Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PUMP DEVICE FOR PUMPING AN ACTIVE FIBER OF AN OPTICAL AMPLIFIER
AND CORRESPONDING OPTICAL AMPLIFIER
It is an object of the present invention to provide a pump device for pumping
an active fiber of an optical amplifier. In particular, it is an object of the
present
invention to provide a pump device for coupling a pump radiation to an an
optical
amplifier adapted to be used in an optical transmission system, for example a
wavelength division multiplexing (WDM) optical transmission system. The
invention
also relates to an optical amplifier that uses the above mentioned pump
device.
Conventional optical fiber amplifiers include active fibers having a core
doped
with a rare earth element. Pump power at a characteristic wavelength for the
rare
earth element, when injected into the active fiber, excites the ions of the
rare earth
element, leading to gain in the core for an information signal propagating
along the
fiber.
Rare earth elements used for doping typically include Erbium (Er),
Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), and Praseodymium (Pr). The
particular rare earth element or elements used is determined in accordance
with the
wavelength of the input signal light and the wavelength of the pump light. For
example, Er ions would be used for input signal light having a wavelength of
1.55pm
and for pump power having a wavelength of 1.48um or 0.98~m; co-doping with Er
and Yb ions, further, allows different and broader pump wavelength bands to be
used.
Traditional pump sources include single mode laser diodes and multi-mode
broad area lasers coupled to the active fiber over single mode and multi-mode
pumping fibers, respectively, to provide the pump power. Single mode lasers
provide
low pump power, typically in the order of 100 mW. Broad area lasers, on the
other
hand, provide high pump power, in the order of 500 mW. These lasers of high
output
power, however, cannot efficiently inject light into the small core of a
single mode
fiber. Consequently, the use of high power broad area lasers requires the use
of
wide core and multi-mode fibers for pumping optical amplifiers. This non-
active
pumping fiber in turn typically inputs the pump power through a coupler and
into the
active fiber, for example into the inner cladding of a double-clad active
fiber, acting as
a multi-mode core for the pump power.
In a double-clad amplifier fiber, pump power is guided into the inner multi
mode cladding of the fiber from which it is transferred into a single mode
core doped
with an active dopant. The double-cladding fiber pumping mechanism is
described
a
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for example in WO 95110868. This document discloses a fiber optic amplifier
comprising a fiber with two concentric cores. Pump power provided by multi-
mode
sources couples transversely to the outer core (equivalent to an inner
cladding) of the
fiber through multi-mode fibers and multi-mode optical couplers. The pump
power
propagates through the outer core and interacts with the inner core to pump
active
material contained in the inner core. This pumping technique is also described
in US
5291501, which illustrates a mono-mode optical fiber with doped core and doped
inner cladding.
A well-known basic amplifying system includes a multi-mode pump source
coupled to an amplification fiber, for example an ErIYb doped double-clad
fiber, via a
conventional fused fiber wavelength division multiplexer (WDM) type coupler.
WDM
couplers behave as multi-mode couplers for the pump power and transmit the
single
mode signals along the amplification fiber substantially without coupling to
the pump
fiber. During the pumping operation, most of the outer modes of the pump power
are
transmitted to the amplification fiber, leaving the inner modes of the pump
power
unused. In the case of a multi-mode or a double cladding amplifier fiber, a
fused fiber
coupler has a theoretical coupling coefficient directly proportional to the
ratio of the
areas of the two fibers constituting the coupler itself. In an ideal case for
two identical
fibers, the coupling coefficient is approximately 50%, but in practice it is
in the range
of 45-48%. This means that only about 45-48% of the total pump power passes
from
the pumping fiber into the inner cladding of the double-clad active fiber,
while the
remaining 52-55% remains in the pumping fiber.
Some systems use two optical fibers having different diameter of cores to
improve the coupling coefficient of the multi-mode coupler. However, such
arrangements often lead to a waste of power due to the difficulty in matching
the
tapering of two cores of different size.
