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CO2 laser heterodyne rangefinders, velocimeters and radars
Inlrared Phys. Vol. 25, No. 1/2, pp. 457 466, 1985
0020 0891/85 $3.00 + 0.00 Pergamon Press Ltd
Printed in Great Britain
CO
2
LASER H E T E R O D Y N E R A N G E F I N D E R S , V E L O C I M E T E R S A N D RADARS K. F. HULME~'~
Royal Signals and Radar Establishment, St Andrews Road, Malvern, Worcs. WR143PS, England
(Received 16 August 1984) A reviewof principles and design features emphasizeshow the choice of transmitter waveformis affected both by the intended application and by the ever-advancingcapabilities of lasers, modulators and receiver signal-processingsystems. Achievementsin application areas are surveyed including the work at RSRE on rangefinders/velocimetersand on Doppler anemometers.
Abstract
1. I N T R O D U C T I O N As soon as laser action was observed, it was realized that these new narrow-linewidth sources would make heterodyne detection possible in the optical and IR spectral range, and that the way was open to make true heterodyne radar systems at these wavelengths. Relative to conventional radars, laser radars were seen to offer extremely narrow beamwidths and extremely large Doppler frequency shifts. Of the heterodyne laser radar systems operating out-of-doors, the greatest attention has been given to those using the CO2 laser which emits at wavelengths in the 10/Jm band nicely inside the longestwavelength window of the optical and IR atmospheric transmission bands. There are good reasons for this attention. The CO2 laser has a very high conversion efficiency of electrical input power into laser power; values up to 20 oj~,have been reported. Also, the long wavelength makes it comparatively easy (relative to visible wavelengths) to attain the wavefront matching of local-oscillator and signal wavefronts needed in the receiver, and enables larger receiver apertures to be used for wavefronts rendered imperfect by atmospheric inhomogenities and speckle effects. Below, we first summarize qualitatively the background theoretical factors which govern any systems design; we then describe the available laser and modulating devices and signal-processing technology that dictate what are practicable systems; finally, we summarize the design features and achievements of various types of heterodyne CO2 laser radars. Rather than quote all the individual papers from the very extensive literature, we refer interested readers to the following recent Conference Proceedings in which references to many earlier individual articles will be found.
SPIE Prec., Vol. 227, C02 Laser Devices and Applications; Conference, Washington, D.C., U.S.A. (1980). (ii) SPIE Proc., Vol. 300, Physics and Technology o.t Coherent InJha Red Radar; Conference, San Diego, Calif., U.S.A. (1981). (iii) SPIE Proe. Vol. 415, Coherent InJi'a Red Radar, Systems and Applications; Conference, Arlington, Va., U.S.A. (1983). (iv) NASA Conf. Publ. No. 2138, Parts i and II; Conference, Williamsburg, Va., U.S.A. (1980). (v) OSA Tech. Dig., Vol. 83, No. 10, 2nd Topical Meeting on Coherent Laser Radar, Technology and Applications; Conference, Aspen, Colo., U.S.A. (1983). (i)
We apologize to authors whose credit has been submerged by this way of referencing, making the excuse that thorough and accurate referencing in a short article on this subject would be very timeconsuming and would give a numeral-bespattered text with a reference list of comparable length. (We apologize also for the possibility that the references underemphasize non-U.S, work.) There is unfortunately an inevitable arbitrariness in selecting examples of coherent CO2 systems in Section 5; Copyright ,:~:Controller, HMSO. London (1985). -I, Deceased. 457
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references are provided there, but I unashamedly emphasize results from my own laboratory RSRE where my most detailed knowledge exists.
2. H E T E R O D Y N E D E T E C T I O N AT 101lm The necessary non-linearity to provide mixing of optical frequencies stems from the fact that photodetectors react to the incident optical power: this is proportional to the s q u a r e of the semiclassical optical field amplitude. More rigorously, the probability of creation of a photoelectron (internal or external) is proportional to the incident optical power. For CO2 laser wavelengths, the creation of individual photoelectrons is not observable in available detectors because detector darkor background-currents produce very large carrier nmnbers within the minimum observational interval set by the time constant of the detector or the measuring circuit their randomness manifesting itself in detector shot noise. One therefore writes detector output amplitude s(t) as proportional to the square of the total tield provided by adding the signal (E~cos¢,)~t) and local oscillator (ELCOS¢OLt) beam amplitudes: s(t) ~ (E~cos~o~t +
ELCOS~OLt)2,
(1)
where the proportionality factor includes the quantum efficiency ~1(the ratio of mean photoelectron creation rate to incident photon flux). The difference-frequency signal power therefore contains it contribution proportional to the product E ; E L . The mean square noise amplitude from the detector will contain a shot-noise contribution proportional to incident optical power and the most sensitive heterodyne detection is achieved if the local-oscillator-induced shot-noise contribution can be made to dominate all other contributions: n 2 ~ E~.
(2)
One therefore obtains, as a limiting case, a ratio of signal-to-noise powers independent of localoscillator power: S N
~1 P :tin' B"
(31
where hv is the photon energy, P the mean signal power (proportional to E~), B the noise bandwidth at detector output, and :¢ - 1 for a reverse-biased photodiode and :~ - 2 for a photoconductor. The heterodyne noise-equivalent power (NEP) per hertz PN - (odn'/tl) is attained by setting (S/N) = 1 and B = 1Hz. W i t h ~ l = 1 , ~ = 1 one o b t a i n s p , ~ = 2 . 1 0 20 W/Hz as the ideal limit. Using(Hg, Cd)Te photodiodes at 77 K, values of about 4 • 10 2o W/Hz have been attained with operating (intermediate frequency) bandwidths extending to the gigahertz range. At the higher temperature of 200 K. l'x values of about 10 ~ W/Hz for bandwidths up to 100 M H z have been reported. Implicitly, we have regarded the detector as a point device. Beams are usually focused onto it. so its spatial extent will be at least a wavelength, and it will be important to obtain cooperative effects from all parts of the detector. This leads to a requirement that the signal and local-oscillator wavefronts be parallel throughout the active part of the detector to within a fraction of a wavelength. For efficient detection, as much of the received signal radiation as possible must be focused onto the detector: this is inconsistent with uniform intensity across the detector for the focused signal radiation; for nonuniform signal intensity, a uniform local-oscillator intensity across the detector will not optimize the S/N ratio. Consequently in practice a mixing efficiency factor fi must be inserted in equation (3). Values of fl as high as 0.8 can be readily obtained.
3. RADAR T H E O R E M S Much of the extensive classical knowledge on signal detectability and waveform choice is useful for CO2 laser radars because individual photoevents are non-observable so that detector output noise is usually Gaussian and the classical results of circuit analysis based on Gaussian noise are valid. The full generality of the classical results is however not as widely appreciated as it should be in the laser radar community.
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3.1. Delectability An important concept here is the "matched filter" which optimally distinguishes a signal with known waveform from Gaussian noise. In principle, the matched filter can be applied to either incident optical waveform or to the waveform emerging from the heterodyne detector; for CO2 laser radars only circuit filters are practicable. Using matched-filter results in equation (3) one then obtains: R-
[v2,] ..... V2o
[~r/e ~hv'
(4)
where the square brackets signify a cycle-averaged value- so that IV2] ..... is half the square of the peak value of the signal envelope at filter output, and VZNOis the mean-square output-noise voltage, at filter output; ¢ is the total received laser signal energy. The significant result of equation (4) is that it shows how system performance is related to received energy. To obtain reliable detection of a signal waveform (low rate for false alarms and for missed signals) the ratio in equation (4) needs to be of the order of 50 (so that of the order of 100 or so photons would need to be received even with near perfect mixing).
