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Claims  |
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What is claimed is:
1. Apparatus for determining the submicroscopic surface characteristics of
an object which is mounted on a stage with the face to be examined exposed
comprising:
means for subjecting said stage to a series of different states of motion;
means for illuminating the exposed areas of said object with coherent
monochromatic radiation of known amplitude and frequency,
said states of motion causing said radiation to be reflected and scattered
from said object in a multiplicity of different frequency components with
the amplitude and phase of each instantaneous overall component determined
by the amplitudes and phases of components scattered from individual
points on said surface,
the scattered radiation contributions of amplitude and phase from said
individual points being representative of the at rest radiation scattered
from each point and the effect of said states of motion thereon; and
means for measuring and recording the amplitude and phase of selected ones
of said different overall frequency components for each different state of
motion so that an image of said object may be constructed.
2. The apparatus as defined in claim 1 wherein said means for subjecting
said stage to motion include means for oscillating said stage about a
displaceable pivot axis, said illuminating means include a laser beam
having a selected wavelength .lambda. and means for focusing said beam,
said object is positioned within the focal volume of said focusing means,
and said measuring and recording means include means for detecting said
different frequency components, means for recording selected instantaneous
attitudes of said stage, means for filtering out undesired frequencies,
means for measuring the amplitude and phase of unfiltered frequency
components, and means for correlating and storing said measuring
amplitudes and phases,
whereby recordings of radiation scattered from points on said object while
in motion may be accumulated and related to radiation scattered from the
object at rest.
3. The apparatus as defined in claim 2 wherein said pivot axis is
displaceable parallel to its original position so that a one-dimensional
image of said object may be constructed and displayed.
4. The apparatus as defined in claim 3 wherein said oscillation is
sinusoidal and said detection is optical homodyne detection so that the
amplitudes of the frequency components of the homodyne signal are given by
Fourier analysis of the expression
##EQU8##
where K is a constant, E.sub.o is the amplitude of said monochromatic
radiation, E.sub..eta. is the amplitude of the radiation scattered from
the .eta..sup.th point on the object, .alpha..sub..eta. is the quiescent
difference in phase at said detecting means between the incident laser
radiation and the radiation reflected from the .eta..sup.th point on the
object, R.sub..eta..sup.p is the distance from the pivot axis position to
the .eta..sup.th point on the object during the p.sup.th measurement,
.theta..sub.m is the maximum angular excursion of the oscillating stage,
and .omega..sub.v is the angular oscillation frequency of the stage.
5. The apparatus as defined in claim 4 wherein said pivot axis is oriented
to a multiplicity of mutually non-parallel attitudes and a two-dimensional
image of said object is constructed by superposition of the multiplicity
of one-dimensional images thus obtained.
6. The apparatus as defined in claim 1 wherein said means for subjecting
said stage to motion include at least two electromechanical transducers
connected one at each end of said stage and means for driving said
transducers and oscillating said stage ends respectively according to the
expressions A.sub.1 Sin .omega..sub.m t and A.sub.2 Sin (.omega..sub.m
t+.pi.) where A.sub.1 and A.sub.2 are respective amplitudes and
.omega..sub.m is the frequency of oscillation, said illuminating means
include a laser beam having a selected wavelength .lambda. and means for
focusing said beam, said object is positioned within the focal volume of
said focusing means, and said measuring and recording means include means
for detecting said different frequency components, means for recording
selected instantaneous attitudes of said stage, means for filtering out
undesired frequencies, means for measuring the amplitude and phase of
unfiltered frequency components, and means for correlating and storing
said measured amplitudes and phases,
whereby recordings of radiation scattered from points on said object while
in motion may be accumulated and related to radiation scattered from said
object at rest.
7. The apparatus as defined in claim 6 wherein said transducers are driven
respectively according to the expressions A.sub.1 =X.theta..sub.m and
A.sub.2 =(D.sub.1 -X).theta..sub.m where .theta..sub.m is the constant
maximum angular excursions of said stage, D.sub.1 is the distance between
the connecting points of said transducers, and X is the distance from the
pivot axis induced by motion of said transducers to one of said connecting
points,
whereby the pivot axis of said stage may be selectively positioned between
said connecting points by varying A.sub.1 and A.sub.2 in conformance with
said last mentioned expressions, permitting a one-dimensional image of
said object to be constructed and displayed.
