JPS61234581A - Frequency stabilized laser having single frequency - Google Patents

Abstract

PURPOSE:To set the length of a resonator, which is kept constant, at an arbitrary point, by applying perturbation, whose speed is less than a constant value. CONSTITUTION:A second resonator-length control system comprises a comparator 11, a flip-flop 12, a perturbation-direction determining circuit 7 and a perturbation applying circuit 8. The fact that the length of the resonator is kept constant by a first resonator-length control system is confirmed, and operation is started. Perturbation, by which the resonator length is gradually increased by the heat generated by a discharge tube, is applied. With respect to the negative perturbation, the sign is determined by the perturbation-direction determining circuit 7. A signal, which is outputted from the perturbation applying circuit 6, is additionally applied to an additive amplifier 5. The resonator length is gradually increased. The signal of light intensity passes the maximum point and minimum point. The flip-flop 12 is operated at this time, and the positive and negative signs of the perturbation-direction determining circuit 7 are reversed. A reversing amplifier 10 is inserted into the first resonator-length control system by using a signal switching circuit 13, and the output of the amplifier is reversed. Thus the instability of the system is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION a) Industrial Application Field The present invention relates to a radar device with a stabilized oscillation frequency.

b) Conventional configuration and its problems Conventional gas laser! 5N! Methods for stabilizing the vibration frequency were broadly divided into the following two types. One is to control the laser resonator length so that the frequency of the optical beats generated during the two-frequency oscillation light obtained by applying a magnetic field to the laser tube is constant. By utilizing the phenomenon that the polarization of adjacent longitudinal modes of oscillated light is orthogonal to each other, and the phenomenon that the intensity ratio of oscillated light of adjacent longitudinal modes changes depending on the cavity length, laser oscillation light can be made orthogonal to each other. A method has been used in which the two polarized light components are detected and the resonator length is controlled so that the ratio of the intensities of the two polarized lights becomes a constant value.

The above two conventional methods have the following drawbacks. In @'s method using optical beats, a laser is emitted by a magnetic field! The uniformity of the ion density that collides with the cathode of the tube is disrupted, and as a result, sputtering does not occur uniformly, and damage to the D electrode may occur locally.
Even the slightest defect in the cathode material, such as impurities, can cause damage to the laser. The drawback is that life is shortened. In addition, in the latter method of keeping the intensity ratio of orthogonal polarized lights constant, in order to improve stability, the intensity ratio is selected so that the intensity of the two polarized lights is approximately T, and when using a laser, the intensity ratio of the two polarized lights is
It is a method that extracts only one of the two polarized lights, and as a result, the intensity of the usable light is half that of laser oscillation light, which has the drawback of large losses.

C) Object of the Invention The present invention obviates the drawbacks of conventional frequency stabilized lasers as mentioned above. The purpose is to realize a frequency-stabilized laser with long life and low loss.

D) Structure of the Invention In a laser having an internal resonator, the resonator length can be changed by changing the temperature of the discharge tube. When the optical beat frequency and the intensity of the oscillated light change according to changes in the laser resonator length, the resonator length can be changed by controlling the temperature of the laser discharge tube so that the optical beat frequency and the intensity of the oscillated light are constant. It is well known that the oscillation frequency is stabilized by keeping it constant. but.

This control is based on an extremely delicate thermal balance, and often requires tightrope adjustments in the control loop.

The present invention is based on the discovery that perturbations can be applied to the control loop that maintains the delicate heat balance described above without destroying the heat balance at speeds below a certain level, and the present invention uses the perturbations to , the above-mentioned constant resonance length is set at any desired point.

The present invention will be explained below using the drawings.

When the laser resonator length is L, the longitudinal mode spacing of the laser is C/21 hertz, and the polarizations of adjacent modes are substantially orthogonal. When the resonator length changes, the intensity of light in each mode changes according to the laser gain curve g, as shown in FIG.

Therefore, as mentioned above, as shown in Figure 1a, stable two-mode oscillation can be obtained by separating the two modes by polarization and controlling their respective intensities to be equal, and this is already in practical use. However, in practical use, the light required is not two modes but a single mode, so only one mode is extracted, resulting in a large loss.

The present invention attempts to minimize loss by controlling the oscillation to occur in the state shown in FIG. 1. In other words, the purpose is to set a control point at the peak position.Originally, peak setting control is usually carried out by superimposing an AC modulation signal on the signal, but AC modulation is a major drawback in lasers. 1. The present invention avoids this and uses the above-mentioned perturbation to set the peak.

As shown in Fig. 3, when one polarized light of the oscillation light of the laser 1 is selected by the polarizer 20 and detected by the detector 2, the detected light intensity signal changes to the second polarized light according to the change in the resonator length.
It changes as shown in figure a. Therefore, if the signal obtained by the detector 2 is amplified by the amplifier 4, the possible bias addition is added by the amplifier 5, and the fan 3 or heater for temperature control is driven, the resonator length is set to a certain value. For example, for 0 or more, which can be fixed at the LO point in Figure 2,
The perturbation direction determining circuit 7 issues a command to determine the perturbation direction, that is, whether to apply perturbation in the direction of lengthening or shortening the resonator length, and the heat balance is determined based on the command of the perturbation direction determination circuit. By additionally applying to the summing amplifier 5 the output of the perturbation applying circuit B, which has the function of applying perturbation while satisfying the condition that the temperature does not increase, the resonator length can be gradually changed while maintaining the thermal balance.

