High-speed versatile optical modulator based on integrated LiNbO ₃ waveguides

Various advanced modulation formats, such as on-off-keying (OOK) with carving techniques, phase-shift-keying (PSK), frequency-shin-keying (FSK), etc, were investigated to obtain enhanced spectral efficiency or receiver sensitivity. Orthogonal modulation techniques with OOK and FSK or OOK and PSK are also useful for optical labeling in packet systems. Recently, we reported a versatile optical modulator consisting of a main Mach-Zehnder interferometer (MZI) which has two sub MZIs embedded in the two arms of the main MZI. The modulator can generate return-to-zero (RZ) OOK, binary phase-shift-keying (BPSK), quadrature phase-shift-keying (QPSK), wideband FSK, continuous-phase FSK, etc. By using this modulator, we demonstrated optical DQPSK modulation at 80Gbit/s. For QPSK modulation, two data signals are applied to the two sub MZ structures, to achieve control of in-phase and quadrature components. The versatile modulator can be also used for ultra high extinction ratio (ER) intensity modulation. The sub MZIs in the main MZI can compensate imbalance between the arms of the main MZI, where the imbalance is mainly due to fabrication error. The ER can be higher than 70dB. By using this technique, we can generate super stable two-tone lightwaves, where we can achieve large spurious suppression ration higher than 40dB.