3.2. WaveJbrm Choice 3.2.1. Accuracy A heterodyne radar system aims to estimate two primary parameters from the target return signal: the r a n g e - - from the round-trip delay; and the velocity from the Doppler shift. Provided certain conditions on signal coherence are satisfied, the accuracies At, AID with which the latter quantities are determined are related to R of equation (4) by the expressions At = 1/t[~'~R ) and
(5) AiD = 1/(~'~R),
where/~' and c¢' are the r.m.s, bandwidths and durations of the signal waveform. Only for simple constant-frequency pulse waveforms is there a relationship between bandwidth and duration. For general waveforms there is no trade-off between accuracies in range and velocity; a long-duration waveform with wideband frequency modulation can give both accurate range and accurate velocity. Since the minimum value of R is dictated by the need for reliable detection, the system requirements on range and Doppler accuracy dictate minimum values of~' and [~' and accordingly affect waveform choice. The choice of waveform and the provision of a matched-filter receiver would be simpler were it not for the Doppler shifts created by relative motion of transmitter/receiver and target. The Doppler sensitivity of the receiver depends greatly on the type of waveform to which it is matched. The situation is summarized by the ambiguity function )~ introduced by Woodward when the theory of waveforms for conventional radars was being developed just after World War II. For a waveform with complex modulation function u(t), the ambiguity function is defined as
Z(~, 4)) -
f_
u(t)u*(t + r)exp(-2~i4)t)dt.
(6)
It gives the response at time r of the matched-filter receiver to a signal centred on t = 0 but having a Doppler frequency shift q~. Ambiguity functions have been computed for an extremely wide range of candidate waveforms and the quantity IZ21 is often shown pictorially.
3.2.2. Doppler effects For 10 #m systems, Doppler shifts ( ~ 200 kHz/m per s) are immense compared with conventional radars ( ~ 20 Hz/m per s), so that it is essential to choose a Doppler-tolerant waveform otherwise system complexity will be high (numerous Doppler channels) or sensitivity sacrificed. By a "Dopplertolerant" waveform we mean one that requires in the receiver only a tolerably small number of
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K . F . HULME
Doppler channels (or preferably only a single channel) to cope efficiently with the range of Doppler shifts likely to be encountered. There are two practically-applicable alternatives: (a) To use a short-duration unmodulated pulse waveform so that the expected Doppler shifts are not large relative to the pulse bandwidth; thus if, for example, a pulse of lOOns duration is used, a bandwidth of +25 M H z (corresponding to radial velocities of _+ 125 m/s) could be covered by, say, five Doppler channels in the receiver (each channel having a matched-filter for a bandwidth of 10 M Hz i; the disadvantage of this approach is that Doppler accuracy is necessarily very poor (or non-existent when a single channel is used for all Doppler shiftst. (b) Use linear-frequency-sweep chirp-codings with bandwidth a few times greater than the maximum Dopplers. A detailed study of the ambiguity functions demonstrates what Fig. 1 merely suggests: that small Doppler shifts are processed by the matched-filter receiver to produce an output time-shifted by an amount proportional to the Doppler shift.
f FREEIUENCY
/ /
J
- -
7•• /
DOPPLER SHIFT
k_ TIME DISPLACEMENT AT FILTER OUTPUT
TIME
>
Fig. 1. Illustrating how for a linear-chirp waveform a small D o p p l e r shift is quasi-equivalent to a time shift.
The detailed analysis shows that output peak amplitude reduces and temporal sidelobes increase with increasing Doppler in such a way that the performance fall-off is graceful rather than abrupt. Since the Doppler-related time shifts are of opposite sign for up-chirp and down-chirp waveforms, it is clearly possible to determine true target range and velocity by determining the apparent ranges in the two cases.
3.3. Fluctuations/Coherelwe So far we have regarded the returned signal as a known invariant quantity. This would be the situation if the target were a smooth highly-reflecting sphere totally illuminated by the transmitter beam, and if there were no random effects due to, for example, atmospheric turbulence. In practice, the target is often more complex and involves mutual.ly-interfering scatterers: it is instructive to consider a model consisting of an array of a large number of random-phase scatterers. The return amplitude at a point is then a sample from a Gaussian process and a large enough change of the relative phases of the scatterers will provide a fresh uncorrelated sample. Such phase changes could be produced by internal motion or rotation of the target, atmospheric turbulence (which would alter both the illuminating wavefront and the return propagation paths), and, for the situation when the target has depth, wavelength change of the transmitter laser; relative phase changes will also occur as the point of observation is moved across the receiver plane (where the intensity fluctuation would be referred to as a speckle-blob pattern). For these reasons, just as in classical radars, we can expect the return signal strength to lluctuate in time and space; instantaneous values will be described by a probability distribution. An important quantity describing temporal effects is the coherence time of the return the time which must be allowed to pass after one observation before another sample becomes uncorrelated in amplitude and phase. Because Doppler shifts are so large, correlation times due to target rotation can be as small as microseconds for 10/~m radars: the coherence time can be estimated from the time it takes for
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461
opposite sides of the illuminated spot on the target to undergo one-half wavelength relative displacement along the sightline due to rotation. Thus a 1 m diameter spot illuminating a target rotating once in 10s gives a coherence time of order 15 #s. The magnitude of the minimum expected coherence time will influence the choice of transmitter waveforms, because the optimized receivers will give a degraded performance if there are phase and amplitude errors in the signal submitted to the matched filter. The probabilistic character of return signal strength must be allowed for in estimating the statistical performance of the system, i.e. the rate at which false alarms are created by receiver noise exceeding a nominated threshold and the missed signal rate where target returns fail to cross the threshold. For a single-pulse system required to have a high degree of reliability, the existence of fluctuations will increase the required transmitter power relative to that needed for a constant amplitude return of the same average magnitude. It will often be advantageous to average over several transmitted waveforms extending over at least a coherence time. Speckle-blob effects limit the useful receiver aperture to about speckle-blob size in the receiver plane. If we assume a far-field situation, the illuminated spot on the target (at range R) has a lateral dimension D, the speckle-blob dimension S is therefore approx. 2R/D; since D is approx. 2R/T, where T is the transmitter aperture, we obtain S ~ T. There is no advantage in collecting signals from more than one speckle-blob because large phase variations will be encountered. Thus, equal transmitter and receiver aperture sizes are often used. Under very severe conditions of optical inhomogeneity of the atmosphere due to turbulence (e.g. long sightlines just above flat surfaces heated by strong sunlight) it is possible for the maximum usable aperture to be set by turbulence rather than speckle. Because the wavefront presented to the receiver aperture is not usually a uniform-amplitude plane wave, and there are variations of phase and amplitude, the phase variations lead to destructive interference and the average detector output will be lower than that calculated on the basis of average power received; relevant correction factors have been evaluated theoretically. For equal transmit and receive circular apertures with the target in the far-field of a transmitter launching a plane wave, the correction factor is 0.46. Atmospheric absorption due to water vapour and atmospheric CO2 affects signal strengths. Absorption coefficients at sea-level can range from less than 0.1 km ~ for cool, dry conditions to more than 0.5 km ~when it is hot and humid; note that even a modest value of 0.23 km ~implies a factor of 10 attenuation for a 5 k m range target and a factor of 100 for a 10km range.
3.4. Aperture Size and Field-oj:view (FO V) An important relationship affecting the design of any single-detector heterodyne system is that relating receiver aperture diameter d to the (far-field) angular field of view 0. For us, it suffices to confine attention to circular apertures and detectors and write this relationship (often referred to as the antenna theorem as 0 ~ 2/d.
(7)
For a given aperture size, the only way to enlarge the FOV above this limitation is to use multiple detectors (each with its own local-oscillator beam and each subject to the specified FOV limitation). Note that the limitation arises from the need to interfere free-space local-oscillator and signal waves at the detector; it does not apply to direct-detection systems, where detector size and details of receiver optics can be modified to change the receiver FOV.
