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Claims  |
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I claim:
1. A system for the near simultaneous analysis and quantitation of selected
multiple polyatomic gases in a gas sample by Raman light scattering
comprising in combination;
(a) laser means capable of producing a polarized laser beam of a selected
wavelength containing a laser cavity said laser cavity containing a plasma
tube and wherein one end of said laser cavity contains a high reflectivity
output coupler mirror;
(b) a gas sampling cell located within said laser cavity between said
plasma tube and said output coupler mirror, said cell having opposing
parallel end windows interconnected by a continuous sidewall, said end
windows and sidewall defining a longitudinal gas chamber oriented such
that, when said laser beam is activated, the laser beam is coincident with
and traverses the axis of said longitudinal gas chamber, said end windows
being positioned to be substantially normal to the axis of the
longitudinal gas cell chamber, said cell also having opposing, aligned
side windows in said sidewall parallel to and on either side of the axis
of said longitudinal gas chamber, said gas cell further containing inlet
and outlet means communicating with said chamber to pass a sample gas
through said cell;
(c) a reflection mirror positioned adjacent to and outside of said gas cell
parallel to and in alignment with said side windows on one side thereof to
capture and redirect a proportion of scattered elastic laser light and
inelastic Raman light through said side windows,
(d) collection lens means positioned parallel to and in alignment with said
side windows outside said gas cell and on the side opposite from said
reflection mirror to collect elastic laser scattered light and inelastic
Raman light passing through said side windows,
(e) laser line rejection filter means positioned to receive the scattered
light passing through said collection lens means, said filter being
selected to reject elastic laser scattered light passing through said
collection lens means while allowing the transmission of inelastic Raman
scattered light,
(f) a rotatable filter wheel containing a series of interference filters
wherein each interference filter is selected to transmit only a single
Raman spectra line of a predetermined wavelength, said filter wheel being
positioned such that, as it rotates, each interference filter will
sequentially receive Raman scattered light passing through said laser line
rejection filter means;
(g) detection and amplification means for sequentially receiving Raman line
signals passing through each of said interference filters and converting
said signals to digital electrical pulses;
(h) processing means for intrepreting said digital electrical pulses and
converting them to visual readouts indicative of the concentration of each
of said selected polyatomic molecular gases in said sample; and
(i) power means to operate said laser means, rotating filter wheel,
detection and amplification means and processing means.
2. A system according to claim 1 wherein said end windows are coated with
an antireflection coating specific to the selected wavelength of the laser
beam.
3. A system according to claim 1 wherein said reflection mirror and output
coupler mirror are of high reflectivity.
4. A system according to claim 3 wherein said reflection mirror has a
radius of curvature and is located relative to the laser beam at a
distance from said beam equal to the radius of curvature of said mirror.
5. A system according to claim 4 wherein said reflection mirror is a
spherical mirror.
6. A system according to claim 4 wherein said reflection mirror is a
cylindrical mirror.
7. A system according to claim 1 wherein said side windows in said gas cell
are coated with a broad band antireflection coating adapted to pass
desired wavelengths of inelastic Raman scattered light.
8. A system according to claim 1 wherein said filter wheel and processing
means are signally connected such that, as single Raman line signals pass
through each interference filter, the processing means determines which
polyatomic gaseous molecule is being analyzed.
9. A system according to claim 8 wherein said filter wheel contains a
series of alternating interference filters and blanks and wherein one of
said interference filters is a reference filter.
10. A system according to claim 1 wherein said output coupler mirror allows
passage of an extracavity laser beam when said laser beam is activated and
wherein means are located to receive said extracavity laser beam signals
and convert said signals to current which is directed to one channel of a
two channel current amplifier-filter for simultaneous correlation with
photocurrent signal in a second channel of said amplifier-filter to
correct Raman signal intensity for random fluctuation in laser optical
power.
