Abstract

Abstract
Dither demodulation of fiber Bragg grating sensors illuminated with multimode light from laser diodes is theoretically and experimentally investigated. Quasi-static temperature and strain sensitivities of 0.09 degrees C/root Hz and 0.6 mu epsilon/root Hz are obtained. We show that it is possible to measure small ac signals that lie outside the feedback loop bandwidth by using a synchronous detection referenced to twice the dither frequency. In this situation, dynamic strain sensitivity of 3.3 n epsilon/root Hz is achieved. (C) 1999 Optical Society of America.

Abstract
A new fiber Bragg grating sensor configuration is presented for simultaneous measurement of strain and temperature. The sensor utilizes the effect of boron codoping on the temperature dependence of the refractive index in germanosilicate fibers. By writing gratings with close wavelengths in undoped and boron doped fibers, different temperature sensitivities are obtained while strain sensitivities remain the same. These gratings are then spliced to obtain a simple sensor head suitable for applications in smart structures and composite materials.

Abstract
A signal processing scheme for fiber Bragg grating sensors based on the utilization of adjacent modes of a multimode laser diode light source is demonstrated which allows high sensitivity to be obtained over a large measurement range, For strain measurements, a range of 4800 mu epsilon is achieved with a resolution of 0.08 mu epsilon/root Hz, yielding a dynamic range of 95 dB.

Abstract
A sensor based on a Sagnac topology in reflection is described. The sensor uses the properties of its frequency response in order to obtain a self-referenced signal proportional to the measured signal. The sensor is a versatile structure for the monitoring of static and dynamic physical parameters, which require a small number of optical components and simple signal processing.

Abstract
Dither demodulation of fibre Bragg grating sensors illuminated with multimode light from cheap compact disc laser diodes is investigated. Quasi-static temperature and strain sensitivities of 0.09 degrees C/root Hz and 0.6 mu epsilon/root Hz are obtained. It is shown that small ac signals outside the loop bandwidth can be measured using synchronous detection at double the dither frequency.