The Setting-Up of an Optical Remote Sensing System for Target Identification: A Laboratory Experiment. Olivier Jerome Dussarrat, D. Fraser Clark, and T. J. Moir.
186
IEEE TRANSACTIONS ON EDUCATION, VOL. 42, NO. 3, AUGUST 1999
The Setting-Up of an Optical Remote Sensing System for Target Identification: A Laboratory Experiment Olivier Jerome Dussarrat, D. Fraser Clark, and T. J. Moir Abstract—The construction of an optical system for vibration measurement using laser doppler velocimetry (LDV) is presented, the basis of which can be used as a laboratory experiment for the classroom. LDV is an optical technique that can be used to determine an object’s velocity by analyzing the frequency content of coherent light reflected from it. It can be used for vibration measurement, target identification, covert surveillance, or studying the flow of fluids. The setting up of such a system in the laboratory can sometimes be difficult if it is performed in a nonsystematic way. This paper describes an approach to configuring a free-space optical system and also demonstrates the optical equivalence of cochannel and multipath interference. Index Terms—Amplitude locked loop, cochannel interference, frequency modulation (FM), interferometer, laser vibrometry, phase locked loop.
I. INTRODUCTION
S
INCE the demonstration of laser doppler velocimetry (LDV) by Yeh and Cummins [1], the technique has been applied to a wide range of situations including the measurement of velocity, range, vibration, and flow rate [2], [3]. Such applications utilize a laser beam focused onto the measurement region and photodetector to collect either the light scattered by the particles present in the flow, or the light reflected from a hard target. By processing the electrical output from the photodetector, information related to the velocity of the target may be obtained. Lasers are ideal for this type of application as they produce a very intense monochromatic beam which can be focussed onto a precise, diffraction-limited spot on the object. Such systems normally use a modified Michelson interferometer [4], [5], Mach–Zehnder [6], or a Twyman–Green interferometer [7]. The aim of this demonstration is to provide the student with a basic understanding of LDV using a Mach–Zehnder interferometer as an optical sensing system to detect the frequency of vibration of a hard target and to show the effect of optical multipath and cochannel interference. A hard target is simulated by a mirror of 100% reflectivity vibrating at a frequency II. PRELIMINARIES The optical system was mounted on a standard vibration isolated optical bench, and constructed using Spindler and Manuscript received November 21, 1997; revised March 5, 1999. O. J. Dussarrat was with the Department of Electronic Engineering and Physics, University of Paisley, Paisley PA1 2BE, U.K. He is now with Philips Semiconductors AG, Telecom Products Group, CH-8045 Zurich, Switzerland. D. F. Clark and T. J. Moir are with the Department of Electronic Engineering and Physics, University of Paisley, Paisley, PA1 2BE, U.K. Publisher Item Identifier S 0018-9359(99)06322-0.
Fig. 1. Optical system.
Hoyer Micro Bench optical components as shown in Fig. 1. It comprises a polarized HeNe laser (5 mW), two beam splitters, an acousto-optic modulator (Bragg Cell operating at 100 MHz), two mirrors, and a photo-detector. The laser output is split and one part of the beam is frequency shifted by the acousto-optic modulator before hitting the target which then Doppler shifts the beam to give a reflected beam which is frequency modulated and directed to the photodiode [8]. The other part of the laser beam impinging on the photo-diode forms a reference signal. At the surface of the detector, the reference signal coherently interferes with the reflected signal from the target to produce a heterodyne detection system. After amplification, the signal from the photo-diode is downconverted to an intermediate frequency. A simple demodulator such as a phase locked loop (PLL) can then be used to demodulate that signal to provide the baseband information which is the frequency of vibration of the target. However, it is well known that the sensitivity of such an optical system is dramatically reduced by cochannel or multipath interference [9] arising from spurious light scattered
0018–9359/99$10.00 1999 IEEE
DUSSARRAT et al.: SETTING-UP OF AN OPTICAL REMOTE SENSING SYSTEM
187
Fig. 2. Laser and photodiode.
