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.

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

Optical microfiber-based temperature sensors have been proposed for many applications in a variety of industrial uses, including biomedical, geological, automotive, and defense applications. This increasing demand for these micrometric devices is attributed to their large dynamic range, high sensitivity, fast-response, compactness and robustness. Additionally, they can perform in-situ measurements remotely and in harsh environments. This paper presents an overview of optical microfibers, with a focus on their applications in temperature sensing. This review broadly divides microfiber-based temperature sensors into two categories: resonant and non-resonant microfiber sensors. While the former includes microfiber loop, knot and coil resonators, the latter comprises sensors based on functionally coated/doped microfibers, microfiber couplers, optical gratings and interferometers. In the conclusions, a summary of reported performances is presented.

https://www.mdpi.com/1424-8220/18/2/461/pdf

A Review of Microfiber-Based Temperature Sensors

Optical microfibers and related structures which incorporate large evanescent field and minimal size offer new opportunities for biosensing applications. In this paper we report the development of an immunosensor based on a tapered microfiber coupler embedded in a low refractive index polymer. Biomolecules adsorbed on the microfiber coupler surface modify the surrounding refractive index. By immobilizing antigens on the surface of the sensing area, the microfiber coupler was able to operate as a label-free immunosensor to detect specific antibodies. We experimentally demonstrated for the first time the sensing ability of this sensor using a fibrinogen antigen-antibody pair. By monitoring the spectral shift in the wavelength domain, the sensor was shown to be capable of detecting the specific binding between fibrinogen and anti-fibrinogen. The detected signal was found to be proportional to the anti-fibrinogen present.

/pdf/microfiber-coupler-based-label-free-immunosensor-3wjr6v4sad.pdf

Microfiber Coupler Based Label-Free Immunosensor

A micro-size optical sensor for simultaneous detection of dual-point seawater temperature based on microfiber double knot resonators (MDKRs) is demonstrated in this paper. To obtain the optimal sensor parameters, the sensing mechanism and spectral characteristics of MDKRs in seawater are investigated theoretically and experimentally. Results show that the MDKRs with short period spectra is more suitable for the simultaneous detection of dual-point seawater temperature, due that it has a long wavelength range and is easy to distinguish resonant wavelengths of two microfiber knot resonators (MKRs). The sensitivities of the cascaded MKRs are −13.86 pm/°C and −5.6 pm/°C, respectively, achieving independent and simultaneous seawater temperature sensing. The sensor proposed here can meet the actual demands of mesoscale ocean phenomena and micro-region detection in seawater, and offer references for developing simple and convenient optical method in ocean detection and other practical application.

Dual-Point Seawater Temperature Simultaneous Sensing Based on Microfiber Double Knot Resonators

https://www.mdpi.com/2079-6374/10/11/179/pdf

State-of-the-Art Optical Microfiber Coupler Sensors for Physical and Biochemical Sensing Applications.

We studied the excitation of whispering gallery modes (WGMs) in a cylindrical microresonator formed by a short length of a multimode fiber. The light was evanescently coupled into the fiber from a micro-cylinder whose tilt angle was varied from perpendicular to parallel with respect to the fiber taper. Narrow WGM resonances are observed in the transmission spectrum of the fiber taper with a maximum Q factor of ~10 5 . Evolution of the transmission spectrum with the change of tilt angle involved changes in the number of WGMs in the spectrum for smaller tilts and the disappearance of WGM resonances at large tilt angles.

Study of whispering gallery modes in a cylindrical microresonator excited by a tilted fiber taper

The paper presents the results of experimental studies of the temperature dependence of a microfibre coupler (MFC) with a waist diameter of ~4 μm covered with a layer of liquid crystal (LC) material. The microfiber coupler is fabricated by fusing together and tapering of two standard telecom fibers using a microheater brushing technique, followed by partially embedding the structure in a low-refractive index UV curable polymer (Efiron PC-363) for stability and later by placing a thin heated LC layer over the polymer-free uniform taper waist region. The temperature dependence of the embedded in polymer MFC sensor before the application of the LC layer demonstrates a redshift of the coupler’s spectrum with an average sensitivity of ~0.5 nm/°C in the temperature range of 14-70 °C. The application of the LC overlay increases the average temperature sensitivity to ~0.7 nm/°C. The demonstrated device offers several advantages such as ease of fabrication and light coupling, the potential for better stability and the possibility of electric field tuning for realizing temperature, electric field, bio-, chemical sensors and tunable add-drop filters for fiber communication systems. Further work is ongoing to explore various tuning mechanisms of the MFC spectrum.

Experimental study of temperature response of a microfiber coupler sensor with a liquid crystal overlay.

A high sensitivity refractive index (RI) sensor based on a tapered small core singlemode fibre (SCSMF) structure is proposed and developed. By tapering the SCSMF section, this sensor has very high sensitivity around the RI of water. Experimentally we have demonstrated a sensitivity of 1988.5 nm/RIU (RI unit) in the RI range from 1.3325 to 1.3407 by tapering a SCSMF section from a diameter of 125 μm to a diameter of 16 μm. The feasibility of using this type of sensor for biosensing application has also been verified by experiments. This was undertaken using a suitable Fibrinogen antigen surface functionalisation, which successfully showed that the sensor can in principle detect differences in the concentration of a Fibrinogen antibody.

Mianzhen Teng

Papers

Study of whispering gallery modes in a cylindrical microresonator excited by a tilted fiber taper

Experimental study of temperature response of a microfiber coupler sensor with a liquid crystal overlay.

A high sensitivity refractometer based on a tapered SCSMF structure and its application to biosensing