JP2689481B2 - Laser device oscillation frequency stabilization method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for stabilizing the frequency intervals of a plurality of laser devices.

(Prior Art) Conventionally, as a method for stabilizing the oscillation frequencies of a plurality of laser devices, the Institute of Electronics and Communication Engineers, entitled "Proposal and Basic Experiment of LD Frequency Interval Locking Method Using Wideband Tunable DBR-LD" by Shimosaka et al Technical Research Report CS87 of Communication Method Study Group
-96. This is the 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 and the frequency swept light whose oscillation frequency is swept, and one of the frequency swept light. By comparing the reference pulse train generated at a time interval corresponding to the resonance frequency interval of the optical resonator obtained by branching the part and passing it through the optical resonator, the difference between the occurrence times of the corresponding pulses of both pulse trains is error. In this method, the plurality of laser devices are controlled so that the error signal has a predetermined constant value as a signal.

(Problems to be Solved by the Invention) By the way, in the above method, when the optical output of the laser device is FSK (Frequency Shift Keying) modulated, and the modulation index thereof is large, it depends on the oscillation frequency interval of the laser device. In order to detect with high accuracy, the pass band width of the low-pass filter that passes the beat signal obtained by combining the output light of the laser device and the frequency-swept light whose oscillation frequency has been swept has been narrowed. However, in this case, there is a problem that two peaks or two beat pulses are generated in one laser device. On the other hand, if the pass band width of the low pass filter is widened so that two peaks or two beat pulses are not generated, there is a problem that the oscillation frequency of the laser device cannot be detected accurately. Also, the pass band width of the low pass filter is narrowed,
Generate two peaks or two beat pulses,
When trying to detect the oscillation frequency of the laser device from the zero point of the differential waveform, multiple zero points occur, or
If the modulation index is large, one of the zero points has a certain width, and the oscillation frequency of the laser device cannot be detected accurately.

An object of the present invention is to solve the above problems and to stabilize the oscillation frequencies of a plurality of laser devices and their intervals with high accuracy.

(Means for Solving the Problem) The present invention relates to the occurrence time of a beat pulse train formed by a beat signal obtained by multiplexing emitted light of a plurality of laser devices to be controlled and a frequency sweep light whose oscillation frequency is swept. And a reference pulse train generated at a time interval corresponding to the resonance frequency interval of the optical resonator obtained by branching a part of the frequency sweep light and passing through the optical resonator, and comparing both pulse trains. In the laser device oscillation frequency stabilizing method, the plurality of laser measures are controlled so that the difference between the occurrence times of the pulses is an error signal, and the error signals have a predetermined constant value. Each beat pulse in the beat pulse train obtained corresponding to the oscillation spectrum of each laser device of the laser device of (1) transmits only the low frequency component of the beat signal. , And, when the envelope detection, after passing through a low-pass filter having a pass band width that the envelope detection output intensity becomes a single peak, the envelope is differentiated to detect the zero point of the differential waveform. , A pulse train obtained by generating a pulse whose zero point is the starting point of the pulse rise. Further, the reference pulse train detects the zero point by differentiating the pulsed electric signal obtained by receiving the frequency swept light through the optical resonator and then receiving the light by the light receiver, and the zero point is defined as the starting point of the pulse rise. It is characterized in that it is a pulse train obtained by generating a pulse.

(Operation) In the present invention, as described above, after the mixed light of the emitted light from each laser device of the plurality of laser devices to be controlled and the frequency sweep light is received by the light receiver, the beat signal obtained there is A low-pass filter that passes only low-frequency components is passed through, and further envelope detection is performed to obtain a pulse-shaped signal output.In the configuration of the present application, the pass-band width of the low-pass filter is frequency-shifted in FSK modulation. As the output light from each laser device is FSK-modulated with a large modulation degree of about m = 2.5, it is clear that the spectrum of the output light from the laser device is clear. Even if two peaks appear at, the output of the low-pass filter does not become a bimodal pulse corresponding to the two peaks in the spectrum, but a single-peak pulse is output. At this time, the width of this single-peaked pulse becomes relatively wide, so that the accuracy at the time of detecting the oscillation center frequency of the laser device deteriorates as it is. However, since it is guaranteed to be unimodal (has such a relatively wide passband width), even if it is differentiated by the differentiating circuit in the next stage, it is guaranteed that the number of zeros is one. Therefore, if the zero point is detected, 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 is detected. Here, since the zero point can be detected with extremely high accuracy, the center frequency of the oscillation frequency of the laser device can be detected with high accuracy. Further, if the same is done for the output from the optical resonator (other than the envelope detection is unnecessary), even if the finesse of the optical resonator is not so high, the resonance frequency of the optical resonator can be improved. It can be detected accurately. Therefore, it is possible to accurately stabilize the oscillation frequencies of the plurality of laser devices to be controlled to the resonance frequency of the optical resonator, and to stabilize the frequency intervals with high precision.

