GB2381593A

GB2381593A - Optical device with optically connected planar lightwave circuits

Info

Publication number: GB2381593A
Application number: GB0125979A
Authority: GB
Inventors: Jolyon Richard Tidmarsh
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: GB0125979D0

Abstract

A hybridised optical device has a first planar lightwave circuit (PLC) 20 having a first waveguide structure. A second planar lightwave circuit (PLC) 21 has a second waveguide structure and mounted on the PLC 20 so that the first waveguide structure is in optical communication with the second waveguide structure. An optical component which maybe a semiconductor optical amplifier (SOA) 22, has a third waveguide structure and mounted on the PLC 20 adjacent to the PLC 21. The SOA 22 is mounted so that the third waveguide structure is in direct optical communication with both the first waveguide structure and the second waveguide structure. Furthermore at least one optical fibre connector 23, 24 is provided on the PLC 20 and positioned in an optical path passing successively through the PLC 20, the SOA 22 and the PLC 21. The optical component may also be an electro-absorption or lithium niobate modulator, an arrayed waveguide grating, or an optical attenuator. The PLC's 20,21 may also incorporate a multiplexer and a demultiplexer, and be based on silicon-on-insulator (SOI) technology.

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 rnisaligmnents. 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 ilO, um, however, and this can result in the length of the SOA varying by as much as 20 am. This is illustrated in Figure 1 of the accompanying drawings in which the angled waveguide 2 is shown extending across the SOA 3, and the variation in the position of each facet is denoted in broken lines at 4. It will be appreciated from this figure that any variation d in the position of an end facet will also result in a change As in the lateral distance s of the waveguide at the end facet from a side edge of the SOA 3.

Furthermore, as shown in Figure 2 of the drawings, lensed optical fibres 5 are usually used to make optical connections to both facets of the SOA 3, 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", IDEE Photonics Technology Letters, Vol. 7, No. 5, May 1995.

However, in many applications of SOAs, 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. 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 10 is placed in the centre of a silicon substrate 11, as shown in side view in Figure 3 of the drawings, and two planar lightwave circuits 12 and 13 are aligned with the opposite facets of the SOA 10 and bonded to the substrate 11. Optical fibres 14 and 15 are connected to the substrate 11 so as to be optically aligned with the waveguides of the circuits 12 and 13. In this case variation in the length of the SOA 10 can be accommodated by moving the circuits 12 and 13 appropriately during assembly. However this arrangement is relatively complex and suffers from the fact that it does not include an alignment reference which can result in 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. This is shown in Figure 4 in which the SOA 3 is shown fitted within a recess 16 in a circuit 17 having a waveguide structure 18. It will be appreciated that, since the recess 16 is oversized, there will normally be significant air

gaps between the SOA 3 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 18 of the circuit 17. Whilst the divergence loss can be reduced by using spot-size converters which 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 coupled are along the same edge of the optical component.

It is an object of the invention to provide a hybridised optical device which can be fabricated in a straightforward manner whilst accommodating tolerances in the dimensions of a hybridised optical component.

According to the present invention there is provided a hybridised optical device comprising a first planar lightwave circuit having a first waveguide structure, a second planar lightwave circuit having a second waveguide structure and mounted on the first planar lightwave circuit such that the first waveguide structure is in optical communication with the second waveguide structure, an optical component having a third waveguide structure and mounted on the first planar lightwave circuit adjacent to the second planar lightwave circuit such that the third waveguide structure is in direct optical communication with both the first waveguide structure and the second waveguide structure, and at least one optical fibre connector on the first planar lightwave circuit.

In such a device the second planar lightwave circuit and the optical component are both mounted on the first planar lightwave circuit, as is the or each optical fibre

connector, so that the assembly process is simplified as compared with the arrangement described with reference to Figure 3 above in that fewer parts are required, and the waveguide structure on the first planar lightwave circuit provides an alignment reference since it is inherently perfectly aligned with the optical axis. Variations in the length of the optical component can therefore be compensated for simply by appropriate relative positioning of the second planar lightwave circuit and optical component relative to one another on the first planar lightwave circuit.

Preferably the optical fibre connector is positioned in an optical path passing successively through the first planar lightwave circuit, the optical component and the second planar lightwave circuit In a preferred embodiment a further optical fibre connector is provided on the first planar lightwave circuit so as to provide an optical path between an optical input and an optical output by way of the first planar lightwave circuit, the optical component and the second planar lightwave circuit.

