Module division multiplexing using combined degenerate modes

Abstract

An optical communication system may include a multiplexer / demultiplexer. The multiplexer / demultiplexer may transmit a first optical data signal between a first propagation mode of a first single-mode fiber and a few-mode fiber. The first propagation mode may have a first effective refractive index. The multiplexer / demultiplexer may transmit a second optical data signal between a second single-mode fiber and a combination of a second propagation mode of the few-mode fiber and a third propagation mode of the few-mode fiber. The second propagation mode and the third propagation mode may have the same effective refractive index different from the first effective refractive index. During propagation within the few-mode fiber, the second optical data signal may be bi-directionally coupled between the second propagation mode and the third propagation mode while substantially isolated from the first optical data signal in the first propagation mode.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of priority under 35 U.S.C. §119 to U.S. Provisional Application No. 63 / 398,251, filed on August 16, 2022, the contents of which are hereby incorporated by reference in their entirety. Technical Field

[0003] The present disclosure relates generally to the multiplexing and demultiplexing of signals in optical fiber, such as by mode division multiplexing. Background Art

[0004] Communication systems may use multiplexing techniques to combine multiple optical signals onto a single optical fiber. Efforts are continually being made to improve multiplexing techniques. Summary of the invention

[0005] In an example, an optical communication system may include a multiplexer / demultiplexer configured to transmit a first optical data signal between a first single-mode optical fiber and a first propagation mode of a few-mode optical fiber, the first propagation mode having a first effective refractive index; and transmit a second optical data signal between a second single-mode optical fiber and a combination of a second propagation mode of the few-mode optical fiber and a third propagation mode of the few-mode optical fiber, the second propagation mode and the third propagation mode having the same effective refractive index different from the first effective refractive index.

[0006] In an example, a method for operating an optical communication system may include: coupling a first optical data signal between a first single-mode optical fiber and a first propagation mode of a few-mode optical fiber using a multiplexer / demultiplexer, the first propagation mode having a first effective refractive index; and coupling a second optical data signal between a second single-mode optical fiber and a combination of a second propagation mode of the few-mode optical fiber and a third propagation mode of the few-mode optical fiber using the multiplexer / demultiplexer, the second propagation mode and the third propagation mode having the same effective refractive index different from the first effective refractive index.

[0007] In an example, an optical communication system may include: a multiplexer configured to: transmit a first optical data signal from a first single-mode optical fiber in a first single-mode optical fiber array to a first propagation mode of the few-mode optical fiber, the first propagation mode having a first effective refractive index; and transmit a second optical data signal from a second single-mode optical fiber in the first single-mode optical fiber array to at least one of a second propagation mode of the few-mode optical fiber and a third propagation mode of the few-mode optical fiber, the second propagation mode and the third propagation mode having the same effective refractive index different from the first effective refractive index; and a demultiplexer configured to: transmit the first optical data signal from the first propagation mode of the few-mode optical fiber to a third single-mode optical fiber in a second single-mode optical fiber array; and transmit the second optical data signal from a combination of the second propagation mode of the few-mode optical fiber and the third propagation mode of the few-mode optical fiber to a fourth single-mode optical fiber in the second single-mode optical fiber array. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 A block diagram of an example of an optical communication system is shown.

[0009] Figure 2 Shown for Figure 1 Side view of an example of a configuration of one or more Metasurface elements of a multiplexer / demultiplexer.

[0010] Figures 3 to 7 Shown for Figure 1 Side views of various examples of multiplexer / demultiplexer configurations.

[0011] Figure 8 A flow chart illustrating an example of a method for operating an optical communication system is shown.

[0012] Throughout the several views, corresponding reference numerals indicate corresponding parts. The elements in the drawings are not necessarily drawn to scale. The configurations shown in the drawings are merely examples and should not be considered limiting in any way. DETAILED DESCRIPTION

[0013] Optical fibers can be designed to confine and guide light within them. For example, a rotationally symmetric optical fiber can have a cross-section that includes a circular core surrounded by a circular cladding of a lower refractive index. In a simple ray optics picture, total internal reflection can occur at the core-cladding interface for a specific range of propagation angles. The use of total internal reflection and materials with low absorption and low scattering can allow light to travel through long lengths of optical fiber. Optical fibers can support a variety of spatial modes, which are eigenstates of the wave equation for a given translationally invariant refractive index profile. In general, when the core diameter increases or the refractive index difference between the core and the cladding increases, the number of propagation modes supported by the optical fiber also increases. When the core diameter or the refractive index difference decreases beyond a specified threshold diameter (which varies with the wavelength of the light and the refractive index of the core and cladding), the optical fiber can only support a single propagation spatial mode (with two polarization states). An optical fiber that only supports a single propagation mode is called a single-mode optical fiber.

[0014] There is a class of optical fibers called few-mode fibers that support more than a single propagation mode. For example, few-mode fibers may support two propagation modes, three propagation modes, four propagation modes, five propagation modes, six propagation modes, ten or fewer propagation modes, twenty or fewer propagation modes, or another suitable finite number of propagation modes.

[0015] The refractive index profile in a rotationally symmetric circular optical fiber depends only on the radial coordinate. In a step-index optical fiber, both the core and the cladding have a constant refractive index (where the core has a higher refractive index than the cladding). Some optical fibers have a parabolic-shaped refractive index profile in which the refractive index decreases quadratically with the radial coordinate. Other optical fibers may have more complex refractive index profiles, such as, for example, a torus-shaped core surrounded by a cladding (e.g., low-refractive index material is disposed inside the torus-shaped core and outside the torus-shaped core). Another example of a few-mode optical fiber may have a cross-section taken in a plane orthogonal to the longitudinal axis of the optical fiber, which includes a core that is elongated along a particular direction, such as an elliptical-shaped core. Other few-mode optical fiber geometries may also be used.

