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.
The recent development of a dense pattern, multiple pass optical cell based on cylindrical mirrors makes possible a differential spectroscopic method that removes (nearly) all common mode features including laser noise, laser distortion, and unwanted optical interference fringes (etalons). The cylindrical mirror cell is similar to other astigmatic cells in that the beam enters through a hole drilled in the center of one mirror. Key differences, however, include the property that for most re-entrant beam trajectories have N passes (where N/2 is odd) through the cell, the N/2 spot is always located at the center of the far mirror. In the differential cell approach, a pellicle beamsplitter located just behind a hole in the far mirror transmits a portion of the beam and reflects the remainder to continue the second set of N/2 passes before exiting through the entrance hole. The two beams - one exiting at the far mirror after N/2 passes, the other exiting at the entrance mirror following N passes - are the reference and sample beams, respectively, applied to a noise canceler circuit. Proof-of-principle experiments reported here using near-infrared measurements of methane absorbance show the differential method does work. The optical system used, however, introduced excessive astigmatism in the beam reflected from the pellicle beam splitter because of the displace of the pellicle from the cell mirror surface. That astigmatism made it difficult to align the return beam for the complete set of N/2 passes and to focus the exit beam onto the photodiode detector. Design improvements are discussed.
KEYWORDS: Sensors, Carbon monoxide, Digital signal processing, Signal processing, Calibration, Environmental sensing, Temperature metrology, Absorbance, Carbon dioxide, Atmospheric chemistry
A compact, diode laser-based sensor has been developed for meteorological balloons to measure atmospheric carbon
dioxide profiles. The sensor achieves a precision of better than 1 ppmv using a novel pressure/temperature
compensating reference cell. This device weighs less than 1 kg and uses less than 4 Watts of battery electrical power.
Turnkey operation is achieved by a digital signal processor. A full description of the sensor and a discussion of its
performance are provided.
KEYWORDS: Digital signal processing, Sensors, Carbon monoxide, Signal processing, Semiconductor lasers, Gas sensors, Oxygen, Modulation, Control systems, Gases
A versatile gas sensor for use in gravitational studies and/or long-term monitoring of biological systems in space is described. The sensor combines two diode lasers in a compact, low power package for quantitative, simultaneous measurements of oxygen, carbon dioxide and potentially, water vapor. Wavelength modulation spectroscopy(WMS) combined with digital signal processor (DSP) control allows this system to meet the stringent weight and power restrictions of a space-borne sensor. The sensor, configured for measurements in plant growth chambers, exhibits a very high level of precision for long-term measurements, with an uncertainty in CO2 concentrations of better than 3 parts per thousand (ppth), and about 5 ppth for ambient oxygen, all at 1 Hz. Allan variance measurements indicate that increasing the averaging time to 100 sec will improve the precision to 0.3 ppth. The dynamic range for CO2 detection exceeds four orders of magnitude.
We describe research leading to a trace gas detection system based on optical absorption using near-IR diode lasers that is intended to provide early warning of incipient fires. Applications include "high loss" structures such as office buildings, hospitals, hotels and shopping malls as well as airplanes and manned spacecraft where convention smoke detectors generate unacceptably high false alarm rates. Simultaneous or near-simultaneous detection of several gases (typically carbon dioxide, carbon monoxide, acetylene and hydrogen cyanide) provides high sensitivity while reducing the chance of false alarms. Continuous measurement of carbon dioxide concentrations also provides an internal check of instrument performance because ambient levels will not drop below ~350 ppm.
KEYWORDS: Digital signal processing, Semiconductor lasers, Sensors, Absorption, Unmanned aerial vehicles, Signal processing, Modulation, Absorbance, Process control, Control systems
A new, near-infrared, diode laser based hygrometer, designed for operation in unmanned aerial vehicles, is presented. This instrument is designed to accurately and rapidly measure moisture from sea level to the lower stratosphere. It is compact, lightweight and requires very little power. All system control and data processing are accomplished using a stand-alone digital signal processor super- controller. The usefulness of this approach for atmospheric measurements as well as other industrial applications will be presented.
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.
Single-frequency near-infrared diode lasers are used to measure atmospheric methane and water vapor. Using high-frequency wavelength modulation methods, sensitive instrumentation with fast time response are designed. Communications lasers operating near 1310 nm probe weak overtone transitions of both molecules; lasers with custom wavelengths at present lack sophisticated packaging, but can achieve much higher sensitivity. We describe two field-tested instruments: an automated, airborne hygrometer with a sensitivity of 8 ppm (by volume) with a one second averaging time, and a fast response methane sensor with a sensitivity of 65 ppb. Improvements to these instruments are outlined, and the effects of laser nonlinearities are noted.
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.
KEYWORDS: Frequency modulation, Modulation, Fermium, Absorption, Signal to noise ratio, Spectroscopy, Semiconductor lasers, Sensors, Signal detection, Interference (communication)
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.
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