CN113466993B - Polarization-maintaining mode group selection type photon lantern and manufacturing and application thereof - Google Patents
Polarization-maintaining mode group selection type photon lantern and manufacturing and application thereof Download PDFInfo
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- CN113466993B CN113466993B CN202110769519.6A CN202110769519A CN113466993B CN 113466993 B CN113466993 B CN 113466993B CN 202110769519 A CN202110769519 A CN 202110769519A CN 113466993 B CN113466993 B CN 113466993B
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/255—Splicing of light guides, e.g. by fusion or bonding
- G02B6/2553—Splicing machines, e.g. optical fibre fusion splicer
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/27—Optical coupling means with polarisation selective and adjusting means
- G02B6/2726—Optical coupling means with polarisation selective and adjusting means in or on light guides, e.g. polarisation means assembled in a light guide
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/27—Optical coupling means with polarisation selective and adjusting means
- G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
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Abstract
The invention discloses a polarization-maintaining mode group selection type photon lantern, wherein N different few-mode optical fibers surround one elliptical coreless optical fiber and are tapered to the output end, and the output endThe end comprises a fused few-mode fiber cladding, a few-mode fiber core leaking to the outer side of the cladding and an elliptical coreless fiber in a fused state to form an elliptical ring-shaped refractive index distribution with birefringence, wherein the major axis and the minor axis of an inner ellipse of the elliptical ring are a y 、a x The major and minor axes of the outer ellipse are b y 、b x (ii) a The output end is welded with an elliptical ring core optical fiber matched with the refractive index distribution of the output end. The invention introduces more than 10 between the output adjacent mode groups through the special capillary structure and the optical fiber arrangement ‑3 The effective refractive index difference of the optical element is larger than 10 generated between the same mode and different polarization states by the stress structure formed after tapering ‑4 The polarization birefringence difference realizes the simultaneous degeneracy of polarization and space modes, maximizes the communication capacity of the optical fiber, and simultaneously reduces the requirement of a transmission system on MIMO-DSP.
Description
Technical Field
The invention belongs to the technical field of optical fiber communication, and particularly relates to a structure of a polarization-maintaining mode group selection type mode group multiplexing/demultiplexing device and application thereof.
Background
In recent years, in order to meet the increasing data traffic demand, the mode multiplexing technology (MDM) has attracted much attention. Unlike conventional Single Mode Fibers (SMFs), few Mode Fibers (FMFs) use the same carrier wavelength over multiple modes by using independent data streams, thereby doubling the fiber capacity. One of the key issues in MDM transmission is crosstalk between the fiber modes, and the design of the mode multiplexing/demultiplexing devices is crucial to reduce this crosstalk. In most MDM experiments, multiple Input Multiple Output (MIMO) Digital Signal Processing (DSP) modules are used to compensate for the multi-modal crosstalk between all spatial channels (i.e., full MIMO). While this approach works effectively even in systems with strongly coupled FMF transmission, the MIMO complexity required to acquire the transmitted information increases rapidly as the number of spatial channels increases. Since differential module delay (DMGD) directly affects the size of the MIMO block, the goal of mode multiplexing/demultiplexing device design is to minimize DMGD.
In a general few-mode fiber (FMF) communication system, a non-mode-selective photon lantern (MNS-PL), a mode-selective photon lantern (MSPL), or a mode group-selective photon lantern (MGS-PL) may be used as a mode multiplexing/demultiplexing device. When a non-mode-selective photon lantern is used, strong coupling will occur for all modes. This strong coupling can cause significant crosstalk between all modes and thus degrade signal quality, requiring higher dimensional MIMO blocks to compensate. The use of the MIMO module greatly increases the cost of data transmission of the MDM system.
However, MIMO blocks are not applicable to short-range communication. On the one hand, short-range communication systems are quite sensitive to power consumption and cost. On the other hand, multimode fiber (MMF) is used in short-distance transmission, and the few-mode fiber (FMF) of longer-distance MDM systems excites a larger number of modes. When the number of modes is large, this will increase the complexity of the MIMO algorithm proportionally, eventually leading to impractical applications. Meanwhile, due to bending and twisting of the optical fiber, spatial modes of the same mode group in the optical fiber are strongly coupled in the transmission process of the multimode optical fiber.
Mode group multiplexing/demultiplexing is performed using a mode group selection type photon lantern without demultiplexing degenerate modes between single specific mode groups, and only non-degenerate mode groups in an optical fiber are used for signal transmission, in which case, the MIMO block is only used for mode coupling compensation within a module, and the dimension of the MIMO block can be greatly reduced.
