Short answer
Distributed Acoustic Sensing (DAS) turns an entire fiber, tens of kilometres long, into a continuous vibration sensor. Fiber Bragg Grating (FBG) sensing uses gratings inscribed at specific points in the fiber to measure strain and temperature with high precision. In short, DAS is for event detection along long routes, FBG for precise point measurement. DuyuSense develops both and combines them per project.
How does DAS work?
DAS works by launching short laser pulses into a fiber and analysing the light scattered back towards the source. The technique is called phase-sensitive optical time-domain reflectometry (Φ-OTDR), and the signal comes from Rayleigh backscatter produced by microscopic inhomogeneities in the glass.
A vibration or acoustic wave near the fiber stretches it on a nanometre scale and shifts the phase of the light returning from that location. The round-trip time gives the position of the event, while the phase change gives its amplitude and frequency. A single interrogator therefore listens to thousands of virtual microphones along the route at the same time.
- Measurand: dynamic strain, vibration and acoustic signals
- Sensor: the standard single-mode fiber itself, no added elements
- Location: calculated from the pulse time of flight
How far can DAS measure, and what is spatial resolution?
A single-channel DAS system typically reaches tens of kilometres; the figure most often cited in the industry is around 40-50 km. Range is limited by fiber attenuation, the number of splices and connectors, the required sampling rate and the signal-to-noise ratio (SNR).
Spatial resolution is the shortest length of fiber over which the system can separate two nearby events, typically a few metres. Gauge length is the length of the fiber segment over which phase is compared: a short gauge length sharpens localisation, a long one raises SNR. Signal processing can shift this trade-off considerably, and DuyuSense's patented DAS signal-processing method is aimed precisely at improving spatial resolution and SNR, suppressing noise and increasing event-detection accuracy.
How does an FBG sensor work?
An FBG is made by inscribing a periodic refractive-index pattern, a few millimetres long, into the fiber core. The grating reflects a single wavelength of broadband light, the Bragg wavelength, and lets the rest of the light travel on.
When the grating is stretched or heated, its period and refractive index change and the reflected wavelength shifts. Near 1550 nm this shift is typically about 1.2 pm per microstrain and about 10 pm per °C. The interrogator tracks the shift and resolves strain at the microstrain level. A strain-free reference FBG is commonly used to separate temperature from strain.
- Measurands: static and dynamic strain, temperature; with suitable packaging, pressure, acceleration, tilt and displacement
- Typical strain resolution: around 1 microstrain class
- No electrical signal at the sensor; immune to electromagnetic interference
How many FBGs can be placed on one fiber?
A single fiber typically carries a few dozen FBGs in series. This is made possible by wavelength-division multiplexing (WDM): each grating is assigned its own wavelength window within the optical band of the source, and the interrogator reads all reflections simultaneously.
The exact count depends on the source bandwidth, the largest expected strain and temperature excursion, and the guard band left between gratings. When more points are needed, multi-channel interrogators or time-division multiplexing (TDM) extend capacity. DuyuSense's patented FBG interrogation system is designed to read many sensors in real time, at high speed and in harsh field conditions.
What is the difference between DAS, DTS and DSS (Brillouin)?
All three are distributed sensing technologies, but they rely on different scattering mechanisms and measure different quantities. DAS uses Rayleigh scattering to capture fast-changing vibration and acoustic signals, while DTS and DSS map slowly changing temperature and strain profiles.
DTS (Distributed Temperature Sensing) is usually based on Raman scattering and responds to temperature only. DSS (Distributed Strain Sensing) mostly uses Brillouin scattering, whose frequency shift depends on both strain and temperature. BOTDR measures spontaneous scattering from one end of the fiber; BOTDA uses stimulated scattering for a stronger signal but needs access to both fiber ends.
- DAS: dynamic events (Hz to kHz), event detection and classification
- DTS: temperature profiles, fire and leak detection, power cable monitoring
- DSS/Brillouin: static strain profiles, ground movement and structural deformation
Can existing telecom fiber be used for sensing?
Yes, for DAS a standard single-mode telecom fiber can in most cases be used directly; one spare (dark) fiber is enough. Cables already laid along railways, pipelines and highways can become a sensing network without new trenching.
