•A composite interference structure for simultaneous measurement of RI and temperature is proposed and experimentally demonstrated.•The proposed structure was constructed with the combination of Sagnac loop mirror and balloon-like interferometer.•As the two sensing structures are based different interference principles, the two interference spectra do not interfere with each other.•Experimental results show that the sensitivities of the RI and temperature can reach 218.56 nm/RIU and 1.7 nm/ °C, respectively.A novel composite interference structure constructed with the combination of Sagnac loop mirror and balloon-like interferometer for simultaneous measurement of refractive index (RI) and temperature is proposed and experimentally demonstrated. A polarization maintaining fiber (PMF) is embedded into the Sagnac loop structure to form a Sagnac loop interferometer, which is highly sensitive to external temperature variations. The balloon-like structure is built with a bend single mode fiber (SMF) to form a modal interferometer, which is sensitive to both external RI and temperature variations. As the two sensing structures are based different interference principles, the two interference spectra do not interfere with each other. By optimizing the bending diameter of the balloon-like interferometer and the length of PMF in the Sagnac loop mirror, separate different resonance wavelengths can be well formed. Hence, the simultaneous measurement of RI and temperature can be realized by monitoring the shift of different resonance wavelengths. Experimental results show that the optimal sensitivities of the RI and temperature can reach up to 218.56 nm/RIU and 1.7 nm/°C, respectively.

A highly sensitive refractive index (RI) sensor based on four-hole grapefruit fiber with surface plasmon resonance (SPR) has been proposed and theoretically investigated. By coating gold film on the wall of large air holes in grapefruit fiber, the RI sensitivity of the proposed sensor could be effectively improved because of SPR effect. It can be found that the thickness of gold film plays an important role for SPR by numerically analyzing the transmission spectra characteristics of the sensor. The simulation results show that the sensitivity of the analyte which has a high RI is higher than that of the low in RI range of 1.33 to 1.43, and an extremely high RI sensitivity of 19,000 nm/RIU can be achieved at 1.43. Due to the excellent performances in high RI sensing range, the proposed sensor has the potential to be used for the measurement of organic chemicals that high-precision RI measurement is required.

•This paper theoretically studied technology of slowing light speed down based on fiber grating, which mainly carried out through numerical simulation about the grating length, refractive index contrast, grating period, loss coefficient, chirp and apodization functions influences on slow light characteristics by using transfer matrix method.•This paper point out the direction that how can we obtain an optimized fiber grating based slow light for the first time.•The proposed theory and method open a new idea in the field of sensing application utilizing fiber grating slow light technology.On the edge of bandgap in a fiber grating, narrow peaks of high transimittivity exist at frequencies where light interferes constructively in the forward direction. In the vicinity of these transmittivity peaks, light reflects back and forth numerous times across the periodic structure and experiences a large group delay. In order to generate the extremely slow light in fiber grating for applications, in this research, the common sense of formation mechanism of slow light in fiber grating was introduced. The means of producing and operating fiber grating was studied to support structural slow light with a group index that can be in principle as high as several thousand. The simulations proceeded by transfer matrix method in the paper were presented to elucidate how the fiber grating parameters effect group refractive index. The main parameters that need to be optimized include grating length, refractive index contrast, grating period, loss coefficient, chirp and apodization functions, those can influence fiber grating characteristics.

A novel refractive index sensor with high sensitivity based on Mach–Zehnder interferometer formed by cascaded two single-mode fiber tapers was proposed and experimentally demonstrated. The dip of the measured spectrum signal caused by Mach–Zehnder interference shifted obviously when the surrounding refractive index changed. The approximate linear relationship between surrounding refractive index and spectrum dip wavelength shift was obtained experimentally. The measurement sensitivity up to 158.4 nm/RIU was showed with the surrounding RI ranged from 1.33 to 1.3792, which meant the measurement resolution about 6.3 × 10−6 could be implemented if wavelength shift measurement resolution of the optical spectrum analyzer is 1 pm. Meanwhile, its ease of fabrication makes itself a low-cost alternative to existing sensing applications.

