Optical Fiber Refractive Index Sensor Research Articles

Core–cladding mode recoupling based fiber optic refractive index sensor

We present a compact and robust fiber optic refractive index sensor based on a tilted fiber Bragg grating (TFBG) inscribed in a thin-core fiber that is spliced to a standard single-mode fiber (SSMF). Due to the core mismatch between the SSMF and the thin-core fiber, cladding modes generated from the TFBG can be back into SSMF for detection. Since the effective refractive index of a cladding mode is dependent on the surrounding refractive index (SRI), the cladding-mode resonances can be used for measurement. We fabricate the sensor probe and test it in a series solution with different SRIs, and the experimental results agree with the theoretical analysis. The repeatability of the sensor is also tested in the same SRI range. We demonstrate its capability and simplicity to use this sensor for a wide range SRI measurement.

侧边抛磨SMS光纤传感器

A new type of optical fiber refractive index sensor was reported.The partial multimode fiber of Singlemode-Multimode Singlemode(SMS)fiber structures were side polished,and then the refractive index sensing properties were analyzed.The results show that multiple loss peaks can be observed in the spectrum of the SMS fiber structures because of the mutual interference by the high order modes of multimode fiber.The spectrum of the SMS fiber structures just drift a few nanometres when the SMS fiber structures are polished to close to the core;but when being side polished to the core of the fiber,the spectrum changes dramatically.After being side polished,the refractive index resolution of the SMS fiber structures will be higher.When the polishing depth is 10μm and the polishing length is 10mm,the refractive index sensitivity of the SMS fiber structure is 703.2nm/RIU.When the polishing depth is 15μm,the refractive index sensitivity of the SMS fiber structure is 798nm/RIU.When the polishing depth is 20μm,the refractive index sensitivity of the SMS fiber structure is up to 1 190nm/URI.A new type of optical fiber refractive index sensor can be realized with a single side polished SMS fiber structure,and the sensitivity of the sensor can be improved by increasing the depth of polishing area.

A different point of view: a continuous tracing of acid-base titration with fiber-optic sensor

Our main aim was to investigate variations in the refractive index of a solution during chemical reaction. The variation of refractive index was measured through the entire acid--base titration process, especially near the equivalence point. To determine the equivalence point, the conductivity of the solution and the refractive index were measured simultaneously. In this preliminary study, sodium hydroxide and hydrochloric acid were used as the base and the acid solution, respectively. To measure the refractive index variation, a fiber-optic refractive index sensor was designed. The fiber-optic probe was dipped into the solution and acted as a refractive index sensor according to Fresnel's fundamental reflection law. A conductometer, a lab-made fiber-optic refractive index sensor ensemble, and a lab-made optical drop counter were each connected to a computer via an analog digital converter and the data acquisition was performed with the LabVIEW program. The equivalence point was derived easily from the refractive index data for the sodium hydroxide solution with different molarities of hydrochloric acid. In our opinion, the measurement of the variation of the refractive index during this kind of chemical reaction is more sensitive than the conductometric measurement.

Fiber-Optic Refractive Index Sensor Based on the Cone-Based Round Structure

Fiber-optic refractive index (RI) sensors have the advantages of low cost, simple optical setup, remote sensing, and simple fabrication. A cone-based round structure, which is the combined structure of the cone structure and the round structure, is successively fabricated by using a cleaving method, a chemical glass etching method, and an electrical arc discharge method. In order to examine the possibility of the RI sensor, the fabricated cone-based round structure is used for the detection of various RIs by measuring the reflected intensities in the target sensing media. In addition, a fiber-optic localized surface plasmon resonance (LSPR) sensor based on the cone-based round structure is fabricated to enhance the sensitivity regarding RI changes. The fabricated fiber-optic LSPR sensor is used to detect the change of the RIs using the shift of peak wavelengths, as well as the change of reflected resonance intensities, by measuring LSPR spectra for gold nanoparticles on it.

