Suppression of Phase Induced Intensity Noise in Recirculating Delay Line Microwave Photonic Filters

High-Q microwave photonic filters may be realised with infinite impulse response (IIR) transfer functions, which are implemented typically with all-optical recirculating delay lines (RDL) operating in the incoherent regime. A key problem with these structures is phase-induced intensity noise (PIIN), which results in SNR degradation. Here we show that by using a hybrid optoelectronic loop instead of an all-optical loop for the RDL, it is possible to reduce the PIIN by up to 28 dB and to also achieve a higher finesse by letting the loop gain approach unity. The significance of the PIIN reduction is shown with the comparison of optoelectronic oscillators implemented with the all-optical RDL and the hybrid optoelectronic RDL, with the latter leading to a reduction in phase noise of 14 dB at 10 kHz frequency offset.