JP4963587B2 - Laser Doppler vibrometer - Google Patents

Description

  The present invention relates to a laser measuring apparatus that performs measurement using laser light, such as a laser Doppler vibrometer.

A laser Doppler vibrometer that measures the vibration and displacement of a measurement object using a Doppler shift generated by the Doppler effect on the laser light reflected by the measurement object is known as a laser measurement apparatus that performs measurement using laser light. (For example, Patent Document 1).
JP-A-7-120304

  In general, in a laser Doppler vibrometer, laser light is condensed using a condensing optical system so as to irradiate a minute spot on the measurement target surface of the measurement target in order to increase spatial resolution. Measure in the state. In addition, the focal length of the condensing optical system is configured to be variable, and the focal length of the condensing optical system is changed according to the distance to the measurement target surface, so that the laser can be used regardless of the distance to the measurement target surface. It was common to allow light to be collected well.

However, when such a focal length is changed, the amount of reflected light that can be observed with the laser Doppler vibrometer may change due to a change in the condensing angle accompanying the change.
Further, for example, when the focal length becomes small as shown in FIG. 3a, the change in the diameter of the spot irradiated on the measurement target surface 300 with respect to the displacement in the optical axis direction is changed compared to the case where the focal length is large as shown in FIG. 3b. Since it becomes large, it becomes difficult to set this spot diameter within a desired range or to maintain it during measurement.

These are disadvantageous for stably maintaining the measurement accuracy of the laser Doppler vibrometer stably.
Therefore, the present invention provides a laser Doppler vibrometer that can perform measurement with high accuracy without changing the focal length of the condensing optical system regardless of the distance from the laser Doppler vibrometer to the measurement target surface. The issue is to provide.

  In order to achieve the above object, the present invention provides a laser Doppler vibrometer that measures vibration of a measurement target surface, a laser light source that emits laser light, a condensing optical system having a fixed focal length, and the laser light source. The laser beam emitted from the laser beam is branched into a first laser beam and a second laser beam, and an interference optical system that emits the first laser beam to the condensing optical system, and the condensing optical system in the optical axis direction. Based on the distance to the measurement target surface measured by the distance measurement unit prior to the start of the measurement of the vibration of the measurement target surface, the moving mechanism that moves, the distance measurement unit that measures the distance to the measurement target surface Thus, the moving mechanism moves the condensing optical system in the optical axis direction so that the distance between the condensing optical system and the measurement target surface of the measurement object matches the focal length of the condensing optical system. It consists of a movement control unit. However, the condensing optical system condenses the first laser light incident from the interference optical system on the measurement target surface and introduces the reflected light reflected by the measurement target surface into the interference optical system. The interference optical system shifts the frequency of the second laser light by a predetermined amount, and also interferes with the second laser light whose frequency is shifted and the reflected light introduced from the condensing optical system. The generated interference light is generated.

  According to such a laser Doppler vibrometer, by moving the condensing optical system in the optical axis direction, regardless of the distance from the laser Doppler vibrometer to the measurement target surface, The laser beam can be favorably condensed on the measurement target surface without changing the distance between the two, and without changing the focal length of the condensing optical system and hence the condensing angle. Therefore, according to the present invention, measurement can be performed with high accuracy without changing the focal length of the condensing optical system regardless of the distance from the laser Doppler vibrometer to the measurement target surface.

  As described above, according to the present invention, measurement can be performed with high accuracy without changing the focal length of the focusing optical system regardless of the distance from the laser Doppler vibrometer to the measurement target surface. A laser Doppler vibrometer can be provided.

Hereinafter, an embodiment of the present invention will be described taking application to a laser Doppler vibrometer as an example.
First, FIG. 1 shows a basic configuration of a laser Doppler vibrometer according to the present embodiment.
As illustrated, the laser Doppler vibrometer includes a laser light source 1, an interference optical system 2, a condensing optical system 3, a reference signal generation unit 4, a photoelectric converter 5, a signal processing unit 6, a moving stage 7, a distance sensor 8, And a movement control unit 9.
The interference optical system 2 includes a first beam splitter 21, a second beam splitter 22, an acoustooptic device 23, a mirror 24, and a third beam splitter 25.
Here, such a laser Doppler vibrometer uses the surface of the measurement object 100 as a measurement object surface, and measures the vibration and transition of the measurement object 100 while rotating the measurement object 100, for example.
In such a configuration, the laser beam having the frequency f0 emitted from the laser light source 1 enters the interference optical system 2 and is divided into two by the first beam splitter 21, and one of the two divided beams is an acoustooptic device. 23, the frequency is shifted by the frequency fM using the reference signal of the frequency fM generated by the reference signal generator 4, and enters the third beam splitter 25 through the mirror 24 as a reference beam of the frequency f0 + fM.

  On the other hand, the other beam divided into two by the first beam splitter 21 is emitted to the condensing optical system 3 having a fixed focal length via the second beam splitter 22, and the object 100 to be measured is collected by the condensing optical system 3. It is condensed on the measurement target surface. Then, the reflected light reflected from the surface of the measurement object 100 and incident on the condensing optical system 3 is introduced into the interference optical system 2 by the condensing optical system 3, and the third beam splitter 25 is passed through the second beam splitter 22. Is incident on.

