JPH03225981A - Method for stabilization of frequency interval of laser apparatus - Google Patents

Abstract

PURPOSE:To stabilize the oscillation frequency interval of a plurality of laser apparatuses at two unequal intervals by a method wherein, out of a reference pulse train obtained when a frequency scanning beam is passed through an optical resonator, pulses which are selected arbitrarily every several pieces are used for a control operation as a new reference pulse train. CONSTITUTION:A beam radiated from a distributed reflection type laser 1 provided with a 1.55mum-band phase control region is transmitted through an isolator 3; after that, it is divided into a first output beam and a second output beam by using a beam splitter 4 in a power ratio of 1:1. The first output beam is passed through a quartz-glass etalon sheet 5 whose reflection factor on both faces has been set; after that, it is incident on a first photodetector 6. A pulse-shaped beam is input to the first photodetector 6 at the time when the frequency of the distributed reflection type laser 1 provided with the 1.55mum-band phase control region coincides with the resonance frequency of the etalon sheet 5 during one cycle of an output signal from a sawtooth wave generator 2. However, the peak voltage of an output from the sawtooth wave generator 2 is adjusted in such a way that the number of pulses in one cycle becomes four. At a control operation, three laser apparatus are stabilized to a first pulse, a second pulse and a fourth pulse.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an oscillation frequency interval stabilization method for stabilizing the interval between oscillation frequencies of a plurality of laser devices used for optical communication or the like.

(Prior Art) The following two methods are conventionally known as methods for stabilizing the frequency intervals of a plurality of laser devices. The oscillation frequency of one laser device is stabilized with respect to the Fabry Velocity resonator, and the oscillation frequency of the other laser device is stabilized with respect to the oscillation frequency of this laser device using the fleece vector range of the Fabry Vero resonator with a different frequency interval. The first method is to stabilize the frequency interval so that it matches the frequency spacing standard given by The frequency is stabilized, and it is combined with the light emitted from several other laser devices, and then this light is combined with the light emitted from a reference laser device whose frequency is swept in a sawtooth pattern with a constant period, and the beat signal is generated. The oscillation frequency interval of each laser device is stabilized by monitoring whether the appearance time of each pulse constituting the pulse train obtained as follows maintains a certain time difference from the appearance time of the pulse corresponding to the above-mentioned stabilized laser device. The second method of doing so (Strebel et al., “I-Fist-O-O-C-E-C-O-C” 85 (100C-EOOC'85) Technical Digest Vol. 3 (1985), page 81)
It is.

However, in the first method, it is necessary to sweep the mirror spacing of the Farley-Perot resonator, which provides a reference for the frequency spacing, and the device becomes larger than when a simple etalon plate is used.

In addition, in the second method, the frequency interval standard is determined from the appearance time interval of each pulse with respect to the frequency change of the reference laser device, which has been measured in advance. It is fully expected that the frequency will change, and it is difficult to say that the frequency interval of each laser device is fixed at the time of control.

On the other hand, recently Shimosaka et al. have proposed a structure that uses an etalon plate as a Fapley-Pero resonator to maintain a strict frequency spacing standard and avoid increasing the size of the device, which was a problem when using a swept-type Fapley-Pero. The one described in IEICE Technical Research Report Vol. 87 C387-96 is known.

In this configuration, the emitted light from the frequency sweeping laser device whose oscillation frequency is periodically swept is split into two, and one of the lights is input into the optical resonator. Pulsed light is emitted when the incident light and the resonant frequency of the optical resonator match. The other light is combined with the light emitted from the laser device to be controlled. If only the low frequency component is extracted from the beat signal of this combined light, a pulsed signal will be obtained when the oscillation frequencies of the laser device to be controlled and the frequency sweeping laser device almost match. By controlling each pulse of the optical resonator output and the pulse obtained from the beat to overlap on the time axis, the oscillation frequency interval of each laser device to be controlled is
It is stabilized to a value equal to the Fries vector range of the optical resonator.

(Problems to be Solved by the Invention) In the above configuration, the fleece vector range of the optical resonator is fixed so that the oscillation frequency intervals during stabilization are equal to each other. On the other hand, in the field of optical communications, attempts are currently being made to improve reception sensitivity and increase transmission distance by inserting semiconductor laser optical amplifiers or optical fiber type optical amplifiers into optical transmission lines. At this time, if the incident light to the optical amplifier is the light emitted from a plurality of laser devices with different oscillation frequencies, if the frequency interval is narrowed to about several G11z or less, the difference between adjacent oscillation frequencies f1 and f2 will be reduced. Near-degenerate four-wave mixing occurs, resulting in crosstalk between adjacent channels, leading to deterioration of reception sensitivity. Regarding near-degenerate four-wave mixing, please refer to IEICE technical research report by Mukai et al., June 20, 1988, 6.
Learn more about the literature listed on pages 9 to 74. The frequency of light generated by near-degenerate four-wave mixing is f+ (f
2 fl) f2 + (f2 - fl), etc. Therefore, if the frequencies of the laser beams incident on the optical amplifier are arranged at a constant frequency interval of f2f1, the amount of crosstalk superimposed on each laser beam is Maximum. Therefore, the laser light frequencies input to the optical amplifier need to be arranged at unequal intervals in order to suppress crosstalk. However, as described above, in the conventional configuration, the interval frequency could only be stabilized at fixed intervals.

