JPS6364380A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To prevent the variation of output frequency based on frequency modulation by controlling the temperature of a semiconductor laser at a fixed value while stopping or weakening a vibration signal which is minutely vibrating laser driving currents only for a preset period. CONSTITUTION:The temperature of a laser diode 11 for a semiconductor laser device is stabilized by a temperature controller 31, and output beams 35, frequency of which is stabilized by stabilizing the temperature, is inputted to a spectroscope 32. A pump optical output and a probe optical output are inputted to photodiodes 16, 17 from the spectroscope 32. A variation component except the variation of oscillation frequency from the laser diode 11 is removed by a divider 15, and inputted to a drive circuit 34. Output beams 35 passing through an optical switch 48 by a half mirror 35a are synchronized with an output from a modulation control circuit 46 stopping or weakening a vibration signal fed to the circuit 34 from a signal source 38 only for a desired period and pass, thus acquiring wavelength stabilizing output beams.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention relates to a semiconductor laser device that can obtain a wavelength (frequency) stabilized laser output, and more particularly relates to a semiconductor laser device that can obtain wavelength (frequency) stabilized laser output, and more specifically, The present invention relates to a semiconductor laser device including means for stabilizing wavelength.

[Conventional technology] In recent years, due to improvements in the performance of semiconductor lasers, frequency stabilization using semiconductor lasers with performance superior to frequency stabilization light sources using gas lasers (He-Ne lasers, etc.) that have been commercialized in the past has become possible. Research on light sources is becoming more active. This type of research has been carried out in close connection with the development of light sources for use in heterodyne/coherent optical communication systems, which are new methods of optical fiber communication. The above optical communication system uses (1) a semiconductor laser with extremely high frequency stability;

(2) Improving the spectral purity of semiconductor lasers.

(3) Wideband, high efficiency modulation technology.

Due to the importance of hats, rapid progress is being made in research and development aimed at finding solutions to these problems. These techniques are also important for high-precision measurement of distance, length, displacement, etc. in coherent optical measurement.

Among these technologies, especially regarding (1) and (2) above, technological progress has been remarkable due to improvements in the semiconductor laser itself.For example, as a laser with a new structure, DBR
(D 1tributed Bran Ref
factor) type (black reflective type) laser, DFB
(Distributed Feed Back)
type (distributed feedback type) laser, etc. This is a laser with a structure that uses a diffraction grating to determine the wavelength, and is a so-called DSM (Dynamic Single Modular Laser).
e) It is about to be put into practical use as a laser. However, there are variations in the laser oscillation wavelength due to temperature and so-called wavelength chirping, in which the wavelength shifts slightly when the semiconductor laser is modulated at high speed.

By the way, in order to stabilize the laser frequency, servo control is an essential requirement. In this servo control, the laser frequency is discriminated by an optical frequency discriminator, and the oscillation is controlled so that an output corresponding to the frequency difference of the laser frequency to be stabilized with respect to the frequency reference of the optical frequency discriminator is obtained at a constant level. There is a need. There are two typical methods for optical frequency discrimination:

(1) A method using a Fabry-Perot interferometer (hereinafter referred to as FP interferometer).

(2) A method that uses absorption spectral lines of molecules or atoms.

FIG. 8 shows a semiconductor laser frequency (wavelength) stabilizing device using a conventional FP interferometer. Here, (11) is a laser diode, (12) is a current drive circuit for driving the laser diode, and (13) is a PID (proportional+integral-sufficient differential) circuit.

(14) is a differential amplifier, (25) is a laser diode (
The variable voltages, sources, ■ and V, for setting the optical output of 11) are the electrical output signals of the photodiodes (16) and (17), respectively, and are applied to the divider (15). (
18) is an FP interferometer, (19) is a mirror, (20) is a beam splitter, and (21) and (22) are spatial filters, for example, pinholes with a diameter of 5 to 10AIII.