4. S T A T U S OF T R A N S M I T T E R A N D S I G N A L - P R O C E S S I N G T E C H N O L O G Y
4.1. Frequency-stable Transmitter The importance of predictable waveforms at detector output stems from the desire to implement matched-filter receivers. The necessary frequency stability of the transmitter and local oscillator relative to one another is very demanding at 10 ~m wavelength corresponding as it does to a carrier frequency of 3- 10 ~3 Hz. Before concerning oneself with the temporal stability of a single output frequency, one first has to ensure that the laser emits a single output line. The laser must operate on a single molecular transition
462
K . F . HULME
line, the transverse mode structure must be controlled to obtain reliably only the simplest lowestorder transverse mode, and only one longitudinal mode must oscillate. Quite soon after the CO2 laser was discovered in the 1960s it was shown that for CW lasers the necessary control was readily achievable in the laboratory. Since that time, great progress has been made in translating these results into reliable field operation to provide robust, long-life, well-controlled CW lasers for use as local oscillators and/or transmitters: CW powers ranging from the 1 W level up to tens of watts and more have been demonstrated, and suitable lasers often of "'waveguide" construction are available commercially using either d.c. or RF excitation. If an unmodulated CW laser is used as a transmitter, only Doppler information will be available. Modulation methods have ranged from simple mechanical chopping to generate pulses of 10 ~s duration to electro-optic or acousto-optic devices. For wide bandwidths and sophisticated modulation formats, the acousto-optic modulator presents favourable opportunities because both frequency (FM) and amplitude modulation (AM) can be readily produced. Bandwidths of tens of megahertzs and rise-times in the fractional microsecond range are readily attainable with singlecrystal Ge acousto-optic modulators using lithium-niobate acoustic transducers. Electro-optic devices can be made with faster rise times, but are most suitable for AM only, and are power-hungry when wide-bandwidth AM coding is used. Ge ACOUSTO-OPTIC BLOCK
r-
FOCUSED | INPUT |
BEAM ~
\sound/ \,,beam/ ~ \ / I~'-.. ',, / . ~ I
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t~ I
BRAGG SCATTERED ~
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~
Electrode f£
Fig. 2. Illustration of how an acousto-optic modulator can be used to produce F M as well as AM.
Figure 2 indicates how an acousto-optic modulator operated in the Bragg regime can produce FM as the transducer input frequency.J2 is varied. Clearly the amplitude of the Bragg-reflected beam can be controlled by varying the acoustic power input. By focusing the input beam into the interaction region and by arranging this focus to be at the focus of the transmitter output lens, the output transmitted beam will not change direction as 12 is varied and the Bragg angle alters. The major advantage of external modulation is that a portion of the CW laser output is available for use as local oscillator; this greatly eases the problem of laser-frequency stabilization because one is concerned only with frequency changes occurring during the round trip to target and back (for 5 km this would amount to 33 ffs). The system thus has a built-in insensitivity to vibrations and temperature changes, so that there is the chance of dispensing with any active laser-stabilization circuitry. It is possible to amplitude-modulate RF-excited waveguide lasers directly by modulating tile RF power supplied rather than running it continuously and this constitutes one type of pulsed source. Another way of producing pulses is to provide constant excitation while repetitively altering the laser cavity Q-factor with, for example, an electro-optic polarization switch: typical pulse lengths obtained are 300ns, with pulse repetition rates of up to 50kHz and average powers amounting to a large fraction of that available when the laser is run CW. As with any sort of pulsed laser, a separate CW local-oscillator laser is needed, and the problem of relative stabilization is non-trivial. Not only must the pulsed and CW lasers be made to emit single modes on the same of the many possible molecular transition lines, but the frequency difference will need to be maintained constant by means of a monitor heterodyne detector and a feedback loop controlling pulsed-laser cavity length. The other major type of CO2 laser is the transversely-excited TEA laser giving out short pulses (initial spike duration a fraction of a microsecond, followed by a tail continuing for, perhaps, a few microseconds). Longitudinal and transverse mode control problems are often more severe than for CW CO2 lasers. One successful method of obtaining the requisite frequency stability has been to use a hybrid structure with a CW gain section and TEA gain section in the same optical cavity: the local oscillator and pulsed lasers can be stabilized onto the P{20) line by using a diffraction grating as one of
CO 2
laser rangefinders,velocimetersand radars
463
the cavity mirrors, and the difference frequency can be stabilized by a monitor heterodyne detector operating a feedback loop controlling laser cavity length; the CW laser output is quenched when the TEA laser output is obtained, so is not, unfortunately, available for local-oscillator use.
4.2. Signal Processing To process the signal waveforms emerging from the heterodyne detector the following circuit functions are often required in addition to the matched filter: waveform spectral analysis (to obtain Doppler shifts and therefore velocities); timing (to determine delays and therefore ranges); pulse-topulse integration (when it is required to increase the S/N ratio to obtain reliable and accurate measurements because signal strengths are too weak to permit single-pulse operation). The advances of electronic capabilities (particularly those stimulated by the needs of conventional radars) have provided sophisticated tools for these tasks. Two device areas are of high importance : digital circuitry, which has provided fast, reliable, compact and low-cost signal-integration capabilities and (with the aid of quartz-crystal frequency standards) precise timing; surface acousticwave devices, which are well-suited to providing compact chirp-waveform expanders and compressors which can be used not only to generate precise modulating waveforms and to precisely compress received signals, but can be applied to spectrally analyse received signals. At present only a minority of CO2 heterodyne laser radars use advanced signal processing, but it can be confidently asserted that more widespread use must follow from the steady advance of electronic circuit technology (particularly large-scale integrated circuitry) which will make practicable more complex systems of higher performance. 5. S E L E C T E D SYSTEMS; DESIGN FEATURES AND RESULTS 5.1. M I T Firepond System (Sullivan L. J., SPIE Proc., Vol. 227, pp. 148 161, 1980) This is notable for its large size and demonstrated ability to track satellites fitted with retroreflectors. Transmitter:
high-stability master oscillator followed by 1 kW power amplifier; CW or chopped-pulse output.
Optical aperture: 48" (1.2 m) giving a limiting beam divergence of 10/~rad. Detector:
(Hg, Cd)Te, 77 K, quadrant photodiodes with bandwidth 1.4 GHz (would cope with target radial velocities up to 7.103m/s), NEP 10- 19 W/Hz.
Signal processing: aimed at precise measurement of large Doppler shifts; frequency synthesizer to mix accurately-known frequencies with incoming signal; banks of crystal filters to determine down-converted frequencies to a limiting resolution of 1 kHz ; sophisticated Doppler acquisition circuitry. The system was equipped with ultra-high-accuracy tracking facility to enable a cooperative satellite (LAGEOS) to be tracked using the quadrant detector signals. The LAGEOS satellite orbited at 6000 km altitude and was a 60 cm sphere; it carried four 10pm retroreftectors made from Ge and located at the vertices of an inscribed tetrahedron. It was possible to determine the rotation period and rotation axis of the satellite from the differing Doppler frequencies (up to 240 kHz apart) provided by the different retroreflectors and from the time interval between successive zeros of the corrected Doppler frequency.
5.2. DREV 1 Hz Hybrid TEA Laser Rangefinder System (Cruickshank J. M., Appl. Opt., Vol. 18, pp. 290 293, 1979) This equipment is notable for using high-power pulses of short duration and is capable of accurate range determination with no Doppler capability. Transmitter:
0,4MW, 500ns (FWHM power), 1Hz; from hybrid CW/TEA cavity. Optical apertures: 10 cm dia separate transmitter and receiver apertures.
464
K . F . HULMF
Detector:
(Hg, Cd)Te 77 K photodiode. NEP 10- ~9 W/Hz.
Signal processing: range determined by crystal-controlled counter from time delay; receiver optimized for zero Doppler. The maximum reported range capability was 32 km against a tree-covered mountain; this was in h fairly low attenuation conditions (12°C, 64 oJo r..). With the rather larger attenuation corresponding to 19.1°C and 64'!o r.h,, the range capability on tree-covered mountains had dropped to 16kin.
5.3. U T R C High p~j System (Del Boca R. L. and Mongeon R. J., SPIE Proc., Vol. 300, pp. 19 32, 1981) A multifunction helicopter-mounted radar was described. A prime purpose was to use a rapidlyscanned beam to detect telephone- or power- line wires that present a hazard to the helicopter's flight intentions; other roles envisaged were to use Doppler for precision hover and navigation and to apply the system for terrain-following. Transmitter:
40 kHz prf; 340 ns pulses; average power 2 W from repetitively Qswitched laser.
Optical aperture: 10 cm; beam scannable by programmable Ge-wedge scanner over 30 ° cone. Receiver:
(Hg, Cd)Te photodiode 77 K.
Signal processing: in wire-avoidance, Doppler shifts up to 20 MHz due to helicopter motion were removed by a tracking receiver; the video-detected return pulses were processed to determine range to discriminate wire returns from other background signals. The system successfully detected transmission lines at a range of 1.6 km. A 1 W CW laser was used to demonstrate navigational capabilities and achieved a velocity resolution of approx. 1 cm/s.
