Multilayer photonic structure for linear and nonlinear processing of light

Abstract

The present disclosure relates to a multilayer photonic platform and a method of forming thereof. The method comprises forming, on a substrate, one or more first photonic layers being made of a first material having a nonlinear material characteristic, wherein each first photonic layer has a respective first layer thickness. The method further comprises forming one or more second photonic layers being made of the first material, wherein each second photonic layer has a respective second layer thickness. Further, the method comprises forming an intermediate cladding layer made of a second material between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers. The method further comprises forming one or more operational layer pairs, by aligning, for each operational layer pair comprising at least one first and at least one second photonic layers, a respective second waveguide structure formed in the at least one second photonic layer with a respective first waveguide structure formed in the at least one first photonic layer.

Description

[0001] MULTILAYER PHOTONIC STRUCTURE FOR LINEAR AND NONLINEAR PROCESSING OF LIGHT

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a multilayer photonic structure. Particularly, embodiments and aspects of the present disclosure relate to a multilayer photonic structure, mode coupling between operational pairs of the multilayer photonic structure, and a method of forming such a photonic structure for efficient engineering of customizable waveguide structures and photonic integrated circuits.

[0004] BACKGROUND

[0005] Photonic integrated circuits (PICs) are devices that manipulate light by confining it in a core material with a high refractive index, surrounded by a cladding material, typically silicon oxide (SiOz), which displays a lower refractive index. These devices are typically fabricated on a substrate also referred to as a substrate wafer in a planar process, where the core material with a defined thickness is arranged. The core material and geometry play a key role in determining what functionalities can be integrated into a single PIC. The silicon-on-insulator (SOI) platform has been extremely successful in PIC development because all the basic optical operations can be implemented in this material. Examples of operations may include low- loss routing, modulation and detection. However, dedicated platforms generally outperform Si in specific operations, such as ultralow-loss routing and nonlinear optics in silicon nitride, detection in germanium, modulation in lithium niobate, and light generation in, e.g., indium phosphide. Therefore, when designing a PIC, the specific trade-offs of the platform of choice have to be considered. To overcome these limitations, numerous efforts have been made to integrate multiple materials and platforms on the same PIC. By combining multiple platforms, the PIC can achieve denser routing, better fiber-to-chip coupling, integration of light sources, or more efficient modulation. Among the different material platforms, silicon nitride (SiaN^ stands out for ultralow-loss- PICs, thanks to the broad transparency window, from visible to mid-infrared, and the compatibility with CMOS processes, which guarantees reliability and scalability. Moreover, SiaN4 displays Kerr nonlinearities and lacks two-photon absorption, which makes it an excellent platform for nonlinear optics, e.g., microcomb generation, noiseless parametric amplification and supercontinuum generation, examples of which are available in the literature. However, the prior art solutions mainly suffer from a fundamental tradeoff between optical confinement, scattering losses, and single-mode operations in silicon nitride.

[0006] Accordingly, there is a need in the field of PICs for development of versatile designs and solutions, which at least address some of the above-mentioned drawbacks.

[0007] SUMMARY

[0008] It is accordingly an object of the present invention to improve the current state of the art and to mitigate at least some of the above mentioned drawbacks.

[0009] These and other objects are achieved by providing a multilayer photonic structure and a method of forming thereof as defined in the appended independent claims. The term exemplary is in the present context to be understood as serving as an instance, example or illustration.

[0010] According to a first aspect of the present invention, there is provided a multilayer photonic platform comprising one or more first photonic layers being made of a first material having a nonlinear material characteristic, wherein each first photonic layer has a respective first layer thickness and comprises a first waveguide structure formed therein and configured to receive and nonlinearly process light. The multilayer photonic platform further comprises one or more second photonic layers being made of the first material, wherein each second photonic layer has a respective second layer thickness and comprises a second waveguide structure formed therein and configured to receive and linearly process light. It should be appreciated that by "a" respective first waveguide structure comprised in each first photonic layer or "a" respective second waveguide structure comprised in each second photonic layer it is intended to mean that there may be one or more of the respective first or second waveguide structures comprised in each first and / or second layers. The multilayer photonic platform comprises one or more intermediate cladding layers made of a second material, each cladding layer being arranged between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers. The multilayer photonic platform further comprises one or more operational layer pairs, each operational layer pair comprising at least one first photonic layer comprising a respective first waveguide structure and at least one second photonic layer comprising a respective second waveguide structure aligned with the first waveguide structure. The respective first and second waveguide structures comprised in each operational layer pair are configured such that the respective first waveguide structure of the at least one first photonic layer is coupled with the respective second waveguide structure of the at least one second photonic layer. In some exemplary embodiments, the one or more first photonic layers may have an anomalous dispersion characteristic and the one or more second photonic layers may have a normal dispersion characteristic.

[0011] According to some aspects and embodiments, each intermediate cladding layer arranged between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers may be a planarized cladding layer overlaying a respective first or second photonic layer.

[0012] In various exemplary embodiments, the photonic platform may be a monolithically integrated photonic platform and the first material may be silicon nitride. In several exemplary embodiments, the second material may be silicon oxide.

[0013] According to some aspects and embodiments, at least one operational layer pair of the multilayer photonic platform may comprise a coupler portion configured to linearly couple the respective first waveguide structure of the at least one first photonic layer with the respective second waveguide structure of at least one second photonic layer in that operational layer pair.

[0014] In several embodiments, the coupler portion may comprise a double inverted taper structure, formed at a coinciding portion of the first and the second waveguide structures. The double inverted taper structure may be configured to linearly couple a fundamental light mode of the first waveguide structure with the second waveguide structure, such that the fundamental light mode may be transferred from the first waveguide structure to the second waveguide structure.

[0015] In several exemplary embodiments, for each operational layer pair, the first waveguide structure may be a nonlinear multimode waveguide structure and the second waveguide structure may be a linear single-mode waveguide structure.

[0016] In several exemplary embodiments, for each operational layer pair, the respective second waveguide structure of the at least one second photonic layer may be arranged above and at least partly extending over the respective first waveguide structure of the at least one first photonic layer.

[0017] In some embodiments, for each operational layer pair, the first layer thickness of the at least one first photonic layer may be at least three times larger than the second layer thickness of the at least one second photonic layer. Ultralow-loss Si3N4 has been demonstrated in single-mode waveguide geometries and in dispersion-engineered, strong-confinement strip waveguides. The latter structure, however, typically results in waveguides that support more than one spatial mode. Single-mode operation is desired to realize optical signal processing and minimize the coupling to higher- order modes in a multimode waveguide geometry. Many practical systems relying on the Kerr nonlinearity will thus require co-integration of linear and nonlinear processing blocks, e.g., in frequency synthesizers and quantum computing. Nonlinear blocks could also be included in systems that normally require linear operation, e.g., in programmable photonic circuits. In a planar integrated platform, with a thickness chosen to satisfy dispersionengineering requirements, the single-mode-condition can only be attained by sufficiently decreasing the width of the waveguide, resulting in increased interaction with the sidewalls and enhanced scattering losses due to roughness. This indicates a fundamental trade-off between optical confinement, scattering loss, and single-mode operation in silicon nitride. The present inventors have realized that the mentioned trade-off can be overcome by integrating two distinct layers i.e. operational layer pairs of Si3N4, individually optimized to perform either linear processing (single-mode waveguide geometry) or nonlinear Kerr applications (dispersion-engineered waveguide geometry). Adiabatic interposers and coupler portions such as tapered couplers may be designed and formed to interface these two layers and the first and the second waveguide structures.

[0018] The present inventors have further realized that advances in multilayer integration would allow bridging between Si3N4 layers featuring different thicknesses, hence effectively overcoming the aforementioned trade-off. Accordingly, dispersion-engineered nonlinear waveguides may be formed in one or more first Si3N4 photonic layers, wherein each first Si3N4 photonic layer has a respective first layer thickness and comprises a first waveguide structure formed therein and configured to receive and nonlinearly process light. The first waveguide structures feature strong optical field confinement.

[0019] Further, one or more second Si3N4 photonic layers are provided in the multilayer structure wherein each second photonic layer has a respective second layer thickness and comprises a second waveguide structure formed therein and configured to receive and linearly process light. The second waveguide structures have waveguide geometries designed to sustain just one spatial mode in either polarization and thus adapted for SM operation for linear processing.