Fig. 3. First beam splitter
S
1:
Fig. 6. Second mirror Fig. 4. Second beam splitter
S
2:
3)
4)
Fig. 5. First mirror
M
1:
5)
from rain, moisture, dust, particles, etc, along the propagation path. On decoding the received signal this type of interference manifests itself as spikes of multiplicative noise in the demodulated signal, decreasing the detection sensitivity which is demonstrated here by incorporating a glass plate of graduated reflectivity in the optical path. Results of the experiment are obtained in the form of oscilloscope traces of the PLL output. III. SEQUENTIAL SETUP PROCEDURE To obtain optimized performance the optical system should be aligned in a systematic way as outlined in the following procedures. 1) The laser beam output is aligned first with the centre of the photodiode receiver which acts as the reference as shown in Fig. 2. The laser axis with the beam where is output can be expressed by the frequency the amplitude of the laser beam, and of the radiation. is placed on the reference 2) The first beam splitter the reflection of from the axis, ensuring that beam splitter, is colinear with the laser output beam as shown in Fig. 3. Due to the limited bandwidth of the detection and amplification circuitry, effects related
6)
M
2:
to scattered light entering the laser cavity were not significant. is positioned so that the The second beam splitter , is colinear with and reflection from it, as shown in Fig. 4. Care should be exercised over the transversal position of the second beam since it could affect the alignment of the system. is aligned so that the beam (reflected The mirror from the mirror and passing the first beam splitter is parallel with As shown in Fig. 5, it is useful for later alignment to introduce a screen and mark the hits as point A. position where is introduced. The beam The second mirror is the reflection from the mirror of Part of is directed to hit another screen of point B whereas due to the splitting action the other part becomes The new beam is of the first beam splitter to hit the redirected by the second beam splitter Adjustment of screen of point C to become is required to ensure that the beam mirror and are parallel as shown in Fig. 6. An alternative (and eventually is to make for adjusting the is directed to sure that the second reference beam is the portion of the beam that is split point C. and reflected by mirrors and at the beamsplitter to hit the target at point C. The acousto-optic modulator (or Bragg Cell) is posientering tioned between the two mirrors. The beam the Bragg Cell is frequency shifted by a frequency (100 MHz) to give the output beam (1) and if the Bragg It is easier to distinguish Cell is switched by a square wave at around 20 Hz, to flicker. The mirror is raised causing the beam vertically above the path. As the Bragg Cell produces and some deviation on the output beam, the mirror the Bragg Cell need to be adjusted until the output beam
188
IEEE TRANSACTIONS ON EDUCATION, VOL. 42, NO. 3, AUGUST 1999
Fig. 7. Bragg cell setup.
is parallel with the reference beam This could to point be done easily by simply directing the beam A as shown in Fig. 7. is returned to its original position, then 7) The mirror is parallel to and gradually adjusted so that the beams pass through points B and C, respectively, as shown in Fig. 8. as 8) The target is placed on the axis of the beam shown in Fig. 9. The target is represented by a mirror mounted on a loudspeaker driven sinusoidally at the The displacement of the mirror is frequency dependent on the amplitude of of displacement and the frequency (2)
is Doppler shifted by the target to The beam directed to the diode by the beam become the beam Note that once the alignment of the system splitter is completed, the Bragg Cell will be driven with a dc voltage to give a permanently diffracted beam. The from the target is colinear with the reflected signal and with after passing and is of the beam form
Fig. 8. Bragg cell and
M
2:
