Stephen So

Pulsed quantum-cascade laser-based sensor for trace-gas detection of carbonyl sulfide

Simultaneous exhaled carbonyl sulfide (OCS) and carbon dioxide concentration measurements in human breath are demonstrated with a compact pulsed quantum-cascade laser-based gas sensor. We achieved a noise-equivalent sensitivity (1sigma) of 1.2 parts per billion by measuring a well-isolated OCS P(11) absorption line in the v3 band at 2057.6 cm(-1) using an astigmatic Herriott cell of 36-m optical path length and 0.4-s acquisition time.

Dual interband cascade laser based trace-gas sensor for environmental monitoring

The development of an interband cascade laser (ICL) based spectroscopic trace-gas sensor for the simultaneous detection of two atmospheric trace gases is reported. The sensor performance was evaluated using two ICLs capable of targeting formaldehyde (H2CO) and ethane (C2H6). Minimum detection limits of 3.5 ppbV for H2CO and 150 pptV for C2H6 was demonstrated with a 1 s integration time. The sensor was deployed for field measurements of H2CO, and laboratory quantification of both formaldehyde and ethane are reported. A cross comparison of the atmospheric concentration data for H2CO with data collected by a collocated commercial H2CO sensor employing Hantzsch reaction based fluorometric detection was performed. These results show excellent agreement between these two different approaches for trace-gas quantification. In addition, laboratory experiments for dual gas quantification show accurate, fast response with no crosstalk between the two gas channels.

LaserSPECks:: laser SPECtroscopic trace-gas sensor networks - sensor integration and applications

We introduce a novel laser spectroscopic trace-gas sensor platform, LaserSPECks that integrates recently developed miniature quartz-enhanced photoacoustic spectroscopy (QE- PAS) gas sensing technology. This universal platform uses infrared laser spectroscopy detect and quantify numerous gas species at part-per-million to part-per-billion (ppm-ppb) concentrations (Curl, 2002). Traditional gas sensing devices capable of the same sensitivity and specificity are several orders of magnitude larger in size, cost, and power consumption. Thus, high resolution gas sensing technology has been difficult to integrate into small, low-power, replicated sensors suitable for wireless sensor networks (WSNs). This paper presents the principles behind laser based trace gas detection, design issues, and outlines the implementation of a miniaturized trace-gas sensor from commerical-off-the-shelf (COTS) components. We report on an early prototype as a proof- of-concept for integration into WSN applications. We also describe a number of ongoing collaborations in utilizing the platform in air pollution and carbon flux quantification, industrial plant control, explosives detection, and medical diagnosis. Furthermore, we discuss experimental performance evaluations to examine general platform requirements for these types of sensors. The results of our evaluation illustrate that our prototype improves upon previous gas sensing technology by two orders of magnitude in measures of power consumption, size, and cost, without sacrificing sensor performance. Our design and experiments reveal that laser-based trace-gas sensors built from COTS can be successfully implemented and integrated within WSN nodes to enable a wide range of new and important sensing applications.

Atmospheric Formaldehyde Monitoring in the Greater Houston Area in 2002

A laser spectrometer based on difference frequency generation (DFG) was deployed for real-time long-term monitoring of HCHO concentrations at an environmental monitoring site located at Deer Park, Texas, in the Greater Houston area. Three HCHO concentration measurements were made during the periods of July 20–31 (period I), August 2–14 (period II), and August 24–September 25 (period III), 2002. In periods I and II, differences in HCHO concentrations are apparent between day and night measurements, with elevated concentrations during daylight hours. Most of the HCHO peak values are less than 20 ppbV except for two intense peaks on August 02 (∼25 ppbV) and August 04 (∼30 ppbV). The formaldehyde concentration levels in ambient air at the measurement site are produced mainly by the photochemical oxidation of volatile organic compounds (VOCs) caused by intense sunlight during periods I and II. This observation was made based on a comparison with the ozone concentration, solar radiation, temperature, relative humidity, and wind speed data obtained from the Texas Commission on Environmental Quality (TCEQ). During period III, data collected by a time-integrating wet-chemical technique are compared to the data collected by the spectroscopic instrument.

