Note: Descriptions are shown in the official language in which they were submitted.
1058~17
CONTINUO~S WAVE GENER~TION OF COHERENT VIBRATIONAL ANTI-STOKES SPECTRA
Background of the Invention
This invention relates to the field of spectroscopy and
more particularly to a method and apparatus in which two coherent,
continuous beams of monochromatic light are transmitted through
; a sample of material at a frequency differential correlated with
the vibrational frequency of a constituent to detect and quan-
titatively measure the constituent.
Description of the Prior Art
In prior art apparatus used for spectroscopic material
analysis, scattere~ light produced by exciting quanta from a pulsed
radiation source at a frequency differential close to the vibrational
frequency of the material is directed through a filtering mechanism
adapted to selectively transmit an anti-Stokes component generated
coherently during scattering. The output of the filtering mechanism
is converted to a detectable signal and displayed.
i One of the major problems with such apparatus is the
difficulty of analyzing mixtures of materials to measure quanti-
tatively materials present in minute amounts. The output signal
` from the filtering mechanism is frequently altered or obscured by
background interference resulting from the non-resonant suscept-
ibility of materials coexistent with the material being analyzed.
The problem is particularly trcublesome when the material being
analyzed is monitored continuously for substantial periods of time.
To alleviate such problems, it has been necessary to provide the
apparatus with costly maintenance standards adapted to prevent
frequency and amplitude instability of the radiation source and to
equip it with highly sensitive forms and combinations of detectors,
pulsed radiation sources, filters, control systems and the like, which
are relatively expensive.
Summary of the Invention
The present invention provides an economical, accurate
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apparatus for spectroscopic material analysis. The apparatus hasradiation source means for generating two coherent, continuous
beams of monochromatic radiation. Such radiation source means
has a tuning means associated therewith for adjusting the fre-
quency difference between said beams of radiation to equal sub-
stantially the vibrational frequency of a preselected constituent
of gaseous material. A projecting means is provided for directing
the beams of radiation through the material to produce scattered
radiation that contains a detectable signal composed of an anti-
Stokes component generated coherently during scattering. A filter-
ing means, adapted to receive the scattered radiation, selectively
transmits the signal to a detecting means, which indicates the
intensity thereof.
Further, the invention provides a method for spectro-
scopically analyzing material comprising the steps of generating
two coherent, continuous beams of monochromatic radiation;
adjusting the frequency difference between the beams of radiation
to equal substantially the vibrational frequency of a preselected
constituent of material; directing the beams of radiation through
the material to produce scattered radiation that contains a detect-
able signal composed of an anti-Stokes component generated coherently
during scattering; filtering the scattered radiation to selec-
~- tively transmit the detectable signal; and indicating the inten-
sity of the signal.
Several known tuning means may be adapted for use with
the above apparatus. Preferably, the tuning means comprises a
pair of high resolution diffraction gratings adjusted to transmit
the two monochromatic light beams at a frequency differential
correlated with the vibrational frequency of a species of the
material. This condition is obtained when
2~ 2 = ~3 and ~ 2 = ~3 ~
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where ~l and ~2 represent, respectively, the frequencies of the
two coherent, continuous beams of monochromatic radiation, ~3 is
the frequency of the coherently generated anti-Stokes component and
~ V is the vibrational frequency of the molecular species. For
a given species, the vibrational spectra exist at a unique set
of frequencies. Each of these spectra can be resonantly enhanced
to produce an anti-Stokes vibrational component of significantly
increased intensity. Identification of the species having a
particular set of vibrational spectra is made positively when
resonant enhancement is detected for an anti-Stokes component
corresponding to at least one vibrational spectral component of
the species.
The frequency and amplitude stabilities of a con-
tinuous wave radiation source are greater than those of pulsed
radiation sources. Hence, a closer correlation exists between
the quantity of material being analyzed and the intensity of
vibrational anti-Stokes components produced by continuous wave
generation. Due to the superior stability of a continuous wave
radiation source, signal collection efficiencies are far greater
than ordinarily expected for the low input power employed there-
by. Accordingly, the accuracy and reliability of the apparatus
are far greater than that obtained by apparatus wherein the
exciting quanta are produced by a pulsed radiation source.
