GB2149993A

GB2149993A - Laser doppler velocimeter

Info

Publication number: GB2149993A
Application number: GB08428254A
Authority: GB
Inventors: Kazuo Kyuma; Toshio Shinnishi; Shuichi Tai; Kiyoshi Yoda; Syotaro Fujino
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1983-11-10
Filing date: 1984-11-08
Publication date: 1985-06-19
Also published as: GB8428254D0; DE3441088C2; DK506284A; GB2149993B; DE3441088A1; DK506284D0

Abstract

A laser Doppler velocimeter for detecting the velocity of a moving object such as a fluid flow comprises: a laser 1; a transmitting optical system 50 which includes optical fibers 2 carrying the light from the laser source, collimating lenses 5 adjacent to the end of optical fibers 2 which roughly collimate the light emitted by the optical fibers 2, and convex lenses 6 for modifying the light beam from the collimating lenses such that its edges are parallel in the measurement region (Fig. 4); a receiving optical system 16 which detects a laser light scattered by the moving object; and heterodyne circuitry which obtains the velocity of the moving object by beating the various components of the radiation received at diode 12. <IMAGE>

Description

SPECIFICATION A laser Doppler velocimeter FIELD OF THE INVENTION The present invention relates to a laser Doppler velocimeter (hereinafter referred to as "LDV") with optical fibers for detecting the velocity of a moving object.

BACKGROUND OF THE INVENTION For measuring velocities or vibrations, the laser Doppler effect has been utilized. The design principle is based on the frequency shift of the laser light scattered by a moving object. The Doppler frequency is measured by observing the beats between the frequency-shifted signal and the original reference light beams. This kind of velocimeter has several advantages such as noncontact sensing, high spatial resolution.

In order to explain a prior art LDV, reference will be made to Fig. 1. The reference numeral 1 designates a laser source, and the reference numeral 2 designates an optical fiber. The reference numeral 3 designates a beam splitter which splits the laser light from the laser source 1 into two beams. The reference numeral 5 designates a micro lens, and the reference numeral 4 designates a transmitting optical probe made up by unifying the micro lens 5 and the optical fiber 2. The reference numeral 8 designates a moving object such as fluid. The reference numeral 9 designates a particle for scattering the laser light in the moving object 8. The reference numeral 11 designates a receiving optical probe made up by attaching the micro lens 5 to the optical fiber 2.The reference numeral 1 2 designates a photo detector such as an avalanche photo diode (hereinafter referred to as "APD"), a pin photo diode (hereinafter referred to as "PINPD"), and a light-electron amplifier. The reference numeral 1 3 designates an optical receiver constituted by a pre-amplifier, a band pass filter, and a main-amplifier. The reference numeral 14 designates a case for including and fixing the probes.

The device will be operated as follows: A laser light emitted from the laser source 1 is split into two beams by the beam splitter 3, and the two laser beams are input to the two transmitting optical probes 4, respectively. The two transmitting optical probes 4 are arranged such that the emitted laser light from each probe crosses with together in the region where the moving object 8 exists, whereby the laser light is scattered by the light scattering particle 9 passing through the intersecting region at the velocity of V, the scattered light receiving a Doppler shift in its frequency. The back scattered light among the scattered lights is received by the receiving optical probe 11, and the laser light received is guided to the photo detector 1 2 through the optical fiber 2.The respective scattered light corresponding to the respective transmitting optical probe 4 receives a Doppler shift which is different from each other relative to the emitted light from the respective transmitting optical probe 4, and these scattered lights are heterodyne-detected by the photo detector 1 2. The detected frequency fD is represented by the following formula, fD= 2 n VI sin 6 2 (1) Where, n refractive index of the moving object 8; A light wavelength in vacuum; 8 intersecting angle of the two output beams from the micro lens; and Vl velocity (V) component in a direction diagonal with the vertical bisector of the optical axises of the two emitted lights.

Using the formula (1), it is possible to obtain the velocity component Vl from the detected frequency f,.