Meanwhile, a stress structure can be introduced into the photon lantern, so that the mode group selection type photon lantern generates a polarization maintaining effect. The common optical fiber type device is designed in a symmetrical cylindrical structure, but in practical application, the device is subjected to mechanical stress to become asymmetrical, and a birefringence phenomenon is generated, so that the polarization state of light is irregularly changed when the light is transmitted in the common optical fiber type device. The main influencing factors of the change are wavelength, bending degree, temperature and the like. Polarization maintaining means that stronger birefringence effect is generated through the design of the optical fiber type device on the geometric dimension, so that the problem of polarization state change can be solved, and the influence of stress on the polarization state of incident light is eliminated. The mode group selection type photon lantern with the polarization maintaining characteristic can fully utilize all modes in the few-mode optical fiber and different polarization states of all the modes.
In the prior art, a mode-selecting photonic lantern (MSPL) cascade Multimode Polarization Controller (MPC) is disclosed in chinese patent documents with publication No. CN108761651A and publication No. 2018, 11/6, and the excitation of a single port OAM mode and multiplexing of multiple OAM modes can be realized by introducing polarization control of multiple modes on a single device, but the use of MPC introduces additional complexity and possible loss, and a plurality of high-order modes exist in a bent optical fiber in MPC at the same time, which may cause strong inter-mode crosstalk, thereby reducing OAM mode purity. Meanwhile, the MSPL can only generate an OAM mode and does not have a polarization maintaining function.
In the second prior art, referring to "Design of elliptical-core mode-selective photonic networks with six modes for MIMO-free mode division multiplexing system" opt-let.42, 4355-4358 (2017) in Xiaowei Sai, yan Li et al, an elliptical core mode selective photonic lantern (EC-MSPL) with six modes is designed, which can be well matched with an elliptical core few-mode fiber (EC-FMF) in a MIMO-free MDM system to release Multiple Input Multiple Output (MIMO) digital signal processing, thereby reducing cost and complexity. However, the elliptical core mode selective photon lantern (EC-MSPL) does not match with the common round optical fiber and the existing polarization-maintaining few-mode optical fiber, and serious loss and mode crosstalk are generated when the connection is carried out.
In the third prior art, according to the "method and device for manufacturing an orbital angular momentum photon lantern" described in chinese patent literature with publication No. CN110208907A and publication date of 2019, 9, 6, the stretching length and the proportion of the cone region part of the photon lantern are controlled during tapering, so that the frequency of the few-mode end normalized frequency obtained after tapering is controlled, and the stretching length is controlled to keep the phase difference of the LP mode at pi/2, so that multiplexing of an OAM mode can be realized, but the precision requirement of the phase termination change of the LP mode realized by the preset stretching proportion and the preset stretching length on the tapering is very high, and the method and device are difficult to realize in the actual manufacturing process.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a polarization-maintaining mode group selection type photon lantern (PMMGS-PL), which is characterized in that a special capillary structure and the arrangement of optical fibers are used during the drawing of the photon lantern (the special capillary structure comprises a glass capillary with an elliptic central hole or air holes symmetrically distributed on two sides of the central hole, and can generate different degrees of deformation in different directions during the tapering process), the optical fiber arrangement comprises optical fiber bundles which are annularly distributed in an elliptic way around a coreless optical fiber and can form refractive index distribution similar to the elliptic annular core optical fiber at an output end, and more than 10 percent of optical fibers with less modes at the output end are introduced into adjacent mode groups -3 While forming stress structures in different directions of the photonic lantern, so that the polarization birefringence difference between different polarization states of the same mode group is larger than 10 -4 The method realizes the simultaneous degeneracy of polarization and space modes, can fully utilize the maximum channel density of the optical fiber, and simultaneously only multiplexes the mode group to reduce the requirement of a transmission system on MIMO-DSP.
The invention provides a polarization-maintaining mode group selection type photon lantern which comprises a tapered glass sleeve, an input end and an output end, wherein the input end is positioned on one side of a large head end of the tapered glass sleeve, and the output end is positioned on one side of a small head end of the tapered glass sleeve; the tapered glass sleeve penetrates through an elliptical coreless fluorine-doped low-refractive-index optical fiber, and the input end comprises N small-mode optical fibers which surround the elliptical coreless fluorine-doped low-refractive-index optical fiber and are distributed according to a circular ring shape, wherein the N small-mode optical fibers are different in diameter; tapering the coreless fluorine-doped low-refractive-index optical fiber from the input end to the output end by N few-mode optical fibers surrounding the coreless fluorine-doped low-refractive-index optical fiber; the output end comprises a few-mode fiber cladding in a fusion state, a few-mode fiber core leaked to the outer side of the few-mode fiber cladding and an elliptical coreless fluorine-doped low-refractive-index fiber in the fusion state, so that an elliptical ring-core tapered fiber bundle with double refraction is formed in the tapered glass sleeve, the refractive index distribution of the elliptical ring-core tapered fiber bundle is an elliptical ring, and the major and minor axes of the inner ellipse of the elliptical ring are a and a respectively y 、a x The major and minor axes of the outer ellipse are respectively b y 、b x (ii) a The outputAnd the end is welded with an elliptical ring core optical fiber matched with the refractive index distribution of the output end.
