All-polarization-maintaining tunable L-band mode-locked fiber laser and cascaded second-harmonic generation

There is an urgent demand for highly stable, broadly wavelength-tunable, and low-repetition-rate pulsed lasers in fields such as cold-atom physics, precision spectroscopy, and bioimaging. However, conventional L-band lasers often suffer from environmental instability and limited tuning range. To address this, we propose an all-polarization-maintaining, wavelength-tunable L-band mode-locked fiber laser, followed by cascaded second harmonic generation. Wavelength tuning from 1570 nm to 1596 nm was achieved by incorporating a rotatable intracavity filter with a bandwidth of 10 nm. The laser exhibits self-starting mode locking and can generate pulses with a pulse width of 87 ps and a repetition rate of 15.6 MHz. Using cascaded periodically poled lithium niobate and \beta -barium borate crystals, we perform frequency conversion on the output pulses to produce tunable near-infrared (778-804 nm) and ultraviolet (389-402 nm) light. With broad wavelength tunability enabling applications such as two-photon excitation fluorescence imaging at 780 nm, laser cooling of calcium ions at 397 nm, and optical pumping in miniaturized rubidium atomic clocks, this laser system can serve as a versatile platform for quantum technologies and ultrafast spectroscopy.