JPH082040B2 - Wavelength synchronization system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention is a wavelength synchronization control in which a combination of oscillation frequencies of a plurality of laser devices possessed by each of a plurality of local stations is controlled so that the respective local stations match each other. Regarding the system.

[Conventional technology]

Conventionally, as a method of wavelength-synchronizing a plurality of local stations, light emitted from a frequency swept light source held in a reference station and light transmitted through an optical resonator thereof are supplied to each local station and sent to each local station. A method to perform frequency interval stabilization control for multiple laser devices in the own station based on these two reference lights (Shimosaka et al., Proceedings of Autumn Meeting of IEICE 1988, Volumes B-1, B- (See page 1-188).

[Problems to be Solved by the Invention]

In the above method, there are three types of reference signals that need to be supplied from the reference station to each local station: frequency sweep light, frequency sweep light transmitted through the optical resonator, and frequency sweep timing signal. Therefore, it is necessary to provide three signal lines from the reference station to each local station, which increases the cable laying cost. Moreover, since the optical cable and the electric line are mixed, when the reference station and the local station are far apart,
The propagation delay difference between signals that should arrive at the same time cannot be ignored.

Further, the light that has passed through the optical resonator through the frequency sweep is a pulse train that reflects the transmission characteristics of the optical resonator, and the generation time within one sweep period corresponds to the resonance frequency of the optical resonator. When the reference station and the local station are far away from each other and it is necessary to pass a long fiber, the occurrence time of the pulse changes due to the influence of the chromatic dispersion of the fiber.

Therefore, the object of the present invention is to reduce the number of signal lines to be provided from the reference station to each local station to one, and to deteriorate the quality of the reference signal to be sent even if the distance between the reference station and the local station becomes long. Not to provide a wavelength synchronization system.

[Means for solving the problem]

The wavelength synchronization system of the present invention includes a plurality of local stations each having a plurality of laser devices having different oscillation frequencies, which are targets for controlling the oscillation frequency, a reference laser device that oscillates at a frequency serving as a control reference, and the reference laser device. A reference station having a first optical branching device for branching the emitted light of the laser device to the plurality of local stations; each of the plurality of local stations branching the outgoing light of the first optical brancher 2 optical splitters, a first optical multiplexer that multiplexes at least a part of the emitted light of each of the plurality of laser devices and the first emitted light of the second optical splitter, and from the outside A reference laser device that sweeps the oscillation frequency in a range including the oscillation frequencies of the plurality of laser devices according to the input signal, a third optical branching device that splits the emitted light of the reference laser device, Light of 3 A second optical multiplexer that combines the first output light of the branching device and the output light of the first optical multiplexer, and a first optical converter that converts the output light of the second optical multiplexer into an electrical signal. Photodetector, and the second optical branching device of the third optical branching device.
Of the output light of the second optical splitter and a third optical multiplexer that multiplexes the output light of the second optical splitter, and a change in the frequency of the output light of the third optical combiner into an amplitude change of the transmitted light. An optical resonator which is converted and whose resonance frequency interval is changed by an applied signal from the outside, and a second which converts the light emitted from the optical resonator into an electric signal
Of the photodetector and the reference laser device at the time of sweeping the oscillation frequency of the reference laser device at the oscillation frequency intervals of the plurality of laser devices detected by the emitted light of the second optical multiplexer at the time of sweeping the oscillation frequency of the reference laser device. A first control device for outputting a control signal for stabilizing an error with respect to a frequency interval reference set by using emitted light corresponding to the resonance peak of the optical resonator, and the first control device. A laser device driving device that changes input signals of the plurality of laser devices in accordance with a control signal from the device, and a control signal applied to the optical resonator so that the DC component of the output of the second photodetector is maximized. And a second control device for outputting.

