平成２１年度　実績報告書

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教育研究プラットフォームIDER研究成果/ポストドクター・GCOE研究員New microwave signal generation device using difference frequency generation in a rectangular waveguide Quang - Hong Ngo D epartment of Systems Innovation, Graduate School of Engineering , Osaka University Okamura Laboratory Abstract I propose a new microwave signal generation device using difference frequency generation in a periodically - poled LiTaO 3 rectangular waveguide. I expect the proposed signal generation scheme can be useful for high - frequency signal generation . Introduction Millimeter - /submillimeter wave s are attractive not only for astronomy, imaging, sensor, and radar but also for broadband communications. The millimeter - /submilli - meter wave signal generation by all - electronic techniques is difficult due to the restriction of electronic device characteristics at high - frequency range over 100 GHz. Difference frequency generation (DFG) based on the second order nonlinear optical effect has been widely used for the signal generation in light frequency ranges. D FG using two coherent lightwaves is also a potential candidate for the microwave/ milli meter - wave signal generation , because i t is possible to generate a microwave/ milli meter - wave signal at a desired frequency by simply adjusting the frequency difference be tween the two coherent lightwaves. Device structure Figure 1 shows the structure of the proposed device. Z - cut LiTaO 3 crystal is used as a nonlinear optical material for DFG . Other nonlinear optical crystals like LiNbO 3 or KNbO 3 are also applicable. The s urfaces of the four side walls of the crystal are covered with a thin metal film to make a rectangular waveguide . Two ends of the crystal are uncovered for lightwave input and generated microwave output. They are also polished to enhance the reflectivity a nd to form a microwave cavity. A periodically - poled structure is constructed for Q P M between the optical signals and the generated microwave signal. The microwave is generated at the design frequency f 3 = f 1 - f 2 by DFG based on the second order nonlinear optical effect when the lightwaves with frequency f 1 and f 2 are launched to the proposed device. The rectangular waveguide is designed as a single - guided mode structure for the generated microwave. The QPM technique is used to compensate for the phase - mismatching between the input lightwaves and the generated micr owave. Resonance effect of a cavity is also utilized for enhancement of the output signal level. Discussion and conclusion The proposed devi ce has the following good characteristics : generated signal can be shifted to a higher - frequency range with a highe r power by simply reducing size of the cross section of the rectangular waveguide, the device has a simple structure and peak generation frequency can be adjusted by tuning the polarization reversal period and resonance condition. A microwave signal is gen erated when a frequency difference between the input optical signals coincides with th e design frequency. However, other input optical signals can pass through it without any disturbance. Due to the DFG characteristics, t he generated power level drasticall y increases as frequency increases. In addition , a pplication of an optica l waveguide structure is attractive for improving the conversion efficiency because of its diffraction - less propagation characteristics. Perfect p hase matching is also possible by u t ilizing the dispersion characteristics of the rectangular waveguide . Based on these advantages, I expect the signal generation scheme to be applicable for generation of high - frequency signal up to Terahertz. D etection of high - speed vector - modulat ion optica l signal is also possible. Fig. 1 . S tructure of the proposed device . 97

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