Photonic Coupler

Abstract

The objective is to achieve reduced wavelength sensitivity, improved power balance, and reduced power loss. SOLUTION: The photonic coupler includes an input coupling section, an output coupling section, and a multi-mode interference (MMI) waveguide section. The input coupling section is adapted to receive an input optical signal along an input waveguide channel. The output coupling section is adapted to output a pair of output optical signals along an output waveguide channel. The output optical signals having an output optical power are split from the input optical signal. The MMI waveguide section is optically coupled between the input coupling section and the output coupling section. Each of the notched waveguide sections may be disposed between the MMI waveguide section and a corresponding one of the input coupling section or the output coupling section, and / or the MMI waveguide section may include a curved sidewall.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 17 / 680,004, filed February 24, 2022, the contents of which are incorporated herein by reference.

[0002] The present disclosure relates generally to photonic devices and particularly, but not exclusively, to photonic splitters. [Background technology]

[0003] A photonic splitter is a device that splits the optical power of an input optical signal into multiple output optical signals in a predictable manner. A beam splitter is an example of a basic photonic splitter that relies on partially transmissive / reflective interface layers (e.g., refractive boundaries, multilayer dichroic films, etc.). A more advanced example of a photonic splitter is the multimode interferometer or multimode interference (MMI) coupler. MMI couplers are popular due to their versatility as power splitters, interferometers, optical switches, etc.

[0004] FIG. 1 shows a conventional 2×2 MMI coupler 100 including two pairs of coupled waveguide ports 105 connected via an MMI region 110. The optical power of an input optical signal 115 is split between the two output ports 105 due to modal dispersion and constructive / destructive interference (multimode beating) between the fundamental and higher order modes of propagation in the MMI region 110. Conventional MMI couplers have linear MMI regions and do not actually provide lossless 50 / 50 power splitting. In fact, MMI couplers are particularly susceptible to fabrication variations that result in lossy power splitting imbalances. Conventional techniques apply adiabatic design principles to the MMI region 110 to improve splitting uniformity and reduce losses. However, these adiabatic design principles typically involve lengthening the MMI region 110 to provide sufficient distance for multimode interference to occur while maintaining a linear geometry. This results in an overall device that is large and may nevertheless not achieve the desired performance specifications (e.g., splitting imbalance, power loss) over the desired operating wavelength range. [Brief description of the drawings]

[0005] Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, in which like reference numerals refer to like parts throughout the various views unless otherwise specified. Not all instances of elements are necessarily labeled, so as to avoid cluttering the drawings where appropriate. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles described. [Figure 1] FIG. 1 (Prior Art) is a block diagram showing a conventional 2×2 optical splitter. [Figure 2A] FIG. 1 is a schematic diagram of a photonic coupler having several optimization features that reduce wavelength sensitivity and improve power splitting balance in a compact form factor, according to one embodiment of the present disclosure. [Figure 2B] 1 is a schematic diagram of a photonic coupler having a non-uniformly dispersive core material filling one or more of the waveguide sections, according to an embodiment of the present disclosure. [Diagram 3]FIG. 1 is a flow diagram illustrating the operation of a photonic coupler as a splitter, according to one embodiment of the present disclosure. [Figure 4] 1 is a heat map illustrating power distribution / splitting within a photonic coupler according to one embodiment of the present disclosure. [Figure 5A] 1 is a plot showing power loss sensitivity versus wavelength of a photonic coupler according to one embodiment of the present disclosure. [Figure 5B] 1 is a plot illustrating power imbalance versus wavelength for a photonic coupler in accordance with one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] Embodiments of systems, apparatus, and methods of operation for photonic couplers / splitters with reduced wavelength sensitivity, 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. However, one of ordinary skill in the art will recognize 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 cases, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

[0007] References throughout this specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0008] The photonic coupler embodiments described herein provide reduced wavelength sensitivity, improved power balance, and / or reduced power loss, all within a compact form factor. It is believed that these advantages / properties are achieved using structural design features that help phase match modal dispersion that occurs in multimode interference (MMI) waveguide sections over shorter distances than is achieved with conventional linear MMI couplers, such as the 2×2 MMI coupler 100.

[0009] 2A is a schematic diagram of a photonic coupler 200 having several optimization features that reduce wavelength sensitivity, increase power splitting balance, and reduce overall power loss, according to one embodiment of the present disclosure. Photonic coupler 200 is a 2×2 coupler and can be operated as a splitter (1×2 coupler) or combiner (2×1 coupler) depending on which of its waveguide channels are stimulated with optical power. Photonic coupler 200 is described with respect to operation as a photonic splitter. However, it should be understood that photonic coupler 200 can be operated in reverse to split optical power in the opposite direction, or function as a photonic combiner in an interferometer.

