Immersed grating

SRON and TNO have developed compact immersed reflection gratings for the TROPOMI-SWIR spectrometer on the Sentinel-5 precursor mission. The gratings combine excellent resolution and efficiency. Diffraction takes place inside the silicon (in immersion) thereby reducing the total spectrometer volume (and mass) by the refractive index cubed, that is, by a factor of 40.

The high efficiency, high resolution, and compactness of our immersed gratings have been tailored for the TROPOMI-SWIR space spectrometer, but can be applied in any SWIR spectrometer. Unique lithographic production processes have been applied that allow superb control over the blaze angle combined with nanometer precision in grating-pattern definition. Our gratings have proven their quality in optical tests in the TROPOMI-SWIR spectrometer breadboard model.

The grating surface area of typically 50x60 mm² is etched in a single silicon substrate. These monoliths are extremely robust against thermal and mechanical shocks. The immersed gratings have line densities around 500 l/mm. The efficiency is optimized by controlling the line density, blaze angle and groove shape.

The immersed gratings have been extensively characterized in the breadboard spectrometer. This has demonstrated that all TROPOMI SWIR performance requirements for high efficiency, low polarization sensitivity and spectral-ghost suppression are met. Qualification models are currently in production.

Breadboard model

A complete spectrometer demonstration/breadboard model has been built in order to test the new technology involved in the TROPOMI-SWIR spectrometer for the Sentinel-5 precursor mission. In the SWIR breadboard model an infrared detector has been integrated.

The SWIR detector was extensively tested and characterized for all key parameters, like noise (including random telegraph pixel noise), dark current (data down to 130 K), linearity, pixel response non-uniformity, memory effect, drift, etcetera. The results have been verified against the TROPOMI-SWIR requirements.

Front-end-electronics for the short-wave infrared detector

The SRON front-end-electronics (FEE) unit is designed to guarantee optimal control and data read-out for the infrared detector in the TROPOMI-SWIR spectrometer. The TROPOMI instrument will be flown as a Sentinel 5 precursor mission.

The radiation hard, latch-up protected, FEE design yields low noise and high stability fulfilling the TROPOMI-SWIR requirements. Interfacing, control and clocking are performed by an ACTEL FPGA. The design comprises fast 14-bit ADCs to digitize the video stream. The stacked board design yields a compact unit of low mass, power and volume.

Link to Sentinel-5 Precursor/TROPOMI-SWIR