An optical communication device

Abstract

Two optical communication devices in which one transmits to, or receives light from, the other. The first optical communication device 110 comprising: a first reflective element to receive light from,

Description

Field of the Invention

[0001] The present application relates to an optical communication system, in particular a Fresnel based optical communication system. Background to the Invention

[0002] With the rise in interest in free-space optical communications, there is a greater need for large aperture magnification / de-magnification optics. As an example, such optics are useful in data transfer, especially since it enables large amount of data to be transferred quickly. One example of such data transfer is Quantum key distribution (QKD), which allows two distant parties to share a key in an information theoretic secure way that is guaranteed by the laws of physics.

[0003] However, although often used, commercial telescopes such as Ritchey-Chretien or Schmidt Cassegrain telescopes are currently standard option.

[0004] Such commercial telescopes are costly and have high manufacturing times, and have .multiple challenges when it comes to reducing costs and / or increasing the rate of production. A major element of this is the polishing process of the mirrors which can in some cases require several months of processing and be prone to sudden failure, requiring scrapping of the mirror, even if best practices are followed. Due to the machines required for this process being of finite size and the polishing times scaling with the area of the optics, there is an upper limit to the size of the mirror which can be processed and what can be feasibly fabricated in a reasonable amount of time. Scaling the size of refractive optics also faces issues as large aperture lenses see a significant increase in the mass and thickness of each element when compared to smaller diameter equivalents. This not only has implications in size, weight and cost but should other factors such as polarisation maintenance be of concern, larger refractive lens thicknesses can have other adverse effects. Therefore, the price and mass of the currently available telescopes provide a steep barrier for entry and limit where optical communication systems may be deployed.

[0005] Furthermore, such telescopes also have comparatively large secondary mirror obscuration, which increases loss. Additionally, due to the large weight of telescope primary mirrors and the driving motors / mounts required to move them, there are inherent limitations on the environments where they can be deployed.

[0006] The inventors have devised the claimed invention in light of the above considerations.

[0007] The embodiments described below are not limited to implementations which solve any or all of the disadvantages of the known approaches described above. Summary of Invention

[0008] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter; variants and alternative features which facilitate the working of the invention and / or serve to achieve a substantially similar technical effect should be considered as falling into the scope of the invention.

[0009] In a first aspect of the present invention, there is provided an optical communication device configured to receive light from or transmit light to another optical communication device, the optical communication device comprising: a Fresnel lens configured to receive light from the other optical communication device, or transmit light to the other optical communication device; a first reflective element configured to receive light from the Fresnel lens when the device is receiving light, or direct light to the Fresnel lens when the device is transmitting light; and a second reflective element configured to receive light from the first reflective element when the device is receiving light, or direct light to the first reflective element when the device is transmitting light.

[0010] In a second aspect of the present invention, there is provided an optical communication system comprising: the device of any preceding claim; and an optical detector configured to detect light when receiving light from the other optical communication device, and / or a light source configured to transmit light when transmitting light to the optical communication device.

[0011] The features and embodiments discussed above may be combined as appropriate, as would be apparent to a person skilled in the art, and may be combined with any of the aspects of the invention except where it is expressly provided that such a combination is not possible or the person skilled in the art would understand that such a combination is self-evidently not possible. Brief Description of the Drawings

[0012] Embodiments of the present invention are described below, by way of example, with reference to the following drawings.

[0013] Figure 1a is 2-D a ray diagram illustrating an optical communication system according to an embodiment of the present application;

[0014] Figure 1b is a 3-D ray diagram of the optical communication system of figure 1a;

[0015] Figure 1c is a simulation of the resultant optical spot produced by the system of figure 1a;

[0016] Figure 2a is a ray diagram illustrating an optical communication system according to another to an embodiment of the present application;

[0017] Figure 2b is a 3-D ray diagram of the optical communication system of figure 2a;

[0018] Figure 2c is a simulation of the resultant optical spot produced by the system of figure 2a;

[0019] Figure 3a is a ray diagram illustrating an optical communication system according to yet another to an embodiment of the present application;

[0020] Figure 3b is a 3-D ray diagram of the optical communication system of figure 3a;

[0021] Figure 3c is a simulation of the resultant optical spot produced by the system of figure 3a.

