The ability to form images through densely scattering media has been highly desirable in medical diagnostics. The "gating" approach utilizes the principle that scattered light will travel farther than unscattered, ballistic light. A laser, with a very short pulse width or coherence length, can make a holographic recording of the light traversing the volume, and only the ballistic light will interfere with the reference beam, creating an image without scatter. Applied to propulsion diagnostics, this technology will yield images from dense flows which are surrounded by a scattering medium through which images cannot be obtained by conventional means. The recent development of photorefractive multiple quantum well (MQW) crystals has provided a breakthrough in recording materials for image gating. These crystals have an extremely high dynamic range, allowing a hologram to be recorded despite noise many orders of magnitude greater than the signal. They are also capable of continuous image capture at kHz repetition rates, making them ideal for flow analysis. North Dancer Labs, Inc. (NDL) will, in Phase I, design and build a test bed for Gated Holographic Imaging (GHI) to demonstrate the proof-of-principle for a high-speed, MQW-based recorder. A copper vapor laser with a dye cell will provide sufficient power and short coherence (spatial and temporal) at kHz recording rates. Beyond simple visualization, NDL will also investigate the ability to provide quantitative data by applying shearing interferometry, Schlieren photography, particle imaging velocimetry, or resonant holography to prove diagnostic capabilities of the GHI system.