11. A method for the near simultaneous and instantaneous determination of
the concentration of multiple polyatomic gas molecules in a gas sample
comprising;
(a) introducing said gas sample into a gas sampling cell located within the
resonance cavity of a laser;
(b) subjecting said gas sample in said laser cavity to a polarized laser
beam of selected wavelength and having sufficient intensity to produce
detectable signals of inelastic Raman scattered light,
(c) capturing and redirecting signals of both inelastic Raman scattered
light and elastic laser scattered light in a plane normal to the axis of
said laser beam by means of a reflection mirror located adjacent to and
outside of said gas cell said reflection mirror being parallel to the axis
of said laser beam,
(d) collecting signals of both inelastic Raman scattered light and elastic
laser scattered light by collection lens means located opposite said
reflection mirror said collection lens means also being parallel to the
axis of said laser beam and in alignment with said reflection mirror,
(e) directing said signals of both inelastic Raman scattered light and
elastic laser scattered light onto a laser line rejection filter wherein
scattered elastic laser light signals are rejected and signals of
inelastic Raman scattered light are transmitted;
(f) subjecting said signals of Raman scattered light to a rotating filter
wheel containing a series of interference filters wherein each
interference filter is specific for the transmission of a single Raman
line,
(g) sequentially sensing single Raman line signals passing through the
interference filters of said filter wheel by detection and amplification
means and converting said signals into digital electrical pulses
(h) sequentially processing said digital electrical pulses in processing
means and converting them to visual readouts indicative of the
concentration of each of said polyatomic molecules in said gas sample
being determined.
12. A method according to claim 11 wherein said polyatomic gases are
members selected from the group consisting of respiratory and anesthetic
gases.
13. A method according to claim 12 wherein said polyatomic gases are
members selected from the group consisting of nitrogen, oxygen, carbon
dioxide and halogenated anesthesia gases.
14. A method according to claim 13 wherein said gases are sampled by means
connected to the airway of a patient.
15. A method according to claim 11 wherein the gas sample is contained in a
gas cell having opposing parallel end windows interconnected by a
continuous sidewall, said end windows and sidewall defining a longitudinal
gas chamber oriented such that, when gas sample is subjected to said laser
beam, the laser beam is coincident with and traverses the axis of said
longitudinal gas chamber, said end windows being positioned to be
substantially normal to the axis of the longitudinal gas cell chamber,
said cell also having opposing, aligned side windows in said sidewall
parallel to and on either side of the axis of said longitudinal gas
chamber, said gas cell further containing inlet and outlet means
communicating with said chamber to pass a sample gas through said cell.
16. A method according to claim 15 wherein said inelastic Raman scattered
light and elastic laser scattered light are captured and redirected by
said reflection mirror wherein said reflection mirror has a radius of
curvature and is located relative to the laser beam at a distance from
said beam equal to the radius of curvature of said mirror.
17. A method according to claim 16 wherein said mirror is a spherical
mirror.
18. A method according to claim 16 wherein said mirror is a cylindrical
mirror.
19. A method according to claim 16 wherein the said end windows in said gas
sampling cell are coated with an antireflection coating specific to the
wavelength of the laser beam.
20. A method according to claim 16 wherein sample gas is continuously
passed through said inlet and outlet means in said gas cell by pump means
located in a gas supply line on the inlet side of said gas cell. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to a method and system for the near simultaneous
analysis of multiple gases by means of Raman scattering wherein the Raman
scattering sample is placed within the laser resonator and a single
detector is utilized for quantitating the Raman signals of each gas being
analyzed. The invention also relates to a system which does not utilize a
spectrometer or spectrograph but rather employes a series of filters which
reject the elastically scattered laser line while passing the particular
Raman lines of interest. More specifically, this invention relates to a
method and system for the detection of multiple respiratory and anesthesia
gases by Raman scattering wherein the incident laser beam passes through
the gas sample placed in the intracavity of a laser and a rotating filter
wheel, containing filters specific for each Raman scatter line of
interest, is used to transmit light onto a single appropriate detector for
quantitating each specific Raman signal, and thus each gas.
