Spectroscopic studies for active laser media development of Ho3+ and Er3+: YLF operating at 3μm range

The lifetime decrease of Er3+ (4I13/2, 4I11/2 and Ho 3+ (5I6, 5I6) levels were investigated in YLF crystals doped with Tb3+, Eu3 and Nd3+ deactivator ions. The critical radius of interaction were determined using the overlap integral method. Each multiplet contribution of the deactivator ion in the nonradiative energy transfer process was determined. Although the microscopic parameters have indicated that Tb3+ and Eu3+ ions introduce strongest deactivation effects in the first excited state of activators, it was concluded that Nd3+ ion is the best deactivator to be used in Er- and Ho-doped YLF. This conclusion was based on the fact that Nd3+ should introduce the smallest population depletion of the second excited-state of activators. Er(1.5, 2.7μm) and Ho(2.1, 2.9μm) fluorescences were investigated in (Er:Nd) and (Ho:Nd):YLF crystals. Short laser pulses excitations delivered by a tunable OPO pumped 2ω-Nd:YAG laser system were used in these investigations. The fluorescent experimental decay curves were fitted by Inokuti-Hirayama, Yokota-Tanimoto and Burshtein models which describe the energy transfer between these ions. The best fit of 4I11/2 (Er3+) level decay was obtained using the Yokota- Tanimoto model when CD-D ∼ 2x10-40 cm6/s. The best fit of 4I13/2 and 5I6 levels were obtained using the Burshtein model when CD-D ∼- 30x10-40 cm6/s. The decay of the 5I7 (Ho3+) level could not be described by the current models. The pumping frequency limit has been improved with the presence of Nd3+ ions in the laser medium. For Er(1%):Nd(1,2%):YLF system, it has increased from 12 to 175 Hz and for Ho(1,5%):Nd(1,2%):YLF this frequency limit goes to 83 Hz. However the luminescence efficiency has decreased with the presence of Nd3+ by a factor of 3.3 for 2.75 μm (Er3+) luminescence and by a factor of 1.6 for 2.9μm. Comparing both systems the Ho:Nd:YLF is more indicated for the laser action test for 3μm operation at frequency > 25 Hz. (author)