WO2015090049A1 - 一种低简并度少模光纤 - Google Patents

一种低简并度少模光纤 Download PDF

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Publication number
WO2015090049A1
WO2015090049A1 PCT/CN2014/081894 CN2014081894W WO2015090049A1 WO 2015090049 A1 WO2015090049 A1 WO 2015090049A1 CN 2014081894 W CN2014081894 W CN 2014081894W WO 2015090049 A1 WO2015090049 A1 WO 2015090049A1
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core
mode
auxiliary
refractive index
main
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French (fr)
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陈明阳
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Jiangsu University
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Jiangsu University
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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
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02042—Multicore optical fibres
    • 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
    • G02B6/36—Mechanical coupling means
    • G02B6/38—Mechanical coupling means having fibre to fibre mating means
    • G02B6/3807—Dismountable connectors, i.e. comprising plugs
    • G02B6/3873—Connectors using guide surfaces for aligning ferrule ends, e.g. tubes, sleeves, V-grooves, rods, pins, balls
    • G02B6/3885—Multicore or multichannel optical connectors, i.e. one single ferrule containing more than one fibre, e.g. ribbon type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06729—Peculiar transverse fibre profile
    • H01S3/06737—Fibre having multiple non-coaxial cores, e.g. multiple active cores or separate cores for pump and gain

Definitions

  • the invention relates to the field of optical fiber communication, and is specifically divided into a small mode optical fiber which can achieve a reduction in mode degeneracy.
  • the fundamental mode of a conventional fiber consists of two degenerate linear polarization modes.
  • the higher-order mode of the fiber consists of two or four degenerate modes, such as the LP U mode and the LP 21 mode, which are composed of four degenerate modes, and LP, according to their symmetry and degeneracy.
  • the 2 mode consists of two linear polarization modes.
  • each linear polarization state consists of two modes. The mode fields of these two modes are different, and the transmission constants are the same. During the transmission process, the two modes will be coupled due to the change of the refractive index distribution of the fiber and the influence of the external environment, and the two modes of energy exchange.
  • the modular multiplexing technology is considered to be a new technology to improve the optical fiber communication capacity.
  • different modes are used as channels for different information, thereby achieving a multiplication of communication capacity [DJ Richardson, JM Fini, and LE Nelson. Space-division multiplexing in optical fibres) Reuse Technology) ⁇ Nat. P/) Oton., 201 3, 7(5): 354-362 ].
  • different modes should be able to achieve relatively independent transmission, thus avoiding crosstalk between modes caused by coupling between modes. For this reason, it is necessary to reduce the degree of degeneracy of higher order modes.
  • Dual-mode fiber has a wide range of applications in mode converters, mode-selective couplers, acousto-optic frequency shifters, and interference fiber sensors.
  • Dual mode fiber means that only two linear polarization fundamental modes, LP, are allowed to be transmitted. And two LP U mode fibers. This also needs to be achieved by increasing the mode birefringence of higher-order modes [W. Jin, Z. Wang, and J. Ju. Two-mode photonic crystal fibers. Opt. Express, 2005, 13 (6) ): 2082-2088] [Double mode photonic crystal fiber and its application, patent number ZL20051 01 1 8576.9].
  • Methods for reducing the high-order mode degeneracy include the use of elliptical core fibers [H. Kubota and T. Morioka. Few-mode optical fiber for mode-division multiplexing. Opt. Fiber Technol. ⁇ 1 , 17(5): 490-494] and photonic crystal fiber [dual-mode photonic crystal fiber and its application, patent number ZL20051 01 1 8576.9]. Its production technology is relatively complicated.
  • the present invention provides a mode-less optical fiber that reduces the high order mode degeneracy.
  • the technical solution of the present invention is a low degeneracy mode-less optical fiber comprising a core and a cladding, the core being composed of a main core and 2 auxiliary cores, the main core being located at the center of the fiber.
