Abstract:We report on a detailed study of the inscription and characterization of fiber Bragg gratings (FBGs) in commercial step index polymer optical fibers (POFs). Through the growth dynamics of the gratings, we identify the effect of UV-induced heating during the grating inscription. We found that FBGs in annealed commercial POFs can offer more stable short-term performance at both higher temperature and larger strain. Furthermore, the FBGs' operational temperature and strain range without hysteresis was extended by the annealing process. We identified long-term stability problem of even the annealed POF FBGs. [7][8][9][10][11][12][13][14][15][16]. 325nm has been employed as a mainstream wavelength for writing grating in PMMA POFs [1][2][3][4][5][8][9][10][11][12][13][14][15]. Other wavelength such as 355nm obtained from a frequency-tripled Nd:YAG laser has been used to write grating in CYTOP fiber developed by Asahi Glass Co. and Keio University [15,16]. On the other hand, 800nm femtosecond pulses from Ti:Sapphire laser or its double frequency was mainly used for point by point direct writing [6] or grating writing with a phasemask [7]. However, the mechanism of index change does not appear to be fully understood [5,13,[18][19][20]. It is believed that more than one process is involved in the photo-induced refractive index changes and hence in the grating formation dynamics [18][19][20]. The widely accepted point is that the principle mechanism of index change is an increase due to the photo-induced polymerization of the unreacted monomers [5,[18][19][20], while laser-induced heating in the irradiated region during the inscription may also contribute to the index change [5]. Previous reports indicated that annealing of the POF before FBG inscription can relieve the frozen-in stress induced by the fiber drawing process [21] and increase the linear operation temperature range of FBGs [22]. However, the effect of annealing on the strain sensitivity performance was not yet considered. Polymer optical FBGs have shown great potential for sensor applications to sense for example temperature and strain with higher sensitivity and wider tunability than its silica counterpart [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Those advantages are due to the lower Young's modulus and higher thermo-optic coefficient of POFs [23,24]. In addition, polymers are clinically

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We present a fibre optical accelerometer based on a balanced double cantilever design that offers improved performance in terms of sensitivity, frequency range and reliability in general. Within a three year project funded by the Danish National Advanced Technology Foundation, we have developed a rugged prototype with a wavelength sensitivity of 20 pm/g within ±1dB over a frequency range from DC to 1 kHz. These units are currently undergoing field tests in monitoring applications, e.g. for wind turbines, energy services, aerospace etc. In particular, we consider sensitive and reliable accelerometers essential for future condition monitoring and structural health monitoring applications.

In this paper we present our latest work on Fiber Bragg Gratings (FBGs) in microstructured polymer optical fibers (mPOFs) and their application as strain sensing transducers in devices, such as accelerometers and microphones. We demonstrate how the cross-sensitivity of the FBG to temperature is eliminated by using dual-FBG technology and how mPOFs fabricated from different grades of TOPAS with glass transition temperatures around 135 o C potentially allow high-temperature humidity insensitive operation. The results bring the mPOF FBG closer to being a viable technology for commercial applications requiring high sensitivity due to the low Young's Modulus of polymer.

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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.

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