M. Volanthen

Distributed grating sensors using low-coherence reflectometry

Distributed 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 /spl mu/m spatial resolution and 5.4 /spl mu//spl epsiv///spl radic/Hz accuracy. A theoretical model of the interrogation technique is derived and the predicted performance limits are examined experimentally.

Distributed and multiplexed fibre grating sensors

A 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 fiber grating sensors and discussion of problem areas

Based on an earlier version presented at OFS13 in Korea, a short review of distributed and multiplexed fiber 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.

Distributed grating sensors: an alternative to multiplexed grating arrays?

Fiber Bragg gratings have been widely used for smart structures sensing applications. Until recently all sensor systems using gratings have only measured the average value of a physical field. The field has been averaged either over the length of a grating or over a length of fiber between gratings. To obtain a spatial image of the physical field, these sensors have been multiplexed to form a sensor array. Recently, fully distributed images of physical fields have been measured along the entire length of a grating. Distributed sensors show great promise for the detection and location of small physical features within structures, such as cracks and hot spots. As the fabrication length of gratings continues to increase, there appears great potential for distributed grating sensors. Distributed grating sensors may be classified as either narrowband or broadband, according to the spectral width of the interrogation source. Both types of sensor are discussed and briefly compared. The various forms of averaging grating sensors are also discussed and their performance is compared with that of distributed grating sensors using two specific smart structures applications. The latest results of our broadband distributed sensor are presented. This distributed sensor may be viewed as an adaptive averaging sensor since the number of interrogation regions, their size, location and the spatial resolution may all be viewed in response to the sensing information. Finally, a sensing network combining the advantages of both multiplexed and distributed sensors is demonstrated and discussed.

Fiber Bragg grating sensors

Three different fiber sensor systems using gratings are presented. Firstly, using an acousto-optic tunable filter and closed loop feedback, arrays of fiber Bragg gratings are being used to monitor small-scale perturbations in composite materials by mapping the strain field around a defect. Gratings are also used as distributed sensors by measuring the wavelength as a function of distance. Low-coherence interferometry selects the location under interrogation and a tunable filter measures the local wavelength. An open loop interrogation technique using a commercially available optical coherence domain reflectometer is demonstrated. The reflectivity of the sensor grating is measured as a function of both distance and wavelength. Gratings 40 cm long are interrogated and several distributed grating sensors are multiplexed in an array. Thirdly, a new sensing concept using subcarrier fiber gratings (SFGs) has been proposed and modelled. The SFG is a periodic reflective array resonant at RF frequencies. The resonance may be measured using subcarrier interferometry. Modelling has demonstrated the SFG to have superior performance over other subcarrier sensors.

Long Fibre Grating Characterisation Using Low Coherence Reflectometry

Optical fibre dispersion compensation, and distributed fibre sensors, make use of long fibre gratings. Their reflectivity and time delay are usually characterised as a function of wavelength using an expensive tunable laser. We have recently achieved similar results with a new low-cost low-coherence reflectometry technique, which is related to earlier techniques for optical fibre characterisation

Lock-in techniques for interrogation of long- and short-gauge length optical fiber sensor arrays

Two complimentary optical fiber strain sensors employing lock-in techniques are presented. The first system interrogates an array of long gauge length sensors, defined by broadband optical reflectors and employs multiplexing in the time domain. The second system operates over shorter gauge lengths using multiple narrowband reflectors and wavelength-division-multiplexing. The first system tracks minima in the amplitude response produced from the superposition of two sinusoidal subcarrier waves. The second uses an acousto-optic-tunable-filter (AOTF) to track the peak reflective wavelength of an array of Bragg gratings. Both systems are constructed using telecommunications components. Together, the systems may be used to examine both line-integrated strain (or temperature) over long gauge lengths and local strain at a number of selected discrete points of particular interest. Lock-in techniques using dithered signals are applicable to sensors having a transfer function containing at least one turning point. This may be a maximum or minimum when observed either in transmission or reflection. The sensor responds to the dither with an amplitude-modulated signal, which permits locking of the interrogation system to the turning point. This provides a real-time response and better noise performance than scanned measurements. High-resolution monitoring of time-varying strain is demonstrated using this method. The long gauge length system has demonstrated a resolution of 3 microstrain over discrete 5 m long sensing sections, with an interrogation time of 0.25 s. When multiplexed to interrogate an array of four sections, intersection crosstalk levels were below minus 50 dB. The short gauge length interrogation system has been demonstrated using both fiber Bragg gratings and an in-line Fabry-Perot cavity as the wavelength selective reflectors. A resolution below 1 microstrain was obtained using the gratings, whereas a resolution of 1.5 multiplied by 10-6 in optical path-length-difference was obtained when interrogating a Fabry-Perot cavity. Simultaneous monitoring of multiple Bragg gratings has also been demonstrated by multiplexing with different dither frequencies. The versatility and the high resolution make the lock-in systems ideal for smart structures applications.