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This invention relates to interferometers, and in particular to a dual beam
interferometer measuring head/probe apparatus that simultaneously measures
both a collimated cylindrical beam around a focused beam to a target(such
as human tissue cells), and detects the multiscattering from the target
with a splitter where a reference arm with matched mirrors allows for
measuring both the intensity and magnitude values of the beam to be
measured, wherein the intensity and magnitude values indicate the imaging
of the target in applications such as OCT(optical coherence tomography)
having medical imaging applications, while improving the signal to noise
ratios.
BACKGROUND AND PRIOR ART
Speckle type noise is generally present in signals recorded when
low-coherence interferometry is applied to characterize targets such as
tissue samples that are surrounded by random media. The speckle noise is
generally comprised of intensity contributions arising from multiple
scattering loops which are collected by the optical system and which
interfere during the temporal coherence interval. Because of the noise
level, prior art types of low-coherence technique lack quantitative
capabilities such as quantifying the optical contrast between the targeted
region and the surroundings and are limited in use.
Optical noise present in low-coherence images is generally determined by
the presence of multiple light scattering trajectories that have similar
lengths as the ballistic component and that are collected by the measuring
head. FIGS. 1A-1C show scattering paths having a total length such that
the paths differ with less than the coherence length of illumination
source will interfere and will generate the background component. FIG. 1A
shows a Path A which refers to a single backscattered signal. FIG. 1B
shows a Path which B refers to multiple forward scattering signals. FIG.
1C shows a path C which refers to multiple scattering signals. FIGS. 1A-1C
show that due to the round-trip geometry of the paths, the actual
penetration depths can be smaller than the actual depth of the target(see
path C). Single backscattering contribution, paths of type A(FIG. 1A), and
mostly forward scattering loops of type B(FIG. 1B) can have similar path
lengths(within the coherence length of the source) and, therefore,
contribute to the recorded signal. However, loops of type B(FIG. 1B)
determine the beam spread and reduces the resolution.
The most difficult problem is to distinguish between paths of types A and
B. The sizes of the scattering centers(scattering particles) in tissue are
usually larger than the wavelengths. Accordingly, there is a strong
forward scattering, which precludes the use of polarization-based methods
to isolate these multiple scattering contributions.
Conventional approaches to reduce the optical noise in low-coherence
techniques are to limit the measurements for targets at sufficiently small
depths, to use low numerical aperture for the probe beam, to work at
wavelengths such that the scattering is reduced, or decreases the
coherence length. Besides reducing the number of multiple scattering
events that are collected, the prior art approaches also affect the
contrast, resolution, and penetration depth of a low coherence technique.
Based on a priori knowledge on the scattering, absorption, and structural
characteristics, one can account for multiple scattering effects of path
types B and C of FIG. 1. In applications where priori information such as
particle size distribution, composition, and spatial location of
scattering particles are known, scattering models can be used to derive
the contribution of multiple scattering. The relative probability to
generate paths of types A, B, and C from layers of thickness L.sub.c at
the depth z(as shown in FIG. 1A), can be calculated if the optical
characteristics such as cross sections, single scattering albedo and phase
function, structural correlation's, layering, optical density of the
surrounding medium are known. In FIGS. 1A-1C, source 40 can be an
illumination light source, 10 is the air medium, 20 is the tissue being
tested and 30 can be the subterranean target within the tissue, with
L.sub.c is the coherence length of the light source, and Z is the depth
within the tissue 20 to the target 30. The air-tissue interface shown in
FIGS. 1A-1C, is only one example, interferometers can also be used in
applications such as defect locations.
FIG. 2 illustrates how the multiple scattering contributions depend on the
targeted depth z. The amount of multiple scattering contributions to the
recorded signal depends not only on the depth value z but also on the
coherence length L.sub.c and the optical characteristics of the medium
between the interface and the targeted depth.
Referring to FIG. 2, probing the medium 20 at a higher depth actually
enlarges the volume probed by OCT(optical coherence tomography). At higher
depths, paths of types B and C become increasingly more probable adding
their contribution to the background noise and decreasing both the axial
and transversal resolution. Thus, the longer the depth the greater the
noise. The complexity precludes a simple estimation of the multiple
scattering background (noise level).
Various types of interferometers have been proposed over the years but fail
to overcome all the problems described above. See for example U.S. Pat.