Brief Description of the Drawings
The invention will more fully understood and further
advantages will become apparent when reference is made to the
following detailed description of the preferred embodiments of
the invention and the accompanying drawings in which:
Figure 1 is a block diagram showing apparatus for
spectroscopic gas analysis;
Figure 2 is a schematic diagram of the apparatus of
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Figure 1.
Figure 3 is a schematic diagram showing an alternate
embodiment of the apparatus of Figure 1.
Description of_the Preferred_Embodiments
Radiation carrying vibrational spectra are found in each
of the visible, infrared and ultraviolet frequency regions. As
a consequence, the invention will function with radiation having
a relatively wide range of frequencies. For illustrative pur-
poses, the invention is described in connection with method and
apparatus for measuring vibrational spectra of gaseous material
scattered by radiation from the visible frequency region. When
applied in this manner, the invention is particularly suited to
detect and to measure quantitatively minor constituents of a
gaseous material such as air. It will be readily appreciated
that the invention can be practiced using radiation from any of the
foregoing frequency regions, and that it can be employed for similar
and yet diversified uses, such as the analysis of vibrational
~ spectra of liquids and solids, the determination of molecular gas
-; constants and the like.
Referring to Figure 1 of the drawings, there is shown
- preferred apparatus for spectroscopic gas analysis. The apparatus,
shown generally at 10, has radiation source means 12 for generating
two coherent, continuous beams 15, 17 of monochromatic radiation. The
radiation source means 12 has associated therewith a tuning means 14
for adjusting the frequency difference between the beams of radiation
to equal substantially the vibrational frequency of a preselected
constituent of material. A projecting means 16 is pro-
vided for directing the beams of radiation 15, 17 through gaseous
material in compartment 18 to produce scattered radiation 20
that contains a detectable signal 22 composed of an anti-Stokes
component generated coherently during scattering. A filtering
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means 23 is adapted to receive the scattered radiation from
compartment 18. The filtering means selectively separates the
signal 22 from the scattered radiation and transmits the signal 22
to a detecting means 24 which indicates the intensity thereof.
More specifically, as shown in Figure 2, the radiation
source means 12 can comprise a dye laser shown generally at 24
adapted to be excited by energy from continuous wave laser 26,
which may be a continuous wave krypton laser, a continuous wave
argon-ion laser, ruby laser or the like. Such dye laser 24 includes
(1) a cell 28 containing dye material and (2) a laser cavity comprised
of a partially transmitting output mirror 30 and an optical element
32 for generating laser radiation. In addition, the dye laser
24 can include a lens 25 and mirror 27 for directing a continuous
.~ wave of radiation 33 into cell 28. The dye materials which are
suitable for use in the dye laser 24 are any of those convention-
ally employed which, when excited, emit light having frequencies
in the transparency range of the gaseous material being analyzed.
Typical dye materials include Rhodamine 6G, Kiton Red, Cresyl
Violet, Nile Blue and the like.
Radiation emitted from the dye material in dye cell 28
is continuously tunable over a wide frequency range. A tuning
means 14 associated with the dye cavity 24 separates the radia-
tion into a pair of coherent beams of monochromatic radiation
~ 2~ which are transmitted from the radiation source means
12 via output mirror 30. Generation of the detectable signal 22 is
most efficient when the radiation emitted from the dye laser 24
has a line width and frequency stability about equal to or less
than the line width of the vibrational spectra of the material
appointed for detection.
The tuning means can include a varying number of optical
components assembled in a variety of combinations. In one em-
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bodiment of the apparatus 10, the tuning means 14 comprises a
beam splitting means 34 for separating radiation from dye cell 28
into a pair of radiation beams, ~ 2 and a pair of diffraction
gratings 36, 38 mounted in autocollimation. The two diffraction
gratings 36, 38 function in the manner of a conventional back
mirror and, in addition, restrict the frequency range of the radia-
tion beams so as to produce within cavity 24 a pair of coherent
beams of monochromatic radiation having narrow line widths. A beam
expanding telescope 29 can, optionally, be disposed in series with
and between dye cell 28 and beam splitting means 34 for enlarging
the width of the beams and improving the efficiency of the gratings.
