WO2025229142A1

WO2025229142A1 - Optical grating coupler

Info

Publication number: WO2025229142A1
Application number: PCT/EP2025/062005
Authority: WO
Inventors: Phoebe TENGDIN; Stephan Gamper; Guilhem ALMA
Original assignee: Miraex SA
Current assignee: Miraex SA
Priority date: 2024-05-02
Filing date: 2025-05-01
Publication date: 2025-11-06
Anticipated expiration: 2026-11-02

Abstract

The present invention relates to an optical grating coupler (10) configured to couple at least two waveguides and allow the propagation of optical beams between them. More specifically, the present invention discloses an efficient and high-performance optical grating coupler (10) designed to propagate light from a first waveguide, preferably a fiber waveguide, into a second waveguide, preferably a nanophotonic waveguide, while maintaining exceptional efficiency.

Description

OPTICAL GRATING COUPLER

[01] The present invention relates to photonics technologies and, more specifically, to an optical grating coupler.

[02] Thin film lithium niobate on insulator (LNOI) is a promising platform for the commercialisation of electro-optical and nonlinear optical devices, including electro-optical modulators, tuneable add/drop filters, second harmonic generation, parametric amplification, and quantum applications such as squeezed state generation and coherent frequency conversion. The recent advancements in etching techniques on this material have enabled the printing of integrated circuits with nanometre-scale dimensions, allowing engineers to exploit the electro-optical and nonlinear properties of LNOI in unprecedented ways. However, one significant challenge remains the high optical losses typically incurred when coupling light on/off the chip into single-mode fibres (SMFs).

[03] Coupling is usually done from an SMF with a Gaussian mode size of ~10 pm into a guided chip mode, which generally has a modal extent of a few 100 nanometres. Due to the limitations of lithographic processes, increasing the mode size of the light on the chip is challenging without resorting to extreme tapers. End-fire coupling involves adapting the mode size using the aforementioned tapers and directly coupling the fibre to the chip from the side. However, this approach has drawbacks, such as limited accessibility to structures in the centre of the chip and demanding alignment tolerances between the edge of the chip and the tip of the fibre.

[04] An alternative approach to coupling light into nanophotonic waveguides is through grating couplers. In this situation, light is coupled into a vertical direction upon exiting the waveguide by an array of scattering units. Current state-of-the-art integrated LNOI grating couplers are mostly limited to uniform, single etch structures that can weakly couple light from a waveguide into a fibre but experience high losses due to scattering in both upward and downward directions. These components typically provide a coupling efficiency of 5-12 dB/facet, which is sufficient for wafer-level testing but limits the device's performance in demanding applications.

[05] CN117031626A relates to a method for optimizing the structure of integrated chirped blazed grating couplers using inverse design techniques. However, the use of the Finite-Difference Time-Domain (FDTD) method at high temporal and spatial resolution, combined with a particle swarm optimization (PSO) algorithm applied to a complex, multi-parameter system (i.e. grating period, etching depth, groove bottom width, optical fiber spatial position, waveguide layer material and transmission wavelength), results in a simulation process that is highly computationally intensive and time-consuming, which moreover may not converge on the most optimal geometrical configuration for the highest coupling efficiency.

[06] It is, therefore, an objective of the present invention to overcome the disadvantages mentioned above of the prior art, at least in part.

[07] Objects of the invention have been achieved by providing an optical grating coupler and a method for making at least one optical grating coupler according to the independent claims. Dependent claims set forth various advantageous embodiments of the invention. [08] Disclosed herein is an optical grating coupler configured to couple at least a first optical waveguide with at least a second optical waveguide, the optical grating coupler being configured to propagate at least one optical beam from said first optical waveguide to said second optical waveguide.

[09] The optical grating coupler comprises:

- A plurality of scattering units being configured to propagate said optical beam from a first direction of propagation to a second direction of propagation, each scattering unit of said plurality of scattering units being aligned in said second direction of propagation, said second direction of propagation being the main direction of propagation of said optical beam within the optical grating coupler;

- A first layer, said first layer comprising a first plane supporting said plurality of scattering units;

[10] Each scattering unit of said plurality of scattering units comprises:

- a structure comprising lithium niobate configured to generate constructive interferences from said optical beam, said structure being repeated a predetermined number of times T according to a predetermined period P along said second direction of propagation, said structure comprising, along said second direction of propagation:

- a first sloped sidewall comprising a dimension of extension, the dimension of extension of said first sloped sidewall forming a first angle Al with said first plane;

- a first stage comprising a dimension of extension DI extending in said second direction of propagation, said first stage being located above said first plane and below said second plane;

- a second sloped sidewall comprising a dimension of extension, the dimension of extension of said second sloped sidewall forming a second angle A2 with said first plane;

- a second stage comprising a dimension of extension D2 extending in said second direction of propagation, said second stage being located above said first plane and below said second plane;

- a third sloped sidewall comprising a dimension of extension, the dimension of extension of said third sloped sidewall forming a third angle A3 with said first plane;

[11] Said first layer comprises a material having a refractive index lower than or equal to the refractive index of the material from which the structure is at least partially made.

[12] The first sloped sidewall extends from the first plane to the first stage, the second sloped sidewall extends from the first stage to the second stage, and, the third sloped sidewall extends to the second stage to the first plane.

[13] The ratio R1 between the height Hl of said first stage regarding said first plane and the height H2 of said second stage regarding said first plane is comprised between 4 and 2, preferably between 3 and 2, and advantageously between 2.5 and 2.

[14] The ratio R2 between the dimension of extension DI of said first stage and the dimension of extension D2 of said second stage is comprised between 0.6 and 0.97, preferably between 0.7 and 0.9, advantageously between 0.80 and 0.90, and more advantageously substantially equals to 0.83. [15] In an embodiment, the first angle Al is comprised between 75 degrees and 80 degrees, the ratio R2 is comprised between 0.8 and 0.9, and the ratio R1 is comprised between 2.5 and 2 such that when the first angle Al and the ratio R2 increase, the ratio R1 decrease to achieve optimal constructive interference on a coupling side of said scattering unit.

[16] In an embodiment, the optical grating coupler further comprises a second layer, said second layer comprising a second plane configured to receive said optical beam from said first optical waveguide, said second layer being on top of or flush with said plurality of scattering units.

[17] In an embodiment, said first angle Al may advantageously be equal to said second angle A2.

[18] In an embodiment, said third angle A3 may advantageously be equal to said second angle A2.

[19] The present invention provides several advantages over existing prior arts. By utilising a double etching scheme for deflecting the scattered beam into a single direction, the present invention allows a maximum efficiency of -1.97dB for TE polarisation at a wavelength of 1550 nm. The present invention has been optimised for a uniform arrangement of scattering units and shows great promise for future improvements through spatial adaptation of the mode, via non-uniform arrangement of scattering elements of the plurality of scattering elements.

[20] The present invention has been designed to achieve high-performance optical devices that offer exceptional directionality while maintaining efficient operation under various conditions.

[21] The efficiency of the present invention is critical in the context of optical communications systems, where minimising signal loss and maximising transmission capacity are crucial considerations. The present invention allows the creation of high-performance optical devices that have the potential to significantly enhance the capabilities of various applications, including fibre optic communication networks, data centres, quantum optical applications, and other advanced optical systems.

[22] The present invention can operate with a wide range of wavelengths, as different applications may require different operating wavelengths. The present invention leverages the unique properties of lithium niobate while mitigating its limitations to achieve high-performance optical devices with improved efficiency across a wide range of wavelengths.

[23] The present invention also relates to an optical device comprising at least one optical grating coupler according to the present invention, a system comprising at least one optical device according to the present invention, and a network comprising at least one system according to the present invention.

[24] Also disclosed herein is a method for making at least one optical grating coupler, wherein said structure comprises a first part and a second part, said first part comprising said first stage and said second part comprising said second stage, the method comprising: realizing said first part using a first lithographic process and realizing said second part using a second lithographic process, and wherein the first lithographic process comprises a first set of etching parameters EPl and the second lithographic process comprised a second set of etching parameters [25] In an advantageous embodiment, at least one value of at least one parameter of EP 1 is different from at least another value of at least one parameter of EP2

[26] Also disclosed herein is a method for making at least one optical grating coupler, said method comprising the following steps: a. Providing a first stack comprising at least: i. The first layer on top of a substrate, the first layer comprising a buried oxide (BOX); ii. A thin film Lithium Niobate (TFLN) on top of said first layer; iii. A hard mask on top said thin film Lithium Niobate; iv. Optionally, a buffer layer located between the thin film Lithium Niobate and the hard mask; b. Putting a resist layer on top of said hard mask; c. Selecting one process among the three following processes: i. First process:

• Forming at least the following elements: a. The first stage; b. A first part of the first sloped sidewall; c. The second sloped sidewall; using a lithographic process and an etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

• Forming the following elements: a. The second stage; b. The last part of the first sloped sidewall; c. The third sloped sidewall; using another lithographic process and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching; ii. Second process:

• Forming at least the following elements: a. At least partially the first sloped sidewall; b. At least partially the third sloped sidewall; using a lithographic process and an etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

• Forming the second stage and the second sloped sidewall by transferring said first sloped sidewall and said third sloped sidewall using another lithographic process and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching; iii. Third process:

• Removing partially the hard mask to keep the first stage protected and to expose the rest of the first stack by a lithographic process and an etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

• Forming at least partially the first sloped sidewall and the third sloped sidewall by another lithographic process and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching;

• Forming, after removal of at least one resist layer used for Deep UV and/or e-beam lithography, the second stage and the second sloped sidewall by another etching process, said another etching process comprising at least one etching.

