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Measurements of pH are important in industry, agriculture, medicine, etc. Two main principles, namely electrochemical and optical ones, have been employed in pH meters and sensors. This paper is aimed at the development of extrinsic fiber-optic sensors of saliva pH. Such sensors have already been used for monitoring pH changes caused by biological processes. In this paper, fiber-optic sensors consisted of inlet and outlet silica fibers transmitting light from a halogen lamp to a sample cell and then to a diode-array spectrometer. Two types of sample cells were used; namely a silica cell with a measurement path of 10 mm, and a special silica capillary cell with a hole diameter of 0.07 mm and measurement path up to 40 mm. This capillary, produced at the Institute, consists of a Bragg mirror applied onto the inner silica capillary wall. Both the sensors were calibrated by using Sorensen buffers with bromothymol blue and a commercial pH meter. The calibration curves were used for the determination of pH of saliva samples collected from one healthy person at different times. It has been found that the fiber-optic sensors provide us with lower pH values than the pH meter. This result can be explain by effects of saliva components on bromothymol blue spectra. By using a correlation line between pH values measured by the sensors and those from the pH meter the sensor reliability of about 0.3 pH units can be estimated.

This paper deals with three approaches for the preparation of arsenic sulfide layers from solutions. First approach employs arsenic sulfide solutions obtained by dissolving powder of arsenic sulfide glass in n-propylamine. Second approach, a novel one, employs amorphous arsenic sulfide precipitated by chemical reaction of arsenic trichloride and ammonium sulfide. The precipitate was dissolved in n-propylamine. Third original approach relays on the same chemical reaction carried out in a mixture of n-propylamine and water that prevents the precipitation. By using all the approaches input arsenic sulfide solutions with a concentration of 0.33 mol/l were fabricated and applied onto glass slides by dip-coating method with withdrawing velocities of 50, 100, 200, and 250 mm/min. Applied layers dried in vacuum at 60 °C for 1 h and thermally treated at 180 °C for 30 s were characterized by optical and atomic force microscopy as well as by transmission spectroscopy in a wavelength range of 300–2500 nm. Refractive indices, thicknesses and band gaps were estimated from measured spectra. A maximum refractive index of about 2.15 at 600 nm and thicknesses up to 220 nm were determined on layers fabricated from input solutions obtained by dissolving of arsenic sulfide glass. Arsenic sulfide layers prepared on the basis of the arsenic sulfide precipitation exhibit refractive indices up around 1.90 and thicknesses up to 410 nm. Photonic band gap values on a level of 2.2 eV have been determined on these layers. On the other hands, composite layers prepared by reaction of arsenic trichloride and ammonium sulfide in the solution of n-propylamine and water exhibited low transparencies, refractive indices around 1.7 and thicknesses of about 2 µm.

Recently optical capillaries modified by Bragg reflection mirrors applied on the inner walls have been investigated for transmitting radiation of MIR lasers. Such capillaries include Bragg and omniguide fibers, holley fibers, or silica Kagome like fibers. Although OmniGuide fibers are commercially available and have been used for delivery of radiation of CO2 fibers at 10.6 μm, novel types of hollow-core fibers are still investigated for MIR applications. In this paper a novel approach for the preparation of capillary optical fibers for MIR region is presented. This approach employs the application of thin layers of arsenic sulfide glass and acrylate polymer from their solutions onto the inner wall of silica capillary. Arsenic sulfide forms high-index and polymers the low-index parts of reflection mirrors. By controlling optical thicknesses of such layers, Bragg mirrors can be obtained. In experiments, input solutions of arsenic sulfide in n-propylamine and UV-curable acrylate in acetone were prepared. Such solutions were applied by dip-coating method on glass slides in order to obtain samples of single layers and multilayer coatings for the determination of thicknesses and refractive indices. Acrylate layers were UV cured and arsenic sulfide layer were thermally treated at 80°C. By passing columns of the input solutions through a silica capillary with a hole diameter of 80 μm and a length of 50 cm multilayer coatings on the inner capillary wall were prepared. The column velocity for each solution was controlled as a main factor influencing the layer thickness. Applied layers were UV cured or thermally treated under a nitrogen flow through the capillary. Coatings of three pairs of the high- and low-index layers were fabricated. Single layers and multilayers applied on planar substrates were characterized by transmission spectroscopy and by optical microscopy. Attenuation coefficients of internally coated capillary fibers of 10-20 dB/m were determined at a wavelength of 1940 nm.

