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.
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 104 to 106 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.
The paper deals with the preparation and characterization of whispering-gallery-mode (WGM) microresonators. Three
types of materials were used for the preparation of these microresonators, namely silica optical fibers,
polydimethylsiloxane polymer and UV-acrylate polymer. In the case of the silica fibers microspheres were prepared by
heating the fiber tip with a miniburner or CO2 laser. Polymer microresonators were applied onto tips of fiber substrates
when they were dipped into polymer and immediately cured. Microresonators with a diameter ranging from 70 to 1000
μm were prepared.
Transmission properties of the prepared microresonators were investigated in experiments where a microresonator was
excited by a red laser at 660 nm by using evanescent field of a coupling element, namely a glass or silica fiber prism.
The output power from the coupling element was detected. Temporal changes of the output power caused by heating the
microresonators were also measured and explained in terms of the microresonator quality factor.
This paper presents new experimental results on the sensitivity of a microstructure fiber (MSF) to toluene and ethanol
in gaseous mixtures with nitrogen. This sensitivity is compared with that determined with a single-capillary fiber (CF).
The results obtained both with the fibers modified by a porous detection layer prepared by the sol-gel method from
tetraethoxysilane are presented. MSF with air holes arranged in one shell and coated with an UV acrylate jacket was
prepared by using the "stack and draw" technique. CF coated with polydimethylsiloxane optical cladding was prepared.
The segments of the prepared fibers were modified by a porous polysiloxane layer applied onto the hole walls. The
sensitivity of the MSF and SCF to gaseous toluene and ethanol in nitrogen was determined from spectral changes of C-H
and O-H absorption bands in a range of 1300-1800 nm. Limits of detection of 0.008 and 0.02 vol.% have been
determined for toluene and ethanol, respectively.
Bending characteristics of long-period gratings (LPGs) written in fiber with parabolic-index cladding are investigated
experimentally. The fiber has a uniform outer cladding and an inner cladding, where the refractive index decreases
quadratically with radius. A recent theoretical analysis has shown that these LPGs can be highly insensitive to bending.
A very small bend-induced wavelength shift of the attenuation band of only 5 nm was predicted for an LPG bent with a
uniform curvature of 100 m-1. Experiments to prove the predicted bend insensitivity are done with LPGs written with a
CO2 laser at one-side and three-side laser irradiation of the fiber. The LPG is bent in an arrangement, where the curvature
is maximum in the LPG's center and decreases linearly along the bent LPG in both directions. Bend-induced wavelength
shifts of an attenuation band appearing at about 1560 nm and attributed to coupling between the fiber modes LP01 and
LP02, are examined for four rotational orientations of the LPG inscribed at three-side laser irradiation. The largest shift of
0.46 nm is obtained for a maximum curvature (in the middle of the LPG) of 11.6 m-1, (or average curvature of 5.8 nm-1)
corresponding to the largest deflection applied on the LPG in the experiments. The depths of the bands of the above LPG
at the -5.5-dB level are observed to vary by less than 0.3 dB over the whole range of applied curvatures.
Long-period gratings (LPGs) in an optical fiber, which was designed and fabricated for achieving high insensitivity of the inscribed LPGs both to bending and to the external index of refraction, are investigated. The fiber has a double cladding, consisting of a uniform outer cladding and a graded-index inner cladding, where the index of refraction decreases approximately parabolically with radius. This fiber is referred to as the parabolic-index cladding (PIC) fiber. Results obtained in the preparation of preforms and drawing PIC fibers are given. The theoretical analysis, carried out by the mode expansion and beam propagation method, yields field distributions and effective indexes of the fundamental core mode LP01 and several low-order circularly symmetric cladding modes LP0m for several external indexes of refraction. Experiments are done with an LPG with a period of 214 μm inscribed with a CO2 laser in the PIC fiber. In experiments, transmission spectra of the LPG immersed in three liquids with refractive indexes in a relatively wide range around the index of silica (water, silica-index-matching liquid, silicone oil) are measured in a temperature range of 25-67.5°C. From experiments it follows, in agreement with the theoretical analysis, that the spectral positions of the investigated attenuation band at &lgr; ≈ 1550 nm (coupling between the LP01 and LP02 mode), determined at temperatures in the above range, are highly insensitive to the external index of refraction.
Fibers with inverted-graded index (IGI) profiles in the core with all refractive-index values above that of silica were drawn from performs doped in the core with germanium oxide which were prepared by the MCVD method. The sensitivity of the fibers to refractive-index changes of the optical cladding was theoretically determined on the basis of measured refractive-index profiles by using ray optics and assuming axial excitation of the fibers. The theoretical results are compared with experimental data measured in model immersing experiments.
KEYWORDS: Optical fibers, Refractive index, Cladding, Step index fibers, Silica, Biological and chemical sensing, Boron, Oxides, Collimation, Graded index fibers
Multimode optical fibers with inverted parabolic refractive- index profiles (IGI fibers) have been shown promising for the fabrication of sensing modules for evanescent-wave chemical detection. Besides IGI fibers with parabolic index profiles, this paper deals with theoretical and experimental investigation of some other types of IGI fibers, particularly with IGI fibers with linear and modified-linear profiles. Refractive-index profiles of the fabricated IGI preforms doped in the core with boron oxide are shown in the paper. The sensitivity of IGI fibers drawn from these preforms to cladding refractive-index changes has been studied by means of the technique of selective excitation of optical models in the fiber immersed in various mixtures of n-butanol and benzyl alcohol. The obtained results are compared with those for reference step-index fibers with uniform pure-silica core. A theoretical model describing the detection process during these measurements has been derived using geometrical optics. On this basis it can be shown that the fiber sensitivity increases in the order of the reference fiber, IGI fiber with linear and modified-linear profiles and IGI fibers with parabolic profiles.
