JP2003121423A - Laser ultrasonic inspection device and laser ultrasonic inspection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser ultrasonic inspection device and a laser ultrasonic inspection method capable of always performing stable and highly-sensitive ultrasonic measurement even when a disturbance such as vibration is applied thereto. SOLUTION: A Fabry-Perot interferometer 40 has two reflecting mirrors 41a, 41b constituting a resonator, and detects the change of a frequency of a second laser beam L2 generated from a cause of an ultrasonic echo by utilizing a resonance characteristic of the resonator. A piezo-element 42 adjusts the position of the reflecting mirror 41b on one side. A function generator 80 generates a voltage having a prescribed waveform, and transmits the voltage to an ultrasonic detection laser 20, to thereby change periodically an oscillation frequency of the second laser beam L2 . A conversion circuit 90 transmits a signal acquired by applying prescribed conversion to the voltage to the piezo- element 42, thereby always allows the frequency at a resonance curve operating point of the interferometer 40 to coincide with the oscillation frequency of the second laser beam L2 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser ultrasonic inspection apparatus and a laser ultrasonic inspection method capable of nondestructively detecting defects inside an inspection object.

[0002]

2. Description of the Related Art The following laser ultrasonic method is known as a method of detecting internal defects and the like of various materials in a non-destructive manner and in a non-contact manner with an inspection object. First, the surface of the inspection object is irradiated with a laser beam to excite ultrasonic waves on the surface or inside of the inspection object. When this ultrasonic wave hits a defect in the process of propagating through the inspection object, a reflected echo of the ultrasonic wave is generated there. On the other hand, the inspection object is irradiated with a laser beam for ultrasonic wave detection in addition to the laser beam for ultrasonic wave generation. When the reflection echo from the defect reaches this irradiation site, ultrasonic vibration is generated on the surface, so the laser beam for ultrasonic detection reflected at the irradiation site undergoes Doppler shift and its optical frequency changes. . This change in optical frequency is converted into a change in transmitted light intensity by, for example, a Fabry-Perot interferometer, and is made incident on the photodetector. As a result, the defect inside the inspection object can be detected as a change in the output signal of the photodetector.

[0003]

By the way, Fabry
The characteristics of the above conversion in the Perot interferometer largely change depending on the distance between the two reflection mirrors that form the resonator inside the Perot interferometer. Therefore, it is necessary to adjust the distance between the two reflecting mirrors with high accuracy. However, since the distance between the reflecting mirrors is easily affected by disturbance such as external vibration, when the disturbance occurs, the distance between the two reflecting mirrors changes, and stable ultrasonic measurement can be performed for a long time. There was a problem that it was difficult.

On the other hand, a laser beam having a single frequency is used as the ultrasonic detection laser beam. In fact, if the laser beam is generated for a long time, the oscillation frequency may fluctuate. . If the oscillation frequency of such a laser beam fluctuates, even if the conversion characteristics of the Fabry-Perot interferometer do not change, the change in transmitted light intensity output from the Fabry-Perot interferometer also fluctuates. , Stable ultrasonic measurement is difficult.

M. Ochiai, etc, Nondestructive Cha
racterization of Materials X, Green et al. (Eds),
2001 Elsevier Science Ltd., pp.305-310, when the position of the reflection mirror in the Fabry-Perot interferometer is displaced, the amount of the displacement is detected and the position of the reflection mirror is adjusted by the piezo element. That technology is disclosed. However, the device to which such a technique is applied is not hard to be affected by the disturbance and does not essentially solve the above problem.

The present invention has been made based on the above circumstances, and a laser ultrasonic inspection apparatus and a laser ultrasonic inspection apparatus which can always perform stable and highly sensitive ultrasonic measurement even when a disturbance such as vibration is applied. The purpose is to provide an inspection method.

[0007]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a first laser device for generating a first laser beam for generating an ultrasonic wave on an inspection object and an inside of the inspection object. A second laser device for generating a second laser beam for detecting an echo of the propagated ultrasonic wave, and two reflecting mirrors facing each other, the second laser beam reflected by the inspection object A Fabry-Perot interferometer that detects a change in the frequency of the second laser beam caused by the echo of the ultrasonic wave by performing multiple reflections between two reflecting mirrors, and the Fabry-Perot interferometer. In the laser ultrasonic inspection apparatus for detecting a defect inside the inspection object based on the detection result by, by adjusting the position of at least one of the reflecting mirrors based on a predetermined signal, Position adjusting means for changing the distance between the two reflecting mirrors and a control signal having a predetermined waveform are generated, and the oscillation frequency of the second laser beam is cycled by sending the control signal to the second laser device. Resonance of the Fabry-Perot interferometer by subjecting the control signal sent from the signal generating means to a predetermined conversion and sending the converted signal to the position adjusting means. And a conversion unit for always matching the optical frequency at the curve operating point with the oscillation frequency of the second laser beam.

