Compact dual polarization couplers for multi-core optical fibers

Abstract

An optical fiber coupler for coupling a plurality of cores of a multi-core optical fiber to an integrated photonic device includes a grating array comprising a plurality of polarizing beam-splitters arranged corresponding to a plurality of cores in the multi-core optical fiber. The optical fiber coupler also includes first and second mode converters extending from first and second sides of each of the plurality of polarizing beam-splitters to receive first and second polarization modes of an optical signal scattered by the polarizing beam-splitters. A plurality of waveguides extend from the end of each mode converter of the mode converter to guide a single polarization mode of one of the optical signals.

Description

[Technical Field]

[0001] This application claims priority to U.S. Provisional Application No. 63 / 146,260, filed February 5, 2021, which is incorporated herein by reference in its entirety.

[0002] This disclosure relates to fiber optic couplers, and more specifically, to grating couplers for multi-core optical fibers. [Previous Technology]

[0003] Fiber optic transmission systems are used in data centers and optical networks to optically connect multiple different optical devices. To increase the capacity of such fiber optic transmission systems, various spatial multiplexing techniques, including multi-core fibers, have been explored. Multi-core fibers are most advantageous if light can be efficiently coupled from all cores. For example, if light from a multi-core fiber is transmitted to an integrated photonic device, such as a photonic integrated circuit, a grating coupler can be used to couple each core of the multi-core fiber to a waveguide in the photonic integrated circuit. Existing grating couplers may include a grating and a waveguide tape for guiding the optical signal from each core of the multi-core fiber to one of the waveguides. Such existing gratings and waveguide tapes are structurally incompatible with certain multi-core fibers. If the core-to-core spacing in the multi-core fiber is too small, for example, existing grating and waveguide tape combinations may be too large to efficiently couple light of two polarization modes from each core to a separate waveguide. As a result, existing grating couplers may cause a loss of polarization diversity in optical signals, which is unacceptable for high-end communication systems. [Summary of the Invention]

[0004] A first aspect of this disclosure includes an optical system comprising: a multi-core optical fiber including a plurality of cores; and an optical fiber coupler. The optical fiber coupler includes a grating array comprising a plurality of polarizing beam splitters disposed on a substrate. The plurality of polarizing beam splitters are arranged to correspond to the plurality of cores in the multi-core optical fiber in a manner to receive optical signals from one of the cores. The number of polarizing beam splitters included in the plurality of polarizing beam splitters is less than or equal to the number of cores in the multi-core optical fiber. The fiber optic coupler also includes a first mode converter extending from a first side of each of the plurality of polarizing beam splitters to receive a first polarization mode of an optical signal scattered by the polarizing beam splitter; a second mode converter extending from a second side of each of the plurality of polarizing beam splitters to receive a second polarization mode of an optical signal scattered by the polarizing beam splitter; and a plurality of waveguides extending from an end of each mode converter to guide a single polarization mode of one of the optical signals.

[0005] The second aspect of this disclosure includes an optical system according to the first aspect, wherein adjacent cores of a plurality of cores are separated from each other by a minimum core-to-core spacing distance of less than or equal to 45µm.

[0006] The third state of this disclosure includes an optical system according to any one of the first to second state samples, wherein the multi-core optical fiber includes a total diameter of less than or equal to 125µm.

[0007] The fourth state of this disclosure includes an optical system according to any one of the first to third states, wherein the multi-core fiber includes at least two cores, such that four polarization modes are guided by a plurality of waveguides extending from the end of each mode converter.

[0008] The fifth state of this disclosure includes an optical system according to any one of the first to fourth states, wherein the multi-core optical fiber includes at least 5 cores, such that 10 polarization modes are guided by a plurality of waveguides extending from the end of each mode converter.

[0009] The sixth state of this disclosure includes an optical system according to any one of the first to fifth states, wherein adjacent cores of the plurality of cores are separated from each other by a minimum core-to-core spacing distance of less than or equal to 35µm.

[0010] The seventh state of this disclosure includes an optical system according to any one of the first to sixth states, wherein the first and second sides of the plurality of polarizing beam splitters are at least as long as the mode field diameter of each of the plurality of cores.

[0011] The eighth state of this disclosure includes an optical system according to any one of the first to seventh states, wherein the mode field diameter is greater than or equal to 5µm and less than or equal to 25µm.

[0012] The ninth state of this disclosure includes an optical system according to any one of the first to eighth states, wherein the first and second mode converters extending from each of the plurality of polarizing beam splitters include dimensions in a direction perpendicular to the propagation direction of the first and second polarizing modes that are less than or equal to 1.2 times the diameter of the mode field.

[0013] The tenth state of this disclosure includes an optical system according to any one of the first to ninth states, wherein the first and second mode converters extending from each of the plurality of polarizing beam splitters include dimensions in the propagation direction of the first and second polarizing modes that are less than or equal to the mode field diameter.

[0014] The eleventh state of this disclosure includes an optical system according to any one of the first to tenth states, wherein each polarizing beam splitter and the first and second mode converters extending therefrom include a combined cross-sectional area of ​​less than or equal to 250 µm2.

[0015] The twelfth state of this disclosure includes an optical system according to any one of the first to eleventh states, wherein the centers of adjacent polarizing beam splitters of the grating array are separated from each other by a minimum grating spacing distance corresponding to the spacing of a plurality of cores.

[0016] The thirteenth state of this disclosure includes an optical system according to any one of the first to twelfth states, wherein each of the plurality of waveguides is separated from the other waveguides in the plurality of waveguides by a minimum waveguide spacing distance of at least 500 nm.

[0017] The fourteenth state of this disclosure includes an optical system according to any one of the first to thirteenth states, wherein a plurality of waveguides are bent such that the plurality of waveguides extend beyond the grating array.

[0018] The fifteenth state of this disclosure includes an optical system according to any one of the first to fourteenth states, wherein at least one of the plurality of waveguides is bent such that, outside the grating array, the extension direction of the at least one waveguide is changed by at least 90° at the mode converter end relative to the initial extension direction.

[0019] The sixteenth state of this disclosure includes an optical system according to any one of the first to fifteenth states, wherein at least one of the plurality of waveguides extends between two of the polarizing beam splitters within the grating array.

[0020] The seventeenth state of this disclosure includes an optical system according to any one of the first to sixteenth states, wherein each polarization mode of the optical signal is coupled to one of a plurality of waveguides with a coupling efficiency greater than or equal to -6.0 dB.

[0021] The eighteenth state of this disclosure includes an optical system according to any one of the first to seventeenth states, wherein each polarization mode of the optical signal is coupled to one of a plurality of waveguides with a coupling efficiency greater than or equal to -10dB and less than or equal to -0.5dB.

[0022] The nineteenth state of this disclosure includes an optical system according to any one of the first to eighteenth states, wherein the fiber coupler couples two polarization modes of each optical signal in a plurality of waveguides with a coupling efficiency bandwidth of greater than or equal to 50 nm around 1550 nm.

[0023] The twentieth state of this disclosure includes an optical system according to any one of the first to nineteenth states, wherein each mode converter of the first and second mode converters extending from each polarizing beam splitter includes a plurality of nanostructures extending along a curved profile toward a waveguide extending from the mode converter.

[0024] The twenty-first state of this disclosure includes an optical system according to any one of the first to twentieth states, wherein the plurality of nanostructures include a minimum feature size of 100 nm.

[0025] The twenty-second embodiment of this disclosure includes an optical device for coupling a multi-core optical fiber to a photonic device, comprising: A polarizing beam splitter grating disposed on a substrate, the polarizing beam splitter grating guiding light of a first polarized mode in a first propagation direction toward a first side of the polarizing beam splitter grating, and guiding light of a second polarized mode in a second propagation direction toward a second side of the polarizing beam splitter grating. The lengths of the first and second sides are greater than or equal to 10 µm and less than or equal to 15 µm. The optical device also includes first and second mode converters extending from the first and second sides of the polarizing beam splitter grating, the first and second mode converters guiding the first and second polarized modes in the first and second propagation directions respectively into first and second waveguides extending from the first and second mode converters. The first and second mode converters include lengths along the first and second propagation directions that are less than or equal to the lengths of the first and second sides of the polarizing beam splitter grating.

[0026] The twenty-third state of this disclosure includes an optical device according to the twenty-second state, wherein the polarizing beam splitter, the first mode converter, and the second mode converter have a combined cross-sectional area of ​​less than or equal to 250 µm2.

[0027] The twenty-fourth state of this disclosure includes an optical device according to any one of the twenty-two to twenty-three states, wherein the first and second sides of the polarizing beam grating include a length greater than or equal to 10 µm.

[0028] The twenty-fifth state of this disclosure includes an optical device according to any one of the twenty-second to twenty-fourth states, wherein the first and second mode converters include lengths greater than or equal to 3µm and less than or equal to 10µm along the first and second propagation directions.

[0029] The twenty-sixth state of this disclosure includes an optical device according to any one of the twenty-second to twenty-fifth states, wherein the first and second mode converters guide the first and second polarization modes to the first and second waveguides, which have an insertion loss of greater than or equal to -0.5dB.

[0030] The twenty-seventh embodiment of this disclosure includes an optical device according to any one of the twenty-two to twenty-sixth embodiments, wherein the polarizing beam splitter is an assembly comprising a grating array including a plurality of polarizing beam splitters, the plurality of polarizing beam splitters being arranged in a manner corresponding to a plurality of cores in a multi-core optical fiber, such that each of the plurality of polarizing beam splitters receives an optical signal from one of the cores.

[0031] The twenty-eighth state of this disclosure includes an optical device according to any one of the twenty-second to twenty-seventh states, and further includes a pair of mode converters extending from a first side and a second side of each of a plurality of polarizing beam splitters, each mode converter of the pair of mode converters directing the polarization mode of one of the optical signals to a waveguide disposed at an end of the mode converter.

