JPS6243497B2 - - 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 vibrator such as a crystal is brought into contact with a sample such as steel,
A so-called non-destructive inspection method using ultrasonic waves has been used in various fields in which a high frequency and high voltage is applied to the vibrator to excite ultrasonic waves in the sample, and defects etc. inside the sample are detected from the propagation status. ing. 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 ultrasonic excitation method is limited in its field of use and environment, and therefore, non-contact ultrasonic excitation and reception techniques are desired for the development of ultrasonic non-destructive testing methods.
In response to such demands, a non-contact method has been proposed in which ultrasonic waves are generated in metal using electromagnetic force (Lorentz force) and then received electromagnetically. It has disadvantages such as the fact that it has to be brought very 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, there is a drawback that the sample is limited to conductive materials such as metals.

By the way, when a strong pulsed laser beam is irradiated onto a sample surface, the material in the surface layer (about a few angstroms) 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 is true, for example, as BPFairand et al.
Literature published in 2015 (Quantitative assessment of
laser induced stress waves generated at
confined surfaces, Applied Physics Letters,
Vol. 25, No. 8, 15 October 1974, P431-433), according to this method, pulsed elastic waves, that is, ultrasonic waves (longitudinal waves) are applied to any sample, not just metals, from a remote location. ) can be excited.

Of course, this excited ultrasonic wave must be detected for flaw detection and other inspection and measurement purposes.
Of course, the detection method must also be non-contact, otherwise 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. In this case, whether the displacement is less than or equal to the wavelength of light,
It is necessary to consider the above separately, and the optical interference method is effective in the following cases. That is, a coherent measuring laser beam is irradiated onto the sample surface, and a phase difference due to mechanical displacement of the sample surface is detected using optical heterodyne or optical homodyne techniques. Measuring minute vibration displacements using the optical homodyne method is described in the section ``Measurement of minute mechanical vibrations using a laser interferometer'' in ``Measurement Theory'', co-authored by Kenichi Iijima and Yasuo Tsuzuki, published by Kyoritsu Shuppan. However, the optical heterodyne or homodyne detection described above is sufficient for continuous signals, but for vibrational displacements that occur only instantaneously, it is necessary to separate and remove mechanical vibration noise and to detect optical quanta involved in displacement measurement. There is a drawback that the S/N is not sufficient due to the lack of . 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 was made in consideration of these points, and its characteristic feature is that one surface of a sample is irradiated with a high-power pulsed laser beam to generate pulsed ultrasonic waves on the sample. Mechanical displacement that occurs on the other surface of the sample when the ultrasonic wave propagates inside the sample and reaches the other surface of the sample is detected by irradiating the other surface of the sample with a measuring laser beam, and the pulsed laser The point is that the time at which the ultrasonic wave will reach the sample surface is predicted from the light irradiation time, and the measurement laser beam is temporarily made high in output only within 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, resulting in S/N
Good non-contact ultrasonic emission and reception can be achieved.
This will be explained in detail below with reference to the illustrated embodiments.

FIG. 1 shows an embodiment of the invention. In the figure, 1 is a pulse laser that instantaneously generates high output through Q-switching.
A pulsed laser beam L ₁₀ is irradiated onto one surface 2a of the plate.
Laser light sources that can output pulsed lasers by 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 power of, for example, 100 MW, and a pulse width.
It is a short time of 20nS, and instantly evaporates and scatters the sample surface layer (apply a protective film such as paint if you do not want to damage it). 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. Sample other side 2
b and causes a minute displacement (indicated by a dotted line).
A part (microscopic part) L ₁₁ of the laser beam L ₁₀ is incident on a photodetector 4 such as a photocell or photomultiplier tube by a beam splitter 3, where it is detected (photoelectric conversion).
be done. The output S ₁ of the photodetector 4 gives information on the time when the sample surface 2 a was irradiated with the laser beam L ₁₀ , but 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 S ₂ is also used for timing the oscilloscope 11 or memory 12. The measurement laser 6 includes ruby,
A Q-switchable device such as a YAG laser (a laser source with a lower output than laser source 1 may also be used) is used. The output laser light L ₂₀ of the laser 6 is incident on the optical modulator 8 driven by the local oscillator 7 at a frequency f _L . The laser beam exiting the optical modulator becomes 0th-order, ±1st-order, ±2nd-order, etc. diffracted light, and among these, the first-order diffracted light L ₂₁ reaches the mirror 9 with a frequency of f ₀ +f _L , and then The light is reflected, passes through the beam splitter 10, and enters the photodetector 11.

