TWI891438B - Edge coupler having suspended waveguide with improved mechanical rigidity - Google Patents

Abstract

The present disclosure relates generally to photonic edge coupler comprising a suspended waveguide having a longitudinal direction and a lateral direction, the suspended waveguide arranged between two longitudinal rows of periodic isolation trenches; an outer cladding; a plurality of lateral support rods arranged 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 over the cavity, wherein the outer cladding is supported by the substrate; and an inverse taper of a second waveguide which is embedded in the first waveguide, wherein each support rod includes a footprint having a first edge width interfacing the suspended waveguide, and a second edge width interfacing the outer cladding, wherein the second edge width is larger than the first edge width.

Description

Edge coupler with suspended waveguide having improved mechanical robustness

This disclosure relates to silicon photonic edge couplers, optical packages, fiber optic couplers, suspended edge coupler structures, and methods for fabricating these structures.

Optical communication links require low-loss light coupling into and out of photonic chips to reduce costs.

Existing approaches attempt to achieve efficient coupling between silicon waveguides and single-mode fibers (SMFs), including surface gratings and spot-size converters (SSCs), such as inverted tapers, suspended edge couplers, and multi-layer edge couplers. However, existing approaches are unsatisfactory.

Surface gratings have limited optical bandwidth and are polarization sensitive, which limits their use in wavelength division multiplexing.

Edge couplers containing inverted tapers offer wide optical bandwidth because their operation is based on mode overlapping. However, due to the proximity of the silicon (Si) substrate to the buried oxide (BOX), modal spreading is limited by substrate leakage. While low coupling loss can be achieved for optical fibers with small mode-field diameters (MFD), it increases significantly for standard single-mode optical fibers. Furthermore, due to the low MFD of these edge couplers, they have very tight alignment tolerances.

Edge couplers containing multilayer spot-size converters help alleviate substrate leakage issues and offer wide alignment tolerances. These devices consist of several layers of low-refractive-index waveguides (e.g., silicon nitride) with cladding materials (e.g., silicon oxide, silicon oxynitride) between them, layered on top of standard waveguide layers (e.g., silicon-on-insulator). However, the stacked structure that forms these multilayer edge couplers increases the complexity of the integration process, while the multiple deposition layers of low-refractive-index waveguides and the cladding layers between them increase the overall film stress across the wafer.

In silicon-on-insulator (SOI)-based photonic platforms, another edge coupler approach uses a silicon oxide top cladding material and a bottom cladding material (also known as a BOX layer) to define coupled waveguides. This is achieved by isolating the silicon oxide top cladding and BOX layers from the silicon substrate through a series of etching steps, creating a suspended oxide waveguide composed of the top oxide cladding and BOX layers above the silicon substrate. The terms "silicon oxide" and "oxide" are equivalent and will be used interchangeably hereafter.

The suspended oxide waveguide is isolated from the rest of the top cladding and BOX layers, as well as the bulk substrate, to prevent substrate leakage. Furthermore, the width of the suspended oxide waveguide can be tuned to match the mode size of the input beam.

Furthermore, because the coupling waveguide is made of silicon oxide, the refractive indices of the silicon fiber and the coupling waveguide are similar, minimizing Fresnel reflections. If the coupling interface is immersed in an appropriate index-matching oil, suspended edge couplers can achieve coupling losses as low as 1.0 dB/plane.

In photonic platforms based on silicon-on-insulator layers, isolation of the silicon oxide coupled waveguides is achieved by first etching the cladding material down to the silicon substrate. This is followed by an isotropic silicon etch, where the structure is etched tens of microns toward the silicon substrate. This allows the coupled waveguides to be defined within the silicon oxide enveloped by air.

The suspended waveguide, hundreds of microns long, is connected to the bulk top oxide cladding by a row of oxide support rods on both sides to prevent collapse.

To avoid disturbing the optical modes propagating through the suspended oxide waveguide, support rods with small feature sizes are desirable. However, this creates a design trade-off, as support rods with wider feature sizes provide improved mechanical support against shock, vibration, and shear effects.

