Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OPTICAL FIBER AMPLIFIER SYSTEM AND METHOD
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of
optical fiber amplifiers for amplifying optical signals.
2. Background Art
Multi-stages fiber amplifiers typically involve
rare-earth doped optical fiber amplifiers optically coupled
in series (i.e., cascaded fiber amplifiers). When coupled
to a cascaded fiber amplifier system, an optical signal,
such as, for example, a pulsed seed laser, can be amplified.
A pulsed seed laser refers to a laser having an emission
state that periodically changes from an on state (radiation
emission) to an off state (no radiation emission). This
1s periodic modulation of the seed laser may be accomplished by
switching the power supply of the laser itself, or by using
an external switch placed at its output. In the above-
referenced design of cascaded fiber amplifiers, when the
seed laser is in the on state, its power radiation is
amplified at least by two fiber amplifiers. Those fiber
amplifiers are usually designed such that the pulse energy
is maximized at the device output.
A fiber amplifier is typically made of an optical
fiber having a rare-earth doped core. A particular gain
spectrum is associated to each type of rare-earth ions (also
called dopant) . In order to amplify light guided in the
fiber core, rare-earth ions have to be in an excited state
of energy. This energy is usually provided by the
absorption of laser pump light guided in the fiber core or
cladding.
When the seed laser is in the off state, no signal
light is incoming in the first amplifier doped core. Rare-
earth ions in the amplifier cascade absorb the pump light
without amplifying the seed radiation. As a consequence,
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the potential gain of the amplifiers increases, and so does
the energy stored in the fiber amplifiers. When the seed
laser is switched on again, part of this stored energy is
transferred to the traveling pulse by stimulated emission.
Rare-earth ions in an excited state spontaneously
emit a photon within an average time span called the
fluorescence lifetime. Even when an amplifier is not seeded
by the master oscillator, spontaneously emitted photons are
guided in the amplifier core and are amplified by
neighboring rare-earth ions still in the excited state. The
resulting light guided in the fiber core is called amplified
spontaneous emission (ASE) . The immediate consequence of
ASE is the depletion of available energy in the doped fiber.
When many amplifiers are used in series, ASE emitted by one
amplifier may act as a seed and furthers depletion of stored
energy in the other amplifiers as well.
SUMMARY OF INVENTION
It is therefore an aim of the present invention to
provide a fiber amplifier system that addresses issues of
the prior art.
It is a further aim of the present invention to
provide a method for amplifying an optical signal.
Therefore, in accordance with the present
invention, there is provided an optical fiber amplifier
system comprising a first optical fiber having a doped core
with a first gain spectral profile upon being pumped, the
first optical fiber being adapted to receive an optical
signal from a light source; a second optical fiber having a
doped core with a second gain spectral profile upon being
pumped, the second optical fiber being optically coupled to
the first optical fiber; a pump light system optically
coupled to at least one of the fibers so as to store energy
in the fibers for a subsequent amplification of the optical
signal from the light source; and an overlapping
configuration between the first gain spectral profile and
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the second gain spectral profile so as to reduce energy
depletion in one of the optical fibers from amplification of
spontaneous emission generated by another of the optical
fibers-
The invention also provides the core of the first
optical fiber doped with neodymium and the core of the
second optical fiber doped with ytterbium.
The invention also provides the pump light system
with a first pump light source optically coupled to the
first optical fiber and a second pump light source optically
coupled to the second optical fiber.
The invention also provides a method for
amplifying an optical signal from a light source coupled to
an optical fiber amplifier system having at least cascaded
two optical fibers, comprising the steps of: i) obtaining a
first gain spectral profile corresponding to one of the
optical fibers; ii) obtaining a second gain spectral profile
corresponding to another of the optical fibers and
associated to the first gain spectral profile so as to
reduce energy depletion in one of the optical fibers from
amplification of spontaneous emission generated by another
of the optical fibers; and iii) emitting an optical signal
in the optical fiber amplifier system; whereby the optical
signal is amplified.
