The invention relates to optical data communications using optical concentrators.
Market research published in 2014 predicted that by 2019 there will be 11.5 billion mobile-connected devices in the world and that these devices will contribute to a tenfold increase in global mobile data traffic between 2014 and 2019. It is anticipated that high concentrations of devices using the RF spectrum will generate so much interference that service quality will be significantly degraded. A key part of the solution to this expected spectrum crunch is to exploit new parts of the electromagnetic spectrum to support mobile wireless communications.
A part of the electromagnetic spectrum not currently used widely for wireless communications is visible light. The possibility of exploiting this part of the spectrum economically is increasing, due to the growing use of light emitting diodes (LEDs) for lighting. Unlike some other lighting technologies, LEDs can be modulated at relatively high frequencies. For example, micro-LEDs can be modulated at frequencies up to 185 MHz. These frequencies are suitable for wireless communications using existing infrastructure. Other potential advantages of visible light communication (VLC) include the lack of electromagnetic interference, the ability to localise light within a space to support many users and the improved cyber-security arising from the fact that light does not penetrate walls.
For indoor VLC lighting LEDs can be used as transmitters with a high signal to noise ratio. Receivers will each need to incorporate a photodetector which converts the modulated light into a modulated electrical signal. To have a large enough bandwidth the photodetectors will typically have to be fairly small (possibly of the order of 100-300 μm diameter). The signal falling on such small photodetectors may be increased using an optical concentrator such as a lens or a compound parabolic concentrator. However, the area of typical concentrators is limited by the conservation of étendue (constant radiance theorem), which means that the maximum gain, Gmax, for a concentrator with a field of view B is given by
where n is the refractive index of the concentrator.
The relationship between the étendue limited maximum theoretical optical gain and the half angle of the field of view of a concentrator is well-known. However, with gains of 1,000 or more the concentrator's aperture will have a diameter of less than 1 cm. Such small apertures will be vulnerable to being accidentally blocked. Furthermore, the high gain reduces the field of view (FOV). A gain of around 1,000 will be associated with a FOV of about 3°. This is likely to cause problems in many practical applications. For example, if VLC were to be used with handheld mobile terminals such as mobile phones or tablets, these would need to have concentrators with relatively large fields of view, for example around 20° or higher. Such fields of view would reduce the maximum theoretical gain of the concentrator to below 20. Unfortunately, a concentrator with a field of view of 20° or more will also have an aperture of 1 mm or less, and will therefore be vulnerable to blocking.
Changing the wavelength of radiation during the concentration process, using fluorescence for example, allows gains and/or fields of view to be achieved which are not constrained by conservation of étendue, and which can therefore be more favourable. Examples of arrangements based on this principle are disclosed for example in GB 2506383A and in ‘High gain, wide field of view concentrator for optical communications’, Steve Collins, Dominic C. O'Brien and Andrew Watt, OPTICS LETTERS, Vol. 39, No. 7, pp 1756-1759 Apr. 1, 2014.
Optical concentration refers to the process of receiving light using a relatively large collecting aperture and concentrating that light onto a much smaller area, such that the photon flux density on the smaller area is larger than the photon flux density on the larger area. There are many applications for concentrators, including in free space optical communications and power generation. In the case of optical communications, light carries an information signal, and an optical receiver uses a concentrator to collect light from the largest area possible and concentrate it on a photo-detector.
The principle of operation of a concentrator 1 comprising a wavelength converting element based on fluorescence is illustrated schematically in
Increasing concentrator gain facilitates high speed data communications by enabling efficient use to be made of small, fast detectors.
It is an object of the invention to provide further improved apparatus and methods for data communications.
According to an aspect of the invention, there is provided a receiver assembly, comprising: a concentration stage configured to receive radiation via an input surface and output concentrated radiation via an output surface, wherein the concentration stage comprises a wavelength converting member configured to convert radiation to longer wavelength radiation; an optical element configured such that if a plane wave of radiation is incident on the optical element a spatial distribution of radiation derived from the plane wave on the input surface of the concentration stage varies as a function of a direction of incidence of the plane wave relative to the optical element; and a plurality of detectors, each detector being configured to detect radiation output from a different portion of the output surface of the concentration stage.
