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本文提出一种在同一段少模光纤上写入两个不同周期
${\varLambda _1}$ 和${\varLambda _2}$ 的长周期光纤光栅构成叠栅的方法, 实现了纤芯基模LP01向高阶纤芯模LP11模式转换的宽带宽的新型的全光纤模式转换器. 利用有限元法和耦合模理论建立了模式转换器的理论分析模型. 数值仿真分析了叠栅中两个子光栅周期间隔、光栅长度、耦合系数等光栅参数对模式转换器的影响. 仿真分析和实验结果表明, 通过改变两个子光栅的周期间隔来改变两个损耗峰的位置, 形成一个损耗峰, 从而可以实现宽带宽的模式转换器, 其10 dB带宽约是单栅的2倍. 与传统的模式转换器相比, 该转换器带宽宽、转换效率高, 尺寸小、抗干扰能力强, 可以在模分复用系统和光通信中得到广泛的应用.Mode-division multiplexing (MDM), as one of the promising techniques for overcoming current limitation of transmission capacity in single-mode fibers (SMFs), has attracted considerable attention. A key component in the MDM system is a mode converter, which makes conversion between the fundamental mode and the higher-order mode. Many mode converters have been demonstrated, such as spatial light modulators, phase plates, silicon-based asymmetrical directional couplers, fiber-based photonic lantern, and long period fiber grating (LPFG). Compared with other methods, mode converter used LPFG is a very feasible technique, which has the advantages of small size, low loss, low backward noise, high coupling efficiency and easy fabrication. However, the limitation of the mode converter is relatively narrow bandwidth. In this paper, a novel broadband all-fiber mode converter is proposed, in which two long period fiber gratings (LPFGs) with different periods are fabricated in the same spatial domain of few-mode fiber to achieve coupling from LP01 to LP11, thus forming superimposed long period fiber gratings (SLPFGs). The influences of grating parameters, such as the interval between two periods, the length of grating and the coupling coefficient on the mode converter, are analyzed by numerical simulation. It is found that the gap between the two resonant wavelengths becomes smaller with the periodic interval decreasing, which can form one rejection band when the gap is small enough, thus a broadband mode converter can be realized. The corresponding bandwidth at a conversion efficiency of 10 dB is about twice that of traditional LPFG. Moreover, with the increase of grating length, the conversion efficiency first increases and then decreases, because coupling efficiency experiences deficient coupling, full coupling and over coupling. The effect of coupling coefficient on converter is similar to that of grating length. According to the numerical results, grating I is fabricated with${\varLambda _1} = 673\;{\text{μ}}{\rm m} $ , 35-period. After that, the platform is rotated 180° and grating II is fabricated with${\varLambda _2} = 688\; {\text{μ}}{\rm m}$ , 35-period by CO2 laser in tow mode fiber (TMF steped-index fiber). The bandwidths of both LPFGs at a conversion efficiency of 10 dB are about 57 nm and 67 nm respectively, while the bandwidth of SLPFG is about 153 nm. The experimental results are in pretty good agreement with the theoretical analyses. In addition, the proposed superimposed structure can also be extended to the conversion of fundamental mode into other high-order core modes. By designing the period of two sub-gratings reasonably, a wide band rejection filter with arbitrary wavelength can be realized. Compared with the traditional mode converter, the converter has the advantages of broad bandwidth, high conversion efficiency and small size, which can be widely used in the mode division multiplexing system and optical communication.-
Keywords:
- superimposed long period fiber gratings /
- few mode fiber /
- mode converter /
- coupling coefficient
[1] Goebel B, Foschini G J, Kramer G, Essiambre R, Winzer P J, Essiambre R 2010 J. Lightwave Technol. 28 662Google Scholar
