Low coherence interferometric system for optical metrology

Low coherence interferometric system for optical metrology
US 20050254059A1
(19) United States
(12) Patent Application Publication (10) Pub. No.: US 2005/0254059 A1
Alphonse
(43) Pub. Date:
(54)
LOW COHERENCE INTERFEROMETRIC
SYSTEM FOR OPTICAL METROLOGY
(76)
Inventor‘
_
C
(21)
comprising: a broadband light source; an optical assembly
Gerard A‘ Alphonse’ Pnnceton’ NJ
(US)
d
Add
(57)
ABSTRACT
A system for optical metrology of a biological sample
_
receptive to the broadband light, the optical assembly con
?gured to facilitate transmission of the broadband light in a
?rst direction and impede transmission of the broadband
coArlrgélglti EHSiBUISES'LLP
light a second direction; a sensing light path receptive to the
55 GRIFFIN ROAD S’OUTH
broadband light from the optical assembly; a ?xed re?ecting
BLOOMFIELD CT 06002
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10 846 445
pp
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device; a reference light path receptive to the broadband
light from the optical assembly, the reference light path
coupled With the sensing light path, the reference light path
(22) Filed:
(51)
(52)
_
NOV. 17, 2005
having an effective light path length longer than an effective
May 14, 2004
light path length ‘of the sensing light path by a selected
Publication Classi?cation
length corresponding to about a selected target depth Within
the biological sample; and a detector receptive the broad
band light resulting from interference of the broadband light
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to provide an electrical interference signal indicative
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thereof
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LOW COHERENCE INTERFEROMETRIC
SYSTEM FOR OPTICAL METROLOGY
[0005] LoW-Coherence Interferometry (LCI) is another
technique for analyZing light scattering properties of a
biological sample. LoW Coherence Interferometry (LCI) is
BACKGROUND
an optical technique that alloWs for accurate, analysis of the
scattering properties of heterogeneous optical media such as
biological tissue. In LCI, light from a broad bandWidth light
source is ?rst split into sample and reference light beams
[0001]
The invention concerns a loW coherence interfero
metric system for optical metrology of biological samples.
The term “biological sample” denotes a body ?uid or tissue
of an organism. Biological samples are generally optically
heterogeneous, that is, they contain a plurality of scattering
centers scattering irradiated light. In the case of biological
tissue, especially skin tissue, the cell Walls and other intra
tissue components form the scattering centers.
[0002] Generally, for the qualitative and quantitative
Which are both retro-re?ected, from a targeted region of the
sample and from a reference mirror, respectively, and are
subsequently recombined to generate an interference signal.
Characteristics of the interference signal are the exploited to
facilitate analysis of the sample. Constructive interference
betWeen the sample and reference beams occurs only if the
optical path difference betWeen them is less than the coher
analysis in such biological samples, reagents or systems of
ence length of the source.
reagents are used that chemically react With the particular
component(s) to be determined. The reaction results in a
physically detectable change in the solution of reaction, for
[0006] US. Pat. No. 5,710,630 to Essenpreis et al.
describes a glucose measuring apparatus for the analytical
determination of the glucose concentration in a biological
instance a change in its color, Which can be measured as a
sample and comprising a light source to generate the mea
measurement quantity. By calibrating With standard samples
suring light, light irradiation means comprising a light
aperture by means of Which the measuring light is irradiated
into the biological sample through a boundary surface
thereof, a primary-side measuring light path from the light
of knoWn concentration, a correlation is determined betWeen
the values of the measurement quantity measured at different
concentrations and the particular concentration. These pro
cedures alloW accurate and sensitive analyses, but on the
source to the boundary surface, light receiving means for the
other hand they require removing a liquid sample, especially
measuring light emerging from a sample boundary surface
a blood sample, from the body for the analysis (“invasive
folloWing interaction With said sample, and a secondary-side
analysis”).
sample light path linking the boundary surface Where the
[0003] Monitoring and evaluating a biological sample
measuring light emerges from the sample With a photode
tector. The apparatus being characterized in that the light
facilitates analysis and diagnosis for patients and research.
Accordingly, a number of procedures and systems have been
employed. Optical monitoring techniques are particularly
attractive in that they are relatively fast, use non-ioniZing
radiation, and generally do not require consumable reagents.
