Method for Measuring Air Pressure with Fiber Optic Gratings

Fiber Bragg grating (FBG) sensors measure air pressure by converting pressure-induced strain into shifts in reflected light wavelength.

Principle of FBG Pressure Sensing

FBG sensors operate by reflecting specific wavelengths of light that shift when the fiber experiences strain. In pressure measurement, external air pressure is applied to a mechanical structure—such as a diaphragm, capillary, or elastic membrane—that transmits axial strain to the FBG embedded in the fiber. This strain changes the grating period and refractive index, causing a measurable shift in the Bragg wavelength, which is proportional to the applied pressure .

Sensor Design and Sensitivity Enhancement

Bare FBGs have low intrinsic pressure sensitivity (around 3.04 pm/MPa), so mechanical amplification structures are commonly used:

• Diaphragms: Thin metal or polymer membranes deform under pressure, transferring strain to the FBG. Sensitivity can range from 43.7 to 175.5 pm/kPa depending on diaphragm thickness .
• Levers or Coaxial Tubes: Mechanical levers or steel tube structures amplify the strain on the FBG, increasing sensitivity above 20 pm/MPa .
• Epoxy-embedded FBGs: Embedding the FBG in a compliant diaphragm material can further enhance response and durability .

Interferometric Integration

FBG sensors can be combined with Fabry-Pérot interferometers (FPI) to improve measurement resolution and allow simultaneous detection of multiple parameters like pressure, temperature, and curvature. In such designs, the FBG reflects specific wavelengths while the FPI cavity provides high-resolution phase-based measurements .

Advantages of FBG-Based Pressure Sensors

• High sensitivity and miniaturization: Suitable for small-scale or biomedical applications.
• Electromagnetic immunity: Ideal for environments with high EMI.
• Multiplexing capability: Multiple FBGs can be inscribed along a single fiber for distributed sensing .
• Durability in harsh environments: With proper packaging, FBG sensors can operate under extreme temperatures and pressures .

Practical Applications

FBG pressure sensors are used in:

• Aerospace and industrial monitoring: Measuring cabin pressure, fuel systems, or pipeline pressures.
• Biomedical devices: Miniaturized sensors for blood pressure or respiratory monitoring.
• Structural health monitoring: Detecting pressure changes in critical infrastructure .

Summary

To measure air pressure with fiber optic gratings, an FBG is embedded in a mechanical structure that converts pressure into strain. The resulting Bragg wavelength shift is detected optically, providing a precise, EMI-immune, and highly sensitive measurement. Sensitivity can be enhanced using diaphragms, levers, or hybrid FBG-FPI designs, making this method suitable for a wide range of industrial, biomedical, and aerospace applications .