FIELD

The present disclosure relates to communicating using electromagnetic signals, for example but not exclusively, in wireless optical communication systems.

BACKGROUND

Wireless communications systems are currently subject to considerable data demands, particularly owing to the popularity of wireless mobile communications. It is predicted that the amount of data being transmitted using radio frequency wireless communication systems will continue to increase to such an extent that there may be a "spectrum crunch" in which the radio frequency (RF) spectrum available for traditional wireless communication methods is no longer sufficient to carry the required volume of data.

Visible light communication (VLC) and other wireless optical communication techniques (for example non-visible light communication such as infrared communication) have emerged as a potential candidate to address the spectrum crunch. Compared with RF communication, VLC operates at an unregulated part of the electromagnetic spectrum and is considered intrinsically safe to be used in electromagnetic interference (EMI) sensitive environments, such as aircraft, hospitals and oil refineries. VLC and other optical techniques may provide a method to enable higher bandwidth data transmission than is currently possible using RF techniques. However, in order for VLC techniques to become more widely deployed, mobile cellular devices need to be VLC enabled, which current commercial considerations mostly rule out because VLC networks have not yet been widely deployed. For example, mobile cellular devices such as smartphones and tablets which are currently on the market will likely require additional hardware and software in order to make use of the higher bandwidth afforded by VLC and other optical communication techniques.

A number of solutions currently exist for providing wireless optical communication. An example of a solution currently available in the marketplace includes PureLiFi™. There are a number of challenges in the wider deployment of wireless optical communication technologies, particularly into the mobile telecommunications marketplace, but also in many other marketplaces such as in the so-called Internet of Things (loT) marketplace. In this respect, it is possible that optical transmitters and receivers for use in wireless optical communication systems will likely need to be appropriately integrated with hardware in a cost effective manner and become accepted by consumers. Consumers tend to have a preference for easy to use technology without requiring significant input from the consumer in order to successfully operate the technology.

Figure 1 illustrates an example of a wireless optical communications system 10 including an optical signal transmitter 12 for transmitting a wireless optical signal 14, which may be modulated for carrying data. The wireless optical signal 14 is divergent so as to provide optical signal coverage over an optical signal area 16, which may support data communications using e.g. LiFi with a number of receivers located within the signal area 16.

In the illustrated system 10, first and second optical receivers 18, 20 are located within the signal area 16 and are each connected to a processor 22 for processing an electronic signal generated by the first and second optical receivers 18, 20. The first optical receiver 18 has a small detection area 24 and the second optical receiver 20 has a large detection area 26. The relatively larger size of the detection area 26 of the second optical receiver 20 is able to detect a larger proportion of the optical signal 14 within the signal area 16 than the small detection area 24 of the first optical receiver 18, which may allow the second optical receiver 20 to have a larger signal to noise ratio (SNR) than the first optical receiver 18. Having a larger SNR may reduce the number of errors occurring during communications between the optical signal transmitter 12 and the second optical receiver 20.

However, the first optical receiver 18 may have a faster response time for detecting the optical signal 14 than the second optical receiver 20, for example, due to the relatively smaller size of the detection area 24 of the first optical receiver 18 compared with the relatively larger size of the detection area 26 of the second optical receiver 20. The faster response rate of the first optical receiver 18 may be due to the smaller size of the detection area 24 of the first optical receiver 18 having a lower capacitance than the larger detection area 26 of the second optical receiver 20.

If the SNR is too small (for example, in the case of the first optical receiver 18) there may be an increased number of communication errors, which may decrease the data transmission rate. If the detection area is too large (for example, in the case of the second optical receiver 20) the response time may be too low, which again, may decrease the data transmission rate. Since wireless optical communications such as LiFi are associated with higher bandwidth applications, the detection area of receivers for receiving wireless optical communications may be associated with affecting the available bandwidth, irrespective of the size of the detection area. SUMMARY

According to an example of the present disclosure there is provided a receiver for a wireless optical communications system. The wireless optical communications system may be configured to modulate at least one spectral component of an optical signal for transmitting data to the receiver. The receiver may comprise an optical element for separating at least one spectral component of the optical signal from at least one other spectral component of the optical signal.

Separating at least one spectral component from at least one other spectral component of the optical signal may allow the spectral components of the optical signal to be distinguished from each other. By separating the optical signal, the receiver may be used in a demultiplexing system or separation system for separating the optical signal. The receiver may be configured to demultiplex or separate out the optical signal based on the at least one spectral component of the optical signal. In use, the receiver may receive an optical signal comprising the at least one spectral component. The receiver may be located within an optical signal area and may be capable of receiving a portion of the optical signal within a detection area of the receiver. The detection area of the receiver may be appropriately dimensioned to allow the receiver to collect sufficient proportion of the optical signal, which may allow the receiver to achieve a desired signal to noise ratio (SNR).

The optical signal may comprise or be formed using optical radiation, such as electromagnetic radiation. The radiation may be non-radiofrequency (RF) radiation. The term "optical" as used herein may mean used, usable or suitable for use with optical radiation or optical signals, such as but not limited to light. The optical radiation may be radiation with a frequency above 300 GHz (or above RF), for example, by making use of at least one of optical communication in at least the following parts of the electromagnetic spectrum: microwave, Terahertz, infrared, visible, ultraviolet, X-ray, and gamma rays. It will be understood that the 300 GHz boundary is merely exemplary and different boundaries may be defined between optical and RF frequency bands. For example, the boundary between optical and RF frequencies may be defined as 25 GHz, 50 GHz, 100 GHz, 150 GHz, 200 GHz, 250 GHz, 300 GHz, 350 GHz, 400 GHz, 500 GHz, 600 GHz, 700 GHz, 800 GHz, 900 GHz, 1 THz, 2 THz, 5 THz, 10 THz, 20 THz, 50 THz, 100 THz, 200 THz, 500 THz, 1 PHz, or indeed any other suitable frequency. The receiver may be an optical receiver.

The receiver may provide a relatively straightforward and cost effective means of separating an optical signal into at least some of its spectral components. The optical signal may carry data, e.g. digital data, in the form of a modulated optical signal. At least one spectral component of the optical signal may be modulated, which may allow the optical signal to simultaneously support data transmission using at least one spectral component of the optical signal. Each spectral component of the optical signal may independently support a modulated signal such that each spectral component may individually carry data, or such that at least one spectral component may carry data, or such that at least one spectral component may carry data that has been modulated in a manner such that the data may be spread across the different spectral components (e.g. such that the data may be recovered by collecting the data by using the different spectral components together), or such that at least two spectral components carry the same data (e.g. for redundancy, or the like). Providing more than one spectral component may allow the optical signal to support a high bandwidth data transmission using the receiver, for example, so as to support a high rate of data transfer using the optical communications system. The receiver may provide a passive approach for separating the optical signal into at least one spectral component of the optical signal, which may simplify the operation of the receiver.