In order to achieve the above object, the present invention irradiates an object to be inspected with a first laser beam to generate ultrasonic waves, while irradiating an object to be inspected with a second laser beam, A Fabry-Perot interferometer detects a change in the frequency of the second laser beam caused by the echo of the ultrasonic wave when the second laser beam is reflected by the inspection object, and inspects based on the detection result. In a laser ultrasonic inspection method for detecting a defect inside an object, a control signal having a predetermined waveform is sent to a laser device that generates the second laser beam to periodically change the oscillation frequency of the second laser beam. And the signal obtained by subjecting the control signal to a predetermined conversion is sent to the position adjusting means for adjusting the distance between the two reflecting mirrors of the Fabry-Perot interferometer. It makes is characterized in that the match at all times an optical frequency in the resonance curve operating point of the Fabry-Perot interferometer to the oscillation frequency of the second laser beam.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a laser ultrasonic inspection apparatus according to an embodiment of the present invention.

The laser ultrasonic inspection apparatus of this embodiment is for nondestructively detecting defects inside the inspection object 2. As shown in FIG. 1, an ultrasonic wave generation laser (first laser apparatus) is used. 10, a single-frequency oscillation laser (second laser device) 20 for ultrasonic wave detection, and three condenser lenses 31a,
31b and 31c and three reflection mirrors 32a and 32b,
32 c, one half mirror 33, Fabry-Perot interferometer 40, photodetector 50, amplifier 60, digital oscilloscope 70, function generator (signal generating means) 80, and conversion circuit 90.
Here, as the inspection object 2, for example, a thin plate-shaped steel material is assumed.

The ultrasonic wave generating laser 10 is used for the inspection object 2
It is a laser for exciting ultrasonic waves inside. As the ultrasonic wave generation laser 10, for example, a YAG laser or CO ₂
Use a high energy pulsed laser such as a laser. Hereinafter, the laser beam L1 emitted from the ultrasonic wave generating laser 10 is also referred to as a “first laser beam”.

The ultrasonic wave detecting laser 20 is a laser for detecting the ultrasonic waves generated in the inspection object 2 by the irradiation of the first laser beam L1 and propagating in the inspection object 2. As the ultrasonic detection laser 20, a laser that emits a laser beam having a single frequency is used. In this embodiment, a single frequency LD pumped YAG laser manufactured by Innolight is used as the ultrasonic detection laser 20. This laser is characterized in that the oscillation frequency of the laser beam can be periodically changed by an external voltage from a function generator or the like. In the following, the laser beam L2 emitted from the ultrasonic wave detecting laser 20 will also be referred to as a "second laser beam".

The first laser beam L1 emitted from the ultrasonic wave generating laser 10 is condensed by the condenser lens 31a, and then is irradiated onto the surface of the inspection object 2 through the reflection mirrors 32a and 32b. At this time, ultrasonic waves are generated at the irradiation point by thermal stress or evaporation reaction force. This ultrasonic wave propagates inside the inspection object 2, but if an internal defect exists in this propagation path, the ultrasonic wave is reflected / scattered even by this internal defect and returns to the surface as an echo. On the other hand, the second laser beam L2 emitted from the ultrasonic wave detecting laser 20 is condensed by the condenser lens 31b, passes through the half mirror 33, and is then irradiated onto the surface of the inspection object 2. Inspection object 2
Since the surface of is a rough surface, the second laser beam L2 is almost isotropically scattered on the surface of the inspection object 2. At this time, when the second laser beam L2 is applied to the returning portion of the ultrasonic echo, the second laser beam L2 scattered there is Doppler caused by the ultrasonic vibration of the surface of the inspection object 2. The optical frequency changes due to the shift.

