a plurality of light sources.

According to a second aspect of the present invention, the combined light of the output lights of a plurality of light sources whose output optical frequencies are set to different values is input, and the optical frequencies set for the plurality of light sources are input. From the optical frequency discriminating means, the optical demultiplexing means for demultiplexing the output light of the plurality of light sources from the output light of the optical frequency discriminating means, and the optical demultiplexing means. Optical frequency error detecting means for detecting the optical frequency error of the output light of each of the plurality of light sources from the demultiplexed light, and a feedback loop for controlling the output optical frequency of each of the plurality of light sources according to the output of the optical frequency error detecting means. In the multi-channel optical frequency stabilization device including the optical frequency discriminating means, the intervals between adjacent optical frequencies of the plurality of light sources are set differently, and the optical frequency discriminating means is set differently. Multi-channel optical frequency stabilizing device which comprises a periodic optical filter whose transmittance varies with a period equal to twice the common divisor of one or divisor thereof intervals of the light frequency is provided.

The second aspect is preferably implemented in combination with the first aspect, but may be implemented independently.

As the periodic optical filter, a periodic filter composed of a Mach-Zehnder interferometer, a Fabry-Perot interferometer or a ring resonator can be used.

According to a second aspect of the present invention, a periodic optical filter is used as an optical frequency discriminating means, and the period or a half of the period is set to a common divisor of different frequency intervals of a plurality of light sources. Set to be one. When the period of the periodic optical filter is set to be a common divisor of the frequency interval of the light source, in order to stabilize the output optical frequency of each light source, the frequency range in which the transmittance characteristics of the periodic optical filter become equal Is used to stabilize each light source.
When the frequency interval of 1/2 of the period of the periodic optical filter is set to be the common divisor of the frequency interval of the light source, not only in the frequency region where the transmittance characteristics of the periodic optical filter become equal, The frequency domain in which the transmittance characteristic is inverted is also used.

As described above, by using the periodic optical filter whose transmittance changes in one of the common divisors of the intervals of the optical frequencies or in a cycle equal to twice the common divisor, the optical frequencies are arranged at unequal intervals. In the case of the above, each light source can be stabilized at a predetermined frequency.

Consider, for example, an optical frequency multiplex communication system using a wavelength band of 1.5 μm as a light source. In such a system, when a dispersion-shifted fiber having a zero-dispersion wavelength near 1.5 μm is used as a transmission line, good transmission characteristics without wavelength deterioration due to dispersion can be obtained even when a high-speed modulated signal is transmitted over a long distance. Obtainable. On the other hand, since the phase matching is achieved between the respective light source wavelengths, a four-wave mixing based on the third-order nonlinearity of the refractive index of the fiber causes a frequency component matching the signal frequency. Therefore, when the light source frequencies are arranged at equal frequency intervals, the four-wave mixed light is generated at the same frequency as the signal frequency, causing crosstalk to the signal and causing deterioration of the transmission characteristics.
Therefore, in order to avoid such a situation, it has been proposed by Shibata et al. And Forzieri et al. To arrange the light source frequencies at unequal intervals. Reference 1: Nobu Shibata, Ralf Brown, Robert Waltz, "Efficiency of Four-Wave Mixed Light Generation in Frequency-Multiplexed Coherent Transmission Systems", IEICE Technical Report OQE86-8
3, September 29, 1986 Reference 2: F. Forghiei, RWTkach, ARChraplyvy and
D. Marcuse, "Reduction of four-wave-mixing cross ta
lk in WDM systems using unequally spaced channel
s ", OFC / IOOC '93 Technical Digest, pp.252-253, Fe
b.21-26, 1993 Among them, in the frequency array example shown in Document 2, a frequency grid of 25 GHz is set, and a multiple value thereof is adopted as the frequency interval. In such a case, 1 cycle or 1/2 cycle of the periodic filter is set to 25G.
It may be set to Hz.

According to the second aspect of the present invention, each frequency interval is a natural number times one cycle or one half cycle of the frequency filter. Therefore, it is possible to stabilize the light source frequencies arranged at unequal frequency intervals while maintaining the optical frequency discrimination sensitivity.

[0018]

FIG. 1 is a block diagram showing a first embodiment of the present invention, showing a plurality of light sources and a multi-channel optical frequency stabilizer for stabilizing the optical frequencies thereof.

