Fiber-optic evanescent field absorption sensor: A theoretical evaluation

A comparative study of the fiber-optic evanescent field absorption sensors based on parabolic, linear, and exponential-linear taper profiles has been carried out. The expressions for the effective evanescent absorption coefficient of the fluid have been derived for a diffuse source as well as a collimated source-microscope objective combination. It has been shown that the sensitivity of the sensor depends on the taper profile and the type of the source used in addition to the numerical aperture of the fiber and the refractive index of the fluid. For a given taper profile, the sensitivity is more in the case of the collimated source-microscope objective combination as compared to the diffuse source illumination. Further, the sensor with the exponential-linear taper profile is more sensitive than those having parabolic and linear profiles in the case of both sources.     

Fiber-optic evanescent field absorption sensor: A theoretical evaluation

Abstract

A comparative study of the fiber-optic evanescent field absorption sensors based on parabolic, linear, and exponential-linear taper profiles has been carried out. The expressions for the effective evanescent absorption coefficient of the fluid have been derived for a diffuse source as well as a collimated source–microscope objective combination. It has been shown that the sensitivity of the sensor depends on the taper profile and the type of the source used in addition to the numerical aperture of the fiber and the refractive index of the fluid. For a given taper profile, the sensitivity is more in the case of the collimated source–microscope objective combination as compared to the diffuse source illumination. Further, the sensor with the exponential-linear taper profile is more sensitive than those having parabolic and linear profiles in the case of both sources.     

Gas sensor response time analysis for tapered porous-clad fibers

The detection of gases with the use of ATR spectroscopy in a multimode step-index-tapered fiber with porous cladding has been described. Geometrical optics has been used to derive an expression for the time-dependent evanescent absorbance as the gas diffuses through the cladding. The ratio of the radii of core and cladding has been assumed to be constant throughout the length of the taper. It has been shown that the relative evanescent absorbance depends on the taper ratio. As the taper ratio increases, the response time of the sensor decreases.     

Surface plasmon resonance based tapered fiber optic sensor with different taper profiles

A theoretical model for surface plasmon resonance (SPR) based tapered fiber optic sensor is proposed with three different taper profiles, namely, linear, parabolic, and exponential-linear. The effect of taper ratio and taper profiles on the sensor’s performance is studied in detail and the design considerations for significantly enhanced sensitivity are reported. The study shows that the exponential-linear taper profile with high taper ratio provides the best performance. The physical reasons behind sensitivity enhancement due to taper ratio and taper profile are given, wherever required.     

Effect of launching condition in response time analysis for few selected tapered porous clad optical fiber gas sensors

In this paper, the effect of launching condition of light and the geometry of the sensing region in the case of detection of gases using attenuated total internal reflection (ATR) technique in multimode step index tapered fiber has been described. Using ray approach the response time of the sensor in case of parabolic, linear and exponential-linear tapered optical fiber have been carried out. It has been shown that launching condition and geometry of the sensing region effect the sensitivity of the sensor. Out of three designs the sensor having exponential-linear tapered with selected ray launching has maximum sensitivity and low response time of the sensor.     

Experimental Studies on the Response of the Fiber Optic Evanescent Field Absorption Sensor

In the present study the response of the fiber optic evanescent field absorption sensor has been analyzed experimentally. The dependence of evanescent absorbance on sample (dye) concentration has been found to be nonlinear, in contradiction to theoretical predictions. The amount of nonlinearity as well as evanescent absorbance has been found to depend on the pH of the sample. As the pH decreases, the amount of nonlinearity and evanescent absorbance decrease. It has been shown that the nonlinearity and high evanescent absorbance at high pH value occur because of the adsorption of the positively charged dye molecules on the surface of the silica core of the plastic-clad silica (PCS) fiber, which is generally used for the sensor. A linear response has been obtained at pH close to 2.0, at which the presence of a tremendous amount of H ions in the sample restricts the adsorption of the dye molecules. The following empirical relation has been found to fit the experimental data: gamma 1.1C 1.6(pH)32C 1 3, where gamma is the evanescent absorption coefficient and C is the concentration of the dye. The first term is due to the simple Beer's law while the second term is the contribution of the pH dependent dye adsorption. In addition, we have found that the increase in core diameter increases the contribution of adsorption.     

Refractometer based on a tapered Mach–Zehnder interferometer with Peanut-Shape structure

A novel refractometer based on tapered Mach–Zehnder modal interferometer (MZI) is proposed and experimentally demonstrated. This sensor is composed of a pair of Peanut-Shape structures and an embedded taper – the former excites high-order cladding modes, while the latter enhances the evanescent field. As the effective refractive index (RI) of cladding is based on the changes of surrounding RI, thus extinction ratio will change due to the alteration of the distribution of power in the fiber which is induced by various differences of core and cladding for RI. As a result, the maximum RI sensitivity of 240.78 extinction ratio/RIU (refractive index unit) is achieved within the range from 1.3334 to 1.4081.     

