Theoretical modeling on resonating the second harmonic for ultraviolet laser generation

The resonance of the second harmonic was proposed to improve conversion efficiency in ultraviolet laser. A schematic to generate the fourth harmonic laser with a compound cavity was modeled based on plane-wave approximation. The simulated results show that the mode-matching condition of the multi-mode lasers depends strongly on both frequency-mode spacing and resonator loss. In the case of the same cavity lengths and losses, the tolerance on the optical cavity length should be maintained on tens or hundreds of nanometer scales. Meanwhile, the TEM₀₀ beam can be achieved due to the best mode-matching at resonance and the beam size increases arising from the cavity detune.