JPH0682882B2 - Method and apparatus for stabilizing oscillation frequency intervals of a plurality of laser devices - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oscillation frequency interval stabilizing device for a plurality of laser devices, which stabilizes the intervals between the oscillation frequencies of the plurality of laser devices.

(Prior Art) Conventionally, as a method of stabilizing the frequency intervals of a plurality of laser devices, the oscillation frequency of one laser device is stabilized with respect to the Fabry-Perot resonator, and A method of stabilizing the oscillation frequencies of other laser devices so that their frequency intervals match the frequency interval criteria given by the free spectral range of another Fabry-Perot resonator (Toba et al. National Conference Proceedings, Volume 2, pages 2-204), or stabilize the frequency of one laser device and combine it with the light emitted from several other laser devices, and further combine this light and frequency with a sawtooth shape with a constant cycle. The stabilized laser is generated by combining the emitted light of the reference laser device that is swept to the pulse laser, and the appearance time of each pulse forming a pulse train obtained as a beat signal. To stabilize the oscillating frequency interval of each laser device by monitoring whether or not there is a constant time difference with respect to the appearance time of the pulse corresponding to the laser (see St. Label et al. -C-O-C ''85 (IOOC-ECOC'85) Technical Digest Vol. 3 (1985) p. 61) is known.

(Problems of the prior art) However, in the above-mentioned first method, it is necessary to sweep and use the mirror interval of the Fabry-Perot resonator that gives the reference of the frequency interval, and the device is more than the case where a simple etalon plate is used. Becomes larger. In the second method,
Since the reference of the frequency interval is calculated as the appearance time interval of each pulse with respect to the frequency change of the reference laser device, it is hard to say that the frequency interval of each laser device can be estimated.

(Object of the Invention) An object of the present invention is to solve the above problems, and instead of sweeping the mirror spacing of a Fabry-Perot resonator,
The frequency of the reference laser device is swept, and the Fabry-Perot resonator can be downsized by using a simple etalon plate.By using the Fabry-Perot resonator as the frequency interval reference, the frequency interval can be stabilized. The purpose is to set.

(Structure of the Invention) In order to achieve the above-mentioned object, the present invention draws each oscillation frequency of a plurality of laser devices to be controlled so that their intervals are substantially constant, and then the plurality of laser devices are controlled. It is characterized in that the oscillating frequency interval is converted into a time interval and the error signal obtained by comparing this time interval with the time corresponding to the frequency interval reference is controlled to be a constant value.

In response to an input signal from the outside, a reference laser device that sweeps the oscillation frequency in a range including the oscillation frequency of a plurality of laser devices that are targets for stabilizing the oscillation frequency interval, and the emitted light from the plurality of laser devices A first optical multiplexer for multiplexing, an optical switch for switching an optical path for outputting an output from the first optical multiplexer to one of optical paths in accordance with a control signal from the outside, and a first optical switch for the optical switch. An optical circuit that multiplexes the output of the reference laser device and the output of the reference laser device, and branches this into a plurality of optical paths; and a high finesse optical resonator that converts a frequency change of the first output of the optical circuit into an amplitude change, A low finesse optical resonator that converts a frequency change of the second output of the optical circuit into an amplitude change, and a second optical multiplexing device that combines the third output of the optical circuit and the second output of the optical switch. And at least the low finesse light Using the output from the resonator, the oscillation frequencies of the plurality of laser devices are brought close to the resonance frequency of the high finesse optical resonator, and then the second optical multiplexing when the oscillation frequency of the reference laser device is swept. Of the oscillation frequency intervals of the plurality of laser devices detected by the output of the resonator, the frequency interval reference set by using the optical output corresponding to the resonance peak of the high finesse optical resonator during the oscillation frequency sweep of the reference laser device. Applied to the optical switch for stabilizing the error with respect to
And a laser device driving device that changes input signals to the plurality of laser devices according to the second or less control signal from the controller. It is characterized by being configured.

(Operation) In the present invention, by adopting the above-mentioned configuration, the reference pulse train of the time difference corresponding to the frequency interval reference determined by the free spectral range of the optical resonator by the combination of the frequency-swept reference laser device and the optical resonator. Generate. Moreover, after the beat light of the emitted light of the reference laser device and the emitted light of the plurality of laser devices to be controlled is received by the photodetector, the low-pass filter is passed to correspond to the frequency intervals of the plurality of laser devices. By generating a pulse train with a time difference and controlling the difference between the generation time of each pulse constituting it and the generation time of the corresponding pulse of the above-mentioned reference pulse train as an error signal, the frequency interval of any number of laser devices is stabilized at the same time. And its frequency spacing is more strictly defined than the optical resonator used.

