CA1103793A - Ring laser gyroscope - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed herein is a ring laser gyroscope of the type which operates with four circularly polarized beams at four mutually distinct frequencies, with two oppositely circularly polarized beams propagating in one direction and the two other oppositely circularly polarized beams propagating in the opposite direction. The ring laser gyroscope comprised two different gas isotopes as the active components of the laser gain medium, there being provided means for applying a magnetic field, whose direction substantially coincides with the beam directions, to the gain medium, i.e. the two gas isotopes, suitably in the form of DC current-carrying coils wound around the tube containing the gain medium, thereby to generate las-ing action at the four frequencies. The complete gyro-scope includes means for detecting beat frequencies re-sulting from combining the beams and means receiving the output from the detecting means for generating output sig-nals which are representative of rotational displacement of the ring laser gyroscope.

Description

This invention relates to ring laser gyroscopes wherein the difference between resonant frequencies of counterrot~ting li~ht beams i~ a measurement of rotation of the laser body. More specifically, thi~ invention has to do with la~er gyroscopes of the so-called four-mode type, wherein the term "mode" is u~ed to di~tinguish a beam from all other beam3 by both different frequency and different, actually opposite direction of propagation.
Ring laser gyroscopes utilizing counterrotating, i.eO oppositely propagating light beams are well-known.
These devices are used for measuring rotation rate~ of the laser body about an axis perpendicular to the plane of the ring laser resonant cavity by detecting the beat frequency which occurs due to a frequency difference between the oppositely propagating beams resulting from the rotation.
; ~owever, for the ring laser gyroscopes to function at low rates of rotation, frequency locking, frequently referred to as "lock-in"~ must be overcome. This phenomenon occurs when two beams of light waves propagating in opposite di rections in a resonant cavity have their slightly different frequencies "pulled" toward each other, so to speak, so that they combine into a single frequency beam of standing light wavec~ so that no useful output can be obtainedO To avoid this phenomenon of "lock-in"~ the frequencies of the oppositely propagating beams mu~t be sufficiently separated one from the other, ~qo that the "pulling together" does not o~cur. The effect~ of lock-in are described in d~tail in Laser ~ , edited by Monte Ross, Academic Press, Inc., New York, N~Y. 1971, pp. 141 to 143.

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One of the ways which has been proposed for eliminating lock-in in a ring laser resonant cavity is to have two pairs of oppositely, i~e. clockwise and counter-clockwi~e, propagating beams, with two oppositely polarized beams in each pair propagating in the cavity simultaneously~
Thus, one pair consist~ of two right circularly polarized light beams, one propagating in the clockwise direction and the other in the counter-clockwise direction. The other pair consists of two left circularly polarized beams which are al90 propagating in opposite directions within the same resonant cavity. Sucn a so-called "four-mode ring laser gyroscope" configuration i9 described in detail in U.S~
Patent No. 3,741,57, issued Jurle 26, 1973, entitled "Laser Gyroscope" byKeimpe Andringa, The structure and operation of a four-mode laser gyroscope is briefly described below.
Disposed in the laser beam path, i.e. within the ; cavity, are reciprocally anisotropic and nonreciprocally anisotropic optically dispersive elements. A reciprocally - anisotropic dispersive element, such as an optical rotator made of quartz crystal, i9 provided to cause different delays, due to different optical indices, to right and left circularly polarized light beams. Thi~ difference in opti-cal index is known as natural optical activity and results in an optical path length difference as seen by one and ~5 the other of a pair of oppo~itely circularly polarized light beams propagating within the ~ame cavity, regardless of the direction of beam propagation~ In addition, a nonreciprocally anisotropic dispersive element, such as a Faraday cell,is provided which presents different optical indices for light beams propagatlng in opposite directions, so that different delays are caused for beams propagating, ,:: . ~ , 1 ~! 3 7 9 ;~

i.e. traveling, in the counterclockwise direction and in the clockwise direction~ regardless of the sense of circular polarization. This is understood to be the result of different path lengths for beams propagating in opposite directions. Therefore, the presence of these two types of optically dispersive, anisotropic elements results in fre-~ncy separation between each resonant mode, wherein a mode is determined by frequency and direction of propagation and sense of circular polarization~ such that all four modes resonate at different frequencies.
Separation between the resonant mode frequencies i9 to be understood to be the phenomenon of conditioning the resonance characteristic of the cavity such that the different modes "see" distinct cavity lengths. In still ~; 15 other terms, a phenomenon of discrimination against other than a predetermined resonant frequency for each mode is ~- involved by establi~hing such resonant frequencyO It ~- can be seen that the effect achieved by the optically dispersive elements is of the natur0 of a tuning phenomenon, though such "tuning" is achieved without physically modi-fying the dimensions of the cavity. As a result, the reso-nant frequencies of the two be~ms traveling, i.e. propagat-ing, in one direction are spaced between the resonant fre-quencies of the two beam~ propagating in the opposite direc-tion, with the two higher frequency modes, or beams7beingcircularly polarized in the same sense but propagating in opposite directionsO Similarly, the two lower frequency mode~, or beams, are circularly polarized in the same sense, - which is oppo ite to the sense of polarization of the other pair, and are al80 propagating in opposite direction3.

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Each pair of such mutually oppositely propagating, identically circularly pola~ized beams may be viewed and understood to operate as a separate one of two distinct la er gyro~copes. As the ring laser gyroscope system is xotated-about an axis which is oriented perpendicularly to the plane of the ring-3haped beam path, the frequency sepa-ration, i.e. the frequency difference,between the higher frequency beam~ will either decrease or increase, while the frequency difference between the two lower-frequency beams will be oppositely affectedO That is, it will either in-; crease or decrease. The output beak signal resulting from combining the two lower frequency beams is subtracted from the output beat signal resulting from combining the two higher frequency beams. The resulting differential frequency :., ~ 15 i8 a substantially linear representation of rotation of :~, the ring laser gyroscope system. Further, the direction of rotation is determined by monitoring one of the pairs of , ~
beams.
~; In order to prevent the undesirable results of the phenomenon known as "hole burning", the four frequenciesassociated with the four resonating modes in the cavity must be substantially ~eparated. The concept of hole burning ;~ involves the population depletion of available light-:, emitting atoms in the gas laser gain medium which can emit radiant light at a given frequency. A laser beam sustained in a laser cavity through stimulated emission depletes the population of available light-emitting atoms about that fre-quency and thereby resultq in a dip or "hole" in the laser gain curveO This hole has a certain bandwidth such that, if two separate beams are operating at frequencies very close to each other, the bandwidths will overlap. This condition .: , .;, ,: ; ,. :

