GB1605217A

GB1605217A - Laser probe for detecting movement of a target

Info

Publication number: GB1605217A
Application number: GB4212473A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1973-09-07
Filing date: 1973-09-07
Publication date: 1984-08-30

Description

(54) LASER PROBE FOR DETECTING MOVEMENT OF A TARGET (71) We, THE PLESSEY COMPANY PLC a British Company of 2/60 Vicarage Lane, Ilford, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a laser probe which may be used for remote, non-contacting, high resolution measurements of velocity, flow, displacement or vibration.

According to the invention there is provided a laser probe for detecting movement of a target, which laser probe comprises a laser source, means for providing two electrical signals in phase quadrature and each derived in dependence upon the doppler phase shift between light transmitted directly from the laser source on the one hand and light deriving from the laser source but reflected from the target on the other hand, first and second multiplicative mixer means fed with the two electrical signals one to each mixer and fed also from a local oscillator with signals in phase quadrature, a signal combiner to which output signals from the first and second multiplicative mixer means are fed; and a phase or frequency detector fed from the signal combiner to provide an output signal the amplitude of which is indicative of target movement.

Preferably, the means for providing two electrical signals in phase quadrature comprises first and second converter means for converting light signals to electrical signals, first and second signal combiners one for each said converter means and from which the two electrical signals in phase quadrature are derived, a beam splitting device for directing part of the light transmitted from the laser and reflected from the target to the first converter means and for directing part of the said light to the second converter means, and an optical phase quadrature device in the light path to either the first or second converter means.

Also preferably, the first and second converter means each include two photodiodes and a polarisation beam splitter. The optical phase quadrature device may be a quarter wave plate and the beam splitting device may be such as to split the received beam in half.

Each signal combiner may be a difference amplifier.

The polarisation beam splitters may each comprise a multi-layer variable dielectric film sandwiched between a pair of 45" prisms.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagram of a known arrangement which could form part of a laser probe not in accordance with the invention, Figure 2 is a diagram of a novel arrangement which forms part of a laser probe in accordance with the invention Figure 3 is a diagram of an electronic signal processing system which can be employed in a laser probe of the present invention and which can process the electrical signals derived from the arrangement shown in Figure 2; Figure 4 shows an auto-balance system; and Figure 5 shows an alternative auto-balance system to that illustrated in Figure 4.

Referring to Figure 1, there is shown an optical/electronic arrangement 2 comprising a laser 4 which transmits light 5 through a polariser 6 and a half wave plate 8 to a polarisation beam splitter 10. At the beam splitter 10, the light from the laser 4 is plane polarised and some of the light 5 passes towards a target 15 as a light beam 12 and some of the light 5 passes towards a mirror 14 as a light beam 16. The light beam 16 passes through a quarter wave plate 18 so that the reflected beam 16 is rotated through a total of 90 by the quarter wave plate 18.

Consequently, when the light 16 returns to the polarisation beam splitter 10, it passes through the beam splitter 10 and strikes a polarisation beam splitter 20.

The light beam 12 which is reflected from the target 15 passes through the quarter wave plate 22 and is deflected by the beam splitter 10 towards the beam'splitter 20. This is because the quarter wave length plate 22 rotates the light 12 through a total of 90".

The light striking the polarisation beam splitter 20 comprises two orthogonally polarised components. The first component has in effect come from the laser 4 (via the mirror 14) and this contains the laser frequency and may also contain spurious frequencies which may have been generated.

The second component will contain the laser frequency but modified in accordance with the speed of movement of the target 15. The beam splitter 20 deflects one light component to a photo-diode 24 and the other component to a photodiode 26. The outputs of the photodiode detectors 24, 26 are subtracted by difference amplifier 28 (see Figure 3) to give a Doppler shift signal which may be used to obtain the speed of movement of the target.

It will be apparent that the system described above with reference to Figure 1 can measure the speed of a target along the optical axis by measuring the beat frequency from the detectors 24, 26. There is however an ambiguity in the direction of movement and, furthermore, a stabilised laser is required as the effect of any laser frequency drift is indistinguishable from that of target velocity.

Also, displacements can be monitored in units of half a wavelength by the beat counting technique, but there is again ambiguity in the direction of displacement and the stabilised laser is again necessary. Furthermore, the system is unsuited to the measurement of displacement or vibrations of amplitude very much less than a wavelength.

The present invention modifies the arrangement described above with reference to Figure 1 and a laser probe in accordance with the invention will now be described solely by way of example with reference to Figures 2-5. Referring especially to Figure 2, it will be seen that many of the parts employed therein are exactly the same as in Figure 1 and they have accordingly been given the same reference number. The operation of these parts is exactly as described in Figure 1 and they will not be described again in detail.

The arrangement shown in Figure 2 additionally comprises a balanced detector module 50, a 50/50 dielectric beam splitter 52 and a quarter wave length plate 54.

