A 440 nm solid-state blue laser produced by aperiodically poled lithium niobate(PPLN)crystal based on cascaded third-harmonic generation(THG)is introduced.According to Sellmeier equation and phase-matching conditions of secondharmonic generation(SHG)and sum-frequency generation(SFG),two different poling periods of PPLN were designed to realize SHG and SFG at the same working temperature,and then the cascaded THG can be realized through SHG and SFG.The fundamental laser was produced by Nd:YAG,and had a 110 n...