Fabrication of long period fiber gratings by electric arc for strain sensing applications

Lately, there has been a huge demand for smart structures. In particular the interest has growth in those structures able to detect deterioration conditions and possible failure. Failure prevention requires an appropriate monitoring and maintenance system. Currently, there are available several types of sensors capable of detecting problems in structures, among them, sensors based on optical fibers have been proposed as they represent a non-invasive technique. Some optical fiber sensors are based on Bragg gratings. A grating is a periodical index perturbation of the fiber core which is most commonly achieved through UV radiation. Another technique used to fabricate the gratings, which has not been studied extensively, is electric arc. Therefore, in this work we propose the use of this technique to fabricate fiber optical sensors based on Long Period Fiber Gratings (LPFG). Manufacturing LPFG through electric arc has the advantage of being quite flexible, inexpensive, present very high temperature stability and can be applied to any type of optical fiber. LPFG with a period of 500 microns and 20 mm of length were fabricated through electric arc on standard monomode fibers with the help of a fusion machine and its spectrum was observed by an Optical Spectrum Analyzer (OSA). This type of LPFG is tunable by changing the fabrication parameters of the electric arc which in turns will vary its sensitivity to measure strain on structures when it is used as a sensor. Also, in this paper a theoretical and analytical examination of arc induced LPFG is presented. Mathematical analysis and simulation of the sensor based on LPFG were carried out using the software MATLAB.