JPS6014298B2 - Ultrasonic transmission and reception method using laser light - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a non-contact ultrasonic reception method using laser beams at both the ultrasonic transmitting end and the ultrasonic receiving end.

A so-called ultraviolet oscilloscope is used to bring a vibrator such as a crystal into contact with a sample such as a steel material, apply high frequency and high voltage to the vibrator to excite ultrasonic waves, and detect defects inside the sample from the propagation status of the ultrasonic waves. Non-destructive testing methods using sound waves have been used in various fields.

However, since this method is a contact method, it is necessary to bring the ultrasonic transmitting end into close contact with the sample surface, or to fill the gap with water or other liquid medium to efficiently inject the ultrasonic waves into the sample. . For this reason, it is practically impossible to perform measurements by bringing the vibrator into contact with a sample such as a steel material that is in a high temperature state. For these reasons, the contact type ultrasonic excitation method is limited in its field of use and environment, so non-contact type ultrasonic excitation and reception technology is desired for the development of ultrasonic non-destructive testing methods. . mosquito)
In response to this demand, a non-contact method has been proposed in which ultrasonic waves are generated in metal using electromagnetic force (Lorentzka) and then received electromagnetically. It has the disadvantages that it must be placed fairly close to the sample surface, making it difficult to use in high-temperature environments, and that fluctuations in the distance between the sample surface and the device strongly affect the signal intensity. Moreover, the sample is limited to conductive materials such as metals. There are some difficulties. By the way, when a strong pulsed laser beam is irradiated onto a sample surface, the material in the very surface layer (about a few amps) of the sample instantly evaporates and scatters, and the reaction force (compressive stress) causes a strong pulsed laser beam to be applied to the sample surface. elastic waves are generated.

This can be seen, for example, in B. P. Fairand et al. 197
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According to this method, pulsed elastic waves, that is, ultrasonic waves (longitudinal waves), can be applied to any sample, not just metals, from a remote location.
can be excited. Naturally, this excited ultrasonic wave must be detected for the purpose of inspecting and measuring deep scratches, but if the detection method is also non-contact, the non-contact ultrasonic excitation will have no meaning.

The ultrasonic waves generated on one side of the sample by irradiation with pulsed laser light then propagate within the sample and reach the other side of the sample.
This causes reflected waves, etc., but at this time, the other surface of the sample protrudes, albeit slightly, and then becomes depressed, ie, mechanical displacement occurs.
If this mechanical displacement is detected by some non-contact method, remote reception of ultrasonic waves is possible. Micromechanical displacements can be measured with light. It is necessary to consider whether this displacement is below the wavelength of light, about the same level, or above the wavelength of light, and the optical interference method is effective in the following cases. That is, a coherent measurement laser beam is irradiated onto the sample surface, and a phase difference due to mechanical displacement of the sample surface is detected using an optical heterodyne or optical homodyne technique. Measuring micro-vibration displacement using the optical homodyne method is described in the section ``Measurement of micro-mechanical vibrations using a laser interferometer'' in ``Measurement Theory'', co-authored by Kenichi Iijima and Yasuo Tsuzuki, published by Kyoritsu Shuppan. However, the above-mentioned optical heterodyne or homodyne detection is sufficient for continuous signals, but for vibration displacement that only occurs instantaneously, it is necessary to separate and remove mechanical vibration noise and to measure displacement. There is a drawback that the S/N ratio is not sufficient due to the lack of photons. In other words, when realizing a non-contact ultrasonic transmission and reception method using laser light, it is not possible to expect good measurement results by simply using pulsed laser light at the transmitting end and measurement laser light at the receiving end. Can not. The present invention is
This method was created by taking into account various points related to the nuclear ultrasound system, and its distinctive feature is that a high-power pulsed laser beam is irradiated onto one side of the sample to generate pulsed ultrasound waves on the sample, and the nuclear ultrasound waves are used as a discussion fee. Mechanical displacement that occurs on the other surface of the sample when it propagates inside and reaches the other surface of the sample is detected by irradiating the other surface of the sample with a measuring laser beam, and from the time of irradiation of the pulsed laser beam. The purpose of this method is to predict the time when the ultrasonic waves will reach the discussion surface and temporarily increase the output of the measurement laser beam for a limited period of time before and after the expected arrival time.

