Tunable Diode Laser Spectroscopy System Research Articles

Phase-locked absorption tomography for retrieving 5 kHz time-resolved tracer profiles in solid fuel combustion

Turbulent mixing processes and species transport govern many important properties like stability and pollutant formation of reactive systems. We propose spectrally resolved absorption tomography as a viable tool to quantitatively assess species transport by imaging tracer pulses at high time resolution. Measurements on a gas-assisted solid-fuel combustor operating in air and oxy-fuel atmospheres are presented. The measurements are conducted using a scanning tomographic imaging system that has previously been validated for gas temperature measurements. As the scanning system itself is not able to achieve the necessary time resolution, a phase-locked tomographic scheme is applied, i.e. each measurement beam is acquired separately for several tracer pulses and phase-averaged. Hence, the full 5 kHz time resolution of the underlying tunable diode laser spectroscopy system can be utilized, granting insight into species transport in the recirculation zones as well as the flame brush. Despite a possible influence of the tracer injections on the inner recirculation zone, it is shown that qualitative differences between methane combustion and gas-assisted solid-fuel operating conditions can be detected, and influences of devolatilization as well as volatile combustion become visible.

Field deployment of a 4320-nm quantum cascade laser-based TDLS system to compare the background CO2 levels in Mt. Abu with foreground measurements in Gandhinagar, India

We report on the design, development, and field deployment of a 10-mW, 4320-nm distributed-feedback quantum cascade laser-based tunable diode laser spectroscopy (TDLS) system in India for in situ measurement of atmospheric CO2. The portable system was deployed at Mount Abu (24.5926° N, 72.7156° E), a hill station in western India, to carry out week-long measurements of background atmospheric CO2 using direct detection. The mean mole fraction was estimated to be 396 ± 8 ppm. The system was then deployed in Gandhinagar (23.2156° N, 72.6369° E), the capital of the state of Gujarat, to make foreground measurements over the next week. The mean mole fraction at this location was 503 ± 27 ppm. The difference between the background levels in Mount Abu and foreground levels in Gandhinagar is evident. The detection limit of the system, as measured from an Allan variance analysis, was determined to be 260 ppb for an integration time of 58 s and a path length of 20 cm, which is sufficient for such measurements. Another compact and light-weight TDLS system was also deployed for water vapor measurement. It consisted of a 1392.54-nm distributed feedback laser driven by custom electronics and a digital signal processor to carry out waveform generation, data acquisition, and postprocessing tasks.

Measurement of atmospheric carbon dioxide and water vapor in built-up urban areas in the Gandhinagar-Ahmedabad region in India using a portable tunable diode laser spectroscopy system.

This paper reports open-path in situ measurements of atmospheric carbon dioxide at Gandhinagar (23.2156°N, 72.6369°E) and Ahmedabad (23.0225°N, 72.5714°E) in the heavily industrialized state of Gujarat in western India. Calibration-free second harmonic wavelength modulation spectroscopy (2f WMS) is used to carry out accurate and fully automated measurements. The mean values of the mole fraction of carbon dioxide at four locations were 438ppm, 495ppm, 550ppm, and 740ppm, respectively. These values are much higher than the current global average of 406.67ppm. A 1mW, 2004-nm vertical cavity surface-emitting laser is used to selectively interrogate the R16 transition of carbon dioxide at 2003.5nm (4991.2585cm-1). The 2f WMS signal corresponding to the gas absorption line shape is simulated using spectroscopic parameters available in the HITRAN database and relevant laser parameters that are extracted in situ from non-absorbing spectral wings of the harmonic signals. The mole fraction of carbon dioxide is extracted in real-time by a MATLAB program from least-squares fit of the simulated 2f WMS signal to the corresponding experimentally obtained signal. A 10-mW, 1392.54-nm distributed feedback laser is used at two of the locations to carry out water vapor measurements using direct absorption spectroscopy. This is the first instance of a portable tunable diode laser spectroscopy system being deployed in an urban location in India to measure atmospheric carbon dioxide and water vapor under varying traffic conditions. The measurements clearly demonstrate the need to adopt tunable diode laser spectroscopy for precise long-term monitoring of greenhouse gases in the Indian subcontinent.

