Frequency-Scanning ϕ-OTDR With Symmetrical Triangular Linear Frequency Modulation Probe Light

A novel frequency-scanning phase-sensitive optical time domain reflectometry (ϕ-OTDR) based on symmetrical triangular linear frequency modulation (LFM) is proposed and demonstrated. By applying pre-distorted triangular driving voltage to the laser, two sets of linear frequency sweeping probe light are generated during the rising and falling edge of the triangular wave, thereby two groups of Rayleigh backscattered (RBS) spectra are obtained, respectively. Cross-correlation calculation is then performed to get the frequency shifts that are generated by the external disturbances, thus enabling quantitative dynamic strain measurement. A high detection sensitivity of 64.2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$p\varepsilon /\sqrt {\text{Hz}} $</tex-math></inline-formula> is obtained with a fiber of 276 m. Unlike the traditional LFM scheme utilizing the sawtooth driving voltage, no sharp frequency change would be generated in the proposed scheme, as a result all the acquired data can be used for processing, which facilitates the detection of higher frequency vibrations. For proof of concept, a 1.2 kHz vibration is successfully retrieved by the proposed system, which turns out to be undetectable using the conventional systems due to the constraints of Nyquist sampling theory. What's more, the demodulated results of the acquired RBS spectra respectively from the rising edges and falling edges could be used to mitigate the demodulation error induced by the spurious correlation peak in cross-correlation calculation, yielding a more accurate frequency shift determination. The proposed method paves the way to achieve high-sensitivity dynamic monitoring in engineering applications.