JPS61130887A - Laser doppler speedometer - Google Patents

Abstract

PURPOSE:To enables the measurement of the speed of an object to be inspected moving at a low speed, by a method wherein a laser light is made incident into an object to be inspected at a specified angle and scattered light therefrom is focused to be splitted into two fluxes, one of which is detected with a photo detector as intact while the other thereof is passed through a polarizer plate to be detected with another photo detector. CONSTITUTION:One photo detector 9' receives scattered lights from the irradiation of linearly polarized laser lights 13 and 13' through a non-polarization beam splitter 14. The linearly polarized scattered lights orthogonal to each other overlap and only the low frequency variation component appears in the output signal of the detector 9' due to the light transmitted through an object 7 to be inspected or changes in the reflectance. The other photo detector 9 receives the scattered lights through a 45 deg. direction polarizer plate 15 via the beam splitter 14. Then, the 45 deg. components of the linearly polarized scattered lights orthogonal to each other are extracted to overlap and as a result, a signal due to a Doppler shift develops together with the low frequency variation component. Them, both signal outputs are inputted into a differential amplifier 16 to obtain a cycle signal of density associated with Doppler shift thereby enabling highly accurate measurement even an low-speed object to be inspected.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field The present invention relates to a velocimeter that non-contactly measures the speed of a moving object or fluid, and particularly to a laser Doppler velocimeter that detects a shift in the frequency of laser light.

(2) Prior Art Conventionally, a laser top-to-speed speed meter has been used as a device for measuring the moving speed of an object or fluid with high accuracy in a non-contact manner. A laser Doppler velocimeter is an effect in which a moving object or fluid is irradiated with a laser beam, and the frequency of the light scattered by the moving object or fluid shifts in proportion to the moving speed (Doppler effect). ) is used to measure the moving speed of the moving object or moving fluid.

An example of a conventional laser Doppler velocimeter is shown in FIG.

1 is a laser, 2 is a collimator lens, 3 is a parallel light beam, 4 is a beam splitter 16 and 6' is a reflection device, 7 is an object or fluid moving in the direction of the arrow at a speed V, 8
9 is a light detector, and 9 is a photodetector.

A laser beam emitted from a laser 1 is made into a parallel beam 3 by a collimator lens 2, split into two binary beams 5 and 5' by a beam splitter 4, and reflected by reflectors 6 and 6', and then An object or fluid 7 moving at V is irradiated with a binary flux at an incident angle θ. Scattered light by the object or fluid 7 is detected by a photodetector via a condenser lens 8t. The frequency of the light scattered by the binary flux undergoes a Doppler shift of +Δf and −Δf in proportion to the moving speed V, respectively. Here, if the wavelength of the laser beam is λ, Δf can be expressed by the following equation (1).

Δf yo V sin θ/λ self...(S+Δf, -Δ
The scattered light beams subjected to the Doppler shift of f interfere with each other and cause a change in brightness on the light receiving surface of the photodetector 9.
The frequency 1 is given by the following equation (2).

y=2Δfw2Vainθ/λ---(2)(2)
From the equation, if the frequency F of the output signal of the photodetector 9 is measured using a spectrum analyzer or the like, the moving speed V of the object or fluid 7 can be determined.

In laser Doppler velocimeters such as the conventional example described above, the moving speed of the object to be measured is slow because the output signal of the photodetector is modulated by a low frequency fluctuation component for the reasons described below (2). When the frequency F of the equation (hereinafter referred to as Doppler frequency) is low, the velocity of the object to be tested cannot be measured.

Generally, when an object is irradiated with highly coherent light such as a laser, the scattered light undergoes random phase modulation due to minute irregularities on the object's surface, forming a so-called speckle pattern on the observation surface. It is known to do.

When using a laser Doppler velocimeter as shown in Figure 1, changes in brightness due to Doppler shift on the detection surface of the photodetector due to the movement of an object or fluid create a speckle pattern. It will be modulated by irregular changes in brightness and darkness due to the flow of water. In addition to this, the output signal of the photodetector is also modulated by changes in the transmittance (or reflectance) of the object under test.

In general, the frequency of changes in brightness and darkness caused by the flow of speckle patterns and the frequency of changes in transmittance of the object being tested are lower frequencies than the Doppler frequency, so the output signal of the photodetector is as shown in Figure 2. is modulated by low frequency fluctuation components. Conventionally, in order to obtain the Doppler frequency from a modulated signal, a method has been used in which the output of a photodetector is passed through a bypass filter to electrically remove low frequency components.

However, when the speed of the object to be tested is slow and the Doppler frequency is low, the frequency difference with the low frequency fluctuation component is small, and the bypass filter cannot be used, resulting in the disadvantage that the speed of the object to be measured cannot be measured.

(3) Summary of the Invention An object of the present invention is to provide a laser Doppler velocimeter that eliminates the drawbacks of the above-mentioned conventional examples and is capable of measuring the speed of a test object moving at low speed with high accuracy. It is in.

The laser Doppler velocimeter according to the present invention is:

A laser as a light source, a jt system that splits the laser light from the laser into two linearly polarized beams perpendicular to each other and makes them incident on the test object at a predetermined angle, and a JT system that focuses the scattered light from the test object. The light beam is split into two light beams, one of which is directly detected by a photodetector, and the other is passed through a polarizing plate and detected by another photodetector.

