High Sensitivity Refractive Index Sensor Research Articles

High Sensitivity Refractive Index Sensor Based on Multicoating Photonic Crystal Fiber With Surface Plasmon Resonance at Near-Infrared Wavelength

A pure silica core photonic crystal fiber with multicoating layers for high refractive index (RI) sensing is proposed. Au is used as the active plasmonic material. Tantalum pentoxide is deposited on Au in order to prevent the oxidation and further enhance the evanescent field. The simulation was preformed through the full-vector finite element method (FEM), and the results show that the proposed fiber sensor can obtain the average wavelength sensitivity of 9180 nm/RIU with the analyte RI range of 1.40–1.44, corresponding to the resolution of 1.09 × 10–5 RIU. Moreover, the maximum amplitude sensitivity of 1739.26 RIU–1 with resolution of 5.75 × 10–6 RIU are also obtained. The designed sensor can be a potential candidate for microfluidic analyte detecting.

Ultrashort and high-sensitivity refractive index sensor based on dual-core photonic crystal fiber

Ultrashort and high-sensitivity refractive index sensor based on dual-core photonic crystal fiber

Coreless side-polished fiber: a novel fiber structure for multimode interference and highly sensitive refractive index sensors.

A novel fiber structure, coreless side-polished fiber (CSPF), is proposed and investigated to implement multimode interference (MMI) and high sensitive refractive index (RI) sensors. For such CSPF, the part of the cladding and the core of a single-mode fiber are side-polished off so as to make the remained cladding a D-shaped multimode waveguide. The excitation and evolution of MMI in the CSPF are simulated numerically. The simulation results show that the high-order modes excited within the D-shaped multimode waveguide are mainly TE0,1 (TM0,1)~TE0,6 (TM0,6) modes. Moreover, the RI sensing characteristics and the influences of residual thickness and dip wavelength on the sensitivity are investigated both numerically and experimentally. The experimental results show that the CSPF with a residual thickness of 43.1 μm can reach an ultra-high sensitivity of 28000 nm/RIU in the RI range of 1.442~1.444. It is also found that the sensitivity can be further increased by reducing the residual thickness and choosing the dip at a longer wavelength. Thanks to the ultra-high RI sensitivity and the ease of fabrication, the CSPF could provide a cost-effective platform to build high-performance fiber devices of various functions.

High Sensitivity Refractive Index Sensor Based on Highly Overcoupled Tapered Fiber-Optic Couplers

In this paper, a simple and compact fiber-optic sensor based on an overcoupled tapered fiber coupler is studied. The coupler is fabricated to be operated well beyond the initial coupling cycles, where the rapid exchange of energy between outputs ports enable the fabrication of a highly sensitive device. The suitability and sensitivity of the proposed scheme is demonstrated by measuring refractive index (RI) variations of sugar concentrations in water. The device presents a linear response in terms of power transmission or wavelength shift versus RI changes. The best achieved sensitivity is 0.442 units of normalized transmission per unit of sugar concentration, with a noise detection limit of 0.003 weight percentage of sugar concentration (wt %). From this result the minimum detectable RI change is estimated as $5 \times 10^{\mathrm {-6}}$ RI unit (RIU). The sensor can be also wavelength-encoded, exhibiting a sensitivity of 2171 nm/RIU, maintaining a linear response in a large range of RI. These experimental results are within the best results reported in the framework of fiber couplers and modal interferometer-based RI sensors.

High Sensitivity Refractive Index Sensor Based on Dual-Core Photonic Crystal Fiber with Hexagonal Lattice.

A refractive index sensor based on dual-core photonic crystal fiber (PCF) with hexagonal lattice is proposed. The effects of geometrical parameters of the PCF on performances of the sensor are investigated by using the finite element method (FEM). Two fiber cores are separated by two air holes filled with the analyte whose refractive index is in the range of 1.33–1.41. Numerical simulation results show that the highest sensitivity can be up to 22,983 nm/RIU(refractive index unit) when the analyte refractive index is 1.41. The lowest sensitivity can reach to 21,679 nm/RIU when the analyte refractive index is 1.33. The sensor we proposed has significant advantages in the field of biomolecule detection as it provides a wide-range of detection with high sensitivity.

