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 tunable filters.
The determination of the wavelenght shift can be carried out using a tunable optical filter. The search of the shift happens through the analysis of the filter output signal, that coressponds to the convolution between the reflected spectrum from the FBG and that one of the filter. The superimposition of the two signals determines the point of maximum intensity of the filter output.
From a measure of its position, therefore, it is possible to go back to the Bragg wavelenght shift. The limits on the precision of the measure are dictated from the signal-to-noise ratio of the signal coming from the convolution between the optical answer of the tunable filter and the FBG. Normally, such an approach has relatively high resolution plus a large working range.
There are three kinds of interrogation systems, based on the tunable filter method:
- The tunable Fabry-Perot Filter
- The tunable acousto-optic filter
- The tunable FBG based filter
the scheme below gives the working principle of a tunable filter.