E. De la Rosa

FIBER SAGNAC INTERFEROMETER TEMPERATURE SENSOR

A modified Sagnac interferometer-based fiber temperature sensor is proposed. Polarization independent operation and high temperature sensitivity of this class of sensors make them cost effective instruments for temperature measurements. A comparison of the proposed sensor with Bragg grating and long-period grating fiber sensors is derived. A temperature-induced spectral displacement of 0.99 nm/K is demonstrated for an internal stress birefringent fiber-based Sagnac interferometer.

Strong green upconversion emission in ZrO2:Yb3+–Ho3+ nanocrystals

Structural and upconversion emission properties of ZrO2:Yb3+–Ho3+ nanocrystals were analyzed as function of Yb3+ concentration. Structural characterization shows a crystallite size up to 80 nm and tetragonal and cubic phase as the main crystalline structures. Strong green (540 nm) and weak red (670 nm) and near infrared (760 nm) emission bands were observed with 968 nm excitation. The upconversion is based on two photons absorption either by the energy transfers from Yb3+ ion or by the excited state absorption. The energy transfer efficiency was calculated to be 50% for 2 mol % of Yb3+ diminishing to less than 20% for higher concentration. The Yb3+ concentration also affects the decay time of the green emission of Ho3+ ion diminishing from 140μs for 2 mol % of Yb to 76μs for higher concentration.

All-fiber absolute temperature sensor using an unbalanced high-birefringence Sagnac loop

We demonstrate a highly sensitive temperature sensor based on a stress-induced high-birefringence-fiber Sagnac loop that uses a Nd-doped-fiber amplified spontaneous emission source. Relative temperature sensing is done in the spectral domain by shifts of a resonant wavelength lambda(r) and absolute temperature sensing by changes in separation between resonances Dlambda . The measured relative change of these parameters with temperature in the range 15-110 degrees C, is (1/lambda(r))(deltalambda(r)/deltaT) = -(1/Dlambda)(deltaDlambda/delta T) approximately (1/Dn)(delta Dn/deltaT)(-0.94 +/- 0.02) x 10(-3)/K, with measured fiber birefringence Dn = 8 x 10(-4) . This gives a wavelength-shift sensitivity of -1.00 nm/K at 1.065 microm and a resonance separation sensitivity of 0.006 nm/K for Dlambda = 6.8 nm . This telemetric point sensor has a loop length of 80 m, an operational bandwidth of more than 50 nm, and a temperature accuracy of better than 1 degrees C.

Role of Yb3+ and Er3+ concentration on the tunability of green-yellow-red upconversion emission of codoped ZrO2:Yb3+–Er3+ nanocrystals

Strong green and red visible emissions were obtained from ZrO2:Yb3+–Er3+ nanocrystals synthesized by sol-gel method and annealed at 1000 °C for 5 h. The average crystallite size was ∼70 nm with tetragonal phase for total concentration lower than 3 mol % and cubic phase for concentration higher than 5 mol %. The color coordinate of the upconverted signal was tailored by controlling the dopant composition that change the red/green ratio dominated by the cross relaxation and energy back transfer process as was demonstrated theoretically and confirmed experimentally. Both coefficients were calculated, C51∼1.02×10−16 and C5b∼6.04×10−17, from the theoretical model based on the rate equations. The highest energy transfer efficiency was η∼64% for 2 mol % of Yb and 2 mol % of Er3+. However, for the highest upconverted signal was only η∼29% obtained for 2 mol % Yb and 1 mol % Er with effective decay time τeff∼438 μs for red and τeff∼290 μs for green band.

Er3+ and Yb3+ concentration effect in the spectroscopic properties and energy transfer in Yb3+/Er3+ codoped tellurite glasses

Spectroscopic properties of Yb3+/Er3+ codoped tellurite glasses as a function of Er3+ and Yb3+ concentration have been investigated. Under 970 nm excitation three strong up-conversion emission bands centred at 525, 546 and 656 nm were observed, and the characteristic near infrared emission band was centred at 1.53 µm. With fluorescence and radiative lifetime the quantum efficiency (QE) of infrared (1.53 µm) and visible upconversion (546 and 660 nm) emissions was calculated. The maximum stimulated emission cross section for 4I13/2 → 4I15/2 transition of Er3+ is 9.7 × 10−21 cm2 for 3/0.5 mol%. The energy transfer (ET) efficiency from Yb3+ to Er3+ (4F5/2) + (4I15/2) → (4 F7/2) + (4I13/2) was calculated, being the maximum ET of 69% for 0.5 mol% of Er3+ with 4.5 mol% of Yb3+. The results indicate that both ET and QE depend mostly on Er3+ rather than on Yb3+ concentration.

