Principle of Fiber Optic Interferometer Sensors

Fiber optic interferometers detect physical changes by measuring phase shifts in light caused by variations in optical path length, which are induced by external perturbations such as strain, temperature, or pressure.

Basic Operating Principle

Fiber optic interferometers operate on the principle of light interference. A coherent light source is split into two or more optical paths, where at least one path interacts with the measurand (the physical quantity to be measured). Changes in the measurand alter the optical path length or the refractive index of the fiber, causing a phase difference between the light beams. When the beams recombine, this phase difference produces an interference pattern, which can be analyzed to quantify the measurand .

Types of Fiber Optic Interferometers

1. Fabry-Perot Interferometer (FPI)
   • Composed of two parallel reflecting surfaces forming a cavity.
   • Interference occurs due to multiple reflections within the cavity.
   • Can be extrinsic (external cavity) or intrinsic (cavity within the fiber).
   • Sensitive to strain, temperature, pressure, and refractive index changes .
2. Mach-Zehnder Interferometer (MZI)
   • Light is split into two separate fiber arms; one arm interacts with the measurand.
   • Phase difference between the arms produces interference at the output.
   • Often used for high-sensitivity measurements and multi-parameter sensing .
3. Michelson Interferometer
   • Light is split and reflected back along the same paths.
   • Interference occurs when the reflected beams recombine.
   • Provides high accuracy and is suitable for displacement, vibration, and temperature sensing .
4. Sagnac Interferometer
   • Light travels in clockwise and counterclockwise directions in a fiber loop.
   • Phase difference arises from rotation or other perturbations affecting the loop.
   • Commonly used in rotation sensing and polarization-based measurements .

Signal Detection and Analysis

The interference signal can be analyzed in terms of intensity, phase, wavelength, or frequency. Advanced techniques such as phase-generated carrier (PGC) demodulation, phase-shifting interferometry (PSI), and white light interferometry (WLI) are used to extract precise measurements with high sensitivity and resolution . These methods allow fiber optic interferometers to achieve sub-nanometer resolution and support multiplexed sensing for multiple parameters simultaneously.

Advantages

Fiber optic interferometers offer high sensitivity, immunity to electromagnetic interference, remote sensing capability, compact size, and multiplexing potential, making them suitable for applications in structural health monitoring, industrial process control, and biomedical sensing .