JPH0954159A - Laser doppler velocimeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an inexpensive and compact laser Doppler velocimeter by reducing the number of optical parts from a conventional laser Doppler velocimeter. SOLUTION: The velocimeter is provided with an optical waveguide element 2, an optical system 5 and a photodetector 12. A laser beam L0 projected from a laser light source is guided from an input terminal 3a to a pair of output terminals 3c and 3d by a Y-shaped diverging-type optical waveguide 3 formed in the optical waveguide element 2. In the optical system 5, a laser beam L1 from one output terminal 3c of the Y-shaped diverging-type optical waveguide 3 is projected to an object 9, and a reflecting laser beam L3 and a laser beam L2 from the other output terminal 3d are let to interfere with each other. Frequency difference is detected by the photodetector 12 from the laser beams interfered in the optical system 5. The photodetector 12 supplies the frequency difference to a demodulator 13 after converting the frequency difference into an electric signal. The demodulator 13 demodulates the electric signal and outputs a demodulated speed signal S0 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser Doppler velocimeter for irradiating a vibrating object with a laser beam and measuring the speed of the object according to the frequency of the laser beam changed by the Doppler effect.

[0002]

2. Description of the Related Art A Doppler velocimeter applying the Doppler effect has been used as a device for measuring the velocity of a vibrating measured object. Everything that moves, including vibration phenomena, always has velocity. Therefore, when a sound or light having a constant frequency is applied to a speedy object, the frequency of the measuring medium changes. This is called the Doppler shift, and the speed of the opponent can be calculated from the shift amount of this frequency.

Among the Doppler velocimeters, the laser Doppler velocimeter measures the velocity of minute vibrations of an object using a laser beam which is light having a single wave. That is, the laser Doppler velocimeter irradiates a vibrating object with a laser beam and measures the velocity of the vibrating object using the frequency changed by the Doppler phenomenon. In this laser Doppler velocimeter, it is necessary to modulate the light in order to detect the directionality of the vibration phenomenon of the object.

FIG. 8 shows a schematic configuration of the laser Doppler velocimeter 30 together with the waveform of the laser beam. First, a laser beam L ₀ having an extremely high frequency is split into two directions by a deflecting beam splitter (PBS) 31. One of them passes through the PBS 32 and the quarter-wave plate 33 and is applied to the vibrating object 34 which is an object to be measured. The return light L ₁ reflected by the vibrating object 34 undergoes a frequency shift proportional to the speed of the vibrating object 34 due to the Doppler effect. The return light L ₁ passes through the quarter-wave plate 33 again, and P
It is incident on BS32. Here, since the return light L ₁ has its polarization direction rotated by 90 ° by the quarter-wave plate 33, P
It is reflected by BS32.

The other of the two, which is divided by the PBS 31, is modulated by the modulator 35 to become a modulated laser beam L ₂ and enters the PBS 36. The modulated laser beam L ₂ and the return light L ₁ reflected by the mirror 37 are reflected by the polarizing plate 38.
, The same polarization components interfere with each other, and a beat signal L ₃ is generated. The beat signal L ₃ is detected by a photodetector 39 such as a photodetector, converted into an electric signal, and then demodulated by a demodulator 40 to obtain a demodulation speed signal S ₀ .

This laser Doppler velocimeter 30 uses an acoustic wave, which is an elastic wave, in a medium in which a light wave propagates as a modulator 35 to generate distortion, and utilizes the change in the refractive index caused by the distortion to generate an optical signal. A bulk wavelength conversion element using an acousto-optic modulator that modulates is used.

The operating principle of the laser Doppler speedometer 30 will be described. The laser beam L ₀ of the frequency f ₀ emitted from the laser light source is transmitted through the PBS 31 to the vibrating object 34.
It is divided into two systems, an incident beam system for the beam and a reference beam system returned inside the device. The laser beam L ₀ that has proceeded to the incident beam system side is further transmitted through the PBS 32 to the vibrating object 34.
And becomes a return light L ₁ that is Doppler-shifted according to the speed of the vibrating object 34, and becomes the quarter-wave plate 3
3, supplied to the mirror 37 via the PBS 32. Here, if the shift frequency is f _D , the Doppler shift is f ₀ ± f _D. On the other hand, the laser beam L ₀ that has proceeded to the reference beam system side has a very high frequency f ₀ that the laser beam itself has, and it is difficult to directly measure it. Therefore, the frequency is f ₀ ± It is modulated with f _M and becomes a modulated laser beam L ₂ .