(23) and (24) are lenses for focusing light similar to microscope objective lenses. Also, laser diode (11)
In order to record the error signal with respect to the oscillation frequency reference (in this case, the resonant frequency of the FP interferometer (18)) as frequency stability evaluation data, the signal input to the telerecorder (28) is filtered through a low-pass filter (26) and an amplifier. (27)
are getting through.

Next, an outline of the operation of the apparatus shown in FIG. 8 will be explained.

The optical output of the laser diode (11) is converted into a parallel beam by a lens (23), passes through a spatial filter (22), and is split into two beams by a beam splitter (20), as shown by the dotted line. One of the light beams continues straight and reaches the FP
Interferometer (18), lens (24) and spatial filter (21)
) and is incident on the foot diode (17). The other beam is reflected by the beam splitter (20), reflected again by the mirror (19), and then incident on the photodiode (16). The electrical output V8 from the photodiode (17) includes the signal component of the difference between the oscillation frequency of the laser diode (11) and the reference frequency, except for the signal component corresponding to the Vf signal of the difference with the frequency reference of the optical frequency discriminator. This includes the optical output fluctuation component, so it is necessary to remove this component. For this purpose, the photodiode (16) obtains the electrical signal ■4 corresponding to the light that does not pass through the FP interferometer (18), and the divider (15) calculates ■, /VA.This allows the optical frequency discriminator to A signal can be obtained from which optical output fluctuation components other than the fluctuation components corresponding to the difference from the frequency reference are removed.In this example, the divider (15) calculates VB/VA, but the subtracter calculates V- A similar effect can be obtained by finding V.

The output of the divider (15) is input to the differential amplifier (14),
It is compared with the reference voltage of the reference voltage source (25), and a signal corresponding to the difference in meat input, that is, an error output, is given to the drive circuit (12) via PID (proportional + integral and sufficient derivative) circuit #1 (13). . As a result, the laser diode (11) is controlled by negative feedback so that the oscillation frequency of the laser diode (11) is constant, that is, the output of the divider (15) is constant.

By the way, the method shown in FIG. 8 has a drawback that the frequency discrimination characteristic of the FP interferometer (18) changes due to temperature fluctuations. Temperature fluctuation of FP interferometer (18) is 10-2 to 1
It is extremely difficult to control the temperature to below 0-3° C., considering the size of the interferometer and the conditions in which it is installed. The following techniques have been proposed to solve the above drawbacks.

(1) The interferometer shall be hermetically sealed to protect it from atmospheric pressure, external temperature, water vapor, etc. (Special Publication No. 60-120584>
The other is set to be negative, and the two interferometers are used so that their temperature characteristics cancel each other out. (Special Publication No. 6O-117693) (3) The interferometer is placed inclined at a set angle θ with respect to the optical axis, and the set angle θ is changed to follow temperature fluctuations to keep the light transmittance of the interferometer constant. (Japanese Patent Publication No. 60-117694) However, the methods shown in (1) to (3) above all require complicated and precise control, and have the disadvantage of being inferior in practicality and reliability. It was difficult to obtain a stability of 10-9 or higher.The square root of Allan variance σ(τ) will be used to express the stability.

