U.S. patent application number 12/562443 was filed with the patent office on 2010-01-14 for hollow-core waveguide-based raman systems and methods.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Marko Klaus Baller, Rui Chen, Peter Joseph Codella, Renato Guida, Xiaoyong Liu, Radislav Alexandrovich Potyrailo, Alexey Vasily Vert, Zhiyong Wang, Anis Zribi.
Application Number | 20100007876 12/562443 |
Document ID | / |
Family ID | 40666698 |
Filed Date | 2010-01-14 |
United States Patent
Application |
20100007876 |
Kind Code |
A1 |
Chen; Rui ; et al. |
January 14, 2010 |
HOLLOW-CORE WAVEGUIDE-BASED RAMAN SYSTEMS AND METHODS
Abstract
Embodiments of the invention include a method for sensing. Other
embodiments include a method for sensing molecules, for example,
homonuclear diatomic molecules. The method may include transmitting
light from a light source through a hollow-core wave-guiding device
that exhibits a low attenuation at predetermined operating optical
frequencies; introducing a gaseous medium between the light source
and the hollow-core wave-guiding device; and detecting molecules
within the gas.
Inventors: |
Chen; Rui; (Clifton Park,
NY) ; Baller; Marko Klaus; (Saarbrucken, DE) ;
Codella; Peter Joseph; (Niskayuna, NY) ; Zribi;
Anis; (Rexford, NY) ; Guida; Renato;
(Wynanskill, NY) ; Vert; Alexey Vasily;
(Schenectady, NY) ; Potyrailo; Radislav
Alexandrovich; (Niskayuna, NY) ; Liu; Xiaoyong;
(Malden, MA) ; Wang; Zhiyong; (Clifton Park,
NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
ONE RESEARCH CIRCLE, PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
40666698 |
Appl. No.: |
12/562443 |
Filed: |
September 18, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12102079 |
Apr 14, 2008 |
7595882 |
|
|
12562443 |
|
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Current U.S.
Class: |
356/301 ;
356/437 |
Current CPC
Class: |
G01N 21/05 20130101;
G01N 21/65 20130101; G01N 2021/655 20130101; G01N 21/0303
20130101 |
Class at
Publication: |
356/301 ;
356/437 |
International
Class: |
G01J 3/44 20060101
G01J003/44; G01N 21/00 20060101 G01N021/00 |
Claims
1. A method for sensing homonuclear diatomic molecules, comprising:
transmitting light from a light source through a hollow-core
wave-guiding device that exhibits a low attenuation at
predetermined operating optical frequencies; introducing a gaseous
medium between the light source and the hollow-core wave-guiding
device; and detecting homonuclear diatomic molecules within the
gas.
2. The method of claim 1, wherein said transmitting light comprises
transmitting light through a photonic crystal fiber.
3. The method of claim 1, wherein the homonuclear diatomic
molecules comprise nitrogen.
4. A method of detecting comprising: transmitting light from a
light source through a hollow-core wave-guiding device; introducing
a material between the light source and the hollow-core
wave-guiding device; and detecting a homonuclear diatomic molecule
within the material.
5. The method of claim 4, wherein the material comprises one of a
flowing liquid, a gaseous medium, and a fluidic medium.
6. The method of claim 4, wherein the material comprises natural
gas.
7. The method of claim 4, wherein the homonuclear diatomic molecule
comprises nitrogen.
8. The method of claim 4, wherein the hollow-core wave-guiding
device comprises a photonic crystal fiber.
9. The method of claim 4, wherein the hollow-core wave-guiding
device exhibits a low attenuation at predetermined operating
optical frequencies.
10. The method of claim 4, wherein the material comprises air.
11. A method for optical measurement of at least one analyte in a
sample, comprising: exciting light into a photonic crystal fiber at
excitation conditions that control non-analyte related emission of
secondary radiation.
12. The method of claim 11, wherein said emission of secondary
radiation is selected from the group consisting of fluorescence
emission from material of the photonic crystal fiber,
photoluminescence emission from material of the photonic crystal
fiber, and Raman emission from material of the photonic crystal
fiber.
13. The method of claim 11, wherein said emission of secondary
radiation is utilized to improve accuracy of measurements.
14. The method of claim 13, wherein said improvement of accuracy of
measurements is provided by corrections for aging of a light
source, repositioning effects, or partial contamination of the
optical elements.
