Note: Descriptions are shown in the official language in which they were submitted.
A METHOD OF TAPPING LIGHT SIGNALS FROM OPTICAL WAVEG~IDE~
TECHNICAL FIELD
The present invention relates to a method of tapping light signals
from an evanescence field surrounding an optical waveguide which
is clad with an encapsulating material, with the aid of a light
conducting probe which includes an optical fiber having a free
end, wherein tapping of the light signals is effected without
stripping the encapsulating material or without breaking the
waveguide, and wherein tapping of said light signals is effected
with a minimized damping loss.
BACKGROUND ART
In present day fiberoptic communication fields, and then in
particular within the telecommunication field, it is desirable to
be able to tap light signals in order to ascertain the traffic
status of the optical fiber. At present, the light signals are
tapped on the fiber with the aid of a permanently attached tapping
device. The fiber is comprised of a light conducting core and a
cladding. In order to make the light signal accessible to the
tapping device, either the cladding is removed at the tapping site
or the fiber is bent. The tapping device functions to tap the light
signal from the core through its evanescence field.
U.S. Patent No. 3 982 123 discloses two methods of tapping a light
signal from an optical fiber without requiring the fiber to be
broken. The inventive concept of this patent is to look into the
~5 fiber so as to ascertain its traffic status, and signal tapping
can be effected anywhere whatsoever without disturbing the
traffic. This is achieved by placing the tapping device, which in
this case is comprised of a material which incorporates a photo-
detector, on a light conducting core or on the fiber so that
tapping of light signals can be effected. The optical fiber is
comprised of a core that has low optical losses and cladding which
has a lower refractive index than the core.
2 ~ 2;~iL~
A first method described in the patent involves removing all, or
practically all of the cladding material from the fiber. The
detector is then placed securely on the light conducting core, the
stripped region of which must be at least three times the wave-
length in the optical fiber.
A second method of tapping light signals is to bend the optical
fiber without removing the cladding material. This enables the
light signals to be extracted through the cladding and captured by
a photodetector. Tapping is effected permanently in both cases.
U.S. Pat. No. US 4 784 452 describes a method in which tapping is
effected with the aid of a tapping device placed on an optical
fiber. This fiber is comprised of a light conducting core and at
least one cladding material. The tapping device, a probe, is an
optical fiber of the same type as the fiber from which the signals
are tapped. This probe has a free end which includes a light
conducting core. In order to tap light signals from the fiber, it
is necessary to remove the cladding so as to expose the core. The
probe is used at this exposed region, with the free end of the
probe placed against the bared part of the fiber. In order to
~0 obtain the best possible tapping effect, it is necessary to adapt
the angle defined by the probe axis and the fiber axis. A coupling
medium connects the region at the probe and the bared part of the
fiber and conducts light signals from the bared part of the fiber
to the probe. The coupling medium, which is a solid and hard
material, fixes the probe in relation to the fiber.
Various experiments have shown that the light conducting core may
be comprised ofpolyimide. In the article "Dependence of Precursor
Chemistry and Curing Conditions on Optical Loss Characteristics
of Polyimide Wageguides" by C.P. Chien and K.K. Chakravorty at
Boeing Aerospace and Electronics, Seattle, USA, SPIE vol 1323,
Optical Thin Films III, New developments (1990), it is disclosed
that polyimide is a good material for the optical fiber core.
Polyimide has good thermal stability and a dielectric index of
3.5, which is compatible with other IC-materials. The material
functions well as a light transmitter, such as in optoelectric
3 ~ ~ s ~
circuits in &H frequency range. The advantage of polyimide is that
when manufacturing cores, the cores can be packed tightly
together. Additional polyimide data is that it has a refractive
index of 1.6 ~1.58-1.62) and optical losses in the core of about
l dB/cm when e~posed to ultraviolet light.
