JPH0228982A - Stabilizing method for gaseous laser - Google Patents

Abstract

PURPOSE:To stabilize frequency by applying flexural load to a front end section, to which an internal mirror is installed, from the lateral direction. CONSTITUTION:The driving voltage of a DC component at several dozen volt and an AC component at several volt of one hundred Hertz is applied to a piezoelectric element 22 from a piezoelectric-element driver circuit 24, and the frequency of a longitudinal mode is shifted within a constant fine range. Rear beams emitted from the rear end section 12R of a laser tube 10 are received by a silicon photo-detector 28, and input to a lock-in amplifier 36 end the primary differential value of a signal is acquired. The primary differential signal is input to a control circuit 38, and currents flowing through a heater 20 are controlled by differential voltage with reference voltage. When a laser output is stabilized, the laser tube 10 is lit, the primary differential value in a Lamb dip is measured in order to determine reference voltage for control, the laser tube 10 is pre-sheeted to the state of thermal equilibrium or more by the heater 20, and the laser tube 10 begins to shrink when heating is stopped. When there is the longitudinal mode near the Lamb dip, control may be started.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for stabilizing a gas laser, and particularly to a method for stabilizing a gas laser, and in particular, an oscillation method suitable for use in a gas laser, which is provided with an internal mirror at the tip protruding from a laser tube to reflect and resonate laser light. The present invention relates to a gas laser stabilization method capable of stabilizing the frequency and light intensity of laser light.

[Conventional technology]

In lasers, it is necessary to stabilize the light intensity and frequency of oscillated laser light. The causes of light intensity fluctuations common to all gas lasers are:
These include changes in excitation, changes in resonator length, and changes in resonator alignment. Among these, stabilization of excitation is usually achieved by constant current control, and the discharge current is detected to control a current control element or to adjust the high voltage. In addition, fluctuations in optical intensity due to changes in the resonator length become a problem when the number of oscillation longitudinal modes is small. Efforts are being made to stabilize the Furthermore, changes in resonator alignment occur primarily due to different thermal expansions of the resonator components and mechanical vibrations, and to alleviate this effect, the resonator can be fixed via leaf springs or bearings. It is being said. In addition, in order to eliminate temperature non-uniformity, methods are used to avoid placing the rod above the laser tube, which is the heating element, and surrounding it with metal, and using forced cooling to prevent temperature non-uniformity. . On the other hand, frequency stabilization of gas lasers is performed by detecting the frequency deviation from a certain reference as an error signal and controlling the laser resonator length. For example, using the laser transition spectrum as a reference, external The laser frequency is modulated and stabilized by changing the position of the mirror in the axial direction and minutely modulating the resonator length. However, in a gas laser in which an internal mirror for reflecting and resonating the laser beam is provided at the tip protruding from the laser tube, the tip is weak in strength, so no force is applied to the tip. No consideration was given to adding . In general, in a gas laser, particularly in an internal mirror type He-N'e laser, the mirror spacing of the resonator changes due to thermal expansion of the laser tube, and the output light intensity fluctuates. In a single mode laser tube, its output light intensity corresponds to its gain profile. Therefore, when the output light is observed, a ram dip appears at the apex of the output light intensity curve as shown in FIG. Its center frequency is stable, coinciding with the center of the spectrum at 0.633 micrometers, and the width of the Lamb dip is on the order of tens of megahertz, compared to the width of the gain profile (hundreds of megahertz to thousands of megahertz). An order of magnitude smaller. Therefore, it is conceivable to stabilize the oscillation frequency of the laser by using the Lamb dip when using the laser transition spectrum as a reference. However, if the ram dip is detected based on the level of the laser output light intensity, there is a risk that the lock point will shift if the laser output light intensity changes significantly.

[Problem to be achieved by the invention]

The present invention makes it possible to stabilize the frequency and light intensity of laser light by applying force to the tip of the laser tube, where the internal mirror is installed, which was previously unthinkable. The first object is to provide a method for stabilizing a gas laser. A second object of the present invention is to provide a gas laser stabilization method that can accurately detect and reliably lock the positions of the ram and dip regardless of level fluctuations in the laser output light intensity. .

