Chromaticity effects in adaptive optics; wavelength a dependence of amplitude compensation

The achievable contrast level for space-based detection of exo-planets will be limited by the stability of the optics. As a consequence, active amplitude and phase compensation will be needed. In order to mitigate these wavefront instabilities, we suggested, as an alternative to classical adaptive optics, the use of a Michelson interferometer equipped with two deformable mirrors. Simulations showed that this set up is able to create a symmetric "dark hole" in an appropriate area of the image plane. However, increasing the bandwidth of the incident light critically alters this nulling performance. A quantitative analysis of this effect will first be presented. An alternative to circumvent this problem is to introduce a dispersive element in one of the legs of the interferometer so that the path length difference does not exhibit the one over wavelength dependence. In the case of the insertion of a gaseous cell, the OPD could then be controlled by pressure variations. The last section of this paper will present a simulation-oriented proof of concept relying on the dispersive properties of nitrogen.