[0016] Each propagation mode supported by a few-mode fiber has three properties. The first property is a spatially varying intensity distribution (or equivalently, an electric field distribution and / or a magnetic field distribution). In some examples, the spatially varying transverse intensity distribution has an intensity that peaks at the core and decreases as the distance away from the core increases. The second property is a spatially varying polarization distribution, such as the direction of the electric field and / or magnetic field throughout the cross section of the optical fiber. In some examples, the polarization distribution varies by mode. The third property is the effective refractive index of the propagation mode. The effective refractive index is a single scalar value that can be calculated based on the spatially varying intensity distribution and the spatially varying polarization distribution, such as by a weighted average over the cross-sectional area of ​​the optical fiber. For example, if there is more optical power in the core than in the cladding (e.g., such as by a higher light intensity), the effective refractive index may be closer to the core refractive index than the cladding refractive index. The effective refractive index may be greater than or equal to the refractive index of the cladding and less than or equal to the refractive index of the core. Depending on the symmetry of the geometry of the core and cladding, the effective refractive index of two or more propagation modes may be equal. For example, modes in rotationally symmetric fibers can always be classified as transverse magnetic (TMmn), transverse electric (TEmn), or hybrid (HEmn / EHmn). In general, for rotationally symmetric fibers, each hybrid mode is doubly degenerate, meaning that there are always two modes, HEmn,a and HEmn,b, with exactly the same effective refractive index. Transverse magnetic and transverse electric are not necessarily doubly degenerate and have low losses in rotationally symmetric fibers.

[0017] Few-mode optical fibers can be used in optical communication systems by employing mode division multiplexing. For example, light beams corresponding to different data channels can be spatially combined and injected into the corresponding propagation modes of the optical fiber. In an ideal optical fiber, light propagates along the optical fiber in a specific mode without coupling into another mode. In practice, perturbations such as bends, twists, distributed or localized stresses, and / or splicing points can cause coupling between different modes. This coupling is called crosstalk between propagation modes.

[0018] One obstacle to adopting mode division multiplexing in few-mode optical fibers is that degenerate modes (i.e., propagation modes with the same effective refractive index) have relatively high crosstalk with each other. For example, during propagation in a few-mode optical fiber, light may be randomly transmitted between propagation modes with equal effective refractive index (in a situation called degeneracy). As a specific example, light injected into the first of two degenerate propagation modes may be randomly transmitted between the two degenerate propagation modes as it propagates along the optical fiber, and may emerge from the optical fiber as a combination of the two degenerate propagation modes, where the relative amount in one mode is a random (or otherwise unpredictable) number between 0% and 100%, and the relative amount in the other mode is 100% minus the random number.

[0019] To overcome the obstacle of high crosstalk between degenerate propagation modes in few-mode fiber, a demultiplexer can combine light from the degenerate modes to form a single data channel. A corresponding multiplexer can emit light into any or all of these degenerate modes. As the light propagates, the propagation can randomize the amount of light in each of these degenerate modes. By combining light from some or all of these degenerate modes to form a single data channel, the effects of degenerate mode crosstalk are reduced or eliminated.

[0020] In an example of an optical communication system, a multiplexer / demultiplexer may transmit a first optical data signal between a first single-mode optical fiber and a first propagation mode of a few-mode optical fiber. The first propagation mode may have a first effective refractive index. The multiplexer / demultiplexer may transmit a second optical data signal between a second single-mode optical fiber and a combination of a second propagation mode of the few-mode optical fiber and a third propagation mode of the few-mode optical fiber. The second propagation mode and the third propagation mode may have the same effective refractive index different from the first effective refractive index. During propagation within the few-mode optical fiber, the second optical data signal may be bidirectionally coupled between the second propagation mode and the third propagation mode while being substantially isolated from the first optical data signal in the first propagation mode.

[0021] Figure 1 A block diagram of an example of an optical communication system 100 is shown.

[0022] The optical communication system 100 may include a multiplexer / demultiplexer 102. The multiplexer / demultiplexer 102 may process optical data signals including digital and / or analog data signals such as optical beams onto which digital and / or analog data signals have been encoded by quadrature amplitude modulation, amplitude and phase shift keying, asymmetric phase shift keying, or another suitable encoding / decoding scheme. The multiplexer / demultiplexer 102 may use a modulator 102M to combine multiple optical data signals from respective single-mode optical fibers 104A, 104B, 104C, ..., 104N onto a single optical fiber, transmit the multiple optical data signals along an arbitrarily long length of optical fiber, and use a demodulator 102D to separate the signals into the original optical data signals and direct the separated signals into respective single-mode optical fibers 110A, 110B, 110C, ..., 110N. The optical communication system 100 may use mode division multiplexing to direct optical data signals to respective propagation modes 106A, 106B, 106C, ..., 106N (or a combination of modes) of at least one mode optical fiber 108, as discussed in detail below.

[0023] The multiplexer / demultiplexer 102 may transmit a first optical data signal between the first single-mode optical fiber 104A and a first propagation mode 106A of the few-mode optical fiber 108. In some examples, the first propagation mode 106A may be a transverse electric mode or a transverse magnetic mode of the few-mode optical fiber 108. The first propagation mode 106A may have a first effective refractive index.

[0024] The multiplexer / demultiplexer 102 can transmit a second optical data signal between the second single-mode optical fiber 104B and a combination of a second propagation mode 106B of the few-mode optical fiber 108 and a third propagation mode 106C of the few-mode optical fiber 108. The second propagation mode 106B and the third propagation mode 106C can have the same effective refractive index that is different from the first effective refractive index. In some examples, the second propagation mode 106B and the third propagation mode 106C can each be a hybrid electrical mode of the few-mode optical fiber 108.