The optical element may be configured to spatially separate at least one spectral component of the optical signal from at least one other spectral component of the optical signal. The optical element may be configured to cause a change in a direction of propagation of at least one of the spectral components of the optical signal so as to separate at least one spectral component of the optical signal from at least one other spectral component of the optical signal.

The optical element may be configured to separate at least one data stream carried by at least one spectral component of the optical signal from at least one other data stream carried by at least one other spectral component of the optical signal. The receiver may be configured to support at least one modulated signal carrying data using at least one or each spectral component(s) independently of each other.

The receiver may be configured to support at least one modulated signal carrying data in a manner such that the data is spread across the different spectral component(s) such as for recovering the data by using or combining the modulated signal(s) from the different spectral components together.

The receiver may be configured to support at least one modulated signal carrying the same data, e.g. simultaneously, using a plurality of spectral components such as for redundancy purposes, or the like.

The optical element may be configured to collect the optical signal over a first area. The optical signal may be incident on the first area in an incident direction. The optical element may separate at least one spectral component from the optical signal by redirecting the at least one spectral component in a different direction to that of the incident direction.

The optical element may be configured to concentrate the at least one spectral component received over the first area. The at least one spectral component may be redirected by the optical element to a second area smaller than the first area. A flux density of the at least one spectral component may be increased by concentrating the redirected at least one spectral component over the second smaller area. The first area may comprise or correspond to a surface area of the optical element. The second area may comprise or corresponds to a cross-section of the optical element. The optical element may comprise a fluorescent component.

The optical element may comprise at least one of a: fluorophore, chromophore, light- sensitive molecule, dye, optical scatterer, quantum dot, liquid crystal, photoluminescent component, and the like. The photoluminescent component may comprise at least one of: a fluorescent, phosphorescent, and Raman component. The optical element may be activated using any appropriate technique, for example, fluorescence resonance energy transfer (FRET), or the like. A person of ordinary skill in the art will recognise that the optical element may be or comprise any appropriate element, substance, object, or the like capable of separating at least one spectral component from at least one other spectral component of the optical signal in any appropriate way.

The fluorescent component may comprise any appropriate atom, molecule, substance or material configured for fluorescence.

The fluorescent component may be configured to be excited, e.g. selectively excited, by at least one spectral component of the optical signal. The fluorescent component may be configured to emit a shifted optical signal, for example a wavelength (e.g. or frequency) shifted optical signal. The receiver may comprise a plurality of fluorescent components. There may be more than one type of fluorescent component. The at least one fluorescent component may be configured to be excited by a spectral band, for example, a predetermined spectral band. The at least one fluorescent component may be resonantly excited by the at least one spectral component. The fluorescent component may not be or may not be substantially activated (e.g. excited) by at least one spectral component of the optical signal. If the fluorescent component is not substantially activated by the at least one spectral component of the optical signal, the fluorescent component may emit a lower intensity shifted optical signal compared to if the example of the fluorescent component being resonantly excited by the at least one spectral component.

The optical element may comprise a plurality of optically active portions. At least one of the optically active portions may be configured to be activated by at least one spectral component of the optical signal. At least one or one other of the optically active portions may be configured to be activated by at least one other spectral component of the optical signal.

The optical element may comprise a first section. The first section may comprise a first optically active portion. The first optically active portion may be configured to be activated by a first spectral component of the optical signal. The optical element may comprise a second section. The second section may comprise a second optically active portion. The second optically active portion may be configured to be activated by a second spectral component of the optical signal.

The first spectral component may comprise a range of spectral components, for example, a range of wavelengths or wavelength band. The second spectral component may comprise a range of spectral components, for example, a range of wavelengths or wavelength band.

The first spectral component may contain at least one wavelength that differs from the second spectral component. The first and second spectral components may define spectral ranges which may or may not overlap. The first and second optically active portions may comprise different fluorescent components for allowing the first and second optically active portions to be activated by different spectral components.

The optical element may comprise a plurality of sections. At least one or each of the sections may comprise at least one respective optically active portion. The sections may be separated by an optical barrier for preventing at least one spectral component in one of the sections from passing to another of the sections.

The optical element may comprise a wavelength-shifting component for wavelength- shifting at least one spectral component of the optical signal. The wavelength-shifting component may comprise a fluorescent component, or the like. The fluorescent component may be configured to absorb radiation having a wavelength or wavelength band and emit radiation at a different wavelength or wavelength band.

The optical signal may be wavelength shifted by the optically active portion. The wavelength shifted optical signal may define a shifted optical signal. The shifted optical signal may define a modified optical signal. The shifted optical signal may comprise at least one fewer spectral component than the optical signal received by the receiver. The optical signal received by the receiver may define an input optical signal.

The wavelength-shifting component may be excitable by an incident optical signal comprising a modulated component. The wavelength-shifting component may be configured to emit a wavelength shifted optical signal comprising or at least partially corresponding to the modulated component in response to excitation by at least one spectral component of the incident optical signal. The modulated component comprised in the incident optical signal may comprise or encode a data component, data stream, or the like. The emitted wavelength shifted optical signal may be similarly modulated e.g. so as to comprise a modulation corresponding to or partially corresponding to or representative of the data component, data stream, or the like. It will be appreciated that if the optical element separates at least one spectral component from at least one other spectral component of the optical signal without using a wavelength-shifting component (e.g. if the optical element instead comprises optical scatterers, or the like), there may be no shift in the wavelength of the spectral component in at least one of the separated spectral components of the optical signal.

The optical element may be configured to direct or transmit at least one, non- wavelength-shifted, spectral component of the optical signal in a first direction. The optical element may be configured to redirect or divert a wavelength-shifted spectral component of the optical signal in a different second direction. The optical element may be configured to direct or transmit at least one, wavelength-shifted, spectral component of the optical signal in a first direction. The optical element may be configured to redirect or divert a non-wavelength-shifted spectral component of the optical signal in a different second direction. The first direction may correspond to an incident direction of the incident optical signal. The second direction may be at an angle to the first direction. The second direction may be the same direction as the first direction.

The non-wavelength-shifted spectral component may traverse the optical element without changing direction. The non-wavelength-shifted spectral component may change direction while traversing the optical element. The wavelength-shifted spectral component may traverse the optical element without changing direction. The wavelength-shifted spectral component may change direction while traversing the optical element.

The optical element may be configured to permit, e.g. selectively permit, transmission of at least one spectral component of the optical signal through the optical element. The optical element may be configured to redirect, divert or concentrate at least one separated spectral component of the optical signal.

