​

Imagine sitting in a spinning chair with your eyes closed—how do you know how fast you're spinning? Traditional gyroscopes rely on a high-speed rotating' rotor' to detect rotation, but modern technology offers a more ingenious solution: letting light race through layers of optical fibers. This is the Fiber Optic Gyroscope (FOG), a cutting-edge device that senses rotation with pinpoint accuracy without any moving parts.

1. What is a Fiber Optic Gyroscope?

Fiber optic gyroscope is an inertial sensor that measures angular velocity by utilizing the variation of light propagation characteristics in a rotating reference frame. Unlike MEMS gyroscopes or mechanical gyroscopes, it contains no rotating mass blocks or mechanical structures. Its core components include a multi-wound fiber optic loop, light source, and photoelectric detector.

Figure 1.1 Single-axis FOGs in different sizes (Source: GUIDENAV)

Fiber optic gyroscopes boast an exceptionally wide measurement range, capable of detecting both extremely slow rotations (such as 0.01°/h ≈ 3×10⁻⁶°/s, or 1% of Earth's rotational angular velocity) and high-speed spins like helicopter propellers (e.g., 600°/s). Like a "smart ruler," they can swiftly measure kilometers-long bridges while discerning micron-level differences, achieving an outstanding balance between dynamic range and precision.

Figure 1.2 Uniaxial, Biaxial, and Triaxial FOG (Source: KVH)

More remarkably, it operates at the speed of light, enabling "instant activation with zero latency." Unlike traditional mechanical gyroscopes that must wait for the rotor to reach a steady state, this "zero-start" advantage is revolutionary in high-tech fields requiring instantaneous response.

Figure 1.3 Small-scale low-precision FOG (Source: NEDAERO and KVH)

Figure 1.4 Comparison of single-axis and three-axis FOG

Table 1.1 Comparison of Single-axis and Three-axis Fiber Optic Gyroscope Selection

Fiber optic gyroscope (FOG) has many advantages, such as no mechanical moving parts, high reliability, instant start-up, high precision and easy integration. It is widely used in aerospace, ship navigation, underwater navigation and high-end inertial measurement system.

Figure 1.5 Typical Applications of FOG (Source: FOG Photonics)

II. Core Principle – Sagnac Effect

The core of fiber optic gyroscope is a simple thought experiment:

Imagine a circular track where two runners start simultaneously from the same point—one runs clockwise and the other counterclockwise. If the track itself rotates, the clockwise runner will reach the finish line first by "facing" the direction of rotation, while the counterclockwise runner will arrive slightly later by "chasing" the direction. Although both cover the same distance, their arrival times differ by a tiny margin.

Figure 2.1 Two runners moving in opposite directions encounter each other at a changing point as the track rotates

In the Sagnac effect, the propagation of light in the ring optical path is completely similar to this process. Although the two beams of light propagating clockwise and counterclockwise follow the same geometric path, the time difference in their arrival at the detector is caused by the rotation of the system during propagation, resulting in a phase difference.

Figure 2.2 Sagnac Effect

In an optical fiber gyroscope, light behaves like two athletes of comparable speed, with the fiber acting as their race track. The essence of this phenomenon in the optical realm is even more remarkable—it transcends the simple superposition observed in classical physics. According to relativity, the speed of light remains constant. What truly changes is the 'effective path' light must traverse within the rotating circuit.

The low-coherence light from the source is split into two beams and injected into the same coiled fiber, with one beam traveling clockwise and the other counterclockwise. When the device is stationary, both beams return simultaneously without interference. However, when the device rotates, the beam traveling clockwise encounters its endpoint continuously 'fleeing' and must cover an extra distance, while the counterclockwise beam's endpoint' approaches' it head-on.

Figure 2.3 Light entering and exiting the optical path

This phase difference is extremely small, measured in picoseconds (trillionths of a second), yet it can be captured by sophisticated optical systems and converted into rotational signals. Experiments demonstrate that the magnitude of this phase difference is directly proportional to the system's rotational angular velocity, allowing the angular velocity to be deduced by detecting phase changes in the interference signal. This phenomenon, known as the Sagnac effect, forms the physical basis for angular velocity measurement in fiber optic gyroscopes.

III. Composition

Fiber optic gyroscope is based on the Sagnac effect to measure the angular velocity of rotation, but the physical principle alone is not enough, but also need a set of specific devices, in order to convert this small optical effect into a readable measurement results.

Overall, the fiber optic gyroscope is not a single device, but a combination of multiple components including a light source, coupler, fiber loop, detector, and signal processing circuit. These components work in concert to enable light propagation and interference within the fiber, ultimately generating an electrical signal related to rotation.

Figure 3.1 Typical open-loop FOG workflow

Figure 3.2 Typical Closed-Loop FOG Workflow

​