JPH10170334A - Vibration measuring instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the vibrational pattern of minor vibrations without providing any vibrating object in a measuring system. SOLUTION: The interference fringe signal of laser light is acquired by receiving light from an optical path to be inspected which guides reflected light from a vibrating body T to be inspected by superimposing light from a reference optical path upon the reflected light. At the time of receiving the light, the wavelength of the laser light emitted from a laser light source 12 is modulated by means of a controller 10 and, at the same time, the phase of the modulated laser light is changed against the vibrational phase of the body T by vibrating the body T at the same frequency as that of the laser light by means of a vibration imparting device 11. While the controller 10 controls in such a way, the amplitude distribution and phase distribution in the vibration of the body T are measured by acquiring an interference fringe signal by means of a solid-state image pickup element 30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring an amplitude distribution and a phase distribution of a vibrating body having a small amplitude.

[0002]

2. Description of the Related Art Measurements utilizing light interference may be used for various measurements due to their high measurement accuracy. Such interferometers include a Twyman-Green interferometer, a Mach-Zehnder interferometer, and a Fizeau interferometer.

Further, in such an interferometer, there is also known a method of measuring a minute vibration of a vibration body to be measured by modulating an optical path length of a reference optical path. For example, the amplitude a (x,
y), irradiating an object that harmonically oscillates at a frequency ω with light of wavelength λ to form an image on a TV camera,
There is a method in which light from the same light source as the irradiation light is directly incident as reference light in the optical path of the reflected light to the camera, interferes with light from an object, and is projected on the camera. Here, (x, y) indicates the coordinate position of the vibration surface. In the interference signal I (x, y, t) obtained at this time, if the intensity of the light from the vibrating object is Io (x, y) and the intensity of the reference light is Ir (x, y), the equation (1. 1) and (1.2).
In these equations, t is time, θo (x, y) is the initial phase of the vibration of the object, and L is the optical path length difference between the optical path of the light from the object and the optical path of the reference light.

[0004]

(Equation 1)

When the interference signal I (x, y, t) is integrated over a sufficiently large number of periods (at least over 10 periods), an intensity distribution M (x, y) represented by the equation (1.3) is obtained. It is known from the prior art that the amplitude distribution can be obtained. Note that J0 is a zero-order Bessel function. This intensity distribution M (x, y) is shown in FIG.
As shown in (1), the maximum intensity becomes 1 at the place where the vibration amplitude a = 0, and the second, third,... Equation (1.3) indicates that the intensity distribution M (x, y) is obtained by calculating the difference in optical path length caused by the vibration of the object (the light source irradiates the object and reflects it.
It is expressed as a function of the amount of change 2a (x, y) of the difference between the optical path length to the V camera (tested optical path length) and the optical path length from the light source through the reference optical path to the TV camera (reference optical path length). It is shown that.

[0006]

(Equation 2)

The above-described vibration measuring method is suitable for measuring a large amplitude distribution of λ / 4 or more.
When (x, y) is small, as can be seen from FIG. 2, the intensity distribution M (x, y) has only a value near the maximum intensity 1, so that it is impossible to detect a minute vibration distribution. difficult.

As a method for compensating for such a defect, a reference light path modulation method is known. In the case of this method, the mirror arranged in the reference light path in the apparatus for performing the above measurement is vibrated at the amplitude b and at the same vibration frequency ω as the vibration body to be measured. Accordingly, the phase term φ (x, y) of the interference signal is expressed by Expression (2.1).

[0009]

## EQU3 ## φ (x, y) = 2a (x, y) sin (ωt + θo (x, y)) − 2b sin (ωt + θr) (2.1)

Here, θr is the initial phase of the reference light.
By transforming equation (2.1), equation (2.2) is obtained.

[0011]

(4) φ (x, y) = 2A (x, y) sin (ωt + θd) (2.2) where A (x, y) ² = a (x, y) ² + b ² -2a (x, y) bcos (θo (x, y) −θr) (2.3) tanθd = (a (x, y) sinθo (x, y) −bsinθr) / (a (x, y) cosθo (x, y) −bcosθr) (2.4)

In the equation (2.1), “modulating the optical path length by oscillating the mirror in the reference optical path with the amplitude b” means “the amplitude of the vibration object to be measured is expressed by the equation (2.2). Vibration at amplitude A (x, y) and reference light path is not modulated ", and a (x, y) is replaced by A (x, y) in equation (1.3). (2.5) and (2.6) are obtained.

