Spectral broadening of incoherent nanosecond pulses in a fiber amplifier

Amplification of incoherent light pulses in a relatively short active fiber is treated. Spectral broadening due to the self-phase modulation effect at negligibly small dispersion has been studied theoretically. The expression for the shape of the output spectrum has been obtained for input pulses of arbitrary temporal and spectral shape at various gain coefficients. The expression is found to be simplified in case of a hyperbolic secant temporal shape. The calculated shapes have been compared with the experimentally measured spectra for Q-switched fiber laser nanosecond pulses amplified in Yb-doped fiber, demonstrating excellent agreement of theory and experiment. The spectrum of the output pulse is shown to be composed of two different-scale structures: a narrow central part copying the initial shape and broad exponential tails that grow with increasing output power.

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