Modeling of 2-um Tm:Ho:ZBLAN diode-pumped cw double-clad fiber lasers

Theoretical modeling of high-power thulium-sensitized holmium fluorozirconate (Tm:Ho:ZBLAN) glass fiber lasers operating on the 2 micrometer transition in holmium is presented. High output power in single-mode can be achieved by pumping the inner cladding of the double clad fiber with a single-mode core with high-power multimode cw diode lasers. The possibility of higher output powers with the double-clad fiber laser (or DCFL) arrangement over the core-pumped fiber laser means that these particular lasers are well suited to a number of medical applications which require output of high power and good beam quality. In a preliminary step to characterize and optimize these devices, a computer model has been developed to analyze the performance of the device when the power coupling coefficients and doping concentrations are varied. It is shown that the form and hence values of the power coupling coefficients which relate to the transfer of power to and from the ensemble of nonabsorbing modes to the ensemble absorbing modes which propagate within the inner cladding and core of the fiber respectively are paramount to the operation of the DCFL. In addition, for a specific doping level the choice of pump wavelength can be made on the grounds of existing core-pumped fiber laser technology.