8. The apparatus as defined in claim 7 and further including a second pair
of transducers connected to said stage so that a line joining said second
pair is orthogonal to a line joining the first pair of transducers; and
means for driving said second pair of transducers respectively according to
the expressions A.sub.3 =Y.theta..sub.m and A.sub.4 =(D.sub.2
-Y).theta..sub.m where A.sub.3 and A.sub.4 are respective amplitudes,
D.sub.2 is the distance between the connecting points of said second pair,
and Y is the distance from the pivot axis induced by motion of said second
pair to one of the connecting points thereof so that by independently
varying X and Y two arbitrary orthogonal axes of oscillation of said stage
are produced which provide respective linearly displaced amplitudes and
phases of scattered reflected radiation and permit construction of
two-dimensional images of said object.
9. The apparatus as defined in claim 1 wherein said means for subjecting
said stage to motion include three electromagnetic transducers connected
to said stage spaced from said object at each of three noncollinear
positions; and
means for driving said transducers and vibrating said stage respectively
according to the expressions A.sub.1 Sin (.omega..sub.m t+.phi..sub.1),
A.sub.2 Sin (.omega..sub.m t+.phi..sub.2) and A.sub.3 Sin (.omega..sub.m
t+.phi..sub.3) where A.sub.1, A.sub.2 and A.sub.3 are respective
amplitudes, .omega..sub.m is the vibration frequency, and .phi..sub.1,
.phi..sub.2, and .phi..sub.3 are respective phases of said vibrations;
said illuminating means including a laser beam having a selected wavelength
.lambda. and means for focusing said beam, said object being positioned
within the focal volume of said focusing means,
said measuring and recording means including means for detecting said
different frequency components;
means for recording selected instantaneous attitudes of said stage;
means for filtering out undesired frequencies;
means for measuring the amplitude and phase of unfiltered frequency
components; and
means for correlating and storing said measured amplitudes and phases,
whereby the relative excursions and phases of the points of connection of
said transducers may be controlled during vibratory motion of said stage
by adjusting A.sub.1, A.sub.2 and A.sub.3 and .phi..sub.1, .phi..sub.2 and
.phi..sub.3 thereby controlling the position of the pivot point and
permitting a two-dimensional image of said object to be constructed and
displayed.
10. The apparatus as defined in claim 9 and further including means for
constructing and displaying said two-dimensional image by providing a
signal output corresponding to the at rest amplitude and phase
contribution from each reflecting point on said object and supplying said
signal to an x-y retention scope having a spot intensity proportional to
E.sub..eta. '.sup.2 where E.sub..eta. ' is the total amplitude of the
radiation reflected by the .eta..sup.th point on said object.
11. A method of determining two-dimensional submicroscopic surface
characteristics of an object comprising the steps of:
subjecting said object to a series of different states of motion,
each state of motion being such that each point on said object experiences
a movement that is different from that experienced by any other point
thereon,
each state of motion also being such that the particular movement of any
point on said object is uniquely related to the distance of that point
from a known stationary reference point;
illuminating said object with coherent monochromatic radiation of known
amplitude and frequency,
the radiation scattered from said object consisting of a multiplicity of
different frequency components as a result of the states of motion of said
object;
measuring the amplitude and phase of selected ones of said different
frequency components for each different state of motion;
calculating from said measured amplitudes and phases and the known
relationship between the contributions to each frequency component from
each said point for each of state of motion, the amplitude and phase of
the radiation scattered from each point on said object when said object is
stationary; and
constructing the shape of said object from said calculations.
12. A method of determining one dimensional submicroscopic surface
characteristics of an object comprising the steps of:
subjecting the object to a series of different states of motion,
each state of motion being such that lines of points on said object
experience a movement different from that experienced by any other
parallel line points thereon,
each state of motion also being such that the particular movement of any
line of points on said object is uniquely related to the distance of that
line of points from a known stationary reference line of points,
illuminating said object with coherent monochromatic radiation of known
amplitude and frequency,
the radiation scattered from said object consisting of a multiplicity of
different frequency components as a result of the states of motion of said
object;
measuring the amplitude and phase of selected ones of said different
frequency components for each different state of motion;
calculating from said measured amplitudes and phases and the known
relationships between the contributions to each frequency component from
each said line of points for each said state of motion, the amplitude and
phase of the radiation scattered from each line of points on said object
when said object is stationary; and
constructing a one-dimensional image of said object from said calculations.