According to another aspect, there is provided a
multi-stage optical fiber amplifier system for amplifying an
optical signal. The optical fiber amplifier system comprises
a first single-pass optical fiber amplification stage having
a first optical fiber with a core doped with a first rare-
earth ion and with a first gain spectral profile upon being
pumped. The first optical fiber amplification stage is
adapted to receive the optical signal. The optical fiber
amplifier system also comprises a second single-pass optical
fiber amplification stage having a second optical fiber with
a core doped with a second rare-earth ion and with a second
T ^tral trofile upon being pumped. The second optical
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fiber is optically coupled to the first optical
fiber for the optical signal amplified in the first optical
fiber and the amplified spontaneous emission generated in
the first optical fiber to be coupled to the second optical
fiber. The optical fiber amplification stages have a
continuous wave pump light system optically coupled to the
fibers to pump the first optical fiber and the second
optical fiber with continuous wave pump light so as to store
energy in the fibers for a subsequent amplification of the
optical signal from the light source. The first and the
second rare-earth ion are different, such that the first
gain spectral profile and the second gain spectral profile
are distinct while overlapping over a wavelength region
including the optical signal wavelength so as to reduce
energy depletion in the second optical fiber from amplified
spontaneous emission when the amplified spontaneous emission
generated by the first optical fiber is coupled to the
second optical fiber. One of the first rare-earth ion and
the second rare-earth ion is neodymium and the other one is
ytterbium.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the
invention, reference will now be made to the accompanying
drawings, showing by way of illustration a preferred
embodiment thereof and in which:
Fig. 1 is schematic view of an optical fiber
amplifier system, in accordance with a first embodiment of
the present invention, to which is optically coupled a light
source; and
Fig. 2 is a graphic representation of gains of a
Ytterbium (Yb) doped amplifier and of a Neodymium (Nd) doped
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amplifier, as a function of the wavelength, for one
embodiment of the optical fiber amplifier system of Fig. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Fig. 1, an optical system 10, in
accordance with a first embodiment the present invention,
having a light source 40 (e.g., seed laser) optically
coupled to a cascaded optical fiber amplifier system 20,
will be described.
The optical fiber amplifier system 20 has a first
amplifier 22 and a second amplifier 26. The first amplifier
22 and the second amplifier 26 are optically coupled (i.e.,
coupled for light transmission therebetween) by way of a
coupler 30 and an isolator 33. The first amplifier 22 and
the second amplifier 26 are optical fibers having respective
rare-earth doped cores. The dopant used in the cores of the
amplifiers 22 and 26 will be described in further detail
hereinafter. The coupler 30 is used to couple the fiber
core of the first amplifier 22 to the fiber core of the
second amplifier 26. Moreover, the coupler 30 couples pump
light coming from a pump source 24 to the fiber of the first
amplifier 22, in order to store energy in the first
amplifier 22 for subsequent amplification of a signal from
the light source 40.
Similarly, a coupler 31 is used to couple the
fiber core of the second amplifier 26 to output optics 36.
The coupler 31 is also used to couple pump light coming from
a pump source 28 to the fiber of the second amplifier 26, in
order to store energy in the second amplifier 26 for
subsequent amplification of a signal from the light
source 40.
The light source 40 has an isolator 32 and
focusing optics 34, through which light beam 42 from the
light source 40 will propagate to enter into the optical
fiber amplifier system 20 via the input end 21 of the first
amplifier 22. Focusing optics 34 are used to adapt the
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light beam 42 so that the light beam 42, when entering the
input end 21 of the first amplifier 22, has proper
dimension. The isolators 32 and 33 are used to reduce back-
and-forth reflections in the system 10.
In the system 10, light beam 42 propagates into
the first amplifier 22, and, as it does, is amplified to a
first intensity level. Light beam 42 then propagates
through the coupler 30, through the isolator 33 and through
the second amplifier 26, where it is amplified to a second
intensity level, higher than the first intensity level.
Then, the amplified laser beam propagates through the
coupler 31 and through output optics 36, by which the
amplified light signal is outputted from the system 10.
