Fiber Optic Gyro (FOG) Teardown

by on April 18, 2022

KVH Fiber Optic Gyro

I recently had an opportunity to take apart a Fiber Optic Gyro (FOG). I have always known the theory behind a FOG and that there was a fiber optic cable that was used as the sensing element, however I was not prepared to actually see the fiber optic cable in the sensor. The unit I have to open is a single axis KVH 225140-1-3. It is a 12V version with RS-232 data output. Back in the day, this was a really nice unit!

Please enjoy these images below as we open up a FOG and get to see the actual fiber.

FOG direction
First step is removing the standoffs on the interface port. Also not the direction of measurement.
FOG Bottom
Lets flip it over to reveal the bottom and the 4 access screws.
FOG warranty void
I guess the warranty is now void…
FOG Bottom removed
Bottom removed and getting our first look inside. Note, the cutout in the PCB over what we will soon see as the light source.
FOG opening
Opening the sensor and exposing the main board.
FOG main board
Main board with the daughter board removed from the black header.
FOG under main board
With the main board removed, we can now get to the sensing element. Note, the cutout that was under the PCB cutout.
FOG sensing element
Lifting the cover things are starting to get exciting. You can see the spool of fiber optic cable. It has a clear gooey material on the top. You can also see near the bottom left where the signal is generated and at the top left where the cable loops through a phase modulator. The couplers for splitting the light to go around the fiber in opposite directions are integrated into the spool (under the clear goey stuff).
FOG fiber spool
Closeup showing the spool of fiber optic cable, and its many loops.
FOG fiber mm
Measuring the fiber shows that the fiber is around 0.2mm wide.
FOG Source
I think this is both the light source and detector
FOG wheel
I think this is a phase modulator for increasing the gyro’s sensitivity to motion

Hopefully you enjoyed seeing this teardown and how actual physical principles are implemented in hardware. Leave comments below if you have more details on how this works.

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