Experimental Researches Of All-Fiber Thulium-Doped Fiber Laser Based On Nonlinear Amplified Loop Mirror

Thulium-doped fiber(TDF) owns a very wide fluorescence spectrum. With different pump methods, thulium-doped fiber lasers(TDFLs) can generate 1650 nm to 2150 nm laser. The 2 μm lasers match the absorption peak of water molecules, thus they have excellent performance in surgery and eyes treatment. TDFLs also have great potential applications in many fields, such as nonlinear frequency conversion, long-distance communication with ultra-low loss, commercial manufacture and so on. This paper studies on an all-fiber passively mode-locked TDFL based on nonlinear amplified loop mirror(NALM), including soliton dynamic patterns of TDFL operating in an all-abnormal dispersion environment and the dissipative soliton resonance(DSR) of TDFL operating in a net-normal dispersion environment. More details are as follows:(1).An all-fiber passively mode-locked TDFL based on NALM with a cavity length of 116 m is constructed. The cavity dispersion is all-abnormal around 2 μm wavelength. When the pump power is over mode-locked threshold, stable mode-locked state with a repetition rate of 1.72 MHz is easily triggered. In fundamental mode-locking, the experimental laser emits soliton mode-locked pulses with a central wavelength of 1965.67 nm and Kelly sidebands can be observed in the optical spectrum. By properly adjusting the polarization controllers(PCs), Kelly sidebands disappears, the optical spectrum becomes smooth and the full width at half maximum increases, the experimental laser is now operating in noise-like(NL) mode-locked state. By further increasing pump power and adjusting the PCs, mode-locked pulses become unstable and start splitting, finally the soliton rain arises. After the triggering of soliton rains, a reverse phenomenon which is called soliton release is observed. By further adjusting the PCs, the drifting solitons decrease and finally disappear. This time, quasi-rectangular pulses generate. With the increase of pump power, the quasi-rectangular pulse has the maximum pulse duration of 33 nsand the maximum pulse energy of 37.7n J, respectively. In this process, the experimental laser always keeps the single-pulse operation.(2).An all-fiber passively mode-lock TDFL based on NALM with a cavity length of 64.3 m is constructed. By using the ultra-high numerical aperture fiber(UHNAF) as the dispersion-managed device, the cavity dispersion is net-normal and its total dispersion parameter is about +0.8585 ps~2@1900 nm. When the pump power is increased over mode-locked threshold, stable mode-locked state with a repetition rate of 3.11 MHz is easily triggered. In fundamental mode-locking, the experimental laser emits rectangular pulses with a central wavelength of 1940.08 nm. Its pulse duration and pulse energy are 1.39 ns and 3.79 n J, respectively. When the cavity matches the dissipative soliton resonance condition, with the increase of pump power, the rectangular pulses keep on broadening and its maximum pulse duration, maximum pulse energy and average peak power are 6.19 ns, 19.5 n J and 3.15 W, respectively. In addition, by properly adjusting the PCs, the central wavelength of rectangular pulses can be tuned from 1940.22 nm to 1969.17 nm, the tunable range is about 28.95 nm.