The development of multilayer mirror technology capable of operating i
n the range of 3-30 nm and the construction of thin membranes with exc
ellent uniformity and strength have made it possible to design and imp
lement a Mach-Zehnder interferometer operating at 15.5 nm. We have tes
ted this interferometer by using a soft x-ray laser as a source, and w
e show its use in probing high-density plasmas.