AN OCEANOGRAPHIC LiDAR SYSTEM

Abstract

Described herein is a LiDAR system for measuring characteristics of an aquatic environment. The system comprises a transmitter module comprising a diode laser source configured to emit pulses of light and direct the pulses of light towards a measuring zone in the aquatic environment. A portion of the emitted pulses of light is returned from the measuring zone as returned light. A receiver module comprises one or more detectors and is configured to receive a portion of the returned light and generate a plurality of detector signals. Each detector signal represents the intensity of the portion of the returned light at a pre-determined wavelength. A processor module is configured to determine a distance between the system and a position at which the emitted pulses of light are returned based, at least in part, on time-of-flight, and determine one or more characteristics of the measuring zone.

Description

AN OCEANOGRAPHIC LIDAR SYSTEMTechnical Field

[0001] The present invention relates to a Light Detection and Ranging (LiDAR) system and in particular to a LiDAR system for measuring characteristics of an aquatic environment.

[0002] Embodiments of the present invention provide a LiDAR system adapted for enabling oceanographic LiDAR applications with spectroscopic capabilities. However, it will be appreciated that the invention is applicable in broader contexts and other applications such as non-oceanographic applications.Background of Invention

[0003] Three-dimensional information about waterbodies has many applications in defence and environmental science. Examples of desirable information include water temperature, salinity, concentrations of specific pigments such as chlorophyll, and concentrations of contaminants such as oil and gas. In the defence sector, this information enables more accurate prediction of how light and sound propagate underwater and can be used to monitor underwater disturbances, moving objects, and shipwrecks. In the environmental sector, this information is useful for monitoring waterway health, forecasting algal blooms, predicting and mitigating coral bleaching events, and refining climate models. Furthermore, knowledge of how ocean temperature distribution affects migration routes and congregation spots of fish allows for more sustainable fishery management to ensure that the exploration of new fishing resources can be undertaken in the most cost-effective manner while still providing the required level of environmental protection.

[0004] Conventional methods for measuring characteristics of aquatic environments have severe limitations. For example, remote sensing by satellites typically provides only surface properties, while strings of sensors deployed from buoys or the like provide only localised information and are prone to fouling and breakage.

[0005] Light Detection and Ranging (LiDAR) technology is widely used for topographic applications including bathymetric mapping. Research-grade "ocean profiling" LiDAR systems can additionally measure other parameters such as turbidity and concentrations of chlorophyll, bubbles and particulates in a column of water. However, these systems employ crystalline orsolid-state laser sources which are large, heavy, inefficient, and generally unsuitable for use in mapping applications where the system must be transported on a water or aerial drone, or a satellite. Furthermore, solid-state lasers are not sufficiently adjustable in excitation wavelength, pulse power, width and frequency so as to accommodate a variety of water conditions. For example, the properties of water can vary greatly, both between geographic locations and within the measurement column, such that different laser parameters are required to optimise measurement range and accuracy.

[0006] Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.Summary of Invention

[0007] According to an aspect of the present invention, there is provided a LiDAR system for measuring characteristics of an aquatic environment, the system comprising: a transmitter module comprising a diode laser source configured to emit pulses of light and direct the pulses of light towards a measuring zone in the aquatic environment, wherein a portion of the emitted pulses of light is returned from the measuring zone as returned light; a receiver module comprising one or more detectors configured to receive a portion of the returned light and generate a plurality of detector signals, wherein each detector signal represents the intensity of the portion of the returned light at a pre-determined wavelength; and a processor module configured to: calculate a time-of-flight based on the detector signals, determine a distance between the system and a position at which the emitted pulses of light are returned based, at least in part, on the time-of-flight, and determine one or more characteristics of the measuring zone.

[0008] In some embodiments, the pre-determined wavelength corresponds with the wavelength of scattered light. In some embodiments, the scattered light comprises Raman scattered light. In other embodiments, the scattered light comprises elastically scattered light. In other embodiments, the scattered light comprises Brillouin scattered light.

[0009] In some embodiments, the pre-determined wavelength corresponds with the wavelength of fluoresced light.

[0010] In some embodiments, the aquatic environment comprises one or more solid objects. The solid object may be part of an ocean floor or a submerged object such as a shipwreck, an aquatic animal, an autonomous underwater vehicle (AUV), a submarine, a container, a mine, and the like.

[0011] In some embodiments, the one or more characteristics of the measuring zone relate to one or more characteristics of the aquatic environment. In some embodiments, the one or more characteristics of the aquatic environment comprise the temperature of the aquatic environment. In some embodiments, the one or more characteristics comprise the salinity of the aquatic environment.

[0012] In some embodiments, the one or more characteristics comprise relative abundances and / or concentrations of specific pigments. In some embodiments, the one or more characteristics comprise relative abundances and / or concentrations of specific pigments. In some embodiments, the one or more characteristics comprise relative abundances and / or concentrations of specific phytoplankton. In some embodiments, the one or more characteristics comprise relative abundances and / or concentrations of algae. In some embodiments, the one or more characteristics of the aquatic environment comprise a presence or relative abundance of organic and inorganic particulates. In some embodiments, the one or more characteristics of the aquatic environment comprise a presence or relative abundance of pollutants or contaminants. In some embodiments, the pollutants or contaminants include oils. In other embodiments, the pollutants or contaminants include microplastics. It will be appreciated that the one or more characteristics of the aquatic environment may also comprise other types of pollutants or contaminants.

