Note: Descriptions are shown in the official language in which they were submitted.
CA 02273582 1999-OS-28
-2-
"OPTICAL COMMUNICATION SYSTEM"
******
The present invention relates to an optical communication system
comprising a transmission switching system.
More particularly, the present invention relates to a system and a
method of transmission switching for an optical communication system
comprising at least a first primary guided optical path for the
transmission of at least one optical signal and at least a first secondary
guided optical path to which the transmission of the optical signal can
be switched in case of degradation of the transmission in the first
primary guided optical path.
In the operation of optical communication systems there is a widely
felt need to minimize the problems which arise when there is a
deterioration of transmission due, for example, to a fault of a device in a
guided optical path (e.g. an optical amplifier) and/or of a device in a
terminal station (e.g. a transmitter or receiver) and/or to the breaking of
an optical cable.
Among the operating systems for optical communication systems,
there are known remote monitoring systems for detecting and locating
the presence of a fault in the system.
EP 0 408 905 describes an optical fibre telecommunications line
comprising active optical fibre amplii'iers. Each active optical fibre
present in the amplifiers is connected to two laser sources of optical
pumping radiation. The first of these two laser sources of optical
pumping radiation is put into operation and the second is kept in
reserve so that it can be put into operation when the first becomes
faulty. The two sources of optical pumping radiation are also connected
to a microprocessor circuit capable of commanding them to send alarm
signals on the state of the amplifier to the terminal stations of the line
CA 02273582 1999-OS-28
-3-
and to receive from these stations control signals for switching
operation between the two sources of optical pumping radiation.
US 5 475 385 describes a telemetry system for locating a fault in an
optical transmission system including optical fibre amplifiers. Each
amplifier has a corresponding source of pumping radiation comprising a
loop system, for automatic gain control, around the fibre of the optical
amplifier. The spontaneous emission of each amplifier is detected and
used to control the corresponding source of pumping radiation and to
maintain the output of the amplifier at a predetermined level. The
transmitter of the system injects a monitoring tone whose presence or
absence is detected by each amplifier. If there is a fault in the
transmission line, the down-line amplifiers, not receiving the said
monitoring tone, inject a corresponding alarm tone. In reception,
suitable devices detect the presence or absence of the monitoring tone
and, in its absence, count the number of alarm tones, if they are
present, thus determining the position of the fault. The absence of the
monitoring tone and of the alarm tones indicates that the fault has
occurred between the last amplifier and the receiving station.
US 5 483 233 also describes a telemetry system for locating a fault
in an optical transmission system including optical fibre amplifiers. Each
amplifier comprises an active optical fibre with a loop system for
automatic gain control (AGC) around it. In the AGC the spontaneous
emission from the amplifier fibre is detected and used to control the
source of pumping radiation in such a way as to maintain the output of
the amplifier at a predetermined level. A monitoring tone is injected by
the transmission unit of the system. Each amplifier detects the
presence of the monitoring tone or of an alarm tone originating from the
preceding amplifier. If one of the tones is detected, the signal is
amplified and transmitted to the next amplifier. If no tone is detected, an
alarm tone corresponding to the amplifier tone is injected into the
CA 02273582 1999-OS-28
-4-
corresponding pumping source. At the receiving unit of the system, the
presence or absence of the monitoring tone, and in its absence the
presence or absence of an alarm tone, is detected. The position of a
fault in the system is determined from this information.
EP 0506163 describes an optical fibre telecommunications line,
comprising two terminal stations, each having a transmitter of optical
signals and a receiver connected operationally by an automatic
protective device capable of switching off the transmitter in the absence
of a received optical signal, and corresponding optical fibre Lines
connecting the transmitter of one station to the receiver of the other
station and including at least one optical amplifier. At least one of the
optical amplifiers has a protective device comprising means of
detecting the presence of an optical signal at the output of the amplifier.
The interruption of light emission in the down-line direction causes the
interruption of emission in the whole line by the intervention of the
automatic protective device of the terminal stations.
EP 0507367 describes an optical fibre telecommunications system
comprising at least one amplifier with an active fibre having a
fluorescent element and a laser pump for supplying light energy to the
active fibre. Detection means for detecting the presence of light energy
are connected to the input of the active fibre. In the absence of light
energy at the input of the active fibre, control means are put into
operation by the said detecting means to interrupt the pump source in
order to interrupt substantially any emission of light energy from the
optical amplifier.
In addition to remote monitoring systems for detecting and locating
the presence of a fault in the system, there are known remote operating
systeriis for switching, in case of a fault, the transmission of an optical
signal, at a predetermined frequency, from a first guided optical path to
a second guided optical path.
CA 02273582 1999-OS-28
-5-
US 5 091 796 describes a communication system comprising a
plurality of stations, a first active line (guided optical path) which
connects adjacent stations of a plurality of stations, a second active line
which by-passes predetermined stations without "dropping" into them,
and a third line for protection, in common between the first active line
and the second active line. A monitoring signal travels continuously
along the protection line. Each station includes an internal circuit for
receiving an input transmission signal from the first active line and for
sending an output transmission signal along this line. Each station also
includes a switching section for the passage of the protection line
without allowing it to drop into each station in normal conditions, and for
connecting, in alarm conditions, the protection line to the internal circuit
so that the input transmission signal is received by the protection line
up-line instead of by the first active line, so that the protection line up-
line from the station is used, instead of the first active line in which, for
example, a fault has occurred. Additionally, the monitoring signal, which
is generated by the output transmission signal, is sent, down-line,
through the protection line.
In the said known systems, a degradation of transmission is dealt
with at the transmitting and receiving stations and also at intermediate
stations or optical amplifiers along a guided optical path. These
therefore require a single operating system which is perfectly
compatible with the guided optical path and with the transmitting and
receiving terminal stations which must, therefore, communicate with
each other.
The inventors have perceived that, in practice, the manufacturers of
terminal stations are frequently different from the manufacturers of
guided optical paths, comprising optical cables and optical amplifiers or
intermediate stations. The said conventional systems therefore require,
at the time of assembly of an optical communication system, a
CA 02273582 1999-OS-28
-6-
laborious process of adaptation for making the terminal stations
compatible (communicating) with the guided optical path.