FIG. 9 shows a method using atomic or molecular absorption spectral lines to discriminate optical frequencies. In this method, for example, a cesium atom, a rubidium atom, or an NH3 A spectrometer (32) is arranged to separate the absorption spectra of molecules such as the above in such a way as to eliminate the spectrum broadening caused by the so-called Doppler effect. This spectrometer (32) includes an absorption cell, a half mirror, a polarizing beam splitter, an optical attenuator, etc., and a laser diode (11).
) contains information on the difference between the oscillation frequency of the laser diode (11) and the reference frequency, that is, absorption spectrum information without Doppler broadening corresponding to each absorption order of many absorption spectrum lines of atoms or molecules at the oscillation frequency of the laser diode (11). It is configured to send out a spectral output (37), that is, a probe light output, and a spectral output (36), that is, a pump light output that includes information on absorption spectrum lines with Doppler spread of atoms or molecules. (16) and (17) are photodiodes that detect the pump light output (36) and the probe light output (37), respectively. The divider (15) connected to the output lines of the photodiodes (16) and (17) is a circuit that calculates the division output V a /V t of the output signal v v of the photodiodes <16) (17). B Yes. The output line (39) of the divider (15) is connected to a lock-in amplifier (33). This lock-in amplifier (33) also connects the vibration signal source (38) to the line (45).
), and is configured to respond to an input signal applied from line (39) and a reference signal applied to line (45), and send an output corresponding to the phase difference between the two signals to line (40). ing. A vibration signal source (38) is connected to the drive circuit (34) through a line (44), a lock-in amplifier (33) is connected through a line (40), and this output line (41) is connected to a laser diode (11). It is connected to the.

(31) is a temperature control device, which detects the temperature of the laser diode (11) using a thermistor (not shown), and controls the temperature of the laser diode (11) to keep this temperature constant.
11) is used to control, for example, a Beltier element (not shown) attached to the controller. Note that the method shown in FIG. 9 is based on, for example, IEICE Technical Report V.

1.82. No. 267, pages 61 to 66 of the paper "Frequency Stabilization of Semiconductor Lasers Using Saturation Absorption Spectroscopy".

FIG. 10 shows the lock-in amplifier (33) in principle, with a signal input terminal (71) connected to the line (39) in FIG. 9, and a reference signal connected to the line (45). It has an input terminal (72). Signal input terminal (71
) is a preamplifier (73) and a bandpass filter (74)
connected to PSD (phase sensitive detection) (75) via
The reference signal input terminal (72) is connected to a bandpass filter (76).
), a phase shifter for phase adjustment (77), and a waveform shaping circuit (78)
) to the PSD (75). P.S.
The output of D (75) is connected to an output terminal (80) via a low-pass filter (79). Note that the output terminal (80) is connected to the output line (40) in FIG. 9.

This lock-in amplifier (33) is connected to the No. 18 source (
38) detects and amplifies the frequency component synchronized with the reference signal given from the output line (45) from the input signal, and sends out a DC output signal corresponding to the phase difference between the input signal and the multi-point signal. .

Next, the operation of the method shown in FIG. 9 will be explained. The temperature of the laser diode (11) is stabilized by feedback control by the temperature control device (31), and its fluctuation range is between 10°C and 10°C.
The temperature will be around 4℃. If the temperature fluctuation is kept below 10-3°C, frequency stability of about 10-6 to 10-8 can be obtained. Output light (35) of laser diode (11)
is input to a spectrometer (32), and a spectral output (37) containing absorption spectrum lines of cesium atoms, rubidium atoms, NHs molecules, etc. is obtained. To obtain this absorption spectral line, an optical system for saturated absorption spectroscopy (not shown) is included. One photodiode (17) receives the spectral output (37), that is, the probe light output, which includes an absorption spectrum line without a broadening due to the Doppler effect, and the other photodiode (16) receives an absorption spectrum line with a broadening due to the Doppler effect. The included spectral output (36), that is, the pump light output is input. One photodiode (1
7), a detection signal V8 containing a fluctuation component corresponding to the absorption spectrum line as an optical frequency discriminator is obtained, but this detection signal Va includes a reference frequency of the absorption spectrum line, for example, the frequency reference point of the absorption spectrum line. It is desirable to remove these fluctuation components, since they include fluctuation components other than fluctuations in the laser output other than fluctuation components corresponding to the difference between the center peak point) and the oscillation frequency of the laser diode (11). The divider (15) is provided for this purpose, and Va/v
By performing the calculation of A, fluctuation components other than the oscillation frequency fluctuation of the laser diode (11) with respect to the frequency reference point of the absorption spectrum line are removed. Of course, this variation component may be removed by a subtracter. In this way, the output line (39) of the divider (15) obtains a signal containing absorption level fluctuation signals due to fluctuations in the oscillation frequency of the laser diode (11) to be stabilized, which is transmitted to the lock-in amplifier (33). It becomes input. The detection signal of the laser frequency fluctuation corresponding to the absorption level fluctuation near the frequency reference point of the absorption spectrum line in the output of the divider (15) is at a minute level and is buried in noise, so it is amplified as is and fed back. Even when used for control, it is difficult to obtain good results.

