A heterogenous vertical optical coupler

Abstract

This document describes a heterogeneous vertical optical coupler which is configured to enable controlled transfer or coupling of optical signals between waveguides formed from different materials and positioned at different configurable vertical heights within a photonic structure. The vertical optical coupler comprises a silicon substrate comprising a first waveguide formed over a central membrane section and extended partially over first and second side regions of the substrate, a support structure configured to suspend a second waveguide above the first waveguide. First and second piezoelectric actuator stacks disposed on side regions and are operative to deform the side regions to vertically displace the first waveguide with respect to the second waveguide to tune an optical coupling ratio between two waveguides.

Description

A HETEROGENOUS VERTICAL OPTICAL COUPLER RELATED APPLICATION

[0001] This application claims the benefit of priority to Singapore patent application no.10202500815S filed on 27 March 2025, the contents of which are hereby incorporated by reference in their entirety for all purposes.TECHNICAL FIELD

[0002] This application relates to a heterogeneous vertical optical coupler which is configured to enable controlled transfer or coupling of optical signals between waveguides formed from different materials and positioned at different vertical heights within a photonic structure.BACKGROUND

[0003] The rapid growth of cloud computing and data-centric applications has spurred the demand and development of large-scale optical switches that are capable of delivering high bandwidth, low latency, and energy-efficient performance. Optical switching architectures fabricated using complementary metal-oxide-semiconductor (CMOS) photonic platforms have been popular as these architectures allow for high-density integration with reduced power budgets. To enhance reconfigurability and further lower the power consumption, vertical optical coupler designs incorporating microelectromechanical systems (MEMS) actuators have recently become popular.

[0004] Vertical optical coupler designs proposed by those skilled in the art typically employ homogeneous waveguide materials such as silicon-on-insulator (SOI) materials, where optical power transfer occurs via evanescent coupling. In this mode, the optical field from one waveguide couples into the evanescent region of the adjacent waveguide, allowing energy to be transferred. However, it was found that such evanescent coupling is highly sensitive to waveguide geometry and coupling length, with efficient transfer only occurring selectively only over discrete coupling lengths where the optical mode “hops” between waveguides. This leads to narrow-band behaviour and imposes strict fabrication tolerances. Furthermore, homogeneous material systems also lack spectral diversity, further constraining the bandwidth and robustness of the coupling mechanism.

[0005] It was also found that MEMs based vertical optical coupler designs tend to introduce additional constraints. While such designs enable low-power and compact displacement control, the actuation range is inherently limited by the pull-in phenomenon, which is a nonlinear instability in electrostatic systems. As the voltage applied increases, the electrostatic force between the movable and fixed electrodes increases at a rate that is faster than the mechanical restoring force. When a critical voltage, known in the art as the pull-in voltage, is exceeded, the movable electrode will then collapse onto the fixed electrode. This pull-in voltage restricts the usable actuation range to approximately one-third of the initial gap distance, significantly limiting the vertical displacement and hence the tuneable range of the optical coupling. Moreover, traditional electrostatic systems often lack an integrated feedback mechanism for precisely sensing the gap distance, leading to uncontrolled or unstable coupling behaviour.

[0006] Hence, those skilled in the art are constantly looking for ways to develop improved vertical optical coupler designs that overcome the limitations associated with conventional homogeneous waveguide systems and electrostatic actuation mechanisms.SUMMARY

[0007] In one aspect, the present disclosure describes a vertical optical coupler that comprises a silicon substrate which in turn comprises a central membrane section, a first side region laterally adjacent to a first end of the central membrane section, and a second side region laterally adjacent to a second end of the central membrane section, wherein the first and second side regions are separated from the central membrane section. The coupler also includes a first waveguide that comprises a silicon nitride (SiN) layer formed over the central membrane section and extends partially over the first and second side regions of the silicon substrate. The coupler also comprises a support structure that is formed on the first side region of the silicon substrate, the support structure configured to support a suspended waveguide structure above the silicon substrate and the first waveguide. The disclosed suspended waveguide structure comprises a second waveguide positioned above the central membrane section of the silicon substrate and vertically separated from the first waveguide by an air gap. The suspended waveguide structure also includes a pair of lateral segments positioned over the first and second side regions of the silicon substrate respectively with the second waveguide being laterallyseparated from each of the lateral segments by a dielectric material. The coupler also has a first piezoelectric actuator stack disposed on the first side region of the silicon substrate and a second piezoelectric actuator stack disposed on the second side region of the silicon substrate, wherein each piezoelectric actuator stack is operative to deform its corresponding side region in response to an applied voltage such that the central membrane section together with the first waveguide are vertically displaced with respect to the second waveguide to tune an optical coupling ratio between the first and second waveguides.