[0022] Figure 4a illustrates a spherical lens;

[0023] Figure 4b illustrates the removal of parts of the spherical lens of figure 4a to form a Fresnel lens;

[0024] Figure 4c illustrates an exemplary Fresnel lens that is equivalent to the spherical lens of figure 4a;

[0025] Figure 4d shows a front view of the Fresnel lens of figure 4c; and

[0026] Figure 5 illustrates the refraction of light through the different parts of the Fresnel lens.

[0027] Common reference numerals are used throughout the figures to indicate the same or similar features. Detailed Description

[0028] Embodiments of the present invention are described below by way of example only. These examples represent the best mode of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved, the description sets forth the functions of the example and the sequence of steps for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0029] Figure 1a shows a 2-D ray diagram of a catadioptric optical communication system 100 according to the embodiment of the present application (termed "Fresnegrain" system in the present application). Figure 2b shows a 3-D ray diagram of the catadioptric optical communication system 100. A catadioptric system is a system which comprises both reflective and refractive elements to form the system. The rays 103 indicate the direction taken by the light when traversing through the system 100 (as explained further below). The rays 103 are simulated using the optical design software Zemax™. The system 100 comprises a Fresnel lens 105 (which forms the refractive element of the system 100), a first reflective element 110 and a second reflective element 115. The Fresnel lens 105 and the first reflective element 110 are arranged opposite one another, as shown in figure 1a. The first reflective element 110 is curved. In particular, the first reflective element 110 is spherical. The first reflective element 110 is of positive power (i.e. the reflective surface is on the convex side of the reflective element 110). The second reflective element 115 is flat. The second reflective element 115 is directly attached or mounted onto the Fresnel lens 105 using a mount and bolts, or glued to the Fresnel lens (i.e. similar to what is commonly seen in corrector plates). In one example, the second reflective element 115 is mounted to the Fresnel lens 105 by cutting a centre piece of the Fresnel lens 105 to accommodate the second reflective element 115. The Fresnel lens 105, the first 110 and the second 115 reflective element together form an optical communication device of the optical communication system 100. In one example, the Fresnel lens 105, the first 110 and the second 115 reflective elements are arranged such that their optical axes 101 are aligned, as shown in figure 1a.

[0030] Both the first reflective element 110 and the second reflective element 115 may be mirrors. In the illustrated example, the Fresnel lens is made from a polymer or plastic. In other examples, the Fresnel lens 105 may be made from glass, or any other suitable material.

[0031] The system 100 can function both as a light collecting system or a light emitting system. For example, when the system is functioning as a light collecting system, light emitted by another optical communication system (which may be same as or different to the system 100 of the present application) initially encounters the Fresnel lens 105. The received light enters the Fresnel lens 105 approximately collimated (as illustrated by the parallels rays in figure 1 a, on the left-hand side). This collimation of the received light is a result of the light originating from a point source (e.g. a light emitting source from another optical communication system) located far away. For example, the system 100 may be a ground-based system that is configured to receive light from another optical communication located on or within a satellite orbiting the Earth.

[0032] A Fresnel lens comprises a series of concentric grooves that are etched on to the surface ofthe lens to produce a ridge structure. Figure 4c shows such a ridge structure of an exemplary Fresnel lens that could be used in the system 100. However, ridge structure ofthe Fresnel lens 105 used in the system 100 (and also in systems 200 or 300, see below) is not limited to that shown in figure 4c. An equivalent spherical lens is shown in figure 4a. The Fresnel lens 4c is produced by removing the non-refractive parts ofthe spherical lens, as shown in figure 4b (dotted lines) leaving the Fresnel lens with a thin profile. As can be seen from figure 4c, the Fresnel lens comprises ridges 405 which corresponds to the curvature ofthe spherical lens but with backward transitions 410 to keep the Fresnel lens flat. Figure 4d shows a front view of the Fresnel lens of figure 4c. The greater the severity of the curvature ofthe spherical lens (or an equivalent curved mirror), the greaterthe density ofthe ridges 405 that are required in the Fresnel lens (i.e. more ridges per inch) to achieve the same refractive power as the spherical lens or the curved mirror. The angle ofthe ridges 405 also governs the refractive power ofthe Fresnel lens at that point. For example, figure 5 shows a Fresnel lens with larger ridge angles at the edges of the lens than compared to the centre of the lens, meaning that the light is refracted to a greater extent at the edges than at the centre ofthe lens. The ridge structure also has an impact on the optical efficiency ofthe Fresnel lens. Further details about the Fresnel lens can be found, for example, in Jui-Wen Pan et al, High concentration and homogenized Fresnel lens without secondary optics element, Optics Communications, 284, 4283 (2011), and Jung Woo Han et al, Clinical Usefulness of Fresnel Ganzfeld Stimulator, Journal ofthe Korean Ophthalmological Society, 56(12), 1874 (2015).

