Patent Description:
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 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 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 result, with no precise alignment necessary.

To transfer the optical signal between dissimilar materials, the heterogeneous approach utilizes tapers whose dimensions are gradually reduced 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 InP or GaN, 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 be cost prohibitive.

Although InP and silicon-based PICs address many current needs, they have some limitations; among them 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 optimal powers that a PIC can handle. To address these limitations, alternate waveguide materials have been considered, such as SiN, TiO<NUM>, TazOs, AIN 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. SiN with bandgap of ~<NUM> eV has refractive index of ~<NUM>, AIN has bandgap of ~<NUM> and refractive index of around ~<NUM>, and SiOz with bandgap of ~<NUM> eV has refractive index of ~<NUM>. For comparison, the refractive index of GaAs and InP is ><NUM>. 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.

There remains, therefore, a need for a method that provides efficient optical coupling between materials (such as, for example, the III-V materials mentioned above, used for active devices, and simple dielectric materials used for waveguides) with dissimilar refractive indices, without requiring prohibitively narrow taper tips. This would allow for scalable integration of materials for the realization of PICs. Ideally, PICs made by such a method would operate over a wide wavelength range from visible to IR and be able to handle high optical power compared to typical Si-waveguide-based PICs.

In this disclosure we call a device or a region of a device active if it is capable of light generation, amplification, modulation and/or detection.

<CIT> describes a structure for a transition from a passive to an active waveguide or vice versa. The structure comprises a passive silica waveguide, a taper section adiabatically "sucking" the optical field guided by the passive silica waveguide in an active polymer rib waveguide, a riser section, which vertically displaces the optical field in the active polymer rib waveguide and an active section with electrodes. At the input of the active section, which follows the riser, the optical field is properly positioned with respect to the electrodes, and highly confined in the active polymer.

<CIT> describes a semiconductor optical amplifier having a main body comprising a PIN structure and tapered portions having a longitudinal axis aligned with the centerline of the main body. The main body and the tapered portions are arranged above a waveguide that receives the light to be amplified.

The present invention includes devices and methods for providing practical and efficient optical coupling between elements comprising materials of different refractive indices, with particular relevance to integrated PICs.

In one embodiment, an optical device according to claim <NUM> is provided.

In another embodiment, a method for making an optical device according to claim <NUM> is provided.

Described herein include embodiments of a method and system 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.

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, wherein like numerals designate like parts throughout, and in which are shown by way of illustration embodiments in which the subject matter of the present disclosure may be practiced.

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 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, as described below with regard to elements <NUM>, <NUM> etc..

Terms "active device" and/or "active region", may be used herein. A device or a region of a device called active is capable of light generation, amplification, modulation and/or detection. We use active device and active region interchangeably meaning either one of them and/or both. This is in contrast to "passive device" and/or "passive region" 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.

<FIG> is a schematic cross-section view of an integrated photonic device <NUM> utilizing butt-coupling and mode conversion for efficient coupling between dissimilar materials. The exemplary cross-section includes a substrate <NUM> that can be any suitable substrate for semiconductor and dielectric processing, such as Si, InP, GaAs, quartz, silicon-on-insulator or other materials known in the art. In the shown embodiment, a layer of second material <NUM> is deposited, grown, transferred, bonded or otherwise attached to the top surface of substrate <NUM> using techniques known in the field. The main purpose of layer <NUM> is to provide optical cladding for material <NUM> (to be described below), 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, layer <NUM> is omitted and substrate <NUM> itself serves as a cladding.

Layer <NUM> is deposited, grown, transferred, bonded or otherwise attached to the top of layer <NUM> if present, and/or to the top of substrate <NUM>, using techniques known in the field. The refractive index of layer <NUM> is higher than the refractive index of layer <NUM> if present, or, if layer <NUM> is not present, the refractive index of layer <NUM> is higher than the refractive index of substrate <NUM>. In one embodiment, the material of layer <NUM> may include, but is not limited to, one or more of SiN, TiO2, Ta2O5, SiO2, and AIN. In some embodiments, other common dielectric materials may be used for layer <NUM>. In other embodiments, a semiconductor material used for layer <NUM> may include, but not be limited to, one or more of Si, GaAs, AlGaAs, InP.

