Ultrasensitive detection of nitric oxide at 5.33 ?m
by using external cavity quantum cascade
laser-based Faraday rotation spectroscopy
Rafał Lewickia, James H. Doty IIIa, Robert F. Curla,1, Frank K. Tittela, and Gerard Wysockib,1
aRice Quantum Institute, Rice University, 6100 Main Street, Houston, TX 77005; andbElectrical Engineering Department, Princeton University, B324
Engineering Quad, Princeton, NJ 08544
Contributed by Robert F. Curl, June 15, 2009 (sent for review May 14, 2009)
A transportable prototype Faraday rotation spectroscopic system
based on a tunable external cavity quantum cascade laser has been
developed for ultrasensitive detection of nitric oxide (NO).
A broadly tunable laser source allows targeting the optimum
Q3/2(3/2) molecular transition at 1875.81 cm?1of the NO funda-
constant minimum NO detection limits (1?) of 4.3 parts per billion
by volume (ppbv) and 0.38 ppbv are obtained by using a thermo-
electrically cooled mercury–cadmium–telluride photodetector and
liquid nitrogen-cooled indium–antimonide photodetector, respec-
tively. Laboratory performance evaluation and results of continu-
ous, unattended monitoring of atmospheric NO concentration
levels are reported.
external cavity laser ? nitric oxide detection ? midinfrared ?
magnetic circular birefringence ? paramagnetic species
ing system for ultrasensitive detection of atmospheric nitric
oxide (NO) based on Faraday rotation spectroscopy (FRS). The
FRS technique as a method for improving sensitivity by reducing
source noise was first reported in the 1980s with a color-center
laser source (1). The recent availability of thermoelectrically
cooled widely tunable continuous wave (CW) external cavity
quantum cascade lasers (EC-QCLs) (2) makes feasible a trans-
portable FRS NO sensor targeting the optimum Q3/2(3/2) NO
absorption line of the fundamental vibration at 5.33 ?m that has
great potential for development into a compact field-deployable
Ultrasensitive trace gas detection is of increasing interest in
various applications including environmental monitoring, indus-
trial emission measurements, chemical analysis, medical diag-
nostics, and security. The combination of midinfrared, contin-
uous wave, high-performance QCL sources with sensitive
spectroscopic measurement techniques is leading to improved
specificity, and lower minimum detection limits (MDLs) for
many molecular species as compared with nonoptical chemical
sensors. This work was made possible by recent advances in QCL
fabrication technology that have resulted in Fabry–Perot QC
laser chips with wide gain bandwidth and high-output power
levels at room temperature (3). These developments permit the
construction of widely tunable EC-QCLs like the one used here.
motivated by the current need for monitoring and quantifying
the significant increase of atmospheric NO concentration levels
due to combustion emissions that are impacting air quality in
urban environments worldwide. NO molecules play a major role
in atmospheric chemistry and significantly contribute to the
formation of photochemical smog and acid rain and to the
depletion of the stratospheric ozone layer (4, 5). Furthermore,
NO at low concentrations in human and mammalian cells is of
great importance in the regulation of biological and physiolog-
ical processes (6). Therefore, the ability to perform accurate
n this article, we describe the development and performance
of a prototype transportable, cryogen-free spectroscopic sens-
quantitative measurements of NO at or below the parts per
billion by volume (ppbv) level is of considerable importance for
a number of real-world applications.
In the vicinity of a Zeeman-split absorption line of the para-
magnetic molecule, the 2 oppositely circularly polarized com-
ponents have different wavelength-dependent complex propa-
gation constants interacting with the Zeeman split ? MJ? ?1
and ? MJ? ?1 NO transition components resulting in magnetic
circular birefringence (MCB) and magnetic circular dichroism.