[Means to achieve the task]

The present invention provides a method for stabilizing a gas laser in which a tip protruding from a laser tube is provided with an internal mirror for reflecting and resonating laser light, in which a bending load is applied to the tip from the lateral direction. By controlling at least one of the magnitude and direction of the bending load, the longitudinal mode, that is, the frequency or the light intensity of the emitted laser light is stabilized, thereby achieving the first object. Further, the bending load is applied by a piezoelectric element. In addition, in a similar method for stabilizing a gas laser, the frequency of the laser beam is varied within a very small range by the AC component of the bending load, thereby obtaining the first differential value of the light intensity depending on the frequency, and calculating the first differential value. The second objective is achieved by fixing the oscillation mode to the ram dip by controlling the temperature of the laser tube according to the temperature.

[Action and effect]

This is a so-called internal mirror type He-Ne laser (λ = 0.633 micrometers), in which an internal mirror is provided at the tip protruding from the laser tube to reflect and resonate the laser beam.
is currently used in a wide range of basic research and optical measurement methods.The inventor conducted an experiment by applying a bending load from the lateral direction to the tip of this internal mirror type He-Ne laser.
Phenomena of longitudinal mode movement and light intensity change were observed.The present invention utilizes these phenomena to stabilize the frequency and light intensity of laser light. Using an experimental apparatus as shown in FIG. Alternatively, a bending load was applied to the rear end portion 12R) from the lateral direction, and the state of longitudinal mode movement was observed. Here, the laser tube 10 is manufactured by Ushio Corporation, UNL-
205R3, UNL-205R, UNL-21OR, U
Using the NL-22OR, the laser tube 10 is fixed with the tip of the fixing screw 16 as shown in FIG.
2 places on the front and back of the main body (thick center part) x equal distance on the same circumference I
Fix it at three points I (120° intervals). The method of applying a load to the tip is to apply a weight from zero to one end of the mirror, perpendicular to the tube axis of the laser tube 10, for both the front end 12F with a built-in front mirror and the rear end 12R with a built-in rear mirror. A mechanical load was applied in a range of 0.6 kgf. As shown in FIG. 2, the load direction was 0 degrees from the center of the laser tube mirror toward the cathode terminal 14, and clockwise rotation was positive and counterclockwise rotation was negative. In order to observe the longitudinal mode movement, the laser output light is input to a spectrum analyzer, the gain profile of the laser light as shown in the upper row of Section 3 is displayed on the oscilloscope, and the displayed longitudinal mode is measured before and after loading. The vertical mode movement amount (frequency movement amount) was calculated. Here, the longitudinal mode spacing was calculated as C/(2L) Hertz (C is the speed of light and L is the laser tube resonator length). The laser tube 10 is fixed in the above state, and the load head is rotated in the load direction mentioned above, from -180 degrees to +1
When observing mode movement in 15 degree increments up to 65 degrees,
For example, the observation results shown in Figure 4 were obtained. In addition,
Here, the load amount is constant. Next, with the load direction being the same (+90 degrees or -90 degrees), a load was applied from zero in units of about 50 or 100 gf, and the longitudinal mode movement at this time was observed. The results were obtained. From a large number of experimental results, the load angle θ and the longitudinal mode movement amount Δf
When we calculated the relationship, we obtained an approximate expression as shown below. Δf=A+B-cos (θ+α)...(1)
Here, A and B are constants (A<<B), and α is the phase. When we calculated the correlation coefficient between this approximate formula and the experimental values, we found that it was 0.
It was found that the correlation was in the range of 847 to 0.988, and that there was a very good correlation. Further, when the relationship between the load amount W and the longitudinal mode movement amount Δf was determined from the same large number of experimental results, the following approximate expression was obtained. Δf=C−W (2) where C is a constant. These phenomena occur when a bending load is applied to the tip 12 of the laser tube 10, so the laser tube 10 is bent, so the laser tube 10 is considered to be a cantilever supported at the position of the fixing screw 16, and this phenomenon can be explained using geometric optics. Material mechanics (cantilever bending)
