An optical system for optical wireless communication

Abstract

A position sensing device is positioned to be separated from a focal plane to which a portion of an incoming light beam is focused. A position sensing signal generated by the position sensing device is used to steer the incoming light beam with respect to a lens arrangement for focusing the portion of the incoming light beam.

Description

[0001] AN OPTICAL SYSTEM FOR OPTICAL WIRELESS COMMUNICATION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of optical wireless communication, and in particular to optical systems for use in optical wireless communication.

[0004] BACKGROUND OF THE INVENTION

[0005] Wireless communications by means of modulated light is becoming increasingly common in use and is commonly called optical wireless communication (OWC).

[0006] Optical wireless communication refers to techniques in which information is communicated in the form of a signal embedded in light emitted by a light source. In this context, light may include any visible or invisible light (such as infrared light). Depending upon the specific wavelengths used, such techniques may also be referred to as coded light, Light Fidelity (LiFi), visible light communication (VLC) or free-space optical communication (FSO).

[0007] Optical wireless communication can be performed with concentrated or collimated beams of light. In such approaches, the transceivers on both sides need to be properly aligned to allow communication in both directions.

[0008] There is therefore a desire to improve mechanisms for aligning a beam generated by one optical wireless communication device with its communication partner.

[0009] An example of a method for aligning a beam generated by an optical wireless communication device with its communication partner is provided in: “Control of fast steering mirror for accurate beam positioning in FSO communication system” by Lahari Sreerama Aamrutha et al, published in 2021 INTERNATIONAL CONFERENCE ON SYSTEM, COMPUTATION, AUTOMATION AND NETWORKING (ICSCAN), IEEE, 30 July 2021, which discloses a method for beam positioning using PID, fuzzy logic controllers and a quad detector. The Quad Detector is used as a position sensor and consists of four photodiodes capable of detecting laser beams on their surfaces and determine the deviation position of the laser spot from its center.

[0010] A further approach for beam steering and adaptive beam control is provided in “Non-Mechanical Beam Steering and Adaptive Beam Control Using Variable Focus Lenses for Free-Space Optical Communications”, by Mai Vuong et al, published in JOURNAL OF LIGHTWAVE TECHNOLOGY, IEEE, USA, vol. 39, no. 24, 8 June 2021. In this paper the authors explore the possibility of using variable focus lenses for the realization of beam steering and adaptive beam control for FSOCs. For this purpose, they employ three variable focus lenses, one on-axis lens for adaptive beam divergence control and two decentered lenses for two-dimensional beam steering and they demonstrate the simultaneous beamsteering and adaptive beam control capability of this variable focus lens-based system over a 104-m long free-space optical link.

[0011] SUMMARY OF THE INVENTION

[0012] The invention is defined by the claims.

[0013] According to examples in accordance with an aspect of the invention, there is provided an optical system for producing a data signal responsive to embedded information carried by an incoming light beam.

[0014] The optical system comprises: a beam steering device configured to controllably steer a direction of the incoming light beam; a beamsplitter configured to split the incoming light beam, after being steered by the beam steering device, into a first light beam and a second light beam that are angled with respect to one another; a lens arrangement configured to focus the first light beam to a first focal point lying in a first focal plane and to focus the second light beam to a second focal point in a second focal plane; a position sensitive device configured to generate a position sensing signal responsive to the position at which the first light beam is incident upon a receiving surface of the position sensitive device, wherein the receiving surface of the position sensitive device is positioned to receive the first light beam and be separated by a non-zero distance from the first focal plane; and a light sensing module positioned to receive the second light beam and configured to generate a data signal responsive to embedded information carried by the second light beam, wherein the beam steering device is configured to steer the direction of the incoming light beam responsive to the position sensing signal generated by the position sensitive device.

[0015] The output of the position sensitive device provides a feedback signal that allows adaptation of the direction of the steered light beam by the beam steering device by means of a control loop. A control loop aims to direct the incoming light beam through the optical axis of the lens arrangement, and where deviation from the optimal direction will cause the first light beam to change position on the light sensing module, thereby giving rise to the feedback signal for adjustment of the control signal for the beam steering device. The non-zero distance (between the receiving surface of the position sensitive device and the first focal plane) is preferably chosen such that this control loop converges for a target detection distance of the optical system. This avoids runaway of the control loop.

[0016] Preferably, the non-zero distance is no less than 0.0.5 mm, e.g., no less than 0.1 mm.

[0017] The lens arrangement may be positioned between the beam steering device and the beamsplitter.

[0018] Alternatively, or additionally, the lens arrangement comprises: a first lens subarrangement positioned between the beamsplitter and the position sensitive device; and a second lens sub-arrangement positioned between the beamsplitter and the light sensing module.

[0019] In some examples, the first focal plane is positioned between the lens arrangement and the position sensitive device.

[0020] In some examples, the non-zero distance between the first focal plane and the position sensitive device is no more than a value calculated by the following formula: f

[0021] ((z,+2fc) 4 / )-i ’W^erefadistance between the first focal plane and the lens arrangement (i. e. , the effective focal distance), L is a target detection distance of the optical system and k is a distance traveled by the first light beam between the beam steering device and the lens arrangement.

[0022] In some examples, the position sensitive device comprises one or more dark areas having a width, and the non-zero distance between the first focal plane and the position sensitive device is no less than a value calculated by the following formula: / 2—f, where W DB is the width of the one or more dark areas, DB is the diameter of the incoming beam; and f is a distance between the first focal plane and the lens arrangement.

