Chao Du

High sensitivity internal refractive index sensor based on a photonic crystal fiber long period grating

ABSTRACT A high sensitivity method is reported for the measurement of the internal refractive index using a photonic crystal fiber long period grating. Long period gratings of different lengths were inscribed in photonic crystal fibers, and the air holes of the fiber had varying refractive indices. A side lobe appeared near the resonant dip through the analysis of the characteristics of transmission spectra showing variation in the refractive index. The resonant dip and its side lobe provided varying sensitivities to the internal refractive index values. The sensitivity of the side lobe was as high as 2343 nm/refractive index unit (RIU), which exceeded the value for the resonant dip (2047 nm/RIU) for refractive indices from 1.3333–1.3792. Due to the high resolution of 8.5 × 10−6 RIU, this method offers promising applications for biological and chemical analysis in which high-precision refractive index measurements are required.

Long-period fiber grating sensor induced by electric-arc discharge for dual-parameter measurement

ABSTRACT A long-period fiber grating sensor induced by electric-arc discharge has been fabricated and demonstrated for the simultaneous measurement of temperature and strain. The proposed sensor was fabricated by inscribing a sing-mode fiber with periodic electric-arc discharge technology that was produced from a commercial fusion splicer. The resonance dips formed by the coupling between cladding modes and core mode have different sensitivity responses, so the simultaneous measurement for multiple parameters was realized by monitoring the wavelength shifts of the resonance dips. Because of the easy fabrication and low cost, the sensor is promising for applications for which simultaneous measurement of temperature and strain is required.

Sensitivity-enhanced single-mode fiber-tapered hollow core fiber-single-mode fiber Mach–Zehnder interferometer for refractive index measurements

ABSTRACT In this paper, a sensitive-enhanced single-mode fiber—tapered hollow core fiber—single-mode fiber Mach–Zehnder interferometer is demonstrated for refractive index sensing. The sensitivity was improved by forming an up-taper at the two splicing joints and concave cone in hollow core fiber. The up-tapered regions served as a more effective mode splitter/combiner, and the tapered hollow core fiber was used to generate a stronger evanescent field to enhance the interaction of light with the analyte. According to the principles of interference between the cladding and fundamental modes, we performed refractive index measurements. The experiments indicated that the proposed sensor has a high refractive index sensitivity of 214.97 nm/RIU in the refractive index range of 1.333–1.379, with a minimum refractive index measurement resolution of 9.3 × 10−5. In addition, the sensor had a low temperature response of 2.96 pm/°C in the range from 50 to 85°C and a low cross sensitivity of 1.377 × 10−5 RIU/°C. The proposed sensor is attractive for its high refractive index sensitivity, easy fabrication, low cross sensitivity, and good mechanical strength, making it of potential value for refractive index measurements for chemical and biological sensing.

Sensitivity-optimized long-period fiber gratings for refractive index and temperature sensing

ABSTRACT A long-period fiber grating sensor was fabricated by periodically changing the structure of single-mode fiber with an electric arc discharge technique. After the fabrication, the refractive index and temperature sensitivities were optimized by etching the cladding with hydrofluoric acid solution. The experimental results illustrate that the thinner cladding shows relatively higher refractive index and temperature sensitivities for the same order cladding mode, which are accordant with that of numerical simulation. After the long-period fiber grating was etched for 15 min, average refractive index sensitivities of 214 nm/refractive index unit (RIU) (1.3333 – 1.3931) and 1987 nm/RIU (1.4115–1.4555) were achieved. An extremely higher refractive index sensitivity of 2731 nm/RIU appears near 1.4555. By systematically studying the temperature sensing characteristic of cladding-etched long-period fiber grating in this work for the first time, the temperature sensitivity can reach as high as 144.23 pm/°C when the ambient temperature changes from 30 to 80°C. This work provides a theoretical reference for the fabrication of a high-sensitivity refractive index and temperature sensor based on arc-induced long-period fiber grating.

High sensitivity fibre surface plasmon resonance sensor based on silver mirror reaction

A novel fibre surface plasmon resonance (SPR) sensor fabricated by a silver mirror reaction is first proposed and demonstrated in this paper. The experimental results showed that the silver film characteristics of the fibre SPR sensing probe are affected by the concentration of silver ammonia solution, and the relations between the concentration of silver ammonia solution and properties of the sensors have been obtained, which is in accordance with the optimal parameters of fibre and silver film through a theoretical simulation. Firstly, a theoretical model of a silver film-based fibre SPR sensing mechanism has been built up. Then the numerical simulations towards the influence of sensing structure, thickness and sensing length of metal films on the sensing system sensitivity have been performed. Finally, the optimal structure parameters of the sensor are obtained. The results show that this fibre SPR sensing system provides a promising platform for sodium chloride solution concentration measurement with a concentration sensitivity of 710.4 nm/%.

Simultaneous measurement of refractive index and temperature based on a long period fiber grating inscribed in a photonic crystal fiber with an electric-arc discharge

Abstract A novel fiber sensor composed by two single mode fibers and long period fiber grating based on a photonic crystal fiber prepared by periodic discharge heating has been experimentally investigated to measure refractive index and temperature. A Mach-Zehnder interferometer was formed due to the presence of two fusion spliced collapsed regions in the photonic crystal fiber. The resonance dip and interference pattern were differently influenced by the ambient disturbance, so the dual-parameters were simultaneously measured by analyzing the characteristics of transmission spectrum. After the experimental measurements, refractive index and temperature sensitivities of 117.28 nm/RIU and −86.29 pm/°C were realized. Therefore, the reported sensor with advantages of easy fabrication, simple structure, and small size has the potential for simultaneous refractive index and temperature measurements involving biochemical sensing applications.

Research and Application of Ice Thickness and Snow Depth Automatic Monitoring System

The ice thickness and snow depth are the basic parameters of conventional weather observation, environmental monitoring, and hydrological services in winter. Moreover, they are also classified as the key observation contents of the polar scientific expedition for analyzing global climate changes. In this paper, an ice thickness measuring method based on temperature gradient differences in air, ice, and water and a snow depth detection method based on differences in optical intensity attenuation induced by infrared light going through air or snow are proposed. Based on these methods, a monitoring system was fabricated to monitor the ice and snow in the Heilongjiang River of China, where the ambient temperature is below −30 °C and the river ice is covered with snow in winter. The results indicate that the system achieved a measurement precision of 0.01 m for both the ice thickness (in the range of 0–2 m) and snow depth detection (in the depth range of 0–0.5 m). The proposed system has the potential to be used for real-time monitoring of the growth, melting, and changing processes of river ice and snow.

Characterization of displacement sensing based on fiber optic microbend losses

ABSTRACT This article theoretically and experimentally characterizes single-mode fiber bending losses and associated sensing. As the bending radius decreased, the losses increased significantly, and shock phenomena were observed in the loss curve. Theoretical analysis shows that these processes were caused by coupling between fundamental models spreading in the optical fiber and whispering gallery mode spreading in the cladding and coating layers. Based on the principles of single-mode fiber bending losses, a loss modulator was designed to bend the single-mode fiber and produce losses due to external displacement. A displacement-loss model was constructed and the results were consistent with the theoretical analysis. The displacement resolving power of the loss modulator was 1 µm. The displacement measurement was from 0–350 µm, resulting in losses from 0–25 dB.