Achromatic prism-based phaseshifter for nulling in the visible

Nulling interferometry is a direct method to detect earth-like planets. To determine whether a planet is earth-like spectrometry is performed on a broadband infra-red (lambda = 4-20 mum) input signal from the planet. The star signal in this region is roughly 10(6) times stronger than the planet signal. Nulling interferometry should decrease the broadband star signal by about this factor of 10(6). This can be performed using an achromatic phaseshifter based on dispersive elements. The design of a complete breadboard under an ESA contract including a prism based (eight prisms in total) dispersive achromatic phaseshifter is presented including error budget and implied tolerances on the mechanical components. Measurements with this breadboard resulted in nulling depths of 3.5.10(5) for polarized laser light and just below 10(3) for polarized visible broadband light in the wavelength range of 530-750nm.