U.S. patent application number 12/323396 was filed with the patent office on 2009-03-19 for optical measurements of properties in substances using propagation modes of light.
This patent application is currently assigned to Tomophase Corporation. Invention is credited to Feiling Wang.
Application Number | 20090073444 12/323396 |
Document ID | / |
Family ID | 37544710 |
Filed Date | 2009-03-19 |
United States Patent
Application |
20090073444 |
Kind Code |
A1 |
Wang; Feiling |
March 19, 2009 |
Optical measurements of properties in substances using propagation
modes of light
Abstract
This application describes designs, implementations, and
techniques for controlling propagation mode or modes of light in a
common optical path, which may include one or more waveguides, to
sense a sample.
Inventors: |
Wang; Feiling; (Medford,
MA) |
Correspondence
Address: |
FISH & RICHARDSON, PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
Tomophase Corporation
|
Family ID: |
37544710 |
Appl. No.: |
12/323396 |
Filed: |
November 25, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11842913 |
Aug 21, 2007 |
7456965 |
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12323396 |
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11200498 |
Aug 8, 2005 |
7259851 |
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11842913 |
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10860094 |
Jun 3, 2004 |
6943881 |
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11200498 |
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60475673 |
Jun 4, 2003 |
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60514768 |
Oct 27, 2003 |
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60526935 |
Dec 4, 2003 |
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60561588 |
Apr 12, 2004 |
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Current U.S.
Class: |
356/369 |
Current CPC
Class: |
A61B 5/0059 20130101;
G01J 3/021 20130101; G01J 3/0224 20130101; G01B 9/0201 20130101;
G01B 2290/40 20130101; A61B 5/0073 20130101; G01J 3/42 20130101;
A61B 5/14558 20130101; G01B 2290/45 20130101; G01N 21/27 20130101;
G01B 2290/35 20130101; A61B 5/1455 20130101; A61B 5/14532 20130101;
G01J 3/02 20130101; A61B 2562/0242 20130101; G01N 21/4795 20130101;
G01N 21/21 20130101; G01B 2290/70 20130101; G01J 3/0218 20130101;
G01N 21/55 20130101; G01N 21/474 20130101; A61B 5/0066 20130101;
G01B 9/02044 20130101; G01N 21/49 20130101; G01B 9/02057 20130101;
G01B 9/02091 20130101; G02B 21/06 20130101; G01N 2021/4792
20130101; G01N 2021/4709 20130101 |
Class at
Publication: |
356/369 |
International
Class: |
G01N 21/17 20060101
G01N021/17 |
Claims
1. A device for optically measuring a sample, comprising: a light
source to produce an input beam for optically probing a sample; a
waveguide having a proximal end to receive the input beam from the
light source and a distal end towards which the received input beam
is guided by the waveguide; a probe head coupled to the distal end
of the waveguide to receive the input beam and to reflect a first
portion of the input beam back to the waveguide and direct a second
portion of the input beam to the sample, the probe head configured
to overlap reflection of the second portion from the sample with
the first portion and to export to the waveguide the reflection as
a reflected second portion; a differential delay modulator in
optical communication with the proximate end of the waveguide to
receive light in the first portion and the reflected second portion
from the proximate end of the waveguide, the differential delay
modulator operable to split the received light into a first beam
and a second beam and to produce variable relative phase delays
between the first beam and the second beam; and a detection module
to detect light that combines the first beam and the second beam
and is output by the differential delay modulator, the detection
module operable to extract information of the sample carried by the
reflected second portion at different depths in the sample based
the variable relative phase delays produced by the differential
delay modulator.
Description
[0001] This application is a continuation application of co-pending
U.S. patent application Ser. No. 11/842,913, filed Aug. 21, 2007;
which is a continuation application of U.S. patent application Ser.
No. 11/200,498, filed Aug. 8, 2005, now U.S. Pat. No. 7,259,851;
which is a continuation application of U.S. patent application Ser.
No. 10/860,094, filed Jun. 3, 2004, now U.S. Pat. No. 6,943,881;
which claims the benefit of the following four U.S. Provisional
Applications:
[0002] 1. Ser. No. 60/475,673 entitled "Method and Apparatus for
Acquiring Images of Optical Inhomogeneity in Substances" and filed
Jun. 4, 2003;
[0003] 2. Ser. No. 60/514,768 entitled "Coherence-Gated Optical
Glucose Monitor" and filed Oct. 27, 2003;
[0004] 3. Ser. No. 60/526,935 entitled "Method and Apparatus for
Acquiring Images of Optical Inhomogeneity in Substances" and filed
Dec. 4, 2003; and
[0005] 4. Ser. No. 60/561,588 entitled "Acquiring Information of
Optical Inhomogeneity and Other Properties in Substances" and filed
Apr. 12, 2004.
The entire disclosures of the above-referenced applications are
incorporated herein by reference as part of this application.
BACKGROUND
[0006] This application relates to non-invasive, optical probing of
various substances, including but not limited to, skins, body
tissues and organs of humans and animals.
[0007] Investigation of substances by non-invasive and optical
means has been the object of many studies as inhomogeneity of
light-matter interactions in substances can reveal their
structural, compositional, physiological and biological
information. Various devices and techniques based on optical
coherence domain reflectometry (OCDR) may be used for non-invasive
optical probing of various substances, including but not limited to
skins, body tissues and organs of humans and animals, to provide
tomographic measurements of these substances.
[0008] In many OCDR systems, the light from a light source is split
into a sampling beam and a reference beam which propagate in two
separate optical paths, respectively. The light source may be
partially coherent source. The sampling beam is directed along its
own optical path to impinge on the substances under study, or
sample, while the reference beam is directed in a separate path
towards a reference surface. The beams reflected from the sample
and from the reference surface are then brought to overlap with
each other to optically interfere. Because of the
wavelength-dependent phase delay the interference results in no
observable interference fringes unless the two optical path lengths
of the sampling and reference beams are very similar. This provides
a physical mechanism for ranging. A beam splitter may be used to
split the light from the light source and to combine the reflected
sampling beam and the reflected reference beam for detection at an
optical detector. This use of the same device for both splitting
and recombining the radiation is essentially based on the
well-known Michelson interferometer. The discoveries and the
theories of the interference of partially coherent light are
summarized by Born and Wolf in "Principles of Optics", Pergamon
Press (1980).
[0009] Low-coherence light in free-space Michelson interferometers
were utilized for measurement purposes. Optical interferometers
based on fiber-optic components were used in various instruments
that use low-coherence light as means of characterizing substances.
Various embodiments of the fiber-optic OCDR exist such as devices
disclosed by Sorin et al in U.S. Pat. No. 5,202,745, by Marcus et
al in U.S. Pat. No. 5,659,392, by Mandella et al in U.S. Pat. No.
6,252,666, and by Tearney et al in U.S. Pat. No. 6,421,164. The
application of OCDR in medical diagnoses in certain optical
configurations has come to known as "optical coherence tomography"
(OCT).
[0010] FIG. 1 illustrates a typical optical layout used in many
fiber-optic OCDR systems described in the U.S. Pat. No. 6,421,164
and other publications. A fiber splitter is engaged to two optical
fibers that respectively guide the sampling and reference beams in
a Michelson configuration. Common to many of these and other
implementations, the optical radiation from the low-coherence
source is first physically separated into two separate beams where
the sampling beam travels in a sample waveguide to interact with
the sample while the reference beam travels in a reference
waveguide. The fiber splitter than combines the reflected radiation
from the sample and the reference light from the reference
waveguide to cause interference.
SUMMARY
[0011] The designs, techniques and exemplary implementations for
non-invasive optical probing described in this application use the
superposition and interplay of different optical waves and modes
propagating along substantially the same optical path inside one or
more common optical waveguides. When one of the optical waves or
modes interacts with the substance under study its superposition
with another wave or mode can be used for the purpose of acquiring
information about the optical properties of the substance.
[0012] The methods and apparatus described in this application are
at least in part based on the recognition of various technical
issues and practical considerations in implementing OCDR in
commercially practical and user friendly apparatus, and various
technical limitations in OCDR systems disclosed by the above
referenced patents and other publications. As an example, at least
one of disadvantages associated to the OCDR system designs shown in
FIG. 1 or described in the aforementioned patents is the separation
of the reference light beam from the sample light beam. Due to the
separation of the optical paths, the relative optical phase or
differential delay between the two beams may experience
uncontrolled fluctuations and variations, such as different
physical length, vibration, temperature, waveguide bending and so
on. When the sample arm is in the form of a fiber-based catheter
that is separate from the reference arm, for example, the
manipulation of the fiber may cause a significant fluctuation and
drift of the differential phase between the sample and reference
light beams. This fluctuation and draft may adversely affect the
measurements. For example, the fluctuation and drift in the
differential phase between the two beams may lead to technical
difficulties in phase sensitive measurements as absolute valuation
of refractive indices and measurements of birefringence.
