Optical: systems and elements – Polarization without modulation
Reexamination Certificate
2000-01-28
2001-05-08
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Polarization without modulation
C359S487030, C359S490020, C359S583000, C359S834000, C359S835000
Reexamination Certificate
active
06229645
ABSTRACT:
FIELD OF INVENTION
The present invention relates to physically combining a Porro (or roof) prism with a thin-film polarizer in a single optical element, one purpose being to simplify alignment of laser-based systems.
BACKGROUND OF THE INVENTION
The roof or Porro prism derives its name from the similarity of its configuration to that of a traditional A-roof house, albeit without eaves. Two faces of the Porro prism are orthogonal and oriented with respect to the rest of the prism so that they form an A-shaped roof over the remaining faces. The remaining faces form a rectangular box(without eaves but having a triangular portion meeting the apex of the roof on two of the four sides) attached to the two faces forming the A-roof. The face directly opposing the faces forming the roof is the input face of the Porro prism.
The Porro prism has the characteristic of retro-reflecting light in one axis. That is, light incident on the Porro prism will be directed back on itself, even as the prism is rotated by several degrees about a line running parallel to the roof crest or apex.
One application of the Porro prism is as an optical element in a laser resonator. A standard polarization output coupler (POC) has been built using at least two separate devices: the conventional Porro prism and a separate thin-film polarizer, or a pair of Porro prisms oriented in an opposing uncrossed geometry. A polarization output-coupling scheme for a laser resonator must either depolarize the circulating laser pulse in the cavity by rotating one of the roof crests of the two Porro prisms from a 90° crossed configuration or use a separate polarizing-depolarizing device, such as a waveplate.
DESCRIPTION OF THE RELATED ART
Various devices are used to change incoming light polarization. In a laser resonator or like device that uses a polarization output coupling arrangement of prisms and polarizers, there is a need to separately align multiple prisms or align a separate polarizer with a prism during assembly and, for systems in extreme environments such as on helicopters, periodically after deployment to maintain performance. Using existing systems, there is a need for continuing physical alignment of the polarizer and its associated hardware. This continuous alignment requirement creates complexities that increase the cost of fabrication and assembly as well as operation.
It is known in the art to use multiple devices to change incoming light polarization. For example, U.S. Pat. No. 5,483,342, “Polarization Rotator with Frequency Shifting Phase Conjugate Mirror and Simplified Interferometric Output Coupler,” issued to David A. Rockwell, on Jan. 9, 1996, describes an interferometer that employs “only” two adjustable elements: a specially shaped prism and a beam deflector that is preferably a Porro prism.
It is also known in the art to achieve polarization changes via coating with optical thin films. U.S. Pat. No. 5,559,634, “Retroreflecting Polarizer,” issued to Michael F. Weber on Sep. 24, 1996, describes a retroreflecting polarizer, having optical thin films coated on structural material of a linear array of multiple isosceles prisms, that divides an incident beam of light, transmitting one polarized component and reflecting the other.
The use of “cemented,” or otherwise joined, polarizing beam splitter “rectangular” prisms is known in the art. U.S. Pat. No. 5,716,122, “Optical Apparatus Using Polarizing Beam Splitter,” issued to Sampei Esaki et al, on Feb. 10, 1998, depicts an optical apparatus for outputting polarized light in generally parallel beams. The apparatus uses a pair, or multiple pairs, of prisms that are cemented together. Each pair of cemented prisms has cross-sections that are right-angled triangles with a vertex angle of less than 45°, or less than 30°. Each of the cemented faces has a “thin-film group” polarizer applied.
It is still accepted in the art to use a prism with at least one other element to effect a polarization change. U.S. Pat. No. 5,717,472, “Polarizing Prism for Panel Type Liquid Crystal Display Front Projector and Optical System Using the Polarized Prism,” issued to Jong Jin Lee on Feb. 10, 1998 provides for a beam separating prism, a triangular prism, and either a twisted nematic (TN) liquid crystal or a _or _wavelength plate. Lee's apparatus provides more efficient light transmittal as well as polarization selection.
Using physically separated multiple prisms for achieving polarizing-output coupling (POC) is also known in the art. U.S. Pat. No. 796,770, “Compact Diode Pumped Solid State Laser,” issued to Eduard Gregor et al on Aug. 18, 1998, describes a POC resonator employing reflections from two opposing uncrossed roof prism mirrors to produce uniform near field and far field beams with near diffraction limited quality.
Means for coating prisms with thin-film polarizers are well known. An improved thin film and method of producing it is described in U.S. Pat. No. 5,071,906, “Polarizing Film and Process for the Production of the Same,” issued to Chikafumi Tanaka et al on Dec. 10, 1991. A thin-film polarizer having superior heat resistance and excellent optical properties is made using iodine and a dichroic dye.
Optical polarizer thin films that are flexible and stretchable are well known. See, for example, U.S. Pat. No. 4,923,758, “Polarizing Prescription Lenses and Method of Making,” issued to Mortimer Marks et al on May 8, 1990.
SUMMARY OF THE INVENTION
Accordingly, it is an aspect of the present invention to provide a polarization selecting optical element constructed from a Porro prism that incorporates a flexable, thin-film polarizer as an integral face of the Porro prism.
It is another aspect of the present invention to eliminate an inherent need for a first optical alignment step as well as a continuous alignment requirement during system operation
It is a further aspect of the present invention to produce a device smaller in physical volume via elimination of the requirement for additional mounts as well as at least two physically separated optical elements.
It is another aspect of the present invention to reduce the complexity of optical systems, in particular, laser systems. This yields a device able to be manufactured and assembled by less skilled employees, thus reducing costs in several areas.
Finally, it is an aspect of the invention to reduce assembly time by removing alignment steps. These aspects have been attained by providing a device combining the best features of two separate elements, a Porro prism and a thin-film polarizer, in a single device, designated the “Hendrix Prism.” Since the Hendrix Prism is “self-contained” in a single element, the need for alignment of separate components is eliminated.
REFERENCES:
patent: 4923758 (1990-05-01), Marks et al.
patent: 5071906 (1991-12-01), Tanaka et al.
patent: 5404820 (1995-04-01), Hendrix
patent: 5483342 (1996-01-01), Rockwell
patent: 5559634 (1996-09-01), Weber
patent: 5716122 (1998-02-01), Esaki et al.
patent: 5717472 (1998-02-01), Lee
patent: 5796770 (1998-08-01), Gregor et al
patent: 5847871 (1998-12-01), Sumida et al.
Curtis Craig
Gottlieb Paul A.
Kehl Dickson G.
McMillan Armand
Spyrou Cassandra
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