[0032] The 29th embodiment of this disclosure includes an optical device according to any one of the 22nd to 28th embodiments, wherein the centers of adjacent polarizing beam splitters of the grating array are separated from each other by a minimum grating spacing distance of less than or equal to 50µm.

[0033] The thirtieth state of this disclosure includes an optical device according to any one of the twenty-second to twenty-ninth states, further comprising holding a multi-core optical fiber at a distance of less than 50 μm above a substrate, such that one of the multiple cores is aligned with one of the multiple polarizing beam splitters of the grating array.

[0034] The thirty-first state of this disclosure includes an optical device according to any one of the twenty-second to thirtieth states, wherein the plurality of cores in the multi-core optical fiber include a minimum core-to-core spacing distance of less than or equal to 45µm.

[0035] The thirty-second state of this disclosure includes an optical device according to any one of the twenty-second to thirty-first states, wherein adjacent cores of a plurality of cores in a multi-core optical fiber are separated from each other by a minimum core-to-core spacing distance greater than or equal to 20µm and less than or equal to 25µm.

[0036] The thirty-third state of this disclosure includes an optical device according to any one of the twenty-second to twenty-third states, wherein the multi-core optical fiber includes a total diameter of less than or equal to 125µm.

[0037] The thirty-fourth aspect of this disclosure includes a method for coupling light from a multi-core optical fiber to a photonic integrated circuit. The method includes the step of transmitting a plurality of optical signals from a plurality of cores of the multi-core optical fiber to a grating array of the photonic integrated circuit. The grating array includes a plurality of polarizing beam-splitters disposed on a substrate, the plurality of polarizing beam-splitters being arranged in a manner corresponding to a plurality of cores in the multi-core optical fiber to receive one of the optical signals. The method includes the step of scattering light from each of the plurality of optical signals at one of the polarizing beam-splitters, such that a first polarization mode and a second polarization mode of the plurality of optical signals are guided in a first and a second propagation direction, respectively. The method includes the step of using a mode converter extending from each of the plurality of polarizing beam-splitters to individually guide each polarization mode of the first and second polarization modes into a plurality of waveguides. The multi-core fiber comprises at least five cores, such that at least 10 different polarization modes are guided into multiple waveguides via multiple mode converters. Adjacent cores in the multiple cores are separated by a core-to-core spacing of less than or equal to 45 µm.

[0038] The thirty-fifth state of this disclosure includes the method according to the thirty-fourth state, wherein the multi-core optical fiber includes a total diameter of less than or equal to 125µm.

[0039] The thirty-sixth state of this disclosure includes a method according to any one of the thirty-fourth to thirty-fifth states, wherein the minimum core-to-core spacing is less than or equal to 35µm.

[0040] The thirty-seventh state of this disclosure includes a method according to any one of the thirty-fourth to thirty-sixth states, wherein the multi-core fiber includes at least two cores, such that at least four different polarization modes are guided into a plurality of waveguides via a mode converter.

[0041] The thirty-eighth state of this disclosure includes a method according to any one of the thirty-fourth to thirty-seventh states, wherein one of the polarizing beam splitter and one of the plurality of mode converters extending therefrom comprises a combined cross-sectional area of ​​less than or equal to 250 µm2 for the mode converter.

[0042] It should be understood that the embodiments presented in the foregoing general description and the following embodiments are merely exemplary and are intended to provide an overview or framework for understanding the nature and features of the claimed item. Additional features and advantages will be set forth in the following embodiments and will be apparent in part from the description or by practice of the embodiments described in the written description and the accompanying drawings.

Implementation Method

[0057] Reference will now be made in detail to embodiments of an optical system including a multi-core optical fiber and an optical fiber coupler for coupling a plurality of cores of the multi-core optical fiber to an integrated photonic device. The optical fiber coupler includes a plurality of polarizing beam splitters, such that the polarizing beam splitters are arranged in a manner corresponding to a plurality of cores in the multi-core optical fiber, such that each polarizing beam splitter receives an optical signal from one of the plurality of cores. First and second mode converters extend from each polarizing beam splitter to waveguides that deliver different polarization modes of the optical signal from the cores to other components of the integrated photonic device. The polarizing beam splitters and the mode converters extending therefrom are designed to be compact to facilitate arrangement in an array manner corresponding to the core arrangement in the multi-core optical fiber. In embodiments, for example, the first and second mode converters associated with each polarizing beam splitter extend from adjacent first and second sides of the polarizing beam splitter to guide the optical signal to a first polarization mode of the waveguide in a first propagation direction and to guide the optical signal to a second polarization mode of the waveguide in a second propagation direction. In one embodiment, the first and second mode converters include lengths along the first and second propagation directions that are less than or equal to the lengths of the first and second sides of the polarizing beam splitter. In another embodiment, each polarizing beam splitter and the first and second mode converters extending therefrom include a combined cross-sectional area less than or equal to 250 µm². In another embodiment, the multi-core fiber is held relative to the multi-core fiber at a coupling angle such that each core of the multi-core fiber is coupled to one of the polarizing beam splitters. In another embodiment, the coupling angle causes the multi-core fiber to extend at a non-zero angle relative to the surface normal of the fiber coupler. In yet another embodiment, the coupling angle causes the multi-core fiber to extend perpendicular to the fiber coupler (e.g., the associated substrate).

[0058] The compactness of each polarizing beam splitter allows for maintaining polarization diversity when coupling light from a particular multi-core fiber to an integrated photonic device. For example, in an embodiment, a multi-core fiber having a minimum core-to-core spacing of less than or equal to 50 µm (e.g., less than or equal to 45 µm, less than or equal to 40 µm, less than or equal to 35 µm, less than or equal to 32 µm, less than or equal to 30 µm, less than or equal to 25 µm, less than or equal to 22 µm) can be coupled to an integrated photonic device by means of a grating coupler described herein, while still maintaining polarization diversity among all cores. That is, even when each core is separated from its adjacent cores in a plurality of cores by less than or equal to 45 µm, two different polarization modes of each of the plurality of cores of the multi-core fiber can be coupled to individual waveguides of the integrated photonic device. In light of this, the grating couplers and mode converters described herein facilitate coupling fibers with sizes (e.g., less than or equal to 125 µm) and relatively high core counts (e.g., greater than or equal to 2 cores, greater than or equal to 5 cores, greater than or equal to 7 cores, greater than or equal to 8 cores) associated with standard single-mode fibers, while still maintaining polarization diversity. The structures of the polarization splitter gratings and mode converters described herein are applicable to systems combining wavelength division multiplexing with spatial division multiplexing and polarization diversity, for high data rate or other space-constrained applications, such as high-density fiber arrays.

[0059] In this specification and the following claims, reference will be made to several terms, which shall be defined as having the following meanings:

[0060] As used herein, the term "about" means that quantities, sizes, formulations, parameters, and other quantities and characteristics are not and need not be accurate, but may be approximate and / or larger or smaller as needed, reflecting tolerances, conversion factors, rounding, measurement errors, and the like, and other factors known to those skilled in the art. When the term "about" is used to describe the values ​​or endpoints of a range, this disclosure should be understood to include the specific values ​​or endpoints referred to. Whether or not the values ​​or endpoints of a range in the specification are listed with "about," the values ​​or endpoints of a range are intended to include two embodiments: one modified by "about" and one not modified by "about." It will be further understood that each endpoint of a range is significant relative to and independent of the other endpoint.

[0061] A multi-core optical fiber, also known as a multi-core portion or "MC," for the purposes of this disclosure, is considered to comprise two or more cores disposed within a cladding matrix forming a common cladding. Each core can be considered to have a higher index core region surrounded by a lower index (refractive index) inner cladding. Each core may include a higher index core region surrounded by one or more lower index inner claddings disposed between each core and the cladding matrix of the common cladding.

[0062] As used herein, a multi-core optical fiber may comprise a plurality of cores, each of which may be defined as the ith-th core (i.e., the first or first, the second or second, the third or third, the fourth or fourth, etc.). The ith-th core may have an outer radius rCi. The ith-th core is positioned within the cladding matrix of the multi-core optical fiber, which defines the common cladding of the multi-core optical fiber. The common cladding contains an outer radius RCC.

[0063] As used herein, the term "adjacent core" refers to the core closest to the reference core. In embodiments, all cores of a multi-core fiber may be equidistant from each other. In other embodiments, cores may be unequally spaced from each other. In embodiments, some core portions will be spaced further from the reference core than adjacent core portions are spaced further from the reference core.

[0064] As used herein, the term “coupling efficiency” is measured and determined as the percentage of optical power transferred between two optical components and calculated as the output power (e.g., measured from a single waveguide) divided by the optical power from the input (e.g., in a specific core of a multi-core fiber).

[0065] As used herein, the term "crosstalk" (when not used to describe crosstalk in multi-core fiber coupling) refers to optical signal coupling that occurs between waveguides of an integrated photonic device, induced by non-ideals present in the fiber-to-chip grating coupler. As described herein, crosstalk coupled to a particular waveguide can be determined by determining the optical power output from an adjacent waveguide (e.g., a waveguide other than the one where crosstalk is being measured) and dividing this measured optical power by the optical power of the signal input to the multi-core fiber coupled to the waveguide where crosstalk is to be measured.

[0066] The mode field diameter (MFD) is measured by radial positioning at the point where the signal intensity drops to e-2 times the peak intensity in the multi-core fiber. Unless otherwise stated, "mode field diameter" or "MFD" refers to the mode field diameter at 1550 nm.

[0067] The directional terms used herein—for example, up, down, right, left, front, back, top, bottom—are for reference only and are not intended to imply absolute orientation.

