Note: Descriptions are shown in the official language in which they were submitted.
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, OPTICAL AMPLIFYING UNIT AND OPTICAL TRANSMISSION SYSTEM
It is an object of the present invention to provide an optical amplifying unit
to
be used for optical telecommunications. The invention also relates to an
optical
transmission system, more particularly a wavelength division multiplexing
(WDM)
optical transmission system, which uses the above-mentioned optical amplifying
unit.
The optical amplifying unit of the invention is also adapted to be used in
analog CATV
systems.
In WDM optical transmission systems, transmission signals including several
optical channels are sent over a same line (that includes at least an optical
amplifier)
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.
Present-day long-distance high-capacity optical transmission systems use
optical fiber amplifiers that, differently from previously used electronic
regenerators,
do not need OEIEO conversion. An optical fiber amplifier includes an optical
fiber of
preset length, having the core doped with one or more rare earths so as to
amplify
optical signals by stimulated emission when excited by pump radiation.
Optical fibers doped with erbium (Er) have been developed for use as both
optical amplifiers and lasers. These devices are of considerable importance
since
their operating wavelength coincides with the third window for optical fiber
communications, around 1550 nm. EP patent application No. 98110594.3 in the
name of the Applicant proposes a thirty-two channels WDM optical transmission
system that uses erbium-doped fiber amplifiers (EDFAs) in the wavelength bands
1529-1535 nm and 1541-1561 nm.
Several methods have been proposed to improve the system performances,
for example in terms of amplification gain and amplification bandwidth.
One technique for improving the system performances consists in co-doping
an erbium-doped amplification fiber with ytterbium (Yb). Co-doping an active
fiber
with erbium and ytterbium not only broadens the pump absorption band from 800
nm
to 1100 nm, offering greater flexibility in selection of the pump wavelength,
but also
greatly increases the ground state absorption rate due to the higher
absorption cross
section and dopant solubility of ytterbium. The ytterbium ions absorb much of
the
pump light and the subsequent cross relaxation between adjacent ions of erbium
and
ytterbium allows the absorbed energy to be transferred to the erbium system.
As
described in Grubb et al., "+24.6 dBm output power ErIYb co-doped optical
amplifier
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pumped by diode-pumped Nd:YLF laser", Electronics Letters, 1992, 28, (13) pp.
1275-1276, and in Maker, Ferguson, "1.56 um Yb-sensitized Er fibre laser
pumped by
diode-pumped Nd:YAG and Nd:YLF lasers", Electronics Letters, 1988, 24, (18),
pp.
1160-1161, the co-doping technique may be applied to efficiently excite fiber
amplifiers and lasers through direct pumping in the long wavelength tail of
ytterbium
absorption spectrum. This pumping is preferably performed by means of diode-
pumped solid state lasers, for example 1047 nm Nd:YLF lasers or 1064 nm Nd:YAG
lasers.
Using an erbium and ytterbium co-doped amplification fiber to amplify
communication signals is further described in European patent application EP 0
803
944 A2 and in US 5,225,925. EP 0 803 944 A2 refers to a multistage Er-doped
fiber
amplifier (EDFA) operating in the wavelength band 1544-1562 nm and comprising
a
first stage that includes Er and AI and a second stage that includes Er and a
further
rare earth element, for example Yb. Such multistage EDFA can have advantageous
characteristics in the cited wavelength band over the all-erbium amplification
systems, e.g. a relatively wide flat gain region,, and relatively high output
power,
without significant degradation of the noise figure. However, the Applicant
noted that
the amplifier proposed in EP 0 803 944 A2 offers no advantages in terms of
number
of transmitted channels, the amplification bandwidth being still limited to
the relatively
narrow (and largely exploited) 1544-1562 nm band. Furthermore, the ENYb second
stage is pumped by means of a diode-pumped Nd-doped fiber laser emitting at
1064
nm. This pump source, largely used for the excitation of mono-modal
amplification
fiber, is relatively expensive and cumbersome.
US 5,225,925 relates to an optical fiber for amplifying or sourcing a light
signal
in a single transverse mode. The fiber comprises a host glass doped with
erbium (Er)
and a sensitizer such as ytterbium (Yb) or iron (Fe). Preferably the host
glass is a
doped silica glass (e.g. phosphate or borate doped). The Applicant noted that
US
5,225,925 proposes an amplification fiber that, due to the shape of its gain
curve, is
particularly adapted for the transmission of a single channel at 1535 nm but
is not
suitable for WDM transmissions. Moreover, such an amplification fiber is
adapted to
be pumped by means of a diode-pumped Nd-doped fiber laser that has the above
mentioned disadvantages.
Neither EP 0 803 944 A2 nor US 5,225,925 address amplification by an ErIYb
co-doped optical amplifier of a signal in a wavelength band different from the
transmission band around 1550 nm.
An improvement of Er/Yb amplification fibers has been obtained by means of
'
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the cladding pumping technique, which consist in pumping the active fiber in
an inner
cladding region surrounding the core, instead that directly in the core.
Cladding
pumping is a technique that allows high power broadstripe diodes and diode
bars to
be employed as efficient, low cost and small dimension pump sources for double-
clad
rare earth doped single-mode fibers. Output powers ranging from several
hundred
milliwatts to several tens of watts may be attained by this technique. A
double-clad
Er/Yb fiber pumped by diode arrays at 980 nm is described, for example, in
Minelly et
al., "Diode-array pumping of Er3+/Yb3+ co-doped fibre lasers and amplifiers",
IEEE
Photonics Technology Letters, 1993, 5, (3), pp. 301-303. The erbium-ytterbium
co-
doped scheme enables much higher ground state absorption for erbium in the
band
about 980 nm than singly-doped erbium fibers, resulting in much shorter
optimum
length. The technique of inserting the pump radiation into a portion of the
fiber
external the core (which can be identically identified as an inner cladding or
an outer
core) is also described, for example, in PCT patent application WO 95110868
and in
US 5696782.
Several methods have also been proposed to increase the number of
channels to be transmitted. One way to increase channel numbers is to narrow
the
channel spacing. However, narrowing channels spacing worsens nonlinear effects
such as cross-phase modulation or four wave mixing, and makes accurate
wavelength control of the optical transmitters necessary. Applicant has
observed that
a channel spacing lower than 50 GHz is difficult to achieve in practice do to
the above
reasons.
Another way to increase the channel number is to widen the usable
wavelength bandwidth in the low loss region of the fiber. One key technology
is
optical amplification in the wavelength region over the conventional 1550 nm
transmission band. In particular, the high wavelength band around 1590 nm, and
precisely between 1565 nm and 1620 nm, is a very promising band for long-
distance
optical transmissions, in that a very high number of channels can be allocated
in that
band. If the optical amplifier for the 1565-1620 nm band must deal with a high
number of channels, the spectral gain characteristics of such amplifier are
fundamental to optimize the system's performances and costs. The use of the
1590
nm transmission wavelength region of erbium-doped fiber amplifiers in parallel
to the
1530 and 1550 wavelength regions, is attractive and has been recently
considered.
As an additional advantage, by employing the 1590 nm wavelength region it is
possible to use dispersion-shifted fiber (DSF) for WDM transmissions without
any
degradation caused by four-wave mixing.
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Several articles in the patent and non-patent literature address amplification
in
the high wavelength transmission band (from 1565 nm up to 1620 nm). All these
documents consider only erbium-doped fiber amplifiers.
The following documents propose several methods to enlarge the usable
bandwidth to the high wavelength transmission band.
US 5,500,764 relates to a Si02-AI203-GeOz single-mode optical fiber (having
a length between 150 m and 200 m) doped with erbium, pumped by 1.55 ~m and
1.47 ~m optical sources and adapted to amplify optical signals between 1.57~m
and
1.61 Vim.
Ono et al., "Gain-Flattened Er3'-Doped Fiber Amplifier for a WDM Signal in
the 1.57-1.60 ~m Wavelength Region", IEEE PHOTONICS TECHNOLOGY
LETTERS, Vol. 9, No. 5, May 1997, pp. 596-599, disclose a gain-flattened Er3+-
doped
silica-based fiber amplifier for the 1.58 ~m band WDM signal; different fiber
lengths
were tested and the authors found that 200 m was the optimum length of EDF
(Erbium-Doped Fiber) for constructing an EDFA with high gain and low noise.
Masuda et al., "~deband, gain-flattened, erbium-doped fibre amplifiers with 3
dB bandwidths of >50 nm", ELECTRONICS LETTERS, 5t" June 1997, Vol. 33, No.
12, pp. 1070-1072, propose a scheme with two-stage erbium-doped fibres and an
intermediate equalizer, obtaining a 52 nm band (1556-1608 nm) for a silicate
erbium-
doped fiber amplifier and a 50 nm band (1554-1604 nm) for a fluoride erbium-
doped
fiber amplifier; in the case of a silicate erbium-doped fiber amplifier, the
two stages
include a 50 m EDF and a 26 m EDF, respectively.
Jolley et al., "Demonstration of low PMD and negligible multipath interference
in an ultra flat broad band EDFA using a highly doped erbium fiber", "Optical
Amplifiers and their Applications" Conference, Vail, Colorado, July 27-29
1998, TuD2-
1/124-127 proposes a broad band EDFA which amplifies signals in the 1585 nm
band
using 45 m of erbium fiber and reaching a maximum external power of more than
+18.3 dBm at 1570.
