Random laser enables speckle-free imaging

Ashley N. Paddock, ashley.paddock@photonics.com

A random laser illumination source that uses multiple spatial modes to mitigate noise could generate speckle-free medical images and advance the fields of endoscopy and microscopy.

Current imaging systems rely on a variety of light sources, including LEDs, specialty lightbulbs and traditional lasers. The brightest of these, traditional lasers, generate a single intense beam of light, called a spatial mode. Photons from that beam can be scattered by a sample under observation, resulting in undesirable background noise, or speckle, on top of the desired image.

Light emerges from a random laser. A team at Yale has created a new illumination source for noise-free imaging. Courtesy of Brandon Redding, Yale.

“When you shine that traditional laser onto a surface, any roughness on the surface causes photons that travel from the surface to a camera to travel slightly different path lengths,” Dr. Michael A. Choma told BioPhotonics. Choma is an assistant professor of diagnostic radiology, pediatrics and biomedical imaging at Yale University School of Medicine. “If the surface roughness is larger than about one wavelength (which isn’t very rough at all), because the source has one spatial mode, the photons interfere with each other to create a random pattern of constructive and destructive interference.”

To generate speckle-free images, Choma and colleagues developed a random laser that generates and emits light in a manner different from traditional lasers. Although it still serves the same function, the new device does not give off image-marring byproducts.

Using multiple spatial modes, such as the light emitted by lightbulbs and LEDs, would mitigate noise, but these sources are not as bright as lasers.

The Yale random laser offers the best of both worlds because it has the brightness of lasers but also operates in many modes, such as lightbulbs, so it can generate speckle-free images.

“We were very excited when our work showed that random lasers have the best of both: low spatial coherence and high power per spatial mode,” Choma said. “Random lasers are unconventional lasers in that they are made from disordered materials that trap light via multiple scattering. Their distinctly structured wavefronts combine to produce emission with low spatial coherence.”

The light from these lasers could provide faster image generation, helping clinicians to better capture fast-moving physiological phenomena, such as the movement of embryo hearts or blood-flow patterns in the eye, as well as the broad swaths of tissue required by current technologies. The lasers also could have applications in consumer electronics; for example, digital light projection systems.

The team currently is refining a prototype random laser it developed for imaging applications.

“We think random lasers have a large translational potential,” he said. In endoscopy, for example, “the goal is to use random lasers as a light source for full-field endoscopic imaging that is faster than that with current light sources.”

The results were reported online in Nature Photonics (doi: 10.1038/nphoton.2012.90).