WO2004010191A1

WO2004010191A1 - Connection to optical backplane

Info

Publication number: WO2004010191A1
Application number: PCT/GB2003/003155
Authority: WO
Inventors: John Michael Goward; Frank Alan Paul Tooley
Original assignee: Terahertz Photonics Ltd
Priority date: 2002-07-23
Filing date: 2003-07-23
Publication date: 2004-01-29
Also published as: AU2003254472A1; GB0217066D0

Abstract

An optical connector (10) including an optical element (16) having a main body portion that has a plurality of waveguides (24), each waveguide (24) including an angled reflector. The element is supported in a holder (18) that can be used to secure the element (16) to another part (12). When the connector is connected to the other part, the arrangement is such that the waveguides in the element (16) are orthogonal to channels, for example waveguides, in the other part and light can be directed along each waveguide (24) onto its angled reflector and from there reflected into the optical channels (15) in or on the other part (12).

Description

CONNECTION O OPTICAL BACKPLANE

Technical field

The present invention relates to a system and method for coupling light between optical components, such as waveguides, on a board or substrate to other such optical components. More specifically, the invention relates to a system and method for coupling optical components that are embedded in or provided on mother and daughter boards in a PCB environment.

Background

As is well known, many communication systems are constructed using a backplane and daughter boards that can be orthogonally connected to that backplane. Standard PCB backplanes use metal traces and connectors to allow daughter boards to be connected thereto. Backplanes are often the bottleneck in a communications system. At present, there is a mismatch between communication rate and processing rate. The International Technology Roadmap for Semiconductors has suggested that in_. order for the industry to maintain its historical scaling trend, chip to next level interconnects are the major challenge that has to be addressed. There is, therefore, a growing demand for increased communication capacity across backplanes.

Despite many efforts to improve backplane connectivity, backplane communication capacity is nearing the limits imposed by the physical connections. The figure of merit for a communication channel, that is the bitrate distance product, is limited in the case of an electrical connection by the unavoidable physical barriers of the line impedance and capacitance. Heroic efforts are required to use electrical connections at high bit rates. For example, a gigabit ethernet requires multilevel coding on four wire pairs, sophisticated digital adaptive filtering techniques and echo- elimination. Pushing up the bitrate of electrical interconnects by advanced techniques such as adaptive equalisation and pre-emphasis is a viable route if the connection density is low as in the case of LANs. However, this becomes increasingly difficult when space is limited such as for the traces and connectors on a PCB.

Backplane communication speed can be improved by the use of optical waveguides instead of the commonly used copper traces. These waveguides are adapted to receive optical signals from corresponding waveguides in the daughterboards, thereby to allow communication between the backplane and the daughter board or one or more optoelectronic devices. This improved speed is possible because the optical waveguides can carry more data than copper traces commonly used. In contrast to electrical systems that offer relatively low bit rates, optical interconnections offer a bitrate distance product that is almost unlimited. However, a problem with optical systems is that alignment tolerances can be significant. Aligning the waveguides correctly is therefore essential to ensure that light is efficiently coupled between them.

Various ways for coupling light between optical elements have been proposed. For example, US 4,750,799 describes a system in which a micro-reflecting mirror is provided on a substrate adjacent to one end of a waveguide. Disposed above the mirror is a light emitting device or a photo-detector. The mirror is positioned so that it folds light through 90° so that optical signals can be coupled between the waveguide and the light emitting device or photo-detector. In this way light can be coupled into the waveguide or light emitted from the waveguide can be detected. There is, however, no disclosure in US 4,750,799 of a mechanism for effectively and efficiently connecting mother and daughter boards that carry optical components that have to be accurately aligned.

US 5,182,787 and US 5,263,111 describe out of plane mirrors that are fabricated integrally with planar waveguides. In each case, a cavity is etched in a planar. waveguide and an inclined sidewall of the cavity is coated with a reflective material. The reflective surface is used to direct light from the waveguides into another plane. A similar arrangement is described in US 5,999,670. However, although the arrangements described in US 5,182,787, US 5,263,111 and US 5,999,670 provide good mechanisms for changing the direction of travel of light and ^■■ so they could be used to direct light from a back plane waveguide to a guide or optical element that lies in another plane, a problem is that the waveguides have to be specially processed in situ to provide the reflective surface. This is a relatively time consuming and costly exercise, thereby making these processes unattractive for the mass manufacture of backplanes.

US 5,375,184 describes a flexible optical coupler for coupling light signals from an optical waveguide on a circuit board or substrate to an optical waveguide on another such board or substrate. In this case, the coupler element is aligned to the waveguides using top surface mounted alignment stops. Once aligned the coupler has to be permanently fixed to the substrates in order to ensure that the alignment is maintained.

