RECEIVERS

TECHNICAL FIELD

[0001] The present disclosure generally relates to the field of optical signal collection and detection. Disclosed embodiments relate to a system and method for signal collection in a spatial estimation system.

BACKGROUND

[0002] Spatial profiling refers to the mapping of an environment as viewed from a desired origin point. Spatial profiles may be useful in identifying objects and/or obstacles in the environment, thereby facilitating automation of tasks.

[0003] One technique of spatial profiling involves sending light into an environment in a specific direction and detecting any light reflected back from that direction, for example, by a reflecting surface in the environment. The reflected light carries relevant information for determining the distance to the reflecting surface. The combination of the specific direction and the distance forms a point or pixel in the representation of the environment. The above steps may be repeated for multiple different directions to form other points or pixels of the representation, thereby facilitating estimation of the spatial profile of the environment within a desired field of view.

[0004] As one example optical signal collection and detection method, a direct detection (DD) method may be employed at a receiver side using avalanche photodiodes (APDs). As the name suggests, collected optical signals, including the intended optical signal at frequency f _c , are directly detected by the APD and converted to an electrical signal for further signal processing.

[0005] Alternatively, homodyne detection may be employed as the optical detection method, in which the collected signal is combined with a local oscillator signal. The combined optical signal may be subsequently detected by a PIN photodiode (PIN PD) and converted to an electrical signal for further signal processing. When the optical frequency of the local oscillator (/LO) is the same as/ _c (i.c. / =fc), the detection method is acknowledged as homodyne detection (HD). When the optical frequency of the local oscillator is different from/e (i.e.fLo *jc), the detection method is acknowledged as heterodyne detection.

SUMMARY

[0006] According to a first aspect of the present disclosure there is provided a spatial estimation system having an operating range including a maximum operating range, the spatial estimation system configured to: provide to an environment first outgoing light with a first reception window ending at a time based on the maximum operating range; provide to the environment second outgoing light with a second reception window ending at a time based on the maximum operating range; provide, during the first reception window, a first local oscillator signal to a homodyne receiver architecture based on or corresponding to the first outgoing light; and provide, during the first reception window, a second local oscillator signal to the homodyne receiver architecture based on or corresponding to the second outgoing light. Wherein: the first reception window and the second reception window overlap in time; and there is a difference in at least one of optical frequency, subcarrier modulation or code modulation between the first outgoing light and the second outgoing light; and wherein the spatial estimation system is further configured to: receive, during the first reception window, a first incoming light signal that comprises reflected first outgoing light and/or a second incoming light signal that comprises reflected second outgoing light; and disambiguate the first incoming light signal and the second incoming light signal based on the difference in at least one of the frequency, subcarrier modulation and code modulation of the first outgoing light and the second outgoing light.

[0007] According to another aspect of the present disclosure, there is provided an optical system for spatial estimation, the system including: a light emitter operatively connected to optical components, configured or collectively configured to provide: optical signals for spatial estimation, the optical signals including: a first signal portion having a first wavelength; and a second signal portion, subsequent to the first portion, having a second wavelength, different from the first wavelength; and at least one local oscillator (LO) signal based on the optical signals, the at least one local oscillator signal including a first LO portion corresponding to the first signal portion and a second LO portion corresponding to the second signal portion; at least one beam director, for receiving outgoing light and directing the outgoing light over free space into an environment remote from the beam director, the outgoing light being provided based on the optical signals, the outgoing light including a first outgoing portion corresponding to the first signal portion and a second outgoing portion corresponding to the second signal portion, the beam director configured to direct the outgoing light in one or more of multiple directions; components to receive reflected light from at least one of the multiple directions, the reflected light including reflected optical signals having at least a first reflection portion corresponding to the first outgoing portion, the components including: at least one optical combiner to combine said local oscillator signal and the reflected light, to provide combined light; at least one optical detector arranged to receive the combined light and provide, based on optical mixing between the first reflection portion and the second LO portion, electrical signals for processing into a spatial estimation of the remote environment.

[0008] According to yet another aspect of the present disclosure, there is provided an optical system for spatial estimation, the system including: a plurality of light emitters operatively connected to optical components, configured or collectively configured to provide: optical signals for spatial estimation, the optical signals including: a first signal portion having a first wavelength; and a second signal portion having a second wavelength that is the same as or different from the first wavelength, the second signal portion being provided simultaneously with the first signal portion; a plurality of local oscillator signals sampled from the optical signals, each local oscillator signal having a first LO portion corresponding to the first signal portion and a second LO portion corresponding to the second signal portion; at least one beam director, for receiving and directing outgoing light over free space into an environment remote from the beam director, the outgoing light being provided based on the optical signals, the outgoing light including a first outgoing portion corresponding to the first signal portion and a second outgoing portion corresponding to the second signal portion, the beam director configured to direct the outgoing light in one or more of multiple directions; components to receive reflected light from at least the one or more of the multiple directions, the reflected light including reflected signals having at least one of a first reflection portion corresponding to the first outgoing portion and a second reflection portion corresponding to the second outgoing portion, the components including: a plurality of optical combiners, each optical combiner to combine a corresponding local oscillator signal and the reflected light, to provide combined light; a plurality of optical detector, each optical detector arranged to receive the combined light from a respective optical combiner and provide, based on optical mixing between either or both of the first and second reflection portions and both of the first and second LO portions, electrical signals for processing into a spatial estimation of the remote environment.

[0009] Further aspects of the present disclosure and embodiments of the aspects summarised in the immediately preceding paragraphs will be apparent from the following detailed description and from the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Fig. 1A illustrates an arrangement of a spatial profiling system with direct detection (DD).

[0011] Figs. IB and 1C illustrate arrangements of a spatial profiling system with homodyne detection (HD) according to some aspects of the present disclosure.

[0012] Fig. 2 illustrates a timing diagram of the spatial profiling system of Figs. IB and 1C.

[0013] Fig. 3 illustrates a timing diagram of a spatial profiling system according to some aspects of the present disclosure.

[0014] Fig. 4 illustrates another arrangement of a spatial profiling system according to some aspects of the present disclosure.

[0015] Fig. 5 illustrates a timing diagram of the spatial profiling system of Fig. 3.

[0016] Fig. 6 illustrates another arrangement of a spatial profiling system according to some aspects of the present disclosure.

[0017] Fig. 7 illustrates a timing diagram of the spatial profiling system of Fig. 6.

[0018] Fig. 8 illustrates another arrangement of a spatial profiling system according to some aspects of the present disclosure.

[0019] Fig. 9 illustrates a timing diagram of the spatial profiling system of Fig. 8.

[0020] Fig. 10 illustrates another arrangement of a spatial profiling system according to some aspects of the present disclosure. DETAILED DESCRIPTION

[0021] Disclosed herein are optical signal collection and detection methods and apparatus for performing optical signal collection and detection. At least certain embodiments of the method and apparatus may be part of or used for a method or system for facilitating estimation of a spatial profile of an environment, based on a light detection and ranging (LiDAR) based technique. “Light” hereinafter includes electromagnetic radiation having optical frequencies, including infrared radiation, visible radiation and ultraviolet radiation. In this specification, “intensity” means optical intensity and, unless otherwise stated, is interchangeable with “optical power”.

[0022] In general, LiDAR involves transmitting light into the environment and subsequently detecting reflected light returned by the environment. By determining the time it takes for the light to make a round trip, the distance of surfaces within a field of view can be determined and an estimation of the spatial profile of the environment may be formed. In one arrangement, the present disclosure facilitates spatial profile estimation based on directing light over one dimension, such as along the vertical direction. In another arrangement, by further directing the one-dimensionally directed light in another dimension, such as along the horizontal direction, the present disclosure facilitates spatial profile estimation based on directing light in two dimensions. The distance to surfaces represents a third dimension in a three-dimensional environment in which a LiDAR system typically operates.

[0023] Fig. 1A illustrates an example arrangement of a spatial profiling system 100A with direct detection (DD). The system 100A includes an optical source 102, a beam director

103, an optical receiver 104 and a processing unit 105. In the arrangement of Fig. 1A, light from the optical source 102 is directed by the beam director 103 in a direction across one or two dimensions into an environment 110 having a spatial profile. In many applications, for example LiDAR, the spatial profile includes a variable depth dimension, transverse to the one or two dimensions. If the outgoing light hits an object or a reflecting surface, at least part of the outgoing light may be reflected (represented in the solid arrows), e.g. scattered, by the object or reflecting surface back to the beam director 103 and received at the optical receiver

104. As the name of “direct detection” suggests, the intensity of the reflected signal is directly detected by the optical receiver that includes a light detector, which may include an avalanche photodiode (APD), and is converted to an electrical signal for further signal processing. The processing unit 105 is operatively coupled to the optical receiver 104 for controlling its operations, to generate a measure of the received signal indicative of the output power received by the light detector, and for performing relevant signal processing including but not limited to determining the distance to the reflecting surface, by determining the round-trip time for the reflected light to return to the beam director 103. The processing unit 105 is also operatively coupled to the optical source 102 for controlling its operations.

