Fiber-optical accelerometers based on polymer optical fiber Bragg gratings

Abstract

Fiber-optical accelerometers based on polymer optical fiber Bragg gratings (FBGs) are reported. We have written 3mm FBGs for 1550nm operation, characterized their temperature and strain response, and tested their performance in a prototype accelerometer. . Bragg gratings written into PMMA POFs could therefore be ideal as the sensing element in fiber-optical accelerometers, because they potentially could provide better sensitivity and a wider dynamic range than silica fibers [2][3][4]. This of course requires that the length of the PMMA POF is short, due to the large loss of PMMA, and that one does not want to operate at high temperature, due to the low melting temperature of PMMA.In order to maximize the sensitivity and the dynamic range of an accelerometer based on FBGs, the outer diameter and the length of the sensing fiber segment should be as small as possible. To this end the phase-mask technique and a 325nm HeCd cw laser have been used to fabricate 3mm FBGs in a commercial PMMA singlemode step-index POF of diameter 115 micron from Paradigm Optics. To minimize the loss problem of PMMA, we have used 1cm POF sections with FBGs in the center and glued them to standard Silica SMF-28 fibers. We use the Paradigm POF directly and we try to anneal it before use, in order to check whether the annealing improves its properties. The annealing is done at 80°C over 2 days. The POF FBGs have been characterized in terms of temperature and strain to find operating regimes with no hysteresis and loss of reflection. Our experiments show that annealing the POF FBG can offer more stable and linear performance at both higher temperatures and larger strain.A prototype accelerometer, equipped with our 3mm POF FBG, has been designed. It will be characterized by a fast commercial wavelength interrogator (kHz) to track the response of the FBG to external vibrations.