  • the two auxiliary auxiliary cores are symmetrically arranged on both sides of the main core, and the center of the main core and the center of two auxiliary cores are on the same straight line; the main core and the auxiliary core
  • the refractive index is the same, the main core boundary is tangent to the adjacent auxiliary core boundary, and the boundaries of two adjacent auxiliary cores are tangent; the auxiliary core diameter is smaller than the main Core diameter, and the auxiliary core is sequentially decreased from the inside to the outside, Medium N is a positive integer.
  • the diameter of the main core should satisfy:
  • the diameter of the auxiliary core should satisfy: d ⁇ 2.451
  • ⁇ i N X is the wavelength of light
  • dad is the cladding refractive index
  • cL is the core diameter, which is the diameter of the ith auxiliary core.
  • the main core diameter should satisfy: ⁇ - .
  • ⁇ is the wavelength of light
  • ⁇ is the cladding refractive index
  • dDT is the core diameter.
  • This refractive index profile reduces the quadratic degenerate higher order mode to a double degenerate mode in which a pair of degenerate mode fields are extended toward the auxiliary core; and the other pair of degenerate modes are distributed away from the auxiliary core, The difference is small when introducing the auxiliary core; thereby, the effective refractive index difference between the two is increased, and the mode field distribution form of the higher-order mode is fixed.
  • the fiber-based fundamental mode still has good circular symmetry, which is beneficial to the connection with ordinary optical fibers.
  • Figure 1 is a schematic cross-sectional view of an embodiment of the present invention
  • Figure 2 shows the mode field distribution of different modes in an elliptical core fiber, where (UV mode, (b) LP u X mode, (c) LP u Y mode, (d) LP : 1 X mode, (e) LP 21 Y mode;
  • Figure 3 is a diagram of the mode field distribution of different modes in the structure shown in Figure 1, where (a P ⁇ mode, (b) LP X mode, (c) LP condimentY mode 2 (d) LP 21 X mode, 1 ⁇ 1 ⁇ , (1 ⁇ . 2 die, ( 8 ) 1 ⁇ 31 die, (h) LP 31 Y die;
  • FIG. 4 is a graph showing the effective refractive index of different modes in the embodiment shown in FIG. 1 as a function of the diameter of the auxiliary core;
  • FIG. 5 is a cross-sectional view showing another embodiment of the present invention.
  • Figure 6 shows the mode field distribution of the different modes of the structure, where (a) LP D1 mode, (b) LPuY mode;
  • a low degeneracy mode-less fiber consisting of a core and a cladding consisting of a main core and 2N auxiliary cores (N is a positive integer), all cores having the same refractive index .
  • the center of the main core is in line with the center of all auxiliary cores.
  • the auxiliary core is located on both sides of the main core, and the main fiber
  • the core boundary is tangent to the boundary of the adjacent auxiliary core, and the boundary of the adjacent auxiliary core is tangent.
  • the distribution has axis symmetry and central symmetry.
  • the diameter of the main core; the auxiliary core adjacent to the main core is labeled as the auxiliary core No. 1
  • the auxiliary core adjacent to the auxiliary core No. 1 is labeled as the auxiliary core No. 2, and so on;
  • L represents the distance between the center of the main core and the center of the adjacent auxiliary core (ie, the No. 1 auxiliary core); indicates the adjacent two auxiliary cores (the auxiliary core i and the i+1 on the same side of the main core) Center spacing of the auxiliary core).
  • the main core diameter should satisfy: ⁇ , the auxiliary core diameter should be full
  • the optical fiber of the present invention can realize dual mode transmission.
  • the main core diameter should satisfy 1.41 - 2.4051.
  • the invention introduces an auxiliary core on both sides of the main core, so that the refractive index distribution of the optical fiber has a double symmetry, so that the symmetry axis of the mode field distribution of the high-order mode is fixed.
  • the main core is at the center of the coordinate axis and that the center of the auxiliary core is on the X axis.
