JPH03229155A - Laser doppler speed indicator - Google Patents

Abstract

PURPOSE:To accurately detect a speed at all times regardless of variation in the wavelength of laser light by reflecting and diffracting emitted laser light and making the two diffracted light beams incident on a moving body at nearly the same angle of intersection. CONSTITUTION:The laser light from a laser diode 1 is made incident on a reflection type diffraction grating 10 through a collimator lens 2. The reflected and diffracted light beams 61, 62 or (+1)st order and (-1)st order which are generated by the grating 10 are made incident on the body 7 to be inspected by mirrors 6 and 6' at the angle theta1 of incidence. The light spots of the diffracted light beams 61 and 62 are put one over the other on the body 7 and formed the image on the photodetection part 9a of a photodetector 9 through a condenser lens 8 to life size. In this constitution, the photodetector 9 outputs a signal corresponding to a Doppler frequency regardless of the variation in the wavelength of the laser light. Therefore, the speed can accurately be detected at all times.

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to a speedometer that detects the speed of a moving object or fluid in a non-contact manner, particularly a laser Doppler speedometer that detects the speed by detecting a shift in the frequency of a laser beam. Regarding the meter.

[Prior art]

Conventionally, laser Doppler velocimeters have been used as devices for measuring the moving speed of objects and fluids in a non-contact manner and with high precision. A laser Doppler velocimeter is an effect in which a moving object or fluid is irradiated with a laser beam, and the frequency of the light scattered by the moving object or fluid shifts in proportion to the moving speed (Doppler effect). This is a device that measures the moving speed of the moving object or moving fluid.

An example of a conventional laser Doppler velocimeter is shown in FIG.

1 is a laser, 2 is a collimator lens, 3 is a parallel light beam, 4 is a beam splitter, 6 and 6' are reflecting mirrors, 7 is an object or fluid moving in the direction of the arrow at a speed V, 8 is a condenser lens, 9 is a photodetector.

The laser beam emitted from the laser 1 becomes a parallel beam 3 by the collimator lens 2, is split into three beams 5 and 5' by the beam splitter 4, and is reflected by the reflecting mirrors 6 and 6', and then is parallelized by the collimator lens 2. A moving object or fluid 7 is irradiated with three beams at an incident angle θ. Scattered light by the object or fluid 7 is detected by a photodetector 9 via a condenser lens 8. The frequencies of the scattered light by the three beams undergo Doppler shifts of +Δf and −Δf in proportion to the moving speed V, respectively. Here, if the wavelength of the laser light is λ, then
Δf can be expressed by the following equation (1).

Δf=Vsinθ/λ ・(1)+Δf,
The scattered lights that have undergone a Doppler shift of -Δf interfere with each other and cause a change in brightness on the light receiving surface of the photodetector 9, and the frequency F thereof is given by the following equation (2).

F=2Δ f=2Vsin θ/λ − (2) Therefore, if the frequency of the output signal of the photodetector 9 (hereinafter referred to as Doppler frequency) is measured, the moving object or moving fluid 7 can be detected based on equation (2). The moving speed V can be determined.

In order to downsize the laser Doppler speedometer, H
It is more advantageous to use a semiconductor laser such as a laser diode than to use a gas laser such as an e-Ne laser as a light source. However, the oscillation wavelength of a semiconductor laser is not stable like that of a gas laser, and the oscillation wavelength changes depending on the temperature. As is clear from the above equation (2), the Doppler frequency F is the wavelength λ of the laser light
Therefore, if the wavelength of the laser light changes, the speed of a moving object or moving fluid cannot be detected accurately.

FIG. 2 is a graph (quoted from the 1987 Mitsubishi Semiconductor Data Book - Optical Semiconductor Element Edition) showing the temperature dependence of the oscillation wavelength of a commercially available laser diode. In the same figure,
The part where the wavelength changes continuously is mainly caused by the temperature change in the refractive index of the active layer of the laser diode, and the ratio is O,'05 to 0.06 nm/
'C. On the other hand, the portion where the wavelength changes discontinuously is due to a phenomenon called longitudinal mode hopping, and changes at a large rate of 0.2 to 0.3 nm/'C.

