Teamed lasers make smaller spots

Focus a laser beam more tightly and it will make smaller marks and scan smaller areas. Scientists have been able to focus light very narrowly on surfaces, but the task has proven more challenging in three dimensions.

Researchers from Boston University have tapped the properties of polarization in order to focus a laser beam more tightly in space. The method could be used to scan objects in finer detail and to make finer features in processes like rapid prototyping and photolithography that use lasers to harden liquid plastic or etch semiconductors.

The electronic field of a light beam vibrates on a plane perpendicular to the path of the beam; when a light beam is polarized the electric field vibrates in only one direction on this plane.

The Boston University researchers focused a laser beam about 1.4 times more tightly by superimposing two ordinary laser beams that were polarized at right angles to each other and focused on two slightly different points along the path of the beam. The interference of the two wavefronts compressed the focal spot.

The technique is easier to use than methods that use optical masks to modify laser beams because such masks are difficult to manufacture, according to the researchers.

The work appeared in the July 28, 2003 issue of Optics Express.