GB2087638A

GB2087638A - Ring laser gyroscope

Info

Publication number: GB2087638A
Application number: GB8131273A
Authority: GB
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1980-10-17
Filing date: 1981-10-16
Publication date: 1982-05-26
Also published as: GB2138585B; JPH02870B2; GB2138585A; DE3141175A1; FR2492522B1; GB2087638B; IT8149504A0; CA1189600A; DE3141175C2; IT1142901B; GB8333405D0; FR2492522A1; NL8104726A; JPS5796581A

Abstract

A laser gyroscope has a non- planar propagation path arranged to provide differential phase shift between pairs of waves of opposite circular polarization and between waves of opposite rotation directions (e.g. using a Faraday rotator). In order to eliminate small amounts of drift due to the different phase shifts or dispersion produced in the propagating waves by the gain medium, means are provided to compensate such phase shifts or dispersion, e.g. by providing (coils 202, 212, 222, 232) a magnetic field axially of the path to effect Zeeman splitting of the laser gain medium and thus provide an equal amount of dispersion to the counter-travelling waves. The amount of magnetic field generated for the Zeeman splitting is preferably controlled in relation to the amount of bias provided by the Faraday rotator. The Faraday rotator consists of a number of cylindrical apertures lying at an angle to the optic axis. A relatively thin (to reduce scattering) rare earth doped glass or similar material of high Verdet constant is used on the rotator. <IMAGE>

Description

SPECIFICATION Ring laser gyroscope 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 comprising the cavity. If the frequencies of the modes are not too different, there is a tendency of the modes to phase lock. For a gyro to be of practical use, it has to overcome this lock-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 instabilities 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 partially 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 differential laser gyro system solves this problem 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 different frequencies are normally generated by using two different optical effects.First, a crystal polarization rotator is used to provide a direction-independent polarization causing the resonant 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 polarization 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 of a piece of optically isotropic material subjected to a longitudinal magnetic field, or it may be realized by applying such a magnetic 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 polarization biased in the opposite direction, the bias being cancelled by subtracting the two outputs.

The operation of a basic four-frequency laser gyroscope is described in our United States patent No.

3,741,657. The reader is also referred to our U.K. patent application no. 7849292 (serial no. 2012101).

U.K. patent no. 1535444, and U.S. patent application no. 141 873 dated 21st April 1 980 (which is a continuation of U.S. application 936164 (23rd August 1978), itself divided from 646308 of 2nd January 1978).

The present invention discloses a laser gyro system which recognises 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 be external sources, such as thermal expansion. In practice, it has been found that the four frequencies are not equally affected by external sources, resulting in different variations of the frequencies. Thus, even though four-frequency gyros have exhibited excellent performance levels under conditions of thermal equilibrium, the practical application of these instruments has, so far, been limited by the presence of thermal sensitivities manifested as unacceptable long-term bias drifts.

One of the primary sources of drift in laser gyroscopes 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 differential variations in the dispersion, or phase shift, seen by each resonant frequency. A further source of drift will be referred to as the Fresnel-Fizeau effect and is due to the dependence of the refractive 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.

According to the present invention there if provided a four-frequency laser gyroscope comprising closed propagation path and containing a gain medium and arranged to produce circularly polarized counter-travelling electromagnetic waves of different frequencies arranged in pairs of first and second polarization sense, and means for providing compensation for gain medium induced changes in phase shift of said waves.

It may comprise non-polarizing means for producing a polarization-dependent phase shift in the waves resulting in a frequency splitting between waves of opposite circular polarization, and nondepolarizing means for producing a direction-dependent phase shift to the waves resulting in a frequency splitting between counter-travelling waves in each of the pairs. The polarization-dependent means may comprise a non-planar resonator and the direction-dependent means may comprise means having a refractive index whose direction-independent components is isotropic. Preferably, the direction-dependent means have a scattering characteristic substantially independent of temperature over the operating temperature range.In the preferred embodiment, the direction-dependent 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 of this thickness of substantially less than one wavelength 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 providing a magnetic field longitudinal to the axis of said gain-medium and having a magnitude and polarity for providing substantially the same amount of gain-medium induced phase-shift to the counter-travelling waves of each pair.