[0013] In some embodiments, the one or more characteristics of the aquatic environment comprise a presence or relative abundance of one or more gases. Examples of the one or more gases include methane, natural gas, and oxygen.

[0014] In some embodiments, the one or more characteristics of the aquatic environment comprise a speed of sound within the aquatic environment.

[0015] In some embodiments, the receiver module comprises a single photon detector. In some embodiments, the receiver module comprises two or more detectors. In some embodiments, at least one of the detectors comprise photomultipliers. In some embodiments,at least one of the detectors comprise silicon photomultipliers (SiPMs). In some embodiments, at least two of the detectors are configured to detect light at different wavelengths. It will be appreciated that standard optical filters such as band-pass filters may be used to define the wavelength(s) that is detected by each of the detectors. In some embodiments, the pulses of emitted light have a wavelength in the range of 350 nm to 580 nm. In a preferred embodiment, the pulses of emitted light have a wavelength in the range of between 405 to 530 nm. In some embodiments, the pulses of emitted light have a wavelength in the range of 530 to 550 nm. In some embodiments, the pulses of emitted light have a wavelength of above 550 nm.

[0016] In some embodiments, the photomultipliers and / or silicon photomultipliers operate in photon counting mode. In other embodiments, the photomultipliers and / or silicon photomultipliers operate in current mode.

[0017] In some embodiments, the diode laser source comprises a GaN-based laser. In some embodiments, the GaN-based laser comprises lnxAlyGai-x-yN. It will be appreciated that other suitable diode lasers may also be used.

[0018] In some embodiments, the diode laser source has a pulse repetition frequency in the kilohertz (kHz) or megahertz (MHz) frequency range. In some embodiments, the diode laser source has a pulse repetition frequency in the range of 0.01 kHz to 10 MHz. In some embodiments, the diode laser has a pulse repetition frequency of about 1 MHz.

[0019] In some embodiments, the diode laser source is wavelength-tuneable. It will be appreciated that the centre wavelength of the diode laser source may be adjusted by a few nanometres by, for example, varying the operating temperature of the diode. It will also be appreciated that the tuneability of the diode laser source may be further increased by utilising other mechanisms such as coupling with volumetric holographic gratings (VHG). A distributed feedback laser may also be employed.

[0020] In some embodiments, the pulses of light have a pulse duration in the nanosecond range. In some embodiments, the pulses of light have a pulse duration in the order of 0.1 ns. In a preferred embodiment, the pulses of light have a pulse duration of 1 to 5 ns. In some embodiments, the pulses of light have a pulse energy of around 0.1 pJ. In some embodiments, the pulses of light have a FWHM laser bandwidth of around 2 nm. In other embodiments, the pulses of light have a FWHM laser bandwidth of less than 1 nm.

[0021] In some embodiments, the pulses of light are scanned across an area of interest.

[0022] In some embodiments, the receiver module comprises beam collimating optics. In some embodiments, the receiver module further comprises spectral filters. In some embodiments, the receiver module comprises beamsplitters. In some embodiments, the beamsplitters may be polarising beamsplitters. In some embodiments, the receiver module comprises telescopes. In some embodiments, the receiver module comprises telescopes. In some embodiments, the receiver module comprises polarisers. In some embodiments, the receiver module comprises optical fibres. In some embodiments, the receiver module comprises mirrors. In some embodiments, the receiver module comprises lenses. In some embodiments, the receiver module comprises apertures. In some embodiments, the receiver module comprises diffractive optics. In some embodiments, the receiver module comprises optical fibres.

[0023] According to another aspect of the present invention, there is provided a LiDAR system for probing a medium, the system comprising: a transmitter module comprising a diode laser source configured to emit pulses of light towards the medium, wherein the diode laser source emits light in the violet-blue-green wavelength range; a receiver module configured to receive returned light, wherein the returned light is returned from interactions between the pulses of light and the medium; and a processor module configured to: determine a distance between the system and a position at which the light is returned at least in part from time-of- flight measurements, and extract information about the medium from the returned light as a function of distance.

[0024] In some embodiments, the medium comprises an aquatic environment. In some embodiments, the aquatic environment comprises one or more solid objects.

[0025] In some embodiments, the interactions between the pulses of light and the medium comprise Raman scattering.

[0026] According to yet another aspect of the present invention, there is provided a method for measuring characteristics of an aquatic environment, the method comprising: configuring a diode laser source to emit pulses of light and direct the pulses of light towards a measuring zone in the aquatic environment, wherein a portion of the emitted pulses of light is returned from the measuring zone as returned light; configuring a receiver module to receivea portion of the returned light and generate a plurality of detector signals, wherein each detector signal represents the intensity of the portion of the returned light at a pre-determined wavelength; and configuring a processor module to: calculate a time-of-flight based on the detector signals, determine a distance between the system and a position at which the emitted pulses of light are returned based, at least in part, on the time-of-flight, and determine one or more characteristics of the measuring zone.

