Combined imaging and spectroscopy component for extreme environments
The French company SILIOS provides a combination of prism and gratings (GRISM) to be integrated in the Near Infrared Spectro-Photometer of ESA’s mission EUCLID. These components allow one camera to be used both for imaging and spectroscopy. Originally designed for the observation of galaxies, GRISMs could also be applied to other components installed in difficult conditions (vacuum, cryogenics, radiation) such as those encountered in lasers or plasmas applications.
In some cases, the observation of galaxies requires a spectral bandpass from 1100nm to 1457nm with a spectral dispersion coefficient around 500. The high number and the low intensity of the observed galaxies requires a transmitted efficiency higher than 72% in the spectral bandpass and a large clear aperture of 136mm diameter.
A grism (also called a grating prism) is a combination of a prism and grating arranged so that light at a chosen central wavelength passes straight through.
The grating should correct some optical aberrations allowing simplification of the instrument optical concept.
The cumulative etching technology allows reaching multilevel stair-like topologies. Such surface topologies are used to produce phase functions for gratings, diffractive laser beam shapers and turbulence phase screens for example. The technology is based on successive masking photolithography and reactive ion etching steps (i.e. successive etching steps through resin masks). Thus, the whole grating is engraved at the same time resulting in a very high uniformity of the groove profile over the full engraved area.
This technology may be applied to other components installed in difficult conditions (vacuum, cryogenics, radiation) such as those encountered in lasers or plasmas applications.
Innovations & Advantages
The advantage of a GRISM is that the same camera can be used for both imaging and spectroscopy. In NISP case, the GRISM are placed onto a rotation wheel to be easily removed when switching to the imaging mode. The GRISMs create dispersed spectrum centered on the object's location in the camera's field of view.
For these GRISMs, very special specifications are required for the grating manufacturing:
- large useful aperture
- low groove frequency
- small blaze angle
- line curvature allowing wavefront correction.
Laser writing photolithography can reach the above requirements. This method consists in writing the grating line by line into a photosensitive resin. However, in space instruments, GRISMs are subject to environmental stresses and the use of resins may not be suitable. Indeed, observation in the near infrared requires a nominal operating temperature of the optical system less than 150K. Differential thermal expansion coefficient between glass prism and resin grating at cryogenic temperatures could cause some issues. Moreover, the properties of this resin in terms of outgassing caused by space vacuum, resistance to cosmic rays and high-energy electromagnetic radiations and ageing are largely unknown and difficult to characterize.
The GRISM developed by SILIOS withstands extreme conditions of space environment without problem. Another advantage of resin-free gratings is the potential extension of the operating spectral range where resins transmission is low.
Current and Potential Domains of Application
This technology will be used in the framework of ESA’s EUCLID mission for the observation of galaxies. This technology may be applied to other components installed in difficult conditions (vacuum, cryogenics, radiation) such as those encountered in lasers or plasmas applications.