JPH02870B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a four-frequency differential type dispersion-equalized ring laser gyroscope device using a non-planar monocycle and a non-depolarizing Faraday bias device. To be effective, laser gyros must overcome lock-in problems that occur at low rotational speeds. Lock-in is caused by the unavoidable scattering of some light from one resonant mode to another due to imperfections in the optical elements that make up the cavity. If the frequencies of these modes are not significantly different, the modes tend to phase lock. In order to put the gyroscope into practical use, this lock-in problem must be overcome. A dual frequency gyro system can avoid lock-in by biasing the gyro so that it operates at a large output frequency relative to zero input rotational speed. To avoid bias accuracy problems, the bias can be oscillated so that bias instability is removed from the output signal by time averaging. However, this concussion method causes lock-in to the gyroscope twice per concussion cycle. This causes the gyroscope to partially lose its phase coherence, thus creating a fractional count error every vibration cycle. This error adds randomly to give a cumulative angular error that increases over time. The four-frequency differential laser gyro device is
By essentially operating two independent gyroscopes in a single stable resonator that share a common optical path but are statically biased in opposite directions by passive biasing elements. This problem has been resolved. In the differential outputs of these two gyros, the biases are canceled and any rotationally generated signals are added, so the sensitivity is twice that of a single dual frequency gyro and problems due to bias drift are avoided. The four different frequencies are typically generated by utilizing two different optical effects. First, a crystal polarization rotator is used to provide direction-dependent polarization, which makes the resonant waves mostly right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP). This polarization rotation occurs because the refractive index of the rotator medium is slightly different for RCP waves and LCP waves. Second, a Faraday rotator is used to provide non-reciprocal polarization rotation by taking advantage of the slightly different index of refraction for waves traveling clockwise than for waves traveling counterclockwise. By this, CW
(Clockwise and CCW (counterclockwise) RCP waves will oscillate at slightly different frequencies, and
CW and CCW LCPs are split similarly, but in opposite directions. This Faraday rotator may be a separate optical element consisting of a piece of optically isotropic material placed in a longitudinal magnetic field, or by applying such a magnetic field to a crystal rotator. It may be possible to realize it. In this way, one laser gyro will operate with right-handed circularly polarized light biased in one direction of rotation, and the other laser gyro will operate with circularly polarized light biased in the opposite direction, with the bias being the difference between the two outputs. is eliminated by subtracting . Basic four-frequency laser gyroscope operation published June 26, 1972 by K. Andringa,
and assigned to the assignee of this application, U.S. Patent No.
Described in No. 3741657. The present invention discloses a laser gyro system that recognizes the problems that limit the stability of current laser gyro systems. Ideally, all four frequencies should be affected equally by external causes such as thermal expansion, so any combined variation in the differential output of a four-frequency gyro should cancel out. . However, in practice, it has been found that the four frequencies are not equally affected by external causes, resulting in different frequency fluctuations. Therefore, although the four-frequency gyro exhibits excellent performance under thermal balance, its practical use is limited by its thermal sensitivity, which is manifested as an unacceptable long-term bias drift. Until now, it has been limited by existence. The present invention discloses a laser gyro system that significantly reduces the sources of drift, thereby reducing the overall gyro output drift by several orders of magnitude compared to other laser gyro systems. One of the major causes of drift is frequency pulling due to the interaction of two resonant modes due to coupling from any scattering sources present, which generally depends on the environment. Another important source of drift is scattering-induced drift due to the relative motion of scattering centers in the cavity. Additionally, there is dispersion-induced drift due to temporal variations in the gain medium;
This causes differential variations in the dispersion, or phase shift, experienced by each resonant frequency. Yet another cause of drift is the Fresnel-Huizeau effect, which is due to the dependence of the refractive index of the gas discharge on the velocity distribution of the gas discharge sampled by the laser mode. The reason is that it can change as the mode moves in response to variations in the ring cavity. These sources of drift are present in all laser gyros and are generally caused by changes in optical path length due to environmentally induced changes in the cavity or optical elements within the cavity. The laser gyroscope of this invention minimizes the physical motion of the cavity using ultra-low thermal expansion materials, thereby reducing the amount of intracavity elements and limiting their form to change the effective optical path length. By minimizing the scattering of all elements, and for purely circularly polarized light, their drift is significantly reduced by using cavity modes. . A non-planar path is used instead of a traditional crystal rotor to provide frequency separation (splitting) between the LCP and RCP waves. A thin Faraday paramagnetic glass slab is used in place of the traditional thick Faraday rotator to provide non-reciprocal separation (split) between the CW and CCW waves. The use of non-planar paths not only eliminates the main source of scattering (crystal rotators) that causes coupling between different waves, but also provides good circular polarization. The use of a glass Faraday rotator avoids elliptical birefringence, thereby maintaining circular polarization. This further eliminates coupling between different waves, since upon reflection a complete LCP wave becomes an RCP wave and therefore no longer couples to the counter-rotating wave. By using the minimum rotor slab thickness that achieves a given amount of rotation, the temperature dependence of any scattering centers introduced by the slab can be minimized. This integrated method for generating and maintaining circularly polarized light eliminates sources of large amounts of drift due to coupling between the various waves, resulting in a more accurate laser gyro. The next performance limit is