W042003 光ファイバセンサによるFRPの成形モニタリング(【W04200】知的材料・構造システムの現状と今後の展開,機械材料・材料加工部門企画,ワークショップ)

It has been proven that fiber-optic refractive-index sensors can be used for monitoring cure process of thermosetting resin. In the present paper, process monitoring of GFRP was conducted by embedded refractive-index sensors. The sensors were embedded in 0° and 90° directions of unidirectional GFRP laminates and in resin of prepregs in order to investigate effect of reinforcing fibers on refractive index of resin. The laminates were manufactured at 140℃ and pressure of 0.5 MPa by a hot-press molding device. The experimental results showed that the effect of reinforcing fibers had remained small until cure process finished, however thermal residual stress caused by reinforcements affected refractive index of prepreg at cooling. Therefore, it was found that cure index of FRP prepregs could be measured by refractive-index sensors. In addition, it appeared that the refractive-index sensors were available to monitor air voids in resin qualitatively during molding.

Optically Heated Long-Period Grating as Temperature-Insensitive Fiber-Optic Refractive-Index Sensor

A temperature-insensitive fiber-optic refractive-index sensor based on optically heated long-period grating (LPG) is presented. The LPG is manufactured in a short section of cobalt-doped optical fiber (COF), which can be thermally stabilized through a pump light. Two fiber Bragg gratings (FBGs) are manufactured in COF and single-mode fiber (SMF), respectively, to monitor the local sensor and surrounding medium temperature. Experimental results demonstrate that this fiber-grating-based miniature refractive-index sensor can efficiently eliminate temperature cross-sensitivity in the temperature range of 5~60°C.

A novel refractive index sensor based on microbend loss

Simulating micro bending loss of SMF through a simple model and contrasting simulated curve and experimental results, confirm that the microbend loss is the main cause of hydrogen-loaded fiber fusion bubbles loss. There is a good linear relationship between the refractive index of liquid and normalized light intensity among the range from 1.3325 to 1.3562. Therefore, we propose a kind of low cost, high resolution optical fiber refractive index sensor.

Surface plasmon resonance based fiber optic refractive index sensor utilizing silicon layer: Effect of doping

We present an experimental study on the surface plasmon resonance (SPR) based fiber optic refractive index sensor utilizing a high index silicon layer between a metal layer and sensing medium using the wavelength interrogation mode of operation. Both n- and p-type silicon have been used. For the metal layer, silver and gold have been used. For a given metal, experimental results predict higher sensitivity of the sensor for the n-type silicon than for the p-type silicon layer. Further, for a given type of silicon, the sensitivity for the gold coated probe is higher than that of the silver coated probe. Numerically, the sensitivity of the n-type silicon with silver as the metal layer is approximately 1.39 times higher than that of the p-type silicon. In the case of gold as the metal layer, the sensitivity of the n-type silicon is approximately 1.50 times that of the p-type silicon. Since the refractive index of both p-type and n-type silicon is the same it appears that the majority charge carriers in silicon play an important role in the sensitivity of the surface plasmon resonance based sensor. The charge carriers are either affecting the field in the analyte region or may be somehow affecting the propagation constant of the surface plasmon wave which is solely due to oscillation of free electrons in the metal layer. In addition, the sensitivity of only the metal coated probes is found to lie between their p-type and n-type silicon coated probes. This suggests that the effect of charge carriers on sensitivity is more than the refractive index of the silicon layer. The effects of charge carriers in silicon, electrons and holes appears to be opposite.

Spatial Frequency Multiplexing of Fiber-Optic Interferometric Refractive Index Sensors Based on Graded-Index Multimode Fibers

Fiber-optic interferometric sensors based on graded-index multimode fibers have very high refractive-index sensitivity, as we previously demonstrated. In this paper, spatial-frequency multiplexing of this type of fiber-optic refractive index sensors is investigated. It is estimated that multiplexing of more than 10 such sensors is possible. In the multiplexing scheme, one of the sensors is used to investigate the refractive index and temperature responses. The fast Fourier transform (FFT) of the combined reflective spectra is analyzed. The intensity of the FFT spectra is linearly related with the refractive index and is not sensitive to the temperature.

