CA1189600A - Dispersion equalized ring laser gyroscope - Google Patents

Abstract

DISPERSION EQUALIZED RING LASER GYROSCOPE
Abstract A ring laser gyroscope system uses a non-planar path and non-depolarizing Faraday bias means to achieve four-frequency differential operation. The use of the non-planar path elimi-nates the crystal rotator normally used to provide reciprocal bias and therefore eliminates scattering surfaces and a major cause of thermal sensitivity. The non-depolarizing Faraday means, such as a thin glass slab having a component of magnetic field along the optic axis of the laser, is used to reduce the corresponding thermal sensitivity of the traditional Faraday bias element normally implemented by a crystal such as quartz.
Eliminating these major sources of drift then allows higher-order sources of drift to be corrected. Zeeman splitting of the laser gain medium is used to provide an equal amount of dispersion to each of the two counter-travelling waves of each wave pair, thus eliminating small amounts of drift due to the different phase shifts produced in the propagating waves by the gain medium.

Description

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Background of the Invention To be useful, a laser gyro has to overcome the lock-in problem that occurs at low rotational rates. Lock-in is caused by unavoidable scattering of some light from one resonator mode into the other by imperfections of the optical elements com-prising the cavity. If the frequencies of the modes are not too different, there is a tendency o the modes to phase lock.
For a gyro to be of practical use, it has to overcome this loc~-in problem. A two-frequency gyro system may avoid lock-in by biasing the gyro so that it operates with a large output frequency for zero input rotation rate. To avoid problems in bias accuracy, the bias can be dithered so that bias instabili-ties may be eliminated from the output signal by time averaging.
This dither approach, however, causes the gyro to go through lock-in twice per dither cycle. This causes the gyro to parti-ally lose its phase coherence; thus an error of a fractional count is made per dither cycle. These errors add randomly giving a cumulative output angular error which increases with time. A
four-frequency diferential laser gyro system solves this pro-blem by essentially operating two independent gyros in a single stable resonator sharing a common optical path, but statically biased in opposite senses by the same passive bias elements.
In the differential output of these two gyros, the bias cancels while any rotation generated signals add, thereby giving a sensitivity twice that of the single two~frequency gyro and avoiding problems due to drifts in the bias~ The four differ-ent fre~uencies are normally generated by using two different optical effects. First, a crystal polarization rotator is used to provide a direction-independent polarization causing the re-sonant waves to be nearly right~hand circularly polarized (RCP) /

and left-hand circularly polarized (LCP). The polarization rotation results from the refractive index of the rotator medium being slightly different for RCP and LCP waves. Second, a Faraday rotator is used to provide non-reciprocal polari-zation rotation, by having a slightly different refractive index for clockwise travelling waves than for counter-clockwise travelling waves. This causes the cw and ccw RCP waves to oscillate at slightly different frequencies while the cw and ccw LCP waves are similarly but oppositely split. This Faraday rotator may be a separate optical element consisting o a piece of optically isotropic material subjected to a longitudinal magnetic field, or it may be realized by applyiny such a mag-netic field to the crystal rotator. Thus, there is a laser gyro operating with right circular polarization biased in one direction of rotation and another with left circular polari-zation biased in the opposite direction, the bias being can-celled by subtracting the two outputs. The operation of a basic four-frequency laser gyroscope is described in United States Patent No. 3,741,657 issued June 26, 1972 to K. Andringa and assigned to the present assignee.

