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 can include one or more
optical
amplifiers, 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 OE/EO 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. This
pump
radiation, when injected into the active fiber, excites the ions of the rare
earth
element, leading to gain in the core for an information bearing signal
propagating
along the fiber.
Rare earth elements used for doping typically include Erbium (Er),
Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), Thulium (Tm) 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
1.55~m
and for pump power having a wavelength of 1.48~m or 0.98um; co-doping with Er
and Yb ions, further, allows different and broader pump wavelength bands to be
used.
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
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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 Er/Yb co-doped optical
amplifier
pumped by diode-pumped Nd:YLF laser", Electronics Letters, 1992, 28, (13) pp.
1275-1276, and in Maker, Ferguson, "1.56 pm 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 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 Er/Yb second stage is pumped by
means of a diode-pumped Nd-doped fiber laser emitting at 1064 nm. This pump
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source, largely used for the excitation of mono-modal amplification fiber, is
relatively
expensive and bulky.
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, for the excitation of Yb ions, 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 Er/Yb
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
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-
cladding 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-cladding 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 to 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
95/10868.
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
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interacts with the inner core to pump active material contained in the inner
core. This
pumping technique is also described in US 5,291,501, which illustrates a mono-
mode
optical fiber with doped core and doped inner cladding.
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 due 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, in
particular 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
transmission wavelength region around 1590 nm in parallel to the 1530 and 1550
wavelength regions of erbium-doped fiber amplifiers, 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.
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 SiOz-AI203-Ge02 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
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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 um 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., "Wideband, gain-flattened, erbium-doped fibre amplifiers with 3
dB bandwidths of >50 nm", ELECTRONICS LETTERS, 5~" 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 conventional line EDFA adapted to amplify
optical signals in the high wavelength band can tipically amplify an optical
signal
having a total 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. A total
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 such that the signal to noise ratio becomes too 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
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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. Presently, special erbium-doped active fibers having a
larger core
diameter are considered for use, which allow obtaining a relative high gain
with fiber
lengths down to 30-40 m.
The Applicant has recently 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 the european patent application No.
EP98117898 filed on 22 September 1998 in the name of the Applicant, it is
proposed
an optical amplifying unit including an erbium-ytterbium co-doped fiber
amplifier in a
single-stage configuration (with bi-directional pumping), or two erbium-
ytterbium co-
doped fiber amplifiers in a double-stage configuration (with co-propagating
pumping
or bi-directional pumping), providing high amplification in the 1575-1602 nm
wavelength region. To reach very high power gains, the proposed amplifying
unit
preferably includes an erbium-doped fiber pre-amplifier and at least a double-
cladding erbium-ytterbium co-doped fiber amplifier. Double=cladding active
fibers
allow high pump performances taking advantage of a multi-mode pumping
mechanism. The used pump lasers 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 the
active fibers.
In the design of the above described amplifying unit, the Applicant has found
that the implementation of a WDM coupler adapted to couple the multimode pump
radiation into the double-cladding fiber is critical. Coupling of the
multimode pump
radiation into the double-cladding fiber is performed preferably by means of
micro
optic (mirror-type) WDM couplers, which have coupling efficiencies much higher
than
those of fused fiber WDM couplers. The WDM coupler must be able to couple the
pump radiation (in the range 920-980 nm) in the internal cladding of the fiber
and the
transmitted signal (in the range 1575-1602 nm) in the core. Thus, the coupler
must
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have optical characteristics that allow, in addition to the required
wavelength
selectivity, a predetermined spatial distribution of the ligth. If a micro-
optic coupler is
used, a focusing lens system able to provide the considered spatial
distribution of the
light is very difficult to implement. Therefore, the use of a double-cladding
active fiber
involves difficulties in achieving a high coupling efficiency between the pump
source
and the active fiber. Moreover, the considered micro-optic coupler has a
relatively
high insertion loss, greater than 1dB at 1550 nm.
According to the present invention, the Applicant has found an alternative
amplifying unit arrangement adapted to be used in the 1565-1620 nm band and
providing advantages over the known amplifying devices. The proposed
amplifying
unit is particularly suitable for use in a WDM transmission system, preferably
as a
booster amplifier.
The Applicant has found that, by pumping single-mode and single-cladding
active fibre co-doped with Er and Yb by means of a first pump source adapted
to
excite Er by a first pump radiation and a second pump source adapted to excite
Yb
by a second pump radiation, a high performance and compact amplifying unit can
be
achieved.
Preferably, the first pump radiation includes a wavelength between 1465 nm
and 1495 nm and is fed to the active fiber in a co-propagating direction (with
respect
to the transmitted signals) and the second pump radiation includes a
wavelength
between 1000 nm and 1100 nm and is fed to the active fiber in a counter-
prpagating
direction (with respect to the transmitted signals).
Preferably, the first pump source is coupled to the active fiber by means of a
micro-optic WDM coupler and the second pump source is coupled to the active
fiber
by means of a fused-fiber WDM coupler.
The amplifying unit of the present invention extends the range of input
signals
to lower powers, with respect of typical booster units. This feature allows,
for
example, the design of a transmission system including a device with not
negligible
losses, for example OADMs (optical Add/drop Multiplexers, i.e. devices for the
insertion and the extraction of optical signals to/from the system) or a
dispersion
compensator, just upstream with respect to the amplifying units. These
additional
losses can in fact be tolerated without sensible worsening of amplification.