For linearly polarized light, which can be considered as a
superposition of right-hand circularly polarized (RHCP) and
left-hand circularly polarized (LHCP) light, propagation for a
distance L through a MCB medium rotates its plane of polar-
ization by an angle ? ? ?nL?/?, where ?n ? nR? nLis the
difference between refractive index for RHCP (nR) and LHCP
(nL) respectively. The refractive index difference, ?n, is propor-
tional to the concentration of the absorber.
The sample cell is surrounded by a solenoid and placed
between 2 nearly crossed polarizers, and the magnetic field is
modulated by using an alternated solenoid current. The Zeeman
splitting caused by the field results in rotation of the plane of
polarization of light, thus modulating the light transmitted
through the second polarizer (analyzer). For low gas concentra-
tions and short optical paths, magnetic circular dichroism is
negligible, and only the MCB signal from the difference between
2 dispersion curves contributes to FRS signal. See supporting
information (SI) for a detailed discussion of FRS.
An alternate FRS approach (7) is to orient the second Rochon
polarizer at 45° to the first. This splits the original beam into 2
equally intense beams of perpendicular polarization; these
is measured as a difference between the 2 signals. Which method
is best depends on many factors: laser power, source noise in the
laser, saturation of the absorption, and detector parameters
(sensitivity, saturation, linearity, etc.). Additional frequency
modulation can be used with any of the FRS methods to
approach quantum noise (QN)-limited performance (8). In this
work, we have used the simple single-detector approach with
nearly crossed polarizers.
In a situation where the main noise source arises from laser
amplitude fluctuations, improved sensitivity through FRS is
achieved by reducing laser source noise by a factor larger than
the simultaneous reduction of signal. Source noise from laser
Author contributions: R.L., R.F.C., F.K.T., and G.W. designed research; R.L., J.H.D., and G.W.
G.W. wrote the paper.
The authors declare no conflict of interest.
1To whom correspondence may be addressed. E-mail: email@example.com or gwysocki@
This article contains supporting information online at www.pnas.org/cgi/content/full/
August 4, 2009 ?
vol. 106 ?
no. 31 ?
amplitude fluctuations is at a minimum when the polarizers are
exactly crossed and initially increases quadratically upon un-
crossing. The FRS signal is null at exact crossing but increases
linearly upon uncrossing. The signal-to-noise ratio (SNR) im-
provement arises because the linear growth of signal is faster
initially than the quadratic growth of noise. Obviously detector
noise is unaffected by the polarizer crossing angle. Quantum or
shot noise grows with uncrossing in the same way as signal. This
will be discussed below. In the work reported here, detector
noise and available laser power are determinative. See SI for
The FRS method (1, 9), although it shares with laser magnetic
resonance spectroscopy (LMRS) the use of magnetic modula-
tion, differs significantly from it. When the 2 approaches were
to be an order of magnitude better than the LMRS technique.
FRS has been used as an extremely sensitive technique for the
detection of paramagnetic molecules such as NO (1, 7, 10–12),
nitrogen dioxide (8, 13), oxygen (14), or hydroxyl radicals (15).
FRS is well suited for atmospheric measurements of free radicals
(16) or for exhaled breath analysis because interference from
diamagnetic species (such as water and carbon dioxide) is
eliminated. Demonstration of FRS for real-time monitoring of
exhaled NO (17) or biogenic NO released from human sweat
(18) have been used to diagnose asthma and other respiratory
disorders and for biomedical and physiological applications,
In this article, we focus on the development of a transportable,
autonomous, cryogen-free FRS NO system targeting the opti-
mum absorption line of its fundamental vibrational transition for
sensitive detection of atmospheric NO. The ability to target the
optimum transition with a very quiet source has made possible
the construction of a highly sensitive instrument, for which there
is a clear improvement path to an even more sensitive and
Selection of Optimum Transition
In the presence of a magnetic field, ro-vibrational transitions, of
molecules that possess a permanent magnetic dipole moment,
undergo Zeeman splitting. The magnetic field breaks the de-
generacy of the molecular rotation states into 2J ? 1 sublevels
labeled by the quantum number M. For low rotational quantum
number J levels of the ground electronic2? state of NO the
angular momenta coupling is close to Hund’s case (a). In case (a)
the2?1/2subsystem is magnetically insensitive, and the2?3/2
subsystem has magnetic dipole moment with g-factor of g ?
