Optical Fiber Refractive Index Sensor Research Articles

Refractive index sensor based on fiber loop ring-down spectroscopy

ABSTRACTAn optical fiber refractive index sensor based on fiber loop ring-down spectroscopy and a tapered fiber was fabricated using an ordinary single mode fiber with an arc fusion splicer. The performance of the sensor was controlled by the parameters of the tapered fiber. A fiber loop ring-down spectroscopy system was employed to enhance the sensitivity and demodulate the transmission spectrum. The results showed that a sensor with a waist diameter of 14 m and a length of 1.2 mm had good optical performance. By monitoring the ring-down time of the system, relatively high sensitivity of 411.576 s/ RIU was achieved with refractive index values from 1.333 to 1.412. This sensor offers few interferences, high sensitivity, easy fabrication, and low cost.

Fiber-optic refractometer based on a phase-shifted fiber Bragg grating on a side-hole fiber.

A fiber-optic refractive index (RI) sensor based on a π-phase-shifted fiber-Bragg-grating (πFBG) inscribed on a side-hole fiber is presented. The reflection spectrum of the πFBG features two narrow notches associated with the two polarization modes and the spectral spacing of the notches is used for high-sensitivity RI sensing with little temperature cross-sensitivity. The side-hole fiber maintains its outer diameter and mechanical strength. The side-hole fiber is also naturally integrated into a microfluidic system for convenient sample delivery and reduced sample amount. A novel demodulation method based on laser frequency modulation to enhance the sensor dynamic range is proposed and demonstrated.

Sensitivity enhancement of fiber-optic refractive index sensor based on multimode interference with gold nanoparticles

In this study, we investigated combinations of fiber-based multimode interference (MMI) sensors with gold nanoparticles (NPs). Gold NPs were synthesized and attached to the MMI sensor region using a silane-coupling agent. Light absorption due to the NPs was confirmed at wavelengths of 560 nm and longer, including the telecommunication band (1300–1600 nm). We examined the variation in the interfered wavelengths with changes in the medium surrounding the MMI sensor, both with and without NPs. The interfered wavelengths were redshifted when the refractive index (RI) increased, and the shift with NPs was almost twice as large as the shift without NPs. We determined the sensitivity of the MMI sensor with NPs to be approximately 218.28 nm/RIU (refractive index unit) in a refractive index range from 1.31535 to 1.35199.

Optical fiber refractive index sensor with temperature calibration ability

提出一种具有温度自校准功能的光纤折射率（RI）传感器，传感头结构由2段很短的多模光纤（MMF）之间夹熔一段对折射率不敏感的光纤布拉格光栅（FBG）构成，传感头总长度为14 mm，FBG可以为折射率测量提供良好的温度校准功能。实验结果证明，该传感器的折射率灵敏度为126 nm。其干涉光谱共振波长的温度灵敏度为35.09 pm/℃，用于温度校准的FBG的温度灵敏度为11.14 pm/℃。相比于普通的折射率传感器，这种具有温度自校准功能的折射率传感器具有良好的实用前景。

A reflective fiber-optic refractive index sensor based on multimode interference in a coreless silica fiber

A reflective fiber-optic refractive index (RI) sensor based on multimode interference (MMI) is presented and investigated in this paper. The sensor is made by splicing a small section of coreless silica fiber (CSF) to the standard single mode fiber (SMF). A wide-angle beam propagation method (WA-BPM) is employed for numerical simulation and design of the proposed RI sensor. Based on the simulation results, a RI sensor with a length of 1.7cm of CSF is fabricated and experimentally studied. Experimental results show that the characteristic wavelength shift has an approximately linear relationship with the RI of the sample. A sensitivity of 141nm/RIU (refractive index unit) and a resolution of 2.8×10−5 are obtained in the RI range from 1.33 to 1.38. As the RI value is higher than 1.38, the sensitivity of the sensor increase rapidly as the RI increase and a maximum sensitivity of 1561nm/RIU can be achieved, corresponding to a resolution of 2.6×10−6. The experimental results fit well with the numerical simulation results.

