404 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 6, MARCH 15, 2008 All-Fiber Wavelength-Tunable and Mode Convertible Bandpass Filter for Optical Interconnections W. Shin, K. Oh, B.-A. Yu, Y. L. Lee, Y.-C. Noh, D.-K. Ko, and J. Lee Abstract—We report an all-fiber wavelength-tunable bandpass filter with multimode to single-mode converting property via seri- ally concatenated structure of a helicoidal long-period fiber grating (HLPFG), a ring-core hollow optical fiber (HOF), and a multimode fiber (MMF). After passing the cascaded HLPFG-ring core HOF- MMF, the bandpass filtering characteristic and the mode conver- sion between multimode and single-mode were realized at the reso- nant wavelength of the HLPFG. Due to the novel spectral function- ality of the HLPFG for directional torsion to the helix, the proposed device also showed wavelength-tunable bandpass filtering opera- tion with low polarization-dependent loss. Index Terms—Optical communication, optical fiber applica- tions, optical fiber device, optical fiber filters. I. INTRODUCTION A S high-speed optical access network technologies are being rapidly developed, the interests and demands in the area of interconnection among local fiber links are growing. All-optical and all-fiber interconnection devices can avoid expensive optic–electronic–optic conversion and furthermore secure high transparency over the data rate and transmission capacity. The interconnection between dense wavelength-division-multiplexing (WDM) system and local area network requires conversion of both the wavelength of a signal and the medium through which the signal propagates. Also, in multimode fiber (MMF)-based local area network systems such as a gigabit ethernet system, the differential modal delay (DMD) has been one of the detrimental effects that limit the transmission capacity [1]. All-optical interconnecting devices for these systems, therefore, require simultaneously wavelength- and mode-selective characteristics that can selec- tively excite higher order modes with a similar group velocity in the MMF, reducing the DMD penalty. The improvement of the DMD effect in MMF links has been achieved by using the mode conversion of hollow optical fiber (HOF) [2]. Optical filters, particularly all-fiber optical filters, are extensively used as key components in fiber-optic communications for WDM and also in fiber-optic sensing. As a band-rejection filter, long-period fiber gratings (LPFGs) have been utilized to flatten the gain of an erbium-doped fiber amplifier. Also, all-fiber bandpass Manuscript received September 19, 2007; revised November 21, 2007. W. Shin, B.-A. Yu, Y. L. Lee, Y.-C. Noh, D.-K. Ko, and J. Lee are with the Advanced Photonic Research Institute, GIST, Gwangju 500-712, Korea (e-mail: dkko@gist.ac.kr; leejm@ gist.ac.kr). K. Oh is with the Department of Physics, Yonsei University, Seoul 120-749, Korea (e-mail: koh@yonsei.ac.kr). 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.2008.916921 filters (BPFs) have been developed in order to take advantage of all-optical structure and relatively low manufacturing cost. Recently, the authors reported an all-fiber BPF based on a null core HOF serially concatenated between a pair of helicoidal long-period fiber gratings (HLPFGs) with flexible transmission control capability [3]. The tunable all-fiber BPF was demon- strated by using an HOF core mode blocker fusion-spliced between a pair of LPFGs with the lowest insertion loss and widest tuning range [4], [5]. However, these devices need the additional electric circuitry for control functionality or require two LPFGs which have exactly the same resonance property. These raise the complexity of system and restrict the possibility of practical applications. It is, therefore, highly desirable to incorporate wavelength-tunable and mode converting capability in a simple fiber-based BPF to cope with demands for securing high transparency over the data rate among the different fiber links. In this study, a wavelength-tunable and mode convertible bandpass filter (WTMC-BPF) is demonstrated with the con- catenated structure of HLPFG–ring-core HOF (rcHOF)–MMF. The proposed device has advantages such as the tunable bandpass filtering capability with mode converting property for reducing the DMD penalty in MMF links, and the simple and robust design of all fiber structure. Moreover, the tunable wavelength selectivity and the mode conversion property were simultaneously realized in a single device with advantages of all-fiber structure such as good optical properties, stable thermal property, and high power endurability. These unique properties of this device are distinguished from the general fiber devices. Characteristics of the proposed WTMC-BPF are described in terms of its operating principle and properties. Also, its design and fabrication process are explained. II. DESIGN AND FABRICATION OF WTMC-BPF The schematic and operation principle of the proposed WTMC-BPF are illustrated in Fig. 1. The device consists of an rcHOF segment serially concatenated between an MMF with a core diameter of 62.5 m and an HLPFG fabricated on a standard single-mode fiber (SMF). The ring-core hollow fiber segment was simultaneously used as the core mode blocker and mode converter due to its broadband mode conversion from the core mode to the radiation mode and the higher order modes excitation with a similar group velocity in an MMF [2]. In Fig. 1, the HLPFG couples light from the fundamental core mode to the phase-matching cladding modes. The rcHOF segment serves as the core mode blocker rejecting the core mode that is not resonant with the HLPFG. The cladding modes from the HLPFG, in contrast to the core mode, further propagate along the cladding of an rcHOF segment. At the 1041-1135/$25.00 © 2008 IEEE