GB2184285A - Ring laser gyroscope - Google Patents

Description

SPECIFICATION Ring laser gyroscope This invention relatesto ring lasergyroscopes and in particularto 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 lasergyroscopes are a significant new class of rotation sensing instruments employing fourwaves oftwo polarization pairs, each polarization pair propagating in opposite circular directions. Such systems are shown and described in U. S. Patent Nos. 3,741,657,3,854,819 and 4,006,989 to Keimpe Andringa and assigned to the present assignee, the specifications ofthose patents being herein incorporated by reference. In such lasersystems, circular polarizationforeach ofthefourwaves 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 rotatorwhich essentially provides a frequency bias (fB). Such a biased four-frequency or multi-oscillator ring laser gyroscope provides a meansforcircumventing thefrequency 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 differentfrequencies are pulled toward each otherto combinein a singlefrequency standing wave.

However, when the frequencies ofthe 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 dueto drifts in the bias and giving a sensitivity twice that of a si ng le two-freq uencv gyroscope. Because the bias need not be dithered, the gyroscope never passes through lock-in. Hence, there are not dither-induced errors to limit instrument performance. Forthis 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 resultsfrom 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 sometimes considered to be a dihedral configuration providing the frequency bias (fus) or polarization splitfrequency difference separating the circularly polarized waves; this frequency is also referred to as a dihedral frequency (AfD). A non-planar electromagnetic wave ring resonator is shown and described in U. S. Patent No. 4,110,045to Irl W. Smith, Jr. and Terry A. Dorschnerand assigned to the present assignee. Second, a Faraday rotator is used to provide non-reciprocal polarization rotation, by having a slightly different refractive indexfor clockwise (cw) traveling wavesthanfor counterclockwise (ccw) traveling waves. This causes the cw and ccw RCP waves to oscillate atslightly differentfrequencies whilethe 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 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 sensetemperature caused variations and then applying a correction factorto the gyroscope output signal. In this invention, the measurement of the polarization split orthe dihedral frequency of a four-frequency laser gyroscope is used to correct the gyroscope output signal for optical powervariations and other error sources, such as loss variations due to aging producing a variation of the polarization split or dihedral frequency.

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 (Afo).

A laser cavity having a closed path with a gain medium produces a plurality of circularly polarized counter-traveling electromagnticwaves 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 determinesthe dihedral frequency which is detected.

A gyroscope output signal, which provides the rotation-induced frequencyshift (AfG) ofthe electromagnetic waves within the closed path, is generated and controlled in a mannerto keep the output signal substantially invariant by means in accordancewith variations in the dihedral frequency (AfD).

One embodiment of the invention discloses a ring laser gyroscope having a cavity comprising a closed path with again medium for the propagation of a plurality of electromagnetic waves in opposite directions, each of the waves being of a differentfrequency. 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 asthe dihedral frequency (AfD). A Faraday rotator produces a direction-dependent phase shiftto the counter-traveling waves for each polarization resulting in a frequency splitting of clockwise and counterclockwise waves referred to asthe Faraday frequency (AfF).

A first detector comprising a high frequency photodiode detects the polarized waves traveling in the same direction which maybe either clockwise or counterclockwise. The frequency detected by the first detector is determined by eitherthe traveling waves in a clockwise direction, AfD + AfFwhich equalsf4-f"orthe traveling waves in a counterclockwise direction, Afo-Afp which equalsfs-fz. 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-inducedfrequencyshift (AfG) and a Faradayfrequency (AfF). Afirstcavityoutput signalequalsthedifferencebetweentheFaradayfrequencyandone-halfoftherotation-inducedfrequency shift (AfF-1/2AfG) which is equivalent to f4-f3. A second cavity output signal equa Is the sum of the Faraday frequency and one-half ofthe rotation-induced frequencyshift (AfF + 1/2AfG) which is equivalenttof2-1.

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 factorforthe gyroscope output signal. The scaler quantity is determined by a ratio of a rate of change of the gyroscope outputto a rate of change of the dihedral frequency.

An alternate embodiment of the invention is disclosed utilizing a feedback network for compensating a lasergyroscope outputsignal for errorsources which produce variations in the dehidral frequency. A laser cavity, the same as in the other embodiment, generates circularly polarized counter-traveling waves. A detectormeansdetectsthetwospacialdirectionsofsaidwavesindependentlywhichcomprisesAfD+AfF, traveling in the clockwise spacial direction and AfD-AfFtraveling in a counterclockwise spacial direction.

