There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Fiber ring interferometer

Gas sensors fabricated with nanowires as the detecting elements are powerful due to their many improved characteristics such as high surface-to-volume ratios, ultrasensitivity, higher selectivity, low power consumption, and fast response. This paper gives an overview on the recent process of the development of nanotechnology and nanowire-based gas sensors. The two basic approaches, top-down and bottom-up, for synthesizing nanowires are compared. The conduction mechanisms, sensing performances, configurations, and sensing principles of different nanowire gas sensors and arrays are summarized and discussed. Meanwhile, an emerging nanowires fabrication method and a self-powered nanowire pH sensor are highlighted. The scientific and technological challenges in the field are discussed at the end of the review. © 2012 Elsevier B.V. All rights reserved.

Nanowire-based gas sensors

Publisher Summary Polarization mode dispersion (PMD) is a linear effect that can be compensated in principle. In an ideal circularly symmetric fiber, the two orthogonally polarized modes have the same group delay. However, in reality, fibers exhibit a certain amount of birefringence because of imperfections in the manufacturing process or mechanical stress on the fiber after manufacture. It is noted that fluctuations in the polarization mode and fiber birefringence produced by the environment lead to dispersion that varies statistically with time and frequency. PMD causes different delays for different polarizations and when the difference in the delays approaches a significant fraction of the bit period, it leads to pulse distortion and system penalties. Environmental changes— including temperature and stress—cause the fiber PMD to vary stochastically in time. PMD, illustrating the basic concepts, the measurement techniques, the PMD measurement, the PMD statistics for first- and higher orders, the PMD simulation and emulation, the system impairments, and the mitigation methods has been summarized in the chapter. Both the optical and the electrical PMD compensations are considered.

Polarization-Mode Dispersion

The impact-induced deposition of Al13 clusters with icosahedral structure on Ni(0 0 1) surface was studied by molecular dynamics (MD) simulation using Finnis–Sinclair potentials. The incident kinetic energy (Ein) ranged from 0.01 to 30 eV per atom. The structural and dynamical properties of Al clusters on Ni surfaces were found to be strongly dependent on the impact energy. At much lower energy, the Al cluster deposited on the surface as a bulk molecule. However, the original icosahedral structure was transformed to the fcc-like one due to the interaction and the structure mismatch between the Al cluster and Ni surface. With increasing the impinging energy, the cluster was deformed severely when it contacted the substrate, and then broken up due to dense collision cascade. The cluster atoms spread on the surface at last. When the impact energy was higher than 11 eV, the defects, such as Al substitutions and Ni ejections, were observed. The simulation indicated that there exists an optimum energy range, which is suitable for Al epitaxial growth in layer by layer. In addition, at higher impinging energy, the atomic exchange between Al and Ni atoms will be favourable to surface alloying.

Nuclear instruments and methods in physics research section B : beam interactions with materials and atoms

In this Letter, we report a novel, to the best of our knowledge, and simple approach for passive quadrature-phase demodulation of relatively long multiplexed interferometers based on two-channel coherence correlation reflectometry. Two-wavelength channels are generated using a single unmodulated CW-DFB diode laser and an acousto-optic frequency shifter. The introduced frequency shift determines the optical lengths of the interferometers. In our experiments, all interferometers have the same optical length of 32 cm corresponding to the π/2 phase difference between channel signals. An additional fiber delay line was introduced between channels to destroy coherence between initial and frequency-shifted channels. Demultiplexing of channels and sensors was performed using correlation-based signal processing. Amplitudes of cross correlation peaks obtained for both channels were used to extract the interferometric phase for each interferometer. Phase demodulation of relatively long multiplexed interferometers is experimentally demonstrated. Experimental results prove that the proposed technique is suitable for interrogating a serial array of relatively long interferometers dynamically modulated with phase excursions exceeding 2π. Simultaneous interrogation and phase demodulation were experimentally demonstrated using an in-line array of low-finesse Fabry-Perot interferometric sensors.

Passive quadrature demodulation of multiplexed interferometric sensors using a CW correlation reflectometer with a single DFB diode laser.

Summary form only given. It is well known that parameters of laser radiation are very sensitive to small influence on a laser cavity. It makes fiber lasers perspective for sensing applications. Fiber laser sensors with a wavelength-encoded output and polarization mode beat frequency encoded output were discussed previously. These fiber laser sensors use at least one broadband cavity mirror. We present a sensor based on a fiber laser in linear configuration with two Bragg gratings used as narrow band cavity mirrors. We discuss a possible application of this technique for liquid leak detection.

http://ieeexplore.ieee.org/iel5/7267/19606/00910294.pdf

A fiber laser sensor with a cavity formed by reference and sensing Bragg gratings

We present two configurations of new fiber optic Bragg grating sensor developed for petroleum hydrocarbon detection. The sensor includes FBG attached to the special polymer swelling in the hydrocarbon presence. This polymer reversibly strains the fiber section with the Bragg grating inside. The first sensor configuration employs a single grating and standard demodulation technique to determine a strain induced shift of the Bragg wavelength. The second sensor configuration make a use of double grating interferometric sensor consisting of a reference grating free from axial strain and a sensitive one, attached to the polymer. A new demodulation technique is based on monitoring of energy of the interference term in the reflection spectrum. We demonstrated experimentally that such configuration is free from temperature influence. The paper presents the results of the swelling-behavior test of the sensitive polymer material and the results of the experimental investigation of the hydrocarbon sensor performance.

Fiber Bragg grating sensor for liquid hydrocarbon detection

We present an inexpensive technique capable to interrogate multiplexed sensors based on ultra-low-reflective Bragg
gratings written in a long standard telecom fiber. The technique is suitable for distributed detection and localization of
disturbances in security and early warning systems for pipeline monitoring, in linear temperature sensors for fire
detectors, etc. It is based on the correlation OTDR principle measuring cross-correlation between a noise-like probe
signal and the signal reflected back from the sensing fiber. In order to simplify the sensor configuration, an unmodulated
CW DFB diode laser was used as a light source. A noise-like probe signal was generated by conversion of phase noise
of the DFB laser into intensity noise with a help of unbalanced Michelson interferometer. Results of the experimental
verification of the proposed technique are presented.

Multiplexing FBG-based sensors using unmodulated continuous wave DFB diode laser

L'invention concerne un dispositif optique de mesure de variations de la pression d'un fluide dans un conduit. Le dispositif comprend une source optique d'emission de lumiere incidente, un capteur a fibre optique avec un interferometre qui sert a moduler la lumiere incidente, un detecteur de sortie qui fournit un signal electrique de sortie representatif des variations de phase de la lumiere modulee detectees par l'interferometre, et des moyens de positionnement pour positionner le capteur a proximite de la paroi d'un conduit de fluide de sorte que le capteur puisse mesurer une deformation de la paroi du conduit. Le dispositif est particulierement utile comme manometre de mesure de la pression sanguine.

Mikhail G. Shlyagin

Papers

Passive quadrature demodulation of multiplexed interferometric sensors using a CW correlation reflectometer with a single DFB diode laser.

A fiber laser sensor with a cavity formed by reference and sensing Bragg gratings

Fiber Bragg grating sensor for liquid hydrocarbon detection

Multiplexing FBG-based sensors using unmodulated continuous wave DFB diode laser

Dispositif optique de mesure de la pression d'un fluide