No. 4,221,486 to Jaerisch et al.; U.S. Pat. No. 4,492,467 to Drain et al.;
U.S. Pat. No. 5,469,259 to Golby et al.; U.S. Pat. No. 5,491,550 to Dabbs;
U.S. Pat. No. 5,619,326 to Takamatsu et al.; U.S. Pat. No. 5,682,240 to
Redlitz; U.S. Pat. No. 5,694,216 to Riza; U.S. Pat. No. 5,696,579 to
Johnson; U.S. Pat. No. 5,716,324 to Toida; and U.S. Pat. No. 5,748,313 to
Zorabedian.
SUMMARY OF THE INVENTION
The first objective of the present invention is to provide a dual beam
low-coherence interferometer with a focused beam and a collimated beam
having identical wavelengths, coherence and path length properties for
imaging applications.
The second object of this invention is to provide a dual beam low-coherence
interferometer with a focused beam and a collimated beam which are used in
real-time applications to reduce background scattering noise and improve
the signal to noise ratio in imaging applications.
The third object of this invention is to provide a dual beam low-coherence
interferometer with a focused beam and a collimated beam to enhance image
resolution and increase penetration depth imaging.
The fourth object of this invention is to provide a dual beam low-coherence
interferometer with a focused beam and a collimated beam for optical
coherence tomography and microscopy applications.
The fifth object of this invention is to measure the effects of a single
backscattered signal, multiple forward scattering signals, and multiple
scattering signals in an OCT(optical coherence tomography) system without
knowledge of optical characteristics such as cross sections, single
scattering albedo and phase function, structural correlation's, layering,
and optical density of the surrounding medium being sampled, and using
layers of thickness L.sub.c at a depth z. Once depth-dependent
contributions of multiple scattering loops are known for a specific
medium, the contributions can be subtracted from measured data for
improved axial resolution and contrast.
A preferred embodiment of the novel dual beam low-coherence interferometer
with improved signal-to-noise ratio includes a low coherence light source,
a first lens for forming a collimated beam from the light source onto a
subsurface target such as but not limited to tumors, abnormal cells in
biological tissues, and defects such as inclusions, cracks, and voids
within composite materials such as ceramics. The novel interferometer
further includes a second lens for forming a focused beam from the light
source onto the target, and a detector for detecting the frequency of the
collimated beam and the frequency of the focused beam from the target. The
collimated beam can alternatively be formed by a collimator. The optical
signal transmission medium within the interferometer can be based on
optical fibers. Alternatively, the transmission medium within the
interferometer can be an open-air based system.
Further objects and advantages of this invention will be apparent from the
following detailed description of a presently preferred embodiment which
is illustrated schematically in the accompanying drawings.
BRIEF DESCRIPTION OF THE FIGURES
FIGS. 1A, 1B, and 1C show the optical noise scattering signals present in
low-coherence images of the prior art.
FIG. 2 shows how multiple scattering contributions depend on the targeted
depth, z.
FIG. 3 shows a component set-up of a preferred embodiment of the dual beam
low-coherence interferometer system.
FIG. 4 shows how the collimated beam CB of FIG. 3 is used to record
multiple scattering paths of type C(FIG. 1C) and the focused beam FB of
FIG. 3, generated by lens 122 is used to select only the paths of type
A(FIG. 1A).
FIG. 5 shows a third embodiment component set-up of the measuring head of
FIG. 3 for a fiber-based interferometer.
FIG. 6 shows a second embodiment set-up of the measuring head of FIG. 3 for
an open-air interferometer.
FIG. 7 shows a graph of a signal to noise ratio versus probing(penetration)
depth in um for single focused beam S/NSB configuration of the prior art
and the subject invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Before explaining the disclosed embodiment of the present invention in
detail it is to be understood that the invention is not limited in its
application to the details of the particular arrangement shown since the
invention is capable of other embodiments. Also, the terminology used
herein is for the purpose of description and not of limitation.
FIG. 3 shows a component set-up of a preferred embodiment of the dual beam
low-coherence interferometer system 100 having improved signal-to-noise
ratios. Referring to FIG. 3, a low-coherence source 105 such as a
superluminescent diode Hamamatsu L3302 having a wavelength of
approximately 830 nm, couples the light through an optical fiber 107 to a
2.times.2 beam splitter 110 such as a Newport, and through optical fibers
112 to an optical head 120 and through optical fiber 114 and to the
reference arm 150 of the interferometer 100. Optical fiber 116 couples
light to a light detector 170 such as a New Focus Nirvana balanced
detector. The measuring head 120 can have a cylindrical geometry and
consist of two convergent lens 122, 124 and a cylindrical waveguide with
two different refractive indices m and n. Convergent lens 122 and 124 can
each be a Newport F-L40B.