caused by a small amount of drift due to higher order effects. The apparatus of the present invention can in turn advantageously use Zeeman separation (bifurcation, splitting) of the gas laser mixture to equalize the dispersion experienced by each wave of the oppositely traveling pair. This allows the four frequency waves to undergo small frequency fluctuations and place them in different parts of the non-linear dispersion curve, thereby stabilizing the differential output. The present invention provides a gain medium phase shift compensation device coupled to a reentrant resonant path for compensating an electromagnetic wave for gain medium induced changes in its phase shift; A laser gyro device is disclosed that uses a substantially scattering center-free circular polarizer to generate circularly polarized counter-progressing waves of different frequencies arranged in pairs of polarization directions. More specifically, a circular polarizer is a non-attenuating polarizer for producing a polarization-dependent phase shift in said waves resulting in a frequency separation (splitting) between waves of opposite circular polarization. , and a non-depolarizing device for producing a direction-dependent phase shift for said waves resulting in a frequency separation between the oppositely traveling waves in each of said pairs. A polarization-dependent device can have a non-planar resonator, and a direction-dependent device can have a refractive index in which the direction-dependent component is isotropic. It is desirable for directionally dependent devices to have substantially temperature independent properties over their operating temperature range. In the configuration example taken, the directionally dependent device has a slab of isotropic material capable of generating a directionally dependent rotation of said wave electromagnetic field in the presence of a magnetic field. , this slab has a thickness that results in a variation in thickness that is substantially less than one wavelength of said wave over the operating temperature range. Preferably, said magnetic field for producing directionally dependent rotation is localized to the immediate area of said slab. Furthermore, the device for compensating the gain medium phase shift includes:
providing a magnetic field longitudinal to the axis of said gain medium having a magnitude and polarity such as to provide approximately the same amount of gain medium-induced phase shift for each pair of counterprogressing waves; We have equipment for this purpose. The invention further provides a device for providing a non-planar closed path for the propagation of circularly polarized electromagnetic waves and also for providing frequency separation of waves of opposite polarization directions, a gain medium disposed in said closed path, a non-depolarizing device for imparting a direction-dependent phase shift to said circularly polarized waves resulting in a frequency separation between oppositely traveling said waves of their respective polarization directions; The present invention provides a gyro device having a device for compensating for unequal gain medium dispersion of waves traveling on the gyro. The invention also provides a scattering particle removal device with a buffle in the area of the electrode. Furthermore, the directionally dependent non-depolarizing device has a slab of isotropic material with a non-zero Verdet constant and a device for applying a magnetic field to this slab and also eliminates any reflected waves. It is advisable to use an absorption device to collect. The dispersion compensator is
applying a magnetic field component to said gain medium of sufficient magnitude to produce a frequency separation of gain and dispersion characteristics approximately equal to the frequency separation between said counterprogressing waves due to said directionally dependent non-depolarizing device; A magnetic field application device is provided for applying a magnetic field along its longitudinal axis. The magnetic field components are also polarized so as to impart approximately equal amounts of gain-medium dispersion to the counter-traveling waves. More specifically, said magnetic field imparting device varies the magnetic field as a function of the average amount of frequency separation of counterprogressing waves by said directionally dependent non-depolarizing device. In one adopted configuration, the magnetic field imparting device includes at least one coil around a portion of the gain medium containing path, the magnetic field generated by the directionally dependent non-depolarizing device into counterprogressing waves. A device is provided for measuring the amount of frequency separation and a device for generating a current in the coil that is proportional to the average value of the frequency separation. The present invention further provides a support container, a device for imparting Faraday rotation disposed within the support container, and a device for absorbing electromagnetic waves disposed on at least one side of the Faraday rotation imparting device. an absorption device for allowing a substantial portion of said wave to pass through said rotation imparting device, and said rotation imparting device further absorbs any reflected portion of said wave through said absorption device. This provides a Faraday rotator that is arranged so as to point in the opposite direction. Other objects and advantages of the present invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings in which like parts are given like reference numerals. The configuration and operation of the laser gyroscope device according to the present invention will be described with reference to FIGS. 1 to 5 at the same time. Gyro block 10
2 is a frame in which this device is configured.
Gyroscope block 102 is preferably constructed of a material with a low coefficient of thermal expansion, such as a glass ceramic material, to minimize the effects of temperature changes on the laser gyroscope device. A suitable commercially available material is sold by the Owens-Illinois Company of the United States under the name "Cer-Vit" material C-101. Also, Schott's "Zerodur" (trade name) can be used. The mechanical block 102 has figures 1 to 4.
There are nine generally planar surfaces as shown in various shapes in the figures. Figures 3 and 4 show the gyro block 102 without other components of the device.
As best shown in the figure, passages 108, 1
10, 112 and 114 are provided. These passages form a non-planar closed propagation path within the laser gyroscope block 102. Mirrors are provided on surfaces 122, 124, 126 and 128 at the intersections of these surfaces and the passages. Substrates 140 and 142 with suitable reflective surfaces constitute mirrors disposed on surfaces 124 and 126, respectively. The mirrored surface also has an optical path length control transducer 16 immediately adjacent surface 128.
