We demonstrate a nonadiabatic microfiber sensor with a taper diameter of few micrometers. The modal interference caused by the abrupt taper results in a sinusoidal spectral response. The wavelength shift arising because of the changes in the external refractive index is found to be significant, achieving a maximum sensitivity of 18681.82 nm/RIU. The measured results show good agreement with the theoretical predictions. The high sensitivity and the simplicity offer the sensor the potential for many real applications.

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Ultrahigh Sensitivity Refractive Index Sensor Based on Optical Microfiber

The turning point of the refractive index (RI) sensitivity based on the multimode microfiber (MMMF) in-line Mach-Zehnder interferometer (MZI) is observed. By tracking the resonant wavelength shift of the MZI generated between the HE(11) and HE(12) modes in the MMMF, the surrounding RI (SRI) could be detected. Theoretical analysis demonstrates that the RI sensitivity will reach ±∞ on either side of the turning point due to the group effective RI difference (G) approaching zero. Significantly, the positive sensitivity exists in a very wide fiber diameter range, while the negative sensitivity can be achieved in a narrow diameter range of only 0.3 μm. Meanwhile, the experimental sensitivities and variation trend at different diameters exhibit high consistency with the theoretical results. High RI sensitivity of 10777.8 nm/RIU (RI unit) at the fiber diameter of 4.6 μm and the RI around 1.3334 is realized. The discovery of the sensitivity turning points has great significance on trace detection due to the possibility of ultrahigh RI sensitivity.

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Refractive index sensitivity characteristics near the dispersion turning point of the multimode microfiber-based Mach-Zehnder interferometer.

A Comprehensive Review of Optical Fiber Refractometers: Toward a Standard Comparative Criterion

Rapid advances in optical microfiber and nanofibers (MNF) based sensors have been driven by powerful industries such as automotive, biomedical and defense, with the increasing demand for highly-sensitive, selective, nonintrusive, fast-response, compact and robust sensors that can perform in-situ measurements at remote and harsh environments. A diverse range of MNF based sensors have been developed for measuring refractive index, bio-chemical, temperature, current, displacement, bend, surface, acceleration, force, rotation, acoustic, electric field and magnetic field. Given the growing interest for this exciting area of scientific research, new designs are emerging continuously at a fast pace. This paper attempts to provide a comprehensive review of all MNF based sensors reported to-date. Sensors are divided according to their morphology and measurand.

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A Review of Microfiber and Nanofiber Based Optical Sensors

A multimode microfiber (MMMF)-based dual Mach-Zehnder interferometer (MZI) is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. By inserting a section of MMMFsupporting a few modes in the sensing arm of the MZI setup, an inline interference between the fundamental mode and the high-order mode of MMMF, as well as the interference between the high-order mode of MMMF and the reference arm, i.e., the dual MZI, is realized. Due to different interference mechanisms, the former interferometer achieves RI sensitivity of 2576.584 nm/RIU and temperature sensitivity of 0.193 nm/°C, while the latter one achieves RI sensitivity of 1001.864 nm/RIU and temperature sensitivity of 0.239 nm/°C, demonstrating the ability to attain highly accurate multiparameter measurements.

Simultaneous measurement of refractive index and temperature using multimode microfiber-based dual Mach-Zehnder interferometer

We report on a functional optical microfiber mode interferometer and its applications for absolute, temperature-insensitive refractive index sensing. A standard optical fiber was tapered down to 10 μm. The central part of the taper, i.e., the microfiber, is connected to the untapered regions with two identical abrupt transitions. The transmission spectrum of our device exhibited a sinusoidal pattern due to the beating between modes. In our interferometer the period of the pattern—an absolute parameter—depends strongly on the surrounding refractive index but it is insensitive to temperature changes. The period, hence the external index, can be accurately measured by taking the fast Fourier transform (FFT) of the detected interference pattern. The measuring refractive index range of the device here proposed goes from 1.33 to 1.428 and the maximum resolution is on the order of 3.7×10−6.

Optical microfiber mode interferometer for temperature-independent refractometric sensing

A Mach–Zehnder interferometer formed in single mode fiber is implemented. The interferometer is built by two mechanically-induced long-period gratings. In addition, a fiber taper in the middle section is inserted. The spectral properties of the whole system are analyzed. Visibility of the interference fringes up to 0.80 (the higher ever reported using mechanically-induced long-period gratings) with fringe spacing in the 4.1 to 0.86 nm range are experimentally demonstrated. The proposed device allows reducing the fiber diameter of the section between gratings with a minimal effect in the interference fringe spacing. The sensitivity of the interferometer to external refractive index changes was also studied. It is experimentally shown that, due to the nature of the cladding mode excited, it is necessary to taper the fiber to improve the system sensitivity to external refractive index. Fiber tapers with different diameter, inserted between the long-period gratings pair were fabricated and tested for measuring external refractive index changes. A maximum resolution of 2.3×10−4 RIU in a refractive index range from 1.36 to 1.402 is achieved.

G.A. Cárdenas-Sevilla

Papers

Optical microfiber mode interferometer for temperature-independent refractometric sensing

Tapered Mach–Zehnder interferometer based on two mechanically induced long-period fiber gratings as refractive index sensor

Mechanically induced long-period fiber gratings on tapered fibers