U.S. patent application number 11/409529 was filed with the patent office on 2008-08-28 for image null-balance system with multisector-cell direction sensing.
Invention is credited to David M. Kane, Phillip Selwyn.
Application Number | 20080205818 11/409529 |
Document ID | / |
Family ID | 39716005 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080205818 |
Kind Code |
A1 |
Kane; David M. ; et
al. |
August 28, 2008 |
Image null-balance system with multisector-cell direction
sensing
Abstract
A light beam is detected/localized by multisector
detector--quad-cell, or 5+ sectors handling plural beams.
Preferences: Beams focus to diffraction limit on the detector,
which reveals origin direction by null-balance--shifting spots to a
central sector junction, and measuring shifts to reach there. One
or more MEMS reflectors, and control system with programmed
processor(s), sequence the spot toward center: following a normal
to an intersector boundary; then along the boundary. One afocal
optic amplifies MEMS deflections; another sends beams to imaging
optics. After it's known which sector received a spot, and the beam
shifts, source direction is reported. The system can respond toward
that (or a related) direction. It can illuminate objects,
generating beams reflectively. Optics define an FOR in which to
search; other optics define an FOV (narrower), for imaging spots
onto the detector. The FOR:FOV angular ratio is on order of
ten--roughly 180:20.degree., or 120:10.degree..
Inventors: |
Kane; David M.; (Tucson,
AZ) ; Selwyn; Phillip; (Falls Church, VA) |
Correspondence
Address: |
PETER I. LIPPMAN
17900 MOCKINGBIRD DRIVE
RENO
NV
89506
US
|
Family ID: |
39716005 |
Appl. No.: |
11/409529 |
Filed: |
April 21, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US06/01056 |
Jan 12, 2006 |
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11409529 |
Apr 21, 2006 |
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60643867 |
Jan 13, 2005 |
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Current U.S.
Class: |
385/16 ;
356/141.2 |
Current CPC
Class: |
G02B 26/0833 20130101;
G01S 3/784 20130101 |
Class at
Publication: |
385/016 ;
356/141.2 |
International
Class: |
G01C 1/00 20060101
G01C001/00 |
Claims
1. Apparatus for detecting, and determining the direction of, an
incident beam of light; said apparatus comprising: a
multiple-sector-cell detector; optics for forming a sharply focused
image of such beam on the multiple-sector-cell detector; and means,
responsive to the multiple-sector-cell detector, for determining
the direction of origin of such beam.
2. The apparatus of claim 1, wherein: the multiple-sector-cell
detector is a quad-cell detector.
3. The apparatus of claim 1, wherein: the optics comprise means for
focusing such beam to a substantially diffraction-limited spot on
the detector.
4. The apparatus of claim 1, wherein: the determining means
interact with the detector in a null-balance mode of operation.
5. The apparatus of claim 4, wherein the direction-determining
means comprise means for: deflecting the sharply focused image to a
central position substantially common to the multiple sectors; and
measuring the amount of deflection required to move the image to
the central position.
6. The apparatus of claim 5, wherein: the deflecting-and-measuring
means comprise an array of one or more MEMS mirrors.
7. The apparatus of claim 5, wherein: the deflecting-and-measuring
means comprise at least one programmed processor for sequencing the
sharply focused image in a logical progression to the central
position.
8. The apparatus of claim 7, wherein the programmed processor
comprises programming for sequencing the sharply focused image to:
first, a boundary between two of the sectors; and then along that
boundary to the central position.
9. The apparatus of claim 8, wherein the programmed processor
comprises: programming for deflecting the beam to the boundary by a
first measured amount, along a track that is generally normal to
the boundary; and then deflecting the beam by a second measured
amount, along the boundary, to reach the central position.
10. The apparatus of claim 9, wherein the processor further
comprises: programming for interpreting the two measured amounts of
deflection to determine the position of the source of the beam.
11. The apparatus of claim 1, further comprising: an array of MEMS
mirrors.
12. The apparatus of claim 11, further comprising: a control system
for operating the MEMS mirrors to deflect the sharply focused image
along the detector.
13. The apparatus of claim 11, further comprising: an afocal
optical element for amplifying the deflecting produced by the MEMS
mirrors.
14. The apparatus of claim 5, further comprising: an afocal optical
element for amplifying the deflecting produced by the deflecting
means.
15. The apparatus of claim 1, further comprising: an afocal optical
element for directing such beam to the image-forming optics.
16. The apparatus of claim 1, wherein: the multiple-sector-cell
detector comprises at least five sectors.
17. The apparatus of claim 16, wherein: the at least five sectors
facilitate detecting, and determining the direction of, plural
incident beams of light.
18. A method for detecting, and determining the direction of, an
incident beam of light; said method comprising the steps of:
receiving the beam on a multiple-sector-cell detector; operating
the detector to determine which of the multiple sectors has
received the beam; and deflecting the beam by a measured amount,
along the detector, to reach a boundary between only two of the
sectors.
19. The method of claim 18, wherein the deflecting step comprises:
deflecting the beam along a track that is generally normal to the
boundary.
20. The method of claim 18, further comprising the step of:
deflecting the beam by another measured amount, along the boundary,
to reach a substantially central position substantially common to
the multiple sectors.
21. The method of claim 20, further comprising the step of:
interpreting the two measured amounts of deflection to determine
the position of the source of the beam.
22. The method of claim 21, further comprising the step of:
reporting, to a human operator or to an automatic apparatus, the
direction of the source of the beam.
23. The method of claim 22, further comprising the step of: further
detecting, determining the source direction of, and reporting, a
second beam.
24. The method of claim 22, further comprising the step of:
reacting to the beam detection by making a response toward the
direction of the source, or to a known related direction.
25. The method of claim 22, further comprising the step of: first
projecting light outward to illuminate an object and generate the
incident beam by reflection from the object.
26. The method of claim 25, further comprising the step of:
reporting, to a human operator or to an automatic apparatus, the
direction of the source of the beam.
27. The method of claim 26, further comprising the step of: further
detecting, determining the source direction of, and reporting, a
second beam.
28. The method of claim 20, further comprising the step of:
reacting to the beam detection by making a response toward the
direction of the source, or to a known related direction.
29. The method of claim 20, further comprising the step of:
projecting light outward to illuminate an object and generate the
incident beam by reflection from the object.
30. Apparatus for detecting, and determining the direction of,
plural incident beams of light; said apparatus comprising: a
multiple-sector-cell detector; first optics for defining a field of
regard within which to search for such beams; second optics for
defining at the multiple-sector-cell detector a field of view that
is within and smaller than the field of regard, and for receiving
such beam, if within the field of view, on the multiple-sector-cell
detector; and means, responsive to the multiple-sector-cell
detector, for determining the direction of origin of such beam.
31. The apparatus of claim 30, wherein: the field of regard and the
field of view respectively subtend angles whose ratio is on the
order of ten.
32. The apparatus of claim 30, wherein: the field of regard and the
field of view respectively subtend angles of roughly 180 and 20
degrees respectively.
33. The apparatus of claim 30, wherein: the field of regard and the
field of view respectively subtend angles of roughly 120 and 10
degrees respectively.
Description
RELATED DOCUMENTS
[0001] This document is based in part upon, and correspondingly
claims priority of, U.S. Provisional Patent Application 60/643,867.