As described above, the oscillation wavelength of a laser diode is extremely unstable, so when using this type of laser as a light source for a laser Doppler speedometer, temperature control using well-known heaters, radiators, temperature sensors, etc. is required. The unit must be installed with a laser diode. However, once these temperature control units are installed,
This is not desirable because the thermometer becomes larger and the price increases.

Furthermore, the above-mentioned longitudinal mode hopping is a phenomenon that occurs due to factors other than temperature fluctuations, and even if a temperature control unit is installed, fluctuations in the oscillation wavelength of the laser diode cannot be completely suppressed.

[Summary of the invention]

The present invention was made in view of the above-mentioned conventional problems, and it is possible to accurately detect the speed of a moving object or moving fluid even if the oscillation wavelength of the laser beam changes, and is a small laser doppler. The purpose is to provide a speedometer.

To achieve this objective, the laser Doppler velocimeter of the present invention makes a laser beam incident on a moving object or a moving fluid at a predetermined angle of incidence, and shifts the frequency of the scattered light from the moving object or moving fluid. A velocimeter that detects changes in the speed of the moving object or moving fluid based on the above-described method includes: a diffraction grating that reflects and diffracts a laser beam emitted from a laser to form first and second diffracted beams; Second
an optical system for causing the diffracted light of and a light receiving means for receiving reflected and scattered light from the moving object or moving fluid illuminated with light, and at least one of the racer and the light receiving means is connected to each optical path of the first and second diffracted light. It is characterized by being located at a location away from the plane of incidence that includes the

According to the present invention, since it has such a configuration, the incident angle θ of the laser light with respect to the moving object or the moving fluid is changed in accordance with the fluctuation of the wavelength λ of the laser light, and thereby the above equation (2) is expressed. The value of sin θ/λ is kept approximately constant. That is, the speed of a moving object or moving fluid can always be accurately detected regardless of wavelength fluctuations of the laser beam. Therefore, a small laser diode (semiconductor laser) whose oscillation wavelength is unstable, such as a laser diode, can be used as a light source, making it possible to provide an unprecedented small laser Doppler velocimeter.

Further, according to the present invention, since at least one of the laser and the light receiving means is provided at a location away from the incident plane including the respective optical paths of the first and second diffracted lights, the speedometer can be made smaller. can. In particular, using reflected diffraction from a diffraction grating,
This is effective because the diffraction grating can be provided close to the moving object or moving fluid when the light receiving means receives reflected and scattered light from a moving object or moving fluid.

Further characteristics and specific embodiments of the present invention will become apparent from the following examples.

〔Example〕

FIGS. 3 and 4 are explanatory diagrams for explaining the principle of speed detection by the laser Doppler velocimeter of the present invention.

Figure 3 shows a transmission type diffraction grating of 1° with a grating pitch of d and a wavelength of λ.
The diffraction angle θ of each order of diffracted light generated by the diffraction grating 10 can be expressed by the following formula.

sinθ=:mλ/d−(3)(m=o,
1.2. ...) Therefore, if each diffraction angle of the 60th order diffracted light other than the 0th order forward light (mO) generated in the diffraction grating 10 is θ, then sin θ, = ±n λ/d ・(4)
(n=1.2...) This is expressed as the following formula. As is clear from equation (4), the ±nn-th order diffraction angle θ7 is the incident laser beam I
The diffraction grating 10 depends on the wavelength λ of
The emission angle of the next beam changes by ±nn from .

FIG. 4 shows a laser Doppler velocimeter incorporating the diffraction grating 10 of FIG. 3 into its optical system. In FIG. 4, other members except the diffraction grating 10 are the same as those shown in the conventional example of FIG. 1, and are given the same reference numerals as in FIG. 1. Therefore, the explanation is omitted here.

In the optical system shown in FIG. 4, the ±nnth forward beams 4142 emitted from the diffraction grating 10 fixed to the optical system are reflected by mirrors 6.6' whose reflective surfaces are parallel to each other, and the ±nnth forward beams are each By all means ±θ. The same angle of incidence θ. The beams are made to be incident on almost the same point on the object 7 to be tested. Note that the reflecting surfaces of the mirrors 6 and 6' are each in a plane perpendicular to the grating arrangement direction of the diffraction grating 10 and the moving direction of the object 7 to be examined.

