Fexible optical backplane

Abstract: An optical backplane is formed by defining channels (grooves) in the plane of a polyimide layer (55), laying optical fibres (56,57) in each channels, retaining the fibres (56,57) using an adhesive and bonding an epoxy resin layer (58) to the polyimide layer (55) to form a flexible structure (54). The flexible structure (54) is made rigid by bonding epoxy glass FR4 layers (60,61) to either side of the flexible structure (54). A flexible ribbon (53) with an optical fibre (57) contained therein is formed as an integral portion of the flexible structure (54) using spacers (59,62) to define a ribbon region and to aid removal of the rigid layers (60,61) in the ribbon region.

Description: This invention relates to an optical backplane for the routing and protection of optical fibres. An optical backplane comprises at least one optical fibre supported by a layer of material. Advanced communication networks require the use of optical fibre technology for the purpose of providing high band width and multiplexing capability and their installation is proceeding rapidly, worldwide. Their use for terrestrial based networks is becoming commonplace and there are also increasing requirements for optical networks on mobile platforms for both civil and military use. Such networks require optical fibre and component management at the exchange/rack level and technologies are evolving to address this market. EP-A-0511779 describes an optical backplane provided with a series of spaced apart grooves of the same depth each containing a number of optical fibres at different depth locations within the groove. Crossover of fibres from one groove to the next takes place by arranging the crossover fibres at different depths in the respective grooves. JP-62123406-A discloses an optical wiring board which is grooved to provide trackways in which one or more optical fibres may be housed below the surface level of the board. It is an object of this invention to provide an optical backplane for the routing and protection of optical fibres and to also provide a method of producing such an optical backplane. According to one aspect of the present invention, there is provided an optical backplane having a first layer of material, a stop layer disposed on the first layer, a second layer diposed on the stop layer, at least two channels defined within the plane of the first layer and at least one optical fibre located within at least one of the channels, wherein at least two of the channels intersect in a crossover region and one of these channels is formed deeper at the crossover point extending into the second layer, so as to permit the optical fibre located in that channel to cross underneath an optical fibre located within the other channel. Preferably each channel may have been formed by ablation of the material and a base for at least one of the channels may be defined by the stop layer. Each optical fibre may be laid in its channel and may be retained in position using an adhesive. Preferably a bend in at least one channel may have a radius greater than 10mm. Conveniently the two channels may intersect substantially at right angles in the crossover region. The channels may be locally widened at the crossover region and the locally widened portion of the channels may be filled with a set resin. Preferably a base of the deeper formed channel may be defined by a stop layer. A further layer of material may be bonded to the first layer to enclose each optical fibre. The further layer of material may be formed from a cured resin which extends into each channel to enclose each optical fibre. Alternatively the material may be formed from polyimide. Preferably the layers form a flexible structure. A rigid layer may be bonded to one side of the flexible structure to provide a rigid structure. A further rigid layer may be bonded to the reverse side of the flexible structure. These rigid layers may be preferably formed from epoxy glass resin. Preferably the layers may be bonded by an epoxy resin. Preferably the layers have at least one aperture containing an optical component connected to at least one of the optical fibres and the layers may have at least one aperture containing an electro-optical component connected to at least one of the optical fibres. Each aperture may be filled with a set resin. A printed circuit may be carried by a surface of at least one of the layers and at least one of the layers may have at least one aperture containing an electronic component connected to the printed circuit. Each aperture may be filled with a set resin. Preferably the flexible structure extends beyond the rigid layer to define a flexible optical ribbon. The flexible optical ribbon may terminate in an optical connector, and at least one optical fibre may be located in a communal channel immediately adjacent