Therefore, in the method shown in FIG. 9, wobbling control is performed. A sine wave alternating current signal with a constant frequency of, for example, 1 kHz is transmitted from the vibration signal source (38) to the drive circuit (3) through the line (44).
4), and a current (I) containing this vibration component as shown in FIG. 11 is supplied from the drive circuit (34) to the laser diode (34).
11). That is, the constant current supplied from the drive circuit (34) to the laser diode (11) is modulated at a level of about 1 μA and 1 kHz, for example. If the current (I) of the laser diode (11) slightly oscillates, the output frequency (light wavelength) of the laser diode (11) changes to the 12th wavelength.
As shown in Figure (a), the frequency is modulated, for example, at least about 108H2 to several tens of MHz. In this way, the laser diode (
When the output frequency of 11) is modulated, the input signal of the input line (39) of the lock-in amplifier (33) containing the level fluctuation information of the absorption spectrum line will contain a wobbling frequency component. In the lock-in amplifier (33), the detection signal corresponding to the laser frequency fluctuation including the frequency component of the double ring is compared in phase with the reference signal on the line (45), and the line (40) is outputted corresponding to this phase difference. I get the output. If the frequency components of the reference signal on line (45) and the input signal on line (39) synchronized therewith match, the error signal obtained on the output line <40> of the lock-in amplifier (33) will be zero. If the phase difference between the two signals becomes negative, the error signal on the output line (40) becomes positive, and if the phase difference becomes positive, the error signal on the output line (40) becomes negative. In other words, this is equivalent to detecting the first-order differential signal of the absorption spectrum. line(
40) is connected to the drive circuit (3=1>, so
The current of the laser diode (11) is controlled by negative feedback using the error output, and a control state in which the error output becomes zero is obtained. The method shown in Figure 9 combines frequency stabilization based on the absorption spectrum line of atoms or molecules, adoption of a wobbling method (modulation method), and temperature control, so frequency stabilization is 10 ~ It will be about 1o-13,
This is improved by 3 to 5 orders of magnitude compared to the FP interferometer system shown in FIG.

[Problems to be solved by the invention] By the way, in the system shown in FIG.
The output frequency of the laser diode (11 ) Spectral spread of the transmission frequency (usually several MHz to several + I (H2)) is not a problem for applications (e.g., light sources for optical bombing, simple wavelength standards, spectroscopy using lasers, precision length measurement, etc.) ).However, it may not be possible to use it as a frequency-stabilized light source for heterodyne/coherent optical communication systems and coherent optical applied measurements.In other words, in these applications, the spectral broadening of the laser diode's oscillation frequency must be at least It is desired to reduce the frequency to below I MHz, and until now there has been no means to achieve this performance as a small and reliable system.

Therefore, the purpose of the present invention is to achieve frequency modulation (wobbling).
A semiconductor that can obtain a laser output without frequency (wavelength) fluctuations based on wobbling control, and can stably switch between frequency stabilization based on wobbling control and frequency stabilization without wobbling control. The purpose of the present invention is to provide a laser device.

