Integrated Optical Instrumentation for Fiber Grating Sensors

Abstract

The objective of this Chapter is the development of fiber compatible, portable, rugged and low cost instrumentation for the processing of the optical signals from Fiber Bragg Grating (FBG) sensors embedded in composite materials in order to monitor local static and dynamic stresses and strains of the structure. The FBG sensor is a grating realised by UV holography1 in an optical fiber that is monomodal at 1300 nm, and reflects radiation whose wavelength satisfies the Bragg relation: λb = 2nΛ (n is the effective refractive index of the fiber, A is the period of the grating)(Fig. 1). The fiber is positioned between two plies of the composite material structure and embedded by the curing process (Fig. 2), with input-output in the external surface of the structure. In this configuration the fiber tracks the stress status of the structure in which it is embedded, and compressive or tensile stresses parallel to the fiber axis at the location of the FBG induce changes of λ B in accordance with the relation: $$\Delta {\lambda _B}/{\lambda _B} \approx (1 - {p_e})\varepsilon $$ (1) Open image in new window Open image in new window where pe is the effective photoelastic constant of the fiber and eis the deformation measured in μstrains. The measurement of this wavelength shift enables the FBG sensor to measure the structures static and dynamic stresses and strains at the location of the sensor.