Gottipaty Rao - Academia.edu (original) (raw)

Papers by Gottipaty Rao

Conference on Lasers and Electro-Optics, 2016

A simple ultra-sensitive real time trace gas detector using a multi-mode Fabry-Perot diode laser ... more A simple ultra-sensitive real time trace gas detector using a multi-mode Fabry-Perot diode laser and cavity ring-down spectroscopy has been developed to detect NO2 with a single shot sensitivity of 530 ppt in 60 μs.

Applied Optics, 2016

We present a simplified cavity ringdown (CRD) trace gas detection technique that is insensitive t... more We present a simplified cavity ringdown (CRD) trace gas detection technique that is insensitive to vibration, and capable of extremely sensitive, real-time absorption measurements. A high-power, multimode Fabry-Perot (FP) diode laser with a broad wavelength range (Δλ laser ∼ 0.6 nm) is used to excite a large number of cavity modes, thereby reducing the detector's susceptibility to vibration and making it well suited for field deployment. When detecting molecular species with broad absorption features (Δλ absorption ≫ Δλ laser), the laser's broad linewidth removes the need for precision wavelength stabilization. The laser's power and broad linewidth allow the use of on-axis cavity alignment, improving the signal-to-noise ratio while maintaining its vibration insensitivity. The use of an FP diode laser has the added advantages of being inexpensive, compact, and insensitive to vibration. The technique was demonstrated using a 1.1 W (λ 400 nm) diode laser to measure low concentrations of nitrogen dioxide (NO 2) in zero air. A sensitivity of 38 parts in 10 12 (ppt) was achieved using an integration time of 128 ms; for single-shot detection, 530 ppt sensitivity was demonstrated with a measurement time of 60 μs, which opens the door to sensitive measurements with extremely high temporal resolution; to the best of our knowledge, these are the highest speed measurements of NO 2 concentration using CRD spectroscopy. The reduced susceptibility to vibration was demonstrated by introducing small vibrations into the apparatus and observing that there was no measurable effect on the sensitivity of detection.

Applied Optics, 2015

We describe and demonstrate a highly sensitive trace gas sensor based on a simplified design that... more We describe and demonstrate a highly sensitive trace gas sensor based on a simplified design that is capable of measuring sub-ppb concentrations of NO 2 in tens of milliseconds. The sensor makes use of a relatively inexpensive Fabry-Perot diode laser to conduct off-axis cavity enhanced spectroscopy. The broad frequency range of a multimode Fabry-Perot diode laser spans a large number of absorption lines, thereby removing the need for a single-frequency tunable laser source. The use of cavity enhanced absorption spectroscopy enhances the sensitivity of the sensor by providing a pathlength on the order of 1 km in a small volume. Off-axis alignment excites a large number of cavity modes simultaneously, thereby reducing the sensor's susceptibility to vibration. Multiple-line integrated absorption spectroscopy (where one integrates the absorption spectra over a large number of rovibronic transitions of the molecular species) further improves the sensitivity of detection. Relatively high laser power ∼400 mW is used to compensate for the low coupling efficiency of a broad linewidth laser to the optical cavity. The approach was demonstrated using a 407 nm diode laser to detect trace quantities of NO 2 in zero air. Sensitivities of 750 ppt, 110 ppt, and 65 ppt were achieved using integration times of 50 ms, 5 s, and 20 s respectively.

SPIE Proceedings, 2011

We report on the development of a new sensor for NO2 with ultrahigh sensitivity of detection. Thi... more We report on the development of a new sensor for NO2 with ultrahigh sensitivity of detection. This has been accomplished by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several km in a small volume cell) with multiple line integrated absorption spectroscopy (MLIAS) (where we integrate the absorption spectra over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity tunable quantum cascade laser operating in the 1601 - 1670 cm-1 range and a high-finesse optical cavity, the absorption spectra of NO2 over 100 transitions in the R-band have been recorded. From the observed linear relationship between the integrated absorption vs. concentration of NO2, we report an effective sensitivity of detection of 10 ppt for NO2. To the best of our knowledge, this is among the most sensitive levels of detection of NO2 to date. A sensitive sensor for the detection of NO2 will be helpful to monitor the ambient air quality, combustion emissions from the automobiles, power plants, aircraft and for the detection of nitrate based explosives (which are commonly used in improvised explosives (IEDs)). Additionally such a sensor would be valuable for the study of complex chemical reactions that undergo in the atmosphere resulting in the formation of photochemical smog, tropospheric ozone and acid rain.

