GB2183956A

GB2183956A - Laser doppler velocimeter

Info

Publication number: GB2183956A
Application number: GB08529010A
Authority: GB
Inventors: Richard Ian Lamming; Neil Anthony Halliwell; Martin Peter Gold; David Neil Payne
Original assignee: University of Southampton
Current assignee: University of Southampton
Priority date: 1985-11-25
Filing date: 1985-11-25
Publication date: 1987-06-10
Also published as: GB2183956B; GB8529010D0

Abstract

A velocimeter comprises a laser diode (10) driven to provide an output beam of sinusoidally varying frequency. The output beam is directed into one port (21) of a four port single mode optical fibre coupler (20). The output beam forms a probe beam (22) which is directed onto a moving surface (5) and a reference beam (23). The reference beam is formed by a portion of the beam which is reflected at the interface (26) with a graded index lens (25) which terminates an output port (22) of the coupler (20). Light backscattered from the moving surface (5) along the direction of the probe beam is recombined with the reference beam (23) at a photodetector (30). The output from the photodetector (30) is demodulated using a sinusoidally-varying signal to give a signal indicative of the speed of movement of the moving surface (5). Feedback control of the laser diode drive is also provided. The invention may be applied to measuring vibrations. <IMAGE>

Description

SPECIFICATION Laser doppler vibrometer The present invention relates to a laser Doppler vibrometer, in particular, to a vibrometer utilising optical fibres.

Laser Doppler velocimetry ("LDV") is a known technique for the measurement of surface motion, particularly vibration. LDV is based on the principle that the frequency of light from, for example, a laser source scattered by a moving surface is shifted due to the Doppler effect. The shift in frequency provides an indication of the speed of movement of the surface. For example, in direct backscattering, the Doppler shift, fD is given by fD=28-U/R where C is the refractive index of the medium, A is the wavelength of the incident light and U is the surface velocity in the direction of the incident beam. In order to determine the velocity at which the surface is moving, it is necessary to measure the Doppler shift unambiguously.This has been achieved using optical heterodyne techniques, that is, light backscattered from the moving surface is combined with a frequency-shifted reference beam at a photodetector to form an optical heterodyne. The detector then produces an output current modulated at the difference (heterodyne) frequency. A frequency-shifted reference beam is utilised so as to provide a carrier frequency at the detector output which is frequency modulated as the surface moves.

The fact that a frequency-shifted reference beam is used means that the velocimeter or vibrometer must include some form of frequency-shifting componment. Existing devices have employed, for example, Bragg cells, Kerr cells or rotating diffraction gratings. Such components are additional to the main velocimeter construction and are often relatively costly.

Conventional frequency-shifting techniques are not readily compatible with the use of optical fibres for two main reasons. Firstly, light which is to form the reference beam would have to leave the fibre to pass through the frequency-shifting device. Secondly, the frequency-shifted reference beam and Doppler-shifted backscattered light would have to follow different optical paths. This second factor would render the velocimeter sensitive to environmental disturbances of the fibre.

Various attempts have been made to overcome the first of these two problems. For example, it has been proposed to provide a frequency-shifted reference beam by ramped phase-modulation using a semiconductor laser diode (Jackson, D.A., Kersey, A.D., Corke, M., and Jones, A.D.C. "Pseudo-Heterodyne detection scheme for optical interferometers", Electronics Letters Vol. 18, pp.1081-1083, 1982) or by sinusoidal modulation of a fibre wrapped around a piezoelectric crystal (Lewis, A.C., Kersey, A.D., and Jackson, D.A. "Non-contact Surface Vibration Analysis Using a Monomode Fibre Optic Interferometer Incorporating an Open Air Path".

Journal of Physics E: Scientific Instruments, Vol. 18, pp.604-607, 1985). However,. these arrangements have to date produced carrier frequencies only of the order of 104Hz. For general engineering use, surface velocities of up to 101 ms--' are to be expected. Velocities of this magnitude give rise to Doppler shifts of around 2.6 x 105Hz. Thus, at least an order of magnitude increase in frequency shift over those obtained hitherto is needed.

