Cavity-enhanced frequency doubling with a third-order quasi-phase-matched PPKTP crystal

Blue lasers are very useful for applications in fundamental sciences and advanced technologies. Although the frequency doubling with a first-order quasi-phase-matched (QPM) crystal is an effective approach to obtain blue and UV lasers, the crystal should be poled with a short period, which is technically difficult to fabricate compared with a long period. Using a third-order QPM can be an alternative way to generate blue light, in which the poling period is 3 times larger than that in first-order QPM. In this work, we report on the generation of a 402.5 nm laser by using cavity-enhanced second-harmonic generation and a periodically poled KTP (PPKTP) crystal with a poling period of 10.02 µm. About 36.49 mW output power and 15.3% conversion efficiency are achieved in our experiment.