# Ring laser
A ring laser is a laser whose resonator is a closed loop of optical path rather than a linear cavity with two end mirrors, so the intracavity light travels around the loop instead of bouncing back and forth[1](https://www.rp-photonics.com/ring_lasers.html). Because a ring resonator has no mirrors hit at normal incidence, it supports two independent propagation directions, clockwise and counterclockwise. Ring lasers are built in two main modes of operation: unidirectional, where light circulates in only one direction to produce a stable single-frequency traveling wave, and bidirectional, where the two counterpropagating beams form the basis of the [ring laser gyroscope](https://www.edgepedia.org/ring-laser-gyroscope)[1](https://www.rp-photonics.com/ring_lasers.html).
| Key fact | Detail |
|---|---|
| Resonator type | Ring resonator with no end mirrors at normal incidence; two possible propagation directions[1](https://www.rp-photonics.com/ring_lasers.html) |
| Unidirectional operation | Enforced with a non-reciprocal loss element such as a Faraday rotator plus polarizer, or an optical isolator; only a tiny loss difference between directions is needed[1](https://www.rp-photonics.com/ring_lasers.html) |
| Main benefit | A traveling wave avoids spatial hole burning in the gain medium, making stable single-frequency operation easy[1](https://www.rp-photonics.com/ring_lasers.html) |
| Gyroscope principle | Sagnac effect: counterpropagating modes acquire slightly different frequencies, and the beat note is proportional to the rotation rate, with scale factor \( S = 4A/(\lambda L) \)[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5) |
| Earth rotation precision | Large frame ring laser gyroscopes reach a relative precision of 3 parts in \( 10^{9} \) in one day of integration[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5) |
| Fundamental noise limit | Angle random walk from spontaneous-emission statistics; RLG performance is nearly at this quantum limit[3](https://ntrs.nasa.gov/api/citations/19900016901/downloads/19900016901.pdf) |
| Chip-scale milestone | A silicon-chip Brillouin ring laser gyroscope measured Earth's rotation (published February 2020)[4](https://authors.library.caltech.edu/records/3erjj-wqv53) |
## Ring resonator versus linear cavity
In a linear (Fabry–Pérot type) cavity, light bounces between two mirrors and the resonance condition requires a standing wave in the gain medium. In a ring cavity, the equivalent condition is that an integer number of wavelengths fit in the total round-trip distance, and the light can circulate as a traveling wave[5](https://puccini.chimica.uniba.it/didattica/corsi/laser/sam-laser-faq/laserioi.htm). The ring shape itself is flexible: the cavity can be planar or nonplanar, a regular or irregular polygon, a zig-zag, or even a closed loop of optical fiber, with sizes ranging from microchips a few millimeters on a side to cavities many meters across[5](https://puccini.chimica.uniba.it/didattica/corsi/laser/sam-laser-faq/laserioi.htm).
**Spatial hole burning.** In a standing-wave laser, the gain medium is fully depleted only at the peaks of the standing wave, leaving regions between peaks where other [longitudinal modes](https://www.edgepedia.org/longitudinal-mode) can build up; this is spatial hole burning, and it drives multimode operation as pumping increases[5](https://puccini.chimica.uniba.it/didattica/corsi/laser/sam-laser-faq/laserioi.htm). A unidirectional ring laser has no standing-wave interference pattern in the gain medium, so this mechanism disappears and single-frequency operation follows readily[1](https://www.rp-photonics.com/ring_lasers.html). A further benefit is higher output power, since the whole active material rather than only the regions around standing-wave maxima contributes to the laser output[6](https://lib.ysu.am/open_books/413756.pdf).
## Unidirectional operation
Left alone, a ring laser may oscillate in both directions at once. Unidirectional operation is enforced by introducing an element with different losses for the two propagation directions, typically a [Faraday rotator](https://www.edgepedia.org/faraday-rotator) combined with a polarizing element such as a Brewster surface of the laser crystal, or a packaged optical isolator[1](https://www.rp-photonics.com/ring_lasers.html). Only a tiny round-trip loss difference is required: in continuous-wave operation, gain saturation drives the round-trip gain to zero, so the less-favored direction then sees slightly negative net gain and cannot build up power[1](https://www.rp-photonics.com/ring_lasers.html).
