GB2307369A

GB2307369A - Laser imaging system

Info

Publication number: GB2307369A
Application number: GB9522835A
Authority: GB
Inventors: Raymond Passmore
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1995-11-08
Filing date: 1995-11-08
Publication date: 1997-05-21
Also published as: GB9522835D0

Abstract

A laser imaging system illuminates a target object (7) with a frequency modulated continuous wave (FMCW) beam of radiation, and generates an intermediate frequency signal by comparing the transmitted signal with a signal reflected from the target by means of combiner (10) and mixer (11). The intermediate frequency signal is filtered by pass band filter (12), the width of the pass band corresponds to the "depth of field" of the viewed scene, and the frequency on which the filter is centred determines the range at which an image is received. Variation of the centre frequency enables the build up of a 3-D image. The system reduces common volume scattering caused by particles in the beam between the imaging system and target object, and is particularly suited to underwater applications when used in conjunction with a pumped frequency doubling system.

Description

A LASER IMAGING SYSTEM The present invention relates to a laser imaging system, particularly but not exclusively of the type where a laser beam is scanned over an object to be imaged and the returned radiation is used to generate an image of the object. The invention is particularly applicable to applications where an object to be imaged is in a translucent medium as may be encountered in underwater applications.

Laser imaging systems are currently known for imaging objects in translucent mediums.

These systems are generally based on one of two techniques. In accordance with a first technique, the object to be imaged is illuminated by a flood beam of light from a pulsed laser, which flood beam is of very short duration. Such a system is disclosed in SPIE VOL 1537 Underwater Imaging, Photography and visibility (1991), Pgs 42-56 "Laser range-gated underwater, imaging including polarisation discrimination": B.A Swartz, the contents of which are herein incorporated by way of reference. In such systems light travels forward from a source in the form of a spherical wall of light centred on the source. An associated detector is gated such that an image is created in a very short duration, the image corresponding to the objects lying within the "wall" of light at a particular instant in time.This technique reduces volume scattering that would otherwise occur in a translucent medium due to the light being scattered back to the detector by the medium, and provides substantial improvement in the signal to noise ratio of the returned signal.

An alternative known type of laser imaging system is disclosed in Applied Opticsfvol 32, No.19/1 July 1993, Pages 3520-3530 "Development and testing of a synchronous scanning underwater imaging system capable of rapid two-dimensional frame imaging": T.J.Kulp et al, the contents of which are also herein incorporated by way of reference.

Such systems use a continuous wave laser in a line scan configuration operating in conjunction with single, multiple or camera type detectors. In such a system a continuous wave laser beam is scanned across a target area and a detector element is scanned in synchronism with the beam, or the image falls on an appropriate detector array, or camera, which is again suitably synchronised such that an image is only received from targets at a specific distance.

With both the above described systems images can be taken from different planes enabling a 3-D representation to be generated. With both systems common volume back scatter can be reduced by introducing an appropriate lateral offset between the transmit and receive apertures. The former system requires high speed switching techniques applied to the transmitter, detector or both.

It is an object of the present invention to provide an improved laser imaging system.

According to the present invention there is provided a laser imaging system comprising: means for generating a frequency modulated continuous wave (FMCW) output signal; means for transmitting a first portion of the output signal to a target area; means for receiving a returned signal from the target area; means for generating an intermediate frequency signal from a second portion of the output signal and the returned signal; and means for generating an image of a target in the target area from the intermediate frequency signal.

By employing an imaging system in accordance with the present invention it is possible to use the coherent nature of the laser light in order to obtain spatial resolution without the need for high speed switching and/or shuttering. This greatly reduces volume back scattering, and because the signal is a coherent continuous wave laser signal, the signal to noise ratio is also substantially improved over systems which employ a short duration illumination pulse but which do not utilise coherent receiver techniques.

Preferably the means for transmitting the first portion of the output signal comprises means for scanning the signal across the target area. Continuous wave transmission enables a low power in the region of tens of watts to be used, which compares with tens of megawatts required by pulsed systems. It is then preferable that the means for receiving the returned signal comprises means for scanning the target area in synchronisation with the means for transmitting the signal. If the transmitting means and receiving means are spaced apart (non-co-axial) this enables the noise generated by (common) volume scattering to be further reduced.

Advantageously the system further comprises a combiner for combining the second portion of the output signal with the received signal, and a mixer for producing the intermediate frequency signal from the output of the combiner.

