GB2374156A

GB2374156A - Aligning array of optical fibres with waveguides of optical device

Info

Publication number: GB2374156A
Application number: GB0108204A
Authority: GB
Inventors: Craig Tombling; Stephen James Fasham
Original assignee: Kamelian Ltd
Current assignee: Kamelian Ltd
Priority date: 2001-04-02
Filing date: 2001-04-02
Publication date: 2002-10-09
Also published as: GB0108204D0

Abstract

The end of an array of optical fibres 27, such as ribbon fibre, is aligned with waveguides 33 in an optical device 31, such as an active semiconductor optical device. The ends of the optical fibres 27 are mounted in grooves 25 in a grooved surface 23 of a first block 21 of a fibre block assembly 20. A second block 22 maybe mounted to the grooved surface 23 of the first block 21 with the fibres 27 mounted between the blocks 21, 22. The grooved surface 23 of the first block 21 extends beyond the second block 22 to form guide surfaces 30. The optical device 31 is supported on a support surface 36 of a bench 32 having guide surfaces 37 extending from the support surface 36. The ends of the optical fibres are aligned with the waveguides 33 in the optical device 31 by moving the fibre block assembly 20 with the guide surfaces 30 of the first block 21 in contact with the guide surfaces 37 of the bench 32. This allows adjustment of the alignment in the direction in which the fibres 27 extend and laterally.

Description

ALIGNMENT OF OPTICAL FIBRES WITH AN OPTICAL DEVICE The present invention relates to the alignment of the end of an array of optical fibres, such as in ribbon fibre, with waveguides in an optical device, such as an active semiconductor optical device or a passive device.

Optical fibres are commonly used to interconnect optical devices of various forms, including passive waveguide structures in silica and silicon and active semiconductor devices. Attaching optical fibres to optical devices is a technical challenge, particularly when attaching an array of optical fibres, for example in the form of ribbon fibre.

A well known way of attaching ribbon fibre to a passive, multi-channel optical device, for example an arrayed waveguide grating (AWG), involves mounting the end of the ribbon fibre in a fibre block assembly. Fig. 1 is an end view of a known form of fibre block assembly 1 comprising two blocks 2 bonded together. An array of fibres 3 are mounted between the two blocks 2 in arrays of v-grooves 4 formed in the opposed bonded surfaces 5 of the blocks 2. The fibre block assembly 1 serves to fix the relative positions of the fibres 3 in the array to a tolerance of, say, around : 0. 5um. The end surface of the fibre block assembly 1 is usually polished to an optical surface, either at normal incidence to the optical axes of the fibres or at an angle, if required.

A known way of using the fibre block assembly 1 of Fig. 1 to couple the optical fibres 3 to a passive optical device such as an AWG is known as"butt coupling". Fig. 2 is a side view illustrating the fibre block assembly 1 butt coupled to a passive optical device 6 in which there are waveguides 7. The polished end surface 8 of the fibre block assembly 1 and the polished surface 9 of the optical device 6 are brought into contact, or close proximity, commonly with a thin layer of optically clear adhesive 10 at the interface between the fibre block assembly 1 and the optical device 6. Prior to curing of the adhesive 10, the fibre block assembly 1 and the optical device 6 may be relatively aligned in the directions perpendicular to the optical fibres 3, that is in the directions up and down in Fig. 2 and into and out of

the paper. However, the alignment in the direction in which the fibres 3 extend is fixed and may not be adjusted.

Butt coupling is unsuitable when it is important to avoid physical contact between the optical fibre and the optical device. This is the case, for example, with many active optical devices, such as those made of a semiconductor such as indium phosphide. Contact is also undesirable for optical devices having a fragile thin film coat.

The purpose of the present invention is to provide for alignment of an array of optical fibres with an optical device which allows alignment in a direction perpendicular to the optical axes of the optical fibres to be achieved more easily and/or which allows adjustment of the alignment along the direction in which the fibres extend.

