GB2376754A

GB2376754A - Optical chip substrate with a channel having a stepped profile

Info

Publication number: GB2376754A
Application number: GB0115364A
Authority: GB
Inventors: Ebrahim Iravani
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-06-22
Filing date: 2001-06-22
Publication date: 2002-12-24
Also published as: AU2002311458A1; GB0115364D0; WO2003001264A2; WO2003001264A3

Abstract

A substrate 1 a for an optical chip 10 has a channel 7 located in a surface 3. The channel 7 contains an optical fibre 13, and has a bottom 23, 25 with stepped profile in order to accommodate excess adhesive. The channel 7 may also have non-uniform widths w1, w2 and an end face 12 in order for a free end 14 of the optical fibre 13 to abut. The end face 12 may also have a recess 27, and extend widthwise at an inclined angle to the axis of the channel 7. The optic fibre 13 may optically connect with an optical component such as a waveguide 5.

Description

IMPROVEMENTS IN MOUNTING OF AN OPTICAL FIBRE Field of the Invention The present invention relates to improvements in mounting an optical fibre, and is particularly, although not exclusively, concerned with mounting of an optical fibre to a substrate for an optical chip.

Background of the Invention An optical chip may have one or more optical components which, for example, either produce photocurrent, emit light in response to an injection of electric current, multiplex or demultiplex optical signals of different wavelengths, or simply transport an optical signal. Typically, the optical chip has a silicon substrate on a surface of which the or each optical component is located. Invariably, the silicon substrate is mounted on an insulator and the optical chip is consequently termed a silicon-on-insulator chip, otherwise referred to as a SOI. One example of an optical chip is an optical transceiver in which a laser diode and a photodiode are located on the substrate surface together with associated waveguides.

It is known in the art to couple an optical fibre to an optical component of an optical chip by mounting the optical fibre in a channel chemically etched in the substrate surface to extend from an edge of the substrate surface to the optical component. When a channel is chemically etched in a silicon substrate the channel has a V-shaped cross section. 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. There are two principal problems in mounting an optical fibre in the channel in the substrate surface.

The first problem is getting the fibre to lie flat in the channel so that the central core of the optical fibre is optimally aligned with the optical component for optical coupling therebetween. One reason for not obtaining a flat lie of the optical

fibre is that adhesive is typically used to secure the optical fibre in place and that no provision is made to accommodate excess adhesive. The excess adhesive then leads to the fibre being displaced from a flat lie. Another reason is that the free end of the optical fibre may be of a greater circumference than the fibre body due to flaring caused on cleavage of the fibre. This is a particular problem when the free end is formed by laser cleaving.

The second problem is preventing the free end of the optical fibre from impacting the optical component and damaging itself and/or the optical component as the fibre is fed along the channel. A previous proposal for addressing this problem has been to provide a stop surface in the channel spaced from the optical component for the free end of the optical fibre to abut before impacting the optical component. One such stop surface is disclosed in US-A-5787214 (Harpin et al/Bookham Technology Ltd. ). A force sensor would typically be used to sense the reaction force created when the free end of the optical fibre contacts the stop surface and to generate a stop signal for stopping further feeding of the fibre. The stop surface is positioned a short distance from the optical component, preferably 5 urn or less, so that the fibre and optical component remain optically coupled.

The present invention proposes to provide means for alleviating these problems.

Summary of the Invention According to a first aspect of the present invention there is provided a substrate for an optical chip having a surface in which a channel is provided for an optical fibre to be received in, the channel having a bottom with a stepped profile.

The stepped profile provides a run-off for excess adhesive used to secure the optical fibre in the channel.

According to a second aspect of the present invention there is provided a substrate for an optical chip having a surface along which a channel extends for receiving an optical fibre, the channel having an end face for a free end of the

optical fibre to be juxtaposed with with a recess being formed in the end face at the surface of the substrate. An optical component can thus be located on the surface at the recess with the end face acting to space the free end of the optical fibre from the optical component to prevent impact damage on the optical component.

