Long-Distance Distributed Acoustic Sensing (DAS)

Abstract

This study explores the implementation of a long-distance Distributed Acoustic Sensing (DAS) system using single-ended inline optical amplification over a 70 km fiber, extending the effective sensing range to 140 km. The phase-sensitive OTDR system was successfully field-tested, detecting a digging event at 72 km. Leveraging Raman amplification and wavelength filtering techniques, the system enhances signal detection and maintains high sensitivity across extended distances. The results demonstrate the system’s effectiveness for long-range acoustic monitoring in real-world scenarios, adhering to the established principles of phase-sensitive DAS technology.

Introduction

Overview of DAS Technology

Distributed Acoustic Sensing (DAS) technology has significantly impacted fields such as oil and gas pipeline monitoring, railway track behavior analysis, and perimeter security, among others. By converting standard optical fibers into distributed acoustic sensors, DAS systems provide continuous and real-rime monitoring capabilities over extended distances with high spatial resolution. The potential to monitor vast infrastructures, such aspipelines and subsea cables, depends heavily on extending the sensing range of DAS systems, which isa current focus of on going research and development.

FOTAS Models and Detection Capabilities

A notable example of DAS technology is the Fiber Optic Disaibuted Acoustic Sensing (FOTAS) system. FOTAS uses optical fibers to detect and analyze acoustic signals along their entire length, enabling real-time monitoring and measurement of acoustic waves, vibrations, and other acoustic events over long distances. The FOTAS system includes several models, such as S10, S50, D5, D30, D50 represented in Figure 1, and the newly developed D70. Inthis context, “S” denotes single-channel systems, while “D” refers to dual-channel systems, with the numbers indicating the respective sensing ranges in kilometers.

Figure 1. FOTAS Models

This study aims to advance the Fiber Optic Distributed Acoustic Sensing (FOTAS) technology by developing the D70 model, an enhancement of the existing D50 model. Our focus is on creatinga phase-sensitive Optical Time-Domain Reflectometry (‹p-OTDR) based DAS system capable of operating over a 70 km single-mode fiber. By utilizing Raman amplification, the system successfully extends the sensing range, enabling effective long-distance acoustic monitoring. The experimental setup, which includesa narrow linewidth laser, acousto-optic modulation, and distributed. Raman amplification,was tested under real-world conditions. The system’s performance was validated through field testing, where it detected a simulated digging event at a distance of 72 km, demonstrating the system’s potential for extensive infrastructure monitoring applications.

The FOTAS technology, including models like S10, S50, D5, D30, D50, and thenewly developed D70, is designed to detect and analyze acoustic signals Over the entire length of optical fibers. These systems are crucial for real-time monitoring and measurement of acoustic waves, vibrations, and other acoustic events over long distances. The D70 model, which we are developing, builds on the principles of the D50 model with enhanced features to funher improve long-distance monitoring capabilities. Thisworkreflects the on going evolution and optimization of the DAS field.

Methodology

Phase Detection

The DAS system emplys a &-OTDR (Phase-sensitive Optical Time-Domain Reflectoinetry) approach, utilizing coherent detection for precise phase measurements. In this setup, the output from a narrow linewidth laser is split into two paths using a 50:50 coupler, creating two separate sensing channels, each responsible for monitoring a 70 km section of the fiber in opposite directions.

For each channel,a 90:10 coupler is used to split the signal, where 90% of the optical power is transmitted down the sensing fiber as the probe signal, and 10% is directed to the reference channel of the balanced photodetector. The probe signal interacts with the fiber, and the backscattered Rayleigh light, which carries phase information related to external perturbations, is coherently mixedwiththe reference signal in the balanced photodetector.

The coherent detection process is governedby thefollowing phase difference equation:

where 2,‹ , E’p and* represents the complex electric field of the reference signal, the complex electric field of the probe signal after Rayleigh backscattering and the complex conjugate respectively.

The balanced photodetector outputs a signal proportional to the interference between the probe and reference fields, allowing the precise extraction of the phase difference AQ. This phase difference is directly related to the acoustic disturbances along the fiber, enabling the system to detect and
localize events with high spatial resolution.

System Architecture and Signal Processing

The architecture includes Raman amplification after the EDFA and before the circulator, which boosts the optical signal within the fiber to extend the sensing range. The amplified signal, containing the Rayleigh backscattered light, is filtered using a Wavelength Division Multiplexer (WDM)to
isolate the 1550mm wavelength before it reaches the balanced photodetector.

The Ramanamplification process enhancesthesignal according tothefollowing equation [I]:

where J,„ , i;„ , 8 , +g,› and z represents the output intensity after Raman amplification, is the input intensity before amplification, the Raman gain coefficient, the power of the Raman pump and the distance along the fiber respectively. This configuration ensures that ihe signal is strong enough to be coherently detected, even over long distances, enabling the system to maintain high sensitivity and accuracy.

Results

Figure 2. FieldTestResult

Conclusion

In this study we presented an enhancement of the existing 50km sensing range to over 70km. In figure 2 the field test and the detected human digging signal at 72km are represented. The next improvement will be the increase the sensing range to 140km fo ra channel which means 280km effective sensing range of dual channel device model (D140) by adding the further 70km standard single mode fiber optic cable with using a erbium doped fiber optic patch cord.The Erbimn doped patch cord will be act as remote pump with the raman pump.

References

[ 1] A. H Hartog, An ıntroduction to Dıslrıbuted Optıcal Fıbre Sensors (CRC Press, 20 17), Chap. 5.

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