APPARATUS FOR OPTICAL COUPLER

There is described an optical coupler having a plurality of waveguides arranged in a plurality of cladding layers, each cladding layer having at least one waveguide. One or more of the vertical distances between waveguides in adjacent cladding layers and transverse distances between waveguides in the same cladding layer may be configured to perform one or more of mode matching with an external light source, maintaining optical coupling between the plurality of waveguides, and ensuring optical efficiency of the optical coupler.

BACKGROUND OF THE INVENTION

The present application relates to optical couplers, and in particular to edge couplers for optical signal communication.

One of the challenges in optical signal communication is the efficient edge coupling of optical fibres and integrated optical waveguides. The difficulty arises from the mismatch of the modal sizes between the different optical mediums. Typically, the fibre mode can be up to ten times larger than that of the integrated waveguide and this difference can result in significant optical coupling loss.

One coupler design involves waveguide segments laterally confined within multiple active optical device layers. However, such designs typically require high degrees of fabrication accuracy. Even small variations (e.g., ranging from 10 nm to 50 nm) in the cross-sectional dimensions of waveguides and/or surrounding cladding dielectric could lead to considerable optical loss.

Thus, there is a need in the art for an improved optical coupler for coupling an optical fibre to a waveguide that at least partially overcomes some of the above identified problems of the related art.

SUMMARY OF THE INVENTION

The present disclosure provides a method and system for an optical coupling device configured to couple an external light source, such as a laser source or a fibre optical cable, to an integrated waveguide on an integrated silicon photonics chip for both classical and quantum photonic applications with relatively high levels of optical efficiency.

In one aspect, the present disclosure provides an optical coupler that provides multiple parameters that may be configured to maintain mode-matching conditions to provide a greater degree of tolerance to dimensional variations in the coupler waveguide and/or surrounding dielectric cladding.

In a further aspect, one of the first optical device layer and the second optical device layer is also the functional optical device layer where the optical circuit elements, including one or more modulators, switches, and detectors, are located. Accordingly, the need for additional guiding or transitional layers for completing the mode transfer from the external light source onto the integrated silicon photonics chip may be obviated.

In a further still aspect, the present disclosure provides a method and system for an optical coupling device that has the flexibility to accommodate fabrication limitations. In some embodiments, the dimensions of the first optical waveguide and those of the second and third optical waveguides may be interchangeable to accommodate fabrication limitations with respect to layer thickness.

The distance between the two or more optical device layers and the distance between the second and third optical waveguides on the second optical device layer may be configured to accommodate fabrication limitations and tolerance errors. Further, the configuration of the two distance values may be configured to mode match external light sources. By way of a non-limiting example, the external light source may be an SMF-28 optical fibre cable which has a relatively large mode field diameter (MFD) of up to 10.4 μm at a wavelength of λ=1550 nm.

According to an exemplary aspect, there is an optical coupler comprising: a semiconductor substrate; a first cladding layer supported over the substrate; a second cladding layer supported over the substrate, the substrate, the first cladding layer, and the second cladding layer defining a facet; a first optical waveguide arranged in the first cladding layer, the first optical waveguide having a first cross-sectional area at the facet defined by a first thickness in a vertical direction and a first width in a transverse direction orthogonal to the vertical direction; a second optical waveguide arranged in the first cladding layer, the second optical waveguide at a first transverse distance from the first optical waveguide, the second optical waveguide having a second cross-sectional area defined by a second thickness and a second width at the facet; and a third optical waveguide arranged in the second cladding layer, the third optical waveguide at a first vertical distance from the first and second optical waveguides, the third optical waveguide having a third cross-sectional area defined by a third thickness and a third width at the facet; and wherein the first transverse distance and the first vertical distance are configured to perform one or more of mode matching with an external light source, maintaining optical coupling between the first, second, and third optical waveguides, and ensuring optical efficiency of the optical coupler.

In any of the above aspects, the first thickness and the second thickness may be substantially identical, and the first width and the second width may be substantially identical.

In any of the above aspects, the third thickness may be greater than the third width, and the first thickness may be less than the first width.

In any of the above aspects, the third thickness may be less than the third width, and the first thickness may be greater than the first width.

In any of the above aspects, the first cross-sectional area and the second cross-sectional area may be configured to optimize mode matching at the facet.

In any of the above aspects, the first cross-sectional area may be defined by a first thickness of less than 300 nm, and the first width may be greater than the first thickness.

In any of the above aspects, the third cross-sectional area may be configured to optimize coupling efficiency.

In any of the above aspects, the third thickness may be greater than 300 nm, and the third width may be less than the third thickness.

In any of the above aspects, the third optical waveguide may have a terminal width that is substantially identical to a width of a routing waveguide of a photonic integrated circuit (PIC).

In any of the above aspects, the PIC may be fabricated onto the second cladding layer such that the third optical waveguide is in optical communication with the routing waveguide of the PIC.

In any of the above aspects, a difference between the first width and the second width, and between the first thickness and the second thickness may be up to 50 nm.

In any of the above aspects, the second cladding layer may be supported above the first cladding layer.

In any of the above aspects, the first cladding layer may be supported above the second cladding layer.

