Cohorent Transmission in optical communications
Why Do We Need Coherent Transmission? Before we had coherent transmission, we would send data via simple on-off keying (OOK). This means that the light is switched on and off to send data. This was very effective for transmitting data until we couldn’t switch the light on and off any faster. With further innovation OOK was pushed further, but we always hit a limit. What we needed was a big jump in data rates and not just incremental steps. We got this jump in bandwidth via higher order modulation. In this case “higher order” just means something more complicated than OOK. Let’s talk first about amplitude modulation. What is Amplitude Modulation? First of all, to modulate something means to exert an influence and change that thing in some way. For example, if you’re singing softly and then you sing louder, you’ve modulated your voice by adding more force (amplitude) to it. Therefore, you have changed (modulated) the power (amplitude) of your voice. Amplitude can be used