JPS61281971A - Flow rate measuring probe - Google Patents

Abstract

PURPOSE:To make it possible to use a probe outdoors by arranging a semiconductor laser element as a measuring light source in the probe itself to make an optical fiber for light introduction unnecessary. CONSTITUTION:A supply voltage 1 is supplied from an external power source to drive a probe 23 for light transmission. The diffused light emitted from a semiconductor laser element 2 arranged in the probe 23 is converged in parallel by a lens system 3 to form a narrow optical beam. A half mirror 4 on the top face and a mirror 5 on the bottom gace are arranged in the optical path at about 45 deg. to the incident optical beam to convert nthis one narrow optical beam to two optical beams parallel with each other, and these optical beams are condensed by a condensing lens system and are allowed to cross each other at a flow rate measuring point P in a fluid, thus preparing for measurement of the flow rate of the fluid by the Doppler effect. If a photo diode 8 is arranged just after an optical mask 7, the diameter of the pinhole of the mask 7 can be extended within a required flow rate measurement precision and the constitution is simplified.

Description

Detailed Description of the Invention (Technical Field) The present invention relates to a laser doppler velocity sensor that measures the flow velocity of a fluid based on a change in the frequency difference caused by the Doppler effect exerted by the fluid between laser beams irradiating the fluid. Regarding the flow rate measurement probe used in the meter (LDV),
In particular, it is designed to be ultra-compact with a simple configuration and easy assembly and adjustment, and to be able to stably and accurately measure the flow velocity of a fluid close to a measurement point in the fluid.

(Prior art and its problems) IMfill of water flow) In this regard, optical methods have recently been used in place of the old mechanical methods such as propeller current meters and electrical methods such as hot wire current meters. The laser Doppler velocimeter (LDv), which was originally developed as an optical measurement method for such flow velocity, has undergone various improvements from the perspective of hydraulic research. The conventional laser Doppler velocimeter measures the flow velocity optically with precision.
Aiming for completely non-contact flow velocity measurement, there were many problems, and it was inferior to hot wire anemometers in terms of ease of use, and along with the cost issue, this hindered its widespread use.

The present inventor has removed the drawbacks of the conventional laser Doppler perosimeter (LDV) and introduced a laser beam with extremely good coherence into the optical method of measuring fluid flow velocity to enable precise measurement of flow velocity. - By introducing an additional fiber to the Doppler velocimeter, the conventional LD
By making the V-globe much smaller, it became possible to measure the flow velocity near interfaces where the density suddenly changes, which was previously difficult to measure, and to measure the flow velocity of plasma flows that deal with minute fluid spaces. We have previously developed a probe for measuring flow velocity described in 1911$3f12.

However, the above-mentioned flow velocity measuring globe using the original fiber has various problems as follows. (1) Conventionally, a laser beam from a separately placed light source is guided into the probe for measuring the principal amount. In order to inject the laser beam emitted from the light source into the original phino (-), the diffused light emitted from the light source is focused, and the input aperture of the optical phino (-) is accurately positioned at the focused point. ``It
, t-I try to use it as effectively as possible, but since the open end face of the core region of the original fiber is extremely small,
Only a part of the emitted laser light can be injected into the core region, and the amount of light lost is quite large. It becomes difficult to maintain the condition, and it takes a lot of time to restore the correct injection position.

(2) - In order to receive a laser beam having a Doppler frequency component corresponding to the fluid flow velocity, a reference source beam that has passed through the fluid flow velocity measurement point to produce the Doppler effect, and the reference source beam It is necessary to place both the scattered light of the measurement light beam and the scattered light of the intersecting measurement light beam at a distance and receive the light with a multiplier conversion element.To do this, it is necessary to place the aperture end of the core region of the source fiber for light reception at a distance from the scattered light of the reference source beam. It was necessary to match the original axis closely with N. However, since the light-receiving end face of the original fiber is extremely small, as in item (1), the reception of scattered light and the amount of light are small, and there is also the problem of positional shift of the original fiber for light reception. ]
As mentioned above, a gas laser element has conventionally been used as a visible range laser light source to accurately illuminate the original fc fiber required for a flow velocity measurement probe, but gas laser elements are susceptible to vibration and shock. It is not suitable for outdoor use, and even though it requires a high voltage power source, it is not suitable for outdoor use.