Further, the polarization-maintaining mode group selection type photon lantern is characterized in that if N is more than 1 and less than or equal to 10, the elliptical ring core optical fiber is a few-mode optical fiber; if N is more than 10, the elliptical ring-core optical fiber is a multimode optical fiber.
The relationship between the major and minor axes of the inner ellipse and the outer ellipse of the elliptical ring is as follows: 0 μm < a y ≤20μm，0μm＜a x ≤a y ，a y ＜b y ≤50μm，0μm＜b x ≤b y 。
The long and short axes of the fiber core of the elliptic coreless fluorine-doped low-refractive-index optical fiber are respectively c y 、c x ，10μm＜c x ＜100μm，c x ＜c y Less than 150 μm; the refractive index difference of the elliptic coreless fluorine-doped low-refractive-index optical fiber relative to the tapered glass sleeve is-9 multiplied by 10 3 And the centers of the light spots generated by the output ends form zero intensity distribution.
The conical glass sleeve is provided with two air holes which are symmetrically arranged in the circumferential direction and run through in the axial direction, and the air holes are used for introducing air when the photon lantern is drawn so as to form an elliptical ring core. The circle centers of the two air holes and the circle center of the central hole of the conical glass sleeve are on the same straight line.
Meanwhile, the invention also provides a manufacturing method of the polarization-maintaining mode group selection type photon lantern, which comprises the following steps: inserting N few-mode optical fibers with different diameters into a glass sleeve in an annular arrangement around an elliptical coreless fluorine-doped low-refractive-index optical fiber, wherein the long and short axes of the fiber core of the elliptical coreless fluorine-doped low-refractive-index optical fiber are c y 、c x ，10μm＜c x ＜100μm，c x ＜c y Less than 150 μm; tapering the glass sleeve into which the optical fiber bundle is inserted to form a tapered elliptical ring core photon lantern; the output end of the formed elliptic ring core photon lantern is a small opening end of a fused conical glass sleeve, and comprises a fused few-mode optical fiber cladding, a few-mode optical fiber core leaking to the outer side of the few-mode optical fiber cladding and an elliptic fused stateA coreless fluorine-doped low refractive index optical fiber; and welding one end of the elliptical ring core optical fiber matched with the refractive index distribution of the output end with the output end.
By applying the polarization-maintaining mode group selection type photon lantern provided by the invention, the simultaneous multiplexing of different polarization and space mode groups can be realized, thereby improving the capacity of a communication system. The specific application is as follows: the polarization-maintaining mode group selection type photon lantern is used as a mode group and a polarization multiplexer, modulated optical signals are input to all the ends of the few-mode optical fibers at the input end of the polarization-maintaining mode group selection type photon lantern, the fundamental mode of the few-mode optical fibers at the input end is excited, modal evolution is realized through taper transition, the fundamental mode is converted into a certain pair of eigenmode pairs corresponding to the elliptical ring core optical fibers at the output end of the polarization-maintaining mode group selection type photon lantern, each fundamental mode signal excited at the input end is coupled with a linear polarization mode group (LPG) in the elliptical ring core optical fiber at the output end after being output through the elliptical ring core tapered optical fiber bundle, the linear polarization mode group (LPG) in the elliptical ring core optical fiber at the output end of the polarization-maintaining mode group selection type photon lantern is corresponding to two polarization states, the same linear polarization mode group (LPG) in the elliptical ring core optical fiber with the matched refractive index distribution is respectively and independently transmitted under different polarization states, and therefore simultaneous multiplexing of different polarization and spatial mode groups is realized.
Compared with the prior art, the invention has the beneficial effects that:
aiming at the problem that in the transmission process of an MDM system in the prior art, the modes of the same mode group in few-mode/multi-mode optical fibers are strongly coupled, so that great crosstalk is generated among all the modes, and the signal quality is deteriorated. On the other hand, different polarization states of the same spatial mode/mode group cannot be multiplexed, and communication channels are wasted, the invention provides a polarization-maintaining mode group selection type photon lantern (PMMGS-PL), wherein a specific capillary structure and the arrangement of optical fibers are used during drawing of the photon lantern, and more than 10 percent of optical fibers with few modes are introduced between adjacent mode groups of the optical fibers with few modes at the output end -3 While forming stress structures in different polarization directions of the photon lantern, so that the polarization birefringence difference between different polarization states of the same mode group is larger than 10 -4 Realizing polarization and spatial modesThe formula simultaneously degenerates, can make full use of the maximum channel density of the optical fiber, and simultaneously reduces the requirement of a transmission system on MIMO-DSP.