[0023] In preferred examples, the light sensing module is positioned to receive the second light beam at the second focal plane.

[0024] The optical system may further comprise a positioning system configured to facilitate controllable adjustment of a distance between the first focal plane and the position sensitive device responsive to at least one target detection distance of the optical system. This approach allows for adjustment of the distance between the lens arrangement and the position sensitive device to adapt the optical system for different target detection distances.

[0025] The positioning system may, for instance, comprise an actuator configured to controllably move the lens arrangement (or a portion thereol) and / or position sensitive device to facilitate control over the distance between the first focal plane and the position sensitive device.

[0026] The beam steering device may be a rotatable mirror.

[0027] The beamsplitter may be a partially transmissive mirror.

[0028] The position sensitive device may be a two-dimensional position sensitive device, i.e., able to monitor or detect a position of the first light beam incident upon the light receiving surface in two-dimensions. One example of a suitable two-dimensional position sensitive device is a quadrant photodetector.

[0029] The beamsplitter may be configured such that the first light beam comprises no less than 5% (e.g., no less than 10%) of the incoming light beam and / or the second light beam comprises no less than 5% (e.g., no less than 10%) of the incoming light beam. In preferred examples, the beamsplitter is configured such that the second light beam comprises no less than 25% (e.g., no less than 50%) of the incoming light beam, for improved strength of the data signal.

[0030] There is also provided an optical wireless communication device comprising any herein disclosed optical system; and a processing system configured to receive the data signal and extract the embedded information from the data signal.

[0031] There is also provided an optical wireless communication system comprising: the optical wireless communication device as herein disclosed; and a second optical wireless communication device configured to generate a light beam, carrying embedded information, and transmit the generated light beam to the optical wireless communication device, wherein the transmitted light beam acts as the incoming light beam for the optical system of the optical wireless communication device.

[0032] When both optical wireless communication devices comprise a beam steering device, a beamsplitter, a lens arrangement and a position sensitive device, as described with reference to the optical wireless communication device, and the light beam generator as described with reference to the second optical wireless device, bidirectional communication is facilitated that uses the beam steering device and beamsplitter as an integral part of both the reception and transmission path of the optical wireless communication devices.

[0033] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] For a beter understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0035] Figure 1 illustrates an optical wireless communication system;

[0036] Figure 2 illustrates a light spot on a position sensing device at the first focal plane;

[0037] Figure 3 illustrates a light spot on a position sensing device distanced from the first focal plane;

[0038] Figure 4 illustrates various parameters;

[0039] Figure 5 illustrates various relationships between a target detection distance and a distance between a first focal plane and a position sensing device;

[0040] Figure 6 illustrates an alternative optical system; and

[0041] Figure 7 illustrating a positioning system.

[0042] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The invention will be described with reference to the Figures.

[0044] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become beter understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

[0045] The invention provides an optical system for an optical wireless communication device. A position sensing device is positioned to be separated from a focal plane to which a portion of an incoming light beam is focused. A position sensing signal generated by the position sensing device is used to steer the incoming light beam with respect to a lens arrangement for focusing the portion of the incoming light beam.

[0046] An optical wireless communication (OWC) device may be configured or designed for communication with another OWC device over a particular range of target detection distance. Example ranges may, for instance, include a range of from: Im to 2m; 10 m to 20 m; 100 m to 250m and so on. Other suitable ranges will be readily apparent to the skilled person.

[0047] FIG. 1 illustrates an optical wireless communication system 100 in which embodiments can be employed for improved contextual understanding. The optical wireless communication system 100 comprises a first optical wireless communication device 110 and a second optical wireless communication device 120 configured to communicate with one another.

[0048] The first optical wireless communication device 110 comprises an optical system 150 and a processing system 160. The optical system 150 is configured to produce a data signal SD responsive to embedded information carried by an incoming light beam 125 (e.g., produced by the second optical wireless communication device 120).

[0049] The optical system 150 comprises a beam steering device 151 configured to controllably steer a direction of the incoming light beam. In the illustrated examples, the beam steering device 151 comprises a tiltable or rotatable mirror. The direction of the incoming light beam is controlled by the processing system 160. Put another way, the operation of the beam steering device may be controlled by the processing system 160, e.g., via a control signal Sc.

[0050] By way of example only, the beam steering device may be a reflective beam steering devices such as a ID or a 2D rotatable mirror or set of mirrors. In alternative examples, the beam steering device may comprise an active metasurface reflector or a reflective optical phase array device, such as a LCoS spatial light modulator (SLMs) that may have phase-only modulation (aka phase light modulator or PLM) or a combination of phase and amplitude modulation.

[0051] By way other examples, the beam steering device may comprise a transmissive device, such as a ID or 2D addressable transmissive twisted nematic liquid crystal cell (as a transmissive PLM), a transmissive ID or 2D active metasurface element, a liquid wedge, or a deformable liquid lens system.

[0052] Other suitable forms and structures for a beam steering device 151 will be readily apparent to skilled person

[0053] The optical system 150 also comprises a beamsplitter 152 configured to split the incoming light beam, after being steered by the beam steering device, into a first light beam and a second light beam that are angled with respect to one another.