To increase the coupling efficiency, the Applicant has considered the
possibility of using micro optic couplers. Micro optic couplers couple optical
beams
using a wavelength selective mirror and a focusing lens system. With this
construction, micro optic couplers obtain much better coupling efficiencies
than
traditional WDM couplers, typically in the range of 89%. Applicant has
remarked that
micro optic couplers have several drawbacks that limit their use for pump
coupling in
fiber amplifiers. In particular, if a single micro optic coupler is used up-
stream of the
active fiber so as to feed the pump radiation to the active fiber in a co-
propagating
direction, the transmission signals passing through the coupler undergo a
power loss
that is much higher than the loss introduced by a fused fiber coupler and that
may be
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excessive (particularly in consideration of the fact that the signals undergo
the
attenuation before being amplified and this leads to an increase of the noise
figure for
the amplifier). Alternatively, if a single micro optic coupler is positioned
down-line
with respect to the active fiber so as to feed the pump radiation to the
active fiber in a
counter-propagating direction, the signal to noise ratio undergoes a reduction
which
again may be excessive. Moreover, due to high coupling efficiency achievable
by
using a micro optic coupler, and then to the high pump power fed into the
fiber, an
inhomogeneous distribution of the population inversion is produced along the
fiber.
More recent systems have attempted to recover the lost pump power in a
conventional fused fiber coupler by means of different pumping schemes using
fused
fiber couplers. In particular, different solutions have been proposed that
include a
second optical coupler in addition to a first optical coupler positioned
according to the
above-described single-coupler arrangement. The second coupler is positioned
at
the opposite end of the active fiber with respect to the first coupler and is
coupled to
the first coupler through a multi-mode pump fiber so as to receive the
residual pump
power (i.e. the fraction of the pump power that has not been directly fed to
the active
fiber by means of the first coupler). The second coupler is then adapted to
couple the
residual pump power to the same active fiber in a counter-propagating
direction, or to
a different active fiber in a co-propagating direction. The proposed pumping
schemes
using the above-mentioned technique to recover the pump power include only
couplers of the fused fiber type. The Applicant observed that the addition of
a second
fused fiber coupler does not significantly improve the total pump power
transfer over
the single-coupler system described above. In fact, the second coupler
receives from
the pump fiber prevalently internal modes left over by the first coupling
operated by
the first coupler, and the transfer of the internal modes into the active
fiber is
inefficient.
EP patent application No 97114622.0 in the name of the Applicant proposes a
technique to improve the total coupling efficiency in the above two couplers
pumping
schemes. The improvement is obtained by interposing, along the pump fiber
connection coupling the first and the second coupler, a mode scrambler, i.e. a
device
that operates a scrambling of the inner modes on the residual pump radiation
so as to
regenerate a high number of external modes that can be efficiently transferred
into
the active fiber through the second coupler. Under ideal circumstances, 50% of
the
pump power signal enters the active fiber at each of the couplers. This would
lead to
a total coupling efficiency for the coupling system of 75%. In practice,
however, the
total coupling efficiency is close to 68%.
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EP patent application No. 97114620.4 in the name of the Applicant proposes
a different solution for the same problem, consisting in the use of two
unequal
couplers manufactured by a fusion biconical tapering technique. In practice,
the first
and the second couplers have different fusing and tapering amounts with the
active
fiber so as to achieve a better coupling efficiency. This further pumping
solution
allows a total coupling efficiency of approximately 66%.
It is an object of the present invention is to provide a pump device that is
adapted to couple pump radiation to the active fiber (or active fibers) with a
better
coupling efficiency with respect to the known optical amplifiers.
The Applicant has found that a very high coupling efficiency can be achieved
by providing a first coupler with a very low insertion loss for the signals
(even if it has
a very low pump coupling efficiency) and a coupler having a high pump coupling
efficiency (even if it has a relatively high insertion loss).
In particular, the Applicant has found that a very high coupling efficiency
can
be obtained by means of a double-coupler arrangement in which a first coupler,
adapted to receive a pump radiation from a pump source and to couple it to the
active fiber, has an insertion loss for optical signals not greater than 0,2
dB, and a
second coupler, adapted to receive a residual fraction of the pump radiation
from the
first coupler and to couple it to the active fiber, has a coupling efficiency
for the pump
radiation not less than 70%.
The couplers can feed the opposite ends of a single active fiber section (in
which case the second coupler counter-pumps the fiber) or can feed two
separate
sections of active fiber. In this case the second coupler preferably pumps the
second section of fiber in a co-propagating direction, while the first coupler
can pump
in either direction the first section.
