2A Commercial (Minimum Configuration) IFOG Electronic CircuitPolarizerSLEDSplitterSplitterSensorCoilDetectorA/DAMPDriverDSPPhase ModulatorDigital or Analog Signal OutputA commercial IFOG consists of a electronic circuit with a lock-in amplifier and several optical components. The optical components include;optical source, detector, splitters, a polarizer, a phase modulator and an optical fiber coil typically utilizing polarization maintaining (PM) fiber and components.
4Disadvantages of Current IFOGs (PM fiber) Slow axisFast axisA PM SplitterIFOG manufacturers typically need to use polarization maintaining fiber (PM)and components to avoid polarization drift in the fiber. This is due to the fact that polarization driftis not distinguishable from rotation signal in a Sagnac interferometer.PM fibers typically have about dB extinction ratio limitations (between fast and slow axis)and this extinction ratio drops dramatically as length of the fiber increases. This prevents lengthof PM fiber in the coil be chosen more than a few kilometers and sensitivity is limited.PM fibers and components are expensive and highly radiation sensitive.It is difficult and costly to make smaller than 2 inches diameter fiber coils with PM fibers.
5Disadvantages of Current IFOGs (Active Type Bias) Commercially available IFOGs use PZT as a phase modulator inside their module. It is a small size and low cost cylindrical piece of piezo-electric crystal with optical fiber wrapped around it. The process however, is mechanical and results in unwanted stresses on the fiber which reduces its longevity. In addition, heat generation in the fiber coil due to mechanically induced effect can not be ignored if high performance is demanded.Electronic CircuitPolarizerSLEDSplitterSplitterSensorCoilDetectorA/DAMPDriverDSPPZT Phase Modulatorstressing and heating the fiberDigital or Analog Signal Output
6Disadvantages of Current IFOGs (Active Type Bias)… Further, the modulation frequency of PZT is limited. For a fiber coil of 100 meters, the modulation rate is about 1 MHz (f~100KHz-Km). This is a near upper limit of piezo-electric crystal materials and consequently length of fiber coil can not be chosen shorter if reduction of fiber’s length and coil size is desired.Electronic CircuitPolarizerSLEDSplitterSplitterSensorSizeDetectorA/DAMPPZTDriverDSPDigital or Analog Signal Output
7Disadvantages of Current IFOGs (Active Type Bias)… In a vibration prone environment, the piezo-electric crystal itself can generate voltage noises that obscure the real signal from these fictitious signals.Electronic CircuitPolarizerSLEDSplitterSplitterSensorCoilDetectorA/DAMPDriverPZTDSPNoise GenerationElectronic noiseDigital or Analog Signal OutputOptical noise
8Disadvantages of Current IFOGs (Active Bias using Lithium Niobate Modulator) Alternatively a lithium niobate modulator can be chosen instead of a PZT. However, Lithium niobate materials are prone to charge ups and drifts when exposed to radiation and vacuum (not suitable for space application). In addition, Lithium niobate modulators are expensive and have high insertion loss.Electronic CircuitPolarizerSLEDSplitterSplitterSensorCoilDetectorA/DAMPDriverDSPLithium niobatePhase ModulatorCharge upsDigital or Analog Signal Output
9Disadvantages of Current IFOGs (Active Type Bias)… Sometimes it is necessary to have the sensing fiber coil separated and placed by a tethered electric and optical cable some distance away from IFOG electronics. In this case the electrical part of connecting cable can pick up noise from surrounding electronics and often can not be chosen longer than few meters.A tethered and powered remote sensor headSensorCoilNoise picked up by a longelectro-optical cable
10In summaryDisadvantages of current IFOG with active bias and PM fiber are;Require expensive PM fiber and componentsPM fiber exhibits extinction ratio limitations specially when fiber’s length exceedsfew kilometersPM fiber and components are radiation sensitiveRequire active bias or phase modulation in fiber coil via a phase modulatorwith its associated issuesPhase modulation is always prone to electronic and optical driftsIn case of tethered cable, noise can limit how far the sensor coil can be placedfrom its electronics