Numerical Analysis of High-Sensitivity Refractive Index Sensor Based on LPFG With Bandpass Transmission

In this paper, a novel surrounding refractive index (SRI) sensor based on the long period fiber grating (LPFG) with cladding refractive index perturbation (C-LPFG) working around the turning-around point is proposed and investigated theoretically. By introducing the incident light into the fiber cladding, the bandpass transmission spectrum of the C-LPFG can be obtained (bandpass C-LPFG). The modified full-vector complex coupled mode theory is then derived to analyze the sensing performance of the bandpass C-LPFG-based SRI sensor. The average sensitivity and the local sensitivity up to 4757.6 and $\sim 6000$ nm/RIU are achieved, respectively, in the SRI range (from 1.33 to 1.36) for the bandpass C-LPFG-based SRI sensor having a small cladding radius. It is demonstrated that the bandpass C-LPFG is more sensitive to the SRI and exhibits an ultra-high sensitivity in contrast to the conventional uncoated LPFG.

A compact and ultra high sensitive RI sensor using modal interference in an integrated optic waveguide with metal under-cladding

We propose a compact and ultra high sensitive refractive index (RI) sensor based on modal interference in an integrated optic waveguide with a metal under-cladding (MUC). It has been shown that the MUC significantly increases the fractional modal power in the ambient medium, affecting the higher order modes much more compared to the lower ones. This results in an ultrahigh dependence of the modal interference on the ambient RI change. The RI sensitivity of the proposed sensor structure is found to vary in the range 5.28–71.94μm/RIU, for the RI range 1.33–1.37, which is the highest reported RI sensitivity, achieved in modal interference based sensors till date.

Utilizing the Metallic Nano-Rods in Hexagonal Configuration to Enhance Sensitivity of the Plasmonic Racetrack Resonator in Sensing Application

A high sensitive plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguides with embedding metallic nano-rods in racetrack resonator has been proposed. The refractive index changes of the dielectric material inside the resonator together with temperature changes can be acquired from the detection of the resonance wavelength, based on their linear relationship. With optimum design and considering a tradeoff among detected power, structure size, and sensitivity, the finite difference time domain simulations show that the refractive index and temperature sensitivity values can be obtained as high as 2610 nm per refractive index unit (RIU) and 1.03 nm/°C, respectively. In addition, resonance wavelengths of resonator are obtained experimentally by using the resonant conditions. The effects of nano-rods radius and refractive index of racetrack resonator are studied on the sensing spectra, as well. The proposed structure with such high sensitivity will be useful in optical communications that can provide a new possibility for designing compact and high-performance plasmonic devices.

High Sensitive Refractometric Sensor Using Symmetric Cladding Modes of an FBG Operating at Mode Transition

We report a high sensitive refractive index (RI) sensor using fiber Bragg grating (FBG) where azimuthally symmetric resonant cladding modes were used for RI sensing. We enhanced the sensitivity by tailoring the effective index of a cladding mode to an optimum value so that the mode has maximum sensitivity around a specific surrounding RI. We tailored the effective index of a clad mode by controlled deposition of a thin polymer layer having RI higher than the cladding. Experiments show that the sensitivity of cladding mode (LP0,12) of a standard FBG with overlay coating to the surrounding medium having RI around 1.333 is ∼ 2600 pm/RIU. The proposed sensing mechanism will be useful for biological sensing application.

High-sensitivity refractive index sensors based on fused tapered photonic crystal fiber

In this paper, a novel liquid refractive index (RI) sensor based on fused tapered photonic crystal fiber (PCF) is proposed. It is fabricated by fusing and tapering a section of PCF which is spliced with two single-mode fibers (SMFs). Due to the fused biconical taper method, the sensor becomes longer and thinner, to make the change of the outside RI has more direct effects on the internal optical field of the PCF, which finally enhances the sensitivity of this sensor. Experimental results show that the transmission spectra of the sensor are red-shifted obviously with the increase of RI. The longer the tapered region of the sensor, the higher the sensitivity is. This sensor has the advantages of simple structure, easy fabrication, high performance and so on, so it has potential applications in RI measurement.

High sensitivity refractive index sensor based on simple diffraction from phase grating.

We present a technique for refractive index sensing using a phase grating structure. A grating under normal incidence can be designed such that the first-order diffracted light travels at a diffraction angle of 90° with respect to the zeroth order. The diffracted light, which is along the direction of periodicity, can further be diffracted from the grating and interfere with the zeroth-order light. Under this condition, the π phase difference that arises between the two interfering beams results in a transmission dip. We can tune this dip wavelength for senor applications, based on the grating equation. This Letter presents both simulation and experimental data that show good agreement with each other.