13. Apparatus for determining the submicroscopic surface characteristics of
an object which is mounted on a stage with the face to be examined exposed
comprising:
means for illuminating the exposed areas of said object with coherent
monochromatic radiation of known amplitude and frequency;
means for establishing a series of different relative states of motion
between said stage and the radiation illuminating said object,
each state of motion being such that each point on said object experiences
a relative movement that differs from that experienced by any other point
thereon,
each state of motion also being such that the particular relative movement
of any point on said object is uniquely related to the distance of that
point from a known stationary reference point,
the radiation scattered from said object consisting of a multiplicity of
different frequency components as a result of each state of relative
motion of said object;
means for measuring the amplitude and phase of selected ones of said
different frequency components for each different state of motion;
means for determining from said measured amplitudes and phases and the
known relationship between the contributions to each frequency component
from each said point for each state of motion the amplitude and phase of
the radiation scattered from each point on said object in the absence of
relative motion;
means for obtaining a signal output corresponding to the amplitude and
phase of the radiation scattered from each point on said object in the
absence of relative motion; and
means for forming a graphic display of said object from said signal
outputs.
14. The apparatus as defined in claim 13 wherein the radiation illuminating
said object impinges thereon at a direction other than normal to said
stage to avoid detection of specularly reflected radiation components.
15. The apparatus as defined in claim 13 wherein said illuminating means is
a laser beam having a wavelength .lambda. and said means for establishing
different states of relative motion include an oscillating mirror-lens
assembly disposed in the path of said beam immediately adjacent said stage
and means for translating said stage transverse to the central direction
of incidence of said laser beam on said object.
16. A system for determining one-dimensional submicroscopic surface
characteristics of an object which is mounted on a stage having a
resolution grid associated therewith comprising:
means for illuminating said object with a laser beam having a selected
wavelength .lambda. and means for focusing said beam so that said object
is within the focal volume of said focusing means;
means for oscillating said stage about a displaceable pivot axis and means
for translating said pivot axis transverse to the central direction of
incidence of said laser beam on said object to different reference grid
positions to provide a multiplicity of different phase and amplitude
values for each reflected frequency component produced by said stage
oscillations;
means for splitting the transmitted laser beam into a reference beam and an
object illuminating beam and for subsequently recombining the reflected
portion of said object illuminating beam with said reference beam to form
co-collimated beams for further processing;
means for detecting said frequency components and filtering out undesired
frequencies; and
means for unambiguously measuring and recording the amplitudes and phases
of selected frequency components of the radiation reflected from said
object to determine the at rest radiation scattered from respective lines
of points on said object so that an image of said object may be
constructed.
17. The system as defined in claim 16 wherein said stage is sinusoidally
oscillated with an instantaneous angular amplitude represented by the
expression .theta..sub.m Sin .omega..sub.v t where .theta..sub.m is the
maximum angular excursion and .omega..sub.v is the angular oscillation
frequency, said beam splitting-recombining means is an optical beam
splitter and a mirror arranged to retroreflect the impinging beam
component that is reflected by the beam splitter, said detecting means
includes a square-law optical homodyne detector for wavelength .lambda.
and an audio frequency electrical signal amplitude detector, said
filtering means is an audio filter with multiple passbands at selected
multiples of .omega..sub.v, and said measuring and recording means include
an electrically controlled translatable retroreflecting mirror mount for
successively increasing and decreasing the propagation path of said
reference beam by .lambda./4 prior to formation of said co-collimated
beams, means for calculating the at rest amplitudes and phases of
radiation reflected from each line of points on said object from the known
relationship between the contributions to each selected frequency
component from radiation reflected from each line of points for motion
with said pivot axis at each resolution grid position of said stage, and
means for constructing the shape of said object from said at rest
amplitude and phase information.
18. The system as defined in claim 17 wherein .theta..sub.m is small so
that the amplitude of the .omega..sub.v frequency component passing
through said audio filter when said pivot axis is at grid position p is
defined substantially precisely by the expression
##EQU9##
wherein K is a constant, N is the number of grid positions, E.sub..eta. is
the total at rest amplitude of the radiation scattered from that portion
of said object which lies at the .eta..sup.th grid position,
.alpha..sub..eta. is the at rest difference in phase at said detector
between said reference beam and the radiation reflected from that portion
of said object which lies at the .eta..sup.th grid position, and J.sub.1
(X.sub..eta..sup.p) is the first order Bessels function with argument
##EQU10##
where R.sub..eta..sup.p is the distance from grid position .eta. to grid
position p, and J.sub.1 (.theta..sub.m) is the first order Bessels
function with argument .theta..sub.m, wherein increasing the propagation
path of said reference beam by .lambda./4 prior to formation of said
co-collimated beams through use of said measuring and recording means
increases the values of all .alpha..sub..eta. '.sup.s by .pi./2, thus
providing frequency component amplitudes given by the expression
##EQU11##
and wherein said measuring and recording means measures and records N
values of A.sub.p.sup.c (.omega..sub.v) and A.sub.p.sup.s (.omega..sub.v)
with said pivot axis at N different grid positions, p, mathematically
inverts said expressions for A.sub.p.sup.c (.omega..sub.v) and
A.sub.p.sup.s (.omega..sub.v) to yield values of E.sub..eta. Sin
.alpha..sub..eta. and E.sub..eta. Cos .alpha..sub..eta. for
1.ltoreq..eta..ltoreq.N, and uses known trigonometric identities to
extract therefrom separate values of all E.sub..eta. '.sup.2 and
.alpha..sub..eta. '.sup.s, thereby permitting construction of a
one-dimensional image.
