Validity Analysis of Holmium-Doped Fiber as Saturable Absorber for Passively Q-Switched Thulium-Doped Fiber Laser

We propose and demonstrate a method for measuring nonlinear transmission (NLT) of holmium-doped fiber (HDF) as saturable absorber (SA) by using two calibrated photodiodes. The HDF with a length of 10 cm is under test as reference, and the NLT measurement is carried out from 1930 to 1970 nm by using a tunable continuous-wave (CW) thulium-doped fiber laser (TDFL). The result finds that modulation depth of HDF is 56% at 1950 nm and decreases down to 37% at both 1930 and 1970 nm. Saturation power is about 20 mW for all measured wavelengths. The validity of the HDF SA for a passively Q-switched TDFL is also studied by taking the measured parameters into account. The laser operates in Q-switched mode from 1944 to 1960 nm, whereas in CW mode beyond the wavelength range due to insufficient modulation depth. The pulse temporal characteristics are also discussed.