For embodiments including the second layer, the method may further include the step of depositing the second layer at least on top of the first sloped sidewall, the first stage, the second sloped sidewall, the second stage, and the third sloped sidewall.

[27] Some optional characteristics that may be used in association or alternatively with embodiments of the invention include:

> According to an example, said second layer comprises silicon oxide and/or any other material having a refractive index lower than the refractive index of the material of the second optical waveguide.

> According to an example, the height Hl of said first stage regarding said first plane is comprised between 200nm and 900nm, preferably between 250nm and 600nm, and advantageously between 300nm and 500nm.

> According to an example, the height H2 of said second stage regarding said first plane is comprised between 50nm and 450nm, preferably between 125nm and 300nm, and advantageously between 150nm and 250nm.

> According to an example, the dimension of extension DI of said first stage is comprised between lOOnm and 400nm, preferably between 200nm and 300nm, and advantageously between 240nm and 280nm.

> According to an example, the dimension of extension D2 of said second stage is comprised between lOOnm and 600nm, preferably between 220nm and 430nm, and advantageously between 275nm and 350nm.

> According to an example, the first angle Al is comprised between 55 degrees and 89 degrees, preferably between 70 degrees and 82 degrees, and advantageously between 75 degrees and 80 degrees.

> According to an example, the second angle A2 is comprised between 55 degrees and 89 degrees, preferably between 70 degrees and 82 degrees, and advantageously between 75 degrees and 80 degrees.

> According to an example, the third angle A3 is comprised between 55 degrees and 89 degrees, preferably between 70 degrees and 82 degrees, and advantageously between 75 degrees and 80 degrees. > According to an example, said first stage is parallel to said second plane.

> According to an example, said first stage is parallel to said first plane.

> According to an example, said second stage is parallel to said second plane.

> According to an example, said second stage is parallel to said first plane.

> According to an example, the dimension of extension DI of said first stage is parallel to said second plane.

> According to an example, the dimension of extension DI of said first stage is parallel to said first plane.

> According to an example, the dimension of extension D2 of said second stage is parallel to said second plane.

> According to an example, the dimension of extension D2 of said second stage is parallel to said first plane.

> According to an example, said first stage is parallel to said second stage.

> According to an example, the dimension of extension DI of said first stage is parallel to the dimension of extension D2 of said second stage.

> According to an example, the first optical waveguide comprises a first refractive index contrast and the second optical waveguide comprises a second refractive index contrast, said first refractive index contrast being lower than said second refractive index contrast.

> According to an example, the first optical waveguide comprises a first confined mode and the second optical waveguide comprises a second confined mode, said first confined mode being wider than said second confined mode.

> According to an example, said structure comprises silicon nitride.

> According to an example, said first layer comprises silicon oxide and/or silicon and/or any metals, preferably gold and/or lithium niobate and/or aluminium oxide.

> According to an example, the predetermined number of time T is higher than 5, preferably than 30, and advantageously than 100.

> According to an example, the predetermined period P is comprised between 700nm and 1400nm, preferably between 800nm and 1200nm and advantageously between 900nm and HOOnm.

> According to an example, the distance D3 between two consecutive scattering units of said plurality of scattering units is comprised between 200nm and 400nm, preferably between 250nm and 350nm, and advantageously between 290nm and 300nm.

> According to an example, said structure comprises a first part and a second part, said first part comprising said first stage and said second part comprising said second stage, and said first part is realized using a first lithographic process and said second part is realized using a second lithographic process.

> According to an example, the size of the repeated structures is adapted to increase the reflection on the side closer to the second waveguide, and decrease reflection on the side further from the second waveguide.

> According to an example, the predetermined period P of at least a portion of the plurality of scattering units is progressively smaller closer to said second optical waveguide. > According to an example, said first layer comprises an additional layer, said additional layer supporting said plurality of scattering units.

> According to an example, said additional layer comprises lithium niobate and/or any metals, preferably gold and/or aluminium oxide.

> According to an example, said additional layer has a thickness H3 comprised between 80nm and 120nm, preferably between 90nm and HOnm and advantageously between lOOnm and 108nm.

> According to an example, said second layer is at least partially in physical contact with said plurality of scattering units.

> According to an example, said second layer is configured to be all around and on top of said plurality of scattering units.

> According to an example, said second layer is coating at least partially said plurality of scattering units.

> According to an example, said second layer is coating said plurality of scattering units.

> According to an embodiment, said plurality of scattering units is coated at least partially by said second layer.

> According to an embodiment, said plurality of scattering units is coated by said second layer.

> According to an example, said first layer is at least partially in physical contact with said plurality of scattering units.

> According to an example, said additional layer is at least partially in physical contact with said plurality of scattering units.

> According to an example, the first lithographic process is configured to use a first lithographic mask and the second lithographic process is configured to use a second lithographic mask, said first lithographic mask being different from said second lithographic mask.

> According to an example, the first lithographic process is configured to use a first exposure and the second lithographic process is configured to use a second exposure, said first exposure being different from said second exposure.

> According to an example, the first lithographic process comprises a first etching time El and the second lithographic process comprises a second etching time E2, and wherein said El is lower or equal than E2.

> According to an example, the first lithographic process comprises a first etching time El and the second lithographic process comprised a second etching time E2, and wherein said E2 is higher or equal than El.

> According to an example, the first lithographic process comprises a first set of etching parameters EP 1 and the second lithographic process comprises a second set of etching parameters EP2, and at least one value of at least one parameter of EPl is different from at least another value of at least one parameter of EP2.

> According to an example, the first optical waveguide comprises at least one optical fibre, preferably said optical fibre comprising a mode field diameter between 5pm and 200pm, preferably 10pm and 100pm, advantageously 10pm and 30pm. > According to an example, the structure of the scattering units comprises curved grating lines.

> According to an example, the structure of the scattering units comprises curved grating lines preferably in a plane parallel to said second layer and/or to said first layer.

> According to an example, the structure of the scattering units is curved in a plane parallel to said second layer.

> According to an example, the structure of the scattering units is curved in a plane parallel to said first layer.

[28] Additional and/or alternative features, aspects and advantages of implementations of the present invention will become apparent from the following description, the accompanying drawings and the appended claims.

[29] For a better understanding of the present invention, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where:

FIG. la illustrates an overview of an optical grating coupler according to an embodiment of the present invention;

FIG. lb schematically illustrates an optical grating coupler according to an embodiment of the present invention; FIG. 1c schematically illustrates an optical grating coupler according to another embodiment of the present invention;

FIG. 2 schematically illustrates a plot of an exponential function and a gaussian function;

FIG. 3a schematically illustrates constructive and destructive interference using a grating coupler with scattering structures according to an embodiment of the invention;

FIG. 3b illustrates plots of the scattering structure dimension DI, D2 dependence on the process angles Al, A2, A3 defining the sloped sidewalls and a plot of the ratio R2 as a function of the process angles Al, A2, A3, which are set as being equal in this example, for a grating coupler according to embodiments of the invention, to achieve optimal constructive interference on a coupling side and destructive interference on the opposite side of the grating coupler;

FIG 3c illustrates plots of the ratio R1 as a function of the process angles Al, A2, A3, defining the sloped sidewalls, which are set as being equal in this example, for a grating coupler according to embodiments of the invention, to achieve optimal constructive interference on a coupling side and destructive interference on the opposite side of the grating coupler;

FIG 3d illustrates (top) a plot of coupling efficiencies for process angle Al ranging from 66 degrees to 85 degrees in steps of 3.6 degrees, shown as a function of the ratio R1 and (bottom) a plot of coupling efficiencies for process angle Al ranging from 72 degrees to 82 degrees in steps of 1.2 degrees, shown as a function of the ratio R2, the process angles Al, A2, A3, being set as being equal as well, for a grating coupler according to embodiments of the invention, to achieve optimal constructive interference on a coupling side and destructive interference on the opposite side of the grating couple; FIG. 4a illustrates a simulation of the present invention according to an embodiment of the present invention; FIG. 4b schematically illustrates a top view of an optical grating coupler according to another embodiment of the present invention;

FIG. 5a illustrates a simulation of the fractional amount of light coupled into the second optical waveguide as a function of the angle of the first optical waveguide according to an embodiment of the present invention;

FIG. 5b illustrates a simulation of the fractional amount of light coupled into the second waveguide from the first waveguide as a function of the wavelength of the light according to an embodiment of the present invention; FIG. 6 illustrates a simulation of the coupling efficiency as a function of the first optical waveguide angle for transverse magnetic (TM) mode according to an embodiment of the present invention.

