WO2021189891A1 - 一种多芯多模光纤 - Google Patents

一种多芯多模光纤 Download PDF

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WO2021189891A1
WO2021189891A1 PCT/CN2020/131831 CN2020131831W WO2021189891A1 WO 2021189891 A1 WO2021189891 A1 WO 2021189891A1 CN 2020131831 W CN2020131831 W CN 2020131831W WO 2021189891 A1 WO2021189891 A1 WO 2021189891A1
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core
optical fiber
layer
cladding layer
mode
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PCT/CN2020/131831
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English (en)
French (fr)
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肖武丰
沈磊
王海鹰
黄荣
张安林
王润涵
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长飞光纤光缆股份有限公司
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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
    • G02B6/028—Optical fibres with cladding with or without a coating with core or cladding having graded refractive index
    • G02B6/0288—Multimode fibre, e.g. graded index core for compensating modal dispersion
    • 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

Definitions

  • the invention relates to a space division multiplexing optical fiber used in an optical fiber communication system, in particular to a multi-core multi-mode optical fiber.
  • the multi-core optical fiber using space division multiplexing (SDM) of multiple spatial channels does not increase the number of optical fiber links.
  • SDM space division multiplexing
  • the unit power consumption can achieve higher transmission capacity, which is more suitable for higher power consumption requirements.
  • High data center interconnection communication further enhances its application potential.
  • multi-core optical fibers there are two main factors limiting the application of multi-core optical fibers, namely bending loss and inter-core crosstalk. Since the multi-core fiber contains multiple cores, the thickness between the core and the outer cladding is small, and the bending loss at this time is large, which affects its transmission performance. On the other hand, there is energy coupling between the cores of the multi-core fiber, which generates inter-core crosstalk, which increases the transmission error rate.
  • multi-core optical fiber The existing research and invention patents of multi-core optical fiber are concentrated on multi-core single-mode fiber or multi-core few-mode optical fiber, and there is less research on multi-core multi-mode optical fiber.
  • most of the transmission distances are relatively short, and the transmission distance of multimode fiber meets the requirements; and compared with long-distance connections, the problem of inter-core crosstalk in short intervals is not serious. Therefore, it is feasible to prepare a multi-core multi-mode fiber that supports short-distance multi-mode transmission.
  • Benefiting from the advantages of VCSEL laser cost and low power consumption, combining multi-core multi-mode fiber and VCSEL laser can greatly reduce the cost of short-distance transmission.
  • the multi-core optical fiber system can be directly connected with silicon photonics and InP chips to achieve high integration and high-density interconnection.
  • the fundamental mode field diameter of the multi-mode fiber is optimized to match the single-mode fiber.
  • the mode field matching center injection causes most of the energy to be injected into the fundamental mode to achieve long-distance quasi-fundamental mode transmission.
  • Multi-mode and single-mode transmission systems each have their own advantages and disadvantages. In the current situation, it is reasonable to use multi-mode fiber and cheap VCSEL light source for short-distance network construction; for long-distance quasi-base mode transmission, the same This kind of optical fiber can reduce complexity and management cost.
  • the bandwidth When the bandwidth is further upgraded, it can be directly transformed into a single-mode transmission system without the need to re-lay optical cables.
  • the mode field diameter is close to that of single-mode fiber.
  • the actual distance between the optical fields of the cores of the multi-core fiber is greater than the core distance, and the crosstalk between cores will be very serious. Small, conducive to space division multiplexing.
  • Mandrel A preform containing a core layer and a partial cladding layer.
  • Radius the distance between the outer boundary of the layer and the center point.
  • Refractive index profile The relationship between the refractive index of the glass of an optical fiber or optical fiber preform (including the core rod) and its radius.
  • the relative refractive index difference ⁇ i is the relative refractive index difference between each layer of the optical fiber (except the cladding) and pure silica:
  • ni is the refractive index of the position i from the center of the fiber core
  • n0 is the refractive index of pure silica material, which is usually the refractive index of the outer cladding of the optical fiber.
  • the inter-mode dispersion in the multi-mode fiber greatly limits the transmission distance that it can support.
  • the relative refractive index difference contribution ⁇ Ge of the Ge-doped fiber core layer is defined by the following equation:
  • n Ge is the Ge dopant of the core, the change in the refractive index of silica glass caused by doping into pure silica without other dopants, where n c is the refractive index of the outer cladding, That is, the refractive index of pure silica.
  • the relative refractive index difference contribution ⁇ F of the fiber core layer F doping is defined by the following equation:
  • n F is the F dopant of the core, the change in the refractive index of silica glass caused by doping into pure silica without other dopants, where n c is the refractive index of the outer cladding layer, That is, the refractive index of pure silica.
  • E is the electric field related to propagation
  • r is the distance from the axis to the electric field distribution point.
