WO2018072763A2 - 新型少模光纤 - Google Patents

新型少模光纤 Download PDF

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WO2018072763A2
WO2018072763A2 PCT/CN2017/117050 CN2017117050W WO2018072763A2 WO 2018072763 A2 WO2018072763 A2 WO 2018072763A2 CN 2017117050 W CN2017117050 W CN 2017117050W WO 2018072763 A2 WO2018072763 A2 WO 2018072763A2
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Prior art keywords
refractive index
mode
optical fiber
filter layer
high refractive
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French (fr)
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WO2018072763A3 (zh
Inventor
王�华
陈明阳
伍小生
李仁华
蔡志民
曹国栋
李申逸
李路明
杨济海
付萍萍
郑美兰
肖辉
褚红亮
李俊
殷芳
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Jiangsu University
Information and Telecommunication Branch of State Grid Jiangxi Electric Power Co Ltd
State Grid Corp of China SGCC
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Jiangsu University
Information and Telecommunication Branch of State Grid Jiangxi Electric Power Co Ltd
State Grid Corp of China SGCC
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Publication of WO2018072763A3 publication Critical patent/WO2018072763A3/zh
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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
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03688—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 5 or more layers
    • 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
    • G02B6/02004—Optical fibres with cladding with or without a coating characterised by the core effective area or mode field radius
    • G02B6/02009—Large effective area or mode field radius, e.g. to reduce nonlinear effects in single mode fibres
    • G02B6/02014—Effective area greater than 60 square microns in the C band, i.e. 1530-1565 nm
    • G02B6/02019—Effective area greater than 90 square microns in the C band, i.e. 1530-1565 nm
    • 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
    • G02B6/02214—Optical fibres with cladding with or without a coating tailored to obtain the desired dispersion, e.g. dispersion shifted, dispersion flattened
    • G02B6/02219—Characterised by the wavelength dispersion properties in the silica low loss window around 1550 nm, i.e. S, C, L and U bands from 1460-1675 nm
    • G02B6/02252—Negative dispersion fibres at 1550 nm
    • G02B6/02261—Dispersion compensating fibres, i.e. for compensating positive dispersion of other 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
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03638—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
    • G02B6/03644—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only arranged - + -
    • 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
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03694—Multiple layers differing in properties other than the refractive index, e.g. attenuation, diffusion, stress properties
    • 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
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/14—Mode converters

Definitions

  • the present disclosure relates to the field of fiber optic communications, for example, to a novel mode-less fiber that is capable of single mode operation and that has low bending loss characteristics for fiber-based molds.
  • Fiber optic cables are often required to operate at small bending radii, while conventional single-mode fibers cannot meet small bending radii.
  • the G.657 optical fiber standard has been proposed internationally.
  • Low-bend loss fibers are typically implemented by reducing the core size of the fiber, introducing a recessed cladding structure, and using aperture-assisted fiber optics [K.Himeno, S.
  • G.657 fiber can work at a bending radius of 7.5mm or even 5mm, its bending loss is still large, and it is difficult to achieve long-term stable operation under a small bending radius.
  • G.657B3 fiber should meet the bending radius of 7.5 and 5mm.
  • the lower bending loss should be less than 0.08 and 0.15 dB/ ⁇ (1550 nm wavelength), respectively.
  • the fiber will still have a large loss after being wound for several times under this small bending radius, which affects the performance of the communication system.
  • the fiber when the normalized frequency is less than 2.405 at the working wavelength, the fiber is a single mode fiber.
  • the fiber can transmit a high-order mode, thereby being a non-single mode fiber.
  • Conventional multimode or low mode fiber due to the presence of higher order modes, can cause severe inter-mode dispersion problems when signal light is transmitted through the fiber, thereby limiting the fiber's communication rate and capacity.
  • single-mode optical fibers are still used as the main transmission medium in fiber-optic communication systems.
  • non-single-mode fiber is used in the fiber-optic communication system, low bending loss transmission can be obtained by effectively increasing the refractive index difference between the fiber core and the cladding, and then single-mode transmission can be realized by matching connection with the single-mode fiber
  • An optical fiber communication system Chinese patent, 201010589018.1, a communication system based on a small mode fiber, ZL201210393511.5.
  • This technology achieves low bending loss, single mode transmission, and low connection loss transmission by relaxing the limits on the number of fiber transmission modes and by suppressing the generation of higher order modes by connecting to single mode fibers.
  • this method requires that the small-mode fiber is connected to the single-mode fiber at both ends and the connection deviation is small, which makes the practical use limited.