GCD~75-17

3 ~3 may be described by the phrase "hole burning competition."
As a result, one of the re~onant mode~3 will dominate and the intensity of the mode, or beam,operating at the adjacent frequency will be sub~tantially reduced or even eliminated.
Hole burning is explained in detail in the text Gas Laser ~S~E~a~YEY by Douglas C. Sinclair and W. Earle Bell, Holt Reinhart and Winston, IncO, New York, ~.Y. 19~9, pp~ 33-350 Accordingly, in order to su~tain lasing action in all four modes in the laser cavity, the ~requency of each mode must be sufficiently separated rom the other three to prevent the effects of hole burning competitionO The frequency spacing must be such that there is no significant overlap between the hole~ burned into the gain curves by each pair of ad~acent resonatiny modes.
To provide sufficient dispersion to avoid hole burning effe~ts between the different beams, the convention-ally used, reciprocally anisotropic crystal disposed in the laser beam path, frequently referred to as a crystal rotator, which exhibits the property of optical reciprocal anisotropy must be undesirably large. Its size contributes to thermal stresses which occur due to thermal gradients within the instrument and temperature changes in the laser system, frequently aggravated by difference~ between coefficients of expansion of the crystal and the remaining laser body.
These stres~es increase linear birefringence in the crystal, which increases coupling between different modes. Coupling, a~ u~ed herein, de~ignates an interaction between different beams propagating in the ~ame direction which result~ in an error source for the output from the ring laser gyroscope.

GCD-75-?7 3~7~313 Typically, the element exhibiting nonreciprocal ani~otropy is a Faraday rotator, also called a Faraday cell, which can be created by winding a coil around the cry~tal and pa3sing a DC current through the coilO The magnitude of the effect achieved by virtue of the property of nonrecip-rocal ani~otropy occurrinq in the cell which--in similarity with the Faraday effect--can be visualized as a twisting action upon the polarized light beams, i9 determined by the length of the cell, the magnitude of the magnetic field, lQ and the Verdet con~tant of the crystal material~ The Verdet constant is defined as rotation per unit length per unit magnetic field strength. It is a material property, i.e.
different materials have different Verdet con~tants associ-ated with them.
In view of the above-discussed output inaccuracies j due to thermal stresses, a conventional crystal is undesir-ably large. Its length, however, is very small when used in a Faraday cell. Then, in order to achieve the required , .
nonreciprocal anisotropy, the magnetic field over the short 2Q length of the cry~tal must be relatively large, typically ovex 1000 Gauqs. Such high field intensity is difficult to control over the short length of the cryqtal element.
The purposes of this invention include reduction in size of the crystal rotator, as well a~ reduction in field intensity of ~he magnetic field in the Faraday cell.

~:31¢i3~7~;~

In accordance with the invention, the foregoing objects are achièved through the utilization of the Zeeman effect, i.e. the imposition of a magnetic field parallel to the laser path over a so-called dual laser gain medium, which phrase denotes a laser gain medium comprising two distinct, separately operative media in the form of distinct gas isotopes within a common resonant cavity. The Zeeman effect resulting from the field causes frequencies of light emitted by atoms in the gain plasma to be shi~ted in such a manner that the frequency of light generated by an atom is either increased or decreased~ Further, these atoms are aligned with the magnetic field, so that all those shifted up in frequency may emit light of one sense of cir-cular polarization in one direction of propagation and of the opposite sense of circular polarization in the opposite direction of propagation. Those atoms which are shifted down in frequency are affected in the same manner, except that the s~nse of circular polarization is reversed for a given direction of propagation~
~0 Thus, due to the Zeeman effect, the gain curve for a given beam generated by the atoms of each isotope in the laser gain medium, i.e. plasma, will be divided into two gain curves. The result is that for one sense of circu-lar polarization, hole burning or source depletion resulting from a light beam propagating in one direction in the laser cavity will not affect the gain curve for a light beam of the same sense of circular polarization propagating in the opposite direction. The use of a dual isotope laser gain plasma results in the fact that the Zeeman effect produces four gain curves. This Zeeman splitting of the gain curve~

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7~3;~

~qubstantially increases the independence of the individual modeA with respect to the effects of hole burning in the ; gain medium. This minimization of the effects of hole burn-ing permits a ~ubstantial reduction in the separation be-tween ~he mean frequency of the two beams of one sense of circular polarization from the mean frequency of the two beams of opposite sense of circular polarization.
Accordlngly, the reciprocally anisotropic disper-sive element, normally a quartz crystal, which accomplishes !
~eparation between right and left circularly polarized light in the ring laser beam path may be substantialLy re-duced in size, as compared to the size in known instruments of the type contemplated, thereby to reduce thermal stresses ~, caused by temperature changes or temperature gradients in -the laser body.
Furthermore, the magnetic field imposed over, i.e~
applied to~the light source, which is the laser gain plasma, ; also acts as a nonreciprocally anisotropic dispersive ele-ment, i.e. it assumes the function of a conventionally used Faraday cell, discu3sed above. Because of the different ; Verdet constants and the increased length associated with the la~er gain medium, the same amount of Faraday splitting, which term is used herein to denote conditionin~ the re~o-nant cavity such that different re~onant frequencie~ are e~-tablished for the two opposite directions of propagation, i~ achieved as with prior art foux-mode gyroscopes with sub-stantially reduced magnetic field intensity.

~L~3~313 In accordance with a broad aspect of the inven-tion, there is provided a ring la~er ~yro~cope which oper-ates with four circularly polarized beam~ at four mutually distinct frequencie~, with two oppositely circularly polar-ized beàm~ propagating in one direction and the two otheroppo~itely circularly polarized beam~ propagating :in the opposite direction, the ring laqer gain medium comprising two different ga~ isotope~ a~ it~ active component.~, wherein means for detecting beat fre~uencie~ resulting from combin-; 10 ing the beam~ are provided and wherein mean~ receiving the output from the detecting means, during operation, generate output 3ignal~ which are repre~qentative of rotational dis-placement of the ring laser gyroscope, there being provided means for app~ying a magnetic field, whose direction ~ub-stantially coincide~ with the beam directions, to the gain medium which includes the two isotope~, thereby to genarate lasing action at the four mutually distinct frequencie~.
In accordance with specific feature~ of one em-bodiment of the invention, use is made of at least one coil which, during operation, carries direct cuxrent, thereby to generate the magnetic field, the coil coaxially surrounding the laser tube containing the gain medium. Then, the mag-netic field, in addition to generating la~qing action at the four mutually distinct frequencies, may assume the function of e~tablishing different re~onant frequencies for oppositely oriented direction~ of beam propagation, regardles~ of the ~ense of circular polari~ation~ In addition, there may be provided a reciprocally ani~otropic disper~ive optical de~