•The paper proposed and demonstrated a novel splicing points tapered SMF-PCF-SMF structure Mach-Zehnder interferometric refractive index sensor for the first time.•The paper built a structure model of splicing points tapered SMF-PCF-SMF structure Mach-Zehnder interferometric refractive index sensor to numerically study the refractive index sensing characteristics, and optimizing the structure parameters to improve the sensitivity.•Experimentally fabricated the splicing points tapered SMF-PCF-SMF structure Mach-Zehnder interferometric refractive index sensor and realized the sensitivity of 260.8 nm/RIU, compared with sensitivity of 224.2 nm/RIU of direct splicing SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4 cm, the sensitivity increased by 16.3%.The paper proposed and studied a Mach-Zehnder mode interferometric refractive index sensor, which is based on splicing points tapered SMF-PCF-SMF (SMF, single-mode fiber; PCF, photonic crystal fiber) structure. For the reason that the effective refractive index of photonic crystal fiber cladding high-order modes near fiber core are more sensitive to surrounding refractive index changes, the refractive index measurement sensitivity of splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer can be enhanced further through tapering the splicing points. Relations between refractive index measurement sensitivity and photonic crystal fiber length and taper waist diameter are studied through numerical simulations and experiments. Simulation and experimental results show that sensitivity will be increased with the increase of photonic crystal fiber length and the decrease of taper waist diameter. In the refractive range of 1.3333–1.3737, splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4 cm and taper waist diameter of 60.4 μm has refractive index measurement sensitivity of 260.8 nm/RIU, compared with sensitivity of 224.2 nm/RIU of direct splicing SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4 cm, the sensitivity increased by 16.3%. The research shows that the sensing structure is with good linearity and repeatability.

•A magnetic field sensor was fabricated and the magnetic field sensing system based on FLRDS and the magnetic field sensor was proposed.•Utilize the tunable-RI of MF to realize the demodulation and the evanescent field formed by etching the fiber combined with the MF to raise the sensitivity.•The sensitivity and the anti-interference ability has been enhanced significantly to fulfill the sensitivity of 12.56 Oe/μs.A novel magnetic field sensing system based on fiber loop ring-down spectroscopy (FLRDS) and etched fiber interacting with magnetic fluid (MF) is proposed and demonstrated for the first time. The enhanced evanescent field effect in the sensing part was achieved by etching the fiber with hydrofluoric acid. The influence of diameters of etched fiber to the performance of the sensor was investigated and discussed. In the sensing system, the etched fiber surrounded by MF was used as the sensing head and on account of the tunable refractive index and absorption coefficient of MF, the transmission spectrum would change with the magnetic field strength. In this letter, the FLRDS sensing system was theoretically modeled and FLRDS technique was utilized to modulate the transmission spectrum. The sensitivity of magnetic field sensing was enhanced significantly. In the experiment, performances of the magnetic field sensing system were tested by applying different measured magnetic field. The final results indicated that a sensitivity of 12.56 G/μs was achieved.

•This paper theoretically studied the technology of slowing light speed down based on fiber double-loop coupled resonator, which mainly carried out through numerical simulation about the loop loss influence on characteristics of slow light by using iterative and parameter matrix method.•A micro-displacement sensing scheme using the loop2 loss and a micro-bending loss modulator is proposed and realized for the first time. In order to implement micro-displacement sensing, a single-mode fiber micro-bending loss modulator was induced into loop2 to produce controllable loss. The slow light delay of 32.0668 ns was achieved in the system.•Finally, the experimental results shows that a 10 μm displacement resolution of slow light micro-displacement sensing in the displacement measuring range of 0–350 μm can be obtained.Slow light technology is becoming a research hotspot because of light-matter interaction enhancement effects, especially sensitivity enhancement effects. This paper theoretically studied the technology of slowing light speed down based on fiber double-loop coupled resonator, which mainly carried on a numerical simulation about the loop loss influence on characteristics of slow light by using iterative and parameter matrix methods. The slow light time delay of 32.0668 ns was achieved in the experimental system. And then a micro-displacement sensing scheme using loop loss and a bending loss modulator was proposed and designed. In order to implement micro-displacement sensing, bending loss effect of single-mode fiber was induced into one loop to produce a controllable loss. Finally, a 2 μm displacement resolution and a 50 ns/mm displacement sensitivity of slow light micro-displacement sensing at the displacement range of 0–350 μm were obtained from the experimental results.

•Proposed a high sensitivity temperature sensor based on isopropanol-filled PCF-LPG.•A temperature sensitivity of 1.356 nm/°C and resolution of 0.015 °C are achieved.•The excellent performance makes it potential for high precision temperature sensing.A high sensitivity measurement method for temperature has been proposed and investigated based on an isopropanol-filled photonic crystal fiber long period grating (PCF-LPG). Due to the high thermo-optic coefficient (TOC) of isopropanol, the sensitivity of the proposed temperature sensor could be effectively improved by filling isopropanol in the air waveguides of PCF. It can be found that the resonant dip will be split in two dips after filling isopropanol and the two dips have different sensitivities to surrounding temperature. Because of PCF-LPG is sensitive to the refractive index (RI) of internal filled liquid, the isopropanol-filled PCF-LPG temperature sensor has a high sensitivities of 1.356 nm/°C in the range of 20–50 °C. The simplicity and the excellent performance of our proposed device make it potential for the applications of high-precision temperature measurement is required.