Abstract:
A path length controller for a ring laser gyro wherein the path length is dithered by dithering of one or more of the mirrors and the mirror or mirrors are controlled by a servo which is selectively biased in response to the servo signal to keep the mirror or mirrors in a position to support the largest amplitude of the principal mode of laser oscillation.

Description:
BACKGROUND OF THE INVENTION 
     It is important to hold the path length of a ring laser gyro at an optimum length wherein the principal mode of oscillation is maximum. The principal mode or wave of the laser will oscillate, and the ring cavity will support such wave over a limited excursion from the optimum path length of the laser. 
     When the path length of the ring laser is optimum, the rate of change of the amplitude of the laser light intensity with respect to the position of any particular mirror is zero. In the prior art, a controlled mirror is dithered at a low frequency and amplitude. The dithering of the mirror causes the amplitude of the envelope of the output signal of the ring laser to be modulated at the dither frequency. The envelope of the output signal is then demodulated at the dither frequency, and the demodulated signal is integrated with the integrated signal used to move the average position of the dithered mirror to produce a maximum amplitude of the output envelope. Such control is typically accomplished by servoing the mirror to a position to dither about the position where the rate of change of the amplitude with respect to the mirror position is zero. 
     The above-described servo causes the demodulated amplitude of the ripple on the envelope of the output signal to approach its minimum value, when the average amplitude reaches its maximum value. Consequently, minimizing the amplitude of the ripple moves the cavity mirror to its optimum position. 
     Causing the demodulated amplitude of the ripple on the envelope of the output signal to approach its minimum value causes the cavity to be properly tuned to its principal mode of oscillation only if the starting position of the mirrors is such that the laser oscillation principal mode can be supported. Should the initial position of the mirror be such that it excites a secondary or undesired mode, the mirror must be moved controllably into a different position where it will excite the principal laser mode. 
     BRIEF DESCRIPTION OF THE INVENTION 
     It is contemplated by this invention to servo the mirror to a position where the rate of change of the laser light amplitude with respect to the mirror position is sufficiently high that such rates only occur in the mirror positions where only the principal mode of oscillation can be supported. 
     Upon start of the operation of the ring laser, the mirror positioning servo is controlled to a position to match a predetermined high rate that ensures that the mirror position is within an excursion range wherein only the principal mode of oscillation can be supported. The bias is then removed, and the servo is then successfully servoed into its optimum position wherein the laser light is at its maximum intensity and in its principal mode of oscillation. 
     It is therefore an object of the invention to maintain the mirror positions of a ring laser such that only the principal mode of oscillation is supported. 
     It is a more specific object of the invention to bias the position of at least one mirror of a ring laser to produce a path length which supports only the principal mode of oscillation of the laser. 
     It is a further object of the invention to provide a method for ensuring that a ring laser operates in only its principal mode of oscillation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects will become apparent from the following description taken together with the accompanying drawings, in which: 
     FIG. 1 is a diagram of a ring laser, together with a biased servo for positioning one of the ring laser mirrors according to this invention; and 
     FIG. 2 is a plot of laser path gain against the position of the controlled mirror. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A ring laser 10 is shown schematically in FIG. 1. It is shown with four mirrors 12,14,16,18, but it could have merely three mirrors or more than four mirrors. The ring laser path is shown partly dashed at 20, and it extends entirely around a closed path between the mirrors. Means are used to excite the laser. For example, it is typically excited by applying voltages between a common cathode 11 and a pair of anodes 13 to produce electric fields in a pair of gain sections to stimulate counterpropagating laser waves. 
     As shown in the graph of FIG. 2, such a ring laser can support several modes of oscillation. As shown at 22 in FIG. 2, a ring laser which is properly designed for use as a ring laser angular rate sensor or gyro typically has a principal mode of oscillation which is of substantially higher intensity than that of any other modes of oscillation. The length of the path of the ring laser may be altered by moving any or all of the mirrors. It is preferable, to simplify the tuning, to move only one mirror 18. There are separated ranges 24 of position of the mirror 18, and hence of the cavity length, wherein the principal mode of oscillation can be supported. By way of example and explanation, two of the ranges 24 are shown in FIG. 2, but there are many more such ranges. 
     When the mirror 18, and hence the cavity length, is positioned outside of the ranges 24, in the ranges 26, other secondary or unwanted modes are created. The secondary modes are shown by the curves 28. 
     To tune the ring laser, mirror 18 is positioned by a piezoelectric crystal or other transducer 30. 