5.4. NOAA Pulsed Doppler Lidar.Jbr Atmospheric Studies and jor W I N D S A 7 (Post M. J. et al. and Lawrence T. R. et al., SPIE Proc., Vol. 300, pp. 60-65, 34 43, 1981) The first of these systems looks upwards, detects returns from atmospheric aerosols (dust etc.) and provides range-resolved Doppler data giving the speed of the wind carrying the aerosols along. Transmitter:
hybrid-TEA, 100mJ pulses nominal duration 3/is, prf to 25 Hz.
Optical aperture : 28 cm; scannable beam. Detector:
(Hg, Cd)Te photodiode 77 K.
Signal processing: 10MHz A/D conversion followed by fast-Fourier transform computation of Doppler shift; frequency resolution 400kHz (corresponding to 2 m/s radial velocity); pulse averaging. When 500 pulses are averaged, range-resolved aerosol returns were typically obtained from up to the tropopause at 10 km altitude; layers of volcanic dust at 16 km altitude have been detected. The system measures only the velocity component along the line of sight, but, by scanning the beam, all the three velocity components of the wind can be determined. Proposals have been made to use an even higher power system (10J pulses, 10Hz prf) looking downwards from the U.S. space shuttle to survey global winds. The beam would be scanned conically to provide three velocity components. The project has the name WINDSAT. Although penetration of clouds would be very limited, such a coherent lidar is viewed as "the only concept now in sight that promises to provide an operational truly global wind determination system in the future". It would provide a wealth of information for meteorological modelers and forecasters.
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5.5. RSRE/RAE Laser True Airspeed System (LATAS) (WoodJield A. A. and Vaughan J. M., AGARDograph, Vol. AG272, pp. 7-1 to 7-18, 1983) The system looks ahead of the aircraft in which it is mounted and senses aerosol-scattered radiation and thereby determines the aircraft's true air speed. Transmitter:
3 W CW waveguide laser P(20) transition.
Optical aperture : 15 cm dia beam focused at a Gaussian beam-waist up to 300 m ahead of the aircraft. Detector:
(Hg. Cd)Te photodiode 77 K.
Signal processing: MESL 25 MHz bandwidth 30 kHz resolution surface acoustic-wave spectrum analyser switchable to cover a frequency range up to 62.5MHz (corresponding to 635 knots airspeed); CCL video integrator to add results from successive 25ps periods. Spectral output data can be obtained at repetition rates from 1/s to 160/s or more depending on signal strength. The compact equipment was mounted in the nose of an HS125 aircraft and the beam pointed to the focus ahead of the aircraft from which aerosol-scattered (and Doppler-shifted) radiation is predominantly received. Aerosol returns from clear air give adequate returns up to many kilometres altitude except just above the top of atmospheric inversion layers at high altitudes. The true airspeed is obtained with an accuracy of about 0.1 m/s. As the aircraft passes through a wind-shear layer in the atmosphere, a small jump in the Doppler shift is observed. Turbulent air motion broadens the frequency width of the return. These effects were observed, for example, as the aircraft was flown through a thunderstorm in the 1982 Joint Airport Weather Study (JAWS) in Colorado, U.S.A.
5.6. RSRE Chirp Rangefinder/Velocimeter (Hulme K. F. et al., Opt. quant. Electron., Vol. 13, pp. 35 45, 1981) The equipment was designed to determine range to 10 m accuracy and velocity to 1 m/s. Transmitter:
Modulated CW laser; laser power 3W; external acousto-optic modulator used to produce transmitted chirp pulse with 4/~s duration and 14MHz bandwidth every 30~ts; chirp waveform derived from surface acoustic-wave device; a sequence of up-chirps is followed by a sequence of down-chirps to provide range and velocity. A fraction of the unmodulated output of the laser is used as a local oscillator.
Optical aperture: 5 cm dia: separate transmitter and receiver apertures. Detector:
(Pb, Sn)Te or (Hg, Cd)Te photodiode 77K.
Signal processing: Detector output signal pulses are compressed in a surface acousticwave device (matched-receiver circuit) to 100 ns pulses; these pulses are envelope-detected and digitized; successive returns are integrated in a 516-stage 16-bit integrator untila threshold is crossed corresponding to reliable range/velocity determination: apparent ranges determined with up-and-down-chirps are used to obtain target true range and velocity. Timing provided by 30 MHz quartz clock. The equipment had a range capability of 5 km with an integration time of 1 s. It was demonstrated that, for a vehicle driven at constant speed towards the equipment, the velocity determined from upand down-chirp apparent ranges agreed closely with the velocity determined from the successive true ranges. The equipment is noteworthy in that it makes use of the sophisticated waveforms developed for conventional radar and that it needs only a single laser for transmitter and local oscillator. Problems
466
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k',,li.
of frequency harmonization are thereby greatly eased since one only detects laser frequency swings occurring in the round-trip time to the target (say 30 izs): consequently the system is not only compact (no separate local oscillator laser and monitor detector etc.) but is tolerant to mechanical vibrations and temperature changes. Nor is it necessary to control the transition on which the laser operates. The transmitted waveforms can in principle be varied by applying different electronic input waveforrns to the acousto-optic modulator thereby conferring a great versatility to the equipment. One cause for concern with long-pulse systems o[ this type is destruction of intra-pulse coherence by target internal motion. It was observed that the signal return at compressor output from trees in leaf was not adversely affected by winds of up to 12 m.'s indicating that coherence was not affected by the considerable wind-created motions of the leaves and branches. Note that the theoretical range accuracy estimated from equation (5) is of the order of Ira: implementing such an accuracy would have strained the speed of available digital integrators; an accuracy of 10m was selected in the design. The theoretical velocity accuracy of about l m/s is attained. Where there are fluctuations in target return strength, they are often sufficiently rapid for the integration of successive returns to provide a good averaging. BEAM- SPLITTER 950/oT Ge/Li NbO 3
B E A M - .1' SPLITTER~'Jf
TRANSDUCER =
95°/° R/'~fL
(
TP,ANSMIT
TELESCOPES ~ =
0
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RECEIVE
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P
(Hg,Cd)T¢ 77OK
TRANSMITTED, RECEIVED AND LOCAL OSCILLATOR OPTICAL BEAMS
,~l..Op,~ UNWANTED BEAMS WITH ACOUSTICALLY -- DELAYED MODULATION
Fig. 3. Path of false-target optical signals in a two-aperture system.
The RSRE system had separate transmit and receive apertures. This was to avoid a major problem in modulated CW systems cross-talk from transmitter to receiver in particular to avoid unwanted optical signals arising from reflections inside the acousto-optic modulator. Weak, timedelayed, reflected acoustic signals cross the input laser beam and produce modulated optical waveforms; as shown in Fig. 3 these can reach the detector via the laser output mirror; they produce weak false-target returns at ranges of the order of I kin. Shaping the modulator, and using irises in the optical path enabled us to reduce these unwanted signals to an acceptably low level in a two-aperture system. However, in a single-aperture system, an additional rather direct path for unwanted signals appears, as shown in Fig. 4, and the acoustic reflections must be reduced at source in the modulator. CO 2 ITTkT~ i---u
B.S. 95%T . \ . ,l, B.S. I , 95O/oR AI, / ~ / i, Z
Gel Li NbO 3 ~ _
~
T/R TELESCOPE / i , _ _ ^ __
TRANSDUCER/i /, "*'-- ~ ~4I1' 11' MIRROR
( ) DETECTOR ~ J (Hg.Cd)T~
POLARIZING BEAM SPUTTER AND QUARTER - WAVE PLATE
P WANTED BEAMS ~ OR-~. UNWANTED'DELAYED' BEAMS ADDITIONAL TO THOSE SHOWN IN FIG.3
Fig. 4. Additional path of false-target optical signals in a single-aperture system.
We have recently shown that the use of In damping layers on the Ge acousto-optic blocks is very effective in reducing these unwanted reflections (Hulme K. F. and Pinson J. T., SPIE Proc., Vol. 415, Paper No. 23, 1983). In has an acoustic impedance close to that of Ge (providing only weak reflected waves from the Ge In interface), it is highly absorbing to ultrasonic waves, and durable, strongly-adhering layers in intimate contact with the Ge can be produced by electrolytic deposition.
,4cknowledgement,~ period of years.