Other related documents include, inter alia: [0002] Kane,
provisional application Ser. No. 60/433,301 and corresponding PCT
application PCT/US03/39535 "OPTICAL SYSTEM"; [0003] Kane,
provisional application Ser. No. 60/381,286 "MEMS BEAM STEERING AND
SHARED APERTURE OPTICAL SYSTEM", also incorporated by reference in
the provisional application and its corresponding PCT application
that are mentioned immediately above; [0004] Kane et al.,
application Ser. No. 10/142,654 "HIGH-SPEED, LOW-POWER OPTICAL
MODULATION APPARATUS AND METHOD", and three provisional
applications incorporated by reference therein, namely 60/289,883,
60/327,759 and 60/327,760. All are wholly incorporated by reference
into this document.
FIELD OF THE INVENTION
[0005] The invention relates generally to notifying people or
automatic equipment of an incident light beam, and its direction
and characteristics--and possibly developing a response--and more
specifically to use of quad cells (four-sector sensing cells) or
other multiple-sector sensing cells, for determining the direction.
The invention may passively detect a light beam that is generated
externally, or may originate an outgoing flash and detect a
reflected beam.
BACKGROUND
[0006] The availability of optical systems that dazzle or blind an
equipment operator, guide a beam-following vehicle to an equipment
platform, or laser-pinpoint an object of interest, represent a
threat to manned and unmanned apparatus. Ultimately, a technology
is required that can perform these functions: [0007] 1. separate
lasers from false-alarm light sources (natural or manmade) [0008]
2. determine the wavelength of the laser [0009] 3. determine the
location of the laser threat [0010] 4. determine the pertinent
laser event temporal and power characteristics for both CW and
pulsed lasers [0011] 5. cover a wide dynamic range of laser powers,
sensitive to energy levels many orders of magnitude lower than
those which are dangerous to vision.
[0012] Conventional detection systems--Current laser-warning
receivers are principally based on detecting where light falls on a
focal plane located behind a large field-of-view (FOV) optic. These
systems are relatively slow in their response, and relatively
inaccurate in terms of their line-of-sight (LOS) measurement of the
location of the incoming laser beam.
[0013] In addition, they are relatively bulky and heavy. Further,
existing laser-detection and -warning receivers provide little if
any spectral information.
[0014] Examples are the present laser-detection systems built by
companies such as BAE systems and Goodrich, particularly the
AN/AVR-2 Laser Detection Set 112 (FIG. 1). They are bulky (not
compatible with installation in an aircraft cockpit), highly
inaccurate in terms of determining location of a laser-beam source,
and yield no spectral information for the incoming beam.
[0015] Companies such as Princeton Scientific Instruments have
developed smaller laser-detection packages 113 that are compatible
with implementation in a small aircraft. Again, however, these
provide no directional or spectral information, and can only detect
average irradiance levels as low as 10.sup.-11 W/cm.sup.2.
[0016] Components not previously associated with laser warning
(except in our own work)--One such device is a four-sector detector
or quadrant detector, familiarly called a "quad cell". Prior to
mention in some of the Kane documents listed above, quad cells to
the best or our knowledge were not used in laser-warning systems
but rather were known primarily for light-beam position control in
industrial machinery.
[0017] In one of those earlier documents, the quad cell was said to
be inferior to a so-called "position-sensing detector" (PSD). A
quad cell is a detector with four discrete photosensitive sectors
arranged within a circular overall detection array, with corners of
each of the four sectors mutually adjoining at the center of the
circle.
[0018] Independent detection-signal leads from the four discrete
sensing areas are brought out separately to independent circuitry,
enabling detection and particularly quantitative comparison of
light levels incident on the four sectors. Conventionally such
comparison is used simply to find ratios of the radiant powers
reaching the different sectors.
[0019] Such ratios are assumed to be due to distribution of light
from a single common source, on the overall detector surface. Based
on that assumption, such ratios are conventionally used to directly
calculate direction of origin of the light.
[0020] To facilitate that kind of operation, conventional systems
defocus the incident light spot so that it spans, speaking very
roughly, about one-third or more of the overall sensor diameter.
The rationale is to provide that at least some of the light will
strike each one of the four sectors-thus enabling routine ratioing
operation based upon the assumption that none of the sectors
receives zero light.
[0021] Quad cell response is very fast, but the pointing accuracy
of such a conventional system is quite poor-particularly in a
low-light-level environment. This is because signal-to-noise
properties of such operation are distinctly unfavorable in
comparison with those of, e.g., a PSD also as conventionally
used.
[0022] Quad cells heretofore have been used in passive sensing
systems exclusively. Thus we are not aware of any prior usage of a
quad cell in a system which emits a probe flash and then analyzes
reflected return.
[0023] Another component previously unknown in laser-warning
systems, except our own earlier development efforts, is an array of
one or more very small mirrors, particularly microelectromechanical
systems ("MEMS") mirrors. The first significant commercial use of
such mirrors was the Texas Instrument Digital Light Projector (DLP)
MEMS array.
[0024] Formed in an array of 1,000-by-1,000 two-axis 10 .mu.m
mirrors, the bi-stable mirrors were controlled open-loop, with the
mirrors stepped from .+-.10.degree. locations at rates on the order
of 10 ms. The mirrors were not analog--more specifically, each one
could only take on one of two positions about either axis--and were
not particularly useful from a wavefront-correction
perspective.
[0025] A more closely related development in MEMS scan-mirror
arrays was in the area of optical switching, where the mirrors
could be controlled open-loop about one or two axes over the entire
range of mirror travel, and thus were "analog" in the sense of
being able to point the mirrors. Lucent in its "Waverunner" optical
switch, and Calient Networks, with its "3-D" MEMS-mirror optical
switch, are good examples of this technology.
[0026] These arrays are typically larger, from millimeters to
hundreds of millimeters, but have millisecond-level step-response
characteristics because they are controlled open-loop. Areal
densities of these arrays are also low, less than fifty percent;
therefore significant modifications to their architecture are
required to obtain an adequate array for an AMBS-quad system.
[0027] Conclusion--Accordingly the prior art has continued to
impede achievement of uniformly excellent laser-alert equipment,
and in particular has failed to make use of quad-cell and MEMS
technologies to enhance laser-alert capabilities. Thus important
aspects of the technology used in the field of the invention remain
amenable to useful refinement.
SUMMARY OF THE DISCLOSURE
[0028] The present invention introduces just such refinement. In
preferred embodiments the invention has several independent aspects
or facets, which are advantageously used in conjunction together,
although they are capable of practice independently.
[0029] In preferred embodiments of its first major independent
facet or aspect, the invention is apparatus for detecting, and
determining the direction of, an incident beam of light. The
apparatus includes a multiple-sector-cell detector. It also
includes optics for forming a sharply focused image of the beam on
the multiple-sector-cell detector.
[0030] (In the bodies of certain of the appended claims the word
"such" is used, in place of "the" or "said", when referring back to
terms introduced in preamble that are not part of the claimed
inventive combination but rather are parts of the environment or
context of the invention. The purpose of this convention is to make
particularly clear which recited elements are within the claimed
invention and which are not, thereby more particularly pointing out
and more distinctly claiming the invention. For example the phrase
"such beam" emphasizes that the light beam is not part of the
invention but only something in the environment to which the
elements of the invention are referred or referenced.)
[0031] In addition this first main aspect or facet of the invention
includes some means, responsive to the multiple-sector-cell
detector, for determining the direction of origin of the beam. For
purposes of breadth and generality in discussing the invention,
these means may be called simply the "determining means".
[0032] The foregoing may represent a description or definition of
the first aspect or facet of the invention in its broadest or most
general form. Even as couched in these broad terms, however, it can
be seen that this facet of the invention importantly advances the
art.
