ABSTRACT
Distributed-feedback quantum-cascade (QC) lasers are expected to form the heart of the next-generation mid-IR laser absorption spectrometers, especially as they are applied to measurements of trace gases in a variety of environments. The incorporation of room-temperature-operable, single-mode QC lasers should result in highly compact and rugged sensors for real-world applications. We report preliminary results on the performance of a laser absorption spectrometer that uses a QC laser operating at room temperature in a quasi-cw mode in conjunction with balanced ratiometric detection. We have demonstrated sensitivities for N(2)O [10 parts in 10(6) volume-mixing ratio for a 1-m path (ppmv-m)] and NO [520 parts in 10(9) volume-mixing ratio for a 1-m path (ppbv-m)] at 5.4 mum. System improvements are described that are expected to result in a 2 orders of magnitude increase in sensitivity.
ABSTRACT
A new laser technology that achieves nearly 100-nm quasi-continuous tuning with only injection-current control in a four-section grating-coupler sampled-reflector laser was used to detect CO and CO(2) simultaneously in room-temperature gas mixtures. The same grating-coupler sampled-reflector laser was used to perform in situ measurements of CO, H(2)O, and OH in the exhaust gases of a CH(4)-air flame. This laser is being evaluated for inclusion in a multispecies combustion-emissions exhaust-analysis sensor, and its operational characteristics as they have an impact on gas sensing are described. Preliminary results suggest that this single laser can be used to replace multilaser sensor configurations for some combustion-emissions monitoring applications.
ABSTRACT
We describe a novel combination of a diode-pumped, wavelength-modulated Er(3+)-doped fiber laser light source with a sensitive noise cancellation circuit for detection of acetylene and ammonia. The laser tuning element, a fiber Bragg grating, was mounted in such a way that it could be strained controllably and rapidly, allowing noise cancellation techniques to be applied to the wavelength-modulated output of the fiber laser. The experimental setup is relatively simple and can be extended to other fiber laser wavelengths for which semiconductor lasers are not readily available by selection of a different fiber Bragg grating and gain medium.
ABSTRACT
Near-IR and visible room-temperature diode lasers in broadly tunable external-cavity configurations are becoming commercially available for gas-sensing applications. Near 1.57 mum, a coincidence of overtone and combination-band transitions from CO, CO(2), OH, and H(2)O is particularly interesting for combustion and combustor emissions monitoring. We report initial observations of the room-temperature absorption of CO and CO(2) made with a commercial external-cavity diode laser.
ABSTRACT
We describe the development of a room-temperature diode sensor for in situ monitoring of combustion-generated NO. The sensor is based on a near-IR diode laser operating near 1.8 mum, which probes isolated transitions in the second overtone (3, 0) absorption band of NO. Based on absorption cell data, the sensitivity for ambient atmospheric pressure conditions is of the order of 30 parts in 10(6) by volume for a meter path (ppmv-m), assuming a minimum measurable absorbance of 10(-5). Initial H(2) -air flame measurements are complicated by strong water vapor absorption features that constrain the available gain and dynamic range of the present detection system. Preliminary results suggest that detection limits in this environment of the order of 140 ppmv-m could be achieved with optimum baseline correction.
ABSTRACT
Recent advances in room-temperature visible diode lasers and ultrasensitive detection techniques have been exploited to create a highly sensitive tunable diode laser absorption technique for in situ monitoring of NO(2) in the lower troposphere. High sensitivity to NO(2) is achieved by probing the visible absorption band of NO(2) with an AlGalnP diode laser at 640 or 670 nm combined with a balanced ratiometric electronic detection technique. We have demonstrated a sensitivity of 3.5 × 10(10) cm(-3) for neat NO(2) in a 1-m path at 640 nm and have estimated a sensitivity for ambient operation of 5 ppbv m (l0 ppbv m at 670 nm), where ppbvm is parts in 10(9) by volume per meter of absorption path length, from measured pressure-broadening coefficients.
ABSTRACT
A dual-beam detection strategy with automatic balancing is described for ultrasensitive spectroscopy. Absorbances of 2 × 10(-7) Hz(-½) in free-space configurations and 5 × 10(-6) Hz(-½) in fiber-coupled configurations are demonstrated. With the dual-beam technique, atmospherically broadened absorption transitions may be resolved with InGaAsP, AlGaAs, and AlGaInP single-longitudinal-mode diode lasers. Applications to trace measurements of NO(2), O(2), and H(2)O are described by the use of simple, inexpensive laser and detector systems. Small signal gain measurements on optically pumped I(2) with a sensitivity of 10(-5) are also reported.
ABSTRACT
Planar laser-induced fluorescence images of OH have been obtained in liquid-fueled spray flames burning heptane, ethanol, and methanol over a range of pressures from 0.1 to 1.0 MPa. In addition to the OH fluorescence, a nonresonant fluorescence interference that increased rapidly with pressure was detected. Examination of the spectrum of this interference indicates that it arises from hydrocarbon fuel-fragment species in the fuel-rich zones of the flame. The pressure dependence of the fluorescence signal is examined in both steady-state and time-dependent analyses, and a model for evaluation of pressure effects and quenching variations in quantitative imaging measurements in nonpremixed flame environments is presented. The results indicate that increased combustor pressure results in a rapid rise of the volume fraction of hydrocarbon fragments and a decrease in the OH volume fraction.
