JPS6076679A - Laser doppler speedometer - Google Patents

Abstract

PURPOSE:To miniaturize a sensor part and to also enable the directional discrimination of a particle, by supplying two sets of laser beams, between which there is predetermined frequency difference, through an optical fiber, and detecting Doppler change due to the particle passing an interference fringe region formed by beams crossing. CONSTITUTION:Two sets of laser beams, which have different linear polarization characteristics and between which the frequency difference is set to a predetermined value by PLL25, outputted from a horizontal Zeeman laser 20 are supplied to a sensor 32 through a single mode optical fiber 31 and aligned to P-polarization by a polarization beam splitter 35 while an interference fringe region 38 is formed through a condensing lens 37. When a particle passes this region 38, scattered beam is generated to be supplied through an optical fiber 40 and frequency difference corresponding to the speed and direction of the particle due to a Doppler phenomenon and beam component of polarity are detected by a signal processing apparatus 50. By this constitution, the miniaturization of a sensor part relating to direct measurement is attained and not only the speed but also the moving direction of the particle can be discriminated.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a laser Doppler velocimeter using optical fibers, and more particularly to a so-called dual beam mode 1' laser Doppler velocimeter.

BACKGROUND ART Conventionally, focusing on the characteristics of laser light which has excellent coherence and monochromaticity, various devices have been proposed for measuring the velocity of moving particles using laser light.

FIG. 1 shows the measurement principle of a dual-heam laser Doppler velocimeter that has become popular in recent years. In this figure, a laser beam from a laser light source 1 such as a laser oscillator having linearly polarized light, such as a helium neon laser or an argon laser, is separated into two parallel laser beams by a beam splitter 2. This laser beam is then focused by a condensing lens 3 and made to intersect at a certain point. In this way, if a scattering object exists in the intersection area, interference fringes will be observed as shown in an enlarged view of FIG. Therefore, when the particle p passes through this intersection region, the scattered light obtained from the particle p has a periodic intensity change corresponding to the brightness and darkness of the interference fringes. Therefore, this scattered light is collected by a condensing lens and sent to a photoelectric converter 5 via a mirror 4.
When converted into an electrical signal by , a burst waveform corresponding to the intensity of scattered light when passing through the interference fringe portion is obtained.

Since the interval between brightness and darkness of the interference fringes is determined by the Lede wave range of the laser beam and its intersection angle, the velocity of the particle p can be determined by measuring the frequency of this burst wave. However, in such a speedometer, since the wavelengths of the two laser beams are the same, the interference fringes formed in the intersection area are stationary, and even if the speed of the particle p passing through the interference fringes can be detected, it is difficult to determine its direction. I couldn't. In order to determine the direction of the particle p, an acousto-optic cell 6 is inserted into the optical path of one of the laser beams, and by driving the acousto-optic cell 6 with an oscillator 7 of fo, the laser beam is It is conceivable that the signal is shifted by a certain frequency (Δfo).

In this way, the interference fringes created in the intersection area will always be Δfo
It will move at a speed corresponding to . Therefore, if there is a particle stationary in the intersection area, the photoelectric converter 5 will provide an output with a frequency equal to Δfo. Further, when the particle p passes through the intersection region, the speed and direction of the particle p can be measured simultaneously based on the frequency shift amount of the burst wave using the frequency Δfo as a reference.

By the way, such a laser Doppler speed needle uses the coherent property of laser light. Therefore, it has been difficult to separate the laser beam processing section and the region to be measured as shown in FIG. Although it is possible to connect the laser oscillator 1 and the beam splitter 2 using a single mode optical fiber without mode conversion, it is possible to connect the laser oscillator 1 and the beam splitter 2 using a single mode optical fiber without mode conversion. Since it is necessary to provide an output oscillator 7 to supply an ultrasonic signal for frequency shift to the acousto-optic cell 6, it has been difficult to downsize the sensor section.

