"High repetition rate frequency combs: ultrafast optics starting with continuous-wave lasers."

"The introduction of mode-locked laser optical frequency combs during the last decade led to revolutionary progress in precision optical frequency synthesis and metrology. Such combs typically operate at repetition rates of hundreds of MHz. Combs at higher repetition rates are of interest for other applications, such as arbitrary waveform generation and radio-frequency signal processing, which do not require full stabilization of the offset frequency. In this talk I discuss two flavors of high repetition rate comb generation starting with a single continuous-wave laser. Continuous-wave lasers subject to strong periodic electro-optic modulation can be formed into optical frequency combs and compressed into trains of picosecond pulses at rates on the order of 10 GHz. Nonlinear optical propagation can extend comb bandwidths into the THz regime, allowing realization of pulse durations at the few hundred femtosecond level, and producing combs with extremely flat and smooth optical power spectra. We have exploited such combs for applications such as line-by-line optical pulse shaping, also known as optical arbitrary waveform generation, and programmable radio-frequency filtering. Generation of combs at much higher repetition rates can be achieved via nonlinear wave mixing in high quality factor (high Q) microresonators. Our group has investigated line-by-line pulse shaping of such combs generated in silicon nitride ring resonators. Our experiments reveal formation of two different types of combs which exhibit strikingly different time domain behaviors. One type can be compressed to nearly bandwidth-limited pulses, which indicates high coherence across the spectrum. A second type exhibits limited compressibility and degraded coherence. Accordingly, understanding and control of the coherence constitutes a topic of significant interest in the further development of microresonator comb generators."