The inventors have also perceived that, in the said conventional
operating systems, in the case of a multiple-wavelength transmission in
a single guided optical path, when a fault occurs along the guided
optical path and it is therefore necessary to switch the transmission of
all the transmitted wavelengths to another guided optical path, the
switching should be controlled by as many terminals as there are _
wavelengths transmitted. Each terminal should therefore check for the
presence of any faults in the guided optical transmission path and, if
necessary, switch the transmission of the corresponding wavelength.
This would require a transmission switching system which is complex
and consequently expensive and unreliable.
In the particular case of loop networks provided with a plurality of
nodes, it has been proposed to provide an additional protection ring
(closed optical path) and to propagate an optical signal simultaneously
in both loops in two different directions of propagation.
EP 0769859 describes a fail-safe loop optical communication
network comprising a first optical communication line, forming a closed
optical path; at least two nodes for injection and extraction of optical
signals connected optically along the line; a second optical
communication line, forming a closed optical path and connected
optically to the nodes for injection and extraction of optical signals. In
the network there are defined a first and a second direction, opposite to
each other, of travel of the optical signals with respect to the position of
the nodes for injection and extraction of optical signals. At least one of
the said nodes also comprises selection means, controlled by the
optical signals, for the selective extraction of the optical signals from
one of the communication lines. At least one of the nodes for injection
and extraction of optical signals also comprises means for the -
CA 02273582 1999-OS-28
-7-
simultaneous injection of at least one optical signal in the first direction
along the first communication line and in the second direction along the
second communication line.
The object of the present invention is to overcome the aforesaid
disadvantages of the conventional operating systems. This is achieved
by means of an optical communication system comprising a
transmission switching system which is universal and independent of
the transmitting and receiving terminal stations.
In a first aspect, the present invention provides therefore an optical
communication system comprising:
- a first transmitter;
- a first receiver,
- a first primary guided optical path having a protected portion;
- a first secondary guided optical path;
- at least an optical amplifier;
- a modulation device, up-line from the said protected portion of the
said first primary guided optical path, for transmitting a first
overmodulation frequency along the said protected portion;
- a detection device, down-line from the said protected portion of the
said first primary guided optical path, for detecting the presence or
absence of the said first overmodulation frequency; and
- an operating device, down-line from the said protected portion of the
said first primary guided optical path, connected operationally to the
said first secondary guided optical path;
the said first transmitter sending at least one optical signal at least
along the said first primary guided optical path and the said first
receiver receiving the said at least one optical signal, in which, when
the said detection device detects the absence of the said first
overmodulation frequency in the said protected portion of the said first
primary guided optical path, the said operating device enables the said
CA 02273582 1999-OS-28
_$_
optical signal to be propagated along the said first secondary guided
optical path up to the said first receiver.
In the present description and in the attached claims, the expression
"guided optical path" is used to mean a path which physically connects
separate points of a system and/or of an optical communication
network, and which is capable of transferring an optical signal in a
guided way from one to another of the said points. This path may
comprise optical waveguides such as optical i:lbres and optical
amplifiers, of the doped fibre type for example.
The expression "protected portion" is used to mean any section of a
guided optical path which is to be protected against any fault in the said
guided optical path.
Preferably said protected portion is comprised between a WDM
multiplexer and a WDM demultiplexer.
Typically, said WDM multiplexer is located up-line from said
modulation device.
Typically, said WDM demultiplexer is located down-line from said
detection device.
In one embodiment, the said at least one optical signal is also sent
along the said first secondary guided optical path and, when the said
detection device detects the presence of the said first overrnodulation
frequency in the said protected portion of the said first primary guided
optical path, the propagation of the said optical signal along the said
first secondary guided optical path is blocked before the said first
receiver.
In another embodiment, the said optical communication system also
comprises a second primary guided optical path having a protected
portion, a second transmitter for sending at least one second optical
signal at least along the said second primary guided optical path, and a
CA 02273582 1999-OS-28
-9-
second receiver for receiving the said at least one second optical
signal.
Preferably, the said second primary guided optical path is
operationally connected to the said first primary guided optical path.
Advantageously, the said at least one second optical signal has a
propagation direction opposite that of the said at least one optical
signal.
Preferably, the optical communication system also comprises a
second secondary guided optical path, connected operationally to the
said second primary guided optical path.
More preferably, the said second secondary guided optical path is
also operationally connected to the said first secondary guided optical
path.
In a first embodiment, the said first primary guided optical path and
the said first secondary guided optical path are connected by a first
beam splitter up-line from the said protected portion of the said first
primary guided optical path.
In a second embodiment, the said first primary guided optical path
and the said first secondary guided optical path are connected by a first
coupler down-line from the said protected portion of the said first
primary guided optical path.
In a third embodiment, the said second primary guided optical path
and the said second secondary guided optical path are connected by a
second beam splitter up-line from the said protected portion of the said
second primary guided optical path.
In a fourth embodiment, the said second primary guided optical path
and the said second secondary guided optical path are connected by a
second coupler down-line from the said protected portion of the said
second primary guided optical path.
CA 02273582 1999-OS-28
-10-
In a fifth embodiment, the said first primary guided optical path and
the said first secondary guided optical path are associated with a first
switch up-line from the said protected portion of the said first primary
guided optical path.
In a sixth embodiment, the said first primary guided optical path and
the said first secondary guided optical path are associated with a
second switch down-line from the said protected portion of the said first
primary guided optical path.
In a seventh embodiment, the said second primary guided optical
path and the said second secondary guided optical path are associated
with a third switch up-line from the said protected portion of the said
second primary guided optical path.
In an eighth embodiment, the said second primary guided optical
path and the said second secondary guided optical path are associated
with a fourth switch down-line from the said protected portion of the
said second primary guided optical path.
Advantageously, at least one of the said first and second primary
guided optical paths comprises, in transmission, an optical transmission
amplifier.
Preferably, at least one of the said first and second secondary
guided optical paths comprises, in transmission, an optical transmission
amplifier.
