Southwest Sciences Inc Laser based gas sensor tutorial

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

Chile decoration 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.

Spectrum of Light

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

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

Submit your custom gas sensing requirements to be evaluated by Southwest Sciences' scientific staff.

Contact Information

Southwest Sciences, Inc.
1570 Pacheco St., Suite E-11, Santa Fe, NM 87505
tel. (505) 984-1322/ fax (505) 988-9230



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