[0054] The optical system 150 also comprises a lens arrangement 153 configured to focus the first light beam to a first focal point lying in a first focal plane Pl and to focus the second light beam to a second focal point in a second focal plane P2. In the illustrated example, the lens arrangement 153 comprises a first lens subarrangement 153A (e.g., a first lens) that focuses the first light beam and a second lens subarrangement 153B (e.g., a second lens) that focuses the second light beam. More particularly, the first lens sub-arrangement 153 A is positioned between the beamsplitter and the position sensitive device. The second lens sub-arrangement 153B is positioned between the beamsplitter and the light sensing module.

[0055] The optical system 150 also comprises a position sensitive device 154 configured to generate a position sensing signal Sp responsive to the position at which the first light beam is incident upon a receiving surface 154A of the position sensitive device. The receiving surface 154A of the position sensitive device is positioned to receive the first light beam and be separated by a non-zero distance dx from (at least) the first focal plane, and preferably from any focal line of the first lens sub-arrangement 153 A. A lens sub-arrangement may comprise a single focal line or a plurality of focal lines, e.g., two orthogonal focal lines as a result of astigmatism of the lens sub-arrangement.

[0056] Examples of suitable position sensitive devices are well known in the art, and include segmented photodetectors (such as quadrant photodetectors), photodiode arrays, CMOS sensors, CCD arrays, other image sensors and so on.

[0057] Another alternative could be staring array or focal-plane array (FPA), which (in effect) is an image sensor with very limited or no dark space between the pixels. Although more expensive, such an array sensors would provide more immediate identification of the source location in a very large field of view, and therefore reduce acquisition time dramatically.

[0058] The optical system 150 also comprises a light sensing module 155 positioned to receive the second light beam and configured to generate a data signal responsive to embedded information carried by the second light beam. Preferably, the light sensing module is positioned to receive the second light beam at the second focal plane.

[0059] The light sensing module may, for instance, comprise an opto-electric photo sensor and a transimpedance amplifier. Other suitable configurations for forming a light sensing module will be apparent to the skilled person, e.g., comprising one or more photodiodes (e.g., an array of photodiodes), or a camera.

[0060] In one example, the light sensing module comprises an optical fiber, having an optical opening (for coupling received light therein) positioned to receive the second light beam. The optical fiber may then direct the coupled light to a dedicated light sensor.

[0061] The data signal SD produced by the light sensing module may then be output off the optical system, e.g., to the processing system. Of course, it will be appreciated that the data signal SD need not be provided to the same processing system that receives the position sensing signal SP (and controls the beam steering device 151). Rather, the data signal may be provided to an entirely separate processing system, e.g., a data processor.

[0062] The beam steering device 151 is configured to steer the direction of the incoming light beam responsive to the position sensing signal generated by the position sensitive device. In this way, the position sensing signal (produced by the position sensitive device 154) effectively acts as an error signal that can be used to steer (e.g., via a control loop) the beam steering device 151.

[0063] More particularly, the position sensing signal may provide an indicator of distance and / or direction of the first light beam with respect to a central point (which can be alternatively labelled a reference point) of the receiving surface 154A. The beam steering device 151 may be configured to steer or guide the incoming light beam such that the first light beam is positioned at a known position with respect to central point of the receiving surface, e.g., centered around the central point of the receiving surface. Appropriate approaches for performing such control are known in the art.

[0064] The central point need not be at the exact center of the receiving surface. Rather, the central point identifies a location for the focused first light beam for which the corresponding focused second light beam provides a maximum available intensity of light to the light sensing module. This can be achieved through appropriate calibration, configuring and / or positioning of the position sensitive device and the light sensing module with respect to the lens arrangement.

[0065] In particular, the central point may be defined by directing a test beam (carrying embedded test information) towards the optical system and steering the incoming test beam using the beam steering device until the strength of the data signal SD is at a maximum. The location of the light spot created by the focused first light beam (being a focused portion of the incoming test beam) on the receiving surface 154A of the position sensing device 154 may define or be the central location.

[0066] The processing system 160 may be configured to receive the data signal and extract the embedded information from the data signal. The extracted embedded information may then be output from the processing system or further processed by the processing system for performing any task for which the optical wireless communication device is designed (e.g., displaying the embedded information, storing the embedded information and so on). The further usage of the embedded information is immaterial for the purposes of the proposed invention. Another possible feature for the first optical wireless communication device, is a beam generating device 170 for generating a light beam 175 for communication with the second optical wireless communication device 120.

[0067] The generated light beam 175 may make use of the same beam steering device 151 to provide an outgoing beam (not shown) that carries modulated information for communicating to the second optical wireless communication device. In such approaches, changing the direction of the incoming light beam (using the beam steering device 151) will similarly cause a change in direction of the outgoing beam.

[0068] In the illustrated example, the beam generating device 170 comprises a beam generator 171 and a mirror 172. The mirror is configured to direct the light beam 175 generated by the beam generator 171 towards the beam steering device 151. The mirror 172 may be located in the path taken by the incoming light beam 125, but appropriately configured to avoid reflection of the incoming light beam. This may, for instance, be performed by way of performing wavelength-selective reflection and / or polarization selective reflection. Other suitable approaches are known to the skilled person.

[0069] The structure and operation of the second optical wireless communication device 120 may be similar / identical to that of the first optical wireless communication device, and is not repeated for the sake of conciseness. Of course, it will be appreciated that the beam generator and / or position sensitive device of the second optical wireless communication device may be configured to complement those of the first optical wireless communication device (which may be important if the beam generator and / or position sensitive devices are wavelength-dependent).