[0013] In various examples described in this application, optical
radiation is not physically separated to travel different optical
paths. Instead, all propagation waves and modes are guided along
essentially the same optical path through one or more common
optical waveguides. Such designs with the common optical path may
be advantageously used to stabilize the relative phase among
different radiation waves and modes in the presence of
environmental fluctuations in the system such as variations in
temperatures, physical movements of the system especially of the
waveguides, and vibrations and acoustic impacts to the waveguides
and system. In this and other aspects, the present systems are
designed to do away with the two-beam-path configurations in
various interferometer-based systems in which sample light and
reference light travel in different optical paths in part to
significantly reduce the above fluctuation and drift in the
differential phase delay. Therefore, the present systems have a
"built-in" stability of the differential optical path by virtue of
their optical designs and are beneficial for some phase-sensitive
measurement, such as the determination of the absolute reflection
phase and birefringence. In addition, the techniques and devices
described in this application simplify the structures and the
optical configurations of devices for optical probing by using the
common optical path to guide light.
[0014] In various applications, it may be beneficial to acquire the
absorption characteristics of the material in an isolated volume
inside the sample. In other case it may be desirable to map the
distribution of some substances identifiable through their
characteristic spectral absorbance. In some OCDR systems such as
systems in aforementioned patents, it may be difficult to perform
direct measurements of the optical inhomogeneity with regard to
these and other spectral characteristics. The systems and
techniques described in this application may be configured to allow
for direct measurements of these and other spectral characteristics
of a sample.
[0015] Exemplary implementations are described below to illustrate
various features and advantages of the systems and techniques. One
of such features is methods and apparatus for acquiring information
regarding optical inhomogeneity in substance by a non-invasive
means with the help of a low-coherence radiation. Another feature
is to achieve high signal stability and high signal-to-noise ratio
by eliminating the need of splitting the light radiation into a
sample path and a reference path. Additional features include, for
example, a platform on which phase-resolved measurements such as
birefringence and absolute refractive indices can be made,
capability of acquiring optical inhomogeneity with regard to the
spectral absorbance, solving the problem of signal drifting and
fading caused by the polarization variation in various
interferometer-based optical systems, and an effective use of the
source radiation with simple optical arrangements. Advantages of
the systems and techniques described here include, among others,
enhanced performance and apparatus reliability, simplified
operation and maintenance, simplified optical layout, reduced
apparatus complexity, reduced manufacturing complexity and
cost.
[0016] Various exemplary methods and techniques for optically
sensing samples are described. For example, one method for
optically measuring a sample includes the following steps. A beam
of guided light in a first propagation mode is directed to a
sample. A first portion of the guided light in the first
propagation mode is directed away from the sample at a location
near the sample before the first portion reaches the sample. A
second portion in the first propagation mode is directed to reach
the sample. A reflection of the second portion from the sample is
controlled to be in a second propagation mode different from the
first propagation mode to produce a reflected second portion. Both
the reflected first portion in the first propagation mode and the
reflected second portion in the second propagation mode are then
directed through a common waveguide into a detection module to
extract information from the reflected second portion on the
sample.
[0017] Another method for optically measuring a sample is also
described. In this method, light in a first propagation mode is
directed to a vicinity of a sample under measurement. A first
portion of the light in the first propagation mode is then directed
to propagate away from the sample at the vicinity of the sample
without reaching the sample. A second portion of the light in the
first propagation mode is directed to the sample to cause
reflection at the sample. The reflected light from the sample is
controlled to be in a second propagation mode that is independent
from the first propagation mode to co-propagate with the first
portion along a common optical path. The first portion in the first
propagation mode and the reflected light in the second propagation
mode are used to obtain information of the sample.
[0018] This application further describes exemplary implementations
of devices and systems for optically measuring samples. One example
of such devices includes a waveguide to receive and guide an input
beam in a first propagation mode, and a probe head coupled to the
waveguide to receive the input beam and to reflect a first portion
of the input beam back to the waveguide in the first propagation
mode and direct a second portion of the input beam to a sample.
This probe head collects reflection of the second portion from the
sample and exports to the waveguide the reflection as a reflected
second portion in a second propagation mode different from the
first propagation mode. This device further includes a detection
module to receive the reflected first portion and the reflected
second portion in the waveguide and to extract information of the
sample carried by the reflected second portion.
[0019] In another example, an apparatus for optically measuring a
sample is disclosed to include a light source, a waveguide
supporting at least a first and a second independent propagation
modes and guiding the light radiation from the light source in the
first propagation mode to the vicinity of a sample under
examination, a probe head that terminates the waveguide in the
vicinity of the sample and reverses the propagation direction of a
portion of the first propagation mode in the waveguide while
transmitting the remainder of the light radiation to the sample,
the probe head operable to convert reflected light from the sample
into the second propagation mode, and a differential delay
modulator that transmits the light in both the first and the second
propagation modes from the probe head and the waveguide and varies
the relative optical path length between the first and the second
propagation modes. In this apparatus, a mode combiner is included
to receive light from the differential delay modulator and operable
to superpose the first and the second propagation modes by
converting a portion of each mode to a pair of new modes. At least
one photodetector is used in this apparatus to receive light in at
least one of the two new modes. Furthermore, an electronic
controller is used in communication with the photodetector and is
operable to extract information of the sample from the output of
the photodetector.
[0020] In yet another example, a device is described to include an
optical waveguide, an optical probe head and an optical detection
module. The optical waveguide is to guide an optical radiation in a
first optical mode. The optical probe head is coupled to the
optical waveguide to receive the optical radiation. The optical
probe head is operable to (1) redirect a portion of the optical
radiation back to the optical waveguide while transmitting the
remaining radiation to a sample, (2) receive and direct the
reflected or backscattered radiation from the sample into the
waveguide, and (3) control the reflected or the backscattered light
from the sample to be in a second optical mode different from the
first optical mode. The optical detection module is used to receive
the radiation redirected by the probe head through the waveguide
and to convert optical radiation in the first and second optical
modes, at least in part, into a common optical mode.
[0021] A further example for a device for optically measuring a
sample includes an input waveguide, an output waveguide and a probe
head. The input waveguide supports a first and a second different
propagation modes and is used to receive and guide an input beam in
the first propagation mode. The output waveguide supports a first
and a second different propagation modes. The probe head is coupled
to the input waveguide to receive the input beam and to the output
waveguide to export light. The probe head is operable to direct a
first portion of the input beam in the first propagation mode into
the output waveguide and direct a second portion of the input beam
to a sample. In addition, the probe head collects reflection of the
second portion from the sample and exports to the output waveguide
the reflection as a reflected second portion in the second
propagation mode. Furthermore, this device includes a detection
module to receive the reflected first portion and the reflected
second portion in the output waveguide and to extract information
of the sample carried by the reflected second portion.
[0022] This application also describes an example of an apparatus
for optically measuring a sample. In this example, a first
waveguide capable of maintaining at least one propagation mode is
used. A light source that emits radiation is used to excite the
propagation mode in the first waveguide. A light director is used
to terminate the first waveguide with its first port, to pass the
light mode entering the first port, at least in part, through a
second port, and to pass the light modes entering the second port,
at least in part, through a third port. The apparatus also includes
a second waveguide that supports at least two independent
propagation modes and having a first end coupled to the second port
and a second end. Notably, a probe head is coupled to the second
end of the second waveguide and operable to reverse the propagation
direction of the light in part back to the second waveguide and to
transmit the remainder to the sample. This probe head is operable
to transform the collected light from the sample reflection to an
orthogonal mode supported by the second waveguide and direct light
in the orthogonal mode into the second waveguide. A third waveguide
is also included which supports at least two independent
propagation modes and is connected to the third port of the light
director to receive light therefrom. A differential delay modulator
is used to connect to the third waveguide to receive light from the
second waveguide and imposes a variable phase delay and a variable
path length on one mode in reference to the other. A fourth
waveguide supporting at least two independent modes is coupled to
the differential delay modulator to receive light therefrom. A
detection subsystem is positioned to receive light from the fourth
waveguide and to superpose the two propagation modes from the
fourth waveguide to form two new modes, mutually orthogonal. This
detection subsystem includes two photo-detectors respectively
receiving light in the new modes.