Since the return light L ₁ reflected by the vibrating object 34 undergoes a Doppler shift, when it interferes with the modulated laser beam L _{2 serving} as the reference light, f _M
Since the beat signal L ₃ having a beat frequency of + f _D is generated,
It is possible to determine the positive / negative of the Doppler frequency, that is, the vibration speed and the vibration direction which are reciprocating motions.

The beat signal L ₃ is further extracted by the frequency (f _D ) that is Doppler-shifted by the photodetector 39, FM demodulated by the demodulator 40, and converted into an electric signal corresponding to the vibration speed of the vibrating object. , Is output as a voltage output.

[0010]

By the way, the bulk wavelength conversion element is expensive because it uses an acousto-optic modulator, and it is difficult to make it compact. Therefore, the laser Doppler velocimeter 30 is expensive and large in size.

Therefore, an optical waveguide having a refractive index larger than that of the crystal is formed in the crystal, the light wave is confined in the optical waveguide, and an electro-optic modulator including a pair of metal thin film electrodes formed near the optical waveguide is used. An optical waveguide type wavelength conversion element (hereinafter referred to as an optical waveguide element) that applies an electric field to the optical waveguide to change the refractive index to modulate the light may be used for the modulator 35. The optical waveguide and the pair of metal thin film electrodes are
Since it can be manufactured by photolithography, it is inexpensive and can be miniaturized.

However, since the number of optical components is still large, it has been difficult to realize low cost and miniaturization.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a laser Doppler velocimeter capable of reducing the number of optical components and realizing low cost and downsizing.

[0014]

In order to solve the above problems, a laser Doppler velocimeter according to the present invention is a branch type optical waveguide for guiding a laser beam from at least one input end to at least a pair of output ends. While irradiating the object with the laser beam from one output end, interfere the reflected laser beam and the laser beam from the other output end,
The frequency difference is detected from this interfered laser beam.

Here, the optical waveguide element is a linear portion parallel to the optical waveguide portion before branching from the input end to the branch portion of the optical waveguide from the branch portion to one output end of the branch type optical waveguide. In addition, a pair of metal electrodes are formed so as to sandwich the optical waveguide of the linear portion.

Further, the present invention is characterized in that a metal coating portion is formed on the branch portion of the branch type optical waveguide.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a laser Doppler velocimeter according to the present invention will be described below with reference to the drawings. In this embodiment, as shown in FIG. 1, a laser beam L ₁ is applied to a vibrating object 8 and the velocity of the object 9 is measured according to the frequency of the laser beam changed by the Doppler effect. The laser Doppler velocimeter 1 as shown in FIG.

This laser Doppler velocimeter 1 is an optical waveguide formed by forming a Y-shaped branched optical waveguide 3 for guiding a laser beam L ₀ emitted from a laser light source from an input end 3a to a pair of output ends 3c and 3d. The object 9 is irradiated with the laser beam L ₁ from one output end 3c of the waveguide element 2 and the Y-shaped branch type optical waveguide 3, and its reflected laser beam L ₃ and the laser beam L ₂ from the other output end 3d. And an optical system 5 for interfering with each other, and a photodetector 12 for detecting a frequency difference from a laser beam interfered by the optical system 5. The photodetector 12 converts the frequency difference into an electric signal and then supplies the electric signal to the demodulator 13. The demodulator 13 demodulates the electric signal and outputs a demodulation speed signal S ₀ .

The optical system 5 includes a lens 6, a polarization beam splitter (hereinafter referred to as PBS) 7, and a quarter wave plate 8.
And a mirror 10 and a polarizing plate 11.

As shown in FIG. 2, the optical waveguide element 2 has one input end 3a formed by thermally diffusing, for example, titanium (Ti) into a crystal substrate made of, for example, lithium niobate (LiNbO ₃ , hereinafter referred to as LN). An optical modulator formed by forming a Y-shaped branched optical waveguide 3 having a pair of output ends 3c and 3d, and including a pair of aluminum (Al) thin film electrodes on the surface of the crystal substrate so as to sandwich the Y-shaped branched optical waveguide 3. 4 is provided.

