JP2006138862A - Laser vibrometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser vibrometer attaining an actual time vibration measurement of a measuring object without adjustment, having ultra-high sensitivity, without the need for a device for extracting vibration signals, such as an optical interferometer. <P>SOLUTION: The laser vibrometer consists of a laser oscillator, a frequency shifter which passes a part of an output light emitted from the laser oscillator, that is the frequency shifter which a frequency shift quantity of a single round trip of a part of the output light coincides with a laser specific relaxation vibration frequency and is made to resonate. The laser vibrometer is equipped with a photodetector, a frequency-modulated wave demodulator and a signal processing device which makes a part of the output light passing through the frequency shifter to be incident on the object to be measured and to make the return light feed back from the object to be measured to the laser oscillator via the light frequency shifter and to process the laser output which is frequency-modulated in resonance fashion and with improved light sensitivity induced by the interference between the returned light and the output light. The vibration state of the measuring object is thus measured in actual time. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a simple and compact laser vibrometer that can measure the vibration of an object to be measured in real time and with extremely high sensitivity without using an optical interferometer and an ultrasensitive detector.

A conventional laser vibrometer basically makes a laser beam output from a laser oscillator incident on a device under test as shown in FIG. 1, and performs a Doppler-shifted scattered light and the laser output according to the vibration of the device under test. A method of measuring the vibration of an object to be measured by causing light to interfere with each other using an optical interferometer such as a Michelson interferometer and measuring a time change of an interference fringe or a beat signal is employed.

However, in this type of laser vibrometer, an optical interferometer that can be adjusted with high accuracy and high stability is indispensable in order to match the wave fronts of the laser output light and the extremely weak scattered light. Since a photodetector and a heterodyne detector are required, there is a problem that the vibration meter is large and extremely expensive.
(K. Otsuka, IEEE J. Quantum Electron. QE-15, 655 (1979)).

An object of the present invention is to provide a laser vibrometer that can achieve real-time vibration measurement of an object to be measured with no adjustment and ultra-high sensitivity without requiring any device for extracting a vibration signal such as the above optical interferometer. It is to provide.

The above problems can be solved by the laser vibrometer of the present invention. That is, after a part of the output light emitted from the laser oscillator passes through the optical frequency shifter, it is made incident on the object to be measured, and the laser output induced by the interference between the return light from the object to be measured and the output light. This is a laser vibrometer that measures the vibration of the device under test by processing the modulated waveform with a photodetector, a frequency modulated wave demodulator, and a signal processing device.

In the laser vibrometer, the return light from the object to be measured is fed back without adjustment in the direction of the oscillation axis of the laser oscillator. The feedback light is twice the frequency shift amount per one pass due to optical frequency shifter as the carrier frequency f _c, is subjected to frequency modulation by the Doppler effect associated with the vibration of the object-receiving surface. The coherent electric field component of this feedback light interferes with the oscillation output light electric field emitted from the laser oscillator. As a result, bright portions and dark portions of interference fringes alternately appear in the laser output mirror at the frequency of the difference between the frequency of the oscillation output optical electric field and the frequency-modulated feedback optical field. Therefore, the transmittance of the output mirror, that is, the loss of the laser oscillator is equivalently modulated at the above difference frequency, and the laser output is subjected to intensity modulation in the form of a frequency-modulated wave having _fc as the carrier frequency (K Otsuka, IEEE J. Quantum Electron. QE-15, 655 (1979)).

The modulated oscillation light is detected by a photodetector and converted into an electrical signal, the electrical signal is detected by a frequency modulation wave demodulator, and the vibration amplitude is displayed and recorded in real time by a signal processing device. The vibration amplitude A _{v (t)} is the oscillation wavelength of the laser lambda, when the phase difference between the carrier wave and the modulation wave frequency f _c and [Delta] [phi (t),
A _v (t) = λΔφ (t)
Given in. Therefore, the vibration amplitude change Av (t) can be measured in real time by detecting the phase difference with the frequency modulation wave demodulator, as in the case of a normal FM receiver.

