Photonic technologies have received considerable attention for enhancement of radio-frequency (RF) electrical systems, including high-frequency analog signal transmission, control of phased arrays, analog-to-digital conversion, and signal processing. Although the potential of radio-frequency photonics for implementation of tunable electrical filters over broad RF bandwidths has been much discussed, realization of programmable filters with highly selective filter lineshapes and rapid reconfigurability has faced significant challenges. A new approach for RF photonic filters based on frequency combs offers a potential route to simultaneous high stopband attenuation, fast tunability, and bandwidth reconfiguration. In one configuration tuning of the RF passband frequency is demonstrated with unprecedented (~40 ns) speed by controlling the optical delay between combs. In a second, fixed filter configuration, cascaded four-wave mixing simultaneously broadens and smoothes comb spectra, resulting in Gaussian RF filter lineshapes exhibiting extremely high (>60 dB) main lobe to sidelobe suppression ratio and (>70 dB) stopband attenuation.Optical frequency combs, generated via self-referenced and stabilized mode-locked lasers, have enabled revolutionary progress in precision optical frequency synthesis and metrology 1-4 . Optical combs are also of tremendous interest for other applications 5 , such as multi-wavelength coherent lightwave communications 6-8 , optical arbitrary waveform generation 9-11 , generation of low-phasenoise 12 or agile ultrabroadband microwaves 13 , and signal processing 6,14 . For these purposes, in which higher pulse repetition rates are desired and only moderate frequency stability is required, comb sources based on strong electro-optic modulation of a continuous-wave laser have seen substantial attention [15][16] . Here we report significant advances in RF photonic filters enabled by the ability to rapidly tune the timing of the comb and shape its power spectrum.

We demonstrate advanced transversal radio frequency (RF) and microwave functions based on a Kerr optical comb source generated by an integrated micro-ring resonator. We achieve extremely high performance for an optical true time delay aimed at tunable phased array antenna applications, as well as reconfigurable microwave photonic filters. Our results agree well with theory. We show that our true time delay would yield a phased array antenna with features that include high angular resolution and a wide range of beam steering angles, while the microwave photonic filters feature high Q factors, wideband tunability, and highly reconfigurable filtering shapes. These results show that our approach is a competitive solution to implementing reconfigurable, high performance and potentially low cost RF and microwave signal processing functions for applications including radar and communication systems.

OBJECTIVES-We present the first clinical imaging of human coronary arteries in vivo using a multimodality OCT and near-infrared autofluorescence (NIRAF) intravascular imaging system and catheter.BACKGROUND-While intravascular OCT is capable of providing microstructural images of coronary atherosclerotic lesions, it is limited in its capability to ascertain compositional/molecular features of plaque, including the definitive presence of a necrotic core. A recent study in cadaver coronary plaque has shown that endogenous NIRAF is elevated in necrotic core lesions. The combination of these two technologies in one device may therefore provide synergistic data to aid in the diagnosis of coronary pathology in vivo.