More preferably, at least one of the said first and second primary
guided optical paths comprises, in reception, an optical receiving
amplifier.
Even more preferably, at least one of the said first and second
secondary guided optical paths comprises, in reception, an optical
receiving amplifier.
Typically, the said modulation device comprises a modulator
associated with each of the said first and second primary guided optical
CA 02273582 1999-OS-28
-11-
paths to generate the said first overmodulation frequency along the said
first and the said second primary guided optical path. Advantageously,
the said modulator is associated with the optical transmission amplifier
of the said first and second primary guided optical paths.
Preferably, the said detection device is associated with each of the
said first and second primary guided optical paths to detect the
presence or absence of the said first overmodulation frequency along
the said first and second primary guided optical paths. More preferably,
the said detection device is associated with the optical receiving
amplifier of the said first and second primary guided optical paths.
Advantageously, the said operating device comprises an electronic
circuit associated with each of the said first and second primary guided
optical paths to switch the transmission of at least one of the said first
and second optical signals according to the presence or absence of the
said first overmodulation frequency in the said first and second primary
guided optical paths. Preferably, the said operating device is associated
with the optical receiving amplifier of the said first and second primary
guided optical paths.
In one embodiment, the said optical communication system
comprises a first plurality of transmitters for sending in the said first
primary guided optical path a plurality of optical signals, each having a
different wavelength from the other optical signals.
In a variant, the said optical communication system also comprises a
second plurality of transmitters for sending in the said second primary
guided optical path a plurality of optical signals, each having a different
wavelength from the other optical signals.
In another embodiment, the said modulation device also comprises a
modulator associated with each of the said first and second secondary
guided optical paths to generate a second overmodulation frequency
along the said first and second secondary guided optical paths.
CA 02273582 1999-OS-28
-12-
Preferably, the said detection device is also associated with each of
the said first and second secondary guided optical paths to detect the
presence or absence of the said second overmodulation frequency
along the said first and second secondary guided optical paths.
A second aspect of the present invention is a method for switching
the transmission of an optical signal from a primary guided optical path
to a secondary guided optical path, comprising the phases of:
a) sending at least a first part of the power of the said optical signal in
the said primary guided optical path;
b) sending an overmodulation frequency in the said primary guided
optical path;
c) detecting the presence or absence of the said overmodulation
frequency in the said primary guided optical path;
d) allowing the propagation of the said optical signal along the said
secondary guided optical path when the absence of the said
overmodulation frequency is detected.
In one embodiment, the phase a) also comprises the sending of a
second part of the power of the said optical signal in the said secondary
guided optical path, and the said method also comprises a phase e) in
which the propagation of the said optical signal in the said secondary
guided optical path is blocked when the presence of the said
overmodulation frequency is detected in the said primary guided optical
path.
In one variant, at least the said secondary guided optical path
comprises an optical receiving amplifier. Preferably the phase e) is
executed by disabling the said optical receiving amplifier.
Advantageously, the phase d) is executed by activating the said optical
receiving amplifier.
In another variant, the said primary guided optical path and the said
secondary guided optical path are associated with a switch. Preferably,
CA 02273582 1999-OS-28
-13-
the phase e) is executed by closing the said switch on the said primary
guided optical path. Advantageously, the phase d) is executed by
closing the said switch on the said secondary guided optical path.
Characteristics and advantages of the invention will now be
illustrated with reference to embodiments represented by way of
example, without restriction, in the attached drawings, in which:
- Fig. 1 is a schematic representation of a first embodiment of a
transmission switching system according to the invention;
- Fig. 2 is a schematic representation of a second embodiment of a
transmission switching system according to the invention;
- Fig. 3 is a schematic representation of the various states of a
transmission switching system according to the invention and of the
possible transitions between one state and the other;
- Fig. 4 is a schematic representation of a third embodiment of a
transmission switching system according to the invention;
- Fig. 5 is a schematic representation of a fourth embodiment of a
transmission switching system according to the invention.
Fig. 1 shows a first embodiment of a transmission switching system
for a bidirectional optical communication system according to the
invention. The said transmission switching system comprises a first and
a second primary (master) guided optical path 1 and 3 along which an
outgoing transmitting apparatus and a return apparatus (not shown)
transmit, respectively, an outgoing optical signal 300 and a return
optical signal 400. The said outgoing and return optical signals are
characterized by one or a plurality of carrier wavelengths and are
modulated at the frequency of an electrical signal containing the
information to be transmitted. In the case of a digital electrical signal,
this modulation frequency corresponds to the transmission bit rate. In
the case of a plurality of carrier wavelengths, a WDM multiplexer is
provided to combine the plurality of modulated wavelengths into a
CA 02273582 1999-OS-28
-14-
signal. The transmission switching system also comprises a first and a
second secondary (slave) guided optical path 2 and 4, to which the
transmission of the said outgoing optical signal 300 and of the said
return optical signal 400, respectively, is switched, if the presence of a
fault is detected along one of the two primary guided optical paths 1
and 3.
The said first and second primary guided optical paths 1 and 3
comprise, respectively, a first and a second primary optical
transmission amplifier 11 and 14 and a first and a second primary
optical receiving amplifier 13 and 16, connected, respectively, by a first
and a second primary optical fibre cable 100 and 110. Typically, the
said first and second primary guided optical paths 1 and 3 also
comprise a certain number of optical amplifiers disposed along the said
first and second primary optical fibre cables 100 and 110. In the
embodiment shown in Fig. 1, there is a primary optical amplifier, 12 and
15, along each of the said primary optical fibre cables 100 and 110
respectively.
In turn, the said first and second secondary guided optical paths 2
and 4 comprise, respectively, a first and a second secondary optical
transmission amplifier 21 and 24 and a first and a second secondary
optical receiving amplifier 23 and 26, connected, respectively, by a first
and a second secondary optical fibre cable 200 and 220. Typically, the
said first and second secondary guided optical paths 2 and 4 also
comprise a certain number of optical amplifiers disposed along the said
first and second secondary optical fibre cables 200 and 220. In the
embodiment shown in Fig. 1, there is a secondary optical amplifier, 22
and 25, also along each of the said first and second secondary optical
fibre cables 200 and 220 respectively.