Performance evaluation of a bilayer SPR-based fiber optic RI sensor with TiO2 using FDTD solutions

We proposed a new bilayer surface plasmon resonance-based fiber-optic refractive index sensor with silver and an over-layer of TiO2. We numerically investigated the optimal thickness of TiO2 over-layer in the proposed sensor and compared its performance to that based on typical bimetallic layers of silver-and-gold in the aqueous media using finite-difference time domain approach. We show that the use of TiO2 over-layer greatly improves the sensor performance in terms of sensitivity and signal-to-noise ratio compared to that with gold as the over-layer. Not only does the TiO2 over-layer offer a cost-effective alternative to gold for overcoming the oxidation problem, but also it allows resonance wavelength-tunability.

Enhanced refractive index sensing characteristics of optical fibre long period grating coated with titanium dioxide thin films

A new type of fibre-optic refractive index sensor based on a long period fibre grating (LPFG) coated with a titanium dioxide (TiO2) thin film was demonstrated.The wavelength shift of the attenuation bands of this LPFG sensor to changes in the refractive index of the external media from 1.30 to 1.64RIU was investigated. In order to optimize the sensor the TiO2 thin film thickness deposited around the LPFGs was varied from 10 to 80nm.It was found that the TiO2 thin film increases the wavelength sensitivity of the LPFG to changes in the surrounding refractive index for values lower and higher than the cladding refractive index. As opposed to the bare LPFG it was shown the possibility to monitor refractive indices lower and higher than cladding refractive index tailoring the TiO2 thickness.An average wavelength sensitivity of 5250nm/RIU was achieved in the range 1.444 to 1.456RIU for a TiO2 thickness of 50nm. In the region between 1.46 and 1.48RIU the average sensitivity of about 825nm/RIU was measured for a 40nm thick film.

Theoretical modeling of lossy mode resonance based refractive index sensors with ITO/TiO₂ bilayers.

A lossy mode resonance (LMR) based fiber optic refractive index (RI) sensor utilizing indium tin oxide (ITO)/titanium dioxide (TiO₂) bilayers has been theoretically investigated. Specifically, the effect of different thickness ratios of bilayers on the sensitivity to surrounding medium RI variations was studied. It was observed that for a specific thickness ratio, a two fold increase in sensitivity can be achieved in comparison to conventional LMR sensors using a single absorbing thin film. The sensitivity can be enhanced by tuning the total bilayer thickness and bilayer thickness ratio for the desired application, which is not possible in conventional LMR sensors.

Temperature-independent optical fiber Fabry–Perot refractive-index sensor based on hollow-core photonic crystal fiber

A novel optical fiber refractive index (RI) sensor, which is based on an intrinsic Fabry–Perot interferometer (IFPI) formed by a section of hollow-core photonic crystal fiber (HCPCF) and standard single mode fibers (SMF), is proposed. The external refractive index is determined according to the maximum fringe contrast of the interference fringes in the reflective spectrum of the sensor. The RI response performance is demonstrated for the measurement of the different RI solutions. The experimental data agree well with the theoretical results. Also the RI resolution and repeatability of ∼1.5×10−5 and ±0.5% in the linear measurement region, are achieved. In addition, the temperature response is tested from 20°C to 120°C, which exhibits excellent thermal stability. Therefore, such an HCPCF-based F–P sensor provides a practical way to measure RI with non-temperature compensation.

High-sensitivity fiber-optic refractive index sensor based on multimode interference using small-core single-mode fiber for biosensing

A fiber-optic refractive index (RI) sensor using multimode interference (MMI) composed of an unclad multimode fiber (MMF) sandwiched by small-core single-mode fibers (SMFs) has been proposed and demonstrated. The small-core SMF at the input port enhances the diffraction of the light outputted to the MMF, and that at the output port provides a steep coupling characteristic of interfered light at the end of the MMF, resulting in the generation of a sharp signal. This leads to high sensitivity to small RI changes. A very high sensitivity of less than 6 × 10−6 at an RI of more than 1.35 has been realized.

High sensitivity optical fiber temperature sensor based on the temperature cross-sensitivity feature of RI-sensitive device

Considerable part of optical fiber refractive index (RI) sensors suffer from the drawback of cross-sensitivity to temperature because of the thermo-optic effect of materials. In this paper, we propose a straightforward method to utilize the temperature cross-sensitivity feature of an optical fiber RI-sensitive device and thus got a high sensitivity temperature sensor. The sensor consists of a single mode fiber-multimode fiber core(MMFC)-single mode fiber structural refractometer encapsulated into a deionized water-filled cylindrical aluminum alloy shell. Benefiting from the larger thermo-optic coefficient difference between water and MMFC compared with the general cladding and core, the wavelength of transmitted spectrum presents enhanced shift when the ambient temperature change and thus get a higher temperature sensitivity. Experimental results show that the enhanced temperature sensitivity is about 358pm/°C, which is almost 30 times that of the inherent temperature sensitivity.