The Applicant has in particular found that a double-coupler arrangement in
which said first coupler is a fused fiber coupler and said second coupler is a
micro
optic coupler can provide a total coupling efficiency up to 85%, much higher
than the
maximum coupling efficiencies achievable with known arrangements.
According to a first aspect, the present invention relates to a pump device
for
coupling a pump radiation into an active fiber of an optical amplifier, said
optical
amplifier being adapted to amplify optical signals, said pump device including
a
multi-mode optical fiber to receive and convey a multi-mode pump radiation, a
first
optical coupler for optically coupling a first fraction of said pump radiation
to said
active fiber, a second optical coupler that is optically coupled to said first
coupler to
receive from said first coupler a second fraction of said pump radiation and
that is
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further optically coupled to said active fiber to feed to said active fiber at
least part of
the second fraction of said pump radiation, characterized in that said first
optical
coupler has an insertion loss for said optical signals less than or equal to
0.2 dB and
said second optical coupler has a coupling efficiency for said pump radiation
of at
least 70%.
Preferably, said first optical coupler is a fused fiber coupler and said
second
optical coupler is a micro optic coupler.
Preferably, the sum of the optical power of the first fraction of said pump
radiation and of the optical power of said at least part of the second
fraction of said
pump radiation is more than 75% of the total optical power of said pump
radiation.
More preferably, said sum is at least 85% of the total optical power of said
pump
radiation.
Preferably, the pump device further comprises a pump optical fiber optically
coupling said second optical coupler to said first optical coupler; said pump
optical
fiber being a multi-modal optical fiber adapted to transmit optical radiation
without
substantial energy transfer between modes
Preferably, said first optical coupler has a first access fiber; a second
access
fiber that is a multi-modal fiber and is optically coupled to said multi-mode
optical
fiber to receive said pump radiation; a third access fiber that is of the same
type of
said first access fiber adapted to be coupled to said active fiber to feed to
said active
fiber the first fraction of said pump radiation; and a fourth access fiber, of
the same
type of said second access fiber, into which said second fraction of said pump
radiation is conveyed; and said second coupler has a first access fiber that
is a multi-
modal fiber optically coupled to said fourth access fiber of said first
optical coupler to
receive the second fraction of said pump radiation; a second access fiber that
is a
double-cladding fiber adapted to be coupled to said second active fiber to
feed to
said second active fiber said at least part of the second fraction of said
pump
radiation, and a third access fiber for conveying said optical signals.
Preferably, said first access fiber of said first coupler is a single-mode
fiber
adapted to be coupled to an optical input to receive said optical signals and
said third
access fiber of said second coupler is a single-mode fiber adapted to be
coupled to
an optical output to feed to said optical output said optical signals.
According to a second aspect, the invention relates to an optical amplifier
including an optical input for the input of optical signals, an optical output
for the
output of said optical signal, an active fiber interposed between said input
and said
output and adapted to amplify said optical signals, a pump source for
generating a
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pump radiation and a pumping device according to the above to optically couple
said
pump radiation to said active fiber.
Preferably, said active fiber is a double-cladding fiber.
Preferably, said active fiber comprises two fiber sections, each coupled to a
respective one of the two couplers.
According to another aspect, the invention relates to an optical amplifying
unit
including two optical amplifiers according to the above arranged in series.
Preferably, said optical amplifying unit further includes a pre-amplifier
arranged in series with said optical amplifiers.
Preferably, said optical amplifying unit further includes at least one noise
rejection filter arranged in series with said optical amplifiers.
According to another aspect, the invention relates to an optical transmission
system including an optical transmitting unit adapted to transmit an optical
signal, an
optical receiving unit to receive said optical signal, an optical fiber link
optically
coupling said transmitting unit to said receiving unit, characterized in that
it further
includes an active fiber positioned along said optical fiber link to amplify
said optical
signal, a pump source to generate pump radiation and a pump device according
to
the above to couple said pump radiation to said active fiber.
