Edge coupler with improved mechanical stiffness of suspended waveguide

Abstract

This disclosure generally relates to a photonic edge coupler comprising: a suspended waveguide having a longitudinal direction and a lateral direction, the suspended waveguide being disposed between two longitudinally arranged periodic isolation trenches; an outer cladding; a plurality of lateral support rods disposed between the isolation trenches and interconnecting the suspended waveguide and the outer cladding; a silicon substrate having a cavity in fluid communication with the isolation trenches, wherein the waveguide and the support rods are suspended above the cavity, wherein the outer cladding is supported by the substrate; and a reverse taper of a second waveguide embedded in the first waveguide, wherein each support rod includes a covering region having a first edge width abutting the suspended waveguide and a second edge width abutting the outer cladding, wherein the second edge width is greater than the first edge width.

Description

Technical Field

[0001] This disclosure relates to structures for silicon photonics edge couplers, optical packages, fiber optic couplers, and levitated edge couplers, as well as methods for manufacturing such structures. Background Technology

[0002] The goal is to couple light into and out of photonic chips with low loss in order to reduce the cost of optical communication links.

[0003] Existing methods attempt to achieve efficient coupling between silicon waveguides and single-mode fibers (SMFs), including surface gratings and patch size converters (SSCs), such as reverse tapers, floating edge couplers, and multilayer edge couplers. However, existing methods are not satisfactory.

[0004] Surface gratings have limited optical bandwidth and are sensitive to polarization, thus limiting their application in wavelength division multiplexing.

[0005] Edge couplers, including the reverse taper, offer wide optical bandwidth because their operation is based on mode overlap. However, mode expansion is limited by substrate leakage due to the proximity of the silicon substrate (Si) to the buried oxide (BOX) layer. Even though low coupling loss can be achieved for fibers with small mode field diameters (MFDs), coupling loss increases significantly for standard single-mode fibers. Furthermore, such edge couplers have very small misalignment tolerances due to their small MFDs.

[0006] Edge couplers, including multilayer patch size converters, help mitigate substrate leakage issues and provide ample misalignment tolerance. These devices consist of several layers of low-refractive-index waveguides (e.g., silicon nitride) with interlayer cladding material (e.g., silicon oxide, silicon oxynitride) and sit on top of a standard waveguide layer (e.g., silicon-on-insulator). However, the stack of layers forming the multilayer edge coupler increases the complexity of the integration process, and the multilayer deposition of low-refractive-index waveguides and interlayer cladding increases the overall thin-film stress across the entire wafer.

[0007] In silicon-on-insulator (SiO2) photonics platforms, an alternative edge coupler approach provides a silicon oxide top cladding material together with a bottom cladding material (alternately referred to as a BOX layer) to define the coupling waveguide. This is achieved by isolating the silicon oxide top cladding and BOX layer from the silicon substrate via a series of etching steps, and forming a suspended oxide waveguide comprising the top oxide cladding and BOX layer over the silicon substrate. The phrase “silicon oxide” and the term “oxide” are equivalent and can be used interchangeably herein.

[0008] The suspended oxide waveguide is isolated from the remaining top cladding, BOX layer, and bulk substrate, thus preventing substrate leakage. Furthermore, the width of the suspended oxide waveguide can be adjusted to match the mode size of the input beam.

[0009] Furthermore, since the coupling waveguide is made of silicon dioxide, the refractive indices of the silicon dioxide fiber and the coupling waveguide will be similar, thus minimizing Fresnel reflections. If the coupling interface is immersed in a suitable refractive index-matching oil, the suspended edge coupler can achieve coupling losses as low as 1.0 dB / endface.

[0010] In silicon-on-insulator (SiIn) photonics platforms, isolation of silicon oxide coupled waveguides is achieved by first etching the cladding material down into the silicon substrate. This is followed by isotropic silicon etching, where the structure is etched towards the silicon substrate at a depth of tens of micrometers. This allows the coupled waveguide to be defined by air-encapsulated silicon oxide.

[0011] The hundreds-of-micrometers-long suspended waveguide is connected to the top oxide cladding of the body by a row of oxide support rods on both sides to prevent collapse.