In any of the above aspects, the first thickness may be substantially identical to the second thickness and the third thickness; and the first width may be substantially identical to the second width and the third width.

Any of the above aspects may further comprise a fourth optical waveguide arranged in the first cladding layer at a second transverse distance from the first optical waveguide, the fourth optical waveguide having a fourth cross-sectional area defined by a fourth thickness and a fourth width at the facet; wherein the first transverse distance, the second transverse distance, and the first vertical distance are configured to perform one or more of mode matching with an external light source, maintaining optical coupling between optical energy transmitted within the first, second, third, and fourth optical waveguides, and ensuring optical efficiency of the optical coupler.

In any of the above aspects, the fourth thickness may be substantially identical to the first thickness, and the fourth width may be substantially identical to the first width.

Any of the above aspects may further comprise a third cladding layer supported over the substrate, the substrate, the first cladding layer, the second cladding layer, and the third cladding layer defining the facet; a fifth optical waveguide arranged in the third cladding layer, the fifth optical waveguide having a fifth cross-sectional area at the facet defined by a fifth thickness in a vertical direction and a fifth width in a transverse direction orthogonal to the vertical direction; and a sixth optical waveguide arranged in the third cladding layer, the sixth optical waveguide at a third transverse distance from the fifth optical waveguide, the sixth optical waveguide having a sixth cross-sectional area defined by a sixth thickness and a sixth width at the facet, the fifth and sixth optical waveguides being at a second vertical distance from the waveguides of an adjacent cladding layer; wherein the first transverse distance, the second transverse distance, the third transverse distance, the first vertical distance, and the second vertical distance are configured to perform one or more of mode matching with an external light source, maintaining optical coupling between optical energy transmitted within the first, second, third, fourth, fifth, and sixth optical waveguides, and ensuring optical efficiency of the optical coupler.

In any of the above aspects, the second cladding layer may be supported above the first cladding layer and the third cladding layer may be supported above the second cladding layer.

In any of the above aspects, the six waveguides in the three cladding layers may be arranged such that they form a hexagonal shape at the facet where a first sum of the first transverse distance and the first width is substantially identical to a second sum of the third transverse distance and the fifth width, the first sum and the second sum being each less than a third sum of the second transverse distance and the second width.

In any of the above aspects, the first and fourth optical waveguides may be configured to optimize optical coupling where the first and fourth thickness are greater than 300 nm, and the first and fourth widths are less than the first and fourth thicknesses; and the second, third, fifth, and sixth optical waveguides may be configured to optimize mode matching where the second, third, fifth, and sixth thicknesses are each less than the second, third, fifth, and sixth widths, respectively.

Like reference numerals are used throughout the figures to denote identical elements and features. While aspects of the invention will be described in conjunction with the illustrated embodiments, it will be understood that it is not intended to limit the invention to such embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Silicon photonics in the implementation of photonic integrated circuits (PIC) has gained increasing traction as a viable technology, particularly in the area of quantum computing, due to its technological maturity, low fabrication cost, high integration density, and compatibility with existing fabrication methodologies. With the advent of the semiconductor fabrication process, the size of integrated circuit elements has been greatly reduced, resulting in higher integration density. However, the increasing size disparity between optical sources such as fibre cables and on-chip waveguides could result in significant optical coupling loss, thereby jeopardizing the signal integrity of the entire system. Thus, optical couplers configured to facilitate optical communication between an external light source (e.g., fibre optical cable or laser source) and an on-chip silicon waveguide at a high optical efficiency becomes a crucial design element in any integrated photonic system.

Methods and systems described herein provide an optical coupling device configured to couple an external light source, such as a laser source or a fibre optical cable, to an integrated waveguide in a PIC for both classical and quantum photonic applications. In one exemplary embodiment, the optical coupler described herein includes one waveguide on a first optical guiding layer and two waveguides on a second optical guiding layer. A vertical distance between the two optical guiding layers, coupled with a horizontal (or transverse) distance between the two waveguides within the second optical guiding layer, may be configured to mode match with the external light source to achieve a desired level of optical efficiency. Further, the two distance values may also be configured to accommodate fabrication tolerance error and/or fabrication limitations with respect to optical guiding layer thickness and waveguide width.

FIGS.1A and1Billustrate a top cross-sectional view and a side cross-sectional view of an exemplary silicon-based photonic device100, respectively. The device100may be a semiconductor chip, a dielectric chip, or any other suitable semiconductor medium. The device100includes one or more cladding layers102supported above a substrate104. The substrate104may be fabricated with any suitable material, including silicon, gallium arsenide (GaAs), indium phosphide (InP), and silicon dioxide for example. The thickness of the substrate104may be dependent upon fabrication technology and/or application. For example, the thickness of a substrate layer104in a typical semiconductor chip ranges from 100 μm to 1000 μm. For clarity and simplicity, the substrate104is omitted from subsequent figures.

It is understood that while only one cladding layer102is shown inFIGS.1A and1Bfor clarity and brevity, embodiments of the present disclosure include two or more cladding layers as described in more detail below. Each cladding layer102may be formed with a suitable cladding material, such as silicon dioxide and gallium aluminum arsenide (GaAlAs) for example. A PIC106is formed in at least one of the cladding layers102. The PIC106includes one or more integrated optical elements such as modulators, switches, and detectors (not shown). The cladding layer102containing the PIC106is also referred to as the main guiding layer, while the other cladding layers102containing one or more optical waveguides are referred to as auxiliary guiding layers. All cladding layers may collectively be referred to as guiding layers.

