Mode-Coupling and Propagation in a Deeply-Tapered Few-Period Long-Period Fiber Grating

In this study, the underlying mechanism for mode coupling and propagation in a strongly-coupled few-period long-period fiber grating (LPG) is firstly revealed both theoretically and experimentally, which is based on the idea that an ultra-short strongly-coupled LPG could be equivalently regarded as cascaded and phase-matched Mach-Zehnder interferometers (MZIs) but with phase-matched two arms. The proposed mechanism and the corresponding analysis method can also be applied to those conventional LPGs (with a distributed weak-perturbation through the whole length of the grating itself), which opens a new way for design and thereafter fabrication of any other kinds of the LPGs such as the LPG-based mode-converters, the multichannel-, the phase-shift- and the high-order LPGs, etc.