vice disposed within the path of the beams for esta~ hing a re~onant frequency for the la~er beam~ which are circularly GCD-~5-17 3'~

polarized in one ~ense and for simultaneou~ly e~tabli~hing a different re~onant frequency for the l~ser beam~ which are circularly polarized in the oppo~ite ~ense, regardle~3 of the direction of beam propagation~ Suitably, the opti cal device i3 an ani~otropic crystal, such as a quartæ cry~-tal.
The invention will become better under~tood from the followi~g detailed de~cription of one embodiment there-of, when taken in conjunction with the drawings, wherein:
Figure I is a schematic illustration of a multi-oscillator, i.e. four-beam ring laser gyroscope, combined with a block diagram of the necessary circuitry to process the information generated, Figure 2 is a graphic representation of the sepa-rate gain curves of each i~otope in a dual isotope gas laser gain medium, to-gether with the combined gain curves of the two isotope~, Figure 3 i~ a graphic representation of the sepa-rate gain curves in a dual isotope ring laser system, with Zeeman frequency splitti~g, Figure 4 schematically illugtrates the establish~
ment of re~onance conditions for the four frequencies associated with each mode of the multi-03cillator ring la~er gyroscope of Figure l; and Figure 5 i~ a ~chematic illustration of the gyro-scope output a~ a function of rotation - rate of the ring laser of Figure 1~

~ ~ 3t~ ~

With reference to Figure 1, the four mode ring laser gyroscope include~ a laser body 12 with a sealed resonant la er cavity 23. The cavity 23, a~ illustrated, provides a rectangular beam path, with mirrors 14, 16, 18, and 20 at its four corners~ The ~ealed cavity 23 is filled with a dual isotope gain medium, such as a helium-neon gas mixture, where the i~otopes neon 20 and neon 22 are the two active isotopes. In the portions of the cavity 23 between the cathode~ 46 and anode~ 48, where the gaseous gain med-ium is electrically excited, it becomes a light-emitting laser plasma which su~tain3 the laser beams at the re~onant frequencies.
Mirrors 14 and 16 are used solely for reflecting the beams in the laser path 24~ Mirror 18 is æecured to a piezoelectric element 20 which moves the mirror in and out, this portion of the structure forming part of the cav.ity length control sy~tem~ Mirror 22 i~ only partially reflec-tive, thereby allowing a .small portion of the light incident on it~ ~urface to pas~. The proportions of light beams pass ing through the mirror 22 arP combined one with the three other~ and processed to provide the de3ired rotational i~-formation a~ the outputO Line 24 represent~ the ring laser heam path for the four modes o circularly polarized light.
The ring la~ex gyroscope is equipped with a recip-rocally anisotropic disper~ive element 26. Matural opticalactivity which occurs wi~hin element 26 upon the circularly polarized light by separating one from the other, due to distinct resonance conditions, the two opposite senses of circular polarization is well-known in the art and may be accomplished with a material such a~ quartz cry~tal oriented such that the beams propa~a~e along its optic axis. Elements . :,,. . :

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28 are electric coils whi.ch, during operation, carry DC
current and thu3 prc)vide a magnetic field superimposed over the plasma gain medium ~ection~ between cathode~ 46 and anodes 48. Coils 28 are wound around the entire sec--tions between the cathode~ and anode~ to apply the magneticfield over substantially the entire gas plasma light source~
The magnetic fields ~et up by the coils 28 are typically about 100 ~au~s and both are oriented in the ~ame direction with respect to the laser path 24, 90 as not to cancel one another, Impo~ition of the magnetic field over the la~er ; beam path creates a condition related to the Faraday rota-tion effect, in the form of nonreciprocally ani~otropic disper~ion which, by distinct re~onant frequencies, differ-entiate~ between the clockwise and the counterclockwise propagating beams. Also, the field ~quperposed over the ex-cited pla~ma provides Zeeman frequency splitting between the light emitted from atom~ in the plasma, so that hole burning e~fects in the gain curves for right and left circularly polarized light beams will be subqtantially xeduced when the lasing frequencies are close together~ The Zaeman ef-fect is thoroughly explained in the text FundameDtals of Optics by Francis A. Jenkins and Harvey E. White, McGraw-Hill, New York, ~.Y. 1957, page3 588 hrough 595~
Line 3G repre~en~ that portion o the counter-cloc~wi~e propagating beams in the multi-o~cillatox ~y~tem which are allowed to pa~ through the partially reflective mirror 22~ The~e beams strike mirror 34 and are reflected - through beam splitter 38 onto a ~ingle photodiode 40. Line 32 representY that portion of the clockwise pro~agating 3Lil~q~a~ 79~
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beams in the system which pass through mirror 22 and strike mirror 36 where they are deflected to beam ;~
splitter 38 and made approximately colinear with line 30. The four beams simultaneously striking photodiode 40 generate several beat ~requencies due to the difference in frequency between all of the individual beams.
The beat frequencies between all of the four beams propagating in four associated modes in the cavity are detected in the photodiode 40~ as described in applicant's U.S. patent No. 4~123,162, issued October 31, 1978. The information generated ; from the beat frequencies between the four oscillating modes is used for determination of the magnitude of rotation of the ring laser system, as well as for `
cavity length control and for determination of the direction~ i.e. the sense of rotation~ A detailed description of how this information is used for these purposes is provided in the above-mentioned patent.
Cavity length control circuitry 42 provides an AC signal along leads 44 to the piezoelectric element 20. This AC signal moves mirror 18 in and out, i.e, back and forth parallel to itself, with this motion resulting in variation of the cavity length of the ring laser. This varies the output from the ring laser system as applied to photodiode 40 at the same frequency as the AC component in leads 44 and thereby provides feedback to the cavity length control circuitry 42. This feedback is processed as described in the above-mentioned patent to control the DC component along leads 44 to optimize the length of the ring laser cavity for maximum output~

