Note: Descriptions are shown in the official language in which they were submitted.
CA 02500876 2013-03-25
PHASE SHIFT OPTICAL LOOP SPECTROSCOPY
Field of the Invention
This invention relates to methods and apparatus for measuring optical
characteristics of a test medium or media. In particular, the invention
relates to use of
the phase shift of light in an optical loop to measure optical characteristics
of a test
medium or media.
Background of the Invention
Measurement of low optical losses of an absorbing medium, such as a gas or a
molecular beam, may be performed by measuring the ring-down time, or decay
time, of
a light pulse as the pulse makes multiple passes through the medium. Such
measurements may be carried out in a ring-down cavity consisting of two or
more
mirrors, between which the light pulse is reflected, and in which the
absorbing medium
under test is disposed. The cavity can also be used to characterize the
mirrors when no
absorbing medium is present. See, for example, Romanini et a/.,1993; Scherer
etal.,
1997; Berden etal., 2000; Lehmann, U.S. Patent No. 5,528,040, issued June 18,
1996.
Cavity ring-down spectroscopy (CRDS) is well established as a gas phase
measurement method. Recently CRDS was shown to be applicable to absorption
measurements on liquid samples, in which a high finesse cavity was either
filled entirely
with a liquid sample (Hallock et al., 2002) or the liquid was contained in a
cuvette (Xu et
al., 2002).
Ring-down spectroscopy using optical fibers rather than a cavity was attempted
by von Lerber et al. (2002), who deposited highly reflective coatings onto
both end facets
of a 10 m optical fiber. Stewart etal., (2001) inserted a gas phase absorption
cell into a
fiber-loop, leading to very high transmission losses. These losses
necessitated the use
of a fiber amplifier, and the sensitivity of measurements using such an active
loop
depended strongly on the amplifier's temporal stability.
1
CA 02500876 2005-03-15
We have previously demonstrated a fiber-loop ring-down technique for
characterizing low-loss processes in optical systems and for spectroscopy of
minute
liquid samples (Loock etal., U.S. Patent No. 6,842,548, issued January
11,2005),
based on measuring the ring-down time of a light pulse injected into the loop.
Although
extremely sensitive, limitations of that technique include slow data
acquisition rate and
high cost of optical components such as fast pulsed lasers.
Summary of the Invention
According to one aspect of the invention there is provided a method for
measuring one or more optical properties of a test medium, comprising:
providing an
optical waveguide loop comprising a test medium; launching in the optical
waveguide
loop intensity-modulated light at a reference phase; detecting a phase of said
light along
the optical waveguide loop; and comparing the detected phase of said light
along the
loop with the reference phase; wherein comparing the detected phase and the
reference
phase provides information about one or more optical properties of the test
medium.
In a preferred embodiment, the optical waveguide loop is passive. In another
preferred embodiment, the optical waveguide is an optical fiber. In various
embodiments, the optical waveguide loop is the test medium, or the optical
waveguide
loop comprises a capillary channel for said test medium.
In another embodiment, the test medium is exposed to an evanescent wave of
light that is guided by the optical waveguide loop. In a further embodiment,
the optical
waveguide loop comprises a cladding, and the test medium is in the cladding.
In one embodiment, the optical property is absorbance. In various embodiments,
the light has at least one wavelength selected from infra-red (IR), visible,
and ultra-violet.
The light may have a wavelength selected from about 200 nm and 2000 nm,
preferably
about 200 nm to about 1700 nm. The test medium may be selected from a gas, a
molecular beam, a liquid, and a solid material. In a preferred embodiment, the
test
medium is a liquid.
In another embodiment, the optical waveguide loop comprises a single-mode
optical fiber, and the method comprises launching in the optical waveguide
loop a single
longitudinal mode of an intensity-modulated light; wherein the phase of the
longitudinal
mode is indicative of one or more optical properties of the test medium. In a
further
embodiment, the method further comprises measuring intensity of said
longitudinal
mode.
2
CA 02500876 2005-03-15
In another embodiment, the test medium comprises a mechanical sensor for
receiving a mechanical force, and the one or more optical properties of the
test medium
provide information about the mechanical force received by the mechanical
sensor. The
mechanical force may be selected from stress and strain.
According to another aspect of the invention there is provided an apparatus
for
measuring one or more optical properties of a test medium, comprising: an
optical
waveguide loop comprising a test medium; an intensity-modulated light source
for
illuminating the loop with light at a reference phase; a detector for
detecting a phase of
said light along the loop; and an analyzer for comparing the detected phase of
the light
with the reference phase of the light; wherein the comparison is indicative of
one or more
optical properties of the test medium. The analyzer may output a result of the
comparison, wherein the result is indicative of one or more optical properties
of the test
medium.
In one embodiment, the apparatus further comprises a device for displaying
and/or storing and/or manipulating data corresponding to at least one of said
reference
phase, said detected phase, and said comparison.
In a preferred embodiment, the optical waveguide loop is passive. In various
embodiments, the optical waveguide loop is an optical fiber or a single-mode
optical
fiber. In a further embodiment, the optical waveguide loop is the test medium.
In some
embodiments the apparatus further comprises a capillary channel for guiding
the test
medium to said light.
In another embodiment, test medium is exposed to an evanescent wave of light
that is guided by the optical waveguide loop. In some embodiments, the optical
waveguide loop comprises a cladding, and the test medium is in the cladding.
In further
embodiments, the test medium or the optical fiber comprises a grating.
In various embodiments, the optical property is absorbance, and in other
embodiments, light has at least one wavelength selected from infra-red (IR),
visible, and
ultra-violet. The wavelength may be between about 200 nm to 2000 nm,
preferably
between about 200 nm to 1700 nm.
In one embodiment, the apparatus comprises a microfluidic device.
In another embodiment, the test medium comprises a mechanical sensor for
receiving a mechanical force, and the one or more optical properties of the
test medium
provide information about the mechanical force received by the mechanical
sensor. The
mechanical force may be selected from stress and strain.
3
CA 02500876 2005-03-15
Brief Description of the Drawings
Embodiments of the invention are described below, by way of example, with
reference to the accompanying drawings, wherein:
Figure 1 is a block diagram of a phase shift fiber loop system according to
the
invention for measuring optical properties of an optical fiber and connector.
Figure 2 is a plot showing a phase shift of the optical signal relative to the
(smooth) reference signal from the function generator, for the setup shown in
Figure 1.
Figure 3 (a) shows a plot of phase angle as a function of concentration for a
26.4
m fiber loop, used to determine a phase angle difference of 00. -36.3 ,
absorption Ao=
0.60 in absence of 1,1'-diethyl-4,4'-dicarbocyanine iodide (DDCI) dye, and gap
between
the fiber ends of d = 42 p.m; (b) shows concentration dependence of phase
angle in
capillary electrophoresis using a microcross. Here, 00. -23.3 , d = 31 jim
and Ao =
0.66 were determined from the fit for a L = 65 m fiber loop.
