CW operation of a 1.064-μm pumped Tm-Ho-doped silica fiber laser

The results from experiments relating to the CW operation of a Tm-Ho-doped silica fiber laser which is pumped with the fundamental output from a Nd:YAG laser are presented. The measured maximum output power from the fiber laser of 11 mW was generated at a slope efficiency of approximately 1.8% for a fiber length of 0.574 m and an output coupling of 10%. An output wavelength of 2170 nm (one of the longest lasing wavelengths to be achieved with the use of a silica host material) was also generated from the Tm-Ho-doped fiber laser when the fiber length was extended to 1.240 m and a 5% output coupling incorporated. The reduced efficiency and increased threshold for the Nd:YAG-pumped Tm-Ho-doped silica fiber laser when compared to previous reports of Ti:sapphire pumping is discussed in detail with the aid of a comprehensive numerical model. The numerical model solves the rate equations for the Tm-Ho-doped silica fiber laser system by taking into account the cross relaxation, energy transfer, and upconversion mechanisms, and it utilizes all published spectroscopic parameters relevant to Tm-Ho-doped silica and Tm-Ho:ZBLAN glass materials. It is established that the excited state absorption relevant to Nd:YAG pumping severely depletes the ³H₄ energy level of Tm³⁺ and consequently hinders the energy transfer process to the ⁵I₇ energy level of Ho³⁺. Optimum dopant concentrations are also established for both Nd:YAG and Ti:sapphire pump schemes