Note: Descriptions are shown in the official language in which they were submitted.
1
A gas cell based on Hollow-Core Photonic Crystal Fiber and its application for
the detection of Greenhouse Gas: Nitrous Oxide
BACKGROUND
Trace-gas sensing is a rapidly growing field of research and has received
considerable attention, especially in the detection and quantification of
greenhouse
gases (e.g., N20 and CO2). It has also applications in non-invasive medical
diagnostics,
environmental monitoring, and homeland security.
Fertilizers used in agricultural fields are major sources of N20. The use of
fertilizer will increase in the next few decades to meet the demand of food
production
as the global population increases. However, optimizing the efficiency with
which
fertilizers produce nutrients, combined with the design of new forms of
fertilizers, can
reduce their emission of N20. It is also important to note that the excess
fertilizer drains
into rivers and lakes due to rain or irrigation and polluting water bodies
[1]. Further, the
emission of N20 is spatially variable in soil because of soil factors that
lead to the
production, consumption, and mobility of the gas. Thus the ability to pin-
point "hot spots"
of N20 emissions will allow one to mitigate soil factors. A farmer can
regulate the use
of fertilizer by measuring the concentration of N20 emitted from agricultural
fields due
to the application of fertilizer at different climate and soil conditions.
This ability will
facilitate the fertilizer industry's worldwide program of 4R (right fertilizer
source, right
rate, right time and right replacement) Nutrient Stewardship management, which
will in
turn improve farm management and finally reduce greenhouse gas emissions.
The current widely used technologies (e.g. GC: Gas Chromatograph,
FTIR: Fourier Transform Infrared spectroscopy, laser spectroscopy using a lead-
salt
detector cooled by liquid nitrogen or thermoelectric cooler, and cavity ring
down
spectroscopy using a quantum cascade laser) to detect trace gases are complex
and
expensive [2]. Thus, a compact and cost effective system that can operate at
room
temperature is in demand. A number of important gases (e.g. CH4, NH3, C2H2,
H25,
N20 and CO2) have overtones of the characteristic absorption (fundamental) and
the
combinations of the overtones bands in the near-infrared (NIR) region (1-2 pm)
of the
electromagnetic spectrum, which matches the emission spectrum of rare-earth
(e.g.
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Erbium) doped fiber [3]. This makes it possible to use passive and active
optical
components available from telecom industries to develop a compact and cost-
effective
device for the detection of trace gases.
The greenhouse effect is caused by the absorption of infrared radiation
(IR) from sunlight by gases such as nitrous oxide (N20). Qualitatively, gases
can be
differentiated by their absorption lines, and quantitatively, their
concentrations can be
determined by measuring the degree of absorption of light directed through a
gas
sample. The absorption of electromagnetic radiation (e.g. IR or NIR) by a gas
is
governed by the Beer-Lambert Law [4]:
= exp(¨aCL)
(1)
where lo is the intensity of the incident optical radiation, I is the
transmitted
optical intensity, a is the absorption coefficient of the gas molecules (an
important
parameter dependent on both the gas species and the wavelength of incident
optical
radiation), C is the concentration of the absorbing molecules and L is the
optical path
length of the gas cell or absorption path length. In general, absorption
spectroscopy
(e.g. FTIR) makes use of incoherent light sources such as incandescent bulbs
to
generate IR radiation. These sources are essentially blackbody radiators, and
complex
optical components are required to collimate and direct the beam through the
sample
with narrow bandwidth. The sensitivity of the above devices is limited by the
physical
length of the gas cell. Highly sensitive spectroscopic techniques to enhance
the
absorption path length have been developed based on the laser, such as
continuous-
wave cavity ring-down spectroscopy (CW-CRDS) and intracavity laser absorption
spectroscopy (ICLAS) [5]. The conventional CRDS technique involves measuring
the
decay time of the laser pulse injected into a high finesse cavity (Fabry-Perot
or Ring
configuration) that contains the gas sample, where the rate of decay of the
pulse
indicates the absorption by the gas sample. One can calculate the
concentration of the
gas sample from the decay time or the ring-down time. On the other hand, in
ICLAS,
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the gas cell is used inside the laser cavity and no external laser is
required. Both CRDS
and ICLAS increase the effective absorption length by several times, compared
to
conventional FTIR systems [5]. As the path length is enhanced, the sensitivity
of the
device increases. Thus, combining advanced detection techniques with a gas
cell with
longer optical path length makes it possible to develop a very highly
sensitive gas
detection system.
In the present Application, details are provided concerning the design of
novel gas cells, and their application for the detection of greenhouse gas;
more
specifically, nitrous oxide (N20).
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a gas
absorption cell apparatus comprising at least one terminal block for coupling
to a first
end of a hollow core photonic crystal fiber to cooperate with a like terminal
block at an
opposing second end of the hollow core photonic crystal fiber to form a fiber-
based gas
absorption cell, wherein each terminal block comprises:
a solid block of air-impermeable material having a main through-bore extending
fully through the block from an optical-coupling end of said block and
opposing fiber-
coupling end of said block;
an optical window fitted in the main through-bore at the optical-coupling end
of
said block to enable admission or exit of light energy to or from said main
through-bore;
an optical fiber chuck fitted in the main through-bore at the fiber-coupling
end of
said block to receive a respective end of the hollow core photonic crystal
fiber and
position said end within the main through-bore in alignment behind said
optical window
for optical communication therewith;
a transverse bore extending into said block from a gas-coupling side thereof
and
intersecting with said main through-bore at an intermediate location between
said ends
of the block to enable admission or evacuation of gas to or from the end of
the hollow
core photonic crystal fiber via said transverse bore; and
external threading carried on the fiber chuck and internal threads provided in
the
main through-bore adjacent the fiber-coupling end of the block, the fiber
chuck being
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fitted with the main through-bore by engagement of the external threading on
the fiber
chuck with the internal threads of the main through-bore;
wherein the external threading carried on the fiber chuck is defined by an
externally threaded carrier through which the fiber chuck axially extends.