     To servo the position of the mirror 18 to the optimum position 32 wherein the maximum laser intensity in the principal mode occurs, it is typical to servo the position of the transducer 30 and mirror 18. A portion of the counter-propagating laser light within the ring laser 10 is extracted by making the mirror 16 only partly reflective. Two photosensors 34, 36 intercept the two counterpropagating rays 35, 37 which are delivered through the mirror 16. Alternatively, only one of the two rays need be intercepted. The outputs of the two photosensors 34, 36 are connected to be summed in the summing amplifier 38. The transducer 30 is dithered by the dither signal source 31 at low amplitude and frequency, usually within the audible range, to produce an alternating signal 39 on the envelope of the light intensity signal at the input of amplifier 38. The output of the amplifier does not oscillate at the light frequency, but is a signal whose amplitude is proportional to the peak intensity of the light and whose frequency is caused by the dither signal. The constant or d.c. component of the signal may be filtered out in the amplifier 38, or it may be filtered through a simple R-C filter 40, 41 to produce a signal which is a measure only of the ripple on the envelope of the light signal. A graph of that ripple is shown at 42, and it is at the frequency of dither of the transducer 30. The dither signal source 31 is connected to the demodulator 44 to demodulate the signal 42. The demodulated signal is shown at 45. The demodulated signal is delivered to the input of an integrator 47 and thence through a power amplifier 49 to the transducer 30. Alternatively, the signal may be delivered through a low-pass filter 46 to the integrator 47. The signal out of the low-pass filter is shown at 48, and it is the average value of the signal 45. Movement of the transducer 30 is in a direction to minimize the amplitude of the signals 42 and 45 and the signal delivered from the amplifier 49, thereby moving the mirror to the position 32 or the position 50. 
     It is not satisfactory to move the mirror to the position 50 because, as shown by the curve 28, the ring laser is then oscillating in its secondary or unwanted modes of oscillation. Such a situation occurs if the mirror is initially in a position within the range 26. 
     To avoid causing the mirror to stabilize at the position 50, it is contemplated by this invention also to deliver the output signal from the demodulator to a summing amplifier 60 in a bias-offset circuit 61. A predetermined amplitude signal is delivered from a potentiometer 62 to the input of the summing amplifier 60. The output of amplifier 60 is delivered through a latch 67 to control the switch 63 when the ring laser is first turned on. The latch control may, alternatively, be a timed control that allows the bias to be connected only for a limited time, or it may be controlled by the power source (not shown), whereby once the switch 63 opens, it will not again close until the power is switched off. 
     Note that the amplitude of the signal 45 is a measure of the slope of the curve of FIG. 2. So long as the amplitude of the signal 45 is less than the amplitude of the signal set on the potentiometer 62, the switch 63 is closed to deliver a predetermined bias signal from the bias source 66 to the input of the integrator 47 . 
     In operation, when the ring laser is started, there is no signal delivered from the demodulator 44 to the amplifier 60, and the voltage delivered from the potentiometer 62 keeps the switch 63 closed. 
     The amplitude of the bias signal from the source 66 is such that the mirror is not servoed to a zero slope such as that shown at 70 or 72 but to a steep slope such as that shown at 77. Note that the slope at 77 must be greater than any slope on the curve 28, and it typically is set about one half the distance up the curve 22. With the switch 63 closed, the mirror 30 is servoed to position 76 which is within the range 24. As soon as the amplitude of the output of the demodulator 44 substantially equals the amplitude of the voltage corresponding to the voltage delivered from potentiometer 62 to the amplifier 60 , the switch 63 opens, and the mirror is immediately servoed to dither about the optimum position 32. 
     The latch 67 prevents the switch 63 from again closing during operation of the ring laser 10 as a gyro. 
     In one form, the latch is closed by the turning on of the power, and it opens after a predetermined time. The predetermined time is adequate to cause the mirror position to be moved into the range 24. When the power to the laser is turned off, the latch is reset so that the signal from 62 may again control and close the switch 63. 
     In still another form, the latch may be connected to be responsive to the output of amplifier 60. Once the signal at the output of amplifier 60 opens the switch 63, the latch 67 is disabled to prevent the reclosing of the switch 63. When the power to the laser is turned off, the latch is reset so that the signal from 62 may again control and close the switch 63. 
     It is to be stressed that modifications may be made in the scale factors of the summing amplifiers, and that the switch 63 may, optionally, be an electronic switch. Various relative speeds of switching may cause the switch 63 to open before the mirror is actually at the position 76. It is only essential that the mirror be within the range 24 when the switch 63 is opened. 
     Although the invention has been described in detail above, it is not intended that the invention be limited by that description, but only by the combination of the specification description and that of the appended claims.