  • the present disclosure provides a novel mode-less fiber that can achieve single mode transmission and maintain low loss transmission at small bend radii.
  • a novel mode-less optical fiber comprising: a core and a cladding surrounding the core, the cladding comprising: a first inner cladding surrounding the core; and a first high refractive index filter layer surrounding the first inner cladding a second inner cladding surrounding the first high refractive index filter layer; a second high refractive index filter layer surrounding the second inner cladding; and an outer cladding surrounding the second high refractive index filter layer.
  • the effective refractive index of the LP11 mode of the optical fiber is greater than the effective refractive index of the cladding defect mode of the optical fiber in the wavelength range of 1260-1625 nm.
  • the radial width a3 of the first high refractive index filter layer and the radial width a5 of the second high refractive index filter layer satisfy: a 3 ⁇ a 5 .
  • the refractive index n1 of the core and the refractive index n6 of the outer cladding satisfy: 0.015>n1-n6>0.007.
  • the radial width a2 of the first inner cladding satisfies: 7.9 ⁇ m ⁇ a 2 ⁇ 4.0 ⁇ m.
  • the radial width a3 of the first high refractive index filter layer satisfies: 8.5 ⁇ m ⁇ a 3 ⁇ 3.5 ⁇ m.
  • the radial width a4 of the second inner cladding satisfies: 4.6 ⁇ m ⁇ a 4 ⁇ 2.4 ⁇ m.
  • the radial width a5 of the second high refractive index filter layer satisfies: 8.5 ⁇ m ⁇ a 5 ⁇ 3.5 ⁇ m.
  • the bending loss of the optical fiber at a wavelength of 1550 nm satisfies: when the bending radius R b ⁇ 7.5 mm, the bending loss of the LP01 mode is less than 1 ⁇ 10-3 dB/ ⁇ ; when the bending radius R b ⁇ 7.5 mm, the bending loss of the LP11 mode is greater than 5 dB. / ⁇ .
  • the cutoff wavelength of the optical fiber is greater than 1.625 [mu]m.
  • the present disclosure provides a novel mode-less fiber solution with a very low bend loss ( ⁇ 1 x 10-3 dB/ ⁇ ) with a bend radius above 7.5 mm.
  • the optical fiber of the present disclosure is a non-single mode fiber in a straight waveguide state, and the core is high-order mode and high-refractive-index filter by bending the fiber at a sufficiently small bending radius.
  • the pattern of the layer is strongly coupled, and the high-order mode can be filtered out to achieve the equivalent single-mode transmission.
  • Single-mode fiber transmission is achieved in the same mode as a single-mode fiber. It can be connected to a single-mode fiber by means of soldering or by active connection.
  • the optical fiber of the present disclosure only needs to achieve the purpose of single mode transmission by appropriately bending the optical fiber, and is suitable for the occasions where a compact optical fiber device and a fiber component are required in various short-distance communication systems.
  • the optical fiber structure of the present disclosure has circular symmetry and can be realized by the existing mature optical fiber fabrication process, and only needs to dope the core and the two high refractive index filter layers, while the refractive indices of other regions are the same as the outer cladding. It simplifies the preparation process of the optical fiber and can effectively reduce the manufacturing cost of the optical fiber.
  • 1 is a radial refractive index profile of an optical fiber according to an embodiment of the present application, wherein a 1-core, a 2-first inner cladding, a 3-first high refractive index filter layer, and a 4-second inner cladding layer are provided. , 5 - second high refractive index filter layer, 6 - outer cladding;
  • FIG. 2(a) is a diagram showing a mode field distribution of an LP01 mode of an optical fiber with a bending radius of 7.5 mm according to an embodiment of the present application;
  • 2(b) is a view showing a mode field distribution of an LP11 mode of an optical fiber with a bending radius of 7.5 mm according to an embodiment of the present application;
  • 2(c) is a view showing a mode field distribution of another LP11 mode of an optical fiber with a bending radius of 7.5 mm according to an embodiment of the present application;
  • FIG. 3 is a schematic diagram of a mode field distribution of a cladding defect mode according to an embodiment of the present application.
  • 4(a) is a graph showing the relationship between the bending loss and the operating wavelength of the LP01 mode of the optical fiber, the comparative optical fiber 1, and the comparative optical fiber 2 of the present embodiment;
  • 4(b) is a graph showing the relationship between the bending loss and the operating wavelength of the LP11 mode of the optical fiber, the comparative optical fiber 1, and the comparative optical fiber 2 of the present embodiment.