FIG. 7a to 7d schematically illustrate some steps of a first method for making at least one optical grating coupler according to an embodiment of the present invention.

FIG. 8a to 8d schematically illustrate some steps of a second method to make at least one optical grating coupler according to an embodiment of the present invention.

FIG. 9a to 9d schematically illustrate some steps of a third method to make at least one optical grating coupler according to an embodiment of the present invention.

[30] The examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the present invention and not to limit its scope to such specifically recited examples and conditions. It will be appreciated that those skilled in the art may devise various arrangements which, although not explicitly described or shown herein, nonetheless embody the principles of the present invention and are included within its spirit and scope.

[31] Furthermore, as an aid to understanding, the following description may describe relatively simplified implementations of the present invention. As persons skilled in the art would understand, various implementations of the present invention may be of a greater complexity.

[32] In some cases, what are believed to be helpful examples of modifications to the present invention may also be set forth. This is done merely as an aid to understanding, and, again, not to define the scope or set forth the bounds of the present invention. These modifications are not an exhaustive list, and a person skilled in the art may make other modifications while nonetheless remaining within the scope of the present invention. Further, where no examples of modifications have been set forth, it should not be interpreted that no modifications are possible and/or that what is described is the sole manner of implementing that element of the present invention.

[33] Moreover, all statements herein reciting principles, aspects, and implementations of the present invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof, whether they are currently known or developed in the future.

[34] With these fundamentals in place, we will now consider some non-limiting examples to illustrate various implementations of aspects of the present invention. [35] In the context of the present invention, the word “sloped” may refer to a surface that deviates from a perpendicular orientation (90 degrees) or an in-plane orientation (180 or 0 degree) relative to the base of a structure. The angle of the deviation can be specified in degrees or radians. In particular, a sloped sidewall can have a uniform or non-uniform inclination. A sloped sidewall can also comprise one or multiple sloped segments. The slope of a sloped sidewall can be linear or curved.

[36] In the context of the present invention, the expression “in physical contact with” means that two surfaces or elements are touching with no intervening material. The expression “at least partially in physical contact with” means that two surfaces or elements are touching optionally with intervening material.

[37] The present invention relates to an optical grating coupler and its applications in optical devices.

[38] According to an embodiment, Figures 1, 2 and 3 are illustrations of cross-sectional views of an optical grating coupler 10 according to various embodiments of the present invention. These figures provide labels for the optimised dimensions of an optical grating coupler 10 according to an embodiment of the present invention.

[39] In the present invention, the word “optical” can refer to phenomena and technologies related to the generation, manipulation, transmission, and detection of electromagnetic waves in the broad spectral range spanning from the far-infrared (approximately 100 terahertz or THz) to the ultraviolet (approximately 10 nanometers or nm). This encompasses various regions of the electromagnetic spectrum; in this context, the word “scattering” refers to the redirection of an optical beam due to collisions with structures in a medium.

[40] The present invention relates to an optical grating coupler 10 configured to couple at least two waveguides and allow propagation of optical beams between them. More specifically, the present invention discloses an efficient and high-performance optical grating coupler 10 designed to propagate light from a first optical waveguide 80, preferably a fiber waveguide, into a second optical waveguide 90, preferably a nanophotonic waveguide, advantageously by changing the direction of propagation of at least one optical beam while maintaining exceptional efficiency.

[41] According to an embodiment, an optical beam 51 coming from the first optical waveguide 80 and propagating according to a first direction of propagation 50 is then propagated according to a second direction of propagation 60 inside the optical grating coupler 10, thanks to scattering units 100. Then, said optical beam is directed into the second optical waveguide 90 by said optical grating coupler 10. Then, inside said second optical waveguide 80, the optical beam 71 is propagated according to a third direction of propagation 70.

[42] According to an embodiment, the angle between the first direction of propagation 50 and the normal to the first layer or second layer, preferably to the first plane or second plane, is comprised between -30 degrees and 30 degrees, preferably between -15 degrees and -2 degrees or between 2 degrees and 15 degrees and advantageously between -10 degrees and -4 degrees or between 4 degrees and 10 degrees.

[43] According to an embodiment, the angle between the first direction of propagation 50 and the third direction of propagation 70 is comprised between 60 degrees and 120 degrees, preferably between 75 degrees and 88 degrees and advantageously between 80 degrees and 86 degrees. [44] According to an embodiment, the angle between the first direction of propagation 50 and the third direction of propagation 70 is comprised between 60 degrees and 120 degrees, preferably between 92 degrees and 105 degrees and advantageously between 94 degrees and 100 degrees.

[45] According to an embodiment, the angle between the first direction of propagation 50 and the second direction of propagation 60 is comprised between 60 degrees and 120 degrees, preferably between 75 degrees and 88 degrees and advantageously between 80 degrees and 86 degrees.

[46] According to an embodiment, the angle between the first direction of propagation 50 and the second direction of propagation 60 is comprised between 60 degrees and 120 degrees, preferably between 92 degrees and 105 degrees and advantageously between 94 degrees and 100 degrees.

[47] this angle satisfies the lowest order Bragg condition, which relates the grating’s pitch p to the effective index n_e/f of the grating:

Where is the wavelength of the radiation, n_ciadding is the effective index of the top cladding layer, i.e. the second layer 20, and 0 is the angle of the first waveguide.

[48] According to an embodiment, the present invention is configured to use a Gaussian beam with a core diameter of 20 um instead of the standard 10 um core diameter for single-mode fibres (SMFs) at the operating wavelength of 1550 nm, which enhances the efficiency of the grating couplers. According to an embodiment, the expanded core diameter of the Gaussian beam used to couple into the second optical waveguide 90 can provide a larger interaction area between the first optical waveguide 80 and the second optical waveguide 90 for enhanced coupling efficiency.

[49] According to an embodiment, the optical grating coupler 10 comprises a plurality of scattering units 100 aligned in said second direction of propagation 60, said second direction of propagation 60 being the main direction of propagation of the optical beam inside the optical grating coupler 10. Advantageously, each scattering unit 100 comprises a structure 110 generating constructive interferences from at least one optical beam.

[50] The optical grating coupler 10 comprises a first layer 30. Said first layer 30 comprises a first plane 31 supporting said plurality of scattering units 100. Preferably, said first layer 30 is located below said plurality of scattering units 100. Advantageously, said first layer 30 is at least partially in physical contact with said plurality of scattering units 100.

[51] According to an embodiment, the optical grating coupler 10 comprises a second layer 20. Said second layer 20 comprises a second plane 21 configured to receive said optical beam 51 from said first optical waveguide 80. Preferably, said second layer 20 is located above said plurality of scattering units 100. Advantageously, said second layer 20 is coating said plurality of scattering units 100. [52] In a variant, the optical grating coupler 10 may not comprise a second layer covering the scattering units. Rather, the scattering units may be surrounded by a fluid such as environmental air, a gas, a gas in a partial vacuum, or a liquid. The end of the first optical waveguide may be immersed in said fluid.

[53] In a variant, the second layer may not completely cover the scattering units, for instance only fill the spaces between scattering units, and for instance have a top surface which is flush with a top surface of the scattering units.

[54] As described hereafter, one of the primary objectives of the present invention is to efficiently radiate light from a first optical waveguide from a first direction of propagation 50 into a third direction of propagation 70, thanks to the optical grating coupler 10, advantageously thanks to the plurality of scattering units 100.

[55] Preferably, the unidirectionality of the structure 110 of the scattering units 100 is achieved through the position and width of each element in the structure 110 of these scattering units 100. The abrupt change in refractive index at each sloped wall causes light to scatter in both upward and downward directions when coming from the second waveguide 80, or causes light to scatter in both an in-plane direction, i.e. the second direction of propagation 60, and a downward direction as illustrated in figure 4, when coming from the first waveguide 70. However, the arrangement of the positions of these elements must be carefully selected such that the light radiating downward is destructively interfered with, while the light radiating upward is constructively interfered with. This can be accomplished by optimising the position and size of the elements of the structure 110 to ensure proper interference conditions are met. Therefore, when an optical beam is coming from an upward direction, its direction of propagation changes into a direction of propagation corresponding to the direction of alignment of the scattering units 100 inside the optical scattering coupler 10, i.e. the second direction of propagation 60. As illustrated, the first direction of propagation 50 is changed into the second direction of propagation 60.