15. The method of claim 13, wherein said measurements include
quantitative analyte determinations
16. The method of claim 15, where said quantitative analyte
determinations include univariate analysis or multivariate
analysis.
17. The method of claim 11, wherein said emission of secondary
radiation is reduced by natural attenuation of the photonic crystal
fiber in spectral regions where said emission of secondary
radiation occurs.
18. The method of claim 17, wherein detection of optical radiation
upon interaction with the sample is accomplished at an excitation
end of the photonic crystal fiber.
19. The method of claim 17, wherein detection of optical radiation
upon interaction with the sample is accomplished at a distal end of
the photonic crystal fiber.
20. The method of claim 19, where an operational lifetime of the
photonic crystal fiber is lengthened by a surface treatment to
reduce deposition effects of contaminants.
21. A method for sensing, comprising: transmitting light from a
light source through a hollow-core wave-guiding device that
exhibits a low attenuation at predetermined operating optical
frequencies; introducing a gaseous medium between the light source
and the hollow-core wave-guiding device; and detecting molecules
within the gas.
22. The method of claim 21, wherein said transmitting light
comprises transmitting light through a photonic crystal fiber.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present invention is a continuation of and claims
priority to U.S. patent application Ser. No. 12/102,079, filed Apr.
14, 2008.
BACKGROUND
[0002] The invention generally relates to a gas sensing system and
method, and more particularly to a hollow-core waveguide-based
Raman system and method.
[0003] Homonuclear diatomic molecules are generally difficult to
detect and measure. Such molecules as nitrogen and hydrogen, for
example, do not absorb light under standard pressure and
temperature conditions. They are, therefore, difficult if not
impossible to detect and quantify with optical absorption based
techniques. Further, oxygen has a weak forbidden absorption band
that is difficult to use for reliable quantitative measurements.
Most common analytical methods are based on low temperature gas
chromatography.
[0004] There are few reliable techniques known for high accuracy
and precision detection and quantification of such molecules. Due
to the symmetry of such molecules, they are Raman active, making it
possible to identify these molecules with high selectivity based on
their Raman spectral peaks.
[0005] Raman sensing is widely applied for detection of various
chemical compounds and biomaterials. Raman spectroscopy measures
the frequency change and intensity of inelastically scattered light
from interaction between molecules and monochromatic light. The
spectral shift of Raman scattering can be associated with the
interaction of an incoming photon and the molecule. The photon
loses or gains energy interacting with specific vibrational,
rotational, or electronic energy states of the molecule. It is
therefore possible to identify molecules from their Raman peak
positions, which indicate various molecular energy levels. Raman
scattering is a function of the incident light power (I.sub.0), the
inverse wavelength (.lamda.) of the incident light to the 4.sup.th
power, the concentration of material in the beam (c), and the
scattering cross section of the molecule (J). In addition, any
experimental setup and/or sample has its own restrictions on the
ability of the instrument to collect and analyze light. This factor
is usually called the instrument factor (K). Therefore, a
simplified equation for the observed Raman Scatter can be expressed
as follows:
R=I.sub.0 c JK/.lamda..sup.4 Eqn. (1)
Clearly, if the experimental conditions are controlled, Raman
spectroscopy should be capable of quantitative analysis, i.e., the
intensity of a Raman signal is proportional to the partial pressure
or concentration of the molecule. The Raman cross section is
multiplied by a concentration factor. For many Raman samples, the
factor is essentially one for a solid or liquid. For a gas sample,
the factor is approximately 4.5.times.10.sup.-5. Therefore, Raman
sensing of low-density media such as gases is very challenging.
Strong laser power around watt level and high gas pressure from 50
to 100 atmospheres are usually employed. Also, quantification of
the concentrations of gas components in a mixture requires strong
signals, especially at low concentrations. Therefore, enhancing
techniques are required to produce and collect Raman scattered
photons from a mixture of gases.
[0006] Raman signals are intrinsically weak, roughly ten to sixteen
orders of magnitude smaller than fluorescence. To achieve lower
detection limits, surface enhanced Raman (SERS) and/or resonance
Raman have been used to improve the Raman signal of certain
chemicals. SERS requires absorbing the target molecules onto a
roughened metal surface. Resonance Raman requires strong coupling
between vibrational and electronic levels; therefore, they are not
universal.
