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A ring laser gyroscope (RLG) consists of a ring laser having two independent counter-propagating resonant modes over the same path; the difference in phase is used to detect rotation. It operates on the principle of the
Sagnac effect The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferomete ...
which shifts the nulls of the internal standing wave pattern in response to angular rotation.
Interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...
between the counter-propagating beams, observed externally, results in motion of the standing wave pattern, and thus indicates rotation.


Description

The first experimental ring laser gyroscope was demonstrated in the US by Macek and Davis in 1963. Various organizations worldwide subsequently developed ring-laser technology further. Many tens of thousands of RLGs are operating in
inertial navigation system An inertial navigation system (INS; also inertial guidance system, inertial instrument) is a navigation device that uses motion sensors (accelerometers), rotation sensors (gyroscopes) and a computer to continuously calculate by dead reckoning th ...
s and have established high accuracy, with better than 0.01°/hour bias uncertainty, and mean time between failures in excess of 60,000 hours. Ring laser gyroscopes can be used as the stable elements (for one degree of freedom each) in an
inertial reference system An inertial navigation system (INS; also inertial guidance system, inertial instrument) is a navigation device that uses motion sensors (accelerometers), rotation sensors ( gyroscopes) and a computer to continuously calculate by dead reckoning ...
. The advantage of using an RLG is that there are no moving parts (apart from the dither motor assembly (see further description below), and laser-lock), compared to the conventional spinning
gyroscope A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining Orientation (geometry), orientation and angular velocity. It is a spinning wheel or disc in ...
. This means there is no friction, which eliminates a significant source of drift. Additionally, the entire unit is compact, lightweight and highly durable, making it suitable for use in mobile systems such as aircraft, missiles, and satellites. Unlike a mechanical gyroscope, the device does not resist changes to its orientation. Contemporary applications of the ring laser gyroscope include an embedded
GPS The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide geol ...
capability to further enhance accuracy of RLG
inertial navigation system An inertial navigation system (INS; also inertial guidance system, inertial instrument) is a navigation device that uses motion sensors (accelerometers), rotation sensors (gyroscopes) and a computer to continuously calculate by dead reckoning th ...
s on military aircraft, commercial airliners, ships, and spacecraft. These hybrid INS/GPS units have replaced their mechanical counterparts in most applications. "Ring laser gyroscopes (RLG) have demonstrated to currently be the most sensitive device for testing rotational motion with respect to an inertial frame."


Principle of operation

According to the
Sagnac effect The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferomete ...
, rotation induces a small difference between the time it takes light to traverse the ring in the two directions. This introduces a tiny separation between the frequencies of the counter-propagating beams, a motion of the
standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect t ...
pattern within the ring, and thus a beat pattern when those two beams interfere outside the ring. Therefore, the net shift of that interference pattern follows the rotation of the unit in the plane of the ring. RLGs, while more accurate than mechanical gyroscopes, suffer from an effect known as "lock-in" at very slow rotation rates. When the ring laser is hardly rotating, the frequencies of the counter-propagating laser modes become almost identical. In this case, crosstalk between the counter-propagating beams can allow for injection locking, so that the standing wave "gets stuck" in a preferred phase, thus locking the frequency of each beam to that of the other, rather than responding to gradual rotation. Forced
dithering Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images. Dither is routinely used in processing of both digital audio and video data, and is often ...
can largely overcome this problem. The ring laser cavity is rotated clockwise and anti-clockwise about its axis using a mechanical spring driven at its resonance frequency. This ensures that the angular velocity of the system is usually far from the lock-in threshold. Typical rates are 400 Hz, with a peak dither velocity on the order of 1 degree per second. Dither does not fix the lock-in problem completely, as each time the direction of rotation is reversed, a short time interval exists in which the rotation rate is near zero and lock-in briefly can occur. If a pure frequency oscillation is maintained, these small lock-in intervals can accumulate. This was remedied by introducing noise to the 400 Hz vibration.''Knowing Machines'', Donald MacKenzie, The MIT Press, (1991). A different approach to avoiding lock-in is embodied in the Multioscillator Ring Laser Gyroscope, wherein what is effectively two independent ring lasers (each having two counterpropagating beams) of opposite circular polarization coexist in the same ring resonator. The resonator incorporates polarization rotation (via a nonplanar geometry) which splits the fourfold-degenerate cavity mode (two directions, two polarizations each) into right- and left-circular-polarized modes separated by many hundreds of MHz, each having two counterpropagating beams. Nonreciprocal bias via the
Faraday effect The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the ...
, either in a special thin Faraday rotator, or via a longitudinal magnetic field on the gain medium, then further splits each circular polarization by typically a few hundred kHz, thus causing each ring laser to have a static output beat frequency of hundreds of kHz. One frequency increases and one decreases, when inertial rotation is present; the two frequencies are measured and then digitally subtracted to finally yield the net Sagnac-effect frequency splitting and thus determine the rotation rate. The Faraday bias frequency is chosen to be higher than any anticipated rotation-induced frequency difference, so the two counterpropagating waves have no opportunity to lock-in.


