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

Abstract

To provide an apparatus for optical coupling and for mode-selective separation or superposition of optical fields, use thereof, and a method for manufacturing waveguide-based optical coupling elements designed for mode-selective separation or superposition of optical fields at additional optical coupling points of optical component portions.SOLUTION: An apparatus comprises at least one waveguide-based optical coupling element having at least three optical coupling points, and at least one optical component portion having at least one additional optical coupling point. At least one of the optical coupling points 100, 370, 380 is optically connected to the at least one additional optical coupling points 410. The waveguide-based optical coupling element 10 bidirectionally transmits light with high efficiency between native modes 120, 260 associated with the first optical coupling point 100 and the second optical coupling point 370, and between native modes 130, 280 associated with the first optical coupling point 100 and the third optical coupling point 380.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention belongs to the field of integrated photonics and micro-optics, and more particularly to micro-optical and nano-optical systems in which light is transmitted between various optical components or between a free-space stretch and an optical component by optical coupling points. In particular, the present invention relates to devices for optical coupling and for mode-selective separation or superposition of optical fields, and their use, and to a method for manufacturing a waveguide-based optical coupling element configured for mode-selective separation or superposition of optical fields at the optical coupling points of optical component parts.

Background Art

[0002] The functionality of integrated optical or micro-optical systems often depends critically on whether the light transmitted, for example, to promote efficient excitation of a particular waveguide mode in the case of waveguide-based components in order to obtain a high coupling efficiency, or to convert the light emitted by a component into a desired field distribution in free space, has a particular spatial distribution and polarization at each optical coupling point. In this case, the distribution and polarization of the light are generally described by a vector mode field that includes both the spatial distribution of the vector electric field E(x,y) and the vector magnetic field H(x,y).

[0003] According to the prior art, in order to adjust the intensity distribution of the mode field, discrete optical elements such as lenses, refractive index distribution type fibers, curved mirrors, or other refractive, diffractive, or reflective optical elements are usually used. In contrast, in order to set the orientation of the field vectors of the electric and magnetic fields, in many cases, polarization operation optical elements such as polarization filters or birefringent optical elements, especially half-wave plates or quarter-wave plates, or appropriate optical fibers (for example, polarization-maintaining optical fibers) are used. These elements need to be appropriately combined with each other in many practical applications, especially to obtain the desired vector mode field distribution at the optical coupling point of the optical component part. First, this leads to a relatively large device, and its installation space is often many times larger than the installation space of the related optical component part. In addition, the individual optical elements must be very accurately aligned with respect to each other and relative to the optical coupling point of the optical component part. This often requires a time-consuming and costly adjustment method, especially an active adjustment method, in which the optical coupling efficiency is continuously measured and optimized during the positioning procedure. Such adjustment methods are only suitable for complex applications and have been recognized for mass production of optical systems or micro-optical systems.

[0004] This problem occurs especially when light from free space, an optical fiber, or an optical component part is intended to be coupled into a particular mode, defined in particular by the polarization direction, of a single-mode waveguide of a further component. In the case of an axially homogeneous waveguide, i.e., a waveguide having a cross-sectional profile invariant in the propagation direction, the term "waveguide mode" denotes a form of electromagnetic field that does not change its lateral spatial dependence during axial propagation. A waveguide mode can have a lower cut-off frequency at which the waveguide mode is guided within each respective waveguide, but below which guiding is no longer possible. The "fundamental mode" denotes a waveguide mode having the lowest cut-off frequency compared to other modes of the same mode family, where the mode family is determined, for example, by polarization. In the case of a waveguide with a step profile, the fundamental mode is generally distinguished by the fact that the lateral intensity distribution belonging to the mode field has a single maximum value in the region of the waveguide core and no zeros in the lateral intensity distribution otherwise.

[0005] In many cases, the guided waveguide modes can be subdivided into two different mode families based on their polarization states, which are particularly called "transverse electric" ("TE") or "transverse magnetic" ("TM"). In this case, for each of the mode families, a field distribution with the lowest cut-off frequency can be determined, so that there are two fundamental modes with different polarization states. Hereinafter, a "single-mode waveguide" is understood to mean a waveguide 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 orientation of the electric field vector belonging to this waveguide mode, and the direction of the lateral component of the electric field, which is often dominant, is often used.

[0006] In the case of a waveguide having a cross-section with a continuous or discrete rotational symmetry shape, such as circular or square, degenerate or substantially degenerate waveguide modes often occur, which have the same or similar propagation constants, and their mode fields are exactly or approximately converted into each other by appropriate rotation. Further, in the case of degenerate modes or substantially degenerate modes, any linear combination of the two mode fields propagates in the axial direction with the same propagation constant as the initial mode, and in the process, its transverse field distribution is completely or almost maintained. The polarization characteristics of the superposition of two degenerate or substantially degenerate waveguide modes can be explained by the relevant polarization states in a manner similar to the superposition of plane waves in free space. When coupling various optical components to each other or linking them to optical fibers, in particular, light from a first single-mode or multimode waveguide having two degenerate modes or substantially degenerate modes of different polarizations needs to be coupled to the two above-mentioned waveguides such that only their respective eigenmodes are excited in each of the two further single-mode or multimode waveguides, each having non-degenerate eigenmodes in any case. As an example, this applies to the coupling of light from an optical fiber, such as a standard single-mode fiber, to a strongly birefringent waveguide on an integrated optical chip, which often has a rotationally symmetric cross-sectional profile of different polarizations, thus degenerate modes, into the fundamental mode, often called TE or TM.

[0007] U.S. Patent No. 7,127,131 discloses an integrated polarization beam splitter fabricated on a planar semiconductor substrate using a planar microstructuring method from a plurality of layers, referred to therein as a "core layer". Thus, the structure requires not only a relatively complex manufacturing process that provides for the processing of at least two layers with high overlay accuracy, but also is limited in its structural shape by a planar, often approximately prism-shaped substructure having parallel bottom and top surfaces, where the centerlines of all waveguides or partial waveguides are arranged in a common plane or in mutually parallel planes. This results in limited functionality, leading to, for example, asymmetric losses in two separated modes. Also, the components described herein are only suitable for separating two polarizations on an optical chip.

[0008] U.S. Patent No. 7,228,015 discloses an integrated optical waveguide that rotates the polarization of an optical field propagating therein by 90°. This is similarly constructed from a plurality of discrete layers, referred to as a "core layer", such that the ideal shape of the optical waveguide having a rectangular cross-section twisted along its longitudinal axis can only be approximated roughly. Thus, this is also subject to limitations in its structural shape by a planar, approximately prism-shaped substructure having parallel bottom and top surfaces, where the centerlines of all waveguides are arranged in a common plane or in mutually parallel planes. Furthermore, this requires a complex manufacturing process that provides for the processing of individual layers by repeatedly applying conventional microstructuring methods, particularly by using planar structuring masks and anisotropic etching processes.

[0009] Watts et al., "Polarization splitting and rotating through adiabatic transitions," in Integrated Photonics Research, A. Sawchuk, ed., Vol. 91 of OSA Trends in Optics and Photonics, 2003, describes an integrated optical structure that combines a polarization beam splitter disclosed in U.S. Patent No. 7,127,131 and a polarization rotation means disclosed in U.S. Patent No. 7,228,015. This thus substantially suffers from the same constraints as the related sub-structures. The fabrication is carried out from a plurality of individual layers, which are applied to a planar semiconductor substrate and processed using conventional planar microfabrication methods.

[0010] Schumann et al., "Hybrid 2D-3D optical devices for integrated optics by direct laser writing," Light Science and Applications, Vol. 3, No. 6, 2014, discloses polymer waveguides fabricated on the surface of a chip by 3D lithography, the polymer waveguides having a twist along their longitudinal axis, thereby enabling rotation of polarization. The structures described herein are only used to interconnect two Si3N4 waveguides on an optical chip.

[0011] Hahn et al., "Polarizing beam splitter integrated onto an optical fiber facet," Optics Express, Vol. 26, No. 25, 2018 describes a polarizing beam splitter fabricated on an optical fiber facet by 3D lithography. The polarizing beam splitter includes a lamellar grating having a grating period on the order of the vacuum wavelength of the light utilized, or a shorter grating period, which is also referred to as a "sub-wavelength lamellar grating." In the lamellar grating, light radiated therein having a polarization called "TE" is coupled to a particular order of diffraction, while another polarization called "TM" passes through the grating with little disruption.

[0012] WO 92 / 00185 discloses the manufacture of an optical waveguide to achieve photoinitiated polymerization of a material at a focus by passing an optical beam from a high-power laser through a lens and focusing it onto a photo-structurable material. Along a path created by moving the focus through the photo-structurable material, strands of polymerized material are manufactured that have a higher refractive index than the surrounding bulk material and can function as an optical waveguide. An optical waveguide device comprising a number of waveguide strands can be manufactured using this method.

[0013] US 2018 / 0314005 discloses a planar integrated polarizing beam splitter comprising a waveguide core made of silicon nitride and configured to split an input optical signal into two waveguide modes of different polarizations. However, this device is a structure fabricated by a planar microstructuring method that has the same constraints described above in relation to US 7,127,131 and US 7,228,015 with respect to the freeform structures used in this application.

[0014] U.S. Patent No. 8,903,205 and U.S. Patent No. 9,034,222 disclose a method and apparatus for using a 3D lithographically fabricated optical freeform waveguide at a target location for interconnecting different optical components. The fact that the freeform waveguide can be easily adapted in terms of position, shape, and size to the position, shape, and size of the optical component portions to be connected is utilized herein.

[0015] Object of the Invention Building on therefrom, it is an object of the present invention 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 manufacturing a waveguide-based optical coupling element configured for mode selective separation or superposition of optical fields at the optical coupling points of optical components, which at least partially overcome the disadvantages and limitations of the prior art.

[0016] In particular, the object of the present invention consists of using an apparatus and a method for coupling light to an optical component portion and / or for coupling light between two or more optical component portions while simultaneously adjusting the spatial mode field distribution and polarization. Further, in the reverse direction, the present invention should enable light to emerge from and be coupled from an optical component portion and provide a specific field distribution and polarization to the light.

[0017] The apparatus should be as compact as possible and should be automatically mass-produced at as low a cost as possible. Further, the apparatus should be accurately aligned in relation to the facet of at least one component without requiring complex adjustment methods, in particular active adjustment methods, and without making the manufacturing process of the optical component portions more difficult, in particular by avoiding the use of complex integrated optical mode field converters or polarization converters.

[0018] The apparatus and method should further enable the separation of spatially overlapping but different polarization field components of the input field and the supply of the separated field components to different, non-spatially overlapping optical waveguides, and this function should be comparable to that of a "polarization beam splitter" of an optical system composed of discrete components.

[0019] Furthermore, in the reverse direction, the apparatus and method should enable the formation of an output field by combining light from different, non-spatially overlapping optical waveguides and superimposing this light in the form of different polarization field components, and this function is equivalent to that of a "polarization beam combiner".

[0020] In particular, the apparatus and method are intended to enable coupling an optical fiber having potentially degenerate eigenmodes with orthogonal polarization directions to two integrated optical waveguides such that light from a first eigenmode of two mutually orthogonal eigenmodes of the optical fiber can be transmitted to a specific fundamental mode of a first integrated optical waveguide, while light from a second eigenmode of the mutually orthogonal eigenmodes of the optical fiber can be transmitted to the fundamental mode of a second integrated optical waveguide.

[0021] Furthermore, reversal of the propagation direction should enable the superimposition of light from two spatially separated optical waveguides in the form of two mutually orthogonal eigenmodes of the optical fiber.

