Tracking telescope using an atomic resonance filter

A tracking system to enable a tracking telescope follow a light source, utilizes an atomic resonance filter positioned in close proximity to said telescope, and imaging optics positioned between the light source and the atomic resonance filter. The atomic resonance filter is positioned such that light signals from the light source pass through the imaging optics and are focused in a region within its interior. A plurality of optical sensors are positioned to detect re-emitted light signals from the atomic resonance filter. The optical sensors convert the detected re-emitted light signals to electrical signals proportional to the intensity of said re-emitted light signals. The telescope is repositioned by using these differences in electrical signals to equalize the re-emitted light signals detected by each of the optical sensors, thereby pointing the telescope directly at incoming light from the light source. Several configurations of the imaging optics, the atomic resonance filter and the optical sensors are possible.

BACKGROUND OF THE INVENTION 
This invention pertains generally to the field of tracking optical 
telescopes and, in particular, to the filters which select particular 
optical wavelengths to be tracked on. Specifically, it pertains to the use 
of an atomic resonance filter to serve this purpose. 
Tracking optical telescopes are commonly used to track moving bodies which 
directly or reflectively emit optical radiation. Frequently, the optical 
tracker employs an optical filter which selects a particular optical 
wavelength to track on, rejecting the rest of the optical spectrum. Such 
an optical filter aids the effectiveness of the tracking telescope by 
rejecting background light and increasing the signal-to-noise ratio (SNR) 
of the optical tracking signal. The standard interference optical filters 
used for selecting a narrow band of optical wavelengths preserve the 
imaging properties of the telescope. Consequently, these optical filters 
preserve the incident angle of the optical signal, which manifests itself 
as a position in the focal plane of the imaging optics. A suitable 
segmented or multi-array sensor can detect the displacement of the 
incident angle and emit electrical signals proportional to this 
displacement. A commonly used electronic servosystem can use these 
electrical signals to reposition the telescope so that the optical image 
is centered on the optical signal. Hence, the use of such a sensor permits 
the telescope to effectively track incoming light signals. 
Recently, the atomic resonance filter (ARF), comprised of vapors of 
specific atoms, has been developed, as a new type of narrow band optical 
filter. (Cf. "Atomic Resonance Filters", Jerry A. Gelbwachs, IEEE Journal 
of Quantum Electronics, Vol. 24, No. 7, July 1988, and U.S. Pat. No. 
4,829,597). Typically, an incoming signal of a specific wavelength 
entering the atomic resonance filter elevates the atoms therein into an 
excited state, which state then deploys in a two-or multi-step cascade, 
emitting light at different wavelengths. A suitable sensor collects the 
light signals at the new wavelength. An appropriate arrangement of optical 
cutoff filters before and after the cell containing the vapors renders 
this device a very effective narrow band optical filter. Unfortunately, 
this class of atomic resonance filters loses the imaging properties of the 
filters and, consequently, atomic resonance filters do not function well 
in a tracking telescope. 
SUMMARY OF THE INVENTION 
It is a principal object of the invention to provide a tracking system for 
a tracking telescope utilizing an atomic resonance filter which overcomes 
the shortcomings of the prior art. 
A tracking system to enable a tracking telescope follow a light source, 
comprises an atomic resonance filter positioned in close proximity to said 
telescope, and an optical imaging means positioned between the light 
source and the atomic resonance filter, in close proximity to said atomic 
resonance filter. The atomic resonance filter is positioned such that 
light signals from the light source pass through the optical imaging means 
and are focused in a region within said atomic resonance filter. A 
plurality of optical sensors are positioned to detect re-emitted light 
signals from the atomic resonance filter. The optical sensors convert the 
detected re-emitted light signals to electrical signals proportional to 
the intensity of said re-emitted light signals. The telescope is 
repositioned by using their differences in electrical signals to equalize 
the re-emitted light signals detected by each of the optical sensors, 
thereby pointing the telescope directly at incoming light from the light 
source. 
In a second aspect of the invention, the atomic resonance filter is 
partitioned into quadrants, and there is one one optical sensor per 
quadrant. 
