It is the simplest line code, directly encoding the bitstream, and is analogous to on-off keying in modulation.
[1] Its drawbacks are that it is not self-clocking and it has a significant DC component, which can be halved by using return-to-zero, where the signal returns to zero in the middle of the bit period.
With a 50% duty cycle each rectangular pulse is only at a positive voltage for half of the bit period.
NRZ (Non-Return-to-Zero) - Traditionally, a unipolar scheme was designed as a non-return-to-zero (NRZ) scheme, in which the positive voltage defines bit 1 and the zero voltage defines bit 0.
An Optical Orthogonal Code (OOC) is a family of (0,1) sequences with good auto- and cross-correlation properties for unipolar environments.