WO2018220389A1

WO2018220389A1 - Concentrator height reduction

Info

Publication number: WO2018220389A1
Application number: PCT/GB2018/051499
Authority: WO
Inventors: Dobroslav TSONEV
Original assignee: Purelifi Limited
Priority date: 2017-06-02
Filing date: 2018-06-01
Publication date: 2018-12-06
Also published as: GB201708861D0; GB201808986D0; GB2565201A

Abstract

An optical detector comprising a plurality of optical gain elements and a respective plurality of photodetectors, each concentrator having a respective optical axis, and wherein the optical axis of a first of the plurality of concentrators has an angle with respect to the respective optical axis of each of the other concentrators of less than 5°.

Description

Concentrator Height Reduction

Field of invention The field of the invention is optical detectors. Background

Optical Wireless Communication (OWC) uses visible, infrared and/or ultraviolet light as the signal carrier for wireless communication. Li Fi is a high speed short range form of OWC. An OWC system typically comprises a light source or sources, such as multiple light emitting diodes LED, and optical receivers that form a wireless network. Signal modulation is used which cannot be detected by the human eye, and thus communication is just as seamless as radio frequency wireless communication systems, allowing the users to be connected where there is OWC enabled light.

Figure 1 is a schematic diagram of a typical OWC network 100. Data originates from a server 101 or the Internet 102 and is sent 103 to a luminaire driver 104. The luminaire driver 104 controls a luminaire 105, which transmits light 106 to a photodetector 107. The photodetector 107 (receiver) detects and demodulates the light signal and the data is transmitted 108 to a local terminal 109. OWC is a complimentary technology that works alongside other wireless technologies such as Wi-Fi. If the light signal to an OWC enabled device is below the receiver's threshold then it will not receive data. In that instance, radio frequency wireless communication systems or cellular networks, if available, will continue to deliver data. However, the moment the device begins to receive sufficient signal from an OWC enabled luminaire, the device will resume high speed communications using light as an additional communications medium. Although OWC can be used to off-load data from existing Wi-Fi networks, implementations may be used to provide capacity for greater downlink demand such that existing wireless or wired network infrastructure may be used in a complementary fashion. OWC has the advantages of greater capacity than Wi-Fi, and OWC is also significantly more secure than other wireless technologies because light can be contained in a physical space. OWC can operate in daylight and even in direct sunlight conditions, as the modulated light can still be detected. OWC relies on detecting the fast changes in light intensity and not on the absolute or slowly varying levels caused by natural disruptions in daylight or sunlight. OWC technology modulates the light at very high rates and sunlight is constant light and therefore can be filtered out at the receiver. OWC can use multiple reflections from surfaces and hence is not exclusively a line of sight technology.

The performance of OWC is typically strongly influenced by the amount of light that is collected. As a result, the optics in such systems are designed for optimal light collection under the presented scenario constraints. A typical device used to collect light for optical receivers in an OWC system is a non-imaging concentrator. Examples of non-imaging concentrators include Total Internal Reflection Concentrators and Compound Parabolic Concentrators CPCs. CPCs allow for optimizing the light collection under the etendue limits imposed from the requirement to have a device with a certain field of view (FOV). Figure 2(a) is a perspective diagram showing a CPC 200. The CPC has an input aperture 201 , an output aperture 202 and parabolic sides 203. The input aperture 201 has a greater area than the output aperture 202 to enable the concentration of light and hence an optical gain. The device has a field of view FOV 204 for which it can receive light. Examples of incident rays are shown, a first ray 205 being within the FOV and hence transmitted to the output aperture 202 and one which is outside the FOV 206. A common problem for such concentrators is that they are restrictively long in the z dimension.

The law of conservation of etendue imposes a maximum gain for the optical concentrator expressed as:

where n is the refractive index of the concentrator material.