Preferably the system further comprises a tunable band pass filter for filtering the intermediate frequency received by the image generating means. The bandwidth of the tunable filter determines the range increment ("gate width") or spatial extent of the sources of reflected energy, and thereby varying the frequency of the pass band causes the image plane to be moved accordingly. As an alternative to a tunable pass band filter, the system could instead comprise a mixer, a voltage controlled oscillator connected to an input of the mixer, and a band pass filter in series with the mixer, for processing the intermediate frequency signal received by the image processing means.Here again, the width of the band pass filter, which would normally be a fixed filter, determines the range increment, but the position of the image plane is now determined by the voltage applied to the voltage controlled oscillator.

In certain applications it may be preferable that the system further comprise means for delaying the second portion of the output signal prior to that portion of the signal being received by the means for generating an intermediate frequency signal. This is advantageous if the transmitting and receiving means are a considerable distance from the means for generating the intermediate frequency, for the delay means will compensate for any delay in the signal travelling to the transmitting means and returning from the receiving means.Such an arrangement is desirable when the complexity of equipment adjacent the transmitter/receiver is to be kept to a minimum, enabling the transmitter/receiver to be connected to the main processing means by an optical fibre, which fibre could be of a considerable length in remote applications, for example, where the transmitter/receiver could be located on a remote vehicle whilst the processing equipment is located in a base station.

In particular applications it is preferable that the system comprises: a laser configuration for receiving a pump signal at a first frequency and generating the output signal at a second frequency distinct from the first frequency; a pump source remote from the laser configuration for providing the pump signal; and an optical fibre link for conveying the pump signal from the pump source to the laser configuration; the optical fibre having good transmission characteristics at the first frequency relative to the transmission characteristics of the fibre at the second frequency. Such a system enables the equipment associated with generating a pump beam at a first frequency to be located remote from the signal transmitting and receiving means when the signal it is desired to transmit is to be at a frequency at which the transmission characteristics of an optical fibre are relatively poor.By using such a system a small microlaser can be located adjacent the signal transmitting means while the pump laser source for the microlaser is located remote from the transmitter. The laser configuration can conveniently employ a solid state laser.

Preferably the laser configuration comprises a laser cavity, a second harmonic generator for doubling the frequency of radiation emerging from the laser cavity and preferably means for frequency modulating the output radiation from the laser cavity.

Such systems employing laser configurations remote from the pump source are particularly advantageous where the laser configuration is arranged to be located in a medium, and wherein the laser configuration provides an output signal at a frequency to which the medium has good transmission characteristics relative to the transmission characteristics of the medium at the pump beam frequency. For example, if the medium is water then transmission characteristics to red are very poor but transmission characteristics to blue/green are good. In contrast, the transmission characteristics of optical fibre are normally very good to red light but poor at blue/green frequencies.

One embodiment of the present invention will now be described by way of example only with reference to the accompanying figures throughout which like reference numerals are used to indicate like parts and of which: Figure 1 illustrates a laser imaging system in accordance with the present invention; Figure 2 illustrates an alternative arrangement of the laser imaging system of Figure 1; and Figure 3 illustrates how a remote laser pump source can be employed with the present invention.

Referring to Figure 1, a laser imaging system is illustrated which comprises a continuous wave laser source 1, the output of which is frequency modulated by FM mixer 2 which receives a sawtooth waveform from signal generator 3. Alternatively a signal from the signal generator 3 could be applied directly to an appropriate laser source by direct cavity modulation, or by a frequency changer/mixer component which could be in the form of a Brag cell. The emergent FMCW signal is applied to input 4.1 of standard directional coupler 4 where it is split between ports 4.2 and 4.3, port 4.4 being isolated (or decoupled). From port 4.2 the signal propagates through a fixed length (L) of optical fibre 5 to emerge from flying spot scanner 6, where it is directed towards target object 7 which could typically be within a range of 20m from the flying spot scanner.

Reflections from the object 7 are received by the flying spot scanner 6 and travel by reciprocal light path to port 4.2 of coupler 4, resulting in reflected energy (less system losses) emerging from ports 4.1 and 4.4, port 4.3 being isolated for reflected energy. The output from ports 4.3 and 4.4 of coupler 4 is fed via respective optical fibres 8 and 9 to combiner 10. The length of optical fibre 8 exceeds the length of optical fibre 9 by a length equivalent to twice the length of optical fibre 5 and therefore compensates for the delay experienced by the signals transmitted and returned along the length of optical fibre 5.The output of combiner 10 is fed into single ended mixer 11 which with linear (or linearised) FM produces an intermediate frequency which is proportional to the two way propagation delay between the scanner 6 and object 7, whilst signal level is proportional to the reflection coefficient of object 7.