The tuning means 14 can additionally comprise a pair of etalons
40, 42, disposed in series with and between the beam splitting
means 34 and diffraction gratings 36, 38, for further restricting
the frequency of the radiation beams. Diffraction gratings 36, 38
are connected through shaft encoded stepping motors 44, 46 to a
control means 48 adapted to vary the rotational velocity of stepping
motor 44 relative to the rotational velocity of stepping motor 46.
Radiation beams ~ 2 are tuned by rotating the diffraction grat-
ings 36, 38 corresponding thereto so that the frequency difference
therebetween equals substantially the vibrational frequency oE a
preselected constituent of gaseous material.
The control means 48 is preferably adjusted so that the
frequency scanning rate of diffraction grating 36 is twlce that of
diffraction grating 38. This adjustment of the control means 48
permits generation of a detectable signal 22 having a substan-
tially constant frequency. A single narrow band pass filter 56
can thus be used to reject unwanted radiation produced during
scattering and selectively transmit the detectable signal 22.
A projecting means comprising mirror 50 is associated
with the dye laser 24. The projecting means introduces the two
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coherent, continuous beams of monochromatic radiation 15, 17
into gaseous material in sample compartment 52 in one direction,
which will be considered to be substantially vertical for con-
venience in referencing directions but may, of course, be in
any direction desired. Raman scattered radiation 20 from the
gaseous material in sample compartment 52 is transmitted via
mirror 54 to the filtering means 23.
Several known filtering means may be used with the
apparatus 10. Preferably, the filtering means 23 is a narrow
band pass interference filter 56 adapted to receive the scattered
light 20 from sample 52. In addition, the filtering means includes
i a lens 60 and a pinhole stop 58 which cooperate to effect
separation of the detectable signal 22 from the scattered radiation
20. The latter includes radiation beams 15 and 17, together
with an anti-Stokes beam which is generated coherently during
scattering. Interference filter 56 is constructed to transmit
radiation within a narrow frequency range centered at the fre-
quency of the anti-Stokes signal 22.
Before describing how the apparatus of Figure 2 can be
used to determine the intensity of signal 22, it would be help-
ful to explain the principles underlying generation of coherent,
continùous vibrational-anti-Stokes spectra.
When two light beams at ~ 1 and ~2 are incident on
a non-linear material, coherent emission at 2 ~1- W2 is gen-
erated through the third-order nonlinear polarization. The third-
order nonlinear susceptibility X (3) associated with this polar-
ization is responsible for the emission. X (3) is composed of
two basic parts, Xnr~(3) a nonresonant part that gives rise
to constant background signal and a resonant part Xr ( ) that
contains resonant denominators that show large enhancement at
1 ~2 en ~1 ~2 = ~v and when ~1 or ~3 apprOach
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an electronic resonance in the material (similar to the resonance
Raman effect). At the peak of the Raman resonance, (3) , which
is normally a sum of real and complex parts, reduces to the com-
plex component that is related to the differential Raman cross
section by the following equation
X"R= ~ ( ~ )
where rR is the normal Raman line width (h~hm ) and ~5/dQ
is the usual spontaneous Raman differential cross section.
The conversion efficiency to the anti-Stokes is given
by the equation.
~ n4 IDX I ZCOh2 ( 1)
where ~ is the refractive index; ~ is the molecular number
density, Zcoh is the coherence length or the distance over
which colinear beams slip out of phase by ~ radians.
The detectable signal 22 from interference filter 56
is focused in the plane of pinhole stop 58 by a lens 60. Lens
60 is adjusted so that the center of the signal 22 is positioned
on the pinhole 62. The intensity of the portion of signal 22
passing through the pinhole 62 is detected by a photomultiplier
64. The output of the filtering means 23, representing signal 22,
; is displayed by an indicating and recording means 66, which can
comprise an oscilloscope and a chart recorder.
The material which can be analyzed by the method and
apparatus of the present invention comprise any gas, liquid or
solid that is transparent to radiation frequencies over some
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portion of the infrared, visible or ultraviolet frequency range.
