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

A liquid level optical fiber sensor with refractive index compensation through cascading double multi-mode interferences

In the present paper, a new kind of concave shaped refractive index sensor (CSRIS) exploiting localized surface plasmon resonance (LSPR) is proposed and numerically optimized. The LSPR effect between polaritons and the core guided mode of designed CSRIS is used to enhance the sensing performance. The sensor is characterized for two types of sensing structures coated with gold (Au) film and Au nanowires (AuNWs), respectively. The influence of structural parameters such as the distance (D) of the concave shaped channel (CSC) from the core, the diameter of the nanowire (dn) and the size (s) of the CSC are investigated here. In comparison to Au film, the AuNWs are shown to significantly enhance the sensitivity and the performance of the designed sensor. An enhanced sensitivity of 4471 nm/RIU (refractive index unit) is obtained with AuNWs, for a wide range of analytes refractive index (na) varying between 1.33 to 1.38. However, for conventional Au film; the sensitivity of 808.57 nm/RIU is obtained for the same range of analytes.

A composed refractive index (RI) sensor based on evanescent field-fiber loop ring down was demonstrated. The ring-down time would be changed with the intensity loss near the tapered single-mode fiber (SMF), which is related to the RI of the test solutions. The feasibility of the sensing scheme was proved theoretically and experimentally. Results showed that the sensitivity of this simple scheme could reach to $-388.581~\mu \text{s}$ per RI unit (RIU) and the detection limit below $2.57\times 10^{-5}$ RIU was achieved. Moreover, a PDMS detection cell was specially designed to enhance the structure’s mechanical strength and the signal’s stability. The proposed RI sensor is attractive due to its high detection sensitivity, fast response, and insensitivity to light source fluctuations beyond what conventional RI sensors can offer.

Multi-point fiber-optic refractive index sensor by using coreless fibers

In this paper, an all-fiber novel fabricated Mach-Zehnder interferometer (MZI) based on refractive index (RI) sensor is presented and simulated. This RI sensor is composed of a single-mode fiber (SMF) with a core diameter of 9 μm and a multimode fiber (MMF) with a core diameter of 105 μm. The microgroove can be etched by femtosecond laser or ground by grinding machine technology. It greatly reduces the limitations of structure fabrication. The RI response of this sensor is obtained by beam propagation method. In the range of 1.33-1.334 RIU, the RI sensitivity is -10373 nm/RIU and the linearity is 99.98% by wavelength demodulation. The intensity demodulation shows a nonlinear trend, and the sensitivity increases with the increase of RI . The maximum RI sensitivity is -9614 dB/RIU, and the fitting is 98.69%. The structure has the advantages of simple manufacture, high sensitivity and good linearity. It is very suitable for the practical applications of RI sensing with high sensitivity.

A Mach-Zehnder interferometer (MZI) based fiberoptic refractive index (RI) sensor is constructed by uniformly tapering standard single mode fiber (SMF) for RI measurement. A custom flame-based tapering machine is used to fabricate microfiber MZI sensors directly from SMFs. The fabricated MZI device does not require any splicing of fibers and shows excellent RI sensitivity. The sensor with a cladding diameter of 35.5 µm and length of 20 mm exhibits RI sensitivity of 415 nm/RIU for RI range of 1.332 to 1.384, 1103 nm/RIU for RI range of 1.384 to 1.4204 and 4234 nm/RIU for RI range of 1.4204 to 1.4408, respectively. The sensor reveals a temperature sensitivity of 0.0097 nm/°C, which is relatively low in comparison to its ultra-high RI sensitivity. The proposed inexpensive and highly sensitive optical fiber RI sensors have numerous applications in chemical and biochemical sensing fields.

A lossy mode resonance (LMR) based fiber optic refractive index (RI) sensor utilizing a layer of fluorine-doped tin oxide (FTO) over the unclad core of the fiber is theoretically analyzed. To enhance its sensitivity, the thickness of the FTO layer is optimized. For further sensitivity enhancement, an over-layer of two different materials, T i O 2 and H f O 2 , is used. The sensitivity of F T O / T i O 2 and F T O / H f O 2 bilayer based LMR sensors increases with the increase in the thickness of the over-layer for a fixed thickness of the bilayer. Further, the sensitivity of the sensor utilizing the F T O / H f O 2 bilayer is found to be more than the sensitivity of the F T O / T i O 2 bilayer sensor. To keep the operating spectral range of the sensor in the visible region, the total thickness of the bilayer is adjusted. It is found that a total bilayer thickness of 30 nm with 90% thickness of H f O 2 gives the maximum sensitivity of 4400 nm/RIU for 1.33 RI of the sensing medium, which is more than three times the sensitivity of the FTO coated LMR sensor. The sensitivity of the proposed LMR sensor utilizing the F T O / H f O 2 bilayer is compared with the other LMR sensors reported in the literature, and it is found that the proposed sensor possesses the highest sensitivity. The other advantage of the proposed sensor is that it works in the visible region, which reduces the cost of the sensor.

