JPH0658986B2 - Laser device oscillation frequency interval stabilization method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser device oscillation frequency interval stabilization method for stabilizing the intervals of oscillation frequencies of a 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: 2-204), or stabilize the frequency of one laser device,
A pulse train to be obtained as a beat signal is formed by combining the light emitted from several other laser devices and further combining this light with the light emitted from the reference laser device whose frequency is swept in a sawtooth shape. A method for stabilizing the oscillating frequency interval of each laser device by monitoring whether the appearance time of each pulse has a constant time difference with respect to the appearance time of the pulse corresponding to the stabilized laser device (Strebel et al. By i o o o e e o o o by
The Sea '85 (IOOC-ECOC'85) Technical Digest Volume 3 (1985) p. 61) is known.

However, in the first method, it is necessary to sweep and use the mirror spacing of the Fabry-Perot resonator that provides the reference of the frequency spacing, which makes the device larger than when using a simple etalon plate. In the second method,
Since the frequency interval standard is calculated as the appearance time interval of each pulse with respect to the frequency change of the reference laser device that has been measured in advance, it is not enough that this interval standard changes significantly during actual control. It is hard to say that the frequency interval of each laser device is expected and fixed at the time of control.

On the other hand, recently, by using an etalon plate as a Fabry-Perot resonator, it maintains a strict frequency interval reference, and as a configuration that avoids the increase in size of the device, which was a problem when using a sweeping Fabry-Perot resonator, The one described in IEICE technical report Vol. 87 CS87-96 by Shimosaka et al. Is known. In this configuration, the emitted light from the frequency sweeping laser device in which the oscillation frequency is swept periodically is branched into two,
One is incident on the optical resonator. When the incident light and the resonance frequency of the optical resonator match, pulsed light is emitted. The other light is combined with the emitted light of the laser device to be controlled. If only the low-frequency component is cut out from the beat signal of this combined light, a pulse-like signal is obtained when the oscillation frequencies of the laser device to be controlled and the frequency sweeping laser device substantially match. Each pulse of said optical resonator output,
By controlling the pulses obtained from the beats to overlap on the time axis, the oscillation frequency interval of each laser device to be controlled is stabilized to a value equal to the free spectral range of the optical resonator.

(Problems to be Solved by the Invention) In the above configuration, in order to improve the frequency interval stability and to narrow the width of each beat pulse, the frequency sweep light and the beat signal of the laser light to be controlled are emitted. Of these, the low-frequency component was selected and this was used directly as the beat pulse. However, the beat pulse obtained by such a method becomes a replica of the emitted light spectrum of the controlled laser device. Therefore, when the laser light emitted from the controlled laser device is FSK-modulated and its modulation index is large, the obtained beat pulse is a bimodal reflection of the modulated light spectral shape. On the other hand, two beat pulses are apparently generated. In the above configuration, since control is performed on a single controlled laser device assuming that only one beat pulse is generated, FSK modulated light with a large modulation index arrives as described above. In that case, there is a drawback that the control becomes out of control.

An object of the present invention is to eliminate the drawbacks of the prior art and to provide a laser device oscillation frequency interval stabilization method that can control even a FSK-modulated laser device to be controlled having a large modulation index. .

(Means for Solving the Problems) In order to achieve the above object, in the present invention, the emitted lights of a plurality of laser devices to be controlled are combined, and the combined light is frequency-swept with an oscillation frequency swept. The beat pulse train obtained by selecting the low-frequency component of the beat signal obtained by combining with the light is compared with the reference pulse train corresponding to the frequency interval reference, and the occurrence time difference of the pulses in the corresponding order of both pulse trains is compared. In the laser device oscillation frequency interval stabilizing method, the plurality of laser devices are controlled so that the error signal becomes a predetermined constant value. After the selection, the envelope is differentiated and half-wave rectified to obtain one pulse of the two pulses obtained for each of the plurality of laser devices. It is characterized in that it is a constituent element of a column.

