1. science
2. physics
3. optics

Spatially resolved multispecies and temperature
analysis in hydrogen flames
Wolfgang Reckers, Lutz H11wel,Gerd GrOnefeld, and Peter Andresen
We report on spatially resolved simultaneous measurements of temperature and majority species
concentrations along a line segment in a premixed laminar H2-air flame. The results are obtained from
Raman and Rayleigh scattering by using a narrow-band KrF excimer laser and a spectrally and spatially
resolving detector system that consists of a high-throughput spectrometer and a gated, intensified,
two-dimensional CCD camera. The data presented here are integrated over 100 laser shots. Absolute
density profiles of N2, 02, H 20, and H 2, as well as temperature profiles at various heights through the
flame, are presented. A discussion of the required calibration procedures and a summary of the
necessary spectroscopicbackground are also included in this paper.
Introduction
During the past decade or so it has been shown by the
combined efforts of many research groups that laserbased diagnostic methods can be applied to a large
variety of combustion systems with exceptional success. This success, and the potential for even more
penetrating and more powerful applications, has its
origin in the basic nonintrusiveness, high sensitivity,
and spatial and temporal resolution that can be
achieved with laser methods. A large number of
specialized approaches have been developed and characterized, and are being applied in many different
situations. Scattering methods such as Rayleigh,
spontaneous, and coherent Raman (such as coherent
anti-Stokes Raman spectroscopy), as well as absorption and fluorescence techniques have been shown to
give useful information. In particular, there has
been much progress in obtaining several independent
data simultaneously, e.g., one- or two-dimensional
detection of specific combustion constituents
or local
multispecies density measurements. In the first case,
variants of laser-induced fluorescence (LIF) techniques have proven to be particularly useful. For
example, two-dimensional probing by LIF'- 3 (soW. Reckers and P. Andresen are with Max-Planck-Institut fir
Str6mungsforschung,
G6ttingen,
Germany;
P. Andresen is also
with the Fakultat fir Physik, Universitat Bielefeld, Bielefeld,
Germany.
L. Htiwel is with the Department
of Physics, Wesleyan
University, Middletown, Connecticut; G. Griinefeld is with the
Laser Laboratorium G6ttingen, e.V., Gottingen, Germany.
Received 1 July 1992.
0003-6935/93/060907-12$05.00/0.
©
1993 Optical Society of America.
called planar LIF) and its quench-free counterpart of
LIF by predissociative states 4 5 have been developed,
and merits and limitations of their applications have
been described. Besides LIF,6 Raman scattering7 8
has also been used in the second case. Recently
some progress has been reported in measuring the
one-dimensional distribution of several combustion
species simultaneously.9 Certain advantages and limitations are associated with pointwise, and one- and
two-dimensional measurements in general. In pointwise techniques, several species can be detected simultaneously by means of efficient spectral filtering.
However, spatial resolution and accuracy are usually
limited by interfering radiation from neighboring
areas. In contrast, two-dimensional methods can
yield superior spatial information that is, however, in
general, constrained to a single species. In this
regard, one-dimensional imaging offers a favorable
compromise, combining good spatial resolution, at
least along a line, with the capability of excellent
spectroscopic filtering and thereby the capability of
simultaneously detecting several different species.
Here we describe experiments in which Raman scattering from all majority species in a H2 -air combustion system was quantitatively measured along an
arbitrary line. The analysis of these experiments
yields spatially and spectroscopically well-resolved
information about the number density variation of
these species along the chosen line. Because of the
complex nature of even a relatively simple case such
as stationary H2 -air combustion 0 and because of the
even more challenging demands of other potential
applications an improved understanding of such systems will come only from measurement techniques
20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS
907
that provide simultaneous information on as many
different aspects as possible. In particular, of utmost importance are both the instantaneous and the
time-averaged spatial distribution of the various major (02, N2 , H2 , H2 0) and minor (mainly OH, NO, etc.)
constituents in relation to geometric parameters and
operation conditions of a given combustion device.
The experimental method chosen here permits the
extraction of one-dimensional density maps of all
majority species and relies on recording wavelengthdispersed images of Rayleigh and Raman scattering
off those molecules along the path of a focused
tunable excimer laser. The current work was stimulated, in part, by recent papers on excimer-laserbased Raman scattering in combustion that clearly
demonstrate the strength and some of the problems
of this method.7 8 This paper is organized to give a
somewhat comprehensive review of this new technique, starting with a survey of the theoretical background needed to use Rayleigh and Raman scattering
as combustion diagnostic tools. Next, aspects of the
experimental setup and experimental procedures are
explained. The discussion is then continued with a
section on the relevant spectroscopy, including a
listing of pertinent cross sections. Then a short
description is offered on how to extract the desired
information from the raw data in terms of onedimensional density and temperature maps. In this
context, the importance of proper calibration procedures is stressed, and ways of implementation are
outlined. Finally, examples of analyzed sets of scattering images are presented with the aim of defining
the performance parameters of our particular approach and the method in general.
Rayleigh and Raman Scattering as Combustion Probes
It is well known that spontaneous elastic and inelastic
scattering of light can be used as a potentially powerful probe of combustion systems."1 One ofthe advantages of light scattering over, e.g., fluorescence detection is the reduction in complications that are due to
quenching; since the scattering is instantaneous there
is negligible collisional interaction. Therefore spatially resolved detection of such scattered light carries
direct information about the spatial distribution of
the pertinent molecules. There are, however, several aspects that make it difficult to use Rayleigh or
Raman scattering as combustion diagnostic tools.
Chief among them are the small cross sections for
Rayleigh scattering and even more so for Raman
scattering, a typical value for Raman cross sections
being 10-30 cm2 /sr for N2 at 488 nm. The wellknown v4 scaling of both Rayleigh and Raman scattering suggests the usage of powerful UV lasers. It is
indeed possible to take meaningful Raman spectra
with a single laser pulse.7 Nevertheless, even under
those optimized excitation conditions, data averaging
over more than one laser pulse may be inevitable.