In the device configured as described above, one detector outputs a signal associated with a Doppler shift modulated by a speckle pattern and a low-frequency fluctuation component caused by a change in transmittance or reflectance of the object to be detected, and the other detector The device outputs only the signal due to the low frequency fluctuation component. By passing the outputs of both signals through a differential amplifier, it is possible to obtain only the periodic signals of brightness and darkness associated with the desired Doppler shift. can be measured.

(4) Embodiment FIG. 3 shows a configuration example of a laser Doppler velocimeter according to the present invention, where 1 is a laser, 2 is a remeter lens, and 6 is a laser doppler velocimeter.
, 6' is a reflecting mirror, 7 is a test object moving in the direction of the arrow at a speed V, 8 is a condenser lens, 9 and 9' are photodetectors, 10
11 is a circularly polarized laser beam, 12 is a polarized beam splitter 115, 13' is a linearly polarized laser beam orthogonal to each other, and 14 is a non-polarized beam splitter 1.
15 is a polarizing plate, 16 is a differential amplifier, 17 is a frequency analyzer, and 18 is a display device. Laser light emitted from the laser 1 passes through the wavelength plate 10 via the collimator lens 2 in a parallel beam. In general, laser light is linearly polarized. % wave plate 10t? By passing through the beam, the beam becomes circularly polarized and becomes the optical laser beam 11. The laser beam 11 is divided into linearly polarized laser beams 15 and 13' having equal intensities and orthogonal to each other by a polarizing beam splitter 12.
divided into The laser beams 13° and 13' each have a velocity V
The object to be detected 7, which is moving at , is irradiated at an incident angle θ. The light that has undergone Doppler shift from the test object 7 enters the non-polarizing beam splitter 14 via the condensing lens 8, a portion of which is reflected and enters the photodetector 9', and the remainder is transmitted. The light then enters the photodetector 9 via the polarizing plate 15. At this time, the orientation of the polarizing plate 15 is set at 45 degrees with respect to the orthogonal linearly polarized laser beams 13 and 13'.

In the above configuration, the photodetector 9' receives the scattered light caused by the irradiation of the mutually orthogonal linearly polarized laser beams 13 and 13' via the non-polarized chi beam sinter 14, and receives the mutually orthogonal linearly polarized scattered light. It becomes a superposition of . Therefore,
No signal due to the Doppler shift appears in the output signal of the photodetector 9', and only low-frequency fluctuation components due to the flow of the speckle pattern and changes in transmittance or reflectance of the object to be detected appear.On the other hand, , the photodetector 9 passes the scattered light through a non-polarized beam sinter 14f;
The light is received through the 1st degree direction polarizing plate, and the 45 degree component of the orthogonal linearly polarized scattered light is extracted and superimposed. Therefore, a signal due to Doppler shift appears together with the If frequency fluctuation component. Here, the differential amplifier 161
If the output signals of the C photodetectors 9' and 9' are input, the fourth
As shown in the figure, only the signal due to the Doppler shift appears, and low-frequency fluctuation components due to the flow of the speckle pattern and changes in the transmittance or reflectance of the object to be examined can be removed.

Next, Figure g5 shows another embodiment of the optical system for irradiating the object to be inspected with laser light. 19 is a non-polarizing beam splitter
120 is a % wavelength plate), and the other numbers are the same as in FIG.

In this embodiment, a laser beam emitted from a laser 1 is converted into a parallel beam by a collimator lens 2), and is split into linearly polarized laser beams having equal intensity and polarization direction by a non-polarizing beam splitter 19. One of the laser beams has its polarization direction changed by 90° through a wavelength plate 20, so that the linearly polarized laser beams 1 have equal intensities and are orthogonal to each other.
3 and 13' are irradiated onto the test object 7 at an incident angle θ.

Hereinafter, the detection method is the same as in the above embodiment.

(5) Effects of the Invention As explained above, the laser Doppler velocimeter according to the present invention is capable of highly accurate velocity measurement using a simple optical means even when the moving speed of the object to be measured is slow. It is a device to

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conventional example of a laser Doppler velocimeter. FIG. 2 is a diagram showing an output signal of a photodetector modulated by a low frequency fluctuation component. 1lf and 13 are diagrams showing an example of the configuration of a laser Doppler velocimeter according to the present invention. FIG. 4 is a diagram showing a signal due to top 2-shift) K. FIG. 5 is a diagram showing another embodiment of the laser beam irradiation optical system. 1...Laser 2...Collimator lens 3...Parallel laser beam 4...Beam splitter 5,5'...Laser beam 6.6'...Reflector 7...Test Object (fluid) 8...Condenser lens 9.9'...Photodetector 10...A wave plate 11...Circularly polarized laser beam 12...Polarized beam splitter 15, 13'...・Linearly polarized laser light 14...Non-polarizing beam splitter 15...Polarizing plate 16 ・Differential amplifier 17...Frequency analyzer 18...Display device 19...Non-polarizing beam splitter 20.・Awakening plate first port M

Claims (1)

[Claims] (1) A means for irradiating linearly polarized laser beams perpendicular to each other onto a moving object or moving fluid from two directions, a means for splitting the scattered light from the moving object or moving fluid into two, and a means for splitting the scattered light from the moving object or moving fluid into two; A laser Doppler velocimeter comprising means for polarizing scattered light and means for detecting the scattered light.