[0022] In this regard, the apparatus should be flexibly applicable to a wide variety of optical integration platforms and should be done without additional discrete optical elements if possible, and the related method is intended to be inserted as seamlessly as possible into procedures from the techniques of optical construction and connection. Summary of the Invention

[0023] This object is achieved by means of an apparatus for optical coupling and for mode-selective separation or superposition of optical fields, the use of the apparatus, and a method for manufacturing a waveguide-based optical coupling element configured for mode-selective separation or superposition of optical fields at the optical coupling point of an optical component part, these having the features of the independent patent claims. Advantageous developments, which can be realized individually or in any desired combination, are provided in the dependent claims.

[0024] The terms “comprising”, “including” or “having”, or any desired grammatical variations thereof, are used hereinafter in a non-exclusive manner. Thus, these terms can relate both to situations in which no further features exist in addition to the features introduced by these terms and to situations in which one or more further features exist. For example, the expressions “A comprises B”, “A includes B” or “A has B” can relate both to a situation in which no further elements other than B exist in A (i.e., a situation in which A consists only of B) and to a situation in which one or more further elements, such as element C, elements C and D or further elements, exist in A in addition to B.

[0025] Furthermore, the expressions “at least one” and “one or more”, and grammatical variations of these expressions, when used in connection with one or more elements or features, are generally intended to express that the element or feature can be provided once or multiple times, and it should be pointed out that they are only used once, for example, at the first introduction of the feature or element. If the feature or element is subsequently mentioned again, the corresponding terms “at least one” or “one or more” generally are not used again without restricting the possibility that the feature or element can be provided once or multiple times.

[0026] Furthermore, terms such as "preferably", "preferably", "in particular", "for example", or similar terms are used below in connection with optional features, but alternative embodiments are not limited thereby. For example, the features introduced by these terms are optional features and are not intended to limit the scope of the claims, in particular the independent claims, by these features. For example, the present invention can also be implemented using different configurations, as will be understood by those skilled in the art. Similarly, features introduced by "in one embodiment of the present invention" or "in a typical embodiment of the present invention" are understood to be optional features without intending that alternative configurations or the scope of the independent claims be limited thereby. Furthermore, these introductory expressions are not intended to cover all possibilities of combining the features introduced thereby with other features, whether the features introduced thereby are optional or not.

[0027] In a first aspect, the present invention relates to an apparatus for optical coupling and for mode selective separation or superposition of optical fields, said apparatus comprising at least at least one waveguide-based optical coupling element, and at least one optical component part having at least one additional optical coupling point, the optical coupling element being at least one first optical coupling point having at least two different guided eigenmodes assigned to a first optical coupling point, at least one second optical coupling point having at least one guided eigenmode assigned to a second optical coupling point, and at least one third optical coupling point having at least one guided eigenmode assigned to a third optical coupling point, having at least three optical coupling points, at least one of the optical coupling points of the waveguide-based optical coupling element is optically connected to at least one additional optical coupling point of the optical component part, the waveguide-based optical coupling element is Between at least one first guided eigenmode assigned to the first optical coupling point and at least one guided eigenmode assigned to the second optical coupling point, and It is configured to transmit light bidirectionally with high efficiency between at least one second guided eigenmode assigned to the first optical coupling point and at least one guided eigenmode assigned to the third optical coupling point.

[0028] The terms "light emission", "emission", or "light" relate to any type of electromagnetic wave that can be guided within an optical waveguide. This includes, in addition to the visible optical range having a vacuum wavelength λ between 400 nm and 800 nm, in particular 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 called the "THz range", and the range of 1 mm ≤ λ ≤ 1 cm is also called the "millimeter wavelength range". Unless otherwise stated, in particular for the dimensions of the structure or to explain the performance indicators of the microstructuring method, such as resolution or accuracy, the numerical values specified below relate to a device configured for a vacuum operating wavelength λ of approximately 1.5 μm. For other operating wavelengths, in particular, taking into account the refractive index of the material used, the specified numerical values can be scaled proportionally to the wavelength.

[0029] The terms "optical coupling point", "optical coupling structure", and "facet" as further used herein each First, it is a region of the light-emitting optical component or the structure of the light-emitting optical component, which is the region where the light finally passes during the above light emission, and Second, it is a region of the light-receiving optical component or the structure of the light-receiving optical component, which is the region where the light first hits when the above light is received.

[0030] Hereinafter, the terms “optical component part” and “optical component” denote optical elements configured to emit, carry, receive, detect, and / or manipulate electromagnetic radiation, while the term “optical system” denotes an apparatus of at least two optical components according to the invention, or a combination between those of one or more apparatuses, where at least one optical component or at least one further additional structure, in particular at least one optical waveguide or at least one micro-optical element, is manufactured in combination with the apparatus according to the invention. Preferably, each optical component used within the scope of the invention is selected from the group comprising an optical fiber, in particular a single-mode or multi-mode fiber made of an organic or inorganic material; a semiconductor-based integrated optical chip, in particular a photodiode, a linear or planar photodiode array, a CCD array or an image sensor, in particular semiconductor-based, preferably silicon or III-V compound semiconductor, or a dielectric material, preferably glass, silicon dioxide, silicon nitride or polymer; a bolometer; a laser, in particular a vertical cavity surface emitting laser (VCSEL) or an edge emitting laser; a superluminescent diode; an optical circuit board; an element for free beam optics, in particular a lens, a beam splitter, an isolator, a mirror or a diffraction grating. Other optical components are conceivable. The optical component may preferably comprise an optical waveguide with a low refractive index contrast, in particular a glass-based optical waveguide, or a waveguide with a medium or high refractive index contrast, in particular a semiconductor-based waveguide. The input coupling or output coupling of light may preferably occur at the edge or surface of the optical component, in particular at the edge of an edge emitting laser, at the edge of a chip, or at the facet of a waveguide-based system, or alternatively on the surface of a surface emitting laser or a surface illuminated photodiode, or on the surface of a waveguide-based chip comprising at least one optical coupling point selected in particular from a grating coupler or a deflecting mirror. However, other methods of input coupling or output coupling of light are possible.

[0031] For low-loss coupling of light to the optical coupling point of an optical component, the light is preferably radiated to the optical coupling point at a defined position and in a defined direction so as to have a defined field distribution. Conversely, at the optical coupling point, light is radiated in a defined direction using a defined field distribution at a defined position. In this context, the terms "vector field distribution" or "field distribution" are understood to mean a combination of the complex vector electric field (E-field) and the magnetic field (H-field) that defines both the intensity distribution and the polarization of the electromagnetic field, and "polarization" refers to the orientation of the corresponding field vectors. Further, the term "orthogonality" of the field distribution is with respect to the orthogonality relations commonly used in integrated optics. See, for example, Katsunari Okamoto, Fundamentals of Opti-cal Waveguides, Academic Press, 2006, pp.154-155.

[0032] The terms "mode field" and "mode field distribution", which show the vector field distribution of the waveguide mode associated with the waveguide cross-section, are used for the field distribution linked to the optical waveguide. As already mentioned, the terms "waveguide mode", "eigenmode", or simply "mode" in the case of a waveguide that is homogeneous in the axial direction denote the form of the electromagnetic field that does not change its lateral spatial dependence in the case of axial propagation. In the case of a more complex waveguide where the cross-sectional profile changes, for example, periodically in the axial direction, the associated mode field can also change periodically accordingly. A waveguide whose cross-sectional profile changes sufficiently slowly in the axial direction, i.e., adiabatically, can often be well approximated and described based on a mode field that changes slowly accordingly, i.e., adiabatically.

[0033] The device of the present invention for optical coupling and for mode selective separation or superposition of optical fields, also simply called an "optical coupling element", comprises a waveguide-based optical coupling element that enables coupling light to an optical component part and / or transmitting between at least two optical component parts, while at the same time adjusting the spatial mode field distribution and polarization. In relation to the optical coupling element, the term "waveguide-based" in this case describes a structure in which light is guided by a waveguide configured at least in part for this purpose. For this purpose, in principle, the waveguide-based optical coupling element can be realized based on any waveguide concept. For this purpose, preferably dielectric waveguides are suitable, but alternatively, metal waveguides, in particular, hollow waveguides for the microwave range, or plasmonic structures can also be used.

[0034] As a result, the waveguide-based optical coupling element is, firstly, suitable for splitting the superposition of at least two mutually orthogonal or substantially orthogonal field distributions present at the first optical coupling point of the optical coupling element while at the same time manipulating the associated spatial field distribution and / or polarization. Secondly, the waveguide-based optical coupling element can operate on the optical signals radiated therein at at least two spatially separated optical coupling points in relation to their field distribution and / or polarization, superimpose the optical signals radiated therein in the form of spatially overlapping partial fields of different modes, and then serve to provide a superposition of the partial fields at at least one output coupling point.

[0035] Furthermore, it is also possible to use a waveguide-based optical coupler as a polarization filter. For this purpose, an optical signal to be filtered in relation to polarization can be coupled to the optical coupler via a first optical coupling point. The desired signal filtered in relation to polarization is then available in one of the guided eigenmodes at the second or third optical coupling point, while the signal components suppressed by the polarization filtering are supplied to termination elements connected to the respective other optical coupling points. The termination element or beam dump is understood to mean a structure that receives and absorbs the incident light without significant back reflection, or radiates the light to the surroundings so that there is no renewed coupling to the waveguide-based optical coupler or one of the optical component parts connected thereto. The power level of the back reflection at the input of the beam dump is preferably at least 10 dB lower than the radiation power, particularly preferably at least 20 dB or 30 dB lower. In a preferred embodiment, the beam dump can be implemented in the form of a continuously tapering taper structure, thereby enabling the light to be emitted, particularly in the direction of the absorption surface. The coupling to the optical component can be carried out directly or, as described above, via at least one connecting waveguide or at least one free beam coupling path.

[0036] Waveguide-based optical coupling elements can be manufactured in situ at the optical coupling points of the optical component part or between at least two optical coupling points of at least one optical component part by a three-dimensional free-form microstructuring method. In this process, in particular, the position, shape and / or size can be adjusted to the position of one optical component part or at least two optical component parts. In the following description, at least two spatially overlapping, orthogonal or substantially orthogonal field distributions supplied to or emitted from the first optical coupling point of the optical coupling element are, in principle, interpreted as fundamental modes of different polarizations, and the associated device then performs the function of a polarization beam splitter or a polarization beam combiner. The term "mode selection" means that, as a result of a suitable adjustment of the design of the waveguide-based optical coupling element, it is also possible to separate two modes with the same polarization but different field distributions using the device according to the invention, any mode in particular.

[0037] What can be utilized to separate modes is that they are guided with different intensities in appropriately formed waveguides, so that separation can be achieved by the geometric branching of the waveguides. In this context, the "strongly guided" mode of a waveguide is understood to mean a waveguide mode having a substantially larger propagation constant, and thus a substantially larger effective refractive index than other modes guided in this waveguide, and thus also the mode also called the "weakly guided" mode. In particular, the strongly guided mode is characterized by adapting substantially more strongly to changes in the trajectory of the waveguide and / or the cross-section of the waveguide along the propagation direction, for example, twist or diameter changes, than the weakly guided mode. It is also conceivable that only the strongly guided mode is present in the waveguide. In many cases, the strongly guided waveguide mode is the fundamental mode, and its electric field is preferentially polarized along the direction in which the waveguide core has its maximum spread.

[0038] As an alternative to the mode separation by a geometrically branching waveguide, a configuration can also be considered in which the coupling of different intensities of the separated modes to parallel waveguides is used in a manner similar to what is known as a "directional coupler". A further option is a method of performing mode conversion by periodically modulating the cross-section of the waveguide axially and opposing the fundamental wave number to the difference in the wave numbers of the modes to be coupled. Thus, it is possible to convert into a form of field in which any desired modes can be separated from each other, particularly in a low-loss and highly reliable manner. Potential application areas in this case are preferably the separation of modes at the facets of multimode fibers or so-called "few-mode fibers" and the input coupling of corresponding optical signals to different optical coupling points in the optical component part. A preferred configuration of the coupling element for this purpose can be determined, inter alia, by a method known as the "topology optimization method", which enables the overall form of the optical coupling element to be numerically optimized rather than individual geometric parameters. The structured geometric shapes obtained thereby generally avoid effective explanations but, at the same time, facilitate the implementation of the device according to the invention for optical coupling and mode separation.