An additional advantage is provided by the use of single-mode couplers to
couple the pump radiation to the active fiber, which allows reduced signal
losses.
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Moreover, the amplifying unit of the present invention has a relatively wide
wavelength amplification band extending above 1565 nm, and it is then
particularly
adapted for use in WDM transmission systems.
According to a first aspect, the present invention relates to an optical
transmission system including:
- an optical transmitting unit to transmit optical signals,
- an optical receiving unit to receive said optical signals,
- an optical fiber link optically coupling said transmitting unit to said
receiving
unit and adapted to convey said optical signals, and
- an optical amplifying unit coupled along said link, adapted to amplify said
optical signals; said optical amplifying unit comprising:
~ an input for the input of said optical signals,
~ an output for the output of said optical signals,
~ an active fiber codoped with Er and Yb, having a first end optically
coupled to said input and a second end optically coupled to said output,
for the amplification of said optical signals,
~ a first and a second pump source for generating a first and, respectively,
a second pump radiation, and
~ a first and a second optical coupler for optically coupling said first pump
source and, respectively, said second pump source to said active fibre,
wherein said first pump radiation includes an excitation wavelength for Er and
said second pump radiation includes an excitation wavelength for Yb.
Said optical amplifying unit has preferably a wavelength amplification band
above 1565 nm.
Advantageously, said first optical coupler is optically coupled to the first
end of
said active fiber for feeding the first pump radiation to the active fiber in
a co-
propagating direction with respect to optical signals and said second optical
coupler
is optically coupled to the second end of said active fiber for feeding the
second
pump radiation to the active fiber in a counter-propagating direction with
respect to
optical signals.
The active fiber is preferably a single-cladding fiber and is preferably a
single-
mode fiber.
The first pump radiation has preferably a wavelength between 1465 nm and
1495 nm and the second pump radiation has preferably a wavelength between 1000
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nm and 1100 nm.
The first optical coupler is preferably a micro-optic WDM coupler and the
second optical coupler is preferably a fused-fiber WDM coupler.
According to a second aspect, the present invention relates to a method for
amplifying optical signals, including the steps of:
- feeding the optical signals to an active fiber codoped with Er and Yb; and
- optically pumping, during the step of feeding the optical signals, the
active
fiber;
wherein said step of optically pumping includes feeding to said active fiber a
first
pump radiation for exciting Er and a second pump radiation for exciting Yb.
Said step of feeding said first pump radiation preferably includes feeding
said
first pump radiation to the active fiber in a co-propagating direction with
respect to
optical signals and said step of feeding said second pump radiation preferably
includes feeding said second pump radiation to the active fiber in a counter-
propagating direction with respect to optical signals.
Said step of feeding to said active fiber a first pump radiation preferably
includes feeding to said active fibre an exciting radiation for Er having a
wavelength
between 1465 nm and 1495 nm.
Said step of feeding to said active fiber a second pump radiation preferably
includes feeding to said active fibre an exciting radiation for Yb having a
wavelength
between 1000 nm and 1100 nm.
Preferably, said active fiber includes a core and a cladding and in said step
of
feeding to said active fiber a first pump radiation and a second pump
radiation
includes feeding said first pump radiation and said second pump radiation into
the
core of said active fiber.
Preferably, said step of feeding the optical signals to the active fiber
includes
feeding to the active fiber optical signals having wavelengths above 1565 nm.
According to a third aspect, the present invention relates to an optical
amplifying unit including:
~ an input for the input of optical signals,
~ an output for the output of said optical signals,
~ an active fiber codoped with Er and Yb, optically connected to said input
and said output, and adapted to amplify said optical signals,
~ a first and a second pump source for generating a first and, respectively,
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a second pump radiation, and
~ a first and a second optical coupler for optically coupling said first pump
source and, respectively, said second pump source to said active fibre,
wherein said first pump radiation includes an excitation wavelength for Er and
said second pump radiation includes an excitation wavelength for Yb.
Preferably, the excitation wavelength for Er is between 1465 nm and 1495 nm
and the excitation wavelength for Yb is between 1000 nm and 1100 nm.
Said first optical coupler is preferably connected between said input and said
active fiber for feeding the first pump radiation to the active fiber in a co-
propagating
direction with respect to optical signals and said second optical coupler is
preferably
connected between said active fiber and said output for feeding the second
pump
radiation to the active fiber in a counter-propagating direction with respect
to the
optical signals.
Said active fiber is preferably a single-cladding and single-mode fiber.
Said first optical coupler is preferably a micro-optic WDM coupler and the
second optical coupler is preferably a fused-fiber WDM coupler.
Preferably, said second pump source comprises a fiber laser including a
further active fiber and adapted to generate said second pump radiation, and a
pump
laser source adapted to pump said further active fiber.
Said further active fiber preferably includes a double-cladding fiber.
Moreover,
said further active fiber preferably includes an optical fiber doped with Yb.
Said fiber laser preferably includes a first and a second Bragg grating
written
on opposite end portions of said further active fiber. said pump laser source
is a
broad-area laser diode.