According to a third aspect of the present invention there is provided a substrate for an optical chip having a surface in which is provided a channel for receiving an optical fibre, the channel having an axis and an end face for a free end of the optical fibre to be juxtaposed with, the end face extending widthwise at an inclined angle to the axis.

According to a fourth aspect of the present invention there is provided a system comprising an optical fibre having a free end and a structure having a surface in which is provided a channel having a bottom with a stepped profile, the optical fibre being mountable in the channel.

According to a fifth aspect of the present invention there is provided a system comprising an optical fibre having a free end and a structure having a surface and a channel which extends along the surface and which has an end face in which a recess is formed at the surface of the substrate, the optical fibre being mountable in the channel so that the free end is located juxtaposed with the end face.

According to a sixth aspect of the present invention there is provided a system comprising an optical fibre having a free end and a structure having a surface in which is provided a channel having an axis and an end face which extends widthwise at an inclined angle to the axis, the optical fibre being mountable in the channel such that the free end is disposed adjacent the end face.

Preferred features of the invention are set out in the dependent claims.

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

Brief Description of the Drawings FIGURE 1 is a schematic, fragmentary cross-sectional side view of an optical fibre mounted to an optical chip in accordance with a first embodiment of the present invention; FIGURE 2 is schematic, fragmentary plan view of the optical chip of the first embodiment; FIGURE 3 is a cross-sectional view along section III-III in FIGURE 2 with the optical fibre omitted; FIGURE 4 is a schematic, fragmentary plan view of an optical fibre cleaved by laser cleaving mounted in the optical chip of the first embodiment; FIGURE 5 is a cross-sectional view along section V-V in FIGURE 4; FIGURE 6 is a schematic, fragmentary cross-sectional side view of an optical fibre mounted to an optical chip in accordance with a second embodiment of the present invention; and FIGURE 7 is a schematic, fragmentary cross-sectional side view of an optical fibre mounted to an optical chip in accordance with a third embodiment of the present invention.

Detailed Description of the Exemplary Embodiments of the Invention In the following description like reference numerals will be used to refer to like features in the different embodiments.

In FIGURES 1 to 3 there is shown an optical chip 10 in accordance with a first embodiment of the invention comprising a substrate 1, preferably of silicon, having an upper surface 3 on which optical components are formed. In this embodiment, a monolithic optical waveguide 5 is formed on the upper surface 3 in a manner known per se.

A groove 7 is etched in the upper surface 3 of the substrate 1 so as to extend from an edge 9 between the upper surface 3 and a side surface 11 to an end face 12 adjacent the waveguide 5. Mounted in the groove 7 is an optical fibre 13 with a free end 14 of the optical fibre 13 juxtaposed with the waveguide 5 for optical coupling therebetween. The optical fibre 13 has a central optical core 15 and an optical cladding 17 and would typically have a diameter of about 125 urn.

The waveguide 5 may be one of several waveguides on the upper surface 3 and may communicate with another optical component, such as a laser diode or photodiode, or simply extend to an edge of the upper surface 3 for optical coupling with another optical fibre.

The groove 7 is formed so as to have a rear section 19 and a front section 21. As shown in FIGURE 1, the rear and front sections 19,21 respectively have a bottom 23,25 with the bottom 25 of the front section 21 being at a greater depth than the bottom 23 of the rear section 19. Thus, the bottom of the groove 7 has a stepped profile.

The advantage of providing a stepped profile to the bottom of the groove 7 is that any excess adhesive used for securing the optical fibre 13 in the groove 7 is able to flow into the deeper front section 21 whilst the optical fibre 13 is supported on the bottom 23 of the rear portion 19. Thus, the optical fibre 13 tends to lie flat in the groove 7 despite the presence of excess adhesive whereby the core 15 of the optical fibre 13 remains aligned with the waveguide 5 for efficient optical coupling therebetween.