Referring to FIG. 3, light that passes through the medium 130 with
refractive index m is further focused by a second smaller aperture lens
124, which generates the focused probing beam FB. Light propagating
through the medium 140 with a higher refractive index n generates a
quasi-collimated beam that propagates along the same optical axis as the
focused beam FB. For ballistic light propagation at depth z, the total
optical path in the measuring head 120 and investigated medium is
Ln+zn.sub.r, and Lm+zm.sub.r, for the collimated beam CB, and the focused
beam FB, respectively. If the difference between these two optical paths
is larger than the coherence length of the illumination source L.sub.c,
the contributions in the two beams (CB, FB) can be independently measured
when appropriate optical path differences are introduced in the reference
arm of the interferometer dc=Ln+zn.sub.r, and df=Lm+zn.sub.r. In the
reference arm 150, modulations of different frequencies F.sub.C and
F.sub.F are introduced at the two mirrors M.sub.C 158, and M.sub.F 154,
and, therefore, the detector 170 will read modulation amplitudes which are
proportional with the backscattered signals generated by the collimated
beam CB, and the focused beam FB, respectively.
Referring to FIG. 3, when the optical head 120 is aimed at the investigated
medium 300 containing multiple scattering centers(as shown in the
preceding figures) and a target 310, the signal from the target 310 is
measured by lock in detection or frequency analysis 180 at the frequency
Ff that is introduced at the mirror Mf 154 such as a Thor Labs
piezoelectric modulator. The distance dF is matched to the depth z of the
target 310. The signal from the detection unit 170 can be frequency
filtered for increasing the sensitivity, and is digitized through a data
acquisition unit 180 such as a National Instruments Labview, and can be
stored and displayed by a personal computer 200 such as an IBM PC.
Simultaneously, a similar detection is performed for the frequency Fc
which is introduced by the piezoelectric modulator-mirror Mc 158, which
matches the optical path difference corresponding to the collimated beam
CB. Computer 200 can also control the frequency generator 190 which output
selected F the frequency Ff for Mirror Mf and the frequency Fc for mirror
Mc. As a result two signals can be recorded in the computer 200
corresponding to backscattered intensities in the collimated beam CB, and
focused beam FB, respectively. Further processing, such as dividing the
focused and the collimated signals, will offer the amplitude of the
scattering from the target relative to scattering from the surrounding
medium enhancing therefore the signal to noise ratio.
In a low-coherence microscopy operation mode, the collimated beam provides
a measure of the overall optical noise corresponding to a geometry where
the reading is performed at the depth z. In the mean time, the focused
beam generates the main reflectance signal and determines the spatial
resolution. Subsequent processing can be developed using the collimated
signal for establishing the real background in the image recorded with
focused beam. According to the specific optical geometry(N/A, z and
diameter of the collimated beam) a multiple scattering contribution per
unit volume can be estimated and subtracted from the main reflectance
signal. In this manner, the effect of multiple scattering is directly
quantified for a specific depth z enhancing therefore the
signal-to-noise-ratio in the low-coherence image.
In a tomographic operation mode, lateral scanning is introduced
simultaneously in both collimated beams(CB) and focused beams(FB). This
permits to account for specific background noise effects at different
locations along the scan.
FIG. 4 shows how the collimated beam CB of FIG. 3 is used to record
multiple scattering paths of type C(previously shown in FIG. 1C) and the
focused beam FB generated by lens 122(FIG. 3) is used to select only the
paths of type A(previously shown in FIG. 1A).
FIG. 5 shows a second embodiment 200 component set-up of the measuring head
of FIG. 3 for a fiber-based interferometer. FIG. 6 shows a third
embodiment set-up 300 of the measuring head of FIG. 3 for an open-air
interferometer that eliminates the optical fibers 107, 112, 114, 116.
Referring to FIGS. 5-6, the interferometer 200 can be constructed using the
basic components of FIG. 3 with the following modifications. A low
coherence superluminescent diode 105 such as a Hamatsu L3302 can be used
that generates a wavelength of approximately 830 nm. Optical fibers 107,
112, 114 and 116 can be connected to a 2.times.2 beam splitter 110 such as
a Newport F-CPL-S22855. The mirrors Mc and Mf can be mounted on
piezoelectric modulators such as ThorLabs AE0203D08 and driven at
frequencies Fc of approximately 1 kHz and Ff of approximately 10 kHz by
modulator drivers such as Burleigh PZ-150M and frequency generators such
as Stanford Research DS340. The collimated beam CB can be formed using a
fiber optics collimator 225 that includes a convergent lens 122(previously
described) and a convergent lens 222. Collimator 225 can be an OzOptics
HPVCO 23-840-S-6.2AS with lens 122 having a diameter of approximately 4
mm. The focused beam FB can be produced by a GRIN lens 224 such as
OptoSigma 024-0440 with a total length of L of approximately 5 mm. This
allows one to scan sample depth z up to approximately 4 mm. Additional
specific selections for the measuring head should allow different
measuring ranges.