This is particularly important in highly integrated multi-input/output coupling schemes, such as those used in photonic integrated circuits, where a closely packed array of suspended edge couplers is used as an optical phase array transmitter/receiver module. In this use case, suspended edge couplers with small feature size oxide support rods are more susceptible to waveguide collapse and/or cracking, as the support rod width can be further reduced due to the cumulative effects of isotropic silicon etching of adjacent waveguides.

Figures 1A, 1B, and 1C illustrate a conventional suspended edge coupler 100 having a rectangular planar surface with oxide support rods.

A conventional suspended edge coupler 100 includes a suspended oxide waveguide 102, multiple rows of isolation trenches 104 on either side of the suspended oxide waveguide 102, a row of oxide support rods with rectangular footprints 106 arranged on either side of the suspended oxide waveguide 102 to provide mechanical support, and nanocones 108 for facilitating adiabatic coupling of light from the suspended oxide waveguide 102 to a silicon waveguide 110.

While many existing methods focus on optimizing coupling performance, few address the mechanical reliability or robustness of suspended edge couplers.

According to a first aspect, an edge coupler includes: a suspended waveguide having longitudinal and lateral dimensions, the waveguide being arranged between two rows of periodic isolation trenches arranged in the longitudinal direction; an external cladding; a plurality of lateral support rods arranged between the isolation trenches and interconnecting the waveguide and the external 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 external cladding is supported by the substrate; and an inverted cone of a second waveguide embedded in the suspended waveguide, wherein each support rod includes a footprint having a first side width interfacing with the waveguide and a second side width interfacing with the cladding, wherein the second side width is greater than the first side width.

According to a second aspect, a method for manufacturing an edge coupler includes forming a top cladding layer on a silicon-on-insulator (SOI) structure, the SOI structure including a silicon layer formed on a buried oxide (BOX) layer formed on a silicon substrate; pattern etching the top cladding layer and the SOI structure with two longitudinally arranged rows of periodic isolation trenches without etching the silicon substrate to define an inner cladding layer between the two longitudinally arranged rows of periodic isolation trenches, an outer cladding layer separated from the inner cladding layer by one of the two longitudinally arranged rows of periodic isolation trenches, and an outer cladding layer. and lateral support rods interconnecting the inner cladding and the outer cladding; isotropically etching the silicon substrate to suspend the inner cladding, thereby providing a suspended waveguide; wherein each support rod includes a cover region having a first side width interfacing with the suspended waveguide and a second side width interfacing with the outer cladding, wherein the second side width is greater than the first side width, and wherein the first waveguide includes an inverted taper of the second waveguide.

100,200:Edge Coupler

102,202: Waveguide

104,204: Groove

106: Coverage Area

206: Support rod

108,208: Cone

110,210: Waveguide

112,114,212,214: Coating layer

116,216:Substrate

118,218: Cavity

222: Inner cladding layer

224: External cladding

w ₁ : first side width

w ₂ : width of the second side

w ₃ : width of the third side, the third side

w ₄ : width of the fourth side, the fourth side

h, D: Dimensions

Figures 1A, 1B, and 1C illustrate a top view, a partially enlarged view, and a front cross-sectional view, respectively, of a conventional suspended edge coupler having a rectangular support rod.

Figures 2A, 2B, and 2C illustrate a top view, a partially enlarged view, and a front cross-sectional view, respectively, of a suspended edge coupler having an isosceles trapezoidal cover support rod according to an embodiment of the present disclosure.

Figure 2D illustrates a top view of the support rod in Figures 2A, 2B, and 2C.

Figures 3A, 3B, 3C, and 3D illustrate support rods of different shapes.

FIG4 illustrates the measured optical loss spectra of a conventional suspended edge coupler having a rectangular support bar design with w ₁ =w ₂ =2 μm, and an isosceles trapezoidal oxide support bar design of one embodiment having design parameters w ₁ =2 μm and w ₂ =10 μm.

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

Embodiments described in the context of one device or method are analogously valid for the other device or method. Likewise, embodiments described in the context of a device are analogously valid for the method, and vice versa.

