U.S. patent application number 10/224199 was filed with the patent office on 2002-12-26 for device for adding and dropping optical signals.
This patent application is currently assigned to Corning O.T.I.. Invention is credited to Grasso, Giorgio, Meli, Fausto, Sanches, Marcos Antonio Brandao, Tamburello, Mario.
Application Number | 20020196495 10/224199 |
Document ID | / |
Family ID | 11374719 |
Filed Date | 2002-12-26 |
United States Patent
Application |
20020196495 |
Kind Code |
A1 |
Grasso, Giorgio ; et
al. |
December 26, 2002 |
Device for adding and dropping optical signals
Abstract
A method for adding at least one added signal to, and dropping
at least one dropped signal from at least one of a first
transmission path and a second transmission path, the method
including (i) creating a first series of signals and a second
series of signals, the signals having distinct wavelengths, the
first series and the second series being mutually staggered in
wavelength, (ii) transmitting the first series to the first
transmission path and the second series to the second transmission
path, and, (iii) providing a wavelength selective switch along at
least one of the first transmission path and the second
transmission path, the wavelength selective switch for adding the
at least one added signal and dropping the at least one dropped
signal.
Inventors: |
Grasso, Giorgio; (Monza,
IT) ; Meli, Fausto; (Piacenza, IT) ; Sanches,
Marcos Antonio Brandao; (Lexington, SC) ; Tamburello,
Mario; (Vimercate, IT) |
Correspondence
Address: |
FOLEY HOAG & ELIOT LLP
PATENT GROUP
ONE POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Assignee: |
Corning O.T.I.
Corning
NY
|
Family ID: |
11374719 |
Appl. No.: |
10/224199 |
Filed: |
August 20, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10224199 |
Aug 20, 2002 |
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09900185 |
Jul 9, 2001 |
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6437888 |
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09900185 |
Jul 9, 2001 |
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09202270 |
May 7, 1999 |
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6288810 |
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09202270 |
May 7, 1999 |
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PCT/EP97/04091 |
Jul 28, 1997 |
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Current U.S.
Class: |
398/82 ;
385/24 |
Current CPC
Class: |
H04J 14/0216 20130101;
H04J 14/0206 20130101; H04J 14/0212 20130101; H04J 14/0213
20130101; H04J 14/0221 20130101; H04J 14/0209 20130101 |
Class at
Publication: |
359/127 ;
385/24 |
International
Class: |
G02B 006/28; H04J
014/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 1996 |
IT |
MI96A001638 |
Claims
What is claimed is:
1. A method for adding at least one added signal to, and dropping
at least one dropped signal from at least one of a first
transmission path and a second transmission path, the method
comprising: creating a first series of signals and a second series
of signals, the signals having distinct wavelengths, the first
series and the second series being mutually staggered in
wavelength, transmitting the first series to the first transmission
path and the second series to the second transmission path, and,
providing a wavelength selective switch along at least one of the
first transmission path and the second transmission path, the
wavelength selective switch for adding the at least one added
signal and dropping the at least one dropped signal.
2. A method according to claim 1, further comprising combining the
signals from the first transmission path and the second
transmission path.
3. A method according to claim 1, where creating includes using a
spectral selection circuit.
4. A method according to claim 1, where creating includes using a
divider to divide said signals into the first series and the second
series.
5. A method according to claim 1, where creating includes using an
optical circulator and a selective reflection circuit.
6. A method according to claim 1, where creating includes using an
optical circulator and at least one Bragg grating filter.
7. A method according to claim 1, where the wavelengths of the
first series and the wavelengths of the second series are separated
by a value lower than the spectral resolution of the wavelength
selective switch.
8. A method according to claim 1, where the at least one signal is
an optical signal.
9. A method according to claim 1, where the first transmission path
and the second transmission path include a fiber optic line.
10. A method according to claim 1, where the wavelength selective
switch is an acousto-optical wavelength selective switch.
11. A method for dropping at least one dropped signal from at least
one of a first transmission path and a second transmission path,
the method comprising: creating a first series of signals and a
second series of signals, the signals having distinct wavelengths,
the first series and the second series being mutually staggered in
wavelength, transmitting the first series to the first transmission
path and the second series to the second transmission path, and,
providing a wavelength selective switch along at least one of the
first transmission path and the second transmission path, the
wavelength selective switch for dropping the at least one dropped
signal from at least one of the first series and the second
series.
12. A method according to claim 11, further comprising combining
the signals from the first transmission path and the second
transmission path.
13. A method according to claim 11, where creating includes using a
spectral selection circuit.
14. A method according to claim 11, where creating includes using a
divider to divide said signals into the first series and the second
series.
15. A method according to claim 11, where creating includes using
an optical circulator and a selective reflection circuit.
16. A method for adding at least one added signal from at least one
of a first transmission path and a second transmission path, the
method comprising: creating a first series of signals and a second
series of signals, the signals having distinct wavelengths, the
first series and the second series being mutually staggered in
wavelength, transmitting the first series to the first transmission
path and the second series to the second transmission path, and,
providing a wavelength selective switch along at least one of the
first transmission path and the second transmission path, the
wavelength selective switch for adding the at least one added
signal from at least one of the first series and the second
series.
17. A method according to claim 16, further comprising combining
the signals from the first transmission path and the second
transmission path.
18. A method according to claim 16, where creating includes using a
spectral selection circuit.
19. A method according to claim 16, where creating includes using a
divider to divide said signals into the first series and the second
series.
20. A method according to claim 16, where creating includes using
an optical circulator and a selective reflection circuit.
Description
DESCRIPTION
[0001] It is an object of the present invention to provide a device
for adding or dropping optical signals of different wavelengths
into or from an optical path and a method for inserting and/or
extracting optical signals of different wavelengths into and from
an optical path.
[0002] In the latest telecommunications technology, it is known to
use optical fibers to send optical information-carrying signals for
long-distance communication.
[0003] Optical telecommunication systems are known that use
wavelength division multiplexing (WDM) transmission. In these
systems several channels, i.e. a number of independent transmission
signals, are sent over the same line by means of wavelength
division multiplexing. The transmitted channels may be either
digital or analog and are distinguishable because each of them is
associated with a specific wavelength.
[0004] EP 668674 (Toshiba) discloses an optical WDM network system
designed to permit communication between any of a plurality of
nodes via a main trunk line. The nodes are interconnected by an
optical fiber ring. Each of the nodes includes an add-drop
multiplexer (ADM) for extracting light of a particular wavelength
among the lights of a plurality of wavelengths transmitted via the
main trunk line into the node and inserting the light of the preset
wavelength from the node into the main trunk line. Each node
further includes an optical receiver for receiving part of the
light extracted by the ADM and a modulator for modulating the light
extracted by the ADM with data to be transmitted and send the light
back to said ADM. In an embodiment, the optical wavelength division
multiplexer and demultiplexer of each of the terminal nodes can be
realized by an ADM filter such as an acousto-optic filter or a
waveguide type lattice filter. In this embodiment, at a node k only
optical signals of wavelengths .lambda.k and .lambda.k' are
directed to an optical WDM whereby the two optical signals are
demultiplexed, while the optical signals of other wavelengths are
transmitted by the ADM filter to the main trunk line.
[0005] Electronics Letters, Vol. 31, No. 6, 16/03/95, pp. 476477
(M. J. Chawki et al.) discloses an experimental optical WDM ring
network using an add/drop multiplexer (ADM) based on a fibre
grating filter. The ring consists of a central node connecting N
secondary nodes using dedicated wavelengths .lambda..sub.i (i=1 . .
. N) over one common fibre. The central node has four transmitters
with a common erbium doped fibre amplifier (EDFA) and a 1:4
demultiplexer with four receivers. The optical wavelength ADM
located in each secondary node is made of a 2:1 coupler, a fibre
grating filter, a second 2:1 coupler and an EDFA. The node
transmitters and receivers are connected to one of the input ports
of the first and second coupler, respectively.
[0006] IEEE Photonics Technology Letters, Vol. 5, No. 7, July 1993,
pp. 825-828 (K. Oda et al.) discloses that optical add/drop
multiplexers (ADM's) can be constructed by optical filters and that
there are many candidates for the filters which could be used for
the ADM's; for example, the acousto-optic tunable filter (AOTF).
The paper teaches that, however, the channel spacing of the AOTF is
basically limited to a few nm, so that the number of channels is
limited to approximately 10 because EDFA gain bandwidth is narrower
than 30 nm. On the other hand, the channel spacing of the
Fabry-Perot filter can be set to less than 1 nm because its finesse
is sufficiently high. Accordingly, the paper proposes an optical
ring network based on optical frequency division multiplexing and
add/drop multiplexers (ADM's), wherein each ADM consists of a fiber
Fabry-Perot filter and an optical circulator.
[0007] WO 96/19884 (Ericsson) discloses a method of configuring
subnodes, or configuring a system of subnodes, in an optical
network ring against both node and fiber failure by means of an
optical add-drop multiplexer (OADM). The network comprises a
working ring and a stand-by ring and each subnode includes
selective optical filter means, optical 2.times.2 switch means and
optical amplifier means. The switching configuration of the optical
2.times.2 switch is controlled by the different available node
states so that it is possible to select either of two measures in
case of a fiber breakage, such as folding the ring or line
switching. According to the inventor of WO 96/19884 the disclosed
OADM structure, besides its advantages in connection with failures,
offers a greater simplicity for adding and removing nodes in an
existing optical network.
