GB1585053A - Laser rangefinder - Google Patents

Description

(54) LASER RANGEFINDER (71) I, SECRETARY OF STATE FOR DEFENCE, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to laser rangefinders.

Prior art laser rangefinders which operate with a pulsed laser time the interval between transmission of a pulse and receipt of that pulse reflected from a target.

Rangefinders are known in the radar field which operate using both pulsed and con tinuous wave (CW) microwave radiation. To a certain extent there exists a similarity be tween pulsed microwave and pulsed laser rangefinder systems but for CW rangefinders the frequency operation of a laser source e.g.

3 X 1013Hz gives rise to many problems not present in microwave source e.g. 3 X 109Hz.

Ideally a rangefinder should not present a health hazard to its users i.e. operators or 'friendly' targets. Unfortunately most of the known pulsed lasers operate in the 1.06 ,am wavelength and this wavelength can cause eye damage. Fortunately eye damage can be mini mised by using lasers operating in the 10.6 um region, i.e. a CO2 laser. Another advantage of operating in the 10.6 ,am region is that atmospheric attenuation of the laser light, due to moisture droplets, etc., is much less than for lasers operating in say the 1.06 ,um wavelength region.

According to this invention a laser rangefinder comprises a laser capable of emitting continuous radiation, a generator of a repeti tive modulating signal which varies in frequency with time at a repetition rate determined by a timing pulse generator, an optical modulator for modulating radiation from the laser with the modulating signal, means for directing radiation from the optical modulator onto a target, means for receiving radiation reflected from a target and directing it onto a detector capable of giving an output representation of radiation received, means for directing a proportion of the laser output onto the detector for mixing with the received reflected radiation, circuitry having frequency time characteristics matched to the generator of a modulating signal for processing the output of the detector to provide a target pulse whose time delay from a timing pulse represents target range, and an integrator for integrating the processed output of the detector.

The generator of a modulated signal may be a pulse generator and a pulse expander capable of giving an output pulse whose frequency varies with time. The circuitry for processing the output of the detector may be a pulse compressor capable of receiving a long pulse whose frequency varies with time and giving a single short output pulse. The pulse expander and compressor may be matched surface acoustic wave (SAW) expander and compressor. The rangefinder may include two matched pulse expanders and compressors having different characteristics, and switching circuitry for switching between the two expanders and compressors, whereby target speed may be determined.

The invention will now be described by way of example only with reference to the drawing accompanying the Provisional Specification which is a schematic diagram of one form of the invention.

As shown in Figure 1 a CO2 laser 1 emits vertically polarised infra red radiation frequency f1 to a partially reflecting mirror 2 which transmits about 90% of the radiation through a quarter wave plate 21 into an optical modulator 3. This modulator 3 comprises a suitably oriented Germanium crystal having flat polished anti-reflection coated end faces, an acoustic absorber 5 on an upper inclined face and a lithium miobate transducer 6 on a lower face. A pulse generator 7 emits a short pulse every 30 us into a surface acoustic wave pulse expander 8. Such expanders are known devices and typically comprise a quartz substrate with two interdigital comb transducers on one flat surface.The comb transducers are so shaped that when the expander receives a short impulse the output is a long burst of frequencies which may vary linearly with time, e.g. a pulse 20 kus long of frequency 16MHz to 14MHz. Output frequency f2 from the pulse expander is amplified in an amplifier and fed into the transducer on the modulator 3 to modulate with the laser radiation in a well known manner. The combined output frequency f l + f2(t) from the modulator 3 is focussed by a lens 10 into the focal surface 11 of a parabolic mirror 12 which transmits the signal frequency fl + f2(t) onto a target (not shown).

Radiation from a target is received on a parabolic receiving mirror 13, and directed by a focal mirror through a quarter wave plate 22 and a half wave plate 23 to a second partial reflecting mirror 13. This second mirror 15 both reflects radiation from the receiving mirror 13 and transmits radiation from the first partial reflecting mirror 2 into an infra red detector 16.

The detector 16 may for example be a PbSnTe, or CdHgTe liquid nitrogen cooled detector. Electrical output from the detector 16 is amplified in an amplifier 17 and fed through a surface acoustic wave compressor 18 similar but opposite in operation to the SAW expander 8. When fed with a long pulse, e.g. a 20 us pulse of frequencies 16 MHz to 14 MHz, the SAW compressor output is a single short pulse e.g. about 0.7 lugs.

