Ultrafast monochromator
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In the diffraction of an ultrashort pulse (in the femtosecond, or lower, time-scale) with a grating, apart spectral filtering, there occurs a time stretching due to the difference in the path length among the rays diffracted, even of equal wavelength.
The reason resides in the action of the grating, that is to depart from the behaviour of a mirror having equal reflected and incidence angles,
- α = β.
The grating directs the rays in directions according to
- mλ = d(sinα − sinβ).
The rays diffracted by adjacent grooves differs in path length of the quantity
- mλ.
The total difference of path length in the diffraction is then computed by subtracting the shorter path from the longer. In internal order diffraction, this is the ray that shines the grating limb opposite to the entrance slit minus to the one closer. A grating illuminated at half intensity for 10 mm with 1200 grooves per mm uses 12000 grooves. At the wavelength of 10 nm and with m = 1, we have that
- ΔOP = 120 μm.
This corresponds to a time difference in the arrival of
fs.
This is often negligible for picosecond pulses but not for those of femtosecond duration.
The ultrafast monochromator aims to preserve the temporal duration of a femtosecond source, by combining more than one grating.
A major application is the extraction without time-broadening of a single laser harmonic of high order from a generated by a few femtosecond laser interacting with a gas target.
The conditions to be fulfilled by the ultrafast monochromator are
- that the different optical path introduced during the dispersion with one grating is removed during the diffraction operated by a second grating;
- that the path length of the rays propagating inside the monochromator are all the same.
This latter condition imposes also the correction of the optical aberrations introduced by the monochromator optical components.
When these conditions are both actuated, the design is compensated for the time broadening and the ultrafast properties of the source are preserved by the monochromator.