the optical connector. Each optical fibre may be located in a laterally spaced individual channel which converges into the communal channel. According to another aspect of the present invention, there is provided a method of manufacturing an optical backplane including forming at least two channels within the plane of a first layer of material, disposing a stop layer on the first layer, disposing a second layer on the stop layer, and locating at least one optical fibre within at least one of the channels, including defining a crossover region for at least two of the channels by forming one of these channels deeper at the crossover point and extending into the second layer so as to permit the optical fibre located in that channel to cross underneath an optical fibre located within the other channel. This method may include forming at least one of the channels by ablating the first layers of material and may include using a mask superimposed over the layer to expose only those areas of the first layers of material that are to be ablated. Preferably this method may include using an Excimer laser to ablate the first layer of material. It may also include using a stop layer to define a base for at least one of the channels. This method may include laying the optical fibre in its channel and applying an adhesive between each optical fibre and its channel. This method may include forming at least one channel with a bend having a radius greater than 10mm. This method may include forming channels such that they are locally wider in the crossover region and may include filling the locally widened portion of the channels with a setting resin. Preferably this method may include bonding a further layer of material to the layer to enclose each optical fibre. In this method, the layers form a flexible structure, and this method may include bonding a rigid layer to one side of the flexible structure and also may include bonding a further rigid layer to the reverse side of the flexible structure. Preferably this method may include introducing an epoxy resin between layers and bonding them together under pressure over the surface area of the layers. This method may include forming at least one aperture in at least one of the layers of material, positioning an optical component within the aperture and connecting that component to at least one of the optical fibres. It may also include forming at least one aperture in at least one of the layers of material, positioning an electro-optical component within the aperture and connecting that component to at least one of the optical fibres. Preferably this method may include forming a printed circuit on a surface of at least one of the layers of material and may include forming at least one aperture in at least one of the layers of material, positioning an electronic component within the aperture and connecting that component to the printed circuit. Preferably this method may also include spacing at least one rigid layer from the flexible structure to which it is to bond with spacer material in specified regions and using a bond inhibiting agent to prevent bonding of the spacer material to either the rigid layers or to the flexible structure, and then removing that portion of the rigid layer in the specified regions to expose those portions of the flexible structure. It may also include defining each channel in the exposed portions of the flexible structure such that the pitch between optical fibres enables interconnection with a multi way ribbon connector. The invention is now described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates in cross sectional view part of a first embodiment of a flexible optical backplane; Figure 2 illustrates in cross sectional view part of a second embodiment of a flexible optical backplane; Figure 3 illustrates in cross sectional view a rigid optical backplane according to the present invention; Figure 4 illustrates in cross sectional view the formation of a channel within a layer of polyimide using a stop layer; Figure 5 illustrates, in cross section view the formation of an optical fibre crossover region of a backplate according to the present invention, within a layer of polyimide using a further stop layer; Figure 6 illustrates in plan view an enlarged channel localised to the crossover region; Figure 7 illustrates in cross sectional diagrammatic view a technique for the calculation of the minimum radius for an optical fibre to bend under another optical fibre in the crossover region; Figure 8 illustrates in cross sectional view the formation of a ribbon enclosing an optical fibre; Figure 9 illustrates in cross sectional view, normal to the longitudinal axis of the ribbon, the optical fibres in position for connection with an optical fibre interconnector; Figure 10 illustrates in plan view the layout of optical fibres