The monitoring of respiratory and anesthetic gases as well as specific
cardiac and pulmonary functions which in turn are based upon the uptake
and production of specific gases has reached a high standard of
technological advancement with the development of sophisticated sensors,
transducers, and computers. These monitoring techniques enable quick
diagnosis and treatment of unfavorable trends in the condition of a
patient and lead to an improved survival rate, early extubation following
surgery and a shorter time in the intensive care unit. Applications of
respiratory and anesthesia gas monitoring include the measurement of
anesthetic uptake, oxygen consumption, and carbon dioxide production These
measurements lead to a more scientific basis for the administration of
anesthesia. A breath-by-breath analysis of multiple respiratory and
anesthesia gases of patients in the operating room, and of respiratory
gases in intensive care and other critical situations can often facilitate
diagnosis and treatment, anticipate and prevent the development of
oncoming problems and otherwise provide instant data for physicians and
other health care personnel to use in therapeutic situations. The same may
be said of the breath-by-breath analysis of gas mixtures used for
noninvasive determination of cardiac output and lung function.
Respiration monitoring of the critically ill patient is now available in
intensive care units. Multiple bed sampling techniques make feasible the
use of an expensive, multiplexed mass spectrometer because it can be
shared among a number of patients. Since the unit is large and not easily
moved from room to room, it is generally placed in a remote location and
lengthy capillary tubes are used to connect the patients. This tube
transport system increases the possibility of gas sample mixing, time
delay, and disconnections and poses inherent limitations for use in
anesthesia, critical care and medical research Mass spectrometry also has
only limited flexibility in the study of gas mixtures. Alternatively,
there are a variety of gas detectors based upon several different physical
principles which, taken together, can measure anesthesia and respiratory
gases. Their problems are: high aggregate cost, bulkiness and poor data
integration into one comprehensive display of patient parameters.
An alternative proposed for use in monitoring several gases in critical
care situations is based on Raman light scattering. The Raman light
scattering effect relies on the interaction of monochromatic light with
the vibrational/rotational modes of molecules to produce scattered light
which is frequency shifted from that of the incident radiation by an
amount corresponding to the vibrational/rotational energies of the
scattering molecules. Since these energies are species-specific, an
analysis of the various frequency components present in the Raman
scattering spectrum provides chemical identification of the gases present
in the scattering volume. The intensity of the various frequency
components or Raman lines provides quantitation of the gases present
providing suitable calibrations have been made. The relative sensitivity
to the different gases remains absolutely fixed, eliminating frequent
calibration requirements.
Raman techniques have been widely used for atmospheric monitoring and for
combustion applications. Sensitivities better than 1 ppm have been
demonstrated. Typical application of Raman scattering analysis coupled
with computer assisted signal processing techniques is reported in Lapp et
al., "Laser Raman Gas Diagnostics", Plenum Press, New York/London, 1974.
Raman scattering analytical techniques are also described in the patent
literature. Chupp, U.S. Pat. No. 3,704,951 teaches laser Raman
spectroscopy utilizing a sampling cell with a multi-pass configuration. A
laser beam enters into the cell configuration of concave mirrors facing
each other such that there is a multiple reflection of the laser beam
between the mirrors to accomplish the required optical power density
enhancement in the sampling area and subsequent signal enhancement. This
device and accompanying technique is limited in that it provides for
analysis through only a single detector. Hence, simultaneous monitoring of
multiple gases is not possible. Moreover, this device is intended for use
primarily with liquids and has only limited application for gases. Also,
the alignment of the mirrors for optimal signal is exceeding delicate.
Finally, the beam size in the sampling region must be quite small to
maintain low sample volume and subsequently high signal response time. A
multimirror approach makes this difficult, if not impossible, given the
optics of such a system.