Fibre optic gyroscope

A related device is the
fibre optic gyroscope A fibre-optic gyroscope (FOG) senses changes in orientation using the Sagnac effect, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed throu ...
which also operates on the basis of the Sagnac effect, but in which the ring is not a part of the laser. Rather, an external laser injects counter-propagating beams into an
optical fiber An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...
ring, where rotation causes a relative phase shift between those beams when interfered after their pass through the fiber ring. The phase shift is proportional to the rate of rotation. This is less sensitive in a single traverse of the ring than the RLG, in which the externally observed phase shift is proportional to the accumulated rotation itself, not its derivative. However, the sensitivity of the fiber optic gyro is enhanced by having a long optical fiber, coiled for compactness, in which the Sagnac effect is multiplied according to the number of turns.


Example applications

*
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* ASM-135 US Anti-satellite missile *
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with the AMI upgrade * EF-111 Raven *
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* MC-130E Combat Talon I and MC-130H Combat Talon II * MQ-1C Warrior *MK39 Ship's Internal Navigation System used in NATO surface ships and submarines *
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(with upgrade) *
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missile. * MH-60R, MH-60S, SH60F and SH60B Seahawk helicopters *
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See also

*
Accelerometer An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...
*
Active laser medium The active laser medium (also called a gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from ...
*
Hemispherical resonator gyroscope The hemispherical resonator gyroscope (HRG), also called wine-glass gyroscope or mushroom gyro, is a compact, low-noise, high-performance angular rate or rotation sensor. An HRG is made using a thin solid-state hemispherical shell, anchored by a ...
*
Laser construction A laser is constructed from three principal parts: *An energy source (usually referred to as the ''Laser pumping, pump'' or ''pump source''), *A ''gain medium'' or ''Active laser medium, laser medium'', and *Two or more mirrors that form an ''o ...
*
Laser science Laser science or laser physics is a branch of optics that describes the theory and practice of lasers. Laser science is principally concerned with quantum electronics, laser construction, optical cavity design, the physics of producing a popula ...
*
List of laser applications Many scientific, military, medical and commercial laser applications have been developed since the invention of the laser in 1958. The coherence (physics), coherency, high monochromaticity, and ability to reach extremely high Radiant flux, powers ...
*
List of laser types This is a list of laser types, their operational wavelengths, and their applications. Thousands of kinds of laser are known, but most of them are used only for specialized research. Overview Gas lasers Chemical lasers Used as directed-ener ...
*
Optical ring resonators An optical ring resonator is a set of waveguides in which at least one is a closed loop coupled to some sort of light input and output. (These can be, but are not limited to being, waveguides.) The concepts behind optical ring Optical cavity, res ...
*
Fibre optic gyroscope A fibre-optic gyroscope (FOG) senses changes in orientation using the Sagnac effect, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed throu ...


References


External links


Canterbury Ring Laser Research Group


* {{cite journal , author=A.D. King , year=1998 , title=Inertial Navigation – Forty Years of Evolution , journal=GEC Review , publisher=
General Electric Company plc The General Electric Company (GEC) was a major British industrial conglomerate involved in consumer and defence electronics, communications, and engineering. It was originally founded in 1886 as G. Binswanger and Company as an electrical good ...
, volume=13 , issue=3 , pages=140–149 , url=http://www.imar-navigation.de/downloads/papers/inertial_navigation_introduction.pdf Gyroscopes Optical devices Laser applications Missile technology Spacecraft attitude control