JPH04111485A - Frequency stabilizing laser beam source - Google Patents

Abstract

PURPOSE:To materialize a laser beam light source for stabilizing frequency of unmodulated output, where the effect of the interference of an optical system does not exist and the oscillated frequency is controlled highly stably, by modulating an electrooptical modulator with a secondary function, and detecting the interfere pattern of an optical system outside the absorbed beams, and correcting. the effect of interference at the center of absorbed beam by the interference pattern. CONSTITUTION:When the output of a timing circuit 19 is 1, a switch 18 selects the output of an oscillator 17, and a switch 14 opens, and a synchronism detection circuit 12 operates as an amplifier, and by the modulated signal having secondary function waveform, the output light frequency of an electrooptical modulator 11 is swept in straight line manner to time, and based on the output of the corresponding photodetector 4, an interference pattern is stored in a memory circuit 13. When the output of the timing circuit 19 is 0, the switch 18 selects the output of an oscillator 16, and the switch 14 closes, and the synchronism detection circuit 12 performs synchronism detection, and based on the said stored interference pattern, in a correction circuit 13, the interference signal is removed from the synchronism detection output.

Description

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

<Prior art> Figure 6 is a block diagram showing a first conventional example of a frequency-stabilized laser light source, in which a semiconductor laser is directly modulated to control its oscillation frequency to the center of the absorption lines of atoms and molecules. It shows. The output light of the semiconductor 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 and converted into an electrical signal, which is input to a synchronous detection circuit 5 consisting of a lock-in amplifier, etc. The oscillation frequency of the semiconductor laser 1 is current-modulated by the output of an oscillator 8. The synchronous detection circuit 5 performs synchronous detection using the output of the oscillator 8 as a reference signal, and the PI control circuit 6 controls the current of the semiconductor laser 1 so that the output of the synchronous detection circuit F#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 of insufficient stability because the acousto-optic modulator is unstable.

In addition, in conventional devices such as those described above, if there is interference in the optical system due to end face reflection, etc., the interference distance changes due to temperature changes in the optical system, resulting in frequency drift.

The present invention was made to solve these problems, and aims to realize a frequency-stabilized laser light source with unmodulated output, which is free from the influence of optical system interference and whose oscillation frequency is highly stably controlled. shall be.

Means for Solving the Problems> A frequency stabilized laser light source according to the present invention includes a semiconductor laser, a first oscillator that generates a sine wave, a second oscillator that generates a quadratic function waveform, and the first oscillator that generates a quadratic function waveform. A changeover switch for selecting one of the second oscillators, a timing generation circuit for controlling the changeover switch, and an electro-optic modulator that is driven by the output of the changeover switch and frequency-modulates the output light of the semiconductor laser; An absorption cell into which the output light of the electro-optic modulator is input and a standard substance having an absorption line at a specific frequency is enclosed, a photodetector that detects the light transmitted through the absorption cell, and a second switch. a memory circuit for storing an interference waveform output from the photodetector at an optical frequency shifted from the absorption line when an oscillator is selected; a synchronous detection circuit that synchronously detects the output of the photodetector using the output of the first oscillator; a correction circuit that corrects the output of the synchronous detection circuit based on the output of the memory circuit; The present invention is characterized in that it includes a control circuit that controls the oscillation frequency of the semiconductor laser to be centered on the absorption line, and is configured to output unmodulated light from the semiconductor laser.

Function> The output optical frequency of the electro-optic modulator is swept linearly with respect to time by a modulation signal having a quadratic function waveform, and an interference pattern is stored in a memory circuit based on the output of the corresponding photodetector. Since the interference signal is removed from the synchronous detection output in the correction circuit based on the interference pattern, frequency drift is eliminated and stable control is performed to the center frequency of the absorption line.

<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. 6 are given the same symbols and the explanation will be omitted.

10 is a beam splitter that separates the output light of the semiconductor laser 1 into two directions; 11 is an electro-optic modulator that inputs the light reflected by the beam splitter 10 and modulates the frequency; 3 receives the output light of the electro-optic modulator 11; 4 is a photodetector for detecting the light transmitted through the absorption cell 3;
2 is a synchronous detection circuit that synchronously detects the output of the photodetector 4 using the output of the oscillator 16; 13 is a memory circuit that stores the interference waveform output from the photodetector 4; and the output of the synchronous detection circuit 12 is the output of the memory circuit. 14 is a switch that inputs the output of the memory correction circuit 13 at one end and opens and closes the control loop; 15 inputs a signal output from the other end of the switch 14; A control circuit 16 generates a sine wave and a synchronous detection circuit 8? controls the current of the semiconductor laser 1 based on the output of the memory correction circuit 13. a first oscillator, 17, serving as a reference signal for I12;
18 is a second oscillator that repeatedly generates a quadratic function waveform and serves as a synchronization signal input to the memory correction circuit 13; 18 is an oscillator 16.1;
7 is a selector switch for selecting one of the outputs; 19 is a switch 14; Changeover switch 18. This is a timing generation circuit that controls the synchronous detection circuit 12 and the memory correction circuit 13.

