Note: Descriptions are shown in the official language in which they were submitted.
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TWO-STAGE BRIGHTNESS CONVERTER
TECHNICAL FIELD
The present description relates to the generation of optical signals and, more
specifically, to brightness conversion.
BACKGROUND OF THE ART
Fiber lasers are nowadays emerging as the most powerful solid-state laser
technology because of their compactness, reliability, efficiency of operation,
and their
high output power levels. A fiber laser can be seen as a single stage
brightness
converter.
One approach for designing a brightness converter uses a laser cavity (see,
for
example, A. Liem et al., "1.3 kW Yb-doped fiber laser with excellent beam
quality"
Proc. of CLEO 2004, CPDD2, Vol. 2, pp. 1067-1068, (2004)). In such a
brightness
converter, a large number of multimode pump diodes are coupled into a rare-
earth-
doped Double Cladding Optical Fiber (DCOF) using a Tapered Fiber Bundle (TFB)
which is also known as a pump combiner. A laser cavity is formed by using a
Fiber
Bragg Gratings (FBGs) at each end of the DCOF to create a laser effect.
Usually, a
high reflectivity FBG is used at the input of the laser cavity and a low
reflectivity FBG
is used at the output of the laser cavity to only partially reflect the signal
and allow
some power extraction from the cavity. Pump power is absorbed by the doped
core of
the DCOF and a high brightness optical signal is generated. Theoretically, the
output
power of the laser is directly proportional to the pump power.
Another approach for designing a brightness converter uses a Master Oscillator
Power Amplifier (MOPA) (see, for example, Y. Jeong, J.K. Sahu et al,
"Ytterbium-
doped large- core fiber laser with 1.36kW continuous- wave output power,"
Optics
Express, V.12 no.25, pp 6088-6092, (2004)). The MOPA configuration is a more
complex brightness converter because it requires more components. A MOPA
consists of a laser diode, which is known as a seed, coupled to a given number
of
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cascaded optical amplifier stages. An optical isolator is typically inserted
between the
laser diode and the first optical amplifier stage as well as in-between the
optical
amplifier stages in order to protect the laser diode and each amplifier stages
from any
back reflection, which could induce damage. The principle of a MOPA does not
rely
on a laser effect. The optical signal from the laser diode is rather amplified
by the
cascade of amplifier stages until the desired output power is obtained. Each
optical
amplifier stage typically consists of a doped optical waveguide pumped using
multiple
pump diodes which are coupled to the optical waveguide using a TFB.
These two configurations have been used to manufacture fiber lasers and,
nowadays,
a fiber laser with an output power of several kilowatts is a reality. However
these
configurations have some limitations.
Regarding the configuration based on a laser cavity, one drawback of this
technique
is related to a practical limitation of the maximum pump power available for
the
system, which is critical in high power fiber lasers. This limitation has two
causes: the
TFB and the maximum brightness produced by a single emitter pump diode. A TFB
is
an optical component which enables the coupling of pump power propagating into
several optical fibers, known as pump arms, into a single fiber, known as the
signal
fiber. However, there is a theoretical limitation to the number of pump arms
that can
be used. The state of the art sets this value to 31. While it is possible to
increase the
number of pump arms by cascading two TFBs, the maximum of available pump arms
is then limited to 49. This limitation in the number of pump arms induces a
limitation in
the maximum pump power available, which is close to 1 kW considering the state
of
the art single emitter pump diodes. Another limitation related to TFBs is its
thermal
limitation. The TFB induces an insertion loss. Lost pump power is then
absorbed by
the package and induces a rise in temperature which may result in a component
failure. Usually, with an insertion loss of the TFB of about 0.1 dB, the
maximum pump
power that may be used for safe operation is about 1 M. Finally, the maximum
pump
power is also limited by the brightness of the pump diodes. The initial
brightness of
the pump diodes should be carefully chosen in order to have an efficient and
powerful
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system. Some pump diodes which are able to deliver much power are commercially
available but their brightness is low with a delivery fiber that is quite
large, i.e. 600 pm
or higher. This complicates the development of an efficient and powerful fiber
laser.