3/[J(J ? 1)] (11, 19, 20). For molecules placed in an axial
magnetic field with light propagating along the axis, the
?MJ? ?1, ?MJ? ?1 components interact with RHCP light and
LHCP light, respectively, whereas the ?MJ? 0 absorption lines
are polarized parallel to the magnetic field and will not be
To analyze the net effect of the magnetic field on the given
ro-vibrational transition, contribution of all allowed transition
components shifted by ?E ? (g?jM?j ? g?jM?j)??BB0 must be
summed. For Q-branch (?J ? 0) transitions, g?J?? g?J?, and all
?MJ ? ?1 components exhibit the same magnetic sensitivity
with opposite signs for ?MJ? ?1 and ?MJ? ?1. This provides
the most efficient summation of all components and thus the
optimum condition for Faraday modulation spectroscopy. The
Q-branch line strength as well as g-factor decreases with increas-
ing J value making the Q3/2(3/2) molecular transition at 1,875.81
cm?1clearly the best choice for sensitive FRS detection of
QCL-based FRS has been used to monitor NO concentrations
previously (11, 12, 18), but the optimum Q3/2(3/2) molecular
transition at 1,875.81 cm?1could not be targeted. Tuning to this
line was made possible for this work by employing a widely
tunable EC-QCL as a spectroscopic source. The total EC-QCL
frequency tuning range between 1,825 and 1,980 cm?1allows
most of the lines within the fundamental absorption band of NO
at 5.2 ?m to be targeted with a single laser source (see Fig. 1).
Thus, isotopic studies of NO can be performed with a single
instrument, because the EC-QCL based FRS spectrometer
provides access to the optimum Q3/2(3/2) molecular transitions
of all stable monosubstituted NO isotopomers with the most
abundant ones being
1,842.76 cm?1, and14N18O at 1,827.13 cm?1. In this work, we
focus on sensitive detection of NO targeting the major isotope
at 1,875.81 cm?1.
14N16O (discussed above),
FRS Experimental Setup
The experimental arrangement of the prototype FRS platform
is schematically shown in Fig. 2. The optical set-up was built on
a 60- ? 90-cm optical breadboard mounted together with all the
dedicated instrumentation on a wheeled cart (model POC001;
Thorlabs) (see SI for details). This FRS system platform has
been transported and operated at 3 different laboratory loca-
tions. A previously described (2) tunable EC-QCL with high-
resolution mode-hop free wavelength tuning capability was used
as the spectroscopic source. In this work, the laser was operated
in a CW mode at ?20 °C and provided a maximum output power
of 2.9 mW at the wavelength coincident with the target NO line.
Fine mode-hop free tuning of up to 2.5 cm?1permitted high-
resolution spectroscopy anywhere within the tuning range. This
was exploited to perform active frequency locking, thus enabling
long-term unattended operation of the FRS NO sensor. In Fig.
3, a HITRAN simulated direct absorption spectrum and a
high-resolution FRS spectrum of the fundamental Q-branch for
for 3 stable NO isotopes accessible within a tuning range of the EC-QCL.
Spectrum of absorption line strengths listed in the HITRAN database
Rochon polarizer; ?/4, quarter wave plate; PC, personal computer.
Schematic diagram of an EC-QCL-based FRS experimental setup. RP,
www.pnas.org?cgi?doi?10.1073?pnas.0906291106Lewicki et al.
10-ppmv NO in N2mixture at reduced pressure are plotted to
demonstrate high spectral resolution of the EC-QCL.
