Abstract:
An Interferometric Fiber Optic Gyro (IFOG) device for high accuracy sensing. An example IFOG includes an integrated optics chip (IOC) and a modulation component that modulates one or more light signals passing thru the IOC according to a bias-modulation waveform. A glitch pattern experienced at front-end components of the IFOG includes frequency content that has approximately zero amplitude at predefined sense harmonics. Frequency content of the bias-modulation waveform is below a predefined threshold value at the predefined sense harmonics.

Description:
BACKGROUND OF THE INVENTION 
     Generally, one of two modulation schemes is used for high accuracy Interferometric Fiber Optic Gyroscope (IFOG): Square-wave modulation or Dual Ramp modulation. Dual Ramp modulation allows the V pi  value of an Integrated Optics Chip (IOC) of the IFOG to be servoed, which provides a highly accurate scale factor. The expense of this modulation scheme is that it produces a glitch pattern at the front-end that has Eigen-frequency first harmonic content. This has the undesirable effect of introducing additional bias into the IFOG. The Square-wave modulation scheme provides a benign glitch pattern at the front-end containing primarily Eigen-frequency second harmonic content, which the IFOG is immune to. However, the Square-wave modulation does not provide a means for servoing V pi . 
       FIGS. 1-1  and  2 - 1  are plots of two prior art bias modulation waveforms and their resulting glitch patterns. These bias modulation waveforms are currently used in high precision IFOG applications. 
       FIGS. 1-2  and  2 - 2  illustrate frequency content of the glitch patterns shown in  FIGS. 1-1  and  2 - 1 . The Square-wave modulation glitch pattern does not have any frequency spikes at the odd Eigen-frequency harmonics and, therefore, will (ideally) contribute no bias due to glitch pick-up during Front-End sampling. The Dual Ramp modulation glitch pattern shows substantial odd harmonic content and will contribute significant bias error in the absence of adequate glitch masking. 
     Another advantage of Dual Ramp modulation over Square-wave modulation is the decrease in Eigen-frequency content over the IOC drive waveform when certain modulation depths are used. Two commonly used modulation depths were investigated here (π/2 and 3π/4).  FIGS. 5-1 ,  5 - 2 ,  6 - 1 , and  6 - 2  show the frequency content for π/2 and 3π/4 modulation depths, respectively. As shown in  FIGS. 5-1  and  5 - 2 , at π/2 modulation depth the Dual Ramp modulation waveform has the same 1st harmonic Eigen-frequency content as Square-wave modulation. As shown in  FIGS. 6-1  and  6 - 2 , the Dual Ramp modulation waveform now has three times less first harmonic Eigen-frequency content than the Square-wave modulation. But the present invention has nine times lower signal level than the Square-wave modulation. These results show that the modulation scheme of the present invention is more favorable than both Square-wave and Dual Ramp modulation, from the perspective of electrical coupling between the IOC drive and Front-End electronics. 
     SUMMARY OF THE INVENTION 
     The present invention provides a modulation scheme for high accuracy Interferometric Fiber Optic Gyroscopes (IFOG) that produces a benign Front-End glitch pattern like Square-wave modulation, while maintaining the benefits of Dual Ramp modulation. 
     An Interferometric Fiber Optic Gyro (IFOG) device for high accuracy sensing. An example IFOG includes an integrated optics chip (IOC) and a modulation component that modulates one or more light signals passing thru the IOC according to a bias-modulation waveform. A glitch pattern experienced at front-end components of the IFOG includes frequency content that has approximately zero amplitude at predefined sense harmonics. Frequency content of the bias-modulation waveform is below a predefined threshold value at the predefined sense harmonics. 