  • the quadratic degenerate higher order modes will form two mode field distributions symmetric along the X axis and symmetric along the Y axis, referred to herein as X and Y modes.
  • the mode field distribution must meet the refractive index distribution requirements of the fiber.
  • two of the four degenerate modes of the quadratic degenerate higher-order mode will exhibit symmetry along the Y-axis, and the energy flow direction will be along the X-axis direction, thereby transferring energy to the auxiliary core, thereby making it
  • the effective refractive index is increased.
  • the other two modes are less different from before the introduction of the auxiliary core.
  • a large refractive index difference is formed between the degenerate modes.
  • energy coupling is easy to occur between modes with similar effective refractive indices. Therefore, after increasing the effective refractive index difference between modes, energy coupling between modes during transmission can be effectively avoided.
  • high-order modes have a fixed mode field distribution that facilitates connection to other fibers and facilitates efficient, selective mode coupling when forming multi-core fiber or fiber couplers.
  • Patterns in elliptical core fibers and high birefringence photonic crystal fibers cause distortion in all modes of the fiber, including the fundamental mode of the fiber.
  • Figure 2 shows the mode field distribution of the fundamental mode and the higher-order mode of the elliptical core fiber. It can be seen from the figure that the mode field deformation in the elliptical core fiber is more serious.
  • Figure 3 shows the mode field distribution of the fiber of the present invention.
  • the fiber fundamental mode still has a similar circular symmetry mode field distribution. This is because, for a round core, the fiber-based mold has circular symmetry. Sex. Since the normalized frequency of the mode-less fiber is large, the fundamental mode energy is mainly concentrated in the core region, and therefore, the auxiliary core has less influence on it.
  • the auxiliary core only has a boundary point tangent to the main core, the area near the main core is still dominated by a matrix material (ie, a cladding), and the matrix material is sufficient to bind the optical fiber base mold, so when the auxiliary core is introduced In hours, the fiber-based fundamental mode will still maintain its original symmetry. Only when the auxiliary core is large enough or the normalized frequency of the main core is small, the mode field distribution will be deformed. It can be known from the fiber theory that when there is no auxiliary core, the LP U mode, the LP 21 mode, and the LP 31 mode are all quadratic degenerate modes.
  • the quadruple degenerate mode is reduced to a double degenerate mode, that is, split into a Y mode and an X mode.
  • the X mode is similar to the front mode field of the auxiliary core, and the Y mode is extended to the auxiliary core, and can be deformed with the increase of the auxiliary core.
  • Figure 4 is a graph showing the effective refractive index of different modes in the embodiment shown in Figure 1 as a function of the diameter of the auxiliary core. as illustrated,.
  • the effective refractive indices of the degenerate modes are equal.
  • the auxiliary core When the auxiliary core is introduced, its quadruple degenerate mode is reduced to a double degenerate mode.
  • the effective refractive index of the LP spiritY mode increases as the diameter of the auxiliary core increases.
  • the effective refractive index of the LP teaspoonX mode does not change substantially with the diameter of the auxiliary core. Therefore, by selecting a suitable auxiliary core, the modes of transmission in the fiber in the same polarization state have a certain refractive index difference. Reduce the coupling between different modes.
  • the auxiliary core should have the same refractive index as the main core. If the refractive index of the auxiliary core is lower than that of the main core, its effect is weakened. If the refractive index of the auxiliary core is higher than that of the main core, the main energy in the main core is easily expanded to the auxiliary core, so that the mode field is severely deformed, and the auxiliary core is formed by being surrounded by the low refractive index material. Separate mode.
  • the set of higher-order modes in which the mode field is extended to the auxiliary core may not be transmitted, and only the double degenerate mode and the quadruple degenerate mode in the circularly symmetric fiber are reduced to reduce the effective refractive index after the degeneracy is reduced.
  • the smaller set of modes which have better similarities to the modes in normal fiber, making it easy to implement low-loss connections.
