JPH03250682A - Frequency stabilized laser light source - Google Patents

Abstract

PURPOSE:To obtain a non-modulation highly stable frequency optical output by controlling the central frequency of oscillation of a semiconductor laser by current modulation to an absorption line frequency highly accurately in a highly stabilized frequency state, and further controlling an oscillation frequency of the semiconductor laser to one shifted a predetermined frequency from the central frequency of output light from the semiconductor laser. CONSTITUTION:A semiconductor laser 1 is current modulated by a modulation frequency fm sinusoidal wave, output light from which is emitted via a beam splitter 2 and is combined by a beam splitter 13 with output light from a semiconductor laser 11 emitted via a beam splitter 12. The light emanating from the beam splitter 13 enters an optical detector 14 and is detected as a beat signal. The output is removed through a low pass filter 161 of a control part 16 in its modulated frequency component, and thereafter is added to bias voltage VB in an adder circuit 162 and is fed back to the semiconductor laser 11 via a PI control circuit 163. Thus, oscillation frequency of the semiconductor laser 11 is controlled to a point shifted by a predetermined frequency from the absorption line with no modulation and is derived externally via the beam splitter 12.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to improving the stability of a 73 frequency stabilized laser light source using a semiconductor laser.

<Conventional branch technique> Figure 4 shows frequency stabilization 1. - A block diagram showing a conventional example of a laser light source, in which a semiconductor laser is directly modulated.

The oscillation frequency is controlled to the center of the absorption line of atoms and molecules.
It shows what is being created. 〒Conductor 1. - The output light from the laser 1 is separated into two directions by a beam splitter 2, and one of the lights enters an absorption cell 3 in which a standard substance is sealed. The light transmitted through the absorption cell 3 is detected by a photodetector 4, converted into an electrical signal, and input to a synchronous detection circuit 5 comprising a lock-in amplifier or the like. The oscillation frequency of the semiconductor laser 1 is current-modulated by the output of the oscillator 8, and the synchronous detection circuit 5 performs synchronous detection using the output of the oscillator 8 as a reference signal.
controls the current of the semiconductor laser 1 so that the output of the synchronous detection port 15 is constant. The oscillation output of the PI control circuit 61 oscillator 8 and the output of the bias current source 9 are added by the adder circuit 7 and input to the semiconductor laser 1. As a result, the oscillation frequency of the semiconductor laser 1 is controlled to the center of the absorption line of the atoms or molecules of the standard material in the absorption cell 3, and the other output light of the beam splitter 2 has a frequency whose absolute value is determined by the atoms or molecules with high precision. becomes.

However, in the above-mentioned apparatus, since the output light is frequency-modulated, the instantaneous frequency is not stable, making it unsuitable for applications such as interferometric measurements, and has the drawback of narrowing the range of applications.

In order to solve these drawbacks, there is a structure in which a non-modulated output is obtained by externally modulating the output light of a semiconductor laser using an acousto-optic modulator and controlling it to atomic and molecular absorption lines.

<Problems to be Solved by the Invention> However, in the case of such a device, there is a problem in that the stability is limited because the acousto-optic modulator is unstable. Further, the acousto-optic modulator has problems in that it consumes a large amount of power, is expensive, and has a large driver.

The present invention has been made to solve such problems, and an object of the present invention is to realize a frequency-stabilized laser light source whose oscillation frequency is highly stably controlled without using an acousto-optic modulator.

<Means for Solving the Problems> The frequency-stabilized laser light source according to the present invention includes a first semiconductor laser and an absorber encapsulating a substance that inputs the output light of the first semiconductor laser and absorbs it at a specific frequency. a cell, a first photodetector that detects light transmitted through the absorption cell, and an oscillator that modulates the current of the first semiconductor laser;
a first synchronous detection circuit that inputs the output of the first photodetector using the output of the oscillator or a signal of an odd multiple frequency thereof as a reference signal; a first control circuit that controls the oscillation frequency of the semiconductor laser to match the absorption line of the absorption cell; a second semiconductor laser; and an optical means that combines the output lights of the first and second semiconductor lasers; a second photodetector that detects the beat signal of the output light output from the optical means; a frequency/pressure conversion circuit that converts the frequency of the signal output from the second photodetector into voltage; a low-pass filter that removes the AC voltage component at the modulation frequency from the output of the positive conversion circuit; and a second M that controls the oscillation frequency of the second semiconductor laser so that the output of the low-pass filter is constant. # circuit, and is characterized in that it is configured such that the output of the second semiconductor laser is not modulated.

