JP2923779B1 - Optical interference device for ultrasonic detection - Google Patents

Abstract

An optical interference method for ultrasonic detection that detects ultrasonic vibration of an object to be measured using light is provided. An optical interferometer is constituted by a biconvex lens (3) inserted between a pair of opposing semi-transmissive concave mirrors (5), and light directly reflected by the semi-transmissive concave mirror (5) and output light from the optical interferometer (5). Are made to interfere with each other in opposite phases to produce interference light having an intensity close to 0, and the interference light is measured for the phase difference between the interference light intensity signal and the external reference signal to detect a change in the optical frequency due to ultrasonic vibration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-contact non-destructive ultrasonic inspection, and more particularly to an apparatus for detecting vibrations caused by ultrasonic waves as a change in the frequency of light using a high-intensity light source.

[0002]

2. Description of the Related Art In a conventional optical interferometer for detecting an ultrasonic wave, as shown in FIG. 5, two opposing semi-transparent concave mirrors 5 and 5 are provided with a radius of curvature F0 (focal length is F0 / 2). Confocal Fabry-Perot optical interferometers facing each other have been used.

In the measurement by the confocal Fabry-Perot optical interferometer, the light from a light source 1
And the reflected light is guided to the interferometer through the lens 3 and the beam splitter 4, and the reflected light from the interferometer is detected by the photodetector 7.
And the change of the optical frequency due to the Doppler shift effect of the ultrasonic wave is measured. The reflected light from the interferometer has three components as shown in FIG. First, the light Eref directly reflected by the input side semi-transmissive mirror, and secondly, the light Eref 4 inside the optical interferometer.
Light E reflected and output n + 3 times (n ≧ 0)
First and third, light E2 reflected 4n + 1 times inside the interferometer.

In general, the diameter of an input light beam is large, and it is difficult to spatially separate these three components. Therefore, all components are detected by a photodetector. By using the confocal mirror, even if the scattered light is reflected from the rough surface, the light E1 automatically overlaps with the light Eref. It is.

However, the light E2 generated at the same time is
f and E1 have no spatial coherence and do not contribute to interference. The interference light intensity signal obtained by the photodetector for the mirror interval L or the optical frequency ν is a periodic signal shown in FIG. At this time, due to the existence of the E2 component that does not contribute to the optical interference, the minimum value of the interference light intensity does not become zero but remains half of the maximum intensity value.

The frequency of the light reflected from the object to be measured is ΔΔ due to the Doppler effect accompanying the ultrasonic vibration of the object to be measured.
It is modulated by f. This frequency change, as shown in FIG. 7, results in a change in the amplitude of the interference light intensity signal, and the ultrasonic vibration can be measured by extracting the AC component of the signal.

In order to detect the change in the optical frequency due to the ultrasonic wave with the maximum detection sensitivity, the interference light intensity signal is amplified by the signal amplifier 8 in FIG. 5 and the control circuit 9 is driven. To maintain the strength, the distance L between the mirrors of the interferometer is
Is controlled by

The detection of ultrasonic waves by a confocal Fabry-Perot optical interferometer is hardly affected by the properties of the surface to be measured.
Also, the interference light intensity signal is not affected by low-frequency mechanical disturbance vibration. Furthermore, since the signal-to-noise ratio is proportional to the square root of the input light intensity to the optical interferometer, using a light source with high brightness leads to an improvement in sensitivity. (Broadban
d optical detection of ul
trasound by optical sideb
and stripping with a conf
ocal Fabry-Perot: Appl. Ph
ys. Lett. vol. 55 p. 1612
(1989))

[0009]

However, in the above-mentioned conventional optical interference device, if the intensity of the input light is increased in order to improve the sensitivity of signal detection, the amount of light input to the photodetector inevitably increases. Become. Therefore, in a region where the photodetector is saturated and loses linearity, sensitivity to a small change in the intensity of the interference light is deteriorated, and the linear region of the photodetector limits the intensity of a light source that can be used. There were challenges. Further, since the interference light intensity also changes due to the intensity change of the light source, there is a problem that the change component of the optical frequency due to the ultrasonic vibration and the change component of the intensity of the light source are mixed and output in the detection signal.

[0010]

In order to solve the above-mentioned problems, the present invention provides an optical interferometer comprising a pair of opposed semi-transmissive concave mirrors and a biconvex lens inserted between the two semi-transparent concave mirrors. High-brightness light phase-modulated by an external signal is input to the optical interferometer, and the light directly reflected by the semi-transmissive concave mirror on the input side and the light output from the interferometer after multiple reflection inside the optical interferometer By interfering in the opposite phase, the light intensity is detected by the photodetector interference light near zero, by measuring the phase difference between this interference light intensity signal and the external signal,
Provided is an optical interference device for ultrasonic detection that outputs a change component of an optical frequency due to a Doppler shift effect of an ultrasonic wave without using a high-intensity light to saturate a photodetector.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing a first embodiment of the present invention, in which a light source 1, a device under test 2, a biconvex lens 3, a beam splitter 4, a pair of semi-transmissive concave mirrors 5, a piezoelectric element 6, a photodetector 7, signal amplifier 8, control circuit 9, oscillator 10, optical phase modulator 11, phase shifter 12,
It comprises a mixer 13 and a biconvex lens 14.