An optical coupler110in accordance with one embodiment of the present disclosure extends from a chip facet108of the photonic device100to the PIC106. The optical coupler110is configured to permit optical communication of an optical signal112from an external light source114, such as an optical fibre or laser source, to a routing waveguide within the PIC106. InFIGS.1A and1B, one end of the optical coupler110is aligned substantially flush with the chip facet108. In other embodiments, the optical coupler110may be recessed at some distance (i.e., 1-5 μm) away from the chip facet108.

An external light source114, such as an optical fibre or laser source, is in optical communication with the optical coupler110at the chip facet108. In embodiments as shown inFIGS.1A and1B, the external light source114is separated from the chip facet108by a free-space gap116. By way of a non-limiting example, the free-space gap116may be approximately 100 nm. Typically, a free-space gap116may happen when a fibre delivering the optical signal112has an incorrectly cleaved end, angled chip facet, or due to physical arrangement of the light source for example.

FIG.2illustrates a partial elevation view of chip facet108from the A-A line inFIG.1Bshowing optical coupler210in accordance with an exemplary embodiment of the present disclosure. The optical coupler210, which could implement the optical coupler110inFIGS.1A and1B, includes three optical waveguides212A,212B, and212C (collectively referred to as optical waveguides212) arranged in two cladding layers202(including a main guiding layer202A and an auxiliary guiding layer202B). In the illustrated embodiment, optical waveguides212A and212B are formed within the main guiding layer202A, while waveguide212C is formed within the auxiliary guiding layer202B. Each of the waveguides212is etched into their respective cladding layers202. A cover cladding layer202C is formed on top of cladding layer202B such that the waveguide212C is encased in cladding material. The dotted lines withinFIG.2show conceptual boundaries between the different layers.

As shown, each of the optical waveguides212is surrounded by cladding material. The cladding material may be of any suitable material that has a lower refractive index compared to the waveguides such that any optical signal112transmitted therewithin is optically confined within the waveguides by means of total internal reflection. By way of non-limiting examples, the cladding layers202may be of silicon dioxide or gallium aluminum arsenide (GaAlAs). The optical waveguides212may, for example, be fabricated from silicon nitride, silicon oxynitride, air, silicon, barium titanate, lithium niobate, indium gallium arsenide phosphide (InGaAsP), gallium arsenide (GaAs), or any suitable material to achieve total internal reflection with the selected cladding material. In one preferred embodiment, all the cladding layers202are fabricated from the same cladding material such that the optical waveguides212are surrounded by a homogenous cladding material with a uniform refractive index. In further embodiments, one or more of the cladding layers202may be fabricated with different cladding materials and present varying refractive indices surrounding one or more of the waveguides212.

FIG.3illustrates a top view of the optical waveguides212shown inFIG.2.FIG.3is intended to show the conceptual longitudinal profiles of the waveguides212and may not accurately depict the specific dimensions of the waveguides and/or physical relationships between the waveguides. For example, any potential spatial overlap as seen from above (e.g., along the y-axis) between waveguides in different cladding layers202are not depicted inFIG.3. The dimensions of the figures may not be proportional and may be exaggerated for illustration purposes.

As can be observed fromFIG.3, each of the optical waveguides212has a tapered profile extending from a first width along the x-axis at the chip facet108to a second terminal width along the x-axis. For example, waveguide212A extends from a first width WA1along the x-axis to a second width WA2along the x-axis over its length along the z-axis. Upon receipt of the input optical signal112at the chip facet108, a coupled mode is formed between the waveguides212and traverses along the waveguides.

In accordance with one aspect of the present disclosure, the physical dimensions of the waveguides212and the spatial relationship between the waveguides may be configured to facilitate mode matching at the chip facet108. Differences between dimensional parameters such as tip widths WA1, WB1, as well as their respective tip thickness (e.g., t1) at the chip facet108may cause mode asymmetry, and thereby negatively impact mode matching and potentially induce undesired optical loss. Thus, in a preferred embodiment, optical waveguides212A and212B are of the same material and therefore have the same refractive indices. Hence, the respective dimensions of the waveguides212A and212B at the chip facet108, namely waveguide tip widths WA1, WB1, and waveguide tip thickness t1are substantially identical. The difference in the refractive index between the waveguide and the surrounding cladding material may also impact the dimensions of the waveguides212. For example, a high refractive index difference may require smaller waveguide width at the chip facet108and, vice versa, wider waveguides may be used for relatively low refractive index differences.

Waveguides212adopt a tapering profile where WA1and WC1are greater than WA2and WC2, respectively, and WB1is less than WB2. In the embodiment shown inFIG.3, WC1is greater than WA1and WB1. For each waveguide, the starting width at the chip facet108and the terminal width may be dictated, at least in part, by the tapering angle. The tapered profile helps to facilitate the transformation of the coupled optical modes from one waveguide to another based on the tapering angle of the waveguides. Thus, in addition to defining the rate of change for the waveguide width, the tapering angle also, at least in part, defines the rate of mode transformation between the waveguides.

