Line focus aperture for optical beams

An aperture useful for spatially filtering a line focus of an optical beam, such as that produced by a cylindrical mirror or lens, comprises a pair of rotating smoothly polished cylinders.

DESCRIPTION 
TECHNICAL FIELD 
The field of the invention is that of an aperture for high-flux optical 
beams formed into a line focus. 
BACKGROUND ART 
In the art of high power lasers, it is often necessary to focus a high 
power beam through an aperture having a certain size, thereby blocking 
undesired optical flux passing along the beam direction at a distance 
greater than the aperture size. In the prior art, rectangular or 
triangular cross section apertures have been used, but these suffer from 
high heat loads caused by the high flux in the optical beams being 
limited. The high temperatures associated with large amounts of power in 
the optical beam can cause severe cooling problems for such devices. 
DISCLOSURE OF INVENTION 
The invention relates to an aperture for spatially limiting a high powered 
beam at a line focus, in which the side members of the aperture are a pair 
of highly polished rotating cylinders.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 2 illustrates an optical aperture constructed according to the 
invention, in which beam 220 is focused to a line focus perpendicular to 
the plane of the paper by a cylindrical lens not shown converging to an 
aperture distance 215 and then diverging thereafter. Distance 215 is the 
distance between outer surfaces of cylinders 210 and 212 rotating about 
axes parallel to the axis of the line focus and also perpendicular to the 
plane of the paper. Radiation in beam 220 which is focused at the focal 
region a distance greater from the axis of the beam than is permitted by 
distance 215 will strike the outer surface of cylinders 210 and 212 at a 
grazing angle and be deflected thereafter outside the diverging beam, one 
such deflection being shown as direction 230 in the drawing. 
The relative size of the cylinders forming the aperture and the angle 
formed by the diverging and converging beam is not critical. A pair of 
cylinders having a large radius will tend to approximate a flat surface 
for the sides of the aperture and a pair of small cylinders will tend to 
approximate the pointed edge shown in FIG. 1. In general, the larger the 
radius the more shallow the angle of scattering, and thus the lower the 
heat load on the surface, but higher heat loads can be compensated for by 
increasing the rate of rotation of the cylinders or by passing a coolant 
fluid through the cylinders. 
The cylinders may be rotated in either direction relative to the path of 
the beam, thus spreading out the heat load caused by scattering the beam 
over the entire cylinder surface instead of concentrating it in point 111 
of the aperture as is characteristic of the prior art. In contrast, the 
prior art aperture shown in FIG. 1 is characterized by a point 111 at the 
boundary of the members 110 and 112 forming the aperture. The high heat 
flux being deposited on those members by the beam tends to concentrate at 
point 111, giving rise to a cooling problem that may be quite severe in 
high power lasers. An aperture having cylindrical members may be further 
improved by passing coolant fluids through the aperture, which may be 
confined to channels near the surface of the cylindrical member or may 
pass through the body of a hollow cylinder, as is convenient. 
The size of the cylinders, the rate of rotation, and the amount of coolant 
fluid flowing through the cylinders will all depend upon the power to be 
dissipated and upon the degree to which the beam must be clipped and 
confined. Those skilled in the art will have no difficulty in tailoring an 
aperture to suit their requirements. 
With all apertures, a portion of the optical flux that is to be removed 
from the beam will be scattered, the direction of scattering depending on 
the relative direction of the incoming flux and the surface of the 
clipping member. The embodiment of FIG. 2, with a high fraction of the 
deflected beam being deflected in a grazing angle, will tend to have the 
undesired radiation closer to the direction of the diverging beam than 
will the prior art device shown in FIG. 1. The use of relatively small 
cylinders will tend to increase the angle at which the scattered radiation 
emerges. 
The cylinders may be polished to a high degree of finish by diamond turning 
and optical polishing techniques, thereby increasing the fraction of 
unwanted radiation which is deflected rather than being absorbed by 
cylinders 210 and 212. 
The distance 215 between the cylinders 210 and 212 may be adjusted by 
incorporating conventional flexible driving means and coolant means so 
that the distance 215 may be readily adjusted. FIG. 2 illustrates 
schematically, i.e. through symbols and without conveying any spatial 
information, a method of rotating the cylinders with belts 252 and 254 
turning shafts 253 and 255, respectively. Power is supplied by 
conventional source 250, represented symbolically be a circle. The spacing 
215 may be adjusted by translating cylinder 210 parallel to its axis by 
any conventional means 240. Coolant fluid may enter one end of the shafts 
or centers of cylinder 210 and 212, travel perpendicular to the paper and 
exit the other end. Conventional means 260 is shown as being connected by 
flexible hoses 261 and 262. Rotating seals to contain the fluid on a shaft 
are well known in the art. All of the foregoing rotating, displacing and 
pumping means are well known in the art and no novelty is claimed for them 
.