JPH0331089Y2 - - Google Patents

Description

[Detailed explanation of the invention] (Industrial application field) This invention allows the wavelength of the output light of a semiconductor laser to be
This invention relates to improving the characteristics of a semiconductor laser wavelength stabilizing device that is stabilized by the absorption line of a standard substance.

(Prior art example) The applicant filed the specification of Japanese Patent Application No.
No. 62-171174) proposed a semiconductor laser wavelength stabilizing device in which the wavelength of the output light is highly stable even instantaneously because the output light is not modulated.
FIG. 5 shows the configuration of this device.

In Fig. 5, the output light of the semiconductor laser LD is
The light is split by the beam splitter BS, the reflected light is taken out as output light, and the transmitted light is input to the acousto-optic modulator UM. This acousto-optic modulator
UM is frequency D by the output of signal generator SG2.
(for example, 80MHz) and diffracts the incident light. This diffraction operation is performed periodically by the signal generator SG1 and the switch SW at a frequency m (for example,
2kHz) is turned on and off. Therefore, the acousto-optic modulator UM periodically outputs the 0th-order diffracted light and the 1st-order diffracted light of the output light of the semiconductor laser alternately. This output light is incident on an absorption cell CL containing a standard substance (for example, Cs), where it is partially absorbed and input into a photodetector PD. The output of this photodetector PD is amplified by amplifier A, and a lock-in amplifier
It is synchronously rectified at frequency m at LA1 and input to PID controller CT2.

Figure 6 shows the output characteristics of lock-in amplifier LA. Since the output light of the semiconductor laser is absorbed by the resonant absorption of the standard material, the output changes depending on the frequency of the incident light. Frequency of resonance absorption of standard material ν _s
Then, as shown in FIG. 6, the output becomes zero at ν _s −D/2. Therefore, lock-in amplifier
When the current or temperature of the semiconductor laser LD is controlled by the PID controller CT2 so that the output of LA1 becomes zero, the frequency of the output light reflected by the beam splitter BS is fixed at ν _s −D/2. Ru.

Note that TB is a constant temperature bath, and the semiconductor laser LD
is stored. CT1 is a PID controller that measures the temperature of the semiconductor laser LD and drives the Peltier element PE so that the temperature is constant.

(Problem to be solved by the invention) However, in such a semiconductor laser wavelength stabilizing device, if the gain of the feedback loop is increased and the operation is started from a point away from the resonance absorption frequency, the frequency of the output light will change. It passes the set value and flies to a point where there is no resonance absorption. Furthermore, if the gain of the feedback loop is lowered, the frequency of the output light will become less stable, although the set value will not be exceeded.

(Purpose of the invention) The purpose of this invention is to synchronize to the set value even if the operation starts from a point outside the resonance absorption of the standard material.
Another object of the present invention is to provide a semiconductor laser wavelength stabilizing device in which the frequency of output light is highly stable.

(Means for Solving the Problems) In order to solve the above problems, the present invention modulates the output light of a semiconductor laser, inputs this output light into an absorption cell, and converts the transmitted light of the absorption cell into a light beam. In a semiconductor laser wavelength stabilizing device configured to detect the wavelength with a detector and feed it back to the semiconductor laser, a variable gain amplifier is installed in the feedback loop.

(Function) The variable gain amplifier is controlled by a signal related to the output of the photodetector, and when the output light of the semiconductor laser deviates significantly from the set value, the gain of the feedback loop is lowered, and when it approaches the set value, the gain of the feedback loop is lowered. Increase the gain of the feedback loop.

(Example) FIG. 1 shows an example of a semiconductor laser stabilizing device according to the present invention. Note that the same elements as in FIG. 5 are given the same reference numerals, and their explanations will be omitted. In Fig. 1, 1 is a variable gain amplifier to which the output of amplifier A is input and its output is input to lock-in amplifier LA1, 2 is a comparator to which the output of amplifier A is input to its inverting input terminal, and 3 is a comparator. 3
is a setting power supply connected between the non-inverting input terminal of and a common potential point. The gain of variable gain amplifier 1 is controlled by comparator 3.

In such a configuration, assuming that the resonance absorption of the absorption cell CL is as shown in Figure 2, the acousto-optic modulator
Assuming that the frequency of the output light of UM is at point P, the amount of transmitted light increases, the output of amplifier A swings significantly in the negative direction, the output of comparator 2 becomes high level, and the gain of variable gain amplifier 1 increases. becomes smaller. Therefore, the operating point slowly moves from P toward the bottom of resonance absorption, that is, toward the frequency ν _s , the amount of transmitted light decreases, and the output of amplifier A gradually increases. At point Q, the output of amplifier A becomes higher than the set power supply 3, the output of comparator 2 becomes low level, and the gain of variable gain amplifier 1 becomes high. Therefore, the output of the semiconductor laser LD is held at point R with high stability.

