1 second and a NO
minimum detection sensitivity of < 1 ppbv. Such high sensitivity, rapid response
measurements are possible with laser absorption spectroscopy in the fundamental absorption
band of NO.University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2005 • 27-32
Development of Quantum-Cascade Laser Based Biosensor Technology
Abstract--The development of new types of sensitive, selective, real-time gas sensors is based on continuous wave and pulsed quantum cascade lasers for various chemical sensing applications, such as medical diagnostics, environmental monitoring, and industrial process control. Tunable laser absorption spectroscopy in the mid-infrared spectral region is a sensitive analytical technique for trace gas quantification. During the past year a nitric oxide (NO) gas sensor was developed based on a novel thermoelectrically cooled, continuous wave, distributed feedback quantum cascade laser operating at 5.45 mm (1835 cm-1) and off-axis integrated cavity output spectroscopy (OA-ICOS) combined with a wavelength modulation technique. Its purpose is to determine NO concentrations at the sub-ppbv levels that are essential for such applications. The sensor employs a 50 cm-long high-finesse optical cavity that provides an effective pathlength of ~ 700 m. A noise equivalent (SNR = 1) minimum detection limit of 0.7 ppbv with a 1 second observation time was achieved.
The development of compact optical sensors for nitric oxide detection is of interest for a number of applications, such as environmental monitoring,1 atmospheric chemistry,2 industrial process control,3 combustion studies,4 and medical diagnostics.5,6 NO is involved in many vital physiological processes in the human body. For example, an elevated level of NO in exhaled breath is correlated with airway inflammation in asthmatic patients.
Knowledge of NO concentrations in the exhaled breath of these patients may allow
health care providers to adjust therapeutic drug dosages.7,8 For medical
diagnostics purposes, it is essential to time-resolve the NO centration as a
function of a breath cycle phase, because corresponding air samples originate in the
different parts of the respiratory tract. This application requires a sensor response time
of 1 second and a NO
minimum detection sensitivity of < 1 ppbv. Such high sensitivity, rapid response
measurements are possible with laser absorption spectroscopy in the fundamental absorption
band of NO.
Distributed feedback quantum cascade lasers (DFB QCLs) operating in a pulsed or continuous wave (CW) mode are promising spectroscopic sources because of their narrow linewidths, single mode operation, tunability, output power, reliability, and compactness. Until recently CW operation of QCLs was possible only at cryogenic temperatures, and room temperature (RT) operation was only realized with pulsed operation at low duty cycle. Recent developments in QCL technology now permit CW operation at room temperature.9,10
A number of approaches utilizing QCLs for the optical sensing of trace NO have
been reported. Background-free Faraday modulation spectroscopy has been demonstrated to be
capable of measuring biogenic nitric oxide with a sensitivity of a few ppbv.11,12
Midinfrared spectrometers based on a QCL and a multipass cell absorption platform achieve
a minimum NO detection limit of 1 ppbv.1,6,13 A technique based on Cavity Ringdown
Spectroscopy (CRDS) reaches a noise equivalent sensitivity at the sub-ppbv level in
several seconds using a comparatively small sample volume because of a long optical
pathlength obtained with ultra-high reflectivity mirrors (R ~ 99.99%).14-16
Another approach is integrated cavity output spectroscopy (ICOS),17,18 which
also uses low-loss mirrors.
In 2001, an ICOS-based biogenic NO sensor utilizing a quantum cascade laser was
reported by our group.19 More recently, we demonstrated a noise equivalent
sensitivity of 10 ppbv in 15 seconds for NO using a compact (~ 5 cm long, 80 cm3 volume) off-axis (OA)-ICOS cell and a liquid nitrogen (LN2)
cooled CW DFB QCL, operated at ~ 5.2 mm.20 The
OA-ICOS approach combined with a wavelength modulation (WM) technique showed a 5-fold
sensitivity enhancement with the same data acquisition time. M. L Silva et al.21
obtained a minimum detection sensitivity of < 1 ppbv in 4 s for NO in human
breath using an ICOS cell and a thermoelectrically cooled (TEC) pulsed QCL.
