A modified virtual source method for off-axis unstable resonators with large equivalent Fresnel number

Physical significance of the ‘edge wave’ in the asymptotic solution of the diffraction by a single slit is expounded in detail. Basing on this concept, we modify the virtual source method to the calculation of the lowest eigenmode mode profile in both the positive and negative branch strip off-axis unstable resonators. The advantages of the method are that it is conceptually simple and requires only minimal computer time specifically for the resonators with large equivalent Fresnel number.