Figure 4 shows a plot of phase angle vs time for 1.0 and 2.0 mM solutions of
DDCI dye placed between two ends of a fiber, forming a fiber loop. The
solutions were
rapidly exchanged and the phase angle was measured. Measurements were taken
every 100 ms and the time resolution of the measurement was sufficient to
monitor the
time taken for the solution to be completely replaced.
Figure 5 is a plot of phase angle vs time showing transient absorption peaks
due
to absorption of DDCI in dimethylsulfoxide (DMSO), recorded by pushing the
analyte
through a 100 pm capillary using a syringe: the upper panel shows peaks at two
concentrations; and the lower panel shows peaks for three different lengths of
plugs of a
1 mM solution. There is significant peak broadening because of the large size
of the
capillary (100/360 m).
Figure 6 is a plot showing capillary electrophoresis separation of two dyes,
ADS
805 WS and ADS 830 WS dissolved in water (buffered by KH2P03, pH - 8.0). 4.5kV
was
used to elute the mixture.
Figure 7 is a schematic diagram of an experimental setup used to investigate
optical losses of a fiber optic cable in the 1.55 jim wavelength region by
determining
optical losses of radiation travelling through the fiber core independently
from the losses
by the cladding.
4
CA 02500876 2005-03-15
Figure 8 is a plot of the phase angle (I) as a function of the angular
modulation
frequency using the setup shown in Figure 7, for broadband and narrow band
excitation sources. The optical decay constant t and the offset phase angle
(00were
determined from the slope.
Figure 9A is a schematic diagram of a fiber-optic strain sensor according to
the
invention.
Figure 9B is a plot of the phase angle as a function of the load on the fiber-
optic
strain sensor of Figure 9A.
Figure 9C is a plot showing the time response of the fiber-optic strain sensor
of
Figure 9A.
Detailed Description of the Preferred Embodiments
Engeln etal. (1996) measured the phase shift associated with an optical decay
in
cavity ring-down spectroscopy (CRDS) and observed that, due to the greatly
enhanced
duty cycle, the accuracy of the technique compared favourably with pulsed
CRDS. To
our knowledge, phase shift CRDS (PS-CRDS) has been used only once since 1996,
when Lewis et al. (2001) reported on the Av = 6 vibrational overtone
transitions of
different hydrocarbons. DeMille et al. (2002) later compared these
measurements with
overtone spectra obtained using conventional CRDS and intracavity laser
photoacoustic
spectroscopy and noted that PS-CRDS yields absorption cross sections about 35%
higher than those obtained with either of the conventional techniques,
possibly indicating
systematic errors in the PS-CRDS system. The lack of interest in PS-CRDS is
likely due
to its shortcomings, which include the inability to deal with multiple
exponential decays,
mode-beating, sensitivity to electrical and optical noise, and difficulty of
measuring small
phase angles in 1 MHz oscillations.
We nevertheless believe that the phase shift technique in general offers a
major
advantage over pulsed (time-resolved) CRDS, in that the time response of
measurements is vastly improved. We believe that the fast time response of the
technique is of particular relevance to liquid phase spectroscopy of very
small samples
that can be changed or cycled rapidly, where measurement time becomes the
limiting
factor in sample throughput. Also, there may be other sources of optical loss
that
change quickly and can not readily be sampled using the conventional pulsed
CRDS
5
CA 02500876 2005-03-15
technique. Such losses could, for example, be due to fast chemical processes
or
mechanical modifications to the cavity geometry.
This invention is based, in part, on the recognition that the advantage of the
phase shift technique may be exploited when adapted to a measurement method
employing an optical wave guide loop. Prior to the invention, such
spectroscopic
techniques could not be carried out on very small liquid phase samples, and
for gas
phase measurements, the fast time response was not important enough to offset
the
drawbacks of the technique. Although Stewart et al. (2001) suggested that a
phase shift
technique might be applied to a measurement method employing an optical fiber
loop,
that suggested method used a microoptic gas cell having high loss (1 dB), and
hence
required an amplifier in the loop. As a result, the technique suggested by
Stewart et al.
(2001) did not involve a passive loop. Moreover, the suggested technique would
be
complex and expensive to implement, and would be vulnerable to complications
such as
gain control and amplifier stability associated with an amplified loop.
According to a broad aspect of the invention, there is provided a method of
measuring one or more optical properties of a test medium by measuring the
phase shift
of light traveling around an optical waveguide loop and through the test
medium. The
invention provides a method by which the phase shift of light in a waveguide
loop can be
used in characterizing the optical properties of a test medium. Preferably,
the optical
waveguide loop is passive, meaning that the loop does not have a device (e.g.,
an
amplifier) for amplifying light.
As used herein, the terms "change in phase", "phase change", "phase shift",
and
"phase angle difference" are equivalent and refer to a change in the phase (I)
of an
optical signal as a function of the test medium through which the signal
travels. A
change in phase may be considered as a temporal shift in a light signal
waveform (the
waveform having a period of 360 degrees) after passing through a test medium,
relative
to a reference. The reference may be, for example, the phase of the light
signal prior to
passing through the test medium. For convenience, the reference phase (e.g.,
phase of
the light signal entering the optical waveguide) may be designated as 0
degrees, and the
phase change of the light signal after passing through the test medium (e.g.,
phase of
the light signal exiting the optical waveguide) may be expressed relative to 0
degrees.
As used herein, the term "test medium" is intended to mean any medium or
material the optical properties of which can be measured in accordance with
the
invention. The test medium is exposed to at least a portion of the light that
is guided by
6
CA 02500876 2005-03-15
the optical waveguide, wherein that portion of light is either within the
waveguide, or
outside of the waveguide (i.e., the evanescent wave). Examples of test media
include,
but are not limited to, the optical waveguide loop itself, a portion of a
second optical
waveguide inserted into the loop, a modified optical waveguide, an optical
connector or
an optical device (e.g., a grating or filter), a sample of a gas, liquid, or
solid material
(e.g., a film or coating, such as a solid or liquid film deposited on the
facet end of an
optical fiber), a molecular beam, or a stationary test medium. Samples of
gases and
liquids may be introduced into the loop using a small conduit for conducting
the sample
therethrough, the conduit intersecting the loop in a manner that allows all or
a portion of
the light to pass through the sample. For example, the conduit may be a tube,
such as a
capillary tube (i.e., a "capillary"), or a flow channel, which may also be
referred to as a
capillary channel or a capillary flow channel. The flow channel may be etched
or
machined into a substrate. It will be appreciated that flow of a sample
through such a
conduit may or may not involve capillary action. Where flow does not involve
capillary
action, flow may be established using any technique known in the art, such as
a pump,
an electrical and/or chemical gradient, a pressure differential, or the like.