According to a second aspect of the invention, there is provided gas
absorption cell apparatus comprising at least one terminal block for coupling
to a first
end of a hollow core photonic crystal fiber to cooperate with a like terminal
block at an
opposing second end of the hollow core photonic crystal fiber to form a fiber-
based gas
absorption cell, wherein each terminal block comprises:
a solid block of air-impermeable material having a main through-bore extending
fully through the block from an optical-coupling end of said block and
opposing fiber-
coupling end of said block;
an optical window fitted in the main through-bore at the optical-coupling end
of
said block to enable admission or exit of light energy to or from said main
through-bore;
an optical fiber chuck fitted in the main through-bore at the fiber-coupling
end of
said block to receive a respective end of the hollow core photonic crystal
fiber and
position said end within the main through-bore in alignment behind said
optical window
for optical communication therewith; and
a transverse bore extending into said block from a gas-coupling side thereof
and
intersecting with said main through-bore at an intermediate location between
said ends
of the block to enable admission or evacuation of gas to or from the end of
the hollow
core photonic crystal fiber via said transverse bore;
wherein the fiber chuck has an externally threaded end disposed outside
the block, a cap with a threaded axial bore extending thereinto from a first
end of the
cap is engaged to the externally threaded end of the fiber chuck via mating of
said
threaded axial bore with the threaded end of the fiber chuck, a seal is
axially
compressed between an end of the axial bore and the threaded end of the fiber
chuck
and is circumferentially tightened around the fiber, which passes through the
threaded
bore of the cap via a smaller axial bore that communicates therewith.
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According to a third aspect of the invention, there is provided a method of
producing a gas absorption cell apparatus comprising at least one terminal
block for
coupling to a first end of a hollow core photonic crystal fiber to cooperate
with a like
terminal block at an opposing second end of the hollow core photonic crystal
fiber to
form a fiber-based gas absorption cell, wherein:
each terminal block comprises:
a solid block of air-impermeable material having a main through-bore
extending fully through the block from an optical-coupling end of said block
and
opposing fiber-coupling end of said block;
an optical window fitted in the main through-bore at the optical-coupling
end of said block to enable admission or exit of light energy to or from said
main
through-bore;
an optical fiber chuck fitted in the main through-bore at the fiber-coupling
end of said block to receive a respective end of the hollow core photonic
crystal fiber
and position said end within the main through-bore in alignment behind said
optical
window for optical communication therewith; and
a transverse bore extending into said block from a gas-coupling side
thereof and intersecting with said main through-bore at an intermediate
location
between said ends of the block to enable admission or evacuation of gas to or
from the
end of the hollow core photonic crystal fiber via said transverse bore; and
said method comprises, for each terminal block of said apparatus, obtaining a
solid block of air-impermeable material, boring the main through-bore through
said solid
block from a first end thereof to an opposing second end, boring the
transverse bore
into said solid block from one side thereof to a point intersecting said main
through-bore
at an intermediate location between said first and second ends of the solid
block,
mounting the optical window in the main through-bore at the first end of the
solid block,
and providing a releasable connection to the main through-bore at the second
end of
the block for removable mounting of the fiber chuck in the main through-bore
in order
to hold the respective end of the hollow core photonic crystal fiber within
the main
through-bore at a position behind the optical window and in fluid
communication with
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the transverse bore to enable optical communication between said optical
window and
said end of the hollow core photonic crystal fiber and admission or evacuation
of gas to
or from the hollow core photonic crystal fiber via said transverse bore.
According to a fourth aspect of the invention, there is provided a method
of adjusting the effective length of a gas absorption cell apparatus
comprising two
terminal blocks for coupling to respective ends of a hollow core photonic
crystal fiber to
form a fiber-based gas absorption cell, wherein:
each terminal block comprises
a solid block of air-impermeable material having a main through-bore
extending fully through the block from an optical-coupling end of said block
and
opposing fiber-coupling end of said block;
an optical window fitted in the main through-bore at the optical-coupling
end of said block to enable admission or exit of light energy to or from said
main
through-bore;
an optical fiber chuck fitted in the main through-bore at the fiber-coupling
end of said block to receive a respective end of the hollow core photonic
crystal fiber
and position said end within the main through-bore in alignment behind said
optical
window for optical communication therewith; and
a transverse bore extending into said block from a gas-coupling side
thereof and intersecting with said main through-bore at an intermediate
location
between said ends of the block to enable admission or evacuation of gas to or
from the
end of the hollow core photonic crystal fiber via said transverse bore; and
said method comprises with the apparatus in an initial state in which an
initial
hollow core photonic crystal fiber runs between the two terminal blocks with
opposing
ends of said hollow core photonic crystal fiber respectively held in the main
through-
bore of the terminals blocks behind the optical windows thereof, decoupling
said
opposing ends of said initial hollow core photonic crystal fiber from said
terminal blocks,
and substituting a replacement hollow core photonic crystal fiber of different
length by
respectively coupling opposing ends of the replacement hollow core photonic
crystal
fiber to said terminal blocks in place of the initial hollow core photonic
crystal fiber.
