WO2018014365A1

WO2018014365A1 - Multi-material waveguide for photonic integrated circuit

Info

Publication number: WO2018014365A1
Application number: PCT/CN2016/091777
Authority: WO
Inventors: Jia Jiang; Dominic John Goodwill; Patrick Dumais
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2016-07-21
Filing date: 2016-07-26
Publication date: 2018-01-25

Abstract

A multi-material waveguide within a photonic integrated circuit is provided.A first waveguide portion (210) formed of a first material, such as silicon, has a tapered end (215). A second waveguide portion (230) formed of a second lower-refractive-index material, such as silicon nitride, has an end region (235) surrounding the tapered end (215). The end region (235) of the second waveguide portion (230) may terminate in an angled facet. A coplanar tapered coupling is there by provided between the two portions. The waveguide may be coupled between two components which also form part of the photonic integrated circuit. Different materials can be used in different sections of the waveguide where they convey an advantage.High refractive index materials can be used where waveguide bends are required, while materials exhibiting low per-unit-length optical loss can be used for longer sections. All portions of the waveguide may be configured as single-mode waveguides.

Description

MULTI-MATERIAL WAVEGUIDE FOR PHOTONIC INTEGRATED CIRCUIT

REFERENCE TO RELATED APPLICATIONS

The present application claims priority from US Patent Application No. 15/215,694, filed on 21 July 2016 and incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention pertains to the field of photonic integrated circuits and in particular to a multi-material waveguide for use within the photonic integrated circuit.

BACKGROUND

Silicon nanophotonic (SiPh) integrated circuits (PICs) with multiple stages, such as optical switches or processors, can require several centimeters of integrated waveguides to connect potentially hundreds or thousands of photonic cells. Because of the potentially long total waveguide length within a PIC, it is desirable to limit the per-unit-length photonic losses in the integrated waveguides.

Silicon strip waveguides can include a fully etched region of silicon adjacent to another material such as silicon dioxide (SiO₂) . Such waveguides can achieve single mode propagation and have a relatively low bending loss, allowing the waveguides to include curved portions of relatively small radius. However, the silicon strip waveguide loss is dominated by scattering losses associated with the side-wall roughness. Typical losses are on the order of about 2.0 dB/cm of waveguide length. This loss rate limits the potential waveguide length within the PIC.

Silicon rib waveguides can be manufactured using partial silicon etching to provide a waveguide having a narrower top portion and a wider bottom portion. These waveguides are usable as a multimode waveguide with a fundamental transverse electric (TE₀) excitation, and have a lower loss than silicon strip single mode waveguides, with some reports showing losses as low as 0.03 dB/cm. However, this type of waveguide exhibits a larger bending loss due to less confinement of the propagated mode. Furthermore, a mode conversion adapter is required to achieve compatibility with single mode devices.

Daoxin Dai and Mao Mao, “Mode converter based on an inverse taper for multimode silicon nanophotonic integrated circuits, ” Opt. Express, Vol. 23 (22) , 28376-28388, 2015, discloses an inverse taper on silicon for providing a mode converter for the connection between multimode silicon nanophotonic integrated circuits and few-mode fibers. However, the application is limited to certain types of mode conversion and interconnection with fibers which are inherently off-chip.

Therefore there is a need for a waveguide for use within a photonic integrated circuit, that obviates or mitigates one or more limitations of the prior art.

This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.

SUMMARY

An object of embodiments of the present invention is to provide a multi-material waveguide within a photonic integrated circuit. In accordance with embodiments of the present invention, there is provided a waveguide which is internal to a photonic integrated circuit. The waveguide includes a first waveguide portion comprising a first light-propagating material, such as silicon. The first waveguide portion has a tapered end. The waveguide also includes a second waveguide portion which is coplanar with the first waveguide portion. The second waveguide portion includes a second light-propagating material, such as silicon nitride, which has a lower refractive index than the first light-propagating material. The second waveguide portion has an end region surrounding the tapered end. This configuration provides a coplanar tapered coupling between the first and second waveguide portions, such that the tapered end and the end region form a transition region for coupling light between the first waveguide portion and the second waveguide portion. The waveguide is coupled between a first component and a second component, which both form part of the photonic integrated circuit. The waveguide may include additional waveguide portions, such that adjacent waveguide portions are formed of different materials. For example, a series of waveguide portions may be alternatingly formed of the first light-propagating material and the second light-propagating material.