The optical element may comprise at least a first side for receiving the optical signal and at least one further side for receiving at least one spectral component of the optical signal. The optical element may be configured to direct, reflect or transmit at least one separated spectral component of the optical signal towards the at least one further side. The incident optical signal may be incident on the first side of the optical element. The incident optical signal and/or a wavelength-shifted spectral component of the optical signal may traverse the optical element such that at least part (e.g. at least one spectral component) may be directed, reflected or otherwise transmitted towards the at least one further side of the optical element. The at least one further side may be a side other than the first side and/or a side of the optical element opposite the first side. At least one spectral component may traverse the optical element, e.g. without any change in direction and/or to a side of the optical element opposite to the first side.

The first side may define a face of the optical element. The further side may define a further face of the optical element. At least one of the sides may be perpendicular to another of the sides. At least one of the sides may be parallel to another of the sides. At least one of the sides may be disposed at an acute or obtuse angle relative to another of the sides. There may be no constraint on the relative angle between adjacent sides of the optical element.

At least one of the sides may be appropriately coated with an anti-reflection coating.

The first side may comprise or correspond to the first area. The further side may comprise or correspond to the second area. The receiver may comprise an optical concentrator for increasing an optical flux density or intensity of the optical signal.

At least one side of the optical element may define at least part of the optical concentrator. The at least one side of the optical element may comprise an internal surface of the optical element. The optical signal may be reflected by the internal surface. The optical signal may undergo total internal reflection within the optical element. The optical element may have an appropriate refractive index (e.g. a refractive index contrast or ratio with respect to an external part of the optical element).

The optical concentrator may be configured to direct at least one spectral component of the optical signal towards or to a further side of the optical element.

The optical signal may be reflected at least once to/from at least one internal surface of the optical element so as to be directed towards or to a further side of the optical element.

The optical concentrator may be configured to increase the optical flux density or intensity of at least one of: the optical signal received by the receiver; and at least one spectral component of the optical signal separated by the optical element.

The receiver may comprise at least one further optical concentrator disposed on a surface of or inside the optical element. The further optical concentrator may comprise any appropriate optical apparatus for increasing the optical flux density or intensity of any optical signal. The further optical concentrator may be shaped so as to increase the optical flux density or intensity of the optical beam as the optical beam passes through the optical concentrator. The optical concentrator may comprise at least one of: a truncated shape, a tapered side or profile, a bevelled side or profile, a cone, a truncated cone, a truncated pyramid, a truncated square based pyramid, and the like.

The receiver may comprise an optical filter for filtering at least one spectral component from the optical signal. The optical filter may be disposed between at least one optical concentrator and the optical element. The optical filter may be disposed on a side of the optical element. The optical filter may be disposed within (e.g. inside) the optical element.

The receiver may comprise an optical signal diverter for changing a direction of propagation of at least one spectral component of the optical signal. The optical signal diverter may comprise at least one of: a mirror, a dichroic mirror, a beam splitter, a prism, and the like. The optical signal diverter may be disposed or provided within, or on at least one surface or side, of the optical element. The optical signal diverter may comprise at least one surface or side of the optical element. The optical element may comprise at least one side oriented to divert at least part of the optical signal. The receiver may comprise at least one of: an optical signal diverter and an optically active portion.

The receiver may comprise at least one detector for detecting at least one spectral component of the optical signal. The detector may be configured to receive the optical signal, for example at least one of the optical signal and the shifted optical signal. The detector may be configured to detect modulations in the optical signal, for example frequency, amplitude or phase modulations, or the like. The detector may measure modulations in the optical signal. The receiver may comprise a plurality of detectors for detecting different spectral components of the optical signal. At least one of the detectors may be configured to detect at least one spectral component. At least one other of the detectors may be configured to detect at least one other spectral component. Any appropriate detector may be provided. The detector may comprise at least one of: a photodetector, photodiode, mini photodiode, photovoltaic cell, semiconductor detector, avalanche photodiode, phototransistor, and the like.

The detector may comprise an optical fibre configured to receive the optical signal. The detector may comprise any appropriate detector for converting the optical signal transmitted via the optical fibre into an electrical signal.

The receiver may comprise a demodulator or processor for demodulating or processing an electrical signal, for example, an electrical signal generated or produced by the at least one detector. The processor may be connected to the at least one detector. The receiver may comprise a plurality of the optical elements. At least one of the optical elements may be configured to separate at least one spectral component from the optical signal such that the at least one spectral component or a wavelength-shifted spectral component of the optical signal may be redirected or redirectable within the at least one optical element and such that at least one other spectral component from the optical signal may be transmitted or transmissible through the at least one optical element to at least one other optical element. The optical elements may be stacked, or otherwise arranged (e.g. with at least one of the optical elements in contact with at least one other of the optical elements). The optical elements may be arranged to define a first side of the receiver. The optical elements may define an assembly of optical elements. At least a first optical element may define a first shape. At least a second optical element may define a second shape. The first and second optical elements may be disposed or positioned in contact with each other for allowing transmission of the optical signal therebetween. Each of the optical elements may comprise a same or different optically active portion. Each optical element may comprise at least one optically active portion. At least one optical element may not comprise an optically active portion. At least one optical element may be configured to separate the optical signal into one spectral component of the optical signal. At least one optical element may be configured to separate the optical signal into more than one spectral component of the optical signal. If the optical elements are stacked or otherwise arrayed or assembled in contact with each other, each optical element may have a different configuration of optically active portions so that each optical element may separate the optical signal into the constituent spectral components of the optical signal, and may also compensate for a loss of signal intensity, for example due to loss of power due to losses such as scattering and/or absorption in the optical elements. If for example a certain spectral component is separated out at a bottom of the stack (e.g. of optical elements), the optical signal intensity may be relatively low at the bottom of the stack compared with the top of the stack (e.g. which may comprise the first side of the receiver for receiving the optical signal). By separating out each spectral component in more than one layer of the stack, the loss of signal intensity may be compensated so that the total optical signal loss (e.g. between the first side and a detector) for each spectral component is approximately the same. According to an example of the present disclosure there is provided a wireless optical communications system comprising at least one receiver according to any example of the present disclosure.

The wireless optical communications system may comprise a transmitter for modulating at least one spectral component of an optical signal. The transmitter may be configured to multiplex, mix or combine a signal (e.g. more than one optical signal having at least one spectral component) into an optical signal comprising at least one spectral component. The at least one receiver may be configured to demultiplex or separate the optical signal comprising the spectral component. The wireless optical communications system may comprise a processor for managing at least one electrical signal generated by a detector, for example a photodetector, for detection at least one spectral component of the optical signal. The wireless optical communications system may comprise at least one of: a modulator for modulating at least one spectral component of the optical signal; and a demodulator for demodulating a modulated electrical signal corresponding to a detected spectral component of the optical signal. Information bits may be processed by the modulator, which may be configured to output at least one modulated symbol.