[56] Indeed, according to an embodiment, due to Lorenz reciprocity of Maxwell’s equations, said optical grating coupler 10 can propagate an optical beam from the first optical waveguide 80 to the second optical waveguide 90 as well as from the second optical waveguide 90 to the first optical waveguide 80. Indeed, the Lorenz reciprocity of Maxwell’s equations is a principle in electromagnetism that states that if two systems are reciprocal, then their electromagnetic responses to each other will be identical when the roles of the sources and observers are interchanged.

[57] The plots of figure 3b and 3c illustrate the dependence of the scattering structure dimensions DI, D2 and the ratios R1 and R2 on the process angles Al, A2, A3 that define the sloped sidewalls to achieve optimal constructive interference on a coupling side and destructive interference on the opposite side of the grating coupler as illustrated in figure 3a. In figure 3b, the bottom-right plot illustrates the dependence of the ratio R2 on the process angles Al, A2, and A3. Within a range of process angles between 72 degrees and 79 degrees, the ratio R2 lies between 0.65 and 1. In general, the ratio R2 increases as the process angles increase to achieve optimal constructive/destructive interference. In figure 3c, the bottom plot illustrates the dependence of the ratio R1 on the process angles Al, A2, and A3. Within a range of process angles between 60 degrees and 90 degrees, the ratio R1 lies between 3.9 and 1.5. In contrast to the relation between the process angles and the ratio R2, the ratio R1 decreases as the process angles increase to achieve optimal constructive/destructive interference. In this example the process angles are equal to each other, however for process angles Al, A2, A3 that differ from each other, the principles underlying the relationship between the structure dimensions DI, D2, or the ratios Rl, R2 and the process angles of the sloped sidewalls remain, whereby the optimal dimensions DI, D2 and the optimal ratios Rl and R2 to slope angle Al, A2, A3 for each configuration of slope angles may be easily obtained by per se known computer modelling and simulation methods.

[58] The plots in figure 3d illustrate the simulation results showing efficiency of the coupler for a range of Rl and R2 values at various process angles. It is shown that for a given process angle, an optimal Rl and R2 can be chosen, in order to fulfil the constructive/destructive interference criteria shown in figure 3a. The top plot shows that the highest coupling efficiency can be achieved when the process angles Al, A2, A3 range from 75 degrees to 85 degrees and the ratio Rl ranges from 2.0 to 2.5 (marked with an arrow in the plot). The bottom plot shows that the highest coupling efficiency can be achieved when the process angles Al, A2, A3 range from 75 degrees to 82 degrees and the ratio R2 ranges between 0.8 and 0.9 (marked with an arrow in the plot). Therefore, for the process angles Al, A2, and A3 ranging from 75 degrees to 80 degrees, the ratio Rl varies between 2.5 and 2.0, while the ratio R2 ranges from 0.8 to 0.9.

[59] The widths DI, D2 of the first and second structures and the ratios Rl and R2 may be adjusted according to the chosen process angles Al, A2, A3 of the sidewalls to optimize constructive/destructive interference for the upward/downward going waves. Since the destructive/constructive interference phenomena must remain in effect and for example, if the structure heights Hl and H2 remain constant (for instance because they may be set by thin film availability and other factors), based on the simulation results shown in figures 3b-3d, DI should be adjusted smaller and D2 should be adjusted larger to retain correct interference for an increasing process angle such that the ratio R2 is chosen between 0.8 and 0.9 depending on the values of the process angles and the ratio Rl . Such adjustment allows to have a highly performant grating coupler even in the presence of the process angle and the ratio Rl, which are subject to the etching process of lithium niobate.

[60] Advantageously, the distances and spacings between the elements of the structure of the scattering units also require optimisation due to the complex nature of the structure. Preferably, no single effective index may fully characterise the effective propagation constant of light in this structure, as its behaviour depends on the dimensions and positions of the scattering units. Therefore, during the development of the present invention, the performance of the structure has been simulated over a wide range of possible dimensions before converging numerically on the presented results to ensure accurate and efficient radiation of light into the upward direction.

[61] The present invention provides exceptional efficiency for light propagation in various applications. According to the present invention, the optical grating coupler 10 is characterised by its ability to efficiently radiate light from a waveguide into a different direction of propagation.

[62] According to an embodiment, the first optical waveguide 80 comprises a first refractive index contrast and the second optical waveguide 90 comprises a second refractive index contrast, said first refractive index contrast being lower than said second refractive index contrast. According to an embodiment, the first optical waveguide 80 comprises a first confined mode 81 and the second optical waveguide 90 comprises a second confined mode 91, said first confined mode being wider than said second confined mode. The first optical waveguide 80 can comprise at least one optical fibre, with a mode field diameter between 5pm and 200pm, preferably between 10pm and 100pm, and advantageously between 10pm and 30pm for optimal performance.

[63] According to an embodiment, the second layer 20 comprises said second plane 21 configured to receive at least partially said optical beam 51 coming from said first optical waveguide 80 and being propagated according to said first direction of propagation 50.

[64] Preferably, said second layer 20 comprises silicon oxide.

[65] According to an embodiment, the first layer 30 can be supported by a substrate 220. Said substrate 220 can comprise silicon oxide. Said first layer can comprise a BOX, i.e. a buried oxide.

[66] According to an embodiment, the first layer 30 comprises a first plane 31 ; Preferably, the first layer 30 comprises said plurality of scattering units 100. Advantageously, said first plane 31 is configured to support said plurality of scattering units 100. Preferably, said first plane 31 is at least partially in physical contact with said plurality of scattering units 100.

[67] According to an embodiment, the first layer 30 comprises a material having a refractive index lower than that of the material used for the structure 110. This design allows for efficient coupling between waveguides by utilising the principle of total internal reflection and constructive interference. The structure 110 comprises lithium niobate and/or lithium tantalate. According to an embodiment, the structure 110 may further comprise silicon nitride. According to an embodiment, the first layer 30 comprises silicon oxide and/or silicon and/or any metals, preferably gold, and/or lithium niobate and/or lithium tantalate and/or aluminium oxide. This preferably enables the use of standard process techniques during manufacturing.

[68] According to an embodiment, the first layer 30 comprises an additional layer 40 supporting the plurality of scattering units. Preferably, said additional layer 40 is configured to be at least partially in physical contact with the plurality of scattering units 100. Advantageously, said additional layer 40 is located between said first layer 30 and said plurality of scattering units 100. The thickness H3 of this additional layer 40 can be adjusted for optimal performance and is comprised between 80nm and 120nm, preferably between 90 and 1 lOnm, and advantageously between 100 and 108nm. According to an embodiment, said additional layer 40 can comprise silicon oxide, and/or silicon and/or any metals, preferably gold, and/or lithium niobate and/or lithium tantalate and/or aluminium oxide.

[69] According to an embodiment, said additional layer 40 comprises said first plane 31. Preferably, said first plane 31 is formed by said additional layer 40. Advantageously, said first plane 31 is defined by said additional layer 40.

[70] According to an embodiment, each structure 110 is configured to generate constructive interferences from at least one optical beam. Preferably, said structure 110 is repeated a predetermined number of times T according to a predetermined period P along said first direction of propagation 30. The predetermined number of times T that the structure 110 is repeated along the second direction of propagation 60 can be adjusted for optimal performance. According to an embodiment, T is higher than 5, preferably higher than 30, and advantageously higher than 100. The predetermined period P between consecutive scattering units 100 can also be adjusted for optimal performance. According to an embodiment, P is comprised between 700nm and 1400nm, preferably between 800nm and 1200nm, and advantageously between 900nm and lOOOnm. The distance D3 between two consecutive scattering units 100 can be adjusted for optimal performance. According to an embodiment, D3 is comprised between 200nm and 400nm, preferably between 250nm and 350nm, and advantageously between 290nm and 300nm.

[71] According to an embodiment, the size of the repeated structures 110 can also be adjusted to lower reflection at the end of the plurality of scattering units 100 according to the second direction of propagation 60 and increase reflection sooner in the plurality of scattering units 100 according to the second direction of propagation 60 for optimal performance.

[72] According to an embodiment, the size of the repeated structures 110 can also be adjusted to increase reflection in the middle of the plurality of scattering units 100 according to the second direction of propagation 60 and lower reflection at the end of the plurality of scattering units 100 according to the second direction of propagation 60 for optimal performance.

[73] According to an embodiment, the size of the repeated structures 110 can also be adjusted to lower reflection at the end of the plurality of scattering units 100 according to the second direction of propagation 60, then increase reflection in the middle of the plurality of scattering units 100 according to the second direction of propagation 60, then lower reflection at the beginning of the plurality of scattering units 100 for optimal performance.

[74] According to an embodiment, the size of the repeated structures 110 can also be adjusted to lower reflection at the beginning of the plurality of scattering units 100 according to the second direction of propagation 60, then increase reflection in the middle of the plurality of scattering units 100 according to the second direction of propagation 60, then lower reflection at the end of the plurality of scattering units 100 for optimal performance.

