JP3440682B2 - Optical fiber laser Doppler velocimeter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber laser Doppler velocimeter, and more specifically, it utilizes a light wave having different optical frequencies to generate a flow velocity component perpendicular to the optical axis and a flow velocity component parallel to the optical axis. The present invention relates to an optical fiber laser Doppler velocimeter that measures simultaneously.

[0002]

2. Description of the Related Art A conventional two-beam differential optical fiber laser Doppler velocimeter is shown in FIG. This fiber optic laser Doppler velocimeter has a Selfoc lens (trade name) at one end.
Polarization-maintaining optical fiber 12 in which 10A and 10B are combined
A and 12B, and a single mode optical fiber 16 having a ball lens 14 coupled to one end thereof. A condenser lens 18 is arranged on the light emission side of the SELFOC lenses 10A and 10B.

According to this optical fiber laser Doppler _velocimeter , the SELFOC lens 10 is provided with light of frequencies f ₀ and f ₀ + f _saw via the polarization maintaining optical fibers 12A and 12B.
When emitted as parallel light from A and 10B, the condenser lens 1
The light is collected, that is, intersected by the light collecting unit by 8.

When the scattering particles pass through the light collecting portion, the passing scattering particles scatter the irradiated light in all directions. A part of the backscattered light from the passing scattering particles is again collected by the condenser lens 1.
8 is collimated into parallel light, is incident on the photodetector 20 via the ball lens 14 and the single mode optical fiber 16, and is subjected to optical heterodyne detection. Then, only the flow velocity component perpendicular to the optical axis is obtained based on this optical heterodyne detection.

With this optical fiber laser Doppler velocimeter,
In order to measure the flow velocity accurately, it is necessary to make the optical axis orthogonal to the flow velocity, and at that time an optical axis shift occurs and a measurement error occurs.
There is a problem.

In order to solve the above problem, Japanese Patent Laid-Open No. 2-136777 uses two photodetectors, and the component perpendicular to the optical axis is a two-beam differential optical fiber laser Doppler flow velocity. It is possible to measure with one photodetector using the principle of the meter, and the component parallel to the optical axis can be measured with the other photodetector using the principle of the reference light fiber optic laser Doppler velocimeter. It is disclosed.

[0007]

A plurality of photodetectors are used in the conventional optical fiber laser Doppler velocimeter for detecting two components, a component perpendicular to the optical axis and a component parallel to the optical axis. Therefore, there is a problem that a complicated optical system is required.

Further, the conventional optical fiber laser Doppler velocimeter for detecting the two components of the flow velocity is a system in which the principles of the differential type and the reference beam type are combined by using two lights of different frequencies, and Since the SN ratio of the dynamic type is lower than that of the reference light type when the signal level is low, there is a problem in that the SN ratio of the conventional flowmeter is not sufficient.

The present invention has been made to solve the above-mentioned problems, and realizes a high SN ratio by measuring both two components of the flow velocity by the principle of the reference light type, and at the same time, the component perpendicular to the optical axis and the optical axis. It is an object of the present invention to provide an optical fiber laser Doppler velocimeter capable of simultaneously measuring a component parallel to and preventing occurrence of a measurement error due to an axis deviation from a measurement target.

[0010]

In order to achieve the above object, an optical fiber laser Doppler velocimeter of the present invention includes an optical frequency shifter for generating and emitting three lights having different frequencies from the light emitted from a light source. , First, second, and third optical fibers for propagating each of the three lights,
Condensing device for condensing the light propagated by the first and second optical fibers to the measurement region, an incident device on which scattered light by the light condensed on the measurement region is incident, and an incident device A detector that superimposes the scattered light incident on the optical path and the light propagated by the third optical fiber to detect the optical heterodyne; and a calculating device that calculates the flow velocity based on the optical heterodyne detection of the detector. It is composed of.

According to the present invention, each of the three lights emitted from the optical frequency shifter and having different frequencies is divided into the first, second, and
And the light propagated through the three third optical fibers and propagated by the first and second two optical fibers are collected in the measured region.

The scattered light due to the light collected in the measured region is incident on the incident device, and the scattered light incident on the incident device and the light propagated by the third optical fiber are superposed to detect the optical heterodyne. To be done.

Here, the frequencies of the three lights are set to f _a > f _b >
When f _c is set, the scattered light due to the light focused on the measurement region undergoes a Doppler frequency shift and its frequency changes. In addition, this Doppler frequency shift includes the xy2 component of the flow velocity.