[0025] In some examples, a cross section of the core of the few-mode optical fiber 108 taken orthogonally to the longitudinal axis of the few-mode optical fiber 108 has a ring shape. In some examples, the first propagation mode 106A includes only light having a polarization orientation parallel to the radial direction or only light having a polarization orientation parallel to the tangential direction. In some examples, the second propagation mode 106B and the third propagation mode 106C may each include at least some light having a polarization orientation angled relative to the radial direction and angled relative to the tangential direction. The ring shape is just one example of the cross-sectional shape of the core of the few-mode optical fiber 108. In some examples, the cross-sectional shape of the core of the few-mode optical fiber 108 may include a plurality of concentric rings as appropriate. In some examples, the cross-sectional shape of the core of the few-mode optical fiber 108 may include one or more step-change refractive index features, such as a feature in which the core material has a first refractive index and the cladding material has a second refractive index different from the first refractive index. In some examples, the cross-sectional shape of the core of the few-mode optical fiber 108 may include one or more gradient index features, such as where the refractive index varies continuously (or in relatively small steps) to define features of the core and cladding. For such gradient index features, the geometry of the gradient index material defines the propagation mode. In some examples, the cross-sectional shape of the core of the few-mode optical fiber 108 may be rotationally symmetric about the longitudinal axis of the few-mode optical fiber 108. Such rotational symmetry may simplify coupling to and from the few-mode optical fiber 108, as the rotational symmetry may eliminate the need to maintain azimuthal alignment when performing coupling. Other configurations may also be used.

[0026] If the second propagation mode 106B and the third propagation mode 106C have the same effective refractive index (such that the second propagation mode 106B and the third propagation mode 106C are degenerate), the second optical data signal may be bidirectionally coupled between the second propagation mode 106B and the third propagation mode 106C. In other words, when the second optical data signal propagates along the length of the few-mode optical fiber 108, the second optical data signal may be randomly transmitted between the second propagation mode 106B and the third propagation mode 106C.

[0027] The second optical data signal may be substantially isolated from the first propagation mode 106A. The first optical data signal may be substantially isolated from the second propagation mode 106B and substantially isolated from the third propagation mode 106C. For the purposes of this document, the phrase "substantially isolated" is intended to mean that the crosstalk between modes that are substantially isolated from each other has a coupling ratio that is less than or equal to a specified threshold, such as -30 dB. Such crosstalk may be caused by propagation along the few-mode fiber 108. For example, the optical coupling from the first propagation mode 106A to the second propagation mode 106B has a signal power in the second propagation mode that may be -30 dB or less (corresponding to a fraction of 0.001 or less) relative to the signal power in the first propagation mode. In fact, if a parasitic optical data signal at -30 dB is added to a desired optical data signal at 0 dB, the parasitic optical data signal at -30 dB may be filtered out as noise, leaving only the desired optical data signal at 0 dB. Other suitable designated thresholds may also be used, including -10dB, -15dB, -20dB, -25dB, -35dB, -40dB, and others.

[0028] In some examples, the multiplexer / demultiplexer 102 includes a multiplexer 102M. The multiplexer / demultiplexer 102M may transmit a first optical data signal from the first single-mode optical fiber 104A to a first propagation mode 106A of the few-mode optical fiber 108. The first propagation mode 106A may have a first effective refractive index. The multiplexer / demultiplexer 102M may transmit a second optical data signal from the second single-mode optical fiber 104B to at least one of a second propagation mode 106B of the few-mode optical fiber 108 and a third propagation mode 106C of the few-mode optical fiber 108. The second propagation mode 106B and the third propagation mode 106C may have the same effective refractive index different from the first effective refractive index. Because the second propagation mode 106B and the third propagation mode 106C may have the same effective refractive index, the second propagation mode 106B and the third propagation mode 106C may be degenerate.

[0029] In some examples, the multiplexer / demultiplexer 102 may include a demultiplexer 102D. The demultiplexer 102D may transmit the first optical data signal from the first propagation mode 106A of the few-mode optical fiber 108 to the third single-mode optical fiber 110A. The demultiplexer 102D may transmit the second optical data signal from a combination of the second propagation mode 106B of the few-mode optical fiber 108 and the third propagation mode 106C of the few-mode optical fiber 108 to the fourth single-mode optical fiber 110B.

[0030] By combining light from the second propagation mode 106B and the third propagation mode 106C to form a single data channel, the demultiplexer 102D may overcome the obstacle of relatively high coupling (or relatively high crosstalk) between the degenerate second propagation mode 106B and the third propagation mode 106C of the few-mode fiber 108 .

[0031] In the previous example, the second propagation mode 106B and the third propagation mode 106C of the few-mode fiber 108 are degenerate. In some examples, the second propagation mode 106B and the third propagation mode 106C are doubly degenerate. In other words, the second propagation mode 106B and the third propagation mode 106C have the same effective refractive index and are the only two propagation modes of the few-mode fiber 108 with this effective refractive index. With this double degeneracy, light from the degenerate modes can be combined into one single-mode fiber (without using multimode fiber) with relatively low loss, such as less than 1.76 dB. For example, the light can be combined by using two polarization states of the fundamental propagation mode. If there are three or more propagation modes with the same or comparable refractive index, combining light from the three or more propagation modes (such as for mode group multiplexing) will produce a loss of 1.76 dB or more, and the loss is comparable to the loss caused by using a beam splitter.

[0032] In some examples, the second propagation mode 106B and the third propagation mode 106C achieve their degeneracy via symmetry. For example, for a few-mode fiber 108 having a refractive index profile that is rotationally symmetric about the longitudinal axis of the few-mode fiber 108, there may be two modes with orthogonal polarization states, the two modes having the same effective refractive index. This is referred to as symmetry-based degeneracy. Due to the symmetry-based degeneracy, the rotational symmetry of the refractive index profile may ensure the existence of two propagation modes having the same effective refractive index. In some examples, the few-mode fiber 108 may include multiple sets of degenerate modes, such as a first pair of symmetry-based degenerate propagation modes having a first effective refractive index and a second pair of symmetry-based degenerate propagation modes having a second effective refractive index different from the first effective refractive index.