Different light-propagating materials can be used in different sections of the waveguide where they convey an advantage. For example, waveguide structures comprising light-propagating materials which have high refractive index and can form high index contrast waveguides can be bent to a desirable degree without causing significant optical losses. Therefore, such light-propagating materials can be used in sections of the waveguide where such bends are required. Light-propagating materials which exhibit low per-unit-length optical losses can be used for relatively long sections of the waveguide. At the ends of the waveguide, waveguide structures comprising appropriate light-propagating materials which can form lower index contrast waveguides with enlarged light modes and which can be directly coupled to optical components which require large input light modes (such as the chip-edge-coupling fiber) can be provided. In addition, an appropriate transition between the waveguides of different light-propagation materials can realize the light coupling in/from different optical components.

It is noted that potential advantages of the silicon nitride waveguide portion include: lower loss for long distance propagation； support of large-mode light propagation for fiber-to-chip coupling or coupling to silicon nitride-based on-chip components； and the ability to transition to silicon waveguide portions via a coplanar tapered coupling.

In various embodiments, both the first waveguide portion and the second waveguide portion are configured as single-mode waveguides. In other embodiments, at least one of the waveguide portions is configured as a multimode waveguide in which light propagation is dominated by an excited fundamental mode.

In various embodiments, the end region of the second waveguide portion terminates in a facet extending inwardly toward the first waveguide portion at a non-perpendicular angle relative to a longitudinal axis of the second waveguide portion. This configuration may mitigate back-reflection at the end region of the second waveguide portion.

In various embodiments, the tapered end of the first waveguide portion terminates at a tip having a width. The width is configured so as to cause an effective refractive index of the first waveguide portion at the tip to match an effective refractive index of the second waveguide portion in a portion of the end region proximate to the tip.

In accordance with other embodiments of the present invention, there is provided a waveguide internal to a photonic integrated circuit. The waveguide is as described above, except that the waveguide is not necessarily coupled between two different components forming part of the photonic integrated circuit. Rather, at least one end of the waveguide may be routed to an edge of the photonic integrated circuit, and there coupled to an external transmission medium such as an optical fiber. In such embodiments, both the first waveguide portion and the second waveguide portion are configured as single-mode waveguides. In other embodiments, one or both of the first waveguide portion and the second waveguide portion is configured as a multimode waveguide.

In accordance with other embodiments of the present invention, there is provided a photonic integrated circuit comprising a waveguide as described above.

In accordance with other embodiments of the present invention, there is provided a method of manufacturing a photonic integrated circuit. The method includes disposing a layer of a first light-propagating material onto a wafer. The method further includes etching the layer of the first light-propagating material to form a first waveguide portion having a tapered end. The method further includes depositing a second light-propagating material onto the wafer and coplanar with the tapered end. The method further includes etching the second light-propagating material to form a second waveguide portion coplanar with the first waveguide portion and having an end region surrounding the tapered end. The method may further include forming a first component of the photonic integrated circuit on the wafer coupled to a first end of a waveguide comprising the first waveguide portion and the second waveguide portion, and forming a second component of the photonic integrated circuit on the wafer coupled to a second end of the waveguide. Additionally or alternatively, the method may further include forming the waveguide so that both the first and second waveguide portions operate as single-mode waveguides.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

FIG. 1 illustrates a photonic integrated circuit layout in accordance with an example embodiment of the present invention.

FIG. 2 illustrates top, side and end views of a coupling between two light-propagating waveguide materials at a coplanar tapered coupling, according to an embodiment of the present invention.

FIG. 3 illustrates a graph showing the effective refractive index of a silicon waveguide as a function of waveguide width, according to an embodiment of the present invention.

FIGs. 4A, 4B, and 4C illustrate top views of three alternative couplings between two light-propagating waveguide materials at a coplanar tapered coupling, according to embodiments of the present invention.

FIGs. 5A to 5D illustrate several example multi-material waveguide arrangements according to embodiments of the present invention.