Obviously this makes true instantaneous imaging of
density traces impossible, which may appear to be a
necessity in turbulent systems. However, it has
908
APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993
been demonstrated that averaged measurements, even
in systems with highly turbulent behavior, can supply
useful information.' 2 Another problem associated
with both scattering processes is the possible interference at the fundamental wavelength (Rayleigh) or
shifted wavelength (Raman) by mechanisms other
than the ones of interest. In the case of Raman
scattering, the displacement of the Raman lines is
obviously characteristic for the specific molecule and,
given a sufficiently high-resolution spectrometer, one
can, in principle, discriminate against light emission
or scattering from most other sources. However,
two practical points stand in the way of a straightforward utilization of a Raman-based diagnostic. First,
the intrinsic weakness of Raman scattering, even
when W light is used, suggests the use of highthroughput spectrometers. For a given detection
setup, the throughput can be increased by using wide
entrance slits. Obviously, wide entrance slits imply
poorer resolution with the concomitant possibility of
interference by neighboring spectral features. Second, and this makes the aforementioned problem
more severe, under conditions of W excitation of
Raman signals with excimer lasers their less-thanperfect bandwidth ( 0.5 cm-' or 15 GHz) and residual broadband contribution make it a real challenge
to avoid resonantly exciting some of the many constituent molecules in such a complex environment as
combustion. Furthermore, in ArF-laser-based diagnostics of combustion systems involving hydrocarbons, broadband emission may be present at all
excitation wavelengths.4 Fortunately such broadband interference problems are less significant in the
context of 248-nm probing of H2 combustion. Great
care in the experimental approach and subsequent
data analysis is, nevertheless, required for extracting
the desired information from measured data. In
particular, since resonance fluorescence is usually
several orders of magnitude stronger than Raman
scattering, the intensity in a given Raman line may be
significantly influenced by line emission features
even if they are relatively far away. The practical
usefulness and problems stemming from this interference have been partially addressed. In particular, it
was found 7 that for the KrF laser at 248.623 nm a
minimum in 02 and OH fluorescence excitation exists
that, under certain conditions, permits the recording
of interference-free Raman-scattering signals from
02, N2, H2, and H2 0. These scattering measurements were obtained from a small volume element
that corresponds to a linear spatial resolution of 0.4
mm. We used a two-dimensional camera as a detection device, and we report here an extension of this
approach to simultaneous measurement of these
Raman signals along a line segment of 4 mm in the
focal region of a tightly focused KrF laser. The next
section describes the experimental arrangement.
Experimental Setup and Procedures
In principle, the experimental approach to obtain the
desired spatially resolved information is similar to
those used in related previous experiments from this
group.3 -5" 3 Figure 1 depicts the schematic experimental setup as used in the experiments described
here. A tunable, narrow-band KrF laser (Lambda
Physik, EMG 150) is focused with a 2-m CaF 2 lens
into a premixed H2 -air flame. The laser has a
typical pulse energy of 200 mJ, a pulse duration of
- 15 ns, and a locking efficiency of
95%, i.e., 95% of
the output energy is delivered in a band of 0.2 cm-'
with a selectable center wavelength, whereas the
remaining 5% of the energy is distributed, according
to the gain profile of this type of laser, between
approximately 248 and 249 nm. The output of the
laser is horizontally polarized to the degree to which
it runs narrow band. Since detection of Raman and
Rayleigh scattering is most efficient in a direction
perpendicular to the laser E vector (and because, for
ease of use, the detection axis is horizontal) we also
rotate the laser polarization by 900 with an appropriate X/2 plate. At the focal point of the focused laser
beam a laminar premixed H 2 -air flame is generated
with a small burner mounted on an x-y-z-translation
stage. This burner has a nozzle with an exit diameter of
2.5 mm.
Gas flows are metered for H 2 and
synthetic air (N2 :0 2 = 4:1) supplies and are typically
adjusted for the data shown here at stoichiometric
ratios of 1.7 standard liters per minute (SLPM) for H2
and 4.8 SLPM for air. Under typical operating
conditions flows are stable to within the uncertainty
of the meter reading (± 5%).
A certain portion of the light, be it from scattering,
spontaneous emission, or LIF, that emerges from the
laser focal line volume is imaged with a pair of quartz
lenses onto the entrance slit of a low f-number ( f/#)
flat-field spectrometer (Oriel, Multispec). The lightgathering lens is chosen with an f/# of 1; the other
lens is selected with an f/# of 4 to match closely the
spectrometer f/# of 3.7. For this experiment, the
spectrometer is mounted so that the entrance slit is
oriented horizontally, i.e., parallel to the laser beam
illuminated flame segment. After dispersion of the
entrance slit image by a grating with 1200 lines/mm
the resulting line spectrum is projected by the refocusing mirror of the spectrometer onto the exit plane,
wavelength
flame
distance/de
^
ditac
OMA 'sIll!
imagingoptics
f
computer
2-mlens
trigger
Fig. 1. Experimental setup for recording one-dimensional Rayleigh and Raman profiles in a H2 -air flame.
channel analyzer.
OMA, optical multi-
where it is intercepted by the photocathode of an 18
mm, proximity-focused image intensifier whose output, in turn, is fiber optically coupled to a twodimensional CCD detector array. In this particular
set of experiments we employed a gated image intensifier that is capable of switching times of 100 ns, a
demagnifying fiber link with an exit-to-entrance diameter ratio of 11:19, and an uncooled 286 x 385 CCD
chip, all incorporated into a slow-scan camera (LaVi-
sion, FlameStar). The camera and the laser are
controlled by an IBM-compatible PC operating under
specialized laser diagnostic software (La Vision).