In another aspect, the invention provides a laser gyro system which employs means coupled to a re-entrant resonant path, for providing compensation to electromagnetic waves for gain medium induced changes in phase shift of said waves and means substantially free of scattering centers for producing circularly polarized counter-travelling waves of different frequencies arranged in pairs of first and second polarization sense.

In a further aspect, a gyroscope comprises means for providing a closed non-planar path for the propagation of circularly polarized electromagnetic waves and also for providing a splitting in frequency to waves of opposite polarization sense, a gain medium disposed in the path, non-depolarizing means for 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. Means for removing scattering particles, which comprises a ba'ffle in the region of electrodes, may also be provided.

Additionally, the direction-dependent non-depolarizing means may 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 longitudinal 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-depolarizing means, the magnetic field component also having a polarity for providing substantially equal amounts of gain-medium dispersion to the counter-travelling waves.More specifically, the magnetic field providing means can vary the magnetic field as a function of the average amount of frequency splitting of counter-travelling waves due to the direction-dependent non depolarizing means. In one preferred embodiment, the magnetic field providing means comprise at least one coil around a portion of the gain-medium containing path, means for measuring the amounts of frequency splitting generated in counter-travelling 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.

In a yet further aspect, the invention provides a Faraday rotator comprising a supporting shell, means for providing a 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 reflected portion of the waves toward the absorbing means.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 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. 1; 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 details of construction of the Faraday rotator device of the laser gyro system shown in Fig. 1; Fig. 6A is a top view of a portion of the laser gyro of Fig. 1 in the region of the Faraday rotator of Fig. 6, and showing a top view of the Faraday rotator; Fig. 7 is a graph showing the power reduction factor 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. by 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.

Referring to Figs. 1-5, the laser gyroscope embodying the present invention comprises a gyro block 102 which forms the frame upon which the gyroscope is constructed. Gyro block 102 is preferably constructed with a material having a low thermal coefficient of expansion such as a glassceramic material to minimize the effects of temperature change upon the laser gyroscope. A preferred commercially available material is sold under the name of Cer-Vit (Trade Mark) material C-101 by Owens-Illinois Company; alternatively Zerodur (Trade Mark) 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 of the faces of gyro block 1 02. The passages define a non-planar closed propagation path enclosed within the 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 1 42 having suitable reflecting surfaces comprise the mirrors positioned upon faces 124 and 126 respectively. The mirror on face 128 is a mirrored surface provided directly adjacent face 128 in front of a path length control transducer 1 60. One of these mirrors should be slightly concave to ensure that the beams are stable and confined essentially to the centre of the passages. Also, a transparent mirror substrate 1 38 having partially transmitting dielectric mirror layers 139 is provided upon face 122 to allow a portion of each beam travelling along a closed path within the gyro block 102 to be coupled into output optics 144.The structure of the output optics 144 is disclosed in United States patent no.4,141,651.

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 polarization exist in the non-planar cavity. With circularly polarized beams, drift due to beam scattering or coupling from one beam to the other is minimized. This reduction 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 polarization, 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 reflecting mirrors are so arranged as to provide a substantially ninety-degree polarization rotation for the various beams. Because beams of right and lefthand 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 beam of left-hand and right-hand circular polarization. In the preferred embodiment, a ninety-degree rotation corresponding to a 1 80 degree relative phase shift is employed although other phase shifts as well may be used depending upon the frequency separation desired.

Polarization rotation will occur as long as the closed propagation path is non-planar. The precise arrangement of the paths will determine the amount of rotation.

