A mirror assembly for a combined radio frequency (rf) and electro-optical system

Abstract

A mirror assembly for a combined Radio Frequency (RF) electro-optical System (figure 1); the mirror assembly 5 having a mirror surface 50 configurable to redirect optical radiation to and / or from an e

Description

US5268680 and US4477814 describe a combined infrared-radar detection system where a radar antenna also serves as a mirror in an infrared (IR) focusing arrangement. This arrangement reduces the physical size of the system. These systems use slot antennas resulting in loss of reflective area within the IR mirror and thus reduction in range performance of the IR sensor. The present invention ameliorates this problem. According to a first aspect of the invention there is provided a mirror assembly for a combined Radio Frequency (RF) electro-optical system; the mirror assembly having a mirror surface configurable to redirect optical radiation to and / or from an electro-optical device of the system; the mirror assembly comprising an electrically conductive material providing the mirror surface; and in which at least a portion of the electrically conductive material providing at least a portion of the mirror surface provides, at least in part, an antenna element electrically couplable to a radio frequency (RF) transmitter and / or RF receiver of the system. According to a second aspect of the invention there is provided a combined RF electro-optical system comprising: an electro-optical transmitter and / or receiver; a radio frequency (RF) receiver and / or RF transmitter; and a mirror assembly comprising a mirror surface configurable to redirect optical radiation to and / or from an electro-optical transmitter and / or receiver; the mirror assembly comprising an electrically conductive material providing the mirror surface; and in which at least a portion of the electrically conductive material providing at least a portion of the mirror surface provides, at least in part, an antenna element which is electrically coupled to the radio frequency (RF) transmitter and / or RF receiver. According to a third aspect of the invention there is provided a mirror assembly for a combined Radio Frequency (RF) electro-optical system; the mirror assembly having a mirror surface configurable to redirect optical radiation to and / or from an electro-optical device of the system; the mirror assembly comprising a substrate having a first side and a second side that face opposite directions; the first side carries an electrically conductive material layer provides the mirror surface; and in which the assembly comprises multiple antenna elements each electrically couplable to a radio frequency (RF) transmitter and / or RF receiver of the system; each antenna element comprised from a different region of the electrically conductive material layer that is electrically isolated from the other regions of the electrically conductive material layer, and each of the regions providing a portion of the mirror surface. The following relates to one or more of the three aspects of the invention. Because the antenna element is optically reflective, the loss of optically reflective area within the mirror is significantly less compared with the prior art system. The mirror assembly may comprise a dielectric substrate having a first side and a second side, the first and second side facing opposite directions; the dielectric substrate carrying the electrically conductive material including the antenna element on the first side. The assembly may further comprise means to carry RF signals through the dielectric substrate between the first and second sides for connection to the radio frequency (RF) transmitter and / or RF receiver and in which the electrically conductive material is electrically coupled through the substrate to the RF transmitter and / or RF receiver. In one arrangement, the mirror assembly includes a feed element positioned on, e.g. carried on, the second side of the dielectric substrate, and in which the antenna is coupled through the dielectric substrate to the feed element through reactive near field electromagnetic coupling, e.g. capacitive coupling. Alternatively, the assembly may comprise one or more electrically conductive vias that extend through the dielectric substrate to electrically connect the antenna. An arrangement that uses electromagnetic coupling is preferred as it avoids deformation of the mirror surface resulting from differing coefficients of thermal expansion between the via material and the substrate. It is also simpler to manufacture and provides a more mechanically robust mirror structure. The antenna may comprise a transmission line on back of substrate - could include connector if mirror assembly not integrated with receiver or transmitter system. The portion of the electrically conductive material layer that provides, at least in part, the antenna element, may be electrically isolated from the remainder of the electrically conductive material layer, e.g. by one or more trenches that extend entirely through the electrically conductive