Optical coupling element for photonic integrated circuit

Abstract

Disclosed is an optical coupling element for enabling evanescent coupling between a PIC waveguide and a glass waveguide that includes an optical waveguide, wherein a first end of the optical waveguide has a first tapered structure to enable first optical coupling with the PIC waveguide, and wherein a second end of the optical waveguide has a second tapered structure to enable second optical coupling with the glass waveguide, wherein each of the first and second tapered structures are configured to enable maximum coupling efficiency between the PIC waveguide and the glass waveguide.

Description

[0001] Title

[0002] Optical coupling element for Photonic Integrated Circuit

[0003] Field

[0004] The present disclosure relates to Photonic Integrated Circuits, and more particularly to micro-optical components for coupling light into and out of Photonic Integrated Circuits (PICs).

[0005] The coupling of light from photonic devices is usually achieved by direct bonding of the photonic device to optical fibres. This is typically achieved by aligning the optical fibre waveguiding core (usually 3-10 microns in diameter) to the edge or facet of the photonic device. The coupling of light from the photonic device to the optical fibre is a major technical challenge due to the demanding submicron alignment tolerances. FIG.1 illustrates conventional packaging of optical fibres to a PIC chip. The optical fibre array is fixed to the edge of the PIC chip using a clear UV-cured epoxy resin. However, epoxies are convenient but not reliable, and are prone to shift over time, which affect the precision alignment of the optical fibre to the PIC waveguide.

[0006] An alternative approach involves the direct coupling of light from the waveguide of the photonic device to another waveguide (often called the optical interposer waveguide), which is placed in close contact with the photonic device. In this case, the waveguides for both photonic devices and optical interposers must be opened or accessible along their light propagation axis. FIG.2 illustrates tapering the waveguide of the photonic device to enable evanescent coupling. The tapering the waveguide of the photonic device induces the light to couple across the gap into the waveguide of the optical interposer. FIG.3 illustrates a side view showing evanescent coupling of light from the tapered photonic waveguide to the optical interposer waveguide. The efficiency of this coupling process depends on a number of factors including the refractive indices (Rl) of both photonic and optical interposer materials, the gap between the two waveguides and the Rl of the material used to fill this gap. A wide range of materials may be used for the optical interposer waveguide and its carrier substrate, including silicon, silicon nitride, glass and polymer. A key requirement for the waveguide is that it is optically transparent at the operating wavelength, and its Rl value is such that it can be controlled to achieve efficient coupling across the gap. Glass is an interposer material of great interest due to its excellent optical properties, relatively low cost and the ability to fabricate high-performance optical waveguides in the substrate or bulk material. Typical methods of fabricating waveguides in glass include epitaxial deposition, laser writing and ion diffusion. The two latter processes have received much attention as they can be scaled for large-volume manufacturing, addressing emerging applications in Artificial Intelligence (Al) and high-speed data centre communication systems.

[0007] FIG.4 illustrates the principle of the laser writing process, where a laser beam is focused inside the glass substrate, and the high-intensity focused beam modifies the material properties, including the Rl value, thus forming an optical waveguide. FIG.5 illustrates the diffusion of silver ions into the glass substrate to form the optical waveguide. This process is ideal for evanescent coupling as it can form optical waveguides close to the surface of the glass. The top-deposited metal mask is used to define the optical waveguide structure in the glass substrate. The ion-diffused waveguide is defined by the high concentration of silver ions, which displace the incumbent sodium ions to modify the Rl value. The ion diffusion process is of most interest for evanescent coupling as it is easier to locate the optical waveguide close to the glass surface, which is a critical requirement for the coupling process.

[0008] FIG.6 illustrates the conventional approach of bonding the photonic device to the glass optical interposer. The coupling length on the photonic device required for efficient evanescent coupling is typically between 2-3 mm. The silicon waveguides (in silicon photonic or Silicon-on-lnsulator (SOI) devices) may be tapered and surface bonded to the optical interposer to achieve high optical coupling efficiencies. The values of better than -2dB coupling loss have been experimentally demonstrated. However, the length of the waveguide taper in the photonic device must be relatively long to achieve this high coupling efficiency (typically between 2-3 mm in length). This requires the use of valuable photonic device surface area to produce these excessively long tapers. This feature is highly undesirable for volume manufacturing of photonic devices as valuable chip area must be used for the coupling process. This condition arises because the Rl value of the ion-diffused glass waveguides is relatively low, and the large Rl difference between both waveguides is relatively large. For example, ion-diffused waveguides in glass have typical Rl values of 1 .4-1 .5, and silicon waveguides have an Rl of 3.3. The ion diffusion process sets limits on the maximum Rl value achievable, which limits the suitability of the technology.