These circularly polarized counter-traveling waves are combined by circuitrythat generates the dihedral frequencywhich is converted to a voltage. A voltage controlled current source is adjusted bytheconverted voltage in accordance with the dihedral frequency and the currentsource controlsthe gain medium of the laser gyroscope cavityAdjusting the lasercavity gain as a function of the dihedral frequency for changes in gyroscope parameters results in a gyroscope outputsignal being compensated bythisfeedback 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 electromagneticwaves of a first polarization sense and a second polarization sense, producing a direction-dependent phase shiftto the waves resulting in a frequency splitting between the counter-traveling waves of the same polarization sense, each of the waves being of a differentfrequency, 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 leasttwo 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 outputsignal 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 electromagneticwaves of a first polarization sense and a second polarization sense, producing a direction-dependent phase shiftto the waves resulting in a frequency splitting between the counter-traveling waves of the same polarization sense, each of the waves being of a differentfrequency, detecting independentlythe polarized wavestraveling 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 counterclockwise polarized waves, said processing means providing signalsfor adjusting the gain medium for compensating an output signal of the gyroscope in accordance with variations in the dihedral frequency.

Otherand furtherfeatures and advantages of the invention will become apparent in connection with the accompanying drawings wherein: Figure 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; Figure2 is a diagram of the gain vs. frequency curvefor a laser gyroscope showing thefour lasing modes of a multi-oscillator ring laser gyroscope and a resulting shift in each of the lasing modefrequencies dueto rotation of the gyroscope; Figure 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 Figure 1, there is shown a block diagram of a laser gyroscope cavity 20 which provides a closed path 30 forthe propagation of a plurality of electromagnetic waves in opposite directions, each of the waves being of a different frequency and referred to as fi, fz, fs and f4. There are four reflectors 34,32,36 and 38for directing the waves around the closed path 30 which provides image rotation by virtue of being a nonplanar ring. The image rotation property, forthis particular geometry of the optical closed path 30, splitsthe resonantfrequencies of the cavity modes. This splitting is referred to as the polarization split or dihedral frequency (AfD).

A Faraday RotatorAssembly 28 provides a direction-dependent phase shift or non-reciprocal polarization rotationforthepropagatingwaves. ThisfrequencysplittingisreferredtoastheFaradayfrequency (AfF). 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 lasergain medium or plasma, sustaining resonant electromagnetic or laserwaves in the closed path 30.

Reflector36 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 electromagnticwaves in the closed path, however, either one of the reflectors 32 and 34 may be used to detect optical leakage signalsfor providing power compensation for the gyroscope output frequency. Reflector 38 is also only partially ref lective, thereby al lowing 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 lasercavityto producethetwo outputs Ga and G2, each one of which represents the difference in frequency between wave pairs havingthe same sense of circular polarization within the cavity 20 as shown in Figure 2. The output reflector38 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 Ti02 and Si02. The beamsplittercoating transmits halfthe incident intensityand reflects the other half. A retro-reflecting prism 39 is used to heterodynethe two beams. This right angle prism is made of fused quartz and has silvered reflective faces. A dielectric coating is used between the silverand fused quarts to obtain minimal phase error upon reflection. A quarterwave plate followed by sheet polarizers are used to separatethe fourfrequencies present in each beam. Awedge 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 coverglass (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 isfully described in U. S. Patent No. 4,141,651 to Irl W. Smith and Terry A. Dorschner and assigned to the present assignee, the specification of this patent being herein incorporated by reference.

The gyroscope block 24 is preferably constructed with a material having a lowthermal coefficientof 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 underthe name of Cer-Vits (Registered Trade Mark) by Owens-Illinois Company ; alternatively, Zerodu rW by Schott Optical Company may be used.

Still referring to Figure 1, a combination of optical signals passing through the partiallytransparent reflector 34 is coupled to high frequency detector 48 which is disposed immediately adjacentto reflector34 ; thiscombinationisthedifferencebetweenadihedralfrequency (ESfD) andaFaradayfrequency (AfF) orAfD- Afp shown diagrammaticallyin Figure 1 by the dotted line 47. The output of the high frequency detector 48 is coupled to a high frequency pre-amplifier 54 which is coupled to a high frequency counter 60 for determining the frequency of AfD-AfF. The output of high frequency counter 60 is coupled to an input of processor61.

