GB2387446A

GB2387446A - Retainer for an optical fibre to an optical chip

Info

Publication number: GB2387446A
Application number: GB0208067A
Authority: GB
Inventors: Ebrahim Iravani
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-04-08
Filing date: 2002-04-08
Publication date: 2003-10-15
Also published as: GB0208067D0

Abstract

A retainer (150) for retaining an optical fibre (13) in a mounting channel (18) extending along an upper surface (24) of a substrate (23) has a lower surface (154) along which a retaining channel (156a) extends in a forward direction from a rear edge (158) of the lower surface. The lower surface (154) is juxtaposed with the substrate upper surface to retain the optical fibre (13) in the mounting channel (18) and to align the respective channels (156a, 18) . The optical fibre (13) is receivable upwardly in the retaining channel (156a) to rest on a contact area (168a) which, at the rear edge, is spaced upward of the lower surface at a first level and extends downwardly from the rear edge towards the lower surface in the forward direction whereby the optical fibre is able to be angled upwardly from the mounting channel through the retaining channel (156a).

Description

1 2387446

IMPROVEMENTS IN MOUNTING

AN OPTICAL FIBRE TO AN OPTICAL CHIP

Field of the Invention

s The present invention relates to improvements in mounting an optical fibre to an optical chip.

Background of the Invention

An optical chip may have one or more circuit elements which, for example, either produce photocurrent, emit light in response to an injection of electric current or multiplex or demultiplex light signals of different wavelengths. The optical chip may be housed in a package having pins for connecting at least some of the circuit 15 elements to a printed circuit board. Such packages are made known in International patent application publication WO00/02079 (Bookham Technology Limited/Yeandle et a/) and US patent No. 6078711 (Bookham Technology Plc/Yeandle et al) and act to provide electrical and environmental shielding for the optical chip. One or more optical fibres extend into the package and are mounted 20 to the optical chip to transmit light to, or from, the circuit element(s).

Typically, the optical chip has a silicon substrate on a surface of which the or each circuit element is formed together with one or more optical waveguides for coupling the circuit element(s) to the optical fibre(s). Invariably, the silicon 25 substrate is mounted on an insulator.

One example of an optical chip is an optical transceiver in which a laser diode and a photodiode are on the substrate surface together with associated waveguides.

Optical transceivers are used for bidirectional communication in access network 30 applications, such as fibre to the kerb or fibre to the cabinet in telecommunications networks, and are designed to work over a temperature range of 0OC to 85 C thereby making them suitable for applications in uncontrolled environments.

It is known in the art to couple an optical fibre to an optical waveguide (or other circuit element) by mounting the optical fibre in a channel extending from an edge of the substrate surface to the waveguide. The mounting channel may be etched, for instance by wet etching with a chemical etchant. When a channel is wet 5 etched in a silicon substrate, the channel has a V-shaped cross section and an end face which slopes away from the edge. This is due to the etching occurring along specific crystallographic planes of the substrate material. The V-shape may have a flat bottom surface depending on the depth of the etch.

to The inclined end face of the channel leads to the optical fibre being spaced farther from the waveguide than is optimal for optical coupling. To address this problem' US patent No. 5787214 (Bookham Technology Limited/Harpin et al) teaches a method of forming the channel so that the end of the waveguide overhangs the inclined end face of the channel to form a so-called "diving board". The diving 15 board arrangement allows the ends of the optical fibre and waveguide to be more closely apposed.

It is the convention to secure the optical fibre in the V-shaped channel with an adhesive or solder. While this provides the optical fibre with resistance to axial 20 withdrawal forces, the optical fibre is not securely held against forces tending to peel the fibre out of the channel in a direction transverse to the fibre axis. To address this, it has been proposed to hold the optical fibre in the V-shaped channel by means of a mechanical constraint or retainer, for example by sandwiching the optical fibre between the constraint and the V-shaped channel.

25 The mechanical constraint may or may not be affixed to the surface of the substrate or the insulator. While such a mechanical constraint alleviates the problem of peel-out of the optical fibre, it suffers the drawback of requiring accurate alignment of the constraint to ensure that it holds the optical fibre without damaging or deforming it.

In US patent No. 6078711 supra there is made known a retainer which is affixed by adhesive or solder to the edge of the substrate surface about the entrance to the channel. In one embodiment, the retainer has an aperture through which the

optical fibre projects into the channel. In another embodiment, the retainer, when affixed to the edge, has an inverted U-shape so that the edge of the substrate and the inner surface of the legs and the base of the retainer define an aperture through which the optical fibre projects into the channel. While these retainers s resist peeling movement of the optical fibre, placing the retainer is operator dependent and a timely procedure. The retainer also easily picks up dirt when it is examined to ensure that it is placed on the fibre the right way round. Moreover, the retainer often does not sit square against the substrate and it is important to have the correct amount of adhesive. If too much adhesive is used, this can overflow 10 into the V-shaped channel on the substrate surface and break the diving board on thermal expansion of the optical fibre. The optical fibre also tends not to lie flat and straight in the Vshaped channel unless this form of retainer is used correctly, which is difficult.

15 The package made known in WOOO/02079 supra has a passageway which extends from an opening or feedthrough in the casing of the package to a cavity defined in the package for housing the optical chip. A ferrule is secured in the passageway and the optical fibre(s) projects into the cavity for mounting to the optical chip through the ferrule. Coupling of the optical fibre to the package in this 20 manner allows a hermetic seal to be formed in the passageway for the optical fibre. As an example, the ferrule is secured to the passageway wall through an adhesive or solder so as to seal off the annulus therebetween. Moreover, a sealing insert can be provided in the ferrule so as to seal between the optical fibre and the ferrule, as disclosed in WOOO/02079. The optical fibre is cleaved to length 25 after being pushed through the ferrule, but prior to the ferrule being secured in the package passageway.