[0027] According to another aspect of the present invention, there is provided a method for probing a medium, the method comprising: configuring a diode laser source to emit pulses of light towards the medium, wherein the diode laser source emits light in the blue-green wavelength range; configuring a receiver module to receive returned light, wherein the returned light is returned from interactions between the pulses of light and the medium; and configuring a processor module to: determine a distance between the system and a position at which the light is returned at least in part from time-of-flight measurements; and extract information about the medium from the returned light as a function of distance.

[0028] In any of the aspects of the invention, the receiver module may be configured to form a multi-channel detector with each channel configured to detect returned light from a different form of interaction between the pulses of light and the medium. By way of example, one channel may be configured to detect Raman scattered light, one channel may be configured to detect elastically scattered light, one channel may be configured to detect Brillouin scattered light and one channel may be configured to detect fluoresced light.Brief Description of Drawings

[0029] Example embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:Figure 1 is a schematic diagram illustrating the basic operation principles of the oceanographic LiDAR system;Figure 2 is a process chart showing the primary steps involved in a typical Raman-LiDAR measurement;Figure 3 is a schematic diagram showing the various components of an embodiment of the system;Figure 4 is a schematic diagram showing the types of signals that may be measured by the system;Figure 5A is an exemplary graph produced by the processor module of the intensity of the detector signals versus signal return time (ns); andFigure 5B is an exemplary graph showing the effect of watertemperature on the Raman shift.Detailed Description

[0030] The LiDAR system described herein may be used to obtain various types of depth- resolved information and / or characteristics of an aquatic environment. Examples of such depth-resolved information and / or characteristics include temperature, salinity, speed of sound within the aquatic environment and relative abundances and / or concentrations of specific phytoplankton, algae, and other specific pigments and contaminants, including oil, microplastics, methane, natural gas, and oxygen. Embodiments of the present invention will be described in the context of oceanographic applications. However, it will be appreciated that the present invention may also be applicable in other aquatic environments such as lakes. In some embodiments, the invention may also be applicable in non-aquatic environments and other applications.

[0031] The aforementioned depth-resolved information and / or characteristics is obtained by spectroscopic techniques coupled with conventional LiDAR. For example, changes in water temperature and salinity cause changes in particular features in and / or around specific Raman emission peaks.

[0032] In one or more embodiments of the present invention, high-power laser diodes are driven in an unconventional way to deliver short (few-ns) pulses at very high (MHz) repetition rate. As diode lasers are robust and compact, they offer improvements over conventional crystalline or fibre lasers with regards to Size, Weight and Power (SWaP). Specifically, diode lasers can deliver high laser power at relatively low size and weight. This is a key advantage for oceanographic LiDAR systems which are typically deployed on aerial drones, water drones, satellites, ships / vessels and other suitable platforms.

[0033] Furthermore, laser diodes (and drivers) are mechanically and electrically interchangeable while providing a variety of wavelengths (in principle any wavelength from350 to 580 nm in the case of GaN). This is in contrast to a typical solid-state laser, which will only operate at a single wavelength (or a pre-determined set of wavelengths) as determined by the specifics of the optical transitions of the gain crystal that it makes use of. Consequently, although new solid state laser wavelengths can be developed for different wavelengths, they are generally not wavelength-versatile and significant work is required for adapting them to different scenarios. For an oceanographic Raman LiDAR system that may require 10 or more excitation wavelengths to cater to different water conditions, suitable solid-state laser sources would be very complex and time consuming to develop. In contrast, a diode laser counterpart can be adapted which uses common electronic and optical systems except the actual emitting element itself, offering "plug-and-play" operation.

[0034] Although there exists a potentially relevant wavelength-versatile solid-state laser in the form of the optical parametric oscillator (OPO), this is a complex laser design that is not suitable for miniaturisation and robust implementation in a portable LiDAR context. Further, only one wavelength can be emitted at a time, and there is a significant switching time due to mechanical and / or thermal adjustments inside the OPO. The OPO wavelength cannot be switched at the timescales that are possible with diode lasers and as required for spectroscopic applications in an aquatic environment. Moreover, there are no OPOs that operate at the high pulse repetition rates required for single photon counting.

[0035] As will be appreciated, the operating regime required for oceanographic LiDAR systems for spectroscopic applications is not a mainstream mode of operating diode lasers and the means of driving diode lasers to achieve the regime are non-trivial. In particular, the design of a laser excitation source for these applications involves balancing several competing requirements. In the case of Raman scattering, because the spectral signal from Raman scattering is at least one order of magnitude lower in relative strength than the elastic signal, Raman LiDAR requires high laser power (i.e. brightness). Existing Raman LiDAR systems use high-powered Q-switched solid-state lasers to maximise received signal strength. Further, the laser output needs to be pulsed with a duration typically in the range of 1 to 20 ns, principally because a shorter duration enables better depth resolution. Finally, Raman LiDAR requires laser pulses emitted at a high pulse repetition frequency for minimising signal acquisition time and for increasing the maximum depth range of the instrument. A pulse repetition frequency(prf) ranging from 100kHz to 10MHz is ideal. In almost all crystalline and fibre lasers suitable for LiDAR applications, an increase in prf is accompanied by a decrease in pulse energy.System Overview