When the optical inter-mirror distance and the amount of change thereof in a loaded state were calculated using a model, theoretical formulas corresponding to the above-mentioned formulas (1) and (2) were obtained and found to be explainable. Note that in the model calculation, the mirrors at both ends of the laser tube were spherical mirrors, but it has been confirmed that similar theoretical formulas can be obtained when using flat mirrors. Next, a load was applied to the tip using a piezoelectric element, and an experiment was conducted to examine changes in light intensity due to bending load. The laser tube fixed state is the same as the device used to measure the fiber mode movement shown in FIGS. 1 and 2, except that the loading device is replaced with a piezoelectric element. Note that the laser tube 10 is UNL-20 manufactured by Ushio Corporation.
5R3 was used, and the discharge current was 5 milliamps. First, the piezoelectric element is connected to the laser tube 1 even when the driving voltage is 0 volts.
The tip was fixed to the side surface of the mirror of No. 0 in a pressurized state (a state where the tip was bent), and the pressurization value at this time was set as P. A load was applied in the direction of 1120 degrees at the front end 12F, and in this state a driving voltage of 0 to 100 V was applied to the piezoelectric element, and changes in light intensity after the laser tube was turned on were observed. The laser output light intensity was measured by receiving the front beam with a silicon photodiode, converting the current value into voltage, and then recording it on a pen recorder. Since the laser tube 205R3 normally oscillates in a single mode, the change in light intensity almost represents the gain profile. Therefore, 18 minutes after the laser tube is turned on, (1) the light intensity IR5 at the ram dip, (2) the amplitude W of the light intensity change, (3
) After considering the shape of the gain profile, we found that
The results shown in Table 1 below were obtained. Table 1 As is clear from Table 1, the light intensity I at the lamb dip
R decreases almost linearly as the load increases. Therefore, it can be seen that the light intensity can be changed by changing the load. Also, to stabilize the light intensity,
On the contrary, it can be seen that it is sufficient to control the bending i weight. From Table 1, it can be seen that the amplitude W of the light intensity change does not change, and furthermore, the shapes of the gain profiles are almost similar, although the ram dip becomes shallower with the load. Therefore, as shown in FIG. 6, the frequency of the laser beam is varied within a minute range by the alternating current component of the bending load, and as a result, as shown in FIG. ...Δ in Figure 6
If I/Δf) is obtained, the first-order differential value becomes zero in the Lamb dip, and the positive and negative values are reversed before and after that. Therefore, by using this signal as an error signal and feedback-controlling the temperature of the laser light, the laser output light intensity can be adjusted. Regardless of level fluctuations, the oscillation mode can be accurately fixed at the position of the ram dip, and the frequency and light intensity of the laser beam can be stabilized at the same time. Note that in the above description, a He-Ne laser was used, but the scope of application of the present invention is not limited thereto, and it is clear that it can be similarly applied to a general gas laser. Furthermore, the means for applying bending load should also be limited to piezoelectric elements.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. This embodiment is a laser output stabilizing circuit capable of simultaneously stabilizing the light intensity and frequency of laser light according to the present invention, and is configured as shown in FIG. This embodiment includes a laser tube 1o in which a front end 12F and a rear end 12R protruding from the laser tube 10 are each provided with internal mirrors for reflecting and resonating laser light. The laser tube 10 is, for example, UNL-205R manufactured by Ushio.
3, and a film-shaped shutter 20, for example, for controlling the temperature is disposed in the center of the main body. A piezoelectric element 22 is disposed on the side of the front end 12F of the laser tube 10 for applying a bending load to the front end 12F. The piezoelectric element 22 includes a piezoelectric element drive circuit 24.
is connected to the piezoelectric element drive circuit 24, and a sine wave oscillator 26 that oscillates a sine wave of, for example, 100 hertz is connected to the piezoelectric element drive circuit 24. Behind the rear end portion 12H (on the left side of the figure), a light intensity of 1
A silicon photodetector 28 is provided to receive the rear beam of the laser beam in order to obtain a second-order differential signal. A current-voltage (IV) converter 30 is connected to the silicon photodetector 28 for converting its output current into a voltage. An AC amplifier 32 is connected to the r-v converter 30,
The AC amplifier 32 has, for example, Q=5 and a cutoff frequency fc.
= 100 [(z) A high-pass filter 34 is connected. The output of the high-pass filter 34 is input to a lock-in amplifier 36.
, a signal synchronized with the AC component of the piezoelectric element drive voltage is
