Interferometric apodization of telescope apertures I. First laboratory results obtained using a Mach-Zehnder interferometer

Aims. We seek to produce apodized apertures for application in stellar coronagraphy to help in direct detections of exoplanets. We show that chromatic apodized apertures of any shape in transmission can be obtained with a specific MZI and we demonstrate this capability in two cases. Methods. The method takes advantage of the capabilities of the MZI, in which the two outputs correspond to the addition and subtraction of the two wave amplitudes in both arms. The result is obtained by re-imaging the entrance aperture of the telescope in the arms of the MZI where two complementary phase masks ± ϕ(x ,y ) are set. At the two outputs of the MZI, the re-imaged apertures interfere, and their transmissions are multiplied respectively by a factor of the form cos[ϕ(x ,y )] and i sin[ϕ(x ,y )]. They correspond to the apodized and anti-apodized complementary outputs. Results. We present the results obtained for two types of apodization. A 1D cosine apodization for a square aperture is obtained by introducing a thin wedge-shaped air film, slightly tilting one of the mirrors of the MZI. A 2D circular symmetric apodization of the form cos[x 2 + y 2 ] is obtained for a circular aperture using two complementary convergent and divergent lenses as phase masks. Aperture transmissions (in intensity) and corresponding point spread functions (PSFs) are given in each case and compared to the theoretical expectations. Conclusions. We have demonstrated the capability of the MZI to produce an apodized aperture. This result is obtained with no loss of photons, considering the fact that there are two complementary outputs. Considerations are given on the wavelength dependence of this technique.