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7.1 Fibre alignment touch-up
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• The fibre coupler can be rotated about the optical axis to align the
FC/APC
key of the coupler to the incident
TA
output, by releasing
the three radial mounting screws. After this mechanical rotation, a
full realignment of the fibre coupling will likely be required, as per
section 7.2.
In both cases a polarimeter measurement is recommended to ensure a
polarisation extinction of
>
20 dB is achieved.
7.1 Fibre alignment touch-up
Minor fibre alignment adjustment may be required when the
TA
system
is first received or any minor or suspected alignment change may have
occurred between the
TA
chip and the fibre coupler.
The amplifier should first be operating normally in free space as described
in the previous chapters. In particular, it is assumed the output is well
collimated both horizontally and vertically and the astigmatism compen-
sation is optimised for the desired
TA
diode operating current. The beam
cross-section should be approximately circular, but achieving a symmetric
aspect ratio may not be possible given the limited range of cylindrical lens
focal lengths available, and variations in
TA
diode astigmatism.
1. The seed power into the
TA
should be set in the range of seed power
recommended in the test report. The
TA
injection current should be
chosen to provide 10 mW before the fibre coupler (or as specified at
Initial FC target
in the test report). The fibre core type should also
be the same as specified in the test report, or have an equivalent
mode field diameter.
2. Measure
P
REF
, the output power from the correct fibre patch cord
connected to the fibre coupler output, and compare to the
Initial FC
target
in the test report for the same test conditions.
3. Referring to figures. 4.1 and 5.1, make very small (typically
1
16
turn
or less) adjustments to the
horizontal
axis of
M4
until the output
power is maximised.