Light coupling and mode-selective separation or superposition of optical fields

Abstract

The invention relates to a device for optical coupling and for mode-selective separation or superposition of light fields, to a use thereof and to a method for producing a waveguide-based optical coupling element (10) designed for mode-selective separation or superposition of light fields at a further optical coupling point (410) of an optical assembly (400). The device comprises at least one waveguide-based optical coupling element (10) having at least three optical coupling points (100, 370, 380) and at least one optical assembly (400) having at least one further optical coupling point (410), wherein the at least one optical coupling point (100, 370, 380) is optically connected to the at least one further optical coupling point (410), and wherein the waveguide-based optical coupling element (10) is designed to efficiently and bidirectionally transport light between eigenmodes (120, 260) associated with a first optical coupling point (100) and a second optical coupling point (370) and between eigenmodes (130, 280) associated with the first optical coupling point (100) and a third optical coupling point (380).

Description

Technical Field

[0001] This invention belongs to the fields of integrated photonics and micro-optics, and particularly relates to micro-optics and nano-optics systems in which light is transmitted between various optical elements or between free-space extensions and optical elements via optical coupling points. Specifically, this invention relates to apparatuses for optical coupling and for mode-selective separation or superposition of optical fields, and their uses, as well as a method for manufacturing waveguide-based optical coupling elements configured for mode-selective separation or superposition at optical coupling points in optical component portions. Background Technology

[0002] The functionality of integrated optical or micro-optical systems often depends decisively on whether the light to be transmitted has a certain spatial distribution and polarization at each optical coupling point; for example, to achieve high coupling efficiency, to facilitate the efficient excitation of certain waveguide modes in the case of waveguide-based components, or to convert the light emitted by the component into the desired field distribution in free space. In this case, the distribution and polarization of light are generally described by vector mode fields, which include the spatial distribution of the vector electric field E(x,y) and the spatial distribution of the vector magnetic field H(x,y).

[0003] According to existing technology, discrete optical elements are typically used to adjust the intensity distribution of the mode field. These optical elements include lenses, gradient-index fibers, curved mirrors, or other refractive, diffractive, or reflective optical elements. In contrast, polarization-manipulating optical elements, such as polarization filters or birefringent optical elements (especially half-wave or quarter-wave plates), or suitable optical fibers (e.g., polarization-maintaining fibers), are typically used to set the direction of the field vectors of the electric and magnetic fields. In many practical applications, these elements need to be properly combined with each other, especially to obtain the desired vector mode field distribution at the optical coupling point of the optical elements. First, this results in a relatively large arrangement, with the installation space typically several times larger than that of the associated optical components. Furthermore, the individual discrete optical elements must be aligned very precisely relative to each other and very precisely relative to the optical coupling point of the optical components. This typically requires time-consuming and expensive adjustment methods, especially active adjustment methods that continuously measure and optimize optical coupling efficiency during the positioning process. Such adjustment methods are complex to apply and are only suitable for the mass production of optical or micro-optical systems.

[0004] This problem arises particularly when light from free space, from optical fibers, or from parts of an optical component is intended to couple to a single-mode waveguide of another element, especially in a particular mode defined by polarization direction. In the case of an axially uniform waveguide (i.e., a waveguide with an invariant cross-sectional profile in the propagation direction), the term "waveguide mode" refers to a form of electromagnetic field that does not alter the lateral spatial dependence in the axial direction during propagation. A waveguide mode may have a lower-limit frequency, which guides the mode down to that frequency in the corresponding waveguide, but it is no longer possible to guide it to a lower frequency. A "fundamental mode" refers to the waveguide mode with the lowest limiting frequency compared to other modes in the same mode family, the mode family being determined, for example, by polarization. In the case of a stepped-profile waveguide, the fundamental mode is distinguished by the fact that the lateral intensity distribution belonging to the mode field has a single maximum value in the region at the center of the waveguide, whereas otherwise there are no zeros in the lateral intensity distribution.

[0005] In many cases, guided waveguide modes can be subdivided into two distinct families of modes based on their polarization states, often referred to as “transverse electric” (“TE”) or “transverse magnetic” (“TM”). In this case, a field distribution with a minimum limiting frequency can be determined for each family of modes, resulting in two fundamental modes with different polarization states. Below, “single-mode waveguide” is understood to refer to a waveguide used for electromagnetic radiation in which at most two mutually orthogonal fundamental modes with different polarizations can propagate along the waveguide axis at the operating frequency. The term “polarization” or “polarization direction” of a waveguide mode describes the direction of the electric field vector belonging to that waveguide mode, with the direction of the dominant transverse component of the electric field often used in many cases.

[0006] In the case of waveguides with cross-sections of continuous or discrete rotational symmetry (e.g., circular or square), degenerate or nearly degenerate waveguide modes often occur. These waveguide modes have the same or similar propagation constants, and their mode fields can be completely or approximately interconverted through appropriate rotation. Furthermore, in the case of degenerate or nearly degenerate modes, any linear combination of the two mode fields will propagate axially with the same propagation constant as the initial mode, and will completely or approximately maintain its transverse field distribution in the process. The polarization characteristics of the superposition of two degenerate or nearly degenerate waveguide modes can be described by the associated polarization states—in a manner similar to the superposition of plane waves in free space. This can also occur when various optical elements are coupled between them or when they are connected to optical fibers, particularly when light from two degenerate or nearly degenerate modes with different polarizations from a first single-mode or multimode waveguide must be coupled into two other single-mode or multimode waveguides with non-degenerate eigenmodes, exciting only the corresponding eigenmode in each of the latter two waveguides. This is the case, for example, when coupling light from an optical fiber with a rotationally symmetric cross-sectional profile (such as a standard single-mode fiber), thereby simplifying different polarization modes into the basic modes (often referred to as TE or TM) of a strongly birefringent waveguide on an integrated optical chip.

[0007] US 7127131B2 discloses an integrated optical polarization beamsplitter fabricated on a planar semiconductor substrate using a planar microstructuring method from multiple layers referred to therein as "core layers". Therefore, this structure not only requires a relatively complex fabrication process involving the processing of at least two layers with high overlap accuracy, but is also limited by the planar structural geometry, which is typically a partially prismatic structure with parallel bottom and top surfaces, where the centerlines of all or part of the waveguides lie in the same or mutually parallel planes. This limits functionality, for example, leading to asymmetric losses in the two separated modes. Furthermore, the components described herein are only applicable to two polarizations on a single optical chip.

[0008] US 7228015B2 discloses an integrated optical waveguide that results in a 90° polarization rotation of the light field propagating therein. It is also composed of multiple discrete layers called "core layers," thus only roughly approximating its ideal form, which has a rectangular cross-section twisted along its longitudinal axis. Therefore, it is also constrained by a planar structural geometry, with a partially prismatic structure having parallel bottom and top surfaces, where the centerlines of all waveguides lie in a common or parallel plane. It also requires a complex fabrication process that repeatedly applies conventional microstructuring methods, particularly using planar structured masks and anisotropic etching processes to process the individual layers.

[0009] Watts et al., in *Polarization splitting and rotating through adiabatic transitions*, *Integrated Photonics Research*, A. Sawchuk, ed., Vol. 91 of OSA Trends in Optics and Photonics, 2003, described an integrated optical structure that combines a polarization beam splitter disclosed in US7127131B2 and a polarization rotation device disclosed in US7228015B2. Therefore, it is essentially subject to the same constraints as the related partial structures. It is fabricated from multiple discrete layers applied to a planar semiconductor substrate and processed using conventional planar microstructuring methods.

[0010] Schumann et al. disclosed in *Hybrid 2D-3D optical devices for integrated optics by direct laser writing*, *Light Science and Applications*, Vol. 3, No. 6, 2014, a polymer waveguide fabricated on a chip surface using 3D photolithography. This polymer waveguide is twisted along its longitudinal axis, allowing polarization rotation. The structure described therein is used only to interconnect two Si3N4 waveguides on an optical chip.

[0011] Hahn et al., in *Polarizing beam splitter integrated onto an optical fiber facet*, *Optics Express*, Vol. 26, No. 25, 2018, described a polarization beam splitter fabricated on the facet of an optical fiber using 3D photolithography. The polarization beam splitter comprises a layered grating with a grating period on the order of the vacuum wavelength of the light used, or with a shorter grating period (also referred to as a "subwavelength layered grating"). In the layered grating, incident radiation with a polarization called "TE" is coupled to a certain diffraction order, while another polarization called "TM" passes through the grating largely undisturbed.

[0012] WO 92 / 00185 A1 discloses the generation of an optical waveguide by focusing a beam from a high-power laser through a lens onto a photostructureable material to achieve photoinitiated polymerization of the material at the focal point. Chains of polymerized material with a higher refractive index than the surrounding bulk material are generated along a path created by moving the focal point through the photostructureable material, and these chains can act as optical waveguides. This method can be used to generate optical waveguide devices comprising multiple waveguide chains.

[0013] US 2018 / 0314005 A1 discloses a planar integrated polarization beamsplitter comprising a waveguide core made of silicon nitride and configured to split an input optical signal into two waveguide modes with different polarizations. However, this arrangement is a structure produced by a planar microstructuring method, which, compared to the free-form structure used in this application, also has the limitations described in the context of US7127131B2 and US 7228015B2.

[0014] US 8,903,205 B2 and US 9,034,222 B2 disclose a method and arrangement for interconnecting different optical components using optical freeform waveguides, which are fabricated at target locations using 3D photolithography. The fact that freeform waveguides can be easily adapted to the location, form, and size of the optical components to be connected, in terms of position, shape, and size, is utilized here.

[0015] Purpose of the invention

[0016] Therefore, the object of the present invention is to provide an apparatus for optical coupling and for mode-selective separation or superposition of optical fields, the use of the apparatus, and a method for generating waveguide-based optical coupling elements configured to perform mode-selective separation or superposition of optical fields at the optical coupling point of an optical element, which at least partially overcomes the disadvantages and limitations of the prior art.

[0017] In particular, the object of the present invention is to use the apparatus and method to couple light to optical component portions and / or couple light between two or more optical component portions while simultaneously adjusting the spatial mode field distribution and polarization. Furthermore, in the opposite direction, the present invention should allow light to couple away from the optical component portions and should allow said light to have a specific field distribution and polarization.

[0018] The device should be as compact as possible and should also be capable of automated mass production at the lowest possible cost. Furthermore, the arrangement should allow for precise alignment with a small plane of at least one component without requiring complex adjustment methods, particularly active adjustment methods, and should not complicate the manufacturing process of the optical components, especially avoiding the use of complex integrated mode field converters or polarization converters.

[0019] The device and method should also allow the separation of spatially overlapping but polarized field components from input fields, and allow the separated field components to be supplied to different, spatially non-overlapping optical waveguides; this function is comparable to a polarization beam splitter composed of discrete components in an optical system.

[0020] Furthermore, in the opposite direction, this arrangement and method should allow for the combination of light from different, spatially non-overlapping optical waveguides, as well as the superposition of light in the form of different polarization field components to form an output field; this function is equivalent to a "polarization beam combiner".

[0021] In particular, this arrangement and method are designed to allow an optical fiber with degenerate eigenmodes having possible vertical polarization directions to be coupled to two integrated optical waveguides in such a way that light from the first eigenmode of two mutually orthogonal eigenmodes of the optical fiber can be converted to a fundamental mode of the first integrated optical waveguide, while light from the second eigenmode of two mutually orthogonal eigenmodes of the optical fiber can be converted to a fundamental mode of the second integrated optical waveguide.

[0022] Furthermore, the opposite direction of propagation should allow light from two spatially separated optical waveguides to be superimposed in the form of two mutually orthogonal eigenmodes of the optical fiber.