19. The system as defined in claim 16 and further including means for
verifying the positions of said pivot axis by establishing a reference
target on said stage positioned outside of said focal volume, deflecting a
portion of said laser beam so as to illuminate said target, and including
in said detecting and measuring and recording means means for heterodyning
radiation reflected from said target with said reference beam to provide a
high frequency signal that is receivable by said detecting means and which
has a phase that depends upon said pivot axis position, and measuring said
phase to precisely determine and verify said pivot axis position from the
value thus obtained.
20. The system as defined in claim 19 wherein said deflected beam is
produced by means of an acousto-optical beam deflector driven at radio
frequency .omega..sub.os, said heterodyning means includes a mirror having
a first reflecting surface for co-collimating said reference beam and the
reflected portion of said object illuminating beam and a second reflecting
surface for co-collimating said reference beam and the reflected portion
of said deflected beam so that both pairs of co-collimated beams impinge
upon said detecting means with the former pair giving rise to an audio
frequency homodyne electrical signal with frequency components near
.omega..sub.v and the latter pair giving rise to a radio frequency
heterodyne electrical signal with frequency components near
.omega..sub.os, and electrical signal frequency filtering means for
separating said radio frequency signal from said audio frequency signal so
that said pivot axis positions can be precisely determined and verified
from the phase of said radio frequency heterodyne signal. |
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Claims |
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Description |
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This invention concerns microscopic investigation of very small objects
and, more particularly, means for and methods of conducting such
investigations without introducing a radiation field that either destroys
the sample or the media or greatly modifies the nature or speed of the
process observed.
The resolution of a conventional microscope is limited to dimensions on the
order of an optical wavelength of the interrogating radiation. Hence, to
study very small objects such as viruses, electron microscopes with
effective interrogation wavelengths in the 10-100 A range must be used.
Since the energy-per-photon of any radiant source is inversely
proportional to .lambda., the radiation used to investigate very small
objects is necessarily very energetic and dissociates or otherwise
destroys the object investigated unless special steps are taken. Thus, a
live virus cannot be studied, and must, in fact, be metal or plastic
coated prior to interrogation by an electron microscope. In addition, use
of an electron microscope requires that the specimen be viewed in a vacuum
thereby placing additional constraints upon its use. Also, a large
fraction of the total time involved in the use of an electron microscope
is consumed in specimen preparation.
Both optical heterodyne and homodyne detection methods enable the change in
frequency of a coherent optical signal to be determined to a very high
degree of accuracy, i.e. frequency changes on the order of one part in
10.sup.10 are readily measurable. This sensitivity has fostered the
development of doppler imaging radar systems in which analysis of doppler
frequency shifts from different surface features of a distant rotating
object are used to resolve or reconstruct the shape of the object with far
greater precision than could be achieved using a telescope with any
reasonable dimensions. In particular, the resolution achieved is much
better than could theoretically be obtained using a telescope with an
aperture as large as the optical radar receiver aperture.
The foregoing electron microscope deficiencies are avoided in the optical
homodyne microscope (OHM) of the present invention by employing the
doppler radar principle wherein optical homodyne detection is used to
detect the phase modulation of optical radiation that is reflected from a
vibrating or rotating body having dimensions smaller than the
interrogating wavelength. By analyzing the resulting sideband spectrum,
information regarding the shape of the object can be obtained.
Accordingly, it is an object of the present invention to provide an optical
microscope which uses radiation of such frequencies as to allow organisms
to be studied without killing them and which does not require special
preparation of the sample or organism to be investigated.
Another object of this invention is to provide an optical microscope which
is capable of using a variety of inspection wavelengths for which samples
exhibit different absorption and reflection properties which cannot be
investigated using electron microscopes.
A further object of this invention is to provide an optical microscope
which is adapted to use visible or near uv radiation for inspection and
quality control of optical components at a much higher resolution than is
presently possible using radiation in these spectral ranges.
A further object of this invention is to provide an optical microscope
which does not require the sample to be placed in a vacuum so that
volatile materials may be examined without having to be coated or cooled
and interactions of solid materials with arbitrary gaseous atmospheres can
be studied in real time.