Hatzenbuhler, U.S. Pat. No. 3,807,862 also teaches a specific application
of Raman spectroscopy in which a fluid sample is subjected to a laser beam
and only a single Raman line is evaluated. In other words, there is no
teaching of a technique for the determination of multiple gases.
Leonard, U.S Pat. No. 3,723,007 is drawn to a method for the remote sensing
of gas concentrations through use of a high-energy pulsed laser and a
mirror telescope, using a grid polychromator. This system requires a laser
output in the 10 kW range and is unsuitable for general application.
Moreover, the use of an expensive spectrometer presents an obstacle in the
way of cost-beneficial production of the device.
A more recent and effective system for the simultaneous detection of
multiple gases is taught in Albrecht, et al., German Pat. No. DE 27 23 939
C2. This patent also utilizes a multi-pass cell to constrain the laser
radiation in a region between two concave mirrors for signal enchancement
but utilizes an unpolarized laser beam to provide a 360.degree. monitoring
geometry for the Raman scattered light. A series of six detectors, each
accompanied by an interference filter comprised of one broad-band and one
gas-specific filter, are provided to collect six separate Raman lines for
the simultaneous monitoring of six different gas components. This method,
while monitoring multiple gases simultaneously, requires six separate
detectors including separate photomultiplier tubes and recording
instruments. Such a complex system is bulky and expensive. Moreover, since
the orientation of the six detectors described in the German patent could
not be expected to exactly image in the same area, the acquisition of all
gas concentrations could not be from exactly the same point in the gas
flow stream.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system and method for
the near simultaneous monitoring of multiple gases by means of Raman
scattering through the use of an intracavity sampling cell and a single
detector receiving Raman line spectra from a rotating filter wheel
containing multiple filters through which Raman scattered light passes
wherein each filter is specific to the gas species being detected.
It is also an object of this invention to utilize a gas sampling cell
located within the resonance cavity of the laser to enhance the laser
power wherein the sampling volume is small and continuous.
Another object of the present invention is to provide a system for
continuous analysis and quantitation of multiple gases through Raman
scattering by means of a combination of an intracavity gas sampling cell
and a spherical reflector to enhance laser power and a rotating filter
wheel located to receive and serially pass species specific Raman line
signals to a single detector which serially quantitates each gas to be
detected.
These and other objects are made possible by means of a Raman scattering
system and method of utilizing that system. The system comprises a laser
source containing a gas sampling cell located within the resonance cavity
of the laser for enhancing the Raman scattering signal. A reflection
mirror is located adjacent to the cell and normal to the laser beam to
capture some proportion of the Raman scattered light solid angle and
direct it towards the collection and detection portions of the system. The
collected Raman scattered light is directed successively onto and through
a collection lens, or a series of antireflection coated collection lenses,
and a laser line rejection filter to attenuate the elastically scattered
laser line and transmit the inelastically scattered Raman lines arising
from the gas molecules being sampled. The Raman lines are then serially
detected by means of a rotating filter wheel containing a series of narrow
band interference filters. Each filter is chosen to pass along one Raman
scatter line corresponding to a discrete molecular species.
A lens, or a series of antireflection coated lenses, behind the filter
wheel is used to focus the transmitted light onto a detector which both
converts the light to an electronic signal and amplifies it by a factor of
10.sup.5 -10.sup.6. Typical of amplification and detection means is a
photomultiplier tube (PMT) used in conjunction with photon counting or
photocurrent electronics, or a variety of solid state photodetectors such
as but not limited to avalanche photodiodes. Thus, as the filter wheel
rotates from filter to filter, various gases are sensed and their
corresponding concentrations determined after suitable calibrations have
been made. With each rotation of the wheel a value for each gas can be
visually displayed or printed out. Hence, the determination of each gas,
while serial, is substantially simultaneous and instantaneous because the
wheel is rapidly rotating, i.e. several hundred revolutions per minute
(rpm).
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of one embodiment of the invention
showing an intracavity gas sampling cell, Raman light collection and
filtering means and a detection system.