In this way, there is less risk of picking up unnecessary noise, and the number of photons carrying minute displacement information can be made sufficiently large, making it possible to achieve non-contact ultrasonic emission and reception with a good S/N ratio. . This will be explained in detail below with reference to the illustrated embodiments. FIG. 1 shows an embodiment of the invention.

In the figure, reference numeral 1 denotes a pulse laser that instantaneously generates a large output by Q-switching, and irradiates one surface 2a of a sample 2, such as a steel material, with pulse laser beams L and o. Laser light sources capable of outputting pulsed laser through Q-switching include ruby lasers and glass lasers. The pulsed laser (also called di-iron laser) generated by these laser light sources has a high output of 10 MW, for example, and a short pulse width of 2 s. (with a protective film)
Instantly evaporates and scatters the layer. As mentioned above, an elastic wave is generated within the sample due to the action of this reaction force, which propagates at a longitudinal wave propagation speed v within the sample with thickness d, and after a time t = d/v has elapsed from the laser beam irradiation. It reaches the other surface 2b of the sample and causes a minute displacement (indicated by dots). A portion (micro portion) L, . The beam is incident on a photodetector 4 such as a photocell or a photomultiplier tube by the beam splitter 3, where it is detected (photoelectrically converted). The photodetector 4 gives information on the time when the sample surface 2a is irradiated with the laser light Lo.
This is delayed by a timer 5 for a certain period of time. Then, the measurement laser 6 is activated using the output S2 of the timer 5 as a trigger signal, and is temporarily oscillated or Q-switched. This trigger signal S2 is also used for timing the oscilloscope 11 or memory 12. As the measurement laser 6, a Q-switchable one such as a ruby or YAG laser (a laser with a lower output than the laser source 1 may be used) is used. The output laser beam L2o of the laser 6 is split into two by the beam splitter 7, and a portion -, reaches the mirror 8, is reflected, returns to the beam splitter 7, passes through this, and enters the photodetector 9. . This laser light-
becomes a reference signal light to be described later. On the other hand, the remainder of the laser beam Lo split by the beam splitter 7 is reflected from the other surface 2b of the sample 2, and the reflected laser beam returns to the beam splitter 7, where it is reflected by the photodetector 9. incident on . Photodetector 9 thus has two inputs L. .
signal S with an amplitude value corresponding to
Outputs 3. This signal S3 is amplified by an amplifier 10 and then displayed on an oscilloscope 11 and/or displayed in a memory 12.
is stored in In this case, if the angular frequency of the measurement laser beam L2o is the amplitude of the reference signal beam L2o, the amplitude of the signal beam 122 reflected by the sample surface 2b is As, and the displacement of the sample surface 2b is Z, then Reference signal light L2, is Arcos t,
Since the signal L2 has a phase difference with respect to this, it can be expressed as a false cos (t + pole false small t).Therefore, the intensity 1 of the output S3 of the photodetector 9, which has been subjected to two-way average detection, is expressed by the following equation. .

1=<{t+ofCOS(t+etc.2Sin〃t>}2>=<~2COS2t+AS2COS2(
t + Imo zSin■〃t) tCOS on the false side of the ten foxes (No t 10th grade zSin misaki t)> = <Kimejucho 2) Juura {MOS2's t + AS2COS2M 10 rods zSi scold)} 10 ~ Kasu side (2
t pole zSi sail) sailing ship side (sheep zSin no sot) ＞=KIME 10$2) 10ArASCOS (pole breast zt)
・……．・[11 In the above equation, ^ is the wavelength of the laser beam 12o, throat is the effective angular frequency when the half-width of the pulsed laser beam Lo is taken as There is a relationship.

