Spectral control of an alexandrite laser for an airborne water-vapor differential absorption lidar system
1994
A narrow-linewidth pulsed alexandrite laser has been greatly modified for
improved spectral stability in an aircraft environment, and its operation has
been evaluated in the laboratory for making water-vapor differential absorption
lidar measurements. An alignment technique is described to achieve the optimum
free spectral range ratio for the two etalons inserted in the
alexandrite laser cavity, and the sensitivity of this ratio is analyzed. This
technique drastically decreases the occurrence of mode hopping, which is
commonly observed in a tunable, two-intracavity-etalon laser system.
High spectral purity (>99.85%) at 730 nm is demonstrated by the use
of a water-vapor absorption line as a notch filter. The effective cross sections
of 760-nm oxygen and 730-nm water-vapor absorption lines are measured at
different pressures by using this laser, which has a finite linewidth of 0.02
cm−1 (FWHM). It is found that for water-vapor absorption
linewidths greater than 0.04 cm−1 (HWHM), or for altitudes
below 10 km, the laser line can be considered monochromatic because the measured
effective absorption cross section is within 1% of the calculated
monochromatic cross section. An analysis of the environmental sensitivity of the
two intracavity etalons is presented, and a closed-loop computer control
for active stabilization of the two intracavity talons in the alexandrite laser
is described. Using a water-vapor absorption line as a wavelength reference, we
measure a long-term frequency drift (≈1.5 h) of less than 0.7 pm in the
laboratory.
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