An interdepartmental team of scientists in applied physics, electrical and biomedical engineering, and diagnostic radiology at Yale University say that there has been an intense and ongoing 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, which can provide the required brightness, exhibit high spatial coherence, and can introduce artifacts like speckle, which corrupts image formation. At the other extreme, low spatial coherence sources, such as thermal sources and light emitting diodes (LEDs) avoid speckle but do not have sufficient power per mode for high-speed imaging.
To combat this, the researchers have developed and demonstrated a new type of semiconductor laser based on a chaotic cavity, which combines low spatial coherence with high power per mode. Such a laser, they say, could enable a wide range of full-field imaging applications, and has the potential to significantly improve the imaging quality of the next generation of high-tech microscopes, laser projectors, photo lithography, holography, and biomedical imaging. (See Figure 1)
Since the group included input from so many disciplines, the team discovered that these lasers are uniquely suited for a wide class of problems in imaging and microscopy, including speckle, a random, grainy pattern, caused by high spatial coherence that can corrupt images when traditional lasers are used. The new, electrically pumped semiconductor laser produces an intense emission, but with low spatial coherence.
“For full-field imaging, the speckle contrast should be less than ~4% to avoid any disturbance for human inspection,” explained Hui Cao, a professor of applied physics. “The standard edge-emitting laser produced speckle contrast of ~50%, while our laser has the speckle contrast of 3%. So our new laser has completely eliminated the issue of coherent artifact for full-field imaging.”