WO2023079306A1

WO2023079306A1 - A receiver for free-space optical signals

Info

Publication number: WO2023079306A1
Application number: PCT/GB2022/052797
Authority: WO
Inventors: Matthias Jäger; Barry Michael Clive
Original assignee: Optect Limited
Priority date: 2021-11-04
Filing date: 2022-11-04
Publication date: 2023-05-11
Also published as: GB2614039A

Abstract

A free-space optical receiver 100 comprising a first optical component 2 having a focal region at a focal length, a second optical component 4 having an entrance plane 34 and an exit plane 36; and a detector 6, arranged to receive light passing through the exit plane 36 of the second optical component 4, wherein the first optical component 2 is catadioptric and the entrance plane 34 of the second optical component 4 is at the focal region of the first optical component 2.

Description

A receiver for free-space optical signals

Field

The present disclosure concerns a receiver for free-space optical signals. More particularly, this disclosure concerns a receiver comprising a first optical component and second optical component configured to focus light onto a detector. The disclosure also concerns an optical transceiver comprising the receiver and a transmitter, and a method of detecting light using an optical receiver, as well as various systems that include the optical receiver.

Background

Free-Space Optical systems (FSO systems) are systems that transmit and/or receive light over free space, that is, where the light propagates at least some distance without being guided in a waveguide. A FSO system will typically comprise an optical transmitter and/or an optical receiver within a housing. Some FSO systems are designed to transmit a signal towards an object or target, which reflects and/or scatters the signal back towards the optical receiver or, in the case of FSO communications systems, the target may transmit a new signal back towards the optical receiver. In other FSO systems, the signal received by the optical receiver is not deliberately transmitted in order to be received (directly or after reflection or scattering) by the receiver; for example, laser warning systems detect light signals in the form of incident laser beams and gun flash detectors detect light signals in the form of flashes emitted by a gun as it is fired.

As used herein, the term “optical signal” refers to light generally, whether or not the light has been modulated or otherwise configured to transmit information.

The optical receiver is designed to focus light that falls within an acceptance angle onto a suitable detector. The degree to which the light can be focused is limited by several factors. A fundamental physical limitation is given by the conservation of etendue. Etendue is a measure of the spread of light, in area and angle, that an optical system can accept. Etendue can be calculated by integrating over an area A and a range of angles 9 through which light arrives at a receiver, whilst taking into account the refractive index n of the medium in which the light propagates. For an infinitesimal surface element dS with a normal n_s, where the surface is crossed by light within a solid angle d(l at an angle 6 with the normal n_s(and hence the area of the light projected in the direction of the propagation of light is given by dScosO), the etendue G can be defined by the following integral:

G = f f n² dScosOd Equation 1

Over an area A and a solid angle defined by a half opening angle a of an aperture, the etendue G is given by:

G = nn² Asin² a) Equation 2

Comparing the etendue at the aperture of a receiver (Gi), and directly above an active area of a detector (G2), one gets:

The maximum aperture area needed for the receiving optics per detector area, for the case of light travelling from air (ni=T) to the detector, may thus be provided by:

Mechanical instability of the components of the system, or scintillation caused by changes in the atmosphere, may impose a lower limit for the acceptance angle, below which it cannot be guaranteed both that (i) light reaches the receiver and that (ii) the light that does reach the receiver is within the acceptance angle of the receiver. However, this lower limit can be improved using active tracking and/or adaptive optics.

The concentration of light in a receiver of an FSO-system may be achieved by using imaging optics, non-imaging optics or a combination thereof. Typical FSO-Systems concentrate the light using lenses and/or a receiving telescope comprising a plurality of optical components; however, these introduce air gaps into the system, which limits the usable etendue, by imposing a maximum angle of incidence on the detector and/or as a result of there being a low refractive index material (for example vacuum or air) directly adjacent to the detector. Having multiple independent optical components separated by air gaps can also introduce losses via unwanted reflection or scattering of the light. Additionally, the physical distance between the components may result in bulky systems which may be difficult to manoeuvre.

Traditional non-imaging optical systems may allow for a higher concentration of light by providing a wider range of incident angles that can be detected by a detector. In an ideal non-imaging system, the maximum angle could be a = 90°. One disadvantage of using only non-imaging optical components in an optical receiver system is that light propagating through the body of the non-imaging optical component can take a wide spread of optical path lengths, due to diffraction, reflection and/or scattering effects. The wide spread of optical path lengths can limit the bandwidth of a signal transmitted by the light.

It is desirable to increase the concentration of light in optical receivers as this may allow (i) a reduction in the size of the detector (which can result in higher bandwidths and lower costs), (ii) an increase in the aperture of the receiving system (reducing geometrical losses from the transmission signal) and (iii) an increase in the acceptance angle (thereby relaxing the need for precise passive or active alignment of the optical components).

The present disclosure seeks to provide an improved optical receiver.

Summary

According to a first aspect of the present disclosure there is provided a receiver for free-space optical signals, having the features set out in claim 1 below.

According to a second aspect of the present disclosure, there is provided an apparatus having the features set out in claim 19 below.

According to a third aspect of the present disclosure, there is provided an optical transceiver having the features set out in claim 20 below.

According to a fourth aspect of the present disclosure, there is provided a method of detecting light, the method having the steps set out in claim 22 below.

Preferred, but optional, features of the present disclosure are set out below and in the dependent claims.