(Examples) Hereinafter, the present invention will be described in detail with reference to Examples. FIG. 1 is a block diagram of an embodiment of the present invention.

Since the control method is divided into two stages, the steps from the first stage will be described here in order. A 1.55 μm wavelength tunable distributed Bragg reflector semiconductor laser (DBR-LD) 1 is a signal applied by a sawtooth wave generator 2 (see 5 in FIGS. 3 (a) and 3 (b)).
0,51), the output light frequency changes in a sawtooth wave shape with respect to time, but in the first stage of control, the sawtooth wave generator is turned off in accordance with a control signal supplied from the control device 3. Therefore, it does not emit light. For the structure and characteristics of the wavelength tunable distributed Bragg reflection type semiconductor laser, see, for example, Electronics Letters Vol. 23, No. 8
Details on the article by Murata et al. On page 403. On the other hand, the 1.55 μm band distributed feedback semiconductor laser (DFB-
The LDs 4, 5 and 6 emit light through the first and second stages of control. Light emitted from the DFB-LDs 4, 5 and 6 passes through the optical isolators 7, 8 and 9, respectively, and then is multiplexed by the first optical multiplexer / demultiplexer 10 having 3 inputs and 2 outputs. The first output of the first optical multiplexer / demultiplexer is connected to the transmission system. The optical switch 11 directs the second output of the first optical multiplexer / demultiplexer 10 to the first optical path 12 according to the electric signal applied by the controller 3. The light passing through the first optical path 12 is branched by the second optical multiplexer / demultiplexer 13 having two inputs and three outputs into three outputs having a power ratio of 1: 1: 1. The first output 34 of the second optical multiplexer / demultiplexer 13 has a free spectral range of 20GH.
After passing through the Fabry-Perot etalon 14 of the z finesse 20, it is converted into an electric signal by the first photodetector 15 and the second
The second output 35 of the optical multiplexer / demultiplexer 13 passes through the fiber type Mach-Zehnder interferometer 16 such that the half cycle of the transmittance change on the frequency axis is 100 GHz, and then the photodetector 17 of FIG. It is converted into an electric signal. The electric signals from the first and second photodetectors 15 and 17 are supplied to the first and second input terminals 3 of the controller 3.
Applied to 1,32. 2 shows the resonance characteristics of the Fabry-Perot etalon 14 and the fiber type Mach-Zehnder interferometer 16. The controller 3 has first and second inputs.
Based on 31 and 32, first, using the resonance characteristic 40 of the Mach-Zehnder interferometer 16, the resonance peaks 41 and 4 of the Fabry-Perot etalon are used.
The oscillation frequency of DFB-LD4,5,6 is pulled up to the vicinity of 2,43. The above is the first stage of control. The second step of the next control will be described. In the second stage, the output optical path from the optical switch 11 is switched from the first optical path 12 to the second optical path 18 according to the control signal supplied from the control device 3, and the sawtooth wave generator 2 is turned on. A sawtooth wave with a repetition frequency of 20 KHz is applied to LD1 and the frequency of DBR-LD1 is swept. DB
The light emitted from the R-LD1 passes through the optical isolator 19 and is then branched into three power ratios of 1: 1: 1 by the second optical multiplexer / demultiplexer path 13. Of these, the first output 34 is transmitted through the Fabry-Perot etalon 14 and then converted into an electric signal by the photodetector 15. Pulsed light is input to the first photodetector 15 when the oscillation frequency of the DBR-LD1 matches the resonance frequency of the Fabry-Perot etalon 14 during one cycle of the output signal from the sawtooth wave generator 2. However, the output waveform of the sawtooth wave generator 2 is adjusted so that the pulse of one cycle corresponds to the resonance peaks 41, 42 and 43 in FIG. The third output 36 is combined with the light of the second optical path of the optical switch 11 in the optical multiplexer 20 and converted into an electric signal in the third photodetector 21, which is not shown in the figure. , Input to low pass filter with cutoff frequency 100MHz. This low-pass filter outputs a pulsed electrical signal when the difference between the frequency of the light emitted from DFB-LD1 and the frequency of the light emitted from DFB-LD4,5,6 is within approximately ± 100 MHz. To be done. The number of pulses is set to DBR-LD1 and DFB-L during one cycle of the output signal of sawtooth wave generator 2.
The difference between the oscillation frequencies of D4, 5 and 6 is equal to the number of times of entering the range of ± 100 MHz, which is three. The output of the third photodetector 21 that has passed through the low-pass filter is applied to the third input terminal 33 of the control device 3. The control device 3 has a third diagram (a).
The input to the first input terminal 31 of the control device 3 shown in FIG.
38. The difference in pulse generation time 52, 53, 54 of the input to the third input terminal 33 of the control device 3 shown in FIG. 38 (b) is used as an error signal, and the magnitudes of these are constant values, zero in this embodiment. Output the control signal. The control signal from the control device 3 is input to the laser device driving devices 22, 23 and 24. The drive current corresponding to the control signal is output from the laser device driver 22, 23, 24 by the DFB-LD.
Injected into 4,5,6. The DBR-LD1, DFB-LD4,5, 6 are temperature-stabilized within a fluctuation range of 0.2 ° C. by temperature control devices 25, 26, 27, 28, respectively.

