David S. Bomse

Frequency modulation and wavelength modulation spectroscopies: comparison of experimental methods using a lead-salt diode laser.

Wavelength modulation spectroscopy (WMS) and one-tone and two-tone frequency modulation spectroscopy (FMS) are compared by measuring the minimum detectable absorbances achieved using a mid-IR lead-salt diode laser. The range of modulation and detection frequencies spans over 5 orders of magnitude. The best results, absorbances in the low-to-mid 10(-7) range in a 1-Hz bandwidth, are obtained by using high-frequency WMS (10-MHz detection frequency) and are limited by detector thermal noise. This sensitivity can provide species detection limits well below 1 part per billion for molecules with moderate line strengths if multiple-pass cells are used. High-frequency WMS is also tested by measuring the absorbance due to tropospheric N(2)O at 1243.795 cm(-1). WMS at frequencies < 100 kHz is limited by laser excess (1/f) noise. Both of the FMS methods, which require modulating the laser at frequencies >/= 150 MHz, give relatively poor results due to inefficient coupling of the modulation waveform to the laser current. The re ults obtained agree well with theory. We also discuss the sensitivity limitations due to interference fringes from unintentional étalons and the effectiveness of étalon reduction schemes.

Frequency modulation multiplexing for simultaneous detection of multiple gases by use of wavelength modulation spectroscopy with diode lasers

Modulation frequency multiplexing provides a straightforward method, analogous to television or radio broadcasting, for performing simultaneous detection of multiple gases by use of wavelength modulation spectroscopy with diode lasers. When fiber-optic coupled lasers are used, our approach guarantees that all beams transit the same optical path and impinge on the same detector. Each laser is modulated at a different frequency and the detector output is processed by a set of lock-in amplifiers, one for each laser, to measure the absorbance encountered by each laser.

Diode laser measurements of trace concentrations of ammonia in an entrained-flow coal reactor.

We used a single-mode Pb-salt diode laser to quantify in situ the amount of ammonia generated during pyrolysis of pulverized coal in an entrained-flow reactor at 1225 K. The combination of wavelength modulation spectroscopy, a Herriott style multiple pass cell, rapid wavelength scanning (to eliminate noise due to turbulence and vibration) and a novel etalon fringe suppression technique provided a minimum detectable absorbance of 2 x 10(-6) (SNR = 1, 1-Hz bandwidth) corresponding to 0.04-ppm ammonia at 1225 K. This is a 4-order-of-magnitude improvement over CO(2) laser based ammonia detectors and is approximately 2000 times more sensitive than electrochemical detection methods (for equal integration times).

Dual-modulation laser line-locking scheme

Dual modulation laser line locking, useful for long-term trace species monitoring, achieves wavelength stability <0.1 ppm and rejects baseline drift in the measured absorbance3000-fold.

Diode laser spectroscopy for on-line chemical analysis

Diode laser spectroscopy provides exceptional sensitivity and selectivity for real-time characterization of reacting systems and gas streams. High frequency wavelength modulation techniques achieve species detection limits that are routinely in the ppm range and can reach sub-ppb levels under favorable conditions. Narrow laser linewidths guarantee selective detection of key species even in the presence of myriad other components. Diode laser spectroscopy is also relatively immune from interference by black body radiation or chemiluminescence. Prototype diode-laser based systems have been demonstrated successfully for trace gas detection in turbulent, high temperature particle-laden streams, for oxygen quantitation in flames, for free radical characterization in a plasma etching reactor and for greenhouse gas flux measurements in air. We also discuss the availability of laser wavelengths, compatibility with fiber optics, cost safety and expectations for new laser development.

Applications of diode laser spectroscopy to environmental and industrial process monitoring

Visible/near-infrared diode lasers are well-suited for use as spectroscopic light sources in detection of a wide variety of gases by optical absorption. The high spectral resolution of these devices permits the selective detection of targeted species, while their characteristics of low cost, room temperature operation, and compatibility with fiber optics make them attractive for instrument development. A partial list of industrially or environmentally significant gases that may be measured by near-IR diode laser spectroscopy includes oxygen, water vapor, methane, acetylene, carbon monoxide, carbon dioxide, hydrogen halides, ammonia, hydrogen sulfide, and nitrogen oxides. This paper describes recent work at Southwest Sciences in development of diode laser-based instrumentation for industrial or environmental monitoring applications. Instrumentation utilizing a 1.393 micrometers DFB diode laser for measurement of trace moisture contamination in high purity process gases is described. In addition, recent laboratory studies to characterize the performance of new types of diode lasers in gas sensing applications are discussed, including vertical cavity surface emitting lasers in the 650 to 960 nm region and antimonide-based lasers in the 2.6 micrometers region.

Photoacoustic detection of intracavity absorption

It is demonstrated that the photoacoustic effect in an external cell is a sensitive resonant detector of intracavity absorption. The detection limits for I2 and Br2 are 3 ng/cm3 and 48 ng/cm3, respectively. For the case of I2 the detection limit using the photoacoustic detector is essentially the same as the detection limit using a fluorescence detector. The sensitive response of photoacoustic detection to IR absorption makes this technique particularly attractive as a potential resonance detector for intracavity absorption with IR lasers.

Portable natural gas leak detector for survey inspections

A diode laser based natural gas leak detector has been developed that can measure methane concentrations over six orders of magnitude, from ambient (1.7 ppm) to pure gas levels. The detection method utilizes a small multipass cell and wavelength modulation absorption spectroscopy. At high methane concentration, various forms of unmodulated absorption spectroscopy are used. The instrument is a handheld unit that operates on less than 2 W of power and weighs 1.4 kg (including battery). A small pump on the unit pulls outside gas into the enclosed optical cell through an extendable probe. The response time of the instrument is approximately 1 - 2 sec.

Frequency-modulation absorption spectroscopy for trace species detection: theoretical and experimental comparison among methods

Theoretical and practical limits for detection of trace concentrations of gas phase species using frequency modulation spectroscopy are described. A variety of frequency modulation schemes are examined, including wavelength modulation (harmonic detection) spectroscopy (WMS) and one-tone and two-tone frequency modulation spectroscopy (FMS). The distinctions among these methods are mostly semantic and all of these techniques can be described by a single theory. The goal of this research is to define guidelines useful for implementing the optimum modulation technique for specific measurement needs. Applying this formalism, expected sensitivities for each method are compared for selected absorption systems. The results suggest that the choice among techniques is most strongly driven by the individual laser tuning characteristics, the absorption linewidth and the detection bandwidth; no individual method is a priori superior. Results of experimental diode laser measurements which confirm these calculations are presented. Predicted minimum detectable concentrations for a representative variety of gas phase species are also shown.

Imaging Sensor for Chemical Species, Temperature and Pressure Measurements

A compact, high speed, in-flight scramjet imaging sensor is being developed for the measurements of local temperatures, pressures and gas concentrations. This sensor is referred to as full laser absorption spectroscopic imager (f-LASIM). Its unique feature is a high speed imaging array detector that, when combined with laser diode wavelength modulation absorption spectroscopy (WMS) and custom electronics, provides an unprecedented combination of temporal and spatial resolution for real-time measurement of combustion gas properties. This paper discusses preliminary results that were obtained to demonstrate the feasibility of the lock-in imaging detector array to perform real-time, high speed, high sensitivity measurements relevant to scramjet engines including measurements of gas concentration, temperature and pressure with high spatial resolution even for weak absorbance levels. This sensor has application not only to scramjet development, but more broadly to all engine diagnostics and other systems where gas concentrations and flow properties need to be measured and understood.