Efficient gain-switched operation of a Tm-doped silica fiber laser

We present the results from experiments relating to a gain-switched Tm-doped silica fiber laser in which a gain-switched Nd:YAG laser is used to pump the ³H₅ energy level of the Tm³⁺ dopant ion. This fiber laser configuration is the first example to our knowledge of a moderate energy gain-switched fiber laser which is pumped with a low-repetition-rate high-energy pulsed laser. For a near-optimized cavity, the gain-switched fiber laser produces a maximum pulse energy of 1.46 mJ at a maximum linear slope efficiency of 20% and a total optical-to-optical efficiency (with respect to the launched energy) of 19%. At low pump energies, the slope efficiency is approximately 40%, however, saturation of the output pulse energy is observed with the increase in the launched pump energy. We also present results from a numerical model that simulates ³H ₅-band pumping and includes all of the known pump excited-state absorption (ESA) mechanisms and, in addition, four cross-relaxation mechanisms have also been included. The calculations establish that the pump ESA mechanism contributes only a small loss factor to the overall efficiency of the laser when the Tm-doped silica fiber laser is pumped at low pump energies, however, as the pump energy is increased, losses due to pump ESA limit the amount of output energy from the fiber laser. The loss mechanism is mainly attributed to pump ESA from the ³H₄ upper laser level to the combined ³F_2,3 energy level at low launched pump energies because of the large absorption cross section for this transition and the relatively long lifetime of the ³H₄ energy level. For harder pumping conditions, the majority of the excited state population resides in the ¹G₄ level, inhibiting in some laser configurations gain-switching of the fiber laser until cessation of the pump pulse itself