[0023] These and other features, system configurations, associated
advantages, and implementation variations are described in detail
in the attached drawings, the textual description, and the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 shows an example of a conventional optical sensing
device based on the well-known Michelson interferometer with
reference and sample beams in two separate optical paths.
[0025] FIG. 2 shows one example of a sensing device according to
one implementation.
[0026] FIG. 3 shows an exemplary implementation of the system
depicted in FIG. 2.
[0027] FIG. 4 shows one exemplary implementation of the probe head
and one exemplary implementation of the polarization-selective
reflector (PSR) used in FIG. 3.
[0028] FIGS. 5A and 5B illustrate another exemplary optical sensing
system that use three waveguides and a light director to direct
light in two modes to and from the probe head in measuring a
sample.
[0029] FIG. 6 illustrates the waveform of the intensity received at
the detector in the system in FIGS. 5A and 5B as a function of the
phase where the detected light intensity exhibits an oscillating
waveform that possesses a base frequency and its harmonics.
[0030] FIG. 7 shows one exemplary operation of the described system
in FIG. 5B or the system in FIG. 3 for acquiring images of optical
inhomogeneity.
[0031] FIGS. 8A and 8B illustrate one exemplary design of the
optical layout of the optical sensing system and its system
implementation with an electronic controller where light in a
single mode is used as the input light.
[0032] FIG. 9 shows another example of a system implementation
where the optical probe head receives light in a single input mode
and converts part of light into a different mode.
[0033] FIGS. 10A and 10B show two examples of the possible designs
for the probe head used in sensing systems where the input light is
in a single mode.
[0034] FIG. 11 shows one implementation of a light director that
includes a polarization-maintaining optical circulator and two
polarization beam splitters.
[0035] FIG. 12 illustrates an example of the optical differential
delay modulator used in present optical sensing systems where an
external control signal is applied to control a differential delay
element to change and modulate the relative delay in the
output.
[0036] FIGS. 12A and 12B illustrate two exemplary devices for
implementing the optical differential delay modulator in FIG.
12.
[0037] FIGS. 13A and 13B illustrate two examples of a mechanical
variable delay element suitable for implementing the optical
differential delay modulator shown in FIG. 12B.
[0038] FIG. 14A shows an exemplary implementation of the delay
device in FIG. 12B as part of or the entire differential delay
modulator.
[0039] FIG. 14B shows a delay device based on the design in FIG.
14A where the mirror and the variable optical delay line are
implemented by the mechanical delay device in FIG. 13A.
[0040] FIG. 15 illustrates an optical sensing system as an
alternative to the device shown in FIG. 5B.
[0041] FIG. 16 shows a system based on the design in FIG. 2 where a
tunable filter is inserted in the input waveguide to filter the
input light in two different modes.
[0042] FIG. 17 shows another exemplary system based on the design
in FIG. 8A where a tunable filter is inserted in the input
waveguide to filter the input light in a single mode.
[0043] FIG. 18 illustrates the operation of the tunable bandpass
filter in the devices in FIGS. 16 and 17.
[0044] FIG. 19A illustrates an example of a human skin tissue where
the optical sensing technique described here can be used to measure
the glucose concentration in the dermis layer between the epidermis
and the subcutaneous layers.
[0045] FIG. 19B shows some predominant glucose absorption peaks in
blood in a wavelength range between 1 and 2.5 microns.
[0046] FIG. 20 illustrates one exemplary implementation of the
detection subsystem in FIG. 3 where two diffraction gratings are
used to separate different spectral components in the output light
beams from the polarizing beam splitter.
DETAILED DESCRIPTION
[0047] Energy in light traveling in an optical path such as an
optical waveguide may be in different propagation modes. Different
propagation modes may be in various forms. States of optical
polarization of light are examples of such propagation modes. Two
independent propagation modes do not mix with one another in the
absence of a coupling mechanism. As an example, two orthogonally
polarization modes do not interact with each other even though the
two modes propagate along the same optical path or waveguide and
are spatially overlap with each other. The exemplary techniques and
devices described in this application use two independent
propagation modes in light in the same optical path or waveguide to
measure optical properties of a sample. A probe head may be used to
direct the light to the sample, either in two propagation modes or
in a single propagation modes, and receive the reflected or
back-scattered light from the sample.
[0048] For example, one beam of guided light in a first propagation
mode may be directed to a sample. A first portion of the first
propagation mode may be arranged to be reflected before reaching
the sample while the a second portion in the first propagation mode
is allowed to reach the sample. The reflection of the second
portion from the sample is controlled in a second propagation mode
different from the first propagation mode to produce a reflected
second portion. Both the reflected first portion in the first
propagation mode and the reflected second portion in the second
propagation mode are directed through a common waveguide into a
detection module to extract information from the reflected second
portion on the sample.
[0049] In another example, optical radiation in both a first
propagation mode and a second, different propagation mode may be
guided through an optical waveguide towards a sample. The radiation
in the first propagation mode is directed away from the sample
without reaching the sample. The radiation in the second
propagation mode is directed to interact with the sample to produce
returned radiation from the interaction. Both the returned
radiation in the second propagation mode and the radiation in the
first propagation mode are coupled into the optical waveguide away
from the sample. The returned radiation in the second propagation
mode and the radiation in the first propagation mode from the
optical waveguide are then used to extract information of the
sample.
[0050] In these and other implementations based on the disclosure
of this application, two independent modes are confined to travel
in the same waveguides or the same optical path in free space
except for the extra distance traveled by the probing light between
the probe head and the sample. This feature stabilizes the relative
phase, or differential optical path, between the two modes of
light, even in the presence of mechanical movement of the
waveguides. This is in contrast to interferometer sensing devices
in which sample light and reference light travel in different
optical paths. These interferometer sensing devices with separate
optical paths are prone to noise caused by the variation in the
differential optical path, generally complex in optical
configurations, and difficult to operate and implement. The
examples described below based on waveguides are in part designed
to overcome these and other limitations.
[0051] FIG. 2 shows one example of a sensing device according to
one implementation. This device directs light in two propagation
modes along the same waveguide to an optical probe head near a
sample 205 for acquiring information of optical inhomogeneity in
the sample. A sample holder may be used to support the sample 205
in some applications. Light radiation from a broadband light source
201 is coupled into the first dual-mode waveguide 271 to excite two
orthogonal propagation modes, 001 and 002. A light director 210 is
used to direct the two modes to the second dual-mode waveguide 272
that is terminated by a probe head 220. The probe head 220 may be
configured to perform at least the following functions. The first
function of the probe head 220 is to reverse the propagation
direction of a portion of light in the waveguide 272 in the mode
001; the second function of the probe head 220 is to reshape and
deliver the remaining portion of the light in mode 002 to the
sample 205; and the third function of the probe head 220 is to
collect the light reflected from the sample 205 back to the second
dual-mode waveguide 272. The back traveling light in both modes 001
and 002 is then directed by light director 210 to the third
waveguide 273 and further propagates towards differential delay
modulator 250. The differential delay modulator 250 is capable of
varying the relative optical path length and optical phase between
the two modes 001 and 002. A detection subsystem 260 is used to
superpose the two propagation modes 001 and 002 to form two new
modes, mutually orthogonal, to be received by photo-detectors. Each
new mode is a mixture of the modes 001 and 002.
[0052] The superposition of the two modes 001 and 002 in the
detection subsystem 260 allows for a range detection. The light
entering the detection subsystem 260 in the mode 002 is reflected
by the sample, bearing information about the optical inhomogeneity
of the sample 205, while the other mode, 001, bypassing the sample
205 inside probe head 220. So long as these two modes 001 and 002
remain independent through the waveguides their superposition in
the detection subsystem 260 may be used to obtain information about
the sample 205 without the separate optical paths used in some
conventional Michelson interferometer systems.