Development of Digital Signal Processor Controlled Quantum Cascade Laser Based Trace Gas Sensor Technology

This paper reports the design and integration of a custom digital-signal-processor (DSP) system into a pulsed quantum-cascade-laser (QCL)-based trace gas sensor to improve its portability, robustness, and operating performance. Specifically, this paper describes the implementation of a custom prototype DSP data acquisition and system controller based on the Texas Instruments TMS320F2812 for embedded control and processing. In addition, the sensor incorporates oversampling by taking advantage of the high-speed conversion capabilities of an analog-to-digital converter, which is embedded within the DSP. A carbon monoxide sensor, employing a thermoelectrically cooled, pulsed 4.6-mum distributed feedback QCL as a mid-infrared radiation source, is used to evaluate the performance characteristics of such a DSP controlled spectroscopic gas sensor

Ppb-level formaldehyde detection using a CW room-temperature interband cascade laser and a miniature dense pattern multipass gas cell

A ppb-level formaldehyde (H2CO) sensor was developed using a thermoelectrically cooled (TEC), continuous-wave (CW) room temperature interband cascade laser (ICL) emitting at 3.59 μm and a miniature dense pattern multipass gas cell with >50 m optical path length. Performance of the sensor was investigated with two measurement schemes: direct absorption (DAS) and wavelength modulation spectroscopy (WMS). With an integration time of less than 1.5 second, a detection limit of ~3 ppbv for H2CO measurement with precision of 1.25 ppbv for DAS and 0.58 ppbv for WMS, respectively, was achieved without zero air based background subtraction. An Allan-Werle variance analysis indicated that the precisions can be further improved to 0.26 ppbv @ 300s for DAS and 69 pptv @ 90 s for WMS, respectively. A side-by-side comparison between two measurement schemes is also discussed in detail.

Recent advances and applications of mid-infrared based trace gas sensor technology

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.

Next generation infrared sensor instrumentation: remote sensing and sensor networks using the openPHOTONS repository

We describe our novel instrumentation architectures for infrared laser spectrometers. Compact, power efficient, low noise modules allow for optimized implementation of cell phone sized sensors using VCSELs, diode, and quantum cascade laser sources. These sensors can consume as little as 0.3W with full laser temperature (<0.001K/Hz1/2) and current control (<2ppm/Hz1/2 noise), photodiode preamplification (<2pA/Hz1/2 noise floor, 1MΩ transimpedance), and digital lock-in amplification with 3 independent channels. We have implemented sensors based on laser absorption spectroscopy, photoacoustic spectroscopy, and Faraday rotation spectroscopy using the openPHOTONS systems, with performance rivaling standalone laboratory measurement instrumentation. Additionally, as openPHOTONS is an open source software repository, this instrumentation can be quickly adapted to new optical configurations and applications. Such modules allow the development of flexible sensors, whether implementing closed path spectrometers, open path perimeter monitoring, or remote backscatter based sensors. This work is also the enabling technology for wireless sensor networks (WSN) of precision sensors, a desirable sensing paradigm for long term, wide area, precision, temporally and spatially resolved studies. This approach can complement existing remote sensing and mapping technologies including satellite observations and sparse networks of flux towers.

Laser Based Chemical Sensor Technology: Recent Advances and Applications

There is an increasing need in many chemical sensing applications ranging from environmental science to industrial process control as well as medical diagnostics for fast, sensitive, and selective trace gas detection based on laser spectroscopy. The recent availability of continuous wave (cw) near infrared diode lasers-, mid-infrared quantum cascade and interband cascade distributed feedback (QC and IC DFB) lasers as mid-infrared spectroscopic sources addresses this need. A number of spectroscopic techniques have been demonstrated. For example, the authors have employed infrared DFB QC and IC lasers for the detection and quantification of trace gases and isotopic species in ambient air by means of direct absorption, cavity-enhanced, and photoacoustic spectroscopy. These spectroscopic techniques offer an alternative to non-spectroscopic techniques such as mass spectrometry (MS), gas chromatography (GC) and electrochemical sensors. The sensitivity and selectivity that can be achieved by both techniques (excluding electrochemical sensors) are similar, but the sensor response time, instrumentation size and cost of ownership for spectroscopic techniques can be advantageous as compared to MS-GC spectrometry.

Ultra-compact multipass laser absorption spectroscopy platform for distributed sensor networks

A prototype three-node wireless sensor network of portable, battery-powered spectroscopic trace-gas sensors equipped with custom 24-pass Herriott cells has been developed. Individual sensor performance and sensor network localization of a gas plume will be reported.