Possible approach for the simultaneous measurement of temperature and strain via first and second order diffraction from Bragg grating sensors

Bragg Grating Sensors (BGS) have generated great interest in recent years because of their potential use in a wide range of applications. BGS form an integral part of the optical fibre structure, being written intracore whilst the fibre is manufactured, and provide absolute wavelength encoding of information that is independent of overall system light levels, but dependent upon strain and temperature effects acting upon the sensor. However, a single measurement of the Bragg wavelength shift cannot distinguish between the effects of temperature and strain; such an ability is of primary importance if fibre optic based sensors are to realise their considerable potential in real-world applications needing, for example, a temperature insensitive or compensated strain gauge. Several discriminating techniques have been demonstrated in recent years; these include use of a second grating element encapsulated in a different material and placed in-line with the first grating and most recently the use of two superimposed fibre grating sensors However, both schemes require two gratings, whilst the latter method requires the overwriting of gratings, which may prove very difficult to achieve if writing occurs during fibre manufacture. Here we propose an alternative technique that uses the second order diffraction from a single, conventionally written BGS when illuminated with a source at approximately half the primary reflecting wavelength. We also present the results of preliminary experimental investigations