PERFORMANCE HETEROGENEOUS LASERS AND ACTIVE COMPONENTS

A device comprises first, second and third elements fabricated on a common substrate. The first element comprises an active waveguide structure supporting a first optical mode and at least one of the modal gain control structures. The second element comprises a passive waveguide structure supporting a second optical mode. The third element, at least partly butt-coupled to the first element, comprises an intermediate waveguide structure supporting intermediate optical modes. If the first optical mode differs from the second optical mode by more than a predetermined amount, a tapered waveguide structure in at least one of the second and third elements facilitate efficient adiabatic transformation between the second optical mode and one of the intermediate optical modes. No adiabatic transformation occurs between any of the intermediate optical modes and the first optical mode. Mutual alignments of the first, second and third elements are defined using lithographic alignment marks.

FIELD OF THE INVENTION

The present invention relates to semiconductor lasers, amplifiers, modulators, and photodetectors. More specifically, certain embodiments of the invention relate to improved performance of heterogeneously integrated lasers, amplifiers, modulators and photodetectors using dissimilar materials that are optically coupled.

BACKGROUND OF THE INVENTION

A photonic integrated circuit (PIC) or integrated optical circuit is a device that integrates multiple photonic functions and as such is analogous to an electronic integrated circuit. The major difference between the two is that a photonic integrated circuit provides functions for information signals imposed on optical carrier waves. The material platform most commercially utilized for photonic integrated circuits is indium phosphide (InP), which allows for the integration of various optically active and passive functions on the same chip. Although many current PICs are realized in InP platforms, there has been significant research in the past decade in using silicon rather than InP for the realization of PICs, due to some superior characteristics as well as superior processing capabilities for the former material, that leverage the investment already made for electronic integrated circuits.

The biggest drawback in using silicon for PICs is that it is an indirect bandgap material which makes it hard to provide electrically pumped optical sources. This problem is generally solved by assembling PICs comprising two or more chips made from dissimilar materials in separate processes. Such an approach is challenging due to a need for very fine alignment, which increases packaging costs and introduces scaling limitations. Another approach to solving the bandgap problem is to bond two dissimilar materials and process them together, removing the need for precise alignment during the bonding of larger pieces or complete wafers of the dissimilar materials, and allowing for mass fabrication. In this disclosure, we use the term “hybrid” to describe the first approach that includes precise assembly of separately processed parts, and we use the term “heterogeneous” to describe the latter approach of bonding two materials and then processing the bonded structure to define the waveguides and other components of interest.

To transfer the optical signal between dissimilar materials, the heterogeneous approach utilizes tapers whose dimensions are gradually changed until the effective mode refractive indices of dissimilar materials match and there is efficient power transfer. This approach generally works well when materials have similar refractive indices as is the case with silicon and InP. In cases where there is larger difference in effective indices, such as between e.g. SiN and GaAs, the requirements on taper tip dimensions become prohibitive limiting efficient power transfer. Specifically, extremely small taper tip widths (of the order of nanometers) may be necessary to provide good coupling. Achieving such dimensions is complex and may not be cost-effective.

Although InP and silicon-based PICs address many current needs, they have some limitations; among them are the fact that the operating wavelength range is limited by material absorption increasing the losses, and the fact that there is a limit on the maximum optical intensities and consequently optical powers that a PIC can handle. To address these limitations, alternate waveguide materials have been considered, such as SiN, TiO2, Ta2O5, AlN or others. In general, such dielectric waveguides have higher bandgap energies which provides better high-power handling and transparency at shorter wavelength, but, in general such materials also have lower refractive indices. E.g. SiN with bandgap of ˜5 eV has refractive index of ˜2, AlN has bandgap of ˜6 eV and refractive index of around ˜2, and SiO2with bandgap of ˜8.9 eV has refractive index of ˜1.44. For comparison, the refractive index of both InP and GaAs is >3. This makes the tapered approach challenging.

The alternative hybrid approach suffers from the drawbacks already mentioned above, namely the need for precise alignment, and correspondingly complex packaging and scaling limitations.

A recent approach to the problems discussed above was presented in U.S. Pat. No. 10,859,764 B2, employing butt-coupling in combination with a mode-converter to allow the heterogenous process to be used without the need for extremely small taper widths. The present invention is directed towards PICs employing butt-coupling in this way, and that include an active device such as a laser, amplifiers, modulators and photodetectors with improved performance. In particular, embodiments described below are concerned with the detailed design of the optical coupling structure and mode control in the active components necessary for creating of high-performance lasers, amplifiers, modulators and photodetectors.