[0023] In this context, the device should be flexible enough to be used on a variety of optical integration platforms and should be implemented as without additional separate optical components as possible. The relevant methods are designed to be seamlessly integrated into the processes and connection technologies of optical construction. Summary of the Invention

[0024] This objective is achieved by an apparatus for optical coupling and mode-selective separation or superposition of optical fields, by using the apparatus, and by a method for generating waveguide-based optical coupling elements configured for mode-selective separation or superposition at the optical coupling points of optical components, having the features of the independent claims. Advantageous developments that can be achieved individually or in any combination are provided in the dependent claims.

[0025] The words “have,” “include,” or “contain,” or any desired grammatical deviation, will be used in a non-exclusive manner below. Thus, these words can relate to a situation where no other features exist besides those introduced by these words, or to a situation where one or more additional features exist. For example, the statements “A has B,” “A includes B,” or “A contains B” can relate to a situation where no other elements besides B exist in A (i.e., A consists only of B), or to a situation where one or more other elements exist in A besides B, such as element C, elements C and D, or even other elements.

[0026] It should also be noted that the expressions “at least one” and “one or more”, and the grammatical variations thereof, when used in conjunction with one or more elements or features, are typically used only once if the expression is intended to indicate that the element or feature may be provided once or multiple times, for example, when the feature or element is first introduced. If the feature or element is subsequently mentioned again, the corresponding terms “at least one” or “one or more” are generally no longer used, without limiting the possibility that the feature or element may be provided once or multiple times.

[0027] Furthermore, the terms "preferredly," "especially," "particularly," "for example," or similar words are used in conjunction with optional features below, and alternative embodiments are not limited in this way. For example, features introduced by these words are optional features and are not intended to limit the scope of the claims, particularly the scope of the independent claims, by these features. For example, as those skilled in the art will understand, the invention can also be implemented using different configurations. Similarly, features introduced by "in embodiments of the invention" or "in exemplary embodiments of the invention" are understood to be optional features and are not intended to limit the scope of alternative configurations or the independent claims herein. Moreover, these introductory expressions are not intended to cover all possibilities of combining the features described herein with other features, whether optional or non-optional.

[0028] In a first aspect, the present invention relates to an apparatus for optical coupling and for mode-selective separation or superposition of optical fields, the apparatus comprising at least:

[0029] - At least one waveguide-based optical coupling element having at least three optical coupling points

[0030] At least one first optical coupling point, having at least two distinct guided eigenmodes assigned to the first optical coupling point.

[0031] At least one second optical coupling point, having at least one guided eigenmode assigned to the second optical coupling point, and

[0032] At least one third optical coupling point, having at least one guided eigenmode assigned to the third optical coupling point.

[0033] - At least one optical component portion having at least one additional optical coupling point;

[0034] Wherein, at least one optical coupling point of the waveguide-based optical coupling element is optically connected to at least one other optical coupling point of the optical component section, and

[0035] Among them, the waveguide-based optical coupling element is configured for efficient bidirectional optical transmission.

[0036] Between at least one first guide eigenmode assigned to the first optical coupling point and at least one guide eigenmode assigned to the second optical coupling point, and

[0037] ο Between at least one second guide eigenmode assigned to the first optical coupling point and at least one guide eigenmode assigned to the third optical coupling point.

[0038] The terms “optical radiation,” “radiation,” or “light” refer to any type of electromagnetic wave that can be guided in a waveguide. This includes, in addition to the visible light range with vacuum wavelengths λ between 400 nm and 800 nm, the UV range of 1 nm ≤ λ ≤ 400 nm, the infrared range of 800 nm ≤ λ < 1 mm, and the microwave range of 1 mm ≤ λ ≤ 1 m. The range of 30 μm ≤ λ ≤ 3 mm is also referred to as the “terahertz range,” and the range of 1 mm ≤ λ ≤ 1 cm is also referred to as the “millimeter-wave range.” Unless otherwise specified, the values ​​specified below, particularly for structural dimensions or performance metrics used to describe microstructuring methods, such as resolution or accuracy, relate to the arrangement configured for a vacuum operating wavelength λ of approximately 1.5 μm. For other operating wavelengths, the specified values ​​may be scaled proportionally to the wavelength, especially taking into account the refractive index of the material used.

[0039] Furthermore, the terms "optical coupling point," "optical coupling structure," and "facet" used in this paper each refer to...

[0040] -First, there is the region of the light-emitting optical component or the region of the structure of the light-emitting optical component, through which the light last passes during emission, and

[0041] - Secondly, there is the area of ​​the light-receiving optical component or the area of ​​the structure of the light-receiving optical component, where the light first shines when it is received.

[0042] Hereinafter, the terms "optical component portion" and "optical component" refer to optical elements configured to emit, transmit, receive, detect, and / or manipulate electromagnetic radiation, while the term "optical system" refers to a device with at least two optical components, or a combination of one or more devices according to the invention, wherein at least one optical element or a combination of at least one additional structure resulting from the combination with a device according to the invention, particularly at least one optical waveguide or at least one micro-optical element. Preferably, each optical component used within the scope of the invention is selected from: optical fibers, particularly single-mode or multimode optical fibers made of organic or inorganic materials; semiconductor-based integrated optical chips, particularly photodiodes, linear or planar photodiode arrays, CCD arrays, or image sensors, particularly semiconductor-based, preferably silicon or III-V compound semiconductors, or dielectric materials, preferably glass, silicon dioxide, silicon nitride, or polymers; calorimeters; lasers, particularly vertical-cavity surface-emitting lasers (VCSELs) or edge-emitting lasers; superluminescent diodes; optical path boards; free-beam optical elements, particularly lenses, beam splitters, isolators, mirrors, or diffraction gratings. Other optical components are conceivable. Optical components may preferably include optical waveguides with low exponential contrast, particularly glass-based waveguides, or waveguides with medium or high exponential contrast, particularly semiconductor-based waveguides. Input or output coupling of light may preferably occur at the edges or surfaces of the optical component; particularly at the edges of edge-emitting lasers, chip edges, or planes of waveguide-based systems; or on the surface of surface-emitting lasers or surface-illuminating photodiodes, or on the surface of waveguide-based chips including at least one optical coupling point, particularly selected from grating couplers or deflectors. However, other methods of input or output coupling of light are possible.

[0043] To couple light to the optical coupling point of an optical element with low loss, the light preferably radiates into the optical coupling point at a defined location and in a defined direction, such that the light has a defined field distribution. Conversely, the optical coupling point is positioned such that the light radiates with a defined field distribution in a defined direction. In this document, the term "vector field distribution" or "field distribution" is understood to refer to the combination of complex vector electric fields (E-field) and magnetic fields (H-field), which define the intensity distribution and polarization of the electromagnetic field, where "polarization" refers to the direction of the corresponding vector field. Furthermore, the term "orthogonality" of the field distribution relates to the orthogonality relations commonly used in integrated optics; see, for example, Katsunari Okamoto, Fundamentals of Optical Waveguides, Academic Press, 2006, pp. 154-155.

[0044] The terms "mode field" and "mode field distribution," which describe the vector field distribution of waveguide modes associated with the waveguide cross-section, are used in relation to the field distribution of optical waveguides. As already mentioned, in the case of a waveguide that is uniform in the axial direction, the terms "waveguide mode," "eigenmode," or simply "mode" refer to the form of the electromagnetic field that does not change its transverse spatial dependence under axial propagation. In the case of more complex waveguides, such as those whose cross-sectional profile varies periodically in the axial direction, the associated mode field may also vary periodically accordingly. Waveguides whose cross-sectional profile varies sufficiently slowly in the axial direction (i.e., adiabatic variation) can generally be described as well approximating a mode field based on a correspondingly slowly varying (i.e., adiabatic variation).

[0045] This device for optical coupling and for mode-selective separation or superposition of optical fields includes a waveguide-based optical coupling element, simply referred to as an "optical coupling element," which allows light to be coupled into and / or transmitted between at least two optical component sections while adjusting the spatial mode field distribution and polarization. Regarding the optical coupling element, the term "waveguide-based" in this context describes a structure in which light is at least partially guided by a waveguide configured for this purpose. In principle, a waveguide-based optical coupling element can be implemented on the basis of any waveguide concept. Dielectric waveguides are preferably suitable for this purpose; alternatively, metallic waveguides, particularly hollow waveguides for the microwave range, or plasma structures can be used.

[0046] Therefore, waveguide-based optical coupling elements are first applicable to splitting the superposition of at least two mutually orthogonal or nearly orthogonal field distributions existing at a first optical coupling point of the optical coupling element, and are applicable to simultaneously manipulating the associated spatial field distribution and / or polarization. Secondly, waveguide-based optical coupling elements can be used to manipulate (with respect to their field distribution and / or polarization) optical signals radiated at at least two spatially separated optical coupling points, so that the optical signals radiated in the form of spatially overlapping partial fields of different modes are superimposed, and then the partial field superposition is provided at at least one output coupling point.

[0047] Furthermore, waveguide-based optical coupling elements can be used as polarization filters. For this purpose, the optical signal to be filtered regarding polarization can be coupled to the optical coupling element through a first optical coupling point. The desired polarization-dependent filtered signal can then be obtained in one of the guided eigenmodes at a second or third optical coupling point, while the signal component to be suppressed by polarization filtering is provided to a terminating element connected to a corresponding other optical coupling point. The terminating element or beam collector is understood as a structure that receives and absorbs incident light without significant back reflection or radiates the light into the surrounding environment, thus avoiding recoupling to the waveguide-based optical coupling element or one of the connected optical components. The power level of back reflection at the input of the beam collector is preferably at least 10 dB lower than the incident power, particularly preferably at least 20 dB or 30 dB. In a preferred embodiment, the beam collector can be implemented in the form of a continuously tapered conical structure, thereby allowing light to be emitted particularly in the direction of the absorbing surface. Coupling with the optical components can be achieved directly, or as described above, through at least one connecting waveguide or at least one free beam coupling extension.

[0048] Waveguide-based optical coupling elements can be generated in situ at the optical coupling point of an optical component or between at least two optical coupling points of at least one optical component using a three-dimensional free-form microstructuring method. In this process, adjustments can be made, particularly regarding the position, shape, and / or size of one or more optical component sections. In the following explanation, at least two spatially overlapping, orthogonal, or nearly orthogonal field distributions fed or emitted at the first optical coupling point of the optical coupling element are, in principle, interpreted as fundamental modes with different polarizations; the associated device then functions as a polarization beamsplitter or polarization beam combiner. The term "mode selectivity" indicates that the device according to the invention can be used to separate any mode, particularly two modes with the same polarization but different field distributions, as a result of appropriate adjustment design of the waveguide-based optical coupling element.

[0049] Modes can be separated by the fact that they are guided at different intensities within a properly formed waveguide, and thus separation can be achieved through geometric deviations of the waveguide. In this paper, a “strongly guided” mode of a waveguide is understood to be a waveguide mode with a significantly larger propagation constant than other modes guided in that waveguide, referred to as “weakly guided” modes, and therefore a significantly larger effective refractive index. In particular, strongly guided modes are distinguished by their substantially stronger modulation to changes in the waveguide trajectory and / or the waveguide cross-section along the propagation direction (e.g., twisting or diameter changes), compared to weakly guided modes. It is also conceivable that only strongly guided modes exist in a waveguide. In many cases, a strongly guided waveguide mode is the fundamental mode whose electric field is directionally polarized primarily along the waveguide core, exhibiting its maximum range.