[0039] For further details regarding the proposed device, reference is made to the following exemplary embodiments.

[0040] In a further aspect, the invention relates to a method for manufacturing a waveguide-based optical coupling element configured for mode-selective separation or superposition of an optical field at an optical coupling point of an optical component part. Specifically, the steps of the method are as follows: a) providing at least one optical component part and positioning at least one further optical coupling point of the at least one optical component part in a coordinate system of a freeform microstructuring unit configured to perform a freeform microstructuring method; b) generating a data set that describes the three-dimensional shape of a waveguide-based optical coupling element in the coordinate system of the freeform microstructuring unit, wherein the waveguide-based optical coupling element is At least one first optical coupling point having at least two different guided eigenmodes assigned to the first optical coupling point, At least one second optical coupling point having at least one guided eigenmode assigned to the second optical coupling point, and At least three optical coupling points, which are at least one third optical coupling point having at least one guided eigenmode assigned to the third optical coupling point, A waveguide-based optical coupling element, Between at least one first guided eigenmode assigned to the first optical coupling point and at least one guided eigenmode assigned to the second optical coupling point, and Between at least one second guided eigenmode assigned to the first optical coupling point and at least one guided eigenmode assigned to the third optical coupling point, configured to transmit light bidirectionally with high efficiency, a process, c) A process of manufacturing a waveguide-based optical coupling element at at least one additional optical coupling point of at least one optical component part by using a freeform microstructuring method.

[0041] The implementation of steps a) to c) does not necessarily need to be executed strictly continuously and may be included in other manufacturing processes that proceed in parallel. In this case, each of steps a) to c) can be executed multiple times, and at least consecutive steps may be executed at least partially in parallel. Further steps, particularly step d) listed below, may additionally be executed. In particular, the dataset generated in step b) may include, in addition to waveguide-based optical coupling elements for mode-selective separation or superposition of optical fields, micro-optical elements such as simple connection waveguides or lenses or mirrors. The design of such simple connection waveguides or micro-optical elements is likewise based on the position and orientation of specific optical coupling points, and these can be manufactured together with the waveguide-based optical coupling elements according to step c). Furthermore, the basic structure of the waveguide-based optical coupling element generated in step c) can be subjected to further subsequent post-processing steps, within which the generated basic structure can, for example, be locally or entirely embedded in an optically low-refractive-index cladding material or a vapor deposition coating can be provided. As an example, dispensing 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. The corresponding dataset may also include, in addition to waveguide-based optical coupling elements for mode-selective separation or superposition of optical fields, micro-optical elements such as simple connection waveguides or lenses or mirrors. The design of such simple connection waveguides or micro-optical elements is likewise based on the position and orientation of specific optical coupling points, and these can be manufactured together with the waveguide-based optical coupling elements in step c).

[0042] In a preferred configuration of the method of the present invention, the following subsequent step d) can preferably be executed after step c): d) A step of at least locally embedding a waveguide-based optical coupling element as a core region into a cladding region adjacent to the waveguide-based optical coupling element, wherein the core region has a refractive index of 1.3 to 1.8 and a refractive index difference of 0.05 to 0.7 occurring between the core region and the cladding region.

[0043] The device is preferably produced in situ, i.e., directly at the target position, by using a microstructuring method configured for this purpose. In this case, the term "microstructuring method" used means a subtractive or additive manufacturing method that can produce three-dimensional structures, preferably free-form structures, having dimensions in the micrometer range and / or millimeter range depending on the operating wavelength of the structures described. The microstructuring method configured for the production of free-form structures is hereinafter referred to as "free-form microstructuring method". The "free form" or "free-form structure" is understood to mean a structure that can have at least locally a surface with any desired curvature within the scope of technical limitations related to resolution and accuracy. Thus, free-form structures are, in particular, different from the structural shapes that can be produced on a planar semiconductor substrate by conventional planar microstructuring methods, such as two-dimensional lithography methods such as thin-film deposition, projection lithography, etc., and combinations of etching processes. In principle, combinations of these conventional planar microstructuring methods lead to prism-shaped three-dimensional structural shapes each having a top surface and a bottom surface substantially parallel to the substrate surface, and the bottom surface and the top surface are identical or very similar in terms of shape, and are interconnected by side walls that are perpendicular to the substrate surface, inclined, or curved inward or outward depending on the respective etching or deposition process. In this case, the shape of the bottom surface and the top surface is substantially specified by the mask used for local etching or deposition, and the mask is often structured by lithography. A multilayer structure consisting of a plurality of prism-shaped substructures can be constructed by repeating the etching or deposition process a plurality of times using different masks, but the additional costs associated with the repetition are enormous, and the quality of the resulting structure is also often limited by the overlay accuracy, so the number of layers is often actually limited to a small number, for example, three. This leads to limitations in the shape of the structures that can be produced at a reasonable cost using conventional microstructuring methods, and thus to functional limitations of the components formed thereby.The additional costs associated with multilayer structuring are often very high and, in particular, when the additional layers involved cannot also be used for other component parts present on the chip, the manufacturing process for the associated optical components becomes much more complex.

[0044] In contrast, freeform structures produced by freeform microstructuring methods are not or are not to the same extent subject to these limitations, since their structural shapes are not limited to combinations of a relatively small number of planar, prism-shaped substructures. This makes it possible to realize non-planar optical coupling elements in which the centerlines of the waveguides constituting the optical coupling element do not lie in a common or parallel plane to each other, especially in waveguide-based optical coupling elements. In this context, it should be noted that freeform structures are often also manufactured from a number of individual layers, for example by using the application of multilayer materials within the scope of 3D printing or by curing the various layers in the case of 3D lithography methods. However, within the scope of reasonable manufacturing costs, it is possible to select a very large number of layers by means of freeform microstructuring methods, so that the approximation of the freeform structure is good and the discretization into individual layers no longer represents a functional limitation of the manufacturable structural shapes.

[0045] In this case, the waveguide-based optical coupling element is preferably constructed from at least 6 layers, particularly preferably from at least 10 layers, and especially from at least 20 or 30 layers. In this context, the layer thicknesses are preferably in the range between 10 nm and 1000 nm, particularly preferably between 30 nm and 500 nm, especially between 50 nm and 300 nm. The freeform microstructuring method used for this purpose enables the production of freeform structures with a precision preferably better than 1000 nm, particularly preferably better than 500 nm, especially better than 100 nm. The resolution of the freeform microstructuring method is preferably better than 3 μm, particularly preferably better than 1 μm, especially better than 500 nm. In this case, the values given in each case relate to the production of freeform structures provided for a vacuum operating wavelength of approximately 1.5 μm. The requirements regarding the dimensions of the freeform structures and thus the precision and resolution of the freeform microstructuring method used for manufacturing purposes can be scaled to other operating wavelengths, particularly taking into account the refractive index of the material used.

[0046] In contrast, WO 92 / 00185 discloses a waveguide-based optical device in which a local increase in the refractive index of a photo-structurable material is obtained purely by lithographic irradiation. In contrast to the present application, the method described in WO 92 / 00185 does not provide a development step in particular, within which the unexposed regions are selectively removed and replaced by a low refractive index cladding material. Thus, the achievable refractive index difference is typically limited to values below 0.025, making the production of compact polarization-sensitive structures significantly more difficult. In particular, the production of a waveguide-based polarization beam splitter with two superimposed partial waveguides does not seem to be feasible according to WO 92 / 00185, since the two orthogonally polarized eigenmodes of the partial waveguides have practically the same effective refractive index due to the low refractive index contrast and are thus guided with equal intensity. Thus, it does not seem possible to separate the orthogonally polarized eigenmodes by branching the partial waveguides if the length of the actually realizable component is maintained.

[0047] In a preferred configuration, the free-form microstructuring method and / or the free-form microstructuring unit that facilitates such a method can be based on a lithography method that uses, in particular, stereolithography or direct writing lithography, preferably three-dimensional direct writing lithography. In this case, an additive manufacturing method or a subtractive manufacturing method can be used. The term "additive manufacturing method" refers to a manufacturing method in which materials are continuously applied to or onto a structure, while the term "subtractive manufacturing method" represents an alternative manufacturing method in which materials are removed from a structure. In a preferred embodiment, the application or removal of materials can be achieved by a lithography method using a suitable photoresist, in particular a negative or positive resist. In this case, a spatial light modulator that enables high-speed patterning can be used in a preferred configuration in the stereolithography method. In a preferred configuration, a multi-photon lithography method can be used as a direct writing lithography method, particularly by using a pulsed laser source. In this case, light pulses having a pulse duration of preferably 10 ps or less, preferably 1 ps or less, particularly preferably 200 fs or less, especially 100 fs or less can be used, preferably at a repetition rate of at least 1 MHz, preferably 10 MHz, particularly preferably at least 25 MHz, especially at least 100 MHz. Particularly suitable for this purpose are laser light sources selected from, in particular, fiber-based femtosecond lasers or pulsed solid-state lasers such as titanium-sapphire lasers or diode lasers that can be combined with a frequency conversion unit 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, the extreme ultraviolet (EUV) radiation range, or the X-ray wavelength range can be preferentially used in the process. In a particularly preferred embodiment, wavelengths of 150 nm to 1700 nm, particularly 300 nm to 1100 nm are used. In the case of pulsed lasers, two-photon, three-photon, or multi-photon absorption effects can be achieved in the desired manner by appropriately selecting the pulse duration and pulse energy.Diode lasers having emission wavelengths of 360 nm to 550 nm, i.e., for example, approximately 365 nm, 385 nm, 405 nm, 550 nm, and 532 nm, are suitable for lithography methods using continuous-wave lasers based on single-photon absorption. To improve the resolution of lithography methods, "stimulated emission depletion" (STED) can be used in the style of the corresponding microscopy method together with a suitable photoinitiator. Further microstructuring methods for manufacturing waveguide-based optical coupling elements are also conceivable, in particular methods that can be based on material extrusion, powder bed fusion, material jetting, binder jetting, selective laser sintering, or electron beam melting. Methods such as metal printing or laser vapor deposition welding can be used, for example, for manufacturing hollow waveguides in the micrometer and millimeter wavelength ranges. Depending on the microstructuring method used, the waveguide-based optical coupling element can include polymers, preferably acrylates, epoxy resins, or fluoropolymers that can be structured optically additively or subtractively, metals, or metal-coated dielectrics. In a preferred configuration, the waveguide-based optical coupling element can include a material different from the material of the optical component part. There can be advantages in performing further post-processing steps for manufacturing the structure, within which the manufactured structure can be locally or entirely embedded in an optically low-refractive-index cladding material or a vapor deposition coating can be provided.

[0048] In particular, by using freeform microstructuring methods, it is possible to manufacture structures having a symmetric or substantially symmetric shape, which can preferably have very similar losses with respect to two separated modes. In this context, a "substantially symmetric structural shape" is understood to mean a three-dimensional shape having a plane of symmetry, axis of symmetry, or center of symmetry, where, as will be explained below, in particular, the complete symmetry can be slightly impaired by adjusting the structural shape used to couple the waveguide-based optical coupling element to the position and direction of at least one optical coupling point assigned to at least one linked optical coupling element. The difference in power loss between the two separated modes is preferably less than 3 dB, particularly preferably less than 2 dB, especially less than 1 dB or 0.5 dB.