FIG. 2 is a schematic diagram showing one complete embodiment of a laser
Raman scattering sampling and detection system.
FIG. 3 is a perspective view of an intracavity gas sampling cell as
utilized in the system shown in FIG. 2 with side windows removed.
DETAILED DESCRIPTION OF THE INVENTION
There is shown in FIGS. 1-3 a complete and preferred embodiment of the
invention.
The systems broadly consists of a laser 10, to be described in detail in
FIG. 2, which directs a polarized laser beam, such as a cw Ar+ laser (tens
of milliwatts extracavity power at 488 nm) through a focusing lens 11 into
a sampling gas cell 12 containing the gases to be analyzed all located
within the resonating cavity of the laser. Alternatively, focusing lens 11
is not necessarily required should the diameter of the laser beam be
sufficiently small to traverse the gas cell and the small size of a
focused beam not need to be imaged onto the entrance slit of a
monochromator.
There are at least two advantages to be gained by placing the sample within
the laser resonator. The intracavity laser power is immediately higher
than the extracavity laser power by a factor of [1+R]/T, where R and T are
the reflectivity and transmission of the laser output mirror 13. The other
advantage is obtained by increasing the reflectivity of the output mirror
13 to further enhance the intracavity power. An intracavity laser Raman
spectrometer, somewhat different from that disclosed in this
specification, is described by Hercher,et al., "Applied Spectroscopy",
Vol. 32, No. 3, (1978) pp. 298-301.
The power of the intracavity laser beam interacting with the gas molecules
can thus be enhanced by a factor up to about 100 within the intracavity
gas cell to provide the necessary high excitation intensity within the
scattering volume at the center of the cell 12. The Raman scattered light,
which is emitted nonisotropically in all directions, is then collected
over as large a solid angle as possible by collection lens 14 located
perpendicular to the axis of the cylinder formed by the incident laser
light. Collection lens 14 may actually consist of a series of optimally
configured lens elements all antireflection coated. One example would be a
fast (f/1.2) camera lens. A reflection mirror 15 captures and redirects a
proportion of the Raman scattered light back into the collections lens 14
and serves to increase the collection of Raman scattered light by a factor
of about 2. The reflection mirror is oriented perpendicularly to the axis
of the laser beam and at a distance from it equal to its radius of
curvature. Such a mirror may be either spherical or cylindrical in shape.
A particular advantage of a preferred embodiment of this invention is that
it is not absolutely necessary to focus the laser beam to a narrow waist
inside the gas cell as in prior art applications. The reason for this is
that a grating spectrometer having a narrow entrance slit is not utilized.
Thus, the optical system need not image a small beam waist upon a narrow
entrance slit. Therefore, focusing lens 11 as found in FIG. 1 is not
necessary in this embodiment. Lens 14 and reflection mirror 15 function
optimally to collect light from a point in the laser beam assuming lens 14
is a spherical lens. Light collection from a specified point with good
stray light rejection, if desired, may be facilitated by use of iris
diaphram 9 optimally placed in the gas cell 12 to reject light from all
other areas of the laser beam.
The collection lens must be properly aligned with respect to the laser beam
as will be better described in conjunction with FIG. 2. Elastically and
inelastically scattered light collected by lens 14 is directed to one or
more serially oriented high rejection laser line filter(s) 16. Filter(s)
16 greatly attenuates the elastically scattered laser line to minimize
interference in the Raman or inelastically scattered light and transmits
the inelastically scattered Raman lines arising from the incident laser
beam interacting with the sample gas and corresponding to the
vibrational/rotional energies of the scattering molecules.
When focusing lens 11 is eliminated and iris diaphram 9 is used to
facilitate light collection from a point in the beam all light rays
exiting collection lens 14 travel parallel to each other and intercept
line rejection filter(s) 16 and interference filter 17 perpendicularly.