[0050] As an alternative to mode separation via geometric deviation waveguides, a configuration is conceivable in which different strong couplings to be separated from the modes to be separated from the parallel-running waveguides are used in a manner similar to a so-called "directional coupler." Another option involves targeted mode conversion by periodically modulating the waveguide cross-section in the axial direction, the fundamental wavenumber of which corresponds to the wavenumber difference of the modes to be coupled. Thus, any desired mode can be converted into a field form that can be separated from each other in a particularly low-loss and reliable manner. Potential applications in this case are preferably mode separation at the end faces of multimode or so-called few-mode fibers, and coupling the input of the corresponding optical signal to different optical coupling points in the optical component section. The preferred configuration of the coupling element for this purpose can be determined, in particular, by a so-called "topology optimization method," in which the entire form of the optical coupling element, rather than a single geometric parameter, can be digitally optimized. The structured geometry thus obtained does not conform to a generally valid description, but it is equally helpful for the implementation of the device for optical coupling and mode separation according to the invention.

[0051] For further details regarding the proposed device, please refer to the exemplary embodiments below.

[0052] On the other hand, the present invention relates to a method for manufacturing a waveguide-based optical coupling element configured to perform mode-selective separation or superposition of optical fields at the optical coupling point of an optical component portion. Specifically, the method comprises the following steps:

[0053] a) Provide at least one optical component portion and position at least one additional optical coupling point of the at least one optical component portion in the coordinate system of a free-form microstructure unit, the free-form microstructure unit being configured to perform a free-form microstructure method;

[0054] b) Generate a three-dimensional dataset of waveguide-based optical coupling elements in a coordinate system describing the microstructured units, where

[0055] - Waveguide-based optical coupling elements have at least three optical coupling points.

[0056] At least one first optical coupling point, having at least two distinct guided eigenmodes assigned to the first optical coupling point.

[0057] At least one second optical coupling point, having at least one guided eigenmode assigned to the second optical coupling point, and

[0058] At least one third optical coupling point, having at least one guided eigenmode assigned to the third optical coupling point.

[0059] -Among them, the waveguide-based optical coupling element is configured for efficient bidirectional optical transmission.

[0060] Between at least one first guide eigenmode assigned to the first optical coupling point and at least one guide eigenmode assigned to the second optical coupling point, and

[0061] ο Between at least one second guide eigenmode assigned to the first optical coupling point and at least one guide eigenmode assigned to the third optical coupling point;

[0062] c) A waveguide-based optical coupling element is generated at at least one additional optical coupling point in at least one optical component section by using a free-form microstructuring method.

[0063] The implementation of steps a) through c) does not need to be strictly sequential, but can also be included in other parallel manufacturing processes. In this case, each of steps a) through c) can be performed multiple times, and at least consecutive steps can be performed at least partially in parallel. Furthermore, additional steps can be performed, particularly step d) listed below. In particular, the dataset generated in step b) may contain simple connecting waveguides or micro-optical elements, such as lenses or mirrors, in addition to waveguide-based optical coupling elements for mode-selective separation or superposition of the optical field. The design of these simple connecting waveguides or micro-optical elements is also based on the location and orientation of certain optical coupling points, and these can be fabricated together with the waveguide-based optical coupling elements according to step c). Furthermore, the basic structure of the waveguide-based optical coupling elements generated in step c) can undergo further post-processing steps, within which the generated basic structure can, for example, be locally or globally embedded with an optical low-refractive-index cladding material or be provided with a vapor-deposited coating. For example, assignment or printing methods or microstructuring methods similar to those used in step c) can be used for the local application of the corresponding cladding material. In addition to waveguide-based optical coupling elements for mode-selective separation or superposition of optical fields, the corresponding dataset may also contain simple connecting waveguides or micro-optical elements, such as lenses or mirrors, wherein the design of the simple connecting waveguides or micro-optical elements is also based on the location and orientation of certain optical coupling points, and these can be fabricated together with the waveguide-based optical coupling elements according to step c).

[0064] In a preferred configuration of this method, subsequent step d) can be performed, preferably after step c):

[0065] d) The waveguide-based optical coupling element is at least partially embedded in the cladding region adjacent to the waveguide-based coupling element as the core region, the core region having a refractive index of 1.3 to 1.8 and a refractive index difference of 0.05 to 0.7 between the core region and the cladding region.

[0066] The device is preferably manufactured in situ using a microstructuring method configured for this purpose, i.e., directly at the target location. In this context, the term "microstructuring method" refers to subtractive or additive manufacturing methods that can produce three-dimensional structures, preferably free-form structures, whose dimensions depend on the operating wavelength in the micrometer and / or millimeter range. The microstructuring method configured to produce free-form structures is referred to below as a "free-form microstructuring method." "Free-form" or "free-form structure" is understood to mean a structure that can at least locally have a surface with any desired curvature (within the technical limitations related to resolution and accuracy). Therefore, free-form structures are particularly different from structural geometries that can be produced on planar semiconductor substrates by conventional planar microstructuring methods, such as combinations of thin-film deposition methods and two-dimensional lithography methods (e.g., projection lithography and etching processes). Typically, combinations of these conventional planar microstructuring methods result in prismatic three-dimensional structure geometries, each having a top and bottom surface substantially parallel to the substrate surface, depending on its respective etching or deposition process. These top and bottom surfaces are identical or very similar in shape and interconnected by sidewalls that are perpendicular, inclined, or arched inward or outward relative to the substrate surface. In this case, the form of the basal and top surfaces is essentially specified by a mask used for local etching or deposition, typically photolithographically structured. Multilayer structures consisting of multiple prismatic local structures can be constructed through repeated etching or deposition processes using different masks; the additional costs associated with repetition are substantial, and the quality of the obtained structure is often limited by stacking accuracy, thus in practice the number of layers is typically limited to a few, such as three. This leads to geometrical limitations on structures produced at a reasonable cost using conventional microstructuring methods, and consequently, functional limitations on the resulting components. The additional costs associated with multilayer structures are often very high and complicate the production process of related optical elements, especially if the associated additional layers cannot be used for other components present on the chip.

[0067] In contrast, free-form structures generated through free-form microstructuring methods are not subject to these limitations, or to the same degree, because their structural geometries are not limited to combinations of a relatively small number of planar, prismatic local structures. This, in particular, makes it possible to generate waveguide-based optical coupling elements with non-planar structures, in which case the centerlines of the waveguides forming the coupling element do not necessarily lie in a common plane or mutually parallel planes. It should be noted that in many cases, free-form structures are also generated from multiple individual layers, for example, through the application of multilayer materials in the realm of 3D printing or through 3D lithography methods that solidify different layers. However, within a reasonable production cost range, free-form microstructuring methods allow for such a large number of layers that this produces a good approximation of the free-form structure, and discretizing into individual layers no longer presents any practically functional limitations on the resulting structural geometry.

[0068] In this case, the waveguide-based optical coupling element preferably consists of at least 6 layers, particularly preferably at least 10 layers, and especially at least 20 or 30 layers. The layer thickness is preferably in the range of 10 nm to 1000 nm, particularly preferably between 30 nm and 500 nm, and especially between 50 nm and 300 nm. The free-form microstructuring method used for this purpose allows for the generation of free-form structures with an accuracy preferably better than 1000 nm, particularly preferably better than 500 nm, and especially better than 100 nm. The resolution of the free-form microstructuring method is preferably better than 3 μm, particularly preferably better than 1 μm, and especially better than 500 nm. In this case, the listed values ​​are related to the generation of free-form structures in each case, which provide a vacuum operating wavelength of approximately 1.5 μm; the size of the free-form structure, and therefore the requirements for the accuracy and resolution of the free-form microstructuring method used for the generation purpose, can be scaled for other operating wavelengths, especially taking into account the refractive index of the material used.

[0069] In contrast, WO 92 / 00185A1 discloses a waveguide-based optical device in which the local increase in the refractive index of the optically structured material is achieved entirely through photolithography. Compared to this application, the method described in WO 92 / 00185A1 does not specifically provide a development step, within which unexposed areas are selectively removed and replaced with a low-refractive-index cladding material. Therefore, the achievable refractive index difference is typically limited to values ​​below 0.025, making the generation of compact polarization-sensitive structures more difficult. Specifically, according to WO 92 / 00185A1, the generation of a waveguide-based polarization beamsplitter comprising two overlapping partial waveguides appears to be unachievable; due to the small exponential contrast, the two orthogonal polarization eigenmodes of the partial waveguides effectively have the same effective refractive index and are thus guided with equal intensity. Therefore, if the practically achievable component length is maintained, separating the orthogonal polarization eigenmodes by bifurcising the partial waveguides appears impossible.

[0070] In a preferred configuration, the free-form microstructuring method and / or the free-form microstructuring units that facilitate this method can be based on photolithography, particularly stereolithography or direct writing lithography, preferably three-dimensional direct writing lithography. In this case, additive manufacturing or subtractive manufacturing methods can be used; the term "additive manufacturing method" refers to a production method that continuously applies material to a structure or onto a structure, while the term "subtractive manufacturing method" describes an alternative production method that removes material from a structure. In a preferred embodiment, material application or ablation can be achieved by photolithography using a suitable photoresist, particularly a negative or positive resist. In this case, a spatial light modulator that allows for rapid prototyping can preferably be configured for stereolithography. In a preferred configuration, multiphoton lithography, particularly by using a pulsed laser source, can be used as a direct writing lithography method. In this case, light pulses can be used, with a pulse duration preferably not exceeding 10 ps, ​​preferably not exceeding 1 ps, particularly preferably not exceeding 200 fs, particularly not exceeding 100 fs, and a repetition rate preferably at least 1 MHz, preferably 10 MHz, particularly preferably at least 25 MHz, particularly at least 100 MHz. Laser sources suitable for this purpose are particularly selected from fiber-based femtosecond lasers or pulsed solid-state lasers such as Ti:sapphire lasers or diode lasers, which can be combined with frequency conversion units, for example, for frequency doubling, sum-frequency generation, or difference-frequency generation. Depending on the lithography method used, wavelengths in the near-infrared, visible, or ultraviolet spectral range, or the extreme ultraviolet (EUV) radiation range, or the X-ray wavelength range can be preferentially used. In particularly preferred embodiments, wavelengths from 150 nm to 1700 nm, especially from 300 nm to 1100 nm, are used. In pulsed lasers, two-photon, three-photon, or multi-photon absorption effects can be achieved in a targeted manner by selecting appropriate pulse duration and pulse energy. Diode lasers with emission wavelengths from 360 nm to 550 nm, i.e., approximately 365 nm, 385 nm, 405 nm, 550 nm, and 532 nm, are suitable for absorption using lithography methods based on single-photon continuous-wave lasers. To improve the resolution of photolithography methods, stimulated emission reduction (STED) can be used in conjunction with suitable photoinitiators, employing appropriate microscopy techniques. Furthermore, other microstructuring methods for fabricating waveguide-based optical coupling elements are conceivable, particularly those based on material extrusion, powder bed fusion, material jetting, binder jetting, selective laser sintering, or electron beam melting. For example, methods such as metal printing or laser deposition welding can be used to fabricate hollow waveguides in the micrometer and millimeter wavelength ranges. Depending on the microstructuring method employed, waveguide-based optical coupling elements can comprise polymers, preferably optically additive or subtractive acrylates, epoxy resins, or fluoropolymers, metals, or metal-coated dielectrics.In a preferred configuration, the waveguide-based coupling element may comprise a material different from that of the optical component portion. Further post-processing steps may be advantageous to produce these structures, within which the resulting structures may be partially or entirely embedded in an optical low-refractive-index cladding material, or may be provided with a vapor-deposited coating.

[0071] The use of free-form microstructuring methods can particularly produce structures with symmetrical or nearly symmetrical geometries, which are preferably likely to have very similar losses for the two separation modes. In this document, "nearly symmetrical structural geometry" should be understood as a three-dimensional form with a plane of symmetry, an axis of symmetry, or a point of symmetry, where perfect symmetry may be slightly compromised by adjusting the structural geometry. This adjustment of the structural geometry is specifically used to couple waveguide-based optical coupling elements to the position and orientation of at least one optical coupling point assigned to at least one optical coupling element to be connected, as described below. The power loss difference between the two separation modes is preferably less than 3 dB, particularly preferably less than 2 dB, and especially less than 1 dB or 0.5 dB.