[0009] US 2022 / 244458 discloses a method for establishing optical coupling between spatially separated first and second planar waveguides includes arranging an optical interconnect on the first planar waveguide. A method for establishing optical coupling between spatially separated first and second planar waveguides includes arranging an optical interconnect on the first planar waveguide. However, the intermediate layer such as TiO2 layer use a chemical-vacuum deposition technique. This approach limits the types of intermediate layers that can be deposited.

[0010] US 2018321445 discloses an optical waveguide that includes opposed end sections for optical radiation to propagate in a longitudinal direction therebetween and an intermediate section extending between the end sections. The intermediate section includes first and second portions superposed in a superposition direction.

[0011] Hence, in view of the above, there is a need for a system and method that increases the Rl value at the region of optical coupling between the silicon photonic device and the optical glass interposer, and reduces the coupling length on the photonic device.

[0012] Summary

[0013] In an aspect of the present invention, there is provided an optical coupling element, as set out in the appended claims, for enabling evanescent coupling between a PIC waveguide and a glass waveguide. The optical coupling element comprises an optical waveguide, wherein a first end of the optical waveguide has a first tapered structure to enable first optical coupling with the PIC waveguide, and wherein a second end of the optical waveguide has a second tapered structure to enable second optical coupling with the glass waveguide, wherein each of the first and second tapered structures are configured to enable maximum coupling efficiency between the PIC waveguide and the glass waveguide.

[0014] In an embodiment of the present invention, the optical coupling element is made from a material consisting of Silicon Nitride.

[0015] In an embodiment of the present invention, the Rl value of the optical coupling element is in the range of 1 .5-3.3.

[0016] In an embodiment of the present invention, the optical waveguide is bonded to a surface of the ion-diffused glass waveguide over the region of evanescent optical coupling.

[0017] In an embodiment of the present invention, the optical waveguide is formed as a chiplet removed from corresponding host wafer by using a selective etch release process, and bonded to the glass waveguide using a Micro Transfer Print (MTP) wafer-level process.

[0018] In another aspect of the present invention, there is provided a method of enabling evanescent coupling between a PIC waveguide and a glass waveguide. The method includes bonding a layer of an optical coupling element onto the surface of an ion-diffused glass waveguide over the region of evanescent optical coupling region, and placing the PIC waveguide over the optical coupling element to facilitate evanescent coupling with the optical coupling element, which in turn couples to the ion-diffused glass waveguide.

[0019] In another aspect of the present invention, the optical waveguide coupling reduces the optical coupling length on the PIC waveguide to less than 1 mm. In another aspect of the present invention, the optical waveguide is arranged in a series of adjacent arrays to include a fan-out configuration to reduce the waveguide pitch at the input section of the PIC waveguide to increase the optical channel density.

[0020] In an embodiment of the present invention, the at least one optical waveguide includes an array of waveguides with variable pitches and tapers at respective ends.

[0021] In an embodiment of the present invention, wherein the array of waveguides forms a fanout configuration with a first waveguide pitch at the first optical coupling with the PIC waveguide and a second waveguide pitch at the second optical coupling with the glass waveguide, wherein the first waveguide pitch is less than the second waveguide pitch. In another aspect of the present disclosure, there is provided an optical coupling element for enabling evanescent coupling between a photonic integrated circuit (PIC) waveguide and a glass waveguide. The optical coupling element includes at least one optical waveguide, wherein each optical waveguide has a first end that has a first tapered structure to enable first optical coupling with the PIC waveguide, and has a second end that has a second tapered structure to enable second optical coupling with the glass waveguide that has a lower Rl than that of the PIC waveguide, wherein the tapered design of the at least one optical waveguide enables maximum coupling efficiency between the PIC waveguide and the glass waveguide, and wherein the optical waveguide is bonded to a surface of the ion-diffused glass waveguide over a region of evanescent optical coupling.

[0022] In an embodiment of the present invention, the optical waveguide is formed as a chiplet removed from corresponding host wafer by using a selective etch release process, and transferred on surface of the glass waveguide.