CA 02273582 1999-OS-28
-15-
Preferably, the said first and second secondary guided optical paths
2 and 4 have the same number of optical amplifiers as the said first and
second primary guided optical paths 1 and 3.
A first beam splitter 31 divides the power of the said outgoing optical
signal 300 into two substantially equal parts, transmitting it both in the
said primary guided optical path 1 and in the said secondary guided
optical path 2. The power level of the said outgoing optical signal 300 at
the input is regulated in the transmitting apparatus in such a way as to
allow for the attenuation of 3 dB caused by the said first beam splitter
31. A first coupler 32 then combines the outputs of the said primary
guided optical path 1 and of the said secondary guided optical path 2.
A receiver is located down-line from said first coupler 32. In the case
of a plurality of carrier wavelengths, a WDM demultiplexer is provided to
separate the signal into a plurality of modulated wavelengths to be sent
to respective receivers.
In turn, a second beam splitter 33 divides the power of the said
return optical signal 400 into two substantially equal parts, transmitting
it both in the said second primary guided optical path 3 and in the said
second secondary guided optical path 4. The power level of the said
return optical signal 400 at the input is also regulated in the return
transmitting apparatus in such a way as to allow for the attenuation of 3
dB caused by the said second beam splitter 33. A second coupler 34
then combines the outputs of the said second primary guided optical
path 3 and of the said second secondary guided optical path 4.
A receiver is located down-line from said second coupler 34. In the
case of a plurality of carrier wavelengths, a WDM demultiplexer is
provided to separate the signal into a plurality of modulated
wavelengths to be sent to respective receivers.
The said optical amplifiers 11-16 and 21-26 are, preferably, optical
amplifiers based on suitably doped active fibres, pumped by a first
CA 02273582 1999-OS-28
-16-
source of optical pumping radiation, for example a laser or a laser
diode. In one variant, the said optical amplifiers 11-16 and 21-26 also
comprise a second reserve source of optical pumping radiation which is
put into operation in case of a fault or degradation of performance of
the first. Alternatively, when a greater pumping power is required, the
said second source of optical pumping radiation operates together with
the first.
Typically, the said active fibre is doped with erbium. The wavelength
of the said optical pumping radiation is selected from the absorption
wavelengths of the dopant used for the active fibre of the optical
amplifier. In the case of erbium, the said wavelength of the said optical
pumping radiation is preferably approximately 1480 nm and/or
approximately 980 nm.
The said first and second primary optical transmission amplifiers 11
and 14 and the said first and second secondary optical transmission
amplifiers 21 and 24 are associated with a modulator which is operated
to modulate, at a first predetermined modulation frequency, the supply
current of the said source of optical pumping radiation. The modulation
of the said supply current is thus transferred to the optical pumping
radiation and, therefore, to the inversion of the population of the dopant
used in the primary optical transmission amplifiers 11 and 14 and in the
secondary optical transmission amplifiers 21 and 24. In this way, the
said optical transmission amplifiers 11, 14, 21 and 24 transmit, at a
predetermined command, a first overmodulation frequency (tone) of the
said optical signals 300 and 400 along the said primary guided optical
paths 1 and 3 or along the said secondary guided optical paths 2 and 4.
In one variant, a modulator modulates the said optical pumping
radiation at the output of the said source by an external modulation
carried out, for example, by means of a conventional electro-optical or
acousto-optical modulator. In a further variant, a conventional optical
CA 02273582 1999-OS-28
-17-
modulator is associated with the said optical transmission amplifiers 11,
14, 21 and 24 (for example, down-line or, more preferably, up-line from
them) in such a way as to supply a modulation at the said first
overmodulation frequency to the outgoing and return optical signals 300
and 400.
To prevent the introduction of noise into the transmission band of the
said optical signals 300 and 400, the said first overmodulafion
frequency is preferably outside the said transmission band. Additionally,
owing to the response times of the dopant of the active fibre of the said
optical transmission amplifiers 11, 14, 21 and 24, the said first
overmodulation frequency is greater than a predetermined value. In the
case of erbium as the active dopant, the said first overmodulation
frequency is preferably in the range from 3 to 100 kHz. More preferably
it is in the range from approximately 5 to 50 kHz. Even more preferably
it is in the range from 5 to 20 kHz.
Each of the said first and second primary optical receiving amplifiers
13 and 16 and each of the said first and second secondary optical
receiving amplifiers 23 and 26 is associated with a device of the
conventional type for detecting the presence or absence of the said first
overmodulation frequency. The said device may, for example, comprise
an opto-electronic receiver (e.g. a photodiode), a filter capable of
selecting the said first predetermined overmodulation frequency, a
conventional peak detector and a conventional comparator circuit (not
shown).
The said device is preferably arranged up-line from the said optical
receiving amplifiers 13, 16, 23 and 26. The said filter is a conventional
electronic filter located after the opto-electronic receiver. At the output
of the said peak detector, the said comparator circuit compares the
received and filtered signal with a predetermined threshold to determine
CA 02273582 1999-OS-28
-18-
the presence or absence of the said first overmodulation frequency
(tone).
Each of the said first and second primary optical receiving amplifiers
13 and 16 and of the said first and second secondary optical receiving
amplifiers 23 and 26 is also associated with an electronic device for
transmitting suitable operating signals (I~, w~, i~) for canying out, as will
be explained below, the transmission switching in the presence of a
fault in a guided optical path or of degradation in the performance of an
optical amplifier or an optical fibre cable (i.e. when the absence of the
said first overmodulation frequency is detected). The said electronic
operating device is a conventional digital circuit.
