JPH0658985B2 - Laser device oscillation frequency interval stabilization method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser device oscillation frequency interval stabilization method for stabilizing the intervals of oscillation frequencies of a plurality of laser devices.

(Prior Art) Conventionally, as a method of stabilizing the frequency intervals of a plurality of laser devices, the oscillation frequency of one laser device is stabilized with respect to the Fabry-Perot resonator, and A method of stabilizing the oscillation frequencies of other laser devices so that their frequency intervals match the frequency interval criteria given by the free spectral range of another Fabry-Perot resonator (Toba et al. National Conference Proceedings, Volume 2, pages 2-204), or stabilizing the frequency of one laser device,
A pulse train to be obtained as a beat signal is formed by combining the light emitted from several other laser devices and further combining this light with the light emitted from the reference laser device whose frequency is swept in a sawtooth shape. A method for stabilizing the oscillating frequency interval of each laser device by monitoring whether the appearance time of each pulse has a constant time difference with respect to the appearance time of the pulse corresponding to the stabilized laser device (Strebel et al. By i o o o e e o o o by
The Sea '85 (IOOC-ECOC'85) Technical Digest Volume 3 (1985) p. 61) is known.

However, in the first method, it is necessary to sweep and use the mirror spacing of the Fabry-Perot resonator that provides the reference of the frequency spacing, which makes the device larger than when using a simple etalon plate. In the second method,
Since the frequency interval standard is calculated as the appearance time interval of each pulse with respect to the frequency change of the reference laser device that has been measured in advance, it is not enough that this interval standard changes significantly during actual control. It is hard to say that the frequency interval of each laser device is predicted and fixed at the time of control.

On the other hand, recently, by using an etalon plate as a Fabry-Perot resonator, it maintains a strict frequency interval reference, and as a configuration that avoids the increase in size of the device, which was a problem when using a sweeping Fabry-Perot resonator, The one described in IEICE technical report Vol. 87 CS87-96 by Shimosaka et al. Is known. This will be described with reference to FIG. In this configuration, the light emitted from the frequency sweeping laser device 1 whose oscillation frequency is periodically swept is split into two by the optical splitter 4, and one of them is incident on the optical resonator 5. When the incident light and the resonance frequency of the optical resonator 5 match, pulsed light is emitted. The other light is combined with the light emitted from the laser devices 8, 9 and 10 to be controlled by the optical multiplexer 15. If only the low-frequency component is cut out from the beat signal of this combined light, a pulse-like signal is obtained when the oscillation frequencies of the laser device to be controlled and the frequency sweeping laser device substantially match. By controlling by the control device 7 so that each pulse of the output of the optical resonator and the pulse obtained from the beat overlap on the time axis, the oscillation frequency interval of each laser device to be controlled becomes the free spectral range of the optical resonator. Stabilized to a value equal to.

The control device 7 is configured as shown in FIG.

(Problems to be Solved by the Invention) In the above configuration, the output pulse train (reference pulse train) when the frequency sweep light is input to the optical resonator and the emitted light of the laser device to be controlled are combined, and Are combined with the frequency sweep light, and the obtained pulses are controlled so that the pulses in the corresponding order of the pulses constituting the beat pulse train consisting of low frequency components overlap on the time axis. Therefore,
When the frequency sweep light changes the frequency sweep range to be swept due to disturbance, the first pulse of the reference pulse train disappears, or a new pulse occurs before the first pulse is generated. Will come off.

It is an object of the present invention to provide a method for stabilizing the oscillation frequency interval of a laser device by eliminating such drawbacks of the prior art and keeping the control operation stable even in the presence of disturbance.

(Means for Solving Problems) In order to achieve the above object, in the present invention, the emitted lights of a plurality of laser devices to be controlled are combined, and the combined light is a frequency sweep light whose oscillation frequency is swept. The beat pulse train obtained by selecting the low-frequency component of the beat signal obtained by combining with the pulse signal is compared with the reference pulse train corresponding to the frequency interval reference, and the occurrence time difference of the pulses in the corresponding order of both pulse trains is calculated. As the error signal, in the laser device oscillation frequency interval stabilization method, wherein the plurality of laser devices are controlled so that the error signal has a predetermined constant value. The difference between the sweep start time of the sweep light and the generation time of the pulse that first occurs among the pulses that make up the reference pulse train that occur within one sweep time is defined. It is characterized in that the frequency range in which the frequency sweep light is swept is controlled so as to substantially match the reference value.