According to an example of the present disclosure there is provided a method for receiving an optical signal in a wireless optical communications system. The method may comprise receiving an optical signal comprising at least one spectral component. The method may comprise separating at least one spectral component of the optical signal from at least one other spectral component of the optical signal. The method may comprise spatially separating at least one spectral component of the optical signal from at least one other spectral component of the optical signal.

The method may comprise causing a change in a direction of propagation of at least one of the spectral components of the optical signal to separate at least one spectral component of the optical signal from at least one other spectral component of the optical signal.

The method may comprise separating at least one data stream carried by at least one spectral component of the optical signal from at least one other data stream carried by at least one other spectral component of the optical signal.

The method may comprise supporting at least one modulated signal carrying data using at least one or each of the spectral component(s) independently of each other.

The method may comprise supporting at least one modulated signal carrying data in a manner such that the data is spread across the different spectral component(s) such as for recovering the data by using or combining the modulated signal(s) from the different spectral components together.

The method may comprise supporting at least one modulated signal carrying the same data, e.g. simultaneously, using a plurality of spectral components such as for redundancy purposes. The method may comprise collecting the optical signal over a first area, the optical signal being incident on the first area in an incident direction. The method may comprise separating at least one spectral component from the optical signal by redirecting the at least one spectral component in a different direction to that of the incident direction.

The method may comprise concentrating the at least one spectral component received over the first area to a second area smaller than the first area.

The method may comprise illuminating the first area using an optical signal having a divergent optical beam.

The method may act to separate at least one spectral component from the divergent optical beam received across the first area. The separated at least one spectral component may be concentrated or otherwise confined within the second area. The method may comprise illuminating the first area using an optical signal having at least one of: a collimated; and focused optical beam.

The collimated or focused optical beam may only illuminate part of the first area e.g. by converging at a spot (or beam spot) on the first area. In this example, the area illuminated by incident optical beam may be less than the second area that may confine the separated at least one spectral component of the optical beam.

The method may comprise using an optical element to separate at least one spectral component from at least one other spectral component of the optical signal.

The optical element may comprise a fluorescent component. The method may comprise activating or exciting the fluorescent component. The method may comprise activating a first optically active portion of the optical element. The first optically active portion may be configured to be activated by a first spectral component of the optical signal. The method may comprise activating a second optically active portion of the optical element. The second optically active portion may be configured to be activated by a second spectral component of the optical signal. The method may comprise activating any optically active portion, for example, if there are more than the first and second optically active portions.

The method may comprise wavelength shifting at least one spectral component of the optical signal.

The method may comprise exciting a wavelength-shifting component of the optical element using an incident optical signal comprising a modulated component so as to emit a wavelength-shifted optical signal comprising or at least partially corresponding to the modulated component. The modulated component may comprise, represent or encode data or a data stream.

The method may comprise directing or transmitting at least one, non-wavelength- shifted, spectral component of the optical signal in a first direction. The method may comprise redirecting or diverting a wavelength-shifted spectral component of the optical signal in a different, second direction. The method may comprise permitting transmission of at least one spectral component of the optical signal through the optical element and redirecting, diverting or concentrating at least one separated spectral component of the optical signal.

The method may comprise optically concentrating the optical signal. Optically concentrating the optical signal may increase an optical flux density or intensity of the optical signal. The method may comprise filtering at least one spectral component from the optical signal.

The method may comprise detecting at least one spectral component of the optical signal.

The method may comprise demodulating or processing an electrical signal generated or produced by at least one detector for detecting at least one spectral component of the optical signal. The electrical signal may be transmitted from the detector to a processor.

The method may comprise demultiplexing or separating at least one spectral component of the optical signal, and extracting data from at least one modulated spectral component of the optical signal. According to an example of the present disclosure there is provided a wireless optical communications system. The system may comprise a transmitter for multiplexing at least one electrical signal carrying at least one data stream into an optical signal comprising at least one spectral component. The transmitter may be configured to modulate at least one spectral component of the optical signal to encode data corresponding to at least part of the at least one data stream. The transmitter may be configured to modulate at least one other spectral component of the optical signal to encode data corresponding to at least another part of the at least one data stream. The system may comprise a receiver for demultiplexing or separating at least one spectral component from at least one other spectral component of the optical signal. The receiver may comprise at least one feature of any receiver described herein. The system may comprise using a processor for managing at least one electrical signal generated by a detector, for example a photodetector. The system may comprise the detector. The detector may be configured to measure or detect at least one spectral component of the optical signal. The system may be configured to extract data from at least one modulated spectral component of the optical signal.

At least one feature of any example, aspect or embodiment of the present disclosure may replace any corresponding feature of any example, aspect or embodiment of the present disclosure. At least one feature of any example, aspect or embodiment of the present disclosure may be combined with any other example, aspect or embodiment of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other examples of the present disclosure will now be described by way of example only and with reference to the following drawings, in which:

Figure 1 is an example of a wireless optical communication system employing various types of receivers;

Figure 2 is a schematic perspective view of part of a receiver for a wireless optical communications system according to an example of the present disclosure;

Figure 3 is a schematic view of a wireless optical communications system according to an example of the present disclosure;

Figure 4 is a schematic cross sectional view of part of a receiver for a wireless optical communications system according to an example of the present disclosure; Figure 5 is a schematic perspective view of the receiver of Figure 4 and illustrating some of the internal components thereof;

Figure 6 is a schematic side view of part of a receiver for a wireless optical communications system according to an example of the present disclosure; Figure 7a is a perspective view of part of a receiver for a wireless optical communications system, optionally comprising a plurality of the parts of the receiver shown in Figure 6, according to a further example of the present disclosure; Figure 7b is an exploded perspective view of part of the receiver of Figure 7a;

Figure 8 is a schematic perspective view of part of a receiver for a wireless optical communications system according to a further example of the present disclosure; Figure 9 is a perspective view of part of a receiver for a wireless optical communications system according to a further example of the present disclosure;

Figures 10a-10c respectively show a schematic perspective view of part of a receiver, and a top elevated view and an additional perspective view of an assembly of receivers for a wireless optical communications system according to a further example of the present disclosure;

Figure 11 is a schematic view of a wireless optical communications system according to a further example of the present disclosure;

Figures 12a-12b are schematic views of different configurations of a MIMO receiver for the wireless optical communications system of Figure 11 ; and

Figures 13a-13b are schematic views of different configurations of a MIMO-WDM receiver for the wireless optical communications of Figure 11.