A test medium
may also be introduced to the optical waveguide loop so as to intercept at
least a portion
of the evanescent wave. In the case where an optical fiber is used for the
optical
waveguide, this may be achieved by, for example, modifying a portion of the
fiber
cladding to accommodate a test medium. For example, a portion of the fiber
cladding
may be removed to expose a test medium to the evanescent wave that resides in
the
cladding close to the interface of the cladding and the fiber core.
As used herein, the term "optical properties" is intended to mean any property
of
a medium that is light-dependent. Examples of optical properties are
absorbance,
scattering efficiency, refractive index, evanescent wave spectrum, and optical
loss.
Optical properties are indicative of, or related to, physical and/or chemical
characteristics
of a medium (e.g., density, structure (such as 1-, 2-, or 3-dimensional
structure)). Thus,
in accordance with the invention, one or more optical properties of a medium
is/are
indicative of one or more physical and/or chemical characteristics of the
medium.
As used herein, the term "optical waveguide" is intended to encompass any
conduit for light. An optical waveguide according to the invention is capable
of being
formed into or provided as a continuous loop, e.g., by joining the two ends of
the
waveguide together, such that a light signal launched in the waveguide travels
around
the loop repeatedly. Thus, as used herein, the term "optical waveguide loop"
refers to a
7
CA 02500876 2005-03-15
loop made of an optical waveguide. The loop is continuous insofar as it
provides a
continuous path for a light signal travelling therethrough; however, the loop
may have an
opening into which a test medium may be inserted. Examples of optical
waveguides are
optical fibers, such as those having a solid core, hollow core (i.e.,
capillary fiber), or
liquid core, and waveguides based on high refractive index fluids. Optical
waveguides
can also be prepared on a substrate such as glass or polymeric material, for
example, in
embodiments where the invention comprises a microchip. Where optical fiber is
employed, such fiber may be selected from commercially available fibers,
including
multi-mode and single mode fibers. The two ends of waveguides such as optical
fibers
are joined using splice connectors, such as any commercially available
connector, fusion
spliced connections, or any other suitable technique known in the art.
Preferably, such
fibers and connections have low transmission loss (e.g., absorbance,
geometrical
mismatch, scattering). In this regard, waveguides based on high refractive
index fluids
are advantageous in that such connectors are not required.
In accordance with the invention, the optimum length of the optical waveguide
loop depends on the desired measurement sensitivity and detection limit.
Generally, as
optical losses within the loop decrease, the loop may be made shorter. For
very small
losses, the loop is preferably as short as possible. In a typical measurement
scenario,
such optical losses would be caused by absorption of the test medium. It is
expected
that for certain applications an entire apparatus according to the invention
may be
fabricated on a microchip. In practice, the minimum length of the loop may be
limited by
factors that contribute to loss of the light signal, such as a small radius of
the bend in the
waveguide (loss increases as radius decreases), high loss of a waveguide
splice (e.g., a
fiber optic splice connector), or high absorbance of a test medium. Loop
length is
discussed in detail in Jakubinek et al. (2004). It should be noted that in
forming the loop,
the optical waveguide can be "wound" into any shape, as may be required for
compactness, etc., of the loop. This is of relevance especially when long
loops are
required.
It is preferred that the optical waveguide loop is a passive loop. As used
herein,
the term "passive loop" refers to a loop that does not have a device (e.g., an
amplifier)
for amplifying light.
In accordance with the invention, the light signal may be of any wavelength
from
about 2000 nm to about 200 nm (i.e., infra-red (IR) to ultra violet (UV)),
preferably from
about 1700 nm to about 200 nm. However, use of wavelengths longer than 1700 nm
8
CA 02500876 2005-03-15
may be problematic due to the high transmission losses in typical waveguide
materials
(e.g., silica). Use of UV may also be problematic because of the degradative
effects of
UV light on optical materials, and comparatively high losses (e.g., 1% per m
of optical
fiber). However, UV is of particular interest in chemical, biochemical,
biological, and
environmental studies, because many compounds and substances of interest
absorb in
this wavelength. In some embodiments of the invention the light signal has a
narrow
bandwidth (e.g., a single colour of light), whereas in other embodiments, the
light signal
is wide band (e.g., white light). Suitable light sources are those light
sources capable of
being modulated at a rate that is comparable to 1/T, where t is the optical
decay
constant of the waveguide loop. The light source may be, for example, a laser,
a laser
diode, or a light emitting diode (LED). In embodiments employing a
spectroscopic
approach wherein ring-down time as a function of wavelength is sought, a
tunable laser
can be employed, and such laser swept to produce laser radiation over a range
of
wavelengths. The light signal may be coupled into the waveguide using any
conventional approach or device, such as, for example, a directional coupler.
However,
in the case of optical fiber, light may be coupled into the fiber simply by
illuminating the
fiber. When using a 1 W laser, the inventors achieved good coupling of light
into an
optical fiber by first focussing the laser light into a power delivery fiber,
and then
connecting this fiber to the loop using a drop of DMSO solvent, which acted as
index
matching fluid. When using a 10 mW infrared laser at 1600 nm, low loss was
achieved
using a 99.5%:0.5% directional coupler, despite the fact that the coupler
introduced a
considerable (4%) insertion loss of light per pass around the loop. Using
evanescent
coupling (Polynkin et al., 2004) these insertion losses were further reduced
and coupling
efficiency was further optimized.
One embodiment of the invention is shown in Figure 1. In this embodiment, a
function generator 2 modulates the intensity of light from a source such as a
laser 4.
The modulated light signal is coupled into an optical wave guide, in this case
an optical
fiber 6, which is formed into a loop using a fiber splice connector 8. As
shown in the
inset, the optical fiber comprises a core 10 and cladding 12. The light signal
traveling
through the fiber loop is detected using a photon detector 14. The detected
light may be
displayed on a suitable device such as an oscilloscope, while the phase angle
difference
between the light entering and exiting the fiber 6 may be determined, for
example, by a
lock-in amplifier 16 or a ratiometer. The data may be stored in and/or
analyzed by a
computer. By measuring the phase angle with respect to a suitable reference
such as
9
CA 02500876 2005-03-15
the phase of the incoming modulated light (see Figure 2), various loss
mechanisms of
the test medium can be characterized. Advantageously, such losses are largely
independent of power fluctuations of the light source. Thus, unlike
conventional single
or multipass-type devices, the invention is not sensitive to the intensity of
the input light
signal, to the input coupling efficiency of the light signal, or to drift of
the light signal
power with, for example, time, temperature, or wavelength. The method of the
invention
is not very sensitive to laser alignment, and a long loop can be provided to
allow for
spatially separate illumination and detection regions.