[75] According to an embodiment, the predetermined period P, in a direction moving away from the second wave guide 90, progressively increases to a maximum value and, optionally, then progressively decreases from the maximum value, configured to adapt the mode of the output of the optical grating coupler to the mode of the first optical waveguide.

[76] The variable period P seeks to match the mode of the output of the grating coupler to the mode of an optical fiber, whereby, uniform scattering elements produce an exponential like shape Eg that does not match the substantially Gaussian shape Gf of the fiber mode as schematically illustrated in the plot of figure 2.

[77] To adapt the fill factor or size of the scattering element, the pitch of the scattering elements should fulfill the following condition:

Where

Theta is the angle of the fiber, n cladding is the index of the cladding, lambda the wavelength of light, and n eff is generally given as the average effective index of the mode in the waveguide, composed of three regions: n eff = (nl*dl +n2 *d2+n3 *d3)/(dl +d2+d3)

Thus, if one adapts the size of the structures at the beginning of the scattering array, one should also adapt the pitch (period) in each region so that one fulfills the equation given above.

It can be shown that the desired variation in scattering structure sizes can be well approximated by a linear chirp, thus we may adopt the following equations to fulfill the criterion above:

Where Lambda i is the adapted period of the z th element and FF _i is the adapted fill factor of the ith element, N is the number of elements, and delta (Lambda/F) is a value that may be adapted using simulation. Individual values of scattering structures may be further optimized using FEM simulations and optimization techniques such as surrogate modelling.

[78] According to an embodiment, said structure 110 comprises a first sloped sidewall 111, a first stage 112, a second sloped sidewall 113, a second stage 114, and a third sloped sidewall 115, according to said second direction of propagation 60.

[79] According to an embodiment, the first sloped sidewall 111 extends from the first plane 31 to the first stage 112.

[80] According to an embodiment, the second sloped sidewall 113 extends from the first stage 112 to the second stage 114.

[81] According to an embodiment, the third sloped sidewall 115 extends from the second stage 114 to the first plane 31.

[82] According to an embodiment, the third sloped sidewall 115 is parallel to the second sloped sidewall 113. Y1

[83] According to an embodiment, said first stage 112 is at least partially in physical contact with said second layer 20, advantageously is coated by said second layer 20.

[84] According to an embodiment, the first sloped sidewall 111 comprises a dimension of extension forming a first angle Al with said first plane 31 according to said second direction of propagation 60. Preferably, said first sloped sidewall 111 is coated by said second layer 20.

[85] According to an embodiment, the first stage 112 comprises a dimension of extension DI extending in said second direction of propagation 60; Preferably, said first stage 112 is located above said first plane 31. Advantageously, said first stage 112 is parallel to said first plane 31. According to an embodiment, said first stage 112 is located above said additional layer 40. Preferably, said first stage 112 is located below said second plane 21. Advantageously, said first stage 112 is coated by said second layer 20.

[86] According to an embodiment, the second sloped sidewall 113 comprises a dimension of extension forming a second angle A2 with said first plane 31 according to said second direction of propagation 60. Preferably, said second sloped sidewall 113 is coated by said second layer 20.

[87] According to an embodiment, the second stage 114 comprises a dimension of extension D2 extending in said second direction of propagation 60; Preferably, said second stage 114 is located above said first plane 31. Advantageously, said second stage 114 is parallel to said first stage 112. According to an embodiment, said second stage 114 is located below said second layer 20. Preferably, said second stage 114 is located above said additional layer 40. Advantageously, said second stage 114 is coated by said second layer 20.

[88] According to an embodiment, said third sloped sidewall 115 comprises a dimension of extension forming a third angle A3 with said first plane 31 according to said second direction of propagation 60. Preferably, said third sloped sidewall 115 is coated by said second layer 20.

[89] The angles Al, A2 and A3 formed by these components with the first plane 31 are designed for efficient coupling. According to an embodiment, the first angle Al is equal to the second angle A2. Advantageously, the third angle A3 may be equal to the second angle A2 for symmetry reasons.

[90] The first angle Al can be adjusted within a specific range for optimal performance. According to an embodiment, Al is comprised between 60 degrees and 85 degrees, preferably between 70 degrees and 82 degrees, and advantageously between 75 degrees and 80 degrees.

[91] The height Hl of the first stage 112 and the height H2 of the second stage 114 can be adjusted within specific ranges to optimise coupling efficiency. According to an embodiment, the ratio R1 between Hl and H2 is comprised between 4 and 2, preferably between 3 and 2, and advantageously between 2.5 and 2.

[92] According to an embodiment, the height Hl is comprised between 200nm and 900nm, preferably between 250nm and 600nm, and advantageously between 300nm and 500nm. According to an embodiment, the height H2 is comprised between 50nm and 450nm, preferably between 125nm and 300nm, and advantageously between 150nm and 250nm.

[93] According to an embodiment, the dimensions of extension DI and D2 of the first stage 112 and second stage 114 can be adjusted to optimise coupling efficiency. According to an embodiment, the ratio R2 between DI and D2 is comprised between 0.6 and 0.97, preferably between 0.7 and 0.9, advantageously between 0.80 and 0.90, and more advantageously substantially equals to 0.83.

[94] The dimensions of extension DI and D2 can also be further specified to specific ranges for optimal performance. According to an embodiment, the dimension of extension DI is comprised between lOOnm and 400nm, preferably between 200nm and 300nm, and advantageously between 240nm and 280nm. According to an embodiment, the dimension of extension D2 is comprised between lOOnm and 600nm, preferably between 220nm and 430nm, and advantageously between 275nm and 350nm. In an advantageous embodiment, the dimension of extension D2 is comprised between 275nm and 285nm.

[95] According to an embodiment, the dimension of extension DI of said first stage 112 is parallel to said second plane 21. According to an embodiment, the dimension of extension D2 of said second stage 114 is parallel to said second plane 21. Advantageously, the dimension of extension DI of said first stage 112 is parallel to the dimension of extension D2 of said second stage 114.

[96] According to an embodiment, the dimension of extension DI of said first stage 112 is parallel to said first plane 31. According to an embodiment, the dimension of extension D2 of said second stage 114 is parallel to said first plane 31.

[97] According to an embodiment, said second plane 21 is parallel to said first plane 31.

[98] According to an embodiment, and as illustrated by Figure 4b, the structure 110 of the scattering units 100 can comprise curved grating lines to focus the light into either the second waveguide 90, i.e. the nanophotonic waveguide, more quickly or to focus the light into the first waveguide 80, i.e. the fibre above, allowing to use a different size of fibre core.

[99] According to an embodiment, the structure 110 can comprise a first part and a second part. The first and second parts of the structure 110 can be realised using different lithographic processes for optimal performance. According to an embodiment, the first part comprises the first stage 112, and the second part comprises the second stage 114, and these parts are realised using different lithographic processes.

[100] The first and second lithographic processes used to realise the first and second parts of the structure can use different masks and/or exposures for optimal performance. According to an embodiment, the first lithographic process uses a first etching time El, and the second lithographic process uses a second etching time E2, with E2 being higher or equal to El for optimal performance. According to another embodiment, E2 is higher or equal than El. The first set of etching parameters EPl used in the first lithographic process can also be different from the second set of etching parameters EP2 used in the second lithographic process for optimal performance.

[101] The present invention offers several advantages over the prior art:

High coupling efficiency: The present invention achieves high coupling efficiencies from optical fibres into lithium niobate on insulator (LNOI) thin film nanophotonic waveguides, surpassing the limitations of current state-of-the-art integrated LNOI grating couplers that can provide a coupling efficiency of only 5-12 dB/facet, typically experiencing high losses due to scattering in both upward and downward directions, and low scattering efficiencies of the individual elements.

Simplicity in design: The present invention maintains simplicity in design, making it easier to implement compared to more complex approaches such as end-fire coupling or using mirror structures beneath the grating couplers.

Ease of fabrication: The present invention can be realised using standard photolithography feature sizes (>200 nm), which is more accessible than advanced nanofabrication processes available for silicon photonics, or using electron beam lithography. The invention also can be realised using etching processes that yield sloped sidewalls, making its implementation feasible in lithium niobate and/or lithium tantalate.

Isolation from coupling of the transverse magnetic (TM) mode: The present invention is configured to provide an isolation from coupling of the TM mode by a coupling coefficient for TM polarised light that can be, for example 8 times less than the TE mode, ensuring efficient and selective coupling between the waveguide and the fibre.

Wavelength adaptability: The present invention can be adapted to various wavelengths, making it suitable for a wide range of applications in telecommunications networks and other fields.

Improved alignment tolerances: The present invention offers improved alignment tolerances compared to endfire coupling, simplifying the process of aligning fibres with the chip.

