Optical wireless communication device and method

Abstract

A device for receiving an incoming optical wireless communication (OWC) signal comprising: a plurality of photodetectors each configured to produce a photo detection signal in response to light being received by the photodetector, wherein the photodetectors comprise single photon detectors configured to detect single photons; wherein the plurality of photodetectors are arranged in a plurality of groups, wherein each group is configured to produce a group response signal representing the photo detection signals produced by the photodetectors within the group in response to receiving light, wherein the plurality of groups are configured to be connected to a common signal output, wherein the device comprises controllable switching circuitry for selectively switching each group response signal onto the common signal output thereby to produce an output signal representative of the OWC signal received by the device and suitable for demodulation and / or decoding by further processing circuitry.

Description

[0001] Optical Wireless Communication Device and Method.

[0002] Field

[0003] The present invention relates to an optical wireless communication device and method.

[0004] Background

[0005] Demand for transmission bandwidth has been impacted negatively by scarcity of the radio-frequency (RF) spectrum, particularly in the fifth generation (5G) and upcoming sixth generation (6G). One solution may be to use the free visible spectrum (380-780 THz) to provide visible light communications (VLCs) using energy-efficient light-emitting diode (LED)-based lighting and Light fidelity (LiFi), using light waves to transmit data instead of radio waves but maintaining consistent high network performance regardless of lighting level.

[0006] Single-Photon Avalanche Diode (SPAD), also known as a Geiger-mode APD (G-APD), is a class of photodetector that can detect low intensity light (down to the single photon) whereupon a SPAD launches billions of electron-hole pair creations in a rapid avalanche process that results in a large current pulse produced at the output of the SPAD. A SPAD can consequently detect the time of a photon arrival event with high temporal resolution (few tens of picoseconds) due to the high speed of avalanche build up.

[0007] A SPAD cell may operate in Geiger-mode with a self-quenching circuit to stop the avalanche ionization, during which the SPAD is unresponsive. The development of SPADs has made it possible to detect extremely weak light phenomena at the photon level with high efficiency and low voltage requirements. SPADs have found applications in consumer and robotics imaging, data and telecom security, advanced driverassistance systems and biophotonics.

[0008] SPAD cells are not normally considered suitable for OWC (optical wireless communications) as they are photodetectors capable of detecting discrete photon arrivals rather than a continuous intensity modulated light signal. In current known configurations, as such Silicon PhotoMultipliers (SiPMs), SPADs and associated circuitry produce insufficient signal to noise ratio (SNR) for OWC signals requiring high orders of Quadrature amplitude modulation (QAM), for example, e.g. Orthogonal frequency-division multiplexing (OFDM) 1024-QAM WiFi.

[0009] It is known that SPADS are typically less suitable for use in OWC analogue signals as they produce a very low amplitude single pulse output on detection of a single photon event. However, SPADs have an advantage over conventional PD or APD (avalanche photodiodes) in sensitivity of light detection which enable successful use for weak light signals such as from signals received over a long distance or low intensity light source. SPAD sensitivity results from high internal gain allowing the SPAD to operate at levels below the noise floor of conventional PD / APDs.

[0010] As shown in Figure 1 , a single discrete SPAD output signal provides a poor representation of a continuous analogue signal as the pulsed output from each photon event is associated with a signal recovery time. Many 1000s of SPADs are currently required to be summed together to improve continuous analogue signal representation.

[0011] Figure 1 depicts a SPAD cell with an incident OWC signal. The incident OWC signal is represented at the quantised (photon) level and also at the continuous optical power level. The power or intensity of the optical signal is proportional to the number of photons.

[0012] The cell has a SPAD 12 connected to a transistor 16 and buffer 14. The cell operates to produce a fixed amplitude discrete pulse when a photon is detected (as illustrated as signal 18). The pulse has a longer duration than the inter-arrival time between individual photons. As such, the discrete pulses output by a single SPAD cell offer a poor representation of a continuous intensity modulated optical signal. Each SPAD cell has a dead time after a photon is detected during which it cannot detect subsequent photons. The dead times may set the bandwidth and dynamic range of each cell.

[0013] In summing 1000s of SPADS together in parallel the amplitude of the signals is scaled by the number of SPADS in the array while the noise level remains low and the SNR is high but unfortunately the analogue signal bandwidth that can be detected is compromised due to the high capacitance of the circuit resulting from the inherent capacitance of each SPAD connected in parallel. For this reason, multi parallel connected SPADs may be problematic with saturation signals - the signal reaches a maximum value and may have a long decay time. Aside of the problems associated with a pulsed output of SPADs as applied to detection of an OWC signal, this saturation of parallel connected SPADs may present a problem in OWC signals with high levels of QAM.

[0014] Undesirable capacitance effects are discussed in WO2011 / 117309. The device described in that document employs a 3 electrode silicon photomultiplier (SiPM) to enable output pulses having short duration. However, that device may retain some capacitance dependence on the number of photocells thereby limiting the total number of photocells that can be used and also limiting the signal amplitude with a consequent limitation on SNR and reception quality of intensity modulated OWC signals.

[0015] The silicon photomultiplier (SiPM), has been developed by interconnecting a large number of SPADs in parallel. Signal Driven Multiplexing is used in systems requiring arrays of SiPMs as a means to sum together the Fast outputs of an SiPM array without loss of the Fast output signal pulse shape or Signal to Noise Ratio. A description of a signal driven multiplexing of Silicon PhotoMultiplier Arrays for imaging applications is given in “AND9772 / D Signal Driven Multiplexing of Silicon Photomultiplier Arrays” by OnSemi. Fast outputs of SiPMs are described in detail in “Introduction to the Silicon Multiplier” by OnSemi.

[0016] Summary

[0017] In accordance with a first aspect, there is provided, a device for receiving an incoming optical wireless communication (OWC) or OWC light signal comprising: a plurality of photodetectors each configured to produce a photo detection signal in response to light being received by the photodetector; wherein the plurality of photodetectors are arranged in a plurality of groups, wherein each group is configured to produce a group response signal representing the photo detection signals produced by the photodetectors within the group in response to receiving light, wherein the plurality of groups are configured to be connected to a common signal output, wherein the device comprises controllable switching circuitry for selectively switching each group response signal onto the common signal output thereby to produce a signal output signal representative of the OWC light signal received by the device and suitable for demodulation and / or decoding by further processing circuitry. The photodetectors may comprise photon detectors. The photon detectors may be configured to produce a photo or photon detection signal in response to being triggered by a photon. The photo detectors may comprise a detecting or sensing element and associated detection circuitry.

[0018] The group response signal may be formed by accumulating the photo detection signals produced by the photodetectors within the group in response to receiving light. The photo detection signals may be generated in response to a photo detection event or in response to detection of a photon.

[0019] The plurality of groups may be configured to have a common ground and / or common supply and / or other common terminal. Each group of the plurality of groups has a common anode, common cathode and / or other common terminal.

[0020] The plurality of groups may be arranged in a plurality of further groups or blocks having a common ground and / or common supply and / or other common terminal, wherein each further group or block comprises two or more groups of the plurality of groups. Selectively switching each group response signal onto the common signal output may comprise switching a bias voltage or supply voltage between further group or block in accordance with a switching scheme. Selectively switching each group response signal onto the common signal output may comprise selectively activating said further groups and / or blocks.

[0021] At least some, optionally all, of the plurality of groups may be configured to be connected to a common signal output in a parallel arrangement. At least some, optionally all of the photodetectors of each group may be provided in a parallel arrangement.

[0022] Each group of photodetectors may be referred to as a photocell or photocell group. Each group may be connected by a dedicated switch to a common signal output line and that switch may be operable in response to receiving an activation signal. The group may be activated in response to one or more photodetectors detecting light or a photon event. The detection of light and / or photon detection event may activate the switching circuitry and the photon voltage signal may be applied to the common signal output.

[0023] The controllable switching circuitry may be configured to switch between the pluralities of groups at a switching frequency such that the output signal comprises a sum of at least part of the group responses, optionally the output signal comprises only part of the group responses.

[0024] The controllable switching circuitry may be configured to switch between a plurality of further groups or blocks, each further group or block comprising two or more of the pluralities of groups, at a switching frequency such that the output signal comprises a sum of at least part of the group responses, optionally such that the output signal comprises only part of the group responses.

[0025] The device may comprise a controller for controlling the controllable switching circuitry. The controller may be configured to control the controllable switching circuitry in accordance with a switching scheme or sequence.

[0026] The switching circuitry may comprise first switching circuitry. The first switching circuitry may be configured to selectively provide an activation or bias signal to activate one or more groups of photodetectors. The first switching circuitry may be configured to sequentially activate one or more groups of photodetectors for detection. The groups of photodetectors may be configured to detect photons only in response to being activated by the switching circuitry. The first switching circuitry may be controllable by an external control signal, for example, from a controller.

[0027] The switching circuitry comprises output switching circuitry for switching one or more groups of photodetectors onto the common signal output while isolating the other groups of the plurality of groups of photodetectors from the common signal output. The first switching circuitry may be controllable by outputs of the plurality of groups.

[0028] The switching circuitry may further comprise simultaneously switching group responses to the common signal output in accordance with a first switching scheme and activating one or more groups of the photodetectors in accordance with a second switching scheme. The first switching scheme may comprise a sequential or other scheme and / or may be activated using an external control. The second switching scheme may be based on internal signal and / or may comprise a threshold based scheme.

[0029] The switching circuitry may comprise controllable circuitry for selectively switching each group response signal onto the common signal output.

[0030] The group response signal may comprises a portion, for example, a rising edge, representing the number and / or intensity of light received by the respective group of photodetectors wherein the controllable switching circuitry is configured to selectively switch the group responses on to the common signal output so that the portion is combined onto output signal.

[0031] The group response signal may comprise an electronic signal. The output signal may comprise an electronic signal.

[0032] The controllable switching circuitry may comprise a plurality of switching elements provided for selectively switching the group outputs to the common output and / or wherein the controllable switching comprises a plurality of switching elements for providing an activation signal, for example, a bias signal, to the one or more groups.

[0033] The output signal may comprise a time varying signal. The output signal may have at least one signal property, for example, a voltage or amplitude that corresponds or is related to property of the incoming optical signal and wherein the demodulation and / or decoding process is based on said property.

[0034] The device may comprise a capacitor associated with each photodetector, wherein each photodetector is coupled to a shared output via its associated capacitor. The photodetector and associated capacitor are configured to produce a photo detection signal comprising a pulse in response to detecting a photon.

[0035] The photodetector may be configured to output short pulses in response to detecting a photon, optionally, up to 100ns, optionally up to 10ns, optionally up to 1 ns in duration.

[0036] The photodetector may comprise or be provided together with associated filtering circuitry configured to filter an output of the photodetector. The filtering circuitry may comprise at least a high pass filter. The filtering circuitry may be configured to filter the output of the photodetector to produce a filtered output for the group response. The filtered output may comprise a pulse. The filtered output may comprise at least a portion, optionally a rising edge, that represents number and / or intensity information of the received light.