[0008] The Applicant has observed that known WDM communication
systems are limited as concerns the number of channels, i.e. the
independent wavelengths that can be used for transmission within
the wavelength band available for signal transmission and
amplification.
[0009] In order to combine and separate signals with different
wavelengths--to combine the signals at the transmission station,
for example, to drop some toward receivers located at intermediate
nodes of the line or to introduce others at intermediate nodes or
to send them to separate receivers at the receiving
station--adjacent channels (in wavelength terms) must be separated
by more than a minimum predetermined value.
[0010] Said minimum value depends on the characteristics of the
components employed in the system, such as the spectral
characteristics of the wavelength selective components (e.g.
bandwidth, center-band attenuation, figure of merit) and the
wavelength stability (thermal and temporal) of the selective
components themselves and of optical signal sources.
[0011] In particular, the Applicant has observed that spectral
selectivity of currently available wavelength selective components
currently available may greatly limit the possibility of adding and
dropping signals in multichannel transmission systems, particularly
when there are signals with close wavelengths, e.g. separated by
less than 2 nm.
[0012] The Applicant found it is possible to add and/or drop in an
optical telecommunication system a number of independent optical
channels greater than that permitted by known techniques, and
closer in wavelength, by employing wavelength selective components
of equivalent characteristics, if the input signals are divided
into two series of staggered wavelengths. The signals of each
series are independently dropped or signals of corresponding
wavelength added, and the signals of the two series are then
combined.
[0013] According to a first aspect, the present invention is
related to a device for adding and dropping optical signals as
claimed in claim 1.
[0014] Preferred embodiments of the device are given in dependent
claims 2 to 4.
[0015] According to another aspect, the present invention relates
to a multichannel optical telecommunication system for the
transmission of optical signals as claimed in claim 5.
[0016] According to a third aspect, this invention relates to a
method for adding/dropping optical signals as claimed in claim
6.
[0017] According to a fourth aspect, the present invention is
related to a device for dropping optical signals as claimed in
claim 7.
[0018] According to a fifth aspect, the present invention is
related to a device for adding optical signals as claimed in claim
8.
[0019] Additional information may be derived from the following
description, with reference to the attached drawings showing:
[0020] in FIG. 1 diagram of an optical telecommunication
system;
[0021] in FIG. 2 diagram of a transmission interfacing unit;
[0022] in FIG. 3 diagram of an optical power amplifier;
[0023] in FIG. 4 diagram of an optical preamplifier;
[0024] in FIG. 5A diagram of an optical demultiplexer;
[0025] in FIG. 5B diagram of a wavelength-selective optical
splitter;
[0026] in FIG. 6 diagram of a bidirectional optical amplifier
[0027] in FIG. 7 diagram of an optical amplifier including a device
for adding and dropping signals;
[0028] in FIG. 8 diagram of a device for adding/dropping
signals;
[0029] in FIG. 9 diagram of a first type of acoustical-optical
switch;
[0030] in FIG. 10 diagram of a second type of acoustical-optical
switch.
[0031] As shown in FIG. 1, a bidirectional optical
telecommunication system with wavelength-division multiplexing,
according to the present invention, comprises two terminal stations
A and B, each of which includes a respective transmission station
1A, 1B and a respective receiving station 2A, 2B.
[0032] In particular, in the version shown in the figure,
transmission station 1A comprises 16 optical signal transmitters
with a first series of wavelengths, indicated with odd-numbered
subscripts, .lambda..sub.1, .lambda..sub.3, . . . , .lambda..sub.31
(included, for example, in the wavelength band of 1530-1565 nm) and
transmission station 1B comprises 16 optical transmitters with a
second series of wavelengths, indicated with even-numbered
subscripts, .lambda..sub.2, .lambda..sub.4, . . . ,
.lambda..sub.32.
[0033] The wavelengths of the second series are selected so that
they are staggered with respect to the wavelengths in the first
series.
[0034] In other words, each pair of wavelengths of one series
encompasses a wavelength of the other series.
[0035] In the present case, the wavelengths of the two series will
be indicated as staggered, more generally, even when the
wavelengths of the signals of each of said series, corresponding to
optical signals emitted by one of the transmission stations 1A, 1B
and propagating in the system in one of the two directions, are
separated (in frequency) by a quantity greater than or equal to 2D,
where D indicates the minimum bandwidth (in frequency) of the
wavelength selective components used in the system to separate the
signals at the various wavelengths.
[0036] The number of independent wavelengths used for the signals
for each transmission station is not limited to the value of 16
indicated in the device described and may assume a different value.
The number of wavelengths, corresponding to the number of optical
channels used for transmission in each direction, may be selected
in relation to the characteristics of the telecommunication system.
In particular, in a telecommunication system according to the
present invention, it is possible, after the system implementation,
to increase the number of channels to increase the transmitting
capacity of the system, e.g. to accommodate an increased traffic
demand, as will be indicated below.
[0037] The wavelengths may be selected so that the corresponding
frequencies are equally spaced within the available spectral
amplification band, so as to utilize said band efficiently.
[0038] It is possible, however, for the frequencies to be totally
or partially unequally spaced, e.g. so as to reduce the effect of
non-linear phenomena, such as four wave mixing (FWM) in optical
fibers used for transmitting the signals.
[0039] The useful amplification band of the amplifiers may also be
constituted of two or more distinct spectral bands separated by
spectral bands not well-suited for signal transmission or
amplification, e.g. due to the particular spectral characteristics
of the amplifiers or optical fibers employed in the
telecommunication system. In this case, the wavelengths of the
communication channels may, for example, be selected such that the
corresponding frequencies are equally spaced within each individual
spectral band, with the separation between adjacent channels
propagating in the same direction greater than or equal (in
frequency) to twice said value D.
[0040] As an example, the wavelengths may assume values between
about 1535 nm and about 1561 nm, where consecutive wavelengths, in
ascending order, are used alternately for each of the two series
.lambda..sub.1, .lambda..sub.3, . . . , .lambda..sub.31 and
.lambda..sub.2, .lambda..sub.4, . . . , .pi..sub.32. The spacing
between the 32 total wavelengths, in this case, is about 0.8
nm.
[0041] The optical transmitters comprised in transmission stations
1A and 1B are modulated, directly or with external modulation,
according to system requirements, in particular in relation with
the chromatic dispersion of the optical fibers in the system, with
their lengths, and with the intended transmission velocity.
[0042] The outputs of each transmitter of transmission stations 1A
and 1B are connected to multiplexers 3A and 3B, respectively, which
combine their optical signals each toward a single output,
connected respectively to the input of optical power amplifiers 5A
and 5B. The outputs of these amplifiers are connected to an input
port of optical circulators 7A and 7B.
[0043] An intermediate port of optical circulators 7A and 7B is
connected to one end of an optical line 8, which connects the two
terminal stations A and B together.
[0044] The optical fiber of optical line 8 is normally a
single-mode optical fiber of the step index or dispersion shifted
type, conveniently included in a suitable optical cable, and has
tens (or hundreds) of kilometers of length between each amplifier,
up to the desired connection distance.
[0045] Inserted along line 8 are bidirectional optical amplifiers
9. Each of them comprises two optical circulators 91 and 92 and two
optical amplifiers 93 and 94, which will be described further on. A
central port of each optical circulator is connected to the optical
fiber of line 8, e.g. through an optical connector, and acts as an
input/output port for the bidirectional amplifier. Optical
amplifier 93 is optically connected between an output port of
optical circulator 91 and an input port of optical circulator 92.
Optical amplifier 94 is optically connected between an output port
of optical circulator 92 and an input port of optical circulator
91.
[0046] Although FIG. 1 indicates two bidirectional optical
amplifiers 9, there may be one or more bidirectional optical
amplifiers in succession, depending on the overall length of the
optical connection and the power in the various sections of it. A
fiber section between a terminal station and an amplifier, for
example, or between two successive amplifiers, may be on the order
of 100 kilometers long.
[0047] Receiving stations 2A and 2B are connected to the output
ports of optical circulators 7A and 7B through preamplifiers 6A and
6B and demultiplexers 4A and 4B.
[0048] The optical circulators are passive optical components,
commonly equipped with three or four access ports placed in an
ordered sequence. After defining a first arbitrarily chosen access
port as "input port", the next ports in sequence will be indicated
as central port and output port. The optical circulators transmit
unidirectionally the radiation input by each of the ports to one
only of the other ports, namely the next one in sequence. The
circulators used in the present invention are preferably of the
polarization-independent type.
[0049] Preamplifier, in the context of the present invention, is an
amplifier dimensioned to compensate the losses of the last section
of optical line and the insertion losses of demultiplexer 4A or 4B,
so that the power level of the signal input to the receiver is
suited to the sensitivity of the device. It is also the task of the
preamplifier to limit signal dynamics, reducing the power level
variations of the signals at the receiver input with respect to the
power level variation of the signals from the transmission
line.
[0050] Demultiplexers 4A and 4B are suited for taking 16 signals
overlapped in a single input port connected to the output of
preamplifier 6A, 6B and separating them on to 16 optical fibers, in
accordance with their respective wavelengths.
[0051] When the optical signals for transmission are generated by
signal sources with their own transmission characteristics (such as
wavelength, modulation type, power) different from those envisaged
for the described link, each transmission station 1A, 1B comprises
interfacing units 901, 903, . . . , 931 and 902, 904, . . . , 932,
respectively, for receiving the optical signals generated by
transmission stations 1A, 1B, detecting them, regenerating them
with new characteristics suited to the transmission system and
sending them to multiplexers 3A, 3B.