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According to a fifth aspect of the invention, there is provided a hollow core
photonic crystal fiber gas absorption cell apparatus comprising a singular
enclosure
that comprises a plurality of exterior walls delimiting a singular interior
space that is
closed or closable in an air-tight manner sealed off from a surrounding
exterior
.. environment by said exterior walls; optical input and output windows bath
installed on
one or more of said exterior walls surrounding the same singular interior
space and
both optically communicating the same singular interior space of the singular
enclosure
with the surrounding exterior environment; first and second fiber supports
both mounted
within the same singular interior space of the singular enclosure, the first
fiber support
residing at a first location proximate the optical input window to support a
first end of a
hollow core photonic crystal fiber inside the singular interior space at said
first location
proximate said optical input window to accept incoming light energy admitted
to the
singular interior space of the singular enclosure via said optical input
window, and the
second fiber support residing at a second location proximate the optical
output window
to support a second end of the hollow core photonic crystal fiber inside the
singular
interior space at said second location proximate said optical output window to
release
light energy from said hollow core photonic crystal fiber to the exterior
environment via
said optical output window; a gas inlet located on one of the exterior walls
surrounding
said same singular interior space, said gas inlet fluidly communicating with
said same
singular interior space and being connected or connectable to a supply of gas
to admit
gas into said same singular interior space; and a gas outlet located on one of
the
exterior walls surrounding said same singular interior space, said gas outlet
also fluidly
communicating with the same singular interior space for evacuating said gas
from said
same singular interior space, whereby said gas admitted into said same
singular interior
space is admissible and evacuatable to and from the hollow core photonic
crystal fiber
via the ends of said hollow core photonic crystal fiber positioned inside said
same
singular interior space by said fiber supports.
According to a sixth aspect of the invention, there is provided a hollow
core photonic crystal fiber gas absorption cell apparatus comprising a
singular
enclosure that comprises a plurality of exterior walls delimiting a singular
interior space
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that is closed or closable in an air-tight manner sealed off from a
surrounding exterior
environment by said exterior walls; optical input and output windows installed
on one
or more of said exterior walls surrounding the same singular interior space
and both
optically communicating the same singular interior space of the singular
enclosure with
the surrounding exterior environment; a hollow core photonic crystal fiber
situated
entirely within the same singular interior space of the singular enclosure
with a first end
of the hollow core photonic crystal fiber situated inside said same singular
interior space
at a first location proximate said optical input window to accept incoming
light energy
admitted to the same singular interior space of the enclosure via said optical
input
window, and a second end of the hollow core photonic crystal fiber situated
inside said
same singular interior space at a second location proximate said optical
output window
to release outgoing light energy from said the hollow core photonic crystal
fiber to the
exterior environment via said optical output window; a gas inlet located on
one of the
exterior walls surrounding said same singular interior space, said gas inlet
fluidly
communicating with said same singular interior space and connected or
connectable to
a supply of gas to admit gas into said same singular interior space and a gas
outlet
located on one of the exterior walls surrounding said same singular interior
space, said
gas outlet also fluidly communicating with the same singular interior space
for
evacuating said gas from said interior space, whereby said gas admitted into
said
interior space is admissible and evacuatable to and from the hollow core
photonic
crystal fiber via the ends of said hollow core photonic crystal fiber
positioned inside said
same singular interior space.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be described in conjunction
with the accompanying drawings in which:
Figure 1A is a schematic illustration a first embodiment gas cell based on
a hollow-core photonic crystal fiber (PCF), where each end of the hollow-core
PCF is
physically supported in an internal chamber of a terminal block through which
gas is
admissible and evacuable.
Figure 1B is a more simplified schematic illustration of the first
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embodiment.
Figure 1C is a photographic image of a prototype gas cell according to
the first embodiment, where a spool of hollow-core PCF has each of its ends
physically
coupled to the respective terminal blocks, which in turn are optically coupled
to
respective lens assemblies.
Figure 1D is another photographic image of the first embodiment
prototype from another viewing angle.
Figure 2 is a schematic cross-sectional view of the terminal blocks of the
first embodiment.
Figure 3A is a photographic exterior view of a prototype of a second
embodiment gas cell using a hollow-core PCF, where the entirety of the PCF
resides
inside a singular air-tight enclosure equipped with gas and optical inputs and
outputs.
Figure 3B is a photographic interior view of the second embodiment
prototype of Figure 3A with a lid of the enclosure removed for illustrative
purpose.
Figure 4 is a schematic illustration of an experimental setup for the
detection of trace gas. PUMP: pump laser; PM-EDF: Polarization-maintaining
erbium-
doped fiber; VOA: Variable optical attenuator; SA: Saturable absorber; FBG:
Fiber
Bragg grating; OSA: Optical spectrum analyzer; CIR: Polarization independent
optical
circulator; PC: Polarization controller and 01: Polarization independent
optical isolator
Figure 5A shows theoretical rotational transition for N20 in the 3v3 band
obtained using Spectral Calculator (GATS).
Figure 5B schematically illustrates an experimental setup using the
second embodiment gas cell to study intracavity laser absorption spectroscopy
(ICAS)
using amplified spontaneous emission (ASE) light inside the cavity.
Figure 6A shows output of the system obtained from the optical spectrum
analyzer (OSA) of the Figure 5 experimental setup as the variable optical
attenuator
(VOA) is adjusted.
Figure 6B shows output of the system obtained from the optical spectrum
analyzer (OSA) of the Figure 5 experimental setup using a hollow-core fiber
(HC19-
1550, NKT) of length 6 m as gas cell inside the cavity filled with 0.5%
acetylene gas.
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The absorption lines are clearly visible in the C band.