In accordance with the invention, there is provided a velocimeter comprising a laser source operable to provide an output beam of sinusoidally-varying frequency; means for dividing the output beam to form a probe beam directed onto a surface whose velocity is to be measured and a reference beam, and means for recombining at a photodetector light backscattered from the moving surface along the direction of the probe beam with the reference beam; the photodetector providing an output signal from which a signal indicative of the speed of movement of the surface along the direction of the incident probe beam can be derived. Preferably the velocimeter includes means for demodulating the output signal from the photodetector by mixing it with a sinusoidally varying signal.

The invention further provides a method of monitoring the speed of movement of a moving surface, the method comprising driving a laser source to provide an output beam of sinusoidallyvarying frequency, dividing the output beam to form a probe beam and a reference beam; directing the probe beam onto the moving surface; recombining at a photodetector light backscattered from the moving surface along the direction of the probe beam with the reference beam; and deriving from the output of the photodetector a signal indicative of the speed of movement of the surface along the direction of the incident probe beam.

A laser Doppler velocimeter in accordance with the invention will now be described in detail, by way of example only, with reference to the drawings, in which: Figure 1 is a schematic diagram of a laser Doppler velocimeter in accordance with the invention; and Figure 2 is a more detailed schematic diagram of the device of Fig. 1.

As illustrated in Figs. 1 and 2, a laser Doppler velocimeter or vibrometer in accordance with the invention comprises a laser light source 10 drivable to provide an output beam of varying frequency. The output beam is directed into one port 21 of a four-port single mode optical fibre coupler 20 having a splitting ratio of 50:50. One of the output ports 23 of the coupler 12 is index matched using a suitable liquid while the other output port 22 forms the velocimeter probe. The port 22 is terminated with a graded index lens 25 which acts as a light collimator.

The light beam from the output port 22 is directed onto the surface 5, the velocity of which is to be measured. Light scattered from the surface 5 is recaptured by the lens 25 and redirected through port 22 into the coupler 20 where it is mixed with reference light reflected from the fibre-lens interface 26. Thus, the light scattered from the surface 5 and that reflected from the fibre-lens interface 26 travels along a common fibre path, rendering the device relatively insensitive to environmental disturbances which effect the reference and probe beams equally. A portion of the combined reference and probe beam is then directed through the fourth port 24 of the coupler 20 into a photodetector 30. The output signal from the photodetector 30 is fed to processor circuitry 40 which provides an output signal indicative of the speed of the surface 5 along the direction of the probe team.

The laser source 10 is a single longitudinal-mode semiconductor laser diode, such as the Hitachi HLP 1400 operating at 830 nm. It has been shown (Dandridge A., and Goldberg, L., Electronics Letters, Vol.18, pp.302-304, 1982) that a change in drive current to a laser diode produces a frequency shift of - 1.0 GHz/mA in the laser diode output. Consequently, sinusoidally modulating the drive current to the laser diode 10 at a frequency Ct)m sweeps the lasing frequency v by an amount v--y+Av sin Wmt (1) where vO is the mean laser frequency and Av in the peak frequency shift. As shown in Fig. 2, the laser diode 10 is driven by means of a crystal oscillator 12 and TTL binary counter 14 whose output is amplified by means of an AGC amplifier 16.The oscillator 12 and counter 14 generate signals at 1.05, 2. 1 and 4.2 MHz. The 1.05 MHz signal is used to modulate the output of the laser diode 10 through the amplifier 16. The 2.1 and 4.2 MHz signals are mixed with the output from the photodetector 30 at two separate detectors 32 and 34 to provide signals representative of J2(m) and J4(0m) for use in the feedback control of the circuit as described in detail below.

The reflected light ER from the fibre-Selfoc interface 26 and the light ET scattered from the surface 5 having been mixed in the coupler 20 interfere at the photodetector 30 to produce an intensity which is modulated by their phase difference. If I is the distance from the fibre-end at the output port 22 to the surface 5, the difference in path lengths of the reference and probe beams is 21 and the current output i(t) from the photodetector 30 can be expressed as

where a is a constant and for practical purposes, that is, small vibration amplitudes fm 47rQAv/C, T=47rvoQ/C (3) Typically, voz106Av so, for small vibration amplitudes, changes in fm can be assumed to be negligible.

Normalising equation (2) for the detector output: i(t) =fl(1 + Kcos(sinw,t -t ,1), (4) where ss is a constant relating to optical power and detector sensitivity, and K is a measure of the scattered-signal amplitude (the "fringe visibility").

