Spectral dynamics of noise-like square pulses generated in a Tm-doped figure-8 fiber laser.

• Noise-like square pulse obtains a square shape in the spectral domain. • The pulse demonstrates a spectral resonance with a 3-dB variation: 10–20 nm. • Simulation approximately reproduced the flat-topped spectra and spectral resonance. • Analysis shows that the flat gain dispersion accounts for the flat spectral peaks. We report the generation of a series of noise-like square pulses (NLSPs) at 2 μm in a figure-8 Tm-doped fiber laser. The NLSP demonstrates a square shape as well in the spectral domain and possesses a resonant property in spectrum similar to that in time. The spectral resonance ranges from 10 nm to about 20 nm under an appropriate control of polarization, an indication of its wide mode-locking regime. The unique spectral dynamics exhibits a strong polarization dependence, subject to the nonlinear mode-locking mechanism. A subsequent numerical simulation shows that a uniform gain dispersion distribution accounts for the forming of the flat-topped spectra. [ABSTRACT FROM AUTHOR]