[0033] There may also be configurations where degeneracy is not achieved via symmetry, for which the effective indices are not the same but differ by less than a specified effective index threshold. Suitable effective index thresholds may include values ​​of 10-4, 10-5, 10-6, 10-7, 10-8, 10-9, 10-10, or other suitable values.

[0034] Figure 2 Shown for Figure 1 FIG. 1 is a side view of an example of a configuration of one or more Metasurface elements of a multiplexer / demultiplexer 102 . Figure 2The configuration and elements shown may be used for the multiplexer 102M, the demultiplexer 102D, or both the multiplexer 102M and the demultiplexer 102D.

[0035] A metasurface element may include an array of nanostructures spaced at sub-wavelength intervals. The nanostructures may control the phase, amplitude, and polarization of light at very high spatial resolution (such as less than the wavelength). In some examples, each nanostructure may have a birefringence value and orientation, where the birefringence value and orientation vary due to the nanostructure. In some examples, a metasurface element may include multiple layers of nanostructures such that incident light passes through a first layer of nanostructures, then through a second layer of nanostructures, and so on. In some examples, two or more layers may be formed directly on top of each other. In some examples, two or more layers may be separated longitudinally by layers or by being formed on separate substrates.

[0036] By varying the size and geometry of the nanostructures, each nanostructure on a Metasurface element can be designed to have anisotropic behavior that can mimic the function of a wave plate and / or polarizer. This birefringent behavior allows the Metasurface element to manipulate the spatial and polarizational degrees of freedom of light with extremely high resolution. For example, the size and / or geometry of the nanostructures can be varied due to the nanostructures. This manipulation of the spatial and polarizational degrees of freedom of light is well suited for mode division multiplexing. The phase and birefringence profiles of the Metasurface element can be designed using numerical optimization techniques such as adjoint analysis. In addition to high spatial resolution, the Metasurface element can provide multiple optical functions on a single Metasurface element and can be relatively easily manufactured using conventional manufacturing techniques. For example, a top-down method for forming a Metasurface element can include growing a thin film on a substrate, coating the thin film with a photoresist, using lithography to define features (using electron beam, photolithography, and / or nanoimprint techniques), etching around the features, adding a cladding material, and adding an optional reflective layer (such as a metal layer). As another example, a bottom-up approach to forming a Metasurface element may include coating a substrate with photoresist, using lithography to define features in the photoresist, growing thin films on the photoresist, stripping the thin films (which may remove all thin films except the photoresist), adding a cladding material, and adding an optional reflective layer (such as a metal layer). Other suitable design and fabrication techniques may also be used. For fabrication of a Metasurface element that includes multiple layers of nanostructures, the top-down approach and / or the bottom-up approach may be repeated for each layer of nanostructures.

[0037] In some examples, the first single-mode optical fiber 104A and the second single-mode optical fiber 104B can be included in a single-mode optical fiber array 202. The single-mode optical fiber array 202 can optionally include a common housing 212, such as a ferrule, that protects the single-mode optical fibers (such as 104A, 104B, ..., 104N and / or 110A, 110B, ..., 110N) in the array 202 during assembly and alignment of the optical communication system 100. The housing 212 can optionally have relatively tight mechanical tolerances. The housing 212 with tight tolerances can be manufactured separately from the single-mode optical fibers, and some of the manufacturing tolerances of the single-mode optical fibers can be relaxed as appropriate. The housing 212 can optionally be included as part of the connector or can be attached to the connector.

[0038] In some examples, the multiplexer / demultiplexer 102 includes one or more Metasurface elements 204A, 204B, 204C, ..., 204N spaced apart along an optical path between the single-mode fiber array 202 and the few-mode fiber 108. Using two or more Metasurface elements 204A, 204B, 204C, ..., 204N sequentially and spaced apart along the optical path may provide additional flexibility for redirecting light rays compared to using a single Metasurface element. For example, using two spaced apart Metasurface elements may allow the multiplexer / demultiplexer 102 to control both the position and propagation angle of a particular light ray; controlling both the position and propagation angle may be challenging with some configurations that use only a single Metasurface element. In a specific example, an incoming light ray (arriving at the multiplexer / demultiplexer 102) can be tracked forward to go to a specific location on the first Metasurface element, an outgoing light ray (outgoing from the multiplexer / demultiplexer 102) can be tracked backward to the second Metasurface element, the first Metasurface element can be configured to redirect the incoming light ray to propagate to a specific location on the second Metasurface element, and the second Metasurface element can be configured to redirect the ray from the specific location on the second Metasurface element to coincide with the outgoing light ray. This is just a specific example. The multiplexer / demultiplexer 102 can optionally include more than two Metasurface elements 204A, 204B, 204C, ..., 204N along the optical path. Increasing the number of consecutive and spaced-apart Metasurface elements 204A, 204B, 204C, ..., 204N may relax some of the manufacturing tolerances and / or alignment tolerances on the Metasurface elements 204A, 204B, 204C, ..., 204N.

[0039] Each Metasurface element 204A, 204B, 204C, ..., 204N is configured to impose a polarization-dependent phase on the first optical data signal and the second optical data signal. The polarization-dependent phase varies with the position on each Metasurface element 204A, 204B, 204C, ..., 204N. For example, the Metasurface elements 204A, 204B, 204C, ..., 204N may include a first region having a first phase value and a second region having a second phase value. The spatially varying polarization-dependent phase of the Metasurface elements 204A, 204B, 204C, ..., 204N may simultaneously perform various optical tasks, such as focusing and beam shaping (e.g., converting light from a first intensity distribution to a second intensity distribution).

[0040] The polarization-dependent phase allows the first optical data signal to be transmitted in the first single-mode optical fiber 104A ( Figure 1 ) and the first propagation mode 106A of the few-mode fiber 108 ( Figure 1 ) between the first and second single-mode optical fibers 104B. Figure 1 ) and the second propagation mode 106B ( Figure 1 ) and the third propagation mode 106C of the few-mode fiber 108 ( Figure 1 ) combinations.

