JP2010071886A - Laser ultrasonic detector, and laser ultrasonic detection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the detection performance in a laser ultrasonic detector. <P>SOLUTION: This laser ultrasonic detector includes: a laser light source 11 for generating ultrasonic wave; a laser light source 12 for detecting ultrasonic wave; a reflected light from an inspecting object; a Fabry-Perot interferometer 13; a detector for detecting an output from the Fabry-Perot interferometer of the reflected light; a resonance length control section; and a laser driving control section 50 for periodically varying the resonance length and driving a pulse laser light source for generating ultrasonic wave synchronously with the periodic variation. The driving control section for laser drives the pulse laser light source for generating ultrasonic wave synchronously with predetermined timings of a rising slope and falling slope of a part of a laser beam for generating ultrasonic wave having transmitted through the Fabry-Perot interferometer, and takes difference between the amount of transmitted light corresponding to the rising slope and the amount of transmitted light corresponding to the falling slope detected by the detector. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a laser ultrasonic detection apparatus and method for detecting ultrasonic waves generated by irradiating a test object with a laser.

Conventionally known is a laser ultrasonic method for generating ultrasonic waves on a material to be inspected in a non-contact manner using a laser and detecting the generated ultrasonic waves, and a detection unit using a Fabry-Perot interferometer is also well known. .
The laser ultrasonic method irradiates a material to be inspected with pulsed laser light, transmits ultrasonic waves using the generated thermal stress or vaporization reaction force, and irradiates the receiving point with another laser light that oscillates continuously. In this technique, the displacement induced by the ultrasonic wave is received using the straightness and coherence.
The laser ultrasonic method is capable of non-contact detection of material cracks and internal defects or evaluation of material properties using ultrasonic waves, and is expected to be applied in various material evaluation fields. Yes.

The principle of the laser ultrasonic method will be described with reference to FIG.
When a laser for ultrasonic wave generation, which is a high-energy pulse laser, is irradiated on the surface of a steel material that is an inspection material, distortion is generated due to thermal expansion and contraction that occurs on the metal surface due to the impact. The generated distortion propagates inside the steel as ultrasonic waves. Next, when the metal surface is irradiated with a continuous laser beam having a single frequency for ultrasonic detection, the reflected light undergoes a frequency change (Doppler shift) corresponding to the vibration of the surface due to the propagated ultrasonic wave. Equation 1 showing the Doppler shift amount (Δf) is shown below.
Δf = 2V / λ (Formula 1)
Where V = surface displacement speed, λ = laser wavelength

  A measuring instrument using the laser ultrasonic method includes a Fabry-Perot interferometer. The Fabry-Perot interferometer operates as a filter that resonates and transmits only a specific frequency. For example, when there is a defective portion inside the steel material, the surface vibration is different from that of a normal steel material, so the Doppler shift amount shows a value different from that of the normal steel material. Therefore, the transmitted light material that passes through the Fabry-Perot interferometer is changed, so that the inspection material can be inspected for cracks and defects, or can be evaluated.

  In the laser ultrasonic method, the state of the inspection object can be observed without bringing anything into contact with the inspection object as long as the laser beam reaches. By using an optical fiber or a mirror, the laser beam can be subjected to ultrasonic inspection relatively easily even for a material to be inspected that is difficult to contact or approach.

  Conventionally, in a laser ultrasonic method, a mirror on one side of a Fabry-Perot resonator is moved in the optical axis direction according to a sinusoidal signal so that laser ultrasonic inspection can be performed in an environment with vibration and temperature change, and a sine wave There has been proposed a method of detecting the Doppler shift amount with the maximum sensitivity by outputting an ultrasonic wave generation laser in synchronization with the period (Patent Document 1 below).

JP 2001-255306 A

  However, in the proposed method, the mirror on one side of the Fabry-Perot resonator is dynamically controlled, so that stable measurement performance can be maintained even in an environment with vibration, but the sensitivity of ultrasonic measurement cannot be improved. .

  In view of the above-described problems, the present invention provides a laser ultrasonic detection apparatus and a laser ultrasonic detection method that maintain stable measurement performance even in an environment with vibration and improve the sensitivity of ultrasonic measurement. Objective.

