JP2007285898A - Laser vibrometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive highly-accurate laser vibrometer for shifting an optical frequency without using an expensive AOM. <P>SOLUTION: In order to detect a displacement portion of an optical Doppler frequency, as a method for constituting optical heterodyne, a vibration element such as an inexpensive piezo element is used, and a frequency of laser light having a prescribed wavelength outputted from a laser light source part and a prescribed frequency generated from the vibration element are mixed together, to thereby shift the frequency of the laser light, and a frequency component wherein a measuring object is vibrated is extracted from a modulated wave through a photodetector, a frequency modulated wave demodulator and a signal processing device, from the modulated wave modulated by the frequency component wherein the measuring object is vibrated by using the prescribed frequency generated from the vibration element as a carrier frequency. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a laser vibrometer that can measure vibrations of an object to be measured with high accuracy in real time using laser light.

Conventionally, as a method for measuring the vibration speed of a vibrating object, laser light is transmitted / received to / from the vibrating object, and the scattered laser light that has undergone frequency shift due to the Doppler effect is caused to interfere with the reference laser light. Methods for detecting the beat frequency and measuring the vibration speed are already known from literatures and catalogs of laser vibrometers that have been commercialized in the past.

As a method of measuring the vibration state of a moving object, there are a method of measuring the “displacement” of the object and a method of measuring the vibration “speed” (m / s) of the object. The vibration “velocity” (m / s) of the moving object is measured. According to the method of measuring the “displacement” of an object, in an environment where the minute vibration of the object is affected by the background noise, the displacement of the background noise is large, the displacement of the object is buried, and the waveform Observation becomes difficult. To eliminate this effect, a high-performance vibration isolation table is required.
On the other hand, in the method of measuring the vibration “speed”, the speed value of the background noise having a low frequency is smaller than the vibration speed of the object, so that the vibration waveform of the object can be easily observed. Objects that vibrate at high speed are less susceptible to background noise when measured with speed output.

The frequency of light is extremely high, for example, 474 THz at the HeNe laser wavelength (633 nm), and signal processing is difficult, so the optical heterodyne interferometry is used instead of directly handling the optical frequency. The laser beam has a characteristic that it is self-mixed and modulated based on interference between the photoelectric field emitted by itself and the return photoelectric field. Optical heterodyne is performed using this self-mixing modulation as a mixer.

Since a light wave is a type of electromagnetic wave, when two light waves with different frequencies are superimposed, similar to the down-conversion principle of converting to an intermediate frequency used in radio equipment using electromagnetic waves, It produces a beat. This beat is called “optical beat” here, and the frequency of the optical beat is called “beat frequency” instead of the intermediate frequency. The light beat can be detected as a periodic change in light intensity.

When some information is given to one of the two lights, information also appears in the light beat correspondingly. Here, information refers to the amount of change given to any of the amplitude, phase, and frequency of light. That is, when the information is superimposed on one of the two lights, when the other light (which is referred to as a reference light) is superimposed on the light on which the information is superimposed, The information can be extracted from the optical beat signal. Such a signal detection method is called optical heterodyne interferometry.

A method of creating two lights having different frequencies for generating an optical beat is based on a method of shifting outgoing light by a certain frequency. Conventionally, an acousto-optic modulator (AOM) has been used for the frequency shift of the light wave. The frequency to be shifted is determined by the ultrasonic frequency supplied to the AOM. The AOM is composed of a piezoelectric medium and an optical medium having an acoustooptic effect for propagating ultrasonic waves. When the ultrasonic wave propagates in the optical medium, the refractive index is dense with a period of the wavelength λs of the ultrasonic wave due to the acoustooptic effect. The grating will travel at the speed of sound, and if the wavelength of the laser incident wave is λ,
Flag condition: 2λsSinθ = λ
When the above condition is satisfied, the laser beam is diffracted in the direction of the angle θ. With diffraction,
Energy transfer occurs between light waves and sound waves, and light wave energy, that is, frequency shift occurs.

