Unclad Fiber Research Articles

An integrated micro-volume fiber-optic sensor for oxygen determination in exhaled breath based on iridium(iii) complexes immobilized in fluorinated xerogels

A novel integrated fiber-optic sensor with micro detection volume is developed and evaluated for O(2) determination on a breath-by-breath basis in human health monitoring applications. The sensing element was fabricated by dip-coating an uncladded optical fiber with [Ir(piq)(2)(acac)]-doped hybrid fluorinated ORMOSIL (organically modified silicate) film, which was prepared from 3,3,3-trifluoropropyltrimethoxysilane (TFP-TriMOS) and n-propyltrimethoxysilane (n-propyl-TriMOS). The sensor was then constructed by inserting the prepared optical fiber into a transparent capillary. A microchannel formed between the optical fiber and the capillary inner wall acted as a flow cell for the sample flowing through. The evanescent wave (EW) field produced on the fiber core surface can excite the O(2)-sensitive fluorophores of [Ir(piq)(2)(acac)] to produce emission fluorescence. O(2) can be sensed by its quenching effect on the emission fluorescence intensity. Spectroscopic properties have been characterized by FTIR and fluorescence measurements. Stern-Volmer and Demas models were both employed to analyse the sensor sensitivity, which is 13.0 with the LOD = 0.009% (3σ) and the response time is about 1 s. By integrating the sensing and detection elements on the optical fiber, the novel configuration showed advantages of easy fabrication and low cost. Parameters of sensitivity, response time, repeatability, humidity effect and temperature effect were discussed in detail. The proposed sensor showed potential for practical in-breath O(2) analysis application due to its advantages of easy fabrication, low cost, fast response, excellent hydrophobicity, negligible temperature interference and suitable sensitivity.

Measurement of chlorophyll in water based on laser-induced fluorescence spectroscopy: Using spiral-wound unclad optical fiber system and partial least squares regression

The feasibility of laser-induced fluorescence (LIF) method for chlorophyll concentration determination in water was investigated in this study. The chlorophyll fluorescence spectra were received by a spiral-wound unclad optical fiber system. Baseline and smoothing correction methods were studied. Statistical equations were established between reference data and fluorescence spectra by linear regression and partial least squares (PLSs) regression. The PLS regression outperformed linear regression by higher correlation coefficient of 0.9996063 and root mean square error value of 0.0221807. Based on the results, it was concluded that laser-induced fluorescence method with a spiral-wound unclad optical fiber system was suitable for chlorophyll concentration determination in water, and PLS regression was a reliable method for fluorescence spectra analysis.

Research for Biofouling Detection Based on Optical Fiber Self-Referencing Technique and ANN

More attention is paid to on-line monitoring of biofouling in industrial water process systems. Combined optical fiber self-referencing technology with artificial neural networks (ANN) technology, real time detection technique for forming thickness and ingredient is put forward, which provides technical support and reliable data for analyzing biofouling influencing factors, contaminant separation and warning. Unclad fiber sensing mechanism and self-referencing fiber optic sensor are presented. Compensation technology based on radial basis function (RBF) neural network and learning algorithm are studied in order to solve the problem of measurement precision and range. Biofouling forming and optical characteristics in industrial real water systems are researched. A new method is provided for the research on biofouling in real water system, which can be used in other fields such as mining, environment protection, medical treatment and transportation of oil, gas and water.

Cross-Comparison of Surface Plasmon Resonance-Based Optical Fiber Sensors With Different Coating Structures

In this paper, optical fiber sensors, based on both surface plasmon resonance (SPR) and localized SPR (LSPR) approaches, have been successfully developed and cross-compared. The SPR sensor has been constructed by coating a thin layer of gold film on a piece of an optical fiber with the cladding having first been removed. Several LSPR sensors, however, have been configured in a slightly different way through coating several unclad fibers, each with a thin layer of gold-nanorods (GNRs) with a different aspect ratio. Subsequently, both SPR and LSPR sensors are exposed to solutions with different refractive index and their corresponding plasmon resonance wavelength shifts have been recorded and cross-compared. The experimental results obtained confirm that the SPR sensor has demonstrated a much higher sensitivity than that of any of the LSPR sensors, irrespective of the variation of the aspect ratio of GNRs coated. The LSPR sensors, however, have demonstrated a much better linearity compared to that of the SPR sensor.

