GB2381325A

GB2381325A - Mounting an optical fibre to an optical chip using a resilient portion

Info

Publication number: GB2381325A
Application number: GB0125759A
Authority: GB
Inventors: Matthew Peter Shaw
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-10-26
Filing date: 2001-10-26
Publication date: 2003-04-30
Also published as: GB0125759D0

Abstract

An optical component, preferably an optical fibre (11), is attached within a mounting channel (13) etched in a silicon substrate (23), using a retainer (15; 115) having a mounting portion (17; 117) and a resilient portion (19; 119). The mounting portion (17; 117) is secured to the substrate (23) or a support therefor and the resilient portion clamps the optical fibre (11) in the mounting channel (13). The resilient portion may be in the form of an arm (19) or a sheet (119).

Description

IMPROVEMENTS IN MOUNTING AN OPTICAL FIBRE TO AN OPTICAL CHIP Field of the Invention The present invention relates to improvements in mounting an optical component to a substrate. In particular, 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. In some applications, the optical chip may be housed in a"package"having pins for connecting at least some of the circuit elements to a printed circuit board. Such packages are described in, for example, International patent application publication WO 00/02079 (Bookham Technology Limited/Yeandle et al.) and US patent No. 6,078, 711 (Bookham Technology Plc/Yeandle et a/.). These packages provide electrical and environmental shielding for the optical chip. Typically, one or more optical fibres extend into the package and are mounted on the optical chip to transmit light to, or from, the circuit element (s).

The optical chip usually has a silicon substrate on a surface of which one or more circuit elements are formed together with one or more optical waveguides for coupling a circuit element to an optical fibre. The silicon substrate is usually mounted on an insulator inside the package.

Correct alignment of the optical fibre with respect to the optical chip is necessary in order that there is optimal optical coupling between the optical chip and the optical fibre. One method of attaching an optical fibre to an optical chip is by mounting the optical fibre in a channel in the surface of the substrate of the optical chip. Typically, such channels are etched, for example by chemical or wet etching, and extend from an edge of the substrate surface to a waveguide. When

etched, the shape of the channel is determined by the way that etching occurs along specific crystallographic planes of the substrate material. Consequently, a channel chemically etched in a silicon substrate has a V-shaped cross section and an end face that slopes away from the edge. The V-shaped channel may have a flat-bottomed surface depending on the depth of the etch.

Usually, the optical fibre is attached to the channel with adhesive or solder.

Attachment of the optical fibre in this way provides the optical fibre with resistance to axial withdrawal forces, that is, forces along the axis of the fibre. However, the optical fibre is not securely held against forces tending to peel the fibre out of the channel.

In order to overcome this problem, a mechanical constraint or retainer may be additionally used to secure the optical fibre in the channel. For example, US patent No. 6,078, 711 proposes the use of a retainer that is attached with adhesive to the edge of the substrate surface at 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 fixed 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. Although these retainers tend to resist peeling movement of the optical fibre, the optical fibre tends not to lie flat and straight in the channel unless the retainer is used correctly, which is difficult. In particular, the positioning of the retainer is operator dependent and tends to be a timeconsuming procedure. If the retainer is not attached accurately then the optical fibre may be damaged or deformed. Furthermore, the retainer may pick up dirt when examined to ensure that it is placed on the fibre in the correct orientation and the retainer may not sit squarely against the substrate. Moreover, it is vital that the correct amount of adhesive is used.

United Kingdom patent application GB 0108747.7 (Bookham Technology Plc.) discloses an improved way of mounting an optical fibre to an optical chip by using a clamping block having a V-shaped groove in its underside which is

mounted on the substrate of the optical chip to hold the fibre in place. This arrangement secures the fibre against forces tending to peel the fibre out of the channel. However, this arrangement still requires adhesive to secure the fibre against axial withdrawal forces.

The current methods of attaching an optical fibre to a channel provided in a substrate of an optical chip all require that an adhesive be used between the optical fibre and the channel. Typically, once the fibre has been placed in the channel, adhesive is applied between the optical fibre and the mouth of the channel. The adhesive is cured, for example, using ultraviolet radiation. However, the adhesive may seep beneath the optical fibre causing the optical fibre to be lifted up in the channel. Therefore, such methods may preclude the provision of a flat-lying optical fibre in the channel, which is necessary for optimum optical coupling at the end of the optical fibre that meets the waveguide, circuit element or other optical component at the end of the channel.

