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Astronomy

Collaboration on Aberration

The 2004 Rudolf Kingslake Medal and Prize-winning paper puts existing technology to good use.

From oemagazine November/December, 2005.
31 January 2006, SPIE Newsroom. DOI: 10.1117/2.5200511.0010

The visual excitement of optics hooked Mark Gruneisen at a young age. But later in life, it was the intellectual excitement that reeled him in.

"When I was a small child, my oldest cousin Larry worked for Kodak," he says. "My family visited and toured the facilities where he gave me a lens as a souvenir. I was intrigued with how the lens would form images on a piece of paper, and later, how lenses could be used in tandem to make telescopes and microscopes." By the time he was in college, and then graduate school, the physics involved in the study of optics had begun to fascinate him.

"As a field of study, optics is both aesthetic and challenging, and conveniently lends itself to quantitative analysis," he says.

Gruneisen is the lead author of the paper named the winner of the 2004 Rudolf Kingslake Medal and Prize: "Programmable diffractive optics for wide-dynamic-range wavefront control using liquid-crystal spatial light modulators." He and colleagues Lewis DeSandre, Jim Rotgé, Raymond Dymale, and Donald Lubin received the award in recognition of the group's "novel use of existing technology."

Gruneisen is principal investigator for the Advanced Wavefront Control Program in the Beam Projection and Compensation Group of the Air Force Research Laboratory's (AFRL; Kirtland AFB, NM) Directed Energy Directorate (DED). Lubin provides technical support to the same group, and DeSandre is deputy of the Precision Engagement Product Line of the DED. Rotgé and Dymale both work for Boeing LTS and are based at AFRL. Rotgé is a senior electro-optic scientist, and Dymale is a senior engineer.

(Left to right) Jim Rotgé, Mark Gruneisen, Donald Lubin, Raymond Dymale, and Lewis DeSandre with their prototype imaging system using programmable diffractive optics technology at Kirtland Air Force Base.
TODD BERENGER, KIRTLAND AFB

The group's current work evolved from an earlier Air Force program that developed optically addressed spatial light modulators (OASLMs) as real-time holographic recording media for compensating aero-optics effects, but a major shortcoming was low diffraction efficiency, says Gruneisen. "Not surprisingly, people weren't interested in optical systems with less than 1% throughput."

A decade ago, the group began studying the feasibility of developing thin optical membranes as deployable primary mirrors for space-based telescopes. "Mechanical and optical tolerances would have to be relaxed," Gruneisen says. "This, in turn, would require a new technology for compensating large dynamic aberrations. Such a technology would need to operate in real time ... but could be significantly slower than that required for compensating aero-optics phenomena."

Achieving the necessary throughput required aberration correction with the 2-D equivalent of blazed diffraction gratings, which, in turn, required computer addressing of the OASLM to create a 2-D modulo-lambda optical path function. A conversation between Gruneisen and Ming Wu (Hamamatsu Corp., Bridgewater, NJ) led to a breakthrough.

"Ming described to me that Hamamatsu Photonics K.K. (Hamamatsu, Japan) was developing a system in which the OASLM could be addressed via a computer VGA output and a display amplitude modulator," Gruneisen says. "I realized that with the computer interface we could generate the 2-D modulo-lambda optical path functions needed for optically efficient compensation of large aberrations."

The result was "a prototype system for high-fidelity wavefront control utilizing programmable diffractive optics," according to the paper's conclusion, compensating a large aberration to produce a near-diffraction-limited planar wavefront.

"To date, we have performed several system demonstrations with liquid-crystal (LC) spatial light modulators mostly centering on compensation of large aberrations associated with primary mirrors in telescope systems," Gruneisen says.

The group has also demonstrated novel concepts including wide-field high-resolution mosaic imaging with standard imaging sensors. Looking ahead, Gruneisen sees the probability that high-pixel-count micro-electro-mechanical systems (MEMS) mirrors will enable frame rates up to several kilohertz (as opposed to the few, 10s of hertz currently achieved with LC spatial light modulators), with even higher optical throughput.

"Widening the temporal bandwidth is an important goal for many Air Force applications associated with compensation of atmospheric aberrations," and mechanical ones as well, he says. "High-speed MEMS mirrors are one approach. Faster LC media, such as the dual-frequency nematic media is another. Recently, with funding from the High-Energy Laser Joint Technology Office, AgilOptics (Albuquerque, NM), in collaboration with the University of Notre Dame (South Bend, IN), demonstrated a novel MEMS mirror operating in excess of 1 kHz to compensate simulated aero-optics turbulence in real time. This is an important step in demonstrating the utility of wide-bandwidth MEMS adaptive optics."

Gruneisen is effusive in his praise of the group's dynamics. "I don't recall that we've ever had a disagreement regarding the direction or results of the work," he says. "We generally work independently on parts of a problem and then collaborate to confirm and piece together results. I would say that a passion for the technology and a common regard for the value of measurement and analysis keep us motivated toward common goals."

Rotgé is responsible for much of the physical optics-based modeling and novel optical system concept development, Gruneisen says. Dymale, "an extremely meticulous optical engineer," supports the project with analytic modeling and experimental measurements and demonstrations. "He is largely responsible for the quality of the measurements and technology performance that comes out of the lab."

Lubin's "invaluable logistical and technical support" allows the senior technical staff to fully utilize their expertise, Gruneisen says. DeSandre, with a background in Fourier optics, adaptive optics, MEMS technology, and high-energy laser development, provides the team with insights "to motivate and direct our efforts."

The Rudolf Kingslake Medal and Prize, first awarded in 1974, recognizes the most noteworthy original paper in the SPIE journal Optical Engineering. The significance of the award is not lost on the recipients.

"We're overwhelmed and grateful to have our paper selected to receive this recognition," Gruneisen says. "This is especially meaningful because this award comes from the professional community of scientists and engineers who are most qualified to critique our work." Moreover, Gruneisen received his PhD in 1988 from the University of Rochester's Institute of Optics, where Kingslake spent much of his career. "It means a great deal to me to receive an award named in honor of such an esteemed member of the institute," he says.

The authors' goal with the award-winning paper was to create a foundation for system concepts enabled by the technology, according to Gruneisen. The group has since completed several demonstrations of new systems, and others have used spatial light modulator technology to create optical tweezers and control angular momentum states of photons.

Gruneisen's work in spatial light modulators takes him to Russia about once a year, he says, and he's made a hobby out of learning the language and studying the country's culture. "I've had wonderful experiences with the technical community there, and it's a real personal growth experience to learn about a culture with a history different from that of our own," he says.

His home in the mountains east of Albuquerque has led to another hobby: xeric landscaping. "It's a real challenge to grow things at 7200 feet elevation during a drought," he says. "These are slow experiments but rewarding ones when something colorful flourishes."

Some of the same meticulous work and patience may be required to advance the spatial light modulator technology, but Gruneisen relishes the process.

"I think the most exciting work is still ahead," he says.