Laser Beam Optics: Calculating Focal Spot of a Gaussian Beam

Whether you’re in the lab or a job shop, you need your laser beam to be a specific size to do the job.

Getting your laser beam to that size is not always simple. In fact, the more I learned about (and experimented with) lasers, the more I realized that there is no one simple equation that can determine how it will behave. There is not one simple value to characterize the laser beam either.

Nevertheless, Sidney Self boiled it down to only a few equations, for spherical Gaussian beams at least.

Using his equations, we have created a calculator that lets you determine the laser spot size and location for a beam that is focused by a lens.

There are several input parameters, such as beam size, divergence, lens focal length, etc. However, several of these are interdependent, so you don’t need to know (or input) all of them. The calculator will always “try” to calculate as many parameters as possible.

For example, if you have a given laser beam and need to determine what the best lens will be for your application, input your laser parameters and a “trial” lens to see what the focus spot will look like. Then, you can adjust your lens parameters to get the ideal focus spot. That’s it.

Although this can be a little daunting at first, it actually provides the most flexibility. Just enter the values you know, and if it can, the calculator will auto-calculate the rest. Once it’s all filled out, click the “calculate” button to get the focus spot size and location, as well as the Rayleigh length.

Click here to calculate your focussed laser beam spot size. For all our laser calculators, click here.