Experiments have been carried out with a silicone elastomer as an
index matching medium for the light conductive core. The article
"Index Matching Elastomers for Fiber optics" by Robert W. Filas,
B.H. Johnson and C.P. Wong at AT&T Bell Laboratories, N.~. USA in
the magazine IEEE, Proc. Electron. Compon. Cont., 3~th, 486-9,
disclose that silicone elastomers are good core index matching
materials. Copolymer reflection as a function of the diphenyl
concentration and temperature is obtained by measuring the
reflection strength of a single mode waveguide whose core has been
encapsulated in an elastomer. It is possible to obtain the same
refractive inde~ on a silicone rubber material as the refractive
index of the core. The silicone rubber can be used as an interface
between different components. Another method is to use the
silicone rubber as protection against moisture and dust, for
instance.
At present, air is used as the refractive medium to the light
conducting core of the lightwave conductor. Air has a much lower
refractive index than polyimide. The refractive index of air is 1,
whereas the refractive index of the polyimide is 1.6 and the
refractive index of the silicone rubber is 1.5.
One drawback with the earlier known solutions is that light
signals are tapped from optical fibers with the aid of permanently
attached devices. This means that light signals are tapped from
the fiber at a specific place thereon at which cladding has been
removed. The earlier solutions are encumbered with a number of
additional drawbacks~ One ofthese drawbacks is that light signals
can only be tapped on fiber waveguides and that it is necessary to
remove cladding from the place at which tapping shall take place.
The tapping device must be placed firmly on the optical fiber at
4 ~ ~` ' 2 '~) ~r ~
that place from which the cladding has been removed. Tapping in
permanent branches results in excessively high losses.
SUMMARY OF THE I~ENTION
The object of the present invention is to eliminate the drawbacks
S that are encountered with earlier known methods for tapping light
signals from optical fibers.
The invention relates to a method of tapping a light signal with
the aid of a light conducting probe directly on an optical
waveguide which is encapsulated in an elastic material. The light
signal tapping method can be carried out on two types of wave-
guide. In one case, the method can be applied on a light waveguide
which lies on a substrate, and in the other case on an optical
fiber. The method is carried out by inserting the probe into
direct contact with the light conducting core of a waveguide, so
as to extract light signals from the evanescence field of the core
without needing to remove encapsulating material or to bend the
waveguide in order to tap the light signals. The probe includes an
optical fiber which is of the same type as the fiber from where
light signals are tapped. The probe fiber has a free fiber end.
The method is as follows:
In a first step, the fiber end of the light conducting probe is
pressed down towards the encapsulated optical waveguide. In a
second step, the fiber end of the probe is pressed into the
encapsulating material 32 while elastically deforming said
material to an extent permitted by the elastic properties of the
material or so that the residual deformation, yield, does not
become permanent. In a final step, the fiber end of the probe is
angled to the optical waveguide 23, so that a part of the light
signal will be ta~en up by the probe. The probe is angled so as to
3~ tap a given light signal. If the angle is changed, another light
signal is obtained in the probe.
~ o~
It should be noted that the fiber end of the light conducting probe
is equally as wide as the optical waveguide, so as to obtain the
best possible tapping. Furthermore, the probe is made from the
same material as the light conducting core or from a material
which has an equally as high or a higher refractive index.
The method also enables the light conducting probe to be fixed
permanently to the optical waveguide, if so desired. Upon
completion of the light signal tapping operation, the probe is
removed without leaving a trace of its earlier presence. When
tapping light signals from light waveguides, no further measures
are required to enable tapping to take place.
Prior to carrying out the first method step on the optical fiber,
the fiber must be placed on a hard supporting surface in order that
tapping can take place. When the fiber is too flexible, it is
impossible to insert a probe onto the fiber without first placing
the fiber on a hard supporting surface.
The invention provides the advantage that light signals can be
tapped without needing to fix the tapping device permanently at a
given tapping site on the optical waveguide. Another advantage is
that it is possible to tap light from the light waveguide when the
waveguide is seated firmly on the substrate. Light signals have
not previously been tapped from substrate mounted light wave-
guides. Thus, it will be seen that light signals can be readily
tapped from such a system. Further advantages lie in the fact that
tapping can be carried out temporarily if so desired, and that the
elastic encapsulation protects against external environmental
influences, such as dust, air and humidity, during a light tapping
process.
Further objects of the invention and advantages afforded thereby
will be evident from the preferred embodiments described below
with reference to the accompanying drawings.