All-fiber microfluidic multimode Mach–Zehnder interferometers as high sensitivity refractive index sensors

We present microfluidic multimode fiber Mach---Zehnder interferometers (MMF-MZIs), suitable for refractive index (RI) measurement. The compact and low-cost all-fiber MMF-MZIs were made by forming two coupling points in series at a localized short section (about 1---5 cm) of a multimode optical fiber, using an electric-arc system, thus allowing the excitation and recombination of core and cladding modes. The MMF-MZIs have been integrated with a home-made microfluidic chip, used as a high sensitivity RI sensor to ascertain the surrounding RI variations. This type of microfluidic lab-on-a-chip MZI-based sensor was shown to be effective in sensing very small quantities of samples and its transmitted light signal could easily be measured. The spectra of all MMF-MZIs were Fourier transformed to obtain and the spatial frequency spectra. The MMF-MZIs exhibited many dominant spatial frequencies for all interferometer lengths and interference was mainly constructed by the core mode and dominant lower order cladding modes when the sensing length was 2---5 cm as well as distributed by the core mode and dominant higher order cladding modes when the sensing length was 1 cm. The transmitted spectra of the MMF-MZIs exhibited various sinusoidal-like interference fringes' amplitudes and peaks changed differently by immersing the MMF-MZIs in different concentrations of sucrose solutions. The microfluidic MMF-MZI sensors have a high resolution of 3.1 × 10ź5 to 7.5 × 10ź5 RIU and a high sensitivity of 160---347 dBm/RIU within RI range from 1.333 to 1.370.

PT-symmetry-induced evolution of sharp asymmetric line shapes and high-sensitivity refractive index sensors in a three-cavity array

It is important to control and tune the Fano-resonance spectra to achieve a large slope with, in addition, a relatively high extinction ratio for low-power optical switching and high-sensitivity sensing. Here, we explore the evolution of sharp asymmetric Fano-like line shapes in a three-cavity array with local parity-time ($\mathcal{PT}$) symmetry. In this three-cavity configuration, a single cavity is coupled to a $\mathcal{PT}$-symmetric combination of two cavities via a common waveguide. The influences of local $\mathcal{PT}$ symmetry on the asymmetric Fano-like line shapes are investigated by monitoring the output transmission spectra at various system parameters. It is found that both the slope and the extinction ratio within the sharp asymmetric line shapes can be significantly enhanced by introducing the $\mathcal{PT}$-symmetric unit, compared with the configuration of two indirectly coupled cavities. Subsequently we discuss the application of such a $\mathcal{PT}$-assisted configuration as a family of high-sensitivity refractive index sensors by numerical analysis. For practical parameters based on microring resonators, the best sensitivity of refractive index sensors is more than five orders of magnitude larger than two indirectly coupled lossy cavities. The proposed scheme can be implemented in current state-of-the-art experiments. This investigation can help us to understand the interplay between the Fano resonance and $\mathcal{PT}$ symmetry.

High sensitivity refractive index sensor based on splicing points tapered SMF-PCF-SMF structure Mach-Zehnder mode interferometer

The paper proposed and studied a Mach-Zehnder mode interferometric refractive index sensor, which is based on splicing points tapered SMF-PCF-SMF (SMF, single-mode fiber; PCF, photonic crystal fiber) structure. For the reason that the effective refractive index of photonic crystal fiber cladding high-order modes near fiber core are more sensitive to surrounding refractive index changes, the refractive index measurement sensitivity of splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer can be enhanced further through tapering the splicing points. Relations between refractive index measurement sensitivity and photonic crystal fiber length and taper waist diameter are studied through numerical simulations and experiments. Simulation and experimental results show that sensitivity will be increased with the increase of photonic crystal fiber length and the decrease of taper waist diameter. In the refractive range of 1.3333–1.3737, splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4cm and taper waist diameter of 60.4μm has refractive index measurement sensitivity of 260.8nm/RIU, compared with sensitivity of 224.2nm/RIU of direct splicing SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4cm, the sensitivity increased by 16.3%. The research shows that the sensing structure is with good linearity and repeatability.

High sensitivity refractive index sensor based on a tapered small core single-mode fiber structure.

A high sensitivity refractive index (RI) sensor based on a tapered small core single-mode fiber (SCSMF) structure sandwiched between two traditional single-mode fibers (SMF28) is reported. The microheater brushing technique was employed to fabricate the tapered fiber structures with different waist diameters of 12.5, 15.0, and 18.8 μm. Experiments demonstrate that the fiber sensor with a waist diameter of 12.5 μm offers the best sensitivity of 19212.5 nm/RIU (RI unit) in the RI range of 1.4304 to 1.4320. All sensors fabricated in this Letter show good linearity in terms of the spectral wavelength shift versus changes in RI. Furthermore, the sensor with the best sensitivity to RI was also used to measure relative humidity (RH) without any coating materials applied to the fiber surface. Experimental results show that the spectral wavelength shift changes exponentially as the RH varies from 60% to 95%. A maximum sensitivity of 18.3 nm per relative humidity unit (RHU) was achieved in the RH range of 90.4% to 94.5% RH.

