Broadband multimode waveguide interface

Abstract

The invention relates to a broadband multimode waveguide interface. A photonic integrated circuit as discussed herein may include one or more multimode waveguide interfaces that define a corresponding transition between a waveguide of the photonic integrated circuit and a free propagation region. In particular, a multimode waveguide interface may include an input waveguide connected to an interference waveguide and a slab waveguide connected to the interference waveguide. The input waveguide and the interference waveguide are positioned and configured to convert a portion of light of a first mode to light of a second mode such that the first mode and the second mode interfere within the interference waveguide. The interference waveguide is configured such that the modes are in phase at an interface between the interference waveguide and the slab waveguide for a first target wavelength and out of phase at the interface for a second target wavelength.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 541,799, filed on September 30, 2023, under 35 U.S.C. §119(e), the contents of which are incorporated herein by reference as if fully disclosed herein. Technical Field

[0003] The present disclosure generally relates to transmitting light in photonic integrated circuits. More specifically, the present disclosure relates to waveguide interfaces designed to reduce the wavelength dependence of diffraction that occurs when light transitions between a waveguide and a free propagation region. Background Art

[0004] Photonic integrated circuits often use waveguides to transmit light between different optical components for various purposes. According to the present application, a photonic integrated circuit can be configured to transmit light having a relatively wide range of wavelengths. In these cases, it may be desirable to configure the various optical components of the photonic integrated circuit to operate similarly across these wavelengths. Summary of the invention

[0005] Embodiments described herein relate to a photonic integrated circuit including a multimode waveguide interface. Some embodiments relate to a photonic integrated circuit having a light source unit and a multimode waveguide interface, the light source unit being operable to generate light of multiple wavelengths across an operating wavelength range. The multimode waveguide interface comprises: an input waveguide positioned to receive the multiple wavelengths of light; an interference waveguide connected to the input waveguide at a first interface; and a slab waveguide connected to the interference waveguide at a second interface. The interference waveguide is configured to convert a portion of the light of the first mode into light of the second mode for each wavelength within the operating wavelength range. The interference waveguide is configured so that for a first target wavelength within the operating wavelength range, the light of the first mode and the light of the second mode will be in phase at the second interface. The interference waveguide is also configured so that for a second target wavelength within the operating wavelength range, the light of the first mode and the light of the second mode will be out of phase at the second interface.

[0006] In some of these variations, the second target wavelength is longer than the first target wavelength. The first mode can be a TE00 mode, and the second mode can be a TE02 mode. In some variations, the input waveguide is centered relative to the interference waveguide. Additionally or alternatively, the interference waveguide includes a tapered section. In some variations, the photonic integrated circuit may include a splitter utilizing a multimode waveguide interface. The splitter may include the input waveguide, the interference waveguide, the slab waveguide, and the plurality of output waveguides.

[0007] Some embodiments relate to a photonic integrated circuit having a light source unit and a multimode waveguide interface, the light source unit being operable to generate light of multiple wavelengths spanning an operating wavelength range. The multimode waveguide interface comprises: an input waveguide having a first width and positioned to receive the light of the multiple wavelengths; an interference waveguide having a second width and connected to the input waveguide at a first interface; and a slab waveguide connected to the interference waveguide at a second interface.

[0008] In some of these embodiments, the first width is narrower than the second width, and the interference waveguide is configured to convert a portion of the light of the first mode into the light of the second mode for each wavelength in the operating wavelength range, so that the first mode and the second mode have a phase difference, and the phase difference has a corresponding half-beat length. The interference waveguide has a certain length, and for the first target wavelength in the operating wavelength range, the length is an even integer number of half-beat lengths. For the second target wavelength in the operating wavelength range, the length is also an odd integer number of half-beat lengths. In some of these variations, the second target wavelength is longer than the first target wavelength. Additionally or alternatively, the first target wavelength and the second target wavelength are separated by a half-beat length. In some variations, the wavelength ratio between the first wavelength and the second wavelength is 1:2.

[0009] In some of these variations, the second target wavelength is longer than the first target wavelength. The first mode can be a TE00 mode, and the second mode can be a TE02 mode. In some variations, the input waveguide is centered relative to the interference waveguide. Additionally or alternatively, the interference waveguide includes a tapered section. In some variations, the photonic integrated circuit may include a splitter utilizing a multimode waveguide interface. The splitter may include the input waveguide, the interference waveguide, the slab waveguide, and the plurality of output waveguides.

[0010] In some embodiments, the first width is wider than the second width, and the interference waveguide is configured to convert a portion of the light of the first mode into the light of the second mode for each wavelength in the operating wavelength range, so that the first mode and the second mode have a phase difference, and the phase difference has a corresponding half-beat length. The interference waveguide has a certain length, and for the first target wavelength in the operating wavelength range, the length is an odd integer number of half-beat lengths. For the second target wavelength in the operating wavelength range, the length is also an even integer number of half-beat lengths. In some of these variations, the second target wavelength is longer than the first target wavelength. Additionally or alternatively, the first target wavelength and the second target wavelength are separated by a half-beat length. In some variations, the wavelength ratio between the first wavelength and the second wavelength is 2:3.