A further problem with methods that use an adhesive to attach an optical fibre to a substrate is that the adhesive tends to expand and contract as the temperature cycles in the operating temperature range of the optical package.

Typically an optical package may operate in the range-40 C to +85 C. Expansion and contraction of adhesive material between the optical fibre and the channel on temperature fluctuation results in flexing of the optical fibre, which is undesirable as it may effect the optical alignment of the optical fibre and optical chip.

It is an aim of the present invention to provide improvements to the way in which an optical component, in particular an optical fibre, is mounted to a substrate. In particular, it is an aim of the present invention to provide a way of attaching an optical component to a substrate that does not require the use of an adhesive between the optical component and the substrate.

Summary of the Invention According to the present invention, there is provided a retainer for retaining an optical component having a generally curvilinear cross section in a mounting channel which extends along the surface of a substrate, the retainer having a mounting portion and a resilient portion, wherein the resilient portion is biased such that it adopts a first clamping position relative to the mounting portion and wherein, in use, with the mounting portion attached to the substrate or a support therefor, interaction between the resilient portion and the optical component prevents the resilient portion adopting said first position and the resilient portion applies a force on the optical component substantially in the direction of the mounting channel.

Thus, the retainer of the present invention is able to retain an optical component in the mounting channel of a substrate by using a resilient portion to bias the optical component downwardly into the substrate on which it is mounted.

In this way, the retainer may secure the optical component against both axial and transverse displacement without the need for an adhesive. Therefore, an optical component secured to an optical chip using the retainer of the present invention will lie flat along the channel in the substrate of the optical chip, providing optimum and readily reproducible alignment of the optical component and optical chip.

The retainer may be used to fix an optical component permanently or temporarily in the mounting channel of a substrate. Preferably, the retainer is intended to be used as a permanent fixing device. However, where the retainer is used as a permanent fixing device, the resilient portion may allow adjustment of the position of the optical component, if desired.

The retainer may be made of any suitable material or combination of materials. Preferably, the retainer is made of a material that enables the resilient portion to retain its resilient properties over the expected operating temperature range of the optical package in which the retainer is to be incorporated. Preferably, the resilient portion of the retainer retains its resilient properties over

the temperature range-40 C to +85 C. Preferably, the retainer is made from spring steel, phosphorous bronze, Be-Cu alloy or a plastic material. More preferably, the retainer is made from spring steel.

Preferably, the optical component retained by the retainer is an elongate element. More preferably, the optical component is an optical fibre. By"optical fibre"is meant any part of an optical cable that has been stripped of any protective buffer layer, polymer layers or sheath to leave only the central optical core surrounded by the optical cladding layer. Light transmitted down the optical fibre travels through the central optical core and is retained therein by total internal reflection at the interface with the optical cladding.

The resilient portion may be any suitable shape. Preferably, the resilient portion provides tangential contact with the optical component having a generally curvilinear cross section. Tangential contact between the resilient portion and the optical component may be provided by virtue of the curvilinear cross section of the optical component, e. g. circular cross section and/or by the resilient portion having a convex clamping surface. Preferably, the resilient portion has two opposed generally parallel arcuate sides. By having tangential contact between the resilient portion and the optical component, there is likely to be less deformation or damage of the optical component.

The mounting portion of the retainer may be of any form and, in use, may be attached to any surface in the package within which the substrate is mounted. By"package"is meant herein a package like that in which an optical chip is housed as disclosed in WO 00/02079. Preferably, the mounting portion of the retainer is attached to the substrate, for example an optical chip. Alternatively, where the substrate is located on a further substrate, for example, where the optical chip is mounted on an insulating material, the mounting portion may be attached to the further substrate. The mounting portion of the retainer may be attached in any suitable manner, for example, using an adhesive or by soldering, welding, clipping, pinning, riveting or, if it is to be attached to a plastics surface of

the package, by heat staking. A retainer may have more than one mounting portion.