At this time, the Doppler frequency F corresponding to the output signal from the photodetector 9 can be expressed by the following equation (5) based on the above equations (2) and (4).

F=2Vsin θ. /λ − (5)
2 n V / d In other words, the Doppler frequency F does not depend on the wavelength λ of the laser beam, is inversely proportional to the grating pitch d of the diffraction grating 10, and is proportional to the order n of the diffracted light and the moving speed V of the test object 7. become. Since the pitch d of the diffraction grating 10 is sufficiently stable, the Doppler frequency F is independent of the wavelength of the laser light and depends only on the velocity V of the object 7 to be examined. Therefore, the photodetector 9 outputs a signal that accurately corresponds to the velocity V of the test object 7 even if the oscillation wavelength of a laser (not shown) changes.

An example of a laser Doppler velocimeter using the detection principle described above will be explained with reference to FIG.

In FIG. 5, the same elements as those shown in FIGS. 1 and 4 are given the same reference numerals, and
Explanation regarding these elements will also be omitted.

Here, a laser diode that emits a laser beam with a wavelength λ=0.78 μm is used as the laser 1, thereby reducing the size of the device.

The laser 1 may be a single mode laser or a multimode laser. Further, the diffraction grating 10 is a reflection type diffraction grating with a grating pitch d=1.6 μm, and is coated with AI;! on the surface of a substrate such as a glass substrate. A reflective film made of Au or the like is periodically formed, or grooves are periodically formed on the surface of a glass substrate, and the above-mentioned reflective film is applied to the groove surfaces. As described above, various types of diffraction gratings such as amplitude type or phase type (relief type) can be used as the diffraction grating 10.

The laser light from the laser diode 1 becomes a parallel beam 3 with a diameter of 2 mmφ by the collimator lens 2.
The light enters the fixed reflection type diffraction grating 10 from a direction perpendicular to the grating arrangement direction t of this diffraction grating 10. Diffraction grating 10
The ±1st-order reflected diffraction lights 61 and 62 generated at It is obliquely incident at an angle θ1. Each of the diffracted lights 61 and 62 forms a light spot of approximately 2 mmφ on the object 7 to be tested, and the respective light spots are superimposed on the object 7 to be tested. The photodetector 9 has a light receiving part 9a with a diameter of 0.8 mm, and the condenser lens 8 focuses the central part 0.8 mm of the light spot formed by the two diffracted lights 61 and 62.
An area of 8 mmφ is imaged at the same size on the light receiving section 9a. Therefore, the photodetector 9 efficiently receives interference light in which reflected and scattered lights generated by the respective diffracted lights 61 and 62 interfere with each other, and photoelectrically converts the received light into the light receiving section 9a. Then, the photodetector 9 is as described in (5) above.
A signal corresponding to the Doppler frequency F independent of the wavelength λ, which is F=2V/d by substituting n=1 in the equation, is output.

Here, as shown in FIG. 6(a), two diffracted lights 61
．． Diffraction grating 1 when 62 completely overlaps on test object 7
The distance between 0 and the test object 7 is h=1!, where l is the distance of the mirror 6.6'. ,/D7:7T7F'T...
(6) becomes. Therefore, when f=30 mm, the wavelength λ of the laser beam is 0.78 μm, and the grating pitch d is 1.6 μm, so h=53.7 mm. Incidentally, assuming that the oscillation frequency of laser diode 1 changes by 1% (change of about 30 degrees Celsius depending on the temperature of laser diode 1), the light spots of the two diffracted lights 61 and 62 will change when the frequency changes to the longer wavelength side. The state becomes as shown in FIG. 6(b), and when the frequency changes to the shorter wavelength side, the state becomes as shown in FIG. 6(C). In Figures 6(b) and (c), the two light spots are approximately 0.
．． It shifts by 8mm.

In both cases shown in FIGS. 6(b) and 6(c), since the photodetector 9 simultaneously receives the scattered light generated by the two diffracted lights 61 and 62, the Doppler frequency F A signal corresponding to is output.

The present invention is a further improvement to the laser Dobbler speedometer as shown in FIG. 5, and it becomes possible to further downsize the speedometer. And, Fig. 6(b), (
As shown in c), the deviation between the two light spots can be kept small. Examples of the present invention will be described in detail below.

