Monolithic mode field adapter and coupler

Abstract

An optical coupler may include one or more gradient refractive index (GRIN) elements having spatially-varying refractive index profiles orthogonal to a propagation direction and one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to a propagation direction. The one or more GRIN elements and the one or more solid propagation elements are joined along the propagation direction to form a monolithic element. The one or more GRIN elements and the one or more solid propagation elements may be configured to couple light between outer faces of the monolithic element distributed along the propagation direction.

Description

MONOLITHIC MODE FIELD ADAPTER AND COUPLERCROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of U. S. Non-Provisional Patent Application 19 / 314,660 filed on August 29, 2025, and claims the benefit of U. S. Provisional Patent Application 63 / 713,325 filed on October 29, 2025; U. S. NonProvisional Patent Application 19 / 314,660 filed on August 29, 2025 also claims the benefit of U. S. Provisional Patent Application 63 / 713,325 filed on October 29, 2025; U. S. Non-Provisional Patent Application 19 / 314,660 and U. S. Provisional Patent Application 63 / 713,325 are both incorporated herein by reference in their entireties.GOVERNMENT LICENSE RIGHTS

[0002] This invention was made with government support under Grant Number W911NF-24- 1-0008 awarded by the Army Research Office (ARO). The government has certain rights in the invention.TECHNICAL FIELD

[0003] The present disclosure relates generally to an optical coupler and, more particularly, to a monolithic all-fiber coupler.BACKGROUND

[0004] It is often desirable to couple optical fibers with different optical mode profiles due to differences in size and / or design. However, existing solutions suffer from precise alignment requirements, a lack of mechanical stability, requirements to modify fibers being coupled (e.g., to provide thermally expanded cores, fiber tapers), and / or bulk optical elements. Further, many existing solutions require a relatively high degree of skill to implement.

[0005] For example, mechanical spacers suffer from mechanical vibrations, which may degrade coupling performance. As another example, modifying the fibers to becoupled requires specialty equipment and expertise. As another example, multimode interference in a gradient index core fiber requires extremely precise lengths of fiber and further requires that the initial and final mode fields are supported by the graded index core fiber. Multimode interference also suffers from focusing light before expanding it under some conditions, which makes it nonviable in high power laser applications.

[0006] There is therefore a need to develop systems and methods to cure the above deficiencies.SUMMARY

[0007] In some embodiments, an optical coupler is provided. The optical coupler may include one or more gradient refractive index (GRIN) elements having spatially-varying refractive index profiles orthogonal to a propagation direction. The optical coupler may include one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to a propagation direction. The one or more GRIN elements and the one or more solid propagation elements may be joined along the propagation direction to form a monolithic element. The one or more GRIN elements and the one or more solid propagation elements may couple light between outer faces of the monolithic element distributed along the propagation direction.

[0008] In some embodiments, at least one of dioptric powers of the one or more GRIN elements, lengths of the one or more GRIN elements, or lengths of the one or more solid propagation elements may be selected to adapt a mode profile of light from a first external optical element coupled to a first of the outer faces to match a mode profile of a second external optical element coupled to a second of the outer faces.

[0009] In some embodiments, the outer faces may be couplable with a first external optical element and a second external optical element.

[0010] In some embodiments, at least one of the first external optical element or the second external optical element may be a hollow-core fiber.

[0011] In some embodiments, at least one of the first external optical element or the second external optical element may be a solid-core fiber.

[0012] In some embodiments, the first external optical element and the second external optical element may be solid-core fibers with at least one of different mode profiles or different mode field diameters.

[0013] In some embodiments, the one or more GRIN elements may be interleaved with the one or more solid propagation elements.

[0014] In some embodiments, the one or more solid propagation elements may include a first solid propagation element, a second solid propagation element, and a third solid propagation element. The one or more GRIN elements may include a first GRIN element between the first and second solid propagation elements along with a second GRIN element between the second and third solid propagation elements.

[0015] In some embodiments, the optical coupler may operate as an imaging system.

[0016] In some embodiments, the optical coupler may operate as a 4F imaging system.

[0017] In some embodiments, the optical coupler may further include one or more anti-reflection treatments on at least one of the outer faces or interfaces within the optical coupler.

[0018] In some embodiments, the one or more anti-reflection treatments may include at least one of an anti-reflection coating, a surface treatment, or an angle-cleaved surface.

[0019] In some embodiments, the optical coupler may further include an exterior connector configured to mechanically couple with a paired connector on a first external optical element. Mechanically coupling the exterior connector to the paired connector may provide optical coupling of light between the first optical element and a first of the outer faces.

[0020] In some embodiments, the exterior connector and the paired connector may be at least one of LC, FC, SC, or ST connectors.

[0021] In some embodiments, the optical coupler may further include an additional exterior connector configured to mechanically couple with an additional paired connector on a second external optical element. Mechanically coupling the additionalexterior connector to the additional paired connector may provide optical coupling of light between the second external optical element and a second of the outer faces.

[0022] In some embodiments, the additional exterior connector and additional the paired connector may be at least one of LC, FC, SC, or ST connectors.

[0023] In some embodiments, the refractive index profiles of the one or more GRIN elements may be parabolic.

[0024] In some embodiments, the one or more solid propagation elements may have uniform refractive index profiles.

[0025] In some embodiments, a method is provided. The method may include fabricating one or more GRIN elements having spatially-varying refractive index profiles orthogonal to a propagation direction. The method may include fabricating one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to the propagation direction. The method may include joining the one or more GRIN elements with the one or more solid propagation elements along the propagation direction to form a monolithic optical coupler. The one or more GRIN elements and the one or more solid propagation elements may be configured to couple light between outer faces of the monolithic optical coupler distributed along the propagation direction.

[0026] In some embodiments, the method may further include joining at least one of the one or more GRIN elements or the one or more solid propagation elements to an external optical element.

[0027] In some embodiments, joining at least one of the one or more GRIN elements or the one or more solid propagation elements to the external optical element may include splicing at least one of the one or more GRIN elements or the one or more solid propagation elements to the external optical element.

[0028] In some embodiments, the method may further include providing one or more anti-reflection treatments on at least one of the outer faces or interfaces within the optical coupler.

[0029] In some embodiments, the one or more anti-reflection treatments may include at least one of an anti-reflection coating, a surface treatment, or an angle-cleaved surface.

[0030] In some embodiments, the method may further include coupling an exterior connector to the monolithic optical coupler. The exterior connector may be configured to mechanically couple with a paired connector on a first external optical element. Mechanically coupling the exterior connector to the paired connector may provide optical coupling of light between the monolithic optical coupler and the first external optical element.