[0049] Generally, when the device according to the invention is used as a polarization beam splitter, at the output coupling point, an extinction ratio better than preferably 6 dB, more preferably better than 10 dB, in particular better than 15 dB or 20 dB can be achieved. In this case, the "extinction ratio" is understood to mean the ratio of the emission power of the required mode at the output coupling point to the emission power of each unwanted mode, and this ratio is often expressed in decibels (dB) by logarithmic transformation. In this case, the relative optical bandwidth of the structure can preferably exceed 1%, more preferably exceed 5%, in particular exceed 10% or 20%. Here, the "relative optical bandwidth" is understood to mean the ratio of the width of the frequency range in which the optical component part reaches the required power index to each intermediate frequency.

[0050] A further advantage of the proposed device is that, preferably with the help of additional connecting waveguides, by in-situ manufacturing, without the need to very precisely align the optical component part with a complex adjustment method, the above device can be coupled to one or more optical coupling points of one or more optical component parts with very low loss. Proceeding from the already fixed optical component parts, the spatial position and direction of the optical coupling points belonging to these optical component parts can be obtained in the first step of the manufacturing method for this purpose and taken into account when designing additional structures such as optical coupling elements and preferably existing connecting waveguides. Thereby, with the necessary position and propagation direction, whether the light is made available or received at the optical coupling point of the waveguide-based optical coupling element and / or at the assigned optical coupling point of an adjacent additional structure, such as a connecting waveguide, by selecting the design of the optical coupling element and optionally existing additional structures, the inaccuracy of the positioning of the optical component part can be compensated by the corresponding adjustment in the form of the claimed device, and thus a high efficiency of the coupling of the optical component part to the optical coupling point and / or to the waveguide mode defined by the optical coupling point is achieved.

[0051] By adjusting the waveguide-based optical coupling element to the position and direction of the optical coupling point of the linked optical components, it is possible to compensate for the inaccuracies in the positioning of the linked optical component parts and obviate the need for very precise alignment of these component parts. To adapt the waveguide-based optical coupling element to the position and direction of the optical coupling point of the linked optical component parts, it is preferably possible to vary the geometric parameters of the optical coupling element, in particular the length of the first waveguide section described below, and / or the exact trajectory of the sub-waveguide. Alternatively or additionally, an additional connecting waveguide or beam shaping element having substantially any desired 3D shape can be connected to the selected optical coupling point of the linked optical coupling element and / or optical component part, and the additional connecting waveguide or beam shaping element can be manufactured together with the optical coupling element at low additional cost by a freeform microstructuring method, making it possible to compensate for the inaccuracies in the positioning of the linked optical component parts.

[0052] In a preferred configuration, the freeform microstructuring method can also be configured to manufacture, for example, what is known as a "photonic wire bond" as disclosed in U.S. Patent No. 8,903,205 or International Publication No. WO 2018 / 083191, in addition to the waveguide-based optical coupling element, preferably within the scope of a common manufacturing process. In particular, when it is intended to manufacture relatively large structures operating at frequencies in the micrometer and millimeter wavelength ranges, other methods, such as three-dimensional printing processes, are also conceivable.

[0053] For further details relating to the method of the present invention, reference is made to the description of the apparatus and the exemplary embodiments.

[0054] Further details and features of the present invention will become apparent from the following description of preferred exemplary embodiments, especially in conjunction with the dependent claims. Here, each feature can be implemented alone or in combination with others. The present invention is not limited to the exemplary embodiments. The exemplary embodiments are schematically illustrated in the following drawings. The same reference numbers in the figures refer to the same or functionally identical elements, or elements corresponding to each other with respect to their functions. Specifically, it is as follows.

Brief Description of the Drawings

[0055]

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Embodiments for Carrying Out the Invention

[0056] FIG. 1 shows a schematic diagram of a preferred exemplary embodiment of the device according to the present invention for optical coupling and for mode selection separation or superposition of optical fields. To explain the functionality of a polarization beam splitter or polarization beam combiner, the exemplary embodiment shows a device according to the present invention comprising a waveguide-based optical coupling element 10 connected to an optical component part 400 having a first optical coupling point 100. In the following example, for the sake of facilitating the use of the waveguide-based optical coupling element 10 as a polarization beam splitter, light is propagated from a first optical coupling point 100 functioning as an input coupling point to a second optical coupling point 370 and a third optical coupling point 380 functioning as output coupling points. In this case, the distinction between “input coupling point” and “output coupling point” is for the sole purpose of a simpler description of the device and should not be construed as a limitation regarding the functionality of the optical component part. Rather, the optical path can be reversed, as a result of which the roles of “input” and “output” are reversed and the use of the device as a polarization beam combiner becomes possible.

[0057] In the illustrated embodiment, the waveguide-based optical coupling element 10 schematically represented in FIG. 1 comprises a first waveguide section 200 having a first waveguide cross-section 110 and a first optical coupling point 100 that functions as an input coupling point in the case of a polarization beam splitter. The first waveguide cross-section 110 present at the first optical coupling point 100 has two mutually orthogonal, possibly degenerate, eigenmodes 120, 130, which are assigned to the first optical coupling point 100, each of which has, for example, an electrical mode field with a dominant, linearly polarized transverse component of the electric field vector, and their intensity distributions 140, 150 can be very similar and can have a significant overlap. In the case of the dominant transverse polarization components in the two mutually orthogonal mode fields, the associated transverse electric field vectors of the transverse components are substantially perpendicular to each other, and the separation of the mode fields at that time is, as a good approximation, equivalent to the separation of the associated linearly polarized light.

[0058] The waveguide-based optical coupler 10 schematically represented in FIG. 1 further comprises a second, branching waveguide section 300, which is adjacent to the first waveguide section 200 and comprises two spatially intersecting partial waveguides 330, 340, which, when considered individually, each have at least two eigenmodes with different polarization directions and very different effective refractive indices, so that the coupling between the waveguide modes appears very weakly during propagation. In principle, modes of the same waveguide with very different effective refractive indices are also called "strongly separated modes". In this case, the difference in the effective refractive indices of 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 only one guided eigenmode, which is generally polarized along a longer range of the waveguide cross-section, is present in each of at least one of the partial waveguides 330, 340. The partial waveguides 330, 340 define two further optical coupling points 370, 380 with associated cross-sections 350, 360. The two further optical coupling points 370, 380 are used as outputs in the case of a polarization beam splitter, i.e., each of the two orthogonal first eigenmodes 120, 130 assigned to the first optical coupling point 100 of the first waveguide cross-section 110 acting as an input is assigned to the waveguide mode of each of the two partial waveguides 330, 340 acting as output waveguides, while the other waveguide modes of each of the partial waveguides 330, 340 acting as output waveguides are not excited, except for unwanted crosstalk.

[0059] The functionality of the device schematically shown in FIG. 1 is based, in particular, on the fact that in the first waveguide section 200, the first waveguide cross-section 110 is continuously converted into a waveguide section 210 that includes the superposition of two waveguide cross-sections 230, 240, where the second waveguide cross-sections 230, 240 each have further eigenmodes 250, 260; 270, 280 with significantly different effective refractive indices. This continuous conversion from the first waveguide cross-section 110 to the second waveguide cross-section 210 can preferably be designed in the process such that there is a continuous deformation of the cross-section of the waveguide-based optical coupling element 10 along the light propagation direction. The first waveguide section 200 preferably has a length of from 0.1λ to 30λ, particularly preferably from 0.2λ to 15λ, especially from 0.2λ to 10λ, while the length of the waveguide-based optical coupling element 10 as a whole, as measured along its maximum length, is preferably less than 50λ, particularly preferably less than 25λ, especially less than 10λ, where λ denotes the vacuum wavelength of the light used and a refractive index of approximately 1.5 is assumed for the material used.

[0060] The waveguide cross-sections 230, 240 are adapted, with respect to shape and size, to the cross-sections of the partial waveguides 330, 340 that the partial waveguides 330, 340 have at the interface between the first waveguide section 200 and the second waveguide section 300. Different from what is schematically shown in FIG. 1, the waveguide cross-sections 230, 240 do not have to be exactly identical to the cross-sections of the partial waveguides 330, 340 at the interface including the waveguide cross-section 210 between the first waveguide section 200 and the second waveguide section 300. Rather, it is sufficient to adapt the cross-sections to each other such that the best possible optical coupling exists between the first waveguide section 200 and the second waveguide section 300. Furthermore, the cross-sections of the various partial waveguides schematically shown as rectangular in FIG. 1 should be understood to be exemplary, and one skilled in the art may also consider the use of other shapes, such as elliptical shapes, which may be found to be more robust in relation to manufacturing inaccuracies under certain circumstances.

[0061] As further schematically illustrated in FIG. 1, the waveguide cross-sections 230, 240 initially superposed in the partial waveguides 330, 340 are adapted to branch continuously into the spatially separated output waveguide cross-sections 350, 360 within the second waveguide section 300. In this case, the branching is made slow enough so that the axial structural cross-section change promotes an adiabatic transition of the mode field and thus a spatial separation with as little interference and loss as possible. Within this separation, the strongly guided modes of the two partial waveguides 330, 340 follow the trajectories of the respective partial waveguides, and in the process, the situation where they hardly couple to the weakly guided modes and are not radiated to a significant extent can be utilized. As a result, first, the optical signals present in the first two orthogonal eigenmodes 120, 130 of the first optical coupling point 100 functioning as an input are converted into the first mode fields 120a, 130a in the plane of the waveguide cross-section 210, and then it becomes possible to assign the first mode fields 120a, 130a to the respective fundamental modes of the two further optical coupling points 370, 380 regarded as outputs with low loss and almost no crosstalk.

[0062] When the first cross-sections 230 and 240 of the partial waveguides 330 and 340 are rectangular as shown in FIG. 1, preferably, in each case, the effective refractive index ne1 of the strongly guided fundamental mode polarized along the long side of the rectangular cross-sections 230 and 240 exceeds the effective refractive index ne2 of the weakly guided fundamental mode polarized along the short side of the rectangular cross-section profile, preferably by more than 0.005, particularly preferably by more than 0.05, and especially by more than 0.1. As an example, this can be achieved by the fact that the partial waveguides 330 and 340 use a very elongated cross-section, for example, an elliptical or rectangular cross-section with a large aspect ratio. The "aspect ratio" of the waveguide cross-section represented by the plan view is understood in this case to mean the maximum possible ratio of two dimensions measured in directions perpendicular to each other in this figure. The aspect ratio is equivalent to the ratio of the sides in the case of a rectangular cross-sectional view and the ratio of the semi-axes in the case of an elliptical cross-sectional view. Depending on the waveguide material selected, the aspect ratio of the cross-sections of the partial waveguides 330 and 340 is preferably more than 1.5, particularly preferably more than 2.5, and especially more than 3.5 or 4.5, at least in segmented sections. Also, a 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 has advantages for dielectric waveguides for the purpose of obtaining a large refractive index difference between the strongly guided mode and the weakly guided mode.

[0063] In the case of the waveguide-based optical coupling element 10 where the core region is preferably generated by lithographic patterning of a polymer material, the refractive index of the core region is preferably between 1.2 and 2, particularly preferably between 1.3 and 1.8, 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. Thus, 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, especially between 0.15 and 0.6. If necessary, the refractive index difference can be set by using an appropriate capping material or cladding material 500, in which the core region of the waveguide-based optical coupling element is completely or partially embedded, where, in the subsequent process steps, the capping material or cladding material 500 can preferably be applied locally or entirely on the core region of the waveguide-based optical coupling element 10 manufactured by using a free-form microstructuring method. In the case of a polymer-based core region, a low refractive index polymer is preferably used as the cladding material 500, particularly, it may be fluorinated and may also have a polysiloxane-based component. The refractive index of the cladding material 500 that at least locally surrounds the waveguide core is preferably between 1.2 and 1.5, particularly 1.3 and 1.45.