In a further aspect of the invention, there is provided an assembly comprising a substrate having a surface in which is formed a mounting channel, an optical component having a generally curvilinear cross section disposed in the mounting channel and a retainer having a mounting portion and a resilient portion extending therefrom, wherein the mounting portion of the retainer is attached to the substrate or a support therefor such that the resilient portion of the retainer contacts the optical component and clamps the optical component within the mounting channel.

The mounting channel may be of any suitable cross section, but is preferably of V-shaped cross section. Furthermore, the mounting channel may have dimensions suitable for placing a single optical component or more than one optical component therein.

The optical component may be an optical fibre and the substrate may be comprised of an optical chip. Alternatively, the optical component may be a ball lens and the substrate comprised of an optical chip.

Where there is more than one optical component in the mounting channel, the optical components may lie side by side or one in front of the other in the channel. Preferably, where more than one optical fibre is present in the mounting channel, the optical fibres lie in the mounting channel with their longitudinal axes in parallel. Where both a ball lens and an optical fibre are present in an assembly according to the present invention, the ball lens is preferably located at one end of the mounting channel between the optical chip and the optical fibre. Where the assembly comprises a mounting channel with more than one component, the retainer may have a resilient arm that contacts both components. Alternatively, more than one retainer may be used.

Preferably, there is a further portion of the optical component projecting from the mounting channel. In particular, where the optical component is an optical fibre, the optical fibre may project from the mounting channel and, for example, extend into the entry portion of a package holding the assembly.

Although it is preferred that no adhesive is used to attach the optical component to the mounting channel, there may be occasions where such an additional attachment is desirable. In this case, adhesive may be added to the upper part of the optical component once it has been secured in position in the mounting channel by the retainer of the present invention. The downward clamping force provided by the resilient arm of the retainer on the optical component is preferably such that there is good contact between the optical component and the walls of the mounting channel. As such, any adhesive applied to the upper part of the optical component and mounting channel once the optical component has been secured by the retainer is unlikely to be able to seep beneath the optical component, thus overcoming the problems of the use of adhesive found in the prior art.

According to another aspect of the present invention, there is further provided a method of retaining an optical component having a generally curvilinear cross section in a mounting channel which extends along the surface of a substrate comprising the steps of: placing said optical component in said mounting channel; placing a retainer having a mounting portion and resilient portion over the optical component and the mounting channel ; and attaching the mounting portion of the retainer to the substrate or to a further substrate in such a way that the resilient portion of the retainer overlies the optical component, wherein the resilient portion applies a force on the optical component substantially in the direction of the mounting channel to clamp the former in the latter.

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

Brief Description of the Figures of Drawings FIGURE 1 is a schematic perspective view of a base of a package housing an optical chip and a first embodiment of retainer; FIGURE 2 is a schematic cross-sectional view along the line A-A of FIGURE 1 ; and FIGURE 3 is a schematic perspective view of a part of a base of a package housing an optical chip and a second embodiment of retainer.

Detailed Description of the Exemplarv Embodiments of the Invention In the following description, like reference numerals are used to identify like features in the different embodiments.

With reference to 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 one or more opto-electronic circuit elements and one or more associated waveguides (not shown), as is known in the art. The base 1 has a recess 3, to the floor of which the optical chip 20 is coupled through a ceramic insulator 21, and includes electrically conducting pins 5 to electrically connect one or more of the opto-electronic circuit elements to a printed circuit board. The base 1 also has a tubular extension 7 through which an optical cable 9 extends, and to which it is coupled. 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 optical chip 20. The package may be formed from Kosar, an alloy of nickel, cobalt and iron.

There is a mounting channel 13 that has been chemically etched in the silicon substrate, for example using the procedure outlined in US patent No. 5,787, 214 (Bookham Technology Ltd./Harpin et a/), the entire contents of which are hereby incorporated herein by reference. The mounting channel 13

extends from the edge 27 of the silicon substrate 23 along a part of the upper surface 29 of the silicon substrate and has walls 33 providing a V-shaped cross section. The mounting channel 13 may have a depth of about 50 to 100 um, preferably about 80 to 100 um. The width of mounting channel 13 at the upper surface 29 of the substrate may be about 140 to 150 um, preferably about 143 to 146 um.