Furthermore the second planar lightwave circuit and the optical component are preferably mounted on the first planar lightwave circuit in such a manner that the optical interface of the second planar lightwave circuit relative to the optical fibre connector is not adversely affected by a change in length of the optical component. To this end the second planar lightwave circuit and/or the optical component may be provided with a guide portion which extends within a recessed track (optical rail) in the first planar lightwave circuit to allow for proper alignment independently of variation in the length of the optical component.

In this case the second waveguide structure of the second planar lightwave circuit conveniently has a first end portion optically aligned with an end portion of the third waveguide structure of the optical component, and a second end portion angled with respect to the first end portion to provide the required alignment for proper optical communication with an optical input and/or output.

In order that the invention may be more fully understood, reference will be made to the accompanying drawings, in which: Figures I to 4 are explanatory diagrams illustrating prior art arrangements; Figure 5 is an axial section through an embodiment of the invention; Figure 6 is a plan view of the embodiment of Figure 5; and Figure 7 is a block diagram illustrating a possible practical implementation of the invention.

A preferred embodiment of the invention will now be described with reference to Figures 5 and 6 of the drawings. In this embodiment the optical component is an SOA, although it will be understood that the invention is not limited to the case in which the optical component is an SOA and that the invention is also applicable to arrangements incorporating other types of optical component, such as an electroabsorption modulator, a lithium niobate modulator, an arrayed waveguide grating and an optical attenuator, for example.

Referring to Figures 5 and 6, the illustrated hybridised optical device comprises a first planar lightwave circuit 20 incorporating a waveguide structure serving as a substrate for alignment of a second planar lightwave circuit 21 and an SOA 22. The circuit 20 incorporates input and output fibre connectors 23 and 24 in the form of V grooves wet etched in the edges of the circuit 20 to ensure efficient optical coupling with optical fibres 25 and 26. Furthermore the circuit 21 also has edges etched to form edge facets 27 and 28, in order to provide efficient optical coupling between the optical fibre 25 and the circuit 21 on the one hand, and between the circuit 21 and the SOA 22 on the other hand.

As shown in Figure 6, the waveguides 29 and 30 on the circuits 21 and 20 incorporate portions which are angled with respect to the edge facets of the fibre

connectors 23 and 24 so as to provide the required optical coupling minimising any optical reflections from the facets. Furthermore the SOA 22 incorporates a waveguide 31 which is aligned with adjacent portions of the waveguides 29 and 30 parallel to an optical axis 32 along which the circuit 21 is also adjustable to compensate for differences in the length of the SOA 22, as described more fully below.

The device is assembled by mounting the SOA 22 on the circuit 20 in conventional manner with conductive pads on the underside of the SOA 22 being soldered to conductive pads on the circuit 20. The circuit 21 is then mounted on the circuit 20 utilising a similar mounting technique with the circuit 21 being aligned relative to the SOA 22 so that the end of the SOA 22 is in proper optical alignment with respect to the etched edge facet 28. This alignment of the circuit 21 relative to the SOA 22 involves movement of the circuit 21 parallel to the optical axis 32 so that any variation in the length of the SOA 22 and hence the position of the circuit 21 does not adversely affect the lateral alignment d of the fibre connector 23 and the waveguide 29 of the circuit 21. It will be appreciated that such adjustment does not cause any change in the lateral alignment d since the adjustment is made along an axis parallel to the fibre axis orthogonal to the lateral alignment.

In one possible implementation the undersides of the circuit 21 and the SOA 22 are formed with rails which fit within a recessed track in the circuit 21 so as to ensure proper alignment, the rails being formed preferably by etching alignment edges on the circuit 21 and the SOA 22. However physical guides need not be provided if appropriate measures are taken to ensure proper alignment during assembly.

Figure 7 is a block diagram of a channel balancer 34 given as an example of an optical circuit arrangement which may be implemented by use of a hybridised device in accordance with the invention. In this case the first circuit 20 incorporates a multiplexer 35, and the second circuit 21 incorporates a demultiplexer 36 and is mounted on the circuit 20 in the manner already described. Also mounted on the circuit 20 are four SOAs 37 such that the appropriate optical paths are formed between the input optical

fibre 25, the demultiplexer 36, the SOAs 37, the multiplexer 35 and the output optical fibre 26.