[0012] A support rod with a small feature size is desirable to avoid disturbing the optical modes propagating through the suspended oxide waveguide. However, this introduces a design trade-off, as a support rod with a wider feature size improves mechanical support against swaying, vibration, and shear effects.

[0013] This is particularly important in densely integrated multi-input / output coupling schemes, such as in photonic integrated circuits, where a closely spaced row of levitated edge couplers is used as an optical phased array transmitter / receiver block. In such applications, levitated edge couplers with small feature sizes and oxide support rods are more prone to waveguide collapse and / or breakage because the support rod width can be further reduced due to the cumulative effect of isotropic silicon etching of adjacent waveguides.

[0014] Figure 1A , Figure 1B Figure 1C shows a conventional suspended edge coupler 100 with an oxide support rod having a rectangular coverage area.

[0015] The conventional suspended edge coupler 100 includes a suspended oxide waveguide 102, multiple rows of isolation trenches 104 located on both sides of the suspended oxide waveguide 102, a row of oxide support rods 106 having a rectangular coverage area and arranged on both sides of the suspended oxide waveguide 102 to provide mechanical support, and a nano-tapered portion 108 for facilitating the thermal coupling of light from the suspended oxide waveguide 102 to the silicon waveguide 110.

[0016] While many existing methods focus on optimizing coupling performance, few address the mechanical reliability or stiffness of suspended edge couplers. Summary of the Invention

[0017] According to the first aspect, an edge coupler includes: A suspended waveguide having a longitudinal direction and a lateral direction, the waveguide being arranged between two longitudinal rows of periodic isolation trenches; outer cladding; Multiple lateral support rods are arranged between the isolation trenches and interconnect the waveguide and the outer cladding; A silicon substrate having a cavity in fluid communication with the isolation trench, wherein the waveguide and the support rod are suspended above the cavity, and wherein the outer cladding is supported by the substrate; and The reverse tapered section of the second waveguide is embedded in the suspended waveguide. Each support rod includes a coverage area having a first edge width that mates with the waveguide and a second edge width that mates with the cladding, wherein the second edge width is greater than the first edge width.

[0018] According to a second aspect, a method for manufacturing an edge coupler, the method comprising: A top cladding layer is formed on a silicon-on-insulator (SOI) structure, the SOI structure including a silicon layer formed on a buried oxide (BOX) layer on a silicon substrate; Without etching the silicon substrate, the top cladding and the SOI structure are patterned and etched such that two longitudinal rows of periodic isolation trenches define an inner cladding located between the two longitudinal rows of periodic isolation trenches, an outer cladding separated from the inner cladding by one of the two longitudinal rows of periodic isolation trenches, and lateral support rods that interconnect the inner cladding and the outer cladding. The silicon substrate is isotropically etched to levitate the inner cladding, thereby providing a levitated waveguide. Each support rod includes a coverage area having a first edge width that abuts the suspended waveguide and a second edge width that abuts the outer cladding, wherein the second edge width is greater than the first edge width, and wherein the first waveguide includes a reverse taper of the second waveguide. Attached Figure Description

[0019] Figure 1A , Figure 1B Figure 1C shows a top view, a partial enlarged view, and a front sectional view of a conventional suspended edge coupler with rectangular support rods; Figure 2A , Figure 2BFigures 2 and 2C show a top view, a partial enlarged view, and a front sectional view of a suspended edge coupler having a support rod in an isosceles trapezoidal coverage area according to an embodiment of the present disclosure. Figure 2D It shows Figure 2A , Figure 2B Top view of the support rod in Figure 2C; Figure 3A , Figure 3B , Figure 3C and Figure 3D Different shapes of support rods are shown; and Figure 4 A design with rectangular support rods is shown (where w 1= w A traditional suspended edge coupler (2=2 µm) and one implementation (where the isosceles trapezoidal oxide support rod design has design parameters of 2 µm) are compared with those of the traditional suspended edge coupler and one implementation (where the isosceles trapezoidal oxide support rod design has design parameters of 2 µm). w 1 = 2 µm and w The optical loss spectrum was measured at 2 = 10 µm. Detailed Implementation

[0020] In the following description, numerous specific details are set forth to provide a thorough understanding of various illustrative and non-limiting embodiments. However, those skilled in the art will understand that embodiments of the invention may be practiced without some or all of these specific details. It should be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the invention. In the accompanying drawings, the same reference numerals or numbers refer to the same or similar functions or features in several views.

