This chirped pulse is then amplified, and subsequently compressed to obtain a high power pulse with a duration nearly equal to the input pulse. Two parallel beams of different wavelengths, incident on the first grating, are still parallel when they leave the second grating, but they have travelled different distances. A grating pair arranged parallel as in fig. A, will introduce a negative group velocity dispersion, i.e. pulses of long wavelength arrive later than short wave pulses. In order to achieve a positive dispersive delay, a more complicated arrangement is necessary.
B shows such an arrangement, normally used in the stretcher stage.
The telescope reverses the sign of the angles so that the beams will hit the second grating at the same angle as they leave the first one. Both stretcher and compressor are normally used in double pass. The gratings are holographically recorded with two highly collimated, clean and homogeneous beams, which give straight and equispaced grooves. Not only the diffraction efficiency is important. In order to avoid thermal effects, the absorption in the grating surface should be as small as possible.