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The Quantum-limited linewidth of a femtosecond frequency comb


For a continuous wave laser, the fundamental Schawlow-Townes limit on the linewidth is determined by spontaneous emission from the gain medium. For a mode-locked laser, the situation is complicated by the presence of strong optical nonlinearities, which couple amplitude noise into phase and frequency noise. With the development and wide application of femtosecond frequency combs based on mode-locked lasers, there is a need for better understanding of their noise properties. Unfortunately, the parameters required to quantitatively predict the quantum noise limit are difficult to predict from first principles. I describe some simple experiments on a Ti:sapphire laser which provided enough information to enable a quantitative calculation of the quantum-limited width of comb lines from that laser. I'll also discuss the implications for optical frequency metrology.