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. Measuring a focal spot size of laser beam is almost an art unto itself since it requires a very high degree of precision in the position of the sensor’s imaging plane and a means of adjusting this position to find the focal plane. The focal spot size can therefore be very small, and when it is, the beam size varies very rapidly along the propagation axis. When you focus a Gaussian beam with a lens of focal length f, the beam waist (or laser spot size equation) becomes: The beam diameter is simply twice the beam radius, and can be measured anywhere along the propagation axis.

Where w 0 is the beam waist (the smallest radius of the Gaussian beam) and z R is the Rayleigh length: If we consider the ideal case of a Gaussian beam, the beam width (or radius, w) along the propagation axis z is defined by the following equation: When a laser beam propagates along its optical path, its diameter is continually changing. 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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