Spatial filter
A laser beam is by definition coherent with respect to longitudinal wavelengths which is determined by the length of the cavity. But coherent light can also exhibit transverse modes. A cross section of laser beam might exhibit a combination of various modes creating an uneven intensity. The spatial filter is a device such as a pinhole used to eliminate transverse modes.
Most lasers now typically possess a single transverse mode though. Unfortunately, because of aberrations due to sub-optimal optical components (lenses, beamsplitters, etc.) or even nonlinear effects (going through an optical parametric amplifier), this single transverse mode beam gets aberrated and since Z-scan requires such a good Gaussian beam we need a way to clean it up. By focusing the beam down, you get the Fourier transform of it in the focal plane. The main point is that higher spatial frequencies (i.e. those responsible for the "noise" on the beam) are further away from the center than the lower spatial frequencies which make up the clean TEM00 mode. By placing an aperture just big enough to let through the low frequencies while cutting of the higher ones, we can clean up the beam.
A pinhole selects for the central TEM0 mode and blocks all the others.
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The result is a clean gaussian distribution of beam power across the cross section. This is desirable for many optical setups such as the Femtosecond Z-Scan Spectrometer