DETAILED DESCRIPTION OF THE DRAWINGS

Figures 2 illustrates part of a wireless optical communications system 110 including a receiver 1 15 for receiving a wireless input optical signal 114 that diverges from a transmitter (not shown). In this example the receiver 115 is in the form of an optical element 1 16 having a cuboid shape having a first side 118 for receiving the wireless optical signal 114. However, it will be appreciated that the optical element 116 need not be cuboid but may be in another shape or form. As such, the invention is not limited to a cuboid optical element 116, which is provided by way of a particularly advantageous example only. In this example the wireless optical signal 1 14 includes four spectral components, + λ ₂ + λ ₃ + λ ₄ . Although four spectral components are used in this particular example, it will be appreciated that the system 1 10 could be readily adapted for use with wireless optical signals 1 14 having other numbers of spectral components. At least one of spectral components may be modulated, for example amplitude modulated, to carry data via the optical signal 114. Having a number of spectral components may allow the optical signal 1 14 to simultaneously carry high bandwidth data via each spectral component, with minimal or zero interference between the respective spectral components. Transmitting multiple data streams via signals with multiple spectral components simultaneously with minimal or zero inter-channel interference may significantly increase the transmission data rate of the optical wireless communication system. It is possible for just one or more than one spectral component to be used for transmitting data in the wireless communications system 1 10. The optical element 1 16 is configured to separate the optical signal 1 14 based on its four spectral components so that each spectral component λι , λ ₂ , λ ₃ , λ ₄ can be spatially distinguished from each other. In this example, the optical element 1 16 separates the optical signal 114 with different wavelengths based on their spectral characteristics, so that each signal component can be decoded independently to achieve wavelength division multiplexing transmission. In this example, a modified optical signal encoding data from each spectral component is respectively directed to a second, third, fourth and fifth side 120 (hereinafter "further sides 120") of the optical element 116. The further sides 120 are perpendicular to the first side 1 18. The optical element 1 16 includes at least one optically active portion (not explicitly shown, but is included in or on the optical element 116) for separating the optical signal 1 14 based on the at least one spectral component of the optical signal 114. The at least one optically active portion is arranged such that the spectral components λι , λ ₂ , λ ₃ , λ ₄ each activate at least one optically active portion so that the optical signal 114 is separated into four modified optical signals 122, each modified optical signal 122 being directed towards a different further side 120 of the optical element 1 16.

In this example the optically active portion takes the form of a fluorescent component (e.g. fluorophores) configured to be excited by the optical signal 1 14 so as to emit a wavelength shifted optical signal 122 (hereinafter "shifted optical signal 122"). The shifted optical signals 122 are wavelength shifted (i.e. lower frequency, higher wavelength) with respect to the optical signal 114 and are denoted by λι', λ ₂ ', λ ₃ ', λ ₄ ' in the figures. The at least one optically active portion is arranged with the optical element 1 16 such that each shifted optical signal 122 is separated (e.g. spatially) to a different further side 120 of the optical element 1 16. In this manner, each shifted optical signal 122 can be individually measured using any appropriate detector disposed in a position to receive the shifted optical signal 122. By separating the input optical signal 1 14 based on the spectral content thereof, the modulated content of each spectral component of the input optical signal 1 14 can be distinguished from each other and the data encoded by the modulations detected, or the carried information data can be decoded, in any appropriate way, for example, using a detector. Examples of arrangements for appropriately separating the wavelength shifted optical signals 122 are described herein.

Figure 3 is a schematic illustration of a wireless optical communications system 110 utilising the receiver 1 15 of Figure 2. The wireless optical communications system 1 10 includes a modulator 1 11 for modulating the optical signal 114 generated by a transmitter 112, for example, for modulating information data streams on to each spectral component of the optical signal 114. The diverging arrows labelled 1 14 in Figure 3 indicate that the optical signal 1 14 is divergent so as to completely illuminate the optical element 1 16 in this example, or at least to illuminate a sufficiently large area of the optical wireless communication system 110 to provide a suitably wide and usable coverage for optical wireless communications. The space between the transmitter 112 and the optical element 116 defines a free-space optical system, as indicated by Figure 3. An input electrical data signal containing information, such as a stream of information bits, is processed by the modulator 11 1 , which outputs the information in the form of one or more modulated electrical signals 113 encoded with the information, such as one or more series of modulated symbols, using any appropriate modulation scheme. An example of a modulation scheme that can be used for transmitting orthogonal subcarriers is orthogonal frequency division multiplexing (OFDM), which has been widely used in modern digital communications systems.

In the present example, the transmitter 112 includes at least one light emitting diode (LED) such as a red-green-blue (RGB) LED (not shown) that can be modulated by the one or more modulated electrical signals 1 13 (e.g. so as to be able to transmit one or more data streams). Each spectral component of the LED used in the transmitter 112 can be individually modulated to produce an optical signal 1 14 with at least one of the individual spectral components capable of carrying data (e.g. at least one data stream).

Upon illumination by the divergent optical signal 1 14, the optically active portion of the optical element 1 16 is excited by the optical signal 1 14 so as to emit a wavelength shifted optical signal (see Figure 2) towards at least one of the photodetectors 136 disposed on the further sides 120 of the optical element 1 16. The photodetectors 136 convert the wavelength shifted optical signal into a photodetector electrical signal 137 corresponding to the detected spectral component(s) in the optical signal 114 and including detected symbols. The photodetector electrical signal 137, i.e. the detected symbols, is/are decoded by a demodulator 139 to recover the data from the photodetector electrical signal 137.

Referring next to Figures 4-5, an example of a possible arrangement for a receiver 1 15 including the optical element 1 16 of Figures 2-3 is illustrated. The optical element 1 16 includes a number of optically active portions, which in this example include fluorophores 124 distributed within the optical element 1 16 and configured to fluoresce in response to excitation by one of the spectral components of the input optical signal 1 14. In this example, the optical element 116 includes sets of fluorophores 124 configured to fluoresce in response to only one of the spectral components, or at least mostly in response to one of the spectral components, of the input optical signal 114. It will be appreciated that there may be some overlap in the absorption spectrum for the sets of fluorophores 124 resulting in some fluorophores 124 being excited by more than one spectral component of the input optical signal 1 14. Each set of fluorophores 124 is located in separate sections 126 of the optical element 116, each section 126 being separated by an optical barrier 128 between adjacent sections 126 for preventing optical signal mixing between the separate sections 126. As best illustrated by Figure 4, each section 126 contains a number of fluorophores 124 (only one fluorophore 124 per section 126 is illustrated for brevity). Upon being excited by the input optical signal 114, the fluorophore 124 emits a shifted optical signal 122 in any direction, which may undergo total internal reflection (TIR) within the section 126 (or may exit any other wall of the section 126) and may exit the section 126 via the respective further side 120 of the optical element 116. The optical element 1 16 acts to concentrate the respective shifted optical signals 122 by virtue of TIR, preventing escape of some of the shifted optical signals 122 from the optical element 1 16. The overlapping spectral components of the input optical beam 114 are separated spatially such that any data encoded by one or more of the spectral components can be detected by measuring the shifted optical signal 122 at the relevant further side 120 of the optical element 116. Sections 126 act as optical concentrators due to the aforementioned TIR effect such that the intensity of the shifted optical signal 122 is increased within the sections 126 due to the shifted optical signals 122 being confined within the sections 126 (which in this example have a smaller cross-sectional area than the area of the first side 1 18).