A still further object of this invention is to provide an optical
microscope which allows the observation of physical processes which occur
at the surfaces of high density data storage media to be observed in real
time without introducing a radiation that either destroys the media or
greatly modifies the nature or speed of the process observed.
Other objects, advantages and novel features of the present invention will
become apparent from the following detailed description thereof when
considered in conjunction with the accompanying drawings in which like
numerals represent like parts throughout and wherein:
FIG. 1 is a schematic drawing of a conventional imaging system;
FIG. 2 is a schematic drawing of an optical homodyne imaging system
explaining the basic principle of operation of the invention;
FIG. 3 is a schematic drawing of a fundamental embodiment of the invention;
FIG. 4 is an enlarged schematic view of an OHM specimen stage arrangement
wherein the stage is oscillated at a frequency .omega..sub.v ;
FIG. 5 shows the position of the components of FIG. 4 after a selected
stage movement;
FIG. 6 is a schematic drawing of an alternate embodiment of the invention
which utilizes heterodyne detection for specimen stage and
retro-reflecting mirror registration;
FIG. 7 is a schematic drawing of an alternate arrangement for moving the
pivot axis relative to the specimen in one dimension;
FIG. 8 is a schematic drawing of another arrangement for positioning and
orienting the specimen with respect to the pivot in two dimensions;
FIG. 9 is a schematic drawing of another embodiment of the invention
wherein the specimen stage is not subject to vibratory motion;
FIG. 10 is a schematic drawing of a further embodiment of the invention
wherein the illuminating beam is not at normal incidence with respect to
the specimen stage; and
FIG. 11 is a schematic drawing illustrating how the various embodiments may
be modified to avoid the detection of specularly reflected radiation
components.
The present invention, in general, concerns a system for detecting phase
modulation of optical radiation that is reflected from a vibrating or
rotating body having dimensions smaller than the interrogating wavelength.
A simplified form of the invention employs a conventional optical homodyne
detection configuration for detecting the phase modulation of coherent
radiation reflected from a moving sample or specimen. The entire specimen
fits within the focal area of the system's focusing lens so that its
structural characteristics are unresolved in the usual optical sense. The
specimen is mounted upon a stage that can be oscillated by angular
vibration about a fixed pivot at a relatively low audio frequency
.omega..sub.v. A fixed stage and vibrating optical system can be used
where specimen damage may result from oscillation. The specimen stage is
arranged so that the distance from the pivot axis to the specimen can be
varied in an identifiable and controllable manner. The sizes and distances
between resolution elements, i.e. elementary areas with fixed known
positions on the surface of the stage, are chosen so as to be as small as
is consistent with the minimum permissible signal-to-noise ratio.
Radiation is scattered from all portions of the specimen, and changes in
the nature of this scattered radiation which result from changes in the
state-of-motion of the specimen are measured and stored. Using all stored
data, the amplitude and phase of the radiation reflected from the portions
of the specimen that lie within each of the resolution elements are
computed. Thereafter, both one-dimensional image information and
depth/reflection phase-shift information are provided as computer outputs.
Two-dimensional image information is obtained by repeating the above
procedure for different angular orientations of the specimen.
The optical homodyne microscope is, in a sense, the microscopic analogue of
doppler imaging radar. FIG. 1 explains the operation of a conventional
optical imaging system. Two stationary reflecting points P.sub.1 and
P.sub.2 which are separated by a distance S are illuminated from the left
with coherent radiation of frequency .nu..sub.o. The scattered radiation
is collected by a lens L.sub.2 and re-imaged by a lens L.sub.1. Were it
not for diffraction effects at the lens apertures, resolvable images of
P.sub.1 and P.sub.2 could be formed, no matter what the value of S.
However, diffraction effects give rise to an unavoidable beam spread,
.theta..sub.d, in the region between lens L.sub.1 and L.sub.2. The
magnitude of this diffractive beam spread is given by the expression,
.theta..sub.d .gtoreq..lambda./D where .lambda. is the wavelength of the
interrogating radiation and D is the lens aperture. It is obvious from
FIG. 1 that if .theta..sub.d exceeds the angle, .theta., subtended by S at
the center of lens L.sub.2, then the two parallel beams that result from
radiation reflected by P.sub.1 and P.sub.2 cannot be distinguished and
distinguishable images of P.sub.1 and P.sub.2 cannot be formed by lens
L.sub.1. Since .theta. is equal to S/f, this means that in order for
P.sub.1 and P.sub.2 to be resolved in the usual optical sense
S/f.gtoreq..lambda./D or S.gtoreq..lambda.f/D. For practically realizable
optical systems f/D.gtoreq.1, so that S.gtoreq..lambda.. Hence, in order
to optically resolve the two points P.sub.1 and P.sub.2, the distance
between them must exceed the wavelength of the incident radiation. If the
separation is less than that, inspection of the image formed will not
allow one to determine that two reflecting points are involved.