[0033] In the system 100, the ridges enable the Fresnel lens 105 to refract light passing through the Fresnel lens 105 such that incoming collimated light is focused towards the first reflective element 110, as shown in figure 1a. Therefore, the grooves and / or ridge structure enable the Fresnel lens 105 to perform the same function as, for example, a curved primary mirror typically found in traditional telescopes. The light focused by the Fresnel lens 105 is then reflected of the first reflective element 110 and is further focused towards the second reflective element 115 as shown. Finally, light is reflected ofthe second reflective element 115 and focused onto an optical detector 120 (e.g. a single photon detector), which may be located behind the first reflective element 110 as shown. Accordingly, the optics ofthe system 100 is configured such that incoming collimated light entering from the left side ofthe system 100 is focused onto the detector 120 to create an optical spot on the detector 120. To enable the light to pass through the first reflective element 110, the first reflective element 110 may comprise an aperture 130 located centrally with respect to the optical axis 101.

[0034] Figure 1c illustrates a resultant optical spot 135 produced by the system 100 (i.e. when functioning as a light collecting system). "IMA" stands for "Image" and indicates the coordinate ofthe reference point located on the image plane. "OBJ" stands for "Object' and indicates the coordinate of the reference point on the object plane. In this case, the reference point used is the chief ray. The y-axis indicates the scale bar, which in this case is set to 40. The units of the scale are millimetres. The diameter ofthe collimated light entering from the left side ofthe figure 1a (i.e. onto the Fresnel lens 105) was set to 400 mm, and the wavelength ofthe light was set to 852 nm. The black solid line 140 indicates the Airy disk, which has a radius of 10.36 pm. The concentric rings around a central spot in the spot 135 are a result of spherical aberrations within the system 100. The resultant optical spot has a root-mean-square (RMS) radius of 3.449 pm, and a geometrical (GEO) radius of 6.298 pm. The RMS radius is the RMS value of radius of the positions the light rays that intercept in the image plane. The GEO radius of a circle in the image plane in which all rays fall. The optical throughput efficiency ofthe system 100 is approximately 72%

[0035] It can be seen the RMS radius ofthe spot 135 is smaller than the Airy radius, meaning that a real system 100 is or will be close to being a diffraction limited system. Note that Zemax™ does not account for diffracted limited behaviour, hence the spot size ofthe spot 135 is shown to be smaller than the Airy disk 140. Producing a diffraction limited or near diffraction limited spot size is important since, in some examples, the detector 120 may not be located immediately after the first reflective element 110. In this case, the focused light / spot 135 will need to be re-collimated such that the light can travel to the detector 120, and this re-collimation process is dependent on the size ofthe spot.

[0036] In the case where the system 100 is operating as a light emitting system, the direction that the light takes through the system 100 is reversed. In particular, light emitted by a light source 125 (e.g. a single photon source) ofthe system 100 is initially reflected off the second reflective element 115 and directed towards the first reflective element 110. The first reflective element 110 reflects the incoming light and directs the light towards the Fresnel lens 105. The light passes through the Fresnel lens 105 and emerges from the system 100 as a collimated beam to be sent to another optical communication system.

[0037] The diameter of the Fresnel lens 105 is larger than the diameter of the first reflective element 110 and the second reflective element 115. Additionally, the diameter of the second reflective element 110 is smaller than diameter than the first reflective element 110. The system 100 therefore uses a comparatively large Fresnel lens 105 acting as the collection aperture to focus incoming light onto smaller reflective elements 110 and 115, which are able to further focus the light onto the detector 120. Since the first reflective element 110 is not used as the collection aperture, it can be substantially smaller and therefore lighter and cheaper than if it were the collection aperture (for example, as used in a traditional telescope). As also mentioned previously, the second reflective element 115 is flat, which enables it to be easily mounted onto the Fresnel lens 105, and also makes it cheap and easy manufacture.