[0014] by superimposing the Doppler frequency shift does not receive the scattered light and the Doppler frequency shift that has received the light detecting optical heterodyne, and interference by the difference frequency (e.g., f _a -f _b or f _b -f _c) A frequency including two components of the flow velocity (for example, x component and y component) is obtained. Therefore, based on the optical heterodyne detection of the detector,
The components can be calculated.

In the present invention, both components of the flow velocity can be measured by the reference light type principle, so that a high SN ratio can be realized.

Further, unlike the conventional laser Doppler velocimeter for measuring one component, it is possible to simultaneously measure a component perpendicular to the optical axis and a component parallel to the optical axis, so that an axis deviation from the measurement target is obtained. It is possible to prevent the occurrence of measurement error due to.

Further, in the invention described in claim 2, the optical frequency shifter is coupled to a single channel optical waveguide, an electrode for generating a surface acoustic wave, and one end of the channel optical waveguide, and is propagated. Light wave is formed so as to be incident substantially parallel to the wavefront of the surface acoustic wave, and the ± 1st-order diffracted light and the non-diffracted light having substantially equal intensities diffracted by the interaction with the surface acoustic wave are propagated respectively. A three-branch optical waveguide including an optical waveguide branched into three;
And a channel optical waveguide for emission light coupled to the other end of each of the branched optical waveguides. At this time,
The optical frequency shifter uses an acousto-optic medium, and 0.1 <2π
Lλ _O / n ₁ Λ ² cos θ _i <10 and 0 <πΔn ₁ L
The electrode and the three-branch optical waveguide can be configured so that the condition of / λ ₀ <2 is satisfied.

Next, the principle of the optical frequency shifter configured as described above will be described. As shown in FIG. 2, three-dimensional xyz coordinates are determined, and for a surface acoustic wave (SAW) propagating at a wavelength Λ, a wave number K _saw , and an angular frequency Ω _s , a wavelength λ _{O in} vacuum,
A light wave having a wave number k _O and an electric field strength E _O (O) is made incident on the wavefront of the SAW at an angle θ _i , and the light wave and the SA with the interaction length L
When a light wave is diffracted by SAW by interacting with W, the electric field intensity E (r, t) of the diffracted light which is the emitted light is expressed by the superposition of the m-th order diffracted light as shown in the equation (1). Be done.

[0019]

[Equation 1]

Where r is the coordinate in the traveling direction of the SAW, t
Is time, m is the order of diffracted light, E _m (z) is the electric field strength in the z-axis direction of m-th order diffracted light, and ω _O is the angular frequency of incident light in a vacuum.

The electric field intensity E _m (z) of the m-th order diffracted light in the equation (1) is expressed by the following (2) which expresses the coupling of a large number of light waves.
It is obtained by solving the differential equation of the equation.

[0022]

[Equation 2]

Where α is (n ₁ Λ sin θ _i ) /
λ _O and Q are the above Klein · Cook parameters represented by 2πLλ _O / n ₁ Λ ² cos θ _i , and Δn ₁ is the SAW.
Of the refractive index n ₁ of the acousto-optic medium induced by E,
_{m + 1} (z) and E _m-1 (z) are the electric field strengths in the z-axis direction of the diffracted light of the m + 1st order and the m−1th order, respectively.

The refractive index n (r, t) of the acousto-optic medium at the point (r, t) is expressed as follows. n (r, t) = n 1 + Δn 1 sin (Ω s t-K saw · r) ··· (3) where, as the light wave is propagated is substantially parallel to incident on SAW wavefront, 0.1 <Q <10 (preferably,
1 ≦ Q ≦ 2) and 0 <πΔn ₁ L / λ ₀ <2 (preferably πΔn ₁ L / λ ₀ ≈1), the high-order diffracted light of ± 2nd order or more is sufficiently small, There is a solution in which the electric field intensity of the non-diffracted light and the electric field intensity of the ± 1st-order diffracted light expressed by the equation (1) become substantially equal, that is, the electric field intensity becomes 1: 1: 1. The wavelength of the light wave, the wavelength of the SAW, the light wave and the SA
There are countless combinations of solutions depending on the interaction length L with W and the strength of SAW, but any combination may be used as long as it satisfies the relationships of the above formulas (1) and (2).