According to a first aspect of the present invention, there is provided a method of aligning the end of an array of optical fibres with waveguides in an optical device, comprising: mounting the end of the array of optical fibres in a fibre block assembly comprising at least a first block, the first block having an array of grooves in a grooved surface of the block, the optical fibres being mounted in the grooves, the first block further having at least one guide surface at a predetermined level with respect to the axes of the array of optical fibres ; supporting the optical device on a support surface of bench having guide surfaces extending from the support surface; and aligning the end of an array of optical fibres with the waveguides in the optical device by moving the fibre block assembly with the guide surfaces of the first block in contact with the guide surfaces of the bench.

The provision of guide surfaces on the fibre block assembly and the bench assists alignment in a direction perpendicular to the optical axes of the optical fibres because the contact between the guide surfaces fixes the alignment in one direction perpendicular to the optical axes of the optical fibres The alignment is achieved simply by bringing the guide surfaces into contact without any active adjustment

being necessary.

Also, the provision of guide surfaces allows the alignment of the end of the array of optical fibres to be adjusted in the direction along which the fibres extend.

The contact between the guide surfaces allows the fibre block assembly to be guided along the bench in a controlled manner during the alignment process. The method also allows the alignment of the array of optical fibres to be adjusted laterally of the array, that is in the direction in which the array repeats. Accordingly, the method allows the end of the array of fibres to be positioned and subsequently fixed at a position in close proximity to the optical device but without physical contact. This is a particular advantage in the case of active semiconductor optical devices and other devices which are fragile, for example because of the presence of an optical thin film.

Nonetheless the method may be used with any type of optical device. The method allows the optical fibres and the optical device to be butt coupled in which case the primary advantage is the fixing of alignment in one direction by the contact between the guide surfaces.

The vertical position of the array of optical fibres relative to the optical device, that is perpendicular to the direction of the optical axes of the fibres and to the direction in which the array repeats, is fixed by the contact between the guide surfaces of the fibre block assembly and the bench. To achieve vertical alignment, preferably the axes of the waveguides of the optical device are at a distance relative to the surface of the optical device on which the optical device is supported on the bench equal to the sum of the distance, if any, of the axes of the optical fibres relative to the guide surface of the fibre block assembly plus the distance, if any, of the guide surface of the bench relative to the support surface. The distance of the axes of the waveguides relative to the surface of the optical device is known, because this may be controlled during manufacture of the optical device. The distance of the axes of the optical fibres relative to the guide surfaces of the fibre block assembly and the distances of the guide surfaces of the bench relative to the support surface may be controlled by suitable design of the fibre block assembly and the bench. In particular, it is possible to control these distances to achieve vertical alignment with a

desirable tolerance of at least : 0. 5 um According to further aspects of the present invention, there are provided a fibre block assembly, a bench and various combinations of a fibre block assembly, a bench and an optical device which are suitable for use in the method according to the first aspect of the present invention To allow better understanding, embodiments of the present invention will now be described by way of non limitative example with reference to the accompanying drawings in which: Fig 1 is an end view of a known fibre block, Fig. 2 is a side view of a fibre block butt coupled to an optical device; Fig. 3 is an end view of a fibre block in accordance with the present invention ; Fig. 4 is a top view of the fibre block of Fig 3 on a bench supporting an optical device ; Fig. 5 is a cross-sectional view of the assembly of Fig. 4 taken along the line IV-IV along which the fibres extend, Fig. 6 is an end view of an alternative fibre block in accordance with the present invention ; and Fig. 7 is a cross-sectional view of the fibre block of Fig. 6 being aligned on a bench supporting an optical component.

A fibre block assembly 20 in accordance with the present invention is shown in end view in Fig. 3 The fibre block assembly 20 comprises a first block 21 and a second block 22 mounted together with opposed surfaces 23 and 24 of the respective blocks 21 and 22 facing one another and in contact. The surface 23 of the first block 21 is grooved with an array of parallel grooves 25 The opposed surface 24 of the second block 22 is also grooved with an array of grooves 26 matching the array of grooves 25 in the first block 21. The grooves 25 formed in the first block 21 are each aligned with a respective groove 26 in the second block 22. The first and second blocks 21 and 22 are bonded to the fibres 27 with zero thickness bonds (4-point contact), hence effectively bonding the first and second blocks 21 and 22 together.

