Patent ID: 12261685

DETAILED DESCRIPTION

Embodiments of a system, apparatus, and method of operation for wavelength-division multiplexer/demultiplexers (mux/demux) having reduced wavelength sensitivities, improved power balance, and reduced power loss are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Embodiments of the photonic device(s) described herein provide improved wavelength sensitives, improved power balance, and/or reduced power loss all within a compact form factor that is orders of magnitude smaller than typical arrayed waveguide grating (AWG) photonic circuits. It is believed these benefits/characteristics are achieved using structural design features that induce optical interference between multiple parallel channels that improve on the design of AWGs in terms of structure and performance.

FIG.2Ais a schematic diagram of an example photonic device200including a curvilinear wavelength-division multiplexer/demultiplexer (mux/demux) divided into multiple functional sub-regions affording improved performance and reduced size, in accordance with an embodiment of the disclosure. Photonic device200is illustrated as a 1×4 wavelength-division demultiplexer, which may be operated as a demux device or a mux device, dependent upon which of its waveguide channels are stimulated with optical power. Photonic device200is described in relation to its operation as a demux; however, it should be appreciated that photonic device200may be operated in reverse to multiplex multiple distinct wavelength channels into one or more multiplexed optical signals. Similarly, photonic device200can be configured for mux/demux of optical signals including more or fewer than four individual channels.

The illustrated embodiment of photonic device200includes an input region205, a dispersive region210, and multiple output regions215. The dispersive region includes multiple sub-regions220, including an input channel section221, in-coupler section223, parallel channel section225, an out-coupler section227, and an output channel section229. Each sub-region220is defined by a respective inhomogeneous arrangement of a first material230and a second material235. The photonic device200can include a surrounding material (not shown inFIG.2A) in which the input region205, dispersive region210, and output regions215are formed as part of a CMOS-compatible fabrication process, for example, as a silicon-on-insulator (SOI) photonic integrated circuit.

Photonic device200can be fabricated in a variety of materials and form factors. In one embodiment, photonic device200is fabricated as a planar waveguide structure disposed within a semiconductor material. First material230is characterized by a higher refractive index core material than second material235. For example, first material230can be or include silicon and second material235can be silicon dioxide. Other example materials include Silicon Nitride (Si3N4), Gallium Arsenide (GaAs), Indium Gallium Arsenide (InGaAs), other III-V semiconductor materials, or the like. Other non-semiconductor materials can also be used. In some embodiments, first material230is characterized by a higher refractive index relative to second material235. In an embodiment, photonic device200is a photonic integrated circuit (PIC) disposed as a planar waveguide in a silicon-on-insulator (SOI) device. Semiconductor manufacturing processes (e.g., CMOS) are well suited for fabricating photonic device200due to its compact form factor and small feature sizes (e.g., micron level dimensions). A demonstrative implementation of photonic device200may have a first, lateral, dimension240of about 2.4 μm (X-axis) by 1.55 to 2.2 μm (Z-axis) and a second, longitudinal, dimension245(Y-axis) of about 6 μm. Of course, other dimensions, fabrication techniques, and component materials may be used. In some embodiments, first dimension240can be from about 1 μm to about 100 μm, from about 5 μm to about 100 μm, from about 10 μm to about 100 μm, from about 15 μm to about 100 μm, from about 20 μm to about 100 μm, from about 25 μm to about 100 μm, from about 30 μm to about 100 μm, from about 35 μm to about 100 μm, from about 40 μm to about 100 μm, from about 45 μm to about 100 μm, from about 50 μm to about 100 μm, from about 55 μm to about 100 μm, from about 60 μm to about 100 μm, from about 65 μm to about 100 μm, from about 70 μm to about 100 μm, from about 75 μm to about 100 μm, from about 80 μm to about 100 μm, from about 85 μm to about 100 μm, from about 90 μm to about 100 μm, or from about 95 μm to about 100 μm, including fractions and interpolations thereof. Similarly, second dimension245can be commensurate with first dimension240or can be different from first dimension240. Advantageously, the footprint of photonic device200in the X-Y plane is as much as two orders of magnitude smaller than a typical AWG. This smaller size improves integration of photonic device200into photonic integrated circuits and other SOI applications.

