Mueller polarimetry for quantifying the stress optic coefficient in the infrared

To observe the polarimetric properties of an uncharacterized infrared transmitting material (IRTM) under various mechanical forces, a Mueller matrix (MM) imaging experiment was augmented with a force apparatus. Principal stress fields were computed from both finite element and closed-form models and spatially aligned with images of birefringence. The slope of the linear relationship between birefringence and principal stress difference is the stress optic coefficient. We discussed the advantages of MM polarimetry for stress optic coefficient measurements. First, no assumptions about the sample’s optical properties are necessary. Second, experimental deviations from the intended in-plane stress field can be identified. Third, independent pixels, over a small but appreciable range of stress values, can be selected to quantify experimental variation and improve statistical significance. To validate our experimental procedures, an N-BK7 sample was characterized at room temperature and compared with the industry-accepted value of 2.77 TPa−1±3% at 589.3 nm. To our knowledge, this is the first report on the stress optic coefficient of N-BK7 in the infrared, which was observed as 2.764±0.1526 TPa−1. The IRTM stress optic coefficient was observed to be 1.948±0.1197 TPa−1. Experimental sources of uncertainty are discussed and quantified.