[0053] For the simplicity of the analysis, consider a thin slice of
the source spectrum by assuming that the amplitude of the mode 001
is E.sub.001 in a first linear polarization and that of the mode
002 is E.sub.002 in a second, orthogonal linear polarization in the
first waveguide 271. The sample 205 can be characterized by an
effective reflection coefficient r that is complex in nature; the
differential delay modulator 350 can be characterized by a pure
phase shift F exerted on the mode 001. Let us now superpose the two
modes 001 and 002 by projecting them onto a pair of new modes,
E.sub.A and E.sub.B, by a relative 45-degree rotation in the vector
space. The new modes, E.sub.A and E.sub.B, may be expressed as
following:
{ E A = 1 2 ( j .GAMMA. E 001 + rE 002 ) ; E B = 1 2 ( j .GAMMA. E
001 - rE 002 ) . ( 1 ) ##EQU00001##
It is assumed that all components in the system, except for the
sample 205, are lossless. The resultant intensities of the two
superposed modes are
{ I A = 1 2 [ E 001 2 + E 002 2 + r E 001 E 002 cos ( .GAMMA. -
.PHI. ) ] ; I B = 1 2 [ E 001 2 + E 002 2 - r E 001 E 002 cos (
.GAMMA. - .PHI. ) ] , ( 2 ) ##EQU00002##
where .phi. is the phase delay associated with the reflection from
the sample. A convenient way to characterize the reflection
coefficient r is to measure the difference of the above two
intensities, i.e.
I.sub.A-I.sub.B=|r|E.sub.001E.sub.002 cos(.GAMMA.-.phi.) (3)
If .GAMMA. is modulated by the differential delay modulator 250,
the measured signal, Eq. (3), is modulated accordingly. For either
a periodic or a time-linear variation of .GAMMA., the measured
responds with a periodic oscillation and its peak-to-peak value is
proportional to the absolute value of r.
[0054] For a broadband light source 201 in FIG. 2, consider the two
phases, .GAMMA. and .phi. to be dependent on wavelength. If the two
modes 001 and 002 experience significantly different path lengths
when they reach the detection system 260, the overall phase angle,
.GAMMA.-.phi., should be significantly wavelength dependent as
well. Consequently the measured signal, being an integration of Eq.
(3) over the source spectrum, yields a smooth function even though
.GAMMA. is being varied. The condition for a significant
oscillation to occur in the measured signal is when the two modes
001 and 002 experience similar path lengths at the location of
their superposition. In this case the overall phase angle,
.GAMMA.-.phi., becomes wavelength independent or nearly wavelength
independent. In other words, for a given relative path length set
by the modulator 250, an oscillation in the measured signal
indicates a reflection, in the other mode, from a distance that
equalizes the optical path lengths traveled by the two modes 001
and 002. Therefore the system depicted in FIG. 2 can be utilized
for ranging reflection sources.
[0055] Due to the stability of the relative phase between the two
modes, 001 and 002, phase-sensitive measurements can be performed
with the system in FIG. 2 with relative ease. The following
describes an exemplary method based on the system in FIG. 2 for the
determination of the absolute phase associated with the radiation
reflected from the sample 205.
[0056] In this method, a sinusoidal modulation is applied to the
differential phase by the differential delay modulator 250, with a
modulation magnitude of M and a modulation frequency of Q. The
difference in intensity of the two new modes is the measured and
can be expressed as follows:
I.sub.A-I.sub.B=|r|E.sub.001E.sub.002 cos [M sin(.OMEGA.t)-.phi.]
(4)
It is clear from Eq. (4) that the measured exhibits an oscillation
at a base frequency of .OMEGA. and oscillations at harmonic
frequencies of the base frequency .OMEGA.. The amplitudes of the
base frequency and each of the harmonics are related to .phi. and
|r|. The relationships between r and the harmonics can be derived.
For instance, the amplitude of the base-frequency oscillation and
the second harmonic can be found from Eq. (4) to be:
A.sub..OMEGA.=E.sub.001E.sub.002J.sub.1(M)|r|sin .phi.; (5a)
A.sub.2.OMEGA.=E.sub.001E.sub.002J.sub.2(M)|r|cos .phi., (5b)
where J.sub.1 and J.sub.2 are Bessel functions of the first and
second order, respectively. Eq. (5a) and (5b) can be used to solve
for |r| and .phi., i.e. the complete characterization of r. We can
therefore completely characterize the complex reflection
coefficient r by analyzing the harmonic content of various orders
in the measured signal. In particular, the presence of the
base-frequency component in the measured is due to the presence of
.phi..
[0057] FIG. 3 shows an exemplary implementation of the system
depicted in FIG. 2. The spectrum of source 201 may be chosen to
satisfy the desired ranging resolution. The broader the spectrum is
the better the ranging resolution. Various light sources may be
used as the source 201. For example, some semiconductor
superluminescent light emitting diodes (SLED) and amplified
spontaneous emission (ASE) sources may possess the appropriate
spectral properties for the purpose. In this particular example, a
polarization controller 302 may be used to control the state of
polarization in order to proportion the magnitudes of the two
modes, 001 and 002, in the input waveguide 371. The waveguide 371
and other waveguides 372 and 373 may be dual-mode waveguides and
are capable of supporting two independent polarization modes which
are mutually orthogonal. One kind of practical and commercially
available waveguide is the polarization maintaining (PM) optical
fiber. A polarization maintaining fiber can carry two independent
polarization modes, namely, the s-wave polarized along its slow
axis and the p-wave polarized along its fast axis. In good quality
polarization maintaining fibers these two modes can have virtually
no energy exchange, or coupling, for substantial distances.
Polarization preserving circulator 310 directs the flow of optical
waves according to the following scheme: the two incoming
polarization modes from fiber 371 are directed into the fiber 372;
the two incoming polarization modes from fiber 372 are directed to
the fiber 373. A polarization-preserving circulator 310 may be used
to maintain the separation of the two independent polarization
modes. For instance, the s-wave in the fiber 371 should be directed
to the fiber 372 as s-wave or p-wave only. Certain commercially
available polarization-preserving circulators are adequate for the
purpose.
[0058] The system in FIG. 3 implements an optical probe head 320
coupled to the waveguide 372 for optically probing the sample 205.
The probe head 320 delivers a portion of light received from the
waveguide 372, the light in one mode (e.g., 002) of the two modes
001 and 002, to the sample 205 and collects reflected and
back-scattered light in the same mode 002 from the sample 205. The
returned light in the mode 002 collected from the sample 205
carries information of the sample 205 and is processed to extract
the information of the sample 205. The light in the other mode 001
in the waveguide 372 propagating towards the probe head 320 is
reflected back by the probe head 320. Both the returned light in
the mode 002 and the reflected light in the mode 001 are directed
back by the probe head 320 into the waveguide 372 and to the
differential delay modulator 250 and the detection system 260
through the circulator 310 and the waveguide 373.
[0059] In the illustrated implementation, the probe head 320
includes a lens system 321 and a polarization-selective reflector
(PSR) 322. The lens system 321 is to concentrate the light energy
into a small area, facilitating spatially resolved studies of the
sample in a lateral direction. The polarization-selective reflector
322 reflects the mode 001 back and transmits the mode 002. Hence,
the light in the mode 002 transmits through the probe head 320 to
impinge on the sample 205. Back reflected or scattered the light
from the sample 205 is collected by the lens system 321 to
propagate towards the circulator 310 along with the light in the
mode 001 reflected by PSR 322 in the waveguide 372.
[0060] FIG. 4 shows details of the probe head 320 and an example of
the polarization-selective reflector (PSR) 322 according to one
implementation. The PSR 322 includes a polarizing beam splitter
(PBS) 423 and a reflector or mirror 424 in a configuration as
illustrated where the PBS 423 transmits the selected mode (e.g.,
mode 002) to the sample 205 and reflects and diverts the other mode
(e.g., mode 001) away from the sample 205 and to the reflector 424.
By retro reflection of the reflector 424, the reflected mode 001 is
directed back to the PBS 423 and the lens system 321. The reflector
424 may be a reflective coating on one side of beam splitter 423.
The reflector 424 should be aligned to allow the reflected
radiation to re-enter the polarization-maintaining fiber 372. The
transmitted light in the mode 002 impinges the sample 205 and the
light reflected and back scattered by the sample 205 in the mode
002 transmits through the PBS 423 to the lens system 321. The lens
system 321 couples the light in both the modes 001 and 002 into the
fiber 372.
[0061] In the implementation illustrated in FIG. 3, the detection
system 260 includes a polarizing beam splitter 361, and two
photodetectors 362 and 363. The polarizing beam splitter 361 is
used to receive the two independent polarization modes 001 and 002
from the modulator 250 and superposes the two independent
polarization modes 001 and 002. The beam splitter 361 may be
oriented in such a way that, each independent polarization is split
into two parts and, for each independent polarization mode, the two
split portions possess the same amplitude. This way, a portion of
the mode 001 and a portion of the mode 002 are combined and mixed
in each of the two output ports of the beam splitter 361 to form a
superposed new mode and each photodetector receives a superposed
mode characterized by Eq. (1). The polarizing beam splitter 361 may
be oriented so that the incident plane of its reflection surface
makes a 45-degree angle with one of the two independent
polarization mode, 001 or 002.