Under the prior art LDV of such construction, the expanding angle of the emitted laser light from the respective transmitting optical probe 4 is large, thereby giving rise to measuring errors caused by the deviations of the distance between the end of the transmitting optical probe 4 and the light scattering particle 9 of the moving object 8. That is, as shown in Fig. 2, if it is presumed that the expanding angle of the emitted laser light from the transmitting optical probe 4 is a, the detected frequencies fa, fb, and fc are represented by the following formulae (2), (3), and (4), respectively when the moving object 8 exists in the respective region a, b, and c where the two emitted lights intersect with together.

fa 2 n Vi ~~~~~ A 2 (2) fb= 2 n vl sin 6 A 2 (3) fc= 2 n V ~~~~~ (4) 2 In this way, the detected frequency fD changes caused by the deviations of the positions of the moving object 8 as shown in the above formulae (2) to (4), resulting in inaccuracy of the measurement.

Another prior art LDV which uses an optical fiber in the pick up is used for measuring the velocity of the blood flow in femoral veins of a rabbit. The optical fiber to be inserted into the veins has a simple structure consisting of the end thereof being cut diagonally. It is possible to measure the velocity in a range of 0.01 to 10 cm/s.

OBJECTS AND SUMMARY OF THE INVENTION The present invention is directed to solve the problems pointed out above, and has for its object to provide a laser Doppler velocimeter capable of decreasing measuring errors caused by the deviations of the distance between the end of the transmitting optical probe and the moving object.

Another object of the present invention is to provide a laser Doppler velocimeter capable of lengthening the distance from the transmitting optical probe to the moving object.

Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

According to the present invention, a convex lens is located in front of the transmitting optical probe so as to collimate the emitted light from the transmitting optical probe approximately in parallel in the neighbour of the measuring region where the moving object exists.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram showing a prior art LDV; Figure 2 is a diagram exemplifying the operation of the LDV of Fig. 1; Figure 3 is a schematic diagram showing one embodiment of the present invention; Figure 4 is a diagram exemplifying the operation of the device of Fig. 3; Figure 5 to Figure 11 are schematic diagrams showing other embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be particularly made to Fig. 3 which shows a first embodiment of the present invention. The reference numeral 50 designates a transmitting optical system for leading the laser light from the laser source 1 to the moving object 8. In the transmitting optical system 50, the reference numeral 3 designates a beam splitter which splits the laser light from the laser source 1 into two beams. The reference numerals 1 spa and 1 sub designate transmitting optical probes both of which consist of an optical fiber 2, a micro lens 5 (collimating lens) provided adjacent to one end of the optical fiber 2, and a convex lens 6 unified with the micro lens 5 through a supporting member 7, respectively.The transmitting optical probes 1 5a and 1 sub operate to collimate the laser light from the optical fiber 2 approximately in parellel, and to further collimate the collimated laser light in parallel in a predetermined region. The two transmitted optical probes 1 spa and 1 sub are arranged such that the emitted lights from the probes 1 spa and 1 sub intersect with together in the predetermined region where the moving object 8 exists.

The reference numeral 60 designates a receiving optical system which detects the laser lights obtained from the output lights of the two transmitting optical probes 1 spa and 1 sub being scattered by the moving object 8 which passes through the predetermined region. In this receiving optical system 60, the reference numeral 1 6 designates a receiving optical probe consisting of an optical fiber 2, a micro lens 5 provided adjacent to one end of the optical fiber 2, and a convex lens 10 unified with the micro lens 5 through a supporting member 7. The reference numeral 12 designates a photo detector for receiving the laser light from the receiving optical probe 16. The reference numeral 1 3 designates an optical receiver.Furtheremore, there is provided a means (not shown) for obtaining the velocity of the moving object 8 from the output of the receiving optical system 60. An optical fiber with its surface polished, or an optical fiber with a micro lens can be used as the receiving optical probe 1 6. The other construction of the device is the same as that shown in Fig. 1.

Fig. 4 shows the positional relation between the micro lens 5 and the convex lens 6, and the path of the emitted light, wherein the convex lens 6 is located at a position of the focus distance f of the convex lens 6 apart from the end of the micro lens 5.

