JPH04248421A - Multidimensional vibrometer - Google Patents

Abstract

PURPOSE:To miniaturize the sensor head of the title vibrometer and, at the same time, to condense a plurality of irradiated rays of light to one point on an object to be measured with a small abbreviation by setting a prism on at least one of the plurality of optical paths. CONSTITUTION:The sensor heads 20a and 20b of two vibrometers 40a and 40a are arranged so that the rays of light emitted from the heads 20a and 20b can advance in parallel with each other. The light emitted from the head 20a reaches one point O on an object to be measured as it is and irradiates the point O and the light emitted from the head 20b irradiates the point O from an oblique direction which is deflected from the light emitted form the sensor 20a by an angle after its optical path is turned by means of a prism 27. Therefore, the sensor heads 20a and 20b can be miniaturized as a whole and, since the optical path of the light emitted from the sensor head 20b is turned by the prism 27, the plurality of rays of light can be condensed to one point with a small abbreviation as compared with another case where a lens is used.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention irradiates a vibrating object to be measured with laser light (irradiation light) emitted from a laser light source,
A laser Doppler vibrometer that obtains a beat signal corresponding to the vibration state of the object to be measured by interfering between the reflected light (object light) reflected from the object to be measured and a reference beam extracted from a part of the laser beam. Regarding.

0002

2. Description of the Related Art A laser beam (object beam) reflected from a vibrating object to be measured and subjected to frequency modulation due to the vibration is caused to interfere with a reference beam of a constant frequency that interferes with the object beam to generate a beat. A laser Doppler vibrometer is known that detects this beating with a photodetector to obtain a beat signal corresponding to the vibration frequency and amplitude of the object to be measured. Among these laser Doppler vibrometers, there is a type called an optical fiber laser Doppler vibrometer that is configured to transmit laser light through an optical fiber in order to provide flexibility in the positional relationship between the main body of the vibrometer and the object to be measured. There are some things.

[0003] The above laser Doppler vibrometer (optical fiber
A laser Doppler vibrometer (including a laser Doppler vibrometer) is a vibrometer that can measure the state of one-dimensional vibration of an object to be measured only in the direction in which it is irradiated with irradiation light.
A multidimensional vibrometer is also known in which the vibrometers are combined to measure vibration states in two or three different directions. Hereinafter, an example of a vibrometer that measures one-dimensional vibration will be explained first, and then a multidimensional vibrometer will be explained.

FIG. 1 is a schematic diagram showing a typical example of a conventional laser Doppler vibrometer using an optical fiber. A laser beam 2 emitted from a laser light source 1 is split into two by a beam splitter 3 into transmitted light and reflected light. This beam splitter 3 is different from a polarizing beam splitter (hereinafter abbreviated as "PBS") and splits the laser beam 2 into two regardless of the polarization direction of the laser beam 2. The light 2a transmitted through the beam splitter 3 reaches the PBS 4. This PBS 4 is originally arranged to transmit polarized laser light 2 (transmitted light 2a), and the transmitted light 2
Light a passes through this PBS 4, is condensed by a fiber input lens 5, and is sent to a polarization-maintaining optical fiber 6 at one end face 6.
The light enters from point a and is transmitted to the sensor head section 20. Here, polarization-maintaining optical fiber refers to a predetermined two orthogonal
A single mode optical fiber that transmits only light that is polarized in one direction.

The light transmitted to the sensor head section 20 by the polarization-maintaining optical fiber 6 is emitted from the end face 6b of the polarization-maintaining optical fiber 6, passes through the λ/4 plate 21, the objective lens 22, and is exposed to the subject. The measurement object 23 is irradiated. Here, it is assumed that the object to be measured 23 is repeatedly vibrating in the direction A-B, which is inclined by an angle θ with respect to the optical path of the irradiated light.

The irradiated light is reflected by the object to be measured 23, and this reflected light (this reflected light is referred to as "object light") passes through the objective lens 22 and the λ/4 plate 21 again to the polarization maintaining optical fiber. 6 from its end face 6b. Here, the light that irradiates the object to be measured 23 is the light that is converted into circularly polarized light by passing through the λ/4 plate 21 once, and the object to be measured 2
Since the object light reflected from 3 passes through the λ/4 plate 21 again, this object light passes through the end face 6b of the polarization maintaining optical fiber 6.
It becomes linearly polarized light whose polarization direction differs by 90 degrees from the light emitted from the . The object light incident on the polarization-maintaining optical fiber 6 is transmitted by the polarization-maintaining optical fiber 6, exits from the end face 6a, and is emitted from the fiber input lens 5.
The beam is collimated by , and enters the PBS 4 . As described above, the polarization direction of this object light has been rotated by 90 degrees, so it is reflected by this PBS 4, and the component of this object light that has passed through the beam splitter 7 enters the photodetector 31 in the signal processing section 30. .