1 am grateful for the benelits of innumerable discussions with colleagues, especially at RSRE, over a
0020 0891/85 $3.00 + 0.00 Pergamon Press Ltd
Printed in Great Britain
CO
2
LASER H E T E R O D Y N E R A N G E F I N D E R S , V E L O C I M E T E R S A N D RADARS K. F. HULME~'~
Royal Signals and Radar Establishment, St Andrews Road, Malvern, Worcs. WR143PS, England
(Received 16 August 1984) A reviewof principles and design features emphasizeshow the choice of transmitter waveformis affected both by the intended application and by the ever-advancingcapabilities of lasers, modulators and receiver signal-processingsystems. Achievementsin application areas are surveyed including the work at RSRE on rangefinders/velocimetersand on Doppler anemometers.
Abstract
1. I N T R O D U C T I O N As soon as laser action was observed, it was realized that these new narrow-linewidth sources would make heterodyne detection possible in the optical and IR spectral range, and that the way was open to make true heterodyne radar systems at these wavelengths. Relative to conventional radars, laser radars were seen to offer extremely narrow beamwidths and extremely large Doppler frequency shifts. Of the heterodyne laser radar systems operating out-of-doors, the greatest attention has been given to those using the CO2 laser which emits at wavelengths in the 10/Jm band nicely inside the longestwavelength window of the optical and IR atmospheric transmission bands. There are good reasons for this attention. The CO2 laser has a very high conversion efficiency of electrical input power into laser power; values up to 20 oj~,have been reported. Also, the long wavelength makes it comparatively easy (relative to visible wavelengths) to attain the wavefront matching of local-oscillator and signal wavefronts needed in the receiver, and enables larger receiver apertures to be used for wavefronts rendered imperfect by atmospheric inhomogenities and speckle effects. Below, we first summarize qualitatively the background theoretical factors which govern any systems design; we then describe the available laser and modulating devices and signal-processing technology that dictate what are practicable systems; finally, we summarize the design features and achievements of various types of heterodyne CO2 laser radars. Rather than quote all the individual papers from the very extensive literature, we refer interested readers to the following recent Conference Proceedings in which references to many earlier individual articles will be found.
SPIE Prec., Vol. 227, C02 Laser Devices and Applications; Conference, Washington, D.C., U.S.A. (1980). (ii) SPIE Proc., Vol. 300, Physics and Technology o.t Coherent InJha Red Radar; Conference, San Diego, Calif., U.S.A. (1981). (iii) SPIE Proe. Vol. 415, Coherent InJi'a Red Radar, Systems and Applications; Conference, Arlington, Va., U.S.A. (1983). (iv) NASA Conf. Publ. No. 2138, Parts i and II; Conference, Williamsburg, Va., U.S.A. (1980). (v) OSA Tech. Dig., Vol. 83, No. 10, 2nd Topical Meeting on Coherent Laser Radar, Technology and Applications; Conference, Aspen, Colo., U.S.A. (1983). (i)
We apologize to authors whose credit has been submerged by this way of referencing, making the excuse that thorough and accurate referencing in a short article on this subject would be very timeconsuming and would give a numeral-bespattered text with a reference list of comparable length. (We apologize also for the possibility that the references underemphasize non-U.S, work.) There is unfortunately an inevitable arbitrariness in selecting examples of coherent CO2 systems in Section 5; Copyright ,:~:Controller, HMSO. London (1985). -I, Deceased. 457
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K.F. HULM[
references are provided there, but I unashamedly emphasize results from my own laboratory RSRE where my most detailed knowledge exists.
2. H E T E R O D Y N E D E T E C T I O N AT 101lm The necessary non-linearity to provide mixing of optical frequencies stems from the fact that photodetectors react to the incident optical power: this is proportional to the s q u a r e of the semiclassical optical field amplitude. More rigorously, the probability of creation of a photoelectron (internal or external) is proportional to the incident optical power. For CO2 laser wavelengths, the creation of individual photoelectrons is not observable in available detectors because detector darkor background-currents produce very large carrier nmnbers within the minimum observational interval set by the time constant of the detector or the measuring circuit their randomness manifesting itself in detector shot noise. One therefore writes detector output amplitude s(t) as proportional to the square of the total tield provided by adding the signal (E~cos¢,)~t) and local oscillator (ELCOS¢OLt) beam amplitudes: s(t) ~ (E~cos~o~t +
ELCOS~OLt)2,
(1)
where the proportionality factor includes the quantum efficiency ~1(the ratio of mean photoelectron creation rate to incident photon flux). The difference-frequency signal power therefore contains it contribution proportional to the product E ; E L . The mean square noise amplitude from the detector will contain a shot-noise contribution proportional to incident optical power and the most sensitive heterodyne detection is achieved if the local-oscillator-induced shot-noise contribution can be made to dominate all other contributions: n 2 ~ E~.
(2)
One therefore obtains, as a limiting case, a ratio of signal-to-noise powers independent of localoscillator power: S N
~1 P :tin' B"
(31
where hv is the photon energy, P the mean signal power (proportional to E~), B the noise bandwidth at detector output, and :¢ - 1 for a reverse-biased photodiode and :~ - 2 for a photoconductor. The heterodyne noise-equivalent power (NEP) per hertz PN - (odn'/tl) is attained by setting (S/N) = 1 and B = 1Hz. W i t h ~ l = 1 , ~ = 1 one o b t a i n s p , ~ = 2 . 1 0 20 W/Hz as the ideal limit. Using(Hg, Cd)Te photodiodes at 77 K, values of about 4 • 10 2o W/Hz have been attained with operating (intermediate frequency) bandwidths extending to the gigahertz range. At the higher temperature of 200 K. l'x values of about 10 ~ W/Hz for bandwidths up to 100 M H z have been reported. Implicitly, we have regarded the detector as a point device. Beams are usually focused onto it. so its spatial extent will be at least a wavelength, and it will be important to obtain cooperative effects from all parts of the detector. This leads to a requirement that the signal and local-oscillator wavefronts be parallel throughout the active part of the detector to within a fraction of a wavelength. For efficient detection, as much of the received signal radiation as possible must be focused onto the detector: this is inconsistent with uniform intensity across the detector for the focused signal radiation; for nonuniform signal intensity, a uniform local-oscillator intensity across the detector will not optimize the S/N ratio. Consequently in practice a mixing efficiency factor fi must be inserted in equation (3). Values of fl as high as 0.8 can be readily obtained.
3. RADAR T H E O R E M S Much of the extensive classical knowledge on signal detectability and waveform choice is useful for CO2 laser radars because individual photoevents are non-observable so that detector output noise is usually Gaussian and the classical results of circuit analysis based on Gaussian noise are valid. The full generality of the classical results is however not as widely appreciated as it should be in the laser radar community.
CO2 laser rangefinders,velocimetersand radars
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3.1. Delectability An important concept here is the "matched filter" which optimally distinguishes a signal with known waveform from Gaussian noise. In principle, the matched filter can be applied to either incident optical waveform or to the waveform emerging from the heterodyne detector; for CO2 laser radars only circuit filters are practicable. Using matched-filter results in equation (3) one then obtains: R-
[v2,] ..... V2o
[~r/e ~hv'
(4)
where the square brackets signify a cycle-averaged value- so that IV2] ..... is half the square of the peak value of the signal envelope at filter output, and VZNOis the mean-square output-noise voltage, at filter output; ¢ is the total received laser signal energy. The significant result of equation (4) is that it shows how system performance is related to received energy. To obtain reliable detection of a signal waveform (low rate for false alarms and for missed signals) the ratio in equation (4) needs to be of the order of 50 (so that of the order of 100 or so photons would need to be received even with near perfect mixing).
3.2. WaveJbrm Choice 3.2.1. Accuracy A heterodyne radar system aims to estimate two primary parameters from the target return signal: the r a n g e - - from the round-trip delay; and the velocity from the Doppler shift. Provided certain conditions on signal coherence are satisfied, the accuracies At, AID with which the latter quantities are determined are related to R of equation (4) by the expressions At = 1/t[~'~R ) and
(5) AiD = 1/(~'~R),
where/~' and c¢' are the r.m.s, bandwidths and durations of the signal waveform. Only for simple constant-frequency pulse waveforms is there a relationship between bandwidth and duration. For general waveforms there is no trade-off between accuracies in range and velocity; a long-duration waveform with wideband frequency modulation can give both accurate range and accurate velocity. Since the minimum value of R is dictated by the need for reliable detection, the system requirements on range and Doppler accuracy dictate minimum values of~' and [~' and accordingly affect waveform choice. The choice of waveform and the provision of a matched-filter receiver would be simpler were it not for the Doppler shifts created by relative motion of transmitter/receiver and target. The Doppler sensitivity of the receiver depends greatly on the type of waveform to which it is matched. The situation is summarized by the ambiguity function )~ introduced by Woodward when the theory of waveforms for conventional radars was being developed just after World War II. For a waveform with complex modulation function u(t), the ambiguity function is defined as
Z(~, 4)) -
f_
u(t)u*(t + r)exp(-2~i4)t)dt.