  • the LP U Y mode has a larger effective refractive index due to the energy being extended to the auxiliary core, and its LP X mode remains in the main core, and its effective refractive index is small.
  • the LPuX mode can be turned off by selecting the appropriate fiber parameters.
  • the main core diameter should meet: ⁇ * ⁇ . That is, when there is no auxiliary core, the fiber should be single mode transmission. As shown in Fig.
  • the LP U Y mode has a part of the energy transmitted in the auxiliary core. Therefore, the introduction of the auxiliary core increases the equivalent core area for the LP U Y mode, thereby This mode can be generated. In addition, for the LP U X mode, the mode field distribution is still similar to that of the introduction of the auxiliary core, so this mode cannot be formed when the core normalization frequency is small.
  • the main core should be combined with the auxiliary core to form a refractive index profile of the LP U Y mode.
  • the optical fiber of the present invention can be realized by a manufacturing method similar to that of a photonic crystal fiber.
  • a high refractive index dielectric column Arranged into a core a cladding is formed by a low refractive index dielectric column, placed in a quartz tube to form a fixed structure, and then drawn to form a desired size of the optical fiber structure. Since there are no air holes, the fiber is produced without complicated control to maintain the air holes, and it is simpler when the fiber is used, such as when connected to a common fiber. It is also simpler to make than an elliptical core fiber that requires a complicated manufacturing process.
  • the fiber of the present invention has a larger core size, which is more advantageous for connection with a common single mode fiber and has a wider bandwidth than the elliptical core fiber.
  • Embodiment 1 is a diagrammatic representation of Embodiment 1:
  • the cross section of the fiber is shown in Figure 1.
  • the main core diameter d negotiate 20 ⁇
  • the auxiliary core diameter d al 6 ⁇
  • the core and cladding refractive index difference is 0.01.
  • the mode field distribution is shown in Figure 3
  • mode The effective refractive index is shown in Figure 4.
  • Embodiment 2 is a diagrammatic representation of Embodiment 1:
  • the cross section of the fiber is shown in Figure 1.
  • the main core diameter d negotiate 10 ⁇
  • the auxiliary core diameter d al 4 m
  • the core and cladding refractive index difference is 0.004.
  • the optical fiber is dual mode transmission.
  • Embodiment 3 is a diagrammatic representation of Embodiment 3
  • the fiber cross section is shown in Figure 5.
  • the main core diameter d negotiate 10 ⁇
  • the auxiliary core diameter d al 4 ⁇
  • the auxiliary core diameter 2 2 3 ⁇
  • the core and cladding refractive index difference is 0.004.
  • the wavelength is 1.42 ⁇ .
  • the fiber is dual-mode transmission.
  • the mode field distribution is shown in Figure 6. It can be seen that the main energy of the fundamental mode is still concentrated in the main core, and the main energy of the LP U Y mode is concentrated in the auxiliary core. .

