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36
Chapter 7. Fibre coupling
aperture. The cylindrical lens may prevent a clear VFL output at the
U-chassis from being observed. The
TA
beam and the VFL beams
should be overlapping at these points. Note that if the VFL output
is very strongly diverging (e.g. larger than 1 cm diameter over 10 to
15 cm propagation), it may be necessary to adjust the fibre coupler
lens. See section 7.3 or check with
MOGL
abs support for how to
proceed.
5. Continue alternately adjusting
M3
to centre the
TA
beam on the
fibre coupler aperture, and adjusting
M4
to centre the VFL beam
on the U-chassis output (or isolator input aperture) until no further
optimisation can be made. The
TA
beam and the VFL beams should
be collinear.
6. Turn off and remove the VFL from the fibre patch cord. Connect the
fibre patch cord output to a power sensor. There should be some
nanowatts or more power detected when you block/unblock the
TA
beam in front of the fibre coupler. If not, check your fibre patch cord
end facets (see section 7.4), or contact
MOGL
abs support for how to
proceed.
7. Walk the mirror pair to optimise output power as follows:
(a) For the
horizontal
axis first, adjust
M4
to achieve maximum
output power from the fibre. Adjust the
horizontal
axis of
M3
to
achieve maximum output power from the fibre. Call this value
P
REF
.
(b) Make a small adjustment to the horizontal axis of
M3
in a clock-
wise direction such that the output power drops by no more than
25%. Take note of how much adjustment was made e.g.
1
16
turn
clockwise.
(c) Next adjust only the horizontal axis of
M4
to optimise the output
power from the fibre.
(d) If the new measured power is greater than
P
REF
, this higher
power is the new reference power
P
REF
. Iterate steps 7b and 7c
until
P
REF
is maximised. It will be necessary to drop the output