Polarization-Dependent Loss and Birefringence in Long-Period Fiber Gratings

Widely used descriptions and relationships for birefringence and polarization-dependent loss (PDL), developed primarily for ultraviolet-induced long-period fiber gratings (LPFGs) written in optical fiber, can be invalid for other types of LPFG. The understanding of PDL is expanded to include LPFGs with birefringence in the core only, in the cladding only, and in both the core and the cladding. Equations that link resonant wavelength separation, one factor that determines PDL, and birefringence for the three categories are presented, along with relevant approximations. Measurement results for two LPFGs fabricated by different techniques are presented that illustrate the effect of birefringence on PDL.