OPTICAL DEVICE

Abstract: A low cost optical device for transmitting or receiving light is formed by securing a light active device (LED or photodiode) (30) on to a hybrid board (10), preferably a thick film circuit, which also provides the interconnections necessary for the integrated circuit driver/receiver (22). The device is readily manufactured and the finished device is extremely well environmentally protected.

Description: -l- OPTICAL DEVICE. The present invention relates to optical devices and more particularly to the design and fabrication of low cost fibre optic link transmitter and receiver components. A fibre optic link transmitter and receiver requires a light emitting diode (LED) or detector and a driver or amplifier circuit associated respectively therewith. It is an object of the present invention to provide a low cost fibre optic link transmitter and receiver component. The present invention provides an optical fibre receiver or transmitter component comprising light active device which may be a receiver or transmitter of light and an associated respective integrated circuit in which the integrated circuit and light active device are mounted on the same hybrid board. In a preferred embodiment the light active device is mounted in a recess in the hybrid board on which the integrated circuit substrate is mounted. Preferably the integrated circuit is covered by a hermetically sealed cover to produce an integrated circuit package on the hybrid board. The light active device is preferably provided with a "pigtail" which is rigidly secured to the upper surface of the cover of the integrated package. In a further preferred embodiment the optical device incorporates an optical connector for attachment to an optical cable. Preferably the hybrid board is a thick film or a plurality of inter-connecting thick film circuits providing a complex connection pattern. Embodiments of the present invention will now be described with reference to the accompanying drawings in which Figure 1 shows a hybrid printed wiring board for the present invention. Figure 2 shows the board of Figure 1 in a -second stage of production with a cut out for an LED r Figure 3 shows the board of Figures 1 and 2 in a third stage of production with the connection legs attached, Figure 4 shows the board of Figures 1, 2 and 3 in a fourth stage of production. Figure 5 shows the board of Figures 1 to 4 following a fifth stage of production to produce the optical fibre receiver and transmitter component according to the present invention, Figure 6 shows a schematic view of a component according to the present invention in cut away form. Figure 7 shows an alternative mounting for the light active device. Figure 8 shows a perspective view of a transmitter module. Figure 9 shows a perspective view of a receiver module. Figure 10 shows a partial cross section of a first optical device assembly according to the present invention and. Figure 11 shows a partial cross section of a second optical device assembly according to the present invention incorporating an optical connector. Referring now to Figure 1, a printed wiring board 10 has a number of tracks 12 on its upper surface each connected to a first pad 14 and to a second smaller pad 16. The tracks 12 and pads 16 are arranged to leave two areas 18, 20 clear for reasons which will become apparent with reference to Figure 2. Only a few of the tracks 12 which would be present on a practical component are shown for clarity. In a practical embodiment the board 10 is a ceramic thick film circuit of typical dimension i inch x 1 inch. In Figure 2 the area 20 has been cut out to give a shape suitable for later reception of an LED (see Figure 5). The invention will be described using an LED and associated integrated circuit. However a photodiode and an integrated circuit for amplification of a received signal may be substituted therefore. On the area 18 an integrated circuit chip 22 is mounted for control of the light emitted from LED 30 (see Figure 5). .Electrical connections are made via gold or aluminium wires 23 of which one only is shown for clarity. In a preferred embodiment the integrated circuit 22 is mounted on a ceramic thick film circuit 24 with appropriate conductor tracking to contact/bond pads for the integrated circuit and optical device (LED) attachment. With reference to Figure 3 in a further stage the board 10 is equipped with connection legs 26 to enable the completed device to be mounted as a standard integrated circuit package. In Figure 4 the integrated circuit is shown after bonding to the thick film circuit 24 and encapsulation with a ceramic inverted lid 28. In Figure 5 the optical component 30, which may be a light emitting device (LED) or a light detection device (photodiode) , is shown fixed to the board 10. This component is hermetically sealed within the package 30. The component package 30 may be soldered or bonded with a suitable adhesive to this board. The glass fibre tail 32 of the component 30 may be treated so as to be solderable to the upper surface of the ceramic lid 28 as shown at 32. Thus the optical *"" component and electronic circuit are fixed rigidly to one another as shown in the elevation view of Figure 5a. The optical component and integrated circuit hybrid may be encapsulated with a low cost plastic enclosure 34 which may or may not be sealed/potted with resin. This allows cable attachment if required or alternatively a fibre pigtail 36 can be used as shown in Figure 6 or a connector attached to the fibre. The mixture of hermetically sealed active components together with plastic encapsulation gives the advantage of high package environmental resistance, reliability and low cost packaging. If higher performance receivers are required then high quality front-end components e.g. GaAs FET; Si JFET; Si MOSFET can be encapsulated under the ceramic lid 28 together with the integrated circuit 22. The construction concept is applicable to any wavelength fibre optic component e.g. 0.8; 0.85; 0.9; 1.3; or 1.6 microns. With reference to Figure 7, an alternative arrangement to the cut out 20 is shown. In this arrangement a heat sink bock 40 (e.g. made of copper) is mounted on top of the board 10 for example by soldering on to a pad on the board and is provided with a hole 42 for reception of the optical component. The arrangement of Figure 7 is further exemplified in Figure 8 which shows an optical transmitter device prior to insertion of the light emitting diode and integrated circuit. With reference to Figure 8 the block 40 is mounted onto a ceramic thick film circuit 44. The light emitting device 46 is mounted in the block 40 and the optical fibre tail (not shown) may be secured to a plate 48 shown in dotted outline which provides a seal for a recess 50 within which the -1- integrated circuit chip 22 (see Figure 2) is mounted. The lid 48 provides a hermetic seal for the integrated circuit 22 which may be further enforced by a secondary seal formed by filling the external enclosure with an encapsulant as described hereinafter with reference to Figures 10, 11. The light emitting device 46 may be provided with power via track 52 and conductive block 40 and via a lead 54 to track 46. With reference now to Figure 9 there is shown a receiver module prior to invention of the photodiode and integrated circuit. The photodiode is mounted in the saddle shaped structure 60 which structure provides a heat sink for the diode. The output lead of the diode is soldered to one of the pads, for example 62 on a ceramic thick film circuit layer 64. The thick film circuit comprises a plurality of layers 63, 64, 66 and 68 which are joined together and enable a greater complexity of connections to the integrated circuit 22 (not shown) which is connected in a recess 70 in layer 66. As in Figure 8 a lid 72 indicated by dotted lines is provided to seal the enclosure 74. The sealing lid is again preferably made from gold plated kovar. A metal plate 76 may be provided underneath the thick film substrate 68 to provide further heat sinking. This plate may also be present in the previous embodiments. With reference now to Figure 10 a partial cross section through Figure 9 is shown including a top cover 80 and a bush 82 which supports the optical fibre "pigtail" 84. The space 86 within the cover 80 may be filled with an encapsulant material to provide additional environmental protection. With reference to Figure 11 the hybrid thick film structure is as for Figure 10 but instead of the bush 82 and "pigtail" 84 the light emitting diode or the photodiode 90 (shown dotted) is enclosed in on^ part of an optical connector 92. Thus connection can be made by use of the other mating part of the optical connector. Again as in Figure 10 the space 86 may be filled with an encapsulating material which provides an additional environmental protection for the integrated circuit 94 and for the connector arrangement. The integrated circuit is therefore hermetically sealed as a stand alone component, it is sealed within the enclosure formed by the ceramic thick film circuit and the lid 72, which enclosure may in all cases be partially or wholly filled with an encapsulant material and i t is protected by the top cover 80 and by any encapsulant 86 introduced therein. The completed device is therefore extremely rugged and environmentally well protected. _ OMPI fr ^ V. -0

