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

Yinxia Meng,Yi Dong,Yuanshuo Bai,Jiang Yang,Weilin Xie,Yuxiang Feng,Ling Zhang,Wei Wei

doi:10.1109/jphot.2021.3094330

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

Highlights

Optical frequency domain reflectometry (OFDR) distributed backscattering interrogator, on account of the advantages including high spatial resolution, high sensitivity and dead-zone free interrogation, has attracted many attentions in fields of such as optical communication network monitoring [1], imaging [2], [3], and distributed sensing [4], [5]
The measurement range is mainly determined by the coherence of the probe laser while the spatial resolution is inversely proportional to the sweep range and is theoretically dictated by the sweep nonlinearity
It is worth noting that a slight temporal shift can be observed between the two locked cases, especially in the beginning of the sweep, inferring a shorter transition from the static to sweep operation in dual-loop case. This is mainly due to the large loop bandwidth, namely the fast loop response enabled by the composite optical phase-locked loop (OPLL) that permits to avoid the underlying phase slips induced by excessive accelerations in the start of the sweep and facilitate the locking process

Summary

Introduction

Optical frequency domain reflectometry (OFDR) distributed backscattering interrogator, on account of the advantages including high spatial resolution, high sensitivity and dead-zone free interrogation, has attracted many attentions in fields of such as optical communication network monitoring [1], imaging [2], [3], and distributed sensing [4], [5]. The inherent nonlinearities in the frequency modulation response can be mitigated a prior by the use of pre-distorted control signals To further address this issue, optical phase-locked loop (OPLL) that permits for simultaneously sweep linearization and coherence enhancement have come into view [21], [22]. The inherit frequency modulation response for commercial fiber lasers acts as a main limiting factor on loop bandwidth, hampering the further suppression of the residual phase error This way, closing the gaps between locking mechanisms and loop delay would allow to take a full advantage of the high coherence. It leads to more than 3 dB dynamic range enhancement in terms of single-trip power loss, corresponding to more than 15 km roundtrip measurement range extension in single-mode fiber (SMF)

Operation Principle

Effect of OPLL in static and dynamic operations

Measurement of Power Loss in OFDR Verification

Conclusions