In laser physics, beam quality refers to how well a laser beam remains focused. The higher the quality, the less divergence in the beam. Beam quality can be affected by a number of factors, such as the type and quality of the laser's components, the type of laser gain medium used, the optical pumping method used, the power level and the mechanical and thermal stresses on the laser.
There is a trade-off between beam quality and beam power. When energy is pumped into the laser's gain medium, which the laser uses to amplify light, some of the energy will convert into heat. Excess heat can result in temperature gradients in the medium that produce a thermal lensing effect, reducing the beam's quality. This makes the type of gain medium used important.
Gas lasers tend to be less vulnerable to thermal lensing because of the lower density of their media, and carbon dioxide lasers can maintain high beam quality at very high power levels. Among solid-state lasers, the highest beam qualities are produced by single-mode fiber lasers and thin disk lasers. Fiber lasers use silicon dioxide glass doped with rare earth ions such as ytterbium or neodymium as their gain medium, and thin-disk lasers use yttrium aluminum garnet crystals doped with trivalent ytterbium ions. These lasers can maintain very high beam quality even at power levels of several kilowatts. At lower power levels, extremely high beam quality can be produced by helium-neon gas lasers and surface-emitting semiconductor lasers.
Other components of the laser's design also are important to beam quality, such as the size and configuration of the laser's optical resonator. Designs that inject light into the gain medium along the path of the laser beam, a technique called end-pumping, produce higher beam quality than side-pumping lasers. Mode cleaners can be used to improve beam quality either by passing the beam through a single-mode fiber or by using a lens to focus an uncollimated beam through a pinhole and into a second lens that recollimates the beam.
Beam quality also is affected by the condition of the laser's components. Solid-state lasers can suffer reduced quality if there are defects in the surface of its medium, which can cause wavefront distortions and scattering. Quality also is harmed by misaligned components, which can be caused by mechanical stresses, temperature changes in the laser's environment or thermal expansion from the laser's own heat. Lens flaws can affect quality, as well.