Advanced Environmental, Chemical, and Biological Sensing Technologies XII, 2015

We report on the measurement of the NO2 concentration in ambient air using a highly sensitive tra... more We report on the measurement of the NO2 concentration in ambient air using a highly sensitive trace gas detector based on a simplified design that uses a multi-mode diode laser and cavity enhanced multiple line integrated absorption spectroscopy. The broad frequency range of the laser covers a large number of absorption lines of NO2 and excites a large number of cavity modes. The off-axis cavity arrangement reduces the detector’s susceptibility to vibration. The detector was calibrated using known concentrations of NO2 in zero air and used to measure the concentration of NO2 in ambient air in our laboratory. A sensitivity of 75 ppt was achieved.

CLEO: 2013, 2013

A violet multi-mode diode laser and cavity ring-down spectroscopy were used to detect trace amoun... more A violet multi-mode diode laser and cavity ring-down spectroscopy were used to detect trace amounts of NO2. The laser excites multiple cavity modes, simplifying the detector alignment and making it less susceptible to vibration.

Conference on Lasers and Electro-Optics 2010, 2010

We demonstrate that employing integrated absorption of multiple lines or summation of the absolut... more We demonstrate that employing integrated absorption of multiple lines or summation of the absolute values of the wavelength modulation spectroscopy signals, sensitivities of ppb can be achieved for the detection of trace gases.

CLEO:2011 - Laser Applications to Photonic Applications, 2011

Employing an external cavity QCL and using OA-ICOS and multiple line integrated absorption spectr... more Employing an external cavity QCL and using OA-ICOS and multiple line integrated absorption spectroscopy techniques, we report an effective sensitivity of detection of ∼28 ppt for NO2 in Zero-Air.

CLEO: 2014, 2014

A simplified trace gas detector using a multi-mode Fabry-Perot diode laser and cavity enhanced ab... more A simplified trace gas detector using a multi-mode Fabry-Perot diode laser and cavity enhanced absorption spectroscopy has been developed and used to detect NO2 at the 100 ppt level.

SPIE Proceedings, 2012

We describe and demonstrate a trace gas detector using a variation of cavity ring down spectrosco... more We describe and demonstrate a trace gas detector using a variation of cavity ring down spectroscopy (CRDS) in which we modulate the laser frequency and use off-axis alignment to simplify the sensor design and make it more suitable for deployment in vibration-prone environments. The use of off-axis alignment reduces the sensitivity to vibration and optical feedback. Modulation of the laser frequency eliminates signal fluctuations that can arise due to laser drift and incomplete averaging of the cavity modes, and thus removes the need for precision locking or frequency stabilization of the laser.

Lasers, Sources, and Related Photonic Devices, 2012

Trace detection of NO2 at ppb-level is demonstrated using a vibration-insensitive variation of ca... more Trace detection of NO2 at ppb-level is demonstrated using a vibration-insensitive variation of cavity ring-down spectroscopy. Modulating the laser frequency and off-axis alignment simplify the detector making it more suitable for in-situ monitoring.

Conference on Lasers and Electro-Optics 2012, 2012

Detection of NO2 at the ppb-level is demonstrated using a vibration-insensitive variation of cavi... more Detection of NO2 at the ppb-level is demonstrated using a vibration-insensitive variation of cavity ring-down spectroscopy. Modulating the laser frequency and off-axis alignment simplify the detector making it more suitable for in-situ monitoring.

