JPH01238081A - Wavelength stabilized laser oscillator - Google Patents

Abstract

PURPOSE:To achieve the stabilization of wavelengths and the correction of a refractive index simultaneously, by arranging a laser interferometer system so that the length of the dead path does not vary, controlling the oscillation wavelength of a laser oscillator in such a way that the wavelength of the laser is always constant. CONSTITUTION:A quarter wavelength plate 14 and a mirror 15 are bonded to one side of a polarizing beam splitter 4 and further, another quarter wavelength plate 16 is bonded to the other side as well. A mirror 6 is disposed so that the mirror faces the bonded face of the foregoing wavelength 16. For example, the mirror is connected to an interferometer 4 by a supporting member 5 consisting of materials such as a super unvar and the like having a smaller linear expansion coefficient, and the distance between the mirror 6 and the interferometer 4 is almost invariable. When oscillation wavelengths of a laser oscillator 1 are controlled by controlling voltages impressed to a heater 3 with a laser wavelength control device 12 so that an output of a displacement measurement device 11 does not vary, the optical path between the interferometer 4 and the mirror 6 is kept at a constant wave number. Thus, the laser wavelength is corrected and stabilized in the environment on the optical path.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser length measuring device, and particularly to an oscillator that can easily and accurately stabilize and correct a laser wavelength.

[Conventional technology]

Conventionally, stabilization of the oscillation wavelength of the laser oscillator itself and correction of the refractive index of the wavelength on the dead path of the laser interference system have been performed independently and separately.

For example, regarding wavelength stabilization of laser oscillators, an overview of the technology can be found in "Gas lasers, solid-state lasers (inquiry; 0pln
sE, p148-156. '84.6) An example of this will be explained using FIG. 2.

FIG. 2 is an example of a general wavelength stabilized laser, and shows the inside of the laser oscillator 1.

The wavelength of the laser light emitted from the laser resonator 2 can be controlled by changing the length of the resonator, so in this example, the temperature is controlled by a heater wrapped around the laser resonator 2, causing resonance due to thermal expansion. This configuration controls the oscillation wavelength by changing the length of the device. In this case, a part of the laser light output is taken out by the beam splitter 24, an error signal detector detects some kind of error signal, and the laser wavelength control device 12 feeds back the error signal to the voltage applied to the heater 3. This stabilizes the wavelength.

In addition, as a technique for correcting the change in the wavelength on the bad path of a laser interference system due to changes in the refractive index of air, for example, a completely independent correction beam is generated near the measurement laser beam, and the correction beam is It is conceivable to monitor the refractive index change of the air on the dead path by configuring the beam to measure an unchanging length, and use the result to correct the measurement result of the change due to the measurement beam.

In this way, it is possible to obtain accurate displacement measurement results in which changes in the refractive index of air are corrected.

[Problem to be solved by the invention]

In the above conventional technology, an interferometer is constructed using a laser oscillator whose wavelength has been stabilized, but in the end, the laser wavelength is disturbed due to changes in the refractive index of air in the dead path part of the interference system, and the wavelength of the laser is disturbed again. As a result, high measurement accuracy could not be obtained unless two steps of stabilization and correction were performed, in which the results were monitored and corrected in some way.

An object of the present invention is to directly detect wavelength disturbances on the dead path of an interference system, and to simultaneously achieve wavelength stabilization and refractive index correction based on the results.

[Means to solve the problem]

The above object is achieved by configuring the laser interference system so that the length of the dead path does not change and controlling the oscillation wavelength of the laser oscillator so that the wavelength of the laser in this portion is always constant.

[Effect]

When the length of the dead path of a laser interference system does not change physically, the intensity of the interference light from the interference system should not change, but if the oscillation wavelength of the laser oscillator changes, When the refractive index of the air on the dead path changes and the laser wavelength on the dead path changes, the phase of the reflected light from the dead path changes, and as a result, the intensity of the interference light 8 changes. By adjusting this change, the dead path can be adjusted. Fluctuations in the wavelength above can be detected.

