Abstract

The effect of the stress-induced birefringence on the polarization-dependent transmission characteristics was thoroughly investigated for a long period fiber grating (LPFG) that was fabricated using a stress relaxation method with a CO₂ laser. Long Period Fiber Gratings (LPFGs ) have been recently developed into a crucial optical component in many applications.^1-4 LPFGs are used in optical communication systems as low-loss in-fiber band-rejection filters¹ and gain flattening devices in Erbium-doped fiber amplifiers (EDFA).² Flexibility for achieving various spectral characteristics, simple and easy fabrication process, low polarization dependency, and low backward reflection are just some of the distinct features of LPFGs.^4,5 The conventional method for fabricating LPFGs is to expose a Ge-doped fiber with UV light by using photosensitivity in the Ge-doped core region of a fiber. It requires complicated hydrogen loading and thermal aging processes, however. A new method for producing LPFGs has recently been proposed. It involves releasing the residual stress of a fiber during fabrication process.⁵ A long period grating structure is formed by heating a fiber locally at the glass transition temperature using a focused CO₂ laser. LPFGs fabricated through this method show low dependence on external environments such as temperature and tensile strain variations for their optical transmission characteristics.⁵ Nonetheless, the origin of polarization-dependent loss (PDL) during the grating fabrication process is not well understood for LPFGs fabricated through the stress relaxation method using a CO₂ laser. Polarization-dependent optical transmission characteristics in LPFGs are a critical factor for making gain flattened EDFA, which is the major application of LPFGs. Understanding the physical origin of PDL in LPFGs is therefore critical in designing or improving the LPFG fabrication process.

© 2003 Optical Society of America