A sensitivity-enhanced optical fiber refractive index sensor based on inline Mach-Zehnder interferometer is proposed. The sensor head is formed by splicing a tapered thin-core diameter fiber between two sections of single mode fibers. The taper (less than 1 mm) plays an important role in improving the sensitivity of the sensor. The sensitivity of refractive index is measured to be 0.447 nm for a 1% change of refractive index in the typical refractive index range of 1.333- 1.3725. The whole fabrication process (including splicing and tapering) can be operated by a commercial fiber splicing machine. The proposed sensor also shows the merits of simple structure, low cost, and easy fabrication.

We report and demonstrate a single-mode asymmetric tapered fiber as a refractive index (RI) sensor based on evanescent field sensing. The proposed setup managed to sense the changes of RI when the tapered region was exposed to different concentrations of Sodium chloride (NaCl). This was possible due to the output interference spectrum shifting in response to the changes of RI. Different environmental sensitivities were achieved by manipulating the transition lengths of the tapered fiber. Experimental results depict RI sensitivities of 3914.7 nm/RIU (refractive index unit) and 3395.7 nm/RIU for taper profiles with down-taper transition lengths of 2 mm and 8 mm, respectively, which is higher than previously reported asymmetric taper RI sensors. Such findings may find itself beneficial in low cost, reliable and simplified sensors.

Mid-infrared refractive index sensor with high sensitivity and wide detection range based on multimode interference in tellurite optical fibers

Exposed-core fiber multimode interference sensor

An all-fibre refractive index sensor with a simple periodical tapers configuration is proposed and investigated experimentally. The proposed fibre refractive index sensor consists of a small core singlemode fibre with tapers periodically fabricated along the fibre using a focused CO2 laser beam, and sandwiched between two standard singlemode fibres. Such a structure can be used for sensing of refractive index by measuring the dip wavelength shift of the multimode interference within the small core fibre cladding. A minimum sensitivity of 125 nm/RIU is measured for a refractive index of 1.33 and a maximum sensitivity of 383 nm/RIU for a refractive index of 1.38. The proposed refractive index sensor benefits from simplicity and low-cost and achieves a competitive sensitivity compared with other fibre-optic sensors.

Surface plasmon resonance refractive index sensor based on photonic crystal fiber covering nano-ring gold film

A plasmonic nanostructure (PNS) which integrates metallic and dielectric media within a single structure has been shown to exhibit specific plasmonic properties which are considered useful in refractive index (RI) sensor applications. In this paper, the simultaneous realization of sensitivity and tunability of the optical properties of PNSs consisting of alternative Ag and dielectric of nanosphere/nanorod array have been proposed and compared by using three-dimensional finite element method. The proposed system can support plasmonic hybrid modes and the localized surface plasmonic resonances and cavity plasmonic resonances within the individual PNS can be excited by the incident light. The proposed PNSs can be operated as RI sensor with a sensitivity of 500 nm/RIU (RIU = refractive index unit) ranging from UV to the near-infrared. In addition, a narrow bandwidth and nearly zero transmittance along with a high absorptance can be achieved by a denser PNSs configuration in the unit cell of PNS arrays. We have demonstrated the number of modes sustained in the PNS system, as well as, the near-field distribution can be tailored by the dielectric media in PNSs.

A refractive index (RI) fiber sensor with low detection limit but large dynamic range is proposed and demonstrated using an exposed core microstructured optical fiber. The exposed-core fiber is highly birefringent due to its asymmetry and also supports multimode propagation; thus, can be used simultaneously as a Mach-Zehnder and Sagnac interferometer. The Mach-Zehnder interference is significantly more phase sensitive to RI due to a longer effective path length difference. This leads to a lower detection limit compared to that for the Sagnac interferometer, which has a larger free spectral range that allows the dynamic range of the RI measurement to be extended. By combining these two interferometers, the proposed sensor achieves a detection limit of as low as 6.02 × 10−6 refractive index units (RIU) while maintaining a large dynamic range from 1.3320 to 1.3465 RIU. The proposed sensor also has the advantages of biocompatibility, low cost, high stability, small size, ability to operate remotely and to be fabricated.