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Adaptive Optics on a Budget

Photonics.com
Jul 2007
ALBUQUERQUE, N.M., July 9, 2007 -- A simple optical clamp that uses a single mirror functions as a sort of "poor man's" adaptive optics device.

Adaptive optics, a system known for its computer control of subdivided, individually angled mirrors, is an efficient but expensive way to correct distortions in laser beams. The mirrors automatically adjust until an undistorted beam is obtained in a way formerly thought unachievable by a single large mirror.
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Jens Schwarz adjusts his group’s newly patented Sandia optical tool that precorrects for laser distortions. (Photo: Randy Montoya)
The new device -- which has received a US patent, issued June 12 -- corrects optical distortions simply through pressure that changes the convexity or concavity of a single reflecting surface. Developed at Sandia National Laboratories, it resembles an inexpensive vise similar to those bolted to many home workshop benches.

“We can’t compensate for small-scale aberrations,” said principal investigator Jens Schwarz, “but certain large-scale beam distortions are correctable with this tool.”

The method has already improved the beam quality of Sandia’s huge Z-Beamlet laser, which can now fire every two hours instead of every four because the device pre-corrects for distortions caused by heat, Schwarz said.

"Similar beam corrections, of course, can be achieved from tens to hundreds of thousands of dollars through traditional adaptive optics," Sandia said in a statement. "Many small reflecting mirrors controlled by a computer can adjust in milliseconds to correct beam distortions reported by sensors farther down the line."

But for the overwhelming majority of laser users who do not need such fine control, deformation of a single mirror through convex or concave deformation applied through only a single actuator may be the ticket, especially when the price is expected to be only a few thousand dollars. The lab said commercial interest in the inexpensive device already has been expressed.

"A reverse use of the technique could deliberately focus the beam to interrogate points of distant interest," Schwarz said. "This use would detect chemical or biological agents introduced at battlefields many miles away, a technique called laser-induced fluorescence spectroscopy. Because the mirror can change the focus of a laser beam quickly and rapidly, a laser beam could interrogate molecules at a variety of distances and the results would be visible through backscattered light."
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Daniel Headley (left), Marc Ramsey and Jens Schwarz examine the performance of one of their optical clamps as it corrects for laser beam distortion in a Z-beamlet laser. (Photo: Randy Montoya)

Distortions happen when new energies are injected in the lasing system to create more powerful beams. These injections are achieved by racks of lamps that flare briefly, like old-fashioned photographic flashbulbs, sending an energy pulse into the laser medium of doped glass in which the beam is forming.

When the laser beam traverses the doped glass, it stimulates the material to release energy that adds to the beam’s strength, an effect known as gain. But the exterior of the doped glass -- closer to the flashlamps -- is unavoidably heated more than its interior. This modifies its refractive index, focusing the beam to a point some meters away instead of allowing it to continue to infinity.

Rather than alter the flashlamps or gain medium, Schwarz, with the aid of Marc Ramsey and Daniel Headley, used a single flexible mirror to pre-correct for the distortion to take place later in the beam’s passage.

The corrective effect is achieved by placing a flat mirror between two concentric rings of different sizes, one stationary and the other free. A screwdriver turned either by fingertips or by motor (the motor raises the price) applies a force to the free ring and bends the mirror a few microns, changing its focal length. The orientation of the large and small rings determines whether the distortion is concave or convex.

The method has been shown to work over a wide range of laser beam energies, ranging from 30 millijoules to 500 joules.

The motivation for the work, said Schwarz, is that “It’s customary to use a static concave mirror -- or a combination of appropriate lenses -- and hope it’s correcting well for distortions in the lensing system you have. But rather than buy a succession of lenses or mirrors, we thought let’s see if we can do the job more simply and inexpensively by using only one mirror with a flexible focal length.”

The device, listed as a “Variable Focal Length Deformable Mirror” will issue as U.S. Patent No. 7,229,178.

Descriptions of the work and its applications were previously reported Applied Physics B: Lasers and Optics (February 2006) and other publications. Other authors in addition to Schwarz, Ramsey and Headley include Ian Smith and John Porter.

For more information, visit: www.sandia.gov


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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