Mark G. Allen

Measurements of CO, CO2, OH, and H2O in room-temperature and combustion gases by use of a broadly current-tuned multisection InGaAsP diode laser.

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.

Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors

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.

Diode laser-based air mass flux sensor for subsonic aeropropulsion inlets

An optical air mass flux sensor based on a compact, room-temperature diode laser in a fiber-coupled delivery system has been tested on a full-scale gas turbine engine. The sensor is based on simultaneous measurements of O(2) density and Doppler-shifted velocity along a line of sight across the inlet duct. Extensive tests spanning engine power levels from idle to full afterburner demonstrate accuracy and precision of the order of 1-2% of full scale in density, velocity, and mass flux. The precision-limited velocity at atmospheric pressure was as low as 40 cm/s. Multiple data-reduction procedures are quantitatively compared to suggest optimal strategies for flight sensor packages.

Absorption measurements of the second overtone band of NO in ambient and combustion gases with a 1.8-µm room-temperature diode laser

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.

Observation of CO and CO 2 absorption near 1.57 µm with an external-cavity diode laser

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.

Thermoelectrically cooled quantum-cascade-laser-based sensor for the continuous monitoring of ambient atmospheric carbon monoxide

We report the first application of a thermoelectrically cooled, distributed-feedback quantum-cascade laser for continuous spectroscopic monitoring of CO in ambient air at a wavelength of 4.6 microm. A noise-equivalent detection limit of 12 parts per billion was demonstrated experimentally with a 102-cm optical pathlength and a 2.5-min data acquisition time at a 10-kHz pulsed-laser repetition rate. This sensitivity corresponds to a standard error in fractional absorbance of 3 x 10(-5).

Planar laser-induced-fluorescence imaging measurements of OH and hydrocarbon fuel fragments in high-pressure spray-flame combustion

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.

Diode laser-based sensors for chemical oxygen iodine lasers

We describe several diode laser-based instruments that can detect important species in chemical oxygen iodine lasers (COIL). Species detected include: water vapor, atomic iodine, and ground state oxygen. The sensors allow non-intrusive, real-time measurements from which one can determine small signal gain and the singlet delta oxygen yield. The water vapor concentrations can also be continuously monitored. The sensitivities of the sensors are sufficient for all the conditions found in typical COIL devices. The room temperature diode lasers are miniature and fiber coupled. Data for all three species are presented.

Simultaneous water vapor concentration and temperature measurements using 1.31-micron diode lasers

This paper reports the development of a compact, inexpensive sensor for simultaneous water vapor concentration and temperature measurements suitable for aeropropulsion exhaust applications. High sensitivity is achieved with an electronically balanced dual detector strategy that circumvents requirements for custom-fabricated lasers operating at specific wavelengths or high-frequency modulation techniques. Using widely available, broadly tunable InGaAsP diode lasers near 1.31 jxm, simultaneous measurements are demonstrated in a fiber-coupled, wavelength multiplexed configuration with a limiting density sensitivity of 1015 cm~3 in a 50-cm path and an rms standard deviation of 42 K over a range from 300 to 1300 K. Initial results suggest the possibility of extending this temperature range to 1900 K and above using other line pairs. (1) where Iv is the monochromatic laser intensity at frequency v, mea- sured after propagating a pathlength t through a medium with an absorbing species number density N. The strength of the absorp- tion is determined by the temperature-dependent line strength S(T), and the line shape function g(v — v()). The line shape function de- scribes the temperature- and pressure-dependent broadening mech- anism of the fundamental line strength. The temperature dependence of the line strength arises from the Boltzmann population statistics governing the internal energy level population distribution of the absorbing species. The single-mode distributed feedback (DFB) diode lasers used in this work are sufficiently narrow in frequency that they may be con- sidered essentially monochromatic with respect to the absorption line shape. The laser frequency may be tuned over a range that en- compasses the entire line shape function so that the resultant trans- mission can be integrated to remove the pressure and temperature dependence of the line-broadening mechanisms. The recorded ab- sorbance, then, is proportional only to the temperature-dependent line strength and the absorbing species number density. It is usu- ally possible to select an absorbing ground state whose line strength is relatively constant over some target temperature range so that the absorbance is a direct measurement of species number density. Separate temperature measurements may be used to correct for tem- perature variations, if necessary. Alternatively, two absorption transitions may be probed (using one or two lasers, depending on the target transition separation and the laser tuning range). The ratio of the integrated absorbance of each transition is a pure function of temperature,2

Fluorescence imaging of OH and NO in a model supersonic combustor

Recent results are presented from an experimental study to develop Planar Laser-Induced Fluorescence (PLIF) diagnostics for application to scramjet combustor development. The measurements are made in a reacting flow shock-tunnel facility which generates Mach 3 air at static conditions of 1500 K and 0.3 atm. Hydrogen or other gases may be injected into the rectangular test section downstream of a rear-facing step. PLIF measurements of NO, naturally present in the reflected shock-heated, freestream air, are used to determine basic flow features, and a technique is demonstrated for determining static temperature variations in nonreacting, non-mixing portions of the flow. pLIF measurements of OH are used to reveal the instantaneous reaction zone interface from a dual round jet injector configuration