DONG AND CHIANG:TEMPERATURE-INSENSITIVE MODE CONVERTERS WITH CO2-LASER WRITTEN LPFGs 1009 1565 These robust mode converters could be used in 1560 mode-division-multiplexing systems and other areas that require cylindrical vector modes. 1550 REFERENCES 1545 -.15-period grating -20-period grating [1]R.Ryf et al.,"Mode-division multiplexing over 96 km of few-mode fiber 1540 30-period grating using coherent 6 x 6 MIMO processing,"J.Lightw.Technol.,vol.30. no.4,pp.521-531,Feh.15.2012. 1535 [2]M.Salsi et al.,"Mode-division multiplexing of 2 x 100 Gb/s channels 153 using an LCOS-based spatial modulator,"Lightw.Technol.,vol.30, 20 30 40506070 80 no.4,Pp.618-623.Feb.15,2012 Temperature (C) (3]S.Ramachandran,P.Kristensen,and M.F.Yan,"Generation and propagation of radially polarized beams in optical fibers,"Opt.Let.. (a) vol.34,no.16,Pp.2525-2527,Aug.2009. 30 [4]A.Witkowska,S.G.Leon-Saval,A.Pham,and T.A.Birks,"All-fiber 29 LP1 mode convertors."Opt.Lett.,vol.33,no.4,pp.306-308. Feb.2008. 28 [5]Y.I.Salamin,"Electron acceleration from rest in vacuum by 27 an axicon Gaussian laser beam,"Phys.Rev.A,vol.73,no.4, 留 Pp.043402-1-043402-6,Apr.2006. -15-period grating [6]Q.Zhan and J.R.Leger,"Microellipsometer with radial symmetry." 24 .20-period grating Appl.Opt,vol.41,no.22,Pp.4630-4637,Aug.2002. 23 30-period grating [7]V.G.Niziev and A.V.Nesterov,"Influence of beam polarization 22 on laser cutting efficiency,"J.Phys.D.Appl.Phys.,vol.32,no.13, 21 PP.1455-1461,Jul.1999. [8]N.Hanzawa.K.Saitoh,T.Sakamoto.T.Matsui.S.Tomita.and 20 0 30 40 50 70 80 M.Koshiba."Asymmetric parallel waveguide with mode conversion for mode and wavelength division multiplexing transmission,"in Proc. Temperature (C) OFC/NFOEC,Mar.2012,pp.1-3,paper OTull.4. (b) [9]D.Dai,Y.Tang,and J.E.Bowers,"Mode conversion in tapered Fig.6.Variations of (a)the resonance wavelengths and (b)the contrasts of submicron silicon ridge optical waveguides."Opi.Exp..vol.20,no.12. the 15-period,20-period,and 30-period gratings with the temperature. [10] vang in andK.$Chang."Mode converter with polymer long-period waveguide grating,"in Proc.Asia Commun. 20-period,and 30-period gratings are only 2.5 Photon.Conf:.Nov.2014,pp.1-3,paper AW4C.4. 1.4,and 1.0 nm,respectively.The temperature sensitivity [11]C.P.Tsekrekos and D.Syvridis,"All-fiber broadband LPo2 mode of the mode converter is much lower than that of a typical converter for future wavelength and mode division multiplexing systems,"IEEE Photon.Technol.Lett.,vol.24.no.18.pp.1638-1641. conventional cladding-mode LPFG.The very low temperature Sep.15,2012. sensitivities of the LPFG mode converters suggest that the [12]K.Lai,S.G.Leon-Saval,A.Witkowska,W.J.Wadsworth,and effective indices of the LPor and LP modes of the fiber are T.A.Birks,"Wavelength-independent all-fiber mode converters," 0pt.Let,vol.32,no.4,pp.328-330,Feb.2007. similarly influenced by the temperature.The slight differences [13]A.Li,X.Chen,A.Al Amin,J.Ye,and W.Shieh,"Space-division in the temperature sensitivities of the three gratings can multiplexed high-speed superchannel transmission over few-mode fiber,' be attributed to the use of different CO2-laser doses in the J.Lightw.Technol.,vol.30,no.24,pp.3953-3964.Dec.15,2012. [14]H.Sakata.H.Sano,and T.Harada,"Tunable mode converter using writing of the gratings,since different CO2-laser doses can electromagnet-induced long-period grating in two-mode fiber"Opt. modify the properties of the fiber glass differently [22]. Fiber Technol.,vol.20,no.3.pp.224-227.Jun.2014. The variations of the contrasts of the three gratings [15]S.Ramachandran,Z.Wang,and M.Yan,"Bandwidth control of long- period grating-based mode converters in few-mode fibers,"Opt.Lett., (i.e.,the depths of the rejection bands shown in Fig.3) vol.27,no.9.Pp.698-700,May2002. with the temperature are shown in Fig.6(b),which indicates [16]L.Gruiner-Nielsen and J.W.Nicholson,"Stable mode converter for conversion between LPo and LP using a thermally induced long that the grating contrast is also insensitive to temperature period grating,"in Proc.IEEE Photon.Soc.Summer Topical Meeting, variations. jul.2012,pp.214-215.paper WC1.2 [17]C.D.Poole,H.M.Presby,and J.P.Meester,"Two-mode fibre spatial- IV.CONCLUSION mode converter using periodic core deformation,"Electron.Lett.,vol.30. no.17,Pp.1437-1438,Aug.1994. We have experimentally demonstrated CO2-laser written [18]H.W.Lee,Y.Liu,and