The photonic lantern with the structure introduces an elliptical ring core structure on the basis of a mode group selection type photonic lantern, can control the effective refractive index difference between different degenerate LP modes by designing the ring core size of the elliptical ring core structure, and simultaneously controls the ellipticity of the elliptical ring core to increase that other high-order vector modes are separated from the adjacent modes except two basic modes, the effective refractive index difference is more than 10 -3 。
Furthermore, the photon lantern with the structure introduces the air hole auxiliary structure on the glass sleeve through drilling along the axial direction, so that an elliptical ring core is easier to generate during drawing of the photon lantern, a more obvious stress double refraction area is generated, and the polarization maintaining characteristic of the photon lantern is further improved.
Drawings
FIG. 1 is a schematic diagram of a polarization maintaining mode group selection type photonic lantern structure with 5 linear polarization mode groups (LPG) except a fundamental mode according to the present invention;
FIG. 2 is a cross-sectional view of the photonic lantern of FIG. 1 at three cut-away locations (a), (b), and (c);
FIG. 3 (a) is a cross-sectional view of the output end of the photonic lantern shown in FIG. 1;
FIG. 3 (b) is a cross-sectional view of the output end of a prior art ring core photonic lantern;
FIG. 3 (c) is a cross-sectional view of the output end of a prior art elliptical core photonic lantern;
FIG. 4 (a) shows the optical field distribution and effective refractive index at different modes at the output end of the photonic lantern of FIG. 1;
FIG. 4 (b) is the optical field distribution and effective refractive index of the ring core photon lantern output end in different modes in the prior art;
FIG. 4 (c) is a graph of the optical field distribution and effective refractive index at the output end of the prior art elliptical core photonic lantern in different modes;
FIG. 5 is a schematic diagram of a transmission experimental facility constructed by applying a polarization-maintaining mode group selection type photon lantern according to the present invention;
fig. 6 is a schematic structural view of embodiment 2 of the present invention.
In the figure: 1-tapered glass sleeve, 2-input end, 3-output end, 4-elliptical coreless fluorine-doped low-refractive-index optical fiber, 5-few-mode optical fiber, 6-elliptical ring core tapered optical fiber bundle, 7-elliptical ring core optical fiber and 8-air hole.
Detailed Description
The invention will be further described with reference to the following figures and specific examples, which are not intended to limit the invention in any way.
The design idea of the polarization-maintaining mode group selection type photon lantern is as follows: n few-mode fibers with different outer diameters and different core diameters surround the coreless fluorine-doped low-refractive-index fiber to be tapered to the output end, the output end comprises a few-mode fiber cladding in a fused state, a few-mode fiber core leaked to the outer side of the few-mode fiber cladding and an elliptical coreless fluorine-doped low-refractive-index fiber in the fused state, and therefore a birefringent elliptical ring-core tapered fiber bundle is formed in the tapered glass sleeve, the refractive index distribution of the elliptical ring-core fiber core formed by the elliptical ring-core fiber bundle is an elliptical ring, and the major and minor axes of an inner ellipse are a y 、a x The major and minor axes of the outer ellipse are b y 、b x (ii) a And the output end is welded with an elliptical ring core optical fiber matched with the refractive index distribution of the output end.
As shown in fig. 1, fig. 2 and fig. 3 (a), the basic structure of a polarization maintaining mode group selection type photonic lantern (PMMGS-PL) provided by the present invention is that the photonic lantern comprises a tapered glass sleeve 1, an input end 2 located at one side of a large end of the tapered glass sleeve 1, and an output end 3 located at one side of a small end of the tapered glass sleeve 1, an elliptical coreless fluorine-doped low refractive index fiber 4 passes through the tapered glass sleeve 1, and the input end 2 comprises N few-mode fibers 5 with different diameters surrounding the elliptical coreless fluorine-doped low refractive index fiber 4 and arranged in a circular ring shape; the input end 2 is tapered from N few-mode fibers 5 to the output end 3 by surrounding the elliptical coreless fluorine-doped low-refractive-index fiber 4, the output end 3 comprises a few-mode fiber cladding in a fusion state, a few-mode fiber core leaking to the outer side of the few-mode fiber cladding and an elliptical non-fiber core in a fusion stateCore-doping fluorine low-refractive-index optical fiber 4, thereby forming a birefringent elliptical ring core-tapered optical fiber bundle 6 in the tapered glass sleeve 1, wherein the elliptical ring core-tapered optical fiber bundle 6 forms elliptical ring core refractive index distribution as an elliptical ring, and the major and minor axes of the inner ellipse are a y 、a x The major and minor axes of the outer ellipse are respectively b y 、b x (ii) a The thickness of the elliptical ring is rho = a x /b x ＝a y /b y And 0.5<ρ<1, ovality of each ellipse η = bx/by = ax/ay, and 1<η<10; and the output end is welded with an elliptical ring core optical fiber 7 matched with the refractive index distribution of the output end. If N is more than 1 and less than or equal to 10, the elliptical ring core optical fiber 7 is a few-mode optical fiber; if N is more than 10, the elliptical ring-core optical fiber 7 is a multimode optical fiber.
