Optimization of the interference spectrum of separated long-period fiber grating pair and its application for multi-channel isolation filter

The interference transmission spectrum of the cascaded long-period fiber gratings, between which inserted a section of fiber, is investigated. Analytic expressions for the interference spectrum are derived, from which insightful physical meanings can be seen, such as the fringe spacing dependences, the maximum peak loss dependences. Based on the design criteria, we used the Levenberg- Marquardt (LM) method to design a flattened spectrum with transmission loss of nearly 50%. A useful device of tunable multi-channel isolation filter was designed by cascading two such identical long period fiber gratings. This device has great potential to be used for wavelength-division-multiplexed optical communication systems.