Optical: systems and elements – Polarization without modulation – Polarization by reflection or refraction
Reexamination Certificate
2001-10-03
2004-01-06
Juba, John (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarization by reflection or refraction
C359S483010, C359S857000, C398S152000, C398S118000
Reexamination Certificate
active
06674576
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention pertains to the direction of electromagnetic radiation towards a target and capture of electromagnetic radiation from a target; especially light.
2. Description of the Prior Art
Various telescope configurations can be used for communicating between terrestrially based terminals through a technique called free-space optical communications (FSOC). Generally, FSOC systems use a laser beam directed from a first opposing terminal and transmit that laser beam to a second opposing terminal. When the two laser beams lock on to each other, they provide two-way communication between the terminals. Of course, terminals need not be based terrestrially; one or both terminals involved in two-way communications can be space-borne. Also, communication can be only in one direction.
The selection of a telescope configuration influences almost all of the performance parameters in an FSOC system. For example, the size, weight, performance, and assembly complexity are all factors that contribute to system effectivity and deployment cost. The selection of the telescope type usually starts by trading off the performance offered by an unobscured telescope against the performance offered by an obscured system. Some of the earliest telescopes ever fabricated utilized refractive elements. These telescopes are very simplistic and can utilize commonly available optical components. Refractive telescope designs provide good overall optical performance and are easy to assemble. However, the materials used in refractive telescope technology are generally heavy, and can be more difficult to athermalize and correct for chromatic aberration. And most importantly, refractive telescopes are mechanically long in length, a trait incompatible with most portable applications. Infrared (IR) materials typically exhibit higher indices of refraction and can be made more lightweight. Use of such IR materials, though, requires additional care during alignment because visible alignment telescopes cannot be used. This can drive up the overall cost of system deployment.
Using reflective surfaces is another approach to forming a telescope. A popular telescope configuration comprises two reflective surfaces that fold an image path into a smaller and more compact package. There are two types of reflective telescope configurations known today; the on-axis configuration and the off-axis configuration. On-axis reflective designs can be easily manufactured and assembled and offer better control of aberrations compared to off-axis designs. There are many design forms that may be considered such as confocal paraboloids, Ritchey-Chrétien and classical Cassegrain types. Many manufacturing options are available such diamond turning, conventional polishing and replication. Substrate material options may also include glass types, metals and composite substrates. In the case of larger telescopes, a lightweighted approach can be employed to minimize the mass of the telescope assembly.
One significant difference between refractive and on-axis reflective configurations is that the reflective design has some of its incoming rays obscured. The effect of obscuring these rays becomes apparent when analyzing the optical throughput of an FSOC system. In these reflective configurations, the center-most rays entering the telescope are obscured from view. And because these, center-most rays have the greatest power density in a typical incoming Gaussian beam profile, much of the power transmitted by the sending side of an FSOC is lost. This makes obscured systems less desirable because of the reduced power efficiency vis-à-vis an unobscured telescope.
The throughput power efficiency of an unobscured telescope may allow the system to use a less powerful and less expensive laser. These types of cost benefits could easily be outweighed by the cost associated with manufacturing an unobscured refractive telescope assembly. Traditional reflective telescopes are cost-effective, but are less efficient optically. Herein lies the basic problem with the known art and the balancing of performance parameters that must be accomplished in any new FSOC system design.
There are reflective telescope designs that provide an unobscured aperture. These designs combine the compact nature of a reflective design with an unobscured aperture like a refractive design, but these are typically off-axis structures. Off-axis reflective designs have the advantage of offering an unobscured aperture. This enables maximum throughput of the Gaussian beam profile for the transmitted signal. Also, these designs have the potential to be athermalized and achromatized with the correct material selection. The off-axis approach suffers from increased field aberrations compared to on-axis systems, and can be more difficult to manufacture, assemble and align.
The structure of the telescope used in an FSOC is not the only system component that affects overall performance. A complete optical throughput budget for each FSOC generally considers several factors that contribute to optical loss. There are several sources of optical loss including material absorption, coating absorption and reflection, scattering, and free-space propagation loss. Polarization can also significantly affect the signal-to-noise ratio in an FSOC system if the design does not account for polarization. All the optical coatings in a system need to be cognizant of the polarization of the light that carries the data. Beam-splitting devices are especially sensitive to polarization effects.
SUMMARY OF THE INVENTION
Ideally, an FSOC system would benefit most if a telescopic device could be constructed that combined the unobstructed view offered by a refractive design together with the compact-size, reduced weight and low-cost of a reflective design. The present invention offers a unique combination of these features. Specifically, the present invention uses a reflective approach in order to collapse the overall size of the telescope. In contrast with traditional reflective designs, the present invention does not include a secondary reflector that can obscure the field-of-view. This one distinguishing feature, as embodied in the present invention, can be applied to FSOC systems or to other applications where electromagnetic radiation needs to be either directed to a remote target or collected from a remote source. It should be noted, that the method and apparatus of the present invention can be utilized throughout the entire electromagnetic spectrum, and that the scope of the present invention should not be limited to that portion of the electromagnetic spectrum comprising light.
Replacing the secondary reflector in the present invention is a specialized filter exhibiting special reflective properties. In a first example embodiment of the present invention, the filter is disposed in front of a primary reflector and allows electromagnetic radiation of a first polarization to pass through the filter. The filter reflects electromagnetic radiation of a second polarization. The effect of this selective reflection capability is to allow appropriately polarized radiation from a distant source to pass through the filter and strike the primary reflector. The primary reflector, having a focal length, reflects the polarized radiation toward the filter. This process of reflection reverses the polarization of the radiation. Because the radiation is now of a second polarization, it is reflected by the filter disposed in front of the primary reflector back toward the primary reflector. Generally, the filter is made sensitive to either right or left handed circularly polarized radiation. In most applications, electromagnetic radiation comprising light waves is utilized.
In a second example embodiment, the filter that is disposed in front of the primary reflector reflects light selectively according to the angle of incidence of the radiation striking the filter. Electromagnetic radiation incident on the filter substantially normal to the filter is allowed to pass through. Electromagn
Carollo Jerome T.
Hoppe Michael J.
Eppele Kyle
Jensen Nathan O.
Juba John
Rockwell Collins, Inc.
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