[0020] According to a second aspect of the present invention, there is provided a method of forming a multilayer photonic platform. The method comprises forming, on a substrate, one or more first photonic layers being made of a first material having a nonlinear material characteristic, wherein each first photonic layer has a respective first layer thickness. The method further comprises forming, in each first photonic layer, a first waveguide structure configured to receive and nonlinearly control light. Further, the method comprises forming one or more second photonic layers being made of the first material, wherein each second photonic layer has a respective second layer thickness. The method further comprises forming, in each second photonic layer, a second waveguide structure configured to receive and linearly control light. The method further comprises forming an intermediate cladding layer made of a second material between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers. Furthermore, the method comprises forming one or more operational layer pairs, by aligning, for each operational layer pair comprising at least one first and at least one second photonic layers, a respective second waveguide structure formed in the at least one second photonic layer with a respective first waveguide structure formed in the at least one first photonic layer such that the respective first waveguide structure of the at least one first photonic layer is linearly coupled with the respective second waveguide structure of the at least one second photonic layer.

[0021] In some exemplary embodiments, the method may further comprise planarizing each intermediate cladding layer formed between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers.

[0022] In several embodiments, the method may further comprise forming, for at least one operational layer pair, a coupler portion for linearly coupling the respective first waveguide structure of the at least one first photonic layer with the respective second waveguide structure of at least one second photonic layer in that operational layer pair.

[0023] In some embodiments, the method may further comprise forming the coupler portion as a double inverted taper structure, at a coinciding portion of the first and the second waveguide structures, wherein a fundamental light mode of the first waveguide structure may be linearly coupled with the second waveguide structure, such that the fundamental light mode may be transferred from the first waveguide structure to the second waveguide structure.

[0024] Further features and advantages of the invention will become apparent when studying the appended claims and the following description. The skilled person in the art realizes that different features of the present disclosure may be combined to create embodiments other than those explicitly described hereinabove and below, without departing from the scope of the present disclosure.

[0025] Further embodiments of the different aspects are defined in the dependent claims.

[0026] It is to be noted that all the embodiments, elements, features and advantages associated with the first aspect also analogously apply to the second aspect of the present disclosure.

[0027] These and other features and advantages of the present disclosure will in the following be further clarified in the following detailed description.

[0028] BRIEF DESCRIPTION OF DRAWINGS

[0029] Further objects, features and advantages of embodiments of the disclosure will appear from the following detailed description, reference being made to the accompanying drawings. The drawings are not to scale.

[0030] Fig. la shows a graph according to some aspects.

[0031] Fig. lb shows a multilayer photonic platform according to several embodiments of the present disclosure.

[0032] Fig. 2a-l show several example multilayer structures according to several embodiments of the present disclosure.

[0033] Fig. 3 shows a coupler according to several embodiments of the present disclosure.

[0034] Fig. 4 shows an example integration of a multilayer photonic integrated circuit according to several embodiments of the present disclosure.

[0035] Fig. 5 shows a flowchart of a method according to several embodiments of the present disclosure. DETAILED DESCRIPTION

[0036] In the present detailed description, embodiments of the present disclosure will be discussed with the accompanying figures. In the following description of exemplary embodiments, the same reference numerals denote the same or similar components. It should be noted that the person skilled in the art will understand that the invention may be practiced without these details and in any other types or variants of the elements or features than the embodiments shown in the appended drawings.

[0037] The following description may use terms such as "top", "bottom", "inner", "outer", "side", "edge", "ridge", "distal", "proximal", "front", "back" etc. These terms generally refer to the views and orientations as shown in the drawings. The terms are used for the reader's convenience only and shall not be construed as limiting.

[0038] In the context of the present invention the directions and orientations such as vertical, horizontal, longitudinal, and lateral directions and extensions need to be interpreted broadly and generally refer to the geometrical extensions of objects in a coordinate system such as a three-dimensional Cartesian coordinate system. The spatial extensions and positions of objects may be defined in at least one plane of the coordinate system e.g. by using x, y, z coordinates and their corresponding angles.

[0039] It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may refer to more than one unit in some contexts, and the like. Furthermore, the words "comprising", "including", "containing" do not exclude other elements or steps. It should be emphasized that the term "comprises / comprising" when used in this specification is taken to specify the presence of stated features, integers, steps, or components. It does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. The term "and / or" is to be interpreted as meaning "both" as well and each as an alternative. The term "obtaining" is herein to be interpreted broadly and encompasses receiving, retrieving, collecting, acquiring, and so forth. It will also be understood that, although the term first, second, etc. may be used herein to describe various elements or features, these elements should not be limited by these terms. These terms are only used to distinguish one element from another.

[0040] Various aspects of the disclosure will hereinafter be described in conjunction with the appended drawings to illustrate and not to limit the disclosure, wherein like designations denote like elements, and variations of the described aspects are not restricted to the specifically shown embodiments, but are applicable on other variations of the disclosure.

[0041] Silicon nitride is a versatile material platform for photonic integration thanks to its wide transparency window, strong Kerr nonlinearity, high refractive index, and absence of two- photon absorption. This allows the development of waveguides with ultralow loss, in the order of ldB / m or lower, which have proven to be advantageous for SiaN^based nonlinear optics applications such as frequency comb generation. Nonlinear applications require a careful design of the waveguide geometry to attain the desired dispersion. Such waveguides feature strong confinement but are typically multimode (MM) waveguides, which leads to fundamental issues when assembled together with linear components for building complex PICs in frequency synthesiser and quantum photonics.

[0042] Linear optics however requires a single-mode (SM)-waveguide design to avoid parasitic coupling to higher order modes. Starting from a dispersion-engineered strong-confinement design, the conventional way to achieve SM operation in SiaNJs to reduce the waveguide width. However, this approach results in a mode with stronger interaction with the sidewalls and increased radiation losses due to surface roughness. As a result, the fundamental tradeoff between single-mode behaviour, dispersion-engineering and losses in strongconfinement SiaN4 waveguides arises. Various embodiments and aspects of the present disclosure strive for providing advantageous devices, implementations and methods for solving the aforementioned problems.

[0043] Fig. la shows, a graph depicting a map 100 of simulated group velocity dispersion coefficient (P2) as a function of core geometry e.g. a core width and / or thickness (height) for a waveguide having varying geometries, dispersion characteristics and configured for linear or nonlinear operations. The map 100 shows the simulated group velocity dispersion coefficient (^2) as a function of the core geometry at an exemplary wavelength 1550 nm. The zero dispersion line 101 denotes the boundary between anomalous dispersion ?2 < 0 above the line 101 and normal dispersion below the line 101. The bottom left corner does not support any mode, hence / ?2 is not defined. The dashed lines 102a - 102c mark the core geometries that either do not support any mode 102a (bottom left corner), support one mode 102b (e.g. TM or TE) or support the fundamental mode 102c in both polarizations (TM and TE). The area between lines 102b and 102c is considered as the single-mode (SM) waveguide. On the right of the SM line 102c, the waveguide is multimode (MM) in at least one polarization.

[0044] The trade-off between dispersion engineering and single-mode operation is analyzed in strip silicon nitride waveguides operating in the telecom wavelength range e.g. 1.5 pm. The term dispersion engineering in the present context is to be construed as the capability of the waveguide to offset the inherent material dispersion for the fundamental light mode. In the near infrared regime (NIR), both silicon nitride and silica exhibit normal dispersion, and for example waveguides having rectangular geometries the overall dispersion of the fundamental mode may be brought close to zero. In silicon nitride strip waveguides, anomalous dispersion is typically achieved when for core thickness of the waveguides corresponding to certain ranges of thickness parameters. For example, the core thicknesses of the waveguide structure may be arranged to be greater than 650 nm for achieving the anomalous dispersion as shown in Fig. la. This type of waveguide exhibits strong optical confinement and may display low losses if the width parameter of the waveguide structures is designed to be sufficiently wide in order to avoid mode interaction with the sidewalls of the waveguide, thereby reducing scattering losses. Typically, the major source of scattering losses within the waveguide structures is the sidewall roughness introduced during fabrication of the waveguides e.g. during an etching process. However, an anomalous dispersion waveguide structure may display multiple TE and TM modes rendering it not suitable for SM operations. Consequently, for achieving SM operations for the above- mentioned range of thicknesses at which the waveguides exhibit anomalous dispersion, the waveguide width parameter needs to be accordingly adjusted. It has been shown that it may be required to reduce the waveguide width parameter to ranges below 850 nm, where the waveguide supports both TEOO and TM00 modes. However, for such geometries, the effective area of the waveguide structure is drastically narrowed, leading to a noticeably increased interaction of the mode with the sidewall roughness of the waveguide structure. As a result, typically waveguide structures having much smaller thickness parameters than the above-mentioned thickness ranges are used in SM operations.

