JPH01292873A - Laser device oscillation frequency stabilization - Google Patents

Abstract

PURPOSE:To stabilize oscillation frequency and its interval of a plurality of laser devices with good accuracy by making each beat pulse to be obtained corresponding to the oscillation spectra of a plurality of laser devices to pass a low-pass filter followed by differentiating their envelopes to detect a zero point of their differentiation wave for making a pulse array having the zero point as a starting point. CONSTITUTION:The outgoing light from the distributed feedback type lasers 8, 9 and 10 transmits photoisolators 11, 12 and 13 for being united with the outgoing light of a wavelength variable laser 1 by the second light collector 15 to be converted into an electric signal by the second photodetector 16 so as to be inputted in the second input terminal 72 of a control device 7. This electric signal is inputted in a low-pass amplifier 401 by the control device 7. This low-pass amplifier 401 plays a role of a low-pass transit filter so that a pulse-shaped electric signal is outputted. Here, the rising position almost accords with the peak position of the pulse and the pulse rising position shows about the central position of the oscillation frequency. Thereby, the oscillation frequency of a laser measure and the oscillation frequency interval can be stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for stabilizing the frequency spacing of a plurality of laser devices.

(Prior art) Conventionally, as a method for stabilizing the oscillation frequency of multiple laser devices, there has been a method called "Broadband wavelength tunable DBR-LD" by Shimosaka et al.
Proposal and basic experiment of LD frequency interval locking method using
There was a method described in the technical research report C387-96 of the Communication Method Study Group of the Institute of Electronics and Communication Engineers. This is based on the generation time of a beat pulse train formed by a beat signal obtained by combining the output light of a plurality of laser devices to be controlled with the frequency swept light whose oscillation frequency has been swept, and the time of occurrence of the beat pulse train of the frequency swept light. The pulse train is compared with a reference pulse train that occurs at a time interval corresponding to the resonant frequency interval of the optical resonator, which is obtained by branching the part and passing it through an optical resonator. As a signal,
This is a method of controlling the plurality of laser devices so that this error signal has a predetermined constant value.

(Problem to be Solved by the Invention) By the way, in the above method, the optical output of the laser device is FSK (Frequency 5-Hift Key
ng) If the modulated light is modulated and the modulation index is large, in order to more accurately detect the oscillation frequency interval of the laser device, the output light of the laser device and the frequency swept light whose oscillation frequency has been swept are combined. The passband width of the low-pass filter that passes the beat signal obtained by oscillating the waves was narrowed. However, in this case, there is a problem that two peaks or two beat pulses are generated for one laser device. - On the other hand, if the passband width of the low-pass filter is widened so as not to generate two peaks or two beat pulses, there is a problem that the oscillation frequency of the laser device cannot be detected with high accuracy. Furthermore, if the passband width of the low-pass filter is narrowed to generate two peaks or two beat pulses, and the oscillation frequency of the laser device is detected from the zero point of the differential waveform, multiple zero points will occur. Or, if the modulation index is large, one of its zero points may have a certain width, which causes the problem that the oscillation frequency of the laser device cannot be accurately detected.

An object of the present invention is to solve the above-mentioned problems and to stabilize the oscillation frequencies of a plurality of laser devices and the intervals therebetween with high precision.

(Means for Solving the Problems) The present invention provides an occurrence time of a beat pulse train formed by a beat signal obtained by combining the emitted light of a plurality of laser devices to be controlled with a frequency swept light whose oscillation frequency has been swept. is compared with a reference pulse train generated at a time interval corresponding to the resonant frequency interval of the optical resonator obtained by branching a part of the frequency swept light and passing it through an optical resonator, and determining the correspondence between both pulse trains. In the method for stabilizing the oscillation frequency of a laser device, the method comprises controlling the plurality of laser devices so that the error signal becomes a predetermined constant value by using the difference in occurrence time of the pulses as an error signal. When each beat pulse in the beat pulse train obtained corresponding to the oscillation spectrum of each laser device of the laser device transmits only the low frequency component of the beat signal and is envelope detected, the envelope detection is performed. After passing through a low-pass filter with a passband width that makes the output intensity unimodal, the envelope is differentiated, the zero point of the differentiated waveform is detected, and a pulse is generated with that zero point as the starting point of the pulse rise. It is characterized by being a pulse train obtained by Further, the reference pulse train is configured such that, after passing the frequency swept light through an optical resonator, a pulsed electric signal obtained by receiving the light with a light receiver is differentiated to detect its zero point, and the zero point is determined as the starting point of the pulse rise. It is characterized by being a pulse train obtained by generating pulses that

