I try to build bidirectional audio communication by fiber optic cable using "Etal P3000" SMD audio transformer. But there is humming sound at normal condition. I think one of mic sound and the other side speaker sound and the other side mic pick up the noise and send back to the first speaker.
How to filter that humming sound.

 

I try to build bidirectional audio communication by fiber optic cable using "Etal P3000" SMD audio transformer. But there is humming sound at normal condition. I think one of mic sound and the other side speaker sound and the other side mic pick up the noise and send back to the first speaker.
How to filter that humming sound.

You have a telephone-bandwith audio transfomer and some fibre-optics. Presumably you have other components as well to make up the system, any one of which could be introducing hum into the system. Without knowing what they are, we would all be guessing what the problem is.
Do you have an unidirectional or omnidirectional microphone?

 

Many audio transformers will pickup hum from nearby AC MAGNETIC FIELDS. That is certain. What is rather confusing is how an audio transformer is being used with fiber optics. I totally do not understand the operation of s fiber optic magnetic transformer.
As for bi-directional transformers, they were common in the telephone industry at one time. So there are quite a few designs available.

The terribly bad news is that my two free circuit websites do not seem to be on line any more. WORSE YET, the "Free information society" site name has been stolen by evil trolls. The Schematics for free site simply can not be found by google.
In addition, we need a much more detailed description of what you are trying to do, because transformers and fiber optics do not go together.