Enhanced directionality: By breaking the symmetry between upward and downward scattered light in a grating coupler design, the present invention provides enhanced directionality, ensuring that most of the light exiting the nanoscale waveguide is sent in the direction of the fibre waveguide.

[102] According to an embodiment, the present invention relates to an optical device comprising at least one optical grating coupler 10 according to the present invention for various applications in optical communication systems, sensors, and other optical devices.

[103] According to an embodiment and as described, the present invention employs a doubly etched geometry to break the symmetry between upward and downward scattered light in a grating coupler design.

[104] During the development of the present invention, the grating coupler 10 has provided simulation and experimental results for unprecedentedly high coupling efficiencies from the optical fibre into lithium niobate thin film nanophotonic rib waveguides. The present invention has reached a simulation coupling efficiency of - 1.97 dB at 1550 nm, with an efficiency better than -3dB across a 30 nm span. The experimental result achieved a coupling efficiency of better than -3.4 dB/coupler at 1550 nm. The design wavelength has been chosen because it is technologically relevant to the telecommunications network and is likely to be very important for future technologies in this field and others.

[105] Advantageously, the present invention is adapted to an etching process in thin film lithium niobate that does not provide vertical sidewalls and can be realised using standard photolithography feature sizes (>200 nm). For the development of the present invention, a 2D finite element (FEM) simulation in the COMSOL Multiphysics Wave Optics module has been utilised to verify and optimise our results. We have completed the 3D design of the structure with an adiabatic linear taper calculated according to the effective index method.

[106] According to an embodiment, Figure 4a displays a cross-sectional view of a simulated nanophotonic waveguide using the multiphysics Finite Element Method (FEM) software COMSOL. Figure 4a illustrates the beam exiting the fibre, i.e. said first optical waveguide 80, at an angle of approximately 8 degrees and passing through a cladding layer of silicon oxide, i.e. said second layer 20, before being scattered by scattering units 100, preferably called directional scattering units 100, into a nanostructured waveguide, i.e. said second optical waveguide 90. This cross-sectional view provides valuable insights into the interaction between the first optical waveguide 80 and the second optical waveguide 90, allowing for a better understanding of the coupling process.

[107] It has to be noted that it is known that the effective index contrast between lithium niobate and silicon oxide is not as high as between silicon and silicon oxide, and this is often cited in the prior art as a reason for the inefficiency of lithium niobate grating couplers. To overcome this issue, the present invention can comprise at least one beam expander to expand the mode size of the first waveguide 80, i.e. the fibre, and thus improve the efficiency of the grating coupler by spreading the beam over a larger area. Preferably, the mode converters enable expansion of the beam but maintain a single mode in the first waveguide 80, i.e. the fibre. By expanding the mode size of the first waveguide 80 using commercially available beam expanders and optimising the dimensions, positioning, and orientation of the scattering units, the efficiency of the present invention can be significantly improved compared to the solutions of the prior arts.

[108] According to an embodiment, Figure 5a illustrates a calculated simulation of the coupling efficiency as a function of the input angle of the first optical waveguide 80; and Figure 5b illustrates a calculated simulation of the coupling efficiency as a function of the wavelength of the light. It has to be noted that the grating coupler 10, according to the present invention, has an efficiency of greater than 50% (better than -3 dB) over a 30 nm span in wavelength.

[109] According to an embodiment, Figure 6 illustrates a calculated simulation of the efficiency of the transverse magnetic (TM) mode coupling into the second optical waveguide, i.e. the nanophotonic waveguide. This allows to demonstrate the rejection of the transverse magnetic (TM) mode at the optimal coupling angle for transverse electric (TE) of 8 times less efficiency, which is desirable for some applications. Indeed, according to an embodiment, the present invention provides an isolation from the coupling of the transverse magnetic (TM) mode by a coupling coefficient for TM polarised light of 8 times less than the transverse electric (TE) mode.

[110] According to an embodiment, the present invention relates to a method for making an optical grating coupler 10 according to the present invention.

[111] According to an embodiment, said method can use commercially available thin film lithium niobate on insulator wafer (NanoLN) with X-cut (for example 600 nm) or Z-cut (for example 700 nm) thick LiNbO3 layer, preferably 4.7 pm buried oxide (thermally grown) on a 525 pm thick Si substrate. [112] According to an embodiment, the method is configured to use a 300 nm thick diamond-like carbon (DLC) film, for example, which is grown via Plasma Enhanced Chemical Vapor Deposition (PECVD) from a methane precursor, as a hard mask material for the physical ion beam etching process. According to an embodiment, the present invention comprises depositing at least a mask layer of silicon nitride.

[113] Preferably, the optical waveguide pattern is structured by Deep Ultraviolet (DUV) stepper photolithography (248 nm for example), or by e-beam lithography, and first transferred into a silicon nitride mask layer using standard fluoride-chemistry-based plasma etching. Advantageously, this intermediate mask has excellent resistance to oxygen plasma etching, which is used to transfer the waveguide pattern into the DLC hard mask. According to an embodiment, with optimised argon ion beam etching (IBE), the etching selectivity between lithium niobate and DLC is up to 3 times, which enables deep etching and steep sloped sidewalls.

[114] According to an embodiment, the method comprises at least two etch steps that take place sequentially, with masks defined separately for each etch step. Since one lithographic process typically cannot produce a strong asymmetry in two dimensions, two lithographic steps are needed to form the structures as previously described.

[115] As it is well-known by the skilled person in the art, a “lithographic process” can refer to a series of techniques used to pattern and/or transfer geometric shapes or structures onto a photosensitive material, such as a photoresist. The primary goal is to create high-resolution, accurate, and reproducible patterns that serve as the foundation for the subsequent fabrication steps. A lithographic process can typically involve the following key steps:

Cleaning: The surfaces are cleaned to remove any contaminants or residues that may interfere with the patterning process.

Coating anti-reflective layer: that can be uniformly applied onto surfaces using spin coating or other methods.

Coating photosensitive materials, called a photoresist: that can be uniformly applied onto surfaces using spin coating or other methods.

Soft baking: The photoresist can be heated to improve its adhesion to the surface and to remove solvents.

Exposure: The photoresist is exposed to ultraviolet (UV) or Deep Ultraviolet (DUV) or extremeultraviolet (EUV) light through a patterned mask, which defines the desired structures or patterns.

Post-exposure baking: The photoresist can be baked after exposure at high temperature for either enhance the chemical reaction initiated by the exposure or stop the chemical reaction.

Development: The exposed photoresist can be chemically treated to selectively remove the unexposed regions, leaving behind the patterned photoresist mask.

Hard baking: The photoresist can be heated again to harden it and improve its resistance to subsequent processing steps. Etching: The substrate is subjected to various etching processes to transfer the patterns from the photoresist mask into the underlying material. This may involve wet and/or dry etching techniques, such as reactive ion etching (RIE) or plasma etching or BeamEtching or Ion Milling, for example.

Resist stripping: The remaining photoresist mask can be removed using a solvent or an oxygen plasma.

Deposition: Layers can be created by deposition using various techniques.

[116] According to a first embodiment, and as illustrated by figures 7a to 7d, the method comprises the following steps:

❖ Providing a first stack comprising at least:

> The first layer 30 on top of a substrate 220, the first layer 30 comprising a BOX;

> A thin film Lithium Niobate (TFLN) 210 on top of said first layer 30;

> A hard mask 200 on top said thin film Lithium Niobate 210;

> Optionally, a buffer layer located between the thin film Lithium Niobate 210 and the hard mask 200;

❖ Putting a resist layer 230 on top of said hard mask 200;

❖ Forming at least the following elements:

> The first stage 112:

> A first part of the first sloped sidewall 111:

> The second sloped sidewall 113: using a lithographic process and an etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

❖ Forming the following elements:

> The second stage 114:

> The last part of the first sloped sidewall 111:

> The third sloped sidewall 115: using another lithographic process and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching;

❖ Depositing the second layer 20 at least on top of the following elements:

> The first sloped sidewall 111;

> The first stage 112;

> The second sloped sidewall 113;

> The second stage 114;

> The third sloped sidewall 115. [117] According to said first embodiment, and as illustrated by figures 7a to 7d, said method can comprise several steps of putting a resist layer 230 on top of some regions of said hard mask 200. Alternatively, a thin film Lithium Tantalate is positioned on top of the first layer 30 instead of the thin film Lithium Niobate.

[118] According to said first embodiment, said method can also comprise a step for removing said hard mask 200.

[119] According to an embodiment, figure 7a illustrates said first stack with the resist layer 230 located on top of some regions of the hard mask 200. The resist layer 230 is configured to protect surfaces of the hard mask 200 that will define later in the process the first stage 112.

[120] According to an embodiment, figure 7b illustrates a step of the process wherein the resist layer 230 and the hard mask 200 have been removed after some lithographic processes and etching processes configured to define the first stage 112, the third sloped sidewall 113 and at least partially the first sloped sidewall 111.