[0004]
Us. Pat. No. 6,226,089 to Hakamata discloses a
source and the photodetector are connected by a reference
light path of de?ned optical length and in that an optic
coupler is inserted into the secondary-side measurement
light path Which combines the secondary-side measuring
light path With the reference light path in such manner that
they impinge on the photodetector at the same location
system for detecting the intensities of backscattering light
thereby generating an interference signal. A glucose con
generated by predetermined interfaces of an eyeball When a
centration is determined utiliZing the optical path length of
the secondary-side measuring light path inside the sample
laser beam of loW coherence emitted from a semiconductor
laser is divided into tWo parts, a signal light beam and a
derived from the interference signal.
reference light beam, Which travel along tWo different opti
BRIEF SUMMARY
cal paths. At least one of the signal light beam and the
reference light beam is modulated in such a Way that a slight
[0007]
The abovementioned and other draWbacks and
frequency difference is produced betWeen them. The signal
de?ciencies of the prior art are overcome or alleviated by the
light beam is projected onto an eyeball, Which has been in
measurement system and methodology disclosed herein.
a predetermined position, and ?rst backscattering light of the
signal light beam generated by the interface betWeen the
for optical metrology of a biological sample. The system
Disclosed herein in an exemplary embodiment is a system
cornea and the aqueous humor is caused to interfere With the
comprises: a broadband light source for providing a broad
reference light beam by controlling the length of the optical
band light; an optical assembly receptive to the broadband
light, the optical assembly con?gured to facilitate transmis
path of the reference light beam. The intensity of ?rst
interference light obtained by the interference betWeen the
?rst backscattering light and the reference light beam is
measured and the intensity of the ?rst backscattering light is
sion of the broadband light in a ?rst direction and impede
transmission of the broadband light a second direction, and
the optical assembly generally maintaining loW coherence of
determined. The absorbance or refractive indeX of the aque
ous humor in the anterior chamber of the eyeball is deter
mined on the basis of the intensities of the backscattering
the broadband light. The system also includes: a sensing
light. Light scattering effects are evident in the near-infrared
range, Where Water absorption is much Weaker than at larger
Wavelengths (medium- and far-infrared). HoWever, tech
broadband light at the biological sample and to receive the
broadband light re?ected from the biological sample; a ?Xed
re?ecting device; a reference light path receptive to the
niques that rely on the backscattered light from the aqueous
humor of the eye are affected by optical rotation due to
broadband light from the optical assembly, the reference
light path con?gured to direct the broadband light at the
cornea, and by other optically active substances. In addition,
other interfering factors include saccadic motion, corneal
?Xed re?ecting device and to receive the broadband light
re?ected from the ?Xed re?ecting device, the reference light
birefringence, and time lag betWeen analyte changes of the
desired biological sample and the intra-ocular ?uids.
path coupled With the sensing light path to facilitate inter
light path receptive to the broadband light from the optical
assembly, the sensing light path con?gured to direct the
ference of the broadband light re?ected from the biological
Nov. 17, 2005
US 2005/0254059 A1
sample and the broadband light re?ected from the ?xed
re?ecting device, the reference light path having an effective
light path length longer than an effective light path length of
the sensing light path by a selected length corresponding to
about a selected target depth Within the biological sample;
and a detector receptive the broadband light resulting from
interference of the broadband light re?ected from the bio
logical sample and the broadband light re?ected from the
the broadband light re?ected from the re?ecting device to
provide an electrical interference signal indicative thereof.
[0010] Also disclosed herein in yet another exemplary
embodiment is a storage medium encoded With a machine
readable computer program code, the code including
instructions for causing a computer to implement the above
mentioned method for optical metrology of a biological
?xed re?ecting device to provide an electrical interference
sample.
signal indicative thereof.
[0011] Further disclosed herein in another exemplary
[0008] Also disclosed herein in an exemplary embodiment
is a method for optical metrology of a biological sample, the
method comprising: providing a broadband light by means
of a broadband light source; facilitating transmission of the
broadband light in a ?rst direction and impeding transmis
sion of the broadband light a second direction, While gen
embodiment is a computer data signal, the computer data
signal comprising code con?gured to cause a processor to
erally maintaining loW coherence of the broadband light;
directing the broadband light by means of a sensing light
implement the abovementioned method for optical metrol
ogy of a biological sample.