WO 02/31567 describes an optical backplane assembly and method of making the same. In this arrangement the backplane includes optical vias that are coupled to optical signal traces in the backplane. Positioned in the optical vias are reflective elements for directing optical signals from the backplane to optical traces in daughterboards. To accurately locate the reflective elements in the vias a process of active alignment is used. This involves using light and various sensors to determine the optical centre of the optical trace in the backplane. Once this is done, the reflective element is carefully positioned so that its reflective surface is at the optical centre of the backplane trace. A disadvantage of this arrangement is that the active alignment process is sensitive and complicated.

^'Furthermore, it cannot be implemented using standard PCB processing technology. This makes its use for the mass manufacture of backplane assemblies impractical.

US 5,195,154 describes the surface mount connectivity of an optical circuit to an optical substrate with a butt coupled optical joint. The connectivity between the waveguides of the optical circuit and the waveguides of the optical substrate, being orthogonal to each other is reportedly achieved through the use of a turn in the substrate optical waveguide, as described in US 5,277,930. The turning element is fabricated using embedded 45° mirrors and Si0₂ waveguides. The optical component and the optical substrate are held in contact using a mechanical clip.

US 6,370,292 uses embedded micro prisms for coupling multimode fibres to embedded waveguides. The limitation of this approach is that there are significant pitch limitations and potential crosstalk problems associated with adjacent channels. There is also no consideration given to the accurate coupling requirement between the embedded waveguide and the external multimode fibre. US 6,317,964 describes yet another system for coupling waveguides. In this case, a complicated series of location grooves is used to align an MT mateable waveguide connector to waveguides at the edge of a substrate. This method requires agitation at an unspecified frequency and amplitude to align the collar and the waveguides. US 6,374,004 uses a collar to butt couple an MT connector to an array of OE devices mounted parallel to the plane of the MT ferrule. Yet another arrangement is described in the paper "The Status and Future of Optical Packaging Technology" by Mikami, 2000 IEMT/IMC Symposium May. In this an optical pin is proposed for turning light about an angle of 90 degrees. A problem with this approach is however that the reflective element cannot maintain the pitch and high density of backplane interconnects. Furthermore, there is no disclosure of how to overcome alignment difficulties.

Whilst some of the known coupling techniques are useful, most can only be used with surface mounted waveguides and not embedded guides. This limits their applicability. A further problem is that none of the known assemblies can be readily manufactured or implemented using standard substrate or PCB manufacturing and assembly techniques. This means that their incorporation into circuits using existing technology is either difficult or impossible. A further problem is that it is difficult to maintain separation of signals at the pitch and high density required for backplane interconnects.- In practice, this means many of the proposed systems suffer from crosstalk between different information channels. Summary of the Invention

According to one aspect of the invention, there is provided an optical connector that comprises: an optical element having a main body portion that includes a plurality of waveguides, each waveguide including an angled reflector, and a holder for carrying the optical element and securing it to another part in such a manner that light can be directed between the reflectors of the waveguides in the optical element and optical elements such as waveguides in or on the other part.

The waveguides of the connector element can be positioned so as to match the pitch and density of waveguides on the other part. In this way, the signals can be accurately directed from the reflective end of each waveguide into the corresponding waveguide of the part to which it is connected, .without significant signal overlap.

Preferably, the other part is a board, such as a backplane or motherboard, or substrate or another optical connector.

Preferably a 2-D array of waveguides is provided, thereby to increase the channel count and therefore the data transmission rate between the optical channels.

Preferably, the 2-D array comprises a plurality of rows and columns of waveguides.

Each reflector may be provided at an end of the corresponding waveguide. Alternatively, each reflector may be embedded within the waveguide.

Preferably, the reflectors are angled at substantially 45 degrees relative to a longitudinal axis of the waveguide. In this way, the element can be used to turn light round by 90 degrees.

The end faces of the waveguides may be prepared using standard laser drilling, etching or mechanical cutting techniques currently used in manufacturing assembly processes. Polishing or honing could further improve the end faces. The reflective ends faces may be coated with one or more materials to control reflective properties.

The waveguides may be multimode or single mode. The waveguides may be deposited within the element, i.e. embedded, or on the surface thereof or may be free standing. Preferably, the optical element is adapted to be received in a cavity on the other part, so that its reflective ends are aligned with the optical component therein or thereon.

The optical element can be formed using manufacturing techniques used to fabricate planar light wave elements, similar in nature to the processes used to fabricate optical die or optical waveguides to be placed on a' PCB.

The element may include a substrate of a material such as silicon, glass or plastic to allow for matching to the substrate or PCB. The size of the substrate is not restricted. Hence, a large number of reflecting elements could be manufactured simultaneously.

The holder and the element may be discrete parts that are secured together. Preferably, the holder and the element are shaped so as to mate together in a predetermined aligned position, thereby to ensure that their relative positions are well defined. A kinematic mounting system may be provided for mounting the element onto the holder. Preferably, the holder is precision moulded and is adapted to be aligned to alignment features on the element. These alignment features can be formed during manufacture of the reflective element and could for example comprise protrusions or bumps for being received in corresponding grooves in the holder. Additionally or alternatively, protrusions or bumps could be formed on the holder for being received in corresponding grooves in the optical element. The holder and the optical element may be integrally formed.

