How DTS Achieves 0.8 m Spatial Resolution: A Deep Dive into Raman-Scattering Temperature Sensing

DTS (Distributed Temperature Sensing) exploits the temperature sensitivity of Raman scattering to turn every meter of a tens-of-kilometers-long fiber into a temperature sensor. Take the LandSub Global DTS as an example: 0.8 m spatial resolution, ±0.5 °C temperature accuracy, and 50 °C/min response sensitivity — several specs ahead of the industry average. This capability moves fire early warning for linear assets such as power cables and utility tunnels from "waiting for a fire" to "watching ahead of time." This article summarizes the engineering insights of the LandSub Global technical team on DTS fundamentals.

The Physics of Raman Scattering: Stokes and Anti-Stokes Light

Let's start with the physics. As light travels through an optical fiber, most photons pass straight through as they would through ordinary glass, but a small fraction exchange energy with the fiber molecules and scatter in other directions. This is Raman scattering.

The direction of that energy exchange decides the fate of the scattered light. When a photon hands part of its energy to molecular vibration, it loses energy and drops in frequency — the scattered light is called Stokes light, with a longer wavelength than the incident light. Conversely, when molecular vibration hands energy to a photon, the photon gains energy and rises in frequency — the scattered light is called anti-Stokes light, with a shorter wavelength.

The behavioral difference between these two components is the key to temperature sensing:

The intensity of anti-Stokes light is highly temperature-sensitive — a small temperature rise makes it noticeably stronger. The intensity of Stokes light barely changes with temperature. This leads to an elegant trick: take the ratio of the two components (anti-Stokes / Stokes). The ratio maps one-to-one to temperature, and "common-mode" interference such as fiber loss and source power fluctuation cancels out naturally in the division. This is the heart of the DTS principle: Raman ratio thermometry.

It is far more reliable than absolute-intensity thermometry. As a laser ages or connector insertion loss changes, the absolute intensity drifts — but the ratio stays rock-steady. This is also the fundamental reason DTS can run for the long term without recalibration.

DTS System Composition: How a Light Pulse Becomes a Temperature Profile

A complete DTS system has three parts: the sensing cable, the interrogator (measurement host), and the host software. The cable "senses," the host "transmits + receives + computes," and the software displays and alarms.

Inside the interrogator, the core chain is:

• Pulse laser — fires nanosecond light pulses into the sensing cable. The narrower the pulse, the better the spatial resolution, but the lower the energy and the shorter the reachable distance — a trade-off that must be balanced.
• Wavelength-division multiplexer (WDM) — separates the returning backscattered light by wavelength, routing Stokes light down one path and anti-Stokes light down another, each into its own detector.
• Photodetector (APD) — converts the two weak optical signals into electrical signals and amplifies them to a level that can be sampled.
• High-speed acquisition and computing module — samples both channels synchronously, converts the time axis to distance (the speed of light in fiber is known: time × speed / 2 = distance), then computes a temperature profile point by point against the Raman-ratio calibration curve.

In every measurement cycle, the system outputs not a single isolated curve but a temperature value for every point along the entire fiber — a full-line scan is typically completed in a few seconds.

Spatial Resolution and Temperature Accuracy: What 0.8 m and ±0.5 °C Mean

Lay the DTS specs on the table, and two numbers reveal a system's true capability. The gap between LandSub Global DTS and the industry average is clear in one table:

Let's take each one apart.

Spatial resolution 0.8 m. Spatial resolution is the minimum distance over which the system can distinguish two adjacent temperature points — the "pixel size" of a temperature camera. The industry average is 1 m; LandSub Global reaches 0.8 m. Don't underestimate that 0.2 m. At failure-prone spots such as cable joints and tray corners, hot spots often sit within a few tens of centimeters. The finer the resolution, the less likely a hot spot gets "smoothed" into the average temperature. 0.8 m means an independent temperature reading roughly every 0.8 m along the line — sharper hot-spot localization, closer to the true peak.

Temperature accuracy ±0.5 °C. The industry is generally ±1 °C; LandSub Global reaches ±0.5 °C. For dynamic rating of cable current-carrying capacity, this half degree is very real: cable load is usually back-calculated from the hot-spot temperature limit (e.g., 90 °C), and the temperature error directly affects the result. The smaller the error, the more accurate the rating and the more fully the cable's potential is used. In a power scenario, this could mean one cable carrying an extra substation's load, or avoiding the purchase of a backup line.