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Fibers, Optical Fiber Cores, And Optical Fiber Bundles (AREA)
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Abstract

一种低简并度少模光纤，包括纤芯和包层（3），所述纤芯由一个主纤芯（1）和2N个辅助纤芯（2）所组成，所述主纤芯（1）位于光纤中心，所述2N个辅助纤芯（2）对称的排布在所述主纤芯（1）两侧，所述主纤芯（1）的中心和2N个辅助纤芯（2）的中心处于同一直线上；所述主纤芯（1）和辅助纤芯（2）的折射率相同，所述主纤芯（1）边界与相邻的所述辅助纤芯（2）边界相切，相邻的两个所述辅助纤芯（2）的边界相切；所述辅助纤芯（2）直径均小于所述主纤芯（1）直径，且所述辅助纤芯（2）直径由内向外依次递减。通过引入辅助纤芯（2），破坏光纤的折射率分布的圆对称性，形成具有二重对称性的折射率分布。这种折射率分布使四重简并的高阶模降为二重简并模，增大两者的有效折射率差，并使高阶模的模场分布形式固定。

Description

说 明 书
一种低简并度少模光纤
技术领域
本发明涉及光纤通信领域， 具体分为可实现降低模式简并度的少模光纤。
背景技术
普通光纤的基模由两个简并的线性偏振模组成。 光纤的高阶模根据其对称性和简并度， 由两个或 4个简并模组成， 如 LPU模， LP21模均由四个简并模组成， 而 LP。2模则由两个线性 偏振模组成。 对于四个简并模组成的高阶模， 其每种线性偏振态均由两个模式组成。 这两个 模式的模场分布不同， 而传输常数相同。 在传输过程中， 这两个模式将由于光纤折射率分布 的变化和外界环境的影响而发生耦合， 两个模式能量发生交换。
模分复用技术被认为是提高光纤通信容量的一种新技术。 在模分复用系统中， 不同的模 式被作为不同信息的通道， 从而实现通信容量的成倍增加 【D. J. Richardson, J. M. Fini, and L. E. Nelson. Space-division multiplexing in optical fibres (光纤中的空分复用技术)■ Nat. P/)Oton.,201 3, 7(5): 354-362 ] 。 为实现有效的模式复用， 不同模式之间应该能够实现 较独立的传输， 从而避免模式间耦合造成传输信号之间的串扰。 为此， 需要降低高阶模的简 并度。 双模光纤在模式转换器、 模式选择耦合器、 声光移频器、 干涉型光纤传感器等方面均 具有广泛的应用。 双模光纤指仅允许传输两个线性偏振基模即 LP。 和两个 LPU模的光纤。 这同样需要通过增大高阶模的模式双折射来实现【W. Jin, Z. Wang, and J. Ju. Two-mode photonic crystal fibers (双模光子晶体光纤） . Opt. Express,2005, 13(6): 2082-2088】 【双 模光子晶体光纤及其应用， 专利号 ZL20051 01 1 8576.9】 。
降低高阶模简并度的方法包括采用椭圆芯光纤【H. Kubota and T. Morioka. Few-mode optical fiber for mode-division multiplexing (用于模分复用的少模光纤） . Opt. Fiber Technol.^ 1 , 17(5): 490-494】 以及光子晶体光纤【双模光子晶体光纤及其应用， 专利号 ZL20051 01 1 8576.9】 。 其制作技术都相对复杂。
发明内容