Figure 7 shows theoretical absorption spectra obtained using Spectral
Calculator (GATS).
Figure 8A shows direct absorption spectra of N20 gas after subtracting
the N2 gas spectrum as background for a 40 meter length of PCF in the first
embodiment
gas cell.
Figure 8B shows direct absorption spectra of N20 gas after subtracting
the N2 gas spectrum as background for a 20 meter length of PCF in the first
embodiment
gas cell.
Figure 9A shows absorption spectra of N20 gas from the Figure 4
experimental setup at two different concentrations.
Figure 9B shows absorption spectrum of 10% N20 from the Figure 4
experimental setup after subtracting the N2 gas spectrum as background, for a
40 meter
length of PCF in the first embodiment gas cell.
Figure10A shows absorption spectra of N20 gas from the Figure 4
experimental setup at two different concentrations.
Figure10A shows absorption spectrum of 10% N20 from the Figure 4
experimental setup after subtracting the N2 gas spectrum as background for a
20 meter
length of PCF in the first embodiment gas cell.
Figure 11 shows absorption spectrum of 500 ppbv N20 gas for a 40 meter
length of PCF in the first embodiment gas cell.
In the drawings like characters of reference indicate corresponding parts
in the different figures.
DETAILED DESCRIPTION
In development of the present invention, a hollow-core photonic crystal fiber
(PCF)
was used to design a gas cell, and was incorporated as an intracavity gas cell
in a fiber
ring laser. The detailed structure of the laser cavity was described in
reference [6]. The
amplified spontaneous emission (ASE) light inside the laser cavity was used
for the
detection. In general, an erbium doped fiber gives a very wide (- 100 nm) ASE
spectrum. A fiber Bragg grating (FBG) with peak wavelength close to one of the
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absorption lines was chosen and the system produced the laser at this
wavelength. The
wavelength was close to the lower side of the emission spectrum, where the
absorption
coefficient of N20 is higher, compared to that in the C and L band regions.
N20 gas has
three fundamental infrared active absorption bands: ui = 1284.9 cm-1 - 7.8 pm;
U2=
588.8 cm-1 - 17 pm; and U3= 2223.8 cm-1 - 4.5 pm. A number of articles,
reporting
results from different spectroscopic techniques such as Fourier Transform
Infrared
Absorption Spectroscopy (FTIR), Intracavity Laser Absorption Spectroscopy (IC
LAS),
and Cavity Ring Down Spectroscopy (CRDS), identify transitions in the overtone
bands
for N20 gas [7-19].
In the experimental system detailed herein, the rotational line in the 3u3
overtone
band (- 1.52 pm band) available from HITRAN was used as a reference line to
develop
the sensing device [20]. The system based on the new gas cell was capable of
detecting
N20 at a concentrations level of sub-ppmv (parts per million by volume). The
efficiency
of the device has been explored using different lengths of hollow-core
photonic crystal
fiber (PCF) and spectroscopic techniques. The system based on the developed
gas cell
will be compact and cost-effective compared to the system based on
conventional gas,
which have larger foot print.
A gas cell that requires a very small amount (- ml) of gas is important for
high
sensitivity laser absorption spectroscopy. In this respect, a hollow-core
photonic crystal
fiber (PCF) is a good candidate because of the high optical-path-to-sample-
volume ratio
[21-23]. In a PCF, light propagates through the hollow core by photonic band
gap
effects, which occur due to the periodic distribution of air holes in the
cladding [24]. The
PCF is very attractive for applications in optical communications, because it
shows very
low attenuation, dispersion, non-linearity, bending loss, and can also guide a
fundamental mode over a wide spectral range without any leakage [25,26]. The
idea of
using a PCF for gas and liquid sensing is relatively new and there is scope to
develop
a new compact device using this fiber [27-29]. An all-fiber gas cell has been
proposed
and demonstrated for gas detection [30]. Recently a number articles have been
published on gas sensors based on PCF in CRD spectroscopy [31,32], wavelength
modulation spectroscopy (WMS) [33], ICAS [34] and Raman spectroscopy [35].
Most
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of the gas cells developed using a PCF are designed to detect a particular
chemical or
gas [31,36-39]. In references [40-43], the dynamics of gas flow in PCF, which
determine
the filling and evacuation time for the PCF-based gas cell, have been
investigated. In
order to reduce the filling and evacuation time and thus increase the response
time of
the detection system, researchers used a number of techniques, such as
splicing the
PCF to a normal single-mode fiber and allowing the gas sample to fill the core
at a
higher pressure [28], drilling holes on the surface of PCF so that gas can
diffuse faster
[44-46], using specially designed mechanical splices [47,48] and finally using
a
specially designed fiber [49]. It is important to note that the commercially
available gas
cell based on PCF from GLO photonics (UK) does not allow one to change the
fiber if
required.