  • a defect mode having a high leakage loss in a bent state is designed, so that a high-order mode of the optical fiber can be leaked by strong coupling with the cladding defect mode, and Maintain low bending loss transmission of the fiber base mode.
  • n 0 (x, y) is the cross-sectional refractive index distribution of the fiber when unbent, the refractive index distribution of the straight waveguide equivalent to n(x, y), x is the elastic coefficient of the material, and Rb is the bending radius of the fiber.
  • n 0 (x, y) is the cross-sectional refractive index distribution of the fiber when unbent, the refractive index distribution of the straight waveguide equivalent to n(x, y)
  • x is the elastic coefficient of the material
  • Rb is the bending radius of the fiber.
  • the mode energy in the core will concentrate toward the side where the refractive index is increased, that is, its mode effective refractive index generally increases as the bending radius increases.
  • the refractive index of the outer cladding also increases with the bending, which eventually causes the cladding defect mode to fail to satisfy the total internal reflection condition and leak.
  • the core has a large refractive index difference with its adjacent inner cladding, that is, its normalized frequency is sufficiently large. This ensures that the fiber-based mold has an extremely low bending loss.
  • the effective refractive index of the higher-order mode is also large, and for this reason, the effective refractive index of the cladding defect mode is also required to be large, thereby facilitating the coupling of the two in a bent state.
  • a larger mode effective refractive index means that the loss of the cladding defect mode in the bent state is lower, which is disadvantageous for the loss of the higher order mode.
  • the high-refractive-index filter layer near the core has a large refractive index, which is advantageous for the coupling of defects and high-order modes, and the high refractive index away from the core.
  • the lower refractive index of the filter layer is beneficial to the large loss of the defect mode in the bent state.
  • the defect modes in the two high-refractive-index filter layers can have similar effective refractive indices under appropriate bending conditions, thereby ensuring A strong coupling occurs, eventually causing the higher order mode to leak through the two layers of the high refractive index filter layer.
  • the first high refractive index filter layer should have enough defect modes to ensure effective coupling with the higher order modes, and at the same time, the binding light capability cannot be too strong, thereby facilitating the leakage of energy in the bent state.
  • the refractive index n3 and the radial width a3 of the first high refractive index filter layer satisfy: 1.7 ⁇ F ⁇ 3.3, and V > F.
  • ⁇ 0 1625 nm.
  • the effective refractive index of the cladding defect mode should be lower than the higher order mode. Because the effective refractive index of the defect mode increases greatly after the fiber is bent, it is easy for the two to achieve the purpose of similar effective refractive index (ie, pattern matching). On the other hand, it is precisely because the effective refractive index of the cladding defect mode is low, which ensures that the effective refractive index and the fundamental mode of the fiber are different in the straight fiber state and the curved state, so that the optical fiber fundamental mode The loss effect is small, ensuring low bending loss transmission of the fiber fundamental mode.
  • the difference in refractive index between the core and the cladding is large, that is, the effective refractive index of the high-order mode is also relatively large, so that the first high refractive index filter layer and the second highest
  • the refractive index filter layer must also have a higher refractive index to achieve matching of its defect mode to the core higher order mode.
  • the greater the refractive index of the first high refractive index filter layer and the second high refractive index filter layer the smaller the bending loss.
  • the refractive index of the first high refractive index filter layer is higher than the refractive index of the second high refractive index filter layer.
  • the defect mode of the first high refractive index filter layer has a higher effective refractive index and is easily coupled with the higher-order mode of the core; on the other hand, the second high-refractive-index filter layer has a lower refractive index and is prone to occur. Leakage; at the same time, since the second high refractive index filter layer is away from the core, its refractive index changes more with the bending of the optical fiber, and thus, in the bent state, it has a similarity to the first high refractive index filter layer.
  • the refractive index guarantees a strong coupling between the modes of the two filter layers. That is, in the bent state, the first high refractive index filter layer functions to couple the high-order mode of the core to the cladding.
  • the second high refractive index filter layer serves to increase the bending loss of the defect mode.
  • the first high refractive index filter layer should still be larger than the refractive index of the second high refractive index filter layer, for which: the first high refractive index filter is required
  • the optical fiber is composed of a core (a first inner cladding layer, a first high refractive index filter layer, a second inner cladding layer, a second high refractive index filter layer, and an outer cladding layer.
  • FIG. 1 is an optical fiber provided by an embodiment of the present application. Radial refractive index profile, wherein 1-core, 2-first inner cladding, 3-first high refractive index filter layer, 4-second inner cladding, 5-second high refractive index filter layer, 6 - an outer cladding layer; reference FIG. 2 is a mode field area distribution of a fiber base mode (ie, LP01 mode) and a high order mode (LP11 mode) of the optical fiber of the present embodiment at a bending radius of 7.5 mm, and FIG.
