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Chapter 8:
Fiber Optic Gyro Development at Fibernetics
Fibernetics started in 1983 with the small-scale manufacturing of fiber optic directional couplers, polarizers, and polarization controllers based on concepts first demonstrated at Stanford University. Sales of these components and small systems such as fiber resonators were largely to research laboratories. In parallel with this manufacturing effort, we conducted research on FOGs. We licensed the intellectual property that developed from this research to Honeywell International, Inc., which began a long-term relationship between the two companies in FOG development. The Honeywell–Fibernetics collaboration slowed down in 2004, but Fibernetics continued its development effort through government contracts and internal funding. Now Fibernetics is poised to manufacture IMUs to serve the highperformance- inertial-system market. This chapter begins with a short overview of our current development, including a phase-modulation scheme and a source-sharing scheme. The so-called occasional-calibration phase-modulation scheme is a combination of a digital phase ramp and dual-ramp phase modulation. The digital phase ramp performs the function of closed-loop operation, and the dual ramp performs two functions: closed-loop operation, and calibration of the phase modulation with respect to the interferogram for accurate SF determination. This design thus provides precise digital representation of the phase difference caused by rotation. The chapter then presents our novel source-sharing system, which makes efficient use of light from a single light source to power all three Sagnac interferometers. It is efficient because light that would normally be thrown away by the first coupler is used to power other Sagnac interferometers. It involves a single light source and three detectors, one for each FOG. The directional couplers are connected in series rather than in parallel, as in other source-sharing schemes. The coupling ratios of the directional couplers can be adjusted to equalize the optical power incident on the various photo-detectors at the output of the FOGs. A calculation of the coupler-splitting ratios that equalize the optical powers at the various photodetectors are presented for three or more FOGs. Although equalizing these optical powers is not critically important in the IMU, it does help balance the SNRs and the control-loop gains of the three FOGs. Finally, a novel FOG architecture is presented that uses a multicore fiber in an effort to further reduce the size of the FOG.
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