mb/pl~ - 14 -il~a3~-7~3 GCD 75-17 Cathodes 46 and anodes 48 are connected to a power supply 52 via leads 50. The cathodes and anodes provide an electrical field over the gaq laser gain medium which is sufficient to maintain stimulated light emi~ion from the gas atoms to ~u~tain the propagation of la~er beams.
The vol~age acro~ cathodes 46 and anodes 48 oscillate~ at a constant frequency controlled by the power supply 52, to vary the output generated in photodiode 40. Thi8 output variation i8 prOCe9Sed in circuitry 52 for determination of the direction of rotation of the gyro~cope system in ac-cordance with the above-mentioned cepe~ g patent a~ e ~3n. The output from photodiode 40 is al90 fed to loyic - circuitry 54 for determination of the magnitude o~ rotation o~ the ring la~er, a is thoroughly discussed in the same patent ~ e~t~
In discus3ing Figures 2, 3 and 4 frequent refer-ence is made to portions of Figure l by way of explanation.
Fisure 2 shows the Doppler-broadened gain curves for a typi-cal dual isotope la~er pla~ma as contained in the tubes sur-rounded by coils 28 o~ Figure l. As mentioned above, the gain medium comprises the two isotopes neon 2Q and neon 22, Line3 62 and 64 represent the gain-Yersus-frequency curves of neon 20 and 22, respectively. Line 66, repxesenting the combined gain curve for the two isotopes, i5 the sum of curves 62 and 64. The curve3 as ~hown in Figure 2 are rep-resentative of a gain curve in a typical dual isotopa ring laser pla~ma, without application of a magnetic field whi~h would cause occurrence of the Zeeman effect.
Zeeman ~plittin ~ as described in undamentals of ~E~ , cause~ ~ach gain curve of Figure 2 to be ~plit into two gain curves separated on~ from the other in fre-~3~3 quency space, a~ i9 ~hown in Figure 3O The magnetic field as set up b~ DC curr.ent through coil~ 28 causes the light-emit~.iny atoms in the la~er gain pla~3ma to be oriented such that any given atom may Pmit right ci.rcularly polarized light in one propagation direction or left circularLy polar.ized light in the opposite propagation direction. Al so . the mag netic field causes the frequency at which light-emitting atoms may emit light to shift either up or down by an amount determined by the magnitude of the field.
CUrYeS 72 an~ 74 in Figure 3 are the gain curves resulting from the splitting of curve 62 in Figure 2. The available light-emitting atoms represented by gain curve 72 may emit light which is left circularly polarized and propa-gates in the clockwise direction or r.i~ht circularly polar-ized and propagates in the counterclockwise direction. Con-ver~ely, the atoms repre~ented by gain curve i4 may emit right circularly polarized l.ight propagating in the counter-clockwise direction and left circularl.y polarized light propagating in the clockwi~e direction. Also, curves 76 and 78 represent the gain for both right and left, respectively~
polarized light propagating in the colmterclockwise direction resulting from splitting of gain cuxve 64 and ].eft and right, re~pectively, circularly polarized light propagating in the clockwise directionO ~ha gain curve~ of Figures 2 and 3 are shown ~or purposes of explanation and are not necessarily drawn to s~ale. The effective Zeeman ~plitting of the gain curves sub~tantia.l!y reduces the hole burning type coupling between the various modes~ As mentioned above, necessary - field magnitudes to accomplish Zeeman splitting ln this four--mode ring laser gyrosco~e are t~pically around 100 Gauss or less~

Figure 4 illustrates the function of the recip-rocally and nonreciprocally anisotropic elements which re Yults in frequency separation between the four beams associ-ated with the four resonating mode~ in the ring laser cavity.
In frequency space, where incxeasing optical frequency is repr,3sented by line 83, line 81 represents the mean resonant frequency of the ring laser cavity. The reciprocally aniso-tropic dispersion element 26 ~natural optical activity crys-tal rotator) in the ring laser path causes frequency ~plit-ting between leEt and right circularly polarized light, i.e.establishing different resonant frequency conditions, as represented by line~ 92 and 90, re~pectively~ Further fre-quency splitting.of the four resonating modes in the re~o-nant cavity is accomplished by nonreciprocally anisotropic dispersion, called Faraday splitting, see above, in the plasma, as the magnetic field causes clockwi~e and counter-clockwise propagating, p~larized light beams to experience different optical indices. Lines 82 and 84 represent the results of Faraday splitting of the left circularly polarized laser beams represented by line 92. In the same manner, lines 86 and 88 show the effects of Faraday splitting on right circularly polarized laser beams repre~ented by line 90. At this point, it should be mentioned that lines 82 and 88 represent frequencies of clockwise propagating beams, as shown in Figure 4~ The lower and upper frequency beam~ thus propagate in the same direction in the la~er cavity. If the magnetic field polarity is rever3ed by reversal of current direction in coils 28, the direction of the extreme frequency will be reversedO

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As shown in Fi~ure 4, the two beams at frequen-cies 82 and 84 at the left-hand side of the figwre whose mode is characterized by left circularly polarized light may be understood to pertain to one ~yroscope/ designated by the legend GY~0 1, while the beam~ of frequencies 8~ and 88 whose light is right circularly polarized similarly form another gyroscope, designated GYR0 2.
Typically, separation resulting from Faraday splitting between the counterrotating beams in GYR0 1 and GYR0 2 is from 500 kilocycles to 1 megahertz. In four-mode ring laser gyroscopes not employing the Zeeman effect frequencies 92 and 90 generally are required to be separated by a distance greater than 200 megahertz, in order to avoid overlap of hole burning in the gain curvesO The Zeeman ef-fect, or the magnetic field causing it in the laser gain medium, makes it possible for the curves 72, 74, 76 and 78 to co-exist at much closer resonant frequencies for a dual iso-tope laser gain medium. The increased independence of the four resonant modes with xegard to hole burning permits the extent of necessary natural optical activity splitting ~re-ciprocally anisotropic dispersion) to be reduced, so that the frequency separation between lines 92 and ~0 may be as small as 10 megahertz. Accordingly, tne crystal element 26 may be reduced in size. Both the size of element 25 and the magnitude of the magnetic field generated by current through coils 28 are optimized, so that the modes associated with frequencies 82, 84, 8~ and 88 minimally affect each other.
- As the ring laser system is rotated about an axis perpendicular to the plane of the laser path in the counter-~ ~23 ~ 9~ GCD-75-17 clockwi~e direction, frequencie~ 82 and 88 will increa~e, while frequencies 84 and 86 will decr~ase. Becau~e the output from the gyroscope is a ~unction of the separation between the frequencies of beams propagating clockwise and counterclockwise in the laser cavity, the output from GYRO 1 will decrease while the output from GYRO 2 will increase.
Conversely, if the laser 3y~tem i~ rotated in a clockwise direction, the outputs from GYROS 1 and 2 will increase and decrease, respectively.
Figura 5 graphically illustrates the output from the gyroscope as a function of rotation rate of the ring laser system. Lines 94 and 9~ repre~ent the outputs for GYROS 1 and 2, respectively, as a function of system rota-tion in inertial space. The output signals from one gyro-scope are substracted from output signals of the other gyro-scope and processed in logic circuitry 54 of Figure 1 to provide a linear net output and a doubled scale factor for system rotation. Point A in Figure 5 represent~ zero rota-tion for the laser system where the outputs of both GYRO 1 and GYRO 2 are approximately equal.
Other embodiments of, and modifications to, the described ring laser system are within the ~cope of this invention. For example, other means of output detection and information processing may be employed, the nu~ber of reflec-tive elements in the ring laser beam path may be changed,and the magnetic field, or fields, for Faraday and/or Zeeman splitting may be implemented using a permanent magnet~ Also, means might be employed within the laser cavity to convert circularly polarized light into linearly polarized light throughout the la~er c vity except in the plaqma area where, during operation, Zeeman splitting occurs~