BRIEF DESCRIPTION OF THE DRAWINGS
path at the biological sample, the sensing light path having
[0012] These and other features and advantages of the
present invention may be best understood by reading the
an effective light path length; and receiving the broadband
light re?ected from the biological sample by means of the
sensing light path. The method also includes directing the
Wherein like elements are numbered alike in the several
broadband light by means of a reference light path at a ?xed
?gures in Which:
re?ecting device, the reference light path having an effective
light path length, the effective light path length of the
reference light path being longer than the effective light path
length of the sensing light path by a selected length corre
sponding to about a selected target depth Within the biologi
cal sample. The method further includes: receiving the
broadband light re?ected from the ?xed re?ecting device by
means of the reference light path; interfering the broadband
light re?ected from the biological sample and the broadband
light re?ected from the ?xed re?ecting device; and detecting
the broadband light resulting from interference of the broad
band light re?ected from the biological sample and the
broadband light re?ected from the re?ecting device to
accompanying detailed description of the exemplary
embodiments While referring to the accompanying ?gures
[0013]
FIG. 1 is a basic all-?ber loW-coherence interfer
ometer (LCI);
[0014] FIG. 2 depicts a plot of the envelope function
G(|:|1) and of the interference signal G(|:|1) cos Els;
[0015] FIG. 3 depicts a range of unambiguous measure
ment for a periodic interference signal;
[0016]
FIG. 4A depicts a minimum con?guration inter
ferometer system in accordance With an exemplary embodi
ment of the invention;
provide an electrical interference signal indicative thereof.
[0017] FIG. 4B depicts a con?guration of an interferom
eter system in accordance With an exemplary embodiment of
[0009] Also disclosed herein in another exemplary
the invention;
embodiment is a system for optical metrology of a biological
sample, the system comprising: a means for providing a
[0018] FIG. 5 depicts an illustration of a splitter-modula
tor module in accordance With an exemplary embodiment;
broadband light by means of a broadband light source; a
means for facilitating transmission of the broadband light in
a ?rst direction and impeding transmission of the broadband
[0019] FIG. 6A depicts a process for fabricating the
splitter-modulator module in accordance With an exemplary
light a second direction, While generally maintaining loW
embodiment;
coherence of the broadband light; and a means for directing
the broadband light by means of a sensing light path at the
biological sample, the sensing light path having an effective
[0020] FIG. 6B depicts a process of fabricating the split
ter-modulator module in accordance With an exemplary
light path length. The system also includes a means for
embodiment;
receiving the broadband light re?ected from the biological
[0021] FIG. 6C depicts a process of fabricating the split
sample by means of the sensing light path; a means for
directing the broadband light by means of a reference light
path at a ?xed re?ecting device, the reference light path
having an effective light path length, the effective light path
length of the reference light path being longer than the
effective light path length of the sensing light path by a
selected length corresponding to about a selected target
ter-modulator module in accordance With an exemplary
embodiment;
[0022] FIG. 7 depicts a miniaturiZed, handheld LCI sys
tem in accordance With an exemplary embodiment;
[0023] FIG. 8A depicts operation of a miniaturiZed, hand
depth Within the biological sample. The system further
held LCI system in accordance With an exemplary embodi
includes: a means for receiving the broadband light re?ected
from the ?xed re?ecting device by means of the reference
light path; a means for interfering the broadband light
ment;
re?ected from the biological sample and the broadband light
embodiment;
re?ected from the ?xed re?ecting device; and a means for
detecting the broadband light resulting from interference of
the broadband light re?ected from the biological sample and
[0024] FIG. 8B depicts operation of a miniaturiZed, hand
held LCI system in accordance With another exemplary
[0025]
FIG. 9 depicts an adaptation of the interferometer
system of FIGS. 4A and 4B With a calibration strip;
Nov. 17, 2005
US 2005/0254059 A1
[0026] FIG. 10A depicts an interface for extension mod
ules in accordance With another exemplary embodiment of
[0033] It should noted that the light Wavelengths discussed
beloW for such methods are in the range of about 300 to
the invention;
about several thousand nanometers (nm), that is in the
[0027] FIG. 10B depicts an interface for extension in
accordance With another exemplary embodiment of the
spectral range from near ultraviolet to near infrared light. In
an exemplary embodiment, for the sake of illustration, a
invention;
Wavelength of about 1300 nm is employed. The term “light”
[0028] FIG. 10C depicts another interface for extension in
accordance With yet another exemplary embodiment of the
restricted to the visible spectral range.
invention;
[0034] It Will also be noted that for a homogeneously
scattering medium for Which a speci?c property, such as the
refractive index, is to be measured, it may be sufficient to
probe at a single depth. In such instances, the desired
information can be obtained from the phase of the interfero
as used herein is not to be construed as being limited or
[0029] FIG. 11 depicts an adaptation of the interferometer
system of FIGS. 4A and 4B for ranging measurements in
accordance With another exemplary embodiment; and
[0030] FIG. 12 depicts another adaptation of the interfer
metric signal, substantially independent of the amplitude.
ometer system of FIGS. 4A and 4B for ranging measure
ments in accordance With yet another exemplary embodi
ment With external probe.
