Fiber optic laser detection and ranging system

One aspect of the invention relates to a laser ranging system. In one embodiment, the invention includes: a laser adapted to transmit a beam of laser radiation to a target area; a plurality of apertures for receiving reflected laser radiation from the target area, wherein each aperture is coupled to a start tank circuit by a start optical fiber, and wherein the start optical fibers are substantially the same length for each aperture in the plurality; an oscillator; a receive phase comparator circuit that determines the phase difference between the output signals from the start tank circuit and the oscillator; a transmitter tank circuit that generates an output signal responsive to the transmission of the laser radiation; a transmit phase comparator circuit that determines the phase difference between the output signals from the transmitter tank circuit and the oscillator; a pulse counter circuit that counts the number of pulses generated by the oscillator between transmission of the laser beam and the receipt of the signal from the start tank circuit; and a distance measuring circuit that calculates the distance from laser ranging system to the target based upon the number of pulses counted by the pulse counter, the phase difference determined by the receive phase comparator circuit and the phase difference determined by the transmit phase comparator circuit.

CROSS REFERENCE TO RELATED APPLICATIONS 
Not Applicable 
STATEMENT ON FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
Not Applicable 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of the invention relates to devices for creating a 
three-dimensional image using a coherent, monochromatic electromagnetic 
radiation (LASER). The invention relates to a new and improved fiber optic 
LADAR (LASER detection and ranging) system. It can be used to provide 
information to guidance systems for vehicles such a guided missiles. This 
system is based upon inputs from optical fibers distributed at various 
locations on the vehicle, and a scanning laser transmitter. A plurality of 
the optical fibers receive and transmit to three photodiode detectors 
intermittent light energy from the scanning LASER energy reflected off the 
target. A plurality of the optical fibers receive individual reflected 
signals from the target and transmit them to the detectors, which provides 
input to a microprocessor which determines the elevational, azimuthal 
direction and range to the target to a high degree of accuracy. The 
individual pulse information can then be combined with other pulses to 
form a three dimensional image. A method for utilizing the system is 
disclosed. 
2. Description of the Prior Art 
The use of optical fibers for transmitting tracking information is known, 
but often has been applied to transmission of such information rather than 
collection thereof. For example, U.S. Pat. No. 4,952,042, issued Aug. 28, 
1990 to Pinson and assigned to The Boeing Company, discloses the use of 
optical fibers for transmitting information obtained by a telescope 
mounted on a gimbal in the forward end of a missile to a camera mounted 
further back in the missile. Similarly, U.S. Pat. No. 5,052,635, issued 
Oct. 1, 1991, to Paulet and assigned to Thomson-CSF, discloses the use of 
optical fibers for transmitting remotely transmitted missile guidance 
information from a sensor on an unpropelled aerodynamic carrier connected 
to the body of the missile by a flexible link. 
U.S. Pat. No. 4,923,276, issued May 8, 1990, to Wells and assigned to 
Teledyne Industries, Inc., discloses a fiber optic telescope including an 
optical train having a plurality of tapered optical fibers arranged in a 
plurality of cascading stages. Wells' optical fibers concentrate and 
magnify the incoming light waves by both their tapered form and the 
cascading stages. Wells requires a large number of tapered optical fibers 
to be combined in a concentrating relationship to amplify the signal 
received for use in obtaining information on location of the source of 
light upon which it is trained. 
The use of optical target detectors utilizing laser light is disclosed in 
U.S. Pat. No. 5,014,621, issued May 14, 1991 to Fox, et al. and assigned 
to Motorola, Inc. This patent utilizes a star coupler to automatically 
align pencil laser beams upon a target, and to track the target based on 
reflections of the laser light. 
The use of missile referenced beamrider guidance links is disclosed in U.S. 
Pat. No. 4,696,441, issued Sep. 29, 1987 to Jones, et al. and assigned to 
the United States of America. Jones et al. discloses a laser beam in which 
the strength of the beam is formed into a gaussian cross section, the beam 
is directed upon a target at short range, detectors on an in-flight 
missile detect and measure the strength of the laser beam, and a guidance 
system guides the missile along the beam by adjusting the guidance 
controls to maximize the detected strength of the beam. The Jones et al. 
missile includes at least one laser detector mounted outside the central 
longitudinal axis of the missile and requires a reference beam. 
A detector device for detecting the presence and originating direction of 
laser radiation is disclosed in U.S. Pat. No. 4,825,063, issued Apr. 25, 
1989 to Halldorsson et al. and assigned to Messcrschmitt-Bolkow-Blohm 
GmbH. The Halldorsson device includes a plurality of discrete light 
collection optics, each discrete optic being capable of gathering laser 
radiation over a certain solid angle, which overlaps the solid angle of 
its neighbors. The discrete optics are mounted together in a head, 
regularly arranged in azimuth aligned planes and elevation aligned planes. 
In Halldorsson's device, first, second and third wave guides are coupled 
to each discrete optic, with all first wave guides having identical 
lengths, shorter than the second and third wave guides. The lengths of the 
second and third wave guides are of increasing length in the direction of 
increasing azimuth and elevation angle, respectively, in order to form 
different transit times. In Halldorsson's device, first, second and third 
detector stages have opto-electrical transducers and are coupled 
respectively to the first, second and third wave guides. Transit time 
measuring circuits are coupled to the first, second and third detector 
stages which determine, respectively, the total time between detection by 
the first detector stage and the second and third detector stages. Based 
upon the times of arrival, the azimuth angle and the elevation angle of 
the incident laser radiation is determined. Halldorsson's system is 
limited by its use of discrete optics, a high speed counter for measuring 
time, and its use of a head for mounting its plurality of neatly aligned 
azimuth and elevation planes of identical discrete optics. Halldorsson's 
degree of accuracy is limited by the measurement only of transit time 
differences. 
