GB2609044A

GB2609044A - Detecting an object in a turbid medium

Info

Publication number: GB2609044A
Application number: GB2110444.3A
Authority: GB
Inventors: Paul Mcgeoch Stephen
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2021-07-20
Filing date: 2021-07-20
Publication date: 2023-01-25
Anticipated expiration: 2041-07-20
Also published as: GB2609044B; GB202110444D0

Abstract

A method and system 2 for detecting an object in a turbid medium comprises sequentially emitting a plurality of pulses of light from a pulsed light source 4 and transmitting each pulse of light through the turbid medium towards a probe volume 42 of the turbid medium, using a gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of the probe volume during a corresponding gate period to thereby generate a corresponding detected signal value, and determining whether an object (60, fig 5) was present in the turbid medium in a path extending from the pulsed light source to the probe volume and from the probe volume to the gated photodetector based on the detected signal values. The method may be used for detecting an object such as a submarine, a mine or an aquatic animal in a turbid medium which comprises water and particulates in the water for scattering the pulses of light, for example wherein the turbid medium comprises seawater. The pulsed light is preferably collimated 40 by a lens 10.

Description

DETECTING AN OBJECT IN A TURBID MEDIUM

FIELD

The present disclosure relates to a method of detecting an object in a turbid medium and an associated system and, in particular though not exclusively, to a method of detecting an object in a body of water such as the sea.

BACKGROUND

It is known to be challenging to reliably detect submerged objects such as submarines in maritime waters such as littoral waters where the depth of water is too great for most electro-optical detection systems and where the waters are too cluttered for acoustic or magnetic detection. Techniques such as direct detection using light detection and ranging (LIDAR) and sonar reflections from the object or magnetic anomaly detection provide only partial solutions.

Electro-magnetic radiation is strongly attenuated and scattered by seawater, so it is difficult to "see" anything from the sea surface if the object of interest is at more than a few metres depth. Not only is the illumination strongly attenuated but it is also scattered by the water, masking the object.

It is also often difficult to "hear anything as acoustic detection techniques suffer from clutter due to scatter, e.g. from the seabed, particularly in complex coastal environments. In this case, the scatter comes from the seabed, the shoreline and the water surface, as well as from any man-made objects. Because these objects are separated from the detector by similar distances to the depth of the object of interest, the scatter signals can mask the return signal from the target object to the detector.

Magnetic Anomaly Detection is also both limited in range and confused by common environmental clutter.

SUMMARY

According to an aspect of the present disclosure there is provided a method of detecting an object in a turbid medium, the method comprising: sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting each pulse of light through the turbid medium towards a probe volume of the turbid medium, each pulse of light being emitted at a corresponding emission time; using a gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of the probe volume during a corresponding gate period to thereby generate a corresponding detected signal value, wherein the corresponding gate period commences at a corresponding start time which is delayed relative to the corresponding emission time by a time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the probe volume and from the probe volume through the turbid medium to the gated photodetector; and determining whether an object was present in the turbid medium in the path during the time of flight of one or more of the pulses of light according to whether the one or more detected signal values corresponding to said one or more pulses of light is less than one or more of the other detected signal values corresponding to one or more of the other pulses of light by more than a threshold difference value.

If a transmitted pulse of light reaches the probe volume, a portion of the pulse of light may be scattered from the probe volume towards the gated photodetector and be incident on the gated photodetector during the corresponding gate period resulting in the generation of a detected signal. If, however, an object interrupts the path, the pulse of light may not reach the probe volume and/or any portion of the pulse of light which is scattered from the probe volume in the direction towards the gated photodetector may be blocked from reaching the gated photodetector such that the corresponding detected signal may be reduced in value. Such a method may be considered to be a shadow detection method in the sense that a reduction in the detected signal value is indicative that an object has interrupted the path that extends from the pulsed light source to the probe volume and from the probe volume to the gated photodetector. Such a method constitutes an indirect detection technique that utilises the scattering of the light in the probe volume of the turbid medium to create a measurable signal. In effect, the method at least partially reduces the masking effects of scattering that may be responsible for degrading the sensitivity of known light detection and ranging methods such as conventional LIDAR methods when used in a turbid medium. Furthermore, the method does not require a range or depth search to detect the presence of an object somewhere in the path.

Optionally, the method comprises forming each pulse of light into a corresponding beam of light which illuminates a corresponding illuminated region through the turbid medium.

Optionally, each beam of light is collimated or substantially collimated. The use of collimated beams of light may increase the detected signal value generated when each pulse of light reaches, and is scattered by, the probe volume and a portion of the scattered light is incident on the gated photodetector. This may increase a signal-to-noise ratio at the gated photodetector. The use of collimated beams of light may at least partially reduce any variation in the detected signal values generated when detecting scattering from different probe volumes of the turbid medium. This may be useful when trying to determine a position of any object in the path.

Optionally, each beam of light is convergent towards the probe volume. The use of convergent beams of light may increase the detected signal value generated when each pulse of light reaches, and is scattered by, the probe volume and a portion of the scattered light is incident on the gated photodetector.

Optionally, the pulsed light source emits each pulse of light as an initial divergent beam of light and the method comprises converting each initial divergent beam of light into the corresponding beam of light, wherein each beam of light is divergent but has a divergence which is reduced relative to the corresponding initial divergent beam of light.

The use of divergent beams of light which have a reduced divergence relative to the corresponding initial divergent beams of light may increase the detected signal value generated when each pulse of light reaches, and is scattered by, the probe volume and a portion of the scattered light is incident on the gated photodetector. This may increase a signal-to-noise ratio at the gated photodetector. The use of divergent beams of light which have a reduced divergence relative to the corresponding initial divergent beams of light may reduce any variation in the detected signal values generated when detecting scattering from different probe volumes of the turbid medium relative to a variation in the detected signal values generated when detecting scattering from different probe volumes of the turbid medium using the initial divergent beam of light emitted from the pulsed light source. This may be useful when trying to determine a position of any object in the path.

