1006 IEEE PHOTONICS TECHNOLOGY LETTERS,VOL.27.NO.9.MAY 1,2015 Temperature-Insensitive Mode Converters With CO2-Laser Written Long-Period Fiber Gratings Jiangli Dong and Kin Seng Chiang,Member;IEEE Abstract-We fabricate mode converters by directly writing alignment problem [8]-101,but the fabrication processes long-period gratings in a two-mode fiber with a CO2 laser. required are complicated and expensive.All-fiber mode These mode converters allow the fundamental LPoi mode to converters compatible with optical fiber links have been be converted to any of the four higher order cylindrical vector modes or the LPj1 modes.The transmission characteristics of demonstrated with the effect of multimode interference in these devices are insensitive to temperature variations and the a multimode fiber (simulation only)[11],the principle of mode conversion efficiency is insensitive to the polarization state adiabatic transition in a photonic-crystal fiber [4].[12],and of the input light.One of our typical gratings,which contains long-period fiber gratings (LPFGs)[3],[13]-[17].Among 15 grating periods,can provide a conversion efficiency >99% these all-fiber devices,LPFGs offer many distinct advantages, over a bandwidth of 34.0 nm in the C-band.These robust mode converters could find applications in mode-division-multiplexing such as low reflection,low loss,and high fabrication and systems and other applications that require cylindrical vector design flexibility.An LPFG mode converter can be formed in modes. a fiber by mechanical means [3],[13],[14]or by exposing the Index Terms-Long-period fiber grating,mode converter, fiber to an ultra-violet (UV)laser [15].a heat source [161,or mode-division multiplexing,cylindrical vector modes. a CO2 laser [17].The early CO2-laser writing method for the fabrication of mode converters [17]involves two steps:the I.INTRODUCTION surface of the fiber is first ablated periodically by a CO2 laser beam and the ablated fiber is next annealed with an electric arc HE transmission capacity of single-mode fibers (SMFs) to form a grating.The resultant grating has a very large index is rapidly approaching the limit and new technologies are being developed to sustain the network traffic growth.Mode- change(~0.1)and suffers from a relatively high insertion loss. In the present study,we employ a direct CO2-laser writing division multiplexing(MDM)is a promising technology for method to fabricate LPFG mode converters in a two-mode increasing the transmission capacity of optical fibers [1],[2] In MDM transmission systems,mode converters are important fiber (TMF),where the CO2 laser can be computer- devices,which can be used,for example,at the sending and programmed to generate the grating profile.This CO2-laser writing method has been applied to the fabrication of sophisti- receiving ends to couple the fundamental mode to selected cated LPFG filters in different types of SMFs (see [18],[19]). high-order modes and vice versa.Mode converters can also be used as elements to generate cylindrical vector beams [3],[4] By properly controlling the amount of radiation on the fiber,the laser beam can induce an index change in the fiber for applications in electron acceleration [5],high-resolution without causing any visible damage to the fiber.Our CO2-laser metrology [6]and material processing [7].In this letter,we written LPFG mode converters are insensitive to temperature demonstrate robust all-fiber mode converters for the conversion variations and the mode conversion efficiency is insensitive between the fundamental mode and the higher-order modes to the polarization state of the input light.One of our typical with potential applications in the aforementioned areas. Mode conversion has been realized for long-haul MDM LPFGs,which contains 15 grating periods,shows a conversion transmission with bulk-optic components,such as phase efficiency higher than 99%over a bandwidth of 34.0 nm plates [1]and spatial light modulators [2].Such mode from 1529.1 to 1563.1 nm.We can obtain the four orthogonal converters can be implemented with readily available discrete LPI modes or the four cylindrical vector modes (the TEoI, TMo.HESY and HE modes)by adjusting the polarization components,but the complication in optical alignment makes it difficult to build compact,low-loss systems with these state of the LPor mode launched into the mode converter. devices.Integrated-optics mode converters based on various II.GRATING FABRICATION mode coupling mechanisms have been proposed to ease the The TMF used in our study was a commercial fiber (Two-Mode Step-Index Fiber,OFS).To determine the pitch Manuscript received December 1,2014;revised January 21,2015:accepted February 16,2015.Date of publication February 24,2015;date of current of the LPFG mode converter,we first wrote a fiber Bragg version April 10,2015.This work was supported by the Research Grants grating (FBG)in the TMF with an excimer laser,which Council through the Hong Kong Special Administrative Region,China, under Project CityU 112113. emitted at the UV wavelength 248 nm.The FBG had a length The authors are with the Department of Electronic Engineering.City of ~20 mm and a pitch of 536 nm.Prior to writing the FBG. University of Hong Kong,Hong Kong (e-mail:jldong2-c@my.cityu.edu.hk; we loaded the TMF with hydrogen at 100 bars and 70 C eeksc@cityu.edu.hk) for 7 days. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. The transmission spectrum of the FBG with both the Digital Object Identifier 10.1109/LPT.2015.2405092 LPor and LP11 modes launched into the fiber is shown 1041-11352015 IEEE.Personal use is permitted,but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.1006 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 9, MAY 1, 2015 Temperature-Insensitive Mode Converters With CO2-Laser Written Long-Period Fiber Gratings Jiangli Dong and Kin Seng Chiang, Member, IEEE Abstract— We fabricate mode converters by directly writing long-period gratings in a two-mode fiber with a CO2 laser. These mode converters allow the fundamental LP01 mode to be converted to any of the four higher order cylindrical vector modes or the LP11 modes. The transmission characteristics of these devices are insensitive to temperature variations and the mode conversion efficiency is insensitive to the polarization state of the input light. One of our typical gratings, which contains 15 grating periods, can provide a conversion efficiency >99% over a bandwidth of 34.0 nm in the C-band. These robust mode converters could find applications in mode-division-multiplexing systems and other applications that require cylindrical vector modes. Index Terms— Long-period fiber grating, mode converter, mode-division multiplexing, cylindrical vector modes. I. INTRODUCTION THE transmission capacity of single-mode fibers (SMFs) is rapidly approaching the limit and new technologies are being developed to sustain the network traffic growth. Mode￾division multiplexing (MDM) is a promising technology for increasing the transmission capacity of optical fibers [1], [2]. In MDM transmission systems, mode converters are important devices, which can be used, for example, at the sending and receiving ends to couple the fundamental mode to selected high-order modes and vice versa. Mode converters can also be used as elements to generate cylindrical vector beams [3], [4] for applications in electron acceleration [5], high-resolution metrology [6] and material processing [7]. In this letter, we demonstrate robust all-fiber mode converters for the conversion between the fundamental mode and the higher-order modes with potential applications in the aforementioned areas. Mode conversion has been realized for long-haul MDM transmission with bulk-optic components, such as phase plates [1] and spatial light modulators [2]. Such mode converters can be implemented with readily available discrete components, but the complication in optical alignment makes it difficult to build compact, low-loss systems with these devices. Integrated-optics mode converters based on various mode coupling mechanisms have been proposed to ease the Manuscript received December 1, 2014; revised January 21, 2015; accepted February 16, 2015. Date of publication February 24, 2015; date of current version April 10, 2015. This work was supported by the Research Grants Council through the Hong Kong Special Administrative Region, China, under Project CityU 112113. The authors are with the Department of Electronic Engineering, City University of Hong Kong, Hong Kong (e-mail: jldong2-c@my.cityu.edu.hk; eeksc@cityu.edu.hk). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2015.2405092 alignment problem [8]–[10], but the fabrication processes required are complicated and expensive. All-fiber mode converters compatible with optical fiber links have been demonstrated with the effect of multimode interference in a multimode fiber (simulation only) [11], the principle of adiabatic transition in a photonic-crystal fiber [4], [12], and long-period fiber gratings (LPFGs) [3], [13]–[17]. Among these all-fiber devices, LPFGs offer many distinct advantages, such as low reflection, low loss, and high fabrication and design flexibility. An LPFG mode converter can be formed in a fiber by mechanical means [3], [13], [14] or by exposing the fiber to an ultra-violet (UV) laser [15], a heat source [16], or a CO2 laser [17]. The early CO2-laser writing method for the fabrication of mode converters [17] involves two steps: the surface of the fiber is first ablated periodically by a CO2 laser beam and the ablated fiber is next annealed with an electric arc to form a grating. The resultant grating has a very large index change (∼0.1) and suffers from a relatively high insertion loss. In the present study, we employ a direct CO2-laser writing method to fabricate LPFG mode converters in a two-mode fiber (TMF), where the CO2 laser can be computer￾programmed to generate the grating profile. This CO2-laser writing method has been applied to the fabrication of sophisti￾cated LPFG filters in different types of SMFs (see [18], [19]). By properly controlling the amount of radiation on the fiber, the laser beam can induce an index change in the fiber without causing any visible damage to the fiber. Our CO2-laser written LPFG mode converters are insensitive to temperature variations and the mode conversion efficiency is insensitive to the polarization state of the input light. One of our typical LPFGs, which contains 15 grating periods, shows a conversion efficiency higher than 99% over a bandwidth of 34.0 nm from 1529.1 to 1563.1 nm. We can obtain the four orthogonal LP11 modes or the four cylindrical vector modes (the TE01, TM01, HEeven 21 , and HEodd 21 modes) by adjusting the polarization state of the LP01 mode launched into the mode converter. II. GRATING FABRICATION The TMF used in our study was a commercial fiber (Two-Mode Step-Index Fiber, OFS). To determine the pitch of the LPFG mode converter, we first wrote a fiber Bragg grating (FBG) in the TMF with an excimer laser, which emitted at the UV wavelength 248 nm. The FBG had a length of ∼20 mm and a pitch of 536 nm. Prior to writing the FBG, we loaded the TMF with hydrogen at 100 bars and 70 °C for 7 days. The transmission spectrum of the FBG with both the LP01 and LP11 modes launched into the fiber is shown 1041-1135 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information