SUMMARY Optical imaging, which offers sufficient spatial resolution, specificity, sensitivity, and temporal resolution, has provided substantial insights into important biological processes at the cellular and molecular levels. Although high quality optical images can be obtained from in vitro cell cultures and/or thin tissue sections, intravital cell imaging in a complex, three-dimensional living tissue environment remains quite challenging. Because biological tissue naturally does not favor the propagation of light, and because of the inevitable presence of strong ambient light in the environment, special challenges arise in virtually every in vivo biomedical optical imaging, namely weak light signals and strong light backgrounds (including the multiply-scattered light in the tissue and the environment light). These challenges have significantly limited the full application of optical imaging in in vivo pre-clinical and clinical investigations. Currently, the detection of weak light signals for optical imaging relies heavily on the use of highly sensitive electronic detectors and electronic amplifiers. Although high-end electronic photo receivers are often very sensitive, the high sensitivity leads to the imaging systems being extremely prone to random photon noise, such environmental background (room) light, which is problematic for practical applications (e.g. in vivo studies and clinical practice). Furthermore, electronic detectors are incapable of distinguishing image-bearing ballistic photons from the multiply-scattered light background, which as a predominant source of noise in optical imaging of biological samples, can be overwhelming and significantly degrade resolution when imaging microstructure deep in tissue. In this proposed project, we will develop a multimodal microscope that utilizes a novel high speed optical parametric amplifier (OPA) to optically amplify weak back-scattered light signals, and demonstrate its capabilities by investigating in vivo cellular apoptosis events in murine skin. As shown by our preliminary data, the OPA will not only provide a high level of signal gain to improve detection sensitivity, but also provide an inherent nonlinear optical gate to both extract imaging-bearing signals and reject the noise sources from environmental photons and multiply-scattered background light. We will systematically explore the benefits afforded by this OPA for multimodal imaging that will include label-free reflectance confocal microscopy and optical coherence tomography. Improvement in resolution, contrast, imaging depth, and reduced photo-damage, will be investigated. The successful completion of this project will demonstrate a high-speed, robust, optical intravital microscope that combines multiple modalities with enhanced performance and new fascinating imaging functions uniquely enabled by the OPA. This intravital microscope will not only enable new biological and clinical studies, but also promote the development of new optical imaging technologies based on optical amplifiers.