CA1125895A - Laser gyro mode locking reduction scheme - Google Patents
Laser gyro mode locking reduction schemeInfo
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- CA1125895A CA1125895A CA319,945A CA319945A CA1125895A CA 1125895 A CA1125895 A CA 1125895A CA 319945 A CA319945 A CA 319945A CA 1125895 A CA1125895 A CA 1125895A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
- G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
- G01C19/58—Turn-sensitive devices without moving masses
- G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
- G01C19/66—Ring laser gyrometers
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- Power Engineering (AREA)
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- Optics & Photonics (AREA)
- Remote Sensing (AREA)
- Gyroscopes (AREA)
- Lasers (AREA)
- Transition And Organic Metals Composition Catalysts For Addition Polymerization (AREA)
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Abstract
ABSTRACT
Weaker secondary laser modes operating slightly above threshold couple with stronger primary counterpropagating laser modes in a ring laser gyroscope and produce a dithering effect which reduces or eliminates lock-in between the primary modes.
In one embodiment, cavity length control circuitry is adjusted to detune the cavity length to allow two secondary opposing modes to be generated within the laser gain medium in the cavity while two primary counterpropagating modes operate well above threshold.
Another embodiment of the invention includes the use of an external laser beam source for generating secondary waves which are introduced into the ring laser path through a partially transmitting mirror which forms one of the corners of the ring laser closed loop path. In still another embodiment, part of one of the primary waves leaves the ring laser path through a partially reflective mirror and is acted upon by an oscillating mirror which doppler shifts the frequency. This doppler shifted mode is then reintroduced into the ring laser path to couple with the primary mode, which causes an oscillation which diminishes lock-in frequency. In all embodiments, the magnitude and frequency of the secondary mode or modes are controlled to optimize lock-in reduction.
Weaker secondary laser modes operating slightly above threshold couple with stronger primary counterpropagating laser modes in a ring laser gyroscope and produce a dithering effect which reduces or eliminates lock-in between the primary modes.
In one embodiment, cavity length control circuitry is adjusted to detune the cavity length to allow two secondary opposing modes to be generated within the laser gain medium in the cavity while two primary counterpropagating modes operate well above threshold.
Another embodiment of the invention includes the use of an external laser beam source for generating secondary waves which are introduced into the ring laser path through a partially transmitting mirror which forms one of the corners of the ring laser closed loop path. In still another embodiment, part of one of the primary waves leaves the ring laser path through a partially reflective mirror and is acted upon by an oscillating mirror which doppler shifts the frequency. This doppler shifted mode is then reintroduced into the ring laser path to couple with the primary mode, which causes an oscillation which diminishes lock-in frequency. In all embodiments, the magnitude and frequency of the secondary mode or modes are controlled to optimize lock-in reduction.
Description
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t _ _
t _ _
2 FIELD OF THE INVENTION
3 This invention relates to ring lasers used as
4 gyroscopes wherein the difference ~etween resonank frequencies of counterpropagating radiant energy or light waves is a measure-~ ment of rotation of the structure in which said propagating 7 waves are traveling.
8 BACKGROU~D OF THE I~VE~TIO~
. . _ _ 9 Ring laser gyroscopes utilizing counterpropagating laser beams are well known. These devices are used for measuring 11 rotation o~ the ring laser gyroscope by combining portions of the 12 counterpropagating modes to generate a beat frequency representa-13 tive of the differences in frequency between the opposing modes.
1~ Incidentally, the term "mode" is used herein interchangeably with the word "wave", and means a resonant traveling wave of radiant 16 energy propagating within a ring laser cavity. As the ring laser 17 body is rotated about an axis having a component perpendicular 18 to the ring laser plane, the frequency of waves propagating in 19 one direction within the cavity will increase while the frequency of waves propagating in the opposite direction will decrease.
21 This change in frequency between the counterpropagating modes 22 results in a change to the beat frequency proportiQnal to the 23 rate of rotation. By monitoring the beat signal, information 24 is obtained about rate of rotation of the ring laser.
Eowever, for the ring laser gyroscope to function 26 at low rates of rotation, frequency locking or "lock-in" must 27 be overcome. This phenomena occurs when two oppositely traveling 28 ___-- ~3 ' ~ :
:
. GCD 7~-5 ~ 25~
1 waves in a resonant cavity with slig'htly different frequencies 2 are pulled -toward each other to combine in a single fre~uency 3 standing wave. The net result is that for low rates of rotation of the ring laser, where fre~uency differences between the two opposing modes are very small, the waves are pulled toye~her ~ such that beat frequency does not change and the gyroscope is 7 insensitive to small rates of rot.ation. The effects of lock-in 8 are described in detail in Laser Applications, edited by Monte 9 Ross, Academic Press, Inc., New York, New York, 1971, in the article entitled, "The Laser Gyro," by Frederick Aronowitz, 11 pages 133-200.
12 It is well known that the principal cause of lock-in 13 coupling is the mutual scattering of energy from each of the 14 beams into the dlrection of the other. This mutual scattering, or backscatter, is explained in detail in Aronowitz, supra, 16 pages 148-153. Briefly, the difference frequency between two 1~ counterpropagating waves in a ring laser is governed by the 18 equation 19 ~ = a + b sin ~
where ~ is the instantaneous phase difference between the 21 counterpropagating waves, a is proportional to the rate of 22 rotation of the ring laser, and b is proportional to the magnitude : 23 of backscattered energy. In the case where a is smaller than b, 24 the beat frequency will be e~ual to zero and the ring laser will '25 be locked in. In order to have a gyroscope output which is 26 representative of rotation of the ring laser body, a must be 27 greater than b.
28 _____ -4-.
8 BACKGROU~D OF THE I~VE~TIO~
. . _ _ 9 Ring laser gyroscopes utilizing counterpropagating laser beams are well known. These devices are used for measuring 11 rotation o~ the ring laser gyroscope by combining portions of the 12 counterpropagating modes to generate a beat frequency representa-13 tive of the differences in frequency between the opposing modes.
1~ Incidentally, the term "mode" is used herein interchangeably with the word "wave", and means a resonant traveling wave of radiant 16 energy propagating within a ring laser cavity. As the ring laser 17 body is rotated about an axis having a component perpendicular 18 to the ring laser plane, the frequency of waves propagating in 19 one direction within the cavity will increase while the frequency of waves propagating in the opposite direction will decrease.
21 This change in frequency between the counterpropagating modes 22 results in a change to the beat frequency proportiQnal to the 23 rate of rotation. By monitoring the beat signal, information 24 is obtained about rate of rotation of the ring laser.
Eowever, for the ring laser gyroscope to function 26 at low rates of rotation, frequency locking or "lock-in" must 27 be overcome. This phenomena occurs when two oppositely traveling 28 ___-- ~3 ' ~ :
:
. GCD 7~-5 ~ 25~
1 waves in a resonant cavity with slig'htly different frequencies 2 are pulled -toward each other to combine in a single fre~uency 3 standing wave. The net result is that for low rates of rotation of the ring laser, where fre~uency differences between the two opposing modes are very small, the waves are pulled toye~her ~ such that beat frequency does not change and the gyroscope is 7 insensitive to small rates of rot.ation. The effects of lock-in 8 are described in detail in Laser Applications, edited by Monte 9 Ross, Academic Press, Inc., New York, New York, 1971, in the article entitled, "The Laser Gyro," by Frederick Aronowitz, 11 pages 133-200.
