Overview
Optical fiber is electrically inert, compact, lightweight, flexible, immune to electromagnetic interference, and resistant to radiation. It can operate in harsh environments that traditional sensors do not support, such as corrosive, high-temperature, or high-humidity conditions. Since its introduction, fiber optic sensing has attracted significant attention and has been studied and applied across multiple fields. There are several demodulation techniques for fiber optic sensing, which can be confusing. This article briefly reviews several common fiber optic sensing methods and compares OFDR with other techniques.
Common fiber optic sensing methods
Unlike traditional sensors that are independent devices, fiber optic sensors can integrate multiple sensing units along a single fiber, forming a monitoring network. Based on the layout of sensing units, fiber optic sensing techniques are generally divided into three categories: point sensors, quasi-distributed sensors, and distributed sensors.
Point sensing: Fiber Bragg Grating (FBG)
Fiber Bragg gratings (FBGs) are a common type of point sensor. An FBG is created by inducing a periodic modulation of the refractive index in the fiber core (for example by UV inscription). It exhibits wavelength selectivity, reflecting specific wavelengths while transmitting others.
An FBG center wavelength is given by the equation shown below.
In the equation, 
is the FBG center wavelength, 
is the effective refractive index of the fiber core, and 
is the grating period. Changes in temperature or strain alter the effective refractive index or the grating period, causing a shift in the FBG center wavelength. By connecting the FBG to an FBG demodulator and analyzing the spectrum, the wavelength shift before and after loading can be measured and converted to physical quantities such as strain, temperature, or pressure using transfer coefficients.
Quasi-distributed sensing
Multiple FBGs can be fabricated or cascaded along a single fiber, so FBG demodulators often use multiplexing techniques to achieve quasi-distributed measurements. For example, wavelength-division multiplexing (WDM) places multiple FBGs with different center wavelengths on the same fiber; a broadband source and a wavelength detector can measure each FBG center wavelength to realize multi-point measurement on a single channel. Spatial-division multiplexing (SDM) can be implemented by using optical switches to route signals to different channels. Time-division multiplexing (TDM) can also be combined with WDM: FBGs with the same wavelength located at different positions produce reflected light that arrives at the detector at different times, allowing separation by time sampling. Combining these methods enables demodulation of large-scale FBG arrays.
The number of FBG sensing points is limited by light source bandwidth, the wavelength shift range of individual FBGs, and mutual interference between adjacent FBG reflection spectra. Spatial resolution is constrained and blind zones exist between sensing points.
Distributed sensing
Distributed fiber optic sensing (DOFS) uses the fiber itself as both the sensing medium and the signal transmission medium. By measuring specific scattered light signals within the fiber, changes in strain or temperature along the fiber can be inferred. A single fiber can provide hundreds to thousands of sensing points simultaneously. Distributed sensing has no blind zones and overcomes the sensor count and spatial resolution limits of FBG multiplexing, enabling continuous distributed measurement.
When light is launched into a fiber, backscattered light is generated. Common backscattering mechanisms include Rayleigh scattering, Raman scattering, and Brillouin scattering.
Rayleigh scattering is elastic; the optical frequency does not shift during scattering. However, when the fiber is subjected to temperature or strain fields, changes in the refractive index distribution cause shifts in the Rayleigh scattering spectrum in the distance domain.
Brillouin scattering involves the coupling among the incident light, the Stokes light, and the acoustic wave field. Due to acoustic diffraction, energy transfers from the incident light to the Stokes light. The frequency difference between the incident and Stokes light, called the Brillouin frequency shift, is a function of temperature and strain, so measuring the Brillouin frequency shift enables temperature and strain sensing.
Raman scattering arises from energy exchange between photons and thermal vibrations of the fiber molecules. If some optical energy converts to thermal vibration, the emitted light has a longer wavelength than the source (Stokes light); if thermal vibration converts to light energy, the emitted light has a shorter wavelength (anti-Stokes light). The intensities of these two scattered components depend on temperature, so the ratio of anti-Stokes to Stokes intensities can be used for temperature measurement.
Distributed fiber optic sensing techniques exploit these scattering mechanisms by measuring changes in scattered signals along the fiber. Common distributed techniques include Rayleigh-based OFDR and DAS, Raman-based ROTDR, and Brillouin-based BOTDR, BOTDA, and BOFDA.
OFDR versus other fiber optic sensing techniques
OFDR versus FBG demodulation
1) Similar sensing principles. FBG demodulators detect shifts in the FBG center wavelength, while OFDR demodulates frequency shifts of Rayleigh backscatter at different fiber positions. In both cases, temperature and strain changes relate to wavelength or frequency shift by similar functional relationships.
2) Distributed versus quasi-distributed. FBG demodulation is quasi-distributed with a limited number of sensing points and blind zones between adjacent sensors. OFDR provides high-resolution distributed measurement without blind zones and supports very high sensor density.
3) Cost. FBG demodulators are typically much less expensive than OFDR demodulators. As sensors, OFDR uses standard single-mode fiber as the sensing element, which is inexpensive. FBG sensors vary in price depending on wavelength and packaging; specialized FBGs can be costly.
4) Measurement range. Typical FBG strain measurement ranges are on the order of several thousand microstrain; OFDR can cover larger strain ranges, exceeding 10,000 microstrain.
5) Sensor deployment. FBG systems require prior placement of FBGs at target locations; if an FBG is not located at the target, it may not capture the desired signal. OFDR measures the entire length of fiber, enabling detection of local variations in strain or temperature and generally yielding higher probability of capturing events of interest.
6) Weak-reflector FBG arrays. OFDR can also demodulate arrays of weak-reflector FBGs. Although OFDR normally uses standard single-mode fiber, it can demodulate uniform weak-reflector FBG arrays by improving signal-to-noise ratio, enhancing measurement stability and interference resistance.
OFDR versus other distributed techniques
Distributed fiber optic sensing can be classified by scattering mechanism into Rayleigh-based, Brillouin-based, and Raman-based techniques. By measurement method, it is also categorized into optical time-domain reflectometry (OTDR) and optical frequency-domain reflectometry (OFDR).
OFDR typically covers sensing lengths of about 100 meters with spatial resolutions on the order of millimeters to centimeters (for example, 1 mm resolution corresponds to 1000 sensing points per meter). Measurement accuracy can reach approximately ±0.1°C and ±1 microstrain, making OFDR suitable for short-range, high-resolution, and high-precision strain and temperature measurements, such as structural health monitoring, composite material fatigue testing, and electric vehicle battery pack temperature monitoring.
OTDR/DAS techniques can reach sensing distances of tens to hundreds of kilometers, but they are limited by probe pulse width; spatial resolution and dynamic range are constrained, making them unsuitable for applications that require both large dynamic range and high spatial resolution.
ROTDR typically measures lengths up to about 10 kilometers with spatial resolution around 1 meter and is mainly used for distributed temperature sensing, such as power cable surface temperature monitoring, fault localization, and fire detection.
BOTDR, BOTDA, and BOFDA can reach measurement ranges of tens of kilometers with spatial resolutions on the order of 1 meter. They are used for long-distance distributed strain and temperature monitoring, such as geotechnical engineering, pipeline health monitoring, and geological hazard monitoring.
Currently, commercial products implementing these distributed fiber optic sensing techniques are available in the Chinese market.