The range of the parameter Q is set to 0.1 as described above.
In the range of <Q <10, an intermediate diffraction phenomenon between the Bragg diffraction (Q >> 1) and the Raman-Nass diffraction (Q << 1) can be obtained, which is an intermediate between the Raman-Nass diffraction and the Bragg diffraction. If the light is diffracted in a typical region, the SA with a frequency lower than the SAW frequency used when diffracting under the Bragg condition
It is possible to diffract using W, and thus S
By lowering the AW frequency, the configuration of the signal processing system can be simplified when the optical frequency shifter is used for optical measurement or the like. Further, in the optical frequency shifter, the frequency of the incident light is shifted by the SAW frequency due to diffraction,
By shifting the frequency by the intermediate diffraction between the Bragg diffraction and the Raman-Nass diffraction as described above, it is possible to obtain a plurality of lights having a small frequency difference.

Further, from the above equation expressing the parameter Q,
Since the interaction length L is proportional to the parameter Q, the interaction length L can be made smaller than in the case of Bragg diffraction. This interaction length L is S of the electrode that generates the SAW.
Since it corresponds to the width in the direction orthogonal to the AW traveling direction, the electrode can be made smaller by reducing the interaction length L, and as a result, the optical frequency shifter can be made smaller.

In the above-mentioned three-branch optical waveguide, the propagating light wave is incident substantially parallel to the SAW wavefront, that is, the angle θ _i formed between the propagating light wave and the SAW wavefront.
Since it is formed so as to be 10, α≈0 in the above equation (2). Therefore, for example, Q = 1, πΔn ₁ L
When / λ ₀ = 1 the above equation (2) is as follows.

[0028]

[Equation 3]

Since the above-mentioned optical frequency shifter uses both the non-diffracted light and the diffracted light when it is actually used in the optical heterodyne measurement method or the like, the optical frequency shifter is designed so that the diffraction efficiency is 100%. It is not necessary, and the output of the optical waveguide branched into three of the three branched optical waveguides should be 33% at most.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the optical fiber laser Doppler velocimeter of the present invention will be described in detail below with reference to the drawings.

As shown in FIG. 3, this embodiment has an optical frequency shifter 30 formed on a substrate 32 of LiNbO ₃ having an acousto-optic effect.

As shown in FIGS. 1 and 3, the optical frequency shifter 30 includes TiN on the surface layer of a substrate 32 of LiNbO _3.
A single-channel optical waveguide 34, a multi-mode optical waveguide 41A, and a three-branch channel optical waveguide 38 including branch multi-mode optical waveguides 41B and 41C, and an outgoing light channel optical waveguide 4
It is equipped with 0A, 40B and 40C. Note that these optical waveguides can be formed by a method such as proton exchange instead of thermal diffusion of Ti.

The channel optical waveguide 34 is composed of a single mode optical waveguide having a width Wo capable of propagating a single mode light wave (guided light), and one end P0 thereof is a semiconductor laser or the like for irradiating coherent light. It is located on one side of the substrate 32 so that the light of the frequency f _opt from the constructed coherent light source 42 is incident.

At the other end of the channel optical waveguide 34, a tapered optical waveguide 36D having a width gradually expanded from Wo to W at a constant inclination angle from the narrow side to the wide side is formed.
Are combined.

A multimode optical waveguide 41A having a width W and a predetermined length capable of propagating multimode light waves is coupled to the wide side of the tapered optical waveguide 36D. To the other end of the multimode optical waveguide 41A, a tapered optical waveguide 36A whose width is gradually reduced from W to Wo at a constant inclination angle from the wide side to the narrow side is coupled. One end of an outgoing light channel optical waveguide 40A is coupled to the narrow side of the tapered optical waveguide 36A.

In order to effectively obtain the non-diffracted light and the ± 1st-order diffracted light, from the substantially central portion of the multimode optical waveguide 41A, θ _m (≈λ) with respect to the multimode optical waveguide 41A.
_O / n ₁ Λ), two branch multimode optical waveguides 41B and 41C having a width W branch symmetrically with respect to the multimode optical waveguide 41A.

The branched multimode optical waveguide 41B has a width Wo capable of propagating a single mode light wave through the tapered optical waveguide 36B having the same shape as the tapered optical waveguide 36A, and a channel light for emitting light. Waveguide 40
A channel optical waveguide 40B for outgoing light, which is bent so as to be parallel to A, is coupled to form a branched multimode optical waveguide 41.
C has a width Wo capable of propagating a single-mode light wave through the tapered optical waveguide 36C having the same shape as the tapered optical waveguide 36A, and is parallel to the channel optical waveguide 40A for emission light. The bent outgoing light channel optical waveguide 40C is coupled.

