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6.1 Procedure for output beam optimisation
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5. The amplifier laser beam divergence and astigmatism depend on
injection current. Output beam optimisation should be conducted
at the expected operating current, typically at or near the maximum
current specified in the system test report. Increase the
TA
current to
the operating current but do not measure the output beam directly,
instead use an external
PBS
and half-wave plate to decrease the
power to a reasonably safe level, for example 100 mW or less.
6. Carefully adjust the
TA
exit lens focus to collimate the major (fast)
axis of the output beam, by using the supplied lens tube spanners
(see figs. 1.2 and 4.2). There is an outer tube spanner for loosening
the retaining lock ring and an inner tube spanner for adjusting the
lens focus. Make sure to retighten the retaining ring once the lens
position has been adjusted. Use an
M
2
beam profiler, or an external
lens with an imaging sensor positioned at the lens focal distance, to
achieve the best collimation. The major axis beam waist on the imag-
ing sensor should be minimised. Note that the cylindrical lens will
be acting on the minor (slow) axis which is typically the horizontal
axis for almost all
MOGL
abs amplifiers.
7. Once the beam is collimated along the major axis, adjust the position
of the cylindrical astigmatism correction lens along the optical axis
to achieve the smallest beam waist along the minor axis. This is the
most critical adjustment as the beam quality is incredibly sensitive
to cylindrical lens rotation (orthogonality of the lens plane to the
optical axis), centring (translation along the minor axis) and position
(translation along the optical axis).
We strongly recommend using an
M
2
measurement system, such as the
MOGL
abs
M2P
, to adjust collimation and astigmatism compensation in or-
der to achieve the best quality output beam and hence achieve optimum
fibre coupling efficiency.