  • a fiber base mode ie, LP01 mode
  • LP11 mode high order mode
  • FIG. 2(a) is the present application.
  • the embodiment provides a mode field distribution diagram of the LP01 mode of the optical fiber at a bending radius of 7.5 mm; and
  • FIG. 2(b) is an LP11 mode of the optical fiber provided by the embodiment of the present application when the bending radius is 7.5 mm.
  • FIG. 2(c) is a schematic diagram of a mode field distribution of another LP11 mode of an optical fiber with a bending radius of 7.5 mm according to an embodiment of the present application; as can be seen from the figure, the LP01 mode is still concentrated on the fiber.
  • the core center region, and the LP11 mode is significantly coupled to the defect mode in the first high refractive index filter layer.
  • the cladding defect mode is mainly distributed in the first high refractive index filter layer, the second inner cladding layer and the second high refractive index filter layer.
  • Fig. 4(a) is the optical fiber of the embodiment, the comparative fiber 1, and the LP01 of the contrast fiber 2.
  • Figure 4 (b) is a graph showing the bending loss of the optical fiber, the comparative fiber 1, and the LP11 mode of the comparative fiber 2 as a function of the operating wavelength of the present embodiment;
  • the LP01 mode has the largest bending loss, but is less than 1 ⁇ 10-4 dB/ ⁇ in the wavelength range shown, ensuring low bending loss transmission of the fiber fundamental mode.
  • the optical fiber is composed of a core, a first inner cladding, a first high refractive index filter layer, a second inner cladding, a second high refractive index filter layer, and an outer cladding.
  • the second high refractive index filter layer has a radial width of 6 ⁇ m
  • the bending loss of LP01 mode is less than 1 ⁇ 10-4dB/ ⁇ in the wavelength range of 1.26 ⁇ 1.625 ⁇ m.
  • the bending loss of LP11 mode is More than 2dB/ ⁇ ensures that it can be effectively filtered out.
  • the mode field diameter was 8.8 ⁇ m.