[0068] Unless otherwise expressly stated, it is not intended to interpret any method described herein as requiring steps to be performed in a particular order, nor that the device requires any particular orientation. Accordingly, where a method request does not actually enumerate the order in which its steps should be followed, or where any device request does not actually enumerate the order or orientation of individual components described, or where the request or specification does not specifically state that the steps will be restricted to a particular order, or where a particular order or orientation of device components is not enumerated, it is not intended to infer any order or orientation in any way. This claim applies to any interpretation that may not be based on the expression of the following: logical questions, operational flows, order of components, or orientation of components related to the arrangement of steps; meanings derived simply and clearly from grammatical organization or punctuation; and numbers or types of embodiments described in the specification.

[0069] As used herein, unless the context explicitly specifies otherwise, the singular forms “a” (a), “an” (an), and “the” (the) encompass plural objects. Thus, by way of example, unless the context explicitly indicates otherwise, a reference to a “a” component includes a state having two or more such components.

[0070] FIG1A schematically depicts an optical transmission system 100. The optical transmission system 100 includes a transmitting optics 110, a receiving optics 115, and a multi-core optical fiber 105 extending between the transmitting optics 110 and the receiving optics 115. Depending on the background of the optical transmission system 100, the transmitting optics 110 and the receiving optics 115 may take various forms. In an embodiment, one or more of the transmitting optics 110 and the receiving optics 115 are integrated photonic devices, wherein at least some components form a photonic integrated circuit disposed on a common substrate. For example, in an embodiment, the transmitting optics 110 includes a first integrated photonic device and the receiving optics 115 includes a second integrated photonic device. In an embodiment, the transmitting optics 110 and the receiving optics 115 may be, for example, a system implemented at a central office (CO), front end, switching center, or the like. In an embodiment, the transmitting optics 110 and the receiving optics 115 may be implemented in a consumer premises equipment (CPE) or other device. The optical transmission system 100 can be used for inter-datacenter communication and intra-datacenter communication. The grating coupler described herein can be used for both pluggable optics and co-packaged optics. Although an example of communication at a wavelength of 1550 nm is described herein, the optical transmission system 100 can operate at other wavelengths (e.g., 1310 nm) consistent with this disclosure.

[0071] In the depicted embodiment, the transmission optical device 110 includes a plurality of transmitters 125 and a plurality of optical communication links 130 coupled to the plurality of transmitters 125. For example, a first transmitter 125-a may be configured to generate a first optical signal coupled to a first optical communication link 130-a. The transmission optical device 110 may include n transmitters 125 and optical communication links 130, enabling the generation of n different optical signals and their coupling to individual optical communication links among the plurality of optical communication links 130. In an embodiment, the plurality of optical communication links 130 includes a plurality of optical fibers (e.g., single-mode or multi-mode optical fibers) that deliver the plurality of optical signals from the plurality of transmitters 125 to a coupler 120. In an embodiment, the plurality of optical communication links 130 includes a plurality of waveguides that deliver the plurality of optical signals from the plurality of transmitters to the coupler 120. Depending on the implementation, each of the plurality of optical signals generated by the plurality of transmitters 125 may include two polarization modes propagated via optical communication link 130.

[0072] In an embodiment, the coupler 120 of the transmission optical device 110 is configured to couple one of a plurality of optical signals generated by a plurality of transmitters 125 to the core of the multi-core optical fiber 105. It should be understood that, in an embodiment, the structure of the coupler 120 may be similar to that of the coupler 135 described herein. That is, the polarizing beam splitter-based coupler described herein can operate in reverse order of the specific example described herein to couple a plurality of waveguides to the multi-core optical fiber 105.

[0073] Depending on the implementation, the multi-core optical fiber 105 may have various lengths. In embodiments, the optical transmission system 100 can be used for both short-distance optical communication systems (e.g., connections within a data center) and long-distance optical communication systems (e.g., connections between data centers, urban environments, submarine environments, such as transcontinental optical communication links). It will be understood that such applications can utilize multi-core optical fibers of different lengths.

[0074] In an embodiment, the multi-core fiber 105 includes a plurality of cores disposed within a cladding matrix forming a common cladding layer. Each core can be considered to have a higher index core region surrounded by a lower index inner cladding layer. Each core may include a higher index core region surrounded by one or more lower index inner cladding layers disposed between each core and the cladding matrix of the common cladding layer. In an embodiment, the coupler 120 includes fiber fan-in devices such as tapered fiber-based couplers, lens coupling systems, waveguide-based couplers, or grating-based couplers to couple optical signals propagating through a plurality of optical communication links 130. In an embodiment, the multi-core fiber 105 includes a plurality of cores corresponding to the number of transmitters in the plurality of transmitters 125, such that each optical signal generated is coupled to a second core of the multi-core fiber 105.

[0075] In an embodiment, the multi-core optical fiber 105 includes a configuration based on additional components of the optical transmission system 100. For example, components such as switches, detectors, and other components of the receiving optics 115 may be configured to operate together with an optical fiber having an outer diameter of 150 µm or less (e.g., 125 µm) and a mode field diameter less than or equal to 15 µm (e.g., 10 µm, 8 µm, etc.). Additionally, to maximize the data transmission rate, it may be advantageous if the multi-core optical fiber 105 includes at least two cores (e.g., more than or equal to three cores, more than or equal to four cores, more than or equal to five cores, more than or equal to six cores, more than or equal to seven cores, more than or equal to eight cores). Confining such a number of cores within the multi-core optical fiber 105 and operating within such size constraints causes the plurality of cores of the multi-core optical fiber 105 to be relatively close to each other. For example, in one embodiment, adjacent cores of the multi-core fiber 105 are separated from each other by a minimum core-to-core spacing of less than or equal to 50 µm (e.g., less than or equal to 45 µm, 40 µm, 35 µm, 32 µm, 30 µm, 25 µm, or 22 µm). Such a core-to-core spacing poses a challenge to coupling optical signals propagating through the multi-core fiber to the receiving optics 115.

[0076] In the depicted embodiment, the receiving optics 115 is an integrated photonic device, wherein a plurality of its components are disposed on a substrate 150. The receiving optics 115 includes a coupler 135, a plurality of optical communication links 145, and a plurality of receivers 140. In the embodiment described herein, the coupler 135 includes a grating coupler comprising a plurality of polarizing beam splitters arranged in a manner corresponding to the core arrangement in a multi-core fiber 105. The ends of the multi-core fiber 105 may be spaced apart relative to the substrate 150 such that each core of the multi-core fiber 105 is aligned with one of the polarizing beam splitters of the grating array, such that one of the optical signals generated by the transmission optics 110 is scattered by one of the polarizing beam splitters. As described herein, one of the plurality of optical signals may comprise multiple polarization modes. In this configuration, a polarizing beam splitter can direct light of a first polarized mode of an optical signal to a first propagation direction and direct light of a second polarized mode of an optical signal to a second propagation direction. The beam splitter array may include mode converters extending from both sides of each of the plurality of polarizing beam splitters to couple each polarization mode of the first and second polarization modes associated with the optical signal to one of the plurality of optical communication links 145. For example, the first optical communication link 145-a may receive the first polarization mode of the first optical signal generated by the transmission optics 110, and the second communication link 145-b may receive the second polarization mode of the second optical signal. In an embodiment, the transmission optics includes n optical communication links 145, where n corresponds to the number of optical signals generated by the transmission optics 110. In an embodiment, the number of optical communication links 145 receiving the optics 115 includes at least twice the number of polarizing beam splitters of the coupler 135. Although in the depicted embodiments, the transmitting optical device 110 and the receiving optical device 115 include the same number of optical communication links (i.e., the transmitting optical device 110 includes n optical communication links 130 and the receiving optical device 115 includes n optical communication links 145), it should be understood that embodiments in which the transmitting optical device 110 and the receiving optical device 115 include unequal numbers of optical communication links are also considered.

[0077] In an embodiment, a plurality of optical communication links 145 of the receiving optics 115 include a plurality of waveguides in a layer formed in (or disposed on) the substrate 150. For example, as described herein, the optical communication links 145 may include a plurality of waveguides formed in the same material layer as the plurality of polarizing beam splitters of the coupler 135. A plurality of receivers 140 are configured to receive and operate on optical signals propagated through the plurality of optical communication links 145. In an embodiment, for example, the plurality of receivers 140 are inputs to optical switches or the like for delivering the plurality of optical signals to different components of the receiving optics 115. In an embodiment, the plurality of receivers 140 convert the optical signals directed thereto into electrical signals for additional operation in photonic integrated circuitry.

[0078] Referring now to FIG1B, the interface between the multi-core optical fiber 105 and the coupler 135 of the optical transmission system of FIG1A is described in more detail according to an exemplary embodiment. In the depicted embodiment, the multi-core optical fiber 105 includes a plurality of cores 160 disposed in a common cladding layer 155. The plurality of cores 160 includes a first core 160-a, a second core 160-b, a third core 160-c, a fourth core 160-d, a fifth core 160-e, a sixth core 160-f, and a seventh core 160-g. The multi-core optical fiber 105 also includes a central axis 165 extending through a longitudinal centerline of the multi-core optical fiber 105. In the embodiment, the multi-core optical fiber 105 is spaced apart from the substrate 150 (e.g., by means of optical fiber units not depicted) such that the end 170 of the multi-core optical fiber 105 is disposed near the coupler 135. In the embodiment, the end 170 of the multi-core optical fiber 105 is secured to the substrate 150 by an adhesive. In one embodiment, the multi-core fiber 105 is positioned relative to the substrate 150 such that the central axis 165 extends at an angle to the surface normal 175 of the substrate 150. In this embodiment, this angle is configured to maximize the coupling efficiency between each of the plurality of cores 160 and one of the plurality of polarizing beam-splitting gratings 180 of the coupler 135. In this embodiment, the angle between the central axis 165 and the surface normal 175 of the substrate 150 is greater than or equal to 5° (e.g., greater than or equal to 7°, greater than or equal to 10°, greater than or equal to 12°) to maximize coupling efficiency and advantageously reduce backscattering of light back into the plurality of cores 160. The structure of the multi-core fiber 105 will be described in more detail herein with reference to FIG1E.

