1. Overview
Different from traditional mechanical gyroscopes with rotating rotors and common vibrating MEMS gyroscopes, the
Fiber Optic Gyroscope (FOG) is a high-precision inertial sensor without any moving mechanical parts. It uses the optical transmission characteristics of coiled optical fibers to accurately measure the angular velocity of rotating objects. With unique advantages of ultra-high precision, excellent stability and instant startup, FOG fills the demand gap of high-end inertial navigation that ordinary sensors cannot meet.
FOG has an extremely wide dynamic measurement range. It can capture extremely slow rotation as subtle as the Earth’s rotation, and also respond quickly to high-speed rotation of aircraft and rotating machinery. It supports zero-delay startup and works stably without the warm-up time required by mechanical gyroscopes, which is critical for high-precision and fast-response industrial and aerospace scenarios.
According to structural configuration, FOG is divided into single-axis and three-axis types. Single-axis FOG features small size, light weight, low cost and simple calibration, suitable for basic single-direction attitude stabilization systems. Three-axis FOG can realize full three-dimensional attitude tracking with higher overall accuracy, but it is larger in size, higher in cost and more complex to calibrate, and is widely used in aircraft, unmanned vehicles, ships and other high-end full-position navigation equipment. Relying on its outstanding comprehensive performance, FOG is widely applied in aerospace, marine navigation, underwater detection and high-precision inertial measurement systems.
2. Core Working Principle: Sagnac Effect
The entire working mechanism of fiber optic gyroscopes is based on the classic Sagnac effect, which can be understood through a simple and intuitive analogy. Imagine two runners starting from the same point on a circular track, one running clockwise and the other counterclockwise. If the entire track rotates meanwhile, the runner moving in the same direction as the track rotation needs to travel a longer distance, while the opposite runner travels a shorter distance, resulting in a tiny time difference in their arrival.
FOG works in the exact same logic with light as the "runner" and coiled optical fiber as the "circular track". The internal light source emits stable low-coherence light, which is split into two identical beams. The two beams travel clockwise and counterclockwise along the fiber loop respectively. When the gyroscope is static, the two beams return at the same time with no phase difference.
Once the device rotates, the optical transmission path changes subtly. One beam of light travels a longer effective path, while the other travels a shorter one, forming a tiny time difference and phase difference. This phase difference is strictly proportional to the rotation angular velocity. By detecting and calculating the optical interference change, the system can accurately solve the real-time rotation speed of the carrier. It is worth noting that this effect originates from the change of effective optical path under rotation, rather than the change of light speed, ensuring ultra-high measurement stability and accuracy.
3. Basic Composition of FOG
Fiber optic gyroscope is a complete optoelectronic integrated system, composed of light source, optical coupler, fiber coil, photodetector and signal processing circuit. All components cooperate precisely to convert tiny optical phase changes into readable and accurate angular velocity electrical signals.
Light Source: It provides stable low-coherence light (such as SLD and ASE light sources) for the whole system. Stable light output is the basic guarantee for suppressing optical noise and ensuring long-term measurement accuracy of FOG.
Optical Coupler & Y-junction Waveguide: As the core optical splitting and combining component, the 2×2 X-type coupler and Y-waveguide split the single beam of light into two reverse propagating beams, and recombine the returned light. Meanwhile, it realizes polarization control and phase modulation, laying the foundation for Sagnac effect detection.
Fiber Coil: It is the core sensing unit of FOG, made of ultra-long optical fiber tightly coiled in multiple turns. More coil turns and longer fiber length can amplify the tiny Sagnac phase difference, effectively improving sensor sensitivity and precision. The winding process and structural stability of the fiber coil directly determine the overall performance of the gyroscope.
Photodetector: It acts as the optical signal receiver of the system. It captures the interfering light beams with phase differences, converts subtle light intensity changes into weak electrical signals, and provides original data for subsequent algorithm solving.
Signal Processing Circuit: It amplifies, demodulates and calibrates the weak original electrical signals, filters out noise interference, and finally calculates and outputs accurate, stable and usable angular velocity data.
4. Advantages, Limitations and Application Positioning
Compared with mechanical gyroscopes and MEMS gyroscopes, fiber optic gyroscopes have unique core strengths. With no mechanical moving parts, they have zero mechanical wear, extremely low long-term drift and excellent working stability. They feature strong vibration and shock resistance, instant startup without delay, and ultra-high measurement accuracy, which can adapt to long-term continuous operation in harsh environments such as aviation, marine and deep-sea scenarios.
Meanwhile, FOG also has obvious application limitations. It requires ultra-long optical fiber and precision optical devices, resulting in larger volume and weight, higher manufacturing cost and relatively higher power consumption. Therefore, it cannot be miniaturized and popularized like consumer-grade MEMS gyroscopes, and is not suitable for tiny battery-powered portable devices.
In the industrial and inertial navigation market, FOG does not replace MEMS gyroscopes, but forms a perfect complementary relationship. MEMS gyroscopes focus on miniaturization, low cost and mass civilian applications, while fiber optic gyroscopes are committed to high-precision, high-stability and high-reliability professional navigation scenarios. It is the core inertial sensor for high-end equipment such as aircraft, ships, deep-sea detectors and high-precision inertial navigation systems, and plays an irreplaceable key role in the high-end inertial sensing field.
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