Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
1999-08-27
2001-03-13
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
Having polarization
Reexamination Certificate
active
06201609
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention in general relates to displacement measurement interferometry and more particularly to displacement measurement interferometers (DMIS) that utilize polarization preserving optical systems to enhance their accuracy by reducing undesirable polarization effects.
In polarization interferometers for the measurement of linear and angular displacements, one or more orthogonally polarized beams are introduced into the interferometer and then are separated by a polarizing beam splitter for travel along reference and measurement legs. Afterwards, the beams are recombined and information about the difference in the optical path lengths they traveled is analyzed to yield displacement. In the process of traveling through the interferometer, perhaps making multiple passes, the polarized beams typically encounter retroreflectors that return them from either fixed or moveable mirrors. Such retroreflectors, which are non- polarization preserving, introduce errors in the accuracy of the measurements obtainable because they do not preserve the state of polarization of the beams to a degree consistent with the demands for high accuracy.
Such non-polarization preserving retroreflectors are well-known and operate to deflect light through 180 degrees such that an incoming beam is exactly reversed in direction, traveling as an outgoing beam parallel to the direction of propagation of the incoming direction and spatially offset with respect to it. The classical retroreflector essentially contains the intersection corner of three mutually perpendicular plane surfaces and is known as the cube corner retroreflector or sometimes the tetrahedron. Here, a ray generally undergoes 3 reflections, one from each 120° sector in the process of entering and exiting the retroreflector. Ideally, the direction of the reflected ray is opposite that of the incident ray but displaced due to a reflection through the retroreflector intersection corner. From the standpoint of polarization effects, the primary problem with the classical retroreflector is that the angles the rays make with the mirror surfaces are skew. Detailed calculations using the Jones matrix formalism, along with the Fresnel reflection formulas, can be used to predict the resultant polarization for different initial polarizations and retroreflector types. From the standpoint of their use in DMI applications where small, linearly polarized beams interact with only small sub-apertures of the retroreflector, the net effect is to rotate the plane of polarization by several degrees (typically 6°). This phenomenon is called Retro Induced Polarization Rotation (RIPR) and misaligns the beam polarization directions with respect to the polarization beam splitter of the interferometer which can cause large periodic errors in the measured interferometric phase. A particularly troublesome periodic or “cyclic” error which occurs in High Stability Plane Mirror Interferometers (HSPMI) produces an error with a frequency at ½ the Doppler shift (as well as other frequencies). It has been shown that this error, which is due to the polarization rotation properties of the retroreflector, can be extremely large and will occur regardless of the beamsplitter quality.
Consequently, it is a primary object of the present invention to provide polarization interferometers utilizing polarization preserving optical systems that provide beam deflection properties without introducing deleterious polarization effects.
It is another object of the present invention to provide displacement measurement interferometers that use polarization preserving optical systems in place of traditional cube corner retroreflectors.
Yet another object of the invention to provide polarization plane mirror interferometers that utilize polarization preserving optical systems to enhance displacement measurement accuracy.
It is still another object of the present invention to provide polarization plane mirror interferometers that utilize internally tilted surfaces for purposes of reducing the effects of undesirable ghost beams on accuracy.
It is yet another object of the present invention to provide polarization plane mirror interferometers that utilize retardation elements in one or more interferometer legs to reduce the effects of ghost images.
Yet another object of the present invention is to provide polarization preserving optical systems for use at multiple wavelengths.
It is yet another object of the present invention to provide multiple-pass (two or more) polarization plane mirror interferometers that utilize polarization preserving optical systems.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the following detailed description is read in connection with the accompanying drawings.
SUMMARY OF THE INVENTION
The present invention relates to polarization interferometric apparatus that utilize polarization preserving optical systems to deviate plane polarized beams through preselected angles without changing their linear state of polarization. The inventive polarization interferometric apparatus have a variety of applications and are particularly suitable for use in distance measuring interferometry (DMI) to achieve higher measurement accuracy than is otherwise possible. This is achieved by reducing undesirable polarization effects that can introduce errors associated with an otherwise present undesirable polarization rotation found in classical retroreflectors. Such interferometers find additional utility in the fabrication of integrated circuits via microlithographic techniques.
While a number of embodiments of the polarization interferometric apparatus of the invention are described, they all share in common the use of interferometer means for receiving at least two beams having orthogonal states of polarization and providing first and second interferometer legs, separating the two beams for travel along the first and second interferometer legs, respectively, and generating exit beams containing information about the respective differences in the optical paths each beam experienced in traveling the first and second interferometer legs. The first and second interferometer legs have optical paths structured and arranged such that at least one of them has a variable physical length, and the optical path length difference between the first and second interferometer legs varies in accordance with the difference between the respective physical lengths of their optical paths and wherein at least one of the first and second interferometer legs comprises a polarization preserving optical system.
The polarization preserving optical systems used in the polarization interferometric apparatus of the invention comprise a plurality of reflecting surfaces arranged such that a change in the direction of propagation of an input beam, normal to both the input beam and an output beam, causes a change in the direction of propagation of the output beam in a direction opposite to the direction of the change in the input beam, and a change in the direction of propagation of the input beam, normal to the input beam and in a plane orthogonal to a normal to both the input beam and the output beam, causes a rotation in the output beam in the plane that is the same as a corresponding rotation of the input beam caused by the change in the direction of propagation of the input beam and wherein the plane of incidence at each of the reflecting surfaces is either orthogonal or parallel to the plane of polarization of an incident beam thereto.
The polarization preserving optical systems are preferably fabricated of a plurality of prismatic optical elements wherein the plurality of reflecting surfaces comprise selected surfaces of the prismatic optical element and preferably operate by total internal reflection.
The plurality of prismatic optical elements are preferably arranged as an integral assembly in which at least one surface of each prismatic optical element contacts at least one surface of another prismatic optical element and in
de Groot Peter J.
Hill Henry Allen
Craufield Francis J.
Kim Robert
Natividad Phil
Zygo Corporation
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