The displacement measuring device 11 takes in the intensity of the interference light 8.

This is a means to accurately interpolate between interference fringes and detect changes in laser wavelength on a dead path with high sensitivity, and a signal corresponding to the detected wavelength change is sent from the laser wavelength control device 12 to the laser. Since the oscillation wavelength of the laser is controlled by feeding back to the heater 3 of the resonator 2, the laser wavelength in this part can be stabilized and standardized with respect to the environment on the dead path.

【Example〕

An embodiment of the present invention will be described below with reference to FIG.

In Fig. 1, 1 is a gas laser such as a He-Nθ laser, which oscillates linearly polarized light with a vibration plane in the 45'' direction relative to the plane of the drawing. The temperature of the laser resonator 2 can be controlled by adjusting the voltage applied to the heater 3 to rA.In general, the oscillation wavelength of a laser changes depending on the length of the laser resonator, so it is possible to control the temperature of the laser resonator 2 by adjusting the voltage applied to the heater 3. It is generally known that the oscillation wavelength is controlled by utilizing the thermal expansion of a resonator, and this embodiment is also an oscillation wavelength control type laser.

In addition, 4 is a polarizing beam splitter, and its - plane has -
Wave plate 14 and mirror 15 are glued together.

A single wavelength plate 16 is also bonded to another surface.

A mirror 6 is disposed at a certain distance away from the adhesive surface of the single-wavelength plate 16, and is connected to the interferometer 4 by a support member 5 made of a material with a small coefficient of linear expansion, such as super amber. Therefore, the distance between the mirror 6 and the interferometer 4 remains almost unchanged. 17 is a polarizing plate; 9
is a photodetector that photoelectrically converts the intensity of the interference light 8 into an electric signal (interference signal 10); 11 is a displacement measuring device that measures the amount of change from the change in the interference signal 10; 12 is the output from the displacement measuring device 11 and the target value This is a laser wavelength control device that controls the complete wavelength of the laser oscillator 1 by adjusting the voltage applied to the heater 3 according to the deviation from the laser oscillator 1.

Next, the operations in FIG. 1 will be explained in order.

First, the linearly polarized light emitted by the laser oscillator 1 and having a vibration plane in the direction of 45 degrees to the plane of the paper enters the polarization beam splitter 4, where it is split into two polarization components: a polarization component parallel to the plane of the paper and a polarization component perpendicular to the plane of the paper. The linearly polarized light having a plane of vibration perpendicular to the paper reflected by the polarizing beam splitter 4 passes through the wave plate 14, is reflected by the mirror 15, and returns to the polarizing beam splitter 4 through the wave plate 14 again, but the wave plate 14 Since the vibration plane of this light that has made one round trip has rotated by 90', it now passes through the polarizing beam splinter 4 and reaches the polarizing plate 17. - On the other hand, linearly polarized light with a vibration plane parallel to the paper plane that first passes through the polarizing beam splitter 4 passes through the - wavelength plate 16 and reaches the mirror 6, where it is reflected and returns to the polarizing beam splitter 4 via the two-wavelength plate 16. Returning, the plane of vibration is still 90' as with light.
Since it is rotating, it is now reflected by the polarizing beam splitter 4 and reaches the polarizing plate 17. These two lights, which overlap when they enter the polarized plate, do not interfere because their polarization planes are perpendicular to each other, but the common components of both lights interfere with each other in the polarizing plate 17', which has a transmission axis in the 45'' direction with respect to one paper plane. Then, interference fringes are generated according to the difference in optical path length between the two lights.This interference light 8 is photoelectrically converted in a photodetector 9, and the output interference signal 10 is taken into a displacement measuring device 11 and first The change in the optical path length difference between the two split beams is measured.

Here, as explained at the beginning, the mirror 15 is glued to the polarizing beam splitter 4, and the other mirror 6 is glued to the polarizing beam splitter 4.
Since it is also connected to the polarizing beam splitter 4 by the support member 5, the relative positional relationship between them should not change physically, and therefore there is no physical change in the optical path length difference, so the displacement measuring device 11 There should be no change in output.