[121] According to an embodiment, figure 7c illustrates a step of the process wherein another hard mask 200 has been deposited at least partially on top of the thin film lithium niobate 210, and wherein another resist layer 240 has been deposited on top of said hard mask 200. Said resist layer 240 and said hard mask 200 cover regions of the patterned thin film lithium niobate 210 that will define the future first stage 112, the future second sloped sidewall 113, the future second stage 114.

[122] According to an embodiment, figure 7d illustrates structures 110 realized by said process with the first sloped sidewall 111, the first stage 112, the second sloped sidewall 113, the second stage 114 and the third sloped sidewall 115.

[123] According to a second embodiment, and as illustrated by figures 8a to 8d, the method comprises the following steps:

❖ Providing a first stack comprising at least:

> A thin film Lithium Niobate (TFLN) 210 on top of said first layer 30;

> A hard mask 200 on top said thin film Lithium Niobate 210;

❖ Putting a resist layer 230 on top of said hard mask 200;

❖ Forming at least the following elements:

> At least partially the first sloped sidewall 111;

> At least partially the third sloped sidewall 115; using a lithographic process and a third etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

❖ Forming the second stage 114 and the second sloped sidewall 113 by transferring said first sloped sidewall 111 and said third sloped sidewall 115 using another lithographic process and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching;

❖ Depositing the second layer 20 at least on top of the following elements:

> The first sloped sidewall 111;

> The first stage 112;

> The second sloped sidewall 113;

> The second stage 114;

> The third sloped sidewall 115.

[124] According to said second embodiment, and as illustrated by figures 8a to 8d, said method can comprise several steps of putting a resist layer 230 on top of some regions of said hard mask 200. Alternatively, a thin film Lithium Tantalate is positioned on top of the first layer 30 instead of the thin film Lithium Niobate.

[125] According to said second embodiment, said method can also comprise a step for removing said hard mask 200.

[126] According to an embodiment, figure 8a illustrates said first stack with the resist layer 230 located on top of some regions of the hard mask 200. The resist layer 230 is configured to protect the surfaces of the hard mask 200 that will define later in the process the first stage 112 and the second stage 114.

[127] According to an embodiment, figure 8b illustrates a step of the process wherein the resist layer 230 and the hard mask 200 have been removed after some lithographic processes and etching processes configured to define the first stage 112 and at least partially the first sloped sidewall 111.

[128] According to an embodiment, figure 8c illustrates a step of the process wherein another hard mask 200 has been deposited on top of the thin film lithium niobate 210, and wherein another resist layer 240 has been deposited at least partially on top of said hard mask 200. Said resist layer 240 covers regions of the hard mask 200 that will define the future first stage 112. Said hard mask 200 covers completely the thin film lithium niobate 210.

[129] According to an embodiment, figure 8d illustrates structures 110 realized by said process with the first sloped sidewall 111, the first stage 112, the second sloped sidewall 113, the second stage 114 and the third sloped sidewall 115.

[130] According to a third embodiment, the method comprises the following steps:

- Providing a first stack comprising at least:

> A thin film Lithium Niobate (TFLN) 210 on top of said first layer 30;

> A hard mask 200 on top said thin film Lithium Niobate 210;

- Putting a resist layer 230 on top of said hard mask 200; - Removing partially the (photo-)resist layer 230 to keep the first stage 112 protected and to expose the rest of the first stack by a lithographic process and an etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

- Forming at least partially the first sloped sidewall 111 and the third sloped sidewall 115 by another lithographic process including putting another resist layer 240 and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching;

- Forming, after removal of at least one resist layer 240 used for Deep UV and/or e-beam lithography, the second stage 114 and the second sloped sidewall 113 by another etching process, said another etching process comprising at least one etching;

- Depositing the second layer 20 at least on top of the following elements:

> The first sloped sidewall 111;

> The first stage 112;

> The second sloped sidewall 113;

> The second stage 114;

> The third sloped sidewall 115.

[131] According to said third embodiment, and as illustrated by figures 9a to 9d, said method can comprise several steps of putting a resist layer 230 on top of some regions of said hard mask 200. Alternatively, a thin film Lithium Tantalate is positioned on top of the first layer 30 instead of the thin film Lithium Niobate.

[132] According to said third embodiment, said method can also comprise a step for removing said hard mask 200.

[133] According to an embodiment, said hard mask 200 can comprise an additional layer made of a different material, such for example a buffer layer.

[134] According to an embodiment, figure 9a illustrates said first stack with the resist layer 230 located on top of some regions of the hard mask 200. The resist layer 230 is configured to protect surfaces of the hard mask 200 that will define later in the process the first stage 112.

[135] According to an embodiment, figure 9b illustrates a step of the process wherein the hard mask 200 has been removed. Another resist layer 240 has been deposited on the top of said hard mask 200 and on at least one sidewall of said hard mask 200. Said resist layer 240 is configured to partially protect surfaces of the hard mask 200 that will define later in the process the first stage 112, the second sloped sidewall 113 and the second stage 114.

[136] According to an embodiment, figure 9c illustrates a step of the process wherein the resist layer 240 has been removed and wherein, after an etching process, the first sloped sidewall 111 and the third sloped sidewall 115 have been partially formed. In this figure, the hard mask 200 is still protecting surfaces that will define later in the process the first stage 112, the second slope sidewall 113, and the first slope sidewall 111. [137] According to an embodiment, figure 9d illustrates structures 110 realized by said process with the first sloped sidewall 111, the first stage 112, the second sloped sidewall 113, the second stage 114 and the third sloped sidewall 115.

[138] According to an embodiment, the optical grating coupler 10 can be realized using various lithographic and etching techniques.

[139] Unless otherwise specified herein, or unless the context clearly dictates otherwise, the term about modifying a numerical quantity means plus or minus ten percent. Unless otherwise specified, or unless the context dictates otherwise, between two numerical values is to be read as between and including the two numerical values.

[140] In the present description, some specific details are included to provide an understanding of various disclosed implementations. The skilled person in the relevant art, however, will recognise that implementations may be practised without one or more of these specific details, parts of a method, components, materials, etc. In some instances, well-known methods associated with lithographic processes have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the disclosed implementations.

[141] In the present description and appended claims, "a", "an", "one", or "another" applied to "embodiment", "example", or "implementation" is used in the sense that a particular referent feature, structure, or characteristic described in connection with the embodiment, example, or implementation is included in at least one embodiment, example, or implementation. Thus, phrases like "in one embodiment", "in an embodiment", or "another embodiment" are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments, examples, or implementations.

[142] As used in this description and the appended claims, the singular forms of articles, such as "a", "an", and "the", may include plural referents unless the context mandates otherwise. Unless the context requires otherwise, throughout this description and appended claims, the word "comprise" and variations thereof, such as "comprises" and "comprising", are to be interpreted in an open, inclusive sense, that is, as "including, but not limited to".

[143] Modifications and improvements to the above-described implementations of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is, therefore, not intended to be limited solely by the scope of the appended claims.

[144] References:

10 An optical grating coupler

20 Second layer

21 Second plane

30 First layer 31 First plane

40 Additional layer

50 First direction of propagation

51 Optical beam from the first optical waveguide

60 Second direction of propagation

70 Third direction of propagation

71 Optical beam in the second optical waveguide

80 First optical waveguide

81 Optical mode of the first optical waveguide

90 Second optical waveguide

91 Optical mode of the second optical waveguide

100 Scattering unit

110 Structure

111 First sloped sidewall

112 First stage

113 Second sloped sidewall

114 Second stage

115 Third sloped sidew all

200 Hard Mask

210 Thin Film Lithium Niobate or Lithium T antalate

220 Substrate

230 Resist layer

240 Another resist layer

Claims

1. An optical grating coupler (10) configured to optically couple at least a first optical waveguide with at least a second optical waveguide, the optical grating coupler (10) being configured to propagate at least one optical beam from said first optical waveguide to said second optical waveguide, the optical grating coupler (10) comprising at least: a plurality of scattering units (100) being configured to propagate said optical beam from a first direction of propagation (50) to a second direction of propagation (60), each scattering unit (100) of said plurality of scattering units (100) being aligned in said second direction of propagation (60), said second direction of propagation (60) being the main direction of propagation of said optical beam within the optical grating coupler (10); and a first layer (30), said first layer (30) comprising a first plane (31) supporting said plurality of scattering units (100); wherein, each scattering unit (100) of said plurality of scattering units (100) comprises a structure (110) comprising or consisting of lithium niobate, said structure (110) being configured to generate constructive interferences from said optical beam on a first side of the first plane facing the first optical waveguide and destructive interferences on a second side of the first plane facing away from the first optical waveguide, said structure comprising, along said second direction of propagation (60):

- a first stage (112) comprising a dimension of extension DI extending in said second direction of propagation (60), said first stage (112) being located above said first plane (31);

- a second stage (114) comprising a dimension of extension D2 extending in said second direction of propagation (60), said second stage (114) being located above said first plane (31);

- a first sloped sidewall (111) extending from the first plane (31) to the first stage (112) comprising a dimension of extension forming a first angle A 1 with said first plane (31);

- a second sloped sidewall (113) extending from the first stage (112) to the second stage (114) comprising a dimension of extension forming a second angle A2 with said first plane (21);

- a third sloped sidewall (115) extending from the second stage (114) to the first plane (31) comprising a dimension of extension forming a third angle A3 with said first plane (31); and wherein:

- said first layer (30) comprises a material having a refractive index lower or equal to the refractive index of lithium niobate;

- a ratio R1 between a height Hl of said first stage (112) regarding said first plane (31) and a height H2 of said second stage (114) regarding said first plane (31) is comprised between 4 and 2;

- a ratio R2 between the dimension of extension DI of said first stage (112) and the dimension of extension D2 of said second stage (114) is comprised between 0.6 and 0.97; and

- the first angle Al is comprised between 65 degrees and 89 degrees.