Figure 2(b) is a cross-sectional view of a typical arrangement of a compound parabolic concentrator 200 and a photodetector 207. An alternative approach to such concentrator designs is the use of a lens with as short a focal distance as possible. However, simple lenses do not approach the maximum efficiency governed by the etendue equation at large fields of view, due to aberrations affecting the quality of the imaging. Such lenses also tend to have some considerable height, and in order to make them smaller, designers can resort to alternative lens structures such as a Fresnel lens or some form of an engineered surface, where micro- structures on top of a flat surface are specially engineered in order to refract the light the same way a conventional lens does. These approaches reduce the lens size, but cannot change the required distance (stemming from the lens focal distance) between the optical element and the photodetector. Arrays of concentrators have been used for angle diversity reception. For this purpose the optical axes of the concentrators are inclined relative to each other, so as to broaden the field of optical reception of the detector. Figure 3 is a schematic diagram of an angular diversity detector 300. It comprises a plurality of concentrators 301 axially aligned with detectors 306, which collect incident light from different angles 302, 303, 304, detect the light and then combine the signals 305. An example of such an array is disclosed in J. M. Kahn and J. R. Barry, "Wireless Infrared Communications", Proceedings of the IEEE, vol. 85, no. 2, February 1997. Arrays of detectors have also been used for interference mitigation, such as in Z. Chen and H. Haas, "Space Division Multiple Access in Visible Light Communications", IEEE International Conference on Communications (ICC), 8 - 12 June 2015 and in Multiple Input Multiple Output systems, such as in S. Rajbhandari et al., "High-speed Integrated Visible Light Communication System: Device Constraints and Design Considerations", IEEE Journal on Selected Areas in Communications, vol. 33, issue 9, September 2015, pp. 1750 - 1757.

Summary According to an aspect, there is provided an optical detector comprising a plurality of optical gain elements and a respective plurality of photodetectors, each optical gain element having a respective optical axis, and wherein the optical axis of a first of the plurality of optical gain element has an angle with respect to the respective optical axis of each of the other optical gain elements of less than 5°.

In an embodiment at least one of the optical gain elements is a passive optical gain element.

In an embodiment at least one of the optical gain elements is a concentrator. In an embodiment at least one of the optical gain elements is an imaging concentrator.

In an embodiment at least one of the optical gain elements is a non- imaging concentrator.

In an embodiment at least one of the optical gain elements is a Compound Parabolic Concentrator CPC.

In an embodiment at least one of the optical gain elements is a total internal reflection concentrator.

In an embodiment at least one of the optical gain elements is a lens.

In an embodiment at least one of the optical gain elements is a Fresnel lens.

In an embodiment at least one of the optical gain elements is a parabolic reflector.

In an embodiment the plurality of optical gain elements are located parallel in a planar array.

In an embodiment the photodetectors are located on a printed circuit board.

In an embodiment, the optical detector further comprises at least one further optical component located at the input aperture of at least one said optical gain element, wherein the further optical component is one of: a prism or a lens.

In an embodiment, the optical detector further comprises at least one further optical component located between an output of at least one said optical gain element and a said respective photodetector, wherein the further optical component is one of: a prism or a lens.

In an embodiment, the optical detector further comprises at least one optical transmitter, located in a cavity between two optical gain elements. In an embodiment, the optical detector further comprises an array of optical transmitters interleaved between the optical gain elements.

In an embodiment, the optical detector further comprises at least one further optical component located at an output of the at least one optical transmitters.

In an embodiment, the optical detector further comprises a trans-impedance amplifier.

In an embodiment, each of the plurality of photodetectors has a respective trans- impedance amplifier.

In an embodiment, signals from at least two of the photodetectors are combined prior to input to the trans-impedance amplifier. In an embodiment, at least one of the photodetectors is a PIN diode.

In an embodiment, at least one of the photodetectors is an avalanche photodiode.

Figures

The above and other aspects of the disclosure will now be explained by way of example only with the aid of the following diagrams:

Figure 1 is a schematic diagram of a LiFi system;

Figure 2(a) is a perspective view of a Compound Parabolic Concentrator CPC according to the prior art;

Figure 2(b) is a cross-sectional view of a typical arrangement of a compound parabolic concentrator and a photodetector;

Figure 3 is an angle diversity receiver according to the prior art;

Figure 4 is a CPC arrangement comprising a plurality of photodetector elements according to an embodiment;

Figure 5 is schematic diagram of a photodiode in a trans-impedance amplifier TIA arrangement; Figure 6 is schematic diagram of a plurality of photodiodes in a trans-impedance amplifier TIA arrangement;

Figure 7 is schematic diagram of a plurality of photodiodes in a trans-impedance amplifier TIA arrangement; Figure 8 is a cross sectional diagram of an array of parabolic concentrators with additional optical components located at the outputs of the concentrators;

Figure 9 is a cross sectional diagram of an array of parabolic concentrators with additional optical components located at the inputs of the concentrators;

Figure 10 is a cross sectional diagram of a concentrator array with integrated transmitters; and

Figure 11 is a cross sectional diagram of a concentrator array with integrated transmitters, with additional optical components for transmitters.