A linear array of optical fibres 27, for example in the form of ribbon fibre, are mounted in the grooves 25 and 26 between the first and second blocks 21 and 22 The first and second blocks 21 and 22 are bonded to the fibres 27 with zero-thickness bonds (4-point contact), hence effectively bonding the first and second blocks 21 and 22 together.

The blocks 21 and 22 are formed from silicon, or any other suitable material.

The grooves 25 and 26 are v-grooves which may be formed in a conventional manner by etching to a crystal face or precision grinding which has a similar or better tolerance than etching. This allows the depth of the grooves 25 and 26 to be precisely controlled. In the fibre block assembly 20 of Fig. 3, the grooves 25 in the first block 21 are the same depth as the grooves 26 in the second block 22. As a result, the fibres 27 are aligned with their axes 28 at the level of the opposed, grooved surfaces 23 and 24 of the first and second blocks 21 and 22.

The grooved surface 23 of the first block 21 extends from either side of the array of grooves 25 beyond the side 29 of the second block 22 to form guide surfaces 30 extending parallel to the axes 28 of the optical fibres 27.

Figs 4 and 5 are a top view and a cross-sectional view, respectively, of the fibre block assembly 20 during alignment with an optical device 31 using a bench 32 (which might sometimes be called a sub-mount or a motherboard).

The optical device 31 has an array of waveguides 33 each centred on a

respective axis 34 proximate to a given surface 35 of the optical device 31. The spacing of the waveguides 33 of the optical device is the same as the spacing of the optical fibres 27 in the fibre block assembly 20. The distance x of the axes 34 of the waveguides 33 from the proximate surface 35 is known. The optical device 31 may be any type of device, but is typically an active semiconductor optical device, for example made from indium phosphide. In this case, the waveguides 33 are formed using epitaxy. During epitaxy, deposition of layers occurs in a controlled manner, so the distance of the axes 34 from the proximate surface 35 formed by the finally deposited layer is precisely known.

The bench 32 is formed with a support surface 36 on which the optical device

31 is supported by the proximal surface 35, where the optical device 31 is fixed in place. Accordingly, the axes 34 of the waveguides 33 are a known distance relative to the support surface 36 of the bench 32 The bench 32 is further formed with a pair of guide surfaces 37 which extend parallel to one another from the support surface 36 in front of the optical device 31.

Between the guide surfaces 37, the bench 32 is formed with a recess 38 so that there is a gap between the guide surfaces 37.

For alignment of the fibre block assembly 20 with the optical device 31, the fibre block assembly is placed with its guide surfaces 30 contacting the guide surfaces 37 of the bench 32. The gap between the guide surfaces 37 is wider than the length of the block 22 between its sides 29. Accordingly, the second block 22 of the fibre block assembly 20 fits in the gap between the guide surfaces 37 of the bench 32 without contacting the bench 32. In contrast, the guide surfaces 30 of the first block of the fibre block assembly 20 extend by a sufficient distance that they overlap the guide surfaces 37 of the bench 32.

The end of the array of optical fibres 3 is aligned with the waveguides 33 in the optical device 31 by moving the fibre block assembly 20 with respect to the bench 32 whilst keeping the guide surfaces 30 of the fibre block assembly 20 in contact with the guide surfaces 37 of the bench 32. This contact allows the fibre block assembly 20 to be slid in a controlled manner during the alignment process. In particular, it allows adjustment in the direction 39 along which the optical fibres extend. It also allows adjustment in the lateral direction 40, that is the direction in which the array of optical fibres 27 repeats. Consequently, the fibre block assembly 20 may be aligned with the ends of optical fibres 27 in close proximity to the optical device 31 but without physical contact.

