Advantages of alternate modulation formats in 40Gb/s transmission

We analyzed linear and nonlinear properties of non-retum-to-zero (NRZ), return-to-zero with 50% duty cycle (RZ50), return-to-zero with 33% duty cycle (RZ33), carrier-suppressed return-to-zero CSRZ, and differential phase shift keying (DPSK) modulation formats in order to predict the comparable advantages of their implementation in 40Gb/s WDM systems. We showed the advantage of RZ type formats over NRZ results from the trade off between better nonlinear tolerance and linear impairments such as inter-channel cross-talk and dispersion penalties. We estimated that in 40Gb/s 0.4 spectral efficiency WDM system on 10 100km spans of standard single mode fiber RZ33 format has Q value 1.5dB higher than NRZ. We also found that in a such system DPSK with controlled chip outperforms RZ33 by 0.8dB Q due to the difference in limiting nonlinear impairments and superior linear properties.