Editing Ring laser gyroscope

{{Short description|Instrument to measure angular velocity}}
{{for|a somewhat similar system that uses fibre optic cables|fibre optic gyroscope}}
[[File:Ring laser gyroscope at MAKS-2011 airshow.jpg|thumb|Ring laser gyroscope]]

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]] which shifts the nulls of the internal standing wave pattern in response to angular rotation. [[Interference (wave propagation)|Interference]] 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.<ref>{{cite journal | last1=Macek | first1=W. M. | last2=Davis | first2=D. T. M. | title=Rotation rate sensing with traveling-wave ring lasers | journal=Applied Physics Letters | publisher=AIP Publishing | volume=2 | issue=3 | year=1963 | issn=0003-6951 | doi=10.1063/1.1753778 | pages=67–68| bibcode=1963ApPhL...2...67M }}</ref>  Various organizations worldwide subsequently developed ring-laser technology further. Many tens of thousands of RLGs are operating in [[inertial navigation system]]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.

[[Image:ring laser interferometer.png|frame|right|Schematic representation of a ring laser setup. At the beam sampling location, a fraction of each of the counterpropagating beams exits the laser cavity.]]

Ring laser gyroscopes can be used as the stable elements (for one degree of freedom each) in an [[inertial reference system]]. 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]]. 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 [[Global Positioning System|GPS]] capability to further enhance accuracy of RLG [[inertial navigation system]]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."<ref>{{cite journal |last1=Beverini |first1=N |last2=Di Virgilio |first2=A |last3=Belfi |first3=J |last4=Ortolan |first4=A |last5=Schreiber |first5=K U |last6=Gebauer |first6=A |last7=Klügel |first7=T |title=High-Accuracy Ring Laser Gyroscopes: Earth Rotation Rate and Relativistic Effects |journal=Journal of Physics: Conference Series |date=2016 |volume=723 |issue=1 |page=012061 |doi=10.1088/1742-6596/723/1/012061 |publisher=IOP Publishing|bibcode=2016JPhCS.723a2061B |hdl=11568/796104 |hdl-access=free }} {{Creative Commons text attribution notice|cc=by3|from this source=yes}}</ref>

==Principle of operation==
According to the [[Sagnac effect]], 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]] 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]] 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&nbsp;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&nbsp;Hz vibration.<ref name="MacKenzie">''Knowing Machines'', Donald MacKenzie, The MIT Press, (1991).</ref>