[0007] One known method for increasing the intensity of a Raman
signal is Coherent anti-Stokes Raman Spectroscopy (CARS), which is
a nonlinear optical method using two or more intense beams of light
to generate anti-Stokes blue-shifted Raman signals. CARS
experiments are not routine and are strongly dependent on the
reproducibility of the performance of expensive lasers. See, for
example, Begley, R. F., et al., "Coherent anti-Stokes raman
spectroscopy, Applied Physics Letters, 25, 387 (1974). Much
emphasis has therefore been placed on improving the interactions
between light and gas analytes, which usually involves a multi-pass
arrangement where the illumination laser beam is focused on the
sample volume from a variety of directions. Gains of 10 to 100's
are about all that is possible from this approach. This approach,
however, has its limitation in that optical mirrors are susceptible
to contamination. Loss of power is inversely proportional to
reflectivity to the power of the number of reflections. Even for
moderately efficient cells, the number of reflections can be
between 25 and 100 therefore even very mild contamination can have
devastating effects on cell efficiency.
[0008] Photonic bandgap fibers are known and commercial products in
certain ranges are available. See www.crystal-fibre.com. These
fibers employ a central hollow core surrounded by a honeycomb
structure. Contrary to traditional fiber optics that relies on
refractive index difference to guide light, photonic band gap
fibers guide light based on the band gap created by periodic
structure of air holes. More than 95% of the light is guided
through the central core. See, G. Humbert, J. C. Knight, G.
Bouwmans, P. S. J. Russell, D. P. Williams, P. J. Roberts, and B.
J. Mangan, "Hollow core photonic crystal fibers for beam delivery,"
Opt. Express 12, 1477-1484 (2004); Russell, P. "Photonic crystal
fibers", Science, 299, 358-262. 2003. The dimensions of both the
core and honeycomb can be customized to yield fiber specifically
tuned to a particular wavelength. Guiding light in the hollow core
holds many applications that were not possible before. It has been
used for IR absorption measurement of weak absorbing gases. See, T.
Ritari, J. Tuominen, H. Ludvigsen, J. Petersen, T. Sorensen, T.
Hansen, and H. Simonsen "Gas sensing using air-guiding photonic
bandgap fibers", Optics Express, Vol. 12, Issue 17, pp. 4080-4087.
Particularly for Raman spectroscopy, the hollow core provides long
interaction lengths between gas and laser while keeping the laser
beam tightly confined in a single mode. The photon intensity inside
the hollow core is very large due to the micron-size space. This
has the potential of greatly enhancing the gas phase spectrum of
nitrogen or any other contained gas. For example, see U.S. patent
publication 2006/0193583. Further, the 226986-3 use of photonic
bandgaps in a Raman device is also known. See, for example, U.S.
Pat. No. 7,283,712 (hereinafter, "the Shaw patent"). The Shaw
patent discloses a gas filled hollow core chalcogenide photonic
bandgap fiber Raman device. The specific Raman device of the Shaw
patent is designed for infrared light. Further, the specific Raman
device of the Shaw patent includes a doped portion.
[0009] One homonuclear diatomic molecule, nitrogen, is a critical
component found in natural gas. The development of an approach that
would enable direct measurement of nitrogen would be critical in
developing an inferential energy meter for the natural gas
industry. It would therefore be advantageous to develop a new
approach to detecting and quantifying homonuclear diatomic
molecules.
SUMMARY
[0010] One embodiment of the invention described herein is directed
to a system for sensing nitrogen. The system includes a light
source and a hollow-core wave-guiding device that exhibits a low
attenuation at predetermined operating optical frequencies. The
hollow-core wave-guiding device is in optical communication with
the light source. The system also includes a gas introduction
system for introducing a gaseous medium between the light source
and the hollow-core wave-guiding device and a detector in optical
communication with the hollow-core wave-guiding device.
[0011] Another embodiment of the invention is directed to a system
for sensing homonuclear diatomic molecules. The system includes a
light source, an undoped hollow-core wave-guiding device in optical
communication with the light source, a lens positioned between the
light source and the undoped hollow-core wave-guiding device, and a
gas introduction system for introducing a gaseous medium between
the light source and the hollow-core wave-guiding device. The
system also includes a detector in optical communication with the
hollow-core wave-guiding device and a filter positioned between the
undoped hollow-core wave-guiding device and the detector.
