GB2289814A

GB2289814A - Laser doppler velocimeter

Info

Publication number: GB2289814A
Application number: GB9409026A
Authority: GB
Inventors: Michael Mclean Ross
Original assignee: Alstom Power UK Holdings Ltd
Current assignee: Alstom Power UK Holdings Ltd
Priority date: 1994-05-06
Filing date: 1994-05-06
Publication date: 1995-11-29
Anticipated expiration: 2014-05-06
Also published as: GB9409026D0; GB2289814B

Abstract

In an improved LDV laser beams A, B inclined to each other are directed to separate (but adjacent) locations on the target 3. Doppler shift of the two beams A, B is extracted, preferably by comparison with respective reference beams, and the velocity is determined from the difference of the Doppler shifts. Pedestal frequencies, i.e. frequency shifts in the reference beams, are preferably incorporated into each comparison to make the velocity determination unambiguous. Modifications of this system use a single laser and an acousto-optic cell (Fig 4, 5) to produce both beams, with and without the pedestal frequency and with and without the reference frequence comparison. <IMAGE>

Description

Laser Doppler Velocimeter This invention relates to laser Doppler velocimeters, that is, devices for measuring the velocity, or at least the modulus of the velocity ("speed") of objects, by means of the Doppler shift of laser light scattered by the moving object or target.

Laser Doppler velocimetry (LDV) provides a means of measuring surface velocity precisely and with a fast response time. It finds application in industrial process control and in traction and braking control in vehicular transport.

One known form of LDV is the differential Doppler velocimeter, one example of which is shown, very diagrammatlically, in Figure 1 of the accompanying drawings.

In this sysem the apparatus, having an optical axis 1, is directed at the surface 3 of the target object. It is assumed that the surface 3 is moving in its own plane at a small angle to the normal (5) to the axis 1. The requirement would be to measure the surface velocity in its own plane and the angle would ideally be zero. Two laser beams A & B, arranged symmetrically about the axis 1, converge to the point on the axis at which it intersects the surface 3. The relative velocity of each beam to the moving surface is different because of the different angle of incidence and consequently light scattered by the surface is subjected to different Doppler shifts for the two beams.

It should be understood that the term "light" should be taken to include any electromagnetic radiation, infra-red, ultra-violet etc, for which laser devices could be designed.

Reverting to Figure 1, it can be shown that the difference of Doppler shift for the two beams is dependent upon the surface velocity transverse to the optical axis.

Thus by measuring the difference of Doppler shift the transverse surface velocity can be determined. This difference of Doppler shift is obtained by the interference of the two beams on the target surface to produce a fringe pattern as shown in Figure 1 (a), which is an enlarged side view of the beam intersection. As the surface moves through the fringe pattern the scattered light has a temporally periodic content with a frequency dependent on the surface velocity. A photo-detector 7 detects the pulse signal and a signal processor 9 determines the surface velocity accordingly.

The original laser beams A & B are conveniently derived from a single laser as shown. It will be apparent that if the two incident beam frequencies are equal and the direction of motion (up or down in the Figure) is not known, the direction of motion will not be determinable. Thus only the modulus of the surface motion velocity can be measured.

To overcome this difficulty a "pedestal frequency" is added in to one of the laser paths. This can be achieved by means of a Bragg cell operating to impose a predetermined frequency shift on the beam frequency. This shift is greater than any possible Doppler shift and a determination of direction becomes a determination of the dopper shift difference signal being greater or less than the pedestal frequency rather than merely, positive or negative.

The measured velocity is proportional to cos +, the angle between the bisector of the projected laser beams and the surface normal. Since 0 is nominally zero this dependence is very weak, this feature being an advantage of the differential system.

This system can achieve a precision of better than 0.1%. However, when the distance to the surface 3 is variable, uncertainty arises through variations in the fringe spacing.

In Figure 1 (a) it is assumed that the beam intersection occurs at the waist of each beam and that the surface 3 coincides with this intersection. If, however, the beam intersection lies off the beam waists, the beams will not be parallel at the intersection and fringe spacing will increase (or decrease) along the axis 1. If the position of the surface on this axis varies, an indeterminate fringe (temporal) frequency will result and the measured velocity will be correspondingly inaccurate.

Accurate alignment of the optical elements is therefore of great importance if only to alleviate this problem.

