Principle of Fiber Optic Microbending Pressure Sensor

Fiber optic microbending pressure sensors operate by converting applied pressure into microbends in an optical fiber, causing measurable light attenuation that correlates with the pressure magnitude.

Working Principle

The core principle of a fiber optic microbending pressure sensor is based on microbending loss in optical fibers. When an optical fiber is subjected to small, controlled bends perpendicular to its axis, some of the guided light in the fiber core escapes into the cladding due to changes in the angle of total internal reflection. This results in a reduction of transmitted light intensity, which is directly proportional to the applied pressure or force (microbending severity) .

Sensor Structure

A typical microbending sensor system consists of:

• Optical Fiber: Usually multimode fibers are preferred for higher sensitivity to bending . The fiber may have its cladding partially etched to enhance sensitivity .
• Bending Mechanism: The fiber is placed between deformable elements, such as corrugated plates or a mode scrambler, which induce periodic or point-specific bends when pressure is applied .
• Light Source: A stable light source, such as a laser diode or LED, injects light into the fiber .
• Photodetector: Measures the intensity of light emerging from the fiber, converting optical changes into electrical signals .
• Signal Processing Unit: Amplifies, filters, and calibrates the detected signal to quantify the applied pressure .

Mechanism of Pressure Detection

When pressure is applied to the sensor:

1. The deformers or plates create microbends along the fiber length.
2. These bends cause mode coupling, where higher-order guided modes radiate out of the fiber core, reducing transmitted light intensity .
3. The photodetector measures the attenuation in light intensity, which is processed to determine the magnitude of the applied pressure . The sensitivity of the sensor can be enhanced by removing the fiber jacket or cladding, which increases the fiber's responsiveness to small pressure changes. For example, etched multimode fibers can achieve sensitivities up to 168.4 dB/cm .

Advantages

• High sensitivity to small pressure changes.
• Immunity to electromagnetic interference.
• Compact and flexible design suitable for harsh environments.
• Capability for distributed sensing along the fiber length. In summary, fiber optic microbending pressure sensors translate mechanical pressure into optical signal attenuation through controlled microbending of the fiber, allowing precise and real-time pressure measurements in various industrial, biomedical, and aerospace applications .