Miniaturized adaptive optics at SPIE Photonics West

For some products, even fairly new ones, you can predict whether the latest versions will be bigger or smaller than their predecessors. Disk and flash drives are getting smaller (or staying the same size while storing more data), TV screens are getting wider, and laptops are getting slimmer.

But if you’d asked me before today to predict the size trend of adaptive optics systems, I wouldn’t have said they’re shrinking.

A perfect parabolic mirror will bring an object into perfect, diffraction-limited focus, provided the wavefronts radiating from the object are parallel and flat. Starlight arriving at a ground-based telescope doesn’t have parallel, flat wavefronts. Fluctuating, uneven refraction caused by atmospheric turbulence bends the wavefronts out of shape. The focus is imperfect.

In 1953 Horace Babcock of the Mount Wilson and Palomar Observatories in California proposed a way—adaptive optics—to compensate for atmospheric blurring . If you could measure the distortions as they happen, and if you could deform the telescope mirror quickly and arbitrarily, you could restore the wavefronts to their flat, parallel state.

Babcock’s idea was ahead of its time. Earth’s atmosphere fluctuates on a time scale of 10 to 100 milliseconds. Measurement and control systems from the 1950s through the 1980s couldn’t keep up. But since the early 1990s, adaptive optics systems have been installed at several observatories, including the two Keck telescopes on Mauna Kea in Hawaii and the Very Large Telescope (VLT) on Cerro Paranal in Chile. The three huge telescopes under development now—the European Extremely Large Telescope (E-ELT), the Thirty Meter Telescope, and the Giant Magellan Telescope—would not be worth building without adaptive optics.

The E-ELT’s primary mirror is 42 meters wide. That’s five times bigger than the primaries in each of the four VLT telescopes. Because I had just read a news story in Nature about the E-ELT, the association of adaptive optics with mirrors the size of swimming pools was freshly established in my mind.

Michael Helmbrecht’s talk at SPIE Photonics West soon wiped out my mistaken prejudice. Helmbrecht is the CEO and owner of Iris AO Inc , a company based in Berkeley, California, that makes miniaturized adaptive optics systems.

The photo shows Iris AO’s PTT111-X deformable mirror. The product fact sheet lists its impressive features, among them its aperture: 3.5 mm. One hundred eleven tiny MEMS (microelectromechanical systems) actuators deform the mirror’s 37 segments.

NASA and the Pentagon are Iris AO’s current customers. After the talk, I asked Helmbrecht what the first commercial applications for his products would be. High-end microscopes for biology was one area. Lasers for semiconductor fab plants was another. As for retinal imaging, an area where bulkier adaptive optics systems are already in use, he said the price of his devices would first have to drop significantly.

Disk and flash drives, big-screen TVs, and laptops have all gotten cheaper. I expect MEMS-based deformable mirrors will, too.