Here, it should be pointed out that there is a misunderstanding that the self-mixing modulation is based on the nonlinearity of the laser such as the saturation effect of the laser. Since the laser is modulated at the difference frequency regardless of the sign, the application of the frequency offset by the optical frequency shifter is an indispensable element for detecting the vibration amplitude and the vibration direction. When the optical frequency shifter is not used, in order to obtain the displacement, it is necessary to count the number of alternating modulation signal maximum values or minimum values with an electronic counter or the like, and the self-light mixing modulation effect cannot be achieved, Real-time measurement is not achieved.

According to the laser vibrometer according to an embodiment of the present invention, vibration in the nanometer region can be measured easily and in real time with an extremely low light intensity feedback rate. In particular, unlike a laser vibrometer based on a conventional optical interferometer, the modulation degree of the laser output is determined only by the light intensity feedback rate, and does not depend on the laser output power at all. There is an effect that a laser vibrometer can be configured.

According to the laser vibrometer according to one embodiment of the present invention, the output light from the laser oscillator is incident on the object to be measured via the optical fiber. It is possible to configure a feedback optical system that can correspond to the measurement of the object to be measured. In particular, in a solid-state laser having a long coherence length, remote sensing of an object to be measured about 1 km away is possible.

According to the laser vibrometer according to one embodiment of the present invention, the output light from the laser oscillator is divided into a plurality of parts, each of which is incident on different objects to be measured, and provided with a feedback optical system that measures the vibrations of the objects to be measured. Therefore, vibrations of a plurality of objects to be measured can be measured almost simultaneously with a single laser by switching the optical path. In addition, by adopting high-speed switching of the optical path by an optical modulator or the like, it is possible to achieve an excellent effect of real-time measurement of vibration amplitude and vibration direction of a three-dimensional object due to incidence of the same vibrator on different places. .

According to the laser vibrometer according to the embodiment of the present invention, since the frequency shift amount in the optical frequency shifter is made to resonate with the relaxation oscillation frequency unique to the laser, an excellent effect of further enhancing the photosensitivity can be obtained. Can do. This effect cannot be realized by a conventional laser vibrometer using an optical interferometer.

As described above, vibration of a large number of objects to be measured and three-dimensional objects can be easily and ultra-highly sensitive in real time without requiring any highly-stabilized optical interferometer or ultra-high sensitivity signal processing system. It is possible to provide a small and inexpensive laser vibrometer that can be measured by the above method.

The vibrometer of the present invention can be applied to a wide variety of optical measurement fields such as simultaneous measurement of the amplitude and direction of weak earthquake vibrations, measurement of turbulent fluid, and surface shape measurement by scanning an object to be measured.

In the first embodiment, in the laser vibrometer described above, output light from a laser oscillator is incident on an object to be measured via an optical fiber. Accordingly, it is possible to smoothly cope with the case where the laser beam cannot be directly incident on the object to be measured.

In the second embodiment, in the above-described laser vibrometer, the output light from the laser oscillator is branched into a large number of pieces, and each of them is incident on different places of different objects to be measured or three-dimensional objects to be measured. In addition, a large number of return lights from the three-dimensional object to be measured are fed back to the laser oscillator.

The third embodiment is such that in the laser vibrometer described above, the frequency shift amount in the optical frequency shifter is made to coincide with the relaxation oscillation frequency inherent to the laser and resonated. Thereby, the further improvement of a photosensitivity can be achieved.

Embodiments of the laser vibrometer according to the present invention will be described below with reference to the accompanying drawings.

(First embodiment)
FIG. 2 is a schematic block diagram showing the laser vibrometer of the first embodiment of the present invention. The laser vibrometer 1 is schematically composed of a laser oscillator 2, a beam splitter 3, an optical frequency shifter 4, a condenser lens 5, a device under test 6, a photodetector 7, a frequency modulation wave demodulator 8, and a signal processing device 9. ing. For example, a photodiode made of InGaAs (indium gallium arsenide) or the like is suitably used as the photodetector 7, and a digital oscilloscope, a digital phosphor oscilloscope, a spectrum analyzer, or the like is suitably used as the signal processing device 9.

