Download Commercial IFOGs

Commercial IFOGs
A Commercial (Minimum Configuration) IFOG
Electronic Circuit
Polarizer
SLED
Splitter
Detector
A/D
Splitter
Sensor
Coil
AMP
Driver
Phase
Modulator
DSP
Digital or Analog Signal Output
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.
Shortcomings of Current IFOGs
Disadvantages of Current IFOGs (PM fiber)
Slow axis
Fast axis
A PM Splitter
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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.
Disadvantages 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 Circuit
Polarizer
SLED
Splitter
Detector
A/D
Splitter
Sensor
Coil
AMP
Driver
DSP
PZT Phase Modulator
stressing and heating the fiber
Digital or Analog Signal Output
Disadvantages 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 Circuit
Polarizer
SLED
Splitter
Splitter
Detector
A/D
AMP
PZT
Driver
DSP
Digital or Analog Signal Output
Sensor
Size
Disadvantages 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 Circuit
Polarizer
SLED
Splitter
Splitter
Sensor
Coil
Detector
A/D
AMP
Driver
PZT
DSP
Noise Generation
Digital or Analog Signal Output
Electronic noise
Optical noise
Disadvantages 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 Circuit
Polarizer
SLED
Splitter
Splitter
Sensor
Coil
Detector
A/D
AMP
Driver
DSP
Digital or Analog Signal Output
Lithium niobate
Phase Modulator
Charge ups
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.
A tethered and powered remote sensor head
Sensor
Coil
Noise picked up by a long
electro-optical cable
In summary
Disadvantages of current IFOG with active bias and PM fiber are;
• 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