12 It is well known that the principal cause of lock-in 13 coupling is the mutual scattering of energy from each of the 14 beams into the dlrection of the other. This mutual scattering, or backscatter, is explained in detail in Aronowitz, supra, 16 pages 148-153. Briefly, the difference frequency between two 1~ counterpropagating waves in a ring laser is governed by the 18 equation 19 ~ = a + b sin ~
where ~ is the instantaneous phase difference between the 21 counterpropagating waves, a is proportional to the rate of 22 rotation of the ring laser, and b is proportional to the magnitude : 23 of backscattered energy. In the case where a is smaller than b, 24 the beat frequency will be e~ual to zero and the ring laser will '25 be locked in. In order to have a gyroscope output which is 26 representative of rotation of the ring laser body, a must be 27 greater than b.
28 _____ -4-.
5~3~5 1 One way of eliminating lock-in i5 to m~chanically 2 oscillate the ring laser body. By osc.illating, or dithering, the ~ laser struc-ture, a rotation rate is superimposed on the gyxoscope 4 such that most of the time a is g,reater than b ~nd -khe ~~ects of b are minimiæed or eliminated. A gyxo employing mechanical dither is discussad in myU.S~ ~atent No. 4,1,15,'004 en-t~-tle* "Co~n~r-7 balanced Oscillatin~ Ring Laser .Gyro" which w.as ~ssued"Sept~x~'~9, ,~918 to,~sJ. Hutc,hings and Virgil E. Sanders and which is assigned to Litton Systemsl ,~c.
9 , Another method of minimizing t~e effects of,lock-in which has been suggested is the directional dither of the 11 magnetic field of a Faraday cell digposed within a ring laser 12 pathO Within the ring laser cavi~, lineaxly polarized laser 13 waves are converted to circularly'polarized light whose vector ,1~ rotates in ~he same'direction as the windin~s in the Faraday cellO
O~5 The circularly polarized light waves are ac~e~ upon by the ~6 magnetic field as they pass through the Faraday cell an~ an 17 increase or decrease in optical path leng~h occurs, depending '~8 upon the direction o~ ~he field and the direction whic~ ~he waves 19 are traveling. After,leaving the Faraday cell, ~he circularly polarized light is converted back to linearly polarized light. :' 21 By oscillating the current in the Faraday cell windings, the 2~ magnetic field oscillates accordingly and varies the optical path 23 lengths of the opposite propagating ~aves in a ~onreciprocal 2~ manner. This also can be used to make a larger than b in the 2~ above e~uation such that the effects o~ ~ock-in are minimizedO
26 ~This magnetic dithering using a Faraday cell is explained in 27 Arono~Jitz, supra, pp. 157 t~rough 1590 28 ' _5_ ~c~
' ' l The above antilock-in techniques are passive, i.e., they 2 are not dependent upon active laser gain media. Also, with ~hese 3 methods the effects seen by waves propagating in one direction 4 in the laser path are equal and opposite to the ef~ect~ on the waves traveling in the opposite direction.
9 , Another method of minimizing t~e effects of,lock-in which has been suggested is the directional dither of the 11 magnetic field of a Faraday cell digposed within a ring laser 12 pathO Within the ring laser cavi~, lineaxly polarized laser 13 waves are converted to circularly'polarized light whose vector ,1~ rotates in ~he same'direction as the windin~s in the Faraday cellO
O~5 The circularly polarized light waves are ac~e~ upon by the ~6 magnetic field as they pass through the Faraday cell an~ an 17 increase or decrease in optical path leng~h occurs, depending '~8 upon the direction o~ ~he field and the direction whic~ ~he waves 19 are traveling. After,leaving the Faraday cell, ~he circularly polarized light is converted back to linearly polarized light. :' 21 By oscillating the current in the Faraday cell windings, the 2~ magnetic field oscillates accordingly and varies the optical path 23 lengths of the opposite propagating ~aves in a ~onreciprocal 2~ manner. This also can be used to make a larger than b in the 2~ above e~uation such that the effects o~ ~ock-in are minimizedO
26 ~This magnetic dithering using a Faraday cell is explained in 27 Arono~Jitz, supra, pp. 157 t~rough 1590 28 ' _5_ ~c~
' ' l The above antilock-in techniques are passive, i.e., they 2 are not dependent upon active laser gain media. Also, with ~hese 3 methods the effects seen by waves propagating in one direction 4 in the laser path are equal and opposite to the ef~ect~ on the waves traveling in the opposite direction.
6 SUMMARY OF THE INVENTION
7 For purposes of this discussion,the two opposing
8 resonant modes in a ring laser cavity which are combined to yield
9 rotational information are referred to as "primary modes". It is an object of this invention to minimize lock-in between ll primary counterpropagating modes in a ring laser cavity by 12 introducing additional modes into the ring laser cavity. These 13 additional modes, or secondary modes, oscillate at frequencies 14 different than the primary modes and couple with the primary modes through the laser gain medium to produce an antilock-in 16 effect.
17 For example, in one embodiment of the invention, four 18 oscillating resonant modes are generated within the ring laser l9 cavity. These four modes may be generated by detuning the laser cavity such that the two primary modes operate at a frequency 21 slightly off-center from the center of the laser gain curve while 22 two weaker secondary modes oscillate at frequencies on the gain 23 curve only slightly above threshold. Threshold i5 define~ as 24 that area on the gain curve where a resonant mode begins to be amplified in the laser gain medium. The secondary modes couple 26 through the active gain medium with the two strong modes to 2~ produce a dither effect on ~ . This dither effect produced 28 ~ 6-~q , . .
~ 395 1 on ~ as a result of coupling the weak and strong modes 2 counteracts the lock-in component of the ecfuation and reduces 3 or eliminates it.
4 Another embodiment of the inven-~ion includes providing a perturbing oscillation from an external laser source. Two 6 laser beams can be injected into the ring laser cavity of a 7 two mode ring laser. The injected modes, one traveling in each 8 direction, experience gain from the laser medium and thus couple 9 with two primary modes generated in the ring laser. These injecte modes, which have different frequencies than the primary modes, 11 couple with the primary modes to accomplish a dithering effect 12 in the difference frequency. The dithering reduces or eliminates 13 coupling between the two counterpropagating primary modes and ~
14 thereby reduces or eliminates lock-in accordingly.
An additional embodiment of the invention includes 16 using a portion of one of the counterpropagating waves of the 17 ring laser as an ext~rnal source. In this case, w~ere two primary 18 modes in the ring laser have sufficient gain to oscillate, 19 a portion of one mode is extracted from the ring laser cavity through a partially reflective mirror. The extracted portion 21 is doppler shifted to alter its resonant frequency and attenuate 22 it, and then injected back into the ring laser. This doppler 23 shifted mode, having a slightly different frequency, recombines 24 with the original primary mode and causes a dikhering ~hich reduces lock-in.
26 It is also an object of the invention to provide means 27 for combining portions of the principal modes to obtain siynals 28 _____ -7-. ~
, ., ' ' ~ s~
representatlve of the ra-te and direction of rotation of the laser gyroscope. Also, the invention includes apparatus for monitoring and optimizing the cavity length of -the laser gyroscope so that the resonant modes oscillate a-t the desired freqency on the gain curve.