  The apparatus or method of the present invention is as described in the following (1) to (8).

(1) an ultrasonic generation laser light source for generating ultrasonic waves by irradiating an object to be inspected with pulsed laser light for ultrasonic generation;
An ultrasonic detection laser light source for detecting the ultrasonic wave by irradiating an object to be inspected with an ultrasonic detection laser beam;
Reflected light from the inspection object of the ultrasonic detection laser light, and a Fabry-Perot interferometer to which a part of the ultrasonic laser light is input,
A detector for detecting the output of the reflected light from the Fabry-Perot interferometer;
A resonance length controller that periodically changes the resonance length of the Fabry-Perot interferometer;
In a laser ultrasonic detection apparatus comprising: a laser drive control unit that periodically changes a resonance length of the Fabry-Perot interferometer and drives an ultrasonic generation pulse laser light source in synchronization with the periodic change;
The laser drive control unit drives the ultrasonic wave generation pulse laser light source in synchronization with a predetermined timing of a rising slope and a falling slope of a part of the ultrasonic wave generation laser light transmitted through the Fabry-Perot interferometer. ,
A laser ultrasonic detection apparatus characterized by taking a difference between a transmitted light amount corresponding to a rising slope and a transmitted light amount corresponding to a falling slope, which is detected by the detector.

  (2) The laser ultrasonic detection device according to (1), wherein the laser drive control unit detects the rise and fall timing at a timing when the ultrasonic detection laser light exceeds a predetermined threshold.

  (3) The laser ultrasonic detection device according to (1) or (2), wherein the periodic change is a sine wave.

(4) an ultrasonic generation laser light source that generates ultrasonic waves by irradiating an object to be inspected with pulsed laser light for ultrasonic generation;
An ultrasonic detection laser light source for detecting the ultrasonic wave by irradiating an object to be inspected with an ultrasonic detection laser beam;
Reflected light from the inspection object of the ultrasonic detection laser light, and a Fabry-Perot interferometer to which a part of the ultrasonic laser light is input,
A detector for detecting the output of the reflected light from the Fabry-Perot interferometer;
A resonance length controller that periodically changes the resonance length of the Fabry-Perot interferometer;
Laser ultrasonic wave using a laser ultrasonic detection device comprising a laser drive control unit that periodically changes a resonance length of the Fabry-Perot interferometer and drives an ultrasonic wave generation pulse laser light source in synchronization with the periodic change. In the sound wave detection method,
Driving an ultrasonic wave generation pulse laser light source in synchronization with a predetermined timing of a rising slope and a falling slope of a part of the ultrasonic light generation laser light transmitted through the Fabry-Perot interferometer,
A laser ultrasonic detection method characterized by taking a difference between a transmitted light amount corresponding to a rising slope and a transmitted light amount corresponding to a falling slope, which is detected by the detector.

  (5) The laser ultrasonic detection method according to (4), wherein the rising and falling timings are detected at a timing when the ultrasonic detection laser beam exceeds a predetermined threshold.

  (6) The laser ultrasonic detection method according to (4) or (5), wherein the periodic change is a sine wave.

  According to the above apparatus or method, the transmitted light quantity detection performance is improved, so that stable measurement performance can be maintained even in an environment with vibration, and more accurate inspection of cracks and defects of inspection materials or material evaluation can be performed. It can be carried out.

Before describing embodiments of the present invention, an outline of the present invention will be described in order to facilitate understanding of the present invention.
FIGS. 1A and 1B are diagrams for explaining a detection method using the Fabry-Perot interferometer (FP interferometer) of the present invention. As shown in (1), a driving unit such as a piezo controller is driven by a sine wave signal of, for example, about 10 Hz generated by a function generator, and the position of the single-sided mirror of the FP interferometer is vibrated by a sine wave. Then, laser light having a frequency f0 is input to the FP interferometer, and the amount of light transmitted from the FP interferometer is detected by an avalanche photodiode (APD) that is a photodetector.

