JP2751521B2 - Laser device frequency interval stabilization method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oscillation frequency interval stabilization method for stabilizing an interval between oscillation frequencies of a plurality of laser devices used for optical communication or the like.

(Prior Art) As a method for stabilizing the frequency interval between a plurality of laser devices, the following two methods are conventionally known. The oscillation frequency of one laser device is stabilized with respect to the Fabry-Perot resonator, and the oscillation frequency of the other laser device is compared with the oscillation frequency of this laser device. The first method is to stabilize it so as to match the frequency interval criterion given by Toba et al., Proceedings of the 1986 IEICE Communication Division National Convention, Volume 2, pages 2-2-204, or one laser device Stabilizes the frequency of the laser beam, multiplexes the beam with some other laser device output light, and multiplexes this light with the reference laser device output light sweeping the frequency in a sawtooth shape at a constant period, and generates a beat signal. It is monitored whether or not the appearance time of each pulse constituting the pulse train obtained as described above keeps a fixed time difference from the appearance time of the pulse corresponding to the stabilized laser device. The second method of stabilizing the transmission frequency interval of each laser device (IOC-EOOC '85 Technical Digest No. 3 by Strebel et al. Volume (1985), p. 61).

However, in the first method, it is necessary to sweep the mirror interval of the Fabry-Perot resonator which gives a reference of the frequency interval, and use the same, and the device becomes larger than when a simple etalon plate is used. Further, in the second method, since the reference of the frequency interval is obtained in the appearance time interval of each pulse with respect to the frequency change of the reference laser device which has been measured in advance, this interval reference is greatly reduced during actual control. It is fully expected that it 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, by using an etalon plate as a Fabry-Perot resonator, it is possible to maintain a strict frequency interval reference, and to avoid the increase in size of the device which was a problem when using a sweep type Fabry-Perot, The one described in IEICE Technical Report Vol. 87, CS87-96 by Shimoita et al. Is known. In this configuration, the output light from the frequency sweeping laser device whose oscillation frequency is periodically swept is split into two, and one of the split light enters the optical resonator. When the incident light coincides with the resonance frequency of the optical resonator, pulsed light is emitted. The other light is multiplexed with the output light of the laser device to be controlled. When only the low-frequency component is cut out of the beat signal of the combined light, a pulse signal is obtained when the oscillation frequencies of the laser device to be controlled and the frequency sweeping laser device substantially 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 equal to the free spectral range of the optical resonator. Stabilized to a value.

(Problem to be Solved by the Invention) In the above configuration, the free spectral range of the optical resonator is fixed so that the oscillation frequency intervals during stabilization become equal to each other. On the other hand, in the field of optical communication, attempts have been made widely to improve reception sensitivity and increase transmission distance by inserting a semiconductor laser optical amplifier or an optical fiber type optical amplifier into an optical transmission line. At this time, when the light incident on the optical amplifier is the light emitted from a plurality of laser devices having different oscillation frequencies, if the frequency interval is narrowed to about several GHz or less, the light of the adjacent oscillation frequencies f ₁ and f ₂ is reduced. Near-degenerate four-wave mixing occurs between the adjacent channels, resulting in crosstalk between adjacent channels, resulting in deterioration of reception sensitivity. For near-degenerate four-wave mixing, see page 69 of June 20, 1988, published by IEICE Technical Report by Mukai et al.
I am familiar with the literature on the page. The frequency of light generated by near-degenerate four-wave mixing is f ₁ − (f ₂ −f ₁ ), f ₂ + (f ₂ −f ₁ )
Therefore, when the frequency of the laser light incident on the optical amplifier is arranged at a constant frequency interval of f ₂ −f ₁ , the amount of crosstalk superimposed on each laser light becomes 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,
Only at certain intervals could the interval frequency be stabilized.