FIG. 3 is a diagram showing another conventional example that eliminates such drawbacks. In this case, the light emitted from the laser light source 1 is split by the polarizing beam splitter 10 into two laser beams lla and 11b with different polarization directions: S-polarized light (indicated by * in the figure) and P-polarized light (indicated by 1 in the figure). However, an acousto-optic cell 6 is arranged on one of the optical paths. and acousto-optic cell 6
The laser beams are shifted by a predetermined frequency (Δfo) by being driven by an oscillator 7, and these laser beams are again combined by a polarizing beam splitter 12 and applied to a single mode optical fiber 13. If the single mode optical fiber 13 is a polarization-maintaining optical fiber, the two laser beams of S-polarized light and P-polarized light can be directly guided to the sensor section 14 provided in the measurement area. In this step, two laser beams with different polarization planes are split again by a polarization beam splitter 15, their polarization planes are aligned by a polarization plane rotator 16, and then crossed by a condenser lens I7, thereby forming an intersection area having moving interference fringes. can.

However, in the case of such a speedometer, the acousto-optic cell 6
The oscillator 7 is required to have a high output to drive the oscillator 7, which poses a problem in that it may adversely affect other signal processing circuits. Furthermore, the structure became complicated, making it impossible to reduce the manufacturing cost.

Purpose of the Invention The present invention solves the problems of the conventional laser Doppler velocimeter, and the measurement position can be extended arbitrarily using an optical fiber, and the sensor part can be miniaturized and operated. It is an object of the present invention to provide a laser Doppler speed needle that has improved performance and can also discriminate the direction of particles.

Structure and Effect of the Invention Invention No. 1: An internal mirror type laser light source that emits laser light having two frequency components with a predetermined frequency difference and having linear polarization characteristics with mutually different directions; A polarizing beam splitter that splits the laser beam output from the optical fiber into two components based on the difference in polarization, and the output of the polarizing beam splitter. a polarization plane rotator that changes the polarization direction of one of the laser beams and aligns it with the polarization direction of the other laser beam; and an optical means that causes the laser beam obtained from the polarization plane rotator and the other laser beam to intersect at one point. , a photoelectric converter that converts scattered light from particles passing through the intersection region into an electrical signal, and a frequency measuring device that measures the frequency of the signal (a frequency measuring device that measures the frequency of the signal obtained from the photoelectric converter); The present invention is characterized by comprising a calculation means for calculating the velocity of the particle by comparing it with the frequency difference of the laser light source.

As described above, in the present invention, the laser light source itself has orthogonal polarization characteristics with mutually different directions, and is a laser light source that generates laser beams with a predetermined frequency difference. Therefore, since the output of the laser light source can be used as is, there is no need to provide an acousto-optic cell or an oscillator to drive it in the vicinity of the laser oscillator in the sensor section, making the structure extremely simple and reducing the cost of the device. be able to. In addition, two laser beams with different frequencies are converted into polarization-maintaining single mode 1,
The optical fiber is extended to the measurement area and is applied to the sensor section.The sensor section can also be made small and lightweight, making it possible to place it at any measurement point and improving operability. can. Furthermore, if the two laser beams are crossed using the optical means provided in the sensor section, interference fringes that move at a predetermined speed are formed, making it possible to determine not only the velocity but also the direction of the particles extremely easily at the same time. Become.

DESCRIPTION OF THE EMBODIMENTS FIG. 4 is a block diagram showing the configuration of a laser Doppler velocimeter showing an embodiment of the present invention. In the present invention, two frequency component elements l and f2 with a predetermined frequency difference Δf have linear polarization characteristics with mutually different directions by utilizing the Zeeman effect.
An internal mirror type laser light source is used that generates a laser beam with a FIG. 4 shows an embodiment in which a frequency-stabilized transverse Zeeman laser 2o is used as a laser light source. This Zeeman laser 20 has a laser tube 2 of an internal mirror type He/Ne laser that oscillates in two to three longitudinal modes.
1, a transverse magnetic field is applied by a magnet 22 to narrow the gain width of the laser tube, thereby converting it into a single longitudinal mode. The laser beam converted into a single-longitudinal mode by this transverse magnetic field has two linearly polarized components L f2 whose directions are orthogonal to each other.
A laser beam consisting of In order to stabilize the oscillation frequency, the light leaking from the high-reflection side reflector of the laser is converted into an electrical signal by a photoelectric converter 24 via a polarizing plate 23 at 45 degrees with respect to the transverse magnetic field direction. Then, the optical beats of these laser beams will cause fl, f2
A photoelectric conversion signal having a frequency difference of is obtained. This signal is applied to a phase-locked 7 quad loop 25 so as to keep the difference frequency constant, and its output is used to drive a small fan 27 via a driver 26. In this way, the temperature of the optical cavity is stabilized due to the integral characteristic of the Fee1-Huck loop, and as a result, the fluctuation of the two frequency difference Δf becomes extremely small. You can get light. The laser output of the transverse Zeeman laser 20 is determined by a condensing lens, such as a Selfo laser.
7 to the optical fiber 31 via the lens 30.