For example, it is assumed that the wavelength λ of the second laser beam L2 is 532 nm and the oscillation frequency f thereof is 5 MHz. Also, the displacement r of the ultrasonic wave on the surface of the inspection object 2
Is given by r = r ₀ exp (iωt). Here, ω = 2πf and r ₀ = 10 nm. In this case, the Doppler shift amount Δf of the second laser beam L2 due to the ultrasonic waves is Δf = 2V / λ = 2r ₀ · 2πf / λ =, where V is the absolute value of the velocity of the ultrasonic wave on the surface of the inspection object 2. 2 (10 × 10 ⁻⁹ ) × 2π × 5 × 10 ⁶ / (532 × 10 ⁻⁹ ) = 1.2 (MHz).

A part of the second laser beam L2 scattered on the surface of the object to be inspected 2 is reflected by the half mirror 33, and then, is passed through the condenser lens 31c and the reflection mirror 32c, and the Fabry-Perot. It is incident on the interferometer 40.

The Fabry-Perot interferometer 40 detects a change in the frequency of the second laser beam L2 caused by the echo of the ultrasonic wave, and has two reflecting mirrors 41a and 41b facing each other and three reflecting mirrors 41a and 41b. It has piezo elements (position adjusting means) 42, 42, 42. The two reflection mirrors 41a and 41b form a resonator, and function as a bandpass filter by multiple reflection of the second laser beam L2 between the two reflection mirrors 41a and 41b. Here, by adjusting the distance between the two reflection mirrors 41a and 41b, the frequency of the light transmitted through this resonator can be adjusted.

Of the two reflecting mirrors 41a and 41b, one reflecting mirror 41a is fixed. The other reflection mirror 41b has three piezo elements 42, 4
2, 42 are provided. Such piezo elements 42, 4
Reference numerals 2 and 42 are for adjusting the position of the reflection mirror 41b. By adjusting the position of the reflection mirror 41b, the distance between the two reflection mirrors 41a and 41b can be changed. In the present embodiment, as will be described later, a signal obtained by subjecting the control signal sent from the function generator 80 to predetermined conversion is sent from the conversion circuit 90 to the piezo elements 42, 42, 42.

A piezo element is provided on each of the two reflecting mirrors 41a and 41b, and both reflecting mirrors 41a and 41b are provided.
By adjusting the position of b, the reflection mirrors 41a, 4a
You may make it adjust the distance between 1b.

Here, the resonance curve in the Fabry-Perot interferometer 40 will be described. FIG. 2 is a diagram showing an example of this resonance curve. In FIG. 2, the horizontal axis represents the optical frequency f of the incident light, and the vertical axis represents the output from the Fabry-Perot interferometer 40, that is, the intensity I of the light transmitted through the Fabry-Perot interferometer 40. As you can see from Figure 2,
The transmitted light intensity I shows a steep peak at a specific optical frequency, but immediately decreases before and after the peak. The optical frequency showing this peak can be changed by adjusting the distance between the reflection mirrors 41a and 41b of the Fabry-Perot interferometer 40. Therefore, the distance between the reflection mirrors 41a and 41b is adjusted so that the optical frequency at the point (resonance curve operating point) A where the slope of the curve shown in FIG. 2 becomes maximum matches exactly with the oscillation frequency of the second laser beam L2. If so, a slight change in optical frequency ± Δf can be converted into a relatively large change in transmitted light intensity ± ΔI. Accordingly, the Fabry-Perot interferometer 40 receives the Doppler shift caused by the ultrasonic vibration of the surface of the inspection object 2 and the second laser beam L2 whose optical frequency is changed.
When is input, the change in optical frequency is output as a change in transmitted light intensity.

The transmitted light intensity output from the Fabry-Perot interferometer 40 is sent to the photodetector 50. Photo detector 5
0 is for converting the transmitted light intensity into an electric signal. As a result, the ultrasonic echo is finally captured as an electrical signal. This signal is amplified by the amplifier 60 and then displayed on the digital oscilloscope 70, and the waveform is recorded if necessary.

When an ultrasonic echo is detected, the defect is detected based on the irradiation position of the second laser beam L2 from the ultrasonic detecting laser 20, the detection timing, and the sound velocity of the inspection object 2 which is known in advance. The position of can be calculated. That is, it becomes possible to detect the internal defect of the inspection object 2 in a non-destructive manner and to specify its position.