[0008] In embodiments of the one aspect, the dielectric material comprises aluminium oxide, silicon dioxide, aluminium nitride or hafnium oxide. Further, the first and second regions are separated from the central membrane section by trenches formed in the silicon substrate.

[0009] In embodiments of the one aspect, the support structure comprises a silicon dioxide post or silicon dioxide pillar. Additionally, a distance of the air gap between the first waveguide and the second waveguide is between 0.20 pm and 1 pm.

[0010] In embodiments of the one aspect, the first and second piezoelectric actuator stacks each comprise a platinum-lead zirconate titanate-platinum (Pt / PZT / Pt) stack, a molybdenumaluminium nitride-molybdenum (Mo / AIN / Ao) stack or a molybdenum-scandium-doped aluminium nitride-molybdenum (Mo / ScAlN / Mo) stack.

[0011] In embodiments of the one aspect, the second waveguide comprises a material selected from the group consisting of lithium niobate (LiNbOs), germanium (Ge), aluminium nitride (AIN), scandium-doped aluminium nitride (ScAlN), silicon nitride (SiN), silicon (Si), poly-silicon (poly-Si) or amorphous silicon.

[0012] In embodiments of the one aspect, a doped silicon region is formed in at least the first side region or the second side region of the silicon substrate, the doped silicon region being laterally adjacent to the central membrane section, wherein the doped silicon region comprises a contact portion coupled to a resistance measurement sensor, such that an electrical resistance of the doped silicon region is measurable to characterize a distance of the air gap.

[0013] According to another aspect, a method for coupling optical signals between a first waveguide and a second waveguide in a vertical optical coupler is described in this disclosure.The disclosed method comprises the steps of providing a silicon substrate comprising a central membrane section, a first side region laterally adjacent to a first end of the central membrane section, and a second side region laterally adjacent to a second end of the central membrane section, wherein the first and second side regions are separated from the central membrane section and forming a first waveguide comprising a silicon nitride (SiN) layer over the central membrane section and partially over the first and second side regions of the silicon substrate. The method also includes the steps of forming a support structure on the first side region of the silicon substrate and supporting a suspended waveguide structure above the silicon substrate and the first waveguide using the support structure. The suspended waveguide structure comprises a second waveguide positioned above the central membrane section of the silicon substrate and vertically separated from the first waveguide by an air gap, and a pair of lateral segments positioned over the first and second side regions of the silicon substrate respectively, wherein the second waveguide is laterally separated from each of the lateral segments by a dielectric material. The method then includes the steps of forming a first piezoelectric actuator stack on the first side region of the silicon substrate and forming a second piezoelectric actuator stack on the second side region of the silicon substrate, and applying a voltage to the first and section piezoelectric actuator stacks to induce deformation of the respective side regions to vertically displace the central membrane section and the first waveguide relative to the second waveguide to adjust an optical coupling ratio between the first and second waveguides.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Various embodiments of the present disclosure are described below with reference to the following drawings:Figure 1 illustrates a front view of a vertical optical coupler in accordance with an embodiment of the present disclosure;Figure 2 illustrates a front view of the vertical optical coupler illustrated in Figure 1 when voltages are applied to the actuators in accordance with embodiments of the disclosure;Figure 3 illustrates a simplified perspective view of the vertical optical coupler illustrated in Figure 1;Figure 4 illustrates a simplified top view of the vertical optical coupler illustrated in Figure 1; Figure 5 a illustrates plots showing the simulated through and diverted power when a heterogeneous waveguide is employed in the vertical optical coupler illustrated in Figure 1; Figure 5b illustrates plots showing the simulated through and diverted power when a homogeneous waveguide is employed in the vertical optical coupler illustrated in Figure 1;Figure 6a illustrates the simulated electric field distribution of a heterogeneous waveguide system;Figure 6b illustrates the simulated electric field distribution of a homogeneous waveguide system;Figure 7a illustrates the simulated variation in through and diverted power as the coupling length changes in a heterogeneous waveguide system;Figure 7b illustrates the simulated variation in through and diverted power as the coupling length changes in a homogeneous waveguide system;Figure 8a illustrates the simulated change in the actuation distance between the first and second waveguides of the vertical optical coupler system illustrated in Figure 1 when the voltage applied to the piezoelectric actuators increases;Figure 8b illustrates the simulated change in the actuation distance between two vertical waveguides in a vertical optical coupler system known in the art when the voltage applied to the electrostatic actuators increases;Figure 9a illustrates the simulated change in through and diverted power of the vertical optical coupler system illustrated in Figure 1 when the actuation distance between the first and second waveguides increases;Figure 9b illustrates the simulated change in through and diverted power of a vertical optical coupler system known in the art when the actuation distance between the two vertical waveguides increases due to the application of electrostatic actuation;Figure 10 illustrates the simulated stress induced in the piezoelectric actuator and silicon substrate layer when the voltage applied to the actuator increases; andFigure 11 illustrates a flowchart showing the process for coupling optical signals between a first waveguide and a second waveguide in the vertical optical coupler as illustrated in Figure 1.DETAILED DESCRIPTION

[0015] The following detailed description is made with reference to the accompanying drawings, showing details and embodiments of the present disclosure for the purposes of illustration. Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments, even if not explicitly described in these other embodiments. Additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0016] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0017] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance as generally understood in the relevant technical field, e.g., within 10% of the specified value.