The output light frequencies of the plurality of light sources 11 to 1n are set to different values, and these output lights are combined by the optical combiner 20 and output. This combined light is used for optical frequency multiplex communication, for example, and a part of it is branched by the optical coupler 22 and input to the multi-channel optical frequency stabilizer.

In the multi-channel optical frequency stabilizer, the transmittance changes with respect to the deviation from the optical frequency set for each of the plurality of light sources 11 to 1n, and the respective frequency fluctuations of the combined light are summarized. Of the optical frequency discriminator 30 for converting the output light of the optical frequency discriminator 30 into output fluctuations of the light sources 11 to 1n, and the optical demultiplexer 40. Demultiplexed light source 11-11n
Error detection circuits 51 to 5n for detecting optical frequency errors (frequency fluctuations) of the respective output lights, and first to control respective output light frequencies of the light sources 11 to 1n according to outputs of the error detection circuits 51 to 5n. Of the periodic optical filter that constitutes the optical frequency discriminator 30 and stabilizes the absolute frequency of the light source, thereby stabilizing the output optical frequency. 31, an optical coupler 23 that combines the output light of the frequency reference light source 31 into a combined light of the output lights of the light sources 11 to 1n, and the output light of the frequency reference light source 31 from the output light of the optical frequency discriminator 30. Error detection circuit 5r for detecting a discrimination error and a second feed bar for controlling the transmission center frequency of the optical frequency discriminator 30 according to the output of the error detection circuit 5r. And a croup. Light sources 11-1
The optical frequency discriminator 30 sets one of the common divisors of the differently set optical frequency intervals or 2 of the common divisors thereof.
It is composed of a periodic optical filter whose transmittance changes at a cycle equal to twice.

In this embodiment, the first feedback loop stabilizes the optical frequency intervals output from the light sources 11 to 1n, and the second feedback loop stabilizes the absolute frequency. Further, due to the transmittance characteristic of the optical frequency discriminator 30, even if the intervals between the adjacent light frequencies of the light sources 11 to 1n are different, the oscillation light frequency thereof can be stabilized.

FIG. 2 is a block diagram showing the second embodiment of the present invention. In this embodiment, the frequency reference light source 3
The output of No. 1 is combined with the output light of each of the light sources 11 to 1n by the optical multiplexer 20, which is greatly different from the first embodiment. In this case, the optical filter 6 is output from the combined output for communication.
The frequency component ( _fr0 ) from the frequency reference light source 31 due to ₀
Is removed.

In the above embodiment, as the optical multiplexer 20, a star coupler may be used as in the conventional example, or a waveguide grating type multiplexer or the like may be used. Optical multiplexer 20
When the star coupler is used as the above, the optical coupler 22 becomes unnecessary, and the other output of the optical multiplexer 20 can be used as the combined output as in the conventional example. Further, as the optical demultiplexer 40, an optical filter, a multi-stage connection MZ filter, an arrayed waveguide grating type optical demultiplexer, or the like can be used.

FIG. 3 is a block diagram showing the third embodiment of the present invention. This embodiment is different from the above-described embodiments in that the multiplexing and demultiplexing of the output lights of the light sources 11 to 1n and the frequency reference light source 31 are realized by the same arrayed waveguide grating type optical multiplexer / demultiplexer 41. By appropriately designing the input / output wavelength characteristics of the arrayed waveguide grating type optical multiplexer / demultiplexer 41, mutually different oscillation frequencies f _{1 to} f _n and fr ₀
When the output lights from the light sources 11 to 1n and the frequency reference light source 31 are input to the input ports a1 to an and ar of the arrayed waveguide grating type optical multiplexer / demultiplexer 41, respectively, they are multiplexed to the output side port b0. Is output. The combined output is branched into two outputs by the optical coupler 30, and one of them is discriminated by the optical frequency discriminator 30. Further, the output of the optical frequency discriminator 30 is input to the port a0 on the input side of the arrayed-waveguide grating optical multiplexer / demultiplexer 41 through the loopback optical path 70. Multiplexed light input to port a0 is the reciprocity of the transmission characteristics of the arrayed waveguide grating type optical demultiplexer 41, to the output side of the port b1~bn and br, optical frequency f ₁ ~f _n and f _r0 The component of is split and output. These demultiplexed discriminated lights are respectively light sources 11 to 1n for the optical frequency discriminator 30 by the error detection circuits 51 to 5n.
Of the optical frequency discriminator 30 with respect to the frequency reference light source 31 is detected by the error detection circuit 5r. The outputs of the error detection circuits 51 to 5n are fed back to the frequency varying means incorporated in the light sources 11 to 1n, and the outputs of the error detecting circuit 5r are fed back to the frequency characteristic varying means incorporated in the optical frequency discriminating means 30. Optical frequencies of ~ 1n are stabilized to a predetermined absolute frequency.