In the vicinity of the branch portion of the three-branch channel optical waveguide 38, an IDT 44 for generating a SAW having a frequency f _saw and an interaction length L with a light wave propagating in the optical waveguide is formed. An oscillator 46 that generates a high frequency is connected to the IDT 44.

Here, the reason why the width of the three-branch channel optical waveguide 38 is widened to W is to prevent the occurrence of diffraction broadening and the excitation of multimode light waves. Further, since the channel optical waveguide and the three-branch channel optical waveguide are coupled by the tapered optical waveguide, the tapered optical waveguide portion also acts as an interaction region between the SAW and the light wave, so that the three-branched channel optical waveguide is lengthened. The interaction length L can be increased without doing so.

One ends of the polarization-maintaining optical fibers 48A, 48B and 48C are coupled to one ends of the outgoing light channel optical waveguides 40A, 40B and 40C of the optical frequency shifter 30, respectively. The other ends of the polarization maintaining optical fibers 48B and 48C are composed of gradient index lenses such as SELFOC lenses, and irradiation measurement probes 50B and 50C that emit the incident light as parallel light are coupled to the other ends. Has been done.

A condenser lens 52 is disposed on the light wave emission side of the irradiation measurement probes 50B and 50C, and the light emitted from the irradiation measurement probes 50B and 50C is condensed on a light condensing portion (second focal point). To be done. As a result, the light emitted from the irradiation measurement probes 50B and 50C intersect at the angle 2φ at the converging portion. The irradiation measurement probe 50B,
50C and the condensing lens 52 act as a condensing device that condenses light on the measured region.

At the first focal position of the condenser lens 52, a light receiving measurement probe 54 which is a ball lens and acts as an entrance device is arranged. One end of a single mode optical fiber 56 is coupled to the light receiving measurement probe 54.

The other end of the polarization maintaining optical fiber 48A and the other end of the single mode optical fiber 56 are connected to the photodetector 58 so as to irradiate the photoelectric conversion surface of the photodetector 58 for heterodyne detection on the photoelectric conversion surface. Are combined. Photo detector 5
Reference numeral 8 is connected to a Doppler frequency analyzer 60 which is a calculation device equipped with a computer for calculating the flow velocity from the signal photoelectrically converted by the photodetector 58.

Next, the operation of this embodiment will be described.
It In FIG. 3, one end P0 of the channel optical waveguide 34
To frequency f_optIncident light wave, and high frequency to the IDT44
And the frequency f_sawInteract with the SAW
If this is done, an intermediate value between Bragg diffraction and Raman-Nass diffraction
Due to the diffraction phenomenon, the SAW frequency f_sawShifted by ±
First-order diffracted light is obtained. ± 1st order diffracted light is branched multi-mode
The non-diffracted light propagates through each of the optical waveguides 41B and 41C.
It propagates through the multimode optical waveguide 41A. As a result,
Zero-order light from the other end P1 of the light output channel optical waveguide 40A
Frequency f which is _optNon-diffracted light of
From the other ends P1 and P2 of the channel optical waveguides 40B and 40C
Frequency f_opt+ F_saw, F_opt-F_sawDiffracted light
Is ejected.

The ± first-order diffracted light having the frequencies f _opt + f _saw and f _opt −f _saw emitted from the optical frequency shifter 30 is collimated from the irradiation measuring probes 50B and 50C via the polarization maintaining optical fibers 48B and 48C. And is condensed by the condensing lens 52 on the condensing portion, that is, intersected.

When the scattering particles pass through the light collecting portion, the passing scattering particles scatter the irradiated light in all directions. A part of the backscattered light from the passing scattering particles is again collected by the condenser lens 5.
The light is collimated through 2 and is incident on the photodetector 58 through the light receiving measurement probe 54 and the single mode optical fiber 56. Non-diffracted light is incident on the photodetector 58 via the polarization maintaining optical fiber 48A.

