ABB GLA131 Series Microportable Analyzers
User Manual
ABB PROPRIETARY INFORMATION
84
Appendix B ABB OA-ICOS Technology
Off-axis integrated-cavity output spectroscopy (OA-ICOS) uses a high-finesse
optical cavity as an absorption cell. 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: 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 (9.8 in) apart, the effective
optical path length is 2,500 m (8,202 ft).
Because the path length depends only on optical losses in the cavity and not on a
unique beam trajectory (such as conventional multi-pass cells or cavity-ringdown
systems), the optical alignment is very robust to allow 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 several to tens of microseconds).
As with conventional tunable-laser absorption-spectroscopy methods:
Laser wavelength selection is based on the selected absorption feature of
the target gas to be measured.
The measured absorption spectra are recorded and used to determine a
quantitative measurement of mixing ratio directly and without external
calibration when combined with the recorded:
o
Measured gas temperature and pressure in the cell
o
Effective path length
o
Known line strength