Referring next to Figure 6, there is illustrated a further example of part of a wireless optical communications system 210 including a receiver 215 for receiving a wireless input optical signal 214. The receiver 215 includes an optical element 216, which optionally is similar to the optical element 1 16 of Figures 3-5. The optical element 216 is advantageously functions as a signal splitter for splitting one or more spectral components of the input optical signal 214. It will be appreciated that optical element 216 may or may not include an optically active portion such as fluorophores. In the presently described example, the optical element 216 does not include fluorophores. However, it will be appreciated that in this and any other examples, it may be possible to provide any appropriate optically active portion such as fluorophores, or the like. Like or similar features of the wireless optical communications system 210 are incremented by 100 as appropriate.

In this example the input optical signal 214 includes a first and second spectral component and enters the optical element 216 via the first side 218 of the optical element 216. In the figure the first spectral component is illustrated in red and the second spectral component λ ₂ is illustrated in blue. The optical element 216 optionally includes fluorophores (not shown) similar to the example of Figures 3-5. The fluorophores act to divert the first spectral component to a further side 220 of the optical element 216, the further side 220 being perpendicular to the first side 218. The first spectral component is is reflected to form a reflected optical signal 222 denoted by λι' in Figure 5. If fluorophores are provided then the reflected optical signal 222 may include at least one of: a non-wavelength-shifted and a wavelength-shifted spectral component.

The optical element 216 includes a beam directing element, which in this example is in the form of a number of dichroic beam splitters 230 disposed within the optical element 216 at 45 degrees to the first side 218 and further side 220, the dichroic beam splitters 230 thus being oriented for guiding (e.g. by reflecting the input optical signal 214 through 90 degrees) the first spectral component (and/or the shifted optical signal 222 for examples that include fluorophores, or the like) to further side 220 and to allow the second spectral component λ ₂ through to an opposite side 221 of the optical element 216 (e.g. by transmission through the beam splitters 230), the opposite side 221 being parallel to and on the opposite side of the optical element 216 to the first side 218.

An optical concentrator 232 is positioned in contact (e.g. for substantially refractive index matching) on the further side 220 of the optical element 216. The optical concentrator 232 is in the form of truncated square based pyramid arranged so that a base 233 of the concentrator 232 is disposed in contact with the further side 220. A truncated portion 234 of the concentrator has a smaller surface area than the base 233. The optical concentrator 232 may act to increase the reflected optical signal 222 intensity by using total internal reflection (to prevent escape of the reflected optical signal 222) within the concentrator 232 to concentrate the input optical signal 214 entering the base 233 into the smaller area of the truncated portion 234.

A photodetector 236 is positioned on the truncated portion 234 of the concentrator 232 and is configured to detect the reflected optical signal 222 and convert the optical signal 222 to a photodetector electrical signal 238 for further processing in any appropriate way (e.g. by demodulation such as that demonstrated by the example illustrated by Figure 3, or the like). The truncated portion 234 acts to concentrate an optical signal from a larger area (within the optical element 216) to the smaller area of the photodetector 236. The receiver 215 further includes an optical filter 240 for filtering out at least one of the first spectral component exiting the optical element 216 via the opposite side 221 and permitting the second spectral component to exit the optical element 216 via the opposite side 221. Using the optical filter 240 to reject at least one signal including at least one spectral component may minimise interference from an optical signal including at least one other spectral component.

Referring next to Figures 7a-7b, there is illustrated a further example of part of a wireless optical communications system 310 including a receiver 315 for receiving a wireless input optical signal 314. The receiver 315 includes a number of optical elements 316, which are each similar to the optical element 216 of Figure 6. The system 310 of Figures 7a-7b optionally comprises a stack of the optical elements 216, such as those shown in Figure 6, which may allow concurrent detection and transmission of more data streams with different spectral components. Like or similar features of the wireless optical communications system 310 are incremented by 100 as appropriate.

In Figure 7a, three optical elements 316 are stacked on top of each other such that a first spectral component of an input optical signal 314 is directed (e.g. via total internal reflection) into a concentrator 332 for detection of a reflected optical signal 322 (e.g. at least one of the first spectral component and the reflected first spectral component λ^) via at least one of several photodetectors 336. The second, third and fourth spectral components λ ₂ , λ ₃ , λ ₄ are transmitted through an optical filter 340 of the optical element 316 on the top of the stack of optical elements 316. The receiver 315 may allow the concurrent transmission and detection of one more data streams with different spectral components. Each of these spectral components λ ₂ , λ ₃ , λ ₄ (and/or the reflected second, third and fourth spectral components λ ₂ ', λ ₃ ', λ ₄ ') are appropriately directed to a set of photodetectors (e.g. photodiodes) 336 in the same manner as described in relation to the first spectral components λι , λ^. The receiver 315 acts to concentrate at least part of the input optical signal 314 entering the receiver 315 via the first side 318 so that at least one of the photodetectors 336 receives sufficient signal to measure a modulated component of at least one of the spectral components of the input optical signal 314 and allow the modulated component to be processed (e.g. by a demodulator). The receiver 315 also acts to separate the input optical signal 314 into its constituent spectral components so as to enable detection/measurement of individual optical signals, each of which corresponding to a spectral component of the input optical signal where each spectral component can be individually modulated e.g. for wavelength division demultiplexing applications in conjunction with a wavelength division multiplexing (WDM) system (e.g. including a transmitter such as illustrated by Figure 3).

Referring next to Figure 8, there is illustrated a further example of part of a wireless optical communications system 410 including a receiver 415 for receiving a wireless input optical signal 414 including a first, second and third spectral component λι , λ ₂ , λ ₃ . The receiver 415 includes a single optical element 416, which is similar to the optical element 216 of Figure 6. Like or similar features of the wireless optical communications system 410 are incremented by 200 as appropriate. The optical element 416 is substantially cube shaped and configured such that the input optical signal 414 entering the optical element 416 via a first side 418 exits via at least one of: different further sides 420 and opposite side 421 of the optical element 416. The geometrical arrangement differs to that of the optical element 216 of Figure 6. In contrast to the example of Figure 6 where there are a number of spaced-apart dichroic beam splitters arranged at 45 degrees to the first side 218 and configured to direct the optical signal 214 towards one of the further sides 220, the present example includes a pair of crossed dichroic beam splitters 430. Each dichroic beam splitter 430 is arranged at 45 degrees to the first side 418 and at 90 degrees to each other such that two of the spectral components are directed towards opposing further sides 420 and another spectral component is directed to an opposite side 421 of the optical element 416 to that of the first side 418. The first spectral component λι' of the reflected optical signal 422 is directed to one of the further sides 420 of the optical element 416, the further side 420 being perpendicular to the first side 418. The second spectral component λ ₂ ' of the reflected optical signal 422 is directed to the opposite side 421 of the optical element 416, the opposite side 421 being parallel to (and on an opposite side to) the first side 418. The third spectral component λ ₃ ' of the reflected optical signal 422 is directed to another of the further sides 420 of the optical element 416, the further side 420 being perpendicular to the first side 418 and parallel to the further side 420 associated with the first spectral component λ^. As with Figures 6-7a-b, a photodetector 436 is provided on an optical concentrator 432 associated with each further side 420 and opposite side 421 of the optical element 416. However, an optical filter 440 is disposed between the further sides 420 and opposite side 421 and the optical concentrator 432 to only permit the correct spectral components through into the optical concentrator 432 for detection by the photodetector 436 and for further processing of the generated electric signal as required. Further, comparing with the receivers shown in Figures 6, 7a-b, the design of the receiver 415 may be considered to be relatively more compact and particularly suited to smaller devices.

