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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
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Google Scholar
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Google Scholar
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Google Scholar
[11] Leon-Saval S G, Fontaine N K, Salazar-Gil J R, Ercan B, Ryf R, Bland-Hawthorn J 2014 Opt. Express. 22 1036
Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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Google 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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图 1 模式转换器的结构图
Figure 1. Schematic of mode converter.
图 2 少模光纤的模式色散特性
Figure 2. Effective refractive indices against wavelength of two modes.
图 3 少模光纤光栅周期与谐振波长的关系
Figure 3. Dependence of grating period and resonant wavelength in few mode fiber.
图 4 单栅与叠栅光谱对比
Figure 4. Spectrum contrast between LPFG and SLPFG.
图 5 不同周期的叠栅仿真
Figure 5. Simulated SLPFG for different periods.
图 6 不同光栅长度的叠栅仿真
Figure 6. Simulated SLPFG for different grating lengths.
图 7 不同耦合系数的叠栅仿真
Figure 7. Simulated SLPFG for different coupling coefficient.
图 8 少模光纤叠栅的实验制备系统
Figure 8. Experiment setup for TMF-SLPFG.
图 9 单栅和叠栅的实验光谱图
Figure 9. Experimental spectrum of LPFG and SLPFG.
图 10 模场观测装置
Figure 10. Schematic of mode profile observation.
图 11 不同波长处的模场图 (a) 1400 nm; (b)1486 nm; (c) 1550 nm
Figure 11. Images of mode profiles located at different wavelength: (a) 1400 nm; (b) 1486 nm; (c) 1550 nm.
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[1] Goebel B, Foschini G J, Kramer G, Essiambre R, Winzer P J, Essiambre R 2010 J. Lightwave Technol. 28 662
Google Scholar
[2] Fernandes G M, Muga N J, Pinto A N 2017 Opt. Express 25 3899
Google Scholar
[3] Zhao Y H, Liu Y Q, Zhang L, Zhang C Y, Wen J X, Wang T Y 2016 Opt. Express 24 6186
Google 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. 30 618
Google Scholar
[5] Von H J, Ryf R, Winzer P 2013 Opt. Express 21 18097
Google 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. 30 521
Google Scholar
[7] Montero-Orille C, Moreno V, Prieto-Blanco X, Mateo E F, Ip E, Crespo J, Linares J 2013 Appl. Optics 52 2332
Google Scholar
[8] Dong J, Chiang K S, Jin W 2015 Opt. Lett. 40 3125
Google Scholar
[9] Dong J, Chiang K S, Jin W 2015 J. Lightwave Technol. 33 4580
Google 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 4355
Google Scholar
[11] Leon-Saval S G, Fontaine N K, Salazar-Gil J R, Ercan B, Ryf R, Bland-Hawthorn J 2014 Opt. Express. 22 1036
Google 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 1713
Google Scholar
[13] Savin S, Digonnet M J, Kino J S, Shaw H J 2000 Opt. Lett. 25 710
Google Scholar
[14] Ramachandran S, Wang Z Y, Yan M 2002 Opt. Lett. 27 698
Google Scholar
[15] Liao C R, Wang Y, Wang D N, Jin L 2010 IEEE Photonics Technol. Lett. 22 425
Google Scholar
[16] Dong J L, Chiang K S 2015 IEEE Photonics Technol. Lett. 27 1006
Google 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. 27 145
Google 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 4708
Google Scholar
[19] Garg R, Thyagarajan K 2013 Opt. Fiber Technol. 19 148
Google 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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