[0038] Figure 2a shows a 2-D ray diagram of catadioptric optical communication system 200 according to another embodiment of the present application. Figure 2b shows a 3-D ray diagram of the catadioptric optical communication system 200. As before, the rays 203 are simulated using the optical design software Zemax™. The system 200 comprises a Fresnel lens 205 (which forms the refractive element of the system 200), and a first reflective element 210 and a second reflective element 215. The first reflective element 210 is flat and is attached to one side of the Fresnel lens 105 as shown in figure 2a. In one example, the first reflective element 210 may be attached on the side opposite the side comprising the concentric grooves of the Fresnel lens 205 (not shown in the figure 2a), such that the light travels through the entire Fresnel lens 205 before reaching the first reflective element 210. Therefore, it is necessary that the first reflective element 210 is flat (i.e. because the Fresnel lens is also flat in the side opposite the side comprising the grooves). As also shown in figure 2a, the second reflective element 215 is located opposite the Fresnel lens 205 and the first reflective element 210. In one example, the Fresnel lens 205, the first reflective element 210 and the second reflective element 215 are arranged such thattheir optical axes 201 are aligned, as shown in figure 2a. In another example, the optical axes 201 of the Fresnel lens 205, the first reflective element 210 and the second reflective element 215 may not coincide, although they may still be aligned with respect to one another. For example, the second reflective element 215 may be at an angle, and in which case the axis 201 of, for example, the Fresnel lens 205 may not coincide with the detector 220 (i.e. the detector 220 will be located at a side to capture the full amount of light coming from the angled second reflective element 215).

[0039] Both the first reflective element 210 and the second reflective element 215 may be mirrors. Alternatively, the first reflective element 210 may be a reflective coating on the Fresnel lens 205. The Fresnel lens may be made from a polymer or plastic. The Fresnel lens 205, the first 210 and the second 215 reflective element together form an optical communication device of the optical communication system 200. In some examples, the Fresnel lens 205 may be made from glass, or any other suitable material.

[0040] As for the system 100, system 200 can function both as a light collecting system or a light emitting system. For example, when the system is functioning as a light collecting system, light emitted by another optical communication system (which may be same as or different to the system 200) initially encounters the Fresnel lens 205 and the first reflective element 210. Light enters the system 200 approximately collimated (as illustrated by the parallels rays in figure 2a, on the left-hand side). The incoming light initially passes through the Fresnel lens 205 and is reflected of the first reflective element 210. The reflected light then passes through the Fresnel lens 205 for the second time and is focused (due to refraction of light passing through the Fresnel lens 205) onto the second reflective element 215. The second reflective element 215 then reflects the incoming light and directs the light towards an optical detector 220 (e.g. a single photon detector) located behind the Fresnel lens 205 and the first reflective element 210. The second reflective element 215 is curved. In particular, the second reflective element 215 is spherical. The second reflective element 215 is of positive power (i.e. the reflective surface is on the convex side of the reflective element 215). Therefore, the Fresnel lens 205, the first 210 and the second 215 reflective elements of the system 200 are arranged like a Schmidt-Cassegrain telescope, where a curved primary mirror is replaced by the Fresnel lens 205 and the first reflective element 210.

[0041] The first reflective element 210 and the Fresnel lens 205 both comprise a respective aperture 230 located centrally with respect to the optical axis 201. The aperture 230 enables light to exit the system 200 onto the detector 220 after being reflected of the second reflective element 215 (or enables light to enter the system 200 - see below). Accordingly, the optics of the system 200 are configured such that collimated light coming from the left side of the system is focused onto the detector 220 to create an optical spot on the detector 220.

