For a somewhat similar system that uses fibre optic cables, see fibre optic 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 between the counter-propagating beams, observed externally, results in motion of the standing wave pattern, and thus indicates rotation.
The first experimental ring laser gyroscope was demonstrated in the US by Macek and Davis in 1963.[1] Various organizations worldwide subsequently developed ring-laser technology further. Many tens of thousands of RLGs are operating in inertial navigation systems and have established high accuracy, with better than 0.01°/hour bias uncertainty, and mean time between failures in excess of 60,000 hours.
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 (RLG) include an embedded GPS capability to further enhance accuracy of RLG Inertial Navigation Systems (INS) 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. A new era for upscaled ring laser gyroscopes started in the 1990s when, thanks to the technological improvements in the production of low loss mirrors, a reflectivity exceeding 99.99% was achieved. Unlocked Earth rotation sensing with a ring laser of about 1 m² of area was demonstrated at the University of Canterbury in Christchurch, New Zealand [2]
Principle of operationEdit
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 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.[3]
A different approach to avoiding lock-in is embodied in the Multioscillator Ring Laser Gyroscope,[4][5] 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 gyroscopeEdit
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.
^Macek, W. M.; Davis, D. T. M. (1963). "Rotation rate sensing with traveling-wave ring lasers". Applied Physics Letters. AIP Publishing. 2 (3): 67–68. Bibcode:1963ApPhL...2...67M. doi:10.1063/1.1753778. ISSN 0003-6951.
^High-Accuracy Ring Laser Gyroscopes: Earth Rotation Rate and Relativistic Effects, N Beverini et al 2016 J. Phys.: Conf. Ser. 723 012061
^Knowing Machines, Donald MacKenzie, The MIT Press, (1991).
^Statz, Hermann; Dorschner, T. A.; Holz, M.; Smith, I. W. (1985). "3. The multioscillator ring laser gyroscope". In Stich, M.L.; Bass, M. (eds.). Laser handbook. Elsevier (North-Holland Pub. Co). pp. 229-332. ISBN0444869271.
^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.) [1] Retrieved 23 October 2019
^. Farnborough. 22–28 July 2002. Archived from the original on 2006-10-17. Retrieved 2008-07-16.
^"Agni-III missile ready for induction". Press Trust of India. 2008-05-07. Retrieved 2008-05-08.
^"India successfully test fires Agni-IV missile". Economic Times India via Press Trust of India. 2014-01-20. Retrieved 2015-10-14.
^"Agni-V missile to take India into elite nuclear club". BBC News. 2012-04-19. Retrieved 2015-10-14.
Weapons and Systems Engineering Department, United States Naval Academy
A.D. King (1998). "Inertial Navigation – Forty Years of Evolution" (PDF). GEC Review. General Electric Company plc. 13 (3): 140–149.
October 14, 2023
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For a somewhat similar system that uses fibre optic cables see fibre optic 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 between the counter propagating beams observed externally results in motion of the standing wave pattern and thus indicates rotation Ring laser gyroscope Contents 1 Description 2 Principle of operation 3 Fibre optic gyroscope 4 Example applications 5 See also 6 References 7 External linksDescription EditThe first experimental ring laser gyroscope was demonstrated in the US by Macek and Davis in 1963 1 Various organizations worldwide subsequently developed ring laser technology further Many tens of thousands of RLGs are operating in inertial navigation systems and have established high accuracy with better than 0 01 hour bias uncertainty and mean time between failures in excess of 60 000 hours nbsp 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 RLG include an embedded GPS capability to further enhance accuracy of RLG Inertial Navigation Systems INS 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 A new era for upscaled ring laser gyroscopes started in the 1990s when thanks to the technological improvements in the production of low loss mirrors a reflectivity exceeding 99 99 was achieved Unlocked Earth rotation sensing with a ring laser of about 1 m of area was demonstrated at the University of Canterbury in Christchurch New Zealand 2 Principle of operation EditAccording 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 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 3 A different approach to avoiding lock in is embodied in the Multioscillator Ring Laser Gyroscope 4 5 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 EditA 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 EditAirbus A320 6 Agni III 7 and Agni IV 8 Agni V 9 ASM 135 US Anti satellite missile Boeing 757 200 Boeing 777 10 B 52H with the AMI upgrade 11 EF 111 Raven 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 12 P3 Orion with upgrade Shaurya missile MH 60R MH 60S SH60F and SH60B Seahawk helicopters Sukhoi Su 30MKI Trident I and Trident II Missiles PARALIGN used for roller alignment International Space Station JF 17 Thunder 13 See also Edit nbsp Wikimedia Commons has media related to 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 gyroscopeReferences Edit Macek W M Davis D T M 1963 Rotation rate sensing with traveling wave ring lasers Applied Physics Letters AIP Publishing 2 3 67 68 Bibcode 1963ApPhL 2 67M doi 10 1063 1 1753778 ISSN 0003 6951 High Accuracy Ring Laser Gyroscopes Earth Rotation Rate and Relativistic Effects N Beverini et al 2016 J Phys Conf Ser 723 012061 Knowing Machines Donald MacKenzie The MIT Press 1991 Statz Hermann Dorschner T A Holz M Smith I W 1985 3 The multioscillator ring laser gyroscope In Stich M L Bass M eds Laser handbook Elsevier North Holland Pub Co pp 229 332 ISBN 0444869271 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 1 Retrieved 23 October 2019 Honeywell s ADIRU selected by Airbus Farnborough 22 28 July 2002 Archived from the original on 2006 10 17 Retrieved 2008 07 16 Agni III missile ready for induction Press Trust of India 2008 05 07 Retrieved 2008 05 08 India successfully test fires Agni IV missile Economic Times India via Press Trust of India 2014 01 20 Retrieved 2015 10 14 Agni V missile to take India into elite nuclear club BBC News 2012 04 19 Retrieved 2015 10 14 Digital Avionics Systems IEEE AIAA 1995 ISBN 0 7803 3050 1 Retrieved 2008 10 16 B 52 Maps Its Way Into New Century fas org 19 Nov 1999 Retrieved 2009 02 24 MK 39 MOD 3A Ring Laser PDF Archived from the original PDF on 2009 02 05 Pakistan Aeronautical Complex Kamra JF 17 Thunder Aircraft www pac org pk Retrieved 2017 02 26 External links EditCanterbury Ring Laser Research Group Weapons and Systems Engineering Department United States Naval Academy A D King 1998 Inertial Navigation Forty Years of Evolution PDF GEC Review General Electric Company plc 13 3 140 149 Retrieved from https en wikipedia org w index php title Ring laser gyroscope amp oldid 1166161204, wikipedia, wiki, book, books, library,
