Yang C.-CCheng C.-HChen T.-HLin Y.-HTseng W.-HChang P.-HWu C.-ICHIH-I WUGONG-RU LIN2021-09-022021-09-02202100303992https://www.scopus.com/inward/record.uri?eid=2-s2.0-85097341475&doi=10.1016%2fj.optlastec.2020.106761&partnerID=40&md5=8d2086457f487752387aff39bf15e34ehttps://scholars.lib.ntu.edu.tw/handle/123456789/580718Ultrathin 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. ? 2020 Elsevier LtdAmplitude modulation; Femtosecond lasers; Fiber lasers; Fibers; Optical pumping; Plasma CVD; Plasma enhanced chemical vapor deposition; Pulsed lasers; Saturable absorbers; Semiconducting germanium; Semiconductor lasers; Semiconductor quantum wells; Stabilization; Erbium doped fiber laser; Femtosecond fiber lasers; Group-IV semiconductors; Laser pulse-width; Modulation coefficients; Pulsed fiber lasers; Saturable absorption; Soliton compression; Mode-locked fiber lasersjournal article10.1016/j.optlastec.2020.1067612-s2.0-85097341475