Note: Descriptions are shown in the official language in which they were submitted.
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AN OPTICAL LIGHT SOURCE
Field of the Invention
This invention relates to an optical light source, an optical amplifier, and a
fibre
laser.
Background of the Invention
There is a demand for an optical light source for pumping optical amplifiers,
lasers
and other amplifying optical devices. There is a related demand for optical
amplifiers that
can output powers of 100mW to l OW, or higher powers, and can amplify many
wavelength
channels simultaneously with high reliability and low cost per wavelength
channel. There
is a related demand for optical amplifiers with low-polarisation dependent
gain.
Conventional optical amplifiers use single-mode optical fibre whose core is
doped
with one or more rare-earth ions such as Erbium. These amplifiers are pumped
by single-
mode pump diodes and hence they provide limited power output that is
insufficient for
multi-channel WDM transmission systems. In addition, conventional amplifiers
are prone
to the failure of pump sources, requiring several pump sources to be contained
within the
amplifier in order to provide certainty of pumping even in the event of pump
failures. The
pump sources have a single-mode waveguiding stripe which operates with high
power
densities. The higher the power density in the stripe, the more difficult it
is to achieve high
reliability. The pump source also need to be wavelength stabilised which is
achieved either
by using Peltier coolers which control the wavelength indirectly via
temperature or by fiber
Bragg grating that provide an optical feedback (~5-10%) at certain wavelength
locking the
output wavelength of the laser.
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The power output of conventional optical amplifiers has recently been
increased by
the introduction of pump modules containing several semiconductor lasers whose
outputs
are wavelength division multiplexed into a single optical fibre. Although the
output
power obtainable from such an optical amplifier containing one of these pump
modules is
sufficient for amplifying many channels simultaneously, the approach is
expensive, is
currently limited in powers to around 1 W, and offers limited pump redundancy.
The cost issue of optical amplifiers is also a problem as the networks expand
into
the metropolitan axeas, the expansion being driven by the insatiable demand
for bandwidth
for Internet, data, mobile phones and cable television. Prior art optical
amplifiers are too
expensive and this is currently limiting the expansion of the networks.
Cladding pumped Ytterbium (Yb) doped fibre lasers operating at around 977nm
have been the subject of significant technical and experimental activity in
recent years.
Despite obvious attractions of such sources - as pumps for erbium doped fibre
amplifiers
(EDFAs) and as stand-alone lasers operating at the shortest wavelength
available from
cladding-pumped silica fibre lasers - there are no reports on practical, user-
friendly,
realizations. The principal requirement for practical implementations of high
power 977nm
fibre lasers is to reach high enough population inversions, since otherwise
emission occurs
on the quasi-four level transition around 1040nm, with laxge reabsorption at
the two-level
977nm transition. Additionally, Yb-doped fibre lasers are known as being
notoriously
noisy, with poor relative intensity noise (RIN) characteristics that
significantly narrow their
range of applications.
Erbium-doped fibre amplifiers (EDFAs) have revolutionized optical
communications over the last ten years. The increasing need for capacity
drives the
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amplification requirements, namely operation over the full C-Band with low
noise and
short transient times and low cost.
The most common approach to EDFA pumping is to use single-mode laser diodes
at 980 or 1480nm. However, a high channel-count means higher output power,
therefore
more laser diodes, wluch increases the cost and complexity of the EDFA.
Cladding-pump
fiber technology offers a cost-effective solution to high power pumping.
However, directly
cladding-pumped EDFAs are sensitized (co-doped) with ytterbium in order to
improve the
pump absorption. Furthermore, additional co-doping with phosphorous is
required for
efficient energy transfer from ytterbium to erbium. Unfortunately phosphorous
leads to
substantial spectral gain narrowing from the blue end of the gain spectrum,
which makes
erbium ytterbium co-doped optical amplifiers less suitable for WDM
applications.
Additionally, compared to traditional EDFAs, ytterbium co-doped directly
cladding-
pumped EDFAs have a higher noise figure, which also holds baclc field
deployment.
It is an aim of the present invention to obviate or reduce the above mentioned
problems.
Summary of the Invention
According to a non-limiting embodiment of the present invention, there is
provided
an optical light source comprising a laser diode, beam shaping optics, and an
amplifying
optical fibre, wherein the amplifying optical fibre comprises a waveguide
comprising a
core and a cladding, wherein the waveguide is doped with a rare earth dopant,
and wherein
the laser diode is able to produce optical pump power which is coupled to the
waveguide
by the beam shaping optics.
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The beam shaping optics may comprise a first lens. The first lens can be
formed on
the end of the amplifying optical fibre.
The beam shaping optics may comprise a second lens. The second lens can be a
cylindrical lens. The cylindrical lens can be a cylindrical microlens which
may have a
shape, such as circular, elliptical or hyperbolic, designed to transform some
particular
given input light distribution into some desired output light distribution.
The cylindrical
lens may have a uniform refractive index profile, or may have a graded
refractive index
profile such as parabolic.
The laser diode can be a multimode laser diode. The laser diode can comprise
at
least one singlemode laser diode. The laser diode can comprise at least one a
diode bar.
The laser diode can comprise at least one diode stack.
The laser diode can emit 0.1 W to SOW of optical pump power. The laser diode
can
emit 0.5 W to 5 W of optical pump power.
The cladding can have an outer diameter in the range l0um to 100um. The
cladding can have an outer diameter in the range l5um to SOum.
The core and/or cladding can be doped with at least one of germanium,
phosphorous, boron, aluminium and fluoride.