[0079] In the depicted embodiment, the end 170 of the multi-core fiber 105 is perpendicular to the central axis 165. It should be understood that, in this embodiment, the multi-core fiber 105 is cut at a non-zero slit angle to reduce reflections at the end 170 of the multi-core fiber 105. It should be understood that similar coupling results can be achieved by polishing the multi-core fiber at an angle so that the fiber end 170 extends substantially parallel to the substrate 150. Such end-face polishing of the multi-core fiber 105 can advantageously result in uniform coupling between the multiple cores and reduce backscattering into the multi-core fiber 105.

[0080] In the embodiment depicted in FIG1B, the coupler 135 is a grating coupler comprising a plurality of polarizing beam splitters 180. In this embodiment, the plurality of polarizing beam splitters 180 are arranged in such a manner that they correspond to a plurality of cores 160 in the multi-core fiber 105. For example, in this embodiment, the plurality of polarizing beam splitters 180 includes a first polarizing beam splitter 180-a, a second polarizing beam splitter 180-b, a third polarizing beam splitter 180-c, a fifth polarizing beam splitter 180-e, a sixth polarizing beam splitter 180-f, and a seventh polarizing beam splitter 180-g. That is, the coupler 135 comprises the same number of polarizing beam splitters as the number of cores in the multi-core fiber 105. The multi-core optical fiber 105 can be spaced apart from the substrate 150, such that one of the seven optical signals propagating through one of the multiple cores 160 is coupled to one pair of optical communication links 145 via a corresponding polarizing beam splitter among the multiple polarizing beam splitters 180. For example, a first pair of optical signals (with orthogonal polarization) propagating through the first core 160-a of the multi-core optical fiber 105 can be guided to the first polarizing beam splitter 180-a of the coupler 135, a second pair of optical signals propagating through the second core 160-b of the multi-core optical fiber 105 can be guided to the second polarizing beam splitter 180-b of the coupler 135, and so on. In an embodiment, the multiple polarizing beam splitters 180 include the same arrangement (e.g., the minimum grating-grating spacing between corresponding points on adjacent gratings can be the same as the minimum core-to-core spacing of the multiple cores 160).

[0081] As described herein, in embodiments, at least one of the plurality of cores 160 of the multi-core fiber 105 may have multiple optical signals propagating through two different polarization modes therethrough. For example, the plurality of optical signals propagating through the plurality of cores 160 may be decomposed into two orthogonal polarization modes: an LP01-x polarization mode and an LP01-y polarization mode. To independently couple the two modes associated with each of the plurality of optical signals to individual optical communication links in the plurality of optical communication links 145, each of the plurality of polarization beam splitters 180 includes an array of scattering elements designed to scatter each polarization mode in different directions. For example, in embodiments, each of the plurality of polarization beam splitters 180 includes a superimposed two-dimensional grating comprising two arrays of scattering elements arranged to guide each polarization mode of each optical signal in different propagation directions. The structure of the plurality of polarizing beam splitters 180 is described in more detail with reference to FIG1C.

[0082] As depicted in FIG1B, one pair of optical communication links 145 extends from each of the plurality of polarizing beam splitters 180. As described herein, the plurality of optical communication links 145 may include a plurality of waveguides formed on a substrate 150. The plurality of waveguides may be positioned to receive one of the orthogonal polarization modes scattered by each of the plurality of polarizing beam splitters 180. As described herein, a pair of waveguides extending from each of the plurality of polarizing beam splitters 180 may extend from a mode converter in contact with the side of each of the plurality of polarizing beam splitters. The mode converter typically serves as a spot size converter for focusing light from the optical signal scattered by the polarizing beam splitter 180, to be compatible with the propagation mode parameters associated with the plurality of waveguides. In this embodiment, waveguides are constructed based on the wavelength of the optical signal propagating through the multi-core optical fiber 105, i.e., the corresponding refractive index of the material used to construct the receiving optical device 115 (e.g., the layer forming the plurality of polarizing beam splitters 180 and the mode converter). In this embodiment, the plurality of polarizing beam splitters 180 are generally square, with their lengths generally corresponding to the mode field diameter of each of the plurality of cores 160 (e.g., greater than or equal to 8 µm, greater than or equal to 10 µm). The plurality of waveguides may include a nominal width of less than or equal to 5 µm (e.g., less than or equal to 500 nm) (e.g., in a plane parallel to the substrate 150). In this way, the mode converter can reduce the spot size associated with the optical signal by at least 10 times (e.g., 20 times) to facilitate coupling with the plurality of waveguides. The structure of the mode converter is described in more detail herein with reference to FIG1C.

[0083] In the depicted embodiment, coupler 135 includes seven polarizing beam splitters 180 and pairs of mode converters and optical communication links 145 extending from each of the plurality of polarizing beam splitters 180. Thus, the receiving optics includes 14 optical communication links 145 that are waveguides extending between each of the plurality of polarizing beam splitters 180 and a plurality of receivers 140. Each waveguide couples a polarization mode associated with one of a plurality of optical signals propagating through multi-core fiber 105 to the plurality of receivers 140. As described herein, the size of the various components of coupler 135 (e.g., the polarizing beam splitters 180 and the mode converters extending therefrom) helps to maintain polarization diversity in the optical signals once coupled to the receiving optics 115.

[0084] The delivery of the plurality of optical communication links 145 is also designed to maximize the coupling efficiency between the plurality of cores 160 and the plurality of receivers 140. In the depicted embodiment, for example, the plurality of optical communication links 145 do not intersect each other as they extend between the coupler 135 and the plurality of receivers 140 to reduce signal loss associated with signal interaction at waveguide intersections. In an embodiment, each of the plurality of optical communication links 145 is separated from each of all other optical communication links in the plurality of optical communication links 145 by at least a minimum waveguide spacing distance 185. In an embodiment, the minimum waveguide spacing distance 185 is greater than or equal to 500 nm (e.g., greater than or equal to 1 µm) to minimize crosstalk between polarization modes of the optical signal during the coupling of the multi-core fiber 105 to the plurality of receivers 140.

[0085] Due to the spacing between the plurality of cores 160 of the multi-core fiber 105, the plurality of polarizing beam splitters 180 are also relatively close to each other. Such close grating-to-grating spacing, coupled with the number of polarization modes coupled to the plurality of receivers 140, necessitates careful design of the delivery of the plurality of optical communication links 145 between the coupler 135 and the plurality of receivers 140. In the depicted embodiment, each of the plurality of optical communication links 145 includes at least one bend 195 in which its extension direction changes so as not to intersect with any other component of the receiving optics 115. In the embodiment, at least one bend 195 of each of the plurality of optical communication links includes a radius of curvature greater than or equal to 5 µm to reduce signal loss occurring at at least one bend 195. In this way, the compactness of the coupler 135 helps maintain the polarization diversity of the optical signals propagating through the multi-core fiber 105, which includes a relatively high core density, while enabling waveguide energy delivery to minimize signal loss. That is, the two polarization modes associated with each optical signal propagating through the multi-core fiber 105 can be coupled to the receiving optics 115 with relatively high coupling efficiency. In an embodiment, each polarization mode associated with one of the plurality of optical signals propagating through the multi-core fiber 105 is coupled to one of the plurality of optical communication links 145 with a coupling efficiency greater than or equal to -6.0 dB. The structure of the plurality of polarizing beam splitters 180 and the mode converters extending therefrom will now be described in more detail.

[0086] FIG1C depicts a close-up view of region 190 of the receiving optical device 115 depicted in FIG1B. The third polarizing beam splitter 180-c is depicted as a superimposed cross grating comprising a first plurality of scattering elements 200 and a second plurality of scattering elements 205. The first plurality of scattering elements 200 comprises an array of nanostructures (e.g., pillars, cavities, or the like) arranged in rows extending perpendicular to the first propagation direction 220. The first plurality of scattering elements 200 may be spaced apart from each other in the rows to scatter light at the wavelength of the optical signal propagating through the multi-core fiber 105 in the first propagation direction 220 toward a first side 210 of the third polarizing beam splitter 180-c. The second plurality of scattering elements 205 comprises an array of nanostructures (e.g., pillars, cavities, or the like) arranged in rows extending perpendicular to the second propagation direction 225. The second plurality of scattering elements 205 may be spaced apart from each other in the rows so as to scatter light at the wavelength of the optical signal propagating through the multi-core fiber 105 in the second propagation direction 225 toward the second side 215 of the third polarizing beam splitter 180-c. Therefore, both the LP01-x and LP01-y modes of the optical signal propagating through the third core 160-c of the multi-core fiber (see Figure 1B) may be scattered and propagated in two different propagation directions attributable to the interaction with the third polarizing beam splitter 180-c.

[0087] In an embodiment, the lengths of the first side 210 and the second side 215 of the third polarizing beam splitter 180-c may correspond to the mode field diameter of the third core 160-c of the multi-core fiber 105, which may be greater than or equal to 5µm (e.g., greater than or equal to 8µm, greater than or equal to 10µm). In an embodiment, the lengths of the first side 210 and the second side 215 are greater than the mode field diameter. A mode converter extends from the first side 210 and the second side 215 to readjust the magnitude of the scattered light so that it propagates downward along one of the plurality of optical signal communication links 145. For example, as shown, a first mode converter 235 extends from the first side 210 of the third polarizing beam splitter 180-c to couple light of a first polarized mode into a first optical communication link 145-a. The second mode converter 240 extends from the second side 215 of the third polarizing beam splitter 180-c to couple light of the second polarized mode to the second optical communication link 145-b. In the depicted embodiment, the first and second optical communication links 145-a and 145-b include waveguides (e.g., strip-loaded waveguides, channel waveguides, ribbed waveguides, or ridge waveguides) formed on the substrate 150. Depending on their thickness and the material constituting the waveguide, the waveguide may include a nominal width 230 (e.g., in a plane parallel to the substrate 150) less than or equal to 1 µm (e.g., less than or equal to 500 nm, less than or equal to 450 nm).

