1226 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 6, JUNE 2005 Dynamic EDFA Gain-Flattening Filter Using Two LPFGs With Divided Coil Heaters Jun Kye Bae, Jinho Bae, Sang Hyuck Kim, Namkyoo Park, and Sang Bae Lee Abstract—We introduce a new type of dynamic erbium-doped fiber amplifier (EDFA) gain-flattening filter using two long-period fiber gratings (LPFGs) equipped with divided coil heaters. By in- dividually controlling each of the 64 segments of coil heaters over the grating, we adjusted the temperature distribution of LPFGs piecewise-uniformly, thus, to maintain optimum gain-flatness for different EDFA operation conditions. By controlling the different cladding modes for each LPFGs, we also achieve enhanced control- lability/flexibility for the filter spectrum of dynamic LPFG. Wide dynamic-range gain/power control with less than 0.8-dB signal rip- ples, over 30 nm was demonstrated. Index Terms—Band-rejection filter, divided heater, long-period fiber gratings (LPFGs), tunable. I. INTRODUCTION G AIN-FLATTENED erbium-doped fiber amplifiers (EDFAs) is one of the key vehicles enabling the increased transmission capacity/distance of dense wavelength-division- multiplexing telecommunication systems [1]. For the gain-flat- tening issue of EDFA, the long-period fiber grating (LPFG) has been considered as one of the most attractive options. Various approaches have been reported for this issue, including, con- catenated LPFGs [2], phase-shifted LPFG [3], and step-changed LPFGs [4]. Still, all these approaches based on passive LPFG filters for gain-flattening have been limited to a predetermined operating condition, exhibiting limitations such as undesirable gain tilt under channel add–drop and/or input signal/pump power changes [1]. Different approaches have been taken for the real- ization of a dynamic filter for dynamic gain-flattening of EDFAs: Metal-coated LPFG [5], tunable microfluidic devices [6], and acoustic optic modulators [1]. But these approaches have their own inherent limitations with respect to long-term stability, flexibility in the spectral shape, or polarization dependencies. In this letter, we describe the successful development of a flexible dynamic EDFA gain-flattening filter using two LPFGs and a divided coil heater. The divided heaters control the tem- perature distribution along the LPFG and tune the refractive Manuscript received February 14, 2005. J. K. Bae is with the Photonics Research Center, Korea Institute of Science and Technology, Seoul 130-791, South Korea, and also with Optical Commu- nication Systems Laboratory, School of Electrical Engineering and Computer Sciences, Seoul National University, Seoul 151-744, South Korea (e-mail: demian@kist.re.kr). J. Bae is with the College of Ocean Science, Cheju National University, Jeju 690-756, South Korea (e-mail: baejh@cheju.ac.kr). S. H. Kim and S. B. Lee are with the Photonics Research Center, Korea Institute of Science and Technology, Seoul 130-791, South Korea (e-mail: demian@kist.re.kr; Ksh625@kist.re.kr, sblee@kist.re.kr). N. Park is with Optical Communication Systems Laboratory, School of Elec- trical Engineering and Computer Sciences, Seoul National University, Seoul 151-744, South Korea (e-mail: nkpark@plaza.snu.ac.kr). Digital Object Identifier 10.1109/LPT.2005.847439 Fig. 1. Dynamic gain-flattening filter with divided coil heaters. index of the LPFGs as piecewise uniform LPFG array. Dif- ferent cladding modes for each LPFG were employed to en- hance the tuning flexibility and performance of the dynamic gain-flattening filters. Gain-flatness control within 0.8 dB over a broad wavelength span ( 30 nm) was demonstrated for a wide range of operation conditions. II. PRINCIPLES/DEVICE SETUP LPFG couples a guided fundamental mode in a single-mode fiber to multiples of forward propagation cladding modes. Since this mode-coupling is also wavelength-selective, the fiber grating could perform as a wavelength-dependent loss element. To get a better control for the frequency response, grating structures composed of several concatenated LPFGs are often used [2]. Concatenated LPFGs with two different cladding modes and independent coupling of core–cladding modes were pre- pared to generate the filter spectrum in the wavelength range 1500–1650 nm. Divided coil heaters were implemented to ad- just the temperature distribution along the pair of LPFGs and, thus, to enable the frequency-tuning of the device as tens of piecewise-uniform LPFG sections. With this implementation, the wavelength-selective mode coupling between core and cladding modes becomes the function of thermal change. Fig. 1 shows the detailed experimental setup. The coil heaters were constructed with individually controllable 64 coil heater sections. Each coil heater element had 12 coil turns with coils of 300- m inner diameter, and length of 2300 m spaced 200 m apart. The controller adjusted the electric power of each coil section individually to build an appropriate temperature distri- bution along the grating. To prevent heat accumulation and at the same time, thermal diffusion to the other sections, a fan cooler was placed above the 1041-1135/$20.00 © 2005 IEEE