The inhomogeneous distribution of first material230and second material235define multiple refractive and/or reflective interfaces in the dispersive region210. Through multiple interactions with the interfaces, photonic device200can at least partially demultiplex an input optical signal250including multiple multiplexed channels (λ1, λ2, . . . λN), where “N” is an integer equal to 2, 3, 4, 5, 6, 7, 8, or more, isolating a first distinct wavelength channel255-1at a first output region215-1of the output regions215. In this context, “partial demultiplexing” refers to an inhomogeneous distribution that isolates a multiplexed signal at an output region215, rather than an individual channel. For example, photonic device200can demultiplex a four-channel input optical signal250into two output signals255that each include two multiplexed channels. In some embodiments, first dimension240and/or second dimension245is determined based at least in part on the number of multiplexed channels included in optical signal250. For example, for multiplexing/demultiplexing transformations, the size of photonic device200can be positively correlated to the number of input channels in optical signal250, with a larger device size being implicated by a larger number of input channels. As such, first dimension240and/or second dimension245can exceed 100 μm based at least in part on the number of input channels. In an illustrative example of a 1×4 demultiplexing device, a size of photonic device200can be about 45 μm×16 μm, 30 μm×16 μm, or the like. A demultiplexing photonic device for a 1×8 transformation can be larger in at least one dimension, for example 45 μm by 32 μm, 30 μm by 32 μm, or the like.

Optical signal250can include multiple distinct wavelength channels255, such that sub-regions220together configure photonic device200to demultiplex the multiplexed optical signal250and to isolate distinct wavelength channels255at respective output regions215. For example, distinct wavelength channels255can be characterized by respective central wavelengths in the ultraviolet, visible, or infrared ranges. For applications in fiber optic communications, infrared wavelengths can be used in wavelength ranges between 1000 nm and 1500 nm. For example, in a 1×4 wavelength-division demultiplexer, input optical signal250can include four distinct wavelength channels255, including a first distinct wavelength channel characterized by a central wavelength of about 1266 nm, a second distinct wavelength channel characterized by a central wavelength of about 1269 nm, a third distinct wavelength channel characterized by a central wavelength of about 1312 nm, and a fourth distinct wavelength channel characterized by a central wavelength of about 1366 nm. In this context, the term “about” refers to a range of values equal to or within ±10% of the stated value. In line with the principles of inverse design, described below, the identification of a number of distinct wavelength channels255and their respective central wavelengths, as well as the composition of first material230and second material235, the size of dispersive region210, and other parameters, can influence the resulting inhomogeneous distribution of first material230and second material235, and thus the overall structure of photonic device200.

The sub-regions220apply respective functional transformations to the optical signal250resulting from the respective inhomogeneous distributions of each sub-region220. The dispersive region210can be optically continuous across lateral dimension240and longitudinal dimension245over the dispersive region210. In this context, “optically continuous” refers to the inhomogeneous arrangement of the first material and the second material being formed from a plurality of islands295(in reference toFIG.2B) of second material235disposed in a matrix of first material230, or vice-versa, where the matrix is coextensive with dispersive region210. In contrast, the component elements of a typical AWG, as illustrated inFIG.1, are discrete and optically isolated from each other (e.g., by an air gap or a dielectric gap material).

Input channel section221can include a respective inhomogeneous distribution of first material230and second material235that defines an input channel260of first material230in contrast to a peripheral region of second material235. As illustrated inFIG.2B, input channel260can correspond to an area of first material230that is substantially free of second material235but can also include a discontinuous and irregular boundary265between input channel260and the peripheral region. Through multiple interactions with boundary265, optical signal250can be coupled into in-coupler section223from input region205.

In-coupler section223comprises a respective inhomogeneous distribution of first material230and second material235that configures the in-coupler section to optically couple the input region205with the parallel channel section225. Similar to the input channel section221, the inhomogeneous distribution of the in-coupler section223can be characterized by a first region270having a higher composition of first material230than of second material235and a second region275having a higher density of second material235than of first material230, and wherein the first region is characterized by a curvilinear boundary265in planar cross-section. In this context, the planar cross-section refers to the plan view illustrated inFIG.2A, sectioning photonic device200in the X-Y plane. WhileFIG.2Aillustrates photonic device200with in-coupler section223and out-coupler section227having ellipsoidal shapes, the respective inhomogeneous distributions of each sub-region220can be a different shape. For example, boundaries265can be curvilinear, rectilinear (as described in more detail in reference toFIGS.3A-3B), and/or polygonal, based at least in part on the outcome of an inverse-design process that optimizes the inhomogeneous distribution of dispersive region210to configure photonic device200(e.g., optimizing output power, signal loss, and/or demultiplexing efficiency).