(Function) As described above, in the present invention, after the mixed light of the emitted light from each laser device of a plurality of laser devices to be controlled and the frequency sweep light is received by the radiator, the beat signal obtained there. A pulse-like signal is output through a low-pass filter that passes only low-frequency components, and then envelope detection is performed to obtain a pulse-like signal output. In the configuration of the present application, the passband width of the low-pass filter is determined by the frequency deviation in FSK modulation. For example, if the output light from each laser device has a large modulation degree of m = about 2.5,
Even if the output light from the laser device is SK modulated and two peaks clearly appear in the spectrum, the output of the low-pass filter is not a bimodal pulse corresponding to the two peaks in the spectrum. Instead, a single peak pulse is output. At this time, the width of these nine single-peak pulses becomes relatively wide, so if this continues, the accuracy in detecting the oscillation center frequency of the laser device will decrease. However, since it is guaranteed to be unimodal (it has such a relatively wide passband width), even if it is differentiated by the next-stage differentiating circuit, it is guaranteed to have only one zero point. Therefore, detecting the zero point means detecting the peak point of the pulsed output from the low-pass filter, that is, the center frequency of the oscillation frequency of the laser device. Here, since the zero point detection can be performed with extremely high precision, the center frequency of the oscillation frequency of the laser device can be detected with high precision. Also, if you do the same thing for the output from the optical resonator (envelope detection is not required, but otherwise), you can determine the resonant frequency of the optical resonator even if the finesse of the optical resonator is not very high. Can be detected with high accuracy. Therefore, the oscillation frequencies of the plurality of laser devices to be controlled can be accurately stabilized to the resonant frequency of the optical resonator, and the frequency intervals can be accurately stabilized.

(Example) Hereinafter, an example of the present invention will be described in detail. 1st
The figure is a configuration diagram of an apparatus implementing one embodiment of the present invention.

A 1.55 pm band wavelength tunable semiconductor laser 1 (hereinafter referred to as a wavelength tunable laser) receives signals 207 and 208 with a repetition frequency of 500 Hz applied by a sawtooth wave generator 2 (input to the control device in FIG. 2 (aXb)). (See the diagram showing the electrical symbols input to the terminals 71 and 72) & Therefore, the frequency of the output light changes with time in a sawtooth pattern. After the light emitted from the wavelength tunable laser 1 passes through an optical isolator 3, it is split by an optical splitter 4 into first and second output lights at a power ratio of 1:1. Among these, the first output light passes through the Fab 1 Le Perot optical resonator 5 and then enters the first photodetector 6 . The first photodetector 6 receives a pulse signal when the frequency of the wavelength tunable laser 1 matches the resonant frequency of the Fapley-Perot optical resonator 5 during one period of the output signal from the sawtooth wave generator 2. The peak voltage of the output of the sawtooth wave generator 2 is adjusted so that the number of pulses with this period becomes three. The electrical signal from the first photodetector 6 is applied to a first input terminal 71 of the control device 7 .

On the other hand, the oscillation frequency and interval thereof are to be stabilized, and the 1.55 pm band distributed feedback laser is FSX modulated with a modulation speed of 400 Mb/s and a modulation index of 2.5.
(Hereinafter referred to as DFB-LD) After passing through optical isolators 11, 12, and 13, the output lights are combined by a first optical multiplexer 14, and this combined light is further branched by an optical splitter 4. The light emitted from the wavelength tunable laser 1 is combined with the second optical multiplexer 15 . The output of the second optical multiplexer 15 is converted into an electrical signal by the second photodetector 16 and input to the second input terminal 72 of the control device 7 . In the control device 7 (details are shown in FIG. 4), this electrical signal is input to a low-pass amplifier 401 (see FIG. 4) having a cut-off frequency of 600 MHz. This low-pass amplifier 401 also plays the role of a low-pass filter, and the difference between the frequency of the emitted light from the wavelength tunable laser 1 and the frequency of the emitted light from the DFB-LD8.9.10 falls within the range of approximately ±600 MHz. During this period, a pulsed electric signal is output (see FIG. 3(a)).

The number of pulses is the output signal of sawtooth wave generator 7 207.20
8 (see Figure 2)
-The difference between each oscillation center frequency of LD8.9.10 is ±
Equal to the number of times it enters the 600 MHz range, which is three. The pulse having the shape shown in FIG.
2 (see FIG. 4), the waveform is converted into the waveform shown in FIG.
is converted into a pulse of the form . Here, the position of the rising edge of the pulse shown in FIG. 3(C) almost coincides with the position of the peak of the pulse shown in FIG. 3(a), and the position of the rising edge of the pulse shown in FIG.
) The rising position of the pulse shown in ) indicates approximately the center position of the oscillation frequency. The waveform in FIG. 3(e) is further shaped into a square wave (FIG. 3(d)) with an amplitude equal to the logic level by a Schmitt trigger circuit 404 (see FIG. 4), and then by an inverter 405 (see FIG. 4). After converting the signal into a square wave (see FIG. 3(e)) with its polarity reversed (see FIG. 3(e)), it is manually input to the pulse generation time difference measuring circuit 406. Note that the electrical signals from the first photodetector 6 are also shown in FIGS. 3(a) to (e).
Waveform shaping similar to that shown in can be performed, but
Since this can be done in the same manner as above, the explanation will be omitted.