High Sensitivity Evanescent Field Refractometer Based on Modal Interference in Micro-Capillary Wall

We present a novel fiber-optic refractive index (RI) sensor based on a short piece of fused-silica micro-capillary (FSC) spliced between single mode fibers (SMFs). The sensing element is the tubing wall of the FSC where multiple modes excited from the lead-in SMF interfere to form fringes that are collected by the lead-out SMF. Two types of sensors with symmetric and asymmetric structures are fabricated and tested. A single lead-in SMF is used in the symmetric structure, whereas two lead-in SMFs are used in the asymmetric structure. Experimental results show that both types of sensors have high RI sensitivities, the symmetric structure sensor has a quadrature response with a higher sensitivity in the tested RI range from 1.33 to 1.37, and the asymmetric structure has a linear RI response with a sensitivity of 580.41 nm/RIU in the same RI range. The RI sensing platform demonstrated here can be further developed as a miniaturized fiber optic biosensor for biological and biochemical analysis.

Optical fiber sensor based on capillary wall for highly-sensitive refractive index measurement

This paper reports a temperature-compensated fiber-optic refractive index (RI) sensor for high sensitivity measurement. The sensor includes a piece of fused-silica capillary (FSC) and a fiber Bragg grating (FBG), both of which are sandwiched by single-mode fibers (SMFs). When light from the lead-in SMF enters into the wall of the FSC that acts as a RI sensing element, multiple modes are excited and interfere to form fringes collected by the lead-out SMF. The FBG is fabricated adjunct to the FSC to compensate its temperature sensitivity. The FSC based sensor prototype is fabricated and sealed in a flow cell to test its performance. Experimental results show that the sensor is highly sensitive to RI, and the sensitivity in the tested RI range from 1.33 to 1.35 is 698.52nm/RIU and from 1.35 to 1.37 is 1061.78nm/RIU. The temperature sensitivity of the FSC is −0.173nm/°C, which is compensated by the FBG. This capillary wall based sensor can be further developed as a miniaturized fiber optic biosensor for biochemical application.

Reflection-based fibre-optic refractive index sensor using surface plasmon resonance

A reflection-based fibre-optic refractive index sensor using surface plasmon resonance (SPR) in a thin metal film sputtered on a bare core of a multimode optical fibre is presented. The sensing element of the SPR fibre-optic sensor is the core of a step-index optical fibre made of fused silica with a gold film double-sided sputtered on the whole core surface, including the core end face. Consequently, a terminated reflection-based sensing scheme to measure the refractive indices of liquids is realized. The sensing scheme uses a wavelength interrogation method and the refractive index of a liquid is sensed by measuring the position of the dip in the reflected spectral intensity distribution. As an example, the aqueous solutions of ethanol with refractive indices in a range from 1.333 to 1.363 are measured. In addition, the increase in the sensitivity of the SPR fibre-optic refractive index sensor with the decrease of the fibre sensing length is demonstrated.

A localized surface plasmon resonance-based optical fiber sensor with sub-wavelength apertures

An optical fiber refractive index sensor based on localized surface plasmon (LSP) resonances within resonant subwavelength apertures in a gold film is demonstrated. The excitation of LSPs within the apertures in the film on the optical fiber endfaces provided the basis of a compact refractive index sensor for liquids. A sensitivity of 755 ± 12 nm per refractive index unit was demonstrated for this sensor.

A Refractive Index Sensor Based on the Resonant Coupling to Cladding Modes in a Fiber Loop

We report an easy-to-build, compact, and low-cost optical fiber refractive index sensor. It consists of a single fiber loop whose transmission spectra exhibit a series of notches produced by the resonant coupling between the fundamental mode and the cladding modes in a uniformly bent fiber. The wavelength of the notches, distributed in a wavelength span from 1,400 to 1,700 nm, can be tuned by adjusting the diameter of the fiber loop and are sensitive to refractive index changes of the external medium. Sensitivities of 170 and 800 nm per refractive index unit for water solutions and for the refractive index interval 1.40–1.442, respectively, are demonstrated. We estimate a long range resolution of 3 × 10−4 and a short range resolution of 2 × 10−5 for water solutions.

