Fiber-Optic Gyroscope
Two laser beams racing in opposite directions around a kilometre of coiled glass — the one fighting the rotation always loses by exactly enough to tell you how fast you're turning.
Measures the angular rate of rotation of whatever it is bolted to. The output is an electrical signal proportional to how fast the platform is turning about the gyro's sensitive axis. Navigation-grade FOGs integrate that rate to compute absolute pointing angles accurate enough to guide rockets, satellites, submarines, and aircraft through GPS-denied environments for hours without position drift.
A superluminescent diode or laser injects light into both ends of a wound fibre coil — typically 100 m to 5 km long. The two counter-propagating beams recombine at a photodetector. When the coil is stationary, the optical paths are identical and the beams interfere constructively at the output port. When the coil rotates at angular rate Ω, the Sagnac effect introduces a phase difference ΔΦ = (8πNA/λc)Ω, where N is the number of turns, A the coil area, λ the wavelength, and c the speed of light. A closed-loop electronic servo holds the output at a null and feeds back a known phase ramp to cancel the Sagnac shift, making the output linear and immune to light-intensity fluctuations.
In plain terms
Imagine two runners on a circular track who start at the same point and run in opposite directions. If the whole track starts spinning, one runner has to cover a tiny bit more ground and the other a bit less. A fiber-optic gyroscope does this with light instead of runners — and it can detect a rotation slow enough to take 100,000 years to complete one full circle.
Also called: FOG, IFOG, interferometric fiber optic gyro, Sagnac gyro.
Why it matters
- Contains no bearings, no spinning mass, and no gas-filled cavity — there is nothing to wear out, eliminating the alignment drift that limits mechanical gyroscopes.
- MTBF figures of 100,000 hours or more are routinely quoted; the Exail UmiX IMU series specifies 120,000 hours.
- Navigation-grade performance (bias instability < 0.01 °/h) is achievable in packages small enough for tactical missiles and satellite attitude-control units.
- The Sagnac phase shift formula contains only fundamental constants and geometry, so scale factor is inherently stable and traceable without periodic re-calibration against an external reference.
Where it lives
- Northrop Grumman LN-200 — tactical-grade IMU (0.5–1 °/hr bias) on hundreds of spacecraft, UAVs, and guided weapons
- Exail Astrix 200 — navigation-grade space gyro (ARW 0.0001 °/√h, bias 0.0005 °/h) on ESA Aeolus, Sentinel-2, and MTG satellites
- KVH DSP-1750 — compact single-axis FOG (bias ≤ 0.05 °/hr, ARW ≤ 0.013 °/√hr) for stabilised camera platforms and antenna pointing
- Boeing 777 and Airbus A380 inertial reference systems — triple-redundant FOG-based INS
Key numbers
- Bias instability (navigation grade): 0.0005–0.01 °/h (Exail Astrix 200 / Astrix 90; ESA-qualified)
- Angle random walk (navigation grade): 0.0001–0.005 °/√h (Exail Astrix series; lower is better)
- Bias instability (tactical grade): 0.5–2 °/h (Northrop LN-200 / KVH DSP-1750)
- Input rate range: ±200 to ±1000 °/s (KVH DSP-1750 high-rate variant)
- Fibre coil length: 100 m – 5 km (Longer coil → greater Sagnac sensitivity)
- MTBF: >100,000 hours (Polaris FOG-50; Exail UmiX: 120,000 hr)
Surprising facts
- A navigation-grade FOG can detect a rotation rate of 0.0001 °/hr — equivalent to one full revolution in roughly 10 million years — indistinguishable from standing still in everyday terms.
- The Sagnac phase shift that a tactical FOG measures is typically a fraction of a wavelength of light — about 10⁻⁸ radians — yet it is read out with better than 1% accuracy.
- FOG coils for inertial navigation can contain up to 5 km of single-mode optical fibre, all wound onto a spool the size of a coffee mug.
- Because the speed of light is the reference, a FOG self-calibrates: a zero rotation rate produces exactly zero phase shift, with no external reference required.
A short history
Georges Sagnac demonstrated optical rotation sensing in 1913 using a free-space ring interferometer, but a practical fibre implementation had to wait for low-loss single-mode fibre. Victor Vali and Richard Shorthill at the University of Utah made the first experimental demonstration of a fibre-optic gyroscope in 1976, using 150 m of fibre and achieving a sensitivity then limited by coherence-length effects. Reciprocity theory by Ulrich (1980) and closed-loop serrodyne readout (McDonnell Douglas, 1980) resolved the key noise and linearity problems. By the mid-1980s, the FOG had reached engineering maturity and entered production for aviation and defence applications, eventually displacing mechanical gyros in most aircraft inertial reference units during the 1990s.
Sources
- Northrop Grumman LN-200 FOG Family — product page with bias class specifications (https://www.northropgrumman.com/what-we-do/mission-solutions/assured-navigation/ln-200-fog-family-advanced-airborne-imu-ahrs)
- Exail Astrix series — space gyroscope datasheet (ARW, bias stability) (https://www.exail.com/product-range/astrix-series)
- KVH DSP-1750 FOG — digital output datasheet (bias ≤ 0.05 °/hr, ARW ≤ 0.013 °/√hr) (https://canalgeomatics.com/wp-content/uploads/2019/11/kvh-dsp-1750-fog-digital-datasheet.pdf)
- Merlo, Norgia & Donati — Fiber Gyroscope Principles (Univ. Pavia); FOG history and Sagnac formula (http://www-9.unipv.it/donati/papers/2c.pdf)