Pengcheng Geng

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.

Design of single-polarization wavelength splitter based on photonic crystal fiber

A new single-polarization wavelength splitter based on the photonic crystal fiber (PCF) has been proposed. The full-vector finite-element method (FEM) is applied to analyze the single-polarization single-mode guiding properties. Splitting of two different wavelengths is realized by adjusting the structural parameters. The semi-vector three-dimensional beam propagation method is employed to confirm the wavelength splitting characteristics of the PCF. Numerical simulations show that the wavelengths of 1.3 μm and 1.55 μm are split for a fiber length of 10.7 mm with single-polarization guiding in each core. The crosstalk between the two cores is low over appreciable optical bandwidths.

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.

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.

A Mach–Zehnder interferometer constructed using lateral offset and a long period fiber grating for two-dimensional bending vector sensing

We report a two-dimensional bending vector sensor based on a Mach–Zehnder interferometer (MZI). The device consists of a strong coupling long period fiber grating (LPFG) and a slight-lateral-offset fusion splicing joint which can generate an interference pattern and keep the resonance dip of the LPFG relatively independent at the same time. The exposure orientation of the LPFG is perpendicular to the lateral-offset orientation, making it capable of identifying two-dimensional bending directions and amplitude simultaneously. Two-dimensional bending vector sensing characteristics, especially in the four orthogonal directions based on the new structured MZI, have been experimentally demonstrated. Temperature cross-sensitivity is eliminated using the different sensitivities of the two resonance dips. The device will have great potential in shape sensing applications due to the advantages of its vector sensing capability, low cost and ease of fabrication.

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.

Simultaneous measurement of temperature and refractive index using a simplified modal interferometer composed of tilted long-period fiber grating and fiber bitaper

A simplified long-period fiber-grating-based Mach–Zehnder interferometer (MZI) for simultaneous temperature and refractive index (RI) measurement is presented. The sensing element is constructed by connecting a waist-enlarged fiber bitaper after a tilted long-period fiber grating written by high-frequency CO2 laser pulses, which has higher writing efficiency than the conventional long-period fiber grating (LPFG). By employing the proposed MZI, the transmission bandwidth for each resonant wavelength of the fiber grating could be conveniently reduced without the presence of the LPFG pair. Experimental results indicate that the MZI has good temperature sensitivity (70.42 pm °C−1) and RI sensitivity (−37.43 nm/RIU, in the RI range from 1.3425 to 1.3750). Besides, the extinction ratio of interference fringes could reach 20 dB at the resonant wavelength. With several advantages such as narrow bandwidth, large extinction ratio, simple structure and ease of fabrication, the proposed device would be a promising candidate for precise large-capacity dual-parameter sensing application.