GB2381592A

GB2381592A - Optical device with recess for optical component

Info

Publication number: GB2381592A
Application number: GB0125977A
Authority: GB
Inventors: Jolyon Richard Tidmarsh; Jim Kalock
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-10-30
Filing date: 2001-10-30
Publication date: 2003-05-07
Also published as: GB0125977D0

Abstract

A hybridised optical device has a planar lightwave circuit 10 incorporating a recess 12 having a bottom surface 18 and side walls 19. An optical component 11, such as a semiconductor optical amplifier (SOA), has a locating portion 16 fitted within the recess 12 and engaging the bottom surface 18. The circuit 10 has waveguides 14 having coupling portions at their ends adjacent to side walls of the recess 12, and the optical component 11 has a waveguide 17 having coupling portions at its ends in optical communication with the coupling portions of the waveguides 14. Furthermore the locating portion 16 of the optical component 11 has etched edge facets in the vicinity of the coupling portions of the waveguide 17 to enable the locating portion 16 to be accurately positioned within the recess 12. Preferably opposite side walls 19 of the recess 12 are inclined inwardly relative to one another to form a taper engaging correspondingly inclined etched edge facets of the locating portion 11 for accurately aligning the locating portion 11 within the recess 12.

Description

"Hybridised Optical Devices" This invention relates to hybridised optical devices, and is concerned more particularly with the alignment of an optical component within a planar lightwave circuit.

Semiconductor optical amplifiers (SOAs) are optical components which are fabricated in substantially the same manner as Fabry-Perot lasers, being formed, for example, on a InP substrate with a waveguide on an epitaxial layer on the substrate.

Increasing interest has been shown in the use of SOAs for the purposes of gating and amplifying optical signals in order to offset losses associated with passive waveguide elements through which the signals have passed, as well as losses due to component misalignments. In the case of a laser diode the waveguide will typically run perpendicularly between cleaved facets of the substrates. However, in the case of a SOA, it is necessary to minimise any optical reflections at the cleaved facets, and accordingly the waveguide is preferably angled at typically 100 to the perpendicular axis.

The SOA facets are typically formed by cleaving of the material in order to simplify the manufacturing process. In this manner the facet position can be controlled to an accuracy of only about 10 urn, however, and this can result in the length of the SOA varying by as much as 20 um. Furthermore any variation in the position of an end facet will also result in a change in the lateral waveguide position at the end facet.

Lensed optical fibres are usually used to make optical connections to both facets of the SOA, each fibre being capable of being individually manipulated and aligned with each facet independently. In this case variations in the length of the SOA do not adversely affect the assembly method or optical coupling efficiency. In this case it is possible to mount the SOA on a silicon chip having etched V grooves in order to assist fibre alignment, as disclosed by Leclerc, Brosson et al.,"High Performance Semiconductor Optical Amplifier Array for Self-Aligned Packaging using Si V-Groove Flip-Chip Technique", IEEE Photonics Technology Letters, Vol. 7, No. 5, May 1995.

However, in many applications, it is required to integrate the SOA in a hybridised optical device, for example within a planar lightwave circuit incorporating a waveguide structure, as well as other optical components, such as multiplexers and the like, for processing optical signals within the waveguide structure. It is particularly important to provide low loss optical coupling between the SOA and the waveguide structure of the planar lightwave circuit, and such low loss coupling is generally achieved by positioning the SOA in accurate alignment with the waveguide structure.

Two different methods have previously been proposed for integration of SOAs in such devices. In one method, as described by Fan and Hooker,"Hybrid Optical Switch using Passive Polymer Waveguides and Semiconductor Optical Amplifiers", Journal of Lightwave Technology, Vol. 18, No. 4, April 2000, the SOA is placed in the centre of a silicon substrate, and two planar lightwave circuits are aligned with the opposite facets of the SOA and bonded to the substrate. Optical fibres are connected to the substrate so as to be optically aligned with the waveguides of the circuits. In this case variation in the length of the SOA can be accommodated by moving the circuits appropriately during assembly. However special alignment techniques must be used to avoid lateral misalignment of the waveguides at the facets between the circuits and the SOA.