(4) As mentioned above, even a slight deviation in the installation position of the original fiber can cause an unmeasurable condition, so it is necessary to set the installation position precisely, and it is necessary to set the installation position outdoors where vibrations and shocks are likely to occur. When using the measurement equipment, great care was required in handling the measurement equipment.

(Objective of the present invention) The object of the present invention is to eliminate the use of original fibers for introducing light, which is the root of the conventional problems mentioned above, and to
An object of the present invention is to provide a flow velocity measuring probe using a laser light source element that can withstand outdoor use.

(Means for solving problems according to the present invention) In order to solve the above-mentioned problems, the current velocity measuring probe of the present invention includes a semiconductor laser element and a laser light beam that is focused by focusing the laser light diverging from the semiconductor laser element. a focusing optical system that splits the laser light beam and refracts it at a substantially right angle to form a pair of parallel laser light beams with different intensities; and a flow velocity measurement of the pair of parallel laser light beams. a condensing optical system that condenses the light at a point l, and a condensing optical system that is located on the axis of the weaker one of the collimated laser beams and passes through at least the flow velocity measurement point, and the source beam is and a photoelectric conversion element that receives light, and is configured such that the flow velocity of the fluid at the flow velocity measurement point l can be measured from the Doppler effect ζ of the fluid flow velocity acting between the collimated laser beams. It is something to do.

That is, first, unlike the gas laser element used in the conventional globe f2 described above, the semiconductor laser element is placed inside the flow velocity measurement probe itself, eliminating the need for a source fiber for light transmission, and transmitting the laser beam into the 5Y : Inject into the fiber! ：lkoyopThe various problems that arose were solved all at once. Next, the focusing optical system is used to form a laser beam by converging the light emitted from the semiconductor laser element and making effective use of all nine diffusing elements, and is composed of, for example, a convex lens, a spherical lens, and a convex lens. , respectively, collimating the diffused source into parallel light, converging the parallel light at a nearby position, and forming the focused beam into a thin source beam, roughly, if necessary, focusing it on the optical axis of the lens system. An optical mask with holes is provided to confine the dimensional beam passing through the lens system to only the pinhole portion. Next, the split refractive optical system is, for example,
an optical glass plate having a smooth surface and a fully reflective bottom surface and placed at an angle of 45° to the original axis of the focusing optics;
Due to the difference between the half mirror formed by the smooth surface and the mirror formed by the fully reflective bottom surface, one laser beam passing through the pinhole is split into two laser beams, and the two laser beams are split at almost right angles. The laser beam is refracted into a parallel laser beam, and then, among the two flat original beams, the half mirror formed by the smooth surface refracts the laser beam, which is reflected and refracted by the mirror formed by the completely reflective bottom surface. The reflected sky-refracted laser light beam is made weaker, for example, with an intensity ratio of about 3, and the laser light beam from the half mirror is used as a reference source, and the laser light beam from the mirror is later scattered. Use as light. Next, the focusing system is composed of, for example, a convex lens, and focuses the two parallel laser beams from the split refractive optical system onto the flow velocity measurement point and causes them to intersect with each other. Next, the photoelectric conversion element is, for example, a photodiode equipped with an optical mask having a pinhole, and the photodiode converts only the reference source and the scattered light within the same solid angle as the reference source through the pinhole. It receives light, converts it into an electrical signal according to the received photoelectric signal, and takes it out as a flow velocity measurement blade.

(Actions of the present invention) The flow rate measuring glove of the present invention configured as described above has the following functions.

(1) By incorporating the semiconductor laser element used as the measurement light source into the LDV glove, the original fiber that was conventionally used for connection to the outside is no longer required, so the source fiber that was generated when injecting the source into the original fiber is no longer required. There is no longer any loss, and the precise adjustment that was required to set the position of the optical 7 eye bar is no longer necessary.