According to a further aspect, the invention relates to a method for coupling
a
pump radiation into an active fiber adapted to amplify optical signals,
comprising the
following steps:
- guiding an optical signal;
- guiding a multimode pump radiation;
- inputting said optical signal and said pump radiation to said active fiber;
said optical signal being input with a predetermined insertion loss and said
pump radiation being input so as to feed a first power fraction to said
active fiber and to obtain a residual power fraction;
- inputting said residual power fraction to said active fiber with a
predetermined coupling efficiency so as to feed a second power fraction to
said active fiber;
characterized in that said insertion loss is lower than or equal to 0.2 dB and
said
coupling efficiency is at least 70%. Preferably, the sum of said first and
second power
fractions is more than 75%, and more preferably at least 85%, of the optical
power of
said pump radiation.
The foregoing general description and the following detailed description are
exemplary and explanatory only and are not restrictive of the invention as
claimed.
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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
optical transmission system of fig. 1;
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;
Fig. 8 is a detailed diagram of a multiplexing section of the optical
transmission system of Fig. 1;
Fig. 9 is a schematic representation of an optical amplifier including the
pumping device of the invention;
Fig. 10 is a schematic representation of an amplification section including
the
amplifier of Fig. 9;
Figs. 11 a and 11 b are schematic representations of a double-cladding fiber
used in the optical amplifier of the invention and of the multi-mode pumping
operation
of a double cladding fiber.
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
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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.
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 or, for
example,
of the type described in EP patent application No. 98110594.3 in the name of
the
Applicant.
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
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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
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 and 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, corresponds 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, corresponds 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, corresponds, according to the
invention, to an amplification wavelength range of erbium/ytterbium-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
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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
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 (TXIRX) 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 WCM 17-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,
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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
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 WCM17-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
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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
(although other rare-earth-doped fiber amplifiers may be used), while
amplifier
section 53 is, according to the invention, an erbiumlytterbium-doped (ErIYb)
fiber
amplifier that will be described in details with reference to fig. 10.
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
low 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,
CA 02282940 1999-09-21
PC770 13
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. 10.
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
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 filter (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 1480/1550 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
CA 02282940 1999-09-21
PC770 14
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.
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
and/or 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 filter 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,
CA 02282940 1999-09-21
PC770 15
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
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 of a
known type or of the type described in EP patent application No. 98110594.3 in
the
name of the Applicant. 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
CA 02282940 1999-09-21
- PC770 16
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
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
CA 02282940 1999-09-21
PC770 17
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,
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 erbiumlytterbium co-doped amplifier adapted to amplify the RB2
band
and will be described in details with reference to Fig. 10.
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
1480/1550
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 subunits coupled upstream one or more of amplifiers
65, 67,
69 for compensating the dispersion of channels in one or more than one of 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 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
CA 02282940 1999-09-21
' - PC770 1 g
- 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
arrangement, five spans of long-distance transmission fiber are used (each
having a
power loss of 0,22 dBlkm 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
CA 02282940 1999-09-21
PC770 19
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,
preferably, an
erbium-doped amplifier of a known type.
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 ~,,, ~,3,
... , 7~,5
separated by 100 GHz and a second sub-band BB" having channels ~,2, ~,4, ....,
7~,s
separated by 100 GHz and interleaved with the channels in the subband 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.
CA 02282940 1999-09-21
- PC770 20
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
channels within the sub-bands BB', BB", RB1', RB1 ", RB2', RB2".
In a preferred configuration, dernultiplexer 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, ... , 7~,5, and WD 95" demultiplexes channels ~,2, ~,a, ... , ~,,s.
Both WD 95' and
WD 95", however, may be 1 x8 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 ~,,~-aa4 and WDs 97' and 97" receive and
demultiplex first sub-band RB2' and second sub-band RB2", respectively, to
produce
channels 7,.ss-~.,2g. 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
Rx 1-Rx 128.
Fig. 9 illustrates an optical amplifier 108 including a pumping device
according
to the present invention. Optical amplifier 108 can be for example used in the
optical
transmission system 1 to amplify signals in the RB2 band and is included in
the
amplifier sections 53 of fig. 4 and 69 of fig. 6.
Amplifier 108 is a bidirectionally-pumped optical amplifier including an input
port 110 for the input of optical signals to be amplified, an output port 120
for the
output of the optical signals after amplification, an active fiber 130 that
optically
couples the input port 110 to the output port 120 and is adapted to amplify
the optical
CA 02282940 1999-09-21
- PC770 21
signals, a pump source 145 for generating pump radiation and a pumping device
140
optically coupling to the active fiber 130 said pump radiation.
