On the optical analysis of the ray path-lengths in the diffraction of femtosecond XUV and soft X-ray pulses

During the diffraction of a femtosecond pulse with a grating, apart spectra l filtering, there occurs a time stretching due to the difference in the pa th length among the rays diffracted, even of equal wavelength. The reason r esides in the action of the grating: to depart from the behavior of the mir ror, that is, alpha = beta, and to direct the rays in directions whose path s are groove-by-groove different of m lambda. The compensation of this broa dening is studied in the following by using a pair of grating in a particul ar mounting. The configuration is studied for the application in the select ion of one single laser harmonics in the framework of high order harmonic g eneration processes.