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Summary of the Invention The present invention discloses a laser gyro system which recognizes the problems that limit the stability of present laser gyro systems. Ideally, any resultant fluctuation in the differential output of a four-frequency gyro should be cancelled since all four frequencies should be affected equally by external sources, such as thermal expansion. In practice, it has been found that the four frequencies are not equally affected by ex-ternal sources, resulting in different variations of the freq-uencies. Thus, even though four-frequency gyros have exhibited excellent performance levels under conditions of thermal equi-librium, the practical application of these instruments has, so far, been limited by the presence of thermal sensitivities mani-fested as unacceptable lon~-term bias drifts.
The present invention discloses a laser gyro system which substantially reduces sources of drift, and thus reduces the overall gyro output drift by several orders of magnitude over other laser gyro systems. One of the primary sources of drift is the frequency pulling resulting from the mutual interaction of two resonant modes due to coupling from any existing scatter sources, which are generally environmentally dependent. Another important source of drift is the scatter-induced drift which is due to the relative motion of scatter centers in the cavity.
Additionally, there is a dispersion-induced drift which is due to variation in time of the gain medium, which causes differ-ential variations in the dispersion, or phase shift, seen by each resonant frequency. A further source of drift will be re-ferred to as the Fresnel-Fizeau effect and is due to the depend-ence of the refractivè index of the gas discharge on the velocity distribution of the discharge sampled by the laser modes, since it may vary as the modes move in response to fluctuations of the ring cavity. These sources of drift are present in all laser gyros, and are generally caused by changes in the optical path length due to environmentally induced changes in the cavity or the optical elements in the cavity. The laser gyro of the present invention substantially reduces these drifts by minimi-zing physical motions of the cavity by using ultra-low-expansion materials, by minimizing changes in effective optical path length by reducing the amount and limiting the type of intra-cavity elements, by reducing the scatter of all the elements, and by using purely circularly polarized cavity modes.
A non-planar path is used instead of the traditional crys-tal rotator to provide the requency split between LCP waves and RCP waves. A thin Faraday paramagnetic glass slab is used instead of the traditional thick Faraday rotator to provide for the non-reciprocal split between the cw and cww waves. Use of a non-planar path not only eliminates a major source of scat-tering (the crystal rotator) which produces coupling between the different waves, but also provides good circular polarization.
Use of a glass Faraday rotator avoids elliptical birefringence and thus maintains the circular polarization. This further eliminates coupling among the different waves since a perfectly LCP wave on reflection will become an RCP wave and thus it will not couple into the counter-rotating wave. Use of the minimum rotator slab's thickness that achieves a predetermined amount of rotation ensures a minimal temperature dependence of any scat-tering centers introduced by the slab. This integrated approach in creating and maintaining circular polarization eliminates the sources of large amounts of drift due to coupling among the waves and results in a more accurate laser gyro.
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The next performance limitation i~ then brought about bysmall amounts of drift due to higher-order effects. The sys-tem of the present invention can now advantageously use Zeeman splitting of the gas laser mixture to equalize the dispersion seen by each wave of the counter travelling pairs. This allows the difference output to be stable as the four-frequency waves experience small frequency variations placing them on different portions of the non-linear dispersion curve.
The present invention discloses a laser gyro system which employs means coupled to a re-entrant resonant path, for pro-viding compensation to electromagnetic waves for gain medium induced changes in phase shift of said waves and means substan-tially free of scattering centers for producing circularly po-larized counter-travelling waves o different frequencies ar-ranged in pairs of first and second polarization sense. More specifically, the circularly polarization means comprise non-depolarizing means for producing a polarization-dependent phase shift in the waves resulting in a frequency splitting between waves of opposite circular polarization, and non-depolarizing means for producing a direction-dependent phase shlEt to the waves resulting in a frequency splitting between counter-travel-ling waves in each of the pairs. The polarization-dependent means may comprise a non-planar resonator and the direction-de-pendent means may comprise means having a refractive index whose direction-independent component is isotropic. Preferably, the direction-dependent means have a scattering characteristic sub-stantially independent of temperature over the operating temper-ature range. In the preferred embodiment, the direction-de-pendent means comprise a slab of isotropic material capable of producing a direction-dependent rotation of the electromagnetic ~,) field of said waves in the presence of a magnetic field and having a thickness resulting in a variation o this thickness of substantially less than one wavelength of of the waves over the operating temperature range. Preferably, the magnetic field for producing direction-dependent rotation is localized to the region immediately adjacent said slab. Additionally, the gain-medium phase shift compensating means comprise means for pro-viding a magnetic field longitudinal to the axis of said gain-medium and having a magnitude and polarity for providing sub-stantially the same amount of gain-medium induced phase-shift to the counter-travelling waves of each pair.
This invention further provides ~or a gyro system compris-ing means for providing a closed non-planar path for the propa-gation of circularly polarized electromagnetic waves and also for providing a splitting in frequency to waves of opposite po-larization sense, a gain medium disposed in the path, non-depo-larizing means or providing a direction-dependent phase shift to the circularly polarized waves resulting in a frequency splitting between counter-travelling ones of the waves of each polarization sense and means for compensating for unequal gain medium dispersion of the counter-travelling waves. The in-vention also provides for scattering particles removing means which comprise a baffle in the region of electrodes. Addition-ally, the direction-dependent non-depolarizing means comprise a slab of isotropic material having a non-zero Verdet constant and means for providing a magnetic field in the slab, preferably, absorbing means are used to collect any reflected waves. The dispersion compensation means comprise means for providing a component of magnetic field in the gain medium along its longi-tudinal axis, and having field components of sufficient magnitude ~' ;^ .:

for producing a frequency splitting of the gain and dispersion characteristics substantially equal to the frequency splitting between the counter-travelling waves due to the non-depolar-izing means. The magnetic field component also has a polarity for providing substantially equal amounts of gain-medium dis-persion to the counter-travelling waves. More specifically, the magnetic field providing means vary the magnetic field as a function of the average amount of frequency splitting of counter-travelling waves due to the direction-dependent non-de-polarizing means. In one preferred embodiment, the magnetic field providing means comprise at least one coil around a por-tion of the gain-medium containing path, means for measuring the amounts of frequency splitting generated in counter-travel-ling waves by the direction-dependent non-depolarizing means, and means for producing a current in the coil proportional to the average value of the frequency split. This invention fur-ther provides for a Faraday rotator comprising a supporting shell, means for providing Faraday rotation disposed within the shell and means for absorbing electromagnetic waves disposed on at least one side of the Faraday rotation means for allowing a substantial portion of the waves to pass through the Faraday rotation means which are further positioned to direct any re-flected portion of the waves toward the absorbing means.