It will of course be appreciated that features described in relation to one aspect of the present disclosure may be incorporated into other aspects. For example, the method of the disclosure may incorporate any of the features described with reference to the apparatus of the disclosure and vice versa. Description of the Drawings

Embodiments of the present disclosure will now be described by way of example only with reference to the accompanying schematic drawings of which:

Figure l is a schematic cross section of an optical receiver according to a first example embodiment;

Figure 2 shows a schematic cross section of a first optical component of the receiver of Figure 1 with ray tracing;

Figure 3 shows a schematic cross section of a second optical component of the receiver of Figure 1 with ray tracing;

Figure 4 shows a schematic cross section of another example of a first optical component having a planar entrance surface with ray tracing;

Figure 5 shows a schematic cross section of an optical system in accordance with another example embodiment;

Figure 6 shows a schematic cross section of an optical transceiver in accordance with another example embodiment.

Figure 7 shows a schematic cross section of an optical receiver in accordance with another example embodiment, including a Fresnel entrance surface;

Figure 8 shows a schematic cross section of an optical receiver in accordance with another example embodiment, including a steerable mirror between the first and second components; and

Figure 9 shows a schematic cross section of an optical receiver in accordance with another example embodiment.

Detailed Description

In its first aspect, this disclosure provides a free-space optical receiver. The receiver includes a first optical component having a focal region at a focal length. The receiver also includes a second optical component having an entrance plane and an exit plane. The receiver also includes a detector, arranged to receive light passing through the exit plane of the second optical component. The first optical component is catadioptric. The entrance plane of the second optical component is at the focal region of the first optical component.

The receiver is for receiving free-space optical signals. As the skilled person will understand, a catadioptric optical component is an optical component that both reflects and refracts light, for example a lens partially coated with a reflective coating.

As used herein, the “focal length” is the distance from the principal imaging plane to the focal point, referred to herein as the “focal region”. The “focal region” is the region at which the highest concentration of light is reached when a narrow beam of on-axis, collimated light is incident on the first optical component. In the case of ideal imaging optics, a distant infinitesimal point on the optical axis of the system would be focused onto another single infinitesimal point on the optical axis of the system (the focal point); however, in real- life optical systems, the focal region is an aberrated focal point because optical aberrations and diffraction cause the light to spread out into a 3-dimensional volume (characterised for example by a point spread function). Thus, the region at which the highest concentration of light is reached will generally be a 3D region.

When one also considers points of the object field that are off-axis and/or not at infinity, the skilled person will understand that there will be a set of the aberrated focal points (focal regions) which together will form an extended region, or a “focal plane”, that is in focus.

The entrance plane of the second optical component is at the focal region. As used herein, “at the focal region” means that the entrance plane of the second optical component may be exactly coincident with the focal region of the first optical component or the entrance plane of the second optical component may be in close proximity to the focal region of the first optical component. Thus, the distance between the entrance plane of the second optical component and the focal region may be less than 10% of the focal length of the first optical component. The distance may be for example from 0.01 mm to 50 mm. The close proximity of the first optical component and the second optical component may lead to a reduced overall receiver size. This may be desirable in human carried and UAV carried devices. Another advantage of the close proximity of the first optical component and second optical component is that it may improve mechanical and thermal stability of the optical receiver. This may be particularly desirable in systems that need to meet military standards, as it may reduce a need for external protective layers.

The first optical component and the second optical component may be in contact with each other. Advantageously this may reduce losses caused by reflections from surfaces of the first and second optical components, which may mitigate the need for anti-reflection coatings. Additionally, contact between the first optical component and the second optical component may (further) increase mechanical stability.

The first optical component and the second optical component may be separated by a non-zero distance. Where this also increases the distance between the focal region and the entrance plane of the second optical component, this may lead to a defocusing of the incident light. The first optical component and/or the second optical component may be configured to be actively separated, so that a variable separation is introduced between the focal region of the first optical component and the entrance surface of the second optical component. Actively separating or moving the first and/or second optical components may be used in a gradient descent method when aligning the optical receiver. The method may include moving the optical components relative to each other and measuring the change in the intensity of the light that reaches the optical receiver.

The first optical component may comprise: a lens, the lens having a front surface and a convex rear surface, and a concave reflector at the convex rear surface of the lens, the concave reflector being arranged to focus light that has passed through the lens from the front surface to the convex back surface back towards the front surface of the lens. Thus, the focusing may result from the combined effects of the surfaces of the lens and of the concave reflector. The front surface of the lens is the surface that is optically closest to the front of the receiver. For example, the receiver may include a window for receiving light and the front surface of the lens may be the surface facing towards the window. The window may be an aperture. The window may be a sheet material.

The entrance plane of the second optical component is at the focal region. The distance between the entrance plane of the second optical component and the focal region may be less than 10% of the distance from the front surface to the back surface of the lens.

The front surface of the lens may be, or may be substantially, planar. Alternatively, the front surface of the lens may be, or may be substantially, convex. Thus, the lens may be for example a plano-convex (or substantially plano-convex) lens or a convex (or substantially convex) lens. Advantageously, use of a Fresnel lens may reduce the thickness of the first optical component. An optical receiver comprising a Fresnel lens may be used for example in a solar concentrating system (since differences in optical path lengths are not a primary concern for that application), or in other similar free-space optical systems. The front surface of the lens may comprise a Fresnel surface. The rear surface of the lens may comprise a Fresnel surface. The focal region of the first optical component, and hence the second optical component, may be on the front side of the lens.

