JPS61102561A - Polarization separation type laser speedometer - Google Patents

Abstract

PURPOSE:To measure excellently a speed even when a body to be measured and a laser oscillation sources are distant from each other to some extent by providing a polarization beam splitter, quarter-wavelength plate, irradiation lens, and a photodetector which obtains a Doppler component. CONSTITUTION:A laser beam 12 emitted from a laser 11 in split by a partial reflecting mirror 13 into about 100:1 intensity proportion. The intense light beam is passed through the polarization beam splitter 14 which transmits only a polarized light beam to illuminate the body 17 to be measured. At such a time, a scattered beam is generated by particles in the object body 17 and returns as a circular polarized beam 12C, but reflected by the polarization beam splitter 14 without being transmitted to change its optical path b6 90 deg.. This vertical polarized light beam 12D and the other weak reference light 12E split by the partial reflecting mirror 13 are photodetected by a photodetector 20 through a partial reflecting mirror 19, then the difference frequency between the vertical polarized beam 12D and reference light 12E, i.e. Doppler frequency is known. Consequently, even when the speed of the object body is measured at a distant point, intense scattered light is obtained, so an accurate measurement is taken.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a polarization separation type laser velocimeter that measures the velocity of an object to be measured, such as a fluid, by irradiating it with laser light.

[Conventional technology]

FIG. 4 is a block diagram showing the configuration of a conventional two-beam differential backscattering type laser current meter, in which 1 is a laser, 2 is a beam splitter, 3 is a converging lens, 4 is a condensing lens,
4a is a mirror, 5 is a photomultiplier, and 6 is a signal processor. The interference fringe pattern created by the converging lens 3 is shown at 7, the output waveform of the photomultiplier 5 is shown at 8, and the output waveform of the signal processor 6 is shown at 7. 9. Further, X represents an optical device, and Y represents a signal processing device.

Now, the laser light generated from the laser 1 is split into two beams by the beam splitter 2, and these two beams intersect by the converging lens 3, creating interference fringes at the intersection. When particles flow through this intersection, periodic changes in the intensity of the NK scattered light related to the particle velocity occur due to the brightness and darkness of the interference fringes. The frequency at this time is called the Doppler frequency. By detecting the periodic change in the scattered light intensity using the photomultiplier 5 and the Doppler frequency using the photoelectric conversion 1-1 signal processor 6, the particle velocity can be determined. Therefore, when it is desired to know the velocity of a fluid, if particles are mixed into the fluid, the velocity of the fluid can be measured from the flow velocity of the particles based on the above-mentioned principle.

[Problem that the invention seeks to solve]

When measuring the fluid velocity of the fluid to be measured using the conventional two-beam differential backscatter type laser current meter as described above, the laser beam generated from the laser 1 is transmitted to the beam splitter 2.
The two beams are split into two beams by the converging lens 3, and interference fringes are created at the intersection. The measurement limit is about 2 meters between the two electrodes, and if the distance is larger than that, the intensity of the scattered light will drop significantly, making it unsuitable for far-field measurements. Furthermore, since the fluid to be measured is irradiated with two parallel laser beams, there are other problems such as the inability to directly measure the velocity component of the fluid to be measured in the traveling direction of the laser beam.

This invention was made in order to solve the above problems, and it is possible to perform good measurements even if the distance between the object to be measured (including the fluid to be measured) and the laser oscillation source is a certain distance, and the object to be measured can be The object of the present invention is to obtain a polarization separation type laser velocimeter that can also measure the velocity component in the traveling direction of the laser beam among the velocity components of the laser beam.

[Means for solving problems]

A polarization separation type laser velocimeter according to the present invention includes a polarization beam splitter that transmits only a horizontally or vertically polarized laser beam and reflects the vertically or horizontally polarized beam, and converts the horizontally or vertically polarized beam into a circularly polarized beam. A 1/4 wavelength plate that focuses the circularly polarized beam and irradiates it onto the object to be measured, and a scattered beam reflected by the object to be measured □! It consists of a light receiver that receives light through the lens, quarter-wave plate, and polarizing beam splitter, and also receives a laser beam from a laser that does not pass through the polarizing beam splitter.