[0041] One or more of the Metasurface elements 204A, 204B, 204C, ..., 204N may operate in transmission. In some examples, a plurality of Metasurface elements 204A, 204B, 204C, ..., 204N are disposed on a surface of a corresponding substrate 206A, 206B, 206C, ..., 206N. The multiplexer / demultiplexer 102 may apply a polarization-dependent phase by transmitting the first optical data signal and the second optical data signal through the substrate and through the plurality of Metasurface elements.

[0042] In some examples, substrates 206A, 206B, 206C, ..., 206N have the same thickness (h). Alternatively, at least two of substrates 206A, 206B, 206C, ..., 206N may have different thicknesses. In some examples, substrates 206A, 206B, 206C, ..., 206N are separated by spacers 208A, 208B, ..., 208M, which may be formed of solid materials. In some examples, the volume between adjacent substrates 206A, 206B, 206C, ..., 206N may be at least partially filled with fluid or solid material 210A, 210B, ..., 210M. Such fluid or solid material may be a high refractive index material of the package surface element 204A, 204B, 204C, ..., 204N, and may optionally reduce the loss caused by reflection when light enters or leaves the substrate. In some examples, the single-mode fiber array 202 can be spaced apart from the first metasurface element 204A by a distance d1. In some examples, the last substrate 206N can be spaced apart from the few-mode fiber 108 by a distance d2, which can be different from the distance d1.

[0043] Figures 3 to 7 Show Figure 1 An example of a configuration of a multiplexer / demultiplexer 102 in which one or more of the metasurface elements operate in a reflective manner. In some examples, such as Figures 3 to 7 In the configuration shown and described below, the plurality of metasurface elements may be disposed on at least one of two opposing surfaces of the substrate. The multiplexer / demultiplexer 102 is configured to impose a polarization-dependent phase by transmitting the first optical data signal and the second optical data signal into the substrate, sequentially reflecting the first optical data signal and the second optical data signal from the plurality of metasurface elements, and transmitting the first optical data signal and the second optical data signal out of the substrate.

[0044] Existence can be adjusted to achieve Figures 3 to 7 The configuration shown in the figure and other suitable configurations not explicitly shown in the figure are at least four factors. The first factor is the layout of the single-mode fiber array and the few-mode fiber. In some examples such as Figure 3 In the configuration of , the single-mode fiber array and the few-mode fiber are located on the same side of the substrate. In other examples such as Figures 4 to 7 In the configuration of , the single-mode fiber array and the few-mode fiber are located on the same side of the substrate. The second factor is the layout of the metasurface elements. In some examples such as Figures 3 to 6 In the configuration of , the metasurface elements are located on the same side of the substrate. In other examples such as Figure 7 In the configuration of , the metasurface elements are located on opposite sides of the substrate. The third factor is the orientation of the input beam. In some examples such as Figure 3 , Figure 4 and Figure 7In the configuration of , the input beam is orthogonal with respect to the substrate. In other examples such as Figure 5 and Figure 6 In the configuration of , the input beam is non-orthogonal with respect to the substrate. The fourth factor is the orientation of the output beam. In some examples such as Figures 3 to 5 and Figure 7 In the configuration of , the output beam is orthogonal with respect to the substrate. In other examples such as Figure 6 In the configuration of , the output beam is non-orthogonal with respect to the substrate. Of the sixteen possible permutations of these factors, five examples are explicitly shown in Figures 3 to 7 It should be understood that, given Figures 3 to 7 , those skilled in the art will readily understand the other eleven arrangements.

[0045] Figure 3 Show Figure 1 1 is a side view of an example of a configuration of the multiplexer / demultiplexer 102.

[0046] exist Figure 3 In the configuration of FIG. 1 , the optical path 302 may extend from the single-mode fiber array 202 through a hole or gap in a reflective layer 306 (such as a metal reflective layer) into the substrate 304. The optical path may extend to a first Metasurface element 308. The first Metasurface element 308 may be embedded or formed in the Metasurface element layer 310, such as by lithography, etching, deposition, and / or growth of material, and the like. An additional reflective layer 312 may be disposed on the Metasurface element layer 310 to increase the reflectivity of the Metasurface element 308. The optical path may travel back and forth between the Metasurface element 308 and the reflective layer 306. The optical path may exit from the substrate 304, through a hole or gap in the reflective layer 306, and arrive at the few-mode fiber 108. Light traversing the optical path 302 may propagate from the single-mode fiber array 202 to the few-mode fiber 108 (in the case of a multiplexer) or from the few-mode fiber 108 to the single-mode fiber array 202 (in the case of a demultiplexer).

[0047] exist Figure 3 In the configuration of FIG. 3 , the single-mode fiber array 202 and the few-mode optical fiber 108 are disposed on the same side of the substrate 304, and the metasurface element 308 is disposed on the opposite side of the substrate 304. The single-mode fiber array 202 and the few-mode optical fiber 108 may be substantially parallel to each other and substantially orthogonal to the plane of the substrate 304.

[0048] Figure 4 Show Figure 1 1 is a side view of an example of a configuration of the multiplexer / demultiplexer 102.

[0049] exist Figure 4In the configuration of FIG. 4 , an optical path 402 may extend from the single-mode optical fiber array 202 to a first Metasurface element 404 disposed on a substrate 406. The first Metasurface element 404 may be embedded or formed in a Metasurface element layer 408 disposed on the substrate 406. A reflective layer 410, such as a metal reflective layer, may be disposed on the Metasurface element layer 408, and the optical path 402 may extend through a hole or gap in the reflective layer 410. An opposing reflective layer 412 may be disposed on an opposite side of the substrate 406 opposite to the Metasurface element layer 408 to increase the reflectivity of the Metasurface element 404. The optical path may travel back and forth between the Metasurface element 404 and the opposing reflective layer 412. The optical path may exit from the substrate 406, pass through a hole or gap in the opposing reflective layer 412, and reach the few-mode optical fiber 108. Light traversing the optical path 402 may propagate from the single mode fiber array 202 to the few mode fiber 108 (in the case of a multiplexer) or from the few mode fiber 108 to the single mode fiber array 202 (in the case of a demultiplexer).