FIGS. 7(A) and 7(B) are schematic diagrams showing a first embodiment of the present invention, with FIG. 7(A) being a front view and FIG. 7(B) being a side view. In FIGS. 7(A) and 7(B), the same elements as those shown in FIG. 5 are given the same reference numerals as those in FIG. 5. As can be understood by comparing the speedometer shown in FIG. 5 with the speedometer of this embodiment, all the components of the speedometer of this embodiment are the same as those of the speedometer of FIG. The improvement of the speedometer is that the arrangement of the laser diode 1, collimator lens 2, and reflective diffraction grating 10 has been changed. The illustration of the mirror 6.6' is omitted in FIG. 7(B).

The characteristic configuration of this embodiment is shown in the side view of FIG. 7(B). As shown in FIG. 7(B), in this example,
The incident plane (
This corresponds to the page of FIG. 7(A). ) in a place away from
A laser diode 1 and a collimator lens 2 are arranged. Then, the diffraction grating 10 is aligned with this plane of incidence (Z
-X plane) and within a plane (z-X plane) perpendicular to both the moving direction (X direction) of the object 7 to be examined.

In this embodiment, the grating lines of the diffraction grating 10 (the grating arrangement direction t
The diffraction grating 10 is tilted so that the direction of the line (corresponding to a groove or the like perpendicular to It is set to .

On the other hand, the optical axis AX of the illumination system including the racer diode 1 and the collimator lens 2 is an axis (2
direction) so as to form an angle W with respect to the direction. This angle W is given with respect to the same plane as the plane (ZY plane) on which the diffraction grating 10 is inclined. Here W=2
It is set at 8.06°.

Here, as shown in FIG. 8, a lattice plane (t-3 plane) formed by the lattice arrangement direction t and the direction S in which the lattice lines g extend
When the laser beam is incident at an angle θ with respect to the perpendicular U, ±
The angle formed by the plane containing the optical path of the first-order diffracted light beams 61.62 and the perpendicular U is α±The bisecting angle (diffraction angle) of the angle formed by the first-order diffracted light beams 61.62 with each other is θ1, and dsinθ = λ (d: pitch of diffraction grating, λ wavelength) sin θ=COS θ, satisfies the relationship: % sin α.

In this example, α=15°, W=28.06'θ−W−α
Therefore, the diffraction angle θ1;2 of the ±1st-order diffracted light 61.62
9°, and the incident plane including the optical path of the ±1st-order diffracted light 61.62 is Z-X, which is perpendicular to the surface of the object to be inspected.
It becomes a plane parallel to the plane.

Therefore, in this embodiment as well, the fixed diffraction grating 10
The 11th order reflected diffraction light 61.6262 generated by the diffraction angle θ
, z29° from the diffraction grating 10, and the mirror 6.6 has its reflecting surface set perpendicular to the moving direction X of the test object 7.
', and obliquely enter the object 7 at an incident angle θ1. Each of the diffracted lights 61 and 62 forms a light spot of approximately 2 mmφ on the object 7 to be tested, and the respective light spots are superimposed on the object 7 to be tested. The photodetector 9 has a light receiving part 9a with a diameter of 0.8 mm, and the condenser lens 8 collects two diffracted lights 61 and 62.
A central 0.8 mm diameter area of the light spot formed by the light spot is imaged at the same size on the light receiving section 9a. Therefore, the photodetector 9 efficiently receives the interference light in which the reflected and scattered lights generated by the respective diffracted lights 61 and 62 interfere with each other through the light receiving section 9a, and photoelectrically converts the interference light. Then, the photodetector 9 outputs a signal corresponding to the Doppler frequency F, which is independent of the wavelength λ and is F=2V/d, which is obtained by substituting n=1 in the equation (5).

In the speedometer shown in FIG.
In the speedometer of this example, the distance between the laser diode 1 and the collimator lens 2 was 53.7 mm.
Since the illumination system verification meter including the 1st-order diffracted light beam 61 and 62 is provided at a location away from the plane of incidence including the optical path of the ±1st-order diffracted light beams 61 and 62, the distance between the test object 7 and the diffraction grating 10 can be shortened to about h:=32 mm. In addition, when configured in this way, the diffracted light 61.6 when the oscillation frequency of the laser diode 1 changes by 1%
The deviation of each light spot due to 2 is 0. It will be 5mm. That is, the deviation of the light spot is also smaller than in the speedometer shown in FIG.