(Operation) In the present invention, the FSK-modulated laser light emitted from the controlled device and the beat signal of the frequency sweep destination are passed through a low-pass filter, and the bimodality as a replica of the FSK-modulated signal obtained after envelope detection is obtained. The waveform of is differentiated and then half-wave rectified. As a result, two pulses are generated for each laser device to be controlled, but the control circuit performs control by considering that only one beat pulse is generated for each laser device to be controlled, and thus control cannot be performed as it is. Normal control operation can be expected by selecting only one of the two pulses and erasing the other.

(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.

1.55 μm wavelength tunable distributed reflection type semiconductor laser (DB
R) 1 has its output light wave number changed in a sawtooth shape with respect to time in accordance with signals 27 and 28 (see FIG. 2) having a repetition frequency of 20 kHz applied by the sawtooth wave generator 2. The light emitted from the DBR1 is transmitted through the optical isolator 3 and then split by the optical branching device 4 into the first and second output lights at a power ratio of 1: 1. Of these, the first output light is incident on the first photodetector 6 after passing through a quartz glass etalon plate 5 having a refractive index of 1.5, a thickness of 1 cm and a reflectance of both sides set to a finesse of 30. . Pulsed light is input to the first photodetector 6 when the frequency of DBR1 matches the resonance frequency of the etalon plate 5 during one cycle of the output signal from the sawtooth generator 2. The peak voltage of the output of the sawtooth wave generator 2 is adjusted so that the number of pulses in one cycle is three. The electric signal from the first photodetector 6 is applied to the first input terminal 71 of the controller 7.

On the other hand, the modulation speed of 100M, which is the target for stabilizing the frequency interval
Light emitted from a 1.55 μm band distributed feedback laser (DFB) 8, 9, 10 that is FSK-modulated with b / s and a modulation index of 5 is transmitted through optical isolators 11, 12, 13 and then the first optical combination. It is multiplexed by the vessel 14. The light emitted from the first optical multiplexer 14 is the second light.
Is combined with the second output light of the optical branching device 4. The output of the second optical multiplexer 15 is converted into an electric signal by the photodetector 16 and then input to a low-pass filter having a cutoff frequency of 100 MHz, which is not shown in the figure. From the low pass filter, the frequency of the light emitted from DBR1 and DFB8,9,10
When the difference in the frequency of the emitted light of is within a range of approximately ± 100 MHz, a pulsed electric signal is output. The number of pulses is ± 1 when the difference between the oscillation frequencies of DBR1 and DFB8,9,10 is 1 cycle per output signal of sawtooth wave generator 27,28 (see Fig.2).
It is equal to the number of times it enters the 00 MHz range, which is three.
The electrical signal from the second photodetector 16 is applied to the second input terminal 72 of the controller (details shown in FIG. 3) 7. Third
In the control device 7 shown in the figure, the input to the first input terminal 71 of the control device 7 shown in FIG. 2 (a) and the second input terminal of the control device 7 shown in FIG. 2 (b) are performed. The pulse generation time difference 24, 25, 26 of the input to 72 is used as an error signal, and a control signal whose magnitude is zero is output.

The pulse generation time difference measurement circuit 33 in FIG. 3 (an example of the circuit is shown in FIG. 4) is arranged as follows:
It outputs a square pulse having a width proportional to the time difference of occurrence of two pulses of a corresponding order (a total of three sets) and having a constant height. However, the output has a function of a positive or negative square pulse depending on which of the two series of pulse trains to which the pulse generated earlier of the above two pulses is input, the details of which are described in the first section. It is shown in FIG.

A portion surrounded by a dotted line in FIG. 4 is a circuit for selecting only the rear half peaks of the bimodal beat pulse corresponding to the FSK modulated light and sending it to the counter. The pulse having the shape shown in Fig. 5 (a) incident on this portion is converted into the waveform shown in Fig. 5 (b) after passing through the differentiating circuit, and is subsequently converted into two pulses of the form shown in Fig. 5 (c) by the diode. To be converted. This pulse is shaped by a Schmitt trigger and a negative logic element into two square waves with an amplitude equal to the theoretical level.
Further, the previous pulse is erased by the logical sum with the next T flip-flop, and the latter pulse is input to the counter of the next stage. By the operation of the circuit in the dotted line described above, the bimodal beat pulse corresponding to the FSK modulation spectrum is converted into a single pulse rising at the time corresponding to the center frequency of the modulated wave.