[0045] This fundamental trade-off between optical confinement, dispersion engineering, and SM operation has been widely discussed and analyzed in the literature. The present inventors have empirically demonstrated that thinner single-mode waveguides display lower losses than thicker single-mode geometries.

[0046] As an example, three particular waveguide geometries: a dispersion-engineered waveguide with core thickness 740 nm and width 1900 nm, a single-mode waveguide with the same thickness and width 800 nm and a thinner single-mode waveguide with core thickness 200 nm and width 1500 nm have been studied. The thickness of the latter waveguide is selected to achieve moderate confinement and maintain a reasonably small critical bending radius of 100 pm. From the characterization of the three core geometries, it has been found that a platform dedicated to linear operation would provide a seven-fold improvement in terms of propagation losses. This analysis motivates the development of the multilayer photonic platform and method of forming thereof presented in this disclosure.

[0047] Fig. lb shows a multilayer photonic platform 1 according to several embodiments and aspects herein. The multilayer photonic platform 1 comprises one or more first photonic layers 11 being made of a first material having a nonlinear material characteristic, wherein each first photonic layer has a respective first layer thickness and comprises a first waveguide structure formed therein and configured to receive and nonlinearly process light. In the example of Fig. lb, for simplicity of illustration, the multilayer photonic platform 1 is shown to comprise only one first photonic layer 11, having a first waveguide structure 11a comprised therein shown in a dashed line 11a.

[0048] Further, the multilayer photonic platform 1 comprises one or more second photonic layers 12 being made of the first material, wherein each second photonic layer 12 has a respective second layer thickness and comprises a second waveguide structure formed therein and configured to receive and linearly process light. In the example of Fig. lb, for simplicity of illustration, the multilayer photonic platform 1 is shown to comprise only one second photonic layer 12, having a second waveguide structure 12a comprised therein shown in a dashed line 12a.

[0049] The multilayer photonic platform 1 further comprises one or more intermediate cladding layers 13 made of a second material, each cladding layer being arranged between each two successively arranged layers of the one or more first photonic layers 11 and / or the one or more second photonic layers 12. In other words, there is arranged a respective cladding layer e.g. 13a between a first photonic layer 11 and a second photonic layer 12 as shown in the example of Fig. lb, or between two successively arranged first photonic layers 11, or between two successively arranged second photonic layers 12 as shown in various examples of Fig. 2 which will be explained further below.

[0050] In several aspects and embodiments, each intermediate cladding layer 13a arranged between each two successively arranged layers of the one or more first photonic layers 11 and / or the one or more second photonic layers 12 may be a planarized cladding layer overlaying a respective first 11 or second photonic layer 12.

[0051] The multilayer photonic platform 1 may further comprise other cladding and buffer layers such as the top cladding layer 13b other than the intermediate cladding layers 13a arranged between the two successively arranged first and / or second photonic layers 11, 12.

[0052] In several embodiments and aspects, the multilayer photonic platform 1 further comprises one or more operational layer pairs 14, such as operational layer pair 14a shown in Fig. lb.

[0053] The operational layer pair 14 comprises at least one first photonic layer such as the first layer 11 comprising a respective first waveguide structure such as waveguide 11a and at least one second photonic layer such as second layer 12 comprising a respective second waveguide structure such as waveguide 12a aligned with the first waveguide structure 11a. The operational layer pair in the present context is to be construed as at least one first photonic layer 11 having a first thickness and comprising at least one respective first waveguide structure 11a and at least one second photonic layer 12 having a second thickness and comprising at least one respective second waveguide structure 12a which is aligned with the at least one first waveguide structure 11a. The first waveguide 11a in the operational layer pair 14 is a dispersion-engineered waveguide and the second waveguide 12a is a SM waveguide aligned with the dispersion-engineered waveguide 11a. The respective first and second waveguide structures 11a, 12a comprised in each operational layer pair 14 are configured such that the respective first waveguide structure of the at least one first photonic layer is coupled with the respective second waveguide structure of the at least one second photonic layer. The operation layer pair may also be referred to as the operational pair, the operational coupled pair, bi-layer integration architecture, or the dual-waveguide in the rest of this description.

[0054] In several aspects and embodiments, the multilayer photonic platform 1 may be a monolithically integrated photonic platform 1. This means that all the layers, waveguides, and other components of the multilayer photonic platform 1 are realized on a single chip. This way, customizable PICs adapted for both nonlinear and linear operations may be readily implemented on a single platform and on a single chip.

[0055] In several aspects and embodiments, the first material may comprise any material in the family of silicon nitrides such as silicon nitride or silicon-rich nitride. In several aspects and embodiments herein, the first material is silicon nitride.

[0056] In several aspects and embodiments, the one or more first photonic layers 11 have an anomalous dispersion characteristic and the one or more second photonic layers 12 have a normal dispersion characteristic.

[0057] In several aspects and embodiments, for each operational layer pair 14, the first waveguide structure 11a is a nonlinear multimode waveguide structure and the second waveguide structure 12a is a linear single-mode waveguide structure.

[0058] In several aspects and embodiments, for each operational layer pair 14, the respective second waveguide structure 12a of the at least one second photonic layer 12 is arranged above and at least partly extending over the respective first waveguide structure 11a of the at least one first photonic layer 11.

[0059] In several aspects and embodiments, for each operational layer pair 14, the first layer thickness of the at least one first photonic layer 11 is at least three times larger than the second layer thickness of the at least one second photonic layer 12. For instance, the first layer 11 in Fig. lb may be referred to as a thick silicon nitride layer having a thickness substantially larger than 650 nm, have anomalous dispersion and adapted for nonlinear operations. The second layer 12 in Fig. lb may be referred to as a thin silicon nitride layer having a thickness around 200 nm, have normal dispersion and adapted for SM operations.

[0060] The proposed operational layer pair 14, 14a has several advantages including but not limited to operating in the ultralow loss regime, resulting in intrinsic Qs> 10M and linear processing waveguides in the sub-dB / cm.

[0061] In Fig. 4, an example of a microcomb demultiplexer, which can be employed in comb-based wavelength division multiplexing transmitters is shown which is designed by taking advantage of the proposed bi-layer integration architecture 14, 14a.

[0062] The present inventors have realized that by carefully designing the multilayer photonic structure 1, one or more operational layer pairs 14 can be formed in the interchanging first and second SiaN4 photonic layers 11, 12, such that dispersion-engineered silicon nitride layers 11 are co-integrated with linear single-mode processing units 12 in each operational layer pair, while preserving ultralow losses. Accordingly, each operational layer pair 14 comprises at least one first photonic layer 11 comprising a respective first waveguide structure 11a and at least one second photonic layer 12 comprising a respective second waveguide structure 12a aligned with the first waveguide structure 11a. The present inventors have also realized that by meticulously aligning and associating the first and second waveguide structures e.g. 11a and 12a as shown in Fig. lb, the first and second waveguide structures formed within the operational layer pair can be coupled. In some aspects and embodiments, combinational modes also referred to as super modes may be generated between the coupled first and second waveguides. In the context of waveguides, the term "combinational mode" refers to a mode of operation where two or more waveguides are coupled together, such that the fields in each waveguide affect the fields in the others. More specifically, a combinational mode in the coupled waveguide pair refers to a situation where energy is transferred back and forth between the first and the second waveguide structures, resulting in a combined wave that is a superposition of the individual waves in each waveguide. This is in contrast to an isolated operation, where each waveguide supports its own independent wave propagation.

