Distributed Acoustic Sensing (DAS) via Optical Frequency Domain Reflectometry (OFDR)
Distributed Acoustic Sensing (DAS) has been attracting considerable amount of attention in recent years for various applications including intrusion detection, railroad monitoring, pipeline protection, seismic profiling, oil and gas well monitoring, underwater acoustic sensing and more. In many such applications sub-meter spatial resolution is necessary in order to allow accurate sampling of the acoustic phase front at high frequencies. For instance, acoustic waves in water propagate at a typical velocity of ~1.5 km⁄s , which means a wavelength of ~15cm for a frequency of 10KHz. All current implementations of DAS are based on fiber-optic reflectometry. Most DAS methods obtain the position information from the time delay of the backscattered signal. The spatial resolution in these methods is determined by the duration of the interrogating pulse. Hence, as in Optical Time-Domain Reflectometry (OTDR), there is a fundamental inherent tradeoff between the spatial resolution and the Signal to Noise Ratio (SNR). Typical OTDR based DAS systems have spatial resolution of ~10m or more. In contrast our approach is based on Optical Frequency Domain Reflectometry (OFDR). OFDR is a well-established method for measuring the reflection profile of an optical fiber. The method is based on transmitting, into the fiber, light whose instantaneous frequency varies linearly with time. The backscattered light from the fiber is then mixed with a reference and detected. The detector output (the beat signal) is Fourier-transformed to yield spatial information. In contrast with time domain reflectometry, OFDR enables excellent spatial resolution alongside with high SNR. In the talk I will describe the various approaches for implementing DAS and some of our work on the development and characterization of OFDR based DAS.