at wavelengths of about 760 nm about 820 nm and about 875 nm with fluctuation minima therebetween design b has fluctuation maxima at about 730 nm about 790 nm about 850 nm and about 890 nm with fluctuation minima therebetween the positions of the fluctuation maxima and minima are arranged such that the ripples of one are about u201c out of phase u201d with the ripples of the other this provides that a pair of mirrors one coated with design a and other with design b can provide and in effect an average gdd that has minimized ripple see dotted curve in fig3 it should be noted that the aggregate gdd imparted by the pair would be about u2212 110 fs 2 fig6 schematically depicts the measured gdd as a function of wavelength for an example of design a at a range of angles of incidence up to 24 u00b0 incidence fig7 schematically depicts the measured ggd as a function of wavelength for an example of design b at a range of angles of incidence up to 31 5 u00b0 incidence it can be seen in each case that the magnitude of ripple in the gdd is about the same as at normal incidence except for the largest incidence angle in each case the ripples shift to longer wavelengths with increasing incidence angle principles of gdd mirror design are described in detail in u s pat nos 6 081 379 and 6 154 138 each assigned to the assignee of the present invention and the complete disclosure of which is hereby incorporated herein by reference software is commercially available from more than.