Wavelength-adjustable mode-locked Tm-Ho co-doped fiber laser from 1839 nm to 1876 nm

We demonstrate a passively mode-locked Tm-Ho co-doped fiber laser using GaAs saturable absorber (SA). The laser has a very broad output spectrum covering from 1700 nm to 2100 nm with output wavelengths adjustable from 1839 nm to 1876 nm. The significant broadening produced by mode-locking is due to the laser-induced self-phase modulation (SPM). Results show that the peak wavelength is red-shifted with the increase of pump power. On the other hand, with the increase of the cavity length, the peak wavelength is blue-shifted. The red-shift generated by the increasing pump power may be due to the reabsorption of the short wavelength part of the laser by Ho3+ ions, while the increase in the blue shift due to the increasing cavity length is due to the absorption loss in the long wavelength direction inside the optical fiber at the broad-band laser since the long wave part of the output has already entered the absorption band of the fiber material. As the length of the fiber increases, the longer wavelength side of the laser output spectrum is absorbed more, causing a blue-shift of the peak wavelength. Therefore, the laser peak wavelength can be adjusted by changing the pump power or cavity length.