Mirror for changing the geometrical form of a light beam

The invention is directed to a mirror having mutually adjacent segments defined by a body such as a cone, sphere, toroid, paraboloid or ellipsoid having a line focus. The segments are displayed in stepwise manner axially or parallelly laterally to suppress interference or for forming an intensity profile.

FIELD OF THE INVENTION 
The invention relates to a mirror for changing the geometrical form of a 
light beam. The invention also relates to a method for using the mirror. 
BACKGROUND OF THE INVENTION 
Mirrors for changing the geometrical form of a beam are widely known. It is 
difficult to generate a beam cross section which has a configuration which 
is essentially linear to rectangular and within which the intensity of the 
beam has a predetermined course, especially is constant in the 
longitudinal direction. Such beam cross sections are necessary especially 
for conducting surface treatment with a laser to name only one essential 
area of application. 
My copending application Ser No. 07/788,997, filed Nov. 7, 1991, which is a 
continuation-in-part application of my patent application Ser. No. 
07/505,177, filed on Apr. 5, 1990 and now abandoned, discloses a mirror of 
the kind referred to above and is incorporated herein by reference. 
The transforming optics assure that the laser beam has a cross section 
which is rectangular to linear when impinging on the work surface and that 
the intensity within this cross section has almost a constant value. If a 
beam with this kind of cross section is guided over the workpiece at 
constant speed and parallel to a pair of edges of the rectangular cross 
section, then the energy profile within the irradiated strip is uniform. 
The lasers used most in practice supply a beam having a cross section which 
is not rectangular and whose intensity distribution is not uniform. For 
this reason, optical arrangements are needed which can suitably transform 
any beam cross section. 
Facet mirrors and integrators are two types of transforming optics which 
are preferably utilized for generating linear geometries with respect to 
the intensity distribution of laser rays. 
Japanese patent publication 63-77178 A discloses a facet mirror having a 
plurality of planar mirrors which are arranged so as to lie tangentially 
to a paraboloidal surface in a mosaic-like manner and which concentrate a 
laser beam on a rectangle having essentially the extent of an individual 
planar mirror. However, a line profile is not obtainable in this way since 
the individual mirrors have no focussing effect. The manufacture of such 
an arrangement is complex. 
Facet mirrors are also disclosed in an article entitled "A Convex Beam 
Integrator" by Stanley L. Ream, published in "Laser Focus", November 1979, 
pages 68 to 71. Multi-facetted mirrors are also disclosed in U.S. Pat. No. 
4,518,232 and in Japanese patent publication 59-151 101 A with the latter 
having spherical facets. 
SUMMARY OF THE INVENTION 
It is an object of the invention to impart a line-shaped to 
rectangular-shaped beam cross section having a predetermined intensity 
distribution to a light beam having any desired cross section with only 
one optical imaging with the effects of interferences occurring at longer 
wavelengths being held as low as possible. The precise production is 
facilitated by the suitability for inherently precise manufacturing 
methods. 
According to a feature of the invention, the mirror is configured out of a 
plurality of segments of rotation bodies mounted one next to the other 
with the segments having respective rotational axes which are at least 
approximately superposed on each other on a line-shaped to 
rectangular-shaped region of an object. The size of the segments is so 
selected that the beam to be converted falls simultaneously on several of 
the segments and that all segments are so formed and aligned that a zone 
of the light beam is precisely reflected once by a segment and is directed 
to a section of the line-shaped to rectangularly-shaped region of the 
object wherein the rotational axes approximately are superposed and all 
zones of the light beam are at least approximately superposed on this 
section with the individual segments being displaced relative to each 
other in a stepwise manner. 
An intense concentration is possible with few segments because of the 
anamorphotic action of the rotationally-symmetrical mirror segments. The 
mirror segments are rotation surfaces which can be inherently manufactured 
with greater precision than planar surfaces. 
The special advantage of this mirror is that the light beam can be broken 
up into several strip-shaped regions which can be superposed on one 
another and directed to a line with only a single reflection of each light 
beam region taking place. This optical conversion of the beam geometry can 
provide an excellent homogeneity of intensity distribution which is 
influenceable by the number of mirror segments with minimum energy losses 
for the optical imaging. 