[0042] Figure 2c illustrates a resultant optical spot 235 produced by the system 200 (i.e when functioning as a light collecting system). The abbreviations "OBJ and "IMA" have their usual meanings (see above). The y-axis indicates the scale bar, which in this case is set to 40. The diameter of the collimated light entering from the left side of the figure 2a (i.e. onto the Fresnel lens 205) was set to 400 mm, and the wavelength of the light was set to 852 nm. The black solid line 240 indicates the Airy disk, which has a radius of 10.93 pm. The concentric rings around a central spot in the spot 235 are a result of spherical aberrations within the system 100. The optical spot 235 has an RMS radius of 3.185 pm, and a GEO radius of 7.467 pm. The optical throughput efficiency of the system 100 was approximately 73%.

[0043] It can be seen the RMS radius of the spot 235 is smaller than the Airy radius, meaning that a real system 100 is or will be close to being a diffraction limited system.

[0044] In the case where the system 200 is operating as a light emitting system, the direction that the light takes through the system 200 is reversed. In particular, light emitted by a light source 225 (e.g. a single photon source) is initially reflected of the second reflective element 215 and directed towards the Fresnel lens 205. The light then passes through the Fresnel lens 205 and is reflected by the first reflective element 210 and passes through the Fresnel lens 205 the second time to emerge as a collimated beam. The collimated beam can then be directed at another optical communication system.

[0045] In the system 200, the diameter of the Frensel lens 205 and the first reflective element 210 is substantially the same. Additionally, the diameter of the second reflective element 210 is smaller than diameter than the first reflective element 110 and the Fresnel lens 205. The advantage of this configuration is that the largest reflective surface (i.e. the first reflective element 210) is optically flat, which increases the manufacturability of the components. Additionally, if a reflective coating is used to form the first reflective element 210, it only requires two elements to create the system 200 (the Fresnel lens 205 and first reflective coating 210 being the first element and the second reflective element 215 being the second element).

[0046] Figure 3a shows a 2-D ray diagram of catadioptric optical communication system 300 according to yet another embodiment of the present application. Figure 3b shows a 3-D ray diagram of the catadioptric optical communication system 300.The system 300 comprises a Fresnel lens 305 (which forms the refractive element of the system 300), and a first reflective element 310 and a second reflective element 315. The first reflective element 310 is flat, and is attached to one side of the Fresnel lens 305 as shown in figure 2a. The first reflective element 305 is attached on the side opposite the side comprising the concentric grooves of the Fresnel lens 305 (not shown in the figure), such that the light travels through the entire Fresnel lens 305 before reaching the first reflective element 310. Therefore, it is necessary that the first reflective element 310 is flat. As also shown in figure 3a, the second reflective element 315 is located opposite the Fresnel lens 305 and the first reflective element 310. The Fresnel lens 305, the first 310 and the second 315 reflective element together form an optical communication device of the optical communication system 300. In one example, the Fresnel lens 305, the first 310 and the second 315 reflective element are arranged such that their optical axes 301 are aligned, as shown in figure 3a.

[0047] Both the first reflective element 310 and the second reflective element 315 may be mirrors. Alternatively, the first reflective element 315 may be a reflective coating. Additionally, the Fresnel lens may be made from a polymer or plastic. In some examples, the Fresnel lens 305 may be made from glass, or any other suitable material.

[0048] As for the systems 100 and 200, system 300 can function both as a light collecting system or a light emitting system. In both cases, the path the light takes is identical to the path taken by light in the system 200 shown in figure 2a. As with system 200, both the first reflective element 310 and the Fresnel lens 305 comprise an aperture 330 located centrally with respect to the optical axis 301. The aperture 330 enables the light to exist the system 300 after being reflected of the second reflective element 315 to be focused onto an optical detector 320 located behind the first reflective element 310 and the Fresnel lens 305 (i.e. when the system is functioning as a light collecting system). The aperture 330 also enable transmission of light from a light source 325 towards the second reflective element 315 (i.e. when the system 300 is functioning as a light emitting system).

