Fabry-Perot scanning and nutating imaging coherent radiometer

An imaging coherent radiometer incorporating a Fabry-Perot interferometer which is scanned or nutated, for detecting and determining location and wavelength of coherent radiation or the coherent absence of radiation in the presence of non-coherent ambient radiation.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for detecting the presence of 
coherent radiation or the coherent absence of radiation in the presence of 
non-coherent background radiation. More particularly, the present 
invention relates to the use of an imaging optical radiometer to make such 
detections and determine the direction and wavelength of such radiation or 
such lack of radiation. 
Imaging optical radiometers, constructed in accordance with the concept of 
this invention are adapted, among other possible uses for detecting and 
determining the wavelength of coherent radiation or the coherent absence 
of radiation. In addition, it can be used to determine the direction of 
arrival of the source of coherent radiation or coherent absence of 
radiation and indicate such position in a display of the field of view. 
Such a device can find application in specific gas cloud detection, oil and 
mineral exploration and detection through Fraunhofer line discrimination 
techniques, and intelligence surveillance. 
Conventional laser receivers use a narrow-band optical filter or 
diffraction gratings in combination with a photodetector, bandpass 
amplifier and thresholded peak detector to detect the presence of coherent 
radiation. This approach has two disadvantages: one, the laser wavelength 
must be known and two, the video bandwidth required to pass nanosecond 
pulses also passes a lot of detector and/or background photon noise. The 
coherent radiometer approach has a broad spectral response and a noise 
integration time limited only by the available observation time. 
The prior art is evidenced by U.S. Pat. Nos. 3,824,018 to R. Crane Jr. and 
U.S. Pat. No. 4,309,108 to E. Seibert, both of which are assigned to the 
same assignee as the present application. The aforementioned patents 
disclose the use of Fabry-Perot etalon interferometers. 
While the prior art devices detect presence, wavelength and direction of 
arrival of coherent radiation from a single source, our contribution is to 
do so for all coherent sources within a scene, resulting in an imaging 
coherent radiometer with longer integration times for sensitivity 
enhancement, to also do so for coherent absence of radiation, and for 
other advantages, as will become apparent as the description proceeds. 
SUMMARY OF THE INVENTION 
The present application is related to U.S. patent application Ser. No. 
884,694 entitled "Imaging Coherent Radiometer" which is assigned to the 
same assignee as the present invention and filed on even date therewith. 
The present invention contemplates the provision of a new and improved 
apparatus using a Fabry-Perot interferometer to detect the presence, 
wavelength and direction of coherent radiation or the coherent absence of 
radiation in the presence of non-coherent radiation. 
The optical path difference or OPD of the unequal path interferometers used 
in the present invention are carefully chosen. Coherent radiation, such as 
that produced by laser light, may be characterized by its unique coherent 
properties: spatial, spectral, temporal and polarization. The temporal 
coherent property is described in terms of coherence length and is the 
property used in the present invention to distinguish coherent radiation 
from noncoherent radiation. This is because it is specific to laser 
radiation and unique relative to a natural background or foreground 
radiation that laser radiation has a long coherence length relative to 
non-coherent radiation. In addition, the coherence length signature of 
laser radiation is not distorted by natural propagation effects. The 
unequal path interferometer OPD is selected so that it is longer than the 
coherence length of the non-coherent background or foreground radiation 
and shorter than the coherence length of coherent radiation. The OPD is 
modulated, with the result that the noncoherent radiation will be 
substantially unmodulated leaving only the coherent laser energy modulated 
at the interferometer output. 
One form of the device to accomplish the objectives of the present 
invention comprises, in combination, an unequal path interferometer which 
is angle scanned or rotated through a limited angle about an axis normal 
to the optical axis of the incoming radiation. The preferred embodiment 
utilizes a Fabry-Perot etalon. 
A row of detectors is located on the side of the etalon opposite the 
incoming radiation and parallel to the axis of rotation. A cylindrical 
lens, with no power in the plane of etalon scan, serves to focus the 
radiation passing through the etalon onto the row of detectors. The output 
from these detectors indicates the varying constructive and destructive 
interference of the recombined coherent radiation components of the 
incoming radiation as the etalon is scanned as described above. The 
signals these detectors produce can be processed for the location and 
wavelength of one or a plurality of coherent radiation sources in a scene 
being scanned. The same analysis can be performed for the coherent absence 
of radiation. 