Another known laser Doppler velocimeter is the reference beam velocimeter a basic version of which is shown diagrammatically in Figure 2. In this arrangement a laser beam A is directed at the target surface 3. A partially reflecting mirror 11 diverts part of the beam to a corner reflector 13, for retransmission through the mirror 11 to a photodetector 7. This is the reference beam. The remainder of the beam passes through the mirror 11 to the target surface 3 and is there scattered, part of it to be reflected by the mirror 11 to the photodetector. If the target surface 3 is moving along the line of the beam there will be a Doppler shift in the beam frequency. The reference beam and the Doppler shifted beam will interfere at the photodetector surface and the photodetector will have a component output at the beat frequency, i.e.

the Doppler frequency shift. This shift is proportional to the target surface velocity in the beam direction and the photodetector output, applied to a signal processor, will indicate accordingly.

As in the differential system, a Bragg cell frequency shifter may be interposed in one or other of the reference beam and the target beam to provide a pedestal frequency. The Doppler shift is then an increase or decrease from this value according to the directon of motion.

It may be noted that the differential system resonds to target motion transverse to the general optic axis while the reference beam system responds to target motion along the optic axis.

The calibration of this arrangement remains constant over many metres of variation of the distance to the moving surface and is not sensitive to small misalignments in the optical elements, unlike the differential system. However, while it is possible to use this arrangement to measure the desired (transverse) component of velocity by inclining the axis of the reference beam LDV at a known angle to the surface, the results are critically dependent on the constancy of this angle.

Both of these known systems have significant disadvantages therefore and it is an object of the present invention to provide a laser Doppler velocimeter which is insensitive to the angle sb (Figure 1), to misalignment of the internal optical components, and to the line of sight distance to the target surface.

According to the present invention, a laser Doppler velocimeter having an optical axis extending through a target region comprises means for directing two laser beams to different parts of the region, the laser beams being differently angled with respect to the optical axis so that, in respect of light scattered from a target moving through the region in a direction transverse to a plane to which the beams are equally inclined, different Doppler shifts are imposed on the scattered light originating from the two laser beams respectively, and means for providing an indication of the frequency difference between the Doppler shifts and thus the speed of the target.

The axes of the two laser beams may be arranged not to intersect.

Each of the two laser beams may be provided by a respective velocimeter unit comprising a laser and an optical system for projecting the laser beam to the target region and collecting light scattered from the target region, each velocimeter unit further comprising means for deriving from the collected scattered light and a reference signal derived from the laser a difference signal indicative of a Doppler shift.

In this case each velocimeter unit may include photodetector means for combining the collected scattered light and the reference signal to provide a difference signal. There may then be included electronic mixing means for mixing the two difference signals to provide a Doppler shift difference signal indicative of target velocity modulus.

There is preferably included means for imposing a predetermined frequency offset between the Doppler shift to permit determination of target velocity, that is, speed and direction.

In a particular arrangement each of the above velocimeter units may include means for imposing a predetermined frequency offset between the projected laser beam and the reference signal, each velocimeter unit thereby producing a difference signal comprising an offset frequency component and a Doppler shift component,the laser Doppler velocimeter,i.e. the overall system, further comprising electrical means for mixing the two difference signals to produce a Doppler shift difference signal comprising a net offset frequency component and a net Doppler shift component,the individual offset frequency components being different so that target directional information is maintained in the Doppler shift difference signal.

In an alternative arrangement, the two laser beams may be derived from a single laser, two reference beams also being dreived from the single laser and a difference signal being derived from each reference signal and light scattered from a respective one of the two laser beams. The two laser beams are preferably derived from the single laser by way of an acousto-optic cell providing beams of difference frequency, means being provided for mixing the difference signals to provide a Doppler shift difference signal comprising an offset frequency component derived from the difference between the output frequencies of the acousto-optic cell, the offset frequency component permitting determination of target direction. There may be respective photodetectors for mixing each reference signal and its respective scattered light.

In a further arrangement, the two laser beams may be drived from a single laser by way of an acousto-optic cell providing beams of different frequency, photo-detector means being arranged to receive and mix light scattered from the target region and originating with the two beams respectively, the output of the photo-detector means providing a Doppler shift difference signal comprising an offset frequency component derived from the difference between the output frequencies of the acousto-optic cell, the offset frequency component permitting determination of target direction.