International audience; We present a generic sol-gel approach for the preparation of a distributed Bragg reflector (DBR) based on combining nanocrystalline Zn2TiO4 with amorphous SiO2 xerogel films. We determined the effects of the annealing temperatures on the structural and optical properties of nanocrystalline Zn2TiO4 films. The increasing size of nanocrystals and film's density caused a regular growth of the films' refractive indices at 632 nm from the value of 1.837–1.960. The acquired data were applied to design the DBR with tailored reflectance at 980 nm and the data predicted by the theoretical model were compared with experimental transmission and reflection spectra. Although the high optical losses caused by the Rayleigh scattering were observed at short wavelengths, the prepared films were fully transparent above 600 nm. The uniform dielectric structure reflecting over 97% of incident light at 980 nm was prepared from eight Bragg's pairs. The normalized spectral bandwidth was about 0.22. We proved the nanocrystalline films exhibiting scattering in UV-VIS spectral region can be used for the construction of the distributed Bragg's reflectors operating in near infrared region. The presented approach can be easily extended to other nanocrystalline films allowing the integration of nanocrystalline luminophores into advanced photonic structures.

This paper presents fundamental characteristics of laboratory designed and fabricated Bragg fibers with air and silica cores at wavelengths of 632, 975, 1064 and 1550 nm. Fibers with the 26- μ m-silica core and 5- or 73- μ m-air cores in diameters and claddings of 3 pairs of Bragg layers were prepared from one preform. The overall transmittance, attenuation coefficients, coupling losses, bending losses, and damage-intensity thresholds were determined using four continuous-wave laser sources with the maximum output power of 300 mW and a pulsed 9 ns laser with the maximum output energy up to 1 mJ. The lowest attenuation coefficient of about 70 dB/km was determined at 1064 nm with the 73- μ m-air-core Bragg fiber. All fibers have been found to exhibit negligible bending losses down to the bending diameters of 5 cm. In comparison with the conventional gradient optical fiber, all the prepared Bragg fibers have approximately six times higher damage intensity threshold of about 30 GWcm−2 and therefore they are very suitable for high power laser radiation delivery.

This paper presents results of the preparation and optical characterization of As 2 S 3 layers by using spin or dip-coating technique and As 2 S 3 solutions in ethylenediamine and n-propylamine as solvents. These solutions were prepared from arsenic sulfide powders obtained from laboratory-prepared bulk glasses. Ethylenediamine and n-propylamine solutions with the same As 2 S 3 concentration of 0.7 mol/l were used for the layer preparation. Layers were prepared by both the dip-coating and spin-coating techniques with velocity of 0.2–30 cm/min and 2000 rpm for 90 s, respectively. Substrates used in these experiments were microscopic slides (refractive index of about 1.49). All applied As 2 S 3 layers were dried in vacuum for 60 min and then heat-treated at 180 °C for 60 s in nitrogen atmosphere. Prepared As 2 S 3 layers were characterized by transmission spectroscopy in the wavelength range of 200–100 nm. Thicknesses of applied layers in the range of 200–1500 nm and their refractive indices ranging from 1.7 to 2.3 at 800 nm were estimated from interference bands in measured transmission spectra using Swanepoel's method. Dispersion curves of the refractive index are shown in the paper. Optical band gaps of prepared layers in the range of 2.15–2.35 eV were determined from measured transmission spectra by using Tauc's extrapolation.

Photonic band gap Bragg fibers can be employed for delivery of high powers in medicine, lighting and heating systems, electricity production, and the development of optical sensors of physical and chemical variables. In this paper, results of testing Bragg fibers with hollow cores for pH detection based on absorbance measurements on solutions filled in the cores. Bragg fibers with claddings of three pairs of high- and low-index layers and hollow cores of 70 µm in diameter were fabricated. Photonic band gap bands of the fibers were estimated theoretically. Aqueous solutions of methyl orange (MO) and hydrochloric acid and bromothymol blue (BTB) in Sörensen buffers were filled into the cores and spectra of MO and BTB were measured for different pH. Calibration curves typical for optical pH sensors were derived from the measured spectra with a reproducibility of about 10%.

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.