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 couplesto preamplifier 52 whose output is coupled to counter 58. The output of counter 58 couples to another input of processor61. Processor61 combinesthetwogyroscopecavityoutputs, G, andG2, withthehighfrequency detected optical signal (, fp-fF) to obtain a compensated gyroscope outputsignal Ofg. Thefrequency output, Gi, from counter 56 equals AfF + 1/2AfG ; likewise, the frequency outputfrom counter 58, G2, equals AfF -1/2AfG. AfG representsthe rotationallyinducedfrequencyshiftoutputofthe multioscillatorringlaser gyroscope. It is determined by the difference between the difference ofthe RCPwaves (f4-f3) andthe difference of the LCP waves (fz-fi). 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 Figure 2, Gland G2 are combined in a sum 62 circuitto produce the signal 2AfF. This signal is coupled to a divide by two 66 circuit, the output of which is AfF, the Faraday frequency. A sum 68 circuit receives atone of its inputs the AfF signal and atanother inputthe Afo- Afp signal from the high frequency counter 60 and provides at its output the dihedral frequency AfD which is fed to multiplier72. A secondinputtomultiplier72isfromscalermemory70. Thescalerquantitystoredinscalermemory70is determined from previous runs of the laser gyroscope system wherein data is taken in orderto determine this scaler quantity.

The scaler quantity (s) provides the correction factorfor producing the compensated, gyroscope output frequency, Afg, as a function of the dihedral frequency which varies with time dueto, forexample, optical power variations. Thus, Afg 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, AfG, is recorded overa 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 respecttothe rate of change of the dihedral frequency, and the resulting scaler quantityisstoredinscalermemory70. Multiplier72multipliesthedihedralfrequency (AfD) bythescaler quantity (s) from scaler memory 70, and thisfactorsAfD is coupled to the sum 74 circuit; a second inputtothe sum 74 circuit is obtained from the difference 64 circuitwhich subtracts Gz from G1 producing an uncompensated AfG signal. The sum 74 circuit produces the power compensated gyroscope output frequency Afg Processor61 may be embodied by electronic devices readily known to one skilled in the art, ordepending upon the availability and type of computer being used in a laser gyroscope system, thefunctions being performed by processor61 may be accomplished within said computer by a software program utilizing the inherent hardware of said computer.

Referring nowto Figure 2, there is shown a laser gain curve as a function of frequency. Four lasing modes orfrequencies of the multi-oscil lator ring laser gyroscope are shown as fa, 2, 3 and f4. An original, four-fol degenerate, longitudinal mode represented byfo 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 thefour distinct lasing frequencies 94-97. Rotation in one direction of the ring laser gyroscope cavity 20, as shown in Figure 1, shifts each of these fourfrequencies by 1/4 Afg in the senses shown in Figure 2 yielding thefour lasing frequencies 1lf2lf3andf4 (asshownbythesolidlines). Frequenciesf1andf4circulateinaclockwisespacialdirectionwhile frequenciesf2 and f3 circulate in a counter-clockwise spacial direction in said cavity 20. However, the frequency splittings, as illustrated in Figure 2, are greatly exaggerated. Typically, the dihedral frequency (AfD) is in the 600 MHz range, the Faraday frequency (AfF) is in the 500 KHz range and the gyroscope output frequency is in the 10 Hz range. The dihedral frequency (AfD) is defined bythefollowing equation: Jfp = 1/2 (f4 + f3)-1/2 (f2 + f, l, wherein 1/2 (f4+f3) is the mean value of the LCP pair of waves and 1/2 (2 + 1) is the mean value of the RCP pair of waves.

The Faradayfrequency (AfF) is defined bythefollowing equation: AfF = /2 (f4-f3) + 1/2 (f2-fl), Based on these equations, itfollowsthat : AfD + AfF = 4-fi which are the traveling waves in a clockwise spacial direction and likewise, AfD-AfF = f3-f2 which are the traveling waves in a counterclockwise spacial direction.