Now, a process of forming the optical waveguide 3 in the optical waveguide device 2 will be described with reference to FIG. First, a liquid resist is spin-coded on the crystal substrate 20 shown in FIG. 3A and thermally dried to form a resist film 21 as shown in FIG. The resist film 21 is irradiated with light L through a photomask 22 as shown in FIG. When the resist film 21 irradiated with the light L through the photomask 22 is developed, as shown in FIG.
A resist pattern as shown in (d) is obtained. The entire surface of this resist pattern is formed by the vapor deposition method as shown in FIG.
A Ti film 23 is formed as shown in FIG. The Ti film 23 formed on the entire surface of the resist pattern is removed only at a desired position as a Ti pattern 23 'as shown in FIG. 3F by removing the Ti film 23 on the resist film 21 by the lift-off method. Remain in. Finally, the Ti pattern 23 ′ is thermally diffused at a temperature of 1000 ° C. or higher for several hours, so that the Ti-diffused optical waveguide 3 can be formed.

Next, the process of forming the optical modulator 4 will be described with reference to FIG.
Will be described with reference to. First, as shown in FIG. 4A, a liquid resist is spin-coded on the crystal substrate 20 on which the Y-shaped branched optical waveguide 3 is formed, and heat-dried to form a resist as shown in FIG. The film 24 is formed.
The resist film 24 is irradiated with light L through a photo mask 25 as shown in FIG. Photo mask 25
When the resist film 24 irradiated with the light L is developed, a resist pattern as shown in FIG. 4D is obtained. The entire surface of this resist pattern is formed by vapor deposition as shown in FIG.
The Al film 25 is formed as shown in FIG. The Al film 25 formed on the entire surface of the resist pattern is separated from the Al film 25 on the resist film 24 by the lift-off method, so that the Al pattern 2 is formed as shown in FIG.
5'is left only in the desired position. Here, Al is placed at a position where one side of the optical waveguide 3 (on the output end 3c side) is sandwiched.
The optical modulator 4 is formed by leaving the pattern 25 '. That is, the optical modulator 4 including a pair of Al thin film electrodes is
One output end 3c from the branch portion 3b of the bifurcated optical waveguide 3
It is formed so as to sandwich the optical waveguide of the straight line portion in a straight line portion parallel to the optical waveguide portion before branching from the input end 3a of the optical waveguide to the branch portion 3b.

First, the laser beam L ₀ emitted from the laser light source and having an extremely high frequency enters from the input end 3a of the Y-shaped branch type optical waveguide 3 and is branched by the branching section 3b.
The light is guided to one output end 3c and the other output end 3d.
Since the optical modulator 4 including the pair of Al thin film electrodes as described above is formed on the one output end 3c side, the modulated laser beam L ₁ is emitted from the output terminal 3c. The unmodulated laser beam L ₂ is emitted from the other output end 3d.

The modulated laser beam L ₁ is reflected by the lens 6,
After passing through the PBS 7 and the quarter-wave plate 8, the vibrating object 9 is irradiated. The reflected laser beam L ₃ from the vibrating object 9 is
Due to the Doppler effect, the frequency shift is proportional to the velocity of the vibrating object. This reflected laser beam L ₃ is 1
Since the polarization direction is rotated 90 ° by the / 4 wavelength plate 8, PB
It is reflected by S7.

The laser beam L ₂ has its optical path bent by the lens 6, is reflected by the mirror 10, and is transmitted through the PBS 7.

The reflected laser beam L ₃ and the laser beam L _{2 which} have passed through the polarizing plate 11 interfere with each other in the same polarization component. Then, the beat signal L ₄ is generated. The beat signal L ₄ can be detected by a photodetector 12 such as a photodetector, converted into an electric signal, and then demodulated by a demodulator 13 to obtain a demodulation speed signal S ₀ .

Here, the modulated laser beam output from one output end 3c of the Y-shaped branch type optical waveguide 3 is applied to the DUT 9, and the laser beam output from the other output end 3d is mirrored. Although the reference light is passed through the reference beam 10, the function of the lens 6 is changed, and the optical path of the modulated laser beam is bent to irradiate the mirror 10 as the reference light, and the laser beam output from the output end 3d is applied to the DUT 9. It may be irradiated.

