Digital holographic (DH) interferometry has been developed in the long-wave infrared spectral range with CO2 lasers and microbolometer arrays. This application has been driven by the European Space Agency’s constant need of techniques for monitoring large displacements of large structures. Here the study focuses on the case of aspheric mirrors, like parabola and ellipses. Usually they are tested through interferometric wavefront error measurements which require expensive null-lenses matching each of the reflectors considered. In the case of monitoring deformation a holographic technique can be considered where the wavefront is compared with itself at different instant. Therefore the optical can be quite simple and easily reconfigurable from one reflector to another.
The advantage of using long wavelength is that large deformations can be measured at once, in addition to being more immune against environmental perturbations. Another advantage of DH at such wavelengths is that the ratio between the wavelength and the pixel size allows reconstructing objects 5 to 10 times larger than with DH in the visible.
In this project we considered first the case of a 1.1 meter diameter parabola for submillimeter range observations. Such specimen shows strong specular reflectivity. We have developed several set-ups with different ways to illuminate the object and to collect rays to form the object beam: either through point source or through and extended diffuser working by reflection of the laser beam. Both possibilities have been compared in terms of fringe quality as well as measurement range. We selected the diffuser illumination for applying the set-up into a large vacuum facility for measuring the deformation of the parabola between 224 and 107 K. Results of this measurement campaign are presented.
A further application has been shown by observation of off-axis ellipse. In this case interferometric testing is difficult to achieve and LWIR DHI with diffuser illumination is found quite simple to implement and gives straightforward results.GSTP HOLODI