针对现有技术的不足， 本发明提供一种降低高阶模简并度的少模光纤。 本发明的技术方 案是， 一种低简并度少模光纤， 包括纤芯和包层， 所述纤芯由一个主纤芯和 2Ν个辅助纤芯 所组成， 所述主纤芯位于光纤中心， 所述 2Ν个辅助纤芯对称的排布在所述主纤芯两侧， 所 述主纤芯的中心和 2Ν个辅助纤芯的中心处于同一直线上； 所述主纤芯和辅助纤芯的折射率 相同， 所述主纤芯边界与相邻的所述辅助纤芯边界相切， 相邻的两个所述辅助纤芯的边界相 切； 所述辅助纤芯直径均小于所述主纤芯直径， 且所述辅助纤芯直接由内向外依次递减， 其 中 N为正整数。
2—4051
进一步， 所述主纤芯直径应满足： ， 辅助纤芯直径应满足： d < 2.4051
" 'H ， 其中， ^i N, X 为光波长， 。re为纤芯折射率， 》dad为包层折射 率， cL为主纤芯直径， 为第 i号辅助纤芯的直径。 进一步， 所述主纤芯直径应满足：
Figure imgf000004_0001
Ά- . 其中： λ为光波长， " 为 纤芯折射率， ^^为包层折射率， d„为主纤芯直径。 本发明的有益效果是： 通过引入辅助纤芯， 破坏光纤的折射率分布的圆对称性， 形成具 有二重对称性的折射率分布。 这种折射率分布使四重简并的高阶模降为二重简并模， 其中一 对简并模模场向辅助纤芯扩展； 而另一对简并模处于远离辅助纤芯的方向分布， 与引入辅助 纤芯时差别较小； 由此， 增大两者的有效折射率差， 并使高阶模的模场分布形式固定。 同 时， 光纤基模仍具有较好的圆对称性， 有利于与普通光纤的连接。
附图说明
图 1为本发明的一种实施例的横截面示意图；
图 2 为椭圆芯光纤中不同模式的模场分布图， 其中（ UV模，（b)LPuX 模， （c)LPuY 模， （d)LP:1X模， （e)LP21Y模；
图 3 为图 1 所示结构中不同模式的模场分布图， 其中 (a P^模， (b)LP X模， （c)LP„Y 模 2 （d)LP21X模， )1^1¥模，（ 1^。2模，（8)1^31乂模， （h)LP31Y模；
图 4 为图 1所示实施例中不同模式的有效折射率随辅助纤芯直径的变化曲线； 图 5 为本发明的另一种实施例的横截面示意图；
图 6 图 1所示结构不同模式的模场分布图， 其中（a) LPD1模，（b)LPuY模；
图中， 1，主纤芯； 2， 辅助纤芯； 3， 基质材料。
具体实施方式
一种低简并度少模光纤， 由纤芯和包层组成， 所述纤芯由一个主纤芯和 2N 个辅助纤芯 所组成 (N 为正整数）， 所有纤芯的折射率均相同。 主纤芯的中心与所有辅助纤芯的中心均位 于同一直线上。 主纤芯中心与相邻辅助纤芯中心的间距 L 满足： L =(cL+dal)/2。 相邻两个辅 助纤芯的中心间距 满足： ,1+1= ((^+4.^/2，这里 i=l~N- 1。 即辅助纤芯位于主纤芯两 侧， 且主纤芯边界与相邻辅助纤芯边界相切， 相邻辅助纤芯的边界相切。 光纤横截面折射率 分布具有轴对称性和中心对称性。 辅助纤芯的直径满足 （1 1〈 ，且有 （1 1>4, 1+1，这里 i=l~N - 1。 主纤芯和辅助纤芯均具有圆对称性。
其中， 为主纤芯的直径； 与主纤芯相邻的辅助纤芯标记为 1号辅助纤芯， 与 1号辅助 纤芯相邻的辅助纤芯标记为 2号辅助纤芯， 依次类推； 表示第 i 号辅助纤芯的直径。 L 表示主纤芯中心与相邻辅助纤芯（即 1 号辅助纤芯）中心的间距； 表示相邻两个辅助纤芯 (位于主纤芯同一侧的 i号辅助纤芯和 i+1号辅助纤芯）的中心间距。
, 2.4052
作为本发明的进一步改进， 主纤芯直径应满足： Ί ， 辅助纤芯直径应满
， 2.4051
足- ^^-^ ， 这里 i=1~N， A为光波长， ；7 为纤芯折射率， 《elad 包层折射率。