[0023] In an embodiment of the present invention, the array of waveguides forms a waveguide pitch transformer by reducing waveguide pitch required at the first optical coupling with the PIC waveguide, in comparison to that required at the second optical coupling with the glass waveguide. In yet another aspect of the present disclosure, there is provided an optical coupling element for enabling evanescent coupling between a photonic integrated circuit (PIC) waveguide and a glass waveguide. The optical coupling element includes an array of waveguides with variable pitches and tapers at respective ends, wherein each optical waveguide has a first end that has a first tapered structure to enable first optical coupling with the PIC waveguide, and has a second end that has a second tapered structure to enable second optical coupling with the glass waveguide that has a lower Rl than that of the PIC waveguide, wherein tapered design of the array of waveguides enables maximum coupling efficiency between the PIC waveguide and the glass waveguide, wherein the array of waveguides forms a fan-out configuration with a first waveguide pitch at the first optical coupling with the PIC waveguide and a second waveguide pitch at the second optical coupling with the glass waveguide, wherein the first waveguide pitch is less than the second waveguide pitch.

[0024] Various embodiments of the present invention provide an optical coupling element that provides improved performance in optical coupling between a photonic device and a glass optical interposer. The optical coupling element includes a layer of SiN, and acts as a refractive index bridge, bridging the relatively large difference between the refractive indices of photonic device waveguide and the glass waveguide. For example, SiN has an Ri of 1 .9, which is an ideal material for this purpose. The optical coupling element improves the performance of glass waveguide interposers which are becoming of great interest for high performance and high-volume photonics manufacturing. The optical coupling element is used to enhance the performance and manufacturability of photonic devices used in a wide range of applications such as communications, Al, AR / VR, healthcare, sensing and diagnostics.

[0025] The optical coupling element is bonded to the glass optical interposer using microtransfer printing (MTP). The transfer printing enables the use of a wide variety of materials, as the glass optical interposer is fabricated on another wafer and picked and placed on the glass waveguide platform. Furthermore, TP allows the material to be placed on a wide variety of platform substrates such as polymer substrates. The deposition techniques can be limited to platform substrates which can survive the vacuum deposition processes (possibly involving high temperature processing).

[0026] BRIEF DESCRIPTION OF DRAWINGS

[0027] 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:-

[0028] FIG.1 illustrates conventional packaging of optical fibres to a PIC chip;

[0029] FIG.2 illustrates tapering the waveguide of the photonic device to enable evanescent coupling;

[0030] FIG.3 illustrates a side view showing evanescent coupling of light from the tapered photonic waveguide to the optical interposer waveguide;

[0031] FIG.4 illustrates the principle of the laser writing process, where a laser beam is focused inside the glass substrate, and the high-intensity focused beam modifies the material properties, including the Rl value, thus forming an optical waveguide;

[0032] FIG.5 illustrates the diffusion of silver ions into the glass substrate to form the optical waveguide;

[0033] FIG.6 illustrates the conventional approach of bonding the photonic device to the glass optical interposer;

[0034] FIG.7A illustrates using an intermediate layer of SiN for evanescent coupling between the photonic device and the optical interposer, in accordance with an embodiment of the present invention;

[0035] FIG.7B illustrates a cross-section view of the intermediate layer between the photonic waveguide and the glass waveguide, in accordance with an embodiment of the present invention;

[0036] FIG.8 illustrates micro-transfer printing of SiN chiplets from a host wafer, in accordance with an embodiment of the present invention; FIG.9 illustrates detailed design of each printed chiplet component, in accordance with an embodiment of the present invention;

[0037] FIG. 10 illustrates detailed design of each printed chiplet component including more than one waveguide, in accordance with an embodiment of the present invention; and

[0038] FIG. 1 1 illustrates detailed design of each printed chiplet component including an array of waveguides to form a fan-out configuration, in accordance with an embodiment of the present invention.

[0039] DETAILED DESCRIPTION OF THE DRAWINGS

[0040] FIG.7A illustrates an optical coupling element 702 for enabling evanescent coupling between a photonic device waveguide 704 and an optical interposer waveguide 706, in accordance with an embodiment of the present invention. In a preferred embodiment, the optical coupling element 702 is an intermediate layer made of SiN, and is hereinafter also referred to as SiN layer 704.

