Dehydroxylation processing and lasing properties of an Nd alumino-phosphate glass
The main high energy high power laser phosphate glasses use Nd3+ ions as dopant. To determine the suitability of a Nd3+-doped glass laser, a knowledge of the spectroscopic properties are required. In particular, the emission cross-section and the quantum efficiency determine the stored energy in the 4F3/2 state and extraction characteristics. Non-radiative processes, if present, reduce the lifetime which leads to a reduction of the stored energy and affect the output laser energy. As energy transfer to OH vibrational modes is an important source of non-radiative losses in these Nd-doped glasses, dehydroxylation of the phosphate glasses is of paramount importance to achieve a good laser performance.
In this collaborative work carried out by the groups led by Alicia Durán (ICV-CSIC) and Rolindes Balda (CFM, UPV/EHU), an aluminophosphate glass composition was selected to carry out a study on the influence of the processing parameters over the dehydroxylation of the glasses, i.e. temperature and time of remelting under N2, viscosity and mass of glass. The glass composition, 13Na2O-13K2O-16BaO-4Al2O3-54P2O5 (mol %), is close to the available commercial phosphate laser glasses for high power lasers such as LG-750, LG770, or LHG-8. Laser experiments were performed using a 5 mm thick plate-shaped sample doped with 2.5 wt% Nd2O3 concentration placed at the center of a 10 cm long confocal symmetrical resonator and oriented at Brewster angle with respect to the resonator axis (Figure 1).
The stimulated emission cross-section calculated from spectral data gives a value of 3.9×10-20 cm2, similar to the one of LG-770 (Shott) glass, in reasonably good agreement with the value estimated from laser threshold data (4±0.5×10-20 cm2). The obtained values for the stimulated emission cross-section, figure of merit (144.3×10-25 cm2s), and quantum efficiency (89%) together with the threshold energy and slope-efficiency of the laser emission at around 1055 nm demonstrate the suitability of this glass for optical amplification (Figure 2). Moreover, the laser emission undergoes detectable changes when the excitation wavelength is tuned along the 4I9/2®4F5/2 pump band due to the crystal field site effects. The observed behavior can be explained if the existence of two distributions of sites for the Nd3+ ions overlapped in energy is taken into account; one of them very wide but with smaller effective cross-section than the other one located close to the center of the former, whose effective-cross section is larger and sharper. The presence of P-O-Al bonds in the glass network could explain the existence of two broad sites distributions for the rare earth in this glass matrix.
Figure 1 Experimental set-up used for laser experiments. The sample is located at the center of a confocal resonator consisting in two concave mirrors of curvature radii 10 cm, one HR and an output coupler of reflectivity 70%. The pump beam is addressed collinear to the resonator axis.
Figure 2: a) Time-evolution of stimulated emission pulses in Nd3+-doped alumino-phosphate glass at two different pump energies. The pump pulse (802 nm wavelength) is at left side of the graph: Black line 28 mJ pump energy, red line 45 mJ pump energy. b) Laser emission energy as a function of pump energy at 802 nm. The threshold energy is 26 mJ and the slope-efficiency is 33 %.