In the situation indicated in FIG. 2, two point reflectors separated by S
again are illuminated by coherent radiation but now these points are
instantaneously moving parallel to the incident radiation at velocities
V.sub.1 and V.sub.2. In addition, a homodyne detection/spectrum analyzer
arrangement provided by a detector 2 and an analyzer 3 is substituted for
image forming lens L.sub.1. This detection system is capable of detecting
and displaying any frequency differences between the reflected radiation
and the incident radiation.
When P.sub.1 and P.sub.2 are moving along the direction of propagation of
the incident beam, the radiation reflected from these points will be
doppler shifted by amounts
##EQU1##
Therefore if V.sub.1 is unequal to V.sub.2, two signals with frequency
sepration
##EQU2##
will appear upon the spectrum analyzer screen. If V.sub.1 -V.sub.2 is
large enough to allow frequency resolution of these electrical signals, it
will be possible to determine from the display that there are at least two
reflecting points involved, no matter what the value of S. For example, if
V.sub.1 -V.sub.2 is equal to 1 cm/sec and visible incident radiation is
used, .DELTA..apprxeq.40 KHz, a value that is very easy to resolve.
Furthermore, if the relative velocity of P.sub.1 and P.sub.2 could be
directly related to their separation, S, measurement of .DELTA.=2 (V.sub.1
-V.sub.2)/.lambda. would allow S to be determined even if S were much less
than .lambda.. Furthermore, the measurement of the heights of the spectrum
analyzer peaks would allow determination of the relative reflectivities of
points P.sub.1 and P.sub.2. Hence, the details of an object which includes
points P.sub.1 and P.sub.2 could be resolved with a resolution exceeding
.lambda., even though this is not possible in the usual optical sense. A
very simple technique for insuring that the relative velocity of P.sub.1
and P.sub.2 is a known linear function of their separation is to attach
these points to a rigid, nonreflecting rod 4 that is rotated about fixed
point 1 at a known angular velocity, .omega.. With this arrangement,
V.sub.1 -V.sub.2 is equal to .omega.S so that S=.lambda..DELTA./2.omega..
The above discussion can be generalized to allow resolution of an
arbitrary number of reflecting points on the surface of a rotating rigid
body. The mathematical formulation of this generalization, for the case of
large microscopic rotating bodies at great distances from the observer,
constitutes the well developed theory of doppler imaging radar.
Applications of such radars allow the shapes of distant rotating objects,
such as satellites or features on the surface of the moon, to be resolved
even though the total dimensions of these objects are much smaller than
could be resolved by an optically perfect telescope with aperture size
equal to that of the radar receiving lens.
The basic principle involved in the operation of the OHM is the same as
that involved in doppler imaging radar. Of central importance for
operation of both systems is the fact that even though an object cannot be
resolved in the usual optical sense, it is generally possible to infer its
shape and optical reflectivity characteristics from the phase and
frequency modulations it produces when coherent radiation is reflected
from it while it is in one or a series of well defined states of motion.
In the case of imaging radar, the motion involved is the rotation of a
large distant object, so that each element on the object's surface moves
through a distance equal to many wavelengths of the incident coherent
radiation during the period of interrogation. In this case, resolvable
doppler shifts are produced by adjacent resolution cells on the object and
something akin to the spectrum analyzer of FIG. 2 can be used to construct
a doppler image, after a single observation. In the case of the OHM,
however, the total extent of the object is in general smaller than an
interrogating wavelength, the imposed motion of the object is periodic,
e.g. vibratory, and the distance moved by the reflecting points on the
object are much smaller than a wavelength. Under these circumstances, a
resolvable doppler frequency shift is not produced in the reflected
coherent radiation but, instead, a phase modulation is produced with
fundamental frequency equal to the vibration frequency of the object. All
reflecting points on the object contribute to this phase-modulated return,
with the contribution from each point being determined by its specific
reflectivity characteristics. It is, therefore, not possible to
reconstruct an image after a single measurement, but instead a series of
measurements must be made in which the state-of-motion, and therefore the
phase-modulation contribution, of each reflecting element is somewhat
different from measurement-to-measurement. As is explained in greater
detail infra, this multiple measurement procedure, if properly carried
out, can result in an unambiguous determination of the amplitude and phase
of the radiation reflected from each individual reflecting element. Hence,
in analogy to the case of doppler imaging illustrated in FIG. 2, with the
OHM an "image" of a submicroscopic object is constructed from a knowledge
of the imposed motions of its reflecting elements and the known
relationship of these motions to the relative spacial positions of the
elements. In contrast to doppler imaging radar, a series of measurements
is made with different, imposed states of motion. This allows unambiguous
image information to be obtained, even though all dimensions and
displacements involved are much smaller than an optical wavelength.