Further, the function generator 80
Generates a control signal (voltage) having a predetermined waveform such as a sine wave or a rectangular wave. The frequency and amplitude of the voltage can be arbitrarily set by the function generator 80. The voltage generated by the function generator 80 is sent to the ultrasonic detection laser 20 and the conversion circuit 90. In the present embodiment, as the ultrasonic wave detecting laser 20, a laser which can periodically control the oscillation frequency of the laser beam by an external voltage is used, and therefore the ultrasonic wave detecting laser 20 is a function generator 80.
The second laser beam L2 whose oscillation frequency changes periodically according to the voltage sent from

The conversion circuit 90 converts the voltage from the function generator 80 into a predetermined signal and sends the converted signal to the piezo elements 42, 42, 42. The piezo elements 42, 42, 42 receive the signal and adjust the position of the reflection mirror 41b to adjust the distance between the two reflection mirrors 41a, 41b.
Here, the contents of the conversion performed by the conversion circuit 90 are mainly
It is to amplify the gain of the voltage sent from the function generator 80.

By the way, the resonance curve of the Fabry-Perot interferometer 40 has reflection mirrors 41a and 41b inside the interferometer.
It varies greatly depending on the distance between them. A specific example will be described. The interval between two adjacent peaks in the resonance curve of the Fabry-Perot interferometer 40 (see FIG. 2) is called the free spectrum range (FSR).
= C / 2d. Here, c is the speed of light, and d is the distance between the two reflection mirrors 41a and 41b. For example,
When the distance d between the reflection mirrors 41a and 41b is 1000 mm, the FSR is c / 2d = 3 × 10 ⁸ / (2 × 1)
= 150 MHz. In addition, the half width Δf _1/2 of the peak of the resonance curve is given by the finesse of the interferometer 40 being F:
It is given by Δf _1/2 = FSR / F. Therefore, when the distance d between the reflection mirrors 41a and 41b changes, the FSR value and the peak half-width of the resonance curve also change.

In general, in the laser ultrasonic inspection apparatus, the distance between the two reflecting mirrors forming the resonator inside the Fabry-Perot interferometer is easily affected by external disturbance such as external vibration, and is stable for a long time. There is a problem that ultrasonic measurement is difficult. That is, conventionally, the distance between the reflection mirrors 41a and 41b is adjusted in advance so that the optical frequency at the resonance curve operating point A matches the oscillation frequency of the second laser beam. However, when the peak position in the resonance curve of the Fabry-Perot interferometer 40 is displaced due to external vibration or the like, the point on the resonance curve corresponding to the oscillation frequency of the second laser beam is no longer the resonance curve operating point A. , The resonance curve deviates to a point where the inclination is small or, in an extreme case, it deviates from the peak. With this, in the Fabry-Perot interferometer 40, a slight change in the optical frequency cannot be converted into a relatively large change in the transmitted light intensity, and the detection sensitivity of the ultrasonic echo decreases.

Therefore, in the present embodiment, the following measures are taken so that the ultrasonic measurement can always be stably performed even when a disturbance such as vibration is applied.

First, as the ultrasonic wave detecting laser 20,
As described above, the one that can control the oscillation frequency of the laser beam by the voltage is used. Then, the function generator 80 sends the output voltage to the ultrasonic wave detecting laser 20 to periodically change the oscillation frequency of the second laser beam emitted by the ultrasonic wave detecting laser 20. This point is different from the conventional laser ultrasonic inspection apparatus that uses the second laser beam having a single frequency that does not change with time. In the present embodiment, the oscillation frequency of the second laser beam from the ultrasonic wave detection laser 20 is constantly and consciously changed.

The function generator 80 also sends the output voltage to the conversion circuit 90 at the same time. The conversion circuit 90 sends a signal obtained by subjecting the output voltage to a predetermined conversion to the piezo elements 42, 42, 42. That is, the converted signal is transmitted to the piezo elements 42, 42, 42.
Then, the peak of the resonance curve is periodically changed by periodically changing the position of the reflection mirror 41b, that is, the distance between the reflection mirrors 41a and 41b. Here, the cycle of the signal that the conversion circuit 90 sends to the piezo elements 42, 42, 42 is the same as the cycle of the output voltage from the function generator 80. Also, the amplitude of such a signal is determined such that the amplitude of the change in the peak of the resonance curve is the same as the amplitude of the change in the oscillation frequency of the second laser beam. As a result, both the oscillation frequency of the second laser beam and the position of the reflection mirror 41b periodically change, but the optical frequency at the resonance curve operating point A of the Fabry-Perot interferometer 40 is always the oscillation frequency of the second laser beam. Can be matched.