In the known systems of the prior art such as that described in the above-referenced U.S. patent no. 3741657, the frequency splitting between beams of right and left-hand circular 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. Hence, 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 stressbirefringence which tends to depoiarize 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 Hz in many cases. Here, 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 1 800 phase-shift experienced by an electromagnetic wave upon reflection is ignored. Since an even number (four) of such reflections is employed, no error is thereby incurred. Suppose, for example, that the beam travelling in passage 110 between face 122 and face 124 is linearly 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 upward but with a slight forward tilt because passage 11 2 drops between face 1 24 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 approximately ninety degrees. Of course, such a rotated linearly polarized beam cannot reinforce itself and resonate along the closed path.Only circularly polarized beams having a frequency 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 splitting, no Faraday rotator or other such element being used.

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 f0 + Af, where Af is proportional to the rate of rotation, and will decrease to a value off0 - Af 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 lockin.

Faraday rotator 1 56 is positioned within a larger diameter portion 113 of passage 11 2 adjacent face 124 as may be seen in the views of FIGS. 2, 4 and 6A. The details of the construction of Faraday rotator 1 56 are seen in the views of FIGS. 6 and 6A. The Faraday rotator means 1 54, preferably formed of the same material as laser gyro block 102, forms the base upon which the structure is constructed.

Rotator mount 1 54 is cylindrical in shape and has several cylindrical apertures of varying diameter at an angle to the longitudinal axis of mount 1 54 for providing support at predetermined locations to all the elements of Faraday rotator 1 56 and for providing a clear path along the longitudinal center axis of mount 1 54. Faraday rotator slab 1 65 is positioned on shelf 1 66 formed by the central portion of mount 1 54. Ring 1 69 prevents lateral movement of slab 1 65. Faraday rotator slab 1 65 may be preferably formed of a rare earth-doped glass or a material of similarly high Verdet constant.A Verdet constant of magnitude in excess of 0.25 min/cm/Oe at the operating wavelength is preferred to reduce the thickness of the slab required to produce the desired amount of frequency splitting. Traditional 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 exhibit a sensitivity to thermal fluxes. This sensitivity may be due to the thermal expansion of the material or to a change in the optical path length due to the temperature dependence of the refractive index of the material. The effective temperature 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 0.5 mm or less is preferred to reduce 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 material to provide for the Faraday rotation and results in the rotator 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 refractive index which introduces elliptical birefringence. This depolarizes the normally 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 rotator to eliminate depolarization of the resonant modes. Operating 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 better.

Faraday rotator slab 165 is held against shelf 1 66 by magnet assembly 188. Two hollow cylindrical permanent magnets 1 86 and 1 87 are positioned end-to-end with like poles adjacent one another at the juncture between the two magnets. The two magnets can be fastened together by any known means, such as solder bonding or welding. The Faraday rotator slab 1 65 is then adjacent one end of the two magnet pair. A longitudinal magnetic field is produced in the slab, but this field attenuates rapidly 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 1 91. Absorber 1 91 is made of a material such as black glass and being antireflection-coated is used to absorb any electromagnetic wave, such as the specular reflections from the Faraday rotator slab, incident on its surfaces. Absorber 1 91 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 circular clip 1 93 against the right side of shelf 166, with spring 1 75 providing a sufficient longitudinal force to keep all the elements tightly in place. On the opposite side of the shelf 166, there is a similar arrangement of elements with the exception of Faraday slab 1 65 and magnet assembly 1 88. Circular clip 1 92 forms the stationary base against which a second absorber 1 90 rests. Spring 174, with one end resting on the left side of shelf 166, pushes spacer 1 96 against absorber 190 and thus keeps all the elements on the left side of shelf 1 66 in their predetermined position.

Rotator mount 1 54 is held in place against the shelf formed by the change of diameter of passages 112 and 113, by helical spring 1 99. 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 1 56 provides for thermal stability, since the optical elements are elastically held against a stable material used for the gyro block.