In this contribution a new method for determination of transmission characteristics and mechanism of optical radiation propagation in Bragg fibers is described. The investigated Bragg fibers are designed for high-power laser radiation delivery. Attenuation coefficient and coupling efficiency are usually measured using simple cutback method for optimum mode matching of the incident radiation to the fiber mode. Our current approach is based on the cut-back technique but we also investigate the dependence of the mentioned parameters on radial position of the excitation mode from the fiber axis. In addition, using the imaging of the measured fiber output face on a CCD camera, the spatial profile of the propagated beam can be obtained. For this reason this method is suitable for delivered laser mode control. The investigated Bragg fiber consisted of the 26um diameter silica core surrounded by three pairs of circular Bragg layers. Each pair is composed of one layer with a high and one layer with a low refractive index being characterized by a refractive-index contrast up to ~0.03. The 1064nm laser beam was focused by a telescope onto the fiber input face. The beam radius in the focal plane was 5um. The Bragg fiber output face was imaged by a 1:6 optical telescope on the CCD camera. The transmitted power and spatial beam profile were registered simultaneously for various offset from the fiber axis. After the fiber shortening, the measurement was repeated and the cut-back was performed. The lowest attenuation coefficient of 0.17dB/m corresponded to a core mode of the delivered laser radiation. In general, the attenuation was higher with a shift from the radial axis of the fiber symmetry. In the case of cladding mode excitation, the attenuation parameter shows a local minimum. This phenomenon was consistent with the refractive index profile of the tested Bragg fiber.

A compact and robust tapered optical fiber microsensor is presented for detection of pH in a range from 5.8 to 7.5 in sub-microliter volumes. The sensor is based on a pH transducer 8- hydroxypyrene-1,3,6-trisulfonic acid trisodium salt immobilized in a xerogel matrix onto the tip of a optical fiber taper with a tip diameter below 20 mm. The sol-gel method and two silicon alkoxides is used for preparing the matrix. A ratio of the fluorescence emission intensities measured at 518 nm after the excitation at 400 and 450 nm is used for evaluating the sensor response to pH. This ratiometric approach enables to reduce effects of ambient light, bleaching of the sensitive layer and geometry of the probe to the fluorescence signal and achieve the resolution of about 0.07 pH units.

Several fiber-optic sensing elements consisting of a U-shaped fiber coated by a detection membrane of polydimethylsiloxane have been prepared with arc radii from 0.3 to 2 mm. Angular distributions of the output optical power from the elements have been measured. A high decrease of the transmitted power of paraxial rays has been observed for elements with arc radii below 1 mm. When elements with arc radii above 1 mm were excited by an inclined collimated beam and brought into contact with toluene solutions a maximum sensitivity of 1 dB/mM was determined. Sensing elements have been tested for monitoring the decomposition of toluene dissolved in water by means of photocatalytic TiO2 nanoparticles applied on silica optical fibers. Differences between concentration data obtained by UV spectrophotometry and those obtained by the element on levels of 10-15 % have been observed.

In this paper we have investigated Bragg fibers for the 1.94 μm laser- radiation delivery generated by a thulium fiber laser with a maximal continuous output power 50W. For such investigation laboratory-designed and fabricated hollow-core Bragg fibers have been employed with different diameters of 5, 40, 56 and 73 μm surrounded by three pairs of circular Bragg layers. Fundamental optical characteristics such as overall transmittance, attenuation coefficient, bending losses, and delivered spatial beam profiles at the wavelength of 1.94 μm for all tested fibers are reported and summarized in this contribution. In the case of laser radiation delivery with the intensity of 65 kW/cm2, the lowest attenuation coefficient of 1.278 dB/m was determined for the Bragg fiber with the inner air-core diameter of 56 μm. Moreover, the bending losses for a small bend diameter of 15 mm reached 0.177 dB only. However delivered laser radiation was highly multimode character.

Bend responses of an LPG inscribed with a CO 2 laser in a conventional single-mode fiber are investigated in an arrangement, where the LPG is inserted into a thin silica capillary bonded to a steel strip. The curvature of the strip, laid on two supports and bent with a micrometer driver in the middle between the supports, decreases linearly with distance from the center of bending to the supports. Experiments are done for a very large range of bend deflections with the LPG in three positions shifted with respect to the center of bending, and two distinct opposite rotational orientations with respect to the plane of bending. Responses obtained for the above positions and orientations are greatly different and some of them show features which have not been reported for LPGs bent with a constant curvature and smaller deflections. They are, particularly, maxima and minima of the central wavelength's shift along with reversals of its direction, or the reappearance of a previously vanished attenuation band with increasing magnitude of bending. Also, it is found that the responses depend not only on the average curvature of the bent LPG, but also on the particular dependence of the curvature along the LPG's length. Some of the results obtained cannot be satisfactorily explained at the present level of knowledge, and a more thorough theoretical analysis is needed.

Absorption and fluorescence spectra in the range from 350 to 1750 nm of several Tm3+- doped optical performs (rods) for optical fiber drawing were measured. Silica-based matrices of Al2O3-GeO2-P2O5-Sb2O3-SiO2 composition doped with several thousands ppm of Tm3+ were characterized. The preforms were fabricated by the Modified Chemical Vapor Deposition and by the solution doping methods. A new method, Genetic Algorithm SPEctra Decomposition was adopted for processing of the measured absorption spectra. This decomposition made it possible to calculate the oscillator strengths of Tm3+ absorption levels. Fluorescence bands of Tm3+ at 800 nm or 1640 nm were found in fluorescence spectra measured on the preform samples when excited at 1064 nm only.

