Acousto-optic Tunable Filters in Two-Mode Fibers

We demonstrate an acousto-optic tunable filter based on a two-mode elliptical-core fiber. The filter is operated at 1090 nm as a broadband notch filter used for spectral flattening and as a bandpass filter with subnanometer bandwidth. When used for spectral flattening the FWHM bandwidth is optimized so that only one acoustic frequency is needed. In this way the spectral variation of a fiber laser source is reduced from 4 dB to less than 0.3 dB within a bandwidth of 6 nm. In the bandpass configuration we obtain a minimum bandwidth of 0.85 nm. Because of a slight polarization dependence of the beat length the bandwidth is 0.91 nm for unpolarized sources. Full acousto-optic mode conversion is obtained at an 8-mW electrical input power to the acoustic transducer. In the bandpass configuration the tuning width is limited by the bandwidth of the required static mode converter. By replacing a uniform mode converter with a linearly chirped one we can increase the tuning range from approximately 10 to 50 nm.