3.1 Light Source

The light source is the starting point of optical signal in fiber optic gyroscope. Its main function is to produce stable light and send it into the system. It is often used low coherence light, such as SLD or ASE light source.

Figure 3.3 FOG light source

The light emitted by the source first enters the coupler, which uniformly splits the light into two beams and guides them separately into subsequent optical paths (fiber optics). Through the coupler, the originally single-path light is allocated to propagate in both clockwise and counterclockwise directions, laying the foundation for subsequent utilization of the Sagnac effect.

3.2 Coupler

An optical fiber coupler is a fiber optic component designed to redistribute optical signals. It encompasses various fiber optic devices such as optical splitters, optical combiners, and optical couplers. This component can either distribute signals from a single fiber to multiple fibers or combine signals from multiple fibers into a single fiber.

(1) X-Type coupler

Figure 3.4 Optical Fiber Coupler

The X-coupler integrates the functions of a splitter and a combiner within a single package. It combines and distributes optical power from two input fibers, then transmits it separately to two output fibers. Also known as a 2×2 coupler.

Figure 3.5 X-type Coupler (2x2)

(2) Y-waveguide

A Y-junction waveguide is a type of Y-shaped coupler featuring a distinctive Y-configuration.

The light from the source is incident on the Y waveguide and split into two beams by the Y-branch waveguide. These beams propagate into the fiber coil in clockwise and counterclockwise directions, respectively. After completing one full cycle within the fiber coil, the beams are recombined into a single beam by the Y-branch waveguide and ultimately reach the photodetector.

In addition to the functions of splitting and combining light, the Y waveguide can also realize the functions of polarization and depolarization, phase modulation and so on.

Figure 3.6 Multifunctional Integrated Optical Device of Lithium Bismuth Oxide (Y Waveguide)

3.3 Optical Fiber Ring

After entering the 2×2 coupler, light travels through a Y-waveguide into the optical fiber loop. This loop, formed by winding an extended fiber, enables light to propagate along a closed path. When light propagates clockwise and counterclockwise simultaneously within the loop, rotation of the fiber creates a slight time difference between the two beams, resulting in a phase difference. This phenomenon, known as the Sagnac effect, forms the core mechanism by which fiber gyroscopes detect rotational information.

To detect minute rotations, optical fibers require lengths of hundreds or even thousands of meters. While such extensive optical paths are impractical, the Sagnac effect reveals that sensitivity is directly proportional to the area enclosed by the optical path. By coiling the flexible fiber multiple times, we can maintain the same effective area while significantly reducing its physical dimensions.

Figure 3.7 Optical Fiber Ring

In fiber optic gyroscopes, optical fibers are typically wound in dozens, hundreds, or even more turns. This is because the phase difference generated by the Sagnac effect depends on the effective area enclosed by the optical path, rather than solely on the size of a single turn. By increasing the number of winding turns, the light propagation distance can be significantly extended within a limited volume, thereby amplifying the time and phase differences caused by rotation.

The principle can be simply understood as: the longer light travels through the fiber loop, the more pronounced the effect of rotation becomes. This explains why high-precision fiber gyroscopes typically feature longer fiber loops, while low-precision or educational devices use relatively shorter fiber lengths. The winding quality of the fiber loop directly impacts the measurement accuracy of the fiber gyroscope, requiring specialized high-precision winding equipment. Fiber gyroscopes not only rely on sophisticated physical principles but also demand extremely stringent manufacturing processes.

FIG. 3.8 Optical fiber loop system

3.4 Photoelectric Detector

In fiber optic gyroscopes, the photodetector is positioned at the end of the optical system. Its primary function is to receive light signals reflected from the fiber loop and convert them into electrical signals. As a device based on the photoelectric effect, the photodetector transforms optical signals into electrical ones. Functioning like the human eye, it enables the detection of both visible and invisible faint signals.

Optical interference occurs when two light beams propagating clockwise and counterclockwise converge again within a system. The rotation creates a phase difference between the beams, causing the intensity of the interference light to fluctuate. Photoelectric detectors utilize this phenomenon to convert minute intensity variations into electrical signals. Since interference signals are typically extremely weak, these detectors must have high sensitivity to ensure subsequent circuits can accurately capture rotation-related information.

Figure 3.9 Photoelectric Detector

3.5 Signal Processing Circuit

The electrical signal output by the photoelectric detector is extremely weak and cannot be used directly. Therefore, a series of signal processing steps are required to obtain the final measurement of the rotational angular velocity. The entire process can be simply divided into the following steps:

Ø Pre-amplification: Boosts extremely weak electrical signals to a suitable level for stable processing by subsequent circuits.

Ø Signal conversion and demodulation: The amplified electrical signal is converted into a signal processable by computer or digital circuit, and the information related to optical phase difference is extracted from it.

Ø Control and Output: The controller calculates the value corresponding to the rotational angular velocity based on the demodulation results, and provides the result to the external system through the output interface.

Figure 3.10: FOG Signal Processing Circuit (Top Circuit Board in the Diagram)

IV. Summary

The defining feature of fiber optic gyroscopes is their use of light rather than mechanical structures to measure rotation, which grants them distinct advantages in critical performance metrics:

Ø High precision: The system measures rotation using optical principles, eliminating reliance on mechanical vibrations for enhanced accuracy.

Ø Stable performance: The absence of high-speed mechanical components inside ensures minimal drift during prolonged operation.

Ø Excellent vibration and impact resistance: Maintains reliable measurement performance even in vibrating environments such as aircraft and ships.

Ø High reliability and long service life: Optical fibers and components show minimal wear, making them ideal for continuous long-term operation.

Despite their superior performance, fiber optic gyroscopes are not universally applicable.

Ø The bulky size and weight: The need to wind long optical fibers makes miniaturization challenging.

Ø High costs: The expenses increase due to fiber materials, optical components, and precision manufacturing processes.

Ø High power consumption: Not ideal for battery-powered micro devices.

Fiber optic gyroscopes are not meant to replace all gyroscopes, but rather to play a pivotal role in fields requiring high precision, reliability, and environmental adaptability. Unlike conventional MEMS (Micro-Electro-Mechanical Systems) gyroscopes, they function more like a 'professional long-distance runner' —prioritizing stability and accuracy over extreme miniaturization and cost-effectiveness, silently maintaining orientation in aerospace, deep-sea, and high-end equipment applications.

The next time you board a plane or imagine a deep-sea probe navigating autonomously, remember that a beam of light might be racing through a delicate fiber optic loop, using its subtle time difference to guide our way.

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