Multichip differential phase encoded optical transmission

We introduce a new multichip differential phase-shift keying (MC-DPSK) family of advanced optical modulation formats, whereby differential phase encoding is simultaneously applied to multiple (>2) consecutive time slots (chips). Four-chip quaternary phase MC-DPSK attains the same spectral efficiency (SE) as differential quaternary phase-shift keying while improving the quantum limit sensitivity by 1.4 dB [22.4 versus 31 photons per bit at 10^-9 bit-error rate (BER)]. A novel coded version of four-chip four-phase MC-DPSK using 16 codewords forming a maximally unbiased base attains 33% better SE than differential binary phase-shift keying, reaching the lowest quantum limit sensitivity to date (14.6 versus 20 photons per bit at 10^-9 BER).