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
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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 amplifying unit according
to
the present invention;
Fig. 10 is a schematic representation of a pump source included in the optical
amplifying unit of Fig. 9;
Figs. 11 a and 11 b are schematic representations of a double-cladding fiber
used for the pump source of Fig. 10 and of the multi-mode pumping operation of
a
double cladding fiber;
Figs. 12 shows a grating writing assembly used to write gratings in the
double-cladding fiber of the pump source of Fig. 10;
Fig. 13 shows the response curve of a fiber laser used for experimental
measurements;
Figs. 14 and 15 illustrate experimental results obtained with an amplifying
unit
according to the invention;
Figs. 16 and 17 are flux diagrams of a method for writing gratings in an
active
fiber used for the pump source of Fig. 10;
Figs. 18a and 18b show schematically the variation of a predetermined
parameter during the grating writing process according to the method of Figs.
16 and
17;
Figs. 19-21 show the simulated performances of a fiber laser used for the
pump source of Fig. 10.
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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 less or 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
adapted to receive a plurality of input channels 16, and a transmitter power
amplifier
section (TPA) 12. The second terminal site 20 preferably includes a receiver
pre-
amplifier (RPA) section 14 and a demultiplexing section (DMUX) 15 adapted to
output
a plurality of output channels 17.
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,
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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 Add/Drop 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 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 and/or 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 falls 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
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advantageously allocated down to 1565 nm and up to 1620 nm.
Adjacent channels, in the proposed one-hundred-twenty-eight (128) channel
system, have preferably a 50 GHz constant spacing. Alternatively, a different
constant spacing may be used, or 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 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 standard system, e.g. a SONET, ATM, IP or SDH system, includes
transmit/receive (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) TX/RX 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.
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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
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 multiplexer, such
as a
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conventional 1x16 planar optical splitter, WM 44 is a forty-eight (48)
channels
wavelength multiplexer, such as a conventional 1x64 planar optical splitter
with
sixteen (16) unused ports and WM 45 is a sixty-four (64) channels wavelength
multiplexer, such as a conventional 1 x64 planar optical splitter. Each
wavelength
multiplexer 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 multiplexer using passive silica-on-
silicon
(SiOz-Si) or silica-on-silica (Si02-SiOz) 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),
cascaded Mach-Zehnder, 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
(although other rare-earth-doped fiber amplifiers may be used), while
amplifier
section 53 is, according to the invention, an erbium/ytterbium-doped (Er/Yb)
fiber
amplifier that will be described in details with reference to Fig. 9.
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 (pre-amplifier) 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
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available from the Applicant. The laser diodes may be coupled to the optical
path of
the pre-amplifiers using 980/1550 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,
which is exploited 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 pump sources in the 980 nm wavelength region may be used. Section
53
will be hereunder described in details with reference to Figs. 9.
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
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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 BB/RB1 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
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
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signals are dropped and/or added, and further amplified by amplifier 67. The
RB2
band is first amplified by amplifier 68, then received by the third OADM stage
77
where predetermined signals are dropped and/or 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,
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
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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 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
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
one at 975 nm and another at 986 nm, or two polarization multiplexed pump
sources
at 980 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,
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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,
depending on the system requirements. From amplifier 68, RB2 band channels are
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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 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
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
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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 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
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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 ~,,-~,,6, each separated by 50 GHz, then router module
91
would split the BB band into a first sub-band BB' having channels 7~,, 7~3,
... , ~,,s
separated by 100 GHz and a second sub-band BB" having channels 7~2, ~,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.
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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, 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,
... , 7~,s,
and WD 95" demultiplexes channels ~,z, 7~4, ... , 7~,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~s4 and WDs 97' and 97" receive and
demultiplex first sub-band RB2' and second sub-band RB2", respectively, to
produce
channels 7~ss-~,zs. Both WD 96' and WD 96" may be 1 x 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 1 x 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-Rx128.
Fig. 9 illustrates an optical amplifier 100 according to the present
invention.
Optical amplifier 100 can be used in the optical transmission system 1, both
in the
amplifier section 53 of Fig. 4 and in the amplifier section 69 of Fig. 6, to
amplify
signals in the RB2 band.
Amplifier 100 is preferably a bidirectionally-pumped optical amplifier and
includes:
- an input port 101 for the input of optical signals to be amplified;
- an output port 102 for the output of the optical signals after
amplification;
- an active fiber 103 having a first end 103a optically coupled to the input
port 101 and a second end 103b optically coupled to the output port 102
and adapted to amplify the optical signals;
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- a first pump source 104 optically coupled to the active fiber 103 by means
of a first optical coupler 105 and adapted to feed a first pump radiation to
the active fiber 103, preferably in a co-propagating direction with respect
to transmitted signals;
- a second pump source 106 optically coupled to the active fiber 103 by
means of a second optical coupler 107 and adapted to feed a second
pump radiation to the active fiber 103, preferably in a counter-propagating
direction with respect to transmitted signals.
Alternatively, by opportunely multiplexing the first pump radiation, the
second
pump radiation and the optical signal, the first pump radiation and the second
pump
radiation may be fed to the active fiber 103 in a same direction, preferably
both in the
co-propagating direction.
Alternatively, a plurality of pump sources may be used in place of the first
pump source and/or the second pump source. This plurality of pump sources may
be
multiplexed either in wavelength (if operating at different wavelengths) or in
polarization.
Amplifier 100 may also comprise a first optical isolator 108 of a known type
positioned between input 101 and the first coupler 105, to allow light
transmission
only from input 101 to coupler 105, and/or a second optical isolator 109 of a
known
type positioned between the second coupler 107 and output 102, to allow light
transmission only from the second coupler 107 to output 102.