The first semiconductor laser becomes highly stable due to current modulation, and the oscillation frequency of the second semiconductor laser is shifted from the center frequency of oscillation of the first semiconductor laser by a frequency corresponding to the setting value of the low-pass filter output. Since there is no modulation, the output frequency is highly stable.

<Example> The present invention will be explained in detail below using the drawings.

FIG. 1 is a block diagram showing a first embodiment of a frequency stabilized laser light source according to the present invention. The same parts as in FIG. 4 are given the same symbols and explanations are omitted. Note that the bias current applied to the semiconductor laser I is omitted. In addition, the part consisting of the synchronous detection circuit 5 and the PI control circuit 6 in FIG.
It is expressed as a control unit 10. 11 is a second semiconductor laser, 12 is a beam splitter that separates the output light of the semiconductor laser 11 into two directions, and 13 is a beam that constitutes an optical means that combines the transmitted light of the beam splitter 12 and the transmitted light of the beam splitter 2. splitter; 14 is a second photodetector that receives the light emitted from the beam splitter 13 and detects a beat signal; 15 is a voltage/frequency that inputs the output of the photodetector 14 and converts the frequency into a voltage; converter, 1
Reference numeral 6 denotes a second control unit which inputs the output of the frequency/voltage converter 15 and whose corresponding control output drives the current of the semiconductor laser 11. FIG. 2 shows the configuration of the second control section 16,
161 is a low-pass filter that removes the modulation component included in the output of the frequency/voltage converter 15; 162 is a bias addition circuit that adds bias voltage v8 to the output of the low-pass filter 161; 163 is an input to the output of the bias addition circuit 162; This is a PI control circuit. The control output of the PI control circuit 163 drives the current of the semiconductor laser 11. The standard substance in the absorption cell 3 is, for example, acetylene.

Hydrogen cyanide, rubidium, cesium, etc. are used. The operation of the apparatus having the above configuration will be explained next.

As explained in the conventional example, the frequency of the 1i conductor laser 1 is controlled to the absorption line of the standard material by current modulation (
3(A)(B)), the semiconductor laser 1 has a modulation frequency f
The current is modulated by a sine wave of 1, and its output e1 is el = E1 s i n (2πf1t+ks i
n2yrfIN t+ψ1)
...Represented by (1). Here, El is the amplitude, fl is the oscillation frequency, k is the coefficient 1ψ is the phase. The output light of such a semiconductor laser is emitted through the beam splitter 2, and is combined with the output light of the semiconductor laser 1, which is emitted through the beam splitter 2, at the beam splitter 3. The light emitted from the beam splitter 13 enters the photodetector 14 and a ζ beat signal is detected. Since the output of the semiconductor laser 2 is not modulated, its oscillation frequency is f2. h2.If the phase is r2, e2=E2s in (2yrf2 is 10P2)・
...(2) becomes. However, the photodetector 1 in which these are combined is
The output current i of No. 4, excluding the f 1 + r 2 component and the DC component, is 1 ()C (e1 + e 2 ) = 2 E 1E 2 COS (2π (fl - f2)
t+ψ1-ψ2±ksin2rf, t) = (3) When this output is passed through the frequency/voltage converter 15, (
A DC voltage corresponding to the frequency (fl-f2) in equation 3) and a sinusoidal voltage corresponding to ks in2πft appear in the output as modulation frequency components.

This output is passed through the low-pass filter 161 of the second control section 16.
After the modulation frequency component is removed in , it is added to the bias voltage Ve in an adder circuit 162 and fed back to the semiconductor laser 1 via a PI control circuit 163 . As a result, the output of the low-pass filter 161 is VB = g (f, -f2) (4
), that is, 1 f2-fl-g (VB)...(5) The frequency of the semiconductor laser 11 is controlled so that (the third
(C)), where g(f) is a function indicating the conversion characteristics of the frequency/voltage conversion circuit, and g"1(V) is its inverse function. In this way, the oscillation frequency of the semiconductor laser 11 is The light is controlled without modulation at a point shifted by a certain frequency from the absorption line, and is extracted from the beam splitter 12 to the outside.