  • ⁇ is the test wavelength
  • D is the distance from the plane of the aperture stop to the end face of the fiber
  • x is the radius of the aperture stop
  • a(x) is the complementary aperture power transfer function
  • the technical problem to be solved by the present invention is to provide a multi-core multi-polished optical fiber in view of the shortcomings of the prior art. It can not only support multi-mode transmission in the wavelength range of 850 nm to 950 nm and single-mode transmission in O band/C band at the same time, but also Optimizing the fiber structure and viscosity matching to reduce attenuation, so that the crosstalk of the fiber and the overall performance of the macrobending and microbending loss of each channel are at a good level.
  • the technical solution adopted by the present invention to solve the above-mentioned problems is: a plurality of core layers and a common outer cladding layer are included. Evenly spaced, the distance between each adjacent core layer is 38 ⁇ 60 ⁇ m for 2 core layers, 4 or 8 is 38 ⁇ 48 ⁇ m, and each core layer is covered with inner cladding layer and sinking cladding layer from the inside to the outside, forming multiple
  • the multiple core layers are evenly distributed on a circle, that is, each core layer is equidistant from the center of the common outer outer layer, and the common outer outer layer diameter of the two or four core layers is 125 ⁇ 1 ⁇ m, the diameter of the common outer cladding layer of 8 core layers is 180 ⁇ 1.5 ⁇ m.
  • the relative refractive index difference ⁇ 3 of the depressed cladding layer is -0.8% to -0.9%.
  • the core layer, inner cladding layer, and depressed cladding layer of each core of the optical fiber are all germanium-fluoride co-doped silica glass layers or fluorine-doped silica glass layers, wherein the relative refractive index of fluorine doping The contribution of the rate difference is -0.02% to -0.9%.
  • the core layer is a graded fluorine doping contribution. From the center of the core layer to the edge of the core layer, the absolute value of the fluorine doping contribution is increasing, and the contribution of the fluorine doping in the center of the core layer is increasing.
  • ⁇ F0 is -0.10% ⁇ 0%
  • the contribution of fluorine doping at the edge of the core layer ⁇ F1 is -0.45% ⁇ -0.10%
  • the ratio of contribution of germanium to fluoride in the inner cladding layer is 0.1 ⁇
  • the optical fiber has a bandwidth of 3500MHz-km or more than 3500MHz-km at a wavelength of 850nm, a bandwidth of 2000MHz-km or more than 2000MHz-km at a wavelength of 950nm, and a bandwidth of 500MHz-km or more than 500MHz-km at a wavelength of 1300nm.
  • the fundamental mode field diameter of each core of the optical fiber at a wavelength of 1310 nm or 1550 nm is 8-12 ⁇ m.
  • the inter-core crosstalk between any two adjacent cores (side cores) in the optical fiber is ⁇ -35dB/km, and the inter-core crosstalk between cores other than adjacent cores ⁇ -55dB/km; preferably, the inter-core crosstalk between any two cores is ⁇ -40dB/km, and the inter-core crosstalk between cores other than adjacent cores is ⁇ -60dB/1km.
  • the attenuation of each channel of the optical fiber at a wavelength of 850nm is less than or equal to 3dB/km, and the attenuation of each channel at a wavelength of 1310nm is less than or equal to 1.2dB/km.
  • the attenuation of each channel of the optical fiber at a wavelength of 850 nm is less than or equal to 2.4 dB/km, and the attenuation of each channel at a wavelength of 1310 nm is less than or equal to 0.6 dB/km.
  • the additional bending loss of the optical fiber caused by winding 2 times with a 7.5 mm bend radius at 850nm wavelength is less than 0.2dB; at 1300nm, the additional bending loss caused by winding 2 times with a 7.5mm bend radius is less than 0.5dB .
  • the single-sided thickness of the coating layer of the optical fiber is greater than or equal to 50 ⁇ m; the drawing tension of the optical fiber is 80-200 g.
  • the beneficial effects of the present invention are: 1.
  • the standard 125 micron fiber diameter is adopted, which maintains compatibility with current devices and occupies the same space and volume as ordinary single-core fibers.
  • the 8-core design guarantees high Features of density, low crosstalk, and low loss.
  • Reasonable core-pack ratio and core-pack structure design enable the optical fiber to simultaneously support multi-mode transmission in the wavelength range of 850nm ⁇ 950nm and single-mode transmission in the O and/or C band. 3.
  • germanium-fluoride co-doped functionally graded material design and reasonable core and cladding ratio structure strictly control the germanium-fluoride ratio, rationally design the internal viscosity match of the fiber, and reduce the defects and profile distortion in the fiber preparation process , Reduce the attenuation coefficient of the optical fiber.
  • the deep fluorine-doped fiber sinking structure is designed, and through the reasonable design of each layer of the fiber, the crosstalk between the cores and the macrobending loss of the fiber are lower, and it greatly reduces the core layer and the edge of the cladding layer. The additional attenuation. 5.