The holder may include alignment marks for use aligning it relative to the other part. Preferably, the holder is adapted to mate or engage with the other part in such a manner that the parts are automatically aligned. To this end, the holder may include alignment means, for example alignment pins for locating it relative to the other part. Additionally or alternatively, the holder may include a solder ball array for the dual purpose of aligning the holder relative to the other part and securing it thereto. Using solder re- flow techniques aids with the alignment of the reflective element relative to the other part.

According to another aspect of the present invention, there is provided an optical connector element comprising a main body portion that includes a plurality of. waveguides, each waveguide having an angled reflector. Preferably, the reflectors are angled at substantially 45 degrees relative to a longitudinal axis of the corresponding waveguide. Preferably each reflector is provided at an end of the corresponding waveguide. Alternatively, each reflector may be embedded within the waveguide .

According to yet another aspect of the invention, there is provided a method of connecting a plurality of first optical channels, preferably waveguides or optical fibres, in a first part to a^' plurality of second optical channels, preferably waveguides or optical fibres, in a second part, the method comprising: placing in a cavity in a selected one of the first or second parts an optical connector having a plurality of waveguides defined therein, each waveguide having an angled and reflective surface; connecting the optical connector to a surface of the selected part, so that the reflective surface of each waveguide lies on an optical axis of the optical channels of the selected part, the optical axis of the waveguides of the element being substantially perpendicular to the optical axes of the optical channels of the selected part and connecting the other one of the parts to the connector so that the optical axis of the waveguides of the element are substantially parallel to the optical axes of the optical channels of that other device. In this way, light reflected from the reflective surface of each waveguide is transmittable between corresponding optical channels in or on the first part and the second part.

By using a dedicated connector element both surface and embedded waveguides can be optically coupled in a simple and straightforward manner.

The connector may be adapted to physically connect the first and second parts in a substantially fixed relationship. The connector may be fixedly secured to the first part. Alternatively, the connector may be adapted to be releasably connected to the first part. The connector may be fixedly secured to the second part. Alternatively, the connector may be adapted to be releasably connected to the second part. By allowing releasable connections between the connector and the first and second parts, one or other of these could be used as a motherboard, with the other being any one of a plurality of optionally connectable daughter boards.

The cavity in the first part can either be a blind or through hole, as typically found in substrates or PCBs. The cavity acts as an initial guide for alignment of the connector relative to the first board or substrate. To allow a finer alignment of the mirror relative to the PCB waveguides, the first board or substrate may include alignment marks, which alignment marks are used to align the connector relative to at least one of the waveguides in or on the first board or substrate. Preferably, the alignment marks used to align the connector are the same as those used to correctly position the optical component, for example a waveguide, in the first board or substrate during manufacture thereof. Using the same alignment marks for location of the waveguides and the connector improves overall alignment of the reflective surface relative to the waveguides.

Preferably, the step of connecting the connector to the first part comprises providing solder on one of the first part and the connector; providing solder pads on the other of the first part and the connector; causing relative movement between the connector and the first part until the solder is substantially aligned with the solder pads and using solder re-flow techniques to connect the two parts.

Brief Description of Drawings

Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which:

Figure 1 is a cross section through a motherboard and a daughter board that are connected using an optical connector;

Figures 2 (a) is a cross section through a reflective element of the optical connector of Figure 1; Figure 2 (b) is a perspective view of the element of Figure 2(a) ;

Figures 3 (a) to (d) show the steps taken to make an alternative to the reflective element of Figures 2 (a) and (b) ;

Figures 4 (a) and (b) are perspective views of other reflective elements for use in the arrangement of Figure

1;

Figure 5 is a flow diagram of the process steps taken to fabricate the elements shown in Figures 4 (a) and (b);

Figures 6(a) and (b) are perspective views of a holder for supporting the element of Figure 4 (a) ;

Figure 7 is a perspective view of the element of Figure 4(a) in the holder of Figures 6(a) and (b) ;

Figure 8 is a perspective view of the holder of Figure 7 mounted on an interposer;

Figure ^'9 is a schematic cross section through the arrangement of Figure 8; . Figure 10 is a perspective view of the arrangement of Figure 8 being connected to a motherboard;

Figure 11 is a plan view of the motherboard of Figure 10;

Figure 12 is a cross-section through a motherboard, in which a plurality of waveguides is formed on an upper surface thereof;

Figure 13 is a cross-section through another motherboard, in which a plurality of waveguides are embedded within that board; Figure 14 is a cross-section through yet another motherboard, in which a plurality of waveguides is formed on an under surface thereof;

Figure 15 is a flow diagram of a process of fabricating a motherboard; Figure 16 is an expanded cross-section of an optical element inserted into a cavity in a motherboard;

Figure 17 is a schematic, partially sectioned side view of the optical element/motherboard arrangement of Figure 16;

Figure 18 is perspective view of another connector arrangement, similar to that of Figure 10, but with a different external connector connected to the optical element, and Figure 19 is perspective view similar to that of

Figure 18, but with yet another different external connector connected to the optical element.

