Sagnac interferometer Fiber Optic Gyroscope ( FOG ) construction and testing

Demonstration of a simple, low cost Sagnac interferometer, also known as a Fiber Optic Gyroscope ( FOG ).

Category:

Science & Technology

Tags:

• Sagnac Interferometer
• Fiber Optic Gyroscope
• Fibre Optic Gyroscope
• FOG
• Harass
• counter-rotating
• laser

[plenum88]

The fringe shift is proportional to the area enclosed by the fiber loop, so the longer the fiber the better. The shortest I have done is 40 meters, you could go shorter but then you will have to amplify the fringe shift a lot. Generally they are around 50-1000 meters long.

[plenum88]

Easy way - calibrate it by running it on a 33 rpm and 45 rpm turntable, determine the voltage at each speed. Hard way - determine the voltage swing on your PIN diode from maximum destructive interference (lowest voltage on multimeter) to maximum constructive interference (highest voltage on multimeter) to find the voltage difference corresponding to a 180 degree phase shift. Then use the Sagnac interferometer simulator on my website to calculate the theoretical phase shift per angular velocity.

[plenum88]

I used a PIN photodiode with a build-in FC connector so the fiber plugs right into it - got it from the lightwavestore in Ottawa for $28 - it is model PIN-1300-FCI. Since the output is primarily DC, you can take the voltage directly off two of the three pins (photovoltaic mode). If you want to amplify the signal further, see the amps that I describe in the part2 and part3 videos on my channel.

[plenum88]

There could be higher vibration sensitivity - I haven't tried PMF to compare. I have tried running powerful magnets along the side of the fibers - no Faraday effect that I can detect. Very high electric fields might have an effect (Pockel's effect or Kerr effect) but you might need specialty fiber. Putting the fiber under tension (stretching) is a simple way of changing its refractive index, and thereby the speed of light in a given arm.

[plenum88]

One important point - although I am using a device commonly known as a "polarization controller", the primary function of adjusting the angle of the paddle is to adjust the phase relationship between the two beams at the output to destructive interference, rather than the polarization.

[plenum88]

I know that it is often suggested to use polarization-maintaining fiber and a polarization filter. However, in experimenting with various setups I have discovered that you can make an adequate Sagnac interferometer (like 20 deg. per hour sens.) with single mode fiber (like SMF-28) and without a polarization filter, which is how I have done it. You can also use a generic source like a DFB or MQW IR laser. It is actually surpising how well it works with such low cost materials!

[vizier87]

what's the minimum length of the fiber Mr Plenum? I'm interrogating one, and I need to make a simple configuration of an IM. Thanks.

[forrest2shay]

@plenum88

Hi, how then do i calculate the rotation rate with the voltage i get from the multimeter? i got stocked here.

Thx

[forrest2shay]

@plenum88

How did you build your photodetector. i have one and i am having problems building it up.

[wazerface]

Thanks for the reply. This low cost build is very inspiring. The great performance when you throw out the filter and PMF is really a neat discovery. Do you think this results in possibly higher sensitivity to vibration because twisting the fiber might affect the interference?

Also, I've been wondering about other methods of inducing asymmetry in light speed. Do you know any physical phenomena that don't directly involve rotation? For example, applying an electric field to GaAs or something?