[0037] The group response may comprise an accumulation of said pulses. Each pulse may have pulse width of the order 1 ns. Each pulse may have a maximum pulse width of 1 ns, optionally, 500 ps. Each group may be configured to output a group response comprising a duration of up to 100 ns, optionally up to 10 ns, optionally up to 1 ns in duration. Each group response may comprise a rising portion characterised by a rise time. The rise time may be up to 100 ns, optionally up to 10 ns, optionally up to 1 ns in duration.

[0038] The controllable switching circuitry may be configured to switch between groups such that the time interval between switching on of each group to the combined signal output is shorter than the rise time of each group response. The switch period may be at least 2 to 10 times shorter, optionally over 10 times shorter than the rise time. The switch period may be at least 2 to 10 times shorter, optionally between 5 to 8 times shorter than the rise time.

[0039] The controllable switching circuitry may be configured to switch between groups such that the time interval between switching on of each group to the combined signal output is less than half the periodic time of the maximum frequency of modulation of the OWC to be received.

[0040] The controllable switching circuitry may be configured to switch between groups such that the time interval between switching on of each group to the combined signal output is less than the decay time of the group response.

[0041] The signal switching circuitry may comprise a plurality of fast response switching devices. The fast response switching devices may be configured to connect a terminal of the group of response or associated circuitry to a readout circuit at a frequency sufficient to capture at least part of the rising portion of the group response. The fast response switching devices may be configured to connect a terminal of the group of response or associated circuitry to a readout circuit at a frequency such that the group is connected for a time shorting than a rise time of the group response.

[0042] The accumulated group response signal may be characterised by a size and / or other signal property that is proportional or representative of a number of photodetectors of the group triggered by incoming light or optical power of the incoming light.

[0043] Each group of at least some, optionally all of the plurality of groups, may have a common signal ground and / or a common bias input. Each group of at least some, optionally all of the plurality of groups may have at least one further common component.

[0044] The group response signal may comprise an accumulated group response signal that is characterised by a size and / or other signal property that is proportional or representative of a number of photodetectors of the group triggered by incoming light or optical power of the incoming light.

[0045] Each group of at least some, optionally all of the plurality of groups may have the same configuration and size. Each group may have a common supply voltage.

[0046] Each group of at least some, optionally all of the plurality of groups may be switched by controlling an activation signal, for example, a bias signal. Each group of at least some, optionally all of the plurality of groups may comprise a further switch on the activation signal, for example, the bias, to provide an additional level of activation control. The activation signal may be applied to allow the photocell photon event signal, representing one or more photon detection events, to be applied to or form part of the signal output line. This may allow additional control options on which group(s) of the plurality of groups are switching and / or which group, for example, how many groups, may be active at substantially the same time.

[0047] The switching circuitry may comprise one or more switching elements or devices configured to switch only when a group response and / or activation signal is above a threshold level. The switching circuitry may be configured to apply a switching sequence, for example, a sequential and / or periodic scheme.

[0048] The scheme and / or a parameter of the scheme, for example, frequency, is selected based on a desired or measured property of the output signal and / or a desired or measured property of the group response. The frequency may be in the range 10Mz to 100GHz.

[0049] The number of photodetectors in each group is selected or based on a desired or measured property of the output signal and / or a desired or measured property of the group response.

[0050] The desired or measured property may comprise a response rise time or other property of a rising portion of the group response. The desired or measured property may comprise a size, amplitude, signal to noise ratio or other measure of signal quality. The desired or measured property may comprise a resistance for measuring device.

[0051] The signal switching circuitry may be configured to output a first group response having a group response duration from one or more first groups at a first time and switch to outputting a second group response from one or more second groups at a second time before the end of the first group response duration.

[0052] The signal switching circuitry may be configured to perform a sampling process comprising obtaining samples of group responses from each group and adding said obtained sample to the output signal.

[0053] The switching circuitry may be configured to couple the signal of each group to the common output for a time period shorter than the group response time, optionally shorter than a rise time of the group response.

[0054] The accumulated group response may comprise a rising portion having a rise time or other property dependent on at least the number of photodetectors in the group and wherein the controllable switching circuitry is configured to switch between groups in in dependence on the group response rise time or other property. The switching may be performed such that only part of the group response rising portion is included in the output signal.

[0055] The rising portion of each group may have a rise time dependent on at least the number of photodetectors in the group and wherein the number of photodetectors in the group are selected to control, for example, restrict the rise time. The rise time may be in the range 1 ps to 100 ps, optionally up to 500 ps, optionally up to 1 ns.

[0056] Each group response signal may have a falling portion and the switching frequency may be selected such that the combined signal is absent of the falling portions of the combined response signals

[0057] The switching frequency may correspond to a switch time period less than the rise time of the group response.

[0058] The device of any preceding claim, wherein at least some, optionally all of the plurality of groups are capacitively coupled to a common group output and wherein the switching circuitry is configured to electronically couple only one photon detector to the group output at a time and / or only one group to the common signal output at a time.

[0059] The device of any preceding claim, wherein the photodetectors are provided as part of photomultiplier modules connected such that each group comprises one or more photomultipliers modules, wherein the signal switching circuitry is configured to connect the one or more photomultiplier modules of each group to a common output and / or connect one or more photomultiplier modules to a common bias signal. The photomultiplier modules may comprise silicon photomultiplier (SiPM) modules. The photomultiplier modules may comprise Multi-Pixel Photon Counter (MPCC) modules. Each group may comprise one or more photomultiplier modules. The switching circuitry may be configured to selectively activate one or more blocks of photomultiplier modules.

[0060] The device of any preceding claim, wherein each photon detector has an intrinsic recovery or dead time and wherein the number of groups is selected and / or a switching frequency is selected so that each photon detector has sufficient time to recover between each triggering event. The common signal output line may be connected to an amplifier and / or other signal measuring circuit or device. The impedance of the amplifier and / or measuring circuit or device may be matched to the impedance of the common signal output.

[0061] The device of any preceding claim, wherein the received OWC signal comprises a unipolar signal and / or the OWC signal comprises a medium to high light intensity signal and / or the OWC signal comprises a DC bias.

[0062] The device of any preceding claim, wherein the OWC signal is modulated in accordance with an OWC data modulation scheme and wherein the device further comprises demodulation and / or decoding circuitry for applying a demodulation and / or decoding process to the signal output.

[0063] The device of any preceding claim, wherein each group produces a group response using a signal driven multiplexing arrangement, optionally, wherein the signal driven multiplexing arrangement comprises pairs of response switches and / or Shottky diode series pairs.

[0064] The device may comprise a photon detection event capturing device or circuit coupled to the common output line configured to capture or otherwise measure at least one photon event signal during the rise time of the photocell response. This may enable the circuitry to detect OWC signals with high fidelity.

[0065] The photon detection event capturing device or circuit may be configured to capture or measure a plurality of photon detection events. The photodetection event signals may be represented on the signal common output line as a cumulative or additive signal. The signal output may be formed by combining rising edges of group responses.

[0066] The switching circuitry may be configured to apply a switching sequence, for example, in accordance with a sequential and / or periodic scheme and / or in accordance with an automatic or threshold-based scheme. The switching circuitry may comprise one or more switching elements or devices configured to switch when a group response and / or activation signal is above a threshold level. The activation sequence or scheme may comprise a periodic handover sequence and / or an automatic scheme, for example, a threshold based or signal driven multiplexing scheme.

[0067] According to a further aspect, there is provided a method of receiving an optical wireless communication signal using a receiver comprising a plurality of photodetectors each configured to produce a photo detection signal in response to receiving light and wherein the plurality of single photon photodetectors are arranged or otherwise provided in a plurality of groups, wherein each group is configured to produce a group response signal from the photo detection signals produced by its photodetectors in response to receiving light wherein each of the plurality of groups is connected to a common output, wherein the method comprises controllably switching groups connected to a common output thereby to provide their group response signal(s) to the common output to produce an output signal for demodulation and / or decoding.

[0068] The method may further comprise demodulation and / or decoding the output signal or a signal derived from the output signal.

[0069] According to a further aspect there is provided a low power analogue sampling method, and related device, of optical wireless communication signals received on a photodetector comprising the use of two or more Single Photon Avalanche Diodes for continual photon detection.

[0070] This aspect may realise a Photon to Analogue Converter, wherein it detects a continuous intensity-modulated incident optical signal (e.g. OWC signal) broken into its constituent quantised units - a stream of discrete time-varying photon arrivals. The aspect may relate to capturing these discrete photon arrivals in the form of rising edges of voltage pulses produced at the output of SPAD cells.

[0071] A plurality of SPAD cells may be used to build up the vertical resolution required to accurately reconstruct the desired modulated signal with high dynamic range and high signal to noise ratio. This is because multiple photons may arrive simultaneously within a narrow time window, so multiple active SPAD cells must be available to capture them. The method and device may comprise two or more SPAD cells connected in parallel to sum the outputs of two or more SPAD cells into a single common output. For example, 10 SPAD cells connected in parallel may produce a combined voltage amplitude up to 10 times greater than that of an individual SPAD cell.

[0072] Connecting SPADs in parallel may minimise power requirements and enable low voltage applications, less than 35 volts. Conventional LiFi receivers may employ APD photodetectors which require up to 100 Volt bias.

[0073] Optionally, SPADs may be summed in a silicon photo multiplier (SiPM), wherein all SPAD outputs are connected to a common SiPM signal output via coupling capacitors. Each SPAD cell may comprise a SPAD operated in Geiger-mode and a self-quenching circuit to stop the avalanche ionization process within a period of around 1ns to 20ns. During this period, also known as the ‘dead time’, the SPAD may be unresponsive to subsequent photon arrivals. After the dead time, the SPAD cell may be reset and ready to detect photons again. Each SPAD cell voltage pulse rising edge may correspond to an individual photon arrival and is followed by an unavoidable slow falling edge decay due to inherent parasitic electronic effects in the SPAD cell. This decay time may be too slow to capture the fast rate of change of high speed OWC signals. The decay time and therefore dead time is extended when SPAD cells are connected in parallel due to the combined capacitances of the SPADs - further exacerbating the capability of the SPAD array to receive high speed OWC signals.

[0074] The device may comprise a photodetector block (an array of SPAD cells divided into Groups) and an electronic block (switching handover circuitry). The electronic switching circuitry may enable fast handover between SPAD Groups, wherein only one SPAD Group is active and connected to a common output at a time. This may allow the fast photon events encoded in the SPAD cell rising edges to be rapidly summed together and offloaded to the common output, while the slow decay component of the electronic circuit response is ignored.

