HORIBA UV Holographic Blazed Replica Grating for TROPOMI SENTINEL-5P satellite

January 31, 2018

The pollution monitoring satellite Copernicus Sentinel-5 Precursor has been successfully launched from the Plesetsk Cosmodrome in Russia on 29th September 2017. The TROPOspheric Monitoring Instrument (TROPOMI) is the satellite instrument on board of the Sentinel-5 Precursor satellite. TROPOMI instrument maps a multitude of trace gases such as nitrogen dioxide, ozone, formaldehyde, sulphur dioxide, methane, carbon monoxide and aerosols – all of which affect the air quality and therefore our health and our climate.

SENTINEL-5P satellite with TROPOMI instrument (left) and first data on gases monitoring (right) Copyright : ESA - ESA/ATG medialab

TROPOMI is a pushbroom spectrometer with spectral bands in the UV, VIS, NIR and SWIR. HORIBA Scientific designed, produced and characterized for the UV channel, in collaboration with TNO, a set of blazed holographic plane reflexion replica gratings based on Jobin Yvon Technology.
A high groove density (2400gr/mm) holographic master grating blazed at 250nm was selected for its high efficiency and low stray light performances. This grating was characterized in efficiency on the TROPOMI UV channel range (270 - 330nm) for both TE and TM polarization (Fig. 1).
The produced Qualification Model (QM), Flight Model and Spare (FM, FS) replica gratings retain the performances of the initial master holographic grating with a cost effective approach :

  • Perfect periodicity, plus excellent micro-roughness of the surface eliminates ghosts and enhances stray light rejection ;
  • Minimal groove errors provide very high resolution.

In addition, owing to their ion-etched, sawtooth smooth and clean groove profile, UV TROPOMI blazed gratings offer higher peak diffraction efficiency than a standard pure holographic grating. The blaze angle is measured by using Atomic Force Microscope (AFM) to a value of 16.5°.

Fig. 1. Diffraction efficiency measurement of the UV TROPOMI 2400gr/mm blazed holographic plane replica grating from 270nm to 330nm, for TE, TM polarization of 0th and 1st order.

The diffracted wavefront was measured with a Fizeau interferometer on the whole grating useful area 64x62mm (Fig.2) :

Fig. 2. Diffracted wavefront measurement of the 1st order 2400gr/mm UV TROPOMI grating measured with a 632nm Fizeau interferometer. Values of ~/4 PV and ~/20 RMS are obtained.

The QM grating has been used for performing successfully environmental tests (temperature cycling from -50°C to +45°C and humidity test at 35°C and 95% RH) according to the TROPOMI space-flight mission requirements (Fig. 3).

Fig. 3. Humidity tests performed during 24 hours on the QM grating with 35°C and 95% RH.

The TROPOMI Sentinel-5 Precursor project allowed HORIBA Scientific to participate to the European Copernicus programme and be selected as grating supplier for Sentinel-3CD and Sentinel-5.