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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.
This paper presents results on using the sol-gel process for the preparation of microstructure fibers (MSFs). In this preparation the sol-gel method is used for the formation of bonds between silica tubes and silica rods in the input stacks for MSF drawing and for filling-in the interstices between these parts. Gels applied on the stacks were prepared from sols composed of tetraethoxysilane, phosphorous oxychloride, boron tribromide, ethanol, HCl and water. Pastes prepared from the sols and solid fabricated through heat treatment of the sols at 1000°C were also used. The sols were applied on the stacks by the dip-coating method and the pastes were applied manually. The formed xerogels were heat-treated at temperatures of up to 1000°C. The MSFs of the grapefruit type were drawn from the stacks at a temperature of about 1900°C. Waveguiding properties of the fibers were examined under their excitation by an inclined collimated beam. It has been found that the sol-gel method is suitable particularly for filling-in the interstices at the end of the stack, which enables pressure control inside holes of the stack during MSF drawing.
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.
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.
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.
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.
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Content accesss denied Microstructure fibers for gas detection
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The paper deals with experimental investigation of capabilities of microstructure fibers (MSFs) for evanescent-wave chemical sensing. It presents results on the sensitivity of MSFs of a “grapefruit” type to gaseous toluene in mixtures with nitrogen which flow through the fiber holes. The sensitivity is determined on the basis of changes of the output optical power from the fiber caused by refractive-index changes of the fiber cladding induced by toluene and determined at 670 nm. Specific changes of spectral absorbance due to C–H overtones of toluene in a range of 1600–1800 nm are also utilized. Results obtained both with unmodified fibers and with fibers modified by the application of methyltriethoxysilane-based xerogel layers onto the hole walls are presented. The results show that fibers modified by the xerogel layers are sensitive to gaseous toluene in the cladding holes. Changes of the output power can also be determined with fibers unmodified by the xerogel layers, namely for saturated toluene mixtures. The presence of toluene in the detection layers was proved by spectral measurements.
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 0i 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 0.56nm/°C are obtained for the LPGs with periods of 1000 and 500μm 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.
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