L. L. Chase

Infrared cross-section measurements for crystals doped with Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/

The absorption and emission cross sections of the transition between the ground spin-orbit multiplet and the lowest excited multiplet were measured for Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/ ions in a variety of crystalline hosts. The materials that were investigated include LiYF/sub 4/, BaY/sub 2/F/sub 8/, Y/sub 3/Al/sub 5/O/sub 12/, LaF/sub 3/, KCaF/sub 3/, YAlO/sub 3/, and La/sub 2/Be/sub 2/O/sub 5/. The absolute magnitudes of the emission cross sections were determined from the absorption spectra, with the aid of the principle of reciprocity. The calculated radiative emission lifetimes derived from these measured cross sections agree well with the measured emission decay times for most materials. The potential use of these rare-earth-doped materials in pulsed laser applications requires that the ground state exhibit adequate splitting to minimize the detrimental effects of the ground state thermal population, and also that the emission cross section be sufficiently large to permit efficient extraction energy. The systems based on Ho/sup 3+/ in the eightfold coordinated sites of LiYF/sub 4/, BaY/sub 2/F/sub 8/, and Y/sub 3/Al/sub 5/O/sub 12/ appear to be the most promising. >

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. >

Laser performance of LiSrAlF6:Cr3+

We have lased the new material, LiSrAlF6:Cr3+ (Cr3+:LiSAF). The single crystals were grown by the horizontal zone melting technique. The spectroscopic properties of Cr3+:LiSAF are similar to those of other low‐field Cr3+‐doped systems, although the emission cross section is strongly π polarized and is also somewhat larger than has been measured for other fluoride hosts. The free‐running lasing wavelength of Cr3+:LiSAF is 825 nm, and the tuning range extends from at least 780 to 920 nm. Using Kr laser pumping, we obtained slope efficiencies of 36% and 14% by utilizing output couplings of 4.8% and 0.8%, respectively. On the basis of these results, the extrapolated maximum efficiency of 53% is determined, to be compared to the quantum defect‐limited value of 78%. It is concluded that a moderate level of excited state absorption (ESA) loss is responsible for the reduced efficiency of the Cr3+:LiSAF system. This contrasts with the related results previously obtained for LiCaAlF6:Cr3+, where it was concluded th...

LiCaAlF/sub 6/:Cr/sup 3+/: a promising new solid-state laser material

LiCaAlF/sub 6/:Cr/sup 3+/ (Cr/sup 3+/:LiCAF) exhibits an intrinsic (extrapolated maximum) slope efficiency of 67%. For comparison, the intrinsic slope efficiencies of BeAl/sub 2/O/sub 4/:Cr/sup 3+/ (alexandrite), Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/ and ScBO/sub 3/:Cr/sup 3+/ were found to be 65, 28, and 26%, respectively. The tuning range of LiCaAlF/sub 6/:Cr/sup 3+/ was determined to be at least 720-840 nm. The conventional spectroscopic properties, such as the absorption, emission, and emission lifetimes as a function of temperature, are reported as well. >

Quantum electronic properties of the Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/ laser

Few of the existing Cr/sup 3+/ vibronic lasers have achieved the slope efficiency and tuning range expected based on their known spectroscopic properties. To discover the cause of this behavior, the performance of chromium-doped gallium fluoride garnet, Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/, as a laser material has been investigated experimentally. The data reported include absorption and emission spectra, emission rates, quantum efficiency, laser wavelength tuning range, laser output slope efficiencies, and excited-state absorption spectra. Similar properties of the alexandrite laser material were studied for comparison. The results indicate that the performance of the gallium fluoride garnet laser is severely limited by Cr/sup 3+/ excited-state absorption (ESA). A model is presented to account for the unexpected nature of the ESA, which appears to be a common problem for all Cr/sup 3+/ vibronic lasers. Criteria are suggested for choosing Cr/sup 3+/ hosts for which the effects of ESA will be minimized. >

Nonlinear refractive-index measurements of glasses using three-wave frequency mixing

We have developed a rapid and accurate method for measuring the nonlinear refractive index of transparent materials. It is based on a version of three-wave mixing in which the frequencies of the two input beams are adjusted to differ by about 60 cm−1. The nonlinear index of a material is measured relative to that of a standard with a known index. Our measurements of 27 glasses are in good agreement with previous results, where available, and with the n2 values calculated empirically from linear index parameters. We also discuss the relationship between the observed nonlinear index and the structure and composition of the glasses.

