New method for strength improvement of silica optical fibers

The reliability and the expected lifetime of optical fibers used in telecommunication technologies are closely related to the chemical environment action on the silica network. To ensure the long-term mechanical strength of the optical fibers, a polymer coating was applied onto the fiber surface during fiber fabrication. This external coating is vital to ensure a long optical fiber lifetime. Its protective action includes several functions, such as to protect glass fiber from any external damage, to limit chemical attack, in particular that of water, and finally to ensure fatigue protection and bending insensitivity, especially during handling and in-service installation. Since the mechanical strength of the fiber is controlled by its surface characteristics, we propose a new method for increasing fiber strength.The silica optical fibers used were 125 μm in diameter, with a 62.5 μm thick epoxy-acrylate coating. Fibers were rolled up around two similar cylinders. Using a screw, these cylinders moved away from one another and thus subjected the fibers to stretching. Submitted to this mechanical loading, the distended fibers were plunged into hot water at 65 or 85 °C and aged for several days. Then, the fibers were removed from the water and various weights were suspended on the fiber ends. Thus, the fibers were subjected to a tensile loading in static fatigue for several days. Just before fiber rupture, the fibers were unloaded and subjected to dynamic tensile tests at different velocities.Result analysis proved that the aging in hot water increased the fiber strength. The Weibull's diagram study shows a bimodal dispersion of defects on the fiber surface and the important role of polymer coating.     

Structural and magnetic studies of Fe2O3/SiO2 granular nanocomposites

Fe₂O₃/SiO₂ nanocomposites were synthesized by mechanical alloying, using Fe and SiO₂ powders as precursors. After 340 h milling, the sample essentially consists of hematite and amorphous silica. TEM images show hematite particles embedded in and surrounded by an amorphous silica matrix. A broad size distribution—5–50 nm—of hematite particles is found, and other group of very small—2–3 nm—unidentified particles are observed. Room temperature Mössbauer spectra show a paramagnetic doublet, which may correspond to a non-crystalline phase in the sample (probably the small unidentified particles), and a sextet corresponding to hematite. Magnetic properties were investigated by measuring hysteresis curves at different temperatures (5–300 K) and by zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves (10 mT). The hysteresis loops were well fitted by a ferromagnetic contribution. No evidence of Morin transition is found down to 5 K.     

Short wave pass and long wave pass dichroic coating at 45° on zinc selenide substrate for dual band thermal imager

In dual band thermal imager dichroic coating plays a vital role in separating 3–5 μm and 7.5–10.5 μm wavelength region for observing better image quality from two different channels. In this work a study has been carried out on the design and fabrication of short and long wave pass dichroic coating at 45° on zinc selenide flat substrate. These dichroic coated optics can be used to separate 3–5 μm (in reflection or transmission channel) and 7.5–10.5 μm (in transmission or reflection channel) wavelength region. An inhomogeneous refractive index profile which is a polynomial of 5th order was considered to design the high and low wave pass dichroic coating on zinc selenide substrate. The inhomogeneous profile was then approximated with five steps from substrate to air medium. These steps were then converted in terms of durable coating materials of six and seven layer stack for short and long wave pass dichroic coating respectively. The coating material combination used was germanium as high index material and IR-F625 as low index material. Result achieved for short wave pass dichroic filter was 94% average transmission in 3–5 μm region and 95% average reflection in 7.5–10.5 μm region. Similarly, result achieved for long wave pass dichroic filter was 95% average reflection in 3–5 μm region and 94% average transmission in 7.5–10.5 μm.     

