Highly efficient, water cooled, resonantly pumped Ho3+ fluoride lasers have been demonstrated at 2 μm. The laser materials, 0.5 at.% Ho:YLF, 0.5 at.% Ho:BYF, and 0.5 at.% Ho:BaYLuF, were resonantly pumped with an IPG Tm:fiber laser operating at 1940 nm. Laser action occurred on the <sup>5</sup>I<sub>7</sub> to the <sup>5</sup>I<sub>8</sub> manifold with center wavelengths of 2062.6 nm, 2074.0 nm, and 2075.2 nm respectively. Continuous wave (CW) operation of 0.5% Ho:BYF achieved a maximum efficiency of approximately 68% with a 50% reflective output coupler. Pulsed operation of this laser system produced 4 mJ pulses at 500 Hz with a slope efficiency of 38%.
Optical cooling of solids, relying on annihilation of lattice phonons via anti-Stokes fluorescence, is an emerging technology that is rapidly advancing. The development of high-quality Yb-doped fluoride single crystals definitely led to cryogenic and sub-100-K operations, and the potential for further improvements has not been exhausted by far. Among fluorides, so far the best results have been achieved with Yb-doped LiYF4 (YLF) single crystals, with a record cooling to 91 K of a stand-alone YLF:10%Yb. We report on preliminary investigation of optical cooling of an LiLuF4 (LLF) single crystal, an isomorph of YLF where yttrium is replaced by lutetium. Different samples of 5% Yb-doped LLF single crystals have been grown and optically characterized. Optical cooling was observed by exciting the Yb transition in single-pass at 1025 nm and the cooling efficiency curve has been measured detecting the heating/cooling temperature change as a function of pumping laser frequency.
In this work we present an overview of the best 2μm laser results obtained in Tm-doped fluoride hosts LiYF<sub>4</sub>(YLF),
LiLuF<sub>4</sub> (LLF) and BaY<sub>2</sub>F<sub>8</sub> (BYF) and we report on the growth, spectroscopy and first laser test emission of a novel
mixed material BaYLuF<sub>8</sub> (BYLF), interesting as a variant of BYF material with a partial substitution of Y<sup>3+</sup> ions by Lu<sup>3+</sup>.
The novel host is interesting mainly because indications are that the mixed crystal would be sturdier than BYF. The
addition of Lutetium would improve the thermo-mechanical properties going into the direction of high power
applications, as suggest from works on YLF and its isomorph LLF. A detailed description of Czochralski growth of
fluoride laser materials is provided, focusing on the growth parameters of the novel BYLF:Tm<sup>3+</sup>12% material grown.
With regard of spectroscopy analysis, we report on the results obtained with BYLF host. Detailed absorption,
fluorescence and lifetime measurements have been performed focusing on the <sup>3</sup>H<sub>4</sub> and <sup>3</sup>F<sub>4</sub> manifolds, the pumping and
upper laser level. Moreover diode pumped CW laser emission at 2 μm has been achieved in BYLF: Tm<sup>3+</sup>12% sample obtaining a slope efficiency of about 28% with respect to the absorbed power.
The development of oxide and fluoride materials as gain materials of choice for solid state lasers ranges from early materials such as Calcium Fluoride and Calcium Tungstate crystals to the now ubiquitous Nd hosts YLF, YAG and Vanadate. Among Tunable laser materials, MgF<sub>2</sub> - an early favorite, gave way to superior oxides such as Alexandrite and Ti:Sapphire only to be followed by development of still newer tunable fluoride media, notably, fluoride colquiriites such as Cr-doped LiSAF and LiCaF. Newer fluoride crystals, such as Barium Yttrium Fluoride BaY<sub>2</sub> F<sub>8</sub> (BYF), KY<sub>3</sub>F<sub>10</sub> (KYF) and the tunable Cr doped LiCaGaF<sub>6</sub> are attractive laser materials, but their growth has not been optimized. Key advantages of two of these new crystals are discussed. Crystal growth results for BYF and Cr:LiCaGaF<sub>6</sub> as well as some material characterization are presented.