In an alternative method, as described by Ogawa Ebisawa et al.,"Hybrid Integrated Four-Channel SS-SOA Array Module using Planar Lightwave Circuit Platform", Electronics Letters, Vol. 34, No. 4,19 February 1998, the SOA is fitted within an oversized recess in a planar lightwave circuit. The recess is made oversized so as to be able to accommodate variations in the length of the SOA, even when at the limits of the cleave tolerances. It will be appreciated that, since the recess is oversized, there will normally be significant air gaps between the SOA and the end walls of the recess, and these air gaps can cause significant optical losses due to the divergence of the optical mode as it propagates through free space, as well as due to lateral refraction of the optical beam resulting in a lateral offset relative to the waveguide structure of the circuit. Whilst the divergence loss can be reduced by using spot-size converters which

However, in many applications, it is required to integrate the SOA in a reduce the divergence of the optical mode, the presence of large air gaps presents serious coupling efficiency problems, and thus renders this method impractical.

Reference is also made to the Applicants'Published British Patent Application No. 2344692A in which an optical component is mounted within a recess in a planar lightwave circuit so that input and output waveguide portions along the same edge of the optical component are aligned with waveguides on the circuit. Such an alignment method is only suitable for use in cases in which the waveguide portions to be optically in such devices. In one method, as described by Fan and Hooker, "Hybrid Optical coupled are along the same edge of the optical component.

Furthermore the Applicants'US Patent No. 5881190 describes an arrangement in which a optical component is aligned within a recess in a planar lightwave circuit such that two adjacent edges of the component abut adjacent location surfaces of the recess so as to provide proper alignment of the component with a waveguide of the circuit.

This is shown diagrammatically in Figure 1 of the accompanying drawings in which the component 1 is shown within the recess 2 adjacent the waveguide 3. In this case the adjacent edges 4 and 5 of the component 1 are intended to engage the location edges 6 SOA. and 7 of the recess 2. However, as shown in the figure, it is possible for the component 1 to rotate and become angularly misaligned as it is manoeuvred into position within the recess 2.

It is an object of the invention to provide an improved arrangement for forming a hybridised optical device.

According to the present invention there is provided a hybridised optical device comprising a planar lightwave circuit incorporating a recess having a bottom surface and side walls, and an optical component having a locating portion fitted within the recess and engaging the bottom surface, wherein the circuit has a waveguide structure having at least one coupling portion adjacent to a side wall of the recess, wherein the

optical component has a waveguide structure having at least one coupling portion in optical communication with the coupling portion of the circuit, and wherein said locating portion of the optical component has at least one etched edge facet enabling

said locating portion to be accurately positioned within the recess, said coupling portion of the optical component being adjacent said etched edge facet.

In the above device the etching of edge facets of the locating portion of the optical component allows the locating portion to be accurately fitted within the recess of the planar lightwave circuit. It will be appreciated that such etching of the edge facets allows much more accurate control of the length of the optical component than is possible with facets simply formed by cleaving of the material of the optical component. This allows the coupling portions of the waveguide structures of the optical component and the planar lightwave circuit to be accurately optically aligned in order to minimise coupling losses, either by virtue of engagement of the etched edge facets with the side walls of the recess or by alignment of the locating portion within the recess using a high accuracy pick and place machine.

The or each etched edge facet of the optical component may be in contact with at least one of the side walls of the recess where it is intended that the locating portion should be a close fit within the recess. Alternatively the or each etched edge facet of the optical component and at least one of the side walls of the recess may be separated by a free space region.

In one embodiment of the invention opposite side walls of the recess are inclined inwardly relative to one another to form a taper engaging correspondingly inclined etched edge facets of the locating portion for accurately aligning the locating portion within the recess. This taper allows the locating portion to be moved into its alignment position without any risk of the optical component rotating out of alignment during this procedure.

In an alternative embodiment the recess has opposite side walls which extend substantially parallel to one another for receiving substantially parallel etched edge facets of the locating portion therebetween. In this case the optical component may be optically aligned using a pick and place machine.