FIG. 3 shows another embodiment of the present invention. In FIG. 3, a plurality of comparators are used to switch the gain of the variable gain amplifier 1 in a plurality of ways. In FIG. 3, 21, 22, and 23 are comparators, the outputs of the amplifier A are connected to their inverting input terminals, and their outputs control the output of the variable gain amplifier 1. 31, 32, 33 are setting power supplies, and comparators 21, 2, respectively.
It is connected to the non-inverting input terminals 2 and 23. Although not shown, the output of the photodetector PD is input to the amplifier A as in FIG. 1, and the output of the variable gain amplifier 1 is input to the lock-in amplifier LA1.
is input. In such a configuration, comparators 21 and 2 are connected at points S, T, and Q in FIG.
2 and 23 become low level, and the gain of the variable gain amplifier 1 is gradually increased. In this way, it is possible to converge to point R more quickly and stably. Note that the gain of the variable gain amplifier may be controlled continuously.

In Figure 4, amplifier A is used as the input to the comparator.
An example using a second-order differential waveform of . Note that the same elements as in FIG. 1 are denoted by the same reference numerals, and explanations thereof will be omitted. In FIG. 4, a signal generator SG1 FM modulates SG2 with a sine wave or a triangular wave. The output of amplifier A is input to lock-in amplifier LA2 and variable gain amplifier 1. lock-in amplifier
LA2 is a signal generator SG1 that also generates an output with a frequency of 2m, which is twice the modulation frequency of the signal generator SG2.
and is synchronously rectified. In this way,
The second derivative of amplifier A is obtained. The output of lock-in amplifier LA2 is input to the inverting input terminal of comparator 2, and the output of comparator 2 provides
The variable gain amplifier 1 is controlled. The output of variable gain amplifier 1 is input to lock-in amplifier LA1. 3 is a setting power supply, which is connected to the non-inverting input terminal of the comparator 2.

In the embodiments shown in FIGS. 1, 3, and 4, the variable gain amplifier 1 is inserted after the amplifier A, but the lock-in amplifier LA1,
It may be inserted after the PID controller CT2.
That is, it may be inserted anywhere within the feedback loop.

In addition, the output light of the semiconductor laser LD is split by a beam splitter BS, the split light is modulated by an acousto-optic modulator UM, and transmitted through an absorption cell CL.
We applied this to a semiconductor laser wavelength stabilization device that detects this transmitted light with a photodetector PD and returns it to the semiconductor laser LD. The present invention may also be applied to a semiconductor laser wavelength stabilizing device configured to detect and feed back to the semiconductor laser. In short, if it is a semiconductor laser wavelength stabilization device that modulates the output light of a semiconductor laser, makes this output light enter an absorption cell, and returns a signal related to the intensity of the transmitted light to the semiconductor laser, The present invention can be applied.

(Effects of the invention) As specifically explained in conjunction with the embodiments above, in the present invention, a variable gain amplifier is inserted between the feedback path between the photodetector and the semiconductor laser, and the gain of the variable gain amplifier is adjusted as described above. It is now controlled by a signal related to the output of the photodetector. Therefore, even if the output light of the semiconductor laser deviates significantly from the set value, it will not jump over the set value and can be maintained at the set value with high stability. Therefore, even if the output light of the semiconductor laser deviates significantly from the set value at the time of startup, it can be converged to the set value, and the wavelength is highly stable.

[Brief explanation of the drawing]

Fig. 1 is a configuration block diagram showing an embodiment of the semiconductor laser wavelength stabilizing device according to the present invention, Fig. 2 is a characteristic curve diagram for explaining the operating point, and Figs. A configuration block diagram showing another embodiment, FIG. 5 is a configuration block diagram showing a conventional example, and FIG. 6 is a configuration block diagram showing a conventional example.
The figure is a characteristic curve diagram showing the output characteristics of a lock-in amplifier. 1...Variable gain amplifier, 2, 21, 22, 2
3... Comparator, 3, 31, 32, 33...
Setting power supply, PD... semiconductor laser, UM... acousto-optic modulator, CL... absorption cell, A... amplifier,
SG1, SG2...Signal generator, LA1, LA2...
lock-in amplifier.

Claims (1)

[Claims for Utility Model Registration] The output light of a semiconductor laser is modulated, the modulated output light is incident on an absorption cell, the transmitted light of this absorption cell is detected by a photodetector, and the photodetector is A semiconductor laser wavelength stabilizing device configured to feed back a signal related to output to the semiconductor laser, further comprising a variable gain amplifier installed between the photodetector and the path of the semiconductor laser, A semiconductor laser wavelength stabilizing device characterized in that a gain is controlled by a signal related to an output signal of the photodetector.