Therefore, in accordance with the pre~ent lnventlo~
there is provided a ring laser gyroscope comprisiny: a ring laser body including reflective surfaces which define a closed loop optical path, means for generating and maintaining at least two counterpropagating in the optical path, whereby frequency differences between the primary modes are representative of angular motion experienced by the ring laser body, means for generating and maintaining at least one secondary resonant traveling mode propagating in the path and coupling with at least one of the primary modes, whereby the effects of lock-in are diminished or eliminated, and means for processing the frequency differences between the primary modes to generate signals representative of angular motion of the ring laser body.
Other objects, features and advantages on the invention will become apparent from consideration of the detailed description and the drawings.
DESCRIPTION OF THF DRAWINGS
_ . _ . ... _ . _ Figure 1 shows a first embodiment of the invention where the cavity length control circuitry adjusts cavity length such that two strong primary modes and two weaker secondary modes are generated in the gain medium.
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- Figures 2 and 3 illustrate how the optical frequency of the resonant cavity may be tuned so that the resonant waves in the cavity operate at desired points on the laser gain curve.
Figure 4 illustrates how detuning a laser cavity to allow weaker secondary modes to couple with stronyer prirnary modes will reduce locking between two opposiny primary modes~
Figure 5 is a second embodiment o:E the invention where secondary modes are generated by an outside laser source and injected into the resonant cavity to couple with the opposing primary modes.
- Figure 6 shows a third embodiment of the invention wherein a portion of one primary mode is extracted from a ring laser resonant cavity, doppler shifted in fre~uency, and then xe-injected back into the cavity to couple with a primary mode.
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~ 5 , 1DETAILED DESCRIPTION OF T~ VENTION
2~s discussed above, the difference frequency or beat 3 freauency which results from combining the two primary opposing resonant modes within a ring laser cavity is governed by the expression 6 ~ = a ~ b sin ~
where ~ is the instantaneous phase difference between the 8 opposite traveling wave~, a is proportional to the rotation rate 9 of the ring laser gyroscope, and _ is proportional to the magnitude of the backscattered energy. The second term on the ll right hand side of the equaltion (b sin ~ ) represents the couplinc 12 which results from backscatter. For small rates of rotation, a 13 is smaller than b and ~ goes to zero. In this situation, the 14 ring laser gyroscope is locked-in and does not yield an output representative of the actual rotation. Thus, at small but finite t 16 rotation rates the ring laser does not ~function well as a gyro-17 scope.
18 By physically doing something to the ring laser such l9 that beat freauency is perturbed sinusoidally, an additional time varying term is added to the above equation to modify it 21 to read 22 ~ - a + b sin ~ + c cos ~t 23 In the new equation, c and ~ represent the amp~itude and , 24 frequency respectively of the perturbation imposed on the difference freauency ~ .
26 Solving this new equation for ~ ~t), a good 27 approximation results in the equation 2~ _____ _9 ~o ___~
~ 5~
1¦ ~ (t) = at ~ ab JO (~) cos (at) 21 If the values of c and ~ are chosen such that JO = zero ¦ this equation reduces to --41 ~ at 51 and the lock-in term of the original difference frequency equation ~¦ is eliminated. In the following discussion of the invention, such an additional perturbing effect on the dif~erence frequency is ~ accomplished ~y introducing additional modes or frequencies into 9 the ring laser cavity to couple with the primary resonant modes.
The effect of these additional perturbing waves, or secondary 11 modes, is described by the addition of the term c cos ~t as 12 explained above. By controlling the magnitude and frequency of 13 the secondary modes, the terms c and ~ may be manipulated to 14 diminish lock-in in the ring laser gyro.
FIGURE 1 shows a ring laser gyroscope 2. The laser 16 body 4 is made of quartz and a sealed cavity 6 within the laser 17 body is filled with 90% helium and 10% neon. Two anode3 8 and 10 18 and two cathodes 12 and 14 are attached to the cavity 6. Th~
19 gas mixture in the areas of the cavity between the cathode 12 and anode 8 and cathode 14 and anode 10 respectively is 21 electrically charged to provide a gas plasma which serves as 22 the amplification medium for generating and amplifying the 23 resonant laser modes within the cavity 6. Three dielectric 24 mirrors 16, 18 and 20 are located at the three corners of the triangular shaped resonant cavity 6. These mirrors comprise 26 multiple layers of dielectric coatings which are well known in 27 the art.
28 ~ 10-29 ____ -., ~
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1 ¦ M~-ror 20 is a partially reflective mirror which allows 2 ¦ ~ @mall ~ e~tage of the rlng laser waves which strike it to ¦~Y~ th~ yh t~he mirror. Portions of the kwo primary coun-ter-~ tl~ modes, which travel in the cavity 6 alony the pa~h 0 ¦ rg~e~e~ by line 22, pass through the mirror and are combined 6 ¦ in ~ ~rl.~m ~tructure within the combiner and photodetector ¦ a~mbl~ 23 -~Q form a fringe pattern. This fringe pattern is ~¦ ~@ggiVQ~ ~y ~hotosensitive detectors and the signals generated - ~¦ ~h@~@ n ~E@ ~ransmitted along leads 24 to standard data reduction 9¦ a~ lo~ie gircuitry 26 which determine rate and sense of rotation.
~¦ A mQre ~@~iled discussion of combining counterpropagating waves ,~21 ~nd pro~sing the information obtained therefrom is included in AronQwitz, supra, pages 139 through 141.
~¦ ~Fter beam frequency is controlled by varying the ~51 eavity length, i.e., the distance that the laser modes travel ¦ i~ eomplç~ing one full loop around the path 22. It is generally 7 ¦ desire~ to adjust or tune the cavity length such that the modes ¦ whi~h may resonate within the cavity are in the center of the 1~ ¦ i~tensity ~istribution curve (gain curve) for the particular 2Q ~ er galn medium. In order to adjust the cavity length, 21 ¦ ~irro~ 16 is attached to the laser body 4 in such a manner that 22 ¦ it ~n mQve in and out. Attached to the back of mirror 16 is a 231 ~$~k of piezoelectric elements. Cavity length control is ~¦ ~compl~hed by oscillating, or dithering, the mirror 16 by 2~1 ~pplyl~ an AC voltage to the piezoelectric elements 28. As ~61 ~he mirror 16 is oscillated at a given frequency, the intensity ~71 ~i~nal generated in the photodetector assembly 23 varies 2~ 1 ~
~9l ~ t I GCD 7~-5 I ~ 3S
1 ¦accordingly and is transmitted along lead 30 to standard closed 2 ¦loop cavity length control circuitry 32. This circuitry 3 ¦ determines where the resonant modes in the cavity are locaked 41 along the gain curve and adjusts the nominal cavity Length by ~¦ increasing or decreasing the DC electric signal provided to the 61 piezoelectric elements 28 along lead 34. A thorough discussion 71 of this type of circuitry is contained in NASA Report No.
~¦ CR-132261, "Design and Development of the AA1300AbO2 Laser Gyro,"
91 by T. J. Podgorski and D. N. Thymian, 1973, pages 10 and 11.
~0¦ For the embodiment of the invention shown in FIGURE 1, 1¦ dithering of the difference frequency between the primary 12¦ counterpropagating modes in the cavity is accomplished by detuning 1~¦ the cavity length. For example, in FIGURE 2 is shown the laser 4¦ gain curve 44, i.e., the intensity distribution of light emitted ~5 ¦ in the laser gain plasma versus the optical frequency of such 16 ¦ emitted light. As is well known in the art, only certain 17 ¦ frequencies may resonate, i.e., be amplified, within the rin~
18 ¦ laser cavity. The frequency spacing between these resonant 19 ¦ modes is determined by the speed of light (c) divided by the 20 ¦path length tL), or the distance a wave makes in co~pleting one 21 ¦ full loop around the laser path.
