2000 Serial FBG Sensors Interrogated With a Hybrid CDM-WDM Scheme

This article presents a new interrogation technique for serial fiber optical sensor networks. A hybrid code-division multiplexing (CDM)-wavelength-division multiplexing (WDM) scheme allows interrogating 2000 serial fiber-Bragg grating (FBG) sensors in a network with a length of 50 m. Furthermore, an interrogation of 477 sensors over a length of 150 m is shown. The work focuses on influences given by the above-mentioned sensor networks, such as reflectance, multiple reflections, and overlapping FBG spectra, as well as on specifications of used equipment. Since CDM relies on orthogonal codes, an autocorrelation function of a code is presented that suits for the interrogation of the previous mentioned sensor networks. A simulation of the interrogation scheme allowed for the study of the overall sensor network performance and the impact of its different components. The promising results were reproduced in a testbed. Furthermore, strain calibration measurements showed a high level of accuracy. The proposed hybrid CDM-WDM interrogation scheme can deal with sensor network lengths from a few meters to the range of a few kilometers with massive numbers of freely distributable sensors in the fiber. It is a promising approach in the field of FBG based optical sensing.