**Monolithic designs.** Nd:YAG crystals cut as nonplanar ring oscillators (NPROs) contain the entire resonator path within a single shaped crystal and achieve unidirectional traveling-wave operation by means of an external magnetic field; such lasers are valued for stable, robust single-frequency output[1](https://www.rp-photonics.com/ring_lasers.html). On integrated photonic chips, unidirectional ring lasers usually require a non-reciprocal loss element such as an optical isolator, whose magneto-optic material must be flip-chip bonded to the silicon waveguide; isolator-free alternatives include spiral geometries, notched whispering-gallery resonators, and coupling a fraction of the counterclockwise power into the clockwise mode through an external reflector or an amplifying S-bend[7](https://optoelectronics.ece.ucsb.edu/sites/default/files/2017-06/Sacher15oe.pdf).
## The ring laser gyroscope
The most prominent bidirectional application is the ring laser gyroscope (RLG), an inertial rotation sensor based on the [Sagnac effect](https://www.edgepedia.org/sagnac-effect): two optical waves traveling in opposite directions around the same closed path experience a rotation-dependent pathlength difference[3](https://ntrs.nasa.gov/api/citations/19900016901/downloads/19900016901.pdf). In an RLG the laser cavity itself is rigidly attached to the sensed frame, with three mirrors in a triangular cavity or four in a square one[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5). The clockwise and counterclockwise waves each satisfy the resonance condition of an integer number of wavelengths in the cavity, so the Sagnac pathlength difference translates into a small frequency difference between the two modes[3](https://ntrs.nasa.gov/api/citations/19900016901/downloads/19900016901.pdf).
The two beams are combined on a detector, and the resulting beat frequency is proportional to the angular rotation rate \( \boldsymbol{\Omega} \) felt by the cavity:
\[ f_{s} = S\,\Omega\cos\theta, \qquad S = 4\frac{A}{\lambda L} \]
where \( A \) is the ring area, \( L \) its perimeter, \( \lambda \) the wavelength, and \( \theta \) the angle between the ring's area vector and the rotation axis[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5). The output is read as a fringe count, traditionally expressed in arcseconds of angular motion, with a scale factor of about 1.5 arcsec per count typical[3](https://ntrs.nasa.gov/api/citations/19900016901/downloads/19900016901.pdf).
**Sensitivity.** The rotation-induced pathlength changes are extremely small. For a typical RLG with optical loop area \( A = 0.01\ \mathrm{m^2} \), sensing a rotation rate of \( 10^{-4} \) deg/hr implies a pathlength difference of about \( 10^{-19} \) m, which is why gyroscope outputs must be integrated over time[3](https://ntrs.nasa.gov/api/citations/19900016901/downloads/19900016901.pdf). The fundamental performance limit is angle random walk caused by statistical fluctuations of spontaneous emissions from the laser, and RLG performance is nearly at this quantum limit[3](https://ntrs.nasa.gov/api/citations/19900016901/downloads/19900016901.pdf).
**Lock-in and bias control.** The main performance limitation is coupling between the two counterpropagating modes: a small fraction of each wave is backscattered at every mirror, and the resulting interference can lock the two frequencies together at low rotation rates[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5). Small-scale RLGs use mechanical dithering to increase the bias between the modes and avoid locking, while large frame rings use the [Earth's rotation](https://www.edgepedia.org/earths-rotation) rate itself as the bias[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5).
## Large frame gyroscopes and geoscience
Large frame RLGs, bolted to the [Earth's crust](https://www.edgepedia.org/earths-crust), are used to measure the Earth's rotation rate and are considered the most sensitive instruments for inertial angular rotation measurements[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5). State-of-the-art large rings reach a relative precision of 3 parts in \( 10^{9} \) for the Earth rotation rate in one day of integration, and shot-noise-limited sensitivity scales with the square of the cavity size[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5). One research instrument built for this purpose is a large ring laser of \( 2.56\ \mathrm{m^2} \) area operating on the \( 2s2 \rightarrow 2p4 \) transition of neon, used to monitor the rotation of the Earth[8](https://opg.optica.org/spotlight/spotlight-modal.cfm?id=403457).
Because the signal is a beat note, the same device combines broad bandwidth, spanning from kilohertz down to DC, with a very large dynamic range; a single instrument can record both microseismic events and high-magnitude nearby earthquakes[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5). Multicomponent systems extend this to full rotation sensing: ROMY, a large multicomponent ring laser, was built to serve both geodesy and geophysics, covering applications from measuring Earth and planetary rotation to rotational ground motions from earthquakes and building vibrations[9](https://academic.oup.com/gji/article/225/1/684/6064309).