According to a broad aspect of -the pr~s~n-t invention, there is provided in combination: means, comprising a housing having a passageway therein, for providing a closed path for electromagnetic waves passing through such passageway; a sup-por-ting shell disposed within the hosuing and aligned with a portion of the passageway; means, attached to, and supported by the shell and disposed in a portion of the passageway, for providing Faraday rotation to the elec-tromagne-tic waves;
means, attached to, and supported by the shell, for absorbing electromagnetic waves, said absorbing means being disposed on at least one side of said ~araday rotation means, said absorbing means having a central region disposed in the pas-sageway adapted to pass a subs-tantial portion of said waves ~or transmission through said rotation means and having an absorbing region displaced from the passageway; and said ro-ta-tion means being further positioned to direct any portion of said waves reflected by the rotation means toward said absorbing region of the absorbing means.

-7a-srief Description of the Drawings Other and further objects and advantages of the invention will become apparent as the description thereof progresses, reference being had to the accompanying drawings in which like numbers refer to like elements and wherein:
FIG. 1 shows a top isometric view taken from a first corner of a laser gyroscope system of the invention;
FIG. 2 is a lower isometric view taken from a second corner of the device shown in FIG. l;
FIGS. 3 and 4 are isometric views of the gyro block taken from a third corner of the device shown in FIG. 1 showing the internal construction and passages of the device therein;
FIG. 5 is a cross-sectional view showing the internal construction of the system shown in FIG. 1 in the region of one of the terminal chambers and mirror substrate;
FIG. 6 is a cross-sectional side view showing the de-tails of construction of the Faraday rotator device of the laser gyro system shown in FIG. l;
FIG. 6A is a top view of a portion of the laser gyro of FIG. 1 in the region of the Faraday rotator o-f FIG. 6, and and showing a top view of the Faraday rotator;
FIG. 7 is a graph showing the power reduction Eactor as a function of the angle of incidence of beams upon an output mirror structure;
FIG. 8A is a graph showing the gain versus frequency of the gaseous laser medium employed with the laser gyro system of FIG. 1 indicating the relative positions of the frequencies of the four beams within the system;
FIG. 8B is a graph showing the phase shift (dispersion) as a function of frequency corresponding to the gain medium of ~' ~ .

FIG. 8A;
FIG. 8C is a graph showing the phase shift (dispersion) versus frequency of a laser medium in the presence of a magnetic field indicating the relative positions of the frequencies of the four beams within the system;
FIG. 9 is an energy level diagram showing the splitting of the energy levels in the presence of a magnetic field.

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Description of the Preferred Embodiments Referring simultaneously to the views of FIGS. 1-5, the construction and operation of a laser gyroscope system in accordance with the teachings of the present invention will be described. Gyro block 102 forms the frame upon which the sys-tem is constructed. Gyro block 102 is preferably constructed wi~h a material having a low thermal coefficient of expansion ~` such as a glass-ceramic material ~o minimize the eEfects of temperature change upon the laser gyroscope system. A pre ferred commercially available material is sold under the name of Cer-Vit~ material C-101 by Owens-Illinois Company; alterna-tively Zerodurn' by Schott may be used.
Gyro block 102 has nine substantially planar faces as shown in the various views of FIGS. 1-4. As shown most clearly in the views of FIGS. 3 and 4 which show gyro block 102 without the other components of the system, passages 108, 110, 112 and 114 are provided between four o the faces of gyro block 102.
The passages define a non~planar closed propagation path with-in laser gyro block 102.
Mirrors are provided upon faces 122, 124, 126 and 128 at the intersections of the passages with the faces. Substrates 140 and 142 having suitable reflecting surfaces comprise the mirrors positioned upon faces 124 and 126 respectively. A
mirrored surface is also provided directly adjacent face 128 in the front of path length control transducer 160. One of these mirrors should be slishtly concave to insure that the beams are stable and confined essentially to the center of the passages.
Also, a transparent mirror substrate 138 having partially trans-mitting dielectric mirror layers 139 is provided upon face 122 to allow a portion of each beam travelling along a closed path / ~

o within the gyro block 102 to be coupled into output optics 144.
The structure of output optics 144 is disclosed in United States Patent No. 4,141,651 issued Feb. 27, 1979 to I. Smith et al, and assigned to the present assignee.
Because passages 108, 110, 112 and 114 define a non-planar propagation path for the various beams within the system, each beam undergoes a polarization rotation as it passes around the closed path. Ideally, only beams of substantially circular po-larization exist in the non-planar cavity of the invention.
With circularly polarized beams, drift due to beam scattering or coupling from one beam to the other is minimized. This re-duction occurs because light of one circular polarization state when scattered is not of the proper polarization to be coupled into and affect the other beams. For other types of light po-larization, this is not the case because there will always be some component of the scattered beam which will couple to other beams.
In the preferred embodiment, the passages and reElecting mirrors are so arranged as to provide a substantially ninety-degree polarization rotation for the various beams. Because beams of right and left-hand circular polarization are rotated in opposite senses by this same amount, independent of their direction of propagation, a frequency splitting between beams of right and left-hand circular polarization must occur in order for the beams to resonate within the optical cavity. This is shown in FIG. 8A as the frequency split between the beams of left-hand and right-hand circular polarization. In the pre-ferred embodiment, a ninety-degree rotation corresponding to a 180 degree relative phase shift is employed although other phase shifts as well may be used depending upon the frequency //