The collimated EC-QCL beam (4 mm in diameter) was split
by a CaF2wedge into 2 independent optical paths. In the main
path, the laser beam propagated through a 50-cm-long optical
gas cell located inside a 44-cm-long solenoid. The gas cell was
placed between 2 nearly crossed MgF2Rochon polarizers (ex-
tinction ratio for both polarizers is ? ? 10?5). When a longitu-
dinal magnetic field was applied, the linearly polarized QCL
as a result of the interaction with paramagnetic NO molecules.
detected by either a midinfrared thermoelectrically cooled
mercury–cadmium–telluride (MCT) photodetector or liquid
nitrogen-cooled indium–antimonide (InSb) photodetector. The
solenoid current was driven at fm? 950 Hz with a commercial
reactive load of the solenoid at ?1 kHz, a series resonant circuit
was constructed matching the modulation frequency fm. The
modulated Faraday rotation resulted in ac amplitude modula-
tion of the transmitted light intensity, which was detected by
using a phase-sensitive lock-in detection at the frequency fm. The
spectrum of detected FRS signal was recorded by a personal
computer. Because signal should exist only when the NO mol-
ecules are present, FRS is considered a zero background tech-
nique. For small rotation angles, the detector signal is directly
proportional to the NO concentration inside the cell. For
calibration of the proportionality constant, 2 cylinders contain-
ing a mixture of 10 ppmv and 96 ppbv of NO in N2were used.
The system gas flow was set to ?300 mL/min.
The second optical branch of the sensor was used as a
reference channel for frequency control of the EC-QCL. The
initial linear polarization of the laser radiation was transformed
into circular polarization by passing it through a quarter wave
directed through a 20-cm absorption gas cell filled with a mixture
of 5% NO by volume in air at 25 Torr. The cell was placed inside
a 10-cm-long solenoid, which was part of the series RLC circuit
formed with the main solenoid. In the alternating axial magnetic
field, the Zeeman modulation signal resulting from magnetic
circular dichroism was observed by a thermoelectrically cooled
MCT photodetector and demodulated by a second lock-in at the
third harmonic of fm. The zero-crossing of the third harmonic
signal was used to lock EC-QCL to the peak of the Q3/2(3/2)
transition of NO.
Results and Discussion
Magnetic Field and Sample Pressure Optimization. A series of ex-
periments were performed to determine the optimum magnetic
field strength, sample gas pressure, and the analyzer offset angle.
The optimum modulation has been calculated (16) and corre-
sponds to a maximum Zeeman shift from the zero-field line
center equal to 1.35 times the half width at half maximum of a
Doppler broadened absorption line. For several different gas
pressures in the 0–200 Torr range, a series of NO spectral
measurements for various solenoid current modulation ampli-
tudes were carried out. The largest FRS signal amplitude was
found experimentally at a pressure of 40 Torr for an ac solenoid
current of 3.5 Armscorresponding to B ? 110 Grmsmeasured
inside the main magnetic coil and 33 Grmsmeasured inside the
reference cell. This current of ?64% of the maximum solenoid
current the 5.5 Arms allowed stable long-term operation. Ac-
cording to simulations reported in ref. 12, these conditions are
?90% of the maximum signal that can be obtained for this line
with stronger magnetic fields and higher sample pressures.
Optimization of an Analyzer Angle. A detailed theoretical analysis
of the magnetic rotation signal and noise sources was described
in refs. 1, 7, 16, and 21. A detailed discussion of these topics very
pertinent to our particular case has been given in ref. 16. The
power transmitted through the analyzer is given by P ? P0[(1 ?
2?)cos2(?) ? ?] (7), where P0is the intensity of light incident at
the analyzer, ? is the polarizer extinction ratio, and ? is an angle
measured between polarization plane of the polarizer and the
analyzer. For further discussion, we replace ? with ? ? ?/2 ? ?.
