A Commercial (Minimum Configuration) IFOG Sensor Coil Polarizer SLED Detector Splitter Phase Modulator Electronic Circuit Driver A 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. A/D DSP Digital or Analog Signal Output AMP
Disadvantages of Current IFOGs (PM fiber) IFOG 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 drift is not distinguishable from rotation signal in a Sagnac interferometer. PM fibers typically have about 20-25 dB extinction ratio limitations (between fast and slow axis) and this extinction ratio drops dramatically as length of the fiber increases. This prevents length of 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. Fast axis Slow axis A PM Splitter
Disadvantages of Current IFOGs (Active Type Bias) Sensor Coil Polarizer SLED Detector Splitter PZT Phase Modulator stressing and heating the fiber Electronic Circuit Driver A/D DSP Digital or Analog Signal Output AMP 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.
Disadvantages of Current IFOGs (Active Type Bias)… Polarizer SLED Detector Splitter Electronic Circuit Driver A/D DSP Digital or Analog Signal Output AMP 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. PZT Sensor Size
Disadvantages of Current IFOGs (Active Type Bias)… Sensor Coil Polarizer SLED Detector Splitter Electronic Circuit Driver A/D DSP Digital or Analog Signal Output AMP In a vibration prone environment, the piezo-electric crystal itself can generate voltage noises that obscure the real signal from these fictitious signals. Noise Generation PZT Electronic noise Optical noise
Disadvantages of Current IFOGs (Active Bias using Lithium Niobate Modulator) Sensor Coil Polarizer SLED Detector Splitter Lithium niobate Phase Modulator Charge ups Electronic Circuit A/D DSP Digital or Analog Signal Output AMP Driver 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.
Disadvantages 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. Noise picked up by a long electro-optical cable A tethered and powered remote sensor head Sensor Coil
In summary Require expensive PM fiber and components PM fiber exhibits extinction ratio limitations specially when fiber’s length exceeds few kilometers PM fiber and components are radiation sensitive Require active bias or phase modulation in fiber coil via a phase modulator with its associated issues Phase modulation is always prone to electronic and optical drifts In case of tethered cable, noise can limit how far the sensor coil can be placed from its electronics Disadvantages of current IFOG with active bias and PM fiber are;