[0031] In some embodiments, the exterior connector may be further configured to mechanically couple with an additional paired connector on a second external optical element. Mechanically coupling to the additional paired connector may provide optical coupling of light between the monolithic optical coupler and the second external optical element.

[0032] In some embodiments, a fiber coupling method is provided. The method may include joining a first fiber to an optical coupler. The optical coupler may include one or more GRIN elements having spatially-varying refractive index profiles orthogonal to a propagation direction. The optical coupler may include one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to a propagation direction. The one or more GRIN elements and the one or more solid propagation elements may be joined along the propagation direction to form a monolithic element. The one or more GRIN elements and the one or more solid propagation elements may couple light between outer faces of the monolithic element distributed along the propagation direction.

[0033] In some embodiments, joining the first fiber to the optical coupler may include splicing the first fiber to the optical coupler.

[0034] In some embodiments, the fiber coupling method may further include joining a second fiber to the optical coupler.

[0035] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention.Brief description of drawings

[0036] The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures.

[0037] FIG. 1A illustrates a simplified schematic view of an optical coupler for coupling light between two external optical elements, in accordance with one or more embodiments of the present disclosure.

[0038] FIG. 1B illustrates an optical coupler providing coupling between a fiber and a photonic integrated circuit (PIC), in accordance with one or more embodiments of the present disclosure.

[0039] FIG. 1C illustrates an optical coupler having a different outer diameter than external optical elements, in accordance with one or more embodiments of the present disclosure.

[0040] FIG. 1D illustrates an optical coupler with anti-reflection treatments on both faces, in accordance with one or more embodiments of the present disclosure.

[0041] FIG. 1E illustrates an optical coupler with an anti-reflection treatment on one of the faces, in accordance with one or more embodiments of the present disclosure.

[0042] FIG. 1F illustrates an optical coupler with angle-cleaved surfaces on all components, in accordance with one or more embodiments of the present disclosure.

[0043] FIG. 2 is a plot of a refractive index profile of a gradient refractive index (GRIN) element, in accordance with one or more embodiments of the present disclosure.

[0044] FIG. 3 is a plot of a uniform refractive index profile of a solid propagation element, in accordance with one or more embodiments of the present disclosure.

[0045] FIG. 4A illustrates a simplified schematic of an optical coupler spliced directly to two external optical elements with a coating, in accordance with one or more embodiments of the present disclosure.

[0046] FIG. 4B illustrates a simplified schematic of an optical coupler with a glass splice protection package, in accordance with one or more embodiments of the present disclosure.

[0047] FIG. 4C illustrates a simplified schematic of an optical coupler with a metal rod and heat-shrink tubing protection package, in accordance with one or more embodiments of the present disclosure.

[0048] FIG. 5A illustrates a simplified schematic of an optical coupler within a female-to-female fiber connector, in accordance with one or more embodiments of the present disclosure.

[0049] FIG. 5B illustrates a cross-sectional view of an optical coupler within the female-to-female connector connected to two external optical elements, in accordance with one or more embodiments of the present disclosure.

[0050] FIG. 5C illustrates a simplified schematic of an optical coupler coupled to a male connector, in accordance with one or more embodiments of the present disclosure.

[0051] FIG. 5D illustrates a simplified schematic of an optical coupler providing a transition between a hollow-core fiber and a solid-core fiber, in accordance with one or more embodiments of the present disclosure.

[0052] FIG. 6A is a simulated plot of beam propagation through the optical coupler, in accordance with one or more embodiments of the present disclosure.

[0053] FIG. 6B is a plot illustrating coupling of light in a first solid propagation element, in accordance with one or more embodiments of the present disclosure.

[0054] FIG. 6C is a plot illustrating coupling of light in a first GRIN element, in accordance with one or more embodiments of the present disclosure.

[0055] FIG. 6D is a plot illustrating coupling of light in a second solid propagation element, in accordance with one or more embodiments of the present disclosure.

[0056] FIG. 6E is a plot illustrating coupling of light in a second GRIN element, in accordance with one or more embodiments of the present disclosure.

[0057] FIG. 6F is a plot illustrating coupling of light in a third solid propagation element, in accordance with one or more embodiments of the present disclosure.

[0058] FIG. 6G is a plot illustrating coupling of light in the optical coupler as a function of wavelength, in accordance with one or more embodiments of the present disclosure.

[0059] FIG. 7A illustrates a simulated plot of beam propagation through an optical coupler with alternating solid propagation and GRIN elements, in accordance with one or more embodiments of the present disclosure.

[0060] FIG. 7B illustrates a simulated plot of mode field diameter through the optical coupler of FIG. 7A, in accordance with one or more embodiments of the present disclosure.

[0061] FIG. 7C illustrates a schematic of light propagation through the optical coupler in FIG. 7A, in accordance with one or more embodiments of the present disclosure.

[0062] FIG. 8A illustrates a simulated plot of beam propagation through an optical coupler with GRIN elements having different refractive index variations, in accordance with one or more embodiments of the present disclosure.

[0063] FIG. 8B illustrates a simulated plot of mode field diameter through the optical coupler of FIG. 8A, in accordance with one or more embodiments of the present disclosure.

[0064] FIG. 8C illustrates a plot of the mode field diameter at an output of the optical coupler, in accordance with one or more embodiments of the present disclosure.

[0065] FIG. 9A illustrates a simulated plot of beam propagation through an optical coupler with separated GRIN elements, in accordance with one or more embodiments of the present disclosure.

[0066] FIG. 9B illustrates a simulated plot of mode field diameter through the optical coupler of FIG. 9A, in accordance with one or more embodiments of the present disclosure.

[0067] FIG. 9C illustrates a plot of the mode field diameter at an output of the optical coupler, in accordance with one or more embodiments of the present disclosure.

[0068] FIG. 10 is a flow diagram illustrating steps performed in a method for fabricating an optical coupler, in accordance with one or more embodiments of the present disclosure.

[0069] FIG. 11 is a flow diagram illustrating steps performed in a method for fabricating and protecting an optical coupler, in accordance with one or more embodiments of the present disclosure.DETAILED DESCRIPTION

[0070] Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to certain embodiments and specific features thereof. The embodiments set forth herein are taken to be illustrative rather than limiting. It should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the disclosure.