Active fiber 103 is a silica fiber co-doped with erbium and ytterbium. Active
fiber 103 is single-mode and has a length preferably comprised between 10 m
and 30
m and a numeric aperture NA preferably comprised between 0.15 and 0.22. The
core
of active fiber 103 includes the following components with the indicated
concentrations:
- AI : between 0.1 and 13 atomic %;
P : between 0.1 and 30 atomic %;
- Er : between 0.1 and 0.6 atomic %;
- Yb : between 0.5 and 3.5 atomic %.
The ratio between erbium and ytterbium concentrations is preferably in the
range between 1:5 and 1:30, for example 1:20.
The first coupler 105 is preferably a micro-optic interferential WDM coupler,
including:
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- a first access fiber 105a optically coupled to the input port 101 to receive
the signals (in the RB2 band channels) to be amplified;
- a second access fiber 105b optically coupled to the first pump source 104
by means of a single-mode optical fiber 110, to receive the first pump
radiation;
- a third access fiber 105c optically coupled to the active fiber 103 to feed
to
the active fiber 103 the optical signals to be amplified together (and in a
same propagation direction) with the first pump radiation .
The first coupler 105 further includes a converging lens system (not shown),
to opportunely direct the light beams among its access fibers, and a selective-
reflection surface (not shown), e.g. a dichroic mirror. The actual inclination
of the
reflection surface inside the coupler depends on the direction of the incoming
optical
beams carrying the signal and the pump radiation. Preferably, the selective-
reflection
surface in coupler 105 is transparent for the wavelengths of the RB2 band
channels
and reflecting for the wavelength of the first pumping radiation. In this way,
the RB2
band channels pass through the reflecting surface substantially without losses
while
the first pump radiation is reflected by the reflecting surface into the core
of the active
fiber 103. Alternatively, the first coupler 105 may include a selective-
reflection surface
that is reflecting for the wavelengths of the RB2 band channels and
transmissive for
the wavelength of the first pumping radiation.
The first coupler 105 has preferably an insertion loss for the optical signals
not
greater than 0.6 dB. For example, the first coupler 105 may be model MWDM-
45/54
made by Oplink.
According to another embodiment, the first coupler 105 may be a fused-fiber
like coupler
The second coupler 107 is preferably a fused fiber WDM coupler including:
a first access fiber 107a optically coupled to the output port 102 to feed to
the output port 102 the amplified signals;
- a second access fiber 107b optically coupled to the second pump source
106 by means of an optical fiber 111, to receive the corresponding pump
radiation;
- a third access fiber 107c optically coupled to the active fiber 103 to
receive
from the active fiber 103 the amplified optical signals and to feed to the
active fiber 103 the pump radiation generated by the second pump source
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106; and
- a fourth access fiber 107d having a free end, that is low-reflection
terminated.
The second coupler 107 may be made by fusing a first fiber defining the first
and the third access fiber 107a, 107c, and a second fiber defining the second
and the
fourth access fiber 107b, 107d.
The second coupler 107 has preferably an insertion loss for the optical
signals
not greater than 0.3 dB.
The first pump source 104 is preferably a semiconductor laser diode,
providing the first pump radiation at a wavelength in the range between 1465
nm and
1495 nrn, adapted to excite the Er ions in the active fiber 103. The pumping
power
provided by the first pump source 104 is preferably comprised between 40 mW
and
150 mW. The first pump source 104 may be, for example, model number
SLA5600-DA supplied by SUMITOMO ELECTRIC INDUSTRIES, Ltd.
Direct pumping of the Er ions, in particular co-directional pumping, is
believed
to originate a pre-amplification of the optical signals in the active fiber
103. This pre-
amplification, in combination with a boosting effect provided by pumping the
Yb ions,
is believed to be the origin of the observed significant performance increase
for the
amplifier, in particular under low input power conditions.
The Applicant has found that pumping directly the Er ions in the 1480 nm
band is preferable with respect to pumping in the 980 nm band. In fact, the
1480 nm
pump radiation, differently from what would occur to a 980 nm pump radiation,
is
believed to be slowly absorbed in the active fiber so as to provide higher
fluorescence
at longer wavelengths (1600 nm). This allows the optical signal power to
progressively rise along the active fiber avoiding an excessive ASE
accumulation.
The proposed amplifier is able, as hereinbelow reported, to amplify optical
signals with very low input powers, down to -25 dBm.
With reference to Fig. 10, the second pump source 106 preferably includes a
fiber laser 112 and a pump laser diode 113. Advantageously, fiber laser 112 is
adapted to generate the second pump radiation at a wavelength in the range
between 1000 nm and 1100 nm, adapted to excite the Yb ions in the active fiber
103.
Fiber laser 112 preferably comprises a double-cladding fiber 114 and a first
and a
second Bragg grating 118, 119. Bragg gratings 118, 119 are written into
opposite
ends of double-cladding fiber 114 and delimit the Fabry-Perot resonant cavity
of the
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fiber laser 112.
-29-
Pump laser diode 113 is optically coupled to one end of the double-cladding
fiber 114 and is adapted to generate an exciting radiation for pumping the
double-
cladding fiber 114. The opposite end of double-cladding fiber 114 is spliced
to fiber
111 for transmitting the second pump radiation to active fiber 103.
Fig. 11 a shows a not-in-scale cross section of double-cladding fiber 114.
Fiber
114 includes a core 115 having a first refraction index n,, an inner cladding
116
surrounding the core 115 and having a second refraction index n2 < n,, and an
outer
cladding 117 surrounding the inner cladding 116 and having a third refraction
index n3
< n2. Core 115, inner cladding 116 and outer cladding 117 are coaxial.