[0062] The system in FIG. 3 further implements an electronic
controller or control electronics 370 to receive and process the
detector outputs from the photodetectors 362 and 363 and to control
operations of the systems. The electronic controller 370, for
example, may be used to control the probe head 320 and the
differential delay modulator 250. Differential delay modulator 250,
under the control of the electronics and programs, generates a form
of differential phase modulation as the differential path length
scans through a range that matches a range of depth inside the
sample 205. The electronic controller 370 may also be programmed to
record and extract the amplitude of the oscillation in the measured
signal characterized by Eq. (3) at various differential path
lengths generated by the modulator 250. Accordingly, a profile of
reflection as a function of the depth can be obtained as a
one-dimensional representation of the sample inhomogeneity at a
selected location on the sample 205.
[0063] For acquiring two-dimensional images of optical
inhomogeneity in the sample 205, the probe head 320 may be
controlled via a position scanner such as a translation stage or a
piezo-electric positioner so that the probing light scans in a
lateral direction, perpendicular to the light propagation
direction. For every increment of the lateral scan a profile of
reflection as a function of depth can be recorded with the method
described above. The collected information can then be displayed on
a display and interface module 372 to form a cross-sectional image
that reveals the inhomogeneity of the sample 205.
[0064] In general, a lateral scanning mechanism may be implemented
in each device described in this application to change the relative
lateral position of the optical probe head and the sample to obtain
a 2-dimensional map of the sample. A xy-scanner, for example, may
be engaged either to the optical head or to a sample holder that
holds the sample to effectuate this scanning in response to a
position control signal generated from the electronic controller
370.
[0065] FIGS. 5A and 5B illustrate another exemplary system that use
waveguides 271, 272, and 273 and a light director 210 to direct
light in two modes to and from the probe head 320 in measuring the
sample 205. A first optical polarizer 510 is oriented with respect
to the polarization axes of the PM waveguide 271 to couple
radiation from the broadband light source 201 into the waveguide
271 in two orthogonal linear polarization modes as the independent
propagation modes. An optical phase modulator 520 is coupled in the
waveguide 271 to modulate the optical phase of light in one guided
mode relative to the other. A variable differential group delay
(VDGD) device 530 is inserted in or connected to the waveguide 273
to introduce a controllable amount of optical path difference
between the two waves. A second optical polarizer 540 and an
optical detector 550 are used here to form a detection system. The
second polarizer 540 is oriented to project both of the guided
waves onto the same polarization direction so that the changes in
optical path difference and the optical phase difference between
the two propagation modes cause intensity variations, detectable by
the detector 550.
[0066] The light from the source 201 is typically partially
polarized. The polarizer 510 may be aligned so that maximum amount
of light from the source 201 is transmitted and that the
transmitted light is coupled to both of the guided modes in the
waveguide 271 with the substantially equal amplitudes. The electric
fields for the two orthogonal polarization modes S and P in the
waveguide 271 can be expressed as:
{ E s = 1 2 E , E p = 1 2 E . ( 6 ) ##EQU00003##
where the electric field transmitting the polarizer is denoted as
E. It should be appreciated that the light has a finite spectral
width (broadband or partially coherent). The fields can be
described by the following Fourier integral:
E=.intg.E.sub..omega.e.sup.j.omega.td.omega.. (7)
For the simplicity of the analysis, a thin slice of the spectrum,
i.e. a lightwave of a specific wavelength, is considered below.
Without loosing generality, it is assumed that all the components,
including polarizers, waveguides, Router, PSR and VDGD, are
lossless. Let us designate the reflection coefficient of the sample
r, that is complex in nature. The p-wave picks up an optical phase,
.GAMMA., relative to the s-wave as they reach the second polarizer
540:
{ E s = 1 2 E , E p = 1 2 rE j .GAMMA. . ( 8 ) ##EQU00004##
The light that passes through Polarizer 540 can be expressed by
E a = 1 2 ( E s + E p ) = 1 2 E ( 1 + r j .GAMMA. ) . ( 9 )
##EQU00005##
The intensity of the light that impinges on the photodetector 550
is given by:
I = E a E a * = 1 4 E 2 [ 1 + r 2 + 2 r cos ( .GAMMA. + .delta. ) ]
. ( 10 ) ##EQU00006##
where phase angle .delta. reflects the complex nature of the
reflection coefficient of the sample 205 and is defined by
r=|r|e.sup.j.delta.. (11)
Assuming the modulator 520 exerts a sinusoidal phase modulation,
with magnitude M and frequency .OMEGA., in the p-wave with respect
to the s-wave, the light intensity received by the detector 550 can
be expressed as follows:
I = 1 + r 2 4 E 2 + r 2 E 2 cos [ M sin ( .OMEGA. t ) + .PHI. +
.delta. ] . ( 12 ) ##EQU00007##
where phase angle .phi. is the accumulated phase slip between the
two modes, not including the periodic modulation due to the
modulator 520. The VDGD 530 or a static phase shift in the
modulator 520, may be used to adjust the phase difference between
the two modes to eliminate .phi.. The waveform of I is graphically
shown in FIG. 4.
[0067] FIG. 6 illustrates the waveform of the intensity I received
at the detector 550 as a function of the phase. The detected light
intensity exhibits an oscillating waveform that possesses a base
frequency of .OMEGA. and its harmonics. The amplitudes of the base
frequency and each of the harmonics are related to .delta. and |r|.
The mathematical expressions for the relationships between r and
the harmonics can be derived. For instance, the amplitude of the
base-frequency oscillation and the second harmonic are found to
be:
A.sub..OMEGA.=0.5|E|.sup.2J.sub.1(M)|r|sin .delta.; (13a)
A.sub.2.OMEGA.=0.5|E|.sup.2J.sub.2(M)|r|cos .delta., (13b)
where J.sub.1 and J.sub.2 are Bessel functions of the first and
second order, respectively. Eq. (13a) and (13b) can be used to
solve for |r| and .delta., i.e. the complete characterization of
r.
[0068] The effect of having a broadband light source 201 in the
system in FIGS. 5A and 5B is analyzed below. When there is a
significant differential group delay between the two propagation
modes there must be an associated large phase slippage .phi. that
is wavelength dependent. A substantial wavelength spread in the
light source means that the phase slippage also possesses a
substantial spread. Such a phase spread cannot be eliminated by a
phase control device that does not also eliminate the differential
group delay. In this case the detected light intensity is given by
the following integral:
I = .intg. { 1 + r 2 4 E ( .lamda. ) 2 + r 2 E ( .lamda. ) 2 cos [
M sin ( .OMEGA. t ) + .PHI. ( .lamda. ) + .delta. ] } .lamda. . (
14 ) ##EQU00008##
It is easy to see that if the range of .phi.(.lamda.) is comparable
to .pi. for the bandwidth of the light source no oscillation in I
can be observed as oscillations for different wavelengths cancel
out because of their phase difference. This phenomenon is in close
analogy to the interference of white light wherein color fringes
are visible only when the path difference is small (the film is
thin). The above analysis demonstrates that the use of a broadband
light source enables range detection using the proposed apparatus.
In order to do so, let the s-wave to have a longer optical path in
the system compared to the p-wave (not including its round-trip
between Probing Head and Sample). For any given path length
difference in the system there is a matching distance between
Probing Head and Sample, z, that cancels out the path length
difference. If an oscillation in I is observed the p-wave must be
reflected from this specific distance z. By varying the path length
difference in the system and record the oscillation waveforms we
can therefore acquire the reflection coefficient r as a function of
the longitudinal distance z, or depth. By moving Probing Head
laterally, we can also record the variation of r in the lateral
directions.
