 4f pulse shaper

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.