[0021] The implementations described in the context of one of the devices or methods are similarly effective for other devices or methods. Similarly, the implementations described in the context of a device are similarly effective for a method, and vice versa.

[0022] Features described in the context of an implementation may be applied accordingly to the same or similar features in other implementations. Features described in the context of an implementation may be applied accordingly to other implementations, even if not explicitly described in those other implementations. Furthermore, additions and / or combinations and / or alternatives described for a feature in the context of an implementation may be applied accordingly to the same or similar features in other implementations.

[0023] It should be understood that the articles “a,” “an,” and “the” used with respect to features or elements include references to one or more features or elements. The term “and / or” includes any and all combinations of one or more associated features or elements. The terms “comprising,” “including,” “having,” and any related terms, as used in the description and claims, are intended to be open-ended and mean that there may be additional features or elements besides those listed. Identifiers such as “first,” “second,” “third,” etc., are used only as labels and are not intended to impose numerical requirements on their objects, nor should they be interpreted in a manner that imposes any relative position or temporal order. The term “as…” may include references to “configured as,” “suitable for,” and “constructed and arranged as,” which are used interchangeably.

[0024] Given existing methods, there remains a need to improve the mechanical stiffness and robustness of levitated edge couplers without compromising their coupling performance. Therefore, this paper discloses a solution for improving the mechanical stiffness of levitated edge couplers using oxide support rods, wherein the width of the support rod at the edge mating with the levitated oxide waveguide is smaller than the width of the support rod at the opposite edge mating with the body cladding.

[0025] Embodiments of this disclosure provide optical edge couplers suitable for use in photonic waveguides constructed on a silicon-on-insulator (SiI) photonics platform.

[0026] Figure 2A , Figure 2B Figure 2C shows various views of an embodiment of a suspended edge coupler 200 with a support rod having an isosceles trapezoidal coverage area.

[0027] The edge coupler 200 includes a suspended or first waveguide 202, a top oxide cladding 212, a bottom oxide cladding 214 (or alternatively, a buried oxide (BOX) layer), a longitudinal row of periodic isolation trenches 204, a lateral row of periodic support rods 206, a reverse taper 208, and a substrate 216.

[0028] The suspended waveguide 202 includes the longitudinal direction ( Figure 2A The z-direction shown corresponds to the axis of optical signal or light propagation, and the lateral direction ( Figure 2AThe waveguide 202 includes opposing sidewalls, each extending along or substantially along the longitudinal direction. The waveguide 202 includes opposing ends, such as a first end and a second end, from which light propagates. The waveguide 202 is arranged or situated between two longitudinal rows of periodic isolation trenches 204, for example, within the same longitudinal row of trenches, each pair of adjacent isolation trenches 204 is separated by a support rod 206. The isolation trenches 204 extend along or substantially along the longitudinal direction. The waveguide 202 includes a lateral dimension D that gradually decreases or narrows along the longitudinal direction; for example, the lateral dimension of the waveguide 202 is larger at its first end than at its second end. In some examples, the minimum lateral dimension of the waveguide 202 can be ≥2µm, as smaller values ​​may result in very thin or completely etched areas between laterally adjacent trenches. This resulting very thin structure may subsequently collapse under an oxide layer approximately 6µm thick. It should be understood that the minimum lateral dimension of waveguide 202 can vary depending on manufacturing specifications.

[0029] The top oxide cladding 212 and the bottom oxide cladding 214 (or BOX layers) (collectively referred to as cladding 212, 214) extend along or substantially along the longitudinal direction. Cladding 212, 214 includes at least two regions: an inner cladding 222 defined between two longitudinally arranged periodic trenches and providing a suspended waveguide 202, and an outer cladding 224 spaced apart from the inner cladding 222 via periodically arranged lateral support rods 206 and providing support for the suspended waveguide 202. The inner cladding 222 and the outer cladding 224 are interconnected by the lateral support rods 206.