Virtually all gases and numerous liquids and solids are trans-
parent to such radiation frequencies. Typical liquids exhibiting
the required transparency to such radiation frequencies comprise:
carbon disulfide, benzene, water, alcohol, carbon tetrachloride,
trimethylene chloride, cineole, hexahydrophenal, decahydroaphthalene,
isoamylphthalate, pentachloroethane, trimethylene bromide, chloroben-
zene, nitrotoluene, aniline, bromoform, methylene iodide, gasoline,
kerosine, vegetable oils, wine, soda and alcoholic beverages, blood
plasma, urine and the like. Representative solids which exhibit
the required transparency to such radiation frequencies comprise
ammonium di-hydrogen phosphate, potassium di-hydrogen phosphate,
borosilicate glass, quartz, fused silica, gallium phosphide,
~ calcium aluminate glass, calcite, rutile, sapphire, strontium
; titanate, lead sulfide, magnesium fluoride, lithium fluoride,
: calcium fluoride, arsenic trisulfide glass, indium phosphate,
gallium arsenic, silicon, sodium fluoride, cadmium sulfide,
cadmium telluride, selenium, germanium, sodium chloride, silver
chloride, potassium chloride, potassium bromide, diamond and the
like.
Coherent, continuous wave (cw), anti-Stokes Raman
scattering has been observed using a fixed frequency pump beam
(at an argon laser wavelength 514.53 nm) and a tunable frequency
Stokes beam (provided by a cw dye laser at 605.41 nm) focused in
a cell containing one atmosphere of methane gas.
The cw dye laser, which was pumped collinearly by an
argon ion laser, consisted of a folded, six-mirror, astigmatically- -
compensated resonator. The total length of the cw dye laser
optical cavity was 1.8 m. A free-flowing jet stream of rhodamine
6G dye in ethylene glycol was pumped with the 514.53 nm argon
laser line. The 514.53 nm beam entered the dye laser cavity
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through a multilayer dielectric mirror which had high
tramsmissivity over the range of 470-530 nm and high re-
flectively over the range of 560-650 nm. A pair of 30-cm-
radius mirrors in the dye laser cavity was used to produce
a focus in a gas cell with Brewster angle windows containing
one atmosphere of methane. All of the mirrors of the dye
laser cavity had high reflectively over the range 560-650 nm.
The above experiment utilized the 1 symmetric
vibrational mode in methane at 2916.7 cm . When the dye
laser was tuned to 605.41 nm, coherent anti-Stokes radiation
at 447.37 nm was generated and transmitted by the second 30-cm-
radius mirror which had high transmissivity at 447.37 nm and high
reflectively at 514.53 nm and 605.41 nm. It was filtered using
a narrow-band-pass interference filter centered at 447.37 nm and
a Corning glass 5-58 filter. The detector was a cooled RCA
8850 photomultiplier tube operated in the pulse counting mode.
Wavelength selection in the dye laser cavity was accom-
plished by use of a prism for coarse tuning and a thin solid etalon
for fine tuning. The prism-etalon combination produced a single-
frequency dye laser line with a width of about 0.1 cm . Fine
tuning of the dye laser wavelength in the vicinity of 605.4 nm
was accomplished by rotation of the solid etalon. A l-m focal-
length Czerny-Turner spectrometer was set to monitor the Stokes
signal at 605.41 nm.
The power measured in the 514.53 nm pump beam was 0.46 W
and the effective power in the Stokes beam was calculated to
-~ be about 36 mW. Using equations (1-3), the power of the coherent
anti-Stokes signal produced by scattering from the ~1 vibrational
mode in methane was calculated to be 2.18 x 10 W corresponding
to 4.90 x 10 photons/sec. which is in good agreement with the
experimentally observed value of 5.72 x 10 photons/sec.