Another form of the device to accomplish the objectives of the present 
invention comprises, in combination, an unequal path interferometer which 
is nutated. The preferred embodiment utilizes a Fabry-Perot 
interferometer. 
Unlike the form just previously described, the present form has an etalon 
which is rotated about two mutually perpendicular axes, each rotation to 
some maximum tilt angle. The two mutually perpendicular scans are at two 
different frequencies so that the processor can distinguish direction 
information for the two axes of the scan. Accordingly, the surface 
described by the intersection of the collimated incoming radiation and the 
etalon's front partially reflecting surface will resemble a Lissajous 
figure with more cycles in one direction than in the orthogonal direction. 
The nutating Fabry-Perot etalon uses a single detector to sense the varying 
constructive and destructive interference of the recombined coherent 
radiation components of the incoming radiation as the etalon is nutated as 
described. The advantage of a nutating system over an angle scan system is 
that only one detector and amplifier channel are required and no 
cylindrical lens is required. The information from the single detector is 
then processed to determine the direction of arrival and wavelength of 
radiation in the scene being viewed. 
The nutating Fabry-Perot etalon system requires more complex computation 
than the angle-scanned system. In some applications, however, it may be 
advantageous to have a very simple sensor head, with the computer located 
remotely where its size is not critical, such as a satellite borne sensor 
and earth based computer. 
There has thus been outlined rather broadly the more important features of 
the invention in order that the detailed description thereof that follows 
may be better understood, and in order that the present contribution to 
the art may be better appreciated. There are, of course, additional 
features of the invention that will be described hereinafter and which 
will form the subject of the claims appended hereto. Those skilled in the 
art will appreciate that the conception on which the disclosure is based 
may readily by utilized as a basis for designing other structures for 
carrying out the several purposes of the invention. It is important, 
therefore, that the claims be regarded as including such equivalent 
structures as do not depart from the spirit and scope of the invention. 
Specific embodiments of the invention have been chosen for purposes of 
illustration and description, and are shown in the accompanying drawings, 
forming a part of the specification.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
It will be appreciated, as the detailed description proceeds, that many 
different forms and combinations of interferometers and coherent energy 
modulators may be employed to carry out the concepts of this invention. 
For the purpose of illustration, a Fabry-Perot interferometer is employed 
in the description which follows. 
Further, while the invention is described herein in reference to detecting 
coherent radiation, it should be understood that it has application as 
well in detecting the coherent absence of radiation. 
An example of the coherent absence of energy is an absorption line. 
Absorption, generally, is the removal of energy from radation by the 
medium through which the radiation is passing. For low or ambient pressure 
gas the absorption line can be very narrow, as narrow as the coherent 
radiation detectable by the apparatus hereinbelow described. 
FIG. 1 shows, in block diagram form, the components of a scanning or 
nutating Fabry-Perot interferometer imaging coherent radiometer. Incoming 
radiation, designated "R", passes through any convenient light collector 
10 whereupon it emerges as a collimated beam of light designated "CR". The 
collimated beam of light "CR", falls incident on a Fabry-Perot etalon 12. 
In a manner described hereinbelow the etalon 12 produces varying 
constructive and destructive interference, designated "I", of the coherent 
energy component of the incoming radiation. 
This interference occurs as the etalon 12 is rotated about one or two 
orthogonal axes. Such rotation is accomplished about one axis by a motor 
16 acting through a first linkage 14 on the etalon 12. Rotation about the 
other axis is provided by a second motor 28 acting through a second 
linkage 22 on the etalon 12. 
The varying constructive and destructive interference is sensed by a 
detector 24 and amplified by an amplifier 26 which signal then enters a 
computer 20. Position signals through lines 18 and 30 from motors 16 and 
28 respectively are proportional to the angle of rotation of the etalon 12 
about its axes. In a manner described hereinbelow the computer 20 then 
compares the signal from the detector 24 with an internally generated 
reference waveform to determine the wavelength of the incoming coherent 
radiation. The computer 20, also in a manner described herein below, 
determines, based upon the position feedback signals through lines 18 and 
30, the direction of arrival of the coherent radiation source in a scene 
being viewed. This information is then conveyed to an observer through a 
display 21. 