[0075] The device and / or method therefore may capture the photonic information i.e. the number of photons which arrived within a given time window - key for Optical Communication - while bypassing the slow electronic response. The slow fall times from parallel connections within a Group may be ignored at the common output because the next Group has already fired & risen quickly (because of medium light intensity). The handover between each SPAD Group may be engineering to a time period (e.g. 10 picoseconds) much shorter than any signalling changes in the incident modulated signal. This may result in the effective common output signal from different switched SPAD groups to be a loose additive sum of multiple SPAD responses distributed across time. Therefore, the device output may produce an analogue voltage signal suitable for demodulation which is proportional to the intensity of the incident optical signal. This switching between Groups may enable continuous real-time photon detection rather than the conventional use of SPADs for imaging applications of photonic events as single snapshots.

[0076] The OWC signals may comprise medium- to high- intensity modulated optical signals. Medium to high light intensity may mean a steady time-varying stream of photons are continuously arriving at the photodetector array. Each Group may output a voltage from 0 to a maximum value (set by Group size) during each switching period. Because groups are constantly being enabled / disabled, saturation of the whole array at once may not be possible. Preferably the OWC signal may be a medium- to high- light intensity OWC signal, preferably from 1 MHz to 100GHz in bandwidth.

[0077] Preferably, each SPAD group may comprise 2 to 100,000 SPADs, depending on the application. For a low power OWC application, 2 to 1000 SPADs may be optimal or for a high throughput LiFi application 1000 to 100,000 SPADs may be employed. The number of SPADs in a group may be in direct relationship to the power requirements and data rate requirements.

[0078] Each SPAD group may be considered to detect and output a sum of photon arrivals.

[0079] Each SPAD group may be identical. Each SPAD group may comprise common switching circuity. Each SPAD group may be use the same voltages. Each SPAD may comprise the same size. Each SPAD group may have the same supply, for example, supply voltage. Each SPAD group may have a common output connection.

[0080] Each SPAD cell within each group may be primed to detect a photon when its group is activated. Each SPAD group may have common ground within the group.

[0081] Handover between SPAD Groups may comprise periodic sequential activation or threshold-based switching between Groups. When a Group is activated, the bias voltage may be applied to the SPAD cells within that Group and the SPADs are primed and will respond to photon arrivals. When the Group is deactivated, the SPADs may be disabled.

[0082] The switching circuity may be implemented with series Schottky diode pairs or transistors.

[0083] The SPAD groups may be activated one at a time by a bias control signal in a periodic handover sequence.

[0084] This periodic sequence between groups may be tuned to match the data rate depending on the application and provide continuous detection required for OWC signals. For example, a bias control period of 100 picoseconds may be set to match the symbol period of a 10 giga bits per second signal. This may enable the common output to continually produce a response from a new Group every 100 picoseconds.

[0085] Alternatively, automatic threshold-based switching may obviate the need for a bias control signal and may be implemented by switching circuitry per group wherein when a group detects sufficiently high intensity optical signal and produces a voltage drop above a threshold for example 0.1 to 0.3 volts. In response to a voltage drop, the switching circuitry may give the sign to switch to another SPAD group. This may be an automatic process of switching to provide the continuous handover between groups and ensure a group is always active with fresh SPADs primed to detect photons corresponding to the optical signal amplitude changing and provide continuous detection required for OWC signals.

[0086] Once switching has occurred from a first activated SPAD group to a second SPAD group, the first activated SPAD group may become deactivated. The automatic nature of switching between groups may be actuated by the incident optical signal continually contacting the SPADs. If a Group output voltage did not cross the switching threshold, the previous Group may stay connected to the output and continues to transfer to the output until another Group crosses the switching threshold.

[0087] Saturation of the photodetector array may be avoided by both methods, i.e. periodic and automatic handover because a single Group is prevented from contributing the output perpetually. It does not matter if a SPAD Group is saturated because another fresh Group in the available pool may be activated subsequently, whilst maintaining high data rates, for example 1 gigabit per second to 10 gigabits per second. This technique may be scalable and only limited by the handover switching time i.e. how fast the output can be switched from one Group to another.

[0088] For both periodic and automatic based switching, a switching rise time of 1 picosecond to 10 picoseconds may allow photon arrival rising edges from the SPAD cells to be captured within the time each Group is activated.

[0089] Each Group may sum active SPAD cells together, scaling the voltage amplitude by the number of active cells in a short time interval. For example, if 10 Cells fire almost simultaneously, the Group output signal may have an amplitude 10x higher than that of a single Cell. The rise time of the Group output may remain short enough to pass the highest frequency component of the expected OWC modulated signal to be received. Since Group rise time is preserved, the combined output signal follows steep signal transitions within a single Group activation period.

[0090] The total number of Groups in the photodetector array may also be large to ensure that individual SPAD Cells within Groups have time to recover from their own long decay times, ready to be fired again by subsequent photons in the signal.

[0091] Switching time to switch between Groups may be at least 2-10 times shorter than the rise time of the expected modulated signal, more preferably over 10 times shorter.

[0092] When a medium to high light intensity optical signal contacts an active SPAD group, an automatic cascade may be triggered whereupon SPAD cells within the group freely respond to photon arrivals. This second activated group may then return to a primed state while a third SPAD group becomes activated to detect a photon event and so on.

[0093] The SPAD may be configured to detect the rising edge of a photonic event. Preferably the decay phase may not be detected or may be excluded by each SPAD. The decay phase of a photonic event may be caused by electronic events, and hence use of conventional electronics may be avoided to further eliminate the decay phase.

[0094] The method may comprise switching circuitry for selectively switching between the SPAD groups to select one or more groups to provide their group response signal(s) to the common output thereby to produce an output signal for demodulation and / or decoding.

[0095] Switching circuitry may be identical within group and controlled by control signal which is the output signal. This signal initiates switching circuitry between group, a photonic event will trigger switching circuitry to switch between SPADs within a group.

[0096] Each SPAD group may have a Fast frequency response comprising a capacitively coupled output that provides a pulse proportional in amplitude to a photocurrent signal. The output from each SPAD group may be switched onto a common output from all the SPAD groups.

[0097] Signal output from each SPAD group may enter a voltage amplifier and thereafter into a circuit for demodulation, for example a Wi-Fi baseband.

[0098] Input impedance of any additional external amplifier may have to be matched to the output impedance of the circuit. Impedance matching may be typical of any RF signal. For optimum operation the impedance may be matched using known methods.

[0099] Preferably output from one or more SPADs may be sampled in a cycle sequence, more preferably by specialised analogue signal driven multiplexing (SDM) circuity.

[0100] Optionally switching may be between the groups to provides bias control between groups and when bias is used common supply between the SPAD groups may be switched too. Device impedance may need to be considered. Impedance matching (input and output) typical of RF signal, for optimal operation. Impedance of output signal may match the impedance of the output circuit, e.g. amplifier, for optimal performance, as known.

[0101] The controllable switching circuitry may be configured to switch between the plurality of groups at a switching frequency such that the output signal comprises a sum of at least part of the group responses, optionally only part of the group responses.

[0102] The switching time may be really the response time needs to be fast enough to catch the rising edge (a photon arrival). The switching time may be sufficiently short and / or the switching frequency may be sufficiently fast that the combined output comprises the rising edge of a group response.

[0103] The device may comprise a photon counting circuit. The method may comprise capturing photons in a photon counting circuit, capturing photon event information in the rise time, so a short switching time may be used to accommodate this.

[0104] As photon information is only indicated in the rising edge, switching from SPAD to SPAD and SPAD group to SPAD group may need to occur before the slow decay portion of the SPAD response starts.

[0105] The controllable switching circuitry may further comprise a plurality of switching elements provided for selectively switching the group outputs to the common output and / or wherein the controllable switching comprises a plurality of switching elements for providing an activation signal, for example, a bias signal, to the one or more groups.

[0106] In accordance with a further aspect there is provided a method of receiving an optical wireless communication signal using a plurality of silicon photomultiplier devices (SiPM) and signal switching circuitry; wherein at least one of the silicon photomultiplier (SiPM) devices comprises a plurality of single photon event photosensitive cells connected in parallel, the SiPM device having a first electrode arranged to provide a bias voltage to the photosensitive cells, a second electrode arranged as a ground electrode for the photosensitive cells, one or more quench elements connected between the photosensitive cells and one of the first and second electrodes, and a third electrode capacitively coupled to the one or more quench elements and to the photosensitive cells and arranged to provide a SiPM device output signal representative of light received by the photosensitive cells; the method comprising electrically connecting at least two of the plurality of the SiPM devices in parallel in a SiPM device group having common signal ground and further electrically connecting the at least two of the plurality of SiPM device output signals by at least one fast response switch to a common signal output conductor; wherein the output signal on the common signal conductor is an analogue signal representation of the optical wireless communication signal.

[0107] The method may comprise connecting the common signal ground of the at least two SiPM devices to the second electrode of each SiPM device.

[0108] The method may comprise measuring the SiPM device output signals with respect to the common signal ground.

[0109] The method may comprise electrically connecting each SiPM device output signal to the common signal output conductor by a respective fast response switch.

[0110] The method may comprise using the signal switching circuitry to switch the at least one fast response switch.

[0111] The method may comprise electrically connecting at least two SiPM devices in parallel to form an SiPM device group using the common signal ground and the common signal output conductor wherein each SiPM group is connected to the common signal conductor by a fast response switch.

[0112] The method may comprise using the signal switching circuitry to control the sequencing of switching of the at least one fast response switch such that at any one instant in time only a desired number of SiPM device output signals and / or a desired number of SiPM device output groups are connected to the common signal output conductor.

[0113] The desired number may be a number less than a threshold number. The method may comprise using the signal switching circuitry to control the sequence of switching of each fast response switch such that only SiPM devices or only groups of SiPM devices which produce a device output signal above a threshold level are switched to connect the device output signal to the common signal conductor.

[0114] The method may comprise using the signal switching circuity to switch the SiPM devices or groups of SiPM devices in a sequence which is at least partly determined by the device output signal level.

[0115] The method may comprise switching the sequence in a time period less than the decay time of the SiPM device output signal or the decay time of the SiPM device group output signal on the common signal conductor so that at least some of the SiPM device output signals and / or at least some of the SiPM device group output signals are additive and / or combined on the common signal conductor.

[0116] The method may comprise using signal switching circuitry comprising a fast response bias switch connected between the first electrode and a bias voltage supply such that the SiPM device or SiPM device group may be selectively switched on or off as part of a sequencing of switching of the SiPM device or SiPM device group.

[0117] The method may comprise switching at least one of the fast response switch or the fast response bias switch such that the time interval between switching on of different fast response switches or different fast response bias switches is less than half the periodic time of the maximum frequency of modulation of the optical wireless communication signal to be received.

[0118] The method may comprise using a fast response switch or using a fast response bias switch which comprises a Schottky diode series pair type switch.

[0119] The method may comprise using a single photon even photosensitive cells which is a Single Photon Avalanche Diode (SPAD).