  • the above optical fiber can be realized by the same manufacturing process as the ordinary single mode fiber.
  • the present disclosure provides a novel mode-less fiber solution with a very low bend loss ( ⁇ 1 x 10-3 dB/ ⁇ ) with a bend radius above 7.5 mm.
  • the optical fiber of the present disclosure is a non-single mode fiber in a straight waveguide state, and the core is high-order mode and high-refractive-index filter by bending the fiber at a sufficiently small bending radius.
  • the pattern of the layer is strongly coupled, and the high-order mode can be filtered out to achieve the equivalent single-mode transmission.

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Abstract

一种新型少模光纤，该光纤包括：纤芯以及包围纤芯的包层；所述包层包括：包围纤芯的第一内包层；包围第一内包层的第一高折射率滤模层；包围第一高折射率滤模层的第二内包层；包围第二内包层的第二高折射率滤模层；包围第二高折射率滤模层的外包层。

Description

新型少模光纤 技术领域
本公开涉及光纤通信领域，例如涉及一种能够单模工作且光纤基模具有低弯曲损耗特性的新型少模光纤。
背景技术
在城域网、局域网、光纤到户以及中短距离通信等应用领域，为了便于铺设或减少所占空间，经常需要光纤光缆在小弯曲半径下工作，而常规的单模光纤无法满足小弯曲半径下工作的要求。为满足对光纤低弯曲损耗的要求，国际上提出了G.657光纤标准。低弯曲损耗光纤通常采用通过减小光纤的纤芯尺寸、引入带凹槽的包层结构、以及采用孔助结构光纤等来实现[K.Himeno，S.Matsuo，N.Guan，and A.Wada，″Low-Bending-Loss Single-Mode Fibers for Fiber-to-the-Home(用于光纤到户的低弯曲损耗单模光纤)，″Journal of Lightwave Technology，2005，23(11)：3494-3499.]。
G.657光纤虽然可以在7.5mm甚至5mm的弯曲半径下工作，但其弯曲损耗仍然较大，难以实现在小弯曲半径下长期稳定的工作，例如G.657B3光纤应满足在7.5和5mm弯曲半径下弯曲损耗应分别小于0.08和0.15dB/匝(1550nm波长)。显然，光纤在此小弯曲半径下被缠绕几圈后仍会产生较大的损耗，影响通信系统性能。
由光纤理论可知，对于阶跃光纤，在工作波长下归一化频率小于2.405时，光纤为单模光纤，当归一化频率大于2.405时，光纤即可传输高阶模，从而为非单模光纤。常规多模或少模光纤由于高阶模的存在，会导致信号光在光纤中 传输时具有严重的模间色散问题，从而限制了光纤的通信速率和容量。为此，目前光纤通信系统中仍采用单模光纤作为主要的传输媒介。
若在光纤通信系统中采用非单模光纤，通过有效增大光纤纤芯与包层的折射率差，来获得低弯曲损耗传输，再通过与单模光纤的匹配连接，可实现单模传输[一种光纤通信系统，中国专利，201010589018.1，一种基于少模光纤的通信系统，ZL201210393511.5]。这种技术通过放宽对光纤传输模式数量的限制，并通过与单模光纤的连接，抑制高阶模的产生，实现了低弯曲损耗、单模传输和低连接损耗传输的几项要求。然而，这种方法要求少模光纤两端连接单模光纤且连接偏差要小，这就使得其实际使用受到了一定的限制。
因此，如何提供一种可以实现单模传输且光纤在小弯曲半径下被缠绕多圈而仍能够保持低损耗传输的光纤是现阶段亟待解决的问题。
发明内容
本公开提供一种新型少模光纤，该光纤可以实现单模传输且在小弯曲半径下保持低损耗传输。
一种新型少模光纤，该光纤包括：纤芯以及包围纤芯的包层，所述包层包括：包围纤芯的第一内包层；包围第一内包层的第一高折射率滤模层；包围第一高折射率滤模层的第二内包层；包围第二内包层的第二高折射率滤模层；包围第二高折射率滤模层的外包层。