[0024] The system with DD may employ multimode fibres and APDs to collect and detect the reflected light. The multimode fibres can collect more signal compared to single mode fibres as they are less sensitive to spatial coherence and commonly have a higher numerical aperture. The APDs have better receiver sensitivity compared to PIN PD, due to possessing a gain stage which amplifies the signal current via avalanche multiplication. However, such an approach may be susceptible to interference from unintended light, such as ambient light, because multimode fibres are prone to collect stray light and APDs have a relatively broad detection bandwidth.

[0025] This issue may be mitigated by employing homodyne detection (HD), which enables a frequency sensitive optical gain to make the LiDAR system more immune to unintended optical sources, such as ambient light and light from other LiDAR systems. As HD depends on the temporal and spatial coherence of the reflected signal, a single mode fibre may be used in place of the multimode fibre, making the system less susceptible to stray light. As HD does not directly measure the intensity of the return signal but measures instead a small AC signal upon a large DC background, a photodiode (PD) may be used in place of the APD for its larger dynamic range and linearity. Furthermore, the measured homodyne signal depends on the square root of the reflected signal power, further improving the dynamic range of the system. Example homodyne receive architectures and associated methods are described in international patent publication WO 2021/000026 Al, the entire content of which is incorporated herein by reference.

[0026] Fig. IB illustrates an example arrangement of a spatial profiling system 100B with HD. In the arrangement of Fig. IB, light from the optical source 102 is also provided to the optical receiver 104. For example, the light from the optical source 102 may first enter a sampler 106A (e.g. an optical coupler or an optical splitter), where a first portion of the light is provided to the beam director 103 as outgoing light for environment sensing and a second portion (e.g. the remaining sample portion) of the light is provided as a local oscillator (LO) signal. The second portion of the light is combined with the reflected light via an optical combiner 106B (e.g. an optical coupler). The combined light is then provided to the optical receiver 104 which detects an optically mixed signal, including any beat signal, of the combined light. The optical receiver 104 includes a light detector for detecting one or more optical inputs (e.g. the combined light) and producing one or more corresponding electrical outputs (e.g. the optically mixed signal, including any beat signal) as the detected signal. The light detector may, for example, be in the form of a PD (not an APD) for single ended detection or be in the form of two PDs for balanced detection.

[0027] In another example, the light from the optical source 102 may first enter an input port of an optical switch and exit from one of two output ports, where one output port directs the light to the beam director 103 and the other output port re-directs the light to the optical combiner 106B at a time determined by the processing unit 105. At least one optical delay (not shown) may be applied to synchronise the local oscillator signal and the reflected light at the optical combiner 106B.

[0028] When light from the optical source 102 is directed to both the beam director 103 and the optical receiver 104, the proportion to the beam director will typically be much smaller, for example 10% or less. An optical amplification stage can amplify the portion to the beam director, to provide sufficient power of output light for spatial profiling.

[0029] In another arrangement of a spatial profiling system 100C with homodyne detection as shown in Fig. 1C, the LO signal may be provided by an optical source 102C separate from the optical source 102. The LO signal from the optical source 102C is provided to the optical combiner 106B and is operatively controlled by the processing unit 105. In one example, the optical source 102C may be controlled to emit light with wavelength channels at the same centre frequency as that of the wavelength channels light emitted from the optical source 102. The processing unit 105 may also control the output power and operation time of the optical source 102C. The optical combiner 106B combines the LO signal and the reflected signal and outputs the combined optical signal to the optical receiver 104.

[0030] In some embodiments, the spatial profiling system includes a modulator 107 for modulating the transmitted optical signals via their intensity, phase or frequency. The modulation may be code modulation based on a coded signal generated by an encoder module. The modulator 107 is placed downstream from the sampler. This way, unmodulated emission from the optical source 102 is provided to the optical receiver 104 as an LO signal. In these embodiments, the processing unit 105 is also configured to correlate the detected signals with the coded signals to calculate a lag between the return optical signal and the transmitted optical signal. In one example, correlation includes determining the crossproducts between the detected signal and time-delayed versions of the coded signal. In another example, correlation includes determining the cross-products between the coded signal and time-delayed versions of the detected signal. The processing unit 105 may in some embodiments be configured to recover the envelope of the detected signal. For example, the processing unit 105 includes an envelope detector, a numerical demodulator or a frequency downconverter, to detect and/or remove the beat frequency in the detected signal prior to determining the correlation. The determined correlation as a function of time delay produces a peak value at a particular time delay. This particular time delay or lag at which the peak correlation value occurs represents the round-trip transit time of the light to the target and back. From this, the distance to the target that reflected the optical signal can be determined.

[0031] A timing and power spectral density diagram of an example form of the homodyne detection system depicted in Figs. IB and 1C is shown in Fig. 2. This form of homodyne detection system transmits light at different centre optical frequencies (or wavelengths). The beam director 103 may direct the light across at least one dimension based on the frequency of the light. In particular, Fig. 2A shows reflected light received at the receiver 104 from three different transmitted light signals, each at a different optical frequency, received within time windows 0-Ti, T1-T2, and T2-T3, respectively. Each of these time windows spans an acquisition period, which is the maximum allowable round-trip transit time for the system to correctly determine the time taken for transmitting an optical signal and receiving the same optical signal back at the receiver, once the signal has been reflected by the environment 110. It will be appreciated that some transmitted optical signals may be received sooner (e.g., transmitted signals that are reflected by objects close to the transmitter) and some transmitted optical signals may be received later (e.g., transmitted signals that are reflected by objects distant from the transmitter). The acquisition period is typically set based on the range of the system 100. For example, if the range of the system is 200 meters, the acquisition period needs to be long enough to receive a reflected optical signal from an object 200 meters away (i.e., long enough for the optical signal to travel 400 meters).

Similarly, if the range of the system is 1000 meters, the acquisition period is long enough to receive a reflected optical signal from an object 1000 meters away (i.e., long enough for the optical signal to travel 2000 meters). [0032] Fig. 2B shows the LO signal provided to the optical receiver 104 (e.g. via the optical combiner 106B) for optical mixing with reflected light. Optical mixing occurs where optical signals of the same or similar optical frequencies interfere with one another. The interference produces an optically mixed signal with characteristics (such as amplitude and/or any beat frequency) detectable in the electrical domain. As seen in this figure, the light signal from the LO remains constant throughout an acquisition period and changes for different acquisition windows. For example, if the three transmitted signals shown in Fig. 2 in this example have three different wavelengths, then for the first acquisition window (0-Ti), the LO signal is maintained at the first wavelength and then for the second acquisition window (T1-T2) the LO signal is maintained at the second wavelength, and for the third acquisition window (T2-T3) the LO signal is maintained at a third wavelength and so on.

[0033] Fig. 2C depicts the three reflected signals upon detection correlated with the coded signal relating to their corresponding transmitted signals. The correlation shows a peak value over time-delay occurring at a time representative of the lag (on the x-axis) between the reflected signals and their corresponding reference signals. Fig. 2D shows the power spectral density of the three corresponding received signals in the frequency domain.

[0034] In the homodyne systems shown in Figs. IB and 1C, the optical receiver 104 mixes the LO and reflected signals only when both these signals are present. Thus, in such homodyne systems the local oscillator signal remains at the same wavelength for the entire duration of an acquisition period AT, for example in each of acquisition windows 0-T1, Tl- T2, or T2-T3. Accordingly, such homodyne systems can have a maximum point per second (PPS) (i.e., number of measured values that can be measured and outputted in one second) of equation 1.

[0035] (equation 1).

[0036] For example, if AT= 2ps, the maximum points -per- second is 500,000 points per second. This assumes a non-overlapping acquisition windows for all points. The inventors of the present disclosure recognise that although homodyne techniques offer benefits to LIDAR sensors, the requirement of having the LO signal incident on the receiver 104 for the entire acquisition period AT is a limitation on the overall points per second throughput.

