Eric J. Zhang

Shot-noise Limited Faraday Rotation Spectroscopy for Detection of Nitric Oxide Isotopes in Breath, Urine, and Blood

Measurement of NO and/or its metabolites in the various body compartments has transformed our understanding of biology. The inability of the current NO measurement methods to account for naturally occurring and experimental NO isotopes, however, has prevented the scientific community from fully understating NO metabolism in vivo. Here we present a mid-IR Faraday rotation spectrometer (FRS) for detection of NO isotopes. The instrument utilizes a novel dual modulation/demodulation (DM) FRS method which exhibits noise performance at only 2 times the fundamental quantum shot-noise level and provides the record sensitivity in its class. This is achieved with a system that is fully autonomous, robust, transportable, and does not require cryogenic cooling. The DM-FRS enables continuous monitoring of nitric oxide isotopes with the detection limits of 3.72 ppbv/Hz1/2 to14NO and 0.53 ppbv/Hz1/2 to15NO using only 45 cm active optical path. This DM-FRS measurement method can be used to improve the performance of conventional FRS sensors targeting other radical species. The feasibility of the instrument to perform measurements relevant to studies of NO metabolism in humans is demonstrated.

Hybrid Faraday rotation spectrometer for sub-ppm detection of atmospheric O 2

Faraday rotation spectroscopy (FRS) of O(2) is performed at atmospheric conditions using a DFB diode laser and permanent rare-earth magnets. Polarization rotation is detected with a hybrid-FRS detection method that combines the advantages of two conventional approaches: balanced optical-detection and conventional FRS with an optimized analyzer offset angle for maximum sensitivity enhancement. A measurement precision of 0.6 ppmv·Hz(-1/2) for atmospheric O(2) has been achieved. The theoretical model of hybrid detection is described, and the calculated detection limits are in excellent agreement with experimental values.

Wavelength modulated multiheterodyne spectroscopy using Fabry-Pérot quantum cascade lasers.

Multiheterodyne spectroscopy implemented with semiconductor Fabry-Pérot lasers is a method for broadband (> 20 cm-1), high spectral resolution (~1 MHz) and high time resolution (< 1 µs/spectrum) spectroscopy with no moving parts utilizing off-the-shelf laser sources. The laser stabilization approach demonstrated here enables continuous frequency tuning (at 12.5 Hz repetition rate) while allowing for multiheterodyne wavelength modulation spectroscopy (WMS). Spectroscopic detection of N2O around 1185 cm-1 is experimentally realized, which shows a direct absorption sensitivity limit of ~1.5⨯10-3/√Hz fractional absorption per mode. This can be lowered using WMS down to 5⨯10-4/√Hz per mode, limited by optical fringes. This approaches the range of sensitivities of standard single-mode laser based spectrometers, which demonstrates that the multiheterodyne method is well-suited for chemical sensing of spectrally broadened absorption features or for multi-species measurements.

Nitric Oxide Isotopic Analyzer Based on a Compact Dual-Modulation Faraday Rotation Spectrometer

We have developed a transportable spectroscopic nitrogen isotopic analyzer. The spectrometer is based on dual-modulation Faraday rotation spectroscopy of nitric oxide isotopologues with near shot-noise limited performance and baseline-free operation. Noise analysis indicates minor isotope (15NO) detection sensitivity of 0.36 ppbv·Hz−1/2, corresponding to noise-equivalent Faraday rotation angle (NEA) of 1.31 × 10−8 rad·Hz−1/2 and noise-equivalent absorbance (αL)min of 6.27 × 10−8 Hz−1/2. White-noise limited performance at 2.8× the shot-noise limit is observed up to ~1000 s, allowing reliable calibration and sample measurement within the drift-free interval of the spectrometer. Integration with wet-chemistry based on acidic vanadium(III) enables conversion of aqueous nitrate/nitrite samples to gaseous NO for total nitrogen isotope analysis. Isotopic ratiometry is accomplished via time-multiplexed measurements of two NO isotope transitions. For 5 μmol potassium nitrate samples, the instrument consistently yields ratiometric precision below 0.3‰, thus demonstrating potential as an in situ diagnostic tool for environmental nitrogen cycle studies.

Isotopic Ratiometry of Nitric Oxide using a Dual-modulation Faraday Rotation Spectrometer

A dual-modulation Faraday rotation spectrometer is employed for isotopic ratiometry of nitric oxide (NO) converted from nitrate/nitrite. Excellent linearity of measured NO to dissolved nitrate is demonstrated. Ratiometry of IAEA-NO-3 standards indicates ~3 % accuracy.

Differential Faraday Rotation Spectrometer for Detection of Nitric Oxide Isotopes

We present a differential dual-modulation Faraday rotation spectrometer for real-time detection of nitric oxide isotopologues. The prototype system provides > 27 dB signal cancellation for optical subtraction of the sample and calibration gas spectra.

Silicon Photonics for On-Chip Trace-Gas Spectroscopy

Natural gas leaks from production wells and pipelines pose a significant environmental risk due to the strong greenhouse effect caused by its main constituent, methane. We present a silicon photonic trace-gas sensing platform, which uses laser spectroscopy to realize cost-effective sensor networks for fugitive methane emission monitoring.

Noise properties in multi-heterodyne spectrometers based on quantum- and interband-cascade lasers

The noise properties of two multi-heterodyne spectrometers based on multi-mode semiconductor lasers (QCLs and ICLs) are investigated to determine their detection performance limitations. Both laser technologies provide well-defined multi-heterodyne beat-note structures suitable for spectroscopic measurements.

Faraday rotation spectroscopy of O 2 using a distributed feedback diode laser and a static magnetic field

Faraday rotation spectroscopy is used for measurement of O2 at atmospheric conditions. Low operating powers (<; 10 W) are achieved by using rare-earth magnets for DC magnetic field generation. A sensitivity of 1.7 ppmv/√Hz is demonstrated.