[0072] Generally, when the device according to the invention is used as a polarization beamsplitter, an extinction ratio preferably better than 6 dB, particularly preferably better than 10 dB, especially better than 15 dB or 20 dB can be achieved at the output coupling point. In this case, "extinction ratio" is understood as the quotient of the emission power in the desired mode and the emission power in the corresponding unwanted mode at the output coupling point, which is typically expressed in decibels (dB) through logarithmic conversion. In this case, the relative optical bandwidth of the structure can preferably be greater than 1%, particularly preferably greater than 5%, especially greater than 10% or 20%. Here, "relative optical bandwidth" is understood as the ratio of the frequency range width (within the frequency range on which the optical component portion achieves the desired power rating) to the corresponding mid-frequency band.

[0073] Another advantage of the proposed device is that, preferably with the aid of an additional connecting waveguide, in-situ generation allows the device to couple to one or more optical coupling points of one or more optical component parts with very low loss, without requiring high-precision alignment of the optical components in complex adjustment methods. To this end, starting from the already fixed optical component parts, the spatial position and orientation of the optical coupling points belonging to these optical component parts can be obtained in the first step of the manufacturing process and can be taken into account when designing the optical coupling elements and preferably the additional structures, such as connecting waveguides. This allows for compensation of errors in the positioning of the optical components by corresponding adjustments to the form of the claimed device, aided by the design of the optical coupling elements and optionally the additional structures, which are selected such that light is available or received at the optical coupling point of the waveguide-based optical coupling element and / or at a designated optical coupling point in an adjacent additional structure (e.g., a connecting waveguide), and has the necessary position and propagation direction, thus achieving high efficiency coupling to the optical coupling points of the optical components and / or to the waveguide modes defined by the optical coupling points.

[0074] By adjusting the position and orientation of the waveguide-based optical coupling element to the optical coupling point of the optical components to be connected, positioning errors of the optical component parts to be connected can be compensated, thereby providing highly precise alignment of these component parts. To adapt the waveguide-based optical coupling element to the position and orientation of the optical coupling point of the optical components to be connected, the geometric parameters of the optical coupling element can preferably be modified, particularly the length of the first waveguide cross-section, and / or the precise trajectory of a portion of the waveguide, as described below. Alternatively or additionally, additional connecting waveguides or beamforming elements, having virtually any desired 3D geometry, can be connected to the optical coupling element and / or selected optical coupling points of the optical components to be connected. These additional connecting waveguides or beamforming elements can be fabricated together with the optical coupling element without the significant additional expenditure of freeform microstructuring methods and allow for compensation of positioning errors of the optical component parts to be connected.

[0075] In a preferred configuration, the free-form microstructuring method can be configured to, preferably within the scope of the common generation steps, generate so-called "photonic wire bonding" in addition to waveguide-based coupling elements, as disclosed, for example, in US 8903 205B2 or WO 2018 / 083191 A1. Other methods, such as three-dimensional printing processes, are also conceivable, especially if the intention is to generate relatively large structures for operation at frequencies in the micrometer and millimeter wavelength range.

[0076] For further details relating to this method, please refer to the description of the apparatus and exemplary embodiments. Attached Figure Description

[0077] Further details and features of the invention will become apparent from the following description of preferred exemplary embodiments, particularly in conjunction with the dependent claims. Here, various features may be implemented individually, or multiple features may be combined together. The invention is not limited to exemplary embodiments. Exemplary embodiments are schematically illustrated in the following figures. The same reference numerals in the figures refer to the same or functionally identical elements or elements that correspond to each other in their function. Specifically:

[0078] Figures 1 to 11 Each of the diagrams illustrates a preferred exemplary embodiment of the apparatus according to the invention for optical coupling and for mode-selective separation or superposition of light fields;

[0079] Figures 12 to 14 Each of the diagrams shows a schematic of the device, which corresponds to a waveguide-based simulation of a polarization beamsplitter with four optical coupling points, and the device can be generated by a free-form microstructuring method on the end face of a multi-core fiber.

[0080] Figure 15 This is a schematic diagram of a passive optical waveguide structure. It belongs to the category of optical polarization multiplexing heterodyne receivers and can be fabricated on the end face of a seven-core optical fiber using a free microstructuring method.

[0081] Figure 16 A schematic diagram of a polarization analyzer structure is shown, which can be generated on the end face of a single-mode fiber using a free-form microstructuring method.

[0082] Figure 17 A schematic diagram of a reflective polarization switch is shown, which exchanges signal components existing in, for example, two mutually orthogonal fundamental polarization modes (so-called LP01 modes) of an optical fiber in a manner similar to a Faraday rotator, and couples the signal components back into the optical fiber in opposite polarization directions.

[0083] Figure 18 and 19 Schematic diagrams of the application of the device according to the present invention are shown, which are in the form of polarization-sensitive image sensors;

[0084] Figure 20 A schematic diagram of a cascade of multiple waveguide-based optical coupling elements is shown, which can be generated together using a free-form microstructuring method;

[0085] Figure 21 and Figure 22 Each diagram shows a schematic of a device in which a waveguide-based optical coupling element is connected to a single-mode fiber through one optical coupling point, while other optical coupling points are mechanically stabilized by additional structures; and

[0086] Figure 23A schematic diagram showing the use of the device according to the invention as a polarizing filter is shown. Detailed Implementation

[0087] Figure 1 A schematic diagram of a preferred exemplary embodiment of a device according to the invention for optical coupling and for mode-selective separation or superposition of optical fields is shown. To illustrate the function of a polarization beamsplitter or polarization combiner, an exemplary embodiment shows a device according to the invention comprising a waveguide-based optical coupling element 10 connected to an optical assembly 400 having a first optical coupling point 100. In the following illustrations, light propagation is achieved from the first optical coupling point 100, serving as an input coupling point, to second optical coupling points 370 and third optical coupling points 380, serving as output coupling points, thus facilitating the use of the waveguide-based optical coupling element 10 as a polarization beamsplitter. In this context, the distinction between "input coupling point" and "output coupling point" is used merely for a simpler description of the device and should not be interpreted as a limitation on the functionality of the component. Rather, the light path can be reversed, thus there is an interchangeability between the roles of "input" and "output," making it possible to use the device as a polarization combiner.

[0088] In the illustrated embodiment, Figure 1 The waveguide-based optical coupling element 10, schematically represented in the diagram, includes a first waveguide portion 200 having a first waveguide cross section 110 and a first optical coupling point 100, which serves as an input coupling point in the case of a polarization beamsplitter. The first waveguide cross section 110, present at the first optical coupling point 100, has two mutually orthogonal, possibly degenerate eigenmodes 120 and 130, which are assigned to the first optical coupling point 100 and each has an electric mode field. These electric mode fields, for example, have a dominant, linear polarization transverse component with an electric field vector, and their intensity distributions 140 and 150 may be very similar and have significant overlap. For the transverse polarization component that dominates in the two mutually orthogonal mode fields, the associated transverse electric field vectors of the transverse component are substantially perpendicular to each other; thus, under a good approximation, mode field separation is equivalent to the separation of the associated linear polarization.

[0089] Figure 1The waveguide-based optical coupling element 10, schematically shown, also includes a second branch waveguide portion 300 adjacent to the first waveguide portion 200 and comprising two spatially intersecting partial waveguides 330, 340. Each of the partial waveguides 330, 340, when considered individually, has at least two eigenmodes with different polarization directions and very different effective refractive indices; therefore, the coupling between waveguide modes is only very weak during propagation. In principle, modes with very different effective refractive indices within the same waveguide are also referred to as “strongly decoupled modes.” In this case, the effective refractive index difference between the two modes is preferably greater than 0.005, particularly preferably greater than 0.05, and very particularly preferably greater than 0.1. Embodiments are also conceivable in which at least one guided eigenmode, typically polarized over a long range along the waveguide cross-section, exists in each of the partial waveguides 330, 340. The partial waveguides 330, 340 define two additional optical coupling points 370, 380 with associated cross-sections 350, 360. In the case of a polarization beamsplitter, two additional optical coupling points 370 and 380 are used as outputs. That is, the two orthogonal first eigenmodes 120 and 130, which are assigned as inputs to the first optical coupling point 100 of the first waveguide cross section 110, are each assigned as output waveguides to the two partial waveguides 330 and 340 of the corresponding waveguide modes. The corresponding other waveguide modes of the corresponding partial waveguides 330 and 340, which are the output waveguides, will not be activated except for undesirable crosstalk.

[0090] Figure 1 The function of the device outlined herein is based in particular on the fact that, in the first waveguide section 200, the first waveguide cross section 110 is continuously transformed into a waveguide cross section 210 comprising two overlapping waveguide sections 230, 240, wherein the second waveguide cross sections 230, 240 each have additional eigenmodes 250, 260; 270, 280 with very different effective refractive indices. This continuous transformation from the first waveguide cross section 110 to the second waveguide cross section 210 can preferably be designed in such a process that the cross section of the waveguide-based optical coupling element 10 continuously deforms along the direction of light propagation. The first waveguide region 200 preferably has a length of 0.1λ to 30λ, particularly preferably from 0.2λ to 15λ, especially from 0.2λ to 10λ, while the entire length of the waveguide-based optical coupling element 10, measured to its maximum extent, is preferably less than 50λ, particularly preferably less than 25λ, especially less than 10λ, where λ represents the vacuum wavelength of the light used, assuming the refractive index of the material used is approximately 1.5.

[0091] In terms of form and size, waveguide cross sections 230 and 240 are adapted to the cross sections of partial waveguides 330 and 340, which have this cross section at the interface between the first waveguide section 200 and the second waveguide section 300. Figure 1As depicted, waveguide cross sections 230 and 240 need not be identical to the cross sections of portions of waveguides 330 and 340 at the interface (including the waveguide cross section 210 between the first waveguide portion 200 and the second waveguide portion 300). Instead, it is sufficient to adapt the cross sections to each other in a manner that provides the best possible optical coupling between the first waveguide portion 200 and the second waveguide portion 300. Furthermore, in Figure 1 The cross-sections of various waveguide sections schematically depicted as rectangles should be understood as exemplary; those skilled in the art may also consider using other forms, such as elliptical forms, which may be found to be more stable in terms of manufacturing inaccuracies in certain situations.

[0092] As in Figure 1 As further schematically shown, the initial superimposed waveguide cross-sections 230 and 240 of the partial waveguides 330 and 340 are fabricated to continuously branch off into spatially non-intersecting output waveguide cross-sections 350 and 360 within the second waveguide section 300. In this case, the branching can be achieved in such a way that the change in the structural cross-section in the axial direction is sufficiently slow to facilitate an adiabatic transition of the mode field, thus enabling spatial separation with minimal interference and loss. In this separation, the strong guided modes of the two partial waveguides 330 and 340 can be utilized such that they follow the trajectories of their respective partial waveguides, and in the process, they are hardly coupled with weak guided modes and do not emit to any noteworthy extent. This makes it possible to transform the optical signals present in the two first orthogonal eigenmodes 120, 130 at the first optical coupling point 100 as input into the first mode fields 120a, 130a in the plane of the waveguide cross section 210, and then, with low loss and low crosstalk, the first mode fields 120a, 130a are assigned to the corresponding fundamental modes at the two other optical coupling points 370, 380 as output.