[0011] In some of these variations, the second target wavelength is longer than the first target wavelength. The first mode can be a TE00 mode, and the second mode can be a TE02 mode. In some variations, the input waveguide is centered relative to the interference waveguide. Additionally or alternatively, the interference waveguide includes a tapered section. In some variations, the photonic integrated circuit may include a splitter utilizing a multimode waveguide interface. The splitter may include the input waveguide, the interference waveguide, the slab waveguide, and the plurality of output waveguides.

[0012] In addition to the exemplary aspects and embodiments described herein, further aspects and embodiments will become apparent by reference to the drawings and by study of the following descriptions. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Reference will now be made to the representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit the present disclosure to an included embodiment. On the contrary, the disclosure provided by the present disclosure is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the embodiments and defined by the appended claims.

[0014] Figure 1 A schematic diagram of a photonic integrated circuit including a waveguide interface is shown.

[0015] Figure 2 A perspective view of a portion of a photonic integrated circuit including a waveguide interface is shown.

[0016] FIG. 3A to FIG. 3C A top view of a variation of a photonic integrated circuit including a multimode waveguide interface as described herein is shown.

[0017] Figure 4 Depicted is a top view of a portion of a photonic integrated circuit including a multimode waveguide interface and an output facet.

[0018] Figure 5 Depicted is a top view of a portion of a photonic integrated circuit including a multimode waveguide interface as part of a beam splitter.

[0019] It should be understood that the proportions and sizes (relative or absolute) of the various features and elements (and sets and subsets thereof), and the boundaries, spacings, and positional relationships presented therebetween, are provided in the accompanying drawings solely to facilitate understanding of the various embodiments described herein, and therefore may not necessarily be presented or shown to scale and are not intended to indicate any preference or requirement for the illustrated embodiments to exclude the embodiments described in conjunction therewith.

[0020] Directional terms, such as "top", "bottom", "upper", "lower", "front", "rear", "above", "below", "above", "below", "left", "right", "vertical", "horizontal", etc., are used with reference to the orientation of some components in some of the figures described below, and are not intended to be limiting. Since components in various embodiments can be positioned in a plurality of different orientations, directional terms are used for illustrative purposes to demonstrate the relative orientation between components of the systems and devices described herein. Directional terms are intended to be interpreted broadly and therefore should not be interpreted to exclude components oriented in different ways. In addition, as used herein, the phrase "at least one of" following a series of items that separate any of the items in the items with the terms "and" or "or" is to modify the list as a whole, rather than modifying each member of the list. The phrase "at least one of" does not require selection of at least one of each of the listed items; rather, the phrase allows for the meaning of including at least one of any of the items in the items and / or at least one of any combination of the items and / or at least one of each of the items in the items. For example, the phrase "at least one of A, B, and C" or "at least one of A, B, or C" each refers to only A, only B, or only C; any combination of A, B, and C; and / or one or more of each of A, B, and C. Similarly, it should be understood that the order in which elements are presented for either a combined or separate list provided herein should not be construed to limit the present disclosure to only the order provided. DETAILED DESCRIPTION

[0021] Reference will now be made in detail to the representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit the embodiments to a preferred embodiment. On the contrary, it is intended to cover alternatives, modifications and equivalents that may be included in the essence and scope of the described embodiments defined by the appended claims.

[0022] The following disclosure relates to a photonic integrated circuit that includes one or more multimode waveguide interfaces that define corresponding transitions between waveguides and free propagation regions of the photonic integrated circuit. Specifically, the multimode waveguide interface may include an input waveguide connected to an interference waveguide and a slab waveguide connected to the interference waveguide. The input waveguide and the interference waveguide are positioned and configured to convert a portion of light of a first mode into light of a second mode so that the first mode and the second mode interfere within the interference waveguide. The interference waveguides are configured so that the modes are in phase at the interface between the interference waveguide and the slab waveguide for a first target wavelength and out of phase at the interface for a second target wavelength.

[0023] Waveguides can be used in photonic integrated circuits to transmit light between different components of the photonic integrated circuit. The waveguide can be shaped to optically constrain light within the waveguide, thereby allowing the waveguide to carry and guide light within the photonic integrated circuit. In some cases, the waveguide may terminate in a free propagation region in which the light is no longer constrained in one or more dimensions. This allows a light beam that was previously constrained in the waveguide to expand in one or more dimensions as it travels through the photonic integrated circuit. In some cases, the interface between the waveguide and the free propagation region can be used to expand the width of the light beam before it leaves the photonic integrated circuit (e.g., via a facet of the photonic integrated circuit). Similarly, optical components such as splitters, multiplexers, etc. can utilize one or more interfaces between the waveguide and the free propagation region.

