| Description: | When Spectra Physics discontinued support for their laser diode power supply, an
alternative was need to extend the usefulness of the Spectra Physics laser head inside the MPLs.
The solution was to use a Coherent laser diode. Unfortunately, the Coherent diode lasers
do not have the exact same characteristics as the Spectra Physics diode lasers. As a
result, the output of the Spectra Physics laser head sometimes produces a second pulse close
in time but lower in intensity from the first main pulse. This is known as ?double pulsing?.
Not all of the Spectra Physics/Coherent combination systems produce a double pulse and the
systems that do produce a double pulse do not always exhibit this behavior. It can be
identified by a second peak at ~.2km. At first glance the signal in the data will resemble
a low aerosol or cloud layer. When the double pulse is weak, the atmospheric signal will
obscure the double pulse signal. At the ARM sites with a sunshade such as TWP and AMF,
the double pulse can be seen when the sunshade closes during solar noon. When the sunshade
closes, the MPL does not see atmospheric signal so the second pulse is evident. At SGP, a
daily window cleaning is part of the routine. During this time the double pulse can be
seen. Without a sunshade or window cleaning it is impossible to say definitively if double
pulse is present.
The first system to produce a double pulse was MPL s/n 008 located at TWP ? C2 Nauru in
February 2005. The second system was MPL s/n 004 located at SGP in August 2005. The
potential for the other systems to produce a double pulse is high. However, as of March 2006,
the MPLs at NSA, TWP ? C1, TWP ? C3 and AMF have not recorded data with the double pulse
artifact.
The ARM program is in the process of replacing all the MPLs with a newer model. After the
new MPLs are deployed the double pulse problem should not be an issue. The old systems
will be relegated as spares. The double pulse problem will return if an old system is
needed to replace a failed new system. |