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Claims  |
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What is claimed is:
1. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation which, in combination, comprises:
at least three unequal path interferometers with the radiation path length
difference in each interferometer being substantially greater than the
coherence length of the incoherent radiation, but substatially less than
the coherence length of the coherent radiation, the average radiation path
length in said three interferometers being different one from the others;
means for detecting radiation transmitted through each of said
interferometers and for generating separate first, second, and third
signals corresponding to the radiation leaving each of said
interferometers;
means for processing said signals through electronic signal processing
elements to detect and determine the wavelength of the coherent radiation,
means for equalizing the radiation intensity between pairs of said
detecting means.
2. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 1 wherein said
means for processing said signals through electronic signal processing
elements includes analog operative elements.
3. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 1 wherein said
means for processing said signals through electronic signal processing
elements includes digitally operative elements.
4. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation which, in combination, comprises:
a Fabry-Perot etalon having at least three regions of different thickness,
said etalon having a first surface positioned to receive coherent
radiation and a second surface, the average distance between said surfaces
being such that the radiation path difference between transmitted portions
of radiation impinging on said first surface is substantially greater than
the coherence length of incoherent radiation but substantially less than
the coherence length of the coherent radiation;
means for detecting radiation transmitted through each of said regions and
generating separate first, second and third signals porportional to the
intensity of radiation leaving each of said regions; and
means for processing said signals thorough electronic signal processing
elements to detect and determine the wavelength of the coherent radiation,
means for equalizing the radiation intensity between pairs of said
detecting means.
5. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 4 further
comprising means for rotating said etalon in a predetermined systematic
manner, means for detecting a phase difference between the detected
signals, and means for determining the direction of the coherent radiation
source corresponding to the detected phase difference and the wavelength
of the coherent radiation.
6. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 4 wherein said
means for processing said signals through electronic signal processing
elements includes analog operative elements.
7. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 4 wherein said
means for processing said signals through electronic signal processing
elements includes digitally operative elements.
8. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 4 wherein said
means for processing said signals through electronic signal processing
elements includes a comparator associated with each two of said detectors,
a plurality of coincident gates associated with said comparators, the
number of said gates being one less than the number of comparators, for
outputting a plurality-bit wavelength code.
9. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 4 wherein said
means for equalizing the radiation intensity includes a beam-splitter
wherein the radiation entering the beam-splitter is divided so that one
portion passes through the beam dividing surface to a first one of said
regions of the etalon and a second portion is reflected from the beam
dividing surface to a second one of said regions of the etalon.
10. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 9 wherein said
beam-splitter has a plurality of dots on the beam dividing surface which
are highly reflective and the spaces therebetween are highly transmitting,
the size of said dots being preselected so that they are smaller than the
atmosphere scintillation spatial structure.
11. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent radiation according to claim 4 further comprising
common aperture means for directing radiation to said first surface so
that the radiation intensities incident on all parts of said surface are
substantially the same.
12. Apparatus for detecting a source of coherent radiation in the presence
of noncoherent ambient radiation which, in combination, comprises:
a Fabry-Perot etalon having at least two regions of different thickness,
said etalon having a first surface positioned to receive coherent
radiation and a second surface, the average distance between said surfaces
being such that the optical path difference between transmitted portions
of radiation impinging on said first surface is substantially greater than
the coherence length of incoherent radiation but substantially less than
the coherence length of the coherent radiation; common aperture means for
directing radiation to said first surface so that the radiation
intensities incident on all parts of said surface are substantially the
same;
means for rotating said Fabry-Perot etalon in a predetermined systematic
manner; and
means for detecting radiation transmitted through each of said regions and
generating separate signals proportional to the intensity of radiation
leaving each of said regions;
means for effectively subtracting said separate signals so that any steady
background signal components caused by noncoherent radiation in said
separate signals are substantially eliminated from the resulting
difference signal, while the variable signal components caused by the
coherent radiation are effectively added;
whereby said variable signal component is of almost periodic form but of
gradually increasing frequency as the impinging coherent radiation makes
an increasing angle of incidence relative to said etalon.
