Amplification and Bandwidth Recovery of Chirped Super-Gaussian Pulses by Use of Gain Shaping in Ytterbium-Doped Fiber Amplifiers

We demonstrate a simple passive method to generate broadband chirped super-Gaussian pulses from high-gain ytterbium-doped fiber amplification system without obvious bandwidth degradation. By use of the inherent gain shaping effect of fiber amplifiers, pulses can recover the initial Super-Gaussian profile and bandwidth with large gain and optical signal-to-noise ratio (OSNR). In this method, fiber amplifier itself works as a spectral filter with flexible transmission profiles and gain, which can be optimized by pump power and fiber length. A spectrally resolved numerical model with amplified spontaneous emission (ASE) is developed for broadband ytterbium-doped double-clad fiber pulse amplifiers. In a proof-of-principle two-stage amplification system, with a shorter fiber preamplifier working in the gain-narrowing region as a pre-shaper, broadband Super-Gaussian pulses resemble themselves after the second amplification stage with the largest total gain and OSNR. Experimental results quantitatively confirm the numerical predictions. The proposed gain shaping method also provides a new way to compensate the pulse deformation and bandwidth narrowing in high-gain fiber chirped pulse amplification systems.