The shift frequency range by AOM is generally in the range of 50 to 300 MHz depending on the medium used for AOM. Therefore, two light waves having this frequency difference are generated between the light transmitted through the AOM and the light not transmitted through the AOM. When these two lights interfere with each other, an optical beat signal having this frequency (50 to 300 MHz) is obtained.

JP-A-8-265262

The AOM used as a frequency shifter in a conventional optical interferometer is limited in the number of manufacturers and is relatively expensive. Further, since the laser beam is diffracted in the angle θ direction, the AOM requires adjustment work to align the wavefronts of the laser beam and the weak scattered light. Furthermore, there was a problem that it was too large as a vibrometer. These problems are particularly noticeable when attempting to measure vibrations of a plurality of objects to be measured.

Accordingly, the present invention has been made in view of the above-described problems, and an object thereof is to provide a laser vibrometer that does not use an AOM, is inexpensive, highly accurate, and can be easily adjusted.

A laser vibrometer according to an embodiment of the present invention is:
A laser light source unit that outputs laser light of a predetermined wavelength;
A polarizing beam splitter;
a λ / 4 plate;
A reflector,
A vibrating element for generating a predetermined frequency;
A half mirror that transmits part of the incident light and reflects the rest,
A photodetector for converting light into an electrical signal;
A frequency modulation wave demodulator;
A signal processing device,
The half mirror, the polarization beam splitter, the λ / 4 plate, the reflection mirror, and the vibration element are sequentially arranged on the optical axis of laser light having a predetermined wavelength output from the laser light source unit, and the polarization beam The measured object is arranged in the optical axis direction reflected by the splitter, and the photodetector is arranged in the optical axis direction reflected by the half mirror.

The photodetector may be arranged on one side in the optical axis direction reflected by the half mirror, and the reflecting mirror may be arranged on the other side.

The vibration element may be a piezo element.

The vibration element may be a crystal resonator.

The waveform oscillated by the vibrator may be a triangular waveform.

The waveform oscillated by the vibrator may be a sawtooth waveform.

The laser light source unit may be composed of a semiconductor laser.

The laser light source unit may be a solid laser excited by a semiconductor laser.

The laser vibrometer according to one embodiment of the present invention is
A plurality of non-polarizing beam splitters for branching the laser light of the predetermined wavelength transmitted through the half mirror into a plurality of lights;
The polarizing beam splitter, the λ / 4 plate, the reflecting mirror, and the vibrating element are sequentially arranged on the optical axes of a plurality of lights generated by the previous non-polarizing beam splitter, and reflected by the polarizing beam splitter. The object to be measured is arranged in the direction of the optical axis, and the photodetector is arranged in the direction of the optical axis reflected by the half mirror.

According to the present invention, it is possible to measure vibrations of an object to be measured with high accuracy in real time at a low cost. In particular, when an optical system is configured to measure the vibrations of a plurality of objects to be measured, it is possible to measure the vibrations of the objects to be measured while maintaining high accuracy while being cheaper.

Hereinafter, a laser vibrometer according to an embodiment of the present invention will be described in detail with reference to the drawings. It should be noted that the present invention is not limited to the following examples, and those skilled in the art to which the present invention belongs can modify or change the present invention without departing from the spirit and spirit of the present invention. .

(First embodiment)
FIG. 1 is a block diagram showing a first embodiment of the laser vibrometer of the present invention. The laser vibrometer of the present invention includes a laser 101, a half mirror 102, a polarizing beam splitter 103, a λ / 4 plate 104, a reflecting mirror 105, a vibrating element 106, a device under test 107, a photodiode 108, Consists of