It is also provided on the front of the 0. One of these mirrors should be slightly concave so that the beam is stably confined almost to the center of the path. A transparent mirror substrate 138 with a partially transparent free-electric mirror layer 139 is also provided on the surface 122 so that a portion of each beam traveling along a closed path within the gyroscope 102 is transmitted to the output optical system 14.
It is now being combined into 4. Output optical system 1
The structure of 44 was created by I. Smith on February 27, 1979.
US Pat. No. 4,141,651 issued to John Smith, Inc. and assigned to the assignee of this application. Because passageways 108, 110, 112, and 114 form non-planar propagation paths for the various beams within the device, each beam undergoes polarization rotation as it circulates through this closed path. Ideally, only substantially circularly polarized beams are present within the non-planar cavities of the present invention. For circularly polarized beams, drift is minimized due to beam scattering or coupling from one beam to another. This reduction occurs because the scattered light of one circularly polarized state is not of a unique polarization such that it can be combined into and influence other beams. For other forms of polarization, this is not the case since there is always a component of the scattered beam that couples into other beams. In the illustrated configuration, the passages and mirrors are arranged to provide approximately 90 degrees of polarization rotation for the various beams. Since the right and left circularly polarized beams are rotated in opposite directions by this same amount regardless of their direction of propagation, the frequency separation (splitting) between the right and left circularly polarized beams is such that the beams are rotated in opposite directions by this same amount regardless of their direction of propagation. must occur in such a way that it resonates. This is shown in FIG. 8A as the frequency separation between the right and left circularly polarized beams. Although in the example configuration taken a 90 degree rotation corresponding to a 180 degree relative phase shift is employed, other phase shifts may also be used depending on the desired frequency separation. . Polarization plane rotation occurs as long as the closed propagation path is non-planar. The configuration of the propagation path itself determines the amount of rotation. In devices known in the prior art, such as that described in the K. Andringer patent mentioned above, the frequency separation between the right and left circularly polarized beams is achieved by propagating a block of solid material of considerable optical thickness. This was achieved by placing the The presence of direct-oscillating solid material in the beam propagation path creates scattering centers that undesirably cause light to be coupled from one beam to another, causing errors in the gyro output. Sometimes. The amount of binding, and therefore the error, is very sensitive to heat. Therefore, the output frequency of such devices is subject to temperature-dependent drifts that cannot be compensated for with a fixed output bias. Furthermore, crystal rotators tend to introduce large amounts of stress birefringence, which reduces the polarization of circularly polarized waves and also contributes to undesired coupling of polarized waves. For this reason, the gyro device has a temporal fluctuation in the output frequency, which is at best several tens of hertz, and in many cases reaches several hundred hertz. In this invention, the solid material traditionally used as a crystal rotor is completely removed from the beam propagation path, thus eliminating the sources of error and drift associated with this material. To help understand how phase shifts occur, it is useful to imagine a linearly polarized beam propagating in an optical path. In this explanation, the 180° phase shift that the electromagnetic wave undergoes upon reflection is ignored. Since we use an even number (4) of such reflections, no errors are introduced. For example, assume that a beam traveling in path 110 between surfaces 122 and 124 is linearly polarized as represented by an upwardly directed electric vector.
When the beam is reflected from the mirror disposed on surface 124, the electric vector is still pointing mostly upwards, but the path 112 is between surface 124 and surface 12.
8, so it is tilted slightly forward. When the beam reflects off the mirror on surface 128, its electric vector is tilted slightly downward and mostly to the left as viewed in FIGS. 3 and 4. beam is surface 126
3 and 4, the electrical vector of the beam in passageway 108 will again be tilted slightly upward and to the left in FIGS. 3 and 4. Surface 122
After reflection from , the electric vector of the beam in passage 110 is still directed to the left, into the plane of the drawing.
It will thus be seen that the beam has undergone a polarization rotation of approximately 90 degrees upon its return to passageway 110. Of course, such a rotated linearly polarized beam cannot self-reinforce and resonate along a closed path. A circularly polarized beam will only resonate if it has a frequency that deviates from the frequency at which it resonates for a planar closed path of the same length. A dual frequency laser gyroscope can be constructed by using a non-planar propagation path to provide a single frequency separation. In such a configuration, Faraday rotators or other such elements are not required. To detect the rotational speed, an output signal is generated by causing the extracted portions of the two beams to beat to form an output signal with a frequency equal to the difference in frequency between the two beams. Occur. At rest, this output signal remains at a certain value f ₀ .
For rotation in one direction, the output signal is f ₀ +△
increases with the value of f (where △f is proportional to the rotational speed), but for rotation in the other direction f ₀ −△
decrease to the value of f. This invention significantly reduces cross-coupling due to backscatter and reduces lock-in range, allowing such laser gyroscopes to be used in many applications without complete elimination of lock-in. . Faraday rotator 156 is located within large diameter portion 113 of passage 112 adjacent surface 124 as seen in FIGS. 2, 4, and 6A. Details of the Faraday rotator 156 are shown in FIGS. 6 and 6A. Faraday rotor fixture 154 is preferably made of the same material as laser radio block 102 and forms the base from which the structure is constructed. Rotor mounting fixture 154
is cylindrical in shape and provided with several cylindrical holes of various diameters at angles to its longitudinal axis to keep all elements of the Faraday rotation 156 in place. Supporting and mounting fixture 15
4 provides an unobstructed path along the central longitudinal axis of the 4. The Faraday rotor slab 165 is connected to the shelf 16 formed by the central portion of the fixture 154.