As outlined in GB-A-2340620 (Bookham Technology Limited/Yeandle et a/), it is preferable for the chip substrate to be arranged in the cavity of the package so that 30 the optical fibre enters the cavity from the ferrule in a plane which is spaced vertically above the plane of the substrate surface. This ensures that the optical fibre adopts a S-bend between the substrate and the ferrule when fixed in the V-

shaped substrate channel, i.e. between the fixing points of the optical fibre. The

different coefficients of thermal expansion of the components of the package results in axial displacement of the ferrule relative to the substrate on temperature cycling. Accordingly, the section of optical fibre between the ferrule and substrate is subject to longitudinal displacement, and the mounting of the optical fibre in the 5 V-shaped channel in the substrate surface is subject to tensile or compressive stress depending on the direction of longitudinal displacement. The provision of the S-bend enables the optical fibre to accommodate such longitudinal displacement so that the tensile or compressive forces experienced at the fixing points are maintained substantially constant. Moreover, while a small additional 10 strain is induced in the optical fibre by the S-bend, the S-bend controls the maximum strain in the optical fibre on axial displacement of the ferrule relative to the substrate. In this way, the mounting of the optical fibre to the substrate is rendered relatively insensitive to temperature fluctuations.

15 Mindful of the above, it would be useful to provide a retainer for retaining an optical fibre in a mounting channel in a substrate which is adapted to let the optical fibre adopt a S-bend configuration.

It is an aim of the present invention to provide such a retainer.

Summary of the Invention

According to a first aspect of the present invention there is provided a retainer for retaining a flexible elongate element of generally curvilinear cross section in a 25 mounting channel extending along an upper surface of a substrate, the retainer having a lower surface along which a retaining channel extends in a forward direction from a rear edge of the lower surface, wherein, in use, the lower surface is juxtaposed with the substrate upper surface to retain the elongate element in the mounting channel and to align the respective channels, wherein the elongate 30 element is receivable upwardly into the retaining channel to rest on a contact area which, at the rear edge, is spaced upward of the lower surface at a first level and extends downwardly from the rear edge towards the lower surface in the forward direction whereby, in use, the elongate element is able to be angled upwardly from

the mounting channel through the retaining channel. The retainer has particular application for retaining an optical fibre in a substrate mounting channel and enabling the optical fibre to adopt a S-bend configuration of the type advocated in GB-A-2340620 supra.

s The sloping contact area may extend downwardly in planar fashion, arcuately or in incremental steps, for example. If arcuate, it is preferably a convex arc.

Preferably, the contact area extends downwardly towards the lower surface monotonically. Preferably, the retaining channel has sidewalls which converge from the lower surface in the upward direction. The convergent sidewalls are advantageous when the retainer is also used to displace the elongate element into the mounting channel, since the sidewalls act to centre the elongate element.

The contact area of the retaining channel is preferably its bottom. This simplifies manufacture of the retainer as the tolerances are reduced, for instance compared to the case where the contact area is selected to be on the convergent sidewalls, particularly when the retaining channel is formed by etching. The bottom of the 20 retaining channel may be a planar surface.

Preferably, the retaining channel is a rear section of a retaining channel structure further comprising a forward section which, in use, is co-extensive with the mounting channel, wherein the forward section has a contact area for the elongate 25 element spaced upwardly from the lower surface at a second level which is spaced downwardly relative to the first level, and wherein the rear section extends forwardly from the rear edge to a forward position at which the contact area is spaced upwardly from the lower surface at a third level which is spaced downwardly relative to the first level no lower than the second level. In this case, 30 the forward section would act to retain the elongate element in the mounting channel with the rear section providing the elongate element with the ability to flex or bend upwardly, e.g. to adopt a S-bend as preferred in optical fibre packages.

Preferably, the second and third levels are at the same level.

Preferably, the forward and rear sections are contiguous at the forward position.

5 The contact area of the forward section may also be its bottom.

The forward section preferably has sidewalls which converge from the lower surface in the upward direction. The provision of such sidewalls in the forward section again enables the retainer to be used to accurately locate the elongate 10 element in the mounting channel The retaining channel may have parallel sidewalls extending downwardly from its bottom to give a generally U-shaped cross section. The convergent sidewalls may then be lower sidewalls and the parallel sidewalls contiguous upper sidewalls to 15 give the retaining channel a funnel-like cross section.

Preferably, the rear section has a lower portion extending upwardly from the lower surface to a level located upwardly of the second level and the lower portion has a minimum lateral width which is at least equal to a lateral width of the forward 20 section at the lower surface. The rear section may have a minimum lateral width which is at least equal to a lateral width of the forward section at the lower surface.

The minimum lateral width of the rear section may be at its bottom.

Preferably, the rear and forward sections of the retainer channel structure are 25 respectively formed in rear and forward sections of the lower surface, the rear section of the lower surface being displaced downwardly relative to the forward section of the lower surface whereby, in use, the forward section of the lower surface is juxtaposed with the substrate upper surface with the rear section of the lower surface overhanging the substrate upper surface. Ideally, the rear and 30 forward sections of the lower surface are contiguous.

The lower surface may have a forward edge and the retaining channel structure extends from the rear edge to the forward edge. The forward section of the lower surface may extend rearwardly from the forward edge.

