Where is the highest power per unit area of a laser beam?

• A. At the aperture, or opening
• B. At the source
• C. Along the beam path
• D. At the target

Answer: (A)

Power per unit area, or irradiance, is the beam power divided by its cross‑sectional area. Right after a laser emits, the beam is confined to the smallest cross section it will have along its path—the aperture. As the beam travels, diffraction causes it to spread and its cross-sectional area grows, so the irradiance decreases. Since the total power stays essentially the same while the area increases, the highest power density occurs where the cross section is smallest—the aperture. Note that adding focusing optics can create an even smaller spot at a target and raise irradiance there, but in simple propagation without extra focusing, the aperture is where the power per unit area is greatest.