It will, of course, be understood that the stepped profile of the bottom of the groove 7 can take other forms for achieving the intended effect. For instance, the bottom 25 of the front section 21 may taper from the rear section 19 towards the end face 12.

Referring to FIGURE 2, it can be seen that the front section 21 has a transverse axis which is slanted to the longitudinal axis of the groove 7. In this manner, the end face 12 is inclined to the longitudinal axis of the groove 7 and, concomitantly, the longitudinal axis of the optical fibre 13. The end face 12 is inclined in this instance because the free end 14 of the optical fibre 13 is cleaved to be inclined to the longitudinal axis of the optical fibre 13. The inclination of the free end 14 of the optical fibre 13 reduces optical signal transmission loss between the optical fibre 13 and the waveguide 5, as outlined in US-A-5787214 supra, the content of which is incorporated herein by reference. The angle of inclination of the free end 14 is substantially equal to that of the end face 12 so that a substantially flush contact is made between the free end 14 of the optical fibre 13 and the end face 12 thereby minimizing impact forces between the two. The end face 12 is inclined so that a normal thereto (when viewed in plan) makes an angle

of 6-10 with the longitudinal axis of the groove 7, more preferably an angle of 6po.

FIGURES 2 and 3 also show that the front section 21 of the groove 7 possesses a width w1 which is greater than the width w2 of the rear section 19 thereby giving the groove 7 a generally T-shape when viewed in plan. FIGURE 3 shows that the rear section 19 of the groove 7 has a generally V-shape crosssectional profile and the front section 21 a generally U-shaped cross-sectional profile. The rear portion 19 may be formed by wet or chemical etching in a manner known per se whereas the front portion 21 may be formed by a dry etch process with well known lithographic techniques, for instance through plasma etching.

Not only does the increased width w1 of the front portion 21 provide an additional reservoir for excess adhesive, it also provides means for enabling the

optical fibre 13 to lie flat in the groove 7 when the free end 14 of the optical fibre 13 has a greater circumference than the bulk, for instance as caused by heating when the free end 14 is cleaved by laser cleaving. FIGURES 4 and 5 show an optical fibre 113 having a flared free end 114 caused by laser cleaving mounted in the groove 7 of the substrate 1. As will be seen, the increased width w1 of the front portion 21 accommodates the flared free end 114 so that the optical fibre 113 lies flatter in the groove 7 than if the groove 7 had a uniform cross-section with the width w2 of the rear portion 19. Of course, the increased depth of the front portion 21 also assists in improving the lie of the optical fibre 113 by accommodating the flared free end 114 of the optical fibre 113.

From FIGURES 1,2 and 4, it can be seen that a recess 27 is etched in the end face 12 of the groove 7. The recess 27 is wet etched at the same time as the V-shaped rear section 19 of the groove 7. The recess 27 is aligned with, and tapers to, the waveguide 5 which is spaced by a distance d from the end face 12. The dimensions of the recess 27 are such that the free end 14; 114 of the optical fibre 13; 113 contacts the end face 12 when fed along the groove 7 whereupon feeding can be stopped through use of an appropriate sensor. In this way, the free end 14; 114 of the optical fibre 13; 113 is prevented from abutting the waveguide 5 and damaging it. The distance d between the end face 12 and the waveguide 5 is selected so that the optical fibre 13; 113 is still optically coupled with the waveguide 5. Preferably, the distance d is no more than about 5 ! -1m, more preferably in the range of 2-3 pm.

In FIGURE 6 there is shown an optical chip 110 in accordance with the present invention which corresponds to the optical chip 10 of FIGURES 1 to 5 other than being provided with a recess 127 which has a stepped profile.

However, the recess 127 functions in the same way as the recess 27 of the optical chip 10 of FIGURES 1 to 5.

In accordance with the present invention, the recess 27; 127 in the end face 12; 112 of the groove 7; 107 may receive a material which has a refractive index which matches that of the core 15 of the optical fibre 13; 113 for optical coupling of

the optical fibre 13; 113 with the waveguide 5; 105 through the index-matching material. The index-matching material may be an epoxy resin or an encapsulant.