  (2) shows the relationship between the temporal movement (a) of the position of the single-sided mirror of the FP interferometer and the output of the laser beam. As can be seen from (2), a large output is obtained at a specific position. Therefore, when the pulsed laser beam for generating ultrasonic waves is irradiated in synchronization with a point where the change rate of the transmitted light amount is maximum, for example, T0, and the frequency of the detection laser light is f0, the transmitted light amount of the reflected FP interferometer is detected. The deviation from the frequency f0 can be detected by being greatly amplified.

  FIG. 2 is a diagram for explaining the principle of the present invention. The present invention was made by paying attention to the fact that the point at which the rate of change in the amount of transmitted light is maximum appears on both the rising and falling slopes of the output of the FP interferometer, and the respective outputs are phase-inverted. The signal-to-noise ratio of the signal is improved by irradiating the pulse laser beam for generating ultrasonic waves in synchronization with the point where the change rate of each transmitted light amount becomes maximum and taking the difference between the respective detection signals.

  SS1 and SS2 in FIG. 2 are Doppler shifts of reflected light due to vibration of the material to be inspected. The Doppler shift SS1 is amplified by the FP interferometer and appears as a change L1a in the amount of transmitted light. Further, the Doppler shift SS2 of the reflected light is amplified by the FP interferometer and appears as a change L1b in the transmitted light amount. Here, when attention is paid to the fact that the time axes of the graphs L1a and L1b of the transmitted light amount changes are opposite to each other, an output signal having a large SN ratio can be obtained by taking the difference between L1a and L1b.

  FIG. 3 is a diagram for explaining the outline of the control unit of the laser ultrasonic detection apparatus of the present invention. The reference light output from the FP interferometer 13 is converted into an electrical signal by the photodetector 20 and input to the control unit 50. On the other hand, the control unit 50 inputs the trigger timing detection unit 55, compares it with a preset threshold value V0, and detects a point exceeding the threshold value as a trigger timing (see T0 to T3 in FIG. 1). A signal output from the trigger timing detection unit 55 is input to the trigger signal generation unit 56. The trigger signal generation unit 56 generates a trigger signal for driving a later-described ultrasonic wave generation laser driving unit based on the output signal from the trigger timing detection unit 55. As a result, the ultrasonic wave generation laser driving unit emits a pulse laser at time points T0, T1, T2, T3,... Shown in FIG.

  On the other hand, the electrical signal output from the photodetector 20 is input to the slope determination unit 54 of the control unit 50. Here, rising, that is, + slope, or falling, that is, -slope is determined, and a slope determination signal is output and input to the signal processing unit 60 described later. The slope determination signal is used when taking a difference between outputs of the FP interferometer.

  As described above, the laser ultrasonic detection device 1 improves the detection sensitivity by alternately subtracting the detection signals using the fact that two kinds of timing conditions at which the sensitivity of the FP interferometer is maximized are alternated. In addition, the SN of the detection signal can be improved.

FIG. 4 is an overall view of the laser ultrasonic detection device 1 of the present embodiment.
The laser light source 11 for generating ultrasonic waves irradiates the inspection object 5 with a high-power pulse laser EL via the mirrors 31a and 31b in order to generate ultrasonic waves on the inspection object 5. The pulse laser EL irradiates a predetermined position on the surface of the inspection object 5 with a predetermined beam shape. As the laser light source 11 for generating ultrasonic waves, for example, a pulse YAG laser (wavelength 1064 nm) having an output of 100 mJ / pulse or more, a pulse CO2 laser, or the like can be used.

  The material to be inspected 5 is suddenly thermally expanded by the pulsed laser light EL and then contracted, whereby distortion occurs, and ultrasonic waves propagate to the material to be inspected 5. The surface of the material to be inspected vibrates due to the propagating ultrasonic waves.

From the ultrasonic detection laser light source 12, the continuous wave laser beam DL passes through the polarization beam splitter 44, is converged by the lens 42, and is irradiated on the inspection object 5 through the mirrors 41 a and 41 b. For example, a continuous wave Ar laser having an output of 5 W or more can be used as the laser light source 12 for ultrasonic detection.
The continuous wave laser beam DL is irradiated to the measurement point on the inspection object 5. The frequency of the reflected light RL of the laser light DL from the material to be inspected 5 undergoes a Doppler shift due to surface vibration. The reflected light RL is converged by the lens 43 via the mirrors 41c and 41d, passes through the polarization beam splitter 45, and enters the Fabry-Perot interferometer 13.