22 ¦ In FIGURE 2, lines 36 and 38 represent the clockwise 23 ¦ and counterclockwise modes respectively which exist at a given 2~ ¦ ~requency when the ring laser cavity is tuned to the center of 25 ¦ the gain curve 44. 1ines 40 and 42 and lines 46 and 48 represent 26 ¦ ~he nearest modes on the optical frequency scale which could 27 ¦ also exist within the cavity, except that no gain medium is 2~ ~ 12-29 ~
..
: .
'~ 5 1 provided which will amplify these other modes within the cavity 6.
2 The intensity level represented by dashed line 50 designates 3 the threshold, or the level above which the laser gain medium 4 will amplify the resonating waves within the cavity.
For the embodiment of the invention in FIGURE 1, detuning ~ of the cavity length is accomplished ~y adjusting the DC componerlt 7 of the electric signal on the piezoelectric elements 28 so that 8 the cavity length is detuned to cause the principal modes 36 9 and 38 to be moved from the center of the gain curve. Sufficient detuning must be done to allow secondary resonant waves which 11 oscillate above threshold to be introduced into the resonant 12 cavity 6. FIGURE 3 shows how the cavity length is adjusted so 13 that resonant modes 36 and 38 are moved off the center of the gain 14 curve 44 sufficiently to allow secondary waves 40 and 42 to 15 oscillate slightly above threshold on the gain curve.
16 Secondary mode 40, which propagates in the cavity in 17 the clockwise direction, will now couple with stronger primary 18 mode 36, which propagates in the cavity 6 well above threshold 19 and in the same direction. This will cause a dither effect on 20 the ~ term in the difference frequency equation. In the same 21 manner the counterclockwise secondary mode 42 combines with 22 primary mode 38 to accomplish a dither effect. The effects of 23 the perturbing modes 40 and 42 are governed by the term c cos ~ t 24 in the above equation. By adjusting the intensity along the gain 25 curve of modes 40 and 42 as well as the requency at which they 26 oscillate, c and ~ in the equation can be controlled to dimish 27 the effects of lock-in, as explained in the above discussion.
28 FIGURE 4 is a graph illustrating how detuning affected ~ _13-, GCD 78~5 . '~ 5 .
1 the locking frequency between pximary modes 36 and 38 in a parti-2 cular experiment. Detuning is defined as tuning the path length of the ring laser such that the optical frequency of the primary modes is changed from the center of the gain curve. Note in FIGURE 4 that lock-in was practically eliminated in one case when 6 the cavity path length was detuned to where the primary modes _ _ 7 were 150 megahertz from the center of the gain curve.
8 Another embodiment of the invention is shown in FIGURE 5.
~ This embodiment includes a two mode ring laser similar to the ring laser gyroscope shown in FIGURE 1. A sealed cavity 52 is 11 provided which contains 90% helium and 10,' neon which, when 12 electrically excited between anodes 54 and cathodes 56, comprises 13 the laser gain medium. Portions of two primary counterpropagating 14 waves in the cavity are processed through a partially transparent dielectric mirror 58 into a combiner and photodetector assembly 60 16 where signals are generated and transmitted to a data reduction 17 and logic circuitry 62. An AC signal generated in the cavity 18 length control circuitry 66 is supplied to a piezoelectric 9 stack 68 which dithers mirror 70, and thereby oscillates the 20 cavity length of the gyroscope. Intensity signals from the 21 combiner and photodetector assembly 60 are transmitted along 22 lead 64 to the cavity length control circuitry 66. Variations 23 in the intensity signal due to the oscillations of the piezo-24- electric stack 68 are processed in the length control circuitry 66.
25 The DC component of the signal transmitted to the piezoelectric 26 stack along lead 72 is adjusted to optimze the cavity length 27 for maximum intensity of the counterpropagating waves therein~
28 _____ -14-: i ' Contrary to the example discussed above relative to FIGURE 1, the cavity length is adjusted so that the resonant modes are operating substantially at the center of the gain curve.
In the embodiment of the invention in FIG~RE 5, perturbing secondary waves at frequencies difEerent rorn the primary resonant modes in the laser yyro are introduced from an external source. The external source in this case is a two mode linear laser 74. Two separate modes generated in the linear laser 74 travel colinearly to the dispersive element 76.
Such dispersive elements are well known in the art and may comprise a grating for diffracting different frequencies different amounts. After passing through the dispersive element 76 one secondary mode 78 is diffracted towards dielectric mirror 80 where it is reflected towards the partially transmitting mirror 82. In passing through mirror 82, mode 78 enters the ring laser cavity 52 in the clockwise - direction and couples with the clockwise primary mode generated in the cavity.
The secondary mode 84 is deflected by the dispersive element 76 towards mirror 86 and then through mirror 82. It enters the cavity 52 traveling in the counterclockwise direction and couples with the counterclockwise primary mode.
Again, the perturbing effect of the secondary modes introduced into the cavity is represented in the difference frequency equation by the term c cos ~t. The difference frequency between secondary modes 78 and 84 is represented by ~.
The amplitude portion c is proportional to the magnitude of signals 78 and 84 and the magnitude of the difference frequencies mb/~ 15 -L
~5l~3~5 1 between the secondary and primary modes in the cavity. The 2 terms c and ~ may therefore be manipulated to diminish lock-in 3 by controlling the transmittance of mirror 82 and the fre~uency 4 and magnitude of the signals generated in the linear laser 74.
FIGURE 6 shows a third embodiment of the invention.
~ This embodiment includes a triangular ring laser gyroscope similar 7 to the ring lasers shown in FIGURES 1 and 5. Cavity length 8 control circuitry ad]usts the piezoelectric stack to maximize 9 intensity of the ring laser gyroscope output. The two opposing modes propagating in the cavity along path 22 have frequencies 11 tuned substantially to the center of the laser gain curve 44 of 12 FIGURES 2 and 3.
lX In the device shown in FIGURE 6, one perturbing 14 secondary mode is introduced in the ring laser cavity and couples with the counterclockwise propagating primary mode. To obtain 16 the secondary mode, a portion of the counterclockwise mode in 17 path 22 passes through the partially transmitting dielectric 18 mirror 88. This transmitted wave 102 then passes through a 19 directional isolator 90. Such directional isolators are well known in the art and operat~ to change the angle of polarization 21 of the traveling waves passing through it~ The ~ode 102 then 22 strikes the dielectric mirror 92, which is attached to a %3 piezoelectric stack 94. An AC voltage is supplied at a selected 24 frequency to the piezoelectric stack 94 from oscillation circuitry 104, causing mirror 92 to oscillate. This oscillation, 26 in turn, doppler shifts the frequency of mode 102 so that after 27 it is deflected from dielectric mirror 98 and reintroduced 26~ _____ 16-~ J
~ 39S
1 throu~h the partially transmitting mirror 88 into the ring 2 laser path, its frequency is change~ relative to the primary mode from which it was extracted. This doppler shif~ed mode, 4 upon xeentering the path 22, couples wikh the counterclockwise primary mode to produce the antilock-in dithering effect on 6 discussed ahove.