[0036] Referring now to Figure 1, there is illustrated a LiDAR system 100 for measuring characteristics of an aquatic environment 300. Figure 2 illustrates the primary steps involved in a typical Raman-LiDAR measurement using the system 100. As used herein, an "aquatic environment" refers to a broad environment within or around a waterbody, including any gas and / or solid object(s) which may be suspended and / or submerged therein. By way of example, an aquatic environment may comprise environments such as oceans, seas, rivers, lakes, ponds, wetlands, estuaries, pools, and even small streams and creeks. The aquatic environment comprises not only the primary liquid medium (e.g. water), but also the biological material contained therein and any solid objects within or surrounding the liquid medium. The solid objects may comprise objects entrained within the liquid medium such as rocks, plants, fish, whales, humans, submarine vessels, autonomous underwater vehicles, submersible objects, containers, garbage or mines / weapons. The solid objects may also comprise materials that contain the liquid medium such as part of an ocean floor, rocks, rock shelves, riverbanks and creek beds.

[0037] The operation of system 100 is described below with reference to Figures 1 and 2. As will be appreciated, although Figure 1 illustrates an embodiment where the system 100 is fully submerged in water, the system 100 may also be disposed on aerial drones, water drones, satellites, ships / vessels and other suitable platforms.

[0038] The system 100 comprises a transmitter module 101 configured to, at step S100, emit pulses of light 201 and direct the pulses of light 201 towards a measuring zone 302 within the aquatic environment 300. At step slOl, the pulses of light 201 interact with the aquatic environment 300 and are then returned as returned light 202. As illustrated in Figure 4, it will be appreciated that returned light 202 may comprise scattered light and / or fluoresced light. In some embodiments, the scattered light comprises Raman scattered light. The scattered light may also comprise elastically scattered light. It will be appreciated that elastically scattered light may also be referred to as "reflected light". In some embodiments, the scattered light comprises other returned light such as Brillouin scattered light and / or light from fluorescence.It will be understood that the amount of returned light 202 depends on factors such as the distance to the position at which the light is returned, the reflectivity of the water and any solid object(s) submerged or suspended therein, the wavelength of the pulses of light 201, the power of the pulses of light 201 and the relevant scattering and / or fluorescing angle(s). It will be appreciated that the amount of returned light detected may be quite small and may be less than 1% of the total light emitted from transmitter module 101.

[0039] The system 100 also comprises a receiver module 102 comprising one or more detectors configured to receive a portion of the returned light at step S102. The receiver module 102 is further configured to generate one or more detector signals 203 at step S103, wherein the or each detector signal 203 represents the intensity of the portion of the returned light at a pre-determined wavelength or a range of wavelengths. The or each pre-determined wavelength may correspond with known wavelength(s) of returned light 202 associated with the different types of light-matter interactions mentioned previously, including elastically scattering, Raman scattering, Brillouin scattering, and / or fluorescence. Other forms of returned light include other inelastic backscattered light. As illustrated in Figure 3, a multichannel receiver capable of receiving light at a various pre-determined wavelength may be used for receiving returned light 202 and generating detector signals 203. It will be appreciated that other pre-determined wavelengths or receiver channels may also be useful.

[0040] The system 100 also comprises a processor module 103 configured to calculate a time-of-flight based on the detector signals 203. Processor module 103 also determines a distance between the system and a position at which the emitted pulses of light are returned based, at least in part, on the time-of-flight. The processor module 103 then determines, at steps S104 and S105, one or more characteristics of the measuring zone 302 as a function of distance. Although illustrated as separate modules in Figure 1, it will be appreciated that the processor module 103 may be partially or wholly integrated with the receiver module 102. For example, the receiver module 102 may leverage processor module 104 to convert the received light into detector signals 203 at step S103. The processor module 104 may be a Field- Programmable Gate Array (FPGA) processing system.

[0041] The measuring zone 302 is schematically illustrated as a rectangle in Figure 1 for simplicity only. It will be appreciated that the location, size and shape of the measuring zone 302 will be determined by parameters of both the transmitter module 101 and thereceiver module 102. In particular, the direction, beam width, divergence and power of the pulses of light 201, as well as the numerical aperture and field of view of the receiver module 102 will in part determine the location, size and shape of the measuring zone 302. In some embodiments, the location, size and shape of the measuring zone 302 can be modified by changing parameters of the transmitter module 101 and / or receiver module 102.Transmitter Module

[0042] Referring now to Figure 3, there is illustrated an exemplary embodiment system 400, which is a specific implementation of system 100 described above. In the system 400, the transmitter module 101 comprises a 465 nm diode laser source 1010 configured to emit pulses of light for LiDAR excitation. The pulses of light are typically polarised or can be polarised if required, for example by utilising conventional polarisers such as a polarizing beamsplitter 1020.

[0043] In some embodiments, light from diode laser source 1010 is already polarised and this polarisation can be used directly. However, in some embodiments, such as where the light is fiber coupled, the output may become depolarised and further polarisation may be required. The beam can further be directed and / or shaped by using conventional optical components such as apertures, lenses and mirrors (e.g. 1012) and spectrally modified by filters 1015. In some embodiments, diode laser source 1010 may be fiber coupled to a length of optical fiber 1017 having a fiber connector 1019 or other fiber end to couple the laser light to free- space.

