External cavity resonant frequency doubling technology has significant potential in precision measurement,quantum optics,and advanced manufacturing,and receives the favor of researchers for its ability to produce single frequency continuous short band lasers with high conversion efficiency and narrow linewidth.In this paper,the principle of external cavity harmonic generation technology and the locking methods of optical resonators are briefly introduced.A comprehensive review is conducted on the research progress of this technology in frequency doubling using semiconductor lasers,fiber lasers and solid state lasers as fundamental light sources.Technical indicators such as wavelength,output power,conversion efficiency,and laser linewidth are highlighted.Additionally,the research achievements are introduced,where a narrow linewidth single frequency fiber laser is utilized as the fundamental light source to achieve single frequency continuous green light output at 532 nm with a power of 10.6 W through frequency doubling.