In a preferred embodiment the waveguide structure of the optical component has two coupling portions in optical communication with coupling portions of the waveguide structure of the circuit adjacent opposite side walls of the recess. This is a particularly convenient arrangement where the optical component is to be optically coupled inline. Furthermore the etched edge facets of the optical component may be recessed in the vicinity of the or each coupling portion, and/or the or each coupling portion of the circuit may be recessed relative to the side walls of the recess.

The invention also provides a method of forming a hybridised optical device comprising a planar lightwave circuit and an optical component, the method comprising forming a recess in the circuit having a bottom surface and side walls, etching the locating portion of the optical component so as to form at least one edge facet, and fitting the locating portion of the optical component within the recess so that at least one coupling portion of a waveguide structure of the optical component in the vicinity of said etched edge facet is placed in accurate optical alignment with at least one coupling portion of a waveguide structure of the circuit.

Preferably the edges of the locating portion of the optical component are formed by dry etching to a depth of approximately 10 um.

In order that the invention may be more fully understood, reference will be made to the accompanying drawings, in which: Figure 1 is an explanatory diagram illustrating a prior art arrangement; Figure 2 is a schematic prospective view of part of a planar lightwave circuit; Figure 3 is a schematic prospective view of an optical component having a locating portion for fitting within a recess in the circuit of Figure 2 to provide a hybridised optical device in accordance with the invention;

Figure 4 is a schematic prospective view during assembly of a further hybridised optical device in accordance with the invention; Figures 5 and 6 are respectively a schematic end view and a schematic plan view of the device of Figures 2 and 3; Figure 7 is an explanatory diagram of a further embodiment in accordance with the invention; Figure 8 shows three successive steps in the assembly of the device of Figures 2 and 3; and Figure 9 is an explanatory diagram illustrating various embodiments in accordance with the invention Figures 2 and 3 show respectively part of a planar lightwave circuit 10 and an optical component 11, such as an SOA or a laser diode for example, which is to be mounted on the circuit 10 to form the hybridised device. To this end the circuit 10 is formed with a trapezium-shaped recess 12 intersecting a waveguide 14 of the circuit 10.

Furthermore the component 11 is formed by dry etching with a trapezium-shaped locating portion 16 provided with a waveguide 17 to be optically coupled to the waveguide 14 of the circuit 10. The recess 12 has a bottom surface 18 and at least three (and preferably four) side walls 19. Within the bottom surface 18 is a recessed slot 20 incorporating a conductive pad 21 to which the locating portion 16 of the component 11 is soldered during assembly.

The locating portion 16 of the optical component 11 also has side walls 22 intended to abut the side walls 19 of the recess 12 on assembly, as well as a top surface 23 intended to abut the bottom surface 18 of the recess when the component 11 is flip bonded to the circuit 10. Preferably the recess 12 is larger than the locating portion 16 so as to allow the locating portion 16 to be introduced into the recess 12 so that the top wall 23 engages the bottom surface 18 of the recess, and then displaced in the direction

of the narrow end of the recess 12 until the narrow end of the locating portion 16 engages the end wall of the recess 12. This enables accurate location of the waveguides 14 and 17 relative to one another.

The locating portion 16 is defined lithographically prior to dry etching to a depth of about 10 pm into the semiconductor material, with the edges being inclined typically by 100 to the central axis. Since the waveguide facets are defined lithographically, the only variation in the length of the waveguide 17 will be caused by the undercutting inherent in the etching process, and this is small, typically less than 1 lam. A suitable dry etching process which may be used is reactive ion etching (RIE) or reactive ion beam etching (RIBE), utilising a CCl2F2/Ar/02 etch system where the material to be etched is InP for example. The ends of the waveguides 14 and 17 are preferably recessed to prevent scratching of their anti-reflection coatings during assembly. Since the waveguide facets are formed by etching they cannot be electrically pumped or otherwise the device will quickly fail. It is therefore necessary to provide transparent, passive waveguides at the interface, which may be fabricated by regrowth or quantum well intermixing, for example.

