Harmonic Order-Dependent Pulsewidth Shortening of a Passively Mode-Locked Fiber Laser With a Carbon Nanotube Saturable Absorber

A pumping-power-dependent high-order harmonic mode-locking erbium-doped fiber laser (HML-EDFL) with a single-wall carbon nanotube (SWCNT)-doped polyvinyl alcohol (PVA)-based saturable absorber is investigated. The HML-EDFL systems using either all-physical-contact (PC) or all-angled-physical-contact (APC) connectors examined in this study show stably mode-locked pulse trains; however, the all-PC connector-based system requires a larger pumping power to initiate the mode-locking and generate a broader pulsewidth of 2.9–5.2 ps at same HML order. The all-APC connector-based HML-EDFL system can provide pulsewidth shortening from 3.8 to 1.9 ps associated with a spectral linewidth broadened from 0.7 to 1.36 nm by increasing the HML order from first to sixth. Either the all-PC or the all-APC connector-based EDFLs can deliver a nearly transform-limited pulse with a time–bandwidth product (TBP) of 0.32. The EDFL passively mode-locked by SWCNT-doped PVA delivers a pulse train with a signal-to-noise ratio (SNR) remaining higher than 40 dB for different HML orders, whereas the side-mode suppression ratio (SMSR) lower than 32 dB inevitably introduces an unequalized peak amplitude phenomenon for adjacent pulses at higher HML operations. A modified Haus' master equation is derived to explain the inverse proportionality of the passive HML-EDFL pulsewidth to the square root of HML order, which is correlated with the shortened carrier recombination time of the SWCNT saturable absorber at higher pumping power or circulating pulse intensity.