[0093] When the initial cross-sections 230 and 240 of some waveguides 330 and 340 are rectangular, such as Figure 1 As shown, such embodiments are particularly recommended, with an effective refractive index n of the strongly guided fundamental mode. e1 In each case, the effective refractive index n is greater than that of the weakly guided fundamental mode. e2Preferably exceeding 0.005, particularly preferably exceeding 0.05, especially exceeding 0.1, wherein the strongly guiding fundamental mode is polarized along the long side of the rectangular cross-sections 230, 240, and the weakly guiding fundamental mode is polarized along the shorter side of the rectangular cross-section profile. This can be achieved, for example, by using highly elongated cross-sections, such as elliptical or rectangular cross-sections with a large aspect ratio, for portions of the waveguides 330, 340. The “aspect ratio” of the waveguide cross-section represented by the planar diagram is thus understood to represent the maximum possible ratio of two ranges measured in mutually orthogonal directions. In the case of a rectangular cross-section diagram, the aspect ratio corresponds to the ratio of the sides; in the case of an elliptical cross-section diagram, it corresponds to the ratio of the half-axis. Depending on the waveguide material chosen, the aspect ratio of the cross-sections of the portions of the waveguides 330, 340 is preferably greater than 1.5, particularly preferably greater than 2.5, especially greater than 3.5 or 4.5, at least in the defined portions. Furthermore, the large refractive index difference between the high-refractive-index core region and the low-refractive-index cladding region of the waveguide-based optical coupling element 10 is beneficial to the dielectric waveguide, thereby obtaining a large effective refractive index difference between the strong guiding mode and the weak guiding mode.

[0094] For the waveguide-based optical coupling element 10, its core region is preferably formed by photolithography of a polymer material, and the refractive index in the core region is preferably between 1.2 and 2, particularly preferably between 1.3 and 1.8, and especially between 1.4 and 1.7. The refractive index of the cladding region is preferably between 1.0 and 1.5, particularly preferably between 1.0 and 1.45. Therefore, the refractive index difference between the core region and the cladding region is preferably between 0.05 and 0.7, particularly preferably between 0.1 and 0.7, and especially between 0.15 and 0.6. If necessary, the refractive index difference can be set by using a suitable cover material or cladding material 500, in which the core region of the waveguide-based optical coupling element is fully or partially embedded, wherein, in subsequent method steps, the cover material or cladding material 500 is preferably applied locally or globally to the core region of the waveguide-based optical coupling element 10 fabricated using a free-form microstructuring method. In the case of a polymer-based core region, a low-refractive-index polymer, particularly a fluorinated polymer or one having a polysiloxane-based component, is preferably used as the cladding material 500. The refractive index of the cladding material 500, at least locally surrounding the waveguide core, is preferably 1.2 to 1.5, particularly 1.3 to 1.45.

[0095] Therefore, in such a device, light can be coupled into the optical coupling element 10 via a first waveguide cross section 110, referred to as the "input surface," and similarly coupled out of the waveguide-based optical coupling element 10 via waveguide cross sections 350 and 360, referred to as the "output surface." In this case, the waveguide-based optical coupling element 10 functions as a polarization beamsplitter. This optical path can also be reversed, thus interchanged between the roles of the input and output surfaces. Consequently, the waveguide-based optical coupling element 10 can also be used to combine two optical signals, each coupled to the respective eigenmodes of two spatially separated optical coupling points 370 and 380, and transferred to the mutually orthogonal eigenmodes of the first optical coupling point 100. Consequently, the waveguide-based optical coupling element 10 can also function as a polarization beam combiner.

[0096] Using a waveguide-based optical coupling element 10, two spatially overlapping eigenmodes 120 and 130 assigned to a first optical coupling point 100 can be spatially separated. These eigenmodes exist at the first optical coupling point 100 and are mutually orthogonal or virtually orthogonal. Optionally, these eigenmodes can be further manipulated by eigenmodes 260 and 280 assigned to a second optical coupling point 370 and a third optical coupling point 380. As a result of this separation and optional further manipulation, the power and / or amplitude and phase initially present in the eigenmodes 120 and 130 assigned to the first optical coupling point 100 can preferably be determined by using a method configured for this purpose (particularly a coherent detection method), and the associated polarization state can be determined thereby. For this purpose, the initially separated eigenmodes 120 and 130 assigned to the first optical coupling point 100 can preferably interfere with each other and / or with an additional reference field. Furthermore, the spatially separated intrinsic modes 260, 280 assigned to the second optical coupling point 370 and the third optical coupling point 380 can also preferably be manipulated such that each mode is configured to excite the intrinsic modes of the component portion or waveguide 430, 440, which are adjacent to the waveguide-based optical coupling element 10 via two other optical coupling points 370, 380 configured as output coupling points. For the component portion 400 connected to the first optical coupling point 100 on the input side, the associated degenerate or non-degenerate intrinsic modes 120, 130 assigned to the optical component portion 400 of the first optical coupling point 100 can thus be split and coupled to the two integrated optical component portions or waveguides 430, 440 connected to the output coupling points of the waveguide-based optical coupling element 10, such that light is converted from the corresponding intrinsic mode of the optical component portion 400 embodied as a waveguide to the intrinsic mode of one of the integrated optical waveguides in each case.

[0097] Figure 1 The apparatus described in the example manner can be modified in various ways. Therefore, in Figure 1In this embodiment, the basic rectangular cross-section of some waveguides 330, 340 is maintained along the propagation direction and adjusted only relative to the lateral position and orientation. In alternative embodiments, the shape and / or size of the cross-section can also be changed along the propagation direction, particularly by continuously transforming the rectangular shape into any other shape, especially a square, ellipse, or circle. Further embodiments are conceivable. In particular, this form can be adapted to the cross-sectional and mode field distribution of the optical coupling points 370, 380 of the optical component portions 430, 440 to be connected, so as to obtain effective coupling. Thus, in addition to mode separation, the device according to the invention can also obtain low-loss connections between at least two optical component portions whose optical coupling points are characterized by mode fields with very different sizes or positions.

[0098] In addition, such as Figure 2 As shown in the exemplary method, Figure 1 Further embodiments of the device depicted are possible, wherein in Figure 1 The waveguide sections 200 and 300 are depicted as clearly distinguishable, seamlessly merging into each other without clear geometric boundaries, or the waveguide sections 200 and 300 are wholly or partially unified. The function of the waveguide-based optical coupling element 10 for polarization beam splitting or polarization beam combining is specifically based on the fact that the waveguide-based optical coupling element 10 includes at least two partial waveguides 330 and 340 that are very close together or spatially overlapping in the first region 600 (e.g., Figure 2 (as shown by reference numeral 301 in the attached figure), and when considered as separate from each other, they at least partially have eigenmodes with very different effective refractive indices, and at least the two partial waveguides 330, 340 are spatially non-intersecting in the second region 610 ... Figure 2 (As indicated by reference numerals 302a and 302b in the accompanying drawings). In this case, the term "partial waveguides very close together" in the first region 600 means that the eigenmodes guided in the two partial waveguides 330 and 340 overlap at least regionally and are therefore able to interact with each other.

[0099] exist Figure 1 and 2 In the device depicted, the two mode fields 120a and 130a are spatially bifurcated only by portions of the waveguides 330 and 340, without any change in polarization direction. In contrast, in other cases, it is desirable to change the polarization direction of the bifurcated mode fields, for example, to convert them into two TE modes of the integrated optical waveguide. Corresponding devices are the subject of some of the exemplary embodiments described below; for example, see... Figure 3 , 8 9 or 10.

[0100] Furthermore, the device according to the invention can also be used to separate any desired modes, particularly two modes having the same polarization but different field distributions. In this embodiment, the form of the waveguide-based optical coupling element 10 is modified accordingly, as described above. In this case, for example, the fact that the modes to be separated are guided with different intensities in a suitably formed waveguide can be utilized in a manner similar to that described above for separating modes with different polarization directions, and separation can ultimately be achieved by geometrically bifurcating the waveguide. Furthermore, implementations in which different strong couplings (in a manner similar to so-called directional couplers) of the modes to be separated into parallel-running waveguides are conceivable. Another option is to selectively convert modes by periodically modulating the waveguide cross-section in the axial direction, the fundamental wavenumber of which corresponds to the wavenumber difference of the modes to be coupled. Thus, any desired mode can be converted into a field form that can be separated from each other with particularly low loss and particularly reliable performance—for example, by means of the concepts described above. Potential applications in this case include, for example, mode separation at the end face of multimode or so-called few-mode fibers, and coupling the input of corresponding optical signals to different optical coupling points in optical component sections. The configuration of the coupling elements corresponding to such objects can be determined, in particular, by a so-called topology optimization method, in which not a single geometric parameter but the entire form of the coupling structure is numerically optimized. The resulting structured geometry avoids generally effective descriptions but also facilitates the realization of the device for optical coupling and mode separation according to the invention.

[0101] In a particular embodiment, the waveguide-based optical coupling element 10 is connected to a plurality of optical components 400, 430, 440. To accommodate the positioning errors of these components, it is advantageous to obtain very accurately the position and orientation of the associated additional optical coupling points 410, 470, 480 in the coordinate system 40 used to generate the free-form microstructure unit of the optical coupling element. For this purpose, it is preferable to use... Figure 1The adjustment marks 411, 412, 471, 472, 481, 482, or alternative structural elements (not shown) on the optical coupling points 400, 430, 440 to be connected, are schematically shown, wherein the position of the adjustment marks or structural elements relative to the other optical coupling points 410, 470, 480 is known very accurately. The adjustment marks 411, 412, 471, 472, 481, 482, or alternative structural elements can preferably be detected by imaging methods, particularly by camera-based methods, which preferably allow the positioning of the adjustment marks 411, 412, 471, 472, 481, 482, or alternative structural elements in three-dimensional space. Confocal imaging methods can also preferably be used in this process. In this case, a portion of the beam path of the free-form microstructured unit can be used, in particular, for the detection of the adjustment marks 411, 412, 471, 472, 481, 482 and the exposure of the waveguide-based optical coupling element 10 to be generated. The detection of adjustment marks 411, 412, 471, 472, 481, 482 or alternative structural elements can be achieved with the highest possible accuracy, wherein the deviation is preferably less than 500 nm, particularly preferably less than 200 nm, and especially less than 100 nm or 50 nm. The positioning accuracy of the waveguide-based optical coupling element 10, fabricated by a free-form microstructuring method, and the additional optical coupling points 410, 470, 480 on the optical coupling points 400, 430, 440 to be connected, is preferably better than 500 nm, particularly preferably better than 200 nm, and especially better than 100 nm or 50 nm. These values ​​relate to the production of the device and structure according to the invention and are provided for a vacuum operating wavelength of approximately 1.5 μm.

[0102] exist Figure 3 In the illustrated embodiment, the optical component portion 400 is configured as a single-mode fiber with a rotationally symmetric refractive index cross section, having two degenerate or nearly degenerate approximately linear polarization waveguide modes (e.g., so-called LP01 modes). In this case, the term "degenerate or nearly degenerate mode" describes waveguide modes with very similar effective refractive indices, typically differing by less than 0.001. In the case of a standard single-mode fiber, the two fundamental modes with different polarizations have very similar intensity distributions; that is, the normalized overlap integral of the spatial intensity distribution is close to 1, preferably greater than 0.9 or 0.95. The waveguide-based optical coupling element 10 allows separation of the two first eigenmodes 120, 130 assigned to the first optical coupling point 100, or separation of mutually orthogonal linear combinations of the two eigenmodes 120, 130, that is, the first eigenmode 120 or the first linear combination of eigenmodes 120, 130 of the optical component portion 400 is converted into the eigenmode 260 of the first integrated optical component portion 430, such as the so-called TE mode. Figure 8In the figure, denoted by reference numeral 71a, the second intrinsic mode 130 or the second linear combination of intrinsic modes 120, 130 of component portion 400 is converted into the intrinsic mode 280 of the second integrated optical component 440, for example again referred to as the so-called TE mode, in Figure 8 The second mode of the output waveguides 430 and 440, denoted by reference numeral 71b, is not excited in this embodiment except for unwanted crosstalk.

