1,721,307 research outputs found
Geiger H. Kent — The family in Soviet Russia
No full textB H. Geiger H. Kent — The family in Soviet Russia. In: Population, 24ᵉ année, n°4, 1969. p. 809
Complexity limitations of optical networks from out-of-band dispersion of grating filters
No full textAccumulated dispersion can limit the number of grating filters in a network. We derive a simple analytical equation for the out-of-band dispersion of any filter, and conclude on the resulting node-limitation of an optical network
Low-cost high-resolution time-domain reflectometry for monitoring the range of reflective points
No full textA new OTDR technique to interrogate the range of reflective markers in an optical fiber (or an electrical cable) is presented. The technique has been developed to monitor strain in sections of a fiber over gauge lengths of several meters. Signal-to-noise analysis shows that current OTDR systems do not fully exploit the spatial resolution theoretically available. The available resolution is explored with a new OTDR technique
In-band dispersion limitations of uniform apodised fibre gratings
No full textApodisation-generated in-band dispersion can limit the useful bandwidth of gratings. We discuss how a trade-off in reflection squareness may be necessary in order to increase the useful bandwidth. Introduction: Wavelength-division multiplexing (WDM) is widely recognised as the technology of choice to utilise the fibre bandwidth. The key enabling technology for WDM is grating-based filters, offering low insertion loss over a wide bandwidth and good suppression of neighbouring channels when properly designed. It has recently been discussed how the dispersion on the edges of the photonic bandgap of a grating both in-band and out-of-band can cause the reflected and transmitted signals to be significantly distorted. In this paper we show that apodisation-induced in-band dispersion can limit the useful bandwidth of gratings considered for current transmission systems. We discuss design rules depending on apodisation and length for an optimisation of the useful bandwidth
Multiplexing methods for fibre optic strain sensors
No full textThe paper briefly reviews a number of methods for multiplexing optical-fibre-based strain sensors. These are intended for application for structural monitoring, for example in aerospace composites or civil engineering structures. Particular emphasis is placed on methods for addressing arrays of sensors using acousto-optic tunable filters and optical time domain reflectometry which have been recently developed in the authors' laboratory
Multiplexed optical fibre strain sensing by cross-correlation of subcarrier interferometric spectra
No full textA novel technique for evaluating strain in multiple long-length sections of optical fibre is described. Sections defined by reflective markers are interrogated with subcarrier interferometry. Path-length changes are monitored using cross-correlation. A resolution of 35µm has been obtained over 5m sensing sections, together with inter-section crosstalk levels below -45dB
Fibre grating pressure sensor with enhanced sensitivity using a glass-bubble housing
No full textA fibre Bragg grating pressure sensor with enhanced sensitivity has been demonstrated, using a grating housed in a glass bubble. This configuration increases the pressure sensitivity and effectively reduces the thermal crosstalk by a factor of 4. A pressure resolution of 0.5% over a full measurement range of 14 MPa has been achieved. This concept could lead to a range of sensors using fibre gratings with mechanical amplifiers to enhance sensitivity
Time-division-multiplexed optical fibre strain-sensor using subcarrier interferometry
No full textA high-resolution quasi-distributed strain sensor employing subcarrier interferometry and time-division multiplexing is reported. The system tracks a null in the subcarrier frequency response of a fibre network containing reflective discontinuities. Time gating selects the sensing section. A resolution better than 2Oµm over 5m lengths of fibre has been obtained
Interrogation of fibre-optic interferometric sensors using an acousto-optic tunable filter
No full textA new technique for the interrogation of interferometric-based fibre-optic sensors using an acousto-optic tunable filter (AOTF) is demonstrated. The scheme involves frequency shift keying (FSK) of the RF drive to the AOTF to track the 3hift of a single fringe peak from an interferometric sensor. Experimental results are presented for a Fizeau interferometer. The technique provides high accuracy, plus a large unambiguous tracking range free from interruptions
Distributed grating sensors using low-coherence reflectometry
No full textDistributed grating sensors have recently been interrogated with low-coherence reflectometry. Initial results have been enhanced using two new and versatile configurations. The first system tracks the wavelength using a closed-loop scheme, while the second system scans the distance using an open-loop approach. Arbitrary strain and temperature profiles along gratings have been examined with 300 µm spatial resolution and 5.4 µ.epsilon/√Hz accuracy. A theoretical model of the interrogation technique is derived and the predicted performance limits are examined experimentally
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