The Applicant has observed that a line EDFA adapted to amplify optical
signals in the high wavelength band can amplify an optical signal having an
input
power of approximately -10 dBm to a maximum power value lower than 19 dBm,
i.e.
with a maximum gain of approximately 29 dB. An input power of approximately -
10
dBm is a proper reference value, being typical for optical amplifiers in long-
distance
transmission systems. Lower input power are not recommended in that, although
EDFAs have higher gains for low power input signals than for high power input
signals, the ASE (amplified spontaneous emission) in this case increases to
values
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- such that the signal to noise ratio becomes to low. On the contrary, signal
input
powers over -10 dBm, obtainable for example to the detriment of transmission
fiber
length, tends to saturate the gain, leading to an undesirable waste of energy.
An
optical transmission system using EDFAs and transmitting sixty-four channels
between 1575 nm and 1602 nm would provide a maximum power per channel, at the
output of the line EDFAs, of about 0.2 dBm and would limit in practice the
maximum
span length to less than 100 km.
The Applicant has further observed that in an erbium-doped active fiber of a
predetermined length, the curve of the gain vs. erbium concentration has an
increase
up to a maximum, corresponding to an optimum value of erbium concentration,
and
then a decrease. Higher gains are obtainable only increasing the length of the
active
region doped with erbium, i.e. increasing the active fiber length. Long-haul
WDM
optical transmission systems for the high wavelength band using conventional
erbium-doped active fibers require fiber lengths of a few hundred meters to
reach a
relative high gain. Actually, special erbium-doped active fibers having a
larger core
diameter are used, which allow obtaining a relative high gain with fiber
lengths down
to 30-40 m.
The Applicant has found that, in the 1565-1620 nm band, transmission
systems including erbium-ytterbium co-doped amplifiers provide very high
performances, in particular they provide higher performances with respect to
erbium-
only doped optical amplifiers. In particular, the Applicant has found that an
optical
amplifying unit including an erbium-ytterbium co-doped fiber amplifier (single-
stage or
multi-stage), optimized in terms of length and doping, can provide very high
gain and
a very flat amplification band (t 0,5 dB) in a wavelength region having a
width of at
least 27 nm and situated above 1565 nm. More in detail, the Applicant has
found that
an optical amplifying unit including an optimized erbium-ytterbium co-doped
fiber
amplifier can provide, in the 1575-1602 nm wavelength region, an output signal
power up to 23 dBm in response to an input of approximately -10 dBm power. The
Applicant has further found that such a high power gain can be efficiently
reached by
including a pre-amplifier, preferably an erbium-doped pre-amplifier, into the
optical
amplifying unit.
Furthermore, the Applicant has observed that, for an erbium-ytterbium co-
doped fiber, the region of increase in the gain vs. erbium concentration curve
is much
more extended than for erbium-doped fibers, and has found that, in the 1565-
1620
nm band, relatively short active fibers may be used. The Applicant has found
that,
depending on system parameters such as the signal power at the input of the
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- amplifying unit and the erbium concentration in the active fiber of the
amplifying unit,
an optimum fiber length of the active fiber can be chosen to minimize the gain
tilt in
the considered wavelength band.
Moreover, the Applicant has found that, in the considered high-wavelength
band, high pump performances can be obtained using erbium-ytterbium double-
clad
fiber, taking advantage of the multi-mode pumping mechanism.
The Applicant has further found that the above-described amplifying unit can
be advantageously used in a long-haul WDM transmission system to obtain high
performances in transmissions in the wavelength region up to 1620 nm. In
particular,
the Applicant has found that a wide-band long-haul WDM transmission system can
be realized by subdividing the wavelength transmission band in three sub-bands
corresponding to 1529-1535 nm, 1541-1561 nm and 1575-1602 nm, and amplifying
the 1575-1602 sub-band by means of optical amplifying units including at least
an
ErIYb co-doped amplifier, preferably combined with an Er-doped pre-amplifier.
Such
a wide band allows, for example, the efficient transmission of sixty-four
channels
spaced by 50 GHz.
Moreover, the Applicant has found that, thanks to the high gain achievable by
means of ErIYb co-doped amplifiers in the high-wavelength band, a WDM
transmission system in the 1575-1602 nm band may include fiber spans having
length greater than or equal to 130 km between subsequent amplification
stages.
According to a first aspect, the present invention relates to an optical
transmission system including an optical transmitting unit adapted to transmit
an
optical signal in a transmission wavelength band above 1570 nm, an optical
receiving
unit to receive said optical signal, an optical fiber link optically coupling
said
transmitting unit to said receiving unit and an optical amplifying unit
coupled along
said link, said optical amplifying unit having an amplification wavelength
band
including said transmission wavelength band and comprising an input for the
input of
said optical signal from said link, an output for the output of said optical
signal into
said link and an optical amplifier interposed between said input and said
output to
amplify said optical signal, said optical amplifier including an amplification
fiber, a
pump source for generating pump radiation and an optical coupler for optically
coupling said pump source and said amplification fiber, characterized in that
said
amplification fiber includes an optical fiber co-doped with erbium and
ytterbium.
In particular, said optical amplifying unit has a power gain greater than 29
dB
when said optical signal has an input power of at least -10.5 dBm and
wavelength
within said amplification wavelength band. Preferably, said optical amplifying
unit has
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- a power gain of at least 31 dB when said optical signal has an input power
of at least
-10.5 dBm and wavelength within said amplification wavelength band. More
preferably, said optical amplifying unit has a power gain of at least 33 dB
when said
optical signal has an input power of at least -10.5 dBm and wavelength within
said
amplification wavelength band.
Preferably, the width of said amplification wavelength band is at least 15 nm
and more preferably at least 27 nm.
Preferably, said lower wavelength limit of said amplification wavelength band
is greater than or equal to 1575 nm.
Preferably, said optical amplifying unit has a gain variation lower than 1 dB
within said amplification wavelength band.
Preferably, said optical transmission system further includes an optical pre-
amplifier interposed between said input of said optical amplifying unit and
said optical
amplifier to pre-amplify said optical signal.
Preferably, said amplification fiber has a core having a concentration of
erbium between approximately 600 ppm and 1000 ppm.
Preferably, said amplification fiber has a core having a ratio between
ytterbium and erbium concentrations between approximately 5:1 and 30:1.
Preferably, said amplification fiber has a length lower than 30 m and more
preferably lower than 13 m.
Preferably, said amplification fiber is a double-clad fiber having a core, an
inner cladding surrounding the core and an outer cladding surrounding said
inner
cladding.
Preferably said optical link includes optical fiber spans having a length of
at
least 130 km.
In a second aspect, the invention relates to a method for transmitting optical
signals, comprising:
- generating an optical signal having a wavelength in a wavelength band, said
wavelength band having a lower limit greater than 1570 nm;
- feeding said signal to an optical link;
- amplifying said signal along said optical link;
- receiving said optical signal from said optical link;
characterized in that said step of amplifying comprises feeding said signal
into
an active fiber co-doped with erbium and ytterbium.
Preferably, said step of feeding comprises feeding said signal into one end of
said active fiber, said active fiber having a length, an erbium concentration
and an
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ytterbium concentration such that the power gain of said optical signal at the
opposite
end of said active fiber is at least 25 dB.
More preferably, said step of feeding comprises feeding said signal into one
end of said active fiber, said active fiber having a length, an erbium
concentration and
an ytterbium concentration such that the power gain of said optical signal at
the
opposite end said active fiber is at least 31 dB.
Preferably, said wavelength band has a width of at least 27 nm.
In a further aspect, the invention relates to an optical amplifying unit for
amplifying optical signals in an optical transmission system, said optical
amplifying
unit having an amplification wavelength band with a lower wavelength limit
greater
than 1570 nm and with a width of at least 15 nm, and including an
amplification fiber,
a pump source for generating pump radiation and an optical coupler for
optically
coupling said pump source to said amplification fiber, characterized in that
said
amplification fiber includes an optical fiber co-doped with erbium and
ytterbium.
The foregoing general description and the following detailed description are
exemplary and explanatory only and are not restrictive of the invention as
claimed.
The following description, as well as the practice of the invention, set forth
and
suggest additional advantages and purposes of this invention.
The accompanying drawings, which are incorporated in and constitute a part
of this specification, illustrate embodiments of the invention, and together
with the
description, explain the advantages and principles of the invention.
Fig. 1 is a block diagram of an optical transmission system consistent with
the
present invention;
Fig. 2 is a qualitative graph of the spectral gain characteristic of the
optical
transmission system of fig. 1, with a designation of the signal transmission
bands
(BB, RB1 and RB2);
Fig. 3 is a more detailed diagram of the multiplexing section of the optical
transmission system in Fig. 1;
Fig. 4 is a more detailed diagram of the transmitter power amplifier section
of
the optical transmission system in Fig. 1;
Fig. 5 is a graph of a filter performance shape of a de-emphasis filter for
the
transmitter power amplifier of the present invention;
Fig. 6 is a detailed diagram of an intermediate station of the optical
transmission system of Fig. 1;
Fig. 7 is a detailed diagram of a receiver pre-amplifier section of the
optical
transmission system of Fig. 1;
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Fig. 8 is a detailed diagram of a demultiplexing section of the optical
transmission system of Fig. 1;
Fig. 9 is a schematic representation of a first embodiment of an optical
amplifying unit according to the invention;
Figs. 10a and 10b show numerical simulation results obtained with the optical
amplifying unit of Fig. 9, in the case of a two channels transmission;
Figs. 11 a, 11 b and 11 c show further numerical simulation results obtained
with the optical amplifying unit of Fig. 9, in the case of a two channels
transmission;
Figs. 12a and 12b are schematic representations of experimental
arrangements used to test the optical amplifying unit of Fig. 9;
Figs. 13a and 13b show experimental results obtained with the experimental
arrangements of Figs. 12a and 12b in the case of a two-channel transmission;
Figs 14a and 14b show experimental results obtained with the experimental
arrangements of Figs. 12a and 12b (but introducing different attenuation) in
the case
of a sixty-four (64) channels transmission;
Fig. 15 is a schematic representation of a second embodiment of the optical
amplifying unit according to the invention;
Fig. 16 is a schematic representation of a third embodiment of the optical
amplifying unit according to the invention; and
Figs. 17a and 17b are schematic representations of a double cladding fibre
used in the optical amplifying unit of the invention and of the multi-mode
pumping
operation of the double cladding fibre.