[0071] Embodiments of the present disclosure are directed to systems and methods providing efficient optical coupling between optical fibers having different mode profiles and / or numerical apertures (NAs). In embodiments, an optical coupler is a monolithic component including a combination of gradient refractive index (GRIN) elements and solid propagation elements. For example, an optical coupler may include one or more GRIN elements that operate as lenses (e.g., elements with dioptric power) and one or more solid propagation elements without dioptric power. In this way, the solid propagation elements may provide a fixed optical path and / or a fixed physical separation between various components within and / or coupled to the optical coupler. For example, light coupled into one of the solid propagation sections may freely expand and propagate to the interior section, which may then focus the light through the other outer solid propagation elements for efficient coupling into another fiber.

[0072] A GRIN element may include any solid-state component having a non-uniform refractive index profile in a plane orthogonal to a propagation direction of light (e.g., an optical axis) designed to provide dioptric power. For example, a GRIN element may include, but is not limited to, a GRIN rod or a GRIN fiber having a core size sufficiently large that the propagation of light may be characterized by free-space formulations rather than optical fiber mode formulations. A solid propagation element may include any solid material having a uniform refractive index profile in a region where light propagates.

[0073] An optical coupler as disclosed herein may include any number or combination of GRIN elements and solid propagation elements. In some embodiments, an optical coupler includes alternating (e.g., interleaved) GRIN elements and solid propagation elements. In this way, the GRIN elements and the solid propagation elements may together operate as a fixed optical system. For example, an optical coupler may include alternating solid propagation elements and GRIN elements arranged as a 4F imaging system. As an illustration, an optical coupler may include two GRIN elements separated by a solid propagation element providing an optical path of twice the focal lengths of the GRIN elements, along with additional solid propagation elements on opposing sides of the GRIN elements providing optical paths equal to the respective GRIN elements.

[0074] The systems and methods disclosed herein may be well suited for a wide range of applications. For example, the systems and methods disclosed herein may provide efficient coupling between any types of optical elements such as, but not limited to, fibers (e.g., hollow-core fibers (HCFs), solid-core fibers, or the like), photonic integrated circuits (PICs), or free-space optical elements. More generally, the systems and methods disclosed herein may be suitable for coupling light to and / or from any optical waveguide or physical structure that guides light, including one or more of a semiconductor laser diode, optical photodetector, an optical waveguide, a splitter, a modulator, a sensor, a silicon photonic waveguide, planar lightwave circuit (PLC), arrayed waveguide grating (AWG), or a grating coupler.

[0075] As an illustration, an optical coupler may provide efficient coupling between HCFs of the same or different design, between an HCF and a solid-core fiber, between solid-core fibers with different core sizes, or between any fiber types generally. As another illustration, an optical coupler may provide coupling to a PIC, where the PIC and the optical coupler are connected by any suitable technique including, but not limited to, epoxy.

[0076] The monolithic construction of an optical coupler as disclosed herein may advantageously provide robust operation with little to no alignment required and may thus not require specialized skills to implement. Further, the systems and methods disclosed herein may be suitable for high-power applications due to the highly-controlled light paths.

[0077] Referring now to FIGS. 1 A-11, systems and methods providing optical fiber coupling are described in greater detail, in accordance with one or more embodiments of the present disclosure.

[0078] FIG. 1A illustrates a simplified schematic view of an optical coupler 100 for coupling light between two external optical elements 102, in accordance with one or more embodiments of the present disclosure.

[0079] In some embodiments, the optical coupler 100 is a monolithic element formed from an alternating series of solid propagation elements 104 and GRIN elements 106 distributed along a propagation direction 108. The solid propagation elements 104 and GRIN elements 106 may be coupled to form a single monolithic element using any technique including, but not limited to splicing or epoxy. In this way, the optical coupler 100 may form a rigid monolithic structure with two opposing faces 110 suitable for coupling with external optical elements 102. In this way, there are no air gaps between the various sections of the optical coupler 100. Further, the optical coupler 100 may provide unidirectional or bidirectional coupling of light between the external optical elements 102.

[0080] Each of the GRIN elements 106 may include a radially-varying refractive index profile in a cross-sectional plane orthogonal to the propagation direction 108, which may produce a dioptric power. In this way, a GRIN element 106 may operate as a lens. A GRIN element 106 may have any spatially-varying refractive index profile such as, but not limited to, a parabolic refractive index profile orthogonal to a propagation direction 108 of light. FIG. 2 is a plot of a refractive index profile of a GRIN element 106, in accordance with one or more embodiments of the present disclosure. In particular, FIG. 2 depicts a parabolic refractive index profile along a direction orthogonal to a propagation direction 108, though this is merely an illustration and not limiting.

[0081] Each of the solid propagation elements 104 may include a uniform refractive index profile in a cross-sectional plane orthogonal to the propagation direction 108 and may thus provide a fixed propagation distance for light within a particular section of the optical coupler 100. FIG. 3 is a plot of a uniform refractive index profile of a solid propagation element 104 along a direction orthogonal to a propagation direction 108, in accordance with one or more embodiments of thepresent disclosure. A solid propagation element 104 may also optionally provide guiding of light through total internal reflection at a perimeter of the rod.

[0082] The optical coupler 100 may include any number or arrangement of alternating solid propagation elements 104 and GRIN elements 106. For example, the optical coupler 100 may include any number of GRIN elements 106 of the same design or different designs. In this way, the dioptric power of any particular GRIN element 106 may be controlled through various properties such as, but not limited to, composition, diameter, refractive index profile, or length along the propagation direction 108. As another example, the optical coupler 100 may include any number of solid propagation elements 104 of the same design or different designs. In this way, properties such as, but not limited to, the composition or length of each of the solid propagation elements 104 may be tailored to provide a desired optical path length. Additionally, the outer diameters of the GRIN elements 106 and the solid propagation elements 104 may be the same or different.

[0083] In some embodiments, the solid propagation elements 104 and the GRIN elements 106 are selected to provide efficient mode field adaptation and coupling of light between two external optical elements 102 having different mode profiles and / or different NAs. In general, the optical coupler 100 may be designed to provide coupling between any external optical elements 102 of any type or size. As an illustration, the optical coupler 100 may be designed to provide efficient coupling between solid-core fibers with different mode field diameters and / or NAs. As another illustration, the optical coupler 100 may be designed to provide efficient coupling between a solid-core fiber and an HCF. As another illustration, the optical coupler 100 may be designed to provide efficient coupling between two HCFs with different mode field diameter, core sizes, designs, or mode profiles. Further, the optical coupler 100 may be designed to provide coupling between a single-mode fiber of any type and a multi-mode fiber of any type. As another illustration, the optical coupler 100 may be designed to provide efficient coupling between a fiber (e.g., an HCF or a solid-core fiber) and a PIC. FIG.1B illustrates an optical coupler 100 providing coupling between a fiber (e.g., an HCF or a solid-core fiber) and a PIC, in accordance with one or more embodiments of the present disclosure. In addition to coupling into or from a PIC, the coupling could be to or from any active or passive optical waveguide or physical structure that guides light, including one or more of: a semiconductor laser diode, optical photodetector, anoptical waveguide, a splitter, a modulator, a sensor, a silicon photonic waveguide, planar lightwave circuit (PLC), arrayed waveguide grating (AWG), and a grating coupler. Such waveguides and devices could be fabricated from silica, doped silica, silicon, lll-V semiconductor materials, polymers, Lithium Niobate (LiNbOs), Indium Phosphide (InP), Silicon Nitride (SiN), chalcogenide glasses, or any hybrid device combination thereof.