Fiber 114 is a silica fiber having the core 115 preferably doped with a high
concentration of Yb, in order to generate the second pump radiation at a
wavelength
suitable for pumping the active fiber 103. Yb concentration in core 115 is
preferably
greater than 0.1 atomic%, more preferably comprised between 0.7 atomic% and
1.5
atomic%.
The concentrations of the other components of core 115 are preferably within
the following ranges:
- Ge : between 0.1 and 20 atomic%;
- AI : between 0.1 and 6 atomic%;
- P : between 0.1 and 20 atomic%.
Pump laser diode 113 is preferably a broad-area laser, with emission
spectrum centered at a wavelength suitable to pump dopant ions in the double-
cladding fiber 114, preferably comprised between 910 nm and 925 nm. Pump laser
diode 113 is preferably provided with an output multi-mode optical fiber 120
having
the core substantially of the same diameter and with the same numeric aperture
of
the inner cladding 116 of active fiber 114, in order to couple the excitation
radiation
into the active fiber 114 with a very high efficiency (near 100%).
As shown in Fig. 11 b, under normal operating conditions, the pump radiation
generated by the pump laser diode 113 is fed into the inner cladding 116 and
is
progressively absorbed by the core 115, exciting the Yb ions. The de-
excitement of
the Yb ions gives rise to stimulated emission in the wavelength range 1000-
1100 nm,
which propagates into the core 115 and amplifies itself. Gratings 118, 119
reflect a
predetermined wavelength in the range 1000-1100 nm (for example 1047 nm),
giving
rise, after multiple reflections, to a high power laser radiation at this
specific
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wavelength which is emitted from the end of fiber 114 opposite to the pump
laser
diode 113.
Fiber laser 112 may be realized by firstly producing the double-cladding fiber
114 with characteristics (length, geometry and composition) optimized
according to
the desired laser performances, and successively writing gratings 118 and 119
on the
opposite ends of fiber 114.
To produce fiber 114, two different preforms (not shown) are used. A first
preform is used to obtain the core 115 and an inner portion of the inner
cladding 116.
The first preform is made by deposing SiOZ, PZOS and AI203 by means of the
known
"chemical vapor deposition" (CVD) method, and then by introducing the rare
earth
ytterbium by means of the known "solution doping" method. The first preform is
then
opportunely worked to reduce its external diameter to a predetermined value.
A second preform of a commercial-type is used to obtain an outer portion of
the inner cladding 116 and the outer cladding 117. The second preform has a
central
region of pure Si02 and a surrounding region of fluoride-doped Si02. The
central
region of the second preform is partly removed so as to obtain a central
longitudinal
hole having a diameter slightly larger than the external diameter of the first
preform,
into which the first preform is introduced. The inner cladding is defined
partly from the
first preform and partly from the second preform.
The three-layer preform so obtained is drawn in the usual way to obtain the
optical fiber 114.
Gratings 118 and 119 may be written by means of a grating writing assembly
130 shown in Fig. 12 and according to the technique hereinbelow described,
developed by the Applicant.
With reference to Fig. 12, the grating writing assembly 130 the pump laser
diode 113 optically coupled to a first end 114a of fiber 114, an optical power
measuring device 131, preferably a power meter, positioned in front of a
second end
114b of fiber 114 and, preferably, an optical band-pass filter 132 interposed
between
the second end 114b of fiber 114 and the measuring device 131.
Measuring device 131 is, for example, a power meter of the type ANDO
AQ2140.
Filter 132 is preferably an interterential filter, centered at the
predetermined
wavelength for the laser emission laser of source 106.
Assembly 130 further includes a processor (PC) 134 adapted to control the
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pump laser diode 113 and the device 131 preferably by means of a DAC (Digital-
Analog Converter) 133 and using a specific software (for example Labview~).
DAC
133 may be for example of the type National Instruments PCI 6110E. As shown in
Fig. 12, processor 134 is further adapted to provide (on a display) the
Pout/PPump
characteristic of laser 106 during grating writing process, according to
information
provided by the measuring device 131.
Moreover, assembly 130 includes a UV writing device 135 suitable to write
gratings 118, 119 on fiber 114. UV writing device 135 includes preferably an
excimer
laser equipment.
The method for writing the first grating 118 is herein described with
reference
to the flux diagram of Fig. 16 and to the schematic representation of Fig.
