Laser Mirror Counteracts Deformation

When photons push against mirrors inside laser cavities, the mirrors get deformed. But scientists now have a way to push back.

A ceramic mirror in development by the Fraunhofer Institute for Applied Optics and Precision Engineering and Ilmenau University of Technology incorporates heating filaments and a piezoelectric layer that reshape the mirror’s face so as to counteract deformation caused by heat transfer from the laser beam.

“We’ve developed a mirror that doesn’t prevent deformation by the laser, but corrects it,” said Fraunhofer researcher Dr. Claudia Reinlein. “By deliberately heating up the mirror to a precisely controlled level, we balance out the unwanted deformation by the laser.”

Thermic-piezoelectric deformable mirror to be used in high-power laser systems. Courtesy of the Fraunhofer Institute for Applied Optics and Precision Engineering.

Mirror deformation can cause the laser beam to lose focus and intensity; this can be a problem especially in cutting and welding applications. Making mirrors more heat resistant works only up to a point, according to Reinlein and colleagues, who decided to take a different approach.

Their mirror has a copper face and built-in temperature sensors and filaments. When a laser beam heats up the mirror, the sensors detect the change. Software calculates how much the mirror is deforming from the heat and sends a corresponding current of electrical power through the filaments. These heat up and balance out the unwanted deformation. A piezoelectric layer on the back of the mirror corrects all further errors that could disrupt the laser beam.

The researchers have to control the system manually, but the mirror should be able to correct deformations automatically in future.

In addition to industrial applications, researchers see the deformable mirror aiding in the defense of satellites against space debris.

Ground-based lasers may be used to change the course of debris to avoid orbital collisions, but such lasers could be sent off course by atmospheric turbulence. A deformable lens could compensate for this after another laser is used to analyze the turbulence (a technique already used in astronomy and known as an artificial guide star).

The prototype deformable lens will be demonstrated at the Optatec trade fair in Frankfurt May 20 to 22.

For more information, visit www.fraunhofer.de.