At least one of the waveguides212in the main guiding layer202A is configured to extend from the chip facet108to the PIC106with a terminating width that is substantially similar to a routing waveguide width of the PIC106. InFIG.3, waveguide212B is shown as the main routing waveguide with a width WB2that substantially matches the width of a routing waveguide within the PIC106. In some embodiments, WB2is greater than each of WA1, WA2, WC1, WC2, and WB1. By way of a non-limiting example, in the case of main routing waveguide212B having a medium refractive index and a thickness of 400 nm, the waveguide tips at the chip facet108may be 100 nm-300 nm in width depending on the MFD of the fibre to be mode matched for and widen to a WB2of 800-1200 nm. The terminating widths WA2and WC2may be less than 200 nm for decoupling end tapers.

In the embodiment shown inFIG.3, each of the waveguides212has a constant tapering angle. In further embodiments, the tapering angle may be dependent upon factors such as material of the waveguide and the permissible length of the waveguide along the z-axis and may be in the form of discrete steps, polynomial order tapering, and other suitable types of tapering profile.

Referring back toFIG.2, at the chip facet108, the tip of each of the waveguides212has a cross-sectional dimension that is substantially quadrilateral having a height along the vertical y-axis that is also the thickness of the waveguide material as denoted by t1, t2, and a width along the transverse x-axis. By way of a non-limiting example, the waveguides in the main guiding layer202A can be constructed using silicon having a layer thickness t1of >300 nm, and the waveguide in the auxiliary guiding layer202B can be constructed using silicon nitride having a layer thickness t2of <300 nm.

In accordance with one aspect of the present disclosure, the cross-sectional profile of the optical waveguides at the chip facet108may be varied to account for differences in layer thicknesses between layers202to maintain substantially the same optical performance. In some embodiments, the tip dimensions of each optical waveguide212may be inversely varied with respect to each other to maintain substantially the same cross-sectional area. For example, a decreased waveguide thickness t2of the waveguide in the auxiliary guiding layer202B may be compensated by increasing the width WC1of waveguide212C by a corresponding amount to maintain substantially similar optical performance. Conversely, when the waveguide thickness t2is increased, the width WC1may be decreased to achieve substantially similar optical performance. Similarly, increased waveguide thickness t1would lead to decreased waveguide width WA1and WB1for waveguides212A and212B, respectively, and vice versa.

As shown, the waveguides from the two cladding layers202A and202B are separated by a vertical distance D1. The waveguides within the same guiding layer202, namely212A and212B in layer202A, are separated by a transverse distance G1. In accordance with a further aspect of the present disclosure, the parameters D1and G1may advantageously be configured to optimize the mode-matching condition to a particular optical source, such as a fibre. By way of non-limiting examples, optical fibres with large MFDs (i.e., up to 10.4 μm) may require larger D1and/or G1(i.e., up to 3.5 μm to 5 μm). The G1and D1distances do not to need to be equal to each other (i.e., D1>G1, G1>D1, or G1=D1). Any configuration of D1and G1may be adopted as long as the mode coupling between waveguides212is preserved.

In some embodiments, D1may be restricted by fabrication capabilities of foundries, typically in the range of 100 nm-2500 nm. The increased thickness of the dielectric or cladding material between the waveguides in adjacent cladding layers as measured by the D1parameter can significantly increase the stress within the device up to the point of structural collapse. Additionally, significantly increasing G1(i.e., 1.5 μm and more) may elevate the optical loss in the structure during transformation of the coupled optical mode. In one aspect of the present disclosure, physical dimension limitations on either D1or G1that could compromise the optical performance can advantageously be compensated, at least to a degree, by adjusting the other parameters. For example, the inability to achieve a large enough D1can be compensated by increasing G1. Correspondingly, should the separation parameter G1between waveguides212A and212B be limited in any way that may result in elevated optical loss, the parameter D1may be increased to compensate for the degradation in optical performance. Advantageously, the optical coupler in accordance with embodiments of the present disclosure is capable of controlling the optical coupling with multiple configurable parameters, such as G1and D1. Hence, embodiments of the optical coupler described herein provide greater tolerance to variations in the waveguide dimensions, fabrication limitations in waveguide spacing, refractive index variations, and mode asymmetry.

FIG.4illustrates a cross-sectional view of an optical coupler410in accordance with another embodiment of the optical coupler inFIG.2. In contrast with optical coupler210, the main routing waveguide412C having layer thickness t2is fabricated into the top cladding layer402B, while auxiliary waveguides412A and412B having a thickness of t1, where t1<t2, is fabricated into the lower cladding layer402A. In some embodiments, t1is <=300 nm and t2>=300 nm. The main waveguide412C, having a greater thickness, has a narrower tip width WC1. The tip widths WA1and WB1of the auxiliary waveguides412A and412B are increased corresponding to the decrease in thickness t1. In one preferred embodiment, waveguides within the same cladding layer, for example waveguides412A and412B, have substantially the same dimensions and both have widths greater than WC1. The embodiment shown inFIG.4, with a potentially smaller separation distance G1between waveguides412A and412B, may better facilitate optical coupling at a higher efficiency.

