Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL AMPLIFIER WITH ACTIVE-FIBER LOOP MIRROR
DESCRIPTION
TECHNICAL FIELD
Tlhis invention relates to optical amplifiers of the kind which have a so-
called loop mirror formed
by a loop of optical fiber with its ends connected to a 3 dB coupler. The
invention is applicable
especially, but not exclusively, to optical fiber amplifiers in which the loop
mirror comprises a rare
earth-doped fiber.
BACKGROUND ART
As explained in United States Patent No. 5,757,541, which issued May 26, 1998
and named B. G.
Fidric as inventor, optical amplifiers ~~re known in which an optical signal
to be amplified is passed
through an active fiber together with pump energy from a separate source, such
as a laser diode.
Vv'hen discussing the prior art, Fidric explained that, whether the optical
signal and the pump
energy were both supplied to the same end of the active fiber, or to opposite
ends of the active
filber, the known amplifiers exhibited non-uniform, non-symmetrical
longitudinal pump excitation.
Also, despite the use of isolators at the input and the output, backreflection
resulted in a certain
amount of pump energy reaching the input source.
Fidric proposed to overcome these problems by means of a loop mirror
arrangement comprising an
a<;tive fiber with its ends connected to two ports of a four-port 3 dB
coupler. The input signal and
pump energy supplied to the other two ports were split into two equal parts by
the coupler and
propagated simultaneously clockwise anal counterclockwise around the loop. The
input and output
signals were applied to, and extracted from, the coupler by means of a
circulator. The fiber was
polarization-maintaining so that the states of the signals propagating through
the loop mirror were
maintained. Maintaining the polarization states was preferable to ensure that
the amplified signal
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was not contaminated by the residual pump light. According to Fidric, this
could be achieved, in
th.e alternative, by means of a polarization controller in the fiber loop, but
that is debatable.
The amplifier's noise figure and gain depend upon the extent to which the
"forward" and
"backward" ASE in the fiber are equal, so it is desirable for the 3-dB coupler
to split both the pump
energy and the input signal precisely into halves. Consequently, the coupler
must be capable of
providing the same 50- 50 splitting over a range which includes both the pump
energy wavelength
and the input signal wavelength.
It is usual to pump an active fiber at different wavelengths, depending on the
application. For
e~;ample, when power output is the main consideration, a pump wavelength of
1480 nm is
preferred. When noise is the main consideration, however, it is preferable to
use a shorter
wavelength, such as 980 nm, because the amplified spontaneous emission (ASE)
produced by the
active fiber is less at that wavelength. Fidric's proposed amplifier will not
be entirely satisfactory
when used with the shorter pump wavelength because, at present, a coupler
capable of providing
precise 50-50 splitting over a range from 980-1600 nanometers is not
available. Even if pump
energy with a wavelength of 1480 nm were used, the problem would persist,
though to a lesser
a};tent.
DISCLOSURE OF INVENTION
An object of a first aspect of the present invention is to eliminate or at
least mitigate the above-
described disadvantages and, to this end, there is provided an optical
amplifier having a loop mirror
comprising an active fiber and a coupler. Preferably, the pump energy is
coupled into the fiber
without passing through the coupler.
In one embodiment of the invention, an optical amplifier comprises a loop
mirror in which a four-
port 3 dB coupler has an. active fiber cormected between second and third
ports, respectively, of the
coupler, the coupler having a first pori to receive an input signal for
amplification, the output signal
being provided at either the first port c>r a fourth port, and at least one
pump means coupled
bc;tween the coupler and the active fiber for injecting pump energy into the
active fiber so as to
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copropagate therein with the input signal, the arrangement being such that the
coupler will split the
input signal into two equal parts which will propagate in opposite directions
within the active fiber.
Preferably, two identical pump means are provided, adjacent respective ends of
the active fiber, for
injecting pump energy into the fiber in opposite directions, and the input
signal and output signal
are coupled to and from the first port by way of a circulator having its
bidirectional port coupled to
the first port. An advantage of using two identical pump means is that the
"forward" and
"backward" ASE in the active fiber can be equalized more readily.
Alternatively, the amplifier may
comprise a first isolator for coupling l:he~ input signal to the first port, a
second isolator for coupling
th,e output signal from the fourth port of the coupler, and a polarization
controller in the loop, in
series with the active fiber, for adjusting; the polarization of the signal in
the loop so that the output
signal will appear at the fourth port.
