TGA200 Trace Gas Analyzer Overview
2.2 Laser Scan Sequence
The laser is operated using a scan sequence that includes three phases: the
modulation phase, the zero-current phase, and the high-current phase, as
illustrated in Figure 2-2. 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 that 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-current 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-current 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-current phase. This stabilizes the laser
temperature quickly, minimizing the effect of the temperature perturbation.
The entire scan sequence is repeated every 2 ms. Each scan is processed to
give a concentration measurement every 2 ms (500 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
Modulation Phase
(Spectral Scan)
High-Current Phase
(Temperature
Stabilization)
Laser
Current
Zero-Current Phase
(Laser Off)
Detector
Response
2 ms
FIGURE 2-2. TGA200 laser scan sequence
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