6 ~ 3`,~
BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1-3 illustrate a first embodiment of the invention and
Figure 4 illustrates a second embodiment.
Figure 1 is a top view of a light waveguide mounted on a silicon
disc.
Figure 2 is an enlarged cross-sectional view of part of the light
waveguide mounted on the silicon disc, taken on the line A-A in
Figure 1, and illustrates a method of tapping light signals from
the light waveguide with the aid of a light conducting probe;
Figure 3 is an enlarged cross-sectional view of a part of the
lightwave conductor mounted on the silicon disc, taken on the line
B-B in Figure 1, and shows a method of tapping light signals from
the light waveguide with the aid of a light conducting probe; and
Figure 4 is a sectional view of an optic fiber and illustrates a
method of tapping light signal from the optical fiber with the aid
of a light conducting probe.
BEST MODES OF C~RRYING OUT THE INVENTION
The Figures of the accompanying drawings are not drawn to scale
and solely show those parts required to obtain an understanding of
the inventive concept.
A first embodiment is illustrated in Figures 1-3.
Figure 1 illustrates an arrangement 20 which is comprised of a
substrate 21, a connecting device 25, a light waveguide 23 and an
optical component 24 which emits or receives light.
Figure 2 is a sectional view of the device20 illustrated in Figure
1, said view being taken on the line A-A in Figure 1 on the optical
component 24. The substrate 21 has applied thereto a very thin
layer 40 which is intended to function as a refractive medium to
a light conducting core 30. A very narrow gap 31 is found between
one end of the core 30 and the component 24. The connecting device
25 is connected directly to the other end of the core 30. The
component 24 and the core 30 are ccvered with an elastic encap-
sulating material 32. The broken line circle 34 illustrates a
light conducting probe 35 which is pressed against the encap-
sulating material 32 and which taps a light signal33 from the core
30. The probe 35 takes up an evanescence field which couples the
light signal 33 frGm the core 30. The light signal 33 taken from
the core 30 by the probe 35 forms a light signal 36 which cor-
responds to the light signal 33 but which is very much leaner in
energy.
Figure 3 is an enlarged cross-sectional view of the light wave-
guide 33 taken on the B-B in Figure 1. Figure 3 illustrates the
light waveguide 23, which is comprised of the light conducting
core 30, the thin layer 40 on the substrate 21 and the encap-
sulating material 32. The refractive index of the encapsulating
material is lower than the refractive index of the core 30. The
layer 40 lies on the substrate 21 and the core 30 lies on top of
the layer 40. The encapsulating material 32 covers everything
which lies on the substrate 21. When the light conducting probe 35
is pressed down towards the core 30, a temporary deformation 41 is
formed in the encapsulating material 32. When the probe 35 is
pressed down to its maximum position in order to achieve an
optimum tap, a very narrow gap 42 is formed between the probe 35
and the core 30.
The substrate 21 illustrated in Figure 1 is a silicon disc of the
kind from which semiconductors are normally comprised. The
substrate 21 may have several light waveguides 23, components 24
and connecting devices 25 mounted thereon. The substrate 21 may
also be comprised of circuit board material, a glass material or
any type of material whatsoever provided that the substrate 21
will have a lower refractive index than the core 30. It is
important that damping in the core 30 is as low as possible .
In the embodiment illustrated in Figures 2 and 3, the light
waveguide 23 is comprised of three parts. These parts are the thin
layer 40 applied to the substrate 21, the core 30 and the encap-
sulating material 32. When the substrate 21 is made of silicon,
the layer 40 is comprised of silicon dioxide. It is necessary that
8 ~ J -
the refractive index of the layer 40 is lower than the refractive
index of the core 30, in order not to conduct light therefrom. In
the case of the Figure 2 and 3 illustrations, the light conducting
~ore 30 is a multimode core, although it may also be produced as
S a single mode core.
The encapsulating material 32 is a silicone elastomer, for
instance silicone rubber. The encapsulating material is intended
to enable light signals to be tapped from the core 30. Since the
encapsulating material is elastic, the probe 35 can be pressed
down towards the core 30. The silicone rubber is optically
conductive. The encapsulating material 32 can be applied to the
substrate 21 and the components ~4 and 25 mounted thereon while
the material is still moldable, whereafter thematerial isallowed
to harden and therewith become elastic.
