A spatial filter is a telescope-like device consisting of two lenses that have a common focus, with a pinhole located at the plane of this focus. The laser beam is focused by the first lens through the pinhole and expands out again until it reaches the second lens which re-collimates it anew.
In the Asterix IV/PALS laser, in total six vacuum spatial filters are spaced along the amplifier chain. Five of them serve for an efficient coupling of the laser beam between adjacent amplifiers, while the sixth, largest, spatial filter transports the beam between the output plane of the last amplifier A5 and the frequency conversion DKDP crystals. All spatial filters employed in the Asterix IV/PALS chain are assembled from thick stainless-steel tubes equipped at each end with a plano-convex lens constituting the vacuum interface. The lenses are mounted on flexible bellows segments making it possible to optically adjust the whole assembly during the initial setup of the laser chain.
The spatial filters deployed in the Asterix IV/PALS laser perform three tasks. These may be illustrated with the help of the following picture.
input beam diameter, and f1 and f2 are the focal lengths of the lenses.
provides nearly optimal coupling of the beam energy between adjacent amplifiers.
As shown in the picture, the output of a laser amplifier constitutes the object plane of the confocal optical system formed by the spatial filter. If (f2/f1) * (f1 + f2) > d1, the intensity pattern from this plane is imaged (= relayed) at the distance d2 equal to (f2/f1) * (f1 + f2) - d1 * (f2/f1)2 beyond the output lens.
In the Asterix IV/PALS laser chain, the initial object plane is constituted by a uniformly illuminated 8-mm aperture located in front of the first spatial filter SF1. This aperture is successively re-imaged by the spatial filters SF1 to SF5 near the exit planes of the individual amplifiers. The arrangement ensures a smooth beam profile on the optical components located in the high-fluence — hence critical — regions near the amplifiers output, minimising the risk of optical damage. The output aperture of the final amplifier A5 is imaged at the entrance plane of the frequency-conversion crystals.
components of the laser.
The job is carried through spatial filtering.
On its focal plane, the input lens produces a diffraction pattern which is a Fourier transform of the light distribution from the object plane. The diffraction pattern is constituted by the spatial frequency spectrum of the object, where a specific frequency is proportional to its distance from the axis of the system. Thus high spatial frequencies, corresponding to small-scale intensity modulations of the object, appear at a large distance from the axis. When a screen with a pinhole is inserted at the focal plane, these high spatial frequencies are eliminated. On the other hand, the low-frequency components constituting the smooth beam profile appear near the axis, and pass through the pinhole unperturbed. The output lens of the spatial filter performs the inverse Fourier transform, projecting the filtered diffraction pattern onto the image plane.
In Asterix IV/PALS, the pinholes of the final spatial filters are typically 2 mm in diameter. It should be also noted that compared to solid state lasers the issue of optical damage by intensity filaments in the beam is less critical, as Asterix IV/PALS employs gas — hence “indestructible” — active medium.