Referring next to Figure 9, there is illustrated a further example of part of a wireless optical communications system 510 including a receiver 515 for receiving a wireless input optical signal 514 including a first, second and third spectral component λι , λ ₂ , λ ₃ (and more spectral components if required). The receiver 515 includes an optical element 516, which is similar to the optical element 416 of Figure 8. Like or similar features of the wireless optical communications system 510 are incremented by 100 as appropriate.

The optical element 516 defines a hexagonal prism shape with a hexagonal first side 518. A detection assembly 542 including a concentrator 532, photodetector 536 and optical filter 540 similar to the example of Figure 8 is disposed on each of the six further sides 520 of the optical element 516 where each further side 520 is perpendicular to the first side 518 (and defining an internal angle of 120 degrees between adjacent further sides 520). The optical element 516 further includes mirrors or beam splitters 530 for appropriately concentrating and/or directing the input optical signal 514 and/or a reflected optical signal 522 to the appropriate detection assembly 542 for a given spectral component. Similar to the other examples, the optical element 516 reflects the first, second and third spectral component λι , λ ₂ , λ ₃ (e.g. to form reflected first, second and third spectral component λι', λ ₂ ', λ ₃ ' of the reflected optical signal 522). The optical filter 540 acts to only permit one of these components through to the appropriate photodetector 536.

Referring next to Figures 10a-10c, there is illustrated a further example of part of a wireless optical communications system 610 including a number of receivers 615 for receiving a wireless input optical signal 614 including a first, second and third spectral component λι , λ ₂ , λ ₃ , λ ₄ (and more spectral components if required). The receiver 615 includes an optical element 616 similar to the optical element 516 of Figure 9. Like or similar features of the wireless optical communications system 610 are incremented by 100 as appropriate.

Each receiver 615 defines an optical element including both a cuboid-shaped portion 644 of optical element 616 and a triangular prism-shaped portion 646 of optical element 616. Both the cuboid-shaped portion 644 and the prism-shaped portion 646 define at least part of a first side 618 of the receiver 615. The prism-shaped portion 646 includes a mirror or beam splitter 630 for directing an input optical signal 614 from the first side 618, via a further side 620 of the prism-shaped portion 646, and into a further side 620 of the cuboid-shaped portion 644 for separating the input optical signal into its constituent spectral components. Four detection assemblies 642 are disposed on further sides 620 of the cuboid-shaped portion 644, each assembly 642 being configured to detect one of the (reflected) spectral components of the input optical signal 614. The cuboid-shaped portion 644 includes further dichroic beam splitters 630 for appropriately transmitting or reflecting the various spectral components of the input optical signal 614 and/or the reflected optical signal 622. Four of the receivers 615 are arranged together so as to form an assembly of receivers 615 with the detection assemblies 642 of each receiver 615 being disposed around a periphery or edge of the assembly of the receivers 615 and the respective prism- shaped portions 646 of each receiver 615 being arranged in contact with each other so that the first side 618 of each prism-shaped portion 646 collectively define a square- shaped section of the first side 618, and so that the combination of all the cuboid- shaped portions 644 and prism-shaped portions 646 defines a square-shaped first side 618 with the detection assemblies 642 being disposed on an edge 648 of the assembly of receivers 615, the edge 648 being perpendicular to the first side 618. In use, the first side 618 defines a relatively large area for collecting the input optical signal 614 and for concentrating the individual spectral components of the input optical signal 614 into the respective detection assemblies 642. The assembly of receivers 615 may be considered to have at least two functions. Firstly, to increase the total detected signal power. Secondly, for each receiver 615, the signal strength detected by each photodetector of the detection assemblies 642 may be slightly different. The longer the signal travels, the lower the remaining signal power due to absorption, scattering, and the like. Thus, the assembly of four receivers 615 can be constructed such that the accumulated signal strength for each of the spectral components is the same by providing different dichroic beam splitters (for different spectral components) at different locations within each receiver 615 so that e.g. red, yellow, green, blue spectral components (as illustrated by Figure 10b) are directed to a different respective photodetector within each receiver 615. For example, in one receiver 615, the red spectral component travels the shortest possible distance to a respective photodetector, and in the other receivers 615, the red spectral component travels a different distance to the respective photodetectors. Collectively, the same or a similar power level for each spectral component can be detected across all of the detection assemblies 642.

Figure 1 1 is a schematic illustration of a wireless optical communications system 710 similar to the wireless optical communications system 1 10 illustrated by Figure 3. The communications system 710 comprises a collimated source and MIMO configuration. The wireless communications system 710 includes a modulator 711 , a receiver 715 including an optical element 716, photodetectors 736 and a demodulator 739 that are similar to or the same as the corresponding features in Figure 3. However in contrast to the example of Figure 3, the communications system 710 includes a transmitter 712 that is configured to produce a non-divergent optical signal 714 e.g. in the form of a plurality of collimated or focused optical signals 714 so that only relatively small areas of the optical element 716 are illuminated by the individual optical signals 714. Due to the collimating or focusing of these optical signals 714, the optical intensity at the optical element 716 (i.e. for a transmitter 714 of similar optical power to the transmitter 1 14) is higher than for the optical element 116 of Figure 3. The higher input optical signal intensity of the non-divergent, e.g. collimated or focused, beam will in turn create a shifted optical signal that is higher in intensity than that produced by the divergent beam of Figure 3, which may increase the signal to noise ratio at the photodetectors 736.

In the present example, the transmitter 712 includes at least one light emitting diode (LED) such as a single LED (not shown) that can be modulated by one or more modulated electrical signals 713. Each LED used in the transmitter 712 can be individually modulated to produce a modulated optical signal 714. In the example of Figure 3 an RGB LED was used. In this example, each single LED can only be modulated across its entire spectrum (whether monochromatic or broadband such as for a blue-white LED) and thus each LED only supports a single modulated signal. In another example, an RGB LED (or similar) can be provided to permit multiple spectral components to be modulated within each optical source.