[0103] like Figure 3 As shown, this can preferably be achieved by continuously twisting portions of the waveguides 330 and 340. Figure 8 , 9 More examples can be found in 10. This structure can also be adjusted in various ways. Therefore, it is preferable that the twisting of the waveguide has already begun in the first region 600 of the waveguide-based optical coupling element 10, in which the waveguide has not yet been separated. Furthermore, other structural geometries of the waveguide-based optical coupling element 10 can be envisioned, which are preferably determined by numerical parameters or topology optimization methods.

[0104] like Figure 4 As exemplarily depicted, a waveguide-based optical coupling element 10 can be fabricated in situ using a three-dimensional freeform microstructuring method at optical coupling point 410 of optical component portion 400 or between at least two additional optical coupling points 410, 470, 480 of at least one optical component portion 400, 430, 440. In this case, the waveguide-based optical coupling element can be adjusted in position, form, and size to the position and orientation of at least two additional optical coupling points 410, 470, 480 of at least one optical component portion 400, 430, 440. Given the need for rapid and high-volume manufacturing, a form that is as simple as possible and has the smallest possible volume is preferred for the waveguide-based optical coupling element 10. For example, simple, continuous regions with little structural detail or topology (having as few so-called "holes" as possible in three-dimensional space) are advantageous in form. The same applies to gratings for light field separation, which often have the problem of resulting in very fine structures with high precision requirements, necessitating complex fabrication steps. Small volume can be achieved by utilizing the waveguide properties present in at least a portion of the optical coupling element. In this case, the volume of the waveguide-based optical coupling element 10 generated by the free-form microstructuring method is preferably less than 1000 μm. 3 Especially preferred is less than 500 μm 3 Especially those smaller than 250μm 3 or 150μm 3Optionally, optional connecting waveguides or other mechanical or optical attachments may be added. These values ​​pertain to a waveguide-based optical coupling element 10 provided for a vacuum operating wavelength of approximately 1.5 μm and comprising a material having a refractive index of approximately 1.5. For other operating wavelengths, the volume of the optical waveguide-based coupling element 10 can be scaled proportionally to the cube of the operating wavelength, taking into account the corresponding refractive index of the material used.

[0105] The manufacturing term "at the optical coupling point" used above describes an embodiment that facilitates optical coupling between the waveguide-based optical coupling element 10 and an additional optical coupling point 410 assigned to the optical assembly section 400. For this purpose, as... Figure 1 As schematically shown, in the region of the first optical coupling point 100, the optical component portion 400 and the waveguide-based optical coupling element 10 can preferably be in direct physical contact. Alternatively, as Figure 4 and Figure 5 As shown, optical transmission between the waveguide-based optical coupling element 10 and additional optical coupling points 410 assigned to the optical component section 400 can be achieved through an additional structure, which is particularly preferably configured to connect waveguides 160 and 170. The connecting waveguides 160 and 170 can preferably be fabricated in situ using a free-form microstructuring method, i.e., directly at a target location that may be particularly associated with the first optical coupling point 100, preferably together with the waveguide-based optical coupling element, thereby advantageously facilitating very precise alignment of the optical elements relative to each other and relative to the waveguide-based optical coupling element 10.

[0106] Figure 4 An exemplary device is shown in which an S-shaped waveguide segment, as a connecting waveguide 160, is inserted between another coupling point 410 of an optical component portion 400 (configured as an optical fiber, particularly a single optical fiber) and a first optical coupling point 100 of a waveguide-based optical coupling element 10, thereby enabling translation and / or rotation of the waveguide-based optical coupling element 10 relative to the rigidly assembled component portion 400. This makes it possible to adjust the position and / or orientation of the waveguide-based optical coupling element 10 relative to a second additional optical coupling point 470 assigned to a separate, securely assembled optical component portion 430, even if the relative positions of components 400, 430 are affected by unavoidable variations caused by manufacturing tolerances. In addition... Figure 4 In addition to the embodiments shown, a waveguide segment (not shown) can be inserted between a second additional optical coupling point 470 of the optical component portion 430 and the associated optical coupling point 370 of the waveguide-based optical coupling element 10. This waveguide segment can be used to further increase the degrees of freedom regarding the positioning of the waveguide-based optical coupling element 10.

[0107] As an alternative or addition, the additional connecting waveguides 160, 170 can also be used to adapt the mode field present at another optical coupling point 410 of the optical component section 400 to the first optical coupling point 100 allocated by the waveguide-based optical coupling element 10. Figure 5 As illustrated in the exemplary embodiment, the additional connecting waveguide 170 can be designed as tapered. In this case, the term "tapered" refers to a waveguide segment having a tapered shape in one direction. In this case, the tapering can be designed such that there is an adiabatic adaptation for the spatial mode distribution with the lowest possible loss, that is, the dominant component of the power of the first mode distribution is converted to the second mode distribution without emission or absorption. Here, the additional connecting waveguide 170 for mode field adaptation can also also have at least a partial multimode configuration, even though the optical component portion 400 and the waveguide-based optical coupling element 10 connected to it both have single-mode optical coupling points. This embodiment can occur in the case of a tapered connecting waveguide 170 connected to an optical fiber, which has a higher refractive index than the optical fiber due to the selected cladding material, but its initial diameter is adapted to the refractive index of the optical fiber. In this embodiment, it is possible, through appropriate design, that effective coupling is still possible, particularly by avoiding the excitation of higher modes in the multimode segment. Figure 4 and Figure 5 Unlike the device depicted, the connecting waveguides 160, 170 and / or the waveguides contained in the optical component section 400 and assigned to the optical coupling point 410 can have non-rotationally symmetric cross sections, such as rectangular or elliptical cross sections of non-degenerate waveguide modes with different polarizations. In this case, the connecting waveguides 160, 170 can be twisted along the propagation direction, thereby enabling them to continuously change the polarization direction of the non-degenerate eigenmodes.

[0108] When the waveguide-based optical coupling element 10 includes at least two partial waveguides 330 and 340, multimode waveguides may also be present. These partial waveguides 330 and 340 contain cross-sections characterized by a high aspect ratio, preferably designed to achieve a large effective difference in refractive index and thus strong decoupling of two different polarization fundamental modes. In this process, in addition to the fundamental mode belonging to the strongly guided fundamental mode, higher modes may be able to propagate. In this case, the excitation of these higher modes can be completely avoided by appropriately shaping the partial waveguides 330 and 340. Figure 6 The schematic depiction shows the spatial separation of the bifurcated partial waveguides 330, 340 and the accompanying strongly guided fundamental modes, which can be achieved, in particular, by using suitable cones 331, 341 to convert the very elongated cross sections 350, 360 of the multimode partial waveguides 330, 340 back to the single-mode cross sections 355, 365.

[0109] Alternatively, higher-mode excitations can be intentionally accepted, and appropriately shaped cones 331 and 341 can be provided. Cones 331 and 341 help guide the interfering mode field back to a well-defined, highly localized output field suitable for the subsequent waveguide's fundamental mode. In order to... Figure 7 In an exemplary embodiment, cones 331 and 341 are configured such that the corresponding lateral offsets 372 and 382 of the centroids 371 and 381 of the cone end faces 355 and 365 are configured to be relative to the centroids 351 and 361 of the corresponding input surfaces 350 and 360, which can preferably be optimized numerically.

[0110] Figure 6 and 7 The cones 331 and 341 shown should be understood as exemplary and can be adjusted in various ways. For example, by utilizing the appropriate curvature of the waveguide trajectory or by using fin-like auxiliary structures applied to the waveguide core, significant extinction of the weakly guided mode can preferably be obtained, thereby increasing the extinction ratio. Furthermore, portions of waveguides 330 and 340 can be incorporated into the cone portions 331 and 341 without explicit geometric boundaries, or can be completely integrated with the latter. Moreover, the corresponding structures can be designed using numerical parameter optimization methods or topology optimization methods and / or based on... Figure 6 and Figure 7 The geometry improves the structure.

[0111] Various possibilities for optically coupling the waveguide-based optical coupling element 10 to optical components Figure 8 , 9 As illustrated in Figures 10 and 10, a device can be envisioned, for example, in which the first optical coupling point 100 of the waveguide-based coupling element 10 does not have direct physical contact with another optical coupling point 410 of the optical component portion 400, and in which there is no connection via additional connecting waveguides 160, 170. Instead, light can propagate through a medium that is at least regionally uniform between the optical coupling points 100, 410. This can facilitate the provision of beam shaping elements 111, 413 for the first optical coupling point 100 of the waveguide-based optical coupling element 10 and / or the other optical coupling point 410 of the optical component portion 400, thereby promoting efficient optical coupling, such as... Figure 9 As shown. Beam shaping elements 111, 413 can preferably be fabricated together with waveguide-based optical coupling elements 10 using a free-form microstructuring method, which is particularly advantageous for very precise alignment with respect to the associated optical coupling points 100, 410.

[0112] Figure 8One embodiment is shown in which optical coupling between two additional optical coupling points 370, 380 of the waveguide-based optical coupling element 10 and integrated optical waveguides 71a, 71b on the optical chip 430 is achieved through additional connecting waveguides 830, 840, tapered structures 850, 860 on the sides of the connecting waveguides 830, 840, and tapered structures 72a, 72b on the sides of the integrated optical waveguides 71a, 71b. The connecting waveguides 830, 840 and the associated tapered structures 850, 860 can be generated in situ (i.e., at the target location) in the same steps of the free-form microstructure method as the waveguide-based optical coupling element 10, and can be aligned with the waveguide structure present on the optical chip 430 with high precision during the process.

[0113] Figure 9 and 10 An embodiment is shown in which two additional optical coupling points 370, 380 of the waveguide-based optical coupling element 10 are not in direct physical contact with the corresponding optical coupling points 74a, 74b of the integrated optical chip 430. Instead, in this case, light propagates through at least a uniform region between the optical coupling points, which may be material-free (vacuum) or filled with certain gases, liquids, or solids, particularly optically transparent covering materials used to protect and stabilize the entire device. To improve coupling efficiency, additional beamforming elements 333, 343 are attached to end faces 350, 360, which are preferably produced together with the waveguide-based optical coupling element 10 using a free-form structuring method and thus can be aligned very precisely with their respective optical coupling points.

[0114] Figure 9 One embodiment is shown in which beam shaping elements 833, 843 are also attached to the facets 73a, 73b of the integrated optical waveguides 71a and 71b on one side of the optical chip 430. The beam shaping elements 833, 843, and beam shaping elements 333, 343 on the side of the waveguide-based optical coupling element 10, increase the alignment tolerance during assembly of the device. Furthermore, to improve the coupling efficiency between another optical coupling point 410 of the optical assembly 400 and the first optical coupling point 100 of the waveguide-based coupling element 10, another beam shaping element 413, 111 can be applied to both optical coupling points 100, 410. Alternatively, the beam shaping elements can be connected only to one side of the respective optical coupling point, such as... Figure 10 As shown.

[0115] As mentioned above, free-form microstructuring methods can particularly produce structures with symmetrical or nearly symmetrical geometries, which may result in very similar losses for both separation modes. Figure 11As illustrated, the geometric representation of the geometrically symmetric structure in this paper includes the three-dimensional form of the symmetry plane 11, where perfect symmetry may be slightly disturbed by possible adjustments to the structural geometry, which may be inevitable, especially regarding the location and orientation of the optical coupling point used to couple the waveguide-based optical coupling element 10 to the optical coupling element to be connected.

[0116] The above embodiments should be understood as examples and in no way reflect the full scope of application of the device according to the invention. Therefore, the waveguide-based optical coupling element 10 can also be combined, in particular, with additional functional optical elements or arrangements of such elements, which can be produced in situ (i.e., at the corresponding target location) together with the corresponding waveguide-based optical coupling element 10 using a microstructuring method, preferably a free-form microstructuring method, and thus can be connected to and / or aligned with the corresponding waveguide-based optical coupling element 10 in a very precise manner. The functional optical elements may preferably further include connecting waveguides, cones, optical power dividers, or optical free-form elements, particularly mirrors, lenses, or other refractive or diffractive components, and in this process can be supplemented by additional auxiliary structures, particularly by further mechanical support structures. Therefore, more complex functional optical devices, for example for polarization analysis, can be generated on a small plane of an optical fiber or integrated optical waveguide or on an extended array of a photodetector such as a camera chip. Figures 12 to 23 Such exemplary embodiments can be found in [the document / relevant text].