Another drawback of the configuration based on a laser cavity is the
achievable beam
quality. The absorption of the pump power propagating into the inner cladding
of the
DCOF is proportional to the ratio between the areas of its core and of its
inner
cladding. In order to accommodate the use of a low brightness pump diode, the
radius of the inner cladding should be increased, which consequently decreases
pump light absorption. In order to counter-balance this effect, one can
increase the
radius of the fiber core. Unfortunately, this solution leads to a degradation
of the
beam quality of the signal.
Finally, another drawback of the configuration based on a laser cavity is
related to
thermal management. In practice, there is propagation loss in the DCOF, which
generates heat. In a laser cavity brightness converter, a low index polymer is
generally used in the outer cladding of the DCOF in order to achieve a proper
Numerical Aperture (NA), which is not achievable with silica. While pure
silica is able
to handle temperatures of up to 1500 C and even higher, low index polymers can
only handle temperatures of up to about 120 C. Accordingly, the pump power
cannot
be increased beyond a certain limit for the polymer not to be subject to
damage
caused by heat.
All of the limitations described above regarding the configuration based on a
laser
cavity also apply to the brightness converter configuration which is based on
a
MOPA. Moreover, there are additional limitations which come from the
complexity of
the scheme. First, because a MOPA consists of a cascade of amplifier stages,
it is
generally more costly compared to the laser cavity configuration. Second,
special
care should to be taken to perfectly control the gain of each amplifier stage
in order to
obtain an optimum and efficient operation. Third, each amplifier stage should
also be
isolated one from another using an optical isolator. However, at the present
time,
there exists no commercially available isolator which is able to handle an
optical
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power of more than tens of watts. When such power levels are reached, MOPAs
are
operated without isolators, which could eventually cause dramatic failures.
Considering the drawbacks of the prior art described above, there exists a
need for a
brightness converter which allows the use of low brightness pump diodes to
achieve
good beam quality.
SUMMARY
According to one aspect, there is provided a two-stage brightness converter. A
first
brightness conversion stage has a first laser cavity having a first optical
waveguide
doped with an active ion defining a first optical band with optical
absorption, a second
optical band with optical absorption and optical gain, and a third optical
band with
optical gain. The first laser cavity is pumped with a pump power having a
wavelength
in the first optical band to generate an intermediate optical signal in the
second
optical band. A second brightness conversion stage which is in cascade with
the first
brightness conversion stage comprises a second optical waveguide doped with
the
same active ion. The second brightness conversion stage is pumped with the
intermediate optical signal to obtain a high brightness optical signal in the
third optical
band.
According to another aspect, there is provided a brightness converter which
comprises a first brightness conversion stage having a laser cavity to
increase the
beam quality of a low brightness pump signal to provide an intermediate
optical
signal, with a high efficiency and a good thermal management; and a second
brightness conversion stage to convert the intermediate optical signal into a
high
brightness optical signal with high efficiency and good thermal management.
The provided two-stage brightness converter allows the generation of an
optical
signal having good beam quality with the use of a low brightness pump diode
while
maintaining efficient thermal management.
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In accordance with one embodiment, the second stage of the two-stage
brightness
converter is a laser cavity and, in accordance with another embodiment, the
second
stage of the two-stage brightness converter uses a configuration based on a
Master
Oscillator Power Amplifier (MOPA).
In a particular case, the two-stage brightness converter uses all-glass Double
Cladding Optical Fibers (DCOFs). Because no low index polymer is used in the
DCOFs, they are able to withstand high temperatures.
Furthermore, the provided two-stage brightness converter allows the use of a
very
low brightness pump diode. Accordingly, compared to techniques of the prior
art, a
two-stage brightness converter allows an increase of the maximum available
pump
power.
The provided brightness converter also allows the use of a single pump source,
thereby requiring only one launching point for the pump power and eliminating
the
need for a Tapered Fiber Bundle (TFB) or any other pump combiner.
According to another aspect, there is provided a pump generator for producing
a
secondary pump power. The pump generator comprises a laser cavity having an
optical waveguide doped with an active ion. The optical waveguide has a first
optical
band with optical absorption, a second optical band with optical absorption
and
optical gain, and a third optical band with optical gain. The pump generator
also
comprises a pump source coupled to the laser cavity for generating a primary
pump
power in the first optical band to pump the laser cavity in order to produce a
secondary pump power in the second optical band. The optical waveguide is
multimode at a wavelength corresponding to the secondary pump power such that
the secondary pump power propagates with multiple modes in the optical
waveguide.