The core can be configured to be a single mode waveguide.
The optical pump power can facilitate optical radiation from the rare earth
dopant
in the waveguide.
The optical radiation from the rare earth dopant in the waveguide can be
coupled to
an amplifying optical device, wherein the amplifying optical device is one of
an optical
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amplifier, a laser or a distributed feedback laser, and wherein the amplifying
optical device
is configured to be pumped by the optical radiation.
The optical radiation from the rare earth dopant in the waveguide can be
coupled to
a plurality of amplifying optical devices via an optical coupler, and wherein
the amplifying
optical devices axe configured to be pumped by the optical radiation.
The cladding may be circular. The cladding may be substantially rectangular.
The
cladding may have a non-circular shape.
The core may be centrally located in the cladding. The core may be offset from
the
centre of the cladding.
The optical radiation from the rare earth dopant in the waveguide can be
coupled to
an optical amplifier and wherein the optical radiation can be used as a pump
source for the
optical amplifier.
The optical radiation from the rare earth dopant in the waveguide can be
coupled to
a plurality of optical amplifiers via an optical coupler, and wherein the
optical radiation can
be used as a pump source for the optical amplifiers.
The amplifying optical fibre can comprise a microstructured mesh surrounding
the
cladding. The microstructured mesh may be sealed at either end of the
amplifying optical
fibre - for example by heating the amplifying optical fibre with an electric
arc, a flame or a
laser. A glass ferrule may be placed onto either end of the amplifying optical
fibre prior to
applying heat. The glass may be silica.
The optical light source can comprise feedback means for providing feedback in
the
waveguide, the waveguide being a laser. The feedback means can be a reflector.
The
reflector can be formed from a cleave in the amplifying optical fibre. The
reflector can be
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a fibre Bragg grating. The reflector can be a dichroic filter. The dichroic
filter may be
deposited on the end of the amplifying optical fibre.
The amplifying optical fibre can be configured as a source of amplified
spontaneous
emission.
The rare eaxth dopant can be contained in the core. The rare earth dopant can
be
contained in the cladding. The rare earth dopant can be contained in both the
core and the
cladding.
The rare earth dopant can be configured in a region surrounding the centre of
the
waveguide. The region surrounding the centre of the waveguide can be a ring
surrounding
the core. The ring can have a thickness in the range 1 to l0um.
The rare earth dopant can comprise Yb and it is preferable that the laser
diode emits
at a wavelength that is absorbed by the Yb. The optical light source may
comprise a
dichroic filter that reflects in the wavelength range 975nm to 9~Onm, and
wherein the
optical light source comprises a second port, the optical light source being
an optical
amplifier for 975nm to 9~Onm radiation. It is preferable that the waveguide is
configured
to emit optical radiation in a wavelength range from 975nm to 980nrn, wherein
the optical
radiation is coupled to at least one erbium-doped optical amplifier via an
optical coupler,
and wherein the optical radiation is used as a pump source for the optical
amplifier. It is
preferred that the Yb is configured in a region surrounding the centre of the
waveguide.
The amplifying optical fibre may comprise an absorber to attenuate unwanted
optical radiation. The absorber may be a saturable absorber or an unsaturable
absorber. It
is preferred that the rare earth dopant is Yb and the absorber is samarium
configured to
absorb unwanted optical radiation occurring in the wavelength region 1020nm to
l OSOnm.
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The absorber may be in the core, the cladding, or in both the core and the
cladding. It is
preferred that the Yb and the absorber is configured in a region surrounding
the centre of
the waveguide. It is preferred that the amplifying optical fibre comprises a
microstructured
mesh surrounding the cladding and that the cladding has an outer diameter in
the range of
15 hum to 75 hum. The cladding may have an outer diameter in the range 25 ~m
to 3 5 ~,m.
The rare earth dopant can comprise Erbium and it is preferable that the laser
diode
emits at a wavelength that is absorbed by the Erbium.
The rare earth dopant can comprise Erbium codoped with Ytterbium, and it is
preferable that the laser diode emits at a wavelength that will be absorbed by
the
Ytterbium.
The rare earth dopant can comprise Neodymium and it is preferable that the
laser
diode emits at a wavelength that is absorbed by the Neodymium.
The rare earth dopant can comprise Thulium and it is preferable that the laser
diode
emits at a wavelength that is absorbed by the Thulium.
The rare earth dopant can comprise Praseodymium and the laser diode emits at a
wavelength that is absorbed by the Praseodymium.
The rare earth dopant can be selected from the group comprising Ytterbium,
Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium and Dysprosium, or
is Erbium codoped with Ytterbium, or is doped with a transition metal or
semiconductor.
The invention also provides an optical amplifier comprising the optical light
source.
The optical amplifier may be configured to have low polarisation dependent
gain.
The invention also provides an optical fibre laser comprising the optical
light
source.
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The invention also provides a method for pumping a plurality of optical
amplifiers
having low polarisation dependent gain, wherein each optical amplifier
comprises a pump
input, the method comprising the steps of providing an optical light source
according to the
present invention, and coupling the optical light source to the pump inputs.
The invention also provides a method for pumping a plurality of fibre lasers
each
comprising a pump input, the method comprising the steps of providing an
optical light
source according to the present invention, and coupling the optical light
source to the pump
inputs.
The invention can also be considered to be a source of amplified spontaneous
emission fox pumping an optical fibre amplif er or laser.