[0013]

(Equation 5)

According to this method, the vibration amplitude a of the test object is
Even when (x, y) is small, by increasing the value of b, the distribution of the vibration amplitude a (x, y) can be changed with high sensitivity to the intensity distribution M (x, y).
Can be replaced by The most effective is that the value of b is the largest when the function Jo ² is the largest and the linear part (b to λ / 1
0). In this region, the intensity distribution M
(x, y) can be approximately expressed as Expression (2.7).

[0015]

[6] M (x, y) = k0 -k 1 A (x, y) ··· (2.7)

When the vibration amplitude a (x, y) of the object to be measured is sufficiently smaller than the mirror amplitude b, A (x, y) is calculated from the equation (2.3) by the equation (2.8). ) Is approximately expressed.

[0017]

(Equation 7)

The composite amplitude A (x, y) is calculated by changing the phase relationship Δθ (x, y) between the vibration of the test object and the mirror.
+ A (x, y) and ba- (x, y). Equation (2.
By substituting 8) into equation (2.7), equation (2.9) representing the intensity distribution is obtained.

[0019]

[Equation 8] M (x, y, Δθ) = k0-k 1 (b-a (x, y) cosΔθ (x, y)) ··· (2.9)

When Δθ (x, y) takes substantially the same value over the entire surface (when the phase distribution θ 0 of the vibration of the object is the same over the entire surface of the object), the vibration of the object and the reference mirror has the same phase (Δθ =
0) and the case of the opposite phase (Δθ = π), the equation (2.10) is obtained, and the small amplitude a of the object
(x, y) can be measured.

[0021]

ΔM (x, y) = M (x, y, π) −M (x, y, 0) = 2k ₁ a (x, y) (2.10)

If Δθ (x, y) cannot be considered to be constant over the entire surface, the initial phase of the reference optical path is θr = 0, π / 2, π,
The values of M (x, y) at 3π / 2 are M0 (x, y) and M1 respectively.
Assuming (x, y), M2 (x, y), and M3 (x, y), these are as shown in equations (2.11) to (2.14).

[0023]

Equation 10] M0 (x, y) = k0 -K 1 (b-a (x, y) cosθ0 (x, y)) ··· (2.11) M1 (x, y) = k0-K 1 (b-a (x, y ) cos (θ0 (x, y) -π / 2)) = k0-K 1 (b + a (x, y) sinθ0 (x, y)) ··· (2.12) M2 (x, y) = k0 -K 1 (b-a (x, y) cos (θ0 (x, y) -π)) = k0-K 1 (b + a (x, y) cosθ0 (x, y) ) ··· (2.13) M3 (x , y) = k0-K 1 (b-a (x, y) cos (θ0 (x, y) -3π / 2)) = k0-K 1 (b −a (x, y) sin θ0 (x, y)) (2.14)

From the above equation, S (x, y) defined as follows:
Is calculated, only the minute amplitude a (x, y) can be extracted as shown in the equation (2.15). The distribution θ0 (x, y) of the initial phase of the vibration of the object at this time is given by the equation (2.1
6). Note that K1 is the slope of the Bessel function J0 and is a known value.

[0025]

S (x, y) = (M1 (x, y) −M3 (x, y)) ² + (M2 (x, y) −M0 (x, y)) ² = (2k ₁ a ( x, y) sinθo (x, y)) 2 + (2k 1 a (x, y) cosθ0 (x, y)) 2 = 4k 1 2 a (x, y) 2 ··· (2.15) θ0 (x, y) = arctan ((M1 (x, y) -M3 (x, y)) / (M2 (x, y) -M0 (x, y)) (2.16)

[0026]

With the above-mentioned method, it is possible to measure minute vibrations. However, in the above-mentioned conventional method, a light source having a constant wavelength is used, and the reference mirror is vibrated. There is a need. By the way, in order to measure the minute vibration of an object as described above, the optical system for measurement must be stable, but when the reference mirror is vibrated, the vibration is transmitted to other optical systems and accurate. Measurement is difficult.

The present invention has been made in view of the above problems, and has as its object to provide a vibration measuring apparatus capable of measuring a vibration pattern of minute vibration without vibrating a mirror.

