Large enhancement in quasiphase matched second-harmonic generation efficiency by an external temperature gradient

We show that for continuous wave second-harmonic generation using quasiphas e matching in periodically segmented waveguides, thermal loading caused by the fundamental and the generated beams degrades the quasiphase matching co ndition which corrupts the spectral quality of the second-harmonic field. W e also show that a carefully designed external temperature gradient leads t o an enhancement of the conversion efficiency and control of the spectral l ine shape of the output field. Compared to earlier work, our simple scheme yields an enhancement of the second- harmonic conversion efficiency by a fa ctor of 2-4 depending on the input fundamental power. (C) 1998 American Ins titute of Physics. [S0003-6951(98)02152-4].