An object of the present invention is to provide a method for stabilizing the frequency intervals of a plurality of laser apparatuses, which eliminates the drawbacks of the prior art and is capable of stabilizing the oscillation frequency intervals of a plurality of laser apparatuses at unequal intervals. It is in.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention sweeps the oscillation frequency by an externally applied signal, and sweeps the oscillation frequency within the sweep range by controlling the oscillation frequency interval. After converting the beat light obtained by combining the frequency swept light including each oscillation frequency of the laser device and the emitted light from the plurality of laser devices into an electrical signal, only the low frequency component is passed. The occurrence time of the beat pulse train, which is an electric pulse train obtained by the above-mentioned electric pulse train corresponding to each oscillation frequency of the plurality of laser devices, coincides with the frequency interval to be controlled by branching a part of the frequency-swept light. The generation time of a reference pulse train obtained by converting a pulse-like change in the intensity of the transmitted light of the frequency-swept light, which is caused by passing the frequency-swept light through an optical resonator having a periodic resonance frequency, into an electrical signal is compared with the occurrence time of a reference pulse train, The time difference between the occurrence time of each pulse and the occurrence time of each of the reference pulses to which each of the beat pulses should be based in the reference pulse train is used as an error signal, and this error signal has a substantially predetermined constant value. In a laser device oscillation frequency interval stabilization method for controlling the oscillation frequency of a plurality of laser devices to be controlled so that
The present invention is characterized in that only pulses selected every few pulses from the reference pulse train are used for control.

(Function) In the present invention, pulses arbitrarily selected every few out of the reference pulse train obtained by passing frequency swept light through an optical resonator are used for control as a new reference pulse train. Therefore, since the frequency interval corresponding to the reference pulse can be freely set within a range that is an integral multiple of the Fries vector range of the optical resonator used, the oscillation frequency interval can be stabilized at an unreasonable interval.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram of a laser device to which a method according to an embodiment of the present invention is applied.

(2) In the distributed reflection laser 1 with a 55 μm band phase control region, the repetition frequency 2 applied by the sawtooth wave generator 2 is
According to the signal 27.28 of 0kl+z (see figure 2),
The frequency of the emitted light changes over time in a sawtooth pattern. The light emitted from the distributed reflection laser 1 with a 1.55 μm band phase control region passes through an optical isolator 3 and is then 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 has a refractive index of 1.5,
After passing through a quartz glass etalon plate 5 with a reflectance on both sides set to have a finesse of 30 at a thickness of 1 cm, the first
is incident on the photodetector 6. The first photodetector 6 receives 1.55μ during one period of the output signal from the sawtooth wave generator 2.
Pulsed light is input when the frequency of the distributed reflection laser 1 with an m-band phase control region matches the resonant frequency of the etalon plate 5, and the number of pulses in one cycle is set to four. The peak voltage of the output of the sawtooth wave generator 2 is adjusted in advance. During control, the three laser devices are stabilized to the first and second 4 pulses. At this time, the frequency intervals of each laser device are stabilized at 15 GHz and 30 GHz, respectively. 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, l, which is the target for stabilizing the frequency interval, is 55 μm.
The emitted light from the band distribution feedback lasers 8, 9.10 passes through the optical isolators 11, 12.13, respectively, and is combined by the optical multiplexer 14 in the rear tube 1.

The light emitted from the first optical multiplexer 14 is sent to the second optical multiplexer 15.
The output light is multiplexed with the second output light of the optical splitter 4. The output of the second optical multiplexer 15 is converted into an electrical signal by the detector 16, and then, although not shown in the figure, the cut-off frequency is 100.
It is input to a MHz low pass filter. From the low-pass filter, the difference between the frequency of the emitted light from the 1.55 μm band distributed reflection laser with phase control region 1 and the frequency of the emitted light from the 1.55 μm band distributed feedback laser 8.9.10 is determined as follows. A pulsed electrical signal is output when the range is approximately ±100M1+z. The number of pulses is 1 per period of the output signal 27.28 of the generator 2 (see Figure 2).
．． Distributed reflection laser with 55 μm band phase control region 1 and 1.
This is equal to the number of times that the difference between the oscillation frequencies of the 55 μm band distributed feedback lasers 8, 9, and 10 falls within the range of ±100 Mllz, which is three. The electrical signal from the second photodetector 16 is applied to a second input terminal 72 of the control device 7 (details shown in FIG. 3).

