Dynamic EDFA gain-flattening filter using two LPFGs with divided coil heaters

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 individually 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 controllability/flexibility for the filter spectrum of dynamic LPFG. Wide dynamic-range gain/power control with less than 0.8-dB signal ripples, over 30 nm was demonstrated.

I. Introduction

Gain-flattened erbium-doped fiber amplifiers (EDFAs) is one of the key vehicles enabling the increased transmission capacity/distance of dense wavelength-divisionmultiplexing telecommunication systems [1]. For the gain-flattening issue of EDFA, the long-period fiber grating (LPFG) has been considered as one of the most attractive options. Fig. 1. Dynamic gain-flattening filter with divided coil heaters. Various approaches have been reported for this issue, including, concatenated 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 realization 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.