7 The magnitude oE the doppler shifted signal 102 ~hich 8 reenters the cavity is represented in the difference frequency 9 e~uation by c. The c term may be controlled by controlling the magnitude of 102. Ways to control this magnitude include 11 controlling the transmittance of the partially transmitting 12 dielectric mirror 88. The ~ term in the difference frequency ~3 equation corresponds to the frequency of oscillation ~
14 transmitted to the piezoelectric stack,94. This term may be easily controlled by simply varying or controlling the frequency 16 of oscillation generated in the circuitry 104. Therefore, by 17 controlling the magnitude and fre~uency of oscillation of mode 102 18 when it reenters the laser cavity and couples with the counter-19 clockwise primary mode, the effects of lock-in may be substantially diminished.
21 Incidentally, a polarizer 96 disposed in the path of 22 mode 102 effectively allows beams of one sense of polarization 23 to pass through while blocking out beams having different 2~ polarization. Polarizer 96 is adjusted to allow beams 102 to pass through. Since the direction isolator 90 has changed the 26 sense of polarization of mode 102, portions of the clockwise 2~ propagating principal mode which pass through mirror 88 hav~
28 _____ -17-29 ~
"~
¦ GCD 78-5 I J~5~3~5 1 ¦different polarization and will be blocked out by the polarizer ~6.
2 ¦ Changes may be made to the above described embodiments 3 ¦of my invention and still be within its scope and ~pirik.
4 ¦Examples of such changes include, but are not limited to, using 5 la rectangular shaped ring laser path, using means other than 6 ¦piezoelectric stacks for oscillating dielectrlc mirrors, using ~¦ alternate cavity length control apparatus, using no cavity 8¦ length control device, and using different means for combining gl and processing primary counterpropagating beams to obtain
17 For example, in one embodiment of the invention, four 18 oscillating resonant modes are generated within the ring laser l9 cavity. These four modes may be generated by detuning the laser cavity such that the two primary modes operate at a frequency 21 slightly off-center from the center of the laser gain curve while 22 two weaker secondary modes oscillate at frequencies on the gain 23 curve only slightly above threshold. Threshold i5 define~ as 24 that area on the gain curve where a resonant mode begins to be amplified in the laser gain medium. The secondary modes couple 26 through the active gain medium with the two strong modes to 2~ produce a dither effect on ~ . This dither effect produced 28 ~ 6-~q , . .
~ 395 1 on ~ as a result of coupling the weak and strong modes 2 counteracts the lock-in component of the ecfuation and reduces 3 or eliminates it.
4 Another embodiment of the inven-~ion includes providing a perturbing oscillation from an external laser source. Two 6 laser beams can be injected into the ring laser cavity of a 7 two mode ring laser. The injected modes, one traveling in each 8 direction, experience gain from the laser medium and thus couple 9 with two primary modes generated in the ring laser. These injecte modes, which have different frequencies than the primary modes, 11 couple with the primary modes to accomplish a dithering effect 12 in the difference frequency. The dithering reduces or eliminates 13 coupling between the two counterpropagating primary modes and ~
14 thereby reduces or eliminates lock-in accordingly.
An additional embodiment of the invention includes 16 using a portion of one of the counterpropagating waves of the 17 ring laser as an ext~rnal source. In this case, w~ere two primary 18 modes in the ring laser have sufficient gain to oscillate, 19 a portion of one mode is extracted from the ring laser cavity through a partially reflective mirror. The extracted portion 21 is doppler shifted to alter its resonant frequency and attenuate 22 it, and then injected back into the ring laser. This doppler 23 shifted mode, having a slightly different frequency, recombines 24 with the original primary mode and causes a dikhering ~hich reduces lock-in.
26 It is also an object of the invention to provide means 27 for combining portions of the principal modes to obtain siynals 28 _____ -7-. ~
, ., ' ' ~ s~
representatlve of the ra-te and direction of rotation of the laser gyroscope. Also, the invention includes apparatus for monitoring and optimizing the cavity length of -the laser gyroscope so that the resonant modes oscillate a-t the desired freqency on the gain curve.
Therefore, in accordance with the pre~ent lnventlo~
there is provided a ring laser gyroscope comprisiny: a ring laser body including reflective surfaces which define a closed loop optical path, means for generating and maintaining at least two counterpropagating in the optical path, whereby frequency differences between the primary modes are representative of angular motion experienced by the ring laser body, means for generating and maintaining at least one secondary resonant traveling mode propagating in the path and coupling with at least one of the primary modes, whereby the effects of lock-in are diminished or eliminated, and means for processing the frequency differences between the primary modes to generate signals representative of angular motion of the ring laser body.
Other objects, features and advantages on the invention will become apparent from consideration of the detailed description and the drawings.
DESCRIPTION OF THF DRAWINGS
_ . _ . ... _ . _ Figure 1 shows a first embodiment of the invention where the cavity length control circuitry adjusts cavity length such that two strong primary modes and two weaker secondary modes are generated in the gain medium.
csm/p~
- Figures 2 and 3 illustrate how the optical frequency of the resonant cavity may be tuned so that the resonant waves in the cavity operate at desired points on the laser gain curve.
Figure 4 illustrates how detuning a laser cavity to allow weaker secondary modes to couple with stronyer prirnary modes will reduce locking between two opposiny primary modes~
Figure 5 is a second embodiment o:E the invention where secondary modes are generated by an outside laser source and injected into the resonant cavity to couple with the opposing primary modes.
- Figure 6 shows a third embodiment of the invention wherein a portion of one primary mode is extracted from a ring laser resonant cavity, doppler shifted in fre~uency, and then xe-injected back into the cavity to couple with a primary mode.
8a -~ csm/j~' '- `
~ ~ , GCD 78--$
~ 5 , 1DETAILED DESCRIPTION OF T~ VENTION
2~s discussed above, the difference frequency or beat 3 freauency which results from combining the two primary opposing resonant modes within a ring laser cavity is governed by the expression 6 ~ = a ~ b sin ~
where ~ is the instantaneous phase difference between the 8 opposite traveling wave~, a is proportional to the rotation rate 9 of the ring laser gyroscope, and _ is proportional to the magnitude of the backscattered energy. The second term on the ll right hand side of the equaltion (b sin ~ ) represents the couplinc 12 which results from backscatter. For small rates of rotation, a 13 is smaller than b and ~ goes to zero. In this situation, the 14 ring laser gyroscope is locked-in and does not yield an output representative of the actual rotation. Thus, at small but finite t 16 rotation rates the ring laser does not ~function well as a gyro-17 scope.
18 By physically doing something to the ring laser such l9 that beat freauency is perturbed sinusoidally, an additional time varying term is added to the above equation to modify it 21 to read 22 ~ - a + b sin ~ + c cos ~t 23 In the new equation, c and ~ represent the amp~itude and , 24 frequency respectively of the perturbation imposed on the difference freauency ~ .
26 Solving this new equation for ~ ~t), a good 27 approximation results in the equation 2~ _____ _9 ~o ___~
~ 5~
1¦ ~ (t) = at ~ ab JO (~) cos (at) 21 If the values of c and ~ are chosen such that JO = zero ¦ this equation reduces to --41 ~ at 51 and the lock-in term of the original difference frequency equation ~¦ is eliminated. In the following discussion of the invention, such an additional perturbing effect on the dif~erence frequency is ~ accomplished ~y introducing additional modes or frequencies into 9 the ring laser cavity to couple with the primary resonant modes.