U.S. Pat. No. 5,784,156 issued Jul. 21, 1998 to Nicholson, describes a 
fiber optic system for laser guided missiles which uses an array of 
apertures disposed along the vehicle in non-uniform locations. Each 
aperture is provided with a set of fibers including start and stop fibers. 
The fibers receive incoming radiation, transmit the radiation as optical 
signals to start and stop detectors. The detectors measure a signal 
strength and time delay related to the length of the start and stop fibers 
through which the optical signals have been transmitted. This information 
is later used by a microprocessor to calculate the direction to the 
radiation source relative to the vehicle. The above mentioned patents are 
incorporated by reference as though set forth in full. 
None of the systems disclosed in these prior art patents provide the 
features disclosed in the following specification. 
BRIEF SUMMARY OF THE INVENTION 
An object of the invention is to provide three-dimensional image 
information to a vehicle guidance system. In one version of the invention, 
a laser transmitter and scanner illuminates the target and optical fibers 
are used to both collect and transmit received optical signals for use in 
an on-board vehicle guidance and flight control system, in which the 
vehicle is intended to home in on its target. 
In one embodiment to the invention, the vehicle is provided with a high 
power laser transmitter coupled with a fiber optic laser receiver 
incorporating collection optics. These structures may be packaged in any 
convenient way as a matter of design choice. For example, if the vehicle 
is intended to be a self-guided missile, then the laser transmitter and 
fiber optics laser receiver may be packaged in a compatible nose cone. By 
way of illustration, important features of the present invention will be 
described with respect to its application in self-guided missiles. 
However, those with skill in the art will appreciate that the invention 
could easily be applied to other vehicles, such as an underwater torpedoes 
or manned vehicles, as a matter of design choice. 
One aspect of the invention relates to a laser ranging system. In one 
embodiment, the invention includes a laser adapted to transmit a beam of 
laser radiation to a target area, a plurality of apertures for receiving 
reflected laser radiation from the target area, each aperture being 
coupled to a start tank circuit by a start optical fiber, the start 
optical fibers being substantially the same length for each aperture in 
the plurality, an oscillator, a receive phase comparator circuit that 
determines the phase difference between the output signals from the start 
tank circuit and the oscillator, a transmitter tank circuit that generates 
and output signal responsive to the transmission of the laser radiation, a 
transmit phase comparator circuit that determines the phase difference 
between the output signals from the transmitter tank circuit and the 
oscillator, a pulse counter circuit that counts the number of pulses 
generated by the oscillator between transmission of the laser beam and the 
receipt of the signal from the start tank circuit, and a distance 
measuring circuit that calculates the distance from laser ranging system 
to the target based upon the number of pulses counted by the pulse 
counter, the phase difference determined by the receive phase comparator 
circuit and the phase difference determined by the transmit phase 
comparator circuit. 
Another aspect of the invention relates to a method for determining the 
range to a target. In one embodiment, the method includes the steps of 
transmitting a beam of laser radiation to a target, generating a signal 
from an oscillator, receiving reflected laser radiation from the target 
through a plurality of apertures, each aperture being coupled to a start 
tank circuit by a start optical fiber, the start optical fibers being 
substantially the same length for each aperture in the plurality, 
generating a signal from the start tank circuit responsive to the received 
laser radiation, determining a receive phase difference between the 
signals from the start tank circuit and the oscillator, generating a 
signal from a transmitter tank circuit responsive to the transmission of 
the laser radiation, determining a phase difference between the signals 
from the transmitter tank circuit and the oscillator, counting the number 
of pulses generated by the oscillator in the time period between 
transmission of the laser beam and receipt of the signal from the start 
tank circuit, and calculating the distance to the target based upon the 
number of pulses counted, the receive phase difference and the transmit 
phase difference. 
Still a further aspect of the invention relates to a laser guidance system 
for a vehicle. In one embodiment, the guidance system includes a laser 
that transmits a beam of laser radiation to a target, a staring array of 
apertures for receiving laser radiation reflected from the target, each 
aperture being coupled to a start tank circuit by a start optical fiber, 
an azimuth tank circuit by an azimuth optical fiber and an elevation tank 
circuit by an elevation optical fiber, the length of the start optical 
fibers being the same for all apertures in the staring array, the length 
of the azimuth optical fibers being related to the azimuth angle of the 
received laser radiation from their respective apertures, and the length 
of the elevation optical fibers being related to the elevation angle of 
the received laser radiation from their respective apertures, an azimuth 
comparator circuit that determines the azimuth phase difference between a 
signal from the start tank circuit and a signal from the azimuth tank 
circuit, an elevation comparator circuit that determines the elevation 
phase difference between a signal from the start tank circuit and a signal 
from the elevation tank circuit, an oscillator, a transmitter tank circuit 
that generates a signal responsive to the transmission of the laser beam, 
a pulse counter that counts the number of pulses from the oscillator 
between the time the laser beam is transmitted and a signal is generated 
by the start tank circuit, a receive phase comparator that determines the 
receive phase difference between a signal from the start tank circuit and 
a signal from the oscillator, a transmit phase circuit that determines the 
transmit phase difference between a signal from the transmitter tank 
circuit and a signal from the oscillator, a circuit that determines a 
three dimensional image of the target based on the azimuth phase 
difference, the elevation phase difference, the number of pulses counted, 
the receive phase difference and the transmit phase difference, and a 
guidance circuit that directs the vehicle to the target based on the three 
dimensional image. 