[0052] In particular, said interfacing units generate optical
working signals with wavelengths .lambda..sub.1, .lambda..sub.3, .
. . , .lambda..sub.31 and .lambda..sub.2, .lambda..sub.4, . . . ,
.lambda..sub.32, respectively, suited to the system requirements as
described below.
[0053] U.S. Pat. No. 5,267,073 by this same Applicant, the
description of which is herein incorporated by reference, describes
interfacing units comprising in particular a transmission adaptor
for converting an optical input signal into a form well-suited for
the optical transmission line and a reception adaptor for
converting the transmitted signal into a form well-suited for a
reception unit.
[0054] For use in the system of the present invention, the
transmission adaptor comprises, preferably, an externally modulated
laser as an output signal generation source.
[0055] The diagram of a transmission interfacing unit 900, of the
type well-suited for use within the context of this invention, is
shown in FIG. 2 in which, for the sake of clarity, the optical
connections are represented by solid lines, while the electrical
connections are represented by broken lines.
[0056] The optical signal, coming from an external source 207, is
received by a photodetector (photodiode) 208, which emits an
electrical signal which is fed to an electronic amplifier 209.
[0057] The electrical signal output by amplifier 209 is fed to a
circuit 210 that drives a modulable laser emitter, designated
overall as 211, that generates an optical signal at the selected
wavelength, containing the information of the incoming signal.
[0058] If appropriate, a circuit 212 for inputting a service
channel may be connected to driving circuit 210.
[0059] Modulable laser emitter 211 includes a continuous emission
laser 213 and an external modulator 214, e.g. of the Mach-Zehnder
type, driven by the output signal of circuit 210.
[0060] A circuit 215 controls the emission wavelength of laser 213,
keeping it constant at the specified value and compensating for any
external disturbances such as temperature and the like.
[0061] Transmission interfacing units of the type indicated are
described in the aforesaid patent and marketed by the Applicant
under the designation TXT/EM-XXX.
[0062] As an alternative, the laser transmitters in transmission
stations 1A and 1B may be laser transmitters operating at the
selected wavelengths, e.g. using DFB lasers at wavelengths
.lambda..sub.1, .lambda..sub.3, . . . , .lambda..sub.31 and
.lambda..sub.2, .lambda..sub.4, . . . , .lambda..sub.32,
respectively.
[0063] Preferably, the wavelength of each source used for the
signals is stable within +/-0.25 nm, more preferably within +/-0.1
nm.
[0064] With reference to FIG. 1, the optical circulators are
components available commercially. A model well-suited for use in
the present invention, for example, is the PIFC-100 produced by
E-TEK DYNAMICS Inc., 1885 Lundy Ave., San Jose, Calif. (USA),
characterized by an attenuation of 0.7 dB in transmission between
two consecutive ports and by a response substantially independent
from polarization.
[0065] Power amplifiers 5A and 5B raise the level of the signals
generated by transmission stations 1A and 1B to a value sufficient
to travel the section of optical fiber separating them from the
receiving station or amplification means with sufficient terminal
power to ensure the required transmission quality.
[0066] A power amplifier well-suited for use in the present
invention will now be described with reference to FIG. 3.
[0067] The power amplifier represented is of the two-stage type. A
first amplification stage comprises an active fiber 32, pumped
counterdirectionally by a pumping source 34 through a dichroic
coupler 33.
[0068] A second amplification stage comprises an active fiber 36,
pumped counterdirectionally by a pumping source 38 through a
dichroic coupler 37.
[0069] An amplifier input 310 is connected through a first optical
isolator 31 to the first amplification stage, and precisely to
active fiber 32, whose output terminates in a branch of dichroic
coupler 33. Pumping source 34 is connected to a second branch of
dichroic coupler 33, while a third branch of the same dichroic
coupler constitutes the signal output of the first stage.
[0070] A second optical isolator is located between the output of
the first stage and an input of active fiber 36 of the second
stage, whose output terminates in a branch of dichroic coupler 37.
Pumping source 38 is connected to a second branch of dichroic
coupler 37, while a third branch of the same coupler constitutes
the signal output of the second stage, which terminates in an
output 320, consisting preferably of a very-low-reflection optical
connector, e.g. an angled connector with reflectivity less than -55
dB. Optical connectors of this type are marketed, for example, by
SEIKOH GIKEN, 296-1 Matsuhidai, Matsudo, Chiba (Japan).
[0071] Output 320 is connected, in the telecommunication system of
FIG. 1, with an optical circulator (7A or 7B). This circulator
permits the unidirectional passage of radiation output by the power
amplifier and prevents radiation from entering by that output. The
circulator is thus equivalent to an additional optical isolator
connected to the amplifier output, particularly in limiting its
interferential noise.
[0072] Active optical fibers 32 and 36 are preferably silica
optical fibers. A rare earth is used as a dopant, preferably
erbium. Aluminum, germanium and lanthanum, or aluminum and
germanium, may be advantageously used as secondary doping
agents.
[0073] The concentration of dopants may correspond, for example, to
an attenuation of around 7 dB/m, for the active fiber in the
absence of pumping.
[0074] In a preferred embodiment, the amplifier described uses
erbium-doped active fibers of the type presented in detail in
patent application EP 677902, in the name of the Applicant, which
is herein incorporated by reference.
[0075] The lengths of active fibers 32 and 36 may be around 7 m and
5 m, respectively.
[0076] For dichroic couplers 33 and 37, fused-fiber couplers may be
used, formed of monomodal fibers at 980 nm and in the 1530-1565 nm
wavelength band, With optical power output variation with respect
to polarization <0.2 dB.
[0077] Dichroic couplers of the type indicated are known and
commercial and are produced, for example, by the aforesaid E-TEK
DYNAMICS.
[0078] Optical isolators 31 and 35 are of the type independent of
the transmission signal polarization, with isolation greater than
35 dB and reflectivity less than -50 dB. The isolators are, for
example, model MDL I-15 PIPT-A S/N 1016 of the firm ISOWAVE, 64
Harding Ave., Dover, N.J. (USA) or model PIFI 1550 IP02 of the
aforesaid E-TEK DYNAMICS.
[0079] Pumping sources 34 and 38 may be, for example, quantum well
lasers with an emission wavelength of .lambda..sub.p=980 nm. The
optical emission power envisaged is around 75 mW for source 34 and
90 mW for source 38.
[0080] Lasers of the type indicated are produced, for example, by
LASERTRON INC. 37 North Avenue, Burlington, Mass. (USA).
[0081] A power amplifier like the one described furnishes, for
example, output power of around 16 dBm, with a noise figure of
around 5 dB.
[0082] The power amplifier described with reference to FIG. 3 uses
counterpropagating pumping for both amplification stages.
Counterpropagating pumping for both stages or for just one of them,
the first stage in particular, are equally possible. The choice of
which configuration to use is left to the skilled in the art,
according to the characteristics of the overall communication
system.
[0083] The optical power amplifier may also be embodied as a
single-stage amplifier, depending on the gain required and the
characteristics of the telecommunication system in which it is to
be used. It is possible, for example, with reference to the device
in FIG. 3, to omit active fiber 36, dichroic coupler 37 and pumping
source 38. This simpler configuration offers less optical output
power and may be sufficient for particular embodiments of the
amplification system, e.g. with a smaller number of communication
channels or with optical fiber sections of limited length
downstream of the amplifier.
[0084] Preamplifiers 6A and 6B of the system in FIG. 1 are, for
example, optical amplifiers of the type that will be described now
with reference to FIG. 4, which represents a two-stage
preamplifier.
[0085] A first amplification stage comprises a first active fiber
44, pumped by a pumping source 42 through a dichroic coupler 43, a
differential attenuator 45, connected to the output of active fiber
44, to attenuate the telecommunication signals without
significantly attenuating the residual pumping radiation, and a
second active fiber 46 pumped by means of said residual pumping
radiation.
[0086] A second amplification stage includes an active fiber 47,
pumped by a pumping source 49 through a dichroic coupler 48.
[0087] An input 410 of the preamplifier, consisting preferably of a
very-low-reflection optical connector, e.g. of the type previously
indicated, is connected to the first amplification stage, and
precisely to a first input of dichroic coupler 43, to a second
input of which pumping source 42 is connected. An output of
dichroic coupler 43 terminates in active fiber 44.
[0088] Input 410 is connected, in the telecommunication system in
FIG. 1, to an optical circulator (7A or 7B). This circulator
permits the unidirectional passage of radiation to the preamplifier
and prevents radiation from exiting that input. The circulator is
thus equivalent to an additional optical isolator connected to the
amplifier input, particularly in limiting interferential noise.
[0089] Differential attenuator 45 is connected between active fiber
44 and active fiber 45. Its function is to attenuate the
telecommunication signals by a predetermined quantity without
significantly attenuating the residual pumping radiation from
active fiber 44. A differential attenuation of the signals with
respect to the pump, in a suitable intermediate position between
two sections of active fiber of an optical amplifier, as described
in patent applications EP567941 and EP695050 in the name of the
Applicant, makes it possible to compress the amplifier dynamics,
i.e. to limit the power variations of the signals output by the
amplifier with respect to the power variations of the input
signals.
[0090] Differential attenuator 45 comprises a dichroic coupler 451
to separate the signals at the telecommunication channel
wavelengths to a first output and the residual radiation at the
wavelength of pumping source 42 to a second output. Said first
output is connected via an optical isolator 452 to a first input of
a dichroic coupler 454. Said second output is connected via a
section of optical fiber to a second input of dichroic coupler 454.
