Optical power compensated ring laser gyroscope
BACKGROUND OF THE INVENTION 
     This invention relates to ring laser gyroscopes and in particular to an apparatus and method for compensating a gyroscope output signal to correct for error sources such as optical power variations which produce variations of a polarization split or a dihedral frequency. 
     Multi-oscillator ring laser gyroscopes are a significant new class of rotation sensing instruments employing four waves of two polarization pairs, each polarization pair propagating in opposite circular directions. Such systems are shown and described in U.S. Pat. Nos. 3,741,657, 3,854,819 and 4,006,989 to Keimpe Andringa and assigned to the present assignee, the specifications of those patents being herein incorporated by reference. In such laser systems, circular polarization for each of the four waves is used. The pair of waves, or beams, propagating in the clockwise direction includes both left-hand circularly polarized (LCP) waves and right-hand circularly polarized (RCP) waves as do those waves propagating in the counterclockwise direction. The separation between the LCP waves and the RCP waves in said referenced patents is provided by a crystal rotator which essentially provides a frequency bias (f B ). Such a biased four-frequency or multi-oscillator ring laser gyroscope provides a means for circumventing the frequency locking or lock-in problem present in all conventional or two-frequency laser gyroscopes. This lock-in phenomenon occurs when two traveling waves propagating in opposite directions in a resonant cavity at slightly different frequencies are pulled toward each other to combine in a single frequency standing wave. However, when the frequencies of the counter-rotating waves are sufficiently separated in frequency, the pulling together does not occur. The four-frequency approach may be described as two independent laser gyroscopes operating in a single stable resonator cavity, sharing a common optical path, but static biased in opposite senses by the same passive bias element. In the differential output of these two gyroscopes, the bias then cancels, while any rotation generated signals add, thereby avoiding the usual problems due to drifts in the bias and giving a sensitivity twice that of a single two-frequency gyroscope. Because the bias need not be dithered, the gyroscope never passes through lock-in. Hence, there are no dither-induced errors to limit instrument performance. For this reason, the four frequency gyroscope is intrinsically a low noise instrument, and it is well suited for applications requiring rapid position update or high resolution. 
     The four different frequencies are normally generated by using two different optical effects. First, a crystal polarization rotator has been used to provide a direction-independent polarization causing the resonant waves to be circularly polarized in two directions. The polarization rotation results from the refractive index of the rotation medium being slightly different for RCP and LCP waves. However, a non-planar ring path is used with this invention which inherently supports only circularly polarized waves without the use of a crystal rotator. The non-planar ring path is sometime-s considered to be a dihedral configuration providing the frequency bias (f B ) or polarization split frequency difference separating the circularly polarized waves; this frequency is also referred to as a dihedral frequency (Δf D ). A planar electromagnetic wave ring resonator is shown and described in U.S. Pat. No. 4,110,045 to Irl W. Smith, Jr. and Terry A. Dorschner and assigned to the present assignee. Second, a Faraday rotator is used to provide non-reciprocal polarization rotation, by having a slightly different refractive index for clockwise (cw) traveling waves than for counterclockwise (ccw) traveling waves. This causes the cw and ccw RCP waves to oscillate at slightly different frequencies while the cw and ccw LCP waves are similarly but oppositely split. Thus, a multi-oscillator laser gyroscope operates with right circular polarized waves biased in one direction of rotation and with left circular polarized waves biased in the opposite direction, the bias being cancelled by subtracting the two outputs. 
     An output signal of a ring laser gyroscope drifts with time due to changes in parameters such as temperature and aging. Direct measurement of these parameters generally is not accurate enough or possible. However, gyroscope output accuracy has been improved by measuring the Faraday frequency to sense temperature caused variations and then applying a correction factor to the gyroscope output signal. In this invention, the measurement of the polarization split or the dihedral frequency of a four-frequency laser gyroscope is used to correct the gyroscope output signal for optical power variations and other error sources, such as loss variations due to aging producing a variation of the polarization split or dihedral frequency. 
     SUMMARY OF THE INVENTION 
     The invention discloses an apparatus and method for improving the performance of a ring laser gyroscope by compensating a gyroscope output signal for error sources such as optical power variations in accordance with variations of a dihedral frequency (Δf D ). 
     A laser cavity having a closed path with a gain medium produces a plurality of circularly polarized counter-traveling electromagnetic waves of a first polarization sense and a second polarization sense and a Faraday rotator produces a direction-dependent phase shift to said waves resulting in a frequency splitting between the counter-traveling waves of the same polarization sense, each of the waves being of a different frequency. A combination of these frequencies determines the dihedral frequency which is detected. 