[0024] In some cases, it may be desirable for a photonic integrated circuit, and any optical system incorporating the photonic integrated circuit, to be able to operate over a wide range of wavelengths. Depending on the intended use of a given optical system (e.g., performing spectroscopic measurements, optical signal transmission or processing, etc.), a light source unit as described herein may be capable of operating to generate light at multiple wavelengths spanning hundreds of nanometers, and the various optical components of the photonic integrated circuit that transmits the light may be designed to accommodate wavelengths spanning some or all of this range. In these cases, it may be desirable for a given optical component to have a similar level of performance regardless of the wavelength of light it receives.

[0025] When a waveguide terminates in a free propagation region (e.g., a slab waveguide) within a photonic integrated circuit, light will diffract as it enters the free propagation region. When light, such as light having a single mode (e.g., the fundamental mode), enters the free propagation region from a waveguide, the diffraction angle of light of a given wavelength depends on the ratio between the wavelength and the mode size of the light when it reaches the free propagation region. Because the waveguide optically confines the light within the waveguide, different wavelengths may have the same or similar mode sizes when entering the free propagation region, which may result in different diffraction angles for light of different wavelengths. This, in turn, may result in beam size in the free propagation region varying with wavelength, which potentially results in wavelength-dependent variations in the performance of components of the photonic integrated circuit.

[0026] To help reduce this wavelength dependency, the waveguide can be sized to be sufficiently narrow (e.g., via tapering) as it approaches the interface with the slab waveguide so that confinement of light in the waveguide is reduced in a waveguide-dependent manner. This can result in the size of the mode of light carried by the waveguide effectively becoming proportional to the wavelength of light across a certain wavelength range. Thus, when the output waveguide is configured to carry a particular range of wavelengths, the width of the waveguide can be selected based on that range of wavelengths.

[0027] Narrowing the output waveguide in this manner can provide improved uniformity of the diffraction angle across a range of wavelengths, but it can also increase back reflections at the interface between the output waveguide and the slab waveguide. Specifically, because modes at certain wavelengths may be poorly confined (e.g., to reduce the wavelength dependence of the diffraction angle), these wavelengths may experience an effective refractive index variation between the optical confinement regions defining the output waveguide and the slab waveguide. This effective refractive index variation can produce back reflections at the interface between the waveguide and the slab waveguide.

[0028] The photonic integrated circuits described herein may include an interface between the waveguide and the free propagation region that is configured to minimize these back reflections while still reducing the wavelength dependence of the diffraction angle across a particular wavelength range.

[0029] The following references Figures 1 to 5 These foregoing embodiments and other embodiments are discussed. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these drawings is for explanation only and should not be construed as limiting.

[0030] Figure 1 A schematic diagram of a photonic integrated circuit 100 as described herein is shown. Specifically, the photonic integrated circuit 100 may be configured to generate or receive light across a certain wavelength range. For example, the photonic integrated circuit 100 includes a light source unit 102, which has one or more light sources and is operable to generate light across a certain wavelength range. In the case where the light source unit 102 includes multiple light sources, the photonic integrated circuit 100 may include one or more multiplexers 106, which are configured to combine the light generated by the light source unit 102 into a single waveguide 104. In this way, the waveguide 104 is optically connected to the light source unit 102. The waveguide 104 may transition to the slab waveguide 107 at an interface (represented by line 108) between the waveguide 104 and the slab waveguide 107 (which acts as a free propagation region). In fact, the waveguide 104 terminates at the interface 108 because the light is no longer constrained in at least one dimension. In general, the light generated by the light source unit 102 will be transmitted to the interface 108 and will be emitted from the waveguide 104 as a divergent beam 110.

[0031] In general, the photonic integrated circuit 100 can be configured such that the waveguide 104 receives light of a plurality of different wavelengths from the light source unit 102. The plurality of different wavelengths span a wavelength range (referred to herein as an "operating wavelength range") that includes a maximum wavelength of the operating wavelength range and a minimum wavelength range of the operating wavelength range. In other words, the light source unit 102 includes at least one light source configured to emit light of the minimum wavelength and at least one light source configured to emit light of the maximum wavelength.

[0032] For example, Figure 1The light source unit 102 shown includes a first light source 102a operable to generate light of a first wavelength λ1, which is the shortest (e.g., minimum) wavelength that can be transmitted to the waveguide 104 during operation of the photonic integrated circuit 100. Similarly, the light source unit 102 includes a second light source 102b operable to generate light of a second wavelength λ N The first light source 102a and the second light source 102b define boundaries of an operating wavelength range of the waveguide 104. The light source unit 102 may include one or more additional light sources operable to emit light of one or more wavelengths within the operating wavelength range.

[0033] The operating wavelength range of the waveguide 104 may depend on the requirements of the optical system in which the photonic integrated circuit 100 is incorporated, and in some cases, the waveguide 104 may receive light of wavelengths that span a relatively wide operating wavelength range. For example, in some variations, the operating wavelength range spans at least 300 nanometers (i.e., the wavelength difference between the maximum wavelength and the minimum wavelength is at least 300 nanometers). In some of these variations, the operating wavelength range spans at least 600 nanometers (i.e., the wavelength difference between the maximum wavelength and the minimum wavelength is at least 600 nanometers). In some of these variations, the operating wavelength range spans at least 900 nanometers (i.e., the wavelength difference between the maximum wavelength and the minimum wavelength is at least 900 nanometers).