[0117] Figure 12 An apparatus 1000 is shown, corresponding to a waveguide-based simulation of a polarization beamsplitter space with four optical coupling points, and can be fabricated on the end face of a multi-core fiber 720 using a free-form microstructuring method. Compared to a single optical coupling element, Figure 12 The device shown in the diagram, in principle, allows for bidirectional separation and / or merging of various polarizations without power loss. In this case, the multi-core fiber 720 functions as at least one optical component portion 400, and the multi-core fiber 720, as shown in the diagram, includes four optical coupling points. The device shown includes four waveguide-based optical coupling elements 10a, 10b, 10c, and 10d arranged on the end faces of the four-core fiber 720, which are interconnected by additional connecting waveguides 310a, 310b, 310c, and 310d. In this case, the fiber cores can each have a circular cross-section. Alternatively, they can also be configured as polarization-maintaining fiber cores with a double rotationally symmetric cross-section—which... Figure 13The following is an overview based on four polarization-maintaining fibers. A waveguide-based optical coupling element 10a distributes two LP01 modes (characterized by different polarization directions) of fiber core 730a between waveguides 310a and 310d, and couples these modes thereto to the corresponding LP01 modes of fiber cores 730b and 730c. Similarly, a waveguide-based optical coupling element 10d distributes two LP01 modes of fiber core 730d (characterized by different polarization directions) between waveguides 310b and 310c, and couples these modes thereto to the still-free LP01 modes of fiber cores 730b and 730c. In this embodiment, waveguide-based optical coupling elements 10a and 10d together function as a polarization beamsplitter, while waveguide-based optical coupling elements 10b and 10c together function as a polarization combiner. These roles can be interchanged by reversing the optical paths, and the entire device can be used as an optical fiber-coupled polarization beamsplitter. In this case, additional tapered structures 170a, 170b, 170c and 170d are used to effectively couple to the corresponding optical fiber core.

[0118] Figure 13 It shows something similar to Figure 12 The device 1050 is based on four individual polarization-maintaining fibers 740a, 740b, 740c, and 740d having associated fiber cores 750a, 750b, 750c, and 750d. In this case, the individual fibers 740a, 740b, 740c, and 740d function as at least one optical component portion 400. In the depicted device 1050, the associated fiber cross-sections exhibit double rotational symmetry due to the additional stress-generating element 760, which is conventional for many polarization-maintaining fibers. The individual polarization-maintaining fibers 740a and 740d are rotated 90° about their longitudinal axis to allow for enhancement of the device's functionality through polarization rotation, in a manner corresponding to the use of an additional half-wave plate in conventional free-beam optics.

[0119] Connecting waveguides 310a, 310b, 310c, and 310d extending between waveguide-based optical coupling elements 10a, 10b, 10c, and 10d Figure 12 and Figure 13 It is depicted as a free-form waveguide with a rectangular cross-section. For example... Figure 14As shown in the depicted device 1060, optical transmission between waveguide-based optical coupling elements 10a, 10b, 10c, and 10d can alternatively be achieved using a so-called whispering-gallery mode. This embodiment utilizes the fact that light can be guided along a convex outer profile of appropriate size along an optically high refractive index region. For example, this allows the optical connection between the waveguide-based optical coupling elements 10a, 10b, 10c, and 10d to be supported by extended structural elements 311a, 311b, 311c, and 311d with higher mechanical stability. The structural elements 311a, 311b, 311c, and 311d can be additionally fastened to a separate mechanical support structure in an area not contacted by guided light without negatively impacting optical transmission performance.

[0120] Figure 15 A passive optical waveguide structure 1100 is shown, belonging to an optical polarization multiplexing heterodyne receiver, and can be fabricated on the end face of a seven-core optical fiber 770 using a free microstructuring method. In this case, the seven-core optical fiber functions as at least one optical component 400. The device 1100 includes two waveguide-based optical coupling elements 10a and 10b for polarization separation, two multimode interference couplers 20a and 20b for coherently superimposing signals contained in their respective polarizations, and additional connecting waveguides 320a, 320b, 320c, 320d and cones 170a, 170b, 170c, 170d, 170e, 170f. For use as a coherent optical polarization multiplexing receiver, the fiber cores 770a, 770b, 770c, 770d, 770e, 770f can be connected to external components as follows:

[0121] - Fiber cores 770a and 770b are connected to the first balanced photodetector;

[0122] - Fiber cores 770d and 770e are connected to the second balanced photodetector;

[0123] - 770g of fiber core not connected;

[0124] - Fiber core 770c is connected to the data signal source, and fiber core 770f is connected to the local oscillator, and vice versa.

[0125] The data signal is decomposed into two signal components via a waveguide-based optical coupling element 10b connected to the data signal input. These components correspond to two degenerate orthogonal polarization eigenmodes LP01 of the relevant fiber core. Similarly, the local oscillator signal is decomposed into two signal components via a waveguide-based optical coupling element 10a connected to the local oscillator input. These components correspond to two degenerate orthogonal polarization eigenmodes LP01 of the relevant fiber core. The polarization directions of these data signal components and the local oscillator signal components are matched by the torsion of waveguides 320a, 320b; 320c, 320d, which are adjacent to the optical coupling elements and superimposed by multimode interference couplers 20a, 20b. The superimposed signal is provided to a balanced photodetector. The polarization-multiplexed data signal is reconstructed by processing (typically digitally) an electrical signal, which can be generated by a photodetector used as a mixer. Any defects that may exist in the distributed structure shown can be compensated for within this processing range.

[0126] Figure 15 The illustrated device 1100 should be understood as exemplary and can be modified in various ways. Thus, a polarization-multiplexed homodyne receiver can be provided by adding additional fiber cores and appropriately enhanced signal superposition, preferably by using 2 to 4 multimode interference couplers. Furthermore, the polarization beamsplitter connected to the local oscillator can be replaced by a simple power divider. This structure can be generated not only at the end face of a multi-core fiber but also using one-dimensional or two-dimensional fiber arrays or other waveguide arrangements, such as integrated optical waveguides. Similar to... Figure 13 The embodiment shown can also use polarization-maintaining fiber in this case.

[0127] Figure 16A polarization analyzer structure 1200 is shown, which can be fabricated on the end face of a single-mode fiber using a free-form microstructuring method, in which case the single-mode fiber serves as at least one optical component 400. Light coupled from the fiber to the structure via another optical coupling point 410 is initially uniformly distributed among four waveguides 810a, 810b, 810c, and 810d. In this case, two waveguide-based optical coupling elements 10a and 10b, rotated 45° relative to each other about their principal axes, are connected to waveguides 810a and 810b, whereby signal components can be separated in associated polarization directions tilted 45° to each other. In this case, the principal axes of the optical coupling elements are defined by the polarization directions of the linear polarizations separated by the optical coupling elements. An optical birefringent waveguide 30 is connected to waveguide 810c, the length of which is chosen such that waveguide 810c behaves like a quarter-wave plate, its alignment determined by the orientation of its rectangular cross-section. The birefringence of waveguide 30 occurs due to its material properties or the shape of its core cross-section, such as a rectangular rather than square, or an elliptical rather than circular cross-section. Another waveguide-based optical coupling element 10c is connected to the output of the birefringent waveguide 30, and its principal axis is twisted by 45° relative to the principal axis of the birefringent waveguide 30, which serves as a quarter-wave plate. The principal axis of waveguide 30 is defined by the polarization direction of the correlated polarization eigenstates of the birefringent waveguide 30, and in the case of a rectangular or elliptical cross-section of waveguide 30, by the direction defined by the side of the rectangular cross-section or the principal axis of the elliptical cross-section. Waveguide 810d remains open at one end and is used only for power measurement.

[0128] If light is coupled from an optical fiber to any desired superposition of two orthogonal polarization modes LP01 corresponding to a certain polarization state, this will produce a characteristic power distribution at outputs 811d, 370a, 380a, 370b, 380b, 370c, and 380c, which can be measured using a properly positioned photodetector. Using this power distribution, the polarization state of the light coupled to the structure can be uniquely reconstructed; see, for example, K. Kikuchi et al., Multi-level signaling in the Stokes space and its application to large-capacity optical communications, Optics Express, Vol. 22, No. 7, 2014. Therefore, a polarization analyzer can be provided relatively cost-effectively with the aid of structure 1200, combined with appropriate data processing and preferred calibration. In this case, a photodiode array (preferably an image sensor) can be used for power detection, which does not require complex adjustments to the emission direction of the plotted output. Alternatively, the various outputs 811d, 370a, 380a, 370b, 380b, 370c, and 380c can be connected to additional optical component parts (such as photodetectors or optical fibers) by using additional waveguides and / or micro-optical elements.

[0129] Due to the use of three optical coupling elements, a quarter-wave plate, and an output 811d (as shown in the image) used only for power measurement. Figure 16 As shown, the polarization state at the output of the structure has been defined. Redundancy information can be used to check the reliability of the measurement. Alternatively, the structure can be further simplified, for example by omitting the output 811d used only for power measurement.

[0130] Figure 17 A reflective polarization switch is shown that interchanges signal components in two mutually orthogonal fundamental polarization modes (so-called LP01 modes) existing in an optical fiber in a manner similar to a Faraday rotator. In this case, it functions as optical component section 400 and couples the signal components back into the optical fiber in opposite polarization directions. Of the three optical coupling points 100, 370, and 380 of the waveguide-based optical coupling element 10, the first optical coupling point 100 is connected to the optical fiber, while the other two optical coupling points 370 and 380 are interconnected via a twisted waveguide 390.

[0131] Figure 18 and 19 The application of the device according to the invention is shown, which is in the form of a polarization-sensitive image sensor 1400, 1450. In this case, Figure 18The individual structure depicted includes a free-form lens 111, whereby externally incident light 112 can initially be coupled to the waveguide-based optical coupling element 10. After the two polarizations are separated, the corresponding signal components are transmitted to two photodetectors 50a, 50b (which are connected to two other optical coupling points 370, 380 of the waveguide-based optical coupling element 10), where they are converted into electrical signals guided by wire 51. The photodetectors 50a, 50b are preferably independent detectors of the image sensor, thus allowing the polarization-sensitive image sensor to... Figure 18 The polarization-sensitive image sensor is constructed by the periodic continuation of the structure depicted in the image, and is used to detect the spatial distribution of incident power based on the separation of two polarizations.

[0132] Figure 19 An array of repeating structures 1400 is shown, which can produce a polarization-sensitive image sensor 1450 or a polarization-sensitive camera with multiple pixels.

[0133] Figure 20 A cascade 1500 of multiple waveguide-based optical coupling elements 10, 10a, and 10b is shown, which can be generated together using a free-form microstructuring method. By using this device, the polarization extinction ratio at optical coupling points 370a, 380a, 370b, and 380b can be improved in particular.

[0134] Figure 21 A device 1600 is shown in which a waveguide-based optical coupling element 10 is connected to a single-mode fiber, at which point the single-mode fiber acts as an optical component 400, and its output is mechanically stabilized by an additional structure 60. At another optical coupling point 410, the optical signal coupled from the single-mode fiber to the cone 170 leading to the waveguide-based optical coupling element 10 is initially split into two signal components, corresponding to the degenerate orthogonal polarization eigenmodes LP01 of the associated fiber core. As a result of twisting the waveguide portions 332, 342, the separated signal components are then matched in their polarization directions, and output coupling occurs at optical coupling points 825, 826 (which have mode distributions regulated by cones 815, 816). The positions of the additional optical coupling points 825, 826 are anchored at the plate-like portion structure 62 of the mechanical support structure 60 supported by pillars 61a, 61b, 61c, 61d. The distance and position of the additional optical coupling points 825, 826 can be freely chosen.

