Solution:

In communications applications the following characteristics of LEDs are important:

Low Cost:

• LEDs have been very low in cost compared to communication lasers. This is highly controversial. Communication LEDs and lasers are not too different in their structures and are comparable in manufacturing cost.

Low Power:

• The maximum light output of an LED has typically been a lot lower than that of a laser (about 100 microwatts). However, recently a new class of LEDs, with an output of up to 75 milliwatts, have become available.

Incoherent Light:

• The light produced is neither directional nor coherent. This means that you need a lens to focus the light onto the end of a fiber. LEDs are not suitable for use with single-mode fiber for this reason (it is too hard to get the light into the narrow core).

Digital Modulation:

• Digital modulation is straightforward. The device “turns on” when the forward voltage applied results in a potential across the junction greater than the bandgap energy required. It extinguishes when the voltage drops below that.

Analogue Modulation:

• LEDs can also be analog modulated quite simply by maintaining a forward bias just larger than the bandgap energy (since the device the response is linear with the current flow).