Measurement of CO2 by Wavelength Modulated Reinjection Off-Axis Integrated Cavity Output Spectroscopy at 2 μm

A high sensitivity wavelength modulated reinjection off-axis integrated cavity output spectroscopy (WM-RE-OA-ICOS) experimental setup was built at a 2 μm band. On the basis of an off-axis integrated output spectroscopy (OA-ICOS), combined with an optical reinjection (RE) approach to improve signal intensity, and wavelength modulation spectroscopy (WMS) to improve the signal-to-noise ratio (SNR) of the system, the experimental study of trace CO2 with high sensitivity was carried out using the setup. The performance was compared and evaluated, and the results show that: Compared with the OA-ICOS, the wavelength modulated reinjection OA-ICOS enhanced the signal intensity by 6.3 times, and the SNR increased 7.2 times from 179 to 1288. The Allan variance results showed that the detection limit of the system is 0.35 ppm when the average system time is 230 s. The setup was used to measure the indoor CO2 concentration for a long time (22 h), and the measured results were in line with the actual concentration change. The proposed method shows good performance enhancement for the OA-ICOS system in trace gas measurements.

Validity of Triply Labelled Water Analysis for Energy Expenditure Measurements in Mice

The Doubly Labelled Water (DLW) method is widely used to determine energy expenditure. In this work, we demonstrate the addition of the third stable isotope, 17O, to turn it into Triply Labelled Water (TLW), using the three isotopes measurement of optical spectrometry. We performed TLW (2H, 18O and17O) measurements for the analysis of the CO2 production (rCO2) of mice on different diets for the first time. Triply highly enriched water was injected into mice, and the isotope enrichments of the distilled blood samples of one initial and two finals were measured by an Off-Axis Integrated Cavity Output Spectroscopy instrument. We evaluated the impact of different calculation protocols and the values of evaporative water loss fraction. We found that the dilution space and turnover rates of 17O and 18O were equal for the same mice group, and that values of rCO2 calculated based on 18O-2H, or on 17O-2H agreed very well. This increases the reliability and redundancy of the measurements and it lowers the uncertainty in the calculated rCO2 to 3% when taking the average of two DLW methods. However, the TLW method overestimated the rCO2 compared to the indirect calorimetry measurements that we also performed, much more for the mice on a high-fat diet than for low-fat. We hypothesize an extra loss or exchange mechanism with a high fractionation for 2H to explain this difference.

Low-Threshold Microlasers Based on Holographic Dual-Gratings

Among the efforts to improve the performances of microlasers, optimization of the gain properties and cavity parameters of these lasers has attracted significant attention recently. Distributed feedback lasers, as one of the most promising candidate technologies for electrically pumped microlasers, can be combined with dual-gratings. This combination provides additional freedom for the design of the laser cavity. Here, a holographic dual-grating is designed to improve the distributed feedback laser performance. The holographic dual-grating laser consists of a colloidal quantum dot film with two parallel gratings, comprising first-order (210 nm) and second-order (420 nm) gratings that can be fabricated easily using a combination of spin coating and interference lithography. The feedback and the output from the cavity are controlled using the first-order grating and the second-order grating, respectively. Through careful design and analysis of the dual-grating, a balance is achieved between the feedback and the cavity output such that the lasing threshold based on the dual-grating is nearly half the threshold of conventional distributed feedback lasers. Additionally, the holographic dual-grating laser shows a high level of stability because of the high stability of the colloidal quantum dots against photobleaching.

In situ observations of stratospheric HCl using three-mirror integrated cavity output spectroscopy

Stratospheric HCl observations are an important diagnostic for the evaluation of catalytic processes that impact the ozone layer. We report here in situ balloon-borne observations of HCl employing an off-axis integrated cavity output spectrometer (OA-ICOS) fitted with a reinjection mirror. Laboratory assessments demonstrated that the spectrometer has a 90 % response time of 10 s to changes in HCl and a 30 s precision of 26 pptv. The instrument was deployed alongside an ozone instrument in August 2018 on a balloon-borne descent between 20–80 hPa (29–18 km altitude). The observations agreed with nearby satellite measurements made by the Earth Observing System Microwave Limb Sounder within 10 % on average. This is the first time that stratospheric measurements of HCl have been made with ICOS and the first time any cavity-enhanced HCl instrument has been tested in flight.