Figure 1 shows the prototype gas cell developed by the present inventors using
a
hollow-core photonic crystal fiber [HC19-1550, core diameter: 20 pm] from NKT
Photon ics to detect trace gases. The system has two lens system assemblies
100, 200,
together with two solid terminal blocks 1, 2, for example whose outer
dimensions may
be 2"x2"x4", and which may be made of aluminum, steel, other metals, or other
gas-
impermeable solids. A very small hole was drilled inside each block, so that
the volume
of the gas admissible inside was very small¨only a few ml. One terminal block
1, 2 is
coupled to one end the hollow-core PCF 40 and is connected to a gas supply 32,
and
the other terminal block is coupled to the other end of the hollow-core PCF 40
and is
connected to a vacuum pump 29. The illustrated embodiment uses the same
identical
design for both terminal blocks 1, 2, so that each be connected to either a
gas supply
or a vaccum pump. During experimental use of the prototype, the hollow-core
fiber 40
was evacuated using the pump 29, and the vacuum level was maintained at
approximately 0.2 mb, when evacuated from one side only. It was important to
adjust
the optimum pressure difference between the two terminal blocks 1, 2 for a
steady gas
flow through the hollow-core PCF 40 from the supply 32. The time to evacuate a
20
meter long PCF was approximately 80 minutes, which led to a slow response time
for
the system. The experiment was repeated with a 40 meter length of PCF. The
increased
fiber length caused a longer gas evacuation and filling time. To improve the
filling time,
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the gas was allowed to diffuse from one end while other side was connected to
the
vacuum pump, and, after some time, gas was allowed to diffuse from both ends.
The front end of each block was fitted with an AR (antireflection) coated
quartz
window 28 for coupling of light to or from the respsective lens assembly, and
the
opposing rear end was fitted with fiber coupling components. Accordingly, the
front and
rear ends are also referred to herein as the optical coupling and fiber
coupling ends,
respectively. The top of block 2 was fitted with a gas on/off switch or
control valve 31,
and a vacuum gauge 30. To eliminate leakage through the fiber chuck 38 at the
rear
end of the block, a specially designed 0-ring and cap assembly 46, 48 was used
as
part of a unique fiber connector. Figure 1D shows how the PCF was inserted
inside the
aluminum block using the custom-designed connector. The advantage of this
system
was that it could maintain a constant vacuum level (e.g. - 0.2 mb) at one end
of the
fiber while allowing gas to diffiuse through the other end. During the filling
process, the
fiber was evacuated from both sides of the PCF for a certain amount of time,
and then
one end of the PCF was maintained at a lower pressure and the other end was
connected to a gas supply in the form of a Tedlar bag filled with N20 gas at a
particular
concentration. [Two gas tanks, one with N20 and other with N2 from Praxair,
Canada,
and a mass flow controller from Omega, was used to prepare specific mixtures
of gas.
Both tanks were connected directly to 100 nm filters in order to remove any
dust
particles contained in the gas]. The optical fiber at the input (and output)
and lenses in
each assembly were mounted on a three-axis stage. As shown in Figure 1A, one
lens
collimated the beam and the other one focused the collimated beam on the tip
of the
PCF through the quartz window.
Figure 2 reveals further structural detail of the terminal blocks 1, 2 and the
coupling
of the hollow core PCF 40 thereto. Each block has a front optical coupling end
10 and
an opposing rear end 12. A main through-bore 14 is drilled longitudinally
through the
block from the front end thereof to the opposing rear end, but is drilled in
multiple
sections of varying diameter. A counter-bored section 16 of greatest diameter
resides
at the front end of the block, and opens into a gas receiving section 18 of
lesser
diameter. A smaller counter-bored section 20 at the rear end of the block is
threaded,
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and is joined to the gas receiving section 18 by a reduced or constricted
section 22 of
lesser diameter than the other sections. A transverse bore 24 is drilled into
the block
from one side thereof, for example the top side 13 in the illustrated example,
and
terminates where it intersects with the gas receiving section 18 of the main
through-
bore 14. The transverse bore 24 is internally threaded where it intersects
with the side
wall 13 of the block, and the smaller counter-bored section 20 at the rear end
12 of te
block is likewise internally threaded. A gas fitting 26 is threaded into the
transverse
bore 24 of each block 1, 2 in air-tight relation therewith, for example using
thread tape
to ensure an air-tight seal. The gas fitting 26 of block 2 is connected to a
vacuum pump
29, vacuum gauge 30 and control valve 31, and the gas fitting 26 of block 1 is
connected
to a gas supply 32.
The quartz window 28 is seated within the large counterbored section 16 at the
front
end 10 of the block. Before and/or after the insertion of the quartz window 28
during
production of the block, a flowable sealant is applied around the peripheral
wall of this
section so as to form an air-tight seal 30 between the window 28 and the wall
of the
main through-bore, thus preventing escape of gas from the neighbouring gas
receiving
section 14 through the front end 10 of the block.
At the rear end 12 of each block, an axially-bored bolt 36 has its externally
threaded
shaft 36a engaged with the internal threading of the rear counter-bored
section 20,
while the larger diameter head 36b of the bolt 36 resides outside the block
beyond the
rear end 12 thereof. The bolt 36 has a central though-bore passing axially
therethrough
from the head to the distal end of the threaded shaft. The fiber chuck 38 is
received
within this through-bore, and extends axially from each end of the bolt 36.
The bolt thus
serves as a carrier of the fiber chuck 38, and the bolt shaft 36a defines an
external set
.. of threads spanning a partial length of the chuck's elongated body in order
to mate with
the rear threaded section 20 of the block's main through-bore 14. The enlarged
head
36b of the bolt remains outside the block for manual or tool-driven rotation
of the bolt
during installation and removal thereof to and from the block. While the bolt
36 may
have a hexagonal head for wrench-driven operation thereof, the prototype in
the
Date Recue/Date Received 2021-01-19
15
drawings instead features a round head, but with a knurled or otherwise
textured outer
peripheral surface for manually gripped turning of the bolt.