The laser 101 may be a semiconductor laser (LD), but it is desirable to use a microchip solid-state laser and oscillate by exciting the end face with the LD. In order to realize a vibrometer with high resolution and optical sensitivity, a high carrier-to-noise ratio (CNR) characteristic is required. CNR depends on the fluorescence lifetime ratio K = τ / τp (τ: fluorescence lifetime, τp: photon lifetime). This is because a microchip solid-state laser can easily realize high photosensitivity because 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). In addition, when LD is used for self-mixing modulation, in which the photoelectric field emitted by itself and the return photoelectric field interfere with each other to modulate the intensity of the laser at the beat frequency, the terminal voltage changes when the return light is received and the characteristics deteriorate. This is because it is not desirable to use LD for self-mixing modulation. The microchip solid-state laser oscillates an LD with an injection current from an LD drive unit (not shown) and excites the solid-state laser with the oscillation light to emit light. The injection current is a constant direct current. is there.

The half mirror 102 transmits a part of the incident light and reflects the remaining light in the direction of a constant angle.

The polarization beam splitter 103 divides the linearly polarized light into P-polarized light and s-polarized light, and transmits the P-polarized light and transmits the S-polarized light when linearly polarized light in an arbitrary direction is incident. Reflect 90 °.

The λ / 4 plate 104 is a ¼ wavelength plate when linearly polarized light is incident and the orientation of the linearly polarized light is 45 ° with respect to the optical axis of the crystal in the ¼ wavelength plate. The light that has passed through it uses the phenomenon that the original linearly polarized light becomes circularly polarized light, and creates circularly polarized light from linearly polarized light or creates linearly polarized light from circularly polarized light.

For example, it is desirable to use a piezo element that has a property of deforming when a voltage, magnetism, or the like is applied, and whose vibration frequency is variable by changing the voltage. The vibration frequency must be a triangular wave or a sawtooth wave whose waveform rises linearly. An optical Doppler shift is used when the laser beam is incident when the sawtooth wave applied voltage rises or when the triangular wave applied voltage rises and falls.

The photodetector 108 uses a photodiode (PD) made of a semiconductor such as InGaAs (indium gallium arsenide), for example. A photodiode is an optical sensor that converts light into electric current and has a property of generating electric current when absorbing light, and has excellent linearity with respect to incident light. It has features such as a wide sensitivity wavelength range, small size, light weight, and long life, and is used for detecting the presence or absence of light, measuring light intensity, and position detection.

The laser beam output from the laser 101 enters the half mirror 102. Part of the laser light incident on the half mirror 102 passes through the half mirror 102 and enters the polarization beam splitter 103. The laser light incident on the polarization beam splitter 103 is separated into P-polarized light and S-polarized light by the polarization beam splitter 103, and the S-polarized laser light passes through the polarization beam splitter 103.

Next, the linearly polarized light (S-polarized light) 112 transmitted through the polarizing beam splitter is converted into circularly polarized light 113 by the λ / 4 plate 104. The circularly polarized light 113 enters the vibration element 106 and is frequency-shifted based on the vibration frequency of the vibration element 106.

The frequency-shifted circularly polarized light 113 is reflected by the reflecting mirror 105 and converted into linearly polarized light (P-polarized light) 114 by the λ / 4 plate 104. Since this linearly polarized light 114 is P-polarized light, it is reflected by 90 ° by the polarizing beam splitter 103 to become linearly polarized light 115 and enters a measured object 107 provided on the optical axis thereof. The linearly polarized light 115 incident on the object to be measured is subjected to frequency fluctuations according to the vibration state of the object to be measured, reflected, and returned to the polarization beam splitter 103.

Since the return light 115 from the object to be measured is P-polarized light, it is reflected by 90 ° by the polarization beam splitter 103 to become linearly polarized light 114 and converted again to circularly polarized light 113 by the λ / 4 plate 104. The circularly polarized light 113 enters the vibration element 106 and is frequency-shifted again based on the vibration frequency of the vibration element 106. The frequency-shifted circularly polarized light 113 is reflected by the reflecting mirror 105 and converted to linearly polarized light (S-polarized light) 112 by the λ / 4 plate 104. Since the linearly polarized light 112 is S-polarized light, it passes through the polarization beam splitter 103 and enters the half mirror 102.

