Sagnac Interferometer (Fiber Gyro)

[Voltux]

How does one determine which direction a sagnac interferometer is turning in a fiber optic gyro? ("FOG")

I have been reading about them and found some DIY examples where the output is taken using a photo-diode. Now my understanding is that these photo-diodes are simply measuring intensity but isn't the output of the interferometer simply a fringe pattern?

Any additional information, sources, references, imagery, or calculations would be incredibly appreciated.

Thank you!

Reference 1 - Sagnac Interferometer: http://www.conspiracyoflight.com/Sagnac/Sagnac.html

Reference 2 - Fringe Pattern Generated (CW + CCW): http://www.conspiracyoflight.com/pd...ed_on_a_Platform_in_Uniform_Motion_(1942).pdf

 

[Ibix]

The output is two light beams with a phase difference between them and you want to measure that phase difference. Depending on how you combine the beams you can get different patterns, and which one you prefer depends on what you're doing with it.

If the beams are perfectly collimated and perfectly parallel then the output is just a field whose brightness depends on the phase difference. If you tilt one of the beams very slightly then you add a phase difference that varies across your output field and you get fringes like in the pictures and you will see them move as the interferometer changes rotation rate.

If you are doing a demo for students, or planning on getting a few photos to publish in a journal then the fringes are preferred. They're easier for humans to read and interpret than the "black cat at night/polar bear in a snowstorm" output of the perfectly aligned configuration. On the other hand, a simple dark/light field is perfect for a photodiode to read.

I haven't completely thought this through, but you may also need some additional optics for the perfectly aligned configuration to be practical,since you want zero spun to give you a half wave path difference so that clockwise/anticlockwise corresponds to brighter and darker. Presumably that's paid off by the simpler output interpretation.