13. Apparatus for detecting a source of coherent radiation in the presence
of noncoherent ambient radiation according to claim 12 wherein said common
aperture means comprises a beam-splitter having a plurality of dots on the
beam dividing surface which are highly reflective and the spaces
therebetween are highly transmitting, the size of said dots being
preselected so that they are smaller than the atmosphere scintillation
spatial structure.
14. Apparatus for detecting a source of coherent radiation in the presence
of noncoherent ambient radiation which, in combination, comprises;
a Fabry-Perot etalon having at least two regions of different thickness,
said two regions being formed of a plurality of interdigitated steps, said
etalon having a first surface positioned to receive coherent radiation and
a second surface, the average distance between said surfaces being such
that the optical path difference between transmitted portions of radiation
impinging on said first surface is substantially greater than the
coherence length of incoherent radiation but substantially less than the
coherence length of the coherent radiation;
means for rotating said Fabry-Perot etalon in a predetermined systematic
manner;
two detector means corresponding to the etalon regions, said detector means
having a plurality of interdigitated elements corresponding to the
interdigitated steps of the etalon which are adjacent or in contact
therewith, said detector means serving to detect the radiation transmitted
through each of said regions and generate separate signals proportional to
the intensity of radiation leaving each of said regions, the width of said
elements being preselected so that they are smaller than the atmosphere
scintillation spatial structure;
means for effectively subtracting said separate signals so that steady
background signal components caused by noncoherent radiation in said
separate signals are substantially eliminated from the resulting
difference signal, while the variable signal components caused by the
coherent radiation are effectively added;
whereby said variable signal component is of almost periodic form but of
gradually increasing frequency as the impinging coherent radiation makes
an increasing angle of incidence relative to said etalon.
15. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation which comprises;
a Fabry-Perot etalon having at least three regions of different thickness,
said etalon having a first surface positioned to receive coherent
radiation and a second surface, the average distance between said surfaces
being such that the optical path difference between transmitted portions
of radiation impinging on said first surface is substantially greater than
the coherence length of incoherent radiation but substantially less than
the coherence length of the coherent radiation;
means for detecting radiation transmitted through each of said regions and
generating separate first, second, and third signals proportional to the
intensity of radiation leaving each of said regions;
means for generating a fourth signal proportional to the difference between
said first and second signals;
means for generating a fifth signal proportional to the difference between
said second and third signals; and
means for generating an output proportional to the ratio between said
fourth and fifth signals.
16. The apparatus of claim 15 wherein said output generating means
comprises: means for generating a sixth signal proportional to said ratio;
and means responsive to said sixth signal for generating an output
proportional to the arccosine of said ratio.
17. The apparatus of claim 16 wherein said etalon has four regions.
18. The apparatus of claim 17 wherein the optical phase difference between
adjacent regions is from approximately 55.degree. to approximately
150.degree..
19. The apparatus of claim 15 wherein said etalon has four regions.
20. The apparatus of claim 15 wherein the optical phase difference between
adjacent regions is from approximately 55.degree. to approximately
150.degree..
21. The apparatus of claim 15 further comprising common aperture means for
directing radiation to said first surface so that the radiation
intensities incident on all parts of said surface are substantially the
same.
22. The apparatus of claim 21 wherein said common aperture means comprises
a beam-splitter having a plurality of dots on the beam dividing surface
which are highly reflective and the spaces therebetween being highly
transmitting, the size of said dots being preselected so that they are
smaller than the atmosphere scintillation spatial structure.
23. Apparatus for detecting a source of coherent radiation in the presence
of noncoherent ambient radiation which, in combination, comprises;
a Fabry-Perot etalon having at least three regions of different thickness,
said regions being formed of a plurality of interdigitated steps, said
etalon having a first surface positioned to receive coherent radiation and
a second surface, the average distance between said surfaces being such
that the optical path difference between transmitted portions of radiation
impinging on said first surface is substantially greater than the
coherence length of incoherent radiation but substantially less than the
coherence length of the coherent radiation;
a plurality of detector means corresponding in number to the number of
etalon regions, said detector means having a plurality of interdigitated
elements corresponding to the interdigitated steps of the etalon which are
adjacent or in contact therewith, said detector means serving to detect
the radiation transmitted through each of said regions and generate
separate signals proportional to the intensity of radiation leaving each
of said regions, the width of said elements being preselected so that they
are smaller than the atmosphere scintillation spatial structure; and
means for processing said signals through electronic signal processing
elements to detect and determine the wavelength of the coherent radiation.
