Measurement of the refractive index of glass fibers by the Christiansen-Shelyubskii method

The Christiansen principle was employed to measure the refractive index of borosilicate glass fibers (13–41 μm diameter) over the visible range. The refractive index for glass fibers at 589.3 nm was measured by temperature and wavelength scan and values obtained were in close agreement. The refractive index for glass fibers as a function of wavelength was measured to an accuracy of < 10⁻⁴. The uniformity of the refractive index for a bundle of fibers of slightly different diameter was calculated using the modified Shelyubskii method and compared to experimental values. Theoretical calculations of the transmission by the present work suggest that, for high optical clarity and transmission of Christiansen cell (or transparent composite consisting of glass fiber and polymer), the refractive index must be controlled to the fifth decimal place. For example, the maximum transmission of a fiber/liquid mixture cell at 25°C can increase from 89 to 97% when the standard deviation is reduced from 13 × 10⁻⁵ to 9 × 10⁻⁵.     

Investigation of semi-insulating oxygen-doped GaAs

We found Oxygen-doped GaAs crystals to be suitable materials for CO₂ laser optical component preparation, with application at 10.6 μm. An optical transmission of 55% in the IR spectrum range, between 2 and 15 μm has been reached for such a GaAs type material. The GaAs crystals that we have analysed were grown by two procedures: Horizontal Bridgman (HB) and Liquid Encapsulated Czochralski (LEC). The HB method has been used for obtaining pure (undoped) crystals, while the oxygen-doped GaAs ingots were grown by LEC technique. The two types of samples processed in the same manner as regards mechanical polishing and chemical etching, which were investigated by Hall measurements, optical transmission spectrometry and elastic recoil detection analysis (ERDA) technique. The GaAs:O (LEC) has near semi-insulating properties as can be observed from the results of the electrical resistivity and Hall effect measurements. The ERDA spectrum shows an intense signal of oxygen in the bulk of GaAs:O (LEC) crystals, while the oxygen signal is not present in the ERDA spectrum of the undoped GaAs (HB). We consider that these results could recommend the ERDA technique as a possible qualitative and quantitative analysis in an ion-beam accelerator for oxygen content in oxygen-doped GaAs crystals. The analysis is not sensitive to the native oxide, as could be seen by measuring GaAs (HB) undoped crystals.     

Growth kinetics of sodium perborate from batch crystallization

The size distribution of sodium perborate crystals was continually monitored using a Malvern sizer during batch crystallization from aqueous solutions carried out under falling supersaturation established at the experiment onset. The growth rate was determined from the time shift of the crystal size distribution expressed in cumulative oversize numbers. The size independent overall growth rate was first order with respect to supersaturation for crystals larger than 150 μm. Crystals between 20 and 150 μm exhibited a significant size-dependent growth rate. Furthermore, the fraction of crystals smaller than 20 μm, formed by primary nucleation, grew extremely slowly or did not grow at all.     

Fabrication, spectroscopy and laser performance of Nd-doped lead-silicate glass fibers

The fabrication of a highly efficient Nd³⁺-doped single-mode fiber laser operating at 1.06 μm is described. The Nd₃₊ is introduced by doping Nd₂O₃ into a multicomponent (flint) lead-silicate glass host, Schott commercial optical glass F7. A fabrication technique for doping rare-earth evenly into commercial optical glasses is demostrated. Spectroscopic properties relevant to laser operation in the Nd³⁺-doped lead-silicate glass fibers were measured and the influence of Pb₂₊ ions on the spectral properties was analyzed. Owing to the long lifetime and large absorption and emission cross-sections of Nd₃₊ in this lead-silicate glass host, a high-performance Nd³⁺-doped lead-silicate fibre laser device operating at 1.06 μm has been successfully demonstrated.     

Fabrication of fluorine-doped silica glasses by the sol-gel method

Fluorine-doped silica glasses are produced by the sol-gel method for optical fiber preforms. In order to dope fluorine into silica glass, fluorinated silicon alkoxide, Si(OC₂H₅)₃F, is titrated into SiO₂ sol solutions. The fluorine content in silica glass depends on: the fluorine concentration in the gel, the specific surface area of SiO₂ particles and the heating rate in the sintering process. Fluorine-doped silica glass with a maximum relative refractive index difference of −0.93% is obtained. Using this technique, optical fibers with a triangular refractive index profile are fabricated with a minimum optical loss of 1.6 dB/km at 1.69 μm wavelength.     

