Miroslav Chomat

Optical fibre sensor coated with porous silica layers for gas and chemical vapour detection

Fibre optic sensors have been conceived in new technologies for detecting very small amounts of chemical, gaseous and biological species. Recently, new polymer-clad silica (PCS) fibres coated with thin porous silica layers have been developed. These porous layers have been prepared by the sol-gel method using the dip-coating technique. In this work, this new material has been investigated for detecting gases and vapours. By choosing a fixed incident angle, variation of light power transmitted through the fibre is detected as the vapour or gas to be detected is sorbed in the porous silica layer. Good sensitivity and reversibility have been obtained for this system. Moreover, both the response time and the desorption time have been found less than 2 min. Chlorinated hydrocarbons such as trichloroethylene, carbon tetrachloride, chloroform, dichloromethane and alkanes such as propane, butane and hexane can be detected with detection limits of 0.6, 1.5, 1.7, 4, 25, 10 and 5% respectively.

Development of organically modified polysiloxanes for coating optical fibers and their sensitivity to gases and solvents

Abstract Materials for coating silica optical fibers which are UV-curable and consist of a siloxane chain modified with reactive acrylate groups are prepared by the sol-gel method. The value of the refractive index of these organically modified polysiloxanes (ORMOCERs) is controlled by chemical modification of input alkoxysilane precursors in the range 1.48 to 1.56. Several sensing structures based on silica optical fibers coated with the ORMOCERs have been designed and fabricated. Fibers with a core of multicomponent glass, with pure silica core, with pure silica core coated with a thin porous Si-O-Ti sol-gel layer and with a composite optical core have been fabricated and characterized by measuring their attenuation. The sensitivity of the ORMOCER coatings to solvents and to CO 2 dissolved in water has been determined in immersion experiments. A new absorption band around 320 nm arising from interaction of the ORMOCERs with SO 2 has been observed, which causes the sensitivity of the ORMOCER layer to gaseous SO 2 .

Optical fiber with novel geometry for evanescent-wave sensing

Abstract First results in the preparation and analysis of an optical fiber with a novel geometry which facilitates the access of chemical species to the evanescent field for sensing purposes are presented. This ‘s-fiber’ is of approximately sectorial cross section with the core located in the carefully rounded vertex of the sector. Using a perturbation method, the dependence of the attenuation coefficient of the fundamental mode in a weakly-guiding, step-index s-fiber on the fiber normalized frequency, vertex angle and cladding thickness are determined. Attenuation coefficients several times higher than in D-fibers are theoretically attainable. Preforms for drawing s-fibers are prepared from standard MCVD preforms by accurate grinding and polishing the preforms to a desired sectorial shape. Multimode s-fibers with core dimension of about 30 μm and cladding size of about 170 μm and exhibiting satisfactory strength have been drawn. Resulting shapes of the fiber and core depend on the shape, structure and composition of the preform, drawing temperature and drawing velocity. Results have proved the feasibility of the chosen approach to the laboratory preparation of s-fibers. In preliminary experiments the sensing ability of the drawn fibers has been examined.

Consequences of TiO2 doping on the optical properties of porous silica layers coated on silica optical fibers

Abstract The paper deals with the investigation of optical properties of porous silica and TiOTi chains – doped porous silica layers coated on silica optical fibers. The porous layers composed of dried gels have been prepared by the sol–gel method from alkoxides. The refractive indices and optical attenuation of the layers have been determined from the measured angular distributions of the output optical power. For this purpose, a model based on geometrical optics has been developed. The interaction of the porous layers with benzene, toluene, xylene and hexane has been investigated by measuring the changes of the output optical power induced by the effect of vapors of the hydrocarbons in air. On the basis of these measurements, the porosity of the layer, partition coefficients and equilibrium adsorption constants of the hydrocarbons have been estimated. The measured decrease of the optical power for the aromatic hydrocarbons and the increase of the output power for the aliphatic hydrocarbon have been explained on the basis of the developed model.

Detection of aromatic hydrocarbons in air and water by using xerogel layers coated on PCS fibers excited by an inclined collimated beam

This paper deals with experimental results on the sensitivity of silica optical fibers coated with detection xerogel membranes to benzene and toluene both in gaseous mixtures and in aqueous solutions. For this purpose thin polysiloxane and methyl-group-doped polysiloxane xerogel membranes are applied on straight segments of PCS fibers. The interaction of these membranes with hydrocarbons is investigated on the basis of changes of the output optical power from the fibers excited by an inclined collimated beam. The results obtained show that the hydrophobic methyl-group-doped xerogel layers exhibit higher sensitivity to hydrocarbons both in the gaseous phase and in solutions. The detection limits are about 230 ppm vol. for toluene in air and about 9 mg/l for toluene in water. Both the membranes show lower sensitivity to benzene than to toluene.

INVERTED-GRADED INDEX FIBER STRUCTURES FOR EVANESCENT-WAVE CHEMICAL SENSING

Abstract The paper deals with a novel multimode fiber-optic structure, the inverted-graded index profile (IGI) fiber, and its feasibility for evanescent-wave chemical sensing. Results of the theoretical analysis of the sensitivity of the IGI fiber to changes of the refractive index and light absorption coefficient of its cladding are shown. Fabrication of IGI fibers doped in the core with GeO 2 or B 2 O 3 is described. The evanescent-wave sensitivity of the prepared IGI fibers to changes of the cladding refractive index and light absorption coefficient is given showing its increase in comparison with reference fibers with cores of pure silica and with the same polymer claddings as the IGI fibers.

Torque and current ripple analytical investigation in space-vector PWM inverter fed induction motor drive under DC-bus voltage pulsation

Analytical investigation of the current and electromagnetic torque ripples in the three-phase voltage source inverter fed induction motor drive under DC-link voltage pulsation is presented in this paper. The analytical expressions for the voltage and current space-vectors as a function of the DC-link voltage pulsation are derived. The analytical results are verified by the experiments.

Influence of space harmonics on properties of six-phase induction machine - Part I. analysis

The paper deals with analysis of the situation in a six-phase induction machine. The method of space vectors is used for this purpose. The necessity to consider higher space harmonic waves of a current layer, flux density and flux in the stator yoke is shown in the case of feeding by unbalanced stator voltages. Equations of a six-phase machine are derived and the relation between symmetrical components of instantaneous values of a six-phase machine with a group of space harmonics of the field in the air gap is shown.

Compensation of unbalanced three-phase voltage supply in voltage source inverter

A control method for the compensation of unbalanced voltage supply that eliminates pulsating current in the DC link of a voltage source inverter is investigated. Such a pulsating current can have detrimental effects on operating properties of the drive as well as on the quality of power supply in the electrical vicinity of the inverter. A feasible range of control parameters is determined based on the required level of immunity against unbalanced voltage supply and on the parameters of the system. The impact of the control method on the power factors and current amplitudes in individual input phases for the available range of control parameters is analyzed and discussed. Examples of numerical simulations illustrate operation of the system under investigation both with and without the compensation method used.

Working regions of adjustable-speed units with doubly fed machines

The working regions of an adjustable-speed generation unit with a doubly fed machine connected to the power grid are determined. The analytical relations for the control range of the rotor voltage phasor due to the maximal permissible values of currents and voltages of the machine and supply cycloconverter are presented. Simulation results are shown to illustrate the operation of the system.