The aforedescribed arrangement 20 is used for tapping light
signals 33 with the light conducting probe 35 inserted directly
onto the optical waveguide. By inserting the probe 35 down through
the elastic encapsulating material, it is possible to come so
close to the core 30 as to enable the evanescence field around the
core 30 to be taken up. This results in practically no losses in
the core 30. It is important that the probe 35 is not inserted too
far into the encapsulating material, because the deformation 41
therein may then be permanent. On the other hand, if the probe 35
is not pressed sufficiently far into the encapsulating material
3~, the probe will be unable to take-up the evanescence field. The
distance between the probe 35 and the core 30 must be of the
correct order of magnitude, less than ~m. In order to obtain the
same distance at each tapping operation, a distance measuring
instrument can be used to obtain the correct distance.
The method includes the steps of:
pressing the fiber end of the light conducting probe down towards
the encapsulated optical waveguide;
pressing the fiber end of the probe 35 into the encapsulating
material 32 while elastically deforming 41 said material to an
extent permitted by the elastic properties of the material; and
9 ~ ?
angling the fiber end of the probe 35 to the optical waveguide, so
that a part of the light signal will be taken up by the probe. The
probe 35 is angled in order to tap a given light signal. If the
angle is changed, another light signal is obtained in the probe.
It should be noted that the fiber end of the light co~ducting probe
35 is equally as wide as the optical waveguide, so as to obtain the
strongest possible light signal 33. Furthermore, the probe 35
shall be manufactured from the same material as the core 30 or from
a material which has an equally as high or a higher refractive
index. The probe 35 may also be manufactured from a plastic fiber.
The method also enables the light conducting probe 35 to be fixed
permanently to the optical waveguide, if so desired. The probe 35
is removed upon completion of a light signal tapping operation,
with no residual deformation of the material at the place where
the probe was inserted. When tapping light signals on light wave-
guide 33, no further measures need be taken to enable tapping to
take place.
Another embodiment of the arrangement is illustrated in Figure 4.
Figure 4 illustrates an optical fiber 1 which is comprised of a
light conducting core 2 and an elastic encapsulating material 3.
The refractive index of the core 2 is higher than the refractive
index of the encapsulating material 3. A probe 35 is pressed into
the encapsulating material 3 down towards the core 2, resulting in
deformation 41 of the encapsulating material 3. When the probe 35
is pressed down to its maximum position at which its best tapping
ability is obtained, a very narrow gap 42 is formed between the
probe 35 and the light conducting core 30. The core 2 may, for in-
stance, be comprised of polyimide and is encapsulated by the
elastic material, which ispreferably silicone rubber. Becausethe
material is elastic, the probe 35 can be inserted into the
encapsulating material 3 and tapping can commence when the probe
35 reaches the evanescence field. The tapping device can be
removed, if so desired. Optical fibers extend between different
telephvne stations or, for instance, between different computers.
The distances concerned may be large and it is necessary at times
;~ 'r ~ . ~ e ~
to go into the fibers and ascertain their traffic status. In the
illustrated embodiment, the optical waveguide is not secured to a
substrate, but is a free lying optical fiber 1.
The method applied to tap light signals from light waveguides 2~
S can also be used to tap light signal from optical fibers. When
tapping signals from optical fibers, the fiber must be placed on
a hard supporting surface prior to the first method step, in order
for tapping to take place. When the optical fiber is unduly
flexible, it is not possible to insert a probe onto the fiber,
unless the fiber rests on a hard supporting surface.
Another advantage afforded by the use of an elastic encapsulating
material, is that a probe can be pressed down into the material and
light signalstapped. Anotheradvantage isthat optical waveguides
according to the aforedescribed embodiment can be manufactured
cheaply and simply. The method enables light signals to be tapped
from optical waveguides on temporary occasions.
It will be understood that the invention is not restricted to the
aforedescribed and illustrated embodiments thereof and that
modifications can be made within the scope of the following
ciaims.