[0010] The illustrated embodiment of photonic coupler 200 includes input coupling section 205, notched waveguide sections 210A and 210B (collectively 210), MMI waveguide section 215, output coupling section 220, and cladding material 222. The illustrated embodiment of input coupling section 205 includes a pair of input waveguide channels I1 and I2 that meet at junction 225 and sidewall corrugation 230A. The illustrated embodiment of output coupling section 220 includes a pair of output waveguide channels O1 and O2 that begin / meet at junction 230 and sidewall corrugation 230B. Notched waveguide sections 210 each include a sidewall notch 235. The illustrated embodiment of MMI waveguide section 215 includes a curved sidewall 240.

[0011] Photonic coupler 200 may be fabricated in a variety of materials and form factors. In one embodiment, photonic coupler 200 is fabricated as a planar waveguide structure disposed in a semiconductor material. The waveguide channels (e.g., input waveguide channels I1 and I2, output waveguide channels O1 and O2, notched waveguide section 210, and MMI waveguide section 215) are fabricated from a core material with a higher refractive index than the surrounding cladding material 222. For example, the waveguide channels may be fabricated from silicon, while the cladding material 222 is silicon dioxide. Other exemplary materials include silicon nitride (Si3N4), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), other III-V semiconductor materials, or others. Other non-semiconductor materials may also be used. In one embodiment, photonic coupler 200 is a photonic integrated circuit (PIC) disposed as a planar waveguide in a silicon-on-insulator (SOI) device. Semiconductor fabrication processes are well suited for fabricating photonic coupler 200 due to its compact form factor and small feature sizes (e.g., micron-level dimensions). An exemplary implementation of photonic coupler 200 may have lateral dimensions of 2.4 um (X-axis) by 1.55-2.2 um (Z-axis) and a longitudinal dimension (Y-axis) of 6 um. Of course, other dimensions, fabrication techniques, and component materials may be used.

[0012] The input coupling section 205 is adapted to receive an input optical signal 250 at its input waveguide channel I1 (or I2). The input waveguide channels are single mode waveguides at their inputs, but facilitate inter-channel evanescent coupling as the input waveguide channels converge towards the junction 225. This evanescent coupling serves to expand the input signal laterally across the photonic coupler 200 and excite higher order modes with power from the fundamental mode as the optical signal propagates towards the MMI waveguide section 215.

[0013] The notched waveguide section 210 is disposed between the MMI waveguide section 215 and either the input coupling section 205 or the output coupling section 220. The notched waveguide section 210A includes a pair of sidewall notches 235 arranged in an opposing configuration (e.g., laterally facing each other). The opposing sidewall notches 235 expand and contract the transverse cross section 255 (e.g., transverse X-axis) of the notched waveguide section 210A as the optical signal travels down the photonic coupler 200 along the longitudinal Y-axis. The sidewall notches 235 can be viewed as outward protrusions or depressions in the waveguide sidewalls (high to low refractive index interface / boundary). This brief expansion, followed by contraction, of the transverse cross section 255 serves a phase matching function between the fundamental mode and higher order modes formed / excited near the junction 225. In other words, the fundamental mode of propagation is temporally slowed down relative to the first higher order mode of propagation, facilitating efficient mode phase matching for improved optical power division between the laterally offset output waveguide channels O1 and O2.

[0014] In addition to the opposing sidewall notches 235, the input waveguide channels I1 and I2 and the output waveguide channels O1 and O2 may also include sidewall corrugations 230A and 230B, respectively. The sidewall corrugations 230 include pairs of opposing sidewall corrugations on each waveguide channel. The corrugations may be implemented as notches, wavy sections, contoured perturbations, etc. The sidewall corrugations 230 appear to provide a phase matching function that counteracts the effects of modal dispersion between the fundamental mode and the first higher order mode. However, the effect provided by the sidewall corrugations 230 is smaller compared to the notched waveguide section 210. Collectively, the notched waveguide section 210 and the sidewall corrugations 230 are referred to as the phase matching section 275. In the illustrated embodiment, the photonic coupler 200 is symmetric about its X and Y axes and is therefore capable of reversible operation (splitter regardless of which waveguide channel is excited). In other embodiments, the phase matching section 275 may be individually optimized for unidirectional operation.

