Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
  • G01C19/58—Turn-sensitive devices without moving masses
  • G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
  • G01C19/72—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers
  • G01C19/721—Details, e.g. optical or electronical details

Definitions

• polarization phenomena that cause erroneous rotation detection by the gyroscope.
• Some polarization errors are caused by light being cross-coupled from one polarization state to another.
• cross-coupling occurs at the coupling of integrated optical circuit with a light source and an optical fiber sensing loop.
• a phase modulator on such integrated optical circuit affects one polarization state of light differently than another for a given signal applied to the modulator.
• a design for suppressing amplitude and intensity type polarization errors in fiber optic gyroscopes uses sophisticated modulation signals.
• the phase modulator or modulators within the sensing loop of the gyroscope act upon the light polarized along the pass axis of the polarizer differently than upon the small amount of light polarized along the reject axis of the polarizer. This situation exists in the case of integrated optical circuit modulators.
• the remaining reject axis light is essentially unmodulated in phase since it takes a different physical path through the integrated optical circuit.
• the waveguide only guides one polarization of light.
• the leakage of the other polarization state of the light through the chip is due to scattered light, which bypasses the phase modulator.
• the present invention which eliminates the resulting polarization errors, applies to both polarization maintaining (PM) and depolarized (SM) type fiber optic gyroscopes.
• Another open-loop processing system suppresses some polarization errors with bias modulation, and suppresses other polarization errors with a modulation signal meeting specific criteria supplied to a second phase modulator located on the opposite side of the sensing loop.
• a closed-looped signal processing system having ramp-like waveforms meeting certain criteria are supplied to modulators located on both sides of the loop, while the bias modulation signal is supplied to a modulator on either one or both sides of the loop.
• the present invention solves the polarization error problem in the situation wherein the phase modulator or modulators do not affect the primary and secondary polarization states in the same way, co-propagating waves also yield a.c. interference terms that fall within the demodulation bandwidth of the gyroscope.
• co-propagating waves also yield a.c. interference terms that fall within the demodulation bandwidth of the gyroscope.
• four types of amplitude polarization errors and one type of intensity polarization errors result.
• the different types of e ⁇ ors are distinguished and have various modulations applied to eliminate them.
• the modulations applied to the various error interferences are also different from the modulation applied to the main signal. This fact allows for the possibility that errors can be suppressed by modulation techniques, while maintaining good signal sensitivity.
• the present invention suppresses these errors.
• Figure 1 illustrates a fiber optic gyroscope with an integrated optical circuit (IOC) having a light splitter and modulators, with various light paths.
• IOC integrated optical circuit
• Figures 2, 3, 4 and 5 show a fiber optic gyroscope having an IOC with various modulator configurations.
• Figures 6a, 6b and 6c show an interferogram, a bias modulation signal, and a waveform for a two-step dual ramp closed loop fiber optic gyroscope, respectively.
• Figures 7a, 7b and 7c show an interferogram, a bias modulation signal and a waveform for a four-step dual ramp closed loop fiber optic gyroscope, respectively.
• Figure 8 is a diagram of the IOC with a light splitter for error classification.
• Figure 9a shows the modulator layout for the IOC of figure 8.
• Figure 9b shows a serrodyne loop-closure signal.
• Figures 9c and 9d show examples of polarization error suppression waveforms.
• Figure 9e is a polarization error suppression waveform invisible to loop closure for ⁇ /2 square wave bias modulation.
• Figure 10a shows a modulator push-pull configuration on the IOC.
• Figure 10b shows a serrodyne waveform that suppresses both amplitude and intensity polarization errors in a push-pull configuration.
• Figures 1 la and 1 lb show IOC configurations for inputting both bias modulation signals to one side and an error suppression signal to the other side.
• Figures 12a and 12b reveal decorrelation schemes for the PM and SM gyroscopes, respectively.
• Figure 13 shows several modulation signal generators connected to one modulator.
