RING LASER GYRO TILT MIRROR DITHER

Abstract: A tilt mirror dither for a ring laser gyro to eliminate lock-in at low rates of rotation. The tilt mirror dither includes means (36, 38) for vibrating two of the ring laser gyro mirrors (30, 28), each in a rotational mode about an axis to tilt the mirrors back and forth, the mirrors being vibrated at the same frequency but 180<o> out of phase to maintain the path length of the laser beams constant. In one embodiment (Fig. 2A) an up-down tilt mirror dither is provided and in another embodiment (Fig. 2B) a side-to-side tilt mirror dither is provided.

Description: Ring Laser Gyro Tilt Mirror Dither Technical Field The present invention relates to a mirror dither for a ring laser gyro and more particularly to a tilt mirror dither for eliminating lock-in at low rates of rotation. Background of the Invention A ring laser gyro for measuring rotation about an input axis includes two monochromatic beams of light which are caused to travel in opposite directions about a closed loop path extending about the input axis. The path is formed by a cavity which is typically polygonal in shape having mirrors disposed at the corners thereof to reflect the beams along the path. As the gyro is rotated, the effective path length for one beam is increased while the effective path length for the other beam is decreased, due to Doppler shifting. A beat frequency is produced in response to heterodyning of the two beams as with a combining prism, the beat frequency signal in turn producing a fringe pattern which is typically detected by a photodiode. The latter produces a sine wave output whose frequency is proportional to the rate of rotation. At very low rates of rotation, errors arise due to "lock-in" effects, whereby no frequency difference is observed. Lock-in arises because of imperfections in the lasing cavity, principally in the mirrors, which produce backscatter from one laser beam into the other laser beam. At low rates of rotation where the frequency splitting between the two beams is small, the coupling of the backscatter from one beam into the other beam causes the two beams to oscillate at the same frequency. This results in a dead band, or lock-in region, in which the gyro output does not track the input. The lock-in threshold rate is determined by the amount of backscatter. When the gyro input rate of rotation exceeds the lock-in threshold rate, the beams separate in frequency and begin to produce the output. Various dither techniques have been employed to eliminate lock-in at low rates of rotation. One such technique is to dither or vibrate the entire body of the ring laser gyro about the input axis. Such body dithers disrupt the environment of the ring laser gyro which does not include any moving mechanical parts. Coupling of the body dither motion to motions caused by the environment, such as shock and vibration, may also be a problem. Further, the output of the ring laser gyro must be sampled synchronously with the body dither in order to cancel the effects of the dither from the gyro output. Another dither technique employed to eliminate lock-in at low rates of rotation is to dither each of the mirrors in a direction perpendicular to the face of the mirror as shown in Hutchings U.S. Patent No.4,281,930. Hutchings requires that all of the mirrors be dithered at the same frequency but out of phase by an amount equal to 360° divided by the number of mirrors. The phase difference between the dither applied to each of the mirrors must be precisely controlled so as to maintain the cavity path length constant. Because all of the gyro mirrors must be dithered as taught by Hutchings, problems in maintaining the cavity path length constant have arisen since the greater the number of mirrors, the more difficult it is to maintain precise control of the phase difference therebetween. Further, when all of the mirrors of the gyro are dithered, the combining optics, associated with the output mirror of the gyro, must be displaced in order to install the driver for dithering the mirror. The displaced optics are typically unstable moving relatively easily so as to produce errors in the output of the gyro. Disclosure of the Invention In accordance with the present invention, the disadvantages of prior ring laser gyros as discussed above have been overcome. The laser gyro of the present invention includes a tilt mirror dither to eliminate errors in the output of the gyro caused by lock-in of the beams. The tilt mirror dither includes means for vibrating two of the ring laser gyro mirrors each in a rotational mode about an axis to tilt the mirrors back and forth, the mirrors being vibrated at the same frequency but 180° out of phase to maintain constant the length of the closed loop path traveled by the beams. In one embodiment of the present invention the rotation axis of each of the mirrors is in a plane perpendicular to the input axis of the gyro resulting in an up-down tilt mirror dither which causes the beams to translate across the surface of the mirrors in a direction parallel to the input axis of the gyro. In another embodiment, the rotation axis of each of the mirrors is parallel to the input axis of the gyro, resulting in a side-to-side tilt mirror dither which causes the beams to translate across the surface of the mirrors in a direction perpendicular to the input axis of the gyro. The tilt mirror dither technique of the present invention eliminates lock-in at low rates of rotation by operating on the backscatter. The result is a Doppler shift in the frequency of the backscatter waves, which are biased away from the primary waves of the laser beams so that coupling effects are minimized and lock-in at low rates of rotation is not present. These and other objects and advantages of the invention, as well as details of an illustrative embodiment, will be more fully understood from the following description and the drawings. Brief Description of the Drawings Fig. 1 is a top view of a cross-section of a ring laser gyro; Fig. 2a is an