A novel, simple, and compact optical fiber directional bending vector sensor based on Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The device consists of a piece of seven-core photonic crystal fiber (PCF) sandwiched between two single mode fibers (SMFs) with a lateral offset splicing joint that covering two cores of PCF. Bending sensitivity of the seven-core PCF based MZI is changed by an axial rotation angle, which shows its capacity for recognizing positive and negative directions. Within a curvature range of -7.05 m-1 to 7.05 m-1, the calculated bending sensitivities of two resonant central wavelengths with opposite fiber orientations are 1.232 nm/m-1 and 1.174 nm/m-1, respectively. This novel MZI is formed by invoking interference between the LP01-like supermode and other higher order supermodes in the core, which leads to insensitive to ambient refractive index (ARI). We have also investigated the transmission characteristics of the sensor with the temperature change.

A one-dimensional micro-bending vector sensor based on two-mode interference has been introduced. This device was fabricated by lateral offset splicing a piece of six-air-hole grapefruit microstructure fiber (GMF) with single mode fiber (SMF). Variation of effective mode index occurred by micro-bending was investigated in simulation and experiment. This device exhibits micro-bending sensitivities of 0.441 nm/m(-1) and -0.754 nm/m(-1) at 0° and 180° bending orientations, respectively. Moreover, this sensor is immune to surrounding refractive index (SRI) and presents a low crosstalk of temperature.

A long period fiber grating (LPFG) fabricated upon the all-solid photonic bandgap fiber by CO 2 laser irradiation was investigated, and its resonance wavelength was at 1335.76 nm with a modulation depth of 15 dB and a 3-dB bandwidth of 2.6 nm. We studied its strain, temperature, and index sensor characteristics, the strain sensitivity of 0.992 pm/με was obtained by using linear fit, and the relationship between the refractive index and wavelength obeyed the distribution of quadratic function. Also, we demonstrated its temperature response was relatively insensitive (21.51 pm/℃).

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interference, and compact size [1]. They are very suitable for applications in space-limited harsh environments [2][3][4][5]. Various types of pressure sensors have been proposed, including those based on fiber Bragg gratings (FBGs) fabricated in SMFs and highly birefringent photonic crystal fibers (PCFs) [5,6], Sagnac interferometer, mode interference of a dual-core PCF [7,8], and a miniature Fabry-Perot cavity [9]. Among them, pressure sensors based on FabryPerot interferometer (FPI) have attracted more attention because they have some advantages such as compactness, higher sensitivity, simple fabrication, and lower cost. The FPI cavity can be formed by various methods, for example using an elastic diaphragm at the fiber tip such as undoped fused silica thin layer, polymer, and graphene film [10-13]; creating a sphere cavity in the splice [14]; and splicing a section of solid-core PCF with SMFs [15]. However, the technique based on an elastic diaphragm always involves multiple components and complex alignment. The mechanical strength and thermal stability of the sensors may be degraded due to the different materials of multiple components. The pressure sensors reported in [14,15] improve the mechanical strength by using all fiber structure. For this type of sensor, cross talk of the temperature is a problem, and it is necessary to compensate for this. Meanwhile, the stability and repeatability should be discussed in the pressure sensing applications. The fiber-tip microcavity pressure sensor [14] demonstrated a pressure sensitivity of 315 pm/MPa and a temperature sensitivity of 1.55 pm/°C, and therefore, the temperature cross talk was calculated to be 5 kPa/°C. The pressure sensor using a solid-core PCFbased FPI [15] showed a low-pressure sensitivity of 5.8 pm/ MPa and a temperature sensitivity of 13 pm/°C, which has a high-temperature cross talk of 2.2 MPa/°C. The pressure sensitivity of an F-P cavity with a hollow-core PCF [16] was also as low as 17.3 pm/MPa; however, the temperature Abstract We demonstrate a novel and compact fiberprobe pressure sensor based on a micro-Fabry-Perot interferometer (FPI). The device is fabricated by splicing both ends of a short-section simplified hollow-core photonic crystal fiber (SHC-PCF) with single-mode fibers. Then, a microchannel is drilled by a femtosecond laser micromachining in the SHC-PCF to allow air to diffuse in. The pressure sensing mechanism is based on the dependence of the air refractive index on pressure. We use both theory and experiment to investigate the sensing characteristics. A micro-FPI with a length of 272 μm demonstrates a pressure sensitivity of 4.071 nm/MPa at 1580 nm and a lowtemperature sensitivity of 1.1 pm/°C at atmospheric pressure. We further study the temperature cross sensitivity of the sensor under different pressures. The sensor also shows strong stability and good reversibility, and may be potentially used in pressure sensing applications.

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We propose a polarization-insensitive ultra-broadband mode filter based on a 3D graphene structure buried in a few-mode optical waveguide. Our experimental device fabricated by burying an L-shaped graphene structure in a three-mode polymer waveguide provides polarization-insensitive filtering of the fundamental mode, while keeping the higher-order modes, with a mode extinction ratio of ∼ 17 d B over the S + C + L band and beyond.

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