5 Preferably, the retaining channel structure consists of the rear and forward sections. Preferably, the retainer is formed from a material which is transparent to ultraviolet radiation. This proves useful when an ultraviolet radiation curable adhesive is to be 10 used to adhere the retainer and/or the elongate element to the substrate.

In a second aspect of the present invention there is provided an assembly comprising a substrate having an upper surface with rear and forward edges in which is formed a mounting channel having a forward end and a rear end, an 15 elongate element of generally curvilinear cross section having a forward section disposed in the mounting channel and a rear section projecting rearwardly from the mounting channel, and a retainer having a lower surface with rear and forward edges in which a retaining channel extends forwardly from the rear edge, the retainer being arranged relative to the substrate so that the lower surface thereof 20 is juxtaposed with the substrate upper surface to cover the mounting channel to retain the forward section of the elongate element therein and that the rear section of the elongate element projects rearwardly upwardly from the mounting channel through the retaining channel.

25 Preferably, the retaining channel is a rear section of a retaining channel structure which further has a forward section which overlies the forward section of the elongate element. The forward and rear sections of the retaining channel structure may each have a bottom at a level spaced upwardly from the lower surface with the bottom of the forward section being at a lower level relative to the 30 level of the bottom of the rear section.

The retainer of the assembly may be a retainer according to the first aspect of the invention.

The elongate element may be an optical fibre and the substrate a substrate of an optical chip.

5 The mounting channel may extend forwardly from the rear edge of the upper surface of the substrate.

Preferably, the rear edge of the lower surface of the retainer overhangs the rear edge of the upper surface of the substrate.

The assembly may further comprise a package having an outer casing, an interior cavity and an inlet through the outer casing to the interior cavity, wherein the substrate is located in the interior cavity such that the upper surface thereof is located downwardly relative to the inlet and wherein the elongate element projects 15 through the inlet.

By way of example, embodiments of the invention will now be described with reference to the accompanying Figures of drawings.

20 Brief Descrintion of the Drawinos Figure 1 is a schematic perspective view of a base of a package housing an optical chip; 25 Figure 2 is a schematic cross sectional side view of the base of the package; Figure 3 is a schematic side view of an optical fibre mounted in a V-shaped groove in a silicon substrate of the optical chip; 30 Figure 4 is an enlarged plan view of the optical fibre mounted in the V-shaped groove with the optical fibre shown in section;

Figure 5 is a cross sectional side view of the optical fibre mounted in the V-shaped groove; Figure 6 is a schematic isometric view of a first retainer mounted to the substrate 5 to retain the optical fibre in the Vshaped groove; Figure 6A is a side view of the first retainer mounted to the substrate with the first retainer shown in section; 10 Figures 7 and 7A are views illustrating use of the first retainer to guide the optical fibre into the V-shaped groove of the substrate; Figure 8 is a front view of the first retainer mounted to the substrate; 15 Figure 9 is a schematic isometric view of a cartridge of the first retainers; Figure 10 is a schematic isometric view of a second retainer in accordance with the invention mounted to the substrate to retain the optical fibre in the Vshaped groove; Figure 11 is a schematic isometric view of the second retainer; Figures 12A-12C are front views of the second retainer guiding the optical fibre into the V-shaped groove on the substrate; Figure 13 is a further schematic isometric view of the second retainer mounted to the substrate; Figure 14 is a side view of the second retainer mounted to the substrate; and Figure 15 is a further side view of the second retainer mounted to the substrate.

Detailed Descriotion of the Exemplary Embodiments of the Invention In the following description of exemplary embodiments of the invention, like

reference numerals are used to identify like features in the different embodiments.

In Figures 1 and 2 there is shown a base 1 of a package for an integrated optical chip 20 having a silicon substrate 23 on which are formed optoelectronic circuit elements and associated waveguides (not shown). The base 1 has a recess 3, to the floor 4 of which the optical chip 20 is coupled through a ceramic insulator 21, 10 and includes electrically conducting pins 5 to electrically connect one or more of the optoelectronic circuit elements to a printed circuit board. The base 1 also has a tubular extension 7 to which an optical fibre cable 9 is coupled through a ferrule 11. Although not shown, the package further includes a lid to cover the recess 3.

The package acts to provide electrical shielding and hermetic sealing for the 15 optical chip 20. To these ends, the package may be formed from Kovar -, an alloy of nickel (Ni), cobalt (Co) and iron (Fe).

As shown more particularly in Figure 2, the ferrule 11 is secured to the inner surface of the tubular extension 7 through an adhesive, soldering or a glass seal 20 15. The optical fibre cable 9 is in turn secured in position in the ferrule 11, for example through use of an insert as detailed in WOOO/02079 supra, the contents of which are hereby incorporated by reference.

Referring to Figures 2 to 5, an optical fibre 13 extends from the ferrule 11 and is 25 mounted in a groove 18 having a uniform V-shaped cross section which extends along an upper surface 24 of the substrate 23 from an edge 25 between the upper surface 24 and a side surface 27. As detailed in WOOO/02079, the portion of the optical fibre 13 projecting from the ferrule 11 and mounted in the V-shaped groove 18 is stripped of its protective plastics outer coating.