Referring to Fig. 1, an optical transmission system 1 includes a first
terminal
site 10, a second terminal site 20, an optical fiber line 30 connecting the
two terminal
sites 10, 20, and at least one line site 40 interposed between the terminal
sites 10
and 20 along the optical fiber line 30.
For simplicity, the optical transmission system 1 hereinafter described is
unidirectional, that is signals travel from a terminal site to the other (in
the present
case from the first terminal site to the second terminal site), but any
consideration
that follow is to be considered valid also for bi-directional systems, in
which signals
travel in both directions. Further, although the optical transmission system 1
is
adapted to transmit up to one-hundred-twenty-eight (128) channels, from the
hereinafter description it will be obvious that the number of channels is not
a limiting
feature for the scope and the spirit of the invention, and more than one-
hundred-
twenty-eight (128) channels can be used depending on the needs and
requirements
of the particular optical transmission system.
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The first terminal site 10 preferably includes a multiplexing section (MUX)
11,
a transmitter power amplifier section (TPA) 12 and a plurality of input
channels 16.
The second terminal site 20 preferably includes a receiver pre-amplifier (RPA)
section 14, a demultiplexing section (DMUX) 15 and a plurality of output
channels 17.
Each input channel 16 is received by multiplexing section 11. Multiplexing
section 11, hereinafter described with reference to Fig. 3, multiplexes or
groups input
channels 16 preferably into three sub-bands, referred to as blue-band BB,
first red-
band RB1 and second red-band RB2, although multiplexing section 11 could
alternatively group input channels 16 into a number of sub-bands greater or
less than
three.
The three sub-bands BB, RB1 and RB2 are then received, as separate sub-
bands or as a combined wide-band, in succession by TPA section 12, at least
one
line site 40 and second terminal site 20. Sections of optical fiber line 30
adjoin the at
least one line site 40 with TPA section 12, RPA section 14, and possibly with
others
line sites 40 (not shown). TPA section 12, that will be later described with
reference
to Fig. 4, receives the separate sub-bands BB, RB1 and RB2 from multiplexing
section 11, amplifies and optimizes them, and then combines them into a single
wide-
band SWB for transmission on a first section of optical fiber line 30. Line
site 40, that
will be later described with reference to Fig. 6, receives the single wide-
band SWB,
re-divides the single wide-band SWB into the three sub-bands BB, RB1 and RB2,
eventually adds and drops signals in each sub-band BB, RB1 and RB2, amplifies
and
optimizes the three sub-bands BB, RB1 and RB2 and then recombines them into
the
single wide-band SWB. For the adding and dropping operations, line site 40 may
be
provided with Optical AddIDrop Multiplexers (OADM) of a known type.
A second section of optical fiber line 30 couples the output of the line site
40
to either another line site 40 (not shown) or to RPA section 14 of second
terminal site
20. RPA section 14, that will be later described with reference to Fig. 7,
also
amplifies and optimizes the single wide-band SWB and may split the single wide-
band SWB into the three sub-bands BB, RB1 and RB2 before outputting them.
Demultiplexing section 15, that will be later described with reference to Fig.
8,
receives the three sub-bands BB, RB1 and RB2 from RPA section 14 and splits
the
three sub-bands BB, RB1 and RB2 into the individual wavelengths of output
channels
17. The number of input channels 16 and output channels 17 may be unequal,
owing
to the fact that some channels can be dropped andlor added in line site (or
line sites)
40.
According to the above, for each sub-band BB, RB1 and RB2 an optical link is
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' PC769 11
defined between the corresponding input of TPA section 12 and the
corresponding
output of RPA section 14.
Fig. 2 is a qualitative graph of the spectral emission ranges of the
amplifiers
used in the optical transmission system 1 approximately corresponding to the
different gain for channels of signals traveling through the fiber link and
the different
allocation of the three sub-bands BB, RB1 and RB2. In particular, the first
sub-band
BB preferably covers the range between 1529 nm and 1535 nm, corresponding to a
first amplification wavelength range of erbium-doped fiber amplifiers, and
allocates up
to sixteen (16) channels; the second sub-band RB1 fall between 1541 nm and
1561
nm, corresponding to a second amplification wavelength range of erbium-doped
fiber
amplifiers, and allocates up to forty-eight (48) channels; and the third sub-
band RB2
covers the range between 1575 nm and 1602 nm, corresponding, according to the
invention, to an amplification wavelength range of erbiumlytterbium-doped
fiber
amplifiers, and allocates up to sixty-four (64) channels. The gain spectral
graph of
the erbium/ytterbium-doped fiber amplifiers is such that, although the 1575-
1602 nm
range offers the best performances in terms .of amplification, channels can be
advantageously allocated down to 1565 nm and up to 1620 nm. More in details,
the
Applicant has observed that a lower limit 1570 nm is preferred for the
allocation of
channels in the RB2 band, due to the shape of the power spectrum curve of
ErIYb
co-doped fibers in this wavelength range.
Adjacent channels, in the proposed one-hundred-twenty-eight (128) channel
system, have a 50 GHz constant spacing. Alternatively, the frequency spacing
may
be unequal to alleviate the known four-wave-mixing phenomenon.
In the erbium amplification band, the RB1 and RB2 bands have a fairly flat
gain characteristic, while the BB band includes a substantial hump in the gain
response. As explained below, to make use of the erbium-doped fiber spectral
emission range in the BB band, optical transmission system 1 uses equalizing
means
to flatten the gain characteristic in that range. As a result, by dividing the
erbium
doped fiber spectral emission range of 1529-1602 nm into three sub-ranges that
respectively include the BB band, RB1 band and RB2 band, optical transmission
system 1 can effectively use most of the erbium-doped fiber spectral emission
range
and provide for dense WDM.
The following provides a more detailed description of the various modules of
the present invention depicted in Fig. 1.
Fig. 3 shows a more detailed diagram of the multiplexing section 11 of first
terminal site 10. The first terminal site 10 includes, in addition to the
multiplexing
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section 11 and the TPA section 12 (not shown in Fig. 3), an optical line
terminal
section (OLTE) 41 and a wavelength converter section (WCS) 42.
OLTE 41, which may correspond to standard line terminating equipment for
use in a SONET, ATM, IP or SDH system, includes transmitlreceive (TX/RX) units
(not shown) in a quantity that equals the number of channels in WDM systems
10. In
a preferred embodiment, OLTE 41 has one-hundred-twenty-eight (128) TXIRX
units.
In multiplexing section 11, OLTE 41 transmits a plurality of signals at a
generic
wavelength. As shown in Fig. 3, in a preferred embodiment OLTE 41 outputs a
first
group of sixteen (16) channels, a second group of forty-eight (48) channels
and a
third group of sixty-four (64) channels. However, as indicated above, the
number of
channels may vary depending on the needs and requirements of the particular
optical
transmission system.
As readily understood to one of ordinary skill in the art, OLTE 41 may
comprise a collection of smaller separate OLTEs, such as three, that feed
information
frequencies to WCS 42. Accordingly, WCS 42 includes one-hundred-twenty-eight
(128) wavelength converter modules WCM1-WCM128.
Units WCM1-WCM16 each receive a respective one of the first group of
signals emitted from OLTE 41, units WCM17-WCM64 each receive one of the
second group of signals emitted from OLTE 41 and units WCM65-WCM128 each
receive one of the third group of signals emitted from OLTE 41. Each unit is
able to
convert a signal from a generic wavelength to a selected wavelength and re-
transmit
the signal. The units may receive and re-transmit a signal in a standard
format, such
as OC-48 or STM-16, but the preferred operation of WCM1-128 is transparent to
the
particular data format employed.
Each WCM1-128 preferably comprises a module having a photodiode (not
shown) for receiving an optical signal from OLTE 41 and converting it to an
electrical
signal, a laser or optical source (not shown) for generating a fixed carrier
wavelength,
and an electro-optic modulator such as a Mach-Zehnder modulator (not shown)
for
externally modulating the fixed carrier wavelength with the electrical signal.
Alternatively, each WCM1-128 may comprise a photodiode (not shown) together
with
a laser diode (not shown) that is directly modulated with the electrical
signal to
convert the received wavelength to the carrier wavelength of the laser diode.
As a
further alternative, each WCM1-128 comprises a module having a high
sensitivity
receiver (e.g., according to SDH or SONET standards) for receiving an optical
signal,
e.g., via a wavelength demultiplexer, from a trunk fiber line end and
converting it to
an electrical signal, and a direct modulation or external modulation laser
source. By
CA 02282941 1999-09-21
PC769 13
the latter alternative, regeneration of signals from the output of a trunk
fiber line and
transmission in the inventive optical communication system is made possible,
which
allows extending the total link length.
Although Fig. 3 shows that the signals are provided and generated by the
combination of OLTE 41 and WCM1-WCM128, the signals can also be directly
provided and generated by a source without limitation to their origin.
