Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 53)

If a new laser is being tested, or if there is any doubt about the identity of the absorption lines, first turn the dual ramp
mode off, and find the ramp A absorption line as described above. Then start dual ramp mode and select the “RefDet A
& B” detector display option. Set the dual ramp parameters as follows so that ramp B will scan the same absorption line
as ramp A:
1) Set the ramp B offset to 0 mA
2) Set the ramp B high current to the same value as the (ramp A) high current
3) Set the ramp B modulation current to the same value as the (ramp A) modulation current
Adjust the Ramp B offset to move ramp B to the correct absorption line, letting the ramp A line lock maintain the first
absorption line in the center of ramp A. It may be necessary to adjust high currents and the modulation currents for
ramp A and ramp B as the ramp B offset is adjusted.
4.3 Laser Mapping
The laser mapping function allows the user to characterize the laser performance. It steps the laser over a range of
temperature and current to measure the target gas absorption lines within the tuning range of the laser.
Configure the plumbing as follows:
Use a tee to connect the reference gas to both the reference inlet and the sample inlet. Set the reference gas flow rate to
at least 10 ml/min. Use a filter and needle valve connected to the suction hose between the analyzer and the sample
pump to admit enough filtered air to set the sample pressure to approximately 20 mbar. This configuration is helpful for
several reasons:
1) Reference gas in the sample cell avoids absorption by water vapor or other gases in the air that can occur if
ambient air is drawn through the sample cell.
2) The stronger absorption lines will absorb virtually all of the light from the laser, giving an indication of the laser’s
multimode power.
3) Some gases, such as nitrous oxide (N
2
O) have a very regular pattern of strong absorption lines that all look alike.
These lines are identified more easily if there is enough absorption for the weaker lines to be seen easily.
Set the parameters for laser mapping:
1) It may be helpful to save a copy of the current parameters (see section 3.6.1.) Several parameters will be changed
to perform the laser map, and saving a copy of the parameter file now will allow the current configuration to be
restored later by reading in the saved parameter file.
2) From the real time screen:
• Zero current: The laser’s threshold current will tend to increase with increasing temperature, so the zero
current must be set below the laser threshold current for the lowest laser temperature to be mapped. When in
doubt it is best to set the zero current to a low value.
• High current: The high current pulse can affect the mode structure of some lasers. Therefore it is best to set the
high current to the normal operating value, if this is known. However, the normal operating value depends on
the laser, and on its temperature and current settings. Also, the laser map covers a wide range of temperature
and current, and the maps are often generated when the optimum temperature and current are unknown. It is
preferable to use a low value for the high current to avoid limiting the range of laser map (the laser is
automatically disabled if the DC current plus the High current would be greater than the maximum laser
current rating). Therefore if the normal operating value is unknown, set the high current to 20 mA, and the
high current counts to 8.
• Detector parameters: The detector gains, offsets, and temperatures must be set to avoid detector saturation
over the range of temperatures and currents to be mapped. The laser’s output power tends to increase at higher
DC current and at lower temperature, so generally the detector parameters should be checked at the lowest
temperature and highest current to be mapped. Set the detector gains to zero and disable the automatic offset
and gain adjustment. Increase the detector temperatures as needed to ensure the detectors do not saturate at
any combination of temperature and current to be used in the laser map. Generally it is best to set the detector
temperatures for relatively low response because a high signal level is not needed for the mapping process,
and a saturated detector response would invalidate the measurement.
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