The active fiber 130 is, for example, a double-cladding fiber co-doped with
erbium and ytterbium and will be hereinafter described in details with
reference to
Fig. 11 a, where a not-in-scale section of optical fiber 130 is shown. Fiber
130
includes a core 139 having a first refraction index n~, an inner cladding 141
that
surrounds the core 139, is coaxial to the core 139 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< nz. As shown in
fig. 11 b,
under normal operating conditions of amplifier 108, while the transmitted
signals are
confined into the core 139, the pump radiation is fed into the inner cladding
141 and
is progressively absorbed by the core 139, exciting the active medium.
The active fiber 130 has, for example, 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 139 of about 5 pm. The core 139 may, for
example
be composed by SiO~IP20~/A1203 co-doped with ErIYb, and may have a weight
percentage of Pz05 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. For example, the
ratio between ytterbium and erbium concentrations is about 20:1. The
refraction
index step between the core 139 and the inner cladding 141 is for example0n =
n~-n2
= 0.013 +I- 0.002 and the refraction index step between the inner cladding 141
and
the outer cladding 142 is for example On' = n2-n3 = 0.017 +~- 0.003 (due
mainly to a
fluoride doping of the outer cladding 142). The core 139 and the inner
cladding 141
define a single-mode waveguide for the conveying of transmission signals,
having for
example a first numeric aperture NA, = 0.19 +I- 0.02, while the inner cladding
141
and the outer cladding 142 define a multi-mode waveguide for the conveying of
pump
radiation, having for example a second numeric aperture NA2 = 0.22 +I- 0.01.
To produce fiber 130, two different preforms (not shown) are used. A first
preform is used to obtain the core 139 and the inner cladding 141 and is made
by
deposing Si02 and P205 by means of the known "chemical vapor deposition" (CVD)
method, and then by introducing the 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 139 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
CA 02282940 1999-09-21
PC770 22
of fluoride-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 130.
Referring again to Fig. 9, the pumping device 140 includes a first optical
coupler 150 that optically couples the pump source 145 to the active fiber 130
and a
second optical coupler 160 optically coupled to the first coupler 150 to
receive from
the first coupler 150 the residual pump radiation (i.e. the fraction of the
pump
radiation that has not been directly fed to active fiber 130 by the first
coupler 150) and
to feed the residual pump radiation to the active fiber 130.
The pump source 145 is preferably a multi-modal laser, providing a pumping
radiation between 920 nm and 980 nm with a pumping power of 400 mW. Pump
source 145 is optically coupled to optical coupler 150 by means of a multi-
modal
optical fiber 170. Pump source 140 may be, for example, model number MECP7PR6
supplied by E-TEK DYNAMICS, INC., 1885 Lundy Ave., San Jose, CA (USA).
The first optical coupler 150 has an insertion loss for the optical signals
not
greater than 0.2 dB and the second optical coupler 160 has a pump coupling
efficiency preferably not less than 70%, and more preferably not less than
80%. In
other words, the optical signals transmitted from input 110 to active fiber
130
undergo, by passing through the first coupler 150, an attenuation not greater
than 0.2
dB and the residual pump radiation is coupled to the active fiber 130 by means
of the
second coupler 160 with a coupling efficiency not less than 70%.
Preferably, the first optical coupler 150 is a fused fiber coupler having an
insertion loss for the optical signals of approximately 0.1 dB, for example a
double
cladding WDM coupler of the type 960/1550 nm or 920/1550 nm. For example,
couplers 124 and 126 are model MW9850-P05 made by the Applicant.
The first coupler 150 has a first access fiber 151, for example a double-
cladding fiber, optically coupled to the input port 110 to receive the signals
to be
amplified; a second access fiber 152, for example a multi-modal (with single
cladding)
fiber, optically coupled to the pump source 140 to receive pump radiation; a
third
access fiber 153 of the same type of first access fiber 151, optically coupled
to the
active fiber 130 to feed to the active fiber 130 the optical signals to be
amplified
together with a first fraction of the pump radiation, which represents
approximately
50% (typically about 48%) of the power of the pump radiation; and a fourth
access
fiber 154 of the same type of the second access fiber 152, that carries a
second
fraction of the pump radiation, which defines the residual pump radiation.
