Limitations Imposed By Spatial Hole Burning On The Single-Frequency Performance Of Unidirectional Ring Lasers

Multi-axial-mode oscillation was caused by spatial hole burning was noticed in the early days of lasers. Various approaches have been taken to eliminate spatial hole burning, in order to achieve single frequency operation, the most widely used being the unidirectional ring laser. This has proved to be an extremely effective way to achieve robust single frequency operation. However, even in such a unidirectional ring laser, it is possible to have some degree of residual hole burning that can limit the maximum single frequency power.Through analysis of the physics of the standing wave region in unidirectional ring laser, the reason of the residual spatial hole burning is found; A simplified analytical expression of maximum single frequency output is obtained by the analysis of a homogeneously broadened four-level system; For end-pumped ring lasers a method of changing the pump beam is proposed to increase the output power. A comparison is made between the experimental results and the numerical calculation to verify the usefulness of this analytical approximation. The obtained result offer theoretical guidance for the designer to predict to some extent limit of maximum single frequency.