JPH088387B2 - Laser device oscillation frequency stabilization method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for stabilizing frequency intervals of a plurality of laser devices and oscillation frequencies thereof.

(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) However, in the above method, the resonance frequency interval of the optical resonator, which is the reference of the oscillation frequency, is extremely stable even when the environmental temperature changes, etc. The frequency itself is stabilized by the above method because, for example, even if a Fabry-Perot resonator made of quartz glass having a relatively small coefficient of thermal expansion is used for the optical resonator, oscillation frequency fluctuation of about 1 GHz / ° C occurs. There is a problem that one laser device group and another laser device group stabilized by the same method may have different oscillation frequencies of the corresponding laser devices. Also, when trying to synchronize the oscillation frequencies of different laser device groups at different points by using different optical resonators having substantially the same resonance frequency interval,
Even if the oscillation frequency interval of each laser device stabilized according to the resonance frequency interval is a slight difference in the resonator length of the optical resonator, the error is suppressed to about the difference between the different optical resonator lengths with respect to the optical resonator length. Therefore, there is a problem in that they substantially match with each other with high accuracy, but the corresponding oscillation frequencies differ greatly if there is a slight difference in the resonator length of the optical resonator.

An object of the present invention is to solve the above problems and stabilize not only the oscillation frequency intervals of a plurality of laser devices but also their oscillation frequencies with respect to a specific frequency reference light.

(Means for Solving the Problem) The present invention relates to a beat pulse train formed by a beat signal obtained by multiplexing emitted light of a plurality of laser devices to be controlled and frequency sweep light whose oscillation frequency is swept. Comparing the occurrence time and the occurrence time of the reference pulse train that occurs 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, The difference between the occurrence times of the corresponding pulses of the beat pulse train and the reference pulse train is used as an error signal, and the oscillation frequencies of the plurality of laser devices are controlled so that the error signal becomes a predetermined constant value. In the laser device oscillation frequency stabilization method, a part of the frequency sweep light and the reference light that is the light emitted from the frequency reference laser device that serves as a reference of the oscillation frequency is further added. The reference pulse train is generated for a delay time relative to the occurrence time of one specific pulse arbitrarily defined in the reference pulse train of the occurrence time of the frequency reference beat pulse formed by the beat signal generated by multiplexing the branched light. A method for stabilizing an oscillation frequency of a laser device, which is characterized by delaying.

(Operation) In the present invention, by adopting the above-described configuration, one of the frequencies corresponding to each pulse of the reference pulse train that is a direct reference of the oscillation frequency of the plurality of laser devices to be controlled is determined. The two frequencies are stabilized so as to match the oscillation frequency of the frequency reference laser device. At the occurrence time of each pulse of the reference pulse train, obtained by combining the emitted light of the plurality of laser devices to be controlled and the oscillation frequency with the swept frequency sweep light of the plurality of laser devices to be controlled. If the difference in the generation time of the corresponding pulses of the beat pulse train with the time difference corresponding to the oscillation frequency interval is controlled to be a predetermined constant value, the frequency interval of an arbitrary number of laser devices is stabilized and The oscillating frequencies are stabilized at a frequency that is separated from the oscillating frequency of the frequency reference laser device by an integer multiple of the resonator spacing of the optical resonator. This means that if the light emitted from the frequency reference laser device is distributed to multiple points and the above-mentioned stabilization is performed at each point, the oscillation frequencies of the multiple laser devices at each point will be the same between the points. Can be done.

(Examples) Hereinafter, the present invention will be described in detail with reference to Examples. First
The figure is a block diagram of an embodiment of the present invention. 1.55 μm wavelength tunable semiconductor laser 1 (hereinafter referred to as wavelength tunable laser)
Is the repetition frequency applied by the sawtooth wave generator 2.
In accordance with the 20 kHz signals 207 and 208 (see FIG. 3), the output light frequency 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 is then transmitted by the optical branching device 4 with the power ratio of 1: 1: 1 to the first, second,
It is divided into a third output light. Of these, the first output light is combined with the light emitted from the frequency reference laser device 6 (see FIG. 2 (a)) that gives the reference of the optical frequency in the first optical multiplexer 5, The light is received by the first light receiving circuit and converted into an electric signal by the same circuit. Although not shown in the figure, after this, it is further input to a low pass filter with a cutoff frequency of 100 MHz. From the low-pass filter, when the difference between the frequency of the emitted light from the tunable laser 1 and the frequency of the emitted light of the frequency reference laser device 6 is within a range of approximately ± 100 MHz, a pulsed electric signal is generated. Is output. This becomes the frequency reference beat pulse (see FIG. 2 (c)). Further, the second output light is transmitted through the Fabry-Perot optical resonator 8 and then is incident on the second light receiving circuit 9. Here, the Fabry-Perot optical resonator 8 is a Fabry-Perot etalon having a free spectral width of 10 GHz (resonator mirror spacing of about 1.5 cm) and a finesse of about 30. In the second light receiving circuit 9, the frequency of the tunable laser 1 is the resonance frequency of the Fabry-Perot optical resonator 8 during one cycle of the output signal from the sawtooth wave generator 2 (see FIG. 2 (b)). The pulsed light is output at the time point that coincides with, but the peak voltage of the output of the sawtooth wave generator 2 is adjusted so that the number of pulses generated during this one cycle is three. In the second light receiving circuit 9, the resonance frequency of the Fabry-Perot optical resonator 8 (FIG. 2 (b))
The pulsed light corresponding to the reference signal is converted into a pulsed electric signal (see FIG. 2D).

