Use of brazing technique for manufacturing of high temperature fibre optical temperature and displacement transducer

In contrast to the case of embedding sensing fibres in composites, very little has been published on embedding fibres in high melting point metallic alloys. In the present work a methodology is presented for manufacturing a fibre optical displacement transducer for high temperature applications. The transducer is based on in fibre Bragg gratings that are metal embedded in a transducer body made of Inconel 600 (a refractory alloy). The fibre embedding is carried out by vacuum brazing at a temperature of 900 degrees C using a silver based brazing alloy. This technique solves many problems in constructing the transducer. During the brazing process the brazing alloy fuses with the metallic coating on the fibre. Because of this and as a consequence of the low thermal expansion of the silica the fibre will end up in axial and radial compression when the alloy around the fibre solidifies and cools down. In the present work cross-sections of embedded fibres are presented and the distribution of elements and phases in the joint is investigated in order to identify the reactions between the brazing alloy and the Ni coating on the optical fibre. Finally, the performance of the joint is illustrated by presenting some in situ optical measurement of Bragg wavelength shifts of the embedded in fibre Bragg gratings. The stress state in a two embedded fibre gratings is also estimated from the measured wavelength shifts.