[0037] Having identified this deficiency, the inventors have devised several arrangements of homodyne detection systems that provide useful alternatives. Embodiments of the present disclosure may allow high scanning speeds and a larger number of points to be measured in one second (i.e., a higher PPS) than embodiments of the homodyne system described above with respect to Figs. IB and 1C. The disclosed embodiments of the homodyne detection systems may therefore support high-resolution LiDAR applications with high scanning speeds and improved received signal quality and dynamic range.

[0038] In general and by way of example, the arrangements modify the homodyne detection systems described above to facilitate embodiments with increased PPS.

[0039] While the description hereinafter refers to systems using optical fibres (such as single mode fibres and multimode fibres), it will be appreciated that the disclosed embodiments are equally applicable, with minor modifications, to systems using optical waveguides (such as single mode waveguides and multimode waveguides).

[0040] In a first embodiment, aspects of the present disclosure implement a system similar to that depicted in Figs. IB and 1C. The optical source 102 may be configured to provide outgoing light at one or more wavelength channels. The optical source 102 may include a light emitter (not shown). For example, the light emitter may be a wavelength- tunable laser of substantially continuous-wave (CW) light intensity, such as a wavelength- tunable laser diode, providing light of a tunable wavelength based on one or more electrical currents (e.g. the injection current into one or more wavelength tuning elements in the laser cavity) applied to the laser diode, to produce a wavelength channel with a centre frequency (fc) . In another example, the light emitter may include a broadband laser source and a tunable spectral filter to provide substantially continuous-wave (CW) light intensity at the selected wavelength.

[0041] The optical source 102 is controlled, for example by the processing unit 105 to selectively provide the outgoing light at one or more wavelength channels. For example the optical source 102 is controlled to provide outgoing light at a first set of one or more wavelength channels for a duration of time and then provide outgoing light at a second set of one or more wavelength channels, different from the first set for the same or different duration of time. In some embodiments the first set and the second set are mutually exclusive.

[0042] Where the wavelength of the optical source is adjusted from the first set or the second set, the homodyne detection technique acts as a time-synchronised spectral filter to facilitate detection of light at the adjusted wavelength(s). In some embodiments, the wavelength adjustment may improve performance such that the described system is more immune to stray light. In some embodiments, wavelength adjustment may improve security such that the described system is more immune to malicious attacks. In some embodiments, wavelength adjustment may facilitate beam direction, such that the described system directs beam based on wavelength. In some examples, such as semiconductor lasers whose emission wavelength is tunable based on carrier effects, the optical source may be wavelength-tunable from the first set to the second set within 5 ms, such as under 500 ps, under 50 ps, under 5 ps or under 0.5 ps. The optical source may be wavelength-tunable within a maximum range of 40 nm, and at a tuning speed within 8 nm/ms, such as under 80 nm/ms, under 800 nm/ms, under 8 nm/ps, or under 80 nm/ps. The light emitter 122 may be temporally incoherent (i.e. include temporally phase noise).

[0043] The optical source 102 may also include at least one depolariser (not shown) to depolarise the outgoing light and output randomly (or in a pseudo-random order) polarised outgoing light (i.e. unpolarised outgoing light). The at least one depolariser may not require any power source, in other words the at least one depolariser may be a passive depolariser.

[0044] As discussed above, in the arrangement of Fig. IB or 1C, an LO signal is provided to the optical receiver 104 for homodyne detection. For example, the emitted light from the emitter may first enter a sampler 106A (e.g. an optical coupler or an optical splitter), where a portion of the outgoing light is provided to an optical combiner 106B, which then provides the portion of the outgoing light to the optical receiver 104 as a LO signal. The arrangement may include at least one optical delay (e.g. before one port or both ports of the optical combiner 106B) to appropriately synchronise the LO signal and the reflected signal at the optical receiver 104.

[0045] As discussed above, the spatial profiling system may include a modulator 107 for modulating the transmitted optical signals via their intensity, phase or frequency. The modulation may be code modulation based on a coded signal generated by an encoder module. The modulator 107 is placed, in the case of Fig. IB, between the sampler 106A and the beam director 103, or in the case of Fig. 1C, between the optical source 102 and the beam director 103. This way, unmodulated emission from the light emitter 102 is provided to the optical receiver 104 as an LO signal. The processing unit 105 is also configured to correlate the detected signals with the coded signals to calculate a lag between the return optical signal and the transmitted optical signal.

[0046] In another example (not shown), the unpolarised light may first enter an input port of an optical switch and exit from one of two output ports, where one output port directs the light to the beam director 103 and the other output port re-directs the light to the optical combiner 106B at a time determined by a processing unit 105 as the LO signal. At least one optical delay (not shown) may be applied to synchronise the local oscillator signal and the reflected light at the optical receiver 104.

[0047] In yet another example (not shown), the LO signal may be provided by a separate LO signal source controlled by the processing unit 105. The separate LO signal source includes an optical source other than the optical source 102 operating at the same or substantially the same centre wavelength as the optical source 102 and may also include a depolariser. The processing unit 105 may control the centre wavelength and power of the emitted light from the separate LO signal source. The processing unit 105 may also control the operational time and duration of the separate LO signal source.

[0048] Although the hardware arrangement may be similar to that shown in Figs. IB and 1C, according to some aspects of the present disclosure, instead of maintaining the LO signal on the optical receiver 104 for the entire acquisition period (AT), the system is configured to provide a first LO signal incident on the optical receiver 104 for a first period shorter than the complete acquisition period, and subsequently provide a second, different LO signal incident on the optical receiver 104 for a second period shorter than the complete acquisition period. For example, the first LO signal may be incident on the optical receiver 104 for the first half

T of the acquisition period, (e.g.,0 to - ) and the second LO signal may be incident on the T optical receiver 104 for the second half of the acquisition period, (e.g., y to 7T). In another example, three different LO signals (first, second and third LO signals) may be sequentially provided, each for one-third of the acquisition period. In yet another example, N different LO signals (first, second, . . . Nth LO signals) may be sequentially provided, each for 1/N of the acquisition period. It should be appreciated that, in some examples, the N different LO signals need not be each provided for an equally divided duration during the acquisition period.

[0049] After each shortened period, the LO signal switches to the next wavelength signal, corresponding to the next set of optical signals transmitted by the emitter. Although the LO signal switches earlier in this embodiment, the detection system may still receive reflected signals corresponding to a given set of transmitted optical signals from the environment in the entire acquisition period (AT) associated with that set of transmitted signals. For example, the receiver 104 may still receive signals reflected by objects distant from the system 100B or 100C after the LO signal has already switched to the next or further wavelength signal. The processing unit 105 may still process these later received reflected signals.

[0050] Accordingly, some reflected signals may optically mix with the current LO optical signal (e.g., where for some reflected signals f _c =./i.o) at the optical receiver 104 while other reflected signals may optically mix with the next or a different LO optical signal (e.g., for such reflected signals, f _c ) at the optical receiver 104. In effect, consecutive acquisition windows are staggered or partially overlapped. In other words, each subsequent acquisition window commences before the preceding acquisition window ends.

[0051] A timing diagram for this example embodiment is depicted in Fig. 3. In particular, Fig. 3A shows reflected light received at the receiver 104 from three different sets of transmitted optical signals received across three respective acquisition windows 0-Ti, Ti/2 - 3Ti/2 and T1-T2. In particular, in the example scenario shown in Fig. 3 A, the reflected signals corresponding to two sets of optical signals transmitted at first and second wavelengths are received in a time overlap manner within the time window Ti/2-Ti, whereas a third set of reflected signals (corresponding to a third set of optical signals transmitted at a third wavelength) are received in the acquisition window T1-T2. Of the first and second sets of reflected signals, one set corresponds to acquisition window 0-Ti and another set corresponds to acquisition window Ti/2 - 3T1/2.

[0052] Fig. 3B shows the LO signal received at the optical receiver 104 for optical mixing with reflected light. As seen in this figure, the LO signal remains constant for half of an acquisition period and changes every half acquisition period. For example, if the three transmitted signals in this example have three different wavelengths i, 2, 3 then for the first half of a first acquisition window (0-Ti), the LO signal is maintained at the first wavelength. Then for the first half of a second acquisition window (Ti/2 - 3T1/2), the LO signal is maintained at the second wavelength. Then during the first half of a third acquisition window (T1-T2), the LO maintains the third signal at the third wavelength.