[0050] exist Figure 4 In the configuration of FIG. 4 , the single-mode fiber array 202 and the few-mode fiber 108 are disposed on opposite sides of the substrate 406. The metasurface element 404 and the single-mode fiber array 202 are disposed on the same side of the substrate 406. The single-mode fiber array 202 and the few-mode fiber 108 may be substantially parallel to each other and substantially orthogonal to the plane of the substrate 406.

[0051] Figure 5 Show Figure 1 1 is a side view of an example of a configuration of the multiplexer / demultiplexer 102.

[0052] exist Figure 5 In the configuration of FIG. 5 , the optical path 502 may extend from the single-mode fiber array 202 through a hole or gap in a reflective layer 506 (such as a metal reflective layer) and through a transparent portion of the Metasurface element layer 508 to the substrate 504. The Metasurface element 510 may be embedded or formed in the Metasurface element layer 508. An opposing reflective layer 512 (such as a metal reflective layer) may be disposed on an opposing side of the substrate 504. The optical path may move back and forth between the Metasurface element 510 and the opposing reflective layer 512. The optical path may exit the substrate 504, through a hole or gap in the opposing reflective layer 512, and arrive at the few-mode fiber 108. Light traversing the optical path 502 may propagate from the single-mode fiber array 202 to the few-mode fiber 108 (in the case of a multiplexer) or from the few-mode fiber 108 to the single-mode fiber array 202 (in the case of a demultiplexer).

[0053] exist Figure 5In the configuration of FIG. 5 , the single-mode fiber array 202 and the few-mode fiber 108 are disposed on opposite sides of the substrate 504. The metasurface element 510 and the single-mode fiber array 202 are disposed on the same side of the substrate 504. The single-mode fiber array 202 can be at an angle (e.g., non-orthogonal) to the plane of the substrate 504. The few-mode fiber 108 can be substantially orthogonal to the plane of the substrate 504.

[0054] Figure 6 Show Figure 1 1 is a side view of an example of a configuration of the multiplexer / demultiplexer 102.

[0055] exist Figure 6 In the configuration of FIG. 6 , the optical path 602 may extend from the single-mode fiber array 202 through a hole or gap in the reflective layer 606 into the substrate 604. The optical path 602 may extend to a Metasurface element 608 disposed on an opposite side of the substrate 604. The Metasurface element 608 may be embedded or formed in a Metasurface element layer 610. An opposite reflective layer 612, such as a metal reflective layer, may be disposed on the Metasurface element layer 610. The optical path 602 may travel back and forth between the Metasurface element 608 and the reflective layer 606. The optical path 602 may exit from the substrate 604, through a transparent portion of the Metasurface element layer 610, through a hole or gap in the opposite reflective layer 612, and to the few-mode fiber 108. Light traversing the optical path 602 may propagate from the single-mode fiber array 202 to the few-mode fiber 108 (in the case of a multiplexer) or from the few-mode fiber 108 to the single-mode fiber array 202 (in the case of a demultiplexer).

[0056] exist Figure 6 In the configuration of FIG. 6 , the single-mode fiber array 202 and the few-mode fiber 108 are disposed on opposite sides of the substrate 604. The metasurface element 608 and the few-mode fiber 108 are disposed on the same side of the substrate 604. The single-mode fiber array 202 can be at an angle (e.g., non-orthogonal) to the plane of the substrate 604. The few-mode fiber 108 can be at an angle (e.g., non-orthogonal) to the plane of the substrate 604.

[0057] Figure 7 Show Figure 1 1 is a side view of an example of a configuration of the multiplexer / demultiplexer 102.

[0058] exist Figure 7In a configuration of FIG. 7 , the optical path 702 may extend from the single-mode optical fiber array 202 through a hole or gap in a reflective layer 706 (such as a metal reflective layer) and through a transparent portion of a Metasurface element layer 708 to the substrate 704. The optical path 702 may extend through the substrate to a Metasurface element layer 710 located on an opposite side of the substrate 704. An opposite reflective layer 712 (such as a metal reflective layer) may be disposed on the Metasurface element layer 710. In this configuration, there are Metasurface element layers 708, 710 on opposite sides of the substrate 704, with Metasurface elements 714 disposed in the Metasurface element layer 708 and Metasurface elements 716 disposed in the Metasurface element layer 710. The optical path 702 may travel back and forth between the Metasurface element 714 and the Metasurface element 716. The optical path 702 may emanate from the substrate 704, through the transparent portion of the metasurface element layer 710, through the holes or gaps in the oppositely reflective layer 712, and to the few-mode fiber 108. Light traversing the optical path 702 may propagate from the single-mode fiber array 202 to the few-mode fiber 108 (in the case of a multiplexer) or from the few-mode fiber 108 to the single-mode fiber array 202 (in the case of a demultiplexer).

[0059] exist Figure 7 In the configuration of FIG. 7 , the single-mode fiber array 202 and the few-mode fiber 108 are disposed on opposite sides of the substrate 704. The metasurface element 714 is disposed on one side of the substrate 704, and the metasurface element 716 is disposed on the opposite side of the substrate 704. The single-mode fiber array 202 may be orthogonal to the plane of the substrate 704. The few-mode fiber 108 may be orthogonal to the plane of the substrate 704.

[0060] Figure 8 FIG. 8 is a flowchart showing an example of a method 800 for operating an optical communication system. The method 800 may be performed by Figure 1 The method 800 is only one method for operating an optical communication system; other suitable methods may also be used.

[0061] At operation 802, the optical communication system may transmit a first optical data signal between a first single-mode optical fiber and a few-mode optical fiber using a multiplexer / demultiplexer. The first optical data signal may be transmitted between a fundamental mode of the first single-mode optical fiber and a first propagation mode of the few-mode optical fiber. The first propagation mode may have a first effective refractive index.