According to yet another aspect, there is provided a method for generating a
high
brightness optical signal. The method comprises: i) Pumping with a pump power
a
laser cavity having a first optical waveguide doped with an active ion
defining a first
optical band with optical absorption, a second optical band with optical
absorption
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and optical gain, and a third optical band with optical gain. The pump power
has a
wavelength in the first optical band; ii) Generating an intermediate optical
signal in the
second optical band in the laser cavity as a result of the pumping; iii)
Pumping with
the intermediate optical signal a second optical waveguide doped with the same
active ion; iv) Obtaining within the second optical waveguide a high
brightness optical
signal in the third optical band as a result of the pumping with the
intermediate optical
signal; and v) Outputting the high brightness optical signal from the second
optical
waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view illustrating a two-stage brightness converter which
is based
on a cascade of two laser cavities;
Fig. 2 is a cross-sectional view of a generic example of a Double Cladding
Optical
Fiber (DCOF);
Fig. 3 is a schematic view illustrating the relative dimensions of the
delivery fiber of
the pump diode, the optical waveguide of the first stage and the optical
waveguide of
the second stage of the brightness converter of Fig. 1;
Fig. 4 is a graph showing the emission cross section and the absorption cross
section
of the rare-earth ion Ytterbium;
Fig. 5 is a graph showing the power distribution along the doped optical
waveguide of
the first stage of an example brightness converter;
Fig. 6 is a graph showing the population inversion along the doped optical
waveguide
of the first stage of the example brightness converter;
Fig. 7 is a graph showing the intrinsic gain spectral density of the doped
optical
waveguide of the first stage of the example brightness converter;
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Fig. 8 is a graph showing the power distribution along the doped optical
waveguide of
the second stage of the example brightness converter;
Fig. 9A is a schematic view illustrating a two-stage brightness converter
which is
based on a cascade of two laser cavities separated by a Mode Field Adapter
(MFA);
Fig. 9B is a schematic view illustrating an example of a Mode Field Adapter
(MFA);
Fig. 10A is a schematic view illustrating a brightness converter which is
based on a
MOPA using cascaded amplifiers; and
Fig. 10B is a schematic illustrating in more detail one of the amplifiers of
the cascade
of the converter of FIG 10A.
It will be noted that throughout the appended drawings, like features are
identified by
like reference numerals.
DETAILED DESCRIPTION
Before describing specific embodiments, the definition of brightness should be
reminded. The brightness of a light source is expressed in W/(sr.m2) and is
defined
as the following:
B= NAz Tc A (1)
where P is the emitted power from an area of size A, and NA is the Numerical
Aperture.
State of the art pump diodes typically deliver a power in the order of 20 W
into an
optical fiber having a core diameter of about 100 m and a NA of about 0.15.
This
leads to a brightness B of 3.6x1010 W/(sr.m2) which is typical for a state of
the art
pump diode. For a low brightness pump diode which delivers around 500 W into
an
optical fiber having a diameter of about 600 pm and a NA of about 0.22, the
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brightness is rather 1.1x1010 W/(sr.m2). A low brightness pump diode typically
delivers more power but with a compromise on the brightness, which is a major
problem when using a single stage brightness converter.
Now referring to the drawings, Fig. 1 shows a two-stage brightness converter
100
which is based on a cascade of a first stage 102 and a second stage 104, each
consisting of a laser cavity. It is noted that the symbol 'x' placed at
various positions
in Fig. 1 denotes a fusion splice between components.
First stage 102 is pumped using a low brightness pump diode 106 which produces
pump power A available at a delivery fiber 108. First stage 102 uses a rare-
earth-
doped Double Cladding Optical Fiber (DCOF) 110 as the gain medium, and more
specifically an Ytterbium-doped silica DCOF. A high reflectivity Fiber Bragg
Grating
(FBG) 112 is fusion-spliced at the input of DCOF 110 and a low reflectivity
FBG 114
is fusion-spliced at the output of DCOF 110, in order to form a laser cavity.
Delivery
fiber 108 is fusion-spliced to FBG 112 for injection of pump power A to first
stage 102.
First stage 102 uses pump power A to generate an intermediate optical signal
B.
Intermediate optical signal B is available at the output of FBG 114.