Then with ? ? ? 1 and ? ? ? 1, the intensity is approximated by
P ? P0(sin2(?) ? ?).
The rotation of the polarization of incident light can be
modulated within a small range of angles ?? by using the
Faraday effect. At any given analyzer angle ?0, the FRS signal
detected at the modulation frequency by using phase-sensitive
detection can be expressed as S ?dP
small ?, a condition valid here, and neglecting power saturation,
the recorded signal becomes S ? ????P02? ? a?P0(where ? is
the proportionality constant combining detector response and
a ? 2???). It is clear that the maximum signal occurs when ?
is set at ?45°. The maximum SNR, however, depends on the
There are 3 major noise contributions present in the FRS
sensor system: (i) laser source noise, (ii) noise from the photo-
detector and its amplifiers, and (iii) angle-dependent fundamen-
tal QN (photodetector noise often is dominated by QN, but will
be in any case angle independent). Source noise is an intrinsic
feature of the laser used. Its noise spectrum is, in general, not
white, having more noise at low frequencies. The level of this
noise received by the photodetector scales proportionally with
the intensity transmitted by the analyzer and can be expressed as
Pd (d being the proportionality coefficient specific for the
particular laser source in use). QN is proportional to?P.
For ? where sin(?) ? ?, the SNR can be expressed as
d???0?? ? ???P0sin(2?). For
?b2? P0c2??2? ?? ? P0
where b is detector noise equivalent power, c?P0??2? ? is the
transmitted QN (where c is related to detector responsivity), and
of the SNR with respect to ? equal to zero results in a quartic
equation for the optimum ?, which has 4 solutions. The real,
positive solution to the quartic is
The optimum ? corresponds to the square root of the Euclidean
sum of 3 terms. Each term is the ratio of the particular noise
with 44-cm active optical path length by using TE-cooled MCT detector and
? ? 7°. Modulation, 110 G at 950 Hz.
A HITRAN simulated direct absorption (Upper) and FRS (Lower)
Lewicki et al.PNAS ?
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vol. 106 ?
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component at the detector, for a situation when the polarizers
are crossed (? ? 0), to the total laser noise incident on the
analyzing polarizer (P0d). For small ? or small laser power P0, the
?optis primarily determined by the ratio of the detector noise to
the incident laser noise ?opt? (b/P0d)1/2, and is independent of
?, whereas at high power, it is determined by the polarizer quality
?opt? (?)1/2. A rather messy-looking expression for the SNR is
obtained when the complete solution for optimum ? is intro-
duced. For small P0, the SNR rises as?P0and is independent
of ?, SNR ? a?P0/?2bd ? c2. There was no evidence in the
experiments reported here that QN was observed, but with this
EC-QCL laser and our InSb detector, we believe QN is near the
threshold of becoming significant. Further increases in P0, cause
?optto decrease until ? becomes limiting. In the limit P03 ?,
SNR ? a/2d??. Thus, at high power, the signal strength,
polarizer, and the laser source noise are ultimately limiting, and
QN is eliminated from SNR. See SI for a detailed discussion.
A series of measurements was taken for the same gas mixture
(10 ppmv of NO in N2) at different analyzer angles to determine
the optimum ?0for the best SNR value of the FRS system. The
data were measured by using the MCT photodetector (with area
of 1 mm2and D* ? 2?1010cm?Hz/W at 5 ?m). The system was
operated in a spectral scan mode, and for each ? the signal
amplitude at the center of the NO line as well as a standard
deviation of the data points away from the absorption center
representing total system noise were measured and analyzed.
The FRS signal amplitude as a function of ? is shown in the Fig.
4A. These data were fitted by a f1(?) ? A?sin(2?) function (A ?