Therefore, the speedometer of this embodiment is compact and has high detection sensitivity.

FIGS. 9(A) and 9(B) are schematic diagrams showing a second embodiment of the present invention, with FIG. 9(A) being a front view and FIG. 9(B) being a side view. In FIGS. 9(A) and 9(B), the same elements as those shown in FIGS. 5 and 7(A) and (B) are shown in FIGS. 5 and 7(A).

The same reference numerals as those in (B) are used. As can be understood by comparing the speedometer shown in FIG. 5 with the speedometer of this embodiment, the configuration of the speedometer of this embodiment is also exactly the same as that of the speedometer of FIG. Also, the improvements in the speedometer of this example are as follows:
Laser diode 1, collimator lens 2. Another point is that the arrangement of the reflection type diffraction grating 10 is changed. In addition, in the speedometers shown in FIGS. 7(A) and 7(B), the diffraction grating 10 was installed obliquely above the photodetector 9, whereas in the speedometer of this embodiment, the diffraction grating 10 was provided diagonally above the photodetector 9. A diffraction grating 10 is provided horizontally to the surface of the object 7 to be tested.

Note that FIG. 9(A) shows the laser diode 1 and collimator lens 2, and FIG. 9(B) shows the mirror 6.6.
Although not shown, the reference numeral 11 in FIG. 9(B) indicates a reflection point of the diffracted light 61 (62) by a mirror 6' (6), not shown.

In this embodiment, the diffraction grating 1o is provided at approximately the same height as the photodetector 9. As described above, the diffraction grating 10 is provided substantially horizontally on the surface of the object 7 to be examined, and the grating arrangement direction t of the diffraction grating 10 coincides with the moving direction X of the object 7 to be examined.

The optical axis AX of the illumination system verification meter including the laser diode 1 and the collimator lens 2 is at a predetermined angle (θ>n with respect to the X-Y plane with respect to the grating line of the 1° diffraction grating, and the laser diode 1 After the laser beam is converted into a parallel beam by the collimator lens 2, it obliquely enters the fixed diffraction grating 1o.Therefore, the directions in which the ±1st-order reflected diffraction beams 61 and 62 generated by the diffraction grating 10 are emitted are as follows: −
Regarding the Y plane, the grating lines of the diffraction grating 1o and the angle (θ
), and the plane of incidence including the optical paths of these ±1st-order diffracted lights 61 and 62 is inclined with respect to the surfaces of the diffraction grating 10 and the object 7 to be measured. Also in this embodiment, the grating pitch d of the diffraction grating 10 is 1.6 μm, and the wavelength of the laser beam is 0.7 μm.
8 μm, the diffraction angle of the ±1st-order diffracted light 61.62 emitted from the diffraction grating 10 is θ, with respect to the z-X plane.
; It becomes 29°.

The reflected diffracted lights 61 and 62 emitted from the diffraction grating 10 are each reflected by mirrors 6 and 6' whose reflective surfaces are installed perpendicular to the grating arrangement direction t, and are reflected at an angle θ1 with respect to the Z-X plane.
The light is obliquely incident on the object 7 at an angle θ with respect to the Z-Y plane. Each diffracted light beam 61.62 is placed on the test object 7 by several mmφ.
, and each light spot is located on the object 7 to be inspected.
Can be superimposed on top. The photodetector 9 has a light receiving part 9a with a diameter of 0.8 mm, and the condenser lens 8 collects two diffracted lights 61.6.
The area in which the light spots formed by the light spots 2 overlap with each other is imaged at the same size on the light receiving section 9a of the photodetector 9. Therefore, the photodetector 9 efficiently receives the interference light in which the reflected and scattered lights generated by the respective diffracted lights 61 and 62 interfere with each other at the light receiving section 9a.
Convert photoelectrically. Then, the photodetector 9 outputs a signal corresponding to the Doppler frequency F, which is independent of the wavelength λ and is F=2V/d obtained by substituting n=1 in equation (5).