Specific Description of Drawings The incorporation of embedded or surface layer optical waveguides into PCBs is a developing technology. This invention is principally, although not exclusively, useful for the manufacture and assembly of substrates or PCBs using conventional manufacturing and assembly techniques. To simplify the manufacture of hybrid electro-optic substrates and PCBs, the system in which the invention is embodied provides a connector that has an insertable reflective element that couples the light emitted from the edge of a fibre, waveguide or optical component in one plane into another plane. This enables parallel waveguides to couple to elements perpendicular to the plane of the waveguide.

Figure 1 shows an optical connector 10 for connecting a motherboard 12 and a daughter board 14 in an orthogonal relationship. The connector 10 is provided to connect the boards 12 and 14 in a relatively fixed or rigid relationship and additionally to optically couple waveguides 15 embedded in or on the motherboard 12 to waveguides (not shown) embedded in or on the daughter board 14. This connector 10 includes a reflector element 16 for implementing the _, optical coupling and a holder 18 for physically holding the reflector element 16 in place relative to each of the boards 12 and 14. Figures 2 (a) and (b) show an example of the reflector element 16. This is a substantially rigid one- piece element. It has a body of optical material through which a plurality of substantially parallel internal waveguides extend. The spacing and number of these waveguides should match that of the waveguides in the mother and daughter boards 12 and 14 respectively. More specifically, the reflector of Figures 2 (a) and (b) comprises a substrate 20 on which is provided an underclad layer 22. On top of the underclad layer, there is provided a plurality of elongate strips made of a core material, which strips are covered by an over clad layer 26. The cladding layers 22 and 26 are provided to confine light within each of the individual strips of core material, thereby to define waveguides 24. It will be appreciated that whilst the waveguides 24 of Figures 2 (a) and (b) are all provided in the same plane, many layers of such guides could be provided so as to form a 2-D array of guides.

Typically the supporting substrate 20 acts as a mechanical support for the waveguides 24 during manufacture and may remain in end use. The substrate 20 can be formed from a range of materials, for example crystalline semiconductor, glass, ceramic or a composite such as FR4 or polymer. The choice will be influenced by matching the thermal and mechanical material characteristics to the rest of the materials used in the assembly and fabrication of the overall system. For multi mode applications a PMMA based polymer system, or a polymer with similar optical properties, comprised of a core and clad material (s) with the required refractive index properties can be used to fabricate the waveguide array. UV sensitive materials can be used to form the core and clad photo-lithographically. However alternate techniques such as etching or ablation could be used on non-UV sensitive materials. For single mode applications either PMMA polymer, or a polymer with similar optical properties, Si0₂ material systems, and III/V semiconductor material systems such as InP could be used to fabricate the waveguide array.

The end 28 of the reflector element 16 is cut or shaped so that the end of each waveguide defines a surface that is at an angle of 45 degrees to its optical axis. The angled surface can then be prepared to define a reflective surface 30, either as part of a singulation process or as a separate process such as polishing, cutting or laser machining. Typically, one or more layers of material, such as metal, is deposited on the angled end face to control its reflectivity. The material used depends on the wavelength of the light being coupled between the two waveguide arrays and should be chosen accordingly. Typically for light of wavelength 850nm silver is used for the reflecting element. Alternatively dielectric layers may be used. A protective barrier layer may be used to coat the reflective element and protect it from mechanical and environmental damage.

In general, for mass manufacturing purposes, a plurality of arrays of waveguides 24 will be formed on a single substrate. These can be separated into the discrete elements shown in Figures 2 (a) and (b) as part of a singulation process. This can be done using techniques known in the art of semiconductor dice fabrication. For example, equipment with a single cutting blade, typically a thin resinoid blade containing a range of embedded diamond grit sizes, mounted 90° to the axis of the substrate can be used to form the sides. This can also be used to prepare the angled optical face of the waveguides. Alternatively, the angled face can be formed by a separate polishing operation. Rather than using a dicer having a single cutting blade, a dual head dicer could be used with one blade positioned so as to cut the straight edge of the element and the second dicing blade mounted at the required angle to prepare the reflecting surface, typically 45° to the axis of the substrate. The resinoid blade could be arranged to prepare the required length of the element while also polishing the face to the required optical finish to accept the reflecting element. As an alternative to shaping an exposed end of the waveguides 24, the reflective surface can be internally fabricated using lithographic processes to generate a 45 degree mirror, with or without the aid of a metal film. The steps for making such an internal mirror are shown in Figures 3 (a) to (d) . As for the element of Figures 2 (a) and (b) , the reflective element of Figure 3 has an under cladding layer 30 that is deposited on a substrate 32. Then an intermediate layer of ra?i"^pτi3i id ha inσ an angled end 36 is patterned on part of the under clad layer 30. Reflective material 38 is deposited over the angled end 36 of the intermediate layer 34. Once the reflective material is in place, waveguide material 40 is then laid down in parallel lines that contact the reflective surface 38. A cladding layer 42 is deposited over both of the waveguide material 40 and the reflector 38 so that both are embedded.

