Tunable Mode-Locked Pulsed Laser Generation at Longer 2.0 μm Region

This work demonstrates simple, wavelength-tunable mode-locked Thulium-Holmium co-doped fiber laser (THDFL) and Holmium doped fiber laser (HDFL) operating at a longer
$2.0~\mu \text{m}$
region. The mode-locking operation was induced by the nonlinear polarization rotation (NPR) mechanism using the combination of an inline optical polarizer and a polarization controller (PC). Meanwhile, the wavelength tuning was done by adjusting the PC in the birefringent Lyot filter. A 3 dB bandwidth of 3.8 nm, a pulse width of 1.12 ps, a fundamental repetition rate of 11.05 MHz, and a signal-to-noise ratio (SNR) of
$\sim 64$
dB were achieved at the operating wavelength of 1996 nm in THDFL. On the other hand, a 3 dB bandwidth of 3.6 nm, a pulse width of 1.40 ps, a fundamental repetition rate of 11.45 MHz, and a SNR of
$\sim 58$
dB were achieved at 2110 nm in HDFL. These mode-locked lasers were highly stable and showed the absence of wavelength and repetition rate frequency drifting when observed over an hour. The center wavelength of the mode-locked THDFL was tunable from 1942 nm to 1996 nm, whereas the HDFL was tunable from 2070 nm to 2110 nm by adjusting the PC. As per the author’s knowledge, the mode-locked THDFL reported in this work has the most straightforward design compared to other tunable mode-locked THDFL. In contrast, the reported HDFL provides a simple design with the longest operating wavelength among other tunable mode-locked HDFLs reported in the literature.