Here is presented and discussed the history, the spectroscopic, the thermo-mechanical and the laser properties of the new old laser crystal Yb:CaF2, of its Sr and Ba isotypes as well as of an Yb and Na codoped compound. It will be shown that Yb:CaF2 is a very particular luminescent material, in which the laser-active center probably consists of a complex hexameric cluster, that it is the most promising Yb-doped fluoride material for large-scale, high-power and high-energy laser systems and that it can compete in several aspects with the currently used Yb-doped oxide crystals and glasses. The opportunity of operating the crystals at cryogenic temperatures and the recently achieved improvements in the field of femtosecond pulsed laser operation and high peak power laser amplification is highlighted and evaluated.

Yb:CaF2 — a new old laser crystal

We present an overview of laser results we obtained with Yb-doped calcium fluoride and its isotype strontium fluoride. In order to study the laser performance in femtosecond and high power regimes, spectral and thermal properties are first discussed including the potential of these crystals at room and cryogenic temperatures. Experimental demonstrations of high-power and ultrashort pulse oscillators and amplifiers are presented and analyzed.

https://hal-iogs.archives-ouvertes.fr/hal-00658197/document

On Yb:CaF2 and Yb:SrF2 : Review of spectroscopic and thermal properties and their impact on femtosecond and high power laser performance

A detailed spectroscopic study of ${\text{Yb}}^{3+}:{\text{CaF}}_{2}$ is made by using laser selective excitation, low-temperature time-resolved fluorescence, and Raman techniques. Three dominant types of ${\text{Yb}}^{3+}$ isolated ion centers are identified at very low $(0.03\text{ }\text{at}\text{.}\text{ }%)$ dopant concentration: two of them with cubic and tetragonal symmetries and the third one not clearly identified but likely of trigonal symmetry. Fluorescence decays strongly differ from one site to another. The overall measured positions of the Stark sublevels of the ${^{2}F}_{7/2}$ and ${^{2}F}_{5/2}$ multiplets are satisfactorily explained by the crystal-field calculations. On the other hand, at higher dopant concentrations, only one dominant class of ${\text{Yb}}^{3+}$ centers occurs which corresponds to some hexameric cluster in which the ${\text{Yb}}^{3+}$ ions site in nearly tetragonal crystal-field environments and whose energy-level positions are discussed and compared with those previously reported in the literature.

Spectroscopy of Yb 3+ :CaF 2 : From isolated centers to clusters

Fluoride materials for optical applications: Single crystals, ceramics, glasses, and glass–ceramics

The origin of light scattering defects was studied in transparent 6 at% Yb:CaF 2 ceramics. Samples were synthesized by a soft chemistry route followed by sintering and hot pressing which leads to highly transparent ceramics with low scattering losses (0.016 cm −1 at 1200 nm). Light scattering defects were studied using scanning transmission electron microscopy (STEM) and high angle annular dark field-STEM (HAADF-STEM) techniques. Energy dispersive X-ray spectroscopy showed a 50% increase in Yb 3+ concentration at grain boundaries. A 3–5 nm thick oxygen rich phase was detected at some grain boundaries by both HAADF-STEM and EDS. The origin of the oxygenized grain boundaries was traced to a 2–15 nm thick oxygenized shell present on the starting powders. Analysis of high resolution HAADF-STEM images revealed that Yb 3+ substitutes into the fluorite lattice as clusters rather than individual ions, but the types of clusters could not be identified by this imaging technique.

Origin of light scattering in ytterbium doped calcium fluoride transparent ceramic for high power lasers

An output pulse energy of 17.3 mJ has been achieved with a diode-pumped Yb:CaF2 regenerative laser amplifier. The bandwidth of the output pulse spectrum was 7.3 nm, being seeded with femtosecond pulses stretched to 2.2 ns. In cw operation a tuning range of 80 nm has been observed. A maximum pulse energy of 44 mJ at a repetition rate of 1 Hz has been obtained in Q-switched mode. The laser damage threshold of a Yb:CaF2 crystal has been determined at a wavelength of 1064 nm and a pulse duration of 10 ns.

Broad-band regenerative laser amplification in ytterbium-doped calcium fluoride (Yb:CaF2)

Summary form only given. Diode-pumped solid-sate laser systems combined with CPA technique are promising devices for generation of highest peak intensities. With regard to the choice of a suitable gain medium its spectroscopic properties determine the scaling of a laser system towards both highest output pulse energy and shortest pulse duration. In order to saturate the absorption with a minimum number of diode lasers a long fluorescence lifetime of the gain medium is desired. Ytterbium-doped fluorides such as Yb:CaF2 and Yb:SrF2 comprise a broad emission spectrum and a comparable long fluorescence lifetime. At a dopant concentration of 3.3% (atm.) we measured a radiation lifetime of 2.2 ms for Yb:CaF2 and 4.6 ms for Yb:SrF2. It shows absorption and emission cross section whereas the latter is determined by reciprocity method. According to ISO 11254-1 standard the damage threshold of Yb:CaF2 was measured to be 53 J cm-2 at a pulse duration of 10 ns (FWHM) and a center wavelength of 1064 nm. We built a diode-pumped Yb:CaF2 and Yb:SrF2 laser with a stable cavity, a resonator length of 1 m and a glass prism for wavelength tuning. A pulsed driven stack of 25 fast axis collimated diode-laser bars for 100 W (quasi-CW) maximum output power each is applied as pump source. The laser operates in quasi-CW operation with a repetition rate of 1 Hz and a pulse duration of 2 ms. Applying a DKDP Pockels cell a quarter wave plate and a thin film polarizer the laser operates as Q-switched laser and regenerative amplifier respectively. Pulses with a duration of 23 ns and 60 mJ pulse energy were generated in Q-switched operation with Yb:CaF2 as gain medium. As seed-pulse source a commercial Ti:Sa oscillator with 1030 nm center wavelength is used. The hard clip bandwidth after stretching to 2 ns is 32 nm. Regenerative amplification will be demonstrated.

Tunable CW and Q-switched operation in Yb:CaF 2 and Yb:SrF 2

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Broad-band regenerative laser amplification in ytterbium-doped calcium fluoride (Yb:CaF2)

Tunable CW and Q-switched operation in Yb:CaF 2 and Yb:SrF 2