Digital Predistortion Techniques for Finite Extinction Ratio IQ Mach–Zehnder Modulators

We present a digital predistortion (DPD) technique to compensate for in-phase and quadrature (IQ) Mach–Zehnder modulators (MZM) with finite extinction ratio (ER). The proposed method significantly improves the system performance by reducing the impairments introduced by the component imperfections. Our technique is performed in two steps: First, the $\arcsin$ predistorts the nonlinear sinusoidal transfer function of the modulator; second, a constrained optimization method, that utilizes the gradient descent algorithm, predistorts the modulator driving signals, thus mitigating the distortion induced by the finite ER. Since the optimal gradient calculation is rather complex, we introduce a simplified polynomial-based approximation that achieves comparable performance. The DPD gain is assessed via numerical simulations by varying modulation format and ER in an optical back-to-back configuration. The results are presented as required optical signal-to-noise ratio versus the modulator output power ( $\text{P}_{\text{MZM}}$ ) normalized with respect to the MZM input power (continuous wave of the laser, $\text{P}_{\text{CW}}$ ). A considerable improvement is showed when compared to the case without DPD or with the $\arcsin$ module for ERs $\leq$  20 dB and 32/64QAM.