A key aspect of the experiment is to optimize the
imaging of the laser-irradiated line through the flame
onto the spectrometer entrance slit and eventually
onto the CCD detector array. This point is of critical
importance for a quantitative analysis of the various
Raman lines, both in terms of relating their intensities to each other as well as deducing information
about spatial density gradients. After several approaches we developed the following scheme that
appears to give a maximum signal together with the
sharpest images achievable with the overall optical
arrangement used. The procedure can be conveniently broken down into several sequential steps:
(1) The spectrometer grating is adjusted to display the appropriate wavelength range from 240 to
320 nm with the help of a Hg pen lamp oriented
parallel to the spectrometer entrance slit. The lamp
is operated to obtain high output at the 253.4-nm
line. Also visible with this setting are the 265-,
289.4-, 296.8-, 302.2-, and 312.6-nm lines of Hg.
(2) The same Hg lamp is used to illuminate from
afar a straight piece of wire with a known diameter.
The wire is positioned vertically along the flame axis,
i.e., pointing to the center of the burner nozzle.
(3) The two imaging lenses shown in the apparatus sketch of Fig. 1 are, at first, crudely adjusted to
project an image of the wire onto the center of the
(horizontally oriented) input slit.
(4) Then careful and repeated adjustments are
made to obtain a dispersed image of the Hg lamp,
which exhibits simultaneously the sharpest possible
shadows on all the Hg lines visible. For this purpose, accurate and reproducible lens positioning is
essential. In particular, the movements along the
optical axis to ensure that the object is at the focal
point and vertical movement to ensure that the image
is projected exactly onto the spectrometer slit must be
made precisely.
(5) Next the KrF laser focus is aimed, with the
help of the turning mirror, as exactly as possible onto
the wire without further adjustments of the lenses.
At this point the height of the laser beam above the
burner nozzle is also established and the wire is
removed. If the flame is then ignited, it was found
that all spatially varying features in the flame Raman
spectra were as sharp as possible.
Once the optical alignment, as described above, is
completed one has, at the same time, a calibration of
20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS
909
the spatial resolution and magnification of the overall
system. The latter is simply obtained from a comparison of the actual width of the shadow-casting wire to
the full width at half-maximum of the corresponding
images at the various Hg lines. With the two lenses
chosen for this series of experiments (fi = 5 cm, f2 =
20 cm) the observed overall magnification, i.e., the
ratio of final image size on the CCD chip to the object
size, was m = 1.8, which also reflects the scale change
introduced by the tapered fiber link between the
image intensifier and the CCD array. This magnification corresponds to a total length of 4 mm being
imaged along the short side of the CCD array, which
comprises 286 pixels or, in other words, a nominal
spatial resolution of 14 pumper pixel. However, the
actual spatial resolution is limited by the modular
transfer function of the image intensifier and the
imaging quality of all other components (in particular, that of the imaging lens with an f/# of 1).
Empirically a minimum spatial resolution of 150
iim along the laser beam was found by analyzing the
intensity change along this direction associated with
a shadow-casting object in the focal plane. Measurements of the laser focal spot size, with the help of the
UV-objective-equipped CCD camera, reveal dimensions of 750 m in the vertical (parallel to the short
side of the monochromator entrance slit) and 200 Am
in the remaining direction (parallel to the optical axis
of the imaging system). Because the imaging lenses
used in this experiment have an actual magnification
of 3.1 [=1.8/(11:19); see above] the actual observed
volume is 150 m x 200 jim x 90 m, the latter value
along the vertical dimension being the portion of the
image passing through the 280-jm-wide entrance
slit.
Spectroscopic Details
One of the many advantages of the above-mentioned
setup is its versatility in obtaining various types of
data that are relevant for diagnostic purposes and the
ease with which a large quantity of such data can be
obtained. To be sure, the ease refers to a situation in
which the experimental setup has already been
aligned, tested, and optimized. Examples of the
kinds of diagnostic data obtainable are the laser beam
profile in the vicinity of the focal zone, excitationemission spectra of the combustion system at hand,
and one- and two-dimensional density maps of the
system based on scattering and emission techniques.
Because of the overriding importance of interference
problems between fluorescence and Raman signals,
this section is devoted to discussing the issue in detail
for the case of a H2 -air flame and focused KrF laser
excitation. For the purpose of establishing conditions that minimize interferences we recorded excitation-emission spectra like the one shown in Fig. 2.
Shown in this figure is the dispersed emission and
scattering intensity of an oxygen-rich H2 -air flame as
a function of the exciting UV laser wavelength.
Along the vertical axis, the wavelength of the scanning KrF laser is displaced as it goes from one end of
910
APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993
the tuning range (248 nm, top) to the other end (249
nm, bottom). At any given wavelength of the laser,
the resulting emission and scattering spectrum of the
flame is recorded and displayed along the horizontal
axis. The span of the horizontal line,245 to 310 nm,
includes the Rayleigh line (leftmost vertical trace)
and 02 and N2 Raman lines, as well as several 02 and
OH LIF features. Intensities are incorporated in
Fig. 2 as false colors; the figure can thus be viewed as
a contour map showing at each point the intensity of
light reemitted by the flame at emission
(horizontal
axis) following excitation by the laser at Xlaser(vertical
axis). The procedure that we used to obtain this
excitation-emission spectrum is as follows:
(1) With the experimental setup described in the
previous section, we focus the KrF laser with a 2-m
lens into a line that passes through the top section of
an oxygen-rich H2 -air flame;
(2) The spectrometer-gated CCD camera detection system is optimized and aligned to record emission from 245 to 310 nm with a slit width of 280 jim
and a 1200-line/mm grating;
(3) The data acquisition software is set to integrate each image along a certain interval along the
spatial direction (the entrance slit direction) and to
average over a predetermined number of laser pulses.
This software control effectively uses the twodimensional CCD camera as a one-dimensional optical multichannel analyzer with high sensitivity;
(4)
As the CCD camera-software
collect emission
spectra (horizontal cuts through Fig. 2) the laser is
tuned at a scan speed of 0.3 cm- 1 /s from 248 to 249
nm.
