Current Issue : July - September Volume : 2017 Issue Number : 3 Articles : 6 Articles
Adaptive optics reconstructors are needed to remove the effects of atmospheric distortion\nin optical systems of large telescopes. The use of reconstructors based on neural networks has\nbeen proved successful in recent times. Some of their properties require a specific characterization.\nA procedure, based in time series clustering algorithms, is presented to characterize the relationship\nbetween temporal structure of inputs and outputs, through analyzing the data provided by the\nsystem. This procedure is used to compare the performance of a reconstructor based in Artificial\nNeural Networks, with one that shows promising results, but is still in development, in order to\ncorroborate its suitability previously to its implementation in real applications. Also, this procedure\ncould be applied with other physical systems that also have evolution in time....
A useful method for eliminating the detrimental effect of laser frequency instability on\nBrillouin signals by employing the self-heterodyne detection of Rayleigh and Brillouin scattering\nis presented. From the analysis of Brillouin scattering spectra from fibers with different lengths\nmeasured by heterodyne detection, the maximum usable pulse width immune to laser frequency\ninstability is obtained to be about 4 Ã?¼s in a self-heterodyne detection Brillouin optical time domain\nreflectometer (BOTDR) system using a broad-band laser with low frequency stability. Applying the\nself-heterodyne detection of Rayleigh and Brillouin scattering in BOTDR system, we successfully\ndemonstrate that the detrimental effect of laser frequency instability on Brillouin signals can be\neliminated effectively. Employing the broad-band laser modulated by a 130-ns wide pulse driven\nelectro-optic modulator, the observed maximum errors in temperatures measured by the local\nheterodyne and self-heterodyne detection BOTDR systems are 7.9 ââ??¦C and 1.2 ââ??¦C, respectively....
This work is an extension to the evaluation and analysis of a two-dimensional cylindrical cloak in the Terahertz or visible range\nspectrum using Finite Difference Time-Domain (FDTD) method. It was concluded that it is possible to expand the frequency range\nof a cylindrical cloaking model by careful scaling of the inner and outer radius of the simulation geometry with respect to cell size\nand/or number of time steps in the simulation grid while maintaining appropriate stability conditions. Analysis in this study is\nbased on a change in the radii ratio, that is, outer radius to inner radius, of the cloaking structure for an array of wavelengths in the\nvisible spectrum. Corresponding outputs show inconsistency in the cloaking pattern with respect to frequency. The inconsistency\nis further increased as the radii ratio is decreased.The results also help to establish a linear relationship between the transmission\ncoefficient and the real component of refractive index with respect to different radii ratios which may simplify the selection of the\nmaterial for practical design purposes. Additional performance analysis is carried out such that the dimensions of the cloak are\nheld constant at an average value and the frequency varied to determine how a cloaked object may be perceived by the human eye\nwhich considers different wavelengths to be superimposed on each other simultaneously....
Fiber Bragg Gratings (FBGs) are among the most popular optical fiber sensors. FBGs are well suited for direct detection of\ntemperature and strain and can be functionalized for pressure, humidity, and refractive index sensing. Commercial setups for\nFBG interrogation are based on white-light sources and spectrometer detectors, which are capable of decoding the spectrum of an\nFBG array. Low-cost spectrometers record the spectrum on a coarse wavelength grid (typically 78ââ?¬â??156 pm), whereas wavelength\nshifts of 1 pm or lower are required by most of the applications. Several algorithms have been presented for detection of small\nwavelength shift, even with coarsewavelength sampling; most notably, the Karhunen-Loeve Transform(KLT) was demonstrated. In\nthis paper, an improved algorithm based on KLT is proposed, which is capable of further expanding the performances. Simulations\nshow that, reproducing a commercial spectrometer with 156 pm grid, the algorithm estimates wavelength shift with accuracy\nwell below 1 pm. In typical signal-to-noise ratio (SNR) conditions, the root mean square error is 22ââ?¬â??220 fm, while the accuracy\nis 0.22 pm, despite the coarse sampling. Results have been also validated through experimental characterization. The proposed\nmethod allows achieving exceptional accuracy in wavelength tracking, beating the picometer level resolution proposed in most\ncommercial and research software, and, due to fast operation (>5 kHz), is compatible also with structural health monitoring and\nacoustics....
In this work, we have applied optical low-coherence reflectometry (OLCR), implemented\nwith infra-red light propagating in fiberoptic paths, to perform static and dynamic analyses\non piezo-actuated glass micro-membranes. The actuator was fabricated by means of thin-film\npiezoelectric MEMS technology and was employed for modifying the micro-membrane curvature,\nin view of its application in micro-optic devices, such as variable focus micro-lenses. We are\nhere showing that OLCR incorporating a near-infrared superluminescent light emitting diode as\nthe read-out source is suitable for measuring various parameters such as the micro-membrane\noptical path-length, the membrane displacement as a function of the applied voltage (yielding the\npiezo-actuator hysteresis) as well as the resonance curve of the fundamental vibration mode. The\nuse of an optical source with short coherence-time allows performing interferometric measurements\nwithout spurious resonance effects due to multiple parallel interfaces of highly planar slabs,\nfurthermore selecting the plane/layer to be monitored. We demonstrate that the same compact\nand flexible setup can be successfully employed to perform spot optical measurements for static and\ndynamic characterization of piezo-MEMS in real time....
Acceleration is a significant parameter for monitoring the status of a given objects.\nThis paper presents a novel linear acceleration sensor that functions via a unique physical mechanism,\nthe resonant optical tunneling effect (ROTE). The accelerometer consists of a fixed frame, two elastic\ncantilevers, and a major cylindrical mass comprised of a resonant cavity that is separated by two\nair tunneling gaps in the middle. The performance of the proposed sensor was analyzed with a\nsimplified mathematical model, and simulated using finite element modeling. The simulation results\nshowed that the optical Q factor and the sensitivity of the accelerometer reach up to 8.857 Ã?â?? 107 and\n9 pm/g, respectively. The linear measurement range of the device is Ã?±130 g. The work bandwidth\nobtained is located in 10ââ?¬â??1500 Hz. The results of this study provide useful guidelines to improve\nmeasurement range and resolution of integrated optical acceleration sensors....
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