Advanced CO2 laser rangefinders

Advanced C02 laser rangefinders K.F. H U L M E F a c t o r s affecting the design of compact heterodyne- and direct-detection systems are contrasted. Recent U K achievements are placed in c o n t e x t : a T E A laser directdetection rangefinder and a chirp-pulse-compression rangefinder-velocimeter are descri bed. K E Y W O R DS: lasers, rangefinding, infra-red

Introduction

The aim o f this brief communication is to compare the design considerations for two types of CO2 laser rangefinders recently developed in the UK for use against natural targets (that is, targets not fitted with devices to enhance the return signal). The first type is a system based on a TEA laser, and uses direct detection (detector output voltage/current proportional to incident optical power). The second type is based on a cw laser externally-modulated with a chirp waveform, and uses heterodyne detection These rangefinders operate in the 10/am wavelength band. They are attractive on grounds of eye safety and have a natural compatibility with 8-13 #m thermal lmagers m atmospheric transmission and in optical components; also, ff heterodyne detection is used, velocity information is available - as well as range - b y virtue of the Doppler effect. TEA

laser r a n g e f i n d e r s u s i n g d i r e c t d e t e c t i o n

Following lmtlal work at RSRE these were further developed at Ferranti Ltd, Edinburgh 1 , and Marconi Avionics Ltd, Borehamwood 2 . Detailed techmcal information on a rangefinder o f this type has been published by Taylor et al 3 .

optical depths o f the order o f metres wall significantly broaden pulses shorter than, say, 10 ns. The duration of TEA laser pulses is therefore close to ideal for transmitter energy economy and for useful range accuracy. The last row of Table 1 refers to fluctuations in signal returns from shot to shot. Here, one seeks to avoid missed signals associated with the small-signal tail o f the probability distribution o f the returns from a given type o f target. Table 1. Design considerations: C O 2 TEA-laser/directdetection rangefinders

Aim

Implication

Solution

Minimize transmitted energy

Use shortest pulse

TEA laser

Range

Short pulse

TEA laser

Averaging required

Aperture averaging

accuracy Minimize speckle fluctuatmns

Figure 1 shows the Ferranti Type 307 equipment It is based on a Marcom TEA laser tube giving a 1/4 MW pulse with duration about 50 ns (FWHM). The detector is photoconductive (Hg, Cd) Te cooled to 77 K. Excluding the battery power supply and the compressed air bottle for detector cooling, the volume is less than 10 -2 m 3 and weight is 7 kg. The receiver aperture is 80 mm diameter and range coverage is from 300 m to 10 km. The factors affecting the design are summarized m Table 1. For direct-detection systems an incident pulse of given energy provides the greatest signal-to-noise ratio If the energy is contained m the shortest pulse consistent with detector response t~me4 . Even if extremely fast, sensitive detectors were available, the fact that many natural targets, for example trees or buildings viewed obhquely, have The author =sat the Royal S=gnalsand Radar Establishment, St Andrews Road, Great Malvern, Worcestersh=re, WR14 3PS, UK. Rece=ved 3 March 1982.

Fig. 1 CO 2 pulsed rangefmder. TEA-laser/d=rect detect=on (Ferrant= Type 307), Receiver aperture diameter 80 mm

0030-3992/82/040213-03/$03.00 © 1982 Butterworth & Co (Publishers) Ltd OPTICS AND LASER TECHNOLOGY . AUGUST 1982

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Targets behaving as arrays o f random-phase scatterers will produce speckle patterns in the receiver plane, and fluctuations can be greatly reduced if the power available in many speckle blobs can be summed. This is possible in a directdetection system where the detector responds to the total power incident, Irrespective of the phase of the contributions being summed. To obtain aperture-averaging o f this type from a Gaussian target in the far-field case, it will suffice to have a receiver aperture several times the transmitter aperture diameter (It being assumed that the transmitter beam divergence corresponds to the dzffractlon limit from the transmitter aperture). Fluctuation effects were observed b y Taylor et al a and their results support the considerations given

Modulated-cw systems with heterodyne detection One of the attractions of heterodyne systems 1s the advantage o f sensitivity over direct detection For the case o f a single non-fluctuating pulse this amounts, In the present context, to a factor of 80 in energy sensitivity4 . This advantage is somewhat eroded when there are signal fluctuations and when one needs to integrate (incoherently) over many pulses. The RSRE modulated-cw system has been described by Hulme et al 4 . The principle is shown m Fig. 2. The concept is to use the output of the cw laser for the maximum permissible time duration ( ~ 1 s) so that the laser power requirement is minimized. One therefore sums the effects of individual pulses repeated as often as possible The code used is a chirp waveform, duration 4 t~s, centre frequency 60 MHz, frequency swing 14 MHz, this is generated and compressed m SAW devices The waveform is transmitted every 33 gs and sufficient returns are integrated incoherently (that is, after rectification to destroy phase information) to provide adequate signal-to-noise ratm for the particular situation. Figure 3 shows the system. The optics head volume is less than 10 -2 m 3 but additional rack-mounted equipment is required. Transmit and recewe apertures are b o t h 50 mm diameter. Target radial velocity is obtained from the