• the present invention for suppressing amplitude and intensity type polarization errors in fiber optic gyroscopes uses sophisticated modulation signals. It requires that phase modulator 11 or modulators 11 and 12 within a sensing loop 15 of gyroscope 10 of figure 1, act upon light 13 polarized along the pass axis of a proton-exchange lithium niobate LiNbO 3 integrated optical circuit 16 (inherently a polarizer) differently than the small amount of light 14 polarized along the reject axis of polarizer 16. This situation exists in certain integrated optic circuit 16 modulators 11 and 12, as illustrated in figure 1 that is not dimensionally to scale.
• the remaining reject axis light 17 is essentially unmodulated in phase since it takes a different physical path through chip 16, than light 13 which goes through the modulating pass axis.
• Waveguide 18 only guides one polarization of light that is light 13. The leakage of the other polarization state of the light through chip 16 is due to scattered light 17, which bypasses phase modulators 11 and 12.
• Source 19 provides light 13 to integrated optic circuit (IOC) 16. Light returning from IOC 16 goes to detector 23 via coupler 21. Detector 13 converts a returned optical signal 34 into an electrical signal. This electrical signal goes to electronics 26.
• a bias generator 35 provides a bias modulation signal on line 37 to modulator 11 and electronics 26.
• FIG. 1 The output of electronics 26 represents rotation rate of loop 15 and goes to a rate indicator 36.
• Figure 1 has some numerical nomenclature common with that of figure 2.
• Figure 2 shows a simplified diagram of an open loop fiber optic gyroscope 20 having proton-exchange LiNbO 3 IOC 16 with a phase modulator 11 attached to one side of loop 15.
• Generator 28 with a bias modulation signal ⁇ , on line 37 drives modulator 11.
• a reference signal from generator 28 is provided to open loop electronics 26 for demodulation purposes.
• That side of loop 15 is where primary clockwise (CW) light wave 22 enters the loop.
• Primary counter-clockwise (CCW) light wave 24 enters the other side of loop 15.
• Spurious CW light waves 25 and spurious CCW light waves 27 pass through the polarizer 16 reject axis before entering loop 15.
• spurious light waves 25 and 27 are not affected by phase modulator 11 whereas both primary waves 22 and 24 are. Wave 24 is affected upon exiting the loop.
• a well-known amplitude type error signal caused by the interference between primary CCW wave 24 and spurious CW wave 25 is canceled by an equal and opposite error caused by an interference between primary CW wave 22 and spurious CW wave 25. Because this polarization error is automatically suppressed by the modulation on one side of the loop, one does not need to carefully ensure that the spurious CW wave 25 interferes incoherently with primary waves 22 and 24.
• FIG. 3 shows a simplified diagram of an open loop fiber gyroscope 30 having IOC 16 with modulators 11 and 12 on both sides of loop 15, which are driven by generators 28 and 29, respectively.
• the bias modulation signal ⁇ , from generator 28 via line 37 is again applied to one side (i.e., modulator 11) of loop 15, which has the effect of suppressing the amplitude type errors associated with spurious CW wave 25.
• a second modulation signal ⁇ , from modulation generator 29 is applied to second phase modulator 12 on the CCW side of loop 15 to suppress the amplitude type polarization errors associated with spurious CCW wave 27 interfering with primary waves 22 and 24.
• Second modulation signal ⁇ 2 has frequency components that do not interfere with the sensor 30 operation.
• ⁇ 2 may, for example, be a sine wave, a triangle wave, or a saw tooth wave of the correct amplitude to suppress amplitude type polarization errors associated with spurious CCW wave 27 interfering with primary waves 22 and 24.
• FIG 4 shows a simplified diagram of a closed-loop fiber optic gyroscope 40.
• IOC 16 has modulator 11 on the CW side of loop 15 and modulator 12 on the CCW side of the loop.
• a bias modulation signal on line 38 via summer 33 from generator 31 is applied to modulators 11 and 12.
• a bias modulation signal can be applied to only one of the modulators on IOC 16.
• a closed-loop, ramp-like signal ⁇ 2 from generator 32 via summer 33 is also applied to modulators 11 and 12.
• the signal applied to modulator 11 is ⁇
• the signal applied to modulator 12 is ⁇ 2 .
• the closed- loop signal magnitudes are set by loop closure electronics 39, which are determined by an electrical signal from detector 23. Detector 23 receives light returning from loop 15 via IOC 16.
