Effect of noise on the power spectrum of passively modelocked lasers

Abstract

Modelocking is a common technique employed to produce ultrashort optical pulses from lasers over a wide range of wavelengths. Actively modelocked lasers, for example, are driven by a stable external frequency reference. In this case the timing fluctuations are related to the modulator signal and can be treated as stationary processes. The theory revealed spectral characteristics that are commonly referred to as a spike and a pair of pedestals, with the spike resulting from a delta-function-like shape at each harmonic owing to the external locking source. One pedestal, or side band, at each harmonic is due to amplitude fluctuations and the second is due to timing-jitter fluctuations. The latter is harmonic-number dependent. For more than a decade, this theory was applied to the fundamentally different case of passively modelocked lasers, but the results were not adequately explained.