The photon lantern of the invention has the following relationship between the major and minor axes of the inner ellipse and the major and minor axes of the outer ellipse: 0 μm < a y ≤20μm，0μm＜a x ≤a y ，a y ＜b y ≤50μm，0μm＜b x ≤b y . The long and short axes of the fiber core of the elliptic coreless fluorine-doped low-refractive-index optical fiber are respectively c y 、c x ，10μm＜c x ＜100μm，c x ＜c y Less than 150 μm; the refractive index difference of the elliptic coreless fluorine-doped low-refractive-index optical fiber relative to the tapered glass sleeve is-9 multiplied by 10 3 And the centers of the light spots generated by the output ends form zero intensity distribution.
The invention provides a manufacturing method of the polarization-maintaining mode group selection type photon lantern, which comprises the following steps:
inserting N few-mode optical fibers 5 with different outer diameters and different core diameters into a glass sleeve around an elliptical coreless fluorine-doped low-refractive-index optical fiber 4 in an annular arrangement, wherein the long and short axes of the fiber core of the elliptical coreless fluorine-doped low-refractive-index optical fiber 4 are c y 、c x ，10μm＜c x ＜100μm，c x ＜c y Less than 150 μm; tapering the glass sleeve into which the optical fiber bundle is inserted (namely tapering the glass sleeve to form a tapered glass sleeve 1) to form a tapered elliptical ring core photon lantern; the output end 3 of the formed elliptic ring core photon lantern is in a molten stateThe output end 3 comprises a few-mode optical fiber cladding in a fused state, a few-mode optical fiber core leaked to the outer side of the few-mode optical fiber cladding and an elliptical coreless fluorine-doped low-refractive-index optical fiber 4 in the fused state; one end of an elliptical ring core optical fiber 7 matching the refractive index profile of the output end 3 is fusion-spliced to the output end 3.
The polarization-maintaining mode group selection type photon lantern is an all-fiber mode group selection type photon lantern capable of realizing simultaneous degeneracy of polarization and space modes, can realize simultaneous multiplexing of different polarization and space mode groups, and provides a foundation for capacity improvement of a communication system. The polarization-maintaining mode group selection type photon lantern is used as a mode group and polarization multiplexer, so that polarization and space mode simultaneous multiplexing can be realized, and the specific method comprises the following steps: the input end of the polarization-preserving mode group selection type photon lantern inputs modulated optical signals to all the few-mode optical fiber ends, the fundamental mode of the few-mode optical fiber at the input end is excited, modal evolution is realized through taper transition, the fundamental mode signals are converted into a pair of eigenmodes corresponding to the output end elliptical ring core few-mode or multi-mode optical fiber of the polarization-preserving mode group selection type photon lantern, each fundamental mode signal excited by the input end is output through an elliptical ring-shaped tapered optical fiber bundle to form a certain linear polarization mode group (LPG) under two polarization states corresponding to the output end elliptical ring core few-mode or multi-mode optical fiber, the output end of the polarization-preserving mode group selection type photon lantern is coupled with a linear polarization mode group (LPG) in the elliptical ring-shaped core optical fiber matched with refractive index distribution, the same linear polarization mode group (LPG) under different polarization states are respectively and independently transmitted, simultaneous multiplexing of different polarization and space mode groups is realized, and the capacity of a communication system is effectively improved.
Example 1:
in the embodiment, N =6, 6 optical fibers at the input end correspond to the fiber core arrangement mode of the polarization maintaining mode group selection type photon lantern (PMMGS-PL) of 4 LP mode groups, fiber cores with different thicknesses are adopted for different LP mode groups, and the same fiber core is adopted for a degenerate mode in the same LP mode group. The fiber cores of the optical fibers are arranged in an elliptical ring shape, and elliptical coreless fluorine-doped low-refractive-index optical fibers are inserted into the center of the ellipse. The fiber core on the central axis of the elliptical ring is sequentially thinned corresponding to the fiber cores on two sides of the circular symmetry mode, and the thick fiber core on the same ring corresponds to the LP mode with the large effective refractive index of the n-th on the ring. The fiber cores on the ring need to meet the relative thickness and arrangement distribution, and the specific fiber core size can be obtained by parameter optimization calculation through simulation software.