(Example) Hereinafter, one example of the present invention will be described in detail. FIG. 1 is a block diagram of an apparatus for carrying out one embodiment of the present invention. The 1.55 μm band tunable semiconductor laser 1 (hereinafter referred to as “tunable wavelength laser”) is a signal 207, 208 having a repetition frequency of 500 Hz applied by the sawtooth fungus wave generator 2 (FIG. 2 (a)).
According to (b) a diagram showing input electrical symbols to the input terminals 71 and 72 of the control device), the frequency of the emitted light changes in a sawtooth shape with respect to time. The light emitted from the wavelength tunable laser 1 passes through the optical isolator 3 and then the optical splitter 4
Is split into a first output light and a second output light with a power ratio of 1: 1. Of these, the first output light passes through the Fabry-Perot optical resonator 5 and then enters the first photodetector 6.
The first photodetector 6 is pulsed when the frequency of the tunable laser 1 matches the resonance frequency of the Fabry-Perot optical resonator 5 during one cycle of the output signal from the sawtooth wave generator 2. Light is output, but the number of pulses in one cycle is
The peak voltage of the output of the sawtooth wave generator 2 is adjusted so that the number becomes 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, it is a target to stabilize the oscillation frequency and its interval, and is FSK-modulated with a modulation speed of 400 Mb / s and a modulation index of 2.5.
Light emitted from the 5 μm band distributed feedback lasers 8, 9 and 10 (hereinafter referred to as DFB-LD) are respectively optical isolators 11 and 12.
After passing through 13, the light is multiplexed by the first optical multiplexer 14, and the combined light is further multiplexed by the second optical multiplexer 15 with the emitted light of the wavelength tunable laser 1 branched by the optical branching fourth. . The output of the second optical multiplexer 15 is converted into an electric signal by the second photodetector 16 and input to the second input terminal 72 of the control device 7. This electric signal is sent to the controller 7 (details are shown in FIG. 4).
Then, it is input to the low-frequency amplifier 401 (see FIG. 4) having a cutoff 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 light emitted from the tunable laser 1 and the frequency of the light emitted from the DFB-LDs 8, 9 and 10 falls within a range of approximately ± 600 MHz. While it is in motion, a pulsed electric signal is output (see FIG. 3 (a)). The number of pulses is ± the difference between the oscillation center frequencies of the tunable laser 1 and the DFB-LDs 8, 9 and 10 in one cycle of the output signals 207 and 208 (see FIG. 2) of the sawtooth wave generator 7. It is equal to the number of times it is in the 600MHz range, which is three. The pulse having the shape shown in FIG. 3 (a) is converted into the waveform shown in FIG. 3 (b) by the differentiating circuit 402 (see FIG. 4), and further, the half-wave rectifying circuit 403 (see FIG. 4) is used. It is converted into a pulse of the form (c).
Here, the rising position of the pulse shown in FIG. 3 (c) almost coincides with the peak position of the pulse shown in FIG. 3 (a), and the position of the pulse shown in FIG. The rising position indicates almost the center position of the oscillation frequency. The waveform shown in FIG. 3 (c) further has a Schmitt trigger circuit 404.
(See FIG. 4) A square wave having an amplitude equal to the logic level (see FIG. 3 (d)) is shaped, and then the polarity is inverted by the inverter 405 (see FIG. 4) (see FIG. 3). (E))
After that, the pulse generation time difference measuring circuit 406 receives the same waveform shaping as that shown in FIGS. 3A to 3E for the electric signal from the first photodetector 6. However, the description will be omitted because it can be performed in the same manner as above.