Applied Optics, 2009

The absorption spectra and wavelength modulation spectroscopy (WMS) of NO 2 using a tunable, exte... more The absorption spectra and wavelength modulation spectroscopy (WMS) of NO 2 using a tunable, external cavity CW quantum cascade laser operating at room temperature in the region of 1625 to 1645 cm −1 are reported. The external cavity quantum cascade laser enabled us to record continuous absorption spectra of low concentrations of NO 2 over a broad range (∼16 cm −1), demonstrating the potential for simultaneously recording the complex spectra of multiple species. This capability allows the identification of a particular species of interest with high sensitivity and selectivity. The measured spectra are in excellent agreement with the spectra from the high-resolution transmission molecular absorption database [J. Quant. Spectrosc. Radiat. Transfer 96, 139-204 (2005)]. We also conduct WMS for the first time using an external cavity quantum cascade laser, a technique that enhances the sensitivity of detection. By employing WMS, we could detect low-intensity absorption lines, which are not visible in the simple absorption spectra, and demonstrate a minimum detection limit at the 100 ppb level with a short-path absorption cell. Details of the tunable, external cavity quantum cascade laser system and its performance are discussed.

Applied Optics, 2010

A trace gas sensor for the detection of nitrogen dioxide based on cavity ringdown spectroscopy (C... more A trace gas sensor for the detection of nitrogen dioxide based on cavity ringdown spectroscopy (CRDS) and a continuous wave external cavity tunable quantum cascade laser operating at room temperature has been designed, and its features and performance characteristics are reported. By measuring the ringdown times of the cavity at different concentrations of NO 2 , we report a sensitivity of 1:2 ppb for the detection of NO 2 in Zero Air.

Applied Optics, 2011

Since the first quantum cascade laser (QCL) was demonstrated approximately 16 years ago, we have ... more Since the first quantum cascade laser (QCL) was demonstrated approximately 16 years ago, we have witnessed an explosion of interesting developments in QCL technology and QCL-based trace gas sensors. QCLs operate in the mid-IR region (3-24 μm) and can directly access the rotational vibrational bands of most molecular species and, therefore, are ideally suited for trace gas detection with high specificity and sensitivity. These sensors have applications in a wide range of fields, including environmental monitoring, atmospheric chemistry, medical diagnostics, homeland security, detection of explosive compounds, and industrial process control, to name a few. Tunable external cavity (EC)-QCLs in particular offer narrow linewidths, wide ranges of tunability, and stable power outputs, which open up new possibilities for sensor development. These features allow for the simultaneous detection of multiple species and the study of large molecules, free radicals, ions, and reaction kinetics. In this article, we review the current status of EC-QCLs and sensor developments based on them and speculate on possible future developments.

Applied Optics, 2011

We report on the development of a new sensor for NO 2 with ultrahigh sensitivity of detection. Th... more We report on the development of a new sensor for NO 2 with ultrahigh sensitivity of detection. This has been accomplished by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several kilometers in a small volume cell) with multiple-line integrated absorption spectroscopy (MLIAS) (where we integrate the absorption spectra over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity quantum cascade laser operating in the 1601-1670 cm −1 range and a highfinesse optical cavity, the absorption spectra of NO 2 over 100 transitions in the R band have been recorded. From the observed linear relationship between the integrated absorption versus concentration of NO 2 and the standard deviation of the integrated absorption signal, we report an effective sensitivity of detection of approximately 28 ppt (parts in 10 12) for NO 2. To the best of our knowledge, this is among the most sensitive levels of detection of NO 2 to date.

Applied Optics, 2009

We describe a technique that enhances the sensitivity of a spectrometer for trace gas detection e... more We describe a technique that enhances the sensitivity of a spectrometer for trace gas detection employing an external cavity continuously tunable CW quantum cascade laser and integrating the absorption spectra across multiple lines of the species. We demonstrate the power of this method by continuously recording the absorption spectra of NO 2 across the R branch from 1628.8 to 1634:5 cm −1. By integrating the resulting spectra, the detection sensitivity of NO 2 is improved by a factor of 15 compared to the sensitivity achieved using single line laser absorption spectroscopy with the same apparatus. This procedure offers much shorter data acquisition times for the real-time monitoring of trace gas species compared to adding repeated scans of the spectra to improve the signal-to-noise ratio.