     A gyroscope output signal, which provides the rotation-induced frequency shift (Δf G ) of the electromagnetic waves within the closed path, is generated and controlled in a manner to keep the output signal substantially invariant by means in accordance with variations in the dihedral frequency (Δf D ). 
     One embodiment of the invention discloses a ring laser gyroscope having a cavity comprising a closed path with a gain medium for the propagation of a plurality of electromagnetic waves in opposite directions, each of the waves being of a different frequency. Left and right circularly polarized counter-traveling electromagnetic waves in the closed path are produced by a non-planar ring. This polarization splitting or frequency bias is also referred to as the dihedral frequency (Δf D ). A Faraday rotator produces a direction-dependent phase shift to the counter-traveling waves for each polarization resulting in a frequency splitting of clockwise and counterclockwise waves referred to as the Faraday frequency (Δf F ) . 
     A first detector comprising a high frequency photodiode detects the polarized waves traveling in the same direction which may be either clockwise or counterclockwise. The frequency detected by the first detector is determined by either the traveling waves in a clockwise direction, Δf D  +Δf F  which equals f 4  -f 1 , or the traveling waves in a counterclockwise direction, Δf D  -Δf F  which equals f 3  -f 2 . A second detector detects at least two output signals of the gyroscope cavity, each of the output signals comprising a different combination of a rotation-induced frequency shift (Δf G ) and a Faraday frequency (Δf F ). A first cavity output signal equals the difference between the Faraday frequency and one-half of the rotation-induced frequency shift (Δf F  -1/2Δf G ) which is equivalent to f 4  -f 3 . A second cavity output signal equals the sum of the Faraday frequency and one-half of the rotation-induced frequency shift (Δf F  +1/2Δf G ) which is equivalent to f 2  -f 1 . 
     The outputs from both detectors are sent to a processor which determines the amount of compensation for the gyroscope output signal based on changes in varying parameters of the gyroscope. The processor comprises a memory for storing a scaler quantity which when multiplied by the dihedral frequency provides a compensation factor for the gyroscope output signal. The scaler quantity is determined by a ratio of a rate of change of the gyroscope output to a rate of change of the dihedral frequency. 
     An alternate embodiment of the invention is disclosed utilizing a feedback network for compensating a laser gyroscope output signal for error sources which produce variations in the dehidral frequency. A laser cavity, the same as in the other embodiment, generates circularly polarized counter-traveling waves. A detector means detects the two spacial directions of said waves independently which comprises Δf D  +Δf F , traveling in the clockwise spacial direction and Δf D  -Δf F  traveling in a counterclockwise spacial direction. These circularly polarized counter-traveling waves are combined by circuitry that generates the dihedral frequency which is converted to a voltage. A voltage controlled current source is adjusted by the converted voltage in accordance with the dihedral frequency and the current source controls the gain medium of the laser gyroscope cavity. Adjusting the laser cavity gain as a function of the dihedral frequency for changes in gyroscope parameters results in a gyroscope output signal being compensated by this feedback network. 
     A method of compensating an output signal of a multi-oscillator ring laser gyroscope comprising the steps of producing in a closed path with a gain medium a plurality of circularly polarized counter-traveling electromagnetic waves of a first polarization sense and a second polarization sense, producing a direction-dependent phase shift to the waves resulting in a frequency splitting between the counter-traveling waves of the same polarization sense, each of the waves being of a different frequency and a combination of the waves forming a dihedral frequency, detecting the dihedral frequency, and controlling an output signal in accordance with variations in the dihedral frequency, said output signal being representative of a rotation-induced frequency shift of the waves within the closed path. 
     The invention futher discloses a method of compensating an output signal of a multi-oscillator ring laser gyroscope comprising the steps of producing in a closed path with a gain medium a plurality of circularly polarized counter-traveling electromagnetic waves of a first polarization sense and a second polarization sense, producing a direction-dependent phase shift to the waves resulting in a frequency splitting between the counter-traveling waves of the same polarization sense, each of the waves being of a different frequency, detecting the polarized waves traveling in the same direction, a combination of the waves traveling in the same direction comprising a dihedral frequency, detecting at least two signals representative of a rotation-induced frequency shift, and processing the detected polarized waves traveling in the same direction with the signals representative of a rotation-induced frequency shift for compensating an output signal in accordance with the dihedral frequency. 