[0084] Further, the optical coupler 100 may have the same or a different outer diameter than coupled external optical elements 102. For example, FIG. 1A depicts an optical coupler 100 coupled to external optical elements 102 having the same outer diameter. FIG. 1C illustrates an optical coupler 100 having a different (here, larger) outer diameter than external optical elements 102, in accordance with one or more embodiments of the present disclosure.

[0085] In some embodiments, the optical coupler 100 includes two outer solid propagation elements 104 and an interior section 112 including an alternating series of GRIN elements 106 and solid propagation elements 104. In this configuration, the interior section 112 may provide mode conversion and the outer solid propagation elements 104 may provide fixed transmission lengths for precise control of the numerical aperture and beam size at both faces 110 of the optical coupler 100 that interface with the external optical elements 102.

[0086] The optical coupler 100 may operate as an imaging system to image light between the faces 110, where the imaging conditions such as, but not limited to, the spot size or numerical aperture associated with each face 110 is controlled to provide coupling into respective external optical elements 102. As an illustration, coupling into a multimode fiber may be provided by matching a spot size and numerical aperture of light at one end of the optical coupler 100 to the core diameter and numerical aperture of the associated multimode fiber. As another illustration, coupling into a single mode fiber may be provided by matching a Gaussian beam width of light at one end of the optical coupler 100 with a mode waist of the associated single mode fiber. It is contemplated herein that the optical coupler 100 may be particularly beneficial for, but not limited to, coupling between HCFs or coupling between an HCF and another external optical element 102 (e.g., a solid-core fiber, a PIC, or the like).

[0087] For example, as depicted in FIG. 1A, the interior section 112 may include two GRIN elements 106 separated by a single solid propagation element 104. Such a configuration may be configured as, but is not required to be configured as, a 4F imaging system.

[0088] In some embodiments, the optical coupler 100 includes an anti-reflection (AR) treatment on any faces 110 and / or between constituent elements. The optical coupler 100 may include any type of AR treatment such as, but not limited to, an AR coating (e.g., a dielectric coating), an AR surface treatment (e.g., random AR micro / nanotexturing, biomimetic feature fabrication, or the like), and / or angle-cleaved surfaces. AR coatings may also be formed using various standard techniques, including chemical etching, physical vapor deposition, ion-beam sputtering, thermal vaporization, e-beam vaporization, advanced vapor deposition, plasma deposition, plasma-assisted reactive magnetron sputtering, ion-assisted electron-beam evaporative deposition, surface-treatment techniques, and any other applicable process or technique known to a person of skill in the art.

[0089] FIGS. 1D-1F depict variations of an optical coupler 100 with different AR treatments 114.

[0090] FIG. 1D illustrates an optical coupler 100 configured as shown in FIG. 1A, but with AR treatments 114 on both faces 110, in accordance with one or more embodiments of the present disclosure. FIG. 1E illustrates an optical coupler 100 configured as shown in FIG. 1C, but with an AR treatments 114 on one of the faces 110, in accordance with one or more embodiments of the present disclosure. In FIGS.1 D and 1 E, the different AR treatments 114 are depicted as coatings, but this is merely illustrative and should not be interpreted as limiting the scope of the present disclosure.

[0091] FIG. 1 F illustrates an optical coupler 100 with angle-cleaved surfaces on all components, in accordance with one or more embodiments of the present disclosure. Such a configuration may reduce back reflections associated with any of the interfaces within the optical coupler 100 or from the faces 110. FIG. 1F further depicts an AR treatment 114 (e.g., a coating) on one of the faces 110.

[0092] Referring now to FIGS. 4A-5B, various techniques and configurations for coupling two external optical elements 102 with an optical coupler 100 are describedin greater detail in accordance with one or more embodiments of the present disclosure.

[0093] In some embodiments, coupling points between the optical coupler 100 and external optical elements 102 (e.g., at the faces 110) are protected to provide mechanical stability and / or continuous optical properties.

[0094] The optical coupler 100 may include various components to protect the coupling points. For example, the coupling points may be coated (or recoated). Such a configuration may be suitable for, but not limited to, applications where the external optical element 102 is an HOF to provide additional protection against damage, contaminants, and / or light leakage. Such a configuration may additionally be suitable for, but not limited to, communications and high-power laser applications to protect the various components and minimize back reflections. FIG. 4A illustrates a simplified schematic of an optical coupler 100 spliced directly to two external optical elements 102, where the external optical elements 102 and the optical coupler 100 are coated (or recoated) with a coating 402, in accordance with one or more embodiments of the present disclosure.

[0095] In some embodiments, one or both coupling points at one or both faces 110 of an optical coupler 100 are protected by one or more splice protection components (e.g., a splice protector package) to provide further protection of the splice points or the optical coupler 100 as a whole. Such a configuration may be particularly suitable for communications applications. The one or more splice protection components may be formed from any suitable material such as, but not limited to, heat shrink tubing, glass, a metal housing, or a ceramic ferrule.

[0096] FIG. 4B illustrates a simplified schematic of an optical coupler 100 spliced directly to two external optical elements 102, where the optical coupler 100 and the splice points are protected by a splice protection package 404 (e.g., splice protector) in the form of a glass splice protector, in accordance with one or more embodiments of the present disclosure.

[0097] In some embodiments, as depicted in FIG. 4B, the external optical elements 102 include a fiber coating 406 (e.g., a low-index fiber coating, or any other suitable fiber coating) that is partially stripped near an end face. The splice protection package 404 may then be connected to the fiber coating 406 using any suitable techniqueincluding, but not limited to, epoxy 408. Any type of epoxy 408 may be used. For instance, a low-index / high-temperature epoxy 408 may reduce and / or minimize an amount of light that is absorbed by the epoxy 408 and thus enable high-power operation since absorption may damage the optical coupler 100 and / or the external optical elements 102. In some embodiments, additional cladding light strippers 410 are provided to remove stray light from any of the splice points.

