Some time ago the transmission and polarisation rotation caused by the Faraday rotator was measured to be
98.8% and 15.6 degrees respectively.  At that time the TGG crystal was located towards one end of the
Faraday rotator - the "output" end. (see F1.jpg)

    After some effort in aligning the Faraday isolator we found that we could get good transmission but not
good isolation and vice versa.  Comparing the Faraday isolator with the unit installed in HAM5 revealed that
the TGG crystal is located in the middle of the magnet assembly.  Looking at the unit being worked on revealed
that the TGG crystal was loose within its holding cylinder.  Why and when it came loose is not obvious.  The
crystal was pushed more towards the centre of the Faraday rotator and the polarisation rotation angle was
re-measured.

    The new measurements suggest a transmission of 99.5% and a polarisation rotation angle of 46.1 degrees.
(see F2.jpg).  The angle plotted in both cases is the angle of rotation of a half waveplate.

    Anecdotally when the TGG crystal was pushed towards the centre, without any time spent optimising the
alignment we observed fairly good transmission and an isolation ratio of 30 dB.

    At present there is some confusion as to whether or not the TGG crystal is meant to be epoxied in place
or is held into place by what is rumoured to be a paramagnetic holder.  In either case the holding mechanism
failed.



  Koji / Gerardo / Peter