Comparison of twin-SSB modulation schemes

We investigated and compared the performances of three different types of optical twin-single sideband (twin-SSB) modulation methods. Twin-SSB can transmit two different signals on lower side-band (LSB) and upper side-band (USB), and has been studied as a technique to improve spectral efficiency. Twin-SSB arranges the spectra side-by-side without any spectral spacing. Dispersion tolerance of twin-SSB is as high as that of conventional SSB schemes. The twin-SSB modulation can be performed by using a dual-polarization quadrature phase shift keying (DP-QPSK) modulator, a QPSK modulator, or a single Mach-Zehnder modulator (MZM). The twin-SSB with the three modulation methods need to be modulated at a limited linear domain of the optical field output function of the modulator. Therefore, if we apply a large electrical modulation signal to the modulators to decrease the optical loss, the nonlinear characteristics of the modulators degrade the performance of the twin-SSB signals. We investigated the modulation performance by changing the amplitude of the electrical modulation signal for the modulators, and investigated the optimal conditions of the twin-SSB modulation methods. We also performed numerical simulations of 50-km standard single mode fiber (SSMF) transmission of the twin-SSB signals with the optimal modulation conditions, and evaluated the error vector magnitude (EVM) performances of the transmitted signals. The results revealed that the EVM performances of the twin-SSB signals using a DP-QPSK modulator and a QPSK modulator were superior to that of a single MZM.