Hybrid optical device

Abstract

The invention concerns a hybrid optical device combining a silicon-on-insulator (SOI) waveguide platform (1) comprising a silicon waveguide (1a) with a thin-film electro-optical (EO) material such as lithium niobate or barium titanate forming a waveguide structure (2). The EO material waveguide structure (2) is bonded in optical contact with the SOI waveguide (1a), and electrodes (6) on its sides enable refractive index modulation via the electro-optical material Pockels effect. A coupling section (7), preferably comprising a tapered SOI waveguide (7a), transfers light between the SOI and EO structures with low loss. The device may include Bragg gratings (4a) or periodically poled sections (4b) in the EO layer for tunable or wavelength-selective modulation. The structure allows fast, reconfigurable light modulation, filtering, and signal routing on a CMOS-compatible platform, suitable for high-speed communication, optical computing, and sensing, offering a compact and efficient electro-optic integration solution.

Description

[0001] HYBRID OPTICAL DEVICE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to electrically controllable optical devices and methods of manipulating and monitoring properties thereof to introduce, change or monitor the functionality of devices guiding electromagnetic radiation, in particular light.

[0004] BACKGROUND TO THE INVENTION

[0005] It is known that specific materials lack inversion symmetry, such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3), barium titanate (BTO, BaTiO3), potassium dihydrogenphosphate (KH2PO4, KDP), deuterated potassium dihydrogen phosphate (KD2PO4, DKDP, KD*P), specific polymer and glass materials as well as, e.g., electric double layer (EDL) constructions, which can be susceptible to a Pockels effect. With Pockels effect, these electro-optical materials show directionally dependent linear variation in the refractive index, occurring in response to an applied electric field. On the other hand, some specific devices utilize other beneficial properties of these materials, such as birefringence, nonlinear polarization, piezoelectric or electro-optic properties in applications such as optical frequency conversion, polarization rotation, optical switching, optical filtering and optical coupling.

[0006] In particular, there is an emerging interest in utilizing electro-optical materials, such as lithium niobate or barium titanate as a functional material in novel optical devices due to the recent commercial availability of the materials as high-quality thin-films. Using an electro-optical material as an integrated photonic material platform enables tight mode confinement, improving the frequency-mixing efficiency by orders of magnitude while at the same time offering additional degrees of freedom for engineering the optical properties by using approaches such as dispersion engineering. Importantly, the large refractive index contrast of electro-optical materials ,such as BTO and LiNbO3, enables realization of photonic integrated circuits with functionalities based on electro-optical materials on a wafer scale.

[0007] In many applications, several components may need to be cascaded one after another, which may cause undesired losses and deteriorating reflections to the optical signal. For example, waveguide Bragg gratings having periodic surface corrugation are commonly used in optical filtering, and multiple Bragg gratings, designed for different wavelengths, may need to be cascaded. Processing physical surface corrugations to electro- optical materials such as barium titanate or lithium niobate, among others, may also be more challenging due to specific material structural properties. This may cause scattering losses to signals carried by wavelengths that are not intended to be affected by a specific grating or other wavelength-selective structure. Unwanted crosstalk between different channels may also occur. Furthermore, fine-tuning the operation of these physical gratings may be difficult after fabrication.

[0008] PURPOSE OF THE INVENTION

[0009] The purpose of the present invention is to provide a hybrid optical device to be used for coupling, modulating, transmitting and / or detection of light using at least one optical wavelength.

[0010] DESCRIPTION OF THE INVENTION

[0011] The optical device of the invention comprises at least one silicon-on-insulator (SOI) structure, a thin-film electro-optical material waveguide structure, wherein said thin- film structure is bonded or attached in contact or in optical proximity with respect to the SOI waveguide. As a preferred embodiment of the invention, the interface between the SOI waveguide and electro-optical material waveguide structure comprises a coupling section to couple the optical signal from the SOI structure to the electro-optical material and back. The device further comprises electrodes, which are placed at least partially on at least two sides of the electro-optical material waveguide to enable electro-optical index modulation using Pockels effect; and, as preferred embodiment of the invention, a complementary, metal-oxide semiconductor (CMOS) or e.g. gallium-nitride (GaN) application-specific integrated circuit (ASIC) to provide control and drive circuitry for the device.

