One cinematic cliché we've all seen is when the hero deflects the villain's dastardly laser beam with a hastily snatched hand mirror, sending it back at his adversary. Physics, ever the wet blanket, says that this is a highly improbable scenario. Focusing high-powered lasers isn't easy. A powerful laser distorts the mirror, throwing the beam off and spreading it out uselessly. To combat this, Fraunhofer is developing a lens that can deform itself to compensate for heating and other distortion factors to keep lasers focused on target.
For most of its history, the laser has been restricted to low-power applications, such as communications, or high-powered applications at very short ranges, such as cutting metal. Now, with lasers acting at far higher powers over much longer distances requiring higher precision, there’s a much greater need for mirrors that are up to the job of directing them.
The ideal situation is one where the laser beam is kept as narrowly focused as possible to place the maximum amount of energy on one spot, or to place the beam more precisely. The problem is that no mirror is perfect. No matter how highly polished or pure the surface, it won’t reflect all the laser energy. Some will be absorbed, heating the mirror so it expands at the point of reflection and the laser beam expands and distorts. Turning up the power to compensate doesn't work because that makes for more heat and more distortion.
One way of handling the deformation problem is to make mirrors that are tougher and more heat resilient, so they don’t distort when the laser hits them, but this approach only goes so far. The new approach being developed by the Fraunhofer Institute for Applied Optics and Precision Engineering IOF is to make a mirror that takes into account the deformation and changes shape to compensate.
“We’ve developed a mirror that doesn't prevent deformation by the laser, but corrects it,” says Dr. Claudia Reinlein, from Fraunhofer IOF. “By deliberately heating up the mirror to a precisely controlled level, we balance out the unwanted deformation by the laser.”
The new mirror is made of ceramic with a copper coating and built-in temperature sensors and heating filaments. When the laser heats a spot on the mirror, the system kicks in the heater and deforms the mirror in the opposite direction, correcting the distortion. Meanwhile, a piezoelectric layer on the back of the mirror fine tunes the deformation. In the finished system, all this will be done automatically.
According to Fraunhofer, one application of such a correctable laser system would be to use high-powered lasers to clear space debris. This wouldn't be a dramatic blow-it-out-of-the-sky job, but would use the beam to push dust-particle sized rubbish away. Since this would mean fighting against atmospheric distortion of the beam from a ground station, the new mirror could compensate to maintain focus across tens of thousands of miles.
The mirror will be on display at the Optatec trade fair in Frankfurt from May 20 to 22.