American Journal of Physics, 2009

Frequency modulation and harmonic detection are extensively employed in a variety of experimental... more Frequency modulation and harmonic detection are extensively employed in a variety of experimental techniques and offer a detection sensitivity limited only by quantum noise. We discuss a simple experiment using wavelength modulation spectroscopy and near infrared diode lasers operating in the 780 nm region to detect rubidium hyperfine spectra. Modulation spectroscopy techniques are employed to improve the signal to noise ratio for the detection of weak signals and thus significantly improve the sensitivity and specificity for detection. By using quantum cascade lasers and wavelength modulation spectroscopy, one can detect trace species at sub-ppm level, which has numerous applications in diverse areas such as medical diagnostics, food processing, industrial applications, and environmental monitoring.

Applied Optics, 2010

We describe and demonstrate a technique that enhances the sensitivity of a spectrometer for trace... more We describe and demonstrate a technique that enhances the sensitivity of a spectrometer for trace gas detection by employing wavelength modulation spectroscopy (WMS) and integrating the absolute value of the recorded spectra across multiple lines of the species. The sensitivity is further enhanced by conducting WMS with large modulation depths. This technique is demonstrated using a continuously tunable external cavity CW quantum cascade laser to record the second harmonic wavelength modulated spectra of NO 2 across the peak of the R-branch from 1629.5 to 1633:9 cm −1. By integrating the absolute value of the resulting spectra, the detection sensitivity of NO 2 is improved by a factor of 40 compared to the sensitivity achieved using single-line WMS with the same apparatus. By using this technique, we achieve a sensitivity of approximately 6 parts in 10 9 (ppb) using a short-path cell (a 1 m absorption cell with two passes).

Conference on Lasers and Electro-Optics, 2016

A simple ultra-sensitive real time trace gas detector using a multi-mode Fabry-Perot diode laser ... more A simple ultra-sensitive real time trace gas detector using a multi-mode Fabry-Perot diode laser and cavity ring-down spectroscopy has been developed to detect NO2 with a single shot sensitivity of 530 ppt in 60 μs.

Applied Optics, 2016

We present a simplified cavity ringdown (CRD) trace gas detection technique that is insensitive t... more We present a simplified cavity ringdown (CRD) trace gas detection technique that is insensitive to vibration, and capable of extremely sensitive, real-time absorption measurements. A high-power, multimode Fabry-Perot (FP) diode laser with a broad wavelength range (Δλ laser ∼ 0.6 nm) is used to excite a large number of cavity modes, thereby reducing the detector's susceptibility to vibration and making it well suited for field deployment. When detecting molecular species with broad absorption features (Δλ absorption ≫ Δλ laser), the laser's broad linewidth removes the need for precision wavelength stabilization. The laser's power and broad linewidth allow the use of on-axis cavity alignment, improving the signal-to-noise ratio while maintaining its vibration insensitivity. The use of an FP diode laser has the added advantages of being inexpensive, compact, and insensitive to vibration. The technique was demonstrated using a 1.1 W (λ 400 nm) diode laser to measure low concentrations of nitrogen dioxide (NO 2) in zero air. A sensitivity of 38 parts in 10 12 (ppt) was achieved using an integration time of 128 ms; for single-shot detection, 530 ppt sensitivity was demonstrated with a measurement time of 60 μs, which opens the door to sensitive measurements with extremely high temporal resolution; to the best of our knowledge, these are the highest speed measurements of NO 2 concentration using CRD spectroscopy. The reduced susceptibility to vibration was demonstrated by introducing small vibrations into the apparatus and observing that there was no measurable effect on the sensitivity of detection.

