Abstract

Recently we reported about laser action on the ³H₄–³H₆ transition of Tm³⁺ ion when it is excited due to energy transfer from four-fold co-ordinated Cr⁴⁺(t) ions in YAG crystal [1,2]. 2.02 um CW oscillation was performed by 1.06 CW Nd laser pumping. The lasing threshold was about 3 W. It is correspond to 15% quantum efficiency of energy transfer from a Cr⁴⁺(t) ion to a Tm³⁺ ion. On the other hand the quantum efficiency was equal to 90 % by the pulsed pumping as it was obtained from measuring of Cr⁴⁺(t) luminescence relaxation time. We supported some cause reducing the quantum efficiency of energy transfer from Cr⁴⁺(t) ions to a Tm³⁺ ions with CW pumping. So far as an excited Tm³⁺ ions density, is needed for lasing, (which we estimated as 10²⁰cm⁻³) is considerably higher than the Cr⁴⁺ ions concentration (it is equal to 3.1*10¹⁷ cm⁻³), excitation migration in Tm³⁺ ions system must take place after excitation transfer from a Cr⁴⁺ ion to a Tm³⁺ ion to reach the necessary excitation density. But a migration rate in Tm3+ system is expected lower than an excitation transfer rate from a Cr⁴⁺ to a Tm³⁺ ion, therefore excited Tm³⁺ ions will be concentrated near a Cr⁴⁺(t) being souse of excitation. So a local saturation of Tm³⁺ ions excitation may take place, that will cause reducing of energy transfer rate from Cr⁴⁺(t) ions to Tm³⁺ ions. In order to evaluate the energy transfer rate by CW pumping we had measured depandences of transmittance of YAG: YAG:Tm³⁺, Cr⁴⁺ and YAG:Cr⁴⁺ crystals upon the pumping power up to the power density corresponding to our lasing experiments.

© 1995 Optical Society of America