Detailed description

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

The present disclosure provides an optical concentrator arrangement for an optical wireless communication system, which enables a reduction in the height of the device (i.e. the extent of the device in a light transmission direction for which light incident on the device causes an electronic signal of maximum amplitude; broadly speaking this is the distance between one or more light input apertures of the device and furthest surface of one or more corresponding photodetectors), whilst maintaining the optical collection and distribution properties of such a device. The advantage is that a reduction in the height of the device reduces the overall form factor of the system and improves convenience of use. It is shown that performance of the system is maintained. Furthermore, an arbitrary height may be used and a detector designed to that specification, rather than the size being determined by the performance requirements.

Figure 4 is a perspective diagram of an array 400 of concentrators according to an embodiment. Four parabolic concentrators 401 , 402, 403, 404 are illustrated. However, the person skilled in the art would appreciate that other numbers of concentrators are possible and the invention is not limited to any given number such as at least three, at least four or at least five.. In the illustrated embodiment, the concentrators are parabolic. The parabolic shape determines the exact height of the concentrator for a specific gain and field of view. A simple approximation of the relationship between the height of the concentrator and the photodetector aperture is a linear proportion. In an embodiment, the concentrators are co-axial, with their axes parallel. Two of the axes 405, 406 of the respective concentrators 403, 404 are illustrated. The axes are the axes of the parabolas at a central point along the axes of longitudinal symmetry of the parabolic concentrators 403, 404. The axes of the concentrators 401 , 402, 403, 404 are substantially parallel. In another embodiment, the axes of the concentrators are arranged at a small angle to one another. A maximum angle of around 5° is typical. An angle of less than 5°, less than 4°, or less than 3° is also possible.

The optical axis of an optical gain element is a line of rotational symmetry through the element. Alternatively, for an optical gain element which has no rotational symmetry, the optical axis may be defined as the direction of a straight line between the centre of the input surface to the surface of the output surface. For an optical detector, the optical axis is a line normal to the plane of the detector or to a light receiving surface of the detector. Other types of optical gain elements may be used in the place of one or more of the respective concentrator, for example a lens, such as a Fresnel lens. An optical gain element is any device which has a larger input area than its output area, thus concentrating light. For a CPC, the shape of the concentrator in the z-dimension (height direction) is defined by a parabola, which determines the exact height of the concentrator for a specific gain and FOV. A simple approximation of the relationship between the height of the concentrator and the photodetector aperture is a linear proportion, i.e., if the photodetector dimensions change by 1/2, then a concentrator providing the same optical gain, would have an input aperture whose dimensions also scale by 1/2 (the gain of the concentrator is effectively the ratio between the input aperture area, which collects light, and the output aperture area, which is coupled to the photodetector). The concentrator height also approximately scales by 1/2, but the exact dimension would be determined by the parabola. Reducing the size of the detector reduces the height of the overall receiver system, however, it also reduces the area by a factor of k², where k is the reduction factor in the x-dimension and the y-dimension. In the example, discussed in this section, k=M2, which makes the collection area 1/4 of the original collection area and the signal becomes 1/4 of the signal picked up by the original detector.

The performance of a receiving system is limited by noise. This may be dominated by shot noise in the photodiode or the thermal noise in the trans-impedance amplifier (TIA). The dominant noise source will depend on the technology and the illumination levels. The shot noise energy density is typically modelled by the following equation:

A

ishot = V^{2 "} R ^" (⁷D + P^■ 0)^■ F^■ G_a (1) where q = 1.60217662 ^χ 10^"19 C is the elementary charge of an electron, /_D is the photodiode dark current, p is the PD responsivity in Amperes/Watt (A/W), Θ is the optical power collected by the PD in W, G_a is the avalanche gain of the PD (G_a=1 for a PIN PD) and F is the excess noise factor specified in the APD datasheet. The symbol

[-7=] means that the shot noise energy density is measured in the units of amps per square root Hertz. The excess noise factor can be calculated as G_a ^E, where E is the excess noise index often specified in the datasheet in place of F, which would vary as a function of G_a. In practice, the performance of the system that employs an optical detector is mainly influenced by two of the parameters in equation 1 - the dark current of the photodiode and the intensity of the received light.