Therefore, an instrument as described herein in the simplest
con?guration of an exemplary embodiment is con?gured for
measurement at a single depth. HoWever, if desired, to probe
for inhomogeneities (local changes of absorption, re?ection,
DETAILED DESCRIPTION OF AN
EXEMPLARY EMBODIMENT
[0031] Disclosed herein, in several exemplary embodi
ments are high-sensitivity loW coherence interferometric
(LCI) systems (instruments) for optical metrology of bio
logical samples including, but not limited to analytes, lipids,
other biological parameters, and the like, such as glucose
and plaques. In an exemplary embodiment the LCI systems
are miniaturiZed for use in a variety of sensing and moni
toring applications, including, but not limited to, trace
chemical sensing, optical properties, medical sensing such
as analyte monitoring and evaluation and others. In an
exemplary embodiment, the instrument is miniaturiZed,
using integrated optics components such as Waveguides,
splitters and modulators on a single substrate such as, but not
limited to, a LiNbO3 (Lithium Niobate) chip. The exem
plary embodiments may also involve the use of a “circula
tor” type of optical component, including of a polariZing
beam splitter and quarterWave plate, Which can be combined
With the light source and detector into a miniature module
that prevents optical feedback into the light source While
doubling the detected light. Alternatively, instead of the
polarizing beam splitter and quarter Wave plate one or more
isolators and a Waveguide coupler may be employed in a
similar module to accomplish the same purpose. Disclosed
herein in the exemplary embodiments are multiple method
ologies and associated systems employed to derive infor
mation from the magnitude and/or phase of an interferomet
or refractive index), the instrument may be con?gured to
measure both the amplitude and the phase of the interfero
metric signal as functions of depth. Described herein in a
?rst exemplary embodiment is a system con?gured to probe
at a ?xed depth, While later embodiments may be employed
for measurement at variable depths and for general imaging
purposes. In any case, emphasis is placed on miniaturiZa
tion, portability, loW poWer and loW cost.
[0035] Finally, it Will also be appreciated that While the
exemplary embodiments disclosed herein are described With
reference and illustration to analyte determinations, appli
cations and implementations for determination of other
analytes may be understood as being Within the scope and
breadth of the claims. Furthermore, the methodology and
apparatus of several exemplary embodiments are also non
invasive, and thereby eliminate the dif?culties associated
With existing invasive techniques.
[0036]
Another important consideration is that, as a tool,
particularly for medical diagnostic applications, the LCI
system of the exemplary embodiments is preferably con?g
ured to be easily portable, and for use by outpatients it must
be small. Moreover, the LCI system 10 is con?gured to be
readily hand-held to facilitate convenient measurements by
a patient Without additional assistance in any location.
[0037] Similarly, applications and implementations that
are invasive may also be readily employed With the appro
ric signal.
[0032] It Will be appreciated that While the exemplary
priate con?gurations. For example, When implemented With
embodiments described herein are suitable for the analysis
adapted for invasive applications.
in comparatively highly scattering, i.e. optically heteroge
neous biological samples, optically homogeneous (that is,
loW-scattering or entirely non-scattering) samples also may
be analyZed provided suitable implementations of the
embodiments of the invention are employed. It may be
further appreciated that the methods discussed herein may
not permit an absolute measurements of a characteristic of a
sample, but rather a relative measurement from a given
baseline. Therefore, calibration to establish a baseline may
be required. For instance, for one exemplary embodiment, a
calibration strip of knoWn refractive index is employed to
facilitate calibration. Other methodologies, such as using a
sample of knoWn index of refraction, or knoWn properties
may also be employed.
an extensible ?ber/guideWire and catheter arrangement or
the like, the embodiments disclosed herein may readily be
[0038]
To facilitate appreciation of the various embodi
ments of the invention reference may be made to FIG. 1,
depicting an all-?ber loW-coherence interferometer (LCI)
system and the mathematical equations developed herein.
Referring also to FIG. 4A, in an exemplary embodiment, an
LCI system 10 includes, but is not limited to tWo optical
modules: a source-detector module 20a and a splitter-modu
lator module 40a, and associated processing systems 60. The
source-detector module 20a including, but not limited to, a
broad-band light source 22, such as a super luminescent
diode (SLD) denoted hereinafter as source or SLD, attached
to a single-mode ?ber 23 or Waveguide, an isolator 24
con?gured to ensure that feedback to the broad band light
Nov. 17, 2005
US 2005/0254059 A1
source 22 is maintained at less than a selected threshold. The
source-detector module 20a also includes an optical detector
28.
