Power spectra of return-to-zero optical signals

Analytical formulas for the power spectra of return-to-zero (RZ) optical signals generated by Mach-Zehnder (MZ) modulators are derived. Pulse duty cycles of 33%, 50%, and 67%, in conjunction with several modulation techniques, including binary ON-OFF keying (OOK), duobinary OOK, and M-ary differential phase-shift keying (DPSK), phase-shift keying (PSK), and quadrature-amplitude modulation (QAM), are considered. Spectral characteristics and bandwidth requirements of these different schemes are compared.