GB2075787A - Measuring velocity by doppler shift of laser radiation - Google Patents

Abstract

A laser doppler velocity measuring system in which a CO2 laser 4 illuminates a volume 2 of interest in the atmosphere, and laser radiation reflected off particles in this volume 2 is mixed on a detector 11 with a local oscillator signal direct from the laser 4, is characterised in that the laser frequency is locked to a desired frequency, e.g. the P20 line, by a feedback signal derived via a highly selective etalon filter 14, whereby the laser cavity is adjusted to give maximum local oscillator signal on the detector 11, which corresponds to the wanted frequency. <IMAGE>

Description

SPECIFICATION Laser doppler velocimetry This invention relates to laser doppler velocimetry (LDV).

In laser doppler velocimetry radiation from a laser is directed into an area of interest and radiation scattered from particles in this area iS detected and processed to determine velocity.

One system of laser doppler velocimetry mea sures velocity along the laser beam by determining the doppler shift of returned radiation. This invention concerns such a laser doppler velocimetry system.

Another laser doppler-velocimetry system measures velocity transverse to laser beams using a time of flight or a fringe crossing process, but is of no concern in the present application.

The advantages of laser doppler velocimetry techniques are that they are non-perturbative and do not disturb the flow under study; they enable measure ment where it would be impossible to use a mechanical or electrical system; they enable measurement in areas remote from the equipment; and good resolution can be achieved.

Such advantages have led to laser dopplervelocimetry systems being used to detect and measure aircraft wake vortices on air fields; weather effects such astornadoesand cloud dynamics; flow in wind tunnels and turbo machinary; and at the other extreme retinal blood flow measurements.

Another potential use of laser doppler velocimetry techniques is aboard aircraft to detect and measure clear air turbulence (CAT) in front of the aircraft during flight. For such a system carbon dioxide (CO2) lasers are used because the laser radiation, at about 10.6 Fm, suffers less atmospheric attenuation than other wavelengths and is eye-safe.

The above uses are described in for example a paper (and is associated references) "Remote Wind Measurement in the Atmosphere Using Laser Doppler Methods" byJ. M. Vaughan, Plenary Paper presented at LASER 79, Munich, Published by IPC Sci. & ech. Press Ltd., England 1979.

One disadvantage of CO2 lasers is the number of frequencies, -so-called laser lines, at which the laser can operate. One d'esirable frequency is the P(20) line at 10.61 um which is separated'from the P(18), and P(22) lines by only 0.02 um. As a result it is difficult'to ensure the desired frequency of operation. For some applications, e.g. ground based systems, it is not necessary to know precisely what the frequency is because the small fractional errors in the value of flow measurement are acceptable.

However for airborne applications, it is essential to known the precise laser frequency, since the effect of small fractional errors of frequency give unacceptably large absolute errors in the measured air speed and turbulence.

According to this invention a laser dopplervelo- cimetry system comprises a laser for providing a beam of radiation, a detectorfordetecting laser radiation, an optical arrangement for directing the laser beam into a volume of interest in the atmos phere, the directing laser radiation scattered from particles in this volume qf interest into the detector, and for mixing a portion of the laser beam with the scattered radiation, and means for processing the output of the detector containing a doppler signal to -indicate velocity of particles within the volume of interest, characterised by a feedback loop, to the laser to control its frequency of operation, the loop comprising a detector for detecting the laser output, an etalon filter arranged between the laser and detector for transmitting the required laser frequency With. large attenuatio.n of other frequencies, and a stabilisation control for using the detector output to control the laser frequency.

The laser may be a continuous wave (CW) or pulsed carbon dioxide (CO2) laser.

The etalon-filter may be a thin plate of zinc selenide coated on both faces with a layer, reflecting -at the wavelength of interest, e.g. of thorium fluoride and zinc sulphide. All surfaces are as flat and parallel as possible to preserve the wavefronts of transmitted radiation.