[0069] FIG. 7 further shows one exemplary operation of the
described system in FIG. 5B or the system in FIG. 3 for acquiring
images of optical inhomogeneity. At step 710, the relative phase
delay between the two modes is changed, e.g., increased by an
increment, to a fixed value for measuring the sample 205 at a
corresponding depth. This may be accomplished in FIG. 5B by using
the differential delay device 530 or the bias in the differential
delay modulator 250 in FIG. 3. At step 720, a modulation driving
signal is sent to the modulator 520 in FIG. 5B or the modulator 250
in FIG. 3 to modulate the relative phase delay between the two
modes around the fixed value. At step 730, the intensity waveform
received in the detector 550 in FIG. 5B or the intensity waveforms
received in the detectors 362,363 in FIG. 3 are measured and stored
in the electronic controller 370. Upon completion of the step 730,
the electronic controller 370 controls the differential delay
device 530 in FIG. 5B or the bias in the differential delay
modulator 250 in FIG. 3 to change the relative phase delay between
the two modes to a different fixed value for measuring the sample
205 at a different depth. This process iterates as indicated by the
processing loop 740 until desired measurements of the sample at
different depths at the same location are completed. At this point,
electronic controller 370 controls the probe head 320 to laterally
move to a new location on the sample 205 and repeat the above
measurements again until all desired locations on the sample 205
are completed. This operation is represented by the processing loop
750. The electronic controller 370 processes each measurement to
compute the values of .delta. and |r| from the base oscillation and
the harmonics at step 760. Such data processing may be performed
after each measurement or after all measurements are completed. At
step 770, the computed data is sent to the display module 372.
[0070] In the above implementations, light for sensing the sample
205 is not separated into two parts that travel along two different
optical paths. Two independent propagation modes of the light are
guided essentially in the same waveguide at every location along
the optical path except for the extra distance traveled by one mode
between the probe head 320 and the sample 205. After redirected by
the probe head 320, the two modes are continuously guided in the
same waveguide at every location along the optical path to the
detection module.
[0071] Alternatively, the light from the light source to the probe
head may be controlled in a single propagation mode (e.g., a first
propagation mode) rather than two different modes. The probe head
may be designed to cause a first portion of the first mode to
reverse its propagation direction while directing the remaining
portion, or a second portion, to reach the sample. The reflection
or back scattered light of the second portion from the sample is
collected by the probe head and is controlled in the second
propagation mode different from the first mode to produce a
reflected second portion. Both the reflected first portion in the
first propagation mode and the reflected second portion in the
second propagation mode are directed by the probe head through a
common waveguide into the detection module for processing. In
comparison with the implementations that use light in two modes
throughout the system, this alternative design further improves the
stability of the relative phase delay between the two modes at the
detection module and provides additional implementation
benefits.
[0072] FIGS. 8A and 8B illustrate one exemplary design of the
optical layout of the optical sensing system and its system
implementation with an electronic controller. An input waveguide
871 is provided to direct light in a first propagation mode, e.g.,
the mode 001, from the broadband light source 201 to a light
director 810. The waveguide 871 may be a mode maintaining waveguide
designed to support at least one propagation mode such as the mode
001 or 002. When light is coupled into the waveguide 871 in a
particular mode such as the mode 001, the waveguide 871 essentially
maintains the light in the mode 001. A polarization maintaining
fiber supporting two orthogonal linear polarization modes, for
example, may be used as the waveguide 871. Similar to systems shown
in FIGS. 2, 3, 5A and 5B, dual-mode waveguides 272 and 273 are used
to direct the light. A light director 510 is used to couple the
waveguides 871, 272, and 273, to convey the mode 001 from the input
waveguide 871 to one of the two modes (e.g., modes 001 and 002)
supported by the dual-mode waveguide 272, and to direct light in
two modes from the waveguide 272 to the dual-mode waveguide 273. In
the example illustrated in FIG. 8A, the light director 810 couples
the light in the mode 001 from the waveguide 871 into the same mode
001 in the waveguide 272. Alternatively, the light director 810 may
couple the light in the mode 001 from the waveguide 871 into the
different mode 002 in the waveguide 272. The dual-mode waveguide
271 is terminated at the other end by a probe head 820 which
couples a portion of light to the sample 205 for sensing.
[0073] The probe head 820 is designed differently from the prove
head 320 in that the probe head 830 converts part of light in the
mode 001 into the other different mode 002 when the light is
reflected or scattered back from the sample 205. Alternatively, if
the light in the waveguide 272 that is coupled from the waveguide
871 is in the mode 002, the probe head 820 converts that part of
light in the mode 002 into the other different mode 001 when the
light is reflected or scattered back from the sample 205. In the
illustrated example, the probe head 820 performs these functions:
a) to reverse the propagation direction of a small portion of the
incoming radiation in mode 001; b) to reshape the remaining
radiation and transmit it to the sample 205; and c) to convert the
radiation reflected from the sample 205 to an independent mode 002
supported by the dual-mode waveguide 272. Since the probe head 820
only converts part of the light into the other mode supported by
the waveguide 272, the probe head 820 is a partial mode converter
in this regard. Due to the operations of the probe head 820, there
are two modes propagating away from the probe head 820, the mode
001 that bypasses the sample 205 and the mode 002 for light that
originates from sample reflection or back scattering. From this
point on, the structure and operations of the rest of the system
shown in FIG. 8A may be similar to the systems in FIGS. 2, 3, 5A,
and 5B.
[0074] FIG. 8B shows an exemplary implementation of the design in
FIG. 8A where an electronic controller 3370 is used to control the
differential delay modulator 250 and the probe head 820 and a
display and interface module 372 is provided. Radiation from
broadband light source 201, which may be partially polarized, is
further polarized and controlled by an input polarization
controller 802 so that only a single polarization mode is excited
in polarization-maintaining fiber 371 as the waveguide 871 in FIG.
8A. a polarization preserving circulator may be used to implement
the light director 810 for routing light from the waveguide 371 to
the waveguide 372 and from the waveguide 372 to the waveguide
373.
[0075] The probe head 820 in FIG. 8B may be designed to include a
lens system 821 similar to the lens system 321, a partial reflector
822, and a polarization rotator 823. The partial reflector 822 is
used to reflect the first portion of light received from the
waveguide 372 back to the waveguide 372 without changing its
propagation mode and transmits light to and from the sample 205.
The polarization rotator 823 is used to control the light from the
sample 205 to be in the mode 002 upon entry of the waveguide
372.
[0076] FIG. 9 shows another example of a system implementation
where the optical probe head 820 receives light in a single input
mode and converts part of light into a different mode. An input
polarizer 510 is used in the input PM fiber 272 to control the
input light in the single polarization mode. A phase modulator 520
and a variable differential group delay device 530 are coupled to
the output PM fiver 273 to control and modulate the relative phase
delay of the two modes before optical detection. An output
polarizer 540 is provided to mix the two modes and the detector 550
is used to detect the output from the output polarizer 540.
[0077] FIGS. 10A and 10B show two examples of the possible designs
for the probe head 820 including a partially reflective surface
1010, a lens system 1020, and a quarter-wave plate 1030 for
rotating the polarization and to convert the mode. In FIG. 10A, the
termination or end facet of polarization-maintaining fiber 372 is
used as the partial reflector 1010. An uncoated termination of an
optical fiber reflects approximately 4% of the light energy.
Coatings can be used to alter the reflectivity of the termination
to a desirable value. The lens system 1020 reshapes and delivers
the remaining radiation to sample 205. The other role played by the
lens system 1020 is to collect the radiation reflected from the
sample 205 back into the polarization-maintaining fiber 372. The
quarter wave plate 1030 is oriented so that its optical axis make a
45-degree angle with the polarization direction of the transmitted
light. Reflected light from the sample 205 propagates through the
quarter wave plate 1030 once again to become polarized in a
direction perpendicular to mode 001, i.e. mode 002. Alternatively,
the quarter wave plate 1030 may be replaced by a Faraday rotator.
The head design in FIG. 10B changes the positions of the lens
system 1020 and the quarter wave plate or Faraday rotator 1030.
[0078] In the examples in FIGS. 8A, 8B, and 9, there is only one
polarization mode entering the light director 810 or the
polarization-preserving circulator from waveguide 871 or 371.
Therefore, the light director 810 or the polarization preserving
circulator may be constructed with a polarization-maintaining
optical circulator 1110 and two polarization beam splitters 1120
and 1130 as shown in FIG. 11. The polarization-maintaining
circulator 1110 is used to convey only one polarization mode among
its three ports, rather than both modes as in the case shown in
FIGS. 3, 5A and 5B. The polarizing beam splitter 1120 and 1130 are
coupled to polarization-maintaining circulator 1110 so that both
polarization modes entering Port 2 are conveyed to Port 3 and
remain independent.
[0079] A number of hardware choices are available for differential
delay modulator 250. FIG. 12 illustrates the general design of the
modulator 250 where an external control signal is applied to
control a differential delay element to change and modulate the
relative delay in the output. Either mechanical or non-mechanical
elements may be used to produce the desired relative delay between
the two modes and the modulation on the delay.