Second stage 104 receives intermediate optical signal B from first stage 102
and
uses it as a pump to generate a high brightness optical signal C. As first
stage 102,
second stage 104 comprises a rare-earth-doped DCOF 120 used as the gain
medium. The rare-earth ion used in DCOF 120 is the same as in DCOF 110, i.e.
Ytterbium. DCOF 120 is also silica based. A high reflectivity FBG 122 is
fusion-
spliced at the input of DCOF 120 and a low reflectivity FBG 124 is fusion-
spliced at
the output of DCOF 120, in order to form a laser cavity. Second stage 104 is
fusion-
spliced to first stage 102 to receive intermediate optical signal B from first
stage 102.
The generated high brightness optical signal C is available at the output of
FBG 124.
Throughout the description, reference is made to the rare-earth ion Ytterbium
which is
used as the dopant in the gain medium. It is however noted that Ytterbium is
used
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herein as an example and that other ions such as Erbium or any other active
ions
may also be used.
Low brightness pump diode 106 typically produces several hundreds of watt of
pump
power A in an optical fiber having a large core diameter, such as 600 pm or
even
more. By using such a low brightness pump diode, only one pump diode 106 is
necessary in this two-stage brightness converter 100 and there is therefore no
need
for a Tapered Fiber Bundle (TFB). This addresses at the same time at least
some of
the problems discussed in the background that are inherent to this component.
Considering a silica fiber doped with Ytterbium, the wavelength of pump diode
106 is
in the wavelength band of 915 to 976 nm. It is noted that while pumping in the
915-
976 nm band is more efficient, pump absorption in Ytterbium-doped optical
fibers is
also possible over a band extending from about 880 to 985 nm.
First stage 102 is designed to produce a laser emission at a wavelength k,
referred to
as intermediate optical signal B. As explained hereinbelow, the wavelength k,
of
intermediate optical signal B is selected in an intermediate optical band of
about 1020
to 1030 nm in this case. Both FGB 112 and FBG 114 have a peak reflectivity at
2 for
the laser cavity to laze at this wavelength and special care is taken in the
design of
FBGs 112, 114 in order to ensure that all the optical modes sustained by
multimode
DCOF 110 are reflected by FGBs 112 and 114.
It is noted that first stage 102 may also be called a pump generator since it
is used to
generate an optical signal used as a pump in second stage 104.
Second stage 104 receives the intermediate optical signal available at the
output of
first stage 102 and uses it as pump. Second stage 104 is designed to produce a
laser
emission at a wavelength A2. This laser emission which is of high brightness
and high
beam quality is referred to herein as high brightness optical signal C. As
explained
hereinbelow, the wavelength A2 is selected at 1080 nm in this case. Both FGB
122
and FBG 124 have a peak reflectivity at 22 to form the laser cavity and
stabilize the
lasing wavelength to X2. It is noted that while the first and second stages
102, 104
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both use an Ytterbium-doped silica DCOF as the gain medium, the dimensions and
characteristics of DCOF 110 and DCOF 120 are different from one another.
Before discussing the specific dimensions of DCOFs 110 and 120, a schematic of
a
generic DCOF is illustrated in Fig. 2. A DCOF may be regarded as a
superposition of
two waveguides, i.e. a signal waveguide and a multimode pump waveguide. A DCOF
comprises a core 202 which is doped with an active ion, an inner cladding 204,
an
outer cladding 206 and a jacket 208. The signal waveguide consists of core 202
and
inner cladding 204 such that the signal is guided in core 202 using inner
cladding
204. The multimode pump waveguide consists of inner cladding 204 and outer
cladding 206. The pump power is guided in inner cladding 204 using outer
cladding
206. Jacket 208 surrounds outer cladding 206. A DCOF is then able to convert
the
low brightness and poor beam quality multimode pump power propagating in the
pump waveguide into a high brightness and high beam quality signal propagating
into
the signal waveguide. The pump power is injected into the pump waveguide and
during its propagation the pump power overlaps the signal waveguide where it
is
absorbed by the active ion. Finally, a stimulated emission takes place into
the signal
waveguide.