Optionally, the method comprises receiving light at the gated photodetector from a field of view in the turbid medium, wherein the field of view includes the probe volume.

Optionally, the probe volume of the turbid medium is defined by the field of view and the duration of the gate period.

Optionally, the illuminated region and the field of view coincide or substantially coincide. For example, the pulsed light source and the gated photodetector may be arranged in a monostatic configuration.

Optionally, the illuminated region and the field of view overlap only in the region of the probe volume. For example, the pulsed light source and the gated photodetector may be arranged in a bistatic configuration.

Optionally, the pulsed light source emits each pulse of light as an initial divergent beam of light and the method comprises using a focusing element such as a lens or a curved mirror so as to convert each initial divergent beam of light into the corresponding beam of light, wherein each beam of light is divergent but has a reduced divergence relative to the corresponding initial divergent beam of light.

Optionally, the pulsed light source is separated from the focusing element along the path by a distance which is equal to, or substantially equal to, a focal length of the focusing element.

Optionally, the method comprises using the focusing element to collect light scattered from the probe volume and to focus the collected light onto the gated photodetector.

Optionally, the gated photodetector is separated from the focusing element by a distance along the path which is equal to, or substantially equal to, a focal length of the focusing element.

Optionally, the method comprises using a further focusing element such as a further lens or a further curved mirror to collect light scattered from the probe volume and to focus the collected light onto the gated photodetector.

Optionally, the gated photodetector is separated from the further focusing element by a distance along the path which is equal to, or substantially equal to, a focal length of the further focusing element.

Optionally, the duration of the gate period is less than 100 ns, less than 10 ns, less than 1 ns or substantially equal to 1 ns. Such a method may detect the scattering of each pulse of light from a very small, well-defined probe volume thereby at least partially suppressing the detection of unwanted signals corresponding to scattered light arriving at the gated photodetector outside the gate period such as light scattered from regions of the turbid medium outside the probe volume and/or light scattered from any other spurious objects or clutter in the illuminated region.

Such a method may also at least partially reject any ambient light, for example sunlight, which is scattered from regions of the turbid medium in the field of view. This may at least partially reduce any noise associated with detection of the ambient light by the gated photodetector. The method may allow the detection of objects with greater sensitivity and/or at greater distances than conventional direct detection LIDAR methods because the method provides greater suppression of clutter due to scattering from the turbid medium.

Optionally, a duration of each pulse of light matches, or is comparable to, the duration of the gate period. Using pulse of light having a duration which matches, or is comparable to, the duration of the gate period minimises the collection of light scattered from regions of the turbid medium outside the probe volume.

Optionally, the duration of each pulse of light is less than 100 ns, less than 10 ns, less than 1 ns or substantially equal to 1 ns.

Optionally, the gated photodetector comprises a gated photomulfiplier tube (PMT), a gated avalanche photodiode (APD), or a gated silicon photomultiplier (SiPM).

Optionally, the gated photodetector is configured to detect or count single photons during each gate period, for example wherein the gated photodetector comprises a gated single-photon detector such as a gated single-photon avalanche photodiode (SPAD).

Optionally, the method comprises filtering light incident on the gated photodetector so as to at least partially suppress or block any ambient light such as any sunlight which is incident on the gated photodetector at wavelengths outside an emission bandwidth of the pulsed light source. Filtering light incident on the gated photodetector in this way may serve to increase a signal-to-noise ratio at the gated photodetector.

Optionally, the method comprises at least partially blocking any ambient light such as a sunlight from reaching the field of view. At least partially blocking any ambient light from reaching the field of view may serve to increase a signal-to-noise ratio at the gated photodetector.

Optionally, the method comprises directing the field of view away from any source of ambient light. Directing the field of view away from any source of ambient light in this way may serve to increase a signal-to-noise ratio at the gated photodetector.

Optionally, the method comprises averaging the detected signal values corresponding to two or more successive pulses of light so as to generate an average detected signal value corresponding to the two or more successive pulses of light. Such a method may allow the detected signal values to be accumulated and averaged over multiple shots for greater measurement sensitivity and/or a greater measurement range.

Optionally, the method comprises determining whether an object was present in the turbid medium in the path during the time of flight of two or more successive pulses of light according to whether the average detected signal value corresponding to said two or more successive pulses of light is less than an average detected signal value corresponding to two or more other successive pulses of light by more than a threshold difference value.

The signal associated with the light scattered from the probe volume can be detected if it is greater than the total noise signal from the background and any internal noise in the gated photodetector. In many cases where the signal and noise levels are similar and there are fluctuations in the signal and/or the noise, it may be necessary to accumulate (or average) the signal and noise over multiple shots. Photodetectors such as PMTs, APDs or SiPMs can detect multiple photons per gate period. In the specific case in which the noise is attributed to less than one detection per gate period, then a gated single-photon detector such as a gated SPAD can be used to detect the signal.

Such gated single-photon detectors produce a count when a photon or internal noise event is detected. In this case, detection occurs over multiple shots (and the accumulated count will be greater in the presence of signal photons than in their absence). This single photon counting mode is dependent on the background level being kept sufficiently low that the very small signal associated with the radiation scattered from the probe volume can exceed the noise. This may be achieved by minimising the background level. Minimising the background level may comprise directing the field of view away from any source of ambient radiation. Minimising the background level may comprise minimising the field of view. Minimising the background level may comprise using one or more optical filters in front of the gated single-photon detector for filtering light before the light is incident on the gated single-photon detector so as to at least partially suppress or block light incident on the gated single-photon detector at wavelengths outside an emission bandwidth of the pulsed light source. Minimising the background level may comprise using a short gate period. Minimising the background level may comprise using a gate period which is comparable to the duration of each pulse of light. For example, minimising the background level may comprise using a gate period which is less than 100 ns, less than 10 ns, less than 1 ns or substantially equal to 1 ns.