2. The optical grating coupler ( 10) of the preceding claim wherein the first angle A 1 is comprised between 70 degrees and 82 degrees, preferably between 75 degrees and 80 degrees.

3. The optical grating coupler (10) of any preceding claim wherein the ratio R1 is comprised between 3 and 2, preferably between 2.5 and 2.

4. The optical grating coupler (10) of any preceding claim wherein the ratio R2 is comprised between 0.7 and 0.9, preferably between 0.8 and 0.9.

5. The optical grating coupler (10) of any preceding claim wherein the ratio R2 is between 0,82 and 0,84, for instance equal to 0.83.

6. The optical grating coupler (10) of any preceding claim wherein the first angle Al is comprised between 75 degrees and 80 degrees, the ratio R2 is comprised between 0.8 and 0.9, and the ratio R1 is comprised between 2.5 and 2 such that when the first angle Al and the ratio R2 increase, the ratio R1 decreases configured to maximize constructive interference on a coupling side of said scattering unit.

7. The optical grating coupler (10) of any preceding claim further comprising a second layer (20), said second layer (20) comprising a second plane (21) configured to receive said optical beam from said first optical waveguide, said second layer (21) being on top of said plurality of scattering units (100).

8. The optical grating coupler (10) of any preceding claim wherein an angle between the first direction of propagation and a direction perpendicular to the first layer or second layer, preferably to the first plane or second plane, is comprised between -30 degrees and 30 degrees, preferably between -15 degrees and -2 degrees or between 2 degrees and 15 degrees and advantageously between -10 degrees and -4 degrees or between 4 degrees and 10 degrees

9. The optical grating coupler (10) of any preceding claim wherein said first angle Al is equal to the second angle A2 and/or wherein said third angle A3 is equal to said second angle A2.

10. The optical grating coupler (10) of any preceding claim wherein the height Hl of said first stage (112) regarding said first plane (31) is comprised between 200nm and 900nm, preferably between 250nm and 600nm, and advantageously between 300nm and 500nm, and wherein the height H2 of said second stage (114) regarding said first plane (31) is comprised between 50nm and 450nm, preferably between 125nm and 300nm, and advantageously between 150nm and 250nm, and wherein the dimension of extension DI of said first stage (112) is comprised between lOOnm and 400nm, preferably between 200nm and 300nm, and advantageously between 240nm and 280nm, and wherein the dimension of extension D2 of said second stage (114) is comprised between lOOnm and 600nm, preferably between 220nm and 430nm, and advantageously between 275nm and 350nm.

11. The optical grating coupler (10) of any preceding claim wherein the dimension of extension D2 of said second stage (114) is comprised between 275nm and 285nm.

12. The optical grating coupler (10) of any preceding claim wherein the first layer comprises an additional layer (40) supporting the plurality of said scattering units, wherein said additional layer is configured to be at least partially in physical contact with said plurality of scattering units (100), advantageously said additional layer being located between said first layer and said plurality of scattering units.

13. The optical grating coupler (10) of the preceding claim wherein a thickness H3 of said additional layer is comprised between 80nm and 120nm, preferably between 90 nm and HOnm, and advantageously between 100 and 108nm.

14. The optical grating coupler (10) of either of the two directly preceding claims wherein said additional layer comprises silicon oxide, and/or silicon and/or any metals, preferably gold, and/or lithium niobate and/or lithium tantalate and/or aluminium oxide.

15. The optical grating coupler (10) of any of the three directly preceding claims wherein said additional layer comprises said first plane.

16. The optical grating coupler (10) of any preceding claim wherein said first stage (112) is parallel to said first plane (31), and wherein said second stage (114) is parallel to said first plane (31).

17. The optical grating coupler (10) of any preceding claim wherein the first optical waveguide comprises a first refractive index contrast and the second optical waveguide comprises a second refractive index contrast, said first refractive index contrast being lower than said second refractive index contrast.

18. The optical grating coupler (10) of any preceding claim wherein the first optical waveguide comprises a first confined mode and the second optical waveguide comprises a second confined mode, said first confined mode being wider than said second confined mode.

19. The optical grating coupler (10) of any preceding claim wherein said structure (110) further comprises silicon nitride and/or silicon.

20. The optical grating coupler (10) of any preceding claim wherein said first layer (30) comprises silicon oxide and/or silicon and/or lithium niobate and/or aluminium oxide.

21. The optical grating coupler (10) of any preceding claim wherein said structure (110) is repeated a predetermined number of times T of a predetermined period P along said second direction of propagation (60), the predetermined number of time T is higher than 5, preferably higher than 30, and advantageously higher than 100.

22. The optical grating coupler (10) of the preceding claim wherein the predetermined period P is comprised between 700nm and 1400nm, preferably between 800nm and 1200nm, and advantageously between 900nm and HOOnm.

23. The optical grating coupler ( 10) of either of the two directly preceding claims wherein the predetermined period P of at least a portion of the plurality of scattering units is progressively smaller closer to said second optical waveguide.

24. The optical grating coupler (10) of any preceding claim wherein a distance D3 between two consecutive scattering units (100) of said plurality of scattering units (100) is comprised between 200nm and 400nm, preferably between 250nm and 350nm, and advantageously between 290nm and 300nm.

25. The optical grating coupler (10) of any preceding claim wherein the predetermined period P progressively increases to a maximum value and then progressively decreases from the maximum value, configured to adapt the mode of the output of the grating coupler to the mode of the first optical waveguide.

26. The optical grating coupler (10) of any preceding claim wherein the optical grating further comprising at least one beam expander to expand a mode size of the first waveguide.

27. A method for making at least one optical grating coupler (10) of any preceding claim, wherein said structure (110) comprises a first part and a second part, said first part comprising said first stage (112) and said second part comprising said second stage (114), the method comprising realizing said first part using a first lithographic process and realizing said second part using a second lithographic process, and wherein the first lithographic process comprises a first set of etching parameters EPl and the second lithographic process comprised a second set of etching parameters EP2, and optionally wherein at least one value of at least one parameter of EP 1 is different from at least another value of at least one parameter of EP2.

28. A method for making at least one optical grating coupler (10) of any of the claims 1 to 26, said method comprising the following steps: a. providing a first stack comprising at least: i. The first layer (30) on top of a substrate (220), the first layer comprising a buried oxide; ii. A thin film lithium niobate (210) on top of said first layer; iii. A hard mask (200) on top said thin film Lithium Niobate (210); iv. Optionally, a buffer layer located between the thin film lithium niobate (210) and the hard mask (200); b. putting a resist layer (230) on top of said hard mask; c. selecting one process among the three following processes: i. first process:

• forming at least the first stage ( 112), a first part of the first sloped sidewall (111) and the second sloped sidewall (113) using a lithographic process and an etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

• forming the second stage (114), the last part of the first sloped sidewall (111) and the third sloped sidewall (115) using another lithographic process and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching; ii. second process:

• forming at least partially the first sloped sidewall (111), and at least partially the third sloped sidewall (115), using a lithographic process and an etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

• forming the second stage (114) and the second sloped sidewall (113) by transferring said first sloped sidewall (111) and said third sloped sidewall (115) using another lithographic process and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching; iii. third process:

• removing partially the hard mask (200) to keep the first stage (112) protected and to expose the rest of the first stack by a lithographic process and an etching process, said lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said etching process comprising at least one etching;

• forming at least partially the first sloped sidewall (111) and the third sloped sidewall (115) by another lithographic process and another etching process, said another lithographic process comprising using Deep UV photo-lithography and/or e-beam lithography, said another etching process comprising at least one etching; • forming the second stage (114) and the second sloped sidewall (113) by another etching process, said another etching process comprising at least one etching.

29. The method of the preceding claim, the method further comprising: depositing a second layer (20) at least on top of the following elements: the first sloped sidewall, the first stage, the second sloped sidewall, the second state, and the third sloped sidewall.