Direct generation of cylindrical vector beams in a 2 µm mode-locked Ho-doped fiber NALM oscillator with an intra-cavity S-waveplate

We demonstrate a stable and compact 2 µm mode-locked Ho-doped fiber oscillator incorporating an intracavity S-waveplate for the direct generation of cylindrical vector beams. The oscillator employs a figure-9 nonlinear amplifying loop mirror (NALM) configuration entirely composed of polarization-maintaining fibers, ensuring environmentally robust and self-starting mode locking. The output mode-locked pulses exhibit a central wavelength of 2090.8 nm, a repetition rate of 33.8 MHz, and a spectral bandwidth of 6.1 nm. After inserting a tunable bandpass filter, a continuous wavelength tuning range from 2024 nm to 2046 nm is achieved while maintaining single-pulse operation and a polarization extinction ratio over 98%. This work presents a simple and efficient approach to generating high-purity vectorial ultrashort pulses in the 2 µm region, providing a promising platform for high-precision micro- and nanofabrication of polymers and transparent materials.