Except for the displacement of the segments and the symmetry axes, mirrors 
corresponding to the invention in their configuration are shown in the 
above-mentioned United States patent application Ser. No. 07/788,977. With 
the introduction of the step-shaped displacement of the segments, the 
coherence conditions for the amounts of light from the individual segments 
are changed to the extent that interference effects are suppressed. 
According to another feature of the invention, the segments are axially 
stepped relative to each other and this enables the rays reflected from 
mutually adjacent segments to the linear focus to include a larger angle 
with each other. In this way, axial interference structures arising in the 
line focus are structured to be finer and in this way are less disturbing 
for the mentioned applications. 
According to still another feature of the invention, the segments with 
their rotational axes are displaced parallelly and laterally in a stepwise 
manner relative to each other. In this way, the interferences can be 
virtually completely avoided. With a larger increased lateral 
displacement, the superposed region becomes rectangular and, 
perpendicularly to the rotational axes (in the case of an application for 
laser machining in the supply direction), a specific intensity profile can 
be generated by providing a displacement of the segments to a specific 
zone of the light beam having a different overall intensity. 
The characteristics described in the above-mentioned U.S. patent 
application Ser. No. 07/788,977 apply also to the mirror according to the 
invention and this application is also for this reason incorporated herein 
by reference.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
FIG. 1 shows a mirror 21 which images an incident light beam 27 on a 
line-shaped region 28 of an object 29. The line-shaped region 28 lies on 
the common rotational axis 20 of the conical mirror segments (22, 23, 24). 
The segments (22, 23, 24) are selected with respect to number, size, axial 
spacing and aperture angle of the cone so that each segment (22, 23, 24) 
images a zone of the incident light beam with a single reflection on the 
line-shaped region 28 as a linear focus and that the total incident light 
beam 27 is detected. 
The foregoing is disclosed in United States patent application Ser. No. 
07/788,977 referred to above. However, with respect to the embodiment of 
FIG. 1, a stepwise axial displacement of the individual segments (22, 23, 
24) with respect to each other is introduced by the insertion of shaded 
intermediate pieces (23a, 24a) of any desired contour. With this feature, 
the angles between the bundle of rays impinging on the line-shaped region 
28 from the individual segments (22, 23, 24) are increased and in this 
way, the interferences between these respective sets of rays are 
structured so as to be tighter and therefore less disturbing. 
The intermediate pieces (23a, 24a) arranged between the mirror segments 
(22, 23, 24) are so configured that they are shaded by the edges (23a', 
24a') of the particular forward segment (23, 24) from the incident light 
beam 27. Also shaded by these edges is a small region 25 of each next 
adjacent segment (22, 23). 
The mirror can be assembled from the individual segments (22, 23, 24) and 
intermediate rings (23a, 24a). 
The mirror can also be machined without difficulty from one piece such as 
by diamond turning. The necessary turning tools for the production (such 
as individual diamonds) of a mirror 21 of the invention always have 
rounded cutting edges. For this reason, pointed recesses (22', 23') as 
shown in FIG. 1 at the connection locations between the segments (22, 23, 
24) and the intermediate pieces (23a, 24a) can be only incompletely 
realized. The projecting pointed edges (23a', 24a') can be manufactured 
even with rounded tools. However, the pointed recesses (22', 23') lie in 
the shaded region of the forward segments and therefore do not participate 
in the imaging process. For this reason, their precise production is not 
important. 
In FIG. 2, the reflective segments (32, 33, 34) are curved so as to be 
convex while in FIG. 3, the reflective segments (42, 43, 44) are curved so 
as to be concave. Otherwise, the mirrors (31, 41) are configured in the 
same manner as mirror 21 of FIG. 1 and the same reference numerals are 
used. 
For the mirrors (31, 41) having curved mirrored segments (32, 33, 34; 42, 
43, 44) as shown in FIGS. 2 and 3, the advantage is provided that the 
number (and therefore the length) of the segments (32, 33, 34; 42, 43, 44) 
on the one hand and the length of the line-shaped region 28 on the other 
hand can be selected independently of each other. 
In correspondence to the desired applications and the specified 
requirements as to the intensity distribution, the segments (32, 33, 34; 
42, 43, 44) in the embodiments of FIGS. 2 and 3 are of circular, parabolar 
or elliptical cross section in the direction of the rotational axis so 
that toroidal, spherical, paraboloidal or ellipsoidal surfaces are 
provided as reflective segments (32, 33, 34; 42, 43, 44). Deviating forms 
can be advantageous for special intensity distributions in the direction 
of the rotational axis. 