The concave reflector may extend across substantially the whole of the convex rear surface of the lens.

The focal region of the first optical component, and hence the second optical component, may be on the rear side of the lens.

The convex back surface of the lens may have a region having a different radius of curvature from that of the rest of the convex back surface. The region having the different radius of curvature may be at the centre of the convex rear surface. The region may be concave, planar or convex. Thus, it may be that the concave reflector extends across substantially the whole of the rear surface of the lens, except for the region having the different radius of curvature. The region may for example cover from 0% to 50% of the surface area. The region may be transmissive to light, e.g. transparent or translucent. (It will be appreciated that transparency of the entrance and exit surfaces may be wavelength dependent. For example, in embodiments where the optical receiver is used for visible light, transmissive portions would be visibly transparent.) The transmissive region may be at or near the optical axis of the first optical component. The transmissive region may be circular. The transmissive region may lie in the centre of the reflective portion of the back surface. The transmissive region may be or comprise an aperture. The focal region may be located at the transmissive region of the back surface of the lens.

The concave reflector may be annular. The concave reflector may cover for example from 50% to 100% of the surface area of the back of the lens.

The lens may have a planar region at the centre of its front surface. There may be a planar reflector at the planar region at the centre of the front surface.

The lens may have a concave region at the centre of its front surface.

There may be a convex reflector at the concave region at the centre of the front surface. The reflector may be at or in close proximity to the optical axis of the first optical component. The reflector may cover for example from 0% to 25% of the surface area of the front surface.

The front surface of the lens may be transmissive apart from at the convex reflector at the concave region. The transmissive portion of the front surface may be annular. The transmissive portion of the front surface may for example cover from 75% to 100% of the surface area of the front surface. The reflector or reflectors on the lens may comprise a metallic layer, for example a silvered layer. The reflector or reflectors may be a dielectric coating, for example a Fabry- Perot coating.

Thus, for example, the first optical component may have: at its front side, a transparent annular surface that is planar or convex; an annular concave mirror on the opposite, back, side; a secondary, convex, mirror on the front side; and a transparent surface at the centre of the back side that may be flat, convex or concave.

The second optical component may be a waveguide or a horn.

The second optical component may comprise a central portion disposed between the entrance plane and the exit plane. The central portion may be cylindrical. The central portion may have two opposite ends, one end having a larger area than the other end. The entrance plane and/or exit plane may have a circular cross section. The central portion may have a shape that can be described by the rotation of a segment of a parabola around an axis. The second optical component may be a compound parabolic concentrator. The central portion may comprise sides which are, in cross-section, segments of a parabola; for example, the central portion may be a truncated paraboloid. Alternatively, the central portion may for example be cuboidal, where the entrance plane and exit plane may for example have a square cross-section, or they may have a hexagonal cross section. This may be advantageous in some applications where multiple detectors are used. The secondary optical component may have a convex entrance surface. The second optical component may be a non-imaging optical component. The central portion may have sidewalls that are curved inwards. Such an arrangement may be suitable for a non-imaging secondary component.

Non-imaging optical components introduce path length (and hence time of flight) differences between different paths that the light takes through them. This in turn limits the possible bandwidth of the system. Rather than all of the optics in the system being nonimaging, having just the secondary optical component being a non-imaging optical component results in the non-imaging part of the system being smaller (for example 100 micrometres to a few millimetres compared with a total system length of centimetres in size), which reduces the bandwidth-limiting effect significantly.

The central portion may be hollow. Alternatively, the central portion may be solid, in which case, the solid centre will comprise transmissive material; for example, in embodiments where the optical receiver is used for visible light, the transmissive material would be visibly transparent, for example a glass or plastics material. The central portion may comprise material that has the same or a similar (for example, within 10%, for example within 5% and 15%) index of refraction as the material of the first optical component in order to reduce unwanted reflection or refraction of light when passing from the first optical component to the second.

The central portion of the second optical component may include a reflective surface, which may result for example from a mirror layer or from total internal reflection of light within the central portion. The reflective surface may be located on the internal side(s) of the central portion. The reflective surface may improve the efficiency of the optical receiver by reducing loss of light, for example due to partial transmission through the side walls. The reflective surface may assist in directing incoming light that is at an angle to the optical axis, towards a detector located at or near the exit plane of the second optical component, to increase the intensity of the light.

The entrance plane of the second optical component may be or include a window. The window may be an aperture. The window may be a sheet material. The shape of the entrance window of the second optical component will then define whether all or part of the light that could be incident on the second component is collected by it. This may provide a limit on the angles of acceptance of the optical receiver, which can be desirable in some applications.

The entrance plane and the exit plane of the second optical component may each comprise a surface. The entrance surface of the second optical component may comprise material that has the same or similar (for example, within 10%, preferably within 5%) index of refraction as the back surface of the first optical component. Having matched refractive indices may reduce unwanted reflections of light when the light is propagating through the optical receiver.

The receiver may comprise a plurality of the second optical components at the focal region of the first optical component.

The exit plane of the second optical component may be or include a window. The window may be an aperture. The window may be a sheet material. The exit plane of the second optical component may comprise a surface. The surface may be transparent. (Again, it will be appreciated that the transparency of the surface may be wavelength dependent.) For example, the surface may comprise visibly transparent material such as a glass or a plastics material.