In the control device 7 shown in FIG. 3, the input to the first input terminal 71 of the control device 7 shown in FIG.
The pulse generation time difference 24, 25° 26 of the input to the second input terminal 72 of the control device 7 shown in b) is used as an error signal,
A control signal is output that makes these magnitudes zero.

Note that the pulse generation time difference measuring circuit 33 (fourth
An example of the circuit is shown in the figure). When the pulses constituting the two input pulse trains are arranged in order of generation time, the corresponding order (However, since only the reference pulse 1.2.4 is used, , the pulse generation time difference measuring circuit 33 ignores the reference pulse that arrives third). Outputs a square pulse. However, depending on which of the two input pulse trains the first of the two pulses mentioned above belongs to, the output has the function of becoming a positive or negative square pulse. Shown in Figure 4. The first, second and third control signals from the control device 7 are input to laser device driving devices 17, 18 and 19, respectively.

The laser drive device 17, 18, and 19 outputs a drive current of 1.55μ according to the control signal to the distributed feedback laser 8.
Injected on 9.10. Note that the temperature control device 2 is used for the 1.55 μm band distributed reflection laser with phase control region 1, the 1.1.55 μm band distributed feedback laser 8, 9, and 10, respectively.
0,21,22.23, the temperature is stabilized within the fluctuation range of ±0.1'C.

In this embodiment, the frequency interval of only three laser devices is stabilized, but the frequency and peak voltage of the output signal from the sawtooth wave generator 2 are adjusted, and the output signal from the etalon plate 5 per cycle is adjusted. By varying the number of pulses, it is possible to stabilize the interfrequency interference of even more laser devices 2 at the same time. Furthermore, by changing the thickness of the etalon plate, the frequency interval can be freely set. Furthermore, in this example, the frequency intervals are set to 15 GIIz and 30 GHz, but by expanding the sweep range of the frequency sweep light and increasing the number of reference pulses, the range of reference pulse selection will be expanded and the frequency interval setting will be more flexible. You can increase the degree.

Further, the laser device to be stabilized is not limited to a semiconductor laser, but any laser device whose oscillation frequency changes in accordance with an external signal can be stabilized.

(Effects of the Invention) As described above, according to the present invention, it is possible to simultaneously stabilize the frequency interval of any number of laser devices, and the frequency interval can be strictly controlled by the optical resonator used. It can be arbitrarily defined within a range that is an integral multiple of the fleece vector range of the resonator.

[Brief explanation of drawings]

3. FIG. 1 is a configuration diagram of a laser device to which the method of the embodiment of the present invention is applied, and FIG. A diagram showing signals, FIG. 2(b) is a diagram showing an electric signal from the second photodetector 16 that is manually input to the control device 7 in FIG. 3 is a block diagram of the control device 7 shown in FIG. 1, and FIG. 4 is a circuit diagram of the pulse generation time difference measuring circuit shown in FIG. 1... Distributed reflection laser with 1.55 μm band phase control region, 2... Sawtooth wave generator, 3, 11, 12.13.
... Optical isolator, 4... Optical splitter, 5... Etalon plate, 6.16... Photodetector, 7... Control device,
8,910-1.55 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 sawtooth wave generator 2. 4

Claims (1)

[Claims] Frequency swept light including each oscillation frequency of a plurality of laser devices whose oscillation frequencies are swept by an externally applied signal and whose oscillation frequency intervals are controlled within the sweep range;
A beat pulse train obtained by converting beat light obtained by combining the light emitted from the plurality of laser devices into an electrical signal and then passing only the low frequency component thereof, the beat pulse train being obtained by combining the light emitted from the plurality of laser devices. The generation time of a beat pulse train, which is an electric pulse train corresponding to each oscillation frequency of the device, and branching a part of the frequency sweep light to pass through an optical resonator having a periodic resonance frequency that matches the frequency interval to be controlled. The generation time of each pulse of the beat pulse train and the occurrence time of each pulse of the beat pulse train are compared with the occurrence time of a reference pulse train obtained by converting the pulse-like change in the passing light intensity of the frequency swept light into an electrical signal, and The oscillation frequencies of the plurality of laser devices to be controlled are controlled so that the time difference between each beat pulse and the generation time of each of the reference pulses to be used as a reference is used as an error signal, and this error signal is approximately a predetermined constant value. A laser device oscillation frequency interval stabilization method for controlling a laser device oscillation frequency interval, characterized in that only pulses selected every few pulses from the reference pulse train are used for control.