Ultrathin amorphous Ge film enabling stabilized femtosecond fiber laser pulsation

Abstract

Ultrathin amorphous Ge (α-Ge) film is synthesized as nonlinear saturable absorber to enable the femtosecond pulsation of erbium-doped fiber laser, which self-starts the fiber laser pulsation with nearly 700-fs pulsewidth at threshold pumping condition. Under high-gain condition, the α-Ge film with thickness of 17 nm exhibits the largest self-amplitude modulation coefficient of 8.9 × 10−4 to further compress the fiber laser pulsewidth to 305 fs. To compare, other Si-doped Ge films are also synthesized by linearly decreasing the Ge/(Ge + Si) composition ratio during plasma enhanced chemical vapor deposition, which reveal ultrahigh Ge/Si composition ratios with bandgap energy tunable from 1.05 to 0.87 eV. All recipes perform relatively comparable mode-locking force for pulsating the fiber laser with corresponding self-amplitude modulation coefficient degraded only by 6 × 10−6 as compared to the pure α-Ge film. Decreasing the Ge/(Ge + Si) composition ratio by 5% only broadens the fiber laser pulsewidth by 8.2% to 330 fs and slightly increases the peak amplitude fluctuation of the pulse-train to 1.81%. These results corroborate the pulsating stability of fiber laser under high pumping condition via the ultrathin α-Ge even with residual Si dopants, indicating that the pure α-Ge can serve as the optimized group-IV semiconductor nonlinear saturable absorption to provide sufficiently large nonlinear saturable absorbance as well as self-amplitude modulation, particularly suitable for stabilization the pulsed fiber laser no matter in the initial mode-locking or in the soliton compression regime.