The optical cable 9 is secured to the inner surface of the tubular extension 7. Preferably a ferrule as described in WO 00/02079, the entire contents of which are hereby incorporated herein by reference, is used to secure the optical cable within the tubular extension 7.

As detailed, for example, in WO 00/02079, at the end of the optical cable 9 nearest the package 1, the outer sheath has been stripped to provide an optical fibre 11. The optical fibre 11 has a diameter in the range about 100 to 150 um, preferably about 125 um, with the core 31 having a diameter in the range of about 5 to 10 um. The optical fibre 11 is arranged to lie in and be aligned with the mounting channel 13. Prior to insertion into the package, the optical fibre 11 may be cleaved using any suitable cleaving apparatus. The particular type of cleaving apparatus used is not important, as will be understood by the skilled person in the art. One type of mechanical cleaver is the FK12 angled fibre cleaver (York Technologies Limited) while laser cleavers are available from Optek or AEA Technology.

A retainer 15 of spring steel has a mounting portion 17 and a resilient portion in the form of an arm 19. The mounting portion 17 is attached to the ceramic insulator 21 by a layer of adhesive 35 between the mounting portion 17 and the upper surface 37 of the ceramic insulator 21. The retainer 15 is positioned over the optical fibre 11 and mounting channel 13 in an orientation such that the longitudinal axis of the resilient arm 19 is substantially normal to the longitudinal axis of the optical fibre 11. The resilient arm 19 has an arcuate configuration. Prior to the retainer 15 being positioned over the mounting channel 13, the resilient arm 19 of the retainer 15 adopts a first position relative to the mounting

portion 17 of the retainer 15. When the retainer 15 is in position in the assembly as shown in FIGURES 1 and 2, the resilient arm 19 is prevented from adopting this first position and is pushed upward relative to the mounting portion 17 by the interaction with the optical fibre 11. As a result, the resilient arm 19 exerts a downward biassing force on the optical fibre 11 at a contact point 25. In this way, the optical fibre 11 is held against the side walls 33 of the mounting channel 13.

The retainer 15 is placed in position over the mounting channel 13 once the optical fibre 11 has been arranged in the mounting channel 13. Any suitable method of positioning the retainer 15 may be used, for example, using a pick-andplace machine. Pick-and-place machines are well known to those skilled in the art and are widely used in the field of electronics. One such machine is the Micron 2 available from ESEC (ZEVATECH). Typically, the pick-and-place machine has a vacuum tool to hold and move the retainer 15 and a load sensor that stops further movement of the retainer 15 when the mounting portion 17 of the retainer 15 contacts the upper surface 37 of the ceramic insulator 21 or the adhesive layer 35 thereon. In an alternative embodiment (not shown), the mounting portion 17 is attached direct to the silicon substrate 23 and the pick-and-place machine has a load sensor that stops further movement of the retainer 15 when the mounting portion 17 contacts the silicon substrate or the adhesive layer thereon.

Referring now to the embodiment shown in FIGURE 3, a retainer 115 has two mounting portions 117 that are each attached to the substrate surface 29 by a layer of adhesive 35. In this embodiment, the retainer 115 "straddles" the mounting channel 13, that is, there is one mounting portion 117 positioned on a part of the substrate surface 29 each side of the longitudinal axis of the channel 13. The resilient portion is in the form of a sheet-like arm or tongue 119 of spring steel having a slightly convex curved portion 140 at the end intended to contact the optical fibre 11. As for the embodiment of the retainer shown in FIGURES 1 and 2, prior to the retainer 115 being positioned over the mounting channel 13, the resilient arm 119 of the retainer 115 adopts a first position relative to the mounting portions 117 of the retainer 115. When the retainer 115 is in position in the assembly as shown in FIGURE 3, the resilient arm 119 is prevented from adopting

this first position and is pushed upward relative to the mounting portions 117 by the interaction with the optical fibre 11. As such, the resilient arm 119 exerts a downward clamping force on the optical fibre 11 at a contact point 25. In this way, the optical fibre 11 is held against the side walls 33 of the mounting channel 13.