[0064] As a result, in this device, light can be coupled into the optical coupling element 10 via a first waveguide cross-section 110 called an "input facet", and can again emerge from and be coupled from the waveguide-based optical coupling element 10 via waveguide cross-sections 350, 360 called "output facets", where the waveguide-based optical coupling element 10 is used as a polarization beam splitter. It is also possible to reverse this optical path so that the roles of the input and output facets are appropriately reversed. As a result, the waveguide-based optical coupling element 10 can also be used to combine two optical signals, each of which is coupled to a respective eigenmode of two further, spatially separated optical coupling points 370, 380 and converted to eigenmodes that are mutually orthogonal at the first optical coupling point 100. As a result, the waveguide-based optical coupling element 10 can also be used as a polarization beam combiner.

[0065] Using a waveguide-based optical coupler 10, it is possible to spatially separate two spatially overlapping eigenmodes 120, 130 assigned to a first optical coupling point 100, these eigenmodes being present at the first optical coupling point 100 and being orthogonal or substantially orthogonal to each other, and optionally, subjecting the eigenmodes to further operations of eigenmodes 260, 280 assigned to a second optical coupling point 370 and a third optical coupling point 380. As a result of this separation and any further operations, preferably by using a method configured for this purpose, in particular a coherent detection method, it is possible to determine the power and / or amplitude and phase originally present in the eigenmodes 120, 130 assigned to the first optical coupling point 100, and thus it is possible to determine the associated polarization state. For this purpose, the first separated eigenmodes 120, 130 assigned to the first optical coupling point 100 can preferably be made to interfere with each other and / or with an additional reference field. Furthermore, the spatially separated eigenmodes 260, 280 assigned to the second optical coupling point 370 and the third optical coupling point 380 can preferably be operated on such that each of the modes is configured to excite the eigenmodes of a component part or waveguides 430, 440 adjacent to the waveguide-based optical coupler 10 via two further optical coupling points 370, 380 configured as output coupling points. In the case of the component part 400 connected to the first optical coupling point 100 on the input side, as a result, it is possible to split the associated degenerate or non-degenerate eigenmodes 120, 130 of the optical component part 400 assigned to the first optical coupling point 100, and thus it is possible to couple them to two integrated optical component parts or waveguides 430, 440 connected to the output coupling points of the waveguide-based optical coupler 10, whereby the light from each of the eigenmodes of the optical component part 400 embedded as a waveguide is converted into one of the eigenmodes of the integrated optical waveguide in each case.

[0066] The apparatus schematically illustrated in FIG. 1 can be modified in many ways. Thus, in FIG. 1, the generally rectangular cross-sections of the partial waveguides 330, 340 are maintained along the propagation direction and are modified only with respect to their lateral position and orientation. In alternative embodiments, it is also possible to vary the shape and / or size of the cross-section along the propagation direction, and in particular, it is also possible to continuously transform the rectangular shape into other shapes, in particular square, elliptical or circular shapes. Further embodiments are also conceivable. In particular, the shape can be adapted to the cross-section of the optical coupling points 370, 380 and the profile of the mode field of the optical component parts 430, 440 to be linked, so that efficient coupling is obtained. Thus, in addition to mode separation, the apparatus according to the invention makes it possible to obtain a low-loss connection between at least two optical component parts characterized by mode fields of very different sizes or positions at the optical coupling points.

[0067] Also, as exemplarily illustrated in FIG. 2, further embodiments of the apparatus schematically shown in FIG. 1 are possible, where the waveguide sections 200, 300, which are depicted in FIG. 1 as clearly distinguishable, merge into each other without a clear geometric boundary or are integrally formed, either in whole or in part. The functionality of the waveguide-based optical coupling element 10 for polarization beam splitting or polarization beam combining is based in particular on the fact that the waveguide-based optical coupling element 10 comprises at least two partial waveguides 330, 340 having at least partially distinct eigenmodes with very different effective refractive indices, where the at least two partial waveguides 330, 340 are very close together or spatially overlapping in a first region 600, as indicated by reference numeral 301 in FIG. 2, and are considered to be separated from each other, while the at least two partial waveguides 330, 340 are spatially separated in a second region 610, as shown in FIG. 2 by reference numerals 302a, 302b. In this case, the term "very close partial waveguides together" in the first region 600 means that the eigenmodes guided by the two partial waveguides 330, 340 overlap at least locally and can thus interact with each other.

[0068] In the apparatuses schematically shown in FIGS. 1 and 2, the two mode fields 120a, 130a are only spatially branched by the partial waveguides 330, 340 without a change in the polarization direction. In contrast, in other cases, it is desirable to change the polarization direction of the branched mode fields, thereby, for example, converting them into two TE modes of an integrated optical waveguide. The corresponding apparatuses are the subject of some of the exemplary embodiments below, see, for example, FIGS. 3, 8, 9, or 10.

[0069] Furthermore, the device according to the present invention can also be used to separate any desired modes, especially two modes having the same polarization but different field distributions. In the present embodiment, as already described above, the shape of the waveguide-based optical coupling element 10 is appropriately changed. In this case, for example, in a manner similar to the above separation of modes having different polarization directions, it is possible to utilize the fact that the modes to be separated are guided with different intensities in a waveguide in which the modes are appropriately formed, and the separation can be achieved as a result by geometrically branching the waveguide. Also, an implementation can be considered in which different strong couplings of the modes to be separated to parallel waveguides (in a manner similar to a so-called directional coupler) are utilized. A further option consists of a target conversion of the modes by a periodic modulation of the axial waveguide cross section, the fundamental wave number of the modulation corresponding to the difference in the wave numbers of the coupled modes. Thus, any desired mode can be converted into a field configuration that can be separated from each other, especially with low loss and high reliability, for example with the help of the above concepts. As an example, the fields of potential use in this case are the separation of modes at the facets of a multimode fiber or a so-called "few-mode fiber" and the input coupling of corresponding optical signals to different optical coupling points in the optical component part. The configuration of the optical coupling element corresponding to such an object can be determined by a method known as a topology optimization method in which, among other things, not the individual geometric parameters but the overall shape of the coupling structure is numerically optimized. The structured geometric shape obtained thereby generally avoids an effective description, but similarly facilitates the realization of the device according to the present invention for optical coupling and mode separation.

[0070] In a specific embodiment, the waveguide-based optical coupling element 10 is connected to a plurality of optical components 400, 430, 440. In order to adapt the shape of the waveguide-based optical coupling element 10 to the inaccuracies in the positioning of these components, it is advantageous to obtain the positions and orientations of further optical coupling points 410, 470, 480 very precisely in the coordinate system 40 of the free-form microstructuring unit used to manufacture the optical coupling element. For this purpose, alignment marks 411, 412, 472, 481, 482, or alternative structural elements (not shown) as schematically illustrated in FIG. 1 on the linked optical coupling points 400, 430, 440 can be used. The relative positions of these alignment marks or structural elements with respect to the further optical coupling points 410, 470, 480 are very precisely known. The alignment marks 411, 412, 471, 472, 481, 482, or alternative structural elements can preferably be detected by an imaging method, in particular a camera-based method, which preferably enables the localization of the alignment marks 411, 412, 471, 472, 481, 482, or alternative structural elements in three-dimensional space. In this process, it is also possible to preferably use a confocal imaging method. In this case, in particular, it is possible to use a part of the optical beam path of the free-form microstructuring unit for both the detection of the alignment marks 411, 412, 471, 472, 481, 482 and the exposure of the manufactured waveguide-based optical coupling element 10. The detection of the alignment marks 411, 412, 471, 472, 481, 482 or alternative structural elements can be carried out with the highest possible accuracy, and the deviation is preferably less than 500 nm, particularly preferably less than 200 nm, especially less than 100 nm or 50 nm. The positioning accuracy of the waveguide-based optical coupling element 10 manufactured by the free-form microstructuring method and the further optical coupling points 410, 470, 480 on the linked optical coupling points 400, 430, 440 is preferably better than 500 nm, particularly preferably better than 200 nm, especially better than 100 nm or 50 nm. These values relate to the manufacture of the device and structure according to the invention provided for an operating wavelength in vacuum of approximately 1.5 μm.

[0071] In the embodiment schematically shown in FIG. 3, the optical component part 400 is configured as a single-mode optical fiber having a rotationally symmetric refractive index profile with two degenerate or substantially degenerate approximately linearly polarized waveguide modes (for example, those known as the LP01 mode). In this case, the term "degenerate or substantially degenerate (plural) modes" refers to waveguide modes whose effective refractive indices are very similar and the difference is typically less than 0.001. In the case of a standard single-mode fiber, the two differently polarized fundamental modes have very similar intensity distributions, that is, the normalized overlap integral of the spatial intensity distributions is close to 1, preferably greater than 0.9 or 0.95. By the waveguide-based optical coupling element 10, it is possible to separate the two first eigenmodes 120, 130 assigned to the first optical coupling point 100 or to separate the linearly orthogonal combinations of the two eigenmodes 120, 130, that is, the first eigenmode 120 of the optical component part 400 or the first linear combination of the eigenmodes 120, 130 is converted into the eigenmode 260 of the first integrated optical component part 430, shown by reference numeral 71a in FIG. 8, for example, the so-called TE mode, while the second eigenmode 130 of the optical component part 400 or the second linear combination of the eigenmodes 120, 130 is converted into the eigenmode 280 of the second integrated optical component 440, shown by reference numeral 71b in FIG. 8, for example, again the so-called TE mode. The second mode of the output waveguides 430, 440, shown, for example, as "TM", is not excited in this embodiment, except for unwanted crosstalk.

[0072] As shown in FIG. 3, this can preferably be achieved by successive twisting of the partial waveguides 330, 340. Further examples can be seen in FIGS. 8, 9, and 10. This structure can also be modified in various ways. Thus, preferably, it is possible to already start the twisting of the waveguide in the first region 600 of the waveguide-based optical coupling element 10, where the waveguides are not yet separated. Also, further structural shapes of the waveguide-based optical coupling element 10 can be considered, which can preferably be determined by numerical parameter or topology optimization methods.

[0073] As illustrated in FIG. 4, the waveguide-based optical coupling element 10 can be manufactured in-situ by a three-dimensional free-form microstructuring method at the optical coupling point 410 of the optical component part 400 or between at least two further optical coupling points 410, 470, 480 of at least one of the optical component parts 400, 430, 440. In this case, the waveguide-based optical coupling element can adapt its position, shape, and size to the position and orientation of at least two further optical coupling points 410, 470, 480 of at least one of the optical component parts 400, 430, 440. Considering high-speed and high-throughput manufacturing, the waveguide-based optical coupling element 10 preferably has the simplest possible shape with the smallest volume. As an example, a simple adjacent region with as few so-called "holes" as possible in three-dimensional space, with little or no topological detail, is advantageous in terms of shape. Similarly, optical gratings suitable for optical field separation often lead to very detailed structures with high requirements regarding accuracy, which require complex patterning processes. By utilizing the waveguide characteristics that are at least partially present in the optical coupling element, a small volume can be achieved. In this case, the volume of the waveguide-based optical coupling element 10 manufactured by the free-form microstructuring method is preferably less than 1000 μm 3 and particularly preferably less than 500 μm 3 and especially less than 250 μm 3 or 150 μm 3 and is optionally added with any connecting waveguides or other mechanical or optical additional structures. These values relate to the waveguide-based optical coupling element 10 provided for an operating wavelength in vacuum of approximately 1.5 μm and including a material having a refractive index of approximately 1.5. For other operating wavelengths, the volume of the waveguide-based optical coupling element 10 can scale in proportion to the cube of the operating wavelength while taking into account the respective refractive indices of the materials used.