The device will be operated as follows: The laser light emitted from the laser source 1 is split into two beams by the beam splitter 3, and are input to the transmitting optical probes 1 spa and 1 sub, respectively. In the respective transmitting optical probe 1 5a and 1 sub, the laser light from the optical fiber 2 is collimated approximately in parallel by the micro lens 5, and the laser light from the micro lens 5 is further collimated in parallel by the convex lens 6.Accordingly, the light emitted from the respective transmitting optical probe 1 spa and 1 sub forms a beam west BW in a predetermined region which includes the focus position of the convex lens 6 at its center, that is, in the region where the moving object 8 exists, thereby forming a parallel light beam in this region.

In the region where the beam west BW is formed, the laser lights emitted from the transmitting optical probes 1 5a and 1 sub intersect with together at an angle of 6. Thus, if there is a moving object 8 (light scattering particle 9) which passes through this intersecting region at the velocity of V, the laser light is scattered with receiving Doppler effect, and the back scattered light among the scaftered lights is received by the receiving optical probe 16.The laser light received by the receiving optical probe 1 6 is guided by the optical fiber 2 to the photo detector 1 2. The scattered lights obtained from the output lights of the two transmitting optical probes 1 spa and 1 sub are heterodyne-detected by the photo detector 1 2. The heterodyne-detected frequency fD is represented by the following formula as described above.

fD= 2 n Vl sin - As the beam west BW is formed in the intersecting region, the detected frequency fD can be represented by the formula (1) at any points in the intersecting region, such as points a,, b,, and c1 shown in Fig. 4, and measuring errors do not arise regardless of the positional deviation of the moving object 8.

Thus, a current of detected frequency fD flows through the photo detector 12, and it is amplified by the optical receiver 1 3. The current is frequency-analized by a spectrum analizer, a F-V converter, or a frequency trucker (not shown) to obtain the velocity component Vl of the moving object 8 using the above formula (1).

Under the embodiment of such construction, a convex lens 6 is provided at the position which is the focus distance f apart from the micro lens 5 of the respective transmitting optical probe 1 spa and 1 sub, and the laser light emitted from the respective transmitting optical probe 1 spa and 1 sub is collimated in parallel in the neighbour of the measuring region where the moving object 8 exists, thereby decreasing measuring errors caused by the deviation of the distance from the end of the respective transmitting optical probe 1 spa and 1 sub to the moving object 8 to a great extent.Furthermore, it is possible to widen the region where the parallel light beam is obtained by using proper ones as the micro lens 5 and the convex lens 6. This also makes it possible to lengthen the distance between the optical probe and the measuring object.

Such device is applicable to the measurement for high temperature materials.

The second embodiment of the present invention will be described with reference to Fig. 5 which shows the construction of one probe of the transmitting optical system and the path of the lights emitted therefrom. The main purpose of this embodiment is to make the laser light from the transmitting optical probe parallel in the region from the focus position of the convex lens to the position of a predetermined distance apart from the focus position.

The convex lens 6 is located at a position apart from the end of the micro lens 5 by a distance I, given by r1 = #0/(n0 #A), #1 = n0 #A r0, (5) where, f focus distance of the convex lens 6; r1 beam radius of the emitted light from the emitting end of the micro lens 5; and O maximum angle of the emitted light from the optical axis.

Now, if it is supposed that a graded index rod lens (GRIN lens) of 0.25 pitch is used as the micro lens 5, the r1, and the O will become rl = Bo/cn, JAI, (6) 6)1 = no 47 r0, (7) where, r0 core radius of the optical fiber used; 80 numerical aperture of the optical fiber used; nO refractive index on the optical axis of the GRIN lens; and ",/'A refractive index distribution constant of the GRIN lens.

When an optical fiber with a micro lens is located at a position apart from the convex lens 6 by the distance l1, it is possible to obtain a parallel light beam in the region from the position of focus distance f apart from the convex lens 6 to the position of distance 12 therefrom at the side of the moving object 8, wherein the 12 is given by (refer to Fig. 5), l1 . f l2 = -- (8) l1 - f The beam spot size (beam diameter) D of the parallel light beam will becomes D = 2f The laser lights emitted from the two transmitting optical probes 1 spa and 15b intersect with together at an angle of 8 in the region where the respective emitted lights become parallel.It is possible to obtain the velocity of the moving object 8 by executing the same position as described above.

