| Summary: | Polarisation-based
wavelength-division multi-/demulti-plexers (WDMs) and
filters have been designed, constructed and evaluated. These are compact devices made
from single-mode optical fibre possessing high linear birefringence.
Ihe WDMs accept two input wavelengths in a common state of polarisation, and
produce orthogonally polarised outputs which are spatially separated using a
polarisation-selective element. The maximum separation demonstrated is 1.5 nm, and
the minimum 0.04 nm, which is two orders of magnitude better than that achieved b
most other in-line WDMs.
A few metres of fibre only are required. Instantaneous cross-
talk values below -15 dB have been measured experimentally, and theoretical analysis
of the devices predicts that values below -28 dB should be attainable. Temperature
sensitivity currently limits long-term cross-talk, but also offers the possibility of tuning
the centre wavelength. Methods of reducing environmental sensitivity include judicious
choice
of fibre type and appropriate packaging of the WDMs. These techniques are
investigated and discussed. Incorporation of the WDMs in optical fibre sensing networks
is demonstrated, as is separation of frequency-shifted stimulated Brillouin scattered
light, generated in a fibre, from the pump frequency.
Filters are constructed
using concatenated sections of birefringent fibre,
positioned between two polarisers. Rotation of the polarisation azimuth between sections
is achieved
by twistíng portions of the fibre to induce circular birefringence. Thus
devices are made from a single strand of linearly birefringent fibre, less than one metre
in
length. Passband widths and separations are selected b changing the number of
twisted sections in the filter, and altering lengths and twist angles of sections. A
minimum passband width ofI0 nm has been demonstrated, at a passband separation
of60 nm. Environmental stability is suíciently good for the devices to be operated as
passive components.
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