Note that the sine wave frequency of the oscillator 16 is 1/of the full width at half maximum of the absorption line.
The amplitude is about 2, and the phase shift due to the electro-optic modulator 11 is π
It should be about rad.

The operation of the apparatus having the above configuration will be explained next. Semiconductor laser 1
A part of the output light passes through the beam splitter 10 and is emitted to the outside, and the reflected light enters the electro-optic modulator 11.

As shown in FIG. 2, in this device, the timing generation circuit 1
Two modes, a sweep mode and a frequency lock mode, are alternately repeated in response to each output. As shown below,
In sweep mode, the interference pattern is stored in memory, and in frequency lock mode, the oscillation frequency of the laser is controlled to the absorption line.

(a) Sweep mode When the output of the timing generation circuit 19 is 1, the sweep mode is set, the selector switch 18 selects the output of the oscillator 17, the switch 14 is open, the synchronous detection circuit 12 operates as an amplifier, and memory correction is performed. The circuit B13 operates as a write memory. The semiconductor laser 1 is driven by a constant current maintained by a control circuit R15.

As a result, the electro-optic modulator 11 is driven by a quadratic function waveform signal that repeats at frequency f2 as shown in FIG. 2(B). An enlarged view of this portion is shown in FIG. 3. The phase θ of the output light of the electro-optic modulator 11 at this time is expressed by the following equation.

2+ψ... (1) where O<t×1/f2 f: optical frequency f2: oscillation frequency of oscillator 17 ψ: initial phase From equation (1), the frequency is f-=(1/2π)・dθ(t) /dt=f+2f
-t+f2 ...(2), however, Ot
1/f2 From equation (2), the optical frequency of the output light of the electro-optic modulator 11 is shifted by f2 compared to the input optical frequency, and changes linearly with time. Figure 4 shows the absorption characteristics (A
) and the output frequency characteristic (B) of the electro-optic modulator 11. In the absorption characteristic of FIG. 4(A), a sinusoidal interference pattern due to interference of the optical system appears. Further, as shown in FIG. 4(B), the output optical frequency of the electro-optic modulator 11 is linearly swept with a width of 2 f 2 between frequencies b1 and b2 shifted from the absorption line. In this state, the transmitted light power of the absorption cell 3 is detected by the photodetector 4, and the oscillator 1
By writing in the memory section of the memory correction circuit 13 in synchronization with the 7 output, the interference pattern 20 in the frequency section between bl and b2 of the absorption characteristic shown in FIG. 4(A) is stored.

(b) Frequency lock mode When the output of the timing generation circuit 19 is 1, the frequency lock mode is entered, the selector switch 18 selects the output of the oscillator 16, the switch 14 is closed, and the synchronous detection circuit 12
performs synchronous detection, and the memory correction circuit 13 operates as a correction circuit, forming a frequency locked loop as a whole.

As a result, the electro-optic modulator 11 is driven by a sinusoidal signal repeating at a frequency of 1, as shown in FIG. 2(B).

The electric field amplitude of the output light of the electro-optic modulator 11 at this time is:
It is expressed by the following formula.

e=E-cos (2gft+m-sin2rf1m-5in2rf1t)=EJo(+EJ1 (m)co
s2g (f+f1)t EJl (m・)C0
g2π(f-fl)t...(3)
where E: electric field amplitude of incident light on the electro-optic modulator 11;

f: Oscillation frequency fl of laser 1: Frequency of oscillator 16 = (absorption line full width at half maximum)/2 Spectrum a o , a 1 , a in Fig. 4(B)
2 corresponds to each frequency component in equation (3). al.

a2 is a side band.

When the absorption line is detected using optical power, it becomes Gaussian due to the influence of Doppler salt distortion. Therefore, when looking at the dimension of the electric field, the absorption line is expressed by the following formula.

G(f)=1-a [exp (-1n2・(f-f2
2 1/2°)/f+] (4) where fo: Absorption line frequency α: Electric field absorption rate Therefore, the electric field amplitude of the light transmitted through the absorption cell 3 is expressed by the following equation.

e=EJo(m)G(f)cos2πft+EJ1 (
m) G(f 1) t-EJl (m) G(f-f
) cos2g (f-fl)t -= (5) When this is detected by the photodetector 4, its detection output I, 0 is I -K (I +2E Jo (m) Jl(m
) GD (f) G (f + f1 ) cos2πflt-2EJ
(m)J (m)G(f)G(f-fl)cos2π
f t-2E J (m)G (f
-fl1 2 ... (6),)
cos4πf1t) in the synchronous detection circuit FI@12.
When synchronously detecting at 2πft, the synchronous detection output ■.

is V = to −G(f)(G(f+f,)−G(f−fl
)) ...(7) becomes,
In equation (7), f is the absorption line frequency f. If you subtract around
It becomes like the dotted line 21 in FIG.

