“…For observation of such small effects, instrumentation needs to be Fabry-Perot type, or high resolution Fourier-spectrometer (resolution $0.005 cm À1 ). [9] Studying the HFS in Ho doped ILF, [10,11] after additional inspection of spectra with new instrumentation and cooling to 2 K, we confirmed previous observation of shoulders on HFS components. Mostly these effects are observed in conditions similar to those ones, which are necessary for hyperfine structure (HFS) observation.…”