GB2105487A - A mounting for an opto- electrical transducer and a method of mounting such a transducer - Google Patents

Abstract

A mounting for an opto-electrical transducer 2, such mounting comprising; two block portions 5 and 6 joined together to form a block, registering channels 12 on the meeting surfaces 7 of the block portions 5 and 6, and an optical fibre end part 3 received in the registering channels 12, the fibre end face being flush with the block surface 4 from which it emerges, said surface 4 providing a mounting site for the opto- electrical transducer 2. The invention further relates to a method of mounting an opto-electrical transducer. <IMAGE>

Description

SPECIFICATION A mounting for an opto-electrical transducer and a method of mounting such a transducer The present invention relates to a mounting for an opto-electrical transducer and a method of mounting such a transducer.

The present invention finds an application in the manufacture of devices comprising a substrate entry, or substrate illuminated, photodiode coupled to an optical fibre end portion.

Substrate or rear entry photodiodes are used to convert received optical power into electrical output signals and find applications in, for example, receivers for fibre optic communication links. Substrate entry photodiodes have various advantages over conventional top entry photodiodes such as higher quantum efficiency and lower capacitance. A fuller discussion of these photodiodes may be found in an article by Lee et a/ entitled "Small area InGa AS/lnP p-i-n photodiodes" published in Electronics Letters 1980 16 (4) pages 155-6.

When mounting a substrate entry photodiode the light has to be supplied through the substrate and the electrical connections made to the other side of the photodiode, thus the mounting must provide the appropriate access to both sides of the photodiode.

A method of mounting a substrate entry photodiode has been described by Lee eft at in the IEEE Journal of Quantum Electronics Vol. QE-17 No. 2 at page 234. This mount provides a relatively large diameter hole for an optical fibre end but does not provide any technique for aligning the photodiode with the fibre end.

Additionally it has been found that the drilling of a fine channel in an appropriately sized block for mounting a photodiode is difficult, and also that the risk of damaging the fibre end portion as it is inserted in such a channel is substantial.

Accordingly the present invention provides a mounting for an opto-electrical transducer, such mounting comprising; two block portions joined together to form a block, registering channels on the meeting surfaces of the block portions, and an optical fibre end part received in the registering channels, the fibre end face being flush with the block surface from which it emerges, said surface providing a mounting site for the opto-electrical transducer. By the fibre end face being flush with the block surface from which it emerges, it is meant that the fibre end face should not be substantially proud or recessed relative to said block surface. Thus the photodiode may be mounted on said block surface, covering the fibre -end face, substantially contiguous to both the surface of the block and the fibre end face.

Such a construction allows the fibre end portion to be accommodated in the registering channels with very little play and for the block and fibre assembly to be secured together with an adhesive or by soldering, thereby enabling the accurate alignment of the fibre end face with the opto-electrical transducer mounted on the block to be maintained throughout the life of the assembly.

The invention also includes a method of mounting an opto-electrical transducer wherein the transducer is coupled to an optical fibre end portion comprising, the steps of forming a channel in a face of each of two portions of a mounting block formed from a material having a similar degree of hardness as the fibre, adhering the two channelled surfaces together about the fibre end portion with the channels in register, said fibre end portion being received in the registering channels, polishing the surface of the block containing the fibre end portion so that the fibre end is rendered substantially flush with the block surface, and adhering the opto-electrical transducer to said polished surface so that the transducer covers the fibre end face.

Polishing is the rendering of the surface smooth by the removal, usually by rubbing, of surface material. The preferred method of polishing the block surface and fibre end face is to lap them with a rotating cloth disc coated with an abrasive paste, such as jeweller's rouge mixed with water.

The method is similar to that employed to polish gem stones.

Since the fibre end face is rendered flush with the block surface, from which it emerges, by the polishing, the block portions are preferably formed from a material having the same degree of hardness as the material forming the fibre. Thus the problem of preferential erosion of either the block or the fibre during polishing may be avoided.

Accordingly, in an embodiment, if the fibre is formed from a glass, the block portions will be formed from quartz i.e. a silica glass.

In order that the invention may be well understood an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a perspective view of a mounting block with a substrate entry photodiode thereon; Figure 2 shows an intermediate stage in the manufacture of mounting blocks; Figure 3 shows a mounting block in use in a p-in/FET hybrid receiver; and, Figure 4 is a horizontal section on A-A' through the mounting block shown in Figure 1.

Referring to Figures 1 and 4, a substantially cuboidal mounting block 1 for a substrate entry photodiode 2 comprises an optical fibre 3 and two cuboidal block portions 5 and 6. The optical fibre 3 is formed from a glass and the block portions 5 and 6 are formed from quartz.

Two channels 12 are formed in faces 7 of the block portions 5 and 6 respectively.

The block portions 5 and 6 are secured together with their faces 7 in abutment and the channels 12 in register, so as to accommodate the optical fibre 3 in the resulting passage way between the block portions 5 and 6 (See Figure 4).