An example of the structure of the optical frequency shifter 4 is as shown in FIG. 11, and is composed of a piezoelectric medium and an optical medium having an acoustooptic effect for propagating ultrasonic waves. The operation principle will be described below. An ultrasonic wave having a frequency fs is generated in the piezoelectric medium by an oscillator. When an ultrasonic wave propagates through an optical medium, an acousto-optic effect causes a refractive index coarse / dense grating having a period of the ultrasonic wave wavelength λs to travel at the speed of sound. When the wavelength of a laser incident wave is λ, the Bragg condition:
2λ _s sin θ = λ
As shown in FIG. 11, the laser beam is diffracted in the angle θ direction. Bragg's condition is equivalent to the momentum conservation law. In the case of FIG. 11, the laser incident wave obtains energy from the ultrasonic wave, and the frequency f _{d of the} diffracted light is f, where the frequency of the incident wave is f.
f _d = f + f _s
It becomes. When the traveling direction of the ultrasonic wave is reversed in FIG. 11, the laser incident wave gives energy to the ultrasonic wave, and the frequency of the diffracted light is determined by the energy conservation law.
f _d = f−f _s
It becomes. These phenomena are called Brulian scattering, and an optical frequency shifter using this phenomenon is an acousto-optic modulator.

In the laser vibrometer 1, the condenser lens 5 is not always necessary. In addition, a configuration in which output light emitted from the opposite side of the laser oscillator 2 except for the beam splitter 3 is detected by a photodetector and vibration is measured using the frequency modulation wave demodulator 8 and the signal processing device 9 is naturally included. .

FIG. 3 is a configuration diagram showing a more specific configuration of the laser vibrometer 1. This laser vibrometer 10 uses a lithium neodymium tetraphosphate laser (LiNdP ₄ O ₁₂ = LNP laser) 12 having an oscillation wavelength λ = 1048 nm and a thickness of 1 mm, which is excited by a semiconductor laser 11 as a laser oscillator, and a beam splitter. A variable optical attenuator 13 was inserted between the optical frequency shifter 4 and the optical frequency shifter 4, and a speaker 14 was used as an object to be measured. As described with reference to FIG. 11, two acousto-optic modulators 15 and 16 are used as the optical frequency shifter. Voltages of frequencies f ₁ and f ₂ are applied to the respective modulators, and the laser incident wave is affected. Ultrasonic waves were propagated in opposite directions. By adjusting the positive and negative frequency shift amounts f ₁ and −f ₂ , the reciprocating frequency shift amount:
_{f c = 2 (f 1 -f} 2)
It was set. As the signal processing device 9, either a spectrum analyzer or a digital oscilloscope was used.

FIG. 4 shows a modulation waveform (FIG. (A)) and a power spectrum (FIG. (B)) of laser output light when a sine wave voltage of 2 V is applied to the speaker 14 to vibrate the speaker. . The feedback rate of the light intensity from the speaker rough surface with an average surface roughness of 100 μm to the laser oscillator was −100 dB. F _c = 2MHz to the power spectrum
As a carrier frequency, a band to be measured specific to frequency modulation is formed. The vibration frequency f _m = 22.6 kHz is obtained from the frequency interval of the measurement band, and the vibration amplitude is A _v = λβ / 2π = 60 nm from the measured value of the FM modulation index β.
It is obtained.

FIG. 5 shows the vibration waveform corresponding to the real-time voltage measured by the digital oscilloscope when the modulation output from the photodetector 7 is detected by the frequency modulation wave demodulator 8. The different applied voltages are shown.

