Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL FIBER PUMPING APPARATUS AND METHOD FOR USE IN PUMPED
OPTICAL FIBER AMPLIFIER AND LASER SYSTEMS
Field of the invention
The invention relates to the field of optical fiber power amplifier and laser
systems
used for optical communications, and more particularly to optical fiber
pumping apparatus
and method for use with a double-cladding fiber in an optical signal amplifier
or laser
configuration.
Background of the invention
Over the past years, there has been a constant need to increase the output
power of
fiber amplifiers and lasers to comply with requirements of modern electronic
communication
techniques, such as Dense Wavelength Division Multiplexing (DWDM) optical
communication, wherein multiple data channels share the available amplifier
output power
for providing simultaneous channels amplification. Optical Inter-Satellite
Links (OISL) is
another growing application wherein a diffraction-limited beam has to be
emitted in free-
space and received thousands of kilometers away. In the latter case,
propagation distances
involved prescribe high-power, and the diffraction-limited quality of the
optical beam
requires the use of single-mode optical fiber amplifiers. For similar reasons,
some particular
applications of Llght Detection And Ranging (LIDAR) require high-power fiber
lasers.
The power output of fiber lasers and amplifiers is directly related to the
absorbed
pump power in the amplifying, rare-earth-doped fiber section, and thus it is
related to the
amount of pump power that can be coupled to a single fiber. The amplified
signal has to be
transversally single-mode in order to have stable amplification and
diffraction-limited output
with high output power. Thus, the amplification doped region must be confined
to a single-
mode core. For optical amplification to occur, the pump must overlap with the
signal in this
single-mode doped core. The coupling of a pump signal into a single-mode core
may be
performed with a small area laser diode. In practice, the diode active area
must be smaller
than the diameter of the single-mode core to allow an efficient coupling.
Reducing the pump
diode active area limits its output power proportionally, which in turn limits
the output power
of the fiber amplifier. A known way to get around this limitation in an
optical fiber amplifier
application is disclosed in U.S. Patent No. 5,721,636 to Erdogan et al., which
consists in
providing a linear array of pump lasers coupled to a plurality of series
connected amplifier
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fiber sections using a coupler an a plurality of routers. Another pumping
approach applied
to a Nd:YAG laser is taught by Fan et al. in a paper entitled "Scalable, end-
pump, diode-
laser-pumped laser" Optics Letters, Vol. 14, No. 19, Oct. 1989, pp. 1057-1059.
In that
paper, there is proposed to couple a multiple-diode pump source to the Nd :YAG
laser,
wherein the beam generated by each laser diode is collimated individually to
be directed to
a large focusing lens providing pump energy injection into the Nd :YAG laser.
Another
known way to increase the pump power of optical fiber lasers and amplifiers
consists in
using a double-cladding rare-earth doped fiber. The high-power, broad beam
pump
radiation generated by the laser diode signal is coupled to a larger multi-
mode inner
cladding inside which a rare-earth doped single-mode core of a higher index of
refraction is
contained. The inner cladding is usually surrounded by an outer cladding
having a lower
index of refraction to prevent radiation from propagating out of the inner
cladding. Such
known optical fiber structure is disclosed in U.S. Patent no. 4,815,079 issued
to Snitzer et
al., which also discloses side coupling configurations for pumping of
radiation into the inner
cladding. In a typical optical fiber application, an input signal injected
within the core is
amplified through pump energy transfer from the inner cladding, while in a
typical optical
fiber laser application, a pair of spaced grating reflectors integrated within
the receiving fiber
forms an optical resonating cavity allowing the pump energy to be converted to
a power
laser beam. Many configurations for double-cladding fiber amplifiers or
lasers, with many
pump/signal multiplexing/demultiplexing approaches are disclosed by Goldberg
et al. in the
paper entitled "High-efficiency side-coupling of light into double-cladding
fibers using
imbedded V-grooves", OFC'96 technical Digest, 1996, pp. 91-92, in
international PCT
application published under No. WO 95/10868 naming Gapontsev et al. as
inventors, in
U.S. Patent No. 5,790,722 issued to Minden et al. and in U.S. Patent No.