[0012] In the operation of the device, light is being coupled into at least one guided mode of at least one waveguide formed in and / or on said silicon-on-insulator (SOI) structure, further coupling at least part of the light into a thin-film electro-optical material waveguide structure. Electrodes, being placed at least partially on the sides the waveguide, are used to create an electric field within the electro-optical material and thus enable electro- optical index modulation using Pockels effect. In this connection, the ASIC circuitry is used to drive and control the electrodes and thus the index modulation within the electro- optical material.

[0013] Optical components with tunable optical properties can be particularly useful in devices where it is beneficial to alter the optical functionality of the device. Specific applications where the tunable optical properties could be useful include high-speed light modulators, nonlinear optical devices, optical filters, couplers, mirrors and anti -reflective structures, elements varying the polarization state or phase of an optical field, elements changing the direction of propagation of light, optical cavity or Fabry-Perot etalon structures, optical mode conversion structures and structures coupling an electromagnetic radiation in or out of a device and between two or more devices or sections guiding electromagnetic radiation in at least one wavelength and propagating in at least one electromagnetic and / or optical mode.

[0014] LIST OF FIGURES

[0015] In the following description, the invention is illustrated in detail with reference to the accompanying drawings, where:

[0016] Figure 1 shows a schematic view of the device of the invention with physical Bragg gratings in the modulation region,

[0017] Figure 2 shows a schematic view of the device of the invention with periodically poled / domain inverted sections in the modulation region,

[0018] Figure 3 shows a schematic view of the device of the invention with periodically poled / domain inverted sections in the modulation region and optical signal routing to and from SOI platform,

[0019] Figure 4 shows a schematic top view of the device of the invention with circulated optical signal through multiple gratings structures,

[0020] Figure 5 shows a schematic view of the device of the invention with combination of physical Bragg gratings and periodically poled / domain inverted sections in the modulation region,

[0021] Figure 6 shows a perspective view of the device of the invention with emphasis on the coupling region and tapered silicon waveguide, and

[0022] Figure 7 shows an embodiment of the device of the invention with exemplary in- put / output-coupling and multiple modulation regions.

[0023] DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention provides a solution to efficient and compact manipulation of in- and out-coupling of light into and out of at least one waveguide of a hybrid optical device by using a waveguide structure with high effective index and high level of established process technology, such as silicon-on-insulator (SOI) platform 1. At least part of the light 10 comprising at least one optical wavelength may be coupled to at least one waveguide positioned in proximity or in contact with the SOI platform, such waveguide exhibiting electrically controllable modulation characteristics for the in-coupled light in terms of altered reflection, transmission, amplification or absorption, which may be substantially wavelength- selective.

[0025] In order to achieve the aforementioned objective, the device of the present invention comprises a silicon-on-insulator (SOI) platform 1, wherein said SOI-platform 1 may have silicon device layer thickness over 1 pm, such as 3 pm, wherein the platform is considered as thick-SOI platform that supports larger mode sizes (easier fiber coupling), lower propagation losses and high power levels. On the other hand, said SOI-platform 1 may comprise thin, a 200-500 nm, most commonly 220 nm, silicon device layer, wherein such device is considered as nano-SOI, supporting for example submicron waveguides for compact photonic circuits. The SOI platform 1 further comprises waveguide structures la for guiding the light internal to the platform and other elements for manipulating the optical signal, such as multiplexer and / or demultiplexer structures. In order to couple the optical signals 10 into and out the SOI platform, it comprises coupling elements 11 wherein the coupling elements 11 may comprise fiber edge coupling (butt coupling) elements, where an optical fiber is directly aligned with the SOI chip’s waveguide facet; grating couplers, where a periodic grating is formed on the SOI surface and fiber is placed on top of the grating structure to diffractively couple the light in SOI structure; vertical mirrors, such as etched or deposited 45° mirrors; or prisms or total internal reflection (TIR) couplers.