The intermediate frequency from the mixer 11 passes through tunable band pass filter 12. The band width of the filter 12 determines the range increment ("gate width") or spatial extent of the object. The frequency at which the pass band occurs determines the "in focus range" of the imager.

The output of the tunable filter is fed to display 13 which is synchronised to a blanking or synchronisation signal from signal generator 3. Remote connection to the flying spot scanner 6 as indicated by broken line 14, is such that an image is generated on the display independance on the filter output and the corresponding pointing direction of the scanner.

As an alternative to the tunable filter 12 a mixer 15 and fixed band pass filter 16 could be inserted in series between the output of the mixer 11 and the display 13, in which case the "in focus range" of the imaging system would be controlled in dependence on a voltage applied to voltage controlled oscillator 17 providing the second input to mixer 15. In this arrangement the range increment would again depend on the width of the pass band of the band pass filter 16.

The sawtooth waveform of the output of the signal generator 3 results in a large transient signal arising due to the discontinuities in the sawtooth waveform. In order to suppress this on the display 13 the display receives a display synchronisation signal from the signal generator 3 which synchronises the readout of the filter output with the frequency modulation cycle. Alternatively the display may monitor the output of the tunable filter for edges on the output signal corresponding to discontinuities in the FMCW signal, and suppress the intensity of the display by applying a signal to the Z input of the display.

The flying spot scanner can be driven either mechanically or acousto-optically in accordance with known techniques and a typical frame rate would be in excess of 2Hz.

The scanner can be operated with any scan pattern commensurate with adequate scene coverage, for example raster, nodding or helical, but a spiral scan geometry is preferred.

The display unit can be used in conjunction with a multi-image store enabling a number of images from adjacent range increments to be superimposed to generate a "multi-slice" or 3-D (type) representation of the object.

One problem which may be encountered with the arrangement in Figure 1 arises from the co-location of the transmit and receive apertures defined by the flying spot scanner 6. This arrangement causes common volume scattering from the region of the medium between the scanner and object, which is common to both the transmit and receive signal paths. In this example where a common or co-axial scanner is used for transmission and reception this region equates to the complete transmission path between the scanner and the target. Although, as already described, the coherent nature of the laser light is used such that an image is only received from an object lying in a particular plane a set distance from the scanner, common volume back scattering results in a reduction in the systematic S/N ratio.This problem can be alleviated, or at least reduced, by adopting the system illustrated in Figure 2 where the transmit and received apertures are laterally offset.

Referring to the arrangement illustrated in Figure 2, the FMCW signal from mixer 2 is received by input 18.1 of coupler 18 which splits the signal between output ports 18.2 and 18.3 of the coupler. The signal from output 18.3 is transmitted by transmit scan optics 19 driven by scanner drive 20 towards an object to be imaged. The scanner drive also drives receive optics 21 in synchronism with the transmit optics 19. The receive optics 21 receive an image via receive lens 22 which can have a wide aperture now that the transmit and receive optics are separate. The signal received by the scan optics 21 is received on input 18.4 of coupler 18 and divided between outputs 18.2 and 18.3. The signals from mixer 2 and from the scan optics 21 appearing at port 18.2 are received by single ended mixer 11 and an intermediate frequency generated. This intermediate frequency signal is used to generate an image of the object in the same way as the Figure 1 embodiment, with the exception that the display is synchronised with the scanner drive 20.

In certain applications (e.g. bomb disposal) it may be desirable or essential, for reasons of safety or capital value at risk, to physically partition the imaging system to enable the scanner to be operated remotely from the processing and display components. This can be achieved by increasing the length of the optical fibre 5 in the Figure 1 arrangement or for example optical fibres 23 and 24 in the Figure 2 arrangement. In the Figure 2 arrangement delay compensation would be required for the signal received by the mixer 11 directly from the mixer 2. This would be achieved by offsetting the centre frequency of the tuneable filter 12, or alternatively by offsetting the centre frequency of VCO 17.

In either the Figure 1 or Figure 2 arrangement, discrete wavelength division multiplex (WDM) duplex channels are incorporated in the opto-electronic system to allow simultaneous two way information transfer of scanner position data, filter output and tuning control.