[0098] FIG. 4C illustrates a simplified schematic of an optical coupler 100 spliced directly to two external optical elements 102, where the optical coupler 100 and the splice points are protected by a splice protection package 404 in the form of a metal rod 412 and heat-shrink tubing 414.

[0099] Referring now to FIGS. 5A-5D, in some embodiments, one or more faces 110 of the optical coupler 100 are mounted to a fiber connector (e.g., a commercial or standardized connector). Such a configuration may enable flexible connections between the optical coupler 100 and the external optical elements 102.

[0100] FIG. 5A illustrates a simplified schematic of an optical coupler 100 within a female-to-female fiber connector 502 along with two unconnected external optical elements 102, in accordance with one or more embodiments of the present disclosure. FIG. 5B illustrates a cross-sectional view of an optical coupler 100 within the female-to-female connector 502 in FIG. 5A connected to two external optical elements 102, in accordance with one or more embodiments of the present disclosure. In FIGS. 5A-5B, the connector 502 may be referred to as a single unit, but may be formed from two exterior connectors (here, female external connectors) suitable for coupling with paired connectors 504 on external optical elements 102 to be coupled. In this configuration, the optical coupler 100 may be readily connected to external optical elements 102 with male connectors 504. However, this is merely illustrative and the optical coupler 100 may be mounted in any connector of any design with any combination of male or female connections. Further, the optical coupler 100 may be spliced to a face 110 at one face 110 (e.g., as depicted in FIGS. 4A-4B) and coupled to a fiber at another face 110 (e.g., as depicted in FIGS. 4A-4B) with a connector 502 (e.g., as depicted in FIGS.5A-5B).

[0101] The optical coupler 100 may be mounted to the connector 502 using any technique known in the art including, but not limited to, laser bonding, and / or bondingwith epoxy 506. As described with respect to FIGS. 4A-4B, any type of epoxy 506 may be used including, but not limited to, a low-index / high-temperature epoxy. As an illustration, FIG. 5B depicts an optical coupler 100 mounted to the connector 502 with epoxy 408, where the faces 110 of the optical coupler 100 are exposed for coupling with the external optical elements 102. Similarly, the external optical elements 102 are connected to the associated male connectors 504 with epoxy 506 to expose associated end faces for coupling with the optical coupler 100. In this configuration, joining the connector 502 with the associated connectors 504 may align the optical coupler 100 with the external optical elements 102 to enable coupling.

[0102] The connectors 502,504 may include any type of couplable connection components including, but not limited to, a commercially available connector, a standardized connector, or a custom connector. For example, the type of connectors 502,504 may include, but are not limited to, LC, FC, APC, SC, ST, or the like. The connectors 502,504 may further be formed as a single-fiber connector or a multi-fiber connector (e.g., a multi-fiber push-on (MPO) connector, multiple single-fiber connectors, or the like). As an illustration, multi-fiber coupling may be achieved by mounting multiple optical couplers 100 to a connector 502 suitable for simultaneously coupling to multiple external optical elements 102 on each end using any coupling technique or combination of coupling techniques.

[0103] FIG. 5C illustrates a simplified schematic of an optical coupler 100 coupled to a male connector 504, where one face 110-1 is spliced to an external optical element 102 (e.g., an HCF or a single-mode fiber), and where another face 110-2 is connectable to another external optical element 102 via the male connector 504, in accordance with one or more embodiments of the present disclosure. In this way, the male connector 504 operates as an end-cap and coupling mechanism to support coupling of another connectorized external optical element 102.

[0104] Referring generally to FIGS. 5A-5C, as described previously herein, one or both of the faces 110 of the optical coupler 100 may have an AR treatment 114 (not explicitly shown) to mitigate reflections at the splice points (e.g., due to an air gap between the optical coupler 100 and the external optical elements 102 in this configuration).

[0105] In some embodiments, an optical coupler 100 is used to transition between different types of fibers prior to a connector Such a configuration may be particularly suitable for, but not limited to, applications with HCFs. As an illustration, FIG. 5D illustrates a simplified schematic of an optical coupler 100 providing a transition between an HCF and a solid-core fiber, where the solid-core fiber is coupled with a connector 504, in accordance with one or more embodiments of the present disclosure. For example, the HCF may be a first external optical element 102 and the solid-core fiber may be a second external optical element 102.

[0106] In this configuration, the optical coupler 100 may provide stable, high-efficiency coupling to the solid-core fiber while also protecting the HCF from damage during the coupling or uncoupling with the connector 504. Further, in the case of a failure, it may be relatively easier to cut and reattach the connector 504 to a solid-core fiber than an HCF.

[0107] Referring now to FIGS. 6A-9C, various simulations of an optical coupler 100 configured as depicted in FIG. 1A are described, in accordance with one or more embodiments of the present disclosure.

[0108] FIG. 6A is a simulated plot of beam propagation through the optical coupler 100, in accordance with one or more embodiments of the present disclosure. In FIG.6A, the horizontal direction corresponds to distance along the propagation direction 108 of light through the optical coupler 100 and the vertical direction corresponds to distance along a direction orthogonal to this propagation direction 108. As shown in FIG. 6A, light from an input fiber at a first face 110-1 expands in a first solid propagation element 104-1 (e.g., an outer solid propagation element 104), is focused by a first GRIN element 106-1 to an intermediate focal point 602 in a second solid propagation element 104-2 (e.g., a part of the interior section 112), and is refocused to a second face 110-2 with a different mode profile by a second GRIN element 106-2 through a third solid propagation element 104-3 (e.g., an outer solid propagation element 104).

[0109] FIGS. 6B-6F depict simulations of coupling in the various sections of the optical coupler 100 from FIGS. 6A. FIG. 6B is a plot illustrating coupling of light in the first solid propagation element 104-1, in accordance with one or more embodiments of the present disclosure. FIG. 6C is a plot illustrating coupling of light in the first GRIN element 106-1, in accordance with one or more embodiments of the presentdisclosure. FIG. 6D is a plot illustrating coupling of light in the second solid propagation element 104-2, in accordance with one or more embodiments of the present disclosure. FIG. 6E is a plot illustrating coupling of light in the second GRIN element 106-2, in accordance with one or more embodiments of the present disclosure. FIG.6F is a plot illustrating coupling of light in the third solid propagation element 104-3, in accordance with one or more embodiments of the present disclosure. In FIGS. 6B-6F, the horizontal direction corresponds to thickness along the propagation direction 108 and the vertical direction corresponds to coupling percentage in fraction form. FIG. 6G is a plot illustrating coupling of light in the optical coupler 100 as a whole as a function of wavelength, in accordance with one or more embodiments of the present disclosure. Taken together, FIGS. 6A-6G demonstrate available tolerances for excellent coupling in the optical coupler 100, which may provide excellent mode field and / or NA adaptation.