As described above, the laser Doppler velocimeter 1 according to the present embodiment has one of the Y-shaped branch type optical waveguides 3 for guiding the laser beam from at least one input end 3a to at least one pair of output ends 3c and 3d. The object 9 to be measured is irradiated with the laser beam from the output end 3c, and the reflected laser beam and the laser beam from the other output end 3d are interfered with each other to detect the frequency difference from the interfered laser beam. As a result, it is possible to reduce the number of optical parts, and to achieve low cost and downsizing.

In the above embodiment, the optical waveguide element 2 in which the Y-branch type optical waveguide 3 having one input channel and two output channels is formed is used. However, as the optical waveguide element, the number of input channels and the number of output channels are used. You may increase the number. For example, FIG. 5 shows an optical waveguide device 30 in which an optical waveguide 31 for one channel input and four channel output is formed. The laser beam incident from the input end 31a is branched by the branching part 31b, further branched by the branching parts 31c and 31d, and then output ends 31e, 31f, 31g and 31h.
Output from An optical modulator 32 including a pair of Al thin film electrodes is formed in the optical waveguide between the branch portion 31c and the output end 31e so as to sandwich the optical waveguide. Also,
An optical modulator 33 including a pair of Al thin film electrodes is formed in the optical waveguide between the branch portion 31d and the output end 31g so as to sandwich the optical waveguide. If this optical waveguide device 30 is applied to the laser Doppler velocimeter as described above,
It is possible to measure the speed of two measured objects.

By the way, the above laser Doppler velocimeter 1
In order to improve the coherence of light, for example, the laser beam incident on the Y-shaped branch type optical waveguide 3 is a laser beam having a single polarization component. In the case of the optical waveguide device 2,
The polarization component of the laser beam used is TE mode parallel to the surface of the crystal substrate. Therefore, the TM mode polarized in the thickness direction of the crystal substrate becomes unnecessary light. If such unnecessary light exists, they interfere with each other, and unnecessary interference light is generated. Since the unnecessary interference light is included in the light demodulated by the laser Doppler speedometer 1, the speed signal obtained by demodulation is limited and the performance of the laser Doppler speedometer 1 is deteriorated. However, in the Y-shaped branch type optical waveguide 3 as described above, even if light having a single polarization is incident on the optical waveguide, the output light includes unnecessary light whose polarization has changed. For example, when the TE mode is incident, the difference between the extinction ratios of the TE mode and the TM mode in the linear waveguide and the branch type optical waveguide is about 1
It will be about 0 dB.

Therefore, in the laser Doppler velocimeter according to the present invention, a metal coating (metal cladding) is formed on the Y-shaped optical waveguide 3 to attenuate the light having an unnecessary polarization component, as shown in FIG. The optical waveguide device 40 may be used.

In this optical waveguide device 40, one input end 3 is formed by thermally diffusing, for example, Ti into a crystal substrate made of, for example, LN.
A Y-shaped branch type optical waveguide 3 having a and a pair of output terminals 3c and 3d is formed, and an optical modulator 4 composed of, for example, a pair of Al thin film electrodes on the surface of the crystal substrate so as to sandwich the Y-shaped branched type optical waveguide 3. And a metal cladding 41 made of Al is loaded on the branch portion 3b of the Y-shaped branch type optical waveguide 3.

In general, in the light wavelength region, the metal behaves as a dielectric substance having a negative permittivity and a large loss due to the inertial effect of charges in the metal, so that the light wave receives propagation loss due to the metal cladding 41. TM mode is T
Since the electromagnetic field distribution penetrates deeper into the metal than in the E mode, it is more affected by the metal cladding 41,
Propagation loss increases. For example, when the Al metal cladding 41 is not loaded, the extinction ratio between the TE mode and the TM mode is about 12 dB, but when the Al metal cladding 41 is loaded, the extinction ratio is about 17 dB. It can be seen that the light having an unnecessary polarization component can be attenuated by the ding 41.