[0088] The first and second mode converters 235 and 240 readjust the size of the polarization modes scattered by the third polarizing beam splitter 180-c for coupling to the first and second optical communication links 145-a and 145-b. Existing grating couplers typically utilize tapered waveguide-based couplers with a length greater than 100 for such mode conversion. The size of such tapered waveguide-based couplers makes it difficult to maintain the polarization diversity of a particular multi-core fiber because the combination of the polarizing beam splitter and the tapered waveguide has too large a surface area coverage to be arranged for coupling to each core of the multi-core fiber. Accordingly, in this embodiment, the first and second mode converters 235 and 240 are iteratively designed using an objective function that satisfies constraints associated with coupling light to the waveguide (at the first and second sides 210 and 215 of the third polarizing beam splitter 180-c). Such constraints limit the size of the first and second mode converters 235 and 240, and the algorithm can output an arbitrary discrete dielectric constant distribution that, given the size constraints, can be converted to the spot size associated with the polarization mode used for coupling into the waveguide. In an embodiment, the first and second mode converters 235 and 240 are designed to have a width along the propagation direction through which the light propagates (i.e., along the first and second propagation directions 220 and 225, respectively) smaller than the mode field diameter of the plurality of cores 160 of the multi-core fiber (e.g., less than or equal to 25µm, less than or equal to 15µm, less than or equal to 10µm, less than or equal to 8µm). In an embodiment, the first and second mode converters 235 and 240 are designed to have a width in a direction perpendicular to the propagation direction of light (e.g., parallel to the first and second sides 210 and 215) that is less than or equal to 2.0 times the mode field diameter (e.g., less than or equal to 1.2 times the mode field diameter) of a plurality of cores of a multi-core optical fiber (e.g., less than or equal to 50µm, less than or equal to 30µm, less than or equal to 20µm). Given such a size constraint, an iterative technique can output a first discretized dielectric constant distribution for the first mode converter 235 and a second discretized dielectric constant distribution for the second mode converter 240 (for coupling each polarization mode to the first and second optical communication links 145-a and 145-b, respectively). Such a discretized dielectric constant distribution can be formed by lithography using a predetermined minimum feature size (e.g., 50nm, 100nm, 200nm).

[0089] In an embodiment, the combination of the third polarizing beam splitter 180-c, the first mode converter 235, and the second mode converter 240 includes a cross-sectional area of ​​less than or equal to 300 µm² (e.g., less than or equal to 250 µm², less than or equal to 225 µm², less than or equal to 200 µm²). Even when the plurality of cores 160 are relatively tightly packed (e.g., including a minimum core-to-core spacing of less than or equal to 45 µm), such a minimum coverage associated with each grating of the coupler 135 facilitates the arrangement of the plurality of polarizing beam splitters 180 in a manner corresponding to the arrangement of the cores 160 of the multi-core fiber 105. Such multi-polarization coupling for each core of the multi-core fiber 105 with such a tightly packed core has not been achieved using conventional tapered waveguide-based mode converters.

[0090] Within the coupler 135, the third polarizing beam splitter 180-c is further away from the other polarizing beam splitters of the plurality of polarizing beam splitters 180 (see FIG. 1B). That is, components of the coupler 135 (e.g., the second polarizing beam splitter 180-b and its associated mode converter and waveguide) are located between the third polarizing beam splitter 180-c and the plurality of receivers 140. In view of this, the first and second optical communication links 145-a and 145-b can be delivered indirectly to avoid intersection with such other components. As depicted in FIG. 1C, the first optical communication link 145-a includes a plurality of bends 245, wherein their extension direction is changed. At its first end 250, the first optical communication link 145-a extends generally parallel to the first propagation direction 220, while at its second portion 255, the first optical communication link 145-a extends in a direction extending at an angle greater than or equal to 90° relative to the first propagation direction 220. That is, by means of the curved waveguide path, the propagation direction of any polarization mode can be changed by more than 90 degrees within the coupler 135 to deliver light around various other components associated with the coupler 135 to avoid intersection and / or signal loss.

[0091] Referring now to FIG1D, a cross-sectional view of the third polarizing beam splitter 180-c through line II-II of FIG1C is schematically depicted. In an embodiment, the cross-sectional view depicted in FIG1D represents the structure of the entire receiving optics 115 described herein. For example, various layers depicted in FIG1D, such as substrate 150, may extend throughout and throughout the receiving optics 115. As depicted, the third polarizing beam splitter 180-c includes substrate 150, a buried oxide layer 260, and an upper layer 265. In an embodiment, substrate 150 includes a silicon substrate, silicon-on-insulator substrate, or any other suitable type of substrate commonly used in photonic integrated circuits. The buried oxide layer may be composed of a dielectric layer such as silicon dioxide. The upper layer 265 may be composed of silicon.

[0092] In the depicted embodiment, the first plurality of scattering elements 200 includes a plurality of cavities 270 formed in the upper layer 265. In the depicted embodiment, each cavity in the plurality of cavities 270 has the same depth. It should be understood that, in the embodiment, the first plurality of scattering elements 200 may include a plurality of cavities 270 with non-uniform depths. In the embodiment, the first and second plurality of scattering elements 200 and 202 may include different depths. In the embodiment, the first plurality of scattering elements 200 are formed by electron beam photolithography using a pattern of photolithography corresponding to a desired distribution of the plurality of cavities 270, followed by a reactive etching step (e.g., reactive ion etching) to remove a portion of the upper layer 265 to form the first plurality of scattering elements 200. In the embodiment, the plurality of cavities extend only partially through the upper layer 265. In the embodiment, the plurality of cavities extend through the entire thickness of the upper layer 265. Although the depicted embodiment implements the first plurality of scattering elements 200 as negative features etched into the upper layer 265, it should be understood that it is also contemplated that the first plurality of scattering elements 200 (and the second plurality of scattering elements 202) be implemented as positive features (e.g., pillars) disposed on top of the upper layer 265. In the embodiment, the third polarizing beam splitter 180-c (and each of the other plurality of polarizing beam splitters 180) includes a cladding layer (not depicted) disposed thereon. Each scattering element of the plurality of scattering elements 200 and 202 of each grating may be encapsulated in the cladding layer (e.g., a silicon oxide layer deposited by plasma-enhanced chemical vapor deposition) to provide appropriate refractive index contrast to facilitate coupling.

[0093] In an embodiment, additional components of the receiving optics 115 may be formed in a layer common to the plurality of polarizing beam-splitters 180 of the coupler 135. For example, in an embodiment, first and second mode converters 235 and 240 extending from each of the plurality of polarizing beam-splitters 180 and the plurality of optical communication links 145 may be formed within or on the upper layer 265 using the same lithography / etching sequence as the first plurality of scattering elements 200. For example, the first and second discretized dielectric constant distributions of the first and second mode converters 235 and 240 for coupling light to the waveguide may be achieved by etching a plurality of apertures in the upper layer 265 in a location determined by the optimization techniques described herein. The upper layer 265 may also be patterned using any suitable technique to form the plurality of optical communication links 145.

[0094] FIG1E depicts a cross-sectional view of the multi-core optical fiber 105 through line II depicted in FIG1B. As depicted, the multi-core optical fiber 105 includes a first core 160-a, a second core 160-b, a third core 160-c, a fourth core 160-d, a fifth core 160-e, a sixth core 160-f, and a seventh core 160-g disposed in a common cladding layer 155. The common cladding layer 155 includes an outer edge 290, which defines the common cladding layer radius Rcc (extending radially between the outer edge 290 and the center line 280 of each longitudinal axis of the multi-core optical fiber 105) and the outer diameter of the multi-core optical fiber 105. In an embodiment, Rcc is less than or equal to 100µm (e.g., less than or equal to 75µm, less than or equal to 65µm, less than or equal to 62.5µm) such that the outer diameter of the multi-core fiber 105 is less than or equal to 200µm (e.g., less than or equal to 150µm, less than or equal to 130µm, less than or equal to 125µm). In an embodiment, the edges of the plurality of cores 160 are spaced apart from the outer edge 290 by at least one minimum core edge to fiber edge distance 295. As depicted in FIG1E, the minimum core edge to fiber edge distance 295 is the minimum distance from a point along the outer circumference of the core to the nearest point along the circumference of the outer edge 290, as determined by a line segment in a plane perpendicular to the longitudinal axis centerline 280, between a point along the outer circumference of the core and the nearest point along the circumference on the outer edge 290. In an embodiment, the minimum core edge to fiber edge distance 295 is greater than or equal to 8 micrometers. In an embodiment, the minimum core edge to fiber edge distance 295 is greater than or equal to 12 micrometers. In this embodiment, the minimum core edge to fiber edge distance 295 is greater than or equal to 15 micrometers. Not intended to be bound by any particular theory, it is believed that the degree of signal loss due to tunneling effects depends on the minimum value of the minimum core edge to fiber edge distance 295.