[0063] The behavior of combinational modes in a two-coupled waveguide system depends on a number of factors, such as the distance between the waveguides, their refractive indices, and the waveguide geometries. These factors can be used to control the properties of the combined wave and tailor it for specific applications in photonics, telecommunications, and other fields.

[0064] In several aspects and embodiments however, the first and second waveguide structures 11a, 12a in the operational layer pair 14, 14a are configured to be linearly coupled such that the light mode 17 as shown in Fig. 3 is adiabatically transitioned from the first waveguide structure having the dispersion-engineered strong-confinement design to the second waveguide structure with SM geometry. In some embodiments and aspects, interposers e.g. coupler structures may be arranged for enabling the adiabatic mode transition.

[0065] Accordingly, by aligning the first and the second waveguides 11a, 12a each having their respective optical properties and being individually optimized for nonlinear anomalous dispersion processing of light and single-mode processing of light respectively, a versatile and compact ultralow loss photonic structure is formed. The presented multilayer photonic structure 1 can be used for implementing customized PICs on a single platform, thus making the fabrication and operation of the devices simpler and much more cost efficient compared to the conventional approaches.

[0066] Figs. 2a-2l show non-limiting example fabrication and implementation processes of the multilayer photonic structures 1 according to several aspects and embodiments herein. In the proposed solution, an advantageous formation of operational layer pairs 14 comprising the first layer 11 and the second layer 12 fabricated in the same material i.e. Si3N4 and having different dispersion characteristics, i.e., anomalous and normal dispersion respectively is achieved. The fabrication process may make use of a silicon (Si) wafer such as a 100 mm-Si wafer as substrate 200, as shown in Fig. 2a. In several examples, the wafer 100 may undergo an oxidation step to obtain a layer 201 of thermal silicon oxide (SiCh). In some examples the SiOa layer 201 may have a thickness around 3 pm. In some examples, stress release structures may be patterned to avoid cracks in the Si N layer (not shown). A first photonic layer 11 having an example thickness of substantially 740 nm of stoichiometric Si3N4 in this example may be deposited on the thermal SiCh layer 201 by means of low-pressure chemical vapour deposition (LPCVD). This first layer 11 is arranged to form the nonlinear layer (NLL) with the dispersion-engineered Si3N4first waveguides. As shown in Fig. 2b, electron beam lithography (EBL) patterning 202 may be used to define the at least one first waveguide 11a, lib, 11c in the first photonic layer 11. In the example of Figs. 2c-2k, three first waveguides 11a, lib, 11c are shown to be formed in a single layer of first photonic layer 11 by way of example.

[0067] In Fig. 21, there are six first waveguides 11a, lib, 11c, lid, lie, Ilf are shown by way of example, wherein the three first waveguides 11a, lib, 11c are formed in layer 111 and three first waveguides lid, lie, Ilf are formed in layer 112. The one or more first waveguide structures formed in each first photonic layer in the example embodiments of Figs. 2c-2l are dispersion-engineered waveguide structures.

[0068] It should be clear to the person skilled in the art that the number of the first waveguides may be adjusted based on the intended design and application and may comprise one or more first waveguides. The patterns and shapes of the first waveguides are also design parameters, which may be decided accordingly.

[0069] In Fig. 2c, the wafer 200 may be etched with inductive coupled plasma etching. A buffer layer 13a of SiOa may be deposited via LPCVD with precursor tetraethyl orthosilicate (TEOS) as shown in Fig. 2d. This buffer layer 13a will form the intermediate cladding layer 13a for the first layer 11 and may serve as a sacrificial layer for a subsequent planarization process which may be performed via chemical mechanical polishing (CMP) to a target thickness as shown in Fig. 2e. A second SiaN4 photonic layer 12 may be deposited afterwards having an example thickness parameter being equal to substantially 200 nm. This layer 12 may serve as the linear layer (LL) which is deposited on the planarized intermediate cladding layer 13a. The second photonic layer 12 may also be patterned though a second EBL exposure for forming the at least one second waveguide 12a, 12b in the second photonic layer 12 as shown in Figs. 2g and 2h.

[0070] In the example of Figs. 2h-2j, two first waveguides 12a, 12b are shown to be formed in a single layer of second photonic layer 12 by way of example. In Fig. 2k, six exemplary second waveguides 12a, 12b, 12c, 12d, 12e, 12f are shown, wherein two second waveguides 12a, 12b are formed in layer 121, two second waveguides 12c, 12d are formed in layer 122, and two second waveguides 12e, 12f are formed in layer 123. The one or more second waveguide structures formed in each second photonic layer in the example embodiments of Figs. 2h-2l are linear single-mode waveguide structures. It should be clear to the person skilled in the art that the number of the second waveguides may be adjusted based on the intended design and application and may comprise one or more second waveguides. The patterns and shapes of the second waveguides are also design parameters, which may be decided accordingly. In several embodiments and aspects, the one or more second waveguides formed in the one or more second photonic layers 12 are aligned with the one or more first waveguides formed in the one or more first photonic layers 11. For instance, in Fig. 2i, the second waveguide 12a is aligned with the first waveguide 11c, wherein the second waveguide 12b is a discrete component. As mentioned earlier, the at least one first photonic layer comprising a respective first waveguide structure e.g. 11c and the at least one second photonic layer comprising a respective second waveguide structure e.g. 12a which is aligned with the first waveguide structure 11c are comprised in the operation layer pair 14a as shown by the bracket 14a in Fig. 2j. Advantageously, the respective first waveguide structure 11c of the at least one first photonic layer is coupled with the respective second waveguide structure 12a of the at least one second photonic layer.

[0071] In some embodiments, the one or more second photonic layers comprises at least two second photonic layers, in other words, the multilayer photonic platform 1 comprises at least two second photonic layers such as layers 121 - 123 in Figs. 2k and 21. Each second photonic layer comprises two or more single-mode second waveguide structures 12a - 12e. The respective first layer thickness of each of the one or more first photonic layers may be at least three times larger than the respective second layer thickness of each of the at least two second photonic layers. Accordingly, each operational layer pair comprises at least one first photonic layer 11 comprising the respective first waveguide structure 11c, and at least two second photonic layers 121 - 123, each second photonic layer comprising a respective second waveguide structure 12a, 12c, 12e configured to be aligned and coupled with the first waveguide structure 11c. In Fig. 2k, planarized cladding layer 13a is formed between first layer 11 and second layer 121. Planarized layers 13c and 13d are formed between the successively deposited second layers 121, 122 and 123. The top cladding layer 13b which is not planarized in this example is formed over the topmost-deposited second layer 123. In some examples, the top cladding layer 13b may be planarized (not shown) for the intended applications such as including optoelectronics on the top layer. The operational pair 14b is formed for the first waveguide 11c aligned with and coupled to an exemplary waveguide stack comprising three, second waveguides 12a, 12c and 12e. As mentioned earlier, the first waveguide 11a - 11c may have a layer thickness at least three times larger than the second layer thickness of the at least one second photonic layer 12a, 12c, 12e. In some embodiments, the first waveguide 11a - 11c may have a layer thickness larger than 600 nm. The at least one second photonic layer and waveguides 12a, 12c, 12e may have a second layer thickness smaller than 300 nm, more preferably around 200 nm.

[0072] The present inventors have realized that introduction of two or more second layers for SM operations leads to expanding the functionality of the PIC chip 1 by increasing the degree of freedom in the waveguide design and improving the optical isolation from the nonlinear layer to the linear layers. Furthermore, a versatile low-loss routing is achieved by utilizing the multilayer SM waveguide structure, where light can be coupled from the nonlinear dispersion-engineered waveguides e.g. 11c to several SM waveguides e.g. 12a - 12f in order to route the light to different parts of the PIC chip 1. For example, in Fig. 2k, light from the dispersion-engineered first waveguide structure 11c is vertically coupled to SM waveguide structure 12a in linear layer 121. The coupled light to the SM waveguide structure 12a may then be routed to one edge of the PIC chip 1 through horizontal coupling to the other SM waveguide structure 12b in the same linear layer 121. In a different example, light from the dispersion-engineered first waveguide structure 11c may be vertically coupled to SM waveguide structures 12a, and 12c. Light may then be horizontally coupled to waveguide 12d for edge extraction and routing. In some examples, light from the dispersion-engineered first waveguide structure 11c may be vertically coupled to SM waveguide structures 12a, 12c, and 12e, which is the top-most SM waveguide in the example of Fig. 2k. The coupled light to the SM waveguide structure 12e may then be routed to the topmost edge of the PIC chip 1 in formed layer 13b. This way light is routed to the top of the PIC chip 1, which may be extracted either via the surface of the PIC chip 1 or through coupling to the adjacent second waveguide structure 12f.