、． ．．・・Cape Mizumemisaki on the side of the Doryo boat holds true, and is 0 for t>27 sleeves.

[In Equation 1', the first and second terms on the right side are DC components, and the third term is AC component. Assuming that this alternating current component is la, since la is information that includes displacement Z, vibration displacement Z can be received by appropriately extracting this. By the way, this la can be expanded into a series of Bessel functions as shown in the following equation. la = A side oS (¥S
in the throat t)t snobbery {J.

(¥) Jugen 2 Kuwarai Wanmono So Jugen 4 (rod) C.

End of S4...・・・．・}．・・・．・・・．・・【
3} {In formula 3, Z is smaller than ^, and in 4, Z
If ノ＾<1, then J.

(￥)Ko1J2(￥)22汀2(Hayabusa)2 J2n (rod) three.

(n>2).

Therefore, 1a3Ar ship {14 t2 shit)2 S throat t}.・・・．・・・． ...'4}, and the displacement Z can be expressed as the square root of the amplitude of the AC signal of the angular velocity.

This two-prong average detection is based on the optical homodyne method, but the difference from the conventional method is that the 3-inch width corresponds to the pulse width of the pulsed laser light LO. Time is the only thing that exists. In this way, the optical homodyne method makes it possible to detect the displacement Z and thus to receive ultrasonic waves in a non-contact manner, but the problem with this method is the intensity and duration of the signal lights L, .

As mentioned above, the duration of the pulse laser Lo is extremely short, and therefore the time during which the displacement Z occurs and the time during which it can be measured are also extremely short. It is necessary to provide sufficient signals, that is, photons, to the photodetector 9 in a short period of time to ensure reliable detection. Furthermore, the displacement Z to be detected is extremely small, and in high-temperature environments, fluctuations in the measurement light can also cause false signals, so it is important to provide some kind of filter function to prevent such things from being detected. . Therefore, in the present invention, measurement is performed for only a minute time, centered on the time when the pulsed ultrasonic waves generated in the sample 2 by the pulsed lasers L and O reach the sample surface 2b and cause mechanical displacement on the surface. Do it like this. The timer 5 is provided for this purpose, and its delay time is set according to the thickness d of the sample 2 and the ultrasonic propagation speed. That is, as mentioned above, the laser beam Lo
When irradiated onto the sample surface 2a at time t=0, the ultrasonic pulse (elastic wave) generated on the surface propagates within the sample at a speed v and reaches the sample surface 2b at time t=t, causing a microscopic wave on the surface. Since this causes a change, the delay time of the timer 5 is set to a minute width of 4 times centered on d/v. To give a specific example, pulse laser 1
If the sample surface 2a is irradiated with pulsed laser beams L and o with a pulse width of 20cm and an output of 10cm at time t:0, the output of the photodetector 4 will be applied almost simultaneously (even with a delay of 0).
．． (about 1 nsec) signal S appears. Sample 2 with this)
Assuming that the thickness d is 3 mm, the propagation speed of ultrasonic waves (longitudinal waves) is about 6 ribs/1 ms, so mechanical displacement occurs on the sample surface 2b after about 5 sec. The measurement laser beam 2o captures this displacement, and for this purpose, it is preferable to set the delay time of the timer 5 to 4 peaks S and the duration of the measurement laser 6 to 2 peaks S. Note that the time required to Q-switch the laser 6 to generate a pulsed laser is approximately several tens of seconds, and this time can also be ignored. The output (continuous output) of the measurement laser light source 6 may be increased to provide sufficient photons to the photodetector 101;
It is effective to output a pulsed laser by Q-switching the laser beam, and if this Q-switching is performed at the above-mentioned timing, there is also the advantage that unnecessary noise is not picked up.

In this way, according to the present invention, detection sensitivity can be sufficiently increased to an absolute level, and noise caused by mechanical vibrations occurring in the measurement sample and noise caused by light fluctuations that occur particularly when measuring in a hot environment, that is, a high temperature environment, etc. can be suppressed. can be effectively removed. FIG. 2 shows a signal processing circuit that uses the output S3 of the photodetector 9 in various ways, and the output B of the peak hold circuit 21 is an analog value indicating the peak (displacement) of the signal S3.