5,659,644 to
DiGiovanni et al. Although those prior references teach the use of a double-
cladding fiber,
all disclose the use of a single pump source, thus limiting the pump power
that could be
transferred to the core via a side coupling with the inner cladding, such as
taugth by
Goldberg et al. and Gapontsev et al., or via an end coupling with the inner
cladding, such
as taught by Minden et al. and DiGiovanni et al.
In order to further increase the output power, it is also known to use laser
diode
linear arrays as disclosed in U.S. Patent No. 5,268,978 issued to Po et al.
and in U.S.
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Patent No. 5,533,163 issued to Muendel, wherein particular optical coupling
devices are
proposed for combining the individual laser beams of the diodes included in
the array into a
single output beam to be pumped into the inner cladding of a double-cladding
optical fiber
laser. Although representing improvements over prior pump devices using
conventional
fibers, the pump power that can be transferred to the double-cladding fiber
with those multi-
pump source devices is inherently limited to the number of laser diodes that
can be
physically integrated in the linear array.
Moreover, apart from fiber manufacturing techniques, the
multiplexing/demultiplexing of the pump and the signal is an important
challenge associated
with the double-cladding fiber amplifier configuration. Since the signal is to
be injected in or
extracted from the single-mode inner core and the pump power is to be injected
in the multi-
mode inner cladding closely surrounding the core, traditional fusion fiber
coupling to one
end of the amplification medium is no more possible. There is still a need for
improved
optical pumping fiber techniques and devices that can further increase the
pump power that
can be transferred to a double-cladding fiber in amplifier/laser applications,
while allowing
reliable and efficient input signal coupling when used in fiber amplifier
applications.
Summary of the invention
It is an object of the present invention to provide apparatus and method for
optical
pumping of a double-cladding fiber in amplifier or laser configuration which
are simple and
reliable, while exhibiting high pump power rating.
It is another object of the present invention to provide apparatus and method
for
optical pumping of a double-cladding fiber in an amplifier configuration that
allow
simultaneous efficient input signal coupling.
According to one or both of the above objects, from a broad aspect of the
present
invention, there is provided an optical fiber pumping apparatus for use with a
double-
cladding fiber in an optical amplifier or laser configuration, said fiber
including a single-
mode core doped with an active material and disposed within an inner multi-
mode cladding
surrounded by an outer cladding, said inner cladding being provided with a
pumping input
portion. The apparatus comprises a plurality of pump sources disposed in a
spatial
configuration for radiating pump energy along an optical axis through a
surrounding
generally annular area, and an optical coupling device having an optical input
portion
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generally aligned with the optical axis to collect the pump energy and having
an optical
output portion aligned with the pumping input portion for transferring the
pump energy to the
inner cladding.
From another broad aspect of the present invention, there is provided an
optical fiber
pumping apparatus for use with a double-cladding fiber length in an optical
amplifier or laser
configuration, said fiber including a single-mode core doped with an active
material and
disposed within an inner multi-mode cladding surrounded by an outer cladding,
said inner
cladding being provided with first and second spaced pumping input portions.
The
apparatus comprises a first set of pump sources disposed in a spatial
configuration for
radiating pump energy along a first optical axis through a surrounding
generally annular
area and a first optical coupling device having an optical input portion
generally aligned with
the first optical axis to collect the pump energy, and having an optical
output portion aligned
with the first pumping input portion for transferring the pump energy to the
inner cladding.
The apparatus further comprises a second set of pump sources disposed in a
spatial
configuration for radiating further pump energy along a second optical axis
through a
surrounding generally annular area and a second optical coupling device having
an optical
input portion generally aligned with the second optical axis to collect the
further pump
energy, and having an optical output portion aligned with the second pumping
input portion
for transferring the further pump energy to the inner cladding.
