Unified optomechanical modeling: thermo-elastic stability of a fiber optic diffractive encoding system

A common mechanical failure in optical systems is inadequate stability in the supporting structure. Thermal stability is crucial for maintaining the alignment of the optical elements and achieving adequate optical performance as the environmental temperature changes. It is the responsibility of the mechanical engineer to provide adequate stability in the mechanical design. Optical engineers assume that their large-displacement non-linear codes are required to analyze the perturbations caused by mechanical deflections. However, the permitted deflections of the optical elements are usually quite small, on the order of microns for structures of meter-sized dimensions. For perturbations of this magnitude it may be shown that a non-linear solver is not required for engineering accuracies. In fact, it can be argued that the optical functions are more linear than the solid mechanics functions, of which the finite element method itself is but a linear simplification. Unified optomechanical modeling provides a vehicle for tracing offending image motions to particular optical elements and their supporting structure. The unified modeling method imports the optical elements’ imaging properties into a finite element structural model of the optical system. It convolves the elements’ motions and their optical properties in a single optomechanical modeling medium, unifying them. This provides the engineer with a tool that discloses each element’s contribution to the offending motions of the image on the detector. This paper presents the theory of unified optomechanical modeling as applied to the thermal stability of the optical image in a Nastran1 finite element model. The steps used in developing a unified optomechanical model are described in detail. Comparisons of the unified modeling technique to both analytical and empirical validation studies are shown.