On the other hand, the light reflected by the beam splitter 3 is optically detected via the following optical path in order to maintain coherence with the object light by making the optical path length almost the same as that of the object light side. The light is input to the vessel 31. That is, the light reflected by the beam splitter 3 (hereinafter referred to as "reference light") passes through the PBS 8, is focused by the fiber input lens 9, and is sent to the polarization maintaining optical fiber 10. It enters from one end surface 10a and is transmitted to the sensor head section 20,
The light is emitted from the other end face 10b of the polarization-maintaining optical fiber 10, is converted into circularly polarized light by passing through the λ/4 plate 24, passes through the reference light lens 25, and is irradiated onto the reference mirror 26. The reference light reflected by the reference mirror 26 passes through the reference light lens 25 again, passes through the λ/4 plate 24, and enters the polarization maintaining optical fiber 10 from its end face 10b. At this time, the reference light has its polarization direction rotated by 90 degrees with respect to the light emitted from the end face 10b of the polarization-maintaining optical fiber 10, as in the case of the irradiation light (object light) described above. The reference light that enters the polarization-maintaining optical fiber 10 from its end face 10b is transmitted through the polarization-maintaining optical fiber 10, and the reference light enters the polarization-maintaining optical fiber 10 from its end face 10a.
The light is emitted from the optical fiber, is collimated by the fiber input lens 9, and enters the PBS 8. Since the reference light incident on this PBS 8 has its polarization direction rotated by 90 degrees as described above, it is reflected by this PBS 8, and the acousto-optic light modulator 1 is driven by the driver 32 in the signal processing section 30.
1, the frequency is shifted, and the component reflected by the beam splitter 7 enters the photodetector 31 together with the aforementioned object light. The object light and the reference light that have entered the photodetector 31 as described above interfere on the photodetector 31, and when this interfered light is detected by the photodetector 31, a beat signal of frequency fb is generated. can get. After this beat signal is amplified by the preamplifier 33, it is input to a signal processing circuit (not shown), and the vibration frequency, etc. of the object to be measured 23 is determined.

In the optical fiber laser Doppler vibrometer constructed as described above, the wavelength and frequency of the laser beam 2 emitted from the laser light source 1 are λ and fo, respectively, and the vibration velocity of the measured object 23 is V( function of time t), the frequency shift amount f of the object light reflected from the object to be measured 23 from the laser light 2 emitted from the laser light source 1
d is fd = (2・V/λ)・cos θ...
(1) Therefore, the frequency fs of the object light is
fs = fo + fd = fo + (
2・V/λ)・cos θ... (2) Further, as described above, the reference light undergoes a frequency shift in the acousto-optic light modulator 11, so if the modulation frequency of this acousto-optic light modulator 11 is fm, the reference light passes through this acousto-optic light modulator 11. Frequency fr of the subsequent reference light
is fr = fo + fm... (3
) becomes.

Since the object light and the reference light interfere on the photodetector 31, a sum frequency and a difference frequency between the above equations (2) and (3) appear; however, on the photodetector 31, The frequency of this difference is detected, and the frequency of this difference fb is
From equations (2) and (3) above, fb = |fr −fs |
=｜fm −(2・V/λ)・cos
θ| ... (4) becomes. The frequency fb of this difference is detected as a beat signal, and as a result, the velocity of the object to be measured 23 in the optical axis direction of the irradiated light Vcos θ
is detected.

As described above, the velocity Vcos θ of the irradiated light in the optical axis direction can be measured. Therefore, by combining two or three of these vibrometers, the two-dimensional or three-dimensional You can know the state of vibration.

