Chu C. Teng

Detection of airborne chemical releases in densely populated urban environments requires precise sensors with high temporal and spatial resolution capable of covering large areas. For this purpose, we present a mobile mid-infrared quantum cascade laser dual-comb spectrometer for identification and quantification of chemical plumes. Field tests with the remote sensor were conducted during daytime in the downtown Boston area over a five day period during which chemical releases were simulated by intermittently emitting non-toxic substances. Open-air sensing was performed with retroreflectors positioned at up to 230 m distance and with sensitivities in the ppm m range for one second of averaging time. The field campaign demonstrates a step toward a semiconductor dual-comb spectroscopic sensor in the mid-infrared fingerprint region, suitable for long-term deployments. These types of sensors will be valuable complements to existing optical sensors for urban hazardous gas leak monitoring, air quality assessments, and localization of clandestine chemical production.

We present the operation of quantum cascade laser frequency combs in an external cavity configuration. Experimental observations show dependence of comb repetition rate and optical spectrum on the external cavity length. The low phase-noise comb regime is extended to a broader range of bias currents, enabling gapless frequency tuning of the comb modes. Dual-comb measurements also confirm improved comb stability in the presence of unwanted optical feedback when operating in an external cavity configuration. These observations indicate that aside from the continuing efforts to assure low and uniform dispersion characteristics of quantum cascade laser frequency combs, the proposed simple approach of adding a broadband external cavity can significantly enhance operation of sub-optimal devices for spectroscopic applications.

Faraday rotation spectroscopy (FRS) employs the Faraday effect to detect Zeeman splitting in the presence of a magnetic field. In this article, we present system design and implementation of radical sensing in a photolysis reactor using FRS. High sensitivity (100 ppb) and time resolved in situ HO₂ detection is enabled with a digitally balanced acquisition scheme. Specific advantages of employing FRS for sensing in such dynamic environments are examined and rigorously compared to the more established conventional laser absorption spectroscopy (LAS). Experimental results show that FRS enables HO₂ detection when LAS is deficient, and FRS compares favorably in terms of precision when LAS is applicable. The immunity of FRS to spectral interferences such as absorption of hydrocarbons and other diamagnetic species absorption and optical fringing are highlighted in comparison to LAS.

In optical spectroscopic systems where unwanted optical scattering cannot be eliminated, Fabry-Pérot etalons cause unpredictable changes in the spectral background. Frequent system calibration is then required to maintain the desired measurement accuracy, which presents a major limitation to the spectrometer. We introduce a computational approach to mitigate the adverse effects of optical fringing without hardware modifications. Motivated by experimental observations of complicated fringe behaviors, we simplify the problem by decomposing the fringe background into component etalons that can be addressed according to their individual characteristics. The effectiveness of the proposed method is demonstrated on a silicon photonic methane sensor, where accurate measurements of methane concentration are obtained from spectral data strongly affected by optical fringes.