These and other features of the present invention will be more apparent 
from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION 
With reference now to the drawings, a new and improved fiber optic LADAR 
system of particular utility for a guided missile will be described. While 
the present invention is described in terms of embodiment as a laser 
guided missile guidance and control system, it is to be understood that 
the invention is not so limited, being applicable to any vehicular 
guidance and control or imaging LADAR system which images a target in 
three dimensions using a LASER scanner and fiber optic receiver. In the 
missile example, the electromagnetic radiation obtained is laser light, as 
is known in the art. It is to be understood that other substantially 
monochromatic radiation could be employed. Also, in the missile example, 
the laser light detected by the receiver of the guidance system has been 
reflected from the target towards which the missile is being guided. It is 
to be understood that the laser or other substantially monochromatic 
radiation may be generated at or by the target, rather than being 
reflected from the target or generated by another platform. In this case 
only azimuth and elevation information of the target is available. Light 
generated and transmitted from the vehicle allows the receiver to generate 
range data. Further, it is preferred that the light source be capable of 
firing rapid on/off or intermittent bursts, as with known laser targeting 
systems. The wavelengths of light useful in the invention range from the 
ultraviolet to the far infrared, and are preferably those wavelengths 
transmitted through the atmosphere with minimum interference by the normal 
components of the atmosphere. 
Referring to FIG. 1, there is shown a guided missile fitted with a LADAR 
guidance system according to one embodiment of the invention. The LADAR 
guidance system comprises three major hardware components, the laser 1, 
the scanner 3, and the receiver. The laser transmitter is composed of the 
laser 1, a telescope 2 and the laser scanner 3. A typical implementation 
would use a high power solid state laser, for example, 50 kilowatts, with 
a high repetition rate, for example, 25 to 50 kHz. The pulse width should 
be as short as possible for optimum operation, for example, 8 nano 
seconds. The output of the laser is passed through a telescope 2 to expand 
the beam and lower the divergence to the desired level. This controls the 
spot size at the target range. The receiver is composed of optical 
collecting elements 5 and 6, a fiber delay spool 7, and the 
electronics/processor board 8. During the search mode, a push broom scan, 
side to side, is used, and the reflected energy is received through the 
pushbroom collecting lenses 5. The energy passes through the fiber optic 
delay lines on the fiber spool 7 and is sensed and processed on the 
electronics/processor board 8. After target acquisition, a circuit in the 
electronics/processor board 8 selector a homing mode. In homing mode, the 
LADAR uses a raster or spiral scan pattern using the scanner 3, and 
receives the reflected energy using the three, overlapping field of view 
homing optics 6. The received energy then follows the identical path as in 
the pushbroom mode, i.e., through the fiber optic delay spool 7 and to the 
electronics/processor board 8. 
The search mode, illustrated in FIG. 2, uses a "push broom" scan, which, in 
one exemplary embodiment, the laser beam is directed forward and 
30.degree. downward from the director of travel of the missile 200. In the 
embodiment shown, the scanning laser illuminates a cross-range row 201 of 
250 spots which overlap at 50% power in clear conditions. However, the 
number of spots and overlap are, obviously, a matter of design choice. If 
the spots are pulsed at a 25 kilohertz rate, this will yield a push broom 
sweep rate of 100 hertz. In the embodiment illustrated, if the forward 
velocity is 100 meters-second, the spots overlap in the velocity direction 
at the 50% power. The receiver operates with a field of view ("FOV") 
matching the laser spot scan, in the illustrated case, 14.degree. azimuth 
and 0.055.degree. elevation at 50% gain. The receiver is capable of 
detecting the cross-range target power centroid to 0.01.degree. accurately 
at a maximum track range, improving to 0.0034 degrees (or the digitization 
limit) at close range. The range may be detected to an accuracy of at 
least 0.002 meters. 
After a target has been identified, the LADAR is switched to the homing 
mode shown in FIG. 3. This mode uses a forward staring field of view of 
the sensor in the missile 300. In one advantageous embodiment, the staring 
sensor is provided with a nominal 3.degree. FOV at the 50% power point. 
The laser scanner is switched to a raster scan, creating a 55 by 55 spot 
square 301, each spot overlapping at 50% power. The receiver then detects 
the target power centroid in azimuth and elevation to 0.003.degree. 
accuracy, decreasing to a digitization limit of 0.007.degree. as the 
signal to noise level increases. The homing mode range accuracy is the 
same as in the search mode. 
Referring to FIG. 8, the laser receiver consists of five subcomponents, the 
collection optics or apertures allowed into the staring overlay 200 and 
pushbroom array 300, the delay lines, the tank circuits, the IF circuits 
402, 409 and 502, and the phase comparators. Each collection optic of the 
staring and push broom arrays feeds into a set of three fiber optic delay 
bundles, a start bundle, an azimuth stop bundle, and an elevation stop 
bundle. The start bundles from each collection optic are coupled to a 
start sensor 401. Similarly, the azimuth stop and elevation stop bundles 
are coupled to corresponding sponsors 402, 403. The fiber optic delay 
lines increase in length linearly from the lower left comer of the FOV of 
the staring array 200 to the upper right hand comer. Thus, in one 
illustrative embodiment, a target received near the lower left comer would 
have zero azimuth and zero elevation delay relative to the start signal, 
and one near the upper right comer would have 18.69 nanoseconds of optical 
delay using four meters of fiber at 2.14 E8 meters-second. Because of the 
overlapping FOVs, and the linearly varying delay line length, the azimuth 
and elevation position is linearly varying with the delay time from the 
start signal power centroid to the azimuth stop and elevation stop signals 
power centroids. By integrating in a longer row of bottom sensors, i.e., 
the push broom array 300, both arrays can use the same sensors and 
electronics with no switching or gimbles. 