Brief Description of the Drawings
Embodiments of the invention will now be described solely by way of example
and
with reference to the accompanying drawings in which:
Figure 1 is a diagram of a light source according to the present invention;
Figure 2 shows the light source coupled to an optical amplifier;
Figure 3 shows the light source coupled to a plurality of optical amplifiers;
Figure 4 shows waveguide comprising feedback means;
Figure 5 shows a ring-doped amplifying f bre;
Figure 6 shows an optical fibre being stretched by the application of heat and
tension;
Figure 7 shows a lens formed on the end of an optical fibre;
Figure 8 shows a second fibre with a curved fibres being spliced to an
amplifying
fibre;
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Figure 9 shows a cylindrical lens on the end of an amplifying fibre;
Figure 10 shows a beam shaping optics comprising a second lens;
Figure 11 shows a microstructured mesh sealed at either end of an amplifying
fibre;
Figure 12 shows a glass ferrule placed onto an amplifying fibre;
Figure 13 shows an amplifying optical fibre with a non-circular cladding;
Figure 14 shows an amplifying optical fibre with an offset core;
Figure 15 shows the absorption and emission spectra for ytterbium ions in
silica
glass;
Figure 16 shows the dependence of threshold power 161 on cladding diameter for
a
silica optical fibre having a ytterbium-doped single mode core;
Figure 17 shows a two-emitter pump module;
Figure 18 shows the output spectra of the pump module;
Figure 19 shows the output power as a function of laser diode current for the
pump
module;
Figure 20 shows a cross-section of a ytterbium-doped jacketed air-clad (JAC)
fibre;
Figure 21 shows a fibre laser comprising the JAC fibre;
Figure 22 shows the output power versus launched power for a fibre laser and
an
amplified spontaneous emissions (ASE) source that comprise the JAC fibre;
Figure 23 shows the temporal behavior of the fibre laser comprising the JAC
fibre;
Figure 24 shows an amplified spontaneous emission (ASE) source comprising the
JAC fibre;
Figure 25 shows the output spectrum of the ASE source;
Figure 26 shows the temporal behavior of the ASE source;
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Figure 27 shows an erbium doped fibre amplifier (EDFA) that is pumped with the
ASE source;
Figure 28 shows EDFA's spectral gain characteristic for two different input
power
levels;
Figure 29 shows the EDFA's spectral noise figure characteristic;
Figure 30 shows the cross-section of ring-doped ytterbium JAG fibre;
Figure 31 shows an ASE source comprising the ring-doped JAC fibre;
Figure 32 shows a fibre laser comprising the ring-doped JAC fibre;
Figure 33 shows the output power as a function of absoxbed power for the ASE
source and fibre laser;
Figure 34 shows the spectral dependence of output power for the ASE source and
the fibre laser;
Figure 35 shows a measurement of relative intensity noise with frequency for
the
ASE source and the fibre laser;
Figure 36 shows an optical amplifier comprising a gain clamping laser diode
and
which amplifier is pumped with the ASE source;
Figures 37 to 40 show the spectral output response of the optical amplifier
when the
input was between two and 32 separate wavelength chaamels;
Figure 41 shows the dependence of gain and noise figure measured as a function
of
total input power;
Figure 42 shows the spectral dependence of gain for different levels of gain
clamping power;
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11
Figure 43 shows the spectral dependence of polarization dependent gain when
the
optical amplifier is pumped with the ASE source and a laser-diode;
Figure 44 shows the power variation at the output of the EDFA when the input
power increased by lSdB;
Figure 45 shows the doping profiles of the fibre shown in Figure 20;
Figure 46 shows the doping profiles of the fibre shown in Figure 30;
Figure 47 shows an amplifying optical device comprising a first port and a
second
pou;
Figure 48 shows an amplifying optical device comprising a thin film filter;
Figure 49 shows an arrangement in which pump power is amplified by the
amplifying optical device of Figure 48;
Figure 50 shows a preform assembly comprising solid rods and capillaries;
Figure 51 shows an optical fibre drawn from the preform assumebly of Figure
50;
Figure 52 shows a preform assembly comprising a non-circular preform; and
Figure 53 shows an optical fibre drawn from the preform assembly of Figure 52.
Detailed Description of Preferred Embodiments of the Invention
Figure 1 shows an optical light source comprising a laser diode 1, a beam
shaping
optics 2, and an amplifying fibre 3, wherein the ampli;Cying fibre 3 comprises
a waveguide
4 comprising a core 5 and a cladding 6, wherein the waveguide 4 is doped with
a rare earth
dopant 7, and wherein the laser diode 1 can produce optical pump power 8 which
is
coupled to the waveguide 4 by the beam shaping optics 2.
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1~
The amplifying fibre 3 is preferably made from silica or silicate glass. The
amplifying fibre 3 can be made from phosphate glass or other soft glasses.
The laser diode 1 can be a multimode laser diode. The laser diode 1 can be a
singlemode laser diode. The laser diode 1 can be a diode bar. The laser diode
1 can be a
diode stack. The laser diode 1 can comprise a combination or a plurality of
lasex diodes,
diode bars and/or diode stacks.
The laser diode 1 can emit O.1W to SOW of optical pump power. The laser diode
1
can emit 0.5 W to 5 W of optical pump power.
The beam shaping optics 2 can comprise a first lens 71. The first lens 71 can
be
formed on the end of the amplifying fibre 3. Examples of forming lenses on the
ends of
fibres by applying tension and heating the fibre in an electric arc can be
found in US Patent
4,589,897, which is incorporated herein by reference. Figure 6 shows the
principle.