High-Sensitivity Refractive Index Sensor Based on Simply Fabricated Single Mode Fiber Tapers

A high sensitivity fiber-optic refractive index sensor based on the bi-conical single mode fiber tapers is proposed and demonstrated. The relationship between the resonance wavelength shift and surrounding refractive index is investigated. The experimental results show that the resonance wavelength linearly shifts toward longer wavelengths with the environmental refractive index ranging from 1.333 to 1.380. The response sensitivity increases with the decrease of the waist diameter of the tapered fiber. The proposed sensor is easily fabricated, compact and may be useful for the chemical and biotechnological industry.

A Sensitivity-Enhanced Refractive Index Sensor Using a Single-Mode Thin-Core Fiber Incorporating an Abrupt Taper

A sensitivity-enhanced fiber-optic refractive index (RI) sensor based on a tapered single-mode thin-core diameter fiber is proposed and experimentally demonstrated. The sensor head is formed by splicing a section of tapered thin-core diameter fiber (TCF) between two sections of single-mode fibers (SMFs). The cladding modes are excited at the first SMF-TCF interface, and then interfere with the core mode at the second interface, thus forming an inter-modal interferometer (IMI). An abrupt taper (tens of micrometers long) made by the electric-arc-heating method is utilized, and plays an important role in improving sensing sensitivity. The whole manufacture process only involves fiber splicing and tapering, and all the fabrication process can be achieved by a commercial fiber fusion splicer. Using glycerol and water mixture solution as an example, the experimental results show that the refractive index sensitivity is measured to be 0.591 nm for 1% change of surrounding RI. The proposed sensor structure features simple structure, low cost, easy fabrication, and high sensitivity.

Miniaturized Long-Period Fiber Grating Assisted Surface Plasmon Resonance Sensor

This paper presents the design and fabrication of a fiber-optic refractive index (RI) sensor. The novel concept employs a long-period fiber grating (LPG) to achieve surface plasmon resonance (SPR) of a single cladding mode at the gold-coated tip of a single-mode fiber. The sensor combines a high level of sensitivity, a miniaturized sensing area and simple intensity-based interrogation, and is intended for biosensing using portable point-of-care devices or highly integrated lab-on-chip systems.

Photonic crystal cavity on optical fiber facet for refractive index sensing

Using a micromanipulation technique, a planar photonic crystal nanocavity made from a thin semiconductor membrane is released from the host semiconductor and attached to the end facet of a standard single-mode optical fiber. The cavity spectrum can be read out through the fiber by detecting the photoluminescence of embedded quantum dots. The modified fiber end serves as a fiber-optic refractive index sensor.

High-sensitivity optical fiber refractive index sensor using multimode interference structure

This letter presents the experimental investigation to increase the sensitivity of an optical fiber refractive index sensor with a multimode interference (MMI) structure. It is confirmed that the interference wavelength can be set in the long-wavelength region by adjusting the sensing-part length. Moreover, it is shown that the fineness of the cores in the input and output fibers is essential to obtain a sharp interference signal. We demonstrate the high sensitivity of an MMI-structured optical fiber sensor by applying it to refractive index measurements of ethanol/water solutions. This refractive index sensor reveals a resolution as fine as 1.8 × 10-5, which is the highest value so far reported in this type and has one order higher sensitivity than a commercially available Abbe refractometer (∼ 10-4).

Refractive Index Sensor for Liquids and Solids Using Dielectric Multilayer Films Deposited on Optical Fiber End Surface

We propose and demonstrate a novel fiber-optic refractive index (RI) sensor using dielectric multilayer thin films (DMFs) deposited on an optical fiber end surface, which rejects the incident light with particular wavelength. We experimentally show that the center frequency of the reflected light spectrum changes in proportion to the RI of six kinds of liquids attached to the fiber end. Its coefficient of + 0.61 nm/RIU (refractive index unit) agrees well with our simulation results based on characteristic-matrix technique. We also show that this sensor is applicable to the RI measurement of solids.