The range to target is acquired by measuring the phase of the output signal 
from the transmit pulse tank circuit 501 against a local oscillator 503, 
and then counting whole oscillator pulses until the receipt of a signal 
from the start tank circuit 404. The signal phase from the start tank 
circuit is also measured against the local oscillator (the "receive 
phase"). Thus, the total range is the transmit phase plus the receive 
phase plus the number of whole pulses. This technique allows range 
measurements to an accuracy of about 0.002 meters. 
The receiver is also designed having a tank circuit with the ability to 
integrate a power centroid versus time signal from the sum of the delay 
line pulses fed to it. The phase of the start tank circuit relative to the 
azimuth and elevation tank circuits is measured. This allows the receiver 
to achieve a linear transfer function from angular position to delay time 
measure by the phase shift. The IF circuit provides a logarithmic power 
receiver output in addition to its squaring function for phase measurement 
by the phase detector. 
FIG. 4 is a schematic plan view of a vehicle, such as a missile 10, 
incorporating an array of apertures 20 each containing one set of a 
plurality of sets of optical fibers 22. At each aperture is disposed at 
least one start fiber and at least one stop fiber, or a single fiber which 
is subsequently split into at least one start fiber and at least one stop 
fiber. Preferably, as shown in FIG. 4A, each aperture 20 contains a set of 
fibers 22 consisting of three, or multiples of three, collinearly disposed 
fibers 27, 28 and 29, as described in more detail below. For simplicity, 
in the figures other than FIG. 4A, the drawing element depicted as "fiber" 
22 is actually a set of three, or multiples of three, fibers, designated 
by reference number 22. The first of these three fibers is a start fiber 
27, the second is an azimuthal stop fiber 28, and the third is an 
elevational stop fiber 29. The number of fibers in a set of fibers may be 
any integral multiple of three fibers, the number variable as needed to 
obtain adequate signal strength. A light receiving, or proximal, end 24 of 
each fiber in a set of three or multiples of three fibers is disposed at 
an individual aperture 20. The light receiving end 24 for a set of three 
fibers may be a single fiber which is split by a fiber splitter into the 
three fibers of a set of fibers 22. 
Each of the plurality of individual optical fibers 27, 28, 29 includes the 
first, light receiving or proximal end 24, and a second, 
detector-attached, distal end 26. The distal end 26 is interfaced with a 
detector unit 50. The proximal light receiving end 24 functions to allow 
light to enter the fiber without the need for additional light gathering 
optical devices, if desired. Preferably, the light receiving end 24 has a 
flat, polished end, as best shown in FIG. 7, and it is the incoming 
radiation that strikes this surface which provides the input to the 
detector 50 and thence the guidance and control system 60, 70 of the 
present invention through communication lines 56, 58, 62. The field of 
view of each individual fiber 27, 28, 29 is determined primarily by its 
fiber numerical aperture, or the gain optics if used. 
The aperture 20 may include a band pass filter, and or protective material 
which acts to protect the fiber from substances or conditions such as heat 
or cold, but any such protective material does not participate in light 
gathering in this version of the invention. As described below, the 
aperture may contain a single fiber end, which subsequently is split into 
three fibers, or it may contain three separate, discrete fibers, or 
multiples of three. Preferably each individual optical fiber 27, 28, 29 
remains as a separate, individual fiber for its entire length. When 
multiples are used the same number of start, azimuth stop and elevation 
stop fibers are used. The range of integral multiples of the three types 
of fibers which might be used is limited only by the size of the aperture 
required to allow each fiber to receive the incoming radiation. 
Practically, the range of multiples may be considered to be between 2 and 
100. Thus, a set of fibers may practically include 3, 6, 9, 12, 15 . . . 
300 fibers. Preferably, each aperture is linked, via the set of optical 
fibers 22, to three individual detectors via the distal fiber ends 26. The 
three individual detectors, to be described below, are preferably 
contained within the detector unit 50 shown in FIG. 1. Preferably if 
multiples of three fibers are used in the set, the same multiple of each 
fiber arrives at each detector. 
Each fiber is preferably made from standard clad optical fiber material 
typically having a diameter of approximately 125 microns to 400 microns. 
These diameters are exemplary only, and various other diameters may be 
used, as will be understood by those in the art. 
Referring now to FIG. 5, each fiber 22 has a characteristic field of view 
as schematically shown in FIG. 2. The field of view of a fiber depends 
primarily upon its numerical aperture. The numerical aperture acts to 
weaken and block incoming light rays or source signals emanating from an 
illuminated source, with the degree of weakening increasing until the 
signal is effectively blocked at an angle that exceeds the angle defined 
by the field of view associated with the numerical aperture. It is 
understood that while FIG. 5 illustrates the field of view in only two 
dimensions, the actual field of view of the fiber 22 is conical, in three 
dimensions. Whenever this specification refers to a field of view, the 
actual field is a three dimensional cone, with the apex of the cone at the 
first end 24 of the fiber 22. Light reaching the first end 24 from within 
the cone is within the field of view of the fiber. 
