Light-emitting diodes: The unknown entities

Light-Emitting Diodes (LEDs) constitute a low-cost choice for optical transmitters in medium-bit-rate. What differentiates such applications from, e.g., backhaul optical networks, is that for low-cost applications, the LED used is generally not as well characterised by the manufacturer as, are for example, optical fibre amplifiers. While for simple modulation formats like on-off keying (OOK) this lack of knowledge is not always an impediment, in any other situation one faces the often puzzling behaviour of commercial LEDs. In this tutorial the main electro-optical characteristics of LEDs are introduced and the models available in the literature are shown to be inadequate when designing optical links based on advanced modulation techniques or even OOK at very high data rates. As a way out of this grid lock a reverse-engineering approach is presented, enabling the introduction of a realistic model for commercially available LEDs.