DTSS-BOTDR (Distributed Temperature and Strain Sensing) is built on Brillouin optical time-domain reflectometry. With a single fiber it measures both strain and temperature at every point along the line — ±5 με strain accuracy, ±0.5 °C temperature accuracy, and a measurement time as short as 1 second. Because it connects to only one end of the fiber, it has become the preferred solution for structural health monitoring of tunnels, bridges and other large structures. This article summarizes the engineering insights of the LandSub Global technical team on DTSS-BOTDR fundamentals.
Brillouin Scattering: A "Conversation" Between Light and Acoustic Phonons
It all starts with Brillouin scattering. Alongside Rayleigh and Raman scattering, it is one of the three major scattering effects in fiber sensing.
The physical picture of Brillouin scattering is fascinating: the atoms in an optical fiber are not still — they are constantly in thermal vibration. This collective vibration manifests in the medium as a periodic acoustic wave, physically known as an acoustic phonon. As the acoustic wave propagates, it squeezes the fiber's refractive index into alternating dense and sparse periodic fringes, forming an equivalent "moving grating." When incident light strikes this moving grating, some photons get "rubbed" by it and produce frequency-shifted scattered light — this is Brillouin scattering.
The key lies in the amount of frequency shift. This shift (the Brillouin frequency shift, on the order of ~11 GHz) is proportional to the acoustic wave velocity, which in turn is determined by the temperature and strain state of the fiber material. Physics thus gives a direct correspondence:
When the strain at a point on the fiber changes, its Brillouin frequency shift changes; when the temperature changes, the shift changes too. Measure the shift point by point along the fiber, and you obtain the strain and temperature distribution of the entire line.
Two things become possible — and difficult — as a result. Possible: a single fiber tens of kilometers long, with every point along it turned into a strain-and-temperature sensor — this is exactly distributed temperature and strain sensing, the core requirement of structural health monitoring. Difficult: the frequency shift is governed by two quantities, temperature and strain, at the same time. How to separate them? A dedicated section below explains the decoupling methods.
BOTDR vs. BOTDA: The Engineering Trade-off Between Single-Ended and Double-Ended
Brillouin distributed sensing has two technical routes: BOTDR (Brillouin Optical Time Domain Reflectometry) and BOTDA (Brillouin Optical Time Domain Analysis). LandSub Global's DTSS products take the former. Both measure via Brillouin scattering, but the implementations differ greatly, and the engineering consequences are entirely different.
BOTDA requires injecting two beams from both ends of the fiber simultaneously — a pump light and a probe light — which interact along the line, and strain and temperature are recovered by analyzing the frequency-domain features of the stimulated Brillouin gain. Its signal-to-noise ratio is high and its measurement is fast, at the cost of requiring equipment access at both ends of the fiber.
BOTDR is a single-ended scheme: it injects pulsed light from only one end and detects spontaneous Brillouin scattering. It sacrifices some signal-to-noise ratio in exchange for a major engineering convenience: the interrogator only needs to connect to one end of the cable — nothing is required at the other end.
Why does this difference matter so much in engineering? Two practical reasons. First, for many existing cables awaiting monitoring, the other end simply has no space, power, or communication conditions — a double-ended scheme is out of the question. Second, and more decisive: a BOTDA link depends on synchronization between the two ends. Once the cable is cut somewhere in the middle, the whole double-ended system goes down until the repair is complete. BOTDR, by contrast, measures from one end — signal is lost beyond the break point, but everything before it (usually the stretch from the equipment room to the break) remains usable. For tunnels, slopes and other sites where accidental damage can happen at any time, BOTDR's robustness is far more reassuring.
Our judgment is clear: in structural health monitoring, single-ended BOTDR may have a lower signal-to-noise ceiling than BOTDA, but its deployment flexibility and fault tolerance make it a better fit for real engineering environments. This is also why LandSub Global productized the BOTDR route and deployed it in large-scale tunnel projects.
How DTSS-BOTDR Measures Strain and Temperature Together: Decoupling Is the Key
As mentioned earlier, the Brillouin frequency shift is a "mixed signal" of two quantities: temperature and strain. To output both parameters simultaneously, you must first decouple them. Three methods are common in engineering, and real projects often combine them.
Method 1: a temperature reference channel. Reserve a loose, unstrained fiber in the cable that senses temperature only and not strain, serving as the temperature reference. Subtract the temperature contribution using the reference channel's frequency shift, then back out the strain. This is the most common and most reliable approach.