material layer, e.g. from the mirror surface to the substrate. In one arrangement the system may comprise multiple antenna elements each provided by a different region or portion of the electrically conductive material, each of the different regions or portions being electrically isolated from one another. For example, the mirror assembly may comprise an array of antenna elements, each antenna element comprised from a different region of the electrically conductive material that is electrically isolated from the other regions of the electrically conductive material; each antenna element is electrically coupled to a separate electrical connector for connection to the radio frequency RF transmitter and / or RF receiver. Each of the different regions of the electrically conductive material may be electrically isolated from the other regions of the electrically conductive material by the one or more trenches. The mirror surface may be planar, concave or convex as required by application of the system. A lithographic etching process may be used to manufacture the one or more trenches. The electrically conductive layer will typically be thinner (have less depth) than the substrate that it is carried on. Typically electrically conductive layer will thinner than the substrate by a factor of at least ten. The electro-optical transmitter and / or receiver may comprise one or more of: a laser or optical sensor, which may be a sensor array. The electro-optical transmitter and / or receiver could operate in any one or more of IR, visible and UV bands. The electro-optical transmitter and / or receiver could be used to provide the function of, for example, passive imaging, optical free space communications, lidar, laser designation. The RF transmitter and / or receiver could be used to provide the function of, for example, RF communication and radar. The invention will now be described by way of example with reference to the following Figures in which: Figure 1 is a schematic of an electro-optical system comprising a dual function mirrorantenna assembly; Figure 2 is a side view cross-section schematic of the mirror-antenna assembly; Figure 3 is a front view of the mirror-antenna assembly; Figure 4 is a side view schematic of a variant embodiment of the mirror-antenna assembly including vias to electrically connect the antenna elements through the substrate; Figure 5 is a schematic of a first variant arrangement of electro-optical system in which the optical sensor / transmitter is positioned directly in front of the concave mirror surface of the mirror-antenna assembly; Figure 6 is a schematic of a second variant arrangement of electro-optical system in which the optical sensor / transmitter is positioned directly behind the mirror surface; Figure 7 is a schematic of a third variant arrangement electro-optical system in which the mirror-antenna assembly has a convex mirror surface; Figure 8A is a schematic front view of a variant mirror-antenna assembly that provides a dual-arm spiral antenna elements; and Figure 8B is a side view schematic of the mirror-antenna assembly of Fig 8A. Figure 1 is a schematic of a combined optical-RF system 1. The RF system may be, for example, a radar system or a RF communication system. The optical system could be used for the purposes of one or more of: active or passive imaging, optical free space communication, lidar, laser designation (transmit or receive). For the purposes of this specification, the term RF should be taken to include frequencies within the micro wave spectrum. The optical sensor and / or optical transmitter 3 may be configured to sense and / or transmit one or more of infra-red, visible and ultraviolet light. The system 1 comprises an aperture 2; an optical sensor and / or optical transmitter 3 (e.g. one or more photo diode detectors and / or laser) hereafter referred to as an optical transducer; and RF transmitter and / or RF receiver 4, in this example a RF transceiver; a dual function mirror-antenna device 5; a further mirror 6; and a first processor 7 configured to carry out the functions of a controller and / or digital signal processor for the optical system and a second processor 8 to carry out the functions of a controller and digital signal processor for the RF system. Advantageously, the dual function mirror-antenna device 5 enables the system 1 to transmit and / or receive optical radiation and RF signal simultaneously through the aperture 2, which is common to both the optical and RF. The mirror-antenna device 5 comprises a planar optically reflective (mirror) surface 5A, forming part of an optical train of the system 1, which together with mirror 6, guides light between the aperture 2 and the optical transducer 3. It will be appreciated that through suitable positioning of the mirror antenna device 5 and optical transducer 3, the mirror 6 may be omitted. The mirror-antenna device 5 also comprises an array of patch antennas connected to the transmitter and / or receiver 4 for transmitting and / or receiving RF signals through the aperture 2. With reference also to Figs 2 and 3, which are not