Fig. 3 shows the relative dimensions of delivery fiber 108 of pump diode 106
(not
shown in Fig. 3), DCOF 110 of first stage 102 (not shown in Fig. 3) and DCOF
120 of
second stage 104 (not shown in Fig. 3). Delivery fiber 108 has core 302 and
cladding
304. DCOF 110 has core 312, inner cladding 314 and outer cladding 316. DCOF
120
has core 322, inner cladding 324 and outer cladding 326. It is noted that FBGs
112
and 114 have dimensions matching DCOF 110 and FBGs 122 and 124 have
dimensions matching DCOF 120. FBGs 112, 114, 122 and 124 are not shown in
Fig. 3 for better clarity.
Pump power A available at delivery fiber 108 is coupled to inner cladding 314
of
DCOF 110. DCOF 110 is designed such that the diameter of inner cladding 314
matches the diameter of core 302 of delivery fiber 108 such that an optimal
coupling
is obtained. In one embodiment, this diameter is 600 pm. The diameter of core
312 is
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in the order of magnitude of 100 m or higher. DCOF 110 therefore operates in
multimode. Because the diameter of core 312 of DCOF 110 is smaller than the
diameter of core 302 of delivery fiber 108, the brightness of intermediate
optical
signal B is improved compared to pump power A.
In addition, the diameter of inner cladding 324 of DCOF 120 is designed to
match the
diameter of core 312 of DCOF 110. By having a ratio between the area of inner
cladding 324 and the area of core 322 that is in the range of tens of
percents, the
diameter of core 322 can be kept small enough to insure a single mode
operation of
DCOF 120. The single mode operation provides a high brightness optical signal
C
that is diffraction limited. It is noted that a multimode DCOF 120 may rather
be used
but that some special care should then be taken to operate DCOF 120 in a
single
mode regime in order to obtain a diffraction limited beam at its output.
Slightly
multimode operation of the second stage is also possible.
It is noted that the parameters of DCOFs 110 and 120 of the embodiment
described
above allows the use of all-glass optical fibers for both DCOF 110 and DCOF
120. It
is however noted that DCOFs using a low index polymer or silicone may still be
used.
It is also noted that the dimensions described above are given as an example
and
that these dimensions may be varied to adapt to any other practical
applications.
The provided brightness converter allows for flexibility in the choice of the
characteristics of the optical waveguides used. When using DCOFs, the
Numerical
Aperture (NA) and the diameter of core 312 of DCOF 110 of the first stage may
be
carefully chosen to maximize core absorption in DCOF 120 of the second stage
while
keeping inner cladding 324 of DCOF 120 of the second stage small enough. The
core
322 of DCOF 120 of the second stage may then be kept small enough for single
mode operation in the second stage, thereby providing a diffraction limited
output
beam.
Fig. 4 shows the absorption cross section 410 and the emission cross section
420 of
the rare-earth ion Ytterbium and is used to describe the operation of the two-
stage
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brightness converter 100 of Fig. 1. The absorption and emission cross sections
410,
420 are related to the optical gain and their study gives us insight on how a
signal is
amplified inside a laser cavity. Fig. 4 also shows three spectral bands that
will be
described below and which correspond to a first optical band 431, a second
optical
band 432 and a third optical band 433. The first optical band 431 extends
between
about 915 and 976 nm and corresponds to an optical absorption band. The second
optical band 432 extends between about 1020 and 1030 nm, where both absorption
and gain may occur. The third optical band 433 extends around 1080 nm which
corresponds to an optical gain band.
In a single stage brightness converter, pumping is typically performed in the
first
optical band 431 extending between 915 and 976 nm where the absorption cross
section is the highest. The lasing wavelength is then set in the third optical
band 433,
i.e. around 1080 nm.
However, one can note that the absorption cross section is not negligible in
the
second optical band 432 extending between 1020 and 1030nm as it is the case in
the
third optical band extending around 1080 nm. In the second optical band 432,
both
absorption and gain are possible. This observation is important for the two-
stage
brightness converter 100 which specifically uses this band. First stage 102 is
pumped
using low brightness pump diode 106 in the first optical band 431, i.e.
between 915
and 976 nm, and is designed to generate intermediate optical signal B with a
wavelength in the second optical band 432, i.e. between about 1020 and 1030
nm.
Pump power A propagates in inner cladding 314 (see Fig. 3) of doped DCOF 110.
Intermediate optical signal B which is generated in the second optical band
432
propagates in core 312 (see Fig. 3). Intermediate optical signal B in the
second
optical band 432 has an improved brightness compared to pump power A. Because
the absorption in the second optical band 432 is not negligible, the
intermediate
optical signal B generated by first stage 102 may be used to pump second stage
104
to generate high-brightness optical signal C in the third optical band 433.