As shown in Figure 2, the upper surface 24 of the substrate 23 is located below the axis of the ferrule 11 whereby the optical fibre 13 adopts a Sbend configuration between the ferrule 11 and the substrate 23. As detailed in GB-A

- - l 2340620 supra, the contents of which are hereby incorporated by reference, the S bend configuration is provided to compensate for the different coefficients of thermal expansion of the ferrule 11 and package casing. The S-bend allows the optical fibre 11 to accommodate the positive and negative longitudinal 5 displacements of the ferrule 11 on temperature cycling of the package 1 without the fixing of the optical fibre 13 to the ferrule 11 and V-shaped groove 18 being over-stressed. Turning to Figures 4 and 5, as detailed in US patent No. 5787214 supra, the 10 contents of which are hereby incorporated by reference, the V- shaped groove 18 is formed in the substrate 23 by etching so that the end of a waveguide 29 which communicates with one or more opto-electronic circuit elements on the substrate 23 overhangs an end face 31 of the V- shaped groove 18. The V-shaped groove 18 has a depth to a base surface 66 (Figure 6) of substantially 50-100,um, 15 preferably substantially 80-100 Em, and a width of substantially 140-150 Am at the upper surface 24, preferably substantially 143-146 m.

As will be understood by particular reference to Figure 5, this "diving board" arrangement allows a core 33 of the optical fibre 13 to be more closely Opposed to 20 the waveguide 29 than would otherwise be the case as the end face 31 is inclined due to the etching occurring along specific crystallographic planes of the silicon substrate material. The stripped optical fibre 13 has a diameter in the range of substantially 100-150 m, preferably substantially 125,um, with the core 33 having a diameter in the range of substantially 5 to 10 m.

In Figures 6 to 8 there is shown a retainer 50 for retaining the optical fibre 13 in the V-shaped groove 18 in the silicon substrate 23. The retainer 50 is in the form of a block 52 having an underside 54 in the surface of which is formed a groove 56.

30 As will be gathered from Figures 6 and 6A, the groove 56 extends from a first edge 58 between the underside 54 and a first side 60 and an opposed second edge 62 between the underside 54 and a second side 64. The retainer groove 56 consists of two sections, a first section 56a which extends inwardly from the first edge 58

and a second section 56b which extends from the first section 56a to the second edge 62.

Referring to Figures 6 and 7, the first section 56a of the retainer groove 56 has a S funnel-shaped cross section comprising a V-shaped mouth portion 57a defined by tapered sidewalls 74 and a U-shaped base portion 57b defined by vertical sidewalls 79 and a planar base surface 68a. The second section 56b has a V-

shaped cross section defined by the tapered sidewalls 74 and a planar base surface 68b.

As will be understood from Figures 6, 6A and 7, the tapered sidewalls 74 are common to both the first and second sections 56a, 56b of the retainer groove 56. It follows that the retainer groove 56 has a uniform V-shaped cross section immediately adjacent the surface of the underside 54 of the retainer 50, with the 15 width w1 of the retainer groove 56 at the urderside 54 being constant along its length. The additional U-shaped base portion 57b of the first section 56a of the retainer groove 56 means the first section 56a has a greater depth than the second section 56b, as shown in Figure 6A, for example.

20 As further shown in Figure 6A, the base surface 68a of the first section 56a of the retainer groove 56 slopes downwardly from the first edge 58 to the base surface 68b of the second section 56b of the retainer groove 56. As clearly shown in Figure 8, the width we of the U-shaped base portion 57b of the first section 56a of the retainer groove 56 is greater than the diameter of the optical fibre 13, the 25 reason for which will become apparent shortly.

In use, a first section 70 of the surface of the underside 54 having the second section 56b of the retainer groove 56 is secured to the upper surface 24 of the silicon substrate 23 so that the respective V-shaped grooves 18, 56b are in 30 registration with one another with the optical fibre 13 captured therebetween. A second section 72 of the surface of the underside 54 having the first section 56a of the retainer groove 56 overhangs the upper surface 24 of the silicon substrate 23.

The retainer 50 acts to oppose forces which tend to peel the optical fibre 13 out of the V-shaped groove 18 in the silicon substrate 23. Moreover, it is easier to place on the silicon substrate 23 than, for example, the retainer made known in US patent No. 6078711 supra.

Moreover, the greater depth and sloped base surface 68a of the first section 56a of the retainer groove 56 enables the optical fibre 13 to curve upwardly into the U-

shaped base portion 57b to adopt the desired S-bend configuration, as shown in Figures 6, 6A and 8. If the retainer groove 56 did not have the recessed U-shaped 10 base portion 57b, the optical fibre 13 would need to bend more severely at its exit point from the retainer groove 56 to adopt the S-bend. This would induce greater strain in the optical fibre 13 than is desirable.

Furthermore, the retainer 50 can be used to guide the optical fibre 13 into the V 15 shaped groove 18 in the silicon substrate 23. When the ferrule 11 is secured in the tubular extension 7 of the package base 1 of Figure 1, the optical fibre 13 is disposed above the upper surface 24 of the substrate 23, e.g. by 250 Am. Referring to Figures 7 and 7A, as a pick-and-place machine (not shown) lowers the retainer 50 with the groove 56 facing towards the upper surface 24 of the 20 substrate 23, the optical fibre 13 is collected in the groove 56 of the retainer 50.

Moreover, if the optical fibre 13 is not aligned with the V-shaped groove 18 of the substrate 23, the tapered walls 74 of the groove 56 of the retainer 50 displace the optical fibre 13 inwardly in the direction of arrow I as the retainer 50 is lowered due to the optical flare 13 experiencing an upward reaction force F1 opposing the 25 downward force F2 exerted on it by the retainer 50. This situation is illustrated schematically in Figure 7A. Continued downward movement of the retainer 50 results in the optical fibre 13 being guided into the V-shaped groove 18 on the substrate 23, the tapered walls 74 of the groove 56 of the retainer 50 acting to reduce the risk of the optical fibre 1 3 being clamped between the upper surface 24 30 of the substrate 23 and the retainer 50.