Features described in the context of one embodiment may be applied accordingly to the same or similar features in other embodiments. Features described in the context of one embodiment may be applied accordingly to other embodiments, even if not explicitly described in those other embodiments. Furthermore, additions, combinations, and/or substitutions of features described in the context of one embodiment may be applied accordingly to the same or similar features in other embodiments.

It should be understood that the articles "a," "an," and "said" used with respect to features or elements include reference to one or more features or elements. The term "and/or" includes any and all combinations of the relevant features or elements. The terms "including," "comprising," "having," and related terms used in the description and claims are intended to be open-ended, meaning that there may be additional features or elements other than the listed features or elements. Designators such as "first," "second," and "third" are used merely 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 sequence on the limitations. The term "to" may include reference to "for," "suitable for," and "constructed and arranged to," and these terms may be used interchangeably.

Given existing approaches, there remains a need to improve the mechanical robustness and stability of suspended edge couplers without compromising coupling performance. Therefore, the suspended edge coupler disclosed herein provides a solution to improve the mechanical robustness of a suspended edge coupler by employing oxide support rods, wherein the support rods on the edge that interfaces with the suspended oxide waveguide are narrower than the support rods on the opposing edge that interfaces with the bulk cladding.

Embodiments of the present disclosure provide optical edge couplers suitable for use in photonic waveguides constructed on a silicon-on-insulator-based photonics platform.

Figures 2A, 2B, and 2C illustrate various views of an embodiment of a suspended edge coupler 200 having an isosceles trapezoidal footprint support rod.

The edge coupler 200 includes a suspended or first waveguide 202, a top oxide cladding layer 212, a bottom oxide cladding layer 214 (also known as a buried oxide (BOX) layer), vertically arranged periodic isolation trenches 204, laterally arranged periodic support bars 206, an inverted taper 208, and a substrate 216.

The suspended waveguide 202 includes a longitudinal direction (the z-direction shown in FIG. 2A ), corresponding to the axis along which optical signals or light propagate, and a transverse direction (the x-direction shown in FIG. 2A ) perpendicular to the longitudinal direction. The waveguide 202 includes opposing sidewalls, each of which extends along or substantially along the longitudinal direction. The waveguide 202 includes opposing ends, such as a first end and a second end, wherein light propagates from the first end to the second end. The waveguide 202 is arranged or interposed between two rows of periodic isolation trenches 204 arranged in the longitudinal direction. For example, each pair of adjacent isolation trenches 204 in the same longitudinal trench row is separated by a support bar 206. The isolation trenches 204 extend along or substantially along the longitudinal direction. The waveguide 202 includes a transverse dimension D that tapers longitudinally, for example, the transverse dimension of the waveguide 202 at a first end is greater than that at a second end. In some examples, the minimum transverse dimension of the waveguide 202 may be 2 μm, as smaller values may result in very thin or completely etched regions between laterally adjacent trenches. The resulting very thin structures may subsequently collapse under the approximately 6 μm thick oxide layer. It should be understood that the minimum lateral dimension of waveguide 202 may vary depending on manufacturing specifications.

The top oxide cladding layer 212 and the bottom oxide cladding layer 214 (or BOX layer) (collectively, cladding layers 212, 214) extend along or substantially along the longitudinal direction. Cladding layers 212, 214 include at least two regions: an inner cladding layer 222, which is defined between two rows of periodic trenches arranged in the longitudinal direction and provides a suspension for waveguide 202; and an outer cladding layer 224, which is separated from the inner cladding layer 222 by periodic lateral support rods 206 and provides support for the suspension for waveguide 202. The inner cladding layer 222 and the outer cladding layer 224 are interconnected by the lateral support rods 206.

