GB2251150A - Laser radar system - Google Patents

Abstract

A laser radar system comprises a laser (11) having a laser cavity (12) between first and second output ports (13, 14). The laser output beam (Lt) is directed through the first port along an optical path (15) onto a target (5). Scattered light (Lr) is collected back along the transmitted optical path into the laser cavity (12). The scattered light (Lr) is amplified in the laser cavity (12) and passes out through the second port (14) of the laser cavity onto a detector (17). A portion of laser light (Lt), forming a local oscillator signal, also emerges through this second port (14) onto the detector (17) on which the beat between the two beams produces a signal used by a signal processor (18) to provide a desired measurement such as a range, velocity, or vibrational frequency value. The laser (11) may be an Ar ion, or CO2 laser, and may be a cw or pulsed operation laser. The first and second ports may be partly, eg 90%, reflecting mirrors. <IMAGE>

Description

Laser Radar System This invention concerns a laser radar system. Such systems are analogous to conventional radar but use a laser beam of energy instead of microwaves.

BACKGROUND Coherent laser radar, also know as lidar, is a widely used technique for remote sensing; Rev Laser Rangefinders, P A Forrester & K F Hulme, Opt & Quantum Electronics, 13, (1981) pp 259-293. Laser output wavelength depends upon the laser used. The term radiation and light are used herein to indicate the output from a laser and include visible and near visible wavelengths. In one conventional arrangement a laser output is directed into a complex optical arrangement, usually called an interferometer, and a transmitting telescope before emerging to impact on a distant target. Scattered radiation from the distant target travels back to the lidar equipment and enters a separate receiving telescope in a standard biaxial arrangement, or the transmitting telescope, now acting as a receiver, in the standard monostatic arrangement.In either case the signal received beam is passed into the interferometer and is combined with an optical local oscillator beam derived either from the original laser or from an additional local oscillator laser. The two beams, comprising the signal beam and local oscillator beam, are then directed to a detector where mixing and optical light beating take place between them. The consequential electrical signal from the detector thus provides information on the light scattering target which may consist of for example of the target range, velocity, vibrational character, scattering strengths etc.

In the conventional optical arrangement very precise control of the optical beams is required in the interferometer and telescopes. These usually incorporate beam splitters, mirrors, lenses, polarising elements etc and are required to adjust the relative beam sizes, wave front curvatures and beam axes etc in order to ensure optimum matching of the local oscillator and signal beams. Such precise matching is essential for efficient light beating and inaccuracies amounting to small fractions of a wavelength can greatly reduce system capability and effectiveness. These optical components are in themselves expensive and the arrangement and adjustment in an optical system requires a high level of skill, is very time consuming and with beams not visible by eye is often very difficult.Additionally the components are quite easily moved out of optimum positioning by vibration or rough handling, thus requiring a very high standard of construction for portable applications and rugged environments.

The present invention has an object of providing a laser system that is relatively easy to align, simple and robust.

STATEMENT OF INVENTION According to this invention a laser radar system comprises a laser for emitting a beam of coherent radiation along an optical path towards a target, a detector for detecting laser radiation scattered back from the target, and means for processing the detector output, characterised by:a double output laser having a laser cavity between a first port and a second port, the laser being arranged to transmit radiation towards the target through the first port, receive scattered returning radiation through the first port into the cavity, and emit both received and generated radiation through the second port onto the detector.

The received scattered signal may be amplified in the laser cavity.

An optical system, eg a telescope, or arrangement of lenses and or mirrors, may be used to direct radiation from the laser to the target, and to direct returning radiation into the laser cavity.

The laser may be a continuous wave (cw), amplitude and or frequency modulated cw, or pulsed laser, a waveguide laser, a dye laser, or a distributed feedback laser. The ports may be partly transmissive mirrors at either end of a laser cavity. The cavity may be a physically constrained chamber or a volume established by interference conditions.

The laser may have its output beam amplified by a laser or optical amplifier such as a travelling wave amplifier.

The invention will now be described by way of example only with reference to the accompanying drawing of which: Figure 1 is a block diagram of a prior art laser radar system, Figure 2 is a block diagram of a laser radar system according to this invention, Figure 3 is a modification of Figure 2, Figure 4 is a modification of Figure 3, Figure 5 is a graph of laser gain against difference frequency, and Figure 6 is a graph of laser gain and bandwidth against laser power which is a function of laser pumping rate.

The prior art laser radar system shown in Figure 1 comprises a laser 1 whose output beam 2 enters an interferometer 3, then a telescope 4, and onto a target 5. Laser light is scattered from the target 5, and a portion Ls is collected by a receiving telescope 7 and passed via the interferometer 3 onto a detector 8. Additionally the detector 8 receives a local oscillator LO from the laser 1 via the interferometer 3. Output from the detector 8 is to a signal processor 9 which calculates a desired function. For example, if the radar system is used to measure range, then the signal processor measures the time of flight of a laser pulse from and to the system, and calculates range to the target. If the laser radar system is measuring target velocity, then the signal processor measures the Doppler shift to the received radiation to calculate target velocity.An advantage of a separate receiving telescope 7 is that it improves the optical isolation of a transmitted beams and the weak scattered beam and hence may improve system sensitivity.

An alternative prior art system, not shown, uses a single telescope for transmission and reception. Such a system then requires a beam splitter in an optical path between telescope and interferometer to separate transmitted and received beams. A disadvantage of this is that higher power lasers may be required to overcome losses in the beam splitter.

However, an advantage is that it does not require alignment of separate transmit and receive telescopes.

