2-Dimensional, Second-Harmonic Interferometer

Magneto-Inertial Fusion Technologies, Inc. (MIFTI) proposes to develop a new 2-D interferometer by improving on the second harmonic interferometer (SHI) scheme which has been used previously only with small diameter (~50μm) laser beams. Resilience to mechanical vibrations and simplified design are the most important SHI properties. When probing larger plasma volumes multiple 1-D interferometer chords are needed. We want to build a simple device that produces 2-D interferograms, instead of standard temporal evolution of 1-D phase shifts. How is the Problem Addressed: The proposed device is a two-color nterferometer with a 2-D footprint. Light from a 1J, 1ns pulsed Nd:YAG laser is expanded with a set of achromatic lenses to a beam size of about 5cmx5cm. The second color is created by frequency doubling in a nonlinear optical crystal, thus a separate reference beam is not needed. Two beams with fundamental and second harmonic frequency propagate co-linearly towards the sample. The sample dispersive properties, related to the physical quantity of interest, are encoded in light phase shifts. After the sample these beams pass through another nonlinear crystal and the second-harmonic interferograms are recorded. 2-D map of the physical quantity of interest is created by unwrapping the interferograms with appropriate software techniques. What is to be done in Phase I: During Phase I MIFTI will: (1) design and assemble a prototype 2-D SHI (2) test the concept feasibility with different dispersive samples (optical grids, meshes, plastic foils, etc.) (3) establish the lowest laser pulse energy required for creating useful interferograms (4) determine the best phase reconstruction methods for unwrapping the interferograms and (5) compare the 2-D SHI performance with a conventional two-path Mach-Zehnder interferometer. Commercial applications and other benefits 2-D SHI can provide inexpensive snapshots of plasma densities over a volume of interest and monitor gas densities inside a cell, for example in laser wake-field acceleration where gas jets are used to provide a tailored target density profile. Microscope interferometry and imaging of biological samples without stain markers is another large application area for miniaturized versions of 2-D SHI. Key Words: 2-D second-harmonic interferometer (SHI), plasma and gas density measurement, optical components testing, microscope interferometry