Moreover,
the brightness improvement in first stage 102 allows keeping the fiber core of
DCOF
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120 small enough to have single mode operation in the third optical band 433,
i.e. at
1080 nm. Second stage 104 therefore generates a laser signal at 1080 nm with a
diffraction limited beam, thanks to the single mode operation of DCOF 120.
As noted above, while pumping in the 915 to 976 nm band is more efficient,
pump
absorption in Ytterbium-doped optical fibers is also possible over a larger
band
extending from about 880 to 985 nm. Accordingly, the first optical band 431
may also
be expended to this range. Similarly, the second optical band 432 may also be
expended from about 1000 to 1050 nm where both absorption and gain exist. The
third optical band 433 may also be expended from about 1060 to 1100 nm where
an
efficient gain is possible.
The two-stage brightness converter 100 addresses at least some of the
limitations
related to the thermal management of fiber lasers. First, in most cases, doped
DCOFs 110 and 120 may be made all-glass with no need for any low index
polymer.
In single-stage brightness converters based on a laser cavity, the doped DCOFs
typically use a low index polymer in order to increase the NA of the pump
waveguide
to match the NA of the delivery fiber of the Tapered Fiber Bundle (TFB), which
is
about 0.46. However, most of the commercially available low brightness pump
diodes
have a delivery fiber with an NA close to 0.22 and since no TFB is used in
this
embodiment, an NA of 0.22 may be achieved in DCOF 110 using a low index glass.
By avoiding the use of a low index polymer in the design of DCOFs 110 and 120,
problems related to thermal failures are eliminated.
Moreover, a two-stage brightness converter 100 provides an improved thermal
management over the prior art configurations described above. The quantum
efficiency of a single-stage brightness converter varies between 85 % and 90%.
With
two-stage brightness converter 100 of Fig. 1, the quantum efficiency will
typically vary
between 88% and 95% for first stage 102, and between 88% and 94% for second
stage 104. As higher quantum efficiency leads to a lower heat generation, low
heat
generation is achieved in each of stages 102 and 104. Even lower heat
generation is
achieved in first stage 102 because of the large core diameter of DCOF 410
which is
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typically close to or greater than 100 pm. As heat generation is inversely
proportional
to the square of the core radius, a low heat generation is achieved in first
stage 102.
It is noted that the diameter of the core of the optical fiber used to
manufacture of
FBGs 112, 114 should match the diameter of the core of DCOF 110, i.e.
typically in
the order of magnitude of 100 pm. One should note that the writing of FBGs
uniformly
across such a large core can be difficult. It is however noted that it is not
mandatory
that the FBGs be uniformly written across the core since a non-uniform
inscription will
cause unequal reflection of the various modes, simply resulting in less
efficient FBGs.
Example:
An example of a specific design of a two-stage brightness converter according
to
Fig. 1 is now given. It should be understood that the parameters of the
converter
specified below are simply given for illustration and that there exist many
other
possible designs of such a two-stage brightness converter.
First stage 102 of the brightness converter should be designed with care in
order to
obtain a laser emission within the second optical band, i.e. between 1020 and
1030 nm. First stage 102 should be designed with the goal of having a maximum
gain
spectral density within the second optical band in order to obtain an
efficient laser
emission.
Such an efficient laser emission within the second optical band, i.e. between
1020
and 1030 nm, is obtained with a strong population inversion within DCOF 102 of
first
stage 102. In order to obtain the desired strong population inversion a short
cavity
and a large ratio of the core diameter to the inner cladding diameter are used
such
that a strong absorption of the pump power is obtained.
In order to obtain a strong absorption of the pump power, one may want to use
a
heavily doped DCOF. It is however noted that incorporating too much rare earth
ions
in a glass matrix may result in the crystallization of the glass matrix with
the
consequence of increasing the background loss. In the case of Ytterbium, a
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concentration of 3x1026 to 6x1026 ions/ms is acceptable. In this example, a
concentration of 3x1 026 ions/m3 is used with a fiber length of 1 meter.
