Abstract
Blue second-harmonic generation (SHG) in waveguides has been investigated intensively over the past five years and has progressed siginificantly. Now it is thought one of the most promising candidates for a compact blue laser that is necessary for higher density optical disk systems. Cerenkov radiation SHG from a proton exchanged LiNbO3 waveguide opened up such research stream in 19871). Although it demonstrated a mW-leve1 blue output using a laser diode, it unfortunately had a serious drawback of poor focusing characteristics because of its large(l6°) Cerenkov angle. Then quasi-phase matching (QPM) SHG frorn a LiNbO3 waveguide followed2)3)4). The QPM SHG has progressed very rapidly with the improvement in the fabrication process for a periodic domain inversion structure. In particular, a segmented KTiOPO4 waveguide5) and a proton exchanged LiTaO3 waveguide6) reported >10mW blue output with the normalized efficiency of around 100%/Wcm2. LiNbO3 might catch up soon since various methods to fabricate a periodic domain inversion structure with a large aspect ratio have been investigated7). However, the small tolerances in temperature (2-3°Ccm) and a fundamental wavelength (0.1-0.2nmcm) are serious problems for a mass production of SHG using a laser diode. In this respect, a Cerenkov radiation SHG is still very attractive because of its large tolerances owing to automatic phase-matching characteristics.
© 1992 Optical Society of America
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