Emitted from the irradiation measuring probe 50B
After the light wave is backscattered by scattering particles,
Single-mode optical fiber 56 via fixed probe 54
FIG. 4 shows the relationship between the wavenumber vectors when the light enters the. Figure
In 4 (1), K_iaIs the wave number of the light incident on the scattering particles.
Ktor, K_saIs the backscatter, that is, in the negative direction of the x-axis
Is the wave vector of the scattered light, and V is the velocity vector
Torr and V_x, V _yIs the x and y components of the velocity vector
is there. In addition, when the crossing angle of the light incident on the scattering particles is 2φ,
Therefore, the light emitted from the irradiation measurement probe 50B
Angle between wave and scattered light, ie wave vector K_iaAnd x
The angle formed with the axis is φ.

Here, the wave number vector K _a obtained by subtracting the wave number vector K _ia from the wave number vector K _sa shown in FIG. 4A.
Corresponds to the Doppler frequency shift f _da , which is the amount of frequency transition received by scattering by the scattering particles.

This Doppler frequency shift f _da is represented by using the following vector inner product using the velocity vector V (= iV _x + jV _y ), the wave number vector K _ia , and the wave number vector K _sa . _{f da = V · K a /} (2π) = V · (K sa -K ia) / (2π) ··· (5) , where the vector K _sa, the approximate size of the K _ia and K , F _da is represented by the following equation. _{f da = (iV x + jV} y) {- iK- (iKcosφ + jKsinφ)} / (2π) = (iV x + jV y) {- iK (1 + cosφ) + jKsinφ} / (2π) = {- (1 + cosφ) V x K + sinφV y K} / (2π) = { - (1 + cosφ) V x + sinφV y} / λ ··· (6) also holds the same relationship as described above for emitted from the irradiation measurement probe 50C lightwave, Doppler frequency shift f _db Can also be derived in the same manner as above, and is therefore expressed as f _db = {− (1 + cos φ) V _x −sin φV _y } / λ (7).

Therefore, the frequencies of the light waves reaching the photoelectric conversion surface of the photodetector 58 are expressed as follows. Scattered light due to the light wave from the irradiation measurement probe 50B f _opt + f _saw + f _da = f _opt + f _saw + {− (1 + cos φ) V _x + sin φV _y } / λ (8) By the light wave from the irradiation measurement probe 50C Scattered light f _opt −f _saw + f _db = f _opt −f _saw + {− (1 + cos φ) V _x −sin φV _y } / λ (9) Non-diffracted light (local light) from the polarization maintaining optical fiber 48A f _{opt Since} the three light waves of scattered light and local light emitted by the light waves emitted from the irradiation measurement probes 50B and 50C are combined and interfered at the photoelectric conversion surface of the photodetector 58, the following three types of beat signals are obtained. Is obtained.

[0051]

[Equation 4]

By computing these beat signals by a computer provided in the Doppler frequency analyzer 60, the following vector components (V _x , V _y ) of the flow velocity can be obtained.

Irradiation measurement probes 50B, 50C
The backscattered light due to the light wave emitted from the above acts as a signal light of intensity P _s after entering the single-mode optical fiber 56, and the non-diffracted light emitted from the outgoing light channel optical waveguide 40A has a local intensity of P _Lo . Since it acts as a light emission,
The beat signal obtained by the interference also gives the component V _y of the flow velocity perpendicular to the optical axis as in the following formula (14).

[0054]

[Equation 5]

That is, in the conventional optical fiber laser Doppler velocimeter, only the V _y component can be measured, but in the present embodiment, the two-dimensional vector component (V _x , V _y ) is measured. It can be measured.

In the above-described embodiment, the backscattered light emitted from the irradiation measuring probes 50B and 50C and scattered by the scattering particles interferes with the local light, so that the intensity of the local light is increased. Further, the S / N can be improved.

In the above, a pair of irradiation measurement probes is used.
An optical system equipped with a measuring probe for receiving light is scattered in the x-axis direction.
A two-dimensional vector placed at a position where it can receive scattered light
The example of measuring the components was explained, but it is the same as this optical system.
Can receive light scattered in the z-axis direction by another optical system of one configuration
3D vector components if further arranged at different positions
(V _x, V_y, V_z) Can be measured.

In the above, as the acousto-optic material, Li is used.
An example using NbO ₃ has been described, but LiTa
Other materials having an acousto-optic effect such as O ₃ and ZnO may be used.

The frequency f_opt+ F_saw, F_opt-F
_sawOf the frequency f_optLocal light
Light, but frequency f_opt+ F_saw, F_optMeasure the light of
Irradiate the area, frequency f_opt-F_sawOf the local light
May be the frequency f_opt-F _saw, F_optMeasuring light
Irradiate the area and frequency f_opt+ F_sawLight as local light
Good.