The construction and performance of a novel enzyme based optical sensor for in situ continuous monitoring of glucose in biotechnological production processes is presented. Sensitive optical coatings are formed from inorganic?organic hybrid polymers (ORMOCER?s) combined with a flurophore (ruthenium complex) and an enzyme, and applied to lenses, declad polymer optical fibre (POF) and polymer clad silica fibre (PCS). The enzyme, glucose oxidase, catalyzes oxidization of glucose to gluconic acid by depleting oxygen. Oxygen consumption is determined by measuring the fluorescence lifetime of metal organic ruthenium complexes which are quenched by oxygen. The coatings developed were designed to adhere to glass and polymer surfaces, to be compatible with enzymes and ruthenium complexes, and were demonstrated both as double- and single-layer structures. The sensor response to gaseous oxygen, dissolved oxygen and dissolved glucose was measured via fluorescence lifetime changes. A best detection limit of 0.5% (vol) has been determined for gaseous O2 with selected ORMOCER? sensing layers. Glucose concentrations were measured to a detection limit of 0.1 mmol L?1 over a range up to 30 mmol L?1. The sensor was usable for 30 days in a bioreactor. The opto-electronic instrumentation and performance in laboratory bioreactors and in an industrial reactor are evaluated.

This paper presents experimental results on transmission characteristics of a Bragg fiber with an air core at a wavelength of 632 nm. The results are compared with those recently reported for the same fiber but at a wavelength of 1064 nm. The cladding of the fiber consists of three pairs of Bragg layers. Each pair is composed of one layer with a high and one layer with a low refractive index. A diameter of the air core is of about 50 μm. A theoretical modelling of Bragg fibers was carried out from which band gap characteristics of the fiber cladding were determined for the refractive-index contrast of the high- and low-index layers on a level of 0.03 and wavelength of 632 nm and 1064 nm. Preforms of the Bragg fiber in the form of tubes were prepared by the MCVD method. Germanium dioxide and phosphorous pentoxide were used as silica dopants for the high-index layers. The low-index layers were fabricated of silica slightly doped with phosphorous pentoxide. The last layer applied was the high-index one. Bragg fibers were drawn from the tubes under controlled temperatures around 2000 °C in order to obtain the fibers with designed dimensions of Bragg claddings and air cores. Results of characterization of prepared fibers with optical microscopy are presented in the paper. The transmittance and attenuation of the fibers at 632 nm were measured with a continuous-wave He-Ne laser as a light source. Spatial distributions of output beams from the fibers were also determined.

Long-period gratings (LPGs) inscribed in a novel fiber, in which the smoothed refractive index of the cladding increases approximately quadratically with radius (IGI fiber), are investigated for temperature sensing. Fabrication of the fiber and LPGs is briefly described and results of a theoretical analysis are given. The analysis yields fields and effective indices of the LP 01 core mode and LP 0 i cladding modes of the IGI fiber. Wavelength-dependent effective indices, computed for 11 lowest-order cladding modes and an external index of 1.33, are incorporated into a diagram of the phase-matching condition covering also coupling through higher harmonics of the grating. Temperature sensitivities of LPGs submerged in water and in a liquid with a refractive index that decreases with temperature and is close to that of the pure-silica fiber cladding are determined. Average sensitivities of 0.60 and 0.56 nm/°C are obtained for the LPGs with periods of 1000 and 500 μm in water. For the same LPGs in the above liquid, average sensitivities of 5.40 (3.95) and 3.76 (3.11) nm/°C are obtained in a 10 °C (20 °C) temperature interval with a lower limit of sensing at about 50 °C. A significant improvement in sensitivity is observed after slightly bending an LPG in the liquid.

Sensing experiments with a long-period grating (LPG) inscribed by a CO 2 laser in a single-mode fiber with an inverted-parabolic index profile in the fiber cladding were done. A single LPG with a period of 500 μm was used in experiments and spectral changes of the attenuation band centered at ≈1520 nm were measured. Responses of the LPG to temperature changes and temperature-induced refractive-index changes were investigated. An average shift of 0.56 nm/°C was obtained for the bare LPG and of 0.86 nm/°C for the LPG recoated with a polymeric layer. An overall shift of the central wavelengths was observed in experiments repeated after two weeks that can be attributed to post-curing processes in the polymeric layer.