It is located on 6. Ring 169 is slab 1
Prevents lateral movement of 65. Faraday rotator slab 165 may be desirably formed of rare earth doped glass or a similarly high Verdet constant material. A Verdet constant greater than 0.25 min/cm/Oe is adopted to reduce the slab thickness required to produce the desired amount of frequency separation. Traditional Faraday rotators have often used thick slabs of fused silica. Any material in the optical path of the counter-rotating beam introduces scattering points that are sensitive to the thermal flux. This sensitivity may be due to thermal expansion of the material or to changes in optical path length due to the temperature dependence of the material's refractive index. It has been found that the effective temperature dependence of the optical path length, and thus the thermally induced drift, is a very clear function of the thickness of the solid material in the optical path of the beam. It is therefore desirable to use slabs that are as thin as possible, with thicknesses of 0.5 mm or less being taken to reduce drift to acceptable levels, so that over the operating range the temperature is mostly less than one wavelength of the laser wave. Otherwise, variations in thickness may occur due to other causes. Commercially available substances are Hoya Optics, Inc. substance number FR−.
5, which is a glass doped with a paramagnetic material to provide Faraday rotation, resulting in a rotor with an isotropic index of refraction. This turned out to be important because the problem with traditional Faraday rotators is that crystalline materials like quartz are anisotropic, resulting in elliptic birefringence. Because there is. This will depolarize the nominally circularly polarized waves and increase coupling between counter-rotating waves. Therefore, it is important to use an isotropic material as a Faraday rotator to eliminate the depolarization of the resonant mode. Operating as close to circularly polarized light as possible reduces mutual coupling and therefore thermally induced drift due to remaining scattering centers. This allows the gyro device to achieve a stability level that accommodates temporal variations in the output frequency of less than a few hertz. Faraday rotor slab 165 is magnet assembly 1
88 against the shelf 166.
Two hollow cylindrical permanent magnets 186 and 187 are arranged end-to-end so that the same magnetic poles are adjacent to each other at the junction between them. The two magnets can be fixed to each other by any known method, such as soldering or welding. The Faraday rotor slab 165 is therefore adjacent one end of the two magnet pairs. A longitudinal magnetic field is generated in the slab, which decays rapidly after a short distance from the slab or magnet. This arrangement has the advantage that there are virtually no stray magnetic fields that can reach the gas discharge region and cause unwanted mode or frequency offsets due to the Zeeman effect. Also, a single magnet can be used to provide the required magnetic field to the slab. The permanent magnet structure could be replaced with several turns of electrical wire, which is energized to establish a magnetic field in the Faraday rotor slab 165. It is the spring 175 that pushes against the magnet structure 188. The other side of spring 175 rests along the periphery of hollow cylindrical spacer 197;
Further, the spacer 197 partially rests on one side of the hollow cylindrical absorbent body 191. The absorber 191 is made of a material such as black glass and has an anti-reflection coating that absorbs all electromagnetic waves incident on its surface, such as specular reflections from the Faraday rotator slab. used for. The absorbent body 191 is held in position by a circular clip 193 that is frictionally stationary along the periphery of the hole 181. Thus, as can be seen, the elements just described form an assembly placed against the right side of the shelf 166 by the circular clip 193, and the spring 175 keeps all the elements in the correct position. It provides enough vertical force to hold it firmly. On the opposite side of shelf 166 is a similar arrangement of elements except for Faraday rotor slab 165 and magnet assembly 188.