The wavelength conversion to longer wavelengths makes it possible to provide a wider field of view for a given level of gain or, conversely, a higher gain for a given field of view, then concentrators which do not use a wavelength converting member. Increasing the degree of concentration makes it possible for the detectors to be made smaller and therefore more efficient, for example faster and/or cheaper. The optical element and the plurality of detectors make it possible to distinguish between radiation incident on the concentration stage from different directions. This ability makes it possible to implement data communications in a multiple input, multiple output (MIMO) mode, thereby increasing the maximum rate of data transfer.
In an embodiment, the wavelength converting member comprises a plurality of wavelength converting elements and a concentration of the wavelength converting elements per unit area when viewed in a direction perpendicular to the input surface varies as a function of position over the input surface. This feature makes it easier to distinguish reliably between radiation incident on the concentration stage from different transmitters (and therefore from different directions) and/or allows transmitter systems to comprise larger numbers of individual transmitters and/or to locate transmitters closer together (thereby improving compactness at the transmitter system). The variation in concentration can be used to vary a sensitivity of the receiver assembly to differences of direction of incidence as function of direction of incidence, such that the receiver assembly can distinguish more sensitively for example between different radiation beams within selected solid angles in comparison with other solid angles.
In an embodiment, the wavelength converting elements are distributed in a medium and a thickness of the medium in a direction perpendicular to a nearest portion of the input surface varies as a function of position over the input surface. Alternatively or additionally, the number of the wavelength converting elements per unit volume varies as a function of position, at least in a direction parallel to the input surface. These approaches, individually or in combination, make it possible flexibly to enhance how radiation from different transmitters can be distinguished, at low manufacturing cost.
In an embodiment, the concentration stage comprises a plurality of radiation guides; each radiation guide has an elongate form with a length that is at least five times longer than all dimensions of the radiation guide perpendicular to the longitudinal axis; each radiation guide comprises a portion of the wavelength converting member and is thereby configured to convert radiation to longer wavelength radiation within the radiation guide; the input surface of the concentration stage comprises at least a portion of an outer lateral surface of each of the radiation guides; and each of the radiation guides is configured to guide the converted radiation to a longitudinal end surface of the radiation guide, the output surface of the concentration stage comprising at least a portion of the longitudinal end surface of each of the radiation guides.
Use of a plurality of elongate radiation guides enables radiation to be concentrated onto a point-like surface, thereby facilitating use of small detectors, which can operate efficiently at high speed and increase bandwidth of communication devices using the assembly. Furthermore, the inventors have recognised that radiation guides having the required geometry are widely available (e.g. optical fibres) and can be cost effectively adapted to achieve the functions of the invention. Radiation is also only output from the radiation guides via the longitudinal ends surfaces, meaning that detectors are only needed in these locations. This provides a convenient geometry for positioning the detectors and supporting electronics for the detectors.
In an embodiment, wavelength converting elements are distributed non-uniformly through a cross-section of the radiation guide. Varying the concentration of the wavelength converting elements provides flexibility to achieve an optimal balance between efficiently converting radiation to longer wavelength radiation (favoured by regions of relatively high wavelength converting element density) and providing a low absorption path for the converted radiation to travel to the detector (favoured by having paths of relatively low wavelength converting element density). The wavelength converting elements may be concentrated for example in regions where it is expected that incident radiation will be focussed by the particular geometry of the radiation guide.
In an embodiment, more than 95% of the wavelength converting elements may desirably be located within an azimuthal angle of 330 degrees relative to the longitudinal axis, averaged over the length of the radiation guide. Such an embodiment provides an angular region (the remaining 30 degrees) in which fewer wavelength converting elements are present, which may facilitate efficient entry of radiation into the radiation guide and subsequent distribution to the wavelength converting elements in other regions.
