1.Institute of Opt-Electronics and Communication Technologies, Jiangsu University, Zhenjiang 212013, China 2.Jiangsu Tian Xing Optoelectronics Technology Co. Ltd., Zhenjiang 212132, China
Fund Project:Project supported by the Key Research and Development Projects (Industry Foresight and Common Key Technologies) of Zhenjiang, China (Grant No. GY2015033) and the Zhenjiang Key Laboratory of Advanced Sensing Materials and Devices, Jiangsu University, China (Grant No. SS2018001).
Received Date:23 October 2018
Accepted Date:23 January 2019
Available Online:01 April 2019
Published Online:20 April 2019
Abstract:In this paper, a novel tunable mode-filter optical fiber consisting of a high-index core and petal-shaped cladding surrounded by a high-index outer ring is proposed. The cladding of the fiber is formed with periodically arranged liquid rods that support cladding modes with effective indexes. These cladding modes form a two-super-mode group. The mode-selection is realized by the coupling between the core mode and the super-mode group. With the petal-shaped cladding, cladding mode can be transmitted at high loss. With the liquid rods, the index-band of super-mode group can be adjusted by external temperature field, thereby achieving the purpose of tunable mode-selective. The super-mode group formed by the LP11 mode of the liquid rods effectively increases its operating bandwidth and temperature tuning range. The numerical simulation results show that the mode-filter fiber with a length of only 71.4 mm can achieve a particular mode loss more than 20 dB, while other modes’ losses are below 1 dB. This special fiber can be used as a mode-filter in the few-mode fiber transmission system to reduce mode crosstalk of converters, multiplexer/demultiplexer, optical switch and optical routing. Keywords:micro-structure optical fiber/ super-mode/ liquid rods/ mode-filter/ loss
可以计算出液体介质柱的吸收损耗. 图5分别给出入射波长$\lambda = 1550 {\rm{ nm}}$时, 考虑和不考虑液体吸收损耗时纤芯LP01模式和LP11的损耗曲线. 可见液体吸收损耗对高损耗的模式影响较小, 而对低损耗的模式影响较大. 为此, 在后面的模式损耗分析中, 均包含液体吸收损耗. 图 5 考虑和不考虑液体吸收损耗两种情况下的纤芯LP01模和LP11模损耗曲线 Figure5. Variation of the core-mode LP01 mode and LP11 mode loss with and without liquid absorption loss.
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3.1.可调谐滤模的性能分析
以上已经阐述了滤模光纤的结构和选择性滤模的原理, 下面我们分析其传输性能. 根据图4结果, 可以得到纤芯四种模式单独处于超模群区间时, 液体柱折射率范围. 其对应的损耗曲线如图6所示. 当LP01, LP11模分别处于超模群区间时, 其损耗可达300 dB/m以上, 而其他模式损耗低于1 dB/m, 因而可实现有效滤模. 而LP21和LP02模虽然在分别处于超模群区间时, 损耗更大, 可达380 dB/m以上, 但是由于两种模式的有效折射率比较接近, 因而在滤除一个模式的同时, 也会使另一模式发生一定的损耗. 例如, 当液体折射率为1.4860—1.4864时, LP02模损耗小于14 dB/m, 而当液体折射率为1.4810—1.4816时, LP21模损耗小于9 dB/m. 根据图4和图6, 我们可以得出四种模式抑制区间的温度改变量都在4 K左右, 滤模器的工作温度范围比较大, 因而有较大的容差, 利于实际操作. 图 6 纤芯四种模式单独处于超模群区间时损耗曲线 (a) LP01模; (b) LP11模; (c) LP21模; (d) LP02模 Figure6. The loss of single core-mode on the super-mode band: (a) The LP01 mode; (b) the LP11 mode; (c) the LP21 mode; (d) the LP02 mode.
