JPH09257755A - Laser ultrasonic inspection apparatus and laser ultrasonic inspection method - Google Patents

Abstract

(57) Abstract: A laser ultrasonic inspection apparatus and a laser ultrasonic inspection method capable of propagating ultrasonic waves in a specific direction with a simple device. SOLUTION: When a laser beam emitted from an optical fiber 13 is diffracted by a diffraction grating 16 and applied to a subject 17,
An ultrasonic wave is generated at the place where the laser beams diffracted on the surface of the subject 17 reinforce each other due to the interference of the multiple branched laser beams. The femtosecond laser is capable of emitting a very short laser beam with a pulse width of about 10 ^-12 to 10 ^-15 seconds, and it is known that it contains a large number of frequency components. Thus, the laser pulse containing a large number of frequency components can be expanded in pulse width by passing through an optical fiber having a dispersibility with respect to the frequency of light. When this is irradiated through a diffraction grating, the interference fringes move and the generated ultrasonic waves propagate in a predetermined direction.

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 for inspecting a defect in a subject by irradiating the subject with a laser beam to generate ultrasonic waves and detecting the ultrasonic waves propagated through the subject. The present invention relates to a device and a laser ultrasonic inspection method.

[0002]

2. Description of the Related Art One of the methods for detecting internal defects of various materials is a so-called laser ultrasonic method. Regarding this, for example, "Ultrasonic TECHNO
May issue (vol.5, No.5, p38 (1993) Nippon Kogyo Shuppan) ". Since this method detects an ultrasonic wave using a laser beam, it can be used for a non-contact inspection for examining an internal state of a material or the like that cannot be inspected by contact. Therefore, it is expected to be applied to quality inspections in iron making processes, welding inspections in steel frame processing, quality inspections of ceramic materials, internal inspections of aircraft parts, and quality inspections of other metals and composite materials.

A typical laser ultrasonic inspection apparatus is, for example, a laser light source (for example, a Q-switch YAG laser) for exciting ultrasonic waves inside a subject, and a probe for detecting ultrasonic waves propagating in the subject. Laser light source (for example, He-Ne laser). When a subject is irradiated with an appropriate excitation laser beam, ultrasonic waves are generated in the subject due to thermal stress or evaporation reaction force. When this ultrasonic wave propagates through the subject, if there is a defect inside, it is reflected or scattered there. The ultrasonic waves propagating through the subject can be detected by observing the phase change or Doppler shift of the probe laser beam.
Then, it is possible to estimate the presence or absence of the internal defect or the position where the defect exists from the time required for the ultrasonic wave to propagate from the generation position of the ultrasonic wave to the detection position, the change in the amplitude, and the like.

However, in the conventional laser ultrasonic devices, the ultrasonic waves generated at the position irradiated with the excitation laser beam propagate in the object while spreading in a spherical wave shape, so that the energy is dispersed and the ultrasonic waves are detected. The proportion of ultrasonic waves that reach to is very small. Further, as a result of propagating while spreading, for example, even if ultrasonic waves reflected or scattered by a defect are detected, it is difficult to specify the exact position of the defect.

Therefore, H. Nishino et al. Proposed a method of concentrating and propagating ultrasonic waves excited by a laser beam in a specific direction in a sample (H. Nishino, Y. Tsukaha.
ra, H. Cho, Y. Nagata, T. Koda and K. Yamanaka `` Generat
ion and Dierctivity control of bulk Acoustic waves
by phase velocity scanning of laser interference
fringes "Jpn.J.Appl.Phys., 34, 1995, p2874-2878).
In this method, as shown in FIG. 4, a laser beam from a single laser light source (Nd: YAG) is split into two beams, and the two beams are irradiated to the same point on the side of the sample (Si ₃ N ₄ ). This causes interference at the sample surface. At this time, the frequency of one of the branched laser beams is slightly changed (80 MHz in the above-mentioned paper) through the TeO ₂ Bragg cell to scan the interference fringes generated on the sample surface at high speed. On the other hand, a knife edge method is used to detect the ultrasonic waves propagated through the sample, and a beam from an argon laser (Argon ⁺ ) irradiated on the bottom surface of the sample is detected by an avalanche photodiode (APD). The detected signal is a bandpass filter (BP
The signal waveform is displayed through F) or directly to a digital oscilloscope.