[0049] As with the system 200, the second reflective element 315 is curved. In particular, the second reflective element 315 is spherical. However, the key difference between the systems 200 and 300 is that in the system 300 the second reflective element 315 has negative power (i.e. the reflective surface is on the concave side of the reflective element 315) as shown in figure 3a. Additionally, compared to the Fresnel lens of the previous systems 100 and 200, the Fresnel lens 305 of the system 300 uses a different ridge structure. The ridge structure of the Fresnel lens 305 differs from the Fresnel lenses 105 and 205 used in the systems 100 and 200, respectively, in that for the Fresnel lens 305 there are different ridge shapes and densities (i.e. higher number of ridges per inch) in different parts of the ring, therefore exhibiting different focal powers in each area. This may be advantageous as the light reflects off the first reflective element 310 twice.. The overall result is that the optics of the system 300 are configured such that collimated light coming from the left side of the system 300 is focused onto the detector 320 to create an optical spot on the detector 320. Therefore, Fresnel lens 305, the first 310 and the second 315 reflective elements of the system 300 are arranged as a Gregorian telescope, where a curved primary mirror is replaced by the Fresnel lens 305 and the first reflective element 310.

[0050] Figure 3c illustrates a resultant optical spot 335 produced by the system 300 (i.e when functioning as a light collecting system). The abbreviations "OBJ and "IMA" have their usual meanings (see above). The y-axis indicates the scale bar, which in this case is set to 40. The diameter of the collimated light entering from the left side of the figure 3a (i.e. onto the Fresnel lens 305) was set to 400 mm, and the wavelength of the light was set to 852 nm. The black solid line 340 indicates the Airy disk, which has a radius of 3.706 pm. The concentric rings around a central spot in the spot 335 are a result of spherical aberrations within the system 300. The optical spot 335 has an RMS radius of 4.352 pm, and a GEO radius of 13.064 pm. The optical throughput efficiency of the system 100 was approximately 80%.

[0051] In contrast to systems 100 and 200, it can be seen that the RMS radius of the spot 335 is bigger than the Airy radius, meaning that a real system will not produce a diffraction limited spot. In other words, the system 300 provides the highest throughput efficiency of all previous systems, but at the expense of bigger spot size. Although a bigger spot size typically results in a degradation of image quality, for the proposed application of the system 300 (i.e. in free-space optical communications), the higher throughput efficiency can be considered more valuable than the minor degradation in image quality (assuming the spot size is small enough to be fully captured by the detector 320).

[0052] As for system 200, the diameter of the Frensel lens 305 and the first reflective element 310 is substantially the same. Additionally, the diameter of the second reflective element 310 is smaller than diameterthan the first reflective element 310 and the Fresnel lens 305. On top of the advantages of given by the system 200, the system 300 also shows the highest throughput efficiency out the previous two systems, but at the expense of having the largest resultant optical spot 335.

[0053] The embodiments described above use a Fresnel lens. In alternative examples, the Fresnel lens may be replaced by an alternative Fresnel element, such as a Fresnel zone plate. For example, the systems 100, 200 and 300 may comprise a Fresnel zone plate in place of the illustrated Fresnel lens. A Fresnel zone plate consists of a series of concentrating rings that alternate between transparent and opaque regions. This causes diffraction of light passing through the transparent part of the zone plate (i.e. light will diffract around the opaque zones) and then constructively interfere to produce the desired effect. Therefore, in contrast to the Fresnel lens (which works by refraction), the Fresnel zone plates work by diffraction of light.

[0054] The present application discloses non-imaging optical system architectures that comprise a large aperture, but also are lightweight, low cost and mass producible, which are suitable, for example, both classical and quantum optical communications. For example, the proposed systems replace the curved primary mirror usually seen in reflective telescopes with a Fresnel lens placed prior to the primary mirror. By transitioning to a catadioptric system the traditionally curved mirror can be either dramatically reduced in size or changed to a flat surface resulting in significant savings in weight and manufacturing times. The Fresnel lens may be a plastic Fresnel lens which may provide further savings in material mass and thickness whilst providing a good resultant wavefront error and collimation ability.

[0055] The proposed systems may reduce the cost and manufacturing cycle times by more than ten times, making large optical apertures accessible to many more users and accelerate the transition to optical communications as the norm for large data transfers. This could be especially useful for smaller satellites which could then take advantage of optical communications to ground which provide high data rates and compact, low power spaceborne transponders.

[0056] Furthermore, the proposed systems all have superior optical performance for nonimaging applications when compared to commercially available imaging telescopes. For example, optical throughput efficiency is measured to be 72% for the Fresnegrain system 100 and the hybrid Schmidt-Cassegrain 200 system, and 80% in the Gregorian for the system 300. The higher optical throughput efficiency is a result of using a smaller secondary reflective element (as compared to the commercially available telescopes). The total length of each of the three systems 100 to 300 is approximately 0.5 meters, making the systems compact and portable. Additionally, unlike traditional telescopes which are optimised for imaging purposes across the visible wavelengths (which also add greater costs and increase manufacturing times), the optics of the proposed system can be optimised for applicable discrete telecoms wavelengths.