It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

Embodiments of the present invention provide for a multi-material waveguide (also referred to as a hybrid waveguide) which forms part of a photonic integrated circuit. The multi-material waveguide can be formed in the photonic integrated circuit for coupling two different components which also form part of the photonic integrated circuit. The waveguide can therefore be used for optical routing between on-chip components. Use of a multi-material waveguide allows for the different advantages of different light propagating materials to be leveraged at appropriate locations along the waveguide length.

In various embodiments, waveguide sections of a first material, such as silicon (Si) , are provided at locations where a bend in the waveguide is required and/or where coupling to a component (such as a silicon-based component) of the photonic integrated circuit is required. Furthermore, waveguide sections of a second, lower loss material, such as silicon nitride (Si_yN_x) are provided where a relatively long and straight portion of the waveguide is required. Additionally or alternatively, waveguide sections of silicon nitride may be provided for coupling the waveguide to compatible components (such as silicon nitride-based components) in the photonic integrated circuit. The silicon nitride material may be Si₃N₄, for example. As such, the multi-material waveguide comprises a low-loss material used for longer sections, thereby limiting propagation losses. The multi-material waveguide may also comprise a material that can accommodate bends of a relatively small bending radius with limited bending loss. Because of this, constraints on both waveguide lengths and waveguide bends can be relaxed, which allows for improved routing capabilities between photonic integrated circuit components. For example, because tighter bends can be accommodated, a greater range of waveguide layout design options are achievable. This may in turn lead to improved photonic integrated circuit layouts and compactness, and/or reduced waveguide propagation losses. As such, the link budget for on-chip optical routing can be improved. The multi-material waveguide may also comprise a material that can be readily coupled to other photonic integrated circuit components. The use of a short end section of the first material for coupling the waveguide to a photonic integrated circuit component allows for the waveguide to connect to the component using a compatible material, for example in the case that silicon nitride cannot be directly coupled to the component.

The light propagation in the waveguide can be transitioned from one light-propagating material to the next using a coplanar tapered coupling as described herein. For the coplanar tapered coupling, two adjacent light propagating materials (and corresponding waveguide portions) are substantially coplanar, with one of the waveguide portions having a tapered end and the other one of the waveguide portions having an end region which surrounds the tapered end. In various embodiments, the tapered end is substantially entirely contained within the end region. However, an upper or lower face of the tapered end is not necessarily contained within the end region, but rather may be adjacent to a cladding layer. The taper can begin at or a short distance from the boundary of the end region.

As a non-limiting example, the coupling length of the taper may be between 10 and 100 μm, and may couple a 0.5 μm silicon waveguide to a 0.8 μm Si₃N₄ waveguide.

In various embodiments, different portions of the waveguide may be coplanar so that, when light propagates through the waveguide, the directions of propagation are restricted to a two-dimensional planar region. The different portions of the waveguide may be formed on top of a common flat surface of a photonic integrated circuit wafer. Coupling points to optical components, external optical fiber, and the like, may also be coplanar with the waveguide.

The waveguide according to the present disclosure may be coupled between a pair of components which are both integral to the photonic integrated circuit (PIC) . Embodiments of the present invention relate to PIC waveguide routing in a low loss and highly compact architecture. Components optically coupled by a waveguide of the present disclosure may include, but are not limited to, grating couplers, photon sources, photodetectors, optical switching cells, reflectors, power splitters, optical amplifiers, modulators, and optical filters. The PIC may be provided by preparing a suitable wafer using lithography techniques such as etching and deposition. The wafer is patterned so that at least the pair of components and the multi-material waveguide coupling the pair of components are formed as features of the same wafer. The portions of the components which interface with the multi-material waveguide may also be coplanar with the waveguide.

FIG. 1 illustrates a photonic integrated circuit layout in accordance with an example embodiment of the present invention. Several different examples of multi-material waveguides are shown. However, it should be appreciated that other waveguide configurations may be provided. For clarity, each multi-material waveguide is shown as comprising a silicon (Si) waveguide portion and a silicon nitride (Si_yN_x) portion. However, other optical materials can be used. Moreover, when a multi-material waveguide includes three or more portions, it is possible to use three or more different optical materials, although for ease of fabrication it may be desirable to limit the number of different materials to a small number, such as two.