Furthermore, although an example using an optical frequency shifter having a three-branch optical waveguide and one IDT has been described above, the optical frequency shifter is not limited to this.

For example, one channel optical waveguide is branched into two to generate light of frequencies f ₀ and f ₀ + f ₁ , and one (for example, frequency f ₀ ) branch waveguide is further branched into two to generate f.
It is also possible to use an optical frequency shifter that generates ₀ and f ₀ + f ₂ , configures an optical waveguide having three emission ends corresponding to lights of these three frequencies, and has an IDT at each branch. .

Further, two-output optical frequency shifters composed of bulk parts may be connected in multiple stages to obtain light of three different frequencies as described above.

The shift of the optical frequency can be similarly performed based on the electro-optical effect, the sweep of the oscillation frequency of the semiconductor, etc., in addition to the acousto-optical effect.

[0064]

As described above, according to the present invention, both two components of the flow velocity can be measured by the principle of the reference light type, so that a high SN ratio can be realized and
Since it is possible to measure the component perpendicular to the optical axis and the component parallel to the optical axis at the same time, it is possible to prevent the occurrence of a measurement error due to the axis deviation from the measurement target.

[Brief description of drawings]

FIG. 1 is an enlarged view of the vicinity of a branch portion of an optical frequency shifter used in an embodiment of the present invention.

FIG. 2 is a diagram showing a diffraction direction of light of an optical frequency shifter used in the embodiment of the present invention.

FIG. 3 is a configuration diagram of an optical fiber laser Doppler velocimeter according to the embodiment of the present invention.

FIG. 4 is a diagram showing a wave number relationship of the optical fiber laser Doppler velocimeter of the present embodiment.

FIG. 5 is a configuration diagram of a conventional optical fiber laser Doppler velocimeter.

[Explanation of symbols]

30 Optical frequency shifter 32 substrates 34 channel optical waveguide 38 3-branched channel optical waveguide 40A, 40B, 40C Channel optical waveguide for outgoing light

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoru Kato, Aichi-gun, Nagakute-cho, Aichi Prefecture, Nagaleta 1 41 Yokomichi, Toyota Central Research Institute Co., Ltd. 1 Yoko 41, Toyota Central Research Institute Co., Ltd. (56) Reference JP-A-7-311215 (JP, A) JP-A-2-136777 (JP, A) (58) Fields investigated (Int.Cl. ⁷⁾ , DB name) G01P 5/00 G01S 17/58

Claims (3)

(57) [Claims] 1. An optical frequency shifter that generates and emits three lights having different frequencies from the light emitted from a light source, and first, second, and third light propagating each of the three lights.
Of the three optical fibers, the light collecting device for collecting the light propagated by the first and second two optical fibers on the measurement area, and the scattered light by the light collected on the measurement area is incident. An incident device, a detector for detecting the optical heterodyne by superposing the scattered light incident on the incident device and the light propagated by the third optical fiber, and calculating the flow velocity based on the optical heterodyne detection of the detector. A fiber optic laser Doppler velocimeter including: 2. The optical frequency shifter is coupled to a single channel optical waveguide, an electrode for generating a surface acoustic wave, and one end of the channel optical waveguide. An optical waveguide which is formed so as to be incident substantially parallel to the wavefront, and which is branched into three parts for respectively propagating ± first-order diffracted light and non-diffracted light having substantially equal intensities diffracted by the interaction with the surface acoustic wave. The optical fiber laser Doppler velocimeter according to claim 1, comprising a three-branch optical waveguide including: and a channel optical waveguide for emission light coupled to the other end of each of the three-branched optical waveguides. 3. An acousto-optic medium is used, where L is the interaction length between the propagating light wave and the surface acoustic wave, λ _O is the wavelength of the light wave in vacuum, n ₁ is the refractive index of the acousto-optic medium, and Λ is the surface. When the wavelength of the elastic wave, θ _i , the angle between the propagating light wave and the wavefront of the surface acoustic wave, and Δn ₁ are changes in the refractive index of the acousto-optic medium induced by the surface acoustic wave, the electrode and the three branches The optical waveguide and 0.1 <2πLλ _O / n ₁ Λ ² cos θ _i <10
The optical fiber laser Doppler velocimeter according to claim 2, wherein the condition 0 <πΔn ₁ L / λ ₀ <2 is satisfied.