Evanescent-Wave Fiber-Optic Sensor: On Power Transfer From Core-Cladding Interface to Fiber End-Face

In this paper, the enhancement of collection efficiency in fiber-optic evanescent-wave (EW) sensors is studied. Both theory and experimental results are presented. The key is to consider the roughness conditions at the end-face of large-core fibers. The theory is based primarily on ray optics, but for the sake of simplicity and accuracy, wave optics is also considered. Fluorescent light is coupled into the core of a partly unclad multimode fiber. Most power is carried to the unclad end-face by tunneling modes. Reflection from this rough end-face, which is modeled as a diffuse source, mixes the initial modes. Bound rays also play an important role, carrying the power to the other end-face. The amount of output power of the bound rays there is calculated. We also study the output power in relation to the surface condition of the far end-face, which may be smooth or rough. The comparison of these cases in terms of output power shows that a rough near end-face enhances the collection of coupled fluorescent light. In contrast, roughening of the far end-face while the near end-face is rough causes a transfer of the initial, mostly tunneling, modes to the radiation modes and decreases the collectable signal.

Plastic evanescent microlaser

Lasing has been demonstrated in a photopumped microcavity via evanescent-wave-coupled gain. A 50 -μm-diameter unclad quartz fiber is coated with a polymer gain layer having a refractive index lower than that of the quartz fiber. Upon pulsed photoexcitaiton, the cylindrical microcavity exhibits whispering-gallery-mode lasing. A spectral analysis shows that the resonance inside the quartz fiber is responsible for the occurrence of lasing in the orange-red region of the spectrum. The proposed scheme allows the cavity resonant modes to achieve optical gain through evanescent-wave coupling. This is a step toward a device structure that facilitates current injection lasing.

Recapturing lost evanescent power by tuning end face total internal reflection capable tunneling modes at a roughened fiber end face

We reveal that the overall evanescent wave (EW) power captured by an unclad multimode fiber employed in a sensing configuration is determined by the tunneling modes, not the guided modes. While enormous in strength, most of this power is inaccessible using traditional EW power enhancers. However, we found that by roughening the fiber end face, this supposedly lost power can be recaptured and thus can boost the detectable power level significantly. Intensive mode mixing events across various mode categories are proposed to interpret the observed phenomenon.

Integration of fiber optic-particle plasmon resonance biosensor with microfluidic chip

This article reports the integration of the fiber optic-particle plasmon resonance (FO-PPR) biosensor with a microfluidic chip to reduce response time and improve detection limit. The microfluidic chip made of poly(methyl methacrylate) had a flow-channel of dimensions 4.0 cm × 900 μm × 900 μm. A partially unclad optical fiber with gold or silver nanoparticles on the core surface was placed within the flow-channel, where the volume of the flow space was about 14 μL. Results using sucrose solutions of various refractive indexes show that the refractive index resolution improves by 2.4-fold in the microfluidic system. The microfluidic chip is capable of delivering a precise amount of biological samples to the detection area without sample dilution. Several receptor/analyte pairs were chosen to examine the biosensing capability of the integrated platform: biotin/streptavidin, biotin/anti-biotin, DNP/anti-DNP, OVA/anti-OVA, and anti-MMP-3/MMP-3. Results show that the response time to achieve equilibrium can be shortened from several thousand seconds in a conventional liquid cell to several hundred seconds in a microfluidic flow-cell. In addition, the detection limit also improves by about one order of magnitude. Furthermore, the normalization by using the relative change of transmission response as the sensor output alleviate the demand on precise optical alignment, resulting in reasonably good chip-to-chip measurement reproducibility.

Optical Fiber LSPR Biosensor Prepared by Gold Nanoparticle Assembly on Polyelectrolyte Multilayer

This article provides a novel method of constructing an optical fiber localized surface plasmon resonance (LSPR) biosensor. A gold nanoparticle (NP) assembled film as the sensing layer was built on the polyelectrolyte (PE) multilayer modified sidewall of an unclad optical fiber. By using a trilayer PE structure, we obtained a monodisperse gold NP assembled film. The preparation procedure for this LSPR sensor is simple and time saving. The optical fiber LSPR sensor has higher sensitivity and outstanding reproducibility. The higher anti-interference ability for response to an antibody makes it a promising method in application as a portable immuno-sensor.

Using ac-Field-Induced Electro-osmosis to Accelerate Biomolecular Binding in Fiber-Optic Sensing Chips with Microstructures