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SUPPLEMENTARY DISCLOSURE

A more specific manner of clefining ~he necessary magnetic Eield intensity will now be described with reerence to the additional drawings in which:
FIG, 6 shows separate gain vs. frequency curves of each isotope in a dual isotope gas laser plasma, together, with acceptable laser wave frequency separation according to the prlor art;
FIG. 6A shows a gain vs. atom velocity curve of the isotope corresponding to the left curve of FIG, 6D
showing the depletion of atoms of that isotope caused by lasing of the four modes; ~ :
FIG. 6B shows a gain vs~ atom velocity curve of the isotope corresponding to the right curve of FIG, 6, showing the depletion of atoms of that isotope caused by lasing of the four modes;
FIG. 7 shows separate gain vs. frequency curves of each isotope in a dual isotope gas laser plasma, together with unacceptable laser wave frequency separation according to the prior art;
FIG. 7A shows a gain vs, atom velocity curve of the isotope corresponding to the left curve of FIG, 7, showing the depletion of atoms of that isotope caused by lasing of the four modes;
FIG. 7B shows a gain vs, atom velocity curve of - ~he isotope corresponding to the right curve of FIG, 7 9 showlng the depletion of atoms of that isotope caused by lasing of the four modes;
FIG, 8 shows separate gain vs, frequency curves foreach isotope in a dual isotope gas laser plasma, showing an insufficient amount of Zeeman frequency splitting, accordlng to the prior art ~ .
mb ~ . - 20 -3~3 FIG. 8A shows a gain vs. atom velocity curve of the isotope corresponding to the lef ~ curves of FIG. 8~
showlng the depletion of atoms of that isotope caused by lasing of the four modes;
FIG, 8B shows a gain vs~ atom velocity curve of the isotope corresponding to the right curves of FIG. 8, showing the deple~ion of atoms of that isotope caused by :
lasing of the four modes;
FIG~ 9 shows separate gain vs. frequency curves for each isotope in a dual isotope gas laser plasma, wherein the magnetic field intensities in the two gain sections are aiding, showing a proper magnitude of Zeeman frequency splitting, according to this invention;
FIG~ 9A shows a gain vs~ atom velocity curve of ~
the isotope corresponding to the left curves of FIG, 9, ~ :
showing the depletion of atoms of that isotope caused by ~ :~
lasing of the four modes;
;~ FIG. 9B shows a gain vs, atom velocity curve of : : -the isotope corresponding to the right curves of PIG~ 9, s~owing the depletion of atoms of that isotope caused by lasing of the four modes;
FIG. 10 shows separate gain vs, frequency curves for each isotope in a dual isotope gas laser plasma, showing the effect of an excess of Zeeman frequency splitting;
FIG. lOA shows a gain vs. atom velocity curve of the isotope corresponding to the left curves of FIG, 10, showing the depletion of atoms of that isotope caused by lasing of the four modes;
FIG, lOB shows a gain vs. atom velocity curve of the isotope correspond~ng to the right curves of FIG, 10, showing the depletion of atoms of that isotope caused by lasing of the four modes;
f~

mbl~ - 21 -~ . .. ...

3'~513 FIG. 11 is identical to FIG, 9 except that the field intensities in the two gain sections are opposing;
FIG. llA shows a gain vs, atom velocity curve of the isotope corresponding to the left curves of FIG, 11, showing the depletion of atoms of that isotope caused by lasing of the four modes;
FIG. llB shows a gain vs~ atom velocity curve of the isotope corresponding to the right curves of FIG, 6, showing the depletion of atoms of that isotope caused by lasing of the four modes.
Figures 6, 6A, 6B, 7, 7A, 7B, 8, 8A, 8B, 9, 9A, 9B, 10, lOA, lOB, 11, llA and llB are included herein to compare this invention with prior art apparatus and to define the upper and lower limits of the intensity of the magnetic Eield applied by coils 28 to the gain medium in this invention.
Figures 6, 6A, 6B, 7, 7A9 7B, 8, 8A, 8B refer to prior art mechanisms. Figures 6, 6A~ 6B, 7, 7A, 7B, for example, could correspond to the,operation of the apparatus of United States patents 3,741,657 and 4,no6,989 with Figures 6, 6A, 6B representing proper operation with a long crystal and adequate frequency separation of the modes to avoid hole burning. Figures 7, 7A, 7B is an inoperative version of such apparatus where a small crystal is used and the frequency separation of the modes due to natural optical activity splitting has been reduced from the order of 400 Mhz to 10 Mhz, 10 Mhz was chosen to compare such apparatus to the apparatus of this invention which does have a natural optical activity splitting on the order of 10 Mhz while still avoiding hole burning, Figures 8, 8A, 8B corresponds to the apparatus of ~his invention e~cept that the intensity of the magnetic field applied to the gain medium is far too low. For mb~ - 22 -~.a~3~