The optical receiver may include a mechanism arranged to move the second optical component with respect to the first optical component. The second optical component may have three degrees of movement, such that the second optical component may move along the x- or y- axis, (i.e. in the plane parallel to the exit surface of the first optical component) or in the z-axis (i.e. along the optical axis of the first optical component). This may enable changes in the range of acceptance angles, which may be used for example to compensate for misalignments from indirect light sources or to follow the movement of a transmitter. The second optical component may have one or more rotational degrees of freedom, for example, the second optical axis may be rotated around the z-axis.

Alternatively or additionally, the optical receiver may further comprise a mirror arranged between the first optical component and the second optical component, the mirror being configured to be adjusted to alter the path of light passing from the first optical component to the second optical component. The moveable mirror may assist in directing the light from the first optical component to the second optical component. For example, the moveable mirror may enable changing of the range of acceptance angles, which may be used for example to compensate for misalignments from indirect light sources or to follow the movement of a transmitter.

The first optical component and second optical component may be monolithic. The first and second optical components may be manufactured such that the entrance plane of the second optical component is integral with the back surface of the first optical component. In other embodiments, the first optical component and second optical component may be attached to each other for example using an adhesive. The adhesive may have a refractive index between the refractive index of the back surface of the first optical component and the refractive index of the entrance surface of the second optical component advantageously reducing reflections at the interface. Alternatively, the second optical component may be deposited on the exit surface of the first optical component for example using an additive or subtractive process such as 3D printing or lithography

The optical receiver may comprise a liquid disposed between the first optical component and the second optical component. Advantageously, the liquid may be an index matching liquid. Advantageously, the liquid may provide a reduction in the reflections at the interfaces of the components of the optical receiver, whilst simultaneously compensating for inaccuracies or unevenness at the interface of the surfaces. The liquid may allow for the compensation of thermal expansion of the components. The liquid may be a glue. Adhesives such as glue may be used to provide structural support. The first and second optical components may be fixed together after applying the index matching liquid between the surfaces of the first and second optical components. The first optical component and the second optical component may be mounted together on a controlled mount. The mount may comprise a gimbal. The gimbal may be configured to be controlled to move the mount to align the first and second optical components. A particularly advantageous arrangement combines a gimbal mount to provide coarse alignment with a steerable mirror between the first component and the second component (and/or the second component and the detector) to provide fine alignment.

It will be appreciated by the person skilled in the art that the detector may be any suitable detector for detecting the light, for example a photodiode.

The optical receiver may further comprise a mirror arranged between the second optical component and the detector, the mirror being configured to be adjusted to alter the path of light passing from the second optical component to the detector.

Preferably the detector is at the exit plane of the second optical component. The close proximity of the second optical component to the detector increases the concentration factor, which means that more of the possible angles of incidence on the detector can be used which is equivalent to using less of those on a larger area detector. Thus, the close proximity of the second optical component to the detector enables the etendue of the system to be increased, or even maximised, compared with prior-art systems. The greater range of angles of incidence on the detector may reduce the size and/or number of detectors required. Advantageously, where the second optical component is non-imaging, the bandwidth available for the received signal can be increased because differences in the optical paths taken by components of the incident light may be reduced compared with prior art systems by having the detector close to the second component and reducing the size of the detector.

The exit plane of the second optical component and a surface of the detector may have the same refractive indices. This may reduce the losses due to reflections at the surface of the detector (which may otherwise be difficult to achieve using alternative compensators, due partly to the large ranges of incidence angles of the light at that surface.) Advantageously, having the same refractive indices may reduce the size of the optical receiver whilst allowing for increased or even maximum etendue at the interface between the optical receiver and the detector. There may be an index matching liquid disposed between the exit plane of the second optical component and the surface of the detector. An increase in the etendue may be used to increase the field of view of the optical receiver and/or to increase the usable entrance aperture without increasing the size of the detector.

At a constant etendue, the increased range of angles and/or the increased refractive index allows for the use of a smaller detector. Smaller detectors may be advantageous because they allow for lower noise levels and higher data rates. Smaller detectors may be used in optical systems such as FSO communication systems, rangefinders or on UAVs. FSO communication systems can benefit from higher bandwidth and therefore a higher data rate. Rangefinders and passive sensing applications can benefit from the lower noise.

The exit plane of the second optical component and/or the surface of the detector may comprise an anti-reflection coating.

The optical receiver may comprise a liquid disposed between the second optical component and the detector. The presence of the liquid can increase the refractive index directly above the detector and hence can increase the etendue, so that more of the possible angles of propagation inside the detector can be used. The liquid may be an index matching liquid. The liquid may provide a reduction in the reflections at the interfaces of the components of the optical receiver, whilst simultaneously compensating for inaccuracies or unevenness at the interface of the surfaces. The liquid may allow for the compensation of thermal expansion of the components. The liquid may be a glue. Adhesives such as glue may be used to provide structural support. The second optical component and the detector may be fixed together after applying the index matching liquid between the surfaces of the second optical component and the detector.

The second optical component may be manufactured such that the exit plane of the second optical component is integral with the entrance surface of the detector. Alternatively, the second optical component and detector may be attached to each other for example using an adhesive. The adhesive may have a refractive index between the refractive index of the exit plane of the second optical component and the surface of the detector advantageously reducing reflections at the interface. Alternatively, the second optical component may be deposited on the surface of the detector using an additive or subtractive process such as 3D printing or lithography.