Optical isolator 452 provides an attenuation of around 1 dB to the
telecommunication signals that transit through it, while the
residual pump radiation is not significantly attenuated. The
optical isolator also blocks the counterpropagating radiation, with
the effect of reducing the amplifier noise. A section of
attenuating optical fiber 454, with predetermined attenuation, can
be connected in lieu of the optical isolator, or preferably in
series with it. The characteristics of this attenuating fiber may
be predetermined according to the indications contained in the two
patent applications cited.
[0091] Dichroic coupler 454 combines the residual pump radiation
with the attenuated telecommunication signals to active fiber 46,
which further amplifies the signals.
[0092] An optical isolator 461 is placed between the output of the
first stage and the input of the second stage.
[0093] An output of said isolator terminates in one end of active
fiber 47, while the other end is connected to a dichroic coupler
48. Pumping source 49 is connected to an input of said dichroic
coupler 48 in such a way as to feed active fiber 48. An output of
dichroic coupler 48 is connected, by means of an optical isolator
462, to an output 420 of the preamplifier.
[0094] Although the pumping scheme described (copropagating for the
first stage and counterpropagating for the second stage) is
preferable, other pumping schemes are equally possible.
[0095] The characteristics and type of components of the
preamplifier may generally be selected according to the previous
indications regarding the power amplifiers described.
[0096] In particular, in the case of the preamplifier, the lengths
of active fibers 44, 46 and 47 may be advantageously around 7 m, 3
m and 6 m, respectively.
[0097] Pumping sources 42 and 49 may be, for example, quantum well
lasers with an emission wavelength of .lambda..sub.p=980 nm. The
optical emission power is envisaged at 65 mW for source 42 and 75
mW for source 49.
[0098] A preamplifier like the one described gives, for example,
output power of 16 dBm, with a noise figure of 5 dB.
[0099] The preamplifier may also be embodied as a single stage
amplifier, depending on the gain required and the characteristics
of the telecommunication system in which it is to be used.
[0100] Multiplexers 3A and 3B of the system in FIG. 1 are passive
optical devices, by which the optical signals transmitted on
respective optical fibers are combined in a single fiber. Devices
of this type consist, for example, of fused-fiber-, planar-optics-,
microoptics-couplers and similar. Suitable multiplexers, for
example, are marketed by the aforesaid firm E-TEK DYNAMICS.
[0101] An example of demultiplexer well-suited for use in the
present invention is indicated in FIG. 5A. The figure represents a
demultiplexer well-suited for use in a system with 16 channels,
i.e. 16 independent wavelengths, for each path direction. A similar
scheme may be employed in cases where the system calls for a
different number of channels. The signals input to a port 500 are
separated by means of a 3 dB splitter, 540, to two groups of
cascaded wavelength selective splitters 550 and 560 (briefly
indicated as selective splitters). Each selective splitter is
capable of routing to a first output the signals applied to one of
its inputs with wavelengths centered around one of the transmission
channels employed in the system and of reflecting to a second
output the signals with wavelengths external to that band. Said
second output of each selective splitter is connected to the input
of a successive selective splitter, so as to form a cascaded
connection. In the device illustrated in the figure, corresponding
to demultiplexer 4B of FIG. 1, group 550 includes selective
splitters 501, 503, . . . , 515, selective around wavelengths,
.lambda..sub.1, .lambda..sub.3, . . . , .lambda..sub.15
respectively, while group 560 comprises selective splitters 517,
519, . . . , 531 selective around wavelengths .lambda..sub.17,
.lambda..sub.19, . . . , .lambda..sub.31, respectively. The device
described is well-suited for use as demultiplexer 4B in the
telecommunication system of FIG. 1. A similar device, using
selective splitters at wavelengths .lambda..sub.2, .lambda..sub.4,
. . . , .lambda..sub.32 may be employed to embody demultiplexer 4A
of the telecommunication system in FIG. 1.
[0102] The selective splitters may preferably be of the type
diagramed in detail in FIG. 5B, having four access optical fibers
(input and output ports) designated 591, 592, 593 and 594,
respectively, and containing in the center a selective reflecting
component 595 which acts as a transmission bandpass filter and a
reflective band-suppression filter, i.e. designed to transmit with
low attenuation (e.g. with attenuation lower than 1.5 dB) signals
with wavelengths within a predetermined band and reflecting (with
attenuation of the same order of magnitude) signals with
wavelengths outside that band. A signal input to fiber 591 of the
selective splitter with wavelength .lambda..sub.p inside the
passing band of component 595, for example, is transmitted to fiber
593 and, similarly, signals at .lambda..sub.p are transmitted from
fiber 594 to fiber 592 or, symmetrically, from fiber 593 to fiber
591 and from fiber 592 to fiber 594. A signal input to fiber 591
with wavelength .lambda..sub.r outside that band, on the other
hand, is reflected to fiber 594 and similarly signals at
.lambda..sub.r proceed from fiber 592 to fiber 593 and
symmetrically from fiber 594 to fiber 591 and from fiber 593 to
fiber 592.
[0103] The band of wavelengths, close to a wavelength of minimal
transmission attenuation, which corresponds, in transmission
through selective reflecting component 595, to an attenuation of no
more than 0.5 dB in addition to the minimal attenuation, will be
indicated hereinafter as "0.5 dB passband" of selective reflecting
component 595 or, by extension, as 0.5 dB passband of the selective
splitter.
[0104] Likewise, the band of wavelengths, close to a wavelength of
minimal reflection attenuation, which corresponds, in reflection
through selective reflecting component 595, to an attenuation of no
more than 0.5 dB in addition to the minimal attenuation, will be
indicated hereinafter as "0.5 dB reflected band" of selective
reflecting component 595 or, by extension, as 0.5 dB reflected band
of the selective splitter.
[0105] The selective splitters are selected in a way that, for each
of them, the wavelength of one of the communication channels is
included in the respective 0.5 dB passband, while the wavelengths
of the remaining communication channels are included in the
respective 0.5 dB reflected band.
[0106] By analogy, the band of wavelengths corresponding in
transmission through the selective splitter to an attenuation of no
more than 20 dB in addition to the minimal attenuation is indicated
as a -20 dB passband of the selective splitter.
[0107] Although described with four access fibers, the selective
splitters suitable for the aforesaid use may have only three access
fibers, the fourth (e.g. the one indicated as 594) remaining
unused.
[0108] Selective splitters of the type indicated and well-suited
for use in the present invention are marketed, for example, by
Optical Corporation of America, 170 Locke Drive, Marlborough, Mass.
(USA).
[0109] Selective splitters of the type indicated are now available,
e.g., with a 0.5 dB passband of about 0.7 nm and a 20 dB bandwidth
of about 2.4 nm.
[0110] Selective splitters based on Mach-Zehnder interferometers
employing Bragg fiber-optic gratings, such as the "Mach-Zehnder
based FBG" model produced by INNOVATIVE FIBER, are also suitable
for use in the present invention.
[0111] Of possible use in the present invention are also, for
example, demultiplexers made, according to the general scheme of
FIG. 5A, with groups of cascaded selective splitters integrated on
a single substrate, such as those produced by the aforesaid Optical
Corporation of America.
[0112] Demultiplexers of the type described may be easily adapted
to operate with a number of channels different from that determined
in the system installation phase. It is possible, for example, to
add one or more selective splitters cascaded with the selective
splitters already present, so as to permit the demultiplexing of
additional wavelengths.
[0113] The Applicant observes that the number of independent
channels transmitted in the system may, through the present
invention, be greater than the number of channels that can be
separated by the available demultiplexers. Thus, for example, with
reference to the example described, a total of 32 channels are
transmitted through the system (16 in each direction) using
demultiplexers adapted to separate 16 channels.
[0114] A bidirectional multichannel optical amplifier 9 according
to the present invention, well-suited for use in the
telecommunication system of FIG. 1, will now be described in
greater detail with reference to FIG. 6.
[0115] Multichannel optical amplifiers 93 and 94 connected between
optical circulators 91 and 92 in such a way as to amplify the
signals propagating from transmission station 1A to receiving
station 2B and, respectively, from transmission station 1B to
receiving station 2A, are embodied as wavelength selective optical
amplifiers and namely selective at the wavelengths .lambda..sub.1,
.lambda..sub.3, .lambda..sub.5, . . . .lambda..sub.29,
.lambda..sub.31 and, respectively, .lambda..sub.2, .lambda..sub.4,
.lambda..sub.6, . . . , .lambda..sub.30, .lambda..sub.32.
[0116] In a first stage of amplifier 93, a dichroic coupler 62
feeds the communication signals coming from an input port 641,
connected to an output port of optical circulator 91, and the
pumping radiation, coming from a first optical pumping source 61
connected to dichroic coupler 62, to a first active optical fiber
63, whose output terminates in an input of a dichroic coupler 671.
A first output of dichroic coupler 671 is connected in input to an
optical isolator 672, while a second output of dichroic coupler 671
is connected to an input of a dichroic coupler 675 by means of an
optical fiber section, so as to constitute a low-attenuation path
for the residual pump radiation downstream of active fiber 63.
[0117] A comb filter is connected between the output of optical
isolator 672 and a second input of dichroic coupler 675 by means of
low-reflectivity connectors 673 and 674.