On the other hand, the 0th order diffracted light L ₂₂ has the original frequency f ₀
The laser beam L 23 passes through the beam splitter 10 and is reflected by the other surface 2b of the sample 2, and the reflected laser beam _L23 returns to the beam splitter 10, where it is reflected and enters the photodetector 11. The photodetector 11 is thus
It is given inputs L ₂₁ and L ₂₃ , performs root mean square detection on these, and outputs a signal S ₃ . This signal S ₃ is transmitted to the amplifier 13
After being amplified by the mixer 14, it is mixed with the drive vibration f _L by the local oscillator 7, and its output is displayed on the oscilloscope 11 and/or stored in the memory 12.

Here, the frequency of the measurement laser beam L ₂₀ is f ₀ ,
Assuming that the amplitude of the first-order diffracted light L ₂₁ is A _L0 , the amplitude of the signal light L ₂₃ reflected on the sample surface 2b is As, and the displacement of the sample surface 2b is Z, the first-order diffracted light L ₂₁ is A _L0 cos {2π(f ₀
+f _L )t}, and the signal light L ₂₃ can be expressed as Ascos {2πf ₀ t+4πZ/λ sin2πfst}. Here, f _s represents the frequency of the ultrasonic wave excited when the sample 2 is irradiated with the pulsed laser 1. Also, λ is the frequency
There is a relationship of λ=c/f ₀ (c: speed of light) at the wavelength of light f ₀ . Therefore, the photoelectric conversion output of the photodetector 11
The intensity i(t) of S ₃ is determined by the root mean square detection as shown in the following equation.

i(t)=<[Ascos {2πf ₀ t+4πZ/λsin2πfst} +A _L0 cos{2π(f ₀ +f _L )t}] ² > =1/2(As ² +1/2A _L0 ² )+AsA _L0 [cos{2πf _Lt +4πZ/λsin2πf _s t} (1) In equation (1), the first and second terms on the right side are DC components, and the third term is an AC component including the displacement Z of the vibrating object. Therefore, the third term is set as iac(t).
When 4πZ/λ<1, iac(t) can be approximately expressed by the following equation.

iac(t)=AsA _L0 [cos2πf _Lt +2πZ/λ {cos2π(f _L +f _s )t−cos2π (f _L −f _s )t}] …(2) The drive frequency f _L is added to the detection signal iac(t) By mixing (heterodyne), it is possible to extract the vibration component iac(t)=AsA _L0 2πZ/λcos2πf _s t (3) which is proportional to the vibration displacement Z. Let the output of the mixer be S ₄ . In equation (3), f _s is the pulsed laser beam L ₁₀
The effective frequency when the half-width of is τ _s ,
There is a relationship between f _s and τ _s as shown in the following equation.

f _s 1/τ _s (4) However, cos2πf _s t holds true between 0t≓2τ _s , and is 0 when t>2τ _s .

This root-mean-square detection is based on the optical heterodyne method, but the difference from the conventional method is that the signal A _s A _L0 2πZ/λcos2πf _s t due to vibration displacement is generated over a period of time corresponding to the pulse width of the pulsed laser beam L ₁₀ This is the only thing that exists.

