Why can the footsteps on the ground tens of kilometers away be heard through a single optical fiber?
Distributed Acoustic Sensing (DAS) can “hear” footsteps, vehicle traffic, or mechanical construction on the ground tens of kilometers away using a standard single-mode optical fiber. This capability stems from the synergistic interplay of optics, wave dynamics, and weak signal processing.
Its core physical mechanisms and implementation principles can be broken down into the following key stages:
I. Core Physical Principle: Optical Fiber as a Continuous “Distributed Microphone”
DAS is typically based on Phase-sensitive Optical Time-Domain Reflectometry (\Phi\text{-OTDR}). The optical fiber itself is not just a transmission medium; its entire core physically acts as a continuous acoustic/vibration-sensitive element.
1. Intrinsic Microscopic Inhomogeneities and Rayleigh Scattering
During the manufacturing of fused silica optical fibers, inherent thermodynamic density fluctuations and minor refractive index perturbations exist in the microscopic crystalline structure of the silica material. When an ultra-narrow linewidth, highly coherent laser pulse is injected into the fiber, it excites continuous Rayleigh backscattered light throughout the fiber at various points.
2. Acousto-Optic Coupling and Photo-elastic Effect
When a person walks on the ground, their footsteps exert impact forces on the ground, generating mechanical waves (primarily longitudinal waves, transverse waves, and slower Rayleigh surface waves) that propagate along the surface.
When these mechanical waves reach the micro-cable laid underground or on the surface:
- The subtle vibrations from the ground cause minuscule geometric deformations in the fiber (axial stretching or compression, i.e., dynamic strain).
- This mechanical strain modulates the local refractive index n of the fiber core through the photo-elastic effect.
A slight deformation in a segment of the optical fiber leads to an optical path difference for the light passing through that region, resulting in the phase modulation (Phase Modulation) of the backscattered Rayleigh light:
\Delta \phi \approx \frac{4\pi n}{\lambda} \xi \cdot \Delta L
Here, \lambda is the laser wavelength, \xi is the photo-elastic correction factor, and \Delta L is the minute length change caused by the vibration.
3. Coherent Interferometric Demodulation
The light pulses emitted by the DAS system possess extremely high spatial coherence (laser linewidth typically in the range of hundreds of Hz to a few kHz). The Rayleigh backscattered light from different scattering points within the pulse width interferes with itself inside the fiber.
By performing high-frequency acquisition and phase unwrapping on the continuous interference signals returned by successive pulses, the demodulation host can accurately reconstruct the frequency, amplitude, and phase evolution of the vibration at that location. Due to its extremely high sensitivity, the detection resolution can reach the nano-strain (n\varepsilon) or even pico-strain (p\varepsilon) level, which is sufficient to reproduce the subtle micro-strain vibration waveforms caused by footsteps on the ground.
II. Why Can It Cover “Tens of Kilometers”?
1. Ultra-low Transmission Loss
Standard communication single-mode silica optical fibers (e.g., G.652 fiber) have extremely low transmission loss at the 1550\text{ nm} wavelength window, typically around 0.18\text{ dB/km} \sim 0.20\text{ dB/km}. This low attenuation characteristic allows the light pulses and their excited weak Rayleigh backscattered light to retain sufficient energy after round-trip transmissions of 40 \sim 50\text{ km} to be received by ultra-low noise photodetectors.
2. Temporal and Spatial Localization Mechanism (Time-of-Flight, ToF)
Since the group velocity of light in the optical fiber core is known (approximately v = c / n \approx 2 \times 10^8\text{ m/s}), the demodulation host uses time-of-flight to directly map spatial locations:
z = \frac{c \cdot t}{2n}
Here, t is the time interval from the emission of the pulse to the reception of the backscattered light. The system typically monitors the entire fiber continuously with a spatial sampling interval (Gauge Length) of 1\text{ m} \sim 10\text{ m}. A 40\text{ km} fiber is equivalent to having thousands to tens of thousands of discrete “acoustic sensors” distributed in parallel along its length.
III. How Are Vibration Signals Reproduced as “Sound”?
When footsteps propagate on the ground, their energy is primarily concentrated in the low-frequency band (approximately 1\text{ Hz} \sim 50\text{ Hz}).
- The Pulse Repetition Frequency (PRF) of a DAS system is limited by the measurement distance. For example, at a 50\text{ km} measurement distance, the round-trip optical time is about 0.5\text{ ms}, allowing for a theoretical maximum PRF of up to 2\text{ kHz}.
- According to the Nyquist sampling theorem, this system is sufficient to capture acoustic and mechanical vibration signals below 1\text{ kHz} without distortion.
- Coupled with adaptive filtering, wavelet transforms, or deep learning pattern recognition algorithms, the system can accurately distinguish and identify acoustic vibration signatures from different sources, such as human walking, running, vehicle passage, and excavator construction.
IV. Technical Classification and Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) Product Line Description
Within the classification of fiber optic sensing technologies, DAS falls under the category of fully continuous Distributed Optical Fiber Sensing (DOFS):
- Continuous Distributed Technology (DAS / DTS / BOTDA / OFDR): Based on the microscopic scattering (Rayleigh, Raman, or Brillouin scattering) inherent in standard single-mode/multi-mode optical fibers, the entire fiber can sense everywhere. This is suitable for large-scale acoustic and vibration sensing in applications like long-distance perimeter security and pipeline integrity monitoring.
- Discrete/Quasi-Distributed Technology (FBG Fiber Gratings): Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)'s core product series focuses on Fiber Bragg Grating (FBG) sensors and demodulators. FBGs are created by photolithographically etching microscopic periodic refractive index gratings at specific points along an optical fiber. Their operating principle is based on wavelength reflection and wavelength shift measurement (\Delta \lambda_B = 2 n_{\text{eff}} \Lambda), primarily targeting industrial-grade high-precision measurements of temperature, strain, stress, displacement, and shape.
Therefore, DAS acoustic demodulation hosts based on the \Phi\text{-OTDR} principle are not part of Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)'s core product series. However, for distributed sensing projects (e.g., high-resolution micro-strain measurement or high-temperature monitoring based on OFDR), Beijing Dacheng Yongsheng Technology Co., Ltd. provides specially packaged distributed fiber protection cables (such as seamless steel pipe armored cables, etc.) to enhance the mechanical protection performance against lateral pressure and impact for bare fibers in harsh industrial environments.