为了得到工作带宽, 我们分别选取液体折射率为1.4927, 1.4892, 1.486和1.4812从而分别滤除LP01模、LP11模、LP21模和LP02模. 这里液体折射率的选择兼顾了抑制模式的损耗须足够大、其他模式损耗又比较低的要求. 四种纤芯模式的损耗曲线如图7所示. 为了实现有效滤模, 要求抑制模式的损耗应不小于20 dB, 而其他模式的损耗均小于1 dB. 可以得出, 在工作波长为1540—1555 nm时, 抑制模式的损耗都可以达到280 dB/m以上, 同时其他模式的损耗都低于14 dB/m. 因此, 滤模光纤的长度可以做到仅为71.4 mm, 便于制备和集成. 图 7 不同液体折射率时, 四种纤芯模式的损耗曲线 (a) ${n_{{\rm{liquid}}}} = {\rm{1}}{\rm{.4937}}$; (b) ${n_{{\rm{liquid}}}} = {\rm{1}}{\rm{.4892}}$; (c) ${n_{{\rm{liquid}}}} = {\rm{1}}{\rm{.486}}$; (d)${n_{{\rm{liquid}}}}$ = 1.4812 Figure7. The loss of four core-mode with various liquid index: (a) ${n_{{\rm{liquid}}}} = {\rm{1}}{\rm{.4937}}$; (b) ${n_{{\rm{liquid}}}} = {\rm{1}}{\rm{.4892}}$; (c) ${n_{{\rm{liquid}}}} = {\rm{1}}{\rm{.486}}$; (d) ${n_{{\rm{liquid}}}} = {\rm{1}}{\rm{.4812}}$.
本文提出花瓣形包层结构, 以增大纤芯模式损耗, 减小光纤长度. 为此, 我们对比了介质柱折射率${n_{{\rm{rod}}}} = 1.4937$时, Chen和Chiang[20]提出的圆形外包层结构光纤与本文提出的花瓣结构光纤的损耗情况. 为了便于对比, 除外包层形状外, 其他结构参数均相同. 图8(a)为圆形外包层结构的LP01模场分布, 图8(b)为花瓣结构光纤的LP01模场分布. 图 8 不同结构光纤的LP01模的模场分布 (a)圆形结构; (b)花瓣结构 Figure8. Field distributions of LP01 mode with various circle structures: (a) Circle structure; ( b) petal-shape structure.
四种模式损耗的对比如图9所示. 从图9(a)中可以看出, 花瓣结构的损耗明显更大. 特别地, 其LP01模的损耗在波长为1550 nm时比圆形结构大100倍, 达到了488 dB/m. 在1550 nm波长处, LP01模的损耗最大, 而LP21模的损耗最小, 对于花瓣结构和圆形结构光纤, 两者的损耗比分别为65066∶1, 60003∶1. 可见, 采用花瓣结构可以在保证抑制模式和其他模式保持足够大的损耗差情况下, 有效减小光纤的长度, 有利于液体介质的填充和温度控制. 图 9 两种MOF的模式损耗对比 (a) LP01模和LP11模损耗曲线; (b) LP21模和LP02模曲线 Figure9. Loss of two MOF: (a) Loss band of the LP01 mode and LP11 mode; (b) loss band of the LP21 mode and LP02 mode.
23.3.两种包层超模群性能对比 -->
3.3.两种包层超模群性能对比
本文以液体柱的LP11模所形成的超模群作为耦合区间. 下面分析其与LP01模所形成超模群区间的不同之处. 如图4所示, LP11模所形成的超模群区间更宽, 同时其超模的斜率曲线更小. 以纤芯LP01模传输为例, 使其分别处于两种超模群区间, 对比损耗. LP01超模群区间设置${n_{{\rm{liquid}}}} = 1.472$, LP11超模群区间设置${n_{{\rm{liquid}}}} = 1.492$, Loss-band为当取光纤长度为71.4 mm时, 抑制模式损耗达到20 dB时所对应的工作区间. 如图10所示, 相比于介质柱LP01模所形成的超模群区间, 介质柱LP11模所形成的超模群区间可以在更宽的温度和波长范围内实现对纤芯LP01模式的滤除. 其主要原因是, LP11模的能量耦合进包层中更多, 因而更容易与相邻介质柱的模式发生耦合. 图 10 纤芯 LP01模式在双超模群时的两种超模群区间LP01模损耗 (a)波长为1550 nm, 温度改变量相同; (b)温度相同, 波长改变 Figure10. Dependence of the loss of the core LP01 mode locating in different two super-mode region: (a) With same temperature variation at wavelength 1550 nm; (b) with various wavelength at the same temperature.