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Fig. 82 Crystal Fibre [148]
More complex index structures can also be constructed by utilising arrangements of holes of different
size in various periodic or nonperiodic structures. In addition, highly asymmetric core fibres can be
fabricated thereby creating fibres with very high level of birefringence.
The technology also makes it possible to create highly nonlinear fibres which can be used for e.g.
super continuum generation [148].
A research team from the Massachusetts Institute of Technology (MIT) is working on the design,
production and characterisation of optoelectronic fibres consisting of metallic, semiconductive and
insulating materials. Basically, optoelectronics encompasses the study, design and manufacture of
hardware devices that convert electrical signals into photon signals and vice versa. Any device that
operates as an electrical-to-optical or optical-to-electrical transducer is considered an optoelectronic
device [149].
In order to result in highly uniform fibres with well-controlled geometries and good optical properties,
they used a fibre-drawing technique from a preform. This preform is made up by a low melting
temperature crystalline conductor (Sn), amorphous semiconductors (As-Se-Te-Sn and As2Se3) and a
high glass transition temperature polymeric insulator (polyetherimide and polyethersulphone). It is
heated up in a furnace and drawn into a fibre. A single fibre can be regarded as an optoelectronic
device, but by modification it can adapt more functionality. Therefore, the researchers developed a
fibre structure that is capable for dual electron-photon transport. This is achieved by a hollow air core
fibre surrounded by a mirror layer, within which the light is confined. The fibres’ clad contains an array
of Sn metal strands that give an ohmic response at the same time. The scientists also succeeded in
developing a light sensitive fibre. The working principle can be described as follows: Sn metal
electrodes in the fibre core contact a photoconductive chalcogenide cylinder which produces a current
upon illumination under bias conditions. As these fibres are flexible, they can be processed into a
woven structure, which then has the ability to identify the location of illumination. Possible application
areas represent fabrics embedded into computer screens or projectors. Thus, instead of using a
mousse for communication with the computer, a light beam can be utilised because the screen can
detect the location where it was hit [150, 151].
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