JP2004333165A - Ultrasonic measuring apparatus using optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To implement an apparatus for measuring various ultrasonic waves through the use of the Doppler shift effect which is not affected by oscillations from the outside and does not require adjustments of a special optical element. <P>SOLUTION: The ultrasonic measuring apparatus comprises a condensing lens for condensing scattering light generated by a laser beam irradiated to an object to be measured; an optical fiber provided with both a partial reflecting mirror at its tip for receiving the incidence of the scattering light from the condensing lens and a partial reflecting mirror in the rear; a Doppler shift sensor having a high-frequency photo-detector for detecting interference light which has passed through the optical fiber; and a controller provided with a control circuit for altering the shape of a piezoelectric element wound on the peripheral surface of the optical fiber, so that the high-frequency photo-detector of the Doppler shift sensor can sense the interference light in an optimum condition. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an apparatus for measuring various ultrasonic waves using the Doppler shift effect, and more particularly, to a high efficiency, small size, unaffected by external vibration, and a special optical element. The present invention relates to an ultrasonic measurement device using an optical fiber that does not require an adjustment operation and can be applied even in a severe environment with vibration.
[0002]
[Prior art]
Generally, to measure an ultrasonic wave, a Doppler shift measurement interferometer (Fabry-Perot interferometer) (100) as shown in FIG. 1 is used. In the Doppler shift measurement interferometer (Fabry-Perot interferometer) (100), when a laser beam is irradiated on the surface of the measurement target (101) from one side, the surface of the measurement target is irradiated by ultrasonic waves generated from the measurement target (101). However, due to the surface vibration of the object to be measured, the frequency of the laser scattered light causes a frequency shift due to the so-called Doppler shift. By measuring the amount of frequency shift of the laser light, the frequency of the It measures sound waves.
[0003]
A Doppler shift interferometer (100) used for measuring such a Doppler shift irradiates a laser beam to an object to be measured, and the laser scattered light frequency-shifted by an ultrasonic wave (103) generated from the laser beam. And a converging lens (104) for condensing light, and two spherical mirrors (105) and (106) having a constant reflectance.
[0004]
The intensity of light passing through the condenser lens (104) is determined by the distance (L) between the two spherical mirrors (105) and (106), the reflectivity of the spherical mirrors (105) and (106), and , Depending on the frequency of the light. Further, the frequency of the scattered light incident on the condenser lens (104) is changed by the ultrasonic wave, and the intensity of the light is changed accordingly. Therefore, the Doppler shift interferometer (100) uses the intensity of the scattered light. The ultrasonic wave can be measured by measuring the height with the photodetector (107).
[0005]
However, for stable ultrasonic measurement, light must be continuously reflected multiple times between the spherical mirrors (105) and (106) along the same path inside the interferometer (100). The parallelism of the spherical mirrors (105) and (106) must be precisely adjusted, and the distance between the spherical mirrors (105) and (106) must always be kept constant. There are points.
[0006]
However, such a Doppler shift interferometer (100) has a problem that it is too sensitive to external vibration and is difficult to be applied in an industrial site in a severe environment because of the required precision of adjustment of its components. .
[0007]
Further, in the conventional interferometer (100), since the measurement efficiency increases as the distance (L) between the two spherical mirrors (105) and (106) increases, the interferometer ( There is a disadvantage that the size of 100) must be increased.
[0008]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION The present invention is directed to an ultrasonic measuring apparatus using an optical fiber, which is small, does not react sensitively to external vibration, is very easy to maintain and repair, and has high efficiency in order to solve the conventional problems as described above. Is to provide.
[0009]
It is another object of the present invention to provide an ultrasonic measuring apparatus which is excellent in applicability in an industrial field and uses an optical fiber more easily even in a severe environment where various vibrations frequently occur.
[0010]
[Means for Solving the Problems]
In order to achieve the object as described above, the present invention provides an apparatus for measuring ultrasonic waves using the Doppler shift effect, for condensing scattered light generated by a laser beam applied to an object to be measured. A high-frequency photodetector for attaching a condensing lens, a partial reflection mirror capable of receiving scattered light from the condensing lens to an end thereof, an optical fiber having a partial reflection mirror attached behind, and detecting interference light passing through the optical fiber A Doppler shift sensor provided with a control circuit for changing the shape of a piezoelectric element in which the optical fiber is surrounded so that the high-frequency photodetector of the Doppler shift sensor can detect interference light in an optimal state. An ultrasonic measurement apparatus using an optical fiber, comprising: a controller.
Note that the high-frequency photodetector is a photodetector having sensitivity in a band of about 1 MHz or more for detecting an ultrasonic signal.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. An ultrasonic measuring apparatus (1) using an optical fiber according to the present invention includes a condenser lens for condensing scattered light generated by a laser beam applied to a measurement target (101) as shown in FIG. . More preferably, the condenser lens is a convex lens.
[0012]
Further, a sensor (20) for detecting Doppler shift of scattered light from the condenser lens is provided behind the condenser lens (10). The Doppler shift sensor (20) is provided with an optical fiber, and the optical fiber (30) is provided with a partial reflection mirror (21) at its tip which receives scattered light from the condenser lens (10). After the partial reflection mirror (21) and one end of the optical fiber (30) are adhered in parallel, the adhered state is maintained by an adhesive such as epoxy bond.