  • the comprehensive performance parameters such as macrobending and microbending loss of each channel of the optical fiber of the present invention are good in the application band.
  • the space division multiplexing technology can be used for short-distance signal transmission of multiple channels, and each mode has a low attenuation coefficient, which can support the space division multiplexing transmission of dense wiring in the data center. 6.
  • the core rod is deposited by the PCVD technology, and then the outer tube is sleeved for wire drawing, which is more capable of preparing complex cross-section optical fiber structures and strictly controlling various structural parameters of the optical fiber, and is suitable for large-scale production.
  • Figure 1 shows the spot size distribution of commercial VCSEL lasers.
  • Fig. 2 is a schematic diagram of a refractive index profile of a core in an embodiment of the present invention.
  • Fig. 3 is a diagram of the relationship between core pitch and inter-core crosstalk according to an embodiment of the present invention.
  • Fig. 4 is a schematic cross-sectional structure diagram of an embodiment of the present invention.
  • Fig. 5 is a schematic cross-sectional structure diagram of a second embodiment of the present invention.
  • Fig. 6 is a schematic cross-sectional structure diagram of a third embodiment of the present invention.
  • the spot size of the multimode fiber-matched VCSEL laser tends to decrease.
  • the core diameter of the multimode fiber must match the spot size.
  • the light spot may be further reduced. Therefore, we limit the radius of the core layer to 12 to 20 ⁇ m, and further to 13 to 17 ⁇ m.
  • the refractive index of the core layer needs to be increased.
  • the greater the relative refractive index difference of the core layer the more serious the inter-mode dispersion, and the smaller the transmission bandwidth.
  • the present invention ensures that the relative refractive index difference of the core layer is low, and at the same time adjusts the relative refractive index of the core layer and the inner cladding layer to ensure the mode field diameter. Controllable.
  • the NA is also large, which can further ensure the coupling efficiency with the optical module and the multimode fiber.
  • the influence of geometric parameters and relative refractive index in the profile design on the fundamental mode field diameter is calculated.
  • the factors that affect the mode field diameter of the optical fiber fundamental mode LP 01 include the core diameter, the relative refractive index difference and ⁇ , The thickness and relative refractive index difference of the inner cladding layer, the thickness and depth of the sinking cladding layer, etc.
  • the main factor affecting the mode field diameter of the LP 01 mode is the difference between the core diameter and the core cladding refractive index.
  • the mode field diameter is proportional to the core diameter and inversely proportional to the difference between the relative refractive index of the core and the cladding.
  • the estimation formula is as follows:
  • the fundamental mode MFD of the actual prepared fiber is generally slightly smaller than that calculated by the empirical formula.
  • the range of ⁇ 1max- ⁇ 2 of the required fiber can be calculated according to the required matching working waveband and mode field diameter.
  • the appropriate core relative refractive index ⁇ 1max and cladding relative refractive index difference ⁇ 2 are determined.
  • viscosity matching and bending resistance determine the relative size of R2 and R3, you can prepare the required compatible laser or fiber, and upgrade to a compatible graded index fiber for single-mode transmission in the future.
  • the number of cores of the multi-core optical fiber is preferably an even number, preferably 2 or 4 or 8 cores, and the centrally symmetrically distributed multi-core distribution facilitates rotary splicing.
  • the 2-core can support two-way transmission of a single fiber, and the 2-core connection scheme is relatively mature and the cost is low. 4 cores can realize single fiber and single wavelength 4 ⁇ 25Gb/s 100Gb/s transmission, 8 cores can realize single fiber and single wavelength 8 ⁇ 50G (PAM4) 400Gb/s transmission, without using parallel such as SR4 or PSM4 Transmission technology, thereby greatly reducing the density of optical fibers in the data center.
  • a deep fluorine-doped fiber depressed structure is designed outside the core, and through the reasonable design of the cross-section of each layer of the fiber, the distance between the core and the cladding edge is greatly reduced.
  • the additional attenuation produced For a certain embodiment, the width of the depressed cladding layer is 4 ⁇ m and the depth is -0.8%.
  • the relationship between the pitch between adjacent cores and the crosstalk (XT) between adjacent cores is calculated theoretically, as shown in FIG. 4. The inter-core crosstalk decreases with the increase of the core spacing.
  • the inter-core crosstalk (XT@850) of the high-order mode at 850nm is much larger than the inter-core crosstalk at 1310nm of the fundamental mode (XT@1310).
  • the inter-core crosstalk of the high-order mode is selected as the maximum value.
  • the crosstalk of the low-order mode is very small, such as XT@850 ⁇ XT@1310 of the fundamental mode, which is much less than -110dB. Therefore, the intercore crosstalk of the actual prepared multi-core fiber is higher than the above
  • the calculated value of XT@850 is much smaller, sometimes even negligible.