A circular clip 192 forms a stationary base;
The second absorbent body 190 comes into contact with this and remains stationary. A spring 174 rests at one end on the left side of the shelf 166 and presses the spacer 196 against the absorber 190, thereby holding all elements in their position on the left side of the shelf 166. The rotor mount 154 is held in place by a helical spring 199 against a shelf formed by the change in diameter of passages 112 and 113. The first smaller diameter turn of spring 199 rests on the body of rotor mount 154, and the other larger diameter end extends around and is in frictional engagement with the wall of bore 113. It matches. Rotor 1
This configuration of elements at 56 is thermally stable because the optical elements are held resiliently against the stable material used in the gyroscope. As previously discussed, the axis of the hole in rotor fixture 154 is at an angle to the longitudinal axis of fixture 154 so that the plane of Faraday rotor slab 165 is also aligned with the longitudinal axis of fixture 154. It forms an angle. This is Faraday rotor slab 165
Since all reflections from the two surfaces of are now blocked and absorbed by the two black glass absorbers,
Helps eliminate coupling between counter-traveling waves. In the rotor of FIG. 6, waves circulating from left to right are reflected from slab 165 and absorbed by the lower portion of absorber 190, and waves circulating in the opposite direction are reflected from slab 165. It is absorbed by the upper part of the absorbent body 191. The two absorbers 190 and 191 and the rotor slab 165 are also anti-reflective coated to further reduce the amount of reflection. In addition to providing frequency separation (splitting) between clockwise and counterclockwise circulating beams,
Faraday rotator 156 performs a second function. Due to the close contact in the area of shelf 166, Faraday rotator 156 prevents the longitudinal flow of gas through passageway 112. Since there can be no net circulation of gas through a closed path, the possibility of circulation of scattering particles carried by the gas is considerably reduced, as is drift due to the Fresnel-Fuizeau effect. Referring again to Figures 1, 3 and 4,
It will be seen that the beam impinging on the partially transparent mirror located on surface 122 is given a low angle of incidence. Each passage 108, 110, 11
The beams traveling in 2 and 114 are circularly polarized beams. The closer the direction in which one of these beams is incident on a reflecting mirror or surface is to the normal, the closer the polarization of the beam transmitted through the mirror surface becomes circular. As the angle of incidence moves away from the normal, the partially transmitted beam begins to take on an elliptical polarization. If the beam in the output optics/detector structure is perfectly circularly polarized, as explained in the aforementioned U.S. Pat. There is virtually no unwanted mutual coupling and interference between the two lower frequency beams. As the ellipticity increases,
Mutual coupling begins to become evident and detector diode 1
It appears as an amplitude modulation in the output signals from 45 and 146. As mentioned above, it has been discovered that the amount of undesired mutual coupling is a nonlinear monotonically increasing function of ellipticity. Mutual coupling has been found to be relatively low for angles of incidence below about 15 degrees. However, the amount of mutual coupling increases very rapidly above this angle of incidence. Mutual coupling within the output optical structure can be removed by suitable polarizing filters, but the available filtered power decreases as the unfiltered mutual coupling increases. As the angle of incidence of each beam at the output mirror increases, the power available at the detector diode for each beam decreases. The power reduction rate, i.e. the ratio of the power available at the detector for a beam at a given angle of incidence to the power available for a beam normal to the specular surface, is determined by the power structure described in the aforementioned U.S. Pat. No. 4,141,651. This is shown in FIG. As can be easily seen, the power reduction rate drops rapidly for angles of incidence greater than about 15 degrees. Therefore, in accordance with one aspect of the invention, the angle of incidence of the beam in passages 108 and 110 with respect to a partially transmissive mirror disposed on surface 122 is less than or equal to 15 degrees. Stated differently, the angle between passages 108 and 110 is 30
degree or less. During operation of the device, the four frequency waves are preferably centered symmetrically about the peak of the gain curve. To this end, a piezoelectric transducer 160 is provided to mechanically position the mirror on surface 128 and adjust the total optical path length within laser gyroscope cavity 102 to properly center the four frequency waves. Optical path length controller 320 extracts signals for operating piezoelectric transducer 160 from detection diodes 145 and 146. These signals have amplitudes that are proportional to the total amplitude of the corresponding Δf ₁ and Δf ₂ signals. Controller 320 generates a difference between these two amplitude related signals. This output difference signal will of course have zero amplitude when the four frequency waves are properly centered on the gain curve. The output difference signal is of one polarity when the four waves are off-center in one direction and of the other polarity when the four waves are off-center in the other direction. The average amplitude signal can be formed by known circuitry, the output of which is coupled to the input lead of piezoelectric transducer 160. Referring still to FIGS. 1, 3, and 4, an electrode for exciting a gaseous gain medium is located within the passageway 108. The center cathode electrode 22 is connected to the negative terminal of an external regulated power source 310 and is also connected to the anode electrodes 32 and 42.
is preferably connected to its positive terminal. The cathode electrode is in the form of a short hollow cylinder with a hollow metal hemisphere attached to the end farthest from the laser gyroscope 102. The cathode electrode is attached to the surface of the gyroscope 102 adjacent the hole 20 in a conventional manner. The anode electrodes 32 and 42 are in the form of metal rods extending into the electrode holes 30 and 40. In this way, the beam passing through the path in which the electrodes are arranged passes through current paths of equal length in opposite directions, thereby avoiding the effects of drag on the beam caused by unequal currents through the gaseous gain medium. is virtually eliminated. However, due to manufacturing tolerances in the positions of the various electrodes, the distances between the negative electrode and the two positive electrodes in the two passages may not be exactly equal. To compensate for this inequality, electrodes 32 and 4
2 to two independent positive terminals of power supply 310 to make the currents unequal between the two positive electrodes and the adjacent negative electrode, thereby compensating for the effects of different drag forces. It is set as. The gaseous gain medium filling passageways 108, 110, 112, and 114 is supplied through gas fill hole 106 from an external gas source. A mixture of ³ He, ²⁰ Ne and ²² Ne in a ratio of 8:0.53:0.47 is collected. Once all the passageways are filled, a seal 107 is applied to the hole 106 to sealingly contain the gas. The details of the structure of the laser gyroscope device in the area of one of the positive electrodes are shown in detail in the cross-sectional view of FIG. The metal electrode 32 is placed within the electrode hole 30, held in place by an electrode seal 33. Electrode 32 extends somewhat more than half way from the surface of gyroscope 102 to passageway 108. Electrode aperture 30 intersects passageway 108, preferably at a right angle. Terminal room 118
is formed inside the surface of the gyro block 102 on which the output optical system structure 144 is placed. Terminal chamber 118 and passageway 108 are coaxial with each other. Since the passage 108 crosses the terminal chamber 118 after slightly exceeding the electrode hole 30, there is a gap 130 between the power exchange passage 30 and the terminal chamber 118.