Measurement of δ18O and δ2H of water and ethanol in wine by Off-Axis Integrated Cavity Output Spectroscopy and Isotope Ratio Mass Spectrometry

There are two officially approved methods for stable isotope analysis for wine authentication. One describes δ18O measurements of the wine water using Isotope Ratio Mass Spectrometry (IRMS), and the other one uses Deuterium-Nuclear Magnetic Resonance (2H-NMR) to measure the deuterium of the wine ethanol. Recently, off-axis integrated cavity output (laser) spectroscopy (OA-ICOS) has become an easier alternative to quantify wine water isotopes, thanks to the spectral contaminant identifier (SCI). We utilized an OA-ICOS analyser with SCI to measure the δ18O and δ2H of water in 27 wine samples without any pre-treatment. The OA-ICOS results reveal a wealth of information about the growth conditions of the wines, which shows the advantages to extend the official δ18O wine water method by δ2H that is obtained easily from OA-ICOS. We also performed high-temperature pyrolysis and chromium reduction combined with IRMS measurements to illustrate the “whole wine” isotope ratios. The δ18O results of OA-ICOS and IRMS show non-significant differences, but the δ2H results of both methods differ much more. As the δ2H difference between these two methods is mainly caused by ethanol, we investigated the possibility to deduce deuterium of wine ethanol from this difference. The results present large uncertainties and deviate from the obtained 2H-NMR results. The deviation is caused by the other constituents in the wine, and the uncertainty is due to the limited precision of the SCI-based correction, which need to improve to obtain the 2H values of ethanol as alternative for the 2H-NMR method.

Measurement of OH radicals using off-axis integrated output spectroscopy (OA-ICOS) at 2.8 µm

<p>The hydroxyl (OH) free radical plays an important role in atmospheric chemistry due to its high reactivity with volatile organic compounds (VOCs) and trace species (CH<sub>4, </sub>CO, SO<sub>2</sub>, etc) [1]. Due to its very short lifetime (~1 s or less) and very low concentration in the atmosphere (in the order of 10<sup>6</sup> cm<sup>-</sup><sup>3</sup>), in situ and direct measurement of OH concentration in the atmosphere is challenging [2].</p><p>We report in this paper our recent work on developing a compact spectroscopic instrument based on off-axis integrated cavity output spectroscopy (OA-ICOS) [3] for optical monitoring of OH radicals. In the present work, OH radicals of ~10<sup>12</sup> OH radicals/cm<sup>3</sup> were generated from continue micro-wave discharge at 2.45 GHz of water vapor at low pressure (0.2-1 mbar), and were used as sample for validation of the developed OA-ICOS approaches. Two experimental approaches are designed for the measurements of OH radicals: (1) OA-ICOS [4] and wavelength modulation enhanced OA-ICOS (WM OA-ICOS) [5]. A distributed feedback (DFB) laser operating at 2.8 µm was employed for probing the Q (1.5e) and Q (1.5f) double-line transitions of the <sup>2</sup>Π<sub>3/2</sub><sub></sub>state at 3568.52382 and 3568.41693 cm<sup>-</sup><sup>1</sup>, respectively. A 1s detection limit of ~2.7×10<sup>10</sup> cm<sup>-3</sup>  was obtained for an averaging time of 125 s using a simple OA-ICOS scheme. This limit of detection is further improved by a factor of 3.4 using a WM OA-ICOS approach.</p><p>The experimental detail and the preliminary results will be presented and discussed.</p><p><strong> </strong><strong>Acknowledgments. </strong>The authors thank the financial supports from the CPER CLIMIBIO program and the Labex CaPPA project (ANR-10-LABX005).</p><p><strong>References</strong></p><p>[1]  U. Platt, M. Rateike, W. Junkermann, J. Rudolph, and D. H. Ehhalt, New tropospheric OH measurements, J. Geophys. Res. <strong>93</strong> (1988) 5159-5166.</p><p>[2]  D. E. Heard and M. J. Pilling, Measurement of OH and HO<sub>2</sub> in the Troposphere, Chem. Rev. <strong>103</strong> (2003) 5163-5198.</p><p>[3]  J. B. Paul, L. Lapson, J. G. Anderson, Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment, Appl. Opt. 40 (2001) 4904-4910.</p><p>[4]  W. Chen, A. A. Kosterev, F. K. Tittel, X. Gao, W. Zhao, "H<sub>2</sub>S trace concentration measurements using Off-Axis Integrated Cavity Output Spectroscopy in the near-infrared", Appl. Phys. B 90 (2008) 311-315</p><p>[5] W. Zhao, X. Gao, W. Chen, W. Zhang, T. Huang, T. Wu, H. Cha, Wavelength modulation off-axis integrated cavity output spectroscopy in the near infrared, Appl. Phys. B 86 (2007) 353-359</p>