The or each pump means may comprise a wavelength multiplexer and a pump energy
source, such
arc a laser diode. A second aspect of the invention concerns automatic gain
control in rare earth-
doped fiber amplifiers. It is desirable to be able to maintain the amplifier
gain at a constant value
o~rer a wide range of input powers. In am article entitled "Gain-clamped Fiber
Amplifier with a
L~~op Mirror Configuration", IEEE Photonics Technology Letters, Vol. 1 l, No.
5, May 1999, Kyo
In.oue explained that it is known to clamp the gain using optical feedback.
According to moue,
drawbacks of such gain-clamped amplifiers include the fact that they cannot be
used for signals
around the oscillation wavelength because such light enters the feedback loop
and is not amplified
efficiently, and laser oscillation light appearing at the output is an
obstacle to system application.
In.oue proposed overcoming these drawbacks by means of a gain-clamped fiber
amplifier in which
the laser cavity is formed by a fiber grating and a loop mirror that comprises
a loop of erbium-
doped fiber connected to two ports of a 3 dB coupler. The grating and the pump
energy are
supplied to a third port of the coupler and the input signal is supplied by
way of a circulator to a
fourth port. The output signal is extracted via the circulator. According to
moue, the signal light
passes through the EDF without entering; the laser cavity, even when its
wavelength is close, or
identical, to the oscillation wavelength.
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This approach is not entirely satisfactory because it requires all of the
components to be
p~~larization-maintaining, which makes the amplifier complicated and expensive
to manufacture.
Another disadvantage is that available v~avelength is limited because the
wavelength of the fiber
grating is within the useful wavelength of the amplifier.
According to the second aspect of the present invention, there is provided an
optical amplifier
comprising a loop mirror formed by a four-port 3 dB coupler and a length of
active fiber having its
ends connected to first and second ports, respectively, of the coupler, the
coupler having a third
part to receive an input signal for amplification, such that the coupler will
split the input signal into
two equal parts which will propagate in opposite directions within the active
fiber, the output signal
leaving the coupler via the third port, and at least one pump means coupled
between the coupler
and the active fiber for injecting pumla energy into the fiber, further
comprising means responsive
to a portion of the amplified signal for reflecting into the loop mirror a
selected wavelength that is
outside a normal operating range of the input signal. An attenuator may be
provided between the
reflecting means and the coupler for adjusting the amount of the amplified
signal reflected back
into the loop mirror.
The reflecting means may comprise a grating, such as a fiber Bragg grating.
Where the loop
comprises an erbium-doped fiber, the selected wavelength may be, for example,
1525 nm. Because
amplifiers embodying the present invention supply the pump energy directly in
to the loop, i.e.
without passing through the coupler, i is possible to use pump energy having a
wavelength of 1480
mn for high power applications and 980 run for applications requiring a low
amplifier noise figure.
The pump means may conveniently cc:>rnprise a wavelength multiplexer
connecting a pump energy
source, for example a laser diode, to the loop.
In accordance with another embodiment of the invention there is provided an
optical amplifier
having a loop mirror including an active fiber and a coupler, optically
coupled to a linear
amplifying section. The linear amplifying section amplifies an input optical
signal, while
simultaneously producing amplified spontaneous emission that is used to
further pump the
amplified optical signal passing through the active fibre in the loop mirror.
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BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the various aspects of the invention will now be described by
way of example only
and with reference to the accompanying drawings, in which:
Figure 1 is a simplified schematic dia,gr<~m of a loop erbium-doped fiber
amplifier having a pair of
isolators at its input;
Fiigure 2 is a simplified schematic diagram of a loop erbium-doped fiber
amplifier having a
circulator at its input;
Fiigure 3 is a simplified schematic diagram of a loop erbium-doped fiber
amplifier with automatic
gain control and which is a modification of the amplifier of Figure 2;
Filgure 4 is a simplified schematic diagr~~rn of an optical amplifier having
an EDFA section and a
loop mirror filter having an active section;
Fiigure 5 illustrates an embodiment of the optical amplifier shown in Fig. 4
including a polarization
controller disposed in the loop mirror; and
Fiigure 6 illustrates yet another embodirrtent of the optical amplifier shown
in Fig. 4.
BEST MODES) FOR CARRYING CIUT THE INVENTION
In the drawings, identical components appearing in different Figures have the
same reference
numbers.
Referring now to Figure 1, an optical fiber amplifier comprises an input port
PIN for receiving an
optical signal to be amplified having a wavelength which, typically, will be
in the range from 1525
mn to 1625 nm, i.e. the third telecommunication window, and an output port
Pour for outputting a
corresponding amplified signal.