K.S.Chiang."Writing of long-period grat- LPFGs in a TMF for achieving conversion between ings in conventional and photonic-crystal polarization-maintaining fibers by CO2-laser pulses,"IEEE Photon.Technol.Lett.,vol.20.no.2. the fundamental mode and the higher-order modes.The Pp.132-134,Jan.15,2008. CO2-laser writing process can be computer-programmed and [19]Y.Gu,K.S.Chiang,and Y.J.Rao,"Writing of apodized phase. the growth of the grating can be monitored in real time.The shifted long-period fiber gratings with a computer-controlled CO2 laser," IEEE Photon.Technol.Lett.,vol.21,no.10,pp.657-659,May 15,2009 transmission characteristics of the gratings are insensitive to [20]J.Dong and K.S.Chiang,"Mode-locked fiber laser with transverse- temperature variations and the mode conversion efficiency is mode selection based on a two-mode FBG."IEEE Photon.Technol. polarization-insensitive.By control of the polarization state of Le,vol.26,no.17,Pp.1766-1769,Sep.1,2014. the input LPor mode,these gratings can function as effective [21]G.Volpe and D.Petrov,"Generation of cylindrical vector beams with few-mode fibers excited by Laguerre-Gaussian beams,"Opt.Commun., LPoI-LPu mode converters or cylindrical vector vol.237,no.1-3,pp.89-95,Jul.2004. mode generators.One of our typical gratings,which contains [22]Y.Liu,H.W.Lee,K.S.Chiang,T.Zhu,and Y.J.Rao,"Glass 15 grating periods,can provide a conversion efficiency higher structure changes in CO-laser writing of long-period fiber gratings in boron-doped single-mode fibers,"J.Lightw.Technol..vol.27.no.7. than 99%over a bandwidth of 34.0 nm in the C-band. Pp.857-863,Apr.1,2009.DONG AND CHIANG: TEMPERATURE-INSENSITIVE MODE CONVERTERS WITH CO2-LASER WRITTEN LPFGs 1009 Fig. 6. Variations of (a) the resonance wavelengths and (b) the contrasts of the 15-period, 20-period, and 30-period gratings with the temperature. 20-period, and 30-period gratings are only 2.5, 1.4, and 1.0 nm, respectively. The temperature sensitivity of the mode converter is much lower than that of a typical conventional cladding-mode LPFG. The very low temperature sensitivities of the LPFG mode converters suggest that the effective indices of the LP01 and LP11 modes of the fiber are similarly influenced by the temperature. The slight differences in the temperature sensitivities of the three gratings can be attributed to the use of different CO2-laser doses in the writing of the gratings, since different CO2-laser doses can modify the properties of the fiber glass differently [22]. The variations of the contrasts of the three gratings (i.e., the depths of the rejection bands shown in Fig. 3) with the temperature are shown in Fig. 6(b), which indicates that the grating contrast is also insensitive to temperature variations. IV. CONCLUSION We have experimentally demonstrated CO2-laser written LPFGs in a TMF for achieving conversion between the fundamental mode and the higher-order modes. The CO2-laser writing process can be computer-programmed and the growth of the grating can be monitored in real time. The transmission characteristics of the gratings are insensitive to temperature variations and the mode conversion efficiency is polarization-insensitive. By control of the polarization state of the input LP01 mode, these gratings can function as effective LP01−LP11 mode converters or cylindrical vector mode generators. One of our typical gratings, which contains 15 grating periods, can provide a conversion efficiency higher than 99% over a bandwidth of 34.0 nm in the C-band. These robust mode converters could be used in mode-division-multiplexing systems and other areas that require cylindrical vector modes. REFERENCES [1] R. Ryf et al., “Mode-division multiplexing over 96 km of few-mode fiber using coherent 6 × 6 MIMO processing,” J. Lightw. Technol., vol. 30, no. 4, pp. 521–531, Feb. 15, 2012. [2] M. Salsi et al., “Mode-division multiplexing of 2 × 100 Gb/s channels using an LCOS-based spatial modulator,” J. Lightw. Technol., vol. 30, no. 4, pp. 618–623, Feb. 15, 2012. [3] S. Ramachandran, P. Kristensen, and M. F. Yan, “Generation and propagation of radially polarized beams in optical fibers,” Opt. Lett., vol. 34, no. 16, pp. 2525–2527, Aug. 2009. [4] A. Witkowska, S. G. Leon-Saval, A. Pham, and T. A. Birks, “All-fiber LP11 mode convertors,” Opt. Lett., vol. 33, no. 4, pp. 306–308, Feb. 2008. 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