[0028]

According to the present invention, a laser light source capable of wavelength modulation is provided.
A vibration imparting device for vibrating the vibration object to be measured, a light splitting means for splitting a laser beam emitted from a laser light source into two, and one of the laser beams split by the light splitting means is irradiated on the vibration body to be measured to be irradiated. A test light path for guiding the light reflected from the vibrating body, a reference light path having an optical path length different from the length of the test light path and guiding the other laser light split by the light splitting means, and a test light path. A light synthesizing unit that superimposes the laser light that has passed through and the laser light that has passed through the reference optical path; And a laser wavelength modulating means for modulating and controlling the output wavelength of the laser light source.

Then, the wavelength of the laser light emitted from the laser light source is modulated by the laser wavelength modulation means, and at the same time, the vibration body is vibrated at the same frequency as the frequency of the laser light thus modulated by the vibration applicator. Then, the phase of the modulated laser light is changed with respect to the vibration phase of the test vibration body. While the control by the laser wavelength modulating means is being performed in this manner, the interference fringe signal is acquired by the signal acquiring means, and the amplitude distribution and phase distribution of the vibration in the vibration body to be measured are measured.

In the case of the vibration measuring apparatus according to the present invention having the above-described configuration, it is possible to measure the minute vibration of the vibration object by simply modulating the laser beam applied to the vibration object. For this reason, it is not necessary to vibrate the mirror as in the related art, and since there is no vibration in the measurement system, highly accurate measurement is possible.

The wavelength modulation amount Δλ of the laser light by the laser wavelength modulating means is represented by the following formula: λ ² / (10) where L is the optical path length difference between the test light path and the reference light path, and λ is the laser light wavelength.
L) <Δλ <λ ² / (4L)

Further, the amplitude distribution and phase distribution of the vibration in the vibrating body to be measured are measured from the average value of the interference fringe signals obtained over a sufficiently long time (at least over a time longer than 10 times the frequency period). Is desirable.

The average value of the interference fringe signal when the phase of the modulated laser beam is matched with the vibration phase of the vibration body to be measured is obtained as first data M0 (x, y). The average value of the interference fringe signal when the phase of the light is shifted by π, π / 2, and 3π / 2 with respect to the vibration phase of the test object, respectively, is the second data M1 (x, y) and the third data. M2 (x, y) and the fourth data M3 (x, y). Then, the equation (M1 (x, y) −M3 (x, y)) ² + (M2 (x, y) −M0 (x,
y)) ² = 4k ₁ ² Calculate the amplitude distribution a (x, y) from a (x, y) ² and obtain the equation θ0 (x, y) = arctan ((M1 (x, y) −M3 (x , y)) / (M
2 (x, y) −M0 (x, y)), the phase distribution θ0 (x, y) can be calculated.

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described. As described above, in the past, the mirror provided in the reference optical path was vibrated to measure the minute vibration of the vibration object to be measured, but when the mirror was vibrated in this way, this vibration affected other optical systems and the like. This increases the measurement error. For this reason, in the present invention, a semiconductor laser is used as a light source, and the minute vibration of the vibration object to be measured is measured without modulating the wavelength of the semiconductor laser to generate solid vibration.

First, in the above equation (1.1), considering the case where the wavelength is changed by Δλ (t), the interference signal I
(x, y, t) is represented by the following equation (3.1).

[0036]

(Equation 12)

Here, the wavelength is expressed by the formula ｛Δλ (t) = Δλsin (ω
(t + θl) (3) Let us consider a case where modulation is performed in a sinusoidal manner so as to satisfy (3.2)｝. This equation (3.2) is converted into equation (3.
By substituting into 1), it can be transformed as shown in Expression (3.3) and Expression (3.4).

[0038]

(Equation 13)

Comparing equation (3.4) with equation (2.1),
It can be seen that this is equivalent to the case where b in equation (2.1) is replaced by (LΔλ / 2λ). Therefore, (LΔλ / 2λ)
However, if Δλ that matches the linear region of Jo ² (λ / 20 to λ / 8) is selected, it is possible to detect the minute vibration a (x, y) of the test object according to the above discussion. become. This condition is expressed as in equation (3.5).

[0040]

(Λ / 20) <(Δλ / 2λ) <(λ / 8) Therefore, λ ² / (10 L) <Δλ <λ ² / (4L) (3.5)

[0041]

Next, a specific embodiment of the vibration measuring device according to the present invention will be described with reference to FIG. This device is a device for measuring the vibration of the vibration body T to be tested, and includes a vibration applicator 11 composed of a piezo element, an amplifier, a microphone, and the like for imparting a predetermined vibration to the vibration body T to be tested. A controller 10 for controlling the operation and a laser light source 12 whose output is also controlled by the controller 10 are provided as shown in the figure.

