--- title: Fiber Optic Sensing Glossary: DAS, FBG, OTDR | DuyuSense url: https://www.duyusense.com/en/guides/fiber-optic-sensing-glossary --- [Home](https://www.duyusense.com/en/) / [Applications](https://www.duyusense.com/en/applications) / Fiber Optic Sensing Glossary: DAS, FBG, OTDR Technical Glossary # What do fiber optic sensing terms mean? A glossary Short answer A fiber optic sensing glossary defines, in one place, how light scatters in a fiber (Rayleigh, Raman, Brillouin), the techniques that read that scatter (OTDR, phi-OTDR, OFDR, BOTDA), the sensor families (DAS, DTS, FBG) and the performance metrics (gauge length, spatial resolution, SNR). DuyuSense uses these terms as a shared language with engineering teams when it develops FBG and DAS systems. Last updated: 2026-09-30 · Prepared by: Dr. Serhat Boynukalın, DuyuSense ## What are the scattering types: Rayleigh, Raman, Brillouin? Fiber optic sensing measures light that scatters inside the glass and travels back to the source. The type of scattering decides which quantity (vibration, temperature or strain) can be read. - **Backscatter**: The small fraction of scattered light that travels back towards the source. Every distributed sensing method works by analysing this signal. - **Rayleigh scattering**: Elastic scattering from microscopic density variations in the glass, with no change in wavelength. It is the strongest backscatter component and the basis of DAS and OFDR. - **Raman scattering**: Inelastic scattering in which light interacts with glass molecules and splits into Stokes and anti-Stokes components. The anti-Stokes intensity depends on temperature, which is why DTS systems use it. - **Brillouin scattering**: Scattering in which light interacts with acoustic waves (phonons) in the glass and shifts in frequency. The shift, about 11 GHz near 1550 nm, changes with both temperature and strain and underpins DSS and DSTS. ## Which techniques read the signal: OTDR, phi-OTDR, OFDR, BOTDA? Time-domain methods (the OTDR family) or frequency-domain methods (OFDR) map backscattered light to a position on the fiber. Brillouin techniques can work from one end or from both ends of the fiber. - **OTDR (Optical Time-Domain Reflectometry)**: The basic method of sending a pulse into a fiber and calculating position from the arrival time of the returning light. Telecom teams use it to locate breaks, splices and loss points. - **Phi-OTDR (phase-sensitive OTDR)**: A technique that uses a narrow-linewidth, coherent laser to track changes in the phase or intensity of Rayleigh backscatter. It is the physical basis of DAS. - **C-OTDR (coherent OTDR)**: An OTDR variant that mixes the backscattered light with a local reference laser (coherent detection). It improves sensitivity and is common in long-range DAS. - **OFDR (Optical Frequency-Domain Reflectometry)**: A method that resolves Rayleigh backscatter in the frequency domain using a swept-wavelength laser. It delivers sub-millimetre spatial resolution, but its range is typically tens to a few hundred metres. - **BOTDA (Brillouin Optical Time-Domain Analysis)**: A technique that injects pump and probe light from both ends of the fiber to measure stimulated Brillouin scattering. It is highly accurate but needs loop access to the fiber. - **BOTDR (Brillouin Optical Time-Domain Reflectometry)**: A technique that measures spontaneous Brillouin scattering from a single end. It is simpler to install, but the weaker signal means longer measurement times than BOTDA. ## What are the distributed sensing types: DAS, DTS, DSS, DVS, DSTS? Distributed sensing types are named after the physical quantity they measure. Quasi-distributed sensing sits between point sensors and fully continuous measurement. - **DAS (Distributed Acoustic Sensing)**: Turns a fiber into thousands of virtual microphones and measures vibration and acoustic signals as dynamic strain. It runs on Rayleigh-based phi-OTDR. - **DVS (Distributed Vibration Sensing)**: Systems that detect the presence and position of vibration, usually from intensity alone. Usage varies by vendor, but the term generally refers to simpler DAS-like systems without phase information. - **DTS (Distributed Temperature Sensing)**: Derives a temperature profile along the fiber from the ratio of Stokes to anti-Stokes Raman light. Fire detection, power cables and wells are typical uses. - **DSS (Distributed Strain Sensing)**: Measures static and slowly changing strain from the Brillouin frequency shift or from Rayleigh-based methods. It suits landslide, dam and pipeline deformation monitoring. - **DSTS (Distributed Strain and Temperature Sensing)**: Refers to Brillouin techniques measuring temperature and strain together. A second, strain-free fiber is usually added to separate the two effects. - **Quasi-distributed sensing**: Measurement with many discrete sensors placed along one fiber, typically an FBG array. It gives high-accuracy data at defined points rather than a continuous profile. ## What do FBG terms mean: Bragg wavelength, FWHM, apodization? A Fiber Bragg Grating (FBG) is a periodic change in refractive index written into the fiber core, and it reflects a single wavelength. The terms below describe how an FBG is written, what its spectrum looks like and how many can share a fiber. - **Bragg wavelength**: The centre wavelength an FBG reflects, given by λB = 2·neff·Λ. Near 1550 nm it shifts by about 1.2 pm/µε with strain and about 10 pm/°C with temperature. - **Grating period (Λ)**: The spacing at which the refractive index change repeats, around 0.5 µm for the 1550 nm band. It directly sets the Bragg wavelength. - **Reflectivity**: The share of light at the centre wavelength that the FBG reflects. WDM arrays favour high reflectivity, while TDM arrays use very low reflectivity (below one percent). - **FWHM (full width at half maximum)**: The spectral width of the reflection peak at half its maximum value. A narrow FWHM lets the interrogator locate the peak more precisely. - **Apodization**: Gradually tapering the amplitude of the index modulation along the grating. It suppresses side lobes and reduces crosstalk between neighbouring FBGs. - **Draw-tower grating (DTG)**: An FBG written while the fiber is being drawn, before the coating is applied. Because the coating is never stripped, it has high mechanical strength and suits long arrays. - **Femtosecond grating**: An FBG written with an ultrashort-pulse laser. Some types can be written through the coating and remain stable at high temperatures. - **WDM (Wavelength-Division Multiplexing)**: Reading many sensors on one fiber by giving each FBG its own wavelength band. The sensor count is limited by the interrogator's spectral bandwidth. - **TDM (Time-Division Multiplexing)**: Separating FBGs of the same or similar wavelength by the arrival time of their reflections. With low-reflectivity gratings, hundreds of sensors can be read on a single fiber. ## What are the performance terms: gauge length, spatial resolution, SNR? Performance terms define what an interrogator measures, how often and in how much detail. When comparing quotes and specifications, check that each term is used with the same definition. - **Interrogator**: The instrument that sends light into the fiber, reads the returning signal and converts it into measurement data. DAS, DTS and FBG each use a different type of interrogator. - **Gauge length**: The length of fiber over which DAS calculates the phase difference. A longer gauge length raises SNR, while a shorter one separates smaller events. - **Spatial resolution**: The shortest distance between two events that can be detected separately along the fiber. For DAS it is typically a few metres and depends on pulse width and gauge length. - **Sampling resolution**: The distance between consecutive measurement points. It can be finer than the spatial resolution, in which case neighbouring channels are not independent. - **Channel**: Each measurement point for which the interrogator produces data along the fiber. A 40 km route sampled every 1 m yields about 40,000 channels. - **SNR (Signal-to-Noise Ratio)**: The ratio of the measured signal to the noise floor, usually given in dB. SNR falls with distance as the backscatter weakens. - **Microstrain (µε)**: One millionth of strain, equal to 1 µm of elongation per metre. Typical FBG strain resolution is in the 1 µε class. - **Strain rate**: The change of strain over time (µε/s or nε/s). Many DAS systems output the phase change directly as strain rate. - **Dynamic range**: The ratio between the largest and smallest signals a system can measure without distortion. It is used both for signal amplitude (dB) and for optical budget (dB of loss). - **Fading**: Deep drops in signal at some positions caused by random interference of Rayleigh backscatter. DAS readings become unreliable there; multi-wavelength interrogation and signal processing reduce the effect. - **Pulse repetition rate**: The number of pulses launched into the fiber per second. Each pulse must travel to the fiber end and back, so fiber length limits the rate (roughly 2 kHz for 50 km) and sets the highest measurable frequency. ## What are the cable and installation terms: tight-buffered, loose-tube, coupling? Cable design and installation quality directly shape the signal a sensor records. The same interrogator can give very different results under different cable and coupling conditions. - **Tight-buffered cable**: A cable in which the fiber is tightly embedded in a buffer layer. It transfers strain well, so DAS and DSS projects favour it. - **Loose-tube cable**: A cable in which the fiber lies free inside a gel-filled tube. It isolates the fiber from strain, which is why telecom and DTS use it widely. - **Engineered fiber**: Fiber with deliberately enhanced Rayleigh backscatter or added point reflectors. In DAS it raises SNR and reduces fading. - **Coupling**: How well the fiber is mechanically bonded to the structure or ground being measured. Poor coupling weakens the