(6)
It gives the response at time r of the matched-filter receiver to a signal centred on t = 0 but having a Doppler frequency shift q~. Ambiguity functions have been computed for an extremely wide range of candidate waveforms and the quantity IZ21 is often shown pictorially.
3.2.2. Doppler effects For 10 #m systems, Doppler shifts ( ~ 200 kHz/m per s) are immense compared with conventional radars ( ~ 20 Hz/m per s), so that it is essential to choose a Doppler-tolerant waveform otherwise system complexity will be high (numerous Doppler channels) or sensitivity sacrificed. By a "Dopplertolerant" waveform we mean one that requires in the receiver only a tolerably small number of
460
K . F . HULME
Doppler channels (or preferably only a single channel) to cope efficiently with the range of Doppler shifts likely to be encountered. There are two practically-applicable alternatives: (a) To use a short-duration unmodulated pulse waveform so that the expected Doppler shifts are not large relative to the pulse bandwidth; thus if, for example, a pulse of lOOns duration is used, a bandwidth of +25 M H z (corresponding to radial velocities of _+ 125 m/s) could be covered by, say, five Doppler channels in the receiver (each channel having a matched-filter for a bandwidth of 10 M Hz i; the disadvantage of this approach is that Doppler accuracy is necessarily very poor (or non-existent when a single channel is used for all Doppler shiftst. (b) Use linear-frequency-sweep chirp-codings with bandwidth a few times greater than the maximum Dopplers. A detailed study of the ambiguity functions demonstrates what Fig. 1 merely suggests: that small Doppler shifts are processed by the matched-filter receiver to produce an output time-shifted by an amount proportional to the Doppler shift.
f FREEIUENCY
/ /
J
- -
7•• /
DOPPLER SHIFT
k_ TIME DISPLACEMENT AT FILTER OUTPUT
TIME
>
Fig. 1. Illustrating how for a linear-chirp waveform a small D o p p l e r shift is quasi-equivalent to a time shift.
The detailed analysis shows that output peak amplitude reduces and temporal sidelobes increase with increasing Doppler in such a way that the performance fall-off is graceful rather than abrupt. Since the Doppler-related time shifts are of opposite sign for up-chirp and down-chirp waveforms, it is clearly possible to determine true target range and velocity by determining the apparent ranges in the two cases.
3.3. Fluctuations/Coherelwe So far we have regarded the returned signal as a known invariant quantity. This would be the situation if the target were a smooth highly-reflecting sphere totally illuminated by the transmitter beam, and if there were no random effects due to, for example, atmospheric turbulence. In practice, the target is often more complex and involves mutual.ly-interfering scatterers: it is instructive to consider a model consisting of an array of a large number of random-phase scatterers. The return amplitude at a point is then a sample from a Gaussian process and a large enough change of the relative phases of the scatterers will provide a fresh uncorrelated sample. Such phase changes could be produced by internal motion or rotation of the target, atmospheric turbulence (which would alter both the illuminating wavefront and the return propagation paths), and, for the situation when the target has depth, wavelength change of the transmitter laser; relative phase changes will also occur as the point of observation is moved across the receiver plane (where the intensity fluctuation would be referred to as a speckle-blob pattern). For these reasons, just as in classical radars, we can expect the return signal strength to lluctuate in time and space; instantaneous values will be described by a probability distribution. An important quantity describing temporal effects is the coherence time of the return the time which must be allowed to pass after one observation before another sample becomes uncorrelated in amplitude and phase. Because Doppler shifts are so large, correlation times due to target rotation can be as small as microseconds for 10/~m radars: the coherence time can be estimated from the time it takes for
c02 laser rangefinders, velocimetersand radars
461
opposite sides of the illuminated spot on the target to undergo one-half wavelength relative displacement along the sightline due to rotation. Thus a 1 m diameter spot illuminating a target rotating once in 10s gives a coherence time of order 15 #s. The magnitude of the minimum expected coherence time will influence the choice of transmitter waveforms, because the optimized receivers will give a degraded performance if there are phase and amplitude errors in the signal submitted to the matched filter. The probabilistic character of return signal strength must be allowed for in estimating the statistical performance of the system, i.e. the rate at which false alarms are created by receiver noise exceeding a nominated threshold and the missed signal rate where target returns fail to cross the threshold. For a single-pulse system required to have a high degree of reliability, the existence of fluctuations will increase the required transmitter power relative to that needed for a constant amplitude return of the same average magnitude. It will often be advantageous to average over several transmitted waveforms extending over at least a coherence time. Speckle-blob effects limit the useful receiver aperture to about speckle-blob size in the receiver plane. If we assume a far-field situation, the illuminated spot on the target (at range R) has a lateral dimension D, the speckle-blob dimension S is therefore approx. 2R/D; since D is approx. 2R/T, where T is the transmitter aperture, we obtain S ~ T. There is no advantage in collecting signals from more than one speckle-blob because large phase variations will be encountered. Thus, equal transmitter and receiver aperture sizes are often used. Under very severe conditions of optical inhomogeneity of the atmosphere due to turbulence (e.g. long sightlines just above flat surfaces heated by strong sunlight) it is possible for the maximum usable aperture to be set by turbulence rather than speckle. Because the wavefront presented to the receiver aperture is not usually a uniform-amplitude plane wave, and there are variations of phase and amplitude, the phase variations lead to destructive interference and the average detector output will be lower than that calculated on the basis of average power received; relevant correction factors have been evaluated theoretically. For equal transmit and receive circular apertures with the target in the far-field of a transmitter launching a plane wave, the correction factor is 0.46. Atmospheric absorption due to water vapour and atmospheric CO2 affects signal strengths. Absorption coefficients at sea-level can range from less than 0.1 km ~ for cool, dry conditions to more than 0.5 km ~when it is hot and humid; note that even a modest value of 0.23 km ~implies a factor of 10 attenuation for a 5 k m range target and a factor of 100 for a 10km range.
3.4. Aperture Size and Field-oj:view (FO V) An important relationship affecting the design of any single-detector heterodyne system is that relating receiver aperture diameter d to the (far-field) angular field of view 0. For us, it suffices to confine attention to circular apertures and detectors and write this relationship (often referred to as the antenna theorem as 0 ~ 2/d.
(7)
For a given aperture size, the only way to enlarge the FOV above this limitation is to use multiple detectors (each with its own local-oscillator beam and each subject to the specified FOV limitation). Note that the limitation arises from the need to interfere free-space local-oscillator and signal waves at the detector; it does not apply to direct-detection systems, where detector size and details of receiver optics can be modified to change the receiver FOV.