[0033] In particular, as will be recalled the conventional use of
multiple-sector-cell detectors (of which essentially the only known
type is a quad cell) has called for a broadly defocused beam image
("spot"), specifically to facilitate a quick and easy determination
of beam-source location. That determination normally is through
proportional response to energy found in the different quadrants
respectively, and the relatively large spot size makes the
likelihood very high that some radiant energy will be initially
found in at least two quadrants.
[0034] The relatively large spot size, however, carries with it
high opto-electronic noise, which in turn degrades the potential
pointing accuracy of the apparatus. Thus in the present invention
the sharply focused beam image is far less noisy and so can yield a
correspondingly far more accurate beam origin.
[0035] In practice of our invention the mechanism for locating that
source location, as will be seen, advantageously is something other
than the proportional response employed conventionally. This
mechanism does not rely upon initially finding irradiance in two or
more quadrants.
[0036] Although the first major aspect of the invention thus
significantly advances the art, nevertheless to optimize enjoyment
of its benefits preferably the invention is practiced in
conjunction with certain additional features or characteristics. In
particular, one relatively preferable condition--for some
situations--is that the multiple-sector-cell detector be a
quad-cell detector.
[0037] Another is that the optics include means for focusing the
beam to a substantially diffraction-limited spot on the detector.
Here the term "substantially" (i.e., "in substance") is included to
make clear that a mere minor or insubstantial departure from a
"diffraction limited" focus provides no escape from the scope of
the invention. In other words, it is not possible to design around
the protection provided by this document merely by using an
inconsequential variation from the diffraction limit.
[0038] Another basic preference is that the determining means
interact with the detector in a null-balance mode of operation. If
this preference is observed, then a subpreference is that the
determining means deflect the sharply focused image to a central
position, substantially common to the multiple sectors, and also
measure the amount of deflection required to move the image to the
central position.
[0039] If this subpreference, too, is observed, then a
still-further-nested subsubpreference is that the
deflecting-and-measuring means include an array of one or more MEMS
mirrors. Another such subsubpreference is that the
deflecting-and-measuring means include at least one programmed
processor for sequencing the sharply focused image in a logical
progression to the central position.
[0040] In this latter case it is yet further preferred that the
programmed processor include programming for sequencing the sharply
focused image to, first, a boundary between two of the sectors; and
then along that boundary to the central position. If this is so,
then it is still further preferred that the programmed processor
include programming for deflecting the beam to the boundary by a
first measured amount, along a track that is generally normal to
the boundary; and then deflecting the beam by a second measured
amount, along the boundary, to reach the central position. Even
further yet, if these latter program features are included, then
preferably the processor further includes programming for
interpreting the two measured amounts of deflection to determine
the position of the source of the beam.
[0041] The inclusion of an array of MEMS mirrors actually is also a
basic preference. When this preference is observed, the apparatus
also further includes a control system for operating the MEMS
mirrors to deflect the sharply focused image along the
detector.
[0042] When MEMS mirrors are included the apparatus also preferably
includes an afocal optical element for amplifying the deflecting
produced by the MEMS mirrors. This particular feature, the afocal
optic, is also applicable to many of the other combinations and
subcombinations of features and preferences discussed above. The
focal elements also directs the beam to the previously mentioned
image-forming optics.
[0043] As an alternative preference to the use a quad cell as
mentioned above, it is preferable that instead the
multiple-sector-cell detector include at least five sectors. These
five sectors, if present, facilitate detecting--and determining the
direction of--plural incident beams of light.
[0044] In preferred embodiments of its second major independent
facet or aspect, the invention is a method for detecting, and
determining the direction of, an incident beam of light. The method
includes the steps of receiving the beam on a multiple-sector-cell
detector.
[0045] It also includes the step of operating the detector to
determine which of the multiple sectors has received the beam. In
addition it includes the step of deflecting the beam by a measured
amount, along the detector, to reach a boundary between only two of
the sectors.
[0046] The foregoing may represent a description or definition of
the second aspect or facet of the invention in its broadest or most
general form. Even as couched in these broad terms, however, it can
be seen that this facet of the invention importantly advances the
art.
[0047] In particular, as can now be appreciated this second main
facet of the invention is complementary to the first, in that this
second facet provides the previously mentioned mechanism for
locating the beam origin. This mechanism avoids reliance upon the
energy proportioning which is basic to origin location in
conventional systems--and which, as explained earlier, is very
inaccurate.
[0048] Also this mechanism does not rely upon finding any initial
measurement state with radiant energy received in two or more
sectors. Furthermore the deflecting step departs very markedly from
prior-art procedures.
[0049] Although the second major aspect of the invention thus
significantly advances the art, nevertheless to optimize enjoyment
of its benefits preferably the invention is practiced in
conjunction with certain additional features or characteristics. In
particular, preferably the deflecting step includes deflecting the
beam (i.e. its focused spot) along a track or path that is
generally normal to the boundary.
[0050] If this preference is observed, then a further set of
preferences, most of them generally nested in this order, includes
the steps of: [0051] deflecting the beam by another measured
amount, along the boundary, to reach a substantially central
position substantially common to the multiple sectors; [0052]
interpreting the two measured amounts of deflection to determine
the position of the source of the beam; [0053] reporting, to a
human operator or to an automatic apparatus, the direction of the
source of the beam; [0054] further detecting, and determining the
source direction of, and reporting, a second beam; [0055]
reporting, to a human operator or to an automatic apparatus, the
direction of the source of the beam; [0056] further detecting,
determining the source direction of, and reporting, a second
beam.
[0057] Another preference that is basic, relative to this second
main aspect of the invention, includes the step of reacting to the
beam detection by making a response toward the direction of the
source, or to a known related direction. Yet another preference
includes the step of projecting light outward to illuminate an
object and generate the incident beam by reflection from the
object.
[0058] In preferred embodiments of a third major independent facet
or aspect, the invention is apparatus for detecting, and
determining the direction of, plural incident beams of light. The
apparatus includes a multiple-sector-cell detector.
[0059] It also includes first optics for defining a field of regard
within which to search for the beams. Furthermore the apparatus
includes second optics defining, at the multiple-sector-cell
detector, a field of view that is within and smaller than the field
of regard.
[0060] These second optics also form a sharply focused image of the
beam, if it is within the field of view, on the
multiple-sector-cell detector. Preferred embodiments of this third
main aspect of the invention also include some means, responsive to
the multiple-sector-cell detector, for determining the direction of
origin of the beam.
[0061] If desired these last-mentioned means maybe simply called
the "determining means". The foregoing may represent a description
or definition of the third aspect or facet of the invention in its
broadest or most general form. Even as couched in these broad
terms, however, it can be seen that this facet of the invention
importantly advances the art.
[0062] In particular, the nested field of view within the larger
field of regard facilitates discrimination of plural and even
multiple incident light beams from one another.
[0063] This two-field characteristic also makes it easier for the
invention to continue monitoring of one beam while also continuing
to watch for others. Moreover it enables an artificially generated
light beam, to be detected, to be more readily distinguished from
natural sources.
[0064] Although the third major aspect of the invention thus
significantly advances the art, nevertheless to optimize enjoyment
of its benefits preferably the invention is practiced in
conjunction with certain additional features or characteristics. In
particular, preferably the field of regard and the field of view
respectively subtend angles whose ratio is on the order of ten.
[0065] Also preferably the field of regard and the field of view
respectively subtend angles of roughly 180 and 20 degrees
respectively. An alternative preference is that they subtend angles
of roughly 120 and 10 degrees respectively.