[0120] The method may comprise demodulating and decoding the SiPM device output signal or the SiPM device group output signal to obtain data comprised in the received optical wireless communication signal. In accordance with a further aspect there is provided an optical wireless communication receiver configured to receive an optical wireless communication signal comprising encoded data, the optical wireless communication receiver comprising: a plurality of silicon photomultiplier devices (SiPM) and signal switching circuitry; wherein at least one of the silicon photomultiplier (SiPM) devices comprises a plurality of single photon event photosensitive cells connected in parallel, the SiPM device having a first electrode arranged to provide a bias voltage to the photosensitive cells, a second electrode arranged as a ground electrode for the photosensitive cells, one or more quench elements connected between the photosensitive cells and one of the first and second electrodes, and a third electrode capacitively coupled to the one or more quench elements and to the photosensitive cells and arranged to provide a SiPM device output signal representative of light received by the photosensitive cells; wherein at least two of the plurality of the SiPM devices are electrically connected in parallel in a SiPM device group having common signal ground and wherein the SiPM device output signals are electrically connected by at least one fast response switch to a common signal output conductor; wherein the output signal on the common signal conductor is an analogue signal representation of the optical wireless communication signal and the receiver further comprises receiving circuitry to demodulate and decode the output signal to obtain the encoded data.

[0121] In accordance with a further aspect, there is provided, a device for receiving an incoming optical wireless communication (OWC) or OWC light signal comprising: a plurality of photodetectors each configured to produce a photo detection signal in response to light being received by the photodetector; wherein the plurality of photodetectors are arranged in a plurality of groups, wherein each group is configured to produce a group response signal representing the photon detection signals produced by the photodetectors within the group in response to receiving light, wherein the plurality of groups are configured to be connected to a common signal output. The device may comprise switching circuitry for switching each group response signal onto the common signal output thereby to produce a signal output signal representative of the OWC light signal received by the device and suitable for demodulation and / or decoding by further processing circuitry. The device may comprise first switching circuitry at an output of each group, the first switching circuitry configured to switch the output to the common signal output in accordance with a first switching scheme. The device may comprise second switching circuitry at an input of each group, the second switching circuitry configured to switch an input, for example, a bias voltage or supply, between the plurality of groups of photodetectors and / or a further plurality of groups of photodetectors in accordance with a second switching scheme. Each further group may comprise one or more of groups of the plurality of groups of the photodetectors. Each further group may comprise a common supply and / or bias input.

[0122] Features of one aspect may be provided as features of another aspect. For example, method claim may be provided as device features and vice versa.

[0123] Brief Description of Figures

[0124] Various aspects of the invention will now be described by way of example only, and with reference to the accompanying drawings, of which:

[0125] Figure 2 shows an arrangement of SPAD detectors

[0126] Figure 3 shows simultaneous photon arrival at a third SPAD and a fourth SPAD followed by a later photon arrival at the third SPAD;

[0127] Figure 4 depicts an example of an output signal based on photon events detected by SPADs;

[0128] Figure 5 is a block diagram of a device, in accordance with an embodiment;

[0129] Figure 6 is a block diagram of a device, in accordance with an embodiment;

[0130] Figure 7 is a schematic diagram of a device, in accordance with an embodiment;

[0131] Figure 8 is a diagrammatic representation of a light signal over time;

[0132] Figure 9 shows a diagrammatic representation of single photon arrivals;

[0133] Figure 10 shows a diagrammatic representation of an individual SPAD Cell Outputs;

[0134] Figure 11 shows a diagrammatic representation of a SPAD Group Output slow fall time due to capacitance of SPADs in parallel, and

[0135] Figure 12 shows a diagrammatic representation of a switching Time to switch between Groups.

[0136] Detailed Description

[0137] The current invention will now be described by way of example only with reference to the examples and figures. The current description relates to methods and devices for high data rate OWC applications particularly to methods and devices for configuring single photon photodetectors comprising summing circuitry to produce high Signal to Noise Ratio for OWC receiver applications.

[0138] Figure 2 depicts a SPAD array having a plurality of photodetectors, in this embodiment SPADs. In the following description the term SPAD is used to refer to a single photon avalanche diode. It will be understood that each SPAD is provided together with required circuity for operation, for example, a readout circuit and / or quenching circuitry is provided. The term SPAD used in the following can therefore refer to a SPAD cell or device that includes additional circuitry. The readout circuit may comprise, for example, a transimpedance amplifier or a series resistor. The photodetectors are configured to produce an electronic photodetection signal in response to light being received by the photodetector. In some embodiments, the readout circuit comprises a measuring circuit for measuring a property of the signal output.

[0139] Figure 2 depicts a plurality of SPADs 22a, .. 22p. The plurality of photodetectors are arranged in a plurality of groups. Each group is configured to produce a group response signal, for example, an accumulated group response signal in response to receiving light, for example, in response to one or more photon detection events. The group response represents In Figure 2, a single group 24 of SPADs is illustrated. The group includes four SPADs 22a, 22b, 22c, 22d. The grouping of SPADs is described in further detail in the following. The SPAD array depicted in Figure 2 may form part of a silicon photon multiplier (SiPM) device. As can be seen from Figure 2, the SPADs are connected in a parallel arrangement. The SPADs of Figure 2 will be understood to include the single photon detector together with a self-quenching circuit. The selfquenching circuit operates to stop an avalanche ionisation process within a period of, for example, 1 ns to 20ns.

[0140] Figure 3 is a circuit diagram of a group of SPADs. Figure 3 depicts a group of SPADs 24 in accordance with an embodiment. As depicted in Figure 3, each SPAD of the group has a common signal output 20. In this embodiment, the SPADs also have a shared bias 28. The shared bias may be an example of, or be provided together with, a shared activation signal. In the present embodiment, the group of SPADs is provided with common components such as a common bias input and a common signal output. In the present embodiment, the group of SPADs is provided as a Silicon Photomultiplier module. A silicon photomultiplier is a photon sensitive device having a plurality of SPADs formed on a common silicon substrate.

[0141] As depicted in Figure 3, each SPAD in the group of SPADs is provided together with an associated capacitor. In the present embodiment, the associated capacitors form part of the SiPM. As such the group includes first SPAD 22a and associated first capacitor 26a, second SPAD 22b and associated second capacitor 26b, third SPAD 22c and associated third capacitor 26c, fourth SPAD 22d and associated fourth capacitor 26d. Each SPAD and associated capacitor are arranged in a parallel circuit arrangement with a shared bias signal 28 and common signal output 20. Each SPAD is capacitively coupled to the common signal output, via their associated capacitor.

[0142] In the present embodiment, it will be understood that each SPAD corresponds to a SPAD device or cell but is referred to as a SPAD for brevity. In the present embodiment, each SPAD device or cell includes a quench resistor. Each SPAD is connected in series with the quench resistor between a cathode and an anode. The SiPM is arranged such that each SPAD of Figure 3 is connected to a common cathode and common anode such that a shared bias 28 can be provided to the group of SPADs. It will be understood that the bias is provided to each SPAD simultaneously by providing a bias signal i.e. a steady voltage or current provided to the SPAD. In the present embodiment, the SiPM operates in Geiger mode in which the common cathode is positively biased with respect to the common anode.

[0143] In addition to the quench resistor, each SPAD is connected to its associated capacitor at a node between the SPAD and quench resistor. As depicted in Figure 3, the capacitors are coupled to a common signal output. By providing a capacitor for each SPAD or SPAD device, a fast output response may be obtained. In particular, a group response can be obtained.

[0144] In operation (in Geiger mode) the group of SPADs produces a photocurrent proportional to the number of SPADs that have detected a photon. It can be understood that either the anode or cathode of the SiPM can be used as a standard output for the SiPM and connected to appropriate read out circuitry. In the present embodiment, the associated capacitor offers an alternative (or additional) output from the SPAD. In operation, in response to receiving a photon, the output via the capacitor outputs a pulse having a rise time and pulse width of the order of 1ns. By accumulating such pulses, the group of SPADs are configured to form a group response proportional to the number of SPADs that have fired and can therefore provide information on the number of photons detected. In embodiments, the group response is coupled directly to a readout circuit, for example, a digitizer circuit for obtaining a digital signal.

[0145] Figure 4 depicts an example of an output signal for a group of SPADs (for example, provided as a SiPM) based on photon events detected by SPADs. Figure 4 depicts simultaneous photon arrival at a third SPAD and a fourth SPAD at a first time followed by a later photon arrival at the third SPAD at second, later time. The y-axis of the plot of Figure 4 is a voltage of a common signal output of the group of SPADs shown in Figure 3. As can be seen from Figure 4, an accumulated signal is formed when simultaneous photons arrive at the third SPAD and the fourth SPAD at the first time. The accumulated signal at the first time includes a first contribution 122 from the third SPAD and a second contribution 124 from the fourth SPAD. The accumulated signal at the second time includes only a contribution 126 from the third SPAD.

[0146] Figure 5 shows a device in accordance with an embodiment. The device has switching circuitry, with SiPM1 to SiPM4 forming a photodetector array with each SiPM connected via a fast switch (such as a Schottky diode series pair) to a common signal output. The fast switch is an example of a switching device provided for each group of SPADs. Together the switching devices form switching circuitry.

[0147] Figure 5 depicts a circuit having a first SiPM 132a, a second SiPM 132b, a third SiPM 132c, a fourth SiPM 132d. Each SiPM can be understood as including at least one group of SPADs that may be, for example, provided on a common substrate. Each SiPM has an SPIM output which corresponds to the common signal output for the SPADs of the SiPM, as described with reference to Figure 2 to 4. Each group of SPADs is configured to output a group response as described above.

[0148] A switching device (referred to in the following as a switch, for brevity) is provided for each SiPM and the output of each SiPM is connected to a common output 136 by a respective switch: first SiPM 132a by first switch 134a, second SiPM 132b by second switch 134b, third SiPM 132c by third switch 134c, fourth SiPM 132d by fourth switch 134d. Each SiPM can be understood as including at least one group of SPADs that may be provided on a common substrate. Each SiPM has an output which is a common signal output for the SPADs of the SiPM. Each switch is a controllable switch having a control signal input (138a, 138b, 138c, 138d). Each switch is configured to be controlled by a control signal received via its respective control signal input. The outputs of each SiPM are themselves coupled to a common output 136 via their respective switch. For example, first SiPM 132a is coupled to the common output 136 via the first switch 134a.

[0149] The first to fourth switches of Figure 5 are controllable switches and form controllable switching circuitry. The switching circuitry is configured to selectively switch between each SiPM output to the common output. In some embodiments, a controller (not shown) may be provided to control the switching circuitry. In some embodiments, the controllable switching circuitry comprises the controller.

[0150] In operation, the outputs of the SiPMs are coupled to the common output line 136 by controlling their respective controllable switches 134a to 134d. The switching of the SiPM is controlled in accordance with a switching scheme. The switching scheme may be a switching sequence, for example, a sequential and / or periodic scheme.

[0151] In the present embodiment, each SiPM corresponds to a controllable group of photodetectors. The controllable group of photodetectors is controllable via a respective controllable switching device. In some embodiments, each group of photodetectors is connected to the common signal output in response to receiving a control signal via the input to the switch. In some embodiments, the SiPM is configured to be connected to a common output in response to one or more of the SPADs of the SiPM detecting a photon.

