JPH0760122B2 - Photoacoustic beam position detection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to adjustment of a beam position of an optical device in an optical system using a laser, and particularly to a photoacoustic generated in an optical element such as a mirror or a lens. The present invention relates to a photoacoustic beam position detection method for detecting a beam position by utilizing the beam position detection.

(Prior Art) In various laser systems, optical elements such as a mirror, a lens, and a polarizing plate are provided in an optical path for guiding a laser beam to a desired work position. In such an optical system, in order to effectively function the optical device, it is necessary to adjust the device according to the purpose, and the work of detecting the position of the beam accompanying it is essential. As a general beam position detection method, in addition to a simple method of printing a beam pattern on a thermal paper or acrylic plate for visualization, a linear array or a four-quadrant detector is inserted in the beam optical path for quantitative measurement. Is used.

On the other hand, it is known that when a sample in a closed container is irradiated with intermittent light, elastic waves are generated due to the photoacoustic effect.
Various measuring instruments using photoacoustic have been developed. For example, a method of measuring laser damage due to photoacoustic effect has been reported because the amount of minute absorbed energy of an optical thin film is proportional to the intensity of a photoacoustic signal for the purpose of improving the yield strength of a high-power laser mirror ( Ueda, Hagiwara, etc. 2
Name: Laser Research, Volume 15, Issue 1, p22-p25, Jan., (1
987)). In addition, the energy released by deformation and destruction of the solid propagates as an acoustic pulse (ultrasonic wave) and is captured to detect the defective portion of the measured material.
Ooustic Emission) measuring devices are commercially available.

(Problems to be Solved by the Invention) In order to obtain Raman laser oscillation in the infrared long wavelength region, the present inventor has developed a multi-beam Raman converter composed of a pair of facing total reflection mirrors made of copper. Proposed to No. 102985. In the Raman converter, it is necessary to obtain a desired Raman conversion medium length, and it is necessary to accurately guide the beam to a predetermined position on the mirror surface and secure the number of beam reflections. Therefore, highly accurate beam position detection and its adjustment have been important tasks.

In such beam irradiation alignment work, if a beam position detector is inserted in the optical path, the beam is cut off one by one, so not only a lot of adjustment time is required, but also
In the case of invisible light such as a CO ₂ laser, it was dangerous. In addition, in the indirect method in which the beam is split by a half mirror to detect the position, there is a problem that the beam intensity is attenuated, which is an obstacle to simplifying the device and realizing automatic control of the laser system. It was

Therefore, the present inventor has paid attention to the fact that when a mirror is irradiated with a pulse laser, an elastic wave due to a photoacoustic effect can be generated inside the mirror even if the mirror is a total reflection mirror, and the propagation time thereof is determined by the photoacoustic detection element. It was found that the position of the beam irradiated onto the mirror surface can be detected without obstructing the beam propagation by observing with.

It is an object of the present invention to provide a beam incident position detection method using photoacoustic that is effective for controlling the laser itself and the laser beam propagation system.

(Means for Solving the Problem) The above problem is to detect the propagation time from the signal waveform of the elastic wave accompanying the photoacoustic effect generated by the pulsed laser beam applied to the optical element such as the mirror and the lens, It can be solved by the present invention which detects the beam position in the plane of the optical element.

Specifically, by the method of detecting the position of the pulse laser beam from the propagation time of any one of the first, second and third peaks of the detected trigger signal output immediately before generation of the elastic wave and the detected elastic wave signal. Can be resolved. Further, a photoacoustic detecting means is arranged around the outer circumference of an optical element such as a mirror, a prism or a polarizing plate, and the generated elastic wave is subjected to signal processing by using a digital waveform analyzer, a time interval counter or the like to determine the position of the beam. Can be detected.

(Operation) For example, when the mirror surface is irradiated with pulsed laser light,
First, a part of light energy is absorbed on the surface and converted into heat energy, and the mirror surface is expanded and distorted to generate elastic waves. The generated elastic waves are longitudinal waves (P waves) and transverse waves (S waves).
Wave), and further propagates in the mirror as a surface wave. If the rise time of the laser pulse is sufficiently shorter than the acoustic pulse width, the propagation time of the P wave having the maximum traveling speed is proportional to its distance. Therefore, it is possible to detect the propagation time of the generated P wave by a sensor or a plurality of sensors attached to the back surface of the mirror, and to detect the position of the beam on the mirror surface from the propagation velocity of the P wave unique to the material.

(Effect of the Invention) According to the present invention, a detector for detecting elastic waves is attached to the side surface or the back surface of an optical element such as a mirror that is already provided without inserting another detector in the beam optical path, The position of the irradiation beam can be detected without interrupting the beam optical path and without attenuating the beam output.
Therefore, the drive device provided in each optical element in the optical system can be operated and automatically controlled so as to irradiate each optical element with a desired beam position accuracy.

(Example) Hereinafter, the present invention will be described in detail based on examples.