  • the inter-core crosstalk in the quasi-fundamental mode transmission state is very low, and the main limiting factor is the inter-core crosstalk in the high-order mode in the multi-mode transmission state.
  • the inter-core crosstalk of XT@850/150m will be reduced by about 8dB.
  • the above transmission When the distance is extended to 300m, the inter-core crosstalk of XT@850/300m will be reduced by about 5dB.
  • the inter-core crosstalk during transmission needs to be less than -30dB, and more preferably less than -40dB. Under the above conditions, the core spacing should be greater than 38 microns.
  • the thickness from the core of the multi-core fiber to the edge of the outer cladding layer needs to be large enough to ensure that the attenuation of the multi-core fiber is small enough.
  • the bending loss and additional loss of the multi-core fiber need to be reduced. 20 ⁇ m. Combined with a deep fluorine doped (-0.7% to -0.9%) sunken cladding, it can ensure that the attenuation, bending loss and inter-core crosstalk of the multi-core fiber are low.
  • the core density of the multi-core fiber is high, and the thickness of the core and cladding is smaller than that of the conventional fiber.
  • the multiple holes distributed in the ring make the wall thickness of the preform thinner. Therefore, in order to ensure the multi-core
  • the geometry of the optical fiber will not deviate too much. It is necessary to ensure that the material viscosity during the drawing process is relatively high. At this time, the drawing tension is 80-200g.
  • the coating material of the optical fiber includes but is not limited to epoxy acrylate or polyacrylate, and special coatings such as low refractive index coatings or high temperature resistant coatings can also be used in some usage scenarios.
  • the single-sided thickness of the coating layer needs to be ⁇ 50 ⁇ m.
  • the multiple cores of the multi-core optical fiber are evenly distributed in the circumferential direction and arranged in a ring shape, as shown in the drawings.
  • the core is a homogenous multimode optical fiber, and each core layer is sequentially covered with an inner cladding layer and a depressed cladding layer from the inside to the outside.