Tension is applied to the amplifying fibre 3 and heat is applied. This results
in a neck 61
being formed in the amplifying fibre 3. The amplifying fibre 3 then separates
into two.
Further application of heat results in the first lens 71 being formed on the
amplifying fibre
3 as shown in Figure 7. An alternative method for forming a spherical lens is
described in
US Patent 4,345,930. Alternatively, a second fibre 81 having a curved surface
82 can be
fusion spliced or joined to the amplifying fibre 3 as shown in Figure 8. This
arrangement
is described in US patent 4,737,006.
The amplifying optical fibre 3 will generally have a circular fundamental mode
and
a laser diode an elliptical mode. The first lens 71 may be a cylindrical lens
91 formed by
polishing the end of the amplifying fibre 3 or the second fibre 81. A
cylindrical lens 91 is
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13
shown in Figure 9 and is further described in US patent 6332053 which is
hereby
incorporated by reference.
The beam shaping optics 2 can comprise a second lens 100 as shown in Figure
10.
The second lens 100 can be a cylindrical lens. The cylindrical lens can be a
cylindrical
microlens which may have a shape, such as circular, elliptical or hyperbolic,
designed to
transform some particular given input light distribution 101 into some desired
output light
distribution 102. The cylindrical lens may have a uniform refractive index
profile, or may
have a graded refractive index profile such as parabolic. Examples of
cylindrical lenses
and their application to coupling to laser diodes can be found in US patent
5,080,706 which
is incorporated herein by reference.
The cladding 6 can have an outer diameter in the range l0um to 100um. The
cladding 6 can have an outer diameter in the range l5um to 50um. The cladding
6 can be
circular. The cladding 6 can be non-circular. Advantageously, a non-circular
cladding 6
can increase the overlap of light propagating in the cladding 6 with the core
5.
The core 5 andJor cladding 6 can be doped with germanium, phosphorous, boron,
aluminium andlor fluoride.
The core 5 can be configured to be a single mode waveguide. Alternatively the
core 5 can be configured to be a multimode waveguide. The core 5 can be
circular, ring-
shaped, elliptical, oval, rectangular, or in the foam of an irregular or a
regular polygon.
The core 5 can be configured centrally with respect to the cladding 6. The
core 5
can be configured off centre with respect to the cladding 6. Advantageously, a
non-circular
cladding 6 can increase the overlap of light propagating in the cladding 6
with the coxe 5.
CA 02465522 2004-04-29 PCT/GB 203 ~ 0 f.
~ a arc ~~o~
14
The optical pump power 8 can stimulate optical radiation 9 from the rare earth
dopant 7 in the waveguide 4. The optical radiation 9 may be amplified
spontaneous
emission. The optical radiation 9 may be dominated by stimulated emission.
Figure 2 shows the optical radiation 9 from the rare earth dopant 7 in the
waveguide
4 coupled to an optical amplifier 20, wherein the optical radiation 9 is used
as a pump
source for the optical amplifier 20. The coupling is achieved using a lens 21.
It is
preferable that the coupling is achieved using an optical fibre coupler.
Figure 3 shows the waveguide 3 coupled to a plurality of amplifying optical
devices
33 via an optical fibre 31, a plurality of optical couplers 32. The optical
radiation 9 is used
as a pump source for the amplifying optical devices 33. The amplifying optical
devices 33
can be optical amplifiers, lasers, distributed feedback fibre lasers or
distributed Bragg
reflector fibre lasers.
The amplifying fibre 3 can comprise a microstructured mesh 111 surrounding the
cladding 6. As shown in Figure 1 l, the microstructured mesh 111 may be sealed
at either
of end 112,1 I3 of the amplifying fibre 3 - for example by heating the
amplifying fibre 3
with an electric arc, a flame or a laser. The first lens 71 may be formed on
the end 1 I2 in.
order to facilitate coupling to a laser diode. The end 113 may be cleaved as
shown in
Figure 11, or fusion spliced to an output fibre (not shown). The cleaved end
provides a fiat
surface for subsequent coating of the end face of the fibre, for example with
a dichroic
mirror.
As shown in Figure 12, a glass ferrule 120 may be placed onto the amplifying
fibre
3 prior to applying heat. A reflecting material 123 may be placed onto the
glass fez~rule.
The reflecting material 123 may be a metal such as chrome, silver or gold, and
the metal
~~,~~~~~~ ~~-~~~~'
;~~~~'~1'~~JT~ ~H~~T (Ri~L~ ~~~
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WO 03/038486 PCT/GB02/04912
may be deposited using electroless plating techniques. This configuration has
advantages
in that pump light not absorbed in the amplifying fibre 3 can be reflected
baclc through the
amplifying fibre 3.
Figure 4 shows feedbaclc means 40 for providing feedback in the waveguide 4,
the
waveguide 4 being a laser: The feedback means 40 can be a reflector. The
reflector can be
formed from a cleave in the amplifying fibre 3. The reflector can be a fibre
Bragg grating.
The reflector can be a mirror. The reflector can be a dichroic mirror.
The amplifying fibre 3 can be configured as a source of amplified spontaneous
emission.
Referring to Figure 1, the rare earth dopant 7 can be contained in the core 5.
The
rare earth dopant 7 can be contained in the cladding 6. The rare earth dopant
7 can be
contained in the core 5 and in the cladding 6.
