What types of optical fibers are used for sensing

Optical fiber sensors are classified primarily into intrinsic and extrinsic types, and further categorized based on the optical parameters they measure, such as intensity, phase, frequency, and polarization.

Intrinsic vs Extrinsic Sensors

Intrinsic optical fiber sensors have the sensing mechanism embedded within the fiber itself. Environmental changes such as temperature, pressure, or strain directly modulate the light traveling through the fiber, without the light leaving the fiber. These sensors exploit the fiber's properties, including changes in refractive index, birefringence, or scattering, to detect physical or chemical variations (e.g., vibration, temperature, or magnetic fields) . Extrinsic optical fiber sensors use the fiber to transmit light to an external sensing element. The light interacts with the environment outside the fiber, and the modulated signal is returned to the fiber for analysis. This configuration is often used for chemical or biological sensing where the fiber guides light to a remote sensing region .

Classification by Optical Parameters

Optical fiber sensors can also be classified according to the optical property they monitor:

• Intensity-based sensors: Measure changes in light intensity due to absorption, scattering, or microbending of the fiber. These are simple and cost-effective but sensitive to alignment and source fluctuations .
• Phase sensors: Detect changes in the phase of light caused by environmental perturbations, often used in interferometric setups for high-precision measurements .
• Frequency sensors: Monitor shifts in the frequency of light, useful in applications like vibration or acoustic sensing .
• Polarization sensors: Measure changes in the polarization state of light, often exploiting birefringent fibers to detect strain, temperature, or magnetic fields .

Additional Considerations

Optical fiber sensors can also be distributed or point sensors. Distributed sensors measure parameters along the entire fiber length, while point sensors measure at discrete locations. They are widely used in structural health monitoring, aerospace, environmental sensing, and biomedical applications due to their small size, immunity to electromagnetic interference, and ability to multiplex multiple sensors along a single fiber . In summary, the classification of sensing optical fibers is multi-dimensional, encompassing intrinsic vs extrinsic configurations, the optical parameter measured, and the spatial sensing arrangement, providing flexibility for diverse industrial, biomedical, and environmental applications .