[0039] The splitter-modulator module 40a includes, but is
not limited to, a Waveguide input 41, a Waveguide output 43,
a splitter/coupler 50, and tWo Waveguide light paths: one
light path, Which is denoted as the reference arm 42, has
adjustable length lr With a re?ecting device, hereinafter a
mirror 46 at its end; the other light path, Which is denoted as
the sensing arm 44, alloWs light to penetrate to a distance Z
in a medium/object and captures the re?ected or scattered
light from the medium. It Will be appreciated that the
from the source-detector module 20 as described herein.
Additional features of a computer system and certain pro
cesses executed therein may be disclosed at various points
herein.
[0044] The processing performed throughout the LCI sys
tem 10 may be distributed in a variety of manners as Will
also be described at a later point herein. For example,
distributing the processing performed in one ore more
modules and among other processors employed. In addition,
processes and data may be transmitted via a communications
interface, media and the like to other processors for remote
processing, additional processing, storage, and database
captured re?ected or scattered light is likely to be only the
so-called “ballistic photons”, i.e., those that are along the
generation. Such distribution may eliminate the need for any
axis of the Waveguide. Provision is also made for one or
more modulators 52, 54 in each of the reference arm 42 and
combining distributed processes in a various computer sys
tems. Each of the elements described herein may have
additional functionality that Will be described in more detail
herein as Well as include functionality and processing ancil
sensing arm 44 respectively.
[0040]
Continuing With FIG. 4B as Well, in another exem
plary embodiment, the source-detector module 20b includes,
but is not limited to, a polariZed broad-band light source 22,
attached to a single-mode ?ber 23. The source-detector
module 20b also includes a polariZing beam splitter 25 With
such component or process as described or vice versa,
lary to the disclosed embodiments. As used herein, signal
connections may physically take any form capable of trans
ferring a signal, including, but not limited to, electrical,
optical, or radio.
polariZation con?gured to facilitate ensuring that feedback
[0045] The light re?ected from the reference mirror 46
(Electric ?eld E,) in the reference arm 42 and the light
to the broad band light source 22 is maintained at less than
a selected threshold. The source-detector module 20b also
includes an optical detector 28.
re?ected or scattered from depth Z Within the biological
sample (Electric ?eld E5) in the sensing arm 44 are com
bined at the optical detector 28, Whose output current is
an quarter Wave plate 26 employed to ensure a selected
[0041] The splitter-modulator module 40b of this embodi
ment includes, but is not limited to, a Waveguide inputs/
output 45, a Y-splitter-combiner 51, and the tWo Waveguide
arms: reference arm 42, and sensing arm 44. Once again,
provision is also made for one or more modulators 52, 54 in
each of the reference arm 42 and sensing arm 44 respec
proportional the combined electric ?elds. For example, in
one instance, the output of the detector is proportional to the
squared magnitude of the total electric ?eld Et=EI+ES.
[0046] The detector current Id is given by:
tively.
[0042]
It Will be appreciated that While certain compo
nents have been described as being in selected modules, e. g.,
20, 40, such a con?guration is merely illustrative. The
various components of the LCI system 10 may readily be
distributed in one or more various modules e.g., 20, 40 as
cally <1), II=11EI*EI* is the detector current due to EI alone,
IS=11ES*ES* is the detector current due to ES alone, and the *
represents the complex conjugate. EI*EI* and ES*ES* repre
sent the optical poWer in the re?ected reference ?eld and
re?ected sensing ?eld, respectively. The quantity at is the
time delay betWeen the reference ?eld EI and sensing ?eld
suits a given implementation or embodiment. Furthermore,
in an exemplary embodiment the Waveguide arms 42, 44
and/or ?bers 23 are con?gured for single-transverse-mode
ES, and is given by:
transmission, and preferably, but not necessarily, polariZa
1,
tion-maintaining Waveguides or ?bers. Furthermore it Will
be appreciated that in any of the exemplary embodiments
disclosed herein the Waveguide and/or ?ber tips of each
component joined are con?gured e.g., angled-cleaved in a
manner to minimiZe re?ection at the junctions.