In a preferred embodiment, the invention comprises launching a modulated
continuous light signal into the optical loop, and recording the phase angle
over a time
constant corresponding to a number of oscillation cycles (i.e., periods or
wavelengths) of
the modulated light signal being averaged. Use of a larger time constant
increases the
number of oscillation cycles averaged, and hence provides a more accurate
measure of
the phase angle difference. Use of a lock-in amplifier conveniently provides
for
adjustment of the time constant.
The embodiment shown in Figure 1 is suitable for applications such as
characterizing loss processes in fiber optic transmission. For example, the
method can
be used to accurately determine the absolute transmission spectrum of an
optical fiber
and of the fiber connector, as well as other optical properties such as
refractive index,
evanescent wave spectrum, and optical loss. Further, the deformation (e.g.,
strain) of a
fiber can be evaluated by determining the effect of the deformation on one or
more such
optical properties of the fiber. Deformation may be caused by a mechanical
force (e.g.,
stress), and/or by physical factors (e.g., temperature, pressure) acting on
the fiber. For
example, deformation may comprise bending the fiber, with a smaller radius of
the bend
associated with greater deformation, and hence greater changes of optical
properties of
the fiber.
In one embodiment, the invention is used to measure one or more optical
properties (e.g., absorbance) of a test medium, using a short optical path
length through
the test medium (e.g, a path length less than about 100 pm, preferably about 1
to 10
pm). A short optical path length can be achieved by using a very small
capillary channel
to introduce the test medium (such as a liquid or a gas) into the loop, for
example. It will
be appreciated that this embodiment requires only very small sample volumes of
test
medium (e.g., in the order of picoliters).
________________________ RIIION=========MMINFROMNI===
___________________________________________
CA 02500876 2005-03-15
In particular, the invention is advantageously used in spectrometry of small
volumes of test media such as fluid (i.e., liquid or gas) samples. It is
desirable to
measure very small samples of a substance, particularly when the substance is
expensive, rare, or toxic. However, previously-known cavity techniques are not
conveniently applied to measurement of very small samples because of the
larger
sample quantities generally required. Further, use of small sample volumes in
accordance with the invention makes possible rapid flushing of the channel,
and hence a
high repetition rate for measurement of subsequent samples (e.g., less than 1
s for a
measurement). Previously known cavity techniques cannot provide such rapid
flushing
of samples because of the large samples required, and hence cannot provide
rapid
measurement of successive samples. Previously known ring-down spectroscopy
techniques, even when applied to measurement of small samples in capillary
channels,
cannot provide rapid measurement of successive samples because of the
considerable
time required for data acquisition and processing. Thus, none of the above-
mentioned
techniques provides rapid measurement of optical properties of very small
fluid samples
of test media.
The phase shift optical loop method of the invention provides for an extremely
sensitive and rapid absorption spectroscopic technique, and as such it is
suitable for
numerous applications, as exemplified by the embodiments described below. It
is noted
that, in contrast to most other absorption measurement techniques, the
sensitivity of the
phase angle measurement (i.e., the change of phase angle with concentration
change)
is larger for weak absorption processes than for strong absorption processes.
Therefore, the invention is well suited to weak absorbers, dilute samples,
and/or short
absorption path lengths.
In another embodiment, the phase shift due to optical losses of an evanescent
wave is used for example to detect the presence of one or more compounds, or
to
measure the absorption spectrum of one or more compounds. This embodiment
takes
advantage of the fact that significant optical energy resides in the cladding
close to the
interface with the core. In such embodiment, the fiber cladding on a section
of the loop
can either be removed, coated, replaced (e.g., with a chemically modified
polymer, such
as a silicon-based polymer), or modified (e.g., chemically), to permit
detection and
recording of the evanescent wave absorption spectrum produced by a compound(s)
exposed to the evanescent wave. In particular, solid phase micro extraction
(SPME)
may be used, in which a polymer coating provides enrichment of the analyte
(e.g., by
11
CA 02500876 2005-03-15
2000 times or more) through extraction of the analyte into the polymer, may be
used.
The partitioning coefficients that govern the efficiency of solid phase
microextraction vary
depending on the class of chemicals and the polymer matrix. For example, Table
1 lists
a number of siloxane-based polymers and corresponding types of analytes for
which
they are suitable, which may be used in accordance with the invention.
Partitioning of a
molecule of interest into the cladding changes its optical properties (for
example,
refractive index, optical absorbance), resulting in a change in the reflection
efficiency of
the cladding and its optical losses. Effects of such changes on the evanescent
wave
can be measured using the phase shift techniques of the invention.
Table 1. Examples of siloxane-based polymers suitable for solid phase micro
extraction
(SPME) polymer coatings on optical fibers.
SPME Coating Application
100pm polydimethylsiloxane For Volatiles
7pm polydimethyislioxane For Nonpolar High Molecular
Weight Compounds
85pm polyacrylate For polar semivolatiles
30pm polydimethylelloxane For Nonpolar Semivolatiles
65pm For Volatiles, Amines, and
polydimethylsiloxane/divinylbenzene Nitroaromatic Compounds
65pm Carbowaxidivinylbenzene For Alcohols and Polar
Compounds
60pm For Amines and Polar
polydimethylsiloxane/divinylbenzene Compounds (HPLC use only)
50pm Carbowsx/tamplated resin For Surfactants (HPLC use
only)
75pm Carboxen/polydimethylsiloxane For Gases and Low Molecular
Weight Compounds
65pm For Volatiles, Amines, and
polydimethylsiloxaneldivinylbenzene Nitroaromatic Compounds
=
50/30 pm divinylbenzene/Carboxen For Flavor Compounds
(Volatiles and Semivolatiles)
85Pm Carbocen/pofydfinethylsilonne For. Gases and Low Molecular
Weight Compounds
70pm Carbowax/divinylbenzene For Alcohols and Polar
Compounds
100pm polydimethyislioxane For Volatiles
50/30pm divinylbenzene/Carboxen For Odor Compounds
12
CA 02500876 2005-03-15
In another embodiment, there is provided a method of measuring polarization-
dependent loss using pulsed polarized laser light as a source and a
polarization-
maintaining fiber for the loop. Polarization-dependent loss is an important
quantity in the
telecommunications industry; however, such measurements are difficult to
undertake
with currently available technology.
According to another aspect of the invention there is provided an apparatus
for
measuring one or more optical properties of a test medium by measuring the
phase
angle difference of the modulation of light guided by the waveguide loop and
passing
through the test medium, relative to a reference phase angle. An example of
such an
apparatus is an absorbance detector.
