 For instance, in a 40 Gbit/s system, one bit corresponds to 25 picoseconds, and light travels 5 mm in a fiber optics or 7.5 mm in free space within that period.Thus the optical path difference between the two beams is 5 mm or 7.5 mm depending on the type of interferometer used.DQPSK is the four-level version of DPSK.