Teledyne LGA-3500 - Laser Gas Analysis system - Manual (Page 22)

Operational Theory Model 3500
Teledyne Analytical Instruments 8
2.2 Principles of Operation
The Model 3500 Laser Gas Analysis System uses laser
spectroscopy to generate a signal based on the composition of a gas
mixture. While similar in nature to other photonic analyzers, the Model
3500 Laser Gas Analysis System offers many advantages over these
technologies.
Traditional online gas analyzers such as the Non-dispersive
Infrared (NDIR) Spectroscopy Online Gas Analyzer are subject to
interference from other constituents in the environment (including dust
and other gas species such as water vapor). This could be especially
severe when the measured gas is of low concentration. However, the
Model 3500 Laser Gas Analysis System series employ advanced Laser
Absorption Spectroscopy (DLAS) gas analysis and measurement
technology, i.e. “single-line” spectroscopic methods.
Conventional infrared spectroscopy uses light sources such as
lamps or LED’s that are normally non-laser and have very broad line-
widths. The absorption spectrum obtained includes not only the spectral
lines from the gas under test, but also those from background gas species
which introduces cross interference. DLAS gas analyzers use diode
lasers that have line-widths of less than 0.0001nm, or only 1/10
6 of that
of the non-laser sources. By selecting a laser that will emit a specific
absorption line close to that of the gas under test and tuning its
wavelength through changing its temperature and driving current, an
absorption spectrum that only covers a single line of the gas under test
can be obtained and eliminates cross interference. See Figure 2-2.
The line-width of a diode laser is much narrower than that of the
gas molecule absorption spectrum. The Laser spectral line is depicted in
the figure as a light solid line.
The diode laser output from the transmitter goes through the
environment under test, gets absorbed by the target gas molecules, and
the resulted attenuated light is collected by the optical sensor in the
receiver unit. The attenuation is in proportion to the concentration of the
target gas. Varying the wavelength of the diode laser within the carefully
selected laser spectral scanning range (Figure 1.2), a gas absorption
spectral line without interference from the absorption spectra of
background dust and other gas species can be obtained for high
precision data analysis. This feature, no interference from background
dust and gases, enables the laser gas analysis systems to be applied on
the spot for online gas analysis. Conventional infrared spectroscopy, due