The multiplexing section 11 includes three wavelength multiplexers (WM) 43,
44 and 45. For the preferred one-hundred-twenty-eight (128) channels system,
each
selected wavelength signal output from units WCM1-WCM16 is received by WM 43,
each selected wavelength signal output from WCM 17-WCM64 is received by WM 44
and each selected wavelength signal output from WCM65-WCM128 is received by
WM 45. WM 43, WM 44 and WM 45 combine the received signals of the three bands
BB, RB1 and RB2 into three respective wavelength division multiplexed signals.
As
shown in Fig. 3, WM 43 is a sixteen (16) channels wavelength multiplexes, such
as a
conventional 1x16 planar optical splitter, WM 44 is a forty-eight (48)
channels
wavelength multiplexes, such as a conventional 1x64 planar optical splitter
with
sixteen (16) unused ports and WM 45 is a sixty-four (64) channels wavelength
multiplexes, such as a conventional 1x64 planar optical splitter. Each
wavelength
multiplexes may include a second port (e.g. 2x16 and 2x64 splitters) for
providing
optical transmission system 1 with an optical monitoring channel (not shown).
As
well, WM 43, 44 and 45 may have more inputs than is used by the system to
provide
space for system growth. A wavelength multiplexes using passive silica-on-
silicon
(Si02-Si) or silica-on-silica (Si02-Si02) technology, for instance, can be
made by one
of ordinary skill in the art. Other technologies can also be used for WMs,
e.g., for
reducing insertion losses. Examples are AWG (Arrayed Waveguide Gratings),
fiber
gratings, and interferential filters.
With reference to fig. 4, the BB, RB1 and RB2 band output from multiplexing
section 11 are received by TPA section 12. The BB, RB1 and RB2 band signals
may
be provided to TPA section 12 from a source other than the OLTE 41, WCS 42,
and
WM 43, 44 and 45 configuration depicted in Fig. 3. For example, the BB, RB1
and
RB2 band signals may be generated and directly supplied to TPA section 12 by a
customer without departing from the intent of the present invention described
in more
detail below.
TPA section 12 includes three amplifier sections 51, 52, 53, each for a
respective band BB, RB1 and RB2, a coupling filter 54 and an equalizing filter
61.
Amplifier sections 51, 52 are preferably erbium-doped two-stages fiber
amplifiers
CA 02282941 1999-09-21
' PC769 14
(although other rare-earth-doped fiber amplifiers may be used), while
amplifier
section 53 is, according to the invention, an erbium/ytterbium-doped (ErIYb)
fiber
amplifier that will be described in details with reference to figs. 9, 15 and
16.
The outputs of amplifiers 51, 52 and 53 are received by filter 54, which
combines the BB, RB1 and RB2 bands into a single wide-band (SWB).
Each of the amplifiers 51 and 52 is pumped by one or two laser diodes to
provide optical gain to the signals it amplifies. The characteristics of each
amplifier,
including its length and pump wavelength, are selected to optimize the
performance
of that amplifier for the particular sub-band that it amplifies. For example,
the first
stage of amplifier sections 51 and 52 may be pumped with a laser diode (not
shown)
operating at 980 nm to amplify the BB band and the RB1 band, respectively, in
a
linear or in a saturated regime. Appropriate laser diodes are available from
the
Applicant. The laser diodes may be coupled to the optical path of the pre-
amplifiers
using 98011550 WDM couplers (not shown) commonly available on the market, for
example model SWDM0915SPR from E-TEK DYNAMICS, INC., 1885 Lundy Ave.,
San Jose, CA (USA). The 980 nm laser diode ,provides a low noise figure for
the
amplifiers compared with other possible pump wavelengths.
The second stage of each amplifier section 51-53 preferably operates in a
saturated condition. The second stage of amplifier section 51 is preferably
erbium
doped and amplifies the BB band with another 980 nm pump (not shown) coupled
to
the optical path of the BB band using a WDM coupler (not shown) described
above.
The 980 nm pump provides better gain behavior and noise figure for signals in
the
tow band region that covers 1529-35 nm. The second stage of amplifier section
52 is
preferably erbium-doped and amplifies the RB1 band with a laser diode pump
source
operating at 1480 nm. Such a laser diode is available on the market, such as
model
FOL1402PAX-1 supplied by JDS FITEL, INC., 570 Heston Drive, Nepean, Ontario
(CA). The 1480 nm pump provides better saturated conversion efficiency
behavior,
which is needed in the RB1 band for the greater number of channels in the
region
that covers 1542-61 nm. Alternatively, a higher power 980 nm pump laser or
multiplexed 980 nm pump sources may be used. Section 53 will be hereunder
described in details with reference to Figs. 9, 15 and 16.
Filter 61 is positioned within the RB1 band amplifier chain for helping to
equalize signal levels and SNRs at the system output across the RB1 band. In
particular, filter 61 comprises a de-emphasis filter that attenuates the
wavelength
regions of the high amplification within the RB1 band. The de-emphasis filter,
if used,
may employ long period Bragg grating technology, split-beam Fourier filter,
etc.. As
CA 02282941 1999-09-21
PC769 15
an example, the de-emphasis filter may have an operating wavelength range of
1541-
1561 nm and have wavelengths of peak transmission at 1541-1542 nm and 1559-
1560 nm, with a lower, relatively constant transmission for the wavelengths
between
these peaks. Fig. 5 illustrates the filter shape or relative attenuation
performance of a
preferred de-emphasis filter 61. The graph of Fig. 5 shows that the de-
emphasis filter
61 has regions of peak transmission at around 1542 nm and 1560 nm, and a
region
of relatively constant or flat attenuation between about 1546 nm and 1556 nm.
The
de-emphasis filter 61 for erbium-doped fiber amplifiers need only add an
attenuation
of about 3-4 dB at wavelengths between the peaks to help flatten the gain
response
across the high band. The de-emphasis filter 61 may have an attenuation
characteristic different from that depicted in Fig. 5 depending on the gain-
flattening
requirements of the actual system employed, such as the dopant used in the
fiber
amplifiers or the wavelength of the pump source for those amplifiers.
Alternatively, the de-emphasis filter 61 may be omitted and the de-emphasis
operation may be obtained in the multiplexing section 11 of the first terminal
site 10
by means of calibrated attenuation.
After passing through the amplifiers of TPA 12, the amplified BB, RB1 and
RB2 bands output from amplifier sections 51, 52 and 53, respectively, are
received
by filter 54. Filter 54 is a band combining filter and may, for example,
include two
cascaded interferential three port filters (not shown), the first coupling the
BB band
with the RB1 band and the second coupling the BBIRB1 bands provided by the
first
filter with the RB2 band.
An optical monitor (not shown) and insertion for a service line, at a
wavelength different from the communication channels, e.g. at 1480 nm, through
a
WDM 148011550 interferential filter (not shown) may also be added at the
common
port. The optical monitor detects optical signals to ensure that there is no
break in
optical transmission system 1. The service line insertion provides access for
a line
service module, which can manage through an optical supervisory channel the
telemetry of alarms, surveillance, monitoring of performance and data,
controls and
housekeeping alarms, and voice frequency orderwire.
The single wide-band output from filter 54 of TPA section 12 passes through a
length of transmission fiber (not shown) of optical fiber line 30 such as 100
kilometers, which attenuates the signals within the single wide-band SWB.
Consequently, line site 40 receives and amplifies the signals within the
single wide-
band SWB. As shown in Fig. 6, line site 40 includes several amplifiers (AMP)
64-69,
three filters 70-72, an equalizing filter (EQ) 74 and three OADM stages 75-77.
CA 02282941 1999-09-21
PC769 16
Filter 70 receives the single wide-band SWB and separates the RB2 band
from the BB and the RB1 bands. Amplifier 64 receives and amplifies the BB and
the
RB1 bands, whereas filter 71 receives the output from amplifier 64 and
separates the
BB band and the RB1 band. The BB band is equalized by equalizing filter 74,
received by the first OADM stage 75 where predetermined signals are dropped
andlor added, and further amplified by amplifier 65. The RB1 band, which has
already passed through de-emphasis filter 61 in TPA 12, is first amplified by
amplifiers 66, then received by the second OADM stage 76 where predetermined
signals are dropped andlor added, and further amplified by amplifier 67. The
RB2
band is first amplified by amplifiers 68, then received by the third OADM
stage 77
where predetermined signals are dropped andlor added, and further amplified by
amplifier 69. The amplified BB, RB1 and RB2 bands are then recombined into the
single wide-band SWB by ~Iter 72.
Amplifier 64, which receives the single wide-band SWB, preferably comprises
a single optical fiber amplifier that is operated in a linear regime. That is,
amplifier 64
is operated in a condition where its output power is dependent on its input
power.
Depending on the actual implementation, amplifier 64 may alternatively be a
single
stage or a multi-stage amplifier. By operating it in a linear condition,
amplifier 64
helps to ensure relative power independence between the BB and RB1 band
channels. In other words, with amplifier 64 operating in a linear condition,
the output
power (and signal-to-noise ratio) of individual channels in the one of the two
sub-
bands BB, RB1 does not vary significantly if channels in the other sub-band
RB1, BB
are added or removed. To obtain robustness with respect to the presence of
some or
all of the channels in a dense WDM system, first stage amplifier (such as
amplifier 64
and amplifier 68) must be operated, in a line site 40, in an unsaturated
regime, before
extracting a portion of the channels for separate equalization and
amplification. In a
preferred embodiment, amplifiers 64 and 68 are erbium-doped fiber amplifiers,
pumped in a co-propagating direction with a laser diode (not shown) operating
at 980
nm pump to obtain a noise figure preferably less than 5.5 dB for each band.
Filter 71 may comprise, for example, a three-port device, preferably an
interferential filter, having a drop port that feeds the BB band into
equalizing filter 74
and a reflection port that feeds the RB1 band into amplifier 66.