[0030] Lateral support rods 206 are arranged or located between adjacent trenches 204. Each support rod 206 extends in the lateral direction and interconnects the suspended waveguide 202 and the outer cladding 224. Thus, multiple lateral support rods 206 arranged or located between isolation trenches 204 interconnect the waveguide 202 and the outer cladding 224 to provide mechanical support to the suspended waveguide 202. Each support rod 206 has opposing ends, such as a first end and a second end, wherein the first end is attached to the suspended waveguide 202 (or the inner cladding 222) and includes a first edge width that abuts against the suspended waveguide 202. w 1, and wherein the second end is attached to the outer cladding 224 and includes a second edge width abutting the outer cladding 224. w 2. Each support rod 206 has a coverage area (see...) Figure 2D The coverage area includes at least the width of the first edge. w 1 and second edge width w 2. Second edge width w 2 is greater than the width of the first edgew 1. In other words, the width of the first edge. w 1 compared to the width of the second edge w 2. Narrow. First edge width w 1 is positioned closer to the waveguide and can be as narrow as possible to prevent mode perturbations, while the second edge width w 2 compared to the width of the first edge w The width is 1 to improve the mechanical rigidity and reliability of the edge coupler structure. The coverage area may also include a third edge width that intersects with adjacent trenches in trench 206. w 3 and fourth edge width w 4. In this disclosure, the term "footprint" includes a reference to an area taken from a plan view.

[0031] Substrate 216 has a cavity 218. Cavity 218 is in fluid communication with isolation trench 204. Waveguide 202 is suspended above cavity 218 and supported by support rods 206 disposed on opposite sidewalls of waveguide 202. Support rods 206 interconnect waveguide 202 and outer cladding 224. At least a portion of outer cladding 224 is non-suspended, for example, supported by substrate 216.

[0032] A reverse taper 208, which may be a reverse taper or part of a second waveguide 210, is embedded in the suspended waveguide 202 and extends at least partially within it in the longitudinal direction, particularly from a midpoint between the first and second ends of the waveguide 202 toward the second end of the waveguide 202. The reverse taper 208 is narrower at the midpoint and wider at the second end of the waveguide 202. As light propagates from the midpoint toward the second end of the waveguide 202, the taper 208 facilitates thermal coupling of light from the suspended waveguide 202 to the reverse taper waveguide 210 coupled thereto.

[0033] In some embodiments, the levitation waveguide 202, cladding 212, 214, and support rod 206 are made of silicon oxide. The substrate 216 and the reverse taper 208 are made of silicon. The trench 204 and cavity 218 contain air. In other embodiments, the reverse taper 208 may be made of silicon nitride (SiN), aluminum nitride (AlN), or any other material having a higher refractive index than the first waveguide 202.

[0034] In some implementations (see) Figures 2A to 2D Each support rod 206 covers an area comprising an isosceles trapezoid. In this disclosure, an isosceles trapezoid is generally understood as a quadrilateral having two parallel sides of unequal size or length and two non-parallel sides of equal size or length. Specifically, in such an embodiment, the first edge width of the profile... w 1 and second edge width w2 corresponds to two parallel sides of unequal size or length, while the width of the third edge of the profile... w 3 and fourth edge width w 4 corresponds to two non-parallel sides of equal size or length.

[0035] In some embodiments, the profile of each support rod 206 comprises a trapezoid. In this disclosure, a trapezoid is generally understood as a quadrilateral having two parallel sides of unequal size or length and two non-parallel sides of unequal size or length. Specifically, in such embodiments, the first edge width of the profile... w 1 and second edge width w 2 corresponds to two parallel sides of unequal size or length, while the width of the third edge of the profile... w 3 and fourth edge width w 4 corresponds to two non-parallel sides of unequal size or length.

[0036] In some implementations (see) Figure 3A and Figure 3B Each support rod 206 has a profile comprising multiple rectangles of different sizes or lengths juxtaposed or stacked, such as two rectangles of different lengths, three rectangles of different lengths, etc.

[0037] In some implementations (see) Figure 3C and Figure 3D ), third edge w 3 and / or the fourth edge w 4. Includes circular arcs, elliptical arcs, parabolic arcs, or hyperbolic arcs. Third edge. w 3 and the fourth edge w 4. Selected from one or more of the examples above, which may be symmetrical or asymmetrical with each other.

