An example of the measured detected signal spectrum is shown in Fig. 6 (a). The maximum detected signal was observed at 15GHz, which was in good agreement with expected characteristics calculated from the designed frequency (15GHz) by the periodic poling. Fig. 6 (b) shows the measured frequency response of the detected signal when the frequency of the light modulation signal was tuned from 13GHz to 19GHz. The measured frequency response coincided with the theoretical frequency response of the detected signal, which was calculated from DFG based on thsecond order nonlinear opticaleffect with QPM in the periodically-poled LiTaO3 microwave rectangular waveguide and the measured microwave resonance characteristics of the fabricated device (Fig. 2).