The operation of a long-period grating inscribed into the core of a single-mode fibre with the cladding having an inverse parabolic-index profile is theoretically investigated. Two-dimensional bidirectional mode expansion and propagation method is used to simulate mutual interaction of rotationally symmetric LP modes in the fibre. Numerical analysis has shown that the inverse parabolic-index profile of the cladding helps increase the sensitivity of a fibre as a refractometric sensor.

The paper deals with the fabrication of long-period gratings (LPG) in special optical fibers with an inverted parabolic-index profile of the cladding by periodically exposing the fibers to radiation of a high-power CO 2 laser. Details on the fiber fabrication and preparation of LPGs with a period of about 1 mm in the fibers are presented. The fabricated LPGs were evaluated on the basis of their transmission spectra determined by measuring the output spectral density of the fiber with the LPGs. It is shown that the position of the band and its strength depend on the laser beam velocity, beam power and the interaction time of radiation with the fiber.

Optical fibers with non-circular cross-sections can be effectively used for pumping of fiber lasers and amplifiers. Noncircular fiber-optic structures can be also employed for a control of propagating light polarization. Details of preparation of three types of optical fibers with non-circular cross-sections are presented. A polarization-maintaining (PM-) fiber with elliptical core was drawn from non-circular preform to study the effect of drawing temperature onto the fiber geometry. A stabilization of fiber rotation during the drawing was successfully tested during preparation of double-clad (DC-) optical fibers with hexagonal and special tailored cross-sections. The conservation of original geometry during the drawing was examined.

The paper presents results on preparation and characterization of highly reflective coatings on planar substrates and inside silica tubes. Coatings are designed for a maximum reflectivity at a wavelength of 550 nm and consist of several pairs of oxide layers. Each pair is composed of one layer with a high refractive index and one layer with a low refractive index with a refractive-index contrast of about 1.1. The layers were prepared by the sol-gel method. High-index layers were applied from a sol based on titanium butoxide while a sol of tetramethoxysilane was use for low-index layers. The sols were deposited onto silica slides or onto walls of silica tubes by using the dip-coating technique. Applied gel layers were thermally treated at temperatures up to 450 °C in order to obtain densified layers with thicknesses 50-100 nm. Coatings with one to five pairs of layers were fabricated. Prepared coatings were characterized by transmission and reflection spectrometry in a wavelength range from 190 to- 1100 nm, by contact profilometry, and by spectral ellipsometry. Thicknesses and refractive indices of coatings were determined from these measurements. For normal light incidence a reflectivity higher than 99% in a wavelength range of 500-650 nm was measured by transmission spectrometry on coatings prepared from four or five pairs of layers. Similar reflectivity values were determined for angles of incidence of 15, 30, 45 and 60 degrees by using reflection spectrometry. Transmission spectra measured on the coated tubes which show interference bands are also presented in the paper.

This paper presents the comparison of experimental characteristics of two types of Bragg fibers, one with a silica core and second with an air core. The claddings of the fibers consist of three pairs of Bragg layers. Each pair is composed of one layer with a high and one layer with a low refractive index with a refractive-index contrast of about 0.03. The diameter of the silica core is of about 30 μm and that of the air core is of about 55 μm. Preforms of the Bragg fibers in the form of a tube have been prepared by the MCVD method by using germanium dioxide, phosphorous pentoxide and fluorine as silica dopants. In the case of the fiber with the air core only Bragg layers were applied inside a substrate tube while in the case of the fiber with the silica core one additional silica layer was deposited over the Bragg layers. The fibers were drawn from the preforms under controlled temperatures in order to obtain fibers with air or solid cores. Results of characterization of prepared fibers by optical microscopy are presented. Several laser sources including a pulse and continuous-wave Nd:YAG lasers at 1064 nm and a laser diode at 1550 nm were used for testing the transmission of laser radiation through the fibers. Transmittances, attenuation coefficients, bending losses and spatial profiles of output beams from the fibers were determined from such measurements.

This paper deals with the detection of toluene dissolved in water by means of straight polymer-clad silica (PCS) detection fibers with short detection regions. The approach is based on the detection of refractive index changes of the polymer cladding induced by toluene. The fibers were excited by an inclined collimated light beam with the aim of finding an alternative to the coiled axially excited fibers which would achieve similar detection sensitivity. Two types of siloxane polymers, thermally curable one with the refractive index of 1.407, and UV curable polymer with the refractive index of 1.423 were used for the modification of fiber properties in the detection area. The polymers were applied as thin layers (5–20 μm) on the silica core of the sensing fibers. Angular distributions and time–responses of the output optical power to changes of toluene concentration in solutions were measured. Sensitivity curves were drawn on the basis of these results. The best value of the detection sensitivity of about 0.006 dB/(mg/l) was achieved with the UV curable siloxane. This value corresponds to the detection limit of toluene in water of about 15 mg/l. Sensing fibers with beveled input ends were prepared. They exhibited the same sensitivity as fibers with the end perpendicular to the fiber axis when they were excited by the uninclined beam.