To fabricate an elliptical ring core fiber optic photonic lantern, as shown in fig. 1 and 2, the same procedure as the main Mode Selection Photonic Lantern (MSPL) was followed and an elliptical coreless fluorine-doped low index fiber was used in place of the central core. The diameter of the elliptical coreless fluorine-doped low-refractive-index optical fiber is determined by the diameters of six surrounding few-mode optical fibers, the row of the long hexagonal fiber with the elliptical section as the circumscribed circle is tangent to the six surrounding few-mode optical fibers respectively, as shown in figure 2, the fiber core of the optical fiber is centered according to the arrangement of the long hexagonal fiber with the elliptical core distance from the circumscribed circle, and 1 elliptical coreless fluorine-doped low-refractive-index optical fiber is centered to form more stable hexagonal fiber core arrangement, so that crosstalk is reduced, and less loss is generated. And meanwhile, the stretching proportion is controlled to control the normalized frequency value of the few-mode optical fiber obtained after tapering, so that the output end of the tapered photon lantern presents the fiber core refractive index distribution of the elliptical ring-core optical fiber, and the tapered photon lantern is better coupled with the ring-core optical fiber to enable the photon lantern to generate a designed mode. The core spacing of the few-mode optical fiber before tapering is controlled to be 248 μm respectively, and the major axis and the minor axis of the elliptical coreless fluorine-doped low-refractive-index optical fiber core are c y ＝90μm、c x =120 μm, refractive index difference relative to cladding of few-9 × 10 -3 And after tapering, zero intensity distribution can be formed in the center of the output end. Such a design would result in an elliptical ring core distribution at the output of the photon lantern, as shown in fig. 3 (a). The optical field output by the output end presents a stable elliptical LP mode, and the polarization birefringence difference between different polarization states of the same mode group is larger than 10 -4 . In the manufacturing process, the few-mode optical fiber is fixed at the input end, the whole optical fiber bundle and the tapered glass sleeve are fixed at the output end, and the tapered glass sleeve on the outer layer and the tapered glass sleeve inside are tapered by controlling the tapering speed and the tapering temperature in the tapering processUnder the condition that the tapering proportion of core is certain, the photon lantern structure size is longer on the direction of perpendicular ground, and the optical fiber external diameter changes more slowly, guarantees the adiabatic evolution and the transmission of mode in the optic fibre simultaneously to make the photon lantern form the structure as shown in fig. 1, and fig. 4 (a) is the light field distribution and the effective refractive index under the different modes of photon lantern output end that this embodiment shows.
Comparative example 1:
based on the contents of the above example 1, 6 kinds of few-mode optical fibers having different core diameters and 1 round coreless fluorine-doped low refractive index optical fiber, which are the same as those of the example 1, were used as a comparative example, and under the same design conditions, the 6 few-mode optical fibers correspond to LP x 01 Mode, LP y 01 Mode, LP 11a Mode, LP 11b Mode, LP 21a Mode, LP 21b Mode(s). As shown in fig. 3 (b), the tapered output end of the photonic lantern shows the core refractive index distribution of the circular ring core optical fiber. The diameters of the inner circle and the outer circle of the ring core are respectively a x ，b x =5.06 μm. The output pattern in this distribution is shown in FIG. 4 (b), where δ n eff ＜1×10 -4 Due to δ n between polarization states eff Small and cannot multiplex different polarization states in the same spatial mode at the same time.
Comparative example 2:
based on the contents of the first embodiment, as a comparative example, 6 few-mode fibers with different core diameters were used, and the 6 few-mode fibers correspond to LP x 01 Mode, LP y 01 Mode, LP 11a Mode, LP 11b Mode, LP 21a Mode, LP 21b Mode(s). As shown in fig. 3 (c), the tapered output end of the photon lantern shows the core refractive index distribution of the elliptical core fiber. The major and minor semi-axes of the elliptical core are respectively a ＝ ，b x =5.06 μm. The output modes in this distribution are shown in FIG. 4 (c), where each mode has a different polarization δ n eff ＜1×10 -4 Due to δ n between polarization states eff Small and cannot multiplex different polarization states in the same spatial mode at the same time.
Example 2:
the invention also provides an air hole auxiliary type polarization maintaining mode group selection type photon lantern (PMMGS-PL), wherein the cross section distribution of the optical fiber arrangement is shown as figure 6, and the photon lantern is different from the basic structure only in that: two air holes 8 which are symmetrically arranged in the circumferential direction and run through in the axial direction are formed in the conical glass sleeve 1, and are used for introducing air when the photonic lantern is drawn so as to form an elliptical ring core. This photon lantern includes toper glass sleeve 1 and is located input 2 of 1 big head end one side of toper glass sleeve and is located the output 3 of one side of the little head end of toper glass sleeve 1, it mixes fluorine low refractive index optic fibre 4 to pass an ellipse centreless in the toper glass sleeve 1, input 2 glass sleeve centre bore diameter is 2a, and 1a is restricted 500 mu m, and there is an air vent respectively in the centre bore both sides, and the diameter is 2r, and 1 is restricted r and 300 mu m, and the centre of a circle of two air vents 8 and the centre of a circle of the centre bore of toper glass sleeve 1 are on same straight line, and the distance of air vent 8 and centre of a hole is equallyd divide and do not be d. The air holes are deformed in the drawing process of the photon lantern, so that the central hole generates an elliptical stress structure, and the refractive index distribution of the optical fibers in the photon lantern can be influenced.