In FIGURE 3, the contact point 25 is behind the mounting portions 117 with respect to the edge 27 of the silicon substrate 23. However, it will be understood that the retainer could be rotated 180 such that the contact point 25 lies between the mounting portions 117 and the edge 27.

It will further be understood that the retainer 115 may have mounting portions 117 which, instead of being on the substrate surface 29, are positioned on opposite sides of the substrate 23 on the upper surface 37 of the ceramic insulator 23.

For both embodiments of the invention described above, the biasing force of the arm 19 ; 119 of the retainer 15 ; 115 is sufficient to retain the optical fibre 11 in place in the mounting channel 13 in a standard shock test but not so great as to crush the optical fibre 11 and significantly reduce or affect its mechanical and optical properties.

It will be understood that the present invention is not limited to the exemplary embodiments which have been described with reference to the accompanying FIGURES but may be varied in many different ways within the scope of the appended claims. As an example, the mounting portion (s) of the retainer of the invention may be attached to any surface of a package containing an optical chip, not just the insulator or the chip substrate as described for the embodiments shown in the FIGURES of drawings. As an example, the mounting portion (s) may be attached to an interior surface of the package such as the lid. The mounting portion (s) may also be attached other than through an adhesive, e. g. through soldering, welding, clipping, pinning, riveting or heat staking to a plastics surface of the package.

Claims

CLAIMS: 1. A retainer for retaining an optical component having a generally curvilinear cross section in a mounting channel which extends along the surface of a substrate, the retainer having a mounting portion and a resilient portion, wherein the resilient portion is biased such that it adopts a first clamping position relative to the mounting portion and wherein, in use, with the mounting portion attached to the substrate or a support therefor, interaction between the resilient portion and the optical component prevents the resilient portion adopting said first position and the resilient portion applies a force on the optical component substantially in the direction of the mounting channel.
2. A retainer according to claim 1, wherein the optical component is an elongate element.
3. A retainer according to claim 2, wherein the elongate element is an optical fibre.
4. A retainer according to any one of the preceding claims, wherein the resilient portion is arcuate.
5. A retainer according to any one of the preceding claims, wherein the resilient portion comprises a resilient arm.
6. A retainer according to any one of claims 1-5, wherein the resilient portion comprises a sheet of spring steel.
7. An assembly comprising a substrate having a surface in which is formed a mounting channel, an optical component having a generally curvilinear cross section disposed in the mounting channel and a retainer having a mounting portion and a resilient portion extending therefrom, wherein the mounting portion of the retainer is attached to the substrate or a support therefor such that the resilient

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portion contacts the optical component and clamps the optical component within the mounting channel.
8. An assembly according to claim 7, wherein the optical component is an optical fibre and the substrate is comprised of an optical chip.
9. An assembly according to claim 7, wherein the optical component is a ball lens and the substrate is comprised of an optical chip.
10. The assembly according to claim 7 or claim 8, wherein there is a further portion of the optical component projecting from the mounting channel.
11. The assembly according to any one of claims 7 to 10, wherein there is tangential contact between the resilient portion and the optical component.
12. A retainer substantially as hereinbefore described with reference to, and as illustrated in, FIGURES 1 and 2, or FIGURE 3, of the accompanying drawings.
13. An assembly of a retainer, an elongate element of generally curvilinear cross section and a substrate substantially as hereinbefore described with reference to, and as illustrated in, FIGURES 1 and 2, or FIGURE 3, of the accompanying drawings.
14. A method of retaining an optical component having a generally curvilinear cross section in a mounting channel which extends along the surface of a substrate comprising the steps of: placing said optical component in said mounting channel ; placing a retainer having a mounting portion and a resilient portion over the optical component and the mounting channel ; and attaching the mounting portion of the retainer to the substrate or to a further substrate in such a way that the resilient portion of the retainer overlies the optical component, wherein the resilient portion applies a force on the optical component substantially in the direction of the mounting channel to clamp the former in the latter.

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15. A method of retaining an optical component having a generally curvilinear cross section in a mounting channel which extends along the surface of a substrate substantially as hereinbefore described with reference to, and as illustrated in, FIGURES 1 and 2 or FIGURE 3 of the accompanying drawings.