The effect of these additional perturbing waves, or secondary 11 modes, is described by the addition of the term c cos ~t as 12 explained above. By controlling the magnitude and frequency of 13 the secondary modes, the terms c and ~ may be manipulated to 14 diminish lock-in in the ring laser gyro.
FIGURE 1 shows a ring laser gyroscope 2. The laser 16 body 4 is made of quartz and a sealed cavity 6 within the laser 17 body is filled with 90% helium and 10% neon. Two anode3 8 and 10 18 and two cathodes 12 and 14 are attached to the cavity 6. Th~
19 gas mixture in the areas of the cavity between the cathode 12 and anode 8 and cathode 14 and anode 10 respectively is 21 electrically charged to provide a gas plasma which serves as 22 the amplification medium for generating and amplifying the 23 resonant laser modes within the cavity 6. Three dielectric 24 mirrors 16, 18 and 20 are located at the three corners of the triangular shaped resonant cavity 6. These mirrors comprise 26 multiple layers of dielectric coatings which are well known in 27 the art.
28 ~ 10-29 ____ -., ~
: . :
..
l GCD 78~5 ~ 5~
1 ¦ M~-ror 20 is a partially reflective mirror which allows 2 ¦ ~ @mall ~ e~tage of the rlng laser waves which strike it to ¦~Y~ th~ yh t~he mirror. Portions of the kwo primary coun-ter-~ tl~ modes, which travel in the cavity 6 alony the pa~h 0 ¦ rg~e~e~ by line 22, pass through the mirror and are combined 6 ¦ in ~ ~rl.~m ~tructure within the combiner and photodetector ¦ a~mbl~ 23 -~Q form a fringe pattern. This fringe pattern is ~¦ ~@ggiVQ~ ~y ~hotosensitive detectors and the signals generated - ~¦ ~h@~@ n ~E@ ~ransmitted along leads 24 to standard data reduction 9¦ a~ lo~ie gircuitry 26 which determine rate and sense of rotation.
~¦ A mQre ~@~iled discussion of combining counterpropagating waves ,~21 ~nd pro~sing the information obtained therefrom is included in AronQwitz, supra, pages 139 through 141.
~¦ ~Fter beam frequency is controlled by varying the ~51 eavity length, i.e., the distance that the laser modes travel ¦ i~ eomplç~ing one full loop around the path 22. It is generally 7 ¦ desire~ to adjust or tune the cavity length such that the modes ¦ whi~h may resonate within the cavity are in the center of the 1~ ¦ i~tensity ~istribution curve (gain curve) for the particular 2Q ~ er galn medium. In order to adjust the cavity length, 21 ¦ ~irro~ 16 is attached to the laser body 4 in such a manner that 22 ¦ it ~n mQve in and out. Attached to the back of mirror 16 is a 231 ~$~k of piezoelectric elements. Cavity length control is ~¦ ~compl~hed by oscillating, or dithering, the mirror 16 by 2~1 ~pplyl~ an AC voltage to the piezoelectric elements 28. As ~61 ~he mirror 16 is oscillated at a given frequency, the intensity ~71 ~i~nal generated in the photodetector assembly 23 varies 2~ 1 ~
~9l ~ t I GCD 7~-5 I ~ 3S
1 ¦accordingly and is transmitted along lead 30 to standard closed 2 ¦loop cavity length control circuitry 32. This circuitry 3 ¦ determines where the resonant modes in the cavity are locaked 41 along the gain curve and adjusts the nominal cavity Length by ~¦ increasing or decreasing the DC electric signal provided to the 61 piezoelectric elements 28 along lead 34. A thorough discussion 71 of this type of circuitry is contained in NASA Report No.
~¦ CR-132261, "Design and Development of the AA1300AbO2 Laser Gyro,"
91 by T. J. Podgorski and D. N. Thymian, 1973, pages 10 and 11.
~0¦ For the embodiment of the invention shown in FIGURE 1, 1¦ dithering of the difference frequency between the primary 12¦ counterpropagating modes in the cavity is accomplished by detuning 1~¦ the cavity length. For example, in FIGURE 2 is shown the laser 4¦ gain curve 44, i.e., the intensity distribution of light emitted ~5 ¦ in the laser gain plasma versus the optical frequency of such 16 ¦ emitted light. As is well known in the art, only certain 17 ¦ frequencies may resonate, i.e., be amplified, within the rin~
18 ¦ laser cavity. The frequency spacing between these resonant 19 ¦ modes is determined by the speed of light (c) divided by the 20 ¦path length tL), or the distance a wave makes in co~pleting one 21 ¦ full loop around the laser path.
22 ¦ In FIGURE 2, lines 36 and 38 represent the clockwise 23 ¦ and counterclockwise modes respectively which exist at a given 2~ ¦ ~requency when the ring laser cavity is tuned to the center of 25 ¦ the gain curve 44. 1ines 40 and 42 and lines 46 and 48 represent 26 ¦ ~he nearest modes on the optical frequency scale which could 27 ¦ also exist within the cavity, except that no gain medium is 2~ ~ 12-29 ~
..
: .
'~ 5 1 provided which will amplify these other modes within the cavity 6.
2 The intensity level represented by dashed line 50 designates 3 the threshold, or the level above which the laser gain medium 4 will amplify the resonating waves within the cavity.
For the embodiment of the invention in FIGURE 1, detuning ~ of the cavity length is accomplished ~y adjusting the DC componerlt 7 of the electric signal on the piezoelectric elements 28 so that 8 the cavity length is detuned to cause the principal modes 36 9 and 38 to be moved from the center of the gain curve. Sufficient detuning must be done to allow secondary resonant waves which 11 oscillate above threshold to be introduced into the resonant 12 cavity 6. FIGURE 3 shows how the cavity length is adjusted so 13 that resonant modes 36 and 38 are moved off the center of the gain 14 curve 44 sufficiently to allow secondary waves 40 and 42 to 15 oscillate slightly above threshold on the gain curve.
16 Secondary mode 40, which propagates in the cavity in 17 the clockwise direction, will now couple with stronger primary 18 mode 36, which propagates in the cavity 6 well above threshold 19 and in the same direction. This will cause a dither effect on 20 the ~ term in the difference frequency equation. In the same 21 manner the counterclockwise secondary mode 42 combines with 22 primary mode 38 to accomplish a dither effect. The effects of 23 the perturbing modes 40 and 42 are governed by the term c cos ~ t 24 in the above equation. By adjusting the intensity along the gain 25 curve of modes 40 and 42 as well as the requency at which they 26 oscillate, c and ~ in the equation can be controlled to dimish 27 the effects of lock-in, as explained in the above discussion.
28 FIGURE 4 is a graph illustrating how detuning affected ~ _13-, GCD 78~5 . '~ 5 .
1 the locking frequency between pximary modes 36 and 38 in a parti-2 cular experiment. Detuning is defined as tuning the path length of the ring laser such that the optical frequency of the primary modes is changed from the center of the gain curve. Note in FIGURE 4 that lock-in was practically eliminated in one case when 6 the cavity path length was detuned to where the primary modes _ _ 7 were 150 megahertz from the center of the gain curve.
