RING LASER ANGULAR RATE SENSOR

Abstract: A ring laser (10) is provided with at least one retro­ reflector (50) external to the ring laser closed-loop path for redirecting energy extracted from one of the counter-­ propagating beams into the opposite traveling counter-­ propagating beam. The redirected energy is modulated by means (55, 56) so as to oppose the normal coupling between the counterpropagating beams which causes lock-in.

Description: The present invention relates to ring laser angular rate sensors according to the preamble of claim 1, and more particularly to means for providing antilocking biasing within such a sensor. Ring laser angular rate sensors are well known in the art of inertial navigational systems. Some examples of ring laser angular rate sensors are shown and described in U.S. patents 3 323 411, 3 373 650, 3 390 606, 3 467 472, and 4 152 071. Ring laser angular rate sensors comprise a pair of laser beams which travel or propagate in opposite directions along a closed-loop path or cavity. Rotation of the sensor causes the frequency of the laser beams to change and exhibit a frequency difference which is proportional to the rotation rate of the sensor. However, at low rotation rates, the laser beams tend to "lock-in" to a common frequency. The "lock-in" phenomenon is thought primarily to be caused by backscattering of the laser beams along the closed-loop path, and particularly at the reflection surfaces defining the closed-loop path. Research and development related to the ring laser angular rate sensor has been primarily directed to apparatuses and methods to avoid, reduce, or correct for the effects of lock-in. US Patent 3,323,411 recognized that the backscattering energy could be neutralized by introducing back into the cavity or closed-loop path a beam of energy equal in amplitude but opposite in phase to the sum of the undesirable reflections which cause the backscattering. Since opposite phase beams neutralize each other, the controlled redirected beam could cancel out the backscatter energy and thus minimize or prevent lock-in. U.S. Patent 3 697 181 which corresponds to the preamble of claim 1, also redirects energy back into the cavity, but the redirected energy is phase and amplitude controlled in response to amplitude modulation of the countertraveling laser beams which is a function of backscattering. U.S.Patent 3,373,650 takes an altogether different approach to minimizing the effects of lock-in. The '650 patent teaches the use of a mechanical rotation or dither which rotationally oscillates the sensor back and forth. This oscillation causes the frequencies between the two countertraveling laser beams to vary in direct relationship to the rotation rate caused by the mechanical dithering. Mechanical dithering is known to reduce lock-in substantially. Departing from the known devices, it is therefore the object of the present invention to provide improved means for reducing lock-in. This object is achieved according to the characterizing features of claim 1. Further advantageous embodiments of the present invention may be taken from the subclaims. According to the present invention a ring laser is provided with at least one retroreflector external to the ring laser closed-loop path for reflecting energy extracted from one of the counterpropagating beams into the opposite traveling counterpropagating beams. The redirected energy is continuously dithered or modulated so as to substantially obviate the effects of lock-in. With respect to the embodiments shown in the attached drawing, the invention shall be further explained. It shows: Figure 1 a schematic diagram showing one embodiment of the present invention. Figure 2 a schematic diagram showing an embodiment of the present invention employing an optical filter. Figure 1 shows a schematic diagram of a laser angular rate sensor including the apparatus of the present invention. There shown is a ring laser angular rate sensor 10 comprising a cavity 15 which includes mirrors 20, 21, and 22 which define an optical closed-loop path. Laser beams 30 and 40 are generated to travel in opposite directions about the closed-loop path defined by mirrors 20, 21 and 22 in a well known manner. Mirror 22 is shown to be partially transmissive thereby extracting a portion of laser beam 40 as designated by arrow 40x, and extracting a portion of laser beam 30 as designated by arrow 30x. Extracted beam 40x propagates along a path toward retroreflector 50 which in turn redirects a beam of energy designated by arrow 40r. Extracted beam 30x propagates along a path toward retroreflector 60 which in turn redirects a beam of energy designated by arrow 30r. Coupled to retroreflectors 50 and 60 are transducing means 55 and 65 respectively. Transducing means 55 is shown controlled by signal generator 56, and transducing means 65 is shown controlled by signal generator 66. Signal generators 56 and 66 may be the same generator, but may be phase separated. Transducing means 55 and 65 may be by way of example a piezoelectric device which causes the point of reflection of beams 40x and 30x to move in a direction parallel to the path traversed by beams 40x and 30x respectively. The operation of the embodiment of the invention shown in FIGURE 1 will now be described. Normal scattering energy from beam 40 into the direction of beam 30 is illustrated in FIGURE 1 by a wave designated 40s; and scattered energy from beam 30 into beam 40 is designated by a wave 30s. As a result of retroreflector 50, beam 40r is injected back into the closed-loop path so as to travel substantially coaxial with laser beam 30 and beam 40s. If beam 40r is unattenuated, beam 40r will be more significant than beam 40s. Therefore, the lock-in rate of the sensor system 10 will be directly related to the energy injected into the closed-loop path, namely, beam 40r. Application of a sinusoidal voltage to transducing means 55 will vary the position of the retroreflector 50 and cause the phase of beam 40r to vary or modulate. If transducing means 55 is capable of varying the position of retroreflector 50 ± one-half of a wave length of the laser light beam, the phase of the laser beam 40r will vary ± π radians. Furthermore, if the signal means 56 is sinusoidal, then beam 40r will sinusoidally vary in phase ± π radians. Because of the modulating phase of the retroreflected beam 40r, the two laser beams will be unlocked for a majority of the time during the low rotation rates. The behavior of the beams due to a modulating phase of beam 40r is somewhat analogous to a modulated frequency difference caused by mechanical dither. The total variation of phase change is not critical but it is desired to be greater than π radians. Depending upon the quality of the retroreflection system, it may be unnecessary to pass beams 40x and 40r through an attenuator 70 having a control input (not shown). The purpose of attenuator 70 is to reduce the amount of energy of beam 40r returned into the closed-loop path so as to have a magnitude somewhat greater than the backscattered energy beam 40s. A factor of 10 is not unreasonable. In a similar manner, retroreflector 60 provides beam 30r so as to be redirected into the closed-loop path to travel substantially coaxial with laser beam 40. Attenuator 71 provides attenuation like attenuator 70. Application of a sinusoidal voltage to transducing means 65 varies the position of retroreflector 60 thereby also capable of unlocking the laser beams. In some circumstances it may be desirable to provide both retroreflected beams 30r and 40r concurrently so as to be the primary scattering waves traveling in opposite directions. Sinusoidally varying these waves will therefore sinusoidally vary lock-in and thus produce an overall lock-in reduction, and thereby improve the performance of the sensor by use of a single modulated retroreflected beam. The system as just described provides a mechanization for preventing mode locking using modulated retroreflected energy. Either retroreflected beam 30r or 40r may be utilized to prevent mode pulling thus avoiding lock-in. The embodiment shown in FIGURE 1 may be implemented in a somewhat different fashion as illustrated in FIGURE 2. Shown in FIGURE 2 is beam 40x which impinges upon a switched optical filter 200. Filter 200 is electrically controlled by control oscillator 210. When no voltage is applied to filter 200, the filter is transmissive, and beam 40x transmits through filter 200 as designated by beam 40x'. Beam 40x' traverses a path length "d" so as to impinge upon a retroreflecting means 50 thereby providing a retroreflected beam 40r'. Beam 40r' is redirected back through filter 200 back into the cavity similar to beam 40r as illustrated in FIGURE 1. On the other hand, when a sufficient voltage is applied to filter 200 by means 210, filter 200 becomes reflective and thereby acts as a retroreflection means causing a retroreflected beam 40r''. Beam 40r'' is redirected into the closed-loop path similar to beam 40r illustrated in FIGURE 1. In operation, the apparatus as shown in FIGURE 2 provides a redirected beam of energy into the closed-loop path having a varying phase as controlled by oscillator 210 and the distance between filter 200 and retroreflection means 50. The change in length between filter 200 and retroreflection means 50 causes a phase change in the beam redirected into the closed-loop path. The phase is dependent upon the total path length 2d, and must be less than an integral number of wave lengths of the laser beam. It is desirable that 2d should be "n + 1/2" wave lengths where n is an integer. The behavior of the system is that as already described, namely, the varying phase forces the two laser beams to be unlocked because of the frequency unlocking action of the retroreflected energy. As is well known, the retroreflection means 50 may be provided by a variety of apparatuses including, among others, phase conjugate mirrors and corner cube prisms and the like.

