Ultra-Deep Multi-Notch Microwave Photonic Filter utilising On-Chip Brillouin processing and Microring Resonators

Microwave notch filters that exhibit wideband transmission and widely tunable notches are desirable, but difficult to implement using modern radio frequency (RF) electronics [1] , [2] . Microwave photonic (MWP) notch filters that utilise on-chip stimulated Brillouin scattering (SBS) [3] are particularly attractive, as they are uniquely capable of delivering notches with fine frequency selectivity. Recently, an RF notch filter with deep rejection was demonstrated by aligning an over-coupled ring resonator with Brillouin gain to generate spectrally localised RF destructive interference [4] . The optical response was tailored to provide unity gain and a π phase shift at a point on one sideband, while leaving the other sideband unchanged. The key advantage of this approach is that destructive interference of the RF output only occurs at specific frequencies, causing ultra-deep notch formation. Since the π phase shift is localised it allows intensity modulation (IM) to be used, increasing the RF passband gain as the upper and lower sidebands are in phase. Although deep RF notches were achieved, previous demonstrations that used single on-chip resonators were constrained to a single notch [4] - [6] , exhibited wide 3-dB notch bandwidths of several GHz [4] and required specific modulator bias [5] , [6] or heater voltages [7] to form destructive interference.