In more detail, FIG. 1 illustrates a first multi-material waveguide 110 coupled between a grating coupler 120 and a first optical component 125, a second multi-material waveguide 130 coupled between a second optical component 140 and a third optical component 145, a third multi-material waveguide 150 coupled between the third optical component 145 and an external optical fiber 160, and a fourth multi-material waveguide 180 coupled between a silicon nitride-based grating coupler 122 and the first optical component 125. The

grating couplers

120 and 122 are examples of optical components. The first multi-material waveguide 110 comprises

silicon waveguide portions

112, 116 coupled to the grating coupler 120 and the first optical component 125, respectively, and a silicon nitride waveguide portion 114 coupled between the two

silicon waveguide portions

112, 116.

The second multi-material waveguide 130 comprises

silicon waveguide portions

131, 138 coupled to the second optical component 140 and the third optical component 145, respectively, and silicon

nitride waveguide portions

132, 137 coupled to the two

silicon waveguide portions

131, 138, respectively. The second multi-material waveguide 130 further comprises a silicon nitride-based optical component 142 coupled between the silicon nitride waveguide portion 132 and a further silicon nitride waveguide portion 133. The second multi-material waveguide 130 further comprises

silicon waveguide portions

134, 136 coupled to the silicon

nitride waveguide portions

133, 137, respectively, and a further silicon nitride waveguide portion 135 coupled between the two

silicon waveguide portions

134, 136. The

silicon waveguide portions

131, 134, 136 comprise bends. In other words, the second multi-material waveguide 130 defines a path between the second component 140 and the third component 145, the path having multiple bends, and the

silicon waveguide portions

131, 134, 136 coinciding with the bends. In other cases, the path may have a single bend. The path may terminate with a grating coupler 199. In a different embodiment, the further silicon nitride waveguide portion 135 may be omitted and the two

silicon waveguide portions

131, 138 may be joined into a single portion. This may be appropriate when the length traversed by the further silicon nitride waveguide portion 135 is small. In yet another different embodiment, the silicon nitride -based optical component 142 is omitted.

In embodiments of the present invention, a multi-mode waveguide may include bends formed of silicon single-mode waveguide sections. The silicon single-mode waveguide sections are used for the bends because the high refractive index contrast between the silicon and the surrounding material, such as silicon oxide (SiO₂) , exhibits a lower bending loss at small bending radii.

The third multi-material waveguide 150 comprises a silicon waveguide portion 152 coupled to the third optical component 145, a silicon nitride waveguide portion 153 coupled to the silicon waveguide portion 152, another silicon waveguide portion 154 coupled to the silicon nitride waveguide portion 153 and comprising a bend, and another silicon nitride waveguide portion 155 coupled between the silicon waveguide portion 154 and the external optical fiber 160.

The fourth multi-material waveguide 180 comprises a silicon waveguide portion 184 coupled to the first optical component 125, and a silicon nitride waveguide portion 183 coupled between the silicon waveguide portion 184 and a silicon nitride-based grating coupler 122. This provides an example in which the multi-material waveguide is used to couple two optical components each of which is based on a different type of optical material. Because optical components based on a particular type of material may be best suited for coupling to a waveguide having the same or a compatible type of material, such embodiments allow for optical components of different types of materials to be connected to each other via a multi-material waveguide having a transition located within the waveguide.

It is noted that, although some embodiments of the present invention provide a multi-material waveguide in which both end sections comprise a first, higher refractive index material such as silicon, in other embodiments, one or both ends of the multi-material waveguide comprise the second, lower refractive index material such as silicon nitride. An end formed of such lower refractive index material may be suitable for directly coupling the waveguide to optical fiber at an edge of the chip.

Between each Silicon waveguide portion and adjacent Silicon Nitride waveguide portion is a transition region, such as transition region 170. The transition region 170 comprises a tapered end of the silicon waveguide portion, surrounded by an end region of the silicon nitride waveguide portion.