[0018] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0019] As used herein, “comprising” means including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0020] As used herein, “consisting of’ means including, and limited to, whatever follows the phrase “consisting of’. Thus, use of the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0021] In the context of various embodiments, the term “disposed on" or “formed on” relates to the placement or deposition of one material or layer onto the surface of another and may involve one or more types of deposition techniques.

[0022] In the context of various embodiments, the directional terms mentioned herein, such as “above” and “below” or “upper” and “lower” refer to directions as described with reference to the drawings. Therefore, the directional terms are only used for illustration and are not meant to limit the present disclosure.

[0023] It should be noted that although the terms first, second and third are used herein to describe various elements, these elements should not be limited by these terms as these terms are meant to only distinguish one element from another element. Thus, the first element described herein could be termed as a second element without departing from this disclosure.

[0024] As used herein, “laterally adjacent” relates to the positioning of a structure next to or beside another structure in the same horizontal plane. In other words, it refers to two or more structures that are arranged side-by-side across a surface of a substrate, with at least part of one structure aligned along a common lateral axis or direction with respect to the other structure.

[0025] As used herein, “vertically separated” relates to the positioning of a first structure above a second structure along a vertical axis, such that at least a portion of the first structure is located at a higher or lower vertical position relative to the second structure.

[0026] Additionally, for the sake of brevity, extensive explanations of conventional techniques of fabricating semiconductor devices and integrated circuits are not described in detail herein. The tasks and processes described herein may also be integrated into a more comprehensive procedure with extra steps of features that are not elaborated upon in this document. Specifically, certain processes of fabricating semiconductor devices are well known to one skilled in the art hence, such processes will be omitted entirely.

[0027] Figure 1 illustrates a cross-section or a front view of a vertical optical coupler in accordance with embodiments of the present disclosure. As illustrated, vertical optical coupler 100 comprises a silicon substrate that includes central membrane section 102b, side region 102a that is laterally adjacent to one side of central membrane section 102b, and side region 102c that is laterally adjacent to an opposing side of central membrane section 102b. In embodiments of the disclosure, side regions 102a, 102c are typically formed from bulk silicon and serve to mechanically support central membrane section 102b. These side regions may also act as anchors for central membrane section 102b and may house electrical or optical routing structures (e.g., vias, electrodes, etc.).

[0028] Optical waveguide 106 may be formed on central membrane section 102b and may also extend partially over parts of side regions 102a and 102c. Waveguide 106 may comprise a silicon nitride (SiN) core waveguide having a rectangular or tapered cross-section optimized for vertical coupling. Specifically, waveguide 106 comprises waveguide central core 106a which is responsible for confining and guiding optical modes. Waveguide 106 also includes vertical extensions 106b on either sides of central core 106a which serve as structural supportor cladding to assist with mode shaping. In some embodiments, dielectric isolation layer 104, such as silicon dioxide (SiCL), aluminum oxide (AI2O3), aluminium nitride (AIN) or hafnium oxide (HfCh), may be disposed between the silicon substrate and waveguide 106 to improve optical confinement and reduce scattering losses. Additionally, dielectric isolation layer 104 may function as an etch-stop or protective barrier during release processes, for example by preventing vapor hydrofluoric acid (HF) or other etchants from attacking the material of waveguide 106. Limiting such unintended etching helps preserve waveguide geometry, thereby maintaining low propagation loss and reliable optical transmission of waveguide 106.

[0029] In further embodiments, doped silicon region 118 may be formed in side regions 102a and / or 102c. Doped silicon region 118 may be patterned and positioned such that it undergoes mechanical stress when the respective side regions are actuated, e.g., when parts of side regions 102a, 102c flexes or deforms due to an external force. This deformation induces a change in mechanical stress within doped silicon region 118, which in turn modulates its electrical resistance. Further, doped silicon region 118 may include a contact portion that is coupled to a resistance measurement sensor (not shown), such that an electrical resistance of doped silicon region 118 is measurable to characterize a distance of the air gap between waveguides 116 and 106.

[0030] By doing so, doped silicon region 118 functions as a piezoresistive sensor that enables electrical readout of the membrane displacement. In particular, variations in the resistance of doped region 118 can be monitored to infer a corresponding displacement or gap value between the two waveguides. Hence, it may be used to facilitate feedback control, active alignment, or sensing functionality in optical packaging and integration scenarios.

