Acoustic sensing over 100km using coherent-correlation OTDR and Manchester coding

Acoustic sensing using Coherent Correlation Optical Time Domain Reflectometry (CC-OTDR) over 100 km of standard Single-Mode Fiber (SMF) with Manchester coding is demonstrated. This technique achieves a spatial resolution of approximately 10 meters. Optical fiber sensing on existing communication links has gained significant interest due to the cost-effectiveness and maturity of optical components. Integrating sensing capabilities into communication fibers allows for efficient monitoring and fault detection, leveraging the existing infrastructure. CC-OTDR, adapted from optical communications, involves sending optical bit sequences into the fiber and cross-correlating the received Rayleigh backscatter signal with the transmitted signal. This method supports variable symbol-rates, enabling the interrogation of fibers at different lengths. However, the AC-coupling of coherent receivers restricts the symbol-rate to high frequencies, limiting long-distance sensing. Manchester coding addresses this by converting each symbol into 1 and -1 codes, allowing lower symbol-rates. This work showcases distributed acoustic sensing over 100 km with a 10-meter resolution and detects acoustic events over 400 meters with a 1-2 meter resolution by adjusting the symbol-rate and bit sequence. This is the first demonstration of CC-OTDR at a 100 km range with high spatial resolution.