Either or both of layers <NUM> and <NUM> can be patterned, etched, or redeposited as is common in the art, before layer <NUM> is bonded on top of the whole or part of the corresponding (<NUM>, <NUM>) top surface. Said bonding can be direct molecular bonding or can use additional materials to facilitate bonding such as e.g. metal layers or polymer films as is known in the art. Layer <NUM> makes up what is commonly called an active region, 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. Layer <NUM> in 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. In yet another embodiment, layer <NUM> uses lower layers <NUM>, <NUM> and/or <NUM> to provide electrical and/or optical confinement and one or more electrical contacts.

In some embodiments, layer <NUM> can be efficiently electrically pumped to generate optical emission and gain. The present invention enables efficient optical coupling between waveguides formed in layer <NUM> and layer <NUM>. Said materials <NUM> can provide additional functionality such as wide-band transparency, high intensity handling, phase shifting by temperature, strain or other tuning mechanisms, combining, splitting, filtering and/or others as is known in the art.

Efficient coupling is facilitated by layer <NUM>, and by layer <NUM>. Layer <NUM> primarily serves as either an anti-reflective or a highly-reflective coating at the interface between layer <NUM> and layer <NUM>. Layer <NUM> serves as an intermediate waveguide that in some embodiments accepts the profile (depicted by dashed line <NUM>) of an optical mode supported by the waveguide for which layer <NUM> provides the core, captures it efficiently as mode profile <NUM>, and gradually transfers it to mode profiles <NUM>, <NUM> and finally <NUM>. Mode profile <NUM> is then efficiency coupled to the waveguide for which layer <NUM> provides the core. In other embodiments, the direction of travel may be reversed, with layer <NUM> efficiently capturing an optical mode supported by the waveguide for which layer <NUM> provides the core and gradually transforming its mode profile to that of a mode supported by the waveguide for which layer <NUM> provides the core.

The refractive index of layer <NUM> can be engineered to facilitate efficient coupling of mode profile <NUM> and to efficiently transform the mode to one with mode profile <NUM> by taking advantage of tapered structures made in layer <NUM> and/or <NUM>. Prior to the present invention i.e. in the absence of intermediate layer <NUM>, the requirements on taper tip width would be, as discussed above, problematic. The use of intermediate layer <NUM>, however, significantly reduces the stringent requirements on taper tip width, allowing efficient transfer between very high refractive index materials (such as e.g. GaAs or GaN in layer <NUM>) to low refractive index materials (such as e.g. SiN, SiO2 in layer <NUM>). Layer <NUM> may comprise a dielectric and/or a polymer.

Differences between the optical modes supported by waveguides in layers <NUM> and <NUM> respectively may or may not be obvious by observation of the mode profiles, but mode overlaps less than <NUM>% could (in the absence of intermediate layer <NUM>) result in significant optical loss. In some cases, it may be considered that losses of up to 1dB are acceptable, but losses greater than that are not. In other cases, a 3dB loss level may be the criterion chosen. The function of layer <NUM> is 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 layer <NUM> for waveguides realized in <NUM> and/or <NUM> can be ambient air (meaning no cladding material is actually deposited) or can be any other deliberately deposited suitable material as shown in <FIG>, including, but not limited to, a polymer, SiO2, SiNx, etc..

One or more lithography alignment marks (not shown in this cross sectional view, but see, for example, <NUM> in <FIG> described below) are present to facilitate precise alignment between the layers formed during various processing steps.

<FIG> depicts two embodiments of the present invention. In one embodiment, the photonics integrated circuit <NUM> comprises substrate <NUM>, optional layer <NUM> that can be deposited, grown, transferred, bonded or attached by other techniques known in the field. The refractive index of layer <NUM> is higher than the refractive index of layer <NUM> if present, or if layer <NUM> is not present, the refractive index of layer <NUM> is higher than the refractive index of substrate <NUM>.

Layer <NUM> is deposited, grown, transferred, bonded or attached to the top surface of <NUM> (or <NUM> if <NUM> is not present) by other techniques known in the field. Layer <NUM> is patterned to form a waveguide and one or more other optical devices, but in this embodiment, part of the layer <NUM> (the part below active layer <NUM>) is preserved through the patterning process, forming layer 202A. Layer <NUM>, which corresponds to layer <NUM> in <FIG>). is bonded on top of layer 202A. Said bonding can be direct molecular bonding or can use additional materials to facilitate bonding such as e.g. metal layers or polymer films as is known in the art. Layer 202a can be used to evanescently influence an optical mode in layer <NUM>, can guide a hybrid mode that is hybridized between layers <NUM> and 202A and/or can serve electrical purposes such as conducting or blocking current.