The third beam splitter 25 combines the incident reference beam and the reflected light in this way, and outputs them to the photoelectric converter 5.
Here, a Doppler shift fD corresponding to the vibration speed of the measurement target surface of the measurement target 100 is generated in the frequency of the reflected light by the measurement target 100, and the frequency of the reflected light is f0 + fD. Therefore, in the photoelectric converter 5, a beat signal of fM ± fD due to the interference between the reference beam and the reflected light is observed in the electric signal obtained by photoelectrically converting the incident light from the third beam splitter 25. Therefore, the signal processing unit 6 performs FM demodulation on the input beat signal at the frequency fM of the reference signal, thereby obtaining a speed signal V corresponding to the vibration of the measurement target surface of the measurement object 100. By analyzing, the displacement of the measurement object surface of the measurement object 100 is obtained. Alternatively, in the signal processing unit 6, the displacement of the measurement target surface of the measurement target 100 can be obtained from the phase difference between the reference signal and the beat signal generated by the difference between the optical path lengths of the reflected light and the reference beam.

  On the other hand, prior to the measurement as described above, the movement control unit 9 uses the distance sensor 8 to measure the distance to the measurement target surface of the measurement object 100 and is obtained from the distance measured by the distance sensor 8. The converging optical system 3 is moved in the optical axis direction using the moving stage 7 so that the distance between the condensing optical system 3 and the measurement target surface of the measuring object 100 coincides with the focal length of the condensing optical system 3. Move to. Here, the moving stage 7 can be configured using a holder that holds the condensing optical system 3, a step motor, and a ball screw mechanism that is driven by the step motor and moves the holder in the optical axis direction.

  Further, the distance sensor 8 is disposed between the interference optical system 2 and the condensing optical system 3, for example, as shown in FIG. 2a, and splits the reflected light output from the condensing optical system 3. 81, the optical path branching device 81, and the condensing optical system 3 can be used to configure a camera 82 that photographs the measurement target surface 300 of the measurement target 100. In this case, the movement control unit 9 determines the amount of movement of the condensing optical system 3 by the phase stage so that the spot diameter of the laser beam formed on the measurement target surface 300 photographed by the camera 82 becomes a desired size. By controlling the moving direction, the condensing optical system 3 is placed on the optical axis so that the distance between the condensing optical system 3 and the measurement target surface of the measuring object 100 coincides with the focal length of the condensing optical system 3. Move in the direction. However, the distance sensor 8 may be any type of sensor as long as it can measure the distance to the measurement target surface.

  Now, according to such a laser Doppler vibrometer, as shown in FIGS. 2A and 2B, by moving the condensing optical system 3 in the optical axis direction, the focal length F of the condensing optical system 3 and thus the converging angle. Without changing the distance, the distance between the condensing optical system 3 and the measurement target surface 300 is kept constant regardless of the distance from the laser Doppler vibrometer to the measurement target surface, and the laser light is excellent on the measurement target surface. Can be condensed.

  Therefore, according to the present embodiment, measurement can be performed with high accuracy without changing the focal length of the condensing optical system 3 regardless of the distance from the laser Doppler vibrometer to the measurement target surface.

It is a block diagram which shows the structure of the laser Doppler vibrometer which concerns on embodiment of this invention. It is a figure which shows the appearance of the focus movement of the laser Doppler vibrometer which concerns on embodiment of this invention. It is a figure which shows the appearance of the focus movement of the conventional laser Doppler vibrometer.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Laser light source, 2 ... Interference optical system, 3 ... Condensing optical system, 4 ... Reference signal production | generation part, 5 ... Photoelectric converter, 6 ... Signal processing part, 7 ... Moving stage, 8 ... Distance sensor, 9 ... Movement Control unit, 21 ... first beam splitter, 22 ... second beam splitter, 23 ... acoustic optical element, 24 ... mirror, 25 ... third beam splitter, 81 ... optical path splitter, 82 ... camera, 100 ... measurement object, 300 ... surface to be measured.

Claims (1)

A laser Doppler vibrometer that measures the vibration of the surface to be measured,
A laser light source for emitting laser light;
A focusing optical system with a fixed focal length;
An interference optical system for branching the laser light emitted from the laser light source into a first laser light and a second laser light, and emitting the first laser light to the condensing optical system;
A moving mechanism for moving the condensing optical system in the optical axis direction;
A camera for photographing the measurement target surface;
A movement control unit for moving the condensing optical system in the optical axis direction in the moving mechanism;
The condensing optical system condenses the first laser light incident from the interference optical system on the measurement target surface, and introduces reflected light reflected by the measurement target surface into the interference optical system,
The interference optical system shifts the frequency of the second laser light by a predetermined amount, and interferes with the second laser light whose frequency is shifted and the reflected light introduced from the condensing optical system. Produce light,
Prior to the start of measurement of vibration of the measurement target surface, the movement control unit is condensed by the condensing optical system on the measurement target surface photographed by the camera based on the output of the camera. A laser Doppler vibrometer, wherein the converging optical system is moved in the optical axis direction by the moving mechanism so that a spot diameter of the first laser beam becomes a predetermined size.