[0118] The comb filter has a passband that includes wavelengths
.lambda..sub.1, .lambda..sub.3, .lambda..sub.5, . . . ,
.lambda..sub.29, .lambda..sub.31 of the signals propagating from
transmission station 1A to receiving station 2B. Wavelengths
.lambda..sub.2, .lambda..sub.4, .lambda..sub.6, . . . ,
.lambda..sub.30, .lambda..sub.32 of the signals propagating in the
system in the opposite direction, on the other hand, are external
to the passband of said comb filter.
[0119] Said comb filter may include, as illustrated in the figure,
an optical circulator 64 with a selective reflection circuit 65
connected to one of its intermediate ports. Said circuit 65
comprises serially connected filters 601, 603, 605, . . . , 629 and
631, with selective reflection at wavelengths .lambda..sub.1,
.lambda..sub.3, .lambda..sub.5, . . . , .lambda..sub.29,
.lambda..sub.31, respectively, and is terminated by a
low-reflectivity termination 650.
[0120] An output of dichroic coupler 675 terminates in a second
active optical fiber 66, which in turn terminates at in input of an
optical isolator 676.
[0121] Said second active fiber 66 is pumped through the residual
pump radiation from first active fiber 63.
[0122] The output of optical isolator 676 is connected to a third
active optical fiber 67. Active fiber 67 is fed with
counterpropagating pumping radiation through a optical pumping
source 69 and a dichroic coupler 68.
[0123] An output of dichroic coupler 68 is connected to an output
port 68, connected to an input port of optical circulator 92.
[0124] In amplifier 93, signals at wavelengths .lambda..sub.1,
.lambda..sub.3, .lambda..sub.5, . . . , .lambda..sub.29,
.lambda..sub.31 input to port 641 are amplified in the first stage
of amplification, transmitted by the comb filter through the
reflection of each signal by one of the selective reflection
filters of circuit 65 and further amplified in the second stage of
amplification.
[0125] Any other signals, or noise, at wavelengths external to the
bands of selective reflection filters 601, 603, . . . , 631, after
passage through the first amplification stage, are transmitted
through circuit 65 without being reflected and are eliminated from
the circuit through low-reflectivity termination 650.
[0126] Multichannel amplifier 94 is similar to multichannel
amplifier 93. For a description of the corresponding parts and the
general functioning of amplifier 94, refer therefore to the
previous description of amplifier 93.
[0127] In amplifier 94, the comb filter has a passband that
includes wavelengths .lambda..sub.2, .lambda..sub.4,
.lambda..sub.6, . . . , .lambda..sub.30, .lambda..sub.32 of the
signals propagating from transmission station IB to reception
station 2A. Wavelengths .lambda..sub.1, .lambda..sub.3,
.lambda..sub.5, . . . , .lambda..sub.29, .lambda..sub.31 of signals
propagating in the system in the opposite direction are external to
the passband of said comb filter.
[0128] This comb filter may comprise, as illustrated in the figure,
an optical circulator 654 with a selective reflection circuit 655
connected to one of its intermediate ports. This circuit 655
includes serially connected filters 602, 604, 606, . . . , 630,
632, with selective reflection at wavelengths .lambda..sub.2,
.lambda..sub.4, .lambda..sub.6, . . . , .lambda..sub.32,
respectively. Reflection circuit 655 is terminated by a
low-reflectivity termination 650.
[0129] Optical amplifiers 93 and 94 described are of the two-stage
type. A first stage of amplification comprises active fiber
sections 63, 653 and 66, 656. Active fibers 63 and 653 are pumped
directly by sources 61 and 651 through dichroic couplers 62 and
652. Active fibers 66 and 656, connected downstream from the comb
filter, are pumped with residual pumping radiation present at the
output of active fibers 63 and 653 by means of the low-attenuation
path created by connecting together dichroic couplers 671, 675 and
681, 685, respectively.
[0130] The signal attenuation by optical isolator 672, optical
circulator 64 and selective reflection circuit 65, connected along
the optical signal path in the section between dichroic couplers
671, 675 and 681, 685, respectively, and the reduced attenuation of
the residual pump radiation compress the signal dynamics in the
first amplifier stage, according to the mechanism previously
illustrated with reference to differential attenuator 45 of the
device in FIG. 4.
[0131] A second stage of amplification comprises active fiber
sections 67 and 675, which are pumped by pumping sources 69 and 659
through dichroic couplers 69 and 659.
[0132] The second stage, operating in saturation, further
compresses the signal dynamics.
[0133] The length of active fiber 67, 657 of the second stage is to
advantage around 2/3 the total length of the active fiber of the
first stage (fiber 63, 66).
[0134] The length of active fiber 66,656, connected downstream from
the comb filter, is to advantage around half the length of active
fiber 63, 653, connected upstream from the comb filter.
[0135] If the active fibers used are of the type previously
indicated with reference to the power amplifier in FIG. 3, for
example, the lengths of active fibers 63 and 653, 66 and 656, 67
and 657 may be around 7 m, 3 m and 6 m, respectively.
[0136] Active fiber 66, 656 may be used to good advantage,
according to the description, to compensate at least in part for
the signal attenuation by the comb filter.
[0137] Said active fiber 66, 656 may be omitted, however,
particularly if the attenuation of the comb filter is sufficiently
low. If fiber 66, 656 is not present in the amplifier, it is also
possible to omit the low-attenuation path for the pumping
radiation, comprising dichroic coupler 671, 675 and 681, 685,
respectively, and the respective connecting optical fibers. In this
case, active fiber 63, 653 is connected directly to the input of
optical isolator 672, 682 and connector 674, 684 directly connects
the input port of optical circulator 64, 654 and the input of
optical isolator 676, 686.
[0138] Optical amplifiers 93, 94, depending on the required gain
and the characteristics of the telecommunication system in which it
is to be used, may also be embodied as single-stage amplifiers. It
is possible, for example, with reference to device 93 of FIG. 6, to
omit the second stage comprising active fiber 67, dichroic coupler
68 and pump source 69. This simpler configuration may be sufficient
to cover shorter sections of optical line.
[0139] Although the embodiment described with reference to FIG. 6
is preferable, particularly in terms of noise figure and output
power, another alternative would be to connect the comb filter
downstream or upstream from the power amplifier, respectively.
[0140] A bidirectional multichannel optical amplifier 9 may be
realized by using, where no otherwise specified, components similar
to those previously described with reference to the devices in
FIGS. 3 and 4.
[0141] Pump sources 61, 69, 651, 659, for example, may be quantum
well lasers with an emission wavelength .lambda..sub.p=980 nm. The
optical emission power envisaged is around 90 mW for each
source.
[0142] Optical connectors 676, 674, 683, 684, for example, are
connectors with reflectivity of less than -40 dB. Connectors of
this type are produced, for example, by the aforesaid firm SEIKOH
GIKEN.
[0143] Selective reflection filters well-suited for use in the
present invention, for example, are distributed Bragg reflection
optical waveguide filters. They reflect the radiation within a
narrow wavelength band and transmit the radiation outside said
band. They consist of a portion of optical waveguide, e.g. optical
fiber, along which an optical parameter, e.g. the refractive index,
has a periodic variation. If the reflected portions of the signal
at each change of index are mutually in phase, constructive
interference occurs and the incident signal is reflected. The
condition of constructive interference, corresponding to maximum
reflection, is expressed by the relationship
2.multidot.I=.lambda..sub.s/n, where I indicates the pitch of the
grating formed by the variations in the index of refraction,
.lambda..sub.s the wavelength of the incident radiation and n the
refractive index of the waveguide core. The phenomenon described is
indicated in the literature as distributed Bragg reflection.
[0144] A periodic variation of the refractive index may be obtained
by known techniques, e.g. by exposing a portion of optical fiber,
deprived of its protective coating, to the interference fringes
formed by an intense UV beam (like that generated by an excimer
laser, a frequency-doubled argon laser or a frequency-quadrupled
Nd:YAG laser) made to self-interfere by a suitable interferometric
system, e.g. by means of a silica phase mask, as described in U.S.
Pat. No. 5,351,321. The fiber, and particularly the core, are thus
exposed to UV radiation of an intensity varying periodically along
the optical axis. In the areas of the core reached by the UV
radiation the Ge--O bonds are partially broken causing a permanent
change in the refraction index.
[0145] The central wavelength of the reflected band can be
determined at will by selecting a grating pitch that results in the
constructive interference relationship.
[0146] With this technique it is possible to obtain filters with a
-3 dB reflected wavelength band of only 0.2-0.3 nm, reflectivity at
the center of the band almost up to 100%, a central wavelength of
the reflected band that can be determined in the construction phase
within +/-0.1 nm and a temperature variation of the central
wavelength of the band not greater than 0.02 nm/.degree. C.
[0147] Optical distributed Bragg reflection filters with a broader
reflection band can be realized by gradually chirping the grating
pitch along its extension between two values, corresponding to the
wavelengths that delimit the desired reflection band.
[0148] Optical fiber distributed Bragg reflection filters with
chirped grating are known, for example, from the article by P. C.
Hill et al. published in Electronic Letters, vol. 30 no. 14, Jul.
7, 1994, pages 1172-74.
[0149] The gradual variation of the grating pitch, in a distributed
Bragg reflection filter, may also be employed to realize filters
capable of compensating for the delay (or advance) of some
chromatic components of an optical signal with respect to others.
For this reason, components of a signal with different wavelengths
must be reflected by different portions of the same grating,
displaced on an optical path so as to compensate for said delay or
said advance.
