Recently, on-chip comb generation based on high quality factor microresonators has been intensively studied due to its simplicity, small size and low cost fabrication. Frequency combs with free spectral ranges (FSRs) of a few tens of GHz are of specific interest for communication applications. However for silicon nitride resonators in this FSR regime a huge pump power is required which far exceeds the maximum available power from current on-chip laser sources. Meanwhile, most research employs microresonators with anomalous dispersion, for which modulation instability is believed to play a key role in initiation of the comb. Comb generation in normal dispersion microresonators has also been reported but is less well understood.;We demonstrate Silicon nitride microresonators with intrinsic Qs up to 17 million at an FSR of 24.7 GHz. These Q values are the highest recorded for Silicon nitride resonators used for comb generation. The frequency comb onset power can be as low as 2.8 mW, within reach of on-chip semiconductor lasers. Furthermore, we report a detailed investigation of few-moded, normal dispersion silicon nitride microresonators, showing that mode coupling can strongly modify the local dispersion, even changing its sign. We demonstrate a link between mode coupling and initiation of comb generation by showing experimentally pinning of one of the initial comb sidebands near a mode crossing frequency. Associated with this route to comb formation, we observe direct generation of coherent, bandwidth-limited pulses at repetition rates down to 75 GHz, without the need to rst pass through a chaotic state. Finally we investigate the four-wave mixing (FWM) process in silicon nano-waveguides by demonstrating an on-chip scheme to scale the bandwidth of the electrooptic (EO) frequency comb lines. With a input of 55 lines from EO frequency comb, it can generate a at-topped frequency comb with over 100 lines in a 5-dB bandwidth.
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