  • the core layer has a radius of R1, the relative refractive index difference of the core layer is ⁇ 1, and the radius of the inner cladding layer is R2 ,
  • the relative refractive index difference of the inner cladding is ⁇ 2, the radius of the depressed cladding is R3, and the relative refractive index difference of the depressed cladding is ⁇ 3.
  • the structural settings and main performance parameters of the optical fibers in the five embodiments of the present invention are shown in Table 1, where the performance parameters of the cores are the average values of multiple cores.
  • Table 1 The structural settings and main performance parameters of the optical fiber of the embodiment

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Abstract

一种多芯多模光纤，包括有多个芯层和共同外包层，其特征在于芯层为2个、4个或8个，各个芯层沿周向等距均布，各相邻芯层的间距2个芯层为38～60μm，4个或8个为38～48μm，各芯层外从内向外依次包覆内包层和下陷包层，构成多芯同质多模光纤，芯层折射率剖面呈抛物线形，α为1.9～2.1，芯层R1为12～20μm，芯层Δ1max为0.7％～1.7％，内包层单边宽度为0.5～2.5μm，Δ2为-0.4％～0.0％，下陷包层单边宽度为3～7μm，Δ3为-0.7％～-0.9％，共同外包层为纯二氧化硅层。不仅能够同时支持850nm～950nm波长范围的多模传输和O波段/C波段的单模传输，而且能优化光纤结构和粘度匹配降低衰减，使光纤的串扰、各信道的宏弯和微弯损耗等综合性能处于良好的水平。

Description

一种多芯多模光纤 技术领域
本发明涉及一种用于光纤通信系统的空分复用光纤，具体涉及一种多芯多模光纤。
背景技术
近年来，随着云计算、大数据、移动互联网的兴起，具有高效服务器间协同以及数据处理能力的数据中心，成为了明显的信息总量和信息密度增长热点，从而对数据中心互连通信速率的提升提出迫切要求。由于数据中心互连通信呈现出设备数量众多、布线复杂、接口密度大等特点，仅仅依靠提高器件调制带宽，增加光纤链路或者具有不同稳定波长输出光源的数量，势必会增加系统运行或维护的成本、功耗、复杂度等，因此，采用新的调制/复用方式增加有限带宽情况下单光纤/波长的传输速率，被看作是提升数据中心互连速率的有效解决方案。而采用多空间信道的空分复用(SDM)的多芯光纤，在不增加光纤链路数量的基础上，理论上单位功耗可以实现更高的传输容量，更加适用于对功耗要求较高的数据中心互连通信，进一步提升了其应用潜力。
当前限制多芯光纤应用的主要因素有两个，分别是弯曲损耗和芯间串扰。由于多芯光纤含有多个纤芯，纤芯到外包层之间的厚度较小，此时的弯曲损耗较大，影响其传输性能。另一方面，多芯光纤的纤芯之间存在能量耦合，产生芯间串扰，使得传输误码率增大。
现有的多芯光纤的研究与发明专利集中在多芯单模光纤或者多芯少模光纤，对多芯多模光纤的研究较少。在数据中心的使用场景，大部分传输距离较近，多模光纤的传输距离满足要求；而且与长距离连接相比，短间隔内的芯间串扰问题并不严重。因此，制备支持短距离多模传输的多芯多模光纤是可行的。受益于VCSEL激光器成本和功耗低的优势，结合多芯多模光纤和VCSEL激光器，可以大大降低短距离传输成本。此外，多芯光纤系统可以与硅光子和InP芯片的直接连接，以实现高度集成和高密度互连。
在多模传输多芯光纤的基础上，结合准基模传输的原理，通过优化多模光纤的基模模场直径，使之与单模光纤匹配。当采用单模激光器时，模场匹配中心注入使绝大部分能量注入到基模中，以此实现长距离的准基模传输。多模和单模传输系统各有各的优点和缺点，在当前情况下，使用多模光纤和便宜的VCSEL光源进行短距离组网建设是合理的；在长距离进行准基模传输，采用同一种光纤可以减小复杂度和管理成本。当带宽进一步升级时，可以直 接改造成单模传输系统，而不需要重新铺设光缆。此外，多模光纤准基模传输时，模场直径接近单模光纤的模场直径，此时多芯光纤各纤芯的光场之间的实际间距是大于芯间距的，芯间串扰会很小，有利于空分复用。
发明内容
为方便介绍发明内容，定义如下术语：