[Effect]

In this invention, only the horizontal or vertically polarized component of the laser beam is passed through the polarizing beam splitter, and the horizontally or vertically polarized beam is further converted into circularly polarized light by the quarter-wave plate, and the beam is focused by a lens before being measured. The body is irradiated. A lens condenses the scattered beam generated at that time, and a quarter-wave plate converts this scattered beam into a vertically or horizontally polarized beam, which is incident on a polarizing beam splitter, where it is reflected and sent to a light receiver. The laser beam enters the beam, mixes with the original laser beam, and obtains a Doppler frequency component as the difference between the two.

〔Example〕

The first panel is a schematic diagram of the configuration of a polarization separation type speedometer showing an embodiment of the present invention, and 11 is a laser which oscillates a laser beam 12 having a specific polarization component. A partial reflecting mirror 13 divides the laser beam 12 at an intensity ratio of about 100:1. Reference numeral 14 denotes a polarizing beam splitter, which allows a specific polarized component of the strong light that has passed through the partial reflecting mirror 13, such as the horizontally polarized beam 12A, to pass therethrough. Reference numeral 15 denotes a 174-wavelength plate which changes a linearly polarized beam into a circularly polarized beam 12B by a birefringence phenomenon. 16 is a lens, and circularly polarized beam 12
Focus B. Further, 12C is a circularly polarized beam, which is a scattered beam generated by reflection of the circularly polarized beam 12B irradiated onto the object 17 to be measured, which travels in the direction shown by the broken line.
The light passes through the /4 wavelength plate 15 and becomes a vertically polarized beam 12D.

A reflecting mirror 18 reflects the reference light 12E consisting of weak light separated by the partial reflecting mirror 13. 19 is a partially reflective mirror,
A part of the reference beam 12H guided by the reflecting mirror 18 is reflected, and further, the vertically polarized beam 12D reflected by the polarizing beam splitter 14 is passed through and both are guided to the light receiver 20. 21 is the main wind tunnel.

Next, the operation will be explained.

A laser beam 12 having a specific polarization component emitted from a laser 11 is reflected by a partial reflecting mirror 13 at an intensity ratio of 100:
It is divided into about 1 pieces. The stronger light beam passes through a polarizing beam splitter 14 which has a characteristic of transmitting only that polarized light beam, passes through a quarter-wave plate 15 and a lens 16, and is irradiated onto the fluid 17 to be measured. At this time, a scattered beam is generated by the particles in the object to be measured 17, and it becomes a circularly polarized beam 12C and returns along the same path, but the circularly polarized beam 1
Since the beam 2C is a vertically polarized beam 12D whose polarization plane differs by 90 degrees from the quarter-wave plate 15, it is not transmitted through the polarization beam splitter 14 but is reflected and its optical path is bent by 90 degrees. This vertically polarized beam 12D and a weak reference beam 12E split by the partial reflection mirror 13 are sent to the light receiver 2 via the partial reflection mirror 19.
0, vertically polarized beam 120 and reference beam 12E
and the difference frequency, that is, the Doppler frequency. In this way, by determining the flow velocity of the object to be measured 17 from the circularly polarized beam 12C emitted from the object to be measured 17, the velocity of the object to be measured 17 can be obtained. Note that the object to be measured 17
It is not limited to fluids.

Figure 2 is an example of measuring the flow velocity in a wind tunnel using the polarization separation type laser velocimeter shown in Figure 1, where the vertical axis represents the fluid velocity (m/s) measured by the present invention, and the horizontal axis represents the fluid velocity (m/s) measured by the Pitot tube. It represents speed (m/s). * indicates the measurement point, and the solid line is the trace of the measured value. As can be seen from this figure, the polarization separation type laser velocimeter according to the present invention can be considered to be in complete agreement with measurement using a Pitot tube.