According to the above-mentioned paper, the incident angle of two laser beams irradiating the sample is θ with the geometrical arrangement as shown in FIG. 5 (the surface on which the exciting laser beam is irradiated is shown horizontally). , The absolute value of the angular frequency difference | ω ₁ −ω ₂ |
_a and the wave number of the laser beam is k (= k ₁ ≈k ₂ ), the scanning speed v of the interference fringes (SIF) moving on the surface of the sample is given by v = ω _a / 2k sin θ (1). From the equation (1), the scanning speed v of the interference fringes is
It can be seen that it depends on the incident angle θ of the two laser beams. The scanning direction of the interference fringes is as follows when ω ₁ <ω ₂ .
The direction is from left to right. Also, ultrasound (BAW)
Is propagated by φ = sin ⁻¹ (c / v) (2) and depends on the scanning speed v of the interference fringes. Therefore, from the expressions (1) and (2), by changing the incident angle θ of the two laser beams, the propagation direction φ of the ultrasonic wave can be changed.
It turns out that it is possible to control. According to the above-mentioned paper, this principle enables non-contact and non-destructive defect detection at a minute level.

[0007]

By the way, the main purpose of the above-mentioned paper is to verify that ultrasonic waves can be propagated in a specific direction by scanning the interference fringes at a high speed. It does not mention specific guidelines for actually inspecting product defects at the manufacturing site. Therefore, it is necessary to solve various problems in order to realize an actual inspection device using the above principle. For example, the method described in the above paper requires two laser beams to excite the ultrasound,
There is a need for optical means to diverge the beam from a single laser source. Further, a means for accurately changing one frequency of the branched laser beam is required. Further, in order to control the propagation direction of the ultrasonic waves, it is necessary to change the incident angles of the two laser beams at the same angle at the same time, so it is expected that precise angle control will be difficult.

The present invention has been made under the above circumstances, and provides a laser ultrasonic inspection apparatus and a laser ultrasonic inspection method capable of propagating ultrasonic waves in a specific direction with a simple device. With the goal.

[0009]

A laser ultrasonic inspection apparatus according to claim 1 for solving the above problems is a laser light source for generating a pulsed laser beam containing a plurality of frequency components, and the pulsed light source. A pulse width expansion control unit made of a substance having a dispersive property for passing laser light, and a diffraction grating that diffracts the laser light that has passed through the pulse width expansion control unit and irradiates the surface of the subject with the diffracted light, And an ultrasonic wave detecting means for detecting ultrasonic vibrations generated in the subject by the irradiation of the diffracted light.

According to a second aspect of the invention, in the first aspect of the invention, the laser light source is a femtosecond laser. According to a third aspect of the present invention, in the first or second aspect of the invention, the pulse width expansion control means is an optical fiber. The invention according to claim 4 is the invention according to claim 1 or 2
In the invention described above, the pulse width expansion control means is a glass rod.

In the laser ultrasonic inspection method according to the present invention, the pulse laser light including a plurality of frequency components is passed through the pulse width expansion control means having a dispersive property so that the frequency is predetermined in time. A pulse laser beam that changes in a ratio, and the interference laser beam diffracted by making the pulse laser beam incident on a diffraction grating arranged on the front surface of the subject move the interference fringes formed on the surface of the subject. Selecting a propagation direction of an ultrasonic wave generated by irradiation of laser light by controlling a moving speed based on the rate of frequency change, and detecting the ultrasonic wave to inspect a defect in the subject. It is characterized by.

[0012]

