Laura D. DeLoach

Evaluation of absorption and emission properties of Yb/sup 3+/ doped crystals for laser applications

The emission and absorption properties of numerous host crystals doped with Yb/sup 3+/ ions have been studied. The hosts which have been selected include LiYF/sub 4/, LaF/sub 3/, SrF/sub 2/, BaF/sub 2/, KCaF/sub 3/, KY/sub 3/F/sub 10/, Rb/sub 2/NaYF/sub 6/, BaY/sub 2/F/sub 8/, Y/sub 2/SiO/sub 5/, Y/sub 3/Al/sub 5/O/sub 12/, YAlO/sub 3/, LuPO/sub 4/, Ca/sub 5/(PO/sub 4/)/sub 3/F, LiYO/sub 2/, and ScBO/sub 3/. Spectral determinations have been made of the resonant absorption and emission cross sections between 850 and 1100 nm, and the emission decay times of the upper laser level have been measured. The emission cross sections have been evaluated using the absorption cross section and principle of reciprocity, and again using the Fuchtbauer-Ladenberg formula. Agreement between the two methods is within 20% for most materials. The results are discussed in the framework of requirements for an effective diode-pumped Yb/sup 3+/ laser system. Ca/sub 5/(PO/sub 4/)/sub 3/F:Yb is predicted to exhibit the most useful laser properties and is expected to be far superior to Y/sub 3/Al/sub 5/O/sub 12/:Yb in many key microscopic parameter values. >

Transition metal-doped zinc chalcogenides: spectroscopy and laser demonstration of a new class of gain media

The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr/sup 2+/, Co/sup 2+/, Ni/sup 2+/, or Fe/sup 2+/ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 /spl mu/m which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 /spl mu/m for Cr and from 2.8-4.0 /spl mu/m for Co, Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures, Cr/sup 2+/ is estimated to have the highest quantum yield at room temperature among the media investigated with values of /spl sim/75-100%. Laser demonstrations of Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity with a Co:MgF/sub 2/ pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 /spl mu/m, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr/sup 2+/ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalcogenides; Co/sup 2+/ may also serve usefully, but laser demonstrations yet remain to be performed.

Cr/sup 2+/-doped zinc chalcogenides as efficient, widely tunable mid-infrared lasers

Transition-metal-doped zinc chalcogenide crystals have recently been investigated as potential mid-infrared lasers. Tetrahedrally coordinated Cr/sup 2+/ ions are especially attractive as lasants on account of high luminescence quantum yields for emission in the 2000-3000-nm range. Radiative lifetimes and emission cross sections of the upper /sup 5/E state are respectively /spl sim/10 /spl mu/s and /spl sim/10/sup -18/ cm/sup 2/. The associated absorption band peaked at /spl sim/1800 mm enables laser-diode pumping of the Cr/sup 2+/ systems. Laser demonstrations with ZnS:Cr and ZnSe:Cr (using a MgF/sub 2/:Co/sup 2+/ laser pump source) gave slope efficiencies up to 30%. Excited-state-absorption losses appear small, and passive losses dominate at present. Tuning experiments with a diffraction grating produce a tuning range covering at least 2150-2800 nm. Laser crystals can be produced by Bridgman growth, seeded physical vapor transport, or diffusion doping. Zinc chalcogenide thermomechanical properties of interest for medium-to-high-power operation compare favorably with those of other host materials, except for the larger refractive-index derivative dn/dT.

Laser and spectroscopic properties of Sr 5 (PO 4 ) 3 F:Yb

Sr5(PO4)3F (S-FAP) has been investigated as a new Yb-doped laser crystal belonging to the apatite structural family. The spectroscopy of the Yb3+ ion and the laser properties of the medium have been investigated. The maximum absorption cross section of Yb in S-FAP is 8.6 × 10−20 cm2, and the maximum emission cross section is 7.3 × 10−20 cm2. The measured emission lifetime of Yb3+ is 1.26 ms. An Yb:S-FAP laser has been demonstrated with a Ti:sapphire laser pump operating at 899 nm. The Yb:S-FAP laser was measured to have slope efficiencies as high as 71%. The spectroscopy and laser studies are reported, as well as certain thermal, mechanical, and optical properties.