• Rotation of loop 15 about an axis pe ⁇ endicular to a plane parallel with the fiber loop winding results in phase shifts between primary waves 22 and 24.
• Interference between the phase-shifted primary waves is detected and passed on as a signal to electronics 39, ramp generator 32, and to modulators 11 and 12.
• This feedback signal tends to bring primary waves 22 and 24 back into phase with each other, during rotation.
• the amount of this feedback signal is an indication of rotation rate of loop 15.
• the interference of light at detector 23 may partially be the result of spurious waves.
• the signal to electronics 39, ramp generator 32, and modulators 11 and 12 may be erroneous and result in inaccurate rotation rate indications. Such inaccurate indications are due to polarization cross-coupling.
• the waveforms of bias modulation signal waveform and the closed loop signal are composed of digital steps, with the time duration of each step equal to the transit time of the light around loop 15.
• E ⁇ represents the average (or expected) value of the enclosed waveform.
• the averaging time is one period of the loop closure.
• This system 40 may have a closed-loop waveform that is a four-step dual ramp waveform.
• An alternative for the closed-loop signal is a dual serrodyne system where separate serrodyne waveforms are applied to modulators 11 and
• Another alternative is to use any kind of closed-loop signal, and then further add waveforms composed of non-interfering frequency components that meet the above noted criteria.
• modulation techniques to suppress polarization error have the following criteria:
• E ⁇ sin( ⁇ J*[cos( ⁇ 2x - ⁇ 2y + ⁇ m - 7)] ⁇ -E ⁇ sin( ⁇ *[cos( ⁇ 2x - ⁇ 2y + ⁇ )] ⁇ and -E ⁇ sin( ⁇ J*[cos( ⁇ lx - ⁇ Iy + ⁇ )] ⁇ .
• ⁇ lx phase modulation signal applied to the x (pass) polarized component by modulator
• ⁇ i phase modulation signal applied to the x (pass) polarized component by modulator
• ⁇ phase modulation signal applied to the y (reject) polarized component by modulator
• ⁇ 2y phase modulation signal applied to the y (reject) polarized component by modulator 12.
• e, polarization extinction ratio for the modulator 11 side of integrated circuit 16.
• e 2 polarization extinction ratio for the modulator 12 side of integrated circuit 16.
• A the change of the amount of light in clockwise wave 22 in the x (pass) polarization axis over the length of fiber in sensing coil 15.
• B the amount of light in clockwise wave 25 in the y (reject) polarization axis cross- coupled over the length of fiber in sensing coil 15 to clockwise wave 22 in the x (pass) polarization axis.
• C the amount of light in clockwise wave 22 in the x (pass) polarization axis cross- coupled over the length of fiber in sensing coil 15 to clockwise wave 25 in the y (reject) polarization axis.
• D the change of the amount of light in clockwise wave 25 in the y (reject) polarization axis over the length of fiber in sensing coil 15.
• I mt is the intensity of the light exiting the loop.
• the first error term, Error 1 is:
• the second error term, Error 2 is:
• This error is one of the two main amplitude type polarization errors.
• the third error term, Error 3 is:
• the fourth error term, Error 4 is:
• Error 1 can cancel Error 2 if the modulation waveforms are chosen correctly.
• the general criterion includes:
• ⁇ m (t) ⁇ ⁇ (t) - ⁇ Xx (t + ⁇ ) + ⁇ 2x (t + r) - ⁇ 2x (t) ⁇ m (t) is the total phase bias impressed on the main beams of the interferometer.
• the signal demodulation D s can often be represented by a multiplication of sm i ⁇ m (01 followed by a low pass filter.
• Error 3 2 e 2 S cos[ ⁇ (t) - ⁇ x (t + r) + ⁇ 2x (t + r) - ⁇ 2 (t) - ⁇ R + f]
• Error 4 2 e, S cos[ ⁇ 2x (t) - ⁇ 2y (t) + ⁇ ]
• Figures 6a, 6b and 6c show the waveforms 59, 60 and 61, respectively, for a dual ramp closed loop fiber optic gyroscope.