In this work, we use a novel and now commercially available TEC, CW, DFB quantum
cascade laser fabricated by our collaborating team from the University of Neuchatel,
Switzerland.9 DFB CW QCL characteristics, such as a narrow laser spectral width
( 3 MHz16),
necessary for efficient laser-to-cavity coupling and high average power, make CW TEC QC
lasers more suitable than pulsed QCLs for ICOS-based sensor platforms for real world
applications, avoiding the size and complications of liquid nitrogen cooling required by
earlier QC lasers. The basic sensor platform is an OA-ICOS configuration with a 50 cm-long
optical cavity. A wavelength modulation technique (harmonic detection) was implemented in
order to reach sub-ppbv levels of NO detection sensitivity.
Methodology and Equipment
A schematic of the ICOS-based sensor is depicted in Fig. 1. A TEC CW DFB QCL, operating at
5.45 mm9, was installed in a compact evacuated QCL
housing. The housing was equipped with a 25 mm diameter CaF2 window and
a single-stage thermoelectric cooler (Melcor Corporation, Type UT8-12-40F1) that provides
thermal control of the QC laser. A minimum QCL temperature of -32°C can be achieved when
the laser housing is at reduced pressures (typically ~ 10-3 to 10-4
Torr) and the hot side of TEC is actively water-cooled. A three-lens collimator (see Fig.
1) was employed to achieve optimum coupling of the QCL radiation to the OA-ICOS cavity.
The first lens after the QCL is a 25 mm diameter ZnSe aspherical lens with an
antireflection coating, and a 12.7 mm effective focal length. This reduces spherical
aberration thereby improving the quality of the beam coupled to the OA-ICOS cell. An iris
diaphragm in the focal point of the second lens serves as a spatial optical filter to
minimize fringes caused by the collimator lenses and the laser housing output window and
reduces back-reflection to the QCL by a factor of 5. The second and third lenses also have
diameters of 25 mm and effective focal lengths of 500 mm and 63 mm, respectively. The
collimated QCL beam incident on the OA-ICOS cavity has a diameter of 2.3 mm. Highly
reflective 50.8 mm diameter concave mirrors (1 m radius of curvature) separated by a 50 cm
stainless steel spacer form the optical OAICOS cavity.
Figure 1. TEC-CW-DFB QCL based OA-ICOS sensor. MCT is a cryogenically-cooled photovoltaic HgCdTe detector, and MCZT is a thermoelectrically-cooled HgCdZnTe photodetector (Vigo System, Model PVMI-10.6).
The same set of mirrors used in our previous study at a frequency of 1920 cm-1
(5.2 mm)20 was employed in this work. The
reflection coefficient for the ultra-low loss mirror set at 1920 cm-1 was
estimated to be R 
99.975 % using a CDRS approach.20 According to the mirror specifications, the
reflection coefficient at a laser frequency of 1836 cm-1, which was used in the
current study, is the same (with a discrepancy of ± 0.005%) according to the manufacturer
(Los Gatos Research, Inc.).
The longer cavity length of 50 cm was chosen in order to enhance the NO detection limit of the OA-ICOS technique. The cavity was aligned off-axis with respect to the laser beam providing improved cavity mode noise suppression. The suppression of cavity mode noise is the critical factor determining the sensitivity of the ICOS technique.18,20 Further mode noise suppression sensitivity enhancement was obtained by averaging the cavity resonances by dithering the cavity length using an assembly of 3 piezo-electric actuators attached to one of the resonator mirrors (See Fig. 1). The mirror was moved back and forth with a frequency of ~ 200 Hz and a maximum translation travel of ~ 15 mm.
The CW QCL utilized in this work was a 3 mm long, 14 mm wide, junction-up mounted DFB laser with a high reflection coating evaporated on its back facet. Spectral output, current and temperature tuning range, and rate of the QCL were evaluated using:
The laser operated at temperatures of up to +20°C with a total tuning range from ~
1827 cm-1 (+20°C) to ~ 1838 cm-1 (-32°C). The temperature tuning
rate was ~ 0.2 cm-1/K. A temperature controller (Wavelength Electronics, Inc.,
Model MPT-10000) provides a long-term temperature stability dT
of about 0.01°C,
limiting the spectral line shift to 0.002 cm-1 which is negligible compared to the NO linewidth
at 200 Torr (0.032 cm-1). At a fixed temperature of the laser thermal sink, the
frequency of the output radiation can be tuned by varying the QCL current. A current
driver (Wavelength Electronics, Inc., Model MPL-2500) was used to operate the QCL.