Quantum confinement and interface structure of Si nanocrystals of sizes 3–5 nm embedded in a-SiO2

Spectroscopic ellipsometry and Monte Carlo simulations are employed to answer the fundamental question whether the energy gaps of Si nanocrystals with sizes in the range of 3–5 nm, which are embedded in amorphous silica, follow or deviate from the quantum confinement model, and to examine their interfacial structure. It is shown that the optical properties of these nanocrystals are well described by the Forouhi–Bloomer interband model. Analysis of the optical measurements over a photon-energy range of 1.5–5 eV shows that the gap of embedded nanocrystals with a mean size of 3.9 nm follows closely quantum confinement theory. A large band gap expansion (0.65 eV) compared to bulk Si is observed. The Monte Carlo simulations reveal a non-abrupt interface and a large fraction of interface oxygen bonds. This, in conjunction with the experimental observations, indicates that oxygen states and the chemical disorder at the interface have a negligible influence on the optical properties of the material in this size regime.     

Visible,near-infrared and infrared optical properties of silica aerogels

Silica aerogel is an excellent thermal insulation material with a low thermal conductivity and a high porosity and has attracted great concern in applications. This paper was to experimentally investigate the optical properties of optically thick silica aerogel in the visible, near-infrared and infrared spectrum region. The fiber-loaded silica aerogel sample was prepared through sol–gel technique and supercritical drying process. Silica fibers were added into the aerogel during the preparation procedure to strength the skeleton of aerogel. As a comparison with the fiber-load silica aerogel, a silica fiber composite sample with the same chemical component and different physical structure was also prepared. A simplified two-flux model neglecting the boundary effect was used to describe the radiation propagation characteristics inside the samples. The spectral normal-hemispherical reflectances, transmittances, and normal emittances of silica aerogel and silica fiber samples were measured and compared in the wavelengths of 0.38–15 μm. Then the spectral optical constants of samples were determined using the experimental data. The spectral absorption and scattering coefficients of silica aerogel were within (0.01 cm⁻¹, 31.0 cm⁻¹) and (1.4 cm⁻¹, 25.8 cm⁻¹). The results showed that the spectrum region where the scattering coefficient is low usually corresponds to a high absorption coefficient. In addition, the total radiation properties of samples were predicted at high temperatures. The analysis of optical properties of silica aerogel is necessary to provide valuable data in applications.     

Gain characteristics and optimization of dual-pump fiber optical parametric amplifier using two-section highly nonlinear fibers

In this paper the gain characteristics of two-pump fiber optical parametric amplifiers (FOPA) with two-section highly nonlinear fibers are analyzed numerically and the parameters of the fibers are optimized to reach broad and flat gain spectra using genetic algorithm. Different from the previous methods, here the space between two pump wavelengths and the parameter β₄ of the fibers are included as a pivotal factor in the optimization. The numerical simulation shows that using two-section practical high nonlinear fibers, the amplifier may reach 110 nm bandwidth covering 1495–1605 nm with 10.5 dB average gain and gain ripple of 0.17 dB, when the total pump power is 1 W.     

Fabrication and Gamma-Ray Irradiation Effect on Optical and Mechanical Properties of Germano-Silicate Glass Fibers with Inner Cladding of B and F Doped Silica Glasses

ABSTRACT

The effect of glass composition, pure silica glass, boron-doped and fluorine-doped silica glasses of inner cladding, in the germano-silicate glass core fibers on optical and mechanical properties upon the γ-ray irradiation was investigated. Enhanced radiation hardening at 1,550 nm was found in both the B- and F-doped fibers after the γ-ray irradiation with dose-rate of 1.25 kGy/h. The radiation-induced attenuation (RIA) was found to increase in the order of the B-doped fiber, the F-doped fiber, and the silica fiber. While no major influence on the refractive index of the fibers after the γ-ray irradiation was found, the residual stress was slightly changed.     

