Note: Descriptions are shown in the official language in which they were submitted.
~38669
This invention relates to optical fibres
having low mode dispersion, particularly as used for
communications systems.
An optical fibre conveys, or transmits, light
from an input end to an output end by the phenomenon of
internal reflection. A fibre generally comprises a core
surrounded by a cladding and the light is retained within
the core by the internal reflection.
One of the faults of conventional optical
fibres is "mode dispersion" which causes pulse spreading.
Differential mode delay can be eliminated if a fibre is allowed
to propagate only a single fundamental mode. However this
requires the fibre to have a small core diameter, making fibre
splicing and connecting difficult. Also, single-mode lasing
sources are required for efficient light insertion into such
a single-mode fibre. The diameter of the core cannot be
arbitrarily increased by reducing the numerical aperture as
too small a numerical aperture requires a thick cladding for
containment of the evanescent wave and also too large a
radius of curvature for low bending losses.
The present invention provides an optical
fibre having a cross-sectional area which enables efficient
coupling and a fairly large numerical aperture and which
substantially eliminates mode dispersion. An optical fibre,
in accordance with the present invention, in addition to a
light transmitting core, also has one or more rings around the
core - each transmitting light, with a ring of "cladding"
material, that is material having a refractory index lower
than the light transmitting material on each side thereof.
An alternative arrangement has a core of"cladding" material
with alternate rings also of "cladding" material, with the
intermediate rings light transmitting. ~- ;
~.,
-- 1 --
103&669
The invention will be readily understood by
the following description of certain embodiments, by way of
example, in conjunction with the accompanying diagrammatic
drawings, in which:-
Figure 1 is a cross-section through one known
form of optical fibre having a stepped refractory index;
Figure 2 illustrates the form of the refractive
index curve for the fibre of Figure li
Figure 3 is a longitudinal cross-section of a
fibre of the form of Figure 1, illustrating light ray propagation
therein;
Figure 4 is a curve illustrating the refractive
index characteristic of another known form of optical fibre;
Figure 5 is a longitudinal cross-section of
the known form of fibre, the refractive index characteristic
of which is illustrated in Figure 4;
Figure 6 is a cross-section through a further
form of optical fibre in accordance with the present invention;
Figure 7 is a curve illustrating the refractive
index characteristic of the fibre of Figure 6;
Figure 8 is a cross-section through yet a
further form of optical fibre in accordance with the present
invention;
Figure 9 is a curve illustrating the
refractive index characteristic of the fibre of Figure 8;
Figures 10 and 11 are curves illustrating
the refractive index characteristics of two further forms of
optical fibre in accordance with the present invent;on.
As previously stated, an optical fibre conveys,
or transmits, light from an input end to an output end by the
phenomenon of internal reflection. One form of optical fibre
has a core 10 and a cladding 11, as illustrated in Figure
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l, with the core lO having one refractive index and the
cladding ll a lower refractive index - as seen in Figure 2.
Waveguiding occurs via internal reflection for all light
rays launched within the full cone angle ~ as illustrated
in Figure 3. The cone angle ~ is given by the numerical
aperture (NA):-
NA - sin ~ = ~n2 _ n2 (1-A)2 - n ~
where n is the refractive index of the core lO and n(l-~) is
the refractive index of the cladding ll.
Typically, for low loss guides, A ~ 1/2% to 4%,
NA ~ 0.15 to 0. 42 and ~ ~ 17 to 50. The core diameter D,
NA, and cladding thickness T, all determine the nature of
modes propagating along the fibre. Thus, for example, a
light ray 12 launched at a large angle to the fibre axis
(near the critical angle ~/2) will experience a large number
of reflections at the core/cladding interface 13, compared
to a light ray 14 entering at a shallower angle. At the
end of a length L of fibre the time delay difference between
these highest and lowest order modes is:-
rS = L n~ ~ 50a ns/km,
where c is the vacuum light velocity, ~ being in %.
This differential mode de1ay is termed mode
dispersion and causes pulse spreading even with monochromatic
light. The frequency - length product band~Jidth of the step-
index fibre is thus limited at about 5 to 40 MHz-km.
Differential mode dispersion is eliminated if
the fibre is allowed to propagate only the single fundamental
(HEll) mode. At a wavelength ~ this occurs if the "V-value"
of the guide satifies lO
V _ ~D . NA < 2.405.
In single mode operation, the fibre's
information capacity is limited essentially by chromatic
material dispersion (about 0.8 to 1 ns/km per 100 A of source
spectral width in the GaAlAs range of 8000 to 8600A). However
this requires D, the core diameter, to be approximately equal
to 1-5~m. This is a small core cross-section and makes
splicing and connecting difficult. As previously stated,
single-mode lasing sources are required for efficient light
insertion into such a single-mode fibre. The diameter D
cannot be arbitrarily increased by reducing the NA, since too
small an NA requires too thick a cladding for containment of the
evanescent wave and too large a radius of curvature for low
bending losses.
An alternative form of optical fibre has a
core with a non-uniform refractive index. This is illustrated
in Figures 4 and 5, Figure 4 showing the core refractive
index decreasing approximately parabolically, at 16. The
cladding has a lower refractive index, as in the fibre of
Figures 1 to 3. As seen in Figure 5 light rays 17 follow
quasi-sinusoidal paths, rather than a zig-zag one. Light
travels a shorter distance in regions of high refractive
index than in regions of low index in a given time and this
tends to equalize the average velocities of the various rays.
The time delay between highest and lowest order modes 17
and 18 respectively is
T = K L n~2 ~ K ~2 ns/km
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where k i5 a number ranging from 1/8 to about 2 depending upon
the accuracy with which the profile is maintained; ~ in %.
