I. What is DAS Polarization Fading?

In a Distributed Acoustic Sensing (DAS) system, the system acquires phase changes of Rayleigh backscattering within the optical fiber through coherent detection, enabling continuous sensing of vibration, acoustic, and strain signals.

However, in practical engineering applications, DAS systems often encounter a phenomenon affecting signal stability — Polarization Fading.

Polarization Fading refers to the continuous change in the state of polarization (SOP) of the backscattered light due to random intrinsic birefringence in the fiber and external perturbations, leading to:

• Polarization mismatch between the local oscillator (LO) light and the backscattered light
  
• Decreased interference efficiency
  
• Random reduction or even complete fading of the detected signal amplitude
  

In long-distance, high-sensitivity DAS systems, polarization fading is the second major core stability challenge, following coherent fading.

II. Causes of DAS Polarization Fading

1️⃣ Intrinsic Fiber Birefringence

Standard single-mode fiber is not an ideal isotropic medium. Manufacturing stress, micro-bends, and cladding non-uniformity introduce random birefringence, causing continuous rotation of the light's SOP during propagation.

As distance increases:

• SOP becomes randomly distributed
  
• Backscattered light polarization becomes unpredictable
  

This is the fundamental physical origin of polarization fading.

2️⃣ Environmental Perturbation-Induced Polarization Drift

In practical deployments, optical fibers are subjected to:

• Temperature fluctuations
  
• Soil pressure
  
• Vibration and shock
  
• Pipeline stress
  

which continuously alter the SOP, causing DAS signal fluctuations over time.

3️⃣ High Sensitivity of Coherent Detection to Polarization

DAS employs a coherent detection structure:

Backscattered light × Local Oscillator light → Interference signal

When their polarizations are misaligned, interference efficiency drops, the equivalent detection gain decreases, thus creating "fading zones".

III. Engineering Impacts of Polarization Fading on DAS Systems

If left unmitigated, polarization fading directly leads to:

• Spatial signal blind spots
  
• Random sensitivity fluctuations
  
• Decreased long-distance monitoring stability
  
• Increased false alarm rate in AI event identification
  

In continuous monitoring scenarios such as rail transit, oil & gas pipelines, and perimeter security, such uncertainty is unacceptable.

IV. Mainstream Mitigation Solutions for DAS Polarization Fading

✅ Solution 1: Polarization Diversity Reception

Simultaneously acquiring the backscattered signal through dual orthogonal polarization channels:

• One channel for X-polarization
  
• One channel for Y-polarization
  

Followed by digital synthesis:

S_total = sqrt(Sx² + Sy²)

Achieving polarization-insensitive detection.

This is currently the most mature and reliable engineering solution.

✅ Solution 2: Polarization Scrambling (Averaging)

Utilizing a high-speed polarization scrambler to rapidly alter the incident light's SOP, allowing the system to sample across multiple polarization states, statistically reducing the probability of fading.

However, this method sacrifices some real-time performance.

✅ Solution 3: Polarization-Maintaining Optical Path (Higher Cost)

Constructing a full-link stable system using PM fiber and polarization-maintaining components. However, implementation cost is extremely high for kilometer-scale DAS, limiting engineering feasibility.

✅ Solution 4: Digital Signal Layer Polarization Compensation Algorithms

Introducing at the FPGA/DSP layer:

• Adaptive gain equalization
  
• State of polarization estimation
  
• Multi-channel fusion algorithms
  

To further eliminate residual fading.

V. Combined Coherent Fading + Polarization Fading Suppression: A Hallmark of High-End DAS

It is important to note:

A truly engineering-grade DAS system must simultaneously address:

• Coherent fading
  
• Polarization fading

Suppressing only one still cannot guarantee full-range stability.

Therefore, high-end DAS architectures typically employ:

• Multi-frequency probing + Coherent fading suppression
  
• Dual-polarization reception + Polarization fading suppression
  
• FPGA real-time fusion algorithms
  

Forming a comprehensive anti-fading system.

VI. Shanghai Kunlian Technology's DAS Polarization Fading Solutions

Shanghai Kunlian Technology has integrated the following into its professional and ultimate-grade DAS systems (e.g., DAS-P250, DAS-U250 series):

• Dual-polarization synchronous acquisition architecture
  
• FPGA real-time polarization fusion processing
  
• Combined coherent + polarization suppression algorithms
  
• 250 MSPS high-speed ADC ensuring complete transient capture
  

This design enables, in long-distance complex environments:

• Continuous stable sensing
  
• Blind-spot-free coverage
  
• High signal-to-noise ratio output
  

Making it widely suitable for:

• Metro and rail transit
  
• Oil & gas pipelines
  
• Perimeter security
  
• Seismic and micro-vibration monitoring
  
• UAV acoustic fingerprint sensing and other applications.