Output from the SAW compressor 18 is passed through an envelope detector 19 to remove the carrier frequency and into a range integrator 20 capable of integrating a large number of successive returns from the target.

The range to the target is determined from the integrator 20 output. Timing pulses are fed to the integrator from the pulse generator.

In operation the rangefinder is pointed at a target (assumed stationary) so that it is illuminated by radiation of frequency fl + f2(t).

The target reflects radiation back onto the receiving mirror from where it is directed to fall on the detector. Radiation direct from the laser also falls on the detector. It can be shown that the detector acts as a mixer when supplied with the two separate signals fl and fl + f2(t) to give an electrical output representing f2(t) only. The detector output, after amplification is passed through the SAW compressor to give a short sharp output pulse which, after passage through the detector enters the integrator.

The integrator may be considered as a store having 1,024 range bins each 50 nanosec wide, each representing 25 feet. A signal received from a stationary target will fall into the same range bin on successive returns since the returns will occur at the same position in time after the store receives its timing pulse from the pulse generator. These stores add successive returns, thus integrating the target, and noise, signal. The signal to noise ratio increases with observation time thus target signals rapidly add whereas noise signals tend to average out to zero.

Reflections from within the rangefinder, e.g.

surface of the Germanium crystal, can travel back into the laser and into the detector 16 to produce a large zero range signal which could swampt the wanted target returns. Such zero range returns are reduced by the quarter wave plates 21, 22 and half wave plate 23. The vertically polarised laser output is changed to right circularly polarised light by the quarter wave plate 21; this right circularly polarised radiation is transmitted onto the target. On reflection from the target the radiation is left circularly polarised and is then converted to horizontally polarised light by the quarter wave plate 22, and to vertically polarised light by the half way plate 23.Internal reflections e.g. from the modulator 3 and lens 10, return through the quarter wave plate 21 emerging as horizontally polarised light and do not therefore mix on the detector 16 either with the wanted target signal or the laser 1 output.

If the target is moving towards or away from the rangefinder the signal received by the detector will be shifted in frequency by the Doppler frequency, from f2(t). Thus output.

from the detector will be f2 + fd, where fd is the Doppler frequency shift. Output from the SAW compressor will have a time error due to fd and as a result successive returns will enter an incorrect range bin. However, if the frequency f2(t) is swept from a high to a lower frequency for a number of returns then swept from a low to a higher frequency for an equal number of returns the integrator will show two target returns whose mean is related to target range and whose difference represents Doppler frequency shift and hence target speed.

WHAT I CLAIM IS:- 1. A laser rangefinder comprising a laser capable of emitting continuous radiation, a generator of a repetitive modulating signal which varies in frequency with time at a repetition rate determined by a timing pulse generator, an optical modulator for modulating radiation from the laser with the modulating signal, means for directing radiation from the optical modulator onto a target, means for receiving radiation reflected from a target and directing it onto a detector capable of giving an output representation of radiation received, means for directing a proportion of the laser output onto the detector for mixing with the received reflected radiation, circuitry having frequency time characteristics matched to the generator of a modulating signal for processing the output of the detector to provide a target pulse whose time delay from a timing pulse represents target range, and an integrator for integrating the processed output of the detector.

2. A laser rangefinder as claimed in claim 1 wherein the generator of a repetitive modulating signal includes a pulse expander arranged to be impulsed by the timing pulse generator.

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

Claims (8)

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

   pulse expander is amplified in an amplifier and fed into the transducer on the modulator 3 to modulate with the laser radiation in a well known manner. The combined output frequency f l + f2(t) from the modulator 3 is focussed by a lens 10 into the focal surface 11 of a parabolic mirror 12 which transmits the signal frequency fl + f2(t) onto a target (not shown).

   Radiation from a target is received on a parabolic receiving mirror 13, and directed by a focal mirror through a quarter wave plate 22 and a half wave plate 23 to a second partial reflecting mirror 13. This second mirror 15 both reflects radiation from the receiving mirror 13 and transmits radiation from the first partial reflecting mirror 2 into an infra red detector 16.

   The detector 16 may for example be a PbSnTe, or CdHgTe liquid nitrogen cooled detector. Electrical output from the detector

   16 is amplified in an amplifier 17 and fed through a surface acoustic wave compressor 18 similar but opposite in operation to the SAW expander 8. When fed with a long pulse, e.g. a 20 us pulse of frequencies 16 MHz to 14 MHz, the SAW compressor output is a single short pulse e.g. about 0.7 lugs.

   Output from the SAW compressor 18 is passed through an envelope detector 19 to remove the carrier frequency and into a range integrator 20 capable of integrating a large number of successive returns from the target.

   The range to the target is determined from the integrator 20 output. Timing pulses are fed to the integrator from the pulse generator.

   In operation the rangefinder is pointed at a target (assumed stationary) so that it is illuminated by radiation of frequency fl + f2(t).

   The target reflects radiation back onto the receiving mirror from where it is directed to fall on the detector. Radiation direct from the laser also falls on the detector. It can be shown that the detector acts as a mixer when supplied with the two separate signals fl and fl + f2(t) to give an electrical output representing f2(t) only. The detector output, after amplification is passed through the SAW compressor to give a short sharp output pulse which, after passage through the detector enters the integrator.

   The integrator may be considered as a store having 1,024 range bins each 50 nanosec wide, each representing 25 feet. A signal received from a stationary target will fall into the same range bin on successive returns since the returns will occur at the same position in time after the store receives its timing pulse from the pulse generator. These stores add successive returns, thus integrating the target, and noise, signal. The signal to noise ratio increases with observation time thus target signals rapidly add whereas noise signals tend to average out to zero.

   Reflections from within the rangefinder, e.g.

   surface of the Germanium crystal, can travel back into the laser and into the detector 16 to produce a large zero range signal which could swampt the wanted target returns. Such zero range returns are reduced by the quarter wave plates 21, 22 and half wave plate 23. The vertically polarised laser output is changed to right circularly polarised light by the quarter wave plate 21; this right circularly polarised radiation is transmitted onto the target. On reflection from the target the radiation is left circularly polarised and is then converted to horizontally polarised light by the quarter wave plate 22, and to vertically polarised light by the half way plate 23.Internal reflections e.g. from the modulator 3 and lens 10, return through the quarter wave plate 21 emerging as horizontally polarised light and do not therefore mix on the detector 16 either with the wanted target signal or the laser 1 output.

   If the target is moving towards or away from the rangefinder the signal received by the detector will be shifted in frequency by the Doppler frequency, from f2(t). Thus output.

   from the detector will be f2 + fd, where fd is the Doppler frequency shift. Output from the SAW compressor will have a time error due to fd and as a result successive returns will enter an incorrect range bin. However, if the frequency f2(t) is swept from a high to a lower frequency for a number of returns then swept from a low to a higher frequency for an equal number of returns the integrator will show two target returns whose mean is related to target range and whose difference represents Doppler frequency shift and hence target speed.

   WHAT I CLAIM IS:- 1. A laser rangefinder comprising a laser capable of emitting continuous radiation, a generator of a repetitive modulating signal which varies in frequency with time at a repetition rate determined by a timing pulse generator, an optical modulator for modulating radiation from the laser with the modulating signal, means for directing radiation from the optical modulator onto a target, means for receiving radiation reflected from a target and directing it onto a detector capable of giving an output representation of radiation received, means for directing a proportion of the laser output onto the detector for mixing with the received reflected radiation, circuitry having frequency time characteristics matched to the generator of a modulating signal for processing the output of the detector to provide a target pulse whose time delay from a timing pulse represents target range, and an integrator for integrating the processed output of the detector.
2. 2. A laser rangefinder as claimed in claim 1 wherein the generator of a repetitive modulating signal includes a pulse expander arranged to be impulsed by the timing pulse generator.
3. 3. A laser rangefinder as claimed in claim 2,

   wherein the circuitry for processing the output of the detector includes a pulse compressor matched in frequency time characteristics to the pulse expander.
4. 4. A laser rangefinder as claimed in claim 3 and including two pulse expanders and two pulse compressors matched to the pulse expanders, one pulse expander having an output whose frequency increases with time whilst the other pulse expander has an output whose frequency decreases with time, and further including means for switching between the two expanders whereby target velocity may be determined.
5. 5. A laser rangefinder as claimed in claims 3 or 4 wherein the pulse expander and pulse compressor are surface acoustic wave devices.
6. 6. A laser rangefinder as claimed in any one of claims 2 to 5 wherein the optical modulator is a Germanium crystal carrying an acoustic transducer.
7. 7. A laser rangefinder as claimed in any one df claims 2 to 6 and including plates for varying the nature of polarisation of the laser radiation whereby radiation received from a target onto the detector does not mix with radiation reflected from surfaces within the rangefinder.
8. 8. A laser rangefinder as claimed in claim 1 constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the drawing accompanying the Provisional Specification.