As described above, in this embodiment, the oscillation frequency of the second laser beam and the oscillation frequency of the second laser beam are constantly controlled by controlling the ultrasonic wave detecting laser 20 and the piezoelectric elements 42, 42, 42 based on the voltage from the function generator 80. The distance between the reflection mirrors 41a and 41b is periodically changed, and the optical frequency at the resonance curve operating point A of the Fabry-Perot interferometer 40 is always matched with the oscillation frequency of the second laser beam. Such a reflection mirror 41
Of course, the distance between a and 41b is less likely to be affected by a disturbance such as an external vibration when the constant control is performed than when the control is not performed at all. In addition, the ultrasonic detection laser 2
There is a problem that the oscillation frequency of the second laser beam of 0 fluctuates, but the oscillation frequency of the second laser beam is always controlled as compared with the case where no control is performed. It is possible to suppress the occurrence of fluctuations. Therefore, the laser ultrasonic inspection apparatus of this embodiment can always perform stable and highly sensitive ultrasonic measurement.

Next, the contents of conversion applied to the control signal from the function generator 80 in the conversion circuit 90 will be described in detail. Generally, when the oscillation frequency of the second laser beam of the ultrasonic detection laser 20 is periodically changed based on the control signal (voltage) v (t) from the function generator 80, the oscillation frequency f1 is f.
It is represented by a certain function G1 of 1 = G1 (v). For example, in the case of the Innolight single frequency LD pumped YAG laser used in this embodiment, the function G1 can be approximated by a simple linear function. In reality, the voltage v has a periodic waveform suitable for continuous use for a long time.
(T) = v ₀ sin (ωt) may be given.

On the other hand, the conversion circuit 90 simultaneously converts the voltage v from the function generator 80 into a predetermined signal α (v) by a conversion function α. Then, the signal α (v) is applied to the piezo elements 42, 42, 42 for adjusting the position of the reflection mirror 41b. In that case, the optical frequency f2 at the resonance curve operating point A of the Fabry-Perot interferometer 40 is , F2 = G2 (α
It is represented by a certain function G2 called (v)). Therefore,
In order for the optical frequency f2 at the resonance curve operating point A of the Fabry-Perot interferometer 40 to match the oscillation frequency f1 of the second laser beam, G1 (v) = G2 (α
(V)). That is, the conversion circuit 90 responds to the voltage v from the function generator 80 by
α ^{(v) =} G2 - 1 is subjected to (G1 (v)) becomes converted,
The converted signal α (v) is converted into piezo elements 42, 42,
It may be applied to 42. Such a conversion circuit 90 can be actually manufactured.

A specific example will be given here. Ultrasonic wave detection
As a laser 20, a single frequency LD pump manufactured by Innolight
A YAG laser (wavelength: 1064 nm) is used. Figure 3
Shows the control characteristics of the oscillation frequency of this laser. The horizontal axis is
The voltage input to the laser, the vertical axis is the laser
2 shows the oscillation frequency of the second laser beam. Where f ₀
Is the oscillating frequency in the uncontrolled state, and is normally THz off.
The value of the vendor. As you can see from Figure 3, with this laser
Is a second laser at a rate of 4MHz per 1V of external voltage.
The oscillation frequency of the beam can be changed linearly
It Now, change the oscillation frequency of the second laser beam.
The amplitude of the movement is ± 0.5MHz, and the frequency of the fluctuation is 1Hz.
Output of the function generator 80
It is assumed that the voltage waveform is determined. At this time, the reflection mirror 4
The following control is performed for the position 1b. example
For example, when the piezo elements 42, 42, 42 are not controlled, reflection
Distance d between mirrors 41a and 41b₀Is 1m and is Fabry
・ Assume that the FSR of the Perot interferometer 40 is 150 MHz.
It The optical frequency at the operating point A of the resonance curve is
In order to always match the oscillation frequency of the
When the oscillation frequency of the beam changes to the maximum,
The FSR must change by the same amount as the change amount. Shi
Therefore, the maximum amplitude of FSR is 150.5 MHz.
And the distance between the reflecting mirrors 41a and 41b at that time is d
₀If + Δd, then (d₀+ Δd) / d₀= 150.5
From / 150, d₀+ Δd = 1000mm + 3.33m
m. That is, regarding the position of the reflection mirror 41b
The amplitude of the fluctuation is ± 3.33 mm, and the frequency of the fluctuation is
The control may be performed so that the number becomes 1 Hz. In other words
For example, the conversion circuit 90 has the position of such a reflection mirror 41b.
Signals for realizing the position control are provided to the piezo elements 42, 4
2 and 42.