The operation of the arrangement as shown in Figures 1 and 2 is substantially the same to the extent that a light beam coming in effect directly from the laser 4 and a light beam reflected from the target and shifted in frequency according to the speed of movement of the target are directed towards the polarisation beam splitter 56. However, in Figure 2, before the light signals reach the polarisation beam splitter 56, they are intercepted by the dielectric 50/50 beam splitter 52. This beam splitter directs 50% of the light received towards the polarisation beam splitter 20 and 50% of the light received towards the quarter wave length plate 54.

Light from the plate 54 passes to the polarisation beam splitter 56 from where it passes to two photodiode detectors 58, 60.

Signals emanating from the detectors 58, 60 are received by a difference amplifier 62, see Figure 3. It will be appreciated that the interference beat signals from the detectors 58, 60 are in phase quadrature because of the orientation of the quarter wave plate 54, the axis of which may be set either parallel or at right angles to that of local oscillator polarisation Referring now especially to Figure 3, there is shown an electronic signal processing system (i.e. a twin channel quadrature homodyne interferometer) for receiving two signals emanating from the difference amplifiers 28, 62. The electronic signal processing system illustrated in Figure 3 is designed to give the sum or a difference of the signals but not to give the two signals together.

The signals are received by two signal conditioners 74, 76 effective to act as low pass filters and balancers for removing the d.c.

component of the signals. The resultant signals are fed to first and second multiplicative mixer means 78,80. Also fed to the means or multipliers 78, 80 are signals from a quadrature oscillator 82 which directs a sin wt signal towards the multiplier 78 and a cos wt signal towards the multiplier 80.

Finally, the resultant signals are fed to a differential amplifier 84 and a phase/frequency demodulator (not shown). The demodulator enables velocity, displacement, flow or vibration measurements to be obtained.

The two signals are processed in the arrangement of Figure 3 in the same way as in single sideband frequency conversion; that is the in-phase and quadrature components of a (electronic) local oscillator (82 in Figure 3) are multiplied by the detector signals, and the resultants are added (or subtracted).

Thus sin 0 sin wt + cos coswt=cos cos wt = cos (wt--O) or sin 0 sin wt cos 0 cos wt = -cos (wt + ) or sin cos wit + cos 0 sin wt = sin sin wt = (wt+) or sin 8 coswt - cos 0 sin wt = - sin (wt Here "wt" is the instantaneous phase of the electronic local oscillator 82 whilst "8" is the instantaneous phase of the optical signal.

The frequency of the oscillator 82 should be chosen to be above the maximum Doppler frequency interest.

The resultant signals is a sinusoid of amplitude proportional to the received optical signal field strength, and phase a linear combination of the optical phase and the electronic local oscillator phase. This signal can be analysed in various ways according to the particular requirement. Thus velocity is derived by addition of a frequency discriminator; no ambiguity exists in direction.

Displacement is dervied by applying the signal to a phase discriminator, using the electronic local as a reference; again there is no directional ambiguity.

As stabilised lasers are complex and expensive items, it is desirable to be able to operate with non-stabilised types which undergo frequency drifts. Certain requirements are easily fulfilled using such types. In particular, vibrations can be measured with amplitudes ranging from much less to much greater than a wavelength provided that d.c. information is not required.

This may be accomplished by following the frequency discriminator by an integrator with a low frequency cut-off to prevent integration of the d.c. component. Noise equivalent displacements of less than 10-12m/ iwzmay be achieved using a 633 nm He-Ne line.

Alternatively, the laser probe can be used to monitor the laser frequency drift, which can then be applied as a correction to results from a parallel system doing measurements of displacement, or velocity.

It is envisaged that the novel system will be found of considerable value for systems operating at the CO2 laser line at 10.6 m, at which wavelength it is particularly difficult to achieve the large depths of modulation required for the implementations of alternative methods.

The two electronic signals received from difference amplifiers 28, 62 are ideally symmetrical about zero potential. Where these signals are small, the asymmetries may become significant and may be removed by applying a d.c. offset. In many situations, particularly where a long atmospheric path is used (as opposed to a self-contained system on an optical cable) the path length will randomly vary by several wavelengths over short periods to time, e.g. one second, even when the target is nominally stationary.

Under these circumstances, it is possible to incorporate circuitry which automatically removes the asymmetries. One such circuit arrangement is shown in Figure 4 and an alternative circuit arrangement is shown in Figure 5.

Referring now to Figure 4, which shows a circuit arrangement for only one signal conditioner 74 or 76, it will be seen that the auto balance system includes two amplifiers 90, 92 and a mean peak-level sensor 94.

Signals leaving the sensor 94 are compared with earth 96 and are subtracted at 98.

Referring now to Figure 5, it will be seen that part of the circuit arrangement has been previously shown in Figure 3. The system further incurs two synchronous detectors 100, 102 and two mean peak-level sensors 104, 106.

The signals leaving the sensors 104, 106 are compared with earth to generate an error.

The error signals are amplified by amplifiers 108, 100 and the resultant signals are fed back into the circuit at 112 and 114.