[2] Fernandes G M, Muga N J, Pinto A N 2017 Opt. Express 25 3899Google Scholar
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[4] Salsi M, Koebele C, Sperti D, Tran P, Mardoyan H, Brindel P, Bigo S, Boutin A, Verluise F, Sillard P, Astruc M, Provost L, Charlet G 2012 J. Lightwave Technol. 30 618Google Scholar
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[10] Sai X, Li Y, Yang C, Li W, Qiu J, Hong X, Zuo Y, Guo H, Tong W, Wu J 2017 Opt. Lett. 42 4355Google Scholar
[11] Leon-Saval S G, Fontaine N K, Salazar-Gil J R, Ercan B, Ryf R, Bland-Hawthorn J 2014 Opt. Express. 22 1036Google Scholar
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[14] Ramachandran S, Wang Z Y, Yan M 2002 Opt. Lett. 27 698Google Scholar
[15] Liao C R, Wang Y, Wang D N, Jin L 2010 IEEE Photonics Technol. Lett. 22 425Google Scholar
[16] Dong J L, Chiang K S 2015 IEEE Photonics Technol. Lett. 27 1006Google Scholar
[17] Wang B, Zhang W G, Bai Z Y, Wang L, Zhang L Y, Zhou Q, Chen L, Yan T Y 2015 IEEE Photonics Technol. Lett. 27 145Google Scholar
[18] Zhao Y H, Liu Y Q, Zhang C Y, Zhang L, Zheng G J, Mou C B, Wen J X, Wang T Y 2017 Opt. Lett. 42 4708Google Scholar
[19] Garg R, Thyagarajan K 2013 Opt. Fiber Technol. 19 148Google Scholar
[20] 江鹏 2016 博士学位论文(秦皇岛: 燕山大学)
Jiang P 2016 Ph.D. Dissertation (Qinhuangdao: Yanshan University) (in Chinese)
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[1] Goebel B, Foschini G J, Kramer G, Essiambre R, Winzer P J, Essiambre R 2010 J. Lightwave Technol. 28 662Google Scholar
[2] Fernandes G M, Muga N J, Pinto A N 2017 Opt. Express 25 3899Google Scholar
[3] Zhao Y H, Liu Y Q, Zhang L, Zhang C Y, Wen J X, Wang T Y 2016 Opt. Express 24 6186Google Scholar
[4] Salsi M, Koebele C, Sperti D, Tran P, Mardoyan H, Brindel P, Bigo S, Boutin A, Verluise F, Sillard P, Astruc M, Provost L, Charlet G 2012 J. Lightwave Technol. 30 618Google Scholar
[5] Von H J, Ryf R, Winzer P 2013 Opt. Express 21 18097Google Scholar
[6] Ryf R, Randel S, Gnauck A H, Bolle C, Sierra A, Mumtaz S, Esmaeelpour M, Burrows C, Essiambre R J, Winzer P J, Peckham D W, McCurdy A H, Lingle R 2012 J. Lightwave Technol. 30 521Google Scholar
[7] Montero-Orille C, Moreno V, Prieto-Blanco X, Mateo E F, Ip E, Crespo J, Linares J 2013 Appl. Optics 52 2332Google Scholar
[8] Dong J, Chiang K S, Jin W 2015 Opt. Lett. 40 3125Google Scholar
[9] Dong J, Chiang K S, Jin W 2015 J. Lightwave Technol. 33 4580Google Scholar
[10] Sai X, Li Y, Yang C, Li W, Qiu J, Hong X, Zuo Y, Guo H, Tong W, Wu J 2017 Opt. Lett. 42 4355Google Scholar
[11] Leon-Saval S G, Fontaine N K, Salazar-Gil J R, Ercan B, Ryf R, Bland-Hawthorn J 2014 Opt. Express. 22 1036Google Scholar
[12] Ali M M, Jung Y, Lim K S, Islam M R, Alam S U, Richardson D J, Ahmad H 2015 IEEE Photonics Technol. Lett. 27 1713Google Scholar
[13] Savin S, Digonnet M J, Kino J S, Shaw H J 2000 Opt. Lett. 25 710Google Scholar
[14] Ramachandran S, Wang Z Y, Yan M 2002 Opt. Lett. 27 698Google Scholar
[15] Liao C R, Wang Y, Wang D N, Jin L 2010 IEEE Photonics Technol. Lett. 22 425Google Scholar
[16] Dong J L, Chiang K S 2015 IEEE Photonics Technol. Lett. 27 1006Google Scholar
[17] Wang B, Zhang W G, Bai Z Y, Wang L, Zhang L Y, Zhou Q, Chen L, Yan T Y 2015 IEEE Photonics Technol. Lett. 27 145Google Scholar
[18] Zhao Y H, Liu Y Q, Zhang C Y, Zhang L, Zheng G J, Mou C B, Wen J X, Wang T Y 2017 Opt. Lett. 42 4708Google Scholar
[19] Garg R, Thyagarajan K 2013 Opt. Fiber Technol. 19 148Google Scholar
[20] 江鹏 2016 博士学位论文(秦皇岛: 燕山大学)
Jiang P 2016 Ph.D. Dissertation (Qinhuangdao: Yanshan University) (in Chinese)
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