Figure 5 shows the rare earth dopant 7 configured in a region 50 surrounding
the
centre of the waveguide 4. The region 50 surrounding the centre of the
waveguide 4 is
shown as a ring 51 surrounding the core 5. The ring 51 can have a thickness 52
in the
range 1 to l0um.
The rare earth dopant 7 can comprise Ytterbium (Yb) and it is preferable that
the
laser diode 1 emits at a wavelength that is absorbed by the Yb. It is
preferable that the
waveguide 4 is configured to emit optical radiation in the wavelength range
970nm to
980nm. It is preferred that the wavelength range is from 975nm to 988nm. The
Yb can be
configured in a region surrounding the centre of the waveguide 4.
Alternatively, the Yb
can be configured in a region that is offset from the center of the waveguide
4 which can be
advantageous to increase the absorption of pump power. The optical radiation
can be
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16
coupled to at least one erbium-doped optical amplifier via an optical coupler,
and wherein
the optical radiation is used as a pump source for the optical amplifier. This
embodiment
has particular advantages for pumping optical amplifiers as wells as lasers
and distributed
feedback lasers. There are advantages of configuring the waveguide 4 as a
source of
amplified spontaneous emission when pumping these devices. These advantages
include
wavelength stability, lower amplitude noise, higher reliability and reduced
cost owing to
their lower power densities in the waveguiding stripes. In addition, it is not
necessary to
temperature stabilise the laser diode which further reduces cost and improves
reliability
because a peltier device is not required. The unpolarised nature of the ASE
output when
used as a source of pump radiation for lasers or amplifiers provides
significant advantages
in terms of noise reduction and reduction in polarisation dependant gain.
The amplifying fibre 3 may comprise an absorber to attenuate unwanted optical
radiation. The absorber may be a saturable absorber or an unsaturable
absorber. It is
preferred that the rare earth dopant is Yb and the absorber is samarium
configured to
absorb unwanted optical radiation occurring in the wavelength region 1020nm to
lOSOnm.
The absorber may be in the core, the cladding, or in both the core and the
cladding. It is
preferred that the Yb is configured in a region surrounding the centre of the
waveguide. It
is preferred that the amplifying fibre comprises a microstructured mesh
surrounding the
cladding and that the cladding has an outer diameter in the range of 15~m to
75~,m. The
cladding may have an outer diameter in the range 25 ~,m to 3 5 ~,m.
Figure 13 shows an amplifying fibre 130 comprising a core 5, a non-circular
cladding 136, an air cladding region 111, and an outer jacket 133. The air
cladding region
111 comprises holes 135, 139 that extend longitudinally along the amplifying
fibre 130.
_,r"!__"_,__~~" - - _ _
' ~ CA 02465522 2004-04-29
- 17
- The holes 135 are formed from the inside of capillaries used to fabricate
the amplifying
fibre I30. The holes 139 are formed from the interstitial spaces between the
capillaries
used to fabricate the amplifying fibre 130. In certain embodiments, the
amplifying fzbre
130 may comprise only holes 135 (if the interstitial holes 139 are closed up
by the
application of a vacuum in the fibre drawing process), or only interstitial
holes 139 (if rods
are used instead of capillaries, or if the capillaries are collapsed by the
application of
vacuum in the fibre drawing process).
Advantages of the non-circular cladding 136 are that it better matches the
near field
of typical laser diodes, and that there will be an increased overlap between
the modes
guided by the non-circular cladding 136 and the core 5. The non-circular
cladding 136 can
be rectangular, square, triangular, D-shaped, or a circular shape comprising
flats that are
machined prior to preform assembly. The dimensions of the non-circular
cladding I36 can
be l Opm to SOO~.m for the minor axis, and I50~n to 1000E,ixn for the major
axis.
Figure 14 shows an amplifying fibre 140 in which the core 5 and region I3I is
offset from the center of the cladding 141. The amplifying fibre I40 is an
example of a
jacketed air-clad (JAC). The amplifying fibre 140 comprises an air cladding
region 142
and an outer jacket 143 that can advantageously be configured to ensure that
the core S is
substantially central with respect to the outer circumference of the outer
jacket 143 (note
the centre lines 149 shown in Figure I4). Having a core that is concentric
with the outside
of the fibre is advantageous for fusion splicing, whilst having a core that is
not central with
respect to the cladding is advantageous because of the increased overlap of
the cladding
modes with the core 5 and/or the optional region 13I that surrounds the core
S. This
~'a~~i~3~',9~~~ ~F ~~=Z.~
CA 02465522 2004-04-29
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18
configuration thus combines the increased mode overlap advantages of an offset
core with
the fusion splicing advantages arising from concentric cores.
The core 5 may comprise the rare-earth dopant 7. Alternatively, or
additionally, the
amplifying fibre 130 may comprise a region 131 that surrounds the core 5 and
this region
131 may comprise the rare-earth dopant 7. Figure 13 also shows an outer region
132 that
surrounds the region 131. The outer region 132 may be doped with a saturable
or an
unsaturable absorber. The region 131 may be doped with Ytterbium ions and the
outer
region 132 may be doped with samarium, and the amplifying fibre 130 used as a
source of
radiation at around 977mn. Such a source can be susceptible to radiation
induced or fed
back at 1035nm to 1060nm, and the samarium is useful to absorb this radiation.
Referring to each of the embodiments described above, the rare earth dopant 7
can
comprise Erbium (Er) and it is preferable that the laser diode 1 emits at a
wavelength that
is absorbed by the Er.