**Fundamental physics.** The GINGER project proposes an array of large frame RLGs on Earth to test the Lense–Thirring effect, a general-relativistic frame-dragging prediction, with 1% accuracy; this requires improving the relative accuracy of Earth rotation rate measurement from 1 part in \( 10^{9} \) toward 1 part in \( 10^{12} \)[2](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5). A prototype called GINGERino has been installed in the Gran Sasso underground laboratory to investigate the properties of a deep underground site for a future apparatus[10](https://arpi.unipi.it/handle/11568/796104).
## Miniaturization and related designs
Chip-scale optical gyroscopes have been a long-standing goal of integrated photonics, since solid-state systems would be robust and shock-resistant. In 2020 a Caltech-led team reported a gyroscope using Brillouin ring lasers on a silicon chip whose stability and sensitivity enabled a measurement of Earth's rotation, a benchmark result for this class of device[4](https://authors.library.caltech.edu/records/3erjj-wqv53). Passive approaches also scale: a large-scale passive laser gyroscope that locks two beams to adjacent modes of a square ring cavity achieved a rotation resolution of about \( 2 \times 10^{-9} \) rad/s at a 1000 s integration time[11](https://arxiv.org/abs/1901.05446).
Ring cavities also serve purposes unrelated to rotation sensing. Bidirectional Kerr-lens mode-locked femtosecond Ti:sapphire ring lasers produce synchronized counterpropagating pulse trains, with observed beat frequencies of 80 to 150 kHz in one demonstration[12](https://www.sciencedirect.com/science/article/pii/S0030401897002101). In fiber systems, ring geometries support mode-locked soliton lasers, where an isolator is usually included to force unidirectional operation, though self-started unidirectional soliton operation without an isolator has been demonstrated through nonlinear polarization rotation[13](https://arxiv.org/abs/0910.5805).
## References
1. [Ring Lasers, RP Photonics Encyclopedia](https://www.rp-photonics.com/ring_lasers.html)
2. [Analysis of ring laser gyroscopes including laser dynamics, The European Physical Journal C (2019)](https://link.springer.com/article/10.1140/epjc/s10052-019-7089-5)
3. [JPL Progress Report 42-101: Optical gyroscopes (ring-laser gyroscope), NASA NTRS](https://ntrs.nasa.gov/api/citations/19900016901/downloads/19900016901.pdf)
4. [Earth rotation measured by a chip-scale ring laser gyroscope, CaltechAUTHORS (2020)](https://authors.library.caltech.edu/records/3erjj-wqv53)
5. [Sam's Laser FAQ, Items of Interest: ring cavity operation](https://puccini.chimica.uniba.it/didattica/corsi/laser/sam-laser-faq/laserioi.htm)
6. [Principles of Lasers, 5th Edition (Svelto)](https://lib.ysu.am/open_books/413756.pdf)
7. [Unidirectional hybrid silicon ring laser, Optics Express 23, 26369 (2015)](https://optoelectronics.ece.ucsb.edu/sites/default/files/2017-06/Sacher15oe.pdf)
8. [The Macek and Davis experiment revisited: a large ring laser interferometer on the neon 2s2 → 2p4 transition, Optica Spotlight](https://opg.optica.org/spotlight/spotlight-modal.cfm?id=403457)
9. [ROMY: a multicomponent ring laser for geodesy and geophysics, Geophysical Journal International (2021)](https://academic.oup.com/gji/article/225/1/684/6064309)
10. [High-Accuracy Ring Laser Gyroscopes: Earth Rotation Rate and Relativistic Effects, University of Pisa repository](https://arpi.unipi.it/handle/11568/796104)
11. [A large scale passive laser gyroscope for Earth rotation sensing, arXiv:1901.05446](https://arxiv.org/abs/1901.05446)
12. [Bidirectional Kerr-lens mode-locked femtosecond ring laser, Optics Communications 141, 53 (1997)](https://www.sciencedirect.com/science/article/pii/S0030401897002101)
13. [Self-started unidirectional operation of a fiber ring soliton laser without an isolator, arXiv:0910.5805](https://arxiv.org/abs/0910.5805)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Waves and optics › Optical technologies and instruments › Interferometers and optical cavities › Optical cavities and resonators › Ring and traveling-wave resonators*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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