[0070] For the system illustrated in FIG. 1, if the beam steering device has been appropriately controlled or steered, responsive to the position sensing signal, then the first light beam will be located parallel to, as well as centered around, the optical axis of the first lens sub-arrangement 153A. This will result in the first light beam (after being focused by the first lens sub-arrangement) being positioned at a central point on the receiving surface 154A of the position sensitive device 154.

[0071] The (position and / or configuration of the) light sensing module 155 is calibrated with the position sensing device 154 such that when the position sensing device receives the first light beam at the central point on the receiving surface 154A thereof, the light sensing module will similarly receive the greatest available or potential intensity of the second light beam. Thus, if the beam steering device is controlled such that the first light beam is positioned at a central point on the receiving surface, then this will similarly target maximizing the amount of the second light beam that is incident on the light sensing module 155 (i.e.,. the light strength and therefore signal strength).

[0072] As previously mentioned, the receiving surface 154A of the position sensitive device is positioned to receive the first light beam and be separated by a non-zero distance dx from a first focal plane. The first focal plane is a plane containing a focal point to which the first light beam is focused. If astigmatism and / or other optical distortions are present, then this focal point is preferably the most distant focal point of all focal points that can be discerned with respect to the first light beam.

[0073] The advantage of the proposed approach is achieved by distancing the position sensing device 154, and more particularly the receiving surface, from the first focal plane.

[0074] More particularly, it has been recognized that an incoming light beam that is laterally displaced from (but parallel to) the optical axis of an “ideal” incoming beam (i.e., an “ideal incoming direction”) will similarly produce a first light beam and a second light beam that are laterally displaced from the lens arrangement. Thus, for the illustrated example, the first light beam will be displaced from the first lens sub-arrangement and the second light beam will be laterally displaced from the second lens sub-arrangement. The laterally displaced incoming light beam would result, if steered in a same manner as the ideal incoming beam in not all of the light of the first / second light beam falling upon the lens arrangement, and therefore being effectively lost. This is, of course, under the assumption that the radius of the first / second light beam at the lens arrangement is equal to or less than the radius of the lens arrangement.

[0075] However, a laterally displaced first light beam would be located, when focused, at a same position on a receiving surface positioned in the focal plane as a (focused) first light beam produced by an ideal incoming light beam. This would prevent accurate correction of the direction of the incoming beam, leading to reduced light strength at the light sensing module.

[0076] FIGS. 2 and 3 each illustrate a light spot created (as a result of a laterally displaced incoming beam) on the receiving surface 134A of a position sensing device 134 for different distances from the first focal plane. In the illustrated example, the position sensing device is a quadrant detector, the quadrants of which are schematically illustrated using dotted lines. FIG. 2 illustrates a scenario in which the receiving surface 134 A is located at the first focal plane. The light spot 200 created as a result of the laterally displaced incoming beam will be centered / located at the center of the receiving surface 134A.

[0077] FIG. 3 illustrates a scenario in which the receiving surface 134A is distanced from the first focal plane. The light spot 300 created as a result of the laterally displaced incoming beam will not be centered at the center of the receiving surface 134A. Thus, a position sensing signal (acting as an error signal) can be generated to correct or move the light spot 300 to be centered at the central point of the receiving surface 134A.

[0078] Thus, by distancing the receiving surface from the first focal plane, then (the center or average of) the part of the laterally displaced beam incident upon the receiving surface will be distanced from the central point. This allows for correction of the beam steering device to achieve improved light strength at the light sensing module 155.

[0079] It has been previously mentioned how the first optical wireless communication device 110 may comprise a beam generating device 170 that makes use of the same beam steering device and that the second optical wireless communication device 120 may contain corresponding features to the first optical wireless communication device 110.

[0080] Thus, the first optical wireless communication device 110 may generate the incoming beam for the second optical wireless communication device and the second optical wireless communication device 120 may generate the incoming beam for the first optical wireless communication device.

[0081] The proposed optical system is particularly advantageous for use in such examples, as the direction of the two incoming beams will be steered such that, for each optical wireless communication device 110, 120, the first light beam converges upon the central point of the receiving surface, even taking into account the change in direction of the incoming light beam as the other optical wireless communication device steers its respective beam steering device.

[0082] Referring to FIG. 1, the beamsplitter may be configured such that the first light beam comprises less of the incoming light beam than the second light beam. This recognizes that the demand for high strength signals for communication is more advantageous than high strength signal for steering the beam.

[0083] In particular examples, the beamsplitter may be configured such that the first light beam comprises no less than 25% of the incoming light beam. In some examples, the beamsplitter may be configured so that the first light beam comprises less than 50% of the incoming light beam. The second light beam may comprise no less than 25%, e.g., no less than 50% of the incoming light beam.

[0084] Further embodiments recognize specific design techniques for selecting an appropriate distance between the first focal plane and the receiving surface that realizes preferred characteristics. In the hereafter described examples, it is assumed that the beam steering device comprises a tiltable mirror.