[0041] The SiN layer 702 may be used as an intermediate material between the high Rl photonic waveguide 704 and low Rl glass optical interposer waveguide 706. SiN is an ideal optical waveguide material as it is transparent across a wide waveguide range. More importantly, it has a relatively high Rl value of 1 .9-2.0, making it an ideal material for evanescent coupling. In an embodiment of the present invention, a thin layer 702 of SiN waveguiding structure is bonded to the surface of the ion-diffused glass waveguide 706 over the optical coupling region. The photonic device 704 may be placed in close contact to the SiN waveguide 702 to facilitate evanescent coupling to the SiN, which in turn couples to the ion-diffused glass waveguide 706. The SiN waveguide 702 may be tapered to achieve coupling in both directions in a manner similar to that used for the photonic waveguide. The use of the intermediate SiN layer 702 reduces the coupling length on the photonic device 704 to less than 1 mm. This is a significant space and a cost-saving factor in producing these photonic devices 704. However, it would be apparent to one of ordinary skill in the art, that the intermediate layer 702 may be made from any other optical waveguide material with an Rl value of greater than 1 .8 up to 3.3.

[0042] FIG.7B illustrates a cross-section view of the optical coupling element 702 between the photonic waveguide 704 and the glass waveguide 706, in accordance with an embodiment of the present invention. The cross-section view shows approximate layer thickness values for each component and the clear epoxy adhesive for bonding the photonic device 704, the intermediate SiN layer 702, and the glass waveguide 706.

[0043] The thickness of various layers of FIG.7B is illustrated below in Table I:

[0044] Table I

[0045] As can be seen, the thickness of SiN chiplet optical interposer 702 has a typical value of 2-4 microns, but the exact layer thickness can be varied to optimise the light coupling efficiency between the different layers. FIG.8 illustrates micro-transfer printing of SiN chiplets 802 from a host wafer 804, in accordance with an embodiment of the present invention. The SiN waveguides are in the form of chiplets 802 removed from their host wafer 804 by the MTP process and transferred or stamped on the surface of glass optical interposer wafer 806. It is to be noted that the SiN chiplets 802 are only required in the region of evanescent optical coupling. Also, the ion diffusion is the most common way to produce this type of waveguide in glass. However, direct laser writing in the glass material and spin-on polymer waveguide on the glass substrate are other approaches.

[0046] The transfer printing enables the use of a wide variety of materials, as the glass optical interposer is fabricated on another wafer and picked and placed on the glass waveguide platform. Furthermore, TP allows the material to be placed on a wide variety of platform substrates such as polymer substrates. The deposition techniques can be limited to platform substrates which can survive the vacuum deposition processes (possibly involving high temperature processing).

[0047] FIG.9 illustrates detailed design of each printed chiplet component 900, in accordance with an embodiment of the present invention. The printed chiplet component 900 includes first and second tapered waveguide structures 904a and 904b located at either end of its optical waveguide 902. Thue first tapered structure 904a enables light to be efficiently coupled from the photonic device to the chiplet 900. The second tapered structure 904b enables light to be efficiently coupled from the chiplet 900 to the glass optical interposer. The coupling of the photonic device and the chiplet 900 is referred to as phase 1 , and the coupling of the chiplet and the glass optical interposer is referred to as phase 2. The tapered structure may be individually designed and optimized for maximum coupling efficiency between each phase, as each phase consists of materials with different Rl values, waveguide dimensions and layer thickness.

[0048] FIG. 10 illustrates detailed design of a printed chiplet component 1000 including more than one waveguide, in accordance with an embodiment of the present invention. The printed chiplet component 1000 is shown to include three optical waveguides. Each waveguide is similar to waveguide 902 with tapered ends. Herein, the three optical waveguides have variable pitch and tapers at respective ends to enable in-coupling with the photonic waveguide and out-coupling with the glass waveguide.

[0049] FIG. 1 1 illustrates detailed design of a printed chiplet component 1 100 including an array of waveguides, in accordance with an embodiment of the present invention. The array of waveguides forms a fan-out configuration that has a first waveguide pitch 1 102 at the first optical coupling with the PIC waveguide and a second waveguide pitch 1 104 at the second optical coupling with the glass waveguide. It can be seen that the first waveguide pitch 1 102 is less than the second waveguide pitch 1104. In an example, the first waveguide pitch 1102 is around 10 microns, and the second waveguide pitch 1 104 is around 50-100 microns.