[0013]
In the partial reflection mirror (21), the efficiency of the Doppler shift sensor (20) increases as the reflectance increases, but when the reflectance increases, the amount of scattered light that can enter the optical fiber (30) increases. Therefore, the reflectance of the partial reflection mirror (21) is 99% or less.
[0014]
The optical fiber (30) is integrated with another partial reflection mirror (22) at the rear, and scatters between the partial reflection mirrors (21) and (22) provided at the front and rear ends of the optical fiber (30). Multiple reflection of light is performed.
[0015]
A half-wave plate (51), a polarizing beam splitter (52), and a high-frequency photodetector (53) are sequentially arranged behind the rear end partial reflection mirror (22). Become.
[0016]
According to the present invention, a controller (60) for automatically adjusting the length of the optical fiber (30) optimally so that the high-frequency light detector (53) of the Doppler shift sensor (20) can detect interference light in an optimal state. Installed.
[0017]
The controller (60) includes a low frequency light detector (43) for receiving a part of the light from the polarizing beam splitter (52), and is electrically connected to the low frequency light detector (43). A control circuit (42); The control circuit (42) is electrically connected to a voltage generator (41), and the voltage generator (41) includes a cylindrical piezoelectric element (30) having an optical fiber (30) surrounding its outer peripheral surface. 40) are electrically connected so as to control the power supplied thereto.
Note that the low-frequency photodetector is a photodetector having sensitivity in a band of about 1 KHz to 100 KHz to be used for controlling a cylindrical piezoelectric element.
[0018]
The operation and effect of the ultrasonic measuring device (1) using the optical fiber according to the present invention having such a configuration will be described in detail.
[0019]
In the ultrasonic measuring apparatus (1) using an optical fiber according to the present invention, first, when a worker or an experimenter irradiates a laser beam to a measurement target (101), scattered light is generated in the measurement target (101) and the scattered light is generated. The scattered light is condensed by the condenser lens (10) and is incident on the polarization maintaining optical fiber (30) through the partial reflection mirror (21).
[0020]
The scattered light that has passed through the partial reflection mirror (21) and entered the optical fiber (30) reaches the partial reflection mirror (22) at the other end. The scattered light thus incident is partially reflected by the partial reflection mirror (22) according to a reflectance set in advance by the partial reflection mirror (22), and is reflected toward the partial reflection mirror (21) at the tip of the optical fiber. A part of the reflected light passes through this, passes through it, passes through a half-wave plate (51) and a polarizing beam splitter (52), and is supplied to a photodetector (53). A shift is detected.
[0021]
On the other hand, the scattered light reflected again by the partial reflection mirror (22) toward the partial reflection mirror (21) is reflected by the partial reflection mirror (21) and again toward the partial reflection mirror (22), as described above. Such a process is repeated.
[0022]
The scattered light reflected by the partial reflection mirrors (21) and (22) thus continuously reciprocates inside the optical fiber (30), and at the same time, a part of the scattered light is transferred to the rear partial reflection mirror. After passing through (22), it is detected by the photodetector (53), and an accurate Doppler shift can be measured.
[0023]
Here, in the present invention, the optical fiber (30) forming the above-mentioned sensor (20) can be bent softly, is small regardless of space or location, and has high applicability. The Doppler shift can be accurately measured without being affected by external vibrations.
[0024]
As described above, the measuring apparatus (1) using the optical fiber (30) according to the present invention measures Doppler shift through the optical fiber (30) and the partial reflection mirrors (21) and (22) attached to both ends thereof. However, in order for the operator to always stably measure the Doppler shift, the length of the optical fiber (30) must always be kept in an optimum state.
[0025]
That is, when the length of the optical fiber (30) becomes longer, the measurement efficiency of the photodetector (53) increases in proportion to the length. However, in order to set the photodetector (53) to the maximum measurement efficiency, the optical fiber (53) is required. 30) The length must be finely adjusted.
[0026]
Therefore, in the ultrasonic measurement using the optical fiber (30) of the present invention, the controller (60) that always keeps the length of the optical fiber (30) in an optimum state works. In order to adjust the length of the optical fiber (30), the controller (60) closely surrounds the cylindrical piezoelectric element (40) at a fixed number of times, and By applying a potential difference between the inner wall and the outer wall of the element (40), the outer diameter of the cylindrical piezoelectric element (40) changes, and the optical fiber (30) closely contacting the piezoelectric element (40) and surrounding the same. The length is also finely adjusted.
[0027]
A change in the length of the optical fiber (30) caused when a constant potential difference is applied to the cylindrical piezoelectric element (40) changes depending on the number of turns of the optical fiber (30) surrounded by the piezoelectric element (40). . The method of maintaining the optical fiber 30 in the optimum length by using the cylindrical piezoelectric element 40 will be described as follows.