FIG. 3 schematically illustrates one embodiment of the invention that
includes a C. W. gas laser 11 having a beam output 12 at wavelength
.lambda. which is substantially focused by a lens 13 onto a specimen stage
14, only generally shown, that has a specimen 15 mounted thereon. Beam 12
is divided by a beam splitter 18 into a reference component 19 that is
directed to a retroreflecting mirror 20 and component 21 which impinges on
stage 14 and specimen 15. The path of component 19 is adjusted by a
piezoelectric transducer 23 on which mirror 20 is mounted, with transducer
23 controlled by a transducer driver 24 which is powered and controlled by
a computer 25. Stage 14 necessarily is subject to repetitive motion such
as rotary vibration about a pivot axis 30 on a bracket 31 by a driver 32.
Radiation reflected from specimen 15 is directed by beam splitter 18 to a
homodyne detector 35 which also receives component 19. Unwanted frequency
components of the output of detector 35 are filtered out by an audio
filter 36 having a center frequency .omega..sub.v which is the preferred
oscillation frequency that stage 14 is driven at. The amplitude of the
.omega..sub.v signal component from filter 36 is detected by an audio
amplitude detector 38 and fed to computer 25. FIGS. 4 and 5 are
enlargements of the stage 14 configuration and show that the stage
comprises a plate 40 which is slidably mounted on a base 41 that is
rotatable about pivot axis 30. Base 41, and therefore pivot axis 30, are
varied in planar position with respect to specimen 15 by a piezoelectric
translation driver 45 which is controlled by computer 25 and is oscillated
rotationally by a vibration driver 46 at frequency .omega..sub.v.
Information on the instantaneous planar positions of any fixed point on
plate 40 relative to pivot axis 30 are read by a translation measuring
device 47 and fed to the computer. A coordinate system which includes the
surface of specimen 15 is defined by a plurality of resolution elements A,
B, C, etc. having known planar positions on the surface of plate 40. FIG.
5 illustrates the condition where base 41 has been translated a distance
.DELTA.R from the initial position shown in FIG. 4. The distance between
resolution elements is uniform and corresponds to the distance between
successive translation steps.
FIG. 6 illustrates an embodiment of the invention that utilizes heterodyne
detection to obtain precise measurement of the specimen stage and
retroreflecting mirror positions. This embodiment includes a weak
acousto-optic beam deflector 50 driven by a deflector driver 51 at a
frequency .omega..sub.os to provide an angularly deflected and frequency
secondary reference beam 52 of amplitude E.sub.r. This beam is used to
interrogate a reference spot indicated at 53 on specimen stage 14 which is
displaced a small but resolvable distance .DELTA.s from specimen 15. An
appropriately wedged retroreflecting mirror 55 is mounted on transducer 23
and aligned in such a manner that reference beam 52 reflected from
reference spot 53 is optically heterodyned with the primary local
oscillator reference beam 19 to provide an output signal of a desired
form. Also added in this embodiment are an RF filter 57 having a center
frequency .omega..sub.os to separate the homodyne signal resulting from
detection of beam component 21 from the heterodyne signal resulting from
detection of secondary reference beam 52 and a phase detector 58 to
extract the quiescent phase and phase modulation index of the secondary
reference beam heterodyne signal. For this process, output from deflector
driver 51 is used for phase reference.
FIG. 7 illustrates an alternate arrangement for varying the distance from
the pivot axis to the specimen wherein a pair of transducers 60 and 61
suitably mounted in a manner not shown and pivotally supporting and
connected to a specimen stage 63 as indicated at 64 and 65 are operated so
as to angularly oscillate stage 63 about a pivot axis 66. The excursions
of transducers 60 and 61 are adjusted so that the distance x between
transducer connection point 64 and pivot axis 66 can be continuously
changed by adjusting the absolute and relative amplitudes of the maximum
transducer excursions. FIG. 8 shows another arrangement for effecting
specimen motion which includes triangularly spaced piezoelectric
transducers 71-73 which are suitably mounted in a manner not shown at one
end and connected to and support a specimen stage 75 at points 76-78 at
the opposite end. By proper operation of the transducers, the relative
excursions of points 76-78 on stage 75 can be accurately controlled during
vibratory motion. This arrangement allows both the angular orientation and
the position of pivot to be controlled at will.