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separation desired. Polarization rotation will occur as long as the closed propagation path is non-planar. The p~ecise ar-rangement of the paths will determine the amount of rotation.
In the known systems of the prior art such as that de-scribed in the above-referenced patent to K. Andringa, the frequency splitting between beams of right and left-hand cir-cular polarization was accomplished with the use of a block of solid material of significant optical thickness disposed in the propagation path. The presence of any solid material directly in the path of beam propagation provides scattering centers from which light may undesirably be coupled from one beam to another causing an error in the gyro output. The amount of coupling, and thus the error, is thermally very sensitive. ~ence, the output frequency of such devices was subject to a temperature dependent drift which could not be compensated for with a fixed output bias. Additionally, a crystal rotator introduces an amount of stress-birefringence which tends to depolarize the circularly polarized waves, further contributing to unwanted coupling of the waves. This results in a gyro system having a variation in time of the output frequency of the order of, at best, tens of Hz and reaching hundreds of ~Iz in many cases.
With the present invention, the solid material which had been used for the crystal rotator has been completely eliminated from the beam propagation path thereby eliminating the sources of error and drift associated with the material.
To aid in understanding how the phase shift occurs, it is useful to imagine a linearly polarized beam propagating around the path. In this description, the 180 phase~shift experienced by an electromagnetic wave upon reflectiGn is ignore~. Since an even number (four) of such reflections is employed, no error /,, .....
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6~Cl is thereby incurred. Suppose, for example, that the beam travelling in passage 110 between face 122 and face 124 is line-arly polarized with the electric vector pointing in the upper direction. As the beam is reflected from the mirror provided upon face 124, the electric vector is still nearly pointed up-ward but with a slight forward tilt because passage 112 drops between face 124 and face 128. As the beam is reflected from the mirror upon face 128, it will be pointing nearly to the left with a slight downward tilt as would be seen in FIGS. 3 and 4.
As the beam is reflected from face 126, the electric vector of the beam within passage 108 would point to the left with a slight upward slope again in the views of FIGS. 3 and 4. After reflection from face 122, the electric vector of the beam within passage 110 still points leftward and into the plane of the drawing. Thus, it may be seen that the beam as it arrives back in passage 110 has experienced a polarization rotation of approxi-mately ninety degrees Of course, such a rotated linearly po-larized beam cannot reinforce itself and resonate along the closed path. Only circularly polarized beams having a Erequency shifted from the frequency at which such beams would resonate for a planar closed path of the same length will be resonant.
A two-frequency laser gyroscope may be constructed using a non-planar propagation path to provide the only frequency split-ting. No Faraday rotator or other such element is required in such an embodiment. To detect the rate of rotation, an output signal is produced by beating the extracted portions of the two beams together to form an output signal having a frequency equal to the difference in frequency between the two beams. At rest, the output signal will remain at some value fO. For rotation in one direction, the output signal will increase to a value fO-~f, /J'~

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where ~f is proportional to the rate of rotation, and will decrease to a value of fO~f for rotation in the other direction.
This invention significantly reduces cross-coupling due to back-scattering so that the lock-in range diminishes permitting such a laser gyroscope to be used in many applications without complete elimination of lock-in.
Faraday rotator 156 is positioned within a larger diameter portion 113 of passage 112 adjacent face 124 as may be seen in the views of FIGSo 2, 4, and 6A. The details of the construction of Faraday rotator 156 are seen in the views of FIGS. 6 and 6A.
The Faraday rotator mount 154, preferably formed of the same ma-terial as laser gyro block 102, forms the base upon which the structure is constructed. Rotator mount 154 is cylindrical in shape and has several cylindrical apertures of varying diameter at an angle to the longitudinal axis of mount 154 for providing support at predetermined locations to all the elements of Faraday rotator 156 and for providing a clear path along the longitudinal center axis of mount 154. Faraday rotator slab 165 is positioned on shelf 166 ormed by the central portion of mount 15~. Riny 169 prevents lateral movement of slab 165. Faraday rotator slab 165 may be preferably formed of a rare earth-doped glass or a ma-terial of similarly high Verdet constant. A Verdet constant of magnitude in excess of 0.25 min./cm./Oe. at the operating wave-length is preferred to reduce the thickness of the slab required to produce the desired amount of frequency splitting. Tradi-tional Faraday rotators have employed a thick slab of material, often fused quartz. Any solid material in the path of the counter-rotating beams will introduce scatter points which ex-hibit a sensitivity to thermal fluxes. This sensitivity may be due to the thermal expansion of the material or to a change in /y . .