[0066] The foregoing features and benefits of the invention will be
more fully appreciated from the following detailed description of
preferred embodiments--with reference to the appended drawings, of
which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0067] FIG. 1 is, in the upper view, Goodrich AN/AVR-2 laser
detection set, and in the lower view a helicopter equipped with a
Princeton Scientific package in an external mount;
[0068] FIG. 2 is a functional block diagram of a preferred
embodiment of a four-mirror prototype afocal MEMS beam-steering
system with a quad-cell sensor, according to the present
invention;
[0069] FIG. 3A is a like diagram, highly conceptual, for a
full-array apparatus according to the invention--and also showing a
beam search and capture sequence;
[0070] FIG. 3B is a like diagram illustrating the problem of
detecting and locating more than one incident beam;
[0071] FIG. 4 is a design drawing for a 120.degree. afocal lens,
suitable for the system of either one of FIGS. 3A and 3B;
[0072] FIG. 5 is a compound illustration including, in the upper
view, a diagram, highly conceptual, of three-layer MEMS mirror
array architecture; and in the lower view a block diagram of
functions performed by the array (numerical values in the blocks
being merely illustrative);
[0073] FIG. 6 is a set of two perspective or isometric views of
prototype MEMS mirrors, capable of independently controllable
motion in the tip, tilt, and piston directions--the upper view
being a single mirror 800 .mu.m square with control elements below,
and the lower view a two-by-two array of mirrors each 400 .mu.m
square;
[0074] FIG. 7 is a graph of AMBS noise sources, expressed in terms
of electrical current (A) as a function of sample frequency
(Hz);
[0075] FIG. 8 is a graph of performance of an afocal MEMS
beam-steering system, in terms of (1) signal-to-noise ratio S/N and
(2) angular measurement uncertainty .DELTA..alpha. in line-of-sight
("LOS") location of the incoming beam, both as a function of
irradiance--for 300 field of view, wavelength of 1 .mu.m, and a
silicon detector (120.degree. and higher FOVs are discussed
elsewhere in this document);
[0076] FIG. 9 is a pair of graphs showing solar-background
irradiance (W/cm.sup.2) vs. sensor zenith angle (radians, left-hand
view) and instantaneous field of view (IFOV in radians, right-hand
view);
[0077] FIG. 10 is a graph of solar-glint irradiance (W/cm.sup.2)
vs. IFOV (radians); and
[0078] FIG. 11 is a diagram, like FIG. 3A, showing search and
capture sequence for a multisector or "Ndrant" sensing cell
(particularly shown is an exemplary ten-sector cell).
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0079] Preferred embodiments of the invention include an afocal
MEMS beam-steering system with a multiple-sector sensing cell. The
multisector cell may have four sectors--i.e., may be a four-sector
cell or so-called "quad cell", as described in the "BACKGROUND"
section of this document--or may have a different number of
sectors. A higher number, in particular, facilitates independent
discrimination and analysis of two or more incident beams; however,
a sensor having three sectors is also workable and within the scope
of the invention.
[0080] The multisector cell (e. g. quad sensor) at least initially
sees the entire field-of-view ("FOV") and receives the incoming
laser beam, preferably focused sharply. Ideally it is focused to a
substantially diffraction-limited spot.
[0081] Use of a sharp focus departs very dramatically and
surprisingly from the conventional practice, introduced in the
"BACKGROUND" section, of defocusing the beam to facilitate
operation of a quad cell. The larger spot size, as mentioned
earlier, enables the system to find some optical energy in each
sector (quadrant), and thereby to drive the beam position rather
straightforwardly to an optical null-balance.
[0082] The larger spot, however, also produces more electrooptical
noise, and thus degrades the signal-to-noise ratio--and with it the
pointing accuracy of the cell. We have realized that far higher
signal-to-noise and therefore far finer pointing accuracy is
attainable by use of a sharply focused spot, and by employing other
means to reach the null-balance condition.
[0083] More specifically, the present invention finds the
null-balance point through a capture process that follows a logical
sequence, exploiting the availability of a programmed
microprocessor for driving a MEMS array of one or more mirrors to
deflect the beam. The result is superb pointing accuracy, orders of
magnitude finer than with a position-sensing detector (PSD) such as
favored in the previous Kane '535 PCT application mentioned
above.
[0084] Preferably the multisector cell receives the beam after
passage through an afocal optical element, reflection by the MEMS
scan-mirror array, and traversal of a focusing optic that provides
the sharp focus mentioned above. The afocal lens effectively
magnifies (in the space outside the optical system) the beam motion
introduced by the MEMS array.
[0085] We have developed a MEMS array for use in this invention, to
over-come many of the shortcomings described in the "BACKGROUND"
section. Features of this new array will be introduced shortly.
[0086] Responding to beam-location signals from the multisector
cell, a closed-loop control system steers the mirror array, driving
the focused laser to the central intersection of the multisector
cell in a time period on the order of 100 .mu.s. Knowledge of the
incoming laser-beam position is then automatically calculated from
the known angles of the MEMS array, and the known magnification by
the afocal lens.
[0087] With the incident beam location thus determined, all of part
of the incident beam is then diverted to a focal-plane array
("FPA") for further analyses, preferably including spectral
analysis. If desired, ongoing positional monitoring of a fraction
of the beam can be performed during the FPA investigation. This
dual functionality can be implemented by insertion of a beam
splitter, or by other techniques for tapping out a beam
fraction.
[0088] This approach offers significant advantages over alternative
detection approaches: [0089] excellent minimum incoming laser
irradiance sensitivity, 10.sup.-13 W/cm.sup.2 at 30 FOV, 10.sup.-2
W/cm.sup.2 at 120.degree. FOV; [0090] improved response time, much
faster than 100 .mu.s step response as limited by the MEMS array,
and 10 ns for the multisector cell alone; [0091] focal-plane array
(FPA) response time in tens of milliseconds; [0092] ability to
drive the incoming beam to a desired location within the local
system, and provide multifunctional capability; [0093] a secondary
sensor can be positioned to accept radiation from the incoming
laser and perform a spectral analysis on the beam; [0094] finer
uncertainty in the desired angle between the laser and the
line-of-sight (LOS)--limited by signal-to-noise ratio (SNR of 10
results in LOS uncertainty of 100 .mu.rad); in current systems by
comparison LOS uncertainty is limited by number of pixels, and an
array of 5,000 pixels by 5,000 pixels is required to achieve 100
.mu.rad uncertainty over 30.degree. FOV; [0095] a
spectral-broadband measurements in the range of 0.5 to 5 .mu.m are
possible in a two-detector shared system; and [0096] our system is
not limited to passive sensing, as was customary with prior quad
cells, but rather is entirely compatible with active systems that
emit a probe light flash and then analyze the reflected return.
[0097] Our system is also not limited to a four-sector or
"quadrant" cell but rather encompasses use of an "Ndrant" or
generalized multisector cell which facilitates detection and
tracking of plural incident light beams concurrently.
[0098] The invention contemplates system-level architecture and
performance for an AMBS-multisector-cell detection system,
leveraging preexisting MEMS arrays.
[0099] AMBS-multisector-cell-sensor overview--Preferred embodiments
of this system include five primary elements: an afocal lens 1
(FIG. 2) magnifies the circular field of regard 7 (i.e., angular
region of interest outside the optical system) that can be
addressed by the two-axis MEMS scan-mirror array 2. The field of
regard typically subtends between thirty and one hundred twenty
degrees, or greater.
[0100] Tip motion Y is rotation about the Z axis. Tilt motion X is
rotation in and out of the plane of the page. So-called "piston"
motion is direct elevation or retraction normal to the plane of the
array 2.
[0101] A reimaging lens 3 brings the incoming laser radiation 8 to
a sharp focus. The focal spot, on the silicon or other
multisector-cell sensor 4, is preferably diffraction limited.
