Fiber modal index measurements based on fiber gratings

A novel method based on fiber gratings for measuring the effective indices of fiber modes is proposed. The effective index difference between the core mode and a cladding mode was obtained by analyzing the interference fringe of a pair of cascaded long-period fiber gratings. In order to extract the core mode index from the measured index difference, an index matching oil i mmersion method is proposed. By analyzing the interaction between the cladd ing mode and the oil applied on the cladding surface, the mode order and th e effective index of the involved cladding mode might be calculated. Experi mental results about the interference fringe shifts induced by the oil inde x and the oil-applied length are also presented.