Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to photoconductors with
improved mechanical strength.
Photoconductive fibres and rods normally consist
of a core material which is transparent for selected wave
lengths and is surrounded by a sheath which is likewise
transparent for selected wave lengthe but has a refractiVe
index lower than that of the core material.
Light entering the fibres at less than a certain
angle to the fibre axis travels along the fibre by total
internal reflection. In order to obtain a good photo-
conductive effect the thickness of the fibre sheath should
correspond to at least one wave length of the light to be
transmitted. In photoconductive fibres in which the
greatest importance is placed on low light loss, for
example in so-called communication fibres or wave guides,
the thickness of the sheath will correspond to several
wave lengths of the light to be transmitted, whereas in
the so-called single mode fibres the thickness of the
sheath amounts to from thirty to a hundred times the wave
length of the light to be transmitted.
If one is not too limited by such parameters
as for example refractive index, percentage light
transmission, and viscosity ratio, the core and sheath
materials will be so chosen as to achieve a mechanically
stable fibre and the sheath will have a lower coefficient
of thermal expansion than the core material. Thus, in
the manufacture of the fibres a compressive resilient
strain will be created in the sheath which makes the
fibres insensitive to tensile stresses and bending strains.
However, the choice of materials is very small,
particularly in communication fibres, because of the high
demands relating to transmissivity both in the core material
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and in the sheath. It is therefore only to be expected
that on occasions fibres of high optical quality are
obtained which have low mechanical strengths.
It i~ an object of the invention to provide
photoconductive fibres with a mechanical strength which
enables the unquestioned further manufacture and use of
the photoconductive fibres. A further object of the
invention is the provision of a simple process for
manufacturing such improved photoconductive fibres.
According to the invention, fibres made of a
core and a sheath material have a second sheath applied
thereto the coefficient of thermal expansion of which is
lower than that of the first sheath or of the combination
of the core and the first sheath.
~5 In fibres which have a fluid a~ the core material
the ~olid sheath, which i8 largely free of gtre89, i8
surrounded by a further sheath with a higher coefficient
of thermal expansion, which is therefore under tensile
stress so that a compressive resilient strain is trans-
mitted to the first sheath. Such a fibre is very sensitive
to stresses and bending strains and is therefore provided
with an additional or third sheath the coefficient Or
thermal expansion of which is lower than that of the
second sheath. In the first and third sheaths compressive
resilient strain is prevalent, while the second sheath is
under tensile stress. ~Sicrocracks on the inner boundary
surface between the fluid and the first sheath and on
the outer boundary surface between the third sheath and
the surrounding medium will accordingly not lead so readily
to breaking of the fibres during tensile stress or bending
strain.
If the fibres are produced by the "rod-pipe"
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process, a rod made of the core material i8 placed within
a pipe made o~ the material of the first sheath ar.d the
rod and pipe are inserted together into a pipe made of
the material of the second sheath; the assembly i8 heated
until it becomes soft, so that it is capable of extrusion
and it is then drawn into fibres.
Capillary tubes forming the sheaths of fluid-
filled fibres are produced in a similar manner in that
the three original pipes of the first, second and third
sheaths are put one inside the other, heated until softened
and drawn out to form a composite capillary tube.
In fibre production by the double-jet drawing
process, the known apparatus is modified in that a further
ring noæzle is arranged concentric to the double jet, this
further ring nozzle being connected to a third crucible
containing the material of the second sheath. The core
material and the materials of the first and second sheaths
flow out simultaneously and are drawn out to form a
composite fibre.
If the fibres are produced from internally
coated tube the coated tube is put into a pipe made of
the material of the second sheath, both are heated together
until soft and are then drawn to form a fibre together
with the core material.
Depending on the intended use of the fibres,
attention should be paid, when selecting the materials
for the second and where applicable the third sheath, to
the refractive index as well as to the coefficient of
thermal expansion and the viscosity ratio.
An advantage of the invention lies in that,
because of the provision of the additional sheath one is
free, regarding the parameter of the coefficient of thermal
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expansion, in the choice of the materials for the core and
fi:rst or optical Gheath and c~l choose combinations, which,
~ithout the second sheath, would not be usable in practice.
Coloured materials may be used for the second sheath -
and where applicable each additional sheath - so that light
wh:ich would otherwise be emitted from the fiber is absorbed.