The optical fiber 31 is a polarization-maintaining single-mode optical fiber, and its end portion extends to the region to be measured through which the particles to be measured pass. A sensor section 32 having an optical means for separating and crossing this laser light is provided in the region to be measured. The sensor section 32 is provided with a polarization beam splitter 33 at the output end of the optical fiber 3 which separates the laser beam into two laser beams based on the polarization direction of the laser beam. One of the optical paths is provided with a polarization plane rotator 35 that rotates the polarization plane of the mirror 34 and the laser beam to align the other polarization plane, and furthermore, the laser beam that is the output of the mirror 34 is rotated to align the polarization plane of the other laser beam. Condensing lens 37 to condense light
is provided. Further, a condenser lens 39 is provided behind the condenser lens 37 to condense the scattered light from the e-striped region 38, and one end of an optical fiber 40 is fixed at the focal position of the condenser lens 39. Since the optical fiber 40 guides the scattered light to the signal processing section, a normal multimode optical fiber can be used. Now, the signal processing section 50 is provided with a photoelectric converter 51 such as an avalanche type photodiode that converts the output of the optical fiber 40 into an electrical signal. The output of the photoelectric converter 51 is given to a bypass filter 53 via an amplifier 52. The bypass filter 53 provides only the high frequency components of the applied electrical signal to the frequency measuring device 54 at the next stage. The frequency measuring device 54 measures the frequency of a given signal, and may be a frequency analyzer that processes the signal in the frequency domain, or a counter type that processes the signal in the time domain or an F/V using frequency negative feedback. It may also be a converter. The output of the frequency measuring device 54 is sent to the signal processor 5
given to 5.

The signal processor 55 outputs the velocity and direction of the particle p by signal processing based on the frequency difference between two laser beams of the transverse Zeeman laser, and stores the frequency difference between the two laser beams in an internal memory. Alternatively, as shown by the broken line in FIG. 4, signal processing may be performed based on the output of the two frequency differences of the photoelectric converter 24. The output of the signal processor 55 is given to a display 56 and displayed.

Next, the operation of the laser Doppler speed hand of the present invention will be explained. As shown in FIG. 4, the transverse Zeeman laser 20 outputs two laser beams that differ by frequency Δf and have orthogonal polarization directions, and these two laser beams are transmitted directly to the sensor section 32 through a single mode optical fiber 31.
given to.

In the sensing section 32, these laser beams are separated by a polarizing beam brick 33. That is, for example, as shown in the figure, P-polarized light parallel to the plane of the paper, indicated by 1, is passed through as is, and S-polarized light, indicated by . The polarization plane of the S-polarized laser beam reflected by the mirror 34 is rotated by the polarization plane rotator 35 and converted into P-polarization. After aligning the polarization directions of the two Rade beams 36a and 36b in this manner, the beams are focused by the condenser lens 37 so as to intersect at one point, as described above, to obtain interference fringes in the intersection region 3B. The pitch of this interference fringe is the frequency fl of the laser beam of the transverse Zeeman laser 20.

f2 and the intersection angle, and at the same time, the interference fringes move corresponding to the frequency difference between the two laser beams in the transverse Zeeman laser 20, that is, the frequency of 8f. Therefore, this intersection region 38 is defined as the particle p
, the scattered light is given to the photoelectric converter 51 of the signal processing section 50 via the condensing lens 37, 39 and the optical fiber 4o, but the frequency of the electric signal obtained from the photoelectric converter 51 depends on the velocity of the particle p. and the direction. For example, as shown in FIG. 5, the direction and speed can be determined based on the frequencies before and after Δ" as the center. For example, as shown in FIG. 5, the spectrum S of the output signal of the photoelectric converter 51 shifts. If the frequency is lower than Δf, the particle p is in the intersection region 3 in the enlarged view shown in FIG.
It is considered that the interference fringe portion of No. 8 has progressed in the b direction.