[0085] A non-zero distance of between 0.15mm and 0.8mm provides good flexibility of design for an optical system designed to receive an incoming beam from a distance of between 0.02m and 2m. A non-zero distance of between 0.05mm and 0.30mm provides good flexibility of design for an optical system designed to receive an incoming beam from a distance of between 0.5m and 50m. A non-zero distance of between 0.05mm and 0.20mm provides good flexibility of design for an optical system designed to receive an incoming beam from a distance of between 2.5m and 250m. Other suitable variations will be apparent from the herein described techniques for selecting or defining the non-zero distance.

[0086] FIG. 4 illustrates various terms for understanding the parameters used in mathematical formula to derive an appropriate distance.

[0087] In particular, FIG. 4 illustrates the following terms: a focal distance f (being the distance between the lens arrangement and the first focal plane F); a target detection distance L (being the target distance between the first and second optical wireless communication devices); a travel distance k (being the distance travelled by the first light beam from the beam steering device 151 to the lens arrangement); a beam angle a (being the angle between the first light beam and the optical axis of the first lens sub-arrangement); a beam diameter DB; a pupil diameter Dp (being the optically transmissive diameter of the first lens sub-arrangement); a mirror rotation MR; a beam displacement dBP (being the displacement or distance, at the first lens sub-arrangement, between the first light beam and the first lens sub-arrangement); and the non-zero distance dx between the first focal plane F and the receiving surface 134 of the position sensing device.

[0088] There is also defined a mirror step value AMR that defines the smallest step in mirror rotation that can be achieved in rotating the tiltable mirror.

[0089] It is noted that the value of the beam angle a and the value of the beam displacement dBP will change responsive to the direction in which the incoming beam is steered by the beam steering device. To this end, there will be a combination of beam angle a and beam displacement dBP at which the focused first light beam will be incident upon the central point of the receiving surface of the position sensing device. This occurs (assuming DB = Dp) when the following equation is true: f dBP= 2f a(l + — ) (1) dx

[0090] Taking into account all possible combinations of angles and beam displacements, and expressing the beam displacement in terms of the finite elementary step of the mirrors as well as the distances L and k and the focal length f, there is a certain value for dx that marks the separation between convergence of the control loop and potential divergence, i.e., run-away of the beam due to mirror adjustments away from the ideal condition. This critical dx value (dxc) can be expressed, under the condition that (L+2k) > 4f, as f dxc= 7 - r - (2)

[0091] ((L + 2k) - 4 / ) - 1V’

[0092] More particularly, equation (2) can effectively be used to define a condition for convergence (i.e., avoidance of runaway and / or an undefined final lateral offset of the first optical beam) for a pair of optical wireless communication devices.

[0093] More particularly, equation (2) can define an upper limit for the value of the non-zero distance dx to achieve or ensure convergence.

[0094] Thus, one approach for determining a value for the distance dx between the first focal plane and the receiving surface of the position sensing device is to select a distance that lies between 0 and the critical dx value dxcfor the largest target detection distance for which the optical system is designed or is to be designed.

[0095] However, an improved approach for determining a value for the distance dx is to further take account of a residual beam offset that will be present after control of the beam steering device has stabilized or entered a steady state or a quasi-steady state (as, in some circumstances, the control of the beam steering device may infinitely oscillate around a time- averaged residual beam offset).

[0096] More particularly, a residual beam offset is a value that represents the relative offset of the steady state beam (i.e., after the control of the beam steering device has stabilized or become steady state) with respect to an idealized beam position - i.e., a distance between the optical axis of the lens arrangement and the central axis of the first light beam. The minimum available residual beam offset is defined at an ideal non-zero distance dxideai, which can be defined by the following equation: under the condition that (L+2*k) > [6 - A / (20)]*f. The value of the residual beam offset will increase with an increasing difference between the non-zero distance dx and the ideal non-zero distance dxideai.

[0097] Equation (3) makes it possible to determine an ideal value for the non-zero distance, e.g., for a known target detection distance, focal length and lens arrangement position.

[0098] The maximum residual beam displacement dni> MAX that may still be present for a steady state beam (i.e., after the control of the beam steering device has stabilized or become steady state) can be expressed as: under the condition that 0 < dx < f / [(L+2*k) / (4*l) - 1] and (L+2*k) > 4*f The maximum residual beam displacement represents the value for the maximum possible beam displacement for a steady state beam.

[0099] The minimum value dxmin for the non-zero distance dx to comply with a requested or tolerated maximum residual beam displacement is given as: under the condition that L > 4*f- 2*k - dni> MAX / AMR.

[0100] The maximum value dxmax for the non-zero distance dx to comply with a requested or tolerated maximum residual beam displacement is given as:

[0101] Equations (5) and (6) can be used to control or define a value for dx that falls between dxmin and dxmax for a desired target detection distance L and / or range of desired target detection distances. By way of example, the value for dx may be identified as a value that falls equidistant between dxmin and dxmax, i.e., an average of dxmin and dxmax. However, other suitable techniques could be used, e.g., picking either dxmin or dxmax.

[0102] Where there is a range of desired target detection distances (between a minimum target detection distance Lmin and maximum target detection distance Lmax), then, when applying equation (5): L = Lmin and, when applying equation (6), L = Lmax.

[0103] Thus, the minimum value dxmin for the non-zero distance can be calculated by processing the smallest target detection distance Lmin using equation (5). The maximum value dxmax for the non-zero distance dx can be calculated by processing the largest target detection distance L max using equation (6).

[0104] It will be clear that the lines for the minimum and maximum dx values cross at a certain value of L. This marks the longest detection distance L possible with the given boundary conditions.