[0074] The term "manufacturing at the optical coupling point" used above represents an embodiment in which optical coupling between the waveguide-based optical coupling element 10 and a further optical coupling point 410 assigned to the optical component part 400 is promoted. For this purpose, and as schematically illustrated in FIG. 1, preferably there can be a direct physical contact between the optical component part 400 and the waveguide-based optical coupling element 10 in the region of the first optical coupling point 100. Alternatively, as shown in FIGS. 4 and 5, particularly preferably configured as connection waveguides 160, 170, light transmission may occur between the waveguide-based optical coupling element 10 and a further optical coupling point 410 assigned to the optical component part 400 via a further structure. The connection waveguides 160, 170 can preferably be manufactured in situ, i.e., directly at a target position that can be associated with the first optical coupling point 100, particularly preferably together with the waveguide-based optical coupling element 10 by a free-form microstructuring method, and thus, as an advantage, very precise alignment of the optical elements with respect to each other and with respect to the waveguide-based optical coupling element 10 becomes easy.

[0075] FIG. 4 exemplarily shows a device in which an S-shaped waveguide segment is inserted as a connection waveguide 160 between a further optical coupling point 410 of an optical component part 400 configured as an optical fiber, particularly a single-mode fiber, and the first optical coupling point 100 of the waveguide-based optical coupling element 10, thereby enabling translation and / or rotation of the waveguide-based optical coupling element 10 relative to the securely assembled optical component part 400 to be achieved. Thereby, even when the relative positions of the optical component parts 400, 430 are subject to inevitable variations resulting from manufacturing tolerances, the position of the waveguide-based optical coupling element 10 can be adapted to the position and / or orientation of a second further optical coupling point 470 assigned to a further securely assembled optical component part 430. In addition to the embodiment illustrated in FIG. 4, a waveguide segment (not shown) that can be used to further increase the degree of freedom associated with the positioning of the waveguide-based optical coupling element 10 can also be inserted between the second further optical coupling point 470 of the optical component part 430 and the associated optical coupling point 370 of the waveguide-based optical coupling element 10.

[0076] Alternatively, or in addition, the additional connecting waveguides 160, 170 can serve to adapt the mode field present at a further optical coupling point 410 of the optical component part 400 to the assigned first optical coupling point 100 of the waveguide-based optical coupling element 10. As exemplarily illustrated in FIG. 5, the additional connecting waveguide 170 can be designed as a taper. In this case, the term "taper" refers to a waveguide segment that tapers in one direction. In this case, the taper can be designed such that there is an adiabatic adaptation of the spatial mode distribution with as little loss as possible, i.e., such that most of the power of the first mode distribution is converted into the second mode distribution and neither emitted nor absorbed. Here, the additional connecting waveguide 170 used for mode field adaptation can have at least a partially multimode configuration, even if both the optical component part 400 connected thereto and the waveguide-based optical coupling element 10 have single-mode optical coupling points. This embodiment can occur in the case of a tapered connecting waveguide 170 connected to an optical fiber, the connecting waveguide having a refractive index contrast greater than that of the optical fiber due to the selected cladding material, but its initial diameter being adapted to the refractive index of the optical fiber. What can be achieved in this embodiment by appropriate design is that efficient coupling remains possible, in particular by avoiding the excitation of higher-order modes in the multimode section. Departing from the apparatus schematically shown in FIGS. 4 and 5, the waveguides included in the connecting waveguides 160, 170 and / or the optical component part 400 and assigned to the optical coupling point 410 can have a non-rotationally symmetric cross-section with non-degenerate waveguide modes of different polarizations, for example a rectangular or elliptical cross-section. In this case, the connecting waveguides 160, 170 can have a twist along the propagation direction, thereby enabling the continuous change of the polarization direction of the non-degenerate eigenmodes.

[0077] When the waveguide-based optical coupling element 10 comprises at least two partial waveguides 330, 340 with cross-sections characterized by a high aspect ratio, preferably to achieve a large difference in effective refractive index and thus a strong separation of two differently polarized fundamental modes, a multimode waveguide can occur. In this process, for the polarization belonging to the strongly guided fundamental mode, a case may occur where higher-order modes can propagate in addition to the fundamental mode. In this case, the excitation of these higher-order modes can be completely avoided by appropriate shaping of the partial waveguides 330, 340. As schematically depicted in FIG. 6, this can be achieved in particular by converting the multimode partial waveguides 330, 340 with very elongated cross-sections 350, 360 back to single-mode cross-sections 355, 365 by means of appropriate tapers 331, 341 after the branching of the partial waveguides 330, 340 and the associated spatial separation of the strongly guided fundamental mode.

[0078] Alternatively, it is possible to intentionally accept the excitation of higher-order modes and provide appropriately formed tapers 331, 341, which promote the induction of an interference mode field back to a well-defined, highly localized output field adapted to the fundamental mode of the subsequent waveguide. For the purpose of configuring the tapers 331, 341 in the exemplary embodiment according to FIG. 7, respective lateral offsets 372, 382 of the geometric centers 371, 381 of the taper end faces 355, 365 with respect to the geometric centers 351, 361 of the respective input faces 350, 360 are set, which can preferably be numerically optimized.

[0079] The tapers 331, 341 illustrated in FIGS. 6 and 7 should be understood as being exemplary and can be modified in many ways. For example, preferably, by an appropriate curvature of the waveguide track or by a fin-shaped auxiliary structure applied to the waveguide core, it is possible to significantly extinguish the weakly guided mode and thus increase the extinction ratio. Further, the sub-waveguides 330, 340 can merge into the taper portions 331, 341 without a distinct geometric boundary or can be fully combined with a distinct geometric boundary. Further, it is possible to design the corresponding structure by a numerical parameter optimization method or a topology optimization method and / or to improve the above structure proceeding from the geometric shapes according to FIGS. 6 and 7.

[0080] Various options for optically coupling the waveguide-based optical coupling element 10 to an optical component are exemplarily illustrated in FIGS. 8, 9, and 10. Thus, for example, a device can be considered in which the first optical coupling point 100 of the waveguide-based optical coupling element 10 is not in physical direct contact with a further optical coupling point 410 of the optical component portion 400 and there is also no connection by additional connecting waveguides 160, 170. Instead, light can propagate between the optical coupling points 100, 410 through at least a locally homogeneous medium. This may desirably involve providing beam shaping elements 111, 413 at the first optical coupling point 100 of the waveguide-based optical coupling element 10 and / or at a further optical coupling point 410 of the optical component portion 400 in order to facilitate efficient optical coupling, as depicted, for example, in FIG. 9. The beam shaping elements 111, 413 can preferably be manufactured together with the waveguide-based optical coupling element 10 by a free-form microstructuring method, which particularly facilitates a very accurate alignment in relation to the associated optical coupling points 100, 410.

[0081] FIG. 8 shows an embodiment in which optical coupling between two further 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 effected by additional connection waveguides 830, 840 in combination with taper structures 850, 860 on the side of the connection waveguides 830, 840 and in combination with taper structures 72a, 72b on the side of the integrated optical waveguides 71a, 71b. The connection waveguides 830, 840 and associated taper structures 850, 860 can be fabricated in situ, i.e., at the target location, in the same process as the waveguide-based optical coupling element 10 by a free-form microstructuring method, and in this process can be accurately aligned with the waveguide structures present on the optical chip 430.

[0082] FIGS. 9 and 10 show embodiments in which two further optical coupling points 370, 380 of the waveguide-based optical coupling element 10 are not in physical direct contact with the associated optical coupling points 74a, 74b of the integrated optical chip 430. Instead, the light, in this case, propagates between the optical coupling points through at least a locally homogeneous region that can be material-free (vacuum) or can be a specific gas, liquid, or solid (in particular, can be filled with an optically transparent capping material for protecting and stabilizing the entire device). To improve the coupling efficiency, further beam shaping elements 333, 343 are attached to the end faces 350, 360, and the beam shaping elements can preferably be fabricated together with the waveguide-based optical coupling element 10 by a free-form microstructuring method and can thus be very accurately aligned with the respective optical coupling points.

[0083] FIG. 9 shows an embodiment in which the beam shaping elements 833, 843 are also attached to the facets 73a, 73b of the integrated optical waveguides 71a and 71b on the side of the optical chip 430, and the beam shaping elements, together with the beam shaping elements 333, 343 on the side of the waveguide-based optical coupling element 10, increase the alignment tolerance when assembling the device. Further, additional beam shaping elements 413, 111 can be applied to the two optical coupling points 100, 410 for the purpose of improving the coupling efficiency between the additional optical coupling point 410 of the optical component part 400 and the first optical coupling point 100 of the waveguide-based optical coupling element 10. As an alternative, as shown in FIG. 10, it is possible to attach the beam shaping element only to one side of each optical coupling point.

[0084] As described above, the freeform microstructuring method makes it possible to manufacture structures having a symmetric or substantially symmetric geometric shape, which may have very similar losses, particularly in two separated modes. As schematically illustrated in FIG. 11, the substantially symmetric structural shape associated with this shows a three-dimensional shape including a symmetry plane 11, where the possible adaptation of the structural shape may slightly disrupt the perfect symmetry, which may be required, in particular, to couple the waveguide-based optical coupling element 10 to the position and orientation of the optical coupling points of the linked optical coupling elements.

[0085] The above embodiments are to be understood as examples and in no way reflect the entire field of use of the device according to the invention. Thus, the waveguide-based optical coupling element 10 can in particular be combined with additional functional optical elements or devices of such elements, which can be manufactured in situ, i.e. at their respective target positions, together with the respective waveguide-based optical coupling element 10 by using a microstructuring method, preferably a free-form microstructuring method, and can therefore be connected and / or aligned to the respective waveguide-based optical coupling element 10 in a very precise manner. The functional optical elements can preferably further comprise connecting waveguides, tapers, optical power splitters, or optical free-form elements, in particular mirrors, lenses, or other refractive or diffractive component parts, and can be complemented in the process by further auxiliary structures, in particular by further mechanical support structures. As a result, it is possible to generate more complex functional optical devices, for example for polarization analysis, on the facets of optical fibers or integrated optical waveguides, or on an extended array of photodetectors such as a camera chip. Such exemplary embodiments are seen in FIGS. 12 to 23.

[0086] FIG. 12 shows an apparatus corresponding to a waveguide-based analogue of a polarizing beam splitter cube having four optical coupling points and manufacturable at the end face of a multi-core fiber 720 by a free-form microstructuring method. In contrast to a single optical coupling element, the apparatus shown in FIG. 12 enables, in principle, the separation and / or fusion of various polarizations in both directions without power loss. In this case, the multi-core fiber 720 serves the role of at least one optical component part 400 and schematically comprises four optical coupling points. The exemplary apparatus comprises four waveguide-based optical coupling elements 10a, 10b, 10c, 10d arranged at the end face of a 4-core fiber 720, which optical coupling elements are interconnected by additional connecting waveguides 310a, 310b, 310c, 310d. In this case, each of the fiber cores can have a circular cross-section. Alternatively, they can also be configured as polarization-maintaining fiber cores having a double rotationally symmetric cross-section, which is schematically shown in FIG. 13 based on four individual polarization-maintaining fibers. The waveguide-based optical coupling element 10a distributes two LP01 modes of a fiber core 730a characterized by various polarization directions between waveguides 310a, 310d, from which the modes are coupled to the respective LP01 modes of fiber cores 730b, 730c. Similarly, the waveguide-based optical coupling element 10d distributes two LP01 modes of a fiber core 730d characterized by various polarization directions between waveguides 310b, 310c, from which the modes are coupled to the still free LP01 modes of fiber cores 730b, 730c respectively. In the present embodiment, the waveguide-based optical coupling elements 10a, 10d function together as a polarizing beam splitter, while the waveguide-based optical coupling elements 10b, 10c are used together as a polarizing beam combiner. These roles can be reversed by reversing the optical path, and the entire apparatus can be used as a fiber-coupled polarizing beam splitter. In this case, additional taper structures 170a, 170b, 170c, and 170d are used for efficient coupling to the respective fiber cores.