At either end of the tuning range, the laser runs
broadband with the resultant emission spectra being
the sum of all individual features seen in the narrowband portion of the scan. Eventually, once the laser
runs completely broadband, the emission spectra do
not change anymore along the vertical direction
toward the top and bottom ends of Fig. 2. The total
accumulation time of the spectra shown in this figure
was 15 min. Several features of this excitationemission spectrum are worth mentioning. The first,
and obvious one, is that an enormous amount of
spectroscopic observations can be accomplished with
such an experimental setup in a short time. Once
measured, such plots allow us to obtain, from vertical
slices, excitation spectra of the system under study
with a variable bandwidth of detection and an arbitrary center emission wavelength. Likewise horizontal profiles reveal the emission spectra of the system
in response to any excitation wavelength (or combination of wavelengths). Second, Raman emission and
resonantly induced fluorescence are easily discernible
in the narrow-band portion of the spectrum as continuous vertical bands in the first instance and isolated,
island-like features in the latter. Third, from spectroscopic pecularities one can rapidly identify which
molecular species is resonantly excited. In this spe-
cific instance 02 emission is characterized by seven
lines to the red of the Rayleigh peak, whose spacings
increase with increasing wavelength. This behavior
is most clearly seen in the broadband portion of the
excitation spectrum (top and bottom of Fig. 2).
These lines correspond to emission from two different
excitation bands of the 02 Schumann-Runge system,
which is accessible with the KrF laser, namely, the
0 - 6 and the 2 - 7 excitation.14 Because of the
rapid predissociation of the 02 B state there is no
noticeable rotational redistribution even at atmospheric pressure, and the observed peaks represent
the two unresolved P amd R lines followingexcitation
of a particular J level. Similarly predissociation of
the v' = 3 state of OH, which is accessible as a 3 -- 0
excitation within the KrF tuning range, assures
equally simple emission features for this molecule.14
In this case, because of the large rotational level
spacing and the II-E nature of the transition, three
clearly separated P-, Q-, and R-branch lines are
observed, which are also indicated in Fig. 2. The
displayed excitation-emission spectrum also clearly
reveals the existence of KrF laser wavelengths at
ble 1 we have summarized the cross-section values for
the relevant major constituents in the context of the
H2 -air combustion and 248-nm excitation. It should
be pointed out here that the usual V4 scaling of the
scattering cross sections can fail to predict the accurate value of the cross section by a wide margin when
resonance enhancement effects are important, as
they are, for example, in the case of UV excitation of
H2 and 02.
fluorescence is observed.
can be labeled as n(i)(x), where j = 1, 2, 3, and 4,
which no resonant OH and
02
These regions are indicated by arrows in Fig 2 and
occur at the following values: X = 248.623 nm (the
wavelength chosen in Ref. 7), X2 = 248.404 nm, and
others. By tuning the laser to these wavelengths, we
can observe Raman scattering against a minimum of
interfering fluorescence. In the experiments described in this paper, we have explored both gap
wavelengths and found that they both give useful
results. Finally, a point not evident from Fig. 2
should be stressed concerning nonlinear effects and
possible interferences that are absent in a simple
one-photon picture. Because of the need for high
laser power densities to compensate for the small
Raman efficiency, multiphoton effects can become
important. In the context of a KrF laser probe of
H2 -air combustion systems, two-photon resonant
Density Maps
The primary information contained in the measured
scattering intensity is the one-dimensional spatial
variation of molecualr densities pertaining to the
majority
species
indicate the molecular species in the sequence listed
above. What is actually measured is a scattering
intensity of the appropriate Raman line, properly
integrated over the associated wavelength range and
corrected for any background, as it is intercepted by a
certain number of pixels viewing a spatial interval
from xi to xi + Axi. That intensity or, more accurately, the number of electrons in those pixels accumulated during a predetermined number of laser pulses
is denoted here by Ni(i). The running subscript i
counts from 1 to 285 and reflects spatial increments
of approximately
zAx = 14 [lm and a total range of
4
mm in the flame, as explained in the section on the
experimental setup. The number Ni(i) is related to
the average density n(i)(xi)of the particular molecule
in the corresponding spatial range as follows:
excitation of H 20 is important. 15 The excited C state
of H2 0 can either fluoresce with a broad emission
band near 400 nm or predissociate into OH(A21),
which subsequently fluoresces, with the strongest
features being those of the v = 0 bands near 308 nm.
Although not visible in Fig. 2, these emission structures are definitely present in less oxygen-rich flames.
As is discussed in detail in the section on calibration
procedures, a quantitative analysis of Raman- and
Rayleigh-scattering data in terms of species density
traces may require that, for the various species
involved, absolute or relative cross sections at the
chosen excitation wavelength are known because only
N2 and 02 can be calibrated directly from room air
(see calibration procedures). There are several important resources in this regard that we have used
and that can prove useful for future experiments.
References 11 and 16-18 have experimental and
theoretical information concerning those kind of data
for various molecules, including H2 , 02, N2, and H2 0,
as well as several carbon-containing species. In Ta-
N2, H 2 O, and H2 . We first
02,
elaborate on how this information can be extracted
from the raw data. Once this information is available it can also be used to calculate temperature
profiles along the same path. The discussion on how
such temperature profiles can be obtained is given in
the section on temperature profiles. If we define the
spatial coordinate along the laser direction as x then
the corresponding densities in the object under study
Ni(i)= TtWiCu(j)Vin(J)(xi)NL,
(1)
Table1. RayleighandRamanCrossSectionsfor A= 248-nmExcitation
Gas
H2
02
Rayleigh
Vibrational
Cross Section
(cm2 )
Raman Cross Section
2
(cm )
3.67 x 10-27a
1 0 -26b
N2
1.384 x
1.555 x
1 0 -26b
H2 0
1.166 x
10
-26b
aE. W. Rothe, Department
4.83 x 10-29c
1.54 x 10-29c
4.84 x 10-29c
of Chemical Engineering,
Wayne
State University, Detroit, Michigan 48202 (private communication, 1991).
bA. Koch, H. Voges, P. Andresen,
H. Schiter,
D. Wolff, W.
Hentschel, W. Oppermann, and E. Rothe, "Planar imaging of a
flame and of internal combustion in an automobile engine using
UV Rayleigh and fluorescence light," (to be published).