[0073] In addition, combination of two or more linear layers 121, 122, 123 allows for super-mode engineering, i.e., exploiting the interaction between the two neighboring waveguides to achieve bandwidth localized anomalous dispersion. In other words, superposition of modes can build a new mode having an anomalous dispersion in a certain engineered bandwidth. Multiple linear layers e.g. 121, 122, 123 can further be utilized for geometrically manipulating the mode to achieve larger modes. This is particularly useful for coupling with standard single mode fibers, or rotating the electrical field of the mode by breaking the planar symmetry of the linear SM waveguides 12a, 12c, 12e.

[0074] Additionally, having two or more successively-deposited linear layers 121 - 123 allows for a larger separation between the nonlinear layer 11 and the top linear layer 123, which improves the optical isolation between these two layers. Larger separation between these layers allows for low loss crossings of the waveguides, which has the advantage of increasing the density of the photonic integrated circuit 1.

[0075] Even further, multilayer integration of first and seconds photonic layers enables the combination of strong confinement waveguides 11a - 11c and the SM waveguides 12a - 12f that can be configured to interact with an optoelectronic layer on top of the photonic chip 1. For instance, the one or more linear layers 121 - 123 may be configured to efficiently couple light to single mode optical fibers (SMF28) via large mode field diameter edge couplers. The light can then be coupled to the nonlinear layer 11 via an interposer 16, 16a where the light is processed via nonlinear processes. The nonlinearly processed light can be transferred via a series of interposers 16, 16a through all the silicon nitride layers including the nonlinear 11 and linear layers 121 - 123 to reach the top layer close to the surface e.g. layer 13b of the photonic structure 1. The top layer 13b may interact with optoelectronic devices such as photodiodes, semiconductor optical amplifiers or electro-optic modulators that can be placed on the top surface of the chip 1 via micro-transfer printing. Here the advantage of multiple linear layer lays in the optical separation between the large mode field diameter linear waveguide 12e, 12f from the top layer 13b interacting with the optoelectronics. This way, structures including metals e.g. metallic portion 15 or other materials that account for high optical loss may be placed on top of the chip, with minimal interaction with the optical modes processed in the photonic chip 1. This opens for the co-integration of control electronics and optoelectronics elements in the photonic integrated circuit l.ln several embodiments, the multilayer photonic platform 1 comprises at least two first photonic layers such as layers 111, 112 in Fig. 21. Each first photonic layer comprises two or more dispersion- engineered first waveguide structures e.g. waveguide structures 11a - 11c in layer 111 and lid - Ilf in layer 112. Further, one or more second photonic layers of the multilayer photonic platform 1 comprises at least two second photonic layers such as layers 121 - 123 in Figs. 2k and 21. Each second photonic layer comprises two or more single-mode second waveguide structures 12a - 12e. The respective first layer thickness of each of the at least two first photonic layers may be at least three times larger than the respective second layer thickness of each of the at least two second photonic layers. The at least one first photonic layer in each operational layer pair comprises at least two first photonic layers 111, 112. Each first photonic layer comprises a respective first waveguide structure 11c, Ilf. The at least one second photonic layer in each operational layer pair comprises at least two second photonic layers 121, 122. Each second photonic layer comprises a respective second waveguide structure 12a, 12c configured to be aligned and coupled with the respective first waveguide structures 11c, Ilf of the at least two first photonic layers.

[0076] In some embodiments, the respective first layer thickness of at least one layer 111 of the at least two first photonic layers 111, 112 may be greater than layer thicknesses of remainder layers 112 of the at least two first photonic layers. The mentioned layer thickness may in some examples be at least 5% or 10% or 20% or 30% or 40% or 50% greater. This way multiple layers of the nonlinear first waveguide structures with slightly different layer thicknesses and dispersion engineering characteristics are monolithically integrated in the same platform 1, wherein each first layer may be customized for an intended target performance, as described with reference to Fig. 21 in the following.

[0077] In Fig. 21, planarized cladding layer 13a is formed between two successive first layers 111 and 112. Planarized layer 13c is formed between the deposited first layer 111 and the second layer 121. Planarized layer 13d is formed between the two successively-deposited second layer 121 and the second layer 122. The top cladding layer 13b which is not planarized in this example is formed over the topmost-deposited second layer 122. The operational pair 14c is formed for the first waveguides 11c and Ilf aligned with and coupled to an exemplary waveguide stack comprising two second waveguides 12a, and 12c.

[0078] The integration of two or more nonlinear layers 111, 112 comprising the first waveguide structures 11a - Ilf further provides advantages similar to integration of two or more linear layers as discussed with reference to Fig. 2k. Even further, an additional advantage is achieved by integrating two or more successively- deposited nonlinear first layers 111, 112 in the PIC chip 1 to monolithically integrate photonic devices such as microcombs tailored with different target performances. These PIC chips 1 may find applications in datacenter interconnects or space communication systems. In these examples, chip-based frequency microcombs can be employed to implement large bandwidth communication channels and distribute optical clock signals over long distances. However, since communication channels and optical clocks have different dispersion engineering requirements, tailored platforms having two or more nonlinear layers 111, 112 prove highly advantageous. For instance in Fig. 21, the layer 111 and first waveguide structures 11a, lib, 11c may have a layer thickness that is slightly larger than the layer 112 and first waveguide structures lid, lie, Ilf. The waveguides 11a, lib, 11c of layer 111 may advantageously be dispersion-engineered for implementing large bandwidth communication channels. The waveguides lid, lie, Ilf of layer 112 may advantageously be dispersion- engineered for implementing and distributing optical clock signals. At least one of the waveguides 11a, lib, 11c of layer 111 in this example is optically uncoupled from the waveguides lid, lie, Ilf of layer 112, thus allowing monolithic integration of multiple devices with different target performances. This way several operational layer pairs can be formed. For instance, one operational layer pair 14c may comprise first waveguide structures Ilf coupled with 11c, and respectively coupled with second waveguide structures 12a and 12c. Another example operational layer pair 14d may comprise the first waveguide 11a coupled to a respective second waveguide 12g aligned therewith, being optically isolated i.e. uncoupled from the other waveguides Ilf, 11c, 12a and 12c in the operational layer pair 14c.

[0079] The monolithic integration of these platforms has the potential of optimizing the form factor and the control for applications where weight and size are relevant. Various on-chip routing may then be achieved by utilizing two or more linearly arranged layers with SM functionality as described above with reference to Figs. 2j - 21. In several embodiments, each operational layer pair 14a - 14d may further comprise at least one other first waveguide structure that is horizontally shifted with respect to the aligned and coupled waveguide structures in that operational layer pair. For example, waveguide structures lib, or lid, or lie as shown in respective Figs. 2h - 21, are arranged to be laterally displaced i.e. horizontally shifted in relation to the aligned and optically coupled respective first 11c, Ilf and second 12a, 12c, 12e waveguide structures. The laterally displaced at least one other first waveguide structure is configured to horizontally couple and transfer the nonlinearly processed light between several dispersion-engineered first waveguides.

[0080] Devising horizontally shifted first waveguide structures with respect to the aligned and optically coupled first and second waveguide structures in each operational layer pair, has an advantage of horizontally coupling and transferring nonlinearly processed light between several dispersion-engineered first waveguides. For instance, light may be horizontally coupled from the nonlinear dispersion-engineered waveguide lib to the aligned and vertically coupled first waveguide 11c in the operational layer pair 14a of Fig. 2j or to the aligned and vertically coupled first waveguide 11a in the operational layer pair 14d of Fig. 21.