FIG.5illustrates a top view of the arrangement of optical waveguides412within the optical coupler410shown inFIG.4. As shown, each of the waveguides412A,412B, and412C has respective widths WA1, WB1, and WC1at the chip facet108. As waveguide412C is the main routing waveguide that is responsible for routing the coupled optical mode onto the PIC106, its width WC2is dictated by the width of the routing waveguide of the PIC106. In some embodiments, WC2is greater than WC1, WA1, WA2, WB1, and WB2. All three waveguides412have tapering profiles where WA1and WB1are greater than or equal to WA2and WB2, respectively, and WC2is greater than or equal to WC1.

As mentioned, the substantially transversely oriented waveguides may provide better mode matching at the chip facet108. Thus, in a further embodiment of optical coupler610shown inFIG.6, all three waveguides612are oriented in this manner. In one preferred embodiment, all waveguides612participating in the cross-section mode building are fabricated from the same material having substantially the same refractive index, and hence have substantially the same waveguide dimensions with WA1, WB1, and WC1being substantially the same (i.e., WA1≈WB1≈WC1) at the chip facet108and t1≈t2<300 nm. The transversely oriented waveguides612may permit larger G1and D1values that are more capable of mode matching with fibres having larger MFDs. However, the improved mode matching comes at the expense of coupling efficiency. This embodiment may be well-suited for optical devices that have an escalator waveguide formed for transforming optical modes onto the PIC106. The escalator could be utilized to improve the coupling efficiency in conjunction with the improved mode-matching profile of the waveguides.

FIG.7illustrates a top view of one exemplary embodiment of optical waveguides612within the optical coupler610shown inFIG.6. The three waveguides612are of substantially the same dimensions at the chip facet108(i.e., WA1≈WB1≈WC1and t1≈t2). Each of the waveguides612maintains its tapering profile where WA1>WA2, WB1>WB2, and WC1>WC2.FIG.7also shows the presence of an escalator waveguide612D, which also has a tapering profile widening from a first end of width WD1which is substantially similar to WC2, and a second end with width WD2which is substantially similar to the routing waveguide width within PIC106. The escalator waveguide612D is typically formed in another cladding layer602and may spatially overlap with, for example, portions of waveguide612C in order to facilitate the mode transformation from waveguide612C onto the escalator waveguide612D.

Alternatively, the waveguides can also all be vertically oriented (e.g., WA1≈WB1≈WC1, and t1=t2>300 nm), which provides improved optical coupling. Such embodiments may be better suited in situations where high coupling efficiency is desired with less demanding mode-matching conditions. The escalator waveguide may not be needed and one of the thicker waveguides of the optical coupler can be configured to match the width of the PIC routing waveguide.

The design choices for G1, D1, tip width, and tip thickness may be configured to improve coupling efficiency, such as through simulation. Other embodiments, such as when the three waveguides (212,412,612) are vertically oriented (i.e., WA1and WB1<t1, WC1<t2) may also be similarly adopted on a mutatis mutandis basis.

FIGS.8A to8Dshow the coupling efficiency (CE) of the optical coupler210, mode matched with a fibre having an MFD of 6.4 μm, as a function of various design parameters generated using Ansys Lumerical MODE®.FIG.8Ashows simulation plots of the CE as a function of transverse distance G1for various thicknesses in the main guiding waveguide. The optimized design (theoretical case with no dimensional variations) shows a maximum CE of −0.1 dB (or 0.1 dB loss) at a transverse distance G1of approximately 2 μm. The other plot lines show that the simulated coupler can tolerate fabrication errors of ±50 nm in the main routing waveguide thickness, while maintaining a CE of greater than −0.15 dB.FIG.8Bshows the simulated optical coupler exhibiting less than 0.05 dB of CE change when introduced with fabrication errors of ±50 nm in the thickness of the auxiliary waveguides at the optimal G1value for the reference design.FIG.8Cshows the simulated optical coupler, at the optimal G1, showing less than 0.5 dB in CE variation in response to ±50 nm in main waveguide tip width variation.FIG.8Dshows the simulated optical coupler, at the optimal G1for the reference design, exhibiting less than 0.2 dB (ranging from −0.1 to −0.3 dB) in CE change when introduced with ±500 nm variation in D1.

FIG.9illustrates another embodiment of the optical coupler910having four waveguides912arranged in two cladding layers902A and902B. As shown, waveguides912A and912B are formed in the cladding layer902A, and waveguides912C and912D are formed within the cladding layer902B. The waveguides912C and912D have thickness t2≥300 nm, and a tip width WC1and WD1<t2. Thus, one of the waveguides912C and912D is configured as the main waveguide having a terminal width substantially similar to that of the routing waveguide in the PIC106. The waveguides912A and912B have thickness t1≤300 nm for improved mode matching.

The distance between the waveguides912A,912B and912C,912D, namely D1, may be defined by the mode-matching condition to be achieved. In some embodiments, D1is between 100 nm and 2500 nm, which may allow the optical coupler910to achieve mode matching with fibres having an MFD of up to 10.4 μm with 0.1 dB optical loss. Waveguides912A and912B are separated by a distance G1, and waveguides912C and912D are separated by a distance G2.

