Open-loop Fiber Optic Gyroscope

Fizoptika Fiber Optic Gyroscope (FOG) is a precise yet most miniature fiber optic rotation sensor. It comprises fiber optic "minimum configuration" sensing assembly and analog processing electronics. The FOG is a robust, reliable, maintenance-free electro-optical device offering all advantages of the optical sensing technology. The main frame of the gyro sensor is made of aluminum alloy or hard plastic to withstand a wide temperature range and high levels of vibration and shocks. Possibility to use plastic housing gives the gyroscope the lowest weight in its size.

Fizoptika FOGs are produced using the spliceless technology when the entire fiber optic assembly is fabricated from a single fiber length. The technology provides the highest quality of the assembly with ZERO excess loss due to the absence of joints between components. The fiber is of a very special (40µ) polarization preserving type to suppress the effects of field rotation and polarization state changes that occur in the fiber. By fine optical tuning the assembly may acquire immunity to electromagnetic interference eliminating the need for heavy shielding. A single miniature analog PCB performs all necessary functions to process the optical signal and provide stability and precision conforming to the performance of the optical assembly.





The open-loop FOG architecture is illustrated by the Figure. The broadband light-emitting diode (SLD module) couples the light into an input/output fiber coupler (II). Then the light passes through a polarizer and a spatial filter which ensure the reciprocity of the counter-propagating light beams through the fiber coil. Another coupler (I) splits the two light beams in the fiber optic coil where they pass through a harmonic modulator (PZT). The modulator is offset from the center of the coil to impress a relative phase difference between the counter-propagating light beams. After passing through the fiber coil, the two light beams recombine and pass back through the polarizer and are directed onto the photodetector. Synchronous demodulation behind the detector converts the rotationally-induced first harmonic signal into a rate proportional voltage.







Some general properties of the open-loop gyroscope may be deduced from fundamental physical principles.

No dead zone or hysteresis. The Sagnac phase responds to rotation nearly instantly and without distortions. That is why the relation between the output voltage and the input angular rate is perfectly proportional. In some ways, the open-loop gyro is an ideal sensor of rotation.

Instant response. Sagnac phase delay is 0.8 µs (light transmission time) to rotation of the sensing coil. The practical limit of the gyro frequency range (1-1.5kHz) comes from the mechanical delay between rotation of the mounting frame and the sensing coil.

Bias immunity to acceleration (gravity). Constant acceleration does not create any phase difference between counter-propagating waves of the ring interferometer.




Detailed information about FOG's major parts and components can be found in Optical Assembly Info Notes.   




The open-loop sensor requires electronics to control SLD current and PZT voltage for signal conditioning and for precise demodulation of the interferometric signal after its conversion from the optical power to the receiver voltage.

For more information regarding the electronics refer to Analog Processing Board OE141-55 Info Notes  and Analog Processing Board OE141-35 Info Notes.  




Detailed information about  fiber optic gyro production technology and quality control can be found here.


DOWNLOAD a full version of Open-Loop Fiber Optic Gyroscope Info Notes.