Optical fibers designed for high power laser radiation delivery represent important tools in medicine, solar systems, or industry. For such purposes several different types of glass optical fibers such as silica, sapphire, or chalcogenide ones as well as hollow-glass fibers, photonic crystal fibers and Bragg fibers have been investigated. Air-core Bragg fibers or photonic crystal fibers offer us the possibility of light transmission in a low dispersive material - air having a high damage threshold and small non-linear coefficient. However, preforms for drawing Bragg fibers can be fabricated by MCVD method similarly as preforms of standard silica fibers. In this paper we present fundamental characteristics of laboratory-designed and fabricated Bragg fibers with air cores intended for delivery of laser radiation at a wavelength range from 0.9 to 1.5 μm. Bragg fibers with different air core diameters of 5, 45 and 73 mm were prepared. The fiber core was surrounded by three pairs of circular Bragg layers. Each pair was composed of one layer with a high and one layer with a low refractive index with a contrast up to 0.03. Several laser sources emitting at 0.975, 1.06, and 1.55 μm were used as radiation sources. Attenuation coefficients, overall transmissions, bending losses, and spatial profiles of output beams from fibers were determined at these wavelengths. The lowest attenuation coefficient of 70 dB/km was determined for the 45 μm and 73 mm air-core fiber when radiation from a laser was launched into the fibers by using optical lenses. However, multimodal transmission has been observed in such condition. It has also been found that bending losses of such fibers are negligible for bending diameters higher than 15 mm.

The paper summarizes results achieved in the Institute of Radio Engineering and Electronics in the field of investigation and fabrication of novel fiber-optic structures for evanescent-wave chemical sensing. Several approaches for increasing the evanescent-wave sensitivity of multimode silica optical fibers to gaseous and liquid chemicals are shown. These approaches make use of the decrease of the core diameter in sectorial and capillary sectorial fibers, modification of trajectories of optical rays in the cores of inverted-graded-index fibers, tailoring of the refractive index of porous layers applied on silica cores, an output mode filter preventing the detection of rays propagating at low axial angles or fibers bent in single or multiple turns.

We have proposed and demonstrated experimentally that an Er-doped twin-core fibre (ErTCF) can be employed as a tracking narrow-band filter in fibre ring lasers for line narrowing and enhancement of the spectral stability. The device mimics a saturable absorber with inhomogeneous broadening. The slow erbium ion saturation dynamics in the ErTCF prevents wavelength hopping whilst it enables the filter to track the operation wavelength on its tuning. We have improved the device operation by utilizing the inherent filtering effect of a twin-core fibre coupler.

We present preparation and spectroscopic characterization of thulium-doped fibers with enhanced 3H4 fluorescence lifetime obtained by codoping with alumina. With an alumina content of 10 mol%, a fluorescence lifetime of 58 μs is achieved. For the first time we study and optimize the thulium-doped fiber amplifier (TDFA) with a dual-wavelength (800 + 1400 nm) pumping scheme, where the pump at 800 nm propagates in the cladding and the pump at 1400 nm propagates in the core. In the study we use numerical simulations and experimentally obtained fiber characteristics. The results are compared with those obtained for numerical simulation of a TDFA pumped at a single wavelength in the 1050 nm band.

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.

Several approaches for fiber-optic evanescent-wave detection of small amounts of liquid hydrocarbons are investigated in the paper. These approaches use polydimethylsiloxane-coated silica (PCS) fibres and detect changes of the fiber attenuation induced by the penetration of hydrocarbons into the fiber coating. The detection ability of bent PCS fibres both in the spectral region from 400 to 600 nm and near-infrared region from 900 to 1700 nm is studied by using available commercial or laboratory spectroscopic techniques. Straight sensing fibres coated with a thin high-refractive index layer and selective excitation of the fibres at 670 nm are investigated as well. A novel approach employing straight fibres coated with a thin siloxane coating and an output mode filter is described as a promising technique for increasing the detection sensitivity of fiber-optic evanescent-wave sensors to petrol fuels.

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.

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.