The embodiments of the invention given above have been described solely by way of example and modifications may be effected.

The laser probe of the present invention may have especial application as a laser radar probe, for measuring wind speeds, clear air turbulence and vibration and for use in machine tool control.

WHAT WE CLAIM IS: 1. A laser probe for detecting movement of a target, which laser probe comprises a laser source, means for providing two electrical signals in phase quadrature and each derived in dependence upon the doppler phase shift between light transmitted directly from the laser source on the one hand and light deriving from the laser source but reflected from the target on the other hand, first and second multiplicative mixer means fed with the two electrical signals one to each mixer and fed also from a local oscillator with signals in phase quadrature, a signal combiner to which output signals from the first and second multiplicative mixer means are fed, and a phase or frequency detector fed from the signal combiner to provide an output signal the amplitude of which is indicative of target movement.

2. A laser probe according to Claim 1 in which the means for providing two electrical signals in phase quadrature comprises first and second converter means for converting light signals to electrical signals, first and second signal combiners one for each said converter means and from which the two electrical signals in phase quadrature are derived, a beam splitting device for directing part of the light transmitted from the laser and reflected from the target to the first converter means and for directing part of the said light to the second converter means, and an optical phase quadrature device in the light path to either the first or second coverter means.

3. A laser probe according to Claim 2 in which the first and second converter means each include two photodiodes and a polarisation beam splitter.

4. A laser probe according to Claim 2 or to Claim 2 and 3 in which the optical phase quadrature device is a quarter wave plate and the beam splitting device is such as to split the received beam in half.

5. A laser probe according to Claim 2 or to Claim 2 and any claim when appendant thereto in which the first and second signal combiners are difference amplifiers.

6. A laser probe according to Claim 3 in which the polarisation beam splitters each comprise a multi-layer variable dielectric film sandwiched between a pair of 45" prisms.

7. A laser probe substantially as herein described with reference to Figures 2, 3 and 4 or 2, 3 and 5 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. local as a reference; again there is no directional ambiguity. As stabilised lasers are complex and expensive items, it is desirable to be able to operate with non-stabilised types which undergo frequency drifts. Certain requirements are easily fulfilled using such types. In particular, vibrations can be measured with amplitudes ranging from much less to much greater than a wavelength provided that d.c. information is not required. This may be accomplished by following the frequency discriminator by an integrator with a low frequency cut-off to prevent integration of the d.c. component. Noise equivalent displacements of less than 10-12m/ iwzmay be achieved using a 633 nm He-Ne line. Alternatively, the laser probe can be used to monitor the laser frequency drift, which can then be applied as a correction to results from a parallel system doing measurements of displacement, or velocity. It is envisaged that the novel system will be found of considerable value for systems operating at the CO2 laser line at 10.6 m, at which wavelength it is particularly difficult to achieve the large depths of modulation required for the implementations of alternative methods. The two electronic signals received from difference amplifiers 28, 62 are ideally symmetrical about zero potential. Where these signals are small, the asymmetries may become significant and may be removed by applying a d.c. offset. In many situations, particularly where a long atmospheric path is used (as opposed to a self-contained system on an optical cable) the path length will randomly vary by several wavelengths over short periods to time, e.g. one second, even when the target is nominally stationary. Under these circumstances, it is possible to incorporate circuitry which automatically removes the asymmetries. One such circuit arrangement is shown in Figure 4 and an alternative circuit arrangement is shown in Figure 5. Referring now to Figure 4, which shows a circuit arrangement for only one signal conditioner 74 or 76, it will be seen that the auto balance system includes two amplifiers 90, 92 and a mean peak-level sensor 94. Signals leaving the sensor 94 are compared with earth 96 and are subtracted at 98. Referring now to Figure 5, it will be seen that part of the circuit arrangement has been previously shown in Figure 3. The system further incurs two synchronous detectors 100, 102 and two mean peak-level sensors 104, 106. The signals leaving the sensors 104, 106 are compared with earth to generate an error. The error signals are amplified by amplifiers 108, 100 and the resultant signals are fed back into the circuit at 112 and 114. The embodiments of the invention given above have been described solely by way of example and modifications may be effected. The laser probe of the present invention may have especial application as a laser radar probe, for measuring wind speeds, clear air turbulence and vibration and for use in machine tool control. WHAT WE CLAIM IS:

1. A laser probe for detecting movement of a target, which laser probe comprises a laser source, means for providing two electrical signals in phase quadrature and each derived in dependence upon the doppler phase shift between light transmitted directly from the laser source on the one hand and light deriving from the laser source but reflected from the target on the other hand, first and second multiplicative mixer means fed with the two electrical signals one to each mixer and fed also from a local oscillator with signals in phase quadrature, a signal combiner to which output signals from the first and second multiplicative mixer means are fed, and a phase or frequency detector fed from the signal combiner to provide an output signal the amplitude of which is indicative of target movement.