DEVICES AND METHODS FOR WAVEGUIDE ALIGNMENT

Abstract

The present disclosure is directed towards aligning a photonic integrated circuit (PIC) through providing a PIC with a marker waveguide, wherein a marker waveguide is a waveguide having: a first end located at the edge of the PIC wherein the first end is coupled to an edge coupling; and a second end coupled to a grating coupler or a device coupler, wherein: the grating coupler or device coupler is configured to receive light and couple the light to the waveguide to illuminate the waveguide to facilitate the correct alignment of the edge coupler to an external component.

Claims

1. A photonic integrated circuit (PIC) comprising a marker waveguide, wherein the marker waveguide comprises: a first end located at the edge of the PIC wherein the first end is coupled to an edge coupling; and a second end coupled to a grating coupler or device coupler, wherein: the grating coupler or a device coupler is configured to receive light and couple the light to the waveguide to illuminate the waveguide to facilitate the correct alignment of the edge coupler to an external component.

2. The PIC of claim 1 comprising two or more marker waveguides and an array of other edge coupled waveguides.

3. The PIC of claim 2, wherein at least two marker waveguides are provided, one at each outer position of the waveguide array.

4. The PIC of claim 3, wherein the marker waveguides are positioned at a fixed pitch.

5. The PIC of claim 4, wherein the fixed pitch matches the pitch of the other edge coupled waveguides.

6. A device comprising the PIC of any preceding claim.

7. The device of claim 6 further comprising a glass or silicon interposer on which the PIC is flipchip mounted.

8. The device of claim 6 or 7, further comprising an integrated micro turning mirror and silicon or glass package capping layer, through which input and output light can be transmitted.

9. A method of aligning a photonic integrated circuit (PIC) with an external optical element comprising: providing the PIC with a marker waveguide, wherein a marker waveguide is a waveguide having: a first end located at the edge of the PIC, wherein the first end is coupled to an edge coupling; and a second end coupled to a grating coupler or a device coupler, wherein: the grating coupler or device coupler is configured to receive light and couple the light to the waveguide to illuminate the waveguide to facilitate the correct alignment of the edge coupler to an external component; illuminating the grating coupler or device coupler of the marker waveguide with a light source to illuminate the marker waveguide; and aligning the PIC using the light emitted by the edge coupler of the marker waveguide.

10. The method of claim 9, wherein the PIC is provided with two marker waveguides.

11. The method of claim 9 or 10, wherein the method comprises flipchip mounting the PIC on a glass or silicon interposer.

12. The method of any one of claims 9 to 11, wherein the light illuminating the grating coupler of the marker waveguide is directed to the grating coupler of the marker waveguide from: above the PIC surface; or below the PIC surface, through the substrate of a device comprising the PIC.

13. The method of any one of claims 9 to 12, wherein light source is comprised within the packaging equipment for packaging the PIC to facilitate the active alignment of the PIC.

14. The method of any one of claims 9 to 13, wherein: the PIC is provided with at least two marker waveguides; and the marker waveguides are provided at each outer position of an array of other edge coupled waveguides.

15. The method of claim 14, wherein the marker waveguides are positioned as outer waveguides with a fixed pitch, wherein the fixed pitch matches the pitch of the other waveguides in the array of waveguides.

16. The method of any one of claims 9 to 15, wherein micro lenses are actively aligned to the edge of photonic device using packaging equipment including a tuning mirror or periscope and imaging camera.

17. The method of any one of claims 9 to 16, wherein the light for illuminating the grating coupler of the marker waveguide is provided through a glass or silicon interposer on which the PIC is flipchip mounted.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:—

[0035] FIG. 1 is side view of a PIC, flipchip mounted on an electrical interposer;

[0036] FIG. 2 is a top view of an alignment element;

[0037] FIG. 3 is a top view of a PIC comprising two alignment elements;

[0038] FIG. 4 is a side view of a PIC being aligned on an Electrical Interposer; and

[0039] FIG. 5 is a side cut-away view of an aligned and packaged PIC.

DETAILED DESCRIPTION OF THE DRAWINGS

[0040] The present disclosure provides a novel means of illuminating optical waveguides to enable precise active alignment of edge coupled waveguide to external optical components such as optical fibres or micro optic elements. The technique involves incorporating connected optical elements in the photonic device design.

[0041] FIG. 2 illustrates an exemplary optical element 10. The optical element comprises a grating coupler 11 connected to a waveguide 12. A ‘grating coupler’ is a region on top of or below a waveguide where there is a grating. Off-resonance light incident on the grating behaves almost the same as it would if it was incident in an area where there is no grating. For specific combinations of incident angles and light frequency, there is resonance, allowing the grating to couple light into a guided mode of the waveguide. Thus, the grating coupler 11 is located on the top or bottom surface of a photonic device and is configured to couple light from an independent light source located above or below the photonic device to the waveguide 12. The independent light source can, for example, be part of the packaging equipment.