Amplifier 66 is preferably a single erbium-doped fiber amplifier that is
operated in saturation, such that its output power is substantially
independent from its
input power. In this way, amplifier 66 serves to add a power boost to the
channels in
the RB1 band compared with the channels in the BB band. Due to the greater
CA 02282941 1999-09-21
PC769 17
number of channels in the RB1 band compared with the BB band in the preferred
embodiment, i.e. forty-eight (48) channels as opposed to sixteen (16), the RB1
band
channels typically will have had a lower gain when passing through amplifier
64. As
a result, amplifier 66 helps to balance the power for the channels in the RB1
band
compared with the BB band. Of course, for other arrangements of channels
between
the BB and the RB1 bands, amplifier 66 may not be required or may
alternatively be
required on the BB band side of line site 40.
With respect to the RB1 band of channels, amplifiers 64 and 66 may be
viewed together as a two-stage amplifier with the first stage operated in a
linear mode
and the second stage operated in saturation. To help stabilize the output
power
between channels in the RB1 band, amplifier 64 and 66 are preferably pumped
with
the same laser diode pump source. In this manner, as described in EP 695049,
the
residual pump power from amplifier 64 is provided to amplifier 66.
Specifically, line
site 40 includes a WDM coupler positioned between amplifier 64 and filter 71
that
extracts 980 nm pump light that remains at the output of amplifier 64. This
WDM
coupler may be, for example, model number SWDMCPR3PS110 supplied by E-TEK
DYNAMICS, INC., 1885 Lundy Ave., San Jose, CA (USA). The output from this
WDM coupler feeds into a second WDM coupler (not shown) of the same type and
positioned in the optical path after amplifier 66. The two couplers are joined
by an
optical fiber 78 that transmits the residual 980 nm pump signal with
relatively low
loss. The second WDM coupler passes the residual 980 nm pump power into
amplifier 66 in a counter-propagating direction.
From amplifier 66, RB1 band signals are conveyed to OADM stage 76, for
example an OADM of a known type. From OADM stage 76, RB1 band signals are
fed to amplifier 67. For the preferred erbium-doped fiber amplifier, amplifier
67 has a
pump wavelength of, for example, 1480 nm from a laser diode source (not shown)
having a pump power in excess of the laser (not shown) that drives amplifiers
64 and
66. The 1480 nm wavelength provides good conversion efficiency for high output
power output compared with other pump wavelengths for erbium-doped fibers.
Alternatively, a high power 980 nm pump source or a group of multiplexed pump
sources, such as two pump sources at 980 nm, or one at 975 nm and another at
986
nm, could be used to drive amplifier 67. Amplifier 67 preferably operates in
saturation to provide the power boost to the signals within the RB1 band, and
if
desired, may comprise a multi-stage amplifier.
After passing through amplifier 64 and filter 71, the BB band enters
equalizing
filter 74. As discussed above, the gain characteristic for the erbium-doped
fiber
CA 02282941 1999-09-21
PC769 1 g
spectral emission range has a peak or hump in the BB band region, but remains
fairly
flat in the RB1 band region. As a result, when the BB band or the single wide-
band
SWB (which includes the BB band) is amplified by an erbium-doped fiber
amplifier,
the channels in the BB band region are amplified unequally. Also, as discussed
above, when equalizing means have been applied to overcome this problem of
unequal amplification, the equalizing has been applied across the entire
spectrum of
channels, resulting in continued gain disparities. However, by splitting the
spectrum
of channels into a BB band and a RB1 band, equalization in the reduced
operating
area of the BB band can provide proper flattening of the gain characteristic
for the
channels of the BB band.
In a preferred embodiment, the equalizing filter 74 comprises a two-port
device based on long period chirped Bragg grating technology that gives
selected
attenuation at different wavelengths. For instance, equalizing filter 74 for
the BB
band may have an operating wavelength range of 1529 nm to 1536 nm, with a
wavelength at the bottom of the valley at between 1530.3 nm and 1530.7 nm.
Equalizing filter 74 need not be used alone and. may be combined in cascade
with
other filters (not shown) to provide an optimal filter shape, and thus, gain
equalization
for the particular amplifiers used in the WDM system 1. Equalizing filter 74
may be
manufactured by one skilled in the art, or may be obtained from numerous
suppliers
in the field. It is to be understood that the particular structure used for
the equalizing
filter 74 is within the realm of the skilled artisan and may include, for
instance, a
specialized Bragg grating like a long period grating, an interferential
filter, or Mach-
Zehnder type optical filters.
From equalizing filter 74, BB band signals are conveyed to OADM stage 75,
which is, for example, of the same type of OADM stage 76, and then to
amplifier 65.
With the preferred erbium-doped fiber amplifier, amplifier 65 has a pump
wavelength
of 980 nm, provided by a laser diode source (not shown) and coupled via a WDM
coupler (not shown) to the optical path for pumping the amplifier 65 in a
counter-
propagating direction. Since the channels in the BB band pass through both
amplifier
64 and amplifier 65, equalizing filter 74 may compensate for the gain
disparities
caused by both amplifiers. Thus, the decibel drop for equalizing filter 74
should be
determined according to the overall amplification and line power requirements
for the
BB band. The amplifier 65 preferably operates in saturation to provide a power
boost
to the signals in the BB band, and may comprise a multi-stage amplifier if
desired.
The RB2 band is received from fiber amplifier 68, which is, preferably, an
erbium doped fiber amplifier pumped with a 980 nm or a 1480 nm pump light,
CA 02282941 1999-09-21
' PC769 1 g
depending on the system requirements. From amplifier 68, RB2 band channels are
conveyed to OADM stage 77, which is, for example, of the same type of OADM
stages 75 and 76, and then fed to amplifier 69. Amplifier 69 is, according to
the
invention, an erbium/ytterbium co-doped amplifier adapted to amplify the RB2
band
and will be described in details with reference to Figs. 9, 15, 16.
After passing through amplifiers 65, 67 and 69 respectively, the amplified BB,
RB1 and RB2 bands are then recombined by filter 72 into the single wide-band
SWB.
Like filter 54 of fig. 4, filter 72 may, for example, include two cascaded
interferential
three port filter (not shown), the first coupling the BB with the RB1 bands
and the
second coupling the BB and RB1 bands provided by the first filter with the RB2
band.
Like TPA section 12, line site 40 may also include an optical monitor and a
service line insertion and extraction {not shown) through, e.g., a WDM
148011550
interferential filter (not shown). One or more of these elements may be
included at
any of the interconnection points of line site 40.
Besides amplifiers 64-69, filters 70-72 and 74, and OADM stages 75-77, line
site 40 may also include a dispersion compensating module (DCM) (not shown)
for
compensating for chromatic dispersion that may arise during transmission of
the
signals along the long-distance communication link. The DCM (not shown) is
preferably comprised of sub-units coupled upstream one or more of amplifiers
65, 67,
69 for compensating the dispersion of channels in the BB, RB1, RB2 bands, and
may
also have several forms. For example, the DCM may have an optical circulator
with a
first port connected to receive the channels in one or more than one of the
three
bands BB, RB1 and RB2. A chirped Bragg grating may be attached to a second
port
of the circulator. The channels will exit the second port and be reflected in
the
chirped Bragg grating to compensate for chromatic dispersion. The dispersion
compensated signals will then exit a next port of the circulator for continued
transmission in the WDM system. Other devices besides the chirped Bragg
grating,
such as a length of dispersion compensating fiber, may be used for
compensating the
chromatic dispersion. The design and use of the DCM section are not limiting
the
present invention and the DCM section may be employed or omitted in the WDM
system 1 depending on overall requirements for system implementation.
After the line site 40, the combined single wide-band SWB signal passes
through a length of long-distance optical transmission fiber of optical fiber
line 30. If
the distance between the first and the second terminal site 10, 20 is
sufficiently long
to cause attenuation of the optical signals, i.e. 100 kilometers or more, one
or more
additional line sites 40 providing amplification may be used. In a practical
CA 02282941 1999-09-21
PC769 20
arrangement, five spans of long-distance transmission fiber are used (each
having a
power loss of 0,22 dB/km and a length such as to provide a total span loss of
approximately 25 dB), separated by four amplifying line site 40.
Following the final span of transmission fiber, RPA section 14 receives the
single wide-band SWB from last line site 40 and prepares the signals of the
single
wide-band SWB for reception and detection at the end of the communication
link. As
shown in Fig. 7, RPA section 14 may include amplifiers (AMP) 81-85, filters 86
and
87, an equalizing filter 88 and, if necessary, three router modules 91-93.
Filter 86 receives the single wide-band SWB and separates the RB2 band
from the BB and RB1 bands. Amplifier 81 is preferably doped with erbium and
amplifies the BB and RB1 bands to help improve the signal-to-noise ratio for
the
channels in the BB and RB1 bands. Amplifier 81 is pumped, for example, with a
980
nm pump or with a pump at some other wavelength to provide a low noise figure
for
the amplifier. The BB and RB1 bands are in turn separated by filter 87.
As with TPA section 12 and line site 40, amplifier 82 and 83 amplify the BB
band and, respectively, the RB1 band, with a 980 nm pumping. To help stabilize
the
output power between channels in the RB1 band, amplifier 81 and 83 are
preferably
pumped with the same 980 nm laser diode pump source, by using a joining
optical
fiber 89 that transmits the residual 980 nm pump signal with relatively low
loss.
