Q-switched thulium–ytterbium co-doped fibre laser using newly developed octagonal shaped inner cladding double-clad active fibre and multi-walled carbon nanotubes passive saturable absorber

Simple and low cost passive Q-switched thulium-ytterbium co-doped fibre lasers (TYFL) operating at 1977.5 nm is demonstrated using 905 nm multimode pumping, in conjunction with a multi-walled carbon nanotubes (MWCNTs) saturable absorber. It uses a new octagonal shaped double-clad thulium ytterbium co-doped fibre, manufactured using a modified chemical vapour deposition process and a solution doping. The Tm³⁺ and Yb³⁺ cladding absorptions of the fibre are 0.325 and 3.3 dB/m at 790 nm and 976 nm, respectively. The MWCNTs composite is prepared by mixing the MWCNTs homogeneous solution with a diluted polyvinyl alcohol polymer solution before it is left to dry at room temperature to produce thin film. The film is sandwiched between two fibre connectors and integrated into the laser cavity for Q-switching pulse generation. The repetition rate of the Q-switching pulse can be tuned from 18.8 to 50.6 kHz while the pulse width reduces from 8.6 to 1.0 μs as the pump power increases from 1.6 to 2.3 W. At the pump power of 2.1 W, the TYFL produces the maximum pulse energy of 5.7 nJ.