The two pulse trains (FIGS. 2A and 2B) thus obtained are input to the pulse generation time difference measurement circuit 406 shown in FIG. 4, and the input pulse generation time differences 204, 205, 2
06 (see FIG. 2) is used as an error signal, and a control signal that makes these magnitudes zero is output. Here, the pulse generation time difference measuring circuit 406 (an example of the circuit is shown in FIG. 5) in FIG. 4 corresponds when the respective pulses forming the two input pulse trains are arranged in the order of generation time. It outputs a square pulse having a width proportional to the time difference of occurrence of two pulses (3 sets in total) in the order and having a constant height. However, the above 2
The pulse that occurs first of the two pulses is input 2
The output has a function of becoming a positive or negative square pulse depending on which one of the series pulse trains it belongs to, the details of which are shown in FIG. The first, second and third control signals from the control device 7 are input to the first, second and third level device driving devices 17, 18 and 19, respectively. Each laser device driving device 17,
From 18 and 19, the drive current corresponding to the control signal is DFB-LD8,
Injected into 9 and 10. Tunable laser 1, DFB-LD
8, 9 and 10 are temperature-stabilized within a temperature fluctuation of ± 0.1 ° C. by temperature control devices 20, 21, 22 and 23, respectively.

In this embodiment, the oscillation frequency intervals of the three laser devices are stabilized, but the frequency and peak voltage of the output signal from the sawtooth fungus generator 7 are adjusted to widen the sweep frequency width per cycle. By increasing the number of pulses emitted from the Fabry-Perot optical resonator 5 by
Further, the oscillation frequency of the laser device and its interval can be stabilized by the resonance frequencies of more optical resonators. Further, the frequency interval can be freely set by changing the interval between the resonator mirrors of the Fabry-Perot optical resonator. 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.

(Effects of the Invention) As described above, according to the present invention, even if the output light from a plurality of laser devices to be controlled is FSK-modulated, the oscillation frequency is accurately stabilized to the resonance frequency of the optical resonator. It is possible to stabilize the frequency interval with high accuracy. Further, even if the finesse of the optical resonator is not very high, the resonance frequency of the optical resonator can be detected, and the oscillation frequency and the oscillation frequency interval of the laser device can be stabilized more accurately.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 (a).
(B) is a figure which shows the electric signal input into the input terminals 71 and 72 of the control apparatus in FIG. 1, and FIG. 3 (a)-(e) is one of the functions of the control apparatus 7 in FIG. Explanatory diagram for explaining the section,
FIG. 4 is a block diagram of the control device 7 in FIG. 1, and FIG.
It is a circuit diagram of a pulse generation time difference measurement circuit in the figure. In FIGS. 1 and 4, 1 ... 1.55 μm band tunable semiconductor laser, 2 ... Serrated 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 ...... 1.55 μm band distributed feedback laser , 14,15
...... Optical multiplexer, 17,18,19 …… Laser device driver, 20,2
1,22,23 …… Temperature control device, 24,25,26 …… Modulation signal input terminal.

Claims (2)

(57) [Claims] 1. An occurrence time of a beat pulse train formed by a beat signal obtained by combining emitted light of a plurality of laser devices to be controlled and oscillation frequency with a swept frequency swept light, and the frequency swept light. The reference pulse trains generated at a time interval corresponding to the resonance frequency interval of the optical resonator obtained by branching a part and passing through the optical resonator are compared, and the occurrence time difference between the corresponding pulses of both pulse trains is error. As a signal, in the laser device oscillation frequency stabilizing method, the plurality of laser devices are controlled so that this error signal has a predetermined constant value, and an oscillation spectrum of each laser device of the plurality of laser devices is provided. Each beat pulse in the beat pulse train obtained corresponding to
Only the low-frequency component of the beat signal is transmitted, and when envelope detection is performed, the envelope detection output strength is passed through a low-pass filter having a pass bandwidth that is a single peak, and then the envelope is A laser device oscillation frequency stabilizing method comprising a pulse train obtained by differentiating, detecting a zero point of the differentiated waveform, and generating a pulse having the zero point as a starting point of pulse rise. 2. The laser device oscillation frequency stabilizing method according to claim 1, wherein the reference pulse train is a pulse obtained by receiving the frequency sweep light through an optical resonator and then receiving the light with a light receiver. 2. The laser device oscillation according to claim 1, wherein the pulse train is obtained by differentiating the electrical signal to detect its zero point and generating a pulse with the zero point as the starting point of the pulse rise. Frequency stabilization method.