The fiber chuck 38 reaches through the reduced diameter section 22 of the
block's
main through-bore into the gas-receiving section 18 thereof, where a free end
40a of
the hollow-core PCF 40 emerges from the fiber chuck 38 on the central axis of
the
block's main through bore. Accordingly, gas introduced or evacuated to or from
the
gas-receiving section of the block's main through-bore 14 via the transverse
bore 24
can enter or exit the hollow-core PCF 40 through the free end 40a thereof. So
that the
gas-receiving section 18 of the block's main through bore defines an air-tight
gas
chamber, an o-ring seal 42 is situated around the fiber chuck 38 between the
distal end
of the carrier bolt's threaded shaft 36a and the inwardly reaching shoulder 44
defined
at the transition between the rear counterbored section 20 of the block's main
through-
bore and the adjacent reduced-diameter section 22 thereof. The o-ring 42 is
axially
compressed against the inwardly reaching shoulder of the block by the distal
end of the
bolt 36 when tightened, whereby the o-ring 42 circumferentially tightens
around the fiber
chuck 38. This cooperates with the sealant-retained position of the quartz
window 28
and the air-tight threaded engagement of the gas fitting 26 with the block to
maintain
an air-tight status of the gas receiving section of the block's main through-
bore.
Accordingly, gas can only enter and exit the block through the gas fitting 26
and the
fiber chuck 38.
To similarly provide an air-tight seal between the hollow-core PCF 40 and the
fiber
chuck 38,the hollow-core PCF 40 passes through a smaller o-ring seal 46 just
outside
a threaded exterior end 38a of the chuck 38, and an interally threaded cap 48
is
threaded onto the chuck 38 in order to axially compress the smaller o-ring
seal 46
against the threaded end of the chuck, and thus circumfernetially constrict
the smaller
o-ring seal 46 around the hollow-core PCF 38. Figure 2 shows the cap 48 and
corresponding seal 46 in exploded positions prior to threading of the cap onto
the fiber
chuck. The cap's threaded axial bore 48a extends into the block-facing inner
end of
the cap 48, but stops short of the opposing outer end of the cap 48. A smaller
axial
bore 48b of the cap continues onward from the threaded bore 48a to the outer
end of
Date Recue/Date Received 2021-01-19
16
the cap, and accommodates passage of the hollow-core PCF fully through the cap
48,
and onward through the chuck 38 and into the block. When the cap 48 is
suffciently
threaded onto the chuck 38, the seal 46 is axially compressed between the
chuck's
threaded end and the closed end of the cap's threaded bore, which casues the
seal 46
to tighten around the hollow-core PCF 40. This maintains the air-tight state
of the
block's gas-receiving section 18 by sealing closed the annular space between
the fiber
40 and the chuck 38 at the threaded outer end 38a thereof. Accordingly, gas
can only
enter and leave the block via the hollow-core PCF 40 and the gas fitting 26.
The forgoing terminal block design is easily manufactured, requiring only a
singular
block of stock material and basic drilling and tapping machinery to bore out
and thread
the main through-bore and transverse bore, while the clutch-carrying bolt 36
enables
simple but removable threaded connection of a conventional off-the-shelf fiber
chuck to
the block. It will be appreciated however that other embodiments may instead
use a
modified fiber chuck with its own external threading to make the removable
threaded
connection to the block, rather than relying on an axially bored bolt or other
chuck-
holding carrier to define the external male threads of this chuck-block
connection.
Connection of the fiber to the block in an air tight manner requires only
passage of the
fiber through the chuck, threading of the carrier bolt 36 to the block, and
threading of
the cap 48 to the chuck. Removal of the fiber 40, for example to swap the
existing fiber
for a replacement fiber of different length to vary the effective length of
the gass cell,
requires only the simple reversal of the installation process, i.e. loosenign
or removal
of the cap 48, removal of the threaded carrier bolt 36 to allow withdrawal of
the chuck
of the block, and withdrawal of the fiber 40 from the chuck 38.
In addition to the first embodiment cell shown in Figure 1, the present
inventor also
developed another gas cell (Figure 3), which in its prototype form consist of
a steel
rectangular box 300 with a removable lid. A PCF 40 of desired length was
placed inside
the box. The steel box was fitted with a gas inlet 302 and a gas outlet 304
for
introduction and evacuation of gas to the interior space of the box. An
optical
breadboard 306 was also placed inside the box on the floor thereof to make it
possible
to mount optical components inside the box. Light was coupled into (and out
of) the
Date Recue/Date Received 2021-01-19
17
PCF through the anti-reflection (AR) coated quartz windows 308, 310 installed
in an
exterior wall of the box (as shown in Figure 3A). The fibers for ASE (or
light) In/Out
were mounted on three-axis translation stages with a combination of lenses, as
described above for the first embodiment cell of Figure 1 but omitted from
Figure 3 for
illsutrative simplicity.
While the illustrated embodiment features placement of the two quartz windows
308,
310 in the same wall of the box 300 to enable convenient mounting of the lens
assemblies on a shared exterior breadboard at a singular side of the box, it
will be
apprecaited that the windows could alternatively be installed in different
walls of the
box. The PCF 40 inside the box was mounted on fiber clamps 312, 314 [HFF003,
Thorlabs] installed on the interior optical breadboard in close proximity to
the quartz
windows 308, 310 so as to hold the ends of the hollow-core PCF 40 in close
relation to
the windows to accept and return the ASE light from and to the exterior lens
assemblies
via said windows. The gas inlet fitting was mounted at one end of the box and
connected to a supply of gas, and the gas outlet fitting was mounted at the
opposing
end of the box and and connected to a vacuum pump. Initial gas in the steel
box (e.g.
ambient air, or remnant gas from a prior test) was first evacuated from one
end of the
box via the outlet 304, while maintaining the inlet closed via a control valve
coupled
thereto, and then the gas under investigation was allowed to enter from other
end of
the box via the input 302 by opening the control valve. It is also possible to
evacuate
and fill the PCF from both ends thereof since both ends of the PCF are open
and reside
inside the same air-tight enclosure.