18a. The
method includes the following steps:
- defining a reflecting surface associated to the active fiber, preferably by
cutting and cleaning (block 200) the second end 114b of fiber 114 in order
to reach a predetermined reflectivity R2 at the interface glass/air,
preferably of about 4%; this reflecting surface has a reflection wavelength
band wider than the reflection wavelength band expected for the first
grating;
- feeding (block 210) pump radiation having an optical power P;~ to the
active fiber 114 by means of the pump laser diode 113 in order to excite
the dopant ions and to give rise to an amplified stimulated emission (ASE)
defining a free-running emission;
- writing (block 220), by means of UV writing device 105, the first grating
118 near the first end 114a of fiber 114, with a spatial period
corresponding to the predetermined laser wavelength yase~; the first
grating 118 has a varying reflectivity R, and defines, together with the
second end 114b of fiber 114, a resonant cavity allowing the stimulated
emission to travel forward and backward in fiber 114 and to output as a
laser emission at the wavelength 7~~ase~;
- scanning (block 230) repeatedly, during the writing step, the power of the
pump radiation in a predetermined power range (possibly starting with
zero power), by driving the pump laser diode 113 by means of processor
134 and DAC 133; the minimum value of the pump radiation power to
have laser emission defines a threshold power P,h which depends on the
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grating intensity; the scanning period may be, for example, 15 - 20 s;
- spectrally filtering (block 240) the optical radiation which is output from
the
second end 114b of fiber 114, by means of filter 132; filtering allows to
suppress residual pump radiation and, at the beginning of the writing
process, the free running radiation;
- measuring (block 250), during the writing and scanning steps, the optical
power of the filtered output radiation, by means of the measuring device
131; measuring the optical power includes obtaining, during a scanning
period, a predetermined number N (for example 10) of optical power
values, each value being obtained by calculating the average value of the
power detected in a predetermined measuring period (for example 2 s);
the predetermined number N of optical power values and the
predetermined measuring period being related to the value of the
scanning period ;
- processing (block 260), preferably by performing a linear regression, the
measured optical power in order to obtain the laser's efficiency ~ and
threshold power P,n; performing the linear regression comprises finding a
straight line which defines a best-fitting of the N last points (corresponding
to the N optical power values obtained during the last scanning period) on
the Po~c~P~~ characteristic, and evaluating the slope and the intersection of
the straight line with the P;~ axis in order to obtain current values of ri
and
Pm;
- checking (block 270) if the efficiency ri is increasing, by comparing the
current value of r~ (ri~~~~, point A in Fig. 18a), i.e. the value of r~
related to
the last scanning period, with the preceding value of r~ (r~prec, point B in
Fig. 18a), i.e. the value obtained in the preceding step of processing and
related to the preceding scanning period; the current value r~~~« of
efficiency r~ is related to the current value of the first grating
reflectivity R,;
- repeating, if the efficiency rl is increasing (T~curr~~prec), the steps of
writing,
scanning, filtering, monitoring, processing and checking (blocks 220-270);
- stopping the process (block 280) when the laser's efficiency ri begins to
degrade, i.e. if the efficiency rt is no more increasing (T~currC~prec) having
reached a limit value y,m,c (point C in Fig. 18a); r~nm,t corresponds to a
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maximum value for the reflectivity R, of the first grating 108 (near 100%)
and is the maximum efficiency obtainable with the considered value of RZ
(4%); if the writing process were continued over this point, r~ would
decrease (point D in Fig. 18a) due to a grating degradation related to
some incoming phenomena, like saturation of defect centers and
reduction of interference fringe contrast;
- evaluating (block 290), according to the efficiency limit value y,m,t, the
final
reflectivity of the first grating 118.
The described process for writing the first grating may have a total duration
of
a few minutes.
The first grating has preferably a reflection wavelength band between 0.3 nm
and 1 nm, more preferably between 0.4 and 0.7 nm.
The reflecting surface used in the first step may alternatively be defined by
a
multi-layer interferential reflecting surface made on the second end 114b, a
separate
portion of fiber including a grating, micro-optic elements like semi-
reflecting mirrors or
lens systems, or similars.
The Applicant has observed that the threshold power Pth is another parameter
that can be used, for example in addition to the efficiency, to establish when
the first
grating writing must be stopped. In fact, the threshold power P,h decreases
during the
writing process and reaches a limit value P,n,~~m~t when the efficiency
reaches its limit
value y,m,c. However, the Applicant has observed that the evaluation of Pth is
more
difficult than the evaluation of rl and that the variations of Pth during the
writing
process are less than the variations of r~. Moreover, the actual value of Pth
is slightly
different from the value obtainable from the linear regression. Therefore, the
Applicant has observed that rt is the preferred parameter to be used in the
checking
step.
Typically, the limit efficiency r~~~m" obtained at the end of the above
process
does not correspond to the maximum efficiency Amax obtainable for the fiber
laser 112
(point E in Fig. 18a). In order to reach the maximum efficiency rtmaX, it is
typically
necessary to write the second grating 119 and to optimize its reflectivity.
Writing only the first grating 118 may be sufficient in some applications in
which the reflectivity of the second end of active fiber 114 allows to define
a laser
cavity with the desired characteristics. For example, the 4% reflectivity of
the second
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-34-
end of active fiber 114 may be sufficient for a "in-air laser", i.e. a laser
whose output
radiation is emitted directly in air.
For the considered use, the Applicant has observed that the presence of a
second grating 119 having a reflectivity of at least 4% allows an improvement
in the
performance of the fiber laser 112.
The Applicant has further observed that the previously described writing
technique is also suitable for writing the second grating 119, even if an
additional
attention must be paid to the second grating 119 actual spectral allocation.
This is
because the active fiber 114, during the writing step, changes its refraction
index, and
the grating wavelength peak then shifts. For remedying to this drawback, the
second
grating 119 is advantageously a relatively large band grating, so that the
peak shifts
are included in the grating's band. Preferably, the ratio between the
reflecting band of
the second grating and the reflecting band of the first grating is between 1,5
and 3. If
the "phase mask" writing technique is used, a grating with an enlarged
reflection
band may be obtained by positioning, in front of the mask, a screen provided
of a slit,
which introduces a predetermined diffraction of the UV radiation.
Moreover, it is preferred to make an a priori evaluation of the possible peak
shift during writing in order to reach an overlap of the peak related to the
first grating
118 and the peak related to the second grating 119. This evaluation can be
made by
estimating the required duration of the writing process and the approximate
shift per
second of the grating peak.