[0044] In some embodiments, the transmitter module 101 comprises a violet-blue-green diode laser source. This wavelength range is desirable due to higher transmission in seawater at these wavelengths. It will be appreciated that the optimal excitation wavelength is different for different types of water, noting that different water types will have different constituents (e.g. temperature, salinity, dissolved organic matter, phytoplankton, suspended inorganic materials), leading to different absorption and scattering characteristics. For example, coastal water attenuates least in the green, while in open ocean waters this occurs in the violet-blue range. Accordingly, it is advantageous to use different excitation wavelengths for different water types.

[0045] The modules can be designed for easy swapping of one module for another. It will also be appreciated that the optimal wavelength for elastic and Raman LiDAR in certain waterbodies may be in the green-yellow-orange range. For oceanographic Raman LiDAR applications for measuring physical properties of the waterbody (e.g. temperature and salinity), an excitation wavelength in the range of 405 nm to 530 nm is the preferable range. However, this might be extended to shorter wavelengths (e.g. down to 350 nm) for other property measurement, such as fluorescence LiDAR in specialist applications, for example to measure the presence and / or concentration of phytoplankton. The excitation wavelength may also extend up to 580 nm in some embodiments. Thus, the pulses of emitted light emitted from the transmitter module 101 may have a wavelength in the range of 350 nm to 580 nm This full range is covered by InAIGaN laser diodes. Also, there is likely other applications where slightly longer wavelengths in the green-yellow-orange range would be useful.

[0046] In some embodiments, the transmitter module 101 comprises a GaN-based diode laser source. In GaN-based laser diodes, the pulse energy and pulse repetition frequency can be optimised relatively independently of each other and are thus very well suited to photon counting oceanographic LiDAR applications. Although commercially available GaN transistors can be used to achieve the fast-switching times, chip design and packaging must be optimised to reduce inductance, for thermal control, and for other reasons.

[0047] As will be appreciated, GaN laser diode may comprise InAIGaN laser diodes. The diode devices include layers of semiconductor with varying bandgap and strain, engineering by adjusting the alloy composition between lnxAlyGai-x-yN, with the addition of various dopants such as Si and Mg to achieve the required n-type or p-type doping. In other embodiments, the transmitter module 101 may be packaged with appropriate beam collimating optics and / or one or more spectral filters to ensure a low divergence and spectrally-pure output beam is produced. It will be appreciated that the transmitter module 101 may also comprise further optical components.

[0048] In some embodiments, the transmission module 101 comprises a diode laser drive circuit. The drive circuit is employed to deliver current pulses with a laser power about 10 times higher than the continuous wave (cw) level (e.g. 45 A), with very fast switching times ( ~ Ins). By way of example, transmission module 101 may comprise a short-pulse commercial LiDAR driver board as a drive circuit, such as those manufactured by Efficient Power Conversion, USA.The LiDAR driver board includes an adjustable nanosecond pulse generator and a GaN fieldeffect transistor (FET) switching a resonant high-voltage high-current output stage that drives the laser diode.

[0049] For the wavelength range of interest for in oceanographic systems (typically violet- blue-green, as will be discussed below), generating and detecting these pulses involves tradeoffs in design. In particular, it is important to maintain high pulse energy and short pulse width whilst increasing pulse frequency. Maintaining a sufficiently high pulse energy is important so as to produce a sufficiently high signal to noise ratio. Furthermore, it is also important for Raman LiDAR systems to accurately measure the return light with high fidelity. It will be appreciated that such driver circuits may be in the form of commercially available evaluation boards, e.g. those that are conventionally used for autonomous vehicle applications. As an example, the EPC9179 Laser Driver Evaluation Board produced by Efficient Power Conversion Corporation (EPC) may be suitable.

[0050] A typical example for the oceanographic LiDAR system employs 3.5ns to 4 ns duration pulses at 1 MHz prf, with pulse energy around 10 nJ to 0.1 pJ. The wavelength of interest would depend on the characteristic of interest, but a typical application would be 430 nm to 465 nm, with 2 nm to 2.5 nm FWHM laser bandwidth. Notably, this laser bandwidth would be considered broad in typical elastic LiDAR application and would consequently give rise to large background noise, which in turn requires the bandwidth of the receiver channels to be restricted using additional components such as monochromators and interference filters. However, in Raman LiDAR applications, the width of the Raman emission features themselves are typically in the range of 8 to 15 nm under relevant excitation wavelengths. The bandwidth of the diode laser itself accordingly does not limit the precision of the measurement.

[0051] Example laser diode characteristics suitable for LiDAR excitation in the present invention are summarised in Table 1 below:Table 1

[0052] As will be appreciated, the laser diodes and associated driving electronics could also generate laser parameters that are more effective for the application (e.g. 1 ns pulses at a selectable range of prfs. Narrower spectral bandwidth (<1 nm) and higher pulse energy). These protocols could be determined during measurement to enable dynamic optimisation.

