Optonet system: a 64 on/off fiber optic sensor network designed for industrial and military applications

ABSTRACTThe OPTONET system monitors up to 64 ON/OFF fiber optic sensors. It is based on Optical Time Domain Reflectometry (OTDR). An optical pulse emitted by a pulsed laser diode is splitted between the sensors through an opticalstar network including delay lines to separate the pulses reflected back from the sensors. A dual wavelength emission boardis implemented to garantee line surveillance and optical path loss compensation, in order to minimize the error rate, andinformation discrepancy. These characteristics also combined with the well known advantages of fiber optic sensors makethis system attractive in military or harsh environment. Various reflective sensors can be connected to the OPTONETsystem ; we describe here sensors developped for a French Navy application. Keywords : OTDR, fiber optic sensor, multiplexing, Rayleigh backscattering, intensity referencing, liquid detection, temperature detection. 1. SYSTEM DESCRIPTION The system developped and described hereafter is dedicated to the monitoring of a French Navy system. The aim was to builda complete fiber optic sensor network demonstrator to evaluate the advantages and the viability of this new type of systemfor a specific Naval application. The overall system, includes ON/OFF sensors, such as limit switches, water detectionsensors, together with analog single point sensors. This paper will only focus on the ON/OFF sensor network.The principle of the system is based on OTDR 1,2,3,4,5 and derived from previous developments. The system has fouroptical outputs and incorporates two detectors and an optical star architecture network (Figure 1). The effort was focused tosecure the information. So a dual wavelength scheme was implemented to achieve optical path loss compensation andoptical line surveillance. In each sensor is incorporated a dichroic filter that is reflective at the chosen reference wavelength.The measurement wavelength pulse is transmitted to the transducer and reflected or not according to the sensor state. A linefailure is detected by the absence of the reflected pulse at the reference wavelength. Secondly the sensor state information is