In an alternative, non-illustrated implementation, the first circuit 20 incorporates a multiplexer, the second circuit 21 incorporates a series of DFB lasers, and a series of electro-absorption modulators are mounted on the first circuit intermediate the DFB lasers and the multiplexer which supplies the resulting output signal to an output optical fibre. In this case, it will be appreciated that no input optical fibre is required.

In another non-illustrated implementation the circuit 21 incorporates a demultiplexer, the circuit 22 incorporates a series of photodiodes, and a series of SOAs are mounted on the circuit 20 intermediate the outputs of the demultiplexer and the photodiodes. In this case an input signal is supplied to the demultiplexer by way of an input optical fibre, and no output optical fibre is required.

The hybridised optical device described is particular advantageous since it allows optical components to be fabricated with single growth steps and cleaved facets, thus minimising the process complexity and improving device yield and performance.

Claims

CLAIMS:

1. A hybridised optical device comprising a first planar lightwave circuit having a first waveguide structure, a second planar lightwave circuit having a second waveguide structure and mounted on the first planar lightwave circuit such that the first waveguide structure is in optical communication with the second waveguide structure, an optical component having a third waveguide structure and mounted on the first planar lightwave circuit adjacent to the second planar lightwave circuit such that the third waveguide structure is in direct optical communication with both the first waveguide structure and the second waveguide structure, and at least one optical fibre connector on the first planar lightwave circuit.

2. A device according to claim 1, wherein said optical fibre connector is positioned in an optical path passing successively through the first planar lightwave circuit, the optical component and the second planar lightwave circuit

3. A device according to claim 1 or 2, wherein a further optical fibre connector is provided on the first planar lightwave circuit so as to provide an optical path between an optical input and an optical output by way of the first planar lightwave circuit, the optical component and the second planar lightwave circuit.

4. A device according to claim 1,2 or 3, wherein one of the optical fibre connectors is arranged to align an optical fibre in direct optical communication with the second waveguide structure of the second planar lightwave circuit.

5. A device according to any preceding claim, wherein the third waveguide structure is optically coupled to the first waveguide structure on one side of the optical component, and the third waveguide structure is optically coupled to the second waveguide structure on the opposite side of the optical component.

6. A device according to any preceding claim, wherein the second planar lightwave circuit and the optical component are mounted on the first planar lightwave circuit in

such a manner that the optical interface of the second planar lightwave circuit relative to one of the optical fibre connectors is not adversely affected by a change in length of the optical component.

7. A device according to claim 6, wherein the second planar lightwave circuit and/or the optical component has a guide portion which extends within a recessed track in the first planar lightwave circuit to allow for proper alignment independently of variation in the length of the optical component.

8. A device to any preceding claim, wherein the second waveguide structure of the second planar lightwave circuit has a first end portion optically aligned with an end portion of the third waveguide structure of the optical component and a second end portion angled with respect to the first end portion to provide the required alignment for proper optical communication with an optical input and/or output.

9. A device according to any preceding claim, wherein the second planar lightwave circuit and/or the optical component is mounted on the first planar lightwave circuit by being soldered to an associated contact on the first planar lightwave circuit.

10. A device according to any preceding claim, wherein the second planar lightwave circuit has at least one etched edge facet by means of which optical communication is established with the optical component and/or an optical input and/or output.

11. A device according to claim 10, wherein the etched edge facet is in the form of a V-groove.

12. A device according to any one of claims 1 to 11, wherein the optical component is a semiconductor optical amplifier.

13. A device according to any one of claims 1 to 11, wherein the optical component is an electro-absorption modulator.

14. A method according to any one of claims 1 to 11, wherein the optical component is a lithium niobate modulator.

15. A device according to any one of claims 1 to 11, wherein the optical component is an arrayed waveguide grating.

16. A device according to any one of claims 1 to 11, wherein the optical component is an optical attenuator.

17. A device according to any preceding claim, wherein one of the planar lightwave circuits incorporates a demultiplexor and the other of the planar lightwave circuits incorporates a multiplexor.

18. A device according to any preceding claim, wherein a plurality of optical components are mounted on the first planar lightwave circuit so as to be in direct optical communication with the first waveguide structure.

19. A device according to any preceding claim, wherein the first and second lightwave circuits are based on silicon-on-insulator (SOI) technology.

20. A hybridised optical device substantially as hereinbefore described with reference to the accompanying drawings.