From another broad aspect of the present invention, there is provided a pumped
optical fiber system comprising a double-cladding fiber in one of an optical
signal amplifier
and laser configuration, said fiber including a single-mode core doped with an
active
material and disposed within an inner multi-mode cladding surrounded by an
outer cladding,
said inner cladding being provided with a pumping input portion. The apparatus
further
comprises a plurality of pump sources disposed in a spatial configuration for
radiating pump
energy along a optical axis through a surrounding generally annular area, and
an optical
coupling device having an optical input portion generally aligned with the
optical axis to
collect the pump energy and having an optical output portion aligned with the
pumping input
portion for transferring the pump energy to the inner cladding.
From another broad aspect of the present invention, there is provided a pumped
optical fiber system comprising a double-cladding fiber in one of an optical
signal amplifier
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and laser configuration, said fiber including a single-mode core doped with an
active
material and disposed within an inner multi-mode cladding surrounded by an
outer cladding,
said inner cladding being provided with first and second spaced pumping input
portions. The
apparatus further comprises a first set of pump sources disposed in a spatial
configuration
for radiating pump energy along a first optical axis through a surrounding
generally annular
area, and a first optical coupling device having an optical input portion
generally aligned with
the first optical axis to collect the pump energy and having an optical output
portion aligned
with the first pumping input portion for transferring the pump energy to the
inner cladding.
The apparatus further comprises a second set of pump sources disposed in a
spatial
configuration for radiating further pump energy along a second optical axis
through a
surrounding generally annular area, and a second optical coupling device
having an optical
input portion generally aligned with the second optical axis to collect the
further pump
energy and having an optical output portion aligned with the second pumping
input portion
for transferring the further pump energy to the inner cladding.
From a further broad aspect of the present invention, there is provided an
optical
fiber pumping method for use with a double-cladding fiber in an optical
amplifier or laser
configuration, said fiber including a single-mode core doped with an active
material and
disposed within an inner multi-mode cladding surrounded by an outer cladding.
The method
comprises the steps of:
i) radiating pump energy along a optical axis from a plurality of sources
disposed in a spatial
configuration and through a generally annular area surrounding the optical
axis; ii)
collecting the pump energy; and iii) transferring the pump energy to the inner
cladding.
Brief description of the drawings
Preferred embodiments of the apparatus and method according to the present
invention will now be described in detail in view of the accompanying drawings
in which:
Fig. la is a schematic representation of a preferred embodiment of an optical
fiber
pumping apparatus for use with a double-cladding fiber in an optical
amplifier/laser
configuration.
Fig. 1 b is a cross-sectional end view of a conventional double-cladding
optical fiber to
be pumped with the apparatus of the present invention.
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Fig. 2 is end view according of the embodiment of Fig. 1 a according to line 2-
2 along
z axis, which shows the annular configuration of the pump sources.
Fig. 3 is a top view of a quasi-collimating lens set with a focusing lens as
part of an
optical coupling device that can be used to build an apparatus according to
the present
invention.
Fig. 4 is a side view of the quasi-collimating lens set shown in Fig. 3
according to line
4-4 along x axis.
Detailed description of the preferred embodiments
According to the different preferred embodiments which will be described later
in
detail, the apparatus of the present invention can act either simultaneously
or independently
as a pump combiner, multiple pump injector, signal coupler or pump/signal
multiplexer/demultiplexer. Referring now to Fig. la, the optical fiber pumping
apparatus
generally designated at 10, comprises a first set of laser diodes 12 used as
pump sources,
which are disposed in a spatial arrangement on a support (not shown). As
better shown in
Fig. 2, the diodes 12 radiates pump energy indicated at 13 along a first
optical axis 14
parallel to z axis, through a surrounding generally annular area, leaving a
middle area
available for other purposes, as explained later in detail. While the set of
sources 12 are
preferably disposed in a simple coplanar arrangement, it is to be understood
that more
complex spatial arrangement for the pump sources can be implemented, provided
the pump
energy is radiated through the generally annular area surrounding the optical
axis 14.