Next, in order to obtain an efficient laser cavity, the choice of the
wavelength of the
pump power is important. As seen in Fig. 4, the first optical band 431, which
corresponds to the typical pumping band of Ytterbium-doped optical fibers,
extends
between 915 and 976 nm. One should note that the closer the wavelength of the
pump is to the wavelength of the laser emission, the more efficient the laser
is.
However, because the absorption bandwidth is very narrow at this wavelength, a
pump wavelength close to 976 nm would require a pump laser diode that is very
stable in wavelength. For this reason, a pump wavelength of 965 nm is rather
selected.
A low brightness pump diode delivering a power of 1000 W at 965 nm in a
delivery
fiber having a core diameter of 400 pm and a NA of 0.22 may be obtained, for
example, from Laserline GmbH located in Germany. The brightness of such a pump
diode is 5x1010 W/(sr.m).
Then, in order to match the diameter of the delivery fiber of a typical low
brightness
laser diode, the diameter of the inner cladding is 400 p.m and the diameter of
the core
is 100 m, for a core to inner cladding diameter of 0,25.
Finally, the lasing wavelength is set to 1027nm.
Figs. 5 to 7 show the result of a simulation of first stage 102 according to
the above
described design. The simulation is performed using the rate equation modeling
(see
Rare-Earth-Doped Fiber Lasers and Amplifiers, 2nd Edition, Michel J.F.
Digonnet,
Marcel Dekker Inc., 2001, p. 341-344)
Fig. 5 shows the power distribution along DCOF 110 of first stage 102; Fig. 6
shows
the population inversion along DCOF 110; and Fig. 7 shows the intrinsic gain
spectral
density of DCOF 110. As can be seen in Fig. 6, the population inversion is
close to
50% which is two to three times stronger than inversions typically used for
classic
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double cladding fiber lasers emitting at wavelengths around 1080 nm. As shown
in
Fig. 7, the maximum intrinsic gain of first stage 102 is reached at a
wavelength of
about 1020 nm which is close to the lasing wavelength of 1027 nm. The nice
match
between the maximum intrinsic gain and the lasing wavelength results in a good
laser
efficiency of 93%, as can be observed in Fig. 5.
Now, second stage 104 converts intermediate optical signal B at 1027 nm into a
high
brightness optical signal C. The diameter of the inner cladding of DCOF 120 of
second stage 104 is selected to match the diameter of the core of DCOF 110 of
first
stage 102. Accordingly, the diameter of the inner cladding of second stage 104
is
100 m. In order to obtain a high-brightness optical signal C that is
diffraction limited,
DCOF 104 is designed to be single-mode. The diameter of the core is 15 pm and
the
numerical aperture is 0.08 which result in a v-number of 3.5 which is close
enough to
2.405 for a single mode operation of DCOF 120. The Ytterbium concentration is
equal
to 3x1026 ions/m3 in order to obtain good laser efficiency while keeping the
background loss at an acceptable level.
Fig. 8 shows the result of a simulation of second stage 104 according to the
above
described design. Again, the simulation is performed using the rate equation
modeling. It shows the power distribution along DCOF 120. An efficiency of 80%
is
observed in second stage 120, which leads to an overall laser efficiency of
75%.
This example two-stage brightness converter produces a power of 750 W, which
gives a brightness of more than 2x1 014 W/(sr.m) for a NA of 0.08. This
corresponds
to an improvement in brightness from pump power A to high-brightness optical
signal
C of a factor higher than 4000.
Fig. 9A shows a variation from the two-stage brightness converter 100 of Fig.
1. Two-
stage brightness converter 900 of Fig. 9A is mostly identical to the
brightness
converter 100 of Fig. 1 and similar components are therefore not repeatedly
described. Two-stage brightness converter 900 comprises a pump diode 906, a
first
stage 902 with a DCOF 910 and a second stage 904 with a DCOF 920. Two-stage
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brightness converter 900 includes an additional component, i.e. a Mode Field
Adapter
(MFA) 930, inserted between first stage 902 and second stage 904.