[0152] In some embodiments, a sequential switching scheme is applied to couple each of the SiPMs to the common output in turn. The scheme is such that only a single SiPM is coupled to the common output at a point in time. The frequency of switching between SiPMs can be in a desired range, for example, between 10 MHz and 100 GHz. As described with reference to Figure 12, the switching frequency may be in dependence on a desired data rate and / or dependent on a property of the incoming optical wireless signal. In some embodiments, the switching scheme is based on a threshold of signals of each group. Such schemes may be referred to as signal driven multiplexing schemes. In such embodiments, the outputs from each group of SPADs (or each SiPM in embodiments in which each SiPM corresponds to a single group) are summed and distributed in time randomly. The switching between groups is dependent on the received signals at each group output and the switching of groups to the common output is dependent on a group response being above a threshold.

[0153] In some embodiments, the switching scheme is based on a combination of threshold based and sequential switching schemes.

[0154] In some embodiments, the switching scheme may comprise activating groups of SPADS in sequence. Once switching has occurred from a first activated SPAD group to a second SPAD group, the first activated SPAD group may become deactivated. In some embodiments, a threshold or signal driven scheme is applied to only active SPAD groups. The scheme may be threshold based, such that if a group output voltage did not cross the switching threshold, the previous group may stay connected to the output and continue to transfer to the output until another group crosses the switching threshold.

[0155] In some embodiments, the SiPM array is arranged so that the SPADs of the SiPM array are operable to produce a fast output. As described with reference to Figure 3, the fast group output may include summed pulse widths formed through associated capacitors of the SPADs.

[0156] Any suitably fast switching device can be used. In some embodiments, the switching device may comprise a diode pair, for example a Shottky diode pair, to allow a fast output signal from SiPMs to be transferred to a common readout node while effectively isolating fast outputs from the remaining pixels. In embodiments, each switching device shown in Figure 5 is a Shottky diode pair. In embodiments, the Shottky diode pair is configured such that in response to receiving an incoming photon, the output of the SiPM produces a positive voltage pulse that turns the first Shottky diode of the pair on while turning the second Shottky diode of the pair off. When the first Shottky diode is turned on, the fast pulse from the SiPM is transferred to the common readout line. The Shottky diode switches may have a voltage drop across them, to attenuate the output signal from the SiPM similar to an insertion loss. The Schottky diode pair may create a symmetry whereby the sum of current at the fast node of the SiPM is constant. The positive signal on the common readout node also acts to reverse bias the Shottky diodes of the other SiPMs therefore suppressing a contribution from any other signals generated by the other SiPMs. As such, the Shottky diode pairs may act to isolate groups of photodetectors.

[0157] As a non-limiting example, the SiPM sensors may be ON Semiconductor C-, J- and R- Series SiPM sensors. Such sensors provide a fast, capacitively coupled output that gives a high speed output signal.

[0158] In further embodiments, the SPAD array is split into time gated SiPM blocks that capture snapshots of the incident signal during predetermined cycle periods.

[0159] Figure 6 depicts an alternative embodiment in which part of the switching circuitry is also provided at a bias switch. As described with reference to Figure 5, the circuit has four sets of SPADs provided in this embodiment, as SiPM1 232a, SiPM2 232b, SiPM3 232c, SiPM4 232d. Each SiPM is connected to a common signal output 236 via controllable switching devices (234a, 234b, 234c, 234d). Each controllable switch is controlled by a respective control signal (238a, 238b, 238c, 238d) as described with reference to Figure 5.

[0160] In the embodiment of Figure 6, each SiPM corresponds to a group of SPADs. In alternative embodiments, a group of SPADs may include the SPADs of more than one SiPM.

[0161] In the embodiment of Figure 6, the SPADs of SiPM1 and the SPADs of SiPM2 are grouped together to form a first further group or block of SPADs. Likewise, the SPADs for SiPM3 and the SPADs for SiPM4 are grouped together to form a second further group or block of SPADs. The first and second further groups or blocks of SPADs are served by a common bias signal. In the embodiment of Figure 6, the bias signals are delivered to their respective groups via further controllable switching devices. The first further group (including the SPADs of SiPM1 and SiPM2) has a further group bias switching device 240a and the second further group (of SiPM3 and SiPM4) has a further group bias switching device 240b. Each bias switching device is controlled by a respective control signal (first control signal 242a and second control signal 242b). Each further group is coupled to a common bias signal input 244 via their respective switch.

[0162] In the embodiment of Figure 6, the bias switching devices and corresponding control signal inputs form part of a controllable switching circuitry. In embodiments, the controllable switching circuitry operates such that a group or block of two or more groups is provided with a bias voltage at a given time such that only one group or block of two or more groups is active at the same time. The bias signal is also referred to as an activation signal. In response to receiving an activation signal the group of SPADs can be considered to be in an active state. In absence of an activation signal, the group of SPADs are considered to be in an inactive state.

[0163] In the present embodiment, the active bias switching is combined with a switching scheme at the output of each SiPM. The scheme at the output may be as described with reference to Figure 5. In accordance with embodiments, the scheme at the output may be a signal driven multiplexing scheme or other threshold based scheme.

[0164] In the embodiment of Figure 6, the bias switches form part of a first switching circuitry and the output switches form part a second switching circuitry. The two switching circuities may operate in accordance with a switching scheme. In some embodiments, the first switching circuitry operates in accordance with a first switching scheme and the second switching circuitry operates in accordance with a second switching scheme. The first switching scheme may be periodic scheme (handover scheme) in which bias signals are selectively provided to groups of SiPMs at a switching frequency. In some embodiment, the periodic scheme applied to the bias circuitry is combined with an automatic (threshold based) scheme applied to the output switching circuitry. In some embodiments, only the bias switching circuitry is provided without the output switching circuitry.

[0165] By providing bias voltage control the device may be better adapted to higher intensity light reception. In some cases, depending on light level the switching of all SiPM within a group may be almost simultaneous and in this circumstance it may be desirable to create multiple groups (or blocks) of SiPM where the groups (or blocks) of SiPM are switched in a more deliberate or sequenced fashion. This may be implemented by for example, bias voltage control of each SiPM group such as shown in Figure 6. The switching of each SiPM group (or block) is then sequenced such that one group (or block) is active at one time, the maximum SiPM group size may be determined by the output signal bandwidth required to successfully represent the bandwidth of the incoming light signal to be detected.

[0166] In some embodiments, sequencing switching of groups of SiPM may be implemented by SDM type circuitry applied to the bias of the SiPMs and the presence of a SiPM output signal at one SiPM group may be used to disable the output from the previous SiPM group. Alternatively, a period sampling type sequence may be applied to control the SiPM bias and therefore the group SiPM signal actively being transferred to the common output.

[0167] In embodiments, the switching circuitry comprises first and second switching circuities. The switching circuitries are configured to switch at a switching frequency such that the output signal comprises a sum of at least part of the group responses, optionally the output signal comprises only part of the group responses. The switching frequency in the range 10MHz to 100GHz. In an embodiment, the switching circuitry comprises first switching circuitry at the input of the groups (or blocks of two or more groups) of SPADs and second switching circuitry at the group output. Different switching schemes may be applied to the first and second switching circuitry. In embodiments, the bias switching circuitry may be configured to switch at a switching frequency in the range 10 MHz to 100 GHz and the output switching frequency may be configured to automatically switch in accordance with a threshold based scheme. In embodiments, the bias switching circuitry may be configured to switch based on an external signal, for example, from a controller and the second switching circuitry is configured to switch based on internal signals from the groups of SPADs.

[0168] In embodiments, the bias control switching circuitry comprises a plurality of switching devices. The switching devices may comprise, for example, a transistor or a semiconductorjunction or other suitable switching device. In the above-described embodiments, the groups of photodetectors are formed by SiPM arrays. In further embodiments, groups of photodetectors may be formed using alternative types of photodetector. In some embodiments, groups of photodetectors are formed using Multi-Pixel Photon Counter (MPCC). In embodiments, the group of photodetectors comprise any group configured or arranged to output a group response with a portion, for example, a rising edge, that represents the number and / or intensity of the receive light by the group.

[0169] In some embodiments, the photodetectors comprise filtering circuitry or are provided together with filtering circuitry to produce a filtered output. The filtering may retain a portion of the output of the photodetector output. For example, in embodiments, the filter outputs a filtered output that includes a pulse or at least a rising edge that includes photon count and / or photon intensity information. The filter may be a high pass filter and / or a DC block.

[0170] In further detail, when the photodetector receives irradiance, that may cause the photodetector to draw more current. If the additional current triggers an avalanche a pulse will be generated at an output. The pulse is a fast signal representing the arrival of a photon and / or a photon detection event. However, if an avalanche is not triggered, a bias current may be raised. This output would be filtered by the high pass filter and therefore not form part of the filtered output. In accordance with embodiments, the high pass filter may comprise a DC block that filters out a constant bias voltage leaving only the time-varying signal. At the filtered output, only signals above a certain frequency are output.

[0171] The output of the SiPM consists of pulses generated by photon arrivals. Due to the passive quenching method used, the pulse's rising edge may be extremely sharp, and for example, have a duration on the order of hundreds of picoseconds. The falling edge is much slower, for example, extending over tens of nanoseconds. When a single photon is detected, it produces a long pulse, making it difficult to distinguish subsequent photons that arrive closely in time, as their pulses tend to merge. A method of addressing this issue is to provide a group of photodetectors configured to output an output that can be considered as a fast output. Such an output has a reduced falling edge. This may be obtained, for example, by filtering. By doing so, the timing of closely spaced photon arrivals may be clearly differentiated, enabling precise ultra-fast timing measurements. From an optical wireless communication perspective, both the standard output and the fast output can be utilized for signal detection, with the fast output offering slightly better performance.

[0172] Figure 7 depicts a receiver, in accordance with an embodiment. In the embodiment of Figure 7, the receiver is configured to control the summing process by dividing the array of SiPMs into time gated SiPM blocks which capture snapshots of the incident signal during predetermined cycle periods. The blocks may be formed of two or more SiPM array or other groups of photodetectors.

[0173] Figure 7 depicts a SiPM array 302. Without limitation, in this embodiment, each SiPM has 2880 SPADs. The SiPM array is coupled to controllable switching circuitry module 304 and a summing module 306. In the present embodiment, the controllable switching circuitry module 304 is configured to switch between groups at 24 GHz. The summing module 306 is configured to receive output from each SPAD group and form an accumulated or summed output. A controller (not shown) may also be provided for controlling the switching circuitry.

[0174] In operation, an optical intensity modulated signal 310 is received at the SiPM array 302. The optical intensity modulated signal is a time-varying optical signal propagating through free space. The signal is incident on the SiPM array 302 and the switching circuitry performs a multiplexing process, for example, in accordance with a switching scheme to form an accumulated signal at module 306. The resulting summed output is an output voltage signal 312. The output voltage signal is at time-varying signal. The output voltage signal or a signal derived from the output voltage signal may be suitable for demodulation

[0175] In accordance with embodiments, the summed output or signals derived from the summed output are provided to further processing circuitry. The further processing circuitry incudes demodulation and / or decoding circuitry configured to demodulate and / or decode the output signal or signals derived from the signal.