Claims: A ring laser angular rate sensor comprising means (15) for generating first and second laser beams in opposite directions along a closed-loop path, means (22) for extracting a portion of said first laser beam from said closed-loop path, first redirecting means for redirecting said extracted portion of said first laser beam to travel along said closed-loop path in the direction of said second laser beam, said redirecting means including retroreflection means (50) and means (55,200) for modulating the phase of said redirected first beam portion, characterized in that said redirecting means is operated by an externally generated oscillating signal (56,210) serving to continuously modulate the phase of the redirected energy in response to said external signal. Sensor according to claim 1 further comprising means (22) for extracting a portion of said second laser beam from said closed-loop path, and second redirecting means for redirecting said extracted portion of said second laser beam to travel along said closed-loop path in the direction of said first laser beam, said second redirecting means including retroreflecting means (60) and (65,200) for modulating the phase of said redirected second laser beam portion, characterized in that said second redirecting means is operated by an externally generated oscillating signal (66,210) serving to continuously modulate the phase of the redirected energy in response to said external signal. Sensor according to claim 1 and 2 further characterized by amplitude control means (70,71) for controlling the amplitude of said redirected first and second laser beam portion. Sensor according to claims 1 and 2, characterized in that said retroreflection means (50,60) are coupled to piezoelectric transducing means (55,65). Sensor according to claim 4, characterized in that said externally generated oscillating signal (56,66) is sinusoidal and is controlling said piezoelectric transducing means (55,65). Sensor according to claims 1 and 2, characterized in that said first and second redirecting means include: optical means (200) capable of being selectively reflective and transmissive in response to a control voltage presented thereto: said optical means and said retroreflection means being in the path of said extracted portion of said laser beam; and oscillating means (210) for periodically providing said control voltage to said optical means (200). Sensor according to claims 1 and 2, characterized in that said redirected first or second laser beam portion travels along said closed-loop path substantially coaxial with said second or first laser beam.

Inventors: UPTON, ROBERT W., JR.
Applicants: HONEYWELL INC.
Application year: 1984
Main classification: H01S3
IPC codes: G01C19, G01P9, H01S3