A first isolator 10 couples the input signal from port P,N to a port A of a
fiber coupler 12, and a
second isolator 14 couples the output sil;nal from port B of the coupler 12 to
amplifier output port
P,~~T. A loop of active optical fiber, specifically an erbium-doped fiber
(EDF) 16 is connected, in
series with a polarization controller 1 ~ a.nd a wavelength-selective coupler
20, between ports C and
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D, respectively, of the coupler 12. 'The I~DF 16 and the coupler 12 constitute
a loop mirror. The
coupler 12 is a 3 dB fiber coupler, which splits the signal at port A into two
equal parts which
appear at ports C and D, respectively, and propagate around the loop in
opposite directions. The
wavelength-selective coupler 20 couples into the loop mirror pump energy from
a source 22. In this
p~°eferred embodiment, the pump source 22 is a laser diode which
supplies pump energy having a
wavelength of 980 nm.
Thus, in operation, the isolator 10 wil l pass the input signal from input
port P,N to port A of the
fiber coupler 12, which will split the signal into two equal parts. Each part
will propagate around,
and be amplified by, the loop mirror and return to ports C and D of the fiber
coupler 12. The
polarization controller 18 will be adjusted so that, when the counter-
propagating signals arrive back
at: ports C and D, the phase difference between them is zero. Consequently,
the coupler 12 will
couple both parts, clockwise and counterclockwise, to exit via port B of the
coupler 12 and isolator
l~l will pass them to the output port Pour of the amplifier. Meanwhile, the
ASE will interfere
destructively in the coupler 12 and, on average, only one half of the ASE will
appear at the output
pert PpUT
Because the pump 22 is coupled directly into the loop mirror, the 3 dB coupler
12 does not have to
be capable of accurately splitting both the input signal and the pump energy.
Consequently, the
wavelength of the pump energy can be 980 nm, which will result in less ASE
being produced in the
EDF 16, with a concomitant reduction in signal-to-noise ratio of the amplifier
as compared with,
for example, that disclosed in US 5,7:>7.,541 which is constrained to using
pump energy having a
wavelength of about 1480 nm so as to avoid imbalance at the coupler I 2.
It should be noted that the polarization controller 18 may be omitted and the
isolators 10 and 14
replaced by a circulator. Also, if desired, two wavelength multiplexers may be
used. Thus, the
amplifier shown in Figure 2 is similar to that shown in Figure 1 in that it
has a loop mirror formed
b;y a coupler 12 and an EDF 16. It differs, however, in that, instead of
isolators, it has a circulator
2~1 having a unidirectional input port I' and a unidirectional output port E
connected to the input
port PiN and a first output port P 1 ouT, respectively, and a bidirectional
port G connected to port A
o:f the coupler 12.
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The circulator 24 provides more than 4~~ dB isolation and 0.8 dB insertion
loss between its ports.
Port B of the coupler 12 is coupled to a secondary output port P2oUT of the
amplifier. Also, a
second wavelength-selective coupler 26 connected between one end of the EDF 16
and coupler port
C injects into the loop energy from a second pump source 28. Both of the pump
sources 22 and 28
supply pump energy with a wavelength of 980 nm. As before, in operation, the
coupler 12 splits the
input signal into two equal parts which propagate in opposite directions
around the loop of erbium-
doped fiber 16. The energy from the two pump sources 22 and 28 also propagates
around the loop
o~E fiber 16. An advantage of using two pumps 22 and 28 is that each will
generate an equal amount
of ASE which will improve the noise figure and gain. In either of the above-
described amplifiers,
while it is preferred to use a pump energy wavelength of 980 nm for low noise
applications, as
e~cplained in the introduction, it is also possible to use a pump energy
wavelength of 1480 nm. Both
wavelengths are used in present day optical amplifiers.
Figure 3 illustrates a modification of the amplifier shown in Figure 2 to
provide automatic gain
control. As before, the amplifier shown in Figure 3 comprises a loop mirror
formed by a 3 dB fiber
coupler 12 and an EDF 16, with the energy from pump sources 22 and 28 coupled
into the loop by
wavelength-selective couplers 20 and 2Ei, respectively, and a circulator 24
whereby the input signal
to be amplified is directed to port A of the coupler 12 and the amplified
signal is directed from port
B of the coupler 12 to primary output port P2«~T of the amplifier. In this
case, however, a Bragg
grating 30 and an attenuator 32 are interposed, in series, between port B of
the coupler 12 and
output port P2ouT of the amplifier.
