GB2072936A - Acoustically dithered ring laser gyro - Google Patents

Abstract

A ring laser gyro has light beams that circulate in opposite directions around the same closed passageway (14). The passageway is filled with a laser medium that is also dielectric. The dielectric medium is caused to move in an oscillatory manner within the passageway by an acoustic dithering mechanism (40) so that the light beams passing through the flowing medium undergo a change in velocity and thus a change in frequency in accordance with the Fresnel-Fizeau effect to eliminate frequency locking. <IMAGE>

Description

SPECIFICATION Acoustically dithered ring laser gyro This invention relates to ring laser gyros and, more particularly, to a ring laser gyro wherein frequency locking of oppositely directed light waves is reduced.

It is well known that for sufficiently high rotation rates the frequency difference between two counter-rotating light waves in a ring laser is directly proportional to the rate at which the laser is rotating. This frequency difference or beat note can thus be used to measure rotation, hence the term ring laser gyro. However, backscattering radiation couples the two oppositely directed light beams and, for low rotation rates, causes the beams to frequency lock. That is, the beat note becomes zero even though the rotation rate of the gyro is not zero. The beat note 0 as a function of the beat rate is mathematically described by: 0=a +bsin(-), where a is essentially the rotation rate and b is the backscattering coefficient. So long as a is much greater than b in the equation above, frequency locking does not occur.However, where a < locking occurs causing f to vanish.

One way to eliminate the mode-locking problem is to frequency modulate the gyro by an optically non-reciprocal technique, e.g., by mechanically dithering the entire system; Such a system for mechanical dithering is shown in U.S.

Letters Patent 4,1 5,004, by Thomas J.

Hutchings and Virgil E. Sanders which issued on September 19, 1978 and is assigned to Litton Systems, Inc.

Another way to eliminate the mode-locking problem is through the utilization of a device which works on the principle of optical dithering.

That is, the oppositely directed light beams are created by the excitation of the laser medium. If the laser is pumped with additional energy beyond that which is needed to produce the first mode pair of light beams, the laser will produce an additional mode pair corresponding to the next longitudinal mode. This so-called four-mode laser is described in a paper entitled "Novel Multioscillator Approach to the Problem of Locking In Tow-Mode Ring-Laser Gyros", by Marlan O. Scully, Virgil E. Sanders and Murray Sargent Ill, Optics Letters, Vol. 3, page 43, August 1978.

Although mechanically dithering and, to a lesser extent, optical dithering of a laser body have been established as a practical design, simpler and more practical devices are of current interest.

According to the present invention, there is provided a ring laser gyro comprising: a laser body having a passageway therein; a laser medium filling said passageway; means for generating two modes of laser oscillation within said laser medium which modes propagate in opposite directions along said passageway; and means for causing oscillatory flow of said laser medium within said passageway, whereby said oscillatory flowing medium causes a frequency difference between said two laser modes to oppose frequency locking of said modes.

The simpler device which can be constructed according to the present invention is an acoustically dithered ring laser gyro in which the dielectric medium which forms the laser medium is caused to flow in an oscillating fashion. As the oppositely directed light beams pass through the oscillating dielectric medium, the medium causes a frequency shift of each of the beams as predicted by the Fresnel-Fizeau effect. By modulating the magnitude and direction of the flow of the dielectric medium, the effects of dithering either mechanically or optically can be minimised.

In 1818, Fresnel deduced from the ether theory that the velocity of light in a dielectric medium V moving along the direction of propagation should be: V=c/nVrn(1 - 1/n2) where n is the refractive index of the medium and Vm is the velocity of the medium. The existence of the Fresnel-Fizeau effect or Fresnel drag has been confirmed by various experimenters including Fizeau who first did so in 1851. For an example of a more recent paper, see "The Ring Laser" by Warren M. Maced and Earl J. McCartney published in Sperry Rand Engineering Review, Vol. 8, Spring 1 966. This paper deals with the use of a ring laser to test the effect of moving dielectrics through the laser path. A second paper which discusses Fresnel drag is entitled "A Precision Measurement of Fresnel Drag In a Ring Laser", by Walter K.

Stowell, Oklahoma State University, Ph.D., 1974, Engineering, electrical, published by Xerox University Microfilms, Ann Arbor, Michigan, No. 75-8899. In this paper, a disk of fused silica is used to demonstrate and measure Fresnel drag.

While ring lasers have been used to measure Fresnel drag, the prior art does not disclose a ring laser using Fresnel drag as a means of frequency modulations to prevent mode locking and thus create a ring laser gyro.