The mode-matching condition to a particular fibre is defined by configurations of D1, G1, and G2. For example, fibres having larger MFDs may require larger D1, G1, and G2. Values of the three waveguide separation distances D1, G1, and G2do not to need to be equal as D1>G1>G2or G1>D1>G2or G2>D1>G1or G2=G1=D1are all possible configurations as long as the coupled mode is preserved. In certain embodiments, D1is restricted by the fabrication abilities of the foundries during fabrication of the optical device. In some embodiments, the value of D1is in the range of 100 nm-2500 nm. Since the presence of thicker dielectric cladding (i.e., increased D1) between waveguides of different cladding layers can increase the stress on the system, up to the structure collapsing, the inability to achieve large (i.e., ≥1 μm) D1thicknesses can be at least partially compensated by increasing at least one of G1and G2. Similarly, increasing G1or G2may induce loss in the structure during transformation of the coupled mode onto the main routing waveguide. Thus, any limitations to increasing G1and/or G2can be at least partially compensated by increasing D1. Advantageously, the mode-matching condition may be controlled with three separate fabrication parameters, namely D1, G1, and G2. The greater degree of mode-matching flexibility means that the optical coupler may provide greater tolerance to variations in waveguide dimensions, fabrication errors to D1, G1, or G2, and refractive index variations in the dielectric cladding or in the waveguiding material.

Similar to other embodiments described herein, the thicker and vertically oriented waveguides912C,912D are better suited to function as the main waveguide, whereas the thinner and more transversely oriented waveguides912A,912B are better suited as auxiliary waveguides configured for mode transformation and to provide better mode matching at the chip facet108. In a preferred embodiment, waveguides within the same layer, such as waveguides912A and912B or912C and912D, have substantially the same dimensions to minimise mode profile distortion and optical loss resulting from aggravated mode mismatch. For example, embodiments of the present optical coupler may have up to 50 nm in height or width variation between waveguides of the same layer and still maintain optical losses of less than 0.2 dB.

Similar to other embodiments described herein, the cladding layers902may be fabricated from the same material having uniform refractive indices or different materials having contrasting refractive indices. For example, the cladding layer902A can be fabricated with silicon and the cladding layer902B can be fabricated with silicon nitride. Alternatively, both layers902A and902B can be fabricated with silicon nitride or any other suitable material.

In further embodiments, the relative positions of the cladding layers902A and902B may be switched such that the vertically oriented waveguides are located below the transversely oriented waveguides.

FIG.10illustrates a top view of one exemplary embodiment of optical waveguides912within the optical coupler910shown inFIG.9. In some embodiments, waveguides within the same cladding layer have substantially the same tip dimensions (i.e., WA1≈WB1with uniform t1and WC1=WD1with uniform t2). Each of the waveguides912has a tapered profile where WA1WA2, WB1≥WB2, WC1≥WC2, and WD2>WD1. As shown, the waveguide912D is the main routing waveguide that is in optical communication with the PIC106, and hence WD2has a dimension substantially equal to that of routing waveguides within the PIC. Any waveguide within the main guiding layer902B, such as waveguide912C, may be configured as the main routing waveguide. In the embodiment shown, WD2>WA1, WA2, WB1, WB2, WC1, and WC2.

FIGS.11A and11Billustrate a partial elevation view of chip facet108from the A-A line inFIG.1Bshowing two-layer optical couplers1120and1150having four substantially identically dimensioned waveguides in accordance with exemplary embodiments of the present disclosure. Each of the four waveguides has substantially identical tip dimensions at the chip facet108, i.e., WA1≈WB1≈WC1≈WD1.FIG.11Ashows optical coupler1120having two thinner and transversely oriented waveguides1122having thicknesses t1and t2of 300 nm or less where WA1≈WB1≈WC1≈WD1≥t1.FIG.11Bshows optical coupler1150having four thicker and more vertically oriented waveguides having thicknesses t1and t2of 300 nm or more where WA1≈WB1≈WC1≈WD1≤t1.

The waveguides1152of optical coupler1150may require smaller critical waveguide dimensions, which in turn may also impose limitations on the maximum values of D1, G1, and G2. In contrast, optical coupler1120may allow the use of waveguides1122of greater dimension, which may provide greater configurability of D1, G1, and G2and be able to achieve better mode matching with fibres having larger MFDs (i.e., up to 10.4 μm).

FIGS.12A and12Bshow top views of two possible longitudinal configurations in the z-direction of the waveguides that have substantially the same tip dimensions, such as waveguides1122and1152shown inFIGS.11A and11B, respectively. As shown inFIG.12A, each of the waveguides1122or1152have substantially the same tip width (i.e., WA1≈WB1≈WC1≈WD1). All waveguides1122,1152are shown to have a tapered profile along the z-axis of the optical coupler, but may have tip dimensions where WA1≥WA2, WB1≥WB2, WC1≥WC2, and WD2≥WD1. At least one of the waveguides1122or1152within the main guiding layer1102B (for example, waveguide1122D,1152D inFIG.12A) serves as the main routing waveguide having a terminating width substantially equal to that of routing waveguides within the PIC106. Optical coupler1150may be used in conjunction with an escalator waveguide1162E for improved coupling efficiency. There may be spatial overlap between waveguide1162E with at least part of waveguide1122C,1152C and/or1122D,1152D to facilitate the mode transformation.