[0034] It should be understood that, depending on the operation of the photonic integrated circuit 100, the waveguide 104 may not receive all of the multiple wavelengths simultaneously. Thus, when the light source unit is described herein as being operable to generate light at multiple different wavelengths (e.g., across the operating wavelength range), the light source unit need not be operated to generate (or in some cases, even be capable of generating) all of these wavelengths simultaneously. The light source unit 102 may generate multiple different wavelengths simultaneously, or may generate different wavelengths (or sets of wavelengths) at different times. When the waveguide is discussed herein as receiving multiple different wavelengths (e.g., across the operating wavelength range), it should be understood that the waveguide 104 may similarly receive these wavelengths simultaneously or at different times (e.g., during different operating modes of the photonic integrated circuit 100). Additionally, the light source unit need not be capable of generating the entire spectrum within the operating wavelength range (e.g., every wavelength between the longest wavelength and the shortest wavelength of the operating range), and in some cases may only generate a discrete number of wavelengths within the operating wavelength range.

[0035] Each light source of the light source unit 102 can be selectively operated to emit a set of corresponding wavelengths of light. Each light source can be any component capable of generating one or more specific wavelengths of light, such as a light emitting diode or a laser. The laser may include a semiconductor laser, such as a laser diode (e.g., a distributed Bragg reflector laser, a distributed feedback laser, an external cavity laser), a quantum cascade laser, etc. A given light source may be single frequency (fixed wavelength) or may be tunable to selectively generate one of a plurality of wavelengths (i.e., the light source may be controlled to output different wavelengths at different times). The group of light sources may include any suitable combination of light sources and may work together to generate light of any wavelength in a plurality of different wavelengths. Therefore, the light beam 110 emitted from the waveguide 104 may include any wavelength or combination of wavelengths within the operating wavelength range at a given time, depending on the photonic integrated circuit 100.

[0036] The waveguide 104 and the slab waveguide 107 (which defines a free propagation region) may each be formed as part of a planar waveguide layer of a photonic integrated circuit. Figure 2 A perspective view of a portion of a photonic integrated circuit 200 as described herein is shown. Specifically, the photonic integrated circuit 200 may include a planar waveguide layer 202 supported by a substrate 204. In some cases, a cladding 206 is positioned between the substrate 204 and the waveguide layer 202 such that the substrate 204 supports the cladding 206 and the cladding 206 supports the waveguide layer 202. The cladding 202 may help confine light within the plane of the waveguide layer 202, and the photonic integrated circuit 200 may include additional cladding layers positioned in contact with other surfaces of the waveguide layer 202. The various layers of the photonic integrated circuits described herein may be formed of any suitable material, depending on the wavelength or wavelengths of light that the waveguides defined in the photonic integrated circuit will carry. For example, in some variations, the waveguide layer (including any waveguides defined in and formed by the waveguide layer) is formed of silicon, silicon nitride, silicon dioxide, etc., the cladding(s) are formed of dielectric material(s) such as silicon dioxide, and the substrate is formed of silicon.

[0037] The waveguide layer 202 may be processed to define the waveguide 220 and the slab waveguide 207 so that the waveguide 220 terminates at the interface between the waveguide 220 and the slab waveguide 207. Specifically, the waveguide layer 202 may be etched or patterned to define a pair of cavities 205a-205b in the waveguide layer 202. These cavities may define the waveguide 220 (e.g., the first cavity 205a defines a first lateral surface of the waveguide 220 and the second cavity 205b defines a second lateral surface of the waveguide 220) and may help confine light within the waveguide 220. In some variations, the photonic integrated circuit includes one or more additional claddings that fill the cavities 205a-205b. In some of these cases, the one or more additional claddings may also cover the top side of the waveguide 220 so that the waveguide is surrounded by the cladding. In other cases, the cavities 205a-205b may remain unfilled to provide an air interface with the lateral side surfaces of the waveguide 220. In some of these cases, the top surface of the waveguide may also be uncovered to provide an air interface with the top surface of the waveguide 220 .

[0038] Thus, when light is introduced into the waveguide 220, the light may be confined within the waveguide 220 due to the refractive index difference between the waveguide 220 and its surrounding materials (e.g., cladding and / or air in contact with the waveguide 220). The cavities 205a-205b terminate at the interface between the waveguide 220 and the slab waveguide 207. Because the cavities 205a-205b no longer function to confine the light in the waveguide 220, the light transferred from the waveguide 220 into the slab waveguide 207 may diffract and propagate freely within the slab waveguide 207. In this way, the slab waveguide 207 acts as a free propagation region. It should be understood that, although the light may freely propagate laterally within the plane of the waveguide layer 220 as it travels through the slab waveguide 207, the light may still be confined vertically so that the light is confined within the plane of the waveguide layer 220.