is formed. A similar configuration is provided for electrode 42 by hole 40, seal 43 and terminal chamber 119. Conventional devices were not provided with a buffer. The terminal chamber extended directly outward from the electrode hole to the surface of the laser radio block. When the electrodes are energized, dust or other unwanted particles, such as those generated by the ion bombardment and spatter of the laser gyroscope, will accumulate around the intersection of the electrodes and the beam path. The suspended particles acted as scattering centers and increased the optical loss of the structure. In contrast, with the present invention, it has been found that dust or other unwanted particles are not suspended in the area of the intersection of electrode holes such as 30 and passageways 108. therefore,
Potential sources of drift are eliminated. As mentioned above, by maintaining good circular polarization, the gyro eliminates all known sources of large amounts of drift. However, there are additional sources that provide a small amount of drift that must be compensated for if the laser gyro is to be used in high performance equipment. This residual drift is a dispersion, a frequency-dependent index of refraction that is related to the gain of the medium used. For He--He gain media, the gain curve has an approximately normal curve shape due to Doppler broadening, and the dispersion curve can be depicted as S-shaped. A dispersion curve represents the amount of optical phase shift that a wave of a particular frequency undergoes due to the presence of a gain medium. As seen in Figure 8B, frequencies below the center frequency fc undergo an opposite phase shift from frequencies above the center frequency fc, and all modes are shifted toward the centerline. I will be moving it. Since the dispersion curve is non-linear, the four modes of the differential gyro are operating in such a way that they have different amounts of dispersion and therefore, with respect to FIG. 8B, different amounts of phase shift.
φ ₁ is the phase shift corresponding to f ₁ , φ ₂ corresponds to f ₂ , φ ₃ corresponds to f ₃ , and φ ₄ corresponds to f ₄ . If the difference (φ ₂ −φ ₁ ) is quantitatively different from the difference (φ ₄ −φ ₃ ), it will depend on the shape of the dispersion curve, which is itself a function of many factors such as temperature, gain, and pressure. A dependent non-zero differential output will be present at rest. If any one of these factors changes, this change will be reflected as a shift of the four modes across the dispersion curve, and this shift will result in a varying differential output due to the nonlinearity of the dispersion curve. become. Therefore, the gyro will have a drift in its output frequency that varies according to various factors. The gyro device of this invention utilizes the Zeeman effect to eliminate drift due to gain medium dispersion. The Zeeman effect involves the separation (splitting) of the spectral lines of the laser gas into two or more components. This frequency separation will result in a separation of the gain curve and the corresponding dispersion curve. This physical mechanism is a quantum mechanical phenomenon in which a magnetic field separates atomic energy levels into several states that have different energies and interact with waves of a given circularly polarized state. This is illustrated in Figure 9, where the left side of the energy diagram shows typical energy levels in the absence of a magnetic field. In this case, _the radiation frequency is f ₀ = _{(E 2 −}
E ₂ )/h. The right side of this diagram shows how the energy levels separate in the presence of a magnetic field. Line 242 shows the energy level transition corresponding to Δm=+1, which is one set of radiation frequencies such as the center frequency for separate dispersion lines 260, f ₊ =f ₀ −g ^BH /h occurs. Line 244 shows the energy level transition corresponding to △m=-1, which is similar to the separated dispersion line 2
yielding the other set of radiation frequencies, such as the center frequency for 50, f _- = f ₀ +g ^BH /h. Here, g=
Lande's G ratio, that is, the gyromagnetic ratio, B = Bohr magneton, and h = Planck's constant. The four circulating modes are determined by the magnetic quantum number m of the neon atom as follows:
The change in Δm has different values.

[Table] The Zeeman effect depends on both polarization and direction.
The reason for this is that the rotation direction of the electric field vector of the light wave when measured with respect to the magnetic field interacts with the spin of the electron whose energy levels are separated (branched) by the magnetic field. Therefore, one of the resulting dispersion lines interacts with an RCP wave traveling in a direction parallel to the direction of the magnetic field and an LCP wave traveling antiparallel to, or opposite to, the direction of the magnetic field; The other dispersion line interacts with the RCP wave traveling in a direction antiparallel to the magnetic field vector and the LCP wave traveling in the same direction as the magnetic field. Since the values of Δm correspond to different atomic transitions, the transitions are separated by an amount equal to 2g ^BH /h by the Zeeman effect. Turning to FIG. 8c, there is shown a diagram of the separate dispersion curves and the corresponding phase shifts of the four modes of the gyro. △m=+
1 line is (f ₂ −
If there is a magnetic field that is lowered by an amount 280 equal to f ₁ ), then the heights of lines 270 and 272 will be equal, ie, the amount of phase shift imparted to f ₁ and f ₂ will be equal. Similarly, lines 274 and 276 are at the same height so that frequencies f ₃ and f ₄ have similar amounts of phase shift.