[0150] Chromatic dispersion, i.e. the delay (or advance) per
wavelength unit of a grating having a pitch that may vary between
two extreme values, depends not only on the width of the reflected
band but also on the length of the grating or, in greater detail,
on a quantity equal to twice the length of the grating multiplied
by the effective index of refraction of the means in which it is
embodied. This quantity corresponds to the difference between the
optical paths of the signal chromatic components which are
reflected close to the two extremes of the grating.
[0151] The use of distributed Bragg reflection filters for
compensating chromatic dispersion is known, for example, from the
aforementioned article by F. Ouellette published in Optics Letters
or from U.S. Pat. No. 4,953,939.
[0152] To compensate for the chromatic dispersion at the
communication signal wavelengths, it is possible to use as
selective reflection filters 601, 603, . . . , 631 and 602, 604, .
. . , 632 optical fiber distributed Bragg reflection filters with
chirped grating.
[0153] In this case, each of the filters will be realized with a
central wavelength and passband width suitable to reflect radiation
corresponding to one of the communication channels, and with
dispersion characteristics that compensate for the chromatic
dispersion of the corresponding communication channel.
[0154] Depending on the conditions under which the device is used,
the filters may be realized in such a way as to provide the
reflected communication signal with a chromatic dispersion equal in
absolute value, and of opposite sign, to that (estimated or
measured) accumulated by the signal through the fiber sections it
has traveled, or such as to overcompensate for the dispersion
accumulated by the signal, so that the dispersion is nullified at a
successive point on the optical signal path, including an
additional section of optical fiber.
[0155] If the amplifier is used under conditions characterized by
significant variations in temperature, it may be advisable to
thermally stabilize fiber optic filters 601, 603, . . . , 631 and
602, 604, . . . , 632.
[0156] The optical output power of an optical amplifier 93 or 94 as
described is, in an example, about 16 dBm under operating
conditions, with circulators 91, 92 connected to the two extremes
and with optical input power of -10 dBm. The noise figure is around
5 dB.
[0157] The Applicant has observed that optical circulators 91 and
92 permit radiation to enter and exit in only one direction for
each of optical amplifiers 93, 94 and precisely only the radiation
propagating from transmission station 1A to receiving station 2B
for amplifier 93 and only the radiation propagating from
transmission station 1B to receiving station 2A for amplifier
94.
[0158] Optical circulators 91 and 92 therefore act as
unidirectional components placed at the input and output of the two
stages of optical amplifiers 93 and 94 and reduce the noise,
particularly that due to counterpropagating spontaneous emission,
Rayleigh and Brillouin scattering and their respective reflections
along the communication line.
[0159] In addition to permitting the bidirectional amplification of
the signals, the bidirectional amplifier described attenuates the
propagating amplified spontaneous emission (ASE) along with the
signals. In amplifiers 93 and 94, the ASE components coming from
inputs 641 and 643 and those generated in active fibers 63 and 653
are removed by the respective comb filters and do not propagate to
active fibers 66 and 656.
[0160] The Applicant has determined that bidirectional amplifier 9
functions stably without oscillations and with negligible
interferometric noise. This is thought to derive from the fact that
the arrangement of the signal wavelengths, along with the spectral
characteristics of the comb filters, prevents the creation of
possible feedback rings, including amplifiers 93 and 94, which
might be formed in the presence of back-reflections along the
optical fibers of line 8, e.g. by connectors of optical circulators
91 and 92 with said optical fiber of line 8.
[0161] An optical amplifier according to the present invention is
well-suited for use not only along communication lines configured
to have low reflections (e.g. employing low-reflection optical
connectors and welds) but also along optical communication lines
already installed and in the presence of components with
non-negligible residual reflectivity, particularly if they are used
along fiber-optic transmission lines in which the amplifier is
connected to the line fibers by means of optical connectors, which
may be of the type that transmit most of the power of the signals
transiting through them, and thus ensure the optical continuity of
said signals, but which under some conditions reflect back a small
portion of them (e.g. in case of an imperfect clamping caused by
incorrect positioning of the two fiber ends inside them).
[0162] Nonetheless, to obtain a high signal/noise ratio in the
transmission along the telecommunication system, such as to permit
transmission at velocities greater than or equal to 2.5 Gb/s, the
optical connections linking an optical amplifier 9 and optical
communication line 8 have preferably a reflectivity of less than
-31 dB, more preferably less than 40 dB. Furthermore, to facilitate
the operations of line installation and maintenance, these optical
connections should be realized with optical connectors.
[0163] The Applicant has determined that an optical amplifier of
the type described minimizes the gain tilt, a phenomenon caused by
the characteristics of the doped fiber and, in particular, by the
relative level of amplified spontaneous emission (ASE) and the
signals along the communication line and in the amplifiers cascaded
along it, which consists of a variation in gain with the wavelength
and results in different gains for the various channels.
[0164] Exploiting the small residual attenuation of the selective
reflection filters in the band transmitted (about 0.1 dB, for
example, for passage in each direction through each Bragg grating
selective reflection filter), it is possible to arrange said
filters, in the selective reflection circuit that is part of the
comb filter, in an order such that it compensates for the
differences in gain.
[0165] In greater detail, the channels subject to less gain can be
attenuated to a lesser degree by connecting the selective
reflection filters related to those channels in proximity to the
extremity of the selective reflection circuit that is connected to
optical circulator 64 (the signals are reflected after passing
through a limited number of selective reflection filters, thus with
less attenuation), and the channels subject to greater gain can be
attenuated to a greater degree by connecting the respective
selective reflection filters in proximity to the opposite extremity
of the selective reflection circuit.
[0166] Should it be necessary to compensate for the gain tilt to a
greater extent than permitted by the selective attenuation provided
by the filters, sections of optical fiber with calibrated
attenuation may be connected between the selective reflection
filters.
[0167] To compensate for a predetermined difference in gain, in
output to an amplifier, between signals of different wavelengths,
the difference in attenuation of the two signals in the comb filter
must generally be greater, in absolute value, than said
predetermined difference in output gain.
[0168] In the configuration described with reference to FIG. 6, the
distances between the filters connected along the selective
reflection circuit increase as the wavelength increases, so that
the attenuation of each channel is attenuated by 0.2 dB more (due
to the double passage) than the adjacent channel at a lower
wavelength.
[0169] In one example, the Applicant evaluated the functioning of a
bidirectional multichannel telecommunication system like the one
described, in a configuration including five sections of optical
fiber 8, each with maximum total attenuation of 26 dB (including
attenuation at the optical splices), connected by four
bidirectional amplifiers 9, each of the type described.
[0170] The Applicant has determined that this communication system
permits the simultaneous transmission of 16 channels in each
direction of propagation at a bit rate of 2.5 Gb/s, with a minimum
signal/noise ratio of 13 dB (measured on an 0.5 nm band).
[0171] In a second example, the Applicant evaluated the functioning
of a bidirectional multichannel communication system like the one
described but configured to operate with 8 wavelengths in each
direction of propagation, with the wavelengths of the signals
propagating in one direction staggered with respect to those of the
signals propagating in the opposite direction. The configuration
considered includes five section of fiber-optic line 8, each with a
maximum total attenuation of 28 dB (including the attenuation of
the optical junctions), connected by four bidirectional amplifiers
9, each of the type described.
[0172] The Applicant determined that said communication system
permits the simultaneous transmission of 8 channels in each
direction of propagation at a bit rate of 2.5 Gb/s, with a minimum
signal/noise ratio of 13 dB (measured on a 0.5 nm band).
[0173] In another example, regarding a communication system similar
to the one in the second example but where the total maximum
attenuation of each fiber-optic line section is 23 dB (including
the attenuation of the optical splices), and in which the four
bidirectional amplifiers include chromatic dispersion compensation
means of the type indicated earlier, the Applicant determined that
it is possible to transmit 8 channels simultaneously in each
direction of propagation at a bit rate of 10 Gb/s, with a minimum
signal/noise ratio of 18 dB (measured on a 0.5 nm band).
[0174] It is known that an optical communication system may assume
the structure of an optical network connecting a number of stations
to each other. Optical network is generally intended here to mean a
set of fiber-optic transmission lines and their respective stations
of interconnection, also known as interchange nodes. In the
interchange nodes the optical signals can be routed from one of the
transmission lines linked to the node to one or more of the other
transmission lines linked to the node. Nodes for adding and
dropping optical signals to or from the network may be positioned
either along the transmission lines or at the interchange nodes.
Some of the transmission lines in this optical network, in
particular, may have a ring structure.
[0175] A particular example of optical network with nodes for
adding or dropping signals is that of a WDM communication system
comprising a fiber-optic line extended between transmission and
receiving stations and intermediate stations for adding/dropping
signals placed along the line. The signals at various wavelengths
emitted by a transmission station propagate along an optical fiber,
possibly through amplifiers, e.g. of the active optical fiber type,
up to an intermediate signal addition/dropping station, which may
be configured in such a way that the radiation to some of the
signal wavelengths is dropped from the communication line and
routed to specific receivers (which, for example, convert the
signals into electrical form), while at the same time radiation ta
one or more of the same wavelengths, generally modulated by
transmission signals (e.g. in electrical form) present at the input
of the intermediate station, is inserted into the communication
line downstream from the dropping point. The optical radiation
output from the intermediate station is transmitted along an
additional section of optical fiber, and possibly through
additional amplifiers and intermediate stations for adding/dropping
optical signals, until it reaches a receiving station.
[0176] Each wavelength constitutes an independent communication
channel. The optical telecommunication system may be configured in
such a way that it transmits optical signals separately between
pairs of stations included between the terminal stations and the
stations placed along the line. It is also possible to transmit
independent signals with the same wavelength along lines without
common sections.
