Beam quality in lasers refers to how well the beam stays focused, affected by factors such as laser components, gain medium, power level, and thermal stress. There is a trade-off between beam quality and power, with gas and solid-state lasers having different vulnerabilities. Other design components and the condition of the laser components also affect beam quality.
In laser physics, beam quality refers to how well a laser beam stays focused. The higher the quality, the smaller the divergence in beam. Beam quality can be affected by a number of factors, such as the type and quality of laser components, the type of laser gain medium used, the optical pumping method used, power level, and 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 gain medium, which the laser uses to amplify light, some of the energy is converted into heat. Excess heat can cause temperature gradients in the medium which produce a thermal lensing effect, reducing beam quality. This makes the type of means of income used important.
Gas lasers tend to be less vulnerable to thermal lenses due to their lower media density, 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 disc lasers. Fiber lasers use silicon dioxide glass doped with rare earth ions such as ytterbium or neodymium as the gain medium, and thin-disc 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, helium-neon gas lasers and surface-emitting semiconductor lasers can produce extremely high beam quality.
Other components of the laser design are also 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 laser beam path, a technique called end-pumping, produce higher beam quality than side-pumped lasers. Mode cleaners can be used to improve beam quality by passing the beam through a single-mode fiber or by using one lens to focus an out-of-collimation beam through a pinhole and into a second lens that re-collimates the beam.
Beam quality is also affected by the condition of the laser components. Solid-state lasers can experience reduced quality if there are defects in the surface of its medium, which can cause wavefront distortion and scattering. Quality is also affected by misaligned components, which can be caused by mechanical stress, temperature changes in the laser environment, or thermal expansion due to the heat of the laser itself. Lens defects can also affect quality.
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