[0081] In several embodiments, each operational layer pair 14a - 14d may further comprise at least one other second waveguide structure that is horizontally shifted with respect to the aligned and coupled waveguide structures in that operational layer pair. For example, waveguide structure 12b in Figs. 2i - 2j or waveguide structures 12b, 12d, 12f in Figs. 2k and 21, are arranged to be laterally displaced i.e. horizontally shifted in relation to the aligned and optically coupled respective first 11c, Ilf and second 12a, 12c, 12e waveguide structures. The laterally displaced second waveguide structures e.g. the second waveguide 12b are devised as discrete components. The laterally displaced at least one other second waveguide structure is configured to horizontally couple and transfer the fundamental light mode between several second waveguides in order to route the light to different parts of the PIC chip 1.

[0082] This way a versatile low-loss routing is achieved by utilizing horizontal coupling of light from the aligned and vertically coupled waveguides to the laterally displaced waveguides or vice versa. For example, in Fig. 2j, light from the dispersion-engineered first waveguide structure 11c is vertically coupled to SM waveguide structure 12a in linear layer 12. The coupled light to the SM waveguide structure 12a may then be routed to one edge of the PIC chip 1 through horizontal coupling to the other laterally displaced SM waveguide structure 12b in the same linear layer 12. Second waveguide structures 12b, 12d and 12f in examples of Figs. 2k and 21 may be similarly utilized for routing the nonlinearly processed light mode throughout the PIC chip 1. Another buffer layer 13b of SiCh may be deposited via LPCVD with (TEOS) as shown in Fig. 2i. This buffer layer 13b will form the cladding layer 13b which may be substantially 3 pm-thick. In the example of Fig. 2j, metallic portions 15 intended to be used as metal heaters 15 may be deposited via a lift-off process of evaporated-platinum . Patterning may be performed via mask-less UV lithography. Note that since the cladding layer 13b is the top cladding layer of the complete multilayer photonic structure 1, no planarization step is needed, even though such a step may be performed for the very top cladding layer 13b if so desired. In some embodiments, several photonic chips i.e. PICs may be fabricated on a single wafer substrate 200, and may be singulated with a combination of SiCh etching and deep Si etching.

[0083] The intermediate cladding layers 13, 13a deposited between the first and / or second photonic layers are configured to be homogeneous layers of SiCh. The inventors have realized that different deposition techniques give rise to different material density and hardness characteristics. The hardness of the material determines the CMP removal rate of that material and thus an interface between two materials in the buffer layer can introduce unwanted residual morphology. The LPCVD deposition of TEOS is a conformal process that produces a layer of high quality SiO2, which copies the etched morphology on the top surface of the PIC (see e.g. Fig. 2d). This surface morphology needs to be removed to have a flat surface for the next layer to be fabricated without any extra losses. Moreover, the surface roughness of the deposited TEOS can be significantly higher compared to the surface roughness of the thermal SiO2 and the deposited Si3N4. This could potentially lead to additional scattering losses in the second photonic layer. Therefore, the planarization step plays an important role in the performance of a multilayer photonic platforms 1 and the PICs.

[0084] As mentioned earlier, CMP has been used advantageously for planarizing the surface morphology of the intermediate cladding layers 13 such as cladding layer 13a and defining the residual thickness on top of the dispersion-engineered waveguides 11a - Ilf. To this end, and in some examples, the removal rate of TEOS on unpatterned wafers may be initially calibrated by measuring the SiO2 thickness across the wafer 200 before and after the CMP with white light reflectance spectroscopy. The removal rate could be used to decide the thickness of the TEOS buffer layer i.e. cladding layer 13 to completely planarize the surface morphology given a pattern density of 1%, comparable to the average pattern density of the presented example designs of the multilayer photonic platform 1 in Fig. 2. Therefore, in some examples and embodiment, dummy structures (not shown) may be added to homogenize the pattern density. A thick buffer oxide layer i.e. the intermediate cladding layer 13 may be deposited to attain a homogeneous residual thickness between the one or more NLL 11 and / or the LL 12.

[0085] This surprisingly, allows reaching the intended target residual thicknesses with an outstanding approximation. For instance, a 389±70 nm residual thickness of the cladding layer 13a on top of the one or more dispersion-engineered layers 11 may be achieved by the proposed solution, wherein a target residual thickness was set to 300 nm. CMP is known to be a process involving chemicals that can affect the optical performance of the waveguides, e.g. KOH, and nanoparticles of SiO2, which can adhere strongly to the surfaces of the waveguides and negatively affect the performance of the PICs. Thus, in some examples in order to minimize the added losses due to CMP, the wafers 200 may be cleaned by submerging in a megasonic bath. In some example embodiments, an RCA cleaning as known in the art may be performed maintaining the wafer 200 in wet conditions. This advantageously helps minimize the number of particles that bond on the wafer 200 surface before being removed by the cleaning process. After polishing, the surface morphology is completely removed and only a step of a few nanometers is present, akin to the waveguide morphology.

[0086] Another advantage of the CMP process is the reduction of the surface roughness of the deposited TEOS buffer layer 13a. The present inventors have performed atomic force microscopy (AFM) scans of the oxidized Si wafer, the LPCVD-deposited Si3N4and the LPCVD- deposited TEOS layers i.e. the cladding layers 13a before and after CMP. The present inventors have based on the mentioned experimental verifications realized that the thermal SiO2 layer 201 displays the best roughness and the first photonic layer 11 of Si3N4 deposited with LPCVD shows a moderate increase in roughness. The TEOS layer 13a after deposition displays the worst surface roughness, approaching the nanometer scale. However, a surprising effect was observed after performing CMP, wherein the residual roughness of the cladding layer 13a was found to be substantially equivalent to that of the thermal SiO2, thus restoring a pristine surface for arranging the second photonic layer 12 of Si3N4.

[0087] According to several aspects and embodiments herein, the multilayer photonic platform 1 and more specifically, at least one operational layer pair e.g. the operational pair 14a as shown in Fig. lb and Fig. 2j of the multilayer photonic platform 1 may comprise a coupler portion 16 as shown in the example of Fig. 3. The coupler portion 16 may be configured to linearly couple the respective first waveguide structure 11c of the at least one first photonic layer 11 with the respective second waveguide structure 12a of at least one second photonic layer 12 in that operational layer pair 14c. In several embodiments, the coupler portion 16 may comprise a double inverted taper structure 16a as also shown in Fig. 4, formed at a coinciding portion 16b of the first 11c and the second 12a waveguide structures. The coupler portion 16 may in some embodiments be a 3D coupler having the double inverted taper structure 16a. The double inverted taper structure may accordingly be configured to linearly couple a fundamental light mode 17 of the first waveguide structure 11c with the second waveguide structure 12a, such that the fundamental light mode 17 is transferred (see arrow 17a in Fig. 3) from the first waveguide structure 11c to the second waveguide structure 12a. The coupler 16 may be configured to adiabatically transfer the mode 17 from the bottom i.e. the first waveguide 11c to the top i.e. the second waveguide 12a in the dual-waveguide 14a. The double tapered geometry 16a also has an advantage of making the coupler 16 broadband and resilient to misalignment. For instance, shifting the top taper i.e. the tapered second waveguide 12a up to e.g. 100 nm in the x direction does not negatively affect the transmission efficiency of the coupler 16a. Nevertheless, an alignment between the top and bottom tapers i.e. the tapered second waveguide 12a and the tapered first waveguide 11c at the coinciding portion 16b of the coupler portion 16a is configured to have an offset less than 100 nm. This way an advantage is achieved for mitigating any extra losses for the transition of the light mode 17 from the first waveguide 11c to the second waveguide 12a in the linearly coupled dual-waveguide 14a. The present inventors have made use of buried SiaN4 markers, detectable with a deep scan of the EBL at 100 keV acceleration. The alignment between the waveguides 11c and 12a of the operational pair 14a was measured with a Vernier scale and scanning electron microscopy (SEM) to achieve the desired alignment in the dual-waveguide 14a. Cutback method was employed to experimentally evaluate the performance of the 3D couplers 16a. The response of different first and second waveguides comprised in the first and second layers 11, 12 and comprised in various operational pairs such as the operational pair 14a were measured. In one example, several paired 3D couplers were cascaded and responses of the measured devices were linearly interpolated. The loss per 3D coupler 16a was obtained as the slope of the fitted data of the device responses. An uncertainty was calculated based on the covariance matrix of the linear fitting of the data. The present inventors observed an outstanding agreement between the experimental data and the FDTD simulations, and thus concluded that the coupling between the two waveguides e.g. waveguides 11c and 12a of the operational layer 14a has negligible losses across a large bandwidth i.e. linear coupling and transmission of the mode 17 from the first waveguide 11c to the second waveguide 12a succefully achieved. This is the first time that a dispersion- engineered silicon nitride layer e.g. the first waveguide 11c of the first layer 11 is cointegrated with a linear single-mode processing unit e.g. the second waveguide structure 12a of the layer 12 while preserving ultralow mode transmission losses.