[0177] In this communication line it is possible to add or drop
signals at various points (nodes) along the line at some of the
communication wavelengths, so that they travel only a portion of
the line extension.
[0178] A scheme of a multichannel optical amplifier comprising a
device of the first type for adding/dropping optical signals will
now be described with reference to FIG. 7.
[0179] The figure represents an optical amplifier 93' suitable for
use in a telecommunication system of the type described with
reference to FIG. 1, in lieu of one or more optical amplifiers 93
of said system. In the example indicated in FIG. 7, amplifier 93'
is suitable for amplifying optical signals propagating from
transmission station 1A to receiving station 2B, at some of
wavelengths .lambda..sub.1, .lambda..sub.3, . . . ,
.lambda..sub.31, unselected, to drop from the optical communication
line signals at the remaining wavelengths .lambda..sub.1,
.lambda..sub.3, . . . , .lambda..sub.31, selected, and to add to
said line new signals at some or all of the selected wavelengths.
The scheme in FIG. 7 may be modified, by known techniques, in such
a way as to adapt it to the wavelengths to be
amplified/dropped/added in each case of interest. It is possible,
for example, to make an amplifier, not represented in the figure,
to amplify optical signals propagating from transmission station 1B
to receiving station 2A at some of wavelengths .lambda..sub.2,
.lambda..sub.4, . . . , .lambda..sub.32, unselected, to drop from
the optical communication line signals at the remaining wavelengths
.lambda..sub.2, .lambda..sub.4, . . . , .lambda..sub.32, selected,
and to add to said line new signals at some or all of the selected
wavelengths.
[0180] Optical amplifier 93' includes one or more amplification
stages between an input port 741 and an output port 742. The
example indicates two amplification stages 71 and 72 that may, for
example, be analogous to the amplification stages previous
described with reference to amplifier 93 of FIG. 6.
[0181] A device 750 for adding/dropping optical signals and
filtering is connected in an intermediate position between input
port 741 and output port 742.
[0182] The position of said device 750 may be determined as
previously indicated in relation to the amplifier of FIG. 6.
[0183] A device for adding and dropping signals to and from an
optical fiber will now be described with reference to FIG. 8. Said
device also performs a signal filtering action.
[0184] It comprises an input port 950 for optical signals at
wavelengths .lambda..sub.1, .lambda..sub.3, . . . , .lambda..sub.31
connected to a first port of a first optical circulator 951. A
second port of the optical circulator is connected to a selective
reflection circuit 959, including optical filters 601, 605, . . . ,
629, cascade-coupled and with selective reflections to wavelengths
.lambda..sub.1, .lambda..sub.5, . . . , .lambda..sub.29,
respectively. The output of selective reflection circuit 959 is
optically connected to a first input of a first wavelength
selective switch 953. A first output of selective switch 953 is
connected to a first port of a second optical circulator 961.
[0185] A second port of optical circulator 961 is connected to a
selective reflection circuit 969, comprising optical filters 603,
607, . . . , 631, cascaded and with selective reflection to
wavelengths .lambda..sub.3, .lambda..sub.7, . . . ,
.lambda..sub.31. The output of selective reflection circuit 969 is
optically coupled to a first input of a second wavelength selective
switch 963. A first output of selective switch 963 is coupled to a
third port of the first optical circulator 951.
[0186] The wavelengths of the series .lambda..sub.1,
.lambda..sub.5, . . . , .lambda..sub.29 are staggered with respect
to those of series .lambda..sub.3, .lambda..sub.7, . . . ,
.lambda..sub.31.
[0187] The separation between the wavelengths of adjacent channels
between each of the two series is at least twice the minimum
separation of adjacent channels between the signals at wavelengths
.lambda..sub.1, .lambda..sub.3, . . . , .lambda..sub.29,
.lambda..sub.31 input to the device.
[0188] A third port of the second optical circulator 961 is
connected to an output port 960.
[0189] Signals at one or more of wavelengths .lambda..sub.1,
.lambda..sub.3, . . . , .lambda..sub.31, coming from outside, e.g.
from transmitters 954, are multiplexed by means of a multiplexer
955 and routed to a second input of the first selective switch 953
through an optical amplifier 956.
[0190] The signals present at a second output of the first
selective switch 953 are demultiplexed by means of a demultiplexer
957 and routed to an external user, consisting, for example of
optical receivers 958 connected to the demultiplexer outputs.
[0191] Signals at one or more of wavelengths .lambda..sub.1,
.lambda..sub.5, . . . , .lambda..sub.29, coming from outside, e.g.
from transmitters 964, are multiplexed by means of a multiplexer
965 and routed to a second input of the first selective switch 963,
possibly through an optical amplifier 966.
[0192] The signals present at a second output of the first
wavelength selective switch 963 are demultiplexed by means of a
demultiplexer 967 and routed to an external user, consisting, for
example, of optical receivers 968 connected to the demultiplexer
outputs.
[0193] The device in FIG. 8 functions as follows.
[0194] The signals input to port 950 are transmitted through
optical circulator 951 to selective reflection circuit 959. The
signals at wavelengths .lambda..sub.1, .lambda..sub.5, . . . ,
.lambda..sub.29 are reflected by filters 601, 605, . . . , 629 to
optical circulator 951, which transmits them to the first input of
selective switch 963. The selective switch is suitable for
transmitting signals at some of the wavelengths, selected by means
of specific control signals, to the second output and for
transmitting the signals at the remaining wavelengths to the first
output The signals reaching the second output are separated onto
different optical paths, based on their respective wavelengths, by
means of demultiplexer 967, and sent to respective receivers
968.
[0195] The signals at the selected wavelengths (included among
wavelengths .lambda..sub.1, .lambda..sub.5, . . . ,
.lambda..sub.29) are thus separated from the remaining signals
input at port 960 of the device. Signals at the same selected
wavelengths, generated by transmitters 964, combined on to a single
optical path by means of multiplexer 965 and amplified, if
required, by optical amplifier 966, are sent by selective switch
963 from the second input port to the first output port of said
switch. At the first output port of selective switch 963,
therefore, the signals at the unselected wavelengths entering from
the first input port of said selective switch are overlapped with
signals at the selected wavelengths coming from transmitters
964.
[0196] Selective switch 963, as a whole, drops the incoming signals
at the selected wavelengths and transmits in output the signals at
the unselected wavelengths, to which the new signals at the
selected wavelengths are added.
[0197] The signals at wavelengths .lambda..sub.1, .lambda..sub.5, .
. . , .lambda..sub.{square root}output by selective switch 963 are
then transmitted through selective reflection circuit 969 (said
circuit 969, in fact, reflects none of wavelengths .lambda..sub.1,
.lambda..sub.5, . . . , .lambda..sub.29) and, through optical
circulator 961 are sent to output port 960.
[0198] The remaining signals input to port 950, with wavelengths
.lambda..sub.3, .lambda..sub.7, . . . , .lambda..sub.31, follow
partially different optical paths.
[0199] With greater detail, these signals reach selective
reflection circuit 959, through optical circulator 951, and pass
through it without being reflected, and thus reach the first input
of selective switch 953. Said selective switch, similar to what was
previous indicated for selective switch 963, drop to the second
output the signals at some of wavelengths .lambda..sub.3,
.lambda..sub.7, . . . , .lambda..sub.31 selected by means of
control signals sent to said selective switch. The signals at the
unselected wavelengths are routed to the first output of the
selective switch, while signals at the selected wavelengths,
present at the fist output of the selective switch, are overlapped
at said first output with said signals at the unselected
wavelengths.
[0200] The signals at wavelengths .lambda..sub.3, .lambda..sub.7, .
. . , .lambda..sub.31 are subsequently transmitted through optical
circulator 961 to selective reflection circuit 969. Filters 603,
607, . . . , 631 in said circuit reflect the signal to optical
circulator 961, which sent them to output 960. Thus at said output
960 signals at wavelengths .lambda..sub.3, .lambda..sub.7, . . . ,
.lambda..sub.31 are overlapped with signals at wavelengths
.lambda..sub.1, .lambda..sub.7, . . . , .lambda..sub.29.
[0201] Radiation at wavelengths intermediate between those
indicated, input from port 950, follows an optical path through
optical circulator 951, selective reflection circuit 959, selective
switch 953, optical circulator 961, selective reflection circuit
969, selective switch 963, optical circulator 951 and is them
reflected back to port 950. This radiation at intermediate
wavelengths is not transmitted, therefore, to output port 960, and
the signals output by the device are filtered of any noise present
at wavelengths intermediate between those of the signals.
[0202] The intermediate wavelength radiation reflected back to port
950 is attenuated, with respect to the input radiation at the same
wavelengths, to an extent corresponding to the sum of the
attenuation of the passed-through optical components. In the
configuration described, for example, with the use of 8 cascaded
selective reflection filters for each selective reflection circuit,
the optical circulators and selective switches of the type
described below, the maximum attenuation is around 9-10 dB.
[0203] Optical circulators 951, 961, selective reflection filters
601, . . . , 631, transmitters 954, 964, multiplexers 955, 965,
demultiplexers 957, 967 and receivers 958, 969 may be of the types
described earlier.
[0204] Optical circulators 956, 966 are suitable for bringing the
signals coming from transmitters 954 to a sufficient power level,
with respect to that of the optical signals coming from input port
950, so that the relative power levels at output port 960 are
equalized.