The speed can be detected by the difference from shift 1 - frequency Δf. Further, when the particle p moves in the direction a through the intersection region 38, the output dregs vector of the photoelectric converter 51 is larger than the shift frequency Δf,
The frequency difference corresponds to the velocity of particle p. After such signal processing is performed by the signal processor 55, the display 5
6 indicates the velocity and direction of particle p.

As mentioned above, when the signal processor 55 is given two frequency difference signals from the photoelectric converter 24,
Even if there is a slight frequency fluctuation in the output of the transverse Zeeman laser 20, the correct speed can be displayed. That is, if Δf shifts slightly as shown by the broken line in FIG. 5, the frequency of the signal provided by the photoelectric converter 51 will also shift accordingly, as shown by the broken line as signal spectrum s1. The frequency difference between them is equal. Therefore, frequency fluctuations of the transverse Zeeman laser can be canceled out, making it possible to measure speed more precisely.

In this embodiment, a backscattering type laser Doppler velocimeter 6 has been described which collects scattered light scattered backward from a laser beam forming interference fringes and uses it as a detection signal. This is easy to handle because the light emitting part and the light receiving part are integrated as shown in Figure 3, but in order to increase the S/N ratio and obtain stronger scattered light intensity, the It is also possible to apply it to a forward scattering type laser Doppler speed needle that collects scattered light and uses it as a signal.

[Brief explanation of the drawing]

Figures 1 and 3 are block diagrams showing a conventional laser Doppler velocimeter, Figure 2 is a diagram showing the state of interference fringes formed in the intersection area of laser beams, and Figure 4 is a laser Doppler velocity meter according to the present invention. A block diagram showing an example of the configuration of the meter,
FIG. 5 is a detection output cass vector diagram showing the velocity signal obtained from the photoelectric converter. 1---Laser 20---Transverse Zeeman laser 13,
31--One wheel one-seventh optical fiber 10°12,
33--Partial optical beam splinter 35-Polarization plane rotator 37--Condensing lens 38-Cross region 40--One optical fiber 50-One signal processing unit 5
1----One photoelectric converter 54--Frequency measuring device 55
------- Signal Processor Patent Applicant Nihon Kagaku Kogyo Co., Ltd. Agent Patent Attorney Yoshiki Okamoto (and 1 other person) Figure 1 Figure 3 Figure 5 Th Direction continuous d direction

Claims (4)

[Claims] (1) An internal mirror type laser light source that emits laser light having two frequency components with a predetermined frequency difference and having linear polarization characteristics with mutually different directions; a polarization-preserving single modulus 1' optical fiber extended to 1'; a polarization beam splitter that splits the laser beam output from the optical fiber into the two components based on the difference in polarization; and a polarization beam splinter. A polarization plane rotator that changes the polarization direction of one of the output laser beams and aligns it with the polarization direction of the other laser beam, and the laser beam obtained from the polarization plane rotator and the other laser beam are made to intersect at one point. an optical means; a photoelectric converter that converts scattered light from particles passing through the intersection region into an electrical signal; a frequency measuring device that measures the frequency of the burst signal obtained from the photoelectric converter; and the frequency measuring device. 1. A laser Doppler velocimeter, comprising: calculation means for calculating the velocity of a particle by comparing the frequency obtained from the above with the frequency difference of the laser light source. (2) The laser Doppler velocimeter according to claim 1, wherein the gas laser light source is a transverse Zeeman laser. (3) The calculation means is given an electrical signal with a difference in two frequencies obtained from the laser light source, and uses the signal as a reference to compare the frequency obtained from the frequency measuring device to calculate the speed. A laser Doppler velocimeter as claimed in claim 1 or 2. (4) The laser templater speed needle according to any one of claims 1 to 3, wherein the optical means is a focusing lens that focuses two laser beams.