The laser light emitted from the laser light source 12 is split by the first beam splitter 13 into the test light A1 and the reference light A2. The test light A1 is reflected by the first mirror 14 and enters the first convex lens 15, where it is condensed on the pinhole 16 and passes therethrough to become divergent light, and irradiates the test vibration body T. The test light A1 reflected from the test object T enters the solid-state imaging device 30 via the second and third convex lenses 17 and 18 and the variable stop 19, and the solid-state image sensor 30 outputs the test light A1. An image signal is obtained.
Note that, as described above, the laser light source 12 is
Is referred to as a test light path.

On the other hand, the reference light A2 is
After passing through the beam expander 22, it is reflected by the second mirror 23 and enters the fourth convex lens 24, where it is condensed on the pinhole 25, passes through it and becomes divergent light, and becomes the second beam splitter. The light 27 is combined with the test light A1 by 27 and enters the solid-state imaging device 30. The optical path of the reference light A2 from the laser light source 12 to the solid-state imaging device 30 is referred to as a reference optical path.

As described above, the test light A1 and the reference light A2 are superimposed and incident on the solid-state imaging device 30, and the solid-state imaging device 30 can obtain interference fringe signals of both lights. The image signal thus obtained by the solid-state imaging device 30 is displayed on the monitor 32 via the image processing device 31. This signal information is fed back to the controller 10.

In the vibration measuring apparatus having the above structure, if the wavelength of the laser light emitted from the laser light source 12 is modulated by the controller 10, the minute vibration of the vibration body T to be detected can be detected as described above. It is possible. A specific method of the vibration measurement will be described below.

(Step 1) First, from the optical path length difference L between the detection optical path and the reference optical path, and the wavelength λ of the laser light emitted from the laser light source 12, the wavelength modulation amount Δλ is obtained in accordance with the above equation (3.5). Decide. For example, the wavelength λ of the laser beam is
Assuming that the optical path length difference is 30 nm and the optical path length difference L is 20 mm, a wavelength modulation amount Δλ = 0.0069 nm = 6.9 pm is obtained.

(Step 2) A current modulation amount ΔI corresponding to the wavelength modulation amount Δλ is determined from the data of the semiconductor laser used for the laser light source 12 measured in advance.
Here, in order to obtain the wavelength modulation amount Δλ = 6.9 pm, that is, to modulate the wavelength of the laser beam to 6.9 pm, for example, the drive current of the laser light source 12 is set to 0.52 mA.
The modulation may be performed, which is a sufficiently large modulation amount.

(Step 3) The frequency of the vibration applied to the test vibration body T by the vibration applying device (piezo element) 11
Determine the signal strength. The oscillation frequency is set to be the same as the frequency of the laser beam modulated by the current modulation amount ΔI.

(Step 4) The frequency and phase of the vibration of the vibration body T to be tested are matched with the frequency and phase of the laser beam from the laser light source 12. That is, the controller 10 causes the vibration applicator 11 and the laser light source 1 to generate vibration and laser light having the same frequency and phase.
2 is controlled. At this time, the current modulation amount of the drive signal applied to the laser light source 12 is ΔI (for example, 0.5
2 mA).

(Step 5) When such control is performed, the average value of interference fringe signals due to light incident on the solid-state imaging device 30 from the test light path and the reference light path is compared with the period of the given signal. Acquired over a sufficiently long time (at least 10 cycles or more), and this is set as first data M0 (x, y).

(Step 6) Next, only the phase of the drive signal applied to the laser light source 12 is shifted by π / 2 with respect to the phase in the above step 4, and the same data as in the step 5 is obtained. It is assumed that two data M1 (x, y).

(Step 7) Further, similarly, only the phase of the drive signal applied to the laser light source 12 is shifted by π and 3π / 2 with respect to the phase in the above step 4, and the same data as in the step 5 is respectively obtained. Acquired and used as third data M2 (x, y) and fourth data M3 (x, y).

(Step 8) From the first to fourth data obtained as described above, the amplitude distribution a (x, y) of the test object T is calculated using the above-mentioned equation (2.15). calculate.

(Step 9) Similarly, the phase distribution θ (x, y) is calculated using the equation (2.16).