Photoluminescence and growth mechanism of amorphous silica nanowires by vapor phase transport

Amorphous silica [SiO_x (1<x<2)] nanowires were fabricated on silicon substrate in an acidic environment by heating the mixture of ZnCl₂, and VO₂ powders at 1100 °C. The length of SiO_x nanowires ranges from micrometers to centimeters, with uniform diameters of 10–500 nm depending on substrate temperature. Room-temperature photoluminescence spectra of the SiO_x nanowires showed two strong luminescence peaks in the red and green region, respectively. The photoluminescence was suggested to originate from nonbridging oxygen hole center (red band), and hydrogen-related species in the structure of SiO_x (green band). The study on chemical reactions and growth of the SiO_x nanowires revealed the formation process of silica nanowires in acidic environment was closely related to the vapor–solid–liquid mechanism.     

Nanoscale near-field infrared spectroscopic imaging of silica-shell/gold-core and pure silica nanoparticles

Spectroscopic near-field imaging of single silica-shell/Au-core and pure silica nanoparticles deposited on a silicon substrate is performed in the infrared wavelength range (λ = 9–11 μm) using scattering-type scanning near-field optical microscopy (s-SNOM). By tuning the wavelength of the incident light, we have acquired information on the spectral phonon–polariton resonant near-field interactions of the silica-shell/Au-core and pure silica nanoparticles with the probing tip. We made use of the enhanced near-field coupling between the high index Au-core and the probing tip to achieve spectral near-field contrast of the thin silica coating (thickness < 10 nm). Our results show that spectroscopic imaging of thin coating layers and complex core–shell nanoparticles can be directly performed by s-SNOM.     

Lasing characteristics of Rhodamine B and Rhodamine 6G as a sensitizer in sol–gel silica

We report lasing characteristics of Rhodamine B (Rh. B) in sol–gel silica under excitation with frequency-doubled Nd:YAG laser and sensitization with Rhodamine 6G (Rh. 6G). The principle of radiative energy transfer (from Rh. 6G to Rh. B) has been utilized as a longitudinally Rh. 6G laser (at 585 nm)-pumped Rh. B laser process in the same sample. Rh. B offers a high photostable and efficient laser dye in sol–gel silica sensitized with Rh. 6G; 75,000 shots as a laser half-lifetime of the sample and 24% efficiency at pumping intensity 0.1 J/cm² of 532 nm. Wavelength shift occurs from 606 to 630 nm in the Rh. B laser with increasing its concentration from 1×10⁻⁴ to 8×10⁻⁴ M. The measured optical gain for Rh. B sensitized with Rh. 6G in sol–gel silica is higher than that in ethanol. A new effect has been observed; at 1×10⁻⁴ M of Rh. B and 0.5×10⁻⁴ M of Rh. 6G mixture, the emitted color of laser is changed by changing the pump intensity of frequency-doubled Nd:YAG laser.     

The microstructure, optical, and electrical properties of sol–gel-derived Sc-doped and Al–Sc co-doped ZnO thin films

Transparent conductive ZnO:Al–Sc (1:0.5, 1:1, 1:1.5 at.% Al–Sc) thin films were prepared on glass substrates by sol–gel method. The microstructure, optical, and electrical properties of ZnO:Sc and ZnO:Al–Sc films were investigated. Results show that Sc-doping alone obviously decreases grain size and degrades the crystallinity; there is an amorphous phase on the surface of ZnO grains; the transmittance spectra fluctuate dramatically with a large absorption valley at about 430–600 nm. However, Al–Sc co-doping can stabilize grain size and improve the microstructure; an average visible transmittance of above 73% is obtained with no large absorption valley; the amorphous phase does not appear. The optical band gaps of ZnO:Sc and ZnO:Al–Sc films (3.30–3.32 eV) are blue-shifted relative to pure ZnO film (3.30 eV). Hall effects show that the lowest resistivity of 2.941 × 10⁻² Ω cm and the maximum Hall mobility of 24.04 cm²/V s are obtained for ZnO:Al–Sc films while ZnO:Sc films do not exhibit any electrical conductivity. Moreover, there is an optimum atomic ratio with Al to Sc of 1:0.5–1 at.%. Although the resistivities are increased compared with that of ZnO:Al film, the Hall mobilities are raised by one order of magnitude.     