The rare earth dopant 7 can comprise Er codoped with Yb, and it is then
preferable
that the laser diode 1 emits at a wavelength that will be absorbed by the Yb.
The rare earth dopant 7 can comprise Neodymium (Nd) and it is preferable that
the
laser diode 1 emits at a wavelength that is absorbed by the Nd.
The rare earth dopant 7 can comprise Thulium (Tm) and it is preferable that
the
laser diode 1 emits at a wavelength that is absorbed by the Tm.
The rare earth dopant 7 can comprise Praseodymium (Pr) and the laser diode 1
emits at a wavelength that is absorbed by the Pr.
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19
The rare earth dopant 7 can be selected from the group comprising Ytterbium,
Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium and Dysprosium, or
is Erbium codoped with Ytterbium, or is doped with a transition metal or
semiconductor.
Cladding-pumping with high-power multimode diode pump sources is the preferred
way to power-scale fibre lasers. In cladding-pumped devices the overlap of the
pump field
with the gain medium is small and therefore a large amount of dopant is
required to absorb
the pump. However before the pumping creates enough gain at 977 nm in a Yb-
doped
laser, undesired gain at longer wavelengths (typically 1035nm to 1100nm) with
weak re-
absorption becomes so high that spurious oscillations cannot be suppressed.
This unwanted
gain restricts fibre length and thus pump absorption, resulting in low slope
efficiency. To
achieve lacing at 977 nm one has to ensure that the gain at 1040 nm is lower
than the
threshold for spurious lacing and that the pump intensity, and thus power, is
high enough to
invert more than 50% of the Yb-ions. Both pump threshold power and gain at
1040 nm
are proportional to the inner cladding area and for a practical device with,
say, a threshold
below 400 mW and a pump absorption of 6 dB, the inner cladding diameter should
be
below 25 ~,m [J. D. Minelly et al., OFC'2000, Paper PD2, Baltimore, USA
(2000)]. For
efficient pump launch into such a small inner cladding its numerical aperture
should be as
high as possible.
In our device we have chosen a jacketed air-clad (JAC) geometry, since it not
only
offers a route to achieving a numerical aperture (NA) of 0.7 or higher, but
also offers the
robustness and reproducibility of conventional silica fibre technology [J. I~
Sahu, et al.
Electron. Lett. 37, 1116 (2001 )] .
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Yb-ions have a strong emission cross-section at 976 nm. Thus using low cost
broad
area pump diodes operating at 915 nm and a double clad fibre, high power
radiation can be
achieved in the wavelength region that is preferred for pumping EDFAs.
Figure 15 shows the absorption spectrum 151 and emission spectrum 152 of Yb-
ions in silica glass. The emission 152 and absorption 151 cross sections at
around 976 mn
are equal so in order to achieve lasing one has to reach a 50% population
inversion.
Transparency pump intensity (i.e. the pump intensity required for a 50%
population
inversion) is approximately 2.5~ 104 W/cm2 or 10 W for a double clad fibre
with 200 ~,m
pump cladding. To make such a source practical one has to employ a high
brightness
pump source.
Figure 16 shows the dependence of threshold power 161 as a function of pump
cladding diameter 162 for a Yb-doped single-mode core in silica glass.
Assuming an
acceptable threshold for such a pump source is around 500 mW, then Figure 16
shows that
the pump cladding diameter 162 should be below 30 hum. From the data presented
in Figure
16, one can conclude that today's commercially-available, pig-tailed, high-
power, broad-
area pump diodes (1.5 - 2.5 W in 100 ~,m diameter fibre) are not suitable for
the practical
realization of cost-effective fibre based pump sources.
The realization of a 976 non fibre pump source using broad-area pump diodes is
even more difficult because of unwanted gain at around 1010 nm to 1080nm (see
Figure
15) which shows that the Yb-doped glass system is quasi four level at these
wavelengths.
There are two main requirements for an efficient laser. First the pump
threshold P~,
should be small compared with the available pump power Pp and second the slope
efficiency r~ with respect to launch power must be high.
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21
In cladding pumped devices the overlap of the pump field with the gain medium
is
small and therefore a large amount of dopant to absorb the pump is required.
However
before the pumping creates enough gain at 978 nm in such a laser, undesired
gain at longer
wavelengths (1030 -1080 in case of Yb-doped fibre lasers) with weak re-
absorption
becomes so high that spurious oscillations cannot be suppressed. This unwanted
gain
restricts fibre length, pump absorption and results in low slope efficiency.
In a homogeneously broadened gain medium such as Yb-doped silica fibres, the
gain G (in dB) can be written as [J. Nilsson et al., Opt. Lett. 23, 355-357
(1998)]
G = kNoAa~a (~,)~[tee (~) + ~a (~)]n2 " as (~)'tL ~ (1)
where k = 4.343, No is the concentration of active ions, L is the fibre
length, ae and
6a are the emission and absorption cross sections, respectively and na is the
fraction of
active ions that are excited. Finally ~d is the value of the normalized modal
intensity
averaged over doped area Ad (in other words if P is the incident pump power
then Pfd is
the average intensity in the doped area). It can be shown that the unwanted
gain at 1030
nm can be expressed as
Gio3o = 0.25G9'6 + 0.72(3aop, (2)
where (3 = ~ds~ap = A~ladding ~Acore ~d a0pp 1S the pump absorption.
The 1030 nm gain is proportional to the cladding-to-core area ratio A~ladding
~Acore
and grows rapidly with pump absorption aopp.