[0031] As shown in Figure 1, suspended waveguide structure 111 comprises optical waveguide 116 and adjacent support / lateral segments 112a and 112b, where segment 112a is laterally adjacent to one side of optical waveguide 116 and segment 112b is laterally adjacent to an opposing side. As illustrated, segment 112a is positioned substantially above side region 102a, optical waveguide 116 is positioned directly above waveguide 106, specifically above waveguide central core 106a and central membrane section 102b, while segment 112b is positioned substantially above side region 102c. Together, segments 112a, 112b provide mechanical support for suspended waveguide structure 111 and may also serve as anchor pointsfor suspending waveguide 116 across the coupling region. Dielectric layer 114, such as silicon dioxide, aluminium oxide, aluminium nitride or hafnium oxide may be deposited, patterned, or selectively etched between or around suspended waveguide structure 111. Dielectric layer 114 serves both as an insulating medium and as a vertical spacer that helps define or stabilize a coupling gap between the two waveguides. Dielectric layer 114 may similarly function as an etch-stop or protective barrier during release processes, for example by preventing vapor hydrofluoric acid (HF) or other etchants from attacking the material of waveguide 116. Further as illustrated, it can be seen that waveguide 116 is laterally separated from each of the support / lateral segments 112a, 112b by dielectric layer 114.

[0032] In embodiments of the disclosure, support structure 110 may be formed on side region 102c. Segment 112b of suspended waveguide structure 111 may in turn be formed on or supported by support structure 110 that provides mechanical stability and anchoring for the suspended elements. In embodiments, support structure 110 may comprise, but is not limited to, a silicon dioxide post or silicon dioxide pillar. In embodiments of the disclosure, waveguide 116 may comprise, but is not limited to, a waveguide whose core is formed from a material selected from the group consisting of lithium niobate (LiNbOs), germanium (Ge), aluminium nitride (AIN), scandium-doped aluminium nitride (ScAlN), silicon nitride (SiN), silicon (Si), poly-silicon (poly-Si) or amorphous silicon. One skilled in the art will recognize that support structure 110 may instead be formed on side region 102a. In such an arrangement, segment 112a of suspended waveguide structure 111 may in turn be formed on or supported by support structure 110 that is formed on side region 102a without departing from this disclosure.

[0033] Waveguide 116 may be vertically aligned with waveguide 106, or specifically waveguide central core 106a to enable vertical optical coupling between the two waveguides. This configuration facilitates efficient evanescent or grating-assisted coupling across the vertical gap d separating the suspended and lower waveguide structures, i.e., waveguides 116 and 106 respectively. The initial vertical separation d between waveguides 106 and 116 may be defined by the thickness and mechanical properties of central membrane 102b, in conjunction with the design of support structure 110. This allows the initial vertical alignment d to be tailored to satisfy specific wavelength ranges, polarization conditions, or modematching requirements.

[0034] As illustrated, actuator 108a may be formed on side region 102a and / or actuator 108b may be formed on side region 102c. Actuators 102a and 102b are configured to enable dynamic tuning of vertical gap d between waveguide 116 and underlying waveguide 106. Actuators 108a and 108b may each comprise a piezoelectric actuator that is configured to apply a controlled force to deform the side regions 102a and 102c respectively, thereby adjusting vertical position d of waveguide 116 relative to waveguide 106. In embodiments of the disclosure, actuators 102a and 102c comprise piezoelectric actuator stacks that may comprise of, but is not limited to, a platinum-lead zirconate titanate-platinum (Pt / PZT / Pt) stack, a molybdenum-aluminium nitride-molybdenum (Mo / AIN / Ao) stack or a molybdenum-scandium-doped aluminium nitride-molybdenum (Mo / ScAlN / Mo) stack.

[0035] In an exemplary embodiment, central membrane section 102b and side regions 102a and 102c may each have a thickness in a range of approximately 60 nm with central membrane section 102b having a width of about 0.9 pm. The gap between central membrane section 102b and side region 102a may be about 1 pm, while the gap between central membrane section 102b and side region 102b may similarly be about 1 pm. Central core 106a of optical waveguide 106 may have with a width in a range of approximately 1 pm and optical waveguide 106 may have an overall width of 4 pm. Central core 106a may have a core thickness in a range of approximately 400 nm, and dielectric layer 104 may have a thickness of about 50 nm.

[0036] Segment 112a, waveguide 116 and segment 112b may have a thickness of approximately 0.4 and waveguide 116 may have a width of about 1 pm. The initial vertical separation d between waveguides 106 and 116 may be in a range of approximately 0 pm to 2 pm to enable efficient vertical optical coupling. Dielectric layer 114 may have a thickness of about 50 nm. It should be noted that these dimensions are provided by way of example and may be varied according to wavelength of operation, material selection, and mechanical design constraints.