This article reports the use of ac-field-induced charges at the corners of microstructures on fiber-optic sensing chips to generate electro-osmotic vortex flows in flow cell channels that can accelerate solute binding on the fiber. The sensing chip made of a cyclic olefin copolymer COC substrate contained a flow cell channel of dimensions 15 mm x 1 mm x 1 mm. A partially unclad optical fiber was placed within the channel. Relief-like strip structures of 25-mum thickness fabricated on the channel bottom were produced with an injection-molding process. The external electric field lines penetrating through the corners of the plastic microstructures induce charges on the corner surfaces to build up electrical double layers. When a high-frequency ac field (approximately 100 kHz) is used to flip the field polarities quickly, neutralization of the induced charge cannot be accomplished. The electrical double layer is therefore sustained. When absorbed charges in the double layer are driven by the external field, electro-osmotic flows are generated. The unclad portion of the fiber was coated with biotin-functionalized gold nanoparticles. The streptavidin solution was filled in the channel from the feeding tube, and the ac field (approximately 50 V/cm) was subsequently turned on for 30 s. The ac-field-induced electro-osmotic flows can accelerate solute transport in the sensing channel to enhance the binding kinetics of streptavidin molecules with biotin probes implanted on the gold nanoparticle surface. As a result, the fiber-optic localized plasmon resonance (FO-LPR) sensing signal becomes steady as soon as the external field is turned off. In contrast, the signal cannot reach steady state until 200-300 s in a typical static sensing cell. A significant reduction in the sensing response time is demonstrated. The binding assay of streptavidin with immobilized biotin on gold nanoparticle-coated sensing fibers was validated using this mixing device. The detection limit for streptavidin of approximately 10(-11) M is close to the reported values obtained using static cells. Similarly, the sensing response time of an orchid Odontoglossum ringspot virus (ORSV) sample was reduced from 1000 to 330 s when an external field was applied to mix the fluid for 60 s, even though the detection limit was maintained.

Preparation of optical fibers based on Ge–Sb–S glass system

Unclad optical fibers based on Ge–Sb–S glass systems were prepared by preform drawing. It was shown that optical losses in fibers depend on the content of limiting impurities in glass. The correlation between the level of optical losses in fibers and the content of gas-forming dissolved impurities and heterophase inclusions in glasses was established. The minimum optical losses were equal to 300 dB/km at 5.35 μm wavelength in fiber from Ge 25Sb 10.5S 64.5 glass prepared using chemical-distillation melt purification.

Simulation of biochemical binding kinetics on the microfluidic biochip of fiber-optic localized plasma resonance (FO-LPR)

A reflection-based localized surface plasma resonance fiber-optic probe for chemical and biochemical sensing, called fiber-optic localized plasma resonance (FO-LPR), has been proposed in the literature. Bio-molecular recognition is detected by the unique optical properties of self-assembled gold nanoparticles on the unclad portions of an optical fiber whose surfaces are modified with a receptor. To enhance the performance of the sensing platform, the sensing element is integrated with microfluidic chips to reduce sample and reagent volume, to shorten response time and analysis time, as well as to increase sensitivity. The main purpose of the present study is to simulate the biochemical assays in the FO-LPR microfluidic chip and to investigate the effects parameters, such as inlet concentrations of analyte or the flowrate on the biochemical binding kinetics. The geometry of the grooved channel is also proposed to enhance the biochemical binding on the unclad optical fiber. The results reveal that the chaotic mixing generated by the grooves enhances the biochemical binding when the injected flowrate is high and, because of that, limits the performance of the molecular mixing. The enhancement of biochemical binding performance was significant, especially at the low injected concentration of analyte.

Quantitative measurement of binding kinetics in sandwich assay using a fluorescence detection fiber-optic biosensor

Fiber-optic biosensors have been studied intensively because they are very useful and important tools for monitoring biomolecular interactions. Here we describe a fluorescence detection fiber-optic biosensor (FD–FOB) using a sandwich assay to detect antibody–antigen interaction. In addition, the quantitative measurement of binding kinetics, including the association and dissociation rate constants for immunoglobulin G (IgG)/anti-mouse IgG, is achieved, indicating 0.38 × 10 6 M −1 s −1 for k a and 3.15 × 10 −3 s −1 for k d. These constants are calculated from the fluorescence signals detected on fiber surface only where the excited evanescent wave can be generated. Thus, a confined fluorescence-detecting region is achieved to specifically determine the binding kinetics at the vicinity of the interface between sensing materials and uncladded fiber surface. With this FD–FOB, the mathematical deduction and experimental verification of the binding kinetics in a sandwich immunoassay provide a theoretical basis for measuring rate constants and equilibrium dissociation constants. A further measurement to study the interaction between human heart-type fatty acid-binding protein and its antibody gave the calculated kinetic constants k a, k d, and K D as 8.48 × 10 5 M −1 s −1, 1.7 × 10 −3 s −1, and 2.0 nM, respectively. Our study is the first attempt to establish a theoretical basis for the florescence-sensitive immunoassay using a sandwich format. Moreover, we demonstrate that the FD–FOB as a high-throughput biosensor can provide an alternative to the chip-based biosensors to study real-time biomolecular interaction.