[0176] Figure 8 depicts an illustrative plot of an incident optical wireless communication signal 402, in particular, showing the variation in the light intensity of the incident signal over time. Figure 9 depicts a corresponding illustration of photon arrival. Each line in Figure 9 corresponds to a photon arrival. For example, a single photon arrival is marked by line 404.

[0177] Figure 10 depicts output of individual SPAD outputs in a group of SPADs (for example, a group of SPADs provided together as a SIPM). Each line in Figure 10 corresponds to a detection of a photon by a SPAD. Each SPAD is configured to output a signal voltage (here indicated by x).

[0178] The output of a SPAD can therefore be characterised as a rapid spike to a peak voltage (x) followed by a fall time. In accordance with embodiments, such an output may correspond to a pulse having a pulse width under 1 ns. The fall time will be understood as significantly longer in duration than the initial rapid spike. The fall time is due to an intrinsic recovery or dead time. During the falling portion, the SPAD is not primed to detect a further photon.

[0179] Figure 11 depicts the output of a group of SPADs. Like in Figure 9 and 10, each SPAD outputs an event.

[0180] As can be seen from the illustrative plot of Figure 11 , a group response from the group of SPADs accumulates over time. The output voltage is summed output from the SPADs in the group. The magnitude of the group response has a maximum value dependent on the number of SPADs in the group. In this embodiment, the group has 10 SPADs so the group response signal may reach a maximum value of 10x (where x is the output voltage of each separate SPAD).

[0181] While Figure 11 depicts a group having 10 SPADs, it will be understood that a group may have more SPADs. Figure 11 depicts a group response in which rising edges (or pulses) produced by each SPAD are combined into a group response, for example, as described with reference to Figure 3. The group response of Figure 11 has a first rising portion 410 and a second falling portion 412. The rising portion has a short rise time relative to the slower fall time. The slower fall time may be due to the capacitance of parallel SPADs. The short rise time may be maintained by restricting the number of SPAD cells in the group. Figure 12 depicts combination of outputs from a plurality of groups of SPADs. Figure 12 depicts a number of group responses produced by different groups. Each group response is characterised by a rising portion (for example, rising portion 414 of example group response) together with a falling portion (for example, falling portion 416). By applying a switching process between the groups of SPADs, for example, as described above, an output signal 418 is obtained. As can be seen, the output signal is a sum of part of the group responses. The combining of group responses can be considered as part of a sampling process in which the outputs of the groups are sampled and added to a combined output signal. The SPAD group scales up the voltage by the number of active SPADs in a short time interval,

[0182] As can be seen, the output is a time-varying analogue signal. In embodiments, by rapidly switching between group outputs, only the rising portions contribute to the combined signal and the slower decay portion is ignored. The group rise time is preserved to allow the combined signal to follow steep signal transitions. The switching rate between groups is such that at least one property of the OWC signal is retained in the output signal. The combined signal may be an analogue electronic signal that has one property related to, for example, proportional to, a corresponding property in the incoming optical signal. For example, the combined signal may have an amplitude or voltage corresponding to the intensity of the incoming optical signal.

[0183] It will be understood that Figure 12 depicts an accumulated signal output of more than one group of SPADs. In embodiments, each group corresponds to a SiPM device, but SPADs within a SiPM may be grouped together. In some embodiments, a maximum number of SPAD cells may be included in a group to maintain a desired short rise time.

[0184] The incoming optical wireless communication signal is a modulated optical wireless communication signal. The incoming optical wireless communication signal carries data. The combined signal is demodulated and / or decoded by demodulation and / or decoding circuity to obtain data modulated onto the optical wireless communication signal.

[0185] The above embodiments refer to medium to high light intensity. Medium to high light intensity may depend on the photodetectors of the device. For example, smaller photodetector arrays will saturate more easily than larger arrays, therefore the threshold for medium to high intensity would be relatively low. In contrast, for SiPMs with larger array sizes, the same intensity would be considered higher. For instance, with OnSemi J-30020 SiPM, significant nonlinearity due to dead time may be measured when the received optical power exceeds 1 pW. For that specific photodetector technology, light having an optical power above 1 pW can be defined as the medium to high light intensity. However, medium to high light intensity may be understood as an irradiance high enough that the plurality of photodetectors experience significant saturation in the absence of switching.

[0186] As described above, a signal driven multiplexing scheme may be provided. The following comments are provided. Under normal circumstances on detection of a light signal received by a SPAD within an SiPM, for example SiPM1 , then the respective switch of SiPM1 is closed and the SiPM1 output signal is connected to the common output. SiPM1 becomes disconnected from the common output when the next SiPM within the SiPM group generates an output signal. If the next SiPM detection event takes place within a sufficiently short time then the decay time of the first SiPM output signal becomes partly additive to the next SiPM output, thereby increasing the signal amplitude of the signal on the common output while maintaining good SNR (signal to noise ratio).

[0187] In this way only one SiPM is connected to the common output of the SDM circuit at any one time and therefore the additive increase of the amplitude of the output signal is not subject to the problems of signal response associated with additive capacitance resulting from parallel connection of multiple SiPM.

[0188] If the bandwidth of the light signal that is to be detected is known, then the SiPM and SiPM group and SDM circuitry configuration is designed to have an operating bandwidth that is suitable for detection of incoming light signal to be received. For example, the number of SiPMs / SPADS within an SiPM group can be selected to set the maximum desired or target SNR for the expected modulation characteristics of the OWC signal.

[0189] In some cases, depending on light level the switching of all SiPM within a group may be almost simultaneous and in this circumstance it may be desirable to create multiple groups of Si PM where the groups of Si PM are switched in a more deliberate or sequenced fashion. This may be implemented by for example, bias voltage control of each SiPM group as shown in Figure 6. The switching of each SiPM group is then sequenced such that one group is active at one time, the maximum SiPM group size being determined by the output signal bandwidth required to successfully represent the bandwidth of the incoming light (or OWC) signal to be detected.

[0190] Sequencing switching of groups of SiPM may be implemented by SDM type circuitry applied to the bias of the SiPMs and the presence of a SiPM output signal at one SiPM group may be used to disable the output from the previous SiPM group. Alternatively, a period sampling type sequence may be applied to control the SiPM bias and therefore the group SiPM signal actively being transferred to the common output.

[0191] Further comments on Figure 12 are provided. The Group output voltage transferred to the common output at the time of switching (regular and periodic if the light intensity is medium to high) forms the ‘sample’ of the signal at that point in time.

[0192] Each single photon detector (SPAD) in a group responds to a single photon arrival at a given time with a short rise time and long fall time decay. The Group sums active Cells together, scaling the voltage amplitude by the number of active cells in a short time interval. For example, if 10 Cells fire almost simultaneously, the Group output signal has an ‘events’ amplitude 10x higher than that of a single SPAD Cell. The rise time of the Group output will remain short enough to pass the highest frequency component of the expected LiFi OWC IM signal to be received. In this example, we have designed the Group size to have a total of 1000 Cells (for an SNR target). This means a signal with 100x (+20 dB) more resolution and dynamic range than a single 10-Cell Group can be reconstructed by the receiver (RX).

[0193] In this example SPAD Groups are activated with the periodic sequential bias enable control signal. This may be as described above, as above, with reference to Figure 6, in which a connection of a common bias signal to one or more groups of SPADS is controlled by control signals.

[0194] Embodiments may provide approximately 100x improvement in SNR compared to a single sum of SPAD Cells. The total number of Groups in the RX array must also be large to ensure that individual SPAD Cells within Groups have time to recover from their own long fall times, ready to be fired again by subsequent photons in the signal

[0195] The (e.g. Schottky diode pairs or transistors) switches may have a voltage drop across them, which attenuates the output signal similar to an insertion loss.

[0196] The method and device of the current embodiment may obviate the need for conventional electronic circuity which is a significant source of noise.

[0197] Due to the method of measurement a combined capacitance and resulting undesirable slow signal bandwidth limitation may be avoided.

[0198] In the example shown in Fig. 7, the Group may have a total of 1000 Cells (e.g. designed for an SNR target). This means a signal with 100 times (+20 dB) more resolution and dynamic range than a single 10-Cell Group may be reconstructed.

[0199] Figure 12 shows a diagrammatic representation of a switching Time to switch between SPAD Groups. This period is very short compared to rise time of each Group Output (at least 2 to 10 times shorter). Since SPAD Group edge rise time is preserved, the combined output signal follows steep transitions within a single Group switch period. The slow fall times from parallel connections within a Group are ignored at the common output because the next Group has already fired & risen (because of the OWC medium light intensity). This SPAD Group output voltage does not cross the switching threshold, so previous Group stays connected to output. Switching time to switch between Groups may be at least 2-10 times shorter than the rise time of the expected modulated signal, more preferably over 10 times shorter. The example shown in Figure 12 simplifies this concept and may have 3 times shorter switching time.

[0200] Being able to capture the photon event signal during the rise time of the photocell response enables the circuitry to detect LiFi signals with high fidelity, (because the undesirable and slow decay time of the SPAD signal is heavily dependent on output circuitry - it is ignored by switch deactivation -and the subsequent fast switching in of further different SPAD single photon event rising signals enables this group switching circuitry to produce an output signal that may represent a continuous photonic event and (assuming sufficiently fast switching) a continuous high frequency signal such as a LiFi signal. As such, a sampling of the rise time of the group responses may be performed.

[0201] When multiple event detections occur, the circuitry enables the SPAD signal outputs to be represented on the signal common output line as a cumulative signal as shown in the Figure. It is this accumulation of switched photon rising output signals on the common signal output that enables accurate representation of a continuously varying received light signal.

[0202] For optimum operation the switching circuitry may therefore needs to operate sufficiently quickly to see the SPAD photon event signal rise time. Some benefits of using SPAD in this circuitry arrangement are very low power consumption, and higher sensitivity, for example much longer optical signal transmission compared with today’s usual photodiode arrangements.

[0203] Further non-limiting concept regarding the device, method are also provided.

[0204] The device may enable the successful use of SPADS for detection of OWC modulated signals.

[0205] In operation a SiPM that receives a light signal and is connected to the common output via its respective switch is disconnected automatically by the respective switch from the common output when the next SiPM within the SiPM group generates an output signal. The decay time of the first SiPM output signal then becomes partly additive to the next SiPM output signal, thereby increasing the signal amplitude of the common output signal while maintaining good SNR - this decay time being partly dependent on the characteristics of the output circuitry.

[0206] In this mode only one SiPM is connected to the common output at any one time and therefore the additive increase of the amplitude of the output signal in this mode is not burdened with a lower performance signal bandwidth associated with an additive capacitance resulting from parallel connection of all of the multiple SiPM in the photodetector. By controlling each individual SiPM switch the number of SiPM outputs that are connected to the common output is controlled and the capacitance of the circuitry and hence the bandwidth of the signal is thus only dependent on the number of SiPM that are enabled at any one time.