In a further arrangement according to the invention, the two laser beams may be derived from a single laser by way of beam-splittng means, photodetector means being provided for mixing scattered light from the two beams, and the outut of the photodetector means providing a Doppler shift difference signal indicative of target speed.

Several embodiments of laser Doppler velocimeter in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a diagram of a known differential Doppler laser velocimeter; Figure 1 (a) is an enlarged view of the laser beam intersection region of Figure 1; Figure 2 is a diagram of a known reference beam laser Doppler velocimeter; Figure 3 is a diagram of one embodiment of laser Doppler velocimeter according to the invention, using separate reference beam LDV units; Figure 4 is a diagram of a second embodiment according to the invention, using a single laser source, Bragg cell beam splitting and separate processing; Figure 5 is a diagram of a third embodiment employing a single laser, Bragg cell beam splitting and composite beam processing; and Figure 6 is a diagram of a fourth embodiment similar to Figure 5 but using simple beam splitting without the introduction of a pedestal frequency.

Figures 1 and 2 have already been described to show typical prior art arrangements of the differential and reference beam types.

In Figure 1 it will be clear that it is essential that the two laser beams A and B coincide at the target region shown enlarged in Figure 1(a). It is the interference between these two beams on the target surface 3 that provides the light fringes.

Movement of the surface 3 transversely through the beam intersection causes a periodic variation of the scattered light which is collected and focused on to the photodetector. The fringe rate is a function of the fringe pitch and the velocity of the target surface. This process does rely on the target surface not being optically flat for there would then be no discerible movement of the fringe pattern.

It can be shown that the fringe spacing (pitch) S is given by:

3 (n (0/a) where X is the wavelength of the laser beams (assumed equal) and 8 is the angle between the beams.

It can also be shown that the difference between the Doppler shift of the two beams is given by:

sn (3 / cos + where ?d is the individual Doppler shifted frequency; is is difference between the Doppler shifted frequencies; V is the velocity of the target surface in its own plane and 4 is the angle by which the optical line of sight is off the normal to the target surface.

It follows from the above that

sfd is measured by the photodetector and following signal processor so that can be determined by knowledge of 8 and > and an assumption that is neglible (if it is not known).

This system has the following advantages; (1) The useful scattered light is not limited by coherence conditions and therefore considerably more light can be collected than in a reference beam system.

(2) The two beams overlap on the surface so that the statistics of the light scattered from each of them are mutually coherent.

(3) Velocity direction can be determined by the use of a single pedestal frequency.

A disadvantage is that the fringe spacing is dependent on the target distance and is critically dependent on the internal alignment of the optical head.

In the reference beam system of Figure 2 the Doppler shift in frequency is given by homodyning the light scattered from the target surface with the reference beam (i.e.

sample of the laser beam).

This gives : A vcosx where X is the laser beam wavelength is the angle between the line of sight and the taget surface.

and is the surface velocity along the line of sight.

It will be clear that the surface must not be square to the line of sight (i.e. o( must not be 900 ) if a measure of the transverse velocity is to be obtained.

This system has the following advantages: (1) The calibration constant 2 is independent of the target distance.

# (2) Homodyne detection between the scatterd light and the coherent laser.

beam provides optimal use of the signal.

Disadvantages are: (1) The useful light scattered from the surface is restricted to that light within the 'cone of coherence' (2) Measurements are very sensitive to variation of o( .

Referring now to Figure 3 this first embodiment of laser Doppler velocimeter according to the invention comprises a target surface 3 moving in its own plane (upwards in the Figure), an optical axis 1 nominally normal to the surface 3 but in fact at an angle sb to the surface normal, and two independent laser doppler velocimeter units 13, 15.

These LDV units referred to as LDV and LDV 2, are each of the reference beam type, such as shown in Figure 2, and have lines of sight equally disposed about the optical axis 1 and separated by an angle /3 . It is important that the two LDV unit beams A and B are distinct at the surface 3that they do not overlap. This can be achieved by arranging that the beams would intersect but at a position beyond the surface 3 (e.g. as shown), or that the beams lie in parallel but spaced apart planes.

In the latter case no axial movement of the target would bring the beams on the surface into coincidence.