[0015] The manufacturability of junction 225 is a consideration when designing photonic coupler 200. The gap (e.g., arc radius) introduced by junction 225 (or 230) is affected by the minimum feature size for a given fabrication process. The size of junction 225 shown in FIG. 2A is merely exemplary and may affect the feature size and profile details of sidewall cutout 235 and sidewall corrugation 230. Inverse design principles may be used to refine or optimize the topological profile and / or feature size of the corrugations and cutouts.

[0016] When the optical wavefront enters the MMI waveguide section 215, higher order propagation modes are excited, which facilitates MMI beating (i.e., constructive and destructive interference) along the longitudinal length of the MMI waveguide section 215, which is the driver of the power division between the output optical signals 260 in the output waveguide channels O1 and O2. Wavelength sensitivity is further reduced by using curved sidewalls 240 along the longitudinal length of the MMI waveguide section 215 to compensate for modal dispersion. The curved sidewalls 240 arc inward toward the central region of the MMI waveguide section 215. In other words, the transverse cross section 265 at the longitudinal center of the MMI waveguide section 215 is narrower than the transverse cross section 270 at the longitudinal periphery of the MMI waveguide section 215. The MMI waveguide section 215 has a narrow waistline that gradually tapers using a curved shape (e.g., a smooth, uninterrupted bend). In one embodiment, transverse cross-section 265 is 0.16 um narrower than transverse cross-section 270, and MMI waveguide section 215 is approximately 1.9 um in length. Of course, these dimensions are exemplary and material dependent.

[0017] Notched waveguide section 210B and output coupling section 220 operate in much the same way that input coupling section 205 and notched waveguide section 210A operate, but inversely. In various embodiments, photonic coupler 200 is symmetric about its X-axis, Y-axis, and / or about both the X-axis and Y-axis. In fully symmetric embodiments, the characterization of a given longitudinal end as input versus output may be entirely arbitrary. In other embodiments, one or more characteristics may be adjusted to result in an asymmetric design with distinct input and output sides.

[0018] Similar to the corrugations and cutouts discussed above, inverse design principles may be applied to design, improve, or optimize any or all of the topological shape, contour (including the curvature of the curved sidewalls 240), or feature size of the photonic coupler 200. For example, an inverse design simulator (aka design model) may be constructed using an initial design, such as the 2×2 MMI coupler 100, or the initial design of the photonic coupler 200, to perform a forward behavioral simulation of the initial design (e.g., using Maxwell's equations of electromagnetism). The output of the forward behavioral simulation is a simulated electromagnetic field response at the output coupling section 220. A particular performance parameter of this output field response may be selected as the parameter of interest (e.g., power loss, power imbalance, etc.) and is referred to as the simulated performance parameter. The simulated performance parameter is used by a performance loss function to calculate a performance loss value, which may be a scalar value (e.g., the mean squared difference between the simulated performance value and the target performance value). The differentiable nature of the design model allows for back-propagation via adjoint simulation of the performance loss error, which is the difference between the simulated output value and the desired / target performance value. The performance loss error (e.g., loss gradient) is back-propagated through the design model during adjoint simulation to generate a structural design error at the input coupling section 205. Back-propagation of the performance loss error facilitates the calculation 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 type, etc.) of the photonic coupler 200. These gradients are output as structural design errors and can then be used by a structural optimizer to perform an iterative gradient descent method (e.g., stochastic gradient descent) that optimizes or refines the initial structural design to generate a modified structural design. The forward and inverse simulations can then be repeated until the performance loss value falls within acceptable design criteria. The above description is merely an exemplary inverse design technique that can be used to improve or optimize the features and topology of the photonic coupler 200. It should be understood that other inverse design techniques can be implemented alone or in combination with other conventional design techniques.

[0019] FIG. 2B is a schematic diagram of a photonic coupler 201 having a non-uniformly dispersive core material 223 filling one or more of the waveguide sections, according to one embodiment of the present disclosure. The photonic coupler 201 is similar to the photonic coupler 200, except that the dispersive core material 223 may include a non-uniform mixture of two or more materials having different refractive indices. For example, the dispersive core material 223 may include a non-uniform arrangement of silicon and silicon oxide features, a non-uniform arrangement of differently doped semiconductor materials, or others. The non-uniformly dispersive core material 223 may include an arrangement or pattern of different refractive material features / portions that collectively aid in phase matching (e.g., spatially targeting and slowing down one mode compared to the other) between the fundamental and higher order modes of propagation. In other words, the particular beating pattern between the fundamental and higher order modes may be advantageously tailored by the non-uniformly dispersive core material 223. In some embodiments, the non-uniformly dispersive core material 223 may be configured to further reduce power splitting imbalance over a wider wavelength range. The inverse design techniques described above can be applied to determine specific material combinations, feature sizes, and feature placements (i.e., patterns) to achieve the desired phase matching functionality through appropriate selection of the performance loss function and target performance values.

