Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2000-10-26
2002-05-07
Ben, Loha (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C623S001210
Reexamination Certificate
active
06384981
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns an optical emitter array with collimation optics in which a number of extended emitters are arranged side-by-side in the x-direction, with a specified divergence &agr;
x
in this direction and a center-to-center separation (P
x
) greater than the emitter size E
x
and in which the collimation optics include a cylindrical lens array with a number of convergent cylindrical-lens surfaces each assigned to an emitter and having its cylindrical axis lying in the y-direction, arranged in front of the emitter array.
PRIOR ART
Such emitter arrays, with a number of optical sources that are linearly extended in the x-direction of width (E
x
) running along the x-axis in the shape of a linear matrix with center-to-center distance (P
x
) appear, for example, in high-power diode laser bars. The combined width of these laser bars lies in the range of a few millimeters. For concrete applications, it is generally necessary to subject these discontinuous beam profiles formed from separate linear sections to a beam shaping such as a homogenization or a geometric cross-section transformation. The first step in this beam preparation is generally collimation, the goal of which is to achieve the greatest possible degree of divergence reduction over the entire active beam cross-section.
The collimation must take into account the strongly anisotropic divergence distribution of semiconductor lasers. Specifically, the divergence angle &agr;
y
in the y-direction (“fast-axis”) is relatively large while the divergence angle &agr;
x
in the x-direction (“slow-axis”) is comparatively small. This situation is handled by using cylindrical lenses to produce parallel rays in the different coordinates. For a laser bar with a number of emission centers, the fast-axis collimation is carried out by a single cylindrical lens lying in the x-direction and, because of the large divergence, positioned as closely as possible in front of the emitter array. For collimation along the slow-axis, a cylindrical-lens array is employed in which a cylindrical lens, whose width corresponds to the center-to-center distance (P
x
) of the emitter, is placed in front of each respective emitter in the beam direction.
A good collimation in the x-direction is fundamentally possible when the focal length (F) of the cylindrical-lens array is as large as possible relative to the emitter width (E
x
). This, however, leads to a large overall length, which is particularly disadvantageous for micro-optic components and is therefore undesirable. This procedure has the additional disadvantage that ray bundles emerging from the emitters overlap one another in the x-direction at a distance (a) in front of the emitter surfaces and, as a result, a separate divergence reduction for individual emitters is no longer possible.
For a separate collimation of the individual emitters, a cylinder-lens array can of course be positioned within the overlap distance, a. From fundamental considerations, a divergence reduction under the assumption of smaller divergence in the slow-axis is only possible when the ratio of emitter size to center-to-center distance, E
x
/P
x
<0.5, holds. For high-power diode laser bars, however, generally E
x
/P
x
≧0.5 so that a separate collimation with a single cylindrical-lens array is fundamentally impossible.
An additional problem is that the beam cross-section still has a discontinuous energy distribution and so the high brightness of the emitter array is of only limited utility due to the great inhomogeneity.
SUMMARY OF THE INVENTION
Based on the problems illustrated above, the task of the invention is to provide an optical emitter array, in particular some laser bars, with collimation optics, which delivers a better beam quality, especially with regard to parallelism and homogeneity, while having a smaller overall length.
For solving this task, the invention proposes that a first cylindrical-lens array with focal length (F
1
) be positioned within the overlap distance (a) in front of the emitters at which the individual ray bundles emerging from the emitters overlap, in which the focal length (F
1
) is smaller then the overlap distance (a) and that a second cylindrical-lens array with focal length (F
2
) be positioned in a telescopic arrangement at a distance (b=F
1
+F
2
) in front of the first spherical-lens array.
According to the invention, the first and second cylindrical-lens arrays form an optical telescope whose eyepiece side is directed toward the emitter. Since the first cylindrical-lens array located there is maximally at the overlap distance (a) the overall result is a very short total length.
Since the front end of the collimation optics is located within the overlap distance (a), a separate collimation of each individual emitter is possible. The particular advantage of the invention consists in that an effective divergence reduction results nonetheless, even when E
x
/P
x
≧0.5.
This advantage of the collimator-telescope arrangement according to the invention has to do with the fact that the first cylindrical-lens array produces a virtual magnification in an image plane of the emitter with width (E
x
) to the width of the center-to-center distance (P
x
). This becomes the effective source size for the second cylindrical-lens array. Because of the constant beam-parameter product according to the Lagrange invariant (aperture x sine (divergence)=constant), the result is a divergence reduction by a factor of the magnification, M, of the telescopic arrangement. The approximation is valid for small angles, which is, however, no problem for the small divergence along the slow-axis.
A further essential advantage of the collimation optics according to the invention consists in that, by magnifying the sources to the center-to-center distance (P
x
), a continuously linear beam cross-section is produced in the x-direction. For a given emitter array, a high brightness is thereby produced. The homogeneous intensity distribution is particularly advantageous as an input beam for subsequent beam transformation devices.
For this embodiment of the invention, the overlap distance (a) observes the relation:
a
≤
(
P
x
-
E
x
)
2
tan
(
α
x
)
.
In this formula, &agr;
x
is the divergence in the x-direction. For the focal length (F
1
) of the first cylindrical-tens array, the following value is set:
F
1
=
a
E
r
p
r
+
1
.
Based on this value, the focal length (F
2
) of the second cylindrical-lens array is calculated as:
F
2
=
F
1
P
r
E
r
.
With the formulas described above, a cylindrical-lens collimator with two successive cylindrical-lens arrays that together form an imaging telescope is described for the first time.
The collimation efficiencies of the embodiment of the invention with an imaging-telescope arrangement mentioned above, can alternatively be realized by a telescopic arrangement with Fourier transform and field lens in which a first cylindrical-lens array (A) is positioned in front of the emitter (E) at a distance (z
1
) that is smaller then the overlap distance (a) at which the individual beams emerging from the emitter (E) overlap and a second cylindrical-lens array (B) is positioned at a distance (T) in front of the cylindrical-lens array (A) whereby the arrays (A and B respectively) together form a biconvex lens with resultant focal length (F
r
=F
A
) and principal-plane distance (T=F
A
).
For the collimation, the same requirement holds with respect to the distance to the first cylindrical-lens array (A) as before with the imaging-telescope arrangement, namely, that ray bundles emerging from neighboring emitters (E) must not overlap.
In contrast to the first variant of the invention, however, no image results from the first cylindrical-lens array (A) but rather a Fourier transformation. The second cylindrical-lens array (B) has, in this arrangement, the function of a field lens.
This embodiment starts with the assumption that the maximal focal length (F
A
) can only be theoretically reac
Ben Loha
Hentze Joachim
Hoffman Wasson & Gitler
Seyrafi Saeed
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