PNNL

Abstract

In this paper we present a prism coupler that is capable of characterizing optical dispersion and thermal index variations (dn/dT) in bulk and thin film materials at measurement wavelengths extending through the mid-infrared (3 to 12 µm). Our research was motivated by the need for precise, rapid, and low cost optical refractive index analysis to facilitate development of new mid-infrared optical materials, assessment of variability in mid-infrared optical materials acquired from commercial sources, and design of optical elements used in advanced, high performance mid-infrared sensing platforms. Such efforts commonly require ±1x10-3 or better absolute index measurement accuracy at measurement wavelengths spanning from the visible to the mid-infrared. Unfortunately most dispersion and dn/dT characterization methods require compromises in accuracy, cost, and timeliness, or cannot access the mid-infrared spectral region where many of the most important sensing and defense applications exist. A prism coupler, implemented at the mid-infrared, was found to provide rapid and cost-effective optical materials metrology with sufficient accuracy to meet most design requirements. We discuss the challenges of integrating the required mid-infrared optical components, including a sensitive mid-infrared detector and the quantum cascade and other infrared laser sources, with a commercial Metricon prism coupler and the calibration steps necessary to achieve the desired measurement accuracy.