1,720,969 research outputs found
Distributed and multiplexed fibre grating sensors
No full textA short review of distributed and multiplexed fibre grating sensor technology is presented, followed by a more detailed account of work at the University of Southampton, including aspects of temperature and strain discrimination and multiplexed and distributed sensing
Distributed and multiplexed fibre grating sensors, including discussion of problem areas
No full textA short review of distributed and multiplexed sensor technology, based on fibre gratings, is given. This is followed by details of more specific work in this area at the University of Southampton, particularly grating fabrication, distributed and multiplexed addressing and important practical aspects such as temperature and strain discrimination. The paper concludes with a short discussion of the problems that must be avoided in order to construct viable systems for engineering requirements
Review of distributed and multiplexed fibre grating sensors and discussion of problem areas
No full textBased on an earlier version presented at OFS13 in Korea, a short review of distributed and multiplexed fibre grating sensor technology is given, followed by details of work at the University of Southampton, including aspects of temperature and strain discrimination and our own methods for multiplexed and distributed sensing. The paper concludes with a short discussion of the problems that should be avoided in order to construct viable systems for engineering requirements
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
Distributed fibre grating sensor for smart structures applications
No full textOptical fibre sensors have been widely used to measure strain and temperature for smart structures applications. Distributed sensors using fibre Bragg gratings have recently been demonstrated. These sensors have many advantages, including high spatial resolution and no dead-zones, allowing a complete spatial image of a measurand to be obtained. Interrogation systems using a tunable narrowband source have been previously demonstrated, but are limited to monotonically varying wavelength profiles. We recently demonstrated a broadband interrogation system for distributed grating sensors, capable of measuring arbitrary strain fields. Low-coherence interferometry selected the location under interrogation and a tunable filter measured the local wavelength. Further to previously published work, two improved systems are presented. The first system uses electronic processing for rapid and accurate wavelength measurement, whereas the second system uses a commercially available optical coherence domain reflectometer (OCDR) as part of the interrogation system. The latest sensing results from both systems are presented. The ultimate performance limits are examined experimentally and compared to the theoretically derived values. Both thermal and strain measurements have been made using single fibre gratings up to 40cm in length. Multiple grating sensors have also been combined into a single sensing network, producing a sensor system that is both distributed and multiplexed
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
Enhanced sensitivity optical fibre sensors using subcarrier gratings
No full textFibre Bragg gratings (FBGs) have been widely used to sense strain and temperature since both parameters change the resonant frequency of the grating. The grating is suited to sensing applications since the resonance is encoded in the reflected wavelength, which is unaffected by amplitude fluctuations in the interrogation system. Recently sensors have been made using long period gratings (LPGs), gratings with a pitch of the order of 250µm (resonant at 400GHz). Similar to the FBG, physical fields effect the resonant frequency, however harmonics of this frequency are interrogated, rather than the fundamental resonance. We propose the first subcarrier fibre grating (SFG), a very long period fibre grating. The SFG is resonant at RF frequencies, determined by the spacing of the reflectors. The resonant frequency may be interrogated by observing the interference of RF modulation carried on a low-coherence optical signal. This interrogation method using subcarrier interferometry is similar to interrogation systems previously used with long-gauge-length optical fibre sensors. In such sensors the value of the measurand is encoded in the subcarrier frequency, a parameter that is also independent of amplitude fluctuations and which may be measured simply, rapidly and accurately. Previous sensors interrogated by subcarrier interferometry have defined the sensing section by a single reflector at each end. The SFG is effectively one such sensing section, but with additional reflectors periodically located between the two end reflectors. Compared with previous subcarrier sensors, the SFG exhibits enhanced sensitivity and provides limited information on the spatial uniformity of the measurand
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
Low-coherence grating characterisation scheme
No full textOptical fibre gratings are being produced for an increasing number of applications. With the advances in grating fabrication technology, advances in grating characterisation schemes are required. A few years ago, only the reflectivity, bandwidth and central wavelength of gratings were needed. Today, applications such as dispersion compensation and distributed sensing require the distributed characteristics of a grating, namely the time delay and reflectivity as a function of wavelength. Until recently, distributed grating measurements have been made using a tunable narrowband laser source. The wavelength to be measured was selected by tuning the laser. The reflectivity was obtained from the returning optical power level and the time delay of the chosen wavelength was obtained using phase measurements. The phase measurements were either optically coherent, or optically incoherent (electrically coherent). These systems only correctly characterise gratings having monotonic wavelength profiles. Their temporal resolution is determined by the local chirp of the grating and the grating wavelength. For dispersion compensation, gratings with a linear, monotonic chirp are required and thus the laser characterisation scheme at first sight appears suitable. We recently demonstrated a novel grating characterisation system using a broadband source. Low-coherence interferometry selected the time delay to be interrogated and a tunable filter measured the local Bragg wavelength. This system has characterised gratings of arbitrary (including non-monotonic) wavelength profiles. The latest results of our low-coherence interrogation system are presented together with results from a theoretical analysis of its performance. The ultimate performance characteristics are predicted and examined experimentally
Low coherence technique to characterise reflectivity and time delay as a function of wavelength within a long fibre grating
No full textA new technique for obtaining both time delay and reflectivity as a function of wavelength within a long fibre Bragg grating is presented. A narrowband wavelength-tunable reflector with 0.18nm bandwidth is used to select the interrogation wavelength, while optical coherence-domain reflectometry is used to obtain the time delay with better than 2ps accuracy
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