Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2001-03-15
2003-02-25
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S619000, C359S636000, C359S638000, C359S833000
Reexamination Certificate
active
06525884
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a beam splitter to divide an electromagnetic beam into at least two partial beams. In particular, the invention refers to such a beam splitter with essentially flat functional elements that divide the beam.
BACKGROUND OF THE INVENTION
There already exist a series of various technologies for the above-cited beam splitter to cause the cited division of the beam. There operation is based on optical properties of a basic dielectric, and especially reflection of the incident beam from optical boundary surfaces is used. Such a beam splitter is shown in
FIG. 1
a.
An incident beam
10
falls on the front outer surface
11
of a dielectric
12
and is partially reflected there
13
. The transmitted beam part
14
then penetrates
15
a rear outer surface
16
of the dielectric
12
and is also partially reflected there
17
. Some of the partially reflected beam
17
then leaves
18
the front outer surface
11
of the dielectric
12
and forms a partial beam bundle
13
,
18
together with partial beam
13
. Note that
FIG. 1
a
is greatly simplified; when partial beam
17
contacts the front outer surface
11
, additional reflection arises what is not represented here.
In these simplified representations, we can see two basic problems with the cited beam splitter. On one hand, so-called ghost images arise due to the displayed double reflection which substantially impair the beam properties of the reflected beam and, in particular, cause beam expansion. In addition, the arising parallel displacement of the penetrating (transmitted) beam produces axial image displacement, which is also frequently undesirable or even unacceptable.
The cited problems have already led to the development of beam splitters in which the thickness of the dielectric (viewed in the direction of the transmitted beam) is extremely thin to minimize the cited effects arising from multiple reflections and parallel displacement of the penetrating beam. Such a membrane beam splitter is schematically illustrated in
FIG. 1
b.
This beam splitter has a membrane
21
in a frame
20
. An incident beam is partially transmitted
23
through the membrane as well as partially reflected
24
. These membrane beam splitters
20
,
21
are designed to be thin enough (approximately two pm nitrocellulose film) so that both reflected beam bundles
24
are practically completely superposed, i.e., there is no beam displacement or beam expansion (as described above). This beam splitter has the disadvantage, however, that is not suitable for the outside UV range; in addition, the interference that usually arises with thin membranes is greatly disturbing when such beam splitters are used.
Furthermore, other beam splitters have been suggested in which the outer surfaces are wedge-shaped to block the reflected partial beams; however, the bend in the optical axis is disadvantageous.
The above-described relevant state-of-the-art is also published in a catalog by Oriel Instruments under the title of
The Book of Photon Tools
in the section “Prism and Beam Splitters, Beam Splitters Technical Discussion”.
FIGS. 1 and 2
are taken from this catalog. The cited disadvantages have also produced a completely different approach which is also described in the catalog. The basic beam splitter principle will now be briefly discussed with reference to the perspective view in
FIG. 2
(with an enlarged section shown in the right half of the figure) of such a beam splitter. With this beam splitter, the beam is divided by means of a point pattern
31
on a highly-reflective material on a dielectric
30
; an aluminum layer in this case. With this beam splitter, part of an incident beam is reflected off the point pattern
31
, and the other part is transmitted through the uncoated areas
32
of the dielectric
30
. By tilting the beam splitter in reference to the optical axis of the incident beam, the incident beam can be divided into partial beams similar to the method shown in FIG.
1
.
All of the above described beam splitter technologies still have substantial disadvantages. On one hand, chromatic, spherical and astigmatic image errors arise, and there is also a relatively large reflection loss and intensity loss as the beam passes through the dielectric. As the beam passes through the dielectric, and initially unpolarized beam becomes partially polarized due to the angle of installation required to divide the beam. The cited disadvantages can at least be reduced by additional measures, however. For example, the intensity of the cited ghost images can be reduced e.g. by using reflection-reducing coatings and/or even sup pressed by blocking out disturbing reflections.
OBJECTS AND ADVANTAGES OF THE INVENTION
It is therefore the problem of the present invention to describe and present a beam splitter of the above-described kind that avoids the cited disadvantages of state-of-the-art beam splitters.
In particular, optical image errors such as the generation of ghost images, parallel displacement between incident and exiting beams as well as axial beam displacement or image haziness are effectively avoided as much as possible.
In addition, the reflection and transmission loss in the cited dielectrics is avoided as much as possible.
Furthermore, the invention prevents in particular the reflected beams from becoming polarized at certain angles of incidence due to reflection, and therefore prevents the exiting partial beams from being polarized in contrast to the incident beam.
In addition, the beam splitter is useful over the entire optical wavelength range and is highly suitable for the ultraviolet range of the spectrum.
Finally, the beam splitter is easier and hence more economical to manufacture than prior-art beam splitters.
SUMMARY OF THE INVENTION
The cited problems are solved with a beam splitter of the initially-cited type in that the functional element has openings or perforations corresponding to a point pattern. The areas of the functional element between the openings are essentially reflective, at least on the side the incident beam, and are essentially designed to form parallel exiting partial beams.
The incident beam is divided by the beam splitter according to the invention as follows: a portion of the incident beam (the portion contacting the membrane at the height of the openings) can freely pass through the beam splitter, and the remaining portion of the incident beam is reflected off the areas of the functional element between the openings. Since the transmitted beam portion is transmitted exclusively in air, an optical active medium such as a dielectric can accordingly be dispensed with; there is therefore no intensity loss, polarization, or imaging errors of the transmitted beam as is the case with state-of-the-art beam splitters as described in the introduction. By suitably selecting a reflective coating on the side of the functional element facing the incident beam, the reflected beam portion can also exit the beam splitter almost without any loss in intensity and without any polarization.
The functional element is preferably designed as a membrane that can be manufactured from a material that is essentially non-transparent to the respective electromagnetic beam or a material that is impermeable to the beam, in particular from a semiconductor material, metal, etc. The membrane with the corresponding openings can be manufactured by micromechanical means in particular, which allows the openings and especially their opening cross-sections to be microscopic so that the beam splitter can be used for a wide range of beam diameters. This advantageously allows the beam splitter to be used in the field of laser optics. For the cross-section of the incident beam to cover a sufficient number of openings to ensure a fixed and predetermined divider ratio, the average distance between two openings can be smaller by at least a factor of 2 to 5 than the diameter of the incident beam.
The functional element is preferably approximately 1-100 &mgr;m thick at the openings. The respective micromechanical manufac
Kraiczek Karsten
Mueller Beno
Agilent Technologie,s Inc.
Epps Georgia
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