According to the frequency stabilized l/-the light source with such a configuration,
The center frequency 11 of the oscillation of the semiconductor laser 1 is controlled by current modulation to the absorption line frequency with high accuracy and in a highly stable frequency state, and the oscillation frequency of the semiconductor laser 11 is adjusted from the center frequency of the output light of the semiconductor laser 1. By controlling the frequency to a constant frequency shift, it is possible to obtain a non-modulated optical output with high frequency stability. Therefore, it is optimal as a light source for coherent optical measuring instruments, interferometric length measuring instruments, etc.

Furthermore, since the absorption line is controlled by current modulation without using an acousto-optic modulator, there is no change in the optical path in the acoustic or optical modulator, and the frequency stability is excellent. Furthermore, it can be made small, inexpensive, and consumes low power.

In the above embodiment, if the oscillation spectrum width of the semiconductor laser 1゜11 is wide < poor S/N ratio, or when the interval between both oscillation frequencies is widened, the input high frequency is It is sufficient to insert a brisger, which is a frequency divider that converts to a lower frequency.

Also, instead of adding a bias voltage to the output of the frequency/voltage converter, the bias frequency may be subtracted by a mixer before the frequency/voltage converter.

Furthermore, by changing the bias voltage, the output optical frequency can be made variable.

Further, the reference frequency for synchronous detection is controlled to the 0 cross point of the first differential signal using fLll, but it can also be controlled to the 0 cross point of the third differential signal using 3fl as the reference frequency. Generally, using a reference frequency that is an odd multiple of fl, it is possible to control the O cross point of an odd-order differential signal.

Furthermore, in each of the above embodiments, a part of the light emitted from the semiconductor laser 1 enters the absorption cell 3 as a pump light, and the other part enters the absorption cell 3 as a thin beam from the opposite direction as a probe light.
Saturation absorption method (Hori, Tsunoda,
Tatsuno, Yabusaki, Ogawa: Frequency stabilization of semiconductor laser using saturation absorption spectroscopy, IEICE Technical Report 0QE82-1.1.6), it is possible to realize a more stable frequency-stabilized laser light source.

<Effects of the Invention> As described above, according to the present invention, a frequency-stabilized laser light source in which the oscillation frequency is highly stably controlled can be realized without using an acousto-optic transducer.

[Brief explanation of drawings]

FIG. 1 is a configuration block diagram showing an embodiment of a frequency-stabilized laser light source according to the present invention, and FIG. 2 is a second control section 1 thereof.
FIG. 3 is an explanatory diagram showing the operation of the device shown in FIG. 1, and FIG. 4 is a construction block diagram showing a conventional example of a frequency-stabilized laser light source. DESCRIPTION OF SYMBOLS 1... First semiconductor laser, 3... Absorption cell, 4...
...first photodetector, 5...first synchronous detection circuit, 6
... first control circuit, 8 ... oscillator, 11 ... second semiconductor laser, 13 ... optical means, 14 ... second photodetector, 15 ... frequency/voltage converter, 161
...Low pass filter, 163...Second control circuit, fl...Oscillation frequency of first semiconductor laser, f2.
...Second half→

Claims (1)

[Claims] a first semiconductor laser, an absorption cell filled with a substance that absorbs the output light of the first semiconductor laser at a specific frequency, and a first photodetector that detects the light transmitted through the absorption cell. an oscillator that modulates the current of the first semiconductor laser; and a first synchronous detection circuit that inputs the output of the first photodetector using the output of the oscillator or a signal of an odd multiple frequency thereof as a reference signal. a first control circuit that controls the oscillation frequency of the first semiconductor laser to the absorption line of the absorption cell based on the output of the first synchronous detection circuit; a second semiconductor laser; and an optical means for combining the output light of the second semiconductor laser, a second photodetector for detecting the beat signal of the output light output from the optical means, and a second photodetector for detecting the beat signal of the output light output from the second photodetector. a frequency/voltage conversion circuit that converts the frequency of a signal into a voltage; a low-pass filter that removes an alternating current voltage component at a modulation frequency from the output of the frequency/voltage conversion circuit; 1. A frequency-stabilized laser light source, comprising: a second control circuit for controlling the oscillation frequency of the second semiconductor laser, and configured such that the output of the second semiconductor laser is unmodulated.