A FOURIER TRANSFORM SPECTROMETER WITHOUT BEAM SPLITTER FOR THE VUV–EUV RANGE

We describe a Fourier transform spectrometer designed to operate down to 60 nm on a synchrotron beam line. As far as we know, there is no such instrument available in the EUV (λ < 140 nm) partly because manufacturing accurate beam splitters remains the major difficulty at these wavelengths. We use a wave front division interferometer instead of an amplitude division one to overcome this difficulty. The interferometer is based on a modified Fresnel bimirror configuration, which is controlled by an original optical system. This system keeps the mirror tilt error to a negligible value during mirror translation, and provides a sensitive interferometric measurement of the mirror translation. The sampling interval is 29 nm (path difference), allowing one to record large band spectra down to λ = 58 nm with spectral resolution δσ = 0.33 cm -1 for 512 K samples (one-sided interferograms). We measured the apparatus function by recording an interferogram from a He–Ne stabilized laser. By studying the white noise in the corresponding spectrum, we found that the sampling error in the interferogram was about 0.4 nm rms, which produces a near-perfect apparatus function. Finally, we recorded the visible/near UV spectrum of an arc mercury lamp for illustration.