JPH0451771B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a microvibration measuring device that non-contactly measures microvibrations on a mirror surface or a surface having an area degree close to that of a mirror surface using a laser beam.

(Prior art) Contact-type piezoelectric vibrators have been widely used for a long time to measure vibrations on the surface of solids and liquids, but recently, similar surface vibrations have been measured non-contact using laser light. Several methods have been reported. One of the representative methods is one that uses a Michelson interferometer.

(Problem to be Solved by the Invention) In the vibration measurement method based on this Michelson interferometer that has been reported in the past, a simple metal Since a vapor-deposited plate half mirror is used, absorption loss of 20% to 30% is unavoidable when the laser beam is reflected by this half mirror or passes through the mirror, and half of the laser beam energy is due to vibration. It had the disadvantage that it could not be used for measurements and the laser light energy was not used effectively.

(Means for Solving the Problems) An object of the present invention is to eliminate the above-mentioned drawbacks.
Using a linearly polarized laser light source, two polarized beam splitters, two quarter-wave plates, a piezoelectric actuator, and two photodetectors (photodiodes), almost 100% of the laser light energy is used effectively. ,
It is an object of the present invention to provide a microvibration measuring device that can significantly suppress noise caused by fluctuations in the output of a laser light source by simultaneously calculating the difference between the output signals of two photodetectors. Embodiments of the present invention will be described in detail below with reference to the drawings.

(Example) FIG. 1 shows the configuration of a measurement system according to an example of the present invention. In the figure, 1 is a linearly polarized laser oscillator such as a He-Ne laser or an Ar laser, and 2
is a laser light beam, 3 and 15 are polarized beam splitters, 4 and 5 are laser light beams, 6 and 11 are quarter-wave plates, 8 and 12 are achromatic lenses, 9 is a plane mirror, and 10 is a piezoelectric Actuator, 13 is a sample, 18 and 23 are plano-convex lenses, 1
9 and 24 are photodetectors, 20 is a voltage actuator control section, 21 and 25 are signal detection sections, and 22 is a difference calculation processing section. Next, its operation will be explained. A laser light beam 2 emitted by a linearly polarized laser oscillator 1 first enters a first polarized beam splitter 3 . At this time, the plane of polarization of the laser beam 2 is made to match the diagonal direction of the incident surface of the polarizing beam splitter 3. In this way, the incident light beam 2 is equally divided into the S-polarized light component (laser light beam 4) serving as the reference optical path and the P-polarized light component (laser light beam 5) serving as the optical path to be measured, with almost no loss. The laser beam 4 is then converted into a circularly polarized beam 7 by passing through a quarter-wave plate 6, and is further focused on the surface of a plane mirror 9 by an achromatic lens 8 with small spherical aberration. is transformed so that it is formed. The plane mirror 9 is attached to a piezoelectric actuator 10, so that the reference optical path length can be precisely controlled within a range of several μm. The laser beam reflected by the plane mirror 9 passes through the achromatic lens 8 and the quarter-wave plate 6 again.
The light beam passes through the polarizing beam splitter 3 and enters the polarizing beam splitter 3 again. At this time, since the laser beam reflected by the plane mirror 9 and returned passes through the quarter-wave plate 6 twice, its plane of polarization is rotated by 90 degrees with respect to the forward laser beam 4. It is P-polarized light. Therefore, the returned laser beam 4 passes through the polarizing beam splitter 3 with almost no loss and becomes a laser beam 14.

On the other hand, the P-polarized laser beam 5 on the optical path to be measured is
Using the same principle as the reference optical path beam 4, it passes through a quarter-wave plate 11 and an achromatic lens 12, is reflected by the surface of the sample 13 placed at the focal point of the achromatic lens 12, and is then reflected again. The light is converted into S-polarized light by passing through the achromatic lens 12 and the quarter-wave plate 11, and then enters the polarization beam splitter 3 again. Then, it is reflected with almost no loss and becomes a laser light beam 14.

Therefore, the laser beam 14 is a combination of the return light from the reference optical path and the return light from the measured optical path. Since the planes of polarization of these two returned lights are perpendicular to each other, no interference occurs. Furthermore, since the amount of light returned from the polarizing beam splitter 3 to the laser oscillator 1 is almost zero, the influence of the returned light from the oscillation of the laser oscillator 1 can be sufficiently suppressed.

Laser light beam 14 is then directed to a second polarizing beam splitter 15 . At this time, the second polarizing beam splitter 15 is arranged at an angle of 45° with respect to the first polarizing beam splitter 3 with the laser beam 14 as the rotation axis. The laser light beam 14 is divided by the polarizing beam splitter 15 into
Laser light beam 16 (polarized beam splitter 15
The amount of light is equally divided into two, with almost no loss, into a P-polarized light component (as seen from the polarizing beam splitter 15) and a laser beam 17 (an S-polarized light component as seen from the polarizing beam splitter 15). Then, since the planes of polarization of the laser beams 16 and 17 are aligned, interference occurs.

The laser beam 16 is focused by a plano-convex lens 18 and converted by a photodetector 19 into an electrical signal proportional to the amount of light. The detected electrical signal is sent to the piezoelectric actuator control section 20 and the signal detection section 21. In the piezoelectric actuator control section 20,
Among the detected electrical signals, low frequency components ranging from DC to several hundred Hz are amplified to an appropriate level and sent to the piezoelectric actuator as a high voltage control signal. Further, in the signal detection section 21, a minute vibration signal of several hundred KHz or more is amplified to an appropriate level, and the signal is sent to the difference calculation processing section 22.

