Editing Ultrafast monochromator (section)

==Diffraction grating==
Diffraction gratings are constructed such that the angle of the incident ray, ''θ<sub>i</sub>'', is related to the angle of the ''m''th outgoing ray, ''θ<sub>m</sub>'', by the expression

:<math> m \lambda = d ( \sin{\theta_i} - \sin{\theta_m})</math>. 

Two rays diffracted by adjacent grooves will differ in path length by a distance ''mλ''. The total difference between the longest and shortest path within a beam is computed by multiplying  ''mλ'' by the total number of grooves illuminated. 

For instance, a beam of width 10&nbsp;mm illuminating a grating with 1200 grooves/mm uses 12,000 grooves. At a wavelength of 10&nbsp;nm, the first order diffracted beam, ''m'' = 1, will have a path length variation across the beam of 120&nbsp;μm. This corresponds to a time difference in the arrival of 400 femtoseconds. This is often negligible for picosecond pulses but not for those of femtosecond duration.