[0028]
When a voltage that increases linearly with time is applied to the piezoelectric element (40) using a voltage generator (41), the length of the optical fiber (30) wound on the piezoelectric element (40) also increases. Becomes longer in proportion to.
[0029]
FIG. 3 shows the voltage increasing with time and the intensity of the interference light. FIG. 3A shows the voltage increasing with time, and FIG. 3B shows the intensity of the interference light. When the photodetector (53) measures the Doppler shift, the most efficient condition is that when there is no Doppler shift, the intensity of the interference light is 1 / (the intensity of the maximum interference light in FIG. 3). This is a condition corresponding to 2. Therefore, the controller (60) reflects a part of the interference light passing through the partial reflection mirror (22) by the polarization beam splitter (52) and measures it by the low-frequency light detector (43). The control circuit (42) electrically connected to the photodetector (43) is preset so as to always output a value corresponding to half of the maximum interference light intensity as an electric signal. The control circuit (42) controls the voltage supplied to the piezoelectric element (40) by controlling the voltage generator (41) with an output value corresponding to half the intensity of the input maximum interference light.
[0030]
If the interference light reflected from the polarizing beam splitter (52) and input to the low-frequency photodetector (43) is input as a new maximum value larger than the conventional value, the control circuit (42) By controlling the voltage generator (41) with an output value corresponding to half of the intensity of the new maximum interference light, the voltage supplied to the piezoelectric element (40) is newly controlled.
[0031]
Accordingly, when the operator performs an experiment, the length of the optical fiber (30) is adjusted to half the intensity of the maximum interference light by the deformation of the piezoelectric element (40). (53) keeps the optimum detection condition.
[0032]
【The invention's effect】
As described above, according to the ultrasonic measuring apparatus (1) using the optical fiber according to the present invention, the ultrasonic wave to be measured is measured by the photodetector (53) while being small, and is not affected by external vibration. By using an optical fiber (30) that does not require a special optical element or other adjustment work, it is easily applied to a work site in a severe environment. Furthermore, the overall size can be made smaller than before, and at the same time the length of the optical fiber (30) can be increased, so that the measurement efficiency and its sensitivity can be significantly increased, and the precision of the Doppler shift measurement can be greatly improved. There is.
[0033]
Although the preferred embodiments according to the present invention have been described above, those skilled in the art can implement various modifications without departing from the technical spirit of the claims. Obviously, it is included in the present invention.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating a conventional Doppler shift measurement interferometer.
FIG. 2 is a configuration diagram of an ultrasonic measurement device using an optical fiber according to the present invention.
FIG. 3 is a diagram showing a correlation between a voltage time change amount and an interference light signal in an ultrasonic measuring apparatus using an optical fiber according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Ultrasonic measuring device 10 ... Condensing lens 20 ... Sensors 21 and 22 ... Partial reflection mirror 23 ... Adhesive 30 such as epoxy bond ... Optical fiber 40 ... Piezoelectric element 41 ... Voltage generator 42 ... Control circuit 43 ... Low frequency light detection Device 51 Half-wave plate 52 Polarizing beam splitter 53 High-frequency light detector 60 Controller 100 Doppler shift measurement interferometer 101 Measurement object 102 Laser beam 103 Ultrasonic wave 104 Condensing lenses 105 and 106 Spherical surface Mirror 107: photodetector

Claims (4)

In an apparatus for measuring ultrasonic waves using the Doppler shift effect, a condensing lens (10) for condensing scattered light generated from a laser beam applied to a measurement object (101), and the condensing lens ( A partial reflection mirror (21) receiving scattered light from 10) is attached to the tip thereof, and an optical fiber (30) provided with a partial reflection mirror (22) is provided behind the mirror, and interference light passing through the optical fiber (30) is detected. A Doppler shift sensor (20) provided with a high-frequency light detector (53) and the optical fiber (30) so that the high-frequency light detector (53) of the Doppler shift sensor (20) can detect interference light in an optimal state. And a controller (60) provided with a control circuit (42) for changing the shape of the piezoelectric element (40) surrounded on the outside of the ultrasonic wave using an optical fiber. Constant apparatus. The controller (60) includes a low-frequency light detector (43) that receives a part of light from a polarization beam splitter (52) provided in front of a high-frequency light detector (53) of the Doppler shift sensor (20). ), Wherein the control circuit (42) is electrically connected to a voltage generator (41), and the voltage generator (41) includes a cylindrical piezoelectric element (40) around which an optical fiber is surrounded. The ultrasonic measuring apparatus using an optical fiber according to claim 1, wherein the ultrasonic power supply is electrically connected so as to control the power supplied to the optical fiber. 2. The partial reflection mirrors (21), (22) and the end face of the optical fiber (30) are adhered in parallel, and then the adhesive state is maintained by an adhesive (23). Or an ultrasonic measurement device using the optical fiber according to 2. A control circuit (42) electrically connected to the low-frequency photodetector (43) outputs an output corresponding to about half of the intensity of the maximum interference light input from the low-frequency photodetector (43). The optical fiber according to claim 2 or 3, wherein a voltage generator (44) is controlled by a value to control a voltage supplied to the piezoelectric element (40). Ultrasonic measuring device.

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