CW.K. Bischel and G. Black, "Wavelength dependence of Raman
scattering cross section from 200-600 nm," AIP Conf. Proc. 100,
181-187 (1983).
20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS
911
where
knowledge of the value of ax
T = zogq.
The various symbols in this equation each express
one particular, more or less separable influence of the
experimental detection scheme on the final observable Ni(J). Read from right to left, the right-hand
side of this equation is ordered to mimick how
photons, which are scattered off molecules of typej,
are propagated through all relevant components,
converted to electrons, and finally collected as a total
charge eNi(i) in the relevant pixels with position index
i. It is understood that all thermally generated
electrons and electrons that are due to any other
emission or scattering features collected in those
same pixels have been properly subtracted. The
significance of the symbols in Eq. (1) is as follows:
NL is the total number of laser photons impinging
upon the system under study during the exposure
time of the CCD array (this may or may not comprise
more than one laser pulse),
Vi is the effective volume from which scattered
photons are imaged onto pixels with spatial index i,
n(J)(xi)is the average density of molecular speciesj
in volume Vi,
wiis the solid angle under which volume element Vi
appears to collection optics,
v&i) is the average differential Rayleigh or Raman
cross section of speciesj averaged over solid angle xi,
t is the overall optical transmission coefficient for
the wavelength range (including the grating efficiency),
7 is the overall detection efficiency and gain of
detector (in electrons per CCD pixel per incoming
photon),
q is the quantum efficiency of the photocathode of
the image intensifier,
g is the gain factor of the image intensifier (including both the microchannel plate and the phosphor
screen);
o is the fraction of light emerging from the phosphor screen that is guided by a fiber-optic taper to the
COD array,
z is the quantum efficiencyof the CCD array.
For the case in which two experiments are performed
in two different objects such that only the density of
the molecular species differs, the results of two such
measurements can be expressed in the simplified
form
Nki'j =
fnkj)(Xi), k = 1, 2,
where a represents the overall conversion and detection efficiency for this type of measurement. If one
of these measurements (say k = 1) is done on an
object with the known density distribution nl(i)(xi),it
is obviously a trivial matter to extract the unknown
density from the observables Nei(i)without requiring
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APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993
n2 (i)(x) =
n1 (i)(x.
(2)
This relationship suggests a powerful calibration
procedure that is outlined in the following section.
In essence, the desired density trace in the combustion system is obtained by a simple comparison with
corresponding density traces in a reference system
with known densities. The cancellation of the overall conversion and detection efficiency factor a eliminates a series of potentially serious sources of errors
and uncertainties such as pixel-dependent gain or
spatial variation of detection efficiency. Because of
the mathematical simplicity of Eq. (2), which is used
to convert the raw data with the help of appropriate
calibration curves into absolute density traces and
because of the relatively large number of data points
involved in this process, the actual data analysis was
performed with automated spreadsheets that import
the necessary raw data from appropriately chosen
traces of the corresponding images and, with a few
key strokes (we used, with good success, Lotus 123
and Quattro Pro with their built-in macrocommand
capabilities), output tables and graphs of the absolute
density traces. Examples of such output are shown
in Fig. 3. Further information relating to these
density profiles can be found in the following section
documenting the underlying raw data.
CalibrationProcedures for 2, N 2 , H 2 0, and H2
As was shown above, the relevant information contained in our data sets, namely the spatial variation of
the density of the majority species 02, N2, H20, and
H2, can be extracted most reliably by comparing
intensities of Raman traces of unknown densities
with those of the same species at known densities
obtained under the same experimental conditions.
In this regard ambient air serves as an almost ideal
calibration object, at least for 02 and N2, and, with
some care, even water-density calibration can be
obtained from the water content of ambient air. H2,
on the other hand, obviously must be treated with a
different approach. Minority components such as
NO or, more difficult yet, OH deserve special consideration; since no attempt was made to measure these
species quantitatively in the current set of experiments they are not considered further here. In the
section on our preliminary experimental results, we
adopted the calibration procedures outlined below
with good success. Further details are given there.
By measuring the barometric pressure po, the ambient temperature To, and the humidity level of the
laboratory air we can easily establish the partial
pressures and, hence, densities of 02, N2, and H20 at
the time of measurement. Then Raman traces of
those air molecules are measured with the completely
aligned and optimized experiment, as described in the
section on experimental procedures. Since the density of the calibration gases is constant along the laser
248.75 Uln
Q211
Ei
W
| |
5
5
.;
P
I
C~~~~~~~~~~~Q11
|
|
|
l
248.25nm
~~~~~~~~~~
nm
~~~~~~~~~~~~248
310 (nm)
290
270
250
m
~~~~~~248.5
fi
N2 Raman + 0 2Fluorescence
02 Raman + 02 Fourescence
Rayleigh
Fig. 2. Excitation-emission spectrum of oxygen-rich H 2 -air flame that was obtained by using a tunable KrF laser excimer laser.
Density, Height z = 8mm
Density, Height z = 2mm
20-
O
1816-
Ray
N2
14
E1
E
12-
PCI
10_
W
a,
a
a)
C1
4
20
1mm
1mm
0
0
1mm
(b)
(a)
Fig. 3. (a) Measured density profiles of all majority species at a height of z = 2 mm above the burner nozzle. (b) Measured density profiles
of all majority species at a height of z = 8 mm above the burner nozzle.
1mm
20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS
913
polarized components, experiences this anisotropy.
The components see different refraction indices and,
therefore, experience different dispersions in the sample. Their absorption in the molecules is also different. Consequently the linear polarization of the
probe beam is slightly rotated, and it acquires a small
amount of ellipticity. The signal detected in polarization spectroscopy arises from analyzing these changes
in the polarization state of the probe beam.
A simple model calculation of the polarization
signal in the limit of weak saturation is made in Refs.
8 and 10. This calculation ignores the effects of
repopulation of the ground state that are due to
fluorescence and collisions. It also assumes that the
length of the pump pulse is short compared with the
ground-state relaxation time and that the upper state
is short-lived enough not to contribute to the induced
anisotropy.
