Gain dependence of the linewidth of Brillouin amplification in optical fibers.

We report a novel, pump-power dependent, linewidth broadening effect in stimulated Brillouin amplification of a continuous-wave probe by a pulsed pump. This behavior is different from the case of two interacting continuous-wave pump and probe fields, where the shape of the logarithmic Brillouin gain spectrum is independent of the pump power. Studying this effect numerically and experimentally and also analytically, we find that for a given width of the pump pulse the Brillouin linewidth grows linearly with the Brillouin logarithmic gain with a slope, which inversely depends on the pulse width. Thus, for example, in a standard single-mode fiber, a 15ns pump pulse, strong enough to generate a gain of 0.5dB, broadens the logarithmic lineshape by ~1.5MHz, while a 45ns pulse, providing the same gain, increases the linewidth by only ~0.5MHz. Since the rising and falling slopes of the shape of the Brillouin gain spectrum are also gain dependent, this effect might challenge the calibration of Brillouin distributed slope-assisted sensing techniques.