[0105] For instance, consider a scenario in which an optical wireless communication device has the following characteristics: a mirror step angle AMD of 50 prad, a focal distance f of 20 mm, a travel distance k of 40 mm, and a maximum tolerated residual beam displacement dBP MAX of 0.6 mm. This corresponds to 10% for an incoming beam having a beam diameter of 6 mm. Following equations (4) and (5), we can determine that, for a desired target detection range of between 0.1 m and 2 m, then: dxmin = 0.13 mm and dxmax = 0.66 mm.

[0106] In a variation, consider a scenario where the maximum tolerated residual lateral beam displacement dBP MAX is instead 1.8 mm (i.e., 30% for an incoming beam having a beam diameter of 6mm). In this variation, dxmin = 0.04 mm and dxmax = 0.75mm.

[0107] As another example, consider a scenario in which an optical wireless communication device has the following characteristics: a mirror step angle AMD of 25 prad, a focal distance f of 80 mm, a beam diameter of 23 mm, a travel distance k of 40 mm, and a maximum tolerated residual beam displacement dBP MAX of 10mm. Following equations (4) and (5), we can determine that, for a desired target detection range of between 50 m and 150 m, then: dxmin = 0.06 mm and dxmax = 0.11 mm. The example values for dxmin and dxmax provided above are rounded to the nearest 2 decimal places.

[0108] FIG. 5 graphically illustrates an approach for determining an allowable range for the value of dx for a desired range of desired target detection distances L. In this scenario, the mirror step angle AMR, focal distance f, travel distance k and maximum tolerated residual beam displacement dup MAX are known.

[0109] A first line 510 illustrates the critical dx value dxcfor different values of the target detection distance. This is calculated using equation (2).

[0110] A second line 520 illustrates the ideal non-zero distance dxideai for different values of the target detection distance. This is calculated using equation (3).

[0111] A third line 530 illustrates the minimum value dxmin for the non-zero distance dx to comply with a requested or tolerated maximum residual beam displacement. This is calculated using equation (5).

[0112] A fourth line 540 illustrates the maximum value dxmax for the non-zero distance dx to comply with a requested or tolerated maximum residual beam displacement. This is calculated using equation (6). It can be seen that the maximum value dxmax falls below the critical dx value dxcfor all values of the target detection distance L.

[0113] From FIG. 5, it is clear that the minimum value for the non-zero distance reduces with increasing distance L. Similarly, the maximum value for the non-zero distance reduces with increasing distance L.

[0114] It is possible to determine a minimum dxl and maximum dx2 value for the nonzero distance dx that falls between the third and fourth lines for all target detection distances within a range of target detection distances LI - L2. In particular, a minimum dxl value for the non-zero distance can be calculated by processing the smallest target detection distance L using equation (5). The maximum dx2 value for the non-zero distance dx can be calculated by processing the largest target detection distance L using equation (6).

[0115] It will be clear that the lines for the minimum and for the maximum dx values cross at a certain value of L. This marks the longest detection distance L possible with the given boundary conditions.

[0116] In an embodiment, the optimized dx value for a chosen Lmin and Lmax is found by identifying the non-zero distance dx (within the range from the minimum value dxmin to the maximum value dxmax) that minimizes the maximum residual offset using equation (4).

[0117] The optimized value dxopt for the non-zero value dx can be calculated as follows: where

[0118] >li = 4f4(20k2+ k(16Lmax+ 4Lmin) + 4Lmax2,+ Ltnin2and

[0119] •^2 ^(2Lmax+ 2Lmin- 24 f)

[0120] FIG. 6 illustrates an alternative configuration for the optical system 650 of the first optical wireless communication device.

[0121] In particular, the lens arrangement here comprises a single lens 653 positioned between the beam steering device 151 and the beamsplitter 152. Thus, the beamsplitter splits an already converging incoming light beam into the first and second light beams. This reduces the number of components required for the optical system, e.g., avoiding use of a plurality of lens sub-arrangements.

[0122] However, use of a beamsplitter can introduce astigmatism into the converging first beam. More particularly, the focal plane can be conceptually viewed as being effectively split into two orthogonal focal lines. This effect is particularly pronounced if the beamsplitter is a plate beamsplitter (which is advantageous to minimize or mitigate the impact of Fresnel reflections). A cube beam splitter will introduce spherical aberrations that can be corrected for with a collimating lens.

[0123] Thus, as a consequence of using a beamsplitter, the approach for determining the non-zero distance dx may be affected, particularly if a minimum dxmin and maximum dxmax value for the non-zero distance dx is to be generated.

[0124] The effective splitting of the focal plane into two orthogonal focal lines (located an inter-focal line distance apart) means that the available range for placement of the position sensitive device should be reduced by that same amount (i.e., by the same inter-focal line distance). More particularly, if the focal line closest to the lens arrangement is at location Fl = F + dF and the second (orthogonal) focal line at location F2 = Fl +AF, then dxmin should be defined relative to F2, while dxmax should be defined relative to Fl.

[0125] Mathematically, this can be expressed as:

[0126] Adx (L1, L2, F1, F2) = dxmax(L2, Fl') - dxminLl, F2)

[0127] = dxmax(L2, Fl) - [dxmin(Ll,Fl)' + (F2 - Fl)] (8) d-xmax(L2^ dxmin(Ll) AF where AF = F2 - Fl and Adx represents the range for dx from F2 + dxmin (LI).