FIG. 4 is a block diagram showing the fourth embodiment of the present invention. In this embodiment, the output light from the light sources 11 to 1n and the output light from the frequency reference light source 31 are not collectively multiplexed, but an optical coupler 25 provided separately from the arrayed waveguide grating type optical multiplexer / demultiplexer 41 is used. The difference from the third embodiment is that the output light of the frequency reference light source 31 is combined with the combined light of the light sources 11 to 1n.

[0026]

EXAMPLES Specific examples of the present invention will be described in detail below.

FIG. 5 is a block diagram showing the first embodiment. In this embodiment, semiconductor lasers 121 to 125,
An optical multiplexer 20 that multiplexes those outputs, an optical coupler 22 that splits the combined outputs into two outputs for use in taking out the output and frequency stabilization, and a frequency reference light source 3
1 and the output light of each of the semiconductor lasers 121 to 125 (optical frequencies f ₁ , f ₂ , f ₃ , f ₄ , f ₅ ) and the output light of the frequency reference light source 31 (optical frequency f _r0 ) are multiplexed. The optical coupler 23, the MZ filter 33 for optical frequency discrimination provided with the MZ filter tuning electrode 32, the oscillator 34 for modulating the MZ filter 33, and the MZ filter 3
Optical demultiplexer 41 for demultiplexing the optical frequency multiplexed signal transmitted through
, Photodetectors 511 to 516 that receive the demultiplexed signal components, and lock-in amplifiers 521 to 521 that extract the fluctuation components of the optical frequencies from the outputs of the photodetectors 511 to 516 using the signal of the oscillator 34 as a reference signal.
26, variable frequency power supplies 111 to 115 that change the bias currents or temperatures of the semiconductor lasers 121 to 125 based on the outputs of the lock-in amplifiers 521 to 525, and the output of the lock-in amplifier 526 based on the output of the lock-in amplifier 526. It is composed of an adder 35 that detects a frequency fluctuation component, adds the output of the oscillator 34, and applies a bias to the MZ filter tuning electrode 32.

The first feature of this embodiment is that the MZ filter 3 is used.
A first feedback loop for stabilizing the frequency intervals of the plurality of semiconductor lasers 121 to 125 using the periodic frequency characteristic of No. 3, and a second feedback loop for stabilizing the MZ filter 33 by the frequency reference light source 31. It is equipped with a loop. Here, the oscillation frequency of the frequency reference light source 31 and another frequency multiple light source (semiconductor laser 121
~ 125), the set frequency interval is also the MZ filter 3
It is set to be a natural number multiple of the period of the frequency transmission characteristic of 3 or a natural number multiple of 1/2 period. As the frequency reference light source 31, for example, a light source whose frequency is stabilized by the absorption line of HCN or C ₂ H ₂ existing in the 1.5 μm wavelength band is used. In such a light source, the transmission characteristics of the MZ filter 33 are stabilized, so that not only the frequency interval can be stabilized, but also the absolute value of the light source frequency can be stabilized.

The second characteristic of the present embodiment is that the frequency arrangements set for the semiconductor lasers 121 to 125 are unequal intervals, and the periodic transmission characteristics of the MZ filter 33 have the unequal intervals. Common divisor of frequency intervals or 2
It is set to double. That is, the frequency intervals of the non-equidistant arrangement are a natural number multiple of the period of the frequency transmission characteristic of the MZ filter 33, or a natural number multiple of 1/2 period. As a result, even if the frequency arrangement is unequal intervals, each can be stabilized.

