RING LASER GYRO FRAME DESIGN RESISTANT TO THERMAL GRADIENT EFFECTS

Abstract: In a ring laser gyro having a ring laser conduit having a laser gas therein, including at least two means for delivering electrical energy to said gas to form electric fields which are oppositely poled relative to said conduit, within gain regions of said conduit where said delivery of electrical energy occurs, to generate counter-propagating laser beams within said ring laser, the improvement comprising: said means for delivering electrical energy being symmetrically mounted within said ring laser conduit to define gain regions and non-gain regions within said conduit, said regions being symmetrically positioned to generate said at least two counter-propagating laser beams; and said laser passages including symmetrically mounted means therein for creating a ratio of gas flow conductance in said gain regions to gas flow conductance in said non-gain regions that is less than unity, whereby gas flow within said conduit is reduced.

Description: RING LASER GYRO FRAME DESIGN RESISTANT TO THERMAL GRADIENT EFFECTS Background of the Invention Field of Invention This invention relates to ring laser gyros and, more particularly, to an improved gyro having a reduced sensitivity to bias caused by thermal gradients. Description of The Prior Art Ring laser gyros use two or more counter-propagating light beams of predetermined zero-angular velocity frequencies to measure the rotational rate about a sensitive axis of a ring laser as a function of the difference in frequency, i.e., beat frequency, between the counter-propagating beams. Such counter-propagating beams are laser beams that are usually stimulated by an electric discharge in a gain medium, such as a suitable mixture of helium and neon. The gas gain medium flows during operation of the ring laser gyro both because of a thermal gradient along gain region of the enclosing conduit and an electrophoretic force directed along the excited plasma discharge. The flow caused by electrophoreεis is called Lang uir flow. The interaction of the light beams with the moving laser gas produces a frequency shift of the counter- propagating beams. The beam propagating in the direction of the gas flow experiences an optical length which is different from that of the beam propagating in the direction opposite to the gas flow. The flow then produces a beat frequency between the counter-propagating beams. That beat frequency bias is present when the ring laser is not experiencing rotation. Rotation of the ring laser about its sensing axis lengthens the path of one of the counter-propagating beams and shortens the path of the other beam. Such variations in the path-lengths of the beams cause the beam frequencies to vary, one increasing and the other decreasing. The beat between those two beams is, except for the beat produced by the gas flow, a measure of the experienced angular velocity. Output information, produced by rotation of the ring laser path may thus contain an error due to the bias caused by the gas plasma flow. It has been discovered that the accuracy of the ring laser gyro is limited, in addition to being limited by Langmuir flow, by thermal gradients within the laser. Thermal gradients may be caused by many factors including the non-uniform ambient temperature to which the laser is exposed and the internal heating within the laser due to resistance of the gas to current flow and by anode heating... Summary of the Invention The invention provides for a means of controlling the ring laser gas plasma flow produced by thermal gradients. It provides an optimal ratio of flow conductance between the gain regions and the non-gain regions of the ring laser conduit. At this optimal ratio, the rhermally induced gas flow vanishes within the gain regions, and therefore the thermal gradient does not affect the operation of the ring laser gyro. One embodiment controls the plasma flow by placing a restriction symmetrically within the gain regions of the ring laser conduit. Another embodiment enlarges the cross-section of the non-gain regions of the ring laser conduit, thus causing the conductance of the non-gain regions to be greater than that of the gain regions. The enlarged non-gain regions enhance the gas flow in such non-gain regions where the flow does not noticeably affect the frequencies of the laser beams. BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of this invention may be had by reference to the specification and drawings, wherein: Fig. 1 is a schematic diagram showing a ring laser gyro with the gain regions of the ring laser conduit marked by a grid pattern; Fig. 2 shows a ring laser gyro in cross-section with a "T" shaped connection between the cathode and the laser conduits; and Fig. 3 illustrates a ring laser gyro again in cross- section showing a "Y" shaped connection between the cathode and the laser conduit. DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 1, a ring laser gyro 10 is mounted in a laser block 11 which embodies the principles of- the present invention. A ring laser path is produced by mounting corner mirrors 12, 14, 16 and 18 on each corner of the block 11. The ring laser conduit comprises passages 20, 22, 24 and 26 formed within the block 11 between mirrors 12, 14, 16 and 18.The ring laser conduit contains a laser gain medium, such as helium and neon, suitable for producing counter-propagating laser beams of predetermined frequency determined by the laser frequencies of the gases and the resonant frequency of the optical cavity. The conduit is dimensioned to support counter-propagating beams of electro-magnetic energy in the passages 20, 22, 24, 26. The ring laser is stimulated by an electrical discharge in the gain regions of the conduit. An electrical discharge is produced between a cathode 30 open to the ring laser conduit and two anodes 32 and 34 which are open to the ring laser conduit and symmetrically positioned about the conduit relative to the cathode. Cathode 30 may be hermetically sealed to the center of one of the side faces of block 11 of the ring laser gyro 10 adjacent to passage 20. The cathode 30 is joined to the passage 20 through auxiliary