FIG. 2 is a diagram showing a state in which irradiation light is irradiated onto one point O on the object to be measured from three different directions. As shown in Figure 2, one point O on the object to be measured is the origin,
The optical axis 41a of the irradiation light emitted from the objective lens 22a of the first vibrometer 40a is taken as the z-axis, and the optical axis 41b of the irradiation light emitted from the objective lens 22b of the second vibrometer 40b is the z-x plane. The objective lens 22 of the third vibrometer 40c
It is assumed that the optical axis 41c of the irradiation light emitted from c is within the yz plane and is also inclined by an angle ψ from the z axis.

[0012] Three vibration meters 40a, 40b, 40c
is arranged as above, the instantaneous vector velocity of one point O on the object to be measured is V, and x of this vector velocity V is
, y, and z directions respectively as Vx , Vy , and Vz
When, the first vibration meter 40a, the second vibration meter 4
The respective velocities V1, V2, and V3 detected by the third vibration meter 40c and the third vibration meter 40c are as follows: V1 = Vz
……
(5) V2 = Vz ・cos ψ+Vx ・sin
ψ…… (6) V3 = Vz ・cos ψ+V
y ・sin ψ... (7) and the above (5)
, (6), (7) From formulas, Vx = (V2 - V1 ・
cos ψ)/sin ψ... (8) Vy
=(V3 -V1 ・cos ψ)/sin ψ...
... (9) Vz = V1
... (10)
becomes.

In this way, the three vibration meters 40a, 40
Using b, 40c, the three-dimensional vector velocity V of the object to be measured is determined. Here, the 3
The dimensional vibrometer has the characteristic that it does not produce measurement errors due to the shape of the object to be measured, which was a drawback of the displacement meters used previously.

[0014]

[Problems to be Solved by the Invention] In the multidimensional vibrometer as described above, the sensor heads 20 of the plurality of vibrometers (Fig.
The problem is how to arrange a plurality of irradiation lights to focus a plurality of irradiation lights on one point O on the object to be measured. FIG. 3 is a diagram showing an example of the arrangement of a plurality of sensor heads. Of the three vibrometers 40a, 40b, 40c shown in FIG. 2, only the sensor heads 20a, 20b corresponding to two vibrometers 40a, 40b are shown here.

FIG. 3(a) shows vibration meters 40a and 40b.
2 is a diagram illustrating a state in which the sensor heads 20a and 20b are tilted relative to each other so that the irradiation light irradiates one point O on the object to be measured. When arranged in this way, the sensor heads 20a and 20b are arranged at an angle ψ to each other, and the adjustment mechanism for accurately adjusting the angle ψ is complicated because it involves a rotation mechanism, and the sensor head as a whole There is a problem that the section becomes large.

Further, in FIG. 3(b), two sensor heads 20a and 20b are arranged parallel to each other, and the irradiation light emitted from these two sensor heads 20a and 20b is directed onto the object to be measured using a lens 50. FIG. 3 is a diagram showing a state in which the light is focused on one point O. With this configuration, in order to adjust the angle ψ, it is only necessary to move the sensor head 20b in parallel to change the distance d between it and the sensor head 20a, and the adjustment mechanism for the angle ψ can be simplified as a parallel movement mechanism. Although the sensor head section as a whole is also miniaturized, there is a problem in that the irradiation light emitted from the sensor head 20b is easily affected by the aberration of the lens 50. In view of the above circumstances, it is an object of the present invention to provide a multidimensional vibrometer in which the entire sensor head section consisting of each sensor head of a plurality of vibrometers is miniaturized and is equipped with a condensing optical system with relatively few aberrations. With the goal.

[0017]

[Means for Solving the Problems] A multidimensional vibrometer of the present invention for achieving the above object includes a laser beam emitted from a laser light source, an irradiation light that irradiates a measured object, and a laser beam emitted from a laser light source. By separating the object light obtained by being reflected from the object to be measured and the reference light for interference, causing the object light and the reference light to interfere, and receiving the interference light obtained by the interference. A plurality of vibrometers are provided to obtain beat signals representing the state of vibration of the object to be measured, and one point on the object to be measured is irradiated with each of the plurality of irradiation lights from different directions. The multidimensional vibration meter includes an exit optical system that emits a plurality of the irradiation lights in the same direction, and a plurality of the irradiation lights that are emitted from the exit optical system so as to be focused on the one point. and a prism disposed on at least one optical path of the plurality of optical paths of the plurality of irradiation lights.