[0012] Another embodiment of the invention is directed to a system
for sensing homonuclear diatomic molecules. The system includes a
light source and a hollow-core wave-guiding device in optical
communication with the light source. The system also includes a gas
introduction system for introducing a gaseous medium between the
light source and the hollow-core wave-guiding device and a detector
in optical communication with said hollow-core wave-guiding device.
The hollow-core wave-guiding device is configured to transmit in
the visible range of the light spectrum.
[0013] Another embodiment of the invention is a method for sensing
homonuclear diatomic molecules. The method includes transmitting
light from a light source through a hollow-core wave-guiding device
that exhibits a low attenuation at predetermined operating optical
frequencies, introducing a gaseous medium between the light source
and the hollow-core wave-guiding device, and detecting homonuclear
diatomic molecules within the gas.
[0014] Another embodiment of the invention is a method for optical
measurement of at least one analyte in a sample. The method
includes exciting light into a photonic crystal fiber at excitation
conditions that control non-analyte related emission of secondary
radiation.
[0015] These and other advantages and features will be more readily
understood from the following detailed description of preferred
embodiments of the invention that is provided in connection with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic view of a sensing system constructed
in accordance with an embodiment of the invention.
[0017] FIG. 2 is a schematic view of another sensing system
constructed in accordance with an embodiment of the invention.
[0018] FIG. 3 is a schematic view of another sensing system
constructed in accordance with an embodiment of the invention.
[0019] FIGS. 4A and 4B are views illustrating a photonic crystal
fiber used in the sensing systems of FIGS. 1-3.
[0020] FIG. 5 illustrates a Raman spectrum of air at one
atmosphere.
[0021] FIG. 6 illustrates Raman spectra of nitrogen gas, nitrogen
gas in air, and helium.
[0022] FIG. 7 illustrates a calibration curve constructed from
multiple measurements of Raman spectra of nitrogen when different
concentrations of nitrogen were introduced into a photonic crystal
fiber.
[0023] FIG. 8 illustrates another calibration curve for nitrogen
gas concentrations were introduced into another photonic crystal
fiber.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0024] Referring to FIG. 1, there is shown a sensing system 10 with
a laser 11, a photomultiplier within a housing 38, and a photonic
crystal fiber assembly within a housing 50. The laser 11 is in
optical communication with the photonic crystal fiber assembly
through a fiber apparatus 12, which optically couples the laser 11
with a lens 16 within an integrated lens housing 14. A photonic
crystal fiber 22 is placed such that one end 22a is within the
integrated lens housing 14 on a side of the lens 16 opposite from
the fiber apparatus 12. Positioned between the lens 16 and the end
22a of the photonic crystal fiber 22 is an inlet 18 to allow a
flowing liquid or gaseous medium 20, such as natural gas, to flow
into the integrated lens housing 14.
[0025] With specific reference to FIGS. 4(A) and (B), the photonic
crystal fiber 22 includes a hollow, or air, core 24 surrounded by a
cladding 26 and a coating 28. The cladding 26 includes numerous
openings 27. The air core 24 provides long interaction lengths
between the natural gas 20 and the laser 11, while also keeping the
beam of the laser 11 tightly confined in a single mode. The photon
intensity inside the air core 24 is large due to the micro-size
space.
[0026] The opposite end 22.sub.b of the photonic crystal fiber 22
of FIG. 1 is in optical communication with a lens 32 within a
second integrated lens housing 30. Positioned between the lens 32
and the end 22.sub.b of the photonic crystal fiber 22 is an outlet
34 to allow the natural gas 20 to flow out of the integrated lens
housing 30. The photomultiplier within the housing 38 is optically
coupled to the photonic crystal fiber 22 through a second fiber 36.
The housing 38 includes a lens 42 positioned between a filter 40
and a detector 44, such as a photomultiplier.
[0027] The photonic crystal fiber 22 is designed to exhibit a low
attenuation at the operating optical frequencies. This may
exponentially enhance Stokes scattering while also reducing the
required power threshold for the excitation photon source for
stimulated Raman scattering to occur. Further, the photonic crystal
fiber 22 provides a long optical path for interaction between the
beam from the laser 11 and chemical molecules, enabling the
confinement of the isotropic Raman photons to a two-dimensional
structure for more efficient signal collection. While the sensing
system 10 has been described as including a photonic crystal fiber
22, any hollow-core, wave-guiding device that exhibits a low
attenuation at the operating optical frequencies would be suitable.
