2 Basic Principles of Fiber Sensing
Strategies for fiber optic sensing can be split into sensitized fiber and intrinsic fiber methods. An example of sensitized fiber is a Fiber Bragg Grating (FBG), used for decades for localized strain and temperature measurement within fibers. FBGs represent an “etched into fiber” sensor. Periodic changes in refractive index across a short length of fiber are made by engraving gratings into the fiber. As the fiber is strained through these sections, changes in the scattered light wavelength can be measured and related to the magnitude of strain by careful observation of the spectral backscatter that arises following insertion of a broad spectral pulse. FBGs represent “local” measurements and each point of measurement must have different grating spacings (to produce unique backscatter wavelength bands), which limits the number of FBGs that can be used on a single fiber to typically less than 200 (Hill and Meltz, 1997).
Unlike FBGs embedded in the fiber (analogous to individual electrodes in an electric resistance tomography string), intrinsic distributed sensing collects data throughout the length of the optical fiber, which can be many kilometers long. Optical fibers are designed to be highly transmissive to light which allows them to carry relatively weak signals over great distances. In principle, fibers are designed to be completely internally refracting for coherent light, obtained by layering glass of decreasing refractive indices from the inner “core” to the outer “cladding”. However, fibers are not 100 percent transparent, and some absorption of photons occurs.
Most distributed sensing relies upon scattering of laser-generated photons by interactions within the electrons of the silica (SiO2) molecules within the optical fiber. When photons are absorbed and re-emitted, a process commonly referred to as scattered, their scattering can take several forms. If the re-emitted photon remains at the same frequency and energy state as the absorbed one, the scattering is termed elastic. The most common elastic scattering is Rayleigh scattering, which is used in Distributed Acoustic Sensing (DAS). If, however, the re-emitted photon returns at a lower (or higher) energy state, the scattering is said to be inelastic. The magnitude of inelastic scattering and its impacts on the frequency and photon energy state can be used to infer the temperature and strain at the scattering location in the fiber. Common forms of scattering used in hydrogeophysical measurements are Rayleigh, Raman and Brillouin scattering. Figure 1 shows a typical spectrum of scattered photons in an optical fiber from an initially single wavelength energy source. When a scattering event occurs, the highest probability of scattering is elastic (Rayleigh). Raman inelastic scattering produces photons that fall into relatively narrow and predictable bandwidths, shifted in frequency either higher or lower. Brillouin scattering produces frequency shifts, and the frequency shift is a function of the strain at the scattering site.
Figure 1 – Diagram representing typical scattering intensity as a function of temperature T, wave length λ and strain ε within an optical fiber. The incident wavelength is defined by the choice of laser and is typically in the near-infrared frequency (800-1500 nm), which is noted as λ0 near the center of the x axis. Raman scattering typically produces a scattered photon at predictable wavelengths of 20-100 nm both longer (Stokes) and shorter (anti-Stokes). The intensity of the shifted backscattered photons is used to estimate fiber temperature. Brillouin scattering employs density-dependent wavelength shifts, and it is the magnitude of the shift that corresponds to changes in glass density due either to a change in temperature or fiber strain.
The minimum length of fiber that returns a signal is a function of the maximum frequency of the detectors, the sensitivity of the detectors and the frequency of the laser repetition rate. The maximum length is constrained by the optical budget (the number of photons injected into a fiber) and the rate of scattering or attenuation of the photon numbers within the optical fiber. The intensity of injected light is limited by non-linear scattering properties which arise at high intensity. Some Distributed Temperature Sensing (DTS) instruments (commonly called “interrogators” in the industry) extend the range of these methods by sequentially injecting differing intensities wherein the highest intensity injections are only employed further from the point of injection to avoid non-linear effects while obtaining readings from greater distances.
Distributed sensing is now widely used for temperature measurement, strain measurement and, most recently, strain rate measurement. In hydrogeology, distributed sensing is widely applied for assessing surface water/groundwater interactions, soil moisture, groundwater flow, heat transport in the subsurface, strain and ground motion related to pumping, tides, and surface loadings. DAS is now poised to become standard practice in seismic refraction and reflection surveying.
Common to all distributed fiber sensing methods are: a low power laser illumination source (commonly in the near infrared), an optical fiber whose light transmission, scattering or length is a function of the property to be measured; and detectors/processors to control the laser firing and to measure the returned light signal. Distributed sensing has many analogies to other remote sensing tools; it is most closely related to lidar (light detection and ranging) where scattering photons from interactions with the land surface are recorded and their time of flight is used to calculate the distance from the laser source to the scattering site.