Birefringence reduction in long period gratings by the fiber-twist method

Summary form only given.Broadband gain-flattened optical amplifiers are essential for increasing the capacity of wavelength division multiplexed transmission systems. Among the several approaches to flatten the gain of an amplifier, gain flattening filters (GFFs) based on long period gratings (LPGs) are strong candidates because of the ease of fabrication, the flexibility in spectral-shaping, and very low back reflection. There are, however, two major problems that had to be overcome to use the LPGs as a GFF: temperature sensitivity and polarization dependency. UV-induced birefringence in hydrogen loaded fibers makes fiber gratings sensitive to the polarization of the incident light. In this paper, we characterize the birefringence in LPGs by the measurements of polarization dependent loss and demonstrate a fiber-twist method for reducing the birefringence LPGs.