Embodiments of the two-layer-four-waveguide configuration of the optical coupler may provide greater tolerance to fabrication error. For example, up to 50 nm in difference in the waveguide widths and thicknesses may be acceptable without significant degradation in optical efficiency performance. Optical couplers1120and1150may be capable of achieving 0.1 dB loss of the coupling efficiency based on the mode overlaps with fibres, and possible mode matching with fibres having up to 10.4 μm MFD. The choice of D1, G1, and G2may be dependent on the MFD for which mode matching is to be performed.FIGS.13A to13Cshow simulation plots of the coupling efficiency (CE) of the optical coupler910, mode matched with a fibre having an MFD of 6.4 μm, as a function of various design parameters generated using Ansys Lumerical MODE®.FIG.13Ashows simulation plots of the CE as a function of transverse distance G1for various thicknesses in the auxiliary guiding waveguides. The reference design (without any variations) exhibits an optimal CE of near −0.1 dB (or 0.1 dB loss) with G1of approximately 1.1 μm. The other two plots show that the CE remains at −0.3 dB or greater with ±50 nm of fabrication error in the auxiliary waveguide thickness.FIG.13Bshows that the simulated optical coupler maintains −0.5 dB or better CE with ±50 nm variation in the main waveguide layer tip width.FIG.13Cshows the simulated optical coupler exhibiting approximately 0.1 dB in CE change with ±500 nm variation in the D1parameter.

FIG.14shows simulation plots of the coupling efficiency (CE) of the optical coupler910as a function of G1for fibres with different MFDs and different D1values generated using Ansys Lumerical MODE®. As shown, the parameters G1and D1may be configured to improve optical coupling depending on different MFDs of the fibre. When mode matching with a fibre with an MFD of 6.4 μm, variations in D1and G1have little impact on the CE. When mode matching with a fibre having a smaller MFD of 4.0 μm, the smallest G1(0.5 μm) and a reduced D1(i.e., the waveguides are closer together transversely and vertically for improved mode matching with a smaller fibre) exhibits the best CE. In contrast, when mode matching with the larger 10.4 μm MFD fibre, the largest G1(3.0 μm) and a larger D1(i.e., the waveguides are further spaced apart to better mode match with a larger fibre) exhibits the best CE.

In further embodiments, the optical coupler in accordance with the present disclosure includes a fifth and a sixth optical waveguide arranged in a third cladding layer as shown inFIG.15. The additional cladding layer and waveguides provide an increased cross-sectional area at the chip facet to enable more efficient mode matching, especially with fibres having large MFDs (e.g., 10.4 am). For the optical coupler1510, the mode-matching condition to a particular fibre is defined by five parameters, including the separation distance D1between the waveguides in the cladding layer1502A and the waveguides of the adjacent cladding layer1502B, the separation distance D2between the waveguides of cladding layers1502B and1502C, and the intra-layer waveguide separation distances G1, G2, and G3within each of the cladding layers1502A,1502B, and1502C, respectively.

The inter-layer separation distances D1and D2are defined by the desired mode-matching condition to be achieved and may be between 100 nm and 2500 nm. In some embodiments, D1and D2should be kept substantially the same for coupling mode symmetry. Larger values of D1and D2may facilitate better mode matching with fibres having larger MFDs, whereas smaller D1and D2dimensions may better facilitate mode transformation with reduced optical loss. In some embodiments where D1<D2, it may be possible to mode match with fibres having larger MFDs (e.g., MDF of 10.4 μm).

The thicker waveguides (i.e., the vertically oriented waveguides) may have thicknesses of t1≥300 nm, and preferably substantially the same width as the routing waveguide width in the PIC106or available layer stack so that at least one may be configured as the main waveguide.

The remaining waveguides in other cladding layers1502B and1502C may be configured as transversely oriented waveguides that have t2and t3of <300 nm for improved mode matching.

Waveguides within the same cladding layer preferably have substantially the same tip dimensions at the chip facet108. Given the same material, differences in tip dimensions between waveguides of different guiding layers (i.e., WA1, WB1with WC1, WD1, or with WE1, WF1) may create mode asymmetry and aggravated mode mismatching, leading to increased optical loss.

Waveguide tip widths and thicknesses of the waveguides in cladding layers1502B and1502C may be different. The tip thickness will directly impact the tip width dimensions as increased thicknesses require a smaller corresponding width. For example, in the case of t2>t3, the tip widths may be configured to satisfy the relationship (WC1=WD1)<(WE1≈WF1), or when t2<t3, the tip width relationship becomes (WC1=WD1)>(WE1≈WF1). However, in embodiments where t1is considerably larger than t2and t3(e.g., t1is twice as much as t2or t3), it may be preferable to keep the waveguides in cladding layers1502B and1502C (i.e.,1512C,1512D,1512E,1512F) of substantially the same tip dimensions.

InFIG.15, three cladding layers1502do not need to be fabricated from the same material. For example, the auxiliary cladding layers1502B and1502C can be constructed with silicon and the main guiding layer1502A can be constructed with silicon nitride.