Efficient transfer of power from layer <NUM> to <NUM> is facilitated by layer <NUM>, where <NUM> facilitates mode transformation between modes supported by layer <NUM> and layer <NUM> (in the same way that layer <NUM> facilitates mode transformation in relation to <FIG>) and by layer <NUM>, which comprises an anti-reflective or highly-reflective coating.

In the embodiment shown in the lower part of <FIG>, the photonics integrated circuit <NUM> comprises substrate <NUM>, optional layer <NUM> that can be deposited, grown, transferred, bonded or attached by other techniques known in the field.

Layer <NUM> is deposited, grown, transferred, bonded or attached by other techniques known in the field to the top of layer <NUM> (or <NUM> if <NUM> is not present). The refractive index of layer <NUM> is higher than the refractive index of layer <NUM> if present, or if layer <NUM> is not present, the refractive index of layer <NUM> is higher than the refractive index of substrate <NUM>.

Layer <NUM> is patterned to form waveguide and other optical devices, but in this embodiment, the part of layer <NUM> underlying layer <NUM> (to be described below) is removed as is the corresponding underlying part of layer <NUM> if present, with techniques known in the art, putting layer <NUM> in direct contact with substrate <NUM>.

The direct contact between layers <NUM> and <NUM> in PIC <NUM> can facilitate better heat extraction through the substrate and/or by forming electrical current paths through layer <NUM>. If the effective refractive index of layer <NUM> is higher than substrate <NUM>, substrate <NUM> can also be used as an optical cladding layer. In some embodiments, layer <NUM> comprises cladding layers that serve to optically isolate the light generated in layer <NUM> from coupling to layer <NUM>. Layers <NUM> and <NUM> are bonded, said bonding can be direct molecular bonding or can use additional materials to facilitate bonding such as e.g. metal layers or polymer films as is known in the art.

<FIG> offers a top-down view <NUM> and several corresponding end-on cross-sectional views 350A, 350B, 350C, 350D of a device according to some embodiments of the present invention.

Top-down view <NUM> shows optional layer <NUM> that covers substrate <NUM> (not visible in this view, but shown in views 350A, 350B, 350C, 350D). The optical mode supported by "active" layer <NUM> is guided through coating layer <NUM> to layer <NUM> that serves to convert the mode for efficient coupling to layer <NUM>. To facilitate that coupling, the dimensions of layer <NUM> are tapered down towards layer <NUM>, as indicated by the relatively small width of the tip <NUM> relative to the width of layer <NUM> shown at the extreme left of the figure. It has been calculated that the requirements on taper dimensions are significantly relaxed up to several hundred nanometers due to the presence of layer <NUM>. For example, a coupling efficiency between <NUM> and <NUM> of or greater than <NUM>% may be achieved, even if the refractive index difference between <NUM> and <NUM> is larger than one, for a tip width of a few hundred nanometers. In contrast, in the absence of layer <NUM>, where layer <NUM> has to be tapered such that its mode may directly couple into layer <NUM>, the dimensions of taper tip <NUM> would have to be much less than one hundred nanometers for a similar coupling efficiency. In another embodiment, a taper is created in layer <NUM> instead of in layer <NUM>. In yet another embodiment, tapers may be created in both layers <NUM> and <NUM> for highly efficient coupling.

In some embodiments (not shown), the taper tip can physically touch layer <NUM>. In yet another embodiment (not shown), the tapering of layer <NUM> extends over the full length of layer <NUM> (to the right in the orientation shown in the figure) so that there is no abrupt termination, but the width variation continues to facilitate more efficient coupling.

One or more lithography alignment marks <NUM> (only one shown for simplicity) are used for precise alignment between various processing steps.

Cross-sectional views 350A, 350B, 350C, 350D correspond to four characteristic locations marked A, B, C and D in top down view <NUM>. Cross-section 350D shows an exemplary cut through a region that comprises layer <NUM> (which typically includes a multilayered active structure) and optional layers 302A and/or <NUM> (as described with regard to 202A and <NUM> in relation to <FIG>). In embodiments where layer <NUM> does not terminate before layers <NUM> and/or <NUM>, of course view 350C would not be found. Cross-section 350B shows a region where the tapered transition between layers <NUM> and <NUM> is formed. As discussed above, the requirements on taper tip dimensions are significantly reduced due to mode conversion carried out by layer <NUM>. Cross-section 350A shows one embodiment of layer <NUM> at the far left of the device as shown in view <NUM>, after optical coupling (assuming optical signal flow from right to left in view <NUM>) is complete. Typical heights and widths of waveguides <NUM>, <NUM>, and <NUM> can range from submicron to several microns, although they are largely dependent on specific material systems and implementations. Optional upper cladding layer <NUM> is shown in views 350A, 350B, 350C, 350D.