In some embodiments, the brightness of pump diode 906 is so poor that the
diameter
of the inner cladding of DCOF 910 of first stage 902 needs to be increased in
order to
obtain an optimum lasing effect in the second optical band. In this particular
scenario,
the ratio between the area of the inner cladding of DCOF 910 and the area of
the
core of DCOF 920 would be decreased such that it becomes smaller than unity
and
absorption of power in DCOF 920 would then decreases. One solution would be to
increase the diameter of core of DCOF 920 but this would then result in an
undesired
multimode operation of DCOF 920. In order to overcome this problem, MFA 930 is
inserted between stages 902 and 904. As can be seen in Fig. 9B which details
MFA
930, a MFA is a section of optical fiber where the diameter is gradually
decreased. It
can be compared to a buffer between an optical fiber with a large diameter and
one
with a smaller diameter. Usually, if the two diameters are close enough, this
adaptation can be done without severe insertion loss.
It is noted that a MFA may alternatively or additionally be inserted between
the pump
diode and DCOF of the first stage in the case of a pump diode having a
delivery fiber
with a large diameter. This allows maintaining a low diameter of the inner
cladding of
the DCOF of the first stage.
Fig. 10A and 10B shows another embodiment of a brightness converter 1000 which
is
based on cascaded Master Oscillator Power Amplifiers (MOPAs). In the
brightness
converter of Fig. 10A, a laser seed 1002 at 1080 nm, is amplified by a cascade
of
amplifiers 1004, 1006 separated by optical isolators 1008. It is noted that
the number
of amplifiers may vary. Brightness converter 1000 varies from state of the art
MOPAs
mostly by the configuration used for each amplifier 1004 and 1006 as shown in
Fig. 10B. Each amplifier 1004 and 1006 has a first stage 1010 and a second
stage
1020. First stage 1010 generates pump power at a wavelength in the 1020-1030
nm
band, which is used to pump second stage 1020. Second stage 1020 consists of
an
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amplifier that amplifies seed laser 1002 using the pump power generated in
first stage
1010.
First stage 1010 comprises a DCOF 1012 doped with an active ion, Ytterbium in
this
case, which is pumped using a low brightness pump diode 1014. High and low
reflectivity FBGs 1016 and 1018 are used to form a laser cavity and stabilize
the
emission wavelength of the laser to A,, which lies in the 1020-1030 nm band.
Because of a laser effect at wavelength A,, a beam brightness improvement is
obtained in first stage 1010 as in the first stage of brightness converter 100
of Fig. 1.
The pump power generated in first stage 1010 is used to pump second stage
1020.
Second stage 1020 is a MOPA and consists of a DCOF 1022 doped with an active
ion, Ytterbium in this case. The pump power is coupled to DCOF 1022 a pump
combiner 1024. Because of the improved beam brightness obtained from first
stage
1010, DCOF 1022 may be operated in single mode, resulting in an optimum
amplification of laser seed 1002. As in the embodiment of Fig. 1, the use of a
two-
stage brightness converter in the MOPA configuration of Fig. 10A allows to
overcome
the pump power limitation because it uses a low brightness pump diodes which
can
typically deliver more power than pump diode with better brightness.
Furthermore, as
in the embodiment of Fig. 1, all-glass DCOFs may be used in the brightness
converter of Fig. 10A, thereby improving thermal management compared to the
configurations of the prior art. Finally, the configuration of Fig. 10A also
does not
require the use of a TFB. A simple pump combiner is rather used, which
addresses at
least some of the limitations related to the use of TFBs.
While the embodiments described herein use optical fibers as waveguides, it is
noted
that any other type of optical waveguides, such as planar optical waveguides,
may
also be used.
Fiber Bragg Gratings (FBGs) may also be replaced by other wavelength specific
mirrors such as thin film filters for example.
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It is also noted that while reference is made herein to optical fibers doped
with
Ytterbium, other active ions may also be used as dopants. For example, Erbium
is
one ion that may be used because it shows a first optical band with absorption
at
980 nm or around 1480 nm, a second optical band between 1520 and 1550 nm
where both absorption and gain take place, and a third optical band with gain
between 1550 and 1600 nm. Erbium may then be used in a first stage to generate
an
intermediate pump in the 1520-1550 nm band from a low brightness pump diode at
980 nm, and in a second stage pumped by the intermediate pump to generate a
signal in the 1550-1600 nm band.
The embodiments described herein use a single high-power low-brightness pump
diode as a pump source. It is however noted that it is still possible to use
instead a
plurality of pump diodes each with lower pump power and combined using a pump
combiner.
The embodiments described above are intended to be exemplary only. The scope
of
the invention is therefore intended to be limited solely by the appended
claims.
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