(2) Conventionally, the purpose was to measure two-dimensional flow velocity, so an optical mask with eight pinholes was used to form two laser beams eccentrically perpendicular to the reference source beam. Was. Therefore, the original that missed the pinhole was not used, but the unexploded F! ALDV
In the 7' lobe, we focused on one-dimensional measurement, and the laser beam was divided into two by a half mirror, so the loss of jf and it was significantly reduced.◇(8) The photoelectric conversion element was converted into LD
Due to the strict specifications placed on the light-receiving part of the v-globe, the outer diameter of the probe itself has become as large as approximately 12 nm, making it somewhat inappropriate for measuring the ultrafine structure of fluids. In order to ensure mechanical strength to withstand insertion into fluid, it was housed in an outer casing with an outer diameter of about 18 fins.
There is no performance deterioration in terms of shape and dimensions, and the optical mask for light reception can be used without using the original fiber for light reception, so
(4) As mentioned above, by not using the original fiber, the FF density for the optical system arrangement is eased, and the amount of light received can be increased. Accurate signal detection is possible even if the optical system is misaligned due to vibration or impact, making it easy to measure fluid flow velocity outdoors. This greatly increases the amount of light that can be used directly for measurement, making it easier to detect the measurement result signal, and
This increase in light intensity makes it possible to clearly detect and measure changes in light intensity due to the Doppler effect even in fluids with low scattering particle content, such as fresh water. This is easy because it can be small.

(6) Since the laser light source used has been changed from the conventional gas laser element to a semiconductor laser element, the high voltage power supply conventionally used to drive the gas laser element is no longer required, which can reduce power consumption accordingly. By not using a gas laser element that is susceptible to oxidation, the flow velocity measurement probe can be made suitable for outdoor measurements.

In particular, the flow velocity measuring glove of the present invention is far superior to conventional gloves for outdoor measurements in the following points.

()) Since the LDV probe according to the present invention does not make contact with the flow velocity measurement point in the fluid, it is possible to accurately measure the desired flow velocity even in the wild without substantially disturbing the fluid flow state.

(8) The glove of the present invention can also measure instantaneous flow velocity, which is a fundamental feature of a laser Doppler velocimeter (LDV), and therefore can also measure flow velocity of turbulent flows that occur in rivers and the like.

(9) Since the glove of the present invention basically performs absolute measurement of flow velocity, there is no risk of deterioration of measurement accuracy due to flow conditions or wear of measuring equipment.

(Example) The present invention will be described in detail below with reference to the drawings.

First, the overall configuration and arrangement of a hazardous fluid flow velocity measurement system using the flow velocity measurement glove of the present invention will be described with reference to FIG.

In the illustrated overall configuration 8, among the flow velocity measuring probes of the present invention, the light transmitting probe 28 is integrated with the light receiving probe 24 facing across the fluid 20 to be measured, It is connected to each component of the external measurement system. That is, two laser beams emitted from a laser light source built in a measurement glove supplied with a power supply voltage 22 from a power supply 21 for the laser light source are focused on a measuring point P in the fluid 20, and then, just in case, the two laser beams are Of the laser source beam, the light component incident on the photodiode in the light receiving probe 24, which is located in the straight direction of the reference source beam, is affected by the Doppler effect caused by the flow velocity of the fluid on the mutual interference between the reference source and the scattered light. The electric signal 25t containing the next beat frequency component generated based on the photoelectric conversion output of the incident light component - the amplifier 26
After amplification, only the beat frequency component due to the Doppler effect is extracted using a bandpass filter 2F, and further,
The frequency-voltage converter 286c extracts an alternating voltage signal proportional to the beat frequency of the beat frequency component, and the arithmetic unit 29 calculates required flow velocity measurement data.

Next, the detailed structure 8 and operation of the flow rate measuring glove of the present invention, which constitutes the main part of the above-mentioned fluid flow rate measuring system, will be explained in detail with reference to FIG. Of the current velocity measuring probes of the present invention whose main parts are schematically shown in FIG.
In this case, the diffused light emitted from the semiconductor laser element 2, which is driven by supplying the power supply voltage 1 from an external power supply, is focused in parallel by a lens system 8 configured as described later to form a narrow original beam. For example, a half-mirror with mutually parallel surfaces configured with a mirrored bottom surface of a transparent glass plate and a mirror 5 on the bottom surface are arranged in the optical path at an angle of approximately 45 degrees to the incident light beam. By doing this, it is converted into two parallel light beams that are parallel to each other, and further,
The light is focused by the light focusing lens system 6 and is made to intersect with each other at the flow velocity measurement point P in the fluid 20, so that the light is focused on the fluid 20 by the Doppler effect.
In preparation for flow velocity measurement.