Bl:3 the optical path lengtn due to the temperature dependence of the refractive index of the material. The effective temper-ature dependence of the optical path length, and therefore the thermally induced drift, has been found to be a strong positive function of the thickness of the solid material in the path of the beams. Thus, it is desirable to use as thin a slab as possible and a thickness of 0.5mm or less is preferred to re-duce drift to an acceptable level resulting in a variation of the thickness due to temperature or other causes substantially less than one wavelength of the laser waves over the operating region. A commercially available material is Hoya Optics, Inc.
material No. FR-5 which is a glass doped with paramagnetic ma-terial to provide for the Faraday rotation and results in a ro-tator having an isotropic refractive index. This was found to be important since a problem of a traditional Faraday rotator is that a crystal material such as quartz has an anisotropic re-fractive index which introduces elliptical birefringence. This depolarizes the nominally circularly polarized waves and leads to increased coupling between the counter-rotating waves. Thus, it is important to use an isotropic material for the Faraday ro-tator to eliminate depolarization of the resonant modes. Oper-ating as close to circular polarization as possible reduces cross-coupling and therefore reduces thermally induced drifts due to any remaining scatter centers. This allows a gyro system to achieve stability levels corresponding to a variation in time of the output frequency of a few Hz or betterO
Faraday rotator slab 165 is held against shelf 166 by magnet assembly 188. Two hollow cylindrical permanent magnets 186 and 187 are positioned end-to-end with like poles adjacent one another at the juncture between the two magnets. The two , ~
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magnets can be fastened together by any known means, such as solder bonding or welding. The Faraday rotator slab 165 is then adjacent one end of the two magnet pair. A longitudinal magnetic field is produced in the slab, but this field attenuates rapi-dly upon moving a short distance away from the slab or magnets.
This embodiment has the advantage that essentially no stray magnetic field is produced which could extend into the gaseous discharge region and, by the Zeeman effect, produce unwanted modes or frequency offset. Alternatively, a single magnet may be used to provide the required magnetic field to the slab.
The permanent magnet structure might also be replaced by a few coils of wire to allow an electrical current to establish a magnetic field in Faraday rotator slab 165. Pushing on magnet structure 188 is spring 175. The other side of spring 175 rests along the periphery of hollow cylindrical spacer 197, which in turn rests partially on one side of hollow cylindrical absorber 191. Absorber 191 is made of a material such as black glass and being antireflection-coated is used to absorb any electromagnetic wave, such as the spec~llar reflections from the Faraday rotator slab, incident on its surfaces. Absorber 191 is held in place by circular clip 193, which rests by friction along the periphery of aperture 181. Thus, it can be seen that the elements just described form an assembly positioned by cir-cular clip 193 against the right side of shelf 166, with spring 175 providing a sufficient longitudinal force to keep all the elements tightly in place. On the opposite side of shelf 166, there is a similar arrangement of elements with the exception of Faraday slab 165 and magnet assembly 188. Circular clip 192 forms the stationary base against which a second absorber 190 rests. Spring 174, with one end resting on the left side of /~^

shelf 166, pushes spacer 196 against absorber 190 and thus keeps all the elements on the left side of shelf 166 in their prede-termined position.
Rotator mount 154 is held in place against the shelf formed by the change of diameter of passages 112 and 113, by helical spring 199. A portion of the first smaller diameter turn of spring 199 rests on the body of rotator mount 154, while the other larger diameter end expands circumferentially and frictionally engages the wall of bore 113. The arrangement of the elements of rotator 156 provides for thermal stability, since the optical ele-ments are elastically held against a stable material used for the gyro block.
As described above, the axes of apertures provided in rotator mount 154 are at an angle with respect to the longi-tudinal axis of mount 154, and thus the plane of Faraday rota-tor slab 165 also describes an angle with respect to the longi-tudinal axis of mount 1540 This contributes to the elimination of coupling between counter-travelling waves since any reflec-tions off of the two surfaces o Faraday slab 165 are now inter-cepted and absorbed by the two black glass absorbers. Waves cir-culating from left to right in the rotator of FIG. 6 will have a reflection from slab 165 intercepted and absorbed by the lower por~ion of absorber 190, while waves circulating in the oppo-site direction will have reflections from slab 165 absorbed by the top portion of absorber 191. The two absorbers, 190 and 191, and rotator slab 165 are also coated with an anti-reflection coating to further reduce the amount of reflections.
Besides providing the frequency splitting between the clockwise and counter-clockwise circulating beams, Faraday ro-tator 156 performs a second function. Because of the close fit ~8:96~