Figure 4 shows this locating and bonding process with respect to a variant embodiment in which like parts denoted by the same reference numerals primed as in the embodiment of Figures 2 and 3. In this case the optical lightwave circuit 10'and the optical component 11'have waveguides 14'and 17'which extend in the direction of the axis of symmetry of the trapezium-shaped recess 12'. Figure 4 shows the component 11'inverted so that its locating portion 16'can be fitted within the recess 12'prior to being appropriately located and bonded within the recess 12'as described above. In this case the waveguide 17'of the component 11'may be optically coupled to the circuit 10' at only one end.

Figure 5 shows an end view of the optical component of Figure 3 bonded to the circuit of Figure 2 by means of a fillet 24 of solder connecting the locating portion 16 to the conductive pad 21.

Figure 6 is a plan view from above of the optical component of Figure 3 bonded to the circuit of Figure 2. In this case the view is shown as though the component 11 is transparent (which, in practice, it will not be) so that the underlying parts of the component 11 and circuit 10 are visible. The alignment of the waveguides 14 and 17 can be seen in this figure. Furthermore it will be appreciated that the inclination of the side walls of the recess and the locating portion 16 can be chosen to provide the required angling of the waveguides at about 100 to the normal at the interface between the component 11 and the circuit 10, in order to minimise any optical reflections at the interface. In Figure 6 the recess is shown as being open along one side, although it will be appreciated that the recess can be closed along all four sides if required.

Figure 8 shows, at (a), (b) and (c), three successive steps in the assembly of the device. In this case the recess 12 in the circuit 10 is shown open-ended, with the locating portion 16 of the component 11 being introduced into the open end of the recess 12 in engagement with the bottom surface 18 of the recess, as shown at (a). However, as already indicated, the recess 12 may be closed on all four sides provided that it is made larger than the locating portion 16 so as to allow the locating portion 16 to be introduced into the recess 12 from above. The component 11 is then displaced in the direction of the arrow 40 towards the end wall 41 of the recess, as shown at (b), with the inverted top wall of the locating portion 16 engaging the bottom surface 18 of the recess, until the narrow end of the locating portion 16 engages the end wall 41, as shown at (c). This enables accurate lateral location of the waveguides 14 and 17 relative to one another. It should be appreciated that, in the fully engaged position shown at (c), there may still be slight gaps remaining between the oppositely inclined edges of the locating portion 16 and the side walls of the recess 12 in certain instances.

In a modification of the invention shown in Figure 7, the planar lightwave circuit 30 has a recess 32 provided with parallel side walls 33 terminated by diverging walls 34 which serve to guide a locating portion 31 of an optical component having parallel side walls into the recess 32. In this case the difference in width of the recess 32 and the locating portion 31 may be only 1 pim, as the tapering walls 34 help to guide the component into the correct position. In this position a waveguide 35 on the

component is optically coupled to a waveguide 36 on the circuit 30. Alternatively such an arrangement can be used for coupling of waveguides 37 and 38, as shown in broken lines in the figure.

In a further, non-illustrated modification of the invention, the shape of the recess and the corresponding locating portion of the optical component is generally that of a rhombus with its sides being parallel to one another and inclined so that the normal to each side is angled at approximately 10 to the waveguide axis.

It is a particular advantage of the described embodiments of the invention that the locating portion of the optical component is defined lithographically, rather than by cleaving, and can therefore fit snugly within the recess in the planar lightwave circuit so as to ensure that the waveguides on the component and the circuit are efficiently optically coupled by virtue of the fact that any gaps between the waveguide facets are minimise. The method is not specific to silicon-based systems, and can be applied to any planar lightwave circuit material system. Furthermore the method can be used to hybridise any inline optical component, such as an electro-absorption modulator or a lithium niobate modulator or a passive optical circuit such as an arrayed waveguide grating (AWG), for example. The shape of the locating portion of the component reduces the tendency for the component to twist as it is moved into alignment.

Figure 9 shows various possible shapes of the locating portion of the optical component which may be used in different embodiments in accordance with the invention. It will be appreciated that in each case the generally rectangular shape of the optical component is indicated by the rectangular external outline 50 whereas the inner hatched area 51 denotes the shape of the locating portion. Apart from the generally wedged shape of the locating portion as already described with reference to Figures 2 to 6 and 8, the illustrated arrangements include locating portions having opposite parallel etched edge facets, as well as arrangements in which etched edge facets are provided only along two adjacent edges (but not along the other two edges of the locating portion) for optical coupling of waveguides provided along those edges.