Ground-state depleted solid-state lasers : principles, characteristics and scaling

A novel class of rare-earth-doped solid-state lasers is described. The ground-state depleted laser is pumped by an intense (more than tens of kW cm−2) narrow-band (less than a few nm) laser source and is characterized by: (1) an unusually low laser ion doping density (5 to 10×1018ion cm−3), (2) an unusually large fractional excited population inversion density (4 to 8×1018 ion cm−3, or >75%), (3) a gain element that is optically thick at the pump wavelength and (4) a gain element that has a substantially uniform gain distribution due to a bleaching of the pump transition at the pump intensity utilized. These features enable efficient room-temperature operation of rare-earth-ion laser transitions terminating on the ground manifold. The relationships between laser parameters (cross-sections, saturation fluences and fluxes, bleaching wave velocities, etc.) are given and laser performance scaling relationships are presented and discussed.

Investigation of the laser properties of Cr/sup 3+/:LiSrGaF/sub 6/

A boule of Cr:LiSGAF (Cr/sup 3+/:LiSrGaF/sub 6/), the Ga analog of Cr:LiSAF, was grown, a laser sample was fabricated, and its efficiency was measured using a laser-pumped laser configuration. The laser performance near 820 nm and related properties of Cr:LiSGAF are reported. The use of Sr instead of Ca results in a red shift of the absorption and emission band (along with an increase in the emission cross section). Insight into the effect of a change in the identity of the substitutional site on the properties of the laser is provided. >

Optical spectroscopy of the new laser materials, LiSrAlF6:Cr3+ and LiCaAlF6:Cr3+

Abstract We have obtained the absorption and emission spectra and the emission lifetimes, of Cr 3+ -doped LiCaAlF 6 and LiSrAlF 6 . The spectral observations can be understood by carefully accounting for the small non-octahedral components of the crystal potential field existing at the substitutional Al site. The polarization properties of the absorption spectra are due to a static t 2u -type component of the field, although the odd-parity dynamical component also contributes significantly to the oscillator strength. An analysis of the spin-orbit components of the 4 T 2 state reveals the importance of a Jahn-Teller distortion in the e g coordinate. Finally, the overall configurational displacement between the ground and excited states is found to be larger in LiSrAlF 6 , compared to LiCaAlF 6 .

Spectroscopy and gain measurements of Nd 3+ in SrF 2 and other fluorite-structure hosts

We investigate the optical properties of Nd3+ in CaF2, SrF2, and BaF2 with the intent of determining whether any of these materials might usefully serve as a laser-pumped-amplifier medium. The Nd3+ impurities tend to cluster at low levels of doping in CaF2, leading to the formation of nonluminescent centers. The addition of La or Y buffer ions to CaF2:Nd serves to increase the luminescent yield, but it also renders the system spectrally inhomogeneous. Although single-ion centers predominate in BaF2, the interstitial fluoride compensator occurs at the next-nearest-neighbor position relative to Nd3+, leading to unsuitably low transition strengths. The interstitial fluoride occurs at the nearest-neighbor site of Nd3+ in SrF2 and thereby induces significant oscillator strength into the 4f–4f transitions by breaking the inversion symmetry. The radiative lifetime of SrF2:Nd is found to be 1470 μsec by measuring the emission lifetime and quantum efficiency; this value was confirmed by Judd–Ofelt analysis of the absorption features. The peak-emission cross section at room temperature was determined to be 1.7 × 10−20 cm at 1036.5 nm. A maximum of 0.20 at. % Nd3+ may be doped into SrF2 without the occurrence of significant Nd clustering. Direct measurements of the gain spectrum in SrF2:Nd reveal the presence of the 4F3/2 → 2G9/2 excited-state absorption, although its effect on the emission cross section is only minor.