Most of these modules perform electrical measurements, but many applications have environmental or physical limitations that make the use of electrical sensors extremely challenging. Fortunately, the inherent characteristics of fiber optic sensors solve or eliminate many of these problems. Learn the fundamentals of fiber optic sensing to see how this new technology addresses many of the challenges faced by electrical sensors.
Fundamentals of Optical Sensing
Traditional electrical sensors use sensors to convert physical phenomena into electrical signals, which are then regulated, digitized, and scaled to the desired value by a data acquisition system. While ubiquitous, these sensors have inherent limitations, and their use is impractical, if not entirely impossible, in certain types of applications. Fiber optic sensors offer an excellent solution to these challenges.
Fundamentally, fiber optic sensors work similarly to electrical sensors, but use light instead of electricity and glass fibers instead of copper wires. While electrical sensors can modulate electrical characteristics such as current, resistance, or voltage, fiber optic sensors modulate one or more properties of light, including intensity, phase, polarization, or wavelength.
Optical sensing technology relies on optical fiber—a type of glass thinner than a human hair—that transmits light within its core. This fiber consists of three main parts: the core, the cladding, and a buffer coating. The cladding reflects stray light back to the core, ensuring transmission through it with minimal light loss. This is achieved by ensuring the core material has a higher refractive index than the cladding, resulting in perfect internal reflection of light. The outer buffer coating protects the fiber from external conditions and physical damage. It can contain many layers, depending on the required robustness.
Figure 1: Cross-section of a typical optical fiber
Although many types of fiber optic sensors exist, the most commonly used is the fiber Bragg grating (FBG). A Bragg grating is a variation of refractive index that is “written” into the core of an optical fiber at periodic intervals called the grating period. When an input optical signal strikes an FBG, the intervals between the gratings cause constructive interference of reflections from each grating, reflecting light at a specific wavelength called the Bragg wavelength (see Figure 2).
Figure 2: Operation of the FBG optical sensor
Variations in strain and temperature affect the effective refractive index (ne) and grating period (Λ) of the FBG, resulting in a shift in the reflected Bragg wavelength (B) according to Equation 1 below. Therefore, the wavelength shift can be measured to determine the corresponding changes in strain and/or temperature. Since both strain and temperature affect the Bragg wavelength (and thus the measurement), temperature compensation is an important consideration for testing over large temperature or strain ranges.
FBGs can be fabricated with various grating periods and therefore various Bragg wavelengths, allowing different FBG sensors on the same fiber to reflect unique light wavelengths. This makes each wavelength distinguishable from one another within the spectral range. The process of distinguishing FBGs on the same fiber based on their respective Bragg wavelengths is called wavelength division multiplexing (WDM). As long as the wavelength shift associated with each measurement does not cause the Bragg wavelength of one FBG sensor to cross the Bragg wavelength of another FBG sensor, dozens of sensors on the same fiber can perform independent measurements.
Solving problems using optical sensing
Anyone who has struggled with noise filtering, shielding, wiring issues, or sensor damage can tell you that electronic sensors have some applications where they are difficult to adapt to. The four most challenging aspects of using electronic sensors are maintaining reliability under harsh electrical conditions, resisting degradation in harsh environments, economically detecting large areas using multiple sensors, and mounting traditional sensors in confined spaces. Each of these challenging problems can be solved by using fiber optic sensors instead of electrical sensors.
High electromagnetic interference and high voltage environment
Electromagnetic interference (EMI) is one of the most common sources of measurement error and failure in electrical sensor systems. Electrical sensor signal measurements under high EMI conditions, such as near high-power generators, motors, or other AC power sources, are particularly prone to distortion. These environments often include high-voltage components that can damage or even destroy conventional sensor systems. Filtering and isolation instruments can mitigate the risks of high EMI and high voltage to some extent, but they have limited levels of noise suppression and isolation.
Similarly, fiber optic sensors are made of glass and are completely non-conductive and electrically passive. This makes them resistant to even the highest levels of EMI and completely unaffected by high voltages or currents in the environment. For example, you can connect fiber optic temperature sensors directly to ultra-high power components such as motor windings, transformers, and power lines for high-precision thermal characterization analysis during operation.