The reflective elements of Figures 2 and 3 can be mass-produced using techniques known in the art for the fabrication of planar lightwave circuits. The cladding layers and the waveguides can be made of polymer or glass materials and can have their refractive indexes tailored to a desired application, thereby to maximise coupling and to cut down on the loss. The reflective surface, however formed, preferably has a maximum transmission of multimode or single mode light in the wavelength range 450 to 1550nm. As noted above, the element can be fabricated as a chip on a large substrate that is singulated out as a part being square or rectangular, after the PLC -fabrication steps have been completed. The total length of the reflecting element can be chosen to accommodate for waveguides being on the surface, embedded within or on the bottom of the PCB.

The use of the reflector element will now be described primarily with reference to the element of Figure 2. It will, however, be appreciated that the element of Figure 3 could equally be used. In use the mirrored end 30 of the reflector element 16 is located in a cavity in the motherboard 12. The element 16 has to be positioned so that light transmitted through its internal waveguides 24 can be reflected from the mirror 30 and directly into the embedded or surface motherboard waveguides 15 and light transmitted from the motherboard waveguides 15 is reflected from the mirror and into the waveguides 24 of the reflective element 16. In the present example, the mirror 30 is angled at 45 degrees and the reflector element waveguides 24 and the embedded waveguides 15 of the motherboard 12 are substantially perpendicular. The non-mirrored or input end of the reflective element 16 is connected to the daughter board 14, the relative alignment of the element 16 and the daughterboard 14 being such that the optical axes of the reflector waveguides 24 and the daughter waveguides are substantially in-line. Hence, light passing through a waveguide in the daughter board is directed into the corresponding waveguide in the reflective element 16, reflected off the mirror 30 and into the corresponding waveguide 15 embedded in the motherboard 12. Correct alignment of the reflective element 16 relative to the motherboard 12 is essential to ensure that light is accurately and efficiently transferred between the mother and daughter boards 12 and 14. To achieve this the reflective element 16 is firstly accurately aligned in the holder 18 and the holder 18 is then aligned with the motherboard 12. To assist with positioning the holder 18 and reflective element 16, alignment features on the reflective element 16 are formed using any suitable technique, such as lithographic techniques or mechanical techniques such as ablation or grinding. Corresponding marks are formed on the holder 18. By ensuring that the reflective element 16 and the holder 18 are accurately aligned and that the holder 18 can have a fixed, pre-determined physical engagement with the motherboard 12, correct alignment of the reflective element 16 and the motherboard 12 can be ensured.

Figures 4 (a) and (b) show examples of reflective elements 50 and 52 on which are formed suitable alignment features for aligning these elements to a corresponding holder. In this case, the alignment features are formed on the back surface ^• of the elements 50 and 52, these features being trenches 44. The trenches 44 extend along an elongate axis of the element 50,52 and so provide an alignment mechanism along the full length thereof. The element 52 of Figure 4(b) has an additional groove 46 that runs perpendicular to the elongate axis of the element 16.

Figure 5 shows some of the steps that are taken to fabricate the elements 50 and 52 of Figures 4 (a) or (b) . The first step in the process is to prepare alignment grooves on the reverse side of the substrate 20. Then, on a front side of the substrate a lower layer of cladding material 22 is deposited. The substrate is then aligned to as to ensure that it is in a pre-determined position suitable for deposition of the waveguide material. Ideally this is such that the waveguides can be deposited substantially parallel to the elongate grooves 44 on the back surface. Then the waveguides 24 are formed, and an upper cladding layer 26 is deposited. Once this is done the element is polymerised.

To mate with the elements 50 or 52 of Figures 4 (a) or (b) , a holder is provided that has ridges that are adapted to mate with the grooves of these elements. Figures 6 (a) and (b) show an example of a holder 54 for mating with the element of Figure 4 (a) . This can be fabricated using precision moulding of plastic parts or any other suitable processing technique. In this example, the holder has two parts 58 and 60 that co-operate to retain the element 50 between them, one of these parts 58 having ridges 62 that are provided to act as alignment features for the element, thereby to provide a means for accurately locating the element 50 relative to the holder 54. Two holes 64 and 66 are defined through an outer surface of the holder 54. These are for applying glue through, thereby to lock the parts together. Each part of the holder 58 and 68 defines through channels 70 that can be used for receiving locating members. During manufacture of the connector, the reflective element 50 is inserted between the two holder parts 58 and 60. The element 50 and the holder parts 58 and 60 are then secured together using any suitable means, for example a UV curable adhesive. Once positioned in the parts 58 and 60 of the holder, part of the reflective element 50, in particular the reflective surface 30, extends from the under surface of the holder, as shown in Figure 7.