Optionally, the method comprises sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting two or more of the pulses of light through the turbid medium towards two or more corresponding different probe volumes of the turbid medium, each pulse of light being emitted at a corresponding emission time. Optionally, the method comprises using a gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of the corresponding probe volume during a corresponding gate period to thereby generate a corresponding detected signal value, wherein the corresponding gate period commences at a corresponding start time which is delayed relative to the corresponding emission time by a corresponding time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the corresponding probe volume and from the corresponding probe volume through the turbid medium to the gated photodetector.

Optionally, the method comprises determining a position of the object in the path based on the plurality of detected signal values.

If an object interrupts the path at a position between a first more proximate probe volume and a second less proximate probe volume, then a first pulse of light may reach the first probe volume and a portion of the first pulse of light may be scattered from the first probe volume and be incident on the gated photodetector, whereas a second pulse of light may not reach the second probe volume and/or a portion of the second pulse of light which is scattered from the second probe volume may be blocked from reaching the gated photodetector such that a second detected signal value corresponding to the second pulse of light may be reduced in value relative to a first detected signal value corresponding to the first pulse of light. Such a reduction in the second detected signal value relative to the first detected signal value is indicative that an object has interrupted the path at a position between the first more proximate probe volume and the second less proximate probe volume. In effect, therefore, transmitting the pulses of light through the turbid medium towards different corresponding probe volumes of the turbid medium and detecting the light incident on the gated photodetector during corresponding different gate periods allows a distance or depth scan to be performed through the turbid medium to allow the position of any object in the path to be determined.

If each beam of light is collimated or substantially collimated, then any variation in the detected signal values corresponding to scattering from different probe volumes of the turbid medium should be reduced and the interruption of the path by an object should result in a reduction in the detected signal values which is detectable in spite of any variation in the detected signal values resulting from scattering from different probe volumes of the turbid medium.

Optionally, the method comprises sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting each pulse of light through the turbid medium towards a plurality of different probe volumes of the turbid medium, each pulse of light being emitted at a corresponding emission time. Optionally, the method comprises using the gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of each different probe volume of the plurality of different probe volumes during a plurality of corresponding gate periods to thereby generate a plurality of corresponding detected signal values for each pulse of light, wherein each gate period commences at a corresponding start time which is delayed relative to the emission time of the pulse of light concerned by a corresponding time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the corresponding probe volume and from the corresponding probe volume through the turbid medium to the gated photodetector. Put another way, the method may comprise "opening the gate" of the gated photodetector multiple times per pulse so as to generate a plurality of corresponding detected signal values from different probe volumes at a plurality of different depths illuminated by a single pulse of light. In effect, the method may comprise sampling the light returning from probe volumes at different depths for each pulse of light.

Optionally, the method comprises determining a position of the object in the path based on the plurality of detected signal values corresponding to the pulse of light concerned.

Such a multiple sampling method may be advantageous because it could provide a more efficient way to perform depth scanning for target location than the depth scanning method described above.

The method may comprise: determining a ratio value between a reference signal value and each detected signal value corresponding to each different probe volume for the same pulse of light; and determining a position of the object in the path based on the plurality of ratio values.

The reference signal value may, for example, be selected from the plurality of detected signal values.

Such a multiple sampling method may be advantageous because it enables an alternative mode of operation in which the detection criterion is based on the ratio of the detected signal values from different probe volumes rather than on a simple comparison to the noise floor of the detector. For example, a detected signal value corresponding to a probe volume at a depth of 25m in sea water might be 1% of a detected signal value from a probe volume at a depth of 5m in sea water when no occluding object is in the space between them. This may enhance the measurement sensitivity of the method. Such a multiple sampling method may also provide some signal processing advantages e.g. in cases where the pulse energy varies significantly from pulse to pulse.

Optionally, the pulsed light source and the gated photodetector are mounted on a platform or a vessel, wherein the platform or vessel is configured to float on a surface of the turbid medium and/or wherein the platform or the vessel is configured to be submerged in the turbid medium.

Optionally, the platform or the vessel is configured to at least partially block ambient light such as a sunlight from reaching the field of view. Optionally, the pulsed light source and the gated photodetector are mounted on an underside of the platform or the vessel.

Optionally, the pulsed light source and the gated photodetector are oriented so that the probe volume is located below the pulsed light source and the gated photodetector. Optionally, the pulsed light source and the gated photodetector are oriented such that the probe volume is located to one side of the pulsed light source and the gated photodetector.

Optionally, wherein the pulsed light source and the gated photodetector are mounted on a vessel which is configured for movement on or through the turbid medium and the method comprises piloting the vessel on or through the turbid medium according to whether an object was determined to be present in the turbid medium in the path during the time of flight of one or more of the pulses of light. Such a method could be used for the purposes of obstacle avoidance. Such a method could be used for the purposes of detecting obstacles in the path of a vessel or a craft such as a ship, a submarine or a remote operated vehicle (ROV) through the turbid medium for the purposes of piloting the vessel or the craft through the turbid medium.

Optionally, the pulsed light source and the gated photodetector are mounted above the surface of the turbid medium. Optionally, the pulsed light source and the gated photodetector are mounted on an aircraft or an airborne platform such a drone which is configured to fly above a surface of the turbid medium.

Optionally, the object is solid.

Optionally, the object is opaque.

Optionally, the object is partially transmissive.

Optionally, the object is at least partially reflective.

Optionally, the turbid medium comprises water and particulates in the water for scattering the pulses of light.

Optionally, the turbid medium comprises seawater.

Optionally, the object comprises a target such as a submarine or a mine.

Optionally, the object comprises an aquatic animal such as a marine animal.

Such a method could be used for monitoring the movements of animals in the turbid medium such as movements of marine animals living in the sea.

Optionally, the turbid medium comprises a fluid stream and the object comprises an object moving in the fluid stream. Such a method could be used for detecting, or monitoring the movements of, objects in a fluid stream in a fluid conduit such as a pipe. Such a method could be used for detecting, or monitoring the movements of, objects in a fluid handling system such as a sewerage pipe.