DETAILED DESCRIPTION

Described herein include embodiments of a platform for realization of photonic integrated circuits using wafer bonding and deposition of dissimilar materials where optical coupling is improved by use of mode conversion and a butt-coupling scheme, and performance is optimized for robust, fabrication tolerant coupling and mode-control in the active device.

The description may use perspective-based descriptions such as top/bottom, in/out, over/under, and the like. Such descriptions are merely used to facilitate the discussion and are not intended to restrict the application of embodiments described herein to any particular orientation. The description may use the phrases “in an embodiment,” or “in embodiments,” which may each refer to one or more of the same or different embodiments. Furthermore, the terms “comprising,” “including,” “having,” and the like, as used with respect to embodiments of the present disclosure, are synonymous.

The term “coupled with,” along with its derivatives, may be used herein. “Coupled” may mean one or more of the following. “Coupled” may mean that two or more elements are in direct physical, electrical, or optical contact. However, “coupled” may also mean that two or more elements indirectly contact each other, but yet still cooperate or interact with each other, and may mean that one or more other elements are coupled or connected between the elements that are said to be coupled with each other. The term “directly coupled” means that two or more elements are in direct contact in at least part of their surfaces. The term “butt-coupled” is used herein in its normal sense of meaning an “end-on” or axial coupling, where there is minimal or zero axial offset between the elements in question. The axial offset may be, for example, slightly greater than zero in cases where a thin intervening layer of some sort is formed between the elements, such as e.g. thin coating layer typically used to provide high-reflectivity or anti-reflectivity functionality. It should be noted that the axes of two waveguide structures or elements need not be colinear for them to be accurately described as being butt-coupled. In other words, the interface between the elements need not be perpendicular to either axis.FIG.1embodiments discussed below are exemplary of such possibilities.

Term “active device” may be used herein. A device or a part of a device called active is capable of light generation, amplification, modulation and/or detection. This is in contrast to what we mean by a “passive device” whose principal function is to confine and guide light, and/or provide splitting, combining, filtering and/or other functionalities that are commonly associated with passive devices. Some passive devices can provide functions overlapping with active device functionality, such as e.g. phase tuning implemented using thermal effects or similar that can provide modulation. No absolute distinction should be assumed between “active” and “passive” based purely on material composition or device structure. A silicon device, for example, may be considered active under certain conditions of modulation, or detection of low wavelength radiation, but passive in most other situations.

FIG.1offers a top-down view of one embodiment of an integrated photonic device100utilizing butt-coupling and mode conversion for efficient coupling between dissimilar materials. Dashed lines A, B, C, D, E, F, and G correspond to cross-sectional end-on views of a device according to some embodiments of the present invention described in more detail with the help ofFIG.4and more specifically end-on-views400A,400B,400C,400D,400E,400F, and400G.

The optical mode151, guided by structures defined in layer101is efficiently coupled to the optical mode153supported by structures defined in layer102or vice-versa as will be described in detail with the help ofFIGS.1,2,3and4.

Layer101makes up what is commonly called an active device, and may be made up of materials including, but not limited to, InP and InP-based ternary and quaternary materials, GaAs and GaAs based ternary and quaternary materials, GaN, GaP, InAs and InSb and their variations and derivatives. Layer101in one embodiment is multilayered, comprising layers providing both optical and electrical confinement as well as electrical contacts, as is known in the art for active devices. Two of such sub-layers101aand101bare present in view shown inFIG.1, with more detailed description presented with the help ofFIG.4. Said multilayers generally providing vertical confinement. Horizontal confinement, in one of the embodiments, is provided by defining a ridge type structure (as also shown in400G and described later). Horizontal confinement can also be provided by generating a strip structure (not shown), by implants or other techniques in the field. In either case, the intent of confinement structures is to control the position and shape of the optical mode, provide guiding and optimize the interaction between the optical mode and injected, generated and/or depleted carriers.

In this top-down view, two sublayers of layer101are shown. Sublayer101aproviding horizontal confinement functionality and sublayer101bcomprising active region of the active device. Active region is the volume in which majority of the interaction between carriers and optical modes takes place and typically comprises quantum wells, quantum dots and/or p(i)n-junctions. Dimensions of both101aand101bcan be optimized along the length of device as e.g. shown inFIG.1where width of the101across-section is larger at the interface with layer103(to be described).