to scale, the mirror-antenna device 5 comprises an optically-reflective layer 50 which provides the optically reflective surface 5A. The optically-reflective layer 50 is provided on a first side 51A of a dielectric substrate 51. Non-limiting examples of suitable material for the dielectric substrate 51 include fused silica and sapphire. The optically-reflective layer 50 is comprised from an electrically conductive material, typically a metal. Non-limiting examples of suitable materials include one or more of copper, aluminium, silver and gold; gold is typically preferred. Non-limiting example methods of forming the optically reflective layer 50 onto the substrate 51 include chemical deposition, vacuum deposition - such as chemical vapour deposition, and spluttering. The first side 51A of the substrate 51 is ground and polished (and / or diamond turned) prior to the deposition of the optically-reflective layer 50 to provide the desire surface geometry and surface roughness requirement for the optically reflective surface 5A. A pattern of trenches 52 are formed through the optically-reflective layer 50 to define a three-by-three array of antenna patches 53, see Fig 3. Each antenna patch 53 is comprised from a different region of the optically reflective layer 50. Each patch 53 is separated from the others by an intermediate region 50A of the optically-reflective layer 50. The trenches 52 may be formed, for example, using conventional mask and etch processes. To ensure electrical isolation of each patch 53 from the others and the intermediate region 50A, the trenches 52 extend entirely through the optically-reflective layer 50, optionally also into the substrate 51. The lateral spacing between the antenna patches 53 is selected based on the expected operating RF band of the system, as will be familiar to those skilled in the art of array antenna design. The lateral width of the trenches 52 is selected to minimise loss of reflective area of the mirror surface 5 A whilst ensuring electrical isolation of the antenna patches 53 from the remainder of the optically conductive layer 50. The substrate 51 has a second side 5 IB which faces an opposite direction to the first side 51A. Provided on the second side 5 IB are a set of feed patches 54 arranged in a three-by-three array. The array of feed patches 54 aligns with the array of antenna patches 53 such that each feed patch 54 lies directly opposite a different antenna patch 53. Also provided on the second side 5 OB of the substrate 50 is a metallic layer 55 serving as a ground plane for the antenna array. Electrical isolation between the feed patches 54 and the ground plane 55 is provided by a dielectric layer 56. Behind the ground plane 55 is provided a printed circuit board 57 providing a combining / dividing network of the antenna array having a common port 57A connected to transmitter and / or receiver 4. Each feed patch 54 is separately connected to the combining / dividing network by vias 58 extending through the insulating layer 56 and ground plane 55. The thickness D of the substrate 51 is selected to place the antenna patch array and feed patch array in sufficiently close proximity to allow for reactive near field electromagnetic coupling, (sometimes referred to as proximity coupling) e,g. capacitive coupling, between each antenna patch 53 and the feed patch 54 lying directly opposite it on the second side 5 IB of the substrate 50. As such, the generation of a current through either the feed patch 54 or antenna patch 5 3 will induce a corresponding current in the other. Thus the generation of a current through a feed patch 54 from a RF transmitter will induce a corresponding current in its corresponding antenna patch 53 which is in turn propagated into space. Inversely, a current induced in an antenna patch 53 by a RF signal that has passed through the aperture 2, will induce a corresponding current in the feed patch 45 for receipt by a RF receiver. The feed patches 54 may be provided by a patterned metallic layer formed on the substrate 51 using any suitable conventional technique. Alternatively, the feed patches 54 may be formed on the insulating layer 56 and positioned against the second side 5 IB of the substrate 51. In a further variant, the ground plane may be interposed between the antenna patches 53 and feed patches 54. Where so, an array of apertures may be provided in the ground plane that are in register with the array of antenna patches and feed patches to allow electromagnetic coupling to occur through the ground plane between the antenna patches 53 and feed patches 54. Figure 4 is a side cross-section schematic of a variant layer construction for the mirrorantenna device 5. The primary difference is the omission of the feed patches 54 and insulator layer 56, and instead the provision of vias 60 that extend entirely through the substrate 50 and ground plane 55 to electrically interconnect each antenna patch 53 directly to the combining / dividing network of the printed circuit board 57. The following applies to either of the examples above. It will be appreciated that the