As an important technology for ultra-high-rate optical communication systems

advanced modulation technology can effectively improve their capabilities to withstand such adverse effects as chromatic dispersion

polarization mode dispersion and nonlinear effect.Minimum Shift Keying（MSK）is a constant envelope modulation technology with many advantages such as large dispersion tolerance and power concentration.This paper proposes an improved modulation scheme for ultra-high-rate optical communication systems

namely PM-MSK modulation scheme

in which MSK modulation is introduced into the Polarization Multiplexing（PM）system.Theoretical analysis and numerical simulation results show that this scheme has better BER performance than RZ and QPSK schemes.