Grooves of RVS blazed transmission grating
The Fraunhofer Institut für Optik und Feinmechanik (IOF) in Jena designed, built and tested a state-of-the-art grating for the Gaia Radial Velocity Spectrometer (RVS). The blazed transmission grating is a 9 mm thick fused silica plate, of which the top surface layer has been carefully etched with an accuracy of about 20 nm in depth to form the ~2 µm deep grating grooves.
The blazed transmission grating has 302.11 grooves per mm. The grooves are clearly visible in this electron microscope image of the grating's surface texture, running parallel from lower left to upper right. Each groove element (unit cell) actually comprises 5 patches of binary index modulation elements over a width of 3.31 µm. The elements that make up a single groove can be seen clearly in this image as the repeated pattern of discrete patches (3 patches run continuously along the groove and two patches are discontinuous along the groove).
The light passing through this sub-wavelength scale texture (the RVS will observe in the near-infrared between 847-874 nm) does not "see" the individual patches, but rather the integrated profile of the patches. The net effect is the same as for gratings with the more regular saw-tooth groove pattern, but this pattern of discrete patches allows for all the optical requirements to be met, including a very high efficiency of >80%, well above the required 70% for the RVS.