provided in the region of shelf 165, Faraday rotator 156 blocks the longitudinal flow of gas through passage 112. Because there can be no net circulation of gas through the closed path, the possibility of circulation of scatter particles carried by the gas is substantially reduced, as are drifts due to the Fresnel-Fizeau effect.
Referring again to the views of FIGS. ], 3 and 4, it may be seen that a low angle of incidence is provided for the beams striking the partially transmitting mirror disposed upon face 122. The beams traveling within each passage 10~, 110, 112 and 114 are circularly polarized. The nearer to normally that one of these beams strikes a reflecting mirror or a surface the nearer to circular will be the polarization of the beam trans-mitted through the mirror surface. As the angle of incidence moves away from the normal, the partially transmitted beams begin to assume an elliptical polarizationO
As explained in the above-referenced Patent No. 4,141,651, if the beams within the output optics and detector structure are entirely circularly polarized, there will be essentially no unwanted cross-coupling and interference between the beams oE
the upper two frequencies and the beams of the lower two fre-quencies within the detector structure. As the amount of el-lipticity increases, cross-coupling begins to become evident and appears as an amplitude modulation upon the output signals from detector diodes 145 and 146. As discussed above, it has been discovered that the amount of the unwanted cross-coupling is a nonlinear monotonically increasing function of the degree of ellipticity. It has been found that the cross-coupling is relatively low for angles of incidence below approximately fif-teen degrees. However, the amount of cross-coupling increases /~

quite rapidly above this angle of incidence. The cross-coupliny within the output optics structure may b eliminated by means of a suitable polarization filter, but the available filtered power decreases as the unfiltered cross-coupling increases. As the angle of incidence of each beam upon the output mirror in-creases, the power available at the detector diodes for each beam decreases. A calculated graph of power reduction factor, the ratio of power available at the detectors at a given angle of incidence to that available for the same beam normal to the mirror surface, is shown in FIG. 7 for the output structure de-scribed in the above-referenced Patent No. 4,141,651. As may readily be seen, the power reduction factor falls rapidly for angles of incidences greater than approximately fifteen degrees.
Hence, in accordance with one aspect of the invention, the angle of incidence of the beams in passages 108 and 110 to the parti-ally transmitting mirror disposed upon face 122 is made to be fifteen degrees or less. Alternately stated, the angle between passages 108 and 110 is thirty degrees or less.
In systems operation, it is desirable that the waves of the four frequencies be centered symmetrically about the peak of the gain curve. To this end, a piezoelectric transducer 160 is provided to mechanically position the mirror on face 128 to adjust the total path length within laser gyro cavity 102 to properly center the four frequencies. Path length control 320 derives a signal for operating piezoelectric transducer 160 from detector diodes 145 and 146 These signals have an ampli-tude in proportion to the total amplitudes of the corresponding ~fl and~ f2 signals. Control 320 generates the difference be-tween these two amplitude related signals. The output differ-ence signal of course has a zero amplitude when the waves of /~' the four frequencies are properly centered upon the gain curve.
The output difference signal is of one polarity when the four waves are off center in one direction and the opposite plurality when the waves are off center in the other direction. The average amplitude signals can be formed by known circuitry, the output of which is coupled to the input leads of piezoe-lectric transducer 160.
Still referring to the views of FIGS. 1, 3 and 4, elec-trodes for exciting the gaseous gain medium are disposed within passage 108. Preferably, center cathode electrode 22 is con-nected to the negative terminal of an external regulated power supply 310 while anode electrodes 32 and ~2 are connected to the positive terminals. The cathode electrode is in the form of a short hollow cylinder capped by a hollow metal hemisphere at the end most distant from laser gyro block 102. It is at-tached by conventional means to the surface of gyro block 102 adjacent aperture 20. Positive electrodes 32 and 42 are in the form of metal rods extending into electrode apertures 30 and 40. With this configuration, the electron current Elows outward toward electrodes 32 and 42 in two opposed directions.
In this manner, because a beam traversing the passages in which the electrodes are located passes through equal lengths of cur-rent flow of opposite direction, the effects of drag on the beam caused by unequal current flow through the gaseous gain medium are substantially eliminated. However, because of manufactur-ing tolerances in the positions of the various electrodes, the distances between the negative and two positive electrodes in the two passages may not be precisely equal~ To compensate for the inequality, electrodes 32 and 42 are connected to two inde-pendent positive terminals of supply 310, so that current flow r ~' between the positive electrodes and thereto adjacent negative electrode may be made unequal and thus compensate for the different drag effects.
The gaseous gain medium which fills passages 108, 110, 112 and 114 is supplied through gas fill aperture 106 from an ex-ternal gas source. A mixture 3He,20Ne and 22Ne in the ratio o~
8:0.53:0.47 is preferred. Once all the passages have been filled, a seal 107 is applied to aperture 106 to contain the gas for sealed-off operation.
The details of construction of the laser gyroscope system in the region of one of the positive electrodes are shown in detail in the cross-sectional views of FIG. 5. Metal electrode 32, held in place by electrode seal 33, is positioned within electrode aperture 30. Electrode 32 extends somewhat more than half way from the surface of gyro block 102 to passage 108.
Electrode aperture 30 intersects passage 108 preferably at a right angle. Terminal chamber 118 is formed into the surface of gyro block 102 upon which is positioned output optics struc-ture 144. Terminal chamber 118 is cylindrical in shape having a diameter at least twice that of passage 108. Terminal chamber 118 and passage 108 are coaxial with one another. Because pas-sage 108 extends slightly beyond electrode aperture 30 before intersecting with terminal chamber 118, a baffle 130 is formed between electrode passage 30 and terminal chamber 118. A simi-lar arrangement is provided for electrode 42 by aperture 40, seal 43 and terminal chamber 119.
In prior art system, no baffle was provided. The terminal chamber extended directly from the electrode apertures out to the surface of the laser gyro block. When the electrodes were excited, dust or other unwanted particles which may be produced .~/

