We can see that if we look at the point where the radius is equal to w (ergo at the theoretical beam width radius), then the intensity is 1/e 2 times the maximum value: In the equation describing the Gaussian beam’s radial distribution, 1/e 2 (13.5% of maximum)Īlthough it might seem to be a weird choice of value, 1/e 2 actually comes from a simplification. A common one is 13.5%, which leads us to our next beam diameter definition: 1/e 2. Some people prefer using other percentages of a beam’s maximum intensity to define its width. The FWHM corresponds to the distance between the two points closest to the peak that have 50% of the maximum irradiance or intensity. The full width at half-maximum, also called half-power beam width, is measured from the distribution curve of the beam’s intensity along a pre-defined axis, passing through the beam’s center, which is also usually its point of maximum intensity.
Here is a quick overview of how they are calculated and how they are related. There are three common beam diameter definitions: FWHM, 1/e 2 and D4σ. Which beam diameter definition do you want to use? Since a focal spot measurement is basically a special case of laser beam diameter measurement, we will concentrate on the latter in this article. It is important to make a distinction between the two cases because the approach to measurement is different. Without the context of what someone wishes to achieve with a measurement, it is easy to get lost in translation.įor some people, the expression spot size of a laser beam means its smallest diameter at the focal plane when focused by a lens, whereas for other people they mean the laser beam’s diameter when it exits the laser, or at any other place where they need to measure it.
Do you want to measure the focal spot size or the beam diameter? Let’s look at each one and why I ask them. Do you have the appropriate instruments?.Which beam diameter definition do you want to use?.Do you want to measure the focal spot or the beam diameter?.When someone asks me how to measure the spot size of their laser beam, I often answer with a few questions, because this type of measurement is not as simple as it can appear at first glance, especially if you want to do it with precision and rigor.
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