In an embodiment, a spatial density of wavelength converting elements in the radiation guide, averaged over the length of the radiation guide, varies as a function of radius relative to the longitudinal axis. This arrangement may be particularly easy to manufacture. In an embodiment, the radiation guide comprises a core of an optical fibre. High quality optical fibres are widely available and can be adapted in a cost-effective manner. For example, in an embodiment the radiation guide further comprises an outer layer on the core of the optical fibre and wherein the conversion of the received radiation to longer wavelength radiation is performed at least partially in the outer layer. The outer layer can be provided simply by replacing the outer cladding of a conventional optical fibre with the outer layer.
According to an aspect, there is provided a data communications method, comprising the following steps: using a concentration stage to receive radiation via an input surface and output concentrated radiation via an output surface; using a wavelength converting member in the concentration stage to convert radiation to longer wavelength radiation; providing an optical element configured such that if a plane wave of radiation is incident on the optical element a spatial distribution of radiation derived from the plane wave on the input surface of the concentration stage varies as a function of a direction of incidence of the plane wave relative to the optical element; and detecting radiation output from the output surface using a plurality of detectors, each detector being configured to detect radiation output from a different portion of the output surface.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols represent corresponding parts, and in which:
Optical concentration can be used to reduce the size of photo-detectors required in free space optical communications applications, particularly where a wavelength converting material is used to convert radiation to longer wavelength radiation within a concentration stage. Smaller photo-detectors have higher bandwidths on average. The inventors have recognised that further increases in channel capacity can be achieved by providing a receiver assembly that uses wavelength conversion and is also adapted to operate in a multiple input, multiple output (MIMO) mode. Examples of how this may be achieved are described below with reference to the figures.
In an embodiment, an example of which is depicted in
The concentration stage 14 comprises a wavelength converting member 6. The wavelength converting member 6 converts radiations to longer wavelength radiation. The wavelength converting member 6 absorbs radiation of a first wavelength or range of wavelengths and re-emits the radiation at a second wavelength or range of wavelengths that is different to the first. The conversion involves shifting power from shorter wavelengths towards longer wavelengths. In an embodiment, the wavelength converting member 6 has a short response time, for example of 1 microsecond or less, optionally 10 nanoseconds or less, optionally 1 nanosecond or less, in order to facilitate high bandwidth data communications.
In an embodiment the wavelength converting member 6 comprises a plurality of wavelength converting elements. The plurality of wavelength converting elements may comprise fluorophores, which operate on the basis of fluorescence. The wavelength converting elements may comprise fluorescent dye. Alternatively or additionally, the wavelength converting elements may comprise quantum dot wavelength converters, for example solution processed quantum dots. Solution processed quantum dots are particularly suitable for this application because they have tuneable absorption and emission characteristics, large luminescence quantum yields and Stokes shifts compatible with minimal re-absorption losses. The one or more wavelength converting elements may optionally be substantially transparent to converted radiation so as to reduce or minimize re-absorption losses.
In an embodiment, the conversion of the received radiation to longer wavelength radiation comprises one or more of the following: conversion of infrared or near-infrared radiation to infrared radiation or near-infrared radiation having a longer wavelength, conversion of UV radiation to visible radiation, conversion of UV radiation to infrared or near-infrared radiation, conversion of visible radiation to visible radiation having a longer wavelength, and conversion of visible radiation to infrared or near-infrared radiation. In one particular embodiment, radiation is absorbed at approximately 475 nm and re-emitted at approximately 600 nm, with a corresponding confinement structure 17 being provided that substantially passes radiation having a wavelength of approximately 475 nm and traps radiation having a wavelength of approximately 600 nm. Such a system may be implemented using the dye Ru(BPY)3 for example. Many other dyes may be used. Alternatively or additionally, quantum dots may be used. For example, Qdot® (Life Technologies Corporation) quantum dots may be used, which are available in various different formats with different absorption and emission characteristics. Qdot® 605, or Qdot® 655, which have respective emission maxima of about 605nm and about 655 nm may be used for example.
