A novel fiber Michelson interferometer (FMI) based on parallel dual polarization maintaining fiber Sagnac interferometers (PMF-SIs) is proposed and experimentally demonstrated for temperature sensing. The free spectral range (FSR) difference of dual PMF-SIs determines the FSR of envelope and sensitivity of the sensor. The temperature sensitivity of parallel dual PMF-SIs is greatly enhanced by the Vernier effect. Experimental results show that the temperature sensitivity of the proposed sensor is improved from −1.646 nm/°C (single PMF-SI) to 78.984 nm/°C (parallel dual PMF-SIs), with a magnification factor of 47.99, and the temperature resolution is improved from ±0.03037°C to ±0.00063°C by optimizing the FSR difference between the two PMF-SIs. Our proposed ultrasensitive temperature sensor is with easy fabrication, low cost and simple configuration which can be implemented for various real applications that need high precision temperature measurement.

The survey focuses on the most significant contributions in the field of fiber optic plasmonic sensors (FOPS) in recent years. FOPSs are plasmonic sensor-based fiber optic probes that use an optical field to measure the biological agents. Owing to their high sensitivity, high resolution, and low cost, FOPS turn out to be potential alternatives to conventional biological fiber optic sensors. FOPS use optical transduction mechanisms to enhance sensitivity and resolution. The optical transduction mechanisms of FOPS with different geometrical structures and the photonic properties of the geometries are discussed in detail. The studies of optical properties with a combination of suitable materials for testing the biosamples allow for diagnosing diseases in the medical field.

A highly-sensitive temperature sensor with controllable sensitivity based on Vernier effect by cascading a tunable extrinsic Fabry-Perot interferometer (FPI) and a fixed reflective Lyot filter (RLF) is theoretically investigated and experimentally demonstrated. The temperature sensitivity can be tuned by modulating the cavity length of the extrinsic FPI and online monitoring the envelope of superimposed spectrum with optical spectrum analyzer (OSA). The FPI works as the reference arm to tune the temperature sensitivity of the sensing system, while the RLF with 1-meter polarization maintaining fiber (PMF) acts as the sensing probe. Experimental results prove that by changing the cavity length of the FPI, the sensitivities of −3.82 nm/°C, −8.33 nm/°C and −14.63 nm/°C can be achieved. Compared with the single sensing element, the sensitivities are magnified by 3.78, 8.25 and 14.49 times. The proposed temperature sensor is feasible to be applied practically in scenarios which require different temperature sensitivities in demanded temperature detection ranges.

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.

The emerging development of sensing technology initiates innovative sensors achieving low-cost to facilitate practical realization. An interesting crush of the work is to propose a simple structural sensor and to analyze the different schemes of the metal coating by stimulated emission of Raman scattering (SERS) intensification. For the first time, we propose a simple geometrical photonic crystal fiber refractive index based sensor (PCF-RIBS) with three different Schemes A, B, and C, i.e., gold (A) layer-coated surface plasmon resonance (SPR) based D-type PCF-RIBS; Au with titanium-di-oxide (TiO${}_2$) layer-coated SPR D-type PCF-RIBS; and Au + TiO${}_2$ grating-coated localized surface plasmon resonance (LSPR) D-type PCF-RIBS. Characterizing the three different Schemes A, B, and C using finite element method simulation shows, a maximum wavelength sensitivity of 48,000 nm/RIU, 52,000 nm/RIU and 75,000 nm/RIU, respectively, for a wide range of analyte-refractive index from 1.33 to 1.45 and operates in the wavelength range from 500–2000 nm. Of all the Schemes, Scheme C is found to excite a relatively larger number of surface-plasmons. Eventually, it exhibits improved sensing performances compared to SPR based Schemes A and B. Consequently, it would turn out to be an appropriate candidate to detect a broad range of biological and chemical sample detection.

We propose a sucrose concentration sensor by utilizing a fiber Sagnac interferometer with no-core fiber (SI-NCF) based on the Vernier effect. The Vernier effect is realized by introducing a single Sagnac interferometer (SI) with a similar free spectral range of SI-NCF. When the NCF is exposed to the external sucrose solution, the cladding state of NCF is changed, which induces the wavelength shift of the SI-NCF. The measured sucrose concentration sensitivity of a single SI-NCF is 2.97 nm/M, and the sensitivity can be improved to -13.84 nm/M with the assistance of the Vernier effect, which is 4.66 times of the single SI-NCF. The sensor has the advantages of high sensitivity, easy-fabrication and cost-effectiveness which can be applied in the field of the food industry, chemistry and agriculture.