[0080] In one implementation, a non-mechanical design may include
one or more segments of tunable birefringent materials such as
liquid crystal materials or electro-optic birefringent materials
such as lithium niobate crystals in conjunction with one or more
fixed birefringent materials such as quartz and rutile. The fixed
birefringent material provides a fixed delay between two modes and
the tunable birefringent material provides the tuning and
modulation functions in the relative delay between the two modes.
FIG. 12A illustrates an example of this non-mechanical design where
the two modes are not physically separated and are directed through
the same optical path with birefringent segments which alter the
relative delay between two polarization modes.
[0081] FIG. 12B shows a different design where the two modes in the
received light are separated by a mode splitter into two different
optical paths. A variable delay element is inserted in one optical
path to adjust and modulate the relative delay in response to an
external control signal. A mode combiner is then used to combine
the two modes together in the output. The mode splitter and the
mode combiner may be polarization beams splitters when two
orthogonal linear polarizations are used as the two modes.
[0082] The variable delay element in one of the two optical paths
may be implemented in various configurations. For example, the
variable delay element may be a mechanical element. A mechanical
implementation of the device in FIG. 12B may be constructed by
first separating the radiation by polarization modes with a
polarizing beam splitter, one polarization mode propagating through
a fixed optical path while the other propagating through a variable
optical path having a piezoelectric stretcher of polarization
maintaining fibers, or a pair of collimators both facing a
mechanically movable retroreflector in such a way that the light
from one collimator is collected by the other through a trip to and
from the retroreflector, or a pair collimators optically linked
through double passing a rotatable optical plate and bouncing off a
reflector.
[0083] FIGS. 13A and 13B illustrate two examples of a mechanical
variable delay element suitable for FIG. 12B. Such a mechanical
variable delay device may be used to change the optical path length
of a light beam at high speeds and may have various applications
other than what is illustrated in FIG. 12B. In addition, the
optical systems in this application may use such a delay
device.
[0084] The mechanical delay device shown in FIG. 13A includes an
optical beam splitter 1310, a rotating optical plate 1320 which may
be a transparent plate, and a mirror or reflector 1330. The beam
splitter 1310 is used as the input port and the output port for the
device. The rotating optical plate 1320 is placed between the
mirror 1330 and the beam splitter 1310. The input light beam 1300
is received by the beam splitter 1310 along the optical path
directing from the beam splitter 1310 to the mirror 1330 through
the rotating optical plate 1320. A portion of the light 1300
transmitting through the beam splitter 1310 is the beam 1301 which
impinges on and transmits through the rotating optical plate 1320.
The mirror or other optical reflector 1330 is oriented to be
perpendicular to the light beam incident to the optical plate 1310
from the opposite side. The reflected light beam 1302 from the
mirror 1320 traces the same optical path back traveling until it
encounters the Beam Splitter 1310. The Beam Splitter 1310 deflects
part of the back traveling light 1302 to a different direction as
the output beam 1303.
[0085] In this device, the variation of the optical path length is
caused by the rotation of the Optical Plate 1320. The Optical Plate
1320 may be made of a good quality optical material. The two
optical surfaces may be flat and well polished to minimize
distortion to the light beam. In addition, the two surfaces should
be parallel to each other so that the light propagation directions
on both sides of the Optical Plate 1320 are parallel. The thickness
of the Optical Plate 1320 may be chosen according to the desirable
delay variation and the range of the rotation angle. The optical
path length experienced by the light beam is determined by the
rotation angle of the Optical Plate 1320. When the surfaces of the
Optical Plate 1320 is perpendicular to the light beam (incident
angle is zero), the path length is at its minimum. The path length
increases as the incident angle increases.
[0086] In FIG. 13A, it may be beneficial to collimate the input
light beam so that it can travel the entire optical path without
significant divergence. The Optical Plate 1320 may be mounted on a
motor for periodic variation of the optical delay. A good quality
mirror with a flat reflecting surface should be used to implement
the mirror 1330. The reflecting surface of the mirror 1330 may be
maintained to be perpendicular to the light beam.
[0087] If a linearly polarized light is used as the input beam 1300
in FIG. 13A, it is beneficial to have the polarization direction of
the light parallel to the incident plane (in the plane of the
paper) as less reflection occurs at the surfaces of Optical Plate
1320 for this polarization compared to other polarization
directions. Antireflection coatings can be used to further reduce
the light reflection on the surfaces of the Optical Plate 1320.
[0088] The beam splitter 1310 used in FIG. 13A uses both its
optical transmission and optical reflection to direct light. This
aspect of the beam splitter 1310 causes reflection loss in the
output of the device due to the reflection loss when the input
light 1300 first enters the device through transmission of the beam
splitter 1310 and the transmission loss when the light exits the
device through reflection of the beam splitter 1310. For example, a
maximum of 25% of the total input light may be left in the output
light if the beam splitter is a 50/50 beam splitter. To avoid such
optical loss, an optical circulator may be used in place of the
beam splitter 1320. FIG. 13B illustrates an example where the
optical circulator 1340 with 3 ports is used to direct input light
to the optical plate 1320 and the mirror 1330 and directs returned
light to the output port. The optical circulator 1340 may be
designed to direct nearly all light entering its port 1 to port 2
and nearly all light entering its port 2 to the port 3 with nominal
optical loss and hence significantly reduces the optical loss in
the device. Commercially available optical circulators, either
free-space or fiber-based, may be used to implement the circulator
1340.
[0089] FIG. 14A shows an exemplary implementation of the delay
device in FIG. 12B as part of or the entire differential delay
modulator 250. A first optical mode splitter 1410 is used to
separate two modes in the waveguide 373 into two paths having two
mirrors 1431 and 1432, respectively. A second optical mode splitter
1440, which is operated as a mode combiner, is used to combine the
two modes into an output. If the two modes are two orthogonal
linear polarizations, for example, polarization beam splitters may
be used to implement the 1410 and 1440. A variable optical delay
line or device 1420 is placed in the upper path to control the
differential delay between the two paths. The output may be coupled
into another dual-mode waveguide 1450 leading to the detection
module or directly sent into the detection module. FIG. 14B shows a
delay device based on the design in FIG. 14A where the mirror 1432
and the variable optical delay line 1420 are implemented by the
mechanical delay device in FIG. 13A. The mechanical delay device in
FIG. 13B may also be used to implement the device in FIG. 14A.
[0090] In the above examples, a single dual-mode waveguide 272 or
372 is used as an input and output waveguide for the probe head
220, 320, or 820. Hence, the input light, either in a single mode
or two independent modes, is directed into the probe head through
that dual-mode waveguide 272 or 372, and the output light in the
two independent modes is also directed from the probe head to the
detection subsystem or detector.
[0091] Alternatively, the single dual-mode waveguide 272 or 372 may
be replaced by two separate waveguides, one to direct input light
from the light source to the probe head and another to direct light
from the probe head to the detection subsystem or detector. As an
example, the device in FIG. 2 may have a second waveguide different
from the waveguide 272 to direct reflected light in two different
modes from the optical probe head 220 to the modulator 250 and the
detection subsystem 260. In this design, the light director 210 may
be eliminated. This may be an advantage. In implementation, the
optics within the probe head may be designed to direct the
reflected light in two modes to the second waveguide.
[0092] FIG. 15 illustrates an example for this design as an
alternative to the device shown in FIG. 5B. In this design, the
probing light is delivered to the sample 205 through one dual-mode
waveguide 1510 and the reflected/scattered light is collected by
the probe head 320 and is directed through another dual-mode
waveguide 1520. With the probe head shown in FIG. 4, the mirror 424
may be oriented and aligned so that the light is reflected into the
waveguide 1520 instead of the waveguide 1510. This design may be
applied to other devices based on the disclosure of this
application, including the exemplary devices in FIGS. 2, 3, 8A, 8B
and 9.
[0093] The above-described devices and techniques may be used to
obtain optical measurements of a given location of the sample at
different depths by controlling the relative phase delay between
two modes at different values and optical measurements of different
locations of the sample to get a tomographic map of the sample at a
given depth or various depths by laterally changing the relative
position of the probe head over the sample. Such devices and
techniques may be further used to perform other measurements on a
sample, including spectral selective measurements on a layer of a
sample.
[0094] In various applications, it may be beneficial to obtain
information about certain substances, identifiable through their
spectral absorbance, dispersed in the samples. For this purpose, a
tunable bandpass filter may be used to either filter the light
incident to the probe head to select a desired spectral window
within the broadband spectrum of the incident light to measure the
response of the sample and to vary the center wavelength of the
spectral window to measure a spectral distribution of the responses
of the sample. This tuning of the bandpass filter allows a variable
portion of the source spectrum to pass while measuring the
distribution of the complex reflection coefficient of the
sample.