Referring nowto Figure 3, there is shown an alternate embodimentfor providing powercompensationfor the laser gyroscope outputfrequency (AfG) 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 outputfrequency substantially invariant or independent of various error sources. One of the reflectors 34 in laser cavity 20 provides the optical signals (AfD-AfF) and (AfD + AfF) shown diagrammatically in Figure 3 by dotted lines 122 and 124; they are detected and amplified bythe highfrequency photodiodesand 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 2AfFand2AfDwhicharecoupledtoahighpassfilter106whereonlythe2AfDsignalisallowedtopassthrough itto 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"n"the frequency at its input2AfDtoany sub-multiplefrequency2/nAfDsuitablenforsaidconverter110, thedesignofWhichisreadilyknowntoone skilled in the art. The frequency-to-voltage converter 110 converts its inputfrequencyto a voltage ; this voltage iscoupled to a voltage difference amplifier 112 which senses a change in voltage at one of its inputs with respectto a voltage reference 14 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 therebyvarying the gain of the gyroscope for providing optical power consumption forthe laser gyroscope outputfrequency (AfG) which in this embodiment is equivalentto Afg in the previous embodiment.

The outputoptics 40 extracts a portion of each wave circulating within the laser cavity 20 to producetwo outputs, G and G2, 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 Figure 2. The details of the embodiment of outputs optics 40 are the same as described for Figure 1. Likewise, the detected lasercavity 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 Gi and Gz. The difference 64circuitsubtracts AfG which equals Afg for the embodiment of Figure 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 artwithout departing from the spirit and scope of the inventive concept.

Claims (35)

CLAIMS Incombination:

1. means having a closed path with a gain medium for producing a plurality of circularly polarized counter-traveling electromagnetic waves of a first polarization sense and a second polarization sense; meansfor producing a direction-dependent phase shiftto said waves resulting in a frequency splitting between said counter-traveling waves of the same polarization sense, each of said waves being of a different frequency and a combination of said waves forming a dihedral frequency; means coupled to said wave producing means for detecting said dihedral frequency; and means for controlling an output signal in accordance with variations in said dihedral frequency, said output signal being representative of a rotation-induced frequency shift of said waves within said closed path.

2. In combination : means having a closed path with a gain medium for producing a plurality of circularly polarized counter-traveling electromagneticwaves of a first polarization sense and a second polarization sense; means for producing a direction-dependent phase shiftto said waves resulting in a frequency splitting between said counter-traveling waves of the same polarization sense, each of said waves being of a different frequency; first detecting means for detecting said polarized waves traveling in the same direction, a combination of said wavestraveling in the same direction comprising a dihedral frequency ; second detecting means for detecting at leasttwo signals, generated by said wave producing means including output optics, representative of a rotation-induced frequency shift ; and means for processing said polarized waves from said first detecting means with said signals from said second detecting means for compensating an output signal in accordance with said dihedral frequency.

3. The combination as recited in Claim 2wherein : said first polarization sense comprises a left circular polarization and said second polarization sense comprises a right circular polarization.

4. The combination as recited in Claim 2wherein : said first detecting means comprises a high frequency photodiode disposed immediately adjacentto a reflector in said wave producing means.

5. The combination as recited in Claim 2wherein : the frequency of said waves in the same direction from said first detecting means comprises eitherthe difference between said dihedral frequency and a Faraday frequency orthe sum of said dihedral frequency and said Faraday frequency.

6. The combination as recited in Claim 2 wherein : a first of said signals from said second detecting means comprises the difference between said Faraday frequency and one-half of said rotation induced frequency shift.

7. The combination as recited in Claim 2 wherein : a second of said signals from said second detecting means comprises the sum of said Faraday frequency and one-half of said rotation-induced frequency shift.

8. The combination as recited in Claim 2wherein said processing means further comprises : meansfor combining said detected signals from said wave producing means with said detected circularly polarized waves traveling in the same direction thereby producing said dihedral frequency; meansforstoring a scalerquantity ; meansfor multiplying said dihedral frequency by said scalerquantity ; and means for summing said rotation-induced frequency shift with an output of said multiplying means, for producing said compensated output signal.