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22
Thus in order to suppress lasing at 1030 nm one has to ensure that Gloso < 40
dB
i.e. when lasing cannot be initiated by spurious reflections or Raleigh
scattering. Taking
the core to cladding diameter ratio equal to 3 and assuming that the single
pass gain at 976
nm is 7 dB (lasing from one cleaved end) then the pump absorption will be in
the region of
6 dB or 75% of available pump power, which is sufficiently high to allow for
an efficient
device. In practical terms, the pump cladding diameter should not exceed 25
,um since in
order to achieve low splice loss to commercial fibres the doped core should be
single-
moded at 976 nm and the typical diameter of a core in a standard telecom-fibre
is in the
region of 8 ~,m.
Figure 17 shows a pump module 171. In order to achieve low threshold and high
efficiency, a pump source 171 was used based on a two-emitter assembly, that
is the pump
source 171 used two laser diode chips whose outputs were combined together and
launched
into the Yb-doped fibre. Similar pump module can be procured Milon Laser Co.
from St.
Petersburg in Russia, or from New Optics Limited based in the United Kingdom.
The New
Optics Limited product has a product name "Ultra-6". Each laser diode is
capable of
delivering up to 2 W of optical power at 915 nm. Launching efficiency into a
30 ~,m
diameter, 0.3 NA optical fibre should be greater than 75%. The optical
spectrum 180 of the
pump module 171 is shown in Figure 18 in which the measured output power 181
is
plotted against wavelength 182. Figure 19 shows the output power 191 measured
as a
function of the laser diode current 192.
There are several lcey requirements for a rare-earth doped fibre that is
intended to
operate in a three level transition: the doped fibre should have high
efficiency (greater than
50°f° and preferably greater than 70%); there should be high
pump absorption; the pump
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23
cladding area should be below 600 ~,m~ (i.e. core-to-cladding diameter ratio
should be
more than 0.3); and the pump cladding numerical aperture (NA) should be high
enough to
allow high coupling efficiency from broad-stripe pump diodes.
Figure 20 shows a jacketed air clad (JAC) fibre 200 that meets these criteria.
The
Yb-doped fibre 200 has a raised index core 201 eo-doped with boron and
germanium, a
pure silica inner cladding 202, a mesh 203 comprising two rings of
longitudinally
extending circular holes 204 and an outer silica jacket 205. The doped core
diameter was 8
~,m and the NA = 0.1. The germanium doping makes the core 201 photosensitive
which is
advantageous for writing fibre Bragg gratings into the core 201 with
ultraviolet light. The
diameter of the pure silica inner cladding 202 was 28 ~,m. To ensure a high NA
the mesh
203 was fabricated with two layers of silica capillary tubes stacked around a
preform inside
a silica jaclcet. The strand thickness - ie the diameters of the silica
capillary tubes in the
resulting JAC fibre 200 was 1 to 2 ~,m. The JAC fibre 200 has a polymer
coating 206 (not
shown) that has a refractive index greater than the refractive index of the
silica jacket 205.
Note that it may actually be beneficial to have a polymer coating with a
refractive index
less than the refractive index of the silica jacket 205. Figure 45 shows the
dopant profiles
450 of the JAC fibre 200 as a function of radius 455. The JAC fibre 200
comprises a
region 451 doped with germania (in order to make the core 201 photosensitive)
and a
region 452 doped with Ytterbium. The region 452 included the core 201 as well
as a zing
453 surrounding the core 201. Note that germania would have been lost from the
centre of
the JAC fibre 200 during the collapsing stages of the (earlier) preform
manufacturing
process, leading to the well-k~lown refractive index dip at the centre of the
fibre 200. This
refractive index dip is not shown in Figure 45. Referring again to Figure 20,
the 915 nm
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24
pump absorption was 1 dB/m. The pump cladding NA (ie the effective numerical
aperture
of pump light transmitting along the pure silica inner cladding 202) was
measured at 0.4
and 0.5 depending on the length of the JAC fibre 200 under test.
There axe two approaches in the development of a high-power pump source. One
is
based on the development of a fibre laser where the pump wavelength is fixed
by a
wavelength selective reflector (such as a fibre Bragg grating or a filter).
Another way is to
configure the fibre as a source of amplified spontaneous emission ASE - ie an
ASE source.
Both approaches have pros and cons: fibre lasers ultimately deliver more power
and are
more efficient, whereas an ASE source is simpler in design, does not require
any
wavelength selective elements and is less noisy.
Figure 21 shows a fibre laser 210 comprising the pump source 171 and the JAC
fibre 200. A laser cavity 211 was formed by a first fibre Bragg grating 212
and a second
fibre Bragg grating 213. The first fibre Bragg grating 212 was written
directly into the core
201 and the second fibre Bragg grating 213 was written into a photosensitive
single mode
optical fibre 216 procured from FiberCore Limited which had a second-mode cut-
off at
920nm and which was sliced to the JAC fibre 200 at splice 214. The
reflectivity of the
second grating 213 was 20% and the reflectivity of the first grating 212 was
15% to 20%.
The length 215 of the cavity 211 was 4m.
Figure 22 shows the output power 221 of the fibre laser 210 versus the
launched
power 222 defined as the power that is coupled into the inner cladding 202
from the pump
source 171. The slope efficiency 223 with respect to launched power 222 was
37%. This
relatively low slope efficiency can be explained by the fact that the device
length was kept
short in order to prevent unwanted lasing at 1030 nm.