In this embodiment, an illumination system including a laser diode 1 and a collimator lens 2, and a detection unit including a condenser lens 8 and a photodetector 9 are installed at a location away from the incident plane including the optical paths of each of the diffracted lights 61 and 62. Since both systems are arranged, Figure 7 (
The speedometer is even more compact than the speedometers shown in A) and (B). For example, in the speedometer of this embodiment, the distance between the diffraction grating 10 and the test object cloth 7 along the incident plane including the optical paths of each of the diffracted lights 61 and 62 can be set to about h' = 23 mm. This is possible, and the distance between the diffraction grating 10 and the test object 7 can be suppressed to 20 mm or less. In addition, in the speedometer of this embodiment, the oscillation frequency of the laser diode 1 is 1
% change, the deviation of each light spot due to the diffracted light 61.62 can be suppressed to about 0.35 mm.

10(A) and 10(B) are schematic diagrams showing a third embodiment of the present invention, FIG. 10(A) is a front view, and FIG. 10(B)
is a side view. Figure 10(A).

In (B), the same elements as the t-element shown in FIGS. 5, 7 (A), (B), and 9 (A), (B) include ), (B) and Figure 9 (A), (B)
The same symbol as the one is the symbol. In addition, Fig. 10 (A
) and (B), the reference numeral 12 indicates a double-sided mirror.

The speedometer of this embodiment is improved over the speedometer of FIG. 5 by using a double-sided mirror 12 and a laser diode 1. Collimator lens 2. This is the point where the condenser lens 8 and the photodetector 9 are arranged horizontally with respect to the surface of the object 7 and the diffraction grating 10.

In this embodiment, the grating arrangement direction t of the diffraction grating 10 and the moving direction X of the test object 7 match, and as described below,
The plane of incidence containing the optical path of each of the diffracted lights 61, 62 is parallel to these directions t, x. This plane of incidence is perpendicular to the surface of the object 7 to be inspected.

The double-sided mirror 12 is formed by depositing reflective films such as Af on both sides of a glass substrate, or by forming a metal plate. The double-sided mirror 12 is installed obliquely between the test object 7 and the diffraction grating 10 so that the reflective surfaces formed on both sides form an angle of 45° with respect to the surface of the test object 7 with respect to the ZY plane. An illumination system including a laser diode 1 and a collimator lens 2 is provided to direct the laser beam to the upper reflective surface of the double-sided mirror 12, and the optical axis AX of this illumination system is set parallel to the grating lines of the diffraction grating 10. be done. On the other hand, the condenser lens 8 and the photodetector 9
A detection system including the double-sided mirror 12 is provided to receive the light reflected by the lower reflective surface of the double-sided mirror 12, and the optical axis of this detection system is set parallel to the optical axis AX of the illumination system. The optical axis AX of the illumination system is bent at a right angle by the reflective surface of the double-sided mirror 12 and intersects perpendicularly to the diffraction grating 10, and the optical axis of the detection system is also bent at a right angle by the reflective surface of the double-sided mirror 12. , intersect perpendicularly to the surface of the test object 7.

In such a configuration, the laser beam from the laser diode 1 is converted into a parallel beam by the collimator lens 2, and is incident on the reflective surface of the double-sided mirror 12. The laser beam reflected by the reflective surface of the double-sided mirror 12 is perpendicularly incident on the fixed diffraction grating 10, and is reflected and diffracted by the diffraction grating 10. Of the reflected diffracted lights generated by the diffraction grating 10, the ±1st-order reflected diffracted lights 61 and 62 have a diffraction angle θ1, as in each of the above embodiments.
exiting from the diffraction grating 10 at 29°, each mirror 6.6
’. Here, the dimensions and arrangement of the double-sided mirror 12 are determined so as not to block the respective optical paths of the ±1st-order diffracted lights 61 and 62 generated by the diffraction grating 10. Further, each reflective surface of the mirrors 6 and 6' is in a plane perpendicular to the grating arrangement direction of the diffraction grating and the moving direction X of the object 7 to be examined.

