MOGlabs MOA, Manual, Page: 13 / 66

3
PBS
) to pick off a fixed (or variable) free-space beam from the seed laser,
which can be used for locking to a frequency reference or monitoring the
seed laser. For the
MOA
series, the seed beam enters through the input
aperture either in free-space or via a fibre coupler, and is then deflected
by mirrors
M1
and
M2
into the
TA
diode.
The seed laser beam must be mode-matched to the
TA
diode. Mode match-
ing can be optimised by ensuring that the reverse propagating
TA
beam,
emitted from the small area ridge waveguide side of the
TA
back towards
M2
and
M1
, is overlapped with the seed beam propagating towards the
TA
. The seed and reverse propagating
TA
beams should have similar beam
size along their paths, and parallel linear polarisation. The
MOGL
abs am-
plifiers have a zero-order half-wave plate installed for adjusting the seed
polarisation to match the
TA
diode. Usually the half-wave plate is installed
in the end-cap of the input isolator in
MSA
systems, or on the chassis for
MOA
systems (see figs. 4.1, 4.3 and 5.1).
The main
TA
output beam propagates through a high-power Faraday iso-
lator. A photodetector (
PD
) monitors the
TA
output to ensure that the
TA
is properly seeded at high operating currents (see chap. 2 for further
information).
The main
TA
output exits through the free-space output port or is reflected
by mirrors
M3, M4
to a fibre coupler. Replacing
M3
with a
PBS
allows dual
beam output with a variable ratio between free-space and fibre-coupled
outputs. See chapter 7 for details on fibre coupling.
For background information on tapered amplifiers, please see Refs. [1, 2].