A laser system according to this invention is shown in Figure 2 to comprise a laser 11 having a laser cavity 12 formed between first and second output ports, eg two partly transmissive front and rear mirrors 13, 14. The laser 11 may be a carbon dioxide or argon ion laser having an output of 100 mwatt and a wavelength of 0.488 um. The front mirror 13 has a transmission of about 90%, and the rear mirror 14 a transmission of about 90%. Forward output Lt from the laser 11 is along an optical path 15 through an optical system 16 to a target 5. The optical system 16 may be a telescope, or other lens system, or an arrangement of mirrors, or a mixture of lenses and mirrors. A small portion Lr of incident laser radiation is scattered back from the target along the optical path 15, through the optical system 16, the front mirror 13 and into the laser cavity 12.

Inside the cavity 12 the radiation Lr interacts with the laser operation and emerges as a signal beam Ls through the rear mirror 14 onto a detector 17. Additionally laser radiation Lt emitted from the cavity 12 is incident on the detector 17; this beam acts as a local oscillator signal (Llo). Both radiation Llo and Ls mix on the detector 17 to produce a signal for a signal processor 18. This signal processor 18 processes the detector signal in the conventional manner.

For example by measuring a time interval to provide a range value, or calculating a Doppler shift to provide a velocity value, or a frequency to provide information about vibration of the target 5.

The detector may be a PIN photo diode or other photo voltaic device or photo conductor. The detector may be separate as shown or may be attached directly to the rear mirror, eg by bonding.

In the system of Figure 2 the transmitted and received beams of radiation are inherently aligned with a reduced number of components compared to the prior art. This makes initial alignment relatively simple, reduces optical losses, and provides a robust system that is readily portable. A further advantage is that the cavity 12 may amplify the received radiation Lr. This is explained further with reference to Figures 5, 6.

Figure 5 shows the gain versus the frequency difference between the transmitted (Lt) and received (Lr) beams for different laser output powers. Larger gain is obtained at lower laser powers. This allows the laser transmitted power to be reduced because the cavity will compensate by amplifying the reduced scattered beam. An advantage of reducing the laser output power is reduced danger to eye damage caused by accidental illumination of operators. For safety reasons the CO2 laser is preferred because it is more eye safe than lasers at other wavelengths.

Unfortunately the noise in the amplified signal increases with increasing gain. Thus high amounts of amplification are best used with detectors of low quantum efficiency or relatively noisy detectors, eg room temperature detectors, where the increased noise is less significant. Low noise detectors, eg cooled detectors, are best used with low amounts of laser amplification and hence lower noise.

For measurement of target velocity, information about bandwidth and laser gain is important. Figure 6 shows bandwidth and gain data for an Ar laser.

Bandwidth is given by { lh ) above threshold = P/(P+Ps) where A is the observed bandwidth for signal gain A is the passive cavity bandwidth, P is laser power. and Ps is laser power at twice threshold pumping rate.

Gain is given by (G,(O))above threshold = ((P+Ps)/P)2 where GT is Ls divided by Lr and is thus the gain for the signal beam as it passes through the laser.

In operation the laser system transmits a beam Lt onto the target 5.

Light is scattered back along the optical path 15, through the optical system 16 and into the cavity 12. In this cavity the laser light at the Lt frequency mixes with the returned beam at Lr frequency. The amplified signal at Lr frequency and a small amount of laser power at Lt frequency emerge from the rear mirror 14 onto the detector 17 where they beat to produce the signal at the difference frequency. This difference signal is then processed as appropriate to provide eg a range value, a velocity value, or vibration value.

A modification of Figure 2 is shown in Figure 3. The system is similar to Figure 2 with like items given like reference numerals. A laser or optical amplifier 19 is arranged between the laser 11 and an optical system 16. Output from the laser 11 is amplified and directed through the optical system 16 onto the target 5 as before. Returning scattered radiation Lr passes through the optical system 16, the optical amplifier 19 and into the laser cavity 12 for amplification as before. The optical amplifier 19 may for example be a travelling wave device.

Alternatively, as shown in Figure 4, the relative positions of optical system 16 and optical amplifier 19 may be reversed.

These two embodiments, Figures 3, 4 are particularly useful for rangefinding where the laser 11 may be a low powered cw CO2 laser and the optical amplifier may be a switched (pulsed) device. Returning radiation Lr passes freely through the amplifier 19 whilst it is quiescent between pulses.

Claims (8)

1. A laser radar system comprising a laser (11) for emitting a beam of coherent radiation (Lt) along an optical path (15) towards a target, a detector (17) for detecting laser radiation (Lr) scattered back from the target, and means (18) for processing the detector output, characterised by:a double output laser (11) having a laser cavity (12) between a first port (13) and a second port (14), the laser (11) being arranged to transmit radiation towards the target (5) through the first port (13), receive scattered returning radiation through the first port (13) into the cavity (12), and emit both received (Lr) and generated radiation (Lt) through the second port (14) onto the detector (17).

2. The system of claim 1 wherein the laser cavity is an amplifying cavity to the received radiation.

3. The system of claim 1 wherein the ports are partly transmissive mirrors (13, 14) at both ends of a laser cavity (12).

4. The system of claim 1 and further comprising an optical system (16) for directing radiation from the laser (11) to the target (5), and to direct returning radiation (Lr) into the laser cavity (12).

5. The system of claim 1 wherein the laser (11) is a continuous wave (cw) laser.

6. The system of claim 1 wherein the laser (11) is a frequency and or amplitude modulated laser.

7. The system of claim 1 wherein the laser (11) is a pulsed laser.

8. The system of claim 1 and further comprising an optical amplifier (19) for amplifying the laser output beam (Lt).