[0018]

[Operation] The multidimensional vibration meter of the present invention is designed to emit a plurality of irradiation lights in the same direction as in the case shown in FIG. 2(b), but in the case shown in FIG. Unlike the above, the plurality of emitted irradiation lights are condensed by a prism instead of the lens 50 in FIG. A condensing optical system with less aberration than in the case where the

[0019]

[Examples] Examples of the present invention will be described below. FIG. 4 is a diagram schematically showing the sensor head section of the multidimensional vibrometer of the present invention. As in the case of FIG. 3, two vibration meters 40a,
40b sensor heads 20a, 20b are shown. Sensor head 2 of two vibration meters 40a, 40b
0a, 20b are those sensor heads 20a, 20
The irradiation light emitted from the sensor head 20a is arranged so that it travels parallel to each other, and the irradiation light emitted from the sensor head 20a directly irradiates one point O on the object to be measured, and the irradiation light emitted from the sensor head 20b The optical path is bent by a prism 27, and the irradiation light emitted from the sensor head 20a is configured to irradiate a point O from a direction inclined by an angle ψ.

In this way, a plurality of sensor heads 20a, 2
Since the sensor heads 20a and 20b are arranged parallel to each other so that the irradiated light emitted from the sensor head 0b travels parallel to each other, the entire sensor head section is miniaturized. Furthermore, as shown in FIG. 3(b), since the optical path is bent by the prism 27 without using the lens 50, multiple irradiated lights can be focused at one point with less aberration than when using a lens. Become. Further, by using the prism 27, by slightly rotating the prism 27 in the direction of the arrow C-D shown in FIG. 4, for example, the optical axis 41b' can be moved to the optical axis 41b. It also becomes possible to finely adjust the irradiation position of the irradiated light on the object to be measured.

Note that when a plurality of irradiation lights are irradiated simultaneously as described above, the irradiation light emitted from one of the plurality of vibrometers is reflected by the object to be measured, and then emitted and reflected from the other vibrometers. This will enter the other vibration meter together with the object light that has been detected, but if this entering light interferes with the light inside the other vibration meter, it will lead to measurement errors. Therefore, it is preferable to provide separate laser light sources (see laser light source 1 in FIG. 1) so that the lights used in each of the plurality of vibrometers do not interfere with each other.

Furthermore, although the above embodiment is an example of a vibration meter using an optical fiber, it goes without saying that the present invention is not only applicable to a vibration meter using an optical fiber.
Generally, the present invention can be widely applied to laser Doppler vibrometers configured to detect multidimensional vibrations.

[0023]

As described above in detail, the multidimensional vibrometer of the present invention includes an exit optical system that emits a plurality of irradiation lights in the same direction, and a plurality of irradiation lights that are emitted from the exit optical system. and a prism arranged on at least one of the plurality of optical paths so that the irradiated light is focused on one point on the object to be measured, so the sensor head as a whole is small. In addition, it becomes possible to focus a plurality of irradiation lights onto a single point on the object to be measured with little aberration.

[Brief explanation of the drawing]

[Figure 1] Schematic configuration diagram showing a typical example of a laser Doppler vibrometer using a conventional optical fiber

[Figure 2] A diagram showing a state in which irradiation light is irradiated to one point O of the object to be measured from three different directions.

[Figure 3] Diagram showing an example of arrangement of multiple sensor heads

[Figure 4
] A diagram schematically showing the sensor head of the multidimensional vibration meter of the present invention.

[Explanation of symbols]

1 Laser light source 2 Laser light 6, 10 Polarization maintaining optical fiber 11 Acousto-optic light modulator 20, 20a, 20b, Sensor head 27
Prism 40a, 40b, 40c Vibration meter 100
Measured object

Claims (1)

[Claims] 1. A reference for causing laser light emitted from a laser light source to interfere with irradiation light that irradiates an object to be measured and object light obtained by reflecting the irradiation light from the object to be measured. a plurality of vibrometers that obtain a beat signal representing the state of vibration of the object to be measured by interfering the object light and the reference light and receiving the interference light obtained by the interference; In a multidimensional vibrometer configured such that one point on the object to be measured is irradiated from different directions by each of the plurality of irradiation lights, the plurality of irradiation lights are directed in the same direction. an exit optical system for emitting light; and an exit optical system disposed on at least one of the optical paths of the plurality of irradiation lights so that the plurality of irradiation lights emitted from the exit optical system are converged on the one point. A multidimensional vibration meter characterized by being equipped with a prism.