The two LDV units each contain a laser of oerating wavelength X , the two lasers being nominally identical, a beam splitting mirror 11, corner reflector 13, focusing optical system and photodetector 7. The output from each LDV unit is then an electrical signal which is applied to signal conditioning means (17,19) the outputs of which are applied to electronic mixing means 21. As shown in Figure 2 the reference beam path incorporates a Bragg cell frequency shifting device 23 which incorporates a predetermined pedestal frequency in that path. The pedestal frequencies are necessarily different in order to achieve their object of permitting determination of the direction of target movement.

If the pedestal frequencies are fol and2 for LDV1 and LDV2 respectively, the (electrical) output signal, referred to as "the (or "said") difference signal" elsewere is given by

+ = sin(/3 LD2l 9 t'a six f0 7 The first term in this signal is of course the pedestal frequency and the second the Doppler shift determined by the particular LDV unit. The latter term is positive or negative according to whether the incident beam is broadly 'against' or 'with' the target motion. After conditioning (17,19) of the difference signals they are applied to an electronic mixer circuit 21 which extracts the difference frequency, i e

fdl fc'a l fit (foe t2) Sin A This signal is referred to as the doppler shift difference signal and has two components, the net offset (or pedestal) component and the net doppler shift component.

It is assumed that

Doppler shifted frequencies and also that

Doppler difference frequency

Consideration of the system of Figure 3 shows that: (1) The dual reference beam method implies that a frequency pedestal in both LDV units is necessary in order to avoid all ambiguities of result.

(2) In order to determine the velocity direction,f01 and foa should be of the same sign (ie a step change in the same direction) and have different values.

(3) For the same enclosed angle between the two projected beams and the same surface velocity, the frequency measured by this system ze the Doppler shift difference frequency, is exactly twice that measured by a prior art Doppler difference LDV.

(4) If the two scattered light signals withinthe LDV units 13 and 15 are mixed directly at a photodetector (without differencing with any reference signal) then the modulus of the velocity component perpendicular to the line of sight can be measured without the use of any pedestal frequency. Such a system will be described with reference to Figure 6.

Velocity direction can then be determined with such direct mixing if just one of the projected beams is shifted in frequency. Such a system will be described with reference to Figure 5.

Several modifications of the system of Figure 3 will now be described.

Figure 4 shows a single laser arrangement in which the projected laser beam is 'sampled' by a beamsplitting mirror BS1, further split by beam splitting mirrors BS2, BS5 and BS6, to produce two reference beams of identical frequency to respective photodetectors 23 and 25. The main laser beam from BS1 is shaped by optics 27 and applied to an acousto-optic cell 29 in such manner as to produce first and second order output beams both therefore of frequency stepped up (differently) from that of the laser. These stepped up signals are transmitted by beam splitters BS2 and BS3 to a lens system 31 for focusing on the target surface 3. Again it is essential that the two beams A and B are directed at different parts of the surface so that there is no cross talk between the Doppler shifted signals. Light scattered from the two locations on the target surface are reflected by BS2 and BS3 to photodectectors 23 and 25 where they produce interference signals with their respective reference signals. The electonic output signals from the photodetectors, the difference signals, are amplified and processed and then applied to a mixer circuit 33.

The two inputs to the mixer are (a) pedestal 1 plus Doppler shift and (b) pedestal 2 minus Doppler shift , so that the mixer output is the difference of pedestals plus the Doppler shift.

This particular arrangement has three advantages. First, only a single radio frequency driver is required. Secondly, the same radio frequency source can be used in the signal processing, thereby eliminating potential uncertainty arising from any frequency instability of the driver. The third advantage derives from the use of diffractive beam splitting. This aids in the correctionof wavelength instabilities of the laser source, thereby allowing inexpensive diode lasers to be used.

A further manifestation of the same inventive concept is illustrated in Figure 5.

In this arrangement no reference beams are employed: the two signal beams from the target being mixed directly on a single photodetector to give the Doppler-shift difference frequency. The single laser beam is passed through beam shaping optics 27 and is then split by an acousto-optic cell. A zero order mode gives an unchanged output frequency (B) while a first order mode (A) gives a higher frequency, equivalent to the laser frequency plus a pedestal frequency. The two beams, shifted and unshifted, are projected to different but adjacent locations on the target surface 3 as before and the scattered light is collected and reflected by the beam splitting mirror BS7 to the photodetector 35. The photodetector output corresponds to the difference frequency which will be the pedestal frequency plus the sum of the doppler shift moduli. Application of the pedestal frequency to the signal processor enables the surface velocity to be determined.