Temperature resolution 0.1 °C. Accuracy and resolution are two different things and are easy to confuse at selection time. Accuracy (±0.5 °C) is the absolute error of the reading — how close the reported temperature is to the true value. Resolution (0.1 °C) is the smallest temperature change the system can distinguish between two adjacent readings — how finely it can see a rising trend. LandSub Global DTS holds both to ±0.5 °C and 0.1 °C respectively. A system with fine resolution but poor accuracy still reads reliably wrong; one with good accuracy but coarse resolution smooths away a slow, small hot-spot rise. For early fire warning, resolution is often the more decisive of the two.

Under multimode fiber the sensing distance is 20 km; under single-mode fiber it reaches 30 km. Long distance and high precision are not mutually exclusive — this depends on the coordination of optical path design and demodulation algorithms.

Response Sensitivity of 50 °C/min: No Fast Temperature Rise Escapes Notice

DTS has another spec that is easy to overlook but literally a matter of life and safety: response sensitivity, i.e., the system's ability to capture the rate of temperature change. The industry average is 30–40 °C/min; LandSub Global reaches 50 °C/min.

Why fuss over this number? Because cable fires are often not gradual heating but sudden events. Overload heating is usually progressive, but once insulation breakdown or short-circuit arcing occurs, the temperature spikes in an extremely short time. A system that can track a 5 °C rise per second and capture a 50 °C rise per minute behaves completely differently from one that can only follow 30 °C/min when facing a fast-developing fire: the former triggers an early warning in the first seconds of the temperature surge, while the latter may have already missed the most critical response window.

DTS alarm logic is therefore usually dual-channel: an absolute temperature threshold handles "overheating," while the rate of temperature rise handles "the spark of a fire." Response sensitivity determines the firepower of the second channel. This is why we treat 50 °C/min as a core selling point of the product — it directly maps to the timeliness of fire early warning.

DTS vs. Point Thermometry and Linear Heat-Sensing Cables

Behind this comparison lies the essential difference between the DTS principle and point-measurement logic. The power industry has historically used two kinds of temperature measurement, each with its own trade-offs versus DTS.

Point thermometry (thermocouples, resistance temperature detectors, infrared guns). High precision, fast response, low cost — but one probe measures one point. A cable trench runs for kilometers; full coverage means deploying thousands of probes, and the cost of power, wiring and maintenance spirals out of control. Worse, wherever no probe is installed remains a blind spot. DTS's value is turning "sampling-point density" from a few per kilometer into more than twelve hundred per kilometer, with the entire line passive and requiring no power.

Linear heat-sensing (heat-sensing) cables. Analog heat-sensing cables along the line can also measure temperature continuously, and cost less than DTS, but they have two clear shortcomings. First, spatial resolution is poor — usually only able to say "some section is overheating," with localization accuracy from meter-level to tens-of-meters. Second, accuracy and response speed are limited, so the localized temperature rise of an early small flame is easily missed. DTS, in contrast, pinpoints to within 0.8 m and delivers complete, analyzable temperature data.

There is also a natural advantage of DTS worth mentioning: the sensing cable carries no current and contains no metal, stays immune in electromagnetic environments, and is intrinsically explosion-proof. It can be laid directly in cable trenches, mine roadways and petrochemical process areas. Installation is flexible too — wrapped around cables, attached inside trays, or embedded in tunnel sidewalls.

Power Cable Fire Early Warning: A Typical Field Scenario

Power cable channels (trenches, tunnels, shafts) are among the highest fire-risk parts of an urban power grid. Cables are dense, heat dissipation is poor, load fluctuates widely, and once a fire starts it spreads extremely fast in a confined space, accompanied by toxic smoke that makes suppression difficult.

The deployment logic of DTS here is clear: lay the sensing cable along the entire cable channel, parallel to or in close contact with the power cables. The system outputs a real-time temperature profile and sets two alarm levels: the first is an absolute-temperature over-limit (overload hazard), the second is a sudden rise in the rate of temperature increase (the spark of a fire). Combined with precise localization, O&M crews can pinpoint the cable well number and joint location where the hot spot sits, shortening the response from "found during an hours-long patrol" to "minute-level localized response."

For deployment details on power cable temperature monitoring, see the Power Cable Temperature Monitoring solution; for product specifications, see the DTS product page.

Fire Protection and Other Applications

Beyond power cables, DTS's typical fire-protection applications include utility tunnels, integrated utility corridors, belt conveyor channels, and temperature monitoring in underground coal mines. These places share common traits: long and narrow, enclosed, and hazardous — conventional sensors are hard to deploy, while a single fiber covers the entire line.

Another trend is "one fiber, multiple uses": a DTS temperature cable often shares different cores of the same cable with a DAS vibration cable, or uses a combined sensing-and-communication cable directly, so one system handles both temperature and vibration. Fire-protection and security needs are met on a single platform, significantly lowering O&M cost.