One of the most common fiber optic sensor (FOS) types used are fiber Bragg gratings (FBG), and the most frequently measured parameter is strain. Hence, FBG strain sensors are one of the most prevalent FOS devices in use today in structural sensing and monitoring in civil engineering, aerospace, marine, oil and gas, composites and smart structure applications. However, since FBGs are simultaneously sensitive to both temperature and strain, it becomes essential to utilize sensors that are either fully temperature insensitive or, alternatively, properly temperature compensated to avoid erroneous measurements. In this paper, we introduce the concept of measured “total strain”, which is inherent and unique to optical strain sensors. We review and analyze the temperature and strain sensitivities of FBG strain sensors and decompose the total measured strain into thermal and non-thermal components. We explore the differences between substrate CTE and System Thermal Response Coefficients, which govern the type and quality of thermal strain decomposition analysis. Finally, we present specific guidelines to achieve proper temperature-insensitive strain measurements by combining adequate installation, sensor packaging and data correction techniques.
We review recent advancements in making high resolution distributed strain and temperature measurements using
swept-wavelength interferometry to observe the spectral characteristics of Rayleigh scatter in optical fibers. Current
methods available for distributed strain or temperature sensing in optical fiber include techniques based on Raman,
Brillouin, and Rayleigh scattering. These techniques typically employ optical time domain reflectometry and are thus
limited in spatial resolution to 0.1 to 1 m. Fiber Bragg gratings can yield higher spatial resolution but are difficult to
multiplex in large numbers for applications requiring wide scale coverage. Swept-wavelength interferometry allows
the Rayleigh scatter amplitude and phase to be sampled with very high spatial resolution (10s of microns). The
Rayleigh scatter complex amplitude can be Fourier Transformed to obtain the Rayleigh scatter optical spectrum and
shifts in the spectral pattern can related to changes in strain or temperature. This technique results in distributed strain
measurements with 1 με resolution or temperature measurements with 0.1 C resolution. These measurements can be
made with sub-cm spatial resolution over a 100 m measurement range or with sub-10 cm resolution over a 1 Km range.
A principle advantage of this technique is that it does not require specialty fiber. Thus, measurements can be made in
pre-installed single mode or multimode fibers, including those used for telecommunication networks. Applications
range from fault monitoring in short range communications networks, structural health monitoring, shape sensing,
pipeline and electrical transmission line monitoring, to perimeter security. Several examples are discussed in detail.
We describe the use of swept-wavelength interferometry for distributed fiber-optic sensing in single- and multimode
optical fiber using intrinsic Rayleigh backscatter. The interrogation technique is based on measuring the spectral
shift of the intrinsic Rayleigh backscatter signal along an unaltered standard telecommunications grade optical fiber
and converting the spectral shift to strain or temperature. This technique shows great utility as a method for highly
distributed sensing over great distances with existing, pre-installed optical fiber. Results from sensing lengths
greater than 1 km of optical fiber with spatial resolutions better than 10 cm are reported.