  Interferometric pump-probe experiments in the extreme ultraviolet (XUV)
domain are experimentally very challenging due to the high phase stability
required between the XUV pulses. Recently, an efficient phase stabilization
scheme was introduced for seeded XUV free electron lasers (FELs) combining
shot-to-shot phase modulation with lock-in detection. This method stabilized
the seed laser beampath on the fundamental ultraviolet wavelength to a high
degree. Here, we extend this scheme including the stabilization of the XUV
beampath, incorporating phase fluctuations from the FEL high gain harmonic
generation process. Our analysis reveals a clear signal improvement with the
new method compared to the previous stabilization scheme.
