A confocal scanning laser holography (CSLH) microscope to non-intrusively measure the three-dimensional temperature and composition of a fluid




Jacquemin, Peter B.

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The Confocal Scanning Laser Holography (CSLH) microscope non-intrusively measures the three-dimensional (3D) temperature and composition of a solid, fluid, or plasma. A unique reconstruction algorithm uses phase-shift data from the recorded holograms and boundary conditions of the specimen to measure the 3D temperature. The CSLH microscope uniquely combines holography with a scanning confocal microscope to determine the phase-shift in a hologram and to reconstruct the 3D temperature. The confocal aspect of the microscope reduces optical aberrations in the hologram and increases sensitivity to a temperature at a scan position in the specimen. The optical design maintains a stationary focal point on the pinhole aperture within the confocal optics during scanning. The CSLH microscope uses a focused laser beam instead of a collimated beam to probe the specimen. The advantage of the focused probe beam over the collimated beam is that different phase-shift data is obtained for each scan position of the probe beam. Another advantage is preventing rotational scanning of the laser about the specimen or rotating the specimen, increasing the number of practical applications. This limits the scan angle to the cone angle of the probe beam only. Reconstruction of the 3D temperature given restricted scanning from a single viewing window places a burden on the reconstruction algorithm to produce low reconstruction error. Three-dimensional reconstruction using methods of tomography prove inaccurate due to the small cone angle. The result is ill-conditioned reconstruction matrices. A unique low reconstruction error algorithm given a single viewpoint window that specifies a particular scanning geometry and requires boundary conditions is derived for the microscope. This research involved the design, building, and evaluation of a specific CSLH microscope intended for fluid flow and heat transfer studies in micro-gravity space based experiments. The fluid specimen used to evaluate the microscope sets a benchmark for resolution, sensitivity, and performance. The reconstruction error is primarily due to measurement error, residual optical aberrations affecting holograms, and vibrations since the reconstruction algorithm error is negligible. Additional knowledge gained includes the understanding of sensitivity to optical alignment as well as methods to accurately determine the phase-shift in a varying fringe contrast hologram. A significant trade-off is that as the cone angle of the probe beam increases, the reconstruction error decreases but the optical aberrations increase. One of the more difficult challenges during scanning is to maintain a fixed focal point on the confocal apertures as the beam is tilted off the optical axis centerline. Further recommended advancements for the microscope are improving the optical lenses to provide pupil planes that are stationary during scanning and the miniaturization of the microscope using diffraction grating lenses instead of glass lenses for more practical applications. Determining the internal temperature of a flame by passing a focused laser beam through the flame is an example of a practical application. The CSLH microscope is uniquely capable of non-intrusively measuring the 3D temperature of a specimen given a single viewpoint window for scanning with applications in the physical and biological sciences.



Holography, Microscopes