However, due to the following two factors, the interferometer 4 and mirror 6
The wavelength of the laser changes by β on the laser optical path between
Since the phase difference changes, the output of the displacement measuring device 11 also changes.

The first factor is that the oscillation wavelength changes as the resonator length changes due to thermal changes in the laser resonator 2.

Second, the wavelength of the laser beam emitted from the laser oscillator 1 is further subtly distributed and further changed in accordance with the refractive index distribution of the air on the optical path through which it propagates and its changes.

In the configuration of this embodiment, there is no problem on the common optical path of the two lights to be interfered because both lights are affected by these factors in the same way, but when the interferometer 4 on the non-common optical path
If the wavelength changes due to these factors on the optical path between the mirror 6 and the mirror 6 (such an optical path is called a dead path), the phase between the two lights will change, and the output of the displacement measuring device 11 will also change. This is because the displacement measurement result of the measurement target, which does not physically change, changes even though it should not change due to the following two error factors, and the result is as shown in this example. This shows that there is a measurement error in the laser displacement meter with this configuration.

These two factors have traditionally been a problem in laser displacement meters, and as mentioned in the conventional example, the wavelength was stabilized and corrected independently, but essentially the wavelength fluctuates. Since the same phenomenon occurs, in this embodiment, it occurs comprehensively due to these two factors.

The goal is to stabilize and standardize the wavelength of the laser on the measurement beam of the interference system by monitoring the final result of this phenomenon, that is, the interference fringes.

Actually, the laser oscillator 1 is oscillated by controlling the voltage applied to the heater 3 by the laser wavelength control device 12 so that the brightness of the interference light 8 does not change, that is, the output of the displacement measuring device 11 does not change. If the wavelength is controlled, a constant wave number is always maintained in the optical path between the interferometer 4 and the mirror 6, so the laser wavelength is calibrated and stabilized in the environment on this optical path.

Therefore, the laser beam is split into two by the beam splitter 18 and guided to the interferometer 20 via the mirror 19.

When an interference system is configured to measure the displacement X of the measurement object 21, if the dead paths of both interferometers are close and the refractive index distribution of the air on both optical paths is the same, the laser wavelength at this time will be the same as that of the air. By receiving the interference fringes from the interferometer 20 with the photodetector 22 and processing them with the displacement measuring device 23, it is stabilized against the refractive index change of the air in the dead path. Moreover, accurate displacement X can be measured.

In this embodiment, a method for stabilizing the wavelength of a gas laser has been described, but it is also possible to stabilize a semiconductor laser or the like if the wavelength control method is considered.

〔Effect of the invention〕

According to the present invention, environmental correction and stabilization of the laser wavelength can be performed at the same time, thereby simplifying the apparatus and improving usability.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing a prior art laser wavelength stabilization method. DESCRIPTION OF SYMBOLS 1... Laser oscillator, 4... Polarizing beam splitter, 5... Support member, 6... Mirror, 11... Displacement measurement device, 12... Laser wavelength control device. Figure 1 1-L-Joma Sobuta 4--I Separate Light Hi" 4 Sufu° Rizota 11-11-Sho 2-I 58 Nori Rent 1 Boat Story 412- Shizufusei, 1#Meki Fig. 2 1---L-length pa starting key device Z-L-length"j! -: Imaging Z5-A: i4 Tsubajime weapon

Claims (1)

[Claims] 1. In a laser oscillator that has means for controlling the oscillation wavelength by changing the temperature of the laser resonator, the injection current, etc., a beam splitting means is provided on the laser beam generated by the laser oscillator to separate the beam into a reference light and a signal light. constitute an interference system in which the beam splitting means and the mirror that reflects the signal light are held by a supporting member so that the optical path length of the signal light does not change; A means for detecting a change in the intensity of the interference light generated in the interference system is provided, and a signal is fed back to the oscillation wavelength control means to stabilize the laser wavelength so that the intensity of the interference light does not change. wavelength stabilized laser oscillator.