Pick-and-place machines are well known to those skilled in the art. For instance, they are widely used in the conventional electronics field. One such machine is the

Micron 2 available from ESEC (ZEVATECH). Typically, the pick-and-place machine would have a vacuum tool to hold and move the retainer 50 and a load sensor to sense when the retainer 50 contacts the upper surface 24 of the substrate 23, or adhesive located thereon, and to stop further movement of the 5 retainer 50.

As shown in Figure 7, the width w1 of the groove 56 of the retainer 50 is preferably greater than the width w2 of the V-shaped groove 18 in the substrate 23. This provides for greater tolerances in the positioning of the optical fibre 13 relative to 10 the V-shaped groove 18 in the substrate upper surface 24.

As shown in Figure 8, it is preferable for the optical fibre 13 to be subject to a 3 point contact with the groove 18 of the substrate 23 and the second section 56b of the retainer groove 56, namely single point contacts 76, 77 with each tapered wall 15 78, 80 of the V-shaped substrate groove 18 and a single point contact 82 with the base surface 68b of the second section 56b of the retainer groove 56. To this end, it is preferable if the base surface 68b of the second section 56b of the retainer groove 56 has a width w3 which is at least as great as the width w2 of the V shaped substrate groove 18. This allows for tolerances in the placement of the 20 retainer 50 on the substrate 23 while still obtaining the preferred 3-point contact on the optical fibre 13.

Alternatively, the retainer groove 56 may be so sized with respect to the V-shaped substrate groove 18 to provide for a 4-point contact on the optical fibre 13, namely 25 the single point contacts 76, 77 as before and two single point contacts with the respective flanks 74 of the V- shaped second section 56b of the retainer groove 56.

In this case, the width of the U-shaped base portion 57b would be made greater than the width of the base surface 68b of the second section 56b of the retainer groove 56.

As further shown in Figure 8, the optical fibre 13 and the first section 70 of the retainer 50 are secured to the silicon substrate 23 and to one another through an adhesive 90, preferably an epoxy resin adhesive. As shown in the inset of Figure

8, the adhesive 90 may space the underside 54 of the retainer 50 from theupper surface 24 of the substrate by a distance d1, for example 10-15 Em.

In addition to the U-shaped base portion 57b of the retainer groove 56 providing 5 the retainer 50 with the space to enable the optical fibre 13 to curve upwardly to adopt the S-bend, it can also act as an overflow reservoir for any excess adhesive used. Furthermore, as the planar base surface 68a slopes upwardly any protuberance thereon is unlikely to come into contact with the optical fibre 13 and act as a fulcrum about which the section of the optical fibre 13 in the V-shaped 10 groove 18 of the substrate 23 is biased upwardly out of the V-shaped groove 18, as might be the case if the first section 56a of the retainer groove 56 was identical to the second section 56b.

Referring to Figures 6 to 8, the retainer 50 may have the following dimensions: A width w in the range of substantially 1000-1 500 m, preferably substantially 1400 Am. À A height h in the range of substantially 500-550 Am. A width w1 for the retainer groove 56 in the range of substantially 250-350 Am, 20 preferably substantially 300 m.

À A width w3 for the U-shaped base portion 57b and the base surface 68b of the V-shaped second section 56b of the retainer groove 56 of substantially 140 170,um. 25 These dimensions for the retainer 50 compare with the following preferred dimensions for the substrate 23: À A substrate height in the range of substantially 500-550,um.

À A substrate length of substantially 16 mm.

30 À A substrate width of substantially 2 mm.

In Figure 9 there is shown a cartridge 100 containing a series of the retainers 50 for use with a pick-and-place machine. The retainers 50 are biased forwardly in

the direction of arrow A by a spring 102. This positions the forwardmost retainer 50 on a sponge 104 against which is it pressed in the direction of arrow B. The sponge 104 has a complementary profile to the underside 54 of the retainer 50 and is impregnated with adhesive. The underside 54 and groove 56 of the retainer S 50 are therefore coated with the adhesive impregnated in the sponge 104. The pick-and-place machine then transfers the forwardmost retainer 50 from the cartridge 100 onto the substrate 23 in the manner shown in Figures 7 and 7A to form the arrangement shown in Figures 6, 6A and 8. As will be realised, the adhesive 90 is applied to the V-shaped groove 18 of the substrate 23 prior to the 10 pick-and-place machine transferring the retainer 50 onto the substrate 23.

Attention is now turned to Figures 10 to 15 which show a further retainer 150 in accordance with the present invention for retaining the optical fibre 13 in the V shaped groove 18 of the substrate 23. The retainer 150 is in the form of a block IS 152 having an underside 154 in the surface of which is formed a groove 156 which extends from a first edge 158 between the underside 154 and a first side 160 to an opposed second edge 162 between the underside 154 and a second side 164.

The groove 156 of the retainer 150 consists of two sections, a first section 156a 20 which extends inwardly from the first edge 158 and a second section 156b which extends from the first section 156a to the second edge 162.

The first section 156a of the retainer groove 156 has a funnel-shaped cross section comprising a V-shaped mouth portion 157a defined by tapered sidewalls 25 174 and a U-shaped base portion 157b defined by vertical sidewalls 179 and a planar base surface 168a. The second section 156b has a V-shaped cross section defined by the tapered sidewalls 174 and a planar base surface 168b.

