Ultra-Wide (a Few Tens THz) Optical Sideband Generation by Large Amplitude Electrooptic Modulation and Its Applications.

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We have developed new electrooptical methods which make it possible to generate extremely wide modulation sidebands. In the first year of this research, we developed novel electrooptic modulators which operate in large amplitude in microwave and millimeter wave regions.We overcome the limitation of velocity miss-matching in a high frequency electrooptic modulator with a novel velocity matching scheme, which makes a modulator efficient in millimeter wave and microwave regions. The velocity matching between modulating microwave and light has been achieved by using higher order microwave resonant modes on a micro strip line. We developed the electrooptic modulator/deflector of an LiTaO_3 crystal at 16 GHz. An experiment on large amplitude phase modulation was carried out with the velocity-matched modulator. As the result, we obtained modulation sidebands of 0.7 THz width, which is a record of modulation sideband at the frequency.In the next year, we tried the combination of a Ti : Sapphire laser and the large amplitude modulator to extend the modulation sideband width . For the first step of this study, an experiment of mode locking using an electrooptic phase modulator was tested with a coupled cavity configuration. For this experiment, we prepare another electrooptic phase modulator whose operating frequency was the internal longitudinal mode spacing of the laser. The modulator was placed within the external cavity. We confirmed the mode locking and obtained ultrashort pulses.We estimate after these experiments that the combination of the velocity matched microwave modulator and a Ti : Sapphire laser will extend the modulation sideband width to several tens tera-hertz.