Design and fabrication of ultra broadband infrared antireflection hard coatings on ZnSe in the range from 2 to 16 μm

In conventional infrared multilayer antireflection coatings (MLAR) materials of fluoride and chalcogenide types are used, which are disadvantaged due to their low mechanical strength and poor stability against humidity and environmental impacts. In this paper, we show that high performance ultra broadband and hard infrared multilayer antireflection coatings on ZnSe substrates in the wavelength range from 2 to 16 μm can be designed and fabricated. Diamond-like carbon (DLC) hard coating as a mechanical and environmental protection layer was proposed and deposited onto MLAR surfaces (MLAR + DLC) using a pulsed vacuum arc ion deposition technique. The thickness of the high optical quality DLC can be optimized in the design simulation to achieve a practically best antireflection and surface protection performance. We show that a germanium thin film (15 nm) between the MLAR and DLC surfaces can be used as a transition layer for optical and material match. The average transmission of the fabricated MLAR+DLC surfaces was 93.1% in the wavelength range between 2 and 16 μm. The peak transmission was about 97.6%, close to the simulated values. The durability and stability against mechanical impacts and environmental tests was improved significantly compared with the conventional infrared windows.     

The effect of cerium oxide argon-annealed coatings on the high temperature oxidation of a FeCrAl alloy

Ceria coatings were applied in order to improve the adherence of alumina scales developed on a model Fe–20Cr–5Al alloy during oxidation at high temperature. These coatings were performed by argon annealing of a ceria sol–gel coating at temperatures ranging between 600 and 1000 °C. The influence of these coatings on the alloy oxidation behaviour was studied at 1100 °C. In situ X-ray diffraction (XRD) was performed to characterize the coating crystallographic nature after annealing and during the oxidation process. The alumina scale morphologies were studied by means of scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The present work shows that the alumina scale morphology observed on cerium sol–gel coated alloy was very convoluted. On the cerium sol–gel coated alloy, argon annealing results in an increase of the oxidation rate in air, at 1100 °C. The 600 °C argon annealing temperature results in a good alumina scale adherence under thermal cycling conditions at 1100 °C.     

Superhydrophobic silica films by sol–gel co-precursor method

Wetting phenomena of water droplets on solid are of crucial concern in our daily life as well as in engineering and science. The present paper describes the room temperature synthesis of superhydrophobic silica films on glass substrates using trimethylethoxysilane (TMES) as a co-precursor. The coating sol was prepared by keeping the molar ratio of tetraethoxysilane (TEOS) precursor, methanol (MeOH) solvent, water (H₂O) constant at 1:38.6:8.68, respectively, with 2 M NH₄OH throughout the experiments and the TMES/TEOS molar ratio (M) was varied from 0 to 1.1. It was found that with an increase in M value, the hydrophobicity of the films increased, however the optical transmission decreased from 88% to 82% in the visible range. The hydrophobic silica films retained their hydrophobicity up to a temperature of 275 °C and above this temperature the films became superhydrophilic. The hydrophobic silica films were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared (FT-IR) spectroscopy, percentage of optical transmission, humidity test and static and dynamic contact angle measurements.     

Temperature and fiber optic spectroscopy composition measurements of heated propanol–acetone droplets

Time-dependent temperatures and compositions within individual fiber-supported droplets initially from about 2–3 mm in diameter were investigated. In the experiments, droplets were composed of mixtures of 1-propanol and acetone. The droplets evaporated in room air, where the air was heated by placing an electrically heated coil underneath the droplets. The experiments employed thin optical fibers to carry light from a UV–vis light source into and out of a droplet. The time-dependent UV absorption spectrum of the liquid between the fiber ends was measured using a spectrometer coupled to one of the fibers. This spectrum yielded real-time information on the composition of the liquid. Droplet temperatures were simultaneously measured using a thermocouple that was immersed into the liquid. Results demonstrate that droplet evaporation follows a multi-stage process and that acetone is preferentially gasified from a droplet.     