[0110] FIGS. 7A-9C illustrate beam propagation through different variations of an optical coupler 100, in accordance with one or more embodiments of the present disclosure.

[0111] FIG. 7 A illustrates a simulated plot of beam propagation through an optical coupler 100 formed as a solid propagation element 104-4, a GRIN element 106-3, a solid propagation element 104-5, a GRIN element 106-4, and a solid propagation element 104-6, in accordance with one or more embodiments of the present disclosure. FIG. 7B illustrates a simulated plot of mode field diameter (MFD) through the optical coupler 100 of FIG. 7A, in accordance with one or more embodiments of the present disclosure. FIG. 7C illustrates a schematic of light 702 propagation through the optical coupler 100 in FIG. 7A, in accordance with one or more embodiments of the present disclosure. In FIGS. 7A-7C, the optical coupler 100 transitions an input MFD of 9.6 pm (e.g., from a standard telecommunication fiber) to an output MFD of 25 pm (e.g., associated with an HCF).

[0112] FIG. 8A illustrates a simulated plot of beam propagation through an optical coupler 100 formed as a solid propagation element 104-7, a GRIN element 106-5 with a refractive index variation of An = 0.03, another GRIN element 106-6 with a refractive index variation of An ~ 0.015, and another solid propagation element 104-8, in accordance with one or more embodiments of the present disclosure. FIG. 8B illustrates a simulated plot of MFD through the optical coupler 100 of FIG. 8A, inaccordance with one or more embodiments of the present disclosure. FIG. 8C illustrates a plot of the MFD at an output of the optical coupler 100, in accordance with one or more embodiments of the present disclosure. In FIGS. 8A-8C, the optical coupler 100 transitions an input MFD of 9.6 pm (e.g., from a standard telecommunication fiber) to an output MFD of 25 pm (e.g., associated with an HCF).

[0113] FIG. 9A illustrates a simulated plot of beam propagation through an optical coupler 100 formed as a solid propagation element 104-9, a GRIN element 106-7 with a refractive index variation of An = 0.03, another solid propagation element 104-10, another GRIN element 106-8 with a refractive index variation of An = 0.03, and another solid propagation element 104-11, in accordance with one or more embodiments of the present disclosure. FIG. 9B illustrates a simulated plot of MFD through the optical coupler 100 of FIG. 9A, in accordance with one or more embodiments of the present disclosure. FIG. 9C illustrates a plot of the MFD at an output of the optical coupler 100, in accordance with one or more embodiments of the present disclosure. In FIGS. 9A-9C, the optical coupler 100 transitions an input MFD of 9.6 pm (e.g., from a standard telecommunication fiber) to an output MFD of 25 pm (e.g., associated with an HCF).

[0114] Referring now to FIG. 10, FIG. 10 is a flow diagram illustrating steps performed in a method 1000 for fabricating an optical coupler 100, in accordance with one or more embodiments of the present disclosure.

[0115] In some embodiments, the method 1000 includes a step 1002 of fabricating one or more GRIN elements 106 having spatially-varying refractive index profiles orthogonal to a propagation direction 108.

[0116] In some embodiments, the method 1000 include a step 1004 of fabricating one or more solid propagation elements 104 having spatially-uniform refractive index profiles orthogonal to the propagation direction 108.

[0117] In some embodiments, the method 1000 includes a step 1006 of joining the one or more GRIN elements 106 with the one or more solid propagation elements 104 along the propagation direction 108 to form a monolithic optical coupler 100. The one or more GRIN elements 106 and the one or more solid propagation elements 104 may be configured to couple light between outer faces 110 of the monolithic optical coupler 100 distributed along the propagation direction 108.

[0118] in some embodiments, the method 1000 indudes a step 1008 of joining at least one of the one or more GRIN elements 106 or the one or more solid propagation elements 104 to an external optical element 102 such as, but not limited to, an HCF, a solid-core fiber, or a PIC.

[0119] Any of the steps 1002-1006 may be performed in any order. For example, the monolithic optical coupler 100 may be fabricated first and then joined to an external optical element as a whole. As another example, the monolithic optical coupler 100 may be fabricated piecewise directly onto an external optical element 102 such as, but not limited to, a fiber.

[0120] Referring now to FIG. 11, FIG. 11 is a flow diagram illustrating steps performed in a method 1100 for fabricating and protecting an optical coupler 100, in accordance with one or more embodiments of the present disclosure.

[0121] In some embodiments, the method 1100 includes a step 1102 of splicing a first fiber (e.g., external optical element 102) to a first outer solid propagation element 104 of an optical coupler 100, where the optical coupler 100 includes an interior section 112 spliced to the first solid propagation element 104 and a second solid propagation element 104, and where the interior section 112 comprises two or more GRIN elements 106 separated by one or more interior solid propagation elements 104. For example, as depicted in FIGS. 4A-4C, the first fiber may be an HCF or a solid-core fiber that is spliced directly to the optical coupler 100.

[0122] In some embodiments, the method 1100 includes a step 1104 of encapsulating a splice point between the first fiber and the first outer solid propagation element 104 with one or more optical coatings. For example, as depicted in FIG. 4A, the splice point may be coated (or recoated) with a coating 402 to provide additional protection against damage, contaminants, and / or light leakage.

[0123] In some embodiments, the method 1100 includes a step 1106 of encapsulating a splice point between the first fiber and the first outer solid propagation element 104 with a splice protector. For example, as depicted in FIG. 4B, the splice point may be protected by a splice protection package 404 in the form of a glass splice protector. As another example, as depicted in FIG. 4C, the splice point may be protected by a splice protection package 404 in the form of a metal rod 412 and heatshrink tubing 414.

[0124] in some embodiments, the method 1100 includes a step 1108 of splicing a second fiber (e.g., external optical element 102) to the second solid propagation element 104. For example, as depicted in FIGS. 4A-4C, the second fiber may be an HOF or a solid-core fiber that is spliced directly to the optical coupler 100.

[0125] In some embodiments, the method 1100 includes a step 1110 of encapsulating a splice point between the second fiber and the second outer solid propagation element 104 with one or more optical coatings. For example, as depicted in FIG. 4A, the splice point may be coated (or recoated) with a coating 402 to provide additional protection against damage, contaminants, and / or light leakage.