The signal S3 amplified by the preamplifier 10 is also branched to an integrator 22 and a Schmitt circuit 23, and the output of the Schmitt circuit 23 starts a pulse generator 24, and a gate controller 25 generates a start pulse Ps'R and a stop pulse PsTP. let PsTR and PsTP join the integrator 22 via /Agate 8 and define its operating time. Therefore, for example, a portion of the signal S3 above a certain level is integrated, and area information A thereof is output from the integrator 22. The calculator 26 uses the area information A and the peak value B to calculate the half width of the signal S3. The sample and hold circuit 27 samples and holds the output of the calculator 26 at a timing when the pulse PsTP is delayed by a predetermined period of time in the delay circuit 28 . Therefore, the output C of the hold circuit 27 becomes pulse width information of the signal S3. In the embodiment, a case has been described in which the ultrasonic waves excited on one surface 2a of the sample 2 first reach the other surface 2b and only the mechanical change caused on that surface is measured. By displaying S and S at the same time, it is possible to accurately measure the propagation velocity of the ultrasonic wave from the time interval between the two.

Accurate propagation velocity information of this ultrasonic wave provides various information regarding the properties of the sample and the like. Also, the other side of the sample 2b
The ultrasonic waves that have reached the surface are reflected there, return to the irradiation surface 2a, and are reflected there again, resulting in multiple reflections, and the other surface 2b of the sample is displaced each time these reflected waves (echoes) arrive. By detecting all the displacements, it is possible to find crystal grain fleas inside the sample from the attenuation constant. In this case, the timer 5 is configured to repeatedly generate the signal S2 at timings near t=t, circle, 5t, . . . . As described above, according to the present invention, the ultrasonic transmitting end and the receiving end are each made of laser light, which enables non-contact transmission and reception of ultrasonic waves, and the laser light at the receiving end is pulsed to provide high output power. In addition, since the time is limited, the S/N can be improved (expected to be improved by more than 3 orders of magnitude).
The practicality of measurements, etc. can be further improved. Compared to non-contact methods that use electromagnetic force, this non-contact method of transmitting and receiving ultrasonic waves using laser light allows the transmitter and receiver to be installed far enough away from the sample, making it suitable for high-temperature samples. It has the advantage that it can be applied without any problem even if the sample is non-metallic.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing an example of a processing circuit for a signal obtained at a receiving end. In the figure, 1 is a pulse laser, 2 is a sample, 3 and 7 are beam splitters, 4 and 9 are photodetectors, 5 is a timer,
6 is a measurement laser, 8 is a mirror, and 11 is an oscilloscope. Figure 1 Figure 2

Claims (1)

[Claims] 1. One surface of a sample is irradiated with a high-power pulsed laser beam to generate pulsed ultrasonic waves in the sample, and the ultrasonic waves propagate inside the sample and reach the other surface of the sample. The mechanical displacement occurring on the other surface of the sample is detected by irradiating the other surface of the sample with a measurement laser beam, and the time from the irradiation time of the pulsed laser beam to the time when the ultrasonic wave reaches the sample surface. A method for transmitting and receiving ultrasonic waves using a laser beam, the method comprising temporarily increasing the output of the measurement laser beam for a limited period of time before and after the predicted arrival time. 2 Mechanical displacement of the other surface of the sample due to ultrasonic waves is detected by homodyne detection of the reflected light of the measurement laser beam irradiated onto the other surface of the sample and the reflected light of the measurement laser beam from the reference signal mirror. The first claim characterized in that
Method for transmitting and receiving ultrasonic waves using laser light as described in Section 1. 3. Ultrasonic waves generated by laser light according to claim 1 or 2, characterized in that a series of mechanical displacements occurring on the other surface of the sample is also detected by echoes of the ultrasonic waves generated inside the sample. The Law of Issuance and Invasion.