[0087] FIG. 13 shows an apparatus 1050 similar to FIG. 12 based on four polarization-maintaining individual fibers 740a, 740b, 740c, 740d with associated fiber cores 750a, 750b, 750c, 750d. At this point, the individual fibers 740a, 740b, 740c, 740d serve the role of at least one optical component part 400. In the apparatus 1050 depicted, and as is conventional for many polarization-maintaining fibers, the cross-section of the associated fibers has a double rotational symmetry as a result of additional stress-generating elements 760. By a 90° rotation about their longitudinal axes of the individual polarization-maintaining fibers 740a and 740d, it is possible to enhance the functionality of the apparatus in terms of the mode of polarization rotation in a way corresponding to the use of additional half-wave plates in conventional free-beam optics.

[0088] The connecting waveguides 310a, 310b, 310c, 310d running between the waveguide-based optical coupling elements 10a, 10b, 10c, 10d are schematically shown as free-form waveguides having a rectangular cross-section in FIGS. 12 and 13. As shown by the apparatus 1060 depicted in FIG. 14, the optical transmission between the waveguide-based optical coupling elements 10a, 10b, 10c, 10d can alternatively be achieved by using so-called whispering gallery modes. This embodiment takes advantage of the fact that light can be guided along a suitably dimensioned convex outer contour of an optically high refractive index region. Thereby, for example, the optical connection between the waveguide-based optical coupling elements 10a, 10b, 10c, 10d can be supported by a wide range of structural elements 311a, 311b, 311c, 311d having higher mechanical stability. The structural elements 311a, 311b, 311c, 311d can be additionally fixed to another mechanical support structure in a region that is not affected by the guided light without adversely affecting the optical transmission characteristics.

[0089] FIG. 15 shows a passive optical waveguide structure 1100 belonging to an optical polarization multiplexed heterodyne receiver and manufacturable on the end face of a 7-core fiber 770 by using a free-form microstructuring method, the 7-core fiber serving in this case as at least one optical component part 400. The device 1100 comprises two waveguide-based optical couplers 10a, 10b for polarization separation, two multimode interference couplers 20a, 20b for the coherent superposition of the signals contained in the individual polarizations, and further connecting waveguides 320a, 320b, 320c, 320d and taper sections 170a, 170b, 170c, 170d, 170e, 170f. For use as a coherent optical polarization multiplexed receiver, the fiber cores 770a, 770b, 770c, 770d, 770e, 770f can be connected to external components as follows: Connect fiber cores 770a, 770b to a first balanced optical detector; Connect fiber cores 770d, 770e to a second balanced optical detector; Fiber core 770g is not connected; Connect fiber core 770c to a data signal source and fiber core 770f to a local oscillator, or vice versa.

[0090] The data signal is decomposed into two signal components corresponding to two degenerate ortho-polarized LP01 eigenmodes of the associated fiber core by connecting the waveguide-based optical coupler 10b to the data signal input. Similarly, the signal of the local oscillator is decomposed into two signal components corresponding to two degenerate ortho-polarized LP01 eigenmodes of the associated fiber core by connecting the waveguide-based optical coupler 10a to the input of the local oscillator. The polarization directions of these data signal components and the signal components of the local oscillator are aligned with each other by the twists of the waveguides 320a, 320b; 320c, 320d adjacent to the optical coupler and superimposed by the multimode interference couplers 20a, 20b. The superimposed signal is supplied to the balanced photodetector. The polarization multiplexed data signal is reconstructed by processing the electrical signal that can be generated by a photodetector functioning as a mixer, generally by digital processing. Potential defects in the structure of the shown distributor can be compensated within the scope of this processing.

[0091] The apparatus 1100 illustrated in FIG. 15 should be understood to be exemplary and can be modified in many ways. Thus, a polarization multiplexed homodyne receiver can be provided by adding additional fiber cores and by appropriately enhanced signal superposition, preferably by using 2 to 4 multimode interference couplers. Further, the polarization beam splitter connected to the local oscillator can be replaced with a simple power splitter. The structure can be fabricated not only on the end surface of the multicore fiber, but alternatively, a one-dimensional fiber array, a two-dimensional fiber array, or other waveguides, such as integrated optical waveguide devices, can also be used. Similar to the embodiment shown in FIG. 13, polarization-maintaining fibers can also be used in this case.

[0092] FIG. 16 shows a polarization analyzer structure 1200 that can be fabricated on the end face of a single-mode fiber by using a free-form microstructuring method, where the single-mode fiber serves as at least one optical component portion 400 in this case. Light coupled into the structure via an additional optical coupling point 410 from the fiber is initially evenly distributed among four waveguides 810a, 810b, 810c, 810d. In this case, two waveguide-based optical coupling elements 10a, 10b, which are rotated 45° with respect to each other about their major axes, are connected to waveguides 810a, 810b, thereby enabling the signal components to be separated in associated polarization directions that are inclined 45° with respect to each other. In this case, the major axis of the optical coupling element is defined by the polarization direction of the linearly polarized light separated by the optical coupling element. An optical birefringent waveguide 30 is connected to waveguide 810c, and the length of the optical birefringent waveguide 30 is selected such that waveguide 810c behaves like a quarter-wave plate with an alignment determined by the orientation of the rectangular cross-section. The birefringence of the optical birefringent waveguide 30 arises due to its material properties or its core cross-sectional shape, which is, for example, rectangular and non-square, or elliptical and non-circular. A further waveguide-based optical coupling element 10c is connected to the output of the optical birefringent waveguide 30, and its major axis is twisted 45° with respect to the major axis of the birefringent waveguide 30 that functions as a quarter-wave plate. The major axis of the optical birefringent waveguide 30 is defined by the polarization direction of the associated polarization eigenstate of the birefringent waveguide 30 and, in the case of a rectangular or elliptical cross-section of the birefringent waveguide 30, results from a direction defined by the sides of the rectangular cross-section or the major axis of the elliptical cross-section. Waveguide 810d remains open at one end and only functions for power measurement.

[0093] When light is coupled from the fiber into the structure in any desired superposition of two orthogonal polarization modes LP01 corresponding to a specific polarization state, a power distribution characteristic of outputs 811d, 370a, 380a, 370b, 380b, 370c, 380c is thereby obtained, and it is possible to measure the power distribution by using appropriately positioned photodetectors. Using this power distribution, it is possible to uniquely reconstruct the polarization state of the light coupled to the structure, for which reference is made, for example, to 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. Thus, with the help of structure 1200 combined with appropriate data processing and preferably calibration, a relatively cost-effective manufacturable polarization analyzer can be provided. In this case, a photodiode array, preferably an image sensor, applied without complex adjustment of the radiation direction of the output shown in the overview can be used for power detection. Alternatively, the various outputs 811d, 370a, 380a, 370b, 380b, 370c, 380c can be connected to additional optical components, such as photodetectors or optical fibers, by using additional waveguides and / or micro-optical elements.

[0094] As schematically shown in FIG. 16, by using three optical coupling elements, a quarter-wave plate, and output 811d used only for power measurement, the polarization state at the output of the structure has already been overly determined. The redundant information can be used to confirm the reliability of the measurement. Alternatively, the structure can be further simplified, for example, by omitting output 811d used only for power measurement.

[0095] FIG. 17 shows a reflective polarization swapper that exchanges signal components present in two orthogonally polarized fundamental modes (so-called LP01 modes) of an optical fiber, which in this case serves the role of the optical component part 400, in the same way as a Faraday rotor mirror, and combines the signal components and returns them to the fiber in the opposite polarization direction. Of the three optical coupling points 100, 370, 380 of the waveguide-based optical coupler 10, the first optical coupling point 100 is connected to the optical fiber, while the two additional optical coupling points 370, 380 are interconnected by a twisted waveguide 390.

[0096] FIGS. 18 and 19 show the application of the device according to the invention in the form of polarization-sensitive image sensors 1400, 1450. In this case, the individual structure schematically shown in FIG. 18 comprises a free-form lens 111, whereby the light 112 incident from the outside can first be coupled to the waveguide-based optical coupler 10. Following the separation of the two polarizations, the corresponding signal components are transferred to two photodetectors 50a, 50b connected to two additional optical coupling points 370, 380 of the waveguide-based optical coupler 10, where they are converted into electrical signals guided via the electric wires 51. The photodetectors 50a, 50b can preferably be individual detectors of the image sensor, so that a polarization-sensitive image sensor can be constructed by continuously repeating the structure schematically shown in FIG. 18 periodically, and the polarization-sensitive image sensor can be used to detect the spatial distribution of the incident power separated according to the two polarizations.

[0097] FIG. 19 shows an array-like repetition of the structure 1400, by means of which a polarization-sensitive image sensor 1450 or a polarization-sensitive camera with a large number of pixels can be manufactured.

[0098] FIG. 20 shows a cascade 1500 of a plurality of waveguide-based optical couplers 10, 10a, 10b that can be manufactured together by using a free-form microstructuring method. By using such a device, it is possible to improve the polarization extinction ratio, particularly at the optical coupling points 370a, 380a, 370b, 380b.

[0099] FIG. 21 shows an apparatus 1600 in which a waveguide-based optical coupling element 10 is connected to a single-mode fiber, at this point serving the role of the optical component part 400, and whose output is mechanically stabilized by an additional structure 60. An optical signal coupled to a taper 170 leading from a single-mode fiber to the waveguide-based optical coupling element 10 at a further optical coupling point 410 is first split into two signal components corresponding to the degenerate orthogonal polarization LP01 eigenmodes of the associated fiber core. As a result of the twisted waveguide sections 332, 342, the separated signal components are then aligned with respect to their polarization directions and output and combined at optical coupling points 825, 826 having a mode profile adjustable by tapers 815, 816. The positions of the further optical coupling points 825, 826 are fixed to a slab-shaped sub-structure 62 of the mechanical support structure 60 carried by the struts 61a, 61b, 61c, 61d. The distances and positions of the further optical coupling points 825, 826 can be freely selected.

[0100] FIG. 22 shows an apparatus 1650 representing an extension of the apparatus 1600 from FIG. 21 for forming an array 80 of input fibers 400a, 400b, 400c, 400d. By selecting the distances of the associated outputs 825a, 826a; 825b, 826b; 825c, 826c; 825d, 826d, the apparatus can also be connected on the output side to a fiber array or a waveguide array located on a chip. In this embodiment, the apparatus 1650 corresponds to a multi-channel polarization beam splitter.

[0101] In an exemplary manner, FIG. 23 shows an apparatus 1700 in which a waveguide-based optical coupler 10 is used as a polarization filter. In the schematically shown apparatus 1700, an optical signal to be filtered with respect to polarization is emitted by an optical component part 400, and at its optical coupling point 410, the waveguide-based optical coupler 10 is connected to its first optical coupling point 100. The desired signal filtered in relation to polarization is then available in one of the guided eigenmodes of the second optical coupling point 370, while the signal components suppressed by the polarization filtering are supplied to a termination element (beam dump) 395 connected to the third optical coupling point 380. In the case schematically shown in FIG. 23, the beam dump is embodied in the form of a continuously tapering taper structure through which light is emitted in a lateral direction, for example, in the direction of an absorption surface. Many aspects of the apparatus schematically shown in FIG. 23 can be modified. For example, the roles of the second optical coupling point 370 and the third optical coupling point 380 can be interchanged without changing the functional principle at all. A configuration is also conceivable in which, at the first optical coupling point 100, light is coupled from free space without physical contact with the optical component part present there, and the third optical coupling point 370 is coupled to the optical component part. The coupling to the optical component can be carried out directly or, as described above, via a corresponding connecting waveguide or a free-beam coupling stretch.

