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

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 ${\rm{\sim}} \text{8.2 GHz}$ at ${\text{164 GHz/s}}$ sweep rate is generated with a peak-to-peak frequency error as low as ${\rm{\sim}} \text{120 kHz}$ . It leads to a dynamic range enhancement of more than ${\text{3 dB}}$ for the measured power loss compared to the case when only piezo loop is applied. This corresponds to ${\rm{\sim}} \text{15 km}$ 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 $\text{28}$ 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.