Taking advantage of the fact that the direction of change in the resonator length can be arbitrarily selected, the resonator length is gradually adjusted to the maximum or minimum point of the oscillation light intensity. When the resonator length approaches the maximum point or minimum point, the direction of change in the resonator length is reversed, making it possible to control the resonator length so that it always exists near the maximum point or minimum point.

FIG. 4 shows an embodiment of the present invention.

In this embodiment, the intensity of one polarized component of the oscillation light of the laser 1 selected by the polarizer 20 is detected by the detector 2 and amplified by the amplifier 4. The maximum value or minimum value of the amplified signal is stored as a peak value in the peak holder 8, and the difference between the peak value and the signal value at a certain point in time appears as the output of the difference amplifier 3. A summing amplifier 5 applies a bias to the difference in signal values, and the output of the summing amplifier 5 is set at a temperature mI.
fjSl
The second resonator length control system, called the resonator length control system, includes a comparator It, a flip-flop +2, and a perturbation direction determining circuit7. It consists of a perturbation applying circuit B. The second resonator length control system starts operation after confirming that the resonator length is kept constant by the first resonator length control system. First, a perturbation is applied so that the resonator length gradually increases due to the heat generated by the discharge tube. Hereinafter, perturbations in this direction will be referred to as negative perturbations, and perturbations in which the resonator length gradually decreases will be referred to as positive perturbations.
Negative perturbation is.

The perturbation direction determining circuit 7 determines the sign, and the signal output by the perturbation applying circuit 6 based on the determined sign is added and amplified! This is done by applying an additional voltage to 15. When a negative perturbation is performed, the cavity length gradually increases, and the light intensity signal changes to a maximum or minimum (Fig. 2a,
LC) point. Whether the maximum or minimum point has been passed can be determined by detecting the deviation of the output of the difference amplifier 8 from zero using the comparator 11. When the passage of the maximum or minimum point is confirmed by the comparator 11, the flip-flop 12 is operated to reverse the polarity of the perturbation direction determining circuit 7, and the signal VJ circuit 1 is activated.
By inserting the inverting amplifier B10 into the first resonator length control system using 3, the output of the amplifier of the first resonator length control system is inverted as shown in tIS2 to prevent the system from becoming unstable. . When the perturbation changes from negative to positive, the light intensity signal passes through the maximum or minimum point again, but when the passage of this point is confirmed by the comparator 11, the perturbation direction is reversed again. In other words, by always knowing the perturbation direction, checking the passage of the maximum or minimum point, determining the direction of the perturbation using that point as the boundary, and performing the perturbation, the light intensity will always be directed towards the maximum or minimum point. As it moves, the first resonator length control system maintains heat balance.
The resonator length is determined by perturbing the second control system.The perturbation applying circuit is an integrator with a sufficiently long time constant, and the perturbation direction determining circuit is configured to change the positive or negative direction according to the flip-flop signal. This is realized by a comparator that generates a constant voltage.

Lamb dips do not clearly appear in ordinary helium-neon laser discharge tubes.

In that case, the setting value of the resonator should be set to the maximum value of the polarized light to be used or the minimum value of the polarized light of the adjacent mode.If a lamb dip appears as shown in Figure 1 C in a discharge tube using isotopes, It is best to choose the minimum value for .

[Brief explanation of drawings]

FIG. 1, FIG. 2, and FIG. 3 are explanatory diagrams of the present invention. FIG. 4 shows an embodiment of the present invention. l...Laser discharge tube 2...Detection = 3...Temperature 31tM device 4...Ring m Device 5...Summing amplifier 6...Perturbation applying circuit 7...Perturbation direction determining circuit 8...Peak Holder 9... Difference amplification base 0... Inverting amplifier 11... Comparator 12...
...Flip-flop 13...Signal switching circuit 20...Polarization Ig...Gain curve L... ...resonator length

Claims (1)

[Claims] Of the oscillation light of a gas laser having an internal cavity using discharge excitation, light having one polarization component is detected, and the light having one polarization component is detected.
a first resonator length control system that controls the resonator length of the gas laser by controlling the temperature of the laser discharge tube so that the intensity of light having two polarization components becomes one set value;
a second resonator length control system having a function of gradually increasing or decreasing the two set values in an arbitrary direction, and the resonator length of the gas laser is controlled by the first resonator length control system. Frequency stabilization characterized in that the constant value is maintained at a constant value and the constant value is selected by the second resonator length control system so that the intensity of the light having the one polarization component is near a maximum value or a minimum value. chemical laser equipment