<FIG> depicts a device according to one embodiment of the present invention where a passive waveguide is configured such that a wavelength selective structure is formed within it. In the shown embodiment, the photonics integrated circuit device <NUM> comprises substrate <NUM>, and optional layer <NUM> that can be deposited, grown, transferred, bonded or otherwise attached to the top surface of substrate <NUM> using other techniques known in the field are used. The refractive index of layer <NUM> is higher than the refractive index of layer <NUM> if present, or if layer <NUM> is not present, the refractive index of layer <NUM> is higher than the refractive index of substrate <NUM>.

Layer <NUM> is deposited, grown, transferred, bonded or attached by other techniques known in the field to the underlying layer <NUM> or, if <NUM> is not present, to substrate <NUM>. Layer <NUM> is patterned to form a waveguide and optionally other optical devices. As shown, a part of layer <NUM> below active layer <NUM> (analogous to <NUM>, <NUM>, <NUM> in previous figures) is patterned to form a frequency selective structure comprising materials 402A and 402B, of different refractive indices. This frequency selective structure can be utilized to generate e.g. a single frequency laser such as a DFB laser or DBR laser, and/or can be used to make mirrors and other structures known to persons skilled in the art.

<FIG> depicts a device according to one embodiment of the present invention where an unbroken or continuous dielectric waveguide extends through active and passive regions. In the shown embodiment, photonics integrated circuit device <NUM> comprises substrate <NUM>, optional layer <NUM> that can be deposited, grown, transferred, bonded or attached to the top surface of <NUM> by other techniques known in the field. The refractive index of layer <NUM> is higher than the refractive index of layer <NUM> if present, or if layer <NUM> is not present, the refractive index of layer <NUM> is higher than the refractive index of substrate <NUM>.

Layer <NUM> is deposited, grown, transferred, bonded or attached by other techniques known in the field to the underlying layer <NUM> or, if <NUM> is not present, to substrate <NUM>. Layer <NUM> is patterned to form a waveguide and optionally other optical devices. As shown, layer <NUM> is continuous along the structure, extending below active layer <NUM>. Layer <NUM> is bonded directly on top of layer <NUM> in this embodiment. The dimensions of layer <NUM> are optimized to facilitate efficient transfer of an optical mode between the waveguides formed by <NUM> and <NUM> respectively.

<FIG> depicts a top-view of a device <NUM> according to one embodiment of the present invention, where boundaries between dissimilar materials are angled to reduce the back reflection. In the shown embodiment, optional layer <NUM> is present, overlying the device substrate (not shown). The optical mode supported by active layer <NUM> is guided through optional coating layer <NUM> to intermediate layer <NUM> that serves to convert the mode for efficient coupling to passive layer <NUM>. To facilitate this transition, the dimensions of layer <NUM> are tapered down towards layer <NUM> as indicated by the relatively small width of the tip <NUM> relative to the width of layer <NUM> shown at the extreme left of the figure. As discussed above, the requirements on taper dimensions are significantly relaxed due to layer <NUM>. Additionally, in this embodiment, one or more of interfaces <NUM>, <NUM> and <NUM> are angled to reduce corresponding back reflection(s).

It is to be understood that optical coupling between modes in active and passive layers is reciprocal, so that, taking <FIG> as exemplary, the structure can be configured to facilitate light transmission from region <NUM> to region <NUM> as explicitly shown, but also to facilitate transmission in the reverse direction, from region <NUM> to region <NUM>. 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.

<FIG> is a process flow diagram of a method according to embodiments of the present invention, showing some of the operations carried out to make integrated devices of the types described above.

Method <NUM> for making the devices need not always include all the functions, operations, or actions shown, or to include them in exactly the sequence illustrated by the sequence from blocks <NUM> through <NUM> as shown. In an exemplary case, however, method <NUM> begins with block, <NUM>, in which a substrate, suitably prepared for subsequent processing steps, is provided. Method <NUM> may then proceed from block <NUM> to block <NUM>, where a first element, comprising one or more dielectric materials, is formed on the prepared substrate, by deposition, growth, transfer, bonding or some other well-known technique.