Highly sensitive refractive index fiber inline Mach–Zehnder interferometer fabricated by femtosecond laser micromachining and chemical etching

A High sensitive refractive index (RI) sensor based on Mach–Zehnder interferometer (MZI) in a conventional single-mode optical fiber is proposed, which is fabricated by femtosecond laser transversal-scanning inscription method and chemical etching. A rectangular cavity structure is formed in part of fiber core and cladding interface. The MZI sensor shows excellent refractive index sensitivity and linearity, which exhibits an extremely high RI sensitivity of −17197nm/RIU (refractive index unit) with the linearity of 0.9996 within the refractive index range of 1.3371–1.3407. The experimental results are consistent with theoretical analysis.

High sensitivity refractive index sensor based on adiabatic tapered optical fiber deposited with nanofilm by ALD.

Atomic layer deposition (ALD) technology is introduced to fabricate a high sensitivity refractive index sensor based on an adiabatic tapered optical fiber. Different thickness of Al2O3 nanofilm is coated around fiber taper precisely and uniformly under different deposition cycles. Attributed to the high refractive index of the Al2O3 nanofilm, an asymmetry Fabry-Perot like interferometer is constructed along the fiber taper. Based on the ray-optic analysis, total internal reflection happens on the nanofilm-surrounding interface. With the ambient refractive index changing, the phase delay induced by the Goos-Hänchen shift is changed. Correspondingly, the transmission resonant spectrum shifts, which can be utilized for realizing high sensitivity sensor. The high sensitivity sensor with 6008 nm/RIU is demonstrated by depositing 3000 layers Al2O3 nanofilm as the ambient refractive index is close to 1.33. This high sensitivity refractive index sensor is expected to have wide applications in biochemical sensors.

Design of a compact and high sensitive refractive index sensor base on metal-insulator-metal plasmonic Bragg grating.

A nanometric and high sensitive refractive index sensor based on the metal-insulator-metal plasmonic Bragg grating is proposed. The wavelength encoded sensing characteristics of the refractive index sensor were investigated by analyzing its transmission spectrum. The numerical results show that a good linear relationship between the Bragg wavelength and the refractive index of the sensing material can be obtained, which is in accordance with the analytical results very well. A high refractive index sensitivity of 1,488 nm/RIU around Bragg resonance wavelength of 1,550 nm was obtained. Besides, the simulation results show that the sensitivity is depended on the Bragg resonance wavelength and the longer the Bragg resonance wavelength, the higher sensitivity can be obtained. Furthermore, the figure of merit of the refractive index sensor can be greatly increased by introducing a nano-cavity in the proposed plasmonic Bragg grating structure. This work pave the way for high sensitive nanometric refractive index sensor design and application.

High-Sensitivity Refractive Index Sensor Based on D-Shaped Photonic Crystal Fiber with Rectangular Lattice and Nanoscale Gold Film

We propose and investigate a D-shaped photonic fiber refractive index sensor with rectangular lattice based on surface plasmon resonance. In such sensor, the nanoscale gold metal film is deposited on the flat surface where it is side polished. Numerical results show that the average sensitivity of Au-metalized surface plasmon resonance (SPR) sensor could reach as high as 8,129 nm/refractive index unit (RIU) in the dynamic index range from 1.35 to 1.41 as well as 2,000 nm/RIU from 1.33 to 1.35. Compared to conventional Au-metalized SPR sensors, the performance of our device is obviously better, and the production process is greatly simplified.

Ultracompact and high sensitive refractive index sensor based on Mach–Zehnder interferometer

We report an ultracompact and high sensitive refractive index (RI) sensor based on Mach–Zehnder interferometer (MZI) structure. The sensor was assembled using flexible poly(trimethylene terephthalate) nanofiber with a diameter of 700nm. The valley wavelength shifts of the output spectra of the sensor caused by the variation of ambient medium around the MZI structure region were measured. In the RI range from 1.34 to 1.39, the highest measured sensitivity of the sensor is about 1100nm/RIU (RI unit), which is 6 times higher than that of S-like abrupt taper based MZI sensors, and the RI detection limit is about 1.8×10−6. The sensor allows the construction of a low-cost, compact, simple, and highly sensitive sensing system.