More efficient second harmonic generation of whispering gallery modes by selective out-coupling

We demonstrate second harmonic generation (SHG) in an $x$-cut congruent lithium niobate (LN) whispering gallery mode resonator. We first show theoretically that independent control of the coupling of the pump and signal modes is optimal for high conversion rates. A scheme based on our earlier work in Ref. [1] is then implemented experimentally to verify this. Thereby we are able to improve on the efficiency of SHG by more than an order of magnitude by selectively out-coupling using a LN prism, utilizing the birefringence of it and the resonator in kind. We report 5.28%/mW efficiency for SHG from 1555.4 nm to 777.7 nm.