Estimation of the laser beam enabling rectangular processing by the inverse diffraction theory

A small rectangular laser spot has been widely studied for laser processing and laser repair technology. It is usually made by converging the input beam with the lens. Faithful reproduction of the laser spot is dependent on NA of the imaging lens. The small rectangular spot can be obtained by high NA lens, which is limited by many factors, such as high energy loss due to the reflection on the surface, large mass and volume, and strong sensitivity to aberrations and misalignment. On the other hand, the beam cannot be faithfully reproduced because of the diffraction with the low NA lens which has no such the limitations. One of the alternative ways to produce small rectangular profiles by using the low NA lens system is to estimate the input beam profile leading to the output profile of the sharp rectangular shape.Before estimation, we first defined spatially broad functions that did not contain high spatial frequencies and have sharp rectangular cross-sectional profile. Then, we calculated the input beam profiles leading to these functions by the inverse diffraction theory. We also confirmed that the quantization for realizable input beam profile could not much affect rectilinearity of the output beam.