Mach–Zehnder modulator condition for a low chirp factor versus DC bias

An analysis of the Mach-Zehnder modulator's chirp factor α versus DC bias is presented. Unlike the well-known belief that α is infinite at 0 and π biases, it is shown that α can be zero at these biases under a careful balance of the modulations in the two modulator arms. Derivation of the condition to achieve this effect is given, and practical verifications are presented, where an excellent agreement with the theory is obtained. It is shown that modulators with a low residual chirp over a wide range of DC bias are achieved by designing the RF driving such that α would be minimized at zero bias instead of at quadrature bias as used currently in the modulator design.