In normal operating conditions of the bidirectional optical
communication system,
- the said outgoing optical signal 300 is transmitted to the input of the
said first beam splitter 31;
- the said return optical signal 400 is transmitted to the input of the
said second beam splitter 33;
- the said source of optical pumping radiation of the said optical
amplifiers 11-16 and 21, 22, 24 and 25 along the said primary and
secondary guided optical paths 1-4 is switched on;
- the said source of optical pumping radiation of the said secondary
optical receiving amplifiers 23 and 26 is switched off;
- the said modulator of the said source of optical pumping radiation of
the said first and second primary optical transmission amplifiers 11
and 14 is active;
- the said modulator of the said source of optical pumping radiation of
the said first and second secondary optical transmission amplifiers
21 and 24 is disabled.
Consequently,
CA 02273582 1999-OS-28
-19-
- the said optical signals 300 and 400 are actually transmitted only
along the said primary guided optical paths 1 and 3, because the
said source of optical pumping radiation of the said secondary optical
receiving amplifiers 23 and 26 is not switched on and therefore
prevents the said optical signals 300 and 400, transmitted by the
said beam splitters 31 and 33 both along the said primary guided
optical paths 1 and 3 and also along the said secondary guided
optical paths 2 and 4, from being recombined, by the said couplers
32 and 34, at the output of the said switching system;
- the said first overmodulation frequency (tone) is present only along
the said primary guided optical paths 1 and 3, because the said
modulator of the said secondary optical transmission amplifiers 21
and 24 is disabled.
In case of a fault in one (or both) of the said first and second primary
guided optical paths 1 and 3, the detection of the absence of the said
first overmodulation frequency by one (or both) of the said primary
optical receiving amplifiers 13 and 16 causes the transmission of the
said optical signals 300 and 400 to be switched immediately to the said
first and second secondary guided optical paths 2 and 4 respectively.
For example, in case of a break in the said first primary optical fibre
cable 100, the said first primary optical receiving amplifier 13 detects,
by means of the said opto-electronic receiver, filter, peak detector and
comparator circuit, the absence, down-line from the break, of the said
first overmodulation frequency in the said first primary optical path 1.
Consequently, the said operating device of the said first primary optical
receiving amplifier 13 switches off its source of optical pumping
radiation and sends the following:
- a first digital operating signal w, to the said second primary optical
transmission amplifier 14, to disable the modulator of its source of
optical pumping radiation;
CA 02273582 1999-OS-28
-20-
- a second digital operating signal i, to the said second primary optical
transmission amplifier 14, to disable its source of optical pumping
radiation;
- a third digital operating signal I, to the said first secondary optical
receiving amplifier 23, to activate its source of optical pumping
radiation.
In tum, since the said primary optical transmission amplifier 14 no
longer transmits the said first overmodulation frequency, the said
second primary optical receiving amplifier 16 detects the absence of the
said first overmodulation frequency in the said second primary optical
path 3. Consequently, the said operating device associated with the
said second primary optical receiving amplifier 16 switches off the
source of optical pumping radiation of the latter and sends the
following:
- a first digital operating signal w2 to the said first primary optical
transmission amplifier 11, to disable the modulator of its source of
optical pumping radiation;
- a second digital operating signal i2 to the said first primary optical
transmission amplifier 11, to disable its source of optical pumping
radiation;
- a third digital operating signal 12 to the said second secondary optical
receiving amplifier 26, to activate its source of optical pumping
radiation.
The said operating devices associated with the said first and the said
second secondary optical receiving amplifier 23 and 26 also send, to
the said second and the said first secondary optical transmission
amplifier 24 and 21 respectively, digital operating signals w3 and w4
respectively, to activate their modulators, and send to the said first and
the said second primary optical receiving amplifier 13 and 16
respectively digital operating signals /3 and I4 respectively, to keep their
CA 02273582 1999-OS-28
-21 -
sources of optical pumping radiation, which have already been
switched off, disabled.
Since both the said sources of optical pumping radiation of the said
first and second secondary optical receiving amplifiers 23 and 26 and
also the said modulators of the said secondary optical transmission
amplifiers 21 and 24 have been activated, the transmission of the said
optical signals 300 and 400 is automatically switched to the said first
and second secondary guided optical paths 2 and 4 respectively,
together with the said first overmodulation frequency (tone).
This is so because, since the said sources of optical pumping
radiation of the said primary optical receiving amplifiers 13 and 16 have
been switched off, the said optical signals 300 and 400, although
transmitted by the said beam splitters 31 and 33 both along the said
primary guided optical paths 1 and 3 and along the said secondary
guided optical paths 2 and 4, are actually transmitted only along the
said secondary guided optical paths 2 and 4.
On completion of the transmission switching, an operator can
intervene if necessary to locate and repair the fault in the said primary
guided optical path 1. At this point, in case of a fault in one of the said
secondary guided optical paths 2 and 4, the transmission can be
switched again, in a similar way, to the said primary guided optical
paths 1 and 3.
In general, in order to carry out the switching, the said operating
device associated with each of the said optical receiving amplifiers 13,
16, 23 and 26 uses the following digital operating signals (Table 1 ):
- a digital operating signal w~ to one of the said optical transmission
amplifiers 11, 14, 21 and 24, to send or not send the said first
overmodulation frequency;
CA 02273582 1999-OS-28
-22-
- a digital operating signal i~ to one of the said optical transmission
amplifiers 11, 14, 21 and 24, to activate or disable the said source of
optical pumping radiation;
- a digital operating signal I~ to another optical receiving amplifier, to
activate or disable the said source of optical pumping radiation.
The different possible states S1-S5 of the switching system
according to the invention are shown schematically in Table 2, where:
- the expression "active" is used to indicate the guided optical paths
along which the said optical signals 300 and 400 are actually
transmitted (as stated above);
- the expression "protection" is used to indicate the guided optical
paths which are not used for the transmission of the said optical
signals 300 and 400, but to which the transmission of the said optical
signals 300 and 400 is switched in case of a fault in the active optical
paths;
- the expression "in service" is used to indicate the guided optical
paths in which the said optical amplifiers 11, 12, 14, 15, 21, 22, 24,
25 (independently of the optical receiving amplifiers) have the said
source of optical pumping radiation switched on;
- the expression "out of service" is used to indicate the guided optical
paths in which both the said optical transmission amplifiers 11, 14
and 21, 24 and the receiving amplifiers 13, 16 and 23, 26 have the
said source of optical pumping radiation switched off.
