Fibre Bragg gratings have moved from the optics laboratory to the bridge deck within a single generation. The grating itself, a periodic modulation of refractive index written into the core of a single-mode fibre with ultraviolet light, became practical once side-writing through the cladding was demonstrated (Meltz et al., 1989), and its use as a sensor followed quickly (Kersey et al., 1997; Rao, 1997). For the engineer responsible for a large structure the attractions are easy to list. The measurand is encoded in wavelength, so the reading does not depend on optical power or connector loss. Many gratings can be read along one fibre. The sensing element is passive, immune to electromagnetic interference and lightning, and can sit kilometres from its interrogator (Li et al., 2004; Majumder et al., 2008).

The same physics that makes the grating a good strain gauge makes it a good thermometer. A change in temperature alters both the refractive index of the glass and the period of the grating, and the reflected wavelength moves exactly as it would under mechanical strain. Near 1550 nm a bare silica grating shifts by roughly 1.2 pm for each microstrain and by about 10 pm for each degree Celsius, so one degree of unaccounted temperature change reads as something like eight to ten microstrain. A bridge member in temperate climates experiences tens of degrees between a winter night and a summer afternoon, while the live-load strain of interest in a girder is often a few tens of microstrain. Without compensation the quantity being sought is buried under the quantity being ignored.

This article reviews how that problem has been addressed. It is concerned less with listing every sensor configuration that has been published than with the assumptions each class of method makes and whether those assumptions hold on a real bridge, where the temperature field is neither uniform nor steady and the monitoring period is measured in decades.

The article is a narrative review of the published literature. It draws on the foundational papers on grating sensors, on the body of work on discriminating strain from temperature that appeared mainly in optics and measurement journals during the 1990s and early 2000s, on standard monographs (Othonos and Kalli, 1999; Glisic and Inaudi, 2007), and on reports of instrumented bridges. No claim of exhaustive coverage is made, and no formal search protocol was followed. The sources were chosen because they are widely cited and because each illustrates a distinct principle. Distributed sensing by Brillouin or Rayleigh scattering faces a related cross-sensitivity but is outside the scope, as are gratings in polymer fibre.

Confusion about compensation often begins with a failure to say what is to be removed. At least three distinct thermal effects reach the interrogator, and they do not all deserve the name of error.

The methods in the literature fall into four families, arranged in Figure 1. The first three act at the sensor and aim at the intrinsic response, with or without the host expansion. The fourth acts on the data and can be directed at any of the three contributions. Table 1 summarises the assumption that each family depends on and the way it most commonly fails in bridge service.

Laboratory demonstrations of discrimination are usually made in an oven or a water bath, with slow uniform temperature changes. A bridge offers neither uniformity nor slowness. Direct sunlight on one web of a steel box produces differences of many degrees across a section within an hour, and a concrete deck lags the air temperature by hours. A reference grating a short distance from the active one, or on the shaded side of the same plate, may be at a different temperature for much of the day. Because the apparent strain per degree is so large, a mismatch of a single degree is enough to mask a typical vehicle response. Reports of instrumented bridges describe temperature references installed beside the strain gratings for this reason (Tennyson et al., 2001; Chan et al., 2006). The practical inference is that the reference should sit in thermal contact with the same surface, under the same cover, and with a comparable thermal mass.

The bond between fibre and structure is a second weak point. Strain reaches the grating through a coating and an adhesive layer whose stiffness changes with temperature and whose creep under sustained strain appears as a slow drift indistinguishable from a thermal offset. Non-uniform strain along the grating, produced by a poorly made bond, broadens or splits the reflection peak and biases the wavelength found by the peak-detection algorithm. Each of these effects has its own temperature dependence, so the sensitivity coefficients measured on a free fibre in the laboratory are not those of the installed sensor. Packaging and installation are accordingly treated as central engineering problems in reviews of fibre sensing for structures (López-Higuera et al., 2011), and calibration of the packaged gauge on a specimen of the host material, over the full service temperature range, is the most dependable source of coefficients.

Finally, the reference against which wavelength is measured can drift. The grating itself decays slowly after writing unless it has been annealed, a process characterised by Erdogan et al. (1994), and the interrogator relies on an internal wavelength reference, commonly a gas cell or an athermal etalon, whose stability sets a floor under every reading. A drift of a few picometres over several years is small by the standards of telecommunications and corresponds to several microstrain. In a monitoring programme meant to detect slow structural change, it is exactly the kind of error that matters, and it affects active and reference gratings alike only when both are read by the same channel in the same sweep.

Several gaps stand out. The literature on discrimination is rich in configurations and poor in long-term evidence: most schemes were demonstrated over hours or days, and very few reports follow a compensated sensor on a structure through many annual cycles with an independent check on its accuracy. There is no widely adopted test method that specifies how the residual thermal error of a packaged grating strain gauge should be measured and declared, in the way that apparent-strain curves are supplied with electrical resistance gauges. The treatment of thermal gradients is largely empirical. And the boundary between sensor compensation and structural interpretation is seldom drawn clearly in published monitoring reports, which makes results from different bridges hard to compare.

Useful work would include agreed procedures for characterising installed sensors, including thermal cycling of the packaged gauge on representative substrates; reporting of residual apparent strain as a stated uncertainty in line with ordinary metrological practice; and designs in which the reference element shares the thermal path of the active one by construction. On the data side, methods that model the thermal response of the structure explicitly, and monitor that model for change, promise more than methods that filter temperature out as noise.

Temperature compensation of grating strain sensors on bridges is a solved problem in principle and an unfinished one in practice. The reference-grating method remains the workhorse because its failure modes are understood and can be designed against, while the more ingenious single-point schemes are limited by matrix conditioning and by the fragility of their sensing heads. What the literature shows most clearly is that the dominant errors on a bridge arise from the installation: temperature differences between sensor and reference, the behaviour of adhesives and coatings, and the long-term stability of the wavelength reference. Progress now depends more on standardised characterisation and long-term field evidence than on new grating configurations, and on stating plainly, in every monitoring report, which thermal contribution has been removed and which has been left in.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.