Note: Descriptions are shown in the official language in which they were submitted.
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The invention concerns a fibre network for the opto-
electronic transmission of data between an arbitrary number of
subscriber stations, such that the network allows the transfer
of data between any stations, each station having a transmitter
and a receiver, a specific address code is allocated to each
transmitter and its associated receiver, and the information
content of the individual subscriBer stations is sampledcycli-
` cally by a common addressing unit, such a network having n
junction points, whereby n ~ 1, and the individual fibre con-
ductors lead from these junction points to the connected sub-
scriber stations.
BACK~ROUND OF THE INVENTION
A fibre network of this kind is described in S~iss
patent 559,990, filed by the present applicant. In this patent
it is assumed that the fibre conductors leading from the in-
dividual subscriber stations to the common junction point exhibit
relatively high attenuation, so that an amplifier is necessary
at least at the junction point. The junction points according
to the earlier application are therefore in the form of
repeaters, a very practical arrangement from the circuitry stand-
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point, and one which also conveniently solves the problem
of distributing the signal. Recent developments in the
field of fibre technology show, however, that glass fibres
with outstandingly low attenuation can be made (e.g. 3-4
dB/km). ~ith these, the repeaters, which despite their
advantages are costly and with regard to reliability are
not always completely dependable, can be dispensed with over
relatively short transmission distances of not more than
about 1 km, and the remaining attenuation (interface atten-
uation, fibre attenuation) can then be effectively overcomewith the aid of the transmitting power of the individual
subscriber stations. In view of this, network structures
can be much simpler and more reliable.
OBJECTS AND BRIEF DESCRIPTION OF THE INVENTION
It is thus an object of the invention to create a
fibre network of the kind described above with n junction
points, without incorporating active components in the
form of repeaters in the junction points, and at the same
time satisfactorily resolving the question of signal dis-
tribution. This object is achieved in that the junctionpoints contain a passive coupling element in the form of a
solid, light-conducting core, the fibres leading from the
transmitters of the subscriber stations and from the address-
ing unit to the junction point are optically connected to
one end face of this coupling element, and the outgoing
fibres leading to the receivers of the subscriber stations
are optically connected to the other end face of this
coupling element, and wherein the relationship of the
length of the coupling element to the active surface area
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of the light-transmitting channels coupled to +he element is
such that the end face to which the receiver fibres are
connected is fully illuminated by the light beam from each
transmitting fibre.
More particularly, the fiber optic network of
the present invention comprises:
a plurality of subscriber stations;
each said station including an optical information
transmitter, an optical information receiver, and a coding
facility means for recognizing a specific one of a plurality
of address codes, the transmitter of each of said plurality
of subscriber stations being adapted to transmit in-
formation when an address code associated with the particu-
lar transmitter is recognized by its corresponding coding
facilities;
common address means for addressing said sub-
scriber stations by periodically transmitting the address
codes of said stations whereby the transmitter of the
addressed subscriber station transmits information to the
remaining said stations;
a passive optical coupling element, said coupling
element comprising a solid light-conducting core having a
pair of opposing first and second end faces, said first end
face being devoid of an anti-reflective coating, said second
end face having a substantially completely reflective coat-
ing to form a mirror surface thereon;
each of said transmitters being optically connected
to a first end of a different one of a first group of in-
dividual light-conducting fibres, a remaining end of each
of said first group of fibres being optically connected
to said first end face of said coupling element;
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each of said receivers being optically connected
to a first end of a different one of a second group of
individual light-conducting fibres, a remaining end of each
of said second group of fibres being optically connected to
said first end face of said coupling element;
said addressing means being optically coupled to a
first end of an individual light-conducting fibre, the
remaining end of said light-conducting fibre being optically
coupled to said frist end of said coupling element;
said coupling element having a length between
said first and second end faces selected to cause said
remaining end of all of said light-conducting fibres to be
fully illuminated by optical energy emanating from said
remaining end of any one of said light-conducting fibres,
whereby transfer of optical information between any pair of
stations is accomplished without transferring the information
through said common addressing means.
BRIEF DESCRIPTION OF THE FIGURES
The invention is explained more fully below with
reference to the drawings 1-3, in which:
Figure 1 is a block diagram showing a fibre net-
work with a common passive coupling element in accordance
with the invention, and a number of subscriber stations.
Figure la is a block biagram showing a fibre
network having groups of stations coupled by a plurality of
passive coupling elements in accordance with the invention.
Figure 2 shows the structure of a passive coupling
element in accordance with the invention, and
Figure 3 is a variant of the passive coupling ele-
ment with a reflective surface at one end face.
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DETAILED DESCRIPTION oF T~E INVENTION
. _ .. .. _
In Figure 1, the number 1 denotes the common,passive coupling element, 2 a number of subscriber stations
(six are shown symbolically), each comprising a receiver E,
a transmitter S and a coding facility CT, and 3 denotes a
common addressing unit AS. The transmitters of the stations
2 and of the addressing unit 3 are each connected optically
via fibre conductors 4 to the end face 1' of coupling
element 1.
The receivers of the stations 2 receive both
addresses cyclically generated by the addressing unit and
information generated by the subscriber stations. When an
address code is emitted by the addressing unit, it always
passes through coupling element 1 and along the fibre con-
ductors leading to all the receivers. As soon as the called
transmitter starts to operate, its ascociated address re-
ceiver is disconnected via line D so that no feedback
phenomena can occur. The information sent out by the trans-
mitter of one station also passes via coupling element 1
to all the receivers, but is accepted only by the station
programmed to the same address. If certain stations are
intended only to receive, the transmitter in these cases can
be omitted. Conversely, if a station is intended only to
transmit, its receiver can be omitted. The numbers of fibre
conductors arriving at, and departing from, coupling ele-
ment 1 can thus be quite different.