[0053] Fig. 3C shows the correlation between the detected signals and the coded signal relating to the corresponding transmitted signals and Fig. 3D shows the power spectral density of the three corresponding received signals in the frequency domain. Although for simplicity Figs. 3A to 3D show only various signals for one and a half acquisition periods, a skilled person would appreciate that the described system can be configured such that the LO signal changes every half acquisition period and remains constant for half of an acquisition period.

[0054] As shown in Fig. 3, in one and a half acquisition periods (1.5 AT) three sets of reflected signals are detected and processed. Accordingly, in this scenario, the maximum PPS is shown in equation 2.

[0055] PPS = 2/ AT (equation 2).

[0056] Thus, by reducing the period of time for which the LO signal maintains a given optical signal, the PPS can be increased. For example, by halving the period of time for which the LO signal maintains a given optical signal, the PPS can be doubled using the same optoelectronic hardware as that shown in Fig. IB and 1C. In this case, the optical delay may be applied before the optical combiner 106B to appropriately synchronise the LO signal with a given time period, which does not cover the entire time period required to receive the reflected signal at the optical combiner 106B.

[0057] The overlap in reflected signals corresponding to two different sets of transmitted signals (shown in Fig. 3A) may be referred to as range ambiguity. Generally speaking, LIDAR systems have a maximum detection range at which a target can be located such that a transmitted pulse can travel to the target, scatter or reflect from the target, and return to the detector while the detector is still arranged to time the transmitted pulse (i.e., during a particular acquisition window). However, in the presently described embodiment, range ambiguity occurs when reflected signals corresponding to a set of signals transmitted by the transmitter at a previous time are received when the LO signal has already switched to allow the detector to detect reflected signals corresponding to the next set of signals transmitted by the transmitter. Fig. 3A illustrates an example of this when reflected signals 302A and 302B overlap. Even if there is only one reflected signal present during the half acquisition window Ti/2-Ti, e.g. signal 302A is present and signal 302B is absent, range ambiguity occurs because the present reflected signal could correspond to either acquisition window 0-Ti or acquisition window Ti/2 - 3T1/2.

[0058] In this case, the reflected signal 302A (which corresponds to a first set of transmitted optical signals) is detected and appears to the system in a reception time window that overlaps with a time window also used for detecting reflected signal 302B (which corresponds to a second set of transmitted signals). In such cases, in order to determine the range at each of the points corresponding to the reflected signals 302A, 302B, the receiver needs to disambiguate the signals. In some embodiments, to overcome this issue, the configuration of the transmitter to transmit at different wavelengths is utilised. This way, when the detector receives the reflected signals 302A and 302B, the difference in wavelengths of the received signals 302A and 302B is used by the system to disambiguate between overlapping received signals 302A and 302B.

[0059] For example, as shown in Fig. 3, the electrical output 304A of optical receiver 104 due to reflected signal 302A during the second half of the first acquisition window (To- Ti) contains an underlying frequency 306. The underlying frequency is the beat frequency that corresponds to the difference in optical frequency between the reflected signal 302A (at the first wavelength) and the second LO signal (at the second wavelength). Further, the electrical output 304A of optical receiver 104 has an envelope 308 that varies over time in correspondence with the intensity of reflected signal 302A. In comparison, the lack of any difference in optical frequency between the reflected signal 302B (at the second wavelength) and the second LO signal (also at the second wavelength) during the first half of the second acquisition window causes no beat frequency in the electrical output 304B due to reflected signal 302B. Further, the amplitude 310 of the electrical output 304B varies over time in correspondence with the intensity of reflected signal 302B. In other words, while the received signals may be within overlapping time windows, they result in electrical output signals of different underlying frequencies.

[0060] The processing unit 105 may be configured to determine, at least when there is time window ambiguity, the acquisition window to which a reflected signal corresponds, based on the underlying frequency (e.g. its presence/absence and/or its value) of the electrical output signal produced by the optical receiver 104. Based on the determined acquisition window to which a reflected signal corresponds, the processing unit 105 is configured to determine its round trip time. For example, the processing unit 105 may account for the lag based on the transmission time of the determined acquisition window. Further, the processing unit 105 may determine a correlation between the electrical output and the coded signal relating to the corresponding transmitted signals. The processing unit 105 may in some embodiments be configured to recover the envelope, for example, by an envelope detector, a numerical demodulator or a frequency downconverter, to detect and/or remove the beat frequency prior to determining the correlation. The signals may be separated in the frequency domain by the difference in the wavelengths of the two signals. [0061] In one example, the transmitter may adopt two different wavelengths such that consecutive sets of transmitted signals have different wavelengths. Alternatively, the transmitter may adopt three or more different wavelengths (e.g., 3 as shown in Fig. 3A-3D) in a cyclic manner. Consecutive transmissions utilise adjacent wavelength channels, for example where the field of view is scanned based on wavelength in a raster arrangement or in other embodiments non-adjacent wavelength channels are used for consecutive transmissions. In some embodiments the wavelengths are selected on a quasi-ransom basis or selected according to a code, for example for security purposes.

[0062] The optical receiver 104 is selected to have an electrical bandwidth (e.g. the 3 dB band) that is approximately equal to, or higher than, the largest difference in optical frequency among the sets of transmitted signals within an acquisition period. Where the transmitter adopts two different wavelengths within an acquisition period, the optical receiver 104 is selected to have an electrical bandwidth that is approximately equal to, or higher than, the difference in optical frequency between the two consecutive sets of transmitted signals. For example, if the consecutive sets of transmitted signals are 20 GHz apart in optical frequency (or 0.16 nm apart in wavelength at 1550 nm), an optical receiver with an electrical bandwidth of approximately 20 GHz or more is selected. A sufficiently high electrical bandwidth facilitates detection of any beat frequency between a received signal and the LO signal, disambiguating signals received within overlapped acquisition windows. A sufficiently electrical high bandwidth also facilitates recovery of the envelope for correlation with the coded signal.

[0063] In another example, to distinguish between two sets of reflected signals, the transmitted optical signals may be distinguishably encoded (e.g., using different orthogonal codes). For example a code modulation (e.g. amplitude, phase, frequency) may be imposed on each of the transmitted optical signals, with a different code used for acquisition different windows, or for each of the different wavelengths, or between at least one or more pairs of wavelength channels where disambiguation functionality is required. At the receiver, the reflected signals are decoded to determine which set of transmitted signals the reflected signals correspond to. For example, a first set of transmitted signals may be encoded using a first code and a second set of transmitted signals may be encoded using a second code. At the receiver, the reflected signals are decoded. If a first code successfully decodes the reflected signal, the receiver determines that the reflected signal corresponds to the first set of transmitted signals. Alternatively, if a second code successfully decodes the reflected signal, the receiver determines that the reflected signal corresponds to the second set of transmitted signals.

[0064] In order to enable this, the spatial profiling system includes an encoder module that is configured to encode the transmitted signals and the receiver includes a decoder module that is configured to decode the received reflected signals. The encoder and decoder may be supplied with the same set of codes and may be synchronized such that the decoder knows which codes were used to encode the transmitted signals. The use of encoded transmitted optical signals may be used instead of or in addition to a determination of the frequency of the returned signal for disambiguation.

[0065] In one example, the encoder module includes a semiconductor optical amplifier (SO A) integrated on a laser diode of the transmitter. The electrical current applied to the SOA may be varied over time to vary the amplification of the CW light produced by the laser over time, which in turn provide outgoing light with a time-varying intensity profile. In another example, the encoder module includes an external modulator (such as a Mach Zehnder modulator or an external SOA modulator) to the laser diode. The decoder module may include a correlator that determines the correlation between the coded signal provided to the encoder and the detected return signal. In another arrangement, instead of having a wavelength-tunable laser, the light source includes a broadband laser followed by a wavelength-tunable filter and the filter also provides the modulation.

[0066] The processing unit 105 matches the encoding of the received signal, as detected and communicated to it by the optical receiver 104. The processing unit 105 may also control the encoding of the transmitted signals. Processing units 105 in different spatial estimation systems may use different encoding, which may assist to reduce interference between spatial estimation systems operating in proximity to each other.

[0067] Fig. 4 illustrates another example system 400 according to aspects of the present disclosure. In this arrangement, the system includes a plurality of optical sources 402 (two shown in this example, as optical sources 402A and 402B), an optical coupler 404, and a plurality of optical receivers 410 (two shown in this example, as optical receivers 410A and 410B).

[0068] The optical sources 402 A, 402B are provided to the beam director 103, which directs the optical signals across one or two dimensions into an environment 110 having a spatial profile. The output optical signals may be provided to the beam director via a common optical fibre.