     The present invention makes the electrical isolation requirement less stringent, because the electrically coupled signal doesn&#39;t cause as much error. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIGS. 1-1  and  2 - 1  illustrate prior art modulation schemes and front-end glitch patterns; 
         FIGS. 1-2  and  2 - 2  show frequency content of the glitch patterns shown in  FIGS. 1-1  and  2 - 1 ; 
         FIG. 3  is an Interferometric Fiber Optic Gyroscope (IFOG) formed in accordance with an embodiment of the present invention; 
         FIG. 4-1  illustrates a modulation scheme and glitch pattern used by the IFOG of  FIG. 3 ; 
         FIG. 4-2  is a harmonic plot of the glitch pattern shown in  FIG. 4-1 ; 
         FIGS. 5-1 ,  5 - 2 ,  6 - 1 , and  6 - 2  show frequency content for the prior art modulation schemes of  FIGS. 1-1  and  2 - 1  at different bias-modulation depths; and 
         FIGS. 5-3  and  6 - 3  show frequency content associated with the modulation scheme shown in  FIG. 4-1  at different bias-modulation depths. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  illustrates an Interferometric Fiber Optic Gyroscope (IFOG)  100  that produces a benign glitch pattern at front-end electronics and has improved electrical isolation. The IFOG  100  includes a light source  102 , a circulator/coupler  104 , a photo detector  106 , an amplifier  108 , an analog-to-digital converter (ADC)  110 , front-end electronics/modulation module  130 , a digital-to-analog converter (DAC)  170 , a second amplifier  173 , and an integrated optics chip (IOC)  142  and a fiber optic loop  144 . The light source  102  sends light waves to the circulator/coupler  104 . The IOC  142  receives light waves from the circulator/coupler  104 , modulates the light waves at a modular component  146  based on a modulation scheme generated by the front-end electronics/modulation module  130  via the DAC  170  and the second amplifier  173 . The modulated light waves circulate in clockwise (CW) and counterclockwise (CCW) directions in the fiber optic loop  144 . The CW and CCW light waves are returned from the fiber optic loop  144  to be combined by the IOC  142  then sent to the circulator/coupler  104 . The combined CW and CCW light waves are passed by the circulator/coupler  104  to the photo detector  106  that generates a sensed voltage (or current) value. The sensed voltage (or current) value is amplified by the first amplifier  108 , converted to a digital signal at the ADC  110  and sent to the front-end electronics/modulation module  130 . The module  130  demodulates the digital signal, generates an output signal based on the demodulated signal, and generates a modulation signal based on the servoed V pi  value and modulation depth. The modulation signal stays the same except that its amplitude changes as V pi  changes. 
     The drive signal cancels the effect of angular rate and so it changes as the rate experienced by the gyroscope changes. The output signal is sent to a data output device  174 . 
     The modulation scheme produced by the modulation module  130  provides a glitch pattern seen at the input of the modulation module  130  that is benign relative to the signal being detected. The glitch pattern is experienced at the output of the photodetector  106  and from there it propagates to the input of the ADC  110 . In other words, the glitch pattern experienced at the front-end electronics/modulation module  130  has a harmonic content that is not in conflict with harmonic components of the light waves sensed by the photo detector  106 . Also, the present modulation scheme allows modulation depth errors (V pi ) to be servoed which provides a highly accurate scale factor. V pi  is the voltage required on the IOC modulation component to create a phase shift of π radians (180°) between the two counter propagating light waves in the coil loop  144 . The scale factor is the constant used to convert the measured signal received by the modulation module  130 , from the ADC  110 , into angle or rate. 
       FIG. 4-1  illustrates an example bias modulation waveform  190  generated by the modulation module  130 . Shown below the bias-modulation waveform  190  is a glitch pattern  192  experienced at the front-end electronics/modulation module  130 . In order to servo V pi , the bias waveform must have at least four states: 
     θ m , 2π−θ m , −θ m , and −2π+θ m  (where θ m =the modulation depth). π is the phase shift voltage value V pi  at the IOC  142 . 
     The Eigen-frequency odd harmonic content of the bias-modulation waveform is below that of both square-wave and dual-ramp modulation (prior art) over the modulation depth range of π/3 to 5π/6, thereby reducing electrical coupling with other components. 
     In order to produce a benign glitch pattern, the pattern does not contain frequency content at the odd harmonics of the Eigen-frequency. Other modulation schemes can be used provided they have glitch patterns with little or zero harmonic component that coincides with a sense frequency component. 
       FIG. 4-2  illustrates a frequency content signal  200  of the glitch pattern  192 . The frequency content signal  200  of the glitch pattern  192  has an Eigen-frequency harmonic that has zero amplitude at the odd harmonics which corresponds to the sense harmonics of the IFOG  100 . The bias-modulation depth for the bias-modulation waveform  190  used to produce the results shown in  FIG. 4-2  is π/2. 
       FIG. 5-3  shows a frequency content signal of the bias-modulation waveform  190  at a bias-modulation depth of π/2 that has a three times lower signal level at the odd harmonics than that in the prior art modulation schemes ( FIGS. 5-1  and  5 - 2 ). 
       FIG. 6-3  shows a frequency content signal of the bias-modulation waveform  190  at a bias-modulation depth of 3π/4 that has a nine times lower signal level at the odd harmonics than that in the Dual Ramp modulation scheme ( FIG. 6-2 ) and a three times lower signal level at the odd harmonics than that in the Square-wave modulation scheme ( FIG. 6-1 ). The results in  FIGS. 5 and 6  indicate that the 
     An example IFOG system that can be modified to execute the example bias-modulation waveform  190  or a modulation scheme that provides comparable benefits is shown and described in U.S. Pat. Ser. No. 7,167,250, which is hereby incorporated by reference. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.