8 Another embodiment of the invention is shown in FIGURE 5.
~ This embodiment includes a two mode ring laser similar to the ring laser gyroscope shown in FIGURE 1. A sealed cavity 52 is 11 provided which contains 90% helium and 10,' neon which, when 12 electrically excited between anodes 54 and cathodes 56, comprises 13 the laser gain medium. Portions of two primary counterpropagating 14 waves in the cavity are processed through a partially transparent dielectric mirror 58 into a combiner and photodetector assembly 60 16 where signals are generated and transmitted to a data reduction 17 and logic circuitry 62. An AC signal generated in the cavity 18 length control circuitry 66 is supplied to a piezoelectric 9 stack 68 which dithers mirror 70, and thereby oscillates the 20 cavity length of the gyroscope. Intensity signals from the 21 combiner and photodetector assembly 60 are transmitted along 22 lead 64 to the cavity length control circuitry 66. Variations 23 in the intensity signal due to the oscillations of the piezo-24- electric stack 68 are processed in the length control circuitry 66.
25 The DC component of the signal transmitted to the piezoelectric 26 stack along lead 72 is adjusted to optimze the cavity length 27 for maximum intensity of the counterpropagating waves therein~
28 _____ -14-: i ' Contrary to the example discussed above relative to FIGURE 1, the cavity length is adjusted so that the resonant modes are operating substantially at the center of the gain curve.
In the embodiment of the invention in FIG~RE 5, perturbing secondary waves at frequencies difEerent rorn the primary resonant modes in the laser yyro are introduced from an external source. The external source in this case is a two mode linear laser 74. Two separate modes generated in the linear laser 74 travel colinearly to the dispersive element 76.
Such dispersive elements are well known in the art and may comprise a grating for diffracting different frequencies different amounts. After passing through the dispersive element 76 one secondary mode 78 is diffracted towards dielectric mirror 80 where it is reflected towards the partially transmitting mirror 82. In passing through mirror 82, mode 78 enters the ring laser cavity 52 in the clockwise - direction and couples with the clockwise primary mode generated in the cavity.
The secondary mode 84 is deflected by the dispersive element 76 towards mirror 86 and then through mirror 82. It enters the cavity 52 traveling in the counterclockwise direction and couples with the counterclockwise primary mode.
Again, the perturbing effect of the secondary modes introduced into the cavity is represented in the difference frequency equation by the term c cos ~t. The difference frequency between secondary modes 78 and 84 is represented by ~.
The amplitude portion c is proportional to the magnitude of signals 78 and 84 and the magnitude of the difference frequencies mb/~ 15 -L
~5l~3~5 1 between the secondary and primary modes in the cavity. The 2 terms c and ~ may therefore be manipulated to diminish lock-in 3 by controlling the transmittance of mirror 82 and the fre~uency 4 and magnitude of the signals generated in the linear laser 74.
FIGURE 6 shows a third embodiment of the invention.
~ This embodiment includes a triangular ring laser gyroscope similar 7 to the ring lasers shown in FIGURES 1 and 5. Cavity length 8 control circuitry ad]usts the piezoelectric stack to maximize 9 intensity of the ring laser gyroscope output. The two opposing modes propagating in the cavity along path 22 have frequencies 11 tuned substantially to the center of the laser gain curve 44 of 12 FIGURES 2 and 3.
lX In the device shown in FIGURE 6, one perturbing 14 secondary mode is introduced in the ring laser cavity and couples with the counterclockwise propagating primary mode. To obtain 16 the secondary mode, a portion of the counterclockwise mode in 17 path 22 passes through the partially transmitting dielectric 18 mirror 88. This transmitted wave 102 then passes through a 19 directional isolator 90. Such directional isolators are well known in the art and operat~ to change the angle of polarization 21 of the traveling waves passing through it~ The ~ode 102 then 22 strikes the dielectric mirror 92, which is attached to a %3 piezoelectric stack 94. An AC voltage is supplied at a selected 24 frequency to the piezoelectric stack 94 from oscillation circuitry 104, causing mirror 92 to oscillate. This oscillation, 26 in turn, doppler shifts the frequency of mode 102 so that after 27 it is deflected from dielectric mirror 98 and reintroduced 26~ _____ 16-~ J
~ 39S
1 throu~h the partially transmitting mirror 88 into the ring 2 laser path, its frequency is change~ relative to the primary mode from which it was extracted. This doppler shif~ed mode, 4 upon xeentering the path 22, couples wikh the counterclockwise primary mode to produce the antilock-in dithering effect on 6 discussed ahove.
7 The magnitude oE the doppler shifted signal 102 ~hich 8 reenters the cavity is represented in the difference frequency 9 e~uation by c. The c term may be controlled by controlling the magnitude of 102. Ways to control this magnitude include 11 controlling the transmittance of the partially transmitting 12 dielectric mirror 88. The ~ term in the difference frequency ~3 equation corresponds to the frequency of oscillation ~
14 transmitted to the piezoelectric stack,94. This term may be easily controlled by simply varying or controlling the frequency 16 of oscillation generated in the circuitry 104. Therefore, by 17 controlling the magnitude and fre~uency of oscillation of mode 102 18 when it reenters the laser cavity and couples with the counter-19 clockwise primary mode, the effects of lock-in may be substantially diminished.
21 Incidentally, a polarizer 96 disposed in the path of 22 mode 102 effectively allows beams of one sense of polarization 23 to pass through while blocking out beams having different 2~ polarization. Polarizer 96 is adjusted to allow beams 102 to pass through. Since the direction isolator 90 has changed the 26 sense of polarization of mode 102, portions of the clockwise 2~ propagating principal mode which pass through mirror 88 hav~
28 _____ -17-29 ~
"~
¦ GCD 78-5 I J~5~3~5 1 ¦different polarization and will be blocked out by the polarizer ~6.
2 ¦ Changes may be made to the above described embodiments 3 ¦of my invention and still be within its scope and ~pirik.
4 ¦Examples of such changes include, but are not limited to, using 5 la rectangular shaped ring laser path, using means other than 6 ¦piezoelectric stacks for oscillating dielectrlc mirrors, using ~¦ alternate cavity length control apparatus, using no cavity 8¦ length control device, and using different means for combining gl and processing primary counterpropagating beams to obtain
10¦ rotational information.
11 I
12
13 I .
14 1~ l 18 I .
19 l 2~
~5 2~ ~ 18-
19 l 2~
~5 2~ ~ 18-
Claims (5)
1. A ring laser gyroscope comprising:
a ring laser body including reflective surfaces which define a closed loop optical path;
means for generating and maintaining at least two counterpropagating primary resonant traveling modes propagating in said optical path, whereby frequency differences between said primary modes are representative of angular motion experienced by said ring laser body;
means for generating and maintaining at least one secondary resonant traveling mode propa-gating in said path and coupling with at least one of said primary modes, whereby the effects of lock-in are diminished or eliminated; and means for processing said frequency differences between said primary modes to generate signals representative of angular motion of said ring laser body.
a ring laser body including reflective surfaces which define a closed loop optical path;
means for generating and maintaining at least two counterpropagating primary resonant traveling modes propagating in said optical path, whereby frequency differences between said primary modes are representative of angular motion experienced by said ring laser body;
means for generating and maintaining at least one secondary resonant traveling mode propa-gating in said path and coupling with at least one of said primary modes, whereby the effects of lock-in are diminished or eliminated; and means for processing said frequency differences between said primary modes to generate signals representative of angular motion of said ring laser body.
