Generally, phase-sensitive optical time-domain reflectometer (φ-OTDR) adopts a single-channel sensing structure, which makes it vulnerable to random interferences and increases the probability of vibration misjudgment in practical applications. In this paper, a dual-channel φ-OTDR based on a two-mode fiber (LP01 mode and LP11 mode) is constructed, and a simple demodulation algorithm is designed accordingly to locate pencil-break vibrations. The purpose of using dual-channel scheme is that the probability of false detection, simultaneously happened in double channels at the same position and at the same time, would be greatly reduced. In signal processing, both the conventional amplitude differential accumulation algorithm (DAA) and the standard variance algorithm (SVA) are employed to process the Rayleigh scattering traces of LP01 and LP11 channels to detect the pencil-break. The results show that the DAA is highly dependent on the parameters of the algorithm and not suitable to be directly used in practical. Due to the strong randomness of Rayleigh scattering, it is found that the pencil break cannot be detected just by the SVA. Thus, a simple method of producing two decision signals is proposed for vibration detection by combining the DAA and SVA, in which the DAA signals of one channel are crossmultiplied with the SVA signals in another channel. The results show that this method shows reliable performance of locating the pencil-break.
In this work, a five-band metamaterial absorber (MMA) for temperature sensing application in terahertz region is analyzed. The MMA is composed of three layers. The bottom layer is the metallic film, the middle dielectric layer is the indium antimonide (InSb) and the top layer is the metallic pattern, in which five resonance peaks are generated. With utilizing the dielectric thermo sensitive property of InSb, the resonant absorption is tunable by varying temperature. The electric current on the MMA is investigated to better understand the physical mechanism of the resonances, revealing the resonances attributed to the high-order magnetic resonances. The multi-band absorber is insensitive to the polarization angle, and be with ultrathin thickness of structure. This design of the MMA provides a new approach for electromagnetic stealth, sensing and imaging.
In this study, we propose a dual-band wide-range tunable terahertz absorber based on graphene and bulk Dirac semimetal (BDS), which consists of a patterned BDS array, dielectric material, continuous graphene layer, and gold mirror. Simulation results show that the absorption at 3.97 and 7.94 THz achieve almost 100%. By changing the Fermi energy of graphene and BDS, the resonance frequency can be tuned between 3.97 and 9.28 THz. In addition, we found that when the background refractive index changes, the absorption is almost the same. This feature will broaden its applications. Finally, the influence of structural parameters and incident angles on device performance is discussed. The proposed absorber may have potential applications in photoelectric sensors and other optoelectronic devices.
In practical application, it is found that single-path phase-sensitive optical time-domain reflectometers (φ-OTDR) is susceptible to noise and random interference, which increases the probability of missing detection over external perturbations by conventional amplitude demodulation. In the work, a dual-channel system based on two fibers extracted from an armored four-core cable is investigated to enhance the robustness of the φ-OTDR. In signal demodulation, by combining the conventional differential accumulation algorithm (DAA) and standard deviation algorithm (SVA) a multipath information fusion algorithm (MIFA) is accordingly proposed to conclude whether the vibration signal is present. The MIFA-based dual-channel φ-OTDR is experimentally demonstrated on a highway of 9 km to position a running vehicle, indicating a considerable performance improvement of vibration identification compared to the DAA and SVA.
Optical frequency-domain reflectance (OFDR) has been widely used in vibration measurement due to its unique advantages over optical time-domain reflectometry (OTDR). It should be noted that, however, OFDR requires long measurement time and shows poor sensitivity when applied to measure vibration signal over long distance. In the work, an algorithm is presented to automatically detect and locate the vibration signals. Firstly, we perform cross-correlation analysis in a moving window between the beating signals without and with vibration, and find the maximum cross-correlation coefficients in all windows to reconstruct them into a cross-correlation curve. Secondly, an automatic decision threshold curve is designed to conclude whether there is any vibration over the sensing fiber. Lastly, the cross-correlation curve is compared with the threshold to locate the vibration. We experimentally test the algorithm in an OFDR system and locate a PZT vibration at 26.96 km, which demonstrates its validity in terms of detecting external disturbances over a relative long distance.
In this paper, we proposed a novel numerical algorithm for nth-order cascaded Raman fiber lasers (CRFLs) with the
combination of genetic algorithm (GA) and shooting method. Although shooting method possesses fast speed in solving
nonlinear two-point boundary-value ordinary differential equations, calculating process may diverge if it is directly
applied in the coupled equations of CRFLs when arbitrarily guessed initial values are out of the domain of convergence.
To overcome the problem, genetic algorithm which has rather strong searching ability in global space is firstly employed
to search for the initial value in convergent domain for each Stokes power; and then, the task of finding the more
accurate initial values is finished by shooting method instead of GA whose searching ability is weak in local region. As
an example, a sixth-order Ge-doped CRFL has been simulated by the novel algorithm. Calculated results show that the
new method can effectively and quickly solve the coupled equations of the CRFL without the problem of divergence.