YTTERBIUM-DOPED APATITE-STRUCTURE CRYSTALS : A NEW CLASS OF LASER MATERIALS

A new class of Yb‐lasers is summarized in this article. The apatite family of crystals, based on the hexagonal structure of the mineral fluorapatite, has been found to impose favorable spectroscopic and laser properties on the Yb3+ activator ion. Crystals of Yb‐doped Ca5(PO4)3F, Sr5(PO4)3F, CaxSr5−x(PO4)3F, and Sr5(VO4)3F have been grown and investigated. Several useful laser crystals have been identified which offer a variety of fundamental laser parameters for designing diode‐pumped systems. In general, this class of materials is characterized by high emission cross sections (3.6–13.1×10−20 cm2), useful emission lifetimes (0.59–1.26 ms), a strong pump band (σabs=2.0–10.0×10−20 cm2), and pump and extraction wavelengths near 900 and 1045 nm, respectively. Efficient lasing has been demonstrated for several of the members of this class of materials, and high optical quality crystals have been grown by the Czochralski method. A summary of the laser parameters and a discussion of the Yb:apatite class of laser...

Laser, optical, and thermomechanical properties of Yb-doped fluorapatite

The laser performance of Yb-doped fluorapatite (Ca/sub 5/(PO/sub 4/)/sub 3/F or FAP), is assessed by employing a Ti:sapphire laser operating at 905 nm as the pump source. We have measured slope efficiencies to be as high as 79%; the residual decrement from the quantum defect-limited efficiency of 87% is accounted for by the presence of passive loss at the 1043-nm laser wavelength. The important spectral properties of Yb:FAP were evaluated, including the absorption and emission cross sections, excited-state lifetime, and ground-state energy-level splitting. The emission and absorption cross sections of Yb/sup +3/ in FAP are found to be substantially larger than those of other Yb-doped media. The thermal, physical, and optical properties of the FAP host are reported as well. >

Vibrational structure in the emission spectra of Yb3+-doped apatite crystals

Abstract We have determined the ground state energy levels of Yb 3+ in crystals with the apatite structure from the low temperature emission spectra of Yb-doped crystals of calcium, strontium and barium fluorophosphates, calcium chlorophosphate and strontium fluorovanadate. Analyses of the data reveal that the previously reported laser transition arises from an electronic level of Yb interacting with the local vibrational modes of the Yb-O bond.

Properties of Cr:LiSrAlF 6 crystals for laser operation

We have performed several physical and optical measurements on the Cr:LiSAF (LiSrAlF(6)) laser material that are relevant to its laser performance, including thermal and mechanical properties, water durabilities, and Auger upconversion constants. The expansion coefficient, Youngs modulus, fracture toughness, thermal conductivity, and heat capacity are all used to determine an overall thermomechanical figure of merit for the crystal. An investigation of the water durability suggests that the cooling solution should be maintained at pH = 7 to ameliorate problems associated with water dissolution. The Auger constant was found to become much more significant at higher Cr doping, in which excited-state migration leads to a substantial increase in the upconversion rate. We propose a design for a 50-W Cr:LiSAF laser system that is based on a detailed knowledge of all the relevant material parameters.

Crystal growth, frequency doubling, and infrared laser performance of Yb/sup 3+/:BaCaBO/sub 3/F

Yb:BaCaBO/sub 3/F(Yb:BCBF) has been investigated as a new laser crystal with potential for self-frequency doubling, Yb/sup 3+/ in BCBF exhibits a maximum absorption cross section at 912 nm of 1.1/spl times/10/sup -20/ cm/sup 2/ with a bandwidth (FWHM) of 19 nm. The maximum emission cross section at 1034 nm is 1.3/spl times/10/sup -20/ cm/sup 2/ with a transition bandwidth of 24 nm. The measured emission lifetime of Yb/sup 3+/ is 1.17 ms. An Yb:BCBF laser has been demonstrated with a Ti:sapphire pump source, and a measured slope efficiency of 38% has been obtained for the fundamental laser output. Single crystal powders of BCBF have been compared with KD/sup +/P for a relative measure of the second-harmonic generating potential, yielding d/sub eff/(BCBF)/spl sim/0.26 pm/V. The phasematching angle has been estimated from the refractive index data for type I second-harmonic generation of 0.517 /spl mu/m light; the predicted angle is 37/spl deg/ from the c-axis. The growth, spectroscopy, laser performance, and linear and nonlinear optical properties of Yb:BCBF are reported.

752 nm wing-pumped Cr:LiSrAlF/sub 6/ laser

The authors have demonstrated a LiSrAlF/sub 6/:Cr/sup 3+/ (Cr:LiSrAlF/sub 6/) laser pumped at 752 nm in the longwave wing of the /sup 4/A/sub 2/ to /sup 4/T/sub 2/ pump band. Measured optical, laser, and thermal properties of LiSrAlF/sub 6/-LiSrCrF/sub 6/ solid solutions indicate that efficient laser pumping can be achieved in the 750-780 nm region, and that AlGaAs laser diode arrays may be used to make efficient high-power diode-pumped tunable solid-state lasers. >