4. S T A T U S OF T R A N S M I T T E R A N D S I G N A L - P R O C E S S I N G T E C H N O L O G Y
4.1. Frequency-stable Transmitter The importance of predictable waveforms at detector output stems from the desire to implement matched-filter receivers. The necessary frequency stability of the transmitter and local oscillator relative to one another is very demanding at 10 ~m wavelength corresponding as it does to a carrier frequency of 3- 10 ~3 Hz. Before concerning oneself with the temporal stability of a single output frequency, one first has to ensure that the laser emits a single output line. The laser must operate on a single molecular transition
462
K . F . HULME
line, the transverse mode structure must be controlled to obtain reliably only the simplest lowestorder transverse mode, and only one longitudinal mode must oscillate. Quite soon after the CO2 laser was discovered in the 1960s it was shown that for CW lasers the necessary control was readily achievable in the laboratory. Since that time, great progress has been made in translating these results into reliable field operation to provide robust, long-life, well-controlled CW lasers for use as local oscillators and/or transmitters: CW powers ranging from the 1 W level up to tens of watts and more have been demonstrated, and suitable lasers often of "'waveguide" construction are available commercially using either d.c. or RF excitation. If an unmodulated CW laser is used as a transmitter, only Doppler information will be available. Modulation methods have ranged from simple mechanical chopping to generate pulses of 10 ~s duration to electro-optic or acousto-optic devices. For wide bandwidths and sophisticated modulation formats, the acousto-optic modulator presents favourable opportunities because both frequency (FM) and amplitude modulation (AM) can be readily produced. Bandwidths of tens of megahertzs and rise-times in the fractional microsecond range are readily attainable with singlecrystal Ge acousto-optic modulators using lithium-niobate acoustic transducers. Electro-optic devices can be made with faster rise times, but are most suitable for AM only, and are power-hungry when wide-bandwidth AM coding is used. Ge ACOUSTO-OPTIC BLOCK
r-
FOCUSED | INPUT |
BEAM ~
\sound/ \,,beam/ ~ \ / I~'-.. ',, / . ~ I
-~.~1
LiNbO 3 / " TRANSDUCER
t~ I
BRAGG SCATTERED ~
MODULATED BEAM
~
~
Electrode f£
Fig. 2. Illustration of how an acousto-optic modulator can be used to produce F M as well as AM.
Figure 2 indicates how an acousto-optic modulator operated in the Bragg regime can produce FM as the transducer input frequency.J2 is varied. Clearly the amplitude of the Bragg-reflected beam can be controlled by varying the acoustic power input. By focusing the input beam into the interaction region and by arranging this focus to be at the focus of the transmitter output lens, the output transmitted beam will not change direction as 12 is varied and the Bragg angle alters. The major advantage of external modulation is that a portion of the CW laser output is available for use as local oscillator; this greatly eases the problem of laser-frequency stabilization because one is concerned only with frequency changes occurring during the round trip to target and back (for 5 km this would amount to 33 ffs). The system thus has a built-in insensitivity to vibrations and temperature changes, so that there is the chance of dispensing with any active laser-stabilization circuitry. It is possible to amplitude-modulate RF-excited waveguide lasers directly by modulating tile RF power supplied rather than running it continuously and this constitutes one type of pulsed source. Another way of producing pulses is to provide constant excitation while repetitively altering the laser cavity Q-factor with, for example, an electro-optic polarization switch: typical pulse lengths obtained are 300ns, with pulse repetition rates of up to 50kHz and average powers amounting to a large fraction of that available when the laser is run CW. As with any sort of pulsed laser, a separate CW local-oscillator laser is needed, and the problem of relative stabilization is non-trivial. Not only must the pulsed and CW lasers be made to emit single modes on the same of the many possible molecular transition lines, but the frequency difference will need to be maintained constant by means of a monitor heterodyne detector and a feedback loop controlling pulsed-laser cavity length. The other major type of CO2 laser is the transversely-excited TEA laser giving out short pulses (initial spike duration a fraction of a microsecond, followed by a tail continuing for, perhaps, a few microseconds). Longitudinal and transverse mode control problems are often more severe than for CW CO2 lasers. One successful method of obtaining the requisite frequency stability has been to use a hybrid structure with a CW gain section and TEA gain section in the same optical cavity: the local oscillator and pulsed lasers can be stabilized onto the P{20) line by using a diffraction grating as one of
CO 2
laser rangefinders,velocimetersand radars
463
the cavity mirrors, and the difference frequency can be stabilized by a monitor heterodyne detector operating a feedback loop controlling laser cavity length; the CW laser output is quenched when the TEA laser output is obtained, so is not, unfortunately, available for local-oscillator use.
4.2. Signal Processing To process the signal waveforms emerging from the heterodyne detector the following circuit functions are often required in addition to the matched filter: waveform spectral analysis (to obtain Doppler shifts and therefore velocities); timing (to determine delays and therefore ranges); pulse-topulse integration (when it is required to increase the S/N ratio to obtain reliable and accurate measurements because signal strengths are too weak to permit single-pulse operation). The advances of electronic capabilities (particularly those stimulated by the needs of conventional radars) have provided sophisticated tools for these tasks. Two device areas are of high importance : digital circuitry, which has provided fast, reliable, compact and low-cost signal-integration capabilities and (with the aid of quartz-crystal frequency standards) precise timing; surface acousticwave devices, which are well-suited to providing compact chirp-waveform expanders and compressors which can be used not only to generate precise modulating waveforms and to precisely compress received signals, but can be applied to spectrally analyse received signals. At present only a minority of CO2 heterodyne laser radars use advanced signal processing, but it can be confidently asserted that more widespread use must follow from the steady advance of electronic circuit technology (particularly large-scale integrated circuitry) which will make practicable more complex systems of higher performance. 5. S E L E C T E D SYSTEMS; DESIGN FEATURES AND RESULTS 5.1. M I T Firepond System (Sullivan L. J., SPIE Proc., Vol. 227, pp. 148 161, 1980) This is notable for its large size and demonstrated ability to track satellites fitted with retroreflectors. Transmitter:
high-stability master oscillator followed by 1 kW power amplifier; CW or chopped-pulse output.
Optical aperture: 48" (1.2 m) giving a limiting beam divergence of 10/~rad. Detector:
(Hg, Cd)Te, 77 K, quadrant photodiodes with bandwidth 1.4 GHz (would cope with target radial velocities up to 7.103m/s), NEP 10- 19 W/Hz.
Signal processing: aimed at precise measurement of large Doppler shifts; frequency synthesizer to mix accurately-known frequencies with incoming signal; banks of crystal filters to determine down-converted frequencies to a limiting resolution of 1 kHz ; sophisticated Doppler acquisition circuitry. The system was equipped with ultra-high-accuracy tracking facility to enable a cooperative satellite (LAGEOS) to be tracked using the quadrant detector signals. The LAGEOS satellite orbited at 6000 km altitude and was a 60 cm sphere; it carried four 10pm retroreftectors made from Ge and located at the vertices of an inscribed tetrahedron. It was possible to determine the rotation period and rotation axis of the satellite from the differing Doppler frequencies (up to 240 kHz apart) provided by the different retroreflectors and from the time interval between successive zeros of the corrected Doppler frequency.
5.2. DREV 1 Hz Hybrid TEA Laser Rangefinder System (Cruickshank J. M., Appl. Opt., Vol. 18, pp. 290 293, 1979) This equipment is notable for using high-power pulses of short duration and is capable of accurate range determination with no Doppler capability. Transmitter:
0,4MW, 500ns (FWHM power), 1Hz; from hybrid CW/TEA cavity. Optical apertures: 10 cm dia separate transmitter and receiver apertures.
464
K . F . HULMF
Detector:
(Hg, Cd)Te 77 K photodiode. NEP 10- ~9 W/Hz.
Signal processing: range determined by crystal-controlled counter from time delay; receiver optimized for zero Doppler. The maximum reported range capability was 32 km against a tree-covered mountain; this was in h fairly low attenuation conditions (12°C, 64 oJo r..). With the rather larger attenuation corresponding to 19.1°C and 64'!o r.h,, the range capability on tree-covered mountains had dropped to 16kin.
5.3. U T R C High p~j System (Del Boca R. L. and Mongeon R. J., SPIE Proc., Vol. 300, pp. 19 32, 1981) A multifunction helicopter-mounted radar was described. A prime purpose was to use a rapidlyscanned beam to detect telephone- or power- line wires that present a hazard to the helicopter's flight intentions; other roles envisaged were to use Doppler for precision hover and navigation and to apply the system for terrain-following. Transmitter:
40 kHz prf; 340 ns pulses; average power 2 W from repetitively Qswitched laser.
Optical aperture: 10 cm; beam scannable by programmable Ge-wedge scanner over 30 ° cone. Receiver:
(Hg, Cd)Te photodiode 77 K.
Signal processing: in wire-avoidance, Doppler shifts up to 20 MHz due to helicopter motion were removed by a tracking receiver; the video-detected return pulses were processed to determine range to discriminate wire returns from other background signals. The system successfully detected transmission lines at a range of 1.6 km. A 1 W CW laser was used to demonstrate navigational capabilities and achieved a velocity resolution of approx. 1 cm/s.
