Optical: systems and elements – Diffraction – Using fourier transform spatial filtering
Reexamination Certificate
2000-09-08
2002-07-16
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
Using fourier transform spatial filtering
C359S619000, C359S624000, C359S641000, C359S669000, C359S900000, C362S259000
Reexamination Certificate
active
06421178
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a device for optical beam transformation of one or more ray bundles, of which some have an oblong cross-section in the x-y-plane, using optical elements made up of optically active interfaces arranged along the beam path. In addition, an element of the invention is a method that concerns the optical transformation of the cross-section of one or more ray bundles of which some have an oblong cross-section in the x-y-plane and show a well-defined convergence distribution.
1. Prior Art
A basic problem in the coupling of high-energy laser beams with fiber optics consists in that the geometric shapes of the beam cross-sections generally differ considerably from the shape of the active fiber cross-section. As an example, semiconductor laser bars, which are found in ever-increasing distribution, have a band-shaped, oblong light emission area, that is, a quasi-linear aperture in which the individual narrowly limited emission centers lay side-by-side in, for example, the x-direction. The divergence of the elementary ray bundles starting out from the emission centers is constant but is considerably smaller in the lengthwise direction of the beam profile, that is, in the x-direction in the coordinate system introduced at the beginning, than in the y-axis lying orthogonal to it. For this reason, of lengthwise axis is from here on referred to as the “slow axis” while the y-axis is referred to as the “fast axis”.
Under the conditions described above, relatively good collimation in the fast axis is quite possible, but the linear profile remains linearly stretched out as before. Limited by this characteristic asymmetry and also in view of the geometric extension of the beam cross-section as well as in view of the divergence distribution, the ray bundle is not amenable for being focused on a symmetric spot suitable for coupling with a circular-symmetric fiber cross-section. Specifically, the numerical aperture of the fiber would then be exceeded in each case. Apart from the fact that the beam cross-section would be even greater at the focus than the active cross-section of the fiber core due to the high divergence, the total reflection angle between core and fiber would be exceeded so that light crosses over into the cladding resulting in considerable loss.
In addition, due to the abovementioned difference between the symmetries of the beam cross-section and the fiber core, relatively large unilluminated zones would appear in the fiber cross-section. This also applies incidentally to the use of laser bar piles, known as “stacks”, in which a number of linear shaped emitters are arranged one on top of another in a stack. The light given off by these is distributed over a larger, mostly rectangular cross-section. Between each of the individual lines however are emissions-free zones, which amounts to having a nonhomogeneous intensity distribution.
In order to achieve a more efficient use and thereby a higher transfer capacity by way of a homogeneous illumination of the fiber cross-section, various solution approaches are already known in the state of the art. With the known arrays or devices, an incoming ray bundle with a beam cross-section stretched out in the x-y-plane is first of all decomposed in one direction, for example along the x-axis, into discrete partials ray bundles. Next, these partial ray bundles are regrouped in the y-direction by refractive or reflective elements so that a desired outgoing cross-section is achieved. For example, in DE 195 14 625 C2 it is proposed that linear shaped sections of the given linear beam cross-section be regrouped on top of one another in a quadratic or rectangular area. With the output ray bundle formed in this manner, a better filling of the circular fiber cross-section area is made possible, whereby a higher capacity is transmittable than with simple collimation or focusing. It is obvious however that the limits of this method or the devices used in it are reached rather quickly since further emission-free zones remain between the regrouped linear sections.
The aperture of the optic fiber is consequently not uniformly illuminated. Depending on the type of emission source, under high transmission capacity a more rapid aging of the fiber can result from local intensity overload. If the local energy density exceeds the destructive threshold of the fiber material, destruction of the fiber can occur.
A further disadvantage comes about in that in decomposing the input ray bundles into discrete partials ray bundles, unavoidable diffraction effects occur at the separation points, which leads to losses. This applies equally to the use of reflective elements, as e.g. stepped mirrors, as well as refractive elements, as e.g. stepped prisms or the like. The efficiency would naturally be likewise limited by these losses.
Another approach is described in DE 196 23 762 A1. By means of the resulting device, a linear input beam profile is transformed into a circular-symmetric beam field by mounting three cylindrical lenses perpendicular to each other in series. Obviously, the output beam for this device has a ring-shaped profile. That means that practically the entire circular inner area, in contrast, represents a dead zone, which is not illuminated. Due to the resulting inhomogeneity of the energy distribution over a fiber cross-section, the transmission capacity is limited just as in the previously mentioned device.
2. Brief Description of the Invention
Starting from the problem described above, the current invention has as its basis the task of producing a beam transformation device as well as a method for beam transformation that makes possible a well-defined intensity distribution in the output ray bundle for which small internal losses occur. In particular, the device and the method should be capable of better coupling light from a light source with a strongly asymmetric beam cross-section, for example from a laser or a laser bar stack with one or more optic fibers.
As a solution to this task, the invention proposes a device for optical beam transformation in which an optical element is shaped as a continuous angle transformation element that has an optical interface that, along the x-axis, has a continuously varying inclination in the beam direction relative to the y-axis.
For the angle transformation element according to the invention, one is dealing with a novel reflective or refractive component whose optically active interface has the form of a surface twisted around an x-axis running at right angles to the beam axis. Pictorially, such a surface comes about if one where to take a surface lying in the x-y-plane and twist the outer ends, for example those in the x-direction, in opposite rotational directions so that the x-axis itself forms the:untwisted, neutral fiber. Thereby the possibility also exists to realize continuously increasing or decreasing inclination angles over the span of the component in the x-direction or also to set variable inclination angles section-wise in different directions. While the first-mentioned shape is similar to a “propeller-like” surface, the second shape has a conic-section-like, furrowed surface.
The angle transformation element according to the invention can equally well be designed as a reflective or refractive element. The refractive element forms virtually a prism element with continuously varying base angles at right angles to the beam direction. With a wave-like progression of the inclination angles, it is possible, for example, to compensate for the intensity pulse with laser diode bars. This is a matter of aberrations of the emitter orthogonal to the expansion direction, which is referred to as “smile” distortion.
This distortion comes about from the warping of the semiconductor chip in its fabrication. As a result, the light output surface, and thereby the beam cross-section, maintains a curved course. If the course of the inclination angle of the angle transformation element is determined at each point by the local deviation of the beam profile from a straight one, a c
Lissotschenko Vitalij
Mikhailov Alexei
Hoffman Wasson & Gitler PC
Jr. John Juba
Lissotschenko Vitalij
Spyrou Cassandra
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