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Chapter 7. Fibre coupling
Some instruments and tools will be helpful in quickly attaining optimum
fibre coupling, including:
1. Suitable fibre patch cord, e.g. end-capped fibre patch cord such
as
MOGL
abs
FPC-EC-780-2
(
PM780-HP
fibre core) for 770 nm to
1100 nm.
2. Optical fibre visual fault locator (VFL, see fig. 7.2).
Figure 7.2:
Visual fault locator (VFL). Injects visible laser light into a fibre,
allowing initial alignment and mode matching.
3. Power meter and sensor, preferably with integrating sphere to avoid
sensor saturation.
Note that power meters with sensors using silicon photodiodes can easily
saturate below their maximum rated power and thus give false readings,
particularly when the entire sensor surface is not illuminated as for a small
beam. Sensors based on integrating spheres are preferred.
Polarisation control
The polarisation of the
TA
output can be aligned to that of a polarisation-
maintaining (
PM
) singlemode fibre in two ways.
• A half-wave retarder can be used, either mounted in the endcap of
the final isolator (see fig. 4.3), or mounted to the chassis just before
the fibre coupler using an optional waveplate holder available from
MOGL
abs. It is unlikely adjustment of the half-wave retarder will
require any substantial fibre coupling realignment. If some small
adjustment seems necessary, refer to section 7.1.