An effective attenuation coefficient for evanescent wave spectroscopy using multimode fiber

An attenuation coefficient for evanescent wave spectroscopy on the core of a multimode step index fiber in contact with an absorbing species is derived in terms of the range of incident angles of the meridional rays to the interface. An expression, for the fractional power outside the core as a function of incident angles is also given. Their ratio is discussed for the case of a fiber sensor having a porous cladding in contact with a gaseous absorbing species and one in which a section of cladding has been removed and replaced by an absorbing liquid.     

基于腐蚀光纤的温度及葡萄糖溶液浓度传感器

提出并制备了一种基于本征倏逝波原理的温度及葡萄糖溶液浓度传感器.通过研究腐蚀包层厚度与透射光谱之间的关系,确定较为合适的腐蚀厚度.将标准单模光纤包层腐蚀至2.4μm,利用光纤倏逝波对外界介质变化敏感的原理,通过测量输出光功率的变化量实现温度及葡萄糖溶液浓度传感.实验结果表明:传感器在1070℃的温度范围内具有9.58×10~(-3) dBm/℃的灵敏度,线性度达到99.36%;在葡萄糖溶液03%的浓度范围内具有0.126dBm/(g/L)的灵敏度,线性度达到97.95%.该传感器的响应时间小于30s,具有操作简便、测量准确度高、重复性好、适用范围广等优点,具备良好的应用价值.     

Fiber-optic sensor based on evanescent wave absorbance around 2.7 μm for determining water content in polar organic solvents

The feasibility of sapphire fiber-optic sensors based on evanescent wave absorption spectroscopy in the infrared range for quantitative determination of water content in polar organic solvents has been investigated. Evanescent wave absorption spectra of sapphire fiber-optic sensors in glycerol, ethanol, and glycol with different water concentrations obtained and analyzed, respectively. Evanescent absorbance of the sensors in those organic solvents has been utilized to implement for in situ monitoring water concentration in organic solvents. The evanescent absorbance of sensors in glycerol and glycol has been found to vary linearly with water content in the range 0–30 % and in ethanol in the range 0–10 %, respectively. The fiber-optic sensors based on evanescent absorbance for monitoring water concentrations in those organic solvents are acceptably accurate, cost-effective, and reliable. Some methods to improve the accuracy of predicated water content in those organic solvents are also suggested. Overall, the results demonstrate that the sapphire fiber-optic sensor based on evanescent absorption spectroscopy is a promising candidate for prediction of water content in polar organic solvents in on-line and remote situation.     

Flow Injection Small-volume Fiber-optic pH Sensor Based on Evanescent Wave Excitation and Fluorescence Determination

A small-volume fiber-optic pH sensor (FOEWS) based on evanescent wave excitation is developed and evaluated. The sensor is simply fabricated by inserting a decladded optical fiber into a transparent capillary tube. A microchannel between the optical fiber and the capillary inner wall was formed and acted as flow cell for solution flowing through. The pH-sensitive fluorophore of fluorescein can be excited by the evanescent wave field produced on the fiber core surface to produce emission fluorescence. pH value was then sensed by its enhancing effect on the emission fluorescence intensity. The response range of the sensor is from pH 2.09 to pH 8.85 and the linear range is from pH 3.25 to 8.85. The proposed sensor has a small detection volume of 2.5 μL and a short response time of 8 s. It has been applied to measure pH values of real water samples and was in good agreement with the results obtained by commercial pH meter.     

A novel probe for an evanescent wave fibre-optic absorption sensor

A novel, highly sensitive probe consisting of a uniform region sandwiched between two linear tapers for an evanescent wave fibre-optic absorption sensor is proposed. The diameter of the uniform region is chosen to be equal to the minimum allowed diameter of the taper's output end. The first taper is used to bring the angles of the rays in the input fibre close to the critical angle of the sensing region while the second taper reconverts the angles into their initial values so that they can propagate in the output fibre. It has been shown that the sensitivity of the sensor depends on the length of the uniform region of the probe, the refractive index of the fluid and the numerical aperture of the fibre. The sensitivity increases with increase in the length of the uniform region and also with decrease in the numerical aperture of the fibre. In addition, the sensitivity of the proposed probe can be further increased by launching the selected rays into the input fibre.     

Effect of the Pd-Au thin film thickness uniformity on the performance of an optical fiber hydrogen sensor

Thin alloy film of Pd and Au, formed by simultaneous electron-beam and thermal evaporation techniques, respectively, is used in the design of an optical fiber hydrogen sensor. The sensor consists of a multimode fiber (MMF) in which a short section of single mode fiber (SMF), coated with the Pd-Au thin film, is inserted. Due to core diameter mismatch, the SMF cladding guides light, allowing the interaction between the sensing layer and the guided light. When the sensor is exposed to hydrogen, the Pd-Au layer refractive index diminishes and causes attenuation changes on the transmitted light. Several samples with different layer thickness uniformity were fabricated and tested in a very simple experimental set-up. We have observed that the sensor signal change is dependant on layer thickness uniformity, since the effective interaction length between the evanescent field and the sensing layer is increased. By contrast, such uniformity practically has no influence on the time response of the sensor. The resulting Pd-Au film can detect 4% hydrogen with a response time of 15 s.     