When the differences in the refraction indices and
absorption coefficients experienced by the two circular components making up the linearly polarized
probe beam are small, i.e., An = n - n- <<1 and
Acx= ux+- a- << 1, the intensity of the probe beam
transmitted through a nearly crossed analyzing polarizer is
+ 02 +b2 +-An
It =o
(~~~)
+ 24
bAaL +
2
(Aot)2 +
+
2 A)21
(L)(An
,
(1)
where Io is the incoming probe intensity, w is the laser
frequency, L is the absorption path length, b (with
b <<1) is an extra ellipticity that is due to a small
background birefringence, caused, for example, by
imperfect cell windows, and Io is a small background
term that is added to account for the finite extinction
ratio of the crossed polarizers. The angle 0 corresponds to the amount that the polarization axis of the
analyzing polarizer deviates from the exact crossed
position.
For a homogeneously broadened medium the frequency dependence of the absorption difference Actis
Lorentzian. With the help of the Kramers-Kronig
relations one obtains a dispersion-shaped profile for
the difference in the refractive indices, An. Inserting these line shapes into Eq. (1), we can write the
formula for the signal intensity as
It= lo+
+-bAt
2
02 +
1
0L
b2
1
-
1 +x
(~~1
2
\2
11
+ I Ao0 LI
I,
4
1 +x2
(2)
where x = (o - o)/y, y is the half-width of the
absorption profile, and AaOis the absorption difference at the line center w = wo. Equation (2) includes
a constant background term ( + 02 + b2)1 , a dispersion-shaped term linear in Aao
0 and 0, and Lorentzian
terms that are proportional to bAa0 and (Aao)2 A
920
Aao
APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993
- a- = -
Pt (JiJf)'
(3)
where J.J are polarization-dependent numerical
factors, which are given in Ref. 8. The cross section
JJiJf is a sum over M of the absorption cross sections
0LJ.
M+ and thus does not depend on the polarization.
ifm
It is proportional to the square of the reduced matrix
element and depends on the Franck-Condon and
H6nl-London factors for the transition. Ip is the
intensity and t the duration of the pump pulse, and
NOis the total number density of the molecules on the
lower state, which depends on the temperature.
Under the same assumptions as above, the signal
intensity can also be calculated for the case in which
the pump beam is linearly polarized in a plane that is
inclined by 450 against that of the probe beam. The
only difference when compared with the result for the
circularly polarized pump light is that the dispersive
and the Lorentzian line-shape terms are interchanged.
For a pump beam polarized in the z direction the
absorption difference Au0' for the orthogonal polarization components of the probe beam will be8
ALt
0 ' = u - a o -(jifNt'
f).
(4)
The polarization-dependent numerical factors C'j.
Jf are given in Ref. 8.
For the case of a circularly polarized pump beam the
factors ZCJjf approach the value of 3/2 for large J values
and AJ = + 1, while for AJ = 0 they decrease rapidly
with increasing J. For a linearly polarized pump
beam the factors A j approach the limit 3/ofor large
J and
J
O and he limit
6/5for
AJ = 0. Con-
sequently, saturation by a circularly polarized pump
wave produces a larger absorption difference for P
and R lines than for Q lines. Linearly polarized light
favors the Q branch lines instead.
3.
x
- OAct
0 L 1+x
1
proper selection of the uncrossing angle 0 and the
window birefringence b makes either the Lorentzian
or the dispersion term dominant.
The difference in the absorption of the two circular
polarization components of the probe beam depends
on the effect of the saturating beam on the orientation of the molecules. For the case of a circularly
polarized pump beam the absorption difference at line
center for a Ji Jf transition is 8
Experimental Setup
A schematic diagram of the experimental system is
presented in Fig. 1. A tunable dye laser (Lambda
Physik FL 2002) was pumped with an excimer laser
(Lambda Physik EMG 103; pulse duration, 14 ns).
To reach the 306-nm wavelength needed for the
optical pumping of OH molecules the output of the
sulforhodamine B dye laser was frequency doubled in
a BBO crystal (Lambda Physik FL37). The linewidth of the frequency-doubled dye laser was 0.12
cm-.
The output of the dye laser was divided into a
integrator (NF Electronic Instruments BX531) for
averaging. The boxcar was triggered from the excimer laser pulse by a fast photodiode.
4.
Measurements
A. Saturation Measurements
Ploter_ JL Printer
Fig. 1. Experimental setup used for the detection of OH by using
PMT, photomultiplier tube; SHG, secpolarization spectroscopy.
ond-harmonic generation.
strong pump beam and a weaker probe beam. The
pump beam was first linearly polarized with a GlanThompson prism and then circularly polarized with a
Fresnel rhomb. Glan-Thompson prisms were also
used as the linear polarizer and analyzer of the probe
beam. The extinction ratio of the polarizer prisms
was better than 10-5.
A geometry with codirectional pump and probe
beams was used. This does not permit the elimination of Doppler broadening. The loss of spectral
resolution is not crucial, however, as the collisional
broadening of the molecular linewidths at atmospheric pressures is of the same order of magnitude as
the Doppler broadening and the laser linewidth used.
The beams were crossed by 60 mrad in order to obtain
improved spatial resolution. The pump and probe
beams were focused into the flame with a 500-mm
focal-length lens placed in front of the polarizers.
This geometry gives a transverse spatial resolution of
less than 100 Rm.
The measurements were made in a premixed acety-
In order to determine a suitable power for the pump
beam for further experiments, saturation measurements were carried out in the R, bandhead of the A
2.-X 2 f(O, 0) transition. The lines measured were
the R1(9) and the R1(8/10) transitions. The two
peaks of the second transition could not be resolved as
their frequency difference was only 0.055 cm-', which
is clearly below the resolution limit for the setup.
The peak intensities of the spectral lines under
study are plotted in Fig. 2 as a function of the pump
beam intensity. According to Eq. (2)a square dependence of the signal intensity on the pump beam
intensity is expected. In the log-log scale, the measured values should form a straight line with a slope
of 2, which is also sketched in the figure. At low
intensities the signal does show an almost constant
increase with the pump beam intensity. At higher
pump intensities, however, a clear saturation behavior of the signal can be recognized. Even with the
lower intensities the signal does not exhibit a square
dependence. This may be due partly to geometric
effects such as the inhomogeneous spatial distribution of intensity in the beam and the incomplete
overlap of the pump and the probe beams.