[0062] At operation 804, the optical communication system may transmit a second optical data signal between the second single-mode optical fiber and the few-mode optical fiber using a multiplexer / demultiplexer. The second optical signal may be transmitted between a fundamental mode of the second single-mode optical fiber and a combination of a second propagation mode of the few-mode optical fiber and a third propagation mode of the few-mode optical fiber. The second propagation mode and the third propagation mode may have the same effective refractive index different from the first effective refractive index.

[0063] In some examples, the first single-mode optical fiber and the second single-mode optical fiber may be included in a single-mode optical fiber array. In some examples, the multiplexer / demultiplexer may include a plurality of metasurface elements spaced apart along an optical path between the single-mode optical fiber array and the few-mode optical fiber. In some examples, the method may further include: directing the first optical data signal and the second optical data signal onto a plurality of metasurface elements. In some examples, the method may further include: applying a polarization-dependent phase to the first optical data signal and the second optical data signal using the metasurface elements. The polarization-dependent phase may vary according to a position on each metasurface element. In some examples, the method may further include: allowing the first optical data signal to be transmitted between the first single-mode optical fiber and the first propagation mode of the few-mode optical fiber via applying the polarization-dependent phase. In some examples, the method may further include: allowing the second optical data signal to be transmitted between the second single-mode optical fiber and a combination of the second propagation mode of the few-mode optical fiber and the third propagation mode of the few-mode optical fiber via applying the polarization-dependent phase.

[0064] In some examples, the second optical data signal can be bidirectionally coupled between the second propagation mode and the third propagation mode during propagation in the few-mode fiber. In some examples, the second optical data signal can be substantially isolated from the first propagation mode during propagation in the few-mode fiber. In some examples, the first optical data signal can be substantially isolated from the second propagation mode and substantially isolated from the third propagation mode during propagation in the few-mode fiber.

[0065] In some examples, a plurality of Metasurface elements may be disposed on a surface of a corresponding substrate. In some examples, applying the polarization-dependent phase may include transmitting the first optical data signal and the second optical data signal through the substrate and through the plurality of Metasurface elements.

[0066] In some examples, the plurality of metasurface elements may be disposed on at least one of two opposing surfaces of the substrate. In some examples, applying the polarization-dependent phase may include transmitting the first optical data signal and the second optical data signal into the substrate, reflecting the first optical data signal and the second optical data signal from the plurality of metasurface elements, and transmitting the first optical data signal and the second optical data signal away from the substrate.

Claims

1. An optical communication system, comprising: A multiplexer / demultiplexer, the multiplexer / demultiplexer being configured to: transmitting a first optical data signal between a first single-mode optical fiber and a first propagation mode of a few-mode optical fiber, the first propagation mode having a first effective refractive index; and A second optical data signal is transmitted between a second single-mode optical fiber and a combination of a second propagation mode of the few-mode optical fiber and a third propagation mode of the few-mode optical fiber, the second propagation mode and the third propagation mode having a second effective refractive index different from the first effective refractive index.

2. The optical communication system of claim 1, wherein the second propagation mode and the third propagation mode are the only two propagation modes of the few-mode optical fiber having the second effective refractive index.

3. The optical communication system of claim 1, wherein the second propagation mode and the third propagation mode have a degeneracy based on symmetry.

4. The optical communication system of claim 1, wherein the few-mode fiber has a refractive index profile that is rotationally symmetric about a longitudinal axis of the few-mode fiber.

5. The optical communication system of claim 1, wherein the first single-mode optical fiber and the second single-mode optical fiber are included in a single-mode optical fiber array.

6. The optical communication system of claim 5, wherein: The multiplexer / demultiplexer includes a metasurface element disposed along an optical path between the single-mode optical fiber array and the few-mode optical fiber; The metasurface element is configured to impose a polarization-dependent phase on the first optical data signal and the second optical data signal; The polarization-dependent phase varies with position on the metasurface element; The polarization-dependent phase is configured such that the first optical data signal is transmitted between the first single-mode optical fiber and the first propagation mode of the few-mode optical fiber; and The polarization dependent phase is configured such that the second optical data signal is transmitted between the second single mode optical fiber and the combination of the second propagation mode of the few-mode optical fiber and the third propagation mode of the few-mode optical fiber.

7. The optical communication system of claim 6, wherein the Metasurface element comprises an array of nanostructures, each nanostructure having a birefringence value and an orientation, the equal birefringence values ​​and the equal orientations varying between nanostructures.

8. The optical communication system of claim 5, wherein: The multiplexer / demultiplexer includes a plurality of metasurface elements spaced apart along an optical path between the single-mode optical fiber array and the few-mode optical fiber; Each metasurface element is configured to impose a polarization-dependent phase on the first optical data signal and the second optical data signal; The polarization-dependent phase varies with position on each metasurface element; The polarization dependent phase is configured to allow the first optical data signal to be transmitted between the first propagation mode of the first single-mode optical fiber and the few-mode optical fiber; and The polarization dependent phase is configured to allow the second optical data signal to be transmitted between the second single mode optical fiber and the combination of the second propagation mode of the few-mode optical fiber and the third propagation mode of the few-mode optical fiber.

9. The optical communication system of claim 8, wherein: The plurality of Metasurface elements are disposed on a surface of a corresponding substrate; and The multiplexer / demultiplexer is configured to impose the polarization-dependent phase by transmitting the first optical data signal and the second optical data signal through the substrate and through the plurality of metasurface elements.

10. The optical communication system of claim 8, wherein: The plurality of Metasurface elements are disposed on at least one of two opposite surfaces of the substrate; and The multiplexer / demultiplexer is configured to impose the polarization-dependent phase by transmitting the first optical data signal and the second optical data signal into the substrate, reflecting the first optical data signal and the second optical data signal from the plurality of metasurface elements, and transmitting the first optical data signal and the second optical data signal out of the substrate.

