We propose and experimentally demonstrate a novel all-optical microwave filter with high frequency selectivity. It is based on a recirculating delay line (RDL) loop in which a semiconductor optical amplifier (SOA) is followed by a tunable narrowband optical filter and a 1$\,\times\,$2 10:90 optical coupler. Converted signal used as a negative tap is generated through wavelength conversion employing the cross-gain modulation (XGM) of the amplified spontaneous emission (ASE) spectrum of the SOA. The converted signal circulating in the RDL loop can realize a high quality factor (Q) response after photo-detection. The 1$\,\times\,$2 10:90 coupler is employed to extract 10% optical power from the loop as output. A frequency response with a high Q factor of 543, a rejection ratio of 40 dB and a shape factor of 15.4 is experimentally demonstrated.
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