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Laser remote sensing represents a powerful technique for investigating many aspects of the environment ranging from probing the upper regions of the atmosphere to measuring the depths of the oceans. It has also been shown to be capable of mapping pollution and may be capable of monitoring crops for stress. Recent, advances in the technology may permit it to be miniaturized and used in new exciting ways.

When a fiber-optic intracore Bragg grating is subject to an appreciable strain gradient, its reflective spectrum will not only be shifted but also be distorted because of the chirp of the grating. We employed the J-matrix formalism to calculate the influence of different strain gradients on the reflective spectra of Bragg gratings and have undertaken experiments to test these calculations. The results of these experiments have confirmed that intracore Bragg gratings can be used to evaluate strain gradients and can be thought of as quasi-distributed strain sensors. This adds a new dimension to structural sensing, permitting measurements in any situation where strain gradients exist. It also provides a warning of any sensor/host debonding.

A fiber-optic strain gauge system for use in structural monitoring and smart-structure applications is described. The strain gauge uses a fiber-optic Bragg grating sensor to measure strain and a passive, wavelength demodulation system to determine the wavelength of the narrow-band, backreflected spectrum from the grating sensor. The fiber-optic strain gauge system permits the measurement of both static and dynamic strains with a noise-limited resolution of 0.44 microstrain/√Hz, a measurement dynamic range of 27.8 dB, and a bandwidth of 250 Hz.

A new distributed strain sensing technique using a fiber optic Bragg grating has been developed and tested. This is the first 'true' distributed strain sensor, to the authors' knowledge, with a high spatial resolution of about 1 mm. Since gratings can be made with a length of tens of centimeters, this new fiber optic measurement technique could have broad applications to smart materials and structures where monitoring of a continuous strain profile over a length of millimeters to tens of centimeters is needed. In this paper three different demodulation approaches are reviewed indicating a trade-off between a relatively simple measurement process for selected types of strain profile and a more complete measurement process that is suitable for any strain profile. Experimental results with different approaches are presented.

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