[0069] Optical signals sampled from the optical sources 402A and 402B are also provided to the optical receiver 410. For example, the optical signal from optical source 402A may first enter a sampler 406A (e.g. an optical coupler or an optical splitter), where a first portion of the light is provided to a modulator 409A a second portion (e.g. the remaining sample portion) of the light is provided to an optical coupler 404A (e.g. a 2x2 coupler). Similarly, the optical signal from optical source 402B may first enter a sampler 406B (e.g. an another optical coupler or an optical splitter), where a first portion of the optical signal is provided to a modulator 409B and a second portion (e.g. the remaining sample portion) of the optical signal is provided to the optical coupler 404A.

[0070] The optical coupler 404A combines the two sampled optical signals from optical sources 402A and 402B and provide a first local oscillator (LO) signal 405A and a second local oscillator (LO) signal 405B .

[0071] The modulators 409A, 409B modulate the transmitted optical signals via their intensity, phase or frequency. The modulation may be code modulation based on a coded signal generated by an encoder module. The modulators 409A and 409B are placed between each of the samplers 406A and 406B and the beam director 103. This way, unmodulated light is provided to the optical receivers 410 as LO signals.

[0072] The processing unit 105 is also configured to correlate the reflected signals with the coded signals to calculate a lag between the return optical signal and the transmitted optical signal. As indicated above, the light output to the beam director 103 from the one or more modulators may be directed into a common optical fibre via an optical coupler 404B for communication to the beam director 103.

[0073] The reflected light 407 A, 407B may be generated by a two output port optical circulator or by splitting return light from the beam director 103. The first LO signal 405A is combined with reflected light 407A via an optical combiner 408A (e.g., an optical coupler). The combined light is then provided to a first optical receiver 410A, which detects an optically mixed signal, including any beat signal, of the combined light. Similarly, the second LO signal 405B is combined with reflected light 407B via an optical combiner 408B (e.g. an optical coupler). The combined light is then provided to a second optical receiver 410B which detects an optically mixed signal, including any beat signal, of the combined light. The optical receivers 410A and 41 OB include a light detector for detecting one or more optical inputs (e.g. the combined light) and producing one or more corresponding electrical outputs (e.g. the optically mixed signal, including any beat signal) as the detected signal. The light detector may, for example, be in the form of a PD for single ended detection or be in the form of two PDs for balanced detection.

[0074] As the optical signal transmitted by the optical sources 402A, 402B are combined by the optical coupler 404B before being directed by the beam director 103 into the environment 110, each receiver 410A, 410B detects two simultaneous optically mixed signals (corresponding to optical sources 402A and 402B).

[0075] Fig. 5 illustrates a timing diagram of this arrangement and in particular for one optical receiver 410, e.g., 410A. In particular, Fig. 5A shows reflected light received at the receiver 410A from four different sets of transmitted optical signals received within time windows 0-Ti and T1-T2. In particular, in the example scenario shown in Fig. 5A, the reflected signals 502A and 502B correspond to first sets of optical signals transmitted by the optical sources 402A and 402B, respectively and reflected signals 502C and 502D correspond to second sets of optical signals transmitted by optical sources 402A and 402B, respectively. Reflected signals 502A and 502B are received with a time overlap in the same acquisition window 0 to Ti (as these correspond to optical signals that were simultaneously transmitted), whereas reflected signals 502C and 502D are received with a time overlap in the next acquisition window Ti to T2.

[0076] Fig. 5B shows the LO signals received at each of the two optical combiners 408A and 408B for mixing with reflected light at the respective optical receivers 410A and 410B. As seen in this figure, in the first acquisition period, each of the optical combiners 408A and 408B receives two LO signals, one of which matches a first optical signal transmitted by first optical source 402A (shown in red) and the other of which matches a first optical signal transmitted by the second optical source 402B (shown in blue). For the next acquisition period, each of the optical combiners 408 A and 408B receives another two LO signals, one of which matches a second optical signal transmitted by first optical source 402A (shown in orange) and the other of which matches a second optical signal transmitted by the second optical source 402B (shown in purple). The LO signals received at each of the optical combiners remains constant for the entire acquisition period. In effect, two acquisition windows are fully overlapped in the same acquisition period. [0077] Fig. 5C shows the correlation between the detected signals of the reflected optical signals and the coded signals relating to their corresponding transmitted signals at the receiver 410A. Fig. 5D shows the power spectral density of the three corresponding received signals in the frequency domain. Although for simplicity Figs. 5 A to 5D show only various signals for two acquisition periods, a skilled person would appreciate that the described system can be configured such that each LO signal changes every acquisition period beyond the first two acquisition periods.

[0078] As shown in Fig. 5A, in the first acquisition window (0-Ti) two sets of reflected signals are detected and processed and in the next acquisition window (T1-T2) the next two sets of reflected signals are detected and processed. Accordingly, in this scenario, the maximum PPS of a single optical receiver is shown in equation 3.

[0079] PPS = 2/ AT (equation 3).

[0080] And the maximum PPS of the system (combining both the optical receivers) is shown in equation 4.

[0081] ^pps = ^ + ^ (equation 4).

[0082] Thus, by using two optical sources 402, two optical combiners 406, and two receivers 410, the PPS can be quadrupled.

[0083] To further distinguish between the two reflected signals detected every acquisition period, the corresponding transmitted signals are distinguishably encoded by the modulators 409A, 409B. For example, the modulators 409A, 409B may employ orthogonal codes. That is, the optical signals from the optical samplers 406A, 406B may be encoded by the corresponding modulators 409A, 409B using orthogonal codes assigned to the respective optical sources. At the detector, the first and second optical receivers 410A, 410B can be associated with first and second modulators 409A, 409B, respectively. For example, as illustrated in Fig. 5A, reflected signals 502A and 502C, which are encoded with a first coded signal, are distinguishably encoded from reflected signals 502B and 502D, which are encoded with a second, different coded signal. Accordingly, at the first optical receiver 410A, when the total reflected signal upon detection is correlated with the first coded signal encoded on optical signals associated with the first modulator 409A, the reflected signal corresponding to the second modulator 409B is supressed, such as reduced in correlation value or eliminated. Similarly, at the second optical receiver 410B, when the total reflected signal upon detection is correlated with the second coded signal encoded on optical signals associated with the second modulator 409B, the reflected signal corresponding to the first modulator 409A is supressed, such as reduced in correlation value or eliminated.

[0084] As mentioned above, the modulators 409 may include a semiconductor optical amplifier (SO A) integrated on a laser diode of the transmitter or an external modulator (such as a Mach Zehnder modulator or an external SOA modulator) to the laser diode. Similarly, each optical receiver 410A, 410B includes a decoder module that is configured to decode the received reflected signals. The decoder module may include a correlator that determines the correlation between the coded signal provided to the encoder and the detected return signal. In order to correlate a given encoder and a decoder the same orthogonal codes are provided.

[0085] Although Fig. 5 shows two optical sources and two optical receiver systems, it will be appreciated that can be generalized to a system that has n optical sources 402 and n optical receivers, which can use an n x n coupler. The resulting performance improvement is an n ² increase in the points-per-second.

[0086] Fig. 6 illustrates another example system 600 according to aspects of the present disclosure. In this arrangement as well, the system includes multiple optical sources 602 (two shown in this example as optical sources 602A and 602B), and multiple optical receivers 610 (two shown in this example as optical sources 610A and 610B).

[0087] The optical signals transmitted by the optical sources 602A and 602B are provided to a beam director 103. The beam director 103 directs the optical signals across one or two dimensions into an environment 110 having a spatial profile. In one example, the beam director 103 is configured to direct light from the multiple optical sources 602 A and 602B to different directions.

[0088] Unlike in system 400, the multiple optical sources in system 600 are not combined before transmission into the environment 110. The system 600 is configured so that the optical receivers 610A and 610B detect light from the respective different directions. For example, reflected light 607 A originating from the optical source 602 A is detectable by optical receiver 610A, but not by optical receiver 610B. Similarly, reflected light 607B originating from the optical source 602B is detectable by optical receiver 610B, but not by optical receiver 610A. For example, light from optical source 602A is directed by beam director 103 in a first direction, and light from optical source 602A is directed by beam director 103 in a second direction, different from the first direction. Further, optical receiver 610A is oriented to capture light returned from the first direction and not the second direction, and optical receiver 61 OB is oriented to capture light returned from the second direction and not the first direction. This way, reflected signals originating from different optical sources 602A and 602B are distinguishable based on the optical receiver having detected the reflected light.