Temperature and current spectral tuning ranges of the QCL are depicted in Fig. 2 together
with a HITRAN based simulated absorption spectrum of an NO - H2O
- N2 mixture. The combined NO absorption line P1/2(11.5),
which is a superposition of two lambda doubling components centered at 1835.57 cm-1,
was selected for concentration measurements as it is the most intense line in the QCL
tuning range. A strong interference from H2O throughout the
entire spectral output range of the QCL can be avoided by using commercially available
Nafion dryer assembly (Perma Pure LLC) with an appropriate length and flow rate to meet
breath analysis requirements. The P1/2(11.5) line was reached at a temperature
of -21°C and a current of ~ 778 mA. Two function generators (Stanford Research Systems,
Model DS345) were utilized for QCL current ramping and for frequency modulation in
applying the wavelength modulation (WM) technique. The QCL ramp and WM frequencies were ~
1 kHz and ~ 50 kHz respectively.
Figure 2. HITRAN-based simulation of an absorption spectrum for a NO - N2 mixture in the tuning range of a TEC CW DFB QCL operating at 5.45 mm. The total pressure of the mixture is 200 Torr; pathlength is 700 m; a concentration of NO is 94.9 ppbv. The NO absorption line P1/2(11.5) at 1835.57 cm-1 (denoted by arrow) was used for the concentration measurements reported in this work.
The ICOS cavity output signal was focused onto a LN2 cooled photovoltaic HgCdTe detector with a built-in transimpedance preamplifier (Kolmar Technologies, Model KMPV8-1-J1/DC) by means of an off-axis aluminum parabolic mirror. After additional amplification (gain factor was 103) and filtering (low-pass filter was set to 1MHz) by a low noise amplifier (Stanford Research Systems, Model SR560), the detector signal was fed into a lock-in amplifier (Stanford Research Systems, Model SR 830) for second harmonic (2f) processing. The output of the lock-in was acquired by a data acquisition card installed in a PC using LabView 7.1. After further averaging, the data were stored in preparation for fitting. Initial feasibility NO concentration measurements with a long cell were made without the wavelength modulation technique. In this case, the amplified photodetector signal was directed straight to the data acquisition card. The QC laser linewidth estimated at ~ 35 ± 3 MHz was determined mainly by the current ripple on the QCL driver current source.
Experimental Activity, Results and Discussion
A calibration mixture (Scott Specialty Gases, Inc.) of a 94.9 ppbv NO in N2
as the balance gas was used for the evaluation of the QCL based NO sensor system.
The NO concentration was certified by the supplier using a chemiluminescence
technique (NO analyzer by GE Analytical Instruments, formerly Sievers). A diluter
was used to obtain up to 5 times lower NO concentration levels. Measurements were
made at a constant flow rate of ~ 100 sccm as adhesion to the ICOS cavity walls may affect
the precision and accuracy of the NO concentration measurements. Pressure levels
were varied from 50 to 200 Torr. A gas flow system described in detail in Bakhirkin et al.20
was utilized for measurement pressure and flow control.
Figure 3. Measured OA-ICOS spectrum of a 94.9 ppbv NO:N2 calibration mixture fitted by a Voigt function. Fit parameters: Lorentzian linewidth is ~ 2.61 × 10-2 cm-1 that is in a good agreement with a HITRAN predicted value of 2.63 × 102 cm-1, Gaussian (Doppler) width is ~ 0.41 × 10-2 cm-1. Minimum detectable NO concentration is 3.2 ppbv, obtained from the deviation of the area under absorption curve dA. Time constant (data acquisition time) is 1 s.
Initial NO concentration measurements with the 50 cm-long ICOS cell were made without applying wavelength modulation. The QC laser output was scanned by current with a frequency of 1 KHz across the absorption line. The cavity output signal was sampled, averaged, and processed using a data acquisition card and PC. Results are depicted in Fig. 3. The solid curve shows a 94.9 ppbv NO absorption line obtained from averaged ICOS signals. An absolute value of the absorption was obtained taking into account an offset resulting from the bimodal characteristic of the QCL. The number of acquired averages of spectra was 1000 and the typical data acquisition time 1 s. The dotted line represents a Voigt fit with parameters: Lorentzian width of ~2.61 × 10-2 cm-1 that is in a good agreement with the HITRAN predicted value of 2.63 × 10-2 cm-1 and a Gaussian linewidth of ~ 0.41 × 10-2 cm-1 determined by the temperature of the gas. Comparison between fitted data and HITRAN simulated spectra yielded an effective optical pathlength of 700 m provided that the line shape obeys a Voigt profile at 200 Torr. This is in a good agreement with our previous results for a compact 5.3 cm long cell obtained with the same set of mirrors that provided a 75 m effective optical pathlength. The effective optical pathlength scales with the physical length of the ICOS cavity. The discrepancy between the measured (~ 700m) and theoretical (~ 2000 m) effective optical pathlength of the OA-ICOS derived from the cavity ringdown time is attributed to several factors, such as: (1) non-homogeneity of the reflectivity across the 2 inch diameter mirror surfaces and, therefore, an uncertainty of the cavity decay time, (2) a large number of higher order transverse modes that are involved in OA-ICOS based gas measurements with higher diffraction losses than for the TEM00 mode which leads to a decrease of the effective optical pathlength, and (3) tilting of the PZT driven mirror which affects the OA-ICOS cavity alignment and, therefore, results in a decrease of the cavity finesse.
