Lasers vs Diodes, modulation speed
I have been wondering why lasers are the preferred sources for super high data rate telecom links.
Lasers have the property of coherence due to their smaller linewidth, much smaller than the LED linewidth.
Lasers work well with fibers and suffer much less distorsion than LEDs.
In free space propagation lasers are collimated beams and have a higher power density over larger distances than LEDs.
However, let's consider a short free space propagation where power density and beam spread are not an issue. Take a laser and a LED of the same central frequency.
Using an external, amplitude modulator, like a chopper, is it possible to modulate faster a laser or a LED?
Will the LED pulses be of poorer quality after a certain chopping speed? Why? Is it due to the different coherence properties of the two sources, laser and LED?
Today, in the market, what is the max speed at which we can ON-OFF key a laser and a LED? Gigabits/s or lower? Is the modulation always direct modulation, acting on the current that operates the laser? Is external modulation too expensive?