Referring now to FIG. 6, a plurality of optical fibers is shown arrayed to 
provide varying degrees of overlap of the fields of view of the individual 
fibers. In FIG. 6, all the fibers have substantially the same field of 
view, but this is not necessarily or preferably the case. The field of 
view, of the various fibers in the arrayed plurality of fibers may be 
selected to provide the degree of accuracy in determination of target 
direction required for a given direction relative to the vehicle. In other 
words, the accuracy in a given direction may be controlled by selection 
of, inter alia, the field of view of the fibers pointing in that 
direction, and by selection of the degree of overlap of adjacent fields of 
view. As suggested by the positions and orientations of the fibers, the 
five fibers near the center of the array, designated as the "a" group in 
FIG. 6, will provide the highest accuracy in determining of the position 
of or direction to a target near the center of their field of view. The 
next outwardly positioned two fibers, designated as the "b" group in FIG. 
6, are both oriented in a different direction and overlap to a different 
degree with the adjacent fibers than do the fibers in the "a" group. These 
"b" group fibers provide a lower accuracy than do the "a" group fibers. 
Finally, the next outwardly positioned two fibers, designated as the "c" 
group, are oriented in yet another direction, and have fields of view with 
less overlap with adjacent fibers in groups "a" and "b." The "c" group 
fibers provide less accuracy than the "b" group fibers. Note that the 
numerical aperture of these groups of fibers may be selected so as to 
increase or decrease the available field of view of any of the fibers 
shown in FIG. 6. Likewise, additional fibers may be added, the fibers may 
be provided with other orientations, fields of view, and degrees of 
overlap with adjacent fiber fields of view, resulting in directionally 
selectable accuracies, in accordance with the selected parameters. 
Referring now to FIG. 7, a schematic drawing is shown of the 
light-receiving end 24 of the fiber 22. As is shown in FIG. 7A, preferably 
the end of the fiber is flat, and most preferably has been polished to a 
high degree so as to avoid distortion or loss of entering optical signals. 
As used in this disclosure, the angle of incidence of incoming radiation 
is defined as the angle formed between the direction of propagation of the 
radiation and the central longitudinal axis of the fiber at or near the 
end of the fiber. It is well known to those in the art of fiber optics 
that the more interaction an optical signal traveling in a fiber has with 
the walls of the fiber, the more the strength of the optical signal is 
attenuated. As is shown in FIG. 7A, an optical signal entering the fiber 
at an angle of incidence at or close to zero degrees will travel through 
the fiber with a minimum of interaction with the walls of the fiber and 
with a minimum of attenuation due to the effect of the refractive index of 
the optical fiber, and so will be attenuated very little by the passage. 
Also shown in FIG. 7A, an optical signal entering the fiber at an angle of 
incidence substantially greater than zero degrees will undergo many 
interactions with the wall of the fiber and will be significantly affected 
by the refractive index of the optical fiber, and so will be attenuated to 
a greater degree than optical signals entering at angles closer to zero 
degrees from the longitudinal axis of the fiber. 
The radiation incident upon the end 24 of the fiber 22 will be further 
attenuated to a small but finite degree as a result of reflection of the 
incident radiation from the outer surface of end 24. The degree of 
attenuation due to reflection will vary with the angle of incidence, and 
other factors known to those in the arts . Such attenuation should also be 
constant for a given fiber at a given wavelength and so provides further 
information to the microprocessor in accurately determining the direction 
to the source. For higher optical gain, a beam contractor may be used as 
seen in FIG. 7B. This is composed of two plano convex lenses 11 and 12. 
The effect is to increase the optical amplitude by the area ratio of lens 
area 12 to 27, 28, 29 fiber receive area. It also increases the angle of 
the light energy on the fiber ends proportionate to their input angle, 
increasing the angular accuracy of the system. 
As FIGS. 7A and 7B suggest, for radiation incident upon the end 24 of the 
fiber 22, the strength of the optical signal initially entering the fiber 
22 will be affected by the angle of incidence of the arriving radiation 
for another reason. If the radiation arrives at an angle of 0.degree., as 
shown in FIG. 7A, the end 24 of the fiber 22 appears to form a round disc 
if the fiber is round. Thus the "target," into which the radiation must 
enter to form an optical signal in the fiber, appears as a circle. By 
contrast, when radiation of the same wavelength arrives at the same end 24 
of the fiber except at an angle substantially greater than zero degrees 
(0.degree.), as also shown in FIG. 7A, less of the incoming radiation can 
enter the end 24 of the fiber 22, simply because the "target" from this 
angle forms an ellipse having an apparent area smaller than a circle. The 
width of the ellipse becomes smaller with increasing angle of incidence. 
Thus, simply because the "target" is smaller, and less radiation will 
enter the fiber. This effect is in addition to the effect of the 
refractive index and other physical variables such as reflection which 
contribute to the attenuation of the signal indexed by the numerical 
aperture. 
As a result of these attenuation effects arising from and related to the 
angle of incidence, optical signals arriving at the detector will have a 
signal strength related to the angle of incidence. The exact relationship 
between angle of incidence and attenuation will vary depending on the 
material from which the optical fiber is made and on the wavelength of the 
incoming radiation, but should be reproducible for a particular fiber and 
laser combination, and being the same for both start and azimuth and 
elevation stop fibers allows accurate determination of the directional 
relationship between the vehicle and the radiating target. This is the 
result of the attenuation of overlapping field of view optics being added 
to the make the composite azimuth and elevation stop signals through their 
respective delay lines. 