In a third aspect of the invention, a tracking system to enable a tracking 
telescope follow a light source comprises an optical imaging means 
positioned between the light source and a fiber-optic bundle is positioned 
near the focal plane of the optical imaging means to collect the focused 
light from said optical imaging means. The fiber-optic bundle is 
subdivided into a plurality of sub-bundles and one atomic resonance filter 
is connected to each of the sub-bundles such that light signals from the 
light source passing through the optical imaging means and focused near 
the fiber-optic bundles are conveyed to each of the atomic resonance 
filters in proportion to the amount of light falling on the fiber-optic 
sub-bundles. At least one optical sensor is positioned after each of the 
atomic resonance filters to detect re-emitted light signals from the 
atomic resonance filters. The optical sensors convert the detected 
re-emitted light signals to electrical signals proportional to the 
intensity of the re-emitted light signals from the atomic resonance 
filters. The telescope is repositioned by using said electrical signals to 
equalize the re-emitted light signals detected by the optical sensors, 
thereby pointing the telescope directly at incoming light from the light 
source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic illustration of a first embodiment of an atomic 
resonance filter system for a tracking telescope. This embodiment relies 
on the short mean free path of the re-emitted light for its effectiveness. 
Referring to FIG. 1, incoming light 11 at an incident angle A passes 
through imaging optics 12, which can be any combination of lenses and 
mirrors positioned on the input side of an atomic resonance filter 14. 
Imaging optics 12 focus the incoming light signals 11 in a region 13 
located inside an atomic resonance filter 14. Imaging optics 12 cause the 
displacement of region 13 at a distance X from the center line 19 of 
atomic resonance filter 14 to be proportional to angle of incidence A of 
the incoming light signal 11. When angle A is zero, displacement X is 
zero. Since the purpose of a trackign system for a tracking telescope is 
to rotate or reposition the telescope until the displacement X is zero, 
thereby pointing the telescope at the incoming light signals, this 
invention is directed to providing a correction for this displacement. 
In region 13, all or most of the incoming light signals 11 are absorbed by 
the vapor inside atomic resonance filter 14. The vapors re-emit the light 
signals at new wavelengths 15a, 15b. Optical sensors 16a, 16b, positioned 
on the output side of atomic resonance filter 14 collect much of the 
re-emitted light signals, and in turn produce electrical signals 17a, 17b 
at their outputs. Optical sensors 16a, 16b are positioned on the output 
side of atomic resonance filter 14 to optimize the difference in the 
detected signals from the re-emitted light 15b, 15b. Because some of the 
re-emitted light is also absorbed by the vapors within atomic resonance 
filter cell 14, the amount of re-emitted light reaching the optical 
sensors 16a, 16b is proportional to the path length of the re-emitted 
light 15a, 15b travelling from region 13 to sensors 16a, 16b. The longer 
the path length of light signals 15a, 15b, the weaker the signals are in 
sensors 16a, 16b, respectively. If the optical light signals 11 are 
displaced in atomic resonance filter 14, there will be a difference in the 
path lengths of re-emitted light 15a, 15b, and consequently a difference 
in the electrical signals 17a, 17b outputted by sensors 16a, 16b. The 
servoelectronics of a tracking telescope system, according to this 
invention, use this difference in the detected signals 17a, 17b to 
reposition the telescope to equalize the light signals detected by each 
optical sensor, and thereby point the tracking telescope at the incoming 
light. Such servo systems are known in the art (Cf. G. J. Thaler, "Design 
of Feedback Systems," Dowden, Hutchinson & Ross, 1973). 
Since the differential signals 17a, 17b result from the physical properties 
of the vapors used in atomic resonance filter, the relative signal 
strengths of the detected signals 17a, 17b can be estimated from the 
properties of the vapors in atomic resonance filter 14. As a 
representative example, assume that: (1) the entering light is blue and 
the re-emitted light is red; (2) the blue light is absorbed approximately 
uniformly over region 13; and (3) the red light 15a, 15b is strongly 
absorbed over the path length between region 13 and sensors 16a, 16b. This 
is shown in the following formulation: 
##EQU1## 
where: .rho..sub.x =the maximum path length from region 13 to a specific 
sensor; 
.rho..sub.n =minimum path length from region 13 to a specific sensor; 
(.rho.)=the weighted means path length for the re-emitted light 15a or 15b 
from the region 13 to either sensor 16a or 16b, respectively; and 
.lambda.=the mean free absorbtion path for the re-emitted light. 
From this it follows that the signals in the two optical sensors 16a, 16b 
are: 
EQU S.sub.a .alpha..epsilon..sup.-.lambda.(.rho..sbsp.a.sup.) ; S.sub.b 
.alpha..epsilon..sup.-.lambda.(.rho..sbsp.b.sup.). 