[0207] SPAD may be understood as single photon avalanche diode. SiPM may be understood as a silicon photo multiplier and may comprise circuitry to sum the output of one or more SPADs producing a combined / summed signal with amplitude greater than an individual SPAD. SPAD outputs are connected in parallel in the SiPM with corresponding increase in capacitance. The chosen size of SiPM may be dependent on performance characteristic required for the expected OWC signal. More than one SiPM may be added into an SiPM group or SiPM block.

[0208] A SiPM Group may correspond to Individual SiPMs combined into a SiPM group within the photo detector.

[0209] SDM may refer to a Signal Driven Multiplexer circuitry that takes the output signal from each SiPM and sums the SiPM outputs using fast switches operated by a switching method which produces an additive output signal on a common output where the additive output signal comprises at least part of the signal output from each SiPM that is receiving a light signal.

[0210] In some embodiments, the OWC signal is a unipolar signal. In some embodiments, the OWC signal has a DC bias. In some embodiments, the OWC signal comprises light having medium- to high- light intensity.

[0211] Without limitation, medium to high light intensity may mean a steady stream timevarying of photons are continuously arriving at the photodetector array. This means that each Group will output a voltage from 0 to a maximum value (set by Group size) during each switching period. An added benefit may be that, because the Groups are constantly being enabled / disabled, simultaneous saturation of the whole array is impossible.

[0212] As described above, the group response may have a rising portion and a falling portion. The rising edge may have a duration referred to a rise time. Without limitation, the rise time may be in the range 1 ps to 100 ps, optionally up to 500 ps, optionally up to 1 ns. The switching frequency may be selected based on the rise time of the group responses. The switching frequency may be such that each group (or one or more groups) is activated and / or coupled to the signal output for a switch time period. In an embodiment, the switching circuitry may be configured so that the switch time period (the switching time to switch between groups) is 7.5 time shorter than rise time of expected signal. In some embodiments the switching period is 3x shorter switching time than the rise time. The switching time period may be between 2 and 10 shorter, optionally 5 and 8 times shorter than the rise time. Optimal switching may be performed at a switching frequency that is between 2 and 10 times faster, more optionally between 5 and 8 times faster than the rise time of each group output.

[0213] Figure 11 shows slow fall time due to capacitance of SPADs of parallel. Figure 12 demonstrates that since the group rise time is preserved the combined output signal rides steep transitions within a single group switch period. The switching time to switch between groups is very short compared to the rise time of each group output. It may have to be at least 7.5 x shorter for idea to work analogue equivalent to ‘sampling’. The slow fall times from parallel connections within a group are ignored at the common output because the next group has already fired and risen quickly (because of medium light intensity). If the group output voltage does not cross the switching threshold, the previous group stays connected to output.

[0214] The following non-limiting comments are provided on a SPAD detector architecture. The front end may implement a method of configuring large arrays of single photon detectors using summing circuitry to produce high SNR for OWC applications. The purpose of the following comments was an investigation to demonstrate a flat 30 dB SNR.

[0215] Tests were performed in which a signal to noise ratio (SNR) improvement was proven with the SDM SiPM array compared to an individual SiPM. This improvement confirms the array is being summed as hypothesised, enabling the 30 dB SNR target to be reached. Measurements were taken across an optimum distance of 2 m. Silicon APDs cannot support this SNR and distance with the same footprint area. SPADs, on the other hand, can be readily condensed into high fill factor mm2arrays (e.g. in Lidar applications).

[0216] In some embodiments, an additional amplifier at the SiPM output to increase the voltage gain. The additional amplifier may raise the shot noise floor further above the noise floor of the instrumentation and allow the SNR to be measured more reliably across the 1 GHz bandwidth (BW).

[0217] Wi-Fi 6 & 7 are trending towards higher throughput via improved spectral efficiency while channel BW remains relatively narrow (e.g. 320 MHz 802.11be). This may be achieved with higher QAM on each subcarrier, demanding higher SNR to differentiate between the denser constellation symbols.

[0218] It will be understood that LiFi and OWC may have high path loss and therefore low SNR since incoherent detection must be used at the receiver. This is because the photon to electron conversion process in photodetectors is too slow to resolve the frequency and phase information of the incident light wave. Coherent optical detection requires specialised lasers and expensive beam combining equipment and is therefore currently limited to research.

[0219] Path loss may limit the signal amplitude and SNR at long distance because there is additive noise in conventional optical front ends. For example, both the APD and TIA (trans impedance amplifier) may add noise to the detected signal. To compensate for high path loss, high optical irradiance is required by either collimating a divergent beam and steering the beam towards the user (to maintain wide field of view - FOV) or increasing the transmit power. However, for LiFi, a narrow laser beam cannot be steered - eye hazard. In addition, laser optical power cannot be increased - eye hazard. In addition, although it is possible to shift laser wavelength to infrared (IR) to boost optical power safely, transmission TX electrical consumption also increases with optical power, draining device battery life.

[0220] Therefore, it is clear that in many instances the TX cannot be changed and instead the performance of the RX must be improved. In addition, lenses or concentrators must be large to achieve high FOV optical gain up to the Etendue limit and cannot be used beyond handset footprint size. In OWC, increased light sensitivity of the optical RX enables increased data rates and increased range of data transmission for the same or less TX power. Increased light collection may be implemented by increasing photodetector (PD) size, but larger PDs have slower frequency response due to increased PD capacitance. Therefore, increasing PD size and dependent capacitance is not a solution for high data rate OWC where high bandwidth is required. Additionally, photodetector (PD) size may be limited by available size in the application e.g. small portable devices. In order to maintain the high speed of individual detectors, hundreds of detectors and differential TIAs must be added together with summing amplifiers to build up a high SNR. Initial estimates indicate the resulting total RX area inflates well beyond available footprints in for example smartphone form factors.

[0221] Conventional receiver sensitivities are therefore inadequate to attain higher SNR with the same (or less) irradiance at long distance.

[0222] The following proposed receiver front end is a SPAD cell, utilising 1000s of single photon avalanche diode (SPAD) cells. Each cell is biased above breakdown in Geiger mode with very high internal gain over 100,000. This increases the gain of the first stage in the RX chain such that the need for a TIA is obviated. The noise figure is therefore set entirely by shot noise from the signal itself. As a result, SPAD RXs can detect low intensity signals below the noise floor of conventional PIN / APD RXs where the dominant noise figure is set at the TIA. For this reason, SPAD-based RXs are said to be shot noise limited. Since wideband shot noise is only induced by the signal, higher signal amplitude provides higher SNR.

[0223] The following description of an embodiment is included. In an embodiment, each SiPM selected has 2880 SPAD cells. The fast output signal is the derivative of the firing of SPAD cells in response to the detection of single photons. The fast output is formed from the sum of all active SPAD cells and so a voltage proportional to the amplitude of the photon flux i.e. received power can be obtained. The more important edge information (photon arrivals) is preserved with a short rise time and the slow fall time component from the parallel dead time constant of the accompanying cell circuitry is removed. The rise time of the SiPM output is set by the rise time of the avalanche formation and the variation in the transit times of signals arriving from different points on the active area of the SiPM. Minimising this transit time spread by careful design of the tracking can improve the rise time. In addition, the rise time is affected by the output impedance of the SiPM and the package.

[0224] The SiPM selected, in accordance with embodiments, has 300 ps rise time (corresponding to an analogue bandwidth of around 0.35 / 300 ps = 1.17 GHz); 600 ps full width half maximum (FWHM) output pulse width; and 5 ns SPAD cell recharge dead time. The SiPM has 2880 SPAD cells on a 1 *1 mm2area. The SiPM is configured to provide two outputs: a fast output and a slow output. The fast output has a capacitance of 1 pF, set by parasitics and the number of SPADs, whereas reading out from the anode directly would have a capacitance >100 pF (the sum of all SPAD capacitances in parallel). There is an additional 2-3 pF added by parasitics from the package.

[0225] The number of SPAD cells within the SiPM cannot be increased beyond 2880 without the 300 ps rise time being compromised due to the summed capacitive load connected to the readout channel. For this same reason, multiple SiPMs cannot be added together in parallel to further increase the output signal amplitude.

[0226] Briefly, each SiPM has an anode and a cathode. Each SPAD device (i.e. SPAD cell) of the SiPM is connected in parallel to a common anode and to a common anode. Each SPAD also has a dedicated capacitor coupled to a third terminal. The third terminal provide a capacitively coupled output also referred to as a fast output. In an embodiment, the fast output from a 1 mm sensor has a capacitance of -1 pF, whereas, reading out from the anode or cathode would have a capacitance of -100 pF.

[0227] As described above the switching device may be formed of Schottky diode pairs. Schottky diode pairs are coupled to a group output of each group of SPADs (for example, to an output of each SiPM. In particular, in some embodiments, the Schottky diode pairs are connected to a fast output of the SiPM. When a first Schottky diode is turned on, the fast pulse of the SiPM is transferred to a common readout node. The diode pair forms a symmetry. The positive signal on the Common Readout node also reverses biases Schottky diodes of the other SiPMs thereby suppressing signals generated by the other SiPMS during the read out. Schottky diodes can be used to implemente a ‘signal driven’ multiplexing scheme that will connect only the SiPM that is activated by incident photons to the multiplexed readout channel. This approach may reduce the effective capacitance and noise at the common node such that single SiPM performance is preserved. Unbiased Schottky diodes may provide the benefits described above, but additional benefits may be achieved by applying a bias to the Schottky diodes themselves.

[0228] For a continuous incident intensity modulated signal, such as that depicted in Figure 8, the SDM may output an analogue signal representing the outputs of a group of SiPMs sampled extremely quickly in a short time snapshot (24 GHz maximum multiplexing rate), The common analogue output signal represents a series of additive multiple analogue responses for that group of SiPMs, in effect it is a partly additive signal of all active SPADs in that group distributed over time. Due to the method of multiplexing and handover between different SiPMs, all active SPADs in that group are not connected in parallel for long and so a combined capacitance and resulting undesirable slow signal bandwidth limitation is avoided.

[0229] The switching between each group of SPADs occurs in a time period much shorter than the duration of the SPAD group signal output and this enables the effective analogue common output signal from different switched SPAD groups to be a loose additive sum of the SPAD responses. The additive response improves the SNR of the analogue output signal while the fact that a limited number of SPADs are switched in circuit at any one time means the disadvantages of higher capacitance with consequent impact on received detection signal bandwidth are reduced to levels below those required for OWC signal detection.

[0230] The following non-limiting comments regarding experimental tests are provided. A device having 120 SiPM arrays is provided. The device is formed of a board having 120 SiPM blocks, 120 switches and each block has 2880 SPADS. In testing a separate single SiPM in its own package, equivalent to one of the board SiPM blocks, the SNR is 20dB. In testing the common output of the 120 SiPM blocks on the board the result is an additional 16dB i.e ~40 times greater.