The invention will now be described by way of example only with reference to-the accompanying drawings of which:- Figure 1 is a view of an aircraft carrying a laser dopplervelocimetry system for measuring air speed and turbulence in front ofthe aircraft; Figure-2 is a diagrammatic view of a CW laser doppler velocimeter system for use on board the aircraft of Figure 1; Figure 3 is a view of an etalon filter used in Figure 2; and Figures 4a-d are waveforms.

An: aircraft 1 Figure 1 carries a laser doppler velocimetry system.which probes the air 2 in front of the aircraft with. a laser beam 3. This system comprises, Figure 2, a carbon dioxide (CO2) laser 4 which emits horizontally polarised infra red radition via a first reflector5 a 98% reflector 6, a.first polarising reflector 7, a W/4 plate 8, an adjustable concave lens 9 and a convex lens 10 into the atmosphere in front of the aircraft 1. The first polarising reflector 7 is set to transmit horizontally polarised radiation and reflect vertically poiarised radiation. The A/4 plate 8 is set with its axis at 45 to convert the transmitted radiation3 in to circularly polarised radiation.

Two per cent of the laser passes through the partial reflector 6 into a detector 11 via a k/2 filter 12, set to convert the radiation to vertically polarised radiation, a 1% bandwidth filter 13, centred own the P branch at 10.6 um; an etalon filter 14, a reflector 15, a partly reflecting mirror 16 and-a lens.17.

Returning laser radiation passes through the lenses 10, 9 and through the B/4 plate.8 where it is converted to vertically polarised radiation. This returning radiation is reflected the polarising reflector7 off a reflector 18, and off the partial reflector 16 into the detector 11.

Output from the detector 11 is both to a stabiliser control 19 and a PZT transducer 20 for controlling the laser 4 cavity length, and also to a surface acoustic wave (SAW) spectrum analyser 21.

The SAW spectrum analyser 21 output is digitised in an analogue to digital (A"fl)- converter 22 whose output is passed through an integrator 23 to a logic unit24.

Figure 3 shows a cross section of the etalon 14. It comprises a thin, about 500 Fm thick, plate 26 of zinc selenide (ZnSe) ground and polished flat then coated with alternate layers of thorium fluoride and zinc sulphide forming reflecting (at desired wavelength) surfaces 27, 28. A desired laserfrequency is at the P(20) line i.-e. at the 10.61 ilm wavelength and thus the ZnSe layer 26 is made 50 x 10.611lem thick. The resultant filter is highly selective with about 60% transmission at the P(20) line and typically less than 6% of the nearby P(18), and P(22) lines (only 0.02 vim away).

Operation of the laser at the P20 line is achieved as follows: The laser4 is capable of emitting infra red at a number of frequencies around 10.6 ym but the 1% band width filter 13 allows only a few lines in the so-called P branch centred around 10.6 ym to pass through tothe etalon filter 14. This etalon filter 14 transmits 10.61 us and heavily attenuates other frequencies. The stabiliser control 19 imparts a ramp voltage on the PZTtransducer 20 to-cause a change in laser cavity length thereby causing the laser 4 to sweep out a range of frequencies. Only the P(20) line will cause a substantial signal at the detector 11, because of the filters 13 and 14.The stabiliser control 19 then adjusts the voltage on the transducer 20 to give maximum detector signal throughout operation of the laser 4, thus locking the laser on the P(20) transition. Practical operation of this device has shown unexpectedly good heterodyning efficiency with no loss of signal that might have arisen due to distortion ofwavefront of the effective local oscilia- tor beam on passing through the etalon filter.

In operation to measure wind shear turbulence, etc. the lens 9 is adjusted to focus the laser beam 3 at the required distance 2 in front of the aircraft 1. For a CW laser the focussing distance is up to about 1 kilometre.

Returning infrared radiation (of opposite circular polarisation to that transmitted) scattered from particles within the focus Volume is focussed by the lenses 10, 9 and converted to vertically polarised radiation by the h/4 plate 8. Thus polarised the radiation is reflected into the detector 11 by the reflectors- 7, 18, 16.

The detector 11 heterodynes the reflected radiation with that direct from the laser 4 to produce a doppler signal. Matching of the wavefronts, from the two sources, onto the detector 11 is desirable for maximum heterodyning efficiency and is achieved in the usual manner by adjustment of the optical components.