[0126] In some embodiments, the method 1100 includes a step 1112 of encapsulating a splice point between the second fiber and the second outer solid propagation element 104 with a splice protector. For example, as depicted in FIG. 4B, the splice point may be protected by a splice protection package 404 in the form of a glass splice protector. As another example, as depicted in FIG. 4C, the splice point may be protected by a splice protection package 404 in the form of a metal rod 412 and heat-shrink tubing 414.

[0127] Any of the steps 1102-1112 may be performed in any order. For example, the optical coupler 100 may be spliced to the first fiber and to the second fiber in any order. As another example, the splice points may be encapsulated with optical coatings before or after being encapsulated with splice protectors. The steps may also be performed in the order presented or in any other suitable order.

[0128] In some embodiments, a fiber coupling method may provide efficient optical connection between fibers and the optical coupler 100. The fiber coupling method may include, but is not limited to, joining a first fiber to the optical coupler 100, where the first fiber may be any type of optical fiber including, but not limited to, an HOF, a solidcore fiber, a single-mode fiber, or a multi-mode fiber. The joining process may establish optical communication between the first fiber and the optical coupler 100 to enable light transmission.

[0129] The joining of the first fiber to the optical coupler 100 may be accomplished through any suitable technique. In some embodiments, the joining may include splicing the first fiber directly to one of the outer faces 110 of the optical coupler 100. The splicing process may involve aligning the first fiber with the optical coupler 100 andcreating a permanent connection through fusion splicing, mechanical splicing, or other suitable splicing techniques. The splicing may provide a low-loss optical connection with minimal back reflection.

[0130] In some embodiments, the fiber coupling method may further include joining a second fiber to the optical coupler 100. The second fiber may be joined to another outer face 110 of the optical coupler 100 to enable bidirectional light coupling or to complete an optical path between the first fiber and the second fiber through the optical coupler 100. The second fiber may be the same type as the first fiber or may be a different type of fiber, such as coupling between an HCF and a solid-core fiber, or between fibers with different mode field diameters.

[0131] The joining of the second fiber may be performed using similar techniques as the first fiber, including splicing the second fiber to the optical coupler 100. The fiber coupling method may enable mode field adaptation between the first fiber and the second fiber through the optical properties of the GRIN elements 106 and solid propagation elements 104 within the optical coupler 100.

[0132] In some embodiments, the fiber coupling method may include preparing the fiber ends before joining. The preparation may include cleaving the fiber ends to provide clean, perpendicular surfaces for optimal coupling. The fiber ends may also be treated with AR treatments to reduce reflections at the coupling interfaces.

[0133] The fiber coupling method may provide mechanical stability and protection for the coupled fibers. In some embodiments, the coupling points may be protected using splice protection packages 404, optical coatings 402, or other protective measures as described in the method 1100. The protection may prevent damage to the coupling interfaces and maintain optical performance overtime.

[0134] The present disclosure provides an optical coupler. The optical coupler may include one or more GRIN elements having spatially-varying refractive index profiles orthogonal to a propagation direction and / or one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to a propagation direction. The one or more GRIN elements and the one or more solid propagation elements may be joined along the propagation direction to form a monolithic element, where the one or more GRIN elements and the one or more solid propagation elements couple lightbetween outer faces of the monolithic element distributed along the propagation direction

[0135] In some embodiments, at least one of dioptric powers of the one or more GRIN elements, lengths of the one or more GRIN elements, or lengths of the one or more solid propagation elements are selected to adapt a mode profile of light from a first external optical element coupled to a first of the outer faces to match a mode profile of a second external optical element coupled to a second of the outer faces. The outer faces may be couplable with a first external optical element and a second external optical element.

[0136] For example, at least one of the first external optical element or the second external optical element may be a hollow-core fiber. In some embodiments, at least one of the first external optical element or the second external optical element is a solid-core fiber. The first external optical element and the second external optical element may be solid-core fibers with at least one of different mode profiles or different mode field diameters.

[0137] In some embodiments, the one or more GRIN elements are interleaved with the one or more solid propagation elements. For example, the one or more solid propagation elements may include a first solid propagation element, a second solid propagation element, and a third solid propagation element, where the one or more GRIN elements include a first GRIN element between the first and second solid propagation elements along with a second GRIN element between the second and third solid propagation elements.

[0138] The optical coupler may operate as an imaging system. In some embodiments, the optical coupler operates as a 4F imaging system. The optical coupler may further include one or more anti-reflection treatments on at least one of the outer faces or interfaces within the optical coupler. For example, the one or more anti-reflection treatments may include at least one of an anti-reflection coating, a surface treatment, or an angle-cleaved surface.

[0139] In some embodiments, the optical coupler further includes an exterior connector configured to mechanically couple with a paired connector on a first external optical element, where mechanically coupling the exterior connector to the paired connector provides optical coupling of light between the first optical element and a firstof the outer faces. The exterior connector and the paired connector may be at least one of LC, FC, SC, or ST connectors. The optical coupler may further include an additional exterior connector configured to mechanically couple with an additional paired connector on a second external optical element, where mechanically coupling the additional exterior connector to the additional paired connector provides optical coupling of light between the second external optical element and a second of the outer faces. For example, the additional exterior connector and additional the paired connector may be at least one of LC, FC, SC, or ST connectors.

[0140] In some embodiments, the refractive index profiles of the one or more GRIN elements are parabolic. The one or more solid propagation elements may have uniform refractive index profiles.

[0141] The present disclosure also provides a method. The method may include fabricating one or more GRIN elements having spatially-varying refractive index profiles orthogonal to a propagation direction, fabricating one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to the propagation direction, and / or joining the one or more GRIN elements with the one or more solid propagation elements along the propagation direction to form a monolithic optical coupler. The one or more GRIN elements and the one or more solid propagation elements may be configured to couple light between outer faces of the monolithic optical coupler distributed along the propagation direction.

[0142] In some embodiments, the method further includes joining at least one of the one or more GRIN elements or the one or more solid propagation elements to an external optical element. For example, joining at least one of the one or more GRIN elements or the one or more solid propagation elements to the external optical element may include splicing at least one of the one or more GRIN elements or the one or more solid propagation elements to the external optical element.

[0143] The method may further include providing one or more anti-reflection treatments on at least one of the outer faces or interfaces within the optical coupler. In some embodiments, the one or more anti-reflection treatments include at least one of an anti-reflection coating, a surface treatment, or an angle-cleaved surface.

[0144] In some embodiments, the method further includes coupling an exterior connector to the monolithic optical coupler, where the exterior connector is configuredto mechanically couple with a paired connector on a first external optical element, and where mechanically coupling the exterior connector to the paired connector provides optical coupling of light between the monolithic optical coupler and the first external optical element. For example, the exterior connector may be further configured to mechanically couple with an additional paired connector on a second external optical element, where mechanically coupling to the additional paired connector provides optical coupling of light between the monolithic optical coupler and the second external optical element.

