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Yale group makes high power laser with low spatial coherence

Chaotic D-cavity laser combines the brightness of traditional lasers with the lower image corruption of LEDs

A new chip-scale, electrically pumped semiconductor laser developed at Yale University in the US could improve the imaging quality of the next generation of high-tech microscopes, laser projectors, photolithography, holography, and biomedical imaging, according to the researchers.

Based on a chaotic D-cavity laser, the technology combines the brightness of traditional lasers with the lower image corruption of LEDs. The laser is described in a paper in the January 19th online edition of the Proceedings of the National Academy of Sciences. Several Yale labs and departments collaborated on the research, with contributions from scientists in applied physics, electrical and biomedical engineering, and diagnostic radiology.

"This chaotic cavity laser is a great example of basic research ultimately leading to a potentially important invention for the social good," said co-author A. Douglas Stone, the Carl A. Morse Professor and chair of applied physics, and professor of physics. "All of the foundational work was primarily motivated by a desire to understand certain classes of lasers - random and chaotic - with no known applications. Eventually, with input from other disciplines, we discovered that these lasers are uniquely suited for a wide class of problems in imaging and microscopy."

There has been an intense search for the ideal light sources for high-speed, full-field imaging applications ranging from next-generation microscopes and laser projectors to digital holography and photolithography. Traditional lasers, although providing the required brightness (i.e., power per mode), exhibit high spatial coherence, which introduces coherent artifacts such as speckle, corrupting image formation. At the other extreme, low spatial coherence sources such as thermal sources and LEDs avoid speckle but lack sufficient power per mode for high-speed imaging.

The new, electrically pumped semiconductor laser offers a different approach. It produces an intense emission, but with low spatial coherence.

"For full-field imaging, the speckle contrast should be less than around 4 percent to avoid any disturbance for human inspection," explained Hui Cao, professor of applied physics and of physics, who is the paper's corresponding author. "As we showed in the paper, the standard edge-emitting laser produced speckle contrast of around 50 percent, while our laser has the speckle contrast of 3 percent. So our new laser has completely eliminated the issue of coherent artifact for full-field imaging."

Co-author Michael A. Choma, assistant professor of diagnostic radiology, pediatrics, and biomedical engineering, said laser speckle is a major barrier in the development of certain classes of clinical diagnostics that use light. "It is tremendously rewarding to work with a team of colleagues to develop speckle-free lasers," Choma said. "It also is exciting to think about the new kinds of clinical diagnostics we can develop."

The laser was fabricated using a commercial laser diode wafer consisting of a PIN junction with gain provided by a GaAs quantum well. The epitaxial structure was grown on an n-type GaAs wafer and consisted of a 1,500nm n-type Al0.55Ga0.45Aλs layer, an undoped 200nm Al0.3Ga0.7As layer with a 10nm GaAs quantum well in the centre, a 1500nm p-type Al0.55Ga0.45As layer, and a 300nm p-type GaAs contact layer. 

With current injection, the AlGaAs quantum well provides optical gain near λ=800 nm. The layered structure results in vertical confinement of light in the undoped Al0.3Ga0.7As layer via index guiding. The fundamental guided mode has the peak intensity at the location of the GaAs quantum well and experiences the highest gain.

'Low spatial coherence electrically pumped semiconductor laser for speckle-free full-field imaging' by Brandon Redding et al: http://www.pnas.org/content/early/2015/01/15/1419672112

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