There is a penalty in this arrangement in that the signal level and quality from the photodetector are poorer since the interfering beams at the photodetector surface arose from different locations on the target surface.

A further embodiment is shown in Figure 6. This is closely similar to that of Figure 5 except that the beam splitter is a simple prismatic device 37 in which there is no frequency change, both output beams having the same laser frequency. Only the modulus of the velocity of the target surface can therefore be obtained ie not the direction of motion.

Claims

1. A laser Doppler velocimeter comprising an optical axis extending through a

target region, means for directing two laser beams to different parts of said region said laser beams being differently angled with respect to said optical axis so that, in respect of light scattered from a target moving through said region in a direction transverse to a plane to which the beams are equally inclined, different Doppler shifts are imposed on said scattered light originating from the two laser beams respectively, and means for providing an indication of the frequency difference between said Doppler shifts and thus the speed of the target.

2. A laser Doppler velocimeter according to Claim 1, wherein the axes of said two laser beams are arranged not to intersect.

3. A laser Doppler velocimeter according to Claim 1 or Claim 2, wherein each of said two laser beams is provided by a respective velocimeter unit comprising a laser, an optical system for projecting the laser beam to said target region and collecting light scattered from said target region, each said velocimeter unit further comprising means for deriving from the collected scattered light and a reference signal derived from the laser a difference signal indicative of a said Doppler shift.

4. A laser Doppler velocimeter according to Claim 3, wherein each said velocimeter unit includes photodetector means for combining the collected scattered light and the reference signal to provide a said difference signal.

5. A laser Doppler velocimeter according to Claim 4, including electronic mixing means for mixing the two difference signals to provide a Doppler shift difference signal indicative of target speed.

6. A laser Doppler velocimeter according to any preceding Claim, including means for imposing a predetermined frequency offset between said Doppler shifts to permit determination of target velocity.

7. A laser Doppler velocimeter according to Claim 3 or Claim 4, wherein each said velocimeter unit includes means for imposing a predetermined frequency offset between the projected laser beam and said reference signal, each velocimeter unit thereby producing a said difference signal comprising an offset frequency component and a Doppler shift component, the laser Doppler velocimeter further comprising electrical means for mixing the two difference signals to produce a Doppler shift difference signal comprising a net offset frequency component and a net Doppler shift component, the individual offset frequency components being different so that target directional information is maintained in said Doppler shift difference signal.

8. A laser Doppler velocimeter according to Claim 1 or Claim 2, wherein said two laser beams are derived from a single laser, two reference beams are derived from said single laser and a difference signal is derived from each reference signal and light scattered from a respective one of said two laser beams.

9. A laser Doppler velocimeter according to Claim 8, wherein said two laser beams are derived from said single laser by way of an acousto-optic cell providing beams of different frequency, means being provided for mixing said difference signals to provide a Doppler shift difference signal comprising an offset frequency component derived from the difference between the output frequencies of said acousto-optic cell, said offset frequency component permitting determination of target direction.

10. A laser Doppler velocimeter according to Claim 9, including respective photodetectors for mixing each reference signal and its respective scattered light.

11. A laser Doppler velocimeter according to Claim 1 or Claim 2, wherein said two laser beams are derived from a single laser by way of an acousto-optic cell providing beams of different frequency, photo-detector means are arranged to receive and mix light scattered from said target region and originating with the two beams respectively, the output of said photo-detector means providing a Doppler shift difference signal comprising an offset frequency component derived from the difference between the output frequencies of said acousto-optic cell , said offset frequency component permitting determination of target direction.

12. A laser Doppler velocimeter according to Claim 1 or Claim 2, wherein said two laser beams are derived from a single laser by way of beam-splitting means, photodetector means are provided for mixing scattered light from the two beams, and the output of the photodetector means providing a Doppler shift difference signal indicative of target speed.

13. A laser Doppler velocimeter substantially as hereinbefore described with reference to any of Figures 3, 4, 5, or 6 of the accompanying drawings.