In one embodiment of the present invention, the
light source 40 is a laser emitting at 1060 nm. The fiber
amplifiers 22 and 26 of the system 20 are designed to
provide gain at the laser wavelength, (i.e., at 1060 nm for
the present embodiment). The fiber amplifiers 22 and 26 of
the system 20 are designed to provide a spectral gain
distribution, with an overlapping configuration between the
spectral gain profiles of the two amplifiers 22 and 26, that
minimizes the amplification by the second amplifier 26 of
the spontaneous emission (SE) and the amplified spontaneous
emission (ASE) produced by the first amplifier 22. One way
to obtain such amplification characteristics is to dope
differently the fiber amplifiers 22 and 26.
Accordingly, in one embodiment of the invention,
the fibers of the first and second amplifiers 22 and 26 of
the system 20 are doped with different dopants. For
instance, the first amplifier 22 is an optical fiber with a
silica core doped with neodymium (Nd). The second amplifier
26 is an optical fiber with a silica core doped with
ytterbium (Yb).
Fig. 2 is a graphic representation of the gain, as
a function of the wavelength, in each one of the fiber
amplifiers 22 and 26, doped as described above. On the
graphic representation of Fig. 2, the x axis 50 corresponds
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to the wavelength, whereas the first y axis 52 corresponds
to the gain of the Yb-doped fiber amplifier in arbitrary
units. Second y axis 53 corresponds to the gain of the Nb-
doped fiber amplifier in arbitrary units. Curve 54
represents the spectral gain of the second fiber amplifier
26, doped with ytterbium (Yb), and curve 56 represents the
spectral gain of the first amplifier 22, doped with
neodymium (Nd). Although in reality the gain amplitude in
the second amplifier 26 may differ from the gain amplitude
of the first amplifier 22, curves 54 and 56 have been
normalized to better show the interrelation between their
respective shapes.
As someone skilled in the art knows, the spectral
gain provided by a fiber amplifier depends on many factors,
among which are the pump wavelength, the pump intensity
delivered to the fiber amplifier, the intensity of the
propagating signal aimed to be amplified, and the intensity
of other propagating "noisy" signals (as, for example,
spontaneous emission). Thus the spectral gain curves
appearing in Fig. 2 are given as examples among many other
examples, and it is understood that their relative shapes
could vary depending on the operating conditions of the
optical fiber amplifier system 20.
Nevertheless, Fig. 2 illustrates some of the
advantages provided by the present invention. It is seen
that gain curves 54 and 56 are overlapping in a wavelength
region where a signal is aimed to be amplified. In the
present embodiment, this wavelength region is located around
1060 nm, which is the wavelength of the light source 40. It
is also seen that the wavelength region (1050 nm to 1080 nm)
where the gain is maximum in the first amplifier 22 is
separated from the wavelength region (970 nm to 980 nm)
where the gain is maximum in the second amplifier 26. This
will result in the minimized amplification by the second
fiber amplifier 26 of any spontaneous emission generated by
the first amplifier 22.
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The spontaneous emission generated by an amplifier
has a spectral profile similar to its spectral gain. Thus,
by designing two amplifiers having distinct spectral gains
but which are arranged in an overlapping configuration over
a wavelength region corresponding to the signal aimed to be
amplified, the amplification by the second fiber amplifier
26 of the spontaneous emission generated in the first
amplifier 22 is minimized. The energy depletion of the
second amplifier 26 due to the amplified spontaneous
emission is reduced.
In the present embodiment, first and second
amplifiers 22 and 26 are single-mode fibers, but the present
invention could be embodied in other types of fibers, such
as slightly multi-mode fibers or double-clad fibers, for
example.
Many variations are contemplated for the system 10
described above. For example, the system 20 is shown in a
pumping counter-propagating configuration but other pumping
configurations (co-co, co and counter) could also be used.
Other dopants could be used, provided the above-described
gain overlapping configuration is present. Similarly, fiber
material other than silica may be used and the pumping could
be performed at other wavelengths. Also, the order of the
first and second amplifiers 22 and 26 could be reversed.
Furthermore, although the described embodiment presented a
cascaded optical fiber amplifier system 20 comprising two
amplifiers, the cascaded optical fiber amplifier system 20
could be provided with several cascaded amplifiers. All of
the above variations are encompassed by the present
description.
