JP2995078B2 - Laser Doppler speedometer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser Doppler velocimeter, and more particularly, to irradiating a laser beam to an object to be measured and detecting the speed and the like of the object to be measured using the Doppler shift of the reflected light. It is suitable for application to a laser Doppler speedometer.

B. Summary of the Invention The present invention relates to a laser Doppler velocimeter that generates reference light with first laser light emitted from one end face of a semiconductor laser and receives second laser light emitted from the other end face. By irradiating the measurement target and separating the optical paths of the second laser light and the measurement light at this time, a beat signal having a high signal-to-noise ratio can be obtained, and the detection sensitivity can be improved accordingly.

C Conventional Art Conventionally, in this type of laser Doppler velocimeter, the speed of a moving body, a vibrating body, a fluid, or the like is measured as a ratio measurement object (that is, the moving speed of the moving body, the vibration speed of the vibrating body, the flow velocity of the fluid ) Can be measured.

That is, as shown in FIG. 2, in the laser Doppler velocimeter 1, the ferroelectric substrate 4
The laser beam LA1 of the helium neon laser 8 is guided to the waveguide 6 formed above, and branched into waveguides 12 and 14 by a 3 dB coupler 10.

The waveguide 12 guides the branched laser light LA2 to the end face of the substrate 4 via the optical modulator 15.

A mirror portion 17 is formed on the end face, and the light beam LA2 branched by the mirror portion 17 is folded back.

The optical modulator 15 has a pair of electrodes 16 arranged with the waveguide 12 interposed therebetween, and the drive signal S1 of the drive power supply 18 is
To modulate the phase of the laser beam LA2.

Accordingly, the laser Doppler velocimeter 1 generates the reference light LA4 by phase-modulating the laser light LA2, and generates the reference light LA4.
The speed of the measured object 19 is detected based on LA4.

On the other hand, the waveguide 14 guides the branched laser beam LA3 through the optical fiber 20 to the lens 22, and irradiates the object 19 to be measured.

Thus, the laser Doppler velocimeter 1 irradiates the laser light LA3 onto the object to be measured 24 and shifts the frequency of the measurement light LA5 reflected by the object to be measured 19 according to the moving speed of the object to be measured 19.

The optical fiber 20 is a measuring light LA reflected by the measured object 19.
5 is received via the lens 22 and guided to the waveguide 14.

Thus, the measurement light LA5 guided to the waveguide 14 is mixed with the reference light LA3 by the 3 dB coupler 10, and the resulting mixed light LA6 is guided to the photodiode 30 via the waveguide 26 and the optical fiber 28.

Thus, by mixing the measurement light LA5 and the reference light LA3, a mixed light LA3 whose light intensity changes according to the beat components of the measurement light LA5 and the reference light LA3 can be obtained.

As a result, the drive signal S1 is output via the photodiode 30.
The reference light LA3 and the measurement light LA5
Can extract the beat component (hereinafter referred to as beat signal) FM of
The beat signal is detected by an FM detection circuit so that the speed of the measured object can be detected.

D Problems to be Solved by the Invention In this type of laser Doppler velocimeter 1, reflection of laser light cannot be avoided at the end face of the optical fiber 2, the incident face of the waveguide 6, and the like.

Of these, the reflected light reflected on the end face of the optical fiber 2 and the incident face of the waveguide 6 destabilizes the oscillation state of the helium neon laser 8 and increases the noise of the beat signal FM accordingly.

Also, the exit surface of the waveguide 14, the entrance surface of the optical fiber 20,
The reflected light reflected from the entrance / exit surface of the lens 22 is the measurement light LA5
Is mixed with the reference light LA4, whereby the unmodulated laser light is mixed and the beat signal FM is distorted.

On the other hand, the coupling loss generated in the optical fibers 2, 20, and 28 lowers the signal level of the beat signal FM.

For this reason, in this type of laser Doppler velocimeter 1, there is a problem that it is difficult to obtain a beat signal FM having a good signal-to-noise ratio, which has a problem that the detection sensitivity is limited.

The present invention has been made in view of the above points, and has as its object to propose a laser Doppler velocimeter capable of improving the detection sensitivity as compared with the related art.

