Multi-pass Fabry-Perot interferometer for Brillouin scattering measurements

Thesis (Ph.D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics. This thesis was digitized by the Institute of Optics in 2014 and was determined to have lapsed into the public domain. If you are the author and have questions about the digitization of your work, please contact Kari Brick, Graduate Program Coordinator for the Institute of Optics, at kbrick@ur.rochester.edu. Other contact information for the Institute is available at http://www.optics.rochester.eduLimits in the available surface flatness and parallelism of conventional Fabry-Perot mirrors generally make it difficult to exceed a finesse of about 70 and a contrast on the order of 10⁴. Recently, a multiple-pass plane-mirror Fabry-Perot interferometer (FPP-M), in which the light beam passes through the same pair of plates a number of times, has been demonstrated by Sandercock to perform better than a single pass FPP. Such an FPP-M is essential for studying Brillouin (Phonon-photon) scattering from solids, specifically for polycrystalline or opaque materials. We present a detailed analysis of such an instrument, taking into account specific surface defects. The advantages of an FPP-M has been clearly established over a single pass version. A major advantage is the increased flexibility afforded by the choice of both the reflectance and the number of passes. This allows one to independently adjust the finesse and contrast for any given plate separation. Since the stability of an FPP used for high resolution work with small signals is critical, we have built an instrument with an unusual, non-kinematic design. The major structural material is Cer-Vit (with an expansion coefficient of 2 x 10⁻⁸/°C), and the only metal used serves to thermally compensate for the expansion of the piezoelectric ceramic elements used for alignment. The mirror spacing is adjustable. No servo-control is used. Yet, the stability is better than fiftieth of an order over a period of days with the original version of the design and several hours with the modified version of the design which was incorporated to facilitate easy access to coarse alignment. Finally, we used the instrument as a three-pass FPP to study the Brillouin shifted spectra from a sample of hot-pressed polycrystalline CaF₂ in which the Brillouin shifted signal is 10⁻⁷ times weaker than the unshifted elastically scattered (Rayleigh) signal. During the process of these studies, we have established a method of studying the elastic properties of a crystalline material using a more easily available polycrystalline sample rather than a hard to grow single crystal sample in the conventional way. This is achieved by probing a volume of the sample with the focussed laser beam which is significantly smaller than the average size of a single grain inside the polycrystalline material