A different approach to avoiding lock-in is embodied in the Multioscillator Ring Laser Gyroscope,<ref>{{cite book |author1=Statz, Hermann |author2=Dorschner, T. A. |author3=Holz, M. |author4=Smith, I. W. |editor1-last=Stich |editor1-first=M.L. |editor2-last=Bass |editor2-first=M. |title=Laser handbook. |date=1985 |publisher=Elsevier (North-Holland Pub. Co) |isbn=0444869271 |pages=[https://archive.org/details/laserhandbook0001arec/page/229 229-332] |language=en |chapter=3. The multioscillator ring laser gyroscope |chapter-url=https://archive.org/details/laserhandbook0001arec/page/229 }}</ref><ref>Volk, C. H. et al., ''Multioscillator Ring Laser Gyroscopes and their applications'', in ''Optical Gyros and their Applications (NATO RTO-AG-339 AC/323(SCI)TP/9)'', Loukianov, D et al. (eds.) [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.856.5890&rep=rep1&type=pdf#page=70] Retrieved 23 October 2019</ref> 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]], 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]] 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]] 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==
*[[Airbus A320]]<ref>{{cite web
 | url=http://www.ainonline.com/Publications/farn/farn_02/farn_02d2_honeywellpg48.html
 | title=Honeywell's ADIRU selected by Airbus
 | date=22–28 July 2002
 | location=Farnborough
 | access-date=2008-07-16
 | archive-url = https://web.archive.org/web/20061017052942/http://www.ainonline.com/Publications/farn/farn_02/farn_02d2_honeywellpg48.html
 | archive-date = 2006-10-17
 }}</ref>
*[[Agni III]]<ref>{{cite news
 | title = Agni-III missile ready for induction
 | url= http://timesofindia.indiatimes.com/India/Agni-III_missile_ready_for_induction/articleshow/3018810.cms
 | publisher = [[Press Trust of India]]
 | date = 2008-05-07
 | access-date = 2008-05-08
 }}</ref> and [[Agni-IV]]<ref>{{cite news
 | title = India successfully test fires Agni-IV missile
 | url=http://articles.economictimes.indiatimes.com/2014-01-20/news/46374897_1_wheeler-island-missile-ring-laser-gyros
 | archive-url=https://web.archive.org/web/20140124075117/http://articles.economictimes.indiatimes.com/2014-01-20/news/46374897_1_wheeler-island-missile-ring-laser-gyros
 | url-status=dead
 | archive-date=January 24, 2014
 | publisher=Economic Times India via Press Trust of India
 | date= 2014-01-20
 | access-date= 2015-10-14
 }}</ref>
*[[Agni-V]]<ref>{{cite news
 | title = Agni-V missile to take India into elite nuclear club
 | url=https://www.bbc.com/news/world-asia-india-17738633
 | publisher=[[BBC News]]| date= 2012-04-19
 | access-date= 2015-10-14
 }}</ref>
*[[ASM-135]] US Anti-satellite missile
*[[Boeing 757|Boeing 757-200]]
*[[Boeing 777]]<ref>{{cite book|url=https://books.google.com/books?id=ZP9VAAAAMAAJ|title=Digital Avionics Systems|publisher=[[IEEE]], [[AIAA]]|year=1995|access-date=2008-10-16|isbn=0-7803-3050-1}}</ref>
* [[B-52H]] with the AMI upgrade<ref>{{cite web
 | url=http://www.fas.org/nuke/guide/usa/bomber/n19991119_992119.htm
 | title=B-52 Maps Its Way Into New Century
 | date=19 Nov 1999
 | access-date=2009-02-24
 | publisher=[[fas.org]]
 }}</ref>
*[[EF-111 Raven]]
*[[F-15 Eagle|F-15E Strike Eagle]]
*[[F-16 Fighting Falcon]]
*[[HAL Tejas]]
*[[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<ref>{{cite web
 |url         = http://www.es.northropgrumman.com/solutions/mk39/assets/mk39.pdf
 |title       = MK 39 MOD 3A Ring Laser
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20090205074409/http://www.es.northropgrumman.com/solutions/mk39/assets/mk39.pdf
 |archive-date = 2009-02-05
}}</ref>
*[[P-3C Orion|P3 Orion]] (with upgrade)
*[[Shaurya (missile)|Shaurya]] missile.
*[[SH-60 Seahawk#MH-60S Knight Hawk (Sierra)|MH-60R]], [[SH-60 Seahawk#MH-60R Seahawk|MH-60S]], [[SH-60 Seahawk#SH-60F Oceanhawk (Foxtrot)|SH60F]] and [[SH-60 Seahawk#SH-60B Seahawk|SH60B]] Seahawk helicopters
*[[Sukhoi Su-30MKI]]
*[[Trident I]] and [[Trident II]] Missiles
*[[PARALIGN]], used for roller alignment
*[[International Space Station]]
*[[CAC/PAC JF-17 Thunder|JF-17 Thunder]]<ref>{{Cite web|url=http://www.pac.org.pk/jf-17|title=Pakistan Aeronautical Complex Kamra – JF-17 Thunder Aircraft|website=www.pac.org.pk|access-date=2017-02-26}}</ref>

== See also ==
{{commons category|Ring laser gyroscopes}}
* [[Accelerometer]]
* [[Active laser medium]]
* [[Hemispherical resonator gyroscope]]
* [[Laser construction]]
* [[Laser science]]
* [[List of laser applications]]
* [[List of laser types]]
* [[Optical ring resonators]]
* [[Fibre optic gyroscope]]

==References==
{{Reflist}}

==External links==
*[http://www.phys.canterbury.ac.nz/ringlaser/about_us.shtml Canterbury Ring Laser Research Group] {{Webarchive|url=https://web.archive.org/web/20160209103652/http://www.phys.canterbury.ac.nz/ringlaser/about_us.shtml |date=2016-02-09 }}
*[http://www.fas.org/man/dod-101/navy/docs/fun/rlg.htm Weapons and Systems Engineering Department, United States Naval Academy]
* {{cite journal
  |author=A.D. King
  |year=1998
  |title=Inertial Navigation – Forty Years of Evolution
  |journal=GEC Review
  |publisher=[[General Electric Company plc]]
  |volume=13 |issue=3
  |pages=140–149
  |url=http://www.imar-navigation.de/downloads/papers/inertial_navigation_introduction.pdf
 }}


[[Category:Gyroscopes]]
[[Category:Optical devices]]
[[Category:Laser applications]]
[[Category:Missile technology]]
[[Category:Spacecraft attitude control]]