[0089] To further or instead distinguish between reflected light originating from different optical sources 602A and 602B, modulators 609A, 609B are used to distinguishably encode the corresponding transmitted signals. The description relating to distinguishable encoding for system 400 similarly applies to system 600. For example, the system 600 may employ orthogonal codes. That is, the optical signals emitted by each optical source may be encoded using orthogonal codes assigned to the respective optical sources. At the detector, the first and second optical receivers 610A, 610B can be associated with the first and second optical sources 602 A, 602B, respectively.

[0090] Optical signals from each of the optical sources 602 are also provided to the optical receivers 610 as LO signals. In particular, optical samplers 603 provide portions of the optical signals from the optical sources 602 to an optical coupler 604 (e.g., a 2x2 optical coupler), which couples the optical signals together. The optical coupler 604 combines optical signals from the optical sources 602 on one side and splits the optical signals on the other side into 2 output ports. In some embodiments the split is even between the output ports. The output optical signal from a first output port of the optical coupler 604 may be provided as a first local oscillator (LO) signal 606A to a first optical receiver 610A and the output optical signal from a second output port of the optical coupler 604 is provided as a second local oscillator (LO) signal 606B to a second optical receiver 610B. Each LO signal 606 is derived from both optical sources 602.

[0091] The first LO signal 606A is combined with reflected light 607A via an optical combiner 608A (e.g. an optical coupler). The combined light is then provided to a first optical receiver 610A which detects an optically mixed signal, including any beat signal, of the combined light. Similarly, the second LO signal 606B is combined with reflected light 607B via an optical combiner 608B (e.g. an optical coupler). The combined light is then provided to a second optical receiver 610B which detects an optically mixed signal, including any beat signal, of the combined light. [0092] Further, the optical sources 602A, 602B emit optical signals with the same wavelengths. For example, optical source 602A may emit three sets of optical signals at three consecutive times having wavelengths i, 2, and 3. Optical source 602B may also emit three sets of optical signals at three consecutive times having the same wavelengths ( i, 2, and X.3). However, optical sources 602A and 602B may each operate to provide consecutive sets of transmitted optical signals, each set for a period shorter than a complete acquisition period, for example half an acquisition period (i.e., AT/2). Further, the operations of the optical sources 602A and 602B may be staggered. That is, wavelength emission from optical source 602B may lag behind that from optical source 602A by, for example, half an acquisition period (i.e., AT/2). Accordingly, the transmitted optical signals from optical sources 602A and 602B may be overlapped in time but non-overlapped in wavelength. In effect, consecutive acquisition windows are staggered or partially overlapped. Each subsequent acquisition window commences before the preceding acquisition window ends.

[0093] The system 600 may also include one or more modulators 609A, 609B, for modulating the transmitted optical signals via their intensity, phase or frequency. The modulation may be code modulation based on a coded signal generated by an encoder module. The modulators 609 A, 609B may be placed between each of the optical samplers (603) for sampling outgoing signals to be provided to the 2x2 optical coupler 604 and the beam director 103. This way, unmodulated light is provided to the optical receivers 610 as LO signals. The processing unit 105 is also configured to correlate the detected signals with the coded signals to calculate a lag between the return optical signal and the transmitted optical signal.

[0094] Further, as the optical signals transmitted by the optical sources 602A, 602B are combined by the optical coupler 604 before being directed into the optical combiners 608A, 608B, each LO signal in this example as well is derived from both optical sources 602 A, 602B in a time-overlapped and wavelength-non-overlapped manner.

[0095] Fig. 7 illustrates a timing diagram of this arrangement and in particular for one optical receiver 610, e.g., 610A. In particular, Fig. 7A shows reflected light received at the receiver 610A from three different sets of transmitted optical signals originating from optical sources 602A and received within time window O-T2. For simplicity, Fig. 7 does not show reflected light received at another optical receiver 610, e.g. 610B, from three different sets of transmitted optical signals originating from optical sources 602B and received within time window O-T2. [0096] In particular, in the example scenario shown in Fig. 7 A, the reflected signal 702 A corresponds to the first sets of optical signals (e.g., at wavelengths i) transmitted by optical source 602A. The reflected signal 702B corresponds to the second set of optical signals (e.g., at wavelength X2) transmitted by optical source 602A, and reflected signal 702C corresponds to the third set of optical signals (e.g., at wavelengths X3) transmitted by optical source 602A.

[0097] Fig. 7B shows the LO signals received at each of the two optical combiners 608A and 608B. For simplicity, only the LO signals received at one of the optical combiners, e.g. 608A, is shown. The LO signals received at the other of the optical combiners, e.g. 608B, is identical.

[0098] In the illustrated example, optical source 602A is configured to emit at the first wavelength (Xi) for half an acquisition period (AT/2). Optical source 602A is configured to subsequently emit at a second wavelength (X2) while optical source 602B is configured to emit at the first wavelength (Xi), both emissions lasting for half an acquisition period (AT/2). Further, optical source 602A is configured to subsequently emit at a third wavelength (X3) while optical source 602B is configured to emit at the second wavelength (X2), both emissions lasting for half an acquisition period (AT/2). Optical source 602A is configured to then cease emission while optical source 602B is configured to emit at the third wavelength (X2) for half an acquisition period (AT/2). These emissions are provided to each optical receiver 610A, 610B as LO signals. In this example, each of the optical receivers 610 is configured to receive three reflected signals (e.g. 702A, 702B and 702C for optical receiver 610A) in three respective acquisition windows (i.e. 0 to Ti, Ti/2 to 3T1/2, and Ti to T2) over two acquisition periods (i.e. 0 to T2).

[0099] Upon detection of a reflected signal, depending on timing of the return, the reflected signal is frequency-matched with the portion of the LO signal that is derived from either the first optical source 602A or the second optical source 602B. If a reflected signal is detected during the first half of an acquisition window, the reflected signal is frequency- matched with the portion of the LO signal derived from the first optical source. If a reflected signal is detected during the second half of an acquisition window, the reflected signal is frequency-matched with the portion of the LO signal derived from the second optical source. For example, as illustrated in Fig. 7, the reflected signal 702A returns during the second half of the first acquisition window (from 0 to Ti), and is therefore frequency-matched with the portion of LO signal derived from the second optical source 602B. Similarly, the reflected signal 702B returns during the second half of the second acquisition window (from Ti/ 2 to 3Ti/2), and is therefore frequency-matched with the portion of LO signal derived from the second optical source 602B. In comparison, the reflected signal 702C returns during the first half of the third acquisition window (from Ti to T2), and is therefore frequency-matched with the portion of LO signal derived from the first optical source 602A. Accordingly, each optical source 602A, 602B maintains emission of the same wavelength for different halves of an acquisition period. In effect, the system 600 provides a frequency-matched LO signal available for optical mixing for an entire acquisition period.

[0100] Fig. 7C shows the correlation between the detected signals of the reflected optical signals and the coded signals relating to their corresponding transmitted signals. Fig. 7D shows the power spectral density of the three corresponding received signals in the frequency domain. Although for simplicity Figs. 7A to 7D show only various signals for two acquisition periods, a skilled person would appreciate that the described system can be configured such that the LO signals define three partially overlapped acquisition windows every two acquisition periods.

[0101] As shown in Fig. 7 A, in the first acquisition period (0-Ti) one set of reflected signals are detected and processed and in the next acquisition period (T1-T2) the next two sets of reflected signals are detected and processed. A return signal in the first half of the each of the three acquisition windows is frequency-matched with the first optical source 602A and a return signal in the second half of each of the three acquisition windows is frequency- matched with the second optical source. Accordingly, in this scenario, the maximum PPS of a single optical receiver is shown in equation 5.

[0102] PPS = 3/2AT (equation 5).

[0103] And the maximum PPS of the system (combining both the optical receivers) is shown in equation 6.

3 3

[0104] PPS = — + — (equation 6).

27i 27i

[0105] Thus, by using two optical sources 602, two optical combiners 606, and two receivers 610, the PPS can be increased by 50% compared to conventional homodyne systems.

[0106] Although Fig. 6 shows two optical sources and two optical receiver systems, it will be appreciated that this can be generalized to a system that has n optical sources 602 and n optical receivers 610, which can use an n x n coupler 604. The resulting performance improvement is a In increase in the points -per- second.

[0107] As described above, the two optical sources 602A and 602B emit optical signals at the same wavelengths (staggered by half an acquisition period). There may be some frequency offset between the two optical sources 602A, 602B for example due to an imprecision in tuning independent optical sources to emit optical signals at the same wavelength.

