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Definition: a parameter for quantifying the beam quality of laser beams
The M2 factor, also called beam quality factor or beam propagation factor, is a common measure for the beam quality of a laser beam. According to ISO 11146, it is defined as the beam parameter product divided by λ/π, the latter being the beam parameter product for a diffraction-limited Gaussian beam with the same wavelength. In other words, the beam divergence is
where w0 is the beam radius at the beam waist and λ the wavelength. A laser beam is often said to be "M2 times diffraction-limited". A diffraction-limited beam has an M2 factor of 1, and is a Gaussian beam. Smaller values of M2 are physically not possible. A Hermite-Gaussian beam, related to a TEMnm resonator mode, has an M2 factor of (2n + 1) in x direction, and (2m + 1) in y direction.
The M2 factor of a laser beam limits the degree to which the beam can be focused for a given beam divergence angle, which is often limited by the numerical aperture of the focusing lens. Together with the optical power, the beam quality factor determines the brightness (more precisely, the radiance) of a laser beam.
For not circularly symmetric beams, the M2 factor can be different for two directions orthogonal to the beam axis and to each other. This is particularly the case for the output of diode bars, where the M2 factor is rather low for the fast axis and rather high for the slow axis.
According to ISO standard 11146, the M2 factor can be calculated from the measured evolution of the beam radius along the propagation direction (i.e., from the so-called caustic). A number of rules have to be observed, e.g. concerning the exact definition of the beam radius and details of the fitting procedure. Alternative methods are based on wavefront sensors, e.g. Shack-Hartmann sensors, which require the characterization of the beam only in a single plane.
Note that the M2 factor, being a single number, cannot be considered as a complete characterization of beam quality. The actual quality of a beam for a certain application can depend on details which are not capture with such a single number.
The concept of the M2 factor does not only allow to quantify the beam quality with a single number, but also to predict the evolution of the beam radius with a technically very simple extension of the Gaussian beam analysis: one simply has to replace the wavelength with M2 times the wavelength in all equations. This is very convenient e.g. for designing the pump optics of diode-pumped lasers. Note, however, that this method works only when a certain definition of beam radius is used which is suitable also for non-Gaussian beam shapes; see again ISO standard 11146 for details.
Bibliography
| [1] | ISO Standard 11146 (2005) |
| [2] | A. E. Siegman, "New developments in laser resonators", Proc. SPIE 1224, 2 (1990) |
| [3] | A. E. Siegman, "Defining, measuring, and optimizing laser beam quality", Proc. SPIE 1868, 2 (1993) |
See also: beam quality, beam parameter product, beam divergence, brightness, Gaussian beams, Spotlight article 2007-06-11
Categories: general optics, metrology