The method for writing the second grating 119 is herein described with
reference to the flux diagram of Fig. 17 and to the schematic representation
of Fig.
18b. The method includes the following steps:
- cutting (block 300) the second end 114b of active fiber 114 to obtain and
end surface inclined with an angle of 7-8° (with respect to a plane
perpendicular to the fiber axis) having a negligible reflectivity;
- feeding (block 310) pump radiation having an optical power P;~ to the
active fiber 114 by means of the pump laser diode 113 in order to excite
the dopant ions of the active fiber 114;
- writing (block 320), by means of UV writing device 105, the second grating
119 near the second end 114b of fiber 114, with a spatial period
corresponding to the predetermined laser wavelength yase~; the second
grating 119 has a varying reflectivity R2 and defines, together with the first
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grating 118 which has a reflectivity R, near 100%, a resonant cavity
allowing the stimulated emission to travel forward and backward in fiber
114 and to output as a laser emission at the wavelength yase~;
- scanning (block 330) repeatedly, during the writing step, the power of the
pump radiation in a predetermined power range (which may be different
from the range used for the first grating writing), by driving the pump laser
diode 113 by means of processor 134 and DAC 133; the minimum value
of the pump radiation power to have laser emission defines a threshold
power P,h which depends on the second grating intensity;
- spectrally filtering (block 340) the optical radiation which is output from
the
second end 114b of fiber 114, by means of filter 132; filtering allows to
suppress residual pump radiation and, at the beginning of the second
grating writing, the possible free running radiation;
- measuring (block 350), during the writing and scanning steps, the optical
power of the filtered output radiation, by means of the measuring device
131; measuring the optical power includes obtaining, during a scanning
period, a predetermined number N' (which may be different from the
predetermined number N used for the first grating writing) of optical power
values, each value being obtained by calculating the average value of the
power detected in a predetermined measuring period; the predetermined
number N' of optical power values and the predetermined measuring
period being related to the duration of the scanning period;
- processing (block 360), preferably by performing a linear regression, the
measured optical power in order to obtain the laser's efficiency r~ and
threshold power Pth; performing the linear regression comprises finding a
straight line which defines a best-fitting of the N' last points
(corresponding
to the N' optical power values obtained during the last scanning period) on
the Po~,/P,n characteristic, and evaluating the slope and the intersection of
the straight line with the P;~ axis in order to obtain current values of r~
and
P,h; the first detected value of rl will be intermediate between zero and the
limit value y,m~t found at the end of the first grating writing;
- checking (block 370) if the efficiency r~ is increasing, by comparing the
current value of r~ (rt~~~~, point A in Fig. 18b), i.e, the value of ~ related
to
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-36-
the last scanning period, with the preceding value of ri (riprec, point B in
Fig. 18b), i.e. the value obtained in the preceding step of processing and
related to the preceding scanning period; the current value ri~U~~ of
efficiency rl is related to the current value of the second grating
reflectivity
R2;
- repeating, if the efficiency rl is increasing (T~curr~~prec)~ the steps of
writing,
scanning, filtering, monitoring, processing and checking (blocks 320-370);
- stopping the process (block 380) when the laser's efficiency r~ begin to
degrade, i.e. if the efficiency ri is no more increasing (l~curr~~prec) having
reached a maximum value l~max (point E in Fig. 18b); rimax corresponds to
an optimum value RZ,oPt for the reflectivity Rz of the second grating 109 (for
example included between 4 and 10%) and represents the maximum
efficiency obtainable for the fiber laser 112; if the writing process were
continued over this point, rl would decrease (point D in Fig. 18b) due to a
grating degradation related to some incoming phenomena, like saturation
of defect centers and reduction of interference fringe contrast;
- evaluating (block 390), according to the maximum value of efficiency rimaX,
the final reflectivity of the second grating 119.
The Applicant has observed that, during the second grating writing process
(R2 increasing), the threshold power P,h progressive decreases and this trend
continues over the optimum value Rz,oP,. Having a lower value of the threshold
power
Pth is an advantage in that it allows lasering with a lower input power. Thus,
a further
improved criterion to optimize the fiber laser 112 performances would be that
of
stopping the process when the best compromise, or a predetermined relation,
between the efficiency ri and the threshold power P~h has been reached.
This compromise may depend on the particular application considered.
Experimental results on amplifying unit 100 performances
Experimental measurements have been carried out on an amplifying unit 100
whose characteristics are hereinbelow described in detail.
An active fiber 103 used in the experiment has a core diameter of 4.3 Vim, a
cladding diameter of 125 Vim, a numeric aperture NA = 0.2 and is composed as
fol lows:
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element Si Al P Er Yb
atomic% 70.8 1.5 25 0.125 2.5
The ratio of Er and Yb concentrations is about 1:20.
The first coupler 105 is an interferential filter model MWDM-45/54 made by
OPLINK. The first coupler 105 has an insertion loss of 0.6 dB.
The second coupler 107 is a fused fiber WDM coupler. The second
coupler 107 is, according to the above, made by fusing a first fiber defining
access
fibers 107a and 107c and a second fiber defining access fibers 107b and 107d.
