Low coherence interferometry for mid IR precision optics manufacturing

We report on an optical, non-contact, thickness measurement system for materials that are opaque at ultraviolet (UV) through near infrared (NIR) wavelengths, such as Germanium and Nano-Composite Optical Ceramics (NCOCs). Measurement options do exist, but they must physically touch the sample or rely on an assumed bulk distribution of material. Additionally, optics are often highly sensitive to contamination and greatly benefit from non-contact metrology. The authors used the Lumetrics Optigauge MIR low coherence interferometry (LCI) system to successfully measure a NCOC. A Silicon (Si) control is used as a reference because it can be measured by both an Optigauge II and the Optigauge MIR-LCI system. In this work, the authors successfully measured and report on materials that are transparent in the mid-infrared (MIR) range. The authors speculate that MIR-LCI will enable wedge, thickness, flatness, and other measurements performed using an Optigauge II system for MIR transparent materials.