[0037] Figure 2 illustrates the deformation that occurs in parts of side regions 102a and 102c when a voltage is applied to actuators 108a and 108b respectively. Specifically, when a voltage is applied to actuator 108a, a piezoelectric deformation is induced in underlying side region 102a of the substrate. Specifically, actuator 108a expands or contracts in response to the applied voltage. This mechanical deformation causes a part of side region 102a to deflect either upwardtoward the suspended waveguide structure 111 or downward away from it. The resulting displacement modifies the vertical distance d between waveguide 106 and suspended waveguide structure 111. Similarly, actuator 108b drives piezoelectric deformation at side region 102c when a voltage is applied. Specifically, actuator 108b expands or contracts in response to the applied voltage. This mechanical deformation causes a part of side region 102c to deflect either upward toward the suspended waveguide structure 111 or downward away from it. These geometrical changes influence the evanescent coupling between waveguide 106 and waveguide 116 by adjusting the vertical separation between these two waveguides, thereby adjusting the proportion of optical power transferred between the two waveguides.

[0038] Figure 3 illustrates a simplified perspective view of the vertical optical coupler previously described in Figures 1 and 2. It should be noted that while the dielectric layers are omitted from this simplified drawing for visual clarity, they may be included in the actual structure as required and as illustrated in Figures 1 and 2. Certain parts of suspended waveguide structure 111 are also omitted for visual clarity.

[0039] As shown in Figure 3, suspended waveguide 116 is separated from waveguide 106 by a vertical gap d, enabling optical coupling that depends on the relative alignment and spacing between the waveguides. As illustrated, input optical signal 301 enters the coupler from an input end of waveguide 116. Depending on the coupling condition, a portion of optical signal 301 is transferred to the central core of waveguide 106, which guides the coupled light to output 303 of waveguide 106. The remaining portion of optical signal 301 continues through waveguide 106 to output 302. These outputs correspond to different optical paths, and the system is configured to selectively direct light based on the coupling ratio between waveguides 116 and 106.

[0040] As mentioned in the previous sections, to dynamically control the coupling ratio between the two waveguides, piezoelectric actuators 108a and 108b may be employed to modulate the deformation of parts of side regions 102a and 102c. This piezoelectrically induced deformation alters the vertical gap d and the curvature of side regions 102a and 102c, thereby tuning the coupling efficiency. Additionally, doped silicon region 118, located on side region 102c, adjacent to actuator 108b, functions as a piezoresistive sensor that detects mechanicalstrain in the substrate, or side region 102c. This allows for electrical monitoring of the deformation that occurs and enables feedback or closed-loop control of the actuator system.

[0041] Figure 4 illustrates a simplified top-down schematic of the vertical optical coupler illustrated in Figure 1. As shown, input optical signal 301 enters from the left side of waveguide 116. A portion of the optical signal propagating through waveguide 116 is then coupled into the core of waveguide 106 whereby the optical coupling condition between the two waveguides depends on the vertical gap d, and the waveguide alignment. The optical signal coupled into waveguide 106 then propagates towards “diverted” output 303. The remaining uncoupled portion continues through waveguide 116 to “through” output 302. It should be noted that this figure omits certain dielectric layers and parts of suspended waveguide structure 111 for visual clarity, but in practice, the waveguides may be embedded or encapsulated in appropriate materials to maintain optical confinement and mechanical support.

[0042] Simulated performance of vertical optical coupler

[0043] Figure 5 a illustrates the simulated through and diverted output power (based on arbitrary units (a.u.)) for a vertical optical coupler design in accordance with embodiments of the disclosure whereby the coupler employs heterogeneous waveguides (SiN / SiN-Si) across the wavelength range of 1.50 pm to 1.60 pm. As shown in the plots, the diverted power (circle) remains consistently high and shows only minimal variation, from 96.11% to 92.68%, which in turn results in a power variation of just 3.57%. This stability across a broad wavelength range demonstrates the broadband nature of adiabatic coupling enabled by the heterogeneous waveguide design. The slight dip in diverted output is expected due to practical limitations, but the overall flatness confirms wavelength-insensitive coupling behavior.

[0044] Conversely, Figure 5b illustrates the simulated through and diverted output power for a vertical optical coupler design in accordance with embodiments of the disclosure whereby the coupler employs homogeneous waveguides (SiN / SiN) across the wavelength range of 1.50 pm to 1.60 pm. In this approach, coupling occurs via evanescent coupling. As shown in the plots, the diverted output (circle) in this case increases significantly as the wavelength increases, ranging from 83.10% to 99.66%, yielding a much larger power variation of 16.62%. This clearly illustrates the wavelength dependence of evanescent coupling, which is lessdesirable for broadband applications. The plots in these two figures underscore the superior broadband performance and wavelength insensitivity of the proposed heterogeneous waveguide-based vertical coupler design.