Growth of Tb3Ga5O12 fiber and bulk crystals using micro-pulling-down apparatus

A micro-pulling-down process, using Ir crucibles and RF heating, has been used to grow single-crystal fiber and bulk crystals of Tb₃Ga₅O₁₂ garnet (TGG). Single crystals ranging up to 450 mm in length have been produced. The crystals were 1–4 mm in diameter and were seeded-grown in the direction close to 1 1 1. The maximal crystal diameter achieved was 10 mm. Dependence of behavior of the solid–liquid interface on the growth parameters (temperature and pulling-rate) is discussed in detail.     

Growth of laser single-crystals Er:YVO4 by floating zone method

High optical quality Er³⁺:YVO₄ laser crystals have been grown by using the floating zone method (FZ). The spectroscopic properties and 3 μm lasing of Er³⁺:YVO₄ were investigated. It is found that the Er³⁺ concentration has a negative effect on the emission of the transition ⁴I_13/2→⁴I_15/2(1.55 μm ), and a positive effect on that of the ⁴I_11/2→⁴I_13/2 transition (2.68 μm). With direct upper-state pumping and a plane-concave cavity a self-terminating laser was achieved at the wavelength of 2.724 μm in the 30 at% Er³⁺ doped sample.     

Effect of Yb content on purity and crystal growth of Ba2NaNb5O15

Growth by the micro-pulling-down technique (μ-PD) of homogeneous and crack-free fiber single crystals with composition Ba₂Na_1−xYb_xNb₅O₁₅ (0<x<0.08) is reported. The effect of Yb³⁺ addition to barium sodium niobate (BNN, x=0), having the tungsten bronze-type structure, was examined by room temperature X-ray powder diffraction and differential thermal analysis coupled to thermogravimetry. In the region of the monophased field the structure is tetragonal from x=0.02 to 0.16. Volume change is mainly by variation of the a-axis length. Addition of Yb³⁺ to BNN could be effective for the production of high optical quality, crack-free, bulk crystals by the Czochralski technique.     

Crystal growth of Ce: PrF3 by micro-pulling-down method

Micro-pulling-down (μ-PD) growth apparatus was modified for fluoride crystals. PrF₃ was grown with various concentrations of Ce³⁺ from 0–100%. The crystals were transparent and colorless (CeF₃) or greenish and 3 mm in diameter and 15–50 mm in length. Neither visible inclusions nor cracks were observed. Radioluminescence spectra and decay kinetics were measured for the sample set at room temperature. In comparison to the Czochralski or Bridgman method, the μ-PD method allows to produce single crystalline material in a faster thus more economic way. Once it is established for the fluoride crystals, it is an efficient tool for exploring the field of new functional fluorides.     

InP/InGaAlAs distributed Bragg reflectors grown by low-pressure metal organic chemical vapor deposition

Long-wavelength vertical cavity surface emitting lasers (VCSELs) are considered the best candidate for the future low-cost reliable light sources in fiber communications. However, the absence of high refractive index contrast in InP-lattice-matched materials impeded the development of 1.3–1.5 μm VCSELs. Although wafer fusions provided the alternative approaches to integrate the InP-based gain materials with the GaAs/AlAs materials for their inherent high refractive index contrast, the monolithic InP-based lattice-matched distributed Bragg reflectors (DBRs) are still highly attractive and desirable. In this report, we demonstrate InP/InGaAlAs DBRs with larger refractive index contrast than InP/InGaAsP and InAlAs/InGaAlAs DBRs. The switching between InP and InGaAlAs layers and growth rate control have been done by careful growth interruption technique and accurate in situ optical monitoring in low-pressure metal organic chemical vapor deposition. A 35 pairs 1.55 μm centered InP/InGaAlAs DBRs has the stopband of more than 100 nm and the highest reflectivity of more than 99%. A VCSEL structure incorporating 35 pairs InP/InGaAlAs DBR as the bottom mirror combined with a 2λ thick periodic gain cavity and 10 pairs SiO₂/TiO₂ top dielectric mirrors was fabricated. The VCSELs lased at 1.56 μm by optical pumping at room temperature with the threshold pumping power of 30 mW.     

Optical properties of the optical fiber containing Co doped ZnO-Al2O3-SiO2 glass-ceramics

Optical characteristics of silica glass optical fibers containing Co²⁺ doped ZnO-Al₂O₃-SiO₂ (ZAS) glass-ceramics prepared by slurry-doping method were investigated. The absorption and emission bands of the fibers were found to be originated from the tetrahedral Co²⁺ in ZnAl₂O₄ crystals in ZAS glass-ceramics particles embedded in the core of the fibers. The crystal field strength of the Co²⁺ ions in the optical fiber was found to be smaller than that of the Co²⁺ ions in the bulk ZAS glass-ceramics.     