Fabrication Quality Analysis of a Fiber Optic Refractive Index Sensor Created by CO2 Laser Machining

This study investigates the CO2 laser-stripped partial cladding of silica-based optic fibers with a core diameter of 400 μm, which enables them to sense the refractive index of the surrounding environment. However, inappropriate treatments during the machining process can generate a number of defects in the optic fiber sensors. Therefore, the quality of optic fiber sensors fabricated using CO2 laser machining must be analyzed. The results show that analysis of the fiber core size after machining can provide preliminary defect detection, and qualitative analysis of the optical transmission defects can be used to identify imperfections that are difficult to observe through size analysis. To more precisely and quantitatively detect fabrication defects, we included a tensile test and numerical aperture measurements in this study. After a series of quality inspections, we proposed improvements to the existing CO2 laser machining parameters, namely, a vertical scanning pathway, 4 W of power, and a feed rate of 9.45 cm/s. Using these improved parameters, we created optical fiber sensors with a core diameter of approximately 400 μm, no obvious optical transmission defects, a numerical aperture of 0.52 ± 0.019, a 0.886 Weibull modulus, and a 1.186 Weibull-shaped parameter. Finally, we used the optical fiber sensor fabricated using the improved parameters to measure the refractive indices of various solutions. The results show that a refractive-index resolution of 1.8 × 10−4 RIU (linear fitting R2 = 0.954) was achieved for sucrose solutions with refractive indices ranging between 1.333 and 1.383. We also adopted the particle plasmon resonance sensing scheme using the fabricated optical fibers. The results provided additional information, specifically, a superior sensor resolution of 5.73 × 10−5 RIU, and greater linearity at R2 = 0.999.

Microfluidic refractive index sensor based on an all-silica in-line Fabry–Perot interferometer fabricated with microstructured fibers

We report a microfluidic fiber-optic refractive index (RI) sensor based on an in-line Fabry-Perot (FP) interferometer, which is formed by a silica tube sandwiched by two microstructured fibers (MFs). The sensor reported here can be fabricated at low cost, possess a robust structure, and has microfluidic capability. The micro-sized holes in the MFs naturally function as microfluidic channels through which liquid samples can be efficiently and conveniently delivered into and out of the FP cavity by a pressure/vacuum pump system for high-performance RI measurement. Due to the microfluidic capability enabled by the MFs, only sub microliter sample is required. We also experimentally study and demonstrate the superior performances of the sensor in terms of high RI sensitivity, good measurement repeatability, and low temperature cross-sensitivity.

Characteristics of optical fiber refractive index sensor based on surface plasmon resonance

AbstractA refractive index optical fiber sensor based on surface plasmon resonance technology is demonstrated. The sensing device is a multimode optical fiber in which half of the core is polished away and a thin‐film layer of gold is deposited. We measure the spectrum‐difference variations and the resonance wavelength shifts for different refractive index solutions. In terms of the experimental data, the characteristics including resolution, sensitivity, and signal‐to‐noise ratio for the sensor are obtained. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:574–576, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27397

Sensitivity enhancement of a surface plasmon resonance based fibre optic refractive index sensor utilizing an additional layer of oxides

We have experimentally demonstrated the capability of metal and different oxide combinations to be used in surface plasmon resonance (SPR) based fibre-optic refractive index sensor by using wavelength interrogation technique. The analysis of the sensor response is carried out using multilayered structure and geometrical optics. The configuration contains copper as a SPR active metallic layer covered by one of the three oxide layers TiO2, SiO2, and SnO2. The thickness of the copper layer is optimized to achieve the most pronounced dip at the resonance condition. The maximum sensitivity is obtained for TiO2 film. Further, increase in the thickness of the TiO2 layer increases the sensitivity of the sensor. The trend of sensitivity obtained by experimental results match qualitatively with the theoretical results obtained using the N-layer model and the ray approach. The additional advantages of oxide layer, apart from sensitivity enhancement, are protection of metallic layer from oxidation, tunability of the resonance wavelength region, biocompatibility and capability of gas sensing.