All-fiber all-optic tunable filter based on the combination of long period grating and photorefractive cladding layer

In this paper, a broadband tunable long period grating (LPG) with an ultra-thin cladding layer using liquid crystal is presented. By chemical etching the long period grating to a very thin diameter (approximately 30 μm), the long period grating becomes extremely sensitive to the surrounding refractive index change so that a wide range tunable filter is realized. By using liquid crystal material (18523) that has a n_o less than the refractive index of cladding, a resonant band of the LPG is tuned by controlling the temperature of the liquid crystal. We obtained a tuning range of 87 nm with a temperature change of only approximately 22 degrees. It is believed that this filter has the largest temperature-dependent tuning effect ever reported. Further experiment will be focused on photo-induced reorientation of the liquid crystal molecules, so that another type of tunable filter via photochemical effect can be realized.