Overview
Distributed fiber optic sensing uses the optical fiber as both the sensing medium and the data transmission medium. By exploiting the properties of light propagation in fiber, continuous sensing can be performed along the fiber length. Changes in the optical signal are demodulated to recover environmental physical quantities such as temperature, strain, and pressure, providing time-varying spatial distribution information of the monitored object.
Distributed fiber sensing principles include interferometric methods and scattering-based methods such as Rayleigh, Brillouin, and Raman scattering. Different methods require different types of distributed fiber sensors. This article focuses on sensor selection for optical frequency domain reflectometry (OFDR), which is based on Rayleigh scattering.
Distributed Fiber Optic Sensors
Distributed fiber optic sensors are passive, compact, flexible, immune to electromagnetic interference, corrosion resistant, and offer high sensitivity. They are used across many fields. As distributed sensing technologies have been adopted more widely, various sensor formats have emerged.
Sensor composition
Common distributed fiber optic sensors available on the market are either bare optical fiber or fibers packaged into distributed sensing cables with protective sheaths and armor. As shown in Figure 1(a), a typical bare optical fiber consists of a core, cladding, and coating. The core and cladding are made of silica with different refractive indices; the core refractive index is higher than the cladding, and light is guided when the incident angle satisfies total internal reflection. The coating is typically an acrylic material whose main role is to protect the fiber from external damage and to increase fiber toughness, extending service life.
Sensor selection for OFDR
OFDR demodulates the Rayleigh backscatter signal in the fiber, so single-mode fiber is used as the sensing element. Different test scenarios call for different types of distributed fiber sensors, but their common characteristic is a single-mode fiber core.
1. Common single-mode fibers used for OFDR
Common single-mode fiber types such as G.652 and G.657 series can be used as OFDR sensors. The G.657 series is bend-insensitive and exhibits lower bending loss than G.652 at the same bend radius. In field deployments or complex structures where fiber bending is unavoidable, G.657 series fibers are generally preferable to G.652 for OFDR applications.
2. Effect of fiber coating on measurements
OFDR senses strain applied to the fiber that has been transferred from the structure under test. In practice, distributed fiber sensors are either bonded to structure surfaces or embedded within structures. The strain measured by OFDR is the strain transmitted to the fiber by the structure, and strain transfer differs with coating materials. Coatings with higher elastic modulus provide better strain transfer.
A comparison of strain transfer among standard fiber with an acrylic coating, PI fiber with a polyimide coating, and bare fiber (coating removed) shows that acrylic coatings provide poor strain transfer, while polyimide coatings transfer strain nearly as well as bare fiber. Therefore, for precision structural strain tests, PI fiber is commonly used because it maintains measurement accuracy while protecting the fiber from damage.
3. Effect of cable sheath on measurements
In harsh field environments, bare fiber is too fragile. Tight-buffered cables are used to improve survivability. Although tight-buffered cables provide greater mechanical strength than bare fiber, they introduce additional strain transfer loss compared to bare fiber, and this loss increases with sheath diameter.
Common tight-buffered cable diameters include 0.9 mm and 2 mm, as well as larger armored cables. When survivability can be ensured, it is advisable to choose the smallest tight-buffered diameter that meets protection requirements to preserve sensing performance.
4. Operating temperature range of fiber sensors
Fiber sensors consist of bare fiber, coating, and sheath. The bare fiber material is silica, which can operate at high temperatures up to around 1000 °C. The practical operating temperature range of a fiber sensor is determined by the temperature tolerance of the coating and the outer sheath.
Acrylic-coated fibers typically operate between -40 and 80 °C and are suitable for ambient temperature strain testing. PI-coated fibers operate between -40 and 300 °C and can be used for strain testing and temperature measurements up to about 300 °C. Gold-coated fibers operate between -70 and 1000 °C and are used for high-temperature strain and temperature measurements.
5. Difference between temperature-sensing fiber and strain-sensing fiber
For OFDR, both strain and temperature changes appear as frequency shifts in the Rayleigh backscatter signal, and the technique cannot inherently distinguish strain from temperature. Therefore, different sensing fiber constructions are used to separate strain and temperature effects.
Temperature-sensing fibers typically use loose-buffered constructions. A loose-buffered temperature fiber commonly consists of a 0.9 mm hollow buffer tube housing a 165 μm PI fiber that can move freely inside the tube. External strain is mechanically decoupled by the buffer tube, so the sensor responds only to temperature changes.
For strain testing in isothermal environments, bare fiber or buffered fiber may suffice depending on the application. For strain testing in environments with varying temperature, temperature-compensated measurements use two fibers: one tight-buffered fiber that responds to both temperature and strain, and one loose-buffered fiber that responds only to temperature. Subtracting the loose-buffered measurement from the tight-buffered measurement yields the strain component from the structure.
Conclusion
A variety of distributed fiber sensors are available for OFDR temperature and strain measurements. First, select sensors based on the physical quantity to be measured. Then choose fiber constructions that ensure survivability in the test environment. Finally, select fiber coatings and cable formats that meet the required measurement accuracy to ensure reliable results.