[0053] As this is a beam-based LiDAR system, it could be employed in a scanned manner to map areas or volumes of environment to understand property variation in further dimensions. As will be appreciated, there is a degree of angular information to the return signal that could in principle be used to form a rudimentary tomography system. Scanning of the transmitter module 101 may be performed by one or more electromechanical actuators (not shown).Receiver Module

[0054] As illustrated in Figure 3, in the embodiment of system 400, the receiver module 102 comprises three detectors 2023, 2024 and 2026. More generally, the receiver module 102 may comprise two or more detectors. In some embodiments, the detectors may include one or more single photon detectors. In some embodiments, at least one of the detectors comprise photomultipliers. In some embodiments, at least one of the detectors comprise silicon photomultipliers (SiPMs). In some embodiments, the outputs of the detectors may be passed to device for pre-processing such as a Teledyne ADQ14 4-channel 1 GSPS digitizer. This device may form part of the processor module described below.

[0055] In the illustrated embodiment, three different spectroscopic channels are formed via beamsplitters 2022 and / or polarising beamsplitters 2023 and detectors in the form of photomultipliers 2023, 2024 and 2026. These are combined with corresponding spectral filters 2025 to detect different wavelengths or ranges of wavelengths within the returned light at the different photomultipliers 2023, 2024 and 2026. The spectral filters may comprise any one or more of bandpass, low pass and high pass filters. These spectroscopic channels may sit behind a collection telescope 2020 and a collection mirror 2030. The angle of the collection mirror 2030 and parameters of the collection telescope 2020 define, in part the location, size and shape of the measuring zone 302. In some embodiments, the different channels are adapted for detecting different types of returned light such as elastic backscattered light, Raman backscattered light and fluoresced light.

[0056] Typically, each receiver channel has a bandwidth of around 1 to 15 nm. Advantageously, the bandwidth of each receiver channel is not required to be spectroscopically limited to reject background emission, as it might be the case in elastic LiDAR applications utilising diode lasers. As mentioned previously, in the violet-green-blue excitation regime, the Raman feature of water typically has a bandwidth that ranges from 8 to 15 nm.

[0057] Referring again to Figures 1 and 2, the detector signals 203 relate to one or more characteristics of the measuring zone 302. The distance between the system 100 and a position at which the emitted pulses of light are returned can be determined based, at least in part, on a calculated time-of-flight. At steps S104 and S105, characteristics of the aquatic environment are determined based on the detector signals 203. As illustrated in Figure 2, depth-resolved oceanographic information may then be determined by the processor 103 based on the detector signals 203. As will be appreciated, in practice, multiple signals are usually processed to extract information about depth-resolved information and / or characteristics of the aquatic environment 203.

[0058] The receiver module 102 may comprise one or more polarising elements such as polarizing beam splitter 2021 for polarising some or all of the returned light prior to detection by the one or more detectors. In some embodiments, the returned light 202 may have the same polarisation as the emitted pulses of light 201. In other embodiments, the returned light 201 may have the perpendicular polarisation to the emitted pulses of light 201. It will be appreciated that each of the parallel and perpendicular components of the returned light 201carries different information about the measuring zone. Accordingly, additional information relating to the measuring zone can be obtained by measuring both the parallel and perpendicular components.

[0059] In some embodiments, the channels in the receiver module can be interchanged to match the excitation wavelength, for example to collect a desired combination of elastic, Raman, fluorescence returns which may be polarised or unpolarised. The "swappable" modules may enable a high degree of versatility in terms of the information extracted about the water column. This swapping of channels may comprise swapping in / out different diode lasers and / or removing or adding one or more optical elements such as lenses, spectral filters and polarizing elements.

[0060] Example receiver components are listed below in Table 2.Table 2Processor Module

[0061] Referring to Figures 1 and 3, the processor module 103 is configured to determine one or more characteristics of the measuring zone 302 as a function of distance. The depth- resolved information may be derived by signal processing software and / or hardware (such as a digitizer 1030) within the processor module 103 to perform functions such as filtering, transforming, interpolation and resampling.

[0062] To determine the distance information, processor module 103 identifies photon peaks and extracts their height and arrival timing (e.g. to 1 ns precision). Noise peaks may be excluded by setting a lower height threshold, and the remaining peaks may be histogrammed by arrival time for a large number of laser shots to retrieve LiDAR traces in counts / ns versus time. The time axis is later converted to distance. The photomultiplers preferably have single photon capability so as to perform photon counting for LiDAR.

[0063] As mentioned above, the one or more characteristics of the aquatic environment may comprise one or both of temperature and salinity. In other embodiments, the one or more characteristics may comprise relative abundances and / or concentrations of specific pigments, specific phytoplankton and / or algae. It will be appreciated that phytoplankton, for example, gives rise to fluorescence. In some embodiments, the one or more characteristics of the aquatic environment comprise a presence or relative abundance of organic and inorganic particulates. In some embodiments, the one or more characteristics comprise a presence or relative abundance of pollutants or contaminants, including, for example, oil and / or gas. In some embodiments, the one or more characteristics of the aquatic environment comprise a speed of sound within the aquatic environment.

