A theoretical study of the Nd: Yb: Er glass laser

This paper discusses the prospects of getting efficient laser generation from Er at 1·54 μm with Nd and Yb co-dopants in an unclad glass rod. Target outputs at room temperature are 50 mJ free-running or 30 mJ Q -switched. Ion-to-ion transfer rates are deduced from fluorescence lifetime measurements on a variety of Li-Mg-SiO 2 base glasses. Good energy transfer can be obtained from Nd to Yb and from Yb to Er. Unfortunately there is also some transfer back from Er to Nd. Measurements on flash-tube spectra and flash-tube efficiency are combined with glass absorption spectra measurements and pumping cavity efficiency calculations to give values of pumping efficiency for various concentrations. By combining the efficiency results with the ion-to-ion transfer results we predict the pump energies needed to get the target output for various rod dimensions and ion concentrations. Pump pulse duration is varied continuously with the energy into the tube to keep down tube loading and give an acceptable tube life. Optimum concentrations (compromised between the various targets) are found to be 4·2×10 20 for Nd, 13×10 20 for Yb, and 0·3×10 20 ions/cm 3 for Er, and the optimum dimensions, length about 3 cm, diameter about 0·3 cm, giving a threshold energy of 60 J and input energies for the target outputs of 80 and 120 J. Limitations on the accuracy of these predictions are discussed. Smaller inputs are obtainable if the design is made more specific to the target output.