Calculate focused spot size - Gaussian FWHM

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The discussion centers on calculating the focused spot size using Gaussian FWHM, specifically referencing a formula for plane waves. The formula provided is approximately 2*wavelength*f/(pi*d0), where f is the focal length and d0 is the collimated beam diameter. Participants note that deriving the Gaussian spot size requires consulting laser optics textbooks, with suggestions to refer to "Born and Wolfe" for detailed explanations. While the formula is deemed relatively straightforward, further clarification on Gaussian calculations remains sought after. Understanding these principles is essential for accurate optical measurements.
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http://www.calctool.org/CALC/phys/optics/f_NA

Does anyone know how they calculate the spot size (Gaussian FWHM) at this website?

For a plane wave, the focused spot size it approx=2*wavelength*f/(pi*d0)
f=focal length of lens
d0=collimated beam diameter before lens

but how do you calculate the spot size of a gaussian?
 
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If you're asking how to derive the formulas used on the website, you'll have to look it up and wade through the details on your own - laser optics textbooks often go through it, and I bet it's in Born and Wolfe in one form or another, but generally it's a relatively simple and approximate formula that you just use.
 
Anyone else have a more detailed answer?
 

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