within the plane of the optical backplane, and Fibre 11 illustrates in plan view the layout of channels within the plane of the polyimide layer. In Figure 1, there is illustrated part of a first embodiment of the present invention including a first layer 10 formed from a polyimide material with a channel 11 defined in the polyimide with the plane of the layer 10 and an optical fibre 12 located within the channel 11. The defining of the channel 11 will be described with reference to Figures 4 and 5 below. A further layer 13 also formed of polyimide material is bonded to layer 10 with epoxy resin 14 and pressure applied over the plane of the layers 10 and 13, in this manner a flexible structure 15 is constructed. An alternative to the construction described with reference to Figure 1 is illustrated in Figure 2 in which is shown in part a second embodiment comprising a first layer 16 formed from a polyimide material with a channel 17 defined in the polyimide with the plane of the layer 16 and optical on fibre 18 located within the channel 17. The defining of the channel 17 will be described with reference to Figures 4 and 5 below. A further layer 19 formed of a curing type material, such as a low temperature (85°C) epoxy resin, is applied under pressure over the plane of the layer 16, in this manner an alternative flexible structure 20 is constructed. It will be appreciated that the flexible structure 20 when described below could be readily replaced by the flexible structure 15 and that the description will only refer to the flexible structure 20 for clarity. When the optical backplane is to be used in conjunction with the passive and/or active components the flexible structures 15 to 20 are made rigid to provide a stable base to which to attach the components. In Figure 3, there is illustrated a flexible structure 20, wherein the flexible structure 20 is bonded to a layer 21 of epoxy glass FR4 on one side of the plane of the flexible structure 20 with an epoxy resin. Bonding of the layer 21 to the flexible structure 20 provides rigidity to the flexible structure 20. Further rigidity for the flexible structure 20 is provided by bonding a further layer 22 of epoxy glass FR4 to the opposite side of the flexible structure 20 with epoxy resin. In this manner a rigid structure 23 is constructed. Passive or active components can be secured to the structure 23 and/or interconnected with optical fibres and optical components within the optical backplane in accordance with conventional printed circuit board and optical connection techniques. The external surfaces of the layers 21 and 22 or alternatively the internal surfaces of the layers 21 and 22 can be machined to provide areas to accommodate these components on the external surface of the layers 21 and 22 or between the layers 21 and 22 and the flexible structure 20. Apertures can be provided between and/or in layers to enable connection of these components to the optical fibres. In the case of active components power is provided using conventional printed circuit board technology. In Figure 4, a channel 24 is formed in a polyimide layer 25 using an Excimer laser in accordance with one of the two following methods. A polyimide layer having a copper mask 26 transposed thereon is exposed to an Excimer laser passing over exposed portions provided by the copper mask 26, in either a time based or number of passes fashion, to ablated the polyimide and form the channel. Alternatively, as illustrated in Figure 4, the channel 24 is defined in the polyimide layer 25 by transposing a copper mask 26 on the surface of the polyimide layer 25 and exposing the etched portions to an Excimer laser to ablated the polyimide layer 25 and form the channel 24. The base 27 of the channel 24 is defined by a copper stop layer 28 which prevents the Excimer laser machining further. When a channel 24 with greater depth in localised areas is required, further ablation with the Excimer laser will remove the stop layer 28 to allow the Excimer laser to penetrate into a further polyimide layer 29 thereby providing greater depth. Greater depth is required for a crossover region 30 as illustrated in Figure 5. Here a first optical fibre 31 is required to pass over a second optical fibre 32 so a greater depth is provided at the crossover point by ablating a first layer of polyimide 33 having a copper mask 34 with an Excimer laser. Ablation with the laser is halted by a first copper stop layer 35. The first process of ablation provides channels 36 for optical fibre 37 and optical fibres 31 and 32, with the exclusion of the crossover region 30, in the polyimide layer 33. In the crossover region 30 the stop layer 35 is further exposed to the Excimer laser until it is burnt away and ablation of a second layer of polyimide 38 allows the necessary depth to provide the crossover region 30 for