An optical fiber coupler is a simple and fundamental component for fiber optic technologies that works by reducing the fiber diameter to hundred nanometers or several micrometers. The microfiber coupler (MFC) has regained interest in optical fiber sensing in recent years. The subwavelength diameter rationales vast refractive index (RI) contrast between microfiber “core” and surrounding “cladding”, a large portion of energy transmits in the form of an evanescent wave over the fiber surface that determines the MFC ultrasensitive to local environmental changes. Consequently, MFC has the potential to develop as a sensor. With the merits of easy fabrication, low cost and compact size, numerous researches have been carried out on different microfiber coupler configurations for various sensing applications, such as refractive index (RI), temperature, humidity, magnetic field, gas, biomolecule, and so on. In this manuscript, the fabrication and operation principle of an MFC are elaborated and recent advances of MFC-based sensors for scientific and technological applications are comprehensively reviewed.

Miniaturized real-time fiber optic sensing systems with high sensing performance are in extreme demand. In this work, we propose a novel photonic quasicrystal fiber sensor in the terahertz region and test its sensing characteristics using the finite element method. The proposed simulated sensor numerically investigates the cancer-infected cells from the normal cells in the human cervix, blood, adrenal glands, and breast based on the difference in their refractive index changes. The effective refractive index of core-guided mode is due to the interaction of light between the refractive index of the fiber material and infiltrated normal and cancer cells, respectively. The proposed sensor exhibits a high birefringence of 0.03, a low dispersion of 0.35 ps/THz/cm, along with a high numerical aperture of 0.99. Besides, the sensor holds a less-effective material loss of 2.53 × 10${}^{-9}$ (dB/cm), a maximum power fraction of 88.10, a maximum relative sensitivity of 82.67%, and an effective mode area of 3.16 mm${}^2$. The results envisage that the proposed sensor displays high sensing performances with a rapid cancer detection mechanism.

Recently, Vernier effect is introduced into fiber optic interferometric sensors to enhance the sensitivity. To achieve a high-quality of the Vernier spectrum, the match of the free spectral range (FSR) and the extinction ratio (ER) in two arms is required. Furthermore, the reference arm should be designed to be different from the sensing arm to avoid crosstalk. In this work, a compact in-line Mach-Zehnder interferometer (MZI) by sandwiching a hollow capillary fiber (HCF) between two sections of no-core fiber (NCF) is proposed for the reference arm in Vernier effect based sensing system. FSR of the in-line MZI can be tuned by modulating the length of the HCF, while the ER can remain high up to ~20 dB, which is equivalent to that of the polarization-maintaining fiber (PMF) based interferometers. Experimental results show that the MZI exhibits low sensitivity to temperature, strain, refractive index (RI) and torsion. When employed as the reference arm cascaded with PMF Sagnac interferometer (SI), the upper and lower envelope are with high extinction ratio (ER). The proposed MZI is a promising candidate for the reference arm in Vernier effect based sensing system with the merits of compact size, good spectral characteristics and insensitivity to ambient environment.

A simple, robust, compact, and highly sensitive temperature sensor based on silicone rubber-sealed non-adiabatic tapered fiber (SR-NATF) is proposed and demonstrated. By combining with the fast Fourier transform (FFT) method, an online Vernier reference interferometer (OVRI) is constructed to enhance the temperature sensitivity. The temperature sensitivity is −1.30 nm/°C for the single SR-NATF in the temperature range of 35 °C – 40 °C. After utilizing the FFT method and cascading an OVRI, the temperature sensitivity can obtain 6.26 nm/°C, which is 4.82 times as that of a single SR-NATF. The proposed sensor is robust, soft and easy-fabrication which can be applied in human body temperature detection. In addition, the proposed method is efficient and convenient for the construction of an online reference arm based on Vernier-amplified sensitivity.