Other methods for eliminating the problem of mode locking may be found in the prior art including U.S. Letters Patent 3,533,014 which describes a method of oscillating the mirrors within a ring laser gyro; U.S. Letters Patent 3,612,690 which describes a method of random electrical dithering; U.S. Letters Patent 3,721,497 which describes a technique for modulating the laser beam electrically, and U.S. Letters Patent 3,743,969 which describes yet another electrical dithering approach.

A preferred embodiment of the present invention is an acoustically dithered ring laser gyro that prevents frequency locking due to radiation backscattering at low displacement rates and eliminates the need for prior art mechanical or optical dithering. The ring laser gyro is formed within a body having passageways configured in a closed loop and filled with a laser medium such as a mixture of helium and neon gas. The laser medium is dielectric and thus susceptible to the Fresnel-Fizeau effect when placed in motion.The motion of the medium causes the acceleration or deceleration of a light beam passing through it depending on whether the medium is moving with or against the direction of the light The acceleration or deceleration of the light beam causes a change in its ultimate frequency so that focusing the beam upon a surface, in combination with a second beam, causes a change in the interference fringes due to the cancellation and/or reinforcement of the electromagnetic energy within the beams. The changing interference fringes are then detected by heterodyne detection to indicate the rotation rate of the ring laser gyro.

The dielectric medium within the laser passageways is placed in motion, through the utilization of an acoustic dithering mechanism, such as a diaphragm or plunger. The diaphragm or plunger may be mounted in a cavity separate and apart from the passageway and caused to oscillate by the use of an electromagnetic coil. Alternatively one may form a diaphragm from a piece of piezoelectric material. An alternative arrangement for creating the flow of the laser medium is to place a toroidally shaped diaphragm or plunger directly within the laser passageway and again drive the diaphragm or plunger by either an electromagnetic coil or a piezoelectric transducer.

Through this arrangement, it is possible to eliminate the need for a mechanical dithering system or an optical system in which third and fourth modes are generated.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a top plan view, partially in section, showing a typical ring laser gyro which may be utilized in the Dresent invention; Figure 2 is a partial section taken along lines 2-2 of Figure 1 showing an acoustic dithering mechanism: Figure 3 is a partial section taken along lines 3-3 of Figure 1 showing another acoustic dithering mechanism; Figure 4 is a partial section taken along lines 4--4 4 of Figure 3; and Figure 5 is a view similar to Figure 3 showing a further acoustic dithering mechanism.

Figure 1 illustrates a typical ring laser gyro 10 which is formed in a body 12, e.g., quartz or an ultra-low expansion material, such as titanium silicate. The laser body 12 is constructed with four passageways 14 arranged to form a closed rectangular path therein. The passageways 14 are sealed to retain a gas mixture consisting of approximately 90% helium and 10% neon in a vacuum of approximately 3 torr, it being understood that atmospheric pressure is approximately 760 torrs.

In accordance with known laser practice, the body 12 is provided with two cathodes 16 and 18 and two anodes 20 and 22 which are secured to the body in a manner that is well known in the art.

A gas discharge is established between cathode 1 6 and anode 20 in passageway 14 as well as between cathode 18 and anode 22. A getter 24 is provided to absorb impurities found within the gas in the passageway 14. Mirrors 28, 30, 32 and 34 are located at the four corners of the optical path formed within the passageway 14 of the ring laser gyro 10 wherein two of the mirrors 28 and 34 are mounted upon photodetection output devices 36 and 38, respectively. The photodetection devices measure the beat frequency of oppositely directed light beams to indicate the rotation of the ring laser gyro 10 by heterodyne detection, as is known.

Mounted upon the outer surface of the laser body 12 are a pair of acoustic dithering mechanisms 40 one of which is best seen in Figure 2, wherein passageway 14 has been rotated 900 for clarity. The acoustic dithering mechanism 40 is formed within a cylindrical housing 42 constructed from a non-magnetic material having a threaded end which engages a threaded aperture 44 within the laser body 12.

The threaded aperture 44 is reduced at its inner end to form a shoulder 46 and chamber 47. The innermost end surface of aperture chamber 47 terminates in a bored opening which is drilled at an angle to form a second passageway 48 into ;he passageway 14. An O-ring 50 is provided between the shoulder 46 and the innermost end of the housing 42 to seal the housing as it is threaded into the laser body 12. The angled arrangement of the second passageway 48 ensures that the oscillatory displacement of the laser medium within passageway 14 will be a laminar flow without injection of cavitation or other anomalies which could affect the Fresnel Fizeau effect.The pair of acoustic dithering mechanism 40 is arranged to produce a push-pull medium flow and thus an oscillatory displacement between the two angled passageways 48 whereby the Fresnel-Fizeau effect occurs only in the relatively short distance therebetween as shown by arrow 49.