[Means for Solving the Problems] In order to solve the above problems and achieve the above objects, the present invention provides a semiconductor laser and a temperature control system for controlling the temperature of the semiconductor laser to a predetermined value by feedback control. a control device and an atomic or molecular absorption spectrum spectrometer, the light for obtaining a signal for detecting a frequency difference between the frequency of the output light of the semiconductor laser and a frequency serving as a frequency reference as an absorption level difference of the absorption spectrum; a frequency discriminator, a vibration signal source that generates a vibration signal for micro-vibrating the driving current of the semiconductor laser, and a vibration signal source connected to the optical frequency discriminator and the vibration signal source, the discrimination signal and the vibration signal being connected to the optical frequency discriminator and the vibration signal source; A phase difference compatible signal forming circuit that outputs a signal (error signal) corresponding to the phase difference between In the semiconductor laser device, the vibration signal supplied from the vibration signal source to the drive circuit is set to a predetermined value. The present invention relates to a semiconductor laser device characterized in that it is provided with a control means for stopping for a certain period of time or for causing an extremely weak drop.

[Operation] The control means according to the present invention controls the temperature of the semiconductor laser based on the temperature control device and the temperature control of the semiconductor laser based on the temperature control device, and the control of atoms or The output frequency (wavelength ) is obtained. If the modulation of the drive current by the vibration signal is stopped or weakened in this state, the laser output frequency will no longer include frequency fluctuations based on the modulation, or will become minute. If the period of stopping or weakening is short, it is possible to obtain a stabilized wavelength (frequency) that substantially continues the effect of the control immediately before the period of stopping or weakening. During this period of stopping or weakening, the drive current is not substantially modulated, so there is no variation in the output frequency based on this, and at this time, the stability of the optical output frequency changes in response to the temperature stability of the semiconductor laser. It's decided.

Therefore, the output light at this time can be extracted and used as a light source for a coherent light application device. In this method, if the stopping or weakening operation by the control means is released, it can be used in the same manner as the conventional method shown in FIG. Furthermore, it is possible to extract and use the frequency outputs of both the period of stopping or weakening and other periods as required. In this case, the same semiconductor laser will be used in a time-division manner.

[Example] Next, a semiconductor laser device according to an example of the present invention will be described.
This will be explained with reference to FIGS. However, parts in FIG. 1 that are common to those in FIG. 9 are designated by the same reference numerals and their explanations will be omitted. The semiconductor laser device shown in FIG. 1 has a modulation control port 11 (46) and a half miller (35a) in addition to the device shown in FIG.
and an optical switch (48). The modulation control circuit (46) is provided as a control means for stopping or weakening the vibration signal supplied from the signal source (38) to the pivot circuit (34) for a desired period, and in this example, the vibration signal is is configured to stop the supply of The optical switch (48) is a laser diode (1
The output light of 1) is branched by a half mirror or the like and passed in synchronization with the stop control by the modulation control circuit (46) to obtain wavelength-stabilized output light.

FIG. 2 shows in detail a portion of the modulation ff111081 times 1 (46) and signal source (38) of FIG. Signal source (38)
includes a Wien bridge oscillator, which is composed of an operational amplifier (50), capacitors (53), (54), resistors (51), (52), (56). This oscillator constitutes a positive period circuit by a series-parallel circuit of R and C, and can oscillate at a frequency where the phase change of this circuit becomes zero. Moreover, the oscillation frequency is stabilized and the distortion of the output waveform is improved by negative feedback by Ri and Rf.

The modulation control circuit (46) includes a control pulse generation circuit (47) that intermittently generates pulses, a switch SW that responds to this output pulse, and an operational amplifier (
A resistor (49) is alternatively connected in parallel to the feedback resistor (52) of the resistor (50).

The oscillation conditions of the Wien bridge oscillator are
When the voltage gain of 0> is determined as A, it is as follows.

In order to cause oscillation, the voltage gain A must be positive, so the value Rf of the resistor (52) and the value Ri of the resistor (51) must be set to satisfy Rf>2Ri. In response to the low level of the control pulse shown in FIG. 4(b) obtained from the control pulse generating circuit (47) shown in FIG. 2, the circuit satisfying the above oscillation conditions is switched S.
When W is turned on and 42 resistors of the value Rf are connected, Rf in the gain equation becomes 1/2 and the oscillation condition is no longer satisfied. Therefore, switch S
It is possible to intermittently generate an oscillation output, that is, a vibration signal, by controlling W on and off.

