We describe a characterization technique for electro-optic (EO) and optical-electric (OE) conversion devices, by using pure two-tone optical signal generation by using precise and high-speed modulation. A Mach-Zehnder modulator (MZM) provides precise and high-speed lightwave control as an EO conversion device, so that it would be good candidate for a reference signal source for conversion efficiency estimation. However, conventional modulators have imperfections, such as imbalance in optical circuits, electrode structures, etc. We have recently developed pure two-tone optical signal generation using high extinction ratio modulators consisting of active trimmers. This technique can be used for calibration of photodetectors, where theoretical radiation power can be defined precisely.