Polarizer devices and methods for making the same

Details Polarizer devices and methods for making the same Polarizer devices and methods for making the same

1999-11-10

2001-06-26

Shafer, Ricky D. (Department: 2872)

Optical: systems and elements

Polarization without modulation

By relatively adjustable superimposed or in series polarizers

C359S490020, C359S490020

Reexamination Certificate

active

06252710

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical polarization devices, and more particularly to non-absorbing, normal-incidence, reflective polarizing optical elements employing a multilayer stack of optically birefringent thin film layers obtainable by oblique vapor deposition in vacuum.
2. Background Information
The prior art includes several well-known light polarizing components. For example, prism polarizers, commonly known as Nicol and Glan polarizers, are based on double refraction of birefringent crystals—usually calcite. These components utilize polarization-selective total internal reflection at the interfaces of two matched prisms made of birefringent crystals. With these polarizers, one of the linearly polarized light components of incident light is removed by total internal reflection, while the orthogonal component is transmitted. These prism polarizes off an extinction ratio of higher than 100,000 and can cover a relatively wide spectral range. For example, calcite polarizers nominally have a useful spectral range of 0.35-2.3 &mgr;m. However, these crystal polarizers are limited in size by the scarcity of calcite of good optical quality. In addition, they tend to be expensive, bulky and extremely sensitive to angle of incidence.
A variation of the Glan type polarizer is the cube polarizer disclosed in U.S. Pat. No. 2,403,731 issued to MacNeille. This polarizer consists of two glass prisms cemented together along their hypotenuses. The diagonal face of one of the prisms is coated with a multilayer dielectric film stack designed to reflect one linear polarization component and transmit the orthogonal component. Although much less costly than crystalline prism polarizers, MacNeille cubes typically exhibit a lower extinction ratio of about 1,000 and can operate over a wavelength band of only about 300 nm.
Additional significant prior art includes dichroic polarizers, such as those commonly referred to as “Polaroid®”, sheet polarizers. These polarizers are based on polarization-selective absorption, or dichroism, which are first discovered by Biot circa 1815. Modern sheet polarizers are based on an invention by Edwin Land circa 1928. This technology is reviewed by the inventor in
Journal of Optical Society America,
41, 957(1951). These widely used sheet polarizers can typically provide an extinction ratio of 1,000 and are of low cost. High extinction ratio is possible but is achieved at expense of transmittance efficiency. Because they are constructed of organic polymers, they possess severe inherent limitations, i.e. they are limited to low optical power handling capability, limited to use at low operating temperature and restricted to use in the visible spectral range.
Another example of a dichroic polarizer recently introduced by Corning Glass Works of Corning, N.Y., is sold under the trademark Polarcor™. This polarizer is made of glass which contains elongated submicroscopic silver particles aligned in the same direction. The silver particles preferentially absorb light polarized along the long axis of the silver particles while transmitting light of the orthogonal polarization. These glass-based filters are durable and may typically withstand a temperature up to 400° C.; however, they exhibit relatively narrow bandwidth and are limited in operating wavelength range from approximately 0.633 to 2.1 &mgr;m.
A further example of a dichroic filter includes a metal film obliquely deposited on a glass substrate as disclosed in U.S. Pat. No. 5,122,907 (hereinafter, the '907 patent) issued to Slocum. This dichroic film is made up of microscopic prolate metal spheroids all aligned in the same direction on the glass surface. A polarizer of this general type, commercially available from Melles Griot Company of Irvine, Calif., for example, operates in the near-infrared spectral range and has a relatively low transmittance of only about 20%.
Other important prior art includes polarizers based on organic cholesteric liquid crystalline (CLC) materials. This art has been taught, for example, by Lee and Jacobs, in a paper published in
Journal of Applied Physics,
Vol. 68, No. 274 (1990). Cholesteric liquid crystalline thin films, with molecular alignment in the so-called planar texture, exhibit wavelength- and circular polarization-selective reflectance over a spectral range which is material-dependent. Such films may operate satisfactorily as circular polarizers in many applications, but because they are organic materials, they absorb strongly in the UV spectral range, limiting their operating range. Moreover, the operating temperature is limited to about 140° C. These films may be used as a transmissive linear polarizers in conjunction with a quarter-wave plate which converts the polarization of transmitted light from circular to linear.
Recently, Iwatsuka et al disclosed polarizing devices based on diffraction grating based on obliquely deposited thin film in U.S. Pat. No. 5,245,471. One of the limitations of such devices is that the 0-th diffracted light beam remains unpolarized.
Thus, a need exists for an improved polarizing optics method and apparatus for polarizing light in a manner which achieves a substantial improvement in efficiency and power handling capacity.
SUMMARY OF THE INVENTION
According to an embodiment of this invention, a light polarizer includes a substrate and a plurality of discrete layers of birefringent film disposed on the substrate in superposed relation to one another. Each one of the discrete layers is substantially planar and defines a planar direction. Each one of the discrete layers has an optic axis and a planar component thereof extending in the planar direction. The planar component of at least one of the plurality of discrete layers is angularly offset from the planar component of an other of the plurality of discrete layers.
As an enhancement, the planar component of each successive one of the discrete layers is angularly offset relative each preceding one of the discrete layers by an oblique, acute angle in one of either a positive or negative direction to provide the light polarizer with either a right-handed chirality or a left-handed chirality for circularly polarizing incident light.
The present invention provides, in a second aspect, a light polarizer including a substrate and a plurality of birefringent layers superposed over one another on the substrate. Each one of the birefringent layers has a quarter-wave thickness t=&lgr;
0
/4, where &lgr;
0
is a characteristic wavelength of the light polarizer. Each one of the birefringent layers is substantially planar, defining a planar direction, and has an optic axis and a planar component thereof extending in the planar direction. An optically isotropic layer of quarter-wave thickness is located between adjacent birefringent layers.
As an enhancement, at least one of the birefringent layers is a composite layer of superimposed sublayers. Each one of the superimposed sublayers has a sublayer planar component of a sublayer optic axis. The sublayer planar components of each one of the superimposed sublayers extend substantially parallel to one another in the planar direction.
A still further aspect of the present invention includes a method of fabricating a polarizer including the steps of providing a substrate and depositing a plurality of layers of birefringent film on the substrate in superposed relation to one another, each one of the plurality of discrete layers being substantially planar and defining a planar direction. Each one of the plurality of layers are deposited from an oblique deposition angle relative the planar direction, to provide each one of the layers with an optic axis having a planar component thereof extending in the planar direction. The deposition angle of at least one of the layers is angularly offset in the planar direction from the deposition angle of an other of the layers to angularly offset the planar component of at least one of the layers from the planar component of an other of the layers.
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