The optical receiver may comprise a mirror configured to be moved to direct light to the receiver. The mirror may enable an adjustable field of view and acceptance angle, for example a wider field of view and acceptance angle, for the optical system.

A housing with an aperture at or near the first optical component of the optical receiver may protect the optical components from external environmental effects or other unwanted radiation sources.

A further aspect of this disclosure provides apparatus that is or includes a communication system, rangefinder, aerial platform, laser warning system, sensing system ( for example remote sensing system), or a solar concentrating system, and that includes a receiver as described above with respect to the first aspect of the disclosure.

Thus, the optical receiver may be used in optical communications systems. The optical receiver may be used in FSO communications systems. Advantageously, the optical receiver may reduce the size and weight of a FSO communication system, compared with prior-art systems. The optical receiver may increase the field of view which may make it easier to align. The optical communication system may be a Free-Space Optical Transceiver. The combination of the transmitter and optical receiver as disclosed herein provides an efficient system which is much faster to align and reduces the size, weight, and power of the optical system compared with prior-art systems. The wide field of view of the receiver mitigates the need to move the receiver for alignment which means that only the transmitter is required to be aligned. The transmitted beam is small compared to the receiving beam and so alignment can be done by moving a mirror only, which can result in faster tracking.

The optical receiver may be used in unmanned aerial vehicles (UAVs, for example drones). The optical receiver may reduce the precision of alignment required for a FSO communication system to successfully operate between UAVs.

The optical receiver may be used for optical imaging systems such as cameras or viewing platforms such as binoculars or telescopes. Refracting telescopes are typically long and heavy, which may prove difficult to manoeuvre, especially in the dark. The present receiver may reduce the size of traditional optical telescopes, making them smaller, lighter and easier to handle.

Another aspect of this disclosure provides an optical transceiver including an optical receiver as described above with respect to the first aspect of the invention and a transmitter. The optical transceiver may include a mirror configured to be adjusted to alter the path of light leaving the transmitter.

An optical transceiver may be used as a rangefinder. Modulated light may be emitted from the transmitter, reflected and/or scattered by one or more external objects and received by the optical receiver of the rangefinder.

A further aspect of the disclosure provides a method of detecting light using a receiver according to the first aspect of the invention, comprising the steps of: receiving light at the first optical component; focusing the light with the first optical component to the focal region; receiving the light at the focal region in the entrance plane of the second optical component; and directing the light through the second optical component to the detector. The received light may propagate to a front surface of the first optical component, where the first optical component is a lens, and through the lens to a concave reflector at the rear surface of the lens, the light being focused by the concave reflector back towards the front surface of the lens.

The light focused by the concave reflector back towards the front surface of the lens may be reflected at the front surface of the lens back towards the back surface of the lens. The light may be received at the entrance plane of the second optical component at the back surface of the lens.

The method may include the step of moving the second optical component with respect to an optical axis of the first optical component to change the range of acceptance angles of the receiver. This may be useful for collecting light that has been reflected off-axis to the optical axis. The second optical component may for example be moved along the x-, y-, or z-axis. The second optical component may have one or more rotational degrees of freedom, for example, the second optical component may be rotated around the z-axis.

Other example embodiments will now be described in further detail with reference to Figures 1 to 9.

Figure 1 shows in cross section an optical receiver 100 including a housing 30 defining an aperture that acts as a window 31 for receiving light into the receiver housing. The receiver 100 also includes a first optical component in the form of a lens 2 comprising a convex lens, with a curved front surface facing towards the window 31, in the form of an entrance surface 12 and a convex rear surface in the form of a curved exit surface 14. In this present embodiment, the material of the lens 2 is glass. The exit surface 14 of the lens 2 is in close proximity to a second optical component 4. The second optical component 4 has an entrance plane 34 located near the exit surface 14 of the lens 2, and an exit plane 36 opposite the entrance plane 34. A detector 6 is located at the exit plane 36. The detector 6 may be a photodiode or any other suitable detector. The close proximity of the lens 2 and the second optical component 4 reduces the size of the optical receiver whilst maintaining its efficiency for receiving and focusing incoming light. Additionally, the close proximity of the lens 2 and second optical component 4 improves mechanical stability of the optical receiver 100.

With reference to Figure 1 and Figure 2, the lens 2 has a transmissive portion 122 at the entrance surface 12 and a transmissive portion 22 at the exit surface 14. The ray tracing 20 shows the light incident on the transmissive portion 122 refracting upon entry into the lens 2. The lens 2 has a concave reflector in the form of a reflecting portion 18 at the exit surface 14 and a convex reflector in the form of a reflective portion 16 at the entrance surface 12. The reflective portion 18 of the exit surface 14 covers 90% of the exit surface. (In other embodiments, the reflective portion 18 may cover for example from 25% to 100%, for example 50% to 99%, for example 85% to 95% of the exit surface.) The reflective portion 18 of the exit surface 14 is annularly shaped such that it surrounds a central portion of the exit surface that is transparent to the incident light. The central part is the transmissive portion 22 of the exit surface 14 and is located at a focal region 24 of the lens 2.

