1. Overview
Both Fiber Optic Gyroscope (FOG) and Ring Laser Gyroscope (RLG) are optical gyroscopes based on the Sagnac effect for inertial navigation and attitude control. However, they differ greatly in working principle, structure and performance.
2. Working Principle
FOG measures the phase difference between two light beams traveling in opposite directions inside coiled optical fiber. Long fiber coils are used to boost sensing sensitivity. RLG works as an optical resonant cavity (mostly triangular or square shape). It detects the frequency difference of counter-propagating light and calculates rotation speed directly by fringe counting, with no complex closed-loop control required.
3. Structure & Material
- FOG: Uses optical fiber as the light transmission medium. Low material cost, but susceptible to temperature change and fiber winding stress. The fiber coil is its core component.
- RLG: Light travels inside a high-precision resonant cavity. It is less affected by external interference and delivers top precision, yet the cavity and ultra-smooth mirrors are expensive to manufacture.
4. Core Performance Comparison
表格
| Item |
Ring Laser Gyroscope (RLG) |
Fiber Optic Gyroscope (FOG) |
| Precision |
Ultra-high, for aerospace & military navigation |
High, fits most inertial navigation systems |
| Cost |
High, complicated cavity processing |
Low, easy mass production |
| Size & Weight |
Bulkier, hard to miniaturize |
Compact and lightweight |
| Start-up Speed |
Requires warm-up & stabilization |
Instant start, no preheating |
| Shock & Vibration Resistance |
Moderate, sensitive to delicate optical parts |
Excellent, all-solid-state structure |
| Typical Applications |
Aerospace, missile, submarine high-precision navigation |
UAV, vehicle navigation, robot, attitude control |
Main strengths of FOG
All-solid design with no moving parts, high impact resistance, adjustable precision by changing fiber length, fast startup, easy digital integration and good cost-performance.
5. Performance in Underwater Navigation
FOG for underwater use
✅ Advantages: Solid structure for deep-sea pressure resistance, easy anti-corrosion packaging, fast startup, compact size, affordable for large-scale deployment ❌ Limitations: Prone to zero-bias drift from temperature variation, long-term stability is moderate, special sealing protection for optical components is needed
RLG for underwater use
✅ Advantages: Outstanding long-term stability, ultra-high positioning accuracy, strong immunity to electromagnetic interference, suitable for long-endurance deep-sea missions ❌ Limitations: Very high cost, bulky size, slow startup, vulnerable to shock, high requirements for waterproof & pressure-resistant encapsulation
Underwater Application Selection
表格
| Scenario |
Recommended Solution |
Reason |
| Military submarines & strategic submersibles |
RLG |
Ultra-high precision and long-time stability |
| ROV / AUV underwater robots |
FOG |
Lightweight, fast startup, low cost and easy integration |
| Seabed survey & scientific research |
RLG / FOG |
RLG for ultra-precise survey; FOG for general tasks |
| Civil underwater monitoring network |
FOG |
Low cost and flexible deployment |
| Diver navigation aids |
FOG |
Small size, instant response and shock resistance |
6. Summary
FOG is a cost-effective, compact and fast-response solution, ideal for projects with strict budget, size and quick-start requirements. RLG represents top-tier long-term precision and stability, mainly adopted in high-end aerospace, defense and high-grade underwater navigation.
Although built on the same fundamental optical principle, FOG and RLG are selected according to actual mission demands. With advances in integrated optics, FOG is replacing RLG in more and more commercial and industrial fields.
Quick takeaway: RLG – Top precision for long-endurance deep-sea & submarine navigation FOG – Light, robust and budget-friendly for underwater robots
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