The planar base surface 168a of the first section 156a of the retainer groove 156 30 slopes downwardly from the first edge 158 to the planar base surface 168b of the second section 156b of the retainer groove 156. The first section 156a of the retainer groove 156 is thus at a greater depth relative to the depth of the second

- l section 156b, with the depth of the first section 156a increasing continuously in a direction towards the first edge 158.

As clearly shown in Figures 12A-12C, the width w13 of the U-shaped base portion 5 157b of the first section 156a of the retainer groove 156 is greater than the diameter of the optical fibre 13. In fact, the width w13 of the U-shaped base portion 157b-corresponds to the width of the second section 156b of the retainer groove 156 at the surface of the underside 154, as evident from Figures 12A-12C.

10 In use, a first section 170 of the surface of the underside 154 having the V-shaped second section 156b of the retainer groove 156 is secured to the upper surface 24 of the silicon substrate 23 so that the respective Vshaped grooves 18, 156b are in registration with one another with the optical fibre 13 captured therebetween. The optical fibre 13 is secured to the V-shaped substrate groove 18 by an adhesive, for 15 example an epoxy resin adhesive. A second section 172 of the surface of the underside 154 having the funnel-shaped first section 156a of the retainer groove 156 overhangs the upper surface 24 of the silicon substrate 23. The second section 172 is stepped from the first section 170 so that, in addition to overhanging the substrate upper surface 24, the second section 172 extends down the side 27 20 of the substrate 23. In this way, the retainer 150 is formed with two fingers 173, 175 on either side of the first section 156a of the retainer groove 156.

The retainer 150 has many attributes in common with the retainer 50 shown in Figures 6 to 8. The retainer 150 acts to oppose forces which tend to peel the 25 optical fibre 13 out of the V-shaped groove 18 in the silicon substrate 23, and it is easier to place on the silicon substrate 23 than the retainer made known in US patent No. 6078711 supra.

Moreover, the funnel-shaped first section 156a of the retainer groove 156 accepts 30 an upward curvature of the optical fibre 13 so that the optical fibre 13 can adopt the preferential S-bend configuration. Furthermore, as the planar base surface 168a slopes upwardly any protuberance thereon is unlikely to act as a fulcrum

which tends to bias the optical fibre 13 upwardly out of the V-shaped substrate groove 18, as outlined above in connection with the first retainer 50.

Furthermore, as shown in Figures 12A-C, it can be used to guide the optical fibre S 13 into the V-shaped groove 18 in the silicon substrate 23. In connection with the latter point, and referring to Figures 12A-C, as a pick-and-place machine (not shown) lowers the retainer 150 towards the substrate 23 the slanted sides 174 of the groove 1 56 tend to centre the optical fibre 13 into the groove 156 for the same reasons described for the retainer 50 with reference to Figures 7 and 7A.

However, an advantage of the retainer 150 over the retainer 50 of Figures 6 to 8 is that the stepping of the overhanging second section 172 from the seating first section 170 means that the optical fibre 13 is in contact with the first section 156a of the groove 1 56 for a greater period of time during displacement of the retainer 15 150 onto the substrate 23. This gives more time for the optical fibre 13 to be centred in the groove 156 and reduces the risk of clamping of the optical fibre 13 between the upper surface 24 of the substrate 23 and the underside 154 of the retainer 150. Moreover, the stepped profile of the underside 154 means that the first section of the groove 156 has a width w11 at the surface of the underside 154 20 which is greater than that of the V- shaped second section 156b of the groove 156.

This allows greater tolerances in the placement of the optical fibre 13 vis-a-vis the V-shaped groove 18 in the substrate 23, as illustrated in Figure 12A.

The base surface 168b of the V-shaped second section 156b of the groove 156 in 25 the retainer 150 should be wide enough to (i) secure a 3-point contact on the optical fibre 13, as discussed hereinabove with reference to the retainer 50 of Figures 6 to 8, and (ii) cover tolerances of-the pick-and-place machine and in the manufacture of the retainer 150.

30 Point (ii) is illustrated in Figure 12B where it can be seen that the groove 156 of the retainer 150 is not centred over the V-shaped groove 18 of the substrate 23.

Nevertheless, the width of the base surface 168b is sufficiently great that there is an overlap of the base surface 168b with the V-shaped groove 18 in the substrate

23 so that the only point contact between the retainer 150 and the optical fibre 13 is made with the base surface 168b. Thus, the optimal 3- point contact on the optical fibre 13 is obtained. In this connection, it is preferable that the base surface 168b have a width which is at least as great as the width of the V-shaped 5 groove 18 in the substrate 23.

As an alternative, the V-shaped second section 156b of the retainer groove 156 may be sized to provide a 4-point contact on the optical fibre 13 with the V-shaped substrate groove 18, as detailed with reference to the retainer 50 of Figures 6 to 8.

Figure 12C shows the ideal position of the retainer 150 on the substrate 23, namely with the respective V-shaped grooves 18,156b centred with one another.

As shown in Figures 12A-C, the fingers 173, 175 are sized so as to be spaced 15 from the insulator 21 when mounted on the silicon substrate 23. However, as will be understood by the reader skilled in the art, the fingers 173,175 may be sized to contact the insulator 23 when mounted to the substrate 23.

The fingers 173, 175 of the retainer 150 have the advantage that the pickand 20 place machine can dip the retainer 150 partially into adhesive before placing it on the substrate 23. As shown in Figure 13, the adhesive 190 applied to the fingers 173, 175 in this manner secures the fingers 173, 175 to the ceramic insulator 21 on which the silicon substrate 23 is mounted. As an alternative, or in addition, the adhesive 190 may be placed on the insulator 21, and optionally the upper surface 25 24 of the substrate 23, before placing the retainer 150. A further alternative would be to solely join the retainer 150 to the substrate 23 by placing high viscosity adhesive, preferably epoxy adhesive, on at least one of them before placement.