The use of an array of concentrators in place of a single concentrator with the same overall area will result in a reduction of the optical detector size in the direction parallel to the axes of the concentrators (the "height" of the concentrators). This is expected to reduce the dark current noise if the same technology is used for the smaller detector. The exact amount, however, depends on the technology. In an Optical Wireless Communication, OWC, scenario, the dark current is not the dominant noise contributor due to the typically high received optical intensity levels. The noise power (variance) is linearly proportional to the received light intensity, so for a device with 1/4 of the area (collecting 1/4 of the light), the noise power reduces by 1/4. At the same time, the signal amplitude is reduced by 1/4, so the signal power (variance) is reduced by 1/16. As a result, the signal-to-noise ratio (SNR), which determines the performance of the communication system, is reduced by 1/4. If the signals from four identical receivers are combined, then the signal amplitude is multiplied by 4, leading to an increase in the signal energy of 16. The noise components from the four receivers are statistically independent, and when combined, produce a noise component with four times the power (variance). As a result, the achieved SNR using four smaller detectors (with /c² of the area each) is equivalent to the SNR achieved by using one larger detector. The benefit of using the four smaller detectors in this case is that the optical concentrator height would be reduced by approximately 1/2.

Typically, a photodetector will be connected to a trans-impedance amplifier. Figure 5 is a schematic diagram of typical arrangement 500 of a photodetector and an amplifier according to the prior art. There is provided a photodiode 501 , an amplifier 502 and resistor 503. With this arrangement, the current variations through the photodiode due to varying levels of incident light may be converted into a voltage value.

Two options are available for amplifiers for the receiver array arrangement according to the present disclosure. In an embodiment, the signals from the photodetectors are combined before the TIA stage. In another embodiment, the TIA stage occurs before combination. Figure 6 is a schematic diagram of the arrangement in which the receiver signals are combined first. The arrangement 600 comprises a plurality of photodiode receivers 601 , which are combined together at the input 602 of the amplifier 603. A resistor 604 is provided. Figure 7 is a schematic diagram of a two stage TIA arrangement 700. As previously, the arrangement 700 comprises a plurality of photodiode receivers 701 , each of which is provided with a respective TIA 702. The signals from the TIAs 702 are combined at the input 703 of the combining TIA 704. With detectors such as avalanche photodiodes APDs, the internal gain of the device enables the information signal to be amplified significantly before getting to the TIA, which can make any noise components from the TIA insignificant.

If the signals from the photodetector are combined before the TIA stage, as in the embodiment of Figure 6, the four detectors are expected to perform equivalently to the larger single detector because the parasitic capacitances of the four detectors add up and the parasitic capacitance is proportional to the device area for PDs of the same technology. The capacitance might not scale down linearly with the device area if the PD packaging is contributing significantly to the parasitic capacitance of the device.

If the four signals are combined after the TIA stage, then each photodetector is connected to its own TIA (see Fig. 7). Due to the smaller capacitance C_d of each smaller photodetector, the bandwidth of the receiver configuration or the TIA gain can be increased. The bandwidth of the receiver can be approximated by the formula:

where GBWP is the gain-bandwidth product of the operational amplifier used for the TIA and R_F is the resistance. In terms of SNR and performance, the two configurations would be equivalent for the same communications bandwidth. In other photodetector technologies, such as PIN PDs, the thermal noise in the TIA is likely to dominate the shot noise in the PD. The TIA thermal noise typically consists of three major independent components and can be calculated as:

where i_A is the input-referred current noise of the operational amplifier, v_A is the input- referred voltage noise of the operational amplifier, k = 1.38064852 ^χ 10^"23 m²kgs^"2K^"1 is the Boltzmann constant and T is the temperature in Kelvin. The third term in the summation in Equation 3 calculates the thermal noise due to the TIA feedback resistor R_F. If the array of PDs is connected in parallel to the same TIA amplifier, then, as already explained above, the noise is equivalent to the noise of the receiver employing a large PD and the signal is the sum of the electrical signals from all the PDs in the arrays, which means the received signal power is also equivalent to the signal power in the receiver employing the larger PD. Hence, the performance of the two systems is equivalent, while the height (z- dimension) of the PD array receiver is lower. If each of the individual detectors is connected to its own TIA, then, as in the case when an APD is used, the parasitic capacitance affecting each individual TIA performance is lower (since the PD is smaller) and the system bandwidth can be improved. The received signal strength at each detector is 1/4 of the signal strength, but when the four signals from the four detectors are combined in a subsequent amplifier stage (see Fig. 7), the resulting signal level is equivalent to the signal level obtained with an individual larger detector. In this case, the noise components coming from the four TIAs are uncorrelated and when combined increase the noise level power (variance) by a factor of 4. Possible ways to mitigate this issue include redesigning the TIA configurations for each photodetector as the smaller PD capacitance allows for larger R_F values (see equation 3) to be used. This reduces the thermal noise component. The smaller parasitic capacitance is also expected to reduce the input referred voltage noise V_A (see equation 3). Therefore, it is possible that the noise figure can be reduced, so that four small detector elements could achieve the same performance as one large detector. How close the two systems' performances are, however, would depend on the specific values of the different system parameters such as parasitic capacitance of the PD, TIA gain-bandwidth product, overall system bandwidth, trans-impedance gain, etc.