FIG. 2 illustrates the geometry of a Fabry-Perot interferometer or etalon. 
It includes a spacer 36, having an index of refraction n, with optically 
polished and exactly parallel sides. Each side has a partially reflecting 
surface 34, 38 disposed thereupon. An incident wave has one component 
which is directly transmitted and another which is twice reflected. One 
beam path length is equal to s plus a plus b. The second beam path has the 
length s plus c. The optical path difference, OPD, through the etalon is: 
##EQU1## 
This can easily be calculated from Snell's Law (Sin.theta.=n Sin .theta.'), 
in view of the geometry of FIG. 2. It is found to be: 
##EQU2## 
where OPD=optical pathlength difference, in waves 
n=index of refraction of etalon spacer material 
d=spacer thickness 
.theta.=angle of incidence 
.theta.'=angle of incidence inside spacer 
.lambda.=wavelength 
The transmission through the etalon is a function of the angle of incidence 
.theta. and the wavelength .theta., .lambda., of the incident radiation 
and the reflectivity and transmission of the etalon surfaces. By summing 
the contributions to the transmitted wave from all possible multiple 
reflections and expanding, one obtains: 
##EQU3## 
Where T.sub.fp =transmission of a Fabry-Perot etalon 
R=reflectivity of etalon surfaces 
.phi.=(2 .pi.)OPD=optical phase difference 
.lambda.=wavelength 
T=transmission of etalon surfaces 
When R is close to unity, there are many multiple reflected wavelets 
contributing to the transmitted wave and the etalon is said to have high 
finesse. High finesse etalons are used for very narrow band spectral 
filters because of their narrow transmission characteristics as a function 
of wavelength. On the other hand, when reflectivity is moderate, the 
etalon is said to have low finesse. This type of etalon is best suited for 
coherent radiation detection. 
When a source is spectrally incoherent it emits light at many wavelengths. 
Therefore, to find the transmission of a Fabry-Perot etalon one must 
average over all wavelengths weighted by the spectral intensity. Referring 
to equation 2 for the transmission of a Fabry-Perot etalon, it is seen 
that when the spectrum of the source is broad (i.e, large variations in 
.phi.), the oscillating terms in the equation for T.sub.fp will average to 
zero. The transmission of the etalon will become a constant, independent 
of the OPD, consequently independent of the angle of incidence, wavelength 
and etalon thickness. Such a source will be unmodulated as these 
parameters are varied. It can be assured that .phi. will go through large 
variations for all but the narrowest, or most coherent, of sources by 
merely making the etalon thickness, d, sufficiently large. 
For a source to be coherent, the variation in .phi. must be small (less 
than .pi.). The spectral width, .DELTA..lambda., of the source must 
satisfy the equation: 
EQU .DELTA..lambda./.lambda.&lt;.lambda./4nd (3) 
where .lambda., .DELTA..lambda., n, .phi. and d have the meaning 
hereinbefore defined. 
A preselected etalon thickness, d, depending upon the spectral range, is 
selected to allow such spatially coherent radiation to be modulated while 
still not modulating the background, or non-coherent, radiation. 
FIG. 3 shows the transmission of a Fabry-Perot etalon T.sub.fp, versus the 
"scanning" angle of incidence, .theta., for a coherent radiation source. 
Point 78, corresponding to a zero angle of incidence, is readily found on 
both sides of this normal incidence point. It can be seen that the 
"frequency" of the quasi-periodic variation from maximum to minimum 
intensity increases the greater the angle of incidence, .theta., becomes. 
Thus, a frequency discriminator or other device can be used to determine 
on which side of .theta.=0 a particular portion of the curve shown in FIG. 
5 is and the magnitude of .theta.. Even if the Fabry-Perot etalon is 
titled to an angular range not including a zero angle of incidence (e.g., 
the range indicated by the bracket 80) such a frequency discriminator or 
other technique can determine the angle of the incoming coherent radiation 
by interpolating where the point 78 would be. 