[0088] The proposed multilayer approach herein is beneficial when a combination of linear and nonlinear operations are designed to be performed on a single PIC chip. As a proof of concept, an exemplary power-efficient microcomb and respective waveguide resonators 18 generated by using an exemplary continuous wave laser light source 18a are shown in the non-limiting example of Fig. 4. The exemplary waveguide resonators of the microcomb 18 are circular resonators, which have been fabricated in the dispersion-engineered first SiaN4 layer e.g. the first layer 11 as previously explained. The microcomb 18 is combined with the linear normal-dispersion layers for SM operations. A demonstration of the stability of the microcomb 18 with a 3D coupler 16a as previously explained in detail with reference to Fig. 3 and demultiplexing of the frequency comb is shown in Fig. 4. The microcomb 18 is fabricated in the first photonic layer 11 having a 3 dB splitter 19 based on a multimode interference (MMI) coupler. The microcomb 18 displays a conversion efficiency > 34% and > 52 lines with power above -20 dBm. The power-efficient microcomb 18 is configured to be combined with a demultiplexer 21 on chip 1 based on an arrayed waveguide grating (AWG) designed to filter nine lines of the frequency comb. The 3dB splitter 19 is incorporated to monitor the frequency comb before it is processed by the AWG 21. The splitter 19 is added after the 3D coupler 16a. This enables a more consistent reference of the comb measured at the monitor port 20a and at the input of the device under test, i.e. the AWG 21. The MMI coupler designed in the LL is characterized to have an example 0.3 ± 0.1 dB of insertion losses. The AWG 21 is designed as a 2x9 channel device with 100GHz channel spacing. One of the two input channels is used as a comb input 20b, the other one is included as a monitor 20c. The nine output channels 20d of the AWG 21 are routed to the edge of the chip 1 and coupled to the measurement setup via tapered fibers (not shown). Before demultplexing the microcomb, the response of the AWG 21 with a broadband laser frequency comb with 250MHz spacing may be characterized to evaluate the response of the AWG 21. In the example characterization test, the AWG 21 shows an insertion loss of 2.5 dB for the best channel. The channel spacing is >90GHz for channels 1-5 and then it drops to 80GHz. The high insertion loss and low channel spacing accuracy may be due to Si3N4residuals at the interface with the waveguides and the free propagation region. In this region, the pattern density and the narrow gaps of 300 nm may lead to more challenges in developing and etching the photoresists, hence leaving behind unetched SiaN4 residuals. Further process calibration for densely patterned areas may drastically improve the performance of the AWG 21. The demultiplexing of the microcomb 18 may be done by tuning the comb so that one of the frequency lines matches the one or more channels of the AWG 21.

[0089] Fig. 5 show a flowchart of a method 500 according to several embodiments and aspects of the present disclosure. The method 500 of forming a multilayer photonic platform 1, comprises forming 501, on a substrate 200, one or more first photonic layers 11 being made of a first material having a nonlinear material characteristic, wherein each first photonic layer has a respective first layer thickness. Further, the method 500 comprises forming 503, in each first photonic layer 11, a first waveguide structure 11a - Ilf configured to receive and nonlinearly process light. The method 500 further comprises forming 505 one or more second photonic layers 12 being made of the first material, wherein each second photonic layer has a respective second layer thickness. The method further comprises forming 507, in each second photonic layer 11, a second waveguide structure 12a - 12f configured to receive and linearly process light. The method further comprises forming 509 an intermediate cladding layer 13, 13a made of a second material e.g. SiCh between each two successively arranged layers of the one or more first photonic layers 11 and / or the one or more second photonic layers 12. The method 500 further comprises forming 511 one or more operational layer pairs 14a - 14c, by aligning, for each operational layer pair 14a - 14c comprising at least one first 111, 112 and at least one second 121 - 123 photonic layers, a respective second waveguide structure 12a - 12f formed in the at least one second photonic layer with a respective first waveguide structure 11a - Ilf formed in the at least one first photonic layer. Thus, the respective first waveguide structure 11a - Ilf of the at least one first photonic layer is linearly coupled with the respective second waveguide structure 12a - 12f of the at least one second photonic layer. In some embodiments, the method 500 may further comprise planarizing 513 each intermediate cladding layer 13, 13a formed between each two successively arranged layers of the one or more first photonic layers 111, 112 and / or the one or more second photonic layers 121, 122, 123.

[0090] In some embodiments, the method 500 may further comprise forming 515, for at least one operational layer pair 14a, a coupler portion 16, 16a for linearly coupling the respective first waveguide structure of the at least one first photonic layer with the respective second waveguide structure of at least one second photonic layer in that operational layer pair.

[0091] In several embodiments, the method 500 may further comprise forming 517 the coupler portion 16 as a double inverted taper structure 16a, at a coinciding portion 16b of the first and the second waveguide structures. Thus, a fundamental light mode 17 of the first waveguide structure 11a - Ilf may be linearly coupled with the second waveguide structure 12a - 12f, such that the fundamental light mode may be transferred from the first waveguide structure to the second waveguide structure.

[0092] In several embodiments, the method 500 may further comprise forming at least two first photonic layers such as layers 111, 112 in Fig. 21. The method 500 may further comprise forming, in each first photonic layer, two or more dispersion-engineered first waveguide structures e.g. waveguide structures 11a - 11c in layer 111 and lid - Ilf in layer 112. Further, the method 500 may comprise forming at least two second photonic layers such as layers 121 - 123 in Figs. 2k and 21. The method 500 may further comprise forming, in each second photonic layer, two or more single-mode second waveguide structures 12a - 12e. The respective first layer thickness of each of the at least two first photonic layers may be at least three times larger than the respective second layer thickness of each of the at least two second photonic layers. The method 500 further comprises forming in each operational layer pair at least two first photonic layers, each first photonic layer comprising a respective first waveguide structure. Further, method 500 comprises forming in each operational layer pair at least two second photonic layers, each second photonic layer comprising a respective second waveguide structure. Further, method 500 comprises aligning the respective second waveguide structure of each second photonic layer with the respective first waveguide structures of the at least two first photonic layers, such that the respective first waveguide structures of the at least two first photonic layers are coupled with the respective second waveguide structures of the at least two second photonic layers. The at least one first photonic layer in each operational layer pair comprises at least two first photonic layers 111, 112. Each first photonic layer comprising a respective first waveguide structure 11c, Ilf. The at least one second photonic layer in each operational layer pair comprises at least two second photonic layers 121, 122. Each second photonic layer comprises a respective second waveguide structure 12a, 12c configured to be aligned and coupled with the respective first waveguide structures 11c, Ilf of the at least two first photonic layers.

[0093] In some embodiments, the respective first layer thickness of at least one layer 111 of the at least two first photonic layers 111, 112 may be greater than layer thicknesses of remainder layers 112 of the at least two first photonic layers. The mentioned layer thickness may in some examples be at least 5% or 10% or 20% or 30% or 40% or 50% greater.

[0094] Additionally, the method 500 may comprise forming, in each operational layer pair, at least one other first waveguide structure arranged to be laterally displaced in relation to the aligned and coupled respective first and second waveguide structures.

[0095] In several exemplary embodiments, the method 500 may additionally or alternatively comprise forming, in each operational layer pair, at least one other second waveguide structure arranged to be laterally displaced in relation to the aligned and coupled respective first and second waveguide structures.