The result of this configuration is excellent rejection of the elastically
scattered laser light and good transmission of the Raman signal with a
subsequently good signal to stray light ratio, but with relatively low
signal.
A particularly preferred approach is to eliminate both focusing lens 11 and
iris diaphram 9 of FIG. 1. In this mode spherical collection lens 14
serves to collect signal from the entire laser beam in gas cell 12. In
practical application collection is from a beam about 1 mm. in diameter
and 15 mm. in length. The result is that the spherical collection lens 14
and reflection mirror 15, if spherical, collect much more light (both
elastic and inelastic) so the signal is much higher. However, they do not
function optimally in this mode because some of the collected light hits
filters 16 and 17 nonnormally resulting in high stray light signal
transmitted to the detector system so that the signal to stray light ratio
is relatively high. One possible alternative to use of a spherical
collection lens 14 would be a clyindrical collection lens in the same
position having an identical focal length and "f" number. Such a lens
would more efficiently collect the laser line and produce a collimated
output for the line rejection filter(s) 16. The same may be said for an
alternative to a spherical mirror 15. That is, it may be a cylindrical
mirror which would more efficiently redirect a solid angle of the complete
length of the laser beam sampled rather than a point.
Since each polyatomic gaseous molecule causes a frequency shift from that
of the incident radiation by an amount corresponding to the
vibrational/rotational energies of that molecule and is species specific,
an analysis of the frequency components present in the Raman scattered
light provides identification and quantification of the gases present in
the scattering volume. Quantification is determined from measured Raman
signal intensities using calibration reference gases and known Raman
scattering cross sections.
The expected Raman signal intensity, I , in photoelectron pulses/second is
given by Equation 1 as follows:
I=2P/.epsilon..rho..sigma.QXS.sub.o T.sub.o T.sub.f T.sub.L T.sub.IF
where p=is the power in the analysis region(assume 2 watts)
.epsilon.=excitation photon energy (@488nm, .epsilon.=4.1.times.10.sup.-19
J)
.rho.=molecular concentration(=2.49.times.10.sup.19 molecules N.sub.2
/cm.sup.2 at STP)
.SIGMA.=Raman scattering cross section for N.sub.2 (5.4.times.10.sup.-31
cm/sr)
Q=PTM quantum efficienty (4%)
X=gas scattering cross section relative to N.sub.2 (1)
S.sub.o =collection optics solid angle (0.27 sr)
T.sub.o =collection optics transmission (0.9)
T.sub.c =focusing optics transmission (0.9)
T.sub.l =laser line filter transmission (0.7)
T.sub.n =interference filter transmission (0.7)
Substituting these values in Equation 1 yields 562,300 photoelectron
pulses/second or counts/second. Assuming the PMT photocathode is 4%
efficient in converting photons to photoelectrons and that the overall
tube gain is 3.8.times.10.sup.6 then each photon induced PMT photoelectron
pulse carries 152,000 electrons which flow as a photocurrent. In the above
example of pure N.sub.2 gas at STP conditions there is a current of
8.547.times.10.sup.10 electrons/second. Given values of 96,500
coulombs/second for the Faraday constant and 6.times.10.sup.23
electrons/mole for Avogadros Number there is obtained a value of
14.times.10.sup.-9 coulombs/second or 14 nanoamps of photocurrent.
In order to quantify each gas species, a filter wheel (described in detail
in conjunction with FIG. 2) containing a series of narrow band
interference filters, is located just behind filter(s) 16. As shown in
FIG. 1 this is represented by a specific interference filter 17 which is
chosen to pass alone one Raman scatter line corresponding to a discrete
molecular species.
Since primary interest on this invention is focused on respiratory and
anesthesia gases the following table illustrates the Raman Stokes
frequency shifts and relative scattering cross sections for those gases of
interest.