Means for Solving the Problem In order to solve the problem, the present invention provides a semiconductor laser 42 that emits laser beams L1 and L2 from both end surfaces of a semiconductor chip, and a first laser beam that is emitted from one end surface of the semiconductor laser 42.
An optical modulator 15 for phase-modulating the laser beam L2 to generate a reference beam L5 and a second laser beam L1 emitted from the remaining one end surface of the semiconductor laser 42 are irradiated to the object 19 to be measured, Target
Measurement optical systems 52, 54 that receive the measurement light L7 reflected at 19,
56, an optical mixer 10 that mixes the measurement light L7 received by the measurement optical systems 52, 54, 56 with the reference light L5 to generate a mixed light L8, and receives the mixed light L8 and outputs a beat signal FM1. Light receiving element 30
And a beat signal processing circuit for detecting the speed of the measured object 19 based on the beat signal FM1, and a measuring optical system 52,
54 and 56 separate the optical paths of the second laser light L1 and the measurement light L7.

F function The first laser light L2 emitted from one end face of the semiconductor laser 42 is phase-modulated to generate the reference light L5, and the second laser light L1 emitted from the remaining one end face is measured by the object 19 to be measured.
The measurement light L7 is obtained by irradiating the second laser light L1 and the measurement light L7 at this time. If the light paths are separated, the light intensity of the measurement light L7 can be improved, and the unmodulated laser light is mixed,
The return of the reflected light to 42 can be effectively avoided, and the signal-to-noise ratio of the beat signal FM1 can be significantly improved accordingly.

G Example Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

In FIG. 1 in which the same reference numerals are given to the corresponding parts in FIG. 2, reference numeral 40 denotes a laser Doppler velocimeter as a whole,
The speed of the measurement target 19 is detected using the laser beams L1 and L2 emitted from the semiconductor laser.

That is, the semiconductor laser 42 couples the semiconductor laser chip directly to the waveguide 44 to guide the laser beam L2 emitted from one end face of the semiconductor laser chip to the waveguide 44.

The waveguide 44 branches the laser beam L2 at the Y branch 46, and guides the branched laser beams L3 and L4 to the waveguide 48 and the optical modulator 15, respectively.

The waveguide 48 is a photodiode that transmits the branched laser light L3.
The laser Doppler velocimeter 40 stably drives the semiconductor laser 42 based on the output signal of the photodiode 50.

At this time, in the coupling portion between the waveguide 48 and the photodiode 50, the waveguide 48 is formed obliquely with respect to the end face of the substrate 4, so that the light reflected on the end face of the substrate 4 causes the waveguide 48 to travel in the opposite direction. It is made so that it does not progress.

On the other hand, the optical modulator 15 generates the reference light L5 by phase-modulating the branched laser light L4, and supplies the reference light L5 to the 3 dB coupler 10.

On the other hand, the lens 52 is arranged shifted by a predetermined amount with respect to the optical axis of the lens 54, receives the laser beam L1 emitted from the remaining one end face of the semiconductor laser chip, and
Inject to 54.

The lens 54 irradiates the measurement target 19 with the laser beam L1, and as a result, the measurement light L7 obtained by regular reflection from the measurement target 19 is incident on a lens 56 arranged symmetrically with the lens 54.

Thus, the laser beam L1 is irradiated directly on the measured object 19 via the lenses 52 and 54, thereby avoiding loss as a result of an optical fiber and measuring light having a large spectral intensity.
L7 can be obtained.

Accordingly, the signal level of the beat signal FM1 can be remarkably improved as compared with the conventional case.

Further, at this time, the lenses 52 and 56 are arranged symmetrically to separate the optical paths of the laser light L1 and the measuring light L7, so that the amount of reflected light of the laser light L1 fed back to the semiconductor laser 42 is much more than before. Can be effectively reduced.

Therefore, the signal-to-noise ratio of the beat signal FM1 can be improved accordingly.

The lens 56 guides the measurement light L7 to the waveguide 58, and mixes the measurement light L7 with the reference light L5 at the 3 dB coupler 10.