     The invention further discloses a method of compensating an output signal of a multi-oscillator ring laser gyroscope comprising the steps of producing in a closed path with a gain medium a plurality of circularly polarized counter-traveling electromagnetic waves of a first polarization sense and a second polarization sense, producing a direction-dependent phase shift to the waves resulting in a frequency splitting between the counter-traveling waves of the same polarization sense, each of the waves being of a different frequency, detecting independently the polarized waves traveling in only a clockwise direction and the polarized waves traveling in only a counterclockwise direction, said polarized waves in each direction comprising a dihedral frequency, and processing both the clockwise and counter-clockwise polarized waves, said processing means providing signals for adjusting the gain medium for compensating an output signal of the gyroscope in accordance with variations in the dihedral frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other and further features and advantages of the invention will become apparent in connection with the accompanying drawings wherein: 
     FIG. 1 is a block diagram of a laser gyroscope cavity coupled to detection and processing electronics according to the invention for compensating the gyroscope output frequency as a function of the dihedral frequency; 
     FIG. 2 is a diagram of the gain vs. frequency curve for a laser gyroscope showing the four lasing modes of a multi-oscillator ring laser gyroscope and a resulting shift in each of the lasing mode frequencies due to rotation of the gyroscope; 
     FIG. 3 is a block diagram of an alternate embodiment of the invention comprising a feedback path for adjusting a discharge control current source and varying the laser cavity gain as a function of the dihedral frequency. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a block diagram of a laser gyroscope cavity 20 which provides a closed path 30 for the propagation of a plurality of electromagnetic waves in opposite directions, each of the waves being of a different frequency and referred to as f 1 , f 2 , f 3  and f 4 . There are four reflectors 34, 32, 36 and 38 for directing the waves around the closed path 30 which provides image rotation by virtue of being a nonplanar ring. The image rotation property, for this particular geometry of the optical closed path 30, splits the resonant frequencies of the cavity modes. This splitting is referred to as the polarization split or dihedral frequency (Δf D ). 
     A Faraday Rotator Assembly 28 provides a direction-dependent phase shift or non-reciprocal polarization rotation for the propagating waves. This frequency splitting is referred to as the Faraday frequency (Δf F ). The cavity 20 further comprises anodes 42 and 44, cathode 46 and a laser gain medium 26 having a helium-neon gas mixture where the two active isotopes are neon-20 and neon-22. The gaseous gain medium 26 is electrically excited by discharge currents generated between anodes 42 and 44 and cathode 46, and it becomes a light emitting laser gain medium or plasma, sustaining resonant electromagnetic or laser waves in the closed path 30. 
     Reflector 36 is attached to a piezoelectric element 37 which moves the reflector in and out as part of a cavity pathlength control system. Reflectors 32 and 34 are used for reflecting the electromagnetic waves in the closed path, however, either one of the reflectors 32 and 34 may be used to detect optical leakage signals for providing power compensation for the gyroscope output frequency. Reflector 38 is also only partially reflective, thereby allowing a small portion of the waves incident on its surface to pass through the reflector and be combined and processed to provide rotational information. 
     The output optics 40 extracts a portion of each wave circulating within the laser cavity to produce the two outputs G 1  and G 2 , each one of which represents the difference in frequency between wave pairs having the same sense of circular polarization within the cavity 20 as shown in FIG. 2. The output reflector 38 has a transmission coating on one side and a beamsplitter coating on the other side. Both coatings are a standard type using alternate layers of TiO 2  and SiO 2 . The beamsplitter coating transmits half the incident intensity and reflects the other half. A retro-reflecting prism 39 is used to heterodyne the two beams. This right angle prism is made of fused quartz and has silvered reflective faces. A dielectric coating is used between the silver and fused quarts to obtain minimal phase error upon reflection. A quarterwave plate followed by sheet polarizers are used to separate the four frequencies present in each beam. A wedge is used between the retro-reflecting prism and the quarterwave plate to prevent reflections from the interfaces from propagating back into the gyroscope cavity and mixing with the counter-rotating beams. A photo-diode cover glass (anti-reflection coated on one side) and a photo-diode package complete the output optics 40. An optical cement is used between the various interfaces to provide adhesion and to minimize reflections. The output optics is fully described in U.S. Pat. No. 4,141,651 to Irl W. Smith and Terry A. Dorschner and assigned to the present assighee, the specification of this patent being herein incorporated by reference. 