TABLE I
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Frequency Relative Scattering
Gas Species Shift (CM.sup.-1)
Cross-Section
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N2 2331 1.0
O2 1555 1.0
CO 2143 0.9
CO2 1285 0.8
1388 1.2
N2O 1285 1.8
2224 0.5
H2O 3652 2.8
Isoforane 772-904 (triplet)
1.8
Enflurane 817 0.5
Halothane 717 1.4
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The Raman scatter light having the selected frequency passes through
intereference filter 17 and is imaged onto a detector by means of a
focusing lens 18. The detector can be any device capable of receiving the
signal and amplifying and processing it into useful data. Represented in
FIG. 1 is a photomultiplier tube (PMT) 19 which is connected to a signal
processing unit 20 which may be a photon counter, a photocurrent amplifier
or other device including a central processing unit or microprocessor
which can further amplify, process and quantitate the Raman signal into
useful analog or digital data which is then displayed on display device
21, e.g., a CRT screen and/or printer. Table I shows that those gases of
interest generally have scattering cross sections >1, that water vapor
does not interfere with the signals and that there is rarely any spectral
overlap between the Raman spectra provided that narrow (1 nm full width
half maximum) band pass interference filters with high (>1,000) out of
band rejection are used.
A preferred embodiment of the invention is illustrated in FIGS. 2 and 3.
The components schematically illustrated in FIG. 1 will be identified by
the same numerals in these figures.
The laser source 10 comprises a power supply 22 connected by lines 23 and
24 to a cathode 25 and anode 26 which surround the ends of and activate a
plasma tube 27 containing argon gas. A radiator 8 and fan system (not
shown)thermally connects and surrounds the plasma tube for cooling
purposes. One end of the laser's resonant cavity, adjacent to the plasma
tube 27, is defined by a high relefectivity mirror 28 and a prism 29 for
wavelength selection. At the end of plasma tube 27 is a Brewster window
30. A dust tight sleeve 31 surrounds the mirror, prism, Brewster window
and end of the plasma tube to protect these components from particulate
and molecular contamination. At the opposite end of the plasma tube is
another Brewster window 32. Brewster windows 30 and 32 cooperate to
transmit the prism selected polarized wavelength of the laser beam being
pumped through the resonating cavity of the laser without substantial
loss.
The remainder of the laser cavity is defined by the gas cell 12 and output
coupler mirror 13. A sleeve 33 connects the plasma tube 27 and Brewster
window 32 with one end of gas cell 12 and another sleeve 34 connects the
output coupler mirror 13 and the other end of the gas cell to prevent dust
or other contamination from entering into the resonance chamber and
subsequently fouling mirrors and windows causing a subsequent attenuation
of intracavity resonance and power.
Mirror 28 is highly reflective. Output coupler mirror 13 also has high
reflectivity for the laser line of interest. Typically it will be about
99% reflective. The extra cavity beam 35 leaving mirror 13 is intercepted
by a beam splitter 36 which directs a small portion of the beam to a
photodiode 37 which is serially connected to laser power supply 22 by
signal line 38 and to one channel of a 2 channel electronic
amplifier-filter combination 63 by signal line 40. The remaining portion
of the beam 35 enters an optical extinguisher (not shown). Via diode 37,
optical feedback to the power supply 22 provides correction for long term
(typically seconds or greater) fluctuations in laser power. Photo diode 37
feedback to the two channel amplifier-filter 63 provides correction for
high frequency fluctuations in the optical output power of the laser (
typically 1 millisecond to 1 second variations).
Since the resonance cavity of the laser is located between mirrors 28 and
13 the placement of gas cell 12 between the end of plasma tube 27 and
mirror 13 is termed "intracavity".