[0095] Alternatively, the broadband light may be sent to the
optical probe head without optical filtering and the spectral
components at different wavelengths in the output light from the
probe head may be selected and measured to measure the response of
the sample around a selected wavelength or the spectral
distribution of the responses of the sample. In one implementation,
a tunable optical bandpass filter may be inserted in the optical
path of the output light from the probe head to filter the light.
In another implementation, a grating or other diffractive optical
element may be used to optically separate different spectral
components in the output light to be measured by the detection
subsystem or the detector.
[0096] As an example, FIG. 16 shows a system based on the design in
FIG. 2 where a tunable filter 1610 is inserted in the input
waveguide 271 to filter the input light in two different modes.
[0097] FIG. 17 shows another exemplary system based on the design
in FIG. 8A where a tunable filter 1710 is inserted in the input
waveguide 871 to filter the input light in a single mode. Such a
tunable filter may be placed in other locations.
[0098] FIG. 18 illustrates the operation of the tunable bandpass
filter in the devices in FIGS. 16 and 17. The filter selects a
narrow spectral band within the spectrum of the light source to
measure the spectral feature of the sample.
[0099] Notably, the devices and techniques of this application may
be used to select a layer within a sample to measure by properly
processing the measured data. Referring back to the devices in
FIGS. 16 and 17, let us assume that the absorption characteristics
of a layer bounded by interfaces I and II is to be measured. For
the simplicity of description, it is assumed that the spectral
absorption of the substance in the layer is characterized by a
wavelength-dependent attenuation coefficient .mu..sub.h(.lamda.)
and that of other volume is characterized by .mu..sub.g(.lamda.).
It is further assumed that the substance in the vicinity of
interface I (II) possesses an effective and wavelength independent
reflection coefficient r.sub.I (r.sub.II). If the characteristic
absorption of interest is covered by the spectrum of the light
source, an optical filter 1610 or 1710 with a bass band tunable
across the characteristic absorption of the sample 205 may be used
to measure the spectral responses of the sample 205 centered at
different wavelengths.
[0100] In operation, the following steps may be performed. First,
the differential delay modulator 250 is adjusted so that the path
length traveled by one mode (e.g., the mode 001) matches that of
radiation reflected from interface I in the other mode (e.g., the
mode 002). At this point, the pass band of filter 1610 or 1710 may
be scanned while recording the oscillation of the measured signal
due to a periodic differential phase generated by the modulator
250. The oscillation amplitude as a function of wavelength is given
by
A.sub.I(.lamda.)=r.sub.Ie.sup.-2.mu..sup.g.sup.(.lamda.)z.sup.1
(15)
where z.sub.I is the distance of interface I measured from the top
surface of the sample 205. Next, the differential delay modulator
250 is adjusted again to change the differential delay so that the
path length traveled by the mode 001 matches that of radiation
reflected from interface II in the mode 002. The measurement for
the interface II is obtained as follows:
A.sub.II(.lamda.)=r.sub.IIe.sup.-2.mu..sup.g.sup.(.lamda.)z.sup.I.sup.-2-
.mu..sup.h.sup.(.lamda.)z.sup.II, (16)
where z.sub.II is the distance of interface II measured from
interface I. To acquire the absorption characteristics of the layer
bounded by the interfaces I and II, Eq. (7) and Eq. (6) can be used
to obtain the following ratio:
A II ( .lamda. ) A I ( .lamda. ) = r II r I - 2 .mu. h ( .lamda. )
z II . ( 17 ) ##EQU00009##
Notably, this equation provides the information on the absorption
characteristics of the layer of interest only and this allows
measurement on the layer. This method thus provides a "coherence
gating" mechanism to optically acquire the absorbance spectrum of a
particular and designated layer beneath a sample surface.
[0101] It should be noted that the pass band of the optical filter
1610 or 1710 may be designed to be sufficiently narrow to resolve
the absorption characteristics of interest and at the meantime
broad enough to differentiate the layer of interest. The following
example for monitoring the glucose level by optically probing a
patient's skin shows that this arrangement is reasonable and
practical.
[0102] Various dependable glucose monitors rely on taking blood
samples from diabetes patients. Repeated pricking of skin can cause
considerable discomfort to patients. It is therefore desirable to
monitor the glucose level in a noninvasive manner. It is well known
that glucose in blood possesses "signature" optical absorption
peaks in a near-infrared (NIR) wavelength range. It is also
appreciated the main obstacle in noninvasive monitoring of glucose
is due to the fact that a probing light beam interacts, in its
path, with various types of tissues and substances which possess
overlapping absorption bands. Extracting the signature glucose
peaks amongst all other peaks has proven difficult.
[0103] The above "coherence gating" may be used overcome the
difficulty in other methods for monitoring glucose. For glucose
monitoring, the designated layer may be the dermis layer where
glucose is concentrated in a network of blood vessels and
interstitial fluid.
[0104] FIG. 19A illustrates an example of a human skin tissue where
the coherence gating technique described here can be used to
measure the glucose concentration in the dermis layer between the
epidermis and the subcutaneous layers. The dermis layer may be
optically selected and measured with the coherence gating
technique. It is known that the superficial epidermis layer, owing
to its pigment content, is the dominant source of NIR absorption.
Because of the absence of blood, however, the epidermis yields no
useful information for glucose monitoring. The coherent gating
technique can be applied to acquire solely the absorbance spectrum
of the dermis layer by rejecting the absorptions of the epidermis
and the subcutaneous tissues. An additional advantage of this
technique is from the fact that dermis exhibits less temperature
variation compared to the epidermis. It is known that surface
temperature variation causes shifts of water absorption, hampering
glucose monitoring.
[0105] FIG. 19B shows some predominant glucose absorption peaks in
blood in a wavelength range between 1 and 2.5 microns. The width of
these peaks are approximately 150 nm. To resolve the peaks, the
bandwidth of the tunable bandpass filter may be chosen to be around
30 nm. The depth resolution is determined by the following
equation:
2 ln ( 2 ) .pi. .lamda. o 2 .DELTA. .lamda. = 60 .mu.m ( 18 )
##EQU00010##
Therefore, the coherence gating implemented with the devices in
FIGS. 16 and 17 or other optical sensing devices may be used to
determine the absorption characteristics of the glucose in tissue
layers no less than 60 .mu.m thick. As illustrated in FIG. 19A,
human skin consists of a superficial epidermis layer that is
typically 0.1 mm thick. Underneath epidermis is the dermis,
approximately 1 mm thick, where glucose concentrates in blood and
interstitial fluids. The above analysis indicates that it is
possible to use the apparatus shown in FIGS. 16 and 17 to isolate
the absorption characteristics of the dermis from that of the
epidermis and other layers.
[0106] It is clear from Eq. (18) that the product of spectral
resolution and layer resolution is a constant for a given center
wavelength .lamda..sub.0. The choice of the filter bandwidth should
be made based on the tradeoff between these two resolutions against
the specific requirements of the measurement.
[0107] The tunable bandpass filter 1610 or 1710 may be operated to
acquire the absorption characteristics of an isolated volume inside
a sample.
[0108] FIG. 20 illustrates one exemplary implementation of the
detection subsystem 260 in FIG. 3 where two diffraction gratings
2010 and 2020 are used to separate different spectral components in
the output light beams from the polarizing beam splitter 361. A
lens 2012 is positioned to collect the diffracted components from
the grating 2010 and focus different spectral components to
different locations on its focal plane. A detector array 2014 with
multiple photodetector elements is placed at the focal plane of the
lens 2012 so that different spectral components are received by
different photodetector elements. A second lens 2022 and a detector
array 2024 are used in the optical path of the diffracted
components in a similar way. In devices shown in FIGS. 5A, 5B, 8A,
and 8B where a single optical detector is used for measurements, a
single grating, a lens, and a detector array may be used.
[0109] In operation, each detector element receives light in a
small wavelength interval. The photocurrents from all elements in
an array can be summed to form a signal which is equivalent to the
signal received in each single detector without the grating shown
in FIG. 3. By selectively measuring the photocurrent from an
individual element or a group of elements in an array, the spectral
information of the sample can be obtained.
[0110] Only a few implementations are disclosed in this
application. However, it is understood that variations and
enhancements may be made.
* * * * *