optical fibres 31 and 32. The base of the crossover region 30 is defined by a second copper stop layer 39 to prevent penetration into a further polyimide layer 40. It will be noted that ablation can be achieved using any form of heat generating means. In Figure 6, two crossover regions 41 are illustrated for optical fibres 42, 43 and 44 in defined channels 45, 46 and 47 in a polyimide layer 48. In the crossover regions 41 the channels 45 and 46 are enlarged to allow greater flow of epoxy resin around and under the optical fibres 42 and 43. Where many optical fibres crossover in close proximity a large communal expanse can be provided for filling with epoxy resin. In Figure 7, the bending of an optical fibre 49 at a crossover region to pass under another optical fibre 50 must not exceed a minimum recommended bend radius, for this application the bend radius of the optical fibre 49 should be great than or equal to 10 millimetres. With reference to Figure 7, the theoretical bend radius can be calculated using the formula, r = x 2 + y 2 2y wherein, r is the radius in millimetres and the dimensions x and y are the bend distances of the core of the optical fibre 49 underneath optical fibre 50 and the distance between the core of the optical fibre 49 from the optical fibre 50, respectively For example, given that the length of a crossover 51 is 7.3 millimetres and that a core bend deflection 52 is 0.15 millimetres then, x= 7.3 4 = 1.825 millimetres y= 0.15 2 = 0.075 millimetres r= 1.825 2 + 0.075 2 2 x 0.075 = 22 2 millimetres In this case the bend radius r is greater than the bend radius minimum of 10 millimetres and therefore acceptable. A further design rule, to reduce the cross talk and loss is to only intersect the optical fibres at right angles relative to one another. The flexible structures 15 or 20 are also used to form flexible ribbons for interconnection with other optical backplanes and/or optical components. In Figure 8, a ribbon 53 is formed as an integral part of a flexible structure 54 by forming a rigid structure from a layer of polyimide 55 having defining channels for optical fibres 56 and 57, an epoxy resin layer 58 bonded to the polyimide 55. As the ribbon 53 will when formed be thinner than the flexible structure 54 it is necessary to machine the polyimide layer 58 to provide an area to retain a spacer 59. Machining can be by way of ablation with Excimer laser of the polyimide layer 55. Spacer 59 is coated with a layer of material which inhibits adhesion to the polyimide layer 55. The flexible structure 54 is made rigid by bonding rigid layers 60 and 61, formed of epoxy glass FR4, to either surface of the flexible structure 54. Rigid layer 61 is also machined to co-operate with spacer 59. The area machined into the rigid layer 61 will not bond to the spacer 59 as the spacer has been coated in a material which inhibits adhesion to the rigid layer 61. The rigid layer 60 is also machined to provide an area to retain a spacer 62 between the machined area and the epoxy resin layer 58. The spacer 62 is also coated with a layer of material which inhibits adhesion to either the epoxy resin 58 on the rigid layer 60. The flexible structure 54 is bonded between the rigid layers 61 and 62 and pressure applied to the structure to aid correct bonding. When curing of the bond is complete a router machine bit 63 is used to cut through the rigid layers 60 and 61 in the area of the spacers 59 and 62. The cut areas will drop away with the spacers 59 and 62 leaving the flexible structure 54 in this region undamaged by the process. There is now provided the ribbon 53 with an optical fibre 57 therein. A number of optical fibres can be enclosed in , a single ribbon, as illustrated in Figure 9. The optical fibres 64 are laid in a communal channel 65 defined in a polyimide layer 66 and bonded to an epoxy resin layer 67. The pitch 68 between each core of adjacent optical fibres 64 depends on the optimum pitch required for interconnection with an optical connector. In this case the pitch of the optical fibres 64 are required to be 250 micrometers for the highest possible yield for connection. Since the optical fibres 64 are 250 micrometers in diameter they are laid abutting one another in the communal channel 65. When the optical fibres are laid in their respective channels as defined in the polyimide layer, the optical fibres can be retained, at least temporarily, in the channels using a rubber based adhesive such as cyanacrylate. Figure 10 illustrates the entire optical backplane with flexible ribbons 69 extending from the rigid backplane 70 and the exposed optical fibres 71 for interconnection with optical connectors. A number of fixing apertures 72 are also shown for securing the optical backplane to a racking system (not shown) Figure 10 illustrates in plan defining channels 73 in a polyimide layer.

