Marie Pospisilova

Fiber-Optic Chemical Sensors and Fiber-Optic Bio-Sensors

This review summarizes principles and current stage of development of fiber-optic chemical sensors (FOCS) and biosensors (FOBS). Fiber optic sensor (FOS) systems use the ability of optical fibers (OF) to guide the light in the spectral range from ultraviolet (UV) (180 nm) up to middle infrared (IR) (10 µm) and modulation of guided light by the parameters of the surrounding environment of the OF core. The introduction of OF in the sensor systems has brought advantages such as measurement in flammable and explosive environments, immunity to electrical noises, miniaturization, geometrical flexibility, measurement of small sample volumes, remote sensing in inaccessible sites or harsh environments and multi-sensing. The review comprises briefly the theory of OF elaborated for sensors, techniques of fabrications and analytical results reached with fiber-optic chemical and biological sensors.

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.

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.

Preparation and optical properties of silica optical fibres with an Al2O3-doped core

Abstract Preforms for drawing optical fibres, whose silica cores are doped with P 2 O 5 and Al 2 O 3 and codoped with Yb 3+ , Er 3+ , Nd 3+ , MgO or the newly studied codopants Cr 3+ and Mn 2+ , are prepared by the extended modified chemical vapour deposition (MCVD) method. A novel approach to the MCVD preparation of porous layers doped with higher P 2 O 5 levels, based on the deposition of this layer during the opposite traverse of the heating burner, is tested. The absorption spectra of the drawn fibres have been measured and the observed absorption bands have been identified as being due to the codopant Mn 2+ , Cr 3+ , Yb 3+ , Nd 3+ , Er 3+ or MgO.

Use of alumina nanoparticles for preparation of erbium-doped fibers

It has been found that optical fibers doped in the core with alumina nanoparticles and erbium ions have approx. three-times lower base-line attenuation than the fibers prepared from a solution of aluminium chloride by using the conventional solution-doping technique. The doping with alumina nanoparticles has not caused a pronounced increase of erbium luminescence at 1530 nm.

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.

Characterization of sensing layer onto the tip tapered fiber

In this paper we present a novel method for the measurement of the thickness of the sensing layer applied on the tip of an optical fiber and fiber tapers. The method is based on analysis of distributions of the fluorescence intensity over this layer. In experiments the fluorescence of the sensing layer prepared by the sol-gel method was investigated be means of a confocal microscope Zeiss LS5 Duo. The fluorescence was excited at 477 nm by an Ar laser and detected in a spectral range from 518 to 600 nm. The fluorescence distribution was determined by scanning the layer in the direction of the taper axis (z-direction) with a step of 500 nm in an overall length of 42 μm and 26 μm. The layer thickness was estimated from the measured distribution of fluorescence intensity. Assumptions of method are that close to the layer boundary the fluorescence intensity decreases with z2, the concentration of fluorescence centers in the layer is homogenous and attenuation of excitation wavelength in the sensing layer is neglected. This method has made possible to investigate sensing layers with thicknesses of about 1 μm.

Optical pH Detection with U-Shaped Fiber-Optic Probes and Absorption Transducers

In medicine knowledge of pH values can provide us with information not only about the patients’ status but also about physiological processes in the patient’s body. Measurements of pH in small-sample volumes and online pH monitoring in vivo can be employed to obtain such information. For such measurements we have developed and investigated U-shaped fiber-optic probes with immobilized pH indicators in this paper. U-shaped probes with a diameter of about 2 mm were prepared. Three different pH indicators, methyl red, methyl orange, and bromothymol blue, were immobilized in two types of matrices, namely, porous silica (PS) and ethylcellulose (EC), and applied on the U-shaped probes. Changes in spectra of transmitted power were measured and calibration curves were determined from these spectra. It has been found that a working pH range of prepared probes was from 3.1 to 7.6. The maximum sensitivity was about 0.1 1/pH unit. Effects of structural relaxations of detection layers and indicator leaching observed in experiments are discussed.

Microstructure fibers for sensing gaseous hydrocarbons

The paper presents new experimental results on the sensitivity of three types of microstructure fibers (MSFs) modified by xerogel layers to aromatic hydrocarbons, namely to toluene. MSFs with air holes with diameters in a range 10 - 50 &mgr;m, arranged in one, two or three rings, were prepared and their segments were used in sensing experiments. Capillary silica fibers (CFs) were also fabricated for reference sensing measurements. Segments of the fabricated MSFs and CFs were modified by thin xerogel layers applied onto the hole walls by the sol-gel method from sols based on tetraethoxysilane or methyltriethoxysilane. For sensing experiments, the segments were fixed in a special cell making it possible to control the excitation of the fiber and flow of gaseous chemicals through the air holes. The sensitivity of the MSFs and SCFs to gaseous mixtures of toluene in nitrogen was determined from spectral changes of the output light from the fibers in a range of 1600-1800 nm. Experimental results show that the sensitivity of MSFs depends on their architecture, particularly on the arrangement, diameters and number of air holes. A detection limit of about 0.007 vol.% of toluene has been achieved.

Effect of Doping Silica Gel Layers with TiOTi-Chains and Cu2+-Ephedrine on the Sensitivity of the Layers to Gases

In this paper the interaction of polysiloxane and TiOTi-doped polysiloxane gel layers containing Cu2+-ephedrine (CuEP) with carbon dioxide, nitrogen and water is investigated. This interaction is studied through changes of the output optical power from an optical fiber coated with the investigated gel layer. The layers are prepared with sols based on tetraethoxysilane and titanium isopropoxide, isopropanol, acetylacetone and Cu2+-ephedrine by the repeated dip-coating method. The applied gel layers are dried at 70°C.Spectral and temporal changes of the output optical power due to effect of the tested gases are measured. The measurements show big changes of the output optical power of fibers coated with CuEP-doped silica gel layers in a wavelength range from 400 to 700 nm, which is in relation with spectral effects of CuEP in alcoholic solutions. Relatively lower changes of the attenuation spectra without any attenuation band are found in the spectra of TiOTi-doped siloxane gel layers containing CuEP. The angular distributions of the output optical power of the coated fibers indicate high optical losses of the layers.