The emission bandwidth from a laser is typically limited by the gain bandwidth of its active medium. Spectral broadening due to optical nonlinearities in the laser cavity can overcome this limitation. This is true of mode-locked pulsed lasers, particularly when they are operated in the noise-like pulse (NLP) regime. NLPs can be modeled as a packet of many fs-scale pulses, with randomly varying amplitudes, durations, and delays. NLPs are distinguished by having a temporally long (ps to ns-scale) pulse envelope with fast noisy substructure (fs-scale), very smooth spectra (averaged over many pulses), and low intra- and inter-pulse coherence. Publishing in JOSA B, a team lead by researchers at the Russian Academy of Science’s Institute of Automation and Electrometry in Novosibirsk has numerically demonstrated that the bandwidth of NLPs from a passively mode-locked fiber laser can exceed the active material gain bandwidth by over a factor of 12. This exceeds the authors’ recent experimental demonstration, where they obtained NLPs with a bandwidth 6-7 times the gain bandwidth of Er doped silica fiber. Their physical model is based on a ring geometry with four sections: a fiber amplifier, a length of anomalous dispersion fiber, a length of zero dispersion fiber, and a fiber with nonlinear loss to provide mode-locking, modeled as a having a loss response like one finds in nonlinear polarization rotation. They found that the broadening most strongly depended on the length of zero dispersion fiber where the pulses undergo significant phase modulation due to the Kerr nonlinearity. Such a spectrally broad, low coherence laser is well-suited for imaging and sensing based on low coherence interferometry (for example, optical coherence tomography), or for pumping ultrabroad, spectrally smooth supercontinuum sources.
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