Coherent light generators – Particular beam control device
Reexamination Certificate
2002-07-22
2004-12-21
Harvey, Minsun Oh (Department: 2828)
Coherent light generators
Particular beam control device
C372S021000, C372S107000
Reexamination Certificate
active
06834062
ABSTRACT:
BACKGROUND OF THE INVENTION
PCT/EP 00/08703 describes a method and apparatus for controlling the level of laser beam energy of a laser beam scanning across a target using at least one laser scanhead, said laser scanhead comprising motor driven deflection means for scanning the laser beam across the target and polarization control means, wherein the polarization control means controls the level of laser beam energy of the laser beam scanning across the target in accordance with the movement of the deflection. In particular, the polarization control means includes one or two Brewster plates or Brewster windows which are rotatable around an axis which is parallel to the direction of the laser beam. The one or two Brewster plates can be rotated at an angle between 0° and 90° to control laser beam transmission between 0% and 100%. Because a laser beam in general includes only one type of polarization, a Brewster plate, set at the correct Brewster angle, can be used to transmit between 0% and 100% of said polarization. The rotation of the Brewster plate is synchronized to the rotation of the deflection means, deflecting the laser beam for targeting a scanned surface.
Reference is made to PCT/EP 00/08703 in its entirety.
FIG. 1
shows a diagram of the laser beam targeting apparatus according to PCT/EP 00/08703. The laser beam targeting apparatus is integrated into a scanning head
32
of a laser scanner into which a linearly polarized laser beam is passed through an aperture
34
. The scanning head
32
comprises a polarization control device
36
which can be either integral or mounted in the beam axis line preceding the scanning head, a beam expansion optic
38
, a Y axis mirror
40
, a X axis mirror
42
, two galvanometric motors
44
and
46
for rotating the two mirrors
40
and
42
, respectively, and an f-theta focusing lens
48
. The polarization control device
36
of the embodiment of
FIG. 1
comprises a first Brewster plate
50
and a second Brewster plate
52
. In an alternative embodiment, this technique could also be used with either pre- or post-objective scanning.
In a further alternative embodiment, a polarization control device comprising a single Brewster plate could be used to achieve basically the same effect, as described above.
In the shown embodiment a carbon dioxide (CO
2
) laser is used for creating a laser beam linearly polarized in a single direction. However, the expert will understand that any other suitable type of laser source may be used. The laser beam enters the scanning head
32
through the aperture
34
and is passed through the polarization control device
36
in which two opposing ZnSe-Brewster plates
50
,
52
are set at the relevant Brewster angles with regard to the laser beam wavelength. The Brewster plates
50
,
52
can be rotated through 90° to attenuate the laser beam thereby, allowing 0% to 100% of the laser beam energy to be transmitted through said Brewster plates
50
,
52
when they are rotated around the laser beam axis from 0° to 90°. For other types of lasers other material might be required for the Brewster plates. Alternative coatings may be used on the face of the Brewster plates which may vary the maximum and minimum levels of transmitivity, the exit polarization, and the required rotation of the Brewster plates. In practice, the maximum transmitivity of a Brewster window of the type described above is “only” 99.98%. However, for the purpose of the present description a transmitivity of 100% may be assumed. Therefore, if in the present text a transmitivity of 100% is indicated, it is referred to the maximum transmitivity of the respective Brewster window, which in the embodiment considered is 99.98%.
The part of the laser beam energy transmitted through the Brewster plates
52
,
50
in this embodiment passes through a beam expansion optic
38
which expands the laser beam diameter and is then deflected off the surface of the Y axis galvanometric motor driven mirror
40
to be then deflected off the surface of the X axis galvanometric motor driven mirror
42
and through the f-theta focusing lens
48
which acts to focus the laser beam to a fine point on a target plane
54
. The intensity of the laser beam energy scanned across the target plane
54
is held under strict control by controlling the rotation of the Brewster plates
52
,
50
as a function of the rotation, position, angular speed of the galvanometric motor driven mirrors
40
,
42
.
It is important that the travelling time to and from maximum required velocity of the combined XY beam position at the target is matched to the transmission curve of the opening and/or closing of the Brewster rotation. In practice, every travelling time of the beam crossing the target plane in the X or Y direction, and importantly the combined XY directions should be defined. It is assumed that this defined travelling time will be determined by the capability in speed of the polarization control device
36
to open and close the Brewster plates. Therefore, if as an example it takes 1 ms for the Brewster plates to open, within an acceptable tolerance, from 0% to 100% and, equally 1 ms to close, then the scanning head
32
and in particular the combined scanning mirrors
40
,
42
should reach the maximum speed in 1 ms. Because coated or enhanced Brewster plates have a transmitivity of 0% when set to the appropriate angle it is not necessary to turn off the laser beam between independent processing or marking steps of the target material.
The PCT application describes the actions of the Brewster windows set at a specific Brewster angle to control laser power by reflecting or transmitting a single directionally polarised carbon dioxide generated laser beam energy. The same method can be applied to any single directionally polarised energy using the correct material for the Brewster windows and the correct Brewster angle specific to the wavelength of said energy.
FIG. 2
details how a laser beam polarised in either the P-pol (parallel) or S-pol (senkrecht or perpendicular) directions can be reflected or transmitted using a single Brewster window
200
.
For illustration purpose both P-polarization and S-polarization are shown in the drawings. However, an expert will understand that in practice a CO
2
laser beam can comprise basically only one type of polarization. With reference to
FIG. 2
if said input polarization is P-pol then the energy will be reflected whilst if said input polarization is S-pol then the energy will be transmitted.
Disadvantageously, said transmitted beam energy will be displaced by a factor calculated by the Brewster angle giving an angle of incidence and by the thickness of the Brewster window itself.
FIG. 3
depicts the Brewster window
200
rotated through 90° where now the P-pol is transmitted and the S-pol is reflected. The beam energy is displaced exactly the same as in
FIG. 2
except that it has now been rotated 90° about the centerline.
FIG. 4
shows how two Brewster windows
450
,
452
aligned together allow for the output beam energy path to be the same as the input beam energy path by the actions of the second Brewster window
452
compensating for the displacement created by the first Brewster window
450
. In reality and depending upon the coating on the Brewster windows
450
,
452
the P-pol reflected off the first Brewster window
450
will be of a very high percentage leaving only a very small percentage to be reflected off the second Brewster window
452
.
FIG. 5
depicts the two Brewster windows
450
,
452
rotated in unison in the same direction. As the rotation increases the P-pol reflected off the first Brewster window
450
decreases and the P-pol transmission increases. Correspondingly as the rotation increases the S-pol transmission through the first Brewster window
450
decreases and the S-pol reflection increases.
It is important to note that the laser beam energy polarization exiting the first Brewster window
450
is rotated with the rotation of said first Brewster window
450
dependant upon its coating. Therefore any P-pol tha
Dullin Peter
Gill Alistair
Hastings Stephen
Hauck Wolfgang
von Jan Peter
Duane Morris LLP
Harvey Minsun Oh
Nguyen Tuan N.
Raylase AG
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