Optical coupler 160 is a micro optics (mirror-type) double cladding WDM
CA 02282940 1999-09-21
PC770 23
coupler having a pump coupling efficiency of approximately 90%. Couplers 160
may
be, for example, model number FWDMCPR1PRS10 supplied by E-TEK DYNAMICS,
INC., 1885 Lundy Ave., San Jose, CA (USA). Coupler 160 includes a first, a
second
and a third access fiber 161, 162, 163, a converging lens system not shown, to
opportunely shape and direct the light beams among its access fibers, and a
reflection-selective surface, e.g. a dichroic mirror, indicated with 164 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 130.
Alternatively,
coupler 160 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 160 has its first access fiber 161 optically coupled, by means of a
pump optical fiber 180, preferably a multi-modal fiber, to the fourth access
fiber 154
of the first optical coupler 150 so as to receive the second fraction of the
pump
radiation, i.e. the residual pump radiation; a second access fiber 162
optically
coupled to the active fiber 130 to receive from the active fiber 130 the
amplified
optical signals and to feed to the inner cladding of the active fiber 130 a
main fraction
(about 90%) of the second fraction of the pump radiation (reflected by mirror
164);
and a third access fiber 163 optically coupled to the output port 120 to feed
to the
output port 120 the amplified optical signals, transmitted through mirror 164.
More in details, in the preferred embodiment here disclosed, the first access
fiber 161 is a multi-mode fiber having a core diameter of 65pm, a cladding
diameter
of 90 ~m and a numeric aperture NA = 0.22; the second access fiber 162 is a
double-
cladding fiber having the same geometrical characteristics of amplification
fiber 108;
and the third access fiber 163 is a single-mode fiber having a core wavelength
cutoff
of 1300 nm t 30 nm, a cladding diameter of 125 pm and a numeric aperture NA =
0.2.
Coupler 160 has an insertion loss of approximately 1.02 dB, measured at
1550 nm, for optical signals passing from the second access fiber 162 to the
third
access fiber 163, and an insertion loss of approximately 0.22 dB, measures at
980
CA 02282940 1999-09-21
- PC770 24
nm, for optical signals passing from the first access fiber 161 to the second
access
fiber 162. Furthermore, coupler 160 has an optical isolation greater than 30
dB,
measured at 980 nm, between the second access fiber 162 and the third access
fiber
163, and an optical isolation greater than 20 dB, measured at 1550 nm, between
the
first access fiber 161 and the second access fiber 162.
The particular coupling arrangement hereinbefore described is adapted to
feed to the active fiber 130 approximately 85% of the pump power generated by
the
pump source 140 and provides therefore a very high efficiency pumping.
The coupling of the residual pump radiation into the active fiber 130 by means
of the second coupler 160 is substantially independent from the mode
distribution of
the pump radiation received by the second coupler 160, so that the pump
optical fiber
180 may be a typical multi-modal fiber and no scrambling device to
redistribute the
energy between optical modes is needed. The residual pump radiation is then
transmitted from the first coupler 150 to the second coupler 160 without a
substantial
energy transfer between modes.
In alternative arrangements not shown, . the second coupler 160 may be
optically coupled to a further active fiber positioned in series to the above-
described
active fiber 130, so as to feed the residual pump radiation to the further
active fiber in
a co-propagation direction (if the second coupler is positioned between the
two active
fibers) or in a counter-propagation direction (if the second coupler is
positioned down-
stream with respect to the further active fiber). The first of the two
alternative
arrangements is preferred in that the further active fiber can compensate the
signal
power loss due to the second coupler.
Pump device 140 has been tested together with the optical amplifier 108, by
feeding an optical signal at 1550 nm to the input 110 with an input power of +
3 dBm
and detecting it at the output 120. Using ErIYb active fiber with a length of
4 m, the
following results have been obtained:
- Output power = 17 dBm
- Noise Figure = 5.8 dB
The Applicant has also tested, in the same experimental conditions but using
an active fiber with a length of 6 m, a pump device including a single micro-
optic
coupler in a co-propagating pumping scheme or in a counter-propagating scheme,
obtaining the following results:
Co-propagating pumping scheme:
- Output power = 16.8 dBm
- Noise Figure = 6 dB
CA 02282940 1999-09-21
' _ PC770 25
Counter-propagating pumping scheme:
- Output power = 17 dBm
- Noise Figure = 7 dB
These results confirm that, in the same conditions in terms of pumping power
and transmission wavelength, the pumping device of the invention offers better
performances with respect to a pump device at least in terms of noise figure.