A novel CO2 gas analyzer based on IR absorption

Carbon dioxide (CO₂) gas analyzer can be widely used in many fields. A novel CO₂ gas analyzer based on infrared ray (IR) absorption is presented sufficiently in this paper. Applying Lambert–Beer Law, a novel space-double-beam optical structure is established successfully. The optical structure includes an IR source, a gas cell, a bandpass filter with a transmission wavelength at 4.26 μm, another bandpass filter with a transmission wavelength at 3.9 μm, and two IR detectors. Based on Redial Basic Function (RBF) artificial neural network, the measuring model of IR CO₂ analyzer is established with a high accuracy. A dynamic compensation filter is effectively designed to improve the dynamic characteristic of the IR CO₂ analyzer without gas pump. The IR CO₂ analyzer possesses the advantages of high accuracy and mechanical reliability with small volume, lightweight, and low-power consumption. Therefore, it can be used in such relevant fields as environmental protection, processing control, chemical analysis, medical diagnosis, and space environmental and control systems.     

Density-modulated alumina-based nanometre films prepared by sol–gel technique

Alumina-based nanometre films produced by a sol–gel method are investigated by wide-angle X-ray scattering and X-ray reflectometry. Every dipping cycle of the sol–gel route results in formation of an X-ray amorphous film exhibiting a density gradient. Both withdrawal speed and drying temperature used as tuning parameters during coating preparation influence the thickness and density of the sol–gel film. An increase of the withdrawal speed results in a thicker film of a lower mean density. An opposite effect is observed with an increase of the drying temperature. The appearance of multilayer Bragg reflections in the reflectivity curves of successive stacked films is discussed in detail. The thermal stability of the density-modulated aluminium oxide-based nanometre multilayers is investigated in a temperature range of 300–1000 °C. Possible applications as X-ray optical elements are outlined. PACS  61.10.Kw; 68.60.Dv; 81.20.Fw     

Electrical properties and structure of thin chromium films

The temperature dependence of the electrical resistance of thin chromium films produced by vacuum condensation at 1· 10^–4 mm Hg on mica, optical glass, and rock salt substrates is considered. The electrical resistance of chromium films condensed on substrates heated to temperatures below 450 °C increases irreversibly on subsequent heating and cooling. Chromium films condensed on substrates heated to 500–600 °C, however, retain stable electrical properties on repeatedly heating and cooling.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 56–60, September, 1971.     

Power transfer characteristics among N parallel single-mode optical fibers

Based on the coupled-mode theory, the power transfer among “- - -” arranged parallel single-mode optical fibers has been investigated. The analysis shows that the distances between each two of the N fibers centers have effects on the coupling coefficient and power transfer. The solution of the coupled equations for three parallel single-mode optical fibers is given, and is studied for different initial conditions comparatively. Numerical simulations show that power transfer will be periodical during coupling among parallel single-mode optical fibers. These results can be extended to multi-parallel single-mode optical fibers.     

Particle size effect on porous Sol–Gel doped cladding U-shaped optical fiber critical micelle concentration (CMC) sensor

In this paper, a sol–gel derived coating U-shaped optical fiber sensors for measurement of critical micelle concentration of surfactants has been studied. The doped sol–gel cladding fiber was used to construct an active cladding U-shaped optical fiber CMC measurement sensor. The porous sol–gel film was deposited a 1000 m core plastic clad silica fiber after removing the cladding of the fiber at the middle portion nearly 6 cm and bending in U-shaped design. CMC detection is based on an adsorption effect in sample solution of sodium dodecylbenzenesulfonate. Three different types of particles size of sol–gel porous cladding were tested in our lab. Test results show that the sensitivity of the small particle size porous sol–gel cladding U-shaped optical fiber in sensing region is higher than that of large particle size sol–gel cladding based U-shaped. PACS 42.81.Bm; 42.81.Pa