OFDR uses a swept optical frequency and coherent analysis of Rayleigh backscatter. It offers dense position information over shorter lengths than route-scale systems, but product specifications must distinguish gage pitch, sampling interval, spatial resolution, measurement resolution and accuracy.
Route-scale sensing versus OFDR
Pulse-based OTDR and phase-OTDR infer position from propagation time and are optimised for long routes. OFDR sweeps optical frequency and analyses coherent beat information, optimising measurement density over shorter lengths. Other long-range systems also use frequency-domain methods, including Brillouin BOFDA/BOFDR, so the purchasing decision is based on the measured quantity and performance trade-off rather than the domain name alone.
How OFDR turns frequency into distance
Unlike pulsed OTDR, OFDR does not directly measure the time of flight of an individual pulse. The laser sweeps its frequency upward continuously (a chirp, at a constant rate γ). The returning field is delayed by propagation, and that delay appears as a beat-frequency component when it is mixed with the current launched field. The beat frequency is proportional to distance. Move the reflector and watch:
The animation uses an illustrative sweep rate, not an ODiSI product specification. The ranging concept is analogous to FMCW radar: a propagation delay becomes a beat-frequency component. Practical OFDR instruments correct sweep non-linearity and other non-ideal effects.
fbeat = γ · 2nL / c — the beat frequency rises
linearly with distance L (γ — sweep rate, n — refractive index of the fiber, c — speed
of light; the factor 2 because the light travels out and back).What sweep bandwidth determines — and what it does not
In the ideal ranging model, wider optical sweep bandwidth improves distance resolution:
Δz ≈ c / (2nΔν) — a useful theoretical relationship, not a complete product-accuracy specification.Practical spatial resolution and measurement performance also depend on sweep linearity and calibration, windowing, coherence, SNR and the correlation or sensing algorithm. Range, density and update rate must therefore be selected from an actual product mode rather than combined from separate maxima.
ODiSI: the fingerprint of a fiber
ODiSI compares the current Rayleigh spectral fingerprint of the sensing fiber with a stored reference and converts the local spectral shift into strain or temperature. The ODiSI 7100 separately specifies gage pitch, strain resolution, instrument accuracy and end-to-end system accuracy.
Dense gage spacing can reveal local strain gradients that a coarser measurement averages. The result still depends on feature width, strain transfer, selected gage pitch, fiber attachment, geometry and SNR.
Illustrative comparison. A coarser measurement can understate a narrow local strain maximum. The difference depends on feature width, strain transfer, selected gage pitch, sensing gauge definition, SNR and attachment quality.
Where high-density sensing creates value
Composites and aerospace
Dense strain maps for test evidence, FEA correlation and design verification, including local concentrations and strain anomalies associated with damage.
Structures and concrete
Local strain profiles and model calibration. A measured strain concentration can indicate cracking or another discontinuity, subject to verified strain transfer and engineering interpretation.
Batteries and energy
Research and validation of temperature or strain distributions where compatible sensing fiber can be integrated into the cell, module or test article.
Materials and additive manufacturing
Residual-strain measurement, warping and curing studies. Mechanical stress requires a validated material and structural model; it is not measured directly.
Precision components and devices
Dense strain or temperature measurements where electrical sensors are impractical; shape measurement requires a suitable sensor geometry and algorithm.
Fiber and component characterisation
OBR reflectometry for loss, reflectance and event analysis with no additional recovery zone beyond its two-point sampling interval.
OFDR at INTERLAB
For new high-definition strain and temperature projects, INTERLAB supplies the current Luna ODiSI 7100. Its modes address three different purchasing priorities:
INTERLAB × Luna Innovations: working together since 2010 — long-term product continuity for selecting current fiber-sensing platforms against real measurement requirements.
ODiSI 7100 supports up to eight channels. Aggregate measurement rate is divided by the number of active channels. Strain resolution is 0.1 µε, instrument accuracy ±1 µε, and system accuracy ±25 µε at 0.65 mm or ±30 µε at other pitches.
OBR 4600 reflectometry provides 10 µm sampling over 30 m, 20 µm over 70 m, or 1 mm in the 2 km extended-reflectometry mode. Its distributed strain/temperature option is typically specified up to 70 m and has separate sensing-resolution specifications. “Zero Dead Zone” means no additional recovery zone beyond the two-point sampling interval.
* Manufacturer maxima refer to specific products, operating modes and channel counts; they are not necessarily available simultaneously. Coverage, spatial granularity, measurement rate and uncertainty are confirmed for the selected configuration and application conditions.
Gage pitch, sampling interval, gauge length, spatial resolution and accuracy are different quantities. The OBR 2 km value applies to extended reflectometry, not 2 km distributed strain/temperature sensing.
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Frequently asked questions
How is OFDR different from DAS if both use Rayleigh scattering?
What lengths do ODiSI 7100 and OBR cover?
Is 0.65 mm the ODiSI measurement accuracy?
What information is needed to select ODiSI or OBR?
Need a dense strain map or high-resolution fiber trace?
Send the specimen or device length, number of channels, target gage pitch or sampling, update rate, strain and temperature range, and planned attachment method. We will identify the compatible ODiSI 7100 or OBR configuration.
Request an ODiSI or OBR configuration →