P0a). For the laser power used, QN remains small compared with
detector or laser noise. Therefore, the QN contribution can be
omitted in the fit function. The total noise vs. ? was fitted by
f2(?) ??D2?sin4(?) ? B2(B ? b, D ? P0d), which is shown in
Fig. 4B. In Fig. 4C, the SNR is calculated for the measurements
(dots) and fitted lines (solid line). The best SNR for a lock-in
time constant of TC ? 1 s occurs at ? of ?7°. However, for better
long-term stability of the system, it is preferable to work with
smaller ? (see Long-Term Measurements for more detailed
discussion). For an experimental demonstration of the effect of
improved photodetector performance on the system SNR, we
have performed measurements using the InSb photodetector
(with 1-mm diameter, and D* ? 1?1011cm?Hz/W at 5 ?m). For
this case, the optimum ? was found to be 2–3° from its crossed
position (the SNR maximum is rather insensitive to angle).
The high-resolution magnetic rotation spectra of NO acquired
with 2 different photodetectors for a certified reference gas
mixture of 96 ppbv of NO in N2are depicted in Fig. 5. The total
pressure was 40 Torr, the solenoid length was 44 cm, and the
temperature was 22 °C. Under these conditions, the fractional
absorption at the Q3/2(3/2) line peak is calculated by using the
HITRAN line strength and pressure broadening coefficient to
be 1.5 ? 10?5and ?max? 4 ? 10?6. Both spectra were recorded
for the same gas sample conditions and show only the 2 strongest
Q3/2(3/2) and Q3/2(5/2) molecular transitions at 1,875.81 cm?1
and 1,875.72 cm?1, respectively. For the MCT (Fig. 5A) 1? MDL
of 4.3 ppbv was obtained for NO concentrations TC ? 1 s, and
? ? 7°. Almost identical MDL (1?) value of 5.4 ppbv was
obtained for a 10-ppmv NO mixture in N2. This demonstrates an
excellent dynamic range of the method with a uniform perfor-
mance for concentrations covering several orders of magnitude.
A significantly improved detection limit was obtained with the
LN2cooled InSb photodetector, resulting in a 1? MDL of 380
pptv for the same TC ? 1 s, and ? ? 3° (Fig. 5B). An equivalent
minimum detectable fractional absorption of 6.7 ? 10?7for the
MCT detector and 5.9 ? 10?8for the InSb detector with TC ?
1 s is lower for significantly shorter optical paths than in current
state-of-the-art direct absorption systems (22).
For either detector, the SNR could be improved with more
laser power. The SNR is proportional to ?P0 in this regime
where the polarizers are not limiting. As laser power is increased,
the optimum ? decreases to keep the total power reaching the
detector constant. Ultimately, light leakage through the polar-
Such light leakage is intrinsic to the polarizers (? ? 10?5), but
also arises from the cell windows spoiling polarization. Taking
both effects into account by an ?eff, this flattening of SNR power
dependence is expected to take place at ?opt??5?eff. If ?effcan
actually be made 10?5and power saturation is negligible, a QCL
laser with P ? 0.5 W could improve the SNR for the InSb
detector by approximately a factor of 7 and for the MCT by a
factor of ?20.
Long-Term Measurements. The scan results of Fig. 5 demonstrate
the excellent sensitivities that can be achieved with a relatively
small time constant. However, scanning introduces the compli-
cation of fairly elaborate data analysis. For purposes such as
atmospheric monitoring, it appears simpler to lock the laser
frequency to the peak of the FRS signal and merely track the
signal magnitude as a function of time. We have explored this
option, and the results are discussed below.
A precise wavelength control of the EC-QCL was imple-
mented to prevent detuning of the laser from the resonance with
the measured values and the fitting curves from A and B shown in C.
An FRS signal amplitude (A) and noise (B) fitted by estimated model functions derived for the FRS technique, with the SNR versus ? calculated by using
(B). Both spectra were measured at the same measurement conditions except
? ? 7° in A, and ? ? 3° in B. TC ? 1 s; modulation, 110 G at 950 Hz.