• Waveform 59 is an interferogram of I ou ⁇ versus ⁇ m ( ⁇ ) ⁇ B as modulation signal 61 incorporated in ⁇ is applied to modulator 11 of gyroscope 42 of figure 5.
• Dual ramp signal 60 is split between ⁇ lx and ⁇ 2x which go
• ⁇ m ⁇ x x (0 - ⁇ i x (t + ⁇ ) + ⁇ 2l (t + r) - ⁇ 2x (t)
• Waveform 63 is an interferogram of I ou ⁇ versus ⁇ m (t).
• Bias modulation signal 65 incorporated in ⁇ Xx is applied to modulator 11 of figure 5.
• Dual ramp signal 64 is split between ⁇ and ⁇ x2 going to modulators 11 and 12, respectively.
• Dimension 66 of waveform 64 is ⁇ .
• Dimension 67 is ⁇ . This is a push- pull operation.
• Bias modulation is a square wave 65 at the proper frequency. The following table describes the various signals for the four-step dual ramp system.
• ⁇ m ⁇ ix (0 - (t + r) + ⁇ x (t + r)- ⁇ ix (0
• Errors 1 and 2 are individually equal to zero.
• SecondTerm Error 3 and Error 4 are individually zero.
• the four-step dual ramp is a case where all four errors individually are modulated to zero, (with a ⁇ ll bias modulation depth).
• Open loop operation is looked at in conjunction with just modulator 11.
• a bias modulation is inco ⁇ orated in ⁇ x which is applied to modulation 11.
• the bias modulation is at the proper frequency.
• Error 3 is not equal to zero, but Error 4 is equal to zero. So one only needs to gamma trim Error 3.
• the next example involves an open loop configuration with carrier suppression applied to modulator 12. ⁇ , includes bias modulation at the proper frequency, which is applied to modulator 11. The low frequency carrier suppression signal is inco ⁇ orated in the ⁇ 2 signal that is applied to modulator 12, where
• fiber optic gyroscopes may have polarization errors.
• the polarization errors can be classified as amplitude-type or intensity-type errors.
• the amplitude-type polarization errors involve interference of cross-coupled waves and a primary wave.
• Cross-coupled waves may be referred to as spurious or secondary waves.
• the primary waves are transmitted through the pass axis of the polarizer.
• the secondary waves are transmitted in the reject axis of the polarizer.
• the intensity-type polarization errors involve interference of two cross-coupled waves. Further, error classification may be done in conjunction with figure 8. There are amplitude-type polarization errors associated with side A of IOC 41. These errors involve interferences between waves cross-coupled at points k, and k 2 and a primary wave. Error 1 involves co-propagating waves and error 2 involves counter-propagating waves. Also, there are amplitude-type polarization errors associated with side B of IOC 41. The errors involve interferences between waves cross-coupled at points k, and k 3 and a primary wave. Error 3 involves counter-propagating waves. Error 4 involves co-propagating waves. Finally, there are intensity-type polarization errors, which are identified as error 5.
• Error 5 involves interferences between two waves cross-coupled at points k 2 and k 3 both of which are located within the gyroscope loop. Errors 1, 1, 3, 4 and 5 may be also referred to types one, two, three, four and five polarization errors, respectively.
• FIG. 9b shows the serrodyne loop closure signal 43, £c(t) , having a peak-to-peak amplitude 55 of phase modulation at 2 ⁇ , 4 ⁇ ,..., n2 ⁇ radians, where n is an integer.
• Signal 43 may instead be a digital phase step signal (i.e., a digitized serrodyne signal).
• the amplitude polarization errors on the A-side experience self-cancellation, as errors 1 and 2 are equal in amplitude, but opposite in sign.
• the amplitude polarization errors on the B-side are suppressed by loop closure at non-zero rates.
• the intensity errors are suppressed by loop closure at non-zero rates.
• the present modulation techniques suppress all errors (or classes).
• Serrodyne loop closure signal £c(t) is applied to modulator 11 and an error suppression modulation es(t) is applied to modulator 12.
• Bias modulation can be applied to either side.
• waveforms 46, 47 and 48 are illustrated in figures 9c, 9d and 9e, respectively.
• the error suppression waveforms should have either low frequency or a frequency close to even multiples of the proper frequency of sensing loop 15.