An example of a suitable epoxy resin is OPTOCAST 3553 (Electronic Materials, Inc.) and examples of suitable encapsulants are WACKER SilGel 612 and GE Silicones RTV615 and RTV655. The use of such a material increases optical transmission and reduces input power. This is because insertion losses are reduced as optical power is not lost in an air gap between the optical fibre 13; 113 and the waveguide 5; 105. Air bubbles in the index-matching material should be avoided as these can cause scattering of the optical signal, e. g. back reflections.

An example of the use of an index-matching material is shown in FIGURE 7. FIGURE 7 shows an optical chip 210 in accordance with the present invention which corresponds to the optical chip 10 of FIGURES 1 to 5 other than having a recess 227 in the end face 212 which extends from the upper surface 203 to the bottom 225 of the front section 221 of the groove 207. The recess 227 is filled with an index-matching material 229, such as an epoxy resin or encapsulant, to the level of the waveguide 205. As shown, some of the index-matching material runs underneath the optical fibre 13. It might therefore be beneficial for the depth of the front section 221 of the groove 207 to be the same as the rear section 219 to avoid or inhibit the material being drawn away from the recess 227.

It is to be understood that the present invention is not limited to the exemplary embodiments described above but may be modified and varied in many different ways within the scope of the appended claims. In particular, the present invention relates to individual features shown in the exemplary embodiments as well as combinations of features. It is also to be noted that the presence in the claims of reference numerals from the Figures of drawings is purely illustrative and not to be taken as having a limiting effect.

Claims

1. A substrate (1; 101; 201) for an optical chip (10 ; 110 ; 210) having a surface (3 ; 103 ; 203) in which a channel (7 ; 107 ; 207) is provided for an optical fibre (13 ; 113) to be received in, wherein the channel has a bottom (23,25 ; 123,125 ; 223,225) with a stepped profile.

2. A substrate according to claim 1, wherein the channel has an end face (12 ; 112 ; 212) for a free end (14 ; 114) of the optical fibre to be juxtaposed with.

3. A substrate (1 ; 101 ; 201) for an optical chip (10 ; 110 ; 210) having a surface (3 ; 103 ; 203) along which a channel (7 ; 107 ; 207) extends for receiving an optical fibre (13 ; 113), wherein the channel has an end face (12 ; 112 ; 212) for a free end (14 ; 114) of the optical fibre to be juxtaposed with and wherein a recess (27 ; 127 ; 227) is formed in the end face at the surface of the substrate.

4. A substrate according to any one of the preceding claims, wherein the channel has a non-uniform width (w1, w2).

5. A substrate according to any one of the preceding claims, wherein the channel has a first section (19 ; 119 ; 219) with a bottom (23 ; 123 ; 223) at a first level and a second section (21 ; 121 ; 221) with a bottom (25 ; 125 ; 225) at a second level which is below the first level.

6. A substrate according to claim 5, wherein the second section of the channel has a greater width (w1) than the width (w2) of the first section.

7. A substrate according to claim 5 or 6 when appended to claim 2 or 3, wherein the second section of the channel is positioned closer to the end face than the first section.

8. A substrate according to claim 7, wherein the second section is juxtaposed to the end face and the bottom of the channel has a downward step which connects the bottoms of the first and second sections.

9. A substrate according to any one of claims 5 to 8, wherein the first section extends to an edge (9 ; 109 ; 209) of the surface.

10. A substrate according to any one of claims 5 to 9, wherein the channel has an axis on which the first and second sections are collinearly arranged.

11. A substrate according to any one of claims 2, 3, 7 or 8, or any one of claims 4 to 6, 9 or 10 when appended to claim 2 or 3, wherein the end face extends widthwise at an inclined angle to the axis of the channel.

