Packaging hygroscopic and stress-sensitive optics for a standard retarder

We discuss the packaging of a standard retarder. The retarder is a double Fresnel rhomb device and we have achieved retardance stability within +/-0.1 degrees over a +/-20 degrees C temperature range. We estimated a 10 year lifetime for storage within normal laboratory temperature and humidity conditions. The retardance of an optical rhomb retarder is sensitive to stress-induced birefringence. We selected a glass with a low stress-optic coefficient to improve optical performance. However, the glass is hygroscopic, and the retardance of the device changes in the presence of water vapor. Thus, for a stable retardance, a package as nearly hermetic to water as possible that does not introduce any stress birefringence is required. We have tested the permeability of the seals, the sensitivity of our optical device to water vapor, and predicted a usable lifetime for the retarder. We discuss the optical materials, water permeability of seal materials and the mathematical models used to predict the lifetime of the device. Also, we discuss the optical design and retarder's placement in the package to minimize stress induced birefringence.