In the laser ultrasonic inspection apparatus of this embodiment,
By constantly controlling the ultrasonic detection laser and the piezo element based on the voltage from the function generator, the oscillation frequency of the second laser beam and the distance between the reflecting mirrors are cyclically changed, and
The optical frequency at the operating point of the resonance curve of the Perot interferometer is always matched with the oscillation frequency of the second laser beam. Therefore, the distance between the reflection mirrors is less likely to be affected by disturbance such as external vibration, and fluctuations in the oscillation frequency of the second laser beam can be suppressed, so that the distance is always stable and high. It is possible to perform sensitive ultrasonic measurement.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the invention.

[0035]

As described above, the laser ultrasonic inspection apparatus according to the present invention constantly controls the ultrasonic detecting laser and the position adjusting means on the basis of the control signal from the signal generating means, so that the second laser beam And the distance between the reflecting mirrors are periodically changed, and the optical frequency at the operating point of the resonance curve of the Fabry-Perot interferometer is always matched with the oscillation frequency of the second laser beam. For this reason, the distance between the reflecting mirrors is less likely to be affected by disturbance such as external vibration, and fluctuations in the oscillation frequency of the second laser beam can be suppressed. When the inspection device is used, stable and highly sensitive ultrasonic measurement can always be performed.

Further, according to the laser ultrasonic inspection method of the present invention, the distance between the reflecting mirrors is less likely to be affected by disturbance such as external vibration, and the oscillation frequency of the second laser beam is the same as the above. Since it is possible to suppress the occurrence of fluctuations, it is possible to always perform stable and highly sensitive ultrasonic measurement.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a laser ultrasonic inspection apparatus according to an embodiment of the present invention.

[Fig. 2] Fabry in the laser ultrasonic inspection device
It is a figure which shows an example of the resonance curve of a Perot interferometer.

FIG. 3 is a diagram showing control characteristics of an oscillation frequency of a single-frequency oscillation laser for ultrasonic detection used in the laser ultrasonic inspection apparatus of this embodiment.

[Explanation of symbols]

2 Inspection object 10 Ultrasonic wave generation laser 20 Single frequency oscillation laser for ultrasonic detection 31a, 31b, 31c Condensing lens 32a, 32b, 32c Reflecting mirror 33 half mirror 40 Fabry-Perot interferometer 41a, 41b Reflecting mirror 42 Piezo element 50 photo detector 60 amp 70 Digital Oscilloscope 80 Function Generator 90 conversion circuit

Claims (2)

[Claims] 1. A first laser device for generating a first laser beam for generating ultrasonic waves on an inspection object, and a second laser beam for detecting echoes of the ultrasonic waves propagated inside the inspection object. Having a second laser device for generating, and two reflecting mirrors facing each other, by multiple reflection of the second laser beam reflected by the inspection object between the two reflecting mirrors, the ultrasonic wave And a Fabry-Perot interferometer for detecting a change in the frequency of the second laser beam caused by the echo of the laser, and a laser for detecting a defect inside the inspection object based on the detection result by the Fabry-Perot interferometer. In the ultrasonic inspection apparatus, position adjusting means for changing the distance between the two reflecting mirrors by adjusting the position of at least one of the reflecting mirrors based on a predetermined signal, A signal generating unit for generating a control signal having a constant waveform and periodically changing the oscillation frequency of the second laser beam by transmitting the control signal to the second laser device, and transmitting from the signal generating unit. The optical signal at the resonance curve operating point of the Fabry-Perot interferometer is changed to the oscillation frequency of the second laser beam by subjecting the generated control signal to predetermined conversion and sending the converted signal to the position adjusting means. A laser ultrasonic inspection device, comprising: 2. An object to be inspected is irradiated with a first laser beam to generate ultrasonic waves, while an object to be inspected is irradiated with a second laser beam, and the second laser beam is reflected by the object to be inspected. Laser ultrasonic inspection that detects a change in the frequency of the second laser beam that sometimes occurs due to the echo of the ultrasonic wave with a Fabry-Perot interferometer, and detects defects inside the inspection object based on the detection result. In the method, a control signal having a predetermined waveform is sent to a laser device that generates the second laser beam, thereby periodically changing the oscillation frequency of the second laser beam and converting the control signal into a predetermined signal. The Fabry-Perot interferometer is transmitted by transmitting the signal obtained by applying the signal to position adjusting means for adjusting the distance between the two reflecting mirrors of the Fabry-Perot interferometer. The laser ultrasonic inspection method for causing the optical frequency of the resonance curve operating point coincides always with the oscillation frequency of the second laser beam.