Tailoring the spatial coherence of random lasers

Abstract

Summary form only given. Spatial coherence is one of the signature properties of traditional lasers. While spatial coherence allows laser emission to be focused to a small volume or collimated over long distances, it is also the cause of deleterious effects such as speckle which has limited the adoption of lasers in most imaging applications. In this work, we show that random lasers have the unique ability to exhibit tunable spatial coherence. We discuss the parameters which dictate the spatial coherence of random lasers and show that a random laser with low spatial coherence can produce speckle-free images. Random lasers are an unconventional laser in which optical feedback is provided by multiple scattering in a disordered medium. The random lasers used in this work consisted of a laser dye solution (Rhodamine 640) interspersed with scattering particles (240 nm diameter polystyrene spheres). The random laser was optically excited using a frequency doubled Nd:YAG laser. To study the spatial coherence of the random laser, we imaged the front of the cuvette containing the random laser solution onto a Young's double slit and recorded the interference fringes in the far-field. The visibility of these fringes provided a measure of the spatial coherence. We repeated the double slit experiment for random lasers with varying concentrations of scattering particles and with varying excitation volumes (controlled by de-focusing the pump laser). We present a summary of these experiments. This study indicates that the spatial coherence of random lasers can be tuned over a wide range by adjusting the scatterer concentration and excitation volume. Based on this study, we engineered a random laser with low spatial coherence (l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> =100 μm, d=300 μm) and compared its ability to produce speckle-free images with a commercial HeNe laser. In the imaging experiment, the random laser or HeNe laser illuminated a U.S. Air Force chart, which was imaged in transmission through a thin scattering film. Using the random laser, the test chart is clearly visible, despite the high background signal introduced by the scattering film. However, using the HeNe laser, high contrast speckle precludes observation of the test chart. We also performed this imaging experiment using an LED which is known to exhibit low spatial coherence and observed a similar image to that obtained with the random laser. Thus, the random laser produces speckle-free images with similar quality to traditionally low spatial coherence sources such as LEDs while maintaining laser level intensity.