24. The apparatus of claim 23 wherein said means for processing said
signals through electronic signal processing elements includes a
comparator associated with each two of said detectors means, a plurality
of coincident gates associated with said detector means, the number of
said gates being one less than the number of detector means, for
outputting a plurality-bit wavelength code.
25. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation which, in combination, comprises;
a Fabry-Perot etalon having eight regions of different thickness, said
etalon having a first surface positioned to receive coherent radiation and
a second surface, the average distance between said surfaces being such
that the optical path difference between transmitted portions of radiation
impinging on said first surface is substantially greater than the
coherence length of incoherent radiation but substantially less than the
coherence length of the coherent radiation;
means for detecting radiation transmitted through each of said regions and
generating first, second, third, fourth, fifth sixth, seventh and eighth
signals proportional to the intensity of radiation leaving each of said
regions;
said first and eighth signals being applied to a first comparator means to
determine the polarity thereof, said second and seventh signals being
applied to a second comparator means to determine the polarity thereof,
said third and sixth signals being applied to a third comparator means to
determine the polarity thereof, and said fourth and fifth signals being
applied to a fourth comparator means to determine the polarity thereof;
and
the outputs from the first and second comparator means being connected to a
first coincidence gate means, the outputs from said second and third
comparator means being connected to a second coincident gate means, and
the outputs from said third and fourth comparator means being connected to
a third coincident gate means, said coincident gate means serving to
compute a three-bit wavelength code.
26. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 25 wherein the
thicknesses of said regions are preselected so that polarity reversals
occur at different wavelengths, to thereby provide a binary code for the
wavelength of the coherent radiation.
27. Apparatus for determining the wavelength of coherent radiation in the
presence of incoherent ambient radiation according to claim 25 further
comprising common aperture means for directing radiation to said first
surface so that the radiation intensities incident on all parts of said
surface are substantially the same.
28. The apparatus of claim 27 wherein said common aperture means comprises
a beam-splitter having a plurality of dots on the beam dividing surface
which are highly reflective and the space therebetween being highly
transmitting, the size of said dots being preselected so that they are
smaller than the atmosphere scintillation spatial structure. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to unequal path interferometers, and more
particularly to a plurality of associated unequal path interferometers.
Interferometers constructed in accordance with the concepts of this
invention are adapted, among other possible uses, for use in detecting,
and determining the wavelength of coherent radiation, as from a laser, in
a quantum of radiation including incoherent radiation. It is particularly
adapted for discriminating coherent radiation from incoherent radiation,
and determining wavelength, from a single pulse or from a continuous wave
of the radiation. In addition, it is adapted to determine the relative
angular position of the source of the radiation. A preferred form of the
apparatus according to the invention includes a Fabry-Perot etalon having
a plurality of regions of different thicknesses.
A Fabry-Perot etalon interferometer consists normally of two plane,
parallel partially reflecting surfaces formed on a solid glass spacer so
that one portion of incident radiation is transmitted directly through
while other portions, being reflected between the partially reflecting
surfaces before emerging, are transmitted over a longer path.
As described in U.S. Pat. No. 3,824,018 to R. Crane, and assigned to the
same assignee as the present application, a Fabry-Perot etalon is adapted
to discriminate coherent radiation by making the optical thickness of the
glass spacer sufficiently less than the absolute coherence length of the
incident radiation (ie. the absolute of the coherence length of the
coherent and incoherent incident radiation) so that the absolute coherent
length will be substantially less than the difference between the lengths
of the paths of the directly and indirectly transmitted radiation (ie. the
optical path difference or OPD). Then, by changing the path length, by
means of tilting the etalon, the intensity of the incoherent radiation
transmitted will remain constant, but the coherent radiation modulates.
