Optical systems often need to operate and survive over a range of temperatures and thermal conditions. Temperature variations adversely affect the performance of an optical system in two ways. First, the thermal expansion and contraction due to thermal variations results in position and shape changes of optical elements, and second, temperature variations change the indices of refraction of optical materials. In addition to performance considerations, thermal effects can create structural failures in optical systems, including yielding or ultimate failure of flexures, debonding of adhesives, and the fracture of glass elements.
Thermal management is a critical aspect of optical system design to ensure performance requirements and integrity are met over the operational and nonoperational service environments. Thermal management includes the use of passive and active control strategies, including appropriate materials selection to maintain temperatures at acceptable levels. For complex systems with demanding performance and environmental requirements, integrated thermal-structural-optical models are beneficial to quantitatively assess thermal-management strategies.
9.1 Thermal Design and Analysis
Thermal design and analysis of high-performance optical systems should begin during the conceptual design stages. First-order analyses and sensitivity studies may be performed using closed-form and simple parametric thermal models accounting for conduction, convection, and radiation modes of heat transfer to identify appropriate thermal-management strategies. Passive thermal control techniques are preferred for their low cost, reliability, and simplicity.
As the thermal design matures, more complex thermal models are developed to predict detailed temperature profiles for a range of operating extremes. These higher-fidelity models may account for detailed geometry, internal heat sources (such as dissipation from electrical components), external heat sources (such as solar and IR fluxes), heat capacitances, joint conductivity, absorptivity/ emissivity, the specularity/diffusivity of coatings and surrounding surfaces, beginning and end-of-life thermal properties, and thermal control solutions such as heaters, insulation, heat pipes, cold plates, and radiators. These analyses are used to ensure that component, assembly, and system thermal requirements are met.
Verification and validation of thermal management strategies is established through coupled thermal-structural-optical performance analyses, along with component-, assembly-, and system-level thermal testing.