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38
Chapter 7. Fibre coupling
ment can affect the profile of the
TA
output and therefore the
coupling efficiency, and;
(b) tiny adjustments to the fibre coupler lens at high current can
drop the fibre coupling efficiency very significantly to the point
where the fibre tip is likely to be damaged. Take great care
and use small well-controlled adjustments to the fibre coupler
lens.
7.3 Fibre coupler collimation
If the
TA
output beam size, astigmatism and/or collimation has changed
compared to the original
MOGL
abs test condition, it will be necessary to
adjust the position of the focusing lens inside the fibre coupler to accom-
modate the changes. Initially when focus is very far from optimised, the
fibre coupler lens collimation can be approximately set using the VFL (see
fig. 7.2) as a light source.
1. Connect the fibre patch cord to the
MSA
(
MOA
) fibre output connector.
2. Connect the
exit
of the fibre to the VFL laser source.
3. Examine the red beam emitted back towards the
TA
diode from the
output fibre coupler.
4. Using the
EX-4
eccentric cam tool or
MGQA
focus tool provided in
your toolkit, adjust the fibre coupler lens collimation (see fig. 7.3)
such that the light exiting the coupler is well-collimated over at least
4 to 5 m distance. The beam can be deflected outside the chassis by
carefully inserting a mirror into the beam path between
M4
and the
fibre coupler.
5. Refer to section 7.2 to redo the fibre coupling alignment.