Xiaopeng Dong

Long-Period Fiber Grating Within D-Shaped Fiber Using Magnetic Fluid for Magnetic-Field Detection

A magnetic sensor utilizing long-period fiber grating (LPFG) written by high frequency CO2 laser pulses in a D-shaped fiber and the magneto-optical effect of magnetic fluid is proposed. Because of the high evanescent field of the grating structure in D-shaped fiber, by immersing the LPFG with a period of 685 μm into a water-based magnetic fluid within a capillary tube, the redshift as high as 33.5 nm of the resonant wavelength of the grating is observed when the external magnetic-field intensity is 189.7 mT, resulting in a sensitivity of 176.4 pm/mT. The hysteresis effect is observed and explained by the characters of magnetic fluid. Such kind of sensors would find potential applications in magnetic sensing fields.

Design of broadband LP01 LP02 mode converter based on special dual-core fiber for dispersion compensation

A novel broadband LP01↔LP02 mode converter for dispersion compensation based on special dual-core fiber is theoretically investigated by using the coupled-mode theory. The simulated mode converter has ∼22u2009u2009nm bandwidth with a conversion efficiency of over 80%. Furthermore, this noncomplete conversion only introduces the insertion loss rather than multipath interference resulting from the residual LP01 mode. Finally, one optimal scheme for broadening the bandwidth of high-efficiency conversion has been proposed by longitudinally tapering the dual-core fiber. The simulation results show that the conversion bandwidth can be improved to ∼31u2009u2009nm by tapering with a scaling range of only 2%.

A tunable polarization-independent comb filter based on high-order mode fiber

This paper presents an all-fiber tunable comb filter based on high-order mode fiber (HOMF). The filter is constructed by splicing a segment of HOMF between two sections of conventional single-mode fibers (SMFs), with an intentionally introduced offset at the first joint between the SMF and HOMF, and without an offset at the second joint between the HOMF and SMF, to optimize the visibility of the transmission spectrum. The optimal transverse offset of the fibers at the first joint and the tunable polarization-independent properties of the filter were studied theoretically and experimentally. An example of using this tunable comb filter in the application of a sensor, based on demodulating the wavelength shift of a fiber Bragg grating, has also been demonstrated. The particular advantages of this all-fiber tunable filter make it valuable in various applications.

A Sensor for Simultaneous Measurement of Displacement and Temperature Based on the Fabry-Pérot Effect of a Fiber Bragg Grating

A temperature-insensitive sensor for micro displacement measurement is proposed based on a novel packaging of an FBG. In this paper, an apodized fiber Bragg grating (FBG) was glued at the specific position on the inner surface of a thin-walled ring. When the ring is deformed, the FBG could be split into two segments of FBGs in the two identical but oppositely directed chirp gradients. In this way, an effective Fabry-Pérot cavity could be induced into the FBG and the resonant peaks could be observed in the reflection spectrum of the FBG. The wavelength separation of the resonant peaks changes linearly with the change of displacement, while it is insensitive to temperature variation. The sensitivity of the wavelength separation versus displacement could be achieved as high as 117 pm mm-1 in the present experiment. Further improvement would be obtained by decreasing the radius or increasing the thickness of the ring. In addition, the ambient temperature could be obtained through a simple calculation, results in a temperature sensitivity of 28.67 pm °C-1. Owing to its compact and temperature independent advantages, the novel displacement sensor has potential application prospect in displacement and pressure measurements, especially in civil engineering structures as the long gauge sensor.

All-fiber tunable bandpass filter based on cascaded twin-core fiber

An all-fiber tunable bandpass filter (TBPF) based on cascaded twin-core fiber (TCF) is theoretically analyzed and experimentally demonstrated. The spectrum of this proposed TBPF can be easily adjusted by bending, twisting, stretching, or changing the length of TCF to achieve the required filter parameters. In the example of a TBPF fabricated with TCF, the center wavelength can be tuned from 1540.8 to 1557.8 nm, while the full width at half-maximum bandwidth of the filter maintains around 6.5 nm.

Novel method for the demodulation of wavelength shift of fiber Bragg gratings

A novel method to detect the small wavelength shift of the fiber Bragg grating used in an electric current sensor with a comb-like fiber filter is proposed in this paper. The wavelength filter was constructed by joining a piece of hi-bi fiber in the loop of a normal fiber Sagnac interferometer, which may result to a periodically varied spectrum in the reflection or transmission light. Therefore the small wavelength shift of the fiber grating caused by the change of the applied magnetic field can be detected by monitoring the intensity of the light traveling through the comb-like filter. With a 0.8-nm spacing filter used in the measurement, a sensitivity better than 0.01 nm for the Bragg wavelength shift of the fiber grating (limited by the noise of the electronic circuit and detection instruments) was obtained in the experiment. The proposed demodulation scheme provides a new approach to the detection of small wavelength shift of fiber grating.

Phase drift compensation for electric current sensor employing a twisted fiber or a spun highly birefringent fiber

We propose a simple, passive signal-processing scheme to compensate for arbitrary phase shift in a Faraday-effect current sensor employing a twisted fiber or a spun highly birefringent fiber. The scheme is based on processing the AC and DC components of the received signals with four detectors in a complementary manner to derive a stable signal that depends only on the Faraday rotation. The effectiveness of the scheme has been demonstrated with an experimental sensor for both twisted and spun highly birefringent fibers.

Discrimination of temperature and axial strain using dispersion effects of high-order-mode fibers

A new approach based on dispersion effects of a high-order-mode fiber (HOMF) is proposed to effectively discriminate changes in both environmental temperature and axial strain. Characterization and theoretical simulation of the HOMF modal interferometer with a SMF(single-mode fiber)-HOMF-SMF structure exhibit an excellent agreement with experimental results. A fast Fourier transform method of interferometry in the spatial frequency domain is adopted to distinguish coupling between different core-guided modes. Distinct phase sensitivities of different dispersion peaks are extracted by employing a novel phase demodulation scheme to realize dual-parameter sensing.

Novel tunable phase-shifted fiber Bragg grating implemented by adjusting gap between two fibers

A novel method to realize the adjustable phase-shifted fiber Bragg grating (FBG) is proposed in this paper. By adjusting the air gap between two identical FBG, the phase shift has been changed, and the transmission characteristics of such phase-shifted FBG can be tuned. Both theoretical simulation and experimental measurement have been carried out to verify and demonstrate the idea. It has been shown the proposed tuning method is effective and the experimental results agree well with the theoretical prediction.

Design and analysis of the high-order mode dispersion compensating fiber

We discussed how to design the typical trip-clad high-order mode fiber (HOMF) profiles to achieve the required dispersion properties based on LP02 mode, to compensate all modern transmission fibers, without sacrificing other important properties, such as effective area. Finally, HOMF compensating 100km eLEAF fiber has been designed. Its dispersion at 1550nm is -1217ps/nm/km, and the relative dispersion slope (RDS) is 0.02nm-1. Only ~345m of HOMF is needed to achieve full dispersion and dispersion slope compensation of the span, while maintaining effective area above 52μm2 over the entire C-band.