Upon illumination by the collimated or focused optical signal 714, the optically active portion of the optical element 716 is excited by the optical signal 1 14 so as to emit a wavelength shifted optical signal towards at least one of the photodetectors 736 for conversion into a photodetector electrical signal 738, and subsequently for demodulation by the demodulator 739. In the present example, the optical element 716 comprises Coumarin6, which absorbs blue and emits green light.

Referring next to Figures 12a-12b, two implementations of the receiver 715 e.g. for the wireless optical communications system 710 of Figure 1 1 are illustrated. In each case, the receiver 715 includes the same optical element 716 (which is similar to the optical element 1 16 illustrated by Figures 2-3) and includes four photodetectors 736, one disposed on each of the four further sides 720 of the optical element 716. The transmitter 712 (see Figure 11) transmits four modulated optical signals 714 that are directed to four spatially distinct spots 717 on the optical element 716. The optical signals 714 can be used in a multiple-input multiple-output (MIMO) configuration for transmitting data across the channels defined by the optical signals 714.

In Figures 12a-12b, there are two different spatial distributions of the spots 717. In Figure 12a, there is a "diamond" shape configuration in which each spot 717 can be considered as being disposed at the points of the diamond. Each spot 717 is adjacent to its respective photodetector 736. In Figure 12b, there is a "rectangular" shape configuration in which each spot 717 can be considered as being disposed at the corners of the rectangle. In this example, each spot 717 is equidistant to two of the photodetectors 736.

Referring next to Figures 13a-13b, there is illustrated part of a wireless optical communications system (e.g. such as the communications system 710) including a receiver 815 for receiving more than one spectral component, in this example, two blue spots 817 and two green spots 819. Thus data can be transmitted by each modulated optical signal by individually modulating each of the LEDs providing the different spectral components of the optical signals.

The receiver 815 includes two stacked optical elements 816 where a first optical element 816a is configured for receiving one of the spectral components (e.g. the blue spectral component) and a second optical element 816b is configured for receiving the other of the spectral components (e.g. the green spectral component). In the present example, the first optical element 816a comprises Coumarin6 and the second optical element 816b comprises Styryl9M, which absorbs green and emits red light.

Although the optical elements 816 are illustrated as not being co-extensive, it is envisaged that in an example the optical elements 816 could well be co-extensive relative to each other. Each optical element 816 includes two photodetectors 836. In this example, the incident optical signals are incident on the first optical element 816a such that the blue spectral components are absorbed by the first optical element 816 so as to emit a shifted optical signal (e.g. to a green spectral frequency). The incident optical signal including the green spectral component passes through the first optical element 816a and is absorbed by the second optical element 816b so as to emit a shifted optical signal (e.g. to a red spectral frequency). In this manner, it is possible to separate different spectral components into different layers of the receiver 815 and thus distinguish between modulated signals provided by the different spectral components, which may increase the data capacity of the wireless optical communications system 810. As with the example of Figures 12a-12b, Figures 13a- 13b employ a "diamond" and "rectangular" configuration of spots 817, 819, respectively. It has been found that providing the two optical elements 816a, 816b may provide a degree of flexibility in terms of the relative positioning the spots 817, 819.

The optical signals 814 can be used in a multiple-input multiple-output (MIMO) wavelength-division multiplexing (WDM) configuration for transmitting data across the channels provided by the different spectral components of the optical signals 814. Providing a MIMO-WDM configuration may improve data capacity of the communications system, which may come at the cost of signal to noise ratio performance. The signal to noise ratio may be improved if the optical signal comprises only one spectral component such as that illustrated by the example of Figures 12a- 12b. The person of ordinary skill in the art will appreciate that the examples described herein may be modified or adapted in any appropriate way, for example, including but not limited to the examples detailed below.

In the example illustrated by Figure 3, each spectral component of the LED used in the transmitter 1 12 can be individually modulated to produce an optical signal 114 with at least one of the individual spectral components capable of carrying data such as digital data. It will be appreciated that other types of optical sources having any range of spectral frequencies can be used in the wireless optical communications system 110 or indeed in any wireless optical communications system of the present disclosure. Examples of possible frequencies include any frequency or range of frequencies selected from at least one of: UV, visible and IR spectral ranges (and/or any from other part of the electromagnetic spectrum). In the example of Figure 3, there is more than one spectral frequency, each of which can be individually modulated to individually carry data independently of each other (and thus increase bandwidth). However in other examples, the optical signal may include one spectral frequency or a range of spectral frequencies where the entire spectral content (whether having one or a range of spectral frequencies) of the transmitter is modulated such that data transmission can only be supported in one channel. At least one of the spectral components of the optical signal 114 can be modulated to carry the signal. In the present example, the optical signal 1 14 includes more than one spectral component (e.g. for wavelength- division multiplexing, or the like). However, one or more of these spectral components can be modulated at any time depending on the particular data capacity requirements at any time.

Although some of the examples describe an example of the optically active portion as including fluorophores, it will be appreciated that any form of optically active element may be used. For example the optically active portion may include, but not be limited to, fluorophores, chromophores, light-sensitive molecules, dye, optical scatterers, quantum dots, liquid crystals, photoluminescent molecules or components, or the like. The skilled person will appreciate that the input optical signal may or may not be shifted in wavelength, depending on the type of optically active portion used. It will be appreciated that in any of the examples the spectral components of the input optical signal may be wavelength shifted and a detector provided for detecting the wavelength shifted optical signal. However, at least some of the input optical signal may still reach the detector, in addition to the shifted optical signal. Thus, one or both of the input and shifted optical signals may contribute to the measurement of the signal of any one spectral component at any one detector of the receiver.

In some examples, dichroic beam splitters, mirrors, prisms, or the like, are used for separating (e.g. by reflection, or the like) at least one spectral component of the optical signal (which may or may not also reflect a wavelength-shifted spectral component) from at least one other spectral component of the optical signal. In any of these examples, it will be appreciated that the optical element may or may not also comprise an optically active portion (such as comprising fluorophores, or the like). It will be appreciated that the optical element may or may not include a dichroic beam splitter, mirror, prism, or the like. Thus, any of the examples described in the present disclosure may include: dichroic beam splitters, mirrors, prisms, or the like for separating the spectral components, or may include the optical active portion for separating the spectral components, or may include any combination thereof. In addition, although certain examples are provided with a certain number of spectral channels, it will be appreciated that the specific examples could be readily adapted for use with different numbers of spectral channels.

Any surface of any component of any example of the present disclosure may be coated in any appropriate way to minimise or maximise signal reflection and/or transmission as appropriate. For example the first side may be coated with an anti-reflection coating to maximise transmission of the input optical signal into the optical element.