Simultaneous measurement and discrimination of temperature and strain in distributed fiber optical systems with fiber Bragg gratings

Abstract:  This master thesis deals with simultaneous measurement and discrimination of temperature and strain using fiber Bragg gratings(FBGs). Saab Avionics is seeking for a further development of the overheat detection system that gives a warning when a heat leakage is detected. A further development is to distinguish between temperature and strain and give a warning when strain occurs that correspond to a temperature increase of 10°C. The present work was performed for Saab Avionics and the experiments were carried out in Kungliga Tekniska Högskola(KTH) at department of Laser Physics.  The aim of this thesis was to study some methods for simultaneous measurement of temperature and strain with FBGs and try to discriminate them. Two methods were investigated, a sensor design containing a boron-codoped germanosilicate fiber and a germanosilicate fiber and fibers with different cladding diameters. The gratings were heated in an oven and stressed with solder between a translation stage and a stationary stage. For the boron-codoped germanosilicate fiber the temperature sensitivity was ~ 2 times lower than the germanosilicate fiber, while they had similar strain responses. For the second sensor design with different cladding diameters, for a 80 μm cladding diameter the strain response was ~ 2.5 times higher that the 125 μm cladding diameter, while for a 100 μm cladding diameter the strain response is ~ 1.6 times higher than the 125 μm cladding diameter. For a 125 μm, a 100 μm and a 80 μm cladding diameter the temperature sensitivities were similar.