Compact optical fiber curvature sensor based on concatenating two tapers

A low-loss, compact, and highly sensitive optical fiber curvature sensor is presented. The device consists of two identical low-loss fused fiber tapers in tandem separated by a distance L. When the optical fiber is kept straight and fixed, no interference pattern appears in the transmitted spectrum. However, when the device is bent, the symmetry of the straight taper is lost and the first taper couples light into the cladding modes. In the second taper, a fraction of the total light guided by the cladding modes will be coupled back to the fundamental mode, producing an interference pattern in the transmitted spectrum. As the fiber device is bent, visibility of the interference fringes grows, reaching values close to 1. The dynamic range of the device can be tailored by the proper selection of taper diameter and separation between tapers. The effects of temperature and refractive index of the external medium on the response of the curvature sensor is also discussed.     

A rigorous analysis of the distribution of power in plastic clad linear tapered fibers

An analytical investigation is presented of the propagation of power in a step-index plastic clad tapered optical fiber operating in the infrared region of the electromagnetic (EM) spectrum. The chosen materials are widely used in low cost optical links. It is assumed that the taper section has a linear profile. Following rigorous analytical approach, the general expressions for power in the core and the cladding sections are derived, and a study is presented of the variation of the relative power along the propagation direction in respect of different (meriodinal and skew) lower order modes. It is observed that, in general, the confinement of power is fairly high in the core section, and the confinement increases with the increase in the taper length and/or mode index. In the cladding region, the confinement decreases for higher taper lengths.     

光纤倏逝波型石英增强光声光谱技术

采用块状光学准直聚焦透镜组的传统石英增强光声光谱(QEPAS)技术存在体积难以缩减,结构稳定性不佳,无法适应空间狭小、振动复杂的特殊环境等缺点.基于此,将光纤倏逝波技术与QEPAS技术相结合,提出了一种新型微纳结构光纤QEPAS痕量气体检测技术.实验中,为了提高QEPAS系统信号幅值,优化了石英音叉与激光束的空间位置、激光波长调制深度,同时对比了两种不同共振频率的石英音叉,最终采用共振频率较低的30.720 kHz石英音叉作为声波探测元件,获得的检测极限为6.25×10~(-4)(体积分数),归一化噪声等效吸收系数为4.18×10~(-7)cm~(-1).W·Hz~(-1/2).     

Influence of tapering process on changes of optical fiber refractive index distribution along a structure

Investigations of internal structure changes along an optical fiber during the manufacture of biconical taper have been described. Basing on the constant volume theory, classification of biconical structures manufactured on a special set-up is presented and discussed. The interferometric tomography method has been used for determination of 3D geometry and refractive index distribution in manufactured optical fiber tapers. The experiments provide detailed information on external (diameter of cladding) as well as internal (core diameter and refractive index profile) changes along the taper region. The results have been discussed in relation to the parameters of the manufacturing process.     

Refractive index and temperature sensing characteristics of an optical fiber sensor based on a tapered single mode fiber/polarization maintaining fiber

An evanescent field optical fiber sensor based on a short section of polarization maintaining fiber spliced with a tapered single mode fiber is proposed and experimentally investigated. We mainly focus on the refractive index(RI) and temperature sensing characteristics of this compact device. The transmission spectrum of the resonance wavelength,induced by the interference between the excited low order cladding modes and core modes,shows the quadratic function relationships with RI and linear relationships with temperature. Thus,the proposal of this simple-to-fabricate,compact,and low cost sensor shows its possible potential in the sensitive detection field.     

Graded-index characteristics in single-crystal fibers

Single-crystal fibers of Ti(3+):Al(2)O (3)have been grown by the laser-heated pedestal growth technique and shown to exhibit radial refractive-index gradients. A refractive-index increase of approximately 12% in the fiber core with respect to the fiber sidewalls has been measured. The index profile can be fitted with a parabolic model. Postgrowth treatment of Cr(3+):Gd(3)Sc (2)Al (3)O(12)fibers has been shown to produce a uniform cladding region with a graded-index core. The core index is some 12.5% larger than the cladding region, with an index profile shape that is approximately parabolic in nature.     

The influence of bulk losses and bulk dispersion on the propagation properties of surface waves in a radially inhomogeneous optical waveguide

The influence of bulk losses and bulk dispersion on the propagation properties of surface waves in a radially inhomogeneous optical fibre is investigated numerically. The fibre is assumed to consist of a homogeneous cladding of either infinite or finite extent and a radially inhomogeneous core of higher permittivity. Bulk losses and dispersion in the core and cladding materials are presented by frequency-dependent complex permittivities. Power series expansions for the electromagnetic field quantities in the core have been used and the attenuation and the phase coefficients for the lower order modes have been calculated numerically. Results are presented for two different permittivity profiles, viz. the parabolic profile and the cubic spline profile, the cladding being of infinite extent in this case. Also, the influence of a finite cladding diameter on the numerical results is discussed.The research reported in this paper has been carried out with partial financial support from the Delfts Hogeschoolfonds.