Modeling of the fringe shift in multiple beam interference for glass fibers

A quadratic model is suggested to describe the fringe shift occurred due to the phase variations of uncladded glass fiber introduced between the two plates of the liquid wedge interferometer. The fringe shift of the phase object is represented in the harmonic term which appears in the denominator of the Airy distribution formula of Fabry-Perot’s interferometer. A computer program is written to plot the computed fringe shifts of the described model.

Cold-Atom Physics Using Ultrathin Optical Fibers: Light-Induced Dipole Forces and Surface Interactions

The strong evanescent field around ultrathin unclad optical fibers bears a high potential for detecting, trapping, and manipulating cold atoms. Introducing such a fiber into a cold-atom cloud, we investigate the interaction of a small number of cold cesium atoms with the guided fiber mode and with the fiber surface. Using high resolution spectroscopy, we observe and analyze light-induced dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms. The latter can be assigned to the modification of the vacuum modes by the fiber.

Broad range pH sensor based on sol–gel entrapped indicators on fibre optic

A broad-range fibre optic pH sensor based on evanescent wave absorption is presented in this paper. This sensor is prepared by immobilizing a mixture of three pH sensitive indicators (dyes): cresol red, bromophenol blue and chlorophenol red onto the unclad fibre surface using a sol–gel cladding technology. Triton is introduced into the sol–gel cladding to improve the cladding quality. Smooth and strong sol–gel cladding with entrapped indicators and triton has been fabricated and observed using a scanning electron microscope (SEM), an energy dispersive X-ray detector (EDX) and an atomic force microscopy (AFM). The pH sensor based the cladding has shown a linear, reversible and repeatable response over a broad range of pH values between 4.5 and 13.0.

Localized Surface Plasmon Coupled Fluorescence Fiber-Optic Biosensor with Gold Nanoparticles

A novel fiber-optic biosensor based on a localized surface plasmon coupled fluorescence (LSPCF) system is proposed and developed. This biosensor consists of a biomolecular complex in a sandwich format of <antibody/antigen/Cy5-antibody-gold nanoparticle (GNP)>. It is immobilized on the surface of an optical fiber where a <Cy5-antibody-GNP> complex forms the fluorescence probe and is produced by mixing Cy5-labeled antibody and protein A conjugated gold nanoparticles (Au-PA). The LSPCF is excited by localized surface plasmon on the GNP surface where the evanescent field is applied near the core surface of the optical fiber. At the same time, the fluorescence signal is detected by a photomultiplier tube located beside the unclad optical fiber with high collection efficiency. Experimentally, this novel LSPCF biosensor is able to detect mouse immunoglobulin G (IgG) at a minimum concentration of 1 pg/mL (7 fM) during the biomolecular interaction of the IgG with anti-mouse IgG. The analysis is expanded by a discussion of the amplification of the LSPCF intensity by GNP coupling, and overall, this LSPCF biosensor is confirmed experimentally as a biosensor with very high sensitivity.

Fiber-optic thermometry and its application

The design of a distributed fiber-optic sensor of radiation heat flow (RHF) is presented. A measuring technique has been developed and results of measuring the “effective” transmission coefficient of an uncladded fiber under its lateral illumination have been obtained. An algorithm for measuring the RHF supplied in the process of testing specimens of materials on the radiant heating facility is proposed. Testing experiments have been performed and a good correlation between the readings of the distributed RHF sensor and the temperature in the specimens of materials measured by thermocouples has been observed.

Less-mode optic fiber evanescent wave absorbing sensor: Parameter design for high sensitivity liquid detection

This paper describes a theoretical way to design a fiber-optic evanescent wave sensor made by a partly unclad commercial optical fibers. The effects of the sensor's main parameters, such as absorbing medium refractive index, diameter of the fiber's bare core and the launching conditions, are analyzed theoretically. The differences of the radiation-field intensity in each optical mode are considered. The sensing region was obtained by HF etching with a silicon support. The prototype sensing devices were tested and partly explained by the calculation results. The methylene blue dye of 3.5 × 10 −7 mol/L concentration was detected with the length of the sensing region of 14 mm. The bromcresol green in de-ionized water was measured in the experiment with 6.0 × 10 −7 mol/L concentration.

Compact modal interferometer built with tapered microstructured optical fiber

A modal interferometer was built with tapered large-mode-area microstructured optical fiber (MOF). The tapering was introduced by slowly stretching the fiber while it was heated with a high temperature flame torch. With this "slow-and-hot" tapering process, the MOF air holes collapse and the taper waist is transformed into a solid unclad multimode fiber. This allows the coupling between the fundamental HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> MOF mode and the HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1m</sub> modes of the solid fiber. The beating of the HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1m</sub> modes makes the transmission spectra of the taper to exhibit an oscillatory pattern. The influence of the taper geometry and wavelength on the interference pattern is discussed. The interferometer can be used for diverse applications