The arrangement of Figure 7 is adapted to be carried on an interposer 72. This has a first opening (not shown) for receiving the protruding end of the reflective element 50 and two sets of smaller holes that correspond to the holes 70 in the holder and are provided for receiving locating pins. To locate the arrangement of Figure 7 on this interposer 72, the protruding end of the reflective element 50 is firstly positioned in the main opening, then locating pins 74 are passed through the through channels 70 in the holder and into the holes in the interposer 72, as shown in Figure 8. The holder/interposer arrangement will be referred to as a carrier 76. The arrangement of the various parts of the carrier 76 should be such that the reflective end 30 of the optical element 50 extends beyond a lower surface thereof.

To allow the carrier 76 to be connected to the motherboard 12, a solder ball arrangement is used, more specifically a solder ball grid array. To this end, provided on an under surface of the interposer 72 are solder pads 78 that are used to receive solder balls 80, as shown in Figure 9. These solder pads 78 are formed with precision in the X-axis and Y-axis and are located so as to correspond with similar features prefabricated on the motherboard 12 along with alignment aids, also known as fiducial marks. Also shown in Figure 9 is a guide pin 82. Optionally, a pair of these pins 82 may be provided to assist with aligning the carrier 76 relative to the motherboard 12.

Figure 10 shows the carrier of Figures 8 and 9 being attached to a motherboard 12. In this case, however, a cover portion 83 is fitted around the reflector element 50 to improve the handlability thereof. In order to allow the reflector element to be connected to the motherboard 12, the motherboard 12 has two rows of solder pads 84 formed on its upper surface and along the sides of the waveguides 15 and a cavity 86 for receiving the reflective end 30 of the element 50. The layout of the motherboard 12 can be more clearly seen in Figure 11. The solder pads 84 on the motherboard 12 form the basis of a solder ball array attach. It will be appreciated that the number of pads 84 may have to be significantly increased depending on the size of the board and the carrier 76, as well as the application. The position of the alignment mark solder pads 84 is such as to correspond with the solder pads on the carrier 76, i.e. on the underside of the interposer 72. Hence, these features 84 are used to define a pre-determined location on the motherboard 12 for connection of the connector, which pre-determined location ensures correct alignment of the reflective element relative to the motherboard 12. Between the solder pads 84 is the cavity 86 that is shaped to receive the end of the reflective element 50. This cavity 86 is formed through the upper surface of the motherboard 12 and beyond the level of the motherboard waveguides 15, so that the ends of the waveguides 15 are exposed. The cavity 86 can be either a through hole as shown in Figures 12 to 14 or a blind hole. In any case, the cavity 86 can be defined using standard PCB techniques. In addition to the solder pads, alignment marks 88 located around the cavity 86 may be provided for use as a means for doing a coarse alignment. It should be noted that the waveguides 15 can be provided on the top surface of the motherboard 12, be embedded therein or provided on an under surface thereof, as shown in Figures 12 to 14. To accommodate for the different height of the waveguides, the length of the reflective element or the amount that it extends from the holder has to be varied depending on the location of the waveguides 15.

. Figure 15 shows an example of the steps that .can be taken to prepare the motherboard 12. This involves firstly depositing an underclad layer on a PCB. Then core waveguide material is patterned to define a plurality of parallel strips. These are aligned relative to copper alignment features on the board. Then an over clad layer is deposited over the core material and the board is polymerised. Once this is done, if the waveguides are to be embedded within the board, suitable layers are deposited over them until the desired depth is reached. Otherwise, the waveguides are covered with a photo- defined solder mask. Then the optical layer is tested. Assuming that the optical performance is acceptable, then the end faces of the waveguide are prepared by forming an optical cavity at a location that is such as to expose the end of the waveguides. This cavity 86 is aligned relative to the waveguides using the same copper alignment marks that were used to locate the waveguides . The cavity 86 may be a through hole or a blind via. Optionally, passive alignment vias are then formed for receiving locating pins, such as those 82 shown in Figure 9. These are aligned using the copper alignment features on the PCB. Then, if the cavity 86 is a blind via, it is back filled with an index-matched resin, which is typically a gel or a heat curable fluid, and the connector is fitted into place. Alternatively, if the cavity 86 is a through hole, the connector is fitted into place first and then the hole is back filled. Figure 16 shows a motherboard with a through hole via that has been backfilled with index-matched material. As well as the steps set out in Figure 15, the motherboard 12 also has to be processed to provide the solder pads and alignment marks 84 and 88. These are typically all formed using lithography and metallisation techniques. The stage at which this is done depends on the location of the waveguides. For surface mounted guides, such as shown in Figures 12 and 14, the solder pads and alignment marks 84 and 88 are deposited first. These can be aligned relative to the copper alignment features in the PCB. Then any or all of the copper alignment features, the solder pads and the alignment marks can be used as alignments in the waveguide deposition process. For embedded waveguides, such as shown in Figure 13 processing is slightly different. In this case, the waveguides are formed on a layer aligned to the copper alignment marks on that layer. The solder pads 84 and the alignment marks 88 are defined in a different layer that is subsequently aligned relative to the layer containing the waveguides; for example in a multi-layer PCB lamination process. In any case, care has to be taken to ensure that the waveguides, solder pads 84 and alignment marks 88 have well defined relative positions. An alternative approach that may increase the alignment between the waveguide position and the solder pads and alignment marks is to define these features, 84 and 88 on the waveguide layer. In this case a cavity, the same size as the interposer 72, will need to be formed in the layers above the waveguide layer to allow the interposer to be joined to the solder pads. To connect the reflective element 50 to the motherboard 12, the array of solder balls 80 and the corresponding solder pads 84 on the board 12 are used. Standard "pick and place" equipment may be used to move the connector towards the motherboard 12 and into the vicinity of the coarse alignment marks 88. The pick and place equipment then uses the solder pads 84 to do a finer alignment and the holder is moved until the reflective element 50 is located in the cavity 86 in the motherboard 12, which itself provides a means for alignment. The element is then moved into the cavity 86 until the solder balls 80 are brought substantially into registration with the motherboard solder pads 84. To improve the accuracy of the positioning, at the same time the alignment pins 82 are inserted into holes (not shown) in the motherboard 12. Once this is done, the solder balls 80 are heated. Heating can be effected by heating of the whole PCB in, for example, a re-flow furnace or vapour phase etc or localised heating using hot air. Heating causes the solder balls 80 or solder paste to ' melt and re-flow, thereby causing the holder carrying the reflective element 50 to bond to the motherboard 12.