The light source 1 uses a continuous wave laser having high coherence. The light from the light source 1 is modulated by the optical phase modulator 11 at a frequency fm higher than the ultrasonic frequency, and the reflected light from the object to be measured is guided to the optical interferometer. The distance between the pair of semi-transmissive concave mirrors 5 is 2
F0 is set, and a biconvex lens 14 having a focal length of F0 / 2 is placed at an intermediate point of the mirror. Thereby, the focal points of the pair of semi-transmissive concave mirrors 5 and the biconvex lens 14 overlap. FIG. 2 shows the relationship between the optical interferometer and the multiple reflected light at this time.

Since the light Eref directly reflected by the mirror surface constituting the semi-transmissive concave mirror 5 and the light E3 reflected inside the interferometer interfere with each other, the feature of causing the scattered light to interfere is maintained. On the other hand, since there is no E2 component generated by the conventional confocal Fabry-Perot interferometer, the minimum value of the interference light intensity is almost zero. The interference light intensity signal obtained by the photodetector 7 is f
The phase of this signal and the modulation signal from the oscillator 10 are detected by the mixer 13 to produce a DC signal corresponding to the phase difference between the two.

Here, the phase shifter 12 is adjusted so that the conversion efficiency into a DC signal is maximized. The interference light intensity signal by the photodetector becomes a periodic signal shown in FIG. 3A with respect to the change of the interval L or the optical frequency ν, and the phase difference signal becomes that shown in FIG. 3B.

The piezoelectric element 6 is driven by the phase difference signal via the control circuit 9 to control the distance between the mirrors, and the point B in FIG. 3B is set as the operating point. Thus, the operating point is fixed to the dark fringe where the intensity of the interference light is almost zero.
Due to the Doppler shift due to the ultrasonic vibration, the amplitude of the phase difference signal changes as shown in FIG. Also,
Since the change in light intensity of the light source does not affect the phase difference signal,
Only the change component of the optical frequency can be accurately detected.

In a second embodiment of the present invention, the first
The continuous laser light of the light source 1 shown in the embodiment
The light is pulse-amplified by 6 and a large output pulse light is irradiated to the device under test 2. The pulse reflected light from the object to be measured is input to the optical interferometer as in the first embodiment. At the same time, the continuous light before amplification is also input to the optical interferometer by the beam sampler 15 and the mirror 17 in order to fix the optical interferometer to the dark fringe. Then, the sensitivity of the measurement is improved by using the high output light. Further, since the dark fringe is the operating point, an increase in the pulsed light amount of the input light does not affect the detection signal.

According to the present invention, since the detection is performed with the dark fringes that cancel each other out due to the interference of light, the light detector does not saturate even when a high-intensity light source is used, and furthermore, the intensity of the light source does not change. It acts as an optical interferometer that detects only changes in optical frequency without being affected.

As described above, the present invention has been described based on the embodiments described in the drawings. However, the present invention is not limited to the above-described embodiments, but may be implemented in any manner unless the configuration described in the claims is changed. can do.

[0019]

According to the present invention, it is possible to improve the signal sensitivity using a high-intensity light source by a Fabry-Perot optical interferometer, which has been impossible in the past, and to obtain an ultra-sensitive device which is hardly affected by variations in the intensity of the light source. Detection of a sound wave vibration signal can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an apparatus according to a first embodiment of the present invention.

FIG. 2 is a conceptual diagram showing light components that interfere with the optical interferometer according to the present invention.

FIG. 3 is a conceptual diagram illustrating the operation principle of the optical interferometer according to the present invention.

FIG. 4 is a block diagram illustrating a configuration of an apparatus according to a second embodiment of the present invention.

FIG. 5 is a block diagram showing a configuration of a conventional Fabry-Perot optical interferometer.

FIG. 6 is a conceptual diagram illustrating the operation principle of a conventional Fabry-Perot optical interferometer.

FIG. 7 is a conceptual diagram showing light components that interfere with a conventional Fabry-Perot optical interferometer.

[Explanation of symbols]

 Reference Signs List 1 light source 2 device under test 3, 14 biconvex lens 4 beam splitter 5 semi-transmissive concave mirror 6 piezoelectric element 7 photodetector 8 signal amplifier 9 control circuit 10 oscillator 11 optical phase modulator 12 phase shifter 13 mixer 15 beam sampler 16 optical amplifier 17 mirror

Claims (1)

(57) [Claims] A pair of opposing semi-transparent concave mirrors;
An optical interferometer is formed by a biconvex lens inserted between the two semi-transmissive concave mirrors, high-intensity light phase-modulated by an external signal is input to the optical interferometer, and light directly reflected by the input-side semi-transmissive concave mirror, The light output from the interferometer due to multiple reflection inside the optical interferometer is caused to interfere in antiphase, and the interference light having a light intensity close to zero is detected by the photodetector. Measuring the phase difference of the light, and outputting a change component of the optical frequency due to the Doppler shift effect of the ultrasonic wave without using the high-intensity light to saturate the photodetector. Interference device.