SUMMARY OF THE INVENTION An object of the present invention is to provide a laser device frequency interval stabilization method capable of stabilizing the oscillation frequency intervals of a plurality of laser devices at unequal intervals by eliminating such disadvantages of the prior art. It is in.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention sweeps the oscillation frequency to a signal applied from the outside, and
Frequency sweep light including the respective oscillation frequencies of a plurality of laser devices whose oscillation frequency intervals are to be controlled in the sweep range, and beat light obtained by multiplexing light emitted from the plurality of laser devices with an electrical signal. After being converted into a beat pulse train obtained by passing only the low-frequency component thereof, the occurrence time of a beat pulse train being an electric pulse train corresponding to each oscillation frequency of the plurality of laser devices, and the frequency sweep light Is obtained by converting a pulse-like change in the transmitted light intensity of the frequency sweep light generated by passing a part of the light through an optical resonator having a periodic resonance frequency corresponding to a frequency interval to be controlled into an electric signal. The occurrence time of a reference pulse train is compared with the occurrence time of each pulse of the beat pulse train, and each beat pulse in the reference pulse train is used as a reference. A laser device oscillation frequency for controlling the oscillation frequency of a plurality of laser devices to be controlled such that the time difference between the occurrence time of each of the reference pulses to be generated is an error signal, and the error signal has a substantially fixed value. In the interval stabilization method, only a pulse selected every few pulses from the reference pulse train is used for control.

(Operation) In the present invention, among the reference pulse trains obtained by passing the frequency sweeping light through the optical resonator, pulses arbitrarily selected every few pulses are used for control as a new reference pulse train. Therefore, the interval of the frequency corresponding to the reference pulse can be set freely within an integral multiple of the free spectrum range of the optical resonator to be used, so that the oscillation frequency interval can be stabilized at an inappropriate interval.

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

In the 1.55 μm band distributed reflection laser 1 with a position control area,
20 kHz repetition frequency applied by sawtooth wave generator 2
According to the signals 27 and 28 of z (see FIG. 2), the frequency of the emitted light changes in a sawtooth manner with respect to time. After the light emitted from the 1.55 μm band distributed control type laser 1 with a phase control region passes through the optical isolator 3, the light splitter 4 divides the light into first and second output lights at a power ratio of 1: 1. this house,
The first output light is 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.
, And then enter the first photodetector 6. In one cycle of the output signal from the sawtooth wave generator 2, the first photodetector 6 includes the 1.55 μm band distributed reflection type laser 1 with a phase control region.
The pulsed light is input at the point in time when the frequency coincides with the resonance frequency of the etalon plate 5. Adjust it. When controlling, the three laser devices are stabilized to the first, second, and fourth pulses among them. At this time, the frequency interval of each laser device is stabilized at 15 GHz and 30 GHz, respectively. The electric signal from the first light detector 6 is applied to a first input terminal 71 of the control device 7.

On the other hand, the output lights from the 1.55 μm band distributed feedback lasers 8, 9, and 10 whose frequency intervals are to be stabilized pass through the optical isolators 11, 12, and 13, respectively, and are then combined by the first optical multiplexer 14. Is done. The output light from the first optical multiplexer 14 is multiplexed with the second output light of the optical splitter 4 by the second optical multiplexer 15. After the output of the second optical multiplexer 15 is converted into an electric signal by the detector 16, the cut-off frequency 10
Input to 0MHz low pass filter. From the low-pass filter, the difference between the frequency of the light emitted from the 1.55 μm band distributed reflection laser 1 with the phase control region and the frequency of the light emitted from the 1.55 μm distributed feedback lasers 8, 9, and 10 is approximately ± A pulse-like electric signal is output when the frequency is in the range of 100 MHz.
The number of pulses is determined by the period of the output signals 27 and 28 of the sawtooth generator 2 (see FIG. 2), the 1.55 .mu.m band distributed reflection type laser with phase control region, and the 1.55 .mu.m band distributed feedback laser 8, 9, 10 Is equal to the number of times that the difference between the oscillation frequencies of each of them falls within the range of ± 100 MHz, that is, three. An electric signal from the second photodetector 16 is applied to a second input terminal 72 of the control device (details are 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. 2 (a) and the second input of the control device 7 shown in FIG. Input terminal
The pulse generation time difference 24, 25, 26 of the input to 72 is used as an error signal, and a control signal is output such that these large values become zero.