As can be seen, when the four frequencies drift across the dispersion curve, or when the dispersion curve changes due to temperature, for example, the dispersion of mode 1 will always be equal to the dispersion of mode 2; The dispersion of mode 3 is likewise equal to the dispersion of mode 4.
Therefore, when there are small changes in operating frequency due to external conditions, the net difference in differential output remains the same. To remove the dispersion drift, the magnetic field for the Zeeman effect ^should be
The formula must be satisfied. By this,
The gyro device will be able to achieve output frequency stability on the order of much less than 1 Hz. Next, looking at Figures 1 to 4, we see that in the interpreted configuration example, the Zeeman separation (bifurcation) of the dispersion curve
It will be appreciated that the magnetic field required for is obtained through the use of coils placed around the path carrying the laser medium. Holes are drilled in the gyroscope block 102 to provide passages 200, 210, 22 for the coils.
0 and 230 are provided. On one side of the cathode 22 are coils 202 and 2 to provide Zeeman separation throughout the laser section of the gyroscope path.
12 is provided, and coils 222 and 232 are provided on the other side of cathode 22. Although the four sets of coils were used to provide a more uniform magnetic field to the laser gas, any other configuration that provides a component of the magnetic field to the laser gas may be used. Coils 202, 212,
222 and 232 are arranged around passageway 108. Preferably, all four coils are controlled by a single source so that the Faraday rotator 1
of magnitude and polarity to produce a magnetic field to cause a separation of the dispersion curves in a magnitude equal to the separation at the frequency of the Faraday bias given by 56 and also in a direction that eliminates the sensitivity of the wave to the gain medium. It is better to supply a current of It is desirable to control the amount of magnetic field generated for Zeeman separation in relation to the amount of Faraday bias provided by the Faraday rotator. Next, looking at FIG. 1, we see that diodes 145 and 1
Output optical structure 144 is shown supporting 46 . The output optical system structure 144 separates the LCP counter-rotating frequency pair and the RCP counter-rotating frequency pair by detecting them with separate diodes. For example, diode 145 is used to provide a signal corresponding to fa, which is the frequency difference (f ₂ −f ₁ ) between the first pair of frequencies, and diode 146 is used to provide a signal corresponding to fa, which is the frequency difference (f 2 −f 1 ) between the first pair of frequencies.
It is used to provide a signal corresponding to fb, which is the frequency difference (f ₄ −f ₃ ) of the frequency pair. diode 1
The outputs of 45 and 146 are connected to a distributed controller 300. At rest, fa=fb, and each difference corresponds to Faraday bias. In the case of rotation, one of the two difference frequencies increases and the other decreases, the amount and direction of the change depending on the direction and speed of rotation. Distributed controller 3
00 has a conventional electronic circuit capable of forming a signal representing the average of the two frequency differences, which measures the Faraday bias even under rotation. Another circuit in distributed controller 300 supplies current to coils 202, 212, 222, and 232 as a function of this Faraday bias signal to equal the frequency separation (split) provided by the Faraday bias. A magnetic field is generated in passage 108 to separate the dispersion curves by an amount. The magnetic field required for dispersion equalization is given by H = Faraday bias / 2g ^Bh = (Faraday bias [Hertz]) / (3.64 × 10 ⁶ ) Oe, and to generate it The current used is proportional to the number of turns in the coil, as is well known in the art. It has been found that a Faraday rotator with this configuration generates a Faraday bias that is inversely proportional to temperature. Because the magnetic field for Zeeman separation is generated by the controller 300 as a function of the measured Faraday bias, dispersion equalization is performed independently of the temperature dependence of the Faraday bias. The controller 300 adjusts the measured Faraday bias via a magnetic field for the Faraday bias, whose polarity is determined by the orientation of the coil winding, and some proportionality constant related to the number of turns of the coil winding. generates a current whose amplitude is controlled as a function of a signal corresponding to the current. Further detailed description of bias control electronics 300 is not necessary as the design of this type of control circuit is well known in the art. Other modifications to the illustrated configuration may be readily made by those skilled in the art without departing from the spirit and scope of the invention. Some embodiments of this invention are listed below. (1) A device for providing a non-planar circuit for the propagation of circularly polarized electromagnetic waves and also for providing frequency separation for waves of opposite polarization direction, arranged in said circuit and as a function of frequency. a gain medium having non-linearly varying gain and dispersion properties; directional dependence on said circularly polarized waves resulting in frequency separation between oppositely traveling said waves of their respective polarization directions; a ring laser gyroscope apparatus comprising: a non-depolarizing device for imparting a symmetrical phase shift; and a dispersion compensating device for compensating for the unequal gain medium dispersion of said oppositely traveling waves. (2) The device according to aspect (1), wherein the non-planar closed path further limits the angle of incidence between adjacent path portions. (3) The device according to aspect (2), further comprising a device for removing scattered particles from the non-planar closed circuit. (4) According to aspect (3), the scattering particle removal device comprises a buffle in the region of the electrode used to electrically excite the gain medium to generate the electromagnetic wave. Device. (5) the directionally dependent non-depolarizer is a slab of isotropic material having a non-zero Verdet constant;
