- The paper presents a DFS system achieving sub-meter resolution and precise splice localization using Golay-coded probes and homodyne coherent detection.
- The study demonstrates coherent detection of 40 Hz acoustic vibrations with minimal propagation loss (0.086 dB/km) over 20km of ultra-low-loss HCF.
- The work validates simultaneous DFS and 1.2 Tbps live traffic operation via WDM, proving strong channel isolation and optimal fiber performance.
High-Resolution Distributed Fiber Sensing in Ultra-Low-Loss Anti-Resonant Hollow-Core Fiber with Live Traffic
Introduction
This paper presents a comprehensive demonstration of high-resolution coherent distributed fiber sensing (DFS) over 20.2 km of ultra-low-loss support tube hollow-core fiber (ST-HCF) under live traffic conditions. The key advances include achieving sub-meter spatial resolution, precisely localizing splices, and the first demonstration of coherent detection of acoustic oscillations in anti-resonant HCF, all while maintaining 1.2 Tbps traffic on an adjacent WDM channel. The work addresses fundamental challenges associated with fiber backscattering in HCF—specifically, exceedingly low Rayleigh backscattering coefficients (RBC)—and the impact of parasitic reflections induced by fiber coupling.
Technical Approach
The fiber under test utilizes a support tube anti-resonant design with capillary structure enhancements to minimize mode coupling and loss, achieving <0.10 dB/km at 1550 nm. Mode field adapters with angle-cut, anti-reflection coatings provide efficient coupling (<0.3 dB per transition) between standard single-mode fiber (SMF) and HCF.
Conventional OTDR is insufficient for high-resolution event localization in HCF due to low RBC and dead zones introduced by strong Fresnel reflections at mode field adapters. The employed DFS system uses a stabilized laser (frequency discriminator based) and Golay-coded probes (up to 500 MBd with a code length of 221 symbols) to maximize measurement dynamic range and SNR. Homodyne coherent detection with real-time processing isolates the weak backscattered signals and enables sub-meter gauge lengths, with repeated code transmission to facilitate high bandwidth (120 Hz) event detection.
Simultaneously, a separate WDM channel at 1.2 Tbps, delivered via a commercial PSI-M transponder, operates at launch powers up to 23 dBm without measurable impact on signal quality (zero UCB at maximum tested power), confirming the DFS does not compromise co-propagating data traffic.
Experimental Findings
Backscattering and Propagation Loss:
OTDR measurements yield a fiber loss of 0.096 ± 0.005 dB/km and estimate RBC for the HCF air core at approximately -94 dB/m, consistent with prior theoretical and experimental reports. The end-to-end system loss—including all adapters and splices—is 2.3 dB, with <0.3 dB attributable to each MFA.
DFS Spatial Resolution:
Coherent DFS achieves a native resolution of 0.3 m (corresponding to the 0.3 m code gauge length), with the empirical full-width-at-half-maximum (FWHM) of localized splice reflections at 60 cm. End-to-end loss assessed via DFS and OTDR methods are in close agreement (0.086 ± 0.005 dB/km), corroborating system accuracy.
Simultaneous Data/DFS Operation:
No increase in uncorrected code blocks is observed on the live traffic channel up to 23 dBm launch power, signifying strong channel isolation and absence of nonlinear or crosstalk penalties induced by the DFS signaling.
Acoustic Sensing:
By mechanically oscillating the fiber near a splice, phase-resolved measurements unequivocally identify induced oscillations at 40 Hz with a clear peak in the power spectral density, even at RBC levels below -100 dB/m. The system’s high-pass filtered phase sensitivity enables detection of modest-frequency vibrations without contamination from low-frequency environmental noise.
Implications and Prospects
The paper unambiguously demonstrates that coherent DFS with sub-meter resolution is feasible in state-of-the-art ultra-low-loss anti-resonant HCF over practical long-haul spans with concurrent Tbps data traffic. The reported loss, resolution, and channel coexistence quantify the respective trade-offs and opportunities for future HCF instrumentations, particularly for embedded or in-line sensing in telecommunication-grade infrastructure.
The approach's instrumentation, specifically the combination of highly stabilized lasers and Golay-coded probes, eliminates the need to degrade coupling quality to suppress reflections. This marks an advance over prior work where high-sensitivity DFS in HCF was only attained over short segments and/or required suboptimal adapter alignments.
Potential future developments include scaling to even longer distances (enabled by code design and laser coherence), higher mechanical bandwidth, and integration with advanced modulation or multiplexing schemes for simultaneous high-capacity data and sensing in next-generation optical backbones. The methodology is directly extendable to in-service health monitoring and smart infrastructure applications, where transparent, distributed acoustic or structural metrics are required with zero impact on data path integrity.
Conclusion
This study establishes the technical feasibility of sub-meter-resolved DFS in ultra-low-loss anti-resonant HCF over 20 km under live high-rate WDM traffic. The system achieves a propagation loss of 0.086 dB/km, robust adapter and splice localization, and coherent acoustic event detection with simultaneous 1.2 Tbps data transmission. These results underscore the readiness of advanced HCF for deployment in applications requiring coexistent transport and high-resolution distributed sensing.