IEEE Photon. Tech. Lett. 23, 1618–1620 (2011)

Microwave photonic filters offer attractive features such as low loss, low sensitivity to electromagnetic interference, and rapid tunability and programmability over large bandwidths. Most of them are based on multiple physical delay lines — a concept that is difficult to scale to large numbers of taps. Recent work has used broadband light sources and a single dispersive element. Because the medium introduces a differential delay between optical frequencies, the multiple optical frequencies function as multiple taps, thus avoiding the use of many physical delay lines. Andrew Weiner and co-workers from the USA now demonstrate a reconfigurable and tunable flat-top microwave photonic filter using an electro-optic frequency comb and a dispersive medium, in which each individual optical frequency component in the comb becomes an independent filter tap. They implemented a flat-top filter by applying positive and negative weights across 32 comb lines, and tuned the filter central frequency by adding a phase ramp to the tap weights. The scheme provides flexible and tunable filter characteristics by programming the amplitude and phase of individual comb lines using an optical line-by-line pulse shaper. The researchers say that increasing the number of comb lines may provide lower pass-band ripple, narrower transition bands and stronger sidelobe suppression.