[0108] Accordingly, in some embodiments, system 600 includes mechanisms to compensate for the frequency offset between the two optical signals. In one example, the two optical sources 602A, 602B are selected to exhibit a frequency difference that is within a few GHz, such as within 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 GHz. Upon detection of the reflected signal, such a frequency offset will manifest as a beat frequency in the electrical output of the optical receivers 610A, 610B, akin to the beat frequency in the electrical output described in the embodiment of Fig. 3. The processing unit 105 may be configured to recover the envelop, for example, by an envelope detector, a numerical demodulator or a frequency downconverter, to detect and/or remove the beat frequency prior to determining the correlation.

[0109] Alternatively, the system is configured to sample at a rate less than the frequency offset and using the technique of under- sampling to alias this measurement into the detection bandwidth. In one embodiment, the processing unit 105 is configured to digitise the electrical output of the optical receivers at a sampling rate at less than the frequency offset. For example, if the frequency offset between the two optical sources 620A and 602B is X GHz (e.g. 10 GHz), the processing unit 105 may digitise the electrical output of the optical receivers 610A, 610B at a sampling rate of less than X GHz (e.g. less than 10 GHz, such as 1, 2, 3, 4 or 5 GHz). The step of under- sampling facilitates in reducing any beat frequency in the detected signal prior to determining the correlation.

[0110] Fig. 8 illustrates another example system 800 according to aspects of the present disclosure. In this arrangement, the system includes a single optical source 802 and a single optical receiver 810. The system also includes two couplers 804, 806.

[0111] The optical signals transmitted by the optical source 802 are provided to a beam director 103, which directs the optical signals across one or two dimensions into an environment 110 having a spatial profile. As discussed above, the described system may include a modulator 803 for modulating the transmitted optical signals via their intensity, phase or frequency. The modulation may be code modulation based on a coded signal generated by an encoder module. The modulator 803 is placed between an optical sampler 805 and the beam director 103. This way, unmodulated emission from the light emitter is provided to the optical receiver 810 as an LO signal. The processing unit 105 is also configured to correlate the detected signals with the coded signals to calculate a lag between the return optical signal and the transmitted optical signal.

[0112] Optical signals from the optical source 802 are also provided to the optical receiver 810. In particular, a portion of the optical signals from the optical source 802 is provided via the optical sampler 805 to a 1 x 2 optical coupler 804. The optical coupler 804 receives a single optical signal from the optical source 802 on one side and evenly splits the optical signal on the other side into 2 output ports. The output optical signal from a first output port of the optical coupler 804 (i.e. an undelayed optical signal) may be provided to a first input port of a second optical coupler 806 (e.g., a 2 X 1 optical coupler). The output optical signal from a second output port of the optical coupler 804 is provided to a fibre delay line 811 to produce a defined fibre-optic transmission delay in the optical signal (i.e. a delayed optical signal). One end of the fibre delay line 811 is connected to the second output port of optical coupler 804 and the other end of the fibre delay line is connected to the second input port of the optical coupler 806.

[0113] The second optical coupler 806 combines the undelayed optical signal and the delayed optical signal received at its two input ports into a single output optical signal which is the LO signal 807.

[0114] The LO signal 807 is combined with reflected light 809 via an optical combiner 808 (e.g., an optical coupler). The combined light is then provided to an optical receiver 810 which detects the optically mixed signal, including any beat signal, of the combined light.

[0115] The delay introduced by the fibre delay line 811 in this example stores the local oscillator signal for a fixed period of time. In one example, the delay time (D) is half the acquisition period, as shown in equation 7.

AT

[0116] D = — — (equation 7).

[0117] For a delay time of D, the length (L) of the fibre delay line 811 can be calculated as shown in equation 8. [0118] L = • D (equation 8).

[0119] c = 299792458 m/s is the speed of light in vacuum, and n = 1.45 is the refractive index of the fibre delay line. For a 2ps acquisition time (Ti), the length of the fibre delay line is a little more than 200m of fibre.

[0120] The system 800 allows the optical source 802 to switch from providing the first set of transmitted optical signals to a second set of transmitted optical signals LO after the delay time, e.g. half the acquisition period (AT/2). However, the LO signal from the first input port of the second optical coupler 608 is provided to the optical receiver 810 for the first half of the acquisition window and the LO signal from the fibre delay line 811 is provided to the optical receiver 810 for the second half of the acquisition window. This way, the optical source can switch to the next set of optical signals after half the acquisition period, but the delayed LO signal arrives at the optical receiver 810 just as the optical source 802 switches. This in effect causes a frequency-matched LO signal to be present at the receiver for optical mixing for the entire acquisition period.

[0121] Fig. 9 illustrates a timing diagram of this arrangement. In particular, Fig. 9A shows reflected light received at the receiver 910 corresponding to three different sets of transmitted optical signals received within time window 0— T2.

[0122] In particular, in the example scenario shown in Fig. 9 A, the reflected signal 902 A corresponds to the first set of optical signals (e.g., at wavelength i) transmitted by optical source 802. The reflected signal 902B corresponds to the second set of optical signals (e.g., at wavelength X2) transmitted by the optical source 802, and reflected signal 902C corresponds to the third set of optical signals (e.g., at wavelength X3) transmitted by the optical source 902.

[0123] Reflected signals 902B and 902C are received in a time overlap manner in the same acquisition period. In this example, the optical receivers 810 is configured to receive three reflected signals (i.e. 902A, 902B and 902C) in three respective acquisition windows (i.e. 0 to Ti, Ti/2 to 3T1/2, and Ti to T2) over two acquisition periods (i.e. 0 to T2). In other words, each subsequent acquisition window commences before the preceding acquisition window ends.

[0124] Fig. 9B shows the LO signals received at the optical receiver 810. As seen in this figure, in a first acquisition window (i.e. 0-Ti), for the first half of that acquisition window, the LO signal 904A is an undelayed portion of the optical signal (of a first wavelength, shown in red) emitted by the optical source 802 in that period and for the second half of that acquisition window, the LO signal 904B is the delayed portion of the optical signal (of the first wavelength, shown in red) emitted by the optical source. The first acquisition window lasts for an acquisition period. Before the first acquisition window ends, a second acquisition window commences, in this example in the middle of the first acquisition window (i.e. Ti/2 - 3Ti/2) lasting for an acquisition period. As seen in Fig. 9B, the LO signal 904C is an undelayed portion of the optical signal (of a second wavelength, shown in blue) emitted by the optical source 802 in that period and for the second half of that acquisition window, the LO signal 904E is the delayed portion of the optical signal (of the second wavelength, shown in blue) emitted by the optical source. Before the second acquisition window ends, a third acquisition window commences, in this example in the middle of the second acquisition window (i.e. Ti - T2) lasting for an acquisition period. As seen in Fig. 9B, the LO signal 904D is an undelayed portion of the optical signal (of a third wavelength, shown in orange) emitted by the optical source 802 in that period and for the second half of that acquisition window, the LO signal 904F is the delayed portion of the optical signal (of the third wavelength, shown in orange) emitted by the optical source. Accordingly, consecutive acquisition windows are staggered or partially overlapped, for example by half an acquisition period. Each acquisition window includes one LO signal corresponding to the undelayed optical signal from the optical source (e.g., LO signal 904D corresponding to the optical signal of the third wavelength in orange) and one LO signal corresponding to the delayed optical signal from the optical source, for example by half acquisition period (e.g., LO signal 904E corresponding to the optical signal of the second wavelength in blue). At any one time, the combined LO signals includes either or both of a currently emitted optical signal from the optical source and a stored optical signal from the optical source. For example, in the first half of the first acquisition window, the combined LO signals include only a currently emitted optical signal 904A. In the second half of the first acquisition window or first half of the second acquisition window, the combined LO signals include both a currently emitted optical signal 904C and a stored optical signal 904B . In the second half of the second acquisition window or first half of the third acquisition window, the combined LO signals include both a currently emitted optical signal 904D and a stored optical signal 904E. In the second half of the third acquisition window, the combined LO signals include only a stored optical signal 904F. [0125] Fig. 9C shows the correlation between the detected signals of the reflected optical signals and the coded signals relating to their corresponding transmitted signals. Fig. 9D shows the power spectral density of the three corresponding received signals in the frequency domain. Although for simplicity Figs. 9A to 9D show only various signals for two acquisition periods, a skilled person would appreciate that the described system can be configured such that the LO signals define three partially overlapped acquisition windows every two acquisition periods.

