= 4f pulse shaper
{title2=Grating–lens–mask–lens–grating}
A <4f pulse shaper> uses a <diffraction grating> to disperse an input pulse, a lens to focus its frequencies at different positions in a shared Fourier plane, a spectral mask or <spatial light modulator> there to modify them, and a second lens and grating to recombine them. In the symmetric idealization the distances are $f,f,f,f$, with the lenses separated by $2f$. Shaping the spatially resolved spectrum implements <spectral pulse shaping> rather than a direct time-domain mask.
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