Performance depends on how well the fiber is mechanically coupled to the ground or structure, the cable type, splice losses and the accuracy of route records. Field fiber characterisation and location calibration are therefore recommended. FBG sensing requires fiber with inscribed gratings; existing telecom fiber does not act as an FBG sensor, although it can carry FBG signals back to the interrogator.
When should you choose DAS and when FBG?
Choose DAS when you need to know where and when something happens along kilometres of route, and FBG when you need an exact, calibrated strain or temperature value at defined points. In many projects the two are complementary: DAS detects and locates the event, FBG delivers quantitative measurement at critical points.
- DAS: railway train tracking and track events, pipeline third-party intrusion detection, perimeter security, seismic and downhole applications
- FBG: Structural Health Monitoring (SHM) of bridges, tunnels and buildings, rail strain and axle counting, power equipment temperature, composite structures
- DTS/Brillouin: temperature profiles along long routes, ground settlement and landslide monitoring
| Property | DAS | FBG | DTS / Brillouin (DSS) |
|---|---|---|---|
| Measurand | Dynamic strain, vibration, acoustics | Strain and temperature (pressure, acceleration, tilt with packaging) | DTS: temperature; Brillouin: strain and temperature |
| Sensing type | Fully distributed | Point or quasi-distributed (gratings in series) | Fully distributed |
| Physical principle | Rayleigh backscatter, Φ-OTDR | Bragg reflection, wavelength shift | Raman (DTS) or Brillouin (BOTDR/BOTDA) scattering |
| Typical range | Tens of km, often cited 40-50 km | Set by the sensor array; sensors can sit km away from the interrogator | Typically a few km to tens of km |
| Spatial resolution | Typically a few metres | Grating length (mm scale), only at sensor points | Typically about 1 m to a few metres |
| Measurement rate | High (Hz to kHz) | High (interrogator dependent, up to kHz class) | Low (seconds to minutes) |
| Typical applications | Railways, pipelines, perimeter security, seismic | Structural Health Monitoring (SHM), rail strain, power equipment | Power cables, fire and leak detection, ground movement |
Frequently asked questions
What is the main difference between DAS and FBG?
DAS uses the entire fiber as a continuous vibration sensor and locates events along tens of kilometres of route. FBG uses gratings inscribed in the fiber to measure strain and temperature precisely at specific points. DAS provides coverage, FBG provides quantitative accuracy, and many projects use both together.
What physical principle is DAS based on?
DAS is based on phase-sensitive optical time-domain reflectometry (Φ-OTDR). It measures phase changes in the Rayleigh backscatter of laser pulses sent into the fiber. The round-trip time gives the event location, and the phase change gives the amplitude and frequency of the vibration.
How many kilometres can DAS monitor?
A single-channel DAS interrogator typically monitors tens of kilometres of fiber, with around 40-50 km the figure most often cited in the industry. Actual range depends on fiber loss, splice count, sampling rate and the required signal-to-noise ratio. Longer routes are covered with multiple interrogators.
What is gauge length and why does it matter?
Gauge length is the length of fiber over which DAS compares optical phase, typically a few metres. A shorter gauge length helps separate nearby events, while a longer one improves signal-to-noise ratio. The right value is chosen according to the wavelength of the monitored signal and the application.
How sensitive is an FBG sensor to strain?
Near 1550 nm, the Bragg wavelength of an FBG typically shifts by about 1.2 pm per microstrain and about 10 pm per °C. With a good interrogator, strain can be resolved at around the 1 microstrain class. Temperature effects are separated using a strain-free reference FBG.
How many FBG sensors can one fiber carry?
With wavelength-division multiplexing (WDM), one fiber typically carries a few dozen FBGs in series, each assigned its own wavelength window. The exact number depends on source bandwidth and the expected measurement range. Multi-channel interrogators and time-division multiplexing increase capacity further.
Can existing telecom fiber be used for DAS?
Yes, standard single-mode telecom fiber can in most cases be used directly for DAS; a single spare fiber is enough. Performance varies with how well the cable is coupled to the ground and with splice losses. FBG sensing needs fiber with inscribed gratings, so existing fiber cannot act as an FBG sensor.
Should I choose DAS, DTS or Brillouin sensing?
Choose DAS for vibration, movement or acoustic event detection, Raman-based DTS when only a temperature profile is needed, and Brillouin-based BOTDA or BOTDR for static strain and temperature along long routes. Where high precision is needed at defined points, FBG is the preferred option.