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