本发 明 光纤可 实现实现双模传输 。 此时 ， 主纤芯直径应满足 1.41 - 2.4051 本发明通过在主纤芯两侧引入辅助纤芯的方法， 使光纤折射率分布具有二重对称， 从而使 高阶模的模场分布对称轴固定。 假设主纤芯位于坐标轴中心， 且辅助纤芯的中心均位于 X 轴。 则四重简并的高阶模将形成沿 X轴对称和沿 Y轴对称的两种模场分布， 这里简称其为 X 模和 Y模。 对于 Y模， 由于辅助纤芯的存在， 光将向辅助纤芯扩展， 从而使此模式的有效折 射率增大。 而对于 X模， 由于模式需要保持其对称性和与 Y模的正交性， 其模式将远离辅助 纤芯。 即 X模的有效折射率与引入辅助纤芯前相近。 由于引入辅助纤芯后光纤折射率分布呈 二重对称， 此时， 模场分布必须符合光纤的折射率分布要求。 这样， 四重简并的高阶模中原 有的四个简并模式中的其中两个模式将呈现沿 Y轴对称， 其能流方向为沿 X轴方向， 从而使 能量向辅助纤芯转移， 使其有效折射率提高。 而另两个模式与引入辅助纤芯前差别较小。 这 样， 简并模之间形成大的折射率差。 由耦合理论， 有效折射率相近的模式间容易发生能量耦 合， 因此， 增大模式间的有效折射率差以后， 可以有效避免传输时模式间的能量耦合。 此 外， 高阶模的模场分布固定， 可以方便与其它光纤的连接， 并有利于组成多芯光纤或光纤耦 合器时实现高效、 有选择性的模式耦合。
椭圆芯光纤和高双折射的光子晶体光纤中的模式会导致光纤中所有模式均发生变形， 包 括光纤基模也具有二重对称性。 如图 2所示为椭圆芯光纤的基模和和高阶模的模场分布图。 由图可见， 椭圆芯光纤中的模式场变形较严重。
图 3给出了本发明光纤的模场分布图。 与椭圆芯光纤和高双折射的光子晶体光纤不同， 光纤基模仍具有类似圆对称的模场分布。 这是因为， 对于圆纤芯来说， 光纤基模具有圆对称 性。 由于少模光纤的归一化频率较大， 基模能量主要集中在纤芯区， 因此， 辅助纤芯对其影 响较小。 特别地， 因为辅助纤芯仅边界点与主纤芯相切， 主纤芯附近区域仍以基质材料 （即 包层） 为主， 基质材料足以束缚光纤基模， 因此当引入的辅助纤芯较小时， 光纤基模仍将维 持其原对称性。 只有当辅助纤芯足够大或主纤芯的归一化频率较小时， 其模场分布才会发生 一定的变形。 由光纤理论可知， 当不存在辅助纤芯时， 其 LPU模、 LP21模、 LP31模均为四重简 并模。 而引入辅助纤芯后， 由图可见， 其四重简并模降为二重简并模， 即分裂为 Y 模和 X 模。 其中 X模与引入辅助纤芯前模场相似， 而 Y模则向辅助纤芯扩展， 并可随辅助纤芯的增 大而发生一定的变形。
图 4 为图 1 所示实施例中不同模式的有效折射率随辅助纤芯直径的变化曲线。 由图可 见，。 简并模的有效折射率相等。 当引入辅助纤芯后， 其四重简并模降为二重简并模。 以 1^„模为例， 其 LP„Y模的有效折射率随着辅助纤芯直径的增大而增大。 而其 LP„X模的有效 折射率基本不随辅助纤芯直径的变化而变化。 因此， 选择合适的辅助纤芯， 即可使同一偏振 态下光纤中传输的模式均具有一定的折射率差， 减小不同模式之间的耦合。
辅助纤芯的折射率应与主纤芯相同。 若辅助纤芯的折射率低于主纤芯， 其作用减弱。 若 辅助纤芯的折射率高于主纤芯， 则易导致主纤芯中主要能量向辅助纤芯扩展， 从而使模场发 生严重变形， 同时辅助纤芯会因为被低折射率材料包围而形成单独的模式。
在实际使用时， 可以不传输模场向辅助纤芯扩展的那组高阶模， 而只传输圆对称光纤中 的二重简并模和四重简并模经降低简并度后有效折射率变化较小的那组模式， 这些模式与普 通光纤中的模式具有更好的相似性， 易于实现低损耗连接。