The alignment may be optimised in a conventional manner, for example by supplying power through either the optical fibres 27 or the optical device 31 and detecting the power output to the other one of the optical fibres 27 and the optical device 31. After alignment, the fibre block assembly 20 is fixed to the bench 32.

The fixing may occur in numerous ways, for example by bonding with glue between

the guide surfaces 30 and 37 or with glue applied through the recess 38 or through channels (not shown) in either or both of guide surfaces 30 or 37, or by butt coupling.

The contact between the guide surfaces 30 and 37 fixes the vertical movement of the fibre block assembly 20 with respect to the bench 32, that is in the direction perpendicular to the axes 28 of the optical fibres 27 and the guide surfaces 30 and also perpendicular to the direction in which the array of fibres 27 repeats. As the axes 28 of the optical fibres 27 are at the same level as the guide surfaces 30, vertical alignment with the waveguides 33 is achieved by the bench 32 being formed with a step 41 between the support surface 36 and the guide surfaces 37 of the bench 32. The step 41 has the same height as the distance x between the axes 34 of the waveguides 33 relative to the proximate surface 35 of the optical device 31. As a result, the axis 28 of the optical fibres 27 is at the same level as the axes 34 of the

waveguides 33 and the optical device 31.

The height of the step 41 may be precisely controlled during manufacture of the bench 32. For example, the support surface 36, the guide surfaces 37 and the recess 38 of the bench 32 may be formed by etching the bench 32 from a piece of silicon. Such etching allows accurate control of the relative levels of the support surface 36 and the guide surfaces 37.

Optionally the optical device 31 overlays the support surface 36, for example by extending the recess 38 to the position indicated by the dotted lines 43. This is to ensure the bench 32 does not limit the progress of the fibre block assembly 20 towards the optical device 31.

There are of course alternative ways to control the vertical alignment of the optical fibres 27 with the waveguides 33 of the optical device 31. One such alternative is the embodiment illustrated in Figs. 6 and 7. This alternative embodiment has many features in common with the embodiment as illustrated in Figs. 3 to 5, so the common features will be given the same reference numerals and the description thereof will not be repeated, for brevity.

The fibre block assembly 20 of the alternative embodiment is shown in end view in Fig. 6. The difference from the embodiment of Figs. 3 to 5 is that the

grooves 25 formed in the first block 21 have an increased depth, whereas the groove 26 formed in the second block 22 have a reduced depth As a result, the optical fibres 27 are mounted between the first and second blocks 21 and 22 with their axes 28 at a level a known distance y relative to the guide surfaces 30 of the first block 21 As previously noted, the depth of the grooves 25 and 26 in the first and second blocks 21 and 22 may be precisely controlled Fig. 7 is a cross-sectional view of the alternative embodiment showing the fibre block assembly 20 positioned on the bench 32. The difference from the embodiment of Figs. 3 to 5 is that the support surface 36 and guide surfaces 37of the bench 32 are at the same level without any step in between. Again, the vertical position of the fibre block assembly 20 relative to the optical device 31 is fixed by the contact between the guide surfaces 30 of the fibre block assembly 20 and the guide surfaces 37 of the bench 32. Vertical alignment of the optical fibres 27 with the waveguides 33 of the optical device 31 is achieved by selecting the distance y of the axes 28 of the optical fibres 27 from the guide surfaces 30 to be equal to the distance of the axes 34 of the waveguides 33 relative to the proximate surface 35 of the optical device 31.

