© 2019 IEEE. Nested Mach-Zehnder modulators, such as DQPSK, DP-mQAm etc., are used in coherent optical communication systems to perform electrical-to-optical data up-conversion and to enable higher order modulation formats in the optical domain. However, their reliability can be affected by drifts in the transfer function, for example due to ambient temperature changes. Maintaining optimum bias voltages (e.g. at quadrature) is a requirement to ensure best performance. We propose a potentially low cost real-time monitoring solution of the biases in nested MZMs using low frequency analogue electronics. This monitoring signal can be used to create an automatic bias control.