The intensity of radiation transmitted through the etalon is a function of
the OPD and of the wavelength of the radiation. The OPD is a function of
the index of refraction of the spacer, of the thickness of the spacer and
the angle of refraction of the radiation passing through the interior of
the spacer. As the etalon is tilted in a scanning mode at a predetermined
rate to vary the optical path lengths, and the OPD, the intensity of the
transmitted radiation, suitably detected by a photodetector at the back of
the etalon, varies in a manner such that the frequency of the radiation as
picked up by the detector decreases and reaches a minimum as the etalon
swings through a position at which the angle of incidence of the impinging
radiation, and hence the angle of refraction is zero. By this means the
etalon is utilized to detect the relative position of the source of the
coherent radiation. Additionally, this etalon may be used to determine the
wavelength of the coherent radiation at a preselected scan rate with
reference to coherent radiation of known wavelength and comparing the
detected frequency-wavelength pattern with the pattern similarly produced
by the coherent radiation whose wavelength is known.
The aforementioned patent also disclosed a two stepped etalon to, in
effect, provide two etalon regions of different thicknesses. As disclosed,
the thicknesses differ by a quarter wavelength of the wavelength of the
coherent radiation to be detected so that the optical path differences of
the two etalon regions differ by a half wavelength. A detector is provided
for each of the two etalon portions. The output signals from the two
detectors, which are made to be of opposite polarity are added, so that
the constant output signal components produced by the incoherent radiation
cancel out and the coherent radiation signals are accentuated. Otherwise,
the detected output is analyzed in the same manner as described with
reference to the single etalon embodiment.
While the device of the aforementioned patent operated with reasonable
success in certain installations having a continuous wave source, my
contribution to the art is a new etalon interferometer, which is an
improvement over such prior devices, which is capable of processing a
single pulse to distinguish the characteristics thereof, and which has
other advantages, as will become apparent as the description proceeds.
SUMMARY OF THE INVENTION
One of the objects of the present invention is the provision of new and
improved apparatus for detecting and/or determining the wavelength of
coherent radiation in the presence of incoherent ambient radiation, which
is compact, efficient, light weight and relatively inexpensive to
fabricate.
Another object of the invention is the provision of apparatus of the
aforesaid character, which covers a wide range of radiation fields and
which has a wide field of view.
As yet another object of the invention, there is provided apparatus of the
type described, which has high sensitivity and which may determine the
relative angular position of the source of radiation.
To accomplish the foregoing and other objectives, the invention
contemplates the provision of new and improved apparatus for determining
the wavelength of coherent radiation in the presence of incoherent ambient
radiation which, in one form thereof, comprises, in combination, at least
three unequal path interferometer means with the radiation path length
difference in each interferometer being substantially greater than the
coherence length of the incoherent radiation, but substantially less than
the coherence length of the coherent radiation. The average radiation path
length in said three interferometer means is different one from the
others. The apparatus further includes means for detecting the radiation
transmitted through each of the interferometers and for generating
separate first, second and third signals corresponding to the radiation
leaving each of the interferometer means. In addition, the apparatus
includes means for processing the signals through electronic processing
elements to detect and determine the wavelength of the coherent radiation.
While the unequal path interferometers may take any one of a number of
various different forms such as a Michelson or Fizeau, for example, a
presently preferred from comprises a Fabry-Perot etalon having at least
three regions of different thickness. The etalon has a first surface,
positioned to receive coherent radiation, and a second surface. The
average distance between the two surfaces is such that the radiation or
optical path difference between the transmitted portions of radiation
impinging on the first surface is substantially greater than the coherence
length of the incoherent radiation, but substantially less than the
coherence length of the coherent radiation.
According to one aspect of the invention, the means for processing the
signals through the electronic processing elements to detect and determine
the wavelength of coherent radiation comprises operative elements, which
in one form thereof may include means for generating a fourth signal
proportional to the difference between the first and second signals, and
means for generating a fifth signal proportional to the difference between
the second and third signals, and means for generating an output
proportional to the ratio between the fourth and fifth signals.
According to another aspect of the invention, the means for processing the
signals through the electronic processing elements to detect and determine
the wavelength of coherent radiation comprises operative elements, which
in one form thereof may include means for generating a fourth signal
proportional to the difference between the first and second signals, and
means for generating a fifth signal proportional to the difference between
the second and third signals, and means for generating an output
proportional to the ratio between the fourth and fifth signals.
