Cascaded Mode Coupling Based On Acousto-optic Interacting With Fiber Grating

All-fiber optical filters have great applications in optical communications and optic sensing systems for their low insertion loss, low cost and compatibility with fibers. With the development of fiber optical systems, capabilities of realizing complex filtering spectra and dynamically tuning are required for all-fiber optical filters, which become a hot topic in fiber optics in recent years.Fiber gratings and acousto-optic tunable filters are two kind of fundamental all-fiber optic filters, whose mechanisms are all based on the mode couplings induced by the refractive index modulation along the fibers. The fiber gratings can realize complex filtering spectra with proper index modulation designs, however, they are difficult in realizing dynamically tuning. On the other hand, the acousto-optical tunable filters have excellent tuning ability, however, they can only realize simple band-block function. In this dissertation, a series of all-fiber tunable filters are proposed and researched based on the cascading coupling processes of fiber gratings and acousto-optic tunable filters, which can realize complex filtering spectra and dynamically tuning simultaneously.Firstly, by parallel connection of two fiber acousto-optic couplings process with different bandwidths, a tunable band-pass fiber acousto-optical filter is proposed and demonstrated theoretically and experimentally.Secondly, by cascading acousto-optical coupling processes with coupling processes of long period fiber gratings (LPFGs) in series, a novel tuning mechanism of the LPFG spectra are proposed and demonstrated theoretically and experimentally, showing that it can realizing dynamically tuning of LPFGs.Thirdly, the filtering characteristics of two cascading acousto-optical coupling processes are investigated theoretically and experimentally, showing that they can realizing spectra of phase-shift LPFGs with any phase-shift distribution.Finally, an analytical method based on no-resonant cascade mode couplings is proposed to analyze the tuning mechanism of acousto-optic modulated fiber Bragg grating (FBG). A novel mechanism of refractive index modulation by applying transverse acoustic wave in polarization-maintaining fiber is proposed. Its feasibility is demonstrated experimentally by its tuning effect of FBGs in polarization maintain fibers.