In general, the vertical alignment of optical fibres 27 of the waveguides 33 of the optical device 31 may be controlled by varying both (1) the level of the axes 28 of the optical fibres 27 with respect to the guide surfaces 30, as in the embodiment of Figs. 6 and 7, and (2) the distance between the support surface 36 and the guide surfaces 37 of the bench 32, as in the embodiment of Figs. 3 to 5. For example, the support surface 36 may be higher than the guide surfaces 37, in contrast to the embodiment of Figs. 3 to 5. Also, in practical embodiments there is likely to be a gap between the first block 21 and the second block 22. In that case, the axes 28 of the fibres 27 will be a predetermined distance below the guide surfaces 30, for example half-way across the gap if the grooves 25 and 26 in the two blocks 21 and 22 are identically sized. In general, vertical alignment is achieved when the sum of the distance, if any, of the axes 28 of the optical fibres 27 relative to the guide surfaces 30 of the first block 21 plus the distance, if any, of the guide surfaces of the

bench 32 relative to the support surface 36 is equal to the distance of the axes 34 of the waveguides 33 from the proximate surface 35. In practice, these distances may be sufficiently controlled to provide accurate vertical alignment. Typical tolerances achievable are of the order : ! : 0. 5 um. Errors which do occur are largely due to eccentricity of the core of the optical fibres 27 and variation in the diameter of the cladding of the optical fibres 27.

Numerous modifications to the described embodiments are envisaged. For example, the end face 42 of the fibre block assembly 20 and the end face 43 of the optical device 31 may be angled in a conventional manner with respect to the direction along which the optical fibres 28 extend, for example across the array of fibres 28 or in the vertical direction. Another possible modification is to omit the second block 22 from the fibre block assembly 20, although the presence of the second block 22 is preferred to assist in accurate positioning of the optical fibres 27 in the fibre block assembly 20.

Other surfaces than the grooved surface 23 of the first block 21 could be used as the guide surfaces of the fibre block assembly 20. For example, specific guide surfaces could be etched or the outside surface of one of the blocks could be used.

However it is preferred to use the grooved surface 23 because this is easy to manufacture simply by making the first block 21 wider than the second block 22, and also because the relative level of the optical fibres 27 from the grooved surface 23 is accurately known and controllable. In contrast if the outer surfaces of the block were used as guide surfaces, the distance from the optical fibres 27 would be harder to determine as the thickness of the blocks 21 and 22 is typically 500um with an error of 50um.

Another possible modification is to introduce physical features on the fibre block assembly 20 and/or the bench 32 which constrain the movement of the fibre block assembly 20 with respect to the bench 32. One example of this is for stops (not shown) to be provided on the bench 32 to prevent the fibre block assembly 20 from being moved too close to the optical device 31. Another example is to provide the first block 21 with additional grooves (not shown) extending parallel to the grooves 25 mounting the optical fibres 27 for lateral alignment of fibre block assembly 20 in the direction in which the array of optical fibres 27 repeats, for example by fitting over matching protrusions on the guide surface 37 of the bench 32.

Claims

1. A method of aligning the end of an array of optical fibres with waveguides in an optical device, comprising: mounting the end of the array of optical fibres in a fibre block assembly comprising at least a first block, the first block having an array of grooves in a grooved surface of the block, the optical fibres being mounted in the grooves, the first block further having at least one guide surface at a predetermined level with respect to the axes of the array of optical fibres; supporting the optical device on a support surface of bench having guide surfaces extending from the support surface ; and aligning the end of an array of optical fibres with the waveguides in the optical device by moving the fibre block assembly with the guide surfaces of the first block in contact with the guide surfaces of the bench.

2. A method according to claim 1, wherein the axes of the waveguides of the optical device are at a distance from the surface of the optical device on which the optical device is supported equal to the sum of the distance, if any, of the axes of the optical fibres relative to the guide surfaces of the first block plus the distance, if any, of the guide surfaces of the bench relative to the support surface.

3. A method according to claim 1 or 2, wherein the guide surfaces of the fibre block assembly are formed by portions of the grooved surface which extend from either side of the array of grooves.

4. A method according to claim 3, wherein the fibre block assembly further comprises a second block mounted with a surface facing the grooved surface of the first block with the fibres mounted between the blocks, the guide surfaces of the first block extending beyond the second block.

5 A method according to claim 4, wherein the surface of the second block facing the grooved surface of the first block has an array of grooves matching the array of grooves in the first block and in which the fibres are mounted.

6. A method according to any one of the preceding claims, wherein the optical fibres are mounted in the grooves with their axes at the level of the grooved surface.