Associated with the first set of laser diodes 12, the apparatus 10 is provided
with a first
optical coupling device schematically represented at 16 in Figs. 1 a and 2,
which has an
input portion generally aligned with the first optical axis 14 to collect the
pump energy, and
having an optical output portion aligned with a first pumping input portion 18
of a
conventional double-cladding fiber length represented at 20.
Turning now to Fig. 1 b, the fiber 20 includes a single-mode core 22 doped
with an
active material and disposed within an inner multi-mode cladding 24 surrounded
by an outer
cladding 26. As well known in the art, materials used to form inner cladding
24 and outer
cladding 26 are selected in a such way that the refraction index of the inner
cladding 24 is
significantly higher than the refraction index of the outer cladding, to
provide complete
reflection of the injected pump energy toward the inner cladding at interface
with the outer
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cladding. In an analog way, the material from which the core 22 is made
exhibits a
refraction index significantly higher than the refraction index of the inner
cladding, ensuring
that the pump energy are in turn transferred to the doped core. For optical
fiber laser
applications, the double-cladding fiber 20 is either coupled to a modular
laser device or is
provided with an integrated pair of spaced grating reflectors forming an
optical resonating
cavity allowing the pump energy to be converted to a power laser beam, as well
known in
the art. The pumping apparatus according to the present invention may also be
adapted to
other fiber structures, such as offset core fiber structures or polygonal-
section core fiber
structure.
While in the example shown in Fig. 2, a number of six (6) laser diodes are
substantially equidistantly distributed about the optical axis 14, other
configurations
involving a different number of sources and/or a different spatial
distribution of the sources,
either symmetrical or asymmetrical, can be contemplated, provided an
appropriate coupling
to the receiving double-cladding fiber is effected. In practice, the number of
pump sources
used, and consequently the pump power available, is only limited by the volume
associated
with each laser diode package and the dimensions of the optical coupling
device 16 used.
In the other hand, it may be desirable in some applications to use a reduced
number of
pump sources according to the same spatial arrangement provided by the present
invention. For example, through known numerical simulation techniques, it is
possible to
design an optical coupling device adapted to match with an coplanar annular
arrangement
of a set of four (4) broad-area laser diodes featuring emission area of 1 X
100,urn and
divergence of about 40 x 12 , which features can be readily found in laser
diodes available
on the marketplace, for example from Uniphase Laser Enterprises, with output
power larger
than 3W. A double-cladding fiber provided with a 200,carc multi-mode inner
cladding
showing a 0.28 numerical aperture, and provided with a centered inner rare-
earth doped
core having a sufficiently narrow diameter to allow single-mode propagation in
the near-
infrared region of the spectrum (about 6,can diameter) has been chosen. The
double-
cladding fiber geometry used is convenient and feasible by the specialty fiber
manufacturing
industry, either with an all-glass or polymer cladding approach.
According to the embodiment shown in Fig. 1, the apparatus may further
comprise a
second set of laser diodes 12' used as further pump sources, which are also
disposed in a
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spatial configuration for radiating further pump energy indicated at 13' along
a second
optical axis 14' through a surrounding generally annular area, in a similar
way as described
before with reference to Fig. 2. Associated with the second set of laser
diodes 12', the
apparatus is further provided with a second optical coupling device
schematically
represented at 16' in Fig. la, which has an input portion generally aligned
with the second
optical axis 14' to collect the further pump energy, and having an optical
output portion
aligned with a second pumping input portion 18' of the double-cladding fiber
length 20. It
can be seen from Fig. 1a that first and second pumping input portion 18 and
18' of the fiber
length 20 are in an opposed spaced relationship, allowing a dual pumping
configuration to
provide more power. While the first and second input portion 18 and 18' are
preferably
located at opposed ends of the fiber length 20, it is to be understood that
other types of
pumping injection principle may be used, such as lateral coupling, especially
in laser
configuration.
In a case where the apparatus 10 as shown in Fig. la is used in a fiber
amplifier
configuration, there is provided an optical signal input device 28 for
injecting an optical
signal to be amplified into a signal input portion of the core 22 shown in
Fig. 1 b, providing
the multiplexing of the optical signal with the pump energy, in a co-
propagation mode.