In the light transmitting glove having the basic configuration as described above, the lens system 8 which diffuses and focuses the light from the semiconductor laser element 2 into a laser beam is, for example, as shown in FIG. Convex lens 9, ball lens 108 and convex lens 1
The convex lens 9 reduces the diffusion angle of the diffused light from the light source, the spherical lens 10 focuses the diffused light to the nearest distance, and the convex lens 11 converts the focused light into parallel light. With the lens system 8 having such a configuration, the amount of diffused light from the semiconductor laser element 2 serving as a light source is concentrated into a narrow original beam and extracted.Next, a half mirror is used.
The housing and mirror 5 transmit the above-mentioned single laser beam,
The light beams are converted into two parallel light beams having different amounts of light and are refracted almost at right angles in the direction of travel. The half mirror 4 and the mirror 5 that perform this function are, for example, 8.5 mm thick and have smooth surfaces with appropriate transmittance.
The bottom surface of the glass plate 111 is made into a mirror surface, and the reflectance of the surface is 2.
By combining the transmission and total reflection at the bottom surface, it is possible to appropriately set the original ratio of the two laser beams formed by the half mirror 4 formed by the front surface and the mirror 5 formed by the bottom surface. However, the reflected beam from the half mirror 4 is usually weaker.

Half mirrors configured with their sides parallel to each other as described above
The half mirror 5 is arranged at an angle of 45° with respect to the incident light beam, as shown in FIG. Weak reference beam B□ and much stronger source beam B from mirror 5
Note that the latter light beam B, converts into fluid 2 and 0.
0 is irradiated and scattered, and a part of the scattered light is taken out along with the reference source beam and used to measure the flow velocity by the Doppler effect. In other words, as shown in Figure 4, the original beam of frequency f0 is incident almost perpendicularly to the direction of the flow of a fluid with a flow velocity U, the flow velocity U of the fluid exerts a Doppler effect on the light beam, and the frequency of the light component traveling straight in the direction of incidence E is the same as that of the incident light. Although the frequency remains the same as f1, the frequency of the scattered light that is scattered by the fluid and goes in another direction is another frequency f, which has changed due to the angle between the direction of incidence and the direction of travel.
becomes.

Therefore, as shown in Fig. 5, both frequencies are f1.
A reference beam B1 and a measuring light beam B having a reference source beam B1 and a measuring light beam B.

When it crosses the total pressure and enters the measuring point P in the fluid,
The light component of the reference light beam B0 that travels in the direction of incidence is the component of the reference light B0 of frequency f0 that travels straight in the direction of incidence, and the component of the measurement light beam B2 that deviates from the direction of incidence by an angle θ and travels in the direction of incidence of the reference source beam. , the frequency fgt according to the intersection angle θ'2 and the flow velocity U - the synthesis with the scattered light component having '+ the acid component, which is the frequency f0゜f, the frequency difference f□~f.
, is sufficiently small, this synthesized light component contains a beat frequency component corresponding to the absolute value If, -f, I of the frequency difference, and the flow velocity u of the fluid can be calculated from this bead frequency. This will happen. The principle of fluid flow rate measurement using the Doppler effect is the same as in the prior art, and detailed explanation thereof will be omitted.

Next, as described above, only the combined light component from the flow velocity measurement point P in the straight direction of the reference beam B0 is transmitted through an optical mask 7 provided with an appropriate aperture as shown in FIG. is extracted from this, captured by a photoelectric conversion element 8 (for example, a photodiode), and converted into an electrical signal.

In the conventional light-receiving glove for measuring flow velocity, the above-mentioned combined light component is received by the aperture end face of the optical fiber and guided to an external photomultiplier, but the aperture diameter of the original fiber is 50~ Extremely small at 200 μm and 8 degrees.
Extremely precise positioning is required to accurately capture the reference beam, and please be aware that if the receiving fiber's position shifts even slightly due to vibration or shock, measurement may be impossible. Roughly speaking, depending on the original incident angle, the aperture end face of an optical fiber may reflect the incident light and make it impossible to capture it, so it is necessary to precisely align the original axes of the receiving fiber and the reference beam. It takes one more IJit to align the fiber. In contrast to the configuration of such a conventional light-receiving probe, if five photoelectric conversion elements, such as photodiodes, are directly incorporated as described above and the required light-receiving components are extracted using an optical mask having pinholes, the optical By arranging the photodiode 8 immediately after the mask 7, the diameter of the pinhole in the optical mask 7 can be set sufficiently large within the required flow rate measurement accuracy, which not only simplifies the configuration but also reduces the number of components. Position i! 11 alignment becomes much easier, and there is no possibility of a situation in which flow velocity cannot be measured due to slight positional deviation or deformation due to vibration or thermal expansion.