The pulse generation time difference measurement circuit 33 (FIG.
In the figure, an example of a circuit is shown. When the respective pulses constituting the two input pulse trains are arranged in the order of generation time, the corresponding order (however, only the first, second and fourth reference pulses are used) The pulse generation time difference measurement circuit 33 ignores the reference pulse arriving thirdly.) A rectangular pulse having a width proportional to the generation time difference of two pulses (total three sets) and a constant height Is output. However, the output has a function of forming a positive or negative square pulse depending on which of the two series of pulse trains the previously generated pulse of the above two pulses belongs to. It is shown in FIG. The first, second, and third control signals from the control device 7 are input to laser device driving devices 17, 18, and 19, respectively.
Drive currents corresponding to the control signals are injected into the 1.55 μm band distributed feedback lasers 8, 9, and 10 from the laser drive units 17, 18, and 19. In addition, the temperature control devices 20, 21, 22, and 23 controlled the temperature of the distributed reflection laser 1 with 1.55 μm band phase control region and the distributed feedback lasers 8, 9, and 10 within ± 0.1 ° C by the temperature control devices 20, 21, 22, and 23, respectively. It has been stabilized.

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 signal is emitted from the etalon plate 5 per cycle. By changing the number of pulses, the frequency intervals of more laser devices can be simultaneously stabilized. Further, the frequency interval can be freely set by changing the thickness of the etalon plate. In the present embodiment, the frequency interval is set to 15 GHz and 30 GHz. However, by expanding the sweep range of the frequency sweep light and increasing the reference pulse, the width of the reference pulse selection is widened and the degree of freedom in setting the frequency interval is increased. Can be increased. Further, the laser device to be stabilized is not limited to a semiconductor laser, and any laser device whose oscillation frequency changes in response to an external signal can be stabilized.

(Effects of the Invention) As described above, according to the present invention, frequency intervals of an arbitrary number of laser devices can be simultaneously stabilized,
In addition, the frequency interval can be strictly determined by the optical resonator used, and arbitrarily defined within a range of an integral multiple of the free spectral range of the optical resonator.

[Brief description of the drawings]

FIG. 1 is a block diagram of a laser device to which the method according to the embodiment of the present invention is applied, and FIG. 2 (a) is an electric signal from a first photodetector 6 input to a control device 7 in FIG. FIG. 2B is a diagram showing an electric signal from the second photodetector 16 input to the control device 7 in FIG. FIG. 3 is a block diagram of the control device 7 in FIG. 1, and FIG. 4 is a circuit diagram of the pulse generation time difference measurement circuit in FIG. 1 .... 1.55μm distributed reflection laser with phase control area
…… Sawtooth wave generator, 3,11,12,13 …… Optical isolator,
4 ... Optical splitter, 5 ... Etalon plate, 6,16 ... Photodetector, 7 ... Control device, 8,9,10 ... 1.55 μm band distributed feedback laser, 14,15 ... Optical multiplexing , 17,18,19 …… Laser device drive device, 20,21,22,23 …… Temperature control device, 24,25,26…
... Error signal, 27, 28... Output waveform from sawtooth wave generator 2.

Claims (1)

(57) [Claims] 1. A frequency sweeping light that sweeps an oscillation frequency by a signal applied from the outside and includes respective oscillation frequencies of a plurality of laser devices to be controlled in the sweep range, and the plurality of lasers. A beat pulse train obtained by converting a beat light obtained by multiplexing light emitted from the device into an electric signal, and then passing only the low-frequency component thereof. The occurrence time of a beat pulse train, which is an electric pulse train corresponding to the frequency, and the frequency generated by branching a part of the frequency sweeping light and passing through an optical resonator having a periodic resonance frequency that matches a frequency interval to be controlled. A pulse-like change in the intensity of the passing light of the sweep light is converted into an electric signal, and the occurrence time of a reference pulse train is compared with the occurrence time of each pulse of the beat pulse train. A time difference between a time and an occurrence time of each of the reference pulses to be used as a reference in the reference pulse train is set as an error signal, and the error signal is controlled to a substantially fixed value. A laser device oscillation frequency interval stabilization method for controlling oscillation frequencies of a plurality of target laser devices, wherein only a pulse selected every few pulses from the reference pulse train is used for control. .