Optical Investigation of Er:Ca2Al2SiO7 and Yb:Ca2Al2SiO7 for Laser Applications in the Near Infrared

Abstract

Single crystals of the aluminosilicate Ca 2 Al 2 SiO 7 (CAS) activated with Er 3+ or Yb 3+ were grown by the floating zone method for several dopant contents. The crystals of good optical quality, were investigated using optical spectroscopy with the aim to assess their potential interest as laser materials in the 1.5 μm (Er:CAS) and 1 μm (Yb:CAS) range, respectively. In both cases, absorption, stimulated emission, and gain cross sections were determined. Radiative and experimental lifetimes were also calculated. In the two compounds, as the two levels involved in the transitions are the ground and first excited states, radiation trapping and energy migration occur. For Er:CAS, emission around 1.55 μm (in the middle of the eye-safe range) may be obtained for a reasonable value of the population inversion ratio β (β = 0.4). The Judd-Ofelt parameters are significantly higher than those of Er:YAG and the nonradiative relaxation of the excited states located above 4 I 13/2 is fast (phonon energy cut-off around 1020 cm -1 ) ensuring efficient population of the 4 I 13/2 emitting level. For Yb:CAS, the absorption and emission cross sections are higher than for Er:CAS. Positive gain cross section is possible for very low β values, for instance β = 0.04 at 1045 nm. Taking into account the pump saturation intensity and the β value, it appears that this compound should be one of the best ytterbium activated laser crystal (if one excepts the apatite-type hosts).