Specifically, amplifier 81 is associated with a WDM coupler, positioned
between
amplifier 81 and filter 87, that extracts the 980 nm pump light that remains
at the
output of amplifier 81. This WDM coupler may be, for example, model number
SWDMCPR3PS110 supplied by E-TEK DYNAMICS, INC., 1885 Lundy Ave., San
Jose, CA (USA). The output from this WDM coupler feeds into a second WDM
coupler of the same type and positioned in the optical path after amplifier
83. The
two couplers are joined by an optical fiber 89 that transmits the residual 980
nm
pump signal with relatively low loss. The second WDM coupler passes the
residual
980 nm pump power into amplifier 83 in a counter-propagating direction. Thus,
amplifiers 81-83, filter 87 and equalizing filter 88 perform the same
functions as
amplifiers 64, 65 and 67, filter 71, and equalizing filter 74, respectively,
of line site 40
and may comprise the same or equivalent parts depending on overall system
requirements.
Amplifier 84 is coupled to filter 86 to receive and amplify the RB2 band.
Amplifier 84 is, for example, an erbium-doped amplifier identical to the
amplifier 68 of
fig. 6. RB2 band channels are then received by amplifier 85 that is, for
example, an
erbium-doped amplifier of a known type.
CA 02282941 1999-09-21
PC769 21
RPA section 14 further comprises a routing stage 90, which permits to adapt
the channel spacing within the BB, RB1 and RB2 bands to the channel separation
capability of demultiplexing section 15. In particular, if the channel
separation
capability of demultiplexing section 15 is for a relatively wide channel
spacing (e.g.
100 GHz grid) while channels in WDM system 1 are densely spaced (e.g. 50 GHz),
then RPA section 14 could include the routing stage 90 shown in Fig. 7. Other
structures may be added to RPA section 14 depending on the channel separation
capability of demultiplexing section 15.
Routing stage 90 includes three router modules 91-93. Each router module
91-93 separates the respective band into two sub-bands, each sub-band
including
half of the channels of the corresponding band. For example, if the BB band
includes
sixteen (16) channels ~,,-~,~s, each separated by 50 GHz, then router module
91
would split the BB band into a first sub-band BB' having channels 7~~, ~,3,
... , ~.ls
separated by 100 GHz and a second sub-band BB" having channels ~,2, ~,4, ....,
~.,s
separated by 100 GHz and interleaved with the channels in the sub-band BB'. In
a
similar fashion, router modules 92 and 93 would split the RB1 band and the RB2
band, respectively, into first sub-bands RB1' and RB2' and second sub-bands
RB1"
and RB2".
Each router module 91-93 may, for example, include a coupler (not shown)
that has a first series of Bragg gratings attached to a first port and a
second series of
gratings attached to a second port. The Bragg gratings attached to the first
port
would have reflection wavelengths that correspond to every other channel (i.e.
the
even channels), while the Bragg gratings attached to the second port would
have
reflection wavelengths that correspond to the remaining channels (i.e. the odd
channels). This arrangement of gratings will also serve to split the single
input path
into two output paths with twice the channel-to-channel spacing.
After passing through RPA section 14, the BB, RB1 and RB2 bands or their
respective sub-bands are received by demultiplexing section 15. As shown in
Fig. 8,
demultiplexing section 15 includes six wavelength demultiplexers (WDs) 95',
95", 96',
96", 97', 97" which receive the respective sub-bands BB', BB", RB1', RB1 ",
RB2' and
RB2" and generate the output channels 17. Demultiplexing section 15 further
includes receiving units Rx1-Rx128 for receiving the output channels 17.
The wavelength demultiplexers preferably comprise arrayed waveguide
grating devices, but alternate structures for achieving the same or similar
wavelength
separation are contemplated. For instance, one may use interferential filters,
Fabry-
Perot filters, or in-fiber Bragg gratings in a conventional manner to
demultiplex the
CA 02282941 1999-09-21
PC769 22
channels within the sub-bands BB', BB", RB1', RB1 ", RB2', RB2".
In a preferred configuration, demultiplexer section 15 combines interferential
filter and AWG filter technology. Alternatively, one may use Fabry-Perot
filters or in-
fiber Bragg gratings. WDs 95', 95", which are preferably eight channel
demultiplexers with interferential filters, receive and demultiplex first sub-
band BB'
and second sub-band BB", respectively. Specifically, WD 95' demultiplexes
channels
~,,, ~.3, ... , ~,,s, and WD 95" demultiplexes channels 7~z, ~,a, ... , ~,,s.
Both WD 95' and
WD 95", however, may be 1x8 type AWG 100 GHz demultiplexers. Similarly, WDs
96' and 96" receive and demultiplex first sub-band RB1' and second sub-band
RB1 ",
respectively, to produce channels 7~"-a,s4 and WDs 97' and 97" receive and
demultiplex first sub-band RB2' and second sub-band RB2", respectively, to
produce
channels a,s5-x,28. Both WD 96' and WD 96" may be 1x 32 type AWG 100 GHz
demultiplexers that are underequipped to use only twenty-four of the available
demultiplexer ports and both WD 97' and WD 97" may be 1x 32 type AWG 100 GHz
demultiplexers that uses all the available demultiplexer ports. Output
channels 17
are composed of the individual channels demultiplexed by WDs 95', 95", 96',
96",
97', 97", and each channel of output channels 17 is received by one of
receiving units
Rx1-Rx 128.
Fig. 9 illustrates an erbium/ytterbium fiber amplifying unit 100 adapted,
according to the present invention, to be used in the optical transmission
system 1 to
amplify the RB2 band. In particular, amplifying unit 100 is a preferred
embodiment
for the amplifier sections 53 of fig. 4 and 69 of fig. 6.
Amplifying unit 100 includes an input 101, an output 102, an erbium fiber pre
amplifier 103 and an erbium/ytterbium fiber amplifier 104. Pre-amplifier 103
and
amplifier 104 are arranged in series and the pre-amplifier 103 is positioned
between
input 101 and amplifier 104 for providing a first amplification to RB2
channels
received at input 101.
Amplifying unit 100 also preferably comprises a first and second isolator 105,
106. The first isolator 105 is positioned between pre-amplifier 103 and
amplifier 104
and is adapted to block light directed from the amplifier 104 towards pre-
amplifier
103. The second isolator 106 is positioned between amplifier 104 and output
102
and is adapted to block light directed from output 102 towards optical
amplifier 104.
Pre-amplifier 103 may be, for example, a single-stage erbium-doped fiber
amplifier of a known type, pumped at 980 nm andlor 1480 nm, depending on the
system requirements. The total length of the erbium-doped active fiber in pre
amplifier 103 is preferably between 80 m and 150 m. Pre-amplifier 103 receives
the
CA 02282941 1999-09-21
PC769 23
RB2 band from input 101 and amplifies the RB2 channels to a first power level,
for
example up to 15-17 dBm. The first amplification performed by pre-amplifier
103
allows reaching a high power level at the output of amplifier 104, as will be
shown
later on. Pre-amplifier 103 also improves the noise figure (NF) of amplifying
unit 100
and permits to equalize the RB2 band channels.
Amplifier 104 is a single-stage fiber amplifier with bi-directional pumping,
including an amplification optical fiber 108 having a preset length.
The Applicant has determined that, in order to minimize the gain tilt in the
considered wavelength band, an optimum total length of the active fiber can be
found, which is dependent on system parameters such as the signal input power
and
the erbium concentration in the active fiber,
The Applicant has found that, with a power level of about 15-17 dBm at the
input of amplifier 104 and with an erbium concentration in the active fiber of
the
amplifier 104 in the range indicated below, a total length of said active
fiber between
10 m and 20 m allows to achieve an acceptably low gain tilt, the minimum gain
tilt
being achieved for a total fiber length of approximately 12 m.
Amplifier 104 further includes a first and a second pump laser 109, 110 for
providing pump radiation to the amplification optical fiber 108, and a first
and a
second WDM optical coupler 111, 112 to couple light from the first and the
second
pump lasers 109, 110 into the amplification optical fiber 108.
Optical fiber 108 is, preferably, a double-cladding fiber of the type
hereinafter
described with reference to Fig. 17a, where a not-in-scale section of optical
fiber 108
is shown. Fiber 108 includes a core 140 having a first refraction index n,, an
inner
cladding 141 that surrounds the core 140, is coaxial to the core 140 and has a
second refraction index n2 < n~, and an outer cladding 142 that surrounds the
inner
cladding 141, is coaxial to the inner cladding 141 and has a third refraction
index n3<
n2. As shown in fig. 17b, under normal operating conditions of amplifying unit
100,
while the transmitted signal is confined into the core 140, the pump radiation
is fed
into the inner cladding 141 and is progressively absorbed by the core 140,
exciting
the active medium. Fiber 108 has preferably an external diameter of the outer
cladding 142 of 90 pm, an external diameter of the inner cladding 141 of 65 ~m
and
an external diameter of the core 140 of about 5 ~.m. Core 140 is preferably
made of
Si02/P205/AI203 co-doped with Er/Yb. More precisely, core 140 preferably has a
weight percentage of P205 greater than 10% (preferably about 20%), a weight
percentage of AI203 less than 2%, a concentration of erbium between 600 ppm
and
1000 ppm and a concentration of ytterbium between 1000 ppm and 20000 ppm. The
CA 02282941 1999-09-21
PC769 24
. ratio between ytterbium and erbium concentrations is preferably between 10:1
and
30:1, more preferably about 20:1. The refraction index difference between the
core
140 and the inner cladding 141 is preferably ~n = n,-n2 = 0.013 +/- 0.002 and
the
refraction index difference between the inner cladding 141 and the outer
cladding 142
is preferably On' = n2-n3 = 0.017 +I- 0.003 (due mainly to a fluorine doping
of the outer
cladding 142). The core 140 and the inner cladding 141 define a single-mode
waveguide for the guidance of transmission signals, having a first numeric
aperture
NA, = 0.19 +!- 0.02, while the inner cladding 141 and the outer cladding 142
define a
multi-mode waveguide for the guidance of pump radiation, having a second
numeric
aperture NA2 = 0.22 +/- 0.01.