Experimental results describing pulsed lasers operating near 3.9 μm on the Ho<sup>3+ </sup>(<sup>5</sup>I<sub>5</sub>-<sup>5</sup>I<sub>6</sub>) transition in highly-doped (> 10 at. %) barium yttrium fluoride (BaY<sub>2</sub>F<sub>8</sub> or BYF) will be presented. The <sup>5</sup>I<sub>5</sub> manifolds in Ho:BYF were pumped using a flashlamp excited, free-running Cr:LiSAF laser tuned to the Ho<sup>3+</sup> absorption peak near 889nm. Ho<sup>3+</sup> concentrations of 10%, 20%, 30% and 40% in BYF were lased in a simple end-pumped resonator. Some similar data was also obtained in 10% and 20% Ho:YLF. The highest 3.9 μm pulse energy obtained in the comparative study was 55 mJ (at ~10% optical-to-optical efficiency) using the 30% Ho:BYF crystal. A dual end-pumped laser in 30% Ho:BYF was also demonstrated, providing a pulse energy of 90 mJ in a near diffraction limited beam (M<sup>2</sup> ~ 1.2). Emission decay data was taken to shed light on the observed dependence of laser efficiency on holmium concentration and excitation density. The lifetimes of both lasing levels (<sup>5</sup>I<sub>5</sub> and <sup>5</sup>I<sub>6</sub>) deviate rather significantly from their low-concentration values. Plausible energy transfer processes that may be responsible for the observed trends in the laser and emission data will also be discussed.
In this work we applied the Thermal Lens (LT) technique to determine the upconversion parameter for Cr<SUP>3+</SUP>:LiSrAlF<SUB>6</SUB>, Cr<SUP>3+</SUP>:LiSrGaF<SUB>6</SUB>, Cr<SUP>3+</SUP>:SrAlF<SUB>5</SUB> and Nd<SUP>3+</SUP>:ZBLAN. The measures for the Cr<SUP>3+</SUP> doped fluorides were accomplished at 15 degrees C to eliminate the suppression effect due to temperature and upon excitation in the cw regime. We used Ar<SUP>+</SUP> laser and dye laser radiation for the Cr<SUP>3+</SUP> doped crystals and 514 nm and 796.7 nm for the Nd<SUP>3+</SUP> doped glasses. We observed a nonlinear increase of the TL signal with excitation power, characterizing a fluorescence quantum efficiency decrease that was attributed to the Auger effect. A theoretical model was developed to obtain (gamma) from the TL data.
Analysis of the process of up-conversion suggest scaling rules for up-conversion based optically written displays. These rules have been demonstrated experimentally and give us confidence in our designs of such displays.
A versatile high power Q-switched Cr:LiSAF laser system has been developed using fiber optics and a low power cw oscillator to remotely control the operating wavelength. This type of system has the advantage of eliminating lossy frequency control elements inside the high power oscillator.
Two key obstacles to the development of high-capacity three-dimensional information mass storage systems using photorefractive crystals are: (1) readout of holograms stored in the conventional manner is destructive, and (2) undoped photorefractive crystals are typically quite insensitive to red and infrared write light from diode lasers (the preferred source). As a consequence, bulky, expensive ion lasers have been necessary for hologram writing. Thus hologram fixing and high infrared sensitivity in these materials are both important if practical systems are to be built. We present our own high-temperature (80-110°C) photorefractive grating writing results for a variety of pure and doped barium titanate (BaTiO<sub>3</sub>) crystals. We found strong fixable secondary gratings that correlated with increasing levels of Fe and Ni in doped crystals. Fixing was not observed in our pure crystals or those doped with V, Rh, or Co. Fixing was enhanced in an iron-doped crystal reduced by high-temperature annealing at low oxygen partial pressures. We also found that the readout diffraction efficiency of a fixed grating in BaTiO<sub>3</sub> is a rapidly increasing function of readout temperature above a threshold temperature, which depends upon the crystal orientation. Lower temperatures lead to longer storage times, as would be expected if the carriers forming the fixed grating have a thermally activated mobility. We also report preliminary experimental results on blue Rh-doped BaTiO<sub>3</sub> crystals with fast and high-gain infrared (840 nm) response.