[0045] Figure 6a illustrates the simulated electric field distribution of a heterogeneous waveguide-based vertical coupler design in accordance with embodiments of the disclosure. As can be seen from this figure, it can be said that the broadband performance of the proposed heterogeneous waveguide system (SiN / SiN-Si) is attributed to its use of adiabatic coupling. In this configuration, the silicon (Si) layer at the bottom plays a key role by supporting a gradual transition of the optical mode from the top SiN waveguide into the bottom SiN-Si composite waveguide. This smooth mode evolution, characterized by a continuous field distribution from the upper SiN to the lower SiN-Si region, allows efficient power transfer across a wide wavelength range. The presence of silicon significantly modifies the refractive index landscape, enabling the system to maintain robust coupling even with wavelength variations which in turn allows for broadband operation.

[0046] Figure 6b illustrates the simulated electric field distribution of a homogeneous waveguide-based vertical coupler design in accordance with embodiments of the disclosure. As can be seen from this figure, this coupler design operates via evanescent coupling. Without the Si layer, the optical power is transferred between the top and bottom SiN layers through discrete 'hops', observable as periodic intensity maxima in the electric field distribution. The distance between these hopping points is highly sensitive to wavelength. As the wavelength changes, the location and efficiency of the coupling shifts, leading to strong wavelength dependence in power distribution across the through and diverted outputs. This lack of robustness to wavelength variation limits the bandwidth performance of the homogeneous system compared to the heterogeneous design.

[0047] Figure 7a illustrates the dependence of the optical power distribution on the coupling length (defined by the length of the waveguide) for a heterogeneous vertical coupler design in accordance with embodiments of the disclosure. The presence of the silicon layer enables adiabatic coupling between the top and bottom waveguides. As a result, the optical power measured at the through and diverted outputs remains relatively stable as the coupling length varies. This indicates that the coupling process is largely insensitive to the length of thewaveguides, meaning that consistent optical performance can be achieved across different coupling region dimensions. Such insensitivity allows the actuator to be designed with a very small footprint and enables the same actuator structure to be reused across different optical layouts without requiring redesign for specific coupling lengths.

[0048] Figure 7b illustrates the dependence of optical power distribution on the coupling length (defined by the length of the waveguide) for a homogeneous vertical coupler design in accordance with embodiments of the disclosure. Based on the plots in this figure, it can be seen that this design exhibits strong dependence on coupling length due to evanescent coupling. In this case, optical power periodically transfers between the two waveguides, and the relative power at the through and diverted outputs changes significantly as the coupling length varies. This behaviour arises because the coupling length must be carefully matched to the wavelength-dependent beat length for efficient power transfer. Consequently, any change in coupling length alters the output power distribution, requiring the actuator design to be adjusted in tandem with the optical design.

[0049] Figure 8a illustrates a plot that compares the vertical gap between the waveguides of the vertical optical coupler against the voltage applied to the piezoelectric actuation mechanism. As shown, it can be seen that the actuation distance varies approximately linearly with the applied voltage. This linear relationship enables continuous and predictable tuning of the gap over the full designed range. As illustrated, applying up to 40 V to the piezoelectric actuator stack produces an actuation displacement of about 1 pm, which corresponds to the designed initial separation between the top and bottom waveguides. Accordingly, piezoelectric actuation allows precise control of the optical coupling ratio.

[0050] Figure 8b illustrates a plot that compares the vertical gap between the waveguides of the vertical optical coupler against the voltage applied to an electrostatic actuation mechanism. The plot exhibits a nonlinear displacement-voltage relationship and is fundamentally limited by the pull-in phenomenon. As the applied voltage increases, the electrostatic force rises faster than the mechanical restoring force, leading to an instability at a critical displacement. Beyond this point, the structure collapses toward the opposing surface and stable actuation are no longer possible. For an initial gap of 1 pm, simulation resultsindicate that the last stable electrostatically accessible displacement is approximately 0.29 pm with actuation distances larger than this value being unstable due to pull-in.

[0051] Figure 9a illustrates the simulated change in through and diverted power of the vertical optical coupler system when the actuation distance between the first and second waveguides increases due to piezoelectric actuation being applied. The plots in this figure show that all power distributed through the through and diverted outputs are realizable across the entire 0 - 1 pm gap. Figure 9b illustrates the simulated change in through and diverted power of the vertical optical coupler system when the actuation distance between the first and second waveguides increases due to electrostatic actuation being applied. As electrostatic actuation suffers from a pull-in instability, as depicted in the shaded region, this renders part of the power distribution range (approximately 0.1 pm to 0.7 pm) physically inaccessible. Once the gap reduces below a critical threshold (~0.7 pm), the actuator collapses and the waveguides abruptly snap together, reaching 0 pm separation. This makes it impossible to access intermediate coupling ratios in that region. Additionally, re-separating the waveguides after pull-in is extremely difficult and may cause permanent adhesion or micro-welding, thus limiting reusability and degrading device reliability.

