Beam diameter is the size of a beam of light or electromagnetic radiation. It can be measured in different ways and is important for laser applications. Beam quality is affected by diameter and divergence, and smaller lasers require improvements in design to maintain high quality.
Beam diameter is a measure of the size of a beam of light or other electromagnetic radiation, such as a laser. It is the diameter on any line that is perpendicular to and intersects the beam axis and is twice the length of the beam radius. For a circular beam, its length is defined as the length of a line segment which passes through the center of the beam and has its endpoints on opposite edges of the beam. If the beam is elliptical, its diameter can be specified as the length of the major or minor axis of the ellipse. If the beam does not have circular symmetry, the width of the beam is often referred to.
Most electromagnetic beams do not have sharp edges, as solid objects do, and beam divergence means that their width is not constant along the entire length of the beam. Therefore, there are several ways to define the diameter of the beam. Beam diameter measurement is done with a device called a laser beam profiler. The point on the beam where the diameter of the beam is narrowest is called the waist of the beam.
Beam diameter is an important attribute of lasers. Rays with a larger diameter experience less beam divergence, which is a measure of how fast the light in the beam spreads out from the beam belt. Beams with low divergence therefore have higher beam quality, a measure of how tightly focused a laser beam remains as it travels. The optical intensity of a beam is the amount of optical power that the beam delivers per unit area on the target, so a laser with low beam divergence will have greater optical intensity than a beam with the same optical power but higher divergence of the radius. This is important for many laser applications, such as cutting, drilling and remote welding in industry and laser microscopy in life sciences.
There is a trade-off between laser beam quality and laser size, as a laser with a smaller lens has a smaller beam diameter and will experience greater beam divergence, all other things being equal. Making a laser smaller, which is often desirable for convenience and cost reasons, while maintaining high beam quality requires improvements in other parts of the design. This can be achieved by using higher quality optical components, optimized resonator design and alignment, and the use of laser gain media less prone to thermal effect distortions such as thermal lenses.
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