The two pulse trains obtained in this way (Fig. 2(a)
, (b) is the pulse generation time difference measuring circuit 40 shown in FIG.
6, the input pulse generation time difference is 204.205
．． 206 (see FIG. 2) is used as an error signal, and a control signal is output so that the magnitude of these signals becomes zero. Here, the fourth
The pulse generation time difference measuring circuit 406 in the figure (an example of the circuit is shown in FIG. 5) detects the two pulses in the corresponding order when the pulses constituting the two input pulse trains are arranged in order of generation time. It outputs a rectangular pulse with a width proportional to the generation time difference of the three sets (total of three sets) and a constant height. However, depending on which of the two input pulse trains the pulse that occurs first of the above two pulses belongs to, the output has the function of becoming a positive or negative square pulse. It is shown in Figure 5. 1st and 2nd from the control device 7)
, and third control signals are input to the first, second) and third laser device driving devices 17, 18, and 19, respectively. A driving current according to a control signal is injected from each laser device driving device 17, 18, 19 to each DFB-LD 8, 9, 10. In addition, wavelength tunable laser 1, DFB-LD8.9.10
The temperature is stabilized within ±0.1°C by temperature control devices 20, 21, 22, and 23, respectively.

In this example, the oscillation frequency interval is stabilized for the three laser devices, but the frequency and peak voltage of the output signal from the sawtooth wave generator 7 are adjusted to widen the sweep frequency width per period. By doing so, by increasing the number of pulses emitted from the Fab 1 Louberot optical resonator 5, the oscillation frequency of the laser device and the interval thereof can be stabilized at the resonance frequencies of even more optical resonators. Also, Fab 1
By changing the interval between the resonator mirrors of the Luperot optical resonator, the frequency interval can be freely set. Further, the laser device to be stabilized is not limited to a semiconductor laser, but any laser device whose oscillation frequency changes in response to an external signal can be stabilized.

(Effects of the Invention) As described above, the present invention allows the oscillation frequency to be accurately stabilized at the resonant frequency of the optical resonator even if the output light from a plurality of laser devices to be controlled is FSK modulated. In addition, the frequency interval can be stabilized with high precision. Furthermore, the resonant frequency of the optical resonator can be detected even if the finesse of the optical resonator is not very high, and the oscillation frequency and oscillation frequency interval of the laser device can be stabilized with higher accuracy.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of an embodiment of the present invention, Fig. 2 (aXb)
is a diagram showing electrical signals input to the input terminals 71 and 72 of the control device in FIG. 1, and FIGS. 3(a) to (e) explain some of the functions of the control device 7 in FIG. 1. FIG. 4 is a block diagram of the control device 7 in FIG. 1, and FIG. 5 is a circuit diagram of the pulse generation time difference measuring circuit in FIG. 4. In FIG. 1 and FIG. 4, 1...1.55 pm band wavelength tunable semiconductor laser, 2
..., sawtooth fungus wave generator, 3, 11, 12, 13...
... Optical isolator, 4... Optical splitter, 5...
...Fabry-Perot optical resonator, 6,16...
- Photodetector, 7... Control device, 71, 72...
...Input terminal of control device 7, 8,9.10...-4,5
511m band distributed feedback laser, 14.15... Optical multiplexer, 17, 1B, 19... Laser device driving device, 20, 21, 22, 23... Temperature control device, 2
4, 25, 26...Modulation signal input terminal.

Claims (2)

[Claims] (1) The generation time of a beat pulse train formed by a beat signal obtained by combining the output light of a plurality of laser devices to be controlled with the frequency swept light whose oscillation frequency has been swept, and the time of occurrence of a beat pulse train of the frequency swept light. comparing a reference pulse train occurring at a time interval corresponding to the resonant frequency interval of the optical resonator obtained by branching the part and passing it through the optical resonator;
A method for stabilizing the oscillation frequency of a laser device, characterized in that the plurality of laser devices are controlled so that the difference in occurrence time between corresponding pulses of both pulse trains is used as an error signal, and the error signal becomes a predetermined constant value. , when each beat pulse in the beat pulse train obtained corresponding to the oscillation spectrum of each laser device of the plurality of laser devices transmits only the low frequency component of the beat signal and is envelope-detected, After passing through a low-pass filter with a passband width that makes the envelope detection output intensity unimodal, the envelope is differentiated, the zero point of the differentiated waveform is detected, and the zero point is used as the starting point of the pulse rise. A method for stabilizing the oscillation frequency of a laser device, characterized in that the pulse train is obtained by generating pulses. (2) In the method for stabilizing the oscillation frequency of a laser device according to claim 1, the reference pulse train has a pulsed shape obtained by passing the frequency swept light through an optical resonator and then receiving the light with a light receiver. The laser device oscillation frequency according to claim 1, which is a pulse train obtained by differentiating an electrical signal, detecting its zero point, and generating a pulse with the zero point as the starting point of the pulse rise. Stabilization method.