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The apparatus 10 which has been described herein can,
of course, be modified in numerous ways without departing from
the scope of the invention. For example, the filtering means 23
can comprise the combination of fixed etalon tuned by controlling
the temperature thereof and a narrow band pass interference filter
having its pass band centered at the frequency of the anti-Stokes
signal 22. One type of fixed etalon which is suitable is comprised
of optically transparent material, such as fused silica, having
opposed surfaces which are polished, flat, parallel and coated
with silver, dielectric material or the like for high reflectivity
at a preselected frequency region. The thickness of the etalon
used in the filtering means 23 can be chosen so that the spectral
range of the etalon is equal to or greater than the full width
of half transmission points of the narrow band pass interference
filter. Fine tuning of the solid etalon used in the filtering means
is affected by providing means for controlling the temperature, and
hence the optical path length, thereof so as to cause the trans-
mission peak for an order to be centered at the frequency of the
anti-Stokes component of signal 22. Such solid etalon preferably
has a finesse chosen so that the f~111 width at half transmission
points thereof is substantially equal to the spectral width of the
anti-Stokes signal 22. The tuning means can be comprised of a
single diffraction grating adapted to generate first and second
beams ~ 2 of monochromatic radiation, the second beam
~1 being derived from the second order of the grating and
having its frequency tuned at twice the rate of the first beam.
An acousto-optic modulator can be disposed in series with and
between telescope 29 and diffraction grating 38 to effect
electronic generation of the radiation beams ~ 2.
As shown in Figure 3, dye laser 24 may comprise a
plurality of cells 28, 28' adapted to be excited by energy from
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continuous wave laser 26, the first cell 28 being constructed
in the aforementioned manner and the second cell 28' being com-
prised of a partially transmitting output mirror 30' and an
optical element 32' including grating 38', for generating laser
radiation. A beam expanding telescope 29' can, optionally, be
disposed in series with and between the dye cell 28' and etalon
40' for improving the efficiency of grating 38'. Radiation from
continuous wave laser 26 is directed through dye material in
dye cells 28, 28' by beam splitting means 35. Each of the cells
28, 28' can be provided with a dye material which, when
excited, emits radiation having frequencies within the trans-
parency range of the material being analyzed, the dye material
of the second cell 28' being further adapted to emit fre-
quencies which overlap vibrational stokes spectra produced when
the material being analyzed is scattered with the frequencies
emitted from dye cell 28. Radiation from dye cell 28' and
~;~ partially transmitting output mirror 30' is directed by mirror
69 to beam splitting means 70 and, optionally, a calibrating
means as hereinafter described.
A calibrating means shown generally at 68 including
beam splitting means 70, reference gas cell 72 and detecting
and recording means 74 can, optionally, be associated with
the apparatus 10 for providing a reference anti-Stokes signal
76 derived from a reference material of the type being analyzed.
The beam splitting means 70 is adapted to direct a portion of
radiation beams 15, 17 through the reference material, which is
contained in cell 72. Scattered radiation produced in refer-
ence material cell 72 is processed by detecting means 74, which is
constructed and operated in the same manner as detecting means
` 30 24. The output of the detecting means 74 represents the
magnitude of the reference anti-Stokes signal 76 for a known
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concentration of reference material. Such output signal can becompared with the output signal of detecting means 24 to
determine the concentration of material in sample compartment 52.
Other similar modifications can be made which fall within
the scope of the present invention. It is, accordingly, intended
that all matter contained in the above description and shown in
the accompanying drawings be interpreted as illustrative and not
in a limiting sense.
In operation of the preferred apparatus, radiation source
10 means 12 generates two coherent, continuous beams 15, 17 of mono-
chromatic radiation. The frequency difference between the radiation
beams 15, 17 is adjusted by tuning means 14 to equal substantially
the vibrational frequency of a preselected constituent of material.
Projecting means 16 directs the radiation beams 15, 17 through the
material to produce scattered radiation 20 containing a detectable
signal 22 composed of an anti-Stokes component generated coherently
during scattering. A filtering means 23 receives the scattered
radiation 20 and selectively separates the signal 22 therefrom.
The resultant signal 22 from the filtering means 23 is displayed by
the indicating and recording means 66.
Having thus described the invention in rather full
detail, it will be understood that these details need not be
strictly adhered to but that various changes and modifications
may suggest themselves to one skilled in the art, all falling
within the scope of the present invention as defined by the
subjoined claims.
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