芯棒：含有芯层和部分包层的预制件。
半径：该层外边界与中心点之间的距离。
折射率剖面：光纤或光纤预制棒(包括芯棒)玻璃折射率与其半径之间的关系。
相对折射率差即Δi为光纤各层(除外包层外)与纯二氧化硅的相对折射率差：
Figure PCTCN2020131831-appb-000001
其中，ni为距离纤芯中心i位置的折射率；n0为纯二氧化硅材料的折射率，通常也是光纤外包层的折射率。
多模光纤中存在的模间色散使其所能支持的传输距离受到大大限制，为降低光纤模间色散，需要将多模光纤的芯层折射率剖面设计成中心至边缘连续逐渐降低的折射率分布，通常我们称其为“α剖面”。即满足如下幂指数函数的折射率分布：
Figure PCTCN2020131831-appb-000002
其中，n1为光纤轴心的折射率；r为离开光纤轴心的距离；a为光纤芯半径；α为分布指数；Δ0为纤芯中心相对包层的折射率。
光纤芯层Ge掺杂的相对折射率差贡献量ΔGe由以下方程式定义：
Figure PCTCN2020131831-appb-000003
其中，n Ge为纤芯的Ge掺杂物，在掺杂到没有其他掺杂物的纯二氧化硅中，引起的二氧化硅玻璃折射率的变化量，其中n c为外包层折射率，即纯二氧化硅的折射率。
光纤芯层F掺杂的相对折射率差贡献量ΔF由以下方程式定义：
Figure PCTCN2020131831-appb-000004
其中，n F为纤芯的F掺杂物，在掺杂到没有其他掺杂物的纯二氧化硅中，引起的二氧化硅玻璃折射率的变化量，其中n c为外包层折射率，即纯二氧化硅的折射率。
光纤各模式的有效面积：
Figure PCTCN2020131831-appb-000005
其中，E是与传播有关的电场，r为轴心到电场分布点之间的距离。
一般，我们以远场可变孔径法来测试光纤的模场直径MFD，确定模场直径的等效公式为：
Figure PCTCN2020131831-appb-000006
其中，λ为测试波长，D为孔径光阑所在平面到光纤端面的距离，x为孔径光阑的半径，a(x)为互补孔径功率传输函数。
本发明所要解决的技术问题在于针对现有技术存在的不足提供一种多芯多磨光纤，它不仅能够同时支持850nm～950nm波长范围的多模传输和O波段/C波段的单模传输，而且能优化光纤结构和粘度匹配降低衰减，使光纤的串扰、各信道的宏弯和微弯损耗等综合性能处于良好的水平。
本发明为解决上述提出的问题所采取的技术方案为：包括有多个芯层和共同外包层，其特征在于所述的芯层为2个、4个或8个，各个芯层沿周向等距均布，各相邻芯层的间距2个芯层为38～60μm，4个或8个为38～48μm，各芯层外从内向外依次包覆内包层和下陷包层，构成多芯同质多模光纤，所述的芯层折射率剖面呈抛物线形，分布指数α为1.9～2.1，芯层的半径R1为12～20μm，芯层中心最大相对折射率差Δ1max为0.7％～1.7％，所述的内包层单边宽度(R2-R1)为0.5～2.5μm，相对折射率差Δ2为-0.4％～0.0％，所述的下陷包层单边宽度(R3-R2)为3～7μm，相对折射率差Δ3为-0.7％～-0.9％，所述的共同外包层为纯二氧化硅层。
按上述方案，所述的多个芯层等距均布在一个圆周上，即每个芯层与共同外包层的中心等距，所述2个或4个芯层的共同外包层直径为125±1μm，8个芯层的共同外包层的直径为180±1.5μm。
按上述方案，所述的下陷包层相对折射率差Δ3为-0.8％～-0.9％。
按上述方案，所述光纤的各芯的芯层、内包层和下陷包层均为锗氟共掺的二氧化硅玻璃层或氟掺杂的二氧化硅玻璃层，其中氟掺杂的相对折射率差贡献量为-0.02％～-0.9％。
按上述方案，所述的芯层为渐变氟掺杂贡献量，从芯层中心位到芯层边缘位，氟掺杂贡献量的绝对值呈递增状，芯层中心的氟掺杂的贡献量ΔF0为-0.10％～0％，芯层边缘的氟掺 杂的贡献量ΔF1为-0.45％～-0.10％；内包层锗氟贡献量之比0.1<|ΔGe/ΔF|<0.9，下陷包层锗氟贡献量之比|ΔGe/ΔF|<0.1。
按上述方案，所述光纤在850nm波长具有3500MHz-km或3500MHz-km以上带宽，在950nm波长具有2000MHz-km或2000MHz-km以上带宽，在1300nm波长具有500MHz-km或500MHz-km以上带宽。
按上述方案，所述光纤的各芯在波长1310nm或1550nm处的基模模场直径为8～12μm。
按上述方案，在波长850nm和1310nm处，所述光纤中的任意相邻两芯(旁芯)之间的芯间串扰<-35dB/km，与相邻芯以外的芯之间的芯间串扰<-55dB/km；优选的，任意两芯之间的芯间串扰<-40dB/km，与相邻芯以外的芯之间的芯间串扰<-60dB/1km。
按上述方案，所述光纤在波长850nm处各信道的衰耗均小于或等于3dB/km，在波长1310nm处各信道的衰耗均小于或等于1.2dB/km。优选的，所述光纤在波长850nm处各信道的衰耗均小于或等于2.4dB/km，在波长1310nm处各信道的衰耗均小于或等于0.6dB/km。
按上述方案，所述光纤在850nm波长处，以7.5毫米弯曲半径绕2圈导致的弯曲附加损耗小于0.2dB；在1300nm波长处，以7.5毫米弯曲半径绕2圈导致的弯曲附加损耗小于0.5dB。
