Fibre optics transmission Distance Limitations - How they extend with what Technique ? ?

Fiber optic lines are extended by determining how far they can go to reach a certain Bit error rate then installing a repeater at that point.

A repeater receives the signal and re transmits it at full power.

Using single mode fibre helps.

Fiber signals can be amplified and repeated to extend range over great distances.

My central offices used BTI 7000s for that purpose.

The only caution is that as with all amplification, some noise is produced and therefore a reduction in signal to noise ratio.

At some point after so many hops, the noise floor will rise to near the level of the desired signal and you will begin to have problems.

I found the BTI 7000 to be an inexpensive and effective solution

There are basically two techniques. Regeneration and optical amplification.

Regeneration involves detecting the signal with an electronic output transducer, recovering the digital data and then sending it out as newly generated optical signal.

Optical amplification keeps the signal in the optical domain but uses scientific electronic magic to boost the optical power of the signal. I have no idea how it works.

Since regeneration goes back into the electronic domain there is the opportunity to do data management functions  - error checking etc. The recovered data signal is synchronised to a new clock, re packaged and made as good as the original output but at the cost of quite a lot electronic processing

Whereas, pure optical amplification just boosts the optical signal. Thus, for a very attenuated signal there are noise issues with keeping the boosted signal within a acceptable data eye.