Fibres with large MFDs (e.g., 10.4 μm) may require larger D1, D2, G1, G2, and G3. The values of the five parameters do not need to be equal and may be configured with any appropriate values as long as the coupled mode is preserved. Separation distances D1and D2are usually restricted by the fabrication abilities of the foundries, but are typically in the range of 100 nm-2500 nm. In one embodiment, one of the most symmetric output modes can be achieved when D1is substantially the same as D2.

The upper value range for D1and D2may be limited as increased dielectric cladding thickness between waveguides in adjacent cladding layers can significantly increase the stress in the system, potentially causing structural collapse. This limitation on D1and D2can be compensated by increasing one or more of G1, G2, and G3. Conversely, any physical limitations on G1, G2, and G3(e.g., large values of G1, G2, or G3may induce optical loss during transformation of coupled modes to the main routing waveguide) can be at least partially compensated by increasing one or both of D1and D2. With the added third cladding layer, the coupled mode may be controlled with even more parameters compared to the two-layer embodiment, thereby allowing for even larger variations in the waveguide dimensions, greater error tolerance in the gaps G1, G2, G3, D1, and D2, and permit greater refractive index variations either in the surrounding dielectric or in the waveguiding material.

The cladding layer in which the main waveguide is fabricated may be any one of the three cladding layers1502.FIGS.16A and16Billustrate two such optical couplers1620and1650where the cladding layer with the vertically oriented main waveguide (i.e., waveguide with thickness >300 nm) is the top and the middle cladding layer, respectively, as opposed to being the bottom cladding layer as shown inFIG.15. In one preferred embodiment as best illustrated inFIG.16A, the waveguides1622within the three cladding layers1602form a hexagonal shape in a cross-sectional arrangement at the chip facet108. Waveguides1622satisfy the dimensional relationship of: G1+WA1≈G3+WE1<G2+WC1, where WA1≈WB1, WC1≈WD,1WE1≈WF1, t1≈t2<t3and D1=D2. This arrangement may provide high coupling efficiency due to improved mode matching as the overall cross-sectional arrangement better matches that of a circular fibre mode and reduces sensitivity to fabrication errors. In some embodiments, it may be desirable to avoid significantly increasing the gap G3(i.e., no more than D1or D2values) between the waveguides1622E and1622F within the main guiding layer1602C as it can lead to increased loss during in-chip mode transformation.

FIGS.17A-17Cillustrate a partial elevation view of chip facet108from the A-A line inFIG.1Bshowing three-layer optical couplers1720,1750,1780in accordance with exemplary embodiments of the present disclosure that include two thick waveguide layers (i.e., thickness >300 nm) and one thin waveguide layer (i.e., thickness <300 nm). The operational properties of1720,1750,1780are substantially similar to those of embodiments 1620, 1650 on a mutatis mutandis basis.

FIGS.18A and18Bshow partial elevation views of chip facet108from the A-A line inFIG.1Bshowing three-layer optical couplers1820,1850in accordance with the present disclosure where all three waveguides are of substantially the same tip dimensions. In optical coupler1820, the main guiding waveguide that is in optical communication with the PIC106may be any one of the waveguides. Specifically, to achieve substantially identical waveguide tip dimensions, the waveguides in all three cladding layers1802are fabricated from the same material with substantially similar refractive indices. For embodiments where the waveguides in the cladding layers1802have different refractive indices even though the thicknesses of the layers are identical (i.e., the waveguides are made of different materials), the tip widths of the waveguides can be adjusted. For the layers with waveguides of higher refractive indices, the tip widths of the waveguides may be decreased; and for the layers with waveguides of lower refractive indices, the tip widths of the waveguides can be increased to maintain optical coupling. In one preferred embodiment, all waveguide tip dimensions are substantially similar with G1≈G2≈G3. The longitudinal profiles of the waveguides along the z-axis of the optical coupler may be substantially similar to those shown inFIGS.12A and12B. Optical coupler1850can be readily introduced on top of any integrated optical device by using an interlayer transition, such as with an escalator waveguide, from an existing layer.

The same design principles of the two- and three-layer embodiments can be extended to other designs with four or more cladding layers.

Simulations have shown that the three-layer optical coupler in accordance with the present disclosure may have tolerances of up to 100 nm in fabrication error in the widths and thicknesses of the cladding layers without compromising optical coupling efficiency.FIGS.19A and19Bshow simulation plots of the coupling efficiency (CE) of the three-layer optical coupler1510, mode matched with a fibre having an MFD of 10.4 μm, as a function of various design parameters generated using Ansys Lumerical MODE®.FIG.19Ashows less than 0.6 dB change in CE at the G1that provides the optimal CE in the reference design when a fabrication error of ±100 nm in auxiliary waveguide thickness is introduced.FIG.19Bshows the simulated optical coupler exhibiting tolerances for fabrication error of ±100 nm in the waveguide tip width for waveguides in one of the cladding layers. As shown, at the optimal G1of approximately 3 μm, the simulated optical coupler exhibits approximately 0.1 dB change in CE.

Although the present disclosure may describe methods and processes with steps in a certain order, one or more steps of the methods and processes may be omitted or altered as appropriate. One or more steps may take place in an order other than that in which they are described, as appropriate.