In embodiments, an optical wireless communication (OWC) device is configured to receive an OWC signal and to extract data from the OWC signal. For example, the OWC device may comprise or form part of an access point (AP) or station (STA). The OWC signal may be transmitted using visible, infrared and/or ultraviolet light.

The OWC device comprises an optical detector. The optical detector comprises a receiver array as described above. The receiver array comprises a plurality of photodetectors (for example, photodiodes) and a plurality of concentrators, such that each of the concentrators is configured to provide light to a corresponding at least one of the photodetectors.

Any suitable non-imaging concentrators may be provided. According to certain embodiments, each non-imaging concentrator may comprise a concentrator that does not produce an image from received light of wavelength(s) of interest, and/or that does not have a single focal point or focal length at wavelength(s) of interest, and/or that substantially does not provide a focusing effect at wavelength(s) of interest.

The optical detector may further comprise at least one amplifier (for example, TIA) as described above. In some embodiments, signals from the photodetectors are combined and the combined signals is provided to the amplifier for amplification. In other embodiments, a signal from each photodetector is amplified by a respective amplifier, and the resulting amplified signals are combined.

The optical detector is configured to combine signals from the plurality of photodetectors (which may be amplified at any appropriate stage). The output of the optical detector is a combined signal that is representative of modulated light received across all of the photodetectors in the receiver array. The combined signal may be referred to as a detection signal.

In certain embodiments, a receiver array (for example, an array of four photodetectors and four concentrators) is used to replace a single photodetector and concentrator. The receiver array has reduced height when compared with the photodetector and concentrator that it replaces. However, the receiver array collects modulated light from a similar area to the single photodetector and concentrator. Light received across the receiver array is used to produce a single output signal. The detection signal output by the optical detector is representative of the combined light received across the photodetectors. The detection signal comprises or represents the combined signals from the plurality of photodetectors.

The OWC device further comprises at least one analogue to digital converter (ADC) which is configured to convert analogue signals into digital signals. In the present embodiment, the detection signal output by the optical detector is an analogue signal. The detection signal is converted into a digital signal by the ADC.

In other embodiments, analogue to digital conversion may be performed at any suitable stage. For example, the ADC may form part of the optical detector. The detection signal output by the optical detector may be a digital signal obtained by performing an analogue to digital conversion of the combined signal from the plurality of photodetectors. The OWC device further comprises demodulation circuitry. The OWC signal received by the OWC device is modulated using an OWC modulation scheme. The demodulation circuitry is configured to perform a demodulation process to demodulate the detection signal to obtain data that has been encoded in the OWC signal using the OWC modulation scheme. In the present embodiment, the detection signal is converted to a digital signal by the ADC, and the resulting digital signal is demodulated by the demodulation circuitry. In other embodiments, the detection signal is itself a digital signal which is demodulated by the demodulation circuitry.

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 data extracted from the detection signal by the demodulation circuitry may comprise any suitable data type, for example packet-based data.

Additional optical components may be added to the arrangements above. For example, prisms or other correction/distribution devices such as lenses, may be used to improve optical concentration or distribution. In an embodiment, optical deflecting elements may be placed between concentrators and receivers. In an embodiment, the optical deflecting element is a prism. Figure 8 is a cross sectional diagram of such an arrangement according to an embodiment, in which prisms are placed adjacent output apertures of the concentrators. Three concentrators, 801 , 802, 803 are illustrated, together with their associated receivers 804, 805, 806. In between two of the concentrators, 801 , 803 and their respective receivers 804, 806, are placed prisms 807, 808. This arrangement has the advantage of improving the overall field of view. The optical deflecting elements (prisms in the embodiment of Figure 8) improve coupling of light between the gain elements (concentrators in the embodiment of Figure 8) and the photodetectors. In an embodiment the photodetectors are arranged in a co- planar configuration. In an embodiment, the photodetectors are located in a printed circuit board, PCB.