Such alternative hollow-core, wave-guiding devices may include
hollow-core capillaries with dielectric coatings for lessening
optical losses within the spectral range of interest. See,
Potyrailo, R. A., Hobbs, S. E., Hieftje, G. M., "Optical waveguide
sensors in analytical chemistry: today's instrumentation,
applications and trends for future development", Fresenius J. Anal.
Chem., 362, 349-373, 1998.
[0028] FIG. 2 illustrates an alternative sensing system
arrangement. Instead of using fiber optics for coupling laser light
to a photonic crystal fiber as shown in FIG. 1, the sensing system
110 of FIG. 2 utilizes direct coupling of the laser light to the
photonic crystal fiber 22. Specifically, the sensing system 110
includes an input housing 121 enclosing a laser 111 in direct
optical communication with a lens 16 in an integrated lens housing
114. The integrated lens housing 114 differs from the integrated
lens housing 14 in the positioning of the lens 16. The photonic
crystal fiber 22 is housed within an enclosure 150, while the
detecting portion of the sensing system 110 is housed within a
detector housing 138. The detector housing 138 includes an
integrated lens housing 130 having a lens 32 in direct optical
communication with a filter 40 and a detector 44. The integrated
lens housing 130 differs from the integrated lens housing 30 in the
positioning of the lens 32. In the sensing systems 10 and 110 of
FIGS. 1 and 2, gas samples enter the photonic crystal fiber 22, and
Raman photons guided in the direction of propagation of the laser
beam (forward Raman scattering) are collected at the exit end
22.sub.b of the photonic crystal fiber 22 and used for quantitative
measurements of the target chemical concentration.
[0029] FIG. 3 illustrates an alternative sensing system 210, which
unlike the previously described sensing systems 10 and 110 utilizes
reflection and not transmission in its detection of homonuclear
diatomic molecules. The sensing system 210 includes a pump laser
211 in optical communication with collimating optics 232 through a
fiber optic cable 212. The laser light exits the collimating optics
232 and enters a splitter 233. The splitter 233 is designed to
transmit the excitation frequency to a collimating optics housing
252 and reflect returning light at different energies to a
photodetector 264. The collimating optics housing 252 includes a
lens 254, focusing optics 256 and an end of the photonic crystal
fiber 22. The housing 252 further includes an inlet 18 and an
outlet 34 for a fluidic medium, such as natural gas. The opposite
end of the photonic crystal fiber 22 (the end not within the
housing 252) is in optical communication with a mirror 258, which
is imbued with high reflectivity at the target chemical Stokes peak
spectral position. Gas samples enter the photonic crystal fiber 22,
and Raman photons guided in the opposite direction of propagation
of the laser (backward Raman scattering) are collected at the exit
end 22 (the end not within the housing 252) of the photonic crystal
fiber 22. The scattered light traverses the focusing optics 256 and
collimating optics 254. The splitter 233 redirects the scattered
light through a filter 260 and focusing optics 262 to the
photodetector 264. The selected wavelengths can then be used for
quantitative measurements of the target chemical concentration. The
mirror 258 can be used to enhance the backward scattered photons by
placing it at the exit end 22 (the end not within the housing 252)
of the fiber 22. If the mirror 258 is designed to be transmissive
at the laser wavelength, it could also act as a Rayleigh filter,
assuming that little laser light is reflected back as it is coupled
to the photonic crystal fiber 22.
[0030] FIG. 5 illustrates the Raman spectra of air at one
atmosphere. As you can see, there is a spike in the spectra at 2331
cm.sup.-1. The spike in the spectra corresponds to nitrogen found
in the air. Compared to measurements done without a photonic
bandgap fiber, a typical enhancement ratio of a few thousand has
been demonstrated. FIG. 6 illustrates the Raman spectra of nitrogen
gas, air, and helium. As is evident from the illustration, nitrogen
peaks are found in the Raman spectra of the nitrogen gas and air,
but not helium, at 2331 cm.sup.-1.
