Two-mode fiber acoustooptic tunable bandpass filter with zero frequency-shift

We propose and experimentally demonstrate a novel configuration for an acoustooptic tunable bandpass filter based on intermodal coupling in a two-mode fiber. The double-pass scheme provides zero frequency-shift, bandwidth narrowing, and enhanced extinction ratio. A 3-dB bandwidth of 2.0 nm, insertion loss of 5.6 dB, and sidelobe suppression ratio of 14 dB were achieved. The wavelength tuning range was greater than 90 nm for the transmission dynamic range larger than 20 dB. Multiwavelength operation of a single device is achieved by simply applying multifrequency electrical signal. Unwanted intensity modulation of the multiwavelength filter is also investigated.