M² is a measurement that was originally designed to determine the quality of a real beam in comparison to a theoretically perfect fundamental mode Gaussian beam (Siegman 1990). Although many lasers are designed to output a fundamental mode Gaussian beam, imperfections and/or cavity design in the laser will often cause superimposed higher-order transverse modes. M² provides an excellent metric for determining the higher-order mode content. In his original paper introducing M², Siegman gives mathematical expressions for the theoretical value of M² based on the weighted content of the various modes in a beam. In this blog post, we will programmatically generate Gaussian beams with multi-mode content and then analyze the M² value of these artificial beams calculated by the DataRay software in order to confirm the accuracy of our algorithms.
As leaders in the laser beam profiling business, we have worked with M² for decades. In this blog post we discuss M², when to use it, and the way it is measured. M² is a very useful measurement for a certain subset of laser beams—those that are predominately Gaussian—and determines how tightly an actual laser beam can be focused in comparison to a theoretically perfect Gaussian beam. Sometimes, customers wish to use M² as a laser beam metric, even when it isn't the most appropriate metric to use for their application. Even if M² isn't the right diagnostic metric for your beam, we would love to work with you and design a custom metric to assess the quality of your beam.