The acoustic dithering mechanism 40 includes a spool 52 inserted into housing 42 about which is wrapped transformer wire 54 connected to terminals 56. A longitudinal bore 58 within the spool 52 receives a rod 60 constructed from corrosion-resistant magnetic steel. The lower end of the rod 60 is connected to a plunger diaphragm 62 which slidablyfits in the chamber 47. A spring 64 urges the plunger 62 in a downward or inward direction. As is well known, an alternating current across terminals 56 will cause the spring loaded plunger 62 to be drawn in an upward direction against the urging of spring 64 into the spool 52 due to the electromagnetic field established by the transformer wires 54. As the field collapses, due to the alternating current, the plunger will be forced down under the urging of spring 64. This will cause the laser medium within passageway 14 to flow in an oscillatory manner due to the pumping action created by the plunger diaphragms 62 of each mechanism 40 as they force medium in and out of the second passageways 48 in a push-pull combination.

An alternative arrangement to the acoustic dithering mechanism 40 is shown in Figure 1 by a second acoustic dithering mechanism 70. As best seen in Figures 3 and 4, the second acoustic dithering mechanism 70 is mounted within a drilled aperture 72 located in the side wall of the laser body 12. The drilled aperture 72 passes through the passageway 14 and is machined at its outermost periphery with a threaded section 74.

The acoustic dither mechanism 70 is formed within a non-magnetic housing 76 which is cylindrically shaped to fit into the aperture 72 as a plug. A bored chamber 78 passes through the housing 76 at right angles to the major axis of the housing. This bore 78 is arranged to be concentric with the passageway 14. Fitted into the bore 78 is a tubular spool 80 formed of non-magnetic material and provided with a recessed cylindrical surface 81 to receive and retain transformer wires 82. The bore 78 within the cylindrical housing 76 substantially reduces the lateral working area within which the acoustic dither mechanism formed in spool 80 may be mounted, as best seen in Figure 4. The spool 80 need not be retained within the bore 78 of housing 76 with any positive retaining device other than bonding as the inner surfaces of the aperture 72 prevent the lateral movement of the housing 76.

The non-magnetic spool 80 is formed with a longitudinal bore 84 which is enlarged at one end to form a central shoulder 86. Slidably mounted within the bore 84 is a tubular plunger 88 formed from magnetic material having a disk diaphragm 90 attached at one end whose centre is provided with an aperture 92 which provides passage for the oppositely directed light beams within the passageway 14. The plunger 88 is retained within the bore 84 by a C-shaped spring clip 94 positioned within a groove in the inner surface of the bore 84. A spring 96 urges the plunger 88 and diaphragm 90 against the clip 94. Application of an alternating current across terminals 98 connected to the transformer wire 82 creates an electromagnetic field which pulls the magnetic plunger 88 into the coil formed by the transformer wire 82 against the urging of spring 96.Removal of the current causes the plunger and diaphragm, 88 and 90, to move in the opposite direction. In this manner, the plunger and diaphragm cause the laminar flow of the laser medium at a frequency established by the signal applied across the terminals 98.' The upper or outermost end of the housing 76 is provided with a groove 100 into which an 0ring 102 is placed. Once the housing 76 has been inserted into the drilled aperture 72 the O-ring 102 seals the passageway 14. The housing 76 is retained within the aperture 72 by a cap 104 which threadably engages the threaded section 74 and mounts the terminals 98 to complete the acoustic dithering mechanism 70. The electromagnetic coil arrangement shown in Figures 3 and 4 may be replaced with a piezoelectric crystal as described below.While only one dither mechanism 70 is shown, it will be understood that a pair of these mechanisms may be used in a push-pull combination as described above. However, the configuration of mechanism 70 lends itself to a single unit application.

Another embodiment of the acoustic dithering mechanism may be seen in Figure 5. In this arrangement, an acoustic dithering mechanism 110 is formed in a counterbored cavity 11 2 located in a side wall of the laser body 12. The counter-bore 112 connects to the passageway 14 through a second passageway 114. This passageway 114 is set at an angle to the passageway 14 to encourage laminar flow of the displaced laser medium as described above with regard to Figure 2. The upper portion of the counterbored cavity 112 is expanded to form a shoulder 11 6. Placed against shoulder 11 6 is a diaphragm 11 8 formed from a disk of piezoelectric material. The outer periphery of the counterbored cavity 112 is threaded to recejve a cap 120 which mounts terminals 122.An O-ring 124 is placed against the upper, outer surface of the piezoelectric diaphragm 118 and the cap 120 is urged against the O-ring to seal the cavity 112.