Refractive index sensitivity of fibre-optic long period gratings coated with SiO2 nanoparticle mesoporous thin films

A fibre-optic refractive index sensor based on a long period grating (LPG) with a nanoassembled mesoporous coating of alternate layers of poly(allylamine hydrochloride) (PAH) and SiO2 nanospheres was demonstrated. PAH/SiO2 coatings of different thicknesses were deposited onto an LPG operating near its phase matching turning point in order to study the effect of the film thickness and porosity on sensor performance. Three aqueous solutions of ethanol, glucose and sucrose of different concentrations were used to allow characterization of the refractive index (RI) over a wide RI range (1.3330–1.4906). The device showed a high sensitivity (1927 nm/refractive index unit) to RI changes with a response time less than 2 s. In particular, the current study suggested that the low RI of the mesoporous film, 1.20 at 633 nm, facilitates the measurement of external indices higher than that of the cladding, extending the range of operation of LPG-based RI sensors. The ability of this device to monitor, in real time, RI changes during a dilution process is demonstrated and discussed.

Surface-plasmon-resonance-based fiber-optic refractive index sensor: sensitivity enhancement

We have experimentally studied the surface plasmon resonance (SPR)-based fiber-optic refractive index sensor incorporating a high-index dielectric layer using the wavelength interrogation method. Silver and gold have been used as SPR active metals followed by a high-index dielectric layer of silicon. Experimental results predict a redshift in the resonance wavelength with the increase in the refractive index of the sensing layer for a given thickness of the silicon layer. Further, as the thickness of the silicon layer increases, the sensitivity of the sensor increases. The upper limit of the silicon film thickness for the enhancement of the sensitivity has been found to be around 10 nm. The experimental results obtained on sensitivity match qualitatively with the theoretical results obtained using the N-layer model and the ray approach. The increase in sensitivity is due to the increase in the electric field intensity at the silicon-sensing-region interface. In addition to an increase in sensitivity, the silicon layer can be used to tune the resonance wavelength and can protect the metal layer from oxidation and hence can improve the durability of the probe.

Refractive index sensing based on Mach–Zehnder interferometer formed by three cascaded single-mode fiber tapers

We report a highly sensitive refractive index (RI) sensor based on three cascaded single-mode fiber tapers, in which a weak taper is sandwiched between the two tapers to improve the sensitivity of the sensor. Experimental results show that the sensitivity of the device is 0.286 nm for a 0.01 RI change, which is about four times higher than that of the normal two-cascaded-taper-based Mach-Zehnder interferometer. In addition, the sensitivity of the device could be enhanced by tapering a longer and thinner middle weak taper. Such kinds of low-cost and highly sensitive fiber-optic RI sensors would find applications in chemical or biochemical sensing fields.

High-Resolution and Temperature-Insensitive Fiber Optic Refractive Index Sensor Based on Fresnel Reflection Modulated by Fabry–Perot Interference

A temperature-insensitive fiber optic sensor for high-resolution measure of refractive index is proposed and demonstrated. The sensor is based on Fresnel reflection modulated by Fabry-Perot interference, which consists of a section of single-mode fiber (SMF) tip coated with a thin film of SU-8 at the end of the fiber tip. The fringe contrast of the interference spectrum reflected from the sensor head is used to determine the extern RI. The experimental results agree well with the theoretical results and show that the fringe contrast is not sensitive to temperature variation. Such a refractive index sensor can offer the refractive index resolution and accuracy of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> and 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , respectively. By simultaneous tracking the wavelength shift of the interference pattern, the sensor also has the potential capability for the simultaneous measurement of liquid RI and temperature.

Multiplexing of fibre optic reflective sensors using Bragg gratings

A multiplexing method for a fibre optic refractive index sensor is presented that is potentially capable of simultaneous measurement from many sensor heads, each of which is a cleaved fibre end. Associated with each sensor head are two fibre couplers and an optical fibre Bragg grating having a unique Bragg wavelength. Experimental characterization of the sensor network through the use of liquids with different refractive indices showed excellent agreement with numerical analysis and a precision of 0.002 RIU and resolution of the order of 10−3 RIU.