Fiber Optic Gyroscope (FOG): Advantages and Disadvantages

This page explores the structure, working principles, advantages, and disadvantages of Fiber Optic Gyroscopes (FOGs).

The measurement of angular velocity is crucial in applications ranging from missile navigation to motion control. Three primary sensors are used for this purpose: fiber optic gyroscopes (FOGs), ring laser gyroscopes (RLGs), and MEMS gyroscopes.

What is a Fiber Optic Gyroscope (FOG)?

• It leverages the Sagnac effect to measure angular velocity.
• Figure 1 illustrates the basic configuration of a fiber optic gyroscope.
• It comprises a passive interferometer where a coupler splits light from the source into two counter-propagating waves (clockwise and counter-clockwise) within the fiber coil. These waves are then recombined on a photodetector (PD) after propagation.
• Due to the Sagnac effect, the light beam traveling against the rotation experiences a slightly shorter path delay compared to the other beam. The resulting differential phase shift is measured using interferometry in the FOG. The Sagnac effect is proportional to the effective area of the closed optical path.
• It’s also known as a fiber optic rotation sensor, a vital tool for navigation, guidance, and control in missiles, aircraft, robots, automobiles, spacecraft, and more.

• Figure 2 shows a circular loop of fiber. The phase shift of a fiber optic gyroscope is expressed as follows:

  ΔΦs = (8 * π * Ω * A * N) / (c * λ)

  Where:

  • N = Number of Turns
  • A = π * (R)^2, where R is the radius of the fiber loop
  • Ω = Earth’s rotation in rad/sec
  • λ = Operating wavelength
  • c = 3 x 10^8

• When combined with an accelerometer, it provides an accurate path of flight or movement for a vehicle or object.

Advantages of Fiber Optic Gyroscope

Here are the benefits of using a Fiber Optic Gyroscope:

• High Sensitivity: Offers superior sensitivity, aiding in determining the Earth’s poles based solely on angular velocity measurements. Its rotational sensitivity (~0.1 °/Hr) is faster and more accurate than conventional gyroscopes.
• Silent Operation: FOGs lack a dither mechanism, making them virtually silent. This is beneficial in applications like submarine navigation.
• High Reliability and Long Lifespan: Being a solid-state device with no moving parts or sealed cavities, it is highly reliable and boasts a longer lifespan. It also eliminates the need for preventive maintenance.
• Low Noise: Delivers high performance thanks to its inherently low random noise.
• Compactness and Cost-Effectiveness: Offers similar compactness and cost advantages to ring laser gyroscopes (RLGs) and MEMS gyroscopes.

Disadvantages of Fiber Optic Gyroscope

Here are the drawbacks of using a Fiber Optic Gyroscope:

• Vibration Sensitivity: Some FOG designs are susceptible to vibrations. This impact can be mitigated by coupling multi-axis FOGs with accelerometers.
• Calibration Required: FOGs require calibration.
• Limited Angular Velocity Detection: Detectable angular velocity is limited to +/- π/2 rad with respect to the phase difference due to the Sagnac effect.
• Susceptibility to Light Intensity Fluctuations: Linearity and scale factor stability of the gyroscope can be deteriorated due to fluctuations in the intensity of incident light on the photodetector.