In accordance with this aspect of the invention, the loop has a test medium
introduced therein. The test medium is a material for which optical properties
are to be
measured. For example, where optical fiber is employed for the optical loop,
the
medium used for index matching in the fiber-splice may be replaced with a test
medium
such as water, organic solvent, etc. Typically, such test medium will have a
refractive
index different from the refractive index of the fiber core. In such an
embodiment, the
space between the two fiber ends acts as a Fabry-Perot cavity. The loss
processes are
then determined by the refractive index of this cavity with respect to the
fiber as well as
by the modes present in the fiber. It is therefore necessary to accurately
determine the
mode structure of the Fabry-Perot cavity and its change as a function of the
refractive
index of the cavity medium. Maintaining a stable mode structure in a
conventional cavity
ring-down laser absorption spectroscopy experiment is challenging, since the
mirrors are
typically spaced by tens of centimeters and the laser pulse coupled into the
cavity
contains a large number of modes. In this embodiment, however, the loop
substantially
simplifies the measurement of the cavity modes if a single mode waveguide is
used.
In one embodiment, the invention provides an absorption detector wherein a
test
medium for absorption measurement is introduced into the optical path of the
optical
loop. This can be accomplished by providing the test medium in, for example, a
capillary
tube or channel or a flow channel, appropriately interfaced with the optical
loop. For
example, depending on the dimensions of the optical waveguide and the
capillary, flow
channel, capillary channel, or the like, the latter may either intersect the
optical
waveguide, or it may pass through the waveguide, via, for example, a hole
through the
waveguide. Further, at least a portion of the optical waveguide loop may be
incorporated into a chip, such as, for example, a microfluidic device. For
example,
13
CA 02500876 2005-03-15
where optical fiber is employed, the splice connector may be replaced with
such a
microfluidic device (e.g., a "lab-on-a-chip" device). Such devices are
provided with
channels having cross-sections in the order of microns, for carrying small
amounts of
analyte solution. The solutions can be separated into their solutes in the
channels. A
microfluidic device thus provides a well-defined small gap between the
waveguide ends.
The waveguide loop intersects one such channel, thereby forming part of a
sensitive,
selective absorption detector. The detection limit for such a device was
experimentally
determined to be about e[l/mol m]*c[mo1/1] d[m] = 10-6. A strongly absorbing
molecule
(e.g., e = 1061/mol m) can therefore be detected at concentrations of several
micromoles
per liter. Improvement of the detection limit may be achieved through, for
example,
using a lower base loss fiber connector and a low loss fiber, or by using a
larger time
constant for the phase angle measurement.
In a variation of this embodiment, polarization-maintaining fibers and
optically
active analytes are used, such that small quantities of absorbing media can be
detected
in a small absorption cell.
In another embodiment the fiber loop is made of single-mode optical fiber and
the
excitation laser has a bandwidth that is comparable to the spectral width of
each mode.
The fiber loop has a mode structure dependent upon the length of the loop and
the
diameter of the core. By selectively exciting a single longitudinal mode of
the loop, the
intensity of light inside the loop and of the emitted light can be increased,
thereby
reducing the time needed for averaging the oscillating signal.
In another embodiment optical properties such as the refractive index can be
measured by selectively exciting a single longitudinal mode in a fiber loop
made of
single-mode optical fiber, using a narrow bandwidth laser and by tracking the
wavelength of the longitudinal mode. The fiber loop has a mode structure
dependent on
the length of the loop and the diameter of the core. The wavelength position
of each
mode depends on the refractive index of the waveguide material; therefore,
changes in
the refractive index may be tracked by monitoring the emitted intensity
together with the
phase angle as a function of wavelength.
In another embodiment, the fiber loop is adapted for measurement of forces
(e.g., stress) and/or physical factors (e.g., temperature, pressure) that
result in
deformation (e.g., strain) of the fiber, by measuring the effect of such
deformation on one
or more optical properties of the fiber. Deformation, such as bending of the
fiber, may
alter one or more optical properties, with a smaller radius of bend associated
with
14
CA 02500876 2013-03-25
greater changes in the optical properties of the fiber. For example, a portion
of the fiber
maybe interfaced with suitable hardware so as to provide a mechanical strain
sensor, as
exemplified below.
The invention is further described by way of the following non-limiting
examples.
Working Examples
Example 1. Phase Shift Fiber Loop Spectroscopy
Introduction
Phase shift optical loop spectroscopy uses an intensity-modulated light source
to
pump the optical loop. This modulated pumping results in a time-varying light
intensity in
the optical loop, and hence the amount of light scattered from the fiber is
also modulated
in time. As will be shown below, the modulation frequencies of the incoming
and emitted
light are identical and the phase angle between the modulated pumping light
signal and
the light scattered from the fiber provides an accurate measure for the
optical loss in the
loop, without the need for extended averaging and exponential fitting.
In these phase-angle ring-down measurements, a continuous wave (cw) laser
beam is intensity modulated in time. This can be done either internally or
externally by
an electro-optical modulator. The time dependence of incoming intensity is
= /0[1+a sin(S2t)] (1)
where a is the modulation depth and Q = 2nf is the angular modulation
frequency.
When such a modulated beam is injected into the loop, the energy density in
the
loop, and hence the light intensity emitted by or scattered from the loop will
be
modulated with the same frequency, 0, but will be phase shifted with respect
to the input
signal (Engeln etal., 1996).
In Equation (2)
CA 02500876 2005-03-15
t
AO= 1 - I/0[1+ a sin(Ot)]exp( t ______________________________ t dt'
0
(2)
a
= {1+ , sin[Ot -arctan(f2r)]
V1+ 02r2
where is the ring-down time of the cavity, it is assumed that the transit time
of light in
the cavity is short compared to the ring-down time and the modulation period.
From the
above equation, the phase shift (1) can be given as
= -arctan(Qr) (3)
and the modulation depth of the emitted light is
a
a' = , (4)
V1+ f22z-2
While the ring-down time can in principle be determined from either the
modulation depth or the phase shift; only phase shift measurements have the
potential
to yield comparable sensitivity, detection limit, and time response to
conventional time-
resolved ring-down spectroscopy.
Methods
The experimental setup consisted of a laser diode (JDS Uniphase SDL-2372-P1,
810 nm 3 nm, max. 2W) current-modulated at frequencies around 200 kHz. The
laser
output was delivered by 1 m of multimode fiber which was coupled to the fiber-
loop
(Fiber-Tech, Optica, AS400/440IRPI, 26 m) with the aid of a drop of
dimethylsulfoxide
(DMSO). The coupling efficiency of such an arrangement is low (-101, but can
readily
be improved using commercial fiber-fiber couplers if necessary. A
photomultiplier tube
(PMT; Hamamatsu 950) was placed at a different location along the loop and
monitored
the light intensity in the loop by detecting photons scattered from the fiber
core and
cladding. In preliminary experiments the ends of a 400/440 pm optical fiber
were
coupled using an x-y-z translation stage to form a loop with low optical
losses. The gap
between the fibers was filled with either water or DMSO containing variable
amounts of
dye. Since the refractive index of DMSO (n = 1.4787) is close to the
refractive index of
the fiber core (n = 1.457), the solvent acted as index-matching fluid and
nearly
eliminated the back reflection at the fiber-solution interface. The lower
refractive index of
water had a surprisingly small effect on the coupling efficiency between the
fiber ends,
16
CA 02500876 2005-03-15
possibly due to a focussing effect at the fiber-water interface. The alignment
was
optimized and characterized using a microscope.