[0126] As shown in Fig. 9 A, in the first acquisition period (0-Ti) one set of reflected signals are detected and processed and in the next acquisition period (T1-T2) the next two sets of reflected signals are detected and processed. A return signal in the first half of the each of the three acquisition windows is frequency -matched with the undelayed LO signal and a return signal in the second half of each of the three acquisition windows is frequency- matched with the delayed LO signal. Accordingly, in this scenario, the maximum PPS of a single optical receiver 910 is shown in equation 9.

[0127] PPS = 3/2 AT (equation 9).

[0128] To distinguish between the different reflected signals detected in neighbouring acquisition windows (e.g., distinguish between reflected signals 902B and 902C), the corresponding transmitted signals are distinguishably encoded. For example, the modulator 803 may employ orthogonal codes. That is, the consecutive optical signals emitted by the optical source 802 may be encoded using orthogonal codes assigned to the respective wavelengths. For example, the first set of optical signals transmitted at the first wavelength may be encoded using one code, whereas the next set of optical signals transmitted at the second wavelength may be encoded using a second code that is orthogonal to the first code. Alternatively or additionally, the third set of optical signals transmitted at the third wavelength may be encoded using a third code that is orthogonal to the second code and/or the first code.

[0129] As mentioned above, the module 803 may include a semiconductor optical amplifier (SO A) integrated on a laser diode of the transmitter or an external modulator (such as a Mach Zehnder modulator or an external SOA modulator) to the laser diode. Similarly, the optical receiver 810 includes a decoder module (not shown) that is configured to decode the received reflected signals. The decoder module may include a correlator that determines the correlation between the coded signal provided to the encoder and the detected return signal. The same orthogonal codes are provided to the modulator 803 and the decoder.

[0130] Fig. 10 illustrates another example system 1000 according to aspects of the present disclosure. In this arrangement, the system includes two optical sources 1002A and 1002B. Light emitted from the optical sources 1002A and 1002B are provided to respective optical samplers (such as 1x2 90/10 couplers) 1004A and 1004B. Each optical sampler 1004 provides a first portion of the light emitted by the respective optical source as a local oscillator signal 1005. An optical combiner 1010 (e.g. a 2x1 coupler) combines the two local oscillator signals 1005A and 1005B as combined oscillator signal 1005C. Each optical sampler 1004 further provides a second portion of the light emitted by the respective optical source as outgoing light. The outgoing light is provided to respective modulators 1006 A and 1006B for combination by an optical combiner 1008 (e.g. a 2x1 coupler). The modulators 1006A and 1006B may be configured to apply subcarrier modulation to the outgoing light. For example, modulators 1006 A and 1006B intensity-modulate the respective outgoing light by different subcarrier frequencies SCI and SC2. The subcarrier frequencies may be orthogonally spaced. For example, modulator 1006A may be configured to apply a rectangular modulation at subcarrier frequency SCI to the outgoing light in the time-domain. The rectangular modulation results in a spectrum following a sine- squared function in the frequency domain. The frequency-nulls of the sine-squared function are based on subcarrier frequency SCI. The frequency-nulls may be arranged to coincide subcarrier frequency SC2 to provide orthogonality. In one example, SCI and SC2 are spaced approximately 100-300 MHz apart, for example, approximately 100, 125, 150, 175, 200, 225, 250, 275 or 300 MHz apart. An optical combiner 1008 (e.g. a 2x1 coupler) receives the modulated outgoing light from modulators 1006 A and 1006B to provide combined outgoing light. The combined outgoing light is provided to the beam director 103 for direction into the environment 110. Light 1011 reflected by the environment 110 is received by the beam director 103 and provided to an optical combiner 1012 for combination with the combined local oscillator signal 1005C as combined light 1013. The combined light 1013 is then provided to an optical receiver 1014, which detects an optically mixed signal, including any beat signal, of the combined light.

[0131] The timing diagrams (not shown) for system 1000 are similar to those of Fig. 5, except that the outgoing light is modulated based on subcarriers rather than coded signals. For example, instead of being intensity-modulated based on a first coded signal as in reflected signals 502A and 502C, the reflected signals in system 100 corresponding to optical source 1002A are intensity-modulated at SCI. Similarly, instead of being intensity-modulated based on a second coded signal as in reflected signals 502A and 502C, the reflected signals in system 100 corresponding to optical source 1002B are intensity-modulated at SC2. Since the combined local oscillator signal 1005C includes both wavelengths emitted by optical sources 1000A and 1000B, reflected light 1011 received by system 1000 when optically mixed with the combined local oscillator signal 1005C produces, upon detection, an electrical signal with beat frequencies equal to the respective subcarrier frequencies. That is, the reflected signals, upon detection by receiver 1014, include both subcarrier frequencies SCI and SC2. The system 1000 may include one or more frequency filters to disambiguate, based on the difference in subcarrier frequencies, the reflected signals corresponding to the respective optical sources. For example, inclusion of a frequency filter configured with centre frequency at SCI selects reflected signals corresponding to optical source 1002A and/or suppresses reflected signals corresponding to optical source 1002B. Similarly, inclusion of a frequency filter configured with centre frequency at SC2 selects reflected signals corresponding to optical source 1002B and/or suppresses reflected signals corresponding to optical source 1002A.

[0132] In the arrangements described above, the systems illustrate some components of the homodyne system. Other components are omitted or not described in detail to avoid obfuscation. However, it will be appreciated that the homodyne system may include additional optical elements. For example, the systems described above may further include a modulator for imparting a time- varying profile on the optical signals, for example a time varying intensity, frequency or phase profile, before providing the optical signals to the beam director 103. Similarly, the systems described with reference to Figs 1-9 may further include an optical amplifier for amplifying the reflected optical signals before they are provided to the optical receiver. In one example, an Erbium-doped fibre amplifier (EDFA) may be used.

[0133] Further still, the system arrangements may include a coaxial transceiver module configured to (a) receive the optical signal via one or more input ports, (b) send the received optical signal via one or more bidirectional ports towards the beam director 103, (c) receive the incoming light from the beam director 103 via the bidirectional port(s) and (d) send the reflected optical signals via one or more output ports to the optical receiver. The optical signal from the coaxial transceiver module may be spatially coherent (but retain phase incoherence if an incoherent light emitter is used). The coaxial transceiver module is arranged such that the outgoing path and the incoming path are spatially arranged to at least partially overlap, while the output is spatially displaced from the one or more input ports. The coaxial transceiver module may be optically coupled with the optical receiver(s) by few-mode fibres or multimode fibres. For example, the coaxial transceiver module may be optically coupled with the optical amplifier via a single mode fibre. Examples of the coaxial transceiver module are disclosed in PCT application no. PCT/AU2018/051175 published as WO 2019/084610 Al on 9 May 2019 the entire content of which is incorporated herein by reference.

[0134] The beam director 103 is configured to (a) direct the optical signals towards the environment into one or more respective outgoing directions, based on the selected wavelength channel, and (b) direct the reflected optical signals from the environment 110 towards the coaxial transceiver module. The beam director 103 may include expansion optics to enlarge the beam size for better divergence characteristics. The beam director 103 may also include one or more dispersive elements, such as grating/s, prism/s and/or grism/s, to provide wavelength-dependent angular dispersion. Examples of beam directors 103 that may be used are disclosed in PCT application no. PCT/AU2016/050899 published as WO 2017/054036 Al on 6 April 2017, PCT application no. PCT/AU2017/051255 published as WO 2018/090085 Al on 24 May 2018, and PCT application no. PCT/AU2018/050961 published as WO 2019/046895 Al on 14 March 2019, and PCT application no.

PCT/AU2019/050437 published as WO 2019/241825 Al on 26 December 2019. The entire content of these patent publications are incorporated herein by reference. At least one characteristic associated with the reflected optical signal includes information for estimation of the spatial profile of the environment associated with the one or more outgoing directions.

[0135] Now that arrangements of the present disclosure are described, it will be apparent that each of the described arrangements may be deployed to increase the points per second limitation of a conventional homodyne system. The arrangement depicted in Fig. IB and 1C that maintains an LO signal on the optical receiver for half the acquisition period, doubles the points per second of the homodyne receiver as compared to a conventional homodyne system. The arrangement depicted in Fig. 4 quadruples the points per second as compared to conventional homodyne systems by employing two optical sources, two optical receivers and an optical coupler. The arrangements depicted in Figs. 6 and 8 increase the points per second of the homodyne receiver as compared to a conventional receiver by 50%.

[0136] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.