Output from the detector 11 to the spectrum analyser is an analogue signal containing the wanted doppler signal from movement of particles within the focussed volume plus detector and pre amplifier noise.

The spectrum analyser 21 takes samples of the detector 11 output and produces, for each sample period, a signal representing amplitude versus frequency as indicated in Figure 4b, i.e. the Fourier Transform of Figure 4a. If a single chirp SAW filter is used in the spectrum analyser 21 samples of the detector output can be processed. However, using multiplex techniques, the complete detector 11 output can be processed with multiple SAW chirp filters. A description of SAW spectrum analysers is given in:- RSRE Research Review Paper No.6, 1978, M. Alldritt et al; also Proc. Radar 77 Conf. IEE p.517, S. J. Alexander et al.

The output from the spectrum analyser 21 is digitised in the AID converter 22 to form a signal such as at Figure 4c, Each element within the digitised signal is then integrated over m.any, e.g.

103, sample periods of the spectrum analyser to provide a signal such as Figure 4d of amplitude versus frequency. Both the value of frequency at peak amplitude and spread of frequencies are useful to indicate air speed turbulence and wind structure and are processed in the logic unit 24 to provide warning of turbulence. Processing of signals such as in Figure 4d are described in J.Phys. D Appl. Phy. 11, 137,1978, A Brown et al.

Since an aircraft is moving at speeds up to about 300 metres/sec there is no doppler ambiguity in the processed signal; all wind velocities will be relatively towards the aircraft.

If the system of Figure 2 is to be used in a static situation e.g. on airfield to measure aircraft wake vortices then there is ambiguity in the doppier signal, i.e. the doppler signal may be positive or negative representing air velocity towards or away from the laser. In this case the laser signal direct to the detector (the local oscillator signal) may be frequency shifted e.g. by passage through a germanium acousto-optic modulator in a known manner.

For measurement of turbulence above, 1 kilometre a pulsed laser system is used. This may use a laser, stabilised to the P(20) line with an etalon filter, to provide a local oscillator signal for heterodyning with the reflected pulsed laser signals. The pulsed laser is also frequency stabilised to the P(20) line.

Claims (12)

1. A laser doppler velocimetry system comprising a laser for providing a beam of radiation, a detector for detecting laser radiation, an optical arrangement for directing the laser beam into a volume of interest in the atmosphere, for directing laser radiation scattered from particles in this volume of interest into the detector, and for mixing a portion of the laser beam with the scattered radiation, and means for processing the output of the.

detector containing a doppler signal to indicate velocity of particles within the volume of interest, characterised by a feedback loop to the laser to control its frequency of operation, the loop comprising a detector for detecting the laser output, an etalon filter arranged between the laser and detector for transmitting the required laser frequency with large attenuation of other frequencies, and a stabilisation control for using the detector outp.utto control the laser frequency.

2. A system according to claim 1 having an additional narrow band filter arranged optically in front of the etalon.

3. A system according to claim 1 wherein the etalon is a thin slice of zinc selenide coated with thin highly reflecting layer on its opposing faces.

4. A system according to claim 3 wherein the laser is a carbon dioxide gas layer.

5. A system according to claim 4 wherein the laser is a pulsed layer.

6. A system according to claim 4 wherein the laser is a continuous wave laser.

7. A system according to claim 4 wherein the detector for stabilising the laser frequency is the same detector that receives laser radiatipn scattered from the particles.

8. A system according to claim 4 wherein the local oscillator signal from the laser to the detector is given a frequency shift.

9. A system according to claim 4 wherein the laser for directing radiation into the atmosphere is a pulsed laser and the local oscillator signal for the detector is from a continuous wave laser, both lasers being frequency controlled using an etalon filter.

10. A system according to claim 4 wherein the stabilisation control includes a transducer for varying the laser cavity length.

11. A system according to claim 4 wherein the transducer is a piezo electric material transducer.

12. A system as claimed in claim 1 constructed arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.