Optionally, the turbid medium comprises a fluid and the object comprises a particle or a biological sample in the fluid. Such a method could be used for detecting, or monitoring the movements of, particles or biological samples when suspended in a fluid.

According to an aspect of the present disclosure there is provided a method of detecting an object in a turbid medium, the method comprising: sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting each pulse of light through the turbid medium towards a plurality of different probe volumes of the turbid medium, each pulse of light being emitted at a corresponding emission time; using a gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of each different probe volume of the plurality of different probe volumes during a plurality of corresponding gate periods to thereby generate a plurality of corresponding detected signal values for each pulse of light, wherein each gate period commences at a corresponding start time which is delayed relative to the emission time of the pulse of light concerned by a corresponding time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the corresponding probe volume and from the corresponding probe volume through the turbid medium to the gated photodetector; and determining a position of the object in the path based on the plurality of detected signal values corresponding to the pulse of light concerned.

Optionally, the method comprises: determining a ratio value between a reference signal value and each detected signal value corresponding to each different probe volume for the same pulse; and determining a position of the object in the path based on the plurality of ratio values.

Optionally, the reference signal value is selected from the plurality of detected signal values.

It should be understood that any one or more of the features of any one of the foregoing aspects of the present disclosure may be combined with any one or more of the features of any of the other foregoing aspects of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

A method of detecting an object in a turbid medium will now be described by way of non-limiting example only with reference to the accompanying drawings of which: FIG. 1A is a schematic illustration of a system for detecting an object in a turbid medium, FIG. 1B is a schematic illustration of an alternative system for detecting an object in a turbid medium; FIG. 2 is a schematic illustration of the system of FIG. 1A mounted on a platform or a vessel floating on a surface of the sea when a laser of the system is non-operational; FIG. 3 is a schematic illustration of the system of FIG. 1A mounted on a platform or a vessel floating on a surface of the sea when a laser of the system is operational; FIG. 4 is a schematic illustration of the system of FIG. 1A mounted on a platform or a vessel floating on a surface of the sea when a laser of the system is operational and an object interrupts a path which extends from the laser to a probe volume of the sea and from the probe volume of the sea to a gated photodetector of the system; and FIG. 5 shows the temporal evolution of various signals during the operation of the system of FIG. 1A.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring initially to FIG. 1A there is shown a system generally designated 2 for detecting an object in a turbid medium. The system 2 includes a pulsed light source in the form of a frequency-doubled 0-switched diode-pumped solid-state Nd:YAG laser 4 and a gated photodetector in the form of a gated single-photon avalanche diode (SPAD) 6. The laser 4 is configured to emit pulses at a wavelength in an emission bandwidth around 532 nm at a repetition rate of 1kHz, wherein each pulse has a duration of the order of 1 ns and an energy of the order of 5 mJ. The system 2 further includes a beam splitter 8, a focussing element in the form of a lens 10, and a processing resource 12 configured for communication with the laser 4 and the gated SPAD 6. Although not shown explicitly in FIG. 1A, it should be understood that the apparatus 2 also includes one or more optical filters in front of the gated SPAD 6 for filtering light before the light is incident on the gated SPAD 6 so as to at least partially suppress or block light incident on the gated SPAD 6 at wavelengths outside the emission bandwidth of the laser 4.

FIG. 2 shows the system 2 mounted on a platform or vessel 20 which floats on a surface 22 of a turbid medium in the form of the sea 24 when the laser 4 is nonoperational. As will be appreciated by one of ordinary skill in the art, the sea 24 contains particulates which scatter light. The floating platform or vessel 20 is illuminated by the sun 26 so that the floating platform or vessel 20 casts a shadow 28 which extends downwardly from the floating platform or vessel 20 into the sea 24. Although the shadow 28 is represented as a triangular outline in FIG. 2, in reality it should be understood that the shape of the shadow 28 may have a differently-shaped outline which may depend not just on the shape of the floating platform or vessel 20, but also on the nature of the scattering of ambient light occurring at the surface 22 of the sea 24 and within the sea 24 itself. The outline of the shadow 28 may be regular or irregular in shape. As will be described in more detail below, the system 2 is configured such that the gated SPAD 6 receives light from a field of view 30 in the sea 24.

In use, ambient light from the sun 26 is scattered at the surface 22 of the sea 24 and/or within the sea 24 before entering the field of view 30. Some of the ambient light entering the field of view 30 is scattered by the sea 24 in the field of view 30 towards the lens 10 which collects said scattered ambient light and focusses the collected ambient light onto the gated SPAD 6 via the beam splitter 8 as shown in FIG. 1A.

FIG. 3 shows the system 2 when the laser 4 is operational so that the laser 4 sequentially emits a plurality of light pulses as an initial divergent beam of light which is reflected by the beam splitter 8 and collimated by the lens 10 so as to form a collimated beam of light 40 which extends downwardly from the lens 10 through the sea 24 so as to illuminate a region 41 of the sea 24 which coincides with the field of view 30 and which includes a probe volume 42 of the sea 24. The collimated beam of light 40 is scattered in the illuminated region 41 of the sea 24. In particular, the collimated beam of light 40 is scattered in the probe volume 42 of the sea 24 to generate scattered laser light 44 in the probe volume 42. As shown in FIG. 1A, a portion of the laser light 44 scattered in the probe volume 42 travels towards the system 2 within the field of view of the system 2 and is collected and focussed by the lens 10 onto the gated SPAD 6 via the beam splitter 8. The processing resource 12 controls the gated SPAD 6 so that the gated SPAD 6 is only enabled to detect light incident on the gated SPAD 6 during a gate period to thereby generate a corresponding detected signal value, wherein the gate period commences at a start time which is delayed relative to the emission time of a corresponding pulse of light emitted by the laser 4 by a time delay which corresponds to a time of flight for the pulse of light to travel along a path which extends from the laser 4 through the sea 24 to the probe volume 42 and from the probe volume 42 through the sea 24 to the gated SPAD 6. Specifically, the processing resource 12 only enables the gated SPAD 6 during a short gate period, TG seconds. This technique is called gating. The start of the gate period is delayed or offset relative to the time of emission of the laser pulse by a time delay or offset TO seconds. The time delay or offset TO is determined by the distance or depth D in metres from the system 2 to the selected probe volume 42 using the formula TO = 2D x n/c, where c is the speed of light in m/s and n is the refractive index of the sea 24, i.e. TO is the round-trip time for a pulse of light to travel from the laser 4 to the probe volume 42 and for the resulting scatter from the probe volume 42 to travel from the probe volume 42 to the gated SPAD 6. The gate period, TG, determines the thickness, W in metres, of the approximately-cylindrical probe volume 42 which is given by W = c x TG/n. For efficient use of the laser radiation and to minimise collection of laser light scattered from regions of the sea 24 which are in the field of view 30 but outside the probe volume 42, TP should not be significantly greater than TG. Using gated detection also means that any noise generated as a result of reflections of the collimated beam of light 40 from any objects in the field of view 30 below the probe volume 42 is rejected.

