4. Detection techniques

4.1 Direct detection

At the receiver, light must be converted into two-state electrical signals before it reaches its destination. The most common detection scheme is based on the use of a simple photodiode, followed by a decision flip-flop. The photodiode converts the optical energy received (number of photons or square of the modulus of the electric field E _s (t ), hence the nickname "quadratic") into a current i (t ), which is directly proportional to it and is called photocurrent, according to :