Claims

CLAIMS:

1. A hybridised optical device comprising a planar lightwave circuit incorporating a recess having a bottom surface and side walls, and an optical component having a locating portion fitted within the recess and engaging the bottom surface, wherein the circuit has a waveguide structure having at least one coupling portion adjacent to a side wall of the recess, wherein the optical component has a waveguide structure having at least one coupling portion in optical communication with the coupling portion of the circuit, and wherein said locating portion of the optical component has at least one etched edge facet enabling said locating portion to be accurately positioned within the recess, said coupling portion of the optical component being adjacent said etched edge facet.

2. A device according to claim 1, wherein said etched edge facet of the optical component is in contact with at least one of the side walls of the recess.

3. A device according to claim 1, wherein said etched edge facet of the optical component and at least one of the side walls of the recess are separated by a free space region.

4. A device according to claim 1,2 or 3, wherein opposite side walls of the recess are inclined inwardly relative to one another to form a taper engaging correspondingly inclined etched edge facets of the locating portion for accurately aligning the locating portion within the recess.

5. A device according to claim 1,2 or 3, wherein the recess has opposite side walls which extend substantially parallel to one another for receiving substantially parallel etched edge facets of the locating portion therebetween.

6. A device according to any preceding claim, wherein said etched edge facet of the optical component is recessed in the vicinity of said coupling portion.

7. A device according to any preceding claim, wherein the waveguide structure of the optical component has two coupling portions in optical communication with coupling portions of the waveguide structure of the circuit adjacent opposite side walls of the recess.

8. A device according to any preceding claim, wherein said coupling portion of the circuit is recessed relative to the side walls of the recess.

9. A device according to any preceding claim, wherein the recess has an end wall which is engaged by an end facet of the optical component.

10. A device according to any preceding claim, wherein the bottom surface of the recess is provided with a conductive contact to which the optical component is connected by soldering.

11. A device according to claim 10, wherein the optical component incorporates an electrical circuit which is electrically connected to the contact.

12. A device according to any preceding claim, wherein opposite edge facets of the optical component are inclined relative to a normal to an optical axis extending through the coupling portions.

13. A device according to any preceding claim, wherein the optical component has an upper portion which overlaps an upper surface of the optical circuit.

14. A device according to any preceding claim, wherein the coupling portions have anti-reflection coatings.

15. A device according to any preceding claim, wherein the circuit is formed by silicon-on-insulator (SOI) technology.

16. A device according to any preceding claim, wherein the optical component comprises a semiconductor optical amplifier (SOA).

17. A method of forming a hybridised optical device comprising a planar lightwave circuit and an optical component, the method comprising forming a recess in the circuit having a bottom surface and side walls, etching the locating portion of the optical component so as to form at least one edge facet, and fitting the locating portion of the optical component within the recess so that at least one coupling portion of a waveguide structure of the optical component in the vicinity of said etched edge facet is placed in accurate optical alignment with at least one coupling portion of a waveguide structure of the circuit.

18. A method according to claim 17, wherein the edges of the locating portion of the optical component portion are formed by etching to a depth of approximately 10 lam.

19. A method according to claim 17 or 18, wherein the edges of the locating portion of the optical component are formed by dry etching.

20. A method according to claim 17,18 or 19, wherein the edges of the locating portion of the optical component are each etched at an angle of approximately 10 to a

normal to adjacent surfaces of the coupling portions. i

21. A method according to claims 17,18 or 19 or 20, wherein the locating portion of the optical component is fitted into the recess and subsequently displaced towards an end wall of the recess so as to provide proper alignment of the locating portion of the optical component within the recess.

22. A hybridised optical device substantially as hereinbefore described with reference to Figures 2 to 8 of the accompanying drawings.

23. A method of forming an hybridised optical device substantially as hereinbefore described with reference to Figures 2 to 8 of the accompanying drawings.