[0135] Figure 22 Device 1650 is shown, which indicates that it comes from Figure 21The device 1600 extends to form an array 80 of input optical fibers 400a, 400b, 400c, and 400d. By selecting the distances between the associated outputs 825a, 826a; 825b, 826b; 825c, 826c; 825d, 826d, the device can also be connected on the output side to an optical fiber array or an on-chip waveguide array. In this embodiment, device 1650 corresponds to a multi-channel polarization beamsplitter.

[0136] In an exemplary manner, Figure 23 A device 1700 is shown, in which a waveguide-based optical coupling element 10 serves as a polarization filter. In the illustrative device 1700, an optical signal to be filtered in terms of polarization is emitted from an optical component section 400, where the waveguide-based optical coupling element 10 is connected to its first optical coupling point 100 at its optical coupling point 410. The desired polarization-filtered signal is then obtained in one of the guided eigenmodes at a second optical coupling point 370, while the signal component to be suppressed by polarization filtering is provided to a terminating element (beam collector) 395 connected to a third optical coupling point 380. Figure 23 In the case shown, the beam collector is embodied in the form of a continuously tapering conical structure through which light is emitted laterally (e.g., in the direction of the absorbing surface). Modifications are possible. Figure 23 Many aspects of the device depicted are described. For example, the roles of the second optical coupling point 370 and the third optical coupling point 380 can be interchanged without altering the functional principle. It is also conceivable that light is coupled in from free space at the first optical coupling point 100 without physical contact with the optical component portion present there, and that the third optical coupling point 370 is coupled to the optical component portion. Coupling with the optical component can be achieved directly or, as described above, through a corresponding connecting waveguide or free beam coupling extension.

[0137] List of reference numerals

[0138]

[0139]

Claims

1. A device for mode-selective separation or superposition of optical coupling and optical fields, comprising: - A waveguide-based optical coupling element (10) having at least three optical coupling points (100, 370, 380), the waveguide-based optical coupling element (10) comprising a core region and a cladding region adjacent to the core region, with a refractive index difference of at least 0.05 between the core region and the cladding region, the waveguide-based optical coupling element (10) being designed as a three-dimensional free-form structure, the free-form structure being a layered structure consisting of at least six layers, thereby ○ At least one first optical coupling point (100) has at least two different guided intrinsic modes (120, 130) assigned to the first optical coupling point (100). ○ At least one second optical coupling point (370) has at least one guided intrinsic mode (260) assigned to the second optical coupling point (370), and ○ At least one third optical coupling point (380) has at least one guided intrinsic mode (280) assigned to the third optical coupling point (380). - At least one optical component portion (400) having at least one additional optical coupling point (410). At least one of the optical coupling points (100, 370, 380) of the waveguide-based optical coupling element (10) is optically connected to at least one additional optical coupling point (410) of the optical component section (400), and Among them, the waveguide-based optical coupling element (10) is configured for efficient bidirectional optical transmission. Between at least one first-guided intrinsic mode (120) assigned to the first optical coupling point (100) and at least one guided intrinsic mode (260) assigned to the second optical coupling point (370), and Between at least one second-guided intrinsic mode (130) assigned to the first optical coupling point (100) and at least one guided intrinsic mode (280) assigned to the third optical coupling point (380), In this context, along a portion of the transition region corresponding to at least three optical coupling points in a waveguide-based optical coupling element, the centerline of the transition region exhibits non-zero torsion, and the cross-section of the transition region undergoes continuous deformation, characterized by monotonic rotation or deformation of the principal constraint axis about the centerline.

2. The apparatus of claim 1, wherein at least one of the second optical coupling point (370) and the third optical coupling point (380) is coupled to an additional optical component portion (430, 440).

3. The apparatus according to claim 1 or 2, wherein the second optical coupling point (370) and the third optical coupling point (380) are spatially separated from each other, and wherein the waveguide-based optical coupling element (10) is configured to separate the optical input field present at the first optical coupling point (100) into partial fields of different polarizations and output coupled optical signals including partial fields at the second optical coupling point (370) and the third optical coupling point (380), or wherein the waveguide-based optical coupling element (10) is designed to superimpose optical signals in the form of spatially overlapping partial fields of different polarization inputs coupled at the second optical coupling point (370) and the third optical coupling point (380), and to provide the superposition at the first optical coupling point (100).

4. The apparatus according to claim 1 or 2, wherein the waveguide-based optical coupling element (10) is configured to separate at least two guided intrinsic modes (120, 130) assigned to the first optical coupling point (100) into different polarizations and subsequently rotate one polarization direction of the separated intrinsic modes (120, 130) to align the polarization direction with at least one guided intrinsic mode (260) assigned to the second optical coupling point (370) and at least one guided intrinsic mode (280) assigned to the third optical coupling point (380).

5. The apparatus according to claim 1 or 2, wherein the waveguide-based optical coupling element (10) comprises at least two partial waveguides arranged close together or spatially intersecting in a first region (600), the partial waveguides having at least partially strongly decoupled eigenmodes when considered in isolation from each other, and the partial waveguides not spatially intersecting in a second region (610).

6. The apparatus of claim 1, wherein the waveguide-based optical coupling element (10) comprises: - At least one first waveguide region (200) adjacent to a first optical coupling point (100) and wherein a first waveguide cross section (110) is continuously transformed into a superimposed second waveguide cross section (210) comprising two waveguide cross sections (230, 240), each having strongly decoupled intrinsic modes (250, 260, 270, 280), wherein at least two guided intrinsic modes (120, 130) assigned to the first optical coupling point (100) are arranged orthogonally to each other in the first waveguide cross section (110), or two mutually orthogonal linear combinations of at least two guided intrinsic modes (120, 130) assigned from the first waveguide cross section (110) to the first optical coupling point (100) are transformed into strongly guided intrinsic modes (260, 280); and - At least one second waveguide region (300) adjacent to the first waveguide region (200), and the initially overlapping waveguide cross sections (230, 240) in the second waveguide region (300) are guided to separate into non-intersecting cross sections (330, 314) defining a second optical coupling point (370) and a third optical coupling point (380).

7. The apparatus according to claim 6, wherein the transformation of the first waveguide cross section (110) in the first waveguide region (200) into the second waveguide cross section (210) is achieved by continuous deformation of the cross section along the direction of light propagation.

8. The apparatus according to claim 6 or 7, wherein the waveguide cross sections (230, 240) respectively form simple connection regions having an aspect ratio of at least 1.

5.

9. The apparatus according to claim 1 or 2, wherein - The first optical coupling point (100) is optically coupled directly or indirectly, either through another waveguide segment or beamforming element, to a waveguide or optical fiber having low exponential differences and degenerate orthogonal eigenmodes; and / or - The second optical coupling point (370) or the third optical coupling point (380) is optically coupled directly or indirectly through another waveguide segment or beamforming element to a waveguide or a semiconductor-based integrated optical waveguide with high exponential difference and strong decoupling of orthogonal eigenmodes.

10. The apparatus according to claim 1 or 2, wherein, The first optical coupling point (100) - Direct physical contact with another optical coupling point (410) of the optical component section (400); or - Not in direct physical contact with other optical coupling points (410) of the optical component section (400), wherein light propagates through a medium that is at least regionally uniform between the first optical coupling point (100) and the other optical coupling point (410).

11. The apparatus according to claim 1 or 2, wherein the first optical coupling point (100) is configured to receive light from free space, or wherein the first optical coupling point (100) comprises a lens configured to couple light from free space.

12. The apparatus of claim 1 or 2, wherein the core region has a refractive index of 1.3 to 1.

8.

13. The use of the device according to claim 1 or 2, as: - A polarizing filter comprising the apparatus according to claim 1 or 2 and a terminating element (395) connected to one of a second optical coupling point (370) or a third optical coupling point (380) of a waveguide-based optical coupling element (10). - A waveguide-based polarization beam splitter having at least one connecting waveguide (310a-310d). - A passive optical waveguide structure for a coherent polarization multiplexing receiver, which incorporates at least two waveguide-based coupling elements (10a, 10b, 10c, 10d) having at least one connecting waveguide (320a-320d). - Passive optical waveguide structure for polarization analyzer, which combines at least two waveguide-based coupling elements (10a, 10b, 10c, 10d) with at least one power divider. - Passive optical waveguide structures for polarization-sensitive image sensors, which combine at least two waveguide-based coupling elements (10a, 10b, 10c, 10d) having at least one free-form micro-optical element.

14. A method for generating a waveguide-based optical coupling element (10), the waveguide-based optical coupling element (10) being configured to perform mode-selective separation or superposition of an optical field at at least one additional optical coupling point (410) of at least one optical component portion (400), the method comprising the steps of: a) Provide at least one optical component portion (400) and position at least one additional optical coupling point (410) of at least one optical component portion (400) in a coordinate system (40) in a free-form microstructure unit, the free-form microstructure unit being configured to perform a free-form microstructure method; b) Generate a dataset that describes the three-dimensional shape of the waveguide-based optical coupling element (10) in the coordinate system (40) of the microstructured unit, wherein - The waveguide-based optical coupling element (10) has at least three optical coupling points (100, 370, 380). ○ At least one first optical coupling point (100) having at least two different guided intrinsic modes (120, 130) assigned to the first optical coupling point (100). ○ At least one second optical coupling point (370), having at least one guided intrinsic mode (260) assigned to the second optical coupling point (370), and ○ At least one third optical coupling point (380) having at least one guided intrinsic mode (280) assigned to the third optical coupling point (380). - Wherein, the waveguide-based optical coupling element (10) is configured for efficient bidirectional optical transmission. ○ Between at least one first-guided intrinsic mode (120) assigned to the first optical coupling point (100) and at least one guided intrinsic mode (260) assigned to the second optical coupling point (370), and Between at least one second-guided intrinsic mode (130) assigned to the first optical coupling point (100) and at least one guided intrinsic mode (280) assigned to the third optical coupling point (380); c) A waveguide-based optical coupling element (10) is generated at at least one additional optical coupling point (410) of at least one optical component portion (400) using a free-form microstructuring method; d) The waveguide-based optical coupling element (10) is embedded, at least partially, as the core region into the cladding region adjacent to the waveguide-based optical coupling element (10), such that a refractive index difference of at least 0.05 occurs between the core region and the cladding region. In this context, along a portion of the transition region corresponding to at least three optical coupling points in a waveguide-based optical coupling element, the centerline of the transition region exhibits non-zero torsion, and the cross-section of the transition region undergoes continuous deformation, characterized by monotonic rotation or deformation of the principal constraint axis about the centerline.

15. The method of claim 14, wherein at least one of the second optical coupling point (370) and the third optical coupling point (380) is optically coupled to the at least one optical component portion (400) or another optical component portion (430, 440), and the position of the at least one optical component portion (400) or the other optical component portion (430, 440) is recorded and taken into account in step b) when generating the dataset.

16. The method of claim 14 or 15, wherein the core region has a refractive index of 1.3 to 1.

8.

17. The method according to claim 14 or 15, wherein the freeform microstructuring method is used to additionally generate at least one additional optical element selected from connecting waveguides (30, 160, 170, 310a-310d, 320a-320d, 830, 840), tapered structures (72a, 72b, 170, 170a-170f, 331, 341, 815, 816, 850, 860), optical couplers (20a, 20b), beamforming elements (111, 411), and mechanical support structures (60).

18. The method according to claim 14 or 15, wherein the free-form microstructuring method is a photolithography method selected from stereolithography and direct-write laser lithography.

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