Distributed temperature sensing (Raman DTS)

Distributed temperature sensing (DTS) turns an ordinary optical fiber into a continuous thermometer. Laser pulses are launched into the fiber, and the spontaneous Raman light scattered back from every point is split into its two sidebands, the Stokes band shifted down in frequency by about 13.2 THz and the anti-Stokes band shifted up by the same amount. The anti-Stokes intensity depends strongly on temperature and the Stokes intensity only weakly, so their ratio gives the local temperature, and the arrival time of the light gives its position, as in optical time-domain reflectometry. A 10 ns pulse resolves about 1 m along the fiber, and a single instrument covers fiber lengths of kilometers to a few tens of kilometers, with temperature resolution of a fraction of a kelvin to about a kelvin after averaging.

Ratio thermometry

The anti-Stokes band comes from scattering off thermally excited vibrations in the glass, whose population follows the Boltzmann factor. With the $\lambda^{-4}$ dependence of scattering included, the ratio of anti-Stokes to Stokes power is

$$R(T) = \left(\frac{\lambda_S}{\lambda_{AS}}\right)^{4}\exp\!\left(-\frac{h\Delta\nu}{kT}\right).$$

For silica, $h\Delta\nu/k$ = 634 K. With a 1550 nm pump the Stokes and anti-Stokes bands sit near 1664 and 1451 nm, and $R$ is 0.199 at 20 °C and 0.316 at 100 °C. The relative sensitivity is

$$\frac{1}{R}\frac{dR}{dT} = \frac{h\Delta\nu}{kT^2},$$

0.74 % per kelvin at 20 °C, falling to 0.45 % per kelvin at 100 °C. Instruments based on a 1064 nm source use bands near 1116 and 1016 nm. Using the ratio cancels launch power, fiber loss common to both bands, and most connector and splice losses, since each affects both wavelengths almost equally.

Locating the measurement

As in OTDR, light returning at time $t$ comes from distance $z = c\,t/(2n_g)$, and a pulse of duration $\tau$ averages over

$$\Delta z = \frac{c\,\tau}{2n_g}.$$

For $\tau$ = 10 ns and a group index of 1.468, $\Delta z$ = 1.02 m; 1 ns would give 10 cm. Shorter pulses carry less energy, and Raman backscatter is roughly three orders of magnitude weaker than Rayleigh backscatter, so the signal is small: DTS instruments average thousands to millions of pulses, and the temperature resolution improves roughly as the square root of the measurement time. A pulse must return from the far end before the next is sent; for 10 km of fiber the round trip is 98 µs, limiting the repetition rate to about 10 kHz. Detectors are usually cooled or high-gain avalanche photodiodes, and optical filters must reject the much stronger Rayleigh light at the pump wavelength.

Graded-index multimode fiber is common for DTS because it captures more backscatter and tolerates higher pulse energy before stimulated Raman scattering distorts the signal; single-mode fiber is used for longer ranges, where its lower modal dispersion preserves spatial resolution.

Calibration and pitfalls

• Differential attenuation: the Stokes and anti-Stokes bands are about 200 nm apart and suffer different fiber attenuation, so the ratio drifts with distance. Instruments correct with a measured or assumed loss difference, or use a double-ended loop that measures from both ends.
• Hydrogen ingress and radiation darken the fiber unevenly with wavelength in harsh environments, such as oil wells, and change the differential loss over time, requiring recalibration.
• Reference sections: a coil held at a known temperature inside the instrument, or a fiber section in a reference bath, anchors the absolute scale.
• Sharp temperature changes shorter than the spatial resolution are averaged and appear smaller than they are.
• Bends and connectors with wavelength-dependent loss create local steps in the apparent temperature.

Applications

DTS monitors temperature along power cables, where it is used to rate cable capacity, along pipelines and in oil and gas wells, in fire detection in tunnels, in dams and in industrial process vessels. It is one of several distributed fiber-optic sensor techniques: Brillouin sensing measures a frequency shift that depends on both strain and temperature, and distributed acoustic sensing uses Rayleigh backscatter phase to detect vibration.

Common questions

Why use the anti-Stokes band and not only the Stokes band?

The Stokes intensity changes little with temperature, while the anti-Stokes intensity follows the thermal population of the vibration. The Stokes band, or Rayleigh backscatter in some designs, serves as a reference that removes loss and power variations.

Is Raman DTS sensitive to strain?

Very little. This distinguishes it from Brillouin sensing, where strain and temperature both shift the signal and must be separated.

What sets the spatial resolution?

The pulse duration, the receiver bandwidth and, in multimode fiber over long distances, modal dispersion, which spreads the pulse as it travels.

References: J. P. Dakin, D. J. Pratt, G. W. Bibby and J. N. Ross, "Distributed optical fibre Raman temperature sensor using a semiconductor light source and detector," Electronics Letters 21, 569 (1985); A. H. Hartog, An Introduction to Distributed Optical Fibre Sensors (CRC Press, 2017); G. P. Agrawal, Nonlinear Fiber Optics, 6th ed. (Academic Press, 2019).