In this embodiment the laser light emitted from the two transmitting optical probes 15a and 15b are collimated to form a parallel light beam in the intersecting region similarly as the embodiment of Fig. 4, and measuring errors do not arise regardless of the positional deviations of the moving object 8 (light scattering particle 9). Furthermore, the region where the laser lights emitted from the transmitting optical probes 15a and 15b become parallel with together is determined by the distance 12 given by the above formula (8), and the 1, is given by the formula (5), whereby the upper limit of the distance of the region is represented by the following formula (10).

Accordingly, it is possible to make the upper limit 12 larger by using a GRIN lens having a large value of n0VA or an optical fiber having a larger value of rO/OO. For example, if it is presumed that n0'mA= 0.978, r0 = 25 x 10-3mm, = = 0.2 rad, f = 100 mm, the 12 becomes 1 295mm. It is especially effective to a velocimeter for high temperature materials. In this case, the beam spot size D becomes 4.9 mum.

The third embodiment of the present invention will be described with reference to Fig. 6 which shows the construction of one probe of the transmitting optical system and the path of the laser light emitted therefrom. The main purpose of this embodiment is to make the laser light from the transmitting optical probe parallel in the region from the center of the convex lens to the focus position thereof.

That is, the convex lens 6 is located at a position 1,' from the micro lens 5, wherein the 1,' is given by l'1 = f - r1 / #1 . (11) It is possible to obtain a parallel light beam in the region from the convex lens 6 to the position of focus distance f apart therefrom. The beam spot size D of the parallel light beam is expressed by o = . (12) Under such construction, the laser lights emitted from the two transmitting optical probes 1 spa and 1 sub always intersect with together at an angle of S in the region where the respective emitted laser light becomes parallel with together. Accordingly, there do not arise measuring errors regardless of the positional deviations of the moving object 8.

When a GRIN lens which has 0.386 of n0VA is used as the micro lens 5, an optical fiber which has 2pm of r0 and 0.2 of 0o is used, and the focus distance f of the convex lens 6 is 700mm, the 1,' becomes 28.8mm, and it is possible to obtain a parallel light beam of beam spot size 1.08mm in the region of 700mm length.

In the above illustrated embodiments the receiving optical probe is located between the two transmitting optical probes, but it can be located outside of the two transmitting optical probes.

Also, both of the two transmitting optical probes and the receiving optical probe are placed on a plain, but they may be placed on different plains, respectively.

Furthermore, fluid is used as a moving object, but rotating or proceeding moving objects of solid or the like can be used. The LDV of the present invention can be modified to measure the 3-D vibrations and fluid flow rates.

The laser propagates in space between the laser source and the beam splitter, but there may be provided an optical fiber therebetween.

The two transmitting optical probes and the receiving optical probe can be unified by a case.

The scattered light is guided to the photo detector through the receiving optical probe, but an APD can be used as the photo detector so that the scattered light may be received directly.

The transmitting optical system and the receiving optical system are arranged at the same side relative to the moving object and the back scattered light among the scattered lights is received to be detected in the above illustrated embodiments. Fig. 7 shows an alteration to this fact wherein the transmitting optical system and the receiving optical system are arranged at the opposite sides relative to the moving object 8, respectively, and wherein the laser light which is forward scattered by the moving object 8 is received by the convex lenses 1 0a and 1 Ob, and the receiving optical probe 16.

The above illustrated embodiments are examples of a differential type optical fiber LDV which uses two transmitting optical probes. But the present invention is applicable to a reference type LDV which uses an optical fiber probe for both of transmitting and receiving. An embodiment of such type is shown in Fig. 8. The transmitting optical system comprises a transmitting and receiving optical probe 20a having the same construction as the transmitting optical probe of Fig. 3, and the receiving optical system comprises the transmitting and receiving optical probe 20a, an optical probe 20b which transmits the scattered light led by the probe 20a and the lights which is reflected at the other end of the transmitting and receiving optical probe 20a and passed through the optical directional coupler 3', and a photo detector 1 2.

