Details of the optical-spectroscopic properties of alexandrite (BeA12O~.Cr +) crystals were studied by different laser-spectroscopy techniques. The temperature dependences of the fluorescence lifetimes and widths of the zero-phonon lines were found to be quite different for Cr + ions in the mirror and inversion crystal-field sites. The results indicate that direct phonon-absorption processes dominate both thermal line broadening and lifetime quenching for ions in the mirror sites while phonon-scattering processes dominate the line broadening of inversion-site ions and leave their lifetime independent of temperature. Tunable-dye-laser site-selection methods were used to obtain the excitation spectra of the Cr'+ ions in inversion sites at low temperature and to identify six types of exchange-coupled pairs of Cr + ions in the lattice. Time-resolved site-selection spectroscopy was used to monitor the energy transfer between Cr + ions in mirror and inversion sites at both low and high temperature. Finally, high-power, picosecond pulse excitation was used to produce two-photon absorption, and the resulting emission spectrum was found to exhibit a new fluorescence band in the 400-nm spectral region.

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The radical suppression of the photodarkening effect and laser performance deterioration via H 2 loading were demonstrated in high-power Yb-doped fiber (YDF) amplifiers. The photodarkening loss at equilibrium was 114.4 dB/m at 702 nm in the pristine fiber, while it vanished in the H 2 -loaded fiber. To obtain a deeper understanding of the impact of photodarkening on laser properties, the evolution of the mode instability threshold and output power in fiber amplifiers was investigated. After pumping for 300 min, the mode instability threshold of the pristine fiber dropped from 770 to 612 W, and the periodic fluctuation of the output power became intense, finally reaching 100 W. To address the detrimental effects originating from photodarkening, H 2 loading was applied in contrast experiments. The output power remained stable, and no sign of mode instability was observed in the H 2 -loaded fiber. Moreover, the transmittance at 638 nm confirmed the absence of the photodarkening effect. The results pave the way for the further development of high-power fiber lasers.

The results of an investigation of thermal effects on the fluorescence spectra of Ti3+ in sapphire are presented. As-grown samples synthesized by the Czochralski method were found to exhibit a fluorescence band characteristic of Ti4+. After thermal annealing this band was greatly reduced and the Ti3+ fluorescence was seen. The fluorescence lifetime was found to be constant between 10 and 300 K and then to decrease significantly as temperature was raised to 500 K. This decrease is shown to be associated with thermal activation to the higher Jahn–Teller component of the excited state. The temperature dependences of the intensities, widths and lifetimes of the two zero-phonon lines were monitored between 10 and 80 K. The lifetimes remained constant as the widths broadened and intensities decreased with temperature.The intensity change is associated with the increased rate of vibronic emission while the line broadening is attributed to the phonon processes coupling spin-orbit components of the ground state and to Raman scattering of phonons.