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Doppler shift b y an up-chirp/down-chirp technique The wavegulde laser was manufactured by Ferrantl (Dundee), at has a rigid lnvar construction with a nominal power of 3 W With less than 1 s integration time, ranging capability typically extends to 7 km or beyond. The design considerations are summarized for this case in Table 2 In an ideal heterodyne system the signal-to-noise ratio of a single pulse depends only on the pulse energy s With a constant power source, one therefore seeks to maximize the pulse duration and the transmitter duty cycle. A suitable choice for range ambiguity IS 5 km corresponding to a pulse repetition interval of 33/as, this choice would limit the chirp pulse duration to 33/as. The choice of pulse duration is, however, more seriously influenced by the posslblhty of target internal motion destroying the coherence of the return pulse and thereby reducing the effectiveness of the compressive receiver. Estimates o f coherence time, and ultimately, the availability of ready-made SAW devices led to the choice of 4/as for pulse duration The required range accuracy of a few metres requires a transmitted bandwidth of order 10 MHz The 14 MHz bandwidth, 4/as chirp device was, conveniently, already available.

43-

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F=g 2 Prmmple of RSRE modulated cw/heterodyne-detectmn CO 2 laser rangefmder

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F=g. 3 RSRE modulated-cw system. The opt)cs head =s mounted on the tripod, and measures 340 mm x 220 mm x 100 ram. Rece)ver and transmitter aperture dmmeters are 50 mm

The use of a linear chirp waveform gave two advantages. Being a frequency-modulated waveform, It avoids the loss of transmitter power inevitable with external amplitude modulation of a cw source. Secondly, the effects of increasing Doppler shifts are comparatively benign, namely a graceful decay of the correlation peak output from the compression filter without serious sidelobe growth, and a Doppler-proportional range error The use of up-chirp

OPTICS AND LASER TECHNOLOGY

. A U G U S T 1982

Table 2. Designconsiderations:modulated-cw/heterodynedetection C02 laser rangefinders Aim

Implication

Solution

Minimize transmitted power

Use longest pulse; add many pulses

4 ~us pulses 30 kHz prf

Range accuracy

Large bandwidth frequency modulated

14 MHz frequency modulated

Cope with moving targets

Chirp (Doppler tolerant)

Up/down gwes range and velocity (< 30 ms-1)

Simple transmitter

Same laser for local oscillator and transmitter

cw laser,

Minim=ze speckle fluctuations

Temporal averaging

Integrate many pulses

Bragg acoustooptic modulator

Available SAW devices affect choice of chirp parameters

The problem of fluctuations cannot be alleviated in a heterodyne system by aperture-averaging because different speckle blobs have differing optical phases. When summed on the detector these contributions are as likely to interfere destructively as constructwely, and fluctuations are not avoided. (Note that for far-field targets, this leads to a useful recewer aperture roughly equal to the speckle blob size, which is m turn roughly equal to the size of the transmitter - It being assumed that the latter is gwing a diffract]on-limited minimum divergence.) However, a convenient averaging process is achieved by incoherently rotegrating pulses over a period long compared with the coherence time.

Conclusion For the rangefinders that we have described, it would be foolhardy to claim that optimum final designs have yet been attained. However, further development will undoubtedly have to take account of the design factors outhned above. Acknowledgement The author acknowledges the benefit of many interactions with those who have advanced the technology to xts present status.

References integration followed by down-chirp (opposite sign range error) m fact enables range and ra&al velocity to be determined. Transmitter design is slmphfied by using the same laser to provide transmitter and local oscillator beams. The need for frequency stabdizatlon is thus pracUcally avoided. One only has to worry about laser frequency changes during the transmit time to-and-from the target. Even under qu]te severe vibration, the effects of laser frequency swings (equivalent to synthetic Doppler) due to cavity length changes are negligible.

OPTICS AND LASER TECHNOLOGY. AUGUST 1982

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Techmcal hterature and data sheets avadable from Ferrantl Ltd, Robertson Avenue, Edinburgh 2 Technical hterature and data sheets from Marcom Avromcs Ltd, Elstree Way, Borehamwood, Hertfordshire 3 Taylor, M.J., Davies, P.H., Brown, D.W., Woods, W.E., Bell, LD., Kennedy, C.J. Pulsed CO2 laser rangefmders, Appl Opt 17 (1978) 885-9 4 Hulme, K.F., Collins, B.S., Constant, G.D., Pinson, J.T. A CO2 laser rangefmder using heterodyne detection and chirp pulse compression, Opt Quant Electr 13 (1981) 35--45 5 Skoinik, M.I. Introduction to Radar Systems, (McGraw-Hill, New York, 1967) Copyright © Controller HMSO, London, 1982

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