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Laser Line Locking & Built-in Verification
Laser Line Locking
The reference cell is used to lock the output wavelength of the laser diode. The
sealed reference cell contains a known concentration of the analyte in a host gas,
which has no absorbance in the spectral range of interest. A block diagram of the
wavelength-locking algorithm employed by the instrument is shown in the figure
below. Two nested levels of temperature control are employed to maintain the
operation of the DFB laser at the proper wavelength. The first level is a simple
PID control loop, which maintains the laser at a target temperature. In the second
level, the outer control loop, the spectra of the analyte samples in the reference
cells are monitored. Minor shifts in the observed peak positions are used as a
feedback signal for the temperature set point of the inner control loop. Thus, the
outer-control loop provides a fine adjustment for the inner-control loop.
Built-in verification
External calibration is time consuming, especially for polar species like water
with long equilibration times. Model 5100 HD uses a sealed reference cell
(containing moisture vapor in a buffer gas that does not absorb in the spectral
range of interest) for quick analyzer verification. The analyzer response is
compared to the value of the analyte concentration in the reference cell and the
analyzer performance is verified continuously.
Measurement
Fiber optics conduct the laser beam through the GRIN lens to the sample
cell. When the beam reaches the mirror at the end of the cell, it is reflected so
that it continues upward back through the cell until it enters the detector. The
detector converts the information from the beam into an electrical signal that is
transmitted to the microprocessor. The software then processes the signal and
determines the analyte concentration.
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Overview
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