[0020] 3 is a flow diagram illustrating a process 300 for the operation of the photonic combiner 200, in accordance with an embodiment of the present disclosure. The process 300 represents the optical splitter function of the photonic coupler 200. However, it should be understood that the photonic combiner 200 may operate in reverse as an optical combiner or a reverse splitter. The order in which some or all of the process blocks appear in the process 300 should not be considered limiting. Rather, one of ordinary skill in the art having the benefit of this disclosure will understand that some of the process blocks may be performed in various orders not illustrated, or even in parallel.

[0021] In process block 305, an input optical signal 250 is received at input waveguide channel I1. Of course, process 300 is equally applicable to an input optical signal being received at input waveguide channel I2. Input waveguide channel I1 is a single mode waveguide configured to receive input optical signal 250 as a single mode signal and direct input optical signal 250 towards notched waveguide section 210A and MMI waveguide section 215 (process block 310).

[0022] As the input optical signal 250 propagates along the input waveguide channel I1, it begins to evanescently couple power across the cladding material 222 proximate the junction 225 to the complementary / adjacent input waveguide channel I2 (process block 315). This evanescent coupling causes the optical power of the input optical signal 250 to spatially spread laterally across the photonic coupler 200. In so doing, as the input optical signal 250 approaches the notched waveguide section 210A, higher order propagation modes begin to be excited. As higher order modes emerge, modal dispersion between the fundamental mode and the first higher order mode also begins to occur. Modal dispersion adversely affects the optical power by spreading it in the longitudinal spatial direction. Thus, in process block 320, sidewall corrugations 230A are placed along the sidewalls of the input sidewall channels I1 and I2 to introduce phase compensation to help match the phase between the fundamental and higher order propagation modes. The impact of the sidewall corrugations 230A on mode phase matching is believed to be relatively small compared to the contribution provided by the notched waveguide section 210A. Some embodiments of the photonic coupler 200 may even omit the sidewall corrugations 230. When both the sidewall corrugations 230A and the notched waveguide section 210A are present, they may collectively affect the relative phase between the fundamental and higher order modes to improve phase matching. In the illustrated embodiment, the sidewall notches 235 and the sidewall corrugations 230A in the notched waveguide section 210A cause the expansion and constriction of the waveguide section (i.e., expansion and contraction in the transverse cross section of the respective waveguide section / channel). In the illustrated embodiment, the input waveguide channel includes a series of small notches that create a series of small expansions and contractions, and the notched waveguide section 210A includes a single pair of sidewall notches 235 that create a single larger expansion and contraction.

[0023] After the notched waveguide section 210A, the optical signal enters the MMI waveguide section 215 where multimode interference beating occurs (process block 325). The beating serves, at least in part, to spread the optical power laterally (X-axis) spatially across the MMI waveguide section 215 in preparation for splitting the optical power into two substantially equal output optical signals O1 and O2 at opposite longitudinal ends of the MMI waveguide section 215. Figure 4 is a heat map illustrating the power distribution and substantially equal power split within the photonic coupler 200.

[0024] Referring to FIG. 2A, the MMI waveguide section 215 also includes curved sidewalls 240. In the illustrated embodiment, the curved sidewalls 250 arc inward toward the central region of the MMI waveguide section 215. In other words, the central transverse cross section 265 of the MMI waveguide section 215 is narrower than the transverse cross section 270 near the periphery of the MMI waveguide section 215. The non-constant transverse width of the MMI waveguide section 215 reduces wavelength sensitivity by compensating for modal dispersion. FIG. 5A is a plot showing a relatively low power loss in the range of 0.2 to 0.34 dB over wavelengths from 1260 nm to 1340 nm. FIG. 5B is a plot showing a relatively low power split imbalance of the photonic coupler 200 between wavelengths from 1260 nm to 1310 nm for an input optical signal 250. The curved sidewalls 240 reduce the wavelength dependence in the MMI beating in the MMI waveguide section 215 (process block 330).

[0025] Returning to Figure 3, output coupling section 220 operates similarly to input coupling section 205. Notched waveguide section 210B and corrugated sidewalls 230B again act to phase match the fundamental and higher order propagation modes as the optical power is transitioned back to the fundamental mode along output waveguide channels O1 and O2 (process block 335). In process block 340, output optical signal 260 is split into substantially equal power halves (see, e.g., Figure 5B) and output as a single mode signal from output waveguide channels O1 and O2 (process block 340).