The diffracted lights 61 and 62 are each reflected by mirrors 6 and 6', and are obliquely incident on the object 7 to be inspected at an incident angle θ1. Each of the diffracted lights 61 and 62 forms a light spot of approximately 2 mmφ on the object 7 to be tested, and the respective light spots are superimposed on the object 7 to be tested. The photodetector 9 has a light receiving part 9a with a diameter of 0.8 mm.
The condensing lens 8, together with the double-sided mirror 12, focuses the central part 0.8 of the light spot formed by the two diffracted lights 61 and 62.
An area of mmφ is imaged at the same size on the light receiving section 9a.

Therefore, the photodetector 9 detects the interference light, in which the reflected and scattered lights generated by the respective diffracted lights 61 and 62 interfere with each other, to the double-sided mirror 1.
2, the light is efficiently received by the light receiving section 9a, and photoelectrically converted. Then, the photodetector 9 is n=1 in the above equation (5).
A signal corresponding to the Doppler frequency F independent of the wavelength λ, which is obtained by substituting F=2V/d, is output.

In this embodiment, a double-sided mirror 12 is installed obliquely between the diffraction grating 10 and the object to be inspected 7, and the laser diode 1. Collimator lens 2. Condensing lens 8. Remove the photodetector 7 and attach the diffraction grating 1.
7 (A), (
The speedometer becomes even more compact than the speedometer shown in B). According to the configuration of this embodiment, it is possible to set the distance between the diffraction grating 10 and the object to be measured to be approximately 20 mm, and when the oscillation frequency of the laser diode 1 changes by 1%, the diffracted light 61.62 The deviation of the light spot is 0.30m
It can be suppressed to about m.

In the first to third embodiments of the present invention described above, ±1st-order reflected diffraction light was used as a pair of diffracted lights illuminating the test object 7, but ±2nd-order and ±3rd-order diffracted light Any pair of diffracted lights of the same positive and negative orders can be used for illumination. It is also possible to use pairs of diffracted lights of different orders.

Further, in the first to third embodiments, the grating arrangement direction t of the diffraction grating 10 coincides with the moving direction X of the test object 7, but the grating arrangement direction t is set to The diffraction grating 10 can also be tilted so as to form a predetermined angle with the moving direction X with respect to the incident plane that includes the optical path of the diffracted light. Also, instead of tilting the diffraction grating 10 in this way, the illumination system (1,
The parallel light beam from 2) can also be obliquely incident on the diffraction grating 10 with respect to this plane of incidence. In either configuration, if the pair of diffracted beams are made to enter the test object 7 at approximately the same angle at approximately the same angle of intersection as the angle formed by each other when they are emitted from the diffraction grating 10, wavelength fluctuations of the laser beam can be avoided. Accurate speed detection becomes possible.

FIG. 11 is a schematic diagram showing an example in which the laser Doppler velocimeter of the present invention is used for facsimile paper feed control.

In FIG. 11, the reference numeral 13 indicates the speedometer of the present invention, and the speedometer 13 has one of the configurations shown in the first to third embodiments. 14 is a belt on which the paper 19 is placed; 15 is a paper feeding unit that supplies the paper 19 onto the belt 14; 16 and 17 are rollers; 16 is a belt roller pivotally supported on the main body; 17;
18 indicates a drive roller, and 18 indicates a drive motor to which the drive roller 17 is attached. The belt 14 is connected to the drive roller 1
7 and the belt roller 16 as shown in the figure,
By rotating the drive roller 17 in the direction of the arrow by the drive motor 18, the drive roller 17 and the belt roller 16 cooperate to move the belt 14.

The paper 19 supplied from the paper feed unit 15 is transferred to the belt 14
It is placed on top and moves in the direction of the arrow as the heald 14 moves. The speedometer 13 irradiates the moving paper 19 with racer light and receives the scattered light generated by the paper 19 with a photodetector.

The state of such irradiation and light reception is as shown in FIGS. 7 to 10. In this embodiment, an output signal from the photodetector of the speedometer 13 (a signal according to the Doppler frequency) is input to the speed detection circuit 110. The circuit 110 detects the moving speed of the paper 19, ie, the moving speed of the belt 14, based on the frequency of the output signal from the photodetector. Information on the speed detected by the circuit 110 is input to a control circuit 111, and the circuit 111 controls the rotational speed of the drive motor 18 via a drive motor driver 112. In this control, the circuit 111 inputs a correction signal to the driver 112 so that the moving speed of the belt 14, that is, the moving speed of the paper 19 is constant, and in response to this signal, the driver 112 adjusts the rotational speed of the drive motor 118. This is to make adjustments. As a result, the feeding speed of the paper 19 becomes almost constant, and it is also possible to cancel periodic speed changes due to eccentricity of the driving roller, which conventionally occurred when speed-running was controlled only by controlling the rotational speed of the driving roller. , paper can be fed more reliably at a constant speed.