  On the other hand, the reference light TL, which is a part of the continuous wave laser beam DL emitted from the ultrasonic detection laser light source 12, is reflected by the polarization beam splitter 44 and further reflected by the polarization beam splitter 45, and is transmitted to the FP interferometer 13. Incident.

  The FP interferometer 13 includes two mirrors 13a and 13b facing each other, and transmits the incident reference light TL and reflected light RL by reciprocating between the mirrors 13a and 13b to increase the amount of light. The reference light TL and the reflected light RL are beams separated by the polarization beam splitter and are transmitted through the FP interferometer 13 without interference.

  The reference light TL that has passed through the FP interferometer 13 is reflected by the polarization beam splitter 16 and is incident on the photodetector 20 formed of an avalanche photodiode. In the photodetector 20, the transmission intensity of the reference light TL is converted into an electric signal S 2, and the electric signal S 2 is input to the control unit 50.

  On the other hand, the reflected light RL that has passed through the FP interferometer 13 passes through the polarization beam splitter 16 and is incident on the photodetector 21 formed of an avalanche photodiode. In the photodetector 21, the transmission intensity of the reflected light RL is converted into an electric signal S 1, and the electric signal S 1 is input to the signal processing unit 60.

  The control unit 50 controls the driving unit 17 to move one mirror of the FP interferometer 13, for example, the mirror 13b in the optical axis direction, thereby changing the resonance length. Further, the oscillation control unit 18 is controlled to determine the laser irradiation timing of the ultrasonic wave generation laser light source 11.

  In the signal processing unit 60, information provided by the reflected light from the member 5 to be inspected is processed to detect internal defects, cracks, and the like of the member to be inspected, and further, material characteristics are evaluated.

FIG. 5 is a diagram illustrating a control unit and a signal processing unit of the laser ultrasonic detection apparatus 1.
The electrical signal S2 input to the control unit 50 is input to the trigger timing extraction unit 55. Since the electrical signal S2 corresponds to the transmission intensity of the reference light TL from the FP interferometer, it is a signal corresponding to the output of the APD 20 in FIG. Therefore, if the point in time when the electric signal S2 exceeds the predetermined threshold voltage TH is taken out, the point in time gives the irradiation point of the ultrasonic light generation laser light source. The threshold voltage TH can be set by the operator by the threshold voltage setting unit 57.

  The output of the trigger timing extraction unit 55 is input to the trigger signal generation unit 56, and the trigger signal generation unit 56 generates a trigger signal that triggers the ultrasonic wave generation laser light source 11 based on the output of the trigger timing extraction unit 55. Is done. The generated trigger signal is input to the oscillation control unit 18 after the trigger timing is adjusted via the delay generation circuit 59. The oscillation control unit 18 drives the ultrasonic wave generation laser light source 11 based on the input trigger signal.

  The electric signal S2 is also input to the slope determination unit 54. The slope determination unit 54 detects a + slope that is the rising edge of the electrical signal S2 and a −slope that is the falling edge of the electrical signal S2 by tracking the change of the control signal S2. The slope determination signal output from the slope determination unit 54 is input to the computer 61 of the signal processing unit 60, and is used as a code for differential processing of the signal S1 input to the computer 61 of the signal processing unit by the computer 61 of the signal processing unit. Used.

FIG. 6 is a diagram illustrating a processing flow in the control unit and the signal processing unit of the present invention.
First, the slope determination unit 54 detects + slope and −slope of an electrical signal corresponding to the reference light output from the photodetector 20 (step S101). Detection of + slope and −slope can be determined by checking whether the value of the electric signal is increasing or decreasing. The slope determination unit 54 transmits a slope determination signal for distinguishing + slope or −slope to the signal processing unit 60.

The signal processing unit 60 receives an electrical signal corresponding to the reflected light. The signal processing unit 60 acquires the waveform L1b from the electrical signal using the −slope determination signal (step S102), and further acquires the waveform L1a from the electrical signal using the − + slope determination signal (step S103). . The signal processing unit 60 subtracts the waveform L1a from the waveform L1b (step S104). The subtracted waveform is twice the normal transmitted light amount. In this way, the detection sensitivity can be increased.
The computer 61 can output the waveform L1a, L1b, the difference processing result of the waveforms L1a, L1b, and the like to the display device 19.