One embodiment of the invention will be described with respect to FIG. 8. 
In this embodiment, the missile is provided with a staring array 200 and a 
pushbroom array 300 to receive incoming laser radiation. 
The staring array 200 includes an array of twelve apertures 201-212 which 
are to be arranged in a forward looking position on the missile. Of 
course, any number of apertures could be provided as a matter of design 
choice. Each aperture in the staring array 200 is provided with at least 
three separate fiber optic cables or lines that connect the aperture to 
the optical sensors 401-403. More specifically, each aperture includes a 
start line that is coupled to optical sensor 401, an azimuth delay line 
coupled to optical sensor 402, and an elevation delay line that is coupled 
to sensor 403. The start line length for all apertures is the same and 
preferably contains no additional length to delay light signals from being 
passed from the apertures to optical sensor 401. 
The apertures 201-212 arranged into azimuthal columns and elevational rows. 
In an azimuthal column, each aperture in the column has the same length 
azimuth delay line. The length of the azimuth and elevation delay lines is 
varied from column to column, or row to row, as the case may be. For 
example, in the left-most column in the staring array 200, the azimuth 
delay lines for apertures 201 and 202 are connected to optical sensor 401 
with no added length to delay the received laser light from reaching the 
sensor 401. One meter of additional length is added for each column so 
that in the right-most column, the azimuth delay lines for apertures 211 
and 212 have 3 meters of additional length. This additional length may be 
conveniently wound around a spool for physical placement in the missile. 
Thus, when laser light strikes the array 200, light from apertures 201 and 
202 will arrive at optical sensor 402 first, followed sequentially by 
light from the other azimuthal columns. Similarly, for the elevational 
rows, apertures 206 and 210, in the lower-most elevational row, are 
provided with an elevation delay line length of 1 meter. The elevation 
delay line length increases from row to row until apertures 203 and 207 
are provided with elevation delay lines of 4 meters. It should be noted 
that here is no "0" length delay line for elevational lines because this 
is reserved for the pushbroom array. 
The pushbroom array 300 includes eight apertures 301-308. As with the 
staring array, the apertures 301-308 are connected by start, azimuth and 
elevation delay lines to the optical sensors 401-403, respectively. 
However, unlike the staring array, the pushbroom array uses the vehicles 
forward velocity to scan the three-dimensional image in one axis. 
Accordingly, there is only one elevational row needed by the pushbroom 
array. Thus, the elevation delay line length is 0, i.e., no additional 
delay, for all apertures in pushbroom array. The samc is true for the 
start line. Since the elevation delay line length is one meter for the 
lowest row in the staring array, the missile can distinguish signals 
received from the staring array from those received by the pushbroom 
array. This will be described in greater detail further herein. 
In an exemplary embodiment, the center lines of the FOV of the apertures in 
the pushbroom array 300 are separated by 1.75 degrees, giving the entire 
array a 14 degree FOV. Each aperture overlaps the center line of its 
nearest neighbors at the 50% power points, yielding good angle resolution. 
Light striking the staring array 200 is passed through the fiber optic 
cables to optical sensors 401-403 as described above. These signals arc 
then processed through the image processing circuit 400. Imaging 
processing circuit 400 contains a start tank circuit 404, an azimuth tank 
circuit 405 and an elevation tank circuit 406, each attached to its 
corresponding optical sensor. The optical sensors and tank circuits will 
be described in greater detail with respect to FIGS. 9A-9C. The outputs 
from the tank circuits are passed to IF amplifiers and then to phase 
comparators. Suitable phase comparators will occur to those skilled in the 
art. The phase is then digitized for use in the image processor according 
to techniques known in the art. 
There is also provided a range finding circuit 500 which is used in 
conjunction with the imaging circuit 400 in order to allow the missile to 
form an accurate three dimensional image of the target, as well as 
determine the exact range of the missile to the target. The range finding 
circuit 500 includes a transmitter tank circuit 501 connected to an 
intermediate frequency amplifier 502. The construction of the transmitter 
tank circuit 501 and IF amplifier 502 are substantially similar to 
corresponding circuits used in the image circuitry 400. The range finding 
circuit also includes an oscillator 503, a receive phase comparator 504 
and transmit phase comparator 505. The receive phase comparator 504 
compares the phase difference between an oscillating signal from the 
oscillator 503 and a signal received on the start line of the staring 
array 200 that initiates an oscillation of start tank circuit 404. 
Similarly, transmit phase comparator 505, compares the difference in phase 
between a signal from oscillator 503 and an outgoing signal detected by 
creating an oscillation of transmitter tank circuit 501. In one 
embodiment, oscillation of transmitter tank circuit 501 is initiated by 
passing optical energy from the outgoing laser beam on to the optical 
sensor 506. When these two phases are combined with the pulse count, the 
total time from transmit to receive can be accurately calculated. 
In operation, an onboard laser is fired in a forward looking direction from 
the missile. When the outgoing laser signal is transmitted, it is also 
passed to optical sensor 506. At the same time, other circuitry on the 
missile (not shown) begins counting pulses from oscillator 503. The phase 
of the broadcast laser signal is compared to the phase of the oscillator 
503 by transmit phase comparator 505. Other circuitry on the missile 
determines the phase broadcast time. The phase broadcast time is shown in 
FIG. 10. FIG. 10 is a timing diagram depicting the phasc difference 
between the signal from the transmitter tank circuit 1000 and the standard 
oscillator signal 1000 from oscillator 503. The phase difference between 
these two signals is the phase broadcast time 1002. 
