All-optical tunable microwave interference suppression filter based on SOA

An all-optical filter structure for interference suppression of microwave signals is presented. The filter is based on a recirculating delay line (RDL) loop consisting of a semiconductor optical amplifier (SOA) followed by a tunable narrowband optical filter, and a fiber Bragg grating connected after the RDL loop. Negative tap is generated in wavelength conversion process based on cross-gain modulation of amplified spontaneous emission spectrum of the SOA. A narrow passband filter with negative coefficients and a broadband all-pass filter are synthesized to achieve a narrow notch filter with flat passband which can excise interference with minimal impact on the wanted signal over a wide microwave range. Experimental results show that measured and theoretical frequency responses agree well and the filter is tunable.