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Notes on Version 3 and Version 4 that lead to Version
5
RW-MOPA ver 3 was based on two
pass experiments done with version 2 with a PORRO HR in the last amp to double
back the beam back through both amps and being picked off by a right angle
prism. The porro was set lower to reflect the beam back thru at a lower
altitude and with a slight tilt to allow pickoff prism to pass first beam and
catch return beam. This two pass arrangement generated 3 to 4 times more power.
But due to short distances the pickoff couldn't catch all of the beam due to
increase divergence in the beam and short angular distances of version two.
RW-MOPA ver 3 corrected this by providing a much larger optical bench and
therefore required a separate case for the laser head and another for the power
supply. At 250 mjoules the two amp two pass was later modified to a two amp 1
two pass and 1 single pass due to damage of right angle prisms as intense hot
spots developed and pitted one right angle prism on return from the second amp
which produced 190 mj. I found damage had occurred to the laser rod in the
second amp (bubble crack appeared). Some of the problem was trying to push 25mj
from this oscillator was allowing some transverse modes to return. The motor q
switch at the rated speed was just producing single pulse and 25mj was pushing
too hard. I now only had effective 3 amps out of the 2 physical ones. The beam
made just a single pass through the 2nd amp next to the oscillator
instead of a porro to bounce a second pass.
rwmopa3.gif diagram of version 3. As you can see I had
to have patience and a lot of alignment paper. Also a trick by drilling a hole
in folded over alignment paper (making it two sided) so I could find and pass
the first beam and find the return one.
Overall the double pass had
problems and this began the design changes for version 4 as 1/4 inch rods are
too narrow for multiple passes and cause possible beam degradation due to non
linear optical rod pumping in various radial rod locations or radial gain
variation from possible overlap beams
Instead of double pass a new
design on four physical stages would be used as a single pass.
rwmopa4.gif diagram of version 4.
So the version 3 laser was gutted
and the optical plate reused and reworked and the laser was rebuilt with the
new configuration. The motor q switch was replaced with a HR/dye cell and the
tank OC element was extracted and placed in a lens tube and gimbal mount.
(Details on the HR/dye cell are covered in version 5 application notes). In
this new configurations I would give up on motor q switches for the excellent
capabilities of saturable absorbers. This would give me better mode control and
pulse control. A lot of work done in the sixties and seventies showed the
benefits of dye q switches. (they are cheap too!)
I sawed off parts of the tank
laser I no longer needed for the oscillator. Also for the amps but kept the
fixed right angle prism which was next to the q switch. I rotated the prism
upside down on amp 2 to allow it to send the beam to amp 3. Also I cut off the
fixed prism mount from some spare tank lasers to use this right angle mount for
the oscillator. A bolt threading into the bench and allowed the left/right
adjustments and rotating the prism controlled altitude. I used two for the
resonator and one more to bend the beam toward amp 1. All left handed beam
directions was handled by the prism rotated to it's upside orientation.
This new configuration yielded
8-12mj q switch pulse at 40 nanoseconds from the oscillator. I aligned up the
beam pass through the amps using alignment paper and if I had powered up the
amps, they would have yielded a total to 190-220 mj output by my calculations.
The design had to be folded to fit the case that had been used before. Two
cases of the same size held the laser head and the other the power supply. But
due to the increase in the number of amps, I needed a larger power supply case
to house all the new caps needed. I decided to redesign the power supply to
increase the power supply current from 40milliamps to 300 milliamps by
purchasing a larger transformer. 3 tank lasers had been taking 15 minutes to
charge up and going to 5 lasers would then take 30 minutes and I felt that was
too long to be waiting. During this phase, I decided to go ahead and redesign
the laser head as well to get rid of extra prisms used in a folded oscillator
and amp stages.
With version 5, I will try
to have all rods tilted a little to cause back reflections to land harmlessly
on component metal and not go back down through previous stages and hit a front
surface of a prisms and come back up again etc. Version 4 could not be adjusted
to prevent this since the right angle prisms on the tank laser body would also
be shifted when the body was shifted to move the rod surface to a tilt to the
beam axis. Reducing the extra prisms by straightening the configuration
hopefully would cut down on reflections as I was trying to trace down what the
alignment paper showed with version 4. I had expected by not using motor q
switches and using a plano-concave resonator and round optics that I would have
this perfect circle of TEM00 mode. I found instead a spot with what looked like
the mode but it also had these little side lobes: 1 above and 1 below with a
faint output. I found this similar to what I experienced with the tank laser
from earlier versions (always some little lobe etc) . During my studies, I knew
that misalign mirrors can make it hard to eliminate higher order transverse
modes and the tank laser used a planar resonator. Plus I had concerns about the
changing angles of the rotating prism. Not having a source to compare my
alignment paper readings, I continued to try and get that perfect spot. One
thing I learned is you can get the occasional tiny dark spot in the lens
expanded view of the beam when reading the alignment paper as this is caused by
diffraction with a dust particle. The lobes I had no answer for except I always
could diminish them by reducing the variable diaphragm aperture and the output
power until they are too weak to show up I guessed. Therefore in version 5 my
main goal was to eliminate the prisms in the oscillator by not using a folded
design and see if that would clear up these lobes. As it turned out it didn't
and now it seems to be a by product of extra reflections from some of the optical
components in the cavity/resonator arrangement.