Improvements as well as new functionalities can be implemented in IR sensors by depositing a stack of thin film layers on top their surface. For this purpose, Safran Reosc has developed and qualified various processes, from conventional deposition methods to planar structuration approaches. In this article, the usefulness of thin film AR coatings are addressed and the latest results obtained on projects like Sentinel-5 and Microcarb IR focal planes are presented and discussed. We also address our progress in pixelization of the coating by structuration of these optical layers. Eventually, the most recent developments of sub-lambda photonic filtering structures are discussed.
Reosc developed pixelated infrared coatings on detector. Reosc manufactured thick pixelated multilayer stacks on IR-focal plane arrays for bi-spectral imaging systems, demonstrating high filter performance, low crosstalk, and no deterioration of the device sensitivities. More recently, a 5-pixel filter matrix was designed and fabricated. Recent developments in pixelated coatings, shows that high performance infrared filters can be coated directly on detector for multispectral imaging. Next generation space instrument can benefit from this technology to reduce their weight and consumptions.
High end applications in infrared may benefit from infrared colorimetry. Actual systems are designed using filter wheel, gratings or dichroic components. Those systems are bulky, do not allow real-time acquisition and are sensitive to stray light.
Sagem & Reosc recently developed technologies to pixelate infrared coating filters at detector level. It allows designing very compact systems, easy to cool down and to significantly reduce ghost images. Optical systems are simplified and can achieve fast acquisition of multi-spectral video.
Numerical simulations have been performed in order to quantify the pattern shape influence on the optical performances at the microscopic scale. The etching processes of multilayer stacks have been developed. Finally, prototypes have been manufactured and tested.
This technology opens up new perspectives in the field of infrared filtering.
For the most demanding infrared applications, fine knowledge of system performance under operational temperature is required. Management of thin film spectral and mechanical behaviour at cryogenic temperature is a key factor to improve infrared system performances.
Reosc recently investigated models and methods to anticipate measure and control effects of low temperature on coated optical components. Solutions have been developed in order to maximize highest performances under operational conditions. Effects of temperature on spectral transmission and flatness have been studied.
This work has been applied to narrow band pass filters and dichroic coatings working in the range 55K to 90K. Measurements show satisfying correspondence with model. On the basis of model prediction, process and design have then been adapted to improve performances at operational temperature. It has been applied to manufacturing of optics for MTG satellites and IASI NG instrument.
PICARD, a Sun observing satellite, has produced more than one million images during its 4-year mission. SODISM is one of three instruments on-board, whose main goal is to measure the solar limb and its spectral dependence from the middle ultraviolet to the near infrared. The very high accuracy (a few milli-arcseconds) needed to measure the solar limb with its spatial and temporal variations makes the instrument very sensitive to small aberrations. In this paper, we will present the impact of various parameters on the solar limb measurement, from simple displacements of mirrors to complex mirror deformations and thermal gradients. A complete scenario has been constructed from these simulations, leading to a model that describes the actual limbs obtained with SODISM. All these simulations will help improving future missions, by assessing the critical parameters affecting the measurement accuracies of such instruments.