FIG. 6 shows the relationship between the light source frequency arrangement and the transmission characteristics of the optical frequency discriminator (MZ filter 33). Here, the MZ filter 3 used as an optical frequency discriminator
The half period of the periodic transmission characteristic of No. 3 is the semiconductor laser 12
Optical frequencies f ₁₀ , f ₂₀ , f ₃₀ , f set to 1 to 125
₄₀ and f ₅₀ are common divisors of the frequency interval, and the frequency interval between the peak frequency and the bottom frequency of the MZ filter 33 is a frequency grid Δf _g , and each frequency interval is 3Δ.
It is assumed that f _g , 6Δf _g , 5Δf _g , and 2Δf _g . Further, the reference light frequency f _r0 from the frequency reference light source 31 is set to f _r0
= F ₅₀ + Δf _g . By matching one peak frequency or bottom frequency of the transmission characteristics of the MZ filter 33 with the reference optical frequency f _r0 , the transmission characteristics of the MZ filter 33 can be stabilized. Further, the output light frequency f ₁ of the semiconductor lasers 121 to 125 is set to the peak frequency or the bottom frequency of the MZ filter 33 whose transmission characteristic is stabilized.
By matching f ₂ , f ₃ , f ₄ , and f ₅ , respectively,
Not only the frequency interval but also the absolute value of the frequency can be stabilized.

The operating principle of this embodiment will be described in more detail. Semiconductor lasers 121 to 1 whose frequency is to be stabilized
The respective output lights of 25 and the frequency reference light source 31 are multiplexed by the optical couplers 22 and 23 and are collectively input to the MZ filter 33. The optical frequencies f _{1 to} f ₅ of the semiconductor lasers 121 to 125 are predetermined frequencies f _{10 to} f _50, respectively.
Set in the vicinity of in advance. On the other hand, MZ Furuta 3
One peak frequency f _r of the transmission characteristics of 3 is set in the vicinity of the frequency reference source f _r0. Furthermore, the transmission characteristic of the MZ filter 33 is finely modulated by supplying the output of the oscillator 34 to the MZ filter tuning electrode 32. The MZ filter tuning electrode 32 is a thin film heater formed on the waveguide of the MZ filter 33, and when a bias current is applied to the thin film heater to generate heat, the center frequency of the filter transmission characteristic can be shifted by the thermo-optic effect. it can. The output of the MZ filter 33 is demultiplexed by the optical demultiplexer 40 and received by the photodetectors 511 to 516. These outputs are locked in amplifiers 521 to 526.
The lock-in detection is performed by using the signal of the oscillator 34 as a reference signal. The outputs of the lock-in amplifiers 521 to 526 are f ₁₀ , f ₂₀ , f ₃₀ , f ₄₀ , and f ₅₀ as shown in FIG.
And f _r0 are set to zero output, and the frequency shift amount δf from that
_{i = f i -f i0 (i} = 1,2,3,4,5, r) becomes the error signal output corresponding to the. In particular, δf _i is sufficiently small f _i0
In the vicinity of, an error signal output proportional to δf _i is obtained,
This output can also be used as an oscillation frequency monitor for each light source. Of these error signals, the lock-in amplifier 5
The output of 26 is the MZ filter 3 from the reference optical frequency f _r0.
3 corresponds to the deviation of the peak frequency _fr , and by feeding this back to the MZ filter tuning electrode 32 through the adder 35, the peak frequency of the MZ filter 33 can be stabilized to the reference optical frequency. Further, the outputs of the lock-in amplifiers 521 to 525 are fed back to the bias currents or operating temperatures of the semiconductor lasers 121 to 125 through the variable frequency power supplies 111 to 115, respectively, so that the semiconductor lasers 121 to 125 are fed.
It is possible to stabilize the respective oscillation frequencies of the above to a predetermined absolute frequency.

FIG. 7 is a block diagram showing the second embodiment. In the first embodiment, the output of the frequency reference light source 31 is combined with the output light of the semiconductor lasers 121 to 125 and is collectively input to the MZ filter 33, whereas in the second embodiment, the semiconductor lasers 121 to 125 are used. The combined light from 125 is MZ
Input to the port C1 of the filter 33, and the frequency reference light source 3
By inputting the output of 1 to port C2,
The combined light is obtained at the port C3. With this configuration, the optical coupler 23 for multiplexing the output light of the frequency reference light source 31 becomes unnecessary, and the number of parts can be reduced.

FIG. 8 is a block diagram showing the third embodiment. This embodiment is different from the first embodiment in the signal path of the output light of the frequency reference light source 31. That is, the output of the frequency reference light source 31 is input to the port C4 of the MZ filter 33 via the optical isolator 37, and the light output from the port C2 is received by the photodetector 516. The output light of the semiconductor laser 121 to 125 (the optical frequency f ₁ ~f ₅₎
Of the output light of the frequency reference light source 31 (optical frequency f _r0 )
Since the propagation directions in the Z filter 33 are different, only the frequency component from the frequency reference light source 31 is input to the photodetector 516, and interference is prevented. In addition, the optical demultiplexer 40
Does not require a port for frequency reference optical demultiplexing, and the configuration can be simplified. The principle of frequency stabilization is the same as in the first and second embodiments.