According to another aspect of the invention, the means for processing the
signals through the electronic processing elements to detect and determine
the wavelength of the coherent radiation comprises digitally operative
elements, which may include in one form thereof, a comparator associated
with each two of said detectors, a plurality of coincident gates
associated with the comparator, the number of said gates being one less
than the number of comparators, for outputting a plurality-bit wavelength
code. Preferably, the thickness of the etalon regions are preselected so
the polarity reversals occur at different wavelengths, to thereby provide
a binary code for the wavelength of the coherent radiation.
It will be appreciated that in some installations problems may occur due to
atmospheric scintillation effects, which destroy the spatial coherence of
the wavefront across the etalon. That is, a stepped-etalon system, of the
type described hereinbefore, relies upon achieving coherent signal
detection and absolute background and background scan noise (incoherent
signal) rejection by means of balanced detectors and signal cancellation
(subtraction) techniques. I have overcome or at least mitigated this
problem in a new and improved manner. According to another aspect of the
invention, I provide means for equalizing the radiation intensity between
the pairs of detectors. The means for equalizing the radiation intensity
between the detectors, in one form thereof is a beam-splitter wherein the
radiation entering the beam-splitter is divided so that one portion passes
through the beam dividing surface to a first one of the regions of the
etalon and a second portion is reflected from the beam dividing surface to
a second one of the regions of the etalon. Thus, a common aperture means
is provided for directing radiation to all portions of the etalon.
Preferably, the beam-splitter has a plurality of dots on the beam dividing
surface, which are highly reflective and the spaces therebetween are
highly transmitting, the size of the dots being preselected so that they
are smaller than the atmosphere scintallation spatial structure. In
another form of the invention, I equalize the radiation intensity between
the detectors by means of providing an etalon having a plurality of
regions of different thicknesses wherein the regions are in the form of a
plurality of interdigitated steps. A plurality of detectors are provided
corresponding in number to the etalon regions, said detectors having a
plurality of interdigitated elements corresponding to the interdigitated
steps of the etalon, that are adjacent or in contact therewith. The widths
of the detector elements are preselected so that they are smaller than the
atmosphere scintillation spatial structure.
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 upon which the disclosure is based
may readily be utilized as a basis for the designing of 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 specifications.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating the geometry of a Fabry-Perot etalon;
FIG. 2 is a schematic diagram of a single step etalon detector apparatus,
such as is known to the prior art;
FIG. 3 is a schematic illustration of the waveforms produced by the
detector of FIG. 2;
FIG. 4 is a schematic illustration of a Fabry-Perot etalon and detectors in
accordance with the present invention;
FIGS. 5 and 5A are schematic illustrations of coherent radiation wavelength
detector apparatus in accordance with the present invention;
FIG. 6 is a schematic illustration of coherent radiation wavelength
detector apparatus similar to FIG. 5, but showing another embodiment of
the invention;
FIG. 7 is a graphic representation of the detected radiation polarity for
the various etalon steps, of the detector apparatus of FIG. 6;
FIG. 8 is a schematic illustration of a beam-splitter employed in
association with the single step etalon detector apparatus of FIG. 2,
according to the present invention;
FIG. 9 is a schematic illustration of a plurality of beam-splitters
employed in association with the coherent radiation wavelength detector
apparatus of FIG. 5;
FIG. 10 is a schematic illustration of a module type beam-splitter in
association with a coherent radiation wavelength detector apparatus,
according to another form of the invention;
FIG. 11 is a schematic illustration of a beam-splitter constructed, in
accordance with the concepts of this invention, for use in conjunction
with the embodiments of FIGS. 2, 5, 6 and 10;
FIG. 12 is a schematic illustration of an interdigital type of coherent
radiation wavelength detector apparatus according to another form of the
invention;
FIG. 13 is an enlarged plan view of the interdigital detector array of the
embodiment of FIG. 12; and
FIGS. 14A and 14B illustrate apparatus to detect the existence of a
coherent source in the presence of incoherent background.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
It will be appreciated that many different forms of unequal path
interferometers may be employed to carry out the concepts of this
invention, such as a Fabry-Perot, Michelson or Fizeau, for example.
However, it is necessary to provide means for varying the radiation or
optical path difference, as will be explained more fully hereinafter. It
has been found that a Fabry-Perot interferometer or etalon is particularly
desirable for carrying out the invention.