When expanded the above equation produces dc terms and a frequency term of more than twice the optical carrier frequency but since the photodetector is ac coupled and of limited bandwidth so cannot respond to these terms which results in (5) By connecting the photodiode to a standard spectrum analyzer the spectrum of an FM signal should appear indicating that the system is properly adjusted. To make the demodulation process easier, the composite signal is down-converted to an such as 10.7 MHz or 455 kHz. intermediate frequency For this application a douple frequency conversion was used. That is, a first conversion down to 10.7 MHz followed by a second conversion down to 455 kHz. Using a PLL for normal FM demodulation (taking the derivative of the phase) gives the can original baseband signal, therefore the PLL output be expressed as the instantaneous frequency
(6) (3) Such a where is the light wavenumber, i.e., signal is frequency modulated (FM). At the surface of the photodetector (heterodyne receiver), the reference signal interferes coherently with the reflected signal from the target [10]. The function of the heterodyne receiver is to convert the optical signal through mixing to radio frequencies for detection. The detector output current is proportional to the input optical intensity, therefore the output is of the form
(4)
in (6) is a dc term and the baseband where information signal. On connecting the demodulator output to an oscilloscope, the sine wave at the baseband frequency corresponding to the frequency of the vibrating target should appear, confirming that the beam has been modulated by the target, and therefore that the frequency of the target can be measured. Typical output traces when the target is vibrating at 30 and 300 Hz are shown in Figs. 10 and 11, respectively. By varying the frequency of the driven loudspeaker, an immediate response to the change of frequency should appear on the oscilloscope. Note that by turning off the drive to the loudspeaker and then by simply shouting near it, the laser system acts as a microphone, and the waveform speech along the beam path appears on the oscilloscope.
DUSSARRAT et al.: SETTING-UP OF AN OPTICAL REMOTE SENSING SYSTEM
189
Fig. 9. Final setup.
Fig. 10.
PLL output for a vibrating target at 30 Hz.
In outdoor applications, particles of rain, moisture, dust, etc., will scatter the optical signal. The loss of signal due to scattering along a different direction to the beam return path will dramatically reduce the signal to noise ratio. Furthermore, should strong scattering occur from a particle or obstacle along the propagation path, multipath or cochannel interference, commonly encountered in conventional FM [11], [12], will arise. In the laboratory, cochannel interference will be simulated using a glass plate of graduated reflectivity aligned on the axis The intensity of spikes of this interference of the beam will vary as a function of the reflectivity. It can be shown that the returned signal has an extra additive term included due to scattering by the atmosphere along the propagation path [9].
Fig. 11. PLL output for a vibrating target at 300 Hz.
Defining the magnitude of this scattering as , at a frequency (corresponding to the laser frequency), then the output from the detector becomes (7) where
(8)
190
Fig. 12.
IEEE TRANSACTIONS ON EDUCATION, VOL. 42, NO. 3, AUGUST 1999
PLL output affected by interference for a vibrating target at 30 Hz.
Fig. 13. PLL output affected by interference for a vibrating target at 300 Hz.
REFERENCES
and
(9)
It can be shown that the modified [9]
output becomes
(10) with (11) The special form of this waveform introduces demodulation problems, with spikes appearing on the waveform in Fig. 12 (with a 30-Hz vibrating target) or Fig. 13 (with a 300-Hz vibrating target) being multiplicative in nature. These spikes dramatically reduce the signal to noise ratio of the output signal and cannot be filtered using traditional techniques such as band pass filtering, as the interference is in band and normally random in nature. IV. CONCLUSIONS It has been shown that the principles of an LDV system used for the identification of targets from their vibration can be realized in the classroom. It offers an insight into the physical and mathematical bases of the techniques. The effects of scattering interference, which occurs in real-life applications have been demonstrated. The alignment of the optical system has been described in a systematic way that should allow the student to implement the arrangement with minimum supervision. ACKNOWLEDGMENT The authors would like to thank J. G. Brown for his valuable comments on the paper.