system 1 may comprise a mechanical system for steering the mirror-antenna device 5 to alter the look direction (azimuth and / or elevation). The trenches 52 may be filled with a dielectric material. In variant embodiments, the arrays may be of sizes other than three-by-three. Although the antenna preferably comprises an array of antenna patches, it may comprise only a single patch element. Where so, depending on the size and form of the mirror, the reflective coating may need not be subdivided by trenches or equivalent. Figures 5-7 illustrate variant systems illustrating mirror devices substantially similar to that described above but with non-planar mirror surfaces facilitating incorporation into different designs of optical train. In the system of Fig 5, the optical transducer 3 is located between common aperture 2 and the mirror-antenna device 501. Mirror 6 is omitted. The mirror-antenna device 501 has a concave mirror surface 501A to reflect light between the aperture 2 and an aperture 3A of the optical transducer 3 facing the mirror surface 501 A. In the system of Fig 6, the optical transducer 3 is located directly behind the mirrorantenna 502, and the optical train includes a concave mirror 60 located directly between the aperture 2 and 502 and the aperture 2, in front of the mirror surface 502A to direct light between the mirror antenna device 502 and optical transducer 3. The mirrorantenna 502 has a concave mirror surface 5 02A and in addition has a central through aperture 502B (mirror-antenna 502 shown in cross-section to illustrate aperture 502B) extending between front and rear sides through which light can pass between the optical transducer 3 and a further mirror 60 of the mirror-antenna 502. In the system of Fig 7 the mirror-antenna 503 has a convex mirror surface 5 03A that faces away from the aperture 2. The system further comprises a dish 600 that performs the function of a parabolic reflector for both optical and RF signals between the aperture 2 and mirror antenna 503. Figures 8A and 8B illustrate an example application of the invention to a spiral antenna. Such antenna have good broadband performance. As such this arrangement suits an application in which the RF system is a broad band RF system adapted to receive and / or transmitted RF signals across a RF band extending over at least three octaves. The mirror-antenna device 504 comprises an optically-reflective layer 550 which provides the optically reflective surface 504A. The optically-reflective layer 550 is provided on a first side 510A of a dielectric substrate 510. Non-limiting examples of suitable material for the dielectric substrate 510 include fused silica and sapphire. The optically-reflective layer 550 is comprised from an electrically conductive material, typically a metal. Non-limiting examples of suitable materials include one or more of copper, aluminium, silver and gold; again gold is preferred. Non-limiting example methods of forming the optically reflective layer 550 onto the substrate 551 include chemical deposition, vacuum deposition - such as chemical vapour deposition, and spluttering. The first side 510A of the substrate 510 is ground and polished (and / or diamond turned) prior to the deposition of the optically-reflective layer 550 to provide the desire surface geometry and surface roughness and improve the reflective characteristics of the optically reflective surface 504A. A spiral pattern of trenches 520 are formed through the optically-reflective layer 550 to define two spiral antenna arms 530A 530B. Each antenna arm 530A 530B is comprised from a different region of the optically reflective layer 50. The trenches 520 may be formed, for example, using conventional mask and etch processes. To ensure electrical isolation of each arm 530A 530B from the other, the trenches 520 extend entirely through the optically-reflective layer 550, optionally also into the substrate 510. The lateral width of the trenches 520 is selected to minimise loss of reflective area of the mirror surface 504A whilst ensuring electrical isolation of the antenna arms 530 from the remainder of the optically conductive layer 550. The substrate 510 has a second side 510B which faces an opposite direction to the first side 510A. Provided on the second side 510B isametallic layer551 serving as a ground plane for the antenna. The thickness D of the substrate 510 is selected to provide the desired separation distance between the antenna arms 530 and the ground plane 551. Extending through the dielectric substrate 510 between the first and second sides 510A 510B are two vias 560 that act as an antenna feed, interconnecting each antenna arm 530 to a balun 570 and thence the transmitter and / or receiver 4. Although the example of Figs 8A and 8B is of an Archimedean spiral, the invention could be readily applied to a logarithmic, or other form of spiral. Similarly, it will be readily appreciated to the person skilled in the art that the invention could also be applied to antenna with sinuous antenna arms (see for example that described in US4658262A), logarithmic or otherwise.