• the proper or eigen frequency is equal to — , where ⁇ is
• loop closure reconstructs the shape of the error suppression modulation waveform, es(t) , at low frequencies.
• error suppression modulation is applied to both modulators 11 and 12 of sides A and B, respectively, of IOC 45.
• the exception is a square wave 48 of figure 9e, which is invisible to loop closure for square wave bias modulation at a modulation depth 58 (i.e., ⁇ a) of — , but provides suppression of polarization errors.
• Square wave 48 may have a
• modulation is applied to side A (modulator 11), the amplitude polarization errors associated with this side experience self cancellation as errors 1 and 2 are equal in amplitude but different in sign.
• the amplitude polarization errors on the B-side (modulator 12) of IOC 45 are suppressed by error suppression modulation es(t) applied to modulator 12.
• bias modulation is applied to side B, the amplitude polarization errors associated with this side experience self cancellation; the errors associated with side A are suppressed by the waveform es(t) reconstructed by the loop closure.
• the intensity polarization errors are suppressed for non-zero rotation rates. For the square wave, the intensity errors are suppressed at all rotation rates including the zero rotation rate.
• the triangular waveform 47 provides for suppression of backscatter errors in addition to the suppression of polarization errors.
• An optimal implementation of a serrodyne loop closure in a push-pull configuration is shown by figure 10a.
• Bias modulation and serrodyne signal £c(t) are applied to modulators 11 and 12 of IOC 50.
• Serrodyne waveform 53 with 4 ⁇ resets 49 acts as error suppression modulation.
• the suppression of amplitude errors takes place for resets 49 having a dimension 59 of n4 ⁇ and the suppression of intensity errors takes place for «2 ⁇ and «4 ⁇ resets, where n is an integer.
• All of the amplitude and intensity polarization errors are suppressed by a serrodyne waveform 53 (of figure 10b) with 4 ⁇ resets at non-zero rates, for the IOC 50 configuration. These errors can also be suppressed by error suppression modulation at all rotation rates provided there is a proper selection of the waveform for the error suppression modulation signals.
• An error suppression waveform is selected to suppress amplitude or intensity polarization errors. Some waveform (e.g., triangular or square wave) can suppress both kinds of polarization errors at the same time.
• Figure 11a shows an IOC 52 wherein both the bias modulation and loop closure signals are input to the A side of IOC 52.
• the bias modulation signal is input to modulator 11a and the loop closure signal is input to modulator 1 lb. Both of these signals may instead be summed by a summer 33 and input to modulator 11 of IOC 79 shown in figure 1 lb.
• the carrier suppression signal from suppression waveform generator 80 is input to modulator 12 of side B of IOC's 52 and 79 of figures 11a and 1 lb, respectively.
• the characteristics of the suppression signal waveform are the same as those for the waveform of the suppression signal input to modulator 12 of IOC 50 in figure 5.
• Figure 13 shows how signals from several signal generators (e.g., first and second modulation signal generators 81 and 82) can be applied to one modulator 11 in an additive or differential manner, by applying each of the signals to each electrode of modulator 11, respectively.
• Decorrelation involves adjusting the lengths of the PM fiber between splices in a PM gyroscope, or splices of a depolarizer or depolarizers of an SM gyroscope and depends on the coherence function of the light source.
• Coherence function is an autocorrelation function.
• the temporal coherence function determines fringe visibility of the interfering waves.
• the delays of light are adjusted so that no two waves at the detector are not correlated.
• Decorrelation is used to suppress remaining errors, if just bias modulation is used to suppress several errors.
• Decorrelation keeps the primary wave and the cross-coupled waves from interfering with one another.
• Figure 12a shows a
• PM gyroscope 44 having PM fiber splices 68. All lengths of PM fiber between splices 68 and the length of IOC are adjusted to provide the needed decorrelation.
• Figure 12b shows an SM (depolarized) gyroscope 54 having splices 72. Particular lengths 74 and 76 of PM fiber are depolarizers. Gyroscope 54 may be designed with only one depolarizer 74 or 76. All lengths of PM fiber between splices 72 and the length of
• IOC are adjusted to provide the needed decorrelation.

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