Abstract
A coherent optical time-domain reflectometer (COTDR) using flexible all-digital orthogonal phase code pulse is proposed for distributed acoustic sensing. All-digital orthogonal phase code pulse with frequency shift and time shift is used as probe. Coherent detection and balance photodetector are used to amplify interference signal and get rid of its direct-current (DC) component. The new scheme needs only single channel detection while keeping sampling frequency. Amplitude triangle modulation and frequency linear sweep modulation waveform are used for system performance investigation. The experiments on 15.4 km optical fiber showed that waveform information can be recovered well. The all-digital orthogonal phase code pulse will provide a flexible solution for different application requirement.
Highlights
Distributed acoustic sensing (DAS) system has a wide application on intrusion detection [1], seismic waves measurements [2], structure health monitoring [3] and pipeline monitoring [4], and so on
Phase extraction is usually realized with the phase generated carrier (PGC) [10], [11], the path-matched interferometry [12], and phase diversity detection [13], [14]
We proposed a coherent optical time domain reflectometry (OTDR) using flexible all-digital orthogonal phase code pulse for distributed sensing
Summary
Distributed acoustic sensing (DAS) system has a wide application on intrusion detection [1], seismic waves measurements [2], structure health monitoring [3] and pipeline monitoring [4], and so on. He et al proposed a similar method with a digital I/Q quadrature demodulating algorithm [20] They used two acousto-optic modulators and a delay fiber to generate heterodyne dual pulse. We proposed a coherent OTDR using flexible all-digital orthogonal phase code pulse for distributed sensing. All-digital orthogonal phase code pulse provides a flexible solution for different requirements from potential applications, such as spatial resolution, signal intensity adjusting related with spatial interval, and so on. PRINCIPLE the light is modulated into digital orthogonal phase code pulse, which is a triple-pulse contain three sub-pulses locating with frequency shift f1, f2 and f3, and time shift W0. The phases of the coherent OTDR signals are the difference value between the phase corresponding to position of sensing fiber and the local reference fiber phase. The data is sampled by a DAQ card with 1GS/s sampling rate and 12-bit resolution
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