[0128] The amount of astigmatism (i.e., the size of AF) is dependent upon the thickness of the beam splitter plate. The precise relationship may be dependent upon, for instance, the material of the beam splitter plate, e.g., the refractive index of the beam splitter plate. Suitable approaches for selecting the material and the thickness in order to maintain an adequate (i.e., at least non-negative and non-zero) range Adx for the non-zero distance dx will be apparent to the skilled person from this understanding.

[0129] Put another way, provided that the size of AF is less than Adx(Ll, L2), then a solution for placement of the position sensitive device is available.

[0130] In some examples, the minimum value for the non-zero distance dx may be further defined or delimited (i.e., in addition to any above-described approach) by component restrictions.

[0131] For instance, a position sensitive device may comprise a dark area or zone surrounding the central point for which if light falls only on this dark area (i.e. a spacing between the respective detectors of the quadrant detector), it is unable to determine the position of the light. The minimum value for the non-zero distance may be selected so that at least some light falls outside of this dark area or zone. For instance, the minimum value may be selected to be no less than 0.1mm (e.g., in addition to any other restrictions set out by the aboveidentified examples).

[0132] As an example, a quadrant detector having a width W separating each pair of quadrants from one another will have a dark area of size W2mm2at the center. For such an example, the minimum value dxmin, w for such a quadrant detector may be defined using the following equation:

[0133] For the avoidance of doubt, it is noted that for a quadrant detector, the minimum value dxmin for the non-zero distance dx may be the largest of any minimum value calculated using any previously described approach, e.g., as calculated using equation (5) or equation (9).

[0134] Table 1 illustrates some example values for parameters in an optical system according to embodiments. The values of the parameters represent example values for non- modifiable parameters (e.g., Lmin, Lmax, f, k, AMR and W), together with appropriate values for parameters including the ideal, minimum and maximum value for the non-zero distance dx).

[0135] 0.02 2 20 20 50 30 0.40 30% 0.15 0.77 0.62

[0136] 0.1 10 40 30 50 30 0.32 30% 0.15 0.55 0.40

[0137] 0.5 50 65 40 25 30 0.17 30% 0.15 0.26 0.11

[0138] 0.5 50 55 40 25 10 0.12 30% 0.07 0.18 0.11

[0139] 2.5 250 120 50 10 10 0.12 30% 0.06 0.17 0.12

[0140] 5 500 150 60 5 10 0.09 30% 0.05 0.15 0.10

[0141] TABLE 1

[0142] In any above-described example of an optical system and / or device that makes use of an optical system, the optical system may further comprise a positioning system configured to facilitate controllable adjustment of a distance between the first focal plane and the position sensitive device.

[0143] In some examples, the positioning system is partially integrated into the lens arrangement. For instance, the lens arrangement may be adjustable to control its focal distance (e.g., comprise a liquid-filled lens). This facilitates adjustment of the position of the first focal plane, and therefore the distance between the first focal plane and the position sensitive device.

[0144] In other examples, the positioning system is configured to manipulate or change a position of the position sensitive device with respect to the lens arrangement (and thereby the first focal plane) by moving the position sensitive device.

[0145] FIG. 7 illustrates a working example of a portion of an optical system having a positioning system 700.

[0146] The positioning system 700 may be configured to control the distance between the first focal plane and the position sensitive device responsive to at least one target detection distance of the optical system.

[0147] Thus, the positioning system may comprise an actuator 710 configured to move the position of the position sensitive device and a control system 720 configured to control the operation of the actuator. The actuator 710 may be configured or calibrated to facilitate accurate control over the distance between the position sensitive device and the first focal plane.

[0148] The actuator 710 here comprises a linear actuator, here: in the form of a worm drive, configured to controllably adjust a position of the position sensitive device and thereby a distance between the first focal plane and the position sensitive device. Other suitable linear actuators and / or positioning systems will be apparent to the skilled person, e.g., pistons, hydraulic arrangements, pneumatic arrangements, screw-based actuators, cam-based actuators and so on.

[0149] In some examples, the positioning system 700 comprises a feedback system (not shown) configured to determine a distance between the actuator 710 and the first focal plane. This can be determined by, for instance, determining a distance between the actuator 710 and the lens (arrangement). The distance between the lens arrangement and the first focal plane can be established based on the focal distance associated with the lens. This facilitates ease of identifying the distance dx. Any suitable distance determining tool may be used.

[0150] The control system 720 may be configured to employ a previously described approach to identify, using the target detection distance(s), a value for the distance between the first focal plane and the position sensitive device. The control system may then use the determined value to control the actuator in order to achieve or actualize the value for the distance.

[0151] For instance, if a single target detection distance is identified, then equation (3) may be applied to determine the value for the distance (with other values being defined by device-specific knowledge, e.g., stored in a memory or storage unit of the optical system or optical wireless communication device).

[0152] As another example, if two target detection distances are identified (e.g., maximum and minimum target distances), then equations (5) and (6) may be used to determine a maximum and minimum value for the distance dx. In particular, the minimum target distance may be used to determine the minimum value for the distance dx and the maximum target distance may be used to determine the maximum value for the distance dx. Any value falling between these two values (e.g., an average value) may be used for the distance dx.

