Range and Bandwidth Extension in Direct-Detection C-OTDR Using Optical Repeaters

Sensing range and response bandwidth are the main parameters that define the performance of distributed optical fiber sensors. In this work, a novel scheme based on the use of in-line optical repeaters is proposed to extend the sensing range of distributed acoustic sensors (DAS) based in direct-detection coherent optical time-domain reflectometry (DD C-OTDR). The proposed regenerators comprise three different stages that reamplify the incoming pulse amplitude, synchronize the acquisition using the incoming pulse and also detect the backscattered signal from the next span. With repeaters placed every 40 km along a fiber under test (FUT), we have achieved a total sensing range above 120 km while keeping a pulse frequency of 2.5 kHz (1.25 kHz bandwidth). At the end of each segment of the FUT we experimentally demonstrate the correct measurement of a 15
$\text{n}\varepsilon $
stimulus, with and an average degradation lower than 3 dB per fiber span. The experimental results show that an extension of the sensing range in DD C-OTDR can be achieved without reducing the frequency response bandwidth or increasing the scheme complexity.