Current modulation to the drive current applied to the laser diode 10 produces both intensity (a) and frequency (Av) modulation of the laser output. The complete expression for the output from the detector 30 therefore becomes i(t)=ss(1 +asinw,t)(l +Kcosifmsince)mt+0Ti), (5) This can be expanded in terms of Bessel functions to

It is known (Kim, B.Y., and Shaw, H.J., "Phase reading.All Fibre-Optic Gyroscope", Optics Letters, Vol.9, pp. 378-3801984), that a signal of this form can be demodulated by mixing it with a square wave fs(t) at fundamental frequency C9m, where fs(t) can be expressed as

After mixing, the signal is bandpass filtered at a frequency 2#m and the vibration information is obtained from the filtered signal using a "frequency tracker", that is, a frequency-to-voltage converter.

However, intensity modulation of the laser diode 10 introduces errors into the output signal from the photodetector 30 and, hence, from the processor circuitry 40. The magnitude of these error signals is dependent upon the scattering efficiency of the surface 5 and the proportion of the light recaptured by the optical fibre at the port 22 of the coupler 20 (over which the operator has no control).

To overcome the discrepancies introduced by these errors, we propose to mix the output signal from the photodetector 30 with a sinusoidal signal f(t) in the form f(t)= 1-Bcosa)rnt (B is an arbitrary constant) (8) In what follows, B has been chosen to be unity.

The mixed signal is then bandpass filtered at a frequency 30, to give i(t)3#m=-ss{K1sin3#mtsin#T+K2cos3#mtcos#T} (9) where Ki=2kJ3(m) (10) and K2=K(J2(#m)+J4(#m)) (11) Since K1 and K2 are both functions of fm, and therefore of Av, they can be equated by choosing the laser drive to give a value of the phase excursion #m=3.0 rad.The filtered signal then becomes i(t)3#m = - ssK1cos(3#mt - #T) (12) Frequency demodulating this output gives the surface velocity d#T - according to dt d0T 47rvo dQ -= (13) dt c dt It can be seen from equation (3) above that the laser frequency excursion Av required to ensure #m=3.0 rads is a function of the distance Q of the port 22 from the surface 5.As the velocimeter is intended particularly for hand-held use, it is necessary to provide an automatic and fast-acting feedback control to adjust the laser drive amplitude to a value appropriate to the instantaneous value of the distance 2. The feedback control can also, advantageously, adjust the laser drive to take account of variations between different laser diodes or environmental effects, such as temperature changes.

For example, the feedback control of #m could be achieved by holding the ratios J2(rn): J4(rn) and J3(#m): JP55(#m) at constant values, where J2(#m), J4(#m), J3(#m) and J5(#m) are the second, fourth, third and fifth harmonic components of the output signal. Signals representative of J2(ç5m) and J4(m) are obtained from detectors 32 and 34 as described above. J3(1m) and J5(m) signals can be obtained in a similar manner.Where 5m is 3.0 rads, suitable values for the ratios J2(0m): J4(0m) and J3(rn): J5(m) are 3.68 and 7.19 respectively.

As mentioned above, the vibrometer of the invention is ideally suited to hand-held use in that it is robust and lightweight and, since it utilises a non-contact method, can be used where access to the vibrating surface is difficult. It can, furthermore, be manufactured cheaply using inexpensive components. Also, a separate frequency shifting device is not needed as the laser diode is itself used to provide a frequency-shifted reference signal.

The use of the laser diode to provide both the probe and reference beams permits the same fibre, in common mode configuration, to be used for both beams so that the device is rendered insensitive to environmental disturbances such as vibrations.

In particular, the sinusoidal phase modulation of the laser diode avoids linearity problems and mixing the detector output with a sinusoidal signal reduces errors due to variations in the characteristics of the surface 5.

Claims

1. A velocimeter comprising a laser source operable to provide an output beam of sinusoidally-varying frequency; means for dividing the output beam to form a probe beam directed onto a surface whose velocity is to be measured and a reference beam; and means for recombining at a photodetector light backscattered from the moving surface along the direction of the probe beam with the reference beam; the photodetector providing an output signal from which a signal indicative of the speed of movement of the surface along the direction of the incident probe beam can be derived.

