Detection of cracks in concrete using near-IR fluorescence imaging.

Structural health monitoring of civil infrastructure is a crucial component of assuring the serviceability and integrity of the built environment. A primary material used in the construction of civil infrastructure is concrete, a material that is susceptible to cracking due to a variety of causes, such as shrinkage, creep, overloading, and temperature change. Cracking reduces the durability of concrete structures, as it allows deleterious environmental agents to penetrate the surface, causing such damage as corrosion of steel reinforcement and delamination of the concrete itself. Conventional crack detection techniques are limited in scope due to issues relating to pre-planning, accessibility, and the need for close proximity to the test surface. Contactless optical image monitoring techniques offer the opportunity to overcome these limitations and have the potential to detect cracks at a distance. Concrete has been reported to have a near-infrared (Near-IR) fluorescence line at a wavelength of 1140 nm when excited with red light. This work investigates the use of fluorescence imaging for the detection of cracks in cementitious surfaces using shallow angle incidence excitation red light. Light oriented at a shallow angle does not excite interior surfaces of cracks, which appear as darker features in images of fluorescing concrete. Artificial cracks with widths of 0.2-1.5 mm were readily imaged using a near-IR camera at distances of 0.5 and 1.3 m. An additional concrete sample with a 0.08 mm wide crack was produced using a flexure apparatus and was also imaged. It is worth noting that the 0.08 mm crack was detected despite its width being below the 0.1 mm pixel resolution of the camera, with the aid of digital image enhancement algorithms.

Mapping Refrigerant Gases in the New York City Skyline.

Cities are now home to more than 50% of the world's population and emit large quantities of pollutants from sources such as fossil fuel combustion and the leakage of refrigerants. We demonstrate the utility of persistent synoptic longwave hyperspectral imaging to study the ongoing leakage of refrigerant gases in New York City, compounds that either deplete the stratosphere ozone or have significant global warming potential. In contrast to current monitoring programs that are based on country-level reporting or aggregate measures of emissions, we present the identification of gaseous plumes with high spatial and temporal granularity in real-time over the skyline of Manhattan. The reported data highlights the emission of chemicals scheduled for phase-out. Our goal is to contribute to better understanding of the composition, sources, concentration, prevalence and patterns of emissions for the purposes of both research and policy.

Cohesion and Coalition Formation in the European Parliament: Roll-Call Votes and Twitter Activities.

We study the cohesion within and the coalitions between political groups in the Eighth European Parliament (2014-2019) by analyzing two entirely different aspects of the behavior of the Members of the European Parliament (MEPs) in the policy-making processes. On one hand, we analyze their co-voting patterns and, on the other, their retweeting behavior. We make use of two diverse datasets in the analysis. The first one is the roll-call vote dataset, where cohesion is regarded as the tendency to co-vote within a group, and a coalition is formed when the members of several groups exhibit a high degree of co-voting agreement on a subject. The second dataset comes from Twitter; it captures the retweeting (i.e., endorsing) behavior of the MEPs and implies cohesion (retweets within the same group) and coalitions (retweets between groups) from a completely different perspective. We employ two different methodologies to analyze the cohesion and coalitions. The first one is based on Krippendorff's Alpha reliability, used to measure the agreement between raters in data-analysis scenarios, and the second one is based on Exponential Random Graph Models, often used in social-network analysis. We give general insights into the cohesion of political groups in the European Parliament, explore whether coalitions are formed in the same way for different policy areas, and examine to what degree the retweeting behavior of MEPs corresponds to their co-voting patterns. A novel and interesting aspect of our work is the relationship between the co-voting and retweeting patterns.

Ultrasensitive, real-time trace gas detection using a high-power, multimode diode laser and cavity ringdown spectroscopy.

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 (Δλlaser∼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 (Δλabsorption≫Δλlaser), 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 (NO2) in zero air. A sensitivity of 38 parts in 1012 (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 NO2 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.

Real-time trace gas sensor using a multimode diode laser and multiple-line integrated cavity enhanced absorption spectroscopy.

We describe and demonstrate a highly sensitive trace gas sensor based on a simplified design that is capable of measuring sub-ppb concentrations of NO2 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 NO2 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.

Extremely sensitive detection of NO2 employing off-axis integrated cavity output spectroscopy coupled with multiple-line integrated absorption spectroscopy.

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 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 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 versus concentration of NO2 and the standard deviation of the integrated absorption signal, we report an effective sensitivity of detection of approximately 28 ppt (parts in 10¹²) for NO2 To the best of our knowledge, this is among the most sensitive levels of detection of NO2 to date.

External cavity tunable quantum cascade lasers and their applications to trace gas monitoring.

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.

High sensitivity detection of NO2 employing cavity ringdown spectroscopy and an external cavity continuously tunable quantum cascade laser.

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.

Enhancement of trace gas detection by integrating wavelength modulated spectra across multiple lines.

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).

Enhanced sensitivity for the detection of trace gases using multiple line integrated absorption spectroscopy.

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 NO2 across the R branch from 1628.8 to 1634.5 cm(-1). By integrating the resulting spectra, the detection sensitivity of NO2 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.

Absorption and wavelength modulation spectroscopy of NO2 using a tunable, external cavity continuous wave quantum cascade laser.

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 (approximately 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.