Determination of the nonlinear thermal expansion coefficient of an epoxy used as an expander and its effects over a Fabry-Perot cavity filled with a polymer

Abstract

In this work, the nonlinear thermal expansion coefficient (TEC) of epoxy clay is determined by means of monitoring the changes induced in the optical thickness of a Fabry-Perot cavity filled with polymer. Here, the epoxy is used to form a long sleeve that holds all elements that form a multilayer interferometric fiber tip filter. Additionally, it is shown that this sleeve acts as an axial strain actuator to change the thickness of a polymer cavity in the order of a few microns. This allows the filter’s spectral fringe pattern to be tuned by several orders of interference, inducing considerable changes in its free spectral range (FSR). Furthermore, it is shown that this filter design has an additional advantage that makes it possible to determine either the TEC or the thermo-optic coefficient (TOC) of another layer material of the filter by using the same measured spectra. Here, as a proof of principle, experimental results of the TEC of silicon were estimated, showing that they are quite close to what was reported in the literature and, therefore, that these can be used for reference purposes. Finally, it is demonstrated that by considering the determined TEC of the epoxy, the overall filter spectrum can be very well modeled, and its main features can be explained for a wide range of temperatures.