A specific example for the use of the novel interferometer refers to
measuring the reflectance of a subsurface target which is immersed in a
multiple scattering medium characterized by a radiation attenuation
length, l. The single scattering signal from the target depends on the
depth z, as defined by SS(z)=A exp(-2 z/l). On the other hand, the
multiple scattering component from a diffusive medium varies as defined by
MS(z)=BF(1-exp(-z/l) and MSC(z)=BC(1-exp(-z/l) for focused and collimated
geometry, respectively. See for example, A. Ishimaru, "Wave Propagation
and Scattering in Random Media", Academic Press 1978. In these
calculations, A, BF, and BC are constants which depend on the specific
diameter of the focusing lens, focal distance, as well as the efficiency
of the detection system(quantum efficiency, amplification, and the like),
and are not important as to showing the signal to noise ratio. The ratio
between the intensity readings in the collimated and focused beam can be
adjusted electronically such that comparable values are obtained. For
example, a typical value could be BC/BF=0.8. The novel dual beam system
permits one to subtract the backscattered intensity in the collimated beam
CB from the corresponding intensity in the focused beam FB. Accordingly, a
signal to noise ratio can be estimated for the classical case and for the
novel dual beam interferometer configuration. Under classical
geometry(focused and single-beam), the signal to noise ratio is given by
SNSB(z)=(SS(z)+MS(z)/MS(z) while in the novel dual beam configuration
S/NDB(z)=(SS(z)+MS(z)-MSC(z)/(MS(z)-MSC(z)). Typical values for
attenuation length could be l=1000 microns and the signal to noise ratios
can be estimated as a function depth z of the target. FIG. 7 presents
these values evaluated for penetration depths up to approximately 10 mm.
As can be seen, a sensible increase in the signal to noise ratio is
obtained for depths around and over the value of the attenuation length.
FIG. 7 shows a graph of a signal to noise ratio versus probing(penetration)
depth in um for single focused beam S/NSB configuration of the prior art
and the novel coaxial beams configuration of the subject invention.
Referring to FIG. 7, this example shows an increase of over a ten time
increase in signal to noise ratio of the subject invention compared to
that of the prior art.
The invention can be used in biomedical optics, tissue characterization and
diagnosis. In a biomedical application, depth-resolved images in tissue
are obtained by scanning the optical head over the region of interest. The
resolution of these images, usually OCT images, is limited by the speckle
noise produced by multiple scattering in tissue. This application can
account quantitatively for such background noise. The ratio of focused and
collimated signals can be less sensitive to local variations in tissue.
The invention can further be used in materials characterization, ceramics,
composites and other granular media as well as subsurface defects
visualization in inhomogenous media.
Since the invention allows for relative measurement, it can be used to
enhance the sensitivity of the measurement of local backscattering
coefficient in materials such as ceramics, composites, and other granular
media. In applications where particle size or density is of interest, the
invention minimizes the influence of multiple scattering and therefore
reduces the data interpretation.
For defect applications, the target 310(FIG. 3) can be a defect such as an
inclusion, void, crack, and the like, in a composite material such as
ceramics and other materials which scatter light. The ratio between
focused and collimated signals enhances the signal to noise ratios. In
this application, the signal corresponds to scattering from the subsurface
defect and the optical noise corresponds to multiple scattering from the
surrounding medium 300.
Although the preferred embodiment of the subject invention is described for
use with air and tissue applications, the invention can be applied to
other applications such as but not limited to defect locations in
nonmedical mediums.
The subject invention can be applicable for high power and other
applications where much smaller wavelengths are desirable, such as but not
limited to extreme ultraviolet(EUV), and soft X-ray regions.
While the invention has been described, disclosed, illustrated and shown in
various terms of certain embodiments or modifications which it has
presumed in practice, the scope of the invention is not intended to be,
nor should it be deemed to be, limited thereby and such other
modifications or embodiments as may be suggested by the teachings herein
are particularly reserved especially as they fall within the breadth and
scope of the claims here appended.
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