passage 36. Anodes 32 and 34 each comprise a conductive electrode coupled to the ring laser passages 22 and 26 through auxiliary passages 38 and 40, respectively. The regions between each anode and the cathode, where laser activity produces counter-propagating laser beams, is designated the gain regions of the ring laser conduit. Such gain regions are shown by the shaded portions of the passages 20, 22, and 26 in Fig. l. The beams thus produced propagate from corner mirror to corner mirror around the closed conduit including through the non-gain regions of the conduit which are unshaded. The ring laser gyro generates an output of combined beams at 42 through a partly transmitting mirror 18. The beats between the extracted beams and their rate are counted. Their rate, without any beats due to temperature gradients and Langmuir flow, is a measure of the angular velocity experienced by the ring laser about its sensing axis. A frequency shift not correlating to the rotation of the ring laser gyro 10 may be caused by the flow of the laser gases within the gain regions of the laser conduit. Such gas flow changes the optical path lengths of the laser beams producing error or bias beats. The gas flow is due to electrophoretic or Langmuir flow and to thermal gradients in the ring laser gas. A ring laser gyro 10 may be subjected to an operating range of more than 150 degrees Centigrade. Such temperature changes, because temperatures take time to stabilize, cause thermal gradients within the laser which cause the laser gas to flow. Previous attempts to control the temperature gradients across a ring laser gyro have been cumbersome and ineffective, and ring laser gyros of the prior art are limited by their bias sensitivity to thermal gradients. In Fig. 2, the laser block 11 is shown in greater detail. It will be seen that restrictive apertures 44 are symmetrically placed at the point of the symmetry formed by the intersection of passage 36 and passage 20. This point of symmetry is formed where the crossed passages or legs join to create a T-shaped configuration. The restrictive apertures 44 are mounted astride passage 36 at the center of passage 20 equidistant from anodes 32 and 34. The restrictions 44 could be mounted between the cathode 30 and the anodes 32, 34,, according to the teachings of this invention, at two other points within passage 20 equidistant from the T-shaped junction of passage 20 and passage 36. Further it will be understood that the counter- propagating laser beams may also be generated by a single anode mounted at passage 36 and two cathodes mounted at passages 32 and 34. Preliminary to describing the invention in detail, it is useful to define certain terms. Flow impedance is defined as the ratio of the forces causing the flow to the ensuing flow velocity.. Flow conductance is the reciprocal of flow impedance. The Poiseuille Law for laminar flow in a circular pipe is: V={1/32 λ -}{D /L}Z1P', where V is the mean velocity; u, is viscosity of the fluid; D is the diameter of the pipe; L is the length of the pipe; and ^ is the pressure differential forcing the flow. Flow conductance is defined as velocity of the gas divided by the pressure differential. It is therefore proportional to the diameter squared divided by the length. Factors causing a pressure differential include, but are not limited to, thermal gradients along the pipe. In Fig. 2 passage 24 is substantially enlarged in cross-section compared to passages 20, 22 and 26. Because of the enlargement, flow of the laser gas, due to thermal gradients, occurs within the non-gain regions, rather than the gain regions, of the ring laser conduit. By increasing the cross-sectional area of the non-gain regions, or decreasing the cross-sectional area of the gain regions, or both, to such an extent that the ratio of the flow conductance of the gain regions to the flow conductance of the non-gain regions is substantially less than unity there is no more net movement of the gas within the gain region. In the preferred embodiment, the ratio of flow conductance in the gain region to flow conductance in the non-gain region is between one-to-two and one-to-four. It is seen that the enlarged portions of the non- gain regions are symmetrically positioned to be symmetrical within the conduit relative to the point of symmetry described above and formed where the crossed passages or legs join to create a T-shaped configuration. That is, every enlargement on one side of that point of symmetry is duplicated on the other side. By symmetrically placing the restriction 44 and/or symmetrically designing the diameters of the gain regions and non-gain regions to achieve the proper ratio of flow conductance, the ring laser gyro 10 experiences no net thermally induced movement of gas molecules within the gain regions where the laser activity takes place. In Fig. 3, a second embodiment of laser block 11 is shown using a Y-shaped junction between passage 36 and passage 20 and an enlarged non-gain region which extends beyond passage 24 into passages 22 and 26. It is seen that passage 36 divides into two legs 64 and 66 with the single leg 36 joined to cathode 30 and the two legs 64 and 66 joined to passage 20 on either side of the restrictive aperture 44 to create the Y-shape. Aperture 44 is thus mounted in the portion 68, of passage 20, between the inner ends of passages 64, 66. Little or no laser activity takes place in the portion 68, but it carries the laser beam. The non-gain regions are fully enlarged in Fig. 3. That is, in addition to enlarging passage 24, that portion of passages 22 and 26 above the passages 38 and 40, respectively, are also enlarged as shown at 70 and 72. Since flow conductance is proportional to diameter squared divided by length, the prolongation of extension of large diameter to non-gain regions 70, and 72 allow the desired high conductance without an excessively large diameter for passage 24. In summary, the flow of gas within the gain regions of the ring laser conduit is reduced by (1) restricting the flow of such gas and (2) reducing the ratio of the flow conductance of the gain regions to the flow conductance of the non-gain regions.