Here, a tunable laser 1 swept by a sawtooth wave 1
The resonance frequency of the Fabry-Perot optical resonator (the frequency shown on the leftmost side of FIG. 2 (b)) that first appears in the oscillation frequency range is generally the oscillation frequency of the frequency reference laser device 6 (FIG. 2). (Frequency shown in (a)) does not match. Therefore, the distributed feedback lasers 11, 12, 13 in the wavelength 1.55 μm band, which are the targets for stabilizing the oscillation frequency and its interval,
The oscillation frequency of (hereinafter referred to as DFB-LD) is based on the oscillation frequency of the frequency reference laser device 6
In order to stabilize the frequency at an integer multiple of the free spectrum interval (resonance frequency interval) of the Perot optical resonator, the variable delay circuit 19 connected to the subsequent stage of the second light receiving circuit 9 is swept by a sawtooth wave. From the second light receiving circuit 9 corresponding to the resonance frequency of the Fabry-Perot optical resonator first appearing in the oscillation frequency range of the wavelength tunable laser 1 (the frequency shown on the leftmost side of FIG. 2B). The output pulse from the first light receiving circuit 7 with respect to the occurrence time of the first pulse (the pulse shown on the leftmost side in FIG. 2D) in one cycle of the sweep by the sawtooth wave in the output pulse ( The positions on the time axis of all output pulses from the second light receiving circuit 9 are delayed by the delay time of the occurrence time shown in FIG. 2 (c). This delayed output pulse train becomes the reference pulse train. The delay time amount is detected by the delay time control circuit 20, and the delay time control circuit 20 detects the variable delay circuit 19
Control the delay amount of. The electric signal (that is, the reference pulse train) from the variable delay circuit is applied to the first input terminal 22 of the controller 21. Here, the variable delay circuit and the delay time control circuit are described in detail in the section of the variable delay pulse generating circuit described on pages 552 to 554 of the Solid State Circuit Handbook (Maruzen).

Next, the third output light of the optical branching device 4 has a wavelength of 1.55 .mu.m DFB-LD11, 12 which is an object for stabilizing the oscillation frequency and the interval thereof, which are multiplexed in the second optical multiplexer 10. , 13 and the third light combiner 14, and the third light receiving circuit
After being received by 15, the same circuit 15 converts the light into an electric signal. Although not shown in the figure, a cutoff frequency of 10
Input to 0MHz low pass filter. From the low pass filter, the frequency of the light emitted from the tunable laser 1
DFB-LD11,12,13 output light frequency difference is about ± 100MH
When it is within the range of z, a pulsed electric signal is output. The number of pulses is the output signal 201 of the sawtooth wave generator 2.
(See Fig. 2) Tunable laser 1 and DFB-LD1 in one cycle
The difference between the oscillation frequencies of 1, 12, and 13 is equal to the number of times that the oscillation frequency falls within the range of ± 100 MHz, which is three. This becomes the reference pulse train (see FIG. 2 (e)). The electric signal (that is, the reference pulse train) from the third light receiving circuit 15 is supplied to the second signal of the control device 21.
Applied to the input terminal 23 of. In the control device 11 whose details are shown in FIG. 3, the input to the first input terminal 22 of the control device 11 shown in FIG. 2 (e) and the control device 11 shown in FIG. The pulse generation time difference Δt1, Δt2, Δt3 of the input to the second input terminal 23 is used as an error signal, and a control signal is output so that these magnitudes become zero. Here, when the light emitted from each DFB-LD is coupled to the third optical multiplexer 14, it is coupled via each of the optical isolators 16, 17, 18 to prevent the oscillation spectrum from being disturbed by reflected return light or the like. I'm out.

