Passively Q-switched femtosecond-laser-written thulium waveguide laser based on evanescent field interaction with carbon nanotubes

Surface channel waveguides (WGs) were fabricated in a monoclinic Tm3+:KLu(WO4)2 crystal by femtosecond direct laser writing (fs-DLW). The WGs consisted of a half-ring cladding with diameters of 50 and 60 μm located just beneath the crystal surface. They were characterized by confocal laser microscopy and μ-Raman spectroscopy, indicating a reduced crystallinity and stress-induced birefringence of the WG cladding. In continuous-wave (CW) mode, under Ti:sapphire laser pumping at 802 nm, the maximum output power reached 171.1 mW at 1847.4 nm, corresponding to a slope efficiency η of 37.8% for the 60 μm diameter WG. The WG propagation loss was 0.7±0.3  dB/cm. The top surface of the WGs was spin-coated by a polymethyl methacrylate film containing randomly oriented (spaghetti-like) arc-discharge single-walled carbon nanotubes serving as a saturable absorber based on evanescent field coupling. Stable passively Q-switched (PQS) operation was achieved. The PQS 60 μm diameter WG laser generated a record output power of 150 mW at 1846.8 nm with η=34.6%. The conversion efficiency with respect to the CW mode was 87.6%. The best pulse characteristics (energy/duration) were 105.6 nJ/98 ns at a repetition rate of 1.42 MHz.