9. In combination : means having a closed path with a gain medium for producing a plurality of circularly polarized counter-traveling electromagnetic waves of a first polarization sense and a second polarization sense; means for producing a direction-dependent phase shiftto said waves resulting in a frequency splitting between said counter-traveling waves of the same polarization sense, each of said waves being of a different frequency; first detecting meansfor detecting said polarized waves traveling in the same direction ; second detecting meansfor detecting at leasttwo signals, generated by said waves producing means including output optics, representative of a rotation-induced frequency shift ; means for processing said signal from said second detecting means to generate a Faradayfrequency ; meansfor combining said polarizedwavesfrom saidfirstdetecting meanswith said Faradayfrequencyto generate a dihedral frequency; meansforstoring a scalerquantity, said scalerquantity being determined from a ratio of a rate ofchangeof said rotation-induced frequency shift to a rate of change of said dihedral frequency; meansfor multiplying said dihedral frequency by said scaler quantity ; and meansforsummingsaidrotation-inducedfrequencyshiftwithanoutputofsaidmultiplingmeans producing a compensated output signal in accordance with said dihedral frequency.

10. The combination as recited in Claim 9wherein : said first polarization sense comprises a left circular polarization and said second polarization sense comprises a rightcircular polarization.

11. The combination as recited in claim 9wherein : said first detecting means comprises a high frequency photodiode disposed immediately adjacent to a reflector in said waves producing means.

12. The combination as recited in Claim 9wherein : the frequency of said waves in the same direction from said first detecting means comprises eitherthe difference between said dihedral frequency and a Faradayfrequencyorthesum of said dihedralfrequency andsaid Faradayfrequency.

13. The combination as recited in Claim 9 wherein : a first of said signals from said second detecting means comprises the difference between said Faraday frequency and one-half of said rotation-induced frequency shift.

14. The combination as recited in Claim 9wherein : a second of said signals from said second detecting means comprises the sum of said Faraday frequency and one-half of said rotation-induced frequency shift.

15. The combination as recited in Claim 9 wherein : said polarizedwavesfrom saidfirstdetecting meanstravel in either a clockwise direction or a counterclockwise direction.

16. In combination: means having a closed path with a gain medium for producing a plurality of circularly polarized countertraveling electromagnticwaves of a first polarization sense and a second polarization sense; means for producing a direction-dependent phase shiftto said waves resulting in a frequency splitting between said counter-traveling waves ofthe same polarization sense, each of said waves being of a different frequency; means for detecting independently said polarized waves traveling in only a clockwise direction and said polarized waves traveling in only a counterclockwise direction, said polarized waves in each direction comprising a dihedral frequency; and meansfor processing both of said clockwise and counterclockwise polarized waves, said processing means providing signalsfor adjusting said gain medium forcompensating an outputsignal in accordance with variations in said dihedral frequency.

17. The combination as recited in Claim 16wherein : said first polarization sense comprises a left circular polarization and said second polarization sense com- prises a right circular polarization.

18. The combination as recited in Claim 16wherein : said detecting means comprises high frequency photo-diodes disposed immediately adjacentto a reflector in said waves producing means.

19. The combination as recited in Claim 16wherein : the frequency of said counterclockwise polarized waves comprises the differences between said dihedral frequency and a Faradayfrequency, and the frequency of said clockwise polarized waves comprises the sum of said dihedral frequency and said Faradayfrequency.

20. The combination as recited in Claim 16wherein said processing meansfurthercomprises : means for combining both of said clockwise and counterclockwise polarized waves for producing said dihedral frequency; meansforconverting said dihedral frequencyto a voltage; and means coupled to said converting meansfor providing said adjusting signalsfor said gain medium.

21. Incombination : means having a closed path with a gain medium for producing a plurality of circularly polarized countertraveling electromagnetic waves of a first polarization sense and a second polarization sense; menasfor producing a direction-dependent phase shiftto said waves resulting in a frequency splitting between said counter-traveling waves of the same polarization sense, each of said waves being of a different frequency; meansfor detecting independently said polarized wavestraveling in only a clockwise direction and said polarized wavestraveling in only a counterclockwise direction; meansfor combining both of said clockwise and counterclockwise polarized waves for producing a dihedral frequency; meansfor converting said dihedral frequency to a voltage ; means coupled to said converting meansfor adjusting a voltage controlled current source in accordance with variations of said dihedral frequency, said current source being coupled to said wave producing means to control said gain medium.

22. The combination as recited in Claim 21 wherein: said first polarization sense comprises a left circular polarization and said second polarization sense comprises a right circular polarization.

23. The combination as recited in Claim 21 wherein: said detecting means comprises high frequency photo-diodes disposed immediately adjacent to a reflector in said waves producing means.