From block <NUM>, method <NUM> may proceed to block <NUM> where a waveguide, and optionally other structures, such as, but not limited to, couplers, filters, resonators, etc. are defined in the first element, the waveguide comprising a core layer (<NUM>, 202A in the case of <FIG>) and optionally a lower cladding layer (<NUM> in the case of <FIG>) (<NUM>, 202A in the case of <FIG>, <NUM> in the case of <FIG>). Subsequent steps (not shown) might include additional material deposition or removal in preparation for step <NUM>.

From block <NUM>, method <NUM> may proceed to block <NUM> in which a second element, typically involving an active semiconductor material, is bonded on the top surface of the structure (layer <NUM> on top of substrate <NUM>, in the case of <FIG>). Said bonding can be direct molecular bonding or can use additional materials to facilitate bonding such as e.g. metal layers or polymer films as is known in the art.

From block <NUM>, method <NUM> may proceed to block <NUM>, where a waveguide (<NUM> in <FIG>, <NUM> in <FIG>) and optionally other structures, such as, but not limited to, couplers, filters, resonators, detectors, amplifiers, optical sources are defined in the second element, Next, at step <NUM>, electrical contacts may be formed in the second element. In embodiments where the second element is used to provide a semiconductor light source, these contacts are used to drive the light source to generate light. In embodiments where the second element is used to provide a photodetector, these contacts may be used to convey the photodetector output signals.

Steps (not shown) subsequent to steps <NUM> and/or <NUM> might include additional material deposition or removal in preparation for step <NUM>, in which a third element, comprising one or more dielectric materials, is formed (by deposition, growth, transfer, bonding or some other well-known technique) in a location between and in contact with the first and second elements, Next, at step <NUM>, an intermediate waveguide is defined in the third element.

Further processing of the various dielectric and/or semiconductor layers, and/or electrical contacts, and the addition and processing of index matching layers, upper cladding, bonding pads, etc may be performed as is known in the art.

Embodiments of the present invention offer many benefits. The integration platform enables scalable manufacturing of PICs made from multiple materials and capable of covering a wide wavelength range from visible to IR and handling high optical power compared to typical Si waveguide-based or InP waveguide-based PICs.

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.

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.

This present invention utilizes a process flow consisting of typically wafer-bonding of a blanket piece of compound semiconductor material on a carrier wafer with dielectric waveguides and subsequent semiconductor fabrication processes as is known in the art. It enables an accurate definition of optical alignment between active and passive waveguides via typically photo lithography step, removing the need for precise physical alignment. Said photo lithography-based alignment allows for scalable manufacturing using wafer scale techniques.

Efficient optical transfer between dissimilar materials is facilitated by using a butt-coupling approach in combination with a mode-converter (the intermediate waveguide) that removes the need for narrow taper tips that are challenging to resolve and fabricate with current state-of-the-art tools.

In some embodiments the active region can utilize the substrate for more efficient thermal sinking, due to direct contact to the substrate with no dielectric in-between. In such embodiments, active region fully defines the optical waveguide in active region and transitions to passive region via the above mentioned butt-coupling.

In some embodiments, the active region creates a hybrid waveguide structure with dielectric layers which can be used, for example, to create a wavelength selective component formed inside the laser cavity for e.g. distributed feedback (DFB) lasers or similar components.

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.

Claim 1:
A device (<NUM>) comprising:
First (<NUM>), second (<NUM>) and third (<NUM>) elements (<NUM>, <NUM>, <NUM>) fabricated on a common substrate (<NUM>) further comprising: a coating layer (<NUM>) between the first and third elements (<NUM>, <NUM>);
wherein the first element (<NUM>) comprises an active waveguide structure supporting a first optical mode (<NUM>), the second element (<NUM>) comprises a passive waveguide structure supporting a second optical mode (<NUM>), and the third element (<NUM>), butt-coupled to the first element (<NUM>) and coupled to the second element (<NUM>), comprises an intermediate waveguide structure supporting intermediate optical modes (<NUM>, <NUM>, <NUM>);
wherein the first optical mode (<NUM>) differs from the second optical mode (<NUM>), a tapered waveguide structure in at least one of the second and third elements (<NUM>, <NUM>) performs adiabatic transformation between the second optical mode (<NUM>) and one of the intermediate optical modes (<NUM>) captured efficiently from the first element (<NUM>); and
wherein mutual alignments of the first, second and third elements (<NUM>, <NUM>, <NUM>) are defined using lithographic alignment marks that facilitate precise alignment between layers formed during processing steps.