FIG. 2 illustrates top, side and end views of a coupling between two light-propagating waveguide materials at a coplanar tapered coupling, also referred to as an in-plane coupling or an inverted optical taper, according to an embodiment of the present disclosure. The illustration is not necessarily to scale. A first waveguide portion 210 includes a first light-propagating material, such as silicon, and has a tapered end 215. A second waveguide portion 230 includes a second light-propagating material, such as silicon nitride, and has an end region 235 surrounding the tapered end 215. The tapered end 215 and the end region 235 form a transition region for coupling light between the first waveguide portion 210 and the second waveguide portion 230. As illustrated, the two

waveguide portions

210, 230 have bottom edges which fall in a common plane. However, the end region 235 has a larger height and width than the tapered end 215, thereby allowing the end region 235 to surround the tapered end 215 on at least three sides.

Both the two

waveguide portions

210, 230 may be disposed over an adjacent cladding layer 250, for example having a planar upper surface. A top cladding layer 255 may also be provided overtop of one or both of the first and

second waveguide portions

210, 230. As illustrated, the cladding layers 250, 255 are interconnected at both sides, thereby providing a contiguous cladding which surrounds (envelops) the

waveguide portions

210, 230. The cladding layers 250, 255 are not shown in the top and side views for clarity. The cladding may have a low refractive index relative to both the first and second light-propagating materials. For example, SiO₂ may be an appropriate cladding material in various embodiments.

As the name suggests, the tapered end 215 progressively narrows as it extends from the first waveguide portion 210 into the transition region and toward the second waveguide portion 230. As illustrated, the tapered end 215 is shaped as a trapezoidal prism. However, other shapes are also possible. For example, the tapered end 215 may be shaped to have cross sections, taken perpendicular to the longitudinal axis of the tapered end 215, which are rectangular, and which have an area which progressively decreases toward the tip 217 of the tapered end 215. The decrease in area can include gradual and/or abrupt decreases. The rate of decrease and/or size of abrupt decreases can be variable along the longitudinal axis.

The rate at which the taper narrows can be configured such that the radiation loss and mode conversion loss are limited or possibly minimized. Generally, lower rates of taper narrowing correspond with lower radiation and mode conversion losses. It is also noted that a lower overall rate of taper narrowing will result in a longer taper, which can impact integrated circuit compactness. However, the transition region encompassing the narrowing taper forms part of the multi-material waveguide, and as such the transition region can be used as part of the waveguide route. Therefore, provided that the required length of the multi-material waveguide is sufficiently high, extra space on the end of the waveguide for accommodating the transition region is not necessarily required. The rate of taper narrowing can be configured to balance loss or link budget requirements against size or circuit space requirements.

As mentioned above, the tapered end can have a variety of shapes, as well as a variety of orientations. For example, the photonic integrated circuit can be provided using lithographic techniques which define a series of layers. For reference, each layer may be viewed as extending in the x-and z-directions (horizontal directions) in a Cartesian coordinate system, with different layers being adjacent in the y-direction (vertical direction) . The taper can progressively narrow in the x-and/or z-direction, while being substantially constant in width in the y-direction. This configuration is compatible with typical fabrication techniques. Alternatively, if fabrication capabilities allow, the taper can progressively narrow in the y-direction while being substantially constant in the x-and z-directions. Both of the above configurations may result in a geometric prism shape. As yet another alternative, the taper can progressively narrow in both the y-direction and one or both of the x-and z-directions, which results in a more conical type of shape for the taper.

In some embodiments, the taper may be curved to at least a limited amount. The end region surrounding the taper may also be correspondingly curved. However, it is noted that curving of the taper may result in a non-negligible amount of optical loss in various embodiments, and as such care may be required when designing curved tapered sections.

The tip 217 at which the tapered end 215 may have a selected width 220, and hence a selected nonzero cross-sectional area. The width 220 is configured to cause an effective refractive index of the first waveguide portion at the tip 217 to match (or approximately match) an effective refractive index of the second waveguide portion in the part of the end region 235 which is proximate to and surrounding the tip 217.

FIG. 3 illustrates a graph showing the effective refractive index of a silicon waveguide as a function of waveguide width. The effective refractive index of the tapered end of a silicon waveguide portion decreases as the tapered end narrows, approximately according to a curve 310. A horizontal line 320 corresponds to the effective refractive index of a silicon nitride waveguide end region, wherein the silicon nitride waveguide is about 0.8 μm wide and 0.4 μm high, and is surrounded by SiO₂. The tip of the tapered end of the silicon waveguide can be configured to have a width for which the effective refractive index of the tapered end matches the effective refractive index of the silicon nitride waveguide end region, as represented by an intersection 315 of the curve 310 and the line 320. This matching of effective refractive indices at the tip may facilitate an improved transfer of light between the silicon waveguide portion and the silicon nitride waveguide portion.