FIG. 1 is a block diagram of the entire apparatus for carrying out the present invention. The laser beam emitted from the CO ₂ laser oscillator 10 passes through the KCl semi-transparent mirror 11, passes through the diaphragm 12 that can shape the beam pattern and change the beam diameter,
A copper total reflection mirror 13 having a diameter of 70 mm and a thickness of 10 mm for carrying out the present invention is reached. On the back side of the mirror 13, two acoustic emission measurement sensors (PZ
T Piezoelectric sensor, resonance frequency 180KHz) 14, 14 'are fixed with insulating adhesive. The photon drag detector 17 detects the generation of a laser pulse in the beam branched by the KCl semi-transparent mirror 11, and becomes a trigger signal for the digital oscilloscope. The first sensor 14 and the second sensor 14 'are fixed to the rear surface of the mirror, and the diameter of the mirror is approximately
An elastic wave propagating in the mirror material is detected by irradiating a 15 mm pulsed beam. The detected signal is amplified by an AE amplifier (frequency characteristic 50 KHz to 2 MHz) 15, 15 'and then introduced into a digital oscilloscope 16 displaying an elastic wave. A digital oscilloscope set with a display of 8 bits and an impedance of 50Ω uses laser light as a trigger to eliminate fluctuations in elastic wave propagation time due to jitter of the laser oscillator (fluctuations in the laser pulse generation time relative to the discharge trigger pulse). There is.

FIG. 2 is a front view (A) and a side view (B) of the mirror stand to which the mirror 13 of FIG. 1 is attached. A slide stage 20 is provided on the mirror stand, and the rotation of the knob 21 moves the mirror on the X axis so that the position of the incident beam can be moved on the mirror surface.

A method for detecting the position of the irradiation beam one-dimensionally using one sensor will be described below.

FIG. 3 shows the position of the irradiation beam in FIG.
It is a waveform of an elastic wave signal obtained by fixing only the sensor 14 and operating the sensor 14. By pulsed laser irradiation, almost stable waveforms are seen up to the first, second, third, and fourth peaks in the initial stage of detection, but with the passage of time, the waveforms are the outer peripheral surface of the mirror. It can be seen that the elastic waves reflected by are interfering and disturbed. Here, the stage 20 of FIG. 2 (A) is moved on the X axis to change the position of the irradiation beam and the distance between the sensor 14 by 5 mm to 45 mm, and the measurement results of the arrival time of each peak are shown in FIG. Shown in. From the first peak to the third peak, the arrival time is detected almost stably even if the position of the beam and the position of the sensor are changed, but it greatly fluctuates at the fourth peak.
From this, the position of the irradiation beam is obtained by using the first peak, the second peak, or the third peak in the early stage of pulse laser irradiation, and the elapsed time ΔT with the trigger signal output immediately before laser irradiation. be able to. That is, if the elastic wave propagation velocity of the mirror material is constant, the trigger signal and the arrival time ΔT to each peak position are
Therefore, the distance between the sensor and the beam can be uniquely obtained. Therefore, two-dimensional beam positions can be detected by using two or more, preferably three or more sensors.

Next, a method for operating the two sensors 14 and 14 'shown in FIG. 1 to detect the position of the irradiation beam will be described. In FIG. 2 (A), an elastic waveform diagram detected at the position b where the position of the irradiation beam is shifted to the sensor 14 side is shown in FIG. In the figure, since the two sensors have different distances with respect to the beam position, the waveforms are greatly deviated, and there is a difference in arrival time at each peak position. However, the waveforms of the first peak and the second peak are almost constant. Here, the respective second peaks of the detection signal are taken to obtain the propagation times ΔT ₁ and ΔT ₂ from the trigger signal, which are multiplied by the propagation velocity of the mirror material to obtain the distances L ₁ and L ₂ to the beam position.
Is required. Next, the beam position is obtained from the intersections of the arcs drawn by the radii L ₁ and L ₂ from the positions of the first and second sensors.

However, if the sensors are arranged symmetrically,
As shown in the figure, when the beam position c is deviated on the Y axis, _two intersections of arcs having radii L ₁ and L ₂ are obtained, and the beam position cannot be determined. Therefore, by arranging the installation conditions of the two sensors within 180 °, one intersection point drawn with radii L ₁ and L ₂ can be obtained inside the sensor, and the beam position can be detected. Furthermore, it is obvious that the two-dimensional position can be detected with high accuracy by disposing three or more sensors.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an entire apparatus for implementing the present invention, FIG. 2 shows configurations of a mirror and a mirror stand according to the present invention, FIG. 1 (A) is a front view, and FIG. ) Is a side view, FIG. 3 is an elastic waveform diagram detected by one sensor at the beam position a in FIG. 2 (A), and FIG. 4 is first to fourth elastic waveform diagrams in FIG. A propagation characteristic diagram detected by moving the beam position about the peak, FIG. 5 is an elastic waveform diagram detected by two sensors at the beam position b in FIG. 2 (A), and FIG. It is explanatory drawing of the position detection method by a sensor. (Explanation of symbols) 10 …… Laser oscillation diagram, 11 …… KCl semi-transparent mirror, 12 ……
Aperture, 13 ... Mirror, 14, 14 '... Photoacoustic detection sensor, 15, 15' ... AE amplifier, 16 ... Digital oscilloscope, 17 ... Photon drag detector, 20 ... Stage, 21 ... Knob .

Claims (1)

[Claims] 1. At least one photoacoustic detecting means is arranged in an optical element, a pulsed laser beam for detecting an irradiation position is irradiated to the optical element to generate an elastic wave, and the pulsed laser beam. Based on the irradiation time of
The irradiation position of the laser beam is detected by obtaining the elapsed time until the detection time of any one of the first, second or third peaks of the elastic wave detected by the photoacoustic detection means. Photoacoustic beam position detection method.