Distributed Optical Fiber Sensing (DFOS) transforms standard fiber optic cables into powerful sensors capable of detecting temperature, strain, and acoustic signals at thousands of measurement points over long distances. This technology is revolutionizing industries from infrastructure monitoring to energy and security. Different sensing techniques include distributed acoustic sensing (DAS), distributed temperature sensing (DTS), and distributed strain sensing (DSS). 

The core of a distributed fiber optic sensing system is the interrogator, a specialized device that launches laser pulses into the fiber and captures the backscattered light. Within the interrogator, high-speed data acquisition boards – or digitizers - play a critical role by rapidly sampling the returning optical signals with high temporal resolution. Digitizers used in DFOS typically operate at sampling rates ranging from 1 to 10 Gigasamples per second (GSPS).

By precise measurement of the time-of-flight and spectral characteristics of the scattered light, the system can localize and quantify changes with high spatial resolution, often down to the meter or sub-meter level. This enables comprehensive monitoring over distances of tens of kilometers with a single fiber.

DFOS systems utilize 1550 nm laser wavelengths, and light scattering within the fiber is classified as either elastic or inelastic, depending on whether the scattered light retains the same energy as the incident light. Elastic scattering, such as Rayleigh scattering, involves no change in wavelength and is primarily used to detect strain and vibrations. In contrast, inelastic scattering - seen in Raman and Brillouin interactions - results in a shift in the light's wavelength due to energy exchange with the fiber material. Raman scattering is only sensitive to temperature, while Brillouin scattering responds to both temperature and strain, making these mechanisms fundamental to the diverse sensing capabilities of DFOS systems.​

Distributed Acoustic Sensing (DAS)

DAS is a fiber-optic sensing technology that transforms standard optical fibers into dense arrays of virtual microphones. It operates by launching coherent laser pulses into the fiber and analyzing the Rayleigh backscattered light. Acoustic or vibrational disturbances along the fiber cause minute changes in the backscattered signal's phase and intensity, which are detected and localized in real-time.

DAS systems can achieve spatial resolutions on the order of meters over tens of kilometers, enabling continuous monitoring without the need for discrete sensors.

Key Features:

• Uses standard single-mode optical fiber
• High spatial and temporal resolution
• Immune to electromagnetic interference
• Enables real-time detection of vibrations, sound waves, and dynamic strain

Application examples:

• Pipeline leak and intrusion detection
• Perimeter and border security
• Seismic and geophysical monitoring
• Railway and traffic surveillance

Distributed Temperature Sensing (DTS)

DTS enables continuous temperature measurement along the entire length of an optical fiber. It operates by sending laser pulses through the fiber and analyzing the Raman backscattered light, which includes both Stokes and anti-Stokes components. The intensity ratio of these components is temperature-dependent, allowing precise temperature profiling at every point along the fiber.

DTS systems can monitor temperature over distances of up to 30 - 70 km with spatial resolutions as fine as 1 meter and temporal resolutions of seconds to minutes.

Key Features:

• Uses standard multi-mode or single-mode optical fiber
• Accurate, real-time temperature profiling
• Immune to electromagnetic interference
• Requires no active electronics in the sensing area

Application examples:

• Fire detection in tunnels and cable trays
• Temperature monitoring in oil and gas wells
• Power cable and transformer monitoring
• Environmental and structural health monitoring

Distributed Strain Sensing (DSS)

DSS enables continuous measurement of strain along the length of an optical fiber. It typically relies on Brillouin scattering, where the frequency shift of the backscattered light is sensitive to both strain and temperature. By analyzing this shift, DSS systems can determine localized strain variations with high spatial resolution.

DSS is capable of monitoring structural deformation over long distances - often tens of kilometers - making it ideal for infrastructure and geotechnical applications.

Key Features:

• Measures both static and dynamic strain
• Uses standard single-mode optical fiber
• High spatial resolution (typically 1 m or better)
• Immune to electromagnetic interference

Application examples:

• Structural health monitoring (bridges, tunnels, dams)
• Ground movement and landslide detection
• Pipeline and embankment monitoring
• Civil and geotechnical engineering​
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• High sampling rate – often in the range 1 to 10 GSPS – is critical for accurate acquisition of high-bandwidth signals and allows for as little as 1 cm spatial sampling step between sensing points along the fiber​
  
• 12- to 14-bit resolution is crucial for ensuring high dynamic range in order to detect weak backscatter signals over long distances
• Multi-channel digitizers support multiple sensing channels and/or polarization states
• Precise triggering and synchronization with the laser source are important for accurate time-of-flight determination
• Real-time onboard FPGA pre-processing helps reduce host PC processing load and minimizes latency as well as dead-time between consecutive measurements
• High-speed data transfers at up to 14 Gbyte/s enable cost-effective GPU post-processing
• Large onboard memory enables recording of long-duration waveforms or high-speed bursts
• Flexible choice of form factors such as PCIe, PXIe, etc. help adjust to environmental conditions​