This page provides a brief tutorial on gas sensing using diode laser absorption spectroscopy and lists many molecules that can be detected with diode laser sensors. Sensors based on these lasers offer advantages including
- Sensitivity, with the capability to measure accurately trace gas concentrations of less than a part per billion;
- Selectivity for a particular gas species, with no observable cross-response from other species;
- Dynamic Range with a real-time response that can be linear over more than four decades of concentration;
- Low Maintenance due to the use of reliable solid state devices and no consumable chemicals;
- Fast Response, with measurement speeds of fractions of a second, or signal averaging to achieve still higher sensitivity.
This page is maintained by Southwest Sciences, Inc., a small business pursuing contract research and development in the physical sciences. Southwest Sciences has been at the forefront of research in the use of diode lasers for measuring trace gases in industrial (stack emissions, fenceline monitoring) and scientific (environmental, atmospheric, and combustion) applications .
Using Spectroscopy for Gas Sensing
Spectroscopy Using a prism, white light from the sun or a light bulb can be split into all the visible colors plus electromagnetic radiation that is invisible to the eye. A molecule, such as methane (CH4) or water vapor (H2O), absorbs light only at certain particular colors (if they are visible) or wavelengths (in the infrared or ultraviolet). These absorbing wavelengths are characteristic of the molecule and are called its spectrum. At infrared wavelengths, the spectrum results from vibrations of the atoms in the molecule, while at visible and ultraviolet wavelengths the spectrum is caused by the electrons orbiting the molecule. The spectrum can be calculated from quantum mechanics or measured in the laboratory.
Usually, the spectrum consists of many wavelengths, at each of which some percentage of the light can be absorbed. Some wavelengths absorb more strongly than others. The percentage of power absorbed at a particular wavelength depends on the number of molecules present, the molecule's "strength" of absorption at that wavelength (called the cross section) and the optical path through the sample.
Laser Measurements While a light bulb can be used to measure gas concentrations, it usually doesn't work very well: splitting apart the different wavelengths is difficult and little power is actually in each wavelength. A laser puts out a single pure color or wavelength, so all its power is concentrated at this single wavelength. When a laser beam goes through a prism, all its light comes out at the same place. The exact wavelength can be tuned slightly by changing the laser temperature or current.
The basic laser measurement experiment is fairly simple. The laser light passes through a gas sample and the laser power transmitted through the sample is detected as a function of laser wavelength. This results in a measurement of one part of the absorption spectrum. There is no need to spread the light out with a prism since the laser is already a pure color. The amount of power absorbed by the gas depends upon the product of the number of molecules present times the cross section times the optical path length. To determine the gas concentration, we measure the amount of power absorbed at a characteristic wavelength, and divide it by the cross section and by the path length.
To make a sensitive instrument (one that measures small concentrations), the instrument designer can
- Measure smaller absorbed power levels. Special techniques such as wavelength modulation, frequency modulation, two tone fm, tone burst spectroscopy, photoacoustic, and "noise canceller" dual beam spectroscopy can measure power changes equivalent to one millionth of the incident power. These techniques are easily implemented with diode lasers.
- Choose a wavelength where the cross section is large. Generally, the cross section is much larger in the mid-infrared than in the near-infrared, but the lasers are much more inconvenient to use.
- Use a long optical path. Special optical cells based on designs by White or Herriott can achieve a long path in a small volume. A new technique called cavity ring down can provide path lengths of more than 10 km from a one meter cell.
Diode Lasers
Diode lasers are manufactured for a variety of purposes, including gas sensing, fiber optic communications, optical storage, and pumping other lasers. Important properties of the lasers include wavelength, power, coherence, cost, operating temperature. Here are some of the diode laser types:
- Galium Nitride laser
- Blue to near UV wavelengths (400-480 nm)
- AlGaInP lasers
- Red (630-690 nm), room temperature, low cost, 10 mW
- AlGaAs lasers
- near-infrared or visible (750-1000 nm), room temperature, low cost, 10 mW
- Vertical Cavity lasers
- near-infrared or visible (650-1680 nm), low cost, room temperature, widely tunable
- InGaAsP communications lasers
- near-infrared (1200-2000 nm), room temperature, fiber-optic, 10 mW power
- Antimonide lasers
- near- to mid-infrared (2000-4000 nm), room temperature or cooled, 1 mW or greater
- Quantum Cascade lasers
- mid-infrared (4000-12000 nm), high power, single frequency, may require cryogenic cooling
- Lead-salt lasers
- mid-infrared (3000-30000 nm), require cryogenic cooling, less than 1 mW
New types are under development. Efforts to improve mid-infrared diode lasers are of special interest to the molecular gas sensing community because of the high sensitivity that can be achieved in this region. The efforts focus on achieving room temperature operation and single frequency output through novel device structures.
The output of a diode laser is a beam of light that is highly monochromatic --that is, it is composed of a single wavelength or color. Thus, even though the power of the laser beam is low, all the light can be directed through the desired measurement region, and the photons are all the correct color to be absorbed by the molecules. This is usually not the case for non-laser based optical measurements.
The laser wavelength depends foremost on the composition and structure of the device. But once a laser has been fabricated, it is possible to tune the output by controlling the laser temperature and current. In practice, it is the current tunability which gives the laser sensor its high sensitivity and fast time response.
Which Gases can be Detected?
Once the molecular cross sections or absorption coefficients are known, the detection sensitivity for a gas can be calculated. Molecular spectroscopy has provided an extensive database of cross sections and wavelengths. The table below gives the detection limits expressed as parts per billion by volume times for an optical path of one meters (ppb) for some commercially important species. Smaller detection limits correspond to more sensitive detection. In all the calculations, we assumed measurement at one atmosphere pressure and room temperature, and that the minimum detectable absorbance of the sensor is 1e-5 of the incident laser power.
Still higher detection sensitivity can be achieved using multipass optics to achieve long optical paths. For instance, with an 50 m optical path, the sensitivity for water vapor at 1390 nm is 60 ppb * (1 m/50 m) = 1.2 ppb. Southwest Sciences demonstrated a moisture sensor exceeding these specifications in an instrument designed for NIST.
Detection Limits
Assuming 1e-5 absorbance, 1 Hz bandwidth and one meter path
Molecule Mid-Infrared Near-Infrared (ppb) lambda (nm) (ppb) lambda (nm) water H2O 2.0 5940 60 1390 carbon dioxide CO2 0.13 4230 700 2040 5500 1570 carbon monoxide CO 0.75 4600 30000 1570 500 2330 nitric oxide NO 5.8 5250 60000 1800 1000 2650 nitrogen dioxide NO2 3.0 6140 340 680 nitrous oxide N2O 0.44 4470 1000 2260 sulfur dioxide SO2 14 7280 methane CH4 1.7 3260 600 1650 acetylene C2H2 3.5 7400 80 1520 hydrogen fluoride HF 10 1310 hydrogen chloride HCl 0.83 3400 150 1790 hydrogen bromide HBr 7.2 3820 600 1960 hydrogen iodide HI 2100 1540 hydrogen cyanide HCN 12 6910 290 1540 hydrogen sulfide H2S 20000 1570 ammonia NH3 0.80 10300 800 1500 formaldehyde H2CO 8.4 3550 50000 1930 phosphine PH3 6.2 10100 1000 2150 oxygen O2 78000 760 ozone O3 11 9500
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Contact Information
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