According to the present invention, when a laser pulse containing a plurality of frequency components is passed through the pulse width expansion control means, the time required for transmitting high frequency light (short wavelength) is short and the frequency is high. The low pulse width (long wavelength) light takes a long time to transmit, so the pulse width of the emerging pulse laser is expanded, and the pulse laser frequency is lower than the high frequency within the pulse width time. Change to frequency. As a result, the rate of change of the laser pulse per unit time is not zero. When such a laser pulse is diffracted by a diffraction grating, interference fringes are formed on the surface of the subject in front of it, and moreover, the interference fringes change at a predetermined rate of change of the frequency of the laser pulse due to the frequency change. It moves on the surface of the subject at a corresponding speed. Due to the movement of the interference fringes, the ultrasonic waves generated in the portions where the lights are intensified propagate in a predetermined direction determined by the moving speed of the interference fringes and the speed of sound in the subject.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of an ultrasonic inspection apparatus according to this embodiment. In the figure,
The laser light source 10 is a laser for ultrasonic excitation, and a femtosecond laser is used here. Laser light source 1
The laser beam emitted from 0 passes through the mirrors 11 and 12 and enters the optical fiber 13 which is the pulse width expansion control means. The laser beam emitted from the optical fiber 13 is a beam diffusion lens 14 and a focusing lens 1.
After passing through 5, the light is diffracted by the diffraction grating 16 and the subject 17
Is irradiated. The diffraction grating 16 of the present embodiment has a large number of thin slits provided in parallel on transparent glass, and the laser beam applied to this slit is split into a large number of laser beams at each slit.
Diffracted by each slit. Due to the interference of the multiple branched laser beams, ultrasonic waves are generated where the laser beams diffracted on the surface of the subject 17 reinforce each other and propagate in the subject. Since the diffraction beams above the central axis of the diffraction grating are unnecessary, the surface of the subject on which these diffraction beams hit is covered by the mask 18.

On the other hand, the ultrasonic wave propagating in the subject is detected by the laser interferometer 20. Well-known laser interferometers such as Fabry-Perot interferometer,
A Twyman-Green interferometer, a Fizeau interferometer, a Mach-Zehnder interferometer, etc. can be used. The output signal of the laser interferometer 20 is sent to the phase detection circuit 21 and then to the oscilloscope 22 to display the signal waveform. Further, the signal is captured by the computer 24 via an appropriate communication line (eg GPIB) 23. Note that FIG. 1 shows a state in which the laser beam of the laser interferometer 20 is applied to the surface of the subject 17, but this is because the ultrasonic waves reflected or scattered by the defect inside the subject, or the subject's 17 This is an arrangement for detecting ultrasonic waves reflected on the back surface. As will be described later, the laser ultrasonic inspection apparatus of the present embodiment can propagate an ultrasonic wave in a specific direction. Therefore, when detecting an ultrasonic wave that directly reaches from the ultrasonic wave generation unit, the propagation direction thereof is The laser beam of the laser interferometer 20 is applied to the surface position of the subject corresponding to.

The femtosecond laser used in this embodiment can emit a laser beam having a very short pulse width of about 10 ^-12 to 10 ^-15 sec. Also,
The beam of a femtosecond laser is known to contain a large number of frequency components within a narrow frequency range. The laser pulse containing a large number of frequency components can be expanded in width by passing through an optical fiber. FIG. 2 is a diagram showing how the pulse width is expanded, where the horizontal axis represents time t and the vertical axis represents intensity I (t). Laser pulse before entering the optical fiber (Fig. 2 (a))
Contains a large number of frequency components, inside a substance that has a dispersive property with respect to the frequency of light, such as an optical fiber, light with a higher frequency (that is, a shorter wavelength) travels faster and reaches the exit end faster. To do. Therefore, the laser pulse emitted from the optical fiber (Fig. 2 (b))
Not only has its pulse width lengthened, but it also changes from a high frequency to a low frequency within the time of its pulse width. Moreover, the rate of pulse width expansion can be easily adjusted by changing the length of the optical fiber. Also,
This means that the rate of change of wavelength per unit time, which will be described later, can be easily adjusted by changing the length of the optical fiber.

Here, it will be described that an ultrasonic wave can be propagated in a specific direction by diffracting a laser pulse whose frequency changes with time in this way by a diffraction grating and irradiating the subject with this. FIG. 3 is an enlarged cross-sectional view of a part of the diffraction grating. When the grating constant of the diffraction grating 16 is p, the wavelength of light is λ, the diffraction direction of the laser beam is θ, and m is an integer, the interfering diffracted light has an angle θ satisfying the condition of p sin θ = mλ (3). Strengthen each other in the direction. If θ is a small angle (actually, this condition is sufficiently satisfied), then θ ≈ mλ / p. In FIG. 1, assuming that the distance between the diffraction grating 16 and the subject 17 is l ₀ and the x axis is as shown in the figure, x = l ₀ tan θ ≈ l ₀ θ (4).