With fibres of the non-uniform, or graded,
refractive index, mode delays of 1/2 to 2 ns/km have been
obtained. Manufacturing tolerances must be extremely tight
if the theoretical limits of 1/64 - 1 ns/km are to be achieved.
A disadvantage of graded index fibres is that they accept
only half as much light from an incoherent source as do step-
index fibres, and also require twice the curvature radius in
10 bends.
In addition to the above disadvantages, graded
index fibres when produced from a concentric double crucibles
require a fast ion diffusion exchange on the fibre drawing step.
Further, soft glasses of attenuation higher than that of
fused silica are used, small cores of 30 to less than 50 llm
diameter are produced, and it is diff;cult to attain a closely
parabolic profile of the refractive index.
Graded index fibres can also be produced by
chemical vapour deposition (CVD) methods, but high precision
20 in dopant concentrations is required.
The present invention uses fibres having a
"stepped" gradient for the refractive index while obtaining
advantages of reduced mode dispersion. Figures 6 and 7
illustrate a fibre in accordance with one feature of the
present invention. The fibre comprises a core 20 and a series
of concentric rings or layers 21. The core 20 and each alter-
nate ring, i.e. rings 22, 24, 26, 28 and 30 are of higher
refractive index than the intervening and outer rings 21, 23,
25, 27, 29 and 31, as will be seen from Figure 7. In the
30 example of Figure 6 the light is conveyed through the core 20
and rings 22, 24, 26, 28 and 30. The thickness of each light
conducting ring is reduced relative to the next inner ring
1038669
and the innermost ring - 22 - is of a thickness slightly less
than the radius of the core 20.
All the rings 22, 24, 26, 28 and 30 have Vr
values defined by the equation
Vr - 2_ ~ .NA
where a and b are the inner and outer radii of a light trans-
mitting ring. To ensure that all modal group velocities are
approximately equal, the V and Vr values of core 20 and rings
22, 24, 26, 28 and 30 should be approximately the same. Hence
the diameter of the core 20 and the inner and outer radii of
the light transmitting rings are such that all areas are equal.
If the core/cladding index differences are held constant, that
is index differences between core 20 and rings 22, 24, 26, 28
and 30 and the cladding rings 21, 23, 25, 27, 29 and 31, then
the light transmitting ring thickness will decrease as radius
increases. It is desirable that the thicknesses of the cladding
rings 21, 23, 25, 27, 29 and 31 be large enough so that
evanescent field coupling between the cores or light transmitting
rings is minimized, as such coupling causes some spreading in
modal velocities.
However, the efficiency of input light
insertion is related to the fraction of cross-sectional area
occupied by core and light transmitting rays, and therefore the
cladding ring thicknesses should not be t~ large. As an
indication, 30 to 50% of the total cross-section of a fibre is
an optimum to be aimed at, for the light transmitting core and
rings.
The number of light conducting rings can
vary, and to some extent is controlled by the NA. A large NA,
for example 0.2, reduces the number of rings, and a smaller NA -
~ 038669
0.1 - permits a larger number of rings. A small NA permits
more and larger rings and a larger light source but light
source must be more collimated than a small one. A larger NA
requires less input light collimation and permits tighter
bends. The arrangement of Figure 6 gives a constant NA, with
varying light transmitting ring thickness.
Figures 8 and 9 relate to an optical fibre
having a light transmitting core and a plurality of light
transmitting rings in which there is a varying refractive
index and constant ring thickness. There is a light trans-
mitting core 35 and light transmitting rings 37 and 39, and
cladding rings 36, 38 and 40.
In both arrangements as in Figures 6 and 8,
the core can be of "cladding" that is of the lower refractive
index and succeeding alternate rings also of cladding, with
intermediate rings of high refractive index. That is, in
Figure 6, the refractive index as indicated in Figure 7 can
be reversed, although an outer ring of cladding will be
required.
In a further alternative, not shown, ring
thicknesses and index differences are both varied suitably.
It is also possible to vary the refractive
index for each ring across the ring thickness. Typical
examples are shown in Figures 10 and 11, for an optical
fibre arrangement as in Figure 8, for example.
Multi ring optical fibres are readily produced
by chemical vapour deposition (CVD), plasma deposition or
flame hydrolysis, all known methods for producing optical
fibres. There is a particular advantage in producing rings
which have a constant refractive index across their thicknessin that the doping can readily be obtained. The doping level
is constant and it is a matter of doping or not doping - so
10386~9
far as each dopant is concerned - for either a light transmitting
ring or a "cladding" ring.
For a graded refractive index it is more
difficult as the doping level must be varied during the
production of a ring. A typical example of a process for CVD
production of an optical fibre is as follows:-
A tube of fused silica is mounted forrotation about its longitudinal axis - the axis vertical.
Oxygen is bubbled separately through reservoirs holding Si C14
and Ge C14 in liquid form, the oxygen picking up a vapour from
the liquid. The oxygen and vapour from each reservoir are
fed to a collecting chamber plus a direct flow of oxygen.
The flows are combined and fed through the fused silica tube.
The tube is rotated and a flame is traversed up and down the
tube. At the heated position in the tube the gases and vapour
dissociate and oxidation of the silicon and germanium occur
with a resultant deposition on the wall of the tube. The
deposition is in the form of a sooty deposit which is fused
onto the wall of the tube in the form of a glassy layer.
Several passes of the flame are made to form a particular ring.
The doping level is adjusted by varying the rate of oxygen
flow through the germanium chloride solution.
After the required number of rings have been
formed, the tube is collapsed, again by passing the flame along
the tube, but with a higher temperature so that the tube softens
and collapses under surface tension forces. Thus the inner
ring becomes the core. The collapsed tube is then pulled into
a fibre in a conventional manner, for example by feeding into
a furnace and pulling from the lower end and winding on a
drum.