FIG. 1 illustrates the geometry of a Fabry-Perot interferometer or etalon.
It comprises a flat glass spacer 10 having partially reflecting surfaces
12, 14. An incident wave has one component which is directly transmitted
and another which is twice reflected. One beam path is determined by the
distance a plus b. The second beam path is determined by the distance c.
The optical path difference:
OPD=(a+b)-c (1)
can be easily calculated from Snell's law (sin .theta.=n sin .theta.') and
the geometry of FIG. 1. It is found to be
OPD=2nd cos .theta.'=2d(n.sup.2 -sin.sup.2 .theta..sup.1/2 (2)
The transmission through the etalon is a function of both the angle .theta.
and the wavelength .lambda. of the incident light. By summing the
contributions to the transmitted wave from all possible multiple
reflections and expanding, one obtains
##EQU1##
where T.sub.FP =transmission of Fabry-Perot etalon
R=reflectivity of etalon surfaces
.phi.=(2.pi./.lambda.) OPD=optical phase difference
.lambda.=wavelength
T=transmission of etalon surface
n=index of refraction of etalon
.theta.=angle of incidence
.theta.'=angle of refraction (in etalon)
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
detection of coherent radiation.
In order to have a high probability of detecting a short laser pulse, a
detector must look in all directions and at all wavelengths all the time.
This requirement necessitates that considerable background radiation must
also be viewed. Natural background radiation may be diffusely reflected
sunlight in the visible and near infrared, or thermal radiation in the far
infrared. In addition, other spurious sources of radiation may also exist,
such as specularly reflected sunlight, lightning flashes, and various
manmade incoherent light sources. A useful detector must be able to
distinguish between a coherent source and background and spurious
radiations.
Properties that may be useful in distinguishing laser radiation from other
radiations are listed in the following table:
______________________________________
SOURCE
NATURAL
BACK- SPURIOUS
LASER GROUND SOURCES
______________________________________
SPECTRAL
COHERENCE Very Low to
(Single Wavelength)
High Low Medium
Spatial Coherence
(can be imagaged to
Very
a point) High Low High
TIME Short
VARIATIONS Pulse Short Pulse
to CW Low Frequency
to CW
______________________________________
All lasers exhibit high spectral and spatial coherence by their very
nature. However, the time properties of the laser can be manipulated by
the user. It is therefore desirable to use spectral and spatial coherence
as the means of discriminating a laser from the background and to use time
characteristics only to enhance the detection of specific types of lasers.
The Fabry-Perot etalon provides a technique for using the spectral and
spatial coherence properties of a laser to distinguish it from the
background.
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 the series expansion 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 equation (3) will all
average to zero. The transmission of the etalon will become a constant,
independent of the optical path difference, i.e., independent of the
angle, 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 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.). One can readily verify that the spectral width,
.DELTA..lambda., of the source must satisfy
.DELTA..lambda./.lambda.<.lambda./4nd (cos .theta.'1)
For example, if nd is 1 mm, and .lambda. is 1 .mu.m, the spectral width of
the source cannot exceed 2.5 A before the oscillating terms start to be
averaged. As a practical matter, the etalon must not be so thick that the
more incoherent lasers, such as GaAs, will be attenuated. A preselected
thickness, depending upon the spectral range, is selected to allow such
lasers to be modulated while still not modulating the background.
Although the background is not modulated by the Fabry-Perot etalon, it is
still present in the form of a "constant" illumination. Variations in the
level of background illumination cannot be distinguished from the
modulation due to a laser source. Unless such background intensity
variations can be removed, they present a serious limitation to the
sensitivity of the sensor. The problem of background intensity variations
(hereinafter called "scan noise" to distinguish them from shot noise) is
not peculiar to the Fabry-Perot technique. It is present to some extent in
all sensor systems that scan a nonuniform scene or view a fluctuating
scene. It is a particularly important limitation in the detection of
continuous or long pulse lasers.
The Fabry-Perot technique provides a simple means of reducing the
background scan noise. FIG. 2 illustrates an etalon 16, having a step 18.