[0043] In order to perform the prescribed functions and
desired processing, as Well as the computations therefore
(e.g., the computations associated With detecting and utiliZ
ing the interference signal, and the like), the LCI system 10,
and more particularly, the processing system 60, may
include, but is not limited to a computer system including
Z
1,-1,
Al
(2)
[0048] Where lS=I1Z and Al=lr—lS and Where Al is the optical
path difference betWeen the reference II and sensing lS arms
42, 44, Z is the selected or desired target depth in the
biological sample, n is the index of refraction in the sample,
and c is the speed of light. Also in Equation (1), v0 is the
center frequency of the light source 22, and G("c) it the
cross-correlation function betWeen the reference and sensing
?elds. Its magnitude is given by:
central processing unit (CPU) 62, display 64, storage 66 and
the like. The computer system may include, but not be
limited to, a processor(s), computer(s), controller(s),
memory, storage, register(s), timing, interrupt(s), communi
cation interface(s), and input/output signal interfaces, and
RAVT )2]
(3)
the like, as Well as combinations comprising at least one of
the foregoing. For example, computer system may include
[0049] Where Av is the FWHM (full Width half maximum)
signal input/output for controlling and receiving signals
frequency bandWidth of the light source 22.
Nov. 17, 2005
US 2005/0254059 A1
[0050] The last term in Equation (1), the interference term,
[0056] Therefore, it Will be readily be appreciated that
is the quantity of interest denoted as i0:
there are tWo types of information, Which can be derived
iD(-c)=2VII|G(-c)| cos Zm/D-c
(4)
[0051] It is convenient to express the interference term i0,
in terms of the center Wavelength k0 and the path difference
al associated With the interferometer, instead of the fre
quency and time delay. Therefore, using voko=c, Where c is
the speed of light in vacuum, Av may be Written in terms of
the Wavelength FWHM bandWidth A7», to obtain:
from the interference signal i0: the envelope G(Al), or its
peak G(Al=0), Which may represent scattering, re?ection,
and absorption; and the more sensitive changes in cost due
to small optical property changes in the sample. In order to
make any such measurements, it is ?rst preferable to sepa
rate the DC components II and IS from G(Al) and cos (1)5 in the
interferometric signal iO described in Equation
[0057]
Referring once again to FIGS. 4A and 4B, broad
band light sources including, but not limited to, SLD’s are
[0052] Where Lc is the coherence length of the light source
and is given by
laser type structures con?gured and designed to operate
substantially Without feedback, e.g., of the order of less than
104, preferably less than 104, more preferably less than
10's. In the presence of feedback, the spectrum of the SLD
light source 22 may be distorted, the coherence is signi?
cantly increased and the spectrum can exhibit very large
ripples and even lasing spikes, and thereby may become
lasers. Therefore, to prevent distortion and maintain spectral
integrity, loW coherence, and broadband characteristics,
re?ections back into the light source 22 are avoided to
maintain a broadband light source 22. Thus, in an exemplary
embodiment of the LCI system, isolation is provided to
alleviate feedback to the light source 22.
[0058]
[0053] A plot of the envelope function G(Al) and if the
interference signal G(Al)cos (I)S is shoWn in FIGS. 2A and
2B respectively, for an interferometer With a light source 22
having center Wavelength KO=1.3 pm and FWHM bandWidth
A>M=60 nm (coherence length Lc=12.4 pm). The detected
interference signal exhibits a maximum When the interfer
ometer is balanced, i.e., When the path difference Al=0. As
the system 10 becomes increasingly unbalanced, e.g., Al#0,
the interference signal exhibits maxima and minima of
decreasing amplitude over a range determined by Al.
Continuing With FIGS. 4A and 4B, in an exem
plary embodiment, the source-detector module 20a, 20b, is
con?gured to prevent the re?ected interferometer light from
reaching the SLD light source 22 and upsetting its operation.
The SLD source 22 is designed and con?gured such that it
is linearly-polariZed. SLDs and lasers are “heterostructures”
semiconductor devices consisting of a thin “active” layer
sandWiched betWeen tWo “cladding” layers of loWer refrac
tive index, all epitaxially groWn on a single crystal substrate
23. One such process for fabrication is knoWn as MOCVD
[0054] It Will be appreciated that the interference signal iO
(metalorganic chemical vapor deposition). One of the clad
ding layers is p-doped, and the other is n-doped. The
substrate 23 is typically n-doped, and the n-cladding layer is
exhibits signi?cant amplitude only over a spatial WindoW of
the ?rst to be deposited on it. The structure forms a p-n
approximately tWice the coherence length LC. As the optical
bandWidth increases, the coherence length Lc decreases and
semiconductor junction diode, in Which the active layer is
caused to emit light of energy equal to its bandgap upon the
the spatial measurement WindoW narroWs. Thus, LCI pro
vides a means for probing samples at precisely de?ned
application of an electric current.