It should be noted that even if the initial position of the connector relative to the motherboard 12 is slightly displaced from the optimum, the surface tension effect of the reflowing solder pulls the part to its ideal location. High accuracy of alignment can be achieved using this technique, even allowing for the parts to be initially misplaced by up to 50% of the pad size. Alignment in the x and y axes is achieved by self centring of the reflective element due to surface tension associated with the solder ball array, which tends to draw the holder into the correct position in the x and y plane, and additionally the alignment pins. Alignment in the z axis can be achieved by the controlled collapse of the solder balls or the use of hard solder balls that do not collapse and maintain the required separation height between the holder and the PCB or through the use of dead stops in the form of non-compressible features on the underside of the reflective element holder (not shown) . Figures 16 and 17 show the reflective element inserted into the cavity in the motherboard, in a correctly aligned position in which the waveguides of the element are aligned with corresponding waveguides of the board, but at substantially 90 degrees thereto.

The solder flow and re-flow techniques described above use the same manufacturing techniques as the principles used in the population of multilayer copper trace PCBs. Therefore, the use of these techniques to align optical components would not require significant modifications to existing technology. This is advantageous. Nevertheless, other techniques for aligning the holder and the motherboard could be used. Alternative approaches for connecting the holder to the motherboard include the use of a solder or adhesive assembly, with contact being made at one or all sides or corners of the holder or reflective element. The use of a solder mask can further enhance the alignment capabilities of the soldering process.

Once the reflector element is inserted into and secured to the motherboard, the holder can be used to physically attach the motherboard to other parts, for example as shown in Figure 1, the motherboard could be connected to a daughter board. This can be done using pins. These pins could be releasable so that the other parts can be disconnected from the motherboard. Alternatively, the daughter board could be permanently fixed to the motherboard. It will be appreciated that whereas the motherboard and the connector are substantially perpendicular to each other, the optical axis of the connector and that of the daughter board waveguides are substantially in line with one another. Hence, the connector provides mechanism for orthogonally connecting the optical components of the two boards. Once the motherboard 12 and daughter board 14 are connected, they are fixedly and rigidly held at right angles to each other. In addition, the waveguides 24 of the daughterboard 14 are correctly positioned so that they are substantially in line with the corresponding waveguides of the reflector element. The reflector element is aligned so that the 45 degree mirrored ends of its waveguides are in line with the waveguides of the motherboard. Hence, light transmitted through the waveguides of the daughter board passes through the reflector element waveguides and is reflected from the mirrored ends thereof into the waveguides of the motherboard, which motherboard waveguides are substantially perpendicular to those of the daughter board.

As an alternative to connecting a motherboard 12 and a daughter board 14, the connector can be used to connect other optical connectors to the motherboard 12. For example, in the arrangement of Figure 10, rather than being connected to a board, the element is connected to a fibre array connector. In this case, the waveguides of the reflective element are matched to both the waveguides of the motherboard and the fibres of the fibre array. The fibre array is connected to the element using, for example pins. Ideally a butt connection is formed, thereby to minimise optical losses. Other connectors can be used, as shown in Figures 18 and 19. Alternatively, the connector bodies shown can be replaced by optoelectronic components such as VCSEL's, detectors or tranceivers .