The relationships between the different possible states of the
switching system according to the invention and the optical amplifiers
and the corresponding digital operating signals are shown in Tables 3-
7. In these tables, the abbreviation AOT is used to mean an optical
transmission amplifier, AO denotes an optical amplifier, and AOR
denotes an optical receiving amplifier, and the index corresponds to the
CA 02273582 1999-OS-28
-23-
numerical reference which indicates the corresponding amplifier in the
description and in the figures.
Additionally, Fig. 3 shows schematically the different states of the
switching system according to the invention and the possible transitions
between one state and another. Table 8 shows the significance of the
transitions.
To summarize, in the first embodiment, shown in Fig. 1, of the
transmission switching system according to the invention,
- the said first overmodulation frequency is present only in the "active"
optical paths; and
- the optical receiving amplifiers of the "protection" guided optical
paths, to which the transmission is to be switched, have their
sources of optical pumping radiation switched off to block the
propagation of the optical signals 300 and 400 and to prevent the
said optical signals 300 and 400, transmitted by the said beam
splitters 31 and 33 both along the said "active" guided optical paths
and along the said "protection" guided optical paths, from being
recombined, by means of the said couplers 32 and 34, at the output
of the said switching system. The said optical signals 300 and 400
are therefore actually transmitted only along the said "active" guided
optical paths.
In the presence of a fault in an "active" guided optical path,
- the optical receiving amplifier of the "active" optical path in which the
fault has occurred detects the absence of the said first
overmodulation frequency down-line from the fault; and
- the transmission of the said optical signals 300 and 400 is
immediately switched to both "protection" guided optical paths by
means of the said digital operating signals w~, i~, I~ associated with the
said optical receiving amplifiers.
CA 02273582 1999-OS-28
-24-
In one variant, the presence of any fault in a protection guided
optical path may be detected by means of a second overmodulation
frequency transmitted by the optical transmission amplifiers 11, 14, 21
and 24 of the protection guided optical path. In this case, the said
modulator associated with the optical transmission amplifiers 11, 14, 21
and 24 shall be operated in such a way that it modulates the said
optical pumping radiation at the said first overmodulation frequency in
the "active" guided optical paths, and at the said second
overmodulation frequency in the "protection" guided optical paths. The
said optical receiving amplifiers will also be associated with further
devices, similar to those illustrated previously, for additionally detecting
the presence or absence of the said second overmodulation frequency.
The considerations mentioned in respect of the said first
overmodulation frequency are also applicable to the said second
overmodulation frequency, which will be different from the said first
overmodulation frequency and outside the transmission band of the
said optical signals 300 and 400. Preferably, it will be in the range from
approximately 3 to 100 kHz. More preferably it will be in the range from
approximately 5 to 50 kHz. Even more preferably it will be in the range
from 5 to 20 kHz.
In case of a fault in the secondary optical path 200, for example,
while this path is in a "protection" state, the said first secondary optical
receiving amplifier 23 will detect the absence of the said second
overmodulation frequency down-line from the fault and will send the
said digital operating signals w3 and i3 to disable the said modulator and
the said source of optical pumping radiation of the said second
secondary optical transmission amplifier 24. In tum, the said second
secondary optical receiving amplifier 26 will detect the absence of the
said second overmodulation frequency in the said second secondary
guided optical path 4 and will send the said digital operating signals w4
CA 02273582 1999-OS-28
- 25 -
and i4 to disable the said modulator and the said source of optical
pumping radiation of the said first secondary optical transmission
amplifier 21. At this point, an operator can intervene if necessary to
locate and repair the fault.
The second embodiment of the invention shown in Fig. 2 differs from
the first embodiment in that the said first and second beam splitters 31
and 33 and the said first and second couplers 32 and 34 are replaced
by a first switch 41, a third switch 43, a second switch 42 and a fourth
switch 44 respectively. The said switches are optical, of the opto-
mechanical type for example, and are operated by suitable electrical
operating signals.
The introduction of the said switches
- makes it possible to transmit the said optical signals 300 and 400
either along the said first and second primary guided optical paths 1
and 3 only, or along the said first and second secondary guided
optical paths 2 and 4 only;
- makes it possible to keep the optical receiving amplifiers of the
protection guided optical path switched on;
- eliminates the necessity of the said digital signals Ij for activating the
said source of optical pumping radiation of the protection optical
receiving amplifiers and for disabling the said source of optical
pumping radiation of the active optical receiving amplifiers;
- makes it necessary to have new digital operating signals to cause
the closing of the switches in the primary guided optical paths 1 and
3 or in the secondary guided optical paths 2 and 4.
The said operating devices associated with the said first primary
optical receiving amplifier 13 and with the said first secondary optical
receiving amplifier 23 must therefore send a digital operating signal f, to
cause the closing of the said second switch 42 either.at the output of the
said first primary guided optical path 1 or at the output of the said first
CA 02273582 1999-OS-28
-26-
secondary guided optical path 2. They must also send a digital operating
signal f3 to cause the closing of the said third switch 43 either at the input
of the said second primary guided optical path 3 or at the input of the
said second secondary guided optical path 4.
In tum, the said operating devices associated with the said second
primary optical receiving amplifier 16 and of the said second secondary
optical receiving amplifier 26 must send a digital operating signal f4 to
cause the closing of the said fourth switch 44 either at the output of the
said second primary guided optical path 3 or at the output of the said
second secondary guided optical path 4. They must also send a digital
operating signal f2 to cause the closing of the said first switch 41 either at
the input of the said first primary guided optical path 1 or at the input of
the said first secondary guided optical path 2.
With the exception of the aforesaid differences, the description and
comments provided in relation to the said first embodiment of the
invention are also applicable to the second embodiment.
In both embodiments of the invention, the total time of one
transmission switching may be less than approximately 50 ms.
Advantageously, it is less than 20 ms.