end view of a mirror illustrating the orientation of the mirror for the up-down tilt mirror dither; Fig. 2b is an end view of a mirror illustrating the orientation of the side-to-side tilt mirror dither; Fig. 3 is a cross-section of the mirror drive for the tilt mirror dither. Best Mode for Carrying Out the Invention The ring laser gyro shown in Fig. 1 includes a body 10, which may be made of quartz, having a cavity 12 therein forming a closed loop path. The cavity 12 has a polygonal shape formed by intersecting gain tubes 14-17 and contains a gas or gases suitable for laser operation such as 90% helium and 10% neon at a pressure of 3 torr. A gas discharge is established between a cathode 18 and a pair of anodes 20 and 22, each of which is in communication with the cavity 12, to produce two counter-rotating laser beams. The beams are reflected around the closed loop path by mirrors 24, 26, 28 and 30 positioned at the corners of the cavity. As the gyro is rotated about an input axis 32 which is perpendicular to the plane in which the beams travel, the effective path length for one beam is increased while the effective path length for the other beam is decreased due to Doppler shifting. A beat frequency which is proportional to the rate of rotation is produced in response to heterodyning of the two beams such as by means of a prism associated with the mirror 26. The beat frequency produces a fringe pattern which is detected by a photodiode 34 providing the output of the gyro. In order to prevent lock-in of the two counter-rotating laser beams, the ring laser gyro employs a tilt mirror dither in which each of the mirrors 28 and 30 is vibrated in rotational mode about an axis to tilt the mirror back and forth. The mirrors 28 and 30 include respective drivers 36 and 38 which are controlled to drive the mirrors at the same frequency but 180° out of phase to maintain constant the length of the closed loop path traveled by the beams. In one embodiment of the present invention providing an up-down tilt dither, the orientation of each of the dithered mirrors is as shown for mirror 30 in Fig. 2a wherein the Z-axis is parallel to the input axis 32 of the gyro. A pair of piezoelectric transducer elements 40 and 42, mounted on opposite sides of the X-axis, are responsive to a drive voltage which may take the form of a sine wave or the like to vibrate the mirror 30 in a rotational mode about the X-axis such that the mirror tilts up and down through a small angle with respect to the Z-axis. The up-down tilt mirror dither causes the laser beams to translate across the surface of the mirror in a direction parallel to the input axis 32 of the gyro. The tilt dithered mirrors 28 and 30 are driven 180° out of phase so that the mirrors tilt the same amount but in opposite directions with respect to the Z-axis. That is, as the mirror 30 is tilting upward, the mirror 28 is tilting downward, the tilt angles of the mirrors with respect to the Z-axis being equal. It is noted that the up-down tilt dither causes the plane of the closed loop path traveled by the laser beams to vary, which in turn varies the input axis of the gyro. However, the resulting variation in the input axis is less than 1 arc second which has a negligible effect on the ring laser gyro operation since the allowable variation is on the order of 5-10 arc seconds. In a second embodiment of the present invention, the orientation of each of the dithered mirrors is as shown for the mirror 30 in Fig. 2b wherein the mirror of Fig. 2a is rotated 90° to provide a side-to-side tilt dither. In this embodiment the piezoelectric transducer elements 40 and 42 are mounted on opposite sides of the Z-axis. The piezoelectric transducer elements are responsive to a drive voltage which may take the form of a sine wave or the like to vibrate the mirror 30 in a rotational mode about the Z-axis such that the mirror tilts from side-to-side through a small angle with respect to the X-axis. The side-to-side tilt mirror dither causes the laser beams to translate across the surface of the mirror in a direction perpendicular to the input axis 32 of the gyro. In this embodiment of the tilt mirror dither, the plane of the closed loop path traveled by the laser beams does not change and there is no variation in the input axis of the gyro. Fig. 3 shows a basic design for the mirror drivers 36 and 38. A mirror, for example, mirror 30, is secured to the outer surface 44 of a diaphragm 46. The diaphragm 46 includes an annular rim 48 which is secured to the inner wall 50 of a frame 52. The diaphragm 46 also includes a centrally located leg 54 which extends inwardly towards the frame and engages a leg 56 extending outwardly from the base 58 of the frame. The leg 56 intersects the base 58 at a point on the axis about which rotation of the mirror is to occur, i.e., the X-axis for the up-down tilt and the Z-axis for the side-to-side tilt dither. Two piezoelectric transducer elements 60 and 62 are mounted on one side of the mirror's rotation axis and on the other side of the axis two more piezoelectric transducer elements 64 and 66 are mounted with the elements 60 and 64 being secured on opposite sides of the base from the elements 62 and 66. The transducer ele¬ ments are mounted having a crystallographic orientation such that with the application of a positive voltage to the elements 60 and 66 by means of leads 68 and 70 and the application of a negative voltage to the element 64 and 62 by means of leads 72 and 74, the frame and thus the mirror 30 is caused to tilt in one direction. With the application of a voltage of the opposite polarity to the leads 68 and 72 the piezoelectric transducer elements cause the mirror to tilt in a second direction opposite the first. The drive voltage applied to leads 68 and 72 takes the form of a sine wave to vibrate the mirror in a rotational mode providing either the up-down tilt mirror dither or the side-to-side tilt mirror dither. A pick- off electrode is connected to the piezoelectric trans¬ ducer element 62 in order to