Claims: An optical backplane having a first layer of material (10, 16, 25, 33, 38, 48), a stop layer (28, 35) disposed on the first layer, a second layer (29, 38) disposed on the stop layer (28, 35), at least two channels (11, 17, 24, 36, 45, 46, 47) defined within the plane of the first layer (10, 16, 25, 33, 48) and at least one optical fibre (12, 18, 31, 32, 37, 42, 43, 44) located within at least one of the channels (11, 17, 24, 36, 45, 46, 47), wherein at least two of the channels (46, 47) intersect in a crossover region (30, 41) and one of these channels (46) is formed deeper at the crossover point extending into the second layer (29, 38) so as to permit the optical fibre (43) located in that channel (46) to cross underneath an optical fibre (44) located within the other channel (47). An optical backplane, as claimed in Claim 1, having a base (27) for at least one of the channels (24), which base (27) is defined by the stop layer (28). An optical backplane, as claimed in any preceding claim, in which optical fibre (12) is laid in its channel (11) and retained in position with an adhesive. An optical backplane, as claimed in any preceding claim, in which a bend in at least one channel (11, 17, 24, 36, 45, 46, 47) has a radius greater than 10mm. An optical backplane, as claimed in any one of Claims 1 to 4, in which the two channels (46, 47) intersect substantially at right angles in the crossover region (41). An optical backplane, as claimed in any one of Claims 1 to 5, in which the channels (46, 47) are locally widened at the crossover region. An optical backplane, as claimed in Claim 6, in which the locally widened portion of the channels (46, 47) is filled with a set resin. An optical backplane, as claimed in any one of Claims 1 to 7, in which a base for the deeper formed channel in the crossover region (30) is defined by a stop layer (39). An optical backplane, as claimed in any preceding claim, including a further layer of material (19) bonded to the first layer (10) to enclose the optical fibre (12). An optical backplane, as claimed in Claim 9, in which the further layer of material (19) is formed from a cured resin which extends into the channel (11) to enclose each optical fibre (12). An optical backplane, as claimed in Claim 9 or Claim 10, in which the material (10, 19) is formed from polyimide. An optical backplane, as claimed in any one of Claims 9 to 11, in which layers (10, 19) form a flexible structure (20). An optical backplane, as claimed in Claim 12, including a rigid layer (21) bonded to one side of the flexible structure (20) to provide a rigid structure. An optical backplane, as claimed in Claim 13, including a further rigid layer (22) bonded to the reverse side of the flexible structure (20). An optical backplane, as claimed in Claim 13 or Claim 14 in which the rigid layers (21, 22) are formed from epoxy glass resin. An optical backplane, as claimed in any one of Claims 12 to 15, in which the layers (10, 19, 21, 22) are bonded by an epoxy resin. An optical backplane, as claimed in any of Claims 13 to 16, in which the layers (10, 19, 21, 22) have at least one aperture containing an optical component connected to at least one of the optical fibres (12). An optical backplane, as claimed in any one of Claims 13 to 17, in which the layers (10, 19, 21, 22) have at least one aperture containing an electro-optical component connected to at least one of the optical fibres (12). An optical backplane, as claimed in Claim 17 or Claim 18, in which each aperture is filled with a set resin. An optical backplane, as claimed in any one of Claims 13 to 19, including a printed circuit carried by a surface of at least one of the layers (10, 19, 21, 22). An optical backplane, as claimed in Claim 20, in which at least one of the layers (10, 19, 21, 22) has at least one aperture containing a component connected to the printed circuit. An optical backplane, as claimed in Claim 21, in which each aperture is filled with a set resin. An optical backplane, as claimed in any one of Claims 14 to 22, in which the flexible structure (20) extends beyond the rigid layer (21, 22) to define a flexible optical ribbon (69). An optical backplane, as claimed in Claim 23, in which the flexible optical ribbon (69) terminates in an optical connector, and at least one optical fibre (64) is located in a communal channel (65) immediately adjacent the optical connector. An optical backplane, as claimed in Claim 24, in which the optical fibre (64) is located in laterally spaced individual channels which converge into the communal channel (65). A method of manufacturing an optical backplane including forming at least two channels (11, 17, 24, 36, 45, 46, 47) within the plane of a first layer of material (10, 16, 25, 33, 38, 48) disposing a stop layer (28, 35) on the first layer, disposing a second layer (29, 38) on the stop layer (28. 