The PMT signal was fed into a fast lock-in amplifier (Stanford Research
Systems
SR 844) and referenced to the driving current of the laser diode. To reduce
radio
frequency interference, all the cables were shielded. The size of the gap
between the
fiber ends was adjusted to about 10% of the diameter of the fiber core and was
experimentally determined to be about 42 lam.
To get the highest sensitivity in the phase-angle measurement, the angular
modulation frequency was set to around = 1/T, i.e., to a phase angle of about
0= 450
(see Equation 3). Due to the inherent time delays in cables and electronic
components,
an offset phase angle was determined. This was easily be done by phase angle
measurements obtained using different concentrations of DDCI analyte as
described
below, or by determining the relative phase as a function of the modulation
frequency SI.
The standard deviation of phase angle measurement depended strongly on the
readout
rate and the intensity of the photon signal and was typically around 0.05
degrees. It was
ultimately restricted by the instrumental limit of the lock-in amplifier
(which in the present
case was 0.02 according to the manufacturer's specifications).
In the capillary electrophoresis measurements the fiber ends were joined by a
commercial 4-way microcross (Upchurch Scientific) instead of the translation
stage. In
the microcross, the two fiber ends (100/140 pill Fiber Tech Optica
AS100/140IRA) were
inserted through opposing holes (150 m) and the capillary ends (Polymicro
Technologies 100/360 gm) were inserted through the other two holes. In this
experiment
the modulation frequency was 150 kHz, the length of the fiber loop was about
65 m, and
the size of the gap between the fiber ends was calculated to be 31 mrn.
Results
Figure 2 shows a typical trace of the time dependence of the intensity
detected
by the PMT. A driving voltage modulated at 200 kHz was supplied to the laser
diode
and also used as a reference signal. The reference is shown as a solid curve
and the
phase shift of the emitted light can be easily seen. The transient signal and
reference
were connected to the lock-in amplifier, which averaged the phase angle
measurement
over a period of 100 ms (variable from 1 ms to 1 s).
17
CA 02500876 2005-03-15
When the clear DMSO solvent was replaced with a solution of DDCI in DMSO at
different concentrations, the phase angle was used to determine the ring-down
time and
hence the absorption in the sample. MCI is not optically stable and its
absorption band
will shift into the visible region after prolonged exposure to the light.
Therefore the
phase-angle averages of only the first 20 s were used in Figure 3.
As mentioned above, there exists an offset to the phase angle arising from
inevitable time-delays in the electronic signal transmission in the electronic
components,
the cables, and the laser diode, as well as light transmission in the power
delivery fiber.
This offset can readily be accounted for by introducing the offset angle, it,
into Equation
(3), resulting in
= ¨ arctan()z)
(5)
where Om is the phase angle measured by the lock-in amplifier.
Substituting the photon lifetime (Brown at al., 2002)
T =
_______________________________________________________________________________
___ (6)
co (-1n(Tsp) aL EDMSOd DDCICd)
into Equation (5), one obtains
= ar c t an
______________________________________ (7)
co (-- aL+ CDNISOd CDDCICC1)
Here eppc,= 3.343x105(M cm)-1 is the extinction coefficient of DDCI (given
with
respect to base e) at its peak absorption wavelength of 825 nm, CDDCI is its
concentration
(M), and d is the width of the cavity formed by the two fiber ends. A similar
term is
introduced to account for the absorption of the solvent.
To simplify the model one can combine the optical loss from the splice
alignment,
the absorption of the fiber and the solvent into a single expression Ao. This
term
represents all optical loss processes other than the analyte's absorption and
is constant
for a given flow system.
DJ,
0 = 0 - arc tan __________________________ (8)
c0(4+ eCd)
As can be seen from Equation (8), the phase angle dependence on
concentration is not linear. An advantage of this non-linearity is that phase-
angle
measurements are more sensitive at low concentrations. The offset angle 0can
be
18
CA 02500876 2005-03-15
determined from a linear fit using Equation (8) and the fact that -ctan(0,-
00) is
proportional to the concentration. Eventually, this procedure will be done
when
calibrating the detector. For small concentrations, the determination of the
offset angle
is not necessary since Om changes approximately linearly with concentration.
In Equation (8), only 00, Ao, and d were unknown and were determined by
fitting
(Figure 3) to give 00= -36.3 , Ao= 0.60, and d = 42 ptrn. For a 26.4 m fiber
loop, the ring-
down time of I' = 224 ns without DDCI is therefore much shorter than expected
for the
light decay in the fiber core (r- lia), which implicates that the detected
photons were
scattered not only from the fiber core, but also from the fiber cladding.
Furthermore,
high-order core and cladding modes, which were biased against in previous
pulsed fiber
loop ring-down spectroscopy (FLRDS) experiments by gating the detector (Brown
etal.,
2002), gave rise to strong and fast intensity decays within the first 100 ns.
Because the lock-in amplifier can only provide one averaged phase angle,
measurements at a number of modulation frequencies need to be made to
characterize
the components associated with the different optical decay processes. However,
since
one needs to Fourier transform these frequency domain measurements in real
time, the
time-resolution of the measurement is affected in an on-line detector. A
simpler way to
solve the problem of multiple optical decays with different time constants is
to increase
the length of the fiber loop and thereby enhance the relative losses of the
high-order
modes and cladding modes over the long-lived low-order core modes.
Figure 4 is a demonstration of the greatly increased time-response of the
phase
shift FLRDS (PS-FLRDS) system over pulsed FLRDS. Here, drops of DDCI with
different concentration were added alternately with pure DMSO solvent between
the two
fiber ends. The rapid change in phase angle between -52 deg to -40 deg and -35
deg
was used together with the calibration of Figure 4 to obtain quantitative and
time-
resolved concentration transients. From the figure the time response was
obtained from
the onset of the leading edge of each peak, and was better than 200 ms. We
consider
this a lower limit since the time needed to displace the existing solution is
on a similar
timescale and may determine the time resolution one can obtain in this
experiment.
For the capillary electrophoresis experiment a microcross was used to couple
the
fiber ends, and the concentration dependence of phase angle is shown in Figure
3. A fit
of the experimental data using Equation (8) yielded Ao= 0.66, q= -23.3 , and
d= 31
gm. The background optical transmission, Ao, was slightly higher compared to
the value
19
CA 02500876 2005-03-15
of Ao= 0.66 for the 400/440 pm fiber experiment, indicating that one can
achieve fair
fiber-fiber coupling efficiencies with a simple and inexpensive commercial
microcross.