In experimental use of the prototype, this process required a long time to
fill and
evacuate the PCF completely. To find the filling and evacuation time for a
particular
.. length of PCF, the output was monitored by observing absorption lines of 1
% C2H2 in
the C band region, where they are very strong. The disadvantage for this
system was
the long response time, which was due to the large volume of the gas cell
compared to
the volume of the PCF. The advantage of the gas cell is that one can drill
holes along
the length on the surface of PCF so that gas can diffuse faster inside the
core of the
fiber, and thus decrease the response time of the system [45,46].
Date Recue/Date Received 2021-01-19
18
While the prototype employed a steel box constructions, any box or container
made
of gas-impermeable solid material (e.g. aluminum, other metals) may similarly
be used
to form a singular enclosure whose interior space contains the entirety of
hollow-core
PCF, or at least the two ends thereof, and the pair of fiber clamps or other
supports for
supporting the ends of the hollow-core PCF in the appropriate positions
optically aligned
behind the quartz windows for optical communiocation therewith. A removable or
openable and closeable lid is preferably employed to allow access to the
hollow-core
PCF for optional replacement thereof, for example to substitute a hollow-core
PCF of
one length for a replacement hollow-core PCF of a different length in order to
easily
change the effective length fo the gas cell, provided that a suitable gasket
or other
sealing means is employed between the lid and the container walls when the lid
is
placed and secured in its closed position.
Below, Applicant presents experimental data based on the first embodiment gas
cell
of Figure 1 for N20.
Figure 4 shows the schematic of the experimental setup used for the detection
of
N20. The first embodiment gas cell of Figure 1 was used inside the cavity. The
unidirectional resonant ring cavity consists of a polarization-maintaining
erbium-doped
fiber (PM-EDF); a variable optical attenuator (VOA) to adjust the loss in the
cavity; an
unpumped PM-EDF as the saturable absorber (SA); a fiber Bragg grating (FBG) of
reflectivity 85.16%, peak wavelength - 1522.22 nm and bandwidth of 0.168 nm,
where
the peak wavelength of the FBG was close to P(12) rotational absorption line
(Figure
5A) of N20; and an all-fiber polarization controller to control the
polarization state of the
light inside the cavity. The detailed cavity design and advanced detection
technique
developed by the present inventors has been described in reference [6]. The
presence
of polarization-maintaining gain fiber and SA increases the stability of the
laser
wavelength.
The advantages of the detection system are: i) The laser generated by the
system
contains a multi-longitudinal mode, which increased the sensitivity of
detection of gases
at lower concentrations; ii) The system is capable of operating at room
temperature
(most of the currently commercially available systems require cooling below
room
Date Recue/Date Received 2021-01-19
19
temperature); iii) Standard optical components available from telecom
industries were
used for developing the device; iv) A gas cell based on hollow-core photonic
crystal
fiber makes the system compact and suitable for conversion into a hand held
device;
and v) The system can be used to detect various other gases (e.g. NH3, H25
etc.) simply
by changing the FBG.
Figure 5A shows the rotational lines in the 3u3 overtone band for N20,
obtained
using Spectral Calculator, GATS [10,20]. Although N20 possesses relatively
strong
absorption lines at - 1522 nm, the gain-coefficient of erbium-doped fiber is
much lower
compared to that in the C and L band regions. Further, erbium-doped fiber is a
homogeneous gain medium at normal temperatures and the lasing wavelength is
determined by the local maximum of the gain curve. For a ring cavity without a
FBG
lasing occurs in the C or L band.
Figure 5B shows an ICLAS system developed using the second embodiment gas
cell of Figure 3, where the gas cell is coupled to a polarization controller
(PC) by a fused
fiber coupler (FFC). The cavity loss could be changed by adjusting the
variable optical
attenuator (VOA), which was adjusted to obtain an almost flat (60% inversion)
spectrum
in the C and L band regions. Once the system has reached this condition
(called
balanced condition), any small change in the cavity loss will switch the laser
from the C
band to the L band or vice versa. Figure 6A shows the switching of the laser
wavelength
from the C band to the L band after a slight adjustment (i.e., increasing the
loss in the
C band region) of the VOA. In order to obtain very high sensitivity in
detection using
ASE inside the laser cavity, it is important to adjust the VOA (and thus the
inversion
level) in such a way that a very small change in attenuation can switch the
laser
between two bands when the laser operates under threshold conditions. A gas
sample
inside the ICLAS cavity also provide attenuation (due to absorption), which is
similar to
the VOA. Figure 6B shows the switching of the laser line from the C band to
the L band
once the second embodiment gas cell of Figure 3 was filled with 0.5% of C2H2.
A few
absorption lines are also visible in the C band, because the gas has many
strong
absorption lines in this region. The absorption coefficient for N20 is lower (-
three
orders of magnitude) than the C2H2 in the C band. Thus, it was not possible to
take the
Date Recue/Date Received 2021-01-19
20
advantage of ASE light available inside the cavity to detect N20 using the
system as
shown in Figure 5B. The OSA (ANDO optical spectrum analyzer) spectra in the
manuscript were collected using LABVIEW program (wavelength resolution:
0.005nm
and intensity resolution: 0.001 dBm) and each spectrum presented is the
average of 10
.. scans.