In this way, the optical heterodyne method makes it possible to detect the displacement Z and, therefore, to receive ultrasonic waves in a non-contact manner.
The problem here is the intensity and duration of the signal lights L ₂₁ and L ₂₃ . Pulsed laser light as mentioned above
The duration of L ₁₀ 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, ie, photons, to the photodetector 11 in such a short period of time to ensure reliable detection. In addition, 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. It is. Therefore, in the present invention, pulsed laser light
Measurement is performed only for a short period of time centered on the time when the pulsed ultrasonic waves generated in the sample 2 by _L10 reach the sample surface 2b and cause mechanical displacement on the surface. The timer 5 is provided for this purpose, and its delay time is set depending on the thickness d of the sample 2 and the ultrasonic propagation speed. That is, as mentioned above, the laser beam L ₁₀ is applied to 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 minute change in the surface, so the timer 5 is delayed. The time is set to a minute width centered on d/v. To give a specific example, if the pulsed laser 1 irradiates the sample surface 2a with a pulsed laser beam L ₁₀ with a pulse width of 20 n sec and an output of 100 MW at time t=0, the output of the photodetector 4 will be applied almost simultaneously (with a delay). (about 0.1n sec) signal S ₁ appears.
Here, assuming that the thickness d of the sample 2 is 30 mm, the propagation velocity of the ultrasonic wave (longitudinal wave) is about 6 mm/1 μsec, so mechanical displacement occurs on the sample surface 2b after about 5 μsec. The measurement laser beam L ₂₀ captures this displacement, but the delay time of timer 5 is 4.
μS, and the duration of the measurement laser 6 is 2 μS.
It is better to Note that the time required to Q-switch the laser 6 to generate a pulsed laser is approximately several tens of nanoseconds, and this time can also be ignored.

In order to provide sufficient photons to the photodetector 11, the output (continuous output) of the measurement laser light source 6 may be increased, but it is effective to Q-switch the light source 6 to output a pulsed laser. If this Q switching is performed at the above-mentioned timing, there is an advantage that unnecessary noise is not picked up. In this way, according to the present invention, detection sensitivity can be raised to a sufficient level, and noise caused by mechanical vibrations occurring in the measurement sample and noise caused by light fluctuations that occur especially when measuring in a hot, high-temperature environment, etc. can be suppressed. can be effectively removed.

FIG. 2 shows a signal processing circuit that uses the output of the mixer 14 in various ways, and the output B of the peak hold circuit 21 is an analog value indicating the peak (displacement) of the signal _S4 . The signal _S4 is also branched to an integrator 22 and a Schmitt circuit 23, and the output of the Schmitt circuit 23 activates a pulse generator 24, causing a gate controller 25 to generate a start pulse P _STR and a stop pulse P _STP . P _STR and P _STP enter integrator 22 via NOR gate 27 and define its operating time. Therefore, for example, a portion of the signal _S3 above a certain level is integrated, and the 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 arithmetic unit 26 at a timing when the pulse P _STP is delayed by a predetermined time in the delay circuit 28 . Therefore, the output C of the hold circuit 27 becomes pulse width information of the signal _S4 .

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. If ₄ and S ₁ are displayed simultaneously, the accurate propagation velocity of the ultrasonic wave can be measured 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. In addition, the ultrasonic waves that have reached the other surface 2b of the sample are reflected there, and the irradiated surface 2a
The other surface of the sample 2b is displaced each time these reflected waves (echoes) arrive, but if all of these displacements are detected, the attenuation constant can be calculated by From this, the crystal grain size inside the sample can be determined. In this case, the timer 5 is configured to repeatedly generate the signal _S2 at timings near t=t, 3t, 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 ratio can be improved (expected to be improved by more than 3 orders of magnitude), and the practicality of ultrasonic flaw detection, inspection, measurement, etc. of specimens can be further increased. . 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 a non-metal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is a block diagram showing an example of a processing circuit for signals obtained at the receiving end. In the figure, 1 is a pulse laser, 2 is a sample, 3 and 10 are beam splitters, 4 and 11 are photodetectors, 5 is a timer, 6 is a measurement laser, 9 is a mirror, and 11 is an oscilloscope.

Claims (1)

[Claims] 1. Irradiate one side of the sample with a high-power pulsed laser beam to generate pulsed ultrasonic waves in the sample, and when the ultrasonic waves propagate inside the sample and reach the other side of the sample, The resulting mechanical displacement is detected by irradiating the other surface of the sample with a measuring laser beam, and the time at which the ultrasonic wave will reach the sample surface is predicted from the irradiation time of the pulsed laser beam, and the time at which the ultrasonic wave reaches the sample surface is predicted. While temporarily increasing the power of the measurement laser beam for a limited period of time before and after the predicted time,
The mechanical displacement of the other surface of the sample due to the ultrasonic waves is detected by heterodyne 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. A method of transmitting and receiving ultrasonic waves using laser light, which is characterized by the following.