To test the capabilities of FLRDS in a more realistic analytical environment,
a fast
transient peak was obtained in two ways. First a syringe was used to inject
the solution
into the capillary, and Figure 5 shows the measured change of phase angle when
two
different concentrations and different sizes of plugs of solution were used.
Secondly, a
capillary electrophoresis separation of two dyes, ADS805WS and ADS830WS
(American Dye Source, Inc., chemical formulae C381-144CIN206S2Na and
C46H51CIN208S2,
respectively) was undertaken, and resulted in an electrochromatogram that
shows the
retention times of not only the original dyes, but also of the degradation
products (Figure
6).
Discussion
To determine the detection limit, the derivative of Equation (8) was
calculated as
follows:
dq) SILedco
(9)
dC c 02 A: + 02 L2
If the detection limit (DL) is defined as 3ac, then
e2 A2 cl2L2
DL= "3cr
(10)
SILedc,
For an uncertainty in the phase angle measurement of as= 0.05 , a detection
limit of 6 jAM was calculated. This value is comparable to the lowest
concentrations of
j.t.M and 15 M shown in Figure 4. Given the sample volumes of 5 nL and 240 pL
respectively, these concentrations correspond to 150x10-15 mol and 3x10-15
mol,
respectively. PS-FLRDS thus enables absorption detection of about 2 billion
molecules
within about 100 ms.
25 A number of improvements can be made to lower the detection limit.
The
accuracy of phase angle measurements affects the detection limit. ao is mainly
decided
by the amount of signal fed into the lock-in amplifier as determined by the
coupling
efficiency. Improving the signal fraction from the core over the cladding
modes
increases Ao, whereas tuning to the peak absorption wavelength (E) of the
analyte and
CA 02500876 2005-03-15
=
optimizing the gap size, d, and fiber length, L, further helps to achieve a
lower detection
limit.
The data collection rate was 10 Hz and the time constant of the lock-in
amplifier
was therefore set to 100 ms. Should it be required one can work at
considerably higher
readout rates - limited ultimately by the modulation frequency to 10 ps - but
for
microfluidic devices, a data acquisition rate of 10¨ 100 Hz is sufficient to
distinguish the
transient peaks.
We note that many of the disadvantages of phase shift CRDS measurements do
not play a large role in PS-FLRDS. While the effective ring-down time (RDT) is
an
average of the RDTs in the fiber core and cladding, and therefore considerably
smaller
than that expected from RDT in the fiber core only, its change with
concentration is
predictable and can be understood using Equation (6). Furthermore, since a
large
number of modes is excited in a multimode fiber, one does not need to be
concerned
with mode-beating or mode-build-up effects. Finally, FLRDS is not so much a
tool for
the measurement of very weak (strongly forbidden) transitions or for extremely
dilute
samples as for the determination of sub-millimolar concentrations in small
liquid
samples. In this application one would use a calibration curve together with
an
independent measurement of the absorption cross section in a large sample to
determine the concentration. Therefore, a possible inaccuracy in the effective
ring-down
time would be corrected for using the phase shift technique of the invention.
Conclusions
In a conventional pulsed laser (10 - 100 Hz) FLRDS measurement, one ring-
down time measurement takes about 30 ¨ 100 s while the effective data
acquisition
takes only a small fraction of that time. PS-FLRDS improves the duty cycle and
data
acquisition rate, and thus enables real-time measurements in analytical
environments.
As demonstrated above, PS-FLRDS is suitable as an online absorption detector
for
capillary electrophoresis with time resolution of 10¨ 100 ms and detection
limit in the
nnicromolar concentration range.
PS-FLRDS is compact and inexpensive. In principle, one could integrate all
electronic components, i.e., the function generator, diode laser driver,
photodiode circuit,
and phase-detector into a single board with a computer interface, thereby
reducing radio
frequency interference an improving performance. PS-FLRDS is also robust and
sensitive. The quasi-continuous measurement of the phase angle eliminates
software
21
CA 02500876 2005-03-15
averaging and exponential fitting, but does not permit a separate
characterization of the
competing optical loss processes. While this did not pose an immediate problem
in this
example, fast optical decays in cladding modes and lossy core modes will have
to be
dealt with as the detection limit of FLRDS is reduced. Commercial fiber-fiber
couplers
can be used to deliver the light signal into the optical loop as well as for
detection of its
intensity in the loop. They will likely reduce the fraction of light
travelling through
cladding modes, and with higher coupling efficiencies also increase the
accuracy with
which the phase angle can be measured.
In certain applications absorption detection at wavelengths shorter than 300
nm
may be desired; however, transmission of commercial fibers is greatly reduced
at
wavelengths shorter than 600 nm. Example 2 illustrates an experimental
arrangement
that conceptually permits FLRDS detection of molecules in the spectral region
used for
telecommunications. At about 1.6 1.1.m optical waveguides have good
transmission, and
many biological molecules show absorption bands due to vibrational overtone
transitions
in this frequency range, which may be interrogated using FLRDS.
Example 2. Near Infrared Phase Shift Optical-Loop Measurement of Optical
Losses
Introduction
In the fiber loop measurement scheme described in Example 1, there is a limit
in
the spectral range to which the technique may be applied. This limit is given
by the use
of the optical waveguide combined with the photomultiplier detector. The
absorption
spectrum of a typical fiber optic waveguide has a maximum transmission
centered
around two spectral regions near 1.35 pill and 1.5 rn, which are separated by
a strong
absorption peak due to excitation of OH overtone vibrations in the waveguide
material.
A third transmission window exists near 800 nm. The transmission then
decreases
dramatically as the wavelength of the guided wave is decreased from the near
infrared
(NIR) through the visible region into the ultraviolet (UV). In example 1 the
photodetector
was a photomultiplier tube with a detection efficiency whose wavelength
dependence is
the inverse of the fiber transmission curve. The detector was most sensitive
in the UV
and visible region, with decreasing sensitivity in the near IR, until at about
850 nm the
detection efficiency decreased to near zero. The combination of these two
wavelength
response curves limits the experiment to a wavelength window from about 780 nm
to
22
CA 02500876 2005-03-15
about 830 nm, and consequently many experiments to date have been conducted on
sample dyes that show strong absorption features in this spectral region.