As will be understood by reference to Figure 14, it is not necessary for the retainer 30 150 to abut against the side 27 of the substrate 23. A gap between the fingers 173,175 and the side 27 of up to 100,um is acceptable and, in fact, would act as a reservoir for the adhesive 190 if it has a suitable viscosity, as in the case of an

epoxy resin adhesive. The retainer 150 would therefore be joined to the ceramic insulator 21 at location C and to the side 27 of the substrate 23 at location D. As shown in Figure 15, adhesive 190 can be applied to the optical fibre 13 after it 5 has been captured between the respective grooves 18, 156 whereupon it will emerge between the fingers 173, 175 and the ceramic insulator 21 and, if the viscosity of the adhesive is suitable, the fingers 173, 175 and the side 27 of the substrate 23. This can be the sole mechanism for adhesion of the fingers 173, 175 to the ceramic insulator 21/substrate 23 or in addition to one of the other 10 mechanisms described with reference to Figures 13 and 14.

The U-shaped base portion 157b of the first section 156a of the retainer groove 156 can also act as an overflow reservoir for excess adhesive.

15 As will be understood by the skilled reader in the art, a series of retainers 150 can be loaded in a cartridge in similar fashion to that shown in Figure 9, the cartridge optionally including an adhesiveimpregnated sponge.

Referring to Figures 10 to 14, the retainer 150 may have the following 20 dimensions: A width w' in the range of substantially 1500-2000 m, preferably substantially 1700 m.

A height h' in the range of substantially 1500-2000 m, preferably substantially 25 1800 Am. A width w13 for the U-shaped base portion 157b, and the V-shaped second section 156b of the groove 156 at the surface of the underside 154, in the range of substantially 150-250 sum, preferably substantially 200 sum.

e A width w11 for the first section 156a of the groove 156 at the surface of the 30 underside 154 in the range of substantially 500-1000 m, preferably substantially 800 m.

. A width w31 for the base surface 168b of the V-shaped second section 156b of the groove 156 in the range of substantially 140-170 Am.

A length I for the second section 172 of substantially 500,um.

A height h" for the step between the first and second sections 170, 172 which is equal to, or less than, the height of the substrate 23, e.g. no more than substantially 500-550 Em, preferably substantially 450 m.

S. A total length 111 of substantially 2250 m.

A depth d12 for the second section 156b of the groove 156 of substantially 60 um. A depth d13 for the U-shaped base portion 157b of substantially 500 m.

An angle a of inclination for the planar base surface 168a of the first section 10 156a of the groove 156 with respect to a plane parallel to the underside 154 of between 25-35 , preferably substantially 30 .

The retainers 50, 150 herein described are preferably made from a material which is transparent to ultraviolet (UV) radiation whereby the adhesive 90, 190 can be 15 set or cured by UV radiation after the retainers 50, 150 have been placed on the substrate 23. As an example, there may be mentioned ceramic materials, such as the glass ceramic MACORm (The Technical Glass Company) and alumina (The Technical Glass Company), and plastics materials, such as an acrylic plastics material, preferably a polymethyl methacrylate (PMMA) and more preferably 20 PerspexlM. The retainers 50, 150 may be formed from a ceramic material by machining, laser cutting, injection moulding or sintering. Use of a ceramic material has the added advantage of enabling use of the so-called "blue light cure system" in which blue light is used in place of UV radiation, as this leads to a maximum duration of 30 seconds for permanent curing of the adhesive. Injection moulding 25 would be the preferred process for manufacturing plastic retainers 50, 150.

For both retainers 50, 150 herein described, increasing the angle of the tapered sidewalls 74, 174 of the grooves 56, 156 (see Figures 7 and 11) reduces the risk of clamping of the optical fibre 13 between the underside 54, 154 of the retainer 30 50, 150 and the substrate upper surface 24. Preferably, the angle of the tapered sidewalls 74, 174 is in the range of substantially 40-50 .

It will be understood that the retainers 50, 150 herein described may be provided with more than one groove 56, 156 so that a plurality of optical fibres 13 can be mounted to the substrate 23, i.e. one groove 56, 156 per optical fibre 13. The substrate 23 may be provided with a corresponding number of V-shaped grooves 5 18. As will be seen, the present invention provides an improved retainer for mounting an optical fibre to a substrate of an optical chip.

10 It will be understood by the skilled reader in the art that the present invention is not limited to the exemplary embodiments which have been described with reference to the accompanying Figures of drawings, but may be varied in many different ways within the scope of the appended claims.

Claims

CLAIMS:

1. A retainer for retaining a flexible elongate element of generally curvilinear cross section in a mounting channel extending along an upper surface of a 5 substrate, the retainer having a lower surface along which a retaining channel extends in a forward direction from a rear edge of the lower surface, wherein, in use, the lower surface is juxtaposed with the substrate upper surface to retain the elongate element in the mounting channel and to align the respective channels, wherein the elongate element is receivable upwardly in the retaining channel to 10 rest on a contact area which, at the rear edge, is spaced upward of the lower surface at a first level and extends downwardly from the rear edge towards the lower surface in the forward direction whereby, in use, the elongate element is able to be angled upwardly from the mounting channel through the retaining channel. 2. A retainer according to claim 1, wherein the contact area extends downwardly towards the lower surface monotonically.

