Operating Manual
3KXG165002R4201_2021_04
ABB PROPRIETARY INFORMATION 19
GLA531 Series LGR-ICOS™ Gas Analyzer
ABB’s OA-ICOS
Off-Axis Integrated-Cavity Output Spectroscopy (OA-ICOS) uses a high-finesse optical cavity as an absorption
cell as shown in Figure 12. Unlike multi-pass detectors, typically limited to path lengths of less than 200 meters,
an OA-ICOS absorption cell effectively traps the laser photon so that, on average, it makes thousands of passes
before leaving the cell.
As a result, the effective optical path length may be several thousand meters using high-reflectivity mirrors and
thus, the measured absorption of light after it passes through the optical cavity is significantly enhanced. For
example, for a cell composed of two 99.99% reflectivity mirrors 25 cm apart, the effective optical path length is
2500 meters.
Because the path length depends only on optical losses in the cavity and not on a unique beam trajectory (like
conventional multi-pass cells or cavity-ring-down systems), the optical alignment is very robust, allowing for
reliable operation in the field. The effective optical path length is determined routinely by simply switching the
laser off and measuring the necessary time for light to leave the cavity (typically tens of microseconds).
As with conventional tunable-laser absorption-spectroscopy methods:
• The laser wavelength selection is based on the selected absorption feature of the target gas to be
measured.
• The measured absorption spectra is recorded and used to determine a quantitative measurement of mixing
ratio directly and without external calibration when combined with the recorded:
ü Measured gas temperature and pressure in the cell
ü Effective path length
ü Known line strength
Figure 12 OA-ICOS Analyzer Schematic Diagram
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