In this example an alternative excitation source and photodetector were used
to
exploit the much higher optical transmission of the fiber optic cable in the
1.55 p.m
wavelength region. As shown in Figure 7, using a tunable (1.5 ¨ 1.62 p.m) NIR
laser
source 20 (ANDO), single mode fibers (Fiberguide), directional couplers 22, 24
(SENKO
and Fiber metric), and either an In-Ga-As photodiode 26 (Thorlabs) or an
optical
spectrum analyser (Agilent 86142B), a 4 m fiber loop setup was realized, which
enabled
the determination of optical losses of radiation travelling through the fiber
core
independently from the losses by the cladding. The phase angle 4),õ was
measured as a
function of the angular modulation frequency C2 of a broadband excitation
source (Av =
200 MHz), and the optical decay constant t7 840 ns and the offset phase angle
4)0= 440
were determined from the slope (Figure 8). When using a narrow band light
source (Av
= 200 kHz) the resultant numbers were very similar, indicating that the
longitudinal mode
structure in the fiber loop was either fluctuating very quickly and/or that
the mode
spacing was small. Note that in both experiments a 90:10 X-coupler was used to
introduce light into the loop and a 99:1 tap was used to direct light out of
the loop.
Despite the fact that these optical devices have inherent losses and that the
two fusion
splices by which they were connected into the loop also had optical losses,
the loss per
pass was calculated to be below 3%.
Similar experiments using a 99:1 X-coupler and 99:1 tap in a much shorter loop
of only L = 1 m gave lower losses of only 0.7% (0.032 dB) per pass, indicating
that there
is a considerable amount of intensity build-up in the loop and also a higher
than
expected optical finesse.
An experimental arrangement such as this therefore allows for very accurate
determination of optical losses in single mode optical devices such as, for
example,
Bragg gratings, couplers, and splices. In addition, the inventors contemplate
introducing
into the loop optical devices whose optical loss characteristics are modified
as a function
of their environment, so as to measure and monitor environmental variables,
for
example. Environmental variables that could be interrogated by such a loop
include, but
are not limited to, temperature (e.g., as it affects the transmission/
reflection
characteristics of a Bragg grating), strain on the fiber, absorption spectrum
of the
evanescent wave, and refractive index of a surrounding medium. We note that
wavelengths around 1.5 Am are particularly suited for unspecific detection of
many
23
CA 02500876 2005-03-15
organic molecules through the vibrational overtone absorption bands of the CH,
NH and
OH stretching vibrations.
Example 3. Use of PS-FLRDS as a Fiberoptic Sensor for Mechanical Strain
An experimental setup similar to that described in Example 2 was used to
investigate the effect of deformation on the optical waveguide loop. A 10 m
loop of 125
pm single mode fiber (Fiberguide) was formed by splicing the fiber ends using
a fusion
splicer. Using a tunable (1.5 - 1.62 pm) NIR laser source (ANDO) which was
coupled
into the loop using a 99.5:0.5 directional coupler (Lightel), and an inline
power monitor
(EigenLight M160) connected to a lock-in amplifier, optical losses of
radiation traveling
through the fiber core were determined independently from the losses caused by
the
cladding. The phase angle (I)m was measured at a fixed angular modulation
frequency SI
of a broadband excitation source ON = 200 MHz). Note that in this experiment a
99.5:0.5 X-coupler was used to introduce light into the loop which was
detected by the
inline power monitor contained within the fiber loop. Additional loss was
introduced by
bending a section of the waveguide loop using a custom made strain sensor.
As shown in Figure 9A, the strain sensor consisted of a sandwich of two
parallel
arrays 100, 200 of cylinders 110, 210, each cylinder being about 12.5 mm in
diameter,
arranged such that the longitudinal axes of the cylinders of the two arrays
were parallel
but offset, and the optical fiber. The fiber 120 was placed between the arrays
perpendicular to the longitudinal axes of the cylinders. Compressing the
waveguide
between the two arrays caused the fiber to bend in a periodic nature as set by
the size
and spacing of the cylinders, and with a bending radius that depended on the
force
applied to the cylinder arrays. Impulse (time) response of the strain sensor
was
determined by dropping a 200 g weight 150 from a height of 3 cm onto the
sensor.
The ring-down time (phase angle) and impulse (time) response of the strain
sensor are shown in Figures 9B and 9C, respectively. In particular, Figure 9B
shows
that ring-down time decreased with increasing load, indicating that the
optical loss
increased with increasing deformation of the fiber. The minimum time response
of the
measurement was 10 ms, set by the 100 Hz data acquisition rate used. From
Figure 9C
it can be seen that the response time of the device is not longer than this
lower
measurement limit.
24
CA 02500876 2005-03-15
Those skilled in the art will recognize, or be able to ascertain using routine
experimentation, variations of the embodiments and examples described herein.
Such
variations are intended to be within the scope of the invention and are
covered by the
appended claims.
CA 02500876 2005-03-15
_
References
Berden, G.; etal., 2000, Cavity ring-down spectroscopy: Experimental schemes
and
applications, International Reviews in Physical Chemisby 19:565.
Brown, R.S.; et al., 2002, Fiber-loop ring-down spectroscopy, J. Chem. Phys.
117:10444.
DeMille, S.; etal., 2002, Comparison of CRDS to ICL-PAS and phase-shift CRDS
spectroscopies for the absolute intensities of C¨H (AvcH=6) overtone
absorptions, Chem. Phys. Left. 366:383.
Engeln, R.; et al., 1996, Phase shift cavity ring down absorption
spectroscopy, Chem.
Phys. Lett. 262:105.
Hallock, A.J.; etal., 2002, Direct monitoring of absorption in solution by
cavity ring-down
spectroscopy, Anal. Chem. 74:1741.
Jakubinek, M.; et al., 2004, Configuration of ring-down spectrometers for
maximum
sensitivity, Can. J. Chem. 82:873.
Lewis, E.; etal., 2001, Phase shift cavity ring-down measurement of C¨H (Av=6)
vibrational overtone absorption,s Chem. Phys. Lett. 334:357.
Polynkin, P.; et al., 2004, Efficient and scalable side pumping scheme for
short high-
power optical fiber lasers and amplifiers, IEEE Photonics Technology Letters
16:2024.
Romanini, D.; etal., 1993, Ring-down cavity absorption spectroscopy of the
very weak
HCN overtone bands with six, seven, and eight stretching quanta, J. Chem.
Phys. 99:6287.
Scherer, J.J.; etal., 1997, Cavity ring-down laser absorption spectroscopy:
History,
development, and application to pulsed molecular beams, Chemical Reviews
97:25.
Stewart, G.; etal., 2001, An investigation of an optical fibre amplifier loop
for intra-cavity
and ring-down cavity loss measurements, Meas. Sci. Technol. 12:843.
von Lerber, T.; et al.; 2002, Time constant extraction from noisy cavity ring-
down signals,
Chem. Phys. Lett. 353:131.
Xu, S.; etal., 2002, Cavity ring-down spectroscopy in the liquid phase, Rev.
Sci. Instr.
73:255.
26