其中，所述纤芯、第一内包层、第一高折射率滤模层、第二内包层、第二高折射率滤模层以及外包层的折射率分别为n1，n2，n3，n4，n5，n6，满足：n1＞n3＞n5＞n6，且有n2＝n4＝n6；所述纤芯与第一内包层之间满足：2.405＜V＜4.4， 其中
Figure PCTCN2017117050-appb-000001
V表示归一化频率，λ 0＝1625nm，a1表示所述纤芯的半径；所述第一高折射率滤模层的折射率n3和径向宽度a3满足：1.7＜F＜3.3，且有V＞F，其中
Figure PCTCN2017117050-appb-000002
λ 0＝1625nm；所述第一内包层、第一高折射率滤模层、第二内包层、第二高折射率滤模层的径向宽度分别为a2，a3，a4，a5。
可选的，在上述光纤中，在1260～1625nm波长范围内，所述光纤的LP11模的有效折射率均大于所述光纤的包层缺陷模的有效折射率。
可选的，在上述光纤中，第一高折射率滤模层与第二高折射率滤模层的折射率之间满足：n3-n5＞(a5/2+a4+a3/2)/Rb，Rb＝7.5mm。
可选的，在上述光纤中，第一高折射率滤模层的径向宽度a3与第二高折射率滤模层的径向宽度a5满足：a 3≥a 5。
可选的，在上述光纤中，纤芯的折射率n1与外包层的折射率n6满足：0.015＞n1-n6＞0.007。
可选的，在上述光纤中，第一内包层的径向宽度a2满足：7.9μm≥a 2≥4.0μm。
可选的，在上述光纤中，第一高折射率滤模层的径向宽度a3满足：8.5μm≥a 3≥3.5μm。
可选的，在上述光纤中，第二内包层的径向宽度a4满足：4.6μm≥a 4≥2.4μm。
可选的，在上述光纤中，第二高折射率滤模层的径向宽度a5满足：8.5μm≥a 5≥3.5μm。
所述光纤在1550nm波长时的弯曲损耗满足：弯曲半径R b≥7.5mm时，LP01模弯曲损耗小于1×10-3dB/匝；弯曲半径R b≤7.5mm时，LP11模的弯曲损耗大于5dB/匝。
所述光纤的截止波长大于1.625μm。
本公开提供一种弯曲半径在7.5mm以上具有极低弯曲损耗(＜1×10-3dB/匝)的新型少模光纤解决方案。在常规的光通信波段(1260～1625nm波长)，本公开光纤在直波导状态下为非单模光纤，通过将光纤在足够小的弯曲半径进行弯曲，使其纤芯高阶模与高折射率滤模层的模式发生强耦合，可以实现滤除高阶模，从而实现等效的单模传输的目的。允许纤芯与包层之间具有高的折射率差，保证了光纤基模的超低弯曲损耗传输，可以实现光纤在小弯曲半径下的多次缠绕仍具有低的弯曲损耗，同时又可以与单模光纤一样实现单模传输工作，既可以通过熔接的方式，也可通过活动连接的方式与单模光纤连接。本公开光纤仅需要将光纤通过适当的弯曲即可实现单模传输的目的，适用于各种短距离通信系统中需要紧凑型光纤器件和光纤组件的场合。本公开的光纤结构具有圆对称性，可以采用现有成熟的光纤制作工艺实现，仅需要对纤芯和两个高折射率滤模层进行掺杂，而其它区域的折射率均与外包层相同，简化了光纤的制备工艺，可有效降低光纤的制作成本。
附图说明
图1为本申请实施例提供的一种光纤的径向折射率分布图，其中1-纤芯、2-第一内包层、3-第一高折射率滤模层、4-第二内包层、5-第二高折射率滤模层、6-外包层；
图2(a)为本申请实施例提供的一种光纤在弯曲半径为7.5mm时，其LP01模的模场分布图；
图2(b)为本申请实施例提供的一种光纤在弯曲半径为7.5mm时，其LP11模的模场分布图；
图2(c)为本申请实施例提供的一种光纤在弯曲半径为7.5mm时，其另一LP11模的模场分布图；
图3为本申请实施例的一个包层缺陷模的模场分布图；
图4(a)为本实施例光纤、对比光纤1、对比光纤2的LP01模的弯曲损耗随工作波长的变化关系曲线图；
图4(b)为本实施例光纤、对比光纤1、对比光纤2的LP11模的弯曲损耗随工作波长的变化关系曲线图。
具体实施方式
本实施例在具有超低弯曲损耗的少模纤芯基础上，设计弯曲状态下具有高泄露损耗的缺陷模，从而使光纤的高阶模既能够与包层缺陷模发生强耦合而泄露掉，又能够保持光纤基模的低弯曲损耗传输。
由光纤的弯曲理论可知，当光纤弯曲时，其结构仍可等效为一直波导，即等效的直波导的折射率分布可表示为：
Figure PCTCN2017117050-appb-000003
这里n 0(x，y)是未弯曲时光纤的横截面折射率分布，n(x，y)等效的直波导的折射率分布，x为材料的弹光系数，Rb为光纤的弯曲半径，这里假设光纤沿x轴正方向弯曲。由公式可见，光纤弯曲后，越远离纤芯中心、弯曲半径越小，其折射率变化越大。由于光通常集中在高折射率区域，因而纤芯中模式能量将向折射率增高的一侧集中，即其模式有效折射率通常随着弯曲半径的增大而增加。同时，在纤芯折射率增大的一侧，外包层的折射率也随弯曲而增大，最终导致包层缺陷模无法满足全内反射条件而发生泄露。