[0038] In some implementations, the third edge w 3 and the fourth edge w 4. They can be symmetrical or asymmetrical.

[0039] In some implementations, the second edge width w 2 can be the width of the first edge. w Multiples of 1. First edge width w 1 and the second edge width w The ratio of 2 can be configured to provide the width of the third edge relative to the adjacent first edge. w An angle greater than 45 degrees between the edges of one of the isolation trenches.

[0040] Figure 2D The isosceles trapezoidal coverage area of ​​the support rod with the following parameters is shown: w 1 = First edge width w 2 = second edge width w 3 = third edge width w 4 = fourth edge width α = The angle between the edge of the third edge and the edge of the groove adjacent to the width of the first edge. h = width of each or adjacent first edge w 1 and second edge width w The dimensions of the groove are 2, and are obtained along the lateral direction (or referred to as "lateral dimensions"). 2d = second edge width w 2 and the width of the first edge w The difference between 1 and 1. Therefore, the relationship between the above parameters can be expressed as follows:

[0041] If the designed angle α value is very small or sharp, the etchant cannot penetrate the sharp corners during manufacturing, resulting in a rounded corner effect at these corners. This rounded corner effect will cause the manufactured support rod to have an edge width greater than the designed edge width.

[0042] In some implementations, an angle α greater than 45 degrees will be provided. Therefore, the first edge width... w 1. Second edge width w 2. The lateral dimension h of each trench is based on the following relationship: .

[0043] It should be understood that other values ​​of angle α can be imagined, such as 90 degrees.

[0044] It should be understood that the above parameters and relationships are applicable to coverage areas of other shapes when properly adjusted.

[0045] Figure 4 The design with a rectangular support rod is shown in the C-band measurement (wherein) w 1= w A traditional suspended edge coupler (2=2 µm) and one implementation (where the isosceles trapezoidal oxide support rod design has) w 1 = 2 µm and w The measured optical loss spectrum (design parameters 2 = 10 µm) was obtained. Compared to conventional designs, the embodiments of this disclosure do not introduce any excessive coupling loss, indicating that it does not disturb light propagation through the suspended oxide waveguide; for example, the optical field does not leak into the support rod. Therefore, the embodiments of this disclosure provide a suspended edge coupler design that improves its mechanical stiffness without reducing the coupling performance of the edge coupler.

[0046] Various methods for fabricating a floating edge coupler are conceivable. In some embodiments, a method for fabricating a floating edge coupler includes: producing a silicon-on-insulator (SOI) structure, wherein a silicon layer is formed on a buried oxide (BOX) layer formed on a silicon substrate. The method further includes: forming a top cladding layer on the SOI structure. The method further includes: patterning the top cladding layer and the SOI structure such that periodic isolation trenches of two longitudinal rows define an inner cladding layer located between the periodic isolation trenches of the two longitudinal rows, an outer cladding layer separated from the inner cladding layer via one of the periodic isolation trenches of the two longitudinal rows, and lateral support struts interconnecting the inner and outer cladding layers. This patterning etching step can be performed without etching the silicon substrate. The method further includes: isotropically etching the silicon substrate to levitate the inner cladding layer, thereby providing a floating waveguide. In the patterning etching step described above, it includes forming each support rod having a covering region having a first edge width abutting the levitation waveguide and a second edge width abutting the outer cladding, wherein the second edge width is greater than the first edge width, and wherein the first waveguide includes a reverse taper of the second waveguide. Other features of the fabricated levitation waveguide have already been described in the preceding paragraphs in conjunction with the accompanying drawings, and therefore will not be described again.

[0047] It should be understood that the above-described embodiments and features are to be considered exemplary and not restrictive. Many other embodiments will be apparent to those skilled in the art from consideration of this specification and from the practice of the invention. Furthermore, certain terms have been used for descriptive purposes and not to limit the disclosed embodiments of the invention.

Claims

1. An edge coupler, the edge coupler comprising: a suspended waveguide having a longitudinal direction and a lateral direction, the waveguide disposed between two longitudinally aligned rows of periodic isolation trenches; an outer cladding; a plurality of lateral support posts disposed between the isolation trenches and interconnecting the waveguide and the outer cladding; a silicon substrate having a cavity in fluid communication with the isolation trenches, wherein the waveguide and the support posts are suspended above the cavity, wherein the outer cladding is supported by the substrate; and a reverse taper of a second waveguide embedded in the suspended waveguide, wherein each support post comprises a footprint having a first edge width interfacing with the waveguide and a second edge width interfacing with the cladding, wherein the second edge width is greater than the first edge width.

