Fiber Optic Strain Sensor: Working, Advantages, and Disadvantages

This article explores the structure, working principle, advantages, and disadvantages of fiber optic strain sensors. It covers both Fiber Bragg Grating (FBG) based sensors and plastic fiber optic strain sensors.

Fiber Bragg Grating as a Strain Sensor

A Fiber Bragg Grating (FBG) reflects a specific wavelength of light. This reflected wavelength shifts in response to changes in temperature and/or strain. The refractive index of the fiber is permanently altered based on the intensity of the light it’s exposed to. This periodic variation in refractive index is what constitutes the fiber Bragg grating.

For accurate strain measurement, it’s crucial to compensate for temperature effects on the FBG. This is typically done by installing an FBG temperature sensor in close proximity to the FBG strain sensor. By subtracting the wavelength shift of the temperature sensor from the wavelength shift of the strain sensor, you can effectively remove the influence of temperature on the data, resulting in a temperature-compensated strain value.

The change in wavelength of an FBG due to strain and temperature can be expressed as follows:

*Δλ_B = { [ (1 - P_e)ε + (α + ((dn_eff /dT)/n_eff )ΔT ]}*λ_B

Where:

• P_e = Effective photo-elastic constant
• α = Thermal expansion coefficient of fiber

Plastic Fiber Optic Strain Sensor

Plastic fiber optic strain sensors offer a cost-effective alternative for strain measurement and structural health monitoring of bridges, skyscrapers, dams, and other large structures. They boast benefits like high resistance to fracture and ease of termination and coupling.

Working Principle:

• The plastic fiber is bonded to the lower surface of the structure (e.g., a suspended bridge) using a specialized adhesive.
• An LED light source is connected to one end of the fiber, and a light detector (e.g., LDR) is connected to the other.
• Under loading conditions, axial strain decreases the curvature of the fiber.
• This increased curvature reduces light intensity as light is lost due to bending within the fiber.

When a metal beam is subjected to transverse loading, the in-phase strain on the lower surface of the beam can be expressed as:

ε = - [(12 * y) / L² ] * δ_ctr

Where:

• y = distance of lower plane of beam from its neutral axis
• L = distance between the supports
• δ_ctr = Beam deflection at its central point

Benefits or Advantages of Fiber Optic Strain Sensors

Fiber optic strain sensors offer several advantages over conventional strain gauges:

• Compact Size: They are small and lightweight.
• High Sensitivity: They can detect even minute changes in strain.
• Easy Installation and Maintenance: Installation is straightforward, and maintenance requirements are minimal.
• Reliable: They provide consistent and dependable performance.
• Longer Life: Fiber optic sensors are designed to last, providing a longer service life than traditional gauges.
• Inexpensive: Relatively low in cost

Drawbacks or Disadvantages of Fiber Optic Strain Sensors

Despite their advantages, fiber optic strain sensors also have certain limitations:

• Thermal Sensitivity: Temperature changes can affect the readings, requiring compensation techniques.
• Demodulation Difficulty: Demodulating the wavelength shift can be complex.
• Discrimination Difficulty: Separating the effects of temperature and strain on the wavelength shift can be challenging.
• Fiber Fragility: Optical fibers are delicate and susceptible to damage during handling and use. Care must be taken to protect them.