[Action]

In the present invention, since only the light whose oscillation frequency is absolutely stabilized is transmitted from the reference station to each local station, the number of cables required to send the reference signal is one optical fiber. Further, since the reference signal to be sent is also DC light, it is not affected by the chromatic dispersion of the optical fiber, and therefore the quality of the reference signal does not deteriorate.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram of one embodiment of the present invention. In this embodiment, the number of local stations is 2 in order to simplify the explanation.
The case where each local station has three controlled laser devices will be described. 1.55 μm band distributed feedback semiconductor laser (DFB) held as a reference laser device in the reference station CS 1
The light emitted from is transmitted through the isolator 2 and then split into two by the half mirror 3. One of the branched lights is input to the first optical branching device 4 having a branching ratio of 1: 1. The other branched light is incident on a glass cell 5 having a length of 50 cm filled with ammonia gas at a pressure of 50 Torr. The light transmitted through the glass cell 5 is converted into an electric signal by the photodetector 6. This electric signal is input to the control circuit 7. The control circuit 7 outputs a control signal such that the level of the input electric signal becomes a constant value, that is, the oscillation frequency of DFB1 is held at a certain point appropriately selected on the absorption curve of ammonia gas. This control signal is converted into a current by the laser driving device 8 and injected into DFB1. By such control, the oscillation frequency of DFB1 is absolutely stabilized at one point on the ammonia absorption curve. In addition, the above-mentioned components 1-8 comprise the reference | standard station CS.

In this embodiment, two local stations LS1 and LS2 are assumed, but since these constituent elements and operations are the same as each other, only one local station LST will be described here. Of the two outputs of the first optical branching device 4 in the reference station CS, the first outgoing light 9 is sent to the first local station LS1.
The first outgoing light 9 is split into first and second outgoing lights 15 and 16 by the second optical branching device 14 having a branching ratio of 1: 1 in the local station LS1. The emitted lights of the controlled laser devices 17, 18, 19 are transmitted through the optical isolators 20, 21, 22 respectively, and then combined with the first emitted light 15 of the second optical branching device 14 by the first optical multiplexer 23. Be waved. On the other hand, the 1.55 μm band tunable distributed block reflection type semiconductor laser (DBR) 11 used as the reference laser device in the local station LS1 follows a sawtooth wave with a repetition frequency of 500 Hz applied by a sawtooth wave generator 12. The oscillation frequency changes in a sawtooth shape with respect to time. The light emitted from the DBR 11 passes through the first optical isolator 13 and is then divided by the third optical branching device 24 into the first and second emitted lights 25 and 26 at a power ratio of 1: 1. Second output light 1 of the second optical splitter 14
After passing through the variable optical attenuator 27, 6 is combined with the second emitted light 26 of the third optical branching device 24 by the third optical multiplexer 28.
The light emitted from the third optical multiplexer 28 is incident on a quartz Fabry-Perot etalon 29 whose thickness and end face reflectance are adjusted so that the finesse is 10 and the free spectral range is 10 GHz. The transmitted light of the Fabry-Perot etalon 29 is transmitted from the pulse train-like light and the reference station CS generated when the frequency of DBR11 matches the resonance frequency of the Fabry-Perot etalon 29 during one cycle of the output signal from the sawtooth wave generator 12. It is composed of DC light corresponding to the received DC light, and the sawtooth wave generator 12 is used so that the number of pulses in one cycle is four.
Adjust the peak voltage of the output of. In addition, the optical variable attenuator 27 is used to prevent the DC optical level included in the Fabry-Perot etalon 29 from exceeding the peak level of the pulsed light.
Adjust the amount of attenuation given to the DC light sent from. The transmitted light of the Fabry-Perot etalon 29 is converted into an electric signal by the second photodetector 30 and then branched into two, one to the second input terminal 33 of the first controller 31 and the other to remove the pulse waveform. Is input to the second control device 34 through the low pass filter 35. The second controller 34 controls the maximum value so that the input DC voltage becomes maximum, and drives the temperature controller 36 by its output control signal to drive the Fabry-Perot etalon 29.
The temperature of the DF is controlled and its resonance frequency is absolutely stabilized.
Absolutely stabilized to the oscillation frequency of B1. First optical multiplexer
The outgoing light of 23 is output by the second optical multiplexer 37 to the third optical splitter 24.
Is combined with the first outgoing light 25 of. The combined light is converted into an electric signal by the first photodetector 38, and then the first control device 31
Is applied to the first input terminal 32 of the.