As described above, the axes of apertures provided in rotator mount 1 54 are at an angle with respect to the longitudinal axis of mount 1 54, and thus the plane of Faraday rotator slab 1 65 also describes an angle with respect to the longitudinal axis of mount 1 54. This contributes to the elimination of coupling between counter-travelling waves since any reflections off of the two surfaces of Faraday slab 1 65 are now intercepted and absorbed by the two black glass absorbers.Waves circulating from left to right in the rotator of FIG. 6 will have a reflection from slab 165 intercepted and absorbed by the lower portion of absorber 190, while waves circulating in the opposite direction will have reflections from slab 1 65 absorbed by the top portion of absorber 1 91. The two absorbers, 1 90 and 1 91, and rotator slab 1 65 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 rotator 1 56 performs a second function. Because of the close fit provided in the region of shelf 166, Faraday rotator 1 56 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. 1,3 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 1 22. The beams traveling within each passage 108, 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 transmitted through the mirror surface. As the angle of incidence moves away from the normal, the partially transmitted beams begin to assume an elliptical polarization.

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 of the upper two frequencies and the beams of the lower two frequencies within the detector structure. As the amount of ellipticity 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 fifteen degrees.However, the amount of cross-coupling increases quite rapidly above this angle of incidence. The cross-coupling within the output optics structure may be 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 increases, 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 described 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 partially 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 1 60 is provided to mechanically position the mirror on face 1 28 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 1 60 from detector diodes 145 and 146. These signals have an amplitude in proportion to the total amplitudes of the corresponding Af1 and Af2 signals. Control 320 generates the difference between these two amplitude related signals. The output difference 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 piezoelectric transducer 1 60.

Still referring to the views of FIGS. 1, 3 and 4, electrodes for exciting the gaseous gain medium are disposed within passage 1 08. Preferably, center cathode electrode 22 is connected to the negative terminals of an external regulated power supply 310 while anode electrodes 32 and 42 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 1 02. It is attached 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 flows 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 current 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 manufacturing 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 compensater for the inequality, electrodes 32 and 42 are connected to two independent positive terminals of supply 310, so that current flow 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 external gas source. A mixture 3He, 20Ne, and 22Ne in the ratio of 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 1 08. Electrode aperture 30 intersects passage 108 preferably at a right angle. Terminal chamber 11 8 is formed into the surface of gyro block 102 upon which is positioned output optics structure 144. Terminal chamber 11 8 is cylindrical in shape having a diameter at least twice that of passage 108. Terminal chamber 11 8 and passage 108 are coaxial with one another.Because passage 108 extends slightly beyond electrode aperture 30 before intersecting with terminal chamber 11 8, a baffle 130 is formed between electrode passage 30 and terminal chamber 118. A similar 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 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 baffle arrangement described it has been found that dust or other unwanted particles will not be suspended 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 polarization, 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 shift that a wave of a particular frequency will experience due to the presence of a gain medium.As can be seen in FIG. 8B, frequencies 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, referring to FIG. 8B, having different amounts of phase shift. , is the phase shift corresponding to ft, 2 corresponds to f2, 3 corresponds to f3 and c, corresponds to f4.If the difference (2 - i) is different in quantity than the difference (4 - 3) there will be non-zero differential output at rest which depends on the shape of the dispersion curve, itself a function of many elements such as temperature, gain and pressure. As any one of these elements changes, this change will be reflected as a shift of the four modes across the dispersion curve which, due to its nonlinear nature, will result in a changing differential oiitput. Thus, the gyro will have a drift in its output frequency which varies according to a variety of factors.