The shape of the wavelength converting member 6 is not particularly limited, making it possible to tailor the shape to suit the particular application in which the receiver assembly 100 is to be incorporated. The receiver assembly 100 can therefore be incorporated into a wide variety of devices with a minimum of visual impact relative to the same devices without the receiver assembly 100. In an embodiment, the wavelength converting member 6 has a thickness that is smaller than the length and/or width of the member 6. In an embodiment, the wavelength converting member 6 has a substantially sheet-like form, for example having a thickness that is at least 10 times, optionally at least 50 times, optionally at least 100 times, smaller than the length and/or width of the member 6. A large collection area (input surface 120) in a relatively small volume device can thus be provided. In an example embodiment, the wavelength converting member 6 is substantially planar.
In an embodiment, the concentration stage 14 comprises a confinement structure 17 that substantially allows passage of radiation having a wavelength suitable for conversion by the wavelength converting member 6 in the concentration stage 14 from the outside of the confinement structure 17 to the inside of the confinement structure 17. The confinement structure 17 further substantially blocks passage of radiation that has been converted by wavelength converting member 6 in the concentration stage 14 from the inside of the confinement structure 17 to the outside of the confinement structure 17. Converted radiation may thus be directed efficiently to the output surface 122 via internal reflections from the confinement structure 17. The confinement structure 17 thus reduces losses.
The confinement structure 17 may comprise two substantially planar elements (e.g. dichroic plates) and the wavelength converting member 6 is located in between the two substantially planar elements. Converted radiation is trapped by the two planar elements and guided towards the output surface 122.
Where the concentration stage 14 comprises a confinement structure 17, the confinement structure 17 may concentrate radiation towards the output surface 122 of the concentration stage 14.
The receiver assembly 100 further comprises a plurality of detectors 42. Each detector 42 detects radiation output from a different portion of the output surface 122 of the concentration stage 14. In the example of
The inventors have found it is often difficult to distinguish between radiation transmitted from different transmitters when the radiation is incident directly on an input surface of a concentration stage. This is because the radiation from the transmitters tends to illuminate the input surface relatively uniformly, which results in a relatively uniform distribution of intensity over the input surface. The relatively uniform distribution of intensity will not vary sufficiently strongly as a function of a location of the transmitter for the detectors 12 to be able to distinguish between transmitters transmitting from different locations.
The inventors have recognised however that the situation can be improved radically by positioning an optical element 102 between the transmitters and the input surface 120, as shown in the example of
In an embodiment, the plurality of detectors 42 are arranged such that if a plane wave of radiation is incident on the optical element 102 each detector 42 receives a respective proportion of an energy from the plane wave and the detectors 42 are arranged such that the respective proportions depend on the spatial distribution of radiation on the input surface 120 and therefore on the direction of incidence of the plane wave.
In an embodiment, the plural detectors 42A-F can distinguish between radiation concentrated onto different regions 111-113 due to absorbance at wavelengths emitted by the wavelength converting member 6 being sufficiently high that detectors nearer to the respective regions 111-113 receive a significantly larger proportion of the incident radiation than detectors located further away. In this embodiment and other embodiments of this type, an absorbance within the concentration stage 14 is arranged to satisfy the following criterion: if a beam of radiation after conversion within the wavelength converting member 6 (i.e. having the wavelength characteristics of radiation that is emitted by the wavelength converting member 6) were to propagate along the entire length of a shortest optical path between two different ones (i.e. at least two different ones) of the plurality of detectors 42A-F, the amplitude of the radiation would be reduced by at least 1%, optionally by at least 5%, optionally by at least 10%, optionally by at least 25% from the start of the optical path to the end of the optical path. An example of such a shortest optical path between two different ones of the plurality of detectors which satisfies the criterion is shown in
In the example of
The arrangement of
The above-described mechanism based on absorbance is not the only mechanism by which the relative proportions of energy arriving at the detectors 42 can be made to vary as a function of the spatial distribution of radiation on the input surface 120. Non-limiting examples of arrangements which can be combined with the absorbance mechanism discussed above (e.g. by using re-absorbance by the wavelength converting elements themselves at least partly as the basis for the absorbance mechanism), or which can be provided separately from that mechanism, are described below.