The 3 dB coupler 10 has one waveguide 60 extending to the end face of the substrate 4 and being connected to the photodiode 30,
The other waveguide 62 extends to the end face of the substrate 4 and is opened.

Thus, the 3 dB coupler 10 allows the mixed light L8 to enter the photodiode 30 and detect the speed of the measured object based on the beat signal FM1 output from the photodiode 30.

In this case, the waveguides 60 and 62 are
Is formed obliquely to the end face of the substrate 4 so that the reflected light reflected on the end face of the substrate 4 does not travel in the waveguides 60 and 62 in the opposite directions.

Therefore, in the semiconductor laser 42, even for the laser light L2 emitted from one end face, feedback of reflected light generated at the end faces of the waveguides 48, 60 and 62 can be effectively avoided.

Further, mixing of unmodulated laser light can be prevented at the same time.

Thus, the signal-to-noise ratio of the beat signal FM1 output from the photodiode 30 can be significantly improved, and the detection sensitivity can be improved accordingly.

According to the above configuration, the reference light L5 is generated by the laser light L2 emitted from one end of the semiconductor laser 42, and the measurement light L7 is emitted by irradiating the measurement target 19 with the laser light L1 emitted from the other end. By separating the optical paths of the laser light L1 and the measurement light L7 at this time, the light intensity of the measurement light L7 can be improved, the unmodulated laser light is mixed, and the reflected light is returned to the semiconductor laser 42. Can be effectively avoided, and the signal-to-noise ratio of the beat signal FM1 can be remarkably improved, thereby improving the detection sensitivity.

In the above embodiment, the case where the reference light and the measurement light are mixed using the 3 dB coupler has been described. However, the present invention is not limited to this, and the mixing may be performed using the Y branch in reverse.

Further, in the above-described embodiment, the waveguides 48, 60 and 62 are formed obliquely with respect to the end face of the substrate 4, thereby effectively avoiding feedback of reflected light generated at the end faces of the waveguides 48, 60 and 62. Although the case has been described, the present invention is not limited to this, and various antireflection means can be widely applied, for example, when attaching a light absorbing member.

Further, at this time, the end face may be simply opened in a practically sufficient range.

H Effect of the Invention As described above, according to the present invention, the first laser light emitted from one end face of the semiconductor laser generates the reference light, and the second laser light emitted from the remaining one end face is generated. By irradiating the object to be measured and separating the optical paths of the second laser light and the measurement light, the light intensity of the measurement light can be improved, the unmodulated laser light is mixed, and the reflected light is returned to the semiconductor laser. Can be effectively avoided.

Accordingly, the signal-to-noise ratio of the beat signal can be remarkably improved, and a laser Doppler velocimeter with improved detection sensitivity can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a laser Doppler velocimeter according to one embodiment of the present invention, and FIG. 2 is a schematic diagram showing a conventional laser Doppler velocimeter. 1, 40 laser doppler velocimeter, 10 3 dB coupler, 4 substrate 6, 12, 14, 26, 44, 48, 58, 60, 62
…… Waveguide, 15 …… Optical modulator, 19 …… Measurement target, 22,
52, 54, 56 ... Lens, 30, 50 ... Photodiode,
42 ... Semiconductor laser.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Yoshio Ohashi 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation (72) Ken-ichi Sato 3-9-17-1 Nishigotanda, Shinagawa-ku, Tokyo (56) References JP-A-62-233767 (JP, A) JP-A-2-679 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01P 3/00-3/80 G01S 7/48-7/51 G01S 17/00-17/95

Claims (1)

(57) [Claims] A semiconductor laser for emitting laser light from both end faces of the semiconductor chip; an optical modulator for generating reference light by phase-modulating the first laser light emitted from one end face of the semiconductor laser; A measurement optical system that irradiates the object to be measured with the second laser light emitted from the other end surface of the semiconductor laser and receives the measurement light reflected by the object to be measured, An optical mixer that mixes the measurement light with the reference light to generate a mixed light, a light receiving element that receives the mixed light and outputs a beat signal, and based on the beat signal, A beat signal processing circuit for detecting a speed, wherein the measuring optical system separates an optical path of the second laser light and an optical path of the measuring light.