1. Date of download: 6/3/2016 Copyright © 2016 SPIE. All rights reserved. Schematic diagram of the proposed method based on heterodyne mixing, laser diode (LD), polarization controller (PC), dual-drive Mach–Zehnder modulator (DDMZM), microwave source (MS), acousto-optic modulator (AOM), optical spectrum analyzer (OSA), photodetector (PD), and electrical spectrum analyzer (ESA). Figure Legend: From: Calibration-free and bias-drift-free microwave characterization of dual-drive Mach– Zehnder modulators using heterodyne mixing Opt. Eng. 2015;55(3):031109. doi:10.1117/1.OE.55.3.031109

2. Date of download: 6/3/2016 Copyright © 2016 SPIE. All rights reserved. Measured electrical spectrum of the heterodyning products: (a) the inset shows a zoom-in of spectra lines of the desired frequency components at around f2, and (b) the corresponding optical spectrum of optical modulated signal under the same condition except with closed MS2. Figure Legend: From: Calibration-free and bias-drift-free microwave characterization of dual-drive Mach– Zehnder modulators using heterodyne mixing Opt. Eng. 2015;55(3):031109. doi:10.1117/1.OE.55.3.031109

3. Date of download: 6/3/2016 Copyright © 2016 SPIE. All rights reserved. Measured electrical power of the desired four frequency components as a function of bias voltage, where the two-tone microwave signal is applied on the upper electrode of DDMZM. Figure Legend: From: Calibration-free and bias-drift-free microwave characterization of dual-drive Mach– Zehnder modulators using heterodyne mixing Opt. Eng. 2015;55(3):031109. doi:10.1117/1.OE.55.3.031109

4. Date of download: 6/3/2016 Copyright © 2016 SPIE. All rights reserved. Measured electrical spectra of the desired frequency components in the case of different modulation frequencies. Figure Legend: From: Calibration-free and bias-drift-free microwave characterization of dual-drive Mach– Zehnder modulators using heterodyne mixing Opt. Eng. 2015;55(3):031109. doi:10.1117/1.OE.55.3.031109

5. Date of download: 6/3/2016 Copyright © 2016 SPIE. All rights reserved. Measured modulation depths of the DDMZM as a function of modulation frequency with our method (red lines and blue lines) and the OSA method (open squares and circles), where the superscript u or l corresponds to the upper or lower electrode of DDMZM. The inset shows the electrical driving amplitude of the DDMZM in the measurement. Figure Legend: From: Calibration-free and bias-drift-free microwave characterization of dual-drive Mach– Zehnder modulators using heterodyne mixing Opt. Eng. 2015;55(3):031109. doi:10.1117/1.OE.55.3.031109

6. Date of download: 6/3/2016 Copyright © 2016 SPIE. All rights reserved. Measured half-wave voltages of the DDMZM as a function of modulation frequency with our method (red and blue lines) and with the OSA method (open squares and circles). Figure Legend: From: Calibration-free and bias-drift-free microwave characterization of dual-drive Mach– Zehnder modulators using heterodyne mixing Opt. Eng. 2015;55(3):031109. doi:10.1117/1.OE.55.3.031109

7. Date of download: 6/3/2016 Copyright © 2016 SPIE. All rights reserved. Error transfer factor as a function of the modulation depth. Figure Legend: From: Calibration-free and bias-drift-free microwave characterization of dual-drive Mach– Zehnder modulators using heterodyne mixing Opt. Eng. 2015;55(3):031109. doi:10.1117/1.OE.55.3.031109