The setup in Figure 4 was developed in order to take the advantage of the ASE
light
inside the cavity in the - 1522 nm band, where N20 has high absorption
compared to
that in the C and L bands [6]. The FBG wavelength was chosen so that the peak
wavelength is close to one of the absorption lines, P (12) rotation line in
the 3v3 band
[Figure 5A], and does not interfere with the absorption lines due to the
presence of CO2
and H20 (Figure 7). Two different lengths (20 m and 40 m) of PCF were used to
make
the gas cell, and the corresponding lengths of the unidirectional ring cavity
were
approximately 40 m and 60 m, respectively [including the length of the PCF].
The
system produced a stable multi-longitudinal mode laser output at room
temperature
with a maximum separation of - 3 MHz for 40 m long PCF. A laser oscillating in
multiple
longitudinal modes is susceptible to mode hopping, but the small length of SA
inside
the cavity was able to eliminate the mode-hopping at normal room temperature
[50,51].
Baev et al. reported properties of multi-longitudinal mode lasers and their
application in
ICLAS [52]. It was found that a multi-longitudinal mode laser provides very
high
sensitivity in ICLAS, if the homogeneously broadened gain bandwidth is larger
than the
absorption linewidth. In fact, the number of photons in a mode that matches
the narrow
band absorption line will decrease following the Beer-Lambert law. The ICAS
produces
a very good absorption spectrum if the absorption line of the gas sample is
larger than
the longitudinal mode separation [52]. In the present system, the absorption
linewidth
was larger than the separation between two longitudinal modes, so many
longitudinal
modes were superimposed within the absorption line. Further, Hansch et al.
[53] also
showed an increase of absorption sensitivity by a factor of 105 due to the
presence of
a number of oscillating modes.
The experimental setup described in Figure 4 produced a multi-longitudinal
mode
laser wavelength, which was selected by the FBG. The laser was kept under the
Date Recue/Date Received 2021-01-19
21
threshold condition, so that the wavelengths related to the ASE light close to
the laser
wavelength were also close to the threshold condition. Thus, the photons
corresponding
to ASE light inside the cavity circulated multiple times and enhanced the
effective path
length of the cavity. In turn, the sensitivity of detection was also enhanced
due to the
large absorption path length.
Figure 8 shows the absorption spectra from the direct absorption of the 10%
N20
(Praxair, Canada, certified concentration of N20: 10% +N2 balance) gas using a
40 m
and 20-meter long PCF after subtracting the reference gas N2, obtained using
the first
embodiment gas cell shown in Figure 1. The gas was allowed to diffuse through
one
end and the other end was maintained at constant vacuum level. The ASE from
the
PM-EDF [Ip=75 mA] was used as the input light for the direct absoprtion
spectroscopic
(DAS) measurement [Note: The laser cavity was not closed]. The output obtained
was
monitored using the OSA. The absorption lines of the gas disappered after a
few cycles
of evacuation and filling with N2. The hollow-core (20 micron diameter) of the
PCF was
surrounded by small, micron-order holes. The longer evacuation time was due to
the
presence of these gas-filled smaller holes. It is anticipated that most of the
laser power
is confined to the core and the effect of absorption due to gas inside the
surrounding
small holes is minimal or not significant. It is also posible to obtain the
DAS spectrum
by using a tunable laser.
Figure 9A shows the transmission spectra of the experimental setup described
in
Figure 4 with the reference gas N2 (Praxair, Canada, Research Grade, Nitrogen
6.0)
and N20 (Praxair, Canada, certified concentration of N20: 10% +N2 balance and
0.1%
+ N2 balance) with 40 m PCF for the gas cell. The absorption spectrum in
Figure 9B
was obtained by subtracting the spectra for N20 (10%) and N2 shown in Figure
9A. The
system described in Figure 4 was operating under threshold condition (Ip=
147.5 mA).
It is to be noted that that the gas cell was flushed with N2 before and after
the scanning
with N20 gas. The experiment was repeated with 20 m long PCF. Figure 10 shows
output spectra obtained using a 20 m long PCF as the gas cell inside the
system in
Figure 4, for 10% and 0.1% N20, respectively.
Date Recue/Date Received 2021-01-19
22
It is clear from the spectra in Figure 8, obtained using DAS, and in Figures 9
and 10,
using ICLAS, that the specificity and sensitivity of detection increases at
the FBG
location. Further, the application of a FBG eliminates any overlap with
absorption lines
due to other gases in the mixture. In the present case, the inventor used the
absorption
line corresponding to the P (12) line of N20. The experiments were performed
with
lower concentrations of N20 gas. The minimum concentration the system can
detect
with 40 m long PCF after a longer evacuation and filling time (- 4 hours) was -
500
ppbv (Figure 11; Praxair, Canada, certified concentration of N20: 520 ppbv+N2
balance).
In summary, presented herein are two gas cells based on hollow-core photonic
crystal fiber for the detection of nitrous oxide gas. In prototype
experimentation, the gas
cell was incorporated inside a laser cavity as an intracavity gas cell. At
present, the
evacuation and filling time for the gas through the PCF may be too long for
use where
fast response is important. The sensitivity of the system can be increased by
improving
the responsivity or by reducing gas evacuation and filling times. In addition,
the laser
cavity supported multi-longitudinal modes, which increased the sensitivity
further. The
system is capable of operating at room temperature and over a wide set of
wavelengths.
One can select the gas absorption line of any trace gas by using a tunable
FBG. The
system can be used as a hand-held device.
Date Recue/Date Received 2021-01-19
23
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Since various modifications can be made in the present invention as herein
above
described, and many apparently widely different embodiments of same made, it
is
intended that all matter contained in the accompanying specification shall be
interpreted
as illustrative only and not in a limiting sense.
Date Recue/Date Received 2021-01-19