In this embodiment of Fig. 8, the velocity of the moving object 8 is obtained from the Doppler shift frequency in the scattered light with reference to the light reflected at the other end of the transmitting and receiving optical probe 20a. The same effects as those discribed above can be obtained.

Furthermore, the laser is split into two beams in the neighbour of the laser source 1 under the above illustrated embodiments. But the laser may be split into two beams after the laser being transmitted by one transmitting optical fiber 2 as shown in Fig. 9. The reference numeral 1 7 designates a reflective mirror.

Two other embodiments wherein the laser is split into two beams after being transmitted by one transmitting optical fiber 2 will be described with reference to Fig. 10 and 11.

In the embodiment of Fig. 10 the laser light from the laser source is transmitted by an optical fiber 2, and it is collimated to form an approximately parallel light beam by the micro lens 5, and the laser light emitted from the micro lens 5 is split into two beams by the beam splitter 3.

The split lights are led to the convex lens 6 by the reflective mirrors 1 7a and 1 7b, respectively, and they are collimated by the convex lens 6 to form a parallel light beam in the neighbour of the moving object 8.

In the embodiment of Fig. 11 the parallel light beam, which is obtained by collimating the laser light in similar manner as in Fig. 10, is further collimated by a convex lens 6 before entering the beam splitter 3, thereby obtaining parallel light beams in a predetermined region after passing through an optical block consisting of the beam splitter 3 and two reflective mirrors 1 7a and 1 7b, which region is located in the neighbour of the moving object 8.

The micro lens 5, the beam splitter 3, the reflective mirror 17, the convex lens 6, and the lens 10 of the receiving optical system can be unified to form a sensor head in the embodiments of Figs. 9 to 11.

Under these embodiments of Figs. 9 to 11 it is possible to decrease measuring errors, and also it is possible to lengthen the distance between the optical probe and the moving object.

These are quite advantageous in practical use.

Besides, single-mode fibers, polarized plain holding fibers, and multi-mode fibers can be used as optical fibers.

As evident from the foregoing description, according to the present invention, a convex lens is located in front of a transmitting optical probe so as to make the laser beam emitted from the transmitting optical probe a parallel light beam, thereby lengthening the distance from the end of the transmitting optical probe to the moving object, and further decreasing measuring errors caused by the deviations of distances to a great extent. Thus, the LDV of the present invention has superior and practical performance for measuring velocities and mechanical vibrations.

Claims

1. A laser Doppler velocimeter for detecting the velocity of a moving object with using laser, which comprises: a laser source which emits a laser light; a transmitting optical system which includes an optical fiber for leading the laser light from thelaser source, a collimating lens which, provided adjacent to the optical fiber, collimates the laser light from the optical fiber approximately in parallel, and a convex lens for collimating the laser light from the collimating lens in parallel in a predetermined region; a receiving optical system which detects the laser obtained from the output light of the transmitting optical system being scattered by a moving object; and a means for obtaining the velocity of the moving object from the Doppler shift of the output light of the receiving optical system.

2. A laser Doppler velocimeter as defined in Claim 1, wherein the transmitting optical system and the receiving optical system are located at the same side of the moving object, and the receiving optical system receives the back scattered light from the moving object.

3. A laser Doppler velocimeter as defined in Claim 1, wherein the transmitting optical system and the receiving optical system are located at the opposite sides of the moving object, respectively, and the receiving optical system receives the forward scattered light from the moving object.

4. A laser Doppler velocimeter as defined in Claim 1, wherein the convex lens is located at a position of the focus distance of the convex lens apart from the end of the collimating lens, and the predetermined region is established such that the center thereof is located at the focus position of the convex lens.

5. A laser Doppler velocimeter as defined in Claim 1, wherein the convex lens is located at such a position that the laser from the collimating lens becomes parallel in a region of a predetermined distance from the focus position of the convex lens at the side of the moving object.

6. A laser Doppler velocimeter as defined in Claim 1, wherein the convex lens is located at such a position that the laser light from the collimating lens becomes parallel in a region from the center of the convex lens to the focus position thereof.