Applied Optics, 2015

We describe and demonstrate a highly sensitive trace gas sensor based on a simplified design that... more We describe and demonstrate a highly sensitive trace gas sensor based on a simplified design that is capable of measuring sub-ppb concentrations of NO 2 in tens of milliseconds. The sensor makes use of a relatively inexpensive Fabry-Perot diode laser to conduct off-axis cavity enhanced spectroscopy. The broad frequency range of a multimode Fabry-Perot diode laser spans a large number of absorption lines, thereby removing the need for a single-frequency tunable laser source. The use of cavity enhanced absorption spectroscopy enhances the sensitivity of the sensor by providing a pathlength on the order of 1 km in a small volume. Off-axis alignment excites a large number of cavity modes simultaneously, thereby reducing the sensor's susceptibility to vibration. Multiple-line integrated absorption spectroscopy (where one integrates the absorption spectra over a large number of rovibronic transitions of the molecular species) further improves the sensitivity of detection. Relatively high laser power ∼400 mW is used to compensate for the low coupling efficiency of a broad linewidth laser to the optical cavity. The approach was demonstrated using a 407 nm diode laser to detect trace quantities of NO 2 in zero air. Sensitivities of 750 ppt, 110 ppt, and 65 ppt were achieved using integration times of 50 ms, 5 s, and 20 s respectively.

SPIE Proceedings, 2011

We report on the development of a new sensor for NO2 with ultrahigh sensitivity of detection. Thi... more We report on the development of a new sensor for NO2 with ultrahigh sensitivity of detection. This has been accomplished by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several km in a small volume cell) with multiple line integrated absorption spectroscopy (MLIAS) (where we integrate the absorption spectra over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity tunable quantum cascade laser operating in the 1601 - 1670 cm-1 range and a high-finesse optical cavity, the absorption spectra of NO2 over 100 transitions in the R-band have been recorded. From the observed linear relationship between the integrated absorption vs. concentration of NO2, we report an effective sensitivity of detection of 10 ppt for NO2. To the best of our knowledge, this is among the most sensitive levels of detection of NO2 to date. A sensitive sensor for the detection of NO2 will be helpful to monitor the ambient air quality, combustion emissions from the automobiles, power plants, aircraft and for the detection of nitrate based explosives (which are commonly used in improvised explosives (IEDs)). Additionally such a sensor would be valuable for the study of complex chemical reactions that undergo in the atmosphere resulting in the formation of photochemical smog, tropospheric ozone and acid rain.

Advanced Environmental, Chemical, and Biological Sensing Technologies XII, 2015

We report on the measurement of the NO2 concentration in ambient air using a highly sensitive tra... more We report on the measurement of the NO2 concentration in ambient air using a highly sensitive trace gas detector based on a simplified design that uses a multi-mode diode laser and cavity enhanced multiple line integrated absorption spectroscopy. The broad frequency range of the laser covers a large number of absorption lines of NO2 and excites a large number of cavity modes. The off-axis cavity arrangement reduces the detector’s susceptibility to vibration. The detector was calibrated using known concentrations of NO2 in zero air and used to measure the concentration of NO2 in ambient air in our laboratory. A sensitivity of 75 ppt was achieved.

CLEO: 2013, 2013

A violet multi-mode diode laser and cavity ring-down spectroscopy were used to detect trace amoun... more A violet multi-mode diode laser and cavity ring-down spectroscopy were used to detect trace amounts of NO2. The laser excites multiple cavity modes, simplifying the detector alignment and making it less susceptible to vibration.

Conference on Lasers and Electro-Optics 2010, 2010

We demonstrate that employing integrated absorption of multiple lines or summation of the absolut... more We demonstrate that employing integrated absorption of multiple lines or summation of the absolute values of the wavelength modulation spectroscopy signals, sensitivities of ppb can be achieved for the detection of trace gases.

