"线偏振光束" can be translated as "linearly polarized light beam" or simply "linearly polarized light" (when the context clearly refers to a beam).
Linearly polarized describes the polarization state where the electric field oscillates in a single fixed plane.
Light beam emphasizes the directional, focused nature of the light, though "light" alone is often sufficient if the context is clear.
Examples in use:
"A linearly polarized light beam was used in the experiment."
"The laser emitted linearly polarized light."