Study Of Laser Beam Width, Beam Quality And Focal Shift

Beam quality is an important index. There are many definitions of beam quality. Also there are some different evaluating ways based on different applications. While passing through a paraxial optical system without aperture, beam propagation factor is only determined by the distributing characteristics of beam itself. Beam propagation factor has nothing to do with the optical system parameter. It reflects the features of near-field and far-field and is mathematically tight. So in certain circumstances, it is an important parameter to evaluate the beam quality. The definition of beam width is significant for the quality of laser beam.A comparative study of different definitions of the beam width is made. For example, the waist width for Gaussian, Hermite-Gaussian, super-Gaussian, flatted-Gaussian and cosh-Gaussian beams are evaluated and compared. It is shown that there are different values of the beam width in accordance with different definitions.Based on three definitions of the power-in the bucket, second-order-moment-based and entropy-based width, the propagations of Cosh-Gaussian beams passing through free space and the focusing properties of them are studied. The propagation of the second-order-moment-based width obeys the ABCD law, the propagation curve of the entropy-based width is approximately a hyperbola, but for the power-in the bucket-based width the ABCD law fails. There are nearly the same focal shifts of Cosh-Gaussian (ChG) beams for the second-order-moment- based width and entropy-based width, whereas it is not the case for the power-in the bucket-based width.By using the entropy-based width, power in the bucket and asymptotic analysis, the M~2 factor of truncated Gaussian beams and the focal shift of Gaussian beams passing through an aperture-lens separated system are studied and compared. It is shown that with increasing truncation parameter d, the difference of the M~2 factor calculated by means of these three methods becomes small. If d≥2. 1, the M~2...