[0231] In testing the decay time of the separate single SiPM block and comparing with the decay time of the common output - it was found that the decay time of the common output is similar to that of the single SiPM block in the array. When there is intense light over the whole array the 120 SiPM blocks act to provide an output signal almost simultaneously - the output signals thereby being additive or cumulative in amplitude. It could be that the switches operate very fast in some form of sequence - the fact that the common output is large suggests the switches are closing. Additionally, because the decay time of the whole array of 120SiPM blocks is of the same order or similar time to the decay time of the separate SiPM then this would suggest that the switches are going open such that only one SiPM is connected at a time i.e. the benefit of the SDM circuit of low capacitance is present.

[0232] The additive response improves the Signal to Noise Ratio of the analogue output signal while the fact that a limited number of SPADs are switched in circuit at any one time means the disadvantages of higher capacitance with consequent impact on received detection signal bandwidth are reduced to levels below those required for OWC signal detection.

[0233] Conventional optical receivers have low fidelity and cannot use SPADs for detecting LiFi signals. The current device may require lower power consumption, (milliwatts rather than Watts) therefore, using the same power as conventional OWC devices, OWC signals can be transmitted across a greater distance without degrading the signal. Furthermore, 100 times lower power than conventional LiFi devices has been observed.

[0234] A reference light source directed to a similar single instance photodetector of part of the board array may show two orders of magnitude of difference with the amplitude output from the SDM board demonstrated improved SNR.

[0235] An evaluation board using an SiPM array employing the SDM technique was tested. The aim of this experimental testing was to:

[0236] 1. Validate the simulation model with measurements

[0237] 2. Validate the SDM method maintains the speed of the LiFi signal detected at each SiPM

[0238] 3. Demonstrate optimised bias voltages, SiPM array size, and board design for SDM

[0239] 4. Determine the degree of SNR that is achievable using this method The SiPMs are mounted in a rectangular array on one side of a 4-layer RF pool printed circuit board (PCB). To avoid PCB parasitics influencing test results, the evaluation board design was simplified to provide only a single multiplexed Fast output, Rout. No additional outputs were routed and there is no on-board amplification of the SDM output. Signal conditioning circuit elements are not included for the purpose of the evaluation. The common readout node connects all the Schottky pair outputs together.

[0240] The term light herein may be used, for example, to refer to electromagnetic waves with wavelengths in a range 1 nm to 2500 nm, which includes ultraviolet, visible light and near-infrared wavelengths. Light may be used to refer to both visible light and non- visible light of any suitable wavelengths.

[0241] Optical wireless communication (OWC), or light communication (LC), can offer advantages over conventional RF wireless communication such as Wi-Fi™, due to the characteristics of the optical channel. Optical signals usually do not penetrate, for example, walls, unlike RF signals, which can provide for increased security. Furthermore, the optical transmissions can be particularly directional in nature.

[0242] In some cases, it may be desirable to adapt protocols from Wi-Fi in OWC, such as packet-based protocols, for example IEEE™ 802.11 ™. This can assist in the integration of technologies such as LiFi and Wi-Fi. However, due to the different properties of the LiFi channel compared to Wi-Fi this is not straightforward.

[0243] Any suitable modulation scheme may be used. For example, orthogonal frequency division multiplexing (OFDM) modulation schemes are used in some embodiments, and the demodulation is from the OFDM modulation scheme. In further embodiments and without limitation, other modulation schemes may be used, for example on-off keying (OOK), phase shift keying (PSK), M-ary pulse amplitude modulation (M-PAM), M-ary quadrature amplitude modulation (M-QAM), Discrete Hartley transformation, Wavelet packet division multiplexing (WPDM), Hadamard coded modulation (HCM), pulse-position modulation (PPM), Colour shift keying (CSK), carrier-less amplitude and phase (CAP), or discrete multi-tone (DMT). The light may be modulated at a modulation rate between 1 kHz and 1 PHz, for example at a modulation rate between 1 MHz and 100 GHz. The modulation scheme may form part of an OWC communication protocol, such that the optical signal is produced according to the OWC communication protocol. The OWC communication protocol may be packet-based. In embodiments, the device may be configured to connect to a network, for example, via an OWC enabled access point.

[0244] In the above-described embodiment, a common signal output is described. It will be understood that a further signal combining module or device may be provided for combining the group responses of the plurality of groups as a combined signal.

[0245] A skilled person will appreciate that variations of the enclosed arrangement are possible without departing from the invention. Accordingly, the above description of the specific embodiment is made by way of example only and not for the purposes of limitations. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

CLAIMS1. A device for receiving an incoming optical wireless communication (OWC) signal comprising: a plurality of photodetectors each configured to produce a photo detection signal in response to light being received by the photodetector, wherein the photodetectors comprise single photon detectors configured to detect single photons; wherein the plurality of photodetectors are arranged in a plurality of groups, wherein each group is configured to produce a group response signal representing the photo detection signals produced by the photodetectors within the group in response to receiving light, wherein the plurality of groups are configured to be connected to a common signal output, wherein the device comprises controllable switching circuitry for selectively switching each group response signal onto the common signal output thereby to produce an output signal representative of the OWC signal received by the device and suitable for demodulation and / or decoding by further processing circuitry2. The device of any preceding claim, wherein the controllable switching circuitry is configured to switch between the pluralities of groups at a switching frequency such that the output signal comprises a sum of at least part of the group responses, optionally the output signal comprises only part of the group responses.

3. The device of any preceding claim, wherein the switching circuitry comprises a plurality of switching devices, for example, Schottky diode pairs and / or transistors and / or fast response switching devices.

4. The device of any preceding claim, wherein the controllable switching circuitry comprises a plurality of switching elements or devices configured to selectively switch outputs of one or more groups to the common signal output and / or wherein the controllable switching comprises a plurality of switching elements or devices for providing an activation signal, for example, a bias signal or supply voltage, to activate one or more groups.

5. The device of any preceding claim, wherein the controllable switching circuitry is configured to switch between the pluralities of groups at a switching frequency such that the output signal comprises a sum of at least part of the group responses, optionally the output signal comprises only part of the group responses.

6. The device of any preceding claim, wherein each group of at least some, optionally all of the plurality of groups, have a common signal ground and / or a common bias input.

7. The device of any preceding claim, wherein the group response signal comprises an accumulated group response signal and is characterised by a size and / or other signal property that is proportional or representative of a number of photodetectors of the group triggered by incoming light or optical power of the incoming light.

8. The device of any preceding claim, wherein common signal output is formed by combining rising edges of group responses and / or wherein the photodetection event signals are represented on the signal common output line as a cumulative or additive signal.

9. The device of any preceding claim, wherein the switching circuitry is configured to apply a switching sequence, for example, in accordance with a sequential and / or periodic scheme and / or in accordance with an automatic or threshold-based scheme.

10. The device of any preceding claim, wherein the switching circuitry comprises one or more switching elements or devices configured to switch when a group response and / or activation signal is above a threshold level.

11. The activation sequence or scheme may comprise a periodic handover sequence and / or an automatic scheme, for example, a threshold based or signal driven multiplexing scheme.

12. The device of any preceding claim, wherein the scheme and / or a parameter of the scheme, for example, a frequency, is selected based on a desired or measuredproperty of the output signal and / or a desired or measured property of the group response,13. The device of any preceding claim, wherein the switching circuitry is configured to selectively switch at a switching frequency in the range 10Mz to 100GHz.

14. The device of any preceding claim, wherein the signal switching circuitry is configured to perform a sampling process comprising obtaining samples of group responses from each group and adding said obtained sample to the output signal.

15. The device of any preceding claim, wherein the signal switching circuitry is configured to output a first group response having a group response duration from one or more first groups at a first time and switch to outputting a second group response from one or more second groups at a second time before the end of the first group response duration.

16. The device of any preceding claim, wherein the switching circuitry is configured to couple the signal of each group to the common output for a time period shorter than the group response time, optionally shorter than a rise time of the group response.

17. The device of any preceding claim, wherein the accumulated group response comprise a rising portion having a rise time or other property dependent on at least the number of photodetectors in the group and wherein the controllable switching circuitry is configured to switch between groups in accordance with a in dependence on the group response rise time or other property.

18. The device of any preceding claim, wherein the switching may be performed such that only part of the group response rising portion is included in the output signal.

19. The device of any preceding claim, wherein the rising portion of each group comprises a rise time dependent on at least the number of photodetectors in the group and wherein the number of photodetectors in the group are selected to control, for example, restrict the rise time, optionally wherein the rise time may be in the range 1 ps to 100 ps.

20. The device of any preceding claim, wherein each group response signal has a falling portion and the switching frequency is selected such that the combined signal is absent of the falling portions of the combined response signals21. The device of any preceding claim, wherein at least some, optionally all of the plurality of the plurality of groups are capacitively coupled to a common group output and wherein the switching circuitry is configured to electronically couple only one photon detector to the group output at a time and / or only one group to the common signal output at a time.

22. The device of any preceding claim, wherein the photodetectors are provided as part of photomultiplier modules, for example, SiPM modules, connected such that each group comprises one or more photomultipliers modules, wherein the signal switching circuitry is configured to connect the one or more photomultiplier modules of each group to a common output and / or connect one or more photomultiplier modules to a common bias signal.

23. The device of any preceding claim, wherein each photon detector has an intrinsic recovery or dead time and wherein the number of groups is selected and / or a switching frequency is selected so that each photon detector has sufficient time to recover between each triggering event.

24. The device of any preceding claim, wherein the common signal output line is connected to an amplifier and / or other signal measuring circuit or device.

25. The device of any preceding claim, wherein the impedance of the amplifier and / or measuring circuit or device is matched to the impedance of the common signal output.

26. The device of any preceding claim, wherein the received OWC signal comprises a unipolar signal and / or the OWC signal comprises a medium to high light intensity signal and / or the OWC signal comprises a DC bias.

27. The device of any preceding claim, wherein the OWC signal is modulated in accordance with an OWC data modulation scheme and wherein the device furthercomprises demodulation and / or decoding circuitry for applying a demodulation and / or decoding process to the signal output.

28. The device of any preceding claim, wherein each group produces a group response using a signal driven multiplexing arrangement, optionally, wherein the signal driven multiplexing arrangement comprises pairs of response switches and / or Shottky diode series pairs.

29. A method of receiving an optical wireless communication signal using a receiver comprising a plurality of photodetectors each configured to produce a photo detection signal in response to receiving light and wherein the plurality of single photon photodetectors are arranged or otherwise provided in a plurality of groups, wherein each group is configured to produce an group response signal from the photo detection signals produced by its photodetectors in response to receiving light wherein each of the plurality of groups is connected to a common output, wherein the method comprises controllably switching groups connected to a common output thereby to provide their group response signal(s) to the common output to produce an output signal for demodulation and / or decoding.

30. The method may further comprise demodulation and / or decoding the output signal or a signal derived from the output signal.

Images