Here, the next signal source (38) is indirectly controlled by the switch SW of the modulation control circuit (46) so that undesirable waveforms such as noise are not generated in the output of the signal source (38). That is, the output of the signal source (38) is connected to the line (44).
) is applied to the drive circuit (34), and the drive current is modulated. At this time, since the semiconductor laser diode is modulated by a minute current, the oscillator built in the signal source (38) is activated as described above to prevent noise from being superimposed on the drive current during on/off control of the switch SW. Controlling the oscillation conditions is extremely important.

By controlling the signal source (38) as described above, for example, a change in drive current of several tens of MHz to 1 μA can be achieved.
Even in a stabilized state of a general semiconductor laser with an oscillation frequency variation of 100HH, the oscillation frequency variation caused by switching the control method by the switch SW is completely eliminated. The waveform of the oscillator output VL in FIG. 2 is not the same as the waveform output from the signal source (38) shown in FIG. 4(a). The output of the signal source (38) is formed by passing the oscillator output VL through a bandpass filter, attenuator, etc. for removing noise (not shown), and has the waveform shown in FIG. 4<a).

FIG. 3 shows the temperature control device (31) of FIG. 1 in detail.
Here, (60) is a Bertier element used to keep the temperature of the laser diode (11) constant, (61) and (62)
) is an operational amplifier, (63) is a drive circuit, R3 is a thermistor used to detect the temperature of the laser diode (11), Rg is a resistor, VC is a constant voltage source, and V is a variable voltage source for temperature setting. Feedback control is performed by detecting a temperature change with thermistor R3, converting this temperature change into a voltage change with an operational amplifier (61), and converting the temperature change into a voltage change with a Vertier element (60) in a drive circuit (63) via an operational amplifier (62). The temperature of the laser diode (11) is maintained at the set temperature by driving the variable voltage source Vr.Instead of the variable voltage source Vr, the offset adjustment function of the operational amplifier (62) may be used, or the thermistor It is desirable that Rs has a bridge configuration in consideration of the stability of temperature detection.

Next, the operation of the apparatuses shown in FIGS. 1 and 2 will be explained with reference to FIG. When the switch SW is turned off at time 1 in Figure 4, a state is obtained that satisfies the oscillation conditions, and Figure 4 (a)
For example, a signal source (3
The output of 8) is given to a drive circuit (34) via a line (44), and the drive current is modulated. The modulated current waveform of the laser diode used for wobbling control based on FIG. In the period t1-t, tS-t7, the ninth
In the case shown in the figure, the drive current is modulated in the same way, and a stabilized frequency can be obtained with the same operation. In this case, the control loop including the lock-in amplifier (33) immediately starts stabilizing operation and stabilizes at t2 after the T3 period, t2-t3
During this period, a stable error signal is obtained from the lock-in amplifier (33). The stability of the output frequency during this period t2 to t3, that is, the stability of the optical frequency corresponding to the approximate center of the wobbling, is good, but as in the case of Fig. 9, the frequency changes periodically, so the coherent It is not possible to use this frequency output for devices that require light. t
At time 3, the switch SW shown in FIG. 2 is turned on,
When the oscillation conditions of the oscillator shown in Fig. 2 are no longer satisfied, the amplitude of the output waveform of the signal source (38) gradually decreases as shown in Fig. 4(a), and the vibration signal is supplied to the drive circuit (34). virtually ceases. For this reason, a lock-in amplifier (3
The control loop including 3) is substantially cut off. From time t3, the temperature control device (31) is in a controlled state only, and this control is stabilized by time t4 after the T period has elapsed. At time t5, the switch sW of the modulation control circuit is turned off again, so that the vibration signal shown in FIG. 4(a) is generated again.