[0039] When light leaves the waveguide 220 at an interface with a free propagation region (e.g., a slab waveguide), the light may diffract at the interface. The diffraction angle of the light diffraction may depend on the wavelength of the light. For example, light of a first wavelength may be diffracted at a first diffraction angle to form a first light beam 230, such as Figure 2 As shown. If light of a different second wavelength is introduced into the waveguide 220 (simultaneously or sequentially with the first wavelength), the light may be diffracted at a different second diffraction angle to form a second light beam 232. As shown, the first light beam 230 has a relatively lower beam divergence than the second light beam 232, and therefore expands at a slower rate. The first light beam 230 and the second light beam 232 may continue to propagate until they reach another component of the photonic integrated circuit 200. Because these light beams may have different beam sizes when they reach another component of the photonic integrated circuit 200, light of different wavelengths may interact with the component differently (e.g., thereby producing wavelength-dependent losses or behaviors).

[0040] Therefore, the photonic integrated circuit described herein utilizes a multimode waveguide interface to reduce wavelength-dependent beam diffraction when the light beam is transferred from the waveguide to the slab waveguide. However, the waveguide used to transmit light is usually designed for single-mode propagation (e.g., propagation in the fundamental mode), and the photonic integrated circuit described herein is configured to transition the single-mode waveguide to an interference waveguide before the interface with the slab waveguide. The interference waveguide can be designed to reduce the wavelength dependence of the diffraction angle of the light entering the slab waveguide from the interference waveguide.

[0041] Specifically, a waveguide interface as described herein may include an input waveguide, an interference waveguide, and a slab waveguide, wherein the interference waveguide is connected between the input waveguide and the slab waveguide. The input waveguide is configured to carry light of a single first mode (e.g., fundamental mode), and the interference waveguide is configured to support light of at least two different modes. The input waveguide and the interference waveguide are configured such that when light enters the interference waveguide from the input waveguide, a portion of the light of the first mode is converted to light of a second higher order mode.

[0042] For example, the center of the input waveguide may be aligned with the corresponding center of the interference waveguide so that the higher order mode is a symmetric mode. For example, the input waveguide may be configured to carry light in the TE00 mode, and the input waveguide and the interferometer waveguide are configured to generate light in the TE02 mode from a portion of the light in the TE00 mode. In this way, the interference waveguide will simultaneously propagate light having both a first mode (e.g., TE00) and a second mode (e.g., TE02 mode). In these cases, the multimode waveguide interface may be configured so that additional higher order modes are not excited, so that the interference waveguide carries only the first mode and the second mode.

[0043] Because different modes of a given wavelength propagate through the waveguide at different speeds, the phase difference between the first mode and the second mode may vary along the length of the interference waveguide. Thus, the first mode and the second mode will form a superposition beam (e.g., representing the first mode superimposed on the second mode) that will have a beam size that varies along the length of the interference waveguide according to the phase difference between the first mode and the second mode. When the interference waveguide terminates at an interface between the interference waveguide and a slab waveguide (acting as a free propagation region), the properties of the beam propagating through the slab waveguide depend at least in part on the characteristics of the superposition beam at the interface. For example, when the first mode and the second mode are in phase (e.g., 0 phase difference between the modes) or out of phase (e.g., π phase difference between the modes) at the interface, the resulting beam will enter the slab waveguide with an approximately Gaussian intensity profile and a flat phase wavefront, which may be desirable in many cases.

[0044] Light of different wavelengths propagates at different speeds, and therefore there is a wavelength dependency in the phase difference between the first mode and the second mode at the interface. For example, for a given length of an interference waveguide, there may be certain wavelengths at which the first mode and the second mode will be in phase, certain wavelengths at which the first mode and the second mode will be out of phase, and the remaining wavelengths will be partially in phase (e.g., neither in phase nor out of phase). Therefore, there may be a wavelength dependency in the beam properties of light entering the slab waveguide from the interference waveguide.

[0045] In one example, if the waveguide is configured to receive light of a certain operating wavelength range, the size of the interference waveguide can be selected to reduce the wavelength dependence. Specifically, the size of the multimode waveguide interface is set to be a different integer multiple of the half-beat length for each wavelength in the first target wavelength and the second target wavelength within the operating wavelength range. As used herein, the "beat length" of the interference waveguide refers to the length of the interference waveguide that will cause the two modes carried by the interference waveguide to have a 2π phase shift. In other words, if the two modes of a given wavelength in the interference waveguide are in phase at a certain point along the length of the interference waveguide, they will be in phase again after one beat length. Similarly, the half-beat length refers to the length of the interference waveguide along which the first mode and the second mode will experience a π phase shift. If the two modes of a given wavelength are in phase at a certain point along the length of the interference waveguide, they will be out of phase (or vice versa) after the half-beat length.