Next, the detailed configuration and function of the first control device 31 will be described. FIG. 3 shows the first control device 31 shown in FIG.
FIG. 4 shows the detailed configuration of the pulse generation time difference measurement circuit 410 in the first controller 31 shown in FIG. In FIG. 3, the electric signal input to the first input terminal 32 is input to the low-frequency amplifier 405 having a cutoff frequency of 600 MHz. The low-pass amplifier 405 also plays the role of a low-pass filter, and the frequency of the light emitted from the DBR 11 and DFB1, 17, 18,
When the difference from the frequency of the emitted light of 19 is within a range of approximately ± 600 MHz, a pulsed electric signal is output. The number of pulses is the output signal 607,608 of the sawtooth wave generator 12 shown in FIG.
(Refer to FIG. 2) The difference between the oscillation center frequency of each of DBR11 and DFB1, 17, 18, and 19 is equal to the number of times within the range of ± 600 MHz, which is three. Hereinafter, these pulses will be referred to as first, second, third and fourth beat pulses, respectively. This pulse is envelope-detected by the envelope detection circuit 406, further shaped by the Schmitt trigger circuit 407 into a square wave having an amplitude equal to the logic level, and subsequently the polarity is inverted by the inverter circuit 408, and then a pulse is generated. It is input to the second input terminal 409 of the time difference measuring circuit 410. Meanwhile, the second
The pulsed electric signal from the photodetector 30 of
After being amplified by 1, the output level is set to a logical level by the Schmitt trigger circuit 402, the polarity is inverted by the inverter circuit 403, and the result is input to the first input terminal 404 of the pulse generation time difference measurement circuit 410. The sawtooth wave generator 12 of FIG.
The number of pulses input to the first input terminal 404 of the pulse generation time difference measurement circuit 410 in one cycle of the output signals 607 and 608 (see FIG. 2) is four as described above. These pulses are called the first, second, third and fourth reference pulses, respectively.