The first, second and third control signals from the control device 7 are input to the laser device driving devices 17, 18 and 19, respectively. A drive current corresponding to the control signal is output from the laser device drive device 17, 18, 19.
Injected into DFB8,9,10. The DBR1, DFB8, 9 and 10 are temperature-stabilized by temperature controllers 20, 21, 22 and 23 within a temperature variation of ± 0.1 ° C.

In this embodiment, only three laser devices are stabilized in frequency intervals, but the frequency and peak voltage of the output signal from the sawtooth wave generator 2 are adjusted to adjust the etalon plate 5 per cycle.
By changing the number of pulses emitted from the laser, the frequency intervals of more laser devices can be stabilized at the same time. Further, the frequency interval can be freely set by changing the thickness of the etalon plate. Further, the laser device to be 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. Further, in this embodiment, the rear pulse is selected from the two pulses of the half-wave rectified output, but the rear pulse is the center frequency of the modulated wave, as is clear from FIG. 5 (c). Since it rises at the time corresponding to, the strict control on the center frequency is possible even during the modulation. Again 2
Control is also possible using the previous of the two pulses.

(Effects of the Invention) As described above, the present invention makes it possible to stabilize the frequency interval for FSK-modulated light. In addition, after selecting only low-frequency components of the beat signal, the envelope is differentiated and half-wave rectified
In particular, if a pulse rearward in time is used, control can be performed after the center frequency is strictly identified.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention, and FIG. 2 (a) is a diagram showing an electric signal from a first photodetector 6 input to a control device 7 in FIG. FIG. 2 (b) is a diagram showing an electric signal from the second photodetector 16 input to the control device 7 in FIG. Further, FIG. 3 is a block diagram of the control device 7 in FIG.
FIG. 5 is a circuit diagram of the pulse generation time difference measuring circuit in FIG. 3, FIG. 5 (a) is a diagram showing a beat pulse shape that is a replica of the spectrum shape of the FSK modulated wave, and FIG. FIG. 5 (a) is a waveform diagram obtained by passing the waveform of FIG. 5 (a) through a differentiating circuit, and FIG. 5 (c) is a waveform diagram obtained by half-wave rectifying the waveform of FIG. 5 (b).
In FIGS. 1, 2 (a), (b) and 3, 1 ... 1.55 μm wavelength tunable distributed reflection type semiconductor laser, 2 ... sawtooth wave generator, 3, 11, 12, 13 ... Optical isolator 4 ... Optical branching device, 5 ... Etalon plate, 6,16 ... Photodetector, 7 ... Control device, 8, 9, 10 ... 1.55 μm band distributed feedback laser, 14, 15 ... Optical multiplexer, 17, 18 , 19 ... Laser device driving device, 20, 21, 22, 23
... Temperature control device, 24,25,26 ... Error signal, 27,28 ... Output waveform from the sawtooth wave generator 2.

Claims (1)

[Claims] 1. A beat signal obtained by combining emitted lights of a plurality of FSK-modulated laser devices to be controlled and combining the combined light with a frequency sweep destination whose oscillation frequency is swept. The beat pulse train to be compared is compared with a reference pulse train corresponding to the frequency interval reference, and the occurrence time difference of the pulses in the corresponding order of both pulse trains is used as an error signal, and the plurality of the plurality of the plurality of beat pulse trains are set so that the error signal has a predetermined constant value. A method for stabilizing an oscillation frequency interval of a laser device, comprising controlling a laser device, wherein only a low frequency component of the beat signal is selected, the envelope is differentiated, and half-wave rectification is performed to obtain the plurality of laser devices. A method for stabilizing an oscillation frequency interval of a laser device, wherein either one of two pulses obtained per one is a constituent element of the beat pulse train.