7. A method according to any one of claims 1 to 5, wherein the optical fibres are mounted in the grooves with their axes at a level a predetermined distance from the grooved surface.

8 A method according to any one of the preceding claims, wherein the array of optical fibres forms a ribbon fibre.

9 A method according to any one of the preceding claims, wherein the guide surfaces of the bench are at the same level as the support surface

10. A method according to any one of claims 1 to 8, wherein the guide surfaces are at a level a predetermined distance from the support surface.

11. A method according to any one of the preceding claims, wherein the guide surfaces of the bench are a pair of surfaces between which the bench has a gap or recess.

12. A method according to any one of the preceding claims, wherein the optical device is an active semiconductor optical device.

13. A method of aligning the end of an array of optical fibres with waveguides in an optical device substantially as hereinbefore described with reference to Figs. 3 to 7 of the accompanying drawings.

14. A fibre block assembly mounting the end of an array of optical fibres, the fibre block assembly comprising ; at least a first block, the first block having an array of grooves in a grooved surface of the block, the optical fibres being mounted in the grooves with their axes at a predetermined level with respect to the grooved surface, wherein the grooved surface extends from either side of the array of grooves to form guide surfaces for guiding movement of the fibre block assembly during alignment of the end of the array of optical fibres.

15. A fibre block assembly according to claim 14, wherein the optical fibres are mounted in the grooves with their axes at the level of the grooved surface.

16. A fibre block assembly according to claim 14, wherein the optical fibres are mounted in the grooves with their axes at a level a predetermined distance from the grooved surface.

17. A fibre block assembly according to any one of claims 14 to 16, wherein the fibre block assembly further comprises a second block mounted to the grooved surface of the first block with the fibres mounted between the blocks, the guide surfaces of the first block extending beyond the second block.

18. A fibre block assembly according to claim 17, wherein the second block has an array of grooves matching the array of grooves in the first block and in which the fibres are mounted.

19. A fibre block assembly according to any one of claims 14 to 18, wherein the array of optical fibres forms a ribbon fibre.

20. A fibre block assembly constructed substantially as hereinbefore described with reference to Figs. 3 to 7 of the accompanying drawings

21. A bench for aligning a fibre block assembly mounting the end of an array of optical fibres with an optical device, the bench having : a support surface for supporting the optical device ; guide surfaces extending from the support surface for guiding movement of the block assembly during alignment of the end of the array of optical fibres, the guide surfaces being at a predetermined level with respect to the support surface.

22. A bench according to claim 21, wherein the guide surfaces are at the same level as the support surface.

23. A bench according to claim 21, wherein the guide surfaces are at a level a predetermined distance relative to the support surface.

24. A bench according to any one of claims 21 to 23, wherein the guide surfaces are a pair of surfaces between which the bench has a gap or recess

25. A bench constructed substantially as hereinbefore described with reference to Figs. 3 to 7 of the accompanying drawings.

26. A bench according to any one of claims 21 to 25, in combination with an optical device having waveguides, the axes of which waveguides are at a predetermined distance from a surface of the optical device.

27. A combination according to claim 26, wherein the optical device is an active semiconductor optical device.

28. A bench according to any one of claims 21 to 25, in combination with a fibre block assembly according to any one of claims 14 to 20

29. A bench according to any one of claims 21 to 25, in combination with both a

fibre block assembly according to any one of claims 14 to 20 and an optical device having waveguides, wherein : the guide surfaces of the bench match the guide surfaces of the fibre block for guiding movement of the fibre block assembly with the guide surfaces of the bench in contact with the guide surfaces of the first block during alignment of the end of the array of optical fibres with the waveguides of the optical device ; and the axes of the waveguides of the optical device are at a distance from a surface of the optical device equal to the sum of the distance, if any, of the axes of the optical fibres relative to the grooved surface of the first block plus the distance, if any, of the guide surfaces of the bench relative to the support surface.

30. A combination according to claim 29, wherein the optical device is an active semiconductor optical device.