Whilst a butt connection is ^'preferred for connecting the optical element to another connector or indeed a daughter board, in some circumstances free space coupling may be the only option. For free space coupling a micro- lens array (not shown) can be attached/bonded to the top or non-mirrored surface of the reflector element. Ideally, this lens should be sized and positioned so that it extends over the end of the waveguide and covers an area that is slightly larger that the cross sectional area of that end of the waveguide. Likewise, another similar lens could be provided at the end of optical element, for example a waveguide in a daughter board. By having the lenses at the ends of the waveguides, slight errors in the alignment of the reflective element and the daughterboard can be accommodated.

The invention provides a prefabricated optical assembly that includes a reflecting surface that allows coupling of light in orthogonal planes. It is principally useful for the manufacture and assembly of substrates or PCBs to be used as optical backplanes using conventional manufacturing and assembly techniques. The first and second boards can therefore be PCBs. To assist in aligning optical components within the assembly, attachment pads are fabricated at the same time as the electrical traces on the PCB or substrate. Fiducial or alignment marks such as crosses can also be defined on the holder or the substrate to assist in the alignment of the reflector element into the holder as well as the assembly of the part into the PCB. Pattern recognition techniques currently used in the placement of components onto PCB boards can also be used for passive alignment of the device to the PCB. Hence, the invention provides a simple and effective mechanism for connecting substrates or boards that include optical components, which can be implemented using existing PCB processing technology.

The reflective element shown in the drawings has various different numbers of waveguides. In practice, the number of reflective element waveguides can be chosen to match those provided in the mother and daughter boards or in another connector element. Alternatively, a standard number of guides may be provided, which may or may not be used depending on the application. An advantage of the system and method in which the invention is embodied is that by using integral waveguides, the reflective element can maintain the pitch and high density of interconnects on the backplane. A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, although the reflecting element and the holder are described herein as being separate parts that are secured together using for example glue, instead the element could be insert moulded to the holder during fabrication. In addition, although the means for locating the holder relative to the motherboard is described as being relatively permanent in that solder is used to join the parts, the connector may be adapted so that it can be selectively releasable. In this way, the parts can be separated. This may be useful for maintenance purposes or to allow re-use of the motherboard and connector with other parts. Also, whilst in the specific description, the element and the holder are aligned using passive alignment, active alignment could equally be used. Likewise, active alignment may be used to align the element and motherboard. Accordingly, the above description of a specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

1. An optical connector that comprises an optical element having a main body portion that includes a plurality of waveguides, each waveguide including an angled reflector, and a holder for holding the optical element and securing it to another part.

2. An optical connector as claimed in claim 1 wherein each reflector is provided at an end of the corresponding waveguide or embedded within that waveguide.

3. An optical connector as claimed in claim 1 or claim 2 wherein the reflector is angled at substantially 45 degrees relative to a longitudinal axis of the waveguide.

4. An optical connector as claimed in any of the preceding claims, wherein the optical element is adapted to be received in a cavity in the other part.

5. An optical connector as claimed in any of the preceding claims wherein the holder and the element are separate elements that are shaped so as to mate together in a pre-determined aligned positior

6. An optical connector as claimed in any of claims 1 to 4, wherein the holder and the optical element are integrally formed.

7. An optical connector as claimed in any of the preceding claims wherein the holder is adapted to mate or engage with the other part in such a manner that the parts are automatically aligned.

8. An optical connector as claimed in claim 7 wherein the holder includes alignment means, for example alignment pins for locating it relative to the other part .

9. An optical connector as claimed in claim 7 or claim 8 wherein the holder includes a solder ball array for aligning the holder relative to the other part and/or securing it thereto.

10. An optical connector element for turning a plurality of beams of light between different optical planes, the element comprising a main body portion that includes a plurality of waveguides, each waveguide having an angled reflector.

11. An optical connector as claimed in claim 10 wherein the reflectors are angled at substantially 45 degrees relative to a longitudinal axis of the corresponding waveguide.

12.- An optical connector as claimed in claim 10 or claim 11, wherein each reflector is provided at an end of the corresponding waveguide.

13. An optical connector as claimed in claim 10 or claim 11, wherein each reflector is embedded within the waveguide .

14. An optical connector as claimed in any of claims 10 to 13, wherein the waveguides are provided in a two dimensional array.

15. A method of connecting a plurality of first optical channels, preferably waveguides or optical fibres or other optical elements, in a first part to^' a plurality of second optical channels, preferably waveguides or optical fibres or other optical elements, in a second part, the method comprising: placing in a cavity in the first part an optical connector having a plurality of waveguides defined therein, each waveguide having an angled reflector; connecting the optical connector to the first part at a pre-determined location, so that the angled reflector of each waveguide lies on an optical axis of one of the optical channels of the first part, and connecting the second part and the connector so that the angled reflector of each waveguide lies on an optical axis of one of the optical channels of the second part.

16. A method as claimed in claim 15 further comprising aligning the optical connector relative to alignment marks on the first part, which alignment marks were used to correctly position the optical channels in that first part during manufacture thereof.

17. A method as claimed in claim lb or ciaim lb, wnerem the step of connecting the second part and the connector is done before the step of connecting the optical connector to the first part.