[0052] As voltages are applied to the piezoelectric stack, mechanical stress is generated within the stack, which induces vertical displacement of the bottom waveguide and hence changes the coupling gap. This actuation mechanism relies on stress transfer between material layers. As shown in Figure 10, optical simulations indicate that the stress increases linearly with the applied voltage in both the piezoelectric and silicon layers. Notably, stress is also manifested in the doped silicon layer beneath the piezoelectric actuator. This induced stress alters the electrical resistivity of the silicon layer. By measuring the current through the doped silicon layer, it is possible to infer the induced stress, and hence, deduce the coupling gap value.

[0053] Figure 10 illustrates process 1100 for coupling optical signals between a first waveguide and a second waveguide in a vertical optical coupler in accordance with embodiments of the present disclosure. Process 1100 begins at step 1102 with process 1100 providing a silicon substrate comprising a central membrane section, a first side region laterally adjacent to a first end of the central membrane section, and a second side region laterally adjacent to a second end of the central membrane section, wherein the first and second sideregions are separated from the central membrane section. At step 1104, process 1100 then forms a first waveguide comprising a silicon nitride (SiN) layer over the central membrane section and partially over the first and second side regions of the silicon substrate. Process 1100 then forms a support structure on the first side region of the silicon substrate at step 1106. At step 1108, process 1100 then supports a suspended waveguide structure above the silicon substrate and the first waveguide using the support structure. In embodiments of the disclosure, the suspended waveguide structure may comprise a second waveguide positioned above the central membrane section of the silicon substrate and vertically separated from the first waveguide by an air gap; and a pair of lateral segments positioned over the first and second side regions of the silicon substrate respectively, wherein the second waveguide is laterally separated from each of the lateral segments by a dielectric material. Process 1100 then forms a first piezoelectric actuator stack on the first side region of the silicon substrate and forms a second piezoelectric actuator stack on the second side region of the silicon substrate. This takes place at step 1110. At step 1112, process 1100 then applies a voltage to the first and section piezoelectric actuator stacks to induce deformation of the respective side regions to vertically displace the central membrane section and the first waveguide relative to the second waveguide to adjust an optical coupling ratio between the first and second waveguides.

[0054] In embodiments of the disclosure, the dielectric material may comprise aluminium oxide or silicon dioxide. Additionally, the first and second regions may be separated from the central membrane section by trenches formed in the silicon substrate and the support structure may comprise a silicon dioxide post or pillar. Further, the distance between the two waveguides may be in the range between 0.2 pm and 1 pm.

[0055] In embodiments of the disclosure, the first and second piezoelectric actuator stacks each comprise a platinum-lead zirconate titanate-platinum (Pt / PZT / Pt) stack, a molybdenumaluminium nitride-molybdenum (Mo / AIN / Ao) stack or a molybdenum-scandium-doped aluminium nitride-molybdenum (Mo / ScAlN / Mo) stack.

[0056] In embodiments of the disclosure, process 1100 may form a doped silicon region in at least the first side region or the second side region of the silicon substrate such that the doped silicon region is laterally adjacent to the central membrane section; and then process 1100 may form a contact portion on the doped silicon region, the contact portion being coupled to aresistance measurement sensor such that an electrical resistance of the doped silicon region is measurable to characterize a distance of the air gap.

[0057] Numerous other changes, substitutions, variations, and modifications may be ascertained by the skilled in the art and it is intended that the present application encompass all such changes, substitutions, variations, and modifications as falling within the scope of the appended claims.

Claims

CLAIMS:

1. A vertical optical coupler, comprising:a silicon substrate comprising a central membrane section, a first side region laterally adjacent to a first end of the central membrane section, and a second side region laterally adjacent to a second end of the central membrane section, wherein the first and second side regions are separated from the central membrane section;a first waveguide comprising a silicon nitride (SiN) layer formed over the central membrane section and extending partially over the first and second side regions of the silicon substrate;a support structure formed on the first side region of the silicon substrate, the support structure configured to support a suspended waveguide structure above the silicon substrate and the first waveguide, the suspended waveguide structure comprising:a second waveguide positioned above the central membrane section of the silicon substrate and vertically separated from the first waveguide by an air gap; and a pair of lateral segments positioned over the first and second side regions of the silicon substrate respectively,wherein the second waveguide is laterally separated from each of the lateral segments by a dielectric material;a first piezoelectric actuator stack disposed on the first side region of the silicon substrate and a second piezoelectric actuator stack disposed on the second side region of the silicon substrate,wherein each piezoelectric actuator stack is operative to deform its corresponding side region in response to an applied voltage such that the central membrane section together with the first waveguide are vertically displaced with respect to the second waveguide to tune an optical coupling ratio between the first and second waveguides.

