Abstract
The spectral properties of the Rayleigh backscattered traces measured by a phase-sensitive optical time-domain reflectometer ( φ OTDR) with direct detection are theoretically and experimentally analyzed. The spectrum of the measured φ OTDR signal is found to be strictly dependent on the spectral shape of the probing optical pulse. Furthermore, the visibility, spatial resolution, fading rate, and correlation spectrum of the traces are analyzed using different detection bandwidths. Results point out that the quality of φ OTDR traces and target spatial resolution are secured only if the electrical bandwidth of the photodetector is broad enough to cover at least 80% of the total power contained in the electrical spectral density function of the measured trace. This means that in the case of using direct detection of the Rayleigh backscattered light induced by rectangular-shaped optical pulses, the minimum bandwidth required for a proper detection of the traces is equal to the reciprocal of the pulse temporal width (which is larger than the pulse spectral width). Although the theoretical analysis and numerical simulations are here experimentally validated for rectangular and sinc-shaped optical pulses, the results and methodology presented in this article can be applied to optimize the direct-detection bandwidth of φ OTDR sensors using any optical pulse shape.
Highlights
P HASE-SENSITIVE optical time-domain reflectometry is a distributed fiber sensing technique based on coherent Rayleigh scattering, widely used in intrusion detection, vibration sensing, and distributed temperatureManuscript received August 13, 2019; revised December 4, 2019 and January 20, 2020; accepted January 30, 2020
As a result of the presented analysis, we can infer the following interesting properties: 1) the optical power spectrum of the backscattered light is identical to the optical power spectrum of the probing optical pulse; 2) the electrical power spectrum of the φOTDR trace measured by direct detection is given by the autoconvolution function of the optical power spectrum of the incident light
The continuous-wave laser light is shaped into pulses using an electro-optic modulator (EOM), driven by a 2.3 GS/s arbitrary waveform generator (AWG) to deliver different pulse shapes
Summary
P HASE-SENSITIVE optical time-domain reflectometry (φOTDR) is a distributed fiber sensing technique based on coherent Rayleigh scattering, widely used in intrusion detection, vibration sensing, and distributed temperature. The profile of a φOTDR trace carries meaningful information about the local conditions along an optical fiber, and most of sensing applications are based on the monitoring of the variations of this longitudinal random pattern. Results demonstrate that a reduced detection bandwidth can impair the visibility, spatial resolution and fading rate of φOTDR traces, as well as broaden the cross-correlation peak of frequency-scanned φOTDR systems, impairing the quality and reliability of the measurements. According to this analysis, the photoreceiver bandwidth is assessed to be large enough when containing 80% of the signal power, which. Represents the minimum required condition to keep impairments at a negligible level
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