According to this system, the signal source (38) is in operation]. During one period, 20 It is possible to obtain a short-term (approximately 1 second or less) frequency proposal below kHz, and during the T2 period when the signal source (38) is not operating, it is stabilized only by the temperature control loop. period stability can be obtained. In addition, during the T1 period, the above 20k
Apart from the stability of the optical frequency corresponding to the approximate center of the wobbling below H2, there is a certain degree of stability that cannot be reduced based on frequency modulation, that is, Wt= of the wobbling In from the relationship of S/N (signal-to-noise ratio). The minimum variation range of the laser oscillation frequency to obtain the effect of wobbling control is 10 MHz to several tens of HH2. Fluctuations of 28H2 or less in the stop period T2 are caused by the above modulation fluctuations (10H2 or less).
H2~several tens) is significantly less than IH2) and can be used in coherent optical application devices. This relationship is shown in Figure 12 (a
) (b).

In this method, frequency outputs with different stability levels can be obtained between period T and period T2, so either one can be extracted and used depending on the purpose, or both can be time-divided. You can also use Also, switch S
By manipulating W, the period T1 can be made extremely long, and modulated output can be generated continuously. Also period T2
It is also possible to make the period of time extremely long and to generate unmodulated output continuously. Therefore, one semiconductor laser device can be used in various ways.

[Modifications] The present invention is not limited to the above-described embodiments, and for example, the following modifications are possible.

1) In order to control the output of the signal source (38>), a control circuit (46a) may be provided on this output line as shown in FIG.
a switch SW for connecting 44) to ground;
It consists of a control pulse generation circuit (47a) that generates the switch control pulse shown in FIG. 6(b) in response to the zero-cross point of the vibration signal of the signal source (38). FIG. 6 shows the operation of a semiconductor laser device including the circuit of FIG. 5, and the vibration signal of FIG. 6(a) is generated only during the T1 period of FIG. 6(b). If it occurs like this, the periods T and T2
Undesirable waveform distortion near the time of switching between the current and the current state will no longer occur.

(2) As shown in FIG. 7, the rise of the control pulse in (b) may be made to coincide with the zero cross of the vibration signal in (a), and the fall may be the same as in the case of FIG. 4.

(3) In the methods shown in Figs. 1 to 4, the vibration signal is stopped during period T2 except for the first part, but even if a vibration signal of extremely small amplitude is generated during the entire period T2. good.

(4) In FIG. 1, the optical switch (48) is turned on in synchronization with the period T2 in FIG. 4, but it may be turned on at any part of the period T2. T and T
The ratio with 2 may be changed arbitrarily within an allowable range based on the desired performance.

(5) A special circuit is provided to bring the output line (40) of the lock-in amplifier (33) to zero level during period T2, or to hold the level of the output line (40) immediately before period T2. Good too.

(6) For stable operation, it is desirable to reduce light reflection to the laser diode (11) as much as possible. For this reason, it is better to employ an optical isolator, apply anti-reflection coating to the end face of the optical element, and perform oblique polishing.

(7) If the laser diode (11) is replaced with a DSM laser diode that has a narrower oscillation frequency spectrum spread than a normal laser diode and can be continuously tuned to optical wavelengths, such as the aforementioned DBR type laser or DFB type laser, the performance will be significantly improved. We can expect improvement.

(8) Also, the photodiode <16) (17) may be replaced with an avalanche photodiode or a photomultiplier tube.

(9) The signal source (38) may be configured using a PLL.

(10) The laser diode (11) may be provided with two output optical paths in order to use the periods T and T2 in a time-sharing manner.

(11) The oscillator of the signal source (38) may be oscillated in phase synchronization with the clock.