按上述方案，所述光纤的涂覆层单边厚度大于等于50μm；所述光纤的拉丝张力为80～200g。
本发明的有益效果在于：1、本发明2芯或4芯设计中，采用标准125微米的光纤直径，与当前器件保持兼容性并与普通单芯光纤所占空间体积相同，8芯设计保证高密度、低串扰、低损耗的特性。2、合理的芯包比例与芯包结构设计，使得光纤能够同时支持850nm～950nm波长范围内的多模传输和O和/或C波段的单模传输。3、采用锗氟共掺的功能梯度材料设计和合理的纤芯和包层比例结构，严格控制锗氟配比，合理的设计了光纤内部的粘度匹配，减少光纤制备过程中的缺陷和剖面畸变，降低了光纤的衰减系数。4、设计了深掺氟的光纤下陷结构，并通过对光纤各层剖面的合理设计，使光纤的芯间串扰和宏弯损耗较低，并极大的降低了芯层距包层边缘较近所产生的附加衰耗。5、本发明的光纤各个信道的宏弯和微弯损耗等综合性能参数在应用波段良好。可使用空分复用技术，进行多个信道的短距离信号传输，每个模式均具有较低的衰减系数，可以支持数据中心密集布线的空分复用传输。6、本发明光纤制备上采用PCVD技术沉积芯棒，然后套外套管进行拉丝，更能制备 复杂剖面的光纤结构并严格的控制光纤的各项结构参数，并适于大规模生产。
附图说明
图1为商用VCSEL激光器光斑大小分布。
图2为本发明一个实施例中一个芯的折射率剖面示意图。
图3为本发明一个实施例的芯间距与芯间串扰的关系图。
图4为本发明一个实施例的截面结构示意图。
图5为本发明第二个实施例的截面结构示意图。
图6为本发明第三个实施例的截面结构示意图。
具体实施方式
下面结合附图和实施例对本发明作进一步的详细说明。
随着传输速率的进一步提升，多模光纤匹配的VCSEL激光器的光斑大小趋于降低。我们测试了市场上主流的10G至100G、850nm～950nm、NRZ与PAM4等不同厂家不同类别的商用多模收发器的发端出光的光斑大小，如图1所示。VCSEL激光器发射的光经过光路后其光斑大小在24～44μm。考虑到实际使用过程中，光纤需要与收发器进行耦合，多模光纤的芯径必须与光斑尺寸相匹配。而随着收发器速率的进一步提升，光斑有可能进一步减小，因此我们将芯层半径范围限制在12～20μm，进一步地限制在13～17μm之间。
在此基础上，为了保证在较大的芯径范围内其MFD仍然能够与标准单模光纤的MFD匹配，需要增大芯层的折射率。然而对多模光纤而言，芯层的相对折射率差越大，模间色散越严重，传输带宽越小。为了平衡高带宽和与多模光纤/VCSEL激光器的耦合损失，本发明在确保芯层相对折射率差较低的同时，通过同时调整芯层和内包层的相对折射率，以保证模场直径的可控。另外，芯包折射率差较大时，NA也较大，可以进一步确保其与光模块和多模光纤的耦合效率。
在光纤剖面设计中，计算剖面设计中的几何参数和相对折射率对基模模场直径的影响，影响光纤基模LP 01的模场直径的因素包括芯层直径、相对折射率差和α、内包层厚度和相对折射率差、下陷包层的厚度和深度等。其中影响LP 01模的模场直径的主要因为是芯径和芯包折射率差，模场直径与芯径成正比，与芯层和包层的相对折射率之差成反比，估算公式如下：
Figure PCTCN2020131831-appb-000007
受下陷包层和制备过程中的偏差等因素影响，实际制备的光纤的基模MFD一般比经验公式计算的稍小一些。基于上述公式，根据所需匹配激光器或者光纤确定芯径后，即可根据所需匹配的工作波段和模场直径，计算出所需光纤的Δ1max-Δ2的范围。再根据与所需匹配光纤芯层折射率的匹配(差异太大则耦合损失过大)和高带宽情况，确定合适的芯层相对折射率Δ1max和包层相对折射率差Δ2。根据掺杂条件、粘度匹配和抗弯曲性能，确定R2和R3的相对大小，即可制备出所需的兼容所需激光器或光纤，同时为未来升级为单模传输的兼容型渐变折射率光纤。
为了使多芯光纤成为数据中心接收的解决方案，多芯光纤的芯数为偶数较佳，首选为2或4或8芯，而中心对称分布的多芯分布便于旋转熔接。2芯可以支持单光纤的双向传输，而且2芯的连接方案已经比较成熟，成本较低。4芯可以实现单光纤和单波长4×25Gb/s的100Gb/s传输，8芯可实现单光纤和单波长8×50G(PAM4)的400Gb/s传输，而无需使用如SR4或PSM4等并行传输技术，从而极大地减小了数据中心的光纤的密度。
为了减少多芯光纤的芯间串扰和衰减，在纤芯外设计了深掺氟的光纤下陷结构，并通过对光纤各层剖面的合理设计，极大的降低了芯层距包层边缘较近所产生的附加衰耗。对某一实施例下陷包层的宽度为4μm，深度为-0.8％，通过理论计算相邻芯间距(pitch)与最近邻芯间串扰(XT)的关系，如图4所示。芯间串扰随着芯间距的增大而减小，其中850nm的高阶模的芯间串扰(XT@850)远大于基模在1310nm的芯间串扰(XT@1310)。计算过程中，850nm多模传输条件下，高阶模的芯间串扰选定的是最大值。假定光纤中的各模式能量平均分布，其中低阶模的串扰很小，如基模的XT@850<XT@1310，远小于-110dB，因此实际制备出的多芯光纤的芯间串扰比上述的XT@850计算值要小得多，有时甚至可以忽略不计。在设计多芯剖面时，准基模传输状态下的芯间串扰很低，主要的限制因素是多模传输状态下高阶模的芯间串扰。