11. The optical communication system of claim 1 , wherein during propagation in the few-mode fiber: The second optical data signal is bidirectionally coupled between the second propagation mode and the third propagation mode; The second optical data signal is substantially isolated from the first propagation mode; and The first optical data signal is substantially isolated from the second propagation mode and substantially isolated from the third propagation mode.

12. The optical communication system of claim 1, wherein: The first propagation mode is a transverse electric mode or a transverse magnetic mode of the few-mode optical fiber; The second propagation mode and the third propagation mode are each hybrid electrical modes of the few-mode optical fiber.

13. The optical communication system of claim 1, wherein: A cross section of the core of the few-mode optical fiber taken orthogonally to the longitudinal axis of the few-mode optical fiber has a rotationally symmetric shape defining a radial direction and a tangential direction; The first propagation mode includes only light having a polarization orientation parallel to the radial direction or parallel to the tangential direction; and The second propagation mode and the third propagation mode each include at least some light having a polarization orientation that is angled relative to the radial direction and angled relative to the tangential direction.

14. A method for operating an optical communication system, the method comprising: coupling a first optical data signal between a first single-mode optical fiber and a first propagation mode of a few-mode optical fiber using a multiplexer / demultiplexer, the first propagation mode having a first effective refractive index; as well as A second optical data signal is coupled between a second single-mode optical fiber and a combination of a second propagation mode of the few-mode optical fiber and a third propagation mode of the few-mode optical fiber using a multiplexer / demultiplexer, the second propagation mode and the third propagation mode having a second effective refractive index different from the first effective refractive index.

15. The method of claim 14, wherein: The first single-mode optical fiber and the second single-mode optical fiber are included in a single-mode optical fiber array; The multiplexer / demultiplexer includes a plurality of metasurface elements spaced apart along an optical path between the single-mode optical fiber array and the few-mode optical fiber; and The method further comprises: directing the first optical data signal and the second optical data signal onto the plurality of metasurface elements; applying a polarization-dependent phase to the first optical data signal and the second optical data signal using the metasurface element, wherein the polarization-dependent phase varies with a position on each metasurface element; allowing the first optical data signal to be transmitted between the first propagation mode of the first single-mode optical fiber and the few-mode optical fiber via applying the polarization dependent phase; and The second optical data signal is allowed to be transmitted between the second single mode optical fiber and the combination of the second propagation mode of the few-mode optical fiber and the third propagation mode of the few-mode optical fiber via applying the polarization dependent phase.

16. The method of claim 14, wherein during propagation in the few-mode fiber: The second optical data signal is bidirectionally coupled between the second propagation mode and the third propagation mode; The second optical data signal is substantially isolated from the first propagation mode; and The first optical data signal is substantially isolated from the second propagation mode and substantially isolated from the third propagation mode.

17. The method of claim 14, wherein: The plurality of Metasurface elements are disposed on a surface of a corresponding substrate; and Imposing the polarization-dependent phase includes transmitting the first optical data signal and the second optical data signal through the substrate and through the plurality of metasurface elements.

18. The method of claim 14, wherein: The plurality of Metasurface elements are disposed on at least one of two opposite surfaces of the substrate; and Applying the polarization dependent phase includes transmitting the first optical data signal and the second optical data signal into the substrate, reflecting the first optical data signal and the second optical data signal from the plurality of metasurface elements, and transmitting the first optical data signal and the second optical data signal away from the substrate.

19. An optical communication system comprising: A multiplexer, the multiplexer being configured to: transmitting a first optical data signal from a first single-mode optical fiber in a first single-mode optical fiber array into a first propagation mode of a few-mode optical fiber having a first effective refractive index, the few-mode optical fiber having a refractive index profile rotationally symmetric about a longitudinal axis of the few-mode optical fiber; as well as transmitting a second optical data signal from a second single-mode optical fiber in the first single-mode optical fiber array to at least one of a second propagation mode of the few-mode optical fiber and a third propagation mode of the few-mode optical fiber, the second propagation mode and the third propagation mode having a second effective refractive index different from the first effective refractive index, the second propagation mode and the third propagation mode being the only propagation modes of the few-mode optical fiber having the second effective refractive index; as well as A demultiplexer, the demultiplexer being configured to: transmitting the first optical data signal from the first propagation mode of the few-mode optical fiber to a third single-mode optical fiber in a second single-mode optical fiber array; as well as The second optical data signal is transmitted from a combination of the second propagation mode of the few-mode optical fiber and the third propagation mode of the few-mode optical fiber to a fourth single-mode optical fiber in the second single-mode optical fiber array.

20. The optical communication system of claim 19, wherein: The multiplexer includes a plurality of first metasurface elements spaced apart along a first optical path between the first single-mode optical fiber array and the few-mode optical fiber; Each first Metasurface element is configured to apply a first polarization-dependent phase that varies with position on each first Metasurface element; The first polarization-dependent phase is configured to cause the first optical data signal to be transmitted from the first single-mode optical fiber to the first propagation mode of the few-mode optical fiber; the first polarization-dependent phase being configured to cause the second optical data signal to propagate from the second single-mode optical fiber to the combination of the second propagation mode of the few-mode optical fiber and the third propagation mode of the few-mode optical fiber; The demultiplexer includes a plurality of second metasurface elements spaced apart along an optical path between the second single-mode optical fiber array and the few-mode optical fiber; Each second Metasurface element is configured to apply a second polarization-dependent phase that varies with position on each second Metasurface element; The second polarization-dependent phase is configured to cause the first optical data signal to be transmitted from the first propagation mode of the few-mode optical fiber to the third single-mode optical fiber; and The second polarization-dependent phase is configured such that the second optical data signal is transmitted from the combination of the second propagation mode of the few-mode optical fiber and the third propagation mode of the few-mode optical fiber into the fourth single-mode optical fiber.

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