One embodiment of a gas sampling cell 12 is shown in detail in FIG. 3 and
consists of a framework having a hollow interior, means for bringing a gas
sample into and out of the interior and windows through which both the
incident laser beam and the scattered Raman light may pass. The shape,
i.e. cylindrical, rectangular, etc., is not important as long as the
functional criteria are met. The cell shown consists of a hollow housing
41 generally cylindrical in shape having its axis oriented to accomodate
the passage of the laser beam through the axial center. At either end of
the housing are optical windows 42 and 43. Windows 42 and 43 are coated
with a highly efficient narrow band antireflection coating, i.e. a
"V"-coating, for the particular wavelength of the laser. "V"-coatings are
multilayer antireflection coatings which reduce the reflectance of an
optical component to near-zero for one very specific wavelength and are
generally intended for use at normal or near normal incidence. Hence,
windows 42 and 43 are parallel to each other and substantially normal to
the axis of the housing and the laser beam. Maximun intracavity power
power is achieved if windows 42 and 43 are slightly tilted to be
non-normal to the laser. By slightly tilted is meant that the windows do
not vary more than about 10.degree. in either direction from a position
perpendicular to the laser beam. Hence the term "substantially normal" is
used. The windows, if tilted, will always be tilted in the same direction
so as to remain parallel to each other. Such coatings will acheive maximum
reflectances of not more than about 0.25% and are generally effective to
allow only about 0.1% reflectance at the specified wavelength. Thus, they
do not appreciably interfere with the transmittances of the laser beam
through the resonating cavity of the laser. The purpose of windows 42 and
43 is two-fold; first, they constrain the sample gas within cell 12 and
thus minimize sample volume and maximize response time, and, second, they
serve to isolate the Brewster window 32 and the output mirror 13 from
possible contamination arising from the gas sample. The hollow housing 41
at the central portion of the cell also contains optical windows 44 and
45. The alignment of these windows is not as critical as the end optical
windows 42 and 43. However, they are preferably parallel to each other.
These optical side windows are also preferably coated with a broad band
antireflection coating. Since these windows must transmit the Raman
scattered light to a detector they must pass the desired wavelengths.
Hence, a high efficiency broad band antireflection coating such as
HEBBAR(tm) is appropriate. V Band and broad band coatings are multilayer
dielectric films, comprising alternate layers of various refractive index
transparent materials, combined in such a way to reduce the overall
reflectance to an extremely low level for the spectral range covered. Over
the broad band range the reflectance will not generally exceed 1.0% and
will generally be below 0.6%. The cell contains an inlet 46 and an outlet
47 for passing the sample gas through the cell. The cell design is very
important in that it allows for a very small volume of gas, typically
between about 0.1 and 1.0 cubic centimeters, to constantly be passed
through the laser beam. On the other hand, it is well adapted for use in a
batch type operation in that only a small sample is required of any given
gas to be analyzed. The inlet 46 is connected via supply line 48 to a
solenoid valve 49 and sample gas is drawn into the cell interior by means
of an air pump 50. Depending upon the position of the solenoid, room air
may be drawn via line 51 into the cell for system calibration against
nitrogen gas. Alternatively, a respiratory gas, or other sample gas, may
be drawn via line 52 from the airway of a patient or other sample source
by air pump 50. Tubing 53 connected to cell outlet 47 conveys sampled gas
out of the cell for disposal or reintroduction into a patient's airway or
for collection and storage.
The gas cell 12 also becomes part of the Raman scattered light collection
system and is located within collection housing 54 as shown in FIG. 2. The
interior of collection housing 54 comprises gas cell 12 oriented with its
axis parallel and coincidental with the intracavity laser beam, a
spherical collection mirror 15, and, if used, an iris diaphram 9. Housing
54 is tubular and is oriented with its axis perpendicular to the laser
beam. Housing 54 telescopes over interface tube 55 via a close frictional
fit. Interface tube 55 is rigidly affixed to vertical translation stage
56. Collection housing 54, interface tube 55 and translation stage 56
function together to provide necessary alignment of gas cell 12 to both
the intracavity laser beam and the collection lens 14 for optimal signal.
It is important that the gas cell be properly aligned and fixed in position
to attain the optimal signal. Therefore, some adjustments are made and
components fixed securely in place at the time of ins | | |