2. Apparatus according to claim 1 which includes means for demodulating the output signal from the photodetector by mixing it with a sinusoidally varying signal.

3. Apparatus according to claim 2 in which the frequency of the sinusoidally-varying signal is the same as the frequency at which the laser output varies.

4. Apparatus according to claim 2 or 3 which includes means for filtering the signal obtained by mixing the photodetector output with the sinusoidally-varying signal.

5. Apparatus according to claim 4 in which the said means for filtering is a bandpass filter operable to pass signals of frequency three times that of the sinusoidally-varying signal.

6. Apparatus according to any preceding claim in which the probe and reference beams follow a common path within the velocimeter.

7. Apparatus according to claim 6 in which the common path is defined by optical fibre means.

8. Apparatus according to claim 7 in which the optical fibre means comprises a 4-port single mode optical fibre coupler; the output beam from the laser source being directed into a first input port of the coupler, and the probe beam exiting from a first output port; the photodetector being arranged to receive light from the second input port of the coupler.

9. Apparatus according to claim 8 in which the first output port is terminated by a graded index lens which acts to reflect a portion of the laser output beam to form the reference beam.

10. Apparatus according to any of claims 1 to 9 in which the laser source is a laser diode.

11. Apparatus according to any preceding claim which includes feedback control means for operating the laser source in dependence on the photodetector output signal.

12. Apparatus according to claim 11 in which the feedback control means includes means for providing a signal indicative of the ratio of two harmonic components of the photodetector outputs; the feedback control means acting to maintain the said signal substantially constant.

13. A velocimeter comprising a laser source operable to provide an output beam; means for dividing the output beam to form a probe beam directed onto a surface whose velocity is to be measured and a reference beam; and means for recombining at a photodetector light backscattered from the moving surface along the direction of the probe beam with the reference beam; the photodetector providing an output signal from which a signal indicative of the speed of movement of the surface can be derived; the probe and reference beams following a common path within the velocimeter.

14. Apparatus according to claim 13 in which the common path is defined by optical fibre means.

15. Apparatus according to claim 14 in which the optical fibre means comprises a 4-port single mode optical fibre coupler; the output beam from the laser source being directed into a first input port of the coupler, and the probe beam exiting from a first output port; the photodetector being arranged to receive light from the second input port of the coupler.

16. Apparatus according to claim 15 in which the first output port is terminated by a graded index lens which acts to reflect a portion of the laser output beam to form the reference beam.

17. A velocimeter substantially as hereinbefore described with reference to the drawings.

18. A method of monitoring the speed of movement of a moving surface, the method comprising driving a laser source to provide an output beam of sinusoidally-varying frequency, dividing the output beam to form a probe beam and a reference beam; directing the probe beam onto the moving surface; recombining at a photodetector light backscattered from the moving surface along the direction of the probe beam with the reference beam, and deriving from the output of the photodetector a signal indicative of the speed of movement of the surface along the direction of the incident probe beam.

19. A method according to claim 18 in which the output signal from the photodetector is demodulated by mixing it with a sinusoidally-varying signal.

20. A method according to claim 19 in which the frequency of the sinusoidally-varying signal is the same as that at which the laser output varies.

21. A method according to claim 19 or 20 in which the signal obtained by mixing the photodetector output with the sinusoidally-varying signal is filtered.

22. A method according to any of claims 18 to 21 in which the probe and reference beams follow a substantially common path.

23. A method according to any of claims 18 to 22 in which a portion of the output from the laser source is reflected back along its path to form the reference beam.

24. A method according to any of claims 18 to 23 in which the laser source is driven in dependence on the output from the photodetector.

25. A method of monitoring the speed of movement of a moving surface, the method comprising driving a laser source to provide an output beam, dividing the output beam to form a probe beam and a reference beam; directing the probe beam onto the moving surface; recombining at a photodetector light backscattered from the moving surface along the direction of the probe beam with the reference beam and deriving from the output of the photodetector a signal indicative of the speed of movement of the moving surface; the probe and reference beams following a substantially common path.

26. A method according to claim 25 in which a portion of the output from the laser source is reflected back along its path to form the reference beam.

27. A method of monitoring the speed of movement of a moving surface, the method being substantially as hereinbefore described with reference to the drawings.