Tunable Diode Laser Based Concentration Measurements of Water Vapour and Methane on a Solid Oxide Fuel Cell

This paper presents concentration measurements of water vapour and methane, taken in-situ on an operational solid oxide fuel cell (SOFC) test rig using tunable diode laser spectroscopy (TDLS). Methane concentration measurements are presented for the TDLS system and are compared with concentration measurements taken using gas chromatography (GC). Furthermore, purge times for the SOFC gas-analysis system have been calculated using TDLS, which are measurements that cannot be obtained directly using GC. Finally, water vapour concentration measurements in the SOFC cathode are shown for different system operating conditions: a dry cathode cycle and during the introduction of water vapour. As GC cannot be used to measure water vapour in the SOFC cathode stream, a direct comparison cannot be made with the TDLS measurements.

Photorespiratory and Respiratory Carbon Isotope Fractionation in Leaves

The photorespiratory pathway in plants is initiated with the uptake of O2 instead of CO2 by the photosynthetic enzyme Rubisco. This generates a compound in the leaf that must then be recycled through enzymatic reactions distributed over the leaf chloroplasts, peroxisomes, and mitochondria, resulting in a net loss of CO2. The pathway was thought to be well understood, but recent studies using mutants lacking expression of the genes coding for peroxisomal malate dehydrogenase (PMDH) have suggested that there are additional mechanisms that alter the stoichiometry of photorespiratory CO2 release to O2 uptake. Any changes in this stoichiometry will likely cause a change in the isotopic composition of the CO2 being released since it would involve additional enzymes or carbon sources. The isotopic composition of the CO2 being released was measured under both photorespiratory and non-photorespiratory conditions, as well as under different isotopic conditions. Measurements were taken using a tunable diode laser spectroscopy system, which allows high frequency in vivo gas exchange measurements of different CO2 isotopologues. Comparing the PIB isotopic signature of wild type Arabidopsis to that of PMDH mutants, may improve our understanding of alternate metabolic pathways in plants. The decarboxylation fractionation seen is significantly higher in PMDH mutant plants relative to wild-type, which can be partially explained by higher rates of photorespiratory decarboxylation seen in previous studies. The photorespiratory fractionation also varied significantly between wild-type and mutant plants.

Effect of optical feedback on a VCSEL TDLAS

This paper describes the effects of optical feedback on the sensitivity of VCSEL tunable-diode laser spectroscopy (TDLS). Three VCSELs, emitting at different wavelengths in the near-infrared, were used. A TDLS system, subjected to optical feedback, exhibited a common signal-to-noise ratio profile for all three lasers. A catastrophic degradation of TDLS sensitivity occurred when feedback exceeded a level which we associate with coherence collapse. The TDLS system had a CH4 minimum detection limit of 7.5 ppmm without optical feedback. Optical feedback of less than ten percent reduced this sensitivity by two orders of magnitude. This reduction of system sensitivity was accompanied by a second-harmonic absorption signal baseline shift which degraded the system accuracy.

Portable optical spectroscopy for accurate analysis of ethane in exhaled breath

We report on a maintenance-free, ward-portable, tunable diode laser spectroscopy system for the ultra-sensitive detection of ethane gas. Ethane is produced when cellular lipids are oxidized by free radicals. As a breath biomarker, ethane offers a unique measure of such oxidative stress. The ability to measure real-time breath ethane fluctuations will open up new areas in non-invasive healthcare. Instrumentation for such a purpose must be highly sensitive and specific to the target gas. Our technology has a sensitivity of 70 parts per trillion and a 1 s sampling rate. Based on a cryogenically cooled lead-salt laser, the instrument has a thermally managed closed-loop refrigeration system, eliminating the need for liquid coolants. Custom LabVIEW software allows automatic control by a laptop PC. We have field tested the instrument to ensure that target performance is sustained in a range of environments. We outline the novel applications underway with the instrument based on an in vivo clinical assessment of oxidative stress.