OPTICAL INTERFACE ARRANGEMENT

Abstract: An optical interface arrangement comprises: a tile (15) carrying an optical component (9) which is mounted substantially at right angles to a closely adjacent circuit board (substrate) (16) which carries an electrical circuit (17). The circuit board (16) has an electrically conductive track (18) extending to an edge of the board (16) adjacent to an edge (20) of said tile (15). The edge surface of the tile (15) has a localised recess (12) carrying an electrically conductive material (10, 11) which extends across the thickness of the tile and along a major surface of the tile (15) to connect with the optical component (9). A short bondwire (21) is provided which links the electrically conductive material (11) in said recess (12) to the adjacent electrically conductive track (18) on the circuit board (16).

Description: Optical Interface Arrangement This invention relates to an optical interface arrangement in which an optical signal is received or transmitted by a photoelectric device. The optical signal will usually be brought to, or transmitted from, the optical arrangement by means of an optical fibre. In the case of a receiver, the photoelectric device will usually be a photodiode, for which an electrical amplifier can be placed after the photodiode to boost the electrical signal if necessary. Such receivers can be used in a wide range of analogue and digital systems to convert the optical signals present in the optical fibre into electrical signals. Digital applications include telecommunications systems. Analogue applications include remote control of microwave transmitters and optically-fed phased array antennas. In such applications the broad-band high-frequency signals are transmitted over optical fibre. The optical modulation frequencies and hence the electrical carrier frequencies of interest are in the microwave and millimetre wave region. At such high frequencies the minimisation of unwanted parasitic elements of the electrical components and electrical connections becomes very important. In a conventional photodiode the optical signal is directed at its surface. Usually such a photodiode is placed on a ceramic tile which is made of dielectric material. Metal tracks on the surface of the tile carry the detected electrical signal. Electrical connection from the tile to the photodiode is made using solder contacts. In a conventional optical receiver design, the photodiode tile is placed horizontally on a flat surface adjacent to the electrical amplifier with both being in the same plane. By making the components small and placing them close together the length of the connecting bondwire can be minimised to maintain a low parasitic inductance and therefore minimise its detrimental effects. Because of the surface entry design of a conventional photodiode the light must enter or leave from a direction orthogonal to the photodiode surface. The interfacing of the optical input from the optical fibre to the photodiode can be performed in several ways. For example, the fibre can be placed such that its axis is orthogonal to the photodiode surface so that the end of the optical fibre is placed close (butt coupled) to the photodiode detecting region and no intervening optical coupling or focussing components are required. Unfortunately, in many applications it is desirable to place the input fibre and an output electrical connector in the same plane as each other and in the same plane as a substrate carrying electrical circuits including the amplifier. In such a case, this can be achieved by leaving the photodiode tile in the same plane as the electrical circuits, and by deflecting the light through 90° down onto the photodiode. The methods used for this technique could include the use of a focussing lens and prism, or a 45° polished fibre end. The manufacture of components and the optical alignment of these methods are more difficult than that of the simple butt coupling method. One way to avoid the 90° deflection of the light into the photodiode is to place the photodiode tile at right angles to the substrate which carries the electrical circuits, but it is difficult to use conventional wire-bonding equipment to bond around such a 90° corner, and also the bondwire can be excessively long, introducing too much parasitic inductance and hence degrading the performance. One known arrangement in which the photodiode plane is at right angles to that of the electrical circuits is disclosed in UK patent application GB 2185151 A, in which the photodiode tiles are formed from a larger substrate which is sawn partway through in a series of parallel lines where the edge of the photodiode tiles will be. Patterning of electrically conductive tracks is then performed which pass around the tile edge into the troughs made by the partial sawing. The substrate is then turned over and sawn up into individual tiles by sawing the rest of the way through the substrate from the opposite face. Thus, the resultant track goes around the edge of the tile and continues partway along the edge of the tile. The length of the bondwire linking the photodiode to the electrical substrate is minimised by such an arrangement, but it has disadvantages in that a relatively thick phototile is required, and the electrically conductive tracks extend only partway along the edge thickness of the tile. The present invention seeks to provide an improved optical interface arrangement. According to a first aspect of this invention, an optical interface arrangement comprises: a tile carrying an optical component which is mounted substantially at right angles to a closely adjacent substrate which carries an electrical circuit and which has an electrically conductive track extending to an edge of the substrate adjacent to an edge of said tile; the interface arrangement being characterised by: the edge surface of said tile having a localised recess carrying an electrically conductive material which extends across the thickness of the tile and along a major surface of said tile to connect with said optical component, and a short bondwire linking the electrically conductive material in said recess to the adjacent electrically conductive track on the substrate. According to a second aspect of this invention, a method of making an optical interface arrangement is characterised by: forming an array of holes through a sheet of tile material; coating the walls of the holes with an electrically conductive material, dividing said sheet into a plurality of smaller tiles such that an edge of each tile passes through a hole to leave a recess on that edge which carries electrically conductive material; mounting an optical component on each tile so that it is electrically connected to said electrically conductive material; mounting each tile at right angles to an adjacent substrate which carries an electrically conductive track; and linking said electrically conductive track to said material in the recess by means of a short bondwire. The invention is further described by way of example with reference to the accompanying drawings in which: Figures, 1, 2, 3 and 4 show steps in the making of an optical interface arrangement, and Figure 5 illustrates diagrammatically a sectional view of such an arrangement. An optical interface arrangement can include an optical component such as a photodiode which acts as an optical receiver or a laser which acts as an optical transmitter, and in each case an optical fibre will be positioned to conduct light towards or away from the optical component. In the case of a photodiode receiver, the diode is connected to an amplifier, and as the arrangement is typically intended to operate at very high frequencies in the microwave region, the amplifier must be positioned closely adjacent to the diode to minimise inductive signal loss by enabling the use of a short bondwire. Similar considerations apply to an optical transmitter. Referring to Figure 1, there is shown therein a sheet 1 of tile material having an array of holes 2, formed therein. By tile material is meant a stable material such as a ceramic, for example alumina or aluminium nitride, on which the photodiode or laser can be directly mounted. Typically the tile material is about 250μm thick. The holes 2 are drilled, eg by a laser, so that the holes pass right through the thickness of the sheet 1 to form circular windows, and each hole typically has a diameter of 300 to 500 m. A metal layer is deposited onto both sides of the sheet 1, eg by sputtering or an electro-plating deposition process. The metal layer may be deposited by a multilayer process, depending on the material of the sheet. Typically, titanium or platinum would form the first layer with a top coating of gold to give good electrical conductivity. The top conductive surface of the sheet is etched to leave a pattern of conductive tracks, eg as illustrated in tile 4, and it is to this top surface that the photodiodes are attached subsequently, one photodiode to each tile. The rear conductive surface of the sheet constitutes a ground plane, and is etched locally around the holes 2 so as to be isolated from the conductive material which coats the walls of the holes 2, the isolating region 5 from which the conductive material has been removed being shown in tile 6. In practice, of course, each tile is identical. The pattern of conductive tracks 3 may be more complex than that shown, and smaller diameter conductive via holes can be provided in each tile to give electrical connection between the two major surfaces of the sheet 1, to permit convenient electrical interconnection and earthing to ground of the photodiode. The track 3 can be of any configuration, eg coplanar or microstrip. The sheet 1 is then sawn along the broken lines 7 and 8 to form a number of individual tiles, all of which are identical. In particular, lines 8 pass through the centre of holes 2, so that each tile has a semicircular recess in one edge, and the walls of this recess are coated with an electrically conductive material which is electrically connected to each track 3, but is electrically isolated from the conductive ground plane on the other side of the tile. Referring to Figures 2 and 3, an individual tile so formed is shown in greater detail. A photodiode 9 is mounted on a conductive track 10 on the front surface, and this conductive track is electrically continuous with the conductive material 11 on the walls of the recess 12. A ground plane 13 extends across the whole of the rear surface of the tile, except for an isolation region 14 which bounds the recess 12. Figure 4 shows a tile 15, which corresponds to the view of the tile shown in Figure 2, and the tile is mounted very closely adjacent to the end edge of a printed circuit board 16, which carries an amplifier 17, and other electrical circuits, not shown, as may be required. The surface of the board 16 carries a conductive track 18 extending from the amplifier 17 to the edge 19 of the board adjacent to the edge 20 of the tile 15. A conductive bondwire 21 extends from the recess 12 to the track 18 so as to make electrical connection therebetween. This bondwire can be extremely short, and this minimises its inductance, and thereby permits very high frequency operation. Referring to Figure 5, an optical interface arrangement is shown. It consists of the tile 15 mounted vertically, and the circuit board 16 mounted at right angles thereto. As before the tile 15 carries a track 10 and a photodiode 9, and is connected via the bondwire 21 to track 18, on the board 16, and which is connected to the amplifier 17. Other electrical circuits, eg circuit 23, are mounted on the board 16 as required, and electrical connection 24 is made by any convenient means to an input/output electrical port 25 mounted in the wall of a housing 26. An optical fibre 27 passes through the opposite wall 28 of the housing, and terminates very close to the surface of the photodiode 9 to make a butt end optical connection therewith.