FIG. 9 is a block diagram showing the fourth embodiment. In this embodiment, without modulating the MZ filter 33, the difference or logarithmic difference component of each light source frequency component obtained from its two output ports C3 and C4 (a component obtained by converting into a logarithm and taking a difference, This is different from the first embodiment in that the error signal component from the predetermined frequency is obtained by using (the same as obtaining the ratio of two). The configuration of this embodiment is similar to that of the first embodiment in that the output light of the semiconductor lasers 121 to 125 and the frequency reference light source 31 are coupled by the optical coupler 23.
Of the output light of the MZ filter 33 and is input to the port C1 of the MZ filter 33. The combined light whose optical frequency is discriminated by the MZ filter 33 is output to the ports C3 and C1 and demultiplexed by the optical demultiplexers 41 and 42, respectively. The demultiplexed outputs of the optical demultiplexer 41 are made photodetectors 511 to 516, and the demultiplexed outputs of the optical demultiplexer 42 are made photodetectors 531 to 531.
6, and each of the frequency components is input to the differential or logarithmic differential amplifiers 514 to 546 to obtain a difference or a logarithmic difference. Among them, the output of the differential or logarithmic differential amplifier 546 corresponding to the frequency error between the frequency reference light source 31 and the MZ filter 33 is fed back to the MZ filter tuning electrode 32, and the frequency characteristic of the MZ filter 33 is adjusted by the frequency reference light source 31. Stabilize to the reference light frequency. The outputs of the differential or logarithmic differential amplifiers 541 to 545 are fed back to the variable frequency power supplies 111 to 115 to stabilize the frequencies of the semiconductor lasers 121 to 125.

FIG. 10 shows the relationship between the frequency arrangement of the semiconductor lasers 121 to 125 and the MZ filter 33 and the difference between the outputs of the MZ filter 33 (error signal output). When a signal is input to the port C1 of the MZ filter 33, the port C
The output characteristic of No. 3 and the output characteristic of the port C4 have a complementary relationship, and ideally the output levels are equal at a frequency at which the transmittance is 1/2 of the peak. The frequency satisfying such a condition (herein referred to as “1/2 output frequency”) is periodically present at the grid frequency Δf _g . Therefore, the oscillation frequencies f ₁₀ and f set in the semiconductor lasers 121 to 125 are set.
₂₀ , f ₃₀ , f ₄₀ , and f ₅₀ and the reference optical frequency f _r0 of the MZ filter 33 are set so as to have this 1/2 output frequency. To this end, first, the semiconductor lasers 121 to 12
5 each one half the output frequency f _r of the oscillation frequency f ₁ ~f ₅ and MZ filter 33 is set in the vicinity of f ₁₀ ~f _50, f _r0 advance, respectively. At this time, from the difference between the outputs of the two ports C3 and C4 of the MZ filter 33, as shown in the lower part of FIG. 10, f _{10 to} f ₅₀ ,
the f _r0 is set to zero output, frequency error _{δf i = f i -f i0 (} i
= 1,2,3,4,5, r).

The present embodiment is characterized in that it is not necessary to modulate the MZ filter 33 as compared with the first to third embodiments. Therefore, it is not necessary to use the MZ filter tuning electrode 32 for the purpose of both modulation and frequency stabilization, and the frequency can be stabilized by feedback controlling the temperature of the MZ filter 33.

