Fiber Optic Pressure Sensors: Working, Advantages, and Disadvantages

This article explains the structure, working principle, advantages, and disadvantages of Fiber Optic Pressure Sensors.

Fiber Optic Pressure Sensor Structure and Working Operation

Fiber optic pressure sensors are generally categorized into two main types: non-interferometric and interferometric.

Figure 1 depicts a simplified structure of a non-interferometric fiber optic pressure sensor.

Figure 1: Fiber Optic Pressure Sensor Structure

As illustrated in the figure, this type of sensor typically consists of three key components:

1. Sensing Head with Metal Diaphragm: This is the pressure-sensitive element.
2. Cable (with Two Multi-Mode Fibers): These fibers transmit and receive light signals.
3. Optoelectronic Systems: These include an LED light source, photodetectors, and signal processing circuitry.

Working Principle

Light emitted from the LED source is transmitted through one fiber optic cable. The light then reflects off the metal diaphragm. This reflected light is captured by the second fiber and directed toward a detector and signal conditioning circuit.

The displacement of the diaphragm is directly proportional to the applied pressure. This displacement modulates the intensity of the light signal detected by the photodetector. Hence, this type of sensor is also known as an intensity-modulated pressure sensor.

The sensor’s response is affected by several parameters:

• Shape, diameter, and thickness of the diaphragm
• Core and cladding diameter of the fiber
• Numerical aperture (NA) of the fiber
• Distance between the two fibers
• Fiber-to-diaphragm distance at zero pressure

Advantages of Fiber Optic Pressure Sensors

Fiber optic pressure sensors offer several benefits, making them suitable for various applications:

• Harsh Environment Operation: They can measure pressure in extreme conditions, including high temperatures (up to 300°C) and pressures (around 300 bar).
• EM Immunity: They are resistant to electromagnetic interference, noise, crosstalk, and ground loops.
• Safe in Hazardous Environments: They eliminate spark and shock hazards, making them suitable for explosive or corrosive environments, thanks to their optical isolation.
• Simple Construction: Non-interferometric types are relatively simple to construct, without requiring complex optical components like single-mode fibers, optical modulators, or beam splitters.
• Low Cost: Generally, they are relatively inexpensive compared to other high-performance pressure sensors.
• Small Size and Mass: Their compact size and light weight make them suitable for applications where space is limited.
• Low Signal Attenuation: They offer low signal attenuation, allowing for remote measurements.
• Long Lifespan: They are designed to function reliably over billions of pressure cycles.
• Performance Characteristics: They can provide high resolution, a high dynamic range, good linearity, and low temperature sensitivity, with multiplexing capabilities.

Disadvantages of Fiber Optic Pressure Sensors

Despite their advantages, fiber optic pressure sensors also have certain drawbacks:

• Fragility: The sensing element and fiber optic cable are fragile and can be easily damaged.
• Poor Compatibility with Some Processes: They may not be compatible with certain harsh process environments.
• Radiation Sensitivity: Exposure to radiation can darken the fiber optic cable, increasing signal attenuation.
• Complex Signal Processing: They may require complex and expensive signal processing equipment.
• Poor Static Pressure Measurement: Static pressure measurements may not be as accurate.
• Sensitivity to Fiber Optic Cable Fluctuations: Intensity-modulated sensors are vulnerable to fluctuations in the transmission characteristics of the fiber optic cables due to environmental and mechanical stressors, detector sensitivity, and the aging and temperature effects on light sources.