[0145] The present disclosure also provides a fiber coupling method. The fiber coupling method may include joining a first fiber to an optical coupler, where the optical coupler includes one or more GRIN elements having spatially-varying refractive index profiles orthogonal to a propagation direction and one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to a propagation direction. The one or more GRIN elements and the one or more solid propagation elements may be joined along the propagation direction to form a monolithic element, where the one or more GRIN elements and the one or more solid propagation elements couple light between outer faces of the monolithic element distributed along the propagation direction.

[0146] In some embodiments, joining the first fiber to the optical coupler includes splicing the first fiber to the optical coupler. The fiber coupling method may further include joining a second fiber to the optical coupler.

[0147] The herein described subject matter sometimes Illustrates different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality Is effectively "associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "connected" or "coupled" to each other to achieve the desired functionality, andany two components capable of being so associated can also be viewed as being "couplable" to each other to achieve the desired functionality.

[0148] It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes. Furthermore, it is to be understood that the invention is defined by the appended claims.

Claims

CLAIMSWhat is claimed:

1. An optical coupler, comprising:one or more gradient refractive index (GRIN) elements having spatially-varying refractive index profiles orthogonal to a propagation direction; andone or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to a propagation direction, wherein the one or more GRIN elements and the one or more solid propagation elements are joined along the propagation direction to form a monolithic element, wherein the one or more GRIN elements and the one or more solid propagation elements couple light between outer faces of the monolithic element distributed along the propagation direction,2. The optical coupler of claim 1, wherein at least one of dioptric powers of the one or more GRIN elements, lengths of the one or more GRIN elements, or lengths of the one or more solid propagation elements are selected to adapt a mode profile of light from a first external optical element coupled to a first of the outer faces to match a mode profile of a second external optical element coupled to a second of the outer faces.

3. The optical coupler of claim 1, wherein the outer faces are couplable with a first external optical element and a second external optical element.

4. The optical coupler of claim 3, wherein at least one of the first external optical element or the second external optical element is a hollow-core fiber.

5. The optical coupler of claim 3, wherein at least one of the first external optical element or the second external optical element is a solid-core fiber.

6. The optical coupler of claim 3, wherein the first external optical element and the second external optical element are solid-core fibers with at least one of different mode profiles or different mode field diameters.

7. The optical coupler of claim 1, wherein the one or more GRIN elements are interleaved with the one or more solid propagation elements.

8. The optical coupler of claim 1, wherein the one or more solid propagation elements include a first solid propagation element, a second solid propagation element, and a third solid propagation element, wherein the one or more GRIN elements include a first GRIN element between the first and second solid propagation elements along with a second GRIN element between the second and third solid propagation elements.

9. The optical coupler of claim 1, wherein the optical coupler operates as an imaging system.

10. The optical coupler of claim 1, wherein the optical coupler operates as a 4F imaging system.

11. The optical coupler of claim 1, further comprising:one or more anti-reflection treatments on at least one of the outer faces or interfaces within the optical coupler.

12. The optical coupler of claim 11, wherein the one or more anti-reflection treatments comprise:at least one of an anti-reflection coating, a surface treatment, or an angle-cleaved surface.

13. The optical coupler of claim 1, further comprising:an exterior connector configured to mechanically couple with a paired connector on a first external optical element, wherein mechanically coupling the exterior connector to the paired connector provides optical coupling of light between the first optical element and a first of the outer faces.

14. The optical coupler of claim 13, wherein the exterior connector and the paired connector are at least one of LC, FC, SC, or ST connectors.

15. The optical coupler of claim 13, further comprising:an additional exterior connector configured to mechanically couple with an additional paired connector on a second external optical element, wherein mechanically coupling the additional exterior connector to the additional paired connector provides optical coupling of light between the second external optical element and a second of the outer faces.

16. The optical coupler of claim 15, wherein the additional exterior connector and additional the paired connector are at least one of LC, FC, SC, or ST connectors.

17. The optical coupler of claim 1, wherein the refractive index profiles of the one or more GRIN elements are parabolic.

18. The optical coupler of claim 1, wherein the one or more solid propagation elements have uniform refractive index profiles.

19. A method, comprising:fabricating one or more gradient refractive index (GRIN) elements having spatially-varying refractive index profiles orthogonal to a propagation direction;fabricating one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to the propagation direction; andjoining the one or more GRIN elements with the one or more solid propagation elements along the propagation direction to form a monolithic optical coupler, wherein the one or more GRIN elements and the one or more solid propagation elements are configured to couple light between outer faces of the monolithic optical coupler distributed along the propagation direction.

20. The method of claim 19, further comprising:joining at least one of the one or more GRIN elements or the one or more solid propagation elements to an external optical element.

21. The method of claim 20, wherein joining at least one of the one or more GRIN elements or the one or more solid propagation elements to the external optical element comprises splicing at least one of the one or more GRIN elements or the one or more solid propagation elements to the external optical element.

22. The method of claim 19, further comprising:providing one or more anti-reflection treatments on at least one of the outer faces or interfaces within the optical coupler.

23. The method of claim 22, wherein the one or more anti-reflection treatments comprise:at least one of an anti-reflection coating, a surface treatment, or an angle-cleaved surface.

24. The method of claim 19, further comprising:coupling an exterior connector to the monolithic optical coupler, the exterior connector configured to mechanically couple with a paired connector on a first external optical element, wherein mechanically coupling the exterior connector to the paired connector provides optical coupling of light between the monolithic optical coupler and the first external optical element.

25. The method of claim 24, wherein the exterior connector is further configured to mechanically couple with an additional paired connector on a second external optical element, wherein mechanically coupling to the additional paired connector provides optical coupling of light between the monolithic optical coupler and the second external optical element.

26. A fiber coupling method, comprising:joining a first fiber to an optical coupler, the optical coupler comprising:one or more gradient refractive index (GRIN) elements having spatially- varying refractive index profiles orthogonal to a propagation direction; and one or more solid propagation elements having spatially-uniform refractive index profiles orthogonal to a propagation direction, wherein the one or more GRIN elements and the one or more solid propagation elements are joined along the propagation direction to form a monolithic element, wherein the one or more GRIN elements and the one or more solid propagation elements couple light between outer faces of the monolithic element distributed along the propagation direction.

27. The fiber coupling method of claim 26, wherein joining the first fiber to the optical coupler comprises:splicing the first fiber to the optical coupler.

28. The fiber coupling method of claim 26, further comprising:joining a second fiber to the optical coupler.

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