这里在保证纤芯足够大的情况下，设置纤芯与其相邻的内包层之间具有大的折射率差，即其归一化频率足够大。由此保证光纤基模具有超低的弯曲损耗。此时，其高阶模的有效折射率也较大，为此要求包层缺陷模的有效折射率也较大，从而有利于两者在弯曲状态下的耦合。较大的模式有效折射率也就意味着包层缺陷模在弯曲状态下的损耗较低，不利于将高阶模损耗掉。为此，我们提出采用双层高折射率滤模层，其中靠近纤芯的高折射率滤模层的折射率较大，有利于其缺陷查与高阶模的耦合，而远离纤芯的高折射率滤模层的折射率较低，有利于其缺陷模在弯曲状态下产生大的损耗。同时，由于弯曲状态下，远离纤芯的区域折射率变化较大，因而在适当的弯曲状态下，两种高折射率滤模层中的缺陷模仍能具有相近的有效折射率，从而保证其发生强的耦合，最终使高阶模经两层高折射率滤模层后发生泄露。
常规单模光纤在工作波长处是单模传输的，即其截止波长小于其工作波长。而本实施例光纤的截止波长大于其工作波长。因而，在直光纤状态下，光纤可 以支持高阶模传输，而在光纤弯曲状态下，其高阶模可产生大的弯曲损耗最终导致其无法形成有效传输，从而实现等效的单模传输。
纤芯的折射率须足够高，以实现基模的超低弯曲损耗的传输。同时避免出现过多的模式出现而影响光纤传输性能。为此要求所述纤芯与第一内包层之间满足：2.405＜V＜4.4，这里：
Figure PCTCN2017117050-appb-000004
V表示归一化频率，λ 0＝1625nm。即其纤芯在1625nm波长及更短的波长范围内都能够支持高阶模的传输。
第一高折射率滤模层应具有足够多的缺陷模，以保证与高阶模产生有效耦合，同时，束缚光能力又不能过强，从而利于其在弯曲状态下能量的泄露。所述第一高折射率滤模层的折射率n3和径向宽度a3满足：1.7＜F＜3.3，且有V＞F。这里：
Figure PCTCN2017117050-appb-000005
λ 0＝1625nm。
包层缺陷模的有效折射率应该低于高阶模。因为当光纤弯曲以后，其缺陷模的有效折射率增加较大，正好使两者容易达到有效折射率相近(即模式匹配)的目的。而另一方面，正是因为包层缺陷模的有效折射率较低，从而保证了其在直光纤状态下和弯曲状态下，有效折射率与光纤基模都相差较大，从而光纤基模的损耗影响较小，保证光纤基模的低弯曲损耗传输。
由于为了保证纤芯基模的低弯曲损耗传输，纤芯与包层的折射率差较大，即其高阶模的有效折射率也比较大，从而使得第一高折射率滤模层和第二高折射率滤模层也必须具有较高的折射率以实现其缺陷模与纤芯高阶模的匹配。而第一高折射率滤模层和第二高折射率滤模层的折射率越大，其弯曲损耗也就越小。为使缺陷模的弯曲损耗增大，这里要求第一高折射率滤模层的折射率比第二高折射率滤模层的折射率高。一方面，第一高折射率滤模层的缺陷模的有效折射率较高，易于与纤芯高阶模发生耦合；而另一方面，第二高折射率滤模层 的折射率较低，易于发生泄露；同时，由于第二高折射率滤模层远离纤芯，其折射率随光纤的弯曲会产生更大的变化，因而，其在弯曲状态下，具有与第一高折射率滤模层相近的折射率，保证了两个滤模层的模式间的强耦合。即在弯曲状态下，第一高折射率滤模层起到将纤芯高阶模耦合到包层的作用。而第二高折射率滤模层起到增大缺陷模的弯曲损耗的作用。在弯曲状态下，沿折射率增加方向，所述第一高折射率滤模层仍然应大于所述第二高折射率滤模层的折射率，为此要求：所述第一高折射率滤模层与所述第二高折射率滤模层的折射率之间满足：n3-n5＞(a5/2+a4+a3/2)/Rb，这里Rb＝7.5mm。
下面结合图形说明本实施例光纤原理与特点。
光纤由纤芯(第一内包层、第一高折射率滤模层、第二内包层、第二高折射率滤模层和外包层组成。图1为本申请实施例提供的一种光纤的径向折射率分布图，其中1-纤芯、2-第一内包层、3-第一高折射率滤模层、4-第二内包层、5-第二高折射率滤模层、6-外包层；参考图2为一种本实施例光纤在弯曲半径为7.5mm时的光纤基模(即LP01模)和高阶模(LP11模)的模场面积分布，图2(a)为本申请实施例提供的一种光纤在弯曲半径为7.5mm时，其LP01模的模场分布图；图2(b)为本申请实施例提供的一种光纤在弯曲半径为7.5mm时，其LP11模的模场分布图；图2(c)为本申请实施例提供的一种光纤在弯曲半径为7.5mm时，其另一LP11模的模场分布图；由图可见，LP01模仍集中在纤芯中心区域，而LP11模明显与第一高折射率滤模层中的缺陷模发生耦合。图3为本申请实施例的一个包层缺陷模的模场分布图，由图可见，包层缺陷模主要集中分布在第一高折射率滤模层、第二内包层、第二高折射率滤模层。为了反映高折射率滤模层的作用，我们对比了本实施例光纤、对比光纤1(相当于本实施例光纤取a5＝0，即不存在第二高折射率滤模层)、以及对比光纤2(相当于本实施例 光纤取a3＝0和a5＝0，即为一种阶跃光纤)三种情况下和LP11模的弯曲损耗曲线。