Lasers have established themselves as revolutionary tools in medicine because they release very bright, highly regular light with a consistent frequency. Yet lasers suffer from speckle, a visual phenomenon that is created when light of one frequency interferes with itself when scattering from a surface. Most other light sources, like traditional bulbs and LEDs, don’t suffer from speckle, but they’re considerably dimmer than lasers.
Researchers at Yale have now resolved the speckle problem with lasers by making “random lasers” with a low spatial coherence that can illuminate a target with a high intensity, while completely avoiding speckle.
A traditional laser emits a single intense beam of light, known as a spatial mode. Photons from that single beam can be scattered by a sample under observation, resulting in random grainy background noise — speckle — on top of the desired image.
One way of mitigating the noise is to use many different spatial modes, such as the light emitted by a LED or light bulb. Unfortunately, these light sources are dim compared with lasers.But random lasers offer the best of both worlds, according to the Yale researchers. They are bright, like lasers, while also having many modes, like a light bulb, so they generate speckle-free images. That is, random lasers are something akin to a light bulb with the intensity of a laser.
“Our random lasers combine the advantages of lasers and the white light sources, and may be used for a wide range of imaging and projection applications,” said Cao.
The light emitted by random lasers could also enable faster image generation. This would help researchers and clinicians better capture fast-moving physiological phenomena — the movements of embryo hearts, perhaps, or blood flow patterns in the eye — as well as broad swaths of tissue in less time than required by current technologies.
Abstract in Nature Photonics: Speckle-free laser imaging using random laser illumination
Top image: Speckle produced by a green laser. credit: Flickr user jurvetson