Figure 9 is a cross sectional view of another embodiment, in which the concentrators have prisms placed over their input apertures. Three concentrators, 901 , 902, 903 are illustrated, together with their associated receivers 904, 905, 906. Prisms 907, 908 are placed over the input apertures of two of the concentrators 901 , 903. This enables wider field of view. In an embodiment, lenses are used instead of prisms. In an embodiment, the lens is a Fresnel lens.

In an embodiment, transmitters may be placed between the concentrators to provide an integrated transceiver. Figure 10 is a cross sectional diagram of such an arrangement according to an embodiment. Five concentrators 1001 , 1002, 1003, 1004, 1005 are illustrated, along with their associated receivers 1006, 1007, 1008, 1009, 1010. In between the concentrators there are located four transmitters 101 1 , 1012, 1013, 1014.

In an embodiment, lenses, prisms or other optical devices are placed over transmitters. Figure 11 is a cross-sectional diagram of such an arrangement, according to an embodiment. As in Figure 10 five concentrators 1 101 , 1102, 1 103, 1 104, 1105 are illustrated, along with their associated receivers 1 106, 1 107, 1 108, 1 109, 1 110. In between the concentrators there are located four transmitters 1 1 11 , 1 112, 1 1 13, 11 14. Over each of the transmitters there is located a lens 1 115, 1 116, 1 117, 1 118. In an embodiment, prisms may be used in the place of lenses. This arrangement enables a shaping of the transmitted power. The transmitters 101 1 , 1012, 1013, 1014 are arranged to generate and transmit light in the direction indicated by the dashed arrows, i.e. substantially opposite to the transmission direction of light incident on the concentrators 1001 , 1002, 1003, 1004, 1005 for which the receivers 1006, 1007, 1008, 1009, 1019 produce the strongest electrical signal.

The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.

Claims

CLAIMS:

1. An optical detector comprising a plurality of optical gain elements and a respective plurality of photodetectors, each optical gain element comprising a non- imaging concentrator and having a respective optical axis; wherein

each of the optical gain elements is arranged to provide light to a corresponding at least one of the photodetectors;

the optical axis of a first of the plurality of optical gain elements has an angle with respect to the respective optical axis of each of the other optical gain elements of less than 5°; and

the optical detector is configured to combine signals from the plurality of photodetectors.

2. An optical detector according to claim 1 , wherein at least one of the optical gain elements is a passive optical gain element.

3. An optical detector according to claim 1 or 2, wherein at least one of the optical gain elements is a Compound Parabolic Concentrator CPC.

4. An optical detector according to claim 1 or 2, wherein at least one of the optical gain elements is a total internal reflection concentrator.

5. An optical detector according to any preceding claim, wherein at least one of the optical gain elements is a parabolic reflector.

6. An optical detector according to any preceding claim, wherein the photodetectors are located on a printed circuit board.

7. An optical detector according to any preceding claim wherein the plurality of optical gain elements are located parallel in a planar array.

8. An optical detector according to any preceding claim, further comprising at least one further optical component located at the input aperture of at least one said optical gain element.

9. An optical detector according to claim 8, wherein the additional optical component is an optical deflecting element.

10. An optical detector according to claim 8, wherein the optical deflecting element is one of: a prism or a lens.

1 1. An optical detector according to any preceding claim, further comprising at least one further optical component located between an output of at least one said optical gain element and a said respective photodetector, wherein the further optical component is one of: a prism or a lens.

12. An optical detector according to any preceding claim, further comprising at least one optical transmitter, located in a cavity between two optical gain elements.

13. An optical detector according to claim 12, further comprising an array of optical transmitters interleaved between the optical gain elements.

14. An optical detector according to claim 12 or claim 13, further comprising at least one further optical component located at an output of the at least one optical transmitters.

15. An optical detector according to any preceding claim comprising a trans- impedance amplifier.

16. An optical detector according to claim 15, wherein each of the plurality of photodetectors has a respective trans-impedance amplifier.

17. An optical detector according to claim 15, wherein signals from at least two of the photodetectors are combined prior to input to the trans-impedance amplifier.

18. An optical detector according to any preceding claim, wherein at least one of the photodetectors is a PIN diode.

19. An optical detector according to any preceding claim, wherein at least one of the photodetectors is an avalanche photodiode.

20. An optical wireless communication (OWC) apparatus comprising:

an optical detector according to any preceding claim;

demodulation circuitry configured to receive a detection signal from the optical detector, wherein

the optical detection signal comprises or represents the combined signals from the plurality of photodetectors; and

the demodulation circuitry is configured to perform a demodulation process to extract data from the detection signal.