The optical mode151supported by active layer101is guided to layer103that serves to convert the mode for efficient coupling to layer102. Optional coating layers (not shown) such as e.g. high-reflectivity and/or anti-reflective coating layers can be used at any of the interfaces between layers, e.g. between layer101and layer103. Layer103serves as an intermediate waveguide that in some embodiments accepts the profile of an optical mode151, captures it efficiently as optical mode152supported by the waveguide for which layer103provided the core and gradually transfers it to the optical mode153for which layer102provides the core utilizing tapers in at least one of the layers102and103. The embodiment shown inFIG.1illustrates tapers in both layers102and103, but similar functionality can be realized with taper only in one of the layers (not shown). Said tapers can be multiplayered, e.g. utilizing multiple etches.

The refractive index and dimensions of layer103can be engineered to facilitate efficient coupling between both modes151and152(utilizing butt-coupling approach) and modes152and153(utilizing adiabatic transition with at least one taper). In some embodiments, the refractive index of layer103is between 1.55 and 1.8, which enables efficient (butt)coupling from layer101whose effective refractive index is generally larger than 3 (if based on GaAs or InP material systems) and various geometries of layer102when the refractive index of layer102is between 1.6 and 2.5. In some embodiments layer103is realized as SiON (silicon oxynitride) layer in which precise refractive index can be tailored during the deposition in the range between the refractive index of SiO2 (˜1.44) and SiN (˜2) materials. In other embodiments, layer103is realized as polymer, many of which have suitable range of refractive indices. In yet other embodiments, other materials with suitable refractive index are utilized.

Layer102in one embodiment is SiN with refractive index of ˜2. In some embodiments, layer102is a SiON layer with refractive index <2. In yet other embodiment, layer102is LiNbO3 with refractive index of ˜2.2. In yet other embodiments, materials such as Ta2O5, Al2O3 or AlN are used for layer102. Note that all the materials refractive indices have wavelength dependencies and approximate numbers are given for near-infrared range.

Dimensions (or more specifically thicknesses) of layers101are generally optimized for the performance of the active devices, while dimensions (or more specifically thickness) of layer102are (is) generally optimized for the performance of the passive devices, while dimensions (both thickness and widths) of layer103are optimized for efficient coupling between modes151and153. Widths of all waveguides, realized in any of the layers, are controllable by the lithography, etch and deposition steps as is known in the art.

The upper cladding layer107for waveguides realized in103and/or102can be ambient air (meaning no cladding material is actually deposited) or can be any other deliberately deposited suitable material, including, but not limited to, a polymer, SiO2, SiN, SiON etc. In all cases, the refractive index of cladding107is lower than the refractive index of both layers102and103. In some embodiments, multiple materials can serve as cladding107with some providing additional functionality such as e.g. surface passivation. In other embodiments (not shown), different claddings can be utilized for waveguides whose cores are defined in layer101,102and/or103.

One or more lithographic alignment marks120(only one shown for simplicity) are used for precise alignment between various processing steps used to define waveguides, contacts and other features.

Each boundary of different layers (e.g. between101and103) can comprise additional thin layers (not shown) that serve as anti-reflective coatings to facilitate more efficient power transfer or provide surface passivation functionality to improve the performance of the active devices.

FIG.2depicts a zoom-in of the top-down view of one embodiment of an integrated photonic device200(as described in relation toFIG.1) to define and describe the interface between active layers201and intermediate layer203. The interface between the two layers201and203in this embodiment is angled to control the corresponding backreflections that can negatively impact performance of components comprising a photonic integrated circuit. Optional coating layers (not shown) such as e.g. high-reflectivity and/or anti-reflective coating layers can be used at any of the interfaces between layers.

The angle225defines the angle between the tangent of the direction of propagation of the mode251inside structure201and the facet (interface toward203). Angle225is primarily utilized to control the backreflection of the mode supported by layer201when it reaches the interface toward203. In one embodiment it is substantially equal to 8°. In yet another embodiment it is between 8° and 20°. In yet other embodiments the angle can be larger than 20°.

In yet other embodiments, where backreflection is actually beneficial for the component performance, the angle is substantially smaller than 8°, and can be substantially equal to 0° if reflection from this interface is utilized as e.g. mirror (not shown).