[0026] The device further comprises a thin-film electro-optical material waveguide structure 2, wherein said electro-optical thin-film structure 2 is bonded or attached in contact or in optical proximity with respect to the SOI platform 1. The electro-optical material waveguide structure 2 comprises at least one waveguide 2a, which may further comprise a ridge or slot waveguide, an ion and / or dopant diffused or implanted waveguide, an annealed proton-exchange waveguide, titanium-diffused waveguide, a proton exchange waveguide and / or an epitaxial thin-film waveguide. The waveguide 2a may further be formed as slab waveguide as a planar thin film, and / or as hybrid structure, wherein thin- film electro-optical material, such as BTO, is bonded or deposited on SOI platform 1 and silicon waveguide is formed on SOI providing the mode confinement and electro- optical material provides the active refractive index -changing properties. Here, the light may be coupled into at least one guided mode of at least one waveguide la formed in / on / under at least one SOI platform 1 and, further coupling at least part of the light into a waveguide structure 2 comprising electro-optical material exhibiting Pockels effect, bonded or attached in contact or in optical proximity with respect to the SOI platform 1 waveguide la or a similar waveguide.

[0027] As a preferred embodiment of the invention, the electro-optical material waveguide structure further comprises a physical Bragg grating 4a and / or multiple poled or otherwise domain inverted sections 4b in order to achieve a periodically alternating and tunable refractive index within said waveguide 2a to create a modulation region 4.

[0028] The device of the present invention further comprises electrodes 6 for controlling the electric field within the electro-optical material. Said electrodes 6 are arranged at least partially on at least two sides of the electro-optical material waveguide structure 2 to enable electro-optical index modulation using Pockels effect within said electro-optical material waveguide structure 2. As a preferred embodiment of the invention, the device further comprises a metal-oxide semiconductor (CMOS) or e.g. gallium-nitride (GaN) application-specific integrated circuit (ASIC) to provide control and drive circuitry for the device, wherein said ASIC is arranged preferably in connection with the SOI platform 1, for example under it, and electrically connected to the SOI platform 1 via electrically conductive connection elements, such as a ball grid array, wire bonds, thru-sili- con vias, or similar connection elements providing electrical connectivity between the ASIC and SOI platform 1. As a preferred embodiment of the invention, the ASIC is arranged in physical separation with the SOI platform 1, wherein both the ASIC and the SOI platform 1 are arranged on an interposer board next to each other for electrical signal distribution using corresponding electrically conductive connection elements.

[0029] As a preferred embodiment of the invention, the device further comprises a coupling section 7 arranged in the interface of the SOI platform 1 and the electro-optical material waveguide structure 2. Here, said coupling section 7 comprises a tapered waveguide section 7a arranged in the SOI platform 1 waveguide la to induce adiabatic coupling to the waveguide 2a of the electro-optical material. The tapering section 7a of the waveguide la is arranged in connection, for example below, the electro-optical material waveguide structure 2, to couple the optical signal from the SOI platform 1 to the electro-optical material waveguide structure 2 and back. As a preferred embodiment of the invention, the tapering section 7a of the waveguide la in SOI platform 1 is aligned with the waveguide structure 2a of the electro-optical material in horizontal direction, wherein in lengthwise direction the tapering section 7a is aligned in such a way that the adiabatic coupling into the electro-optical material is achieved before the light propagates to the modulation region 4.

[0030] As a preferred embodiment of the invention, but not specifically depicted, the device further comprises V-grooves formed with e.g. etching on the SOI platform 1, and / or V- groove array aligned next to the device to couple light into and / or out of the device from a waveguide edge. As a further preferred embodiment of the invention, the device further comprises an etched, substantially 45-deg (or other angle, such as wet-etched crystal plane) mirror or a grating coupler to couple light in and / or our from top / bottom of the device, or any combination of the before mentioned / a derivative of them.

[0031] According to an aspect of the present invention, optical coupling can be arranged to operate with more than one transverse mode, wavelength and / or polarization state simultaneously, utilizing the birefringence of the (electro-optical material) waveguide(s) where applicable. Additionally, when the Pockels effect of at least one waveguide comprising Pockels active material is used to modulate or change the effective index of at least one propagating transverse mode, and when an electrical field for the modulation is applied substantially perpendicular to the plane overlapping at least one wave pattern across at least one waveguide cross section, the spatial variation of mode antinodes(s) can be used to address specific transverse mode(s) at specific location(s).