example, in United States patent 3,973,851, issued August 10, 1976 to Ferrar, the ield was less than one Gauss~
Aside from the fact that the field intensity is too low to prevent hole burning, it is also so low that the earth's magnetic field would interfere with its operation for its intended purpose which is to equalize the gain between clockwise and counterclockwise propagation.
Figures 9, 9A, 9B correspond to the proper operation of the apparatus of this invention. Note in Figures llA~
llB, the region of competition for atoms~ shown shaded in the figures, is minimized, Figures 10, 10A, 10B corresponds to the apparatus of this invention except that the intensity of the magnetic field is far too high.
Thus, by comparing the figures, the range of acceptable field intensity to produce a Zeeman effect of appropriate magnitude to allow relatively small natural optical activity splitting without hole burning may be dlscerlled.
Zeeman splitting, as described in Fundamentals of Optics, supra, results in each gain vs, optical fre~uency curve of Figures 4, and 5 to be split into two curves shifted ln frequency space as shown in Figures 8, 9 and lO~
The magnetic field elements 28 cause the light emitting atoms in the laser gain plasma to be oriented such that any given atom may emit by stimulated emission a right circularly polariæed light wave ;n one direction or a left circularly polarized light wave in the opposite direc~ion, Figures 6, 7, 8, 9, 10 and ll show typical plots of gain vs. optical frequency for the isotopes neon 20 and neon 22. Obviously if other elements or isotopes were used, their frequency range would be different, Actually these b ' ~h` ' ' a^, ~
mb/~ - 23 -~3~
curves are only the portion of a normal distribution curve, where the galn exceeds one, and the laser will oscillate~
Figures 6A, 7A, 8A, 9A, lOA and llA are gain vs, atom velocity distribution for neon 20 in the clockwise (+) and counterclockwise (-) directions of the laser path, The graphs show the total available atoms as a function of velocity and how the various optical wave modes deplete and compete for the various available velocities. The ` shaded region shows where competition occurs, and the dips in the curves demonstrate the "holes" which are "burned"
in the distribution by the four modes of optical wave propagation.
Figures 6B, 7B, 8B, 9B, lOB and llB are the corresponding gain vs. atom velocity distribution for neon 22, Figures 6, 6A, 6B and 9, 9A, 9B and 11, llA, llB
situations where hole burning is avoided, The remaining figures show inoperative situations because of hole burning, Figures 6~ 6A, 6B correspond to the prior art without Zeeman effect, Figures 9, 9A, 9B, 11, llA, llB
correspond to the appara~us of this invention~
Consider now the prior art represented by Figures 6, 6A, 6B.
Curves 100; 102 are gain vs, optical frequency curves for neon 20 and neon 22, respectively, The maximum gains for these two gases occur 875 Mhz apart, and the laser cavity is tuned to the mid frequency fO between those points 104, 106. The natural optical activity splitting must be large. Typically it is about 400 Mhz, and it must be larger than about 200 Mhz, The Faraday separation between clockwise and counterclockwise propagating optical waves is on the order of 0.4 Mhz. Note in U, S, patents 3,741,657 and 4,006,989 the frequency splitting is about mb[J~ - 24 -. .. . :. : ..

;3~93 200 Mhz, the quartæ is about 4 mm long and the field strength is 2000 Gauss~ Figures 6~ 6A, 6B show operation where the reciprocal frequency splitting is 400 Mhz The frequencies are labeled on the abscissa wherein "L" means left polarized,"R" means right polarized, "CW" means clockwise, and "CCW" means counterclockwise The separation of RCW and RCCW~ and the separation of LCw and Lccw are exaggerated.
Turning now to FIG. 6A, it is seen that the available velocities depleted by the four modes are sufficiently separated that they do no~ substantially compete for atoms. The "holes" 108, 110, 112, 114 do not substantially overlap. The velocity at points 108, 110 is proportional to the difference in frequency between that of point 104 (FIG. 6) and points 116. The velocity at poin~s 112, 114 is proportional to the difference in frequency between tha~ of point 109 and points 118c The regions of competition for atoms is m-lnimal as represented by the shaded zones 120, 122~ 124, 126, 128 FIG 6B is a similar graph for neon 22, Note that the hole positions are identical, but they correspond to different modes because the frequencies of 116, 118 are less than that of point 106, The velocity at points 130, 132 is proportional to the difference in frequency between that of point 106 and points 116. The regions of competition for atoms is minimal as represented by the shaded zones 138, 140, 142, 144, 146.
Thus, the apparatus used for Figures 6, 6A, 6B is operative to minimize hole burnin~, and all four modes will lase. To achieve this compensation, however, the crystal is relatively long and the magnetic field is very strong.
At such high fields ~1000-2000 Gauss), field control is very difficult.
'`~
mb~ - 25 -~`3 ~

If the crystal were shortened in the apparatus corresponding to Figures 6, GA, 6B, to provide a natural optical activity splitting of, for exampLe, only 10 ~Ihz (as in this invention), the non-reciprocal Fara~ay separation could not occur in the crysta:L because the crystal would be too short (on the order of 0,4 mm) to concentrate sufficient magnetic Eield intensity in the crystal, An external Faraday section would be needed to obtain even minimal non-reciprocal separation, Figures 7, 7A, 7B correspond to such a situation.
In FIG, 7, the difference between frequencies 150 and 152 is on the order of 10 Mhz, The distance from the frequency of point 104 and that of points 1509 152 are a]most the same, i,e,, 432,5 Mhz and 4b~2,5 Mhz, Thus, the "holes" 154 and 156 9 and the holes 158, 160 are almost on top of each other in Figure 7A for neon 20. The competition for atoms between R cw and LCcw modes and between L w and Rc~ modes is very strong, and only one mode in each pair will lase. The shaded areas 162, 164 representing competition between two modes, is very great, Similarly~ the difference between the frequency corresponding to point 106 and that of points 150, 152 are also 442,5 Mhz and 432.5 Mhz, and hole burning occurs, Notice that holes 17n, 172 and 174, 176 are almost on top of eacll other, The R and L modes in neon 22 compete ccw ccw for atoms as shown by the shaded area 180, The L and R w modes also compete for atoms as shown by the shaded area 178~ Only one mode of each pair will ]ase, Keeping the crystal short and the Faraday field as in Figures 7, 7A, 7B, but applying only a small amount of magnetic field to the gain medium produces Zeeman splitting as shown in Fig. 8. The neon 20 gain vs.
frequency curve of Figures 6 and 7 is shifted up and down mb/J~ - 26 -~3'~

in frequency a small amount to produce two gain vs frequency curves 200, 202 symmetrical about ~he crossover polnt 204. Similarly, the neon 22 gain vs. frequency curve of Figures 6 and 7 is shifted up and down in frequency a small amount to produce two gain vs. frequency curves 206, 208 symmetrical about crossover point 210.
The crossover points 204, 210 and 875 Mhz apart and symmetrically positioned relative to Fo. The amount of Zeeman shift is 1,8 Mhz per Gauss of applied field. ~ote that with 1 Gauss maximum of U~ S~ patent 3,973,851, the amount of Zeeman shift would be negligible, and it likely would not be seen if drawn to scale in Figure 8, Curves 200 and 206, which have shifted downward, describe the gain vs. frequency for the LCw and RCcw 202 and 208, which have shifted upward~ describe the gain vs. frequency for the RCw and LCCW modes.
FIG 8A is a graph of the atom velocity distribution of neon 20. The difference in frequency between that of peak point 220 and the frequency of 226 is too close to the difference in frequency between that of peak point 224 and that of 230. Consequently "holes" 240, 242 and holes 244, 246 are too close together, and only two modes will oscillate.
FIG~ 8B shows the corresponding velocity distribu-tion for neon 22. The frequency difference between that of peak point 232 and 226 is too close to the difference between that of peak point 234 and 230. The coupling between modes is excessive, as shown by the cross-hatched areas of Figures 8A and 8B, and only two modes will lase.
Figures 9, 9A, 9B show conditions for the optimum adjustment of field intensity according to this invention.

