JPH0193190A - Two-frequency stabilized laser with high frequency optical beat - Google Patents

Abstract

PURPOSE:To make a transacted numerical value nearly equal to zero so as to supply a high frequency optical beat stably in such a state as the effect of a temperature drift is small by a method wherein difference between a reference frequency and an optical beat frequency is obtained and the frequency difference is converted into voltage. CONSTITUTION:An inner mirror type laser discharge tube 1 placed in a magnetic field induced by magnets N and S oscillates laser rays which are made to be detected by a detector 4 after passing through a polarizer 4, whereby an optical beat can be obtained, which is composed between polarized lights which intersect each other at a right angle. The optical beat is made to vary corresponding to the change of a resonator length. A reference frequency can be obtained from an output signal of a crystal oscillator 11, a frequency difference between the reference frequency and the optical beat frequency can be obtained through a frequency difference counting device 9, and the frequency difference is inputted into a U circuit when it is positive, and inputted into a D circuit when it is negative, and made to be converted into a control signal through a frequency-voltage convertor 11. The control signal is made to be amplified through a power amplifying circuit 13 so as to be applied to an electric heater 2 for controlling a temperature of a laser discharge tube wall, so that the resonator length can be controlled. The control signal is formed basing on the frequency difference, so that the effect of temperature drift can be ignored.

Description

[Detailed description of the invention] a) Industrial application field Unreleased III is a method for stabilizing the frequency with high accuracy in a two-frequency Zeeman laser by matching the optical beat frequency with a reference frequency created by another oscillator. This invention relates to a frequency-stabilized laser that performs.

b) Conventional technology and its problems Conventional devices of this type are based on frequency-to-voltage conversion of the optical beat frequency to generate control signals, but such devices have the following drawbacks. ing.

When the relationship between the optical beat frequency f8 of the '2-frequency Zeeman laser and the oscillation frequency f[ of the laser beam is determined for some laser discharge tubes, the results shown in FIG. 1 are obtained. That is, discharge tubes A and B.

Although the central values of the optical beat frequencies of C are significantly different.

The ratio of the change in f8 to the change in fL is constant q regardless of the laser discharge tube.

- On the other hand, a two-frequency Zeeman laser is used as a light source for an interferometric distance meter, and the optical beat frequency may limit the operating speed of the distance meter. It may be desirable to use a discharge tube.

When creating an rtAg4 voltage signal with a converter that converts frequency into voltage, there is an effect of the temperature characteristics of the electronic elements that make up the converter, but this effect becomes more severe as the center value of the optical beat frequency becomes higher. Become.

Controlling the resonator length aS- Before the start of operation, the output voltage of the converter changes periodically according to the mode hop, but as can be seen from Figure 1, the range of change in the optical beat frequency is independent of the laser discharge tube. Since the frequency is constant, the change in the output voltage due to an increase in the temperature of the converter's electronic elements due to energization is more significant as the center value increases. Figure 2 a shows the output voltage when the center frequency is low, and b shows the output voltage when the center frequency is high. This shows how vc changes.

In the method of creating a reference frequency, finding the difference in optical beat frequencies, and making corrections to eliminate this difference, there is no effect of changes in the characteristics of the element after the control operation has started; Sometimes it becomes a big problem. That is, at the start of operation, it is necessary that the voltage value of the IT control signal changes sufficiently in accordance with changes in the output voltage of the converter, but this becomes impossible if the operating point moves due to temperature drift. Optical beat frequency is 800KHz
If it's in place, it's drifting! Although F can be ignored, lNi
When the value exceeds 1z, it becomes difficult to ensure that the am operation always starts reliably in general electronic devices. Therefore, in order to create a stabilized laser using a laser discharge tube with a high optical beat frequency, extremely difficult measures against temperature drift are required.

C) Means to solve the problem The length of the laser resonator is controlled so that the optical beat frequency approaches the separately determined reference frequency infinitely.
The present invention calculates the difference between the reference frequency and the optical beat frequency, and converts this difference between frequency and voltage.The frequency and conversion voltage handled are always close to zero, and it operates in a low frequency region where the influence of temperature drift is small. Therefore, one problem is fundamentally solved.

d) Effect of Emission 1 By the above means, it is possible to completely eliminate the occurrence of malfunctions at the start of operation of the frequency-stabilized Zeven laser having a high optical beat frequency, and it is convenient as a light source for high-speed interferometric length measurement equipment. It becomes possible to stably supply a laser with a high optical beat frequency.

e) Example 17S3 is a schematic configuration diagram showing an apparatus according to an example of the present invention.

