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Spectroscopy for Gas Sensing

With the help of a prism, white light from the sun or even a light bulb can be split into different wavelengths. The visible ranges are those that can be observed in a rainbow. The invisible wavelength ranges attach themselves to the visible ones, for example, ultraviolet light joins to short wavelengths and infrared light to long ones, namely the red tone. A gas molecule such as methane (CH4) or water vapor (H2O) absorbs light only at specific wavelengths (or “colors”) in the infrared.

These absorbing wavelengths, like a fingerprint, are characteristic of the molecule and are called its absorption spectrum. The infrared part of an absorption spectrum results from vibrations and rotations of the molecule’s atoms whereas in the visible and ultraviolet spectral range, it is caused by the transition of electrons orbiting the molecule. The spectrum can be calculated with quantum mechanics and may be measured in the laboratory.

Usually, the absorption spectrum consists of many ‘lines’ of various wavelengths, each from which a part of light will incidently be absorbed. Certain absorption lines are significantly more pronounced and therefore more suitable for measurement. The amount of light absorbed at a particular wavelength depends on the concentration of the substance, the molecule's ’strength’ of absorption at that wavelength (called the absorption cross-section) and length of the optical path through the sample.

In principle, it is difficult to use the white light from a bulb to measure the concentration of certain gases because it has a continuous spectrum. The radiation energy is distributed only to a narrow fraction of the actually relevant wavelengths of the absorption lines. Moreover, the presence of other types of gases can easily alter expected measurement results. A laser for gas sensing on the other hand emits a single pure color (or wavelength), where all of its power is concentrated – its emission-spectrum is just a very sharp and strong line. If a laser beam is directed through a prism, all of its light is refracted at the same angle. For measuring a certain gas absorption line, the emission wavelength of a laser is scanned across the line of interest using varying current or temperature.This technique efficiently focuses the laser power on that characteristic feature.

Specific advantages of VCSELs in Absorption Spectroscopy:

  • Wide tuning range: simplifies system design and enables the measurement of blurred features under high-pressure conditions
  • Fast tunability: enables time-resolved measurements of combustion processes
  • Low operating current and voltage: allows for low power consumption and compact electrical circuitry
  • Efficient measuring principle: makes eye-safe power levels possible
 

Which Gases can be detected?

Once the absorption spectrum is determined for a particular gas, i.e. the molecular cross-sections or absorption coefficients, its detection sensitivity can be calculated. These data are well known for various gases.
The table below gives the detection limits expressed in concentrations (ppb) with an optical path of one meter for some commercially important gas types. Smaller detection limits correspond to more sensitive detection. Measurement at normal atmosphere pressure and room temperature is assumed, as well as a minimum detectable absorption of sensor 10-5 of the radiated laser power.