  Coherent extreme ultraviolet (XUV) radiation produced by table-top
high-harmonic generation (HHG) sources provides a wealth of possibilities in
research areas ranging from attosecond physics to high resolution coherent
imaging. However, it remains challenging to fully exploit the coherence of such
sources for interferometry and Fourier transform spectroscopy (FTS). This is
due to the need for a measurement system that is stable at the level of a
wavelength fraction, yet allowing a controlled scanning of time delays. Here we
demonstrate XUV interferometry and FTS in the 17-55 nm wavelength range using
an ultrastable common-path interferometer suitable for high-intensity laser
pulses that drive the HHG process. This approach enables the generation of
fully coherent XUV pulse pairs with sub-attosecond timing variation, tunable
time delay and a clean Gaussian spatial mode profile. We demonstrate the
capabilities of our XUV interferometer by performing spatially resolved FTS on
a thin film composed of titanium and silicon nitride.