Claims

1. A mirror assembly for a combined Radio Frequency (RF) electro-optical system; the mirror assembly having a mirror surface configurable to redirect optical radiation to and / or from an electro-optical device of the system; the mirror assembly comprising a substrate having a first side and a second side that face opposite directions; the first side carries an electrically conductive material layer provides the mirror surface; and in which the assembly comprises multiple antenna elements each electrically couplable to a radio frequency (RF) transmitter and / or RF receiver of the system; each antenna element comprised from a different region of the electrically conductive material layer that is electrically isolated from the other regions of the electrically conductive material layer, and each of the regions providing a portion of the mirror surface.

2. A mirror assembly according to claim 1 further comprising means to carry RF signals through the dielectric substrate between the first and second sides to electrically couple each antenna element to the radio frequency (RF) transmitter and / or RF receiver.

3. A mirror assembly according to claim 2 wherein the means to carry RF signals through the dielectric substrate comprises mulitple electrically conductive feed elements positioned on the second side of the dielectric substrate, and in which each antenna element, when the assembly is in operation, is coupled through the dielectric substrate to a different one of the feed element through reactive near field electromagnetic coupling in order to transfer the RF signals through the dielectric substrate.

4. A mirror assembly according to claim 2 comprising multiple electrically conductive vias extending through the dielectric substrate between the first side and second side to electrically connect each antenna element to the radio frequency (RF) transmitter and / or RF receiver of the system.

5. A mirror assembly according to any previous claim wherein the regions of the electrically conductive material layer are each electrically isolated from the remainder of the electrically conductive material layer by one or more trenches that extend entirely through the electrically conductive material layer.

6. A mirror assembly according to any previous claim wherein each antenna element is a spiral or sinuous antenna element.

7. A combined RF electro-optical system comprising:an electro-optical transmitter and / or receiver;a radio frequency (RF) receiver and / or RF transmitter; anda mirror assembly comprising a mirror surface configurable to redirect optical radiation to and / or from an electro-optical transmitter and / or receiver;the mirror assembly of any previous claim, and in which each of the antenna elements are, when the system is in operation, electrically coupled to the radio frequency (RF) transmitter and / or RF receiver.

8. A combined RF electro-optical system comprising:an electro-optical transmitter and / or receiver;a radio frequency (RF) receiver and / or RF transmitter; anda mirror assembly comprising a mirror surface configurable to redirect optical radiation to and / or from an electro-optical transmitter and / or receiver; the mirror assembly comprising an electrically conductive material providing the mirror surface; and in which at least a portion of the electrically conductivematerial providing at least a portion of the mirror surface provides, at least in part, an antenna element which is electrically coupled to the radio frequency (RF) transmitter and / or RF receiver.

9. A mirror assembly for a combined Radio Frequency (RF) electro-optical5 system;the mirror assembly having a mirror surface configurable to redirect optical radiation to and / or from an electro-optical device of the system; the mirror assembly comprising an electrically conductive material providing the mirror surface; and in which at least a portion of the electrically conductive material10 providing at least a portion of the mirror surface provides, at least in part, anantenna element electrically couplable to a radio frequency (RF) transmitter and / or RF receiver of the system.A

Images