In some embodiments, the functionality of the coplanar tapered coupling can be described as follows. The effective refractive index of the taper gradually reduces as the taper narrows, until it is close to, or matches, the effective refractive index of the end region of the second waveguide portion in the vicinity of the taper. This allows the optical mode to gradually couple between the taper of the first waveguide portion and the end region of the second waveguide portion, thereby lessening or completely suppressing a backreflection. The optical mode expands along the taper, and then gradually couples into the surrounding material (i.e. the second waveguide portion) . The expanded optical mode is therefore transferred and confined in the second waveguide portion and propagates along the second waveguide portion at a lower loss.

This configuration may allow a major portion or substantially all light carried by the waveguide to be transferred between the two waveguide portions. Light transferred into the second waveguide portion may be substantially entirely confined in the second waveguide portion and propagated thereby with a lower amount of loss than would be realized in a comparable length of the first waveguide portion. At an opposite end of the second waveguide portion, another similar transition region may be used to re-couple the light into a third waveguide portion, for example formed of the same material as the first waveguide portion. It is believed that, based on current silicon technology, a properly configured silicon taper may provide an optical connection between the two waveguide portions that approaches losslessness (i.e. 100％coupling efficiency) .

Referring again to FIG. 2, the end region 235 of the second waveguide portion 230 terminates in a facet 240 extending inwardly toward the first waveguide portion 210. As illustrated, the facet 240 is non-perpendicularly angled relative to a longitudinal axis 232 of the second waveguide portion 230. In particular, the facet 240 and edge 237 of the second waveguide portion 230 intersect at an acute angle 242. Alternatively, the angle 242 may be obtuse.

The end view in FIG. 2 illustrates the first waveguide portion 210 entering into the interior of the second waveguide portion 230 through the facet 240. The facet 240 at which the end region 235 terminates needs not be straight as illustrated in FIG. 2.

FIG. 4A illustrates an alternative facet arrangement having a pair of planar,

angled facets

410, 420 on opposite sides of an end region 425 of a second waveguide portion 404 coupled to a tapered first waveguide portion 402. The first facet 410 meets a first the side edge 432 of the second waveguide portion 404 at a first obtuse angle 434, and the second facet 420 meets an opposing side edge 436 of the second waveguide portion 404 at a second obtuse angle 438. The

facets

410, 420 may meet at a point 427, or alternatively a rounded or blunted face may be substituted for the point 427. To reduce or prevent retro-reflection, the

facets

410, 420 may form an acute or obtuse angle at the point 427, that is, other than 90 degrees. Although the facet arrangement is illustrated as being symmetrical about the longitudinal axis, alternative facet arrangements may be asymmetric about the longitudinal axis.

FIG. 4B illustrates a facet arrangement which is similar to that of FIG. 4A, except that the point 427 is replaced with a blunted face 428.

FIG. 4C illustrates a facet arrangement according to another embodiment of the present invention. The facet arrangement includes a first facet 460 which is substantially perpendicular to the first side edge 432 of the second waveguide portion, and a second facet 465 which meets with the first facet and which forms an obtuse angle with the second side edge 436 of the second waveguide portion. The second facet 465 may be planar, curved, or include plural planar sections.

As such, embodiments of the present invention provide an angled facet at a terminus of the second waveguide portion. The angled facet is disposed in the same plane as the coplanar tapered coupling between two waveguide materials. The angle of the facet is non-perpendicular relative to the longitudinal axis of the second waveguide portion, which is the direction of signal propagation in the waveguide. The angle of the facet is configured to mitigate backreflection effects in the vicinity of the coupling.