[0046] For a given interference waveguide, each wavelength has a different beat length that is effectively proportional to that wavelength. For example, if the interference waveguide has a length of one beat length for a first wavelength, the interference waveguide will have two beat lengths for a second wavelength that is half the length of the first wavelength, and three beat lengths for a wavelength that is one-third the length of the first wavelength. Thus, for the purposes of this disclosure, the ratio between two wavelengths will be considered to be the same as the ratio between the beat lengths of these wavelengths in a given interference waveguide.

[0047] The multimode waveguide interface described herein is positioned in a photonic integrated circuit to receive multiple wavelengths spanning an operating wavelength range (e.g., from a light source unit as described herein). The multimode waveguide interface includes an interference waveguide that is sized so that for a first target wavelength within the operating wavelength range, a first mode and a second mode are in phase at an interface between the interference waveguide and the slab waveguide. Additionally, the interference waveguide is sized so that for a second target wavelength within the operating wavelength range, the first mode and the second mode are out of phase at an interface between the interference waveguide and the slab waveguide. In some cases, the first wavelength and the second wavelength may correspond to two wavelengths generated by the light source unit, so that the interference waveguide may receive the first target wavelength and the second target wavelength during operation of the photonic integrated circuit. In other cases, one or both of the target wavelengths need not correspond to any wavelength in the multiple wavelengths generated by the light source unit, but may alternatively be any wavelength that is at least as long as the shortest wavelength of the operating wavelength range and at least as short as the longest wavelength of the operating wavelength range. Thus, the interference waveguide is designed to perform in a certain manner with respect to the first target wavelength and the second target wavelength, regardless of whether the light source unit is capable of generating light of the first wavelength and the second wavelength.

[0048] The first target wavelength and the second target wavelength can be selected to reduce the wavelength dependence of the diffraction angle across the operating wavelength range for a given operating wavelength range. For each wavelength, it depends on the phase difference between the first mode and the second mode at the interface between the interference waveguide and the slab waveguide. Specifically, for a given wavelength, when the first mode and the second mode are in phase, the size of the superposition beam is minimum, and when the first mode and the second mode are out of phase, the size of the superposition beam is maximum. Therefore, if the first target wavelength is smaller than the second target wavelength, the shorter first wavelength will have a smaller superposition beam size than the longer second wavelength, which can reduce the wavelength dependence of the diffraction angle of these wavelengths.

[0049] In order to synchronize the modes of the first target wavelength, the interference waveguide has a length corresponding to a first integer number of half-beat lengths. Similarly, in order to phase the modes of the second target wavelength, the interference waveguide has a length corresponding to a second integer number of half-beat lengths. If the difference between the first integer number and the second integer number is 1 (e.g., the first wavelength and the second wavelength are separated by a half-beat length), there will be no wavelengths that are in phase or out of phase between the first target wavelength and the second target wavelength. In these cases, the superposition beam size can increase monotonically between the first target wavelength and the second target wavelength, which can reduce the wavelength dependence of the diffraction angle across all wavelengths between the first target wavelength and the second target wavelength. Therefore, if the first target wavelength and the second target wavelength are selected to be close to the minimum wavelength and the maximum wavelength of the operating wavelength range, the multimode waveguide interface can provide a more consistent diffraction angle across the entire operating wavelength range. For example, in some variations, the first target wavelength and the second target wavelength are selected as the minimum wavelength and the maximum wavelength of the operating wavelength range, respectively. In other variations, the first target wavelength is within 20% of the value of the minimum wavelength, and the second target wavelength is within 20% of the value of the maximum wavelength.

[0050] In the case where the first target wavelength and the second target wavelength are separated by half a beat length, the ratio of the beat lengths (and thus the wavelength ratio of the first target wavelength to the second target wavelength) will be N:N+1 (because the longer wavelength will necessarily have a smaller number of half beat lengths). Therefore, the value of N may depend on the bandwidth of the operating wavelength range. For example, N of 1 results in a wavelength ratio of 1:2 between the first target wavelength and the second target wavelength (e.g., the second target wavelength is twice as long as the first target wavelength). Although for a wider operating wavelength range, there may be two target wavelengths within the operating wavelength range that meet this ratio, a smaller operating wavelength range may require a larger N (e.g., a ratio of 2:3, a ratio of 3:4, etc.). Therefore, for different operating wavelength ranges, the multimode waveguide interface can be designed to have different sizes.

[0051] The first number and the second number of beat lengths may depend on the configuration of the multimode waveguide interface. Figure 3AA first variation of a photonic integrated circuit 300 including a multimode waveguide interface as described herein is shown. Specifically, the photonic integrated circuit 300 includes a waveguide layer 302 that is patterned to define an input waveguide 320, an interference waveguide 325, and a slab waveguide 310 (e.g., using a first cavity 305a and a second cavity 305b such as discussed herein). In this variation, a first width W1 of the input waveguide 320 is narrower than a second width W2 of the interference waveguide 325. In these cases, when the input waveguide 320 (which is centered relative to the interference waveguide 325) transitions to the interference waveguide 325, a second mode (e.g., a TE02 mode) is generated such that the first mode is in phase with the second mode at the interface between the input waveguide 320 and the interference waveguide 325. In these variations, the interference waveguides 325 will be sized to have a length L that is an even integer multiple of the half-beat length for a first (e.g., shorter) target wavelength, such that the first target wavelength will be in phase at the interface between the interference waveguides 325. For a second (e.g., longer) target wavelength, the length L is also an odd integer multiple of the half-beat length, such that the second target wavelength will be out of phase at the interface between the interference waveguides 325. Thus, with Figure 2 Compared to the waveguide interface shown, light entering the slab waveguide 310 (represented by the example first beam 330 and the second beam 332) can be diffracted at more similar diffraction angles. In the case where the first target wavelength and the second target wavelength are separated by a half-beat length, these interference waveguides 325 can be designed to have a ratio of N:N+1, where N is an odd integer (e.g., 1:2, 3:4, 5:6, etc.).