The angle230defines the angle between the direction of the propagation of the mode251inside the structure201and the angle of the direction of the propagation of the mode252inside the structure203. Said angle is an optimization parameter for coupling efficiency between the modes supported by layer201and203and is related to the choice of the angle225and/or the refractive indices of used materials in layers201and203and their respective claddings. In most embodiments, said angle230is between 0° and 45°, but the precise value is a result of numerical optimization which can be done using e.g. commercial electromagnetic software.

Precise vertical alignment (up/down inFIG.2) between the axis defined by the direction of the propagation of the wave inside the structure201and the center of the waveguide203at the interface to201is an optimization parameter where such offset can be positive (up inFIG.2), negative (down inFIG.2) and/or substantially equal to 0 (no offset). Such optimization is straightforward to perform with numerical software to maximize the performance of the transition together with optimizing the angle225.

A specific region240is outlined inFIG.2corresponding to what we call wall region. Wall region240has enlarged dimensions for at least one of the layers201and203(and illustrated for both in the embodiment shown inFIG.2), compared to regions outside the wall region. The enlarged dimensions of wall region serve to improve the performance of higher power designs by increasing the area of the facet which can dissipate heat, reduce the optical intensities and/or carrier densities. Furthermore, the increase in size helps with reducing the thermal impedance and consequently the temperature at the facet further improving the performance. In the embodiment, as sketched inFIG.2, the optical mode251supported in layer201and the optical mode252supported in layer203are not guided inside the wall region240(due to larger cross-sectional dimensions), but are slowly diverging. Typically wall is of limited thickness245so the effect of the divergence has minimal impact on the coupling efficiency between251and252. In some embodiments, the total wall thickness245is less than 10 μm. In yet other embodiments, the total wall thickness is less than 2 μm. In other embodiments, the total wall thickness is a function of mode size, wavelength of operation and/or average intensities of the optical field and can be optimized in wider range of up to 100s of μm or more. Total thickness245is an optimization parameter that depends on wavelength of operation, mode251and mode252sizes when guided, optical intensity at the facet, carrier density at the facet, and can be optimized using commercial electromagnetic software to provide high level of optical transmission.

FIG.3depicts a zoom-in of the top-down view of one embodiment of an integrated photonic device300(as described in relation toFIG.1) to define and describe the interface between active layers301and intermediate layer303optimized for high-power operation. The interface between the two layers301and303in this embodiment is angled to control the corresponding backreflections that can negatively impact performance of components comprising a photonic integrated circuit. Optional coating layers (not shown) such as e.g. high-reflectivity and/or anti-reflective coating layers can be used at any of the interfaces between layers.

The angle325defines the angle between the tangent of the direction of propagation of the mode351inside structure301and the facet (interface toward303). Angle325is primarily utilized to control the backreflection of the desired modes supported by layer301when it reaches the interface toward303. In one embodiment it is substantially equal to 8°. In yet another embodiment it is between 8° and 20°. In yet other embodiments the angle can be larger than 20°.

In yet other embodiments, where backreflection is actually beneficial for the component performance, the angle is substantially smaller than 8°, and can be substantially equal to 0° if reflection from this interface is utilized as e.g. mirror (not shown).

The angle330defines the angle between the direction of the propagation of the mode351inside the structure301and direction of the propagation of the mode352inside the structure303. Said angle is an optimization parameter for coupling efficiency between the modes supported by layer301and303and is related to the choice of the angle325and/or the refractive indices of used materials in layers301and303and their respective claddings. In most embodiments, said angle330is between 0° and 45°, but the precise value is a result of numerical optimization which can be done using e.g. commercial electromagnetic software.

Precise vertical alignment (up/down inFIG.3) between the axis defined by the direction of the propagation of the wave inside the structure301and the center of the waveguide303at the interface to301is an optimization parameter where such offset can be positive (up inFIG.3), negative (down inFIG.3) and/or substantially equal to 0 (no offset). Such optimization is straightforward to perform with numerical software to maximize the performance of the transition together with optimizing the angle325.

A specific region340is outlined inFIG.3corresponding to what we call flared wall region. Flared wall region340has enlarged dimensions for at least one of the layers301and303(and illustrated for both in the embodiment shown inFIG.3), compared to regions outside the flared wall region. The enlarged dimensions of flared wall region serve to improve the performance of higher power designs by increasing the area of the facet which can reduce the optical intensities and/or carrier densities. Furthermore, the increase in size helps with reducing the thermal impedance and consequently the temperature at the facet further improving the performance. In the embodiment, as sketched inFIG.3and in-contrast to embodiment sketched inFIG.2, the optical mode351supported in layer301outside the flared wall region340and the optical mode352supported in layer303outside the flared wall region340are gradually transformed to modes351aand352a, both of which have larger effective area and consequently lower optical intensities enabling higher powers.