[0050] In an embodiment of the present invention, the spacing or pitch between waveguides in glass may be set by the Rl value of the glass waveguide. As this is relatively low (approx 1.5), adjacent waveguides must be spaced far apart, minimum pitch would typically be 50 microns. However, the optical waveguide 1100 has a relatively large Rl value (approx 1.9), enabling the waveguide pitch in this layer to be much closer, typically 10 microns or lower. Therefore, the optical waveguide 1 100 can serve a second function, acting as a waveguide pitch transformer, reducing the waveguide pitch from the glass waveguide from 50 to 10 microns where the optical waveguide 1100 couples light to the photonic device. This is important as there is a need to reduce the pitch of waveguides at the photonic device interface to increase the number of optical channels. This is often called the Optical Shoreline Density of Edge Bandwidth Density. This is required for applications such as Al where lots of input / output optical data channels are required. Therefore, the optical waveguide 1100 serves two functions 1 ) coupling light between the glass waveguide and photonic device waveguide (vertical direction) and changing the waveguide pitch between the glass waveguide and photonic device waveguide (horizontal or in-plane direction).

[0051] 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.

[0052] The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.

Claims

Claims:

1. An optical coupling element for enabling evanescent coupling between a photonic integrated circuit (PIC) waveguide and a glass waveguide, comprising: at least one optical waveguide, wherein each optical waveguide has a first end that has a first tapered structure to enable first optical coupling with the PIC waveguide, and has a second end that has a second tapered structure to enable second optical coupling with the glass waveguide that has a lower Rl than that of the PIC waveguide, wherein the tapered design of the at least one optical waveguide enables maximum coupling efficiency between the PIC waveguide and the glass waveguide, and wherein the optical waveguide is bonded to a surface of the ion-diffused glass waveguide over a region of evanescent optical coupling.

2. The optical coupling element as claimed in claim 1 , wherein the optical waveguide is formed as a chiplet removed from corresponding host wafer by using a selective etch release process, and transferred on surface of the glass waveguide.

3. The optical coupling element as claimed in claim 2, wherein the chiplet is bonded to the glass waveguide using a Micro Transfer Print (MTP) wafer-level process.

4. The optical coupling element as claimed in claim 1 , wherein each optical waveguide is made from a material consisting of Silicon Nitride and the optical coupling element reduces an optical coupling length on the PIC waveguide to less than 1 mm.

5. The optical coupling element as claimed in any preceding claim, wherein the Rl value of the optical waveguide is in the range of 1 .5-3.3.

6. The optical coupling element as claimed in any preceding claim, wherein the at least one optical waveguide includes an array of waveguides with variable pitches and tapers at respective ends.

7. The optical coupling element as claimed in claim 6, wherein the array of waveguides forms a fan-out configuration with a first waveguide pitch at the first optical coupling with the PIC waveguide and a second waveguide pitch at the second optical coupling with the glass waveguide, wherein the first waveguide pitch is less than the second waveguide pitch.

8. The optical coupling element as claimed in claim 7, wherein the array of waveguides forms a waveguide pitch transformer by reducing waveguide pitch required at the first optical coupling with the PIC waveguide, in comparison to that required at the second optical coupling with the glass waveguide.

9. A method of enabling evanescent coupling between a PIC waveguide and a glass waveguide, the method comprising: bonding an optical coupling element as claimed in claims 1 -8 onto the surface of an ion-diffused glass waveguide over the region of evanescent optical coupling region; and placing the PIC waveguide over the optical coupling element to facilitate evanescent coupling with the optical coupling element, which in turn couples to the ion-diffused glass waveguide.

10. The method as claimed in claim 9, wherein the optical coupling element reduces the optical coupling length on the PIC waveguide to less than 1 mm.

11. The method as claimed in claim 9, wherein the at least one optical waveguide includes an array of waveguides with variable pitches and tapers at respective ends.

12. The method as claimed in claim 1 1 , wherein the array of waveguides forms a fan-out configuration with a first waveguide pitch at the first optical coupling with the PIC waveguide and a second waveguide pitch at the second optical coupling with the glass waveguide, wherein the first waveguide pitch is less than the second waveguide pitch.

13. An optical coupling element for enabling evanescent coupling between a photonic integrated circuit (PIC) waveguide and a glass waveguide, comprising: an array of waveguides with variable pitches and tapers at respective ends, wherein each optical waveguide has a first end that has a first tapered structure to enable first optical coupling with the PIC waveguide, and has a second end that has a second tapered structure to enable second optical coupling with the glass waveguide that has a lower Rl than that of the PIC waveguide, wherein tapered design of the array of waveguides enables maximum coupling efficiency between the PIC waveguide and the glass waveguide, wherein the array of waveguides forms a fan-out configuration with a first waveguide pitch at the first optical coupling with the PIC waveguide and a second waveguide pitch at the second optical coupling with the glass waveguide, wherein the first waveguide pitch is less than the second waveguide pitch.

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