The gyro system shown 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 different 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 fro = (E2 - E1)/h, where E2 and E, 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 Am = + 1 that will give rise to one set of the radiating frequencies, such as the center frequency for split dispersion line 260, f+ = f0 - gBH/h. Lines 244 show the energy level transitions corresponding to Am =-1 that give rise to the other set of radiating frequencies, such as the center frequency for split dispersion line 250, f = f0 + gBH/h, where g = Lands 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. Direction Polarization Delta m 1 clockwise LCP +1 2 counter-clockwise LCP -1 3 counter-clockwise RCP +1 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 electric field vector of the light wave as measured about the magnetic field interacts with the spin of the electrons whose energy levels are split by the field.Thus, one of the resulting dispersion lines interacts with a RCP wave that travels in a parallel direction to the direction of the magnetic 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 Am correspond to different atomic transitions, 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 magneticfield H is such that the Am =+1 line is lower in frequency than the Am =--1 line by an amount 280 equal to (f2 - f1) then we will have line 270 and line 272 equal in height, that is, the amount of phase shift provided to f1 and f2 will be equal.Similarly, line 274 and line 276 will be at the same height, with the result that the frequencies f3 and f4 will have a similar amount of phase shift. It can be seen that as the four frequencies 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 similarly be equal to that of mode 4. Thus, as external conditions create small changes in the operating frequencies, the net difference 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 = (3.64 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 provides a component of the magnetic field to the lasing gas can be used.Coils 202, 212, 222 and 232 are disposed around passage 1 08. 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 1 56 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 rotator.

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 - f1) of the first frequency pair, while diode 146 is used to provide a signal corresponding to fb, the frequency difference (F4 - f3) of the second frequency pair. The outputs of diodes 1 45 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 amounts 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 dispersion control 300 provides 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)/(3.64 x 106)0e, and the current used to produce it depends proportionally on the number of turns of the coils, as is wellknown in the art.

It is found that the Faraday rotator of the present embodiment 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.

The laser gyroscope described substantially reduces sources of drift, and thus reduces the overall gyro output drift by several orders of magnitude over other laser gyro systems.

In particular the drifts discussed earlier are substantially reduced by minimizing 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 crystal rotator to provide the frequency 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 scattering (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 coupie 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 scattering centres 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.

The next performance limitation is then brought about by small amounts of drift due to higherorder effects. The system 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.

Claims

1. A four-frequency laser gyroscope comprising closed propagation path containing a gain medium and arranged to produce circularly polarized counter-travelling electromagnetic waves of different frequencies arranged in pairs of first and second polarization sense, and means for providing compensation for gain medium induced changes in phase shift of said waves.

2. A gyroscope according to claim 1 arranged to produce, in a non-depolarizing manner, a polarization-dependent phase shift in said waves resulting in a frequency splitting between waves of opposite circular polarization.

3. A gyroscope according to claim 2, in which the propagation path is non-planar whereby the said polarization dependent phase shift is produced.

4. A gyroscope according to claim 3, in which said non-planar path providing means further provides for limiting the angle of incidence between adjacent path segments.

5. A gyroscope according to claim 1, 2, 3 or 4 including non-depolarizing means for producing a direction-dependent phase shift to said waves resulting in a frequency splitting between countertravelling waves in each of said pairs.

6. A gyroscope according to claim 5, in which the said direction-dependent means comprise means having a refractive index whose direction-independent component is isotropic.

7. A gyroscope according to claim 5 or 6, in which said direction-dependent means has a scattering characteristic substantially independent of temperature over the operating temperature range.

8. A gyroscope according to claim 5, 6 or 7, in which the said direction-dependent means comprise a slab of isotropic material having a non-zero Verdet constant and means for providing a magnetic field in said slab.

9. A gyroscope according to claim 8, in which the said direction-dependent means further comprise means for absorbing electromagnetic waves disposed on either side of said slab with the slab being positioned to direct any reflected waves to the absorbing means.

10. A gyroscope according to claim 8 or 9, in which the slab and absorbing means are held in place against a low thermal expansion frame by elastic means.