In the remaining part of this disclosure, term wall region includes both the wall region as described with the help ofFIG.2and flared wall region as described with the help ofFIG.3.

Prior to the present invention i.e. in the absence of intermediate layer103/203/303, the requirements on taper tip width for direct transfer between layer101/201/301and102/202/302would be problematic due to the difference in their effective refractive index. Taper tips as narrow as 100 nm or less would have to be resolved in higher refractive index material101/201/301. Electrical pumping of such taper tips can also be challenging further negatively impacting device performance in come embodiments. The use of intermediate layer103/203/303that is butt-coupled, albeit with angled interface in some embodiments, to layer101/201/301, however, significantly reduces the stringent requirements on taper tip widths, allowing efficient transfer between high refractive index materials (layer101/201/301) to low refractive index materials (layer102/202/302). No adiabatic transformation occurs between the optical modes supported by elements101/201/301and103/203/303at the butt-coupled interface.

Cross-section400A shows one embodiment at the far left of the device as shown inFIG.1, after optical coupling to layer402(assuming optical signal flow occurs from right to left inFIG.1) is complete. Details on how exemplary device is fabricated will be described with the help ofFIG.5. Layer405is the substrate, and can be any suitable substrate for semiconductor and dielectric processing, such as Si, InP, GaAs, quartz, sapphire, glass, GaN, silicon-on-insulator or other materials known in the art. Layer404is an optional layer, whose main purpose is to provide optical cladding for waveguide defined in layer402, if necessary to form an optical waveguide. Optical waveguides are commonly realized by placing higher refractive index core between two lower refractive index layers to confine the optical wave. In some embodiments, layer404is omitted and substrate405itself serves as a cladding. Layer402is waveguide core material for the passive devices. Layer408, whose refractive index is lower than the refractive index of layer402, overlays layer402and underlays layers401and403(not shown in cross-section400A, but visible in cross-sections400B-400G andFIG.5). Layer408serves to planarize the surface such that both bottom surfaces of layer403and401are planar and at same height from the substrate405. In some embodiments, layer408covers top surface of layer402(as sketched inFIG.4) with some remaining thickness, typically smaller than 250 nm. In other embodiments (not shown), the thickness of layer408is such that its top surface is at the same height as the top surface of layer402, i.e. no remaining thickness of layer408on top of layer402. Layer407serves as top cladding if thickness of layer408above top surface of layer402is not sufficient to fully confine the tail of the optical mode. Layer407can comprise multiple materials, each providing cladding functionality to respective waveguides formed in layers401,402and403, including also surface passivation functionality.

Cross-sections400B and400C show the region in which tapers in both layers402and403serve to transition from optical mode153present in cross-section400A to optical mode152present in cross-section400D. In some embodiments (not shown) taper can be in only one of the layers402or403. The transformation utilizes adiabatic taper between the two layers, with dominant transition happening when there is phase matching between mode dominantly residing in layer402and mode dominantly residing in layer403. As this phase matching can be engineered to happen at larger waveguide widths due to the relatively small difference in effective indices between these two layers, the need for very fine taper tips can be eliminated. In some cases, tapers as wide as e.g. 200 nm or wider can support efficient transmission enabling high yield fabrication even if standard lithography is utilized. In other cases, narrower tapers, e.g. with width approaching 100 nm, can be utilized which can also be fabricated using high-quality DUV lithography enabling high-throughput fabrication

Transition between cross-sections400D and400E can be abrupt as described with the help ofFIG.2, or can be gradual as described with the help ofFIG.3. In both cases, the mode is dominantly residing inside the layer403, while at least two of the layers404,405,407and408provide cladding functionality, depending on the thickness of the layer408and if layer404is present.

Transition between cross-section400E and cross-section400F utilizes butt-coupling as described with the help ofFIGS.1-3, and as will be further described with the help ofFIG.5. Wall structure at cross-section400E comprises of layer403, with only tails of the mode being in the cladding materials (at least two of layers404,405,407and408), while the wall-structure at cross-section400F comprises three sublayers401a,401band401cforming layer401and cladding materials404,405,407and408. The three sublayers401a,401band401care described in more detail with the help of cross-section400G. This transition between400E and400F comprises the wall region.