The increase of the spectral linewidth with increasing laser intensity was also studied by observing the
R1(9) line. Apart from power broadening, the measured linewidth (FWHM) was composed of a laser
linewidth of 0.12 cm-', an estimated collisional width
of 0.07 cm-' (see Ref. 11), and a Doppler width of
-0.25 cm-'. The width of the R,(9) line was found
to be 0.5 cm-' at a 9-gJ pump pulse energy, and it
slowly decreased to 0.28 cm-' when the pulse energy
was down to 0.3 [LJ. Decreasing the laser intensity
further did not lead to any noticeable decrease in the
width of the line. In the experiments below the
lene-oxygen flame with a nozzle diameter of 1.0 mm
and a Bunsen-type propane-air flame with an orifice
diameter of 10 mm. The flames were attached to a
translation stage, which permitted movement in the
horizontal and the vertical directions by a stepping
motor control. From the flame, the probe beam
traveled through the analyzer, a collimating lens, and
several spatial filters before it reached a photomultiplier tube (EMI QB9558). To minimize the amount
of background light from the flame at the detector, a
double monochromator (Jobin-Yvon Ramanor U 1000)
was used for filtering the probe light.
A microcomputer (HP 9000/216) was used to control the measurement and to file the data. The
computer gave triggering pulses for the excimer laser
and controlled the wavelength scanning of the monochromator and the dye laser. The photomultiplier
signal was amplified with a preamplifier (NF Electronic Instruments BX-31) and sent to a boxcar
I2
100
A
0
Ak
o
A
0
:3
.
_
0
i0)
v)
10-2
0
0
A R2 (8/10) transitions
0
I1.
o- .
104
R1 (9) transition
-
.
105
106
Pump Intensity (W/cm 2 )
Fig. 2. Signal intensity as a function of the pump beam intensity.
20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS
921
pump pulse energy was chosen to be 1.5 p. ( 106
W/cm2 ). The pulse energy of the probe beam was
always kept at 50 nJ.
B.
Spectral
Measurements
of OH
Acetylene-OxygenFlame
With the experimental apparatus described above,
various measurements of the R branches in the
A 2Y,-X 2 fl(0, 0) band were made. The upper curve
in Fig. 3 shows a part of this band in an acetyleneoxygen flame. The bandheads of the R, and R2
branches are shown. This curve was recorded by
averaging 32 pulses per data point with a frequency
step of 0.1 cm-'. The frequency resolution of the
scan can be seen by observing the R1(7) and R1(11)
lines, which lie 0.432 cm-' apart and are clearly
resolved. On the other hand, the R1(14) and R2 (11)
transitions, with a frequency difference of only 0.33
cm-l, are not resolved. In the measured spectrum
the signal-to-noise ratio at the bandhead was better
than 1000:1. The OH concentration in the flame
was estimated to - 1015-1016 molecules/cm3 . From
the observed signal-to-noise ratio and the estimated
OH concentration, a detection limit of better than
1014 OH molecules/cm3 can be inferred.
To demonstrate the dependence of the polarization
signal on the uncrossing angle 0 of the analyzer, the
measurement was repeated with the polarizer and
analyzer slightly uncrossed. The results are shown
in Fig. 3 (middle and lower curves). According to the
calculation for the case of a circularly polarized pump
beam the signal contains a dispersion-shaped term,
which is linear in 0. By opening the analyzer, this
term quickly becomes dominant. It changes sign
with the sign of 0, as can be seen by comparing the
two lower curves in Fig. 3. They are recordings with
opposite opening angles of the analyzer. Because
the analyzer is slightly opened, part of the probe beam
energy leaks to the detector, which enhances the
background signal considerably. The slope of the
background signal in the two lower spectra of Fig. 3 is
due to the wavelength dependence of the gain of the
dye. All subsequent measurements were made with
the angle 0 as close to 0 as possible to minimize the
background noise.
Figure 4 demonstrates the effects of the pump
beam polarization on the signal. The upper spectrum was recorded with a circularly polarized pump
beam. The P, and P2 lines come out strong and the
Q lines are weak, just as the theory predicts. The
lower spectrum in Fig. 4 shows the same 3-nm
wavelength range, but this time with a linearly
polarized pump beam. The polarization planes of
the pump and probe beams make an angle of 450 with
each other. Now the Q, and Q2 lines are clearly
P, 1
2
3
4
P2
1
5
6
1
2
3
4
2
3
4
5
Cb
'7n
S
.55
C
(D
C:
Q0
1 2
3
4
5
02
6
7
241
8
9
BI5 7
10
8
3
11
9
10
12
11
12
121~~~~~~~~~~~~~~~~~
306.3
Fig. 3.
306.5
306.7
306.9
Wavelength
(nm)
R1 and R 2 bandheads of the A
2
X-X
307.1
2
307.3
l(0, 0) transition
in
an acetylene-oxygen flame. The upper curve was measured with
the probe beam polarizers crossed. The lower curves show the
dispersive line shapes, when the uncrossing angle of the analyzer
was approximately 0 = 0.4° for the middle curve and 0 = -0.3° for
the lower curve.
922
APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993
308.0
308.5
309.0
309.5
310.0
310.5
Wavelength
(nm)
Fig. 4. Comparison of polarization signals in OH obtained by
using a circularly polarized pump beam (upper spectrum) and a
linearly polarized pump beam (lower spectrum).
distinguished, but the P1 and P2 lines are barely seen,
again according to the theory.