Claims: CLAIMS;- 1. An optical device for transmitting or receiving light via an optical fibre including a light active device and an associated electrical circuit in which the integrated circuit and light active device are mounted on the same hybrid board. 2. An optical device as claimed in claim 1 in which the hybrid board comprises one or more thick film circuits to provide connection between output pins and said integrated circuit and said light active device. 3. An optical device as claimed in claim 2 in which there are a plurality of thick film circuits and in which the integrated circuit is mounted within a recess formed in the plurality of thick film circuits' and in which the enclosure formed in said plurality of thick film circuits is hermetically sealed by a lid. 4. An optical device as claimed in claim 3 in which the light active device is mounted in a thermally conductive mount secured to the bottom one of said plurality of thick film circuits. 5. An optical device as claimed in claim 4 in which a metal plate is secured to the outside surface of the bottom one of said plurality of thick film circuits to provide additional heat sinking for the light active device. OMPI sfry. 6. An optical device as claimed in claim 3 in which the plurality of thick film circuits is provided with a cover, in which the cover provides a bush for protection of a pigtail optical fibre connected to said light active device. 7. An optical device as claimed in claim 3 in which the light active device is an integral part of one half of an optical fibre connector, which is secured to one of said plurality of thick film circuits. f OMP IP

Inventors: MABBITT ALLEN WILLIAM, MORGAN PETER JOHN, MABBITT, ALLEN, WILLIAM, MORGAN, PETER, JOHN
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1983
Main classification: H01L33
IPC codes: G02B6, H01L31, H01L33