Note that although the shape of layer403is sketched as a rectangle, in some other cases, the shape of the layer403could be modified e.g. slanted sidewall and/or rib geometry to provide better overlap between optical modes depicted in151and152(not shown).

Cross-section400G shows one embodiment of cross-section at the far right of the device as shown inFIG.1when the optical mode451(151inFIG.1) is guided by structures realized in layer401comprising sublayers401a,401band401c. Sublayer401ain some embodiments comprises at least one of the contact layers (either p-contact or n-contact) and optionally a corresponding cladding layer (p-cladding or n-cladding) with the optical mode being laterally confined by etch defining the mesa or ridge. The corresponding metal409a(either p-metal or n-metal) is deposited on top of sublayer401awith optional cladding layer and parts of sub-layer401bserving to provide reduced internal loss by controlling the overlap between the optical mode451and metal409aand contact layers of sublayer401a. Sublayer401acan comprise additional layers such as bandgap smoothing layers, etch stop layers, graded layers, electron blocking layers, etc. to provide improved performance or facilitate more robust fabrication as is known in the art of semiconductor device design and fabrication.

Sublayer401bcomprises active region of the active device. In some embodiments the active region comprises of quantum well, quantum dot, pn junction and/or pin junction layers with optional separate-confinement heterostructure (SCH) layers. Sublayer401bcan comprise additional layers such as second cladding layers, bandgap smoothing layers, etch-stop layers, graded layers, etc. to provide improved performance and facilitate more robust fabrication as is known in the art of semiconductor device design and fabrication. The width of sublayer401b, as sketched in horizontal direction in cross-section400G, is greater than the width of sublayer401a. In other embodiments, the width of sublayer401bis substantially equal to the width of sublayer401a(not shown) accounting for process related limitations (sidewall angle, subsequent lithography step alignment precision, different etch rates for different material compositions, etc).

Sublayer401ccomprises second contact layers (opposite polarity than sub-layer401a) and optional superlattice layers to facilitate bonding and/or prevent dislocations. Sublayer401ccan also comprise additional layers such as cladding layers, bandgap smoothing layers, etch-stop layers, graded layers, etc. to provide improved performance and facilitate more robust fabrication as is known in the art of semiconductor device design and fabrication. The width of sublayer401c, as sketched in horizontal direction in cross-section400G, is greater than the width of either sublayer401aand sublayer401b. Metal409bis laterally offset from the optical mode451(whose lateral confinement is defined by at least one etch) leading to very low or negligible optical loss due to the contact metal regardless of the thickness of the respective cladding layers, in contrast to case of metal409awhere cladding thickness directly impacts the overlap between the mode451and metal409a. Layer402can be unpatterned below the optical mode (as shown in cross-section400G), or can be patterned (not shown) to provide additional functionalities, e.g. frequency-selective feedback to the optical mode451via evanescent field.

FIG.5is a schematic cross-section view of one embodiment of an integrated photonic device500. Functional layers501to507(unless explicitly defined differently) correspond to functional layers101to107as described in relation toFIG.1and functional layers501to509a(unless explicitly defined differently) correspond to functional layers401to409aas described in relation toFIG.4.

The exemplary cross-section includes a substrate505that can be any suitable substrate for semiconductor and dielectric processing, such as Si, InP, GaAs, quartz, sapphire, glass, GaN, silicon-on-insulator or other materials known in the art. In the shown embodiment, a layer of second material504is deposited, grown, transferred, bonded or otherwise attached to the top surface of substrate505using techniques known in the field. The main purpose of layer504is to provide optical cladding for material502, if necessary to form an optical waveguide. Optical waveguides are commonly realized by placing higher refractive index core between two lower refractive index layers to confine the optical wave. In some embodiments, layer504is omitted and substrate505itself serves as a cladding.

Layer502is deposited, grown, transferred, bonded or otherwise attached to the top of layer504if present, and/or to the top of substrate505, using techniques known in the field. The refractive index of layer502is higher than the refractive index of layer504if present, or, if layer504is not present, the refractive index of layer502is higher than the refractive index of substrate505. Layers504and/or502can be patterned, etched, planarized and/or redeposited as is common in the art.

