MOGlabs MOA, Manual, Page: 34 / 66

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Chapter 5. MOA: external seed alignment
5.2 Reverse propagating
TA
diode beam collimation
Please
do not
proceed with the
TA
block adjustment steps below unless
there has been substantial misalignment or the
TA
diode has been replaced.
1. Block any seed light from entering the amplifier chassis. Referring
to the
Unseeded
column in the test report table
Guide for seed
coupling at low and high tapered amplifier current
, set the
TA
diode
injection current to a few mW.
2. Remove
M2
. The reverse propagating
TA
beam is collimated with an
input lens mounted in a flexure translation mount. Using the flexure
alignment screws (see fig. 1.2), adjust the input lens
x − y
position
so that the reverse propagating beam exits parallel to the chassis
horizontal plane and parallel with the chassis long edge. It will be
necessary to remove the input end plate from the main chassis. For
older
MOA
chassis this cannot be completed so skip this step.
3. Allow the reverse propagating beam to propagate over 4 to 5 m, in-
serting a mirror into the beam path to deflect the beam as required.
Adjust the reverse propagating beam divergence by rotation of the
TA
input lens mount using the supplied lens tube spanners (see fig. 1.2
and fig 4.2). There is an outer tube spanner for loosening the re-
taining lock ring and an inner tube spanner for adjusting the lens
focus. Make sure to tighten the retaining ring once the lens position
has been adjusted. The criteria for collimation will be a beam spot
that is as small as possible at a distance of 4 to 5 m.
4. Re-install
M2
. For the
MOA
, remove any input Faraday isolator. Pre-
align the seed laser beam and the
TA
beam using
M1
and
M2
. The
beams should be collinear.
NOTE: Do not allow the
TA
reverse propagating beam to propagate
into the seed laser without an isolator between the
TA
and the seed
laser if there is more than about 1 mW in the reverse propagating
beam; it could damage the seed laser diode.