FIG. 4 depicts, generally, a Fabry-Perot etalon 40 useful as an 
angle-scanned interferometer. 
The etalon structure itself comprises a transparent spacer 42, such as 
glass, with optically polished and exactly parallel partially reflective 
surfaces 48 and 44. Since a relatively "low finesse" etalon is desired, 
surfaces 48 and 44 are preferably between about 40 to 60 percent 
reflecting. 
The etalon 40 is mounted in any conventional manner, not shown, so as to 
allow rotation about any axis parallel to its surfaces 48 and 44 and 
substantially through the center of the etalon. Such an axis is shown as 
50. 
To provide the imaging capability of an imaging coherent radiometer the 
present invention provides a row 46 of detectors 45. 
The row of detectors 46 is located on the plane through the axis of 
rotation 50 and normal to the etalon 40 when the angle of rotation of the 
etalon about that axis is zero. In addition, the row of detectors is 
located on a plane parallel to the etalon surface 44 when the angle of 
rotation of the etalon 50 about its axis of rotation 50 is zero. The 
cylindrical lens 47, one focal length from the row of detectors 46, is 
disposed between the row of detectors 46 and the etalon 40 and serves to 
focus the collimated radiation, "CR", which passes through the etalon 40 
as a line onto the row of detectors 46. The cylindrical lens 47 has no 
power in the plane of the etalon scan. The line of radiation focused by 
the cylindrical lens 47 is incident on the row of detectors 46. 
As the etalon rotates each detector detects radiation and produces a signal 
in a manner previously described in U.S. Pat. No. 3,824,018. Referring to 
FIG. 3 and the discussion in reference thereto, it can be seen that the 
detector produces a sinusoidal signal of increasing frequency as the angle 
of incidence, .theta., of the incoming light, "L", increases. 
The rate at which this frequency increases is a function of the wavelength 
of the coherent light incident on the etalon. Through known 
auto-correlation techniques the signal from the detector is compared with 
internally generated waveforms of known wavelengths in the computer 20 of 
FIG. 1. Such comparisons are done to determine the wavelength of the 
incident coherent radiation. 
The direction, from the detector, of the source of coherent radiation is 
also determined. As previously described, a frequency discriminator or 
other device can readily determine the angle of incidence, .theta., of the 
curve shown in FIG. 3. Once this is known the position indicator 52 of 
FIG. 4 is used to determine the angle of rotation of the etalon. The angle 
of incidence to the etalon, coupled with the angle of rotation of the 
etalon yields the angle, from the detector, of the source of coherent 
radiation. 
The location, in the scene, of the source of coherent radiation in the 
direction along the axis of rotation 50 (the vertical direction as shown 
in FIG. 4) is determined by noting which of the detectors 45 in the row of 
detectors 46 is detecting coherent radiation. It can be understood that by 
providing a row of detectors 46 having in excess of the three detectors 45 
shown in FIG. 4 greater resolution and sensitivity of the radiometer can 
be achieved. 
It can thus be appreciated that the exact location of a source of coherent 
radiation in a scene being viewed can be determined. Such a position is 
described in terms of angle of incidence, .theta., and position on the 
axis of the detector row 46. The position information can then be 
displayed in any convenient manner, such as the display 21 of FIG. 1. 
As can be appreciated by those skilled in the art, detection and processing 
of a plurality of coherent radiation sources of the same or different 
wavelengths can proceed simultaneously. 
FIG. 5 shows, generally, a nutating interferometer 56. In the embodiment 
depicted a Fabry-Perot etalon is used. 
The Fabry-Perot etalon comprises a glass spacer 62 with optically polished 
and exactly parallel sides. On each side of the etalon is disposed a 
partially reflecting surface 54 and 58. Again, a relatively "low finesse" 
etalon is desired, hence surfaces 54 and 58 are preferably between about 
40 to 60 percent reflecting. 
The etalon is mounted in a gimbaled housing so as to allow it to rotate 
freely about point "P". The etalon rotates about two orthogonal axes, axis 
66 and and axis 68, at two different frequencies. One frequency is 
considerably higher than the other. Accordingly, the collimated radiation, 
designated "CR", describes a Lissajous figure at its points of 
intersection with the partially reflective surface 54. 