3. A retainer according to claim 1 or 2, wherein the retaining channel has 20 sidewalls which converge from the lower surface in the upward direction 4. A retainer according to any one of claims 1 to 3, wherein the retaining channel has a bottom which is the contact area.

25 5. A retainer according to claim 4, wherein the bottom is planar.

6. A retainer according to any one of the preceding claims, wherein, from the lower surface to the contact area, the retaining channel has a minimum lateral width which is at least as great as a diameter of the elongate element.

7. A retainer according to any one of the preceding claims, wherein the retaining channel is a rear section of a retaining channel structure further comprising a forward section which, in use, is co-extensive with the mounting

channel, wherein the forward section has a contact area for the elongate element spaced upwardly from the lower surface at a second level which is spaced downwardly relative to the first level, and wherein the rear section extends forwardly from the rear edge to a forward position at which the contact area is 5 spaced upwardly from the lower surface at a third level which is spaced downwardly relative to the first level no lower than the second level.

8. A retainer according to claim 7, wherein the second and third levels are at the same level.

9. A retainer according to claim 7 or 8, wherein the forward and rear sections are contiguous at the forward position.

10. A retainer according to any one of claims 7 to 9, wherein the forward 15 section has a bottom which forms its contact area.

11. A retainer according to any one of claims 7 to 10, wherein the forward section has sidewalls which converge from the lower surface in the upward direction. 12. A retainer according to any one of the preceding claims, wherein the retaining channel has parallel sidewalls extending downwardly from its bottom to give a generally U-shaped cross section.

25 13. A retainer according to claim 12 when appended to claim 3, wherein the convergent sidewalls are lower sidewalls and the parallel sidewalls are upper sidewalls and wherein the upper and lower sidewalls are contiguous to give the retaining channel a funnel-like cross section.

30 14. A retainer according to any one of claims 7 to 11, wherein the rear section has a lower portion extending upwardly from the lower surface to a level located upwardly of the second level and wherein the lower portion has a minimum lateral

width which is at least equal to a lateral width of the forward section at the lower surface. 15. A retainer according to any one of claims 7 to 11, wherein the rear section 5 has a minimum lateral width which is at least equal to a lateral width of the forward section at the lower surface.

16. A retainer according to claim 14 or 15, wherein the minimum lateral width of the rear section is at its bottom.

17. A retainer according to any one of claims 7 to 11 or 14 to 16, wherein the rear and forward sections of the retainer channel structure are respectively formed in rear and forward sections of the lower surface, the rear section of the lower surface being displaced downwardly relative to the forward section of the lower 15 surface whereby, in use, the forward section of the lower surface is juxtaposed with the substrate upper surface with the rear section of the lower surface overhanging the substrate upper surface.

18. A retainer according to claim 17, wherein the rear and forward sections of 20 the lower surface are contiguous.

19. A retainer according to any one of claims 7 to 11 or 14 to 18, wherein the lower surface has a forward edge and the retaining channel structure extends from the rear edge to the forward edge.

20. A retainer according to claim 19 when appended to claim 17 or 18, wherein the forward section of the lower surface extends rearwardly from the forward edge.

21. A retainer according to any one of claims 7 to 11 or 14 to 20, wherein the 30 retaining channel structure consists of the rear and forward sections.

22. A retainer according to any one of the preceding claims, wherein the retainer is formed from a material which is transparent to ultraviolet radiation.

l 23. An assembly comprising a substrate having an upper surface with rear and forward edges in which is formed a mounting channel having a forward end and a rear end, an elongate element of generally curvilinear cross section having a S forward section disposed in the mounting channel and a rear section projecting rearwardly from the mounting channel, and a retainer having a lower surface with rear and forward edges in which a retaining channel extends forwardly from the rear edge, the retainer being arranged relative to the substrate so that the lower surface thereof is juxtaposed with the substrate upper surface to cover the 10 mounting channel to retain the forward section of the elongate element therein and that the rear section of the elongate element projects rearwardly upwardly from the mounting channel through the retaining channel.

24. An assembly according to claim 23, wherein the retaining channel is a rear 15 section of a retaining channel structure which further has a forward section which overlies the forward section of the elongate element.

25. An assembly according to claim 24, wherein the forward and rear sections of the retaining channel structure each have a bottom at a level spaced upwardly 20 from the lower surface with the bottom of the forward section being at a lower level relative to the level of the bottom of the rear section.

26. An assembly according to claim 23, 24 or 25 in which the retainer is a retainer according to any one of claims 1 to 22.

27. An assembly according to any one of claims 23 to 26 in which the elongate element is an optical fibre.

28. An assembly according to any one of claims 23 to 27 in which the mounting 30 channel extends forwardly from the rear edge of the upper surface of the substrate.

Al 29. An assembly according to claim 28 in which the rear edge of the lower surface of the retainer overhangs the rear edge of the upper surface of the substrate. 5 30. An assembly according to any one of claims 23 to 29 further comprising a package having an outer casing, an interior cavity and an inlet through the outer casing to the interior cavity, wherein the substrate is located in the interior cavity such that the upper surface thereof is located downwardly relative to the inlet and wherein the elongate element projects through the inlet.

31. A retainer substantially as hereinbefore described with reference to, and as illustrated in, Figures 6 to 8 or 10 to 15 of the accompanying drawings.

32. An assembly of a retainer, an elongate element of generally 15 curvilinear cross section and a substrate substantially as hereinbefore described with reference to, and as illustrated in, Figures 6 to 8 or 10 to 15 of the accompanying drawings.