FIG. 1 is a diagram for explaining a detection method using an FP interferometer as a premise of the present invention. FIG. 2 is a diagram for explaining the principle of the present invention. FIG. 3 is a diagram for explaining the outline of the control unit of the present invention. FIG. 4 is a diagram illustrating a processing flow in the control unit and the signal processing unit of the present invention. FIG. 5 is an overall view of the laser ultrasonic detection apparatus 1 of the present embodiment. FIG. 6 is a diagram for explaining a control unit of the laser ultrasonic detection apparatus 1. FIG. 7 is a diagram for explaining the principle of the laser ultrasonic method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser ultrasonic detection apparatus 5 Inspected material 11 Laser light source for ultrasonic generation 12 Laser light source for ultrasonic detection 13 Fabry-Perot interferometer 17 Drive part 18 Oscillation control part 19 Display apparatus 44, 45 Polarization beam splitter 50 Control part 51 Function Generator 54 Slope determination unit 55 Trigger timing extraction unit 56 Trigger signal generation unit 57 Threshold voltage setting unit 59 Delay generation circuit 60 Signal processing unit

Claims (8)

An ultrasonic generation laser light source that generates ultrasonic waves by irradiating a pulse laser beam for generating ultrasonic waves to a pair to be inspected;
An ultrasonic detection laser light source for detecting the ultrasonic wave by irradiating a laser beam for ultrasonic detection to a pair to be inspected;
A Fabry-Perot interferometer through which a part of the ultrasonic laser beam passes by irradiating the ultrasonic detection laser beam;
A resonance length controller that periodically changes the resonance length of the Fabry-Perot interferometer;
An ultrasonic generation pulse is generated at a predetermined timing of each of the rising and falling slopes of the ultrasonic detection laser beam that has passed through the Fabry-Perot interferometer by periodically changing the resonance length of the Fabry-Perot interferometer. A laser drive controller for driving the laser light source;
A signal processing unit that takes a difference in the amount of reflected light of the ultrasonic detection laser light detected when the ultrasonic generation pulse laser light is irradiated;
A laser ultrasonic detection apparatus comprising:   2. The laser ultrasonic detection device according to claim 1, wherein the laser drive control unit detects the rising and falling timings at a timing at which the ultrasonic detection laser light exceeds a predetermined threshold.   3. The laser ultrasonic wave according to claim 1, wherein the difference is obtained by subtracting the intensity of the ultrasonic detection laser beam detected at the fall from the intensity of the ultrasonic detection laser beam detected at the rise. Sound wave detection device.   The laser ultrasonic detection device according to claim 1, wherein the periodic change is a sine wave. Ultrasound generation laser light source for generating ultrasonic waves by irradiating the pair to be inspected with ultrasonic pulsed laser light, and ultrasonic waves for detecting the ultrasonic waves by irradiating the pair to be inspected with laser light for ultrasonic detection. A laser ultrasonic detection method using a laser ultrasonic detection device comprising: a detection laser light source; and a Fabry-Perot interferometer that irradiates the ultrasonic detection laser light and passes a part of the ultrasonic laser light. And
An ultrasonic generation pulse is generated at a predetermined timing of each of the rising and falling slopes of the ultrasonic detection laser beam that has passed through the Fabry-Perot interferometer by periodically changing the resonance length of the Fabry-Perot interferometer. Drive the laser light source,
A laser ultrasonic detection method including a step of calculating a difference in the amount of reflected light of the ultrasonic detection laser light detected when the ultrasonic generation pulse laser light is irradiated.   6. The laser ultrasonic detection method according to claim 5, wherein the rising and falling timings are detected at a timing when the ultrasonic detection laser beam exceeds a predetermined threshold.   The laser ultrasonic wave according to claim 5 or 6, wherein the difference is obtained by subtracting the intensity of the ultrasonic detection laser beam detected at the fall from the intensity of the ultrasonic detection laser beam detected at the rise. Sound wave detection method.   The laser ultrasonic detection method according to claim 5, wherein the periodic change is a sine wave.

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