[0032] According to an aspect of the present invention, data rate can be increased by means of longitudinal phase modulation using interference of more than one wavelength and / or transverse mode. In this case, the intensity of the light incident on the detector side of the optical transmission is modulated longitudinally with respect to more than one detector cell / unit by means of interference of more than one wavelength and / or transverse mode. The modulation of the interference pattern may be achieved by altering the optical path length for a transmission line of at least one wavelength and / or transverse mode. In addition, polarization rotation at one or both ends of the transmission line may be used to introduce additional symbol(s) to facilitate the data transfer.

[0033] According to an aspect of the present invention, integrated neuroevolution / neuro-ge- netic algorithm(s) may be used for generation or optimization of concurrent self-opti- mized transmission method and protocol. Good-known, but not necessarily optimum “root hints” for generic protocols may be used as a starting point for optimization - and as a fallback in case the integrated neuroevolution / neuro-genetic optimization results in unoptimized performance with compared to the generic solution. An optimized transmission method and protocol may comprise, e.g., varying number of amplitude, polarization, wavelength, and phase states between at least one transmission channel(s) applied in the system. There may also be unused / spare transmission channel(s) to be used for redundancy. The number of channels in use or number of amplitudes, polarization, wavelength, and phase states may also vary depending on e.g. thermal conditions. As a preferred embodiment of the invention, the device further comprises at least one optical detector, such as a Germanium (Ge) detector, wherein, as a further embodiment of the invention, the detector may be located on a CMOS or similar surface arranged in connection with the electro-optical material waveguide structure 2 for minimum number of signal vias to be introduced through the electro-optical material layer or other waveguide / slab material(s).

[0034] According to another aspect of the present invention, the device further comprises a 45- deg rotated polarization maintaining (PM) fiber to a SOI waveguide to apply edge coupling with enlarged mode size in vertical direction, or any other waveguide with substantially similar mode size(s) in both vertical and horizontal direction. In addition, polarization-dependent routing and rotation for guiding light in fiber using both polarizations may be used to increase the density of data transmission with respect to the applied transmission line(s).

[0035] According to another aspect of the present invention, the device further comprises an end-coupling or, e.g., a beam-folding mirror or a grating coupler to couple substantially coherent light comprising at least one optical wavelength , into the SOI platform 1 as a first element comprising first waveguide, wherein the first element may further comprise an asymmetric branch coupler. Therein, the light 10 propagating in the first waveguide is then substantially coupled to an adjacent second waveguide 2a in a electro-optical material as the second element, wherein the second element may further comprise at least one wavelength- and / or mode-selective reflective grating structure 8. The said wavelength- and / or mode-selective reflective grating structure 8 may be mode-converting and / or be induced via Pockels effect by means of, e.g., an applied electric field. The second waveguide may further comprise one or more of such wavelengths- and / or mode-selective reflective grating structures 8, in addition to structures exhibiting other optical functionality. The residual light further propagating in the second waveguide 2a may then be coupled back to a waveguide la in the first element and rotated substantially 180 degrees to reverse the direction of propagation and coupled to a third waveguide in the second element, parallel to the second waveguide 2a. This third waveguide may comprise similar functionality as the second waveguide 2a, but preferably with other operating wavelength(s). The process of coupling light back and forth between the first and two elements and the waveguides the two elements can be repeated more than one time in order to form a functional structure where the light may propagate multiple times through one or two of the elements.