Vapor phase epitaxy of pure VO2 and V1−xCrxO2

The vapor phase epitaxy of thin epilayers of VO₂ and V_1−xCrxO₂ on TiO₂ transparent substrates is described. Chemical vapor deposition occurs by reacting a (VOCL₃/CrO₂Cl₂/H₂O/H₂) mixture at about 800°C using argon as a carrier gas. The preparation of pure VO₂ requires special care to make it homogeneously stoichiometric and to obtain steep concentration profiles at the TiO₂/VO₂ interface. Layers were obtained which had electrical and optical properties comparable to the best bulk crystals grown by other techniques. Homogeneous solid solutions of V_1−xCr_xO₂ epilayers were also grown for the first time in the range o < x < 0.17. Chromium concentration and homogeneity were determined by electron microprobe analysis. The separation coefficient k was also found to vary with x. It is close to unity below x = 0.001 and above this value Cr is incorporated more easily. High quality heteroepitaxial layers (1 cm² area, 1 to 30 μm thickness) of V_1−xCr_xO₂ have for the first time allowed the measurement of the optical absorption coefficient.     

Infrared fibers based on Te-As-Se glass system with low optical losses

Unclad optical fibers based on high-purity Te-As-Se glasses prepared by chemical and physical methods of purification have been drawn. The optical, thermal and mechanical properties of glasses and fibers were investigated. The minimum optical losses were 0.07 dB/m at 7.3 μm for Te₂₅As₄₀Se₃₅ glass fiber and 0.04 dB/m at 6.7 μm for Te₂₀As₃₀Se₅₀ glass fiber. Sixty five percent of input power of a tunable CO₂ laser emitting at 9.3 μm was transmitted through a 1 m long fiber with diameter of 900 μm.     

Wavelength selective materials modification of bulk As2S3 and As2Se3 by free electron laser irradiation

In this study we report first measurements of wavelength-selective infrared-induced materials modification of bulk As₂S₃ and As₂Se₃. These materials are currently being considered as candidate materials for infrared optical fiber transmission in the range of 1–10 μm. Our study is aimed at modifying oxygen, hydrogen and carbon impurities bound to chalcogenide constituent elements in the materials to reduce absorption. Tunable infrared radiation from the W.M. Keck Free Electron Laser (FEL) at Vanderbilt was used to excite specific vibrational modes, S–O–H and CH_x modes in bulk As₂S₃ and Se–H, CH_x and S–H₂ modes in bulk As₂Se₃. Changes in vibrational mode amplitudes are monitored by measuring the intensity of the Fourier transform infrared (FTIR) spectra before and after irradiation at appropriate wavelengths. By tuning wavelengths to hydrogen vibrational modes, we find evidence that hydrogen is released and/or redistributed athermally. In particular, following irradiation at specific resonant wavelengths, vibrational mode amplitudes as monitored by FTIR associated with CH_x are significantly reduced in bulk As₂S₃ and As₂Se₃ samples. In As₂S₃, the changes in CH_x modes are reversed by heat treatment at 115°C for 35 min in nitrogen atmosphere.     

Viscosity matching of new PbF2–InF3–GaF3 based fluoride glasses and ZBLAN for high NA optical fiber

New multicomponent PbF₂–InF₃–GaF₃ bulk glasses have been investigated. They show lower phonon energy (540 cm⁻¹) in comparison with 580 cm⁻¹ for ZBLAN. Large PbF₂ concentration provided glasses with high refractive index up to 1.582 and the viscosity curves revealed an excellent thermal compatibility with ZBLAYN glass. A multimode fiber with a numerical aperture of 0.51, a loss of 0.85 dB/m at 1.3 μm was fabricated using the rotational casting method.     

Growth of nanofibrous barium carbonate on calcium carbonate seeds

Fibrous barium carbonate (BaCO₃/witherite) crystals 50–100 nm in diameter and several microns in length were grown on calcium carbonate (CaCO₃) seeds at temperatures as low as 4 °C. The BaCO₃ fibers were deposited onto calcite rhombs or CaCO₃ films using the polymer-induced liquid-precursor (PILP) process, which was induced with the sodium salt of polyacrylic acid (PAA). The structure and morphology of the resultant fibers were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and polarized light microscopy (PLM). Fibers were successfully grown on calcite seeds of various morphologies, with a range of barium concentrations, and PAA molecular weight and concentration. Two categories of fibers were grown: straight and twisted. Both types of fibers displayed single-crystalline SAED diffraction patterns, but after examining high-resolution TEM lattice images, it was revealed that the fibers were in fact made up of nanocrystalline domains. We postulate that these nanocrystalline domains are well aligned due to a singular nucleation event (i.e., each fiber propagates from a single nucleation event on the seed crystal) with the nanocrystalline domains resulting from stresses caused by dehydration during crystallization of the highly hydrated precursor phase. These BaCO₃ fibers grown on calcite substrates further illustrate the robustness and non-specificity of the PILP process.     