The etalon is scanned about two axes, 66 and 68, so that the computer, 20, 
of FIG. 1, can distinguish from which axis direction information is 
coming. The lowest frequency should be equal to or greater than one cycle 
per total field of view dwell time. Expressed symbolically: 
EQU T&lt;.phi.R/V 
where: 
T=radiometer field of view dwell time 
0=total radiometer field of view 
R=distance to scene being viewed 
V=velocity of radiometer scan relative to the scene being viewed 
The scan frequency, in one direction or about one axis, becomes&gt;.sup.1 /T. 
The second scan must be at a frequency high enough to minimize cross-talk 
in the computer 20 between the signals generated by the two scans. The 
ratio of the two scan frequencies is a function of the cross-talk and a 
desire to minimize scan frequencies in order to minimize bandwidth. The 
latter factor in turn will enhance signal to noise ratios. A satisfactory 
ratio of scan frequencies is 5.times.to 10.times.. 
To provide the imaging capability of an imaging coherent radiometer the 
nutating interferometer form of the present invention provides a single 
detector 64 which is substantially the same size as the etalon. 
The detector 64 may rotate with the etalon, in which case it is preferably 
positioned directly behind the etalon; or the detector may be stationary 
in which case some axial spacing between the etalon and detector is 
required. 
The nutating produces a variety of angles of incidence in both the x and y 
directions. The result of a nutating scan interferometer with a single 
detector 64 behind it is that the detector 64 scans a given scene in both 
the horizontal and vertical directions simultaneously. No imaging lens is 
required when a nutated etalon is used. Directional information in two 
orthogonal directions is obtained by processing the signals from a single 
detector relative to the synchronizing signals from the two orthogonal 
etalon rotations. 
If a simple light collecting optical arrangement is used an optional lens 
60 located between the etalon and the detector 64 is used. The lens 60 
acts as a "light bucket" to ensure that all light coming out of the etalon 
is directed to the detector 64. It can be understood by those skilled in 
the art that in optical arrangements, such as that shown in FIG. 1, where 
a telescope or light collector 10 is employed the lens 60 is not 
necessary. 
Referring now to FIG. 6, two position indicators 74 and 76, for the X and Y 
axes respectively, provide signals proportional to the amount of rotation 
of the etalon 56 about the X or Y axes. 
At each moment in time the etalon 56 is rotating simultaneously about the X 
and Y axes. As the etalon 56 is rotating, the detector 64 produces the 
characteristic waveform shown in FIG. 3 for each of the x and y directions 
when coherent radiation is incident on the etalon. This waveform is then 
processed by the computer 20 of FIG. 1 in the manner discussed in 
reference to FIG. 4 to yield information as to the direction of arrival 
and wavelength of the coherent radiation incident on the etalon 56. 
Position indicator 76 gives the angle of rotation of the etalon 56 about 
the Y axis. This information, coupled with the information on angle of 
incidence from the waveform of FIG. 3 is processed by the computer 20 
shown in FIG. 1, to give direction of arrival information for the coherent 
source, or sources, in the scene being viewed in the X direction. 
Likewise, the angle of rotation from position indicator 74 coupled with 
the waveform of FIG. 3 processed in the y direction gives the direction of 
arrival information in the Y direction for a coherent source, or sources, 
in the scene being viewed. Such information can then be displayed in any 
convenient manner, such as the display 21, shown in FIG. 1. 
It can be appreciated by those skilled in the art that the detection and 
processing of a plurality of coherent radiation sources of the same or 
different wavelengths can be accomplished simultaneously by the device 
disclosed. 
The coherent absence of energy is detected and processed in a manner 
similar to that described hereinabove for coherent energy. The waveform 
generated when the coherent absence of energy is being scanned will have a 
polarity opposite that shown in FIG. 3 since the cross-correlation 
operations used in processing the data are linear functions. 
Having thus described the invention with particular reference to the 
preferred forms thereof, it will be obvious to those skilled in the art to 
which the invention pertains, after understanding the invention that 
various changes and modifications may be made therein without departing 
from the spirit and scope of the invention, as defined by the claims 
appended hereto.