24. The combination as recited in Claim 21 wherein: the frequency of said counterclockwise polarized waves comprises the difference between a dihedral frequency and a Faraday frequency, and the frequency of said clockwise polarized waves comprises the sum of said dihedral frequency and said Faraday frequency.

25. The combination as recited in Claim 21 wherein: said adjusting means provides compensation for an output signal by varying said gain medium.

26. Apparatuscomprising : means for supporting in a closed path two pairs of electromagnetic waves, said waves in each of thetwo pair of waves traveling in opposite directions, a first pair of said two pairs of waves having a first sense of polarization and a first pair of frequencies and a second pair of said waves having a second opposite sense of polarization and a second pair of frequencies, the difference between the mean of the first pair of frequencies, and the mean of the second pair of frequencies varying as a function of time ; meansfor producing a signal related to the difference between the mean value of the first pair offrequencies and the mean value of the second pair offrequencies ; means responsive to the produced signal foradjusting the apparatus in accordancewith the produced signal to produce an output signal substantially invariantwith variations in the difference between the mean of the first pair of frequencies and the mean of the secnd pair of frequencies and representative of the difference between the difference between the first pair of frequencies and the difference between the second pair offrequencies.

27. 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 electro magneticwaves of a first polarization sense and a second polarization sense; producing a direction-dependent phase shiftto said waves resulting in a frequency splitting between said counter-traveling waves of the same polarization sense, each of said waves being of a different frequency and a combination of said frequencies forming a dihedral frequency ; detecting said dihedral frequency; and controlling an outputsignal of said waves producing means in accordance with variations in said dihedral frequency, said output signal being representative of a rotation-induced frequency shift of said waveswithin said closed path.

28. 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 shiftto said waves resulting in a frequency splitting between said counter-traveling waves of the same polarization sense, each of said waves being of a differentfrequency ; detecting said polarized waves traveling in the same direction, a combination of said waves in the same direction comprising a dihedral frequency; detecting at least two signals, generated by said wave producing means including output optics, representative of a rotation-induced frequency shift; and processing said polarized waves from said first detecting means with said signals from said second detecting means for compensating said output signal in accordance with said dihedral frequency.

29. The method as recited in claim 28wherein : said step of detecting at least one direction of said circularly polarized counter-traveling waves comprises a frequency of eitherthe difference between a dihedral frequency and a Faraday or a frequency of the sum of said dihedral frequency and said Faradayfrequency.

30. The method as recited in Claim 28 wherein : said step of detecting at leasttwo signals comprises a first of said signals being the difference between said Faraday frequency and one-half of said rotation-induced frequency shift and a second of said signals being the sum of said Faraday frequency and one-half of said rotation-induced frequency shift.

31. The method as recited in Claim 28 wherein said processing step further comprises the steps of: combining said detected signals from said waves producing means with said detected circularly polarized waves traveling in the same direction for producing said dihedral frequency; storing a scaler quantity determined from a ratio of a rate of change of the gyroscope output signal to a rate of change of said dihedral frequency ; multiplying said dihedral frequency by said scalerquantity; and summing a rotation-inducedfrequencyshiftwith said dihedral frequency multiplied bysaid scalerquantity for producing said compensated outputsignal.

32. 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 shiftto said waves resulting in a frequency splitting between said counter-traveling waves of the same polarization sense, each of said waves being of a differentfrequency ; detecting independently said polarized waves traveling in only a clockwise direction and said polarized wavestraveling in only a counterclockwise direction, said polarized waves in each direction comprising a dihedral frequency ; and processing both of said clockwise and counterclockwise polarized waves, said processing means provid ing signals foradjusting said gain medium in said waves producing meansfor compensating an output signal of said gyroscope in accordance with variations in said dihedral frequency.

33. The method as recited in Claim 32wherein : said step of detecting independently said polarized counter-traveling waves comprises detecting a frequ- ency of a first wave being the difference between a dihedral frequency and a Faradayfrequencyand afrequ- ency of a second wave being the sum of said dihedral frequency and said Faraday frequency.

34. The method as recited in Claim 32wherein said processing step further comprises the steps of : combining both of said clockwise and counterclockwise polarized waves for producing said dihedral frequency; converting said dihedral frequencyto a voltage; and adjusting said gain medium of said gyroscope in accordance with said voltage for compensating said gyroscope outputsignal.

35. A ring laser gyroscope substantially as described hereinbefore with reference to Figure 1 or 3 of the accompanying drawings.