LI-COR LI-7820, Instruction Manual

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A-1 LI-7820 N₂O/H₂O Gas Analyzer
Appendix A.
Measurement overview
To measure gases, the analyzer injects laser light into a V-shaped optical cavity
defined by mirrors 1, 2, and 3 (
Figure A-1 below). The mirrors are highly reflective
— photons may circulate many times before escaping to strike the sample pho-
todiode. This long effective path-length enhances sensitivity by creating many oppor-
tunities for the sample gas flowing through the cavity to absorb light.
Figure A-1. Simplified schematic of the LI-7820 flow path through the optical bench. The
phase adjuster is a volume in the path between the laser and optical bench that is tied to a
pressure control mechanism. Changing the pressure of that volume alters the refractive index
which subsequently modifies the phase of the laser.
The instrument ramps the laser drive current so the laser scans a range of
wavelengths that span absorption features of the gases. A fraction of the photons are
allowed to leak from the cavity back into the laser, creating optical feedback that
narrows the laser output. This, in turn, couples a large fraction of the laser power
to the cavity and locks the laser frequency precisely to the center of a cavity res-
onance mode.
During a scan, the instrument creates a dense grid of fixed cavity resonance modes
that are equally spaced within a few kHz. The laser jumps discretely from one cav-
ity resonance mode to the next.