[0036] According to one aspect of the present invention and referring in particular to Figure 7 a Silicon-Photonics Circuit, such as the SOI platform 1 is used for coupling light in the heterogeneously integrated device, such as the electro-optical material waveguide structure 2. The input coupler 9 may be a conventional edge coupler or a surface coupler, e.g. based on total internal reflection (TIR). The input waveguide la in the Silicon-on-insulator platform 1 is preferably single-mode and it may be tapered to improve light coupling between the waveguide and an optical fiber. The input waveguide la is followed by an adiabatic asymmetric coupling section 7, which is also in the SOI platform 1. The input waveguide la and the adiabatic asymmetric coupling section 7 locate in the SOI platform 1, after which the vertical coupling section 7 is employed to “lift” light from the SOI platform 1 to the electro-optical material waveguide structure 2. In this example, the electro-optical material waveguide structure 2 comprises one or more tunable physical Bragg gratings 4a and / or multiple poled or otherwise domain inverted sections 4b operating at one or more wavelengths and reflecting light as needed. In other words, physical Bragg gratings 4a and / or multiple poled or otherwise domain inverted sections 4b are modulating light reflection. After reflection, light is coupled back to the SOI platform 1 and further to the output port in the SOI platform 1. The output coupler 12 in the SOI platform 1 may be similar as the input coupler 9.

[0037] Note that in the abovementioned example the passive waveguide devices locate in the SOI platform 1 and only the active mode-converting tilted Bragg gratings 4a and / or multiple poled or otherwise domain inverted sections 4b locate in the electro-optical material waveguide structure 2. In comparison, all the sub-components may be mono- lithically integrated in a electro-optical material waveguide structure 2. Furthermore, in this particular device both the electro-optical material waveguide structure 2, the vertical coupling section 7 between Si and -electro-optical material waveguide structure 2 and the Silicon waveguide la at the common end of the adiabatic asymmetric coupler are designed to support two lateral modes.

Claims

Claims1. A hybrid optical device, comprising a silicon-on-insulator (SOI) platform (1) comprising at least one waveguide (la); a thin-film electro-optical material waveguide structure (2); and electrodes (6) arranged at least partially on at least two sides of the electro-optical material waveguide structure (2) for enabling electro-optical index modulation via the Pockels effect, characterized in that the electro-optical material waveguide structure (2) is bonded or attached in contact or in optical proximity with the SOI platform (1), the device further comprises a coupling section (7) configured to couple at least part of light (10) guided in the SOI waveguide (la) into the electro-optical material waveguide structure (2) and back.

2. The hybrid optical device of claim 1, characterized in that the coupling section (7) comprises a tapered waveguide (7a) arranged in the SOI platform (1) to achieve adiabatic coupling to the electro-optical material waveguide structure (2).

3. The hybrid optical device of claims 1 or 2, characterized in that the SOI platform (1) is a thick-SOI platform having a silicon device layer thickness greater than 1 pm, preferably substantially 3 pm.

4. The hybrid optical device of any one of the preceding claims 1 - 3, characterized in that the device further comprises an application-specific integrated circuit (ASIC), such as a CMOS or GaN ASIC, electrically connected to the SOI platform (1) for providing control and drive circuitry to the electrodes (6).

5. The hybrid optical device of any one of the preceding claims 1 - 4, characterized in that the electro-optical material waveguide structure (2) comprises at least one of: a ridge waveguide, an ion-diffused or dopant-diffused waveguide, an annealed proton-exchange waveguide, a titanium-diffused waveguide, a proton-exchange waveguide, an epitaxial thin-film waveguide, or a slab waveguide.

6. The hybrid optical device of any one of the preceding claims 1 - 5, characterized in that the electro-optical material waveguide structure (2) further comprises a physical Bragg grating (4a) and / or multiple poled or domain-inverted sections (4b) to create a periodically alternating and tunable refractive index.

7. The hybrid optical device of any one of the preceding claims 1 - 6, characterized in that the SOI platform (1) further comprises coupling elements (11) selected from: fiber edge couplers, grating couplers, etched or deposited mirrors, prisms, total internal reflection couplers, or V-groove couplers.

8. The hybrid optical device of any one of the preceding claims 1 - 7, characterized in that optical coupling is arranged to operate with more than one transverse mode, wavelength and / or polarization state simultaneously.

9. The hybrid optical device of any one of the preceding claims 1 - 8, characterized in that the electro-optical material of the waveguide structure (2) comprises lithium niobate, barium titanate, lithium tantalate, potassium dihydrogen phosphate, or a derivative thereof.

10. The hybrid optical device of any one of the preceding claims 1 - 9, characterized in that the device further comprises at least one optical detector, such as a germanium detector, located on a CMOS or similar surface in connection with the electro-optical material waveguide structure (2).

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