Example 3:
the polarization-maintaining mode group selection type photon lantern designed by the invention is used as a communication link of the mode multiplexing and demultiplexing device.
In order to verify the transmission performance of the photonic lantern of the present invention, this embodiment 2 separately excites six modes before the multiplexing mode. After the six modes are multiplexed by the polarization-maintaining mode group selection type photon lantern, the QPSK signals are transmitted on the 1km elliptical ring core optical fiber without MIMO data, the schematic diagram of the experimental device is shown in figure 5, wherein the tunable laser is a continuous wave laser with tunable C-band wavelength, the modulator is a C-band Mach-Zehnder modulator, the bit sequence generator generates PRBS signals, and the modulated optical signals excite different modes through the 1 x 6 coupler. After the optical signals in different modes pass through the delay period, a certain time difference is generated, after the optical signals are combined by the polarization beam combiner, the phase difference is introduced by the polarization controller, different polarization states in different modes are controlled, and the optical signals are coupled into the polarization-maintaining mode group selection typeThe system is characterized in that the photon lantern couples different modes in different optical fibers to the same few-mode optical fiber for mode and polarization state multiplexing transmission, and then receives different mode signals in a time-sharing manner after another photon lantern is used for demultiplexing. A unipolar non-return-to-zero (NRZ) QPSK signal is generated by an IQ modulator driven (2) by two pseudo-random binary sequence (PRBS) signals 15 -1 and 2 20 -1). The 1550nm QPSK modulated signal is split into 6 channels, which are then time delayed to simulate 6 independent QPSK signals. LP x 11a And LP y 11a The corresponding signals are combined by a polarization beam combiner, and polarization is controlled by a polarization controller before being coupled to the elliptical ring core optical fiber. The fiber-based polarization controller is adjusted to align the two polarizations with the major or minor axis of the elliptical ring core fiber. The optical signals aligned by the polarization controller are coupled to the input end of the polarization mode group selection type photon lantern, and the polarization mode group selection type photon lantern is used as a multiplexer to transmit each spatial mode and polarization in the same elliptical ring core optical fiber. After propagating through the 1km elliptical ring core fiber, the signal enters a demultiplexing stage, which receives only one mode at a time. By using another partial mode group selection type photon lantern as a demultiplexer, the beam can be coupled into a Single Mode Fiber (SMF). Finally, the received signal is directed to a coherent receiver, coherently received and captured by a real-time oscilloscope. Conventional digital signal processing is applied to single-polarization single-mode coherent detection systems using off-line processing. Digital signal processing consists of only a retiming block, a single equalizer, frequency offset, and carrier phase recovery.
While the present invention has been described with reference to the accompanying drawings, the present invention is not limited to the above-described embodiments, which are illustrative only and not restrictive, and various modifications which do not depart from the spirit of the present invention and which are intended to be covered by the claims of the present invention may be made by those skilled in the art.
Claims (10)
1. A polarization-maintaining mode group selection type photon lantern comprises a conical glass sleeve, an input end and an output end, wherein the input end is positioned on one side of a large head end of the conical glass sleeve, and the output end is positioned on one side of a small head end of the conical glass sleeve; the tapered glass sleeve penetrates through an elliptical coreless fluorine-doped low-refractive-index optical fiber, and the input end comprises N small-mode optical fibers which surround the elliptical coreless fluorine-doped low-refractive-index optical fiber and are distributed according to a circular ring shape, wherein the N small-mode optical fibers are different in diameter; the method is characterized in that:
tapering the coreless fluorine-doped low-refractive-index optical fiber from the input end to the output end by N few-mode optical fibers surrounding the coreless fluorine-doped low-refractive-index optical fiber;
the output end comprises a few-mode fiber cladding in a fusion state, a few-mode fiber core leaked to the outer side of the few-mode fiber cladding and an elliptical coreless fluorine-doped low-refractive-index fiber in the fusion state, so that an elliptical ring-core tapered fiber bundle with double refraction is formed in the tapered glass sleeve, the refractive index distribution of the elliptical ring-core tapered fiber bundle is an elliptical ring, and the major and minor axes of the inner ellipse of the elliptical ring are a and a respectively y 、a x The major and minor axes of the outer ellipse are respectively b y 、b x ；
The output end is welded with an elliptical ring core optical fiber matched with the refractive index distribution of the output end.