The
first fiber is a SM (single-mode) fiber having a core diameter of 3.6 Vim, a
cladding
diameter of 125 ~m and a numeric aperture NA = 0.195. The second fiber is a SM
fiber having a core diameter of 3.6 ~,m, a cladding diameter of 125 wm and a
numeric
aperture NA = 0.195. Both SM fibers are of the type CS 980 produced by
Corning.
The second coupler 107 has an insertion loss of 1 dB.
The first pump source 104 is a laser diode adapted to provide a pump
radiation power of 50 - 70 mW at 1480 nm. Fiber 110 is a SM fiber.
The second pump source 106 has been made by the Applicant and is
adapted to provide a pump radiation power of 500-650 mW at 1047 nm. Fiber 111
is
a SM fiber. Broad area diode laser 113 is adapted to provide a radiation power
of 800
mW at 915 nm. The Applicant observes that a much higher saturation power of
the
amplifier could probably be obtained by using a more powerful broad area diode
laser.
Active fiber 114 in the second pump source has, in its core 115, the following
composition, detected by means of a SEM analysis.
element Si Ge Al P Yb
atomic% 89.40 2.78 1.17 5.93 0.72
AI concentration has been chosen relatively high in order to obtain a high
concentration of Yb. Ge concentration is relatively low, due to the high value
of the
CA 02321439 2000-09-29
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refraction index determined by the high concentration of AI and Yb. P has been
added in order to reduce the numeric aperture (NA) of the fiber.
The length of active fiber 114 is 10 m and its bending diameter is about 40
mm. The Applicant has observed that this value of the bending diameter
represent
the best compromise between absorbing efficiency and induced losses in the
fiber.
The length of the resonant cavity (i.e. the distance between the first and the
second grating 118, 119) is approximately 10 m.
The active fiber 114 has an external diameter of the outer cladding 117 of
about 90 pm, an external diameter of the inner cladding 116 of about 45 ~m and
an
external diameter of the core 115 of about 4.5 pm. The refraction index step
On = n,-
n2 between the core 115 and the inner cladding 116 is about 0.0083 and the
refraction index step 0n' - n2-n3 between the inner cladding 116 and the outer
cladding 117 is about 0.067. The core 115 and the inner cladding 116 define a
single-
mode waveguide for the conveying of transmission signals, having a first
numeric
aperture NA, of about 0.155, while the inner cladding 116 and the outer
cladding 117
define a multi-mode waveguide for the conveying of pump radiation, having a
second
numeric aperture NA2 of about 0.22.
Gratings 118, 119 have been realized by the method previously described.
Gratings 118, 119 have a Bragg wavelength of 1047 nm. The first grating 118
has a
reflectivity of about 99% at peak wavelength and the second grating 119 has a
reflectivity less then 10% at the same wavelength.
Fig. 13 shows the response curve of fiber laser 112. In particular, Fig. 13
shows the dependence of the optical power P°~t of the emitted laser
radiation on the
pump power P;~ provided by laser diode 113. According to the obtained curve,
the
laser source has an efficiency rt=81,5% and a threshold power P,h=99 mW.
Fig. 14 shows the insertion losses due to the passive components of the
amplifier 100 placed, respectively, between the input 101 and the first end of
the fiber
103 (i.e. the first optical isolator 108 and the first optical coupler 105),
and between
the second end of the fiber 103 and the output 102 (i.e. the second optical
coupler
107 and the second optical isolator 109). The characteristics of Fig. 14 have
been
obtained by means of an optical spectrum analyzer.
Fig. 15 shows the gain curves of the amplifying unit 100, for a wavelength
scanning of the input signal from 1575 nm to 1620 nm. The different curves in
Fig. 15
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refer to input signal powers within the range -25 dBm and 10 dBm. It can be
noticed
that, for input signal power greater than 0 dBm, the amplifying unit 100
provides
output power greater than about 18 dBm and can then be used as a booster
amplifier. In particular, at 10 dBm of input signal power, the unit provides
up to 22
dBm output power with a maximum gain variation less than 1 dB in the RB2 band.
When amplifier 100 is used as a booster unit, with an input signal of 10 dBm
or more, the gain curve exhibits a maximum variation less than 1 dB in the RB2
band.
Moreover, the amplifier 100 exhibits a gain extended beyond the RB2 band,
as far as 1620 nm.
Numerical results of grating writing method simulation
Figs. 19-21 illustrates numerical results obtained by simulating the above
described grating writing method on an active fiber 114 having the
characteristics
hereinabove listed in the experimental measurement.
Fig. 19 shows the dependence of optical output power P°~t on the
pump
optical power P;~ for different values of the first grating reflectivity
during the first
grating writing process. The resonant cavity is defined by the first grating
118 and the
second end 114b (4% reflectivity) of fiber 114. It can be observed the
progressive
increase of the fiber laser efficiency and the progressive decrease of the
threshold
power Pth with the increase of the first grating reflectivity.
Fig. 20 shows the dependence of efficiency rl and threshold power P,h of fiber
laser 112 on the first grating reflectivity during the first grating writing
process. Each
point on the rl and P,h characteristics corresponds to a straight line of Fig.
19
Fig. 21 shows the dependence of efficiency rl and threshold power Pth of fiber
laser 112 on the second grating reflectivity during the second grating writing
process,
in the assumption of a first grating reflectivity of 99%. A maximum in the
efficiency
curve is detectable for a second grating reflectivity of about 4%, having a
value
greater than 80%. If the best compromise (between r~ and Pth) criterion is
used, the
writing process should advantageously be stopped when the second grating
reflectivity is between 4% and 10%.
