B. Rami Reddy

Optical sensing techniques for temperature measurement

Temperature is an important parameter that needs accurate measurement. Theoretical descriptions of the fluorescence ratio method, fluorescence lifetime sensing, and interferometric methods for temperature measurement are given. Fluorescence lifetime sensing calibration plots have been developed for temperature measurement from 20°C to 600°C using Er(3+)-doped glass, and from 20°C to 90°C using Sm(3+)-doped CaF(2). Lifetime sensing results of Pr(3+)-doped YAG and Ho(3+)-doped fluoride crystals for temperature measurement are also summarized. Mach-Zehnder interferometer measurements revealed that the passage of a 300 mW laser beam of 915 nm changed the temperature of the Yb(3+)-doped YAG crystal by 7.1°C. The interferometer technique is useful for measuring absolute temperature changes in laser cooling studies.

Ultraviolet upconversion in thulium-doped fluorozirconate fiber observed under two-color excitation.

Ultraviolet upconversion signals at 293, 351, and 366 nm were observed from thulium-doped fluorozirconate fiber pumped with a 458-nm Ar(+) laser. The upconversion signals intensity was enhanced 5x when the fiber was simultaneously pumped with a 458-nm Ar(+) laser and a 585-nm dye laser. There is evidence of the formation of defect centers under simultaneous excitation by visible and near-infrared lasers (458 and 750 nm), and there is no evidence of color-center formation when both of the exciting beams are in the visible (458 and 585 nm).

Energy transfer and upconversion studies in yttrium–aluminum–garnet crystal doped with thulium and holmium under near-infrared laser excitation

Upconversion emission from Tm3+ and Ho3+ ions in a yttrium–aluminum–garnet crystal was observed on resonant excitation of the 3H4 level of Tm3+ and the 5I4 level of Ho3+ with a Ti:sapphire laser at room temperature and 10 K. The upconversion and the energy transfer mechanisms between Tm3+ and Ho3+ ions are discussed. A rate equation model is developed which supported the energy transfer mechanism.

Photon-avalanche upconversion in thulium-doped lutetium aluminum garnet

Strong blue fluorescence at 487 nm corresponding to the (1)G(4) --> (3)H(6) transition was generated from Tm(3+)-doped lutetium aluminum garnet on excitation with a 618-nm dye laser as a result of a photon-avalanche upconversion mechanism.

Design of a high temperature sensing system using luminescence lifetime measurement.

Temperature sensing system has been designed and fluorescence lifetime measurements have been performed to estimate the temperature. (4)S(3/2) lifetime of LaF(3):Er(3+) and (5)S(2) lifetime of LaF(3):Ho(3+) have been measured as a function of temperature. Lifetime versus temperature calibration plots have been made up to 1000 degrees C.

Persistent spectral hole burning in europium-doped sodium tellurite glass

We have prepared sodium tellurite glasses doped with europium. Emission intensity of the europium-doped glass is at least 500× more than that of the undoped glass. Fluorescence features exhibited dependence on the pump laser wavelength. Persistent spectral hole burning was observed in the F07→D05 transition of Eu3+. Multiple hole burning was possible even if the chemicals were melted in ambient air to make the glass. The hole burning mechanism is also explained. Electron spin resonance measurements confirmed the existence of defect centers.

Interferometric measurement of laser heating in praseodymium-doped YAG crystal

Temperature measurement is required for many applications but can be difficult in some cases. Laser heating or cooling studies demand accurate measurements of temperature changes. A Michelson interferometer configuration has been used to investigate laser heating in solids. An analytical formula was derived to estimate the temperature change from the fringe count by taking into account the temperature dependence of the sample length and refractive index. When 115 mW of a focused Ar+ laser beam (488 nm) passes through a Pr(3+)-doped YAG sample, its temperature increased by 11.7±1.0 K along the beam path due to nonradiative relaxation. The power dependence of the fringe count/movement was recorded. The temperature change was estimated by the interferometric method and is in agreement with that measured by a thermocouple.

Effect of the addition Y2O3 on persistent spectral hole burning in europium doped sodium borate glass

Persistent hole burning is observed in a europium doped sodium borate glass from 10 to 100 K. Hole burning efficiency increased 18× when 5% yttrium oxide was added to the glass mixture. Hole burning was observed up to 300 K in Eu3+, Y3+ Co-doped glass.

Spectroscopic characterization of erbium doped glass ceramic

Erbium doped oxyfluoride glass was synthesized from the molar composition 10.1% Na2CO3 −20.2% PbO −33.7% GeO2 −33.6% TeO2 −2.4% ErF3 by melt quenching technique. The Judd-Ofelt intensity parameters were estimated as Ω2 = 10.8 × 10−20, Ω4 = 1.17 × 10−20, and Ω6 = 4.32 × 10−20 cm2. Radiative transition probabilities and lifetimes were also calculated. Differential scanning calorimetry (DSC) was used for thermal analysis of the sample. Nanocrystals were induced in the glass by heat-treatment. Strong room temperature upconversion emissions were observed at 415, 540, 554 and 667 nm from Er3+ doped sample under 972 nm Ti-sapphire laser excitation. X-ray diffraction (XRD) measurements revealed the presence of NaErF4 crystallites 35 nm in the glassy matrix. The concentration of nanocrystals is found to be low in the middle of the sample and higher close to the surface. Time correlated single photon counting (TCSPC) was used to measure Er3+ lifetimes.

Energy upconversion in holmium doped lead–germano–tellurite glass

Holmium doped lead–germano–tellurite glass was prepared by the melt quenching technique. The Judd–Ofelt intensity parameters were estimated as Ω2=7.6×10−20, Ω4=12.9×10−20, and Ω6=2.5×10−20 cm2. Radiative transition probabilities and lifetimes were also determined for some of the levels. Room temperature upconversion emissions have been observed from Ho3+ at 497 nm under 532 nm laser excitation, and at 557 and 668 nm under 762 nm laser excitation. The upconversion emission mechanisms were found to be due to a step wise excitation process. Upconversion emission intensity enhanced in a heat treated glass.