12. A substrate (1 ; 101 ; 201) for an optical chip (10 ; 110 ; 210) having a surface (3 ; 103 ; 203) in which is provided a channel (7 ; 107 ; 207) for receiving an optical fibre, the channel having an axis and an end face for a free end of the optical fibre to be juxtaposed with which extends widthwise at an inclined angle to the axis.

13. A substrate according to claim 12, wherein the channel has an enlarged section (21 ; 121 ; 221) at the end face.

14. A substrate according to claim 13, wherein the channel has a bottom (23, 25 ; 123, 125 ; 223, 225) which steps down to a lower level at the enlarged section.

15. A substrate according to claim 2 or any one of claims 4 to 11 when appended to claim 2, or any one of claims 12 to 14, wherein the channel extends along the surface and a recess (27 ; 127 ; 227) is formed in the end face at the surface.

16. A substrate according to claim 3, or any one of claims 4 to 11 when appended to claim 3, or claim 15, wherein the recess is formed in a central part of the end face.

17. A substrate according to claim 3, or any one of claims 4 to 11 when appended to claim 3, or claim 15, or claim 16, wherein the recess extends from the surface to a position part way down the end face.

18. A substrate according to claim 3, or any one of claims 4 to 11 when appended to claim 3, or claim 15, or claim 16, wherein the recess extends from the surface to the bottom of the channel.

19. A substrate according to claim 2, or any one of claims 4 to 11 when appended to claim 2, or any one of claims 12 to 14, wherein the surface is provided with an optical component (5 ; 105 ; 205) at the end face.

20. A substrate according to claim 2, or claim 3, or any one of claims 4 to 11 when appended to claim 2 or 3, or any one of claims 15 to 18 further comprising an optical component (5 ; 105 ; 205) on the surface for the free end of the optical fibre to be optically coupled with, the optical component being aligned with the channel and located adjacent the recess.

21. A substrate according to claim 19 or claim 20, wherein the optical component is an optical waveguide.

22. A substrate according to claim 20, wherein a material (229) having a refractive index capable of optically coupling the optical component with the optical fibre is located in the recess.

23. A substrate according to claim 19, wherein a material (229) having a refractive index capable of optically coupling the optical component with the free end of the optical fibre is positioned in the channel at the end face.

24. A system comprising an optical fibre (13 ; 113) having a free end (14 ; 114) and a structure (1 ; 101 ; 201) having a surface (3 ; 103 ; 203) in which is provided a channel (7 ; 107 ; 207) having a bottom (23,25 ; 123,125 ; 223,225) with a stepped profile, the optical fibre being mountable in the channel.

25. A system comprising an optical fibre (13 ; 113) having a free end (14 ; 114) and a structure (1 ; 101 ; 201) having a surface (3 ; 103 ; 203) and a channel (7 ; 107 ; 207) which extends along the surface and which has an end face (12 ; 112 ; 212), the optical fibre being mountable in the channel so that the free end is juxtaposed with the end face, wherein a recess (27 ; 127 ; 227) is formed in the end face at the surface of the substrate.

26. A system comprising an optical fibre (13 ; 113) having a free end (14 ; 114) and a structure (1 ; 101 ; 201) having a surface (3 ; 103 ; 203) in which is provided a channel (7 ; 107 ; 207) having an axis and an end face which extends widthwise at an inclined angle to the axis, the optical fibre being mountable in the channel so that the free end is juxtaposed with the end face.

27. A system according to claim 24,25 or 26, wherein the structure is a substrate for an optical chip or an optical chip.

28. An optical chip comprising a substrate according to any one of claims 1 to 23.

29. A substrate for an optical chip substantially as herein described with reference to, and as illustrated by, Figures 1 to 5, Figure 6 or Figure 7 of the accompanying drawings.

30. An optical chip substantially as herein described with reference to, and as illustrated by, Figures 1 to 5, Figure 6 or Figure 7 of the accompanying drawings.

31. A system substantially as herein described with reference to, and as illustrated by, Figures 1 to 5, Figure 6 or Figure 7 of the accompanying drawings.