# What is a "fiber optic gyroscope" (FOG)? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [October 22, 2026, 8:39am UTC](https://forum.ofscn.net/t/what-is-a-fiber-optic-gyroscope-fog/3124 "2026-10-22T08:39:18Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\ **Post date:** [October 22, 2026, 8:39am UTC](https://forum.ofscn.net/t/what-is-a-fiber-optic-gyroscope-fog/3124/1 "2026-10-22T08:39:18Z")
Why do missiles and airplanes use coiled fiber optics for navigation? ---
**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\ **Post date:** [October 22, 2026, 8:40am UTC](https://forum.ofscn.net/t/what-is-a-fiber-optic-gyroscope-fog/3124/2 "2026-10-22T08:40:05Z")
The “coiled optical fibers” seen in the navigation systems of missiles and aircraft have a core component called a **Fiber Optic Gyroscope (FOG)**. It is the key sensor used in modern Inertial Navigation Systems (INS) to measure the angular velocity (rotational attitude) of a carrier. Its working principle and the physical basis of its structural design are as follows: * * * ### 1. Core Physical Principle: Sagnac Effect The fundamental operating mechanism of a fiber optic gyroscope is based on the **Sagnac Effect** , discovered by the French physicist Georges Sagnac in 1913: 1. **Light Path Propagation Mechanism** : A beam of coherent light is split into two beams of equal intensity, which propagate in both **clockwise (CW)** and **counter-clockwise (CCW)** directions along the same closed optical path. 2. **Introduction of Optical Path Difference by Rotation** : 3. **Relationship between Phase Difference and Angular Velocity** : When the two coherent light beams converge and interfere at the exit, a small relative phase difference \Delta \Phi\_s is generated: By measuring the phase changes in the interference signal using high-precision optoelectronic detectors and modulation-demodulation circuits, the real-time angular velocity of the missile or aircraft can be accurately calculated in reverse. Integration of this velocity can then yield the pitch, roll, and yaw attitude angles of the flight vehicle. * * * ### 2. Why Must It Be “Coiled Many Times”? As can be seen from the theoretical formula, the Sagnac phase difference \Delta \Phi\_s is directly proportional to the **total fiber length L** and the **loop radius R** : - **Enhancing Sensing Sensitivity** : If only a single loop of fiber is used, the phase difference caused by Earth’s rotation or minor angular velocities within a conventional airborne size (e.g., a diameter of a dozen centimeters) is extremely weak, far below the noise floor of ordinary optical systems. - **Signal Amplification Factor** : To achieve the tactical or strategic accuracy required for inertial navigation (e.g., bias stability of 0.01^\circ/\text{h} or higher), engineering practice involves precisely winding long lengths of optical fiber (typically specialized polarization-maintaining fiber, PMF), hundreds to thousands of meters, into a coil using special symmetrical winding methods (such as quadrupole or octupole symmetry winding). This effectively amplifies the interference phase difference by thousands of times or more. - **Canceling Environmental Disturbances** : The special symmetrical coiling process also effectively cancels out non-reciprocal thermal stress noise (Shupe Effect) caused by external temperature gradients. * * * ### 3. Technical Advantages of Fiber Optic Gyroscopes Compared to Traditional Mechanical Gyroscopes The widespread adoption of fiber optic gyroscopes in the navigation systems of missiles and aircraft is mainly due to the following engineering characteristics: - **All-Solid-State with No Mechanical Moving Parts** : Unlike traditional mechanical gyroscopes, there are no high-speed rotating rotors and bearing structures internally. This provides extremely strong resistance to high overloads (capable of withstanding impact accelerations of tens of thousands of g’s during missile launch) and a very high Mean Time Between Failures (MTBF). - **Extremely Short Startup Time** : They can enter the measurement state within milliseconds of power-on, eliminating the need for the motor spin-up preheating process required by traditional mechanical gyroscopes, thus adapting to rapid emergency launch requirements. - **Wide Dynamic Range and High Linearity** : Capable of handling both extreme angular velocity maneuvers and large-range, small-scale attitude corrections. * * * ### Technical Attribution Statement Fiber Optic Gyroscopes (FOGs) and their specialized fiber optic sensing coils belong to the category of **Inertial Navigation and Inertial Measurement (IMU)** systems. Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) focuses its technology and core product lines on **Fiber Bragg Grating (FBG) Sensing Technology**, including FBG temperature sensors, strain sensors, displacement sensors, 3D force sensors, and FBG demodulators for structural health monitoring and industrial physical quantity measurement equipment. **We do not develop or manufacture FOGs or the fiber optic sensing coils used in inertial navigation systems.**