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6. Output beam optimisation
WARNING
Do not operate the amplifier above the
Maximum current, unseeded
without an appropriately coupled input seed. Operation exceeding that
condition can cause fatal structural degradation of the
TA
diode. Refer to
the
Maximum current, unseeded
specified in the amplifier test report.
The beam profile of a
TA
output beam generally looks unpleasant. It is
usually irregular, cross-shaped (like two orthogonal ellipses) and may have
two or three stripes. To some extent, appearances are deceiving because
of the logarithmic response of human vision. For a better appreciation of
the output beam, please use a beam profiler or imaging sensor (e.g. a weak
reflection, viewed on the
CMOS
sensor of a webcam with lens removed).
Typically, the
TA
output will be fibre-coupled to produce a high-quality
TEM
00
Gaussian beam. A simple measure of the beam quality is known as
the
M
2
factor (or beam quality factor)
M
2
=
ω
0
θ
π
λ
where
ω
0
is the waist radius and
θ
is the half-angle of the beam divergence.
The beam quality factor provides an indication of how well the beam can
be focused, compared to an ideal diffraction-limited Gaussian beam. For a
TEM
00
beam, the value will be one, and beams with larger
M
2
will produce
larger spot sizes.
MOGL
abs can provide measurements of
M
2
for your laser system upon
request. We find a more meaningful measurement is how much of the total
power can be coupled out of a single-mode optical fibre, and hence we
measure and optimise the fibre-coupled output as well as the total output
power.
Please
do not
proceed with any output beam optimisation unless there has
been substantial misalignment (e.g. due to mishandling during shipping,
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