High Sensitivity Refractive Index Sensor Research Articles

Investigation of the high sensitivity RI sensor based on SMS fiber structure

A high sensitive refractive index (RI) sensor based on a Single-mode Multimode Single-mode (SMS) fiber structure was used and experimentally demonstrated. The dip in the measured spectrum signal caused by the modal interference shifted obviously when the surrounding RI changed. The approximate linear relationship between surrounding RI and spectrum dip wavelength shift was obtained experimentally. The measurement sensitivity up to 286.2nm/RIU was showed with the surrounding RI range from 1.33 to 1.3775, which means the measurement resolution about 3.5×10−6 could be implemented if the wavelength shift measurement resolution of the Optical Spectrum Analyzer is 1pm.

Long period fiber grating and high sensitivity refractive index sensor based on hollow eccentric optical fiber

We have demonstrated experimentally a long period fiber grating (LPFG) written in a hollow eccentric optical fiber (HEOF). A high-quality LPFG with no structural deformation was fabricated using a high-frequency CO2 laser with a low energy density of 0.896J/mm2. The LPFG writing parameters showed only a weak dependence on orientation of the fiber core to the writing beam. The liquid-filled HEOF LPFG has a high refractive index sensitivity of 1005nm/RIU over a refractive index range of 1.422–1.441. The good performance and easy fabrication of the HEOF LPFG indicate that it is a promising platform for novel in-fiber microfluidic devices.

Liquid refractive index sensor based on slow light in slotted photonic crystal waveguide

A high sensitive and compact refractive index sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. This design is worked on a Mach–Zehnder interferometer (MZI) configuration with S-PhCW as the measuring arm, which can be used to detect any changes in refractive index that correspond to different concentration of the measuring liquid. Combining the slow light enhancement in photonic crystal waveguide (PhCW) with the advantage of excellent optical confinement in slot waveguide, the sensitivity of this simple scheme can reach to 2.3×109nm/RIU with the active region of only 1mm long.

A High Sensitivity Index Sensor Based on No-Core Fibers

A simple, small-size, compact and high-sensitivity refractive-index sensor composed of a short no-core fiber (NCF) about 20 mm in length sandwiched between two pieces of single-mode fibers is proposed in this paper. The index measurement is experimentally demonstrated with the sensitivity of 7792.85 nm/ RIU in the range from 1.440 to 1.454 and 227.14 nm/ RIU in the range from 1.300 to 1.430. This sensing mechanism is based on the induced multi-mode interfering wavelength shift in the no-core fiber when the reflective index of the fiber outside is changed.

High sensitivity LPG Mach–Zehnder sensor for real-time fuel conformity analysis

A high sensitivity refractive index sensor based on the combination of mechanically induced long period gratings (LPG) and fiber tapers was developed for real-time fuel quality analysis. The sensor was built in a Mach–Zehnder configuration by employing a pair of in-series gratings. In order to enhance sensor sensitivity, the region between both LPGs was tapered down from 125 to 10 µm. The system was tested by measuring water concentration in ethanol and ethanol concentration in commercial gasoline. The tapered sensor has shown an average sensitivity of 930 nm/RIU, 18 times higher than the non-tapered version. The resolution limit of the system using spectral interrogation was estimated to be 0.06% of ethanol dissolved in gasoline. For the purpose of real-time monitoring, an interrogation system based on white light interferometry (WLI) and virtual instrumentation was employed to evaluate ethanol evaporation in water, avoiding the use of spectral analysis. The WLI system, using phase tracking techniques, enabled us to record the evolution of the ethanol concentration in water with a resolution of 0.005% (v/v).

Compact Long-Period Fiber Gratings With Resonance at Second-Order Diffraction

Compact 5-mm-long 100-μm-period long-period fiber gratings (LPFGs) are fabricated by femtosecond laser direct writing. Nearly symmetrical photomodified grating segments located at the center of a fiber core lead to well-defined transmission spectra. The LPFGs can couple the guided mode to high-order cladding modes of second-order diffraction, resulting in a high refractive index (RI) sensitivity of 5265 nm/RI units in the RI range of 1.433 ~ 1.443 and a low-temperature sensitivity of -15.52 pm/°C from 20°C to 500°C. The latter property is desired for crosstalk-free high-sensitivity RI sensors.

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.