In this embodiment, the oscillation frequencies of the first to third DFBs 17, 18 and 19 are controlled so that the oscillation frequencies of the DFBs 17, 18 and 19 are adjusted to the second to fourth optical resonance frequencies of the optical resonator, respectively. To do. Therefore, in the control device 31, the second,
Control is performed so that the occurrence times of the third and fourth beat pulses substantially coincide with the occurrence times of the second, third, and fourth reference pulses, respectively. Hereinafter, the details will be described. FIG. 4 is a detailed configuration diagram of the pulse generation time difference measurement circuit 410 in FIG. The above-mentioned reference pulse train and beat pulse train (second
Figures (a) and (b) are the first and second input terminals, respectively.
When input to 404,409, the first and second counters 501,5
The square wave shown in FIGS. 2 (c) to (e) is output from 02. In FIG. 2, the solid line is the output of the first to third output terminals of the first counter 501 made by the reference pulse train, and the dotted line is the second output made by the beat pulse train.
Of the counter 502 from the first to third output terminals.
In these square waves, the occurrence time of each reference pulse and the beat pulse is its rising time, and the occurrence time of the next pulse is its falling time. However,
The square wave (Fig. 2 (e)) rising by the fourth reference pulse and the fourth beat pulse, which are the last pulses, is the output signal of the sawtooth wave generator 2 in Fig. 1 (Fig. 2 607). , 60
(See 8) The start time of the next cycle of the sawtooth wave is the fall time. Regarding the outputs of these counters 501 and 502, the first output is the first exclusive-OR (EX
-OR) circuit 503, the second output to the second exclusive-OR circuit 504, and the third output to the third exclusive-OR circuit 50.
Entered in 5. At this time, these first to third excludes
From the sive-OR circuits 503, 504, and 505, two square pulses are output, as shown in FIGS. 2 (f) to (h). Therefore, the pulse selecting circuit 506 selects and outputs only the leading one of these rectangular pulses, that is, the earlier rectangular pulse. Furthermore, which of the first reference pulse and the first beat pulse has occurred first in the next pulse order determination circuit 507, and similarly, which of the second and third pulses has occurred earlier? If a beat pulse is generated earlier than the reference pulse, a square pulse of positive voltage (the pulse width is the same as the output of the pulse selection circuit) is output, and the beat pulse is output for the reference pulse. When is after, it outputs a square pulse of negative voltage. This is because the oscillation frequency of DFB is controlled by the injection current to DFB, and the direction of the change in the current fed back to DFB must be changed. This pulse sequence determination circuit 507
Are output from the first to third integrators 411 and 41 of FIG. 3, respectively.
2,413 is integrated within 2 to 3 times the sweep time of the sawtooth wave generator 2 shown in FIG. 1 to determine whether the pulse width of the square pulse and the temporal relationship between the reference pulse and the beat pulse are positive or negative. Convert to negative voltage magnitude. The outputs of these integrators 411, 412, 413 are individually connected to the subsequent stages, respectively, and the laser device driving device 3 including the circuit shown in FIG.
It is connected to the input terminals 701 of 9, 40 and 41. The input terminal 70
Superimpose the signal from 1 on the DC bias current for DFB.
Drive DFB.

By performing the above operation also on the second local station LS2, the combination of the same resonance frequency of the Fabry-Perot etalon 29 controlled to have the same resonance frequency by the emitted light of DFB1 supplied from the reference station CS (2 counting from the resonance frequency that matches the oscillation frequency of DFB1 on the frequency axis
Up to the fourth combination), the oscillation frequencies of the three DFBs 17, 18, 19 of each set are stabilized.

Especially, because the oscillation frequency of the light supplied from the reference station CS to each of the local stations LS1 and LS2 is absolutely stabilized, and this is used to absolutely stabilize the resonance frequency of the optical resonator that serves as the reference for control of each local station. Therefore, all the laser devices controlled by each local station are absolutely stabilized. Moreover, even the interval frequency is absolutely stabilized between the local stations to the same value.

DFB1, DFB17, 18, 19 and DBR11 are the first to
The temperature fluctuation width is set to 0 by the fifth temperature control device 42, 43, 44, 45, 46.
The temperature is stabilized within 2 ℃.

In the above embodiment, three laser devices are set as one set, two sets are simultaneously stabilized, and two sets of laser devices having the same combination of oscillation frequencies are generated. However, the number of laser devices and the number of laser devices per set are not limited to these values, and the former adjusts the frequency and peak voltage of the output signal from the sawtooth wave generator 12 and the Fabry-Perot etalon 29 per cycle. The latter can be increased by increasing the number of pulses emitted from, and the latter by increasing the number of branches of the second optical branching device 4. Further, the frequency interval can be freely set by changing the thickness of the Fabry-Perot etalon 29. Furthermore, the laser device to be stabilized is not limited to the semiconductor laser, and any laser device whose oscillation frequency changes according to a signal from the outside can be stabilized.

