Optical: systems and elements – Single channel simultaneously to or from plural channels – By refraction at beam splitting or combining surface
Reexamination Certificate
1998-09-29
2001-04-03
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By refraction at beam splitting or combining surface
C359S638000, C359S490020
Reexamination Certificate
active
06212014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of optical beamsplitters, and to an improved midwave infrared polarizing beamsplitter cube and method of fabricating such cubes.
BACK GROUND OF THE INVENTION
A polarizing beamsplitter cube, first described by Mary Banning in “Practical Methods of Making and Using Multilayer Filters” J. Opt. Soc.Amer., Vol. 37, pages 792-797, (147) comprises two 45 degree prisms, one coated with a thin film polarizing coating on the hypotenuse, and the second cemented to the coated hypotenuse of the first prism to form a cube. This article described beamsplitter fabrication for visible wavelengths. Standard optical cements, having approximately the same index of refraction as the prisms, are typically used to join the prisms together. However, this technique cannot be used in the infrared region of the spectrum, because infrared transmitting materials have a much higher index of refraction. The index of refraction mismatch between all known optical cements and infrared transmitting materials creates total internal reflection (TIR) at the prism-to-prism interface, so that there is little or no transmission through the resulting cube.
One method of fabricating an infrared polarizing beamsplitter that minimizes TIR has been described in U.S. Pat. No. 4,733,926 to Title. This patent discloses join the prisms are joined together via optical contacting, by applying a relatively thick layer of prism material over a thin film polarizing coating. This provides a surface that can be optically polished and then, through Van der Wahls forces, can be brought into optical contact with the other optically polished prism hypotenuse. However, this reference does not disclose the use of a Germanium polarization coating or the use of any special refractive index materials between the prisms.
Beamsplitters of the type shown in the Title patent are difficult, time-consuming, and costly to manufacture. In order to obtain optical flatness, the prism that has the thin film polarization coating must have an additional coating of the prism material applied over the polarization coating. This adds another coating step to the fabrication process, increasing labor and material costs. Additionally, this layer must be optically polished which requires additional handling by experienced optical and quality personnel. This additional coating process adds to the complexity of the coatings and provides an extra step that could potentially damage the previously applied polarization coating. Also, this extra coating adds thickness and therefore weight to the polarization coating, which increases the tendency of the polarization coating to delaminate and flake from the prism.
Further, optical contacting methods produce only marginal transmission performance because there is still a small air gap between the two prisms. In order to defeat total internal reflection at the prism interface, the air gap created from the two optically polished surfaces must be a very small fraction of the wavelength of light. For optically contacted prisms, acceptable beamsplitter performance requires at least a surface flatness of &lgr;/100, which is difficult to meet with optical polishing. Finally, optical contacting only creates a semi-permanent bond between the prisms, which can come apart over time.
Additional beamsplitting devices known in the prior art are shown in U.S. Pat. No. 4,411,492 to Bleuge, U.S. Pat. No. 4,431,258 to Fye, U.S. Pat. No. 4,597,630 to Brandstetter et al., U.S. Pat. No. 4,627,688 to Kobayashi et al., U.S. Pat. No. 5,243,465 to Fein, U.S. Pat. No. 5,351,152 TO Kuo et al., U.S. Pat. No. 5,440,424 to Wu et al., U.S. Pat. No. 5,579,159 to Ito, U.S. Pat. No. 5,596,451 to Handschy et al., U.S. Pat. No. 5,610,765 to Colucci, and patents U.S. Pat. Nos. 5,646,778 and 5,657,164 to Schuman. Similarly, none of these devices solve the problems experienced with making beamsplitter cubes that operate effectively in the infrared range.
Thus, there is a need for an improved infrared range beamsplitter and for an improved manufacturing method for making such beamsplitters.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the invention to provide a bonding method for joining two prisms together so that the ensuing interface defeats total internal reflection.
It is also a general object of the invention to provide an improved beamsplitter assembly incorporating a sealed liquid interface between prismatic elements.
Another object of the invention is to provide a bonding method for prismatic elements of a beamsplitter which reduces optical and mechanical fabrication tolerances, which is relatively inexpensive to fabricate, which provides better polarizing performance, and which provides an improved method of fabricating infrared polarizing beamsplitter cubes.
Still further objects and advantages will become apparent upon consideration of the ensuing description and accompanying drawings.
The Midwave Infrared (MWIR) Polarizing Beamsplitter Cube of the present invention may be used to separate p and s polarization states of infrared radiation, such as radiation in the 3000 nm-5000 nm wavelength region, into two orthogonal paths. The polarization of the radiation incident on any face of the cube will be predominantly transmitted straight through the cube. The s polarization of the radiation incident on any face of the cube will be predominantly reflected orthogonal to the incident direction.
In a preferred embodiment, the polarizing beamsplitter cube comprises two ZnSe 45° prisms, an adhesion layer and a thin-film polarization coating layer that are deposited onto the hypotenuse of the first prism, a refractive index fluid layer that provides an interface between the prisms that defeats total internal reflection when the prisms are joined together, and a urethane adhesive that encapsulates the fluid. The adhesion layer may consist of 100 Å thorium fluoride and 50 Å chromium, while the polarization coating may be constructed from alternating layers of zinc sulfide (ZnS) and germanium (Ge) which are deposited by vapor deposition. The refractive index fluid is preferably Cargille M Series Refractive Index Liquid (n(&lgr;=5893 A)=1.78), and the Urethane adhesive (UA) is Hardmans Kalex® Urethane. The refractive index fluid is placed on the hypotenuse of the first prism which has been coated by vacuum deposition, with the adhesion and polarization coating. The second prism is placed on top of the first prism to form a transparent interface that defeats total internal reflection. A bead of urethane adhesive is used to permanently seal the index fluid between the first and the second prisms.
REFERENCES:
patent: 3704934 (1972-12-01), Holme
patent: 4411492 (1983-10-01), Bluege
patent: 4431258 (1984-02-01), Fye
patent: 4597630 (1986-07-01), Brandstetter
patent: 4627688 (1986-12-01), Kobayashi
patent: 4733926 (1988-03-01), Title
patent: 5173432 (1992-12-01), Lefkowitz
patent: 5243465 (1993-09-01), Fein
patent: 5351152 (1994-09-01), Kuo
patent: 5440424 (1995-08-01), Wu
patent: 5579159 (1996-11-01), Kabushik
patent: 5596451 (1997-01-01), Handschy
patent: 5610765 (1997-03-01), Colucci
patent: 5646778 (1997-07-01), Shuman
patent: 5657164 (1997-08-01), Shuman
patent: 5683480 (1997-11-01), Taniguchi
patent: 5719705 (1998-02-01), Machol
patent: 6018418 (2000-01-01), Pan et al.
Finlan J. Michael
Flood Kevin M.
Gerard Mary E.
Lehman, Jr. John G.
Epps Georgia
LSA, Inc.
Reed Smith Hazel & Thomas LLP
Seyrafi Saeed
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