Next, the performance of the current velocity measurement probe of the present invention will be examined in comparison with a conventional current velocity meter in an indoor waterway and results of actual measurements in an actual river.

The Doppler effect 1 that the fluid flow velocity exerts on the frequency of the incident light, which is the operating principle of the flow velocity measurement probe of the present invention, causes the beat frequency generated in the fluid transmitted light to be the drag-grain shift amount ν.

Since they are equal to 1, the flow velocity U is λ · θ U−/2ns1nTIν, where λ is the wavelength of the incident light, n is the refractive index of the medium, and is the intersecting angle of the incident beam θ.

It is expressed by the following formula.

-1. The hot film velocimeter (HFV), which has been conventionally used to measure fluid flow velocity, has an equilibrium between the amount of heat generated by the current and the amount of heat dissipated by the flow when a thin wire of a friendly metal is spread in a fluid and heated by passing an electric current through it. Flow velocity meters that measure flow velocity based on the change in resistance due to the cooling effect of the flow of a thin metal wire, which changes temperature, can perform point measurements over a wide measurement range from low flow velocities to high flow velocities, and have excellent frequency response. However, it is not suitable for measuring actual rivers because it is not easily soiled, requires a long cable for thermal current, and tends to generate heat and bubbles. It was said that there was no such thing. Indoor waterway ζ
Here, the results of simultaneously measuring flow velocity fluctuations of the same fluid using the flow velocity measuring probe of the present invention and such a hot film anemometer are shown in FIGS. 7(a) and (b), respectively, and the power spectra are shown. The measured results are shown in FIG. 8 (al and Φ). It is clear from Fig. 7 that the two flow velocity fluctuation measurement results are in good agreement, but the power spectra are in good agreement in the low to medium frequency range, but the probe of the present invention has ri 2. The peak in the high frequency region that appears near OHz does not appear prominently in the hot film anemometer, and the probe of the present invention can obtain a sufficient level of electrical output. Since the electrical output is weak, the signal-to-noise ratio deteriorates, causing errors in measurements in the high frequency range, which is recognized as the difference in the following results. From the above comparison results, it is clear that the flow rate measurement glove of the present invention has the required frequency response characteristics required for flow rate measurement in indoor waterways.

- 10,000, compared with the CM-2 type electric current meter, which is also commonly used in the past, that is, a current meter that indicates the power generation output based on the rotation of the flow of a propeller located in the fluid as a flow velocity value. I conducted the measurement results in 8.
Using the glove of the present invention and the 0N-2 type electric current meter, the flow velocity fluctuations of the same fluid were simultaneously measured in the low flow velocity range. The results are shown in Figure 9 ((Rotation) and (1)), respectively. The results of measuring the power spectrum in the flow velocity region are shown in the 10th
The results of measuring the flow velocity fluctuations in the high flow velocity region are shown in Figure 11 (a) and (1).
12(a) and Φ) respectively show the results of measuring the power spectrum in the high flow velocity region.

To compare the respective measurement results, Fig. 9(a),
Although the flow velocity fluctuations in the low flow velocity region shown in (b) are almost the same, the power spectra in the low flow velocity region shown in Figure 10 (al) and (b) have little in common between the two. The attenuation of the latter is particularly remarkable in the high frequency range of 0.2 Hz.
Regarding the flow velocity fluctuations in the high flow velocity region shown in a) and (b), they are almost in agreement, but Fig. 12 (a)
.

■) The power spectrum in the high flow velocity region shown here is the problem.All the peaks that exist in the latter are also present in the former. S does not appear, indicating a sharp decrease in the spectrum in the latter high frequency range. Note that the former, that is, the measurement results using the probe of the present invention, are I
Hz, the level does not decrease to around 1-Hz, as per the theory.
It shows a power spectrum close to a power law.

