It sounds like you have a wide-band
modem, an electrical interface to a Light Emitting
Diode. Some have bandwidths up to 2.5GHz using solid-state
LASER diodes, where the digital signal is a complex
set of subcarriers with different phase relationships.
The simpler types are purely analog, and by using
low-capacitance LEDs they ensure a wide bandwidth
up to say 100MHz. The light intensity received at
a photo-diode is more or less proportional to the
analog signal fed to the LED at the transmitter.
It is not a question of using so much bandwidth,
as having the bandwidth available, the limiting
factor being not the fibre but thebandwidth of the
electrical to light converter, the LED. Intrinsic
parasitic capacitance and inductance limit thebandwidth
at that point. The bandwidth of a monomode fiber
is at least 10GHz, but the bandwidth of a purely
analog LED is far less as noted above. I have seen
another interesting application of these devices
where it was required to isolate a variable frequency
HF oscillator from frequency drift caused by load
impedance "pulling". The oscillator fed an analog
opto-isolator (integrated LED and photo-diode).
Due to the coupling being optical, the isolation
was > 100dB.