To produce fiber 108, two different preforms (not shown) are used. A first
preform is used to obtain the core 140 and the inner cladding 141 and is made
by
deposing Si02 and Pz05 by means of the MCVD method (or by another known
"chemical vapor deposition" (CVD) method) into a pure silica preform, and then
by
introducing Aluminium and rare earths erbium and ytterbium by means of the
known
"solution doping" method. The first preform is opportunely worked so as to
obtain
preset geometrical ratios between the core 140 and the inner cladding 141.
A second preform of a commercial-type is used to obtain the outer cladding
142. The second preform has a central region of pure Si02 and a surrounding
region
of fluorine-doped Si02. The central region of the second preform is removed so
as to
obtain a central longitudinal hole in which the first preform is introduced.
The three-
layer preform so obtained is drawn in the usual way to obtain the optical
fiber 108.
Referring again to fig. 9, pump lasers 109 and 110 are multi-mode broad-area
lasers with an emission wavelength included in the wavelength range 920-980
nm,
for example at 920 nm, each adapted to provide a pump power of approximately
400
mW to optical fiber 108. Pump lasers 109, 110 may be, for example, model
number
MECP7PR6 supplied by E-TEK DYNAMICS, INC., 1885 Lundy Ave., San Jose, CA
(USA).
The first and the second coupler 111, 112 are micro optic (mirror-type) WDM
couplers positioned at the opposite ends of optical fiber 108. Couplers 111,
112 may
be, for example, model number FWDMCPR1PRS10 supplied by E-TEK DYNAMICS,
INC., 1885 Lundy Ave., San Jose, CA (USA). Couplers 111 and 112 include
respective first, second and third access fibers, indicated in the two cases
with 111a,
111 b, 111 c and 112a, 112b and 112c. Each of couplers 111 and 112 further
includes
a converging lens system, not shown, to opportunely shape and direct the light
beams among its access fibers, and a selective-reflection surface, e.g. a
dichroic
CA 02282941 1999-09-21
PC769 25
mirror, indicated respectively with 111d and 112d and represented
schematically with
an oblique segment. The actual inclination of the dichroic mirror inside the
coupler
depends on the direction of the incoming optical beams carrying the signal and
the
pump radiation. The selective-reflection surface in couplers 111 and 112 is
transparent for the wavelengths of the RB2 band channels and reflecting for
the
wavelength of the pumping radiation. In this way, the RB2 band channels pass
through the reflecting surface substantially without losses while the pump
radiation is
reflected by the reflecting surface into the cladding of the amplification
fiber 108.
Alternatively, each coupler 111, 112 may include a selective-reflection
surface that is
reflecting for the wavelengths of the RB2 band channels and transmissive for
the
wavelength of the pumping radiation.
Coupler 111 has its first access fiber 111 a optically coupled to the output
of
pre-amplifier 103 (through isolator 105) to receive the pre-amplified RB2 band
channels, its second access fibers 111 b optically coupled to the pump laser
109 to
receive the pump radiation and its third access fiber 111 c optically coupled
to the
input of amplification fiber 108 to feed to fiber 108 both the RB2 band
channels and
the pump radiation, in a same propagation direction.
Coupler 112 has its first access fiber 112a optically coupled to output 102
(through isolator 105) to transmit to output 102 the amplified RB2 band
channels, its
second access fiber 112b optically coupled to the pump laser 110 to receive
the
pump radiation and its third access fiber 112c optically coupled to the output
of
amplification fiber 108 to receive from amplification fiber 108 the amplified
RB2 band
channels and to feed to fiber 108 the pump radiation generated by pump laser
110 in
an opposite propagation direction with respect to the transmitted signals.
The first access fibers 111a and 112a are single-mode fibers having a core
wavelength cutoff of 1300 nm t 30 nm, a cladding diameter of 125p.m and a
numeric
aperture NA = 0.2; the second access fibers 111 b and 112b are multi-mode
fibers
having a core diameter of 65 Vim, a cladding diameter of 90 ~m and a numeric
aperture NA = 0.22; and the third access fibers 111 c and 112c are double-
cladding
fibers having the same chemical and geometrical characteristics of
amplification fiber
108, but no active ion doping.
Couplers 111 and 112 have an insertion loss of approximately 1.02 dB,
measured at 1550 nm, for optical signals passing from the first access fiber
111 al112a to the third access fiber 111 cl112c, or vice versa, and an
insertion loss of
approximately 0.22 dB, measured at 980 nm, for optical signals passing from
the
second access fiber 111 bl112b to the third access fiber 111 cl112c, or vice
versa.
CA 02282941 1999-09-21
PC769 26
Furthermore, couplers 111 and 112 have an optical isolation greater than 30
dB,
measured at 980 nm, between the first access fiber 111 a/112a and the third
access
fiber 111 c1112c and an optical isolation greater than 20 dB, measured at 1550
nm,
between the second and the third access fiber.
Figs. 10a and 10b show a numerical simulation of a two-channel transmission
through amplifying unit 100. The two channels have been chosen at 1575 nm and
1602 nm, i.e. at the extremes of the RB2 band, both with an input power (at
the input
101 ) of -13.5 dBm. In particular, figs. 10a and 10b show the estimated
spectrum and
the estimated power levels of the two channels, represented in each figure by
means
of two different cross marks, at the output of pre-amplifier 103 and,
respectively, at
the output of amplifier 104.
Figs. 11 a, 11 b and 11 c show further numerical results related to the
transmission through optical amplifier 104 of the above two channels at 1575
nm and
1602 nm. In particular, this calculation simulates the transmission of the two
channels through a line amplification section including an ErIYb amplifier 104
not
preceded by pre-amplifier 103. Figs. 11 a, 11 b and 11 c illustrate the
dependence of
the ASE (Amplified Stimulated Emission) and signal power on the length of the
active
ErIYb fiber (z) for different input signal powers, respectively -12 dBm per
channel, +5
dBm per channel and +10 dBm per channel. The results show that, in the absence
of
pre-amplifier 103, i.e. by using only Er/Yb co-doped amplifying fibers to
amplify the
RB2 band channels, signals with a power per channel of -12 dBm achieve a
relatively low gain, so that limitations on the input signal power range are
imposed.
This is deemed to be due to the high fluorescence peak at 1536 nm of the Er/Yb
co-
doped fiber. In practice, if low signal powers in the bandwidth 1575 nm - 1602
nm
are fed directly to amplifier 104, the signal gain is saturated by the
accumulation of
amplified spontaneous emission (ASE), obtaining a low output power. The
results
then show that the use of a pre-amplifier, e.g. an erbium-doped pre-amplifier,
is
advantageous to ensure high saturated output power in the proposed
configuration,
since the use of only ErIYb co-doped amplifying fibers impose a lower limit on
the
input signal (approximately 0 dBm) in order to ensure high amplifier
efficiency and
saturated output power. On the other hand, using the proposed combined
configuration (Er-doped pre-amplifier and ErIYb co-doped power amplifier)
allows
high efficiency and high saturated output power also for input powers down to -
20
dBm. This strong saturation avoids excessive amplified spontaneous
accumulation
outside the signal bandwidth.
An alternative amplification stage that, however, is less preferred than the
CA 02282941 1999-09-21
_ PC769 27
amplification scheme hereinbefore described, may include a cascaded connection
of
Er/Yb co-doped amplifying fibers, with the addition of filters to suppress the
amplified
spontaneous emission. This arrangement may be advantageously used as a power
amplifier stage with relatively high-input power signals, but may suffer,
according to
the above, a substantial waste of energy, in particular in the first stage
that receives
input signals of very low power level and that tends then to generate high
values of
amplified spontaneous emission.
Figs. 12a and 12b show experimental setups used to test amplifying unit 100.
The experimental setup in Fig. 12a includes pre-amplifier 103, an optical
generator
150 to launch optical signals into the input of pre-amplifier 103 and an
optical
spectrum analyzer 152 to detect the spectrum at the output of pre-amplifier
103. In
particular, generator 150 is, preferably, an array of 64 lasers each
generating a
respective wavelength in the RB2 band. In the experimental setup of fig. 12b,
amplifier 104 has been added in series to pre-amplifier 103, and optical
spectrum
analyzer 152 now detects the spectrum at the output of amplifier 104.
Attenuation
filters 151 and 151' may be added in the experimental setups of Figs. 12a and
12b for
the safety of optical spectrum analyzer 152. Furthermore, a power meter (not
shown)
may be added in the experimental setups of Figs. 12a and 12b to monitor the
optical
power at the output of pre-amplifier 103 and, respectively, of amplifier 104.
Figs. 13a and 13b show the spectra detected at the output of pre-amplifier
103 and, respectively, of the amplifier 104, by means of optical spectrum
analyzer
152, in response to an input signal including two channels at 1575 nm and 1602
nm
with a total power of approximately -10.5 dBm (equivalent to a power of
approximately -13.5 dBm per channel). No attenuation filter is included in
this case
because the optical power at the output of pre-amplifier 103 and,
respectively, of
amplifier 104, is not dangerous for the spectrum analyzer 152.
In the spectra of Figs. 13a and 13b, the power peaks of the two detected
channels are superimposed to the power emission of pre-amplifier 103 and,
respectively, to the combined power emissions of pre-amplifier 103 and
amplifier 104.