[0102] The multisector-cell sensor can be a quad cell, i.e. can
have four sectors; alternatively it can have only three sectors, or
can have five or more. Use of an even number of sensors is favored
to somewhat simplify the microprocessor sequential logic employed
in driving the spot to the central position, since signals from
diametrically opposite sectors are most straightforwardly
balanced.
[0103] The sensor is used first to detect incoming laser
irradiance, commonly in an essentially static so-called "staring"
mode, and usually with the apparatus pointed straight out along the
central axis of the afocal lens, to survey the entire initial field
of view (FOV). Alternatively the initial operation may be in a
repetitive-scanning mode--or after a moving source is first
detected may generally follow the source. Then, after detection of
incoming irradiance, the sensor is used to determine incoming laser
line-of-sight location in a null-balance mode.
[0104] The latter is accomplished by driving the MEMS mirrors to
force the focused laser spot to the center position on the quadrant
detector, while monitoring the corresponding MEMS-mirror positions.
The driving of the spot to the center is accomplished in two stages
as outlined earlier, and as will be further detailed shortly.
[0105] Overall system closed-loop control is accomplished with a
combined digital-analog controller 5 that operates in a
proportional-integral-derivative (PID) mode. The scan angles
.theta..sub.X and .theta..sub.Y, read from the controls of the MS
scan-mirror array 2 (similar to the greatly enlarged prototype
four-mirror subarray), then yield the incoming laser beam LOS
relative to the AMBS assembly.
[0106] In accordance with preferred embodiments of this invention,
the quad cell (or other multisector cell) can observe the entire
FOV (FIG. 3A). The first step in the so-called "capture sequence"
15 is the search mode 11, 12 in which the irradiance is initially
detected in, e. g., sector #1 of the detector face 4 (shown
enlarged, 15).
[0107] The MEMS mirror array next drives 12 in a vertical (as
illustrated) direction, i.e. parallel to the boundary between
sectors #1 and #4, to sector #2--until the laser is detected by
that sector. The apparatus sensitively positions the focused beam
to equalize, as nearly as possible, the signal from the two sensor
sectors #1 and #2. Since the spot has been made extremely small (to
its diffraction limit), any error in this balance can be made
insignificant.
[0108] Then, with the irradiance spot spanning 16 the boundary
between sectors #1 and #2, the MEMS array drives 13 the spot along
that boundary to the intersection of all the sectors (thus in a
quad cell to the central intersection of the four quadrants) and
maintains a continuing lock on that position 14. The MEMS-mirror
angles, known to <1 mrad relative to the sensor line--of sight,
provide the incoming laser beam angle.
[0109] A significant benefit of the AMBS multisector-cell sensor is
its ability to detect and address multiple engagements 25 (FIG. 3B)
or in other words multiple incident light beams. As with a single
engagement, the individual sharply focused spots 11, 21 impinge
upon the multisector cell--which initially, in most cases, is in
staring mode.
[0110] If the arrivals of the two beams 8, 28 are sufficiently
separated in time, particularly by an interval greater than the
duration of the capture sequence, then discrimination of the two
sources (and their respective focused spots 11, 21) is greatly
simplified. In that case, with respect to the first beam 8 the
system can already be in a distinctly different mode of operation
(the lock mode) when the second beam 28 (FIG. 3B) is focused 29
onto the multisector-cell sensor 4 and thereby detected.
[0111] Programming is very easily made sophisticated enough to
memorize the location of the first spot 11 while performing a
second capture sequence for the second spot 21. Since operating the
mirrors to move the second spot 21 also moves everything else in
the field of view, the first spot 11 is shifted away from the
central intersection during the second capture sequence. One of the
many logical-processing options includes then reverting to a check
of the first spot position, which may have changed during analysis
of the second spot.
[0112] If the detector is made to see a small field of view (e. g.
with a zoom focusing lens 3) and if it is desirable to minimize
confusion of the detection system by the first spot during scanning
for other light sources, then a lock on the first spot 11 can be
maintained with some of the mirrors in the MEMS array while
diverting the remaining mirrors to the second capture sequence for
the second spot 21. (This option is available only for arrays
having more than one mirror.) Additional beams arriving later can
likewise be captured and locked.
[0113] Discrimination between the identities and effects of the
different beams can be enhanced in various ways. For example in
some special situations the system can impose small positional
modulations on the different beams, but at different
frequencies--and can apply synchronous detection to keep track of
them independently.
[0114] Whether or not the beams arrive simultaneously, in the case
of adequate incoming LOS angle between the beams the focus falls
onto different sectors (e. g. quadrants). Logic in the system
distinguishes this condition, for initially only one quadrant would
be illuminated for a single incident beam.
[0115] In response to substantially similtaneous plural arrivals, a
capture sequence similar to that described above is invoked. The
laser beam focused into a particular arbitrarily chosen sector (e.
g., quadrant #1) is driven to the central intersection first, and
then the second beam is likewise driven to that intersection
later.
[0116] Generally speaking, the number of different logical
situations and logical-processing strategies in use of our
invention can become rather high and quite complicated, depending
on many factors. Such factors include the angles between beams,
which portions of the multisector-sensor cell intercept the various
beams respectively, the kinds and purposes of the beams, the kinds
and motions of platforms, the political and other practical
relationships between the different beam-source platforms, and
whether the host platform of our invention is staffed or only
automatic equipment.
[0117] This list is not complete; indeed yet many other
considerations can come into play. Therefore it is not possible to
definitively state what the best logical-processing sequences are
likely to be. Given the information in this document, however,
people of ordinary skill in this field and especially programmers
can develop logical processing appropriate to the applicable
particular combinations of all the known considerations.
[0118] As noted above the detector is not necessarily a quad cell
but may instead have "N" sectors, where "N" is a number other than
four. In such a case the multisector cell may be very loosely
denominated an "Ndrant".
[0119] The probability of plural spots 11, 21 falling fortunately
into different sectors is enhanced by building the sensor with more
than four sectors. Increasing the number of sectors thus
facilitates collecting additional information to support the
detection and LOS measurement of simultaneous plural incident beams
8, 28. Operation with an Ndrant sensor is detailed later in this
document.
[0120] Table 1 provides a predicted AMBS-quad-sensor performance
summary of minimum irradiance-detection capability and associated
angular-measurement uncertainty, for the incoming laser-beam. Given
the information in this present document, extension to the
multisector-cell case is straightforward for any particular "N" of
sectors; indeed, in Table 1 only the right-hand column is variable
with number of sectors.
[0121] The data of Table 1 analyze silicon and
mercury-cadmium-telluride detectors at 30.degree. and 120.degree.
FOV for wavelengths of 0.4 .mu.m, 1 .mu.m and 5 .mu.m. A broadband
0.5 to 5 .mu.m operating range is realistic.
[0122] The MEMS array is already broadband, with a gold or silver
coating. A dichroic beam splitter separating the bands spectrally
allows a single system to feed both a visible-region silicon
detector and an infrared mercury-cadmium-telluride detector.
TABLE-US-00001 TABLE 1 AMBS-quad-sensor performance sensor
configuration minimum LOS angular (all angles are wavelength
.lamda. irradiance uncertainty FOV at 1 kHz) (.mu.m) (W/cm.sup.2)
(mrad) 1 Si detector, 30.degree. 0.4 2 10.sup.-13 0.5 2 Si
detector, 120.degree. 0.4 2 10.sup.-12 2 3 Si detector, 30.degree.