In Figures 9A, 9B notice that the region of coupling of the modes, as lndicated by the shaded regions, is minimized.

mb~ - 27 -The "holes" of the four modes are sufficien~ly separated so that they all will lase. Note that the competing regions for gain atoms are substantially the same as in Figures 6A, 6B, Figures 10, lOA, lOB show conditions wherein the apparatus of this invention is using an excessive field intensity Note that the L and R "holes" in FIG lOA
c c.w cw are too close together, they are closely coupled as indicated by the large hatched area, and any one of those two modes will lase. Similarly in FIG. lOB, the L w and RCcw "holes" are too close together, and they are closely coupled as indicated by the large hatched area, and only one of the two modes will lase.
With the fields of coils 28 aiding as shown, the fields not only produce Zeeman effect, but they also produce sufficient non-reciprocal anisotropic Faraday effect without additional Faraday cells, With the fields of coils 28 in the two gain sec~ions opposing, the Faraday effect is minimiæed, and if the two gain sections are substantially identical~ and if the field intensitles are substan~ially identical, the Faraday effect is cancelled, and an additional non-recip-rocal anisotropic element must appear in the loop. Note, however, that the ~eeman effect i5 unchanged from Figures 9, 9A, 9B except that the modes are interchanged as shown in Figures 11, llA, llB~
The minimum allowable magnetic field intensity is above the value where the RCw and LCw mode pair and the R and L mode pair are sufficiently coupled to ccw ccw extinguish one mode of each pair.

~ mb/~

~?3~313 The maximum allowable magnetic fleld lntensity ;~
is,below the value where the LCcw and RCw mode pair and the R and L mode pair are sufficiently coup~ed to c cw cw extln~ulsh one mode of each pair~

" ' - . ~
~. , ~ ;`

~ mb~ ~ - 29 -

Claims (36)

1. A ring laser gyroscope which operates with four circularly polarized beams at four mutually distinct frequencies, with two oppositely circularly polarized beams propagating in one direction and the two other oppositely circularly polarized beams propagating in the opposite direction, the ring laser gain medium comprising two different gas isotopes as its active components, wherein means for detecting beat frequencies resulting from combining the beams are provided and wherein means receiving the output from the detecting means, during operation, generate output signals which are representative of rotational displacement of the ring laser gyroscope, characterized by means for applying a magnetic field (H), whose direction substantially coincides with the beam directions, to the gain medium which includes the two isotopes, thereby to generate lasing action at the four mutually distinct frequencies.

2. Ring laser gyroscope according to Claim 1, characterized by at least one coil (28) which, during operation, carries direct current, thereby to generate the magnetic field (H), the coil coaxially surrounding the laser tube containing the gain medium.

3. Ring laser gyroscope according to Claim 1 or Claim 2, characterized in that the magnetic field (H), in addition to generating lasing action at the four mutually distinct frequencies, assumes the function of establishing different resonant frequencies for oppositely oriented dir-ections of beam propagation, regardless of the sense of circular polarization.

4. Ring laser gyroscope according to claim 1, characterized by a reciprocally anisotropic dispersive optical device (26) disposed within the path of the beams for establishing a resonant frequency for the laser beams which are circularly polarized in one sense and for simultaneously establishing a different resonant frequency for the laser beams which are circularly polarized in the opposite sense, regardless of the direction of beam propagation.

5. Ring laser gyroscope according to Claim 4, characterized in that the optical device (26) is an aniso-tropic crystal.

6. Ring laser gyroscope according to Claim 5, characterized in that the anistropic crystal (26) is a quartz crystal.

7 . A ring laser gyroscope comprising:
reflective means for at least partially confining radiant energy waves to propagate in a closed-loop path;
means for generating at least a plurality of oppositely circularly polarized resonant radiant energy waves propagating in each direction within said closed loop, said generating means comprising a mixture of at least two different gas isotopes as active components; and means for applying a magnetic field, whose direction substantially coincides with the direction of said propagating waves, to said generating means, whereby the coupling effects due to hole burning in the generating means are diminished.

8 . The ring laser gyroscope defined in Claim 7 wherein the frequencies of said waves propagating in one direction in said path increase, while the frequencies of waves traveling in the opposite direction decrease as said gyroscope is rotated about an axis.

9 . The ring laser gyroscope defined in Claim 8 further comprising:
means for detecting beat frequencies resulting from combining said waves: and means receiving the output from said detecting means for generating signals representative of rotational displacement of said gyroscope.

10 . The ring laser gyroscope defined in Claim 7 further including reciprocally anisotropic dispersive means positioned in the path of said propagating waves for separating resonant frequencies of oppositely polarized resonant waves.

11. The ring laser gyroscope defined in Claim 7 wherein said plurality of waves comprises a pair of right circularly polarized counter-propagating resonant waves, and a pair of left circularly polarized counter-propagating resonant waves within said closed loop path.

12. The ring laser gyroscope defined in Claim 7 including nonreciprocally anisotropic dispersive means positioned within said path for separating resonant frequencies of waves propagating in one direction from resonant frequencies of waves propagating in the opposite direction in said path.

13 . A ring laser gyroscope comprising:
reflective means for at least partially confining light waves to propagate in a closed-loop path;
means for generating at least a pair of right circularly polarized resonant light waves and a pair of left circularly polarized resonant light waves, with the two waves of each pair propagating in opposite directions within said closed loop path, said generating means comprising a mixture of at least two different gas isotopes as active components;
means for aligning radiant energy wave emitting atoms in said generating means and increasing or decreasing frequency of oscillation of said atoms, whereby said emitting atoms emit circularly polarized light waves of one sense of polarization in one direction of said path and the opposite sense of polarization in the opposite direction of said path;
means for detecting beat frequencies resulting from combining said waves;
means receiving the output from said detecting means for generating signals representative of rotational displacement of said gyroscope; and means for varying the length of said closed-loop path to optimize said output from said detecting means.

