A cascaded filter combining active and passive filters is proposed. The active filter acts as a low-coherence infinite- impulse-response (IIR) filter and achieves a sharp frequency response. The low-coherence IIR filter is realized by employing the cross-gain modulation (XGM) of the amplified spontaneous emission (ASE) spectrum of the semiconductor optical amplifier (SOA). The passive filter is an n-section unbalance Mach-Zehnder (UMZ) structure, which is used to increase free spectral range (FSR) and Q factor further. The low-coherence IIR filter cascaded with one section of UMZ passive filter is experimentally demonstrated, and a Q factor of 1268 is obtained.