Optimal linear operation point control of silicon-based Mach-Zehnder modulator

We propose and demonstrate a control system to automatically bias the silicon carrier-depletion-based Mach-Zehnder modulator (MZM) at the optimal linear operation point for microwave photonics links. In this system, the RF modulation signal to drive the MZM is superposed with a 5 MHz low-frequency two-tone dither signal. The powers of fundamental harmonic (FH) and third-order intermodulation distortion (IMD3) components caused by the low-frequency dither signal are extracted to calculate the spurs-free dynamic range (SFDR) of the link. The control system sweeps the reverse bias voltage on the PN junction and the static optical path difference between the two modulation arms point-by-point, and then screens out the optimal linear modulation point with the highest SFDR at the dithering frequency. To validate the linearization effect at the RF modulation frequency, the SFDR measurement is implemented at 500 MHz. The SFDR at the linear operation point found by the control system is 107.5 dB·Hz^2/3 at 500 MHz, which is 6 dB better than that measured at the quadrature point.