A laser diode should be biased slightly above its threshold current (i.e. the current required to turn the laser on). A laser is responsible for sending digital data through a fiber optic cable, which means there are only two required output values of the laser: logic 0 and 1. Sending a logic 0 does not mean the laser has to be completely off. It only needs to generate a logic 1 that has a higher optical power than 0. The laser will achieve better performance and a higher data rate if you modulate the laser above its threshold current rather than modulate above and below it. The graph below shows the recommended operating area.