BRAGG GRATING SENSING INSTRUMENT USING A TUNABLE FABRY-PEROT FILTER TO DETECT WAVELENGTH VARIATIONS

A two-Bragg-grating sensing system, demodulated with the use of a tuna ble Fabry-Perot filter to detect the associated wavelength Variation h as been developed. To show the practicality of the system, a low cost ELED (with an output of 15 mu W at 1550 mu m) was used as the light so urce, incorporating readily available Bragg gratings centred around th is wavelength. The relevant parameters considered in designing such a system have been analysed and are discussed in detail. With the use of a digital Gaussian filter to limit the noise, under the same conditio ns, the repeatability obtained was close to the optimum level achievab le (with the 12-bit A/D card used), and the repeatability of the Bragg wavelength, determined with individual data points being averaged ove r ten measurements was 7 pm, when the signal to noise ratio was as low as 30:1. Temperature and strain sensitivities of 8.8 pm degrees C-1 a nd 1.15 pm/mu strain have been demonstrated with such an instrument. T he potential for achieving a higher repeatability of the measurement i s discussed.