2. The edge coupler of claim 1, wherein, The footprint of each support post comprises a trapezoid having two parallel sides of unequal dimensions corresponding to the first edge width and the second edge width.

3. The edge coupler of claim 2, wherein, The trapezoid is an isosceles trapezoid.

4. The edge coupler of claim 1, wherein, The footprint of each support post comprises a plurality of juxtaposed differently sized rectangles.

5. The edge coupler of claim 1, wherein, The footprint of each support post comprises a third edge width and a fourth edge width interfacing with adjacent ones of the isolation trenches, wherein the third edge and / or the fourth edge comprises a circular arc, an elliptical arc, a parabolic arc, or a hyperbolic arc.

6. The edge coupler of claim 1, wherein, The footprint of each support post comprises a third edge width and a fourth edge width interfacing with adjacent ones of the isolation trenches, and wherein the third edge width and the fourth edge width are asymmetric with respect to each other.

7. The edge coupler of any one of claims 1 to 6, wherein, The dimensions of the first edge width and the second edge width are based on wherein, w 1 corresponds to the first edge width, w 2 corresponds to the second edge width, and h corresponds to a lateral dimension of a corresponding one of the isolation trenches adjacent to the first edge width and the second edge width.

8. The edge coupler of any one of claims 1 to 7, wherein, The suspended waveguide comprises a lateral dimension that tapers along the longitudinal direction.

9. The edge coupler of any one of claims 1 to 8, wherein, The isolation trenches comprise air.

10. A method for fabricating an edge coupler, the method comprising: forming a top cladding on a silicon-on-insulator (SOI) structure, the silicon-on-insulator structure comprising a silicon layer formed on a buried oxide (BOX) layer, the buried oxide layer formed on a silicon substrate; without etching the silicon substrate, patterned etching the top cladding and the SOI structure such that two longitudinally aligned rows of periodic isolation trenches define an inner cladding located between the two longitudinally aligned rows of periodic isolation trenches, an outer cladding separated from the inner cladding via one of the two longitudinally aligned rows of periodic isolation trenches, and lateral support posts interconnecting the inner cladding and the outer cladding; isotropically etching the silicon substrate to suspend the inner cladding to provide a suspended waveguide; wherein each support post comprises a footprint having a first edge width interfacing with the suspended waveguide and a second edge width interfacing with the outer cladding, wherein the second edge width is greater than the first edge width, and wherein a first waveguide comprises a reverse taper of a second waveguide.

11. The method of claim 10, wherein, The coverage area of ​​each support rod comprises a trapezoid with two parallel sides of unequal size, the two parallel sides corresponding to the first edge width and the second edge width.

12. The method of claim 11, wherein, The trapezoid is an isosceles trapezoid.

13. The method of claim 10, wherein, The coverage area of ​​each support rod comprises multiple juxtaposed rectangles of different sizes.

14. The method of claim 10, wherein, The coverage area of ​​each support rod includes a third edge width and a fourth edge width that mate with an adjacent isolation trench in the isolation trench, wherein the third edge and / or the fourth edge includes a circular arc, an elliptical arc, a parabolic arc, or a hyperbolic arc.

15. The method of claim 10, wherein, The coverage area of ​​each support rod includes a third edge width and a fourth edge width that mate with an adjacent isolation trench in the isolation trench, wherein the third edge width and the fourth edge width are asymmetrical relative to each other.

16. The method of any one of claims 10 to 15, wherein, The dimensions of the first edge width and the second edge width are based on wherein, w 1 corresponds to the first edge width, w 2 corresponds to the second edge width, and h corresponds to a lateral dimension of a corresponding one of the isolation trenches adjacent to the first edge width and the second edge width.

17. The method of any one of claims 10 to 16, wherein, The suspended waveguide includes lateral dimensions that gradually decrease along the longitudinal direction.

18. The method of any one of claims 10 to 17, wherein, The isolation trench includes air.

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