Spectral characteristics of single-mode-fiber-based cascaded acousto-optic filters

In this paper, two cascaded single-mode-fiber-based acousto-optic fiber grating schemes have been proposed and a comparative study on their spectral characteristics has been performed. By serially employing two acoustic transducers with the same geometrical and physical settings, a cascaded configuration of acousto-optic fiber gratings has been established. Experimental results indicate that the mode coupling efficiency has been improved due to the presence of the acoustic transducers in a serial configuration. Moreover, another cascaded acousto-optic tunable filter scheme is implemented based on a reflection configuration, in which an acousto-optic tunable filter is employed with a thin silver film coated at the fiber end as an efficient reflector for both of the core and cladding modes. And, moreover, spectral fringes resulting from the intermodal interference between the fundamental core mode and cladding modes have been experimentally observed, which is basically in accordance with our theoretical simulation result. Further theoretical investigation of its transmission spectral response to the acousto-optical coupling coefficient reveals the possibility to control the interference spectrum through adjusting the acousto-optic coupling strength of a single acoustic fiber grating, which is an important feature for the acoustic-grating-based fiber interferometers to be used in practical applications.