A microwave photonic filter with passband and stopband interchanged

A microwave photonic filter with passband and stopband interchanged is proposed and demonstrated. The structure is based on a recirculating delay line loop comprising a semiconductor optical amplifier (SOA) and a tunable narrowband optical filter, converted signal used as negative tap is generated employing the cross-gain modulation of the amplified spontaneous emission spectrum of the SOA. The filter structure can be operated as a high quality factor or notch filter with flat passband depends on the central wavelength of the optical filter aligning with or detuning from the pump wavelength.