     The gyroscope block 24 is preferably constructed with a material having a low thermal coefficient of expansion, such as a glass-ceramic material to minimize the effects of temperature change upon the laser gyroscope cavity 20. A preferred commercially available material is sold under the name of Cer-Vit® by Owens-Illinois Company; alternatively, Zerodur® by Schott Optical Company may be used. 
     Still referring to FIG. 1, a combination of optical signals passing through the partially transparent reflector 34 is coupled to high frequency detector 48 which is disposed immediately adjacent to reflector 34; this combination is the difference between a dihedral frequency (Δf D ) and a Faraday frequency (Δf F ) or ΔF D  -Δf F  shown diagrammatically in FIG. 1 by the dotted line 47. The output of the high frequency detector 48 is coupled to a high frequency preamplifier 54 which is coupled to a high frequency counter 60 for determining the frequency of ΔF D  -Δf F . The output of high frequency counter 60 is coupled to an input of processor 61. Gyroscope cavity output 22 is coupled to preamplifier 50 whose output is coupled to counter 56. The output of counter 56 couples to an input of processor 61. Similarly, gyroscope cavity output 23 couples to preamplifier 52 whose output is coupled to counter 58. The output of counter 58 couples to another input of processor 61. Processor 61 combines the two gyroscope cavity outputs, G1 and G2, with the high frequency detected optical signal (Δf D  -ΔF F ) to obtain a compensated gyroscope output signal Δf g . The frequency output, G 1 , from counter 56 equals Δf F  +1/2Δ f G  ; likewise, the frequency output from counter 58, G 2 , equals Δf F  -1/2Δf G . Δf G  represents the rotationally induced frequency shift output of the multi-oscillator ring laser gyroscope. It is determined by the difference between the difference of the RCP waves (f 4  -f 3 ) and the difference of the LCP waves (f 2  -f 1 ). The 1/2 factor results from each detector of the output optics 40 sensing one of the two circular polarizations, thus detecting the frequency shift of the frequencies of that particular circular polarization, as shown in FIG. 2. G 1  and G 2  are combined in a sum 62 circuit to produce the signal 2Δf F . This signal is coupled to a divide by two 66 circuit, the output of which is Δf F , the Faraday frequency. A sum 68 circuit receives at one of its inputs the Δf F  signal and at another input the ΔF D  -Δf F  signal from the high frequency Δf d  counter 60 and provides at its output the dihedral frequency Δf D  which is fed to multiplier 72. A second input to multiplier is from scaler memory 70. The scaler quantity stored in scaler memory 70 is determined from previous runs of the laser gyroscope system wherein data is taken in order to determine this scaler quantity. 
     The scaler quantity (s) provides the correction factor for producing the compensated, gyroscope output frequency, Δf g , as a function of the dihedral frequency which varies with time due to, for example, optical power variations. 
     Thus, Δf g  is maintained substantially invariant or independent of changes due to optical power variations and other inherent laser cavity losses. During a test run of the laser gyroscope, the gyroscope output, Δf G , is recorded over a period of time; similarly, the dihedral frequency is recorded over the same period of time. Then, the scaler quantity is calculated as the ratio of the rate of change of the gyroscope output with respect to the rate of change of the dihedral frequency, and the resulting scaler quantity is stored in scaler memory 70. Multiplier 72 multiplies the dihedral frequency (Δf D ) he scaler quantity (s) from scaler memory 70, and this factor s Δf D  to the sum 74 circuit; a second input to the sum 74 circuit is obtained from the difference 64 circuit which subtracts G 2  from G 1  producing an uncompensate Δf G  signal. The sum 74 circuit produces the power compensated gyroscope output frequency Δf g . 
     Processor 61 may be embodied by electronic devices readily known to one skilled in the art, or depending upon the availability and type of computer being used in a laser gyroscope system, the functions being performed by processor 61 may be accomplished within said computer by a software program utilizing the inherent hardware of said computer. 