The assembly comprising the block portions 5 and 6 and the optical fibre 3 is secured together by an epoxy resin. The optical fibre 3 extends from a location remote from the mounting block 1 through the block 1, in the passage way defined by the channels 12, to the fibre end face flush with a surface 4 of the block 1. The surface 4 is smooth and even, and extends across both block portions 5 and 6. The substrate entry photodiode 2 is secured by an optically transparent adhesive to the face 4 contiguous with and covering the optical fibre end face (not shown).

A step 9 is formed in the block portion 5 between the edge of the surface 4 and the surface of the block portion 5 parallel to the surfaces 7.

Two metal deposits 8 are formed in the step 9, one metal deposit 8 being located at each end part of the step 9. Two electrically conductive wires 14, formed from gold, communicate between the photodiode 2 and each of the metal deposits 8.

The surface of the block portion 6 parallel with the surfaces 7 is coated with a metal deposit (not shown).

The detailed structure of the mounting block 1 and the advantages associated with the various features will become clear from the following description of a method of making the mountings of the present invention.

Two strips of quartz 10 and 11,2 mm x 1 mm in cross-section, are cut to a suitable length, and machine grooved fine channels 12 are machined into one surface of each. These channels 12 are used as locations for optical fibres. One of the strips also has a 0.5 mm step 9 cut into one of the edges of surface 4 to give a recess into which nichrome and gold are sputtered through a mask to form areas of deposited metal 8. A specially machined tungsten mask is used to give the required dimensional accuracy to these areas 8 and to ensure that they are precisely positioned with respect to the channels 12.The face opposite the channels 12 on the non-stepped strip 10 is also coated with a deposit of nichrome and gold by sputtering so as to enable the finished assembly to be secured by soldering to form part of an assembly, for example the receiver package shown in Figure 3.

Optical fibres 3, for example, of the type formed from silica glass, are prepared by stripping the protective plastics coating off at least the end millimetre of fibre. These stripped end portions are then located in the channels in one quartz strip and held in position with epoxy resin adhesive.

The ends of the fibres just protrude from one end of the channels. A thin film of epoxy resin adhesive is applied to the channelled face of the other quarts strip and the two halves are clamped together so that the fibres are held securely in position in the registering channels 1 2. The epoxy resin is now cured to give maximum strength, and surface 4 of the assembly from which the fibres just protrude, is polished to give a suitable finish to the ends of the fibres. Alternatively, if it is desired to avoid the use of epoxy resin to secure the quartz strips together about the fibre end portions, the following method may be employed. The faces of the quartz strips 10 and 11 in which the channels 12 have been formed are coated with a deposit of nichrome and gold by sputtering.The stripped end portions of the fibres are coated with a metallic deposit by one of a number of known techniques, such as vacuum deposition of aluminium on the fibre or the application of a metal resinate to the fibre. The strips and fibres are then secured together with the fibre end portions held securely in the registering channels, by soldering with a lead Indium alloy. The surface 4 of the assembly, from which the fibres just protrude is polished to give a suitable finish to the ends of the fibres as before.

The aforesaid alternative method of adhering the assembly of fibres and strips together by soldering is preferred when the reliability of the photodiode is of primary importance since the photodiodes may be damaged by vapours evolved from epoxy resins after such resins have been cured.

The polishing operation would remove any metal deposited on the surface 4, but having the metal deposits 8 located in the step 9 overcomes this problem. The quartz strips are finally separated into individual blocks 1 by diamond sawing along the planes indicated by chain lines 1 3 in Figure 2. These pianes pass through the middle of the metal deposits 8 as shown in Figure 2, thus each block 1 has a metal deposit 8 at each end of the step 9. After sawing, the blocks 1 are cleaned to remove any residues from the polishing and sawing operations and inspected for damage to the end face of the fibre.

A specially designed assembly jig attached to a movable stage under a low power stereo microscope is used for the assembly of the photodiode 2 to the mounting block 1.

The active area of the diode is typically 70 *m diameter on a substrate which is 500 flm square.

However, the optical fibre typically has an outer diameter of 125 ,um and a core diameter of 50 i'm therefor, alignment of the diode is critical.

The microscope has a device attached to it which superimposes a cross on to the image. The fibre is aligned so that the centre of the polished end face coincides with the centre of the cross, and a small amount of an optically transparent epoxy resin is applied using a fine probe. The photodiode is picked up in the correct orientation and then gently positioned into the epoxy. A probe is used to manipulate the photodiode so that the centre of the active area is positioned under the cross in the microscope field of view. It is obviously very important that there is no movement of the block or the jig at this stage, as this would result in misalignment of the diode.

Once the diode is in position, the assembly jig with the block still in, it is carefully placed in an oven to cure the epoxy.