A part of the linearly polarized light 112 incident on the half mirror 102 passes through the half mirror 102 and enters the laser 101 and returns. The returned laser light is self-mixed with the photoelectric field emitted from the laser 101.

A part of the self-mixed modulated laser light is then reflected by the half mirror 102 and is incident on the photodetector 108. Thereafter, the frequency-modulated wave demodulator and the signal processing device detect the vibration of the object to be measured as the displacement of the carrier frequency, perform arithmetic processing, digitize, and perform video processing.

Here, when the vibration amplitude length of the vibration element is Sa (m) and the vibration frequency is fm (Hz), the vibration velocity V of the vibration element 106 Doppler shifted by the vibration element 106 through the optical system shown in FIG. Is represented by the following formulas (1) and (2).

V = Safm (sawtooth wave) (1)
V = 2Safm (triangular wave) (2)

The carrier frequency fc is determined by the vibration amplitude length Sa, the vibration frequency fm, and the wavelength λ of the laser light of the vibration element, and the following carrier frequency fc is derived as shown in equations (3) and (4).

Carrier frequency fc = 4V / λ = (4Sa / λ) fm (sawtooth wave) (3)
Carrier frequency fc = 4V / λ = (8Sa / λ) fm (triangular wave) (4)

By the optical system shown in FIG. 1, a vibration frequency component fd (t) of the device under test is superimposed on the carrier frequency fc, and a frequency modulation wave demodulator (not shown) having the photodetectors 108 and fc as the carrier frequency. The vibration amplitude can be detected from the vibration frequency component fd (t) of the object to be measured. In the case of a sawtooth wave, the vibration frequency component fd (t) of the detected object to be measured is signal-processed by a signal processing device (not shown) to monitor the vibration state of the object to be measured in real time. Can do. When oscillating with a triangular wave, the polarity of the frequency shift at the time of rising and falling is reversed, and the polarity of the double-tone voltage corresponding to the vibration of the object to be measured is reversed. By electrically inverting the polarity of the regulated voltage, the vibration of the object to be measured can be continuously measured in real time.

(Second Embodiment)
FIG. 1 is a block diagram showing a second embodiment of the laser vibrometer of the present invention. The second embodiment of the laser vibrometer of the present invention is different from the first embodiment of the laser vibrometer of the present invention as follows.

The half mirror 122 has a 90 ° angle different from that of the half mirror 102 of the first embodiment, and the photodiode 108 is disposed on one side in the optical axis direction reflected by the half mirror 122 and the reflecting mirror 125 is disposed on the other side. Is arranged.

A part of the light emitted from the laser 101 is reflected by the half mirror 122, enters the reflecting mirror 125, and is reflected. This is used as reference light.

On the other hand, part of the return light subjected to the frequency shift by the vibration element 106 and the target 107 is reflected by the half mirror 122. The return light and the reference light are combined and incident on the photodetector 108, and a beat signal is electrically extracted by photomixing of the return light and the reference light. That is, instead of “self-mixing modulation” in which laser “emitted light” and “return light” interfere with each other to modulate the intensity of the laser at the beat frequency as in the first embodiment, “return light” and “reference” "Light" is superimposed and photomixing is performed with a photodetector to extract the beat frequency as an electrical signal. Thereafter, the frequency-modulated wave demodulator and the signal processing device detect the vibration of the object to be measured as the displacement of the carrier frequency, perform arithmetic processing, digitize, and perform video processing.

In the above-described second embodiment of the laser vibrometer of the present invention, since the reference light and the return light are combined and the beat wave is converted into an electric signal, the optical system can be configured regardless of the light source.