Fig. 10 illustrates an amplifying unit 100 which is a preferred embodiment for
the amplifier sections 53 of fig. 4 and 69 of fig. 6 and is adapted to amplify
the RB2
band channels up to 22 dBm. Amplifying unit 100 substantially comprises a pre-
amplifier 103 and a double-stage amplifier 104, each stage of amplifier 104
being of
the same type of amplifier 108 of Fig. 9.
In details, amplifying unit 100 includes an input 101, an output 102, an
erbium
fiber pre-amplifier 103, an erbiumlytterbium fiber amplifier 104, a first
optical isolator
105 and a second optical isolator 106. 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 the channels of RB2 band
received
at the input 101. Pre-amplifier 103 may be, for example, a single-stage erbium-
doped fiber amplifier of a known type, pumped at 970-980 nm. Pre-amplifier 103
receives the RB2 band from the input 101 and amplifies the RB2 channels to a
first
power level, up to 15-17 dBm. The first amplification performed by pre-
amplifier 103
is important to reach a high power level at the output of amplifier 104. Pre-
amplifier
103 also improves the noise figure (NF) of the amplifying section 100 and
permits to
equalize the RB2 band channels.
The first isolator 105 is positioned between pre-amplifier 103 and amplifier
104 and is adapted to block light directed from amplifier 104 towards pre-
amplifier
103. The second isolator 106 is positioned between amplifier 104 and output
102
and is adapted to block the light directed from output 102 towards optical
amplifier
104. The first or the second isolator 105, 106 may be differently positioned
or
omitted, or other isolators may be added between the input 101 and the output
102,
depending on system requirements.
Amplifier 104 is a double stage amplifier, each stage having a bi-directional
pumping. Amplifier 104 includes a first and a second amplification stage 108',
108"
which are substantially the same as optical amplifier 108 of the invention and
whose
parts are indicated with the same reference numbers as in Fig. 9 and are
distinguished by apex. First and second amplification stages 108', 108" differ
only in
the length of the respective active fibers, which is preferably 16 m for fiber
130' and
CA 02282940 1999-09-21
' PC770 26
' 18 m for fiber 130".
A noise rejection filter 107 is preferably positioned between the two
amplification stages 108' and 108" to suppress part of the amplified
spontaneous
emission of fiber 130' and 130".
The pump device of the invention, with respect to a pump device including a
single dichroic coupler (which offers better performances with respect to pump
devices using fused fiber couplers) provides output signal powers
substantially of the
same order but allows a better distribution of the pump radiation into the
active fiber
offering better performances in terms of noise figure, in particular thanks to
the more
homogeneous population inversion and the lower insertion loss.
It should be readily ascertained by those skilled in the art that the
invention is
not limited by the embodiments described above which are presented herein as
examples only. Pumping device 140, optical amplifier 108, amplifying unit 100
and
transmission system 1, in fact, may be modified in various ways within the
scope of
protection as defined by the appended patent claims.
In particular, the pump device of the invention can also be advantageously
used to pump a multi-modal active fiber (or more than one multi-modal active
fibers)
instead of a double cladding fiber. In this case, the access fibers of the
first and the
second coupler 150, 160 that are directly coupled to the active fiber are
multi-modal
fibers.
Furthermore, in a less preferred embodiment, the order of the two couplers
may be changed, so that the fused fiber coupler feeds pump radiation to the
active
fiber in a counter-propagation direction with respect to the transmitted
signals, and
the micro optic coupler feeds the residual pump radiation to the active fiber
in a co-
propagation direction with respect to the transmitted signals.
Moreover, the use of the pump device and of the optical amplifier of the
invention of the invention in not restricted to the particular signal
transmission
wavelength band and pump radiation wavelength band hereinbefore considered and
can be used, for example, to pump active fibers operating in the whole
emission
range of ErIYb, between about 1525 and 1620 nm.
