Recent advances in the development of trace gas sensors based on the use of quantum cascade lasers
(QCLs) for the sensitive, selective detection, quantification and monitoring of small molecular gas
species with resolved spectroscopic features will be described. High detection sensitivity at ppbv and
sub-ppbv concentration levels require detection sensitivity, enhancement schemes such as multipass
absorption cells, cavity enhanced absorption techniques, or quartz enhanced photo-acoustic
absorption spectroscopy (QEPAS). These three spectroscopic methods can achieve minimum detectable absorption losses in the range from 10-8 to 10-11 cm-1/√Hz. Two recent examples of real world applications of field deployable PAS and QEPAS based gas sensors will be reported, namely the monitoring of ammonia concentrations in exhaled human breath and major urban environments.
Faraday Rotation Spectroscopy (FRS) is a technique for the sensitive and selective detection of paramagnetic molecules
or radicals such as NO, NO2, O2 or OH-. Moreover FRS is suitable for atmospheric measurements due to the insensitivity
to non-paramagnetic interfering molecules such as H2O and CO2. Experimental results of an FRS sensor for the NO2detection employing an external-cavity quantum cascade laser (EC-QCL) are reported. The CW EC-QCL exhibits modehop
free (MHF) tuning between 1600 cm-1 and 1650 cm-1. This allows targeting the optimum 441←440 Q-branch NO2transition at 1613.25 cm-1. A rotation of the polarization state of the initially linearly polarized laser light is observed
when an AC magnetic field is applied to the NO2 cell, placed between two nearly crossed Rochon polarizers. This
rotation of the polarization state is proportional to the NO2 concentration and can be determined by a photodetector
located after the second polarizer. For long-term continuous measurements a second branch consisting of a detector and
reference cell filled with 0.2 % NO2 in N2 is used to lock the laser to the selected NO2 transition. A minimum detection
sensitivity (1σ) of 1 parts per billion (ppbv) was obtained for a 1 sec lock-in time constant (TC).
Madhuri Thakur, Dmitry Melnik, Heather Barnett, Kevin Daly, Christine Moran, Wei-Shun Chang, Stephan Link, C. Theodore Bucher, Carter Kittrell, Robert Curl
A wide-field four-channel fluorescence imager has been developed. The instrument uses four expanded laser beams to image a large section (6 mm×9 mm). An object can be sequentially illuminated with any combination of 408-, 532-, 658-, and 784-nm lasers for arbitrary (down to 1 ms) exposure times for each laser. Just two notch filters block scattered light from all four lasers. The design approach described here offers great flexibility in treatment of objects, very good sensitivity, and a wide field of view at low cost. There appears to be no commercial instrument capable of simultaneous fluorescence imaging of a wide field of view with four-laser excitation. Some possible applications are following events such as flow and mixing in microchannel systems, the transmission of biological signals across a culture, and following simulations of biological membrane diffusion. It can also be used in DNA sequencing by synthesis to follow the progress of the photolytic removal of dye and terminator. Without utilizing its time resolution, it can be used to obtain four independent images of a single tissue section stained with four targeting agents, with each coupled to a different dye matching one of the lasers.
Nitric oxide detection with Faraday rotation spectroscopy offers excellent
sensitivity and high specificity together with outstanding long term system performance. Development of a
transportable, cryogen-free, prototype field instrument based on mode-hop-free external cavity quantum
cascade laser targeting the optimum NO Q(3/2) transition at 1875.8 cm-1 is reported . The system shows a
minimum detection limit of 5.4 ppb with a 1sec. lock-in time constant. Continuous, unattended NO
monitoring with >1 hour white noise limited averaging times is reported.
Recent advances in the development of sensors based on infrared quantum cascade lasers for the detection of trace gas
species is reported. Several selected examples of applications in environmental and industrial process monitoring as well
as in medical diagnostics using quartz enhanced photoacoustic spectroscopy and laser absorption spectroscopy will be
described.