Similarly to the laser beam 16, the laser beam 17 passes through a plano-convex lens 23 and a photodetector 24.
It is converted into an electrical signal proportional to the amount of light. Subsequently, the electrical signal is amplified to an appropriate level by the progress detection section 25 and sent to the difference calculation processing section 22.

FIG. 2 shows an interference pattern A detected by the photodetector 19 (laser light beam 16 generated by interference).
(change in brightness and darkness of the amount of light) and the interference pattern B of the photodetector 24. With the interferometer having the configuration described so far, complementary patterns having mutually opposite phases as shown in FIG. 2 can be obtained. Then, the piezoelectric actuator 10 and the piezoelectric actuator control section 20 described above set the operating point of the interferometer to the maximum sensitivity (interference The light intensity is maintained at point C, where the amount of light is intermediate (brightness). In this case, if a minute vibrational displacement D exists on the surface of the sample 13 with respect to the wavelength of the laser light source used, the output waveform E1 of the signal detection section 21
and the output waveform E2 of the signal detection section 25 are vibration waveforms having opposite phases to each other, as shown in FIG.

On the other hand, in a normal laser light source, output fluctuations of several percent or less occur over a band of several Hz to several MHz. This output fluctuation can be considered a type of noise in microvibration measurements based on a Michelson interferometer. However, as described above, when the operating point of the interferometer is maintained at C by the piezoelectric actuator 10 and its control unit 20, this output fluctuation is displayed as an in-phase waveform in the outputs of the signal detection units 21 and 25. appear. Therefore, the signal detection section 21
If the difference calculation processing unit 22 performs a difference calculation on the output signals of

FIG. 3 shows an example of an actually measured waveform.
This is a Q-switch YAG laser beam (output number mJ,
When a pulse width of approximately 5 ns) is irradiated onto the center of a 20t x 60φ circular surface of an aluminum sample, the photoacoustic vibration displacement that occurs at the center of the circular surface opposite to the irradiation point is measured using this microvibration measuring device. This is the detected waveform. F is the output waveform of the signal detection section 21, and is approximately
Output fluctuation noise of the 1.3MHz laser oscillator can be seen. G is a direct wave longitudinal wave vibration waveform. H is the output waveform of the difference calculation processing unit 22, and it can be seen that the output fluctuation of the laser oscillator 1 is suppressed and the amplitude of the vibration waveform is doubled. 11 and 12 are electrical inductions during YAG laser oscillation.

In addition, the amplitude of the microvibration to be measured is measured using the laser oscillator 1.
The reason for specifying the wavelength to be approximately one-tenth or less of the wavelength of
As can be seen from the figure, since the operating curve of the interferometer is a sinusoidal curve, as the minute vibration amplitude increases, the fidelity of the detected waveform to the vibration waveform decreases. Therefore, even larger amplitudes can be measured if fidelity is not much of a concern.

(Effects of the Invention) As explained above, according to the present invention, when measuring minute vibrations on the surface of an object using a laser beam in a non-contact manner, the energy of the laser beam can be used without wasting approximately 100%.
%, and by simultaneously calculating the difference between the output signals of two photodetectors, the output fluctuation noise of the laser light source can be sufficiently suppressed, and a high signal-to-noise ratio (S/N) can be obtained. There are advantages that can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of a measurement system according to an embodiment of the present invention, FIG. 2 is a diagram showing an interference pattern obtained by two photodetectors, and FIG. 3 is a diagram showing an example of the measurement system according to the present invention. It is a figure which shows the obtained micro vibration waveform. 1... Laser oscillator, 2, 4, 5, 7, 14,
16, 17... Laser light beam, 3, 15... Polarizing beam splitter, 6, 11... Quarter wavelength plate, 8, 12... Achromatic lens, 9...
...Flat mirror, 10...Piezoelectric actuator, 1
3... Sample, 18, 23... Plano-convex lens, 19,
24... Photodetector, 20... Piezoelectric actuator control section, 21, 25... Signal detection section, 22... Difference calculation processing section.

Claims (1)

[Claims] 1 Using a laser beam as a light source, approximately 10 of the laser light source wavelength on a surface with a mirror surface or near-mirror surface accuracy.
In a device for non-contact measurement of minute vibrations with an amplitude of less than 1/2, the laser beam is passed through a first polarizing beam splitter, one of which is passed through a quarter wavelength plate, and a plane mirror provided on a piezoelectric actuator. means for reflecting the laser beam, and means for irradiating the other beam onto the sample through a quarter-wave plate; and means for transmitting the laser beam reflected from the reflecting mirror and the laser beam reflected from the sample through the first polarizing beam splitter. , the first polarized beam splitter includes means for splitting into two directions by a second polarized beam splitter tilted at 45 degrees, means for converting each of the split laser beams into an electrical signal, and a means for converting each of the split laser beams into an electrical signal. and means for controlling the piezoelectric actuator to maximize sensitivity using the electrical signal obtained by the electrical signal converting means. A microvibration measuring device.