Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
2001-06-05
2002-09-24
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S490020, C359S494010, C359S900000
Reexamination Certificate
active
06456434
ABSTRACT:
FIELD OF INVENTION
The present invention relates to the field of optics, in particular, to optical devices for spatially separating or combining orthogonally polarized optical beams, in particular, to optical devices used as optical beam polarizers or analyzers in the optics of ultraviolet, visible, and infrared radiation, including laser emission. More specifically, the invention relates to birefringent polarizing two-beam prisms.
BACKGROUND OF THE INVENTION
According to commonly accepted rule, orientation of the light-wave electric field determines its polarization direction, and the plane of the electric vector and the light propagation direction are referred to as a polarization plane. If electric field oscillations occur only in that plane, and the plane itself has a constant spatial position, such light is referred to as having linear or planar polarization (or simply polarized). If the wave electric vector rotates around the light propagation direction (i.e., around the wave vector), such light can have either elliptical or circular polarization. For nontonochromatic light, i.e., for one containing a number of frequency components, the temporal changes in the amplitude and spatial position of its resulting electric vector can be absolutely arbitrary, and such light is referred to as unpolarized.
Linearly polarized light beams have found general application in optics, laser engineering, technology, e.g., for precision processing of metals (cutting, drilling etc.), in photochemistry for resonance excitation of molecules and atoms, in biology for similar purposes, in communication engineering, etc. The preference is given to polarized light due to higher accuracy of interaction of such light with materials. Such high interaction accuracy results from the complexity and anisotropy in the inner structure of the aforementioned materials. For example, most of the devices widely used in optics and communication engineering for entering information into a light beam, such as electrooptical and acoustooptical modulators, operate with linearly polarized light because of the pronounced anisotropy of optical properties in the crystals these devices are based upon. Fiber optics communication engineering is a field where polarized light has a constantly increasing application. Anisotropic fibers for polarized light and low-noise polarization amplifiers have been developed. In principle, polarized radiation is used for effective transformation of laser frequencies in nonlinear crystals and for selection of optical radiation frequencies by anisotropic tunable acoustooptical and electrooptical filters. The use of polarized light is required for operation of binary polarization switchers/modulators, polarization multiplexers and, in general, in any optical devices for which anisotropic interaction of light with the materials is advantageous.
There are a number of devices that can be used for light polarization. These include dichroism dye based polarizers, purely crystalline polarizers, interference polarizers, polarizers based on isotropic materials that use the effects of light reflection and light refraction at the Brewster angle, etc.
However, special accent is made on prism-type polarizers that have a specific geometry and are made of optically anisotropic crystalline materials. The reason for making such accent is due to the special properties of these polarizers. As a rule, they are crystalline polarizers that exhibit high extinction (ratio of the useful and unnecessary orthogonally polarized light components) of polarized beams, low optical losses, and high resistance to high-power optical radiation, especially laser radiation.
For better understanding the principles of the present invention, it would be advantageous to briefly describe the structure of conventional polarizing prisms. The basics of polarizing devices are described, for example, in
Handbook of Optics, Vol. II, Devices Measurements and Properties
, McGraw-Hill, Inc., 1995, pp. 3.1-3.70, New York, San Francisco, Montreal, Tokyo, Toronto.
Polarizing prisms are made only of birefringent crystals that have no cubic crystal symmetry. In such crystals, the light is split into two orthogonally polarized beams which, upon exit from the crystal, are in general case spatially separated both with respect to the exit points and the propagation angles. However, for many reasons (small separation angles or distances, unavoidable frequency dispersion of the prism, reflection optical losses and technologically uncomfortable beam exiting geometry) simple single crystal prisms are replaced for combinations thereof that are referred to as polarizing prisms. Polarizing prisms are usually made of a relatively cheap and abundant calcite (CaCO
3
). Recently a wide range of artificially grown birefringent crystals have been developed for polarizing prism applications. Such crystals are, for example, TiO
2
, YVO
4
, KNbO
3
, KTiOPO
4
, &agr;-BaB
2
O
4
, PbMoO
4
, TeO
2
, Te, Se, etc. However, the general use of these materials is precluded by their high cost, complexity of manufacturing compound prisms therefrom or insufficiently pronounced optical anisotropy (birefringence).
Advanced polarizing prisms usually contain two or more trihedral prisms made of optically uniaxial crystals of tetragonal, hexagonal, or trigonal symmetry having similar or different optical axis orientations and bonded to each other with transparent substances (cements) or separated from each other with a thin air or vacuum gap. Cement-free gaps are often used in prisms for short-wave radiation or high-power laser beams.
Prisms are subdivided into one-beam prisms, from which only one linearly polarized light beam exits, and two-beam prisms, that produce two light beams polarized in mutually perpendicular planes (orthogonally polarized beams). The former type prisms operate on the basis of the total internal reflection principle. A nonpolarized incident beam is split in the prism into two orthogonally polarized beams. One of these beams undergoes total internal reflection at the prism bounding and is deflected, while the other beam passes through the bounding for further use or processing. Such prisms are known as the Nicol, Glazebrook, Hartnack-Prazmowsky, Ahrens, etc., prisms.
FIG. 1
shows some of these prisms. (a), (b) and (c) are Glan-type prisms known as the Glan-Thompson (a), Lippich (b) and Frank-Ritter (c) prisms. The second row in
FIG. 1
shows Nicol-type prisms, i.e., the conventional Nicol prism (d), the Nicol-Halle form prism (e), and the Hartnack-Prazmowsky prism (f). The optical axes of the prisms are shown in with double arrows.
Variations in the structure of the prisms is normally accompanied by changes in the prisms' names. For example, the air-gap Glan-Thompson prisms are referred to as the Glan-Foucault prisms, and the air-gap Lippich prisms as the Glan-Taylor prisms. In practice, any of these prisms can be referred to as a Glan prism. The air-gap Nicol prisms are referred to as the Foucault prisms. There also are combinations of three bound prisms, the so-called “double” prisms. The double Glan-Thompson prisms are referred to as the Ahrens prisms.
FIG. 2
shows various types of two-beam polarizing prisms. The optical axes of the two parts of Rochon (a), Sënarmont (b), and Wollaston (c) prisms are perpendicular to each other. The Foster (d) and beam-splitting Glan-Thompson (e) prisms have parallel optical axes. In this respect these prisms are similar to one-beam polarizing prisms, but their shape is changed so the two beams propagate in specific directions without noticeable losses.
The need for the great variety of existing polarizing prisms (not all of them are shown here) stems from the impossibility of designing a prism having universal parameters. Each polarizing prism has its individual advantages and drawbacks that determine its applicability. Prisms are characterized by a number of parameters, such as angular separation of the beams and frequency dispersion of the aforementioned angular separation, angular aperture, extinction, spe
Jr. John Juba
Schwegman Lundberg Woessner & Kluth P.A.
Solid Optics Inc.
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