Remote location of the scanner, relative to the processing means and display of the imaging system, can successfully be achieved by the above techniques providing the medium between the scanner and the object to be imaged has good transmission characteristics at frequencies at which the optical fibres 5, 23 and 24 also have good transmission characteristics. Optical fibres normally have good transmission characteristics at frequencies in the infrared range, but poor transmission characteristics at the blue/green frequencies. This is not a problem if the object to be imaged is in air, for red/infrared radiation propagates with relatively little loss through air, and other gases. However, red/infrared radiation cannot be transmitted any great distance through water, where blue/green frequencies must be used in order to obtain good transmission characteristics.This is unfortunate, for the present invention is ideally applicable to remote imaging of underwater objects where the display and processing equipment can be mounted aboard a surface vessel with a simple scanner located aboard an underwater vehicle connected to the surface vessel by an optical fibre link. This is a particularly desirable arrangement in mine clearance operations where "one shot", expendable underwater vehicles may be used and where it is therefore desirable to minimise the equipment carried by the underwater vehicle.

Referring to Figure 3 there is illustrated an arrangement which enables the present invention to be employed in applications where it is desirable that the scanner be remotely located, but where the medium between the scanner and a target object exhibits poor transmission characteristics to radiation at those frequencies which can be transmitted with relatively low loss along an optical fibre link between the scanner and the rest of the imaging system.

Referring to Figure 3, a laser pump source 25 generates a pump beam at 850nm which is transmitted via optical fibre 26 to a remote location where it pumps ndYLF microlaser 27, which could for example be located on an underwater vehicle. The length of the resonator cavity of the microlaser is controlled by local frequency modulator 28 and provides frequency modulated continuous wave excitation centred on 1040nm. This passes through second harmonic generator 29 which doubles the frequency producing an FMWC signal output 30 centred on 520nm. The output 30 is fed into the coupler 4 of the Figure 1 arrangement (or 18 of the Figure 2 arrangement). The returned signal from the flying spot scanner 6 of Figure 1 (or the received scan optics 21), being fed through the coupler to mixer 11 to generate the intermediate frequency which is then transmitted via the optical fibre link to the tunable filter and display means.

Claims

1. A laser imaging system comprising: means for generating a frequency modulated continuous wave (FMCW) output signal; means for transmitting a first portion of the output signal to a target area; means for receiving a returned signal from the target area; means for generating an intermediate frequency signal from a second portion of the output signal and the returned signal; and means for generating an image of a target in the target area from the intermediate frequency signal.

2. A system as claimed in claim 1 wherein the means for transmitting the first portion of the output signal comprises means for scanning the signal across the target area.

3. A system as claimed in claim 2 wherein the means for receiving the returned signal comprises means for scanning the target area in synchronisation with the means for transmitting the signal.

4. A system as claimed in any preceding claim wherein the transmitting means and receiving means spaced apart.

5. A system as claimed in any preceding claim further comprising a combiner for combining the second portion of the output signal with the received signal, and a mixer for producing the intermediate frequency signal from the output of the combiner.

6. A system as claimed in any preceding claim further comprising a tunable band pass filter for filtering the intermediate frequency received by the image generating means.

7. A system as claimed in any one of claims 1 to 5 further comprising: a mixer; a voltage controlled oscillator connected to an input of the mixer; and a band pass filter in series with the mixer, for processing the intermediate frequency signal received by the image processing means.

8. A system as claimed in any preceding claim comprising means for delaying the second portion of the output signal prior to that portion of the signal being received by the means for generating an intermediate frequency signal.

9. A system as claimed in any preceding claim comprising: a laser configuration for receiving a pump beam at a first frequency and generating the output signal at a second frequency distinct from the first frequency; a pump source remote from the laser configuration for providing the pump beam; and an optical fibre link for conveying the pump beam from the pump source to the laser configuration; the optical fibre having good transmission characteristics at the first frequency relative to the transmission characteristics at the second frequency.

10. A laser system as claimed in claim 9 wherein the laser configuration comprises a solid state laser.

11. A laser system as claimed in claim 9 or 10 wherein the laser configuration comprises a laser cavity and a second harmonic generator for doubling the frequency of radiation emerging from the laser cavity.

12. A laser system as claimed in claim 9, 10 or 11 wherein the laser configuration comprises means for frequency modulating output radiation from the laser cavity.

13. A laser system as claimed in any one of claims 9 to 12 wherein the laser pump source generates radiation having a wavelength in the range 750 nm to 950 nm and wherein the laser configuration produces radiation having a wavelength in the range 480 to 560 nm.

14. A laser imaging system as claimed in any one of claims 9 to 13 wherein the laser configuration is arranged to be located in a liquid medium, and wherein the laser configuration provides an output signal at a frequency to which the medium has good transmission characteristics relative to the transmission characteristics of the medium to the frequency of the pump beam.

15. A system as claimed in claim 14 wherein the medium is water.

16. A laser imaging system substantially as hereinbefore described with reference to, or as illustrated in, the accompanying figures.