The device according to aspect (1), further comprising a device for applying a magnetic field to the slab. (6) said directionally dependent non-depolarizing device further comprising devices for absorbing electromagnetic waves disposed on either side of said slab, said slab absorbing any reflected waves from said slab; The device according to aspect (5), wherein the device is arranged so as to lead to the absorption device. (7) The device according to aspect (6), wherein the slab and the absorption device are pressed against a low thermal expansion frame and held in the correct position by an elastic device. (8) Embodiment (5) wherein said device for generating a magnetic field in said slab consists of two magnets arranged adjacent to said isotropic slab and having like magnetic poles connected to each other. ). (9) The dispersion compensation device includes a magnetic field applying device for applying a magnetic field component to the gain medium along its longitudinal axis, and the magnetic field component is applied to the directionally dependent non-depolarizing device. said magnetic field components have a magnitude for producing a frequency separation of said gain and dispersion characteristics approximately equal to the frequency separation between said counter-progressing waves due to said magnetic field components having approximately an equal amount of gain. - The device according to aspect (1), wherein the device has polarity for imparting medium dispersion to the wave traveling in the opposite direction. (10) The device according to aspect (8), wherein the magnetic field application device varies the magnetic field as a function of the average amount of frequency separation of counterprogressing waves by the directionally dependent non-depolarizing device. (11) said magnetic field imparting device comprises at least one coil around a portion of a gain medium receiving path, the amount of frequency separation produced in counterprogressing waves by said directionally dependent non-depolarizing device; The device according to aspect (10), comprising a device for measuring the average value of the frequency separation, and a device for generating in the coil a current proportional to the average value of the frequency separation. (12) According to aspect (11), the magnetic field applying device includes a plurality of coils arranged throughout a portion of the accommodation path that accommodates the active portion of the gain medium. Device. (13) a support container; a rotation imparting device for imparting Faraday rotation disposed within the support container; and a device for absorbing electromagnetic waves disposed on at least one side of the Faraday rotation imparting device, an absorption device for allowing a substantial portion of said wave to pass through said rotation imparting device, and said rotation imparting device further absorbs any reflected portion of said wave into said rotation imparting device; The Faraday rotator device is placed so that it points toward the (14) The support container is made of a low thermal expansion material and is provided with a plurality of stoppers, and the rotation imparting device and the absorption device are pressed against the stoppers by an elastic device. The device according to aspect (13), wherein the device is held in the correct position by

[Brief explanation of drawings]

FIG. 1 is a top isometric view from a first corner of the laser gyroscope device of the present invention. 2 is a bottom isometric view from a second corner of the apparatus shown in FIG. 1; FIG. 3 and 4 are isometric views of the gyroscope from a third corner of the apparatus shown in FIG. 1, showing the internal structure and passageways of the apparatus. FIG. 5 is a sectional view showing the internal structure of the device shown in FIG. 1 in the region of one terminal chamber and mirror substrate. 6 is a side sectional view showing details of the structure of the Faraday rotator device of the laser gyro device shown in FIG. 1. FIG. 6th
Figure A is a top view of a portion of the laser gyroscope of Figure 1 in the area of the Faraday rotator of Figure 6, showing the top surface of the Faraday rotator.
FIG. 7 shows the power reduction rate as a function of the angle of incidence of the beam on the output mirror structure. FIG. 8A is a diagram showing the frequency versus gain of the gas laser medium used in the laser gyro device of FIG.
It shows the relative position of the frequencies of the four beams within the device. Figure 8B shows the phase shift (dispersion) as a function of frequency for the gain medium of Figure 8A.
FIG. Figure 8C shows the frequency versus phase shift (dispersion) of the laser medium in the presence of a magnetic field.
FIG. FIG. 9 is an energy level diagram showing separation of energy levels in the presence of a magnetic field. In these drawings, 102 is a gyroscope block, 108, 110, 112, 114 are passages (forming a non-planar closed path), 144 is an output optical system, and 1
56 is a Faraday rotor, 154 is a fixture for this (support container), 165 is a Faraday rotor slab,
188 is a magnet assembly, 186, 187 are permanent magnets, 174, 175 are springs, 190, 191 are absorbers, 192, 193 are circular clips, 181 is a hole, 22 is a center cathode electrode, 32, 42 are anode electrodes , 30 is an electrode hole, 118 is a terminal chamber,
130 is Batsuful, 202, 212, 222, 2
32 indicates a coil.

Claims (1)

[Scope of Claims] 1. Means for providing a closed circuit for a plurality of electromagnetic waves passing through the housing, including a housing having a passage therein, and a support provided within the housing and aligned with a portion of the passage. and means attached to and supported by the support and disposed in a portion of the passageway to impart a Faraday rotation to electromagnetic waves; and means attached to and supported by the support to absorb electromagnetic waves. a central region disposed on at least one side of the Faraday rotation means and disposed within the passage through which most of the electromagnetic waves can pass through the Faraday rotation means; and an absorption region offset from the passage. Absorbing means having: a ring laser gyroscope, wherein the Faraday rotation means is arranged to direct electromagnetic waves reflected by the means towards an absorption region of the absorbing means. 2. Claim 1, wherein the support is made of a low thermal expansion material having a plurality of stops, and the Faraday rotation means and the absorption means are held against the stops by elastic means.
Ring laser gyroscope as described in section.