Spot event detection along a large-scale sensor based on ultra-weak fiber Bragg gratings using time-frequency analysis.

A simple scheme for interrogating a 5 m long photonics device and its potential applications to quasi-distributed fiber sensing is proposed. The sensor consists of an array of 500 identical, very weak fiber Bragg gratings (FBGs). The gratings are 9 mm long and have been serially written in cascade along a single optical fiber. The measurement system is based on a combination of optical time domain reflectometry and frequency scanning of the interrogating pulse. The time-frequency analysis is performed by launching an optical pulse into the sensor and retrieving and analyzing the back-reflected signal. The measurement of the temperature, length, and position of spot events along the sensors is demonstrated with good accuracy. As both spatial and temperature resolution of the method depend on the input pulse duration, the system performance can be controlled and optimized by properly choosing the temporal duration of the interrogating pulse. A spatial resolution of 9 mm (ultimately dictated by one grating length) has been obtained with an 80 ps optical pulse, while a temperature resolution of less than 0.42 K has been demonstrated using a 500 ps incident pulse. The sensor proposed proves to be simple, robust, and polarization insensitive and alleviates the instrumentation complexity for distributed sensing applications.