Method 2: independent two-parameter calibration. Calibrate fiber samples in two dimensions — temperature and strain — in the lab to build a complete "frequency-shift–temperature–strain" mapping table. After measuring the frequency shift in the field, look up the solution in the table using the known initial temperature or strain boundary conditions.
Method 3: signal-characteristic separation. In engineering, temperature change is usually slow (hour-scale), while strain change is relatively faster. Applying frequency separation to the frequency-shift time series can also partially split the two quantities. This method serves as a supplement, covering the blind spots of the first two.
Each method has its place. LandSub Global builds this decoupling flow into the standard processing chain of the LS-DTSS series, letting users choose according to their engineering conditions. The accuracy of decoupling directly determines whether "simultaneous strain and temperature measurement" is a slogan or a real capability.
Reading the Key Specs: ±5 με, ±0.5 °C and 1-Second Measurement Time
When selecting a BOTDR system, three numbers deserve close scrutiny. Compared with the industry average, the gap is direct:
| Specification | Market average | LandSub Global DTSS-BOTDR |
|---|---|---|
| Strain measurement accuracy | ±10–±20 με | ±5 με |
| Measurement time | ≥20 s | ≤1 s |
Let's interpret each number.
Strain accuracy ±5 με. με is microstrain — one part per million. The industry average for strain accuracy is ±10–±20 με; LandSub Global reaches ±5 με, twice as good. For concrete structures, 5 με corresponds to the early stage of crack initiation, completely invisible to the naked eye. Doubling the accuracy means the warning window advances by an order of magnitude of deformation.
Temperature accuracy ±0.5 °C. Strain and temperature are coupled; poor temperature accuracy inflates the strain error after decoupling. ±0.5 °C temperature capability guarantees the credibility of the strain data. The sensing distance reaches 30 km under single-mode fiber — long-distance scenarios (such as highway tunnel clusters and river-crossing bridges) still hold these two accuracy specs, a result of hardware chain and algorithm working together.
Measurement time ≤1 s. A measurement cycle is the whole process from one pulse emission to outputting the strain-and-temperature profile of the entire fiber. The common industry level is 20 seconds or more; LandSub Global compresses it to within 1 second. For static structural monitoring, the difference between 1 and 20 seconds may seem minor — but in landslide monitoring, where fast deformation must be captured, a shorter cycle means catching abrupt changes earlier.
Compared with Conventional Strain Gauges: A Point vs. a Line
The most traditional tool in structural monitoring is the resistive strain gauge, mature after decades of use. But it has an inescapable shortcoming: one gauge measures one point. The box girder of a large bridge has tens of thousands of critical locations; covering them all with gauges is unrealistic, yet deploying only at a few cross-sections risks missing where the real problem is.
The strength of distributed sensing is exactly coverage. BOTDR turns the entire length of the fiber into strain-and-temperature sensors — equivalent to tens of thousands of "strain gauges" laid continuously over tens of kilometers, all read simultaneously. This brings two practical gains: first, the probability of missed detection drops sharply; second, once an anomaly is found, it can be localized to within a few tens of centimeters, making re-inspection highly efficient.
The cost must also be stated clearly. Strain gauges still hold an advantage in dynamic measurements requiring very high sampling frequency (such as impact and high-frequency fatigue), and their single-point precision is higher. So our engineering advice is: use DTSS-BOTDR for large-scale surveys, and point sensors for verification at critical locations — the two complement each other rather than replace each other.
Applications: How Tunnels, Bridges and Slopes Are Guarded
Tunnel structural health monitoring. Strain anomalies caused by lining deformation, surrounding-rock pressure and water leakage all leave their traces on the fiber. A structural health monitoring project on a highway tunnel with one of the largest cross-sections built at the time is a representative application of the DTSS series in large-scale tunnel projects. The complex utility environment inside a tunnel happens to be a showcase for the advantages of single-ended deployment.
Bridge structural health monitoring. Girder deflection, bearing displacement and pier settlement — the tiny deformations accumulated over the long term are exactly what BOTDR excels at capturing. Monitoring data can connect to bridge management systems, turning "periodic checkups" into "real-time monitoring."
Slope and geohazard monitoring. Slope instability is usually preceded by a sustained slow-deformation phase, and ±5 με accuracy is enough to capture such precursors. Fiber laid along the slope face or lattice beams gives early warning once the deformation trend turns abnormal, buying time for evacuation.
For specific cases and deployment details, see the Case Center and the Structural Health Monitoring solution. For product configuration and detailed specifications of the LS-DTSS series, see the DTSS-BOTDR product page.