1 8 10.sup.-14 1 4 Si detector, 120.degree. 1 1 10.sup.-12 4 5 MCT
detector, 30.degree. 1 9 10.sup.-11 1 6 MCT detector, 120.degree. 1
1 10.sup.-9 6 7 MCT detector, 30.degree. 5 3 10.sup.-11 6 8 MCT
detector, 120.degree. 5 5 10.sup.-10 30
[0123] Preliminary performance requirements for an operational
visible silicon-based AMBS quad sensor, or multisector cell, appear
as Table 2. TABLE-US-00002 TABLE 2 AMBS-sensor top-level
performance requirements Parameter Operational System 1 field of
view 120.degree. circular 2 detection wavelength .lamda. = 0.5 to 1
.mu.m 3 minimum detectable irradiance level 1 .times. 10.sup.-13
W/cm.sup.2 (MDIL), S/N = 1 (at .lamda. = 1 .mu.m and 1000 Hz
bandwidth) 4 angular uncertainty at MDIL <1 mrad (1.sigma. at 1
.mu.m) 5 MEMS closed-loop step response, <4% <100 .mu.s
[0124] The AMBS-multisector-cell-sensor system of this invention
has five major functional elements, introduced above. Table 5
outlines relevant system parameters for those elements. Some
details of these elements follow.
[0125] The afocal lens 1 (FIG. 2) is similar to other AMBS lenses
discussed in the earlier patent documents mentioned in the
"BACKGROUND" section. It can take the form of a 120.degree. FOV
afocal lens design with entrance pupil 31 (FIG. 4) of diameter
3.333 mm, first stage 32 of focal length f=20 mm, and second stage
33 of focal length 3f. This design is intended for coupling to a
two-axis MEMS array 34 having a mechanical scan angle of
.+-.10.degree. and .phi.=14 mm, a beam splitter 35 disposed along a
substantially collimated segment of the optical path, a detector
reimaging lens 36 of focal length f.sub.D=35 mm, and a
multisector-cell detector 37.
[0126] Afocal lens and beam-splitter assemblies have been
demonstrated and in some cases are commercial, off-the-shelf
("COTS") modules. We have not yet performed custom development of a
front-end design for a final operational embodiment of our
invention, but believe that such development is wholly
straightforward. TABLE-US-00003 TABLE 3 Operational MEMS
scan-mirror array design summary Parameter Requirement 1 individual
scan-mirror clear aperture 1 mm .times. 1 mm 2 total mirrors in
array, distributed along X-Y array of 36 .times. 50 X-Y axis with
all scan axes parallel 3 mechanical scan angle about .theta..sub.x,
.theta..sub.y .+-.8.degree. 4 minimum first mode frequency about
.theta..sub.x, >1000 Hz .theta..sub.y and z 5 minimum mirror
radius about either >5 m X or Y axis 6 mirror reflectivity,
.lamda. = 0.5 .mu.m to 5 .mu.m >95% 7 mirror to substrate areal
density >95% 8 embedded rotational sensor requirements 8.1
angular range about .theta..sub.x and .theta..sub.y .+-.8.degree.
8.2 angular resolution about .theta..sub.x and .theta..sub.y 150
.mu.rad 8.3 angular measurement error about .theta..sub.x and 150
.mu.rad .theta..sub.y, 1.sigma. 8.4 measurement bandwidth 100 kHz 9
scan-mirror control 9.1 step overshoot after settling time <4%
of step size 9.2 time to settle within allowable error <100
.mu.s 9.3 MEMS voltage driver CMOS on-chip or equivalent 9.4 MEMS
local-loop control 10 kHz closed-loop control 9.5 electrical
command interface digital serial 10 environment 10.1 operational
temperature -30 to 80.degree. C. 10.2 shock loading TBD
[0127] As already noted the MEMS scan-mirror array 2 is a two-axis
array. For prototype work--i.e. during ongoing development--such an
array advantageously has all electronics, other than the embedded
capacitive rotation sensors, off-chip.
[0128] In a final operational system, however, all drive
electronics, high-voltage amplifiers and inner-loop PID controllers
for each mirror in the array are very advantageously on-chip (as
opposed to separate boxes of electronics off-chip). Ideally each
mirror in the array is independently addressable through a serial
interface.
[0129] Table 3 outlines requirements and goals for the MEMS
scan-mirror array. Commands to the array are ideally applied
through a digital serial interface 46 (FIG. 5), with its elements
51-59--resulting in output mirror motion 47. A final operational
array should have the following on-chip functions: [0130]
multiplexing (within the MEMS beam-steering controller 44, FIG. 5
lower view) [0131] demultiplexing 51 [0132] calibration look-up
table 53 [0133] D/A converter 55 [0134] proportional, integral,
derivative (PID) controller 57 for each mirror [0135] high-voltage
HEMS actuator driver 58 [0136] embedded rotation sensors 59.
[0137] Immediately below the array of mirror pads 41 is a physical
layer 42, which may be called the "MEMS actuators and embedded
rotation sensors" layer. This layer 42 includes the
actuators-and-sensors block 59.
[0138] Within the interface 46, mirror motion is also fed back 47'
to the PID controllers. This return serves particularly to
implement the integral and differential aspects of the control--as
is generally understood in the related field of electronic control
systems, and accordingly is not further detailed here.
[0139] Below the actuator/sensor layer 42 is another physical layer
43, which includes in particular a CMOS mixed-signal PID controller
57 and high-voltage circuits 58. Remaining circuit blocks 44, 51-56
may be distributed as between the lower two physical layers 42, 43;
or the main-logic controller 44 may be elsewhere in the chip.
[0140] The controller 44 sends multiplexed commands, for all the
mirrors, to the submirror layers 42, 43. After demultiplexing 51,
the system carries control data 52 for each mirror
independently.
[0141] Each mirror, furthermore, has been calibrated independently.
The calibration, stored in and applied from a lookup table ("LUT")
53, considers not only mechanical variations within the mirror
actuators and sensors 59, but also optical nonlinearities and
variations elsewhere in the system, particularly in the afocal lens
1. The many individual mirror-control signals from the LUT 53
accordingly are corrected for all known perturbations from ideal
operation.
[0142] The remainder of the system 54-59, 47, 47' too--although
illustrated as unitary--is multiple, i.e. provides a separate,
independent control-signal channel for each mirror. Following the
digital-to-analog converter block 55, analog mirror-control signals
56 flow to the individual PID control blocks 57.
[0143] These analog signals 56 control electrical signals from the
earlier-mentioned high-voltage block 58, which in turn produce
mechanical signals from the previously mentioned
actuators-and-sensors block 59. These mechanical signals physically
move 47 the mirrors.
[0144] We have developed prototype MEMS can-mirror units (FIG. 6)
with tip, tilt and piston capability--fabricated for us by a
vendor. They have very high fill-factors.
[0145] One such mirror unit (upper view) has extended pads 142 for
electrical characterization in the prototype phase, fully covered
with a low-inertia micromirror 141, 800 .mu.m (0.8 mm) square.
While the entire mirror is plainly very thin for minimum inertia, a
particularly remarkable feature of the design is that each of the
visible side faces 144 in actually a thicker, stiffening truss, 15
.mu.m tall. Another completed prototype is a two-by-two array of
actuators 243 (lower view), 0.4 .mu.m on a side, with a two-by-two
array of micromirrors 241 batch transferred.
[0146] This implementation of the actuators is based on preengaged
vertical comb drives in silicon-on-insulator ("SOI") format, and a
gimballess de-sign demonstrated previously in large tip-tilt
devices. (This design actually does have gimbals of a sort, but not
macroscopic ones; they are truly microgimbals, each supporting just
one of the micromirrors in the array.)
