Precise Optical Modulation Using Extinction-Ratio and Chirp Tunable Single-Drive Mach-Zehnder Modulator

We analyzed the complex transfer function and modulation spectrum of the Mach-Zehnder (MZ) modulator with optical power and phase change imbalances. From the calculations, we found that the characteristic operation can be obtained in three cases: high extinction ratio, zero-chirp, and straight-line trajectory in the IQ plane. However, the results also indicate that we cannot achieve an operation equivalent to that of the ideal modulator, due to the small parasitic chirp in a single-drive modulator. To control the extinction ratio and chirp parameter individually, we propose an integrated modulator with some tunable Y-branches. By using the optical-power-imbalance tunable modulators, we experimentally demonstrate the characteristic operations and obtain the modulation spectrum equivalent to the ideal MZ modulator. However, compared to the ideal amplitude modulator, MZ modulators exhibit an intrinsic nonlinearity in the sinusoidal response. Therefore, we used a third-order nonlinearity compensation method using the superposition of electrical third-order harmonics. The measured spectrum consisted of only first-order modulation sidebands, which is equivalent to the ideal double-sideband suppressed-carrier modulation.