A compact source for a distributed acoustic sensor using a miniaturized EYDFA and a direct digital synthesis module

Distributed Acoustic Sensing (DAS) is a ubiquitous technique which enables concurrent, real-time measurement of fault or event-induced vibrations over long distances. Although there has been focused research in increasing the performance of DAS based on Phase-Sensitive Optical Time Domain Reflectometry (Φ-OTDR), the cost of conventional schemes remains high due to the complexity of the opto-electronic components in the sources used in the interrogator for high coherent Rayleigh scattering visibility, which rely on optical amplifiers designed for wideband telecom networks and multi-purpose waveform generators. However, probes in DAS use narrow linewidth lasers, whose fluctuations are well below the bandwidth of a single ITU grid and the driving waveforms can be generated by compact RF sources. In this contribution, we propose and experimentally demonstrate the design of a compact DAS interrogator using a miniaturized Erbium-Ytterbium-Doped Fiber Amplifier (EYDFA) commonly used in CATV networks together with an integrated Direct Digital Synthesis (DDS) module which can generate readily programmable waveform probes with a bandwidth of up to 1.4 GHz. The DDS module is suitable for use with any digital acquisition system for real-time acquisition of traces. Optical pulse probes generated with the DDS an a miniaturized EYDFA were used to obtain coherent Rayleigh backscattering traces with high SNR and interference visibility, allowing the measurement of a generic vibration at the end of a 10-km fiber. The proposed technique enables the simplification of DAS systems and paves the way toward their scalable development for wider use in among others environmental, seismic and structural health monitoring systems.