FIG. 9 illustrates an embodiment in which the direction of the impinging
focused laser beam is vibrated by a vibrating mirror-lens assembly 80 in
lieu of vibrating the specimen for instances where mechanical simplicity
is desired. Mirror-lens assembly 80 may be oscillated by conventional
means, not shown, at a frequency .omega..sub.v as indicated at 81. By
means of a plurality of mirrors 82 and focusing lens 83, mirror-lens
assembly 80 brings laser beam component 21 to a focus on the surface of a
specimen stage 85. The rotation axis 86 of mirror-lens assembly 80 lies on
the surface of specimen stage 85, within the focal region 87 of lens 83.
The position of specimen 15 on the surface of specimen stage 85 relative
to rotation axis 86 is controlled and varied by a mechanical translator
88, which is in turn controlled by computer 25. Specimen 15 remains
entirely within focal region 87 during this translation process. FIG. 10
shows an embodiment in which the interrogating beam impinges on the
specimen in a direction other than normal to the specimen stage surface to
avoid detection of specularly reflected radiation components. A beam
splitter 90 directs a beam component 91 toward a vibrating stage 92 at
such an angle as to permit only non-specular reflections from specimen 15
to be achieved at a collimating lens 93 and directed to detector 35. An
additional beam splitter 94 is required to direct beam component 21 to the
detector. In FIG. 11 detection of the specularly reflected radiation
component is avoided by forming a collimated illumination beam 96 through
use of a pair of focusing lenses 97 and 98 and a small pickoff mirror 99
positioned at their common focal point. In this arrangement, all radiation
that is specularly reflected from stage 14 is refocused upon pickoff
mirror 99 and, therefore, never reaches detector 35. On the other hand,
radiation that is diffusely reflection from specimen 15 and then captured
by lens 98 is collimated between lens 98 and a beam splitter 100, and all
this radiation therefore reaches detector 35, except for the very small
fraction that is intercepted by pickoff mirror 99. A portion of laser
output beam 12 is split off by a second beam splitter 103 and directed by
a mirror 104 and beam splitter 100 so as to serve as a reference beam at
the surface of detector 35.
Operation of the invention can best be described with reference to the
fundamental system and specimen stage embodiments of FIGS. 3-5. In
preparation for one-dimensional image construction, specimen 15 is placed
over a one-dimensional resolution element grid on the surface of
translatable, rotatable plate 40 in FIG. 4. The specimen is illuminated by
coherent laser beam 21 and the reflected radiation is collected by lens 13
and recombined with a sample of the transmitted laser beam, 19, at beam
splitter 18. The combined beam is then allowed to strike square law
homodyne detector 35. The entire specimen fits inside the focal area of
lens 13 so that its structural characteristics are unresolved in the usual
optical sense. The stage on which the specimen is mounted is angularly
vibrated at a relatively low audio frequency, .omega..sub.v, about stage
pivot axis 30 so that a homodyne signal with frequency components that are
multiples of .omega..sub.v appears at the output of detector 35. All
frequency components except the fundamental, at frequency .omega..sub.v,
are then removed from this homodyne signal by audio filter 36 and the
desired .omega..sub.v frequency component is amplified by audio amplifier
38. The magnitude of this component is supplied as input to computer 25.
In order to construct a one-dimensional image the following sequence of
operations is carried out, under control of computer 25:
(1) with the pivot axis a distance R.sub.o from the first resolution
element, the amplitude of the .omega..sub.v frequency component of the
homodyne receiver output is measured and the result stored;
(2) the distance between beam splitter 18 and mirror 20 is increased by
.lambda./8 by applying an appropriate voltage to piezoelectric transducer
23;
(3) the measurement defined in step 1 is repeated, the results are stored,
and mirror 20 is returned to its original position;
(4) the distance between pivot axis 30 and all resolution elements is
increased by the distance between adjacent elements, .DELTA.R;
(5) steps 1-4 are repeated N times where N is the number of resolution
elements;
(6) from all of the stored data, the amplitude, E.sub..eta., and phase,
.alpha..sub..eta., of the radiation reflected from those portions of
specimen 15 lying within each of the N resolution elements is computed;
and
(7) both one-dimensional image information, i.e. the E.sub..eta. 's, and
depth/phase-shift information, i.e. the .alpha..sub..eta. 's, are provided
as computer output.
Two dimensional image information can be obtained by repeating the above
procedure for different angular orientations of the specimen relative to
the rotation axis.
To understand how the N measurements described above allow computer 25 to
construct a one-dimensional image of specimen 15, it is necessary to
consider, in detail, the nature of the homodyne signal at the output of
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