The present invention resides in a photoconductive ,
fiber or rod comprisng a core and a sheath having a lower
reEractive index than the core, the sheath being surrounded
by at least one further sheath, the coefficient of thermal
expansion of which is lower than that of the first sheath
or of the combination of the core and the first sheath,and
being further characterized in that either (1) the core
comprises a fluid which is contained within a capillary
tube which includes three sheaths extending conoentrically
to the longitudinal axis of the tube, with the coefficient
of thermal expansion of the material of the inner sheath
and of the outer sheath ~eing lower than that of the mater-
ial of the sheath lying therebetween, or (2) the photo-
conductive fiber or rod comprises a core in which the re-
fractive index decreases from the inside outwardly, said
core being surrounded by a first sheath which has a higher
coefficient of thermal expansion than the core and is sur-
rounded by a second sheath which has a lower coefficient
of thermal expansion than the first sheath.
By one aspect of this invention therefore, there is
provided a photoco~ductive fiber or rod comprising a core
and a sheath having a lower refractive index than the core,
this sheath being surrounded by at least one further sheath,
the coefficient of ~hermal expansion of which is lower than
that of the first sheath or of the combination of the core
~nd the first sheath, and further characterized in that
the core camprises a fluid which is contained within a
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capillary tube which includes three sheaths extending con-
centrically to the longitudinal axis of the tube, with the
coefficient of thermal expansion of the material of the inner
sheath and of the outer sheath being lower than that of the mater-
ia:l of the sheath lying therebetween.
In another aspect, the present invention resides in a
photoconductive fibre or rod comprising a core in which the
re~Eractive index decreases from the inside outwardly, said
core being surrounded by a first sheath which has a higher
coefficient of thermal expansion than the core and is sur-
rounded by a second sheath which has a lower coefficient of
thermal expansion than the first sheath.
The invention will now be described by way of example
with reference to the accompanying drawings in which:-
Figure 1 is a perspective view, greatly enlarged, ofa photoconductive fiber with a core 1, a first sheath 2 and
a second sheath 3,
Figure 2 is a perspective view, greatly enlarged~of a
photoconductive fiber with a fluid core 1, a first sheath 2,
a second sheath 3 and a third sheath 4, and
Figure 3 is a perspective view, again greatly enlarged~
of a photoconductive fiber with a core 1, a..first sheath 2
and a second sheath 3 coloured black.
EXAMPLE 1
Photoconductive fibres of about 90 ~ m diameter were
drawn from a glass core having a thermal coefficient of
expansion of 93 x 10 7 per C., a first sheath having a
thermal coefficient of expansion of 96 x 10 7 per C and a
second sheath with a thermal coefficient of expansion of 53
x 10 7 per C. The refractive index of the second sheath
may either be greater th~n or less than that of the first
sheath.
With these fibres the bending r~dius, i.e. that at
which the fibre breaks, was redu oe d by a factor of O.6
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in relation to a fibre of the same thickness without the
second sheath.
Example 2
A photoconductive fibre was drawn by the rod-
pipe process and consisted of a fluid core, a first
sheath with a thermal coefficient Or expansion of 41.5 x
10 7 per C. and a refractive index nd = 1.487, a second
sheath with a thermal coefficient of expansion of 55 x
10 7 per C. and a refractive index nd = 1.508, and a
third sheath with a thermal coefficient of expansion of
41 x 10 7 per C. and a refractive index nd = 1.542
Example 3
- A photoconductor was drawn by the double-jet
drawing process and consisted Or a glass core with a
thermal coefficient of expansion of 93 x 10 7 per C.,
a first sheath with a thermal coefricient Or expansion of
96 x 10 7 per C. and a second, black sheath with a thermal
coefficient Or expansion Or 57.5 x 10 7 per C.
The invention can also be applied to so-called
"gradient fibres" which do not specifically consist of a
highly refractive core and a sheath with low refraction,
but which include a graduated coating having a higher
refractive index on the inside and lower refractive index
on the outside. Gradient fibres can be of quartz glass
in particular. Even those kno~m fibres which are often
fragile in themselves can be made more resistant
mechariically in that the fibre constitutes a core which
is surrounded by a first sheath which has a higher
coefficient of thermal expansion than the core or fibre, and
this first sheath is surrounded by a second sheath which has
a lower coefficient of thermal expansion than the first
sheath.