It is input as a reference signal. The first-order differential value of the signal obtained by the lock-in amplifier 36 is input to the control circuit 38. This control circuit 38 includes a differential amplifier (not shown), and the differential amplifier outputs a voltage difference from a reference voltage (ideally zero volts). The current flowing through the heater 20 is controlled by this output. Here, the reason why the control circuit 38 is provided with a differential amplifier is because
Theoretically, the first differential value at the Lamb dip should be zero volts, but in reality there is a DC component of several volts, and the reference voltage of the differential amplifier is set to this DC component value. settings to remove the DC component. A silicon photodetector 40 is provided in front of the tip portion 12F (on the right side of the figure) for receiving the front beam and detecting the light intensity. The output of this silicon photodetector 40 is converted into a voltage signal by an I-V converter 42, and then input to a pen recorder 44 to monitor stability. The operation of the embodiment will be explained below. In order to obtain the first-order differential value of the laser output light intensity, the piezoelectric element 22 is supplied with a drive voltage of, for example, several tens of volts for a DC component and several volts for an AC component of 100 hertz, using a piezoelectric element drive circuit 24.
The frequency of the longitudinal mode is shifted within a certain minute range. On the other hand, the laser beam emitted from the rear end 12R of the laser tube 10 is received by the silicon photodetector 28, and after converting its current value into voltage by the IV converter 30, it is passed through the AC amplifier 32 and the high-pass filter 34. The signal is input to the lock-in amplifier 36 via the signal. This lock-in amplifier 36 includes the sine wave oscillator 26.
Therefore, the signal synchronized with the AC component of the piezoelectric element drive voltage is
The signal is input as a reference signal, and the lock-in amplifier 36 calculates the first differential value of the signal. The obtained first-order differential signal is input to the control circuit 38,
A built-in differential amplifier outputs the difference voltage from the reference voltage. This output controls the current flowing through the heater 20, for example in the range of zero to 100 milliamps. When stabilizing the laser output, as shown in FIG. 8, after several tens of seconds have elapsed since the laser tube 10 was turned on (step 110), the control base 1! To find out the voltage, measure the first differential value at the lamb dip (step 112.11).
4). Thereafter, the laser tube 10 is pre-sheeted to a temperature above the thermal equilibrium state using the heater 20 (steps 116 and 118). Next, when the heating is stopped, the laser tube 10 begins to contract (step 120), and the control may then be started when the longitudinal mode exists near the ram dip (steps 122 and 124). According to this embodiment, the laser output light intensity (IV converter 42
As is clear from Figure 9, which shows an example of the relationship between the output) and its first derivative value (output of the high-pass filter 34), the reference voltage 1. It can be seen that the ram dip is captured accurately at 8 volts. As shown in FIG. 10, the slope of the one-stroke differential value of the laser light intensity with respect to time at the lamb dip is assumed to be ΔI/ΔW. Assuming that the period of variation of the first-order differential value is W, since the laser tube of the embodiment oscillates in a single mode, the period W is considered to correspond to the fir mode interval of 1260 MHz. Therefore, the frequency fluctuation amount Δf corresponding to the fluctuation amount ΔV of the first-order differential value near the lamb dip is expressed by the following equation. Δf[Mt[z] = (1260/W)・(ΔW/Δwork) ×ΔV[volt] ...(3) Also, Fig. 11 shows an example of the stable state of the laser output light intensity after the start of control. , the light intensity stability is the fluctuation rate with respect to the average light intensity, and the frequency stability is the laser frequency 4.74X1
Defined as the rate of fluctuation with respect to 0" Hertz, the stability for 10 minutes is ±0.034 to ±0.087% in light intensity.
, it was possible to achieve a stability of ±0.41 to 1.33×10' at the frequency. In addition, the stability for one hour is ±0.121 to 0.393% for light intensity and ±0.41 to ±0.41% for frequency.
A stability of 1.33×10 −′ could be achieved. Compared to the case where the level of laser light intensity is used as a control signal, it was confirmed that the stability for 10 minutes is a fixed degree, but the stability for 1 hour is higher. . In this embodiment, since the frequency is oscillated in order to obtain the first-order differential value, it is impossible to reduce the frequency fluctuation (light intensity fluctuation) below the amplitude of this oscillation. In the above embodiment, the temperature of the laser tube 10 was controlled by the heater 20, but the method of controlling the temperature of the laser tube is not limited to this, for example, controlling the rotation speed of a cooling fan may be used. Also good.

