Novel acousto-optic tunable comb filter based on cascaded etched fiber structure

A novel all fiber acousto-optic tunable comb filter based on the interference between core mode and cladding mode LP₁₁ is proposed and demonstrated. The device has cascaded 3-segment structure formed by HF etching from one piece of commercial single mode fiber. The tunable ability is realized by acoustic frequency adjustment. This paper details the structure and the working principle as well as the dependence of working bandwidth on outer diameters of the first and third segments, the dependence of peak interval on length of the second segment. The device extinction ratio may be improved by optimizing the length ratio between the first and the third segments for suppressing the acoustic attenuation and by using gradual transition between the segment-diameters to reduce the acoustic reflection.