(Third embodiment)
FIG. 3 is a block diagram showing a third embodiment of the laser vibrometer of the present invention. The laser vibrometer of the present invention measures vibrations of a plurality of objects to be measured simultaneously. The laser light emitted from the laser 101 passes through the half mirror 102 and is branched into a plurality by a plurality of non-polarized light 109, 209, and n9. Thereafter, each branched light is transmitted through the polarization beam splitter 103, the λ / 4 plate 104, the reflecting mirror 105, the vibrating element 106, and the device under test 107, as in the first embodiment. Via.

At this time, in order to generate different carrier frequencies from the branched light, the vibration elements 106, 206, and n6 have different oscillation frequencies. When the vibration element is a piezo element, the voltage of each piezo element may be changed.

Since the vibration states of the devices under test 107, 207, and n7 are superimposed on different carrier frequencies fc, each device under test is detected by detecting the frequency displacement or amplitude displacement included in each carrier frequency. Detect the vibration frequency.

As described above, when an optical system for measuring vibrations of a plurality of objects to be measured is used, when a conventional AOM is used, the same carrier frequency is branched by a beam splitter in one AOM, and a plurality of objects to be measured are used. It was made to enter. However, by using an inexpensive vibration element such as a piezo element without using an expensive AOM, it is possible to use a plurality of carrier frequencies according to the number of objects to be measured, which is inexpensive and highly accurate vibration measurement. Is possible.

1 is a schematic configuration diagram of a laser vibrometer according to a first embodiment of the present invention. The schematic block diagram of the laser vibrometer which concerns on the 2nd Embodiment of this invention. Laser vibrometer for measuring vibrations of a plurality of objects to be measured according to the second embodiment of the present invention

Explanation of symbols

101 Laser 102, 122 Half mirror 103, 203, n3 Polarizing beam splitter 104, 204, n4 λ / 4 plate Linearly polarized wave 105, 125, 205, n5 Reflecting mirror 106, 206, n6 Vibration element 107, 207, n7 Target 108 Photodiode (photodetector)
109, 209, n9 Non-polarizing beam splitter 111, 112, 114, 115 Linearly polarized circularly polarized light 113 Circularly polarized light

Claims (9)

A laser light source unit that outputs laser light of a predetermined wavelength;
A polarizing beam splitter;
a λ / 4 plate;
A reflector,
A vibrating element for generating a predetermined frequency;
A half mirror that transmits part of the incident light and reflects the rest,
A photodetector for converting light into an electrical signal;
A frequency modulation wave demodulator;
A signal processing device,
The half mirror, the polarization beam splitter, the λ / 4 plate, the reflection mirror, and the vibration element are sequentially arranged on the optical axis of laser light having a predetermined wavelength output from the laser light source unit, and the polarization beam A laser vibrometer, wherein the object to be measured is arranged in an optical axis direction reflected by a splitter, and the photodetector is arranged in an optical axis direction reflected by the half mirror. The laser vibrometer according to claim 1, wherein the photodetector is disposed on one side in the optical axis direction reflected by the half mirror and a reflecting mirror is disposed on the other side. The laser vibration meter according to claim 1, wherein the vibration element is a piezo element. The laser vibrometer according to claim 1, wherein the vibration element is a crystal resonator. The laser vibrometer according to claim 1, wherein a waveform oscillated by the vibrator is a triangular waveform. The laser vibrometer according to claim 1, wherein the waveform oscillated by the vibrator is a sawtooth waveform. The laser vibrometer according to claim 1, wherein the laser light source unit includes a semiconductor laser. The laser vibrometer according to claim 1, wherein the laser light source unit is a solid laser excited by a semiconductor laser. A plurality of non-polarizing beam splitters for branching the laser light of the predetermined wavelength transmitted through the half mirror into a plurality of lights;
The polarizing beam splitter, the λ / 4 plate, the reflecting mirror, and the vibrating element are sequentially arranged on the optical axes of a plurality of lights generated by the previous non-polarizing beam splitter, and reflected by the polarizing beam splitter. A laser vibrometer, wherein the object to be measured is arranged in an optical axis direction and the photodetector is arranged in an optical axis direction reflected by the half mirror.

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