[Brief explanation of the drawing]

Fig. 1 is a front view showing the configuration of an experimental apparatus for explaining the present invention in detail, Fig. 2 is a side view, and Fig. 3 is a gain profile of a single mode laser and its first derivative. Figure 4 is a diagram showing an example of the relationship between load direction and longitudinal mode movement; Figure 5 is a diagram showing an example of the relationship between load amount and longitudinal mode movement; FIG. 6 is a diagram showing an example of the relationship between frequency fluctuations due to alternating current components of bending load and laser output light fluctuations, and FIG. 7 is a block diagram showing the configuration of an embodiment of the laser output stabilization circuit according to the present invention. , FIG. 8 is a flowchart showing the procedure up to the disclosure of stabilization control in the above embodiment, and FIG. 9 is a line showing an example of the relationship between the laser output light intensity and its first derivative value in the above embodiment. 10 is a diagram showing an example of the relationship between the laser output and its first derivative value during thermal expansion of the laser tube. FIG. 11 is a diagram showing the stable state of the laser output light intensity in the above example. . 20... Heater, 22... Piezoelectric element, 24
...Piezoelectric element drive circuit, 26...Sine wave oscillator, 28...Silicon photodetector, 34...High-pass filter, 36...Lock-in amplifier, 38...Control circuit.

Claims (4)

[Claims] (1) A method for stabilizing a gas laser in which a tip protruding from a laser tube is provided with an internal mirror for reflecting and resonating laser light, which includes applying a bending load to the tip from the lateral direction, and bending the tip. A method for stabilizing a gas laser, comprising stabilizing the longitudinal mode, that is, the frequency, of an oscillated laser beam by controlling at least one of the magnitude and direction of a load. (2) The method for stabilizing a gas laser according to claim 1, wherein the bending load is applied by a piezoelectric element. (3) A method for stabilizing a gas laser in which a tip protruding from a laser tube is provided with an internal mirror for reflecting and resonating laser light, which includes applying a bending load to the tip from the lateral direction, and bending the tip. 1. A method for stabilizing a gas laser, comprising stabilizing the intensity of an emitted laser beam by controlling at least one of the magnitude and direction of a load. (4) A method for stabilizing a gas laser in which a tip protruding from a laser tube is provided with an internal mirror for reflecting and resonating laser light, comprising: applying a bending load to the tip from a lateral direction; The frequency of the laser beam is varied within a minute range by the alternating current component of the load, thereby obtaining the first derivative value of the light intensity according to the frequency, and controlling the temperature of the laser tube according to the first derivative value. A method for stabilizing a gas laser characterized by fixing the oscillation mode to the ram dip.