FIGs. 5A to 5D illustrate several example multi-material waveguide arrangements according to embodiments of the present disclosure. The illustrations are not necessarily to scale. Silicon and silicon nitride materials are specified for purposes of illustration, although other materials can also be used. Each arrangement includes at least one coplanar tapered coupling between adjacent silicon and silicon nitride waveguide portions. The silicon nitride portions have end regions surrounding the tapered ends of the silicon waveguide portions. The silicon nitride waveguide portions are also illustrated having angled facets. Free ends of the silicon waveguide portions may be coupled to other waveguide portions or components of the photonic integrated circuit in which the waveguide portions are integrated. Free ends of the silicon nitride waveguide portions may be coupled to other waveguide portions or to an optical fiber via an edge coupling of the photonic integrated circuit.

FIG. 5A illustrates a pair of tapered

silicon waveguide portions

502, 506, coupled by a silicon nitride waveguide portion 504.

FIG. 5B illustrates a silicon waveguide portion 514 having a “C” -shaped bend and coupled at opposite ends to two silicon

nitride waveguide portions

512, 516.

FIG. 5C illustrates a silicon nitride waveguide portion 522 coupled to a silicon waveguide portion 524 having a bend, such as an “L” -shaped bend.

FIG. 5D illustrates a silicon waveguide portion 532 coupled to a silicon nitride waveguide portion 534, which is further coupled to another silicon waveguide portion 536 having a bend. An opposite end of the other silicon waveguide portion 536 is coupled to another silicon nitride waveguide portion 538. The other silicon nitride waveguide portion 538 is coupled to an additional silicon waveguide portion 540.

In a single mode or multimode waveguide as described herein, the second light-propagating material may have a relatively lower refractive index than the first light-propagating material. The second light-propagating material may also be selected to have a lower loss than the first light-propagating material. Suitable pairs of first and second materials (respectively) include, but are not necessarily limited to: silicon and silicon nitride, silicon and silicon oxynitride, silicon and silica glass, two different silica glass materials having different refractive indices, silicon and polymer, two polymer materials having different refractive indices, diverse composition gallium arsenide (GaAs) based materials having different refractive indices, diverse composition indium phosphide (InP) materials having different refractive indices, or the like.

In various embodiments, the multi-material waveguide, including both the first and second waveguide portions, is configured for single mode propagation of a predetermined range of optical signals. This configuration may be provided by appropriate design on the waveguide shape. The taper length and taper tip size may also be selected so as to optimize the coupling efficiency between the two waveguide portions and the coupling length. The coupling length refers to the length of the taper and/or transition region, and the corresponding space required for same within the photonic integrated circuit. In some embodiments, where space in the circuit is at a premium, coupling efficiency can be reduced to allow for a smaller coupling length. Likewise, where space in the circuit is available, coupling length can be increased to allow for a corresponding increase in coupling efficiency.

In other embodiments, some or all sections of the multi-material waveguide may be capable of multi-mode propagation.

In various embodiments, the multi-material waveguide, including both the first and second waveguide portions and the coplanar tapered coupling between these portions, is configured to support both transverse electric (TE) polarization and transverse magnetic (TM) polarization by controlling the waveguide shapes such as height and width.

As such, an on-chip routing waveguide which is polarization independent may be provided. It is noted that such polarization independence can be difficult to achieve using other types of waveguides, such as waveguides using interlayer couplings to connect waveguide sections formed of different materials. Interlayer couplings are avoided in the present invention by use of the coplanar tapered coupling. Furthermore, avoiding the use of interlayer couplings allows for an easier fabrication process, because certain steps such as controlling gaps between adjacent light-propagating layers and planarization processes can be simplified or avoided.

Embodiments of the present invention can be provided on a wafer using a lithography process. For example, a silicon layer may be disposed on the wafer (for example overtop of a lower cladding layer) and etched to form one or more silicon waveguide portions with tapered ends. A silicon nitride material may then be deposited on the wafer so that it is coplanar with the silicon waveguide portions. Deposition may be performed for example using plasma-enhanced chemical vapor deposition (PECVD) , low-pressure chemical vapor deposition (LPCVD) , or another deposition technology. The silicon nitride material may then be etched to form silicon nitride waveguide portions having end regions which surround the tapered ends. A layer of cladding material, such as SiO₂ or another material having a refraction index which is lower than that of both silicon and silicon nitride, is deposited on the waveguide portions as a top cladding. Embodiments of the present invention provide a method of manufacturing a multi-material waveguide and/or photonic integrated circuit comprising same, the method being generally described as above.