Substituting the equation (3) for obtaining the position where the interference fringes are formed on the x-axis, x = ml ₀ λ / p (m = 0, ± 1, ± 2, ...) (5 ). That is, ultrasonic waves are generated on the surface of the subject 17 at positions on the subject that satisfy this condition. by the way,
From the equation (5), dx / dt = v = (ml ₀ / p) · dλ / dt (6) Therefore, if the wavelength λ (frequency) of the laser pulse is temporally changed, The interference fringes that are formed
It becomes possible to scan at the speed obtained by the equation (6). As described above, the magnitude of dλ / dt can be easily set to a desired value by changing the length of the optical fiber. Then, if the interference fringes can be scanned at the velocity v, the generated ultrasonic waves can be propagated in the direction of the angle φ, as described in relation to the above-mentioned formula (2).

Next, calculation is performed based on concrete numerical values.
Controlling the sound velocity c in the subject from 5850 m / sec (for iron) to 11750 m / sec (for Si ₃ N ₄ ) and controlling the ultrasonic propagation direction φ within the range of 30 ° to 60 ° And The scanning speed v of the interference fringes required to propagate the ultrasonic wave in a predetermined direction in the subject is (2)
From the equation, 6755 m / sec when the sound velocity in the subject is 5850 m / sec and the propagation direction angle φ is 60 °, and 23500 m when the sound velocity in the subject is 11750 m / sec and the propagation direction angle φ is 30 °. / Sec. If p = 50 μm and l ₀ = 1 m and the center interference fringes are considered (m = 1), then in equation (6), v = 2 × 10 ⁴ · dλ / dt (7) That is, dλ / dt = v ÷ 2 × 10 ⁴ (8).

Therefore, the scanning speed v of the interference fringes is set to 675.
To obtain 5 m / sec, dλ / dt = 3.38 ×
10 ^-1 m / sec, interference fringe scanning speed v is 23500 m
/ Sec, dλ / dt = 1.175 m /
The wavelength may be changed at a rate of sec. That is, when the subject is steel (iron) and the rate of change of the wavelength of the laser beam is 3.38 × 10 ⁻¹ m / sec, the ultrasonic wave propagation direction φ is 60 °, and the rate of change of the wavelength is Can be controlled so that the propagation direction becomes smaller. On the other hand, when the subject is Si ₃ N _{4 and} the rate of change of the wavelength of the laser beam is 1.175 m / sec, the ultrasonic wave propagation direction φ is 30 °, and the rate of change of the wavelength is smaller than this. By doing so, it is possible to control the propagation direction to increase.

Next, the specifications of the specific femtosecond laser and the optical fiber for expanding the pulse width necessary to realize the above-mentioned rate of change of wavelength will be described. The femtosecond laser has a property that Δt · Δν becomes approximately 1 when the pulse width is Δt and the difference between the maximum frequency component and the minimum frequency component included in this pulse is Δν. Δλ is the wavelength difference corresponding to Δν, and c is the speed of light (1
( ^{8 8} m / sec), Δν = c / Δλ (9), and therefore from this relationship and Δt · Δν≈1, Δt · c / Δλ≈1, Δλ≈c · Δt (10). . A femtosecond laser has a pulse width Δt of 10
^Since there are from ⁻¹² sec to 10 ⁻¹⁵ sec, the wavelength difference Δλ corresponding to these pulse widths is from 10 ⁻⁴ m to 10 ⁻⁷ m from the equation (10).

For example, the wavelength difference Δλ between the maximum wavelength and the minimum wavelength of the included wavelength components is 1 μm (Δt is 10 ⁻¹⁴ se)
Consider the case where the wavelength change rate dλ / dt is set to 0.336 m / sec using the femtosecond laser of c). From dλ / dt = 0.336, Δt = 10 ^-6 /0.336 = 2.98 × 10 ^-6 se
c. Therefore, the optical fiber 13 of FIG. 1 has a pulse width of 10 ⁻¹⁴ sec to about 2.98 μsec.
It can be seen that the one that can be spread is used. Using the same femtosecond laser, the rate of change in wavelength dλ
When / dt is 1.175 m / sec, Δt = 10 ⁻⁶ /1.175 = 0.85 × 10 ⁻⁶ se from dλ / dt = 1.175.
c. Therefore, for the optical fiber 13 in FIG. 1, the pulse width is 10 ⁻¹⁴ sec to about 0.85 μsec.
What can be spread is used.

