Interferometry for Refractive Index and Thermal Defocus Determination

From head and weapon mounted systems on the ground to space-based surveillance for ballistic missile defense, infrared (IR) electro-optic (EO) sensors are needed to overmatch emerging threats. A robust and reliable United States based supply chain for IR optic lens materials is essential to produce optics that perform in these critical defense areas. Unfortunately, the IR lens material supply chain is constrained by high-cost refractive index measurement systems and third-party testing.To overcome this issue, ALLVAR is proposing a novel interferometric method for direct refractive index measurement. This novel system and method are projected to reduce refractive index measurement system and testing costs by 75% while providing dual use for determining thermal defocus of refractive optic assemblies. The novel system has already been proven and tested in the visible spectrum.The goal of this project is to demonstrate that interferometry is a fast and economical method for certifying index of refraction in infrared lens materials. To achieve this goal, the index of refraction measurement concept must be matured by applying the proven technology to the infrared wavelengths and operational temperatures of interest. Then the total combined uncertainty of the apparatus and measurement method must be determined to ensure that a lens material’s refractive index can be certified to 0.001. The present study will determine the various uncertainty contributions to the refractive index measurement and use uncertainty calculations and measurements to explore the solution trade space for each subsystem function. A system will be assembled to measure Type A uncertainty at a single wavelength and temperature. The results of this study will pave the way for a full-scale demonstration across relevant wavelengths and temperatures in a Phase II effort. If successful, this novel measurement technique will impact the entire infrared optic supply chain by enabling certification of refractive index lens materials and thermal defocus of lens assemblies. The result will be a more robust and resilient infrared sensor supply chain.