〔The invention's effect〕

As described above, according to the present invention, even if the reference station and each local station are distant from each other, it is possible to perform wavelength synchronization without deterioration of the control operation, and moreover, the signal line provided between the reference station and the local station. Can be reduced to one and the economic effect is great.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention. FIG. 2 is an explanatory diagram of the operation of the pulse generation time difference measuring circuit in the same embodiment. FIG. 2 (a) is a reference pulse train, FIG. 2 (b) is a beat pulse train, and FIG. The first outputs of the counters 501 and 502 in FIG. 4, the second output of the counters 501 and 502 in FIG. 4, and the fourth output of the counters 501 and 502 in FIG.
Third output of counters 501 and 502 in the figure, FIG. 2 (f),
(G) and (h) are the outputs of the exclusive-OR circuits 503, 504, 505 in FIG. 4, respectively. FIG. 3 is a block diagram of the first controller in the same embodiment, FIG. 4 is a block diagram of the pulse generation time difference measuring circuit in FIG. 3, and FIG. 5 is a structure of a laser device driving device in the same embodiment. It is a figure. CS …… Base station, LS1, LS2 …… Local station, 1,17,18,19…
… 1.55 μm band distributed feedback semiconductor laser, 2,13,20,21,22
...... Optical isolator, 3 ... Half mirror, 4,14,24 ...
… Optical splitter, 5 …… Gas cell, 6,30, 38 …… Photodetector,
7,31,34 …… Control device, 8,39,40,41 …… Laser device driver, 11 …… 1.55μm wavelength tunable distributed Bragg reflection semiconductor laser, 12 …… Sawtooth wave generator, 23, 28,37 …… Optical multiplexer, 27 …… Optical variable attenuator, 29 …… Fabry Perot etalon, 35 …… Low pass filter, 36,42,43,44,45,46…
… Temperature control device, 401 …… Low-pass amplifier, 402,407 …… Schmidt trigger circuit, 403,408 …… Inverter circuit, 404,4
09 …… Input terminal of pulse generation time difference measurement circuit, 405 ……
Low-pass amplifier with cut-off frequency of 600MHz, 406 ... Envelope detection circuit, 410 ... Pulse generation time difference measurement circuit, 411, 412, 413 ...
… Integrator, 501,502 …… Counter, 503,504,505 …… excl
usive-OR circuit, 506 ... Pulse selection circuit, 507 ... Pulse sequence determination circuit.

Claims (1)

[Claims] 1. A plurality of local stations each having a plurality of laser devices of different oscillation frequencies, whose oscillation frequencies are to be controlled, a reference laser device that oscillates at a frequency that serves as a control reference, and the reference laser device. A reference station having a first optical branching device for branching outgoing light of the first optical branching device to the plurality of local stations; each of the plurality of local stations branching the outgoing light of the first optical branching device; An optical branching device, at least a part of the emitted light of each of the plurality of laser devices, and the second
A first optical multiplexer for multiplexing the first output light of the optical branching device of FIG. 1 and a reference for sweeping the oscillation frequency in a range including the oscillation frequencies of the plurality of laser devices according to an input signal from the outside. A laser device, a third optical splitter that splits the outgoing light of the reference laser device, a first outgoing light of the third optical splitter, and an outgoing light of the first optical multiplexer. A second optical multiplexer, a first photodetector for converting the outgoing light of the second optical multiplexer into an electric signal, a second outgoing light of the third optical branching device, and the second optical detector. A third optical combiner for combining the second output light of the second optical branching device and a frequency change of the output light of the third optical combiner into an amplitude change of the transmitted light and An optical resonator in which the resonance frequency interval is changed by an applied signal,
A second photodetector for converting the emitted light of the optical resonator into an electric signal, and the plurality of laser devices for detecting the emitted light of the second optical multiplexer when the oscillation frequency of the reference laser device is swept. A control signal for stabilizing the error with respect to the frequency interval reference set using the emitted light corresponding to the resonance peak of the optical resonator at the time of the oscillation frequency sweep of the reference laser device of the oscillation frequency interval of a constant value. A first control device for outputting, a laser device driving device for changing input signals of the plurality of laser devices according to a control signal from the first control device, and a direct current component of an output of the second photodetector. And a second control device that outputs a control signal applied to the optical resonator so as to maximize the wavelength.