(Operation) In the present invention, of the pulses constituting the reference pulse train emitted from the optical resonator, there is a difference between the generation time of the first pulse counted from the start of the sweep and the start time of the sweep. It is controlled so as to be in the vicinity of a constant value. This prevents the disappearance of the first pulse and the occurrence of another pulse before the original first pulse. Therefore, by using the configuration of the present invention, it is possible to configure a stable system in which control is not lost even when a disturbance is received.

(Examples) Hereinafter, the present invention will be described in detail with reference to Examples. FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a block diagram of the control device in FIG. 1, and FIG. 3 (a) is a concrete example of the pulse generation time difference measuring circuit in FIG. It is a schematic circuit diagram. 1 and 2
Since the operation of the portion excluding the area surrounded by the dotted line in FIG. 3 (a) has been already described in the section (Prior Art), the dotted line portion in FIG. 3 (a) will be described here.

A portion surrounded by a dotted line in FIG. 3 (a) is a circuit for preventing disappearance or new generation of the reference pulse. In this portion, the time difference between the start point of the sweep cycle and the occurrence time of the pulse that first occurs within one sweep time in the reference pulse train is measured. When the measurement result deviates from the preset reference value by a certain amount or more, the amount of deviation from the reference value of the measurement result is used as an error signal in the active region of the 1.55 μm wavelength tunable distributed reflection type semiconductor laser (DBR) 1 The basic operation is to return to the injection current.

The T flip-flop is driven by the first pulse of the reference pulse corresponding to the pulse generated at the starting point of the sweep cycle and the optical resonator output pulse, which is input to the dotted line portion. At this time, the output of the T flip-flop becomes a square wave having the above two pulses as a starting point and an ending point. Since the height of the square wave is equal to the logic level E _L and is always constant, the output E _L Δt corresponding to the time difference Δt between the two pulses is obtained by integrating this square wave by the integrator in the next stage. . This integrator output is input to two comparators having the characteristics shown in FIG. 3 (b). The exclusive OR of the outputs of these comparators opens and closes the switching gates in the control loop. By this operation, the time difference corresponding to the voltages E ₁ and E ₂ shown in FIG. 3 (b)
With respect to t ₁ and t ₂ (where E ₁ = E _L t ₁ and E ₂ = E _L t ₂ ), the time difference Δt between the two pulses is in the range of 0 ≦ Δt ≦ t ₁ and t ₂ ≦ Δt. The control loop closes and the error signal is fed back to DBR1.

In this embodiment, although only three laser devices are frequency-space stabilized, the frequency of the output signal from the sawtooth generator,
By adjusting the peak voltage and changing the number of pulses emitted from the etalon plate 5 per cycle, it is possible to stabilize the frequency intervals of more laser devices at the same time. Further, the frequency interval can be freely set by changing the thickness of the etalon plate. 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, the frequency intervals of an arbitrary number of laser devices can be stabilized at the same time, and the frequency intervals can be strictly defined by the optical resonator used. it can. Furthermore, the reference pulse prevents disappearance or new pulse generation during control,
More stable control is possible.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention. 2 is a block diagram of the control device 7 in FIG.
3A is a circuit diagram of the pulse generation time difference measuring circuit in FIG. 3, and FIG. 3B is the input / output characteristics of the comparators 1 and 2 shown in FIG. 3A. In FIGS. 1, 2 and 3 (a), (b), 1 ... 1.55 μm band wavelength tunable semiconductor laser, 2 ... 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 multiplexer, 17,
18, 19 ... Laser device driving device, 20, 21, 22, 23 ... Temperature control device, 401 ... Input / output characteristic of comparator 1, 402 ... Input / output characteristic of comparator 2.

Claims (1)

[Claims] 1. A low-frequency component of a beat signal obtained by combining emitted lights of a plurality of laser devices to be controlled and combining the combined light with a frequency sweep light whose oscillation frequency is swept. The beat pulse train obtained by selection is compared with a reference pulse train corresponding to the frequency interval reference, and the difference between the occurrence times of the pulses in the corresponding orders of both pulse trains is used as an error signal so that this error signal has a predetermined constant value. In the laser device oscillation frequency interval stabilization method characterized by controlling the plurality of laser devices, in the sweep start time of the frequency sweep light, each pulse constituting the reference pulse train generated within one sweep time Among them, the frequency range in which the frequency sweep light is swept is controlled so that the difference in the generation time of the first pulse is almost equal to the predetermined reference value. Device oscillation frequency interval stabilizing method.