High Sensitive Refractive Index Sensor Based on Cladding Mode Recoupled Chirped FBG

A simple but highly sensitive refractometer based on chirped fiber Bragg grating (CFBG) is proposed and investigated experimentally. Intensive reflected cladding modes can be excited within a large wavelength range through a multimode fiber CFBG structure. Experimental results show that the intensity of reflected cladding modes in CFBG is modulated by surrounding refractive index (RI) changes, which provide a high sensitive and cost-effective solution for RI measurement.

Design of refractive index sensors based on the wavelength-selective resonant coupling phenomenon in dual-core photonic crystal fibers

Design strategies for high-sensitivity refractive index sensors based on the principle of wavelength-selective resonant coupling in dual-core photonic crystal fibers are presented. Phase matching at a single wavelength can be achieved between an analyte-filled microstructured core and a small core with a down-doped rod or one small air hole in the center, thus enabling selectively directional resonant-coupling between the two cores. The transmission spectra of the output light presents a notch at the index-matched wavelength, yielding a resonant wavelength depending on the refractive index of the analyte. Numerical simulations demonstrate that both of the two proposed sensors can be used for highly sensitive detection of low-index analyte. In particular, the configuration realized by introducing the fiber with a small air hole in one core can be used to the detection of the analyte index around 1.33 and the sensitivity reach to 1.2×10(4) nm per refractive index unit (RIU). In addition, the detection limit is as low as 2.5×10(-7) RIU at n(a)=1.33.

High Sensitivity Refractive Index Sensor Based on a Wet-Etched Fused Fiber Coupler

We propose a novel method for fabricating fused fiber couplers whose transmission spectrum and sensitivity can be controlled in the wet-etching process. A wide tuning of the transmission spectrum during wet etching was observed experimentally. It was demonstrated that etched fused fiber couplers with reduced waist size had high sensitivity to the refractive index of an external medium. The sensitivity achieved by the present fiber coupler sensor was a 1125 nm/refractive index unit.

Extremely High Sensitive Plasmonic Refractive Index Sensors Based on Metallic Grating

This work shows that a grating-based surface plasmon (SP) resonance sensing system can exhibit extremely high sensitivity to detect a small change of refractive index in an analyte. The corresponding sensitivity can be much higher than that of the prism-based systems. Both analytical calculation and rigorous coupled-wave analysis are used to study the angular sensitivity of the system. It is found that the system’s sensitivity can be over 600° per unit index change if (1) first-order diffracted wave is chosen to excite SP mode, (2) large SP resonant angles are used in the operation, and (3) grating filling factor is selected to be varied between 0.3 and 0.7. Furthermore, the sensing system has the best performance for detecting low-index analyte with a small change of refractive index.

Thinned fiber Bragg gratings as refractive index sensors

In this work, highly sensitive refractive index measurements have been experimentally demonstrated by using thinned fiber Bragg grating (FBG) sensors. When the cladding diameter is reduced, significant changes in the effective refractive index occur due to surrounding medium refractive index modifications, leading to Bragg wavelength shifts. Uniformly thinned FBGs have been obtained by using wet chemical etching in hydrofluoric acid solutions. In order to prove sensor sensitivity, experimental tests have been carried out by using glycerine solutions with well-known refractive indices. Obtained results agree well with the numerical analysis carried out by using the three-layer fiber model. If the cladding layer is completely removed, resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for the outer refractive index around 1.450 and 1.333, respectively, are possible. Finally, a novel approach based on the selective etching along the grating region has been analyzed, leading to high-sensitivity refractive index sensors based on intensity measurements.

Thinned and micro-structured fibre Bragg gratings: towards new all-fibre high-sensitivity chemical sensors

In this work, recent developments of a new class of all-fibre and high-sensitivity refractive index sensors based on thinned and micro-structured fibre Bragg gratings (FBGs) are reported. Uniform and non-uniform thinned FBGs (ThFBGs) have been proposed for refractive index measurements providing self temperature referencing with performances adequate to the state of the art. Furthermore, micro-structured FBGs have been developed as advanced refractive index sensors able to provide low-cost and high-sensitivity refractive index sensors for chemical sensing. Here, analysis, fabrication and characterization of the proposed optoelectronic devices are presented. Finally preliminary studies on two-defect micro-structured gratings are presented.

Thinned Fiber Bragg Gratings as High Sensitivity Refractive Index Sensor

In this work, the numerical and experimental analysis on the use of thinned fiber Bragg gratings as refractive index sensors have been carried out. Wet chemical etching in a buffered hydrofluoric acid solution was used for sensor fabrication. Experimental characterization for an almost full etched cladding sensor is presented demonstrating good agreement with numerical results and resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for outer refractive index around 1.45 and 1.333, respectively.