These experimental results confirm the numerical results of Figs. 10a, 10b. In
particular, the experimental results of Fig. 13b show that, in all the RB2
band, the
amplifying unit 100 can amplify a -10.5 dBm input power signal (such as a
signal
including two -13.5 dBm channels) to reach a total saturation output of
approximately
23 dBm. In other words, a power gain greater than 33 dB has been obtained for
input
signals in the RB2 band and with an input optical power of approximately -10
dBm.
The gain spectrum curve in the RB2 band of the cascaded pre-amplifier 103 and
CA 02282941 1999-09-21
PC769 2g
amplifier 104 arrangement is approximately flat and has a variation of 0.55 dB
between the two ends of the RB2 band at 1575 nm and 1602 nm.
Figs. 14a and 14b show the experimental power spectra detected with the
arrangements of Figs. 12a and 12b in the case of a sixty-four (64) channels
transmission and with a total input signal power of approximately -9.5 dBm. In
this
case, attenuation filters 151 (20 dB attenuation) and 151' (25 dB attenuation)
have
been added for the safety of optical spectrum analyzer 152. A saturation
output
power of approximately 22 dBm has been detected at the output of amplifier 104
by
the power meter with the arrangement of Fig. 12b. This power is slightly lower
than
that detected in the case of a two channel transmission. The observed power
difference in the two experiments is probably due to the different
characteristics of the
pre-amplifier and of the optical connectors used in the two experiments and
not to the
difference in the number of channels composing the signals. This leaves room
for
further optimization, for example by improving the amplifier design or by
increasing
the pump power, so as to reach higher saturated output powers. In particular,
the
Applicant has determined that, by increasing the pump power above 400 mW and
by
optimizing the optical connections, output powers up to 26 dBm can be reached
with
an input power of approximately -10 dBm.
The experimental results of Figs. 13 and 14 show that, when the optical
signals have an input power around approximately -10.5 dBm or -9.5 dBm and
wavelengths between 1575 nm and 1602 nm, the optical amplifying unit of the
invention allows a power gain greater than 25 dB. In particular, with the
optical
amplifying unit of the invention operating in the mentioned conditions (i.e.
with an
input power of at least -10.5 dBm), a power gain greater than 31 dB, and in
particular greater than 33 dB, has been demonstrated. These values are much
higher than the observed 29 dB limit of erbium-doped amplifiers in the same
conditions.
In Fig. 15 a second embodiment of an amplifying unit for use in the optical
transmission system 1 to amplify the RB2 band is illustrated. Amplifying unit
100' of
Fig. 15 differs from amplifying unit 100 of fig. 9 only in the structure of
erbium/ytterbium amplifier, here indicated with 104'. The other optical
components of
amplifying unit 101' are indicated with the same reference numbers as the
corresponding components in fig. 9.
Amplifier 104' is a double stage amplifier with co-propagating pumping,
including a first and a second amplification fiber 113, 114 doped with erbium
and
ytterbium, a first and a second pump laser 115, 116 to provide pump radiation
to the
CA 02282941 1999-09-21
PC769 2g
first and, respectively, the second amplification optical fiber 113, 114, a
first and a
second micro optic (mirror-type) WDM optical coupler 117, 118 to optically
couple the
first and, respectively, the second pump laser 115, 116 to the first and,
respectively,
the second amplification optical fiber 113, 114, and, preferably, a noise
rejection filter
119, positioned between the two amplification stages, to suppress part of the
amplified spontaneous emission of amplification fibers 113, 114.
First and second amplification fiber 113, 114 may for example include two
stretches of a same erbiumlytterbium fiber, preferably of the same type of
fiber 108 of
Fig.9. Fiber lengths may be chosen depending on the system requirements.
Pump lasers 115, 116 are multi-modal lasers, for example of the same type of
pump lasers 109, 110, and provide a pumping radiation between 920 nm and 980
nm
with a pumping power of approximately 400 mW.
Couplers 117, 118 are micro optics couplers of the same type of couplers 111
and 112 in fig. 9. In particular, each of the micro optic couplers 117 and 118
includes
a converging lens system and a selective-reflection surface that is, for
example,
transparent for the wavelengths of the RB2 band channels and reflecting for
the
wavelength of the pumping radiation. In this way, the RB2 band channels pass
through the reflecting surface substantially without losses while the pump
radiation is
reflected by the reflecting surface into the cladding of the amplification
fiber 113 and,
respectively, 114, in the same direction with respect to the transmitted
signals.
Noise rejection filter 119 is positioned between the amplification fiber 113
and
the coupler 118 and is adapted to filter the amplified spontaneous emission of
amplification fibers 113, 114 so as to improve the performances of amplifying
unit
100' in terms of output power and in terms of channel equalization. Another
rejection
filter (not shown) may be positioned between isolator 106 and output 102, in
alternative or in addition to filter 119. Amplifying section 100' can provide
a saturation
output power of approximately 22 dBm with an input signal power of
approximately -
10 dBm.
Fig. 16 shows a third embodiment of an amplifying unit adapted to be used in
system 1 to amplify the RB2 band. Amplifying unit 100" of Fig, 15 differs from
amplifying units 100 of Fig. 9 and 100' of Fig. 15 only in the structure of
erbium/ytterbium amplifier, here indicated with 104". The other optical
components
of amplifying unit 100" are indicated with the same reference numbers as the
corresponding components in fig. 9 and fig. 15.
Amplifier 104" is a double stage amplifier with bi-directional pumping,
including a first and a second amplification fiber 120, 121 co-doped with
erbium and
CA 02282941 1999-09-21
PC769 30
ytterbium, a first and a second pump laser 122, 123 to provide pump radiation
to the
first and, respectively, the second active fiber 120, 121, a first and a
second WDM
optical coupler 124, 125 to bi-directionally couple the first pump laser 122
to the first
fiber 120, a third and a forth WDM optical coupler 126, 127 to bi-
directionally couple
the second pump laser 123 to the second fiber 121, and a noise rejection
filter 128 to
suppress part of the amplified spontaneous emission of fiber 120, 121.
First and second active fibers 120, 121 are preferably double-clad fibers of
the
same type of fiber 108 in Fig. 9, having respective lengths that may be chosen
depending on the system requirements. Pump lasers 122, 123 are multi-modal
lasers, for example of the same type of pump lasers 109, 110, providing a
pumping
radiation between 920 nm and 980 nm with a pumping power of 400 mW.
Couplers 124-127 are adapted to feed the pump radiation into the inner
cladding of the active fibers 120, 121. Couplers 124 and 126 are preferably
fused
fiber WDM couplers of the type 96011550 nm or 92011550 nm. For example,
couplers
124 and 126 are model MW9850-P05 made by the Applicant. In details, coupler
124
is interposed between pump laser 122 and fiber.120 and coupler 126 is
interposed
between pump laser 123 and fiber 121. Each of the coualers 124. 126 has a
first
access fiber to receive the RB2 band channels, a second access fiber to
receive the
pump radiation, a third access fiber to feed to the corresponding
amplification fiber
120, 121 both the RB2 band channels and approximately 50% (typically about
48%)
of the power of the pump radiation and a fourth access fiber in which the
residual
pump radiation (approximately 50% of the power of the pump radiation) is
conveyed.
Preferably, the first and the third access fibers are opposite end portions of
a same
double-clad fiber, while the second and the fourth access fibers are opposite
end
portions of a same multi-modal fiber.
Couplers 125 and 127 are preferably micro optic (mirror-type) WDM couplers
of the same type of couplers 111 and 112 of fig. 9, each positioned on the
opposite
side of the corresponding fiber amplifier 120, 121 with respect to the
corresponding
fused fiber coupler 124, 126. Each micro optic coupler 125, 127 has a first
access
fiber optically coupled to the active fiber 120 and, respectively, 121; a
second access
fiber optically coupled to the fourth access fiber of the corresponding fused
fiber
coupler 124, 126 by means of a respective optical fiber 130, 131, to receive
the
residual pump radiation and to transmit it into the respective active fiber
120, 121 in a
counter-propagating direction, with a loss not greater than 0.3 dB, more
precisely
about 0,22 dB; and a third access fiber for conveying the amplified
transmission
signals. Each micro optic coupler 125, 127 has a signal pass-band including
the RB2
CA 02282941 1999-09-21
- PC769 31
band so that the RB2 band channels are transmitted with a very low loss.
The particular coupling arrangement hereinbefore described is adapted to
feed to each fiber amplifier 120, 121 about 85% of the pump power generated by
the
respective pump laser 122, 123 and provides therefore a very high efficiency
pumping.
Like the amplifying section 100' of Fig. 15, the amplifying section 100" is
adapted to provide a saturation output power up to 22 dBm with input powers of
approximately -10 dBm.
It will be understood that various changes in the details, materials, steps
and
arrangements of parts that have been described and illustrated above in order
to
explain the nature of the invention may me made by those of ordinary skill in
the art
within the principle and scope of the present invention as expressed in the
appended
claims.
For example, a transmission system can be made having a transmission
wavelength band which includes all and only the RB2 band, or part of the RB2
band
or a band including the RB2 band. A transmission system adapted to transmit
only in
the RB2 band may for example comprise a transmitting unit for generating sixty-
four
channels spaced by 50 GHz at 2,5 Gbitls, an optical link coupled at one end to
the
transmitting unit and a receiving unit coupled to another end of the optical
link.
The optical link may have fiber spans even longer than 130 km between
subsequent amplification stages, each fiber span introducing an approximate
loss of
28 dB (using fibers with loss about 0, 22 dBlkm).