In, operation of the amplifier of Figure 3, the input signal is amplified by
the loop mirror as before
and the amplified signal appears at port B as a reflected signal. One half of
the ASE produced in the
erbium-doped fibre (EDF) 16 will appear at port A of the coupler 12 and hence
appear in the output
signal coupled by the circulator 24 to output port Pl~~~r. The other half of
the ASE will appear at
port B and be applied to the Bragg grating 30, which will reflect that part of
the ASE which has the
same wavelength as the grating. It should be noted that the ASE will have
wavelengths ranging
throughout and beyond the useful band of the input signal. Hence, it is
possible to use a Bragg
grating having a reflection wavelength outside the useful operating band of
the EDFA, though it is
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desirable to avoid the wavelength commonly used for surveillance purposes. In
the preferred
embodiment, the Bragg grating has a reflection wavelength of 1525 nm.
The 1525 nm component of the ASE reflected to the coupler 12 by the Bragg
grating 30 will pass
into the loop again and be amplified. One half of the amplified reflected
component will reappear at
coupler port B, and be passed back to the Bragg grating 30. Consequently,
lasing conditions exist
b.°tween the reflecting Bragg grating a0 and the loop mirror which
includes amplification by virtue
o:f the EDF 16. The lacing is controlled lby the attenuator 32 between the
Bragg grating 30 and port
B of the fibre coupler 12. Hence, adjusting the attenuator 32 adjusts the gain
of the EDFA, i.e. the
g;~in at which it is clamped.
Referring to Fig. 4, there is shown an alternative embodiment comprising an
optical amplifier
including an erbium doped fiber amplifier (EDFAj section 33 and a loop mirror
filter (LMF) 34,
wherein the LMF 34 removes at least some of the amplified spontaneous emission
(ASE) generated
within the EDFA section 33.
The EDFA 33 comprises a first wavelength selective coupler 40, for combining
the input signal
with energy from a pump source 42 and applying it to one end of an erbium-
doped fiber (EDF) 44.
R~ithin the EDF 44, pump energy is transferred to the input signal causing
amplification in known
manner. The other end of the E DF 44 is connected to a wavelength
demultiplexer 46 which extracts
residual pump energy and supplies the amplified signal to input port PAN of
the LMF 34.
The LMF 34 comprises a circulator 48, a coupler 50 having four ports
identified as A, B, C and D,
and a loop of optical waveguide 52 connected between ports C and D of the
coupler 50, which is
preferably a 3-dB fiber coupler, such as a 1550 nm 3-dB fiber coupler. The
loop of optical
waveguide comprises an optical fiber 52 A, such as that marketed as type SMF-
28 by Corning Inc.,
and an active section, such as an erbium-doped fiber 52B. Typically, the total
length of linear
active fibre 52 is as short as possible i:o .avoid polarization fluctuations.
For example, the total
length is typically on the order of 5 meters, and preferably is under 10
metres. Advantageously, the
linear active fibre 52 is short relative to optical fibres used in nan-linear
loops, which for example,
ane in the order of hundreds of meters. T'he other two ports A and B of the
coupler 50 are connected
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to a bi-directional port G of the circulator 48 and a first output port PIoUT
of the LMF 34,
respectively. The circulator 48 has a unidirectional input port F and a
unidirectional output port E
connected to the input port P,N and a second output port P2~»~~r respectively.
Preferably, the
circulator 48 has more than 45 dB isolation and 0.8 dB insertion loss between
its ports.
Optionally, the performance of the optical amplifier 16 is enhanced,
especially for low input signal
p~~wer, by inserting an isolator (not shown) between the output of EDFA 33 and
the input of LMF
3~~ to avoid or reduce Rayleigh backscattering.
In another embodiment, an improvement in overall noise figure is achieved by
using polarization
rr~aintaining components including palarization-maintaining fiber.
Additionally or alternatively, a
polarization controller (not shown) and an inline polarizer are conveniently
added between the
circulator 48 and the coupler 50 to improve eliminating ASE from the signal.
If a polarization
controller is used, the EDFA need not: use polarization-maintaining components
or be constrained
to provide a particular state of polarization.
