Microresonator combs for coherent optical communications

Licentiate thesis, 2016

This thesis covers the study of microresonator-based optical frequency combs for use in coherent optical communication links. Current links typically utilize several free-running lasers to achieve wavelength-division multiplexing (WDM). Having an integrated frequency comb as a source instead could potentially simplify the transmitter while at the same time open up new possibilities on the digital signal processing (DSP) side in the receiver. The thesis goes into some detail to describe what requirements there are on the combs to be useful for this application. Specifically the power per line and related flatness is analyzed. Additionally a large part of the thesis is dedicated to describe the working principle of microresonators. The linear low-power regime is described in detail while a simple Lugiato-Lefever model is analyzed for the nonlinear high-power regime. The appended papers describe two different microresonators with different initialization mechanisms; one where the dispersion is locally tuned using higher order modal interactions while the second one is seeded with three coherent pump waves. The first paper demonstrates that such combs fulfill the criteria for being light sources in long-haul communication systems. This is demonstrated by using it to send data over a more than 6000 km long transmission link.