When there is interference in the optical system as described above, a sinusoidal interference pattern is superimposed on the absorption line, so the actual Vo becomes as shown by the solid line 22 in FIG. This interference pattern has the true center frequency f of the absorption line. When the interference distance changes, the interference pattern moves on the frequency axis, and the frequency at which the laser beam is locked changes sinusoidally, resulting in poor frequency stability. In order to prevent this, in the present device, the acquisition section of the memory correction circuit 13 corrects the output of the synchronous detection circuit 12 based on the interference no<turn stored in the memory section in the aforementioned sweep mode. Here, the interference pattern appears repeatedly at regular intervals as shown below.

f = c/2nl =-(
8) SR However, C: Speed of light 1: Interference distance n: Refractive index Using this repetition, it is possible to easily estimate the interference level at the center of the absorption line (peak, valley, somewhere in between, etc.). Based on the output of the memory correction circuit 13 corrected in this manner, the control circuit 15 can lock the oscillation frequency of the semiconductor laser to the center of the absorption line without being affected by the interference pattern. As a result, an unmodulated optical output without frequency drift is extracted via the beam splitter 10.

According to the frequency stabilized laser light source having such a configuration, since external modulation is performed using an electro-optic modulator, unmodulated output light can be obtained.

In addition, the electro-optic modulator is modulated with a quadratic function to detect the interference pattern of the optical system outside the absorption line, and the interference effect at the center of the absorption line is corrected using the interference pattern, increasing frequency stability and optical Frequency drift no longer appears even if the system temperature changes.

In the above embodiment, the oscillation waveform of the oscillator 17 is a downwardly convex quadratic function waveform, but it may also be an upwardly convex quadratic function waveform.

Further, the number of sweeps of the quadratic function waveform in the sweep mode can be set arbitrarily.

In addition, in order for the synchronous detection circuit 12 to perform synchronous detection using the output of the oscillator 16 even in the sweep mode, in the frequency lock mode, it is sufficient to simply subtract the output of the memory correction section from the output of the synchronous detection circuit 12. The calculations in can be made simpler.

Furthermore, in the above embodiment, the reference frequency for coherent detection is f
l was used to control the first-order differential signal to 0 cross point, but with 3f1 as the reference frequency, the third-order differential signal was controlled to 0 cross point.
It can also be controlled to 0 point. 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.

Further, the standard substance for the absorption cell is not limited to C2H2.

<Effects of the Invention> As described above, according to the present invention, a frequency-stabilized laser light source with a non-modulated output, which is not affected by optical system interference and whose oscillation frequency is highly stably controlled, can be realized with a simple configuration. be able to.

[Brief explanation of the drawing]

FIG. 1 is a configuration block diagram showing an embodiment of the frequency-stabilized laser light source according to the present invention, FIGS. 2 to 5 are explanatory diagrams showing the operation of the device shown in FIG. 1, and FIG. 6 is a frequency-stabilized laser light source. FIG. 2 is a configuration block diagram showing a conventional example of a light source. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 3... Absorption cell, 4... Photodetector, 11... Electro-optic modulator, 12... Synchronous detection circuit, 13... Memory correction circuit, 15...・Control circuit, 16...first oscillator, 7...second oscillator,...switching unit 517-j Figure 4 Figure 4

Claims (1)

[Claims] a semiconductor laser; a first oscillator that generates a sine wave;
A second oscillator that generates a quadratic function waveform, a changeover switch that selects either the first or second oscillator, a timing generation circuit that controls this changeover switch, and a timing generation circuit that is driven by the output of the changeover switch. an electro-optic modulator that frequency-modulates the output light of the semiconductor laser; an absorption cell in which the output light of the electro-optic modulator enters and encapsulates a standard substance having an absorption line at a specific frequency; a photodetector that detects light; a memory circuit that stores an interference waveform output from the photodetector at an optical frequency shifted from the absorption line when the changeover switch selects the second oscillator; a synchronous detection circuit that synchronously detects the output of the photodetector using the first oscillator output when the changeover switch selects the first oscillator; and an output of the synchronous detection circuit as an output of the memory circuit. and a control circuit that controls the oscillation frequency of the semiconductor laser to be centered on the absorption line based on the output of the correction circuit, and outputs unmodulated light from the semiconductor laser. A frequency-stabilized laser light source characterized by comprising: