GB2164221A

GB2164221A - Optical distance measuring system

Info

Publication number: GB2164221A
Application number: GB08520594A
Authority: GB
Inventors: Dr Nicholas Harvey Rutt
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-08-24
Filing date: 1985-08-16
Publication date: 1986-03-12
Also published as: GB8520594D0; GB2164221B

Abstract

A laser ranging system in which a tunable pulsed laser (1) has its frequency locked to, but offset from, that of a frequency stable c.w. reference laser (8) by a feed back loop. The reference laser (8) is used to produce a heterodyne signal from the echo signal. The tunable laser (1) comprises a mirror (4) moved by a piezoelectric element (5) under the control of a frequency offset lock system (11), fed by the detected (10) product (beam splitter 9) of the outputs of the pulsed and reference lasers. <IMAGE>

Description

SPECIFICATION Optical Distance Measuring System The present invention relates to the measurement of distance optically by means of the technique known as lidar in which a pulse of light is emitted and the time taken for it to travel to and return from a target is measured.

Lidar systems are well-known, but if it is desired to measure the line of sight component of velocity of an object by measuring a change in the frequency of the returning light relative to that of the outward going light, then it is necessary to use a laser having very stable frequency characteristics as a light source. Even if the Doppler effect is not used to provide a measurement of the line of sight component of the velocity of the object, high frequency stability is required if high sensitivity is sought by the use of a heterodyne detection system.

It is an object of the present invention to provide an optical distance measuring system of the lidar type in which the very high frequency stability in the light source is obtained by means of a feedback system.

According to the present invention there is provided an optical distance measuring system, comprising a laser arranged to provide a pulsed ranging beam of optical radiation having a nominal frequency, a frequency stabilised constant wave reference laser the frequency of which bears a known relationshi p to that of the nominal frequency of the pulsed laser, a feed-back loop arranged to maintain the frequency of the pulsed laser in the known relationship to that of the reference laser, means for combining radiation from the pulsed laser with that from the reference laser to provide an optical reference signal including time datum points, means for combining the reference optical signal with pulses of radiation returning from a body the distance of which is to be measured to provide a heterodyne signal, means for measuring the time interval between a time datum point relating to a particular pulse of the ranging beam and its associated returning pulse to determine the range of the body.

There may be included also means for measuring changes in the frequency of the returning pulse thereby to provide a measure of the line of sight component of velocity of the body relative to an observer.

Preferably, the pulsed laser is a carbon dioxide laser operating at a nominal frequency of 10.6 p., and the reference laser also is a carbon dioxide laser of the waveguide type.

The invention will now be described by way of example, with reference to the accompanying drawings, in which, Figure lisa schematic diagram of a part of a laser ranging system embodying the invention, and Figure 2 is a schematic circuit diagram of a frequency offset lock system used in conjunction with the part of the laser ranging system shown in Figure 1.

Referring to Figure la a transversely excited carbon dioxide laser 1 which is adapted to operate in the TEMoo mode has an optical cavity 2 which can be varied in length so as to vary the wavelength of the emitted light by some 250 MHz about the nominal frequency of such lasers. This is achieved by moving one of the mirrors 3 and 4 which make up the optical cavity 3 by means of a piezoelectric drive unit 5. Pulses of light emitted by the laser 1 pass through a beam splitting mirror 6 and thence to a transmitting optical system 7. Light from a frequency stabilised cw carbon dioxide laser 8 is mixed with a proportion of that from the laser 1 provided by means of the beam splitter 6 by a second beam splitter 9, and allowed to fall upon a wide band room temperature infra red detector 10.

The detector 10 produces an output signal which is applied to a frequency offset lock system 11, to be described later, which produces a control signal which is applied to the piezoelectric mirror drive unit 5 so as to move the mirror 4 until the frequency of the laser 1 is offset from that of laser 8 by a desired amount, known as the intermediate frequency. Light returning from a target 12 enters a receiving optical system 13 and passes thence to a combining system 14, where it is mixed with light from the laser 8 and passed to an infra red detector 15 which produces a heterodyne output signal. Light from the laser also is mixed with light from the laser 8 and passed to the combiner 14. The separation in time between a range-zero signal and the pulse return signal is measured to provide an indication of the range of the target 12.The frequency of the signal from the detector 15 is equal to the Doppler change in the frequency of the light in the returning pulses relative to the frequency of the light of the laser 8. Hence the speed of the target along the line of sight between the user of the laser ranging system and the target 12 can be deduced. A stationary target produces a signal at the intermediate frequency rather than at DC, which is much more convenient for electrons processing. The frequency offset lock system 11 stabilises this parameter, permitting the use of a narrow intermediate frequency band width for non Doppler systems, which improves their performance, and also improves the accuracy of the measurement of velocity by means of the Doppler effect when this is used.

Referring to Figure 2 the frequency offset lock system 11 comprises a low noise amplifier 20 and a high-pass filter 21, which act to remove pulse envelope components fr6m the signal from the detector 10, and so isolate the heterodyne signal; a low-pass filter 22 to which the pre-processed heterodyne signal from the filter 21 is applied and which is arranged to have a cut-off frequency just below one half of the mode spacing of the cavity of the laser 8, thus removing any inter-mode beats and any heterodyne signal from a second laser mode and enabling the system to lock on to the mode which is closest to the line centre of the laser 8, should it be of a multi-mode type; an amplitude limiter 23; an amplifier 24; a cosine discriminator 25 consisting of a 2-way in phase power divider 26, and a double balanced mixer 28 which has two power level setting attenuators 28' and 28" associated with it; and a gated integrator 29. The output from the gated integrator 29 is applied to the mirror drive unit 5 which consists of an amplifier 30 which has a feed-back loop of the appropriate characteristics, and a power amplifier 32.

The output voltage from the double balanced mixer 28 can be written as Vll=V2 cos(wt) cos[(wt)+0)l where V is the input voltage of the divided signal, the angular frequency of the heterodyne signal and O is the phase delay introduced by the phase shifter 27. If the phase shifter 27 is a simple delay line of length Land velocity factor m, then =2nf/Lmc where f is the frequency of the heterodyne signal and c is the velocity of light. The output from the gated integrator 29 is given by the equation

where At=t2-t1. When t > > this this reduces to V2 Vo=O5ll 2 V2 cos(2rif/Lmc) 2 The output voltage from the gated integrator 29 is thus a cosine function of the input frequency, and VO=0 when 0 =err/2. This zero crossing is used as the frequency lock point, and can be adjusted by altering the length of the delay line which forms part of the cosine divider 25.

Claims

1. An optical distance measuring system, comprising a laser arranged to provide a pulsed ranging beam of optical radiation having a nominal frequency, a frequency stabilised constant wave reference laser the frequency of which bears a known relationship to that of the nominal frequency of the pulsed laser, a feed-back loop arranged to maintain the frequency of the pulsed laser in the known relationship to that of the reference laser, means for combining radiation from the pulsed laser with that from the reference laser to provide an optical reference signal including time datum points, means for combining the reference optical signal with pulses of radiation returning from a body the distance of which is to be measured to provide a heterodyne signal, means for measuring the time interval between a time datum point relating to a particular pulse of the ranging beam and its associated returning pulse to determine the range of the body.

2. An optical distance measuring system according to claim 1 including means for measuring changes in the frequency of the returning pulse thereby to provide a measure of the line of sight component of the velocity of the body relative to an observer.

3. An optical distance measuring system according to claim 1 or claim 2 wherein the feedback loop includes means for isolating a particular mode of the pulsed laser, means for limiting the amplitude of the heterodyne signal, a cosine discriminator to which the amplitude-limited heterodyne signal is applied, and a gated amplifier, the output from which is applied to means for altering the position of a mirror which forms part of the optical cavity of the laser, thereby to alter the length of the optical cavity of the pulsed laser and so maintain the desired relationship between the wavelength of the pulsed ranging beam and the radiation from the reference laser.

4. An optical distance measuring system according to any preceding claims wherein the pulsed and reference lasers utilise carbon dioxide as the lasing medium.

5. An optical distance measuring system substantially as hereinbefore described with reference to the accompanying drawings.