Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 58)

In some cases the minimum concentration noise may be at a different laser temperature than the minimum reference
transmittance. First, if the DC current is near the laser threshold current, the laser’s optical power output may be
reduced significantly at higher laser temperatures (lower DC current). This can be verified by looking at the sample
detector signal as a function of laser temperature. This is shown in Fi
for our example. gure 4-9
Figure 4-9. Typical Sample Detector Signal as a Function of Laser Temperature
0
5
10
15
20
25
30
35
40
45
102.5 103 103.5 104 104.5 105 105.5
Laser Temperature (K)
Samp
le Sign
al (
m
V)
In this case it may be possible to compensate for the reduced laser power be reducing the detector temperatures (see
section 4.5.2). If adjusting the detector temperatures results in low concentration noise at the laser temperature of
minimum reference transmittance, this is the optimal laser temperature.
The other condition that can give a different optimum laser temperature for reference transmittance and concentration
noise is that the position of a mode hop may also move with laser temperature. If the laser has a mode hop near the
absorption line, the concentration noise may increase as the mode hop approaches the line. In this case, it may be
necessary to choose a laser temperature that gives a compromise between reference transmittance and concentration
noise. If there is no laser temperature that gives satisfactory performance, it may be necessary to choose another
absorption line.
For dual ramp operation, follow the process described above, but also record the ramp B offset, ramp B reference
transmittance, and ramp B concentration noise. Ideally ramp A and ramp B will have the same optimum laser
temperature. In some cases it may be necessary to set the laser temperature between the optimum temperatures for ramp
A and ramp B to achieve acceptable performance for both.
4.4.2 Zero Current
The laser current must be reduced below the lasing threshold briefly at the start of each spectral scan (described in
section 1.2.2) to measure the detector response with no laser emission. If the zero current is set too high, the laser will
emit some energy when it should be off, and the TGA will calculate the wrong transmittance. This will cause an error in
the reported concentration. This problem could be avoided by simply setting the zero current to 0 mA to guarantee the
laser is off. However, both current and temperature affect the laser’s emission frequency, and the laser's temperature is
affected by its current. The laser’s temperature falls slightly when the current is reduced, so the temperature must be
stabilized at the start of each spectral scan. Overdriving the current, as discussed in section 4.4.3, can help to stabilize
the laser temperature more quickly, but setting the zero current as high as possible minimizes the temperature
perturbation.
Set the laser zero current by temporarily disabling line lock and automatic gain and offset correction, and then pressing
<Alt+Z> to start an algorithm to determine the optimum laser zero current. This algorithm will set the laser zero current
to zero, and then increase it in steps until the sample detector's response during the zero current phase starts to increase.
This indicates that the laser’s threshold current has been reached and the laser is now emitting. The program will then
reduce the laser zero current slightly and increment it again, using a smaller step size. When the sample detector's zero-
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