[0147] The fabrication process is derived from the multilevel-beam
SOI-MEMS process. For small array elements, back-side etching can
be eliminated.
[0148] Three masks are used for deep-reactive-ion etching ("DRIE")
into the device layer of an SOI wafer, to achieve isolated sets of
vertical comb drives with "up" or "down" actuation. Low-inertia
micromirrors are fabricated in a separate SOI wafer in a
three-level selective DRIE process.
[0149] Individual thinned micromirror plates, stiffened by a
backbone of thicker trusses (including the side walls as mentioned
above), were transferred and bonded onto the actuators using
custom-fabricated microtweezers. Batch bonding and alignment of
multiple mirrors for large-scale, high-fill-factor arrays is a
preferred technique for fabricating our invention.
[0150] Adequate illuminance sensitivity for meaningful deployment
of our invention requires transferring sufficient optical input
power from the afocal lens to the multisector-cell detector. Our
quantitative analyses of this system translate this requirement
into a specification for MEMS array size of roughly 36.times.50 mm,
assuming a favorable fill factor that is well over 95%.
[0151] As indicated above, prototype small elements (one unit
mirror, and a two-by-two prototype subarray) of a MEMS array have
been demonstrated, but a full-size array (e. g. 36.times.50 mm as
just noted) remains to be developed. We estimate the cost of such
an effort--for an array with all electronics on-chip--at roughly $2
million to $4 million.
[0152] As to the reimaging lens at the multisector cell, tradeoffs
are advisable regarding f/number and minimum detectable irradiance
level (MDIL). Given that MDIL is a driving requirement for best
overall performance, the smallest possible f/number design should
be developed to minimize the required quadrant-detector size.
[0153] Noise-equivalent power ("NEP") is driven by detector size,
due to the resulting resistance and capacitance. We consider a
custom reimaging lens to be most highly preferred, although
commercial alternatives bear investigation.
[0154] Coming to the multisector cell detector itself, one
preferred embodiment of the invention uses a commercially available
silicon quadrant detector (quad cell)--particularly as this
configuration can be fabricated most promptly and, probably, at
lowest cost. Currently, the largest commercial detector that has
been found with the required low-noise characteristics is the 7
mm.times.7 mm Photonic Detectors Inc. model PDB-C206.
[0155] In the longer term, and with particular emphasis on ability
to sense and locate plural (even multiple) incident light beams,
the "Ndrant" (i.e., higher-order multisector) forms of the sensor
cell are more highly preferred. Although a quad cell, and even a
lower-order (i.e. tri-sector) cell, can deal with plural beams, an
Ndrant of six, eight or more sectors offers greater speed,
stability and pointing accuracy.
[0156] A key parameter in usefulness of a laser-alert system is
minimum detectable irradiance level ("MDIL"), and this in turn
depends strongly on signal-to-noise ratio ("S/N" or "SNR"). We have
prepared a MathCad model of SNR in our sensor system, with these
five noise contributors: [0157] 1. feedback-resistor noise
I.sub.rf.sub.--.sub.OpAmp 64 (FIG. 7); [0158] 2. interelectrode
resistor noise I.sub.R.sub.--.sub.Det 63; [0159] 3. dark-current
noise I.sub.dark.sub.--.sub.current 62; [0160] 4. voltage amplifier
noise current I.sub.voltage.sub.--.sub.Amp 61; and [0161] 5.
shot-noise current I.sub.shot 65.
[0162] It is essential to consider the currents associated with
each of these noise terms as a function of sample frequency, the
total root-sum-square ("RSS") noise current, I.sub.Tot 67 and
particularly in comparison with the signal current I.sub.Laser 66
(a constant current level in FIG. 7) from an average laser
irradiance of, typically 10.sup.-13 W/cm.sup.2 at .mu..sub.m=0.4
.mu.m (Table 4).
[0163] Thus as shown the signal 66 when compared with the total
noise 67 yields, for this case, SNR exceeding unity at sample
frequencies under 700 Hz.
[0164] The detector-amplifier voltage noise 61 is a function of
sample frequency, FOV, MEMS mirror angle, the op-amp voltage and
feedback resistance, entrance-pupil aperture and f/number. The
dark-current noise 62 is a function only of sample frequency and,
of course, dark current.
[0165] The interelectrode resistor noise 63 is a function of the
frequency, temperature, FOV, MEMS angle, and again the aperture and
f/number. Feed-back-resistor noise 64 is a function of frequency,
temperature, and the op-amp feedback resistance.
[0166] The signal 66 is a function of the laser irradiance and
wavelength, FOV, mirror angle, and the aperture. The shot-noise
current 65 depends upon those same parameters and the sample
frequency.
[0167] In all configurations, the detector-amplifier voltage noise
61 ultimately dominates the noise terms at higher frequency, driven
by the detector capacitance and resistance. This in turn drives the
design to minimize the resulting detector size, for it determines
the performance limit. TABLE-US-00004 TABLE 4 SNR-model example
sensor configuration Parameter Value pupil diameter 40 mm
wavelength 0.4 .mu.m FOV 30.degree. MEMS mechanical scan angle
.+-.10.degree. temperature 77.degree. K afocal magnification 0.75
reimaging-lens f# 0.25 reimaging-lens focal length 24 mm quad-cell
diameter 18 mm quad-cell capacitance 3 .times. 10.sup.-10 Farad
quad-cell resistance 1.3 .times. 10.sup.7 .OMEGA. quad-cell
parallel resistance with 1.2 .times. 10.sup.7 .OMEGA. amplifier
MEMS array size 71 mm .times. 100 mm
[0168] We have analyzed MDIL performance for the
AMBS-multisector-cell-sensor system, for 30.degree. and 120.degree.
FOV systems respectively, for .lamda.=0.4 .mu.m and 1 .mu.m
assuming silicon detectors, and .lamda.=1 .mu.m and 5 .mu.m
assuming mercury-cadmium-telluride detectors. Performance for the
silicon detector at .lamda.=1 .mu.m (FIG. 8) is noteworthy.
[0169] Plotted against laser irradiance on a log-log scale, angular
measurement uncertainty 71 in the incoming LOS, for f/0.25, appears
as descending straight lines at about 45.degree. (with
corresponding SNR 72 at about the same slope but ascending). These
modeling results were obtained for measurement bandwidth of 1
kHz.
[0170] Minimum detectable irradiance level occurs for SNR >1, or
in other words at SNR greater than the unity level 79. The ordinate
scale in FIG. 8 is the SNR only; in other words, LOS uncertainty is
not marked on the graph--but at top that uncertainty is very high,
1.11710.sup.3; and at bottom, 4.36710.sup.-6.
[0171] Similarly LOS uncertainty 73 for f/0.5 is roughly one-half
order lower, with corresponding SNR 74 one-half order higher.
Yielding like results but with still-higher LOS uncertainty and
lower SNR are the same four data sets 75, 76 and 77, 78
respectively, but assuming measurement bandwidth of 10 kHz.
[0172] Thus four configurations were modeled: f/numbers of 0.25 and
0.5, with measurement bandwidths of 1 kHz and 10 kHz--and with
entrance-pupil diameters of 2.5, 20 and 40 mm. For 2.5 mm diameter,
the entrance pupil 80 corresponds to the crossover points between
the SNR and corresponding LOS-uncertainty curves, at laser
irradiance of roughly 610.sup.-17.
[0173] While most of our analyses discussed in this document draw
attention to theoretical sensitivity, another very important set of
criteria relates to ability of the invention to discriminate
between artificial light-beam sources and natural
sources--especially important when the latter are equal or greater
in brightness or irradiance, or both. Thus natural sources pose a
potential for false alarms, as well as for blocking our invention
from generating its full expected response to incident laser beams
and the like.