As with input channel section221, boundary265of in-coupler section can be irregular (e.g. defined by multiple islands295of second material235disposed in a matrix of first material230, or vice-versa). In this way, the curvilinear, rectilinear, polygonal, etc., shape of first region270can be developed during an iterative inverse design process, starting from an initialization design that includes discrete binary regions of first material230and second material235. As such, the schematic diagram ofFIG.2Acan be understood as an initial design for the inverse design process, an exemplary result of which is shown inFIG.2B. Through optimization of the placement of islands295of a characteristic feature size (e.g., limited by manufacturability constraints of a given SOI system) of second material235, the inhomogeneous distribution of in-coupler section223can be defined, which can diverge from the initialization design and result in irregular boundaries265. In some embodiments, first dimension240and/or second dimension245is determined based at least in part on the characteristic feature size. For example, for multiplexing/demultiplexing transformations, the size of photonic device200can be positively correlated to the characteristic feature size, with a larger device size being implicated by a larger characteristic feature size. As such, first dimension240and/or second dimension245can exceed 100 μm based at least in part on the characteristic feature size.

The manufacturability of dispersive region210including characteristic and/or minimum feature sizes of islands295of second material235is a consideration when designing photonic device200. The shape and configuration of boundary265is affected by the minimum feature size of a given fabrication process. The shapes illustrated inFIG.2Aare merely demonstrative of an initial design and may affect the feature size and contour details of structures of sub-regions220. Inverse design principles may be used to refine or optimize the topological contours and/or feature sizes of the islands295and boundaries265.

Parallel channel section225includes multiple channels280defined by a respective inhomogeneous distribution of first material230and second material235. Channels280can be optically intercoupled to permit electromagnetic interference therebetween, for example, where dispersive region is optically continuous, permitting electromagnetic fields to propagate across boundary265, as described in more detail in reference toFIGS.4A-4B. As such, channels280can be formed of first material230between irregular boundaries265formed of second material235. In some embodiments, continuous paths of first material230are formed between channels280, as illustrated inFIG.2B. In this way, channels280differ from waveguides115of AWG100, which are optically isolated. As in AWG100, channels280can be characterized by different path lengths between in-coupler section223and out-coupler section227. In contrast to AWG100, however, the inverse design process can result in irregular increments of path length between channels280.

Out-coupler section227includes a respective inhomogeneous distribution of first material230and second material235that configures out-coupler section227to optically couple output channel section229with parallel channel section225. As with in-coupler section223, out-coupler section can be substantially ellipsoidal, rectangular, polygonal, or the like, defined by boundaries265, which can be irregularly defined by multiple islands295of second material235. As in AWG100, out-coupler227can induce interference in the optical signal, propagated through sub-regions220preceding out-coupler section227in the optical path from input region205to output regions215, that at least partially isolates first distinct wavelength channel255-1at first output region215-1. The combined effect of the multiple interactions between optical signal250and the interfaces defined by first material230and second material235, results in the isolation of first distinct wavelength channel215-1at first distinct wavelength channel255-1, as illustrated inFIG.4A.

The respective inhomogeneous distribution of out-coupler section227is characterized by a third region285having a higher density of first material230than of second material235and a fourth region290having a higher density of second material235than of first material230, where the two regions define a curvilinear periphery in planar cross-section, similar to first region270of in-coupler section223.

Output channel section229includes multiple output channels291defined by a respective inhomogeneous distribution of first material230and second material235. As with parallel channel section225, output channels291are defined by irregular boundaries265and are substantially free of second material235. In some embodiments, islands295of second material235can be disposed between boundaries265in output channels291, based at least in part on the results of inverse design optimization of dispersive region210. In this way, output channels291can be formed of first material230between boundaries265formed of second material235.

