Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
2001-03-16
2002-05-28
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S490020, C359S490020, C359S506000, C359S580000, C359S900000
Reexamination Certificate
active
06396630
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to optical components, which influence the polarization state of incident polarized light. More in particular transmissive phase retarding elements with reduced dimensions and enhanced functionality for large laser beam applications are the subject of this invention.
BACKGROUND OF THE INVENTION
Phase retarders are optical components, which shift the phase between the two polarization components of an incident polarized laser beam. The introduction of a 90 degree phase shift is generally described as a linear-to-circular transformation. A phase shift of 180 degrees is typically described as changing the rotation of linearly polarized light. Phase retarding elements can be subdivided in reflecting and transmissive types. As the names imply, with reflecting phase retarders, the incident light is reflected back from the retarder element with a shifted phase, whereas with transmissive types, the phase shift is introduced as the incident light passes through the element.
For increased efficiency, phase retarders are required to have a high optical throughput. For a reflecting type retarder, the reflectivity of the surface should be as high as possible. For a transmissive phase retarder, the transmission of light through the element should be as high as possible.
For high power laser applications, the phase retarder needs to be able to withstand the high optical power of the incident laser beam. Additionally, high power laser applications typically involve a relatively large diameter laser beam. A typical beam in CO
2
laser optics can be from several millimeters up to several centimeters in diameter. Prior art phase retarders have needed to be commensurately large, resulting in bulky and expensive solutions in the past.
Prior Art 90 Degrees Phase Retarders
FIG. 1
illustrates a prior art phase retarder. The illustrated optical arrangement provides for a quarter wave plate (90 degrees phase shift) using a copper mirror
5
that operates in reflection mode at mid- or far-infrared (IR) wavelengths. Mid- and far-IR wavelengths typically range from 2-20 &mgr;m. Mirror
5
is comprised of metal substrate
1
, upon which is coated multiple thin film layer
2
. Metal substrate
1
is used because metals generally provide a high reflection coefficient. The multilayer coating
2
provides the phase shifting function. In the illustrated example, coating
2
is designed such that a laser beam
3
incident on the element surface at an angle of 45 degrees is converted into a reflected beam
4
at the same angle. The shift between the two polarized components of incident beam
3
is shifted by 90 degrees when the incident light is linearly polarized under an angle of 45 degrees with respect to the input plane of mirror
5
. Note that the operation of the illustrated prior art phase retarder requires a redirection of the propagation path of the incident light beam
3
. This is disadvantageous because of the need for critical alignment of various optical elements using such devices. Any adjustment of one optical element, such as the illustrated phase retarder, would result in a misalignment of all the other optical components of a system. If yet another polarization state of the infrared beam is desired (requiring yet another phase retarder), the complete system would need to be redesigned to allow for another optical clement introducing yet another redirection of the propagation path. Hence the prior art reflection type phase retarders have the disadvantage of requiring critical alignment between elements and make optical system design and redesign difficult.
When more flexibility is required in an optical arrangement, it is preferable that the phase retarder operates in transmission mode, to avoid the above discussed disadvantages. One prior art approach to transmission mode phase retarders uses birefringent materials. These types of retarders exploit the dependency of the refractive index of orientation of the polarization components of the incoming light beam. A phase shift is introduced between the polarization state aligned with the fast axis of the birefringent material with the smallest refractive index and the polarization state aligned with the slow axis, where the index of refraction is the highest. Because the two orthogonally polarized incident waves of the optical beam travel through the birefringent material at different speeds, there will be a phase shift between the two waves when they emerge from the material. By choosing an appropriate thickness for the birefringent material, the required phase shift can be implemented. For the mid- and far-IR wavelength applications, however, birefringent materials are expensive and are not heat resistant for high optical powers due to residual absorption. This limits their applicability in high power applications. Additionally, because the dimensions of the phase retarder scale with the beam size of the impinging optical beam, the result is very expensive and large devices are required to typical application beam sizes.
Prior Art 180 Degrees Phase Retarders
One common high power laser application is in the field of laser machining and laser cutting. In such applications, a high power, focused laser beam is used to cut or scribe a work piece (typically a metal work piece, although laser cutting is also employed with plastics, paper, and other materials). It is well known in laser cutting applications that the cutting profile or width of the cut produced by powerful laser radiation (e.g. CO
2
) optical radiation at &lgr;=10.6 &mgr;m) depends on the polarization orientation of the beam with respect to the cutting direction. This phenomenon is illustrated in
FIG. 2
a
. As shown, the cutting width
12
is widest when the cutting direction is aligned with the polarization orientation of the cutting beam, as represented by orientation indicator
15
. By contrast, the cutting width
11
is narrowest when the cutting direction is parallel to the polarization of the beam, as indicated by
14
, and the cutting width
13
is intermediate when the polarization orientation is at some acute or obtuse angle to the cutting direction, as indicated by
13
.
It is known in the prior art that uniform cutting results can be obtained when the direction of polarization of linear polarized light kept parallel to the cutting direction. This is illustrated in
FIG. 2
b
where tie cutting width
16
,
17
,
18
is uniform because the polarization orientation of the incident light
19
,
20
,
21
is maintained parallel to the cutting direction. Such a system requires that the polarization direction of the cutting beam is dynamically aligned, i.e. rotated, during the cutting process. This requires that the phase retarder can be rotated. One prior art solution to the need to rotate the phase retarder is the use of mirrors to rotate the entire optical system around its optical axis. Such a system is disadvantageous because of the large size required for the optical set up. Furthermore, the need for critical alignment of the mirrors requires that the optical system be mechanically isolated from vibration during operation. These disadvantages add to the cost of the cutting tool.
A preferable approach to aligning the polarization orientation of a high power laser beam is through phase shifting of the polarization components. One such prior art approach is illustrated in
FIG. 3. A
transmissive type phase retarder is illustrated comprised of a multi-layer
35
coated phase shifting plate
30
. Use of the illustrated half-wavelength plate or a combination of two quarter-wavelength plates allows rotating the plane of polarization of a light beam very effectively, as taught by Born and Wolf in Principles of Optics. London, England, 1975 at pp. 52-59. The theory and fabrication methods of plates with multi-layer coatings
35
are well known. See, e.g. W. H. Southwell, “Multilayer Coating Design Achieving a Broad Band 90° Phase Shift,” Appl.Opt., 8-1980, pp. 2688-2692; T. N. Krylova, “The Reflection of Light from
Kotov Vladimir
Stiens Johan
Jr. John Juba
Rose Research, LLC
Slater & Matsil L.L.P.
Spyrou Cassandra
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