How to Measure a Focused Laser Spot Size

In short A focused spot is small, short and intense. A collimated beam of diameter D focused by a lens of focal length f gives a 1/e² spot of 4 λ f M² ÷ (π D): often 5–50 µm, with a depth of focus of a fraction of a millimetre and an irradiance of MW/cm². […]
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