In an embodiment, the wavelength converting member 6 comprises a plurality of wavelength converting elements and a concentration of the wavelength converting elements per unit area when viewed in a direction perpendicular to the input surface 120 varies as a function of position over the input surface 120. The variation in concentration per unit area may be such that an average concentration of the wavelength converting elements per unit area, averaged over at least 5% of the input surface 120, varies by at least 1%, optionally by at least 5%, optionally by at least 10% as a function of position over the input surface 120.
In an embodiment, as depicted schematically in
Alternatively or additionally, the number of wavelength converting elements per unit volume within the medium containing the wavelength converting elements may be made to vary as a function of position, at least in a direction parallel to the input surface 120. The variation in number per unit volume may be such that an average number per unit volume, averaged over at least 5% of the medium, varies by at least 1%, optionally by at least 5%, optionally by at least 10%, optionally by at least 25%, optionally by at least 50%, optionally by at least 75%, as a function of position, at least in a direction parallel to the input surface 120. An example arrangement is shown in
Each radiation guide 4 comprises a portion of the wavelength converting member 6 and radiation is accordingly converted to longer wavelength radiation within each radiation guide 4. Cross-sectional views showing example distributions of wavelength converting elements are shown in
The input surface 120 of the concentration stage 14 in this embodiment comprises at least a portion of an outer lateral surface 8 of each of the radiation guides 4.
Each of the radiation guides 4 guides the converted radiation, for example by total internal reflection, to a longitudinal end surface 2 (shown in
In an embodiment, wavelength converting elements are distributed non-uniformly through a cross-section of each of one or more of the radiation guides 4. For example, a spatial density (number per unit volume), averaged over the length of each radiation guide 4, varies as a function of position in the cross-section.
In an embodiment, examples of which are illustrated in
In the examples of
The outer layer 21 may be provided along the whole length of each of one or more of the radiation guides 4 or along only a portion of the whole length of each of one or more of the radiation guides 4. As shown in
In the example of
In an embodiment, an example of which is shown in
In the particular example of
In an embodiment each of one or more of the radiation guides 4 comprises an elongate region which comprises substantially no wavelength converting elements. In the case where the radiation guide 4 is formed from the core 20 of an optical fibre, the elongate region may conveniently be provided by the core 20 itself (or a portion of the core). This is the case in the examples of
In an embodiment, as depicted in
In an embodiment, each of one or more of the radiation guides 4 has a circular cross section along its whole length. The cross section may alternatively be elliptical, square, rectangular or any other regular or irregular shape which is capable of effectively guiding radiation. Each radiation guide 4 may be straight or curved along its longitudinal axis.
The optical element 102 may be configured to provide the desired functionality (i.e. such that if a plane wave of radiation is incident on the optical element 102 a spatial distribution of radiation derived from the plane wave on the input surface 120 of the concentration stage 14 varies as a function of a direction of incidence of the plane wave relative to the optical element 102) in a variety of different ways. In one embodiment, the optical element 102 comprises a lens. The lens may be configured to focus incident light for example. The lens may take various forms. In an embodiment, the lens comprises a Fresnel lens. In an embodiment the lens comprises a diffractive lens. Configurations in which the optical element 102 is substantially planar, which may be facilitated for example using a diffractive lens or a Fresnel lens, are desirable because this enables the optical element 102 to be provided in a particular compact form. The overall compactness of the receiver assembly 100 can therefore be increased. This is particularly desirable where it is desired to provide a substantially flat or planar receiver assembly 100, for example.