Description of Reference Numerals

[0102] 10, 10a - 10d (waveguide-based) optical coupler 11 Plane of symmetry 20a, 20b Multimode interference coupler 30 Birefringent waveguide behaving like a quarter-wave plate 40 Coordinate system of the lithography system 50a, 50b Photodetector 51 Electric wire 60 Mechanical support structure 61a - 61d Struts of the mechanical support structure 62 Sub-structure of the mechanical support structure carried by the struts 71a, 71b Integrated optical waveguide Taper structures 72a, 72b, 170, 170a - 170f, 331, 341, 815, 816, 850, 860 Facets 73a, 73b of the integrated optical waveguide Optical coupling points 74a, 74b of the integrated optical waveguide Fiber array 80 Optical coupling points of the optical coupling elements 100, 370, 370a - 370c, 380, 380a - 380c Waveguide cross - section (facet) at the optical coupling points of the optical coupling elements 110, 350, 360 Beam - shaping elements 111, 333, 343, 413, 833, 843 Incident light 112 Orthogonal eigen - modes at the first optical coupling points 120, 130; 120a, 130a Intensity distribution of the electric - field vector 140, 150 Connection waveguides 160, 310a - 310d, 320a - 320d, 830, 840 Waveguide sections of the optical coupling elements 200, 300 Waveguide cross - section consisting of the superposition of waveguide cross - sections 210 Waveguide cross - sections 230, 240 Orthogonal eigen - modes at the second and third optical coupling points 250, 260; 270, 280 Cross - section of the superposition of partial waveguides in the waveguide region 301 Cross - sections of partial waveguides in the waveguide region 302a, 302b Extensive structural elements for the induction of whispering - gallery modes 311a - 311d Partial waveguides or (twisted) waveguide segments of the optical coupling elements 330, 340, 332, 342, 390 Geometric centers 351, 361 of the waveguide cross - sections 350, 360 Single - mode cross - sections of the taper structures 331, 341 355, 365 Geometric centers 371, 381 of the cross - sections 355, 365 Lateral offset of the geometric centers 371, 381 with respect to the geometric centers 351, 361 372, 382 Terminal element (beam dump) 395 Optical component parts 400, 430, 440 Optical fibers 400a - 400d Optical coupling points of optical component parts 410, 470, 480 Adjustment marks 411, 412, 471, 472, 481, 482 Capping material or cladding material 500 Waveguide region of the optical coupling element where partial waveguides are very close to each other or spatially overlap Waveguide region of the optical coupling element where partial waveguides are spatially separated Multicore fibers 720, 770 Cores of multicore fibers 730a - 730d, 770a - 770g Polarization - maintaining fibers 740a - 740d Fiber cores 750a - 750d Stress - generating element 760 Connection waveguides 810a - 810d Additional optical coupling points 811d, 825a - 825d, 826a - 826d Additional functional optical elements or devices of such elements, manufactured in - situ with waveguide - based optical coupling elements 1000, 1050, 1060, 1100, 1200, 1300, 1400, 1450, 1500, 1600, 1650, 1700

Claims

1. An apparatus for optical coupling and for mode selection separation or superposition of optical fields, the apparatus comprising: a waveguide-based optical coupler element (10) having at least three optical coupling points (100, 370, 380); at least one optical component portion (400) having at least one additional optical coupling point (410); the waveguide-based optical coupler element (10) includes 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 coupler element (10) being designed in the form of a three-dimensional freeform structure, the freeform structure having a structure approximated by a layer structure of at least six layers; at least one first optical coupling point (100) having at least two different guided eigenmodes (120, 130) assigned to the first optical coupling point (100); at least one second optical coupling point (370) having at least one guided eigenmode (260) assigned to the second optical coupling point (370); and at least one third optical coupling point (380) having at least one guided eigenmode (280) assigned to the third optical coupling point (380); at least one of the optical coupling points (100, 370, 380) of the waveguide-based optical coupler element (10) is optically connected to the at least one additional optical coupling point (410) of the optical component portion (400); the waveguide-based optical coupler element (10) is configured to transmit light bidirectionally with high efficiency between at least one first guided eigenmode (120) assigned to the first optical coupling point (100) and at least one guided eigenmode (260) assigned to the second optical coupling point (370); and configured to transmit light bidirectionally with high efficiency between at least one second guided eigenmode (130) assigned to the first optical coupling point (100) and at least one guided eigenmode (280) assigned to the third optical coupling point (380).

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

3. The second optical coupling point (370) and the third optical coupling point (380) are spatially separated from each other, The waveguide-based optical coupling element (10) is configured to separate an optical input field existing at the first optical coupling point (100) into partial fields of different polarizations, and output and combine optical signals constituted by the partial fields at the second optical coupling point (370) and the third optical coupling point (380), or The waveguide-based optical coupling element (10) is designed to superimpose optical signals in the form of spatially overlapping partial fields of different polarizations input and combined at the second optical coupling point (370) and the third optical coupling point (380), and provide the superimposition at the first optical coupling point (100). The apparatus according to claim 1 or 2.

4. The waveguide-based optical coupling element (10) separates at least two guided eigenmodes (120, 130) assigned to the first optical coupling point (100) into different polarizations, and then, with respect to the at least one guided eigenmode (260) assigned to the second optical coupling point (370), and with respect to the guided eigenmode (280) assigned to the at least one third optical coupling point (380), is configured to rotate the polarization of the separated eigenmodes (120, 130) in one direction to facilitate aligning the directions of polarization. The apparatus according to any one of claims 1 to 3.

5. The waveguide-based optical coupling element (10) includes at least two partial waveguides, the partial waveguides being arranged adjacent to or spatially intersecting each other in a first region (600), the at least two partial waveguides having at least partially strongly separated eigenmodes when viewed separately from each other, and the partial waveguides being spatially separated in a second region (610). The apparatus according to any one of claims 1 to 4.

6. The waveguide-based optical coupling element (10) includes at least one first waveguide region (200) adjacent to the first optical coupling point (100), and at least one second waveguide region (300) adjacent to the first waveguide region (200), and In the first waveguide region (200), a first waveguide cross-section (110) is continuously converted into a second waveguide cross-section (210) that includes an overlap of two waveguide cross-sections (230, 240) each having strongly separated eigenmodes (250, 260, 270, 280). The at least two guided eigenmodes (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 the at least two guided eigenmodes (120, 130) assigned to the first optical coupling point (100) from the first waveguide cross-section (110) are converted into strongly guided eigenmodes (260, 280) of the two waveguide cross-sections (230, 240). The apparatus according to any one of claims 1 to 5, wherein in the second waveguide region (300), initially overlapping waveguide cross-sections (230, 240) are guided away from each other into separated cross-sections (330, 340) that define the second optical coupling point (370) and the third optical coupling point (380).

7. The apparatus according to claim 6, wherein the conversion of the first waveguide cross-section (110) to the second waveguide cross-section (210) in the first waveguide region (200) is performed by a continuous deformation of the cross-section along the optical propagation direction.

8. The apparatus according to claim 6 or 7, wherein each of the waveguide cross-sections (230, 240) forms a single continuous region having an aspect ratio of at least 1.

5.

9. The first optical coupling point (100) is optically coupled, directly or indirectly, via a further waveguide segment or beam shaping structural element, to a waveguide having a low refractive index contrast and degenerate or substantially degenerate orthogonal eigenmodes, or to an optical fiber (400), and / or The second optical coupling point (370) or the third optical coupling point (380) is optically coupled, directly or indirectly, via a further waveguide segment or beam shaping structural element, to a waveguide having a high refractive index contrast and strongly separated orthogonal eigenmodes, or to a semiconductor-based integrated optical waveguide (71a or 71b). The apparatus according to any one of claims 1 to 8.

10. The first optical coupling point (100) is either in physical direct contact with a further optical coupling point (410) of the optical component part (400), or not in physical direct contact with the further optical coupling point (410) of the optical component part (400), and the light is propagated between the first optical coupling point (100) and the further optical coupling point (410) through at least a locally homogeneous medium, the apparatus according to any one of claims 1 to 9. **Claim 11** The apparatus according to any one of claims 1 to 10, wherein the first optical coupling point (100) is configured to receive light from free space, or the first optical coupling point (100) comprises a lens configured to couple light from the free space. **Claim 12** The apparatus according to any one of claims 1 to 11, wherein the core region has a refractive index of 1.3 to 1.

8. **Claim 13** A polarization filter comprising the apparatus according to any one of claims 1 to 12 and a termination 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 multiplex receiver in a combination of at least two waveguide-based coupling elements (10a, 10b, 10c, 10d) having at least one connecting waveguide (320a - 320d). A passive optical waveguide structure for a polarization analyzer in a combination of at least two waveguide-based coupling elements (10a, 10b, 10c, 10d) having at least one power splitter. Use of the apparatus according to any one of claims 1 to 12 as a passive optical waveguide structure for a polarization-sensitive image sensor in a combination of at least two waveguide-based coupling elements (10a, 10b, 10c, 10d) having at least one free-form micro-optical element. **Claim 14** A method for manufacturing a waveguide-based optical coupling element (10) configured for mode selection separation or superposition of an optical field at at least one further optical coupling point (410) of at least one optical component part (400). a) providing the at least one optical component part (400) and positioning the at least one further optical coupling point (410) of the at least one optical component part (400) in a coordinate system (40) of a free-form microstructuring unit configured to perform a free-form microstructuring method; b) generating a data set describing the three-dimensional shape of the waveguide-based optical coupling element (10) in the coordinate system (40) of the free-form microstructuring unit, wherein the waveguide-based optical coupling element (10) has at least three optical coupling points (100, 370, 380), wherein at least one first optical coupling point (100) has at least two different guided eigenmodes (120, 130) assigned to the first optical coupling point (100), wherein at least one second optical coupling point (370) has at least one guided eigenmode (260) assigned to the second optical coupling point (370), and wherein at least one third optical coupling point (380) has at least one guided eigenmode (280) assigned to the third optical coupling point (380), wherein the waveguide-based optical coupling element (10) is configured to transmit light bidirectionally with high efficiency between at least one first guided eigenmode (120) assigned to the first optical coupling point (100) and at least one guided eigenmode (260) assigned to the second optical coupling point (370), and between at least one second guided eigenmode (130) assigned to the first optical coupling point (100) and at least one guided eigenmode (280) assigned to the third optical coupling point (380); c) manufacturing the waveguide-based optical coupling element (10) at the at least one further optical coupling point (410) of the at least one optical component part (400) by using the free-form microstructuring method; d) embedding the waveguide-based optical coupling element (10) at least locally as a core region in a cladding region adjacent to the waveguide-based optical coupling element (10), wherein a refractive index difference of at least 0.05 occurs between the core region and the cladding region.

15. 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 part (400) or a further optical component part (430, 440), and the position of the at least one optical component part (400) or the position of the further optical component part (430, 440) is registered when generating the data set and taken into account in step b), the method according to claim 14.

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

8.

17. The method according to any one of claims 14 to 16, wherein the free-form microstructuring method is used to additionally manufacture at least one further optical element selected from connection waveguides (30, 160, 170, 310a - 310d, 320a - 320d, 830, 840), taper structures (72a, 72b, 170, 170a - 170f, 331, 341, 815, 816, 850, 860), optical couplers (20a, 20b), beam shaping structural elements (111, 411) and mechanical support structures (60).

18. The method according to any one of claims 14 to 17, wherein the free-form microstructuring method is a lithography method selected from stereolithography, in particular by using a spatial light modulator, and direct writing laser lithography, in particular multi-photon polymerization.

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