Propane-AirFlame
The measurements on the R branches were repeated
in a propane-air flame. Figure 5 shows the spectrum measured with a circularly polarized pump
beam and a linearly polarized probe beam. The
spectral resolution achieved in these measurements
was roughly the same as in those given above, with
the R1 (7) and R1 (11) lines resolved and the R1 (14) and
R2 (11) lines unresolved. The signal-to-noise ratio
was 400:1, which is somewhat lower than in the
acetylene-oxygen flame because of the lower OH
concentration in the Bunsen flame. The differences
in the flame temperatures shows up as differences in
the intensity distributions among the rotational lines
of the spectra of Figs. 3 and 5.
C.
Measurements
C
a)
C
-3
-2
-1
0
1
2
3
Distanceacross burner (mm)
Fig. 6. Spatial distribution of the OH signal in an acetyleneoxygen flame.
of the Concentration Distribution
Acetylene Flame
In order to obtain a map of the spatial distribution of
the OH radical in the acetylene-oxygen flame the
laser was tuned to the strong R1 (8/10) transitions at
306.372 nm. With the step-motor-controlled translation stage the flame could be moved relative to the
fixed laser beams in both the horizontal and the
vertical directions. Horizontal cuts of 3.2 mm in
length were run across the flame at various heights.
The step size in the horizontal scan was 50 m. In
the vertical direction the flame was moved in steps of
0.5 mm from the base to the top of the reaction zone.
Above that height the step size was increased to 1
mm. Each measurement point corresponds to a sum
of 32 laser shots. The horizontal cuts at the different heights in the flame are collected in Fig. 6 to give a
picture of the distribution of OH in the acetyleneoxygen flame.
It should be noted that the measured intensity
depends not only on the concentration of the OH
molecule, but also on the temperature of the flame in
the measurement point. Therefore, Fig. 6 does not
really show the concentration distribution of the OH
molecules,but only the intensity distribution of the
R1 (8/10) lines in the flame. The intensity of the OH
polarization spectrum is strongest in the reaction
zone or flame front, which forms a thin cone that
shows strong emission from many radicals. After
reaching a maximum at the top of the conical reaction
zone, the OH signal drops slowly in the upper part of
the flame.
Bunsen Burner
The same measurement was also repeated for the
Bunsen burner, which was enclosed in a steel pipe to
prevent flickering. The fuel was propane and the air
holes were fully opened. In each horizontal scan the
burner was moved in 128 steps over a distance of 19.2
mm, which gives a spatial resolution of 150 prm,
approximately the size of the probe beam diameter in
the flame. In Fig. 7 one can again see a distinct
reaction zone in which the signal intensity from OH is
cn
0)
1a
C
C
0k4
306.3
306.5
306.7
306.9
307.1
307.3
Wavelength (nm)
Fig. 5. Polarization spectrum around the R1 and B2 bandheads in
a propane-air flame, which was measured using a circularly
polarized pump beam.
-8
-6
-4
-2
0
2
4
6
8
Distanceacrossburner(mm)
Fig. 7. Spatial distribution of the OH signal in a propane-air
flame.
20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS
923
highest. In contrast to the acetylene-oxygen flame,
however, here the signal intensity reaches a maximum before the top of the reaction zone and goes
down quickly in the outer reaction zone.
5.
Conclusions
In this paper, the potentials of the use of polarization
spectroscopy for combustion diagnostics have been
investigated. In particular, this well-established
technique of nonlinear laser spectroscopy was applied
to the detection of OH molecules in an acetyleneoxygen and a propane-air flame. The study of OH
was largely motivated by the important role this
radical plays as a transient reaction product in all
hydrocarbon flames.
Polarization spectroscopy is based on the interaction of two polarized laser fields in a sample. The
fields are tuned into resonance with electronic transitions in the molecules under study. Because of the
resonant nature of the interaction, high detection
sensitivities can be achieved. The overall sensitivity
of the method is improved further by the fact that the
signal propagates as a coherent laser beam, which can
easily be collected onto a detector. From the observed signal-to-noise ratio and the estimated concentration of OH in the flames, a detection limit of better
than 1014 OH molecules/cm 3 could be inferred. By
using higher-quality optics and longer averaging times
or higher pulse repetition rates, we can expect further
improvement of the detection sensitivity.
The measurement point in polarization spectroscopy is defined by the overlap volume of the two
interacting laser beams. Probe volumes of less than
1 mm3 can be achieved easily with crossed probe and
pump beams. In this paper, the spatial resolution
capability of the method was demonstrated by measuring the spatial distribution of the OH signal in the
two flames.
In the optical pumping process the behavior of the
electronic transitions of the molecule depends critically on the polarization of the pump beam. The
possibility of distinguishing between R, Q, and P lines
in the spectra by varying the pump beam polarization
was demonstrated by measurements that were made
with linearly and circularly polarized beams. This
feature has a particular advantage in the assignment
of complex molecular spectra.
The polarization signal reflects the distribution of
the molecular population on the lower electronic level
of the transition and, therefore, also permits the
temperature of the measurement point to be extracted from the spectra. This aspect of the method
will be elaborated further.
An important problem in the use of polarization
spectroscopy in flame studies of a more quantitative
nature is collisional relaxation. Measurements of
absolute concentrations of radicals would require a
knowledge of the collisional depolarization of the
magnetic sublevels. Also, when strong saturation of
924
APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993
the optical transition is present, the relaxation rates
of two electronic levels as well as of the optical
coherence need to be known. Furthermore, an accurate prediction of the spectra would require a more
elaborate theory that would include saturation effects.
The use of polarization spectroscopy in combustion
diagnostics offers the possibility of achieving a high
spatial resolution paired with a high detection sensitivity. The measurement signal is, however, sensitive to changes in the polarization of the probe beam
caused, for example, by imperfections in the optical
components or by particles in the flame. Polarization spectroscopy makes use of only two laser beams
of the same frequency and needs no phase matching
of the beams, in contrast to some other nonlinear
spectroscopic methods. Also, the use of a broadband
laser and optical multichannel detection would permit single-shot measurements to be made in turbulent environments. Polarization spectroscopy may
thus provide a useful and simple alternative to other
laser techniques in combustion analysis.
Financial support from the Jenny and Antti Wihuri
Foundation is gratefully acknowledged.
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fluorescence
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