QPS Photronics (Montreal, QE, Canada) offers sensor arrays with 100 Fiber Bragg Gratings (FBGs) in each array, with the total length of the fiber measuring 20 kilometers. Through a front-end switch matrix, 16 of these fiber arrays can be connected together to form a Star Network that contains a total of 1,600 sensors. This fiber array network can be interrogated using Time Division Multiplex (TDM). Under TDM, FBGs are separated by fixed distances of between 10 and 100 meters. These FBGs have characteristics such as center wavelengths, bandwidths, and reflectivity.
The TDM interrogation unit, based on a high-power broadband source, sends a narrow pulse of light energy down each fiber channel. Light reflection from the first FBG arrives first back at the detector, followed by reflections from the other FBGs in the array. The receiver contains a time-gated detection circuit with an edge filter to convert light signal back to the wavelength domain with the necessary signal processes to eliminate noise arising from the interference between neighboring FBGs. As the light source continues to send pulses of light, the extent of the changes and the location of each sensor where the change occurs can be determined.