Inverse design principles may be applied to design, refine or optimize any or all of the topological shapes, contours (including the curvatures of sidewalls), or feature sizes of photonic device200. For example, an inverse design simulator (aka design model) may be configured with an initial design such as 1×4 AWG100, or an initial design of photonic device200(e.g., based at least in part on the schematic illustrated inFIG.2A), to perform a forward operational simulation of the initial design (e.g., using Maxwell's equations for electromagnetics). The output of the forward operational simulation is a simulated field response at output regions215. Specific performance parameters of this output field response may be selected as parameters of interest (e.g., power loss, power imbalance, etc.) and are referred to as simulated performance parameters. The simulated performance parameters are used by a performance loss function to calculate a performance loss value, which may be a scalar value (e.g., mean square difference between simulated performance values and target performance values). The differentiable nature of the design model enables a backpropagation via an adjoint simulation of a performance loss error, which is the difference between the simulated output values and the desired/target performance values. The performance loss error (e.g., loss gradients) is backpropagated through the design model during the adjoint simulation to generate a structural design error at input region205. Backpropagation of the performance loss error facilitates the computation of additional performance gradients, such as structural gradients that represent the sensitivity of the performance loss value to changes in the structural material properties (e.g., topology, material types, etc.) of photonic device200. These gradients are output as a structural design error, which may then be used by a structural optimizer to perform an iterative gradient descent (e.g., stochastic gradient descent) that optimizes or refines the initial structural design to generate a revised structural design. The forward and reverse simulations may then be iterated until the performance loss value falls within acceptable design criteria. The above description is merely an example inverse design technique that may be used to refine or optimize the features and topology of photonic device200. It is appreciated that other inverse design techniques alone, or in combination with other conventional design techniques, may also be implemented.

FIG.2Bis a schematic diagram of a photonic device201with an inhomogeneous distribution of first material230and second material235in one or more functional sub-regions220, in accordance with an embodiment of the disclosure. Photonic device201is an example result of an inverse design optimization using the schematic diagram illustrated for photonic device200ofFIG.2A. For example, dispersive region210can include an inhomogeneous distribution of silicon and silicon oxide features (e.g., islands295), an inhomogeneous distribution of differently doped semiconductor material, or otherwise. The inhomogeneous distribution of first material230and second material235can include an arrangement or pattern of different refractive material features/portions that collectively apply multiple transformations to optical signals received at input region205to demultiplex constituent distinct wavelength channels255and isolate individual channels255at respective output regions215, or vice versa. The inverse design techniques described above may be applied to determine the specific material combinations, feature sizes, and feature arrangement (i.e., pattern) to achieve the desired phase matching function via appropriate selection of the performance loss function and target performance values.

FIG.3Ais a schematic diagram of an example photonic device300configured as a rectilinear wavelength-division multiplexer/demultiplexer, divided into multiple functional sub-regions320affording improved performance and reduced size, in accordance with an embodiment of the disclosure. Photonic device300is an example of photonic devices such as photonic device200that are designed at least in part using an inverse design process that can be initialized using a binary mapping of first material230and second material235. The schematic diagram of photonic device300is an example of such a binary mapping, resulting in an inhomogeneous distribution of first material230and second material235illustrated inFIG.3B. Photonic device300is illustrated as a wavelength-division multiplexer, configured to receive four distinct wavelength channels355at input regions305and, through the application of multiple transformations across sub-regions320of dispersive region310, to generate multiplexed output signal350at output region315. It is understood, however, that photonic device300can operate as a wavelength-division demultiplexer as well as a multiplexer, for example, by coupling a multiplexed optical signal into output region315to generate four distinct wavelength channels355at input regions305.

As with photonic device200, photonic device300includes an input channel section321, an in-coupler section323, a parallel channel section325, an out-coupler section327, and an output channel section329, together making up at least a subset of the functional sub-regions of photonic device300. The structures of sub-regions320, as with sub-regions220, can be optically continuous, formed by disposing islands295of second material235in a matrix of first material230or vice-versa, in a manner compatible with CMOS deposition/etch processes. Similar to photonic device200, photonic device300can include channels380that are optically intercoupled and input channels360that are optically intercoupled, resulting in improved optical performance and reduced footprint (represented by lateral dimension340and longitudinal dimension345) that can be orders of magnitude smaller than typical AWG devices.