According to a third embodiment shown in Fig. 4, the optical
switching system according to the invention comprises two beam
splitters 31 and 33 and two switches 42 and 44. The beam splitter 31
divides the power of the said outgoing optical signal 300 to transmit it
both in the said first primary guided optical path 1 and in the said first
secondary guided optical path 2, while the beam splitter 33 divides the
power of the said return optical signal 400 to transmit it both in the said
second primary guided optical path 3 and in the said second secondary
guided optical path 4. The switches 42 and 44, however, are closed,
respectively, at the output of the said first primary guided optical path 1
or of the said first secondary guided optical path 2 and at the output of
CA 02273582 1999-OS-28
-27-
the said second primary guided optical path 3 or of the said second
secondary guided optical path 4, according to whether the transmission
of the signal takes place in the primary guided optical paths 1 and 3 or
in the secondary guided optical paths 2 and 4. In other words, in normal
operating conditions, the switches 42 and 44 are closed at the output of
the active guided optical paths so that they can subsequently switch, in
case of a fault, to the output of the protection guided optical paths.
In a similar way to that of the first and second embodiments, the
presence of any fault in an active guided optical path is detected by
means of a device which detects the presence or absence of an
overmodulation frequency along the active guided optical paths. When
the absence of the said overmodulation frequency is detected, electronic
operating devices associated with the optical receiving amplifiers 13, 16,
23 and 26 cause the closing of the said switches 42 and 44 on the
protection guided optical paths.
The fourth embodiment of the invention, shown in Fig. 5, differs from
the first embodiment shown in Fig. 1 in that it relates to a transmission
switching system for a unidirectional optical communication system.
The transmission switching system shown in Fig. 5 comprises a
conventional transmitter (not shown) for sending an optical signal 300
having a predetermined carrier wavelength, a conventional receiver
(not shown), a beam splitter 31, a coupler 32, a primary guided optical
path 1 and a secondary guided optical path 2.
In normal operating conditions, the optical signal 300 is actually
transmitted, as stated previously, along the said primary guided optical
path 1 while, in case of a fault along the said primary guided optical
path 1, the transmission of the said optical signal 300 is switched to the
said secondary guided optical path 2.
In a similar way to that of the first embodiment, the presence of any
- fault along the primary guided optical path 1 is detected by means of a
CA 02273582 1999-OS-28
- 28 -
device, associated with a primary optical receiving amplifier 13, which
detects the absence or the presence of an overmodulation frequency
along the said primary guided optical path 1. Until the presence of the
said overmodulation frequency is detected, a secondary optical
receiving amplifier 23 is kept switched off to block the propagation of
the signal in the said secondary guided optical path 2. However, when
the absence of the said overmodulation frequency is detected, an
electronic operating device, associated with the primary optical
receiving amplifier 13, prepares for the disabling of the said receiving
amplifier 13 and sends to the said secondary optical receiving amplifier
23 a digital operating signal I, to activate its source of optical radiation
which has been disabled up to this moment. In this way, the optical
signal 300 is propagated up to the said receiver along the secondary
guided optical path 2, thus making it possible to obtain automatic
switching of its transmission.
In this embodiment, the additional switching to the secondary guided
optical path 2 of the transmission of the first overmodulation frequency
can be can-ied out by means of a suitable system of telemetry, for
example.
Alternatively, in a similar way to that of the third embodiment shown
in Fig. 4, the transmission switching system shown in Fig. 5 may
comprise, in place of the coupler 32, a switch 42 for switching the
transmission of the signal 300 from the active guided optical path to the
protection guided optical path.
In case of a fault in a guided optical path, the optical switching
system according to the invention therefore permits a fast switching of
the transmission of an optical signal to another guided optical path
independently of the transmitting and receiving apparatus and of the
location of this fault. The switching is carried out exclusively by means
of the said modulator associated with the said optical transmission
CA 02273582 1999-OS-28
- 29 -
amplifiers and by means of the said devices for detecting the absence
of the said first overmodulation frequency and of the said digital
operating signals associated with the said optical receiving amplifiers.
The fault may be located, independently, by a conventional remote
monitoring system of an optical communication system, for example by
means of the appropriate service signals transmitted by the transmitting
and receiving apparatus along the said guided optical paths.
The transmission switching time is also independent of the presence
and number of the amplifiers 12, 15, 22 and 25 along the guided optical
paths, since the speed of transmission of the fault information coincides
with the speed of propagation of the optical signals 300 and 400, in
other words the speed of light in the transmission medium.
In the case of a multiple wavelength transmission (WDM), in which a
plurality of signals at different wavelengths are transmitted in each of
the said primary guided optical paths 1 and 3, the optical switching
system according to the invention also makes it possible to carry out
switching, in case of a fault along a guided optical path, of all the
signals at different wavelengths to the said secondary guided optical
paths 2 and 4. This eliminates the necessity, present in conventional
systems of operating an optical communication system, of switching
these signals at different wavelengths with a number of switching
systems equal to that of the different wavelengths transmitted.
CA 02273582 1999-OS-28
-30-
piaital o erating signals of or~tical receivincp am~lif~ss for ca~prina out
SIGNAL STATE COMMAND
w, active/disabledrequest for activation/disabling
of the first
overmodulation frequency from
AOR,3 to
AOT,4. _
w2 active/disabledrequest for activation/disabling
of the first
overmodulation frequency from
AOR,6 to
AOT".
w3 active/disabledrequest for activation/disabling
of the first
overmodulation frequency from
AOR~ to
AOT24.
w4 active/disabledrequest for activation/disabling
of the first
overmodulation frequency from
AOR~ to
AOT2,.
i, active/disabledrequest for activation/disabling
of the
source of optical pumping radiation
from
AOR,3 to AOT,4.
i2 active/disabledrequest for activation/disabling
of the
source of optical pumping radiation
from
AOR,s to AOT".
i3 active/disabledrequest for activation/disabling
of the
source of optical pumping radiation
from
AOR23 to AOT24.
i4 active/disabledrequest for activationldisabling
of the
source of optical pumping radiation
from
AOR26 to AOT2,.
CA 02273582 1999-OS-28
-31 -
I, active/disabledrequest for activationldisabling
of the
source of optical pumping radiation
from
AOR,3 to AOR~.