FIG. 3 is a schematic diagram explaining the invention in detail;
The same numbers as in the figure indicate the same parts, but to explain again, 1
2 is a laser beam, 14 is a polarizing beam splitter, and a birefringent crystal with a right triangular cross section is sandwiched between 2
15 is a 1/4 wavelength plate, which has a thickness that provides an optical path difference of 1/4 wavelength of the laser beam 12 used. It is a precision optical element that has the property of converting linear (horizontal or vertical) polarized light into circular (right-handed or left-handed) polarized light through the phenomenon of birefringence. 12B is the 174 wavelength plate 15
A circularly polarized beam 17 is polarized to the right by a scattering particle 17 in the object to be measured. 12C is a circularly polarized beam that is scattered by the object to be measured 17 and polarized to the left, and 12D is a vertically polarized beam generated when the circularly polarized beam 12C passes through the quarter-wave plate 15.

Next, the operation will be explained.

Since the incoming laser beam 12 is horizontally polarized, it passes through the polarizing beam splitter 14 and becomes a circularly polarized beam 12B by the quarter-wave plate 15, which strikes the object 17 to be measured. At this time, the circularly polarized beam 12B is scattered and becomes a circularly polarized beam 12C with a 180° phase shift, which passes through the quarter-wave plate 15 and becomes a vertically polarized beam 12D.

〔Effect of the invention〕

As explained above, the present invention includes a polarizing beam splitter that transmits only a horizontally or vertically polarized laser beam and reflects the vertically or horizontally polarized beam, and a l/4 polarized beam that converts the horizontally or vertically polarized beam into a circularly polarized beam. A wavelength plate, a lens that condenses the circularly polarized beam and irradiates it onto the object to be measured, and a lens that collects the scattered beam reflected by the object to be measured.
In addition to receiving light through an IU long plate and a polarizing beam splitter, it is also equipped with a receiver that receives a laser beam from a laser that does not pass through the polarizing beam splitter, so it is robust even when measuring the speed of a measured object in a far field. Accurate measurements can be made because scattered light is obtained. Therefore, it has the advantage of being able to perform measurements over a wide area, such as measuring outdoor airflow or measuring flow velocity in a large wind tunnel. Furthermore, it can also be used to measure the velocity of objects other than fluids.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the configuration of a polarization separation type laser velocimeter showing an embodiment of the present invention, FIG. The figure is a detailed schematic diagram explaining the present invention, and FIG. 4 is a block diagram showing the configuration of a conventional two-beam differential backscattering type laser velocimeter. In the figure, 11 is a laser, 12 is a laser beam, 12A is a horizontally polarized beam, 12B and 12C are circularly polarized beams, and 12D
is a vertically polarized beam, 12E is a reference beam, 13 is a partial reflecting mirror, 14 is a polarizing beam splitter, 15 is a quarter-wave plate,
16 is a lens, 17 is a constant fluid,] 8 is a reflecting mirror, 19 is a partial reflecting mirror, and 20 is a light receiver. Patent Applicant Takeshi, Director of Aerospace Technology Research Institute 1) Shun Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] In a laser velocimeter that measures the speed by irradiating a laser beam from a laser onto a measured object and receiving the scattered beam, only the horizontally or vertically polarized beam of the laser beam passes through, and the vertically or horizontally polarized component is reflected. A polarizing beam splitter that converts the horizontally or vertically polarized beam that has passed through this polarizing beam splitter into a circularly polarized beam, and a quarter-wave plate that converts the horizontally or vertically polarized beam that has passed through this quarter-wave plate into a circularly polarized beam. A lens that irradiates the object to be measured, and a scattered beam reflected by the object to be measured is received via the lens, a quarter-wave plate, and a polarizing beam splitter, and the beam is emitted from the laser and passes through the polarizing beam splitter. What is claimed is: 1. A polarization separation type laser velocimeter, comprising: a light receiver that receives a laser beam that does not generate light and obtains a Doppler component.