A mid-IR gas sensor using difference frequency generation was developed to measure trace levels of biological carbon monoxide (CO). A periodically-poled lithium niobate crystal is pumped by a continuous wave Ti:Sapphire laser and a compact diode pumped Nd:YAG laser operating at 864.86 and 1064.6 nm, respectively. The strong IR transition R(6) at 2169.2 cm-1 is chosen for convenient CO detection without interference from other gas species. Carbon monoxide is collected and flowed into a multipass cell with an effective optical path length of 18.3 m. Using such an experimental arrangement, we detected the generation of CO at levels of 30 ppb during a 30 min period from living vascular smooth muscle cells in basal state.
A portable room-temperature diode laser based on mid-infrared gas sensor was developed for open path measurements of methane in ambient air. This sensor is based on a continuous wave difference-frequency generation in periodically poled LiNbO3 near 3.3 micrometers , pumped by a solitary Fabry Perot type diode laser at 810 nm and a distributed Bragg reflector (DBR) diode laser at 1083 nm. IR frequency tuning between 3045 and 3170 cm-1 was performed by tuning the solitary GaAlAs diode laser. In-situ and open path detection of methane in atmosphere with sensitivity of 18 ppbm/Hz1/2 was performed and detection sensitivity was limited by intensity noise.
Recent progress in design of diode-laser-pumped CW tunable narrowband mid-infrared sources based on difference-frequency generation (DFG) is addressed. Application of two such tunable sources to high-resolution spectroscopy and sensitive atmospheric trace detection of methane near 3.2 micrometers is reported. Methane detection limit in air of 12 ppb- m/(root)Hz is reported based upon the signal-to-noise ratio observed in the direct absorption spectra. Performance characteristics of these sources are examined including tuning range, phase matching bandwidth, output power, and amplitude stability. Applications of diode-laser-pumped CW tunable DFG may include atmospheric trace detection of several major hydrocarbons such as ethane, ethylene, and benzene, and toxic air pollutants such as carbon monoxide, nitric and nitrous oxide, sulfur dioxide, and methyl chloride.
We report the results of our efforts to increase the infrared output power from a cw difference frequency generation source based on the nonlinear material AgGaS2 for use in infrared kinetic spectroscopy. Experimental and theoretical infrared conversion efficiencies are compared as a function of pump power, signal power, and focusing conditions. The predicted and observed IR yields agree well for the unfocused beam; however, the agreement is poor (experimental conversion a factor of 3 to 5 less than theory) for all focused conditions. The highest IR output powers obtained corresponded to experimental conditions that are slightly overfocused compared to the theoretical optimum. Thermal loading of the AgGaS2 decreases the parametric conversion efficiency when the power in one of the input lasers beams exceeds approximately 800 mW. Infrared powers of 10 to 30 (mu) W have been achieved near 5.2 micrometers . Future improved crystals should yield powers at the AP 500-(mu) W level.
The suitability of III-V single-mode cw diode lasers for difference-frequency generation of tunable IR radiation has been explored by mixing a red single-mode diode laser with a tunable single-mode cw Ti:Al2O3 laser in AgGaS2. More than 1 (mu) W of cw tunable, ((lambda) approximately equals 5 micrometers ), narrowband coherent radiation has been generated by using type I noncritical phasematching. The feasibility of a more compact, solid state cw laser source based on the mixing of two single-mode diode lasers (808 and 690 nm) as pump sources in AgGaS2 has also been demonstrated (infrared power generated approximately equals 3 nW). Techniques to increase the infrared difference-frequency output power level such as the use of a high-power optical semiconductor amplifier or an external buildup cavity for the nonlinear mixing crystal have been investigated. As much as 47 (mu) W of cw infrared radiation and 89 (mu) W of pulsed infrared radiation, tunable around 4.3 micrometers have been generated by mixing the outputs of a high-power tapered semiconductor amplifier at 858 nm and a Ti:Al2O3$ laser at 715 nm in AgGaS(subscript 2. The GaAlAs tapered traveling-wave amplifier delivered up to 1.5 W of diffraction- limited cw power into the nonlinear crystal. Recent progress in generating cw infrared radiation near 3.2 micrometers by mixing the outputs of an extended cavity diode laser near 800 nm (pump wave) and a compact diode-pumped Nd:YAG laser at 1064 nm (signal wave) in AgGaS2 with an external enhancement cavity to resonate the signal wave inside the nonlinear mixing crystal is also described.
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