11. A gyroscope according to claim 8, 9 or 10; in which the means for producing a magnetic field in the slab comprise two magnets with like poles connected together and positioned adjacent the slab.

12. A gyroscope according to any one of claims 8 to 11, in which the slab has a thickness substantially less than the diameter of the beam formed by the said waves.

1 3. A gyroscope according to any one of claims 8 to 11, in which the slab has a thickness resulting in a variation of said thickness of substantially less than one wavelength of said waves during normal operation.

14. A gyroscope according to any one of claims 8 to 13, including means for generating a magnetic field for producing direction-dependent rotation, the field being localized to the region immediately adjacent said slab.

15. A gyroscope according to any one of the preceding claims, in which the compensating means comprise means for providing magnetic field longitudinal to the axis of said gain medium, said magnetic field having a magnitude and polarity for providing substantially the same amount of gain medium induced phase shift to the counter-travelling waves of each of said pairs.

1 6. A gyroscope according to any one of claims 1 to 14, in which the compensating means comprises means for providing a component of magnetic field in the gain medium along its longitudinal axis, the said magnetic field component having a magnitude for producing a frequency splitting of said gain and dispersion characteristics substantially equal to the frequency splitting between said countertravelling waves due to said direction-dependent non-depolarizing means, and said magnetic field component having a polarity for providing substantially equal amounts of gain-medium dispersion to said counter-travelling waves.

1 7. A gyroscope according to claim 16, in which said magnetic field providing means is arranged to vary the magnetic field as a function of the average amount of frequency splitting of countertravelling waves due to said direction-dependent non-depolarizing means.

18.,A gyroscope according to claim 17, in which the said magnetic field providing means comprise at least one coil around a portion of the gain medium containing path, means for measuring the amounts of frequency splitting generated in counter-travelling waves by said direction-dependent non-depolarizing means, and means for producing a current in the coil proportional to the average value of the said frequency splitting.

1 9. A gyroscope according to claim 18, in which the said magnetic field providing means comprise a plurality of coils positioned throughout the portion of said path containing the active portion of said gain medium.

20. A gyroscope according to any one of the preceding claims, further comprising means for removing scattering particles from said path.

21. A gyroscope according to claim 20, in which the removal means is arranged for removing contaminating particles in the region of electrodes, said electrodes being used to electrically excite the gain-medium to generate said electromagnetic waves.

22. A gyroscope according to claim 21, in which said scattering particles removing means comprise a baffle in the region of the said electrodes.

23. A four-frequency laser gyroscope substantially as herein described with reference to the accompanying drawings.

24. In combination: means coupled to a re-entrant resonant path, for providing compensation to electromagnetic waves for gain medium induced changes in phase shift of said waves; and means for producing circularly polarized counter-travelling waves of different frequencies arranged in pairs of first and second polarization sense, said polarizing means being substantially free of scattering centres.

25. In combination: means for providing a closed non-planar path for the propagation of circularly polarized electromagnetic waves, said means further providing a splitting in frequency to waves of opposite polarization sense; a gain medium disposed in said path and having gain and dispersion characteristics that vary non-linearly as a function of frequency; non-depolarization means for providing a direction-dependent phase shift to said circularly polarized waves resulting in a frequency splitting between counter-travelling ones of said waves of each polarization sense; and means for compensating for unequal gain medium dispersion of said counter-travelling waves.

26. A Faraday rotator comprising a supporting shell, means for providing Faraday rotation disposed within said shell and means for absorbing electromagnetic waves disposed on at least one side of said Faraday rotation means and for allowing a substantial portion of said waves to pass through said rotation means, said rotation means being further positioned to direct any reflected portion of said waves toward said absorbing means.

27. A rotator according to claim 26, in which said supporting shell comprises a low thermal expansion material having a plurality of stops and said rotation means and said absorbing means are held in place against said stops by elastic means.

28. A Faraday rotator substantially as herein described with reference to the accompanying drawings.