Faraday rotation spectrum of Q3/2(3/2) and Q3/2(5/2) transitions of NO
www.pnas.org?cgi?doi?10.1073?pnas.0906291106 Lewicki et al.
the NO Q3/2(3/2) transition. A computer-based active feedback
loop provides simultaneous control of the 3 independent laser
parameters: the EC length, the diffraction grating angle, and
QCL injection current. The demonstrated active wavelength
locking of broadly tunable EC-QCLs provides solid ground for
construction of versatile sensing systems that can simultaneously
target multiple analytes in different significantly separated spec-
Preliminary tests used this approach for NO concentrations
with a sampling port located outside the building. The atmo-
spheric air was measured alternately with certified mixture of 96
ppbv NO in N2and pure N2used for calibration purposes. If
there were no offset from the baseline (caused, we believe, by
pickup of the modulation into the laser) with pure N2, only
1-time calibration with a known sample would be required for
fixed laser power, modulation current, analyzer angle, and
detector, because the signal proportionality constant depends
only on these quantities. The data collected with the room-
temperature MCT detector (? ? 7°) are shown in Fig. 6. The
data points were acquired every 3 s with TC ? 1 sec. During
measurements of the outside air, a number of sharp peaks of NO
concentration, primarily related to automobile activity, are
clearly visible. These outdoor air measurements demonstrate the
FRS system capability for continuous unattended operation.
However, the apparent sensitivity is obviously much poorer than
those obtained with the scans of Fig. 5A. A 1? MDL of 7 ppbv
for the MCT detector was estimated by calculating signal am-
plitude and standard deviation by using the NO and N2time
series. The MDL observed in the absorption line-locked mode is
?2 times higher than MDL determined from the spectral
measurements in Fig. 5 (all measurement parameters were
identical). A similar trend was observed with InSb detector. We
believe that the MDL increase is related to the limited precision
of a relatively slow computer-based active feedback control of
time, as can be seen clearly in the time intervals where pure N2
was flowing. The offset could be caused by electronic pick-up of
the 950-Hz modulation current either into the detection system
or the laser driver system or both. This is the real sensitivity
limitation in long-term monitoring, because other factors that
can influence the FRS signal, including laser power or magnetic
field amplitude, can be reliably controlled and/or monitored.
The sensor stability was studied to explore the origin of
background fluctuations and to optimize sensor operating pa-
rameters for improved long-term system performance and the
best detection limit. As described in the previous section, the
level of detector thermal noise has a major impact on the FRS
system SNR, but because of its random nature, it has only a
minor effect on a long-term system performance. Previously the
lower detectivity of thermoelectrically cooled MCT detectors
was compensated by setting the ? further from its crossed
position to increase signal. At the same time a larger fraction of
laser noise, which is not white noise, will be incident on the
in the laser intensity will appear in the signal. Thus, the FRS
system baseline will be subject to fluctuations and instabilities of
the laser source. To study this effect, a series of long time
measurements were performed for different ?, and the Allan
variance was calculated for each time series (23). These Allan
plots calculated for time series recorded for 100-ppbv NO in N2
with ? of 4, 6, 8, and 10° are shown in the Fig. 7. With increasing
?, a slow drift characteristic for time windows ?100 s becomes
dominant. These results can make sense if the laser is being
amplitude modulated at 950 Hz. Laser power drift is probably
too small to give this large a fractional baseline shift. Previous
experience with pickup suggests that it is likely that the magni-
tude of the 950 Hz modulation of laser and/or possibly detector
is varying with time.
We conclude that despite the improved SNR for short mea-
surement times, larger ? with its higher laser power on the
detector causes a significant contribution of the laser noise/drift,
which strongly affects long-term system stability and detector
performance. It is necessary to reduce ? at the cost of short-term
SNR to provide optimum long-term stability while preserving
gas sensor system with ? set at 4°. A dotted line shows response of an ideal
random noise-limited system.
laser frequency locking to the NO absorption line at 1,875.81 cm?1with MCT
photodetector. Reference [NO] ? 96 ppbv. TC ? 1 se; acquisition every 3 s.