[0064] In the embodiment illustrated in Figure 3, the processor takes the form of a separate personal computer 1031 utilizing Matlab programming software 103. In general, the processor module 103 may be integral to the system (such as an integrated circuit, microprocessor or system-on-chip device) or may utilize in part a connected computer device such as a PC 1031. In some embodiments, the system may comprise an on-board processor configured to perform a basic level of processing and be connectable to an external processor (e.g. PC device) for performing higher level data processing.

[0065] Referring now to Figure 5A, there is illustrated an exemplary graph of return signals 203 versus signal return time obtained from a LiDAR measurement utilising the present invention in a 50 m swimming pool. As can be clearly observed, distinct features can be discerned from each of the elastic return signal 4012 and the Raman return signal 4013. For example, the water surface 4011 and the pool end 4014 both give rise to sharp peaks in the elastic return signal 4012. In the Raman return signal 4013, the water surface 4011 and the pool end 4014 similarly correspond with distinct but different features. Both the Raman returnsignal 4013 and the elastic return signal 4012 attenuate with time at a rate that is characteristic to the water type. Therefore, the length of the swimming pool can be extracted from the return signals based on calibration with a cuvette 4010 with a known length that is filled with the same water as the water in the pool.

[0066] Some water properties, including water depth, can be determined from a single detector signal. More typically, the extraction of water properties requires more than one signal, in which case known methods such as inversion algorithms may be used to extract various characteristics of the aquatic environment. These can be elastic, Brillouin, Raman or fluorescence returns having polarisation parallel to or perpendicular to the polarisation of the excitation light. The return signals may also be unpolarised. Inversion algorithms can involve the use of ratios, differences or normalisation of the various signals. Data from other sensors, for example in-water or above-water sensors (e.g. colour measurements from satellites) may be used incorporated in the inversion algorithm. Bio-optical models which give relationships between various water properties may also be used for the inversion. For example, extraction of temperature profiles can be accomplished by taking a ratio of Raman signals at two carefully selected wavelengths. This ratio, calibrated and instrument-corrected, provides a temperature measurement. A similar method applies to determining salinity and the speed of sound. As it will be appreciated, there are many ways in which signals can be processed and combined to extract various characteristics of interest.

[0067] As an example, turning briefly now to Figure 5B, there is illustrated a graph showing the Raman shift of water at various temperatures. As can be seen, water with a temperature of 12 degrees Celsius gives rise to a much broader and pronounced feature than that associated with water at 25 degrees Celsius and 45 degrees Celsius. As mentioned previously, it will be appreciated that the Raman signal may be polarised or unpolarised, and that the parallel and perpendicular components of the polarised Raman signal may carry different information about the aquatic environment.

[0068] Example results from an embodiment of the invention are summarised in Kitzler, O., Taylor, C. J., Li, Z., Dawes, J. M., Pask, H. M., Spence, D. J., & Downes, J. E. (2024). Photon counting marine LiDAR using blue laser diode excitation. Optics Express, 32(26), 45969-45977. https ; / / doi.org / 10.1364 / OE.S 3523. The contents of this publication are incorporated herein by way of cross-referenceInterpretation

[0069] Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "processing," "computing," "calculating," "determining", analyzing" or the like, refer to the action and / or processes of a computer or computing system, or similar electronic computing device, that manipulate and / or transform data represented as physical, such as electronic, quantities into other data similarly represented as physical quantities.

[0070] In a similar manner, the term "controller" or "processor" may refer to any device, portion of a device or plurality of devices that processes electronic data, e.g., from registers and / or memory to transform that electronic data into other electronic data that, e.g., may be stored in registers and / or memory. A "computer" or a "computing machine" or a "computing platform" may include one or more co-located or distributed processors.

[0071] Reference throughout this specification to "one embodiment", "some embodiments" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases "in one embodiment", "in some embodiments" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0072] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

[0073] In the claims below and the description herein, any of the terms "comprising", "comprised of", "which comprises" or similar are open terms that mean including at least the elements / features that follow, but not excluding others. Thus, the term "comprising" and its variations, when used in the claims or description, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A andB. Similarly, any of the terms "including", "which includes", "that includes" or similar as used herein are also open terms that also mean including at least the elements / features that follow the term, but not excluding others. Thus, "including" is synonymous with and means "comprising".

[0074] It should be appreciated that in the above description of exemplary embodiments of the disclosure, various features of the disclosure are sometimes grouped together in a single embodiment, Fig., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claims require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this disclosure.

[0075] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the disclosure, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0076] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the disclosure may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0077] Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms "coupled" and "connected", along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled"may mean that two or more elements are either in direct physical, electrical or optical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

[0078] Embodiments described herein are intended to cover any adaptations or variations of the present invention. Although the present invention has been described and explained in terms of particular exemplary embodiments, one skilled in the art will realize that additional embodiments can be readily envisioned that are within the scope of the present invention.

Claims

The claims defining the invention are as follows:

1. A LiDAR system for measuring characteristics of an aquatic environment, the system comprising: a transmitter module comprising a diode laser source configured to emit pulses of light and direct the pulses of light towards a measuring zone in the aquatic environment, wherein a portion of the emitted pulses of light is returned from the measuring zone as returned light; a receiver module comprising one or more detectors configured to receive a portion of the returned light and generate a plurality of detector signals, wherein each detector signal represents the intensity of the portion of the returned light at a predetermined wavelength; and a processor module configured to: calculate a time-of-flight based on the detector signals, determine a distance between the system and a position at which the emitted pulses of light are returned based, at least in part, on the time-of-flight, and determine one or more characteristics of the measuring zone.

