Intra-cavity and/or inter-cavity sub-resonators with dielectric coated mirrors have anti-reflection and high-reflection coatings to attain high efficiency third and higher order harmonics generation of laser radiation. Main-resonators and sub-resonators enhance the fundamental and harmonic oscillations, respectively, which result in high conversion efficiency of third harmonics and higher. Such improved lasing efficiency may be implemented in high power lasers for improved power performance to attain over 8 W of UV output power with a high repetition rate. The use of intra-cavity THG techniques provide high efficiency by using existing high intensities of the input fundamental frequencies available in the intra-cavity region of the laser to avoid damage to components of the laser, such as the laser crystal. The THG utilizes a double-pass and/or multi-pass arrangement, which results in about a 60% increase in output power compared to a single-pass configuration. Also, blocking of UV energy generated in a return pass is performed to eliminate damage by the UV to the components in the main cavity.