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such as by ion bombardment and sputtering of the laser gyro block would collect around the intersection of the electrode aperture and beam passageways. The suspended particles acted as scattering centers increasing the optical loss of the structure. In contrast, with the present invention it has been found that dust or other unwanted particles will not be sus-pended in the region of the intersection of electrode apertures, such as 30, and passage 108. Thus a potential source of drift is eliminated.
As discussed above, by maintaining good circular polari-zation, the gyro eliminates all known sources contributing large amounts of drift. There is, however, an additional source that contributes a smaller amount of drift which must be compensated if the laser gyro is to be used in a high performance system. This remaining drift is due to dispersion, that is, a frequency dependent index of refraction associated with the gain of the medium used. For a He-Ne gain medium, the gain line is approximately Gaussian in shape due to Doppler broadening; the dispersion curve can be described as sigmoid.
The dispersion curve expresses the amount of optical phase shiEt - that a wave of a particular frequency will experience due to the presence of a gain medium. As can be seen in FIGo 8B, frequen-cies below center frequency fc experience a phase shift opposite to that of frequencies above center frequency fc resulting in all modes being shifted toward line center. This is the mode pulling effect. Since the dispersion curve is nonlinear, the four modes of a differential gyro will be operating on points having different amounts of dispersion and correspondingly, re-ferring to FIG. 8B, having different amounts of phase shift.

~1 is the phase shift corresponding to fl, ~2 corresponds to f2, v~
.~

6~

corresponds to f3 and ~4 corresponds to f4. If the difference (~2-~1) is different in quantity than the difference (~4-~3), there will be a non-zero differential output at rest which depends on the shape of the dispersion cur~e, itself a function of many elements such as temperature, gain and pressure.
As any one of these elements changes, this change will be re-flected as a shift of the four mo~es across the dispersion curve which, due to its nonlinear nature, will result in a changing differential output. Thus, the gyro will have a drift in its output frequency which varies according to a variety of factors.
The gyro system of the present invention uses the Zeeman effect to eliminate the drift due to the gain medium dispersion.
The Zeeman effect refers to the splitting of the spectral lines of the lasing gas into two or more components. This frequency splitting results in a splitting of the gain curve and its corresponding dispersion curve. The physical mechanism is the quantum mechanical phenomenon in which a magnetic field splits the atomic energy levels into several states which have differ-ent energies and which interact with waves of predetermined circular polarization states. This is illustrated in FIG. 9 where on the left side of the energy diagram there is shown a typical energy-state level in the presence of no magnetic field. In this case, the radiating frequency is fo=(E2-El)/h, where E2 and El are the two energy levels, and h is Planck's constant. The right side of the diagram shows how the energy levels are split in the presence of a magnetic field. Lines 242 show the energy level transitions corresponding to ~ m-+l that will give rise to one set of the radiating frequencies, such as the center frequency for split dispersion line 260, f+=fo-gBH/h. Lines 244 show the energy level transitions .~ J~

corresponding to ~ m=-l that give rise to the other set of radiating frequencies, such as the center frequency for split dispersion line 250, f_=fo~gBH/h, where g=Land~ G-ratio or gyro-magnetic ratio, B=Bohr magnetron and h=Planck's constant. The four circulating modes have different values of the change m of the magnetic quantum number m of the neon atom, as follows:
Mode No. DirectionPolarization Delta m 1 clockwise LCP +l