2. The system of claim 1, wherein the pre-determined wavelength corresponds to the wavelength of scattered light.

3. The system of claim 2, wherein the scattered light comprises Raman scattered light.

4. The system of claim 2, wherein the scattered light comprises elastically scattered light.

5. The system of claim 2, wherein the scattered light comprises Brillouin scattered light.

6. The system of claim 1, wherein the pre-determined wavelength corresponds to the wavelength of fluoresced light.

7. The system of any one of the preceding claims, wherein the aquatic environment comprises one or more solid objects.

8. The system of any one of the preceding claims, wherein the one or more characteristics of the aquatic environment comprise one or both of temperature and salinity.

9. The system of any one of the preceding claims, wherein the one or more characteristics of the aquatic environment comprise relative abundances and / or concentrations of specific pigments, specific phytoplankton and / or algae.

10. The system of any one of the preceding claims, wherein the one or more characteristics of the aquatic environment comprise a presence or relative abundance of organic and inorganic particulates.

11. The system of any one of the preceding claims, wherein the one or more characteristics of the aquatic environment comprise a presence or relative abundance of pollutants or contaminants.

12. The system of any one of the preceding claims, wherein the one or more characteristics of the aquatic environment comprise a presence or relative abundance of one or more gases.

13. The system of any one of the preceding claims, wherein the one or more characteristics of the aquatic environment comprise a speed of sound within the aquatic environment.

14. The system of any one of the preceding claims, wherein the receiver module comprises a single-photon detector.

15. The system of any one of the preceding claims, wherein the receiver module comprises two or more detectors.

16. The system of claim 15, wherein at least one of the detectors comprise photomultipliers.

17. The system of claim 15 or claim 16, wherein at least one of the detectors comprise silicon photomultipliers (SiPM).

18. The system of any one of claims 15 to 17, wherein at least two of the detectors are configured to detect light at different wavelengths.

19. The system of any one of the preceding claims, wherein the pulses of emitted light have a wavelength in the range of 350 nm to 580 nm.

20. The system of claim 19, wherein the pulses of emitted light have a wavelength in the range of between 405 to 530 nm.

21. The system of any one of the preceding claims, wherein the diode laser source comprises a GaN-based laser.

22. The system of any one of the preceding claims, wherein the diode laser source has a pulse repetition frequency in the kilohertz (kHz) or megahertz (MHz) frequency range.

23. The system of claim 22, wherein the diode laser source has a pulse repetition frequency in the range of 0.01 kHz to 10 MHz.

24. The system of claim 22, wherein the diode laser source has a pulse repetition frequency of above 1MHz.

25. The system of any one of the preceding claims, wherein the diode laser source is wavelength-tuneable.

26. The system of any one of the preceding claims, wherein the pulses of light are in the nanosecond (ns) range.

27. The system of any one of the preceding claims, wherein the receiver module comprises one or more polarising elements for polarising the returned light prior to detection by the one or more detectors.

28. The system of any one of the preceding claims, wherein the receiver module comprises any one or more of beam collimating optics, spectral filters, beamsplitters, telescopes, polarisers, optical fibres, mirrors, lenses, apertures, diffractive optics and / or polarising beamsplitters.

29. The system of any one of the preceding claims, wherein the pulses of emitted light is scanned across an area of interest.

30. A LiDAR system for probing a medium, the system comprising:a transmitter module comprising a diode laser source configured to emit pulses of light towards the medium, wherein the diode laser source emits light in the violet- blue-green wavelength range; a receiver module configured to receive returned light, wherein the returned light is returned from interactions between the pulses of light and the medium; and a processor module configured to: determine a distance between the system and a position at which the light is returned at least in part from time-of-flight measurements; and extract information about the medium from the returned light as a function of distance.

31. The system of claim 30 wherein the medium comprises an aquatic environment.

32. The system of claim 30 or claim 31 wherein the interactions between the pulses of light and the medium comprise Raman scattering.

33. A method for measuring characteristics of an aquatic environment, the method comprising: configuring a diode laser source to emit pulses of light and direct the pulses of light towards a measuring zone in the aquatic environment, wherein a portion of the emitted pulses of light is returned from the measuring zone as returned light; configuring a receiver module to receive a portion of the returned light and generate a plurality of detector signals, wherein each detector signal represents the intensity of the portion of the returned light at a pre-determined wavelength; and configuring a processor module to: calculate a time-of-flight based on the detector signals, determine a distance between the system and a position at which the emitted pulses of light are returned based, at least in part, on the time-of-flight, and determine one or more characteristics of the measuring zone.

4. A method for probing a medium, the method comprising: configuring a diode laser source to emit pulses of light towards the medium, wherein the diode laser source emits light in the blue-green wavelength range; configuring a receiver module to receive returned light, wherein the returned light is returned from interactions between the pulses of light and the medium; and configuring a processor module to: determine a distance between the system and a position at which the light is returned at least in part from time-of-flight measurements; and extract information about the medium from the returned light as a function of distance.

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