[0096] As mentioned earlier, a SiaN4 platform designed for Kerr nonlinear optics may display a fundamental trade-off between loss, confinement, and multimode behavior, which hinders ultralow-loss linear operations. In the proposed solution, these limitations are advantageously overcome by integrating at least two stacked layers of SiaN4 i.e. the dualwaveguide operational layers such as operational pair 14a. The integration which may be a monolithic integration is configured for operating in the ultralow-loss regime and tailored for nonlinear and linear operations, respectively. The present inventors have demonstrated that the linear and nonlinear platforms such as layers 11 and 12 and their one or more respective first and second waveguides e.g. 11c and 12a are integrated without significant degradation of the waveguide propagation losses and with negligible transition losses of the fundamental mode 17 between the two layers. The proposed solutions, methods, implementations, and fabrication processes herein, open an avenue of opportunities for high-performance linear processing of Kerr nonlinear sources, ranging from quantum optics to chip-scale synthesizers. Furthermore, the multilayer structure would allow for the development of low-loss fibercoupling interfaces and a decrease in reflections to slab waveguides of higher-refractive index materials, thus facilitating the dense heterogeneous integration with ultralow-loss silicon nitride for high-performance PICs.

[0097] It will be appreciated that the above description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. While specific examples have been described in the specification and illustrated in the drawings, it will be understood by those of ordinary skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure as defined in the claims. Furthermore, modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof.

[0098] Therefore, it is intended that the present disclosure not be limited to the particular examples illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out the teachings of the present disclosure, but that the scope of the present disclosure will include any embodiments falling within the foregoing description and the appended claims. Reference signs mentioned in the claims should not be seen as limiting the extent of the matter protected by the claims, and their sole function is to make claims easier to understand.

Claims

CLAIMS1. A multilayer photonic platform comprising: one or more first photonic layers being made of a first material having a nonlinear material characteristic, wherein each first photonic layer has a respective first layer thickness and comprises one or more first waveguide structures formed therein and configured to receive and nonlinearly process light; one or more second photonic layers being made of the first material, wherein each second photonic layer has a respective second layer thickness and comprises one or more second waveguide structures formed therein and configured to receive and linearly process light; one or more intermediate cladding layers made of a second material, each cladding layer being arranged between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers; wherein the multilayer photonic platform further comprises one or more operational layer pairs, each operational layer pair comprising at least one first photonic layer comprising a respective first waveguide structure, and at least one second photonic layer comprising a respective second waveguide structure aligned with the first waveguide structure; and wherein the respective first and second waveguide structures comprised in each operational layer pair are configured such that the respective first waveguide structure of the at least one first photonic layer is coupled with the respective second waveguide structure of the at least one second photonic layer.

2. The multilayer photonic platform according to claim 1, wherein the one or more first photonic layers have an anomalous dispersion characteristic and the one or more second photonic layers have a normal dispersion characteristic.

3. The multilayer photonic platform according to any one of claims 1 or 2, wherein each intermediate cladding layer arranged between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers is a planarized cladding layer overlaying a respective first or second photonic layer.

4. The multilayer photonic platform according to any one of claims 1-3, wherein the photonic platform is a monolithically integrated photonic platform.

5. The multilayer photonic platform according to any one of the preceding claims, wherein the one or more first waveguide structures formed in each first photonic layer are dispersion-engineered waveguide structures; and wherein the one or more second photonic layers comprises at least two second photonic layers, each second photonic layer comprising two or more single-mode second waveguide structures; wherein the at least one second photonic layer in each operational layer pair comprises at least two second photonic layers, each second photonic layer comprising a respective second waveguide structure configured to be aligned and coupled with the first waveguide structure.

6. The multilayer photonic platform according to any one of claims 1-4, wherein the one or more first photonic layers comprises at least two first photonic layers, each first photonic layer comprising two or more dispersion-engineered first waveguide structures; and wherein the one or more second photonic layers comprises at least two second photonic layers, each second photonic layer comprising two or more single-mode second waveguide structures; wherein the at least one first photonic layer in each operational layer pair comprises: at least two first photonic layers, each first photonic layer comprising a respective first waveguide structure; and wherein the at least one second photonic layer in each operational layer pair comprises: at least two second photonic layers, each second photonic layer comprising a respective second waveguide structure configured to be aligned and coupled with the respective first waveguide structures of the at least two first photonic layers.

7. The multilayer photonic platform according to claim 6, wherein the respective first layer thickness of at least one layer of the at least two first photonic layers is greater than layer thicknesses of remainder layers of the at least two first photonic layers.

8. The multilayer photonic platform according to any one of the preceding claims, wherein each operational layer pair further comprises at least one other first and / or second waveguide structures arranged to be laterally displaced in relation to the aligned and coupled respective first and second waveguide structures.

9. The multilayer photonic platform according to any one of the preceding claims, wherein the first material is silicon nitride.

10. The multilayer photonic platform according to any one of the preceding claims, wherein at least one operational layer pair of the multilayer photonic platform comprises: a coupler portion configured to linearly couple the respective first waveguide structure of the at least one first photonic layer with the respective second waveguide structure of at least one second photonic layer in that operational layer pair.

11. The multilayer photonic platform according to claim 10, wherein the coupler portion comprises a double inverted taper structure, formed at a coinciding portion of the first and the second waveguide structures; wherein the double inverted taper structure is configured to linearly couple a fundamental light mode of the first waveguide structure with the second waveguide structure, such that the fundamental light mode is transferred from the first waveguide structure to the second waveguide structure.

12. The multilayer photonic platform according to any one of the preceding claims, wherein, for each operational layer pair, the first waveguide structure is a nonlinear multimode waveguide structure and the second waveguide structure is a linear single-mode waveguide structure.

13. The multilayer photonic platform according to any one of the preceding claims, wherein, for each operational layer pair, the respective second waveguide structure of the at least one second photonic layer is arranged above and at least partly extending over the respective first waveguide structure of the at least one first photonic layer.

14. The multilayer photonic platform according to any one of the preceding claims, wherein, for each operational layer pair, the first layer thickness of the at least one first photonic layer is at least three times larger than the second layer thickness of the at least one second photonic layer.

15. A method of forming a multilayer photonic platform, the method comprising: forming, on a substrate, one or more first photonic layers being made of a first material having a nonlinear material characteristic, wherein each first photonic layer has a respective first layer thickness; forming, in each first photonic layer, one or more first waveguide structures configured to receive and nonlinearly process light; forming one or more second photonic layers being made of the first material, wherein each second photonic layer has a respective second layer thickness; forming, in each second photonic layer, one or more second waveguide structures configured to receive and linearly process light; forming an intermediate cladding layer made of a second material between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers; forming one or more operational layer pairs, by aligning, for each operational layer pair comprising at least one first and at least one second photonic layers, a respective second waveguide structure formed in the at least one second photonic layer with a respective first waveguide structure formed in the at least one first photonic layer; such that the respective first waveguide structure of the at least one first photonic layer is coupled with the respective second waveguide structure of the at least one second photonic layer.

16. The method according to claim 15, wherein the method further comprises: planarizing each intermediate cladding layer formed between each two successively arranged layers of the one or more first photonic layers and / or the one or more second photonic layers.

17. The method according to any one of claims 15 or 16, wherein the method further comprises: forming at least two first photonic layers; forming, in each first photonic layer, two or more dispersion-engineered first waveguide structures; forming at least two second photonic layers; forming, in each second photonic layer, two or more single-mode second waveguide structures; forming in each operational layer pair: at least two first photonic layers, each first photonic layer comprising a respective first waveguide structure; and at least two second photonic layers, each second photonic layer comprising a respective second waveguide structure; aligning the respective second waveguide structure of each second photonic layer with the respective first waveguide structures of the at least two first photonic layers; such that the respective first waveguide structures of the at least two first photonic layers are coupled with the respective second waveguide structures of the at least two second photonic layers.

18. The method according to any one of claims 15 - 17, wherein the method further comprises: forming, in each operational layer pair, at least one other first and / or second waveguide structure arranged to be laterally displaced in relation to the aligned and coupled respective first and second waveguide structures.

19. The method according to any one of claims 15 - 18, wherein the method further comprises: forming, for at least one operational layer pair, a coupler portion for linearly coupling the respective first waveguide structure of the at least one first photonic layer with the respective second waveguide structure of at least one second photonic layer in that operational layer pair.

20. The method according to claim 19, wherein the method further comprises: forming the coupler portion as a double inverted taper structure, at a coinciding portion of the first and the second waveguide structures; wherein a fundamental light mode of the first waveguide structure is linearly coupled with the second waveguide structure, such that the fundamental light mode is transferred from the first waveguide structure to the second waveguide structure.

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