As was pointed out above, the transmission of the Fabry-Perot etalon is
constant for the incoherent background. Therefore, the background is
transmitted equally through both halves of the etalon. By using two
identical detectors 20, 22, one behind each half of the etalon, and
differencing their outputs, the background level can be subtracted out.
This technique can reduce the background level by more than two orders of
magnitude. Scan noise is reduced by the same factor.
For coherent illumination, the transmission of the etalon is a function of
the optical path difference (OPD). Therefore, the transmission through the
two halves of the etalon can be made to be different by the proper choice
of the OPD of the step. If we choose the step to have an OPD of
approximately a half wavelength, then the transmission of the two halves
of the etalon will be out of phase for coherent light. That is, if one
side has maximum transmission, the other will have minimum transmission.
The signal levels from the two detectors 20, 22 are shown schematically in
FIG. 3. The signals are passed through a differential amplifier 23. As a
result, the out of phase coherent radiation is reinforced, while the in
phase background is cancelled, as shown by the bottom curve. It will be
seen that the stepped etalon technique provides a powerful method for
reducing the scan noise by several orders of magnitude. This technique
rejects incoherent sources no matter what their time characteristics may
be. Thus, even short bursts of incoherent illumination from lightning,
shell bursts, etc., will be suppressed. This is because the Fabry-Perot
laser detector is truly a coherent light modulator and not just a
radiometer.
DETERMINATION OF WAVELENGTH
The transmission of the Fabry-Perot etalon is wavelength dependent because
the optical phase difference, .phi., is wavelength dependent. If .phi.
could be measured, then the wavelength could be readily determined.
However, one actually measures the transmission and, therefore, the cosine
of .phi.. Measuring the cosine of .phi.does not yield a unique value for
.phi. because any multiple of 2.pi. can be added to .phi. without changing
the cosine.
This ambiguity can be overcome by measuring the change in .phi., rather
than .phi. itself. This can be done by varying .theta. (i.e., scanning the
etalon) or by varying the optical thickness "nd". When the laser radiation
occurs in the form of short pulses, the scanning technique requires many
pulses to unambiguously determine the wavelength. If wavelength is to be
determined for a single short pulse of laser illumination, I have found it
desirable to vary the etalon thickness rather than the angle. The
thickness variation can be a simple step in the etalon as described above.
While the addition of another step is sufficient to allow the wavelength
to be determined, the addition of two steps preserves symmetry and
provides redundancy. An etalon 24, having steps 26, 28, 30 and detectors
32, 34, 36, 38 is illustrated in FIG. 4.
To show how such an etalon can be employed to measure wavelength, consider
the signals from each channel when the multistepped etalon 24 is
illuminated by a laser radiation of intensity I.
Let
##STR1##
where: d is the average thickness of the etalon
s is the height of each step
.theta.' is the angle of refraction in the etalon
n is the index of the etalon.
Then the signals from the various detectors can be written as
##EQU2##
The transmission of the etalon, T.sub.FP, is approximately a constant plus
an oscillatory function of .phi., as can be seen from equation (3). The
only difference between signals from different channels is that the
modulation of the coherent laser signal by the etalon has been shifted in
phase. Taking the difference between adjacent channels nulls out the
background and leaves only the coherent laser signal modulated by the
differenced etalon transmission.
##EQU3##
These signals are also oscillatory in nature. By measuring the phase shift
between channels, the value of .beta. can be determined. However, one must
make the steps in the etalon sufficiently small, and in some cases it is
desirable to keep .beta. less than .pi.. This allows the wavelength to be
determined unambiguously because arccosine B is a single valued function.
The wavelength can then be determined from
.lambda.=4ns cos .theta.'/.beta..apprxeq.4ns/.beta. (7)
when n and s are known quantities.
Because .theta.' is such a small angle, the cosine of .theta.' is very
nearly unity. The cosine term can be set equal to unity for all but the
largest fields of view. The error can be further reduced by using an
average value for cosine .theta.'.
It will be appreciated that instead of using an etalon having a plurality
of steps to provide the optical path difference, it is also possible to
use an etalon having regions fabricated of different materials to thereby
vary the index (n) of the etalon or it is possible to vary the cosine
.theta.' factor by using etalons at different angles of incidence or by
changing .theta.' with time (scanning) the etalon to thereby provide an
optical path difference.
