Stress Analysis in Optical Systems

Abstract

Structural failures can occur when loading conditions cause stress and strain within a material that exceeds the ultimate strength. Similarly, the performance of an optical system can be adversely affected by poorly characterized loading conditions and the resulting stress fields. Rigorous stress analysis is crucial to ensuring a specified performance can be achieved under varying environmental conditions. This work presents stress measurements and analysis for two projects: i) measurement of the stress-optic coefficient of N-Bk7, a glass commonly used for optical components, and ii) a payload design for a high-altitude ($\approx$ 30 [km]) balloon deployment of an Infrared Channeled Spectro-Polarimeter (IRCSP). The N-Bk7 stress analysis is functionally based on the relationship between stress and measured polarimetric response. Stress in optical systems induces birefringence, where the index of refraction is dependent on the polarization of the incident light. Measuring the retardance of a material can therefore help determine how the stress is affecting the index of refraction of the material. This effect is quantified by the stress optic coefficient. For this work, a Rotating Retarder Mueller Matrix Imaging Polarimeter (RRMIP) was used to measure the linear retardance of a diametrically loaded sample of N-Bk7 at a wavelength of 1550 [nm]. These retardance measurements were used to compute the N-Bk7 stress optic coefficient as compared to industry-reported values. Prior to the 2021 deployment of IRCSP on a high-altitude balloon, a fully autonomous system was developed to control the image acquisition, focal plane temperature, and humidity of the instrument. Operating this optical system at high altitudes required analysis of the varying environmental conditions to design an instrument enclosure that met both optical and safety specifications. Finite Element Analysis (FEA) was used to show efficacy of the mechanical design under expected flight loads to earn flight approval from Columbia Scientific Balloon Facility (CSBF). This enclosure has been apart of three successful balloon deployments of IRCSP. Additional work to design the electronics for a PID-controlled thermo-electric cooler in also included.