Lgr's Off-Axis Integrated-Cavity Output Spectroscopy (Off-Axis Icos) - Abb Lgr 915-0011 User Manual

LGR's Off-Axis Integrated-Cavity Output Spectroscopy (Off-Axis ICOS)

Off-Axis ICOS uses a high-finesse optical cavity as an absorption cell as shown in Figure 74. Unlike
multi-pass detectors, which are typically limited to path lengths of less than two-hundred meters, an
Off-Axis ICOS absorption cell effectively traps the laser photon so that, on average, they make
thousands of passes before leaving the cell.
As a result, the effective optical path length may be several thousands of 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
spaced by 25 cm, the effective optical path length is 2500 meters.
Figure 74:
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 wavelength of the laser is turned over a selected absorption feature of the target species.
 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:

Measured gas temperature and pressure in the cell

Effective path length

Known line strength
UGGA User Manual
Schematic Diagram of an Off-Axis ICOS Instrument
About Gas Analyzers
65