12 active/disabledrequest for activationldisabling
of the
source of optical pumping radiation
from
AOR, fi to AOR~.
13 activeldisabledrequest for activation/disabling
of the
source of optical pumping radiation
from
AOR23 to AOR,3.
14 active/disabledrequest for activation/disabling
of the
source of optical pumping radiation
from
AORZ6 to AOR,6.
where:
AOR = Optical receiving amplifier
AOT = Optical transmission amplifier
CA 02273582 1999-OS-28
-32
TABLE
I~iaaram of the states shown in Fig 3 for the optical switchinr~~~ystem
according to the invention
STATE OPTICAL PATH OPERATING TRANSMISSION
STATE STATE
S1 P IS AT
S IS PR
S2 P IS PR
S IS AT
S3 P IS AT
S FS PR
S4 P FS PR
S IS AT
S5 P FS //
S FS //
where:
P - Primary
S - Secondary
IS - In Service
FS = Out of Service
AT = Active
PR = Protection
CA 02273582 1999-OS-28
-33-
Relationshic~between the state S1 of the switching~ystem according to
the invention and the states of the or~tical amralifiers and of the
Primary optical AOT" On
path 1 A0,2 On
AOR,3 On
w, Active
i, Active
I, Disabled
Primary optical AOT,4 On
path 3 A0,5 On
AOR,6 On
w2 Active
i2 Active
12 Disabled
Secondary opticalAOT2, On
path 2 AO~
On
AOR23 Off
w3 Disabled
i3 Active
13 Active
Secondary opticalAOT24 On
path 4 A0z5 On
AORzs Off
-34-
Image
CA 02273582 1999-OS-28
-35-
Primary optical AOT" On
path 1 A0,2 On
- AOR,3 Off
w, Disabled
i, Active
I, Active
Primary optical AOT,4 On
path 3 A0,5 On
AOR,6 Off
w2 Disabled
i2 Active
IZ Active
Secondary optical AOT2, On
path 2 A022 On
AOR23 On
w3 Active
i3 Active
13 Disabled
Secondary optical AOT24 On
path 4 A025 On
- - AOR26 On
-36-
Image
CA 02273582 1999-OS-28
-37-
Relationship between the state S3 of the switchi~q s~rstem according to
the invention and the states of the optical amdlifiers and of the
Primary optical AOT" On
path 1 A0,2 On
AOR,3 On
w, Active
i, Active
I, Disabled
Primary optical AOT,4 On
path 3 A0,5 On
AOR,6 On
w2 Active
i2 Active
12 Disabled
Secondary optical AOT2, Off
path 2 AO~
Off
AOR23 Off
w3 Disabled
i3 Disabled
13 Active
Secondary optical AOT24 Off
path 4 AO25 Off
AOR26 Off
-38-
Image
CA 02273582 1999-OS-28
-39-
to the invention and the states of the optical ai~nlifiers and of the
Primary optical AOT" Off
path 1 AO,Z Off
AOR,3 Off
w, Disabled
i, Disabled
1, Active
Primary optical AOT,4 Off
path 3 A0,5 Off
AOR,e Off
w2 Disabled
i2 Disabled
12 Active
Secondary optical AOT2, On
path 2 AO~
On
AOR23 On
w3 Active
i3 Active
13 Disabled
Secondary optical AOT24 On
path 4 AO25 On
AOR26 On
-40-
Image
CA 02273582 1999-OS-28
-41 -
A 7
Relationship between the state S5 of the switching syrstem according to
Primary optical AOT" Off
path 1 A0,2 Off
AOR,3 Off
w, Disabled
i, Disabled
I, Disabled
Primary optical AOT,4 Off
path 3 AO,S Off
AOR,s Off
w2 Disabled
i2 Disabled
f2 Disabled
Secondary opticalAOT2, Off
path 2 AOn
Off
AO R23 Off
w3 Disabled
i3 Disabled
13 Disabled
Secondary opticalAOT24 Off
path 4
A025 Off
AOR~ Off
- 42 -
Image
CA 02273582 1999-OS-28
- 43 -
Transitions shown in Fia. 3 between the different states of the o tp ical
TRANSITION CAUSES OF TRANSITION EFFECT
T,3 Fault in the S/PR opticalThere is no switching
and the
path. S optical path goes FS.
T3, The S/PR optical path There is no switching
is and the
restored by external S optical path returns
to the IS
command. state.
T~ Fault in the P/AT opticalThe switching system is
no
path when the S/PR longer operative.
optical
path was already FS.
T53 Restoration of the Transmission is possible
P/AT
optical path by externalagain. The first path
to be
command. reset becomes active.
T~ Fault in the S/AT opticalThe switching system is
no
path when the P/PR longer operative.
path
was already FS.
T~ Restoration of the Transmission is possible
S/AT
optical path by externalagain. The first path
to be
command. reset becomes active.
T42 Restoration of the There is no switching.
P/PR The P
optical path by externalpath returns to the IS
state in
command. PR. To return to being
P/AT an
external command is required.
T2, . Fault in the P/PR opticalThere is no switching.
The
path. P/PR optical path goes
FS.
CA 02273582 1999-OS-28
-44-
T,4 Fault in the P/AT opticalSwitching takes place.
The S
path. optical path becomes AT
and
the P path goes FS.
T23 Fault in the SLAT opticalSwitching takes place.
The P
path. optical path becomes AT
and
the S path goes FS.
U,z Forced switching from Switching takes place.
The S
outside. optical path becomes AT
and
the P path becomes PR.
U2, Forced switching from Switching takes place.
The P
outside. optical path becomes AT
and
the S path becomes PR.
A, Switching system switchedThe AOR of the P guided
on when the optical optical paths takes priority
signal over
arrives and all paths that of the S guided optical
are IS.
paths, and therefore becomes
AT.
A2 Switching system switchedBoth the P and the S optical
on in the absence of paths are FS. Those which
an are
optical signal and restored first will become
when all AT.
paths are FS.
where:
P/AT Active primary path
-
P/PR Protection primary
- path
S/AT Active secondary path
-
S/PR Protection secondary
- path
IS - In service
FS - Out of service
