Campbell Scientific TGA100A Overview - Manual (Page 4)

TGA100A Trace Gas Analyzer Overview
OV2.2 Laser Scan Sequence
The laser is operated using a scan sequence that includes three phases: the zero
current phase, the high current phase, and the modulation phase, as illustrated
in Figure OV2.2-1. The modulation phase performs the actual spectral scan.
During this phase the laser current is increased linearly over a small range
(typically +/- 0.5 to 1 mA). The laser’s emission wavenumber depends on its
current. Therefore the laser’s emission is scanned over a small range of
frequencies (typically +/- 0.03 to 0.06 cm
-1 ).
During the zero current phase, the laser current is set to a value below the
laser’s emission threshold. “Zero” signifies the laser emits no optical power; it
does not mean the current is zero. The zero current phase is used to measure
the detector’s dark response, i.e., the response with no laser signal.
The reduced current during the zero phase dissipates less heat in the laser,
causing it to cool slightly. The laser’s emission frequency depends on its
temperature as well as its current. Therefore the temperature perturbation
caused by reduced current introduces a perturbation in the laser’s emission
frequency. During the high current phase the laser current is increased above
its value during the modulation phase to replace the heat “lost” during the zero
phase. This stabilizes the laser temperature quickly, minimizing the effect of
the temperature perturbation. The entire scan sequence is repeated every 2 ms
(500 scans per second).
Modulation Phase
(Spectral Scan)
High Current Phase
(Temperature
Stabilization)
Laser
Current
Zero Current Phase
(Laser Off) Used in
Calculation
Omitted
Detector
Response
2 ms
FIGURE OV2.2-1. TGA100A Laser Scan Sequence
OV-4