Hi,
The simplest way is probably to simply purchase a transmitter and receiver, it two transceivers. They will convert the electrical signal to optical signals and back again so you only have to worry about the electrical signals.
You'll also have to think about how they connect to the optical cable, which might vary a bit, but should be found in the manufacturer's data sheets.
If you only have to do 9600 bps though you may want to build up something yourself but because it is far away you might have to use a laser diode rather than an LED. That could get a little more complicated because of the driver.
There's also a chance you might need a repeater but if that's not possible then it would probably be better to buy something that already does the conversions.