Exposed-core fiber multimode interference sensor

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

A novel refractive index (RI) sensor based on the multimode interference effect in an exposed-core tellurite microstructure optical fiber (TMOF) has been designed and implemented in this paper. With different liquid analytes covered onto TMOF core surface, the interference between any two specific modes experiences a fringe shift (or phase change). The exposed-core optical fiber design provides direct access to the evanescent field, allowing RI measurement with a sensitivity of up to 2919.4 nm/RIU. The proposed sensor offers a higher sensitivity while maintaining the fiber robustness and a much wider RI detection range (reaching to 1.58) than its existing counterparts based on multimode interference effect of traditional silica optical fiber. It is also reliable, cost-effective and offers a possibility for label-free biosensing.

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

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

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

An ultra high-sensitivity temperature and refractive index (RI) sensor based on surface plasmon resonance (SPR) effect in D-shape high birefringence photonic crystal fiber (PCF) is presented and discussed by the finite element method (FEM). The designed PCF is polished to enhance the interaction of the evanescent field with the external medium, ultimately improving the sensing performance. The influence of three different plasma materials, gold, silver and aluminum, on the sensing performance was investigated. Finally, the chemically stable plasma material gold is selected to coat on flat surface of D-shape PCF for SPR sensing, realizing the sensitive real-time monitoring of RI and temperature. The sensing performance was greatly improved, with a wide RI (1.43 to 1.50) and temperature (36 °C to 86 °C) detection range, and the corresponding maximum sensitivity was 44850 nm/RIU and -16.875 nm/°C, respectively. Furthermore, the sensitivity is not particularly sensitive to the sizes of the designed air holes, which leads to a sensor with high structure tolerance. Numerical results show that the coating thickness not only affects the sensitivity but also the detection range of the sensor, so we can tune the RI and temperature detection range by adjusting the gold film thickness to meet specific detection requirements. The sensor with enhanced sensitivity, tunable detection range, and high fabrication tolerance has a wide application prospect in the field of life science research, chemical production, and environmental monitoring.

Intensity-modulated refractive index sensor based on the side modes of fiber Bragg grating

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 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.

Femtosecond laser written Bragg gratings have been written in various exposed-core microstructured optical fibres (ECMOFs) for the first time. The EC-MOFs have different core diameters ranging from 2.7 μm to 12.5 μm and offer different degrees of core access depending on application requirements. Writing a Bragg grating on an open core fibre allows for refractive index based sensing, with a view to multiplexed biosensing. Smaller core fibres are shown both experimentally and theoretically to provide a higher sensitivity, with experimental sensitivities demonstrated up to 6.4 nm/RIU over the refractive index range from 1.333 (water) to 1.377 (isopropanol).

Graphene-based metamaterial absorber for refractive index sensing applications in terahertz band

In this work, we present an ultra-high sensitivity (S) refractive index (RI) sensor for biological applications. Silicon on insulator (SOI) ring resonator (RR) based RI sensors have been extensively investigated. Light propagation is studied in the small SOI RRs based on sub-wavelength grating (SWG) waveguides with aqueous claddings. For the device analysis, the two-dimensional conformal transformation method is jointly used with the effective index method. Furthermore, FEM simulations of this structure have been also performed in both the air and water claddings. These good-matched results are further compared with experimental and FDTD results in other valid references. Having found a high data matching between our results and experimental results, we repeat our calculation for different side-widths (d=150-225 nm) and ring radii (7-10 µm). For each ring radius, the best amount of side-width is found. Then bulk refractive index sensing experiments are simulated using changing cladding RI in the range of nc=1.332-1.350 RIU. These RI changes correspond to concertation changes of glucose-water solutions in the cladding. Finally, we obtain the highest sensitivity of 24946 nm/RIU in the case of a radius 7 µm, 500 nm width, 220 nm height, 150 nm side-width, and T of 50 nm between the grating arrays on the RR. It is worth noting that this proposed structure shows 150 times sensitivity enhancement compared to a standard SOI RR-based biosensor with the same radius, w, and h. In addition, the RI sensor suggested in this work shows ultra-high sensitivity in a dynamic range of 1.332-1.352 RIU. This range of RI variation covers almost all the RI variations of the blood components. These results show excellent potential for employing the proposed SOI RR based on SWG waveguides in a liquid-biopsy analysis of patients with cancer. These might pave the way toward the early detection of cancers.

We report on a novel all-fiber refractometer sensor based on multimode interference (MMI) effects. The operating mechanism is based on the self-imaging phenomena that occur in the multimode fiber (MMF) section, which basically replicates the field at the input of the MMF on the output of the MMF for a specific wavelength. However, the longitudinal position of this image is highly dependent on the MMF diameter (D), since there is D² dependence on the longitudinal position of this image. For the refractive index measurement a section of no-core multimode fiber, whose cladding is air, is surrounded by the liquid sample. The liquid sample now works as the cladding medium and as a result of the Goes-Hanchen shift the effective width (fundamental mode width) of the No-Core fiber is increased. As a result, the maximum coupling resulting from the imaging phenomena occurs at a different wavelength, and this can be used to measure the refractive index of the liquid. Using this scheme we can achieve a resolution on the order of 1x10^-5 for a refractive index range from 1.333 to 1.434. The device was used here to measure refractive index in liquids, but can also be applied for measuring concentration of liquids. These sensors are promising and attractive in chemical and biotechnological applications because of their high sensitivity, immunity to electromagnetic interference, and compact size.

A novel sensing peak identification method for high accuracy refractive index (RI) sensing is proposed. The implementation takes the intensity of interference maximum as the characteristic to distinguish interference peaks, tracking the sensing peak continually during a RI changes, with high measurement accuracy and simple computation. To verify the effect of the method, the extrinsic Fabry–Perot interferometer (EFPI) sensor has been fabricated using the large lateral offset splicing technique. In the RI range from 1.346 to 1.388, the measurement range of the EFPI with the proposed method reaches at least 6 times larger than that of EFPI with the wavelength tracking method and the largest measurement error is −4.47 × 10−4. The EFPI refractive index (RI) sensor identified the sensing peak is believed to play an important role in RI, concentration and density sensing, etc., for superior performance.