In order to improve sensitivity toward the NO target limit of 1 ppbv required for breath
analysis, wavelength modulation spectroscopy (WMS)13,22-25 was added to the
OA-ICOS-based sensor platform. In this case, the QCL current is modulated at a high
frequency n, and the wavelength is tuned slowly across the
spectral line of interest by means of a current ramp with a frequency W(n >> W).
The detector output is processed by a lockin amplifier referenced to the modulation
frequency 1f or 2f (first and second harmonic detection respectively). This type of
detection results in a significant improvement in the signal-tonoise-ratio (SNR).22
The amplitude of a WMS-based signal scales linearly with gas concentration,23
which facilitates calibration and a quantitative gas concentration measurement.
The OA-ICOS-based sensor can also benefit from applying WMS as reported in Refs. 20, 26, and 27. In the present work, WMS parameters (amplitude, modulation frequency and modulation function) and the gas pressure inside the ICOS cavity were optimized in order to maximize the signal. Sinusoidal modulation at n together with a triangular ramp at W was found to result in the optimum SNR. The frequencies were W = 1 KHz and n = 50 KHz, respectively. The lock-in amplifier output was averaged using a data acquisition card and LabViewbased software. A WMS signal for calibrated NO concentration was fitted using a general linear fit procedure,28 as shown in Fig. 4. A 1 s deviation if the amplitude as a fit result corresponds to 0.7 ppbv.
Figure 4. Averaged measured (2f ) signal for a NO concentration of 23.7 ppbv and fitting curve, obtained using a general linear fitting procedure.28 The standard deviation, s, of the fit coefficients corresponds to 0.7 ppbv.
Conclusions
An OA-ICOS-based NO sensor, which exploits the recent availability of a CW TEC DFB
QCL operating at 5.45 mm and a 50 cm-long high finesse
optical cavity, demonstrated the feasibility of detecting and quantifying NO
concentrations at a level of 1 ppbv in 1 s. The OA-ICOS technique yields a minimum NO detection
sensitivity of ~ 3 ppbv in 1 s. A combination of OA-ICOS and a 2f WMS technique leads to
minimum NO detection levels of ~ 0.7 ppbv. Additional improvement in sensitivity
can be obtained by using a more powerful (~50 mW) single frequency CW QCL10
operating at 1900.1 cm-1 (optimum frequency for interference free NO
detection21) and recently available 50 ppm ultra-low loss mirrors (instead of
the 250 ppm mirrors used in this work).
Acknowledgments
We acknowledge the assistance of Dr. Anatoliy Kosterev and Prof. R. F. Curl in providing
support and numerous useful discussions. The authors also acknowledge Prof. J. Faist of
the Institute of Physics, University of Neuchatel, Switzerland for providing the TEC
quantum cascade laser used in this work. Financial support of the work performed by the
Rice group was provided by the Institute for Space Systems Operations (ISSO), the National
Aeronautics and Space Administration (NASA), the Texas Advanced Technology Program, the
National Institute of Health via a sub contract from Physical Sciences, Inc., and the
Office of Naval Research via a sub-award from Texas A&M University and the Welch
Foundation.
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Publications
Bakhirkin, Y. A., A. A. Kosterev, R. Curl, F. K. Tittel, D. A. Yarekha, L. Hvozdara, M.
Giovannini, and J. Faist. "Subppbv Nitric Oxide Concentration Measurements Using cw
Thermoelectrically Cooled Quantum Cascade Laser-Based Integrated Cavity Output
Spectroscopy," Appl. Phys. B 82 (2006): 149-54; Online First (Nov. 2005).
