In order to guarantee an optimal performance of a FBG sensor it is necessary to use interrogation techniques through which it is possible to obtain precise evaluation of the Bragg wavelength shift, correspondent to the variation of the physical parameters. The availability of measurement systems is limited by requirements like frequency, sensor multiplexibility, resolution, sample rate and ambient conditions.
The commonly used interrogation techniques are based on five different principles, and one of them is the resort to edge filters.
This method is based on the use of an edge filter which has a linear relationship between wavelenght shifts and the output intensity changes of the filter, as shown in the graph below. By measuring the intensity change, the wavelenght shift induced by the measurand is obtained. The measurement range is inversely proportional to the detection resolution.
The edge filter based method like shown in the scheme below is often used in order to eliminate the influences on the measure of the fluctuatuions of intensity of the luminous source and the fiber connections. The determination of the wavelenght shift is carried out through the analysis of the relationship between the intensity of edge filter output signal IF and the reference one is IR , correspondent to the signal reflected from the sensor. The relation between the ratio, the intensities and the variation of the waveleght is: