Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 14)

1.2.2 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 1-3. 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 during the zero phase 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. Fifty consecutive scans are averaged and processed to give a concentration
measurement every 100 ms (10 Hz sample rate).
Laser
Current
Zero Current Phase
(Laser Off)
Omitted Used in Calculation
2 ms
High Current Phase
(Temperature
Stabilization)
Modulation Phase
(Spectral Scan)
Detector
Response
Figure 1-3. TGA100 Laser Scan Sequence
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