The construction of coupling element 1 is shown
schematically in more detail in Figure 2. It comprises
essentially a solid cylindrical shaped glass core 1, the
end face of the incoming fibre conductors 4 and the out-
going fibre conductors 5 (six of each are shown symbolically)
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being optically matched to its end face 1' and 1 ". The
diameter D of the core is governed by the total area of the
fibre conductors connected to end face 1' or 1''. The
length L must be such that the light from each fibre 4 in-
cident on surface 1' is distributed as uniformly as possible
among all the fibres 5 in plane 1 " . Owing to the direc-
tional characteristic of the fibre ends, all the fibre con-
ductors are mutually decoupled on the transmitter side (and
also on the receiving side). Coupling exists only in the
direction of radiation A of the light, i.e. always between
one fibre on the input side and all fibres on the output
side (beam divider). In order to avoid light losses along
the coupling element, the outer surface 6 of core 1 can be
provided with a mirror surface or coated with a layer of
glass havir.g a refractive index somewhat lower than that of
the body of the core 1. The light distribution along the
core 1 can be made over more uniform by dishing the end faces
1' and 1'' slightly with increasing radius.
The division of power in the coupling element 1
gives rise to interface attenuation. With a basic attenua-
tion of 1 dB/fibre end, this amounts to about 12 dB with 10
subscriber stations, for example, and some 20 dB with 50
stations. To this must be added the transmission losses
LED/fibre and fibre/photodiode, as well as losses in the
fibres themselves (LED meaning light emitting diode).
Assuming low-attenuation fibres, with 50 subscriber stations
and fibre lengths up to 1 km, for example, this gives a
total transmission attenuation of some 30-40 db, which in
relation to the available transmitting power of a subscriber
station is quite within acceptable limits.
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Instead of the transmitter and receiver fibres
leading separately to the coupling element, as shown in
Figure 1, if circumstances so require, the fibre comductors
of neighboring stations can be combined into one on the
transmitting and/or receiving sides, or the transmitter and
receiver of one station can be linked via a single fibre.
The ends of a fibre adjacent the transmitter and receiver of
the same station would then have to be split in a Y shape,
after the manner of the beam divider described above, to
separate optically the transmission and receiving paths. If
the incoming and outgoing conductors of each station are com-
bined in a single fibre, the ends of the transmitter fibres
attached to end face 1' of coupling element 1 are at the same
time the input of the receiver fibres, as shown in Figure 3.
In order that the receiver fibres are fully illustrated by
each transmitter fibre, the opposite end face 1'' must be in
the form of a mirror (7), and appropriately dished as shown
by the broken lines at 7a, if necessary. The power division,
and hence the transmission attenuation of the coupling element,
remains virtually the same as with the fibre coupling arrange-
ment depicted in Figure 2. The Y-connection, however, causes
an increase in attenuation of at least 3 dB, at least in the
transmitting direction, but on the other hand, the total
number of fibre conductors is halved.
A final possibility, which is mentioned here only
incidentally, is to provide each transmitter and receiver
with a separate fibre and attach all the transmitter and
receiver fibres to the same end face, instead of to the
opposite ends of the coupling element 1 as shown in Figure 1
and Figure 2, and to provide the other end face wi~h a
mirror surface, as above in the case of common fibres for
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transmitter and receiver (of Figure 3), possibly also
dishing this face as shown in broken lines at 7a as required.
Each transmitter fibre will then illuminate not only all
the receiver fibres, but also all the transmitter fibres.
The optically effective receiving area is then only half
that of the coupling element, which is equivalent to further
attenuation of 3 dB. On the other hand, the coupling
element is only half as long, and illumination of the fibre
ends can more easily be made homogeneous.
There is a fundamental difference between the
coupling elements shown in Figure 2 and 3. Whereas the
element in Figure 2 acts both as a beam divider and a power
divider, only division of power takes place in the element
of Figure 3. In order to obtain an optically symmetrical
Y-type coupling element, for example, it is necessary to
combine three elements of the kind shown in Figure 2, while
one element is sufficient with the configuration shown in
Figure 3.
If the locations of the various subscriber stations
tend to be concentrated in groups (Figure la), these groups
can be brought together in separate networks with their own
coupling elements at the function points. The individual
coupling elements are then linked to each other by fibre
conductors, although these links will need to incorporate re-
peaters 8, particularly with a relatively large number of
subscriber stations i.e. higher losses in the central coupling
elements.
The structure of the network described is simple
and straightforward. If one subscriber station fails, only
the links connected to it are interrupted, while traffic
between all the other stations can continue unhindered.
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Fibre bundles can also be used instead of single fibres.
Breakage of a fibre then has practically no effect on the
quality of transmission. The fibre network can be freely
e~tended to include a considerable number of subcriber
stations (e.g. 50-100).
With modern low-attenuation multi-anode fibres,
the transmission attenuation occurring in the fibre networks
e~tending up to about 1 km can easily be overcome with the
transmitting power of the individual subscriber stations, and
therefore no repeaters are required. With the proposed
star-shaped network, if one subscriber station fails, only
the links connected to this station are affected, while
traffic between the other stations continues unhindered.
Coupling the fibre conductors optically to the subscriber
stations by means of a simple, passive coupling element
represents a substantial reduction in cost, compared to the
earlier arrangement with repeaters.
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