The lateral support rods 206 are arranged or inserted between adjacent trenches 204. Each support rod 206 extends laterally and interconnects the suspended waveguide 202 and the outer cladding layer 224. Therefore, the plurality of lateral support rods 206 are respectively arranged or inserted between the isolation trenches 204, interconnecting the waveguide 202 and the outer cladding layer 224, and providing mechanical support for 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 suspension waveguide 202 (or the inner cladding 222) and includes a first side width _w1 that contacts the suspension waveguide 202, and the second end is attached to the outer cladding 224 and includes a second side width _w2 that contacts the outer cladding 224. Each support rod 206 has a footprint (see FIG. 2D ) that includes at least the first side width _w1 and a second side width _w2 . The second side width _w2 is greater than the first side width _w1 . In other words, the first side width _w1 is narrower than the second side width _w2 . The first side width _w1 is located closer to the waveguide and is as narrow as possible to prevent mode perturbations, while the second side width _w2 is wider than the first side width _w1 to improve the mechanical robustness and reliability of the edge coupler structure. The footprint may further include a third side width _w3 and a fourth side width _w4 , each of which contacts the adjacent trench 204. In this disclosure, the term "footprint" includes reference to an area viewed from a plan view.

The substrate 216 provides a cavity 218. The cavity 218 is in fluid communication with the isolation trench 204. The waveguide 202 is suspended from the cavity 218 and supported by support rods 206 arranged on opposite side walls of the waveguide 202. The support rods 206 interconnect the waveguide 202 and the outer cladding 224. At least a portion of the outer cladding 224 is non-suspended, for example supported by the substrate 216.

The inverted cone 208, which may be part of the inverted cone waveguide or the second waveguide 210, is embedded in the suspended waveguide 202 and extends longitudinally therein for the longest portion thereof, particularly from a midpoint between the first and second ends of the waveguide 202 toward the second end of the waveguide 202. The inverted cone 208 is narrower at the midpoint and wider at the second end of the waveguide 202. The cone 208 facilitates adiabatic coupling of light from the suspended waveguide 202 to the coupled inverted cone waveguide 210 as light propagates from the midpoint toward the second end of the waveguide 202.

In some embodiments, the suspended waveguide 202, cladding layers 212 and 214, and support rod 206 are made of silicon dioxide. The substrate 216 and inverted cone 208 are made of silicon. The trench 204 and cavity 218 contain air. In other embodiments, the inverted cone 208 may be made of silicon nitride (SiN), aluminum nitride (AlN), or any other material with a higher refractive index than the first waveguide 202.

In some embodiments (see Figures 2A-2D), the footprint of each support rod 206 comprises an isosceles trapezoid. In the present disclosure, an isosceles trapezoid is generally understood to be a quadrilateral having two parallel sides of unequal size or length and two non-parallel sides of equal size or length. In particular, in these embodiments, the first side width _w1 and the second side width _w2 of the outline correspond to the two parallel sides of unequal size or length, while the third side width _w3 and the fourth side width _w4 of the outline correspond to the two non-parallel sides of equal size or length.

In some embodiments, the outline of each support rod 206 comprises a trapezoid. In this disclosure, a trapezoid is generally understood to be a quadrilateral having two parallel sides of unequal size or length and two non-parallel sides of unequal size or length. Specifically, in these embodiments, the first side width _w1 and the second side width _w2 of the outline correspond to the two parallel sides of unequal size or length, while the third side width _w3 and the fourth side width _w4 of the outline correspond to the two non-parallel sides of unequal size or length.

In some embodiments (see FIG. 3A and FIG. 3B ), the outline of each support rod 206 includes the plurality of parallel or stacked rectangles of different sizes or lengths, for example, two rectangles of different lengths, three rectangles of different lengths, etc.

In some embodiments (see FIG. 3C and FIG. 3D ), the third side w ₃ and/or the fourth side w ₄ include arcs of a circle, an ellipse, a parabola, or a hyperbola. The third side w ₃ and the fourth side w ₄ selected from one or more of the above examples may be symmetrical or asymmetrical relative to each other.

In some embodiments, the third side _w3 and the fourth side _w4 may be symmetrical or asymmetrical with respect to each other.

In some embodiments, the second side width w ₂ may be a multiple of the first side width w _1. The ratio of the first side width w ₁ to the second side width w ₂ may be configured to provide an angle between the third side and an edge of one of the isolation trenches adjacent to the first side width w ₁ , the angle being greater than 45 degrees.