[0095] In the depicted embodiment, a plurality of cores 160 are arranged in a hexagonal lattice, wherein a fourth core 160-d extends through the centerline 280 of each longitudinal axis. Each core of the plurality of cores 160, together with the fourth core 160-d and another core of the plurality of cores 160, forms an equilateral triangle such that each core comprises two adjacent cores separated from each other by a minimum core-to-core spacing distance 285. In the embodiment, the centerline of each core of the plurality of cores 160 is separated from the centerlines of the two adjacent cores by a minimum core-to-core spacing distance 285. In the embodiment, the minimum core-to-core spacing distance 285 is greater than or equal to 20 µm to contribute to relatively low crosstalk between the plurality of cores 260. In the embodiment, the minimum core-to-core spacing distance 285 is less than or equal to 45 µm (e.g., less than or equal to 40 µm, less than or equal to 35 µm, less than or equal to 30 µm, less than or equal to 25 µm, less than or equal to 22 µm). Such a low minimum core-to-core spacing helps to increase the data transmission rate by including a relatively large number of cores in the multi-core fiber 105. However, as described herein, difficulties also arise when coupling the multi-core fiber 105 to an integrated photonic device via a grating coupler.

[0096] In an embodiment, each of the plurality of cores 160 includes a relatively higher index core region surrounded by one or more relatively low index inner cladding layers. The refractive index distribution of each of the plurality of cores 160 can be designed to achieve a mode field diameter at a specific wavelength suitable for a particular application. In an embodiment, the mode field diameter of each of the plurality of cores 160 is greater than or equal to 5 µm (e.g., greater than or equal to 8 µm, greater than or equal to 10 µm) to facilitate coupling to standard single-mode fibers in existing optical interconnects. As described herein, the mode field diameter of each of the plurality of cores 160 may correspond to the size of a plurality of polarizing beam splitters 180 of coupler 135.

[0097] Although the embodiments described herein with reference to FIG1A to 1E include a multi-core fiber 105 with seven cores, such that 14 different polarization modes are coupled to 14 waveguides on the receiving optics 115, it should be understood that embodiments with different numbers and arrangements of cores are considered and are within the scope of this disclosure. For example, in one embodiment, the multi-core fiber 105 includes only three cores arranged at the vertices of an equilateral triangle (e.g., each core is separated from another core by the minimum core-to-core spacing 285 discussed above). The equilateral triangle may be centrally located within a common cladding layer 155 such that the centerline of each longitudinal axis extends through the center of the equilateral triangle. In another example, the multi-core fiber 105 includes four cores arranged in a 2×2 configuration, wherein each core is located at a corner of a square. In this example, each core may be separated from the other cores by two minimum core-to-core spacings 285. In another example, the multi-core fiber includes four cores arranged in a 1x4 configuration. In embodiments, the multi-core optical fiber 105 may include more than seven cores (e.g., eight cores arranged in a 2x4 configuration, each core being separated from two of the other cores by a minimum core-to-core spacing of 285). Embodiments in which the multi-core optical fiber 105 includes cores with unequal spacing (e.g., the closest cores may be separated by different distances) are also contemplated. The multi-core optical fiber 105 may contain any number of cores consistent with this disclosure (e.g., 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15).

[0098] In certain applications with relatively high data rate requirements, it may be advantageous to combine a relatively large number of cores in the multi-core fiber 105. For example, it may be advantageous to combine 5 or more cores (e.g., 6, 7, 8, 9, or 10 cores) in the multi-core fiber 105. It may also be advantageous to keep the outer diameter of the multi-core fiber 105 within certain requirements to facilitate coupling with other components of the optical transmission system 100. For example, in an embodiment, the multi-core fiber 105 includes an outer diameter of 125 µm (i.e., Rcc = 62.5 µm) to facilitate compatibility with standard single-mode fiber optic assemblies. In addition to other requirements (e.g., the spacing between core edges to fiber edges), such a large number of cores in an fiber of such size constrains the minimum core-to-core spacing between adjacent cores. As described herein, reducing the minimum core-to-core spacing can create difficulties in maintaining polarization diversity due to the size constraints associated with grating couplers.

[0099] The extremely miniaturized polarizing beam splitter and mode coupler described herein help maintain polarization diversity of multiple optical signals propagating through multi-core optical fibers with densely packed cores. For example, Figures 2A to 2C depict exemplary grating couplers 300, 305, and 310 configured to couple different multi-core optical fibers to an integrated photonic device. The circles in each of Figures 2A to 2C depict the arrangement of the multi-core fiber cores, and each grating coupler 300, 305, and 310 is designed to couple to an integrated photonic device. For example, the grating coupler 300 of Figure 3A is designed to couple a multi-core optical fiber comprising a plurality of cores 315 (e.g., first core 315-a, second core 315-b, third core 315-c, fourth core 315-d, fifth core 315-e, sixth core 315-f, seventh core 315-g, and eighth core 315-h) arranged in a 2×4 configuration, such that the center of each core is separated by a minimum core-to-core spacing distance 320, which is less than or equal to 30µm (e.g., 25µm).

[0100] The grating coupler 300 includes a plurality of polarizing beam splitters 325 arranged in a manner corresponding to a plurality of cores 315 (e.g., adjacent gratings in the plurality of polarizing beam splitters 325 are spaced apart from each other by a minimum core-to-core spacing distance corresponding to a minimum core-to-core spacing distance 320). The polarizing beam splitters 325 may be square and include a length greater than or equal to the mode field diameter of the plurality of cores 315. For example, in an embodiment, each core of the plurality of cores 315 includes a mode field diameter of 8µm, and the polarizing beam splitter 325 includes a square of 10µm × 10µm. The polarizing beam splitters 325 may structurally correspond to the plurality of polarizing beam splitters 180 described herein with reference to FIG1A to 1E, and scatter light from different polarization modes in different directions. The grating coupler 300 further includes a pair of mode converters 330 (e.g., similar to the first mode converter 235 and the second mode converter 240 described herein) extending from each polarizing beam splitter 325 to couple polarizing modes to a pair of waveguides 335 extending from the pair of mode converters 330. In embodiments, the mode converters are iteratively configured to have arbitrary dielectric constant distributions within a given predetermined size constraint. For example, in an embodiment, each of the pair of mode converters 330 includes a width of 5 µm in the propagation direction of light propagating therethrough (e.g., each mode converter may include a rectangular shape of 10 µm × 5 µm). Thus, the combination of one polarizing beam splitter 325 and the pair of mode converters 330 includes a combined cross-sectional area of ​​200 µm². As depicted, this compactness facilitates coupling 16 waveguides to different polarizing modes propagating through a plurality of cores 315 while still maintaining a minimum waveguide-to-waveguide spacing 340 to avoid significant crosstalk. In the embodiment, the minimum waveguide-to-waveguide spacing distance 340 is greater than or equal to 1µm.

[0101] Figure 2B depicts another grating coupler 305, which is designed to couple a plurality of cores 345 (first core 345-a, second core 345-b, third core 345-c, fourth core 345-d, fifth core 345-e, sixth core 345-f, and seventh core 345-g) to an integrated photonic device. The plurality of cores 345 are arranged in a hexagonal lattice arrangement such that each core of the plurality of cores 345 is separated from two other cores by a minimum core-to-core spacing distance 350 (e.g., 25µm) less than or equal to 30µm. The grating coupler 305, as described with reference to FIG2A as the grating coupler 300, includes a plurality of polarizing beam splitters 325, having a pair of mode converters 330 extending from the side of each polarizing beam splitter 325 to couple two polarization modes from one of the plurality of cores to a pair of waveguides 335 extending from the pair of mode converters 330. Although the minimum core-to-core spacing 350 of 25µm is relatively low, polarization diversity is still maintained when seven cores are coupled to an integrated photonic device.

[0102] Figure 2C depicts another grating coupler 310, designed to couple a plurality of cores 355 (first core 355-a, second core 355-b, third core 355-c, fourth core 355-d, fifth core 355-e, sixth core 355-f, seventh core 355-g, and eighth core 355-h) to an integrated photonic device. The plurality of cores 355 are arranged in a 2×4 configuration, such that the center of each core is separated by a minimum core-to-core spacing distance 360 ​​of less than or equal to 25µm (e.g., 22µm). That is, in Figure 2C, the multi-core fiber includes a core arrangement similar to the example in Figure 2A, but the minimum core-to-core spacing distance is 3µm less (reduced from 25µm to 22µm). The grating coupler 310, as described with reference to the grating coupler 300 in FIG2A, includes a plurality of polarizing beam splitters 325, having a pair of mode converters 330 extending from the side of each polarizing beam splitter 325 to couple two polarization modes from one of the plurality of cores to a pair of waveguides 335 extending from the pair of mode converters 330. Although the minimum core-to-core spacing 350 of 22µm is relatively low, polarization diversity is still maintained when eight cores are coupled to an integrated photonic device.

[0103] A notable difference between the exemplary grating couplers 300 and 310 depicted in Figures 2A and 2C is the orientation of the polarizing beam splitter 325 and the pair of mode converters 330. In Figure 2A, the final propagation direction of the coupled light in the waveguide 335 outside the grating coupler 300 is downward (e.g., the negative y-direction depicted in Figure 2A), perpendicular to the length direction of the rows of the grating array. The corners 365 of each polarizing beam splitter 325 extending between the pair of mode converters 330 point downward, parallel to the direction of the light guided by the waveguide outside the grating coupler 300.

[0104] In FIG. 2C, the relative orientation of the polarizing beam splitter 325 is adjusted to compensate for the reduced grating spacing. As depicted in FIG. 2C, the final propagation direction of the coupled light in the waveguide 335 outside the grating coupler 310 is lateral (e.g., the negative x direction depicted in FIG. 2C), parallel to the length direction of the rows of the grating array. The polarizing beam splitter 325 is rotated relative to the orientation depicted in FIG. 2A, so that corner 365 no longer points downwards, but points to the side closer to the final propagation direction of the light coupled to the waveguide. Due to this rotation of the polarizing beam splitter 325, its side containing the mode converter is no longer adjacent to the side of other polarizing beam splitters 325 containing the mode converter, thereby providing more space between the gratings for the arrangement of waveguides.