ANALOG APPARATUS FOR DETERMINING WAVELENGTH
For purposes of illustrating the apparatus, an example apparatus will be
described for the 0.4 to 1.1 .mu.m spectral region. It will be appreciated
that the apparatus can be used in other spectral regions by suitably
changing the material and dimensions.
FIG. 5 schematically illustrates analog circuitry for detecting the
wavelength of coherent radiation. A multiple step solid etalon 24 is
employed. This may be approximately 50 .mu.m thick, in the 0.4 to 1.1
.mu.m region, for example. It has four parts, each of slightly different
thickness. In this example, the steps are approximately 0.1 .mu.m. When
the etalon 24 is illustrated by a single short pulse of laser radiation,
the output of each of the four optical detectors 32, 34, 36, 38 is
likewise just a short pulse. These pulses are subtracted by difference
amplifiers 40, 42, 44. The difference signals pass to peak detectors 46,
48, 50 where they are converted into levels that are held long enough to
be further processed by the electronics. For each pulse that is detected,
three signal levels are thus generated, corresponding to the three
differences between adjacent channels. There are two simple ways to
convert these levels into wavelengths.
If R is made sufficiently small that the higher terms in equation (3) are
small, and can be neglected, T.sub.FP can be considered to be sinusoidal
in nature. Neglecting all terms of R.sup.2 and higher order gives for the
difference:
##EQU4##
Then by forming the ratio
(S.sub.12 +S.sub.34)/2S.sub.23 =cos .beta. (9)
.beta. and .lambda. can be determined. To determine the wavelength
unambiguously the steps in the etalon can be made sufficiently small that
.beta. is always less than .pi.. Specifically, for the 0.4 to 1.1 .mu.m
region, .beta. is restricted to
55.degree..ltoreq.B.ltoreq.150.degree. (10)
where the arccosine function is most linear.
This method makes the calculation of wavelength extremely simple and is
easily implemented as shown in FIG. 5. The outputs of peak detectors 46
and 50 are summed. A conventional ratio module 52 provides a signal
corresponding to cos .beta.. This is converted to reciprocal wavelength by
a conventional arccosine module 54. An energy level circuit 56 may also be
employed to measure pulse energy. To make the higher order terms
sufficiently small to achieve good accuracy, some sensitivity must be
sacrificed by making R small. The optimum value for R is 41% to 50%. If
only coarse wavelength is desired, or if reduced sensitivity is tolerable,
this method is very simple to implement as real-time hardware.
If R is chosen to have its optimum value of 41%, then equation (6) is not
easily solved for cosine of .beta.. Nevertheless, since there are three
equations, the value of .beta. can be uniquely determined. One way of
determining .beta. is to form two ratios, such as S.sub.23 /S.sub.12 and
S.sub.34 /S.sub.12. As best seen in FIG. 5A, the outputs 1, 2 and 3 from
peak detectors 46, 48 and 50 (FIG. 5) are fed to radio modules 47 and 49.
For every pair of ratios there exists a unique value of cosine .beta. and,
therefore, of .lambda.. To find this wavelength, a "lookup" table 51 is
constructed. Then for each pair of ratios measured by the sensor, one
simply looks up the correct wavelength in the table, just as one would
look up a cosine function. If the field position is known, this
information could be added to the lookup table to give the wavelength with
greater accuracy. This is the preferred implementation.
For continuous wave (CW) signals, the phase shift between the various
channels can be measured in the same manner as described above. If the
sensor is scanned, the different signals will oscillate in time. The phase
shift between the different channels can then be measured directly. This
will allow a very accurate determination of wavelength. The frequency of
the signal can be used to obtain location information.
DIGITAL APPARATUS FOR DETERMINING WAVELENGTH
It is noted that in the form of the invention illustrated in FIG. 5, analog
ratios are computed to normalize the intensity of the light. All data are
then extracted from the ratios that are proportional to the transmission
of the etalon. I have found that it is possible to use digital signal
processing techniques. If the difference in intensities across a step in
the etalon is taken, the polarity of the signal is independent of the
background intensity, as well as of the signal intensity. Changes in
polarity are caused only by changes in the optical phase difference of the
two coherent light paths. Furthermo | | |