Figure 2D shows that the isosceles trapezoidal footprint of the support bar has the following parameters: _w1 = first side width, _w2 = second side width, _w3 = third side width, _w4 = fourth side width, α = angle between the third side and the edge of one of the trenches adjacent to the first side width, h = the dimension of each or one of the trenches adjacent to the first side width _w1 and the second side width _w2 , and measured in the transverse direction (or referred to as the " _transverse dimension"), 2d = the difference between the second side width _w2 and the first side width _w1 . Therefore, the relationship between _the aforementioned parameters can be expressed as follows: w2 = 2d + w1 , d = 0.5 ₍ w2 - w1 ) _,

If the angle α is designed to be very small or sharp, the etchant won't be able to penetrate the sharp corners during manufacturing, causing these corners to be rounded. This rounding effect will cause the manufactured support bar to have a wider side width than the designed side width.

In some embodiments, an angle α greater than 45 degrees is provided. Thus, the first side width w ₁ , the second side width w ₂ , and the transverse dimension h of each trench are based on the following relationship: arctan >45°.

It should be understood that other values of angle α are contemplated, such as 90 degrees.

It should be understood that the above parameters and relationships are applicable to floor plans of other shapes with appropriate adjustments.

Figure 4 shows the optical loss spectra measured in the C-band for a conventional suspended edge coupler with a rectangular support rod design (where w ₁ = w ₂ = 2 μm) and an embodiment of the present invention with isosceles trapezoidal oxide support rods (with design parameters of w ₁ = 2 μm and w ₂ = 10 μm). Compared to the conventional design, the disclosed embodiment does not incur any additional coupling loss, indicating that it does not disrupt light propagation through the suspended oxide waveguide. For example, the light field does not leak into the support rods. Therefore, the disclosed embodiment provides a suspended edge coupler design that improves the mechanical robustness of the edge coupler without degrading its coupling performance.

Various methods of fabricating a suspended edge coupler are contemplated. In some embodiments, the method includes fabricating a silicon-on-insulator (SOI) structure, wherein a silicon layer is formed on a buried oxide (BOX) layer, which is formed on a silicon substrate. The method also includes forming a top cladding layer on the SOI structure. The method also includes pattern etching the top cladding layer and the SOI structure to form two rows of periodic isolation trenches arranged vertically to define an inner cladding layer located between the two rows of periodic isolation trenches, an outer cladding layer separated from the inner cladding layer by one of the periodic isolation trenches, and lateral support rods connecting the inner and outer cladding layers. This pattern etching step can be performed without etching the silicon substrate. The method also includes isotropically etching the silicon substrate to suspend the inner cladding layer, thereby providing a suspended waveguide. The pattern etching step includes forming each support rod, which includes a planar surface having a first side width in contact with the suspended waveguide and a second side width in contact with the outer cladding layer, wherein the second side width is greater than the first side width, and wherein the first waveguide includes an inverted taper of the second waveguide. Other features of the fabricated suspended waveguide have been described in the previous paragraphs with respect to the figures, and therefore, these features will not be repeated.

It should be understood that the embodiments and features described above are to be considered illustrative rather than restrictive. Many other embodiments will become apparent to those skilled in the art from consideration of this specification and practice of the invention. Furthermore, certain terminology is used for descriptive purposes only and is not intended to limit the disclosed embodiments of the invention.

200:Edge Coupler

202: Waveguide

204: Groove

206: Support rod

208: Cone

210: Waveguide

D: Dimensions

Claims (18)