2. The vertical optical coupler according to claim 1 , wherein the dielectric material comprises aluminium oxide, silicon dioxide, aluminium nitride or hafnium oxide.

3. The vertical optical coupler according to claims 1 or 2, wherein the first and second regions are separated from the central membrane section by trenches formed in the silicon substrate.

4. The vertical optical coupler according to any one of claims 1 to 3, wherein the support structure comprises a silicon dioxide post or silicon dioxide pillar.

5. The vertical optical coupler according to any one of claims 1 to 4, wherein a distance of the air gap between the first waveguide and the second waveguide is between 0.20 pm and 1 pm.

6. The vertical optical coupler according to any one of claims 1 to 5, wherein the first and second piezoelectric actuator stacks each comprise a platinum-lead zirconate titanateplatinum (Pt / PZT / Pt) stack, a molybdenum-aluminium nitride-molybdenum (Mo / AIN / Ao) stack or a molybdenum-scandium-doped aluminium nitride-molybdenum (Mo / ScAlN / Mo) stack.

7. The vertical optical coupler according to any one of claims 1 to 6, wherein the second waveguide comprises a material selected from the group consisting of lithium niobate (LiNbOs), germanium (Ge), aluminium nitride (AIN), scandium-doped aluminium nitride (ScAlN), silicon nitride (SiN), silicon (Si), poly-silicon (poly-Si) or amorphous silicon.

8. The vertical optical coupler according to any one of claims 1 to 7, wherein a doped silicon region is formed in at least the first side region or the second side region of the silicon substrate, the doped silicon region being laterally adjacent to the central membrane section, wherein the doped silicon region comprises a contact portion coupled to a resistance measurement sensor, such that an electrical resistance of the doped silicon region is measurable to characterize a distance of the air gap.

9. A method for coupling optical signals between a first waveguide and a second waveguide in a vertical optical coupler, the method comprising:providing a silicon substrate comprising a central membrane section, a first side region laterally adjacent to a first end of the central membrane section, and a second side region laterally adjacent to a second end of the central membrane section, wherein the first and second side regions are separated from the central membrane section;forming a first waveguide comprising a silicon nitride (SiN) layer over the central membrane section and partially over the first and second side regions of the silicon substrate;forming a support structure on the first side region of the silicon substrate;supporting a suspended waveguide structure above the silicon substrate and the first waveguide using the support structure, the suspended waveguide structure comprising:a second waveguide positioned above the central membrane section of the silicon substrate and vertically separated from the first waveguide by an air gap; and a pair of lateral segments positioned over the first and second side regions of the silicon substrate respectively, wherein the second waveguide is laterally separated from each of the lateral segments by a dielectric material;forming a first piezoelectric actuator stack on the first side region of the silicon substrate and forming a second piezoelectric actuator stack on the second side region of the silicon substrate; andapplying a voltage to the first and section piezoelectric actuator stacks to induce deformation of the respective side regions to vertically displace the central membrane section and the first waveguide relative to the second waveguide to adjust an optical coupling ratio between the first and second waveguides.

10. The method according to claim 9, wherein the dielectric material comprises aluminium oxide, silicon dioxide, aluminium nitride or hafnium oxide.

11. The method according to claims 9 or 10, wherein the first and second regions are separated from the central membrane section by trenches formed in the silicon substrate.

12. The method according to any one of claims 9 to 11, wherein the support structure comprises a silicon dioxide post or silicon dioxide pillar.

13. The method according to any one of claims 9 to 12, wherein a distance of the air gap between the first waveguide and the second waveguide is between 0.20 pm and 1 pm.

14. The method according to any one of claims 9 to 13, wherein the first and second piezoelectric actuator stacks each comprise a platinum-lead zirconate titanate-platinum (Pt / PZT / Pt) stack, a molybdenum-aluminium nitride-molybdenum (Mo / AIN / Ao) stack or a molybdenum-scandium-doped aluminium nitride-molybdenum (Mo / ScAlN / Mo) stack.

15. The method according to any one of claims 9 to 14, wherein the second waveguide comprises a material selected from the group consisting of lithium niobate (LiNbOs),germanium (Ge), aluminium nitride (AIN), scandium-doped aluminium nitride (ScAlN) , silicon nitride (SiN), silicon (Si), poly-silicon (poly-Si) or amorphous silicon.

16. The method according to any one of claims 9 to 15, further comprising the step of:forming a doped silicon region in at least the first side region or the second side region of the silicon substrate such that the doped silicon region is laterally adjacent to the central membrane section; andforming a contact portion on the doped silicon region, the contact portion being coupled to a resistance measurement sensor such that an electrical resistance of the doped silicon region is measurable to characterize a distance of the air gap.

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