Further, the wavelength difference Δλ is 100 μm (Δt is 1
When the wavelength change rate dλ / dt is set to 0.336 m / sec using a femtosecond laser of 0 ⁻¹² sec), Δt = 10 ⁻⁸ /0.336 is obtained from dλ / dt = 0.336. = 2.98 × 10 ⁻⁴ se
c. Therefore, as the optical fiber 13 in FIG. 1, a fiber that can widen the pulse width from 10 ⁻¹⁴ sec to about 298 μsec may be used. Using the same femtosecond laser, the wavelength change rate dλ / dt is 1.1.
In the case of 75 m / sec, Δt = 10 ⁻⁴ /1.175 = 0.85 × 10 ⁻⁴ se from dλ / dt = 1.175
c. Therefore, for the optical fiber 13 of FIG. 1, the pulse width is from 10 ⁻¹⁴ sec to about 85.1 μsec.
What can be spread is used.

As described above, it is possible to control the propagation of ultrasonic waves in only a single direction. The ultrasonic wave propagating only in a specific direction in this way has a high energy density, and therefore the sensitivity when used for defect detection is improved. It becomes easy to check the existence and identify the position. There is also an advantage that the propagation direction of the generated ultrasonic waves can be arbitrarily controlled by a simple operation of preparing a plurality of optical fibers having different lengths in advance and exchanging the optical fibers through which the laser pulse is passed, if necessary.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the invention. For example, in the above embodiment, the optical fiber is used as the pulse width expansion control means, but what can be used as the pulse width expansion control means is not limited to this, and may be any light transmissive material having light dispersibility, such as glass. You can also use a stick.

[0025]

As described above, according to the present invention, in order to realize the high-speed scanning of the interference fringes on the surface of the object required for propagating the ultrasonic waves in a specific direction,
It does not require two laser beams as in the past, but simply provides pulse width expansion control means for expanding the pulse width of the laser beam of the femtosecond laser, and a single laser beam obtained through this laser beam. Is passed through the diffraction grating and the diffracted light is irradiated onto the subject, which simplifies the device configuration and reduces the need for subtle adjustments, while using, for example, an optical fiber as pulse width expansion control means. If you do, by preparing optical fibers with different lengths,
A laser ultrasonic inspection apparatus and a laser ultrasonic inspection method capable of easily controlling the propagation direction of ultrasonic waves can be provided.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a state in which a pulse width is expanded and a frequency is temporally changed when a laser pulse containing a plurality of frequency components is passed through an optical fiber having a dispersibility.

FIG. 3 is an enlarged view in which a part of the diffraction grating is enlarged.

FIG. 4 is a schematic configuration diagram of a conventional laser ultrasonic inspection apparatus.

FIG. 5 is a diagram showing how interference fringes move when two laser beams having different frequencies are applied to one point.

[Explanation of symbols]

 10 Laser Light Source (Femtosecond Laser) 11, 12 Mirror 13 Optical Fiber 14 Beam Diffusing Lens 15 Focusing Lens 16 Diffraction Grating 17 Subject 18 Mask 20 Laser Interferometer 21 Phase Detection Circuit 22 Oscilloscope 23 Communication Line (GPIB)

Claims (5)

[Claims] 1. A laser light source for generating a pulsed laser beam containing a plurality of frequency components, a pulse width expansion control means made of a dispersible substance for passing the pulsed laser light, and a pulse width expansion control means. A diffraction grating that diffracts the laser light and irradiates the surface of the subject with the diffracted light; and an ultrasonic wave detection unit that detects ultrasonic vibration generated in the subject by the irradiation of the diffracted light. A laser ultrasonic inspection apparatus characterized in that 2. The laser ultrasonic inspection apparatus according to claim 1, wherein the laser light source is a femtosecond laser. 3. The laser ultrasonic inspection apparatus according to claim 1, wherein the pulse width expansion control means is an optical fiber. 4. The laser ultrasonic inspection apparatus according to claim 1, wherein the pulse width expansion control means is a glass rod. 5. A pulsed laser beam containing a plurality of frequency components is passed through a pulse width expansion control means having dispersibility to obtain a pulsed laser beam whose frequency changes at a predetermined rate with time. Is incident on a diffraction grating arranged on the front surface of the subject to move the interference fringes formed by the laser light diffracted on the surface of the subject, and the moving speed of the interference fringes is controlled based on the ratio of the frequency change. A laser ultrasonic inspection method characterized in that the defect in the subject is inspected by selecting the propagation direction of the ultrasonic wave generated by the irradiation of the laser beam and detecting the ultrasonic wave.