Characterization of beam quality in OMEGA is currently carried out using shearing interferometry, which is limited by (1) its inability to perform gradient measurements in more than two directions, (2) its sensitivity only to low-order phase errors, and (3) its low spatial resolution. Conventional phase-shifting interferometers (e.g., Mach-Zehnder) provide greatly improved measurement capability, but are extremely sensitivity to mechanical shock and transmitted vibration because they utilize separate test and reference optical paths that must be aligned to within a fraction of the wavelength of the light being used. Such interferometers are difficult and time-consuming to setup, align, and maintain, and are costly due to the number of optics required for the dual-path design.

In contrast, common path interferometers such as the point-diffraction type are much less sensitive to environmental disturbances such as mechanical vibration, temperature fluctuations, and air turbulence. In our earlier work, we demonstrated that the LCPDI, a phase-shifting point-diffraction interferometer utilizing a liquid crystal (LC) cell as the active electro-optical element, is not only significantly more robust and accurate than a phase-shifting Mach-Zehnder interferometer, but also is considerably smaller and uses fewer optical elements. Such attributes make this device of special interest not only for diagnostic applications on OMEGA, but also for other applications in the commercial, military, and industrial sectors where size and cost are critical issues. One important application example of the latter is in microgravity fluid physics experiments intended to be conducted by NASA on the International Space Station. Here the minute size, weight, and power requirements of LCPDI devices coupled with their inherent physical robustness make these "interferometers on a chip" the ideal candidate for such a demanding application.

The imaged (test) area of the LCPDI device is substantially larger than the reference area (a microsphere embedded within the LC fluid gap). The portion of the beam that passes through the LC fluid must be attenuated using an absorbing dye in order to obtain sufficient contrast for analysis of output images. The lack of available dyes with both sufficient solubility in the LC host and a lmax near 1053 nm led us to synthesize a group of dyes based on zero-valent transition metal dithiolenes that were capable of satisfying preliminary device requirements for use in OMEGA. However, until recently device fabrication issues have prevented us from being able to produce an LCPDI with sufficient contrast to evaluate its potential as an OMEGA diagnostic tool.

Under an externally funded, parallel research project with NASA Glenn Research Center to develop visible-region LCPDI devices for microgravity fluid physics experiments, we have recently found that polymeric microspheres produce an extraordinary improvement in both interference fringe quality and device yield when used as the central reference element in the LCPDI instead of glass microspheres. The photomicrographs below show the difference in contrast between a glass microsphere (a) and a polymer microsphere (b) in the same LCPDI device. We believe the increased compressibility of the polymer sphere is one important factor contributing to this dramatic improvement in device quality.

(a)	(b)

In addition to continuing to refine the device manufacturing process, we plan to continue development of additional near IR dye materials so as to allow us to further increase the maximum dye concentration in the LC host and thus prepare devices with shorter optical paths. Shorter path lengths will allow both faster response times due to reduced viscoelastic effects and higher fringe contrast due to improved long-range ordering of the LC molecules. Improvements in these areas would open up the possibility of using these devices in commercial applications where fast response time is an important factor (e.g., in interferometeric analysis at video frame rates).

Another materials research aspect of interest is the ability to manipulate the absorption band in transition metal dithiolene dyes so as to extend their absorption range out to 1.5 µm while maintaining their high solubility in liquid crystal hosts. Numerous applications for modulation and switching devices operating at this technologically important wavelength exist in both the rapidly growing telecommunications industry and the military sector, and there are currently no available dyes that possess the necessary combination of physical and chemical properties to be useful in such devices.

K. L. Marshall, B. Klehn, B. Watson, and D. W. Griffin, “Recent Advances in the Development of Phase-Shifting Liquid Crystal Interferometers for Visible and Near-IR Applications,” in Advanced Characterization Techniques for Optics, Semiconductors, and Nanotechnologies , edited by A. Duparré and B. Singh (SPIE, Bellingham, WA, 2003), Vol. 5188, pp. 48 - 60.

M. J. Guardalben, L. Ning, N. Jain, D. J. Battaglia, and K. L. Marshall, "Experimental Comparison of a Liquid-Crystal Point-Diffraction Interferometer (LCPDI) and a Commercial Phase-Shifting Interferometer and Methods to Improve LCPDI Accuracy," Appl. Opt. 41 (7), 1353-1365 (2002).

K. L. Marshall, I. A. Lippa, S. Kinsella, M. S. Moore, S. M. Corsello, and A. Ayub, "Chiral Transition Metal Dithiolene Dye Complexes and Their Potential Applications in Liquid Crystal Devices," OSA Annual Meeting and Exhibit 2001, Long Beach, CA, 14-18 October 2001.

K. L. Marshall and S. D. Jacobs, "Near-Infrared Dichroism of a Mesogenic Transition Metal Complex and its Solubility in Nematic Hosts," Mol. Cryst. Liq. Cryst. 159, 181-196 (1988).