Dear SLA 250 users
During the past two days there have been many messages connected by the laser
reliability thread in this news group. There have been many scenarios given
for Laser Remelts, some with great results, some with ambivalence and others
with seemingly less than expected results.
In the Model ' 74 laser tube used on the SLA250 for both the 20mW and 40mW
version there is a need on occasion to reduce(Remelt) the Cadmium that has
condensed during the normal operation of the laser in a section of the plasma
tube called the Cadmium Condenser Area. During normal operation in a HeCd
laser the Vaporous Cadmium is transported down the laser capillary and upon
exiting the bore is collected on the colder outer tube glass wall. The
condensation process in some cases resembles a crystalline growth (much like
in the semiconductor epitaxial growth process) and can form as nearly single
crystal whisker which can grow into the optical path causing a power
degradation. By heating up this section the growth melts into a pool (non
crystalline) and is no longer a problem .
As the growth is subject to normal crystalline growth parameters, Heat/wall
Temperature, Gas Density, Vapor Density, etc. each laser has a unique
structure and a unique period of time before clipping the laser beam. In
general, with the current bulk of 74 lasers installed, the period prior to
the first Remelt is typically 300 to 800 hours. Assuming a complete Remelt
(may require several sequential pushes of the Remelt button) you should
expect each subsequent requirement for Remelt to be approximately the same
period as the first as the above parameters controlling this growth are then
established for the given laser.
I have attached a graph (I hope it translates through the NET) which shows
one of the many lifetest lasers currently being tested at 3D and Omnichrome.
Note the Output power remains relatively constant until a rather rapid power
loss begins to occur. The typical power degradation is two consecutive days
of gradual and increasing power loss. In order to completely restore the
laser 1 or more (up to 5) Remelts may need to be performed. During the
Remelt process the laser head fan is turned off and about 20 Watts of heat is
applied to the condenser area. The rather unpleasant smell sometimes
noticeable (yea right) is the binders used in the Nomex wrapping over this
section of the tube. This Nomex is the same as that used by Fire Fighters,
Race Car Drivers etc. It is a very safe and stable insulating material. We
perform 5 Remelts during the manufacturing process to both test the process
and burn the majority of these binders away.
1. Does more Remelts mean lower lifetime? NO. As you have read in the
Mail the lifetimes of the 74 laser has risen over the past 4 years from ~2500
hours to over 5000 hours average.
2. Why does my laser need realignment after a Remelt? It should not.
After performing several Remelts (remember 20Watts with no cooling) the
resonator has experienced a different thermal load. After the fan comes on
you should wait about 15 min. prior to thinking your laser needs alignment.
The worst thing to do is to immediately realign the laser after a Remelt.
In 15 min. the laser will probably be at a lower power than in the
Please feel free to contact me regarding any additional questions in this
Director OEM Sales
Phone 909 517 2104
Fax. 909 591 8340
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