(Step 10) The calculation results of the amplitude distribution a (x, y) and the phase distribution θ (x, y) thus calculated are displayed on the monitor 32 or the like.

(Step 11) Steps 3 to 10 are repeated by changing the condition of the vibration applied to the vibration object to be tested.

[0057]

As described above, according to the present invention,
The wavelength of the laser light emitted from the laser light source was modulated by the laser wavelength modulating means, and at the same time, the vibrating device was vibrated at the same frequency as the frequency of the laser light thus modulated by the vibration imparting device. The phase of the laser light is changed with respect to the vibration phase of the vibration body to be tested, and while the control by the laser wavelength modulation means is performed in this manner,
Since the interference fringe signal is acquired by the signal acquiring means and the amplitude distribution and the phase distribution of the vibration in the vibration object to be measured are measured,
There is no vibration in the measurement system, and highly accurate measurement is possible.

[Brief description of the drawings]

FIG. 1 is a configuration explanatory view showing an example of a vibration measuring device according to the present invention.

FIG. 2 is a graph showing the relationship between vibration intensity and amplitude when vibration of a vibration object to be measured is measured using an interference system.

[Explanation of symbols]

 Reference Signs List 10 controller 11 vibration imparting device 12 laser light source 13 first beam splitter 27 second beam splitter 30 solid-state imaging device 31 image processing device 32 monitor

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hitoshi Wada 4-10-9, Yawata, Aoba-ku, Sendai, Miyagi Prefecture (72) Inventor Makoto Nara 1-7-11 Nishioi, Shinagawa-ku, Tokyo Nikon Technical Studio Co., Ltd.

Claims (4)

[Claims] 1. A laser light source capable of wavelength modulation, a vibration imparting device for vibrating an object to be measured, a light splitting means for splitting a laser beam emitted from the laser light source into two, and split by the light splitting means. A laser light path for irradiating one of the laser beams onto the vibration body to be tested and guiding the light reflected from the vibration body to be tested; and a light path length different from the length of the light path to be tested. A reference light path for guiding the other laser light split by the splitting means, a light combining means for overlapping the laser light passing through the test light path with the laser light passing through the reference light path, and a laser overlapped by the light combining means Signal acquisition means for receiving light to obtain an interference fringe signal of the superposed laser light; signal processing means for performing signal processing on the interference fringe signal obtained by the signal acquisition means; and an output wave of the laser light source Laser wavelength modulation means for controlling the modulation of the laser light. The laser wavelength modulation means modulates the wavelength of the laser light emitted from the laser light source, and the same as the frequency of the laser light thus modulated by the vibration imparting device. Vibrating the test vibration body at a frequency, and changing the phase of the modulated laser light with respect to the vibration phase of the test vibration body, while the control by the laser wavelength modulating means is being performed; A vibration measuring device for measuring an amplitude distribution and a phase distribution of the vibration in the vibration object to be measured from the interference fringe signal obtained by the signal acquiring means. 2. The wavelength modulation amount Δλ of the laser light by the laser wavelength modulation means is represented by the following formula: λ ² / (10L) <Δλ <λ ² / (4L) where L: the optical path to be measured and the reference optical path The vibration measuring apparatus according to claim 1, wherein the optical path length difference is set within a range determined by a wavelength λ of the emitted laser light. 3. An amplitude distribution and a phase distribution of vibration in the vibration body to be measured are measured from an average value of interference fringe signals obtained by the signal acquisition means over a long period of time at least ten times the frequency period. The vibration measuring device according to claim 1 or 2, wherein: 4. An average value of the interference fringe signal when a phase of the modulated laser light is made to coincide with a vibration phase of the vibration body to be measured is obtained as first data M0 (x, y). The average value of the interference fringe signal when the phase of the modulated laser beam is shifted by π, π / 2, and 3π / 2 with respect to the vibration phase of the vibration body to be tested is represented by second data M1 (x, y), the third data M2 (x, y) and the fourth data M3 (x, y), and the following equation (M1 (x, y) −M3 (x, y)) ² + (M2 (x, y) y) -M0 (x, y)) ² = 4k
₁ ² Calculate the amplitude distribution a (x, y) from a (x, y) ² and obtain the following equation: θ0 (x, y) = arctan ((M1 (x, y) −M3 (x, y)) / (M2 (x, y) −
The vibration measuring apparatus according to any one of claims 1 to 3, wherein a phase distribution θ0 (x, y) is calculated from M0 (x, y)).