2 counter-clockwise LCP -1

3 counter-clockwise RCP +l

4 clockwise RCP -1 The Zeeman effect is both polarization and direction dependent.
The reason for this is that the sense of rotation of the elec-tric field vector of the light wave as measured about the mag-netic field interacts with the spin of the electrons whose energy levels are split by the field. Thus, one of the re-sulting dispersion lines interacts with a RCP wave that travels in a parallel direction to the direction of the maqnetic field and a LCP wave which travels in an anti-parallel direction, that is, opposite the direction of the magnetic field, while the other dispersion line interacts with a RCP wave travelling in a sense anti-parallel to the magnetic field vector and an LCP wave travelling in a direction in the same direction as the magnetic field.
Since the values of ~m correspond to different atomic tran-sitions, these transitions are split by an amount equal to 2gBH/h by the Zeeman effect. Referring now to FIG. 8C, there is shown a diagram of the split dispersion curves and the corresponding phase shifts of the four modes of the gyro. If the magnetic field H is such that the ~m=+l line is lower in frequency than thea m=-l line by an amount 280 equal to (f2-fl) then we will have line 270 and line 272 equal in height, that is, the amount of phase shift provided to fl and f2 will be equal. Similarly, line 274 and line 276 will be at the same height, ~ith the re-sult that the frequencies f3 and f4 will have a similar amount of phase shift. It can then be seen that as the four fre~uen-cies drift across the dispersion curve or the dispersion curve changes due to, e.g., temperature, the dispersion of mode 1 will always be equal to that of mode 2, and that of mode 3 will simi-larly be equal to that of mode 4. Thus, as external conditions create small changes in the operating frequencies, the net dif-ference in a differential output will remain the same. To remove the dispersion drift, the magnetic field for the Zeeman effect must satisfy the following expression: Faraday bias =
2gBH/h=(3.6~ MHz/Gauss)H. This results in a gyro system capable of achieving a stability of the output frequency of the order of much less than one Hz.
Referring now to FIGS. 1-4, it can be seen that in the preferred embodiment the magnetic field necessary for Zeeman splitting of the dispersion curves is obtained by use of coils disposed around the passage that carry the lasing medium.
Bores are drilled into gyro block 102 to provide passages 200, 210, 220 and 230 for the coils Coils 202 and 212 are provided on one side of cathode 22 while coils 222 and 232 are provided on the other side of cathode 22 in order to provide Zeeman splitting throughout the lasing portion of the gyro path. Four sets of coils are used to provide a more uniform magnetic field to the lasing gas, however, any other arrangement that orovides a component of the magnetic field to the lasing ~as can be used.
Coils 202, 212, 222 and 232 are disposed around passage 108.

Preferably, all four coils are controlled by a single source to provide a current of such magnitude and polarity to generate a magnetic field in the passages for creating the splitting of the dispersion curves equal in magnitude to the splitting in frequency of the Faraday bias provided by Faraday rotator 156 and in the direction that removes the sensitivity of the waves to the gain medium.
It is preferable to control the amount of magnetic field that is generated for the Zeeman splitting in relation to the amount of Faraday bias provided by the Faraday rotatorO
Referring now to FIG. 1, there is shown output optics structure 144 supporting diodes 145 and 146. Output optics structure 144 separates the LCP counter-rotating frequency pair from the RCP counter-rotating frequency pair with each pair being detected by a separate diode. For instance, diode 145 is used to provide a signal corresponding to fa, the frequency difference (f2-fl) of the first frequency pair, while diode 146 is used to provide a signal correponding to fb, the frequency difference (f4-f3) of the second frequency pair. The outputs of diodes 145 and 146 are connected to dispersion control 300.
At rest, fa = fb and each difference corresponds to the Faraday bias. In the presence of rotation, one of the two difference frequencies increases and the other decreases, the amount and sense of change being dependent on the direction and rate of rotation. Dispersion control 300 has conventional electronic circuitry to enable the forming of a signal representing the average of the two frequency differences and thus it measures the Faraday bias even under rotation. Further circuitry in dis-persion control 300 pro~ides a current to coils 202, 212, 222 and 232 as a function of this Faraday bias signal to create a magnetic field in passage 108 for splitting the dispersion curve by an amount equal to the frequency split obtained by the Faraday bias. The magnetic field needed for dispersion equalization is given by H=Faraday bias/2gBh = (Faraday bias in Hz)/(3.6~x106) Oe, and the current used to produce it depends proportionally on the number of turns of the coils, as is well-known in the art.
It is found that the Faraday rotator of the present embod-iment produces a Faraday bias having a characteristic that is inversely proportional to temperature. Through control 300, the magnetic field for the Zeeman splitting is generated as a function of the measured Faraday bias and thus the dispersion equalization is made independent of the temperature dependence of the Faraday bias. Control 300 generates a current whose amplitude is controlled as a function of a signal corresponding to the measured Faraday bias, through some proportionality constants accounting for both the relationship of the magnetic field, whose polarity depends on the sense of the coil windings, to the Faraday bias and the number of turns in the coil windings.
A more detailed description of bias control electronics 300 is not needed, since the design of such control circuits is well-known in the art.
Other modifications to the described embodiments will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. Accordingly, it is intended that this invention be not limited except as defined by the appended claims.

/J~7 ..'.~

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In combination: means, comprising a housing having a passageway therein, for providing a closed path for electrormag-netic waves passing through such passageway; a supporting shell disposed within the housing and aligned with a portion of the passageway; means, attached to, and supported by the shell and disposed in a portion of the passageway, for provid-ing Faraday rotation to the electromagnetic waves; means, attached to, and supported by the shell, for absorbing electro-magnetic waves, said absorbing means being disposed on at least one side of said Faraday rotation means, said absorbing means having a central region disposed in the passageway adap-ted to pass a substantial portion of said waves for transmis-sion through said rotation means and having an absorbing region displaced from the passageway; and said rotation means being further positioned to direct any portion of said waves reflec-ted by the rotation means toward said absorbing region of the absorbing means.

2. The combination of Claim 1 wherein: said supporting shell comprises a low thermal expansion material having a plurality of stops; and said rotations means and said absorbing means are held in place against said stops by elastic means.