Whilst the laser 4 sequentially emits the pulses of light, ambient light 32 is scattered by the sea 24 in the field of view 30 towards the lens 10 which collects and focusses said scattered ambient light onto the gated SPAD 6 via the beam splitter 8. As shown in FIG. 5, the processing resource 12 generates a trigger signal which triggers the laser 4 to emit a pulse of light 50 at an emission time. The processing resource 12 also generates a gate signal 52 which defines a short gate period TG after a time delay of TO seconds relative to the emission time of the pulse of light 50. As a consequence of the scattering of the laser light in the selected probe volume 42, the gated SPAD 6 generates a detected signal or pulse 54 during the gate period TG. The duration of each pulse of light and the corresponding gate period TG are minimised so as to minimise the amount of scattered ambient light 32 which is detected by the gated SPAD 6 during the gate period and thereby maximise the signal-to-noise ratio. In practice, it is believed that a pulse of light and a gate period TG of approximately 1 ns in duration may be appropriate for a range of 10 m or more in water.

Referring now to FIG. 4 there is shown the system 2 when the laser 4 is operational and a submerged object 60 interrupts the path which extends from the laser 4 through the sea 24 to the probe volume 42 and from the probe volume 42 through the sea 24 to the gated SPAD 6. The submerged object 60 not only attenuates or prevents the collimated beam of light 40 from reaching the probe volume 42, but also attenuates or prevents any scattered laser light from travelling from the probe volume 42 to the gated SPAD 6. Referring back to FIG. 5, the presence of the object 60 in the path at least partially reduces or suppresses the detected signal or pulse 54 resulting in the gated SPAD 6 generating a detected signal 56.

From the foregoing description, it should be understood that the detected signal or pulse 54 arises as a consequence of scattering of the pulse of light 50 in the probe volume 42 and that the presence of the detected signal or pulse 54 is indicative of the absence of any object in the path which extends from the laser 4 through the sea 24 to the probe volume 42 and from the probe volume 42 through the sea 24 to the gated SPAD 6. Consequently, the method of detecting an object in a turbid medium described above may be considered to be a shadow detection method. Such a method constitutes an indirect detection technique that utilises the scattering of the light in the probe volume of the turbid medium to create a measurable signal. In effect, the method at least partially reduces the masking effects of scattering that may be responsible for degrading the sensitivity of known light detection and ranging methods such as conventional LIDAR methods when used in a turbid medium such as the sea.

Furthermore, the method does not require a range or depth search to detect the presence of an object such as the submerged object 60 somewhere in the path.

As will be appreciated by one of ordinary skill in the art, the system 2 of FIG. 1A may be considered to be a monostatic system in which the field of view 30 and the illuminated region 41 coincide as a result of the arrangement of the laser 4, the gated SPAD 6 and the beam splitter 8. In a variant (not shown) of the monostatic system 2 of FIG. 1A, the beamsplitter 8 may be replaced by a circulator (not shown) to reduce optical losses and improve the measurement sensitivity and/or extend the measurement range of the system.

FIG. 1B shows a bistatic system generally designated 102 for detecting an object in a turbid medium. The bistatic system 102 includes a pulsed light source in the form of a frequency-doubled Q-switched diode-pumped solid-state Nd:YAG laser 104 and a gated photodetector in the form of a gated single-photon avalanche diode (SPAD) 106. The laser 104 is configured to emit pulses at a wavelength in an emission bandwidth around 532 nm at a repetition rate of 1kHz, wherein each pulse has a duration of the order of 1 ns and an energy of the order of 5 mJ. The bistatic system 102 further includes a first focussing element in the form of a first lens 110a, a second focussing element in the form of a second lens 110b, and a processing resource 112 configured for communication with the laser 104 and the gated SPAD 106. The second lens 110b collects light from a field of view 130 and focusses the collected light onto the gated SPAD 106. Although not shown explicitly in FIG. 1B, it should be understood that the apparatus 102 also includes one or more optical filters in front of the gated SPAD 106 for filtering light before the light is incident on the gated SPAD 106 so as to at least partially suppress or block light incident on the gated SPAD 106 at wavelengths outside the emission bandwidth of the laser 104.