The reflecting portion 16 at the entrance surface 12 is located at the optical axis of the lens 2, at the centre of the entrance surface 12. In this example, the reflective portion 16 of the entrance surface 12 covers 10% of the surface of the entrance surface 12. (In other embodiments, the reflective portion 16 of the entrance surface 12 may for example cover from 0% to 50% of the internal surface area of the entrance surface 12.) The reflective portion 16 is curved in the opposite direction to the curvature of the transmissive portion 122 of the entrance surface 12: the surface 12 of the lens 2 is concave at the reflective portion 16, and so the reflective portion 16 itself, on the surface 12 and facing into the lens 2, is convex. The curvature of the reflective portion 16 directs the reflected light from the reflective portion 18 of the exit surface 14 towards the transmissive portion 22 of the exit surface 14, where the focal region 24 of lens 2 is located. At the transmissive part 22 of the exit surface 14, the light passes to the entrance plane 34 of the second optical component 4.

The propagation direction of the light will now be described with reference to the ray tracing in Figure 2 which shows the beam path of collimated light falling within the acceptance angle 102 of the receiver. The collimated light 20 enters the lens 2 at the transmissive portion 122 of the entrance surface 12 and propagates through the lens 2 to the exit surface 14. The light 20 is reflected by the reflective portion 18 of the exit surface 14 to the reflective portion 16 of the entrance surface 12 of the lens 2. The light 20 is further reflected by the reflective portion 16 of the entrance surface 12 to the transmissive portion 22 of the exit surface 14 at the focal region 24. The combination of the refractive index of the material of the lens 2, curvature of the entrance surface 12 and exit surface 14, the curvature of the reflective portions 16, 18, and the thickness of the lens 2, ensures that the collimated beam is directed to the focal region 24.

The second optical component will now be described in more detail with reference to Figure 3. Figure 3 shows a schematic cross sectional view of the second optical component 4 with ray tracing to show examples of the path taken by incident light. The second optical component 4 has a central portion 38 which connects the entrance plane 34 and exit plane 36. The entrance plane 34 and exit plane 36 have complimentary shapes in the present embodiment: each of the entrance plane 34 and exit plane 36 have a circular cross section. In this example, the diameter of the entrance plane 34 is greater than the diameter of the exit plane 36. In the present example, the central portion 38 has a curved side wall 40 connecting the entrance plane 34 and the exit plane 36. The central portion 38 is approximately parabolic in shape, having two circular ends, wherein the diameter of one end is wider than the opposite end, and has the wall 40 between them. The second optical component 4 is a single solid component of a constant refractive index. The surface of the sidewall 40 reflects incoming light, thereby assisting in directing the light to the exit plane 36 and increasing the intensity of the light that reaches the detector 6. Points in focus from the first optical component form a focal plane 34’ located at the entrance plane 34 of the second optical component 4.

Figure 4 shows a schematic cross section of an alternative first optical component in the form of a plano-convex lens 2’, with ray tracing 20 to show the beam path of collimated light falling within the acceptance angle 102 of the plano-convex lens 2’. The plano-convex lens 2’ has a flat front entrance surface 12’ and a curved rear exit surface 14’. The planoconvex lens 2’ has a concave reflector in the form of reflecting portion 18’ at the exit surface 14’ and a convex reflector in the form of reflective portion 16’ at the centre of the flat entrance surface 12’. The reflective portion 18’ of the exit surface 14’ covers 90% of the exit surface. (In other embodiments, the reflective portion 18’ may cover from 25% to 100%, for example 50% to 99%, for example 85% to 95% of the exit surface). The reflective portion 18’ of the exit surface 14’ is again annularly shaped such that it has a central part that is transparent to the incident light. The central part is a transmissive portion 22’ which is located at the focal region 24’ of the lens 2’.

The lens 2’ has a transparent portion 112’ at the entrance surface 12’. The convex reflective portion 16’ is located at or near the position of the optical axis of the lens 2’. The reflective portion 16’ of the entrance surface 12’ covers 7% of the surface area. (In other embodiments, the reflective portion 16’ may cover from 0% to 50% of the entrance surface). The transparent portion 112’ is planar, i.e. flat. The curvature of the reflective portion 16’ directs the reflected light from the reflective portion 18’ of the exit surface 14’ towards the central part 22’ of the exit surface 14’, where the focal region 24’ of lens 2’ is located. At the central part 22’ of the exit surface, the light arrives at the entrance plane 34 of the second optical component 4. Figure 5 shows a cross section of an optical system 200 comprising an optical receiver 100 within a housing 202 in accordance with another embodiment. In example embodiments of the invention, the optical system 200 forms part of a larger apparatus 250, which in example embodiments is a communication system, rangefinder, sensing system, aerial platform, laser warning system, solar concentrating system, or transceiver. The optical receiver 100 has a first optical component 2 which is a lens, in accordance with the description above, and a plurality of second optical components 4a, 4b, 4c. The housing has an opening 204 for incident light to enter the optical receiver 100 via the entrance surface 12 of the lens 2. In this example, there are three second optical components 4a, 4b, 4c arranged to be in contact with the exit surface 14 at the focal region 24 of the lens 2. (In other examples, there may be more detectors arranged at the focal region 24 of the lens 2.) In this example, each second optical component 4a, 4b, 4c has an independent detector 6a, 6b, 6c. The plurality of second optical components 4a, 4b, 4c may be used to increase the range of acceptance angles of the whole system and/or assist in aligning the system by comparing the strength of the signal received by each of the detectors 6a, 6b, 6c. In some examples, there may be a single detector 6 for all of the second optical components 4a, 4b, 4c.