The process of forming the metal cladding 41 will be described with reference to FIG. First, as shown in FIG. 7A, a liquid resist is spin-coded on the crystal substrate 20 on which the Y-shaped branched optical waveguide 3 is formed, and heat-dried to form a resist as shown in FIG. 7B. The film 42 is formed. The resist film 42 is irradiated with the light L through the photomass 43 as shown in FIG. 7C.
By developing the resist film 42 irradiated with the light L through the photomask 43, a resist pattern as shown in FIG. 7D is obtained. An Al film 44 is formed on the entire surface of this resist pattern by a vapor deposition method as shown in FIG. Al film 44 formed on the entire surface of the resist pattern
The Al film 44 on the resist film 42 is peeled off by the lift-off method, so that as shown in FIG.
The l pattern 44 'remains only at a desired position. This A
The l pattern 44 ′ becomes the metal cladding 41. In addition to Al, the metal cladding 41 contains N
Thermal diffusion may be performed using i, Cu or the like.

Although the Y-shaped branch type optical waveguide 3 is formed by thermally diffusing Ti, it may be formed by thermally diffusing Ni and Cu.

In addition to producing the Y-shaped branched optical waveguide 3 by the thermal diffusion method, LN is added to a heated benzoic acid solution.
The optical waveguide may be manufactured by using a proton exchange method, in which Li + → H + is exchanged by immersion to form a high refractive index layer to obtain an optical waveguide, an ion exchange method, an ion implantation method, or the like.

Although LN was used for the crystal substrate 20,
Besides N, tantalum lithium oxide (LiTaO ₂ ) and KTi
OPO ₄ may be used.

Although the optical modulator 4 is made of Al, it may be made of Ti-Au or Cr-Au instead of Al.

The Y-shaped branch type optical waveguide 3 and the optical modulator 4 are also provided.
Although the metal pattern of the metal cladding 41 is deposited by the vapor deposition method, the metal pattern 41 may be deposited by the sputtering method instead of the vapor deposition method.

The Y-shaped branch type optical waveguide 3 and the optical modulator 4 are also provided.
Although the metal pattern of the metal cladding 41 is formed by the lift-off method, it may be formed by an etching method other than the lift-off method.

[0041]

The laser Doppler velocimeter according to the present invention irradiates an object with a laser beam from one output end of a branch type optical waveguide that guides the laser beam from at least one input end to at least a pair of output ends. At the same time, the reflected laser beam and the laser beam from the other output end are interfered with each other, and the frequency difference is detected from the interfering signal, so that the number of optical components can be reduced, and the cost and the size can be reduced.

Further, by loading the metal coating on the branch portion of the branch type optical waveguide, unnecessary light of the optical waveguide element can be attenuated and deterioration of the performance of the laser Doppler velocimeter can be prevented.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of a laser Doppler velocimeter according to the present invention.

FIG. 2 is a schematic view of an optical waveguide device used in the above embodiment.

FIG. 3 is a process drawing of forming a Y-shaped branched optical waveguide used in the optical waveguide device.

FIG. 4 is a process drawing of an optical modulator used in the optical waveguide device.

FIG. 5 is a schematic view of a modification of the optical waveguide device.

FIG. 6 is a schematic view of an optical waveguide device having a metal cladding.

FIG. 7 is a process drawing of forming the metal cladding.

FIG. 8 is a schematic configuration diagram of a conventional laser Doppler velocimeter.

[Explanation of symbols]

 1 Laser Doppler Velocity Meter 2 Optical Waveguide Element 3 Y-Branched Optical Waveguide 4 Optical Modulator 5 Optical System 12 Photodetector 13 Demodulator

Claims (3)

[Claims] 1. A laser Doppler velocimeter for irradiating a vibrating object with a laser beam and measuring the speed of the object according to the frequency of the laser beam changed by the Doppler effect. An optical waveguide element formed by forming on a crystal substrate a branched optical waveguide for guiding at least a pair of output ends from an end, and irradiating the object with a laser beam from one output end of the branched optical waveguide, A laser Doppler velocity including an optical system for causing the reflected laser beam and the laser beam from the other output end to interfere with each other, and a detection unit for detecting a frequency difference from a signal obtained by the interference by the optical system. Total. 2. A linear part of the optical waveguide from the branch part to one output end of the branch type optical waveguide, which is parallel to the optical waveguide part before branching from the input end to the branch part, The laser Doppler velocimeter according to claim 1, further comprising: an optical waveguide element formed by forming a pair of metal electrodes so as to sandwich the waveguide. 3. The laser Doppler velocimeter according to claim 1, wherein a metal coating portion is formed on a branch portion of the branch type optical waveguide.