Beam Properties and Quality Factor of VCSELs

For many industrial and scientific laser applications, such as materials processing, fiber coupling, beam collimation, and beam focusing, it is necessary to have a good laser beam quality. In general, propagating laser beams deviate from an ideal Gaussian intensity distribution. The degree of deviation can be conveniently quantified by a quality factor M (called the “M-squared” factor or the “times diffraction limit number”). This factor has been defined such that M = 1 for an ideal Gaussian beam. Real laser beams have factors greater than one. For example helium neon lasers have typically an M factor of less than 1.1. The M factor of single-mode laser diodes is in the range of 1.1 to 1.5. For multi-mode lasers, M can be as high as 20 and more. Therefore, the beam quality is important for many laser measurements and optical designs, where the M factor cannot be neglected.