Layer508, whose refractive index is lower than the refractive index of layer502, overlays layer502and underlays layers501and503serves to planarize the patterned surface of layer502. In some embodiments, the planarity of the top surface of layer508is provided by chemical mechanical polishing (CMP) or other etching, chemical and/or mechanical polishing methods. In other embodiments, the planarity is provided because of the intrinsic nature of the method by which layer508is deposited, for example if the material of layer508is a spin-on glass, polymer, photoresist or other suitable material. The planarization may be controlled to leave a layer of desired, typically very low, thickness on top of the layer502(as shown inFIG.5), or to remove all material above the level of the top surface of the layer502(not shown). In the case layer508is left on top of layer502, the target thicknesses are in the range of 10 nm to several hundreds of nm, with practical thickness includes the typical across wafer non-uniformity of the planarization process. In some embodiments, spin-on material is used to planarize and is then etched back resulting with improved across wafer uniformity compared to typical CMP processes.

Layer501is bonded on top of the whole or part of the corresponding (508,502) top surface. Said bonding can be direct molecular bonding or can use additional materials to facilitate bonding such as e.g. polymer films as is known in the art. Layer501comprises multiple sublayers501a,501band501cas described with the help ofFIG.1-4. As mentioned prior, layer502can be fully removed below layer501(not shown) with e.g. patterning and etch prior to deposition of layer508.

The present invention enables efficient optical coupling between waveguides formed in layer501and layer502that is facilitated by layer503. Layer503serves as an intermediate waveguide that in some embodiments accepts the profile (depicted by line550) of an optical mode supported by the waveguide for which layer501provides the core, captures it efficiently as mode profile551, and gradually transfers it to mode profiles552, and finally553. Mode profile553is efficiently guided in the waveguide for which layer502provides the core. In other embodiments, the direction of travel may be reversed, with layer503efficiently capturing an optical mode553supported by the waveguide for which layer502provides the core and gradually transforming its mode profile to that of a mode550supported by the waveguide for which layer501provides the core. Both the thickness and the refractive index of layer503can be engineered to facilitate efficient coupling between both: modes550and551(utilizing butt-coupling approach) and modes551and553(utilizing adiabatic transition with one or more tapers as illustrated with the help ofFIG.1).

Prior to the present invention i.e. in the absence of intermediate layer503, the requirements on taper tip width in layer501to facilitate efficient transfer of the optical mode to layer502would be, as discussed above, problematic with challenging lithography and challenging pumping of the active region in the taper. The use of intermediate layer503, however, significantly reduces the stringent requirements on taper tip width, allowing efficient transfer between very high refractive index materials (such as e.g. GaAs in layer501) to low refractive index materials (such as e.g. SiN in layer502).

Differences between the optical modes supported by waveguides in layers501and502respectively may or may not be obvious by observation of the mode profiles, but mode overlaps less than 100% and vertical offset (inFIG.5) between modes550and553could (in the absence of intermediate layer503) result in significant optical loss. In some cases, it may be considered that losses of up to 1 dB are acceptable, but losses greater than that are not. In other cases, a 3 dB loss level may be the criterion chosen. The function of layer503is to keep optical loss due to imperfect mode overlap below whatever is determined to be an acceptable level in a given application.

The upper cladding layer507for waveguides realized in503and/or502can be ambient air (meaning no cladding material is actually deposited) or can be any other deliberately deposited suitable material as shown inFIG.5, including, but not limited to, a polymer, SiO2, SiN, SiON etc. In some embodiments same material is used for layer507and layer508.

In some embodiments, layer508is not present and both layer501is bonded and layer503is deposited on top of a patterned layer102and lower cladding504. In such embodiments, there is no planarization step. This can provide for a simplified processing flow (removing the need for planarization) at the expense of generally reduced coupling efficiency due to lower coupling efficiency between modes550and551, and also more challenging requirements on the taper tip widths in layer502to facilitate adiabatic transition from mode551to mode552and finally mode553.

FIG.6shows a cross-section600of one embodiment of present invention used to optimize the modal gain. Cross-section600is similar to cross-section400G described above, with functional layers601to609bcorresponding to functional layers401to409bas described in relation to cross-section400G. A difference is the introduction of additional layers610and611which serve to control the modal gain in the active region of layer601. In some embodiments, the waveguide structure defined in layer601supports more than one optical mode, two of which (651and652) are illustrated inFIG.6, but more than two can be supported. Layers610and611have high optical loss, e.g. they are metal, their bandgap is below the photon energy and/or they are made from any other suitable material characterized by high loss constant at the wavelength of interest. The position of layers610and611relative to optical modes651and652can be optimized such that the impact of layers610and611on modal loss is different between modes651and652due to different mode overlaps with said regions. By optimizing the mode shapes and position of610and611, modal gain between various supported modes can be optimized to facilitate better device performance, especially in the case of high-power lasers and amplifiers in which stronger injection currents are typically used. Layers610and611can be continuous along the length of the structure (which is surface normal to the cross-section600), or they can be suitably patterned. In some embodiments, only one of the layers610and/or611is present (not shown). Modal gain control structures, as illustrated inFIG.6, are called pad structures for modal gain control.

