We report the successful growth and the laser cooling results of Yb<sup>3+</sup>-doped single fluoride crystals. By investigating
the mechanical and thermal properties of Yb-doped BaY<sub>2</sub>F<sub>8</sub> and LiYF<sub>4</sub> crystals and using the
spectroscopic data we collected from our samples, the theoretical and experimental cooling efficiency of fluoride
crystals are evaluated and compared with respect to those of ZBLAN. Two different methods, a thermal camera
and a fluorescence intensity ratio technique, have been used to monitor the temperature change of the samples.
The temperature change is clearly exponential, as expected from theory, and the temperature drops are 6.3
K and 4 K for Yb:LiYF<sub>4</sub> and Yb:BaY<sub>2</sub>F<sub>8</sub> respectively in single-pass configuration, corresponding to a cooling
efficiency of about 2% and 3%. This last value is slightly larger than that observed in Yb-ZBLAN in similar
We present the results of an investigation of the spectroscopic properties of Ce<sup>3+</sup>:BaY<sub>2</sub>F<sub>8</sub> (BYF), which is a potential laser
material with an emission wavelength range from 320 nm to 360 nm. We have employed a time-resolved pump-probe
technique to investigate the polarization-dependent absorption and emission properties, and the dynamic color centre
formation process. We observe strong absorption from colour centres with millisecond and second lifetimes that will
certainly prevent laser action with the crystals used here. Evidence suggests that there may be potential gain in this crystal
if long-lived colour centres can be reduced.
Scheelite-like disordered double sodium-lanthanum molybdate NaLa(MoO<sub>4</sub>)<sub>2</sub> single crystals co-doped with Er<sup>3+</sup> and different concentrations of Ce<sup>3+</sup> have been grown by Czochralski technique. The 300K spectroscopic properties of the excited states <sup>4</sup>S<sub>3/2</sub>, <sup>4</sup>I<sub>11/2</sub> and
<sup>4</sup>I<sub>13/2</sub> of Er<sup>3+</sup> ions in grown crystals as a function of Cerium concentration have been investigated. The lifetime of <sup>4</sup>I<sub>11/2</sub> have been found to reduce from 130 μs in the solely Er-doped sample to approximately 3.5 μs, when the concentration of Ce reaches 50 at. %. At the same time the lifetime of <sup>4</sup>I<sub>13/2</sub> level at 300 K remains unchanged (3.7 ms) up to the concentration of Ce in crystal equal to 10 at. %, and reduces by less than 30% (2.6 ms) in the 50 at. % Ce co-doped sample. This result looks very promising from point of view of obtaining low-threshold 1.5 μm laser oscillation.
A review on the results achieved by our group in the development of novel solid-state lasers for Lidar applications at 2 μm is presented. These lasers, based on fluoride crystals (YLF<sub>4</sub>, BaY<sub>2</sub>F<sub>8</sub>, and KYF<sub>4</sub>) doped with Tm and Ho ions, are characterized by high-efficiency and wide wavelength tunability around 2 μm. Single crystals of LiYF<sub>4</sub>, BaY<sub>2</sub>F<sub>8</sub>, and KYF<sub>4</sub> codoped with the same Tm<sup>3+</sup> and Ho<sup>3+</sup> concentrations were successfully grown by the Czochralski method. The full spectroscopic characterization of the different laser crystals and the comparison between the laser performance are presented. Continuous wave operation was efficiently demonstrated by means of a CW diode-pumping. These oscillators find interesting applications in the field of remote sensing (Lidar and Dial systems) as well as in high-resolution molecular spectroscopy, frequency metrology, and biomedical applications.
The goal of the VIRGO program is to build a giant Michelson type interferometer (3 kilometer long arms) to detect gravitational waves. Large optical components (350 mm in diameter), having extremely low loss at 1064 nm, are needed. Today, the Ion beam Sputtering is the only deposition technique able to produce optical components with such performances.
Consequently, a large ion beam sputtering deposition system was built to coat large optics up to 700 mm in diameter. The performances of this coater are described in term of layer uniformity on large scale and optical losses (absorption and scattering characterization).
The VIRGO interferometer needs six main mirrors. The first set was ready in June 2002 and its installation is in progress on the VIRGO site (Italy). The optical performances of this first set are discussed. The requirements at 1064 nm are all satisfied. Indeed, the absorption level is close to 1 ppm (part per million), the scattering is lower than 5 ppm and the R.M.S. wavefront of these optics is lower than 8 nm on 150 mm in diameter. Finally, some solutions are proposed to further improve these performance, especially the absorption level (lower than 0.1 ppm) and the mechanical quality factor Q of the mirrors (thermal noise reduction).