Gaussian Beam parameters

When dealing with a Gaussian laser beam, in multiple applications, you don’t simply use it as is. You generally focus it into a small concentrated tight spot in order to make the power density high enough to have a certain outcome from its interaction with a material, like cutting, marking, or performing surgery for example. How tight and small a beam can be focused is called the spot size. Around that point (called the waist) and along the propagation axis, the behavior of the beam can be described by three parameters: the wavelength, the actual spot size, and the position along that axis. With these 3, you can determine the beam diameter, the Rayleigh range, the beam size at the Rayleigh length, the radius of curvature of the beam wavefront, and the divergence. Naturally at the waist, the beam diameter is the same as the spot size and the radius of curvature would be infinite since the wavefront is flat. Looking at the equations below, one would notice that the beam diameter at Rayleigh length can be simplified to √2 times the spot size. You could also do the math yourself using the Rayleigh length as the position on the propagation axis to get the same result. The divergence mentioned here is not suited for a far field, meaning if you want the diameter at multiple times the Rayleigh length, you would be better served with our divergence calculator instead. Similarly, if what you want to calculate is the spot size and you don’t have that info, try this calculator instead.