On-Wafer High-Frequency Measurement of Mach–Zehnder Modulation Chips Based on Fiber-Free Coupling and Low-Frequency Photodetection

A self-referenced electrical method is proposed for measuring the high-frequency response of electrooptic modulator Mach-Zehnder modulator (MZM) chips based on fiber-free optical coupling and low-frequency photodetection, in which the intrinsic frequency responses including modulation depth and half-wave voltage are both extracted. With the help of two-tone modulation, the wideband combined response of the microwave adapter network and the MZM chip can be extracted by referencing the fixed difference-frequency components in the low-frequency region. Then, microwave fixture removal is implemented for de-embedding the uneven degradation response of the adapter network in terms of the transmission attenuation and the impedance mismatch. Finally, the power leveling technique is used to retrieve the incident microwave power to obtain the intrinsic modulation depth and half-wave voltage of the MZM chip. Frequency response of MZM chip is experimentally obtained up to 70 GHz with fiber-free coupling and sub-MHz photodetection. Our method features fixed low-frequency photodetection for on-wafer characterization of high-speed MZM chips, and it is very promising for automatic and noninvasive probing tests, thanks to the large photosensitive area optical coupling.