FIG. 11 is a block diagram showing the fifth embodiment. In this embodiment, the semiconductor lasers 121 to 1 are used.
25 and the MZ filter 33, and the combined light is discriminated at the optical frequency through the MZ filter 33 for port C.
The configuration for outputting to C3 and C4 is the same as that of the fourth embodiment.
This embodiment is different from the fourth embodiment in that an arrayed waveguide grating type optical demultiplexer 44 with 7 inputs and 7 outputs is used as the optical demultiplexer. With this configuration, the functions of the two optical demultiplexers 41 and 42 in the fourth embodiment can be put together.
The number of parts can be reduced. The arrayed-waveguide-grating optical demultiplexer 44 has left-right reciprocal demultiplexing characteristics, and by inputting the output light of the port C3 of the MZ filter 33 to the port b1, the ports a2, a3, a4, a5, a6, The frequency components of f ₁ , f ₂ , f ₃ , f ₄ , f ₅ and f _r0 are output to a7. Also, the port C of the MZ filter 33
By inputting the output light of 4 to the port a1,
2, b3, b4, b5, b6, and b7 have f ₁ and f, respectively.
The frequency components of ₂ , f ₃ , f ₄ , f ₅ and f _r0 are output. These demultiplexed lights are detected by the photo detectors 511 to 51.
6, 531 to 536, and stabilizes the frequencies of the semiconductor lasers 121 to 125 and the MZ filter 33 as in the fourth embodiment.

FIG. 12 is a block diagram showing the sixth embodiment. In this embodiment, the output light of the frequency reference light source 31 can be input to the port C2 of the MZ filter 33 as shown in FIG.
Different from the fourth embodiment shown in FIG. With this configuration, similarly to the second embodiment shown in FIG. 7, the optical coupler 23 for multiplexing the output light of the frequency reference light source 31 becomes unnecessary, and the number of parts can be reduced.

In FIG. 12, the optical demultiplexers 41 and 42 are shown separately, but an arrayed-waveguide grating type optical demultiplexer having input / output terminals of at least twice the number of optical multiplexes is used. Therefore, due to its periodicity, one optical demultiplexer 41, 42
Can play the role of.

FIG. 13 is a block diagram showing the seventh embodiment. In this embodiment, the output light of the frequency reference light source 31 can be input to the port C2 of the MZ filter 33 as shown in FIG.
It is different from the fifth embodiment shown in FIG. In this configuration, similarly to the second embodiment shown in FIG. 7 and the sixth embodiment shown in FIG. 12, the optical coupler 23 for multiplexing the output light of the frequency reference light source 31 is unnecessary, and the number of parts is reduced. Can be reduced.

In each of the sixth and seventh embodiments, the MZ filter tuning electrode 32 is feedback-controlled to stabilize the frequency of the MZ filter 33.
The temperature of the MZ filter 33 may be controlled and the temperature control circuit may be feedback-controlled.

Although the specific examples of the first and second embodiments have been described in the above examples, similar configurations are possible in the third and fourth embodiments.

[0043]

As described above, in the multi-channel optical frequency stabilizing device of the present invention, in addition to controlling the frequency intervals of a plurality of light sources according to the cycle of change in transmittance of the periodic optical filter, By stabilizing the transmission center frequency of the filter, not only the optical frequency interval of the light source but also the absolute frequency can be stabilized.

Further, the frequency intervals of the light sources are set to be unequal intervals by setting the frequency interval of the light source to be a value which is a natural number multiple of the cycle of the transmittance change of the periodic optical filter or its 1/2 cycle. Even in the case, the oscillation frequency interval of each light source can be stabilized. Therefore, it is possible to improve frequency stability in the case of performing frequency-division multiplex transmission while avoiding generation of four-wave mixed light.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

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

FIG. 3 is a block configuration diagram showing a third embodiment of the present invention.

FIG. 4 is a block configuration diagram showing a fourth embodiment of the present invention.

FIG. 5 is a block diagram showing the first embodiment.

FIG. 6 is a diagram showing a relationship between a light source frequency arrangement and a transmission characteristic of an MZ filter.

FIG. 7 is a block diagram showing a second embodiment.

FIG. 8 is a block diagram showing a third embodiment.

FIG. 9 is a block diagram showing a fourth embodiment.

FIG. 10 is a diagram showing a relationship between a frequency arrangement of a semiconductor laser and an MZ filter and a difference between outputs of the MZ filter.

FIG. 11 is a block diagram showing a fifth embodiment.

FIG. 12 is a block diagram showing a sixth embodiment.

FIG. 13 is a block diagram showing a seventh embodiment.

FIG. 14 is a block diagram showing a conventional example.

FIG. 15 is a diagram showing an operating principle of a conventional example.