In operation, the amplif ed optical signal leaving the circulator 48 via port
G and entering port A of
the coupler 50 is a coherent signal and h.as a certain phase. The coupler 50
splits the signal equally
into 50 per cent signals CW and CCW which leave the coupler 50 via its output
ports C and D,
respectively, so that they propagate in opposite directions around the loop of
fiber 52. The signal
CW propagating clockwise (as shown) in the loop 52 will be in phase with the
signal at port A. The
signal CCW leaving port D and propagating counterclockwise will be phase-
shifted through ~/2
radians relative to the clockwise signal CW. This phase difference is
attributed to the fact that light
coupled across a coupler undergoes a ~c/2 radian phase shift relative to light
coupled straight
through. When the CW and CCW si~;nals arrive back at the opposite ports D and
C, respectively,
they pass through the fibre coupler 50 again to produce an output signal at
port A substantially
edual to the sum of the CW and the CC~' signals and having a phase shift of
~/2 radians relative to
th.e input signal. In other words, since the CW and CCW components propagate
through the same
optical path, a relative phase shift therebetween is not produced. However, a
phase shift of ~/2 is
observed between the input and output signals due to the presence of the
coupler 50. This reflected
output signal appearing at port A is a result of the interference within the
coupler 50. It is directed
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from port A to port G of the circulator 48, where it is output via output port
P2ouT. In theory, there
should be no output of the signal from output port Ploi~T because the whole
signal energy should be
reflected in the loop mirror formed by fibre 52 and the coupler 50. In
practice, there may be some
slight leakage because the coupler 50 will not split at exactly 50 per cent.
Similarly, the ASE noise that is generated by the EDFA 33, is split into 50
percent signals
corresponding to CW and CCW components that propagating in opposite directions
around the
loop of the optical fibre 52 and appear at port A along with the reflected
signal. However, when the
amplified signal and ASE pass through 'the active fibre 52b in the linear
active loop 52, the ASE is
used to pump the amplified signal, thus effectively filtering out some of the
ASE and
simultaneously further amplifying the amplified signal. In other words, the
presence of the active
fibre within the linear loop results in ~,nc~rgy transfer from the ASE to the
optical signal, thus
reducing the amount of ASE at the signal wavelength and simultaneously
compensating for the
LMF insertion loss i.e., the insertion loss resulting from the incorporation
of the circulator 48 and
coupler 50. The more ASE accompanying the amplified signal the more energy is
transferred from
the ASE to the signal. Since the amplifying section 33 generates more ASE for
lower input power
levels than higher, the instant invention is particularly useful for low power
applications.
Advantageously, the presence of the erbium-doped fiber 52B reduces the amount
of ASE at the
signal wavelength and simultaneously increases the signal power to improve the
signal-to-noise
ratio. Of course, the erbium-doped section of the optical waveguide 52 could
be replaced with
mother active section. :Eor example, a fibre doped with another rare earth, or
combination of rare
a:girths, is also within the scope of the instant invention.
Referring to Figure 5, there is shown a preferred embodiment of the amplifier
16, wherein the
linear LMF 34 includes a polarization controller 60. Preferably, the
polarization controller 60 uses
v,~riable birefringence to control the power levels between PlouT and P2ouT
and/or to determine the
amount of ASE filtering. For example, the polarization controller 60 could be
adjusted to direct all
o f the amplified signal to the fourth pore. B of the coupler 50, to P 1 ouT,
or alternatively could be
adjusted to tap some of the optical signal, say about 10%. In the former case,
the circulator 48
could be replaced by an isolator 62, as shown in Figure 6. A second isolator
(not shown) might then
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be inserted between the coupler 50 and the output port P 1 ouT to prevent
backreflected light into the
E:DFA, hence improving the noise figure. ,All other components are similar to
the components
dc;scribed with respect to Fig. 4.
Ir~IDUSTRIAL APPLICABILITY
An advantage of embodiments of the invention, in which the pump energy is not
supplied via the
coupler 12, is that the coupler does not need to provide precise 50- 50
splitting over as wide a
range of wavelengths. Consequently, ,:t conventional 3-dB coupler can be used.
In addition,
pumping directly into the loop avoids the need for expensive polarization-
maintaining fiber.
Consequently, the amplifiers may require less manufacturing time and be less
costly than known
loop mirror amplifiers while providing better technical performance.
An advantage of the amplifiers described with respect to Figure 3, as compared
with, for example,
that disclosed by moue et al. is that it does not require a polarization-
maintaining fiber, and other
polarization-maintaining components., nevertheless any of the embodiments
described above, the
active fiber, 3 dB coupler, isolators/circulator, and wavelength-selective
couplers could be
polarization-maintaining so as to further reduce leakage of the amplified
signal to the other output
ports of the 3 dB coupler.
In amplifiers that embody the gain-controlling aspect of the invention, the
wavelength of the fiber
grating 30 is outside the useful spectrum of the amplifier, so the amplifier
may be operated
throughout its useful range.
An advantage of optical amplifiers including a loop mirror filter is that they
work well with low
power input signals, and yield improved signal-to-noise ratios.