Properties of ZrO2---Al2O3 composite powders prepared from Zr---Al metallo-organic compounds

Zr---Al metallo-organic compounds (zircoaluminates), having (CH₂)₄COOH, (CH₂)₁₂CH₃ and (CH₂)₂NH₂ as the organofunctional groups, were treated preliminary by (1) spray-drying, (2) gelation of addition of 10% NH₄OH aqueous solution followed by spray-drying and (3) rotary evaporation under a reduced pressure. After the treatment they were heated in air to prepare ZrO₂---Al₂O₃ composite powders. The IR and DTA profiles for the treated compounds indicated that the procedures modified the structures for the zircoaluminates. The stability of tetragonal ZrO₂ for the ZrO₂---Al₂O₃ composite powder were dependent on the modification in the structure for the zircoaluminates. Balloon shaped particles, 0.5–2 μm in diameter, were obtained through procedure (1) and spherical particles, 1–4 μm in diameter, through (2). Tetragonal ZrO₂ grains, 0.1–0.2 μm in diameter, were dispersed in the particles when heated at 1400°C.     

Nanoindentation of glass wool fibers

The nanoindentation technique is used to analyze the depth dependence of the hardness and the reduced elastic modulus of bulk glasses and glass wool fibers (4-12 μm in diameter) of calcium aluminosilicate composition. In spite of the fiber geometry and the delicate sample mounting-technique, nanoindentation proves to be a relatively accurate method that provides reproducible data for both hardness (H) and reduced elastic modulus (E_r) of thin glass fibers. It is found that H and E_r are generally lower for the fiber than for the bulk sample. Within a given fiber, both H and E_r are approximately constant with increasing indentation depth. However, both of these parameters decrease with diminishing fiber diameter. This trend is attributed to an increase of the free volume of the fibers with decreasing fiber diameter, i.e. to an increase of the fictive temperature.     

Growth of heavily C-doped GaAs/AlGaAs MQW structures by MOVPE for 2–3 μm normal incidence photodetectors

The growth and intersubband optical properties of high quality heavily doped p-type GaAs/AlGaAs multiple quantum well (MQW) structures are reported. The MQWs were fabricated by the atmospheric pressure metalorganic vapor phase epitaxy process using liquid CCl₄ to dope the wells with C acceptors (N_a ≈ 2 × 10¹⁹ cm⁻³). A constant growth temperature was maintained for the entire structure while different V/III ratios were used for the well and barrier regions. By this process it is possible to achieve both high C doping densities in the wells and to simultaneously obtain good quality AlGaAs barriers. Fourier transform infrared spectroscopy measurements on heavily doped 10-period MQW structures reveal a new absorption peak at 2 μm with an effective normal incidence absorption coefficient of 4000 cm⁻¹. Photocurrent measurements on mesa-shaped diodes show a corresponding peak at 2.1 μm. The photodiodes exhibit a symmetrical current-voltage characteristic and a low dark current, which are indicative of a high quality MQW structure and a well-controlled C doping profile. The 2 μm absorption represents the shortest wavelength ever reported for any GaAs/AlGaAs or InGaAs/AlGaAs MQW structure and should be very useful for implementing multicolor infrared photodetectors.     

Operating Limits for Stable Growth of Silicon Fibers with Diameter Less Than 150 μm by Modified μ-PD Method

The μ-PD method originally developed for oxide crystals has been modified and applied for filamentary silicon crystal growth. Our main modification of μ-PD method is concerned with an arrangement of melt permeable feeder which is inserted into the nozzle. The feeder finishes by a sharp tip the diameter of which (is almost the same as that of the desired semiconductor fiber, i.e., less than 150 μm. Silicon fibers were grown from the small liquid pool at the end of the feeder. Three types of crucible-die arrangement were designed and tested. The best results were obtained with the help of inclined insert made of graphite fibers because of its ability to quench oscillations and longer operation life. Fiber crystals, 100 μm in diameter and 70 mm in length, have been grown successfully. Small meniscus stability, operating limits of μ-PD method and silicon carbide formation during the growth process are discussed.