Connections are made between the photodiode 2 and the horizontal regions of the metal deposits 8 on the block using fine gold wires 14. A finished assembly is shown in Figure 1. A protective coating of a silicone RTC rubber may be applied to the photodiode so as to cover the active area thereof in order to protect the diode from harmful outside influences. This coating is not shown in the drawings.

It is possible at this stage to functionally test the photodiode and to actively check the accuracy of alignment between fibre and photodiode before the mounted diode is finally incorporated into an assembly. It will be appreciated that other techniques for mounting the photodiode to the block may be employed and that the above is an example to illustrate how the features of the block allow a photodiode to be mounted in a convenient and accurate manner. In particular it should be noted that incorporation of the fibre end portion in the block prior to the mounting of the photodiode enables the fibre end face to be polished, thereby giving a better light transmission across the fibre/photodiode substrate interface.Whereas if the fibre were to be inserted in the channels after mounting of the photodiode, it would be more difficult to prepare the end face of the fibre and to ensure that the end was flush with the mounting block surface 4.

In order to further illustrate the advantages of the above described mounting block 1 , the incorporation of the block into a receiver package will be described with reference to Figure 3.

A p-i-n/FET receiver package using a conventional top-entry photodiode has been described by D. Smith, in Electronics Letters 1 980 16 pages 69-71. This comprises a thick film hybrid circuit plus a photodiode mounted in a solidsidewall metal dual-in-line package. The fibre exits horizontally from the package. In order to convert this design to use a substrate entry photodiode the mounting block 1 is attached to a gold plated area within the package, with the metal coated face down using either an epoxy adhesive or a iow melting point solder. Wire connections are finally made from the block to the hybrid circuit As the photodiode is now in the vertical plane, these wires are connected to the region of the metal deposits 8 on the face of the step 9, which is perpendicular to the region to which the wires from the diode were attached.

Claims (16)

1. A mounting for an opto-electrical transducer, such mounting comprising; two block portions joined together to form a block, registering channels on the meeting surface of the block portions, and an optical fibre end part received in the registering channels, the fibre end face being flush with the block surface from which it emerges, said surface providing a mounting side for the opto-electrical transducer.

2. A mounting as claimed in Claim 1 wherein the two block portions are formed from a material having a similar degree of hardness to the material from which the optical fibre is formed.

3. A mounting as claimed in Claim 2 wherein the fibre is formed from a glass and the block portions are formed from quartz.

4. A mounting as claimed in Claim 1 wherein at least one area of the block surface is coated with a metal depost.

5. A mounting as claimed in Claim 4 wherein a step formed between the block face from which the fibre end emerges and an adjacent block face, has at least one part of its surface coated with a metal deposit.

6. A mounting as claimed in Claim 4 wherein a surface of the block adjoining the surface from which the fibre end emerges is coated with a metal deposit thereby enabling the block to be secured by soldering as a part of a further assembly.

7. A mounting as claimed in any of the preceding claims wherein the component parts of the mounting are secured together by an epoxy resin.

8. A mounting as claimed in any of Claims 1 to 6 wherein the registering channels, the meeting surfaces of the block portions and the fibre end portion are covered with a metallic deposit, and said component parts of the mounting are secured together by solder.

9. A device comprising an opto-electrical transducer secured by an epoxy resin on a mounting as claimed in any one of the preceding claims.

10. A device comprising a substrate entry photodiode on a mounting as claimed in any one of Claims 1 to 8 and 17.

1 A method of mounting an opto-electrical transducer wherein the transducer is coupled to an optical fibre end portion comprising the steps of forming a channel in a face of each of two portions of a mounting block formed from a material having a similar degree of hardness as the fibre, adhering the two channelled surface together about the fibre end portion with the channels in register, said fibre end portion being received in the registering channels, polishing the surface of the block containing the fibre end face so as the fibre end face is rendered substantially flush with the block surface, and adhering the opto-electrical transducer to said polished surface so that the transducer covers the fibre end face.

12. A method of mounting an opto-electrical transducer as claimed in Claim 11, wherein at least one area of the block surface, is coated with a metal deposit.

13. A method of mounting an opto-electrical transducer as claimed in either Claim 11 or 12 wherein an epoxy resin is used to adhere together the assemblage comprising the two block portions, the fibre end portion and the optoelectrical transducer.

14. A method of mounting an opto-electrical transducer as claimed in either Claim 11 or 12 wherein the registering channels, the meeting surfaces of the block portions and the fibre end portions, are coated with a metallic deposit and said component parts of the mounting are thereafter secured together by soldering.

1 5. A device including a substrate entry photodiode mounted by a method as claimed in anyoneofClaimsll, 12, or 16.

16. A method of mounting an opto-electrical transducer substantially as herein described with reference to and as shown in the accompanying drawings.

1 7. A mounting for an opto-electrical transducer substantially as herein described with reference to and as shown in the accompanying drawings.