[1] Y. Yeh and H. Z. Cummins, Appl. Phys. Lett., vol. 4, pp. 176–178, 1964. [2] L. E. Drain, The Laser Doppler Technique. New York: Wiley, 1980. [3] A. L. Scawloc, Optical Remote Sensing. Berlin, Germany: SpringerVerlag, Springer Series Opt. Sci., vol. 39, p. 383, 1983. [4] A. A. Michelson, Studies in Optics. Chigado, IL: Univ. Chicago Press, Phoenix Sci. Series, 1927. [5] R. H. Belansky and K. H. Wanser, “Laser-Doppler velocimetry using a bulk optic Michelson interferometer, a student laboratory experiment,” Amer. J. Phys., vol. 61, no. 11, pp. 1014–1019, 1993. [6] O. S. Heavens and R. W. Ditchburn, Insight into Optics. New York: Wiley, 1991. [7] J. Schwider and O. Falkenstorfer, “Twyman-Green interferometer for testing microspheres,” Opt. Eng., vol. 34, no. 10, pp. 2972–2975, 1995. [8] G. J. Troup, Optical Coherence Theory, Recent Developments. London, U.K.: Spottiswoode, Ballantyne and Co. Ltd., 1967. [9] O. J. Dussarrat, T. J. Moir, and D. F. Clark, “Application of a PLL and ALL noise reduction for an optical sensing system,” in Proc. SPIE Opt. Inspection Micromeasurements, Lasers, Opt., Vision for Productivity and Manufacturing II, Micropolis, Besoncon, France, June 10–14, 1996. [10] D. F. Clark and T. J. Moir, “Application of a PLL and ALL noise reduction process in optical sensing systems,” IEEE Trans. Ind. Electron., vol. 44, no. 1, pp. 136–139, Feb. 1997. [11] M. Corrington, “Frequency modulation distortion caused by common and adjacent channel interference,” RCA Rev., FM Distortion, 1945, p. 523. , “Frequency modulation distortion caused by multipath transmis[12] sion,” in Proc. IRE, Dec. 1945, pp. 878–889. [13] J. Leonelly, “Dual-use applications of laser remote sensing to the military battlefield and environmental monitoring,” Proc. SPIE, vol. 2112, pp. 240–351, 1994.
Olivier Jerome Dussarrat received the B.Eng. degree in electrical engineering from the University of Paisley, U.K., in 1995. He received the Ph.D. degree in 1999 from the University of Paisley in the field of laser optics and communications. He is currently employed with Philips Semiconductors, Zurich, Switzerland, where his main activity is concerned with yield improvement in semiconductor production. Dr. Dussarrat is an Associate Member of the Institute of Electrical Engineers.
DUSSARRAT et al.: SETTING-UP OF AN OPTICAL REMOTE SENSING SYSTEM
D. Fraser Clark received the bachelor’s degree with first class honors in electrical engineering and electronics from the University of Glasgow, U.K., in 1982. He subsequently pursued further research in the field of integrated optics in the Department of Electrical Engineering and Electronics. He receivd the Ph.D. degree in 1985, with a thesis based upon research related to the development of high-frequency resonant electrode modulators using lithium niobate. He took up a research position in the Department of Electrical and Electronic Engineering at the University of Strathclyde, where his research interests developed in the field of fiber optic sensors, nonlinear optical effects and silicon micromachining. Since 1990, he has been a Lecturer in the Department of Electronic Engineering and Physics at the University of Paisley, where his research interests include waveguide analysis and modeling, silicon micromachined gas sensors and optical communications. Dr. Clark is currently a member of the Institute of Electrical Engineers and the IOP.
191
T. J. Moir was born in Dundee, U.K., and received the BS.C and Ph.D. degrees in 1979 and 1983, respectively, from Sheffield Hallam University, U.K. During the BS.C. degree course which was in control engineering, he was a sponsored student with GEC Electrical Projects Ltd./Industrial Control, Rugby Warwickshire. From 1983 to 1984 he was a Research Assistant in the Department of Electrical and Electronic Engineering, University of Strathclyde, U.K., where he worked on self-tuning control systems and adaptive filtering as applied to dynamic ship positioning. Since 1984, he has been with the University of Paisley, U.K., first as a Lecturer and from 1986 as a Senior Lecturer. His research interests are in the areas of control systems as applied to communication problems and adaptive filtering of speech signals. Dr. Moir is a Chartered Engineer, a member of the Institute of Electrical Engineers, and a European Engineer.
