BACKGROUND OF THE INVENTION 
     This invention relates to ring laser angular rate sensors. In particular, it relates to the avoidance of lock-in in such sensors by moving at least one mirror which defines the ring laser optical closed-loop path in such a manner so as to cause the average error produced by wave scattering effects to be reduced to zero. 
     Reference is hereby made to U.S. Pat. No. 3,373,650 entitled, &#34;Laser Angular Rate Sensor&#34; and U.S. Pat. No. 3,467,472 entitled, &#34;Random Bias For A Ring Laser Angular Rate Sensor&#34; both by J. E. Killpatrick, and U.S. Pat. No. 3,390,606 entitled, &#34;Control Apparatus&#34; and U.S. Pat. No. 4,152,071 entitled, &#34;Control Apparatus&#34; both by T. J. Podgorski, all of which are assigned to the same assignee as the present application. Reference is also hereby made to a publication entitled, &#34;The Laser Gyro&#34; by Frederick Aronowitz, Laser Applications Vol. 1, 1971. 
     The references cited above illustrate the principles and problems of ring laser angular rate sensors well known in the art. A ring laser angular rate sensor usually comprises two countertraveling waves or beams in the form of laser beams which propagate about an optical closed-loop path defined by a plurality of wave reflecting means or mirrors. In such sensors there exists a phenomenon known as lock-in which is primarily attributed to wave or light scattering, at the mirrors, of the impinging wave. The light scattering causes the phenomenon known as lock-in which is a major source of angular rate information error produced in such sensors. 
     In the aforementioned patents, a method known as dithering is utilized to reduce the effects of lock-in. These techniques are well known and have their advantages and disadvantages well known in the art. Such dithering techniques utilize the concept of providing an alternating bias to the frequency difference between the counterpropagating waves. Doing so allows a marked improvement in ring laser angular rate sensors which enhances their capabilities for obtaining low error angular rate information. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides a method and apparatus applicable to ring lasers for modulating scattered waves reflected from the mirrors which define the ring laser optical closed-loop path in such a way to reduce the average error produced by lock-in effects to low levels approaching an average of zero. In the present invention the scattered waves are modulated at a constant rate and an integer multiples of 2π radian phase change. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an optical closed-loop path employed in a ring laser. 
     FIG. 2 is a schematic block diagram showing one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Shown in FIG. 1 is a diagrammatic representation of a ring laser optical closed-loop path in the form or a triangle defined by the orientation of mirrors 1, 2, and 3. As illustrated in FIG. 1, counterpropagating laser beams will travel along the optical closed-loop path along laser beam path segments 10, 11, and 12. Beam segment 10 impinges upon and reflects from mirror 1 at an angle α with respect to the normal to mirror 1; beam segment 11 impinges upon and reflects from mirror 2 at an angle β with respect to the normal to mirror 2; and beam segment 12 impinges upon and reflects from mirror 3 an angle γ with respect to the normal to mirror 3. 
     Assume that mirrors 2 and 3 both move to the left, as illustrated in FIG. 1 by a distance D parallel to the surface of mirror 1. Also assume that mirror 2 moves a distance B along a direction parallel to the surface of mirror 2, and mirror 3 moves a distance C along a path parallel to the surface of mirror 3. 
     Displacement of mirrors 2 and 3 by D causes the impinging laser beam to move a distance D across mirror 1. Simple optics and wave mechanics dictate that the source of backscattered waves reflected from mirror 1 relative to a point on the impinging beam is displaced by a distance, 
     
         ΔL.sub.1 =2D sin α                             (1) 
    
     In turn, the phase of the backscattered wave at mirror 1 changes by ##EQU1## where λ is the wave length of the impinging wave, and Δφ 1  is the number of 2π radian phase changes. 
     Similarly, the lateral motions B and C of mirrors 2 and 3 generate phasing shifts in the backscattered waves reflected from mirrors 2 and 3 of ##EQU2## 
     If mirror motions of mirrors 2 and 3 are constrained to motion along a path perpendicular to the surfaces of mirrors 2 and 3 respectively, and using simple geometry and trigonometry, the following constraints are defined: 
     
         β=D sin (90°-α-β)=D sin γ     (5) 
    
     
         C=D sin (90°-α-γ)=D sin β          (6) 
    
     With the above constraints, and assuming again that the distance D moved by mirrors 2 and 3 is identical, then ##EQU3## From equation (2), ##EQU4## 
     Consider now the effect of dithering the mirrors with rapid triangular push-pull motion normal to the surfaces of mirrors 2 and 3. Assume that the push-pull motion is such that the optical path length remains substantially constant similar to that depicted in FIG. 1. If the peak-to-peak amplitude of Δφ 1 , Δφ 2 , and Δφ 3  can all be made integer multiples of 2π radians, the backscatter look-in errors will all average to zero as will be subsequently shown. 
     As is well known, backscatter lock-in errors are sine/cosine functions of the backscattered phase angles as illustrated in the above referred to publication. The modified lock-in equation including modulation of the scattered waves is: 
     