For appropriate computation of the mirror (21, 31, 41), the angles of 
incidence of the laser beam 27 as well as the geometrical characteristics 
thereof can be varied within a wide range. 
In FIG. 4, the application of a mirror 51 for irradiating the surface of a 
workpiece 59 in a linear work region 58 on the rotational axis 50 of the 
mirror 51 is shown. The light beam 57 is then imaged neither parallelly 
nor paraxially to the rotational axis 50; instead, the light beam is 
imaged divergently with a source point 60 on the rotational axis 50. 
Accordingly, the light beam 57 is imaged by the five segments (52 to 56) 
of the mirror 51 on the line-shaped work region 58 with a jump of the 
image from the end of the work region 58 to its beginning being provided 
at the rearward edges (52a' to 56a') of the segments (52 to 56). The work 
region 58 is in this case an interval on the rotational axis 50. 
Inclinations and lengths of the conical segments (52 to 56) are so 
selected that the corresponding focal lines on the rotational axis 50 are 
all the same length and have the same position. The suitable measurements 
can be calculated easily with known numerical computing processes. 
The mirror 51 is built up in a step-like manner as the mirrors (21, 31, 41) 
shown in FIGS. 1 to 3. The mirror 51 includes intermediate pieces (52a to 
55a) disposed between the different mirror segments (52 to 56) and shaded 
by the respective forward mirror segments (52 to 55). In this mirror 51 
also, a portion of each of the rearward segments (53 to 56) is shaded by a 
corresponding one of the forward segments (52 to 55). A shaded region 61 
of segment 56 is identified in the drawing as exemplary. Although the 
light beam 57 emanating from the source point 60 does not impinge axially 
parallel on the mirror 51, the advantages discussed with respect to FIGS. 
1 to 3 are provided here. 
FIGS. 5 and 6 illustrate a variation of the mirror 21 in that the segments 
(52, 53, 54) with their rotational axes (522, 523, 524) are displaced in a 
stepwise manner laterally relative to each other. FIG. 5 shows a 
perspective view while FIG. 6 shows a front elevation view together with a 
resulting intensity profile 500. 
Segments (52, 53, 54) of rotational bodies such as in the above figures are 
displaced with their rotational axes (522, 523, 524) parallelly in the 
lateral direction with respect to a center axis 520. The line foci (582, 
583) of the individual segments (52, 53) lie on a corresponding one of the 
rotational axes (522, 523) in a rectangularly-shaped region 528 on the 
object 29. A displacement of 0.1 to 0.3 mm is typical. A displacement of 
approximately the width of the individual line foci of the segments (52, 
53, 54) is adequate for suppressing disturbing interferences. With a 
larger displacement, a rectangularly-shaped region can be illuminated with 
a targeted intensity profile. This takes place in that different 
intensities are imparted to mutually adjacent individual lines. The 
variation of the intensities can take place over the segment width or 
simply by utilizing the fact that the segments (52, 53, 54) can be 
illuminated differently according to the profile of the light beam 27. 
Certain freedoms are available because mutually adjacent lines (582, 583) 
must not be generated from neighboring segments (52, 53, 54). The 
displacement of each segment (52, 53, 54) can be selected independently as 
shown in FIG. 6. 
The limits are obtained in that the number of the segments (52, 53, 54) and 
therefore the number of mutually adjacent lines (582, 583) must be held 
small so that the intensity drop at the ends of the lines (582, 583) is 
not flattened too intensely by diffraction phenomena. 
In FIG. 6, the segments (52, 53, 54) are drawn having the same sector 
angles for emphasis. The size of the sectors (52, 53, 54) is to be so 
selected that the light beam 27 can be fully collected thereby as in the 
previous embodiments and so that no gap occurs. 
The manufacture of a mirror 21 having a lateral displacement of the 
rotational axes is realized by displacing the body of the mirror in the 
holder of a lathe after each individual segment (52, 53, 54) is machined. 
Other forms of the displacement are for example the vertical displacement 
which effects a defocussing of the contributions of the individual 
segments and the displacement by tilting the rotational axes. 
It is understood that the foregoing description is that of the preferred 
embodiments of the invention and that various changes and modifications 
may be made thereto without departing from the spirit and scope of the 
invention as defined in the appended claims.