Polysiloxane porous layers doped with TiO- and phenyl-groups were fabricated by the sol-gel method. Starting sols were prepared from alkoxides using catalysis by HCl or HF. Stable SiO-TiO-sols were obtained by using HF. Porous gel layers with thicknesses of 0.3–1.1 μm were coated on fiber surfaces or silicon wafers. Layers with the refractive index of 1.38–1.48 were prepared. The fractional porosity of the layers was estimated to be 0.1–0.35. The interactions of the layers with vapor or liquid chlorinated hydrocarbons, alcohols or aromatic hydrocarbons have been studied in immersing experiments by measuring the output light intensity from the fibers. The observed changes of the output intensity could be correlated to changes of the refractive index of the layer caused by the penetration of the tested chemicals into the layer pores.

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.

In this contribution we demonstrate the effect of the nanostructured optical fiber core matrix, doped with erbium and Al 2 O 3 , on the resulting optical properties. Several optical fibers with nanostructured cores were drawn from preforms prepared by different techniques, i.e., by conventional doping from solution of erbium and aluminium chlorides, by deposition of the dispersed alumina nanoparticles with either Er 3+ ions or Er 2 O 3 . Reference bulk samples were prepared by the solid-state approach and thermally treated by similar way as optical fibers. Prepared optical fibers and bulk samples were investigated by the absorption spectroscopy. Reference samples were analyzed by the X-ray diffraction for the determination of the crystalline properties of formed nanostructures. It was found that nanocrystals inside the optical fiber core matrix improves the homogeneity and decreases the basic optical attenuation. Nanostructured alumina inside the fiber core matrix enhances the absorption properties of Er 3+ ions.

This paper presents results of preparation and characterization of Bragg fibers with silica and air cores designed for delivery of laser radiation at 1060 nm. The fibers consist of cores with a refractive index equal to that of air or silica which is surrounded by three pairs of Bragg layers. Each pair is composed of one layer with a high and one layer with a low refractive index and is characterized by a refractive-index contrast up to 0.03. Three structures of Bragg fibers are presented in the paper, namely the structure with a silica core of 26 μm in diameter, structure with an air core of 5 μm in diameter and that with an air core of a diameter of 72 μm. Preforms of the Bragg fibers in the form of a rod or tube have been prepared by the MCVD method using germanium dioxide, phosphorous pentoxide and fluorine as silica dopants. The fibers have been drawn from the preforms under controlled temperatures in order to obtain fibers with air or solid cores. Results of characterization of prepared fibers with optical microscopy and by measuring their refractive-index profiles, losses and angular distributions of the output optical power are presented. The characterization of fibers for delivery radiation of a Nd:YAG laser with nanosecond pulses at 1060 nm, namely the transmission, attenuation coefficient, spatial profiles of transmitted beams, and bending losses are also presented. Fiber damage thresholds in a range 25-30 GW/cm 2 have been determined.

In this contribution we present numerical study of propagation of the multimode pump radiation in the inner cladding of the developed double-clad fiber. Field evolution was simulated using full vector finite element beam propagation method. Due to reeling of the fiber on the spool, the curvature of the fiber is involved in the model. The longitudinal dependence of power distribution was then analyzed. The simulations showed that after the length of propagation of about 40 mm the field becomes homogeneously distributed in the structure. Over 90 % of pump energy is absorbed in 3 m of the rare-earth-doped fiber with core-absorption of 2000 dB/m and cladding-core diameter ratio of 18.5. This confirms suitability of the tailored cross section for effective pump absorption along the rare-earth-doped fiber.

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.

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.

The thermophoretic efficiency in the modified chemical vapour deposition process has been experimentally studied under conditions of changing temperature and velocity field in the silica deposition tube. The cooling of the tube by a stream of air, deposition under the reverse torch traverse and induction of a circular flow in the tube have been studied. Results of the experiments were explained on the basis of the Walker thermophoretic model and under the assumption that the amount of the deposited glass is determined by chemical equilibrium. Considerable increase of the overall thermophoretic efficiency was observed when cooling by the air stream was used. The derived theoretical model can account for these results.

Laser structuring is rapidly developing manufacturing technique for broad spectrum of industrial branches, e.g. aerospace, power engineering, tool- and mould making, and automotive. It enables to prepare work pieces and products with very fine micro structures achieving a far better degree of details than conventional structuring techniques like etching or eroding. However, the state of art in laser structuring shows a crucial deficit. Used systems contain no metrology setup to detect the shape geometry (depth and length) and contour accuracy during the process. Therefore, an innovative in-line metrology technique based on low coherence interferometry for laser structuring systems has been investigated and described in the paper. In this contribution we present our results in the research of wideband and highpower light sources for the proposed low-coherence interferometric measurement system. The system can be incorporated into a structuring workplace equipped with a Q-switched ytterbium-doped fiber laser at 1064 nm for material processing. In the paper we focus on two wideband sources for such a measurement system. The first source is based on a superluminescent diode and the second one is based on an amplified spontaneous emission in a double-clad ytterbium-doped fiber. An example of results measured with the proposed in-line metrology system is presented.