Figure 6 shows a cross section of an optical system 300 in accordance with another embodiment. The optical system 300 is a Free-Space Optical Transceiver (FSOT). In this example, there is an optical receiver 100 within a housing 302. The housing 302 protects the components from external influences such as radiation or other environmental effects. The optical system 300 also includes an optical transmitter 400. The housing 302 has two windows; a transmitting opening 410 and a receiving opening 304.

The optical receiver 100 has the features of the optical receiver of the other example embodiments described above. The optical receiver 100 is located near the receiving opening 304 such that the light enters via the entrance surface of the first component of the optical receiver 100. The optical transmitter 400 has a light source 402, in this example a laser, a moveable mirror 404, and a window 408. The window 408 may be any material that is transparent to the wavelength of the incident light. In other examples, the window 408 may be a filter. The light 406 from the light source 402 is reflected from the moveable mirror 404 through the window 408 and exits the transmitting opening 410. The window 408 is located close to the transmitting opening 410 to ensure that the light exits the housing 302. The moveable mirror 404 may be moved to change the direction of the beam, by motors for example. The moveable mirror 404 may be used to adjust for known misalignments and/or used to periodically introduce small misalignments to determine the direction into which a misalignment needs to be corrected. The moveable mirror 404 can for example be rotated along two axis. The mirror 404 can be moved by a motor, moved as part of a Micro-Electro- Mechanical Systems (MEMS) or moved by an actuator (not shown).

An example method for establishing a two-way link between two such transceivers is as follows:

(1) Align each transceiver to a target position. This may be manual or automated alignment, and may be to a previously known or agreed upon position or a position provided by an external system, for example a UAV or a camera with an image identification system.

(2) Scan the transceivers in a field of view around those initial positions , each transceiver sending a series of identifier codes as it moves.

(3) Each transceiver monitors for the identifier codes transmitted by the other transceiver and, when one is received, adds the transmission time of that identifier code to its own transmissions.

(4) When a transceiver receives an identifier code from the second transceiver that includes the time delay information, use the time delay information to orient the transceiver to the orientation it was in when it sent the identifier code that was received by the other transceiver.

(5) If no link is established after a timeout period, the process may restart from step (2) with the position at which the current orientation points as the initial position.

An example method of maintaining the alignment between two transceivers is to misalign (for example periodically, or on a user instruction) one of the transceivers in all directions, for example in by a circular movement of the transmit direction. The already existing data channel may then be used by the other transceiver to communicate at which misalignment position the signal was strongest and the transceiver may then be moved to that position for further transmissions. One or both transceivers may also or alternatively measure its own movements (for example measured using accelerometers on-board the transceiver) and either compensate itself for those movements by changing its position or by sending information about its movements so that the other transceiver can change its position to compensate for them.

Figure 7 shows a schematic cross section of an optical receiver 500 in accordance with another embodiment. The first optical component 502 of this embodiment includes a Fresnel lens. The lens 502 has a front entrance surface 512 that is a Fresnel surface and a rear exit surface 514 that is a convex lens surface. The Fresnel lens 502 has a transmissive portion 542 at the entrance surface 512 and a transmissive portion 522 at the otherwise reflective exit surface 514. The Fresnel lens 502 has a convex reflective portion 516 at the entrance surface 512 which is located at the optical axis of the Fresnel lens 502, at the centre of the entrance surface 512. In this example, the reflective portion 516 of the entrance surface 512 covers 7% of the entrance surface 512. (In other embodiments, the reflective portion 516 may cover from 0% to 50% of the surface area of the entrance surface 512. The reflective portion 516 is curved.)

The second optical component 504 has an entrance plane 534 located near the exit surface 514 of the Fresnel lens 2, and an exit plane 536 opposite the entrance plane 534 where a detector may be positioned.

Figure 8 shows a schematic cross section of an optical receiver 600 in accordance with another embodiment. The first optical component 602 of this embodiment is a lens with a front entrance surface 612 and a rear exit surface 614. The lens 602 has a transmissive portion 642 at the entrance surface 612 and a transmissive portion 622 at the otherwise reflective exit surface 614. In this embodiment, there is a mirror 650 that can be moved by an actuator 652. (In other embodiments, the mirror may be moved by other actuators or the actuator 652 may be part of a MEMS.) When incident light exits the transmissive portion 622 of the exit surface 614, the light will be reflected away by the mirror 650 towards a second optical component 604. In this example, the mirror 650 is angled to direct the light towards the second optical component 604 which is at a 90° angle with the optical axis of the first optical component 602. The second optical component 604 has an entrance plane 634 and an exit plane 636 opposite the entrance plane 534 where a detector is positioned.

Figure 9 shows a schematic cross section of an optical receiver 700 in accordance with another embodiment. In this embodiment, the second optical component is located on the same side of the first optical component as the entrance surface of the first optical component, i.e. the front side of the lens 702. The first optical component of this embodiment is a lens 702 with a front surface 712 which is transmissive to incident light, and a rear surface 714 which has a reflector 736 covering the whole of its surface. The first and second surfaces 712, 714 are convex, and so the reflector 736 is concave. When light is incident on the lens 702, the light is reflected by the reflector 736 and directed to a central part 724 of the front surface 712, where the focal region of the lens is located. At the central part 724 of the front surface 712, the light arrives at the entrance plane 734 of the second optical component 704. Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

In some examples, an optical system may comprise more than one optical receiver.