7. A laser Doppler velocimeter as defined in Claim 1, wherein the transmitting optical system comprises a beam splitter which splits the laser light from the laser source, and two transmitting optical probes which, each consisting of an optical fiber, a collimating lens, and a convex lens unified with together, are located such that the respective output light crosses with each other in a predetermined region, wherein the receiving optical system receives the laser lights obtained from the outputs of the two transmitting optical probes being scattered by the moving object which passes through the predetermined region, and wherein the velocity obtaining means obtains the velocity of the moving object from different Doppler shifts in the two scattered lights.

8. A laser Doppler velocimeter as defined in Claim 7, wherein the receiving optical system comprises a receiving optical probe having a lens system including a convex lens and an optical fiber, and a photo detector for receiving the laser light from the receiving optical probe.

9. A laser Doppler velocimeter as defined in Claim 7, wherein the receiving optical system comprises a lens system including a convex lens, and a photo detector for receiving the light from the lens system directly.

10. A laser Doppler velocimeter as defined in Claim 1, wherein the transmitting optical system comprises a transmitting and receiving optical probe which consisting of an optical fiber, a collimating lens, and a convex lens, and wherein the receiving optical system comprises said transmitting and receiving optical probe which receives again the light obtained from the output of the transmitting and receiving optical probe being scattered by the moving object, an optical probe which leads the scattered light led by the transmitting and receiving optical probe and the light obtained from the laser light guided by the transmitting and receiving optical probe being reflected at the other side of the transmitting and receiving optical probe, guided again thereby, and passing through an optical directional coupler, and a photo detector which receives the laser light from the optical probe, and wherein the velocity obtaining means obtains the velocity of the moving object from the Doppler shift in the scattered light with reference to the reflected light.

11. A laser Doppler velocimeter as defined in Claim 7, wherein the convex lens and the collimating lens of the transmitting optical system are unified with together through a supporting member.

1 2. A laser Doppler velocimeter as defined in Claim 7, wherein the emitted light from the laser source leads to the transmitting optical system by space propagation.

1 3. A laser Doppler velocimeter as defined in Claim 7, wherein the emitted light from the laser source leads to the transmitting optical system through an optical fiber.

14. A laser Doppler velocimeter as defined in Claim 8, wherein the transmitting optical probe in the transmitting optical system and the receiving optical probe in the receiving optical system are unified with together.

1 5. A laser Doppler velocimeter as defined in Claim 9, wherein the transmitting optical probe in the transmitting optical system, and the lens system and the photo detector in the receiving optical system are unified with together.

16. A laser Doppler velocimeter as defined in Claim 1, wherein the transmitting optical system comprises an optical fiber for leading the laser light from the laser source, a collimating lens for collimating the light from the optical fiber approximately in parallel, a beam splitter for splitting the light frdm the collimating lens, two reflective mirrors each for reflecting the light split by the beam splitter, and two convex lenses for collimating the lights from the mirrors in parallel in a predetermined region.

1 7. A laser Doppler velocimeter as defined in Claim 16, wherein the transmitting optical system comprises an optical fiber, a collimating lens, a beam splitter, two reflective mirrors, and two convex lenses, and wherein the transmitting optical system is unified with the receiving optical system so as to constitute a sensor head.

18. A laser Doppler velocimeter as defined in Claim 1, wherein the transmitting optical system comprises an optical fiber for leading the laser from the laser source, a collimating lens for collimating the laser from the optical fiber approximately in parallel, and a convex lens for collimating the light from the collimating lens to obtain parallel lights in each predetermined region after passing through an optical block including a beam splitter and two reflective mirrors.

19. A laser Doppler velocimeter as defined in Claim 18, wherein the transmitting, optical system comprises an optical fiber, a collimating lens, a convex lens, a beam splitter, and two reflective mirrors, and wherein the transmitting optical system is unified with the receiving optical system so as to constitute a sensor head.

20. A laser Doppler velocimeter substantially as herein described with reference to any of Figs. 3 to 11 of the accompanying drawings.