Self-mode-locked 2 μm Tm(3+)-doped double-clad fiber laser with a simple linear cavity

We demonstrate the self-mode-locking operation of a thulium (Tm)-doped fiber laser (TDFL) with a simple linear cavity. Since the laser cavity does not include any specific mode-locker, we experimentally investigate and analyze the self-mode-locking mechanism. The mode-locking operation is attributed to the combination of the self-phase modulation effect and the weak saturable absorption of the high-concentration Tm-doped fiber. The mode-locked TDFL operates at a central wavelength of 1985.5 nm with the 3 dB spectral linewidth of 0.18 nm. The self-mode-locking generates a large pulse energy of 32.7 nJ with a pulsed repetition rate of 2.05 MHz and is stable with a radio-frequency signal-to-noise ratio of more than 54 dB. To the best of our knowledge, it is the first demonstration of a 2 μm Tm-doped fiber laser mode-locked by such technique.