In use, the laser 104 sequentially emits a plurality of light pulses as an initial divergent beam of light which is collimated by the first lens 110a so as to form a collimated beam of light 140 which is directed through the sea 24 so as to illuminate a region 141 of the sea 24 which includes a probe volume 142 of the sea 24. The illuminated region 141 of the sea 24 and the field of view 130 of the sea 24 overlap only in the region of the probe volume 142. The collimated beam of light 140 is scattered in the sea 24 and is scattered in the probe volume 142 of the sea 124 to generate scattered laser light 144 in the probe volume 142. As shown in FIG. 18, the second lens 110b then collects a portion of the scattered laser light 144 which is generated in the probe volume 142 and focusses the collected light onto the gated SPAD 106. The processing resource 112 controls the gated SPAD 106 so that the SPAD 106 is only enabled to detect light incident on the gated SPAD 106 during a gate period to thereby generate a corresponding detected signal value, wherein the gate period commences at a start time which is delayed relative to the emission time of a corresponding pulse of light emitted by the laser 104 by a time delay which corresponds to a time of flight for the pulse of light to travel along a path which extends from the laser 104 through the sea 24 to the probe volume 142 and from the probe volume 142 through the sea 24 to the gated WAD 106. In all other respects, the system 102 operates in the same way as the system 2. As will be appreciated by one of ordinary skill in the art, in contrast to the monostatic system 2 of FIG. 1A in which the location of the probe volume 42 depends only on the time delay between the time of emission of a pulse of light from the laser 4 to a start time of the corresponding gate period, in the bistatic system 102 of FIG. 18, the time delay between the time of emission of a pulse of light from the laser 104 to a start time of the corresponding gate period must be selected so as to locate the probe volume 142 at the intersection of the illuminated region 141 and the field of view 130 i.e. in the spatial region where the illuminated region 141 and the field of view 130 overlap.

Although preferred embodiments of the disclosure have been described in terms as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will understand that various modifications may be made to the described embodiments without departing from the scope of the appended claims. For example, for use in seawater, the wavelength of the pulsed light source may be in the range 450 -550 nm, 490 -510 nm, or around 500 nm.

The detected signal values may be accumulated and averaged over multiple shots for greater measurement sensitivity. The signal associated with the laser radiation scattered from the probe volume can be detected if it is greater than the total noise signal from the background and any internal noise in the gated photodetector. In many cases where the signal and noise levels are similar and there are fluctuations in the signal and/or the noise, it may be necessary to accumulate (or average) the signal and noise over multiple shots. Photodetectors such as PMTs, APDs or SiPMs can detect multiple photons per gate period. In the specific case in which the noise is attributed to less than one detection per gate period, then a gated single-photon detector such as a gated SPAD like the gated SPADs 6, 106 can be used to detect the signal. Such gated single-photon detectors produce a count when a photon or internal noise event is detected. In this case, detection occurs over multiple shots (and the accumulated count will be greater in the presence of signal photons than in their absence). This single photon counting mode is dependent on the background level being kept sufficiently low that the very small signal associated with the radiation scattered from the probe volume can exceed the noise. This may be achieved by a combination of one or more of the following features: the apparatus 2, 102 may be configured to direct the field of view 30, 130 away from any source of ambient radiation; the apparatus 2, 102 may be configured to minimise the field of view 30, 130; the apparatus 2, 102 may include one or more optical filters in front of the gated single-photon detector for filtering light before the light is incident on the gated single-photon detector so as to at least partially suppress or block light incident on the gated single-photon detector at wavelengths outside the emission bandwidth of the laser; and use of a short gate period.

The method may comprise performing a range or distance search to determine the position of an object in the path. Specifically, the method may comprise sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting the pulses of light through the turbid medium towards different corresponding probe volumes of the turbid medium, each pulse of light being emitted at a corresponding emission time. The method may comprise using a gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of the corresponding probe volume during a corresponding gate period to thereby generate a corresponding detected signal value, wherein the corresponding gate period commences at a corresponding start time which is delayed relative to the corresponding emission time by a corresponding time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the corresponding probe volume and from the corresponding probe volume through the turbid medium to the gated photodetector. The method may comprise determining a position of the object in the path based on the plurality of detected signal values.

If an object interrupts the path at a position between a first more proximate probe volume and a second less proximate probe volume, then a first pulse of light may reach the first probe volume and a portion of the first pulse of light may be scattered from the first probe volume and be incident on the gated photodetector, whereas a second pulse of light may not reach the second probe volume and/or a portion of the second pulse of light which is scattered from the second probe volume may be blocked from reaching the gated photodetector such that a second detected signal value corresponding to the second pulse of light may be reduced in value relative to a first detected signal value corresponding to the first pulse of light. Such a reduction in the second detected signal value relative to the first detected signal value is indicative that an object has interrupted the path at a position between the first more proximate probe volume and the second less proximate probe volume. In effect, therefore, transmitting the pulses of light through the turbid medium towards different corresponding probe volumes of the turbid medium and detecting the light incident on the gated photodetector during corresponding different gate periods allows a depth scan to be performed through the turbid medium to allow the position of any object in the path to be determined. Moreover, if each beam of light is collimated or substantially collimated, then any variation in the detected signal values corresponding to scattering from different probe volumes of the turbid medium should be minimised and the interruption of the path by an object should result in a reduction in the detected signal values which is detectable in spite of any variation in the detected signal values resulting from scattering from different probe volumes of the turbid medium. It may also be easier to perform such a range or depth search with a monostatic system than a bistatic system because, in the case of a monostatic system there is no need to vary the direction of the beam of light to illuminate different probe volumes or to vary the direction of the field of view to collect light from different probe volumes.

In an alternative variant, the method may comprise sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting each pulse of light through the turbid medium towards a plurality of different probe volumes of the turbid medium, each pulse of light being emitted at a corresponding emission time. The method may comprise using a gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of the plurality of different probe volumes during a plurality of corresponding gate periods to thereby generate a plurality of corresponding detected signal values for each pulse of light, wherein each gate period commences at a corresponding start time which is delayed relative to the emission time of the pulse of light concerned by a corresponding time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the corresponding probe volume and from the corresponding probe volume through the turbid medium to the gated photodetector. Put another way, the method may comprise "opening the gate" of the gated photodetector multiple times per pulse so as to generate a plurality of corresponding detected signal values from different probe volumes at a plurality of different depths illuminated by a single pulse of light. In effect, the method may comprise sampling the light returning from probe volumes at different depths for each pulse of light.

The method may comprise determining a position of the object in the path based on the plurality of detected signal values corresponding to the pulse of light concerned. Such a multiple sampling method may be advantageous because it could provide a more efficient way to do the depth scanning for target location than the depth scanning method described above.