In this embodiment, only three laser devices are stabilized in frequency interval, but the frequency interval of more laser devices can be stabilized by changing the number of pulses emitted from the sawtooth wave generator 2. The frequency interval can be freely set by changing the thickness of the Fabry-Perot etalon used. The laser device to be further stabilized is not limited to the semiconductor laser, and can be stabilized if it is a laser device whose oscillation frequency changes according to a signal from the outside.

(Effects of the Invention) As described above, according to the present invention, it is possible to simultaneously stabilize the frequency interval of an arbitrary number of laser devices, and the frequency interval is set to an arbitrary value depending on the optical resonator used. And can be strictly defined. Also, by introducing the first stage of control, the oscillation frequency of each laser device can be easily pulled to the desired resonance peak of the optical resonator.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention, FIG. 2 is a resonance characteristic of a Fabry-Perot etalon 14 and a Mach-Zehnder interferometer 16 used in the embodiment, and FIG. 3 (a) is a control in the second stage of control. FIG. 3B is a diagram showing a time change of the first input 31 to the device 3, and FIG. 3B is a view showing a time change of the third input 33 to the control device 3 in the second stage of control. 1, 2, and 3 (a) and (b), 1 ... 1.55 μm band wavelength distributable Bragg reflection semiconductor laser, 2 ... sawtooth wave generator, 3 ... control device, 4, 5 , 6 ... 1.55
μm band distributed feedback semiconductor laser, 7,8,9,19 ... Optical isolator, 10 ... 3-input 2-output optical multiplexer / demultiplexer, 11 ... Optical switch,
13 ... 2-input 3-output optical multiplexer / demultiplexer, 14 ... Fabry-Perot etalon, 15, 17, 21 ... Photodetector, 16 ... Fiber type Mach-Zehnder interferometer, 20 ... Optical multiplexer, 22, 23, 24 ... Laser device drive Device, 25,26,27,28 ... Temperature control device, 40 ... Resonance characteristic of Mach-Zehnder interferometer 16, 44 ... Fabry-Perot etalon 14
Resonance characteristics of 50, 51 ... Output waveform from sawtooth generator, 5
2,53,54… Error signal.

Claims (2)

[Claims] 1. The oscillation frequencies of a plurality of laser devices to be controlled are drawn so that the intervals between them are substantially constant, and then the oscillation frequency intervals of the plurality of laser devices are converted into time intervals. An oscillation frequency interval stabilizing method for a plurality of laser devices, characterized in that an error signal obtained by comparing the time interval with a time interval corresponding to a frequency interval reference is controlled to have a constant value. 2. A reference laser device that sweeps the oscillation frequency in a range including the oscillation frequencies of a plurality of laser devices that stabilize the oscillation frequency interval in response to an external input.
A first optical multiplexer for combining lights emitted from the plurality of laser devices, and an optical path switching light for outputting an output from the first optical multiplexer to one of optical paths according to a control signal from the outside. A switch, an optical circuit that multiplexes a first output from the optical switch and the output of the reference laser device, and branches the optical output into a plurality of optical paths; and an amplitude change of a frequency change of the first output of the optical circuit. A low finesse optical resonator for converting to a low finesse optical resonator for converting a frequency change of the second output of the optical circuit into an amplitude change, a third output of the optical circuit and a second output of the optical switch. A second optical multiplexer that multiplexes the outputs and at least the outputs from the low finesse optical resonator are used to bring the oscillation frequencies of the plurality of laser devices close to the resonance frequency of the high finesse optical resonator. Later the reference laser device Corresponding to the resonance peak of the high finesse optical resonator during the oscillation frequency sweep of the reference laser device in the oscillation frequency intervals of the plurality of laser devices detected by the output of the second optical multiplexer during the oscillation frequency sweep A control device for outputting a first control signal and a second or less control signal applied to the optical switch for stabilizing an error with respect to a frequency interval reference set by using an optical output to And a laser device driving device that changes input signals to the plurality of laser devices according to the second or lower control signals from the device. .