Claims: Claims 1. An optical interface arrangement comprising: a tile (15) carrying an optical component (9) which is mounted substantially at right angles to a closely adjacent substrate (16) which carries an electrical circuit (17) and which has an electrically conductive track (18) extending to an edge of the substrate (16) adjacent to an edge (20) of said tile (15), characterised by the edge surface of said tile (15) having a localised recess (12) carrying an electrically conductive material (10, 11) which extends across the thickness of the tile and along a major surface of said tile (15) to connect with said optical component (17), and a short bondwire (21) linking the electrically conductive material (11) in said recess (12) to the adjacent electrically conductive track (18) on the substrate (16). 2. An optical interface arrangement according to Claim 1 characterised by the tile comprising a ceramic material. 3. A method of making an optical interface arrangement characterised by forming an array of holes (2) through a sheet (1) of tile material; coating the walls of the holes with an electrically conductive material (11), dividing said sheet into a plurality of smaller tiles (15) such that an edge (8) of each tile passes through a hole to leave a recess (12) on that edge which carries electrically conductive material (11); mounting an optical component (9) on each tile (15) so that it is electrically connected to said electrically conductive material; mounting each tile (15) at right angles to an adjacent substrate (16) which carries an electrically conductive track (18); and linking said track (18) to said electrically conductive material (11) in the recess by means of a short bondwire (21). 4. A method according to Claim 3 and characterised by the tile material comprising a ceramic material. 5. A method according to Claim 3 and characterised by said coating consisting of a plurality of metal layers applied sequentially. 6. A method according to any one of Claims 3 to 5 and characterised by both faces of the tile material being coated with electrically conductive material. 7. A method according to any one of Claims 3 to 6 and characterised by each of the holes being substantially circular. 8. A method according to Claim 7 and characterised by the diameter of each hole being greater than the thickness of the tile material.

Inventors: LANGLEY LLOYD, LANGLEY, LLOYD
Applicants: LANGLEY LLOYD, MARCONI CASWELL LTD, LANGLEY, LLOYD, MARCONI CASWELL LIMITED
Application year: 2000
Main classification: G02B6
IPC codes: G02B6, H01L31