[0205] Wavelength selective switches 953, 963 are optical
components suitable for transmitting optical signals between the
two inputs and the two outputs based on the signal wavelengths and
on appropriate control signals. By means of said control signals,
input to each of these switches, it is possible to modify their
transmission state, independently for each wavelength, in one
of-the following two modes: bar mode, corresponding to the direct
connection of said first input with said first output, and cross
mode, corresponding to the connection of said first input with said
second output and of said second input with said first output,
respectively.
[0206] A selective switch is to advantage a 2.times.2 wavelength
selective acousto-optical switch with polarization-independent
response.
[0207] Integrated acoustooptical devices are known whose
functioning is based on the interaction between light signals,
propagating in waveguides made on a substrate of a birefringent
photoelastic material, and acoustic waves propagating on the
surface of the substrate, generated by means of appropriate
transducers. The interaction between a polarized optical signal and
an acoustic signal produces a conversion of signal polarization,
i.e. rotation of the mutually orthogonal TE (transverse electrical)
and TM (transverse magnetic) polarization components.
[0208] In said acoustooptical devices it is possible to tune the
spectral response curve by controlling the frequency of the
acoustic waves, making the devices suitable for use as switches in
wavelength division multiplexed optical telecommunication networks.
With these devices the signal selection can be modified without
varying the component wiring. They also permit the simultaneous
switching and selection of different signals or channels, if the
acoustic wave propagating on the substrate surface is the
superposition of different acoustic waves. In fact, the switches
perform the combined switching of the signals at the wavelengths
corresponding to the frequencies applied simultaneously to the
electrodes of the electroacoustic transducers.
[0209] If a channel with a given wavelength is selected, input
optical signals at that wavelength are routed to the corresponding
cross state output (switch in the cross state). The unselected
signals are routed from an input to the corresponding direct output
(switch in the bar state).
[0210] FIG. 9 shows one embodiment of an acoustooptical switch 201.
The switch includes a substrate 101 of birefringent photoelastic
material, consisting of lithium niobate (LiNbO.sub.3).
[0211] Substrate 101 contains two branches of optical input
waveguide 102 and 103, whose extremities 104 and 105 lodge the two
input ports 202 and 204, to which respective connecting optical
fibers 106 and 107 can be connected through known connection or
"pigtailing" devices, schematically represented in the figure.
[0212] To allow connection to said optical fibers (with diameter of
around 250 microns), ports 106 and 107 are preferably at least 125
.mu.m apart.
[0213] Substrate 101 contains two polarization selective elements
108 and 109, a conversion stage 110 and two output optical
waveguide branches 111 and 112, bearing at extremities 113, 114 the
respective output ports 203, 205, through which respective output
optical fibers 115, 116 are connected.
[0214] Polarization selective elements 108 and 109 consist
preferably of polarization splitters, realized by means of
evanescent-wave directional couplers that can separate into two
output waveguides two respective polarizations fed to a common
input and, respectively, combine in a common output waveguide two
respective polarizations fed to two separate input waveguides. In
particular, each of them comprises a central optical waveguide, 117
and 118 respectively, and pairs of input and output optical
waveguides 119, 120, 121, 122 and 123, 124, 125, 126
respectively.
[0215] Conversion stage 110 comprises two parallel optical
waveguide branches 127 and 128, connected to output waveguide pair
121 and 122 of polarization splitter 108 and to input waveguide
pair 123 and 124 of polarization splitter 109. It also comprises an
acoustic waveguide 129, containing optical waveguide branches 127
and 128 and an electroacoustic transducer 130, formed of a pair of
interdigital electrodes for generating an RF surface acoustic
wave.
[0216] Conveniently, transducer 130 is placed in an acoustic
waveguide 131 communicating with acoustic waveguide 129, so as to
form an acoustic coupler.
[0217] Acoustic absorber 133 is placed at the extremity of another
acoustic waveguide 132, for receiving the acoustic signal from
acoustic waveguide 129. Acoustic waveguides 129, 131 and 132 are
delimited by zones 150, 151, 152 and 153 in correspondence to which
the substrate is doped so as to make the propagation velocity of
the acoustic waves higher than in guides 129, 131 and 132,
confining the acoustic signal in the guides.
[0218] The complex formed by electroacoustic transducer 130,
acoustic waveguides 129, 131, 132 and the optical waveguides
contained in acoustic guide 129 constitutes an acoustooptical
converter 140, by which the acoustic signal interacts with the
optical signals.
[0219] The switch of FIG. 9 functions in the following manner.
[0220] When no voltage is applied to electroacoustic transducer
130, the switch is in the off-state and in direct transmission
condition (bar-state), in which there is direct correspondence
between input ports 202 and 204 and output ports 203 and 205,
respectively.
[0221] The light signals enter by ports 202 and 204 and enter
polarization splitter 108, where the TE and TM polarization
components are separated into waveguides 121 and 122, pass
unaltered through branches 127 and 128 of conversion stage 110 and
are then sent to polarization splitter 109, where the polarization
components are recombined, sending the signals to waveguides 125
and 126, so that the signals entering by ports 202 and 204 exit
unchanged from ports 203 and 205.
[0222] Applying an appropriate switching signal to the electrodes
of transducer 130, the switch is placed in the on-state and passes
into cross-transmission condition (cross-state) for the selected
wavelengths, in which input ports 202 and 204 are in correspondence
with crossed output ports 205 and 203, respectively.
[0223] To that end, transducer 130 generates an RF surface acoustic
wave with acoustic driving frequencies f.sub.ac (about 174.+-.10
MHz for devices operating around 1550 nm and 210.+-.10 MHz for
those operating around 1300 nm) corresponding to the optical
resonance wavelengths at which the TE TM or TM TE polarization
conversions take place for one or more predetermined signal
wavelengths, for which switching is required.
[0224] The light signals enter polarization splitter 108, where
polarization components TE and TM are separated and pass through
branches 127 and 128 of conversion stage 110, where those of
signals at the aforesaid predetermined wavelengths are transformed
into their orthogonal components.
[0225] Polarization components TE and TM are then sent to
polarization splitter 109 so that the selected polarization
components coming from input port 202 exit from output port 205,
along with the unselected components coming from port 204, and the
selected polarization components coming from input port 204 exit
from output port 203, along with the unselected components coming
from port 202.
[0226] In this way the signals, which undergo a polarization
conversion in conversion stage 110, are guided to the cross-state,
producing the total switching function, while those that have not
interacted with the acoustic wave pass unaltered.
[0227] In a particular form of embodiment, illustrated in FIG. 10,
substrate 101 also contains a compensator 160, comprising two
parallel optical waveguide branches 161 and 162, connected at one
extremity to input ports 202 and 204 and at the other extremity to
branches 119 and 120 of polarization splitter 108.
[0228] The two optical waveguide branches 161 and 162 are contained
within acoustic waveguide 129 of an acoustooptical converter 164,
similar in structure to converter 140 described earlier, whose
components are indicated with the same references.
[0229] In this embodiment, the input signals with the TE and TM
polarization components travel through branches 161 and 162 of
compensator 160 and, when converter 164 is switched on, are
transformed into their orthogonal components, remaining
combined.
[0230] Subsequently the signals enter a converter 140 contained on
the same substrate and similar to that of the device in FIG. 9. In
it the TE and TM polarization components are converted back into
their original polarization state.
[0231] In that embodiment, the frequency shifts generated in the
two TE and TM components of the signal as a result of
acoustooptical interaction in the conversion stage should be
compensated by the opposite shifts that take place in the
compensation stage.
[0232] In an acoustooptical switch of the type indicated, made by
the Applicant, the bandwidth for an attenuation of 20 dB, in
correspondence to the cross-state, is around 2 nm.
[0233] The maximum attenuation of said switch is around 5 dB,
including the attenuation of the respective optical
connections.
[0234] Said acoustooptical switch can therefore be employed in the
device in FIG. 8 if the separation between adjacent channels at
wavelengths .lambda..sub.3, .lambda..sub.7, . . . , .lambda..sub.31
and between adjacent channels at wavelengths .lambda..sub.1,
.lambda..sub.5, . . . , .lambda..sub.29 is greater than or equal to
2 nm.
[0235] On the whole, acoustooptical switches of the type indicated
may be employed in the device of FIG. 8, if the separation between
adjacent channels, at all the wavelengths .lambda..sub.1,
.lambda..sub.3, . . . , .lambda..sub.31 of the signal entering the
device, is greater than or equal to 1 nm.
[0236] If the device of FIG. 8, having selective switches of the
type described, is employed in an optical telecommunication system,
the selective switches may be controlled by the supervision system
of the communication system. In this way it is possible to rapidly
reconfigure the wavelengths and number of channels to be dropped or
added at the various nodes of the system, to satisfy changing
requirements of traffic or breakdowns.
[0237] As a alternative, it is possible to employ, in lieu of the
selective switch 953, 956, devices of another type that permit the
adding and dropping of signals at some selected wavelengths to and
from an optical line. These devices may also be of the passive
type. In this case, the selection of wavelengths relative to the
signals dropped and/or added to and from the line if made by
selecting passive components with the desired spectral
characteristics. A configuration of this type, al though a change
in channel selection requires the replacement of optical
components, offers the advantage of lower cost.
[0238] The Applicant observes that in the described device of FIG.
8 it is possible to use wavelength selective switches 953, 963 with
a relative low wavelength resolution. In fact, it is sufficient for
the selective switches to be suitable for separating signals
differing in wavelength by double the minimum wavelength distance
between signals input to the device.
* * * * *