Shecheng Gao

Fiber-optic bending vector sensor based on Mach–Zehnder interferometer exploiting lateral-offset and up-taper

A simple, compact, and highly sensitive optical fiber directional bend sensor is presented. This device consists of a lateral-offset splicing joint and an up-taper formed through excessive fusion splicing method. The lateral-offset splicing breaks the cylindrical symmetry of the fiber and defines a pair of directions along which the bending response of the Mach-Zehnder interferometer transmission spectrum is different and thus could be used for bending vector measurement. For a curvature range from -3 to 3 m(-1), the bending sensitivities at 1463.86 nm and 1548.41 nm reach 11.987 nm/m(-1) and 8.697 nm/m(-1), respectively.

Long-Period Fiber Grating Cascaded to an S Fiber Taper for Simultaneous Measurement of Temperature and Refractive Index

A novel, low-cost, and simple dual-parameter measurement scheme based on a cascaded optical fiber device composed of a long-period fiber grating (LPFG) and an S fiber taper Mach-Zehnder interferometer (SFT-MZI) is proposed and demonstrated. The crosstalk problem is solved as different resonance peaks of the LPFG and the MZI possess different refractive index (RI) and temperature sensitivities. Experimental results show distinctive spectral sensitivities of -52.57 nm/refractive index unit (RIU) and 45.87 pm/°C by the LPFG, and 311.48 nm/RIU (in the RI range of 1.33-1.37) and 12.87 pm/°C by the SFT-MZI. The simultaneous measurement of external RI and the temperature is experimentally demonstrated by the sensor. The RI and temperature calculated by the sensor matrix agree well with the actual RI and temperature in the experiment.

Two-dimensional bending vector sensing based on spatial cascaded orthogonal long period fiber.

A novel bending vector sensor based on spatial cascaded orthogonal long period fiber gratings (SCO-LPFGs) written by high-frequency CO(2) laser pulses has been proposed, and two-dimensional bending vector sensing characteristics based on the simple SCO-LPFGs have been experimentally demonstrated. A three-dimensional orthogonal sensing coordinate system has been established, and the measurement results of the proposed SCO-LPFGs sensor based on the above coordinate system is given, and furthermore both of curvature and bending-direction could be intuitively solved according to the three-dimensional orthogonal sensing coordinates. The research work presented in this paper would be helpful to improve the practicability of fiber vector sensors due to the distinguished characteristics such as simple structure, low-cost, ease of fabrication.

Fiber in-line Mach–Zehnder interferometer based on near-elliptical core photonic crystal fiber for temperature and strain sensing

A fiber in-line Mach-Zehnder interferometer is fabricated by selectively filling liquid into one air hole of the innermost layer of a photonic crystal fiber (PCF). The refractive index of the liquid is so close to that of the background silica in the wavelength range of 1300-1600 nm that the two-mode PCF evolves into multimode PCF with an elliptically shaped core. Due to the different propagation constants, interference can occur between the fundamental mode and higher-order modes of the liquid-filled PCF. Such a device is applied in temperature and strain measurements with high sensitivities of 16.49 nm/°C and -14.595 nm/N, respectively.

Simultaneous measurement of temperature and force with high sensitivities based on filling different index liquids into photonic crystal fiber

A double-filled photonic crystal fiber (PCF) was fabricated by filling liquids of different indexes into two air holes in the cladding. The core mode coupled to the local cladding modes LP(01) and LP(11) in the 1310 and 1550 nm wavebands, respectively. Due to the unique characteristics of the mode coupling, the resonant peaks in different resonance areas shifted to the opposite directions with the variations of the temperature or the force. The double-filled PCFs achieved in this work showed useful applications in the simultaneous measurement of both the temperature and the force.

Simultaneous strain and temperature measurement by cascading few-mode fiber and single-mode fiber long-period fiber gratings

A hybrid optical fiber structure of two cascading long-period fiber gratings (LPFGs), respectively, inscribed on a segment of few-mode fiber (FMF) and single-mode fiber (SMF), is proposed and experimentally demonstrated. This structure could be used for simultaneous measurement of strain and temperature. The FMF-LPFG exhibits an opposite temperature response and higher strain sensitivity compared to that of the SMF-LPFG, which can improve the measurement resolutions. Experimentally, the strain and temperature sensitivities of the proposed sensor are -2.9  pm/με and -17.6  pm/°C, respectively, for the FMF-LPFG; for the SMF-LPFG, these are -1.47  pm/με and 46.4  pm/°C, respectively. The maximum errors are ±7.98  με and ±0.54°C for strain and temperature, respectively.

Highly Sensitive In-Fiber Refractive Index Sensor Based on Down-Bitaper Seeded Up-Bitaper Pair

We report a novel highly sensitive in-fiber refractive index (RI) sensor based on the Mach-Zehnder interferometer (MZI). This sensor is composed of an up-fusion-bitaper (UFBT) pair and an embedded down-stretching-bitaper (DSBT), where the UFBT excited high-order cladding modes and the DSBT enhanced the evanescent field. By employing the interaction between the strong evanescent field of high-order cladding mode and ambient environment, an RI sensitivity of 86.565 nm/cm/RIU is achieved in the RI range from 1.3332 to 1.4140. This sensitivity is about an order of magnitude higher than that of the conventional and taper-based improved in-fiber MZIs.

Orthogonal Single-Polarization Single-Core Photonic Crystal Fiber for Wavelength Splitting

A novel photonic crystal fiber (PCF) with a single core, rather than two cores, for wavelength splitting application has been proposed through the full-vector finite-element method. The light located in the 1.31- and 1.55- μm bands propagates through orthogonal polarization states in the proposed fiber. For a propagation distance of 20 mm, the 20-dB bandwidth for the same x- or y-polarized modes within the 1.31- and 1.55- μm wavelength bands could, respectively, reach 20.5 and 45.3 nm, which is much wider than that of a two-core PCF.

Microfiber-Enabled In-line Fabry–Pérot Interferometer for High-Sensitive Force and Refractive Index Sensing

A microfiber-enabled Fabry-Pérot interferometer (FPI) constructed by splicing a section of microfiber between two cleaved standard single-mode fibers (SMFs) with unique relative fiber cross section relationship has been proposed and experimentally demonstrated. The opening air cavity between the two SMF ends connected by the microfiber serves as an FP cavity and also a direct sensing head. The sensing characteristics of the FPIs with different cavity lengths and microfiber diameters have been studied. A force sensitivity as high as 167.41 nm/N (~200 pm/με) and a high refractive index (RI) sensitivity of 1330.8 nm/RIU (around a RI of 1.33) have been achieved by using the microfiber-based FPI with ~21 μm cavity length and ~44 μm microfiber diameter. Such a device has several merits such as simple configuration, compactness and reliability in operation owing to the extremely low thermal cross-sensitivities.

Simultaneous Force and Temperature Measurement Using S Fiber Taper in Fiber Bragg Grating

An ultracompact optical fiber sensor based on an S fiber taper Mach-Zehnder interferometer (SFT-MZI) embedded in fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous force and temperature measurement. Experimental results show distinctive spectral sensitivities of -48.56842 nm/N (~54.97 pm/με) and 14.71 pm/°C by the SFT-MZI, and 1.30588 nm/N and 10.13 pm/°C by the FBG. The SFT-MZI, exhibiting an opposite force response as compared with that of the FBG, is highly sensitive to force. The force and temperature calculated by the sensor matrix agree well with the actual force and temperature in the experiment.