[0026] 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. Although specific embodiments and examples for the invention have been described herein for illustrative purposes, those skilled in the art will recognize that various modifications are possible within the scope of the invention.

[0027] These modifications can be made to the invention in light of the above detailed description. In general, the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed herein. Rather, the scope of the invention should be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.

Claims

1. A photonic splitter an input coupling section adapted to receive an input optical signal along an input waveguide channel; an output coupling section adapted to output at least two output optical signals along output waveguide channels, the output optical signals having an output optical power divided from the input optical signal; a multi-mode interference (MMI) waveguide section optically coupled between the input coupling section and the output coupling section; and a notched waveguide section each disposed between the MMI waveguide section and a corresponding one of the input coupling section or the output coupling section, each of the notched waveguide sections including a pair of opposing sidewall notches that expand and contract a transverse cross-section of the corresponding one of the notched waveguide sections along a longitudinal axis of the photonic splitter.

2. The photonic splitter of claim 1 , wherein the photonic splitter comprises a 2×2 photonic splitter having a pair of input waveguide channels that includes the input waveguide channel.

3. The photonic splitter of claim 2 , wherein each of the notched waveguide sections is disposed proximate a junction of either the input waveguide channel or the output waveguide channel.

4. The photonic splitter of claim 1 , wherein the MMI waveguide section includes curved sidewalls.

5. The photonic splitter of claim 4 , wherein the curved sidewalls arc inwardly toward a central region of the MMI waveguide section.

6. The photonic splitter of claim 4 , wherein a transverse cross-section of the MMI waveguide section is narrower at a longitudinal center of the MMI waveguide section than at a longitudinal periphery of the MMI waveguide section.

7. The photonic splitter of claim 1 , further comprising sidewall corrugations disposed along the input and output waveguide channels.

8. The photonic splitter of claim 7 , wherein the sidewall corrugations include pairs of opposing sidewall corrugations on each of the input and output waveguide channels.

9. 10. The photonic splitter of claim 1, wherein the photonic splitter comprises a planar waveguide disposed in a semiconductor material surrounded by a semiconductor oxide material.

10. The photonic splitter of claim 1 , wherein the input waveguide channel and the output waveguide channel are single mode waveguides.

11. 2. The photonic splitter of claim 1, wherein the notched waveguide section disposed proximate to the input coupling section comprises a phase matching section that slows down a fundamental mode of propagation relative to a first higher order mode of propagation within the photonic splitter.

12. A photonic coupler, an input coupling section adapted to receive an input optical signal along an input waveguide channel; an output coupling section adapted to output a pair of output optical signals along output waveguide channels, the output optical signals having an output optical power divided from the input optical signal; a multi-mode interference (MMI) waveguide section optically coupled between the input coupling section and the output coupling section, the MMI waveguide section including curved sidewalls that arc inwardly toward a central region of the MMI waveguide section.

13. The photonic coupler of claim 12 , wherein the photonic coupler comprises a 2×2 photonic coupler having a pair of input waveguide channels that includes the input waveguide channel.

14. 14. The photonic coupler of claim 13, further comprising notched waveguide sections each disposed between the MMI waveguide section and a corresponding one of the input coupling section or the output coupling section, each notched waveguide section including a pair of opposing sidewall notches that expand and contract a transverse cross-section of a corresponding one of the notched waveguide sections along a longitudinal axis of the photonic coupler.

15. 15. The photonic coupler of claim 14, wherein each of the notched waveguide sections is disposed proximate a junction of either the input waveguide channel or the output waveguide channel.

16. 15. The photonic coupler of claim 14, wherein at least one of the MMI waveguide section or the notched waveguide section comprises a non-uniformly dispersive core material having a pattern selected to tailor a modal phase between a fundamental mode of propagation and a higher order mode of propagation.

17. 13. The photonic coupler of claim 12, wherein a transverse cross-section of the MMI waveguide section is narrower at a longitudinal center of the MMI waveguide section than at a longitudinal periphery of the MMI waveguide section.

18. The photonic coupler of claim 12 , further comprising sidewall corrugations disposed along the input waveguide channel and the output waveguide channel.

19. 20. The photonic coupler of claim 18, wherein the sidewall corrugations include pairs of opposing sidewall corrugations on each of the input and output waveguide channels.

20. The photonic coupler of claim 12 , wherein the photonic coupler comprises a planar waveguide disposed in a semiconductor material.

21. The photonic coupler of claim 12 , wherein the input waveguide channel and the output waveguide channel are single mode waveguides.

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