[0031] Photonic crystal fibers can be designed to operate in the
visible as well as the infrared parts of the spectrum. The
transmission band and its width depend on the materials used to
fabricate the fiber and the periodic hollow structure used to
confine light in the air core. Fibers designed to transmit in the
near infrared are typically useful for telecommunication as well as
some gas sensing applications such as Near Infrared Absorption
Spectroscopy (NIRS). NIR photonic crystal fibers can also be used
for Raman detection of gases and fluids in general, but the Raman
signals are much weaker and can be easily overwhelmed. In fact,
Raman intensity is proportional to the inverse of the fourth power
of the excitation wavelength. Thus, operating at longer wavelengths
in the NIR has detrimental effects on the Raman signal and the
analytical instrument sensitivity and detection accuracy if the
objective is to design a gas analyzer. Embodiments of the invention
include a photonic crystal fiber selected to transmit in the
visible range, where Raman scattering is much stronger than in the
NIR, and the bandwidth is chosen such that the laser excitation
wavelength and the Stokes lines of the target analyte molecules
fall within the transmission band of the fiber. This enables
utilization of the fiber in both forward and backward scattering
modes where the Raman photons are either collected on the
excitation laser launch side or the opposite side.
[0032] It should be appreciated that the emission of secondary
radiation from a primary radiation excitation at a photonic crystal
fiber may be used in the optical measuring at least one analyte in
a sample. For example, the primary radiation excitation of light
into a photonic crystal fiber may be done at excitation conditions
that control non-analyte related emission of secondary radiation.
The secondary radiation may be, for example, a fluorescence
emission, photoluminescence emission, or a Raman emission from
material of the photonic crystal fiber. It should be further
appreciated that the emission of secondary radiation may be reduced
by natural attenuation of the photonic crystal fiber in spectral
regions where the emission of secondary radiation occurs. Also,
detection of optical radiation upon interaction with the sample may
be accomplished at either an excitation end of the photonic crystal
fiber or its distal end. Further, an operational lifetime of the
photonic crystal fiber may be lengthened by a surface treatment to
reduce deposition effects of contaminants.
[0033] Additionally, the emission of secondary radiation may be
used to improve the accuracy of measurements. For example, by
providing corrections for aging of a light source, repositioning
effects, or partial contamination of the optical elements the
accuracy of measurements may be improved. Corrections are provided
using spectral ratiometric or spectral multivariate corrections. In
spectral ratiometric corrections, the intensity of the secondary
emission at a wavelength that corresponds to the secondary emission
mostly from an analyte (for example nitrogen gas) is normalized by
the intensity of the secondary emission at a wavelength that
corresponds to the secondary emission mostly from the photonic
crystal fiber.
[0034] In spectral multivariate corrections, the spectral profile
(several wavelengths) of the secondary emission at a wavelength
that corresponds to the secondary emission mostly from an analyte
(for example nitrogen gas) is corrected by the spectral profile of
the secondary emission that corresponds to the secondary emission
mostly from the photonic crystal fiber. The measurements that may
have their accuracy improved may include, for example, quantitative
determinations, such as univariate analysis or multivariate
analysis.
EXAMPLE
[0035] For quantitation of nitrogen using a photonic fiber, a Raman
microscope has been employed. Raman excitation was achieved with an
argon ion laser emitting at 514 nm laser (16 mW). For experiments,
a 4-centimeter long photonic crystal fiber was used. FIG. 7
illustrates the calibration curve constructed from multiple
measurements of Raman spectra of nitrogen gas when different
concentrations of nitrogen gas were introduced into the photonic
crystal fiber. FIG. 8 illustrates a calibration curve for a broader
range of nitrogen gas concentrations than in FIG. 7 that were
introduced into a longer photonic crystal fiber than used for FIG.
7.
[0036] While the invention has been described in detail in
connection with only a limited number of embodiments, it should be
readily understood that the invention is not limited to such
disclosed embodiments. Rather, the invention can be modified to
incorporate any number of variations, alterations, substitutions or
equivalent arrangements not heretofore described, but which are
commensurate with the spirit and scope of the invention. For
example, another embodiment includes light being collected from the
fiber and transmitted through, or reflected off of, a dispersive
element and detected by a CCD. This arrangement enables collection
of a spectrum. This differs from other embodiments that require
collection of a limited spectral range through a filter. This
embodiment, unlike the other embodiments described herein, will
work for quantitative determinations being some measure of the
baseline. Additionally, while various embodiments of the invention
have been described, it is to be understood that aspects of the
invention may include only some of the described embodiments.
Accordingly, the invention is not to be seen as limited by the
foregoing description, but is only limited by the scope of the
appended claims.
* * * * *
References