[0057] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. Variants should be considered to be included into the scope of the invention.

[0058] Any reference to 'an' item refers to one or more of those items. The term 'comprising' is used herein to mean including the method steps or elements identified, but that such steps or elements do not comprise an exclusive list and a method or apparatus may contain additional steps or elements. 5

[0059] Further, as used herein, the term "exemplary" or "example(s)" is intended to mean "serving as an illustration or example of something".

[0060] Further, to the extent that the term "includes" is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term "comprising" as "comprising" is interpreted when employed as a transitional word in a claim. 10

[0061] It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable modification and alteration of the above devices or methods for purposes of describing the aforementioned aspects, but one of ordinary skill in the art can recognize that many 15 further modifications and permutations of various aspects are possible. Accordingly, the described aspects are intended to embrace all such alterations, modifications, and variations that fall within the scope of the appended claims.

Claims

1. An optical communication device configured to receive light from or transmit light to another optical communication device, the optical communication device comprising:a Fresnel element configured to receive light from the other optical communication device, or transmit light to the other optical communication device;a first reflective element configured to receive light from the Fresnel element when the device is receiving light, or direct light to the Fresnel element when the device is transmitting light; anda second reflective element configured to receive light from the first reflective element when the device is receiving light, or direct light to the first reflective element when the device is transmitting light.

2. The device of claim 1, wherein the second reflective element is flat.

3. The device of any preceding claim, wherein the second reflective element is attached to the Fresnel element.

4. The device of any preceding claim, wherein the first reflective element is spherical.

5. The device of claim 4, wherein the first reflective element has positive power.

6. The device of any preceding claim, wherein a diameter of the Fresnel element is bigger than a diameter of the primary reflective element and a diameter of the secondary reflective element.

7. The device of any preceding claim, wherein a diameter of the second reflective element is smaller than a diameter of the first reflective element.

8. An optical communication device configured to receive light from or transmit light to another optical communication device, the optical communication device comprising:a Fresnel element configured to receive light from the other optical communication device, or transmit light to the other optical communication device;a first reflective element configured to reflect light passing through the Fresnel element back through the Fresnel element; anda second reflective element configured to receive light from the Fresnel element when the device is receiving light or to direct light to the Fresnel element when the device is transmitting light.

9. The device of claim 8, wherein the first reflective element is attached to the Fresnel element.

10. The device of claim 8 or claim 9, wherein the first reflective element is flat.

11. The device of any of claims 8 to 10, wherein the second reflective element is spherical.

12. The device of claim 11, wherein the second reflective element has positive power.

13. The device of claim 12, wherein the system is arranged in a Schmidt-Cassegrain telescope configuration.

14. The device of claim 11, wherein the second reflective element has negative power.

15. The device of claim 14, wherein the system is arranged in a Gregorian telescope configuration.

16. The device of any of claims 8 to 15, wherein a diameter of the Fresnel element is substantially equal to a diameter of the first reflective element.

17. The device of any of claims 8 to 16, wherein a diameter ofthe second reflective element is smaller than a diameter ofthe first reflective element and a diameter ofthe Fresnel element.

18. The device of any of claims 8 to 17, wherein the first reflective element is a reflective coating.

19. The device of any of claims 1 to 17, wherein the first reflective element is a mirror.

20. The device of any of any preceding claim, wherein the second reflective element is a mirror.

21. The device of any preceding claim, wherein the Fresnel element is made from a polymer.

22. The device of any preceding claim, wherein the total length of the system is approximately 0.5 meters.

23. The device of any preceding claim, wherein the Fresnel element is a Fresnel lens.

24. The device of any of claims 1 to 22, wherein the Frensel element is a Fresnel zone plate.

25. An optical communication system comprising:the device of any preceding claim; andan optical detector configured to detect light when receiving light from the other optical communication device, and / or a light source configured to transmit light when transmitting light to the optical communication device.14

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