[0052] Figure 3B A second variation of a photonic integrated circuit 340 is shown, which may be similar to Figure 3A The photonic integrated circuit 300 may be configured as shown in FIG. 1 , except that the first width W1 of the input waveguide 320 is wider than the second width W2 of the interference waveguide 325. In these cases, when the input waveguide transitions to the interference waveguide 325, a second mode (e.g., a TE02 mode) will be generated, such that the first mode and the second mode are out of phase at the interface between the input waveguide 320 and the interference waveguide 325. In these variations, the interference waveguide 325 will be sized to have a length L that is an odd integer multiple of the half-beat length for a first (e.g., shorter) target wavelength, such that the first target wavelength will be in phase at the interface between the interference waveguides 325. For a second (e.g., longer) target wavelength, the length L is also an even integer multiple of the half-beat length, such that the second target wavelength will be out of phase at the interface between the interference waveguides 325. In the case where the first target wavelength and the second target wavelength are separated by a half-beat length, these interference waveguides 325 may be designed to have a ratio of N:N+1, where N is an even integer (e.g., 2:3, 4:5, 6:7, etc.).

[0053] Although the interference waveguide 325 Figure 3A and Figure 3B 325 as having a constant width W2 across its length L, it should be understood that in some cases, the width of the interference waveguide 325 may vary along one or more portions of its length. For example, Figure 3C A variation of a photonic integrated circuit 350 is shown, which may be Figure 3B 325a, which is positioned between a first section having a first straight section 325b (e.g., which is connected to the input waveguide 320) and a second straight section 325c (e.g., which is connected to the slab waveguide 310). In these cases, the tapered section 325a can be used to help generate the TE02 mode and / or expand the size of the interference waveguide 325, which can help further reduce back reflections at the interface between the interference waveguide 325 and the slab waveguide 310.

[0054] The multimode waveguide interfaces described herein can be used at any number of interfaces within a photonic integrated circuit. For example, Figure 4 An example of a photonic integrated circuit 400 is shown that utilizes a multimode waveguide interface to expand the width of a light beam before it is emitted from an output facet 412. Specifically, a waveguide layer 402 may be patterned to define an input waveguide 420, an interference waveguide 425, and a slab waveguide 410 (e.g., using a first cavity 405a and a second cavity 405b such as discussed herein). The slab waveguide 410 may extend between the interference waveguide 425 and the output facet 412 such that light exiting the interference waveguide 425 may diverge within the plane of the waveguide layer 402 before reaching the output facet 412. In some cases, the output facet 412 may be shaped to have a curve 414 that is configured to further change the divergence of the light as it exits the photonic integrated circuit 400. The use of a multimode waveguide interface may provide a more consistent beam divergence as the wavelength across the operating wavelength range or the light exiting the photonic integrated circuit 400 through the output facet 412 varies.

[0055] Figure 5 An example of a photonic integrated circuit 500 utilizing a multimode waveguide interface as part of an optical splitter 506 is shown. Specifically, a waveguide layer 502 may be patterned to define an input waveguide 520, an interference waveguide 525, and a slab waveguide 510 (e.g., using a first cavity 505a and a second cavity 505b such as discussed herein). The waveguide layer 502 is also patterned to define a plurality of output waveguides 512a to 512e (e.g., using a plurality of cavities 514a to 514f).

[0056] In these cases, the slab waveguide 510 acts as a free propagation region to optically couple light from the input waveguide 520 to the plurality of output waveguides 512a to 512e. Light introduced into the slab waveguide 525 from the input waveguide 520 (via the interference waveguide 525) will pass through the free propagation region to split the light between the plurality of output waveguides 512a to 512e. Although the boundaries of the slab waveguide 510 are Figure 5 506, but it should be understood that the actual boundary of the slab waveguide 510 may be positioned at any portion of the waveguide layer 502 that will not affect the operation of the beamsplitter 506. Figure 5 The multimode waveguide interface may help reduce wavelength-dependent variations in the way light is split between the output waveguides 512a to 512e across the operating wavelength range.