In addition, the pulse generation time difference measuring circuit 33 (fourth
An example of a circuit is shown in the figure), when the pulses forming the two input pulse trains are arranged in the order of the derivation time, the width proportional to the time difference of occurrence of the two pulses (three sets in total) of the corresponding order. And outputs a square pulse of 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 earlier generated pulse of the above two pulses belongs. The first, second and third control signals from the control device 21 are transmitted to the first, second and third laser device driving devices 24, 25, respectively.
Entered in 26. Driving currents corresponding to the control signals are injected from the laser device driving devices 24, 25 and 26 into the DFB-LDs 11, 12 and 13, respectively. The wavelength tunable laser 1 and the DFB-LDs 11, 12, and 13 have temperature fluctuations of ± 0.1 due to the temperature controllers 27, 28, 29, and 30, respectively.
The temperature is stabilized within ℃.

In the present embodiment, the oscillation frequency and the frequency interval thereof are stabilized for the three laser devices, but the frequency of the output signal from the sawtooth generator 7 and the peak voltage are adjusted so that the sweep frequency width per cycle is adjusted. By increasing the width, by increasing the number of pulses emitted from the Fabry-Perot optical resonator 3, it is possible to stabilize the oscillation frequency of the laser device and its interval at a higher resonance frequency of the optical resonator. Further, the frequency interval can be freely set by changing the thickness of the Fabry-Perot optical resonator (interval between resonator mirrors). 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, if the frequency reference laser device 6 which gives the reference of the optical frequency is a laser device in which the oscillation frequency is absolutely stabilized, the laser device which is the object of stabilizing the oscillation frequency and the interval thereof (DFB-LD11, The oscillation frequency of 12, 13) is also absolutely stabilized. As an example of a laser device in which the oscillation frequency is absolutely stabilized, the oscillation frequency of the semiconductor laser is
There is a semiconductor laser that is absolutely stabilized by locking it to the NH3 molecular absorption line, and it is described in "Lightwave Transmission LD Light Source Frequency Stabilization" on pages 10-360 of Tohba et al. More on the paper entitled "Circuit Examination".

(Effects of the Invention) As described above, according to the present invention, the frequency intervals of an arbitrary number of laser devices are stabilized, and their oscillation frequencies are changed from the oscillation frequency of the frequency reference laser device to the resonance of the optical resonator. Stabilized to frequencies separated by an integer multiple of the instrument spacing. Further, if the emitted light from the frequency reference laser device is distributed to a plurality of points and the above-mentioned stabilization is performed at each point, the oscillation frequencies of the plurality of laser devices at each point can be made to match between the points. .

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention, and FIG. 2 is an explanatory diagram for explaining a laser device oscillation frequency stabilizing method of the present invention. Further, FIG. 3 is a block diagram of the control device 11 in FIG. 1, and FIG. 4 is a circuit diagram of the pulse generation time difference measuring circuit in FIG. In FIGS. 1, 2 and 3, 1 ... Wavelength tunable laser, 2 ... Sawtooth generator, 3, 16, 17,
18 ... Optical isolator, 4 ... Optical splitter, 5 ... First optical multiplexer, 6 ... Frequency reference laser device, 7 ... First light receiving circuit, 8 ... Fabry-Perot resonator, 9 ...... Second
Light receiving circuit, 10 …… Second optical multiplexer, 11,12,13 …… 1.55
μm band distributed feedback laser, 14 ... third optical multiplexer, 15 ...
… Third light receiving circuit, 19 …… Variable delay circuit, 20 …… Delay time control circuit, 21 …… Control circuit, 22,23 …… Input terminal of control device 11, 24,25,26 …… Laser device drive Equipment, 27, 28, 2
9,30 ... Temperature control device 31,32,33 ... Modulation signal input terminal.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H04B 10/14

Claims (1)

[Claims] 1. An 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 frequency swept light whose oscillation frequency is swept, and the frequency swept light. Of the beat pulse train and the reference pulse train by comparing the occurrence time of the reference pulse train occurring at the time interval corresponding to the resonance frequency interval of the optical resonator obtained by branching a part of In a laser device oscillation frequency stabilizing method, the difference between the occurrence times of corresponding pulses is used as an error signal, and the oscillation frequencies of the plurality of laser devices are controlled so that the error signals have a predetermined constant value. Combining the reference light, which is the light emitted from the frequency reference laser device serving as the reference of the oscillation frequency, and the light obtained by further branching a part of the frequency sweep light. The laser device is characterized in that the reference pulse train is delayed by a delay time with respect to the occurrence time of one specific pulse arbitrarily defined in the reference pulse train of the occurrence time of the frequency reference beat pulse formed by the generated beat signal. Oscillation frequency stabilization method.