These experimental results confirmed the operating principle and demonstrated vibration measurements in the nanometer range in real time. With this laser vibrometer, vibration amplitude measurement in the range of 0.08 μm (micrometer) to 0.8 mm (millimeter) was achieved in the audio frequency band of 20 Hz to 20 kHz. The corresponding measurement speed range is 10 μm (micrometer) / second to 10 cm (centimeter) / second.

A prominent feature of this vibrometer is that vibration measurement in the nanometer region can be achieved in real time with a minimum light intensity feedback factor ε of −100 dB by using a microchip solid-state laser with a resonator thickness of 1 mm. . The intensity modulation degree M of the laser is K = τ / τ _p
(Τ: fluorescence lifetime, τ _p : photon lifetime) M∝Kε ^1/2
It is represented by In the microchip solid-state laser, the photon lifetime proportional to the resonator length can be shortened by 5 to 7 orders of magnitude compared to the fluorescence lifetime (about 100 μs), and thus it is possible to easily realize amazing light sensitivity. If K is constant, the intensity modulation degree of the laser is determined only by the light intensity feedback rate and does not depend on the output power of the laser at all.

A further distinctive feature realized by using self-light mixing modulation in the laser is that the optical frequency shifter resonates by matching the frequency shift amount by the optical frequency shifter with the relaxation oscillation frequency unique to the laser. It is possible to improve drastically further.

The relaxation oscillation frequency f _R is
f _R ∝ (P / Pth-1) ^1/2
(Where P: laser excitation power, Pth: laser threshold excitation power)
Therefore, by providing a mechanism for adjusting the excitation power to the laser, a resonance effect can be easily provided.

As described above, the relaxation oscillation frequency f _R is adjusted to an arbitrary value independently by changing the excitation power to the laser, and the optical frequency shift amount (carrier frequency) is changed by changing the frequency of the ultrasonic wave. it can. Using this feature, in the experiment of the second embodiment to be described later, by adjusting to f _R = f _c = 2 MHz, the enhancement of the light sensitivity of 20 dB is achieved, and the light intensity feedback rate of −120 dB is obtained. The experimental results of 4 to 5 were reproduced. In this case, since the maximum frequency shift of the frequency modulation is sufficiently smaller than the resonance width of the relaxation oscillation, the linearity is not impaired. Similar photosensitivity enhancement effects were demonstrated in all the following examples. An optical frequency shifter used in a conventional vibrometer based on a Michelson interferometer simply gives a frequency offset, and does not exhibit the effect of enhancing the optical sensitivity in principle.

(Second embodiment)
FIG. 6 is a block diagram showing a real-time sound reproducing device 17 to which the laser vibrometer of the second embodiment of the present invention is applied. In the sound reproducing device 17, in the vibrometer of the first embodiment, the output of the digital disk player 18 is applied to the speaker 14, and the speaker is vibrated with an amplitude that is hardly audible (sound pressure level <20 dB SPL). The vibration signal of the speaker was detected by the frequency modulation wave demodulator 8, and clear music could be reproduced from the other speakers 19 in real time. By using the resonance effect, real-time reproduction was achieved only with a simple frequency modulation wave demodulator with a light intensity feedback rate of -120 dB and without using any complicated device such as an electronic counter or a frequency analyzer. FIG. 7 shows the vibration waveform on the speaker surface when the volume of music suddenly changes.

(Third embodiment)
The laser vibrometer 20 shown in FIG. 8 is the laser vibrometer 10 according to the first embodiment of the present invention in which the laser oscillation light is incident on the DUT 6 via the optical fiber 21. The vibration measurement result is equivalent to the laser vibrometer of the first embodiment.

(Fourth embodiment)
The sound reproducing apparatus of FIG. 9 is obtained by irradiating the device under test 14 with laser oscillation light via the optical fiber 21 in the sound reproducing apparatus 17 of the second embodiment of the present invention. The sound reproduction result is equivalent to that of the sound reproducing device of the second embodiment.

