Analysis of second-order gratings in grating-assisted couplers

Summary form only given. Recently, acoustics-induced fiber vibration has been developed to control the reflectivity level of a fiber Bragg grating reflector. The acoustic wave could induce lateral vibration and form micro-bending which works as a long-period fiber grating. Surprisingly, the reflection spectrum of the reflector showed the effect of second-order gratings in the co-directional coupling. Since the properties of second-order gratings in grating-assisted directional couplers have not yet been discussed, we use the Floquet-Bloch theory to analyze the co-directional coupling caused by second-order gratings.