[0105] In this way, various different combinations of core arrangements can be accommodated by carefully selecting the orientation of the polarizing beam splitter 325 relative to the final direction in which light is guided on the integrated photonic device. In the embodiments described herein, the polarizing beam splitter 325 has a common orientation, and the paired mode converters 330 extend from the same side of each polarizing beam splitter 325. It should be understood, and contemplated, that the polarizing beam splitters in the grating coupler may have different orientations (e.g., such that corner 365 points in different directions) and / or that the mode converters 330 extend from the grating to different sides of the grating are also considered and within the scope of this disclosure, wherein it should be understood that in such embodiments, the polarizing beam splitter may be individually designed depending on its orientation relative to the desired propagation direction.

[0106] An example grating coupler is manufactured to couple 14 polarization modes associated with 7 cores of a multi-core fiber, which has the structure of the multi-core fiber 105 depicted in Figure 1E. The plurality of cores 160 have a mode field diameter of 8 µm at 1550 nm, and a minimum core-to-core spacing of 32 µm. The outer diameter of the multi-core fiber is 125 µm.

[0107] A grating coupler 400 (as depicted in FIG3A) is fabricated on a silicon-on-insulator substrate. The grating coupler 400 includes a buried oxide layer (e.g., corresponding to the buried oxide layer 260 depicted in FIG1D) having a thickness of 3µm and a silicon layer (e.g., corresponding to the upper layer 265 depicted in FIG1D having a thickness of 250nm). The silicon layer is patterned by electron beam lithography, with a minimum feature size of 100nm. Two lithography steps can be used (full etching of the silicon layer or partial etching of 120nm). Reactive ion etching is performed to form a polarizing beam splitter grating and scattering elements of the waveguide. The grating is partially etched through the silicon layer, while the mode converter and waveguide are formed by etching the full thickness of the silicon layer. A waveguide with a nominal width of 450nm is formed in the lithography and etching steps. After etching, the wafer is covered with a 1µm silicon dioxide layer by plasma-enhanced chemical vapor deposition.

[0108] Figure 3A depicts the fabricated grating coupler 400. The grating coupler 400 includes seven two-dimensional gratings 405, which are generally square in shape and 10µm in length. Figure 3B depicts a close-up view of one of the two-dimensional gratings 405. The two-dimensional gratings 405 are designed using a finite-difference time-domain method. The gratings are designed for a 10° coupling angle. Each superimposed grating of the two-dimensional gratings 405 is orthogonal and is guided with directional light of different polarization modes in a direction extending approximately 83.1° relative to each other to bear the 10° coupling angle. As depicted in Figures 3A and 3B, the grating coupler 400 includes a mode converter 410 extending from the adjacent sides of the two two-dimensional gratings of each two-dimensional grating 405. The mode converter 410 connects each output of one of the two-dimensional gratings 405 to a 450nm silicon waveguide 415. On an area of ​​only 10µm x 5µm, a gradient-based optimization software (called Stanford Photonic Inverse Design Software (SPINS)) is used to design mode converters 410. Thus, the combined cross-sectional area of ​​each 2D grating 405 and its corresponding pair of mode converters 410 is 200µm². As depicted in Figure 3B, each mode converter 410 includes a plurality of vias 412 extending into the silicon layer, representing a discretized dielectric constant distribution. The vias 412 typically extend in a plurality of curved profiles, each having a tapered shape that bends toward the waveguide 415 as the distance from one of the 2D gratings 405 increases.

[0109] The test apparatus 420 depicted in Figure 3C is used to model and test the grating coupler 400 depicted in Figures 3A and 3B. A multi-core fiber 425 is spaced apart from the grating coupler such that each two-dimensional grating 405 is aligned with the core of the multi-core fiber 425. Optical signals from each core are coupled to one of its waveguides 415 via one of the two-dimensional gratings 405. A plurality of one-dimensional gratings 430 are arranged at the ends of each waveguide 415 to couple light into a plurality of single-mode fibers 435 for measurement.

[0110] A finite-difference time-domain ("FDTD") simulation was performed on the test setup 420. Figure 3D plots 440 depict the coupling efficiency curves for each mode coupled to one of the single-mode optical fibers 435. The FDTD simulation coupling efficiency for each mode is -3.8 dB with a bandwidth of 57 nm. The mode converter 410 is simulated with an insertion loss of -0.5 dB, which is relatively low considering their extremely small coverage area. In the embodiment, the grating coupler described herein has a 3 dB coupling efficiency bandwidth of greater than or equal to 50 nm around 1550 nm.

[0111] Light from a tunable continuous laser source is coupled into one of the single-mode optical fibers 435, coupled to one of the waveguides 415 via one of the multiple one-dimensional gratings 430, and coupled to one of the cores of the multi-core optical fiber 425 via one of the two-dimensional gratings 405 of the grating coupler 400. Physical testing of the grating coupler 400 is performed by coupling light from the multi-core optical fiber 425 to one of the cores of the grating coupler 400. A detector collects light from the multi-core optical fiber 425 to determine the coupling efficiency of the grating coupler 400 for each polarization mode. The single-mode optical fiber is aligned with one core of the multi-core optical fiber 425 to guide the light to a detector, which includes a power meter and an optical spectrometer. The measured coupling efficiency and inter-mode crosstalk between the fourth core (e.g., corresponding to the fourth core 160-d in Figure 1E) and the sixth core (e.g., corresponding to the sixth core 160-g in Figure 1E) are depicted in the coupling efficiency plot 444 of Figure 3E. As shown in the figure, the grating coupler 400 achieves a 3dB bandwidth of 48nm, ranging from 1505nm to 1553nm. The highest coupling efficiency of the second core 160-b is -4.4dB, while the lowest coupling efficiency of the first core 160-a is -6.0dB. These differences can be explained by the non-uniform spacing between the core and the wafer surface, as the fiber is not split at a specific angle. On the other hand, the results show that the maximum polarization-dependent loss is only 0.4dB, which can be attributed to a small amount of asymmetry in the fabrication device and the different bending radii of the feed waveguide. The maximum crosstalk measured at 1509nm is -36.4dB, which is -28.4dB lower than the coupling efficiency at this wavelength.

[0112] As is evident from the above description, fiber optic couplers can be used to couple multiple cores of a multi-core optical fiber to an integrated photonic device. The fiber optic coupler includes multiple polarizing beam splitters and multiple mode converters extending from multiple sides of each polarizing beam splitter. Topology optimization techniques can be used to design the polarizing beam splitters and mode converters to achieve compactness and compatibility with multi-core optical fibers having tightly packed core structures. The compactness of the grating coupler assembly described herein helps maintain polarization diversity while still maintaining relatively high coupling efficiency and low crosstalk when coupling multi-core optical fibers to an integrated photonic device. The grating couplers described herein may be suitable for advanced communication systems where wavelength division multiplexing is combined with spatial division multiplexing and polarization diversity to achieve high data rates.

[0113] It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Therefore, this specification is intended to cover modifications and variations to the various embodiments described herein, provided that such modifications and variations fall within the scope of the appended claims and their equivalents. [Simplified Explanation of the Diagram]

[0043] This specification includes accompanying drawings to provide further understanding, and these accompanying drawings constitute a part of this specification. The drawings are illustrations of selected embodiments of this disclosure and, together with the description, serve to illustrate the principles and operation of the methods, products, and compositions contained in this disclosure, wherein:

[0044] FIG1A schematically depicts an optical transmission system according to one or more embodiments described herein, including a multi-core optical fiber and a grating coupler for transmitting a plurality of optical signals between a first integrated photonic device and a second integrated photonic device;

[0045] FIG1B schematically depicts the multi-core fiber and grating coupler of the optical transmission system depicted in FIG1A according to one or more embodiments described herein;

[0046] FIG1C schematically depicts the polarizing beam splitter of the grating coupler depicted in FIG1B and a pair of mode converters extending therefrom, according to one or more embodiments described herein;

[0047] FIG1D schematically depicts a cross-sectional view of a portion of the grating coupler depicted in FIG1C according to one or more embodiments described herein;

[0048] FIG1E schematically depicts a cross-sectional view of the multi-core optical fiber depicted in FIG1B according to one or more embodiments described herein;

[0049] FIG2A schematically depicts a grating coupler according to one or more embodiments described herein, comprising a plurality of polarizing beam splitters arranged in a manner corresponding to a first plurality of cores of up to a plurality of optical fibers;

[0050] FIG2B schematically depicts a grating coupler according to one or more embodiments described herein, comprising a plurality of polarizing beam splitters arranged in a manner corresponding to a second plurality of cores of up to a multi-core optical fiber;

[0051] FIG2C schematically depicts a grating coupler according to one or more embodiments described herein, comprising a plurality of polarizing beam splitters arranged in such a manner corresponding to a third plurality of cores of up to a multi-core optical fiber.

[0052] (FIC in the original text) Figure 3A depicts an image of a grating coupler according to one or more embodiments described herein, the grating coupler comprising a plurality of polarizing beam splitters arranged in a manner corresponding to a 7-core multi-core optical fiber having a minimum core-to-core spacing distance of 32µm.

[0053] FIG3B depicts an image of one of the polarizing gratings of the grating coupler depicted in FIG3A, according to one or more embodiments described herein;

[0054] FIG3C depicts an image of a photonic integrated device (integrated photonic or photonic integrated) and a multi-core optical fiber used to test the grating coupler depicted in FIG3A, according to one or more embodiments described herein;

[0055] Figure 3D depicts a plot of simulated coupling efficiency versus wavelength achieved during testing of the grating coupler depicted in Figure 3A using the integrated photonic device depicted in Figure 3C, according to one or more embodiments described herein; and

[0056] FIG3E depicts the coupling efficiency curves of 14 polarization modes of a multi-core optical fiber depicted in FIG3A and FIG3C, respectively, using one or more embodiments described herein, according to the grating coupler depicted in FIG3A and the crosstalk curves between the two cores, according to one or more embodiments described herein. [Biomaterial Storage]

[0115] Domestic storage information (please note in order of storage institution, date, and number): None. International storage information (please note in order of storage country, institution, date, and number): None.