In the example embodiments of the first optical component of Figures 1 and 2, the transmissive portion 22 of the rear surface 14 of the first optical component 2 comprises material such as glass or plastics material. In some examples, there may be a coating on the transmissive portion 122 of the entrance surface 12, the transmissive portion 22 of the exit surface 14 or the entrance plane 34 of the second optical component 4. In some examples, there may be a filter between the first optical component 2 and the second optical component 4 and/or between the second optical component 4 and the detector 6 and/or at the window 31.

In the example embodiment of Figure 3, the second optical component 4 has a central portion 38 that is a solid. In alternative embodiments, the central portion 38 may be hollow with a reflective internal surface of the side wall 40. The entrance plane 34 and exit plane 36 of the second optical component may be or have an aperture or a hole. The central portion 38 of the second optical component 4 may have a cylindrical shape, where the entrance plane 34 and exit plane 36 have a circular cross section. In other examples, the central portion 38 may have a cuboidal shape, where the entrance plane 34 and exit plane 36 have a rectangular cross section. The cross sectional area of the entrance plane 34 may be greater than the cross sectional area of the exit plane 36. In other examples, the cross sectional area of the entrance plane 34 and exit plane 36 may be the same. Further, in some examples, there may be an elongate waveguide region having the same shape as the entrance aperture of the second optical component (e.g. cylindrical, square or square) to increase the length of the second optical component.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A free-space optical receiver comprising:

(a) a first optical component having a focal region at a focal length;

(b) a second optical component having an entrance plane and an exit plane; and

(c) a detector, arranged to receive light passing through the exit plane of the second optical component; wherein the first optical component is catadioptric and the entrance plane of the second optical component is at the focal region of the first optical component.

2. A receiver as claimed in claim 1, wherein the first optical component comprises: i. a lens, the lens having a front surface and a convex rear surface, and ii. a concave reflector at the convex rear surface of the lens, the concave reflector being arranged to focus light that has passed through the lens from the front surface to the convex back surface back towards the front surface of the lens.

3. A receiver as claimed in claim 2, wherein the front surface of the lens is or is substantially planar, substantially convex or comprises a Fresnel surface.

4. A receiver as claimed in any of claims 1 to 3, wherein (i) the focal region of the first optical component and (ii) the second optical component are on the front side of the first optical component.

5. A receiver as claimed in any of claims 2 to 4 in which the concave reflector extends across substantially the whole of the convex rear surface of the lens.

6. A receiver as claimed in any of claims 1 to 3, wherein (i) the focal region of the first optical component and (ii) the second optical component are on the rear side of the first optical component.

7. A receiver as claimed in claim 6, wherein the first optical component is a lens, and the lens has a planar region at the centre of its convex rear surface, and the second optical component is at the planar region.

8. A receiver as claimed in claim 7, wherein the concave reflector extends across substantially the whole of the rear surface of the lens, except for the planar region.

9. A receiver as claimed in any preceding claim, wherein the first optical component is a lens, and the lens has a planar region at the centre of its front surface.

10. A receiver as claimed in claim 9 in which there is a planar reflector at the planar region at the centre of the front surface. A receiver as claimed in any of claims 1 to 8, wherein the first optical component is a lens, and the lens has a concave region at the centre of its front surface. A receiver as claimed in claim 11 in which there is a convex reflector at the concave region at the centre of the front surface. A receiver as claimed in any preceding claim wherein the second optical component comprises a hollow central portion disposed between the entrance plane and the exit plane. A receiver as claimed in any of claims 1 to 12 wherein the second optical component comprises a solid central portion disposed between the entrance plane and the exit plane. A receiver as claimed in any preceding claim wherein the second optical component is a non-imaging optical component. A receiver as claimed in any preceding claim comprising a plurality of the second optical components at the focal region of the first optical component. A receiver as claimed in any preceding claim, further comprising a mirror arranged between the first optical component and the second optical component, the mirror being configured to be adjusted to alter the path of light passing from the first optical component to the second optical component. A receiver as claimed in any preceding claim, further comprising a mirror arranged between the second optical component and a detector, the mirror being configured to be adjusted to alter the path of light passing from the second optical component to the detector. Apparatus comprising a communication system, rangefinder, aerial platform, laser warning system, sensing system, or solar concentrating system, wherein the apparatus comprises a receiver as claimed in any of claims 1 to 18. An optical transceiver including a receiver in accordance with any of claims 1 to 18 and a transmitter. An optical transceiver as claimed in claim 20 including a mirror configured to be adjusted to alter the path of light leaving the transmitter. A method of detecting light using a receiver in accordance with any of claims 1 to 18, comprising the steps of: receiving light at the first optical component; focusing the light with the first optical component to the focal region; receiving the light at the focal region in the entrance plane of the second optical component; and directing the light through the second optical component to the detector. A method as claimed in claim 22, wherein focusing the light with the first optical component to the focal region comprises the steps of

- the received light propagating to a front surface of a lens and through the lens to a concave reflector at the rear surface of the lens, the light being focused by the concave reflector back towards the front surface of the lens. A method as claimed in claim 23, wherein the light focused by the concave reflector back towards the front surface of the lens is reflected at the front surface of the lens towards the back surface of the lens and the light is received at the entrance plane of the second optical component at the back surface of the lens. A method as claimed in any of claims 22 to 24 comprising the step of moving the second optical component with respect to an optical axis of the first optical component to change the range of acceptance angles of the receiver.