A Robust Serial FBG Sensor Network with CDM Interrogation Allowing Overlapping Spectra

Abstract

Massive optical sensor networks gained a lot of attention in recent years. They offer new advances in the fields of smart structures and health monitoring. All serial optic al sensor networks rely on multiplexing techniques that provide huge amounts of sensors in a single optical fiber . Wavelength-division multiplex (WDM) which has been established in many applications, is restricted to the spectral width of the used light source that needs to be shared by several non-overlapping fiber-Bragg-g ratin (FBG) spectra. Time-division multiplex (TDM) uses short impulses and relies on different sensor rou nd trip delays to distinguish each single FBG. These short impulses and long round trip times lead to a low si gnal-to-noise ratio (SNR). Optical frequencydomain reflectometry (OFDR) offers a high spatial resolutio n of FBGs but only within a short fiber length. This contribution deals with a code-division multiplex (CD M) interrogation technique that provides numerous sensors in a single optical fiber, a better SNR, and a long rang e of distributed sensing points. It requires codes with good autocorrelation behavior which is characte ized by certain criteria. The detectable criteria are limited which narrows significantly a search for best pos sible codes for the interrogation system. In this contribution, practical implementation limits such as the rigger timing and the achievable SNR are studied. Based on the introduced SNR definitions for CDM and WDM system , a direct comparison is possible and it shows the superiority of the proposed CDM scheme. A networ k with 25 sensors operating at the same wavelength can provide a 2 .67 dB improvement compared to WDM