2. The ring laser gyroscope described in Claim 1 wherein said means for generating and maintaining primary modes and said means for generating and maintaining at least one secondary mode further comprise:
electrically charged gas plasma gain medium means for generating and amplifying said primary and secondary modes;
means controlling the length of said optical path for tuning the length of said path such that at least two weaker counterpropagating secondary modes and at least two stronger counterpropagating primary modes are generated and amplified within said gain medium means, whereby said secondary modes couple with said primary modes to diminish lock-in between said primary modes.
electrically charged gas plasma gain medium means for generating and amplifying said primary and secondary modes;
means controlling the length of said optical path for tuning the length of said path such that at least two weaker counterpropagating secondary modes and at least two stronger counterpropagating primary modes are generated and amplified within said gain medium means, whereby said secondary modes couple with said primary modes to diminish lock-in between said primary modes.
3. The ring laser gyroscope described in Claim 1 wherein said means for generating at least one secondary mode further comprises:
means external to said ring laser body for generating at least one secondary mode having a frequency different from said primary modes; and means for introducing said external mode or modes into said optical path to couple with at least one primary mode, whereby an oscillation produced between said secondary and primary modes reduces lock-in effects between primary modes.
means external to said ring laser body for generating at least one secondary mode having a frequency different from said primary modes; and means for introducing said external mode or modes into said optical path to couple with at least one primary mode, whereby an oscillation produced between said secondary and primary modes reduces lock-in effects between primary modes.
4. The ring laser gyroscope described in Claim 1 wherein said means for generating and maintaining at least one secondary mode further comprises:
means for extracting a portion of at least one of said primary modes from said closed loop path;
oscillating frequency modifying means disposed in the path of said extracted mode for modifying the frequency of said extracted mode; and means for introducing said frequency modified mode into said path to couple with at least one of said primary modes, whereby said coupling causes an oscillation which reduces lock-in between said primary modes.
means for extracting a portion of at least one of said primary modes from said closed loop path;
oscillating frequency modifying means disposed in the path of said extracted mode for modifying the frequency of said extracted mode; and means for introducing said frequency modified mode into said path to couple with at least one of said primary modes, whereby said coupling causes an oscillation which reduces lock-in between said primary modes.
5. The ring laser gyroscope described in Claim 1 further comprising means controlling the length of said optical path for tuning the path length to control the optical frequency of said primary modes.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US90991978A | 1978-05-26 | 1978-05-26 | |
| US909,919 | 1978-05-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1125895A true CA1125895A (en) | 1982-06-15 |
Family
ID=25428045
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA319,945A Expired CA1125895A (en) | 1978-05-26 | 1979-01-19 | Laser gyro mode locking reduction scheme |
Country Status (18)
| Country | Link |
|---|---|
| JP (1) | JPS596520B2 (en) |
| AU (1) | AU521186B2 (en) |
| BE (1) | BE874859A (en) |
| BR (1) | BR7901915A (en) |
| CA (1) | CA1125895A (en) |
| CH (1) | CH645718A5 (en) |
| DE (1) | DE2920429A1 (en) |
| DK (1) | DK215579A (en) |
| ES (1) | ES479075A1 (en) |
| FR (1) | FR2426887A1 (en) |
| GB (1) | GB2021851B (en) |
| GR (1) | GR66808B (en) |
| IL (1) | IL56658A (en) |
| IT (1) | IT1115066B (en) |
| NL (1) | NL181953C (en) |
| NO (1) | NO152272C (en) |
| NZ (1) | NZ189487A (en) |
| SE (1) | SE448320B (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2120839A (en) * | 1982-05-19 | 1983-12-07 | Raytheon Co | Ring laser gyroscope |
| DE3412016C2 (en) * | 1984-03-31 | 1986-12-11 | Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt e.V., 5000 Köln | Ring laser |
| US4783169A (en) * | 1985-08-09 | 1988-11-08 | Litton Systems, Inc. | Control of a ring laser gyro cavity according to a preselected model |
| CN103674003B (en) * | 2012-09-07 | 2016-06-01 | 中国航空工业第六一八研究所 | A kind of electromagnetic drive type shaking laser gyroscope mechanism |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3697181A (en) * | 1967-03-14 | 1972-10-10 | Sperry Rand Corp | Ring laser having amplitude and phase controlled crossed-beam anti-locking feedback |
| US3741657A (en) * | 1971-03-03 | 1973-06-26 | Raytheon Co | Laser gyroscope |
| US3846025A (en) * | 1973-04-02 | 1974-11-05 | S Wilber | Frequency sensitive laser rotary motion sensor |
| US4152071A (en) * | 1976-08-02 | 1979-05-01 | Honeywell Inc. | Control apparatus |
| CA1085031A (en) * | 1976-11-08 | 1980-09-02 | Litton Systems, Inc. | Laser gyro with phased dithered mirrors |
-
1979
- 1979-01-19 CA CA319,945A patent/CA1125895A/en not_active Expired
- 1979-01-26 NZ NZ189487A patent/NZ189487A/en unknown
- 1979-02-02 GR GR58245A patent/GR66808B/el unknown
- 1979-02-06 AU AU43981/79A patent/AU521186B2/en not_active Ceased
- 1979-02-13 IL IL56658A patent/IL56658A/en unknown
- 1979-02-28 NL NLAANVRAGE7901581,A patent/NL181953C/en not_active IP Right Cessation
- 1979-03-14 FR FR7906502A patent/FR2426887A1/en active Granted
- 1979-03-15 BE BE0/194032A patent/BE874859A/en not_active IP Right Cessation
- 1979-03-20 IT IT48410/79A patent/IT1115066B/en active
- 1979-03-29 BR BR7901915A patent/BR7901915A/en unknown
- 1979-03-29 SE SE7902823A patent/SE448320B/en not_active IP Right Cessation
- 1979-03-30 ES ES479075A patent/ES479075A1/en not_active Expired
- 1979-04-16 JP JP54045512A patent/JPS596520B2/en not_active Expired
- 1979-04-20 GB GB7913805A patent/GB2021851B/en not_active Expired
- 1979-05-16 NO NO791638A patent/NO152272C/en unknown
- 1979-05-19 DE DE2920429A patent/DE2920429A1/en not_active Withdrawn
- 1979-05-25 DK DK215579A patent/DK215579A/en unknown
- 1979-05-25 CH CH492179A patent/CH645718A5/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| AU521186B2 (en) | 1982-03-18 |
| IL56658A (en) | 1981-07-31 |
| DE2920429A1 (en) | 1979-11-29 |
| JPS596520B2 (en) | 1984-02-13 |
| SE7902823L (en) | 1979-11-27 |
| BR7901915A (en) | 1979-12-04 |
| CH645718A5 (en) | 1984-10-15 |
| BE874859A (en) | 1979-07-02 |
| ES479075A1 (en) | 1979-06-01 |
| GB2021851A (en) | 1979-12-05 |
| NL181953C (en) | 1987-12-01 |
| SE448320B (en) | 1987-02-09 |
| GR66808B (en) | 1981-04-30 |
| NO152272B (en) | 1985-05-20 |
| FR2426887A1 (en) | 1979-12-21 |
| GB2021851B (en) | 1982-07-21 |
| AU4398179A (en) | 1979-11-29 |
| IT1115066B (en) | 1986-02-03 |
| NL181953B (en) | 1987-07-01 |
| FR2426887B1 (en) | 1984-06-22 |
| DK215579A (en) | 1979-11-27 |
| IT7948410A0 (en) | 1979-03-20 |
| NL7901581A (en) | 1979-11-28 |
| NO791638L (en) | 1979-11-27 |
| JPS54155794A (en) | 1979-12-08 |
| NZ189487A (en) | 1982-12-07 |
| NO152272C (en) | 1985-08-28 |
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