Attached to the terminals 1 22 are spring-loaded contacts 126 which contact opposite ends of the piezoelectric diaphragm 11 8. By impressing a voltage across the terminals 122, the piezoelectric diaphragm is displaced as a voltage is established across it to increase or decrease the volume of cavity 112 and thus establish an oscillatory flow of the laser medium within passageways 114 and 14.

As best seen in Figure 5, it is again desirable to utilize two acoustic dithering mechanisms 110 having the second passageways 114 thereof arranged at conversing angles to the passageway 14 such that the first acoustic dithering mechanism 110 will cause the flow of the laser medium in one direction while the second acoustic dithering mechanism 110 causes a flow in the second direction. In this manner, the two dither mechanisms 110 are arranged in a pushpull configuration to enhance the oscillatory flow of the laser medium as shown by arrow 49.

The various acoustic dithering mechanisms described above including mechanisms 40, 70 and 110 may be varied within the configurations shown or other configurations which will become apparent to those skilled in the art. Whether an electromagnetic coil or a piezoelectric transducer is utilized to urge the laser medium into an oscillatory flowing motion is a matter of design preference. The important feature of the present invention is the realization that, by placing the laser medium into a flowing condition, the ring laser gyro 10 may be acoustically dithered. This acoustical dithering eliminates the need for either mechanical or optical dithering known in the prior art.As described above, the flowing medium causes the velocity of the oppositely directed light beams to change as that electromagnetic energy passes through the flowing medium due to the index of refraction in accordance with the Fresnel Fizeau effect. While the prior art utilized ring lasers to study the Fresnel- Fizeau effect or Fresnel drag, the prior art did not teach the utilization of that effect to dither a ring laser gyro and thereby prevent mode locking caused by the tendency of the oppositely directed light beams to lock at similar frequencies when the displacement rate of the gyro is small.

The acoustic dithering mechanisms described by the present invention are comparable to mechanical dithering in that both the amplitude and frequency may be varied and controlled. It has been found that optical dithering, wherein a third and fourth mode are generated optically, cannot be controlled to the same degree as the amplitude and frequency are fixed by the parameters of the system. Thus, the foregoing describes an arrangement in which the advantages of control found in a mechanical system are retained while the numerous expensive and cumbersome elements of a mechanical dithering system have been eliminated through the utilization of simplistic acoustic mechanisms.

Claims (12)

1. A ring laser gyro comprising: a laser body having a passageway therein; a laser medium filling said passageway; means for generating two modes of laser oscillation within said laser medium which modes propagate in opposite directions along said passageway; and means for causing oscillatory flow of said laser medium within said passageway, whereby said oscillatory flowing medium causes a frequency difference between said two laser modes to oppose frequency locking of said modes.

2. A ring laser gyro as claimed in claim 1, wherein said means for causing the oscillatory flow of said laser medium comprises a displaceable wall element.

3. A ring laser gyro as claimed in claim 2, wherein said wall element extends toroidally about said passageway.

4. A ring laser gyro as claimed in claim 2; wherein said means for causing oscillatory flow includes a second displaceable wall element arranged to be driven in a push-pull sequence with the first-mentioned wall element.

5. A ring laser gyro as claimed in claim 2 or 4, wherein the or each wall element is within a chamber spaced from said passageway and joined thereto by a second passageway.

6. A ring laser gyro as claimed in claim 5, when appended to claim 4, wherein the wall elements are in respective chambers joined to the firstmentioned passageway by respective second passageways, which converge towards the firstmentioned passageway.

7. A ring laser gyro as claimed in any one of claims 2 to 6, wherein the or each wall element has electromagnetic driving means.

8. A ring laser gyro as claimed in claim 2, wherein said wall element is diaphragm means having piezoelectric transducer driving means.

9. A ring laser gyro comprising: a laser body having a passageway therein; a laser medium filling said passageway; means for exciting said laser medium to generate light waves in opposite diredtions around said passageway of said ring laser; and means for causing said laser medium to flow in first one and then another direction so that the Fresnel-Fizeau effect on said oppositely directed light waves in said flowing laser medium creates a frequency difference within said oppositely directed light waves that Qpposes frequency locking of said oppositely directed waves.

10. A ring laser gyro as claimed in claim 9, wherein said means for flowing said laser medium includes oscillatory diaphragm means.

11. A ring laser gyro as claimed in claim 10, wherein said oscillatory diaphragm means includes a pair of acoustic dither mechanisms mounted in chambers juxtaposed to said passageway and joined thereto by second passageways arranged to cqnverge towards said first-mentioned passageway.

12. A ring laser gyro substantially as hereinbefore described with reference to Figures 1 and 2, or 1, 3, 4, or 1 and 5 of the accompanying drawings.