Although the present invention has been described with reference to specific features and embodiments thereof, it is evident that various modifications and combinations can be made thereto without departing from the invention. The specification and drawings are, accordingly, to be regarded simply as an illustration of the invention as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations or equivalents that fall within the scope of the present invention.

Claims

A waveguide internal to a photonic integrated circuit, the waveguide comprising:

a first waveguide portion comprising a first light-propagating material and having a tapered end；

a second waveguide portion coplanar with the first waveguide portion and comprising a second light-propagating material and having an end region surrounding the tapered end,

wherein the waveguide is coupled between a first component of the photonic integrated circuit and a second component of the photonic integrated circuit.
The waveguide according to claim 1, further comprising a third waveguide portion comprising the first light-propagating material or a third light-propagating material and having a second tapered end,

wherein the second waveguide portion, or a further waveguide portion comprising the second light-propagating material and forming part of the waveguide, is coplanar with the third waveguide portion and has a further end region surrounding the second tapered end.
The waveguide according to claim 2, wherein the first waveguide portion is operatively coupled to the first component, and the third waveguide portion is operatively coupled to the second component.
The waveguide according to claim 3, further comprising at least one additional waveguide portion located between and coplanar with the second waveguide portion and the further waveguide portion, the additional waveguide portion comprising the first light-propagating material and having a bend.
The waveguide according to claim 4, wherein the additional waveguide portion comprises tapered ends for coupling with other portions of the waveguide, said other portions comprising the second light-propagating material and having additional end regions surrounding the tapered ends of the additional waveguide portion.
The waveguide according to claim 1, wherein the first waveguide portion comprises a bend.
The waveguide according to claim 6, further comprising a third waveguide portion comprising the second light-propagating material, wherein the first waveguide portion further comprises a second tapered end opposite the tapered end, and wherein the third waveguide portion is coplanar with the first waveguide portion and has a further end region surrounding the second tapered end.
The waveguide according to claim 1, wherein the second waveguide portion is operably coupled to the second component.
The waveguide according to claim 1, wherein both the first waveguide portion and the second waveguide portion are single-mode waveguides.
The waveguide according to claim 1, wherein the end region of the second waveguide portion terminates in a facet extending inwardly toward the first waveguide portion, the facet having at least one portion forming an acute or obtuse angle with a longitudinal axis of the end region of the second waveguide portion.
The waveguide according to claim 1, wherein the first light-propagating material has a higher refractive index than the second light-propagating material.
The waveguide according to claim 1, wherein the first light-propagating material is silicon.
The waveguide according to claim 1, wherein the second light-propagating material is silicon nitride.
The waveguide according to claim 1, wherein the tapered end terminates at a tip having a width, the width causing an effective refractive index of the first waveguide portion at the tip to match an effective refractive index of the end region of the second waveguide proximate to the tip.
The waveguide according to claim 1, further comprising a cladding layer adjacent to one or both of the first waveguide portion and the second waveguide portion.
A waveguide internal to a photonic integrated circuit, the waveguide comprising:

a first waveguide portion comprising a first light-propagating material and having a tapered end；

a second waveguide portion coplanar with the first waveguide portion and comprising a second light-propagating material and having an end region surrounding the tapered end,

wherein both the first waveguide portion and the second waveguide portion are single-mode waveguides.
The waveguide according to claim 16, wherein the second waveguide portion is routed to an edge of the photonic integrated circuit for coupling with an external optical signal transmission medium.
A photonic integrated circuit comprising the waveguide according to claim 17.
A photonic integrated circuit comprising the waveguide according to claim 1.
A method of manufacturing a photonic integrated circuit, comprising:

disposing a layer of a first light-propagating material onto a wafer；

etching the layer of the first light-propagating material to form a first waveguide portion having a tapered end；

depositing a second light-propagating material onto the wafer and coplanar with the tapered end；

etching the second light-propagating material to form a second waveguide portion coplanar with the first waveguide portion and having an end region surrounding the tapered end； and

forming a first component of the photonic integrated circuit on the wafer coupled to a first end of a waveguide comprising the first waveguide portion and the second waveguide portion, and forming a second component of the photonic integrated circuit on the wafer coupled to a second end of the waveguide.