The method may comprise: determining a ratio value between a reference signal value and each detected signal value corresponding to each different probe volume for the same pulse; and determining a position of the object in the path based on the plurality of ratio values.

Such a multiple sampling method may be advantageous because it enables an alternative mode of operation in which the detection criterion is based on the ratio of the detected signal values from different probe volumes rather than on a simple comparison to the noise floor of the detector. For example, a detected signal value from a probe volume at a depth of 25m in sea water might be 1% of a detected signal value from a probe volume at a depth of 5m in sea water when no occluding object is in the space between them. This may enhance the measurement sensitivity of the method. Such a multiple sampling method may also provide some signal processing advantages e.g. in cases where the pulse energy varies significantly from pulse to pulse.

Moreover, if each beam of light is collimated or substantially collimated, then any variation in the detected signal values corresponding to scattering from different probe volumes of the turbid medium should be reduced and the interruption of the path by an object should result in a reduction in the detected signal values which is detectable in spite of any variation in the detected signal values resulting from scattering from different probe volumes of the turbid medium. It may also be easier to perform such a range or depth search with a monostafic system than a bistafic system because, in the case of a monostatic system there is no need to vary the direction of the beam of light to illuminate different probe volumes or to vary the direction of the field of view to collect light from different probe volumes.

In other embodiments, the system 2, 102 may be configured to emit a beam of light which is convergent towards the probe volume 42, 142 and to collect scattered light which is divergent from the probe volume 42, 142.

In other embodiments, the system 2, 102 may be configured to emit each pulse of light as an initial divergent beam of light which is converted into the corresponding beam of light for illuminating the turbid medium, wherein each beam of light is divergent but has a divergence which is reduced relative to the corresponding initial divergent beam of light.

The gated photodetector may be a gated single photon detector of any kind. The gated photodetector may comprise a gated photomultiplier tube (PMT), or a gated avalanche photodiode (APD).

The turbid medium may comprise a body of water other than the sea.

The pulsed light source and the gated photodetector may be located above the surface of the turbid medium.

The pulsed light source and the gated photodetector may be mounted on an aircraft or an airborne platform such a drone which is configured to fly above a surface of the turbid medium.

The probe volume may be located to one side of the pulsed light source and the gated photodetector. Such a method could be used for the purposes of piloting a craft or a vessel such as a ship, a submarine or a remote operated vehicle (ROV) through the turbid medium to detect obstacles in the path of the craft or the vessel The object may be solid.

The object may be opaque.

The object may be partially transmissive.

The object may be at least partially reflective.

The object may comprise a target such as a submarine or a mine.

The object may comprise an aquatic animal such as a marine animal. Such a method could be used for monitoring the movements of animals in the turbid medium such as movements of marine animals living in the sea.

The turbid medium may comprise a fluid stream. The object may comprise an object moving in the fluid stream. Such a method could be used for detecting, or monitoring the movements of, objects in a fluid conduit such as a pipe. Such a method could be used for detecting, or monitoring the movements of, objects in a fluid handling system such as a sewerage pipe. The turbid medium may comprise a fluid. The object may comprise a particle or a biological sample in a fluid. Such a method could be used for detecting, or monitoring particles or biological samples when suspended in a fluid.

Each feature disclosed or illustrated in the present specification may be incorporated in any embodiment, either alone, or in any appropriate combination with any other feature disclosed or illustrated herein. In particular, one of ordinary skill in the art will understand that one or more of the features of the embodiments of the present disclosure described above with reference to the drawings may produce effects or provide advantages when used in isolation from one or more of the other features of the embodiments of the present disclosure and that different combinations of the features are possible other than the specific combinations of the features of the embodiments of the present disclosure described above.

The skilled person will understand that in the preceding description and appended claims, positional terms such as 'above', 'along', 'side', etc. are made with reference to conceptual illustrations, such as those shown in the appended drawings.

These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.

Use of the term "comprising" when used in relation to a feature of an embodiment of the present disclosure does not exclude other features or steps. Use of the term "a" or "an" when used in relation to a feature of an embodiment of the present disclosure does not exclude the possibility that the embodiment may include a plurality of such features.