Whispering-Gallery-Mode (WGM) optical microresonators have attracted growing attention because of their big potential for chemical and biological sensing. Recently, optical bottle microresonators have been fabricated from short sections of optical fibers. These double-neck bottle-shaped microresonators have some features distinguishing them from spherical microresonators. They support non-degenerate whispering-gallery modes that exhibit two well-separated spatial regions with enhanced field strength. The free spectral range (FSR) of such microresonators is about one order of magnitude smaller than that of microsphere resonators of equal diameters. It means that these microresonators have much longer optical path-lengths and can be employed for highly sensitive detection. The paper deals with the preparation and characterization of bottle optical microresonators fabricated from silica optical fibers. A simple 2D numerical modelling has been used to investigate basic spectral characteristics of microresonators with hexagonal cross-sections. Cylindrical bottle microresonators are prepared from optical fiber Corning SMF-28, while microresonators with hexagonal cross-section are prepared from an experimental hexagonal silica fiber. This novel type of bottle microresonators is expected to have much simpler mode structure than cylindrical bottle microresonators. There are compared two methods for the preparation of bottle microresonators in this paper. The first method is based on a combination of controlled local heating and tapering the fiber by the use of carbon dioxide laser Coherent GEM SELECT 50. The second method is based on weak tapering of the silica optical fiber with glass processing unit Vytran GPX-3400.

This paper deals with the preparation of spherical silica whispering-gallery-mode (WGM) microresonators and with their resonance spectra measured in air and in acetone vapors. Spherical microresonators with a diameter ranging from 320 to 360 micrometers have been prepared by heating the tip of a silica fiber by a hydrogen-oxygen burner. Details of this preparation are shown on spherical and spheroidal microresonators. The prepared microspheres were excited by a fiber taper and their resonance spectra were measured and Q factors estimated. Changes in the resonance spectra of the microspheres due to their contact with acetone vapor heated to 55 °C or with liquid acetone have been observed. These changes are explained by interaction of acetone with silica and by temperature changes of the microspheres.

This paper deals with a steering-wheel microstructure optical fibre designed prepared and tested for detection of gaseous analytes. The inner structure of the steering-wheel microstructure fibre consists of a thin silica core that is surrounded by three cladding holes. Numerical simulations showed that the evanescent wave of the guided fundamental mode at a wavelength of 1550 nm penetrates into the cladding holes. The calculated overlap of the evanescent wave of guided mode with the cladding holes of 0.78% can suitably be employed for gaseous analytes detection. The prepared steeringwheel microstructure fibre was experimentally tested for detection of toluene vapors flowing in the cladding holes of the fibre. It has been proved that this type of microstructure fibre can be used for detection of gaseous analytes such as toluene in nitrogen or in air in concentrations of about 0.1 mol.%.

The paper deals with the preparation and characterization of whispering-gallery-mode silica spherical microresonators and with effects of liquid acetone, ethanol, and xerogel layers applied onto these microresonators on their resonance spectra. Microrespheres with diameters ranging from 320 to 360 μm have been prepared by heating a tip of a silica fiber with a hydrogen-oxygen burner. The microspheres were excited by a fiber taper or a bulk prism and their resonance spectra were measured. Values of the Q factor from 10 4 to 10 6 have been determined from these spectra. In experiments, it has been found that short contact of microspheres with acetone causes a shift of resonance dips due to surface effects caused by acetone. A decrease of the Q factor has been observed with a microresonator onto which a xerogel silica layer was applied by the sol-gel method. A very high decrease of the Q factor has been observed when the silica microresonator was brought in contact with liquid ethanol.

International audience; Samples of pure and rare earth (RE)-doped nanocrystalline Zn2TiO4 (RE = Eu, Er, Tm) have successfully been prepared by the single batch sol-gel method in a form of powders. The powders were annealed up to 1200 °C and analyzed by DTA and XRD methods. Sizes of formed nanocrystals were calculated from the Scherer equation and verified by the SEM analysis. Activation energies of crystallization were calculated using the Kissinger, Ozawa and Augis-Bennett approximations and reaction mechanisms were proposed evaluating the Avrami exponents from the Johnson-Mehl-Avrami equation. It has been found that RE elements particularly substitute zinc ions in the crystal lattice and are fully soluble up to 0.5 at.% related to the some of cations. Furthermore, the results have shown that the presence of RE elements blocks the crystallite growth and supports the nucleation process leading to the formation of smaller and more uniform nanocrystals compared to the undoped Zn2TiO4.