Laserbeam quality

Using a Gaussian beam is preferred because of its minimum divergence angle and the ability to achieve the minimal focus diameter. Differences to Gaussian shape can be due to

With a given divergence angle (i.e. knowing the focal length of the lens), the fundamental mode alone produces the theoretically smallest possible beam waist (green curve). If beam quality worsens (red curve), the beam waist increases. If divergence is fixed, beam waist increases linearly by the factor M2 compared to the underlying Gaussian.

In the focus (beam waist) of a diffraction-limited beam (i.e., at the location where the beam radius reaches its minimum), the optical wavefronts are flat. Any scrambling of the wavefronts, e.g. due to optical components with poor quality, spherical aberrations of lenses, thermal effects in a gain medium, diffraction at apertures, or by parasitic reflections, can spoil the beam quality. For monochromatic beams, the beam quality could in principle be restored e.g. with a phase mask which exactly compensates the wavefront distortions, but this is usually difficult in practice, even in cases where the distortions are stationary. A more flexible approach is to use adaptive optics in combination with a wavefront sensor.

Beam qualityformula

These distortions lead to a larger beam waist compared to Gaussian beams when the same focal lens is used. This results in a lower maximum achievable power density in the focal point.

where Θ0 and D0 are the divergence and waist of a higher mode beam and M2 is greater than 1 and is named the “Beam Propagation Ratio” per the ISO 11146 standard. When a Gaussian laser beam is focused, the focused spot diameter is defined by

Beam qualityM2

The beam quality of a laser is an important aspect of laser characterization. It can be defined in different ways, but is normally understood as a measure of how tightly a laser beam can be focused under certain conditions. The most common ways to quantify the beam quality are:

where d00 is the ideal focused spot diameter, f is the focal length of the focusing lens, and D00 is the input beam waist and is placed one focal length from the lens as shown in the figure. However, when a multimode beam is focused, Equation becomes

Beam qualityradiology

What is laser beam quality factor? How to evaluate laser beam quality? Why laser beam quality degrades during propagation? This article will share a bit it.

where D00 is the diameter of the beam waist, and λ is the wavelength. Higher mode beams often start with a larger beam waist, D0, and/or have a faster divergence Θ0. In this case Equation becomes