Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 55)

When the upper current is reached, the data file will be closed and the laser temperature will be incremented. A new file
will be collected for each laser temperature, and each file will contain a scan of the DC current.
The data files can then be processed by the user to make a plot of reference detector signal vs. DC current.
4.4 Optimizing Laser Parameters
Normally the laser parameters are adjusted only when a new laser is installed, or after transporting the system or
warming and recooling the laser. These parameter settings optimize the performance for a specific absorption line, so
these steps should be performed after the correct absorption line is chosen (see section 0).
4.4.1 Laser Temperature
If the TGA’s laser were perfect, it would emit at only one frequency (single mode). This emission frequency would
depend only on the injection current, and the emission frequency could be tuned over a wide range. In fact, the real
laser’s emission frequency is dependent on both its current and temperature, it always emits some of its optical energy
at other frequencies (multi-mode), and its emission frequency can be tuned over only a small range before it jumps to a
different frequency (mode hop). The multi-mode power and the mode hop characteristics of a laser may change
dramatically with temperature. Because both temperature and current determine the emission frequency, changing the
current can compensate for a change in temperature. The goal in setting the laser temperature is to find the combination
of temperature and current that minimizes multi-mode operation and avoids mode hops.
In principle, this is straightforward, but it is complicated by the iterative nature of the process. All of the other laser
parameters must be set to reasonably appropriate values in order to evaluate the laser temperature, but the optimum
value of some of those parameters depend on temperature. To begin, set the other laser parameters as follows:
• Set the zero current as described in section 4.4.2, but then reduce it by approximately 20% before setting the
other parameters. This will help to avoid confusion caused by the laser’s lower threshold current at lower
temperature.
• Set the high current count and the omitted data count to their maximum values
• Set the high current as described in section 4.4.3
• Set the modulation current as described in section 4.4.5
After setting these parameters to these preliminary values, set the laser temperature as described in this section, and then
proceed with the final optimization of the other parameters.
Display the 10 Hz concentration in Graph 1, Sample detector signal in Graph2, and DC Current in Graph 3. Enable the
line locking function and the detector offset and gain adjustment function. Note the laser operating temperature, the
laser DC current, the reference detector’s percent transmittance at the center of the ramp (displayed at the top of the
reference detector transmittance graph near the lower right corner), and the concentration noise. It is helpful to record
these values in a notebook.
gives an example of this process. Table 10
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