Interference Patterns in Optical Fibers: Causes and Solutions

[cyclone24]

Hey all

I am using optical fibers as a read-out medium form a PCB. I see interference in the data through this fiber. Can someone shed a light on the interference patterns on the optical fiber? Usually one should not see any electronic noise in fibers. How true is it and how to get around it? It is an SC-SC type fiber.

Thanks.

 

[NobodySpecial]

You mean interference in the RF sense - not an interference pattern in the light output right?

You can still have pickup and electrical noise in the transmitter and receiver - especially in the photodiode and amplifier on the receiving end.

 

[cyclone24]

Yes, in the RF sense. So are there any simple ways to suppress the noise? Thought there won't be electrical noise on the fibers, because its an 'optical fiber'.

 

[cyclone24]

anybody??

 

[berkeman]

Yes, in the RF sense. So are there any simple ways to suppress the noise? Thought there won't be electrical noise on the fibers, because its an 'optical fiber'.

There is no RF interference with the light in the fiber. Any noise pickup would be in the electronics either at the TX end or the RX end. I've used fiber to decouple noise from very high voltage immunity test equipment, but you have to do things right at the RX end to be able to see a clean signal.

Can you describe your setup more, and tell us what the interfering source is? What is the equipment monitoring the RX signal?

 

[cyclone24]

My setup consists of a small PCB with preamps acquiring digital data onto another PCB that stores the data counts. There is a 1 meter long LVDS cable connecting these two PCBs. Tx, Rx optical fibers are connected from the 2nd PCB (where the data is stored) to the PCI card in a computer.

Now there is a strong RF source which is the radiating element very close to this setup which couples to DC circuit, during the RF's duty cycle. I tried Faraday cage shielding and used decoupling filters. Still there are traces of interference.

Whenever the RF is active, it corrupts the data and the noise is picked up (I guess) on these fibers and received (Rx) by the computer.

I wanted to know the mechanism of noise transfer on fibers and how to suppress it or divert it, if it is not the RF interfering with the light.

Thanks

 

[berkeman]

My setup consists of a small PCB with preamps acquiring digital data onto another PCB that stores the data counts. There is a 1 meter long LVDS cable connecting these two PCBs. Tx, Rx optical fibers are connected from the 2nd PCB (where the data is stored) to the PCI card in a computer.

Now there is a strong RF source which is the radiating element very close to this setup which couples to DC circuit, during the RF's duty cycle. I tried Faraday cage shielding and used decoupling filters. Still there are traces of interference.

Whenever the RF is active, it corrupts the data and the noise is picked up (I guess) on these fibers and received (Rx) by the computer.

I wanted to know the mechanism of noise transfer on fibers and how to suppress it or divert it, if it is not the RF interfering with the light.

Thanks

If you disconnect the fiber, do you still see the noise at the RX end?

 

[berkeman]

And to elaborate more on the setup of mine that I mentioned. I ended up needing to move the RX end at least 15m away from the source, I had to run it off of battery power with no external connections to anything, and I needed to enclose it in a Faraday cage of conductive metal screening. Once I did that, I had no noise signal with no fiber attached, and I had a valid signal with the fiber attached.

There are lots of ways that noise can get into a measurement...

 

[cyclone24]

No, I don't see it...

 

[berkeman]

No, I don't see it...

Okay, that would imply that the problem is at the TX end. Can you post a schematic of the TX circuit & PCB layout?

 

[cyclone24]

Will try...thanx!