Dynamic Range Enhanced Optical Frequency Domain Reflectometry Using Dual-Loop Composite Optical Phase-Locking

Abstract

We report on a dynamic range enhanced optical frequency domain reflectometry distributed backscattering interrogator based on dual-loop composite optical phase-locked loop (OPLL). Exploiting simultaneously an acousto-optic frequency shifter based an external modulation loop and a piezo based direct modulation loop, the proposed composite OPLL allows offering a larger loop bandwidth and gain, permitting a more efficient coherence enhancement as well as sweep linearization. A high fidelity frequency sweep of <inline-formula><tex-math notation= LaTeX >${\\rm{\\sim}} \\text{8.2 GHz}$</tex-math></inline-formula> at <inline-formula><tex-math notation= LaTeX >${\\text{164 GHz/s}}$</tex-math></inline-formula> sweep rate is generated with a peak-to-peak frequency error as low as <inline-formula><tex-math notation= LaTeX >${\\rm{\\sim}} \\text{120 kHz}$</tex-math></inline-formula>. It leads to a dynamic range enhancement of more than <inline-formula><tex-math notation= LaTeX >${\\text{3 dB}}$</tex-math></inline-formula> for the measured power loss compared to the case when only piezo loop is applied. This corresponds to <inline-formula><tex-math notation= LaTeX >${\\rm{\\sim}} \\text{15 km}$</tex-math></inline-formula> extension for the measurement range of Rayleigh backscattering without any spatial resolution penalties. Fourier transform-limited spatial resolution has been demonstrated at a range window more than about <inline-formula><tex-math notation= LaTeX >$\\text{28}$</tex-math></inline-formula> times of the intrinsic coherence length of the adopted fiber laser. The proposed method provides a straightforward optimization of the real-time sweep control and is expected to be a useful tool in industrial and commercial applications.