A sensitivity-amplifying optical fiber sensor using cascaded Sagnac interferometers (CSIs) was designed and experimentally demonstrated for the dual-parameter measurement of strain and temperature. The proposed CSIs sensor consisted of two Sagnac interferometers (SIs) in which two identical PANDA polarization-maintaining fibers (PMFs) for just slightly different lengths were inserted. An envelope with a period of 123.6 nm, which was amplified more than 10 times than the single SI, was observed in the interference spectrum. Experimental results showed that the strain sensitivity increased from 32.91 pm/μϵ for the single SI to 336 pm/μϵ for the CSIs, while the temperature sensitivity increased from −1.38 nm/°C for the single SI to −14.86 nm/°C for the CSIs, correspondingly. We also found that the envelope of the CSIs shifted in opposite directions when the longer or shorter PMF was selected as the sensing arm. Higher sensitivity could be realized by selecting longer PMF as the sensing arm. Our proposed CSIs sensor possesses the merits of simple structure, high sensitivity, low cost, and low hysteresis effect, which make it a competitive candidate for strain and temperature monitoring.

A dual D-type photonic crystal fiber (PCF) was proposed in this work to measure the refractive index and temperature of seawater. Two golden layers were designed to be coated on the two polished planes in the dual D-type PCF. Two confinement loss peaks in the transmission spectrum appeared due to the surface plasmon resonances which were inspired on the two golden layers. In order to achieve two-parameter sensing, one of the two polished planes was further coated with a polydimethylsiloxane (PDMS) layer which was temperature-sensitive. Seawater was assumed to be coated on the outer surface of the PCF. Numerical results by using the finite element method showed that the measurement sensitivity of the refractive index of seawater reached 1371 and 1228 nm/RIU, while the measurement sensitivity of temperature reached −0.3 and −1.06 nm/ °C, respectively. Finally, we obtained the transfer matrix expression which could be used to measure the refractive index and temperature of seawater simultaneously. The designed dual D-type PCF, which is simple in structure, highly sensitive and two-parameter measuring, could be a promising candidate for the monitoring of seawater.

A novel highly sensitive D-shaped photonic crystal fiber-based surface plasmon resonance (PCF-SPR) sensor for dual parameters of refractive index and temperature detecting is proposed. A PCF cladding polishing provides a D-shape design with a gold (Au) film coating for refractive index (RI) sensing (Core 1) and a composite film of silver (Ag) and polydimethylsiloxane (PDMS) for temperature sensing (Core 2). Comsol Multiphysics 5.5 is used to design and simulate the proposed sensor by the finite element method (FEM). The proposed sensor numerically provides results with maximum wavelength sensitivities (WSs) of 51,200 and 56,700 nm/RIU for Core 1 and 2 as RI sensing while amplitude sensitivities are −98.9 and −147.6 RIU⁻¹ with spectral resolution of 1.95 × 10⁻⁶ and 1.76 × 10⁻⁶ RIU, respectively. Notably, wavelength sensitivity of 17.4 nm/°C is obtained between −20 and −10 °C with resolution of 5.74 × 10⁻³ °C for Core 2 as temperature sensing. This sensor can efficiently work in the analyte and temperature ranges of 1.33–1.43 RI and −20–100 °C. Due to its high sensitivity and wide detection ranges, both in T and RI sensing, it is a promising candidate for a variety of applications, including chemical, medical, and environmental detection.