Based on the above results, it is recognized that the 0N-2 type electric current meter cannot follow short-period fluctuations in flow velocity due to the rotational characteristics of the propeller, etc., whereas the glove of the present invention can track over a wide frequency range. It is clear that accurate flow rate measurements can be made using

Furthermore, according to the results of measuring the flow velocity profile in an actual river by sequentially changing the installation depth of the current velocity measurement probe of the present invention, it was found that the characteristics of the flow field in an actual river could be almost faithfully grasped by the probe of the present invention. It is recognized that this is possible.

In other words, it was difficult for conventional current meters to capture short-period fluctuations in flow velocity, and there were considerable limitations in elucidating the periodic characteristics of river turbulence. For example, we were able to confirm the existence of flow velocity fluctuations ranging from short periods of about 0.4 seconds to long periods of several tens of seconds.

(Effect of the invention) Above #1. As is clear from the description, according to the present invention,
The flow velocity measuring glove, which measures fluid flow velocity based on the Doppler effect using a laser source beam, has a much simpler configuration than conventional ones, making it easy to install each component without the need for positional precision vI41M as in the conventional case. In addition, it can be used to measure fluid flow velocity in indoor waterways and actual rivers, and can be used to measure fluid flow velocity over a wide frequency range, which was previously difficult to measure, and to measure flow velocity fluctuations over a wide periodic range. The special effect of being able to carry out these tasks with great fidelity is obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing a configuration example of the current velocity measuring probe of the present invention, FIG. 2 is a diagram schematically showing a configuration example of the focusing optical system in the light transmission globe, and FIG. The figure is a diagram schematically showing an example of the configuration of the split refractive optical system in the light transmission globe, FIG. 4 is a diagram schematically showing the principle of fluid flow velocity measurement based on the Doppler effect, and FIG. Figure 6 is a diagram schematically showing an aspect of fluid flow velocity measurement based on the Doppler effect using the probe of the present invention. Figures 7 (a) and (to)) are characteristic curve diagrams showing the measurement results of flow velocity fluctuations by the non-invention glove 2 and the conventional hot film anemometer, respectively, and Figures 8 (a), 8 and (b) are the same power Characteristic curve diagram showing the measurement results of spectra, FIG. 9(a)
3 and (b) are characteristic curve diagrams showing the measurement results of low-frequency range dawn velocity fluctuations by the probe of the present invention and the conventional 0M-2 type flowmeter 1, respectively, and Fig. 10 (a) ji5 and (
b) is the same (characteristic curve diagram showing the measurement results of the power spectrum in the low frequency range, respectively, and Figure 11 ((roll) and middle) is the characteristic curve diagram showing the measurement results of the flow velocity fluctuation in the high frequency range, respectively. Curve diagram, 12th
Figures (a) 2 and (bl are characteristic curve diagrams respectively showing the measurement results of the power spectrum in the high frequency range. l...Power supply voltage 2...Semiconductor laser element 3.6...Lens system 4... No-7 mirror 5... Mirror 7... Optical mask 8.
・・Photoelectric conversion element 9, 11 ・・Convex lens 10・
... Ball lens 20 ... Fluid 21 ... Power supply for laser light source 22 ... Power supply voltage 23 ... Glove for light transmission 24 ... Probe for light reception 25 ... Electric signal 26 ... Amplifier 2)...Band filter 28...Frequency-voltage converter 29...Arithmetic unit Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5
I--■ ρ

Claims (1)

[Scope of Claims] 1. A semiconductor laser element, a focusing optical system that focuses laser light diverging from the semiconductor laser element to form a laser light beam, and splits the laser light beam and refracts it at approximately right angles. a split refractive optical system that forms a pair of parallel laser beams with different intensities, a focusing optical system that focuses the pair of parallel laser beams on a flow velocity measurement point, and a focusing optical system that focuses the pair of parallel laser beams on a flow velocity measurement point; a photoelectric conversion element located on the axis of the weaker laser light beam and receiving the laser light beam that has passed through at least the flow velocity measurement point; A probe for measuring flow velocity, characterized in that it is configured to be able to measure the flow velocity of the fluid at the flow velocity measurement point using the Doppler effect. 2. The probe according to claim 1, characterized in that the split refraction optical system is constituted by a mutually parallel half mirror and a mirror, each of which is arranged at an angle with respect to the optical axis of the laser beam. Probe for measuring flow velocity. 3. The probe according to claim 2, wherein the intensity of the laser beam refracted by the half mirror is made weaker than the intensity of the laser beam refracted by the mirror.