Kosterev, A. A., Y. A. Bakhirkin, and F. K. Tittel. "Ultrasensitive Gas Detection by
Quartz-Enhanced Photoacoustic Apectroscopy in the Fundamental Molecular Absorption Bands
Region," Appl. Phys. B 80 (2005): 133-8.
Presentations
Bakhirkin, Y. A., and F K. Tittel. "Room-Temperature Continuous-Wave Quantum Cascade
Laser Based Nitric Oxide Measurements using Integrated Cavity Output Spectroscopy,"
SPIE Optics East Conf.: Infrared to Terahertz Technologies for Health and the Environment,
Boston, MA, Oct. 23-26, 2005.
McCurdy, M., S. So, G. Wysocki, Y. Bakhirkin, and F. K. Tittel. "A Versatile Breath
Collection Device for Laser-Based Trace Gas Detection," 22nd Annual Houston Conf. on
Biomedical Engineering Research, Houston, TX, Feb. 10-11 2005.
Tittel, F. K., A. A. Kosterev, Y. Bakhirkin, G. Wysocki, and S. G. So. "Semiconductor
Laser Based Trace Gas Sensor Technology Recent Aadvances and Applications,"
Mid-Infrared Coherent Sources, Barcelona, Spain, Nov. 6-11, 2005.
Tittel, F. K., A. A. Kosterev, Y. Bakhirkin, G.Wysocki, S. G. So, and R. F. Curl.
"Recent Advances of Quantum and Interband Cascade Laser Based Trace Gas Sensor
Technology," MIOMD-VII 2005, Lancaster, UK, Sept. 12-14 2005.
Tittel, F. K., A. A. Kosterev, Y. Bakhirkin, G. Wysocki, S. So, and R. F. Curl.
"Ultra Sensitive Gas Detection by Quartz Enhanced Photoacoustic Spectroscopy,"
Gordon Research Conf. on Photoacoustic and Photothermal Phenomena, Trieste, Italy, June
26-July 1, 2005.
Tittel, F. K., A. A. Kosterev, Y. Bakhirkin, S. G. So, G. Wysocki, and R. F. Curl.
"Semiconductor Laser Based Trace Gas Sensor Technology: Advances and
Challenges," Intl. Congress on Optics and Optoelectronics, Warsaw, Poland, Aug.
28-Sept. 2, 2005.
Tittel, F. K., A. A. Kosterev, Y. Bakhirkin, G. Wysocki, T. Ajtai, and, S. So.
"Recent Developments of Chemical Sensors Based on Quartz Enhanced Photoacoustic
Spectroscopy: Fundamentals and Applications," SPIE Optics East Conf.: Infrared to
Terahertz Technologies for Health and the Environment, Boston, MA, Oct. 23-26, 2005.
Tittel, F. K., A. A. Kosterev, Y. Bakhirkin, G. Wysocki, T. Ajtai, S. So, and R. F. Curl.
"Trace Gas Sensing Based on Quartz Enhanced Photoacoustic Spectroscopy," Intl.
Congress of Acoustics, Budapest, Hungary, Aug. 29-Sept. 2, 2005.
Funding and Proposals
Tittel, F. K. "Development of Advanced Mid-Infrared Laser Based Gas Sensors,"
NASA- JSC-JPL, June 1, 2003-Dec. 31, 2006, $367,333.
---. "High Resolution Spectroscopy with Lasers," Welch Foundation, June 1,
2003-May 31, 2006.
---. NASA-JSC Graduate Fellowship for Matt McCurdy, July 1, 2005-June 30, 2006, $24,000.
---. "Photonic Technologies for Early Detection of Human Disease," NASA-National
Cancer Institute, Rice U. PI, April 1, 2002-June 30, 2005, $367,515.
---. "Quantum Cascade Laser Based Sensors for Chemical and Environmental
Analysis," Texas Advanced Technology Program, Rice U. PI., Jan. 1, 2003-Aug. 30,
2006. $187,380.
---. "Quantum Cascade Laser Photoacoustic Sensor for Chemical Warfare Agent
Detection," Pacific Northwest National Laboratory, Rice U. PI, Jan. 3, 2005-Sept. 30,
2006, $210,000.
---. "Ultra-Sensensitive Detection of Aerosol Precursors Including Ammonia,"
Aculight, Botthell, WA, Sept. 9, 2005-March 26, 2006, $26,000.
Institute for Space Systems Operations - Y2005 Annual Report
Copyright © 2006
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