signal even with the best interrogator. - **Splice**: A permanent fusion joint between two fibers. Each splice adds a small optical loss and a possible reflection point. - **APC connector**: A connector whose end face is polished at about 8 degrees. It keeps back reflection very low, so it is the standard choice for coherent DAS systems. **Core fiber optic sensing terms and units** | Term | What it measures / means | Typical unit | |---|---|---| | Microstrain | Elongation per unit length | µε | | Bragg wavelength | Centre wavelength reflected by an FBG | nm | | Brillouin frequency shift | Frequency shift that depends on temperature and strain | GHz (changes in MHz) | | Gauge length | Fiber length over which DAS computes phase difference | m | | Spatial resolution | Shortest distance between two separately detected events | m | | Strain rate | Change of strain over time | µε/s or nε/s | | SNR | Ratio of signal to noise floor | dB | ## Frequently asked questions ### What is the difference between spatial resolution and sampling resolution? Spatial resolution is the shortest distance between two events that can be detected separately along the fiber. Sampling resolution is the spacing between consecutive data points. A DAS system may output data every metre while its spatial resolution is a few metres, in which case neighbouring channels are not independent measurements. ### Are phi-OTDR and C-OTDR the same thing? Not exactly. Phi-OTDR is the general name for tracking phase or intensity changes in Rayleigh backscatter. C-OTDR describes the coherent detection approach, where the returning light is mixed with a reference laser. Many modern DAS systems extract phase through coherent detection, so the two terms often appear together. ### When is OFDR used? OFDR is used for short-range measurements that need sub-millimetre spatial resolution. Typical applications include composite part testing, laboratory measurements, shape sensing and characterising photonic components. Because its range is usually tens to a few hundred metres, DAS or Brillouin methods are preferred for routes several kilometres long. ### Can DAS and DTS run on the same fiber? Yes, in most cases, either on separate fibers in the same cable or on one fiber with suitable wavelength separation. DAS reads Rayleigh scattering and DTS reads Raman scattering. Cable choice matters: DAS benefits from tight-buffered designs that transfer strain, while DTS benefits from loose-tube designs that isolate the fiber. ### What causes fading in DAS? Fading happens when light returning from many scattering points in the fiber interferes randomly, so the signal drops close to zero at some positions. Phase readings at those points become unreliable. Multi-wavelength interrogation, engineered fiber and advanced signal processing reduce the impact of fading. ### Why is apodization needed in an FBG? Apodization suppresses the side lobes in an FBG spectrum. Side lobes can overlap with reflections from neighbouring FBGs on the same fiber and cause the peak position to be misread. Apodized gratings help dense WDM arrays use tighter channel spacing while keeping measurements accurate. ## Related solutions [### What is the difference between DAS and FBG?What is the difference between DAS and FBG? Phase-OTDR, Bragg wavelength, spatial resolution, range, WDM multiplexing and DTS/Brillouin compared in one guide.Read the guide](https://www.duyusense.com/en/guides/das-vs-fbg) [### How is fiber optic sensing used for landslide monitoring?How DAS, FBG and Brillouin strain sensing monitor landslides, rockfall, dams, tunnels and tailings dams, with early warning thresholds and a sensor comparison.Read the guide](https://www.duyusense.com/en/solutions/landslide-geohazard-monitoring) [### How are wind turbine blade loads measured with fiber optic FBG?FBG blade load and root bending moment sensing, tower and monopile SHM, and DAS/DTS offshore export cable monitoring for wind turbines. 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DuyuSense railway R&D and signaling advice.Read the guide](https://www.duyusense.com/en/solutions/railway-fiber-optic-monitoring) [### How does subsea fiber-optic sensing with DAS work?How subsea fiber-optic DAS detects anchor drag and fishing-gear strikes on power and telecom cables, and divers, ROVs and small vessels near ports and harbours.Read the guide](https://www.duyusense.com/en/solutions/subsea-fiber-optic-sensing) [### How does fiber optic earthquake early warning work?How DAS turns existing telecom fiber into a dense seismic array and FBG sensors measure ground acceleration: P-wave detection, early-warning uses, limits.Read the guide](https://www.duyusense.com/en/solutions/earthquake-early-warning-fiber-optic) ## Ready for fiber sensor solutions? Our technology tracks heat, bending and strain across railways, borders, building integrity and aviation — and our team is ready for B2B collaboration and project partnerships. 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