(5th Example)
The laser vibrometer 23 of FIG. 10 is the same as the laser vibrometer 10 of the first embodiment of the present invention, except that the output light from the laser oscillator is divided into a plurality of parts and incident on different speakers 14 to switch the optical path. The vibration of the plurality of speakers 14 is measured with one laser. Vibration measurement equivalent to that of the laser vibrometer 10 of the first embodiment was demonstrated for each speaker. Similar results were obtained even when an optical fiber was used as in the fourth example. FIG. 10 shows three speakers, but a larger number may be used. In addition, by employing high-speed switching of the optical path by an optical modulator or the like, it is possible to measure a large number of vibrating bodies almost simultaneously. Furthermore, the incidence of the same vibrator on different locations can also provide an effect superior to the laser vibrometer 10 in real time measurement of vibration amplitude and vibration direction of different locations of a three-dimensional object represented by an aircraft wing or the like. .

It is a schematic block diagram of the conventional laser vibrometer. It is a schematic block diagram which shows the laser vibrometer as 1st Example of this invention. It is a block diagram which shows the more concrete structure of the laser vibrometer of 1st Example of this invention. In the laser vibrometer of 1st Example of this invention, it is a figure which shows the experimental result which measured (a) the modulation waveform of laser output light, and (b) the corresponding power spectrum. It is a figure which shows the experimental result which measured the vibration waveform of the speaker using the laser vibrometer of 1st Example of this invention. It is a block diagram which shows the audio | voice reproduction apparatus as 2nd Example of this invention. It is the vibration waveform of the speaker caught with the audio | voice reproduction apparatus of 2nd Example of this invention. It is a block diagram of the optical fiber access type | mold laser vibrometer as 3rd Example of this invention. It is a block diagram of the optical fiber access type audio | voice reproduction apparatus as 4th Example of this invention. It is a block diagram of the laser vibrometer corresponding to a several to-be-measured object as 5th Example of this invention. It is a conceptual diagram explaining an example of the structure of the optical frequency shifter used for this invention with the effect | action.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 10, 20, 23 ... Laser vibrometer 2 ... Laser oscillator 3 ... Beam splitter 4 ... Optical frequency shifter 5 ... Condensing lens 6 ... To-be-measured object (outside this invention)
DESCRIPTION OF SYMBOLS 7 ... Optical detector 8 ... Frequency modulation wave demodulator 9 ... Signal processing apparatus 11 ... Semiconductor laser 12 ... LNP solid state laser 13 ... Variable optical attenuator 14, 19 ... Speaker 15, 16 ... Acousto-optic converter 17, 22 ... Audio | voice Playback device 18 ... Digital disc player 21 ... Optical fiber

Claims (5)

A laser oscillator;
A frequency shifter that allows a part of the output light emitted from the laser oscillator to pass therethrough, and makes a frequency shift amount of one round trip of the part of the output light coincide with a relaxation oscillation frequency unique to the laser; ,
A part of the output light that has passed through the frequency shifter is incident on the object to be measured, and return light from the object to be measured is fed back to the laser oscillator via the optical frequency shifter, and the return light and the output light A laser vibrometer comprising a photodetector, a frequency-modulated wave demodulator, and a signal processing device that process a laser output that is resonantly frequency-modulated with improved photosensitivity induced by interference with
A laser vibrometer that measures the vibration state of the object to be measured in real time. The laser vibrometer according to claim 1, wherein an optical fiber is provided between the frequency shifter and the object to be measured. 3. The laser vibrometer according to claim 1, wherein a relaxation oscillation frequency unique to the laser is changed by changing an excitation power of the laser. 4. The laser vibrometer according to any one of claims 1 to 3, wherein the frequency shift amount is changed by propagating an ultrasonic wave to the frequency shifter. An optical system is provided that branches the output light emitted from the laser oscillator into a large number, enters each of them into a plurality of different objects to be measured, and feeds back each return light from the plurality of objects to be measured to the laser oscillator. The laser vibrometer according to any one of claims 1 to 4, wherein

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