Internal mirror-shaped laser discharge tube placed in the magnetic field of magnets N and S! When the oscillated light is detected by the detector 4 through the polarizer 3, an optical beat between orthogonal polarized lights is obtained. This optical beat changes periodically according to changes in the resonator length. The optical beat signal is amplified by a high frequency amplification@$5, and is waveform-shaped by a comparator 6 into a square wave for TTL.

On the other hand, the alternating signal generated by the crystal oscillator 7 is frequency-divided by a divide-by-1 counter 8 to produce a signal giving a reference frequency.

The value of this reference frequency is set using code switches 8a and 8b.
It will be held in The frequency of the output of the comparator 6, that is, the frequency difference between the optical beat frequency and the reference frequency, is determined by the frequency difference counter 9. If the one frequency difference is positive, a pulse corresponding to the difference frequency appears on the up line U, and a negative In this case, a pulse appears in the down circuit &lD.The pulse train appearing in the 0 line U becomes a positive pulse train by the positive polarity differentiating circuit 10a, and the pulse appearing in the 1st line becomes a negative pulse by the negative polarity differentiating circuit 10b. Depending on whether the frequency difference is positive or negative, a pulse train can be obtained that has positive or negative electrical polarity and one frequency is proportional to the difference frequency. A converter 11 performs frequency-voltage conversion of this pulse train to generate a control signal.

The converter can be constructed from an integrating circuit with a short time constant. The control signal passes through the adder circuit 12, is amplified by the power amplifier circuit 13, and is supplied to the electric heater 2. The resonator length is controlled by controlling the tube wall temperature of the laser discharge tube. At this time, the target value of the control can be determined by the bias signal generator 14, and the resonator length can be kept constant and the optical beat frequency can be kept constant by using a system of 0 or more.

The electric charge of the pulse train obtained by the differentiating circuits 10a and 10b is used to generate a perturbation voltage signal proportional to the amount of soil stored in the defoliator 15.
It is added to the control signal, and the bias level is gradually shifted. This movement is carried out in the direction where there are many positive (negative) pulses, and there are fewer pulses, and continues until the number of positive (negative) pulses becomes equal to the number of negative (positive) pulses. When the number of pulses is large, the movement speed is fast;
As the number decreases, the speed gradually decreases, and when negative (positive) pulses begin to appear, the movement becomes even slower, and by the time the positive and negative pulses become equal, the movement speed becomes infinitely small.

If the bias level moves too fast, it will cause disturbances in the control system, so the slower the bias level is, the smoother the control will be. The ideal is !IIs, where it gradually moves slowly as it approaches, but the above perturbation is exactly as it should be.

As described above, by creating a control signal based on the difference frequency, the influence of temperature drift can be ignored, and by using a method of creating a built-up perturbation voltage, it is possible to obtain a frequency-stabilized laser device in which perturbation does not cause disturbance. Can be done.

[Brief explanation of the drawing]

1 and 2 are explanatory diagrams of the present invention, and FIG. 3 is a schematic configuration diagram showing a stabilized laser device according to an embodiment of the present invention. 1 ・・・・・・・・・Laser discharge tube 2 ・
・・・・・・・・・・・・Electric heater −-3・・・・・・・・・・・・Polarizer 4
・・・・・・・・・Detector 5 ・・・・・・
...High frequency amplifier 6 ......
・Comparator 7......Crystal oscillator 8......Division counter 8a,
8b... Code switch 9... Frequency difference counting device 10a
......Positive polarity differential circuit 10b...
・・・・・・Negative polarity differential circuit 1.1 ・・・・・・
......Frequency-voltage converter 12...
... Adder circuit 13 ...... Power amplifier circuit 14 ...... Bias voltage generator 15 ......・・・ Integral
Container N, S...... Magnet U...
・・・・・・・・・Up circuit D ・・・・・・
・・・・・・Down circuit - one inside 214

Claims (1)

[Claims] In a laser that controls the length of the resonator so that the optical beat frequency between polarized lights having different frequencies and which are orthogonal to each other is a specific value, the optical beat frequency between the polarized lights and the reference frequency created by a crystal oscillator etc. A difference frequency detection means for determining the frequency of the difference, a frequency-voltage conversion means for converting the frequency of the difference into a control signal such as a voltage, and amplifying the control signal to control the temperature of the laser discharge tube using an electric heater or the like. A system consisting of means for controlling the length of the resonator makes the length of the resonator almost constant, and also generates electric power having positive and negative electrical polarity and high and low frequencies depending on the positive and negative and magnitude of the frequency of the difference. A pulse train is created, and the charge of the electric pulse train is gradually integrated to create a perturbed voltage signal, and the perturbed voltage signal is added to the control signal to gradually increase the average value of the control signal, that is, the resonator length. A frequency stabilized laser device characterized by having a moving function.