[Explanation of symbols]

 11-1n Light source 111-115 Variable frequency power supply 121-125 Semiconductor laser 20 Optical multiplexer 21 Star coupler 22-25 Optical coupler 30 Optical frequency discriminator 31 Frequency reference light source 32 NZ filter tuning electrode 33 MZ filter 34 Oscillator 35 Adder 40, 42, 43 Optical demultiplexer 41 Array waveguide grating type optical demultiplexer 44 Array waveguide grating type optical demultiplexer 51-5n, 5r Error detection circuit 511-516, 531-536 Photodetector 521-526 Lock-in Amplifier 541-546 Differential or logarithmic differential amplifier 60 Optical filter

Continuation of the front page (51) Int.Cl. ⁶ Identification code Reference number within the agency FI Technical display area H04J 14/00 14/02 // H01S 3/18

Claims (7)

[Claims] 1. A combined light of output lights of a plurality of light sources whose output optical frequencies are set to different values is input, and each of the plurality of light sources transmits a deviation from the set optical frequency. Optical frequency discriminating means for changing the rate, optical demultiplexing means for demultiplexing the output light of each of the plurality of light sources from the output light of the optical frequency discriminating means, and the plurality of demultiplexed light of the optical demultiplexing means An optical frequency error detecting means for detecting an optical frequency error of each output light of the light source, and a feedback loop for controlling an output optical frequency of each of the plurality of light sources according to the output of the optical frequency error detecting means are provided. In a channel optical frequency stabilizing device, a frequency reference light source whose output optical frequency is stabilized, and output light of this frequency reference light source are input to the optical frequency discriminating means for discrimination thereof. A multi-channel optical frequency stabilizer, comprising: a discrimination error detecting means for detecting a discrimination error from a force; and a control means for controlling a transmission center frequency of the optical frequency discriminating means according to an output of the discrimination error detecting means. Device. 2. The intervals of the optical frequencies adjacent to each other of the plurality of light sources are set differently, and the optical frequency discriminating means sets one of the common divisors of the differently set intervals of the optical frequencies or the common divisor of the common divisors. The multi-channel optical frequency stabilizing device according to claim 1, further comprising a periodic optical filter whose transmittance changes at a cycle equal to twice. 3. A means for modulating the transmission characteristic of the optical frequency discriminating means with a modulation signal, wherein the optical frequency error detecting means and the optical discriminating error detecting means respectively receive the demultiplexed light of the optical demultiplexing means. 3. The multi-channel optical frequency stabilizing device according to claim 1 or 2, further comprising: a light receiving unit for performing phase-locked detection of the output of the light receiving unit by the modulation signal. 4. The optical frequency discriminating means includes an optical filter having a structure in which complementary outputs are obtained at the two output ports, and the optical demultiplexing means outputs the output light from each of the two output ports of the optical filter. At least one of the optical frequency error detection means and the optical discrimination error detection means, and the same light source as the light receiving means provided corresponding to the two output ports of the optical filter. 1 or 2 including means for outputting a difference or ratio component of the two light receiving means corresponding to the optical frequency component of
The multi-channel optical frequency stabilizer described. 5. The optical demultiplexing means includes an arrayed waveguide grating type optical demultiplexer provided with a plurality of input / output ports facing each other, and one set of the arrayed waveguide grating type optical demultiplexer facing each other. 5. The two output ports of the optical filter are connected to the input / output port, and the light receiving means is connected to an input / output port other than the one set of input / output ports of the arrayed waveguide grating type optical demultiplexer. The multi-channel optical frequency stabilizer described. 6. The optical demultiplexing means is formed as the same element as the means for multiplexing the output light of the plurality of light sources.
6. The multi-channel optical frequency stabilizing device according to any one of 1 to 5. 7. The composite light of the output light of a plurality of light sources whose output optical frequencies are set to different values is input, and each of the plurality of light sources is transmitted with respect to the deviation from the set optical frequency. Optical frequency discriminating means for changing the rate, optical demultiplexing means for demultiplexing the output light of each of the plurality of light sources from the output light of the optical frequency discriminating means, and the plurality of demultiplexed light of the optical demultiplexing means An optical frequency error detecting means for detecting an optical frequency error of each output light of the light source, and a feedback loop for controlling an output optical frequency of each of the plurality of light sources according to the output of the optical frequency error detecting means are provided. In the channel optical frequency stabilizing device, the intervals between adjacent optical frequencies of the plurality of light sources are set differently, and the optical frequency discriminating means is set differently. Multi-channel optical frequency stabilizing device which comprises a periodic optical filter whose transmittance varies with a period equal to twice the common divisor of one or divisor thereof frequency interval.