[0057] The above description uses specific nomenclature to provide a thorough understanding of the embodiments for explanation. However, it will be apparent to those skilled in the art that, after reading this specification, the embodiments can be practiced without specific details. Therefore, for the purpose of illustration and description, the foregoing description of the specific embodiments described herein is presented. They are not intended to be exhaustive or to limit the embodiments to the precise form disclosed. It will be apparent to those of ordinary skill in the art that, after reading this specification, many modifications and variations are possible in view of the above teachings.

Claims

1. A photonic integrated circuit, comprising: a light source unit operable to generate light of a plurality of wavelengths spanning an operating wavelength range; an input waveguide positioned to receive light of the plurality of wavelengths; an interference waveguide connected to the input waveguide at a first interface; and a slab waveguide connected to the interference waveguide at a second interface, wherein: The interference waveguide is configured to convert a portion of light in a first mode to light in a second mode for each wavelength within the operating wavelength range; The interference waveguide is configured such that for a first target wavelength within the operating wavelength range, light in the first mode and light in the second mode will be in phase at the second interface; and The interference waveguide is configured such that for a second target wavelength within the operating wavelength range, light in the first mode and light in the second mode will be out of phase at the second interface.

2. The photonic integrated circuit according to claim 1, wherein: The second target wavelength is longer than the first target wavelength.

3. A photonic integrated circuit according to claim 1 or claim 2, wherein: The first mode is the TE00 mode, and the second mode is the TE02 mode.

4. The photonic integrated circuit according to any one of claims 1 to 3, wherein: The input waveguide is centered relative to the interference waveguide.

5. The photonic integrated circuit according to any one of claims 1 to 4, wherein: The interference waveguide includes a tapered section.

6. The photonic integrated circuit according to any one of claims 1 to 5, further comprising: An optical splitter, wherein the optical splitter comprises: the input waveguide; the interference waveguide; said slab waveguide; and Multiple output waveguides.

7. A photonic integrated circuit, comprising: a light source unit operable to generate light of a plurality of wavelengths spanning an operating wavelength range; an input waveguide having a first width, the input waveguide positioned to receive light of the plurality of wavelengths; an interference waveguide having a second width and connected to the input waveguide at a first interface; and a slab waveguide connected to the interference waveguide at a second interface, wherein: the first width is narrower than the second width; The interference waveguide is configured to convert a portion of light in a first mode into light in a second mode for each wavelength within the operating wavelength range, such that the first mode and the second mode have a phase difference having a corresponding half-beat length; and The interference waveguide has a length, wherein: For the first target wavelength within the operating wavelength range, an even integer number of half-beat lengths; as well as For a second target wavelength within the operating wavelength range, the length is an odd integer number of half beats.

8. The photonic integrated circuit of claim 7, wherein: The second target wavelength is longer than the first target wavelength.

9. A photonic integrated circuit according to claim 7 or claim 8, wherein: The first mode is the TE00 mode, and the second mode is the TE02 mode.

10. The photonic integrated circuit according to any one of claims 7 to 9, wherein: The input waveguide is centered relative to the interference waveguide.

11. The photonic integrated circuit according to any one of claims 7 to 10, wherein: The first target wavelength and the second target wavelength are separated by a half-beat length.

12. The photonic integrated circuit according to any one of claims 7 to 11, further comprising: An optical splitter, wherein the optical splitter comprises: the input waveguide; the interference waveguide; said slab waveguide; and Multiple output waveguides.

13. The photonic integrated circuit according to any one of claims 7 to 11, wherein a wavelength ratio between the first wavelength and the second wavelength is 1:

2.

14. A photonic integrated circuit, comprising: a light source unit operable to generate light of a plurality of wavelengths spanning an operating wavelength range; an input waveguide having a first width, the input waveguide positioned to receive light of the plurality of wavelengths; an interference waveguide having a second width and connected to the input waveguide at a first interface; and a slab waveguide connected to the interference waveguide at a second interface, wherein: The first width is wider than the second width; The interference waveguide is configured to convert a portion of light in a first mode into light in a second mode for each wavelength within the operating wavelength range, such that the first mode and the second mode have a phase difference having a corresponding half-beat length; and The interference waveguide has a length, wherein: For the first target wavelength within the operating wavelength range, an odd integer number of half-beat lengths; as well as For a second target wavelength within the operating wavelength range, the length is an even integer number of half beats.

15. The photonic integrated circuit of claim 14, wherein: The second target wavelength is longer than the first target wavelength.

16. A photonic integrated circuit according to claim 14 or claim 15, wherein: The first mode is the TE00 mode, and the second mode is the TE02 mode.

17. A photonic integrated circuit according to any one of claims 14 to 16, wherein: The input waveguide is centered relative to the interference waveguide.

18. A photonic integrated circuit according to any one of claims 14 to 17, wherein: The first target wavelength and the second target wavelength are separated by a half-beat length.

19. The photonic integrated circuit according to any one of claims 14 to 18, further comprising: An optical splitter, wherein the optical splitter comprises: The input waveguide: the interference waveguide; said slab waveguide; and Multiple output waveguides.

20. The photonic integrated circuit of any one of claims 14 to 19, wherein a wavelength ratio between the first wavelength and the second wavelength is 2:3.

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