另外，数据中心的98％的传输距离在150m以内，则相对于XT@850/1km，XT@850/150m的芯间串扰将减小约8dB，考虑到超大型数据中心的使用情况将上述传输距离扩展至300m，XT@850/300m的芯间串扰将减小约5dB。传输时的芯间串扰需要小于-30dB，更优的需要小于-40dB，在上述条件下，芯间距应大于38微米。
多芯光纤的纤芯到外包层边缘的厚度需要足够大，以保证多芯光纤的衰减足够小，需要降低其弯曲损耗和附加损耗，至少需要大于10μm，较佳的大于15μm，更佳的大于20μ m。结合深掺氟(-0.7％～-0.9％)的下陷包层，可以保证多芯光纤的衰减、弯曲损耗和芯间串扰都较低。
多芯光纤的纤芯密度高，其纤芯与包层的厚度较常规的光纤更小，且环状分布的多个孔使得预制棒的壁厚偏薄，因此拉丝过程中，为了确保多芯光纤的几何不会发生太大的偏移，需要保证拉丝过程中的材料粘度较高，此时拉丝张力为80～200g。
光纤的涂覆层材料包括但不限于环氧丙烯酸酯或聚丙烯酸酯，部分使用场景下还可以使用低折射率涂料或耐高温涂料等特种涂料。为了保证衰减和机械性能，涂覆层的单边厚度需要≥50μm。
多芯光纤的多个纤芯沿周向等距均布呈环状排布，如附图所示。纤芯为同质的多模光纤，各芯层外从内向外依次包覆内包层和下陷包层，所述芯层半径为R1，芯层相对折射率差为Δ1，内包层的半径为R2，内包层相对折射率差为Δ2，下陷包层半径为R3，下陷包层相对折射率差为Δ3。
本发明5个实施例光纤的结构设置和主要性能参数见表1，其中的纤芯的性能参数为多个纤芯的平均值。
表1 实施例光纤的结构设置和主要性能参数
Figure PCTCN2020131831-appb-000008
Figure PCTCN2020131831-appb-000009

Claims (10)

  1. 一种多芯多模光纤，包括有多个芯层和共同外包层，其特征在于所述的芯层为2个、4个或8个，各个芯层沿周向等距均布，各相邻芯层的间距2个芯层为38～60μm，4个或8个为38～48μm，各芯层外从内向外依次包覆内包层和下陷包层，构成多芯同质多模光纤，所述的芯层折射率剖面呈抛物线形，分布指数α为1.9～2.1，芯层的半径R1为12～20μm，芯层中心最大相对折射率差Δ1max为0.7％～1.7％，所述的内包层单边宽度(R2-R1)为0.5～2.5μm，相对折射率差Δ2为-0.4％～0.0％，所述的下陷包层单边宽度(R3-R2)为3～7μm，相对折射率差Δ3为-0.7％～-0.9％，所述的共同外包层为纯二氧化硅层。
  2. 按权利要求1所述的多芯多模光纤，其特征在于所述的多个芯层等距均布在一个圆周上，即每个芯层与共同外包层的中心等距，所述2个或4个芯层的共同外包层直径为125±1μm，8个芯层的共同外包层的直径为180±1.5μm。
  3. 按权利要求1或2所述的多芯多模光纤，其特征在于所述的下陷包层相对折射率差Δ3为-0.8％～-0.9％。
  4. 按权利要求1或2所述的多芯多模光纤，其特征在于所述光纤的各芯的芯层、内包层和下陷包层均为锗氟共掺的二氧化硅玻璃层或氟掺杂的二氧化硅玻璃层，其中氟掺杂的相对折射率差贡献量为-0.02％～-0.9％。
  5. 按权利要求4所述的多芯多模光纤，其特征在于所述的芯层为渐变氟掺杂贡献量，从芯层中心位到芯层边缘位，氟掺杂贡献量的绝对值呈递增状，芯层中心的氟掺杂的贡献量ΔF0为-0.10％～0％，芯层边缘的氟掺杂的贡献量ΔF1为-0.45％～-0.10％；内包层锗氟贡献量之比0.1<|ΔGe/ΔF|<0.9，下陷包层锗氟贡献量之比|ΔGe/ΔF|<0.1。
  6. 按权利要求1或2所述的多芯多模光纤，其特征在于所述光纤在850nm波长具有3500MHz-km或3500MHz-km以上带宽，在950nm波长具有2000MHz-km或2000MHz-km以上带宽，在1300nm波长具有500MHz-km或500MHz-km以上带宽。
  7. 按权利要求1或2所述的多芯多模光纤，其特征在于所述光纤的各芯在波长1310nm或1550nm处的基模模场直径为8～12μm。
  8. 按权利要求1或2所述的多芯多模光纤，其特征在于在波长850nm和1310nm处，所述光纤中的任意相邻两芯之间的芯间串扰<-35dB/km，与相邻芯以外的芯之间的芯间串扰<-55dB/km。
  9. 按权利要求1或2所述的多芯多模光纤，其特征在于所述光纤在波长850nm处各信道的衰耗均小于或等于3dB/km，在波长1310nm处各信道的衰耗均小于或等于1.2 dB/km。优选的，所述光纤在波长850nm处各信道的衰耗均小于或等于2.4dB/km，在波长1310nm处各信道的衰耗均小于或等于0.6dB/km。
  10. 按权利要求1或2所述的多芯多模光纤，其特征在于所述光纤在850nm波长处，以7.5毫米弯曲半径绕2圈导致的弯曲附加损耗小于0.2dB；在1300nm波长处，以7.5毫米弯曲半径绕2圈导致的弯曲附加损耗小于0.5dB；所述光纤的拉丝张力为80～200g。
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