The use of reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

CLAIMS1. A method of detecting an object in a turbid medium, the method comprising: sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting each pulse of light through the turbid medium towards a probe volume of the turbid medium, each pulse of light being emitted at a corresponding emission time; using a gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of the probe volume during a corresponding gate period to thereby generate a corresponding detected signal value, wherein the corresponding gate period commences at a corresponding start time which is delayed relative to the corresponding emission time by a time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the probe volume and from the probe volume through the turbid medium to the gated photodetector; and determining whether an object was present in the turbid medium in the path during the time of flight of one or more of the pulses of light according to whether the one or more detected signal values corresponding to said one or more pulses of light is less than one or more of the other detected signal values corresponding to one or more of the other pulses of light by more than a threshold difference value.
2. The method of claim 1, comprising forming each pulse of light into a corresponding beam of light which illuminates a corresponding illuminated region of the turbid medium.
3. The method of claim 2, wherein each beam of light is collimated or substantially collimated.
4. The method of claim 2, wherein each beam of light is convergent towards the probe volume. 30
5. The method of claim 2, wherein the pulsed light source emits each pulse of light as an initial divergent beam of light and the method comprises converting each initial divergent beam of light into the corresponding beam of light, wherein each beam of light is divergent but has a divergence which is reduced relative to the corresponding initial divergent beam of light.
6. The method of any one of claims 2 to 5, comprising receiving light at the gated photodetector from a field of view in the turbid medium, wherein the field of view includes the probe volume.
7. The method of claim 6, wherein the probe volume of the turbid medium is defined by the field of view and the duration of the gate period.
8. The method of claim 6 or 7, wherein the illuminated region and the field of view coincide or substantially coincide or wherein the illuminated region and the field of view overlap only in the region of the probe volume.
9. The method of any preceding claim, wherein the duration of the gate period is less than 100 ns, less than 10 ns, less than 1 ns or substantially equal to 1 ns.
10. The method of any preceding claim, wherein a duration of each pulse of light matches, or is comparable to, the duration of the gate period, for example wherein the duration of each pulse of light is less than 100 ns, less than 10 ns, less than 1 ns or substantially equal to 1 ns.
11. The method of any preceding claim, comprising at least one of: filtering light incident on the gated photodetector so as to at least partially suppress or block any ambient light such as any sunlight which is incident on the gated photodetector at wavelengths outside an emission bandwidth of the pulsed light source; at least partially blocking any ambient light such as a sunlight from reaching the field of view; or directing the field of view away from any source of ambient light.
12. The method of any preceding claim, comprising: averaging the detected signal values corresponding to two or more successive pulses of light so as to generate an average detected signal value corresponding to the two or more successive pulses of light; and determining whether an object was present in the turbid medium in the path during the time of flight of two or more successive pulses of light according to whether the average detected signal value corresponding to said two or more successive pulses of light is less than an average detected signal value corresponding to two or more other successive pulses of light by more than a threshold difference value.
13. The method of any preceding claim, wherein the gated photodetector comprises a gated photomultiplier tube (PMT), a gated avalanche photodiode (APD) or a gated silicon photomultiplier (SiPM), or wherein the gated photodetector is configured to detect or count single photons during each gate period, for example wherein the gated photodetector comprises a gated single-photon avalanche photodiode (SPAD).
14. The method of any preceding claim, comprising: sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting the pulses of light through the turbid medium towards different corresponding probe volumes of the turbid medium, each pulse of light being emitted at a corresponding emission time; using the gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of the corresponding probe volume during a corresponding gate period to thereby generate a corresponding detected signal value, wherein the corresponding gate period commences at a corresponding start time which is delayed relative to the corresponding emission time by a corresponding time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the corresponding probe volume and from the corresponding probe volume through the turbid medium to the gated photodetector; and determining a position of the object in the path based on the plurality of detected signal values.
15. The method of any one of claims 1 to 13, comprising: sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting each pulse of light through the turbid medium towards a plurality of different probe volumes of the turbid medium, each pulse of light being emitted at a corresponding emission time; using the gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of each different probe volume of the plurality of different probe volumes during a plurality of corresponding gate periods to thereby generate a plurality of corresponding detected signal values for each pulse of light, wherein each gate period commences at a corresponding start time which is delayed relative to the emission time of the pulse of light concerned by a corresponding time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the corresponding probe volume and from the corresponding probe volume through the turbid medium to the gated photodetector; and determining a position of the object in the path based on the plurality of detected signal values corresponding to the pulse of light concerned.
16. The method of claim 15, comprising: determining a ratio value between a reference signal value and each detected signal value corresponding to each different probe volume for the same pulse; and determining a position of the object in the path based on the plurality of ratio values, wherein the reference signal value is selected from the plurality of detected signal values.
17. The method of any preceding claim, wherein the pulsed light source and the gated photodetector are mounted on a platform or a vessel, wherein the platform or the vessel is configured to float on a surface of the turbid medium and/or wherein the platform or the vessel is configured to be submerged in the turbid medium and, optionally, wherein the pulsed light source and the gated photodetector are mounted on an underside of the platform or the vessel and, optionally, wherein the platform or the vessel at least partially blocks any ambient light such as a sunlight from reaching thefield of view.
18. The method of any one of claims 1 to 16, wherein the pulsed light source and the gated photodetector are mounted on an aircraft or an airborne platform such a drone which is configured to fly above a surface of the turbid medium.
19. The method of any preceding claim, wherein the pulsed light source and the gated photodetector are oriented so that the probe volume is located below the pulsed light source and the gated photodetector or wherein the pulsed light source and the gated photodetector are oriented such that the probe volume is located to one side of the pulsed light source and the gated photodetector.
20. The method of any preceding claim, wherein the pulsed light source and the gated photodetector are mounted on a vessel which is configured for movement on or through the turbid medium and the method comprises piloting the vessel on or through the turbid medium according to whether an object was determined to be present in the turbid medium in the path during the time of flight of one or more of the pulses of light.
21. The method of any preceding claim, wherein the object is at least one of: solid; opaque; partially transmissive; or at least partially reflective.
22. The method of any preceding claim, wherein the turbid medium comprises water and particulates in the water for scattering the pulses of light, for example wherein the turbid medium comprises seawater and, optionally, wherein the object comprises a target such as a submarine or a mine or wherein the object comprises an aquatic animal such as a marine animal.
23. The method of any one of claims 1 to 21, wherein the turbid medium comprises a fluid stream and the object comprises an object moving in the fluid stream or wherein the turbid medium comprises a fluid and the object comprises a particle or a biological sample in the fluid
24. A method of detecting an object in a turbid medium, the method comprising: sequentially emitting a plurality of pulses of light from a pulsed light source and transmitting each pulse of light through the turbid medium towards a plurality of different probe volumes of the turbid medium, each pulse of light being emitted at a corresponding emission time; using a gated photodetector to detect, for each pulse of light, any light which is incident on the gated photodetector from the direction of each different probe volume of the plurality of different probe volumes during a plurality of corresponding gate periods to thereby generate a plurality of corresponding detected signal values for each pulse of light, wherein each gate period commences at a corresponding start time which is delayed relative to the emission time of the pulse of light concerned by a corresponding time delay which corresponds to a time of flight for the pulse of light to travel along a path extending from the pulsed light source through the turbid medium to the corresponding probe volume and from the corresponding probe volume through the turbid medium to the gated photodetector; and determining a position of the object in the path based on the plurality of detected signal values corresponding to the pulse of light concerned.
25. The method of claim 24, comprising: determining a ratio value between a reference signal value and each detected signal value corresponding to each different probe volume for the same pulse; and determining a position of the object in the path based on the plurality of ratio values, wherein the reference signal value is selected from the plurality of detected signal values.