2. The polarization-maintaining mode group selection type photon lantern of claim 1, characterized in that N is more than 1 and less than or equal to 10, and the elliptical ring core fiber is a few-mode fiber.
3. The polarization maintaining mode group selection type photon lantern of claim 1, wherein N > 10, said elliptical ring core fiber is multimode fiber.
4. The polarization-maintaining mode group selection type photonic lantern of claim 1, wherein the relationship between the major and minor axes of the inner ellipse and the major and minor axes of the outer ellipse is: 0 μm < a y ≤20μm，0μm＜a x ≤a y ，a y ＜b y ≤50μm，0μm＜b x ≤b y 。
5. The polarization maintaining mode group selection type photon lantern of claim 1, wherein the core major and minor axes of said elliptical coreless fluorine-doped low refractive index optical fiber are c y 、c x ，10μm＜c x ＜100μm，c x ＜c y Less than 150 μm; the refractive index difference of the elliptic coreless fluorine-doped low-refractive-index optical fiber relative to the tapered glass sleeve is-9 multiplied by 10 3 And the centers of the light spots generated by the output ends form a zero-intensity distribution.
6. The polarization-maintaining mode group selection type photonic lantern of claim 1, wherein the tapered glass sleeve is provided with two air holes which are symmetrically arranged in the circumferential direction and pass through in the axial direction, and used for introducing air when the photonic lantern is drawn so as to form an elliptical ring core.
7. The polarization-maintaining mode group selection type photon lantern of claim 6, wherein the centers of the two air holes and the center of the central hole of the tapered glass sleeve are on the same straight line.
8. A method of making a polarization maintaining mode group selective photonic lantern according to any of claims 1-7, comprising the steps of:
inserting N few-mode optical fibers with different diameters into a glass sleeve in an annular arrangement mode around an elliptical coreless fluorine-doped low-refractive-index optical fiber, wherein the long axis and the short axis of a fiber core of the elliptical coreless fluorine-doped low-refractive-index optical fiber are respectively c y 、c x ，10μm＜c x ＜100μm，c x ＜c y ＜150μm；
Tapering the glass sleeve into which the optical fiber bundle is inserted to form a tapered elliptical ring core photon lantern;
the output end of the formed elliptic ring core photon lantern is a small opening end of a conical glass sleeve in a fusion state, and comprises a few-mode optical fiber cladding in the fusion state, a few-mode optical fiber core leaked to the outer side of the few-mode optical fiber cladding and an elliptic coreless fluorine-doped low-refractive-index optical fiber in the fusion state;
and welding one end of the elliptical ring core optical fiber matched with the refractive index distribution of the output end with the output end.
9. The application of the polarization-maintaining mode group selection type photon lantern is characterized in that the polarization-maintaining mode group selection type photon lantern prepared by the preparation method of claim 8 and obtained according to any one of claims 1 to 7 realizes the simultaneous multiplexing of different polarization and space mode groups, so that the capacity of a communication system is improved.
10. The application of the polarization-maintaining mode group selective photon lantern according to claim 7, wherein the polarization-maintaining mode group selective photon lantern is used as a mode group and a polarization multiplexer, modulated optical signals are input to all the few-mode optical fiber ends at the input end of the polarization-maintaining mode group selective photon lantern, the fundamental mode of the few-mode optical fiber at the input end is excited, modal evolution is realized through taper transition, the fundamental mode is transformed into a certain pair of eigenmodes corresponding to the elliptical ring core optical fiber at the output end of the polarization-maintaining mode group selective photon lantern, each fundamental mode signal excited at the input end is coupled with a certain linear polarization mode group (LPG) in two polarization states corresponding to the elliptical ring core optical fiber at the output end formed after the elliptical ring core tapered optical fiber bundle is output, the linear polarization mode groups (LPG) in the elliptical ring core optical fiber matched with refractive index distribution at the output end of the polarization-maintaining mode group selective photon lantern are independently transmitted, and thus the simultaneous multiplexing of different polarization modes and spatial group modes is realized.
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| CN114966988B (en) * | 2022-06-17 | 2023-07-07 | 燕山大学 | Oval core photon lantern supporting eight modes in S+C+L wave band |
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| WO2008007743A1 (en) * | 2006-07-12 | 2008-01-17 | The Furukawa Electric Co., Ltd. | Polarization retaining optical fiber, manufacturing method of polarization retaining optical fiber connector, and polarization retaining optical fiber connector |
| JP6529925B2 (en) * | 2016-04-12 | 2019-06-12 | 日本電信電話株式会社 | Mode multiplexer / demultiplexer, optical transmission system and relay transmission system |
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