Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1992-12-09
2003-07-22
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S23700G, C359S371000
Reexamination Certificate
active
06596982
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a system for suppressing reflections from an infrared focal plane array.
2. Brief Description of the Prior Art
Optical countermeasure suppression (OCCM) on scanned infrared focal plane arrays (IRFPAs) has traditionally required the inclusion of a prismatic element to deflect incoming radiation out of the system optical axis, thereby preventing reflected (outgoing) radiation from passing back out through the system optics. This constitutes at least one part of every known suppression scheme.
The most common prismatic element or “optical wedge” includes an infrared-transmissive prism. This simple wedge has two kinds of aberration, namely chromatic and monochromatic. The chromatic aberrations are inversely proportional to the Abbe dispersion (a parameter of the material from which the gratings are made) while the monochromatic aberrations are caused by the difference in Snell's law of refraction for different portions of the ray bundle due to the relatively large height differences between “high” and “low” sides of the wedge. The chromatic aberrations are tolerable, however the monochromatic aberrations are not. The worst of the monochromatic aberrations is coma, with some astigmatism and spherical aberrations. The coma can be fairly well corrected for one image position by decentering an element in the imager. However it remains under- or over-corrected at other image positions. The spherical aberration is minimized by a focal shift and astigmatism does not become dominant until the image position is more than about 0.015 inches off-axis.
Current scanned focal planes extend no more than about 0.006 inch off-axis in the scan direction, but the next generation of time-delay-integrate (TDI) arrays will extend farther out to in excess of 0.040 inch off-axis. Furthermore, the requirement that the optics be decentered leads to a significant increase in system complexity with concomitant increases in cost and weight. It is therefore necessary to, in some way, avoid the coma problem to avoid the significant loss in resolution in the off-axis region beyond about 0.015 inch.
The only alternative solution to the optical wedge aberration difficulty of which applicants are aware is the use of binary optic elements or stepped gratings. This solution has two major problems associated therewith. The first problem is that a stepped grating is, at best, an approximation to a blazed grating. The degree of approximation required to satisfy the minimum requirements for this application still leads to undiffracted (transmitted) losses of at least 0.3 percent compared to the blazed grating. The second major problem is that highly sophisticated methods are required to fabricate a stepped grating meeting the above noted minimum requirements. Line width resolution of at least one micron (0.0004 inch) and registration accuracy of at least 0.1 micron (0.00004 inch) must be maintained, thereby demanding the use of state of the art photolithography and etching equipment.
SUMMARY OF THE INVENTION
The invention relates to the application of blazed diffraction gratings to replace the optical wedge in IRFPA OCCM applications. In a simple embodiment, at least two gratings are used. The first grating, closest to the IRFPA (within 0.050 inch) primarily deflects the incoming radiation—currently the function of the optical wedge. This grating is a replacement for the one large wedge of the prior art and includes a plurality of side by side microwedges somewhat in the shape of sawteeth which result in a first grating. This first grating, in itself, resolves the coma problem since the large displacement of rays passing therethrough from the wide end to the narrow end has been eliminated. The coma is still present in each microwedge, however it is smaller since the wedges are smaller and the coma is further cancelled out by the use of a plurality of wedges. The first grating embodies a blaze angle, &thgr;, selected (ideally) to prevent both grating and focal plane specular reflections from passing back through the system optics.
The minimum deflection angle required to assure that no reflections will re-enter the system optical bundle is defined by:
&phgr;
min
=tan
−1
[(4×
f
sys
2
−1)
−1/2
+l
scan
/(2×
h
cs
)]
where &phgr;
min
is the minimum allowed deflection angle, f
sys
is 1/(2×NA
sys
) where NA
sys
is defined as the sine of the half-angle of the circularly symmetric optical cone focussed at the IRFPA, l
scan
is the length of the optical scan at the IRFPA surface and h
cs
is the cold shield height. This relationship also approximately defines the minimum blaze angle required if no grating reflections can be tolerated within the optical bundle. However, the grating condition can be relaxed somewhat due to its distance from the focal plane. The new condition is that the apparent reflectivity of the grating surface(s) remain below the specified maximum reflectivity allowed for the application. Apparent reflectivity is defined as that reflectivity which would be evidenced by a flat, specular surface placed normal to the ray bundle at a focal plane. To have an acceptable apparent reflectivity, one or more of three conditions must be met by a surface not at a focal plane, these being:
1. It must be specular and tilted with respect to the optical axis by an angle greater than that of any portion of the incoming ray bundle.
2. It must be well out of focus, or
3. Its actual reflectivity must be small.
If the first condition is met, the other two have no first-order significance since no rays can reflect out of the system. If condition 1 is not met, then some rays can exit the system. For this situation to be acceptable, the reflected bundle must either be highly divergent (condition 2) or must contain little energy (condition 3).
Under these conditions the minimum blaze angle requirement is replaced by a minimum distance from the grating to the focal plane and the blaze angle can be set based upon the deflection angle. The relationship to be satisfied is
z
min
≈(8×&dgr;×
f
sys
2
/&pgr;)×(
r
s
/r
app)
)
1/2
+F
2
/Z
where z
min
is the minimum distance of the grating from the focal plane, &dgr; is the design wavelength, f
sys
is the system f-number as defined hereinabove, r
s
is the grating surface reflectivity, r
app
is the desired apparent reflectivity, F is the system focal length and Z is the distance at which the system is focussed.
The relationship between blaze angle and deflection angle for a single grating is
sin &phgr;=
n×
sin[&thgr;−sin
−1
(sin &thgr;/
n
)]
for a grating located on the upper side of the substrate and
sin(&phgr;+&thgr;)=
n×
sin &thgr;
if the grating is located on the lower side of the substrate, where &thgr; is the blaze angle, &phgr; is the deflection angle and n is the index of refraction of the grating substrate. The preferred embodiment is with the grating on the upper surface of the substrate.
Blaze height is defined by the relation
h=&dgr;
/(
n
−1)
where h is the blaze height, &dgr; is the nominal wavelength for deflection and n is the index of refraction of the grating substrate. Selection of a substrate material with index of refraction near 2 will provide a blaze height requirement of about one wavelength.
Finally, grating period is defined by the relation
p=h
/tan &thgr;
where p is the grating period, h is the blaze height and &thgr; is the blaze angle.
The opposite face of the grating substrate may be anti-reflection coated or may support, for example, a cold filter bandpass coating, the second grating described hereinbelow or a microlens array to isolate regions of the IRFPA from irradiation, further limiting reflections. This surface must also satisfy the apparent reflectivity criteria discussed above.
The grating monochromatic aberrations are tolerable, so no correction is absolutely required. The grating chromatic aberration is quite large,
Kennedy Howard V.
Skokan Mark R.
Allen Stephone B.
Baker & Botts L.L.P.
Raytheon Company
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