由图4可见，图4(a)为本实施例光纤、对比光纤1、对比光纤2的LP01模的弯曲损耗随工作波长的变化关系曲线图；图4(b)为本实施例光纤、对比光纤1、对比光纤2的LP11模的弯曲损耗随工作波长的变化关系曲线图；本实施例光纤的LP01模的弯曲损耗最大，但在图示波长范围内均小于1×10-4dB/匝，保证了光纤基模的低弯曲损耗传输。而当其波长达到1.3μm以上时，其LP11模的弯曲损耗超过2dB/匝，保证可被弯曲后滤除。而不论是对比光纤1还是对比光纤2，其LP11模的弯曲损耗均要低得多，从而难以实现宽带滤除高阶模的目的。因此，采用本实施例的双滤模层结构，可以有效提高其LP11模的弯曲损耗，从而在宽波长范围内实现等效的单模传输。实施例：
光纤由纤芯、第一内包层、第一高折射率滤模层、第二内包层、第二高折射率滤模层和外包层组成。纤芯的半径a1＝5μm，纤芯与第一内包层的折射率差n1-n2＝0.01，第一高折射率滤模层的径向宽度为6μm，其与外包层的折射率差n3-n6＝0.005，第二高折射率滤模层的径向宽度为6μm，其与外包层的折射率差n5-n6＝0.003。光纤的弯曲半径为7.5mm时，在1.26～1.625μm波长范围内，其LP01模弯曲损耗均小于1×10-4dB/匝；而当光波长在1.3μm波长以上时，其LP11模弯曲损耗均大于2dB/匝，保证了其可以被有效滤除。在1.31μm波长时，其模场直径为8.8μm。
以上光纤可采用与普通单模光纤相同的制作工艺实现。
工业实用性
本公开提供一种弯曲半径在7.5mm以上具有极低弯曲损耗(＜1×10-3dB/匝)的新型少模光纤解决方案。在常规的光通信波段(1260～1625nm波长)，本 公开光纤在直波导状态下为非单模光纤，通过将光纤在足够小的弯曲半径进行弯曲，使其纤芯高阶模与高折射率滤模层的模式发生强耦合，可以实现滤除高阶模，从而实现等效的单模传输的目的。

Claims (10)

  1. 一种新型少模光纤，包括：纤芯以及包围所述纤芯的包层；
    所述包层包括：包围纤芯的第一内包层；包围第一内包层的第一高折射率滤模层；包围第一高折射率滤模层的第二内包层；包围第二内包层的第二高折射率滤模层；以及包围第二高折射率滤模层的外包层；
    其中，纤芯、第一内包层、第一高折射率滤模层、第二内包层、第二高折射率滤模层以及所述外包层的折射率分别为n1、n2、n3、n4、n5、n6，满足：n1＞n3＞n5＞n6，且有n2＝n4＝n6；所述纤芯与第一内包层之间满足：2.405＜V＜4.4，其中
    Figure PCTCN2017117050-appb-100001
    V表示归一化频率，λ 0＝1625nm，a1表示所述纤芯的半径；所述第一高折射率滤模层的折射率n3和径向宽度a3满足：1.7＜F＜3.3，且有V＞F，其中
    Figure PCTCN2017117050-appb-100002
    所述第一内包层、第一高折射率滤模层、第二内包层以及第二高折射率滤模层的径向宽度分别为a2，a3，a4，a5。
  2. 根据权利要求1所述的新型少模光纤，其中，在1260-1625nm波长范围内，所述光纤的LP11模的有效折射率均大于所述光纤的包层缺陷模的有效折射率。
  3. 根据权利要求1所述的新型少模光纤，其中：第一高折射率滤模层与第二高折射率滤模层的折射率之间满足n3-n5＞(a5/2+a4+a3/2)/Rb，Rb＝7.5mm。
  4. 根据权利要求1所述的新型少模光纤，其中：第一高折射率滤模层的径向宽度a3与第二高折射率滤模层的径向宽度a5满足a 3≥a 5。
  5. 根据权利要求1所述的新型少模光纤，其中：纤芯的折射率n1与外包层的折射率n6满足0.015＞n1-n6＞0.007。
  6. 根据权利要求1所述的新型少模光纤，其中：第一内包层的径向宽度a2满足7.9μm≥a 2≥4.0μm。
  7. 根据权利要求1所述的新型少模光纤，其中：第一高折射率滤模层的径向宽度a3满足8.5μm≥a 3≥3.5μm。
  8. 根据权利要求1所述的新型少模光纤，其中：第二内包层的径向宽度a4满足4.6μm≥a 4≥2.4μm。
  9. 根据权利要求1所述的新型少模光纤，其中：第二高折射率滤模层的径向宽度a5满足8.5μm≥a 5≥3.5μm。
  10. 根据权利要求1所述的新型少模光纤，其中：在1550nm波长时，所述光纤的弯曲损耗满足：弯曲半径R b≥7.5mm时，LP01模弯曲损耗小于1×10-3dB/匝；弯曲半径R b≤7.5mm时，LP11模的弯曲损耗大于5dB/匝。
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