Through the available middle area defined by the spatial arrangement of the
set of pump
sources, an optical input signal, such as a signal coming from an inner single-
mode core
having an appropriate divergence, i.e. not larger than the divergence of the
multi-mode inner
cladding, can be either injected inserted or extracted without interfering
with the diode pump
distribution. Moreover, feedback mechanisms, and other potentially interesting
optical
elements, can be located within this middle area to adapt the signal output to
some
particular applications. It is to be understood that the signal input portion
of the core 22 may
be located
thereon in a such manner to inject the signal in a counter-propagation
direction with
reference to the pump energy transferred to the inner cladding. For example,
the optical
signal input device 28 shown in Fig. 1 a may be disposed in front of the
second input portion
18', with the pump energy being provided by a single set of laser diodes 12
and optical
coupling device 16.
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Conveniently, the signal input portion of the core is surrounded by the first
signal
pumping input portion 18 in the example shown. In such fiber amplifier
configuration, the
optical coupling device 16 is further used to collect and transfer the optical
signal to the core
22. As a result, an amplified optical signal is generated at an optical signal
output portion of
the core 22, which is located adjacent to pumping input portion 18' in the
example shown in
Fig. la, providing the demultiplexing or extraction of the amplified optical
signal with the
pump energy, in a counter-propagation mode. The second coupling device 16' is
further
used to collect and direct the amplified signal shown by arrow 29 which can be
further
redirected as desired.
According to an alternate, more basic embodiment, a single set of laser diodes
12
and a single optical coupling device 16 can be combined with the double fiber
length 20, to
reduce the cost of the apparatus for applications requiring moderate levels of
pump power.
Such basic apparatus can also be used in a fiber amplifier configuration, by
providing an
optical signal input device 28 in a same arrangement as shown in Fig.1a,
providing the
multiplexing of the optical signal with the pump energy, in a co-propagation
mode. As a
result, an amplified optical signal is generated at an optical signal output
portion of the core
22, which amplified signal can be redirected as desired.
Referring to Figs. 3 and 4, the optical design of an optical coupling device
used in the preferred embodiments described above will now be described in
detail. The
optical coupling device generally designated at 16 comprises a plurality of
quasi-collimating
sets 30 of lenses, only one of such set 30 is illustrated in Figs. 3 and 4 for
the ease of
illustration. Each lens set 30, which can be qualified as collimating lens set
for all practical
purposes, is disposed in front of a respective pump diode located at point 32
in Figs. 3 and
4, for redirecting the pump energy into a plurality of pump energy beams 41.
The lens set
30, which is preferably anamorphic, is made of an aspheric lens 34 in series
with three
cylindrical lenses 36, 38 and 40 in the example shown, according to a plano-
convex
aspheric geometry. The optical coupling device 16 further comprises in series
with the lens
set 30 a larger focusing lens 42 used for converging the pump energy beams 41
to the
pumping input portion 18 or 18' of the inner cladding of the fiber 20 as shown
in Fig. 1a. The
focusing lens 42 is preferably of a the plano-convex aspheric type, and has an
optical axis
being aligned with the optical axis 14 or 14'. As for the single-mode signal
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injection/extraction, the use of a plano-convex aspheric focusing lens alone
may not be
sufficient to insure adequate collimating and coupling. However, with an
appropriate quasi-
collimating lens set adapted to the signal properties, one can expect signal
losses to be
smaller than 10 %. The available diameter for signal collimating/coupling in
the middle area
of the focusing lens 42 is approximately 40 mm when a large focusing lens with
a diameter
of 80 mm is used. With respect to the four-diode configuration described
above, an
estimated coupling efficiency of about 80 % can be achieved using the proposed
optical
coupling device. It is to be understood that other appropriate optical designs
can be used to
implement the optical coupling device 16 or 16'. It can be appreciated that
the pump power
available is only limited by the volume associated with each pump diode
package, the
volume of the each lens set 30 and the diameter of each large focusing lens
42.
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