The broadcast laser signal is then reflected off a target, returned to the 
missile and then detected by either the staring array 300 or the pushbroom 
array 200 depending on the mode of operation of the missile. Since there 
is no delay in the start line of either array, the signal from the start 
line is passed through optical sensor 401 through tank circuit 400 and 
eventually to receive phase comparator 504 where it is also compared 
against the signal from oscillator 503. This is depicted in the timing 
diagram shown in FIG. 11. FIG. 11 shows the timing diagram for the 
received pulse 1100 detected by the start lines of the staring array and 
the clock signal 1000. The phase difference, or the phase receive time 
1102 is shown in the figure. 
With this information, the missile can then calculate the time to the 
target based upon the number of cycles counted from oscillator circuit 503 
plus the phase broadcast time 1002 and the receive broadcast time 1102. 
With this information, the range to the target can be calculated as 
accurately as 1.879.times.10.sup.3 meters. 
It is preferable that the longest delay time of any optical fiber is not 
more than about 60.degree. of the ring frequency of the tank circuit that 
it is coupled to. This is because it is desirable that all energy from all 
apertures is passed to the tank circuits during the time the outputs from 
the tank circuits are initially rising in response to this received 
energy. In other words, the energy received from reflected laser pulse 
appears as a single pulse input to the tank circuits. In this way, the 
phase shift of the output from the azimuth and elevation tank circuits, 
relative to the output signal from the start tank circuit, will be 
directly proportional to the power centroid of the received pulse. Since 
the power centroid of the received pulse is determined by the amount of 
laser energy received at each aperture in the array, the phase shift can 
be used to determine the azimuth and elevation of the incoming laser 
radiation, or, in the case of the pushbroom array, the azimuth of the 
incoming radiation. 
This information can, in turn, be used by image processing electronics on 
the missile to generate an image of the target. More specifically, each 
laser pulse received by the missile apertures will have a particular 
azimuth and elevation determined by the phase shift of the signals 
generated by the azimuth and elevation tank circuits relative to the phase 
of the signal generated by the start tank circuit. This azimuth and 
elevation data is, in turn, used to specify a particular pixel on the 
image created by the missle from its field of view. Equipment and methods 
for creating an electronic image from the azimth and elevation data is 
known to those of skill in the art and will not be described in detail 
herein. The imaging equipment provided on the missile also stores 
information related to the amplitude of the incoming laser radiation for 
the particular pixels identified by the azimuth and elevation information 
discussed previously. This information is used to generate gray scale 
information for the images according to known techniques. Moreover, the 
range information determined from the pulse count, transmit and receive 
phase information determined for each pixel, can be combined with the 
azimuth and elevation information to allow the missile to generate a 
three-dimensional image of the target area. In one advantageous 
embodiment, the length of each laser pulse is approximately 20 
milliseconds. 
As seen in FIG. 9A, the start fibers are guided to imaging optics 33a which 
images the front-face ends of these wave guides on a photo diode 34a, 
which is part of the start detector 30. The photo diode 34a is coupled in 
series with a damped resonant circuit comprised of a resistor 35a, coil 
36a, and capacitor 37a. Additionally diode 34a and filter circuit 35a, 36a 
and 37a are coupled to an amplifier 38a at the output of which amplifier 
current I.sub.S flows. When a laser pulse impinges on the first, receiving 
end 24 of a group of start fibers in their apertures, an optical signal is 
transmitted over the start optical fibers receiving the signal. This pulse 
has the current response over time as shown in FIG. 9A and labeled 
L.sub.S. This is shown in detail in FIG. 12, curve 1200. The damped 
resonant circuit has a ringing frequency which matches the pulse width of 
the incoming radiation. The damped resonant circuit output is a signal 
having set oscillation frequency with the amplitude and phase in 
accordance with that of the laser radiation striking the photo diode. The 
phase is set by the power centroid of the signal L.sub.S. This is shown in 
detail in FIG. 13, curve 1300. This output constitutes a first, start 
input to a circuit for determining the time of arrival of incoming 
radiation and its amplitude. 
In the preferred embodiment, the device comparing the start and each of the 
stop signals is a phase comparator chip. This output I.sub.S from the 
start damped oscillation circuit 35a 36a and 37a constitutes one input leg 
to each of the comparators. 
In FIG. 9B the azimuth stop fibers 28 are guided to imaging optics 33b 
which images the front-facc ends of these wave guides on a photo diode 
34b, which is part of the azimuth stop detector 31. The photo diode 34b is 
coupled in series with a damped resonant circuit comprised of a resistor 
35b, coil 36b, and capacitor 37b. Additionally diode 34b and filter 
circuit 35b, 36b and 37b are coupled to an amplifier 38b at the output of 
which amplifier current I.sub.AS flows. When a laser pulse impinges on the 
first, receiving end 24 of a group of azimuth stop fibers in their 
apertures, an optical signal is transmitted over the azimuth stop optical 
fibers receiving the signal . This pulse has the current response over 
time as shown in FIG. 9B and labeled L.sub.AS. This is shown in greater 
detail in FIG. 12, curve 1201. Here the stretch out of the pulse due to 
the delay line length is seen. The damped resonant circuit has a ringing 
frequency which matches the wavelength of the start circuit 9A. The damped 
resonant circuit output is a signal having a set wavelength with an 
amplitude and phase in accordance with that of the laser radiation 
striking the photo diode. The phase is set by the power centroid of signal 
L.sub.AS, which has been time delayed from signal L.sub.S by the fiber 
delay line 28. It is shown in detail in FIGS. 13, curve 1301. This output 
I.sub.AS constitutes an azimuth stop input to a comparator for determining 
the apparent phase difference and time of arrival of the azimuthal 
component of the incoming radiation. 
