A. V. Kir’yanov

Resonant and thermal changes of refractive index in a heavily doped erbium fiber pumped at wavelength 980 nm

We report a theoretical and experimental study of the refractive index variation in a heavily doped erbium silica fiber within the spectral range 1500–1580nm under the pumping at the wavelength 980nm. The two main contributions in the refractive index change are addressed—the resonant part determined by the saturation effect in the fiber and the thermal part stemming from the fiber heating due to the excited-state absorption and Stokes losses. We demonstrate that the thermal contribution in the resultant refractive index change is a notable value, which is the feature of erbium fibers with a high concentration of erbium ions.

Powerful visible (530–770 nm) luminescence in Yb,Ho:GGG with IR diode pumping

Powerful visible luminescence in a Gadolinium Gallium Garnet (GGG) crystal, co-activated with Yb3+ (~15 at.%) and Ho3+ (~0.1 at.%) ions, is investigated under CW laser diode pumping (lambda = 938 and 976 nm). The main visible emission band is observed in the green with its peak at lambda ~540 nm) and measured to be about 10% with respect to Yb3+ IR luminescence (lambda ~1000 nm). Red (lambda ~650 nm) and near-IR (lambda ~755 nm) emission bands are also observed but are weaker (about 3-5%). Analysis of the crystal absorption and luminescence spectra allows one to conclude that Yb3+ - Ho3+ stepwise up-conversion is the mechanism explaining the phenomenon. Ho3+ ions embedded in the crystal in small concentration are shown to form an effective reservoir for energy transferred from the excited Yb3+ subsystem and to be an efficient source of the visible emission.

Excited-state absorption in erbium-doped silica fiber with simultaneous excitation at 977 and 1531 nm

We report a study of the excited-state absorption (ESA) in erbium-doped silica fiber (EDF) pumped at 977 nm, when the fiber is simultaneously excited by signal radiation at 1531 nm. We show, both experimentally and theoretically, that ESA efficiency at 977 nm gets strongly enhanced only in the presence of signal power. Experimentally, this conclusion is supported through the detection of upconversion emission, a “fingerprint” of the ESA process, and through the measurements of the EDF nonlinear transmission coefficient for the pump wavelength, which is sensitive to the ESA value. It is shown that the experimental data are precisely modeled with an advanced five-level Er3+ model developed for the EDF.

Spectral dependence of the excited-state absorption of erbium in silica fiber within the 1.48–1.59μm range

We report the study of the excited-state absorption (ESA) spectrum of Er3+ in erbium-doped silica fiber (EF) within the telecom band, 1.48–1.59μm. Two experimental techniques, the detection of up-conversion emission following the ESA process and the measurement of the EF nonlinear transmission coefficient, both supplied with a theoretical background, allow obtaining a mutually consistent spectral behavior of the ESA parameter, which is the ratio of ESA to ground-state absorption cross section, at 1.48–1.59μm excitation.

Nonsaturable absorption in alumino-silicate bismuth-doped fibers

We report an experimental and theoretical investigation of fluorescence decay and transmission coefficient at ∼1 μm pumping for a series of bismuth (Bi) doped alumino-silicate fibers with different concentrations of Bi centers. By modeling the experimental data, we show that the excited-state absorption (ESA) and up-conversion (UC) processes are responsible for a growth of nonsaturable absorption and deviation from exponential the fluorescence decay in the fibers with an increase in Bi centers content. Assuming that Bi centers ensemble is composed of subsystems of single and paired centers, and an increase in partial weight of the latter with a Bi centers concentration growth, we can successfully explain the experimental data. Also, these assumptions allow us to estimate the constants characterizing the ESA and both homogeneous and inhomogeneous UC processes in the fibers.

Ground-state absorption saturation and thermo-lensing effect as main sources of refractive index non-linear change in Cr4+:YAG at CW 1.06 μm excitation

Non-linear change of refractive index in Cr 4þ :YAG is investigated experimentally by the Z-scan technique (CW case) at wavelength 1.06 lm. Theoretical analysis of the processes resulting in non-linear refraction in Cr 4þ :YAG allows one to conclude that the main contributions in non-linear change of the crystal refractive index are the effect of Cr 4þ centers population perturbation due to ground-state (GS) absorption saturation and the thermo-lensing effect. 2002 Published by Elsevier Science B.V.

Fabrication and characterization of new Yb-doped zirconia-germano-alumino silicate phase-separated nano-particles based fibers

New zirconia-germano-alumino silicate, nano-particles based, Ytterbium doped fibers are obtained through the conventional modified chemical vapour deposition and solution doping techniques. The start fiber preforms are characterized by means of electron micro probe, energy dispersive x-ray, and electron diffraction analyses, revealing the presence of phase-separated nano-sized Ytterbium-rich areas in the core, while the final fibers are inspected in the sense of spectroscopy and laser properties.

Phase-modulated beams technique for thin photorefractive films characterization

The phase-modulated beams technique is developed for nonlinear thin photorefractive films characterization. In the Raman–Nath diffraction approximation, the formulas are deduced, allowing us to measure the amplitude of phase grating recorded in a film and its nonlinear refractive index n2. The method is applied for studying Langmuir–Blodgett multilayer thin (∼0.6 μm) films of Bacteriorhodopsin at wavelength 633 nm.

Ce-doped and Ce/Au-codoped alumino-phospho-silicate fibers: Spectral attenuation trends at high-energy electron irradiation and posterior low-power optical bleaching

We report a study of attenuation spectra’ transformations in a couple of Cerium (Ce) doped alumino-phospho-silicate fibers (one of them codoped with gold (Au)), occurring under irradiation by a beam of high-energy β-electrons. The experimental data reveals an essential effect of β-irradiation upon the absorptive properties of the fibers, given by noticeable susceptibility of Ce ions in Ce3+/Ce4+ valence states to the treatment, arisen as growth followed by saturation of induced absorption. We also report posterior bleaching of the β-darkened fibers, also in terms of attenuation spectra’ transformations, at exposing them to low-power green (a He-Ne laser) and UV (mercury lamp) light, the effect never reported for Ce-doped fibers. It is shown that both phenomena are less expressed in Ce fiber codoped with Au than in Au-free one and that the spectral changes in the former are more regular and plain vs. irradiation dose and bleaching time. Possible mechanisms responsible for the phenomena and their impact at using such fibers for dosimetry and other applications are discussed.

Enhancing type-II optical second-harmonic generation by the use of a laser beam with a rotating azimuth of polarization

We propose a frequency converter which exploits laser pulses with changing state of polarization. The use of such pulses as a pump source for an extracavity doubling crystal is shown to result in the large conversion efficiency. These pulses can also provide a fairly good tool of monitoring the shape of the harmonic pulse. It is shown that temporal variations of the polarization direction experienced by the pump pulse in this system may cause considerable shortening of the harmonic pulse.