Current Issue : October - December Volume : 2018 Issue Number : 4 Articles : 5 Articles
The work presented in this paper shows that Fiber Bragg Grating (FBG) optical fiber sensors\ncan potentially be used as receivers in a long-range guided wave torsional-mode pipe inspection\nsystem. Benefits over the conventional pulse-echo method arise due to reduced total travel distance\nof the ultrasonic guided wave reflections, since reflections from defects and structural features do not\nneed to propagate a full round trip back to the transmitting collar. This is especially important in\npipe configurations with high attenuation, such as coated and buried pipelines. The use of FBGs as\nreceivers instead of conventional piezoelectric or magnetostrictive elements also significantly reduces\ncabling, since multiple FBG receivers can be placed along a single optical fiber which has a diameter\non the order of only around 100 �¼m. The basic approach and sample results are presented in the\npaper. Additionally, a brief overview of some topics in ultrasonic guided waves is presented as a\nbackground to understand the inspection problem presented here....
We propose a nanotube-based erbium-doped fiber laser that can deliver conventional\nsoliton (CS) and stretched pulse (SP) based on D-shaped fiber saturable\nabsorber (DF-SA) where evanescent-field interaction works. The novel\nNanotube-based Fiber Laser can generate SP or CS by tuning pump power\nand polarization controller (PC) properly. The net cavity dispersion of laser is\nslightly negative. In our experiment, by optimizing the PC in the cavity, CS\nand SP can be obtained at the central wavelengths of 1530.6 nm and 1530.3\nnm due to on carbon nanotubes and the spectral filtering effect induced by\nnonlinear polarization rotation. Although the acquired CS and SP nearly have\nthe same central wavelengths, they show distinct optical spectra, 3-dB bandwidths.\nThe proposed fiber laser with switchable CS and SP is attractive for\nultrashort pulse generation and fast measurements in practical applications....
Brillouin optical time domain analysis is the sensing of temperature and strain changes\nalong an optical fiber by measuring the frequency shift changes of Brillouin backscattering. Because\nfrequency shift changes are a linear combination of temperature and strain changes, their discrimination\nis a challenge. Here, a multicore optical fiber that has two cores is fabricated. The differences between the\ncoresââ?¬â?¢ temperature and strain coefficients are such that temperature (strain) changes can be discriminated\nwith error amplification factors of 4.57 ââ??¦C/MHz (69.11 Ã?¼/MHz), which is 2.63 (3.67) times lower than\npreviously demonstrated. As proof of principle, using the multicore optical fiber and a commercial\nBrillouin optical time domain analyzer, the temperature (strain) changes of a thermally expanding metal\ncylinder are discriminated with an error of 0.24% (3.7%)....
We demonstrated a kind of long-period fiber grating (LPFG), which is manufactured with\na thermal diffusion treatment. The LPFG was inscribed on an ultrahigh-numerical-aperture (UHNA)\nfiber, highly doped with Ge and P, which was able to easily diffuse at high temperatures within a\nfew seconds. We analyzed how the elements diffused at a high temperature over 1300 ââ??¦C in the\nUHNA fiber. Then we developed a periodically heated technology with a CO2 laser, which was able\nto cause the diffusion of the elements to constitute the modulations of an LPFG. With this technology,\nthere is little damage to the outer structure of the fiber, which is different from the traditional LPFG,\nas it is periodically tapered. Since the LPFG itself was manufactured under high temperature, it can\nwithstand higher temperatures than traditional LPFGs. Furthermore, the LPFG presents a higher\nsensitivity to high temperature due to the large amount of Ge doping, which is approximately\n100 pm/ââ??¦C. In addition, the LPFG shows insensitivity to the changing of the environmentââ?¬â?¢s refractive\nindex and strain....
A multiuser Ultra Wide Band (UWB) channel suffers seriously from realistic\nimpairments. Among this, multipath fading and interferences, such as\nMultiple Access Interference (MAI) and Inter Symbol Interference (ISI),\nthat significantly degrade the system performance. In this paper, a polar\ncoding technique, originally developed by Arikan, is suggested to enhance\nthe BER performance of indoor UWB based Orthogonal Frequency Division\nMultiplexing (OFDM) communications. Moreover, Interleave Division\nMultiple Access (IDMA) scheme has been considered for multiuser detection\ndepending on the turbo type Chip-By-Chip (CBC) iterative detection\nstrategy. Three different models as Symmetric Alpha Stable (SS), Laplace\nmodel and Gaussian Mixture Model (GMM), have been introduced for approximating\nthe interferences which are more realistic for UWB system. The\nperformance of the proposed Polar-coded IDMA OFDM-based UWB system\nis investigated under UWB channel models proposed by IEEE 802.15.3a\nworking group and compared with Low Density Parity Check (LDPC)-coded\nIDMA OFDM-based UWB system in terms of BER performance and complexity\nunder the studied noise models. Simulation results show that the\ncomplexity of the proposed polar-coded system is much lower than LDPC-coded\nsystem with minor performance degradation. Furthermore, the proposed\npolar-coded system is robust against noise and interferences in UWB indoor\nenvironment and gains a significant performance improvement by about 5\ndB compared with un-coded IDMA-OFDM-UWB system under the studied\nnoise models....
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