Brilliant blue, green and orange–red emission band on Tm3+-, Tb3+- and Eu3+-doped ZrO2 nanocrystals

Tm3+-, Tb3+- and Eu3+-doped ZrO2 nanocrystals were prepared by a facile precipitation method with a hydrothermal process. Structural characterization showed a crystallite size ranging from 30 to 40 nm, and monoclinic and tetragonal zirconia phases were observed depending on the dopant concentration. The monoclinic phase was dominant for 0.5 mol% of Tb3+ and Eu3+, and the tetragonal phase was 100% stabilized for 2 mol% of Tm3+ and Tb3+. The structure of emission bands associated with Eu3+ confirms the substitution of Zr4+ located at C1 and D2h symmetry sites for the monoclinic and tetragonal phases. The emission of three primary colours, red, green and blue, was obtained from Eu3+, Tb3+ and Tm3+, respectively, which makes this nanophosphor an excellent candidate for use in photonics applications. The emitted signal was analysed as a function of ion concentration and the optimum concentration was determined.

Red, green, blue and white light upconversion emission in Yb3+/Tm3+/Ho3+ co-doped tellurite glasses

Several Yb3+/Tm3+/Ho3+ co-doped transparent TeO2–ZnO–Na2O–Yb2O3–Ho2O3–Tm2O3 glasses were prepared and luminescence properties were characterized. Simultaneous red, green and blue (RGB) emission were obtained after excitation at 970 nm. Colour emission was tuned from multicolour to white light with colour coordinate (0.32, 0.33) matching very well with the white reference (0.33, 0.33). Changes in colour emission were obtained by varying the intensity ratios between RGB bands that are strongly concentration dependent because of the interaction of co-dopants. The colour tunability, high quality of white light and high intensity of the emitted signal make these transparent glasses excellent candidates for applications in solid-state lighting.

Blue-green upconversion emission in ZrO2:Yb3+ nanocrystals

Strong blue-green cooperative upconversion emission was observed under infrared excitation in 70 nm average crystallite size ZrO2:Yb3+ nanocrystals prepared by sol-gel process. The structural characterization was performed by x-ray diffraction and high resolution transmission electron microscopy suggesting that crystalline phase of nanoparticles is controlled by active ion concentration. The cooperative absorption coefficient η∼2.5×10−22 is four orders of magnitude larger than the ones reported from bulk crystals. The highest emission intensity was obtained from the doped sample at 4 mol % and was the result of the simultaneous relaxation of two excited Yb ions. Decay time of the upconverted signal τCUC≈0.310 ms is half from the near infrared effective decay time confirming the cooperative process among Yb ions. Such strong cooperative effect is explained in terms of interaction enhancement due to the diminishing of Yb–Yb separation promoted partly by the surface recombination of nanocrystals.

Thermoluminescence and optically stimulated luminescence properties of nanocrystalline Er3+ and Yb3+ doped Y3Al5O12 exposed to β-rays

The thermoluminescence (TL) and optically stimulated luminescence (OSL) characterization of Er3+ and Yb3+ doped Y3Al5O12 nanocrystalline samples prepared by the precipitation process and exposed to β-rays are discussed. The TL as well as the OSL were two orders of magnitude higher in Er3+ doped than in Yb3+ specimens. The charge trapping and the radiative thermally stimulated recombination processes in Y3Al5O12 : Er3+ involve four trapping states at 166, 243, 342 and 424 °C, but just two trapping levels at 219 and 413 °C for Y3Al5O12 : Yb3+ at a heating rate of 10 °C s−1. The photostimulation with 470 nm light causes in both phosphors a radiative recombination of the optically free charge carriers belonging to the same trapping states. The TL and the OSL as a function of radiation dose behaviour were linear in the 10–100 Gy dose range. The results provide evidence of the potential uses of these materials in radiation storage and dosimeter devices.

Synthesis and photoluminescence of Y 2 O 3 : Yb 3+ -Er 3+ nanofibers

The influence of the solvent ratio in the hydrothermal synthesis of co-doped Y2O3:Yb3+-Er3+, via CTAB complex, is presented. The nanophosphors for two different ethanol/water solvent ratios, 1:1 and 1:3, shown the following features: the samples exhibit cubic phase, TEM image of the sample synthesized using a 1:3 solvent ratio shows a homogeneous nanofibers morphology in opposition with the sample obtained from a 1:1 solvent ratio which reveals different morphologies prevailing big irregular particles. In both cases the absorption spectra are similar as well as the upconversion emission. These results suggest that the solvent ratio plays an important role in the morphology and on the luminescence properties of the nanostructured materials.