In FIG. 9C the elevation stop fibers 29 are guided to imaging optics 33c 
which images the front-face ends of these wave guides on a photo diode 
34c, which is part of the elevation stop detector 32. The photo diode 34c 
is coupled in series with a damped resonant circuit comprised of a 
resistor 35c, a coil 36c, and a capacitor 37c. Additionally diode 34c and 
filter circuit 35c, 36c and 37c are coupled to an amplifier 38c at the 
output of which amplifier current I.sub.ES flows. When a laser pulse 
impinges on the first, receiving end 24 of a group of elevation stop 
fibers in their apertures, an optical signal is transmitted over the 
elevation stop optical fibers receiving the signal. This pulse has the 
current response over time as shown in FIG. 7C and labeled L.sub.ES. The 
damped resonant circuit has a ringing frequency which matches the 
wavelength of the circuit 9A. The damped resonant circuit output is a 
signal having a set wavelength with an amplitude and phase in accordance 
with that of the laser radiation striking the photo diode. The phase is 
set by the power centroid of signal L.sub.ES, which has been time delayed 
from signal L.sub.S by the fiber delay line 29. It is shown in detail in 
FIGS. 13, curve 1301. This output I.sub.ES constitutes an elevation stop 
input to a comparator for determining the apparent phase difference and 
time of arrival of the elevational component of the incoming radiation 
signal. 
The damped ringing circuit has the same resonant frequency as that 
described for the start circuit. As can be seen in FIGS. 9B and 9C the 
longer length of optical fibers 28 and 29 delays the arrival of the laser 
pulse on the receiver optics 33b, 33c and consequently on the photo diodes 
34b, 34c. This is shown in greater detail in FIG. 12. The damped 
oscillation is consequently correspondingly delayed, and the signal 
produced from the damped circuit appears with a phase shift relative to 
that from the start detector. This is shown in FIG. 13. The difference 
between the phases of the start signal and each of the azimuth and 
elevation stop signals provide information on the azimuth angle and 
elevation angle of the incident radiation, and thus to the direction to 
the source of the radiation detected by the system. 
The output of the tank circuits is fed into IF amplifier stages. These 
narrow the bandwidth to eliminate noise, output a signal proportional to 
the log of the amplitude, and then amplify and chop the signals until they 
become square waves as shown in FIG. 15. At this point each signal has 
very low noise, and has the phase shift preserved. The phase comparator 
functions by differencing the two signals (i.e, start channel signal 1500 
and with the azimuth or elevation signals, both represented by wave 1501) 
whose phase is to be compared. This is shown in FIG. 15, with the 
resultant signal 1502 having a duty cycle proportional to the phase shift. 
The differenced signal 1402 is then integrated over a number of cycles, 
typically 10. This further decreases the noise level, increasing the phase 
measurement accuracy. The phase is then the 10 cycle integral compared to 
a constant signal. The integrated signal is digitized for input into the 
computer image processing. 
The laser transmitter works by firing a solid state laser through the 
scanner mechanism. The scanner mechanism shown in FIG. 14 consists of 4 
rotatable prisms. The first set of prisms, 1401 and 1402, controls the 
high-speed scan rate of the beam. If the two prisms are out of phase as in 
FIG. 14A, and both rotating in the same direction, they will produce a 
circle 1406. If they are in phase as in FIG. 14B, they will point straight 
ahead. Thus, a spiral can as seen in FIG. 3A can be produced by spinning 
prisms 1402 and 1401 in the same direction, and slowly varying their phase 
relative to each other. Spinning prism 1402 in the opposite direction of 
prism 1401 can make a line scan. This produces a scan seen in FIG. 2. The 
second set of prisms 1403 and 1404 are used to point the centerline of the 
scan pattern. If prisms 1403 and 1404 are in phase as shown in FIG. 14A, 
the centerline of the scan pattern will be straight ahead. If the prisms 
1403 and 1404 are out of phase as in FIG. 14C, the pattern will be 
deflected off centerline. Thus for the push broom scan seen in FIG. 2, 
prisms 1403 and 1404 are out of phase deflecting the centerline of the 
scan pattern downward, and prisms 1401 and 1402 are spinning in opposite 
directions, creating the line scan. For the spiral scan seen in FIG. 3A, 
prisms 1403 and 1404 are in phase, deflecting the centerline of the scan 
pattern straight forward, and prisms 1401 and 1402 are spinning in the 
same direction with a slowly changing phase relationship. For the raster 
scan seen in FIG. 3, prisms 1403 and 1404 are spinning slowly in opposite 
directions, slowly line scanning the centerline of the high speed scan 
pattern, while prisms 1401 and 1402 are spun rapidly in opposite 
directions creating a line scan. It can be seen that this arrangement of 
prisms gives great flexibility in scan patters, and will even allow prisms 
1403 and 1404 to cause a scan pattern to track a target across the filed 
of view of the receiver, while prisms 1401 and 1402 create a small conical 
scan pattern on the target.