Milos Hayer

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.

Twin-core fiber design and preparation for easy splicing

Several methods of preparing twin-core fibers (TCFs) that can be easily spliced to standard single-core single-mode fiber are proposed. Unlike the conventional TCF preparation methods that are used to fabricate a TCF with both cores placed symmetrically with respect to the fiber axis, these methods result in twin-core fibers that have one core in the fiber center. Experimental results obtained with the TCF fabricated by using one of the designs are presented.

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.

Effect of preparation of sol-gel coatings on the strength of optical fibres

SiO2, TiO2, ZrO2, and TiO2-ZrO2 thin coatings were applied by sol-gel method on quartz fibres during the fibre drawing. Durable layers with the thicknesses up to 0.7 μm were obtained. The layer smoothness and thickness were determined using electron microprobe analyzer and electron or optical microscopy. In order to investigate a decrease in the strength of sol-gel coated fibres a novel method employing silica capillaries was developed. The fibre strength decrease was explained by the influence of water penetrating as a result of the coating process into the flaws on the fibre surface.

Organopolysiloxanes as Chemically Sensitive Coatings for Optical Fibers

Various types of UV-curable organically modified siloxanes have been synthesized by the sol-gel method with the aim of fabricating chemically sensitive coatings for silica optical fibers. The refractive index of the coating material can be tailored in the range from 1.46 to 1.56 and sensitivity towards CO2 is achieved by incorporation of amino groups. The interaction of the cured layers with CO2 or with hydrocarbons has been studied in immersion experiments. Both the reaction of CO2 with incorporated amino groups and the penetration of hydrocarbons into the layer induce changes of the light absorption coefficient and the refractive index of the coating which are detected by measuring the output light intensity from the fiber.

Sol-Gel Fabrication and Properties of Silica Cores of Optical Fibers Doped with Yb3+, Er3+, Al2 O3 or TiO2

Optical cores of preforms for drawing optical fibers doped with Er3+ and Yb3+ were fabricated by the sol-gel method with the aim of increasing the thickness of glass layers coated in a single coating cycle and to determine the relation between the preparation conditions and optical properties of the fibers. Al2O3-P2O5-SiO2 and TiO2-P2O5-SiO2 glasses have been studied as matrices for entrapping the rare-earth elements. Input sols have been prepared from silicon and titanium alkoxides, AlCl3, ErCl3, YbCl3, POCl3, water and a modifier under acidic catalysis of HCl. The sols were coated on the inner wall of a silica substrate tube and the gel layers were sintered at high temperatures up to 2000°C after which the tube was collapsed into the preform. Continuous and homogenous glass films with the maximum thickness of about 8 μm were fabricated. The influence of high-temperature heat treatment of the layers on their composition and optical attenuation was observed. The amplified stimulated emission of Er3+ around 1.55 μm was measured under the excitation of the fibers by an Nd : YAG laser at 1.064 μm.

Improvement of the sectorial fiber for evanescent-wave sensing

Abstract Several problems associated with a fiber of sectorial cross section for evanescent-wave sensing, the s-fiber, have been addressed and the results achieved are reported. The approximate theoretical analysis of the s-fiber sensitivity has been extended from single-mode to lowmultimode fibers with the aim of determining the influence of the fiber shape and launching conditions. The technological research has been aimed at fabricating preforms of the s-fibers with a structure as close as possible to the theoretical one. The drawing temperature has been optimized to 1900 °C. A novel s-fiber structure making possible its excitation by a low-multimode fiber joined to the s-fiber incorporated in a matched-size circular capillary, the capillary s-fiber, has been developed. The evanescent-wave sensitivity of the fibers has been determined by immersing the fiber core in aqueous solutions of methylene blue.

Dielectric annular core fiber for optical sensing

The paper deals with a special type of sensing optical fiber, the annular core fiber. This fiber is composed of a narrow annular low-loss glass optical core and glass optical cladding which are applied onto the inner wall of a silica capillary. In the paper transmission characteristics are determined theoretically from the solution of the Helmholtz equation as well as experimentally from measurements of the near field and angular distributions of the output power from the fiber excited by an inclined collimated beam. The sensitivity of the fiber to gaseous toluene is determined under the excitation of the fiber by an inclined beam at 670 nm, when refractive-index changes in the evanescent field are responsible for the sensitivity. These measurements are compared with those carried out with reference capillary silica fibers and PCS fibers. It is shown that the annular core fibers have the highest sensitivity in comparison with the reference fibers. A detection limit of 0.06 % can be determined from the sensitivity measurements.

Capillary optical fibers modified by xerogel layers for chemical detection

Xerogel layers have been used in the development of fiber-optic chemical sensors with the aim of enhancing the interaction of detected chemicals with light waves transmitted in the fiber. In most of fiber-optic sensors xerogels layers have been applied onto the fibers. This paper shows an alternative approach, in which xerogel detection layers are applied onto the walls of holes of silica capillaries. Single capillaries as well as multiple capillaries of microstructure fibers were used as substrates for the application of xerogel layers. Sensitivities of the prepared layers to vapours of water and toluene were determined experimentally. In these experiments, light was coupled into the capillaries and the xerogel layers were brought into contact with gaseous mixtures of the chemicals and nitrogen. Spectral changes of the output light from the capillaries were measured in a wavelength range from 1300 to 1800 nm. Results of these measurements are shown and compared with those obtained for layers applied onto silica fibers. These results show that capillary fibers modified by xerogel layers offer novel means for increasing the detection sensitivity.