[0059] The structure is called heterostructure because the
active and clad layers are made of different material. This is
in contrast With ordinary diodes in Which the p-n junction is
locations Within the sample.
[0055] It is noteWorthy to appreciate that the phase, (1)5, of
the interference signal iO changes by 2st (from a maximum
formulated betWeen similar materials of opposite doping.
to a minimum then to another maximum) as Al varies from
The use of heterostructure has made it possible to con?ne the
0 to A0. Therefore, a small change in Al results in a large
phase change. It Will be further appreciated that the phase of
the interference signal i0 is highly sensitive to small changes
of optical properties of the mediums, such as refractive
indices, or depth Z. Thus, While moderate to large changes
may readily be observed by measuring the magnitude of the
envelope G(Al), small changes are best detected by measur
ing the phase (1)5 of the interference signal i0. It Will be further
appreciated that all the desired information is contained in
the range from 0 to 2:1. For values of Al>7»O, the interference
signal i0 is repetitive. Thus, the range from 0 to 2:1 as
indicated in FIG. 3 is a range for Which the desired
information can be measured Without ambiguity. It may also
be noted hoWever, that if the coherence length Lc is short
enough that the amplitude difference betWeen the main peak
and secondary peaks is measurable, then phase measurement
beyond 275 may be realiZed.
electrical carriers to Within the active region, thus providing
high ef?ciency and enabling operation at room temperature.
In many heterostructures, light is emitted in both TE polar
iZation (the electric ?eld in the plane of the layer) and TM
polariZation (electric ?eld perpendicular to the layer).
[0060]
HoWever, useful effects are obtained When the
active layer is suf?ciently thin such that quantum mechani
cal effects become manifest. Such thin layers are called
“quantum Well” (QW) layers. Furthermore, the active layer
can be “strained”, i.e., a slight mismatch (of about 1%) With
respect to the substrate crystal lattice can be introduced
during the deposition of the QW layer. The strain can modify
the transition characteristics responsible for light emission in
bene?cial Ways. In particular, the light is completely polar
iZed in the TE mode if the strain is compressive. Thus, it is
noW possible to make a linear polariZed laser or broadband
Nov. 17, 2005
US 2005/0254059 A1
SLD by compressive strain of the active layer. In an exem
plary embodiment, such a linearly-polariZed light source 22
is employed.
[0061] In one exemplary embodiment, as depicted in FIG.
4A, the light from the light source 22 is directed through an
isolator 24 con?gured to transmit light in one direction,
While blocking light in the opposite direction. The light is
directed to a splitter/coupler 50 of the splitter-modulator
module 40a. The source-detector module 20a also contains
a detector 28 to receive from the splitter/coupler 50.
[0062] In another exemplary embodiment as depicted in
FIG. 4B, the linearly-polariZed light from the SLD light
source 22 is collimated With lenses 27 and applied to a
splitter 25. If a basic 50/50 splitter 24 is employed, half of
the returned light goes to the detector 28 and the other half
is directed to the SLD light source 22. Once again, in this
con?guration an isolator 24 may be employed to prevent
feedback to the light source 22. Similarly, as stated earlier,
in another exemplary embodiment, the splitter 25 is a
polariZing beam splitter 25 operating in cooperation With a
quarter Wave plate 26, employed to prevent feedback light
from reaching the light source 22. The polariZing beam
splitter 25 facilitates the elimination of feedback to the SLD
light source 22 by redirecting substantially all the re?ected
light from the splitter-modulator module 40b to the detector
28.
QW. The FWHM spectrum is of the order of 2% to 3% of
the central Wavelength emission. ASLD light source 22 With
1.3 pm center Wavelength emission and operating at 10 mW
output poWer at room temperature Would have a bandWidth
of about 40 nm and Would require about 200 mA of current.
In an exemplary embodiment, for continuous Wave (cW)
operation at room temperature, the SLD light source 22 may
be mounted on an optional thermoelectric cooler (TEC) 32
a feW millimeters larger than the SLD light source 22 chip
to maintain the temperature of the light source 22 Within its
speci?ed limits. It Will be appreciated that the SLD light
source 22 and associated TEC 32 peripherals in continuous
operation Would have the largest poWer consumption in the
LCI system 10. HoWever, Without the TEC 32, the SLD
junction temperature Would rise by several degrees under the
applied current and Would operate at reduced ef?ciency.