An edge coupler includes: a suspended waveguide having longitudinal and lateral directions, wherein the suspended waveguide is arranged between two rows of periodic isolation trenches arranged in the longitudinal direction, and an external cladding layer; a plurality of lateral support rods arranged between the isolation trenches and interconnecting the suspended waveguide and the external cladding layer; a silicon substrate having a cavity in fluid communication with the isolation trench, wherein the suspended waveguide and The support rods are suspended above the cavity, wherein the outer cladding is supported by the silicon substrate; and an inverted cone of a second waveguide is embedded in the suspended waveguide, wherein each of the support rods includes a cover region having a first side width for engaging the suspended waveguide and a second side width for engaging the outer cladding, wherein the second side width is greater than the first side width. The edge coupler of claim 1, wherein the coverage area of each support rod comprises a trapezoid having two parallel sides of unequal dimensions corresponding to the first side width and the second side width, respectively. An edge coupler as described in claim 2, wherein the trapezoid is an isosceles trapezoid. An edge coupler as described in claim 1, wherein the coverage area of each support rod includes a plurality of parallel rectangles of different sizes. The edge coupler of claim 1, wherein the footprint of each support bar comprises a third side width and a fourth side width, engaging with the adjacent isolation trench, wherein the third side and/or the fourth side comprises an arc of a circle, an ellipse, a parabola, or a hyperbola. The edge coupler of claim 1, wherein the footprint of each support bar includes a third side width and a fourth side width that engages with the adjacent isolation trench, and wherein the third side width and the fourth side width are asymmetric with respect to each other. The edge coupler of any one of claims 1 to 6, wherein the dimensions of the first side width and the second side width are based on arctan >45°, wherein _w1 corresponds to the first side width, _w2 corresponds to the second side width, and h corresponds to the lateral dimension of one of the isolation trenches adjacent to the first side width and the second side width. The edge coupler of claim 1, wherein the suspended waveguide comprises a lateral dimension that tapers along the longitudinal direction. The edge coupler of claim 1, wherein the isolation trench comprises air. A method for manufacturing an edge coupler, the method comprising: forming a top cladding layer on a silicon-on-insulation structure, the silicon-on-insulation structure comprising a silicon layer formed on a buried oxide layer formed on a silicon substrate; patterning the top cladding layer and the silicon-on-insulation structure with two vertically arranged rows of periodic isolation trenches without etching the silicon substrate to define: an inner cladding layer between the two vertically arranged rows of periodic isolation trenches; and an inner cladding layer between one of the two vertically arranged rows of periodic isolation trenches. An outer cladding layer separated from the inner cladding layer; and lateral support rods connecting the inner cladding layer and the outer cladding layer; isotropically etching the silicon substrate to create a suspension of the inner cladding layer, thereby providing a suspended waveguide; wherein each of the support rods includes a covering region having a first side width joined to the suspended waveguide and a second side width joined to the outer cladding layer, wherein the second side width is greater than the first side width, and wherein the suspended waveguide includes an inverted taper of the second waveguide. A method for manufacturing an edge coupler as described in claim 10, wherein the footprint of each of the support rods comprises a trapezoid having two parallel sides of unequal dimensions corresponding to the first side width and the second side width, respectively. A method for manufacturing an edge coupler as described in claim 11, wherein the trapezoid is an isosceles trapezoid. A method for manufacturing an edge coupler as described in claim 10, wherein the coverage area of each of the support rods includes a plurality of parallel rectangles of different sizes. The method of manufacturing an edge coupler as recited in claim 10, wherein the footprint of each support bar includes a third side width and a fourth side width that engages with the adjacent isolation trench, wherein the third side and/or the fourth side include arcs of a circle, an ellipse, a parabola, or a hyperbola. The method of manufacturing an edge coupler as recited in claim 10, wherein the footprint of each support bar includes a third side width and a fourth side width that engages with the adjacent isolation trench, and wherein the third side width and the fourth side width are asymmetric with respect to each other. A method of manufacturing an edge coupler as claimed in any one of claims 10 to 15, wherein the dimensions of the first side width and the second side width are based on arctan >45°, wherein _w1 corresponds to the first side width, _w2 corresponds to the second side width, and h corresponds to the lateral dimension of one of the isolation trenches adjacent to the first side width and the second side width. A method of manufacturing an edge coupler as claimed in claim 10, wherein the suspended waveguide includes a lateral dimension that tapers along the longitudinal direction. A method of manufacturing an edge coupler as claimed in claim 10, wherein the isolation trench comprises air.