A type of D-shaped photonic crystal fiber sensor based on surface plasmon resonance (SPR) is proposed for refractive index sensing and analyzed by the finite element method. The SPR effect between surface plasmon polariton modes and fiber core modes of the designed D-shaped photonic crystal fiber is used to measure the refractive index of the analyte. Numerical results show that the sensor can detect a range of refractive index ranging from 1.33 to 1.38. When the thickness of metal film is $t=20\mathrm{nm}$, the maximum sensitivity of $\mathrm{10,493}\mathrm{nm}/\mathrm{RIU}$ is obtained with a very high resolution of $9.53\times {10}^{-6}\mathrm{RIU}$. The good sensing performance makes the proposed sensor a competitive candidate for environmental, biological, and biochemical sensing applications.

The polarization filter characters of a gold-coated and liquid-filled photonic crystal fiber are studied using the finite element method. Results show that the resonance strength and wavelengths are different in two polarized directions. Filling liquid of refractive index $n=1.33$ (purified water) in holes in longitudinal direction can increase the loss of core mode polarized in the y-direction around the resonance peak. The resonance strength is much stronger in y-polarized direction than in x-polarized direction. The resonance strength can achieve 508dB/cm in y-polarized direction at the communication wavelength of 1311nm in one of our structures. Moreover, the full width half maximum is only 20nm. Such a small number makes such photonic crystal fibers promising candidate to filter devices. A liquid filled PCF of the small hole in the fiber core is designed and we find that filling liquid increases the resonance strength peak by thirty eight percent for the y-polarized resonance point.

A biocompatible and robust fiber surface plasmon resonance (SPR) temperature sensor was fabricated based on an alcohol-filled hollow-core fiber. The fabrication process of this designed sensor included Ag film coating, liquid injection and fusion splicing, which was low-cost and efficient. Due to the high refractive index sensitivity of SPR effect and the high thermal optical coefficient of alcohol, the designed sensor performed well in temperature sensing, whose linear sensitivity reached as high as 1.16 nm/°C in the range of 35.5 °C ~ 70.1 °C. Being small in size, low in fabrication cost and highly sensitive in performance, this sensor is suitable for temperature detection during biological and chemical reactions, and has the potential to realizing multi-parameter or distributed temperature measurement.

We propose a D-shaped photonic crystal fiber (PCF) refractive index sensor with ultrahigh sensitivity and a wide detection range. The gold layer is deposited on the polished surface, avoiding filling or coating inside the air holes of the PCF. The influences of the gold layer thickness and the diameter of the larger air holes are investigated. The sensing characteristics of the proposed sensor are analyzed by the finite element method. The maximum sensitivity can reach $\mathrm{31,000}\mathrm{nm}/\mathrm{RIU}$, and the refractive index detection range is from 1.32 to 1.40. Our proposed PCF has excellent sensing characteristics and is competitive in sensing devices.

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.