Any-point bias control of Mach-Zehnder modulators using digital filters
Feedback control is used to maintain Mach-Zehnder modulators at a desired bias point,
since heating and aging effects in its electro-optic materials cause a drift in its transfer function
over time. Considering that the desired bias point in the MZM transfer curve varies among
different applications, the control method should have an adjustable setpoint. The proposed
method seeks to achieve this with the minimum number of components possible.
SIMULINK models were used to obtain a better understanding of MZM biasing. Then,
after comparing available commercial products and a literature review, it was decided to apply
a known technique whereby a low-frequency dither signal is applied to the MZM DC input,
the output of the MZM is measured and a PID controller adjusts the MZM DC voltage to
maintain a constant ratio of the dither tone’s 2nd harmonic and fundamental. This method is
independent of optical power and allows the bias to be set to any point in the transfer curve.
To eliminate the need for standalone filters for each frequency and an additional IC for
obtaining the power ratio, digital filtering, processing and PID control are all done within a
microcontroller. A prototype board was built and the bias control measurements are presented.
The application of this method to Si MZM is also briefly discussed