As described in more detail in reference toFIG.2A, photonic device300includes rectangular shaped first region370and second region385, as well as linear input channels360, channels380, and output channel(s)390. In this way, photonic device300is termed a “rectilinear” device. In typical optical element configurations, right angles can induce optical effects, such as reflections, destructive interference, or the like, that impair performance. Such effects are typically avoided by avoiding right angles in a photonic device design. With photonic device300, however, boundary265can be irregular, and islands295of second material235can be disposed to attenuate and/or eliminate adverse optical effects of reflections and interference.

FIG.3Bis a schematic diagram of a photonic device301with an inhomogeneous distribution of first material230and second material235in one or more functional sub-regions320, in accordance with an embodiment of the disclosure. Together the inhomogeneous distribution of first material230and second material235configure photonic device301to act as a 4×1 wavelength-division multiplexer/demultiplexer. Photonic device301is an example of a structure resulting from an inverse design process optimizing the schematic diagram ofFIG.3A. As described in more detail in reference toFIG.2A, inverse design optimization of a binary mapping of first material230and second material235, where the binary mapping includes functional sub-regions as defined inFIG.3A, can result in the distribution shown in dispersive region310. As an iterative optimization technique, inverse design processes can generate different inhomogeneous distributions based at least in part on optimization convergence criteria and the initialization configuration of dispersive region310. In this way, the binary mapping of materials230and235in photonic device301are understood to represent a non-limiting example embodiment.

FIG.4Ais a field intensity diagram showing electromagnetic field strength as a function of position in the curvilinear multiplexer/demultiplexer ofFIG.2A. Similarly,FIG.4Bis a field intensity diagram showing electromagnetic field strength as a function of position in the rectilinear multiplexer/demultiplexer ofFIG.3A.FIGS.4A-4Brepresent simulation results generated using the structures shown inFIG.2BandFIG.3B, with darker coloration indicating greater field intensity. As illustrated, photonic device201isolates a distinct wavelength channel255to a respective output region215-1(FIG.4A) and photonic device301generates a multiplexed output signal350including a distinct wavelength channel355at output region315(FIG.4B).

As shown, dispersive regions210and310can be optically continuous in at least two dimensions, as indicated by the nonzero values of field intensity over sub-regions220and320. Advantageously, the structures illustrated inFIGS.2B and3B, being exemplary of photonic devices of the present disclosure, can improve multiplexing/demultiplexing performance in a photonic device in a smaller footprint than what is otherwise possible for AWGs that rely on mutually optically isolated waveguides115and phase-mismatch interference to perform wavelength-division mux/demux.

FIG.5is a flow chart illustrating an example process500of the photonic devices200and300ofFIG.2AandFIG.3A, in accordance with an embodiment of the disclosure. Process500describes a wavelength-division demultiplexing function of photonic device200; however, it should be appreciated that photonic device200can also operate in reverse as a multiplexer. Photonic device200can operate as a multiplexer in accordance with the described order of process operations505-535. The order in which some or all of the process blocks appear in process500should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel.

At process block505, input optical signal250is received at input region205. Input region205couples optical signal250into input channel260that guides optical signal250toward in-coupler section223as part of process block510. As described in more detail in reference toFIG.2A, propagation of input optical signal250can include multiple interactions with reflective/refractive interfaces between first material230and second material235, for example, at boundaries265.

At process block515, optical signal250is coupled into parallel channel section225, across multiple channels280, as illustrated inFIG.4AandFIG.4B. Also illustrated inFIG.4A, electromagnetic interference between channels280can be extensive, at process block520, due at least in part to the presence first material230that optically intercouples channels280.

At process block525, out-coupler section227conducts electromagnetic radiation including distinct wavelength channels255from parallel channel section225to output channel section229. As illustrated inFIG.4A, the geometry of third region285and the orientation of output channels291relative to third region285induces electromagnetic interference that isolates first distinct wavelength channel255-1at first output region215-1. In this way, for a multiplexed optical signal250including multiple distinct wavelength channels255, photonic device200can be configured by the inhomogeneous distribution of first material230and second material235in dispersive region210to output multiple individual distinct wavelength channels255at respective output regions215(process block535).

The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.