[0153] The target detection distance(s) may be user-defined. Thus, the optical system may comprise an input interface configured to receive a signal carrying the target detection distance(s) defined at a user interface. This allows a user to (re)configure the optical system for a desired target detection distance or range. As another example, the target detection distance(s) may be determined based on a determined distance between the optical wireless communication device (carrying the optical system) and the (other) optical wireless communication device that produces the incoming light beam for the optical system. This distance can be determined by a distance determining system, e.g., that performs a time-of-flight measurement for a received incoming light beam or performs some other mechanism for determining the distance. By way of example, the distance determining system may determine a difference between a timestamp carried be embedded information carried by the incoming light beam and a current timestamp to determine a distance of the other optical wireless communication device from the optical wireless communication device.

[0154] More particularly, the target detection distance may be set or defined as the determined distance between the two optical wireless communication devices.

[0155] In alternative embodiments, the control system 720 is used to configure, adjust or modify the position of the position sensitive device with reduced residual beam offset (e.g., as indicated in equation (4) without direct knowledge of the actual value of the target detection distance. Rather, the modification can be based upon feedback from the signal(s) Sp as the position sensitive device is moved and / or from the signal strength from the data sensor SD. In this way, dynamically an optimal detection configuration with minimum residual beam offset and maximum data signal strength is established for any detection distance L (as long as L is in the feasible range given the further system parameter values).

[0156] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

[0157] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0158] If the term "adapted to" is used in the claims or description, it is noted the term "adapted to" is intended to be equivalent to the term "configured to". If the term "arrangement" is used in the claims or description, it is noted the term "arrangement" is intended to be equivalent to the term "system", and vice versa.

[0159] Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An optical system (150) for producing a data signal responsive to embedded information carried by an incoming light beam (125), the optical system comprising: a beam steering device (151) configured to controllably steer a direction of the incoming light beam; a beamsplitter (152) configured to split the incoming light beam, after being steered by the beam steering device, into a first light beam and a second light beam that are angled with respect to one another; aposition sensitive device (154) configured to generate a position sensing signal (SP) responsive to the position at which the first light beam is incident upon a receiving surface (154A) of the position sensitive device, wherein the receiving surface of the position sensitive device is positioned to receive the first light beam; and a light sensing module (155) positioned to receive the second light beam and configured to generate a data signal (SD) responsive to embedded information carried by the second light beam, wherein the beam steering device is configured to steer the direction of the incoming light beam responsive to the position sensing signal generated by the position sensitive device the optical system further characterized in that it comprises: a lens arrangement (153, 653) configured to focus the first light beam to a first focal point lying in a first focal plane (F) and to focus the second light beam to a second focal point in a second focal plane and wherein the receiving surface of the position sensitive device is positioned to be separated by a non-zero distance (dx) from the first focal plane.

2. The optical system of claim 1, wherein the lens arrangement is positioned between the beam steering device and the beamsplitter.

3. The optical system of claim 1 or 2, wherein the lens arrangement comprises:a first lens sub-arrangement positioned between the beamsplitter and the position sensitive device; and a second lens sub-arrangement positioned between the beamsplitter and the light sensing module.

4. The optical system of any of claims 1 to 3, wherein the first focal plane is positioned between the lens arrangement and the position sensitive device.

5. The optical system of any of claims 1 to 4, wherein the non-zero distance between the first focal plane and the position sensitive device is no more than a value calculated f by the following formula:(L+2fc)where f is a distance between the first focal plane andthe lens arrangement, L is a target detection distance of the optical system and k is a distance traveled by the first light beam between the beam steering device and the lens arrangement.

6. The optical system of claim 5, wherein the non-zero distance between the first focal plane and the position sensitive device is a value that falls within a range of a first calculated value ± 10%, wherein the first calculated value is calculated by the following formula: 7 -f— .( (L+2fc) - (6- V20) / l -17. The optical system of any of claims 1 to 5, wherein the position sensitive device comprises one or more dark areas having a width, and the non-zero distance between the first focal plane and the position sensitive device is no less than a value calculated by the following formula: 2 — f, where W is the width of the one or more dark areas, DB is the diameter of the incoming beam; and f is a distance between the first focal plane and the lens arrangement.

8. The optical system of any of claims 1 to 7, wherein the light sensing module is positioned to receive the second light beam at the second focal plane.

9. The optical system of any of claims 1 to 8, further comprises a positioning system configured to facilitate controllable adjustment of a distance between the first focal plane and the position sensitive device responsive to at least one target detection distance of the optical system.

10. The optical system of any of claims 1 to 9, wherein the beam steering device is a rotatable mirror.

11. The optical system of any of claims 1 to 10, wherein the beamsplitter is a partially transmissive mirror.

12. The optical system of any of claims 1 to 11, wherein the position sensitive device is a quadrant photodetector.

13. The optical system of any of claims 1 to 12, wherein the beamsplitter is configured such that the first light beam comprises no less than 25% of the incoming light beam and the second light beam comprises no less than 25% of the incoming light beam.

14. An optical wireless communication device (110) comprising: the optical system (150) of any one of claims 1 to 13; and a processing system (160) configured to receive the data signal and extract the embedded information from the data signal.

15. An optical wireless communication system comprising: the optical wireless communication device of claim 14; and a second optical wireless communication device configured to generate a light beam, carrying embedded information, and transmit the generated light beam to the optical wireless communication device, wherein the transmitted light beam acts as the incoming light beam for the optical system of the optical wireless communication device.

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