CLEO:2011 - Laser Applications to Photonic Applications, 2011

Employing an external cavity QCL and using OA-ICOS and multiple line integrated absorption spectr... more Employing an external cavity QCL and using OA-ICOS and multiple line integrated absorption spectroscopy techniques, we report an effective sensitivity of detection of ∼28 ppt for NO2 in Zero-Air.

CLEO: 2014, 2014

A simplified trace gas detector using a multi-mode Fabry-Perot diode laser and cavity enhanced ab... more A simplified trace gas detector using a multi-mode Fabry-Perot diode laser and cavity enhanced absorption spectroscopy has been developed and used to detect NO2 at the 100 ppt level.

SPIE Proceedings, 2012

We describe and demonstrate a trace gas detector using a variation of cavity ring down spectrosco... more We describe and demonstrate a trace gas detector using a variation of cavity ring down spectroscopy (CRDS) in which we modulate the laser frequency and use off-axis alignment to simplify the sensor design and make it more suitable for deployment in vibration-prone environments. The use of off-axis alignment reduces the sensitivity to vibration and optical feedback. Modulation of the laser frequency eliminates signal fluctuations that can arise due to laser drift and incomplete averaging of the cavity modes, and thus removes the need for precision locking or frequency stabilization of the laser.

Lasers, Sources, and Related Photonic Devices, 2012

Trace detection of NO2 at ppb-level is demonstrated using a vibration-insensitive variation of ca... more Trace detection of NO2 at ppb-level is demonstrated using a vibration-insensitive variation of cavity ring-down spectroscopy. Modulating the laser frequency and off-axis alignment simplify the detector making it more suitable for in-situ monitoring.

Conference on Lasers and Electro-Optics 2012, 2012

Detection of NO2 at the ppb-level is demonstrated using a vibration-insensitive variation of cavi... more Detection of NO2 at the ppb-level is demonstrated using a vibration-insensitive variation of cavity ring-down spectroscopy. Modulating the laser frequency and off-axis alignment simplify the detector making it more suitable for in-situ monitoring.

Applied Optics, 2009

The absorption spectra and wavelength modulation spectroscopy (WMS) of NO 2 using a tunable, exte... more The absorption spectra and wavelength modulation spectroscopy (WMS) of NO 2 using a tunable, external cavity CW quantum cascade laser operating at room temperature in the region of 1625 to 1645 cm −1 are reported. The external cavity quantum cascade laser enabled us to record continuous absorption spectra of low concentrations of NO 2 over a broad range (∼16 cm −1), demonstrating the potential for simultaneously recording the complex spectra of multiple species. This capability allows the identification of a particular species of interest with high sensitivity and selectivity. The measured spectra are in excellent agreement with the spectra from the high-resolution transmission molecular absorption database [J. Quant. Spectrosc. Radiat. Transfer 96, 139-204 (2005)]. We also conduct WMS for the first time using an external cavity quantum cascade laser, a technique that enhances the sensitivity of detection. By employing WMS, we could detect low-intensity absorption lines, which are not visible in the simple absorption spectra, and demonstrate a minimum detection limit at the 100 ppb level with a short-path absorption cell. Details of the tunable, external cavity quantum cascade laser system and its performance are discussed.

Applied Optics, 2010

A trace gas sensor for the detection of nitrogen dioxide based on cavity ringdown spectroscopy (C... more A trace gas sensor for the detection of nitrogen dioxide based on cavity ringdown spectroscopy (CRDS) and a continuous wave external cavity tunable quantum cascade laser operating at room temperature has been designed, and its features and performance characteristics are reported. By measuring the ringdown times of the cavity at different concentrations of NO 2 , we report a sensitivity of 1:2 ppb for the detection of NO 2 in Zero Air.