(12) In Fig. 2, a so-called Terman oscillator is configured with Rr-(X) and R1=o, and focusing on the amplitude condition A=3, oscillation is performed by controlling the power supply voltage of the operational amplifier (50), for example. May be controlled. This power supply voltage control is performed by the fourth
This is done in synchronization with the control pulse shown in Figure (b).

(13) A subtracter may be provided in place of the divider (15) in FIG. 1 to obtain V Va.

[Effects of the Invention] As is clear from the above, in the present invention, periods in which the driving current of the semiconductor laser is modulated and controlled and periods in which the modulation is stopped or weakened are arranged alternately. For this reason, during the period when modulation is stopped or weakened, it is possible to obtain optical output without frequency fluctuations caused by modulation. Also, is the state stabilized during the modulation control period modulation stopped or weak death stage I? It becomes possible to substantially continue up to 51, and the stability is improved even during this period. Therefore, the output light of the semiconductor laser during the modulation stop or weakening period can be used as a light source for a coherent optical application device. Furthermore, if there is a device that can be used for modulated output or a device that requires modulated output, the output light of the semiconductor laser during the modulation period can be used as a light source for these devices. That is, it becomes possible to use one semiconductor laser device as two types of light sources, expanding the range of applications.

[Brief explanation of drawings]

1 is a block diagram showing a semiconductor laser device according to an embodiment of the present invention, FIG. 2 is a circuit diagram showing the signal source and modulation control circuit of FIG. 1, and FIG. 3 is a circuit diagram showing the temperature control device of FIG. 1. 4 is a waveform diagram showing the state of each part in FIGS. 1 and 2, FIG. 5 is a block diagram showing a modified example of the signal source and modulation control circuit, and FIGS. 6 and 7 are A waveform diagram for explaining the operation of the modified example, FIG. 8 is a block diagram showing a conventional semiconductor laser device, FIG. 9 is a block diagram showing another conventional semiconductor laser device, and FIG. 10 is a block diagram of the conventional semiconductor laser device. FIG. 11 is a block diagram schematically showing the lock-in amplifier; FIG. 11 is a waveform diagram showing the driving current of the laser diode shown in FIG. 9; Figure 12 is a diagram showing the frequency fluctuation range when the frequency is stabilized, Figure 12 (a) shows the optical frequency fluctuation range due to wobbling, and Figure 12 (b) shows the optical frequency when wobbling is stopped. It is a figure showing a fluctuation range. (11)...Laser diode, (31)...Temperature control device, (33)...Lock-in amplifier, (34)
...Drive circuit, (38)...Signal source, (46)...
- Modulation control circuit, (48>...optical switch.

Claims (3)

[Claims] (1) includes a semiconductor laser, a temperature control device that controls the temperature of the semiconductor laser to a predetermined value through feedback control, and an atomic or molecular absorption spectrum spectrometer, and the frequency of the output light of the semiconductor laser and the frequency reference; an optical frequency discriminator that obtains a discrimination signal for detecting a frequency difference with a frequency as an absorption level difference in the absorption spectrum; and a vibration signal source that generates a vibration signal for micro-vibrating the driving current of the semiconductor laser. , a phase difference compatible signal forming circuit connected to the optical frequency discriminator and the vibration signal source and outputting a signal (error signal) corresponding to the phase difference between the discrimination signal and the vibration signal; a drive circuit connected to the signal forming circuit and the vibration signal source, and supplying the semiconductor laser with a drive current that is modulated by the vibration signal and controlled by negative feedback with a signal corresponding to the phase difference; What is claimed is: 1. A semiconductor laser device, comprising: a control means for stopping or extremely weakening a vibration signal supplied from the vibration signal source to the drive circuit for a predetermined period of time. (2) The semiconductor laser device according to claim 1, wherein the vibration signal source includes an oscillator, and the control means is means for switching oscillation conditions of the oscillator. (3) The semiconductor laser device according to claim 1 or 2, wherein the phase difference signal forming circuit is a lock-in amplifier.