Bias-independent Frequency Response Measurement of Electro-optic Modulation Chips Utilizing Fixed Low-frequency Photodetection and Three-port Microwave De-embedding

In this work, we propose a bias-independent and self-calibration method for extracting the intrinsic frequency responses including modulation depth and half-wave voltage of electro-optic modulator (EOM) chips utilizing fixed low-frequency photodetection and three-port microwave de-embedding. The frequency response of electrical-to-optical (E-O) combined with the microwave adapter network (MAN) can be obtained by analyzing two fixed-low-frequency intermodulation tones in the case of two-tone modulation, free of bias control and optical-to-electrical (O-E) frequency response calibration. Then, the frequency response of MAN can be removed with the three-port microwave de-embedding, which allows the extracting of the modulation depths of the high-speed EOM chip. Finally, the intrinsic characteristic of half-wave voltage for the EOM chip can be achieved by retrieving the incident microwave power with power leveling calibration. In the experimental demonstration, an EOM chip in the frequency range of 40GHz is measured by detecting and analyzing the two intermodulation tones only at 400kHz and 800kHz, which enables wideband measurement with sub-MHz narrow-wideband signal processing. Our measurement features self-calibration of O-E frequency response and on-chip characterization capability, providing bias-independent analysis for measuring high-speed EOM chips due to the insensitivity to the bias voltages of the EOM chip. Moreover, thanks to the fixed low-frequency photodetection, the proposed method is promising to significantly improve the off-chip optical coupling efficiency with the assistance of a low-frequency free-space photodetector with a large sensitive area to enable efficient automatic probing.