362 / CLE0'97 / THURSDAY AFTERNOON 0 22 0 2 0 -0 1 8 -0 1 6 -0 1 4 --I , , , . I , , I . . t 140 30 35 40 45 50 55 60 r, mm CThL9 Fig. 2 Dependence of the output power relative changing APiP and maximum output P, , , on output mirror curvature radius r; power changing APiP was caused by pump wavelength variation inside interval of 805 . . . 8 11 nm.output power instability on pumping wavelength variation APIP -21% was observed.The capability of neodymium doped glass to store a high level of energy depends on the concentration of ions, which can be as high as 4-8 10,' ions/cm3. However it is known that dipole-dipole interaction between neighbors ions Nd3+ rise with population, implying a decrease of the lifetime emission of the lasing excited state 4F,iz.1 This increased spontaneous emission becomes an important mechanism loss when using a pumping with duration comparable to the lifetime. Furthermore high densities of inversion are associated to an additional loss mechanism that corresponds to the interaction between two neighboring excited 4F,i, ions. In this process, one ion is further excited to a higher level, while the other returns to the ground ~t a t e .~,~ Although the highly excited state can relax to 4F3,2 via nonradiative process, this Auger upconversion mechanism limits the energy storage capability of the media and should be taken into account when using laser or diode pumping. This is the motivation of this study.Basically, this loss depends quadratically on the population N,(4F3,,) and appears in the usual rates equations as a -yNf term. After the initial buildup of the excited state population NI, its relaxation follows the law:where 6 = Y T~N~~, and T~ is the 4F3,2 lifetime at low excitation. O u a m s m s m d Tvm (In) CThLlO Fig. 1 Time evolution of pump intensity and fluorescence 1.06 mm of excited state 4F3,2. Tcmpr U51 CThLlO Fig. 2 Decay ofthe fluorescence after pumping at low and high excitation. Ercirdpopulruon (1019mdm'1 CThLlO Fig. 3 Data of Auger constant obtained for different excitation. The 750-nm pump wavelength (0) create apopulation on the excited states *S,,, and 4F,,2 and the 782.5 (0), 789 m (0) populate the excited states 4F,,2, ' H, , , . The absorption cross section at 750 nm is greater by one order of magnitude at 782.5 nm. The dependence is linear only for 750 nm pumping suggesting ESA effects at the other wavelengths.We have developed an experimental setup to quantify this effect. We use different types of glass rods pumped by a free running alexandrite laser. This laser delivers up to 5 Joules pulses in typically 150 ps and is tunable from 750 nm to 790 nm.We first monitor the temporal evolution of the inversion population NI (Fig. 1) for different pump fluences and wavelength. Figure 2 shows after the pumping at high excitation level, a faster decrease of the fluorescence around 1.06 pm.We quantify 6 from the fluorescence measurements using Relation (1). The determination of the parameter y needs however the knowledge of the initial population Nlo. We have developed a n...