5.4. NOAA Pulsed Doppler Lidar.Jbr Atmospheric Studies and jor W I N D S A 7 (Post M. J. et al. and Lawrence T. R. et al., SPIE Proc., Vol. 300, pp. 60-65, 34 43, 1981) The first of these systems looks upwards, detects returns from atmospheric aerosols (dust etc.) and provides range-resolved Doppler data giving the speed of the wind carrying the aerosols along. Transmitter:
hybrid-TEA, 100mJ pulses nominal duration 3/is, prf to 25 Hz.
Optical aperture : 28 cm; scannable beam. Detector:
(Hg, Cd)Te photodiode 77 K.
Signal processing: 10MHz A/D conversion followed by fast-Fourier transform computation of Doppler shift; frequency resolution 400kHz (corresponding to 2 m/s radial velocity); pulse averaging. When 500 pulses are averaged, range-resolved aerosol returns were typically obtained from up to the tropopause at 10 km altitude; layers of volcanic dust at 16 km altitude have been detected. The system measures only the velocity component along the line of sight, but, by scanning the beam, all the three velocity components of the wind can be determined. Proposals have been made to use an even higher power system (10J pulses, 10Hz prf) looking downwards from the U.S. space shuttle to survey global winds. The beam would be scanned conically to provide three velocity components. The project has the name WINDSAT. Although penetration of clouds would be very limited, such a coherent lidar is viewed as "the only concept now in sight that promises to provide an operational truly global wind determination system in the future". It would provide a wealth of information for meteorological modelers and forecasters.
co2 laser rangefinders,velocimetersand radars
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5.5. RSRE/RAE Laser True Airspeed System (LATAS) (WoodJield A. A. and Vaughan J. M., AGARDograph, Vol. AG272, pp. 7-1 to 7-18, 1983) The system looks ahead of the aircraft in which it is mounted and senses aerosol-scattered radiation and thereby determines the aircraft's true air speed. Transmitter:
3 W CW waveguide laser P(20) transition.
Optical aperture : 15 cm dia beam focused at a Gaussian beam-waist up to 300 m ahead of the aircraft. Detector:
(Hg. Cd)Te photodiode 77 K.
Signal processing: MESL 25 MHz bandwidth 30 kHz resolution surface acoustic-wave spectrum analyser switchable to cover a frequency range up to 62.5MHz (corresponding to 635 knots airspeed); CCL video integrator to add results from successive 25ps periods. Spectral output data can be obtained at repetition rates from 1/s to 160/s or more depending on signal strength. The compact equipment was mounted in the nose of an HS125 aircraft and the beam pointed to the focus ahead of the aircraft from which aerosol-scattered (and Doppler-shifted) radiation is predominantly received. Aerosol returns from clear air give adequate returns up to many kilometres altitude except just above the top of atmospheric inversion layers at high altitudes. The true airspeed is obtained with an accuracy of about 0.1 m/s. As the aircraft passes through a wind-shear layer in the atmosphere, a small jump in the Doppler shift is observed. Turbulent air motion broadens the frequency width of the return. These effects were observed, for example, as the aircraft was flown through a thunderstorm in the 1982 Joint Airport Weather Study (JAWS) in Colorado, U.S.A.
5.6. RSRE Chirp Rangefinder/Velocimeter (Hulme K. F. et al., Opt. quant. Electron., Vol. 13, pp. 35 45, 1981) The equipment was designed to determine range to 10 m accuracy and velocity to 1 m/s. Transmitter:
Modulated CW laser; laser power 3W; external acousto-optic modulator used to produce transmitted chirp pulse with 4/~s duration and 14MHz bandwidth every 30~ts; chirp waveform derived from surface acoustic-wave device; a sequence of up-chirps is followed by a sequence of down-chirps to provide range and velocity. A fraction of the unmodulated output of the laser is used as a local oscillator.
Optical aperture: 5 cm dia: separate transmitter and receiver apertures. Detector:
(Pb, Sn)Te or (Hg, Cd)Te photodiode 77K.
Signal processing: Detector output signal pulses are compressed in a surface acousticwave device (matched-receiver circuit) to 100 ns pulses; these pulses are envelope-detected and digitized; successive returns are integrated in a 516-stage 16-bit integrator untila threshold is crossed corresponding to reliable range/velocity determination: apparent ranges determined with up-and-down-chirps are used to obtain target true range and velocity. Timing provided by 30 MHz quartz clock. The equipment had a range capability of 5 km with an integration time of 1 s. It was demonstrated that, for a vehicle driven at constant speed towards the equipment, the velocity determined from upand down-chirp apparent ranges agreed closely with the velocity determined from the successive true ranges. The equipment is noteworthy in that it makes use of the sophisticated waveforms developed for conventional radar and that it needs only a single laser for transmitter and local oscillator. Problems
466
K . F . Ht
k',,li.
of frequency harmonization are thereby greatly eased since one only detects laser frequency swings occurring in the round-trip time to the target (say 30 izs): consequently the system is not only compact (no separate local oscillator laser and monitor detector etc.) but is tolerant to mechanical vibrations and temperature changes. Nor is it necessary to control the transition on which the laser operates. The transmitted waveforms can in principle be varied by applying different electronic input waveforrns to the acousto-optic modulator thereby conferring a great versatility to the equipment. One cause for concern with long-pulse systems o[ this type is destruction of intra-pulse coherence by target internal motion. It was observed that the signal return at compressor output from trees in leaf was not adversely affected by winds of up to 12 m.'s indicating that coherence was not affected by the considerable wind-created motions of the leaves and branches. Note that the theoretical range accuracy estimated from equation (5) is of the order of Ira: implementing such an accuracy would have strained the speed of available digital integrators; an accuracy of 10m was selected in the design. The theoretical velocity accuracy of about l m/s is attained. Where there are fluctuations in target return strength, they are often sufficiently rapid for the integration of successive returns to provide a good averaging. BEAM- SPLITTER 950/oT Ge/Li NbO 3
B E A M - .1' SPLITTER~'Jf
TRANSDUCER =
95°/° R/'~fL
(
TP,ANSMIT
TELESCOPES ~ =
0
4
RECEIVE
) DETECTOR
P
(Hg,Cd)T¢ 77OK
TRANSMITTED, RECEIVED AND LOCAL OSCILLATOR OPTICAL BEAMS
,~l..Op,~ UNWANTED BEAMS WITH ACOUSTICALLY -- DELAYED MODULATION
Fig. 3. Path of false-target optical signals in a two-aperture system.
The RSRE system had separate transmit and receive apertures. This was to avoid a major problem in modulated CW systems cross-talk from transmitter to receiver in particular to avoid unwanted optical signals arising from reflections inside the acousto-optic modulator. Weak, timedelayed, reflected acoustic signals cross the input laser beam and produce modulated optical waveforms; as shown in Fig. 3 these can reach the detector via the laser output mirror; they produce weak false-target returns at ranges of the order of I kin. Shaping the modulator, and using irises in the optical path enabled us to reduce these unwanted signals to an acceptably low level in a two-aperture system. However, in a single-aperture system, an additional rather direct path for unwanted signals appears, as shown in Fig. 4, and the acoustic reflections must be reduced at source in the modulator. CO 2 ITTkT~ i---u
B.S. 95%T . \ . ,l, B.S. I , 95O/oR AI, / ~ / i, Z
Gel Li NbO 3 ~ _
~
T/R TELESCOPE / i , _ _ ^ __
TRANSDUCER/i /, "*'-- ~ ~4I1' 11' MIRROR
( ) DETECTOR ~ J (Hg.Cd)T~
POLARIZING BEAM SPUTTER AND QUARTER - WAVE PLATE
P WANTED BEAMS ~ OR-~. UNWANTED'DELAYED' BEAMS ADDITIONAL TO THOSE SHOWN IN FIG.3
Fig. 4. Additional path of false-target optical signals in a single-aperture system.
We have recently shown that the use of In damping layers on the Ge acousto-optic blocks is very effective in reducing these unwanted reflections (Hulme K. F. and Pinson J. T., SPIE Proc., Vol. 415, Paper No. 23, 1983). In has an acoustic impedance close to that of Ge (providing only weak reflected waves from the Ge In interface), it is highly absorbing to ultrasonic waves, and durable, strongly-adhering layers in intimate contact with the Ge can be produced by electrolytic deposition.
,4cknowledgement,~ period of years.
1 am grateful for the benelits of innumerable discussions with colleagues, especially at RSRE, over a