Long-term NO concentration measurements performed with active
the detector, resulting in increased laser source drift and poorer long-term
Allan plots obtained for different ? values. Larger ?s raise power at
Lewicki et al. PNAS ?
August 4, 2009 ?
vol. 106 ?
no. 31 ?
sufficient minimum detection limit required for the particular
An analyzer offset angle of 4° was used to effectively suppress
slow system drift and to achieve extended long-term stability. By
preceded and followed by a 100-ppbv NO mixture was recorded
over ?9 h (see Fig. 8 Lower). The data were collected with TC ?
1 s, and Fig. 8 Lower shows data points acquired every 3 s The
Allan variance calculated for these data are shown together with
an ideal white noise-limited system response in Fig. 8 Upper. The
system now shows good stability that allows for averaging times
of up to ?4,000 s. The SNR is reduced by a factor of ?2 with
respect to the optimum angle of 7°. However, this reduction in
SNR is only observed at time scales ?100 s. The 4° angle
improves SNR for ?100-s averaging times. Such a tradeoff can
be made for applications in which long-term stability is critical
and short-term response and sensitivity is less important.
We can think of 2 ways to overcome the drift problem and
restore optimum performance. The most straightforward, but
rather difficult, method is to track down and eliminate ground
loops and electromagnetic pickup, which cause the modulation
of the laser by the magnet current. Increased suppression of
those effects together with occasional calibration with zero gas
and a calibrated NO mixture should provide reasonable long-
term stability of the system. We believe the most robust method
for achieving long-term stability is to return to scanning over the
line. A series of short scans would be acquired and averaged. The
NO concentration would be obtained by least-squares fitting
the amplitude parameter of the known line shape (Fig. 5) to the
observed traces. With this approach, the instrument should have
zero offset and should require only very infrequent calibration.
The laser-based FRS instrumentation described in this work
offers an effective method for the detection of NO with excellent
sensitivity and selectivity. This technology can be extended to
other atomic and molecular species with an open shell config-
uration such as nitrogen dioxide or hydroxyl radicals. The use of
a broadly tunable EC-QCL allows the selection of the optimum
line for the FRS detection of NO and provides flexibility in
selecting the required laser wavelength to perform sensitive
detection of other paramagnetic species within the laser tuning
range (e.g., other NO isotopes). The mode hop-free frequency
tuning with a frequency-locking algorithm available with our
EC-QCL technology in conjunction with cryogen-free detectors
allowed for the long-term unattended NO monitoring capability.
The sensitivity obtained with this polarization technique
achieves parts-per-billion detection levels by using only 44-cm
effective optical path length. This is comparable with other
sensitive spectroscopic techniques that have been used in the
midinfrared region requiring significantly longer optical path
lengths (22, 24). This property lends itself to the reduction of
sample volume to dimensions restricted only by the laser beam
diameter and the detection chamber length. With careful engi-
neering of the current prototype the physical dimensions of the
sample chamber can be reduced to ?25 mL (with cell diameter
of 8 mm), which at pressure of 40 Torr, corresponds to only ?1.3
mL of gas sample collected at atmospheric pressure. If the
reduction of the sample volume is not critical, the FRS technique
allows for further enhancement of the sensitivity by means of a
multipass configuration (13, 25).
The development of high-power QCLs is progressing rapidly.
QC lasers with an internal grating producing nearly 1 W of CW
power are already technically feasible. A FRS sensor based on
ACKNOWLEDGMENTS. This work was supported by the National Science
Foundation through a subaward from Princeton University (Mid-InfraRed
Technologies for Health and the Environment Engineering Research Center),
the Department of Energy through a subaward from Aerodyne Research Inc.,
and the Robert Welch Foundation.
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