[0066] Advantageously, in yet another exemplary embodi
ment, the utiliZation of a TEC 32 may readily be avoided
Without incurring the effects of signi?cant temperature rise
by pulsed operation of the SLD light source 22. Pulsed
operation has the further advantage of reducing the SLD
electrical poWer requirement by a factor equal to the pulsing
duty cycle. Moreover, for selected applications of digital
technology and storage, only a single pulse is suf?cient to
generate an interference signal and retrieve the desired
information. Therefore, for example, With pulses of duration
10 us and 1% duty factor, the LCI system 10 of an exemplary
[0063] The splitter 25 transmits the horiZontally polariZed
embodiment can average 1000 measurements per second
light to the quarter Wave plate 26, Which coverts the light to
another polariZation, (for example, circular polariZation).
LikeWise, the returning, circularly polariZed light is received
Without causing the SLD light source 22 temperature to rise
signi?cantly. Thus, for loW poWer consumption, the LCI
system 10 should preferably be designed for the SLD light
by the quarter Wave plate 26 and is reconverted to a linear
source 22 to operate in a pulsed mode With a loW duty cycle
polariZation. HoWever, the linear polariZation opposite, for
example, vertical. The vertically polariZed light is transmit
and Without a TEC 32. In such a con?guration the source
detector module 20 Would be on the order of about 2
centimeters (cm)><2 cm><1 cm.
ted to the polariZing beam splitter 25, Which directs all of the
light to the detector 28. Advantageously, this approach
transmits substantially all of the light i.e., the interference
signal, to the detector 28. Whereas embodiments employing
the isolator 24 transmits approximately half of the light to
the detector 28.
[0064] The polariZing beam splitter 25 is a device that
transmits light of one polariZation (say the horiZontal, or
TE-polariZed SLD light) and re?ects at 90° any light of the
other polariZation (e.g., vertical or TM-polariZed). The quar
ter-Wave plate 26 is a device that converts a linearly polar
iZed incident light to circular polariZation and converts the
re?ected circularly-polariZed light to a linearly-polariZed of
the other polariZation Which is then re?ected at a 90° angle
by the polariZing beam splitter 25 to the detector 28.
Therefore, essentially all the light transmitted by the light
source 22 is re-polariZed and transmitted to the splitter
modulator module 40b and all the re?ected light from the
sample and re?ecting device 48 is de?ected by the polariZ
ing beam splitter 25 to the detector 28. Advantageously, this
doubles the light received at the detector 28 relative to the
other embodiments, and at the same time minimiZes feed
back to the SLD light source 22.
[0065] In an exemplary embodiment an SLD chip for the
light source 22 has dimensions of approximately 1 mm><0.5
[0067] The splitter-modulator module 40a, and 40b of an
exemplary embodiment includes a splitter/coupler 50 and
Y-splitter/combiner 51 respectively, With a “reference” arm
42 and a “sensing” arm 44, the reference arm 42 having a
slightly longer optical path (for example, 1 to 3 mm for
measurements in biological tissues) than the sensing arm 44.
The optical path difference betWeen the tWo arms 42, 44 is
con?gured such that the LCI system 10 balanced for the
chosen probing depth Z. Provision is also made to include a
modulator m1 52 and m2 54 in the reference arm 42 and
sensing arm 44 respectively.
[0068] In an exemplary embodiment, the splitter/coupler
50, Y-splitter/combiner 51 reference arm 42 and a sensing
arm 44 are formed as Waveguides in a substrate. HoWever,
other con?gurations are possible, including but not limited
to separate components, Waveguides, optical ?ber, and the
like. The substrate 23 for this module should preferably, but
not necessarily, be selected such that the Waveguides of the
arms 42, 44 and modulators 52, 54 can be fabricated on/in
it by standard lithographic and evaporation techniques. In
one exemplary embodiment, the Waveguides of the arms 42,
44 are fabricated by thermal diffusion of titanium or other
suitable metal that increases the index of refraction of the
mm><0.1 mm (length><Width><thickness), and emits a broad
substrate, evaporated through masks of appropriate Width
for single transverse-mode operation. In another exemplary
band light typically of up to 50 mW upon the application of
embodiment, the Waveguides are formed by annealed proton
an electric current of the order of 200-300 mA. The light is
exchange in an acid bath. This process raises the refractive
TE-polariZed if the active layer is a compressively strained
index in the diffusion region, thus creating a Waveguide by
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