         ψ=θ+Ω.sub.L sin (ψ+ω.sub.D t)    (10) 
    
     where 
     θ  incremental rotation input 
     Ω L    lock-in rate 
     ψ  gyro output phase angle 
     ω D    triangular modulation of phase of the scattered wave 
     If the backscattered waves are modulated integer multiples of 2π, then ##EQU5## where M is the integer multiple of Δφ 2  and Δφ 3 , and N is the integer multiple associated with Δφ 1 . 
     Consider now the special case solution where M=1 and N=2. This selection requires an equilateral triangle where, 
     
         α=β=γ=30°                          (12) 
    
     and 
     
         T=1/3                                                      (13) 
    
     In these circumstances, the push-pull mirror modulations along the normal to mirrors 2 and 3 will have to be λ√3, peak-to-peak to meet all of the imposed constraints. 
     The triangular dither frequency of modulating the mirrors should be reasonably high compared with the actual lock-in rate of the sensor. For example, a typical lock-in rate is 90 Hertz (2π phase changes/sec.) A typical push-pull mirror dither frequency should then be some reasonable multiple thereof, say 10-fold, or 900 Hertz. If (i) mirrors 2 and 3 are modulated at a constant rate, and if (ii) the phase modulation of the scattered waves is an integer multiple of 2π radians, the average error rate becomes zero. 
     If mirrors 2 and 3 are modulated at a constant rate so as to provide a constant phase change rate ω, and for a time interval ωt=0 to 2π, the average composite error rate for mirrors 2 and 3 becomes: ##EQU6## The average error rate for mirror 1 being one-half that of equation (15) because the phase rate change of mirror 1 is twice the ω rate due to modulation of mirrors 2 and 3. With triangular modulation, ω switches back and forth between plus and minus ω. Hence, the average value of the error is reduced to zero over a complete dither cycle or dither modulation of the phase of the backscattered waves. This gives an effective zero bias effect and thus zero lock-in error. 
     In practice of the invention, mirrors 2 and 3 are modulated perpendicular to their respective surface sufficient to move mirrors by a displacement D to meet the above necessary constraints. In order for mirror 2 to move by D, mirror 2 must be moved a distance D cos γ, similarly mirror 2 must be moved D cos β. The value of D is determined from modified equation (2) by the expression: ##EQU7## 
     FIG. 2 is a block diagram showing an implementation of the invention of the present application. There shown is a ring laser angular rate sensor 200 having mirrors 1, 2, and 3. Mirror 2 is coupled to transducer 220 and mirror 3 is coupled to transducer 330. Transducer 220 has a control input 221 electrically connected to signal means 250 through summing means 260. Transducing means 330 has a control input electrically connected to signal means 250 through summing means 265 and inverting means 270. 
     Signal means 250 provides a triangular wave signal which is presented to transducing means 220 and 230 for modulating mirrors 2 and 3 respectively along a path perpendicular to the surfaces thereto. Inverter means 270 inverts the signal provided by signal means 250 so that mirrors 2 and 3 vary in position in opposite sense so as to substantially maintain the optical path length constant. (Inverter means 270 could be eliminated by an appropriate arrangement of electrical connections between transducer 230 and signal means 250.) 
     It is desirable to maintain the path length constant through a more precise path length control means 280 for providing average position control of mirrors 2 and 3 through transducing means 220 and 230 respectively. As illustrated in FIG. 2, the path length control means 280 provides a signal input to transducing means 230 through summing means 265 which sums the output of path length control means 280 and the output of inverting means 270. Similarly, the output of path length control means 280 is provided to transducing means 220 through summing means 260. 
     In operation, path length control means 280 varies the average position of mirrors 2 and 3 through a feedback control system well known in the art. Usually, path length control means 280 is responsive to beam intensity of at least one of the counterpropagating laser beams of sensor 200. Signal means 250 provides a triangular wave signal of sufficient amplitude to modulate the positions of mirrors 2 and 3 to phase modulate the backscattered waves an integer multiple of 2π radians in a manner as already described. 
     There are, of course, many modifications which may be made to the invention as presented. Although the invention of the present application has been shown particularly with a triangular shaped ring laser angular rate sensor, the principles of the present application may be applied to other forms.
The present invention provides a method and apparatus applicable to ring lasers for modulating scattered waves reflected from the mirrors which define the ring laser optical closed-loop path in such a way to reduce the average error produced by lock-in effects to low levels approaching an average of zero.