Claims: CLAIMS 1. In a ring laser gyro having a ring laser conduit having a laser gas therein, including at least two means for delivering electrical energy to said gas to form electric fields which are oppositely poled relative to said conduit, within gain regions of said conduit where said delivery of electrical energy occurs, to generate counter-propagating laser beams within said ring laser, the improvement comprising: said means for delivering electrical energy being symmetrically mounted within said ring laser conduit to define gain regions and non-gain regions within said conduit, said regions being symmetrically positioned to generate said at least two counter-propagating laser beams; and said laser passages including symmetrically mounted means therein for creating a ratio of gas flow conductance in said gain regions to gas flow conductance in said non-gain regions that is less than unity, whereby gas flow within said conduit is reduced. 2. In a ring laser gyro, as claimed in claim 1, wherein: said symmetrically mounted means in said conduit is restrictive means for controlling gas flow within regions where laser activity takes place. 3. In a ring laser gyro, as claimed in claim 1, wherein: said symmetrically mounted means in said conduit include symmetrically positioned enlarged cross-section said non-gain regions to increase the flow conductance of such non-gain regions. 4. In a ring laser gyro, as claimed in claim 1, wherein: said symmetrically mounted means creates a ratio flow conductance of gain regions to flow conductance of non-gain regions of between one-to-two and one-to- four,such that the sensitivity of said ring laser gyro to thermal gradients is reduced. 5. In a ring laser gyro, as claimed in claim 2, in which: said two means for delivering electrical energy to said gas comprise one common cathode and two anodes symmetrically mounted in electrical contact with said laser gas, said anodes being symmetrically spaced from said cathode about said ring laser conduit, and in which the regions of said conduit between said cathode and said anodes define said gain regions; and said restrictive means is mounted in said conduit at a point of symmetry adjacent said cathode and equidistant from said anodes. 6. In a ring laser gyro, as claimed in Claim 5, wherein: said non-gain regions formed in said conduit between said nodes is formed larger than said gain region. 7. In a ring laser gyro, as claimed in Claim 5, additionally comprising: a connection between said cathode and said gain regions having a T-shape formed by crossed legs; and said restrictive means mounted in said T-shape connection at the point where said legs cross to form said point of symmetry symmetrical to said cathode and said anodes. 8. In a ring laser gyro, as claimed in Claim 2, additionally comprising: two cathodes and one anode symmetrically mounted within said passages to form said at least two laser means; and at least a pair of said restrictive means mounted within said gain region of said conduit equidistant from a point of symmetry symmetrical to said cathodes and said anode. 9. In a ring laser gyro, as claimed in Claim 8, wherein: said conduit includes gain and non-gain passages; and said non-gain passages have portions that are symmetrically positioned about said conduit relative to the anode and said portions are larger in cross-sectional area than said gain passages. 10. In a ring laser gyro, as claimed in Claim S, wherein: the ratio of the flow conductance of said non-gain passages to the flow conductance of said gain passages is between two and four to one.

Inventors: FRIDRIC BERNARD G, FRIDRIC, BERNARD, G.
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS INC.
Application year: 1988
Main classification: G01C
IPC codes: G01C19, H01S3