     Referring now to FIG. 2, there is shown a laser gain curve as a function of frequency. Four lasing modes or frequencies of the multi-oscillator ring laser gyroscope are shown as f 1 , f 2 , f 3  and f 4 . An original, four-fold degenerate, longitudinal mode represented by f 0  is split into a left-circularly polarized (LCP) mode 90 and a right-circularly polarized (RCP) mode 92 as a result of the reciprocal image rotation feature of a non-planar ring. Each polarization is further split by the non-reciprocal Faraday rotator resulting in the four distinct lasing frequencies 94-97. Rotation in one direction of the ring laser gyroscope cavity 20, as shown in FIG. 1, shifts each of these four frequencies by 1/4Δf G  in the senses shown in FIG. 2 yielding the four lasing frequencies f 1 , f 2 , f 3  and f 4  (as shown by the solid lines). Frequencies f 1  and f 4  circulate in a clockwise spacial direction while frequencies f 2  and f 3  circulate in a counter-clockwise spacial direction in said cavity 20. However, the frequency splittings, as illustrated in FIG. 2, are greatly exaggerated. Typically, the dihedral frequency (Δf D ) is in the 600 MHz range, the Faraday frequency (Δf F ) is in the 500 KHz range and the gyroscope output frequency is in the 10 Hz range. The dihedral frequency (Δf D ) is defined by the following equation: 
     
         f.sub.D =1/2(f.sub.4 +f.sub.3)-1/2(F.sub.2 +f.sub.1), 
    
     wherein 
     1/2(f 4  +f 3 ) is the mean value of the LCP pair of waves and 
     1/2(f 2  +f 1 ) is the mean value of the RCP pair of waves. 
     The Faraday frequency (Δf F ) is defined by the following equation: 
     
         Δf.sub.F =1/2(f.sub.4 -f.sub.3)+1/2(f.sub.2 -f.sub.1), 
    
     Based on these equations, it follows that: 
     Δf D  +Δf F  =f 4  -f 1  which are the traveling waves in a clockwise spacial direction and likewise, 
     Δf D  -Δf F  =f 3  -f 2  which are the traveling waves in a counterclockwise spacial direction. 
     Referring now to FIG. 3, there is shown an alternate embodiment for providing power compensation for the laser gyroscope output frequency (Δf G ) by changing the gain of the laser cavity via a feedback network 120 as a function of variations in the dihedral frequency and thereby maintaining the gyroscope output frequency substantially invariant or independent of various error sources. One of the reflectors 34 in laser cavity 20 provides the optical signals (ΔF D  -Δf F ) and (ΔF D  +ΔF F ) shown diagrammatically in FIG. 3 by dotted lines 122 and 124; they are detected and amplified by the high frequency photodiodes and preamplifiers 100 and 102, respectively, resulting in the electrical equivalent of these optical signals. The outputs of both high frequency photodiodes and preamplifiers 100 and 102 are each coupled to a mixer 104. Mixer 104 generates the signals 2Δf F  and 2Δf D  which are coupled to a high pass filter 106 where only the 2Δf D  signal is allowed to pass through it to frequency divider 108. The output of frequency divider 108 is coupled to a frequency-to-voltage converter 110. The frequency divider 108 divides down by a factor &#34;n&#34; the frequency at its input 2Δf D  of any sub-multiple frequency (2/nΔf D  suitable n for said converter 110, the design of which is readily known to one skilled in the art. The frequency-to-voltage converter 110 converts its input frequency to a voltage; this voltage is coupled to a voltage difference amplifier 112 which senses a change in voltage at one of its inputs with respect to a voltage reference 114 provided at a second input to said amplifier 112. The output of voltage difference amplifier 112 is coupled to a dual voltage controlled current source 116 which varies the potential between the anodes 42 and 44 and the cathode 46 of laser cavity 20 thereby varying the gain of the gyroscope for providing optical power compensation for the laser gyroscope output frequency (Δf G ) which in this embodiment is equivalent to Δf g  in the previous embodiment. 
     The output optics 40 extracts a portion of each wave circulating within the laser cavity 20 to produce two outputs, G 1  and G 2 , each one of which represents the difference in frequency between wave pairs having the same sense of circular polarizations within the laser cavity 20, as shown in FIG. 2. The details of the embodiment of outputs optics 40 are the same as described for FIG. 1. Likewise, the detected laser cavity outputs 22 and 23 are fed to preamplifiers 50 and 52, respectively, which are connected to counters 56 and 58, respectively, producing the two outputs G 1  and G 2 . The difference 64 circuit subtracts G 2  from G 1  producing the compensated output frequency Δf G  which equals Δf g  for the embodiment of FIG. 3. 
     This concludes the description of the embodiments of the invention described herein. However, many modifications and alterations will be obvious to one skilled in the art without departing from the spirit and scope of the inventive concept. Therefore, it is intended that the scope of this invention be limited only by the appended claims.