利用光纤中 LP„模的两组模式的有效折射率不同的特点， 还可以实现具有双模传输特性 的光纤。 即这种光纤中仅存在基模和 LPUY模， 而其 LPUX模被截止。 这是因为 LPUY模由于 能量被扩展到辅助纤芯， 其有效折射率较大， 而其 LP„X模仍保持在主纤芯中， 其有效折射 率较小。 选择合适光纤参数即可使 LPuX模截止。 对于双模光纤要求： 其主纤芯直径应满足： ϋ *ϋ 。 即在无辅助 纤芯时， 光纤应为单模传输。 如图 3(c)所示， LPUY模有部分能量在辅助纤芯中传输， 因 此， 辅助纤芯的引入对于 LPUY模来说， 其等效的纤芯面积增大， 从而使该模式能够产生。 另对于 LPUX模来说， 其模场分布仍与引入辅助纤芯时相似， 因此， 当纤芯归一化频率较小 时， 此模式仍无法形成。 主纤芯应与辅助纤芯组成足以形成 LPUY模的折射率分布。 本发明光纤可以采用与光子晶体光纤相似的制作方法来实现。 比如将高折射率介质柱 排布成纤芯， 用低折射率介质柱组成包层， 将其放置于石英管中形成固定结构， 再经拉丝形 成所需尺寸的光纤结构。 由于不存在空气孔， 光纤的制作时不需要复杂的控制以保持空气 孔， 在光纤使用如与普通光纤连接时也更为简单。 制作时也比需要复杂制作工艺的椭圆芯光 纤更为简单。 实现双模传输时， 本发明光纤的纤芯尺寸更大， 从而更有利于与普通单模光纤 的连接， 且带宽比椭圆芯光纤宽。
实施例一：
光纤横截面如图 1所示。 主纤芯直径 d„=20 μη， 1号辅助纤芯直径 dal=6 μιη， 纤芯与包 层折射率差 0.01。 在波长为 1.55μιη时， 其模式场分布如图 3所示， 模式的有效折射率如图 4所示。
实施例二：
光纤横截面如图 1所示。 主纤芯直径 d„=10 μη， 1号辅助纤芯直径 dal=4 m， 纤芯与包 层折射率差 0.004。 在波长为 1.39 1.73 μιη范围内， 光纤均为双模传输。
实施例三：
光纤横截面如图 5所示。 主纤芯直径 d„=10 μη， 1号辅助纤芯直径 dal=4 μιη， 2号辅助 纤芯直径 42=3 μιη， 纤芯与包层折射率差 0.004。 在波长为 1.42〜 .9 μιη范围内， 光纤均为双 模传输。 其模场分布如图 6所示， 可见基模主要能量仍集中在主纤芯中， 而其 LPUY模的主 要能量集中在辅助纤芯中。
上述附图仅为说明性示意图， 并不对本发明的保护范围形成限制。 应理解， 这些实施例只是 为了举例说明本发明， 而非以任何方式限制本发明的范围。

Claims

权 禾 要 求 书
1. 1. 一种低简并度少模光纤， 包括纤芯和包层， 其特征在于， 所述纤芯由一个主纤芯和 2N个辅助纤芯所组成， 所述主纤芯位于光纤中心， 所述 2N个辅助纤芯对称的排布在所述 主纤芯两侧， 所述主纤芯的中心和 2N个辅助纤芯的中心处于同一直线上； 所述主纤芯和辅 助纤芯的折射率相同， 所述主纤芯边界与相邻的所述辅助纤芯边界相切， 相邻的两个所述辅 助纤芯的边界相切； 所述辅助纤芯直径均小于所述主纤芯直径， 且所述辅助纤芯直接由内向 外依次递减， 其中 N为正整数。
2. 根据权利要求书 1所述的一种低简并度少模光纤， 其特征在于， 所述主纤芯直径应满
足： 、( ™ ， 辅助纤芯直径应满足： 、 《™― ^ ， 其中，
N， 为光波长， 。 1(：为纤芯折射率， lad为包层折射率， 为主纤芯直径， 为第 i号辅 助纤芯的直径。
3. 根据权利要求书 1所述的一种低简并度少模光纤， 其特征在于， 所述主纤芯直径应满
1_41 ^ .405A
d— <
足： '™ '^'^ ， 其中： 为光波长， 。 re 纤芯折射率， ad为包 层折射率， £1„1为主纤芯直径。
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