It has been found that thin layers of aerogels applied by the sol-gel method on silica optical fibers change their optical properties due to the interaction with gases such as hydrocarbons. This paper deals with modification of the sensitivity and selectivity to vapors of hydrocarbons and chlorinated hydrocarbons of siloxane aerogel layers based on their doping with TiOTi chains and/or immobilizing in them Cu-ephedrine and Cu-phthalocyanine. The sensing fibers were prepared by the sol-gel application of porous siloxane and doped-siloxane layers on bare cores of PCS fibers. By using the excitation of the fiber with inclined laser beams it is experimentally shown that there are positive or negative changes of the output optical power from the fibers due to bringing the prepared layers into contact with vapors of aliphatic, aromatic and chlorinated hydrocarbons in nitrogen. These effects may be related to a difference between the refractive index of the layer and refractive index of liquid hydrocarbon or hydrocarbon its derivative. It is shown experimentally that doping porous siloxane layers with copper complexes changes their refractive index and optical losses changing thus their sensitivity and selectivity to hydrocarbons and hydrocarbon derivatives.
In this paper we describe the detection of polychlorinated biphenyls (PCBs) which is based on the measurement of changes of optical absorption at 400 nm of the medium in an aerobic bioreactor with immobilized cells Pseudomonas species 2. The rate of production, composition and the concentration of yellow intermediates are influenced by concentration and composition of PCB mixtures, concentration of cells and by the methods of immobilization. The method was applied in the detection of commercial mixture D103. It was found that the advantageous carriers were inorganic or organic-inorganic matrices, which sorbed PCBs and a cell outgrowth from their surface was low. In water contaminated with transformer oil and chlorinated hydrocarbons the detection limit is 10-2 gD103/kg. In transformer oil the upper limit for degradation of D103 by sodium dehalogenation (1.5 gD103 /kgoil) was determined also in the presence of the same concentration of trichloroethylene. The employment to of a liquid core waveguide spectrophotometer instead of a diode array spectrophotometer increased the sensitivity of the measurement of yellow intermediates by a factor of 100. An extrinsic fiber-optic sensor was used for in-situ measurement during biodegradation of PCBs in bioreactors.
The application of a novel sensing fiber with an inverted parabolic-index profile to the detection of refractive-index changes of a medium surrounding the fiber core is investigated. The detection experiments were made with a set- up consisting of a fiber connector-based coupler allowing the excitation of the sensing fiber by a light source of a relatively small size placed on the fiber axis in a preset distance from the fiber end face, and an optical cell containing a declad segment of the sensing fiber in an immersion whose refractive index could be varied in a known way by heating. Various light sources, such as outputs of multimode and single-mode fibers were tested as more or less accurate approximations to the ideal point light source. For overcoming problems with the profile imperfections in the central core region appearing in some of the produced fibers and originating from the fiber manufacturing technology, the excitation of the fiber by an inclined beam from a bevelled single-mode fiber was studied as well. For comparison, PCS fibers were used in the experiments, too. The results have shown that in an index range slightly below the index of silica, higher sensitivity can be achieved with the novel fiber than with PCS fibers. The possibility of shifting the measuring range down to a more practical lower index values by using inclined-beam fiber excitation and confinement of the index profile to the outer region at the core will be shown.
This paper deals with two novel modifications of the inverted- graded index (IGI) fibers, namely with IGI fibers with a truncated inverted-linear refractive-index profile and with a truncated inverted parabolic refractive-index profile which is localized only to part of the core. Preforms for drawing the fibers were prepared by the MCVD method by using boron oxide for decreasing the refractive index of silica. Fibers were drawn from the prepared preforms and coated with polydimethylsiloxane polymeric cladding. Fabrication of the fibers and its relation to characteristics of the prepared fibers such as refractive-index profiles are described. The experimental sensitivity of the prepared IGI fibers to changes of the refractive index of the cladding was determined in immersion experiments under the excitation of the fiber with an inclined collimated beam. Results of ray-optics theoretical analysis of these sensing structures for the above excitation conditions are given, which show the possibility of further increasing the sensitivity to refractive-index changes of the optical cladding of IGI fibers even for claddings with a refractive index value of about 1.41 (commercial PCS fibers). The obtained theoretical and experimental results show higher evanescent-wave sensitivity of the two prepared novel sensing IGI fibers in comparison with standard PCS fibers.
The paper examines the fabrication and properties of silica optical fibers suitable for the development of evanescent- wave fiber-optic sensors. Attention is given to factors increasing the fraction of power transmitted in the evanescent wave. The effects of the refractive-index profile and fiber bending in the evanescent-wave area and their relation to the sensor sensitivity are investigated. A simple thermometer is developed on the basis of these effects. Approaches for the modification of optical properties in the evanescent-wave area are examined. In this part, porous layers prepared by the sol-gel method based on the silicon and titanium alkoxides and standard polysiloxane polymer coatings are described. The sol-gel derived layers are investigated from the point of view of preparing an optical core with a porous part. The possibilities of controlling the refractive index, as well as absorption coefficient of the coatings by immobilizing pH indicators and Cu2+-phthalocyanine are shown together with examples of using evanescent-wave sensing structures for the detection of organophosphorous compounds and pH measurements.
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