The data communications system 130 comprises a transmitter assembly 140 comprising a plurality of transmitters 141-143 and an encoder 145. In this embodiment a single encoder 145 is depicted but it will be understood that the encoder 145 could be implemented as plural separate encoder units, for example one encoder unit per transmitter. The encoder 145 applies a modulation to radiation transmitted by the transmitters 141-143 in order to transmit information. Each of the plurality of transmitters 141-143 is configured to transmit beams of radiation onto the optical element 102 of the receiver assembly 100 in a direction which is different to the direction of any of the other transmitters. As described above, these differences in direction of incidence of the radiation onto the optical element 102 will result in characteristic spatial distributions of the radiation on the input surface 120 which will enable the receiver assembly 100 to distinguish between radiation transmitted by different transmitters 141-143 even when the radiation is received at the same time. The data communications system 30 can therefore operate in a MIMO mode and thereby achieve increased bandwidth communications.
Thus, the transmitter assembly 140 may comprise at least a first transmitter 141 and a second transmitter 142 (in the particular example shown the transmitter system 140 comprises first, second and third transmitters 141-143). The first transmitter 141 and the second transmitter 142 (and any further transmitters 143) are configured to transmit beams of radiation onto the optical element 102 of the receiver assembly 100 from different directions. The transmitter assembly 140 transmits first information (an example of a unit of information) in radiation from the first transmitter 141 and second information (and further example of a unit of information) in radiation from the second transmitter 142 (and a further unit of information in radiation from each of any further transmitters 143 that are provided). The decoder 150 of the receiver assembly 100 is able to distinguish, due to the properties of the receiver assembly 100 discussed above, between first information and second information, due to the fact that the first information originates from radiation incident on the optical element 102 from a first direction (from the first transmitter 141) and the second information originates from radiation incident on the optical element 102 from a second direction (from the second transmitter 142), different from the first direction. In the particular example shown the decoder 150 is further able to distinguish third information (from the third transmitter 143) from radiation incident from a third direction.
In accordance with the above, a data communications method is made possible which comprises the following steps. A concentration stage is used to receive radiation via an input surface and output concentrated radiation via an output surface. A wavelength converting member in the concentration stage converts radiation to longer wavelength radiation. An optical element is provided which is such that if a plane wave of radiation is incident on the optical element a spatial distribution of radiation derived from the plane wave on the input surface of the concentration stage varies as a function of a direction of incidence of the plane wave relative to the optical element. Radiation output from the output surface is detected using a plurality of detectors. Each detector detects radiation output from a different portion of the output surface.
The optical element is used to increase a variance in amplitude of radiation incident on the input surface relative to the case if the optical element were not present. The optical element is defined by reference to a plane wave but it is understood that in use the radiation from different transmitters may not necessarily take the form of a plane wave at the optical element. Nevertheless, configuring the optical element as described will increase the efficiency with which radiation from different transmitters can be distinguished using the plurality of detectors.
The method further comprises using a transmitter assembly to transmit plural different radiation beams, each radiation beam having a different unit of information modulated thereon, and being incident onto the optical element from a different direction. The method further comprises decoding the detected radiation in such a way as to distinguish between the plural different units of information.
In further embodiments, one or more further concentration stages may be provided. In such embodiments, one or more further wavelength converting members may also be provided, each incorporated into one or more of the further concentration stages. Having a plurality of wavelength converting materials may be useful for example where it is desired for the transmitter to send signals in a plurality of different wavelength bands. In such a scenario each of the wavelength converting members could be configured to absorb radiation in a different one of the transmitted wavelength bands. Wavelength converting members that have fluorophores with absorption peaks may be particularly well suited to such embodiments.
Light incident on the concentration stage (largely the blue component of the white LEDs) is converted to green light which propagates in the plane of the confinement structure to the detectors at either end. Each detector receives a combination of modulated light ‘carrying’ the signal from both LED-1 and LED-2. The detector converts this light into an electrical signal, and subsequent signal processing allows the data streams from LED-1 and LED-2 to be recovered.
Number | Date | Country | Kind |
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1612740.9 | Jul 2016 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/GB2017/052096 | 7/17/2017 | WO | 00 |