14 . The ring laser gyroscope system defined in Claim 13 wherein said gas isotopes include neon 20 and neon 22.

15. The ring laser gyroscope system defined in Claim 13 including polarization anisotropy means positioned within said closed-loop path for separating.
frequencies of said oppositely polarized resonant waves.

16. The ring laser gyroscope defined in Claim 13 including directional anisotropy means for.
separating frequencies of said resonating waves propagating in opposite directions within said path.

17. The ring laser gyroscope system defined in Claim 16 wherein said means for aligning wave emitting atoms includes a magnetic field induced in substantially all of said wave generating means.

18. The ring laser gyroscope defined in Claim 17 wherein said directional anisotropy means includes said magnetic field induced in said wave generating means.

19. A four mode ring laser gyroscope comprising:
reflective means for confining radiant energy waves to propagate in a closed-loop path;
means for generating at least a pair of right circularly polarized counter-propagating resonant light waves and a pair of left circularly polarized counter-propagating resonant light waves within said closed-loop path, said generating means comprising a mixture of at least two different gas isotopes as active components;
means for inducing a magnetic field into substantially all of said generating means and substantially parallel to said propagating waves, whereby the coupling effects due to hole burning in said generating means are diminished;
reciprocally anisotropic dispersive means positioned in the path of said propagating waves for separating resonant frequencies of oppositely polarized resonant waves;
means for detecting beat frequencies resulting from combining said waves;
means receiving the output from said detecting means for generating signals representative of rotational displacement of said gyroscope; and means for varying length of said closed-loop path to optimize the output from said detecting means.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

20. A four mode ring laser comprising:
reflective means for at least partly confining a pair of right-circularly-polarized clockwise and counter-clockwise propagating radiant energy waves and a pair of left-circularly-polarized clockwise and counter-clockwise propagating radiant energy waves to propagate in a closed loop path;
an enclosure enclosing said closed loop path;
at least two gas isotopes comprising a gain medium within said enclosure;
means for optically pumping said gain medium in at least one gain section of said path;
reciprocally anisotropic dispersion means in said path for separating the frequency of right-circularly-polarized radiant energy waves from the frequency of left-circularly-polarized radiant energy waves;
means for applying a substantially uniform magnetic field in the direction of said path to at least one said gain section, the intensity of said magnetic field being above the value where the right-circularly polarized and the left-circularly-polarized clockwise-propagating radiant energy mode pair and the right-circularly-polarized and left-circularly-polarized counter-clockwise-propagating radiant energy mode pair are sufficiently coupled in each pair to extinguish one mode of each pair, and the intensity of said magnetic field being below the value where the left-circularly-polarized counter-clockwise-propagating and the right-circularly-polarized clockwise-propagating radiant energy mode pair and the right-circularly-polarized counter-clockwise-propagating and left-circularly-polarized clockwise-propagating mode pair are sufficiently coupled in each pair to extinguish one mode of each pair,

21. Apparatus as recited in claim 20 wherein the frequencies of said waves propagating in one direction in said path increase while the frequencies of waves traveling in the opposite direction decrease as said ring laser is rotated about an axis;
means for detecting the different polarized and different propagating modes of said waves to produce signals representative of said different modes of said waves;
means for combining said signals to produce beat frequency signals;
means for detecting said beat frequency signals and means for producing signals which are a measure of the rotational displacement and rotational displacement rate of said laser about said axis.

22. Apparatus as recited in claim 20 and further comprising means in said loop for producing non-reciprocal anisotropic dispersion of frequency between said clockwise and counter-clockwise propagating modes.

23. Apparatus as recited in claim 22 in which said non-reciprocal anisotropic dispersion means is a Faraday rotator.

24. Apparatus as recited in claim 22 in which said isotopes are neon 20 and neon 22.

25. Apparatus as recited in claim 20 in which said reciprocally anisotropic dispersion means is a material having natural optical activity splitting.

26. Apparatus as recited in claim 25 in which said reciprocally anisotropic dispersion means is a quartz crystal.

27. Apparatus as recited in claim 20 in which there are two symmetrically positioned gain sections, and said gain sections are optically pumped by applying a voltage between the two ends of each of said sections.

28. Apparatus as recited in claim 27 in which said magnetic field is applied to the gain medium in both said gain sections.

29. Apparatus as recited in claim 28 in which the polarities of said magnetic fields are in the same direction around said loop.

30. Apparatus as recited in claim 28 in which the polarities of said magnetic fields are in opposite direction around said loop.

31. Apparatus as recited in claim 30 in which said gain sections are substantially identical, and the magnetic field intensities applied to said gain sections are substantially the same, and further comprising means in said loop for producing non-reciprocal anisotropic dispersion of frequency between said clockwise and counter-clockwise propagating modes.

32. Apparatus as recited in claim 29 in which said non-reciprocal anisotropic dispersion means is a Faraday rotator, in which said reciprocally anisotropic dispersion means is a quartz crystal, and said isotopes are neon 20 and neon 22.

33. Apparatus as recited in claim 32 in which said reciprocal optical dispersion may be as small as ten Mhz.

34. Apparatus as recited in claim 33 in which the total intensity of said magnetic fields is on the order of 100 Gauss.

35. Apparatus as recited in claim 34 wherein the frequencies of said waves propagating in one direction in said path increase while the frequencies of waves traveling in the opposite direction decrease as said ring laser is rotated about an axis;
means for detecting the different polarized and different propagating modes of said waves to produce signals representative of said different modes of said waves;
means for combining said signals to produce beat frequency signals;
means for detecting said beat frequency signals;
and means for producing signals which are a measure of the rotational displacement and rotational displacement rate of said laser about said axis.
36. A four mode ring laser gyro comprising:
reflective means for at least partly confining a pair of right-circularly-polarized clockwise and counter-clockwise propagating radiant energy waves and a pair of left-circularly-polarized clockwise and counter-clockwise propagating radiant energy waves in a closed loop path;
an enclosure enclosing said closed loop path;
at least two gas isotopes comprising a gain medium within said enclosure;
means for optically pumping said gain medium in all gain sections of said path;

Claim 36....continue.

reciprocally anisotropic dispersion means in said path for separating the frequency of right circularly-polarized radiant energy waves from the frequency of left-circularly-polarized radiant energy waves by a chosen predetermined value;
means for applying as a function of said predetermined frequency separation a substantially uniform magnetic field in the direction of said path to all of said gain sections, the intensity of said magnetic field in each gain section being at a value to separate the gain curves of each said isotope into two gain curves separated by an amount to minimize hole burning competition for said predetermined frequency separation.