Claims

1. An optical device comprising: A multi-core optical fiber consists of multiple cores; And an optical fiber coupler, comprising: a grating array including a plurality of polarizing beam splitters disposed on a substrate, the plurality of polarizing beam splitters being arranged in a manner corresponding to the plurality of cores in the multi-core optical fiber to receive an optical signal from one of the plurality of optical fibers, wherein the number of polarizing beam splitters included in the plurality of polarizing beam splitters is less than or equal to the number of cores in the multi-core optical fiber; and a first mode converter extending from a first side of each of the plurality of polarizing beam splitters to receive a first polarization mode of the optical signal scattered by the polarizing beam splitters; A second mode converter extends from a second side of each of the plurality of polarizing beam splitters to receive a second polarization mode of the optical signal scattered by the polarizing beam splitters; and a plurality of waveguides extend from the end of each mode converter of the mode converter to guide a single polarization mode of one of the optical signals, wherein both the first mode converter and the second mode converter include a plurality of nanostructures extending along curved profiles toward one of the plurality of waveguides extending from the first mode converter and the second mode converter, respectively.

2. The optical device as claimed in claim 1, wherein adjacent cores of the plurality of cores are separated from each other by a minimum core-to-core spacing of less than or equal to 45 µm.

3. The optical device as claimed in claim 1, wherein the multi-core optical fiber includes a total diameter of less than or equal to 125µm.

4. The optical device as claimed in claim 1, wherein the multi-core optical fiber comprises at least two cores such that four polarization modes are guided by a plurality of waveguides extending from the ends of each mode converter.

5. The optical device as claimed in claim 1, wherein the multi-core optical fiber comprises at least 5 cores, such that 10 polarization modes are guided by the plurality of waveguides extending from the ends of each mode converter.

6. The optical device as claimed in claim 1, wherein adjacent cores of the plurality of cores are separated from each other by a minimum core-to-core spacing of less than or equal to 35 µm.

7. The optical device as claimed in claim 1, wherein the first and second sides of the plurality of polarizing beam splitters are at least as long as the mode field diameter of each of the plurality of cores.

8. The optical device as claimed in claim 7, wherein the mode field diameter is greater than or equal to 5µm and less than or equal to 25µm.

9. The optical device as claimed in claim 7, wherein the first and second mode converters extending from each of the plurality of polarizing beam splitters include a dimension in a direction perpendicular to the propagation direction of the first and second polarizing modes, which is less than or equal to 1.2 times the diameter of the mode field.

10. The optical device as claimed in claim 9, wherein the first and second mode converters extending from each of the plurality of polarizing beam splitters include a dimension in the propagation direction of the first and second polarizing modes that is less than or equal to the mode field diameter.

11. The optical device as claimed in claim 1, wherein each polarizing beam splitter and the first and second mode converters extending therefrom comprise a combined cross-sectional area of ​​less than or equal to 250 µm².

12. The optical device as claimed in claim 1, wherein the centers of adjacent polarizing beam splitters of the grating array are separated from each other by a minimum grating spacing distance corresponding to a spacing of the plurality of cores.

13. The optical device as claimed in claim 1, wherein each of the plurality of waveguides is separated from the other waveguides in the plurality of waveguides by a minimum waveguide spacing distance of at least 500 nm.

14. The optical device as claimed in claim 1, wherein the plurality of waveguides are bent such that the plurality of waveguides extend beyond the grating array.

15. The optical device as claimed in claim 1, wherein at least one of the plurality of waveguides is bent such that, outside the grating array, an extension direction of the at least one waveguide is changed by at least 90° at a mode converter end relative to the original extension direction.

16. The optical device as claimed in claim 1, wherein, At least one of the plurality of waveguides extends between two polarizing beam splitters within the grating array.

17. The optical device as claimed in claim 1, wherein each polarization mode of the optical signal is coupled to one of the plurality of waveguides with a coupling efficiency greater than or equal to -6.0 dB.

18. The optical device as claimed in claim 1, wherein each polarization mode of the optical signal is coupled to one of the plurality of waveguides with a coupling efficiency greater than or equal to -10 dB and less than or equal to -0.5 dB.

19. The optical apparatus as claimed in claim 1, wherein the fiber couplers couple two polarization modes of each optical signal in the plurality of waveguides with a coupling efficiency bandwidth of greater than or equal to 50 nm around 1550 nm.

20. The optical device as claimed in claim 1, wherein the first mode converter and the first mode converter are substantially rectangular.

21. The optical device as claimed in claim 1, wherein the plurality of nanostructures include a minimum feature size of 100 nm.

22. An optical device for coupling a multi-core optical fiber to a photonic device, comprising: One substrate; A polarizing beam splitter is disposed on the substrate. The polarizing beam splitter guides light of a first polarized mode in a first propagation direction toward a first side of the polarizing beam splitter and guides light of a second polarized mode in a second propagation direction toward a second side of the polarizing beam splitter, wherein the lengths of the first and second sides are greater than or equal to 10µm and less than or equal to 15µm; and first and second mode converters extending from the first and second sides of the polarizing beam splitter, the first and second mode converters guiding light of a second polarized mode in the first and second propagation directions toward a second side of the polarizing beam splitter. The first and second polarization modes are respectively guided into the first and second waveguides extending from the first and second mode converters, wherein the lengths of the first and second mode converters along the first and second propagation directions are less than or equal to the lengths of the first and second sides of the polarization split grating, wherein each of the first and second mode converters includes a plurality of nanostructures extending along curved profiles toward one of the plurality of waveguides extending from the first and second mode converters.

23. The optical device as claimed in claim 22, wherein the polarizing beam splitter, the first mode converter, and the second mode converter comprise a combined cross-sectional area of ​​less than or equal to 250 µm².

24. The optical device as claimed in claim 23, wherein the first and second sides of the polarizing beam splitter include a length greater than or equal to 10 µm.

25. The optical device as claimed in claim 22, wherein the first and second mode converters include lengths greater than or equal to 3 µm and less than or equal to 10 µm along the first and second propagation directions.

26. The optical device as claimed in claim 22, wherein the first and second mode converters direct the first and second polarization modes to the first and second waveguides, having an insertion loss of greater than or equal to -0.5 dB.

27. The optical apparatus of claim 22, wherein the polarizing beam splitter is an assembly of a grating array comprising a plurality of polarizing beam splitters, the plurality of polarizing beam splitters being arranged in a manner corresponding to a plurality of cores in the multi-core optical fiber, such that each of the plurality of polarizing beam splitters receives an optical signal from one of the cores.

28. The optical device as claimed in claim 27, further comprising a pair of mode converters extending from the first and second sides of each of the plurality of polarizing beam splitters, each mode converter of the pair of mode converters directing a polarization mode of one of the optical signals to an end of each mode converter of the pair of mode converters.

29. The optical device as claimed in claim 28, wherein the centers of adjacent polarizing beam splitters of the grating array are separated from each other by a minimum grating spacing distance of less than or equal to 50 µm.

30. The optical apparatus of claim 27 further includes holding the multi-core optical fiber above the substrate at a distance of less than 50 μm, such that one of the plurality of cores is aligned with one of the plurality of polarizing beam splitters of the grating array.

31. The optical device as claimed in claim 30, wherein the plurality of cores in the multi-core optical fiber includes a minimum core-to-core spacing of less than or equal to 45 µm.

32. The optical device as claimed in claim 31, wherein adjacent cores of the plurality of cores in the multi-core optical fiber are separated from each other by a minimum core-to-core spacing distance greater than or equal to 20µm and less than or equal to 25µm.

33. The optical device as claimed in claim 30, wherein the multi-core optical fiber includes a total diameter of less than or equal to 125 µm.

34. A method for coupling light from a multi-core optical fiber to a photonic integrated circuit, the method comprising the steps of: transmitting a plurality of optical signals from a plurality of cores of the multi-core optical fiber to a grating array of the photonic integrated circuit, the grating array including a plurality of polarizing beam-splitters disposed on a substrate, the plurality of polarizing beam-splitters being arranged in a manner corresponding to the plurality of cores in the multi-core optical fiber to receive one of the optical signals; scattering light from each of the plurality of optical signals using one of the polarizing beam-splitters, such that a first polarization mode and a second polarization mode of the plurality of optical signals are respectively guided in a first and a second propagation direction; and individually guiding each polarization mode of the first and second polarization modes into the plurality of waveguides using a mode converter extending from each of the plurality of polarizing beam-splitters, wherein: The multi-core fiber includes at least five cores, such that at least 10 different polarization modes are guided into the plurality of waveguides via the plurality of mode converters; and adjacent cores in the plurality of cores are separated by a core-to-core spacing of less than or equal to 45µm, wherein the mode converters extending from each of the plurality of polarization beam splitters include a plurality of nanostructures extending along curved profiles toward a respective waveguide of the plurality of waveguides extending from the mode converters.

35. The method as described in claim 34, wherein the multi-core optical fiber comprises a total diameter of less than or equal to 125 µm.

36. The method as described in claim 34, wherein the minimum spacing between cores is less than or equal to 35µm.

37. The method as described in claim 34, wherein the multi-core optical fiber comprises at least two cores, such that at least four different polarization modes are guided to the plurality of waveguides via the mode converters.

38. The method as described in claim 34, wherein one polarizing beam splitter of the polarizing beam splitter and a pair of mode converters of the plurality of mode converters extending therefrom comprise a combined cross-sectional area of ​​less than or equal to 250 µm².

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