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 interferometric scanning.
The use of this method can guarantee optimal results in the measures of temperature and deformation, in dynamic and nearly static regime. The normalize interference signal coming from a scanning interferometer is given by:
where I0 is the incident light intensity, ø(t) is the thermal phase shift and ??B is the phase shift due to the reflected signal which can be written as:
where ?Y is the temperature variation or the deformation experienced by the sensor, ?LSI is the interferometer optical path difference and ?x is the FBG sensor sensitivity. Form this, measuring the phase shift ??B, it is possible to recover the temperature variation or the strain experienced by the FBG sensor. Even in this case it is mandatory to carefully evaluate the trade-off between sensitivity and measure range.
The Figure above gives the principle of the interferometric scanning method
The interferometric scanning scheme is likely to achieve the highest resolution among the current detection methods.