Characterization of Dynamic Scattering Media and Optical Fibers Using Intensity Correlations

Purdue University, West Lafayette IN

Investigators

Abstract

0323037 Webb Spectroscopy makes optical sensing and imaging a powerful tool relative to other modalities. A large number of domains of interest have significant degrees of scatter, thereby precluding direct interpretation of optical measurements using standard holographic or spectroscopic techniques. For instance, using light it may be possible to detect tumors at an early stage and to determine blood chemistry using safe and inexpensive instruments, with correct interpretation of the influence of scatter. Environmental sensing applications include measurements of or within aerosols, turbid water, sea ice, and snow. There has also been significant recent interest in random media with optical gain, which may find application in displays and for inexpensive lasers. Coherent light within all these domains produces speckle, typically observed as random intensity variations or graininess in a spatial image. While efforts have been made in some applications to reduce the confounding impact of speckle, such as in optical coherence tomography, speckle has the potential to determine important properties of the scattering medium. Webb and Weiner have used frequency correlations of laser speckle to characterize scattering material. This characterization involved determining impulse response or scattering parameters that can be used to form spatial images, such as with optical diffusion tomography, and to extract quantitative spectroscopic data. Of particular note, Webb and Weiner's group recently showed that it is possible to use third order speckle intensity correlations in frequency to directly determine the impulse response of a scattering medium, independent of a model, extending the famous second order intensity interferometer concept of Hanbury-Brown and Twiss. Speckle fields for static media have received significant attention. However, many important systems are dynamic, prime examples being in biological and environmental sensing. There are significant opportunities to exploit dynamic media speckle data, and a very simple instrument will be demonstrated to allow the scattering properties to be determined by exploiting the motion of the scatterers. Furthermore, by combining acoustic and speckle modalities, a larger parameter space for the characterization and imaging of scattering media will be explored. Speckle data in the presence of an acoustic wave will yield data related to the mechanical properties of the domain. This will provide for enhanced contrast in, for example, soft tissue imaging. The third order speckle frequency correlation concept has application far beyond statistical light transport. The PIs will demonstrate a simple and comprehensive approach to characterize optical fiber. Speckle correlations will be used to characterize multimode fibers, important in sensing and local area networks. Third order correlations will also be used in a new measurement scheme to easily and completely characterize polarization mode dispersion, a limiting factor in modern single mode communication fibers. The broad scientific impact from this study of laser speckle in dynamic scattering media will primarily be in the areas of biomedical imaging, environmental sensing, and in the chemical industry. The application of correlation techniques to optical fibers offers the opportunity for additional impact in the area of sensors and telecommunications. Two Ph.D. students will participate in the research, thereby receiving training in optical measurements important for a range of industrial and research applications. Each summer, an undergraduate student will be included in the research group.