Applied Optics, 2011

Since the first quantum cascade laser (QCL) was demonstrated approximately 16 years ago, we have ... more Since the first quantum cascade laser (QCL) was demonstrated approximately 16 years ago, we have witnessed an explosion of interesting developments in QCL technology and QCL-based trace gas sensors. QCLs operate in the mid-IR region (3-24 μm) and can directly access the rotational vibrational bands of most molecular species and, therefore, are ideally suited for trace gas detection with high specificity and sensitivity. These sensors have applications in a wide range of fields, including environmental monitoring, atmospheric chemistry, medical diagnostics, homeland security, detection of explosive compounds, and industrial process control, to name a few. Tunable external cavity (EC)-QCLs in particular offer narrow linewidths, wide ranges of tunability, and stable power outputs, which open up new possibilities for sensor development. These features allow for the simultaneous detection of multiple species and the study of large molecules, free radicals, ions, and reaction kinetics. In this article, we review the current status of EC-QCLs and sensor developments based on them and speculate on possible future developments.

Applied Optics, 2011

We report on the development of a new sensor for NO 2 with ultrahigh sensitivity of detection. Th... more We report on the development of a new sensor for NO 2 with ultrahigh sensitivity of detection. This has been accomplished by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several kilometers in a small volume cell) with multiple-line integrated absorption spectroscopy (MLIAS) (where we integrate the absorption spectra over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity quantum cascade laser operating in the 1601-1670 cm −1 range and a highfinesse optical cavity, the absorption spectra of NO 2 over 100 transitions in the R band have been recorded. From the observed linear relationship between the integrated absorption versus concentration of NO 2 and the standard deviation of the integrated absorption signal, we report an effective sensitivity of detection of approximately 28 ppt (parts in 10 12) for NO 2. To the best of our knowledge, this is among the most sensitive levels of detection of NO 2 to date.

Applied Optics, 2009

We describe a technique that enhances the sensitivity of a spectrometer for trace gas detection e... more We describe a technique that enhances the sensitivity of a spectrometer for trace gas detection employing an external cavity continuously tunable CW quantum cascade laser and integrating the absorption spectra across multiple lines of the species. We demonstrate the power of this method by continuously recording the absorption spectra of NO 2 across the R branch from 1628.8 to 1634:5 cm −1. By integrating the resulting spectra, the detection sensitivity of NO 2 is improved by a factor of 15 compared to the sensitivity achieved using single line laser absorption spectroscopy with the same apparatus. This procedure offers much shorter data acquisition times for the real-time monitoring of trace gas species compared to adding repeated scans of the spectra to improve the signal-to-noise ratio.

American Journal of Physics, 2009

Frequency modulation and harmonic detection are extensively employed in a variety of experimental... more Frequency modulation and harmonic detection are extensively employed in a variety of experimental techniques and offer a detection sensitivity limited only by quantum noise. We discuss a simple experiment using wavelength modulation spectroscopy and near infrared diode lasers operating in the 780 nm region to detect rubidium hyperfine spectra. Modulation spectroscopy techniques are employed to improve the signal to noise ratio for the detection of weak signals and thus significantly improve the sensitivity and specificity for detection. By using quantum cascade lasers and wavelength modulation spectroscopy, one can detect trace species at sub-ppm level, which has numerous applications in diverse areas such as medical diagnostics, food processing, industrial applications, and environmental monitoring.

Applied Optics, 2010

We describe and demonstrate a technique that enhances the sensitivity of a spectrometer for trace... more We describe and demonstrate a technique that enhances the sensitivity of a spectrometer for trace gas detection by employing wavelength modulation spectroscopy (WMS) and integrating the absolute value of the recorded spectra across multiple lines of the species. The sensitivity is further enhanced by conducting WMS with large modulation depths. This technique is demonstrated using a continuously tunable external cavity CW quantum cascade laser to record the second harmonic wavelength modulated spectra of NO 2 across the peak of the R-branch from 1629.5 to 1633:9 cm −1. By integrating the absolute value of the resulting spectra, the detection sensitivity of NO 2 is improved by a factor of 40 compared to the sensitivity achieved using single-line WMS with the same apparatus. By using this technique, we achieve a sensitivity of approximately 6 parts in 10 9 (ppb) using a short-path cell (a 1 m absorption cell with two passes).