In addition, as mentioned above, the speedometer of the present invention is very small and has a small number of parts, so it is low cost.
It can be effectively used in devices such as facsimiles.

In FIG. 11, an image is written on paper 19 that is fed at a constant speed by a print head (not shown). In this example,
Since the image is written while the paper 19 is moving very accurately (movement in the sub-scanning direction when writing the image), high-quality printing is possible.

In the embodiment shown in FIG. 11, a facsimile machine is used as an example, and the present speedometer is used as an image writing (printing) device, but conversely, the present speedometer can also be used as an image reading device.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to accurately detect the speed even if the wavelength of the laser beam varies, and furthermore, it is possible to further downsize the speedometer. Furthermore, the deviation of the light spot due to the two diffracted lights due to the wavelength fluctuation of the laser light is also reduced, and there is an effect that a stable detection signal can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a conventional example of a laser Doppler velocimeter. FIG. 2 is a graph showing the temperature dependence of the oscillation frequency of a racer diode. FIG. 3 is an explanatory diagram of the opening grating. FIG. 4 is an explanatory diagram of a laser Doppler velocimeter using a diffraction grating. FIG. 5 is a specific configuration diagram of a laser Doppler velocimeter using a diffraction grating. FIG. 6 is an explanatory diagram of the deviation between two spots. FIG. 7 is a schematic diagram showing a first embodiment of the present invention. FIG. 8 is an explanatory diagram regarding oblique incidence of laser light onto a diffraction grating. FIG. 9 is a schematic diagram showing a second embodiment of the present invention. FIG. 10 is a schematic diagram showing a third embodiment of the present invention. FIG. 11 is a schematic diagram showing an example in which the speedometer of the present invention is used to control facsimile paper feeding. 1...Racer diode 2...Collimator lens 3...Parallel laser beam 4...Heam splitter 61.62...Diffracted light 6.6'...Mirror 7...Test object 8...Condensing lens 9...Photodetector 9a...Light receiving section 10...Diffraction grating 11...Reflection point 12...Double-sided mirror 13...Laser Doppler speed meter Torazen transfer number The case of 赫叡1'1生 さすこ Tc(''c) Fig. 5 (A) CB) Mao 6 Fig. υ (A') (A)

Claims (1)

[Scope of Claims] (1) A laser beam is made incident on a moving object or moving fluid at a predetermined angle of incidence, and the moving object or moving fluid is detected based on the frequency shift of scattered light from the moving object or moving fluid. In a speedometer that detects changes in speed, the laser beam emitted from the laser is reflected and diffracted to
a diffraction grating that forms a diffracted light beam, and a diffraction grating that intersects the first and second diffracted beams at an intersection angle that is approximately the same as the angle that the first and second diffracted beams make with each other when they are emitted from the diffraction grating. The laser includes an optical system that makes the light enter the moving fluid, and a light receiving means that receives reflected and scattered light from the moving object or the moving fluid illuminated by the first and second diffracted lights by the optical system, A laser Doppler velocimeter characterized in that at least one of the light receiving means is provided at a location away from an incident plane that includes each optical path of the first and second diffracted lights. (2) The optical system causes the first and second diffracted lights to enter the moving object or the moving fluid at substantially the same angle as the diffraction angle of each diffracted light by the diffraction grating. A laser Doppler velocimeter according to scope (1). (3) The laser Doppler velocimeter according to claim (2), wherein the first laser is a semiconductor laser. (4) The laser Doppler velocimeter according to claim (3), wherein the light receiving means is provided to receive reflected and scattered light from the moving object or moving fluid. (5) The first and second diffracted lights are of ±nth order (n=1, 2,
The laser Doppler velocimeter according to claim (4), characterized in that the laser Doppler velocimeter is composed of diffracted light of (3, . . . ).