[0174] Some operational environments for our invention are
relatively remote from the earth--for example, in high orbits
suited to space-station operations and even interplanetary
platforms. Other environments encompass near-earth aircraft
operation and even earth-based stationary facilities.
[0175] Each of these operating environments is susceptible to its
own respective interferants. Thus for instance deployment of our
invention in combat aircraft may be more vulnerable to sunlight
reflection from the ground or from water, and other kinds of
deployment may be more readily disrupted by sunlight received
directly.
[0176] Particularly illustrative of system discrimination
capability is the d. c. irradiance at the sensor entrance pupil due
to solar energy reflected from the ground, in the wavelength range
850 to 905 nm (our "Laser BeamRider" regime). Naturally this
distracting source is progressively less significant as the sensor
is pointed higher, accounting for the down-ward slopes of the
curves 83, 84 (FIG. 9) for 10 and 4 mrad respectively.
[0177] Both curves are impressive, showing system-response numbers
lower than the laser irradiance limit 81 by an order of magnitude
and more--for sensor zenith angle only one to three times the
minimum zenith angle 82 for the sun itself. (As the IFOV or zenith
angle increases, the area imaged by the sensor increases, and
resulting solar background irradiance also increases.)
[0178] Also promising are the solar background irradiances 88, 87,
86 as a function of sensor IFOV for earth reflectivities of 10%,
20% and 40% respectively--at a sensor zenith angle of 10.degree..
These values too are substantially below the laser irradiance 81.
Since we want to detect irradiance on the order of
1.times.10.sup.-13 W/cm.sup.2 with a field-of-view of 120.degree.,
temporal filtering is required.
[0179] An analogous mode of interference is solar glint from water,
which also potentially generates system false alarms. Glint is a
function (FIG. 10) of sensor field-of-view and solar zenith angle.
Given that the sun is an extended source of 10 mrad, it can be
discounted as a false alarm. Low-pass temporal filtering will
eliminate glint as a noise source.
[0180] Our invention relies upon a programmed AMBS control unit,
customized to include closed-loop input from the multisector-cell
detector and laser-capture logic. In prototype work the control
unit typically includes a PC operator interface, a high-speed
digital FPGA command to the MEMS array, and a D/A interface to an
analog PID controller and MEMS voltage driver. For production of an
operational system, in the interest of enhanced compactness,
reliability and speed these elements are ideally incorporated into
an ASIC with custom operator interface.
[0181] Suitable quad-cell detectors for final, operational practice
of our invention are available on a COTS basis. Nevertheless this
component of the invention has two characteristics that we have not
fully resolved:
[0182] First is the problem of detecting low-level laser signals
against large background noise sources. A full evaluation of this
problem remains TABLE-US-00005 TABLE 5 AMBS-quad-sensor operational
approach AMBS-quad- sensor element and parameter Value 1 afocal
lens 1.1 field of view 120.degree. 1.2 pupil diameter 6 mm 1.4
magnification 6:1 2 MEMS scan-mirror array 2.1 MEMS scan-mirror
array size 36 mm .times. 50 mm 2.2 MEMS mirror size 1 mm .times. 1
mm 2.3 MEMS scan angle, mechanical 2-axis, .+-.5.degree. 3
reimaging lens 3.1 f# 0.5 3.2 focal length 18 mm 4 quadrant
detector, silicon PDI, PDB-C206 4.1 detector size 7 mm .times. 7 mm
(16.degree. FOV) 4.2 detector performance, NEP @ 1000 Hz
.apprxeq.10.sup.-12 W 5 AMBS control unit 5.1 user input and
control digital controller 5.2 closed-loop control PID control
to be performed. We strongly believe that such an evaluation should
begin with noise-sensitivity analyses, and demonstration of system
operation with a variety of background noise sources.
[0183] Second, we have not yet elaborated the Ndrant (lower- and
higher-order multisector cell detector) aspects of the invention to
the same extent as the quad cell. A basic analysis of such a
detector and its capture-sequence details follows here:
[0184] In principle the number of sectors may be any number greater
than two. Higher numbers of sectors, however, facilitate detecting
and localizing greater numbers of incident light
beams--concurrently or even simultaneously.
[0185] The ideal number depends upon the probable number of
incident beams that may be encountered, and their probable angular
separation. The principal limiting considerations are the cost of
manufacture and the resulting complexity of the electronics and
logic.
[0186] Very generally these adverse factors are minor in comparison
with other costs and complications, particularly since the
electronics are usually implemented in monolithic form--and
particularly when balanced against potential loss or damage of
equipment if an incident beam escapes detection. Hence a preferred
number of sectors is typically in the range of ten to one hundred,
inclusive.
[0187] Accordingly, use of a multisector-sensor cell with ten
sectors, #1 (FIG. 11) through #10, can greatly aid in timely
detection and alert for incident-beam focused spots 311, 321, 331,
341, 351 etc. As in the simpler case of a quad cell (FIG. 3B), the
beams one at a time--in turn--are detected, driven to null at the
center intersection, their corresponding mirror angles read to
memorize locations, and then released so that the apparatus is
available for the succeeding beam or beams.
[0188] More specifically, after spots 311 and 321 have been
processed the apparatus can turn its attention to spot 331. As
before this spot is driven to a sector boundary, preferably but not
necessarily the nearest one--i.e. the boundary between sectors #1
and #2.
[0189] Ordinarily but not necessarily the preferred path 332 to the
boundary is normal to the boundary. From the intersection 336 of
the path with the boundary, the spot 331 is next driven 333 to the
center intersection 334.
[0190] Once the mirror readings have been stored for spot 331, the
system can turn to another incident-beam focused spot 341. It then
repeat substantially the same process but with respect to the
sector boundary appropriate to that spot.
[0191] It will be understood that if all the beams appear at
substantially the same time, the order of processing of the plural
incident-beam spots is largely arbitrary. Otherwise the spots are
best taken up in order of appearance.
[0192] Another preferred embodiment of our invention relates to
so-called "active" sensing of potentially hostile platforms such
as, for example, guided missiles. As mentioned earlier, this
variant of the invention, rather than passively sensing incoming
light beams, first emits an outgoing light flash and then monitors
reflections of the flash.
[0193] Such a system is particularly effective in generating and
detecting retroreflections from a remote optical system that is
optically homing on our own host platform. Such a remote optical
system necessarily includes a front-end optic pointed toward our
host, and behind that optic a detector of some kind.
[0194] The detector is commonly based on silicon, or other
materials such as mercury-cadmium-telluride--depending on
wavelength--and typically mounted in or otherwise surrounded by a
metallic matrix. Both the detector and matrix are ordinarily very
reflective, and the front-end optic of the remote system
essentially ensures effective optical coupling between the optical
system of our invention and those highly reflective components.
[0195] Hence retroreflection is an efficient mode for locating
hostile remote platforms. A drawback is the need to further reveal
the exact location of our own platform by our pulse excitation
which is retroflected; however, there are several known techniques
for minimizing this problem, including release of chaff or decoys,
as well as evasive action.
[0196] The retroreflection mode can be particularly useful in both
ranging and velocity determination, for the remote system. Our
several patents and other publications in the area of light
detection and ranging ("LI-DAR") provide extensive details that are
applicable in the exploitation of information obtained by these
"active" forms of our present multisector-cell sensing
invention.
[0197] It will be understood that the foregoing disclosure is
intended to be merely exemplary, and not to limit the scope of the
invention--which is to be determined by reference to the appended
claims.
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