Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1999-04-27
2001-07-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C356S450000
Reexamination Certificate
active
06256102
ABSTRACT:
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.
1
A), 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
c
at the depth z(as shown in FIG.
1
A), 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
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
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
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
Font Frank G.
Lee Andrew H.
Steinberger The Law Offices of Brian S.
University of Central Florida
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