[0042] The light coupled by the grating coupler 11 to the waveguide 12 is directed to an edge coupler 13 by the optical waveguide 12. Light from the edge coupler can then be viewed using any suitable light sensor, e.g. such as a camera. Preferably, an infrared (IR) imaging camera is used as wavelengths in the range of 1300-1550 nm are preferred.

[0043] Thus, by coupling the other end of an edge coupled waveguide 12 to a grating coupler 11 it becomes possible to illuminate the waveguide 12 using an external light source. This enables precise viewing and positioning of edge coupler 13 to facilitate accurate alignment and packaging of the external optical components. For the sake of brevity, an edge coupled waveguide which is also provided with a grating coupler can be referred to as a marker waveguide.

[0044] In a preferred embodiment the invention makes use of two surface coupling structures or grating couplers to couple light into optical waveguide to enable packaging of optical components, such as fibers and micro lenses. The invention can make use of any surface feature that will scatter light into the waveguide when illuminated from the top or bottom of the photonic device. Packaging requires very low levels of coupled light so the packaging camera can ‘see’ the waveguides to start the alignment process, which can be termed ‘first-light’. In the present application the use of gratings which diffract light into the waveguide can be replaced by surface micro mirrors or an etched cavity of the like. It was found structures may be more suited than gratings for photonic devices using InP or SiN materials, as grating are difficult to implement in these semiconductor materials.

[0045] Furthermore, the present invention makes use of one or more ‘alignment wave guides’ which are optical waveguides located outside a set of waveguides, with the functional or usable waveguides positioned in-between these two outer alignment waveguides. This configuration enables not only lateral (x and y) alignment, but also angular alignment, which is essential for precise packaging of optical components such as fiber arrays or micro lens arrays.

[0046] FIG. 3 shows an example of a PIC 20 using the optical element 10 shown in FIG. 2. As illustrated in FIG. 3, when packaging an array of functional optical edge coupled waveguides 24 elements, one or more of marker waveguides 21 can be provided in addition to the array. Preferably at least two marker waveguides 21 are provided to ensure correct alignment. More preferably, the at least two marker waveguides 21 are provided such that at least one marker waveguide 21 is located at each outer position of the waveguide array 24 as shown in FIG. 3.

[0047] These marker waveguides 24 have a fixed pitch. Preferably this fixed pitch has the same value as the other (functional) waveguides in the photonic device. These marker waveguides are used to precisely locate the positions of all the waveguides. This is possible because all the waveguides can be accurately pitched (i.e. spaced and/or angled) with respect to each other as they are defined using a sub-micron photolithographic process. In this arrangement, the elements enable a method of illumination and active alignment of arrays of external optical components.

[0048] In situations where the photonic device can be mounted up-side-down or flipchip packaged. This will require coupling to the alignment waveguides through the device backside substrate, and a clear optical path must be made through the substrate in order for the light to reach the scattering structure and couple into the alignment waveguides.

[0049] With reference to FIG. 4, the grating coupler 21 for these marker waveguides 24 can be illuminated from the top 31 or bottom 32 of the photonic device—the latter configuration enables light to be transmitted through the device substrate to illuminate the grating. It should be noted that illumination through the substrate enables active alignment when the photonic device is flipchip mounted up-side-down on an electrical interposer. For silicon photonics, operating wavelengths are typically 1300 nm or above, which is within the optical transparency band of the silicon substrate.

[0050] FIG. 4 shows one embodiment of a design of PIC in accordance with the present disclosure. The PIC is flipchip mounted on a silicon or glass interposer substrate. The grating coupler 21 for the marker waveguides 24 can be illuminated from the top though the device substrate, or from the bottom through the silicon or glass interposer substrate. The light source used to illuminate the grating coupler can be part of the packaging equipment and separate from the photonic device. A micro lens array 33 is used to achieve beam expansion and collimation. The micro lens alignment equipment includes a turning mirror or periscope 34 and an imaging system 35. This enables real-time accurate active alignment of the micro lens 33 to the array of optical waveguides in the PIC.

[0051] FIG. 5 shows a micro lens array 33 being actively aligned to the edge of the flipchip mounted PIC 40. The micro lens array is used to achieve beam expansion and collimation. In this embodiment, the package includes micro prism 35 located after the micro lens array. This micro prism 35 is used to turn the direction of the edge emitted light, directing the light upwards and out of the photonic package. In this embodiment, the photonic package can incorporate a silicon or glass capping layer 36, enabling transmission of the emitted from the capped or sealed package, where the package can be hermetic sealed at wafer-level using e.g. a solder seal 37 between the package base and cap 36.

[0052] In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

[0053] The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.