detect the voltage developed on the transducer as the mirror vibrate . This ' voltage represents the actual displacement of the mirror 30 and is used as a feedback signal in the control circuit for the mirror drivers. The control circuit which maintains the 180° phase difference between the mirrors is shown in detail in the copending application Serial No. 462,548. As the mirrors 28 and 30 tilt back and forth, either in an up-down direction or a side-to-side direction, the laser beams travel back and forth across the surface of the mirrors. This results in scatter center displacement with respect to the translated standing wave field modes, hence satisfying the phase shift requirements of phase modulation. In addition, the displacement between the scatter groups of the mirrors changes with time. It is the vector summation of these scatter groups which gives rise to a magnitude of lock-in. The oscillation of the mirrors causes the net scatter vector to be time modulated so as to further reduce lock-in. With only two of the mirrors being dithered and the remaining mirror or mirrors being stationary, one obtains an amplification of the scatter variation. That is, for a given amplitude of vibration, the laser beam translates a greater distance across the face of the mirrors where only two of the mirrors are dithered than where all of the mirrors are dithered. Further, the phase relationship between the two dithered mirrors and thus the cavity path length are more easily controlled than in gyros wherein all of the mirrors are dithered. Since the dithered mirrors 28 and 36 do not include any combining optics such as are associated with the mirror 26, for the output of the gyro, the shock and temperature stability of the ring laser gyro is also greatly improved over those gyros wherein each of the mirrors is dithered. The mirror dither technique also has several advantages over the body dither biasing technique. Although the mirror dither technique is not purely optical since the mirrors are driven by piezoelectric transducer elements, the displacement of the mirrors is extremely small compared to the displacement imparted to a body dithered laser gyro, the mirror displacement being typically on the order of 20 inches peak to peak. Further, because of the high dither frequency of the mirrors which may be on the order of 9 Khz, coupling of the mirror motion to motions caused by the environment, such as shock and vibration, is minimized. Another advantage of the tilt mirror dither over the body dither is that the output of the ring laser gyro employing the mirror dither may be sampled asynchronously with respect to the mirror dither. Although only two mirrors of the ring laser gyro are shown being tilt dithered, any number of the mirrors may be dithered at the same frequency and at a phase difference of 360° divided by the number of dithered mirrors in order to maintain the path length constant.

Claims: Claims 1. A ring laser gyro for measuring rotation about an input axis, the gyro having a body with a polygonal shaped cavity therein forming a closed loop path through which two counter-rotating beams travel, said gyro including a mirror disposed at each corner of the cavity for reflecting the beams about said path and means for vibrating at least two of said mirrors, each of said mirrors being vibrated in a rotational mode about an axis to tilt the mirror back and forth, said vibrated mirrors eliminating errors in the output of said gyro caused by lock-in of the beams. 2. The ring laser gyro of claim 1 wherein two of said mirrors are vibrated at the same frequency and 180° out of phase to maintain constant the length of the closed loop path traveled by the beams. 3. A ring laser gyro for measuring rotation about an input axis, the gyro having a body with a polygonal shaped cavity therein forming a closed loop path through which two counter-rotating beams travel, said gyro including a mirror disposed at each corner of the cavity for reflecting the beams about the path and means for vibrating at least two of said mirrors, each of said mirrors being vibrated in a rotational mode about an axis in a plane perpendicular to the input axis of the gyro to tilt the mirror back and forth, said vibrating mirrors eliminating errors in the output of the gyro caused by lock-in of the beams. 4. The ring laser gyro of claim 3 wherein said beams translate across the surface of the vibrated mirrors in a direction parallel to the input axis of the gyro. 5. The ring laser gyro of claim 3 wherein said tilting means varies the input axis of the gyro by not more than 1 arc second. 6. The ring laser gyro of claim 3 wherein two of said mirrors are vibrated at the same frequency and 180° out of phase to maintain constant the length of the closed loop path traveled by the beams. 7. A ring laser gyro for measuring rotation about an input axis, the gyro having a body with a polygonal shaped cavity therein forming a closed loop path through which two counter-rotating beams travel, the closed loop path defining a plane perpendicular to said input axis, the gyro including a mirror disposed at each corner of the cavity for reflecting the beams about the path and means for vibrating at least two of said mir¬ rors, each of said mirrors being vibrated in a rotational mode about an axis which is parallel to the input axis of the gyro to tilt the mirror back and forth, said vibrated mirrors eliminating errors in the output of the gyro caused by lock-in of the beams. 8. The ring laser gyro of claim 7 wherein said beams translate across the surface of the vibrated mirrors in a direction perpendicular to the input axis of the gyro. 9. The ring laser gyro of claim 7 wherein two of said mirrors are vibrated at the same frequency and 180° out of phase to maintain constant the length of the closed loop path traveled by the beams.

Inventors: STJERN DARYL C, STJERN, DARYL, C.
Applicants: SUNDSTRAND OPTICAL TECH, SUNDSTRAND OPTICAL TECHNOLOGIES, INC.
Application year: 1985
Main classification: G01C19
IPC codes: G01C19