Duobinary modulation (DB) is implemented in optical communications by using three logical states, where:
A binary zero is represented by the absence of a laser pulse; binary 1s can be represented by a laser pulse with altered phase, based on the previous symbols in the following manner.
Phase of a binary symbol is shifted by π if there is an odd number of binary 0 between two binary 1.
It can be combined with RZ or NRZ rule.
Making the pulse length shorter than the duration of a logical symbol 1 causes the power falling to zero between two (or more) logical 1s. For data containing long chains of binary 1, it is a practical solution for synchronization issues.
Advantages of DB:
Transceiver’s construction:
The DB transmitter consists of an amplitude dual-arm MZM with two electrical inputs:
In the following figure, +1 corresponds to phase 0 and -1 corresponds to phase π or its odd multiples.