Compositions – Light transmission modifying compositions – Inorganic crystalline solid
Reexamination Certificate
1999-07-26
2003-01-21
Tucker, Philip (Department: 1712)
Compositions
Light transmission modifying compositions
Inorganic crystalline solid
C252S582000, C423S508000
Reexamination Certificate
active
06508960
ABSTRACT:
BACKGROUND OF THE INVENTION
It has been estimated that over sixty percent of the combat aircraft losses occurring since the 1960's can be attributed to use of infrared responsive surface to air and air to air missiles. Moreover the existence of newer more sophisticated generations of these missiles including the usually hostile SAM 16, SAM 17 and SAM 18 missiles is now known in the western world. Missiles of these latter types are understood to include countermeasures capabilities making the traditional hot flare and similar basic defensive measures against heat seeking missile attack of limited or little value. Although improved aircraft defensive measures based on laser energy sources have been used to some degree with respect to such missile weapons, until recently laser based infrared countermeasures have been laser source-limited, that is limited in both the available output power level and the spectral coverage achievable. In a very real sense therefore the missile and missile countermeasures battle scene has recently been biased in favor of the missile and its seeker by these power level and spectrum limitations.
As late as 1997 for example the best available solid state laser infrared source for missile countermeasures use operated in the range of five watts of average power level and provided little energy output in portions of the infrared spectrum known to be considered in the sensor of later design missiles. Although laser materials based on a certain class of chalcopyrite alloys have recently made it possible to exceed this 1997 power level by a factor of four and to achieve peak powers in the range of a hundred million watts per square centimeter in a nonlinear optical crystal material, even higher power levels and operation in yet inaccessible portions of the infrared spectrum are viewed as desirable improvements in the missile defense art. The present invention addresses this area of need and provides an infrared capability that is useful in areas other than the missile defense field.
Other needs for the present invention are also believed to exist within the military art. Following the decrease in tensions between major world powers in the 1990's, the threat of chemical or biological weapons used by smaller potential adversaries has emerged as a remaining and ongoing concern for the United States and other free world military forces. With regard to such chemical or biological weaponry it is known for example that one chemical warfare agent now available to most potential adversaries, i.e., the mustard gas of World War I infamy, provides a readily detectable and remotely sensible signature in a specific region of the infrared portion of the electromagnetic spectrum. This signature is, however, somewhat limited in bandwidth and therefore requires access to parts of the infrared spectrum which are not conveniently available with many laser sources. Similar limited spectrum signatures are believed to exist for other chemical and biological warfare agents. The remote, safe distance, sensing of such agents is of clear desirability in protecting the people and equipment necessary to a military operation. However the variety of threats posed by potential chemical and biological weapons now suggests that access to virtually unlimited areas of the infrared spectrum is desirable in the development of chemical and biological warfare defensive apparatus.
From a third perspective, an equal or perhaps even greater military interest in the infrared spectrum is prompted by the presence of windows of reduced atmospheric absorption located in certain specific bands of the infrared spectrum, especially for example in the 2-6 micrometer wavelength band and in the 8-12 micrometer wavelength band. These windows are believed to offer opportunity for communication, surveillance, and other military and civilian uses not currently considered feasible. The current situation in infrared spectrum applications may in fact be comparable with the somewhat recent advent of increased limited spectrum coverage and spectrum agility in the radar utilized microwave frequency parts of the electromagnetic spectrum, a development which has for example made spectral distinction between rain, snow and sleet possible in a weather radar system. In addition to military uses there of course exists numerous communication, detection and object-illumination applications in the non military world which can be benefited by efficient access to specific and possibly newly available portions of the infrared spectrum.
As a practical matter however infrared emitters usable in the most desirable infrared emission source, i.e., usable in the solid state stimulated emission coherent output devices such as the semiconductor laser, generate outputs at certain specific wavelengths. These wavelengths are moreover separated by infrared and other spectral regions in which no desirable efficient direct emission source is available. The gas-based carbon dioxide laser is a non-solid state example of this situation in that such lasers are for example known to have strong emission lines residing at wavelengths of 9.3 and 10.6 microns. Emissions at wavelengths falling between these two wavelengths or at specific wavelengths above and below these wavelengths is significantly less.
The use of wavelength changing devices, devices based on the nonlinear optic characteristics of certain single crystal semiconductor materials, offers one approach for providing energy at otherwise inaccessible spectral locations. Prior to the early 1970's there was in fact little access to the wavelengths greater than 4 microns with the available ruby, Neodidium, YAG, Lithium, Argon and other laser materials of common usage—even with the use of the then available nonlinear and wavelength changing materials. In a similar manner, outside the infrared range an absence of sources in the 1 to 2.5 micron range of wavelength, especially for applications needing tunability, was difficult even when using wavelength mixing arrangements. The utility of a wavelength halving apparatus may be appreciated by, for example, considering that halving the wavelength (doubling the frequency) of the 10.6 micron emission line from a carbon dioxide laser provides an output at the wavelength of 5.3 microns, a wavelength at the extreme end of the two to six micrometer window where the most advanced missiles operate, a wavelength which is inaccessible to most laser materials.
The expression “nonlinear optic characteristics” when used in connection with the materials of such wavelength changing devices is generally understood to relate to the properties of crystal materials in which light transmission characteristics are intensity-dependent, i.e., materials in which the optical refractive index, n, is a function of the electric field strength vector, E, of the light wave. This representation is of course based on a Maxwell's equation model of light and the understanding that light energy is fairly described in terms of electric field strengths. The light wave index of refraction, n(E), is moreover represented as the sum of terms in an infinite series expansion of electric field strength vectors taken to the powers or exponents of zero, one, two and so on with each series term also including a factor of the form n
0
, n
1
, n
2
and so on representing a refractive index. In mathematical symbols this relationship may be expressed as:
n
(
n E
)=
n
0
+n
1
E+n
2
E
2
+n
3
E
3
+ . . . (1)
or alternately as:
n
(
E
)=
n
0
+&Dgr;n
(1a)
&Dgr;n=
2&pgr;/
n
o
[&khgr;
(2)
E+&khgr;
(3)
E
2
+&khgr;
(4)
E
3
. . . ] (1b)
The material property of interest is &khgr;
(2)
. The zero exponent E term, i.e. the n
0
term in the equation 1 series, corresponds to the refractive index used in traditional linear optics, the optics considered in entry level physics courses. The nonlinear materials of interest in the present invention are identified as chi two or second order nonlinear materia
Goldstein Jonathan T.
Ohmer Melvin C.
Zelmon David E.
Hollins Gerald B.
Kundert Thomas L.
The United States of America as represented by the Secretary of
Tucker Philip
LandOfFree
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