35), and locating at least one optical fibre (12, 18, 31, 32, 37, 42, 43, 44) within at least one of the channels (11, 17, 24, 36, 45, 46, 47), including defining a crossover region (30, 41) for at least two of the channels (46, 47) by forming one of these channels (46) deeper at the crossover point and extending into the second layer (29, 38) so as to permit the optical fibre (43) located in that channel (46) to cross underneath an optical fibre (44) located within the other channel (47). A method, as claimed in Claim 26, including forming at least one of the channels (24) by ablating the first layer of material (25). A method, as claimed in Claim 27, including using a mask (26) superimposed over the layer (25) to expose only those areas of the first layer of material (25) that are to be ablated. A method, as claimed in Claim 27 or Claim 28, including using an Excimer laser to ablate the first layer of material (25). A method, as claimed in any one of Claims 27 to 29, including using a stop layer (28) to define a base (27) of at least one of the channels (24). A method, as claimed in any one of Claims 26 to 30, including laying each optical fibre (12) in its channel (11) and applying an adhesive between each optical fibre (12) and its channel (11). A method, as claimed in any one of Claims 26 to 31, including forming a bend in at least one channel (11, 17, 24, 36, 45, 46, 47) with a radius greater than 10mm. A method, as claimed in any one of Claims 26 to 32, including forming channels (45, 46) such that they are locally wider in the crossover region (41). A method, as claimed in Claim 33, including filling the locally widened portion of the channels (45, 46) with a setting resin. A method, as claimed in any one of Claims 26 to 34, including bonding a further layer of material (19) to the layer (10) to enclose each optical fibre (12). A method, as claimed in Claim 35, in which the layers (10, 19) form a flexible structure (20), including bonding a rigid layer (21) to one side of the flexible structure (20). A method, as claimed in Claim 36, including bonding a further rigid layer (22) to the reverse side of the flexible structure (20). A method, as claimed in any one of Claims 35 to 37, including introducing an epoxy resin between layers (10, 19, 21, 22) and bonding them to together under pressure over a surface area of the layers (10, 19, 21, 22). A method, as claimed in Claims 36 or 37, including forming at least one aperture in at least one of the layers of material (10, 19, 21, 22), positioning an optical component within the aperture and connecting that component to at least one of the optical fibres (12). A method, as claimed in any one of Claims 36 to 39, including forming at least one aperture in at least one of the layers of material (10, 19, 21, 22), positioning an electro-optical component within the aperture and connecting that component to at least one of the optical fibres (12). A method, as claimed in any one of Claims 36 to 40, including forming a printed circuit on a surface of at least one of the layers of material (10, 19, 21, 22). A method, as claimed in Claim 41, including forming at least one aperture in at least one of the layers of material (10, 19, 21, 22), positioning a component within the aperture and connecting that component to the printed circuit. A method, as claimed in any one of Claims 36 to 42, including spacing at least one rigid layer (60, 61) from the flexible structure (54) to which it is to bond with spacer material (59, 62) in specified regions and using a bond inhibiting agent to prevent bonding of the spacer material (59, 62) to either the rigid layer (60, 61) or to the flexible structure (54), and then removing that portion of the rigid layer (60, 61) in the specified regions to expose those portions of the flexible structure (53). A method, as in Claim 43, including defining each channel in the exposed portions of the flexible structure (53) such that the pitch between optical fibres enables interconnection with a multi way ribbon connector.

Inventors: MOSS CLAIRE JULIA, MURPHY MICHAEL MATTHEW DR, SHARP PAUL TIMOTHY, TYLER STEVE GEOFFREY, MOSS, CLAIRE JULIA, MURPHY, MICHAEL MATTHEW, DR., SHARP, PAUL TIMOTHY, TYLER, STEVE GEOFFREY
Applicants: BAE SYS ELECTRONICS LTD, BAE SYSTEMS ELECTRONICS LTD.
Application year: 1997
Main classification: G02B6
IPC codes: G02B6