Absolute Prism-Assisted Maker Fringe Measurements of the Nonlinear Profile in Thermally Poled Silica

Abstract

Since the first measurements of a sizeable nonlinearity in thermally poled silica [1] were reported, there has been considerable interest in developing silica-based frequency converters and electrooptic devices. It is generally well accepted that the nonlinearity in thermally poled silica is confined to a thin (1-15 μm) layer on the glass sample that was exposed to the anode side during poling. However, reports differ markedly as to the shape (exponential [1], or Gaussian [2]) and thickness (a few to 10 μm and even 700 μm) [1-3] of this nonlinear region. In practical waveguide devices, a precise knowledge of these parameters is critical to understand and maximize the spatial overlap between this region and the optical mode. Yet to date, no standard method is available to measure these quantities reliably. Earlier, we made initial measurements of the nonlinearity using a prism pair [4]. Difficulties with the homogeneity of the poled region limited the maximum angle that could be explored, and a profile could not be definitively inferred. In this paper we describe 1) improvements in this prism-assisted Maker fringe technique and 2) a theoretical analysis that permits the first absolute measurement of the nonlinearity profile in poled silica.