FIG.7offers a three top-down views700A,700B and700C of certain embodiments of an integrated photonic devices utilizing modal gain control structures. Functional layers701to709bcorrespond to functional layers401to409bas described in relation to cross-section400G described with the help ofFIG.4and to functional layer601to609bas described in relation to cross-section600described with the help ofFIG.6.

In top-down view700A, tapered structures in sublayer701aenable change in the modal gain and/or number of supported modes inside the layer701, with narrower sublayer701agenerally supporting lower number of modes and/or pushing the modes further out (up/down in view700A). Metal709adeposited on top of layer701acan follow the taper structure (as shown in view700A), or can have uniform width (not shown). Modal gain control structures, as illustrated inFIG.7, are called taper structures for modal gain control. Taper structures for modal gain control can be combined with one or more of pad structures for modal gain control, two of which (710and711) are illustrated in embodiment shown in view700A. Various other combinations of pad and taper structures for modal gain control can be utilized with intent to have different impact to the modal gain of different modes. In embodiment shown in view700A, the effect of modal gain control structures is significantly weaker for mode751as its tail only slightly interacts with said structures, while the effect is significantly stronger for exemplary mode752as substantial part of the mode interacts with said structures.

In top-down view700B, fin structures712and713are realized in layer701a. By optimizing the dimensions of the fin structures, the impact on modes can be varied as suggested by the different overlaps between the fin structures712and713and optical modes751and752. Fins can be non-uniform (as sketched in view700B), can be uniformly periodic, pseudo-random or any other arrangement.

In top-down view700C, fin structures714and715are realized in layer701b. By optimizing the dimensions of the fin structures, the impact on modes can be varied as suggested by the different overlaps between the fin structures714and715and optical modes751and752. Fins can be non-uniform (as sketched in view700C), can be uniformly periodic, pseudo-random or any other arrangement. In general, the effect of fin structures714and715is smaller than the effect of fin structures712and713, as they are typically further from the optical mode center, but additional optimizations, such as width of layer701adefining the mesa/ridge, can be utilized to control the strength of fin structures.

In the remaining part of this disclosure, term modal gain control structure includes at least one of the mode gain control structures as described with the help ofFIG.6cross-section600andFIG.7top-down views700A,700B and700C which include pad structures, taper structures and fin structures for modal gain control. It is obvious to one skilled in the art that various combinations of said structures can be designed to enable precise modal gain control without limitation to their number, size, placement and orientation.

Embodiments of the present invention offer many benefits. The integration platform enables scalable manufacturing of PICs made from multiple materials and capable of handling high optical power compared to typical Si waveguide-based or InP waveguide-based PICs. The high-power operation is especially supported by the wall structures and modal gain control structures.

Previous approaches have generally used taper structures in order to transfer an optical mode from an active device to a passive device, where a width of compound semiconductor region is adiabatically tapered down to sub-micron size. However, a required width of the taper tip decreases rapidly to tens of nanometer sizes as the difference in refractive indices increases. The present invention deploys a butt coupling scheme to eliminate the need of a very small taper size in the compound semiconductor waveguide, which eases fabrication of such structures and can improve the performance removing the limitations associated with pumping very narrow structures.

Other approaches have relied on die attachment of pre-fabricated optical active devices to passive waveguides. This requires very stringent alignment accuracy which is typically beyond what a typical die-bonder can provide. This aspect limits the throughput of this process as well as the performance of optical coupling.

It is to be understood that optical coupling between modes in active and passive layers is reciprocal, so that, takingFIG.5as exemplary, the structure can be configured to facilitate light transmission from region501to region502, but also to facilitate transmission in the reverse direction, from region502to region501. In is to be understood that multiple such transitions with no limitation in their number or orientation can be realized on a suitably configured PIC.

Embodiments of the optical devices described herein may be incorporated into various other devices and systems including, but not limited to, various computing and/or consumer electronic devices/appliances, communication systems, sensors and sensing systems.

It is to be understood that the disclosure teaches just few examples of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.