Diode-pumped Er3+:LiYF4 lasers for medical applications

In a first part we report on a Ti:sapphire pumped Er 3+ :YLF laser. The dopant concentration and the polarisation of the pump beam is optimised. The highest slope efficiency is obtained with the polarisation of the pump beam parallel to the c-axis and a dopant concentration of 20 at % Erbium. Slope efficiencies up to 50 % are achieved. This value clearly exceeds the Stokes limit of 35 %. Further, we report on an Er3 + :BYF laser pumped with two polarisation-coupled diode lasers. The dependence of slope efficiency on the reflectance of the resonator mirrors, on the focus of the pump light, and on the resonator length is investigated. The best slope efficiency of η = 24 % is obtained for a nearly hemiconcentric resonator with a reflectance of R in .R out = 97,6 % and a pump beam focused with a lens of f = 20 mm. To obtain a higher pump power, we investigate the coupling of sixteen diode lasers, each one operated to emit a maximum power of 800 mw. The diode laser emission is collimated and then focused using aspherical lenses. The point spread function of this system is investigated. Laser action in a longitudinally pumped Er 3+ :YLF is already achieved using only one single diode laser.