The difference in the signals for the two optical sensors 16a, 16b is: 
##EQU2## 
Referring to FIG. 1, a typical cell of atomic resonance filter 14 may have 
the following dimensions: L=3 inches; a=3 inches; R=4.5 inches. The mean 
free path for absorption of the re-emitted light 15a, 15b is 1 inch. 
Calculated from the above equations, as a function of the displacement X, 
the difference in signals for the two optical sensors 16a, 16b is: 
______________________________________ 
X 0 1.00 2.00 3.00 .fwdarw. .infin. 
.increment. 
0 -0.72 -0.95 -0.99 .fwdarw. -1 
______________________________________ 
These differences are sufficient for tracking. 
This embodiment is not limited to the dimensions of the example presented 
above, or to the use of two optical sensors. A tracking telescope requires 
a minimum of three sensors 16a, 16b, and generally four are used. However, 
a matrix of many small sensors, such as photodiodes or CCD arrays, may 
also serve as the sensors for this tracking telescope system. 
FIG. 2 illustrates a second embodiment of the present invention in which 
the optical tracking telescope system utilizes an atomic resonance filter 
having a vapor cell which is optically partitioned into quadrants, each 
quadrant having its own set of optical sensors. In the design of FIG. 2, 
incoming light 21 at an incident angle A passes through imaging optics 22, 
which can be any combination of lenses and mirrors positioned on the input 
side of an atomic resonance filter 23. The incoming light 21 is focused by 
imaging optics 22 into an atomic resonance filter 23, which is optically 
divided into quadrants by a partition 24, to provide differential tracking 
signals. Partition 24 extends most of the way to the top of atomic 
resonance filter 23, but leaves a small gap to allow for equalization in 
the distribution of the vapors used in atomic resonance filter 23. 
Partition 24 serves to confine re-emitted light to the quadrant in which 
it originated. Each quadrant of atomic resonance filter 23 has its own 
optical sensor 25 on its output side to detect the re-emitted light. As 
described above with respect to FIG. 1, the amount of re-emitted light 
collected in each quadrant will be proportional to angle of incidence A of 
incoming light 21. The difference of the electrical signals from the 
output of four sensors 25 is used to steer a telescope so that the light 
is equally distributed among the four quadrants, and hence point the 
telescope at the incoming light. The imaging optics 22 focuses the 
incoming light 21 in such a manner that when the telescope is pointing at 
the light source, and the angle of incidence A of the incoming light 21 is 
0 (zero), equal portions of the incoming light will illuminate each of the 
four quadrants. 
FIG. 3 illustrates a third embodiment of the present invention in which a 
tracking system for a tracking telescope utilizes a multi-segmented 
fiber-optic bundle, wherein each segment of the fiber-optic bundle 
transmits incoming light into its own atomic resonance filter. In FIG. 3, 
the incoming light 31 passes through imaging optics 32 at angle of 
incidence A and is focused by said imaging optics 32 on to a fiber-optic 
bundle 33 which is positioned near the focal plane of imaging optics 32. 
Fiber-optic bundle 33 is split into a plurality of sub-bundles 34, only 
two of which are shown, to provide differential optical signals. Each 
fiber-optic sub-bundle 34 is connected to the input side of an atomic 
resonance filter 35. An optical sensor 36 is positioned on the output side 
of each atomic resonance filter 35 to detect the re-admitted light. The 
output signals 37 from optical sensors 36 are used to track the incoming 
light. 
In the embodiment of FIG. 3, the imaging optics 32 focuses the incoming 
light 31 slightly in front of the fiber-optic bundle 33 and consequently 
distributes the incoming light on fiber-optic bundle 33 such that the 
light signals in each of the sub-bundles 34 is proportional to the angle 
of incidence A of the incoming light 31. When the displacement X is not 
zero, more light is collected in one sub-bundle, and less in the others. 
The differences are detected to form error signals 37, which drive the 
telescope to zero the differences. These differential signals 37 are used 
to steer the telescope to point at the incoming light 31. When the angle 
of incidence A of the incoming light 31 is zero (0), the light signals in 
each of the sensors 36 are equal. 
At least three fiber-optic bundles 33 are required to track the incoming 
light, and typically a tracking telescope would use four segments. 
According to this invention, various embodiments may include any number of 
sub-bundles and each filter 35 having a corresponding sensor 36.