' CA 02465522 2004-04-29
. 2S
Figure 23 shows the temporal dependence of the output power 221 of the fibre
laser
210 which clearly demonstrates beating of longitudinal modes as evidenced by
the noise
peaks 232. The low Q-value of the laser cavity and relatively long device
length have
resulted in temporal instability of the output signal 221. The characteristic
time 233 is set
by the laser cavity length 215 and. in this example the characteristic time
233 is equal to 40
ns which corresponds to the cavity roundtrip time. Such an instability might
be acceptable
for a pump source intended to be used in EDFA but will significantly restrict
range of
possible applications of fibre-based pumps.
Figure 24 shows a high power ASE source 240 comprising the pump source 171,
the 3AC fibre 200. The configuration of the ASE source 240 is almost identical
to that of
the fibre laser 210 except there are no gratings and the output end 241 of the
source 240 is
angle-cleaved.
The output power 224 of the ASE source 240 is shown plotted against launched
power 222 in Figure 22. The slope efficiency 225 with respect to the launched
power 222
is 27%. Figure 2S shows the normalised intensity 251 of the ASE source 240 as
a function
of wavelength 253. There is a strong output centred at around 977nm with a
spectral width
252 of around 3nm. The output of the ASE source is situated at the peak of the
980nm
absorption band of erbium ions in silica glass. Moreover, the output of the
ASE source
will have a spectral characteristic that will be substantially stable with
respect to ambient
temperature fluctuations thus removing the need for external wavelength
stabilisation (eg
provided by fibre Bragg gratings) as is commonly used in sources for pumping
EDFAs and
other erbium-doped devices.
k_ 2-:.. ~. 5
CA 02465522 2004-04-29
~ 26
' - Figure 26 shows the normalised output power 251 as measured over time 252.
The
maximum output power available from the ASE source 240 was 400 mW. The ASE
source 240 provides relatively high power, has a stable output wavelength with
temperature
and time, and provides a low-noise output that has none of the beating that
was observed in
Figure 23. Note that the parameters of the JAC fibre 200 had not been
optimised and
further development of doped fibres as well as JAC fibres will result in
significant increase
of output power up to 1500 mW or higher.
Figure 27 shows an erbium doped amplifier EDFA 270 that was pumped by the
ASE source 240. The EDFA 270 comprises tap couplers 27I, photodiodes 272,
isolators
273, WDM couplers 274, erbium doped single mode fibre 275, control electronics
276 and
a variable optical attenuator 277. Signal light is input at the input port 278
and output at
the output port 279. Pump power 2711 was delivered by the 978 nm ASE fibre
source 240
via a 1 x 4 pump sputter 2710. The pump sputter 2710 was constructed from
optical fibre
couplers.
The gain and noise figure of the EDFA 270 was measured as a function of
wavelength 281 at signal input power levels of-1 IdBm and-3ldSm. Figure 28
shows the
gain 282 measured at -11 dBm and the gain 283 measured at -3 ldBm. Figure 29
shows the
noise figure 291 measured at -11 dBm. Surprisingly, the gain and noise figure
characteristics were nearly identical to those obtained when pumping with a
commercially-
available 980nm semiconductor pump source designed specifically for pumping
EDFAs.
Advantageously, the fibre pump source 240 is capable of pumping up to four
EDFAs
providing a saturated power of 13 dBm.
AR~~~~~'~ ~~~~~
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27
Figure 30 shows a preferred embodiment of a ring-doped JAC fibre 300. The JAC
fibre 300 comprises a core 301 that is doped with Germania, a rare-earth doped
region 302
surrounding the core 301 that is doped with Yb, a silica inner cladding 303,
longitudinally
extending holes 307, a thin glass mesh 304 where the mesh 304 has a wall
thickness 305
that is around O.Sum to 2um - ie comparable to the intended wavelength of
operation, and
a supporting silica jacket 306. The diameter of the JAC fibre 300 is
approximately
125um. The design results in very low pump leakage from the silica inner
cladding 303
and hence provides a high effective numerical aperture. The core is single-
moded with a
cut-off of 950 nm. In order to suppress unwanted gain at 1040 nm we have
utilized ring-
doping of Yb ions [J. Nilsson et al., Opt. Lett. 23, 355-357 (1998)], [A. S.
Kurkov et al.,
OAA Technical Digest, OMA4-1 (2001)]. The pump absorption is 6 dB/m.
Figure 46 shows the dopant profiles 460 of the JAC fibre 300 as a function of
radius 465. The JAC fibre 300 comprises a region 461 doped with germania (in
order to
make the core 301 photosensitive) and a region 462 doped with Ytterbium that
surrounds
the core 301. Note that diffusion mechanisms during the preform manufacturing
process
can lead to diffusion of the germania into the region 462 and diffusion of
Ytterbium into
the region 461. Note also that germania would have been lost from the centre
of the JAC
fibre 300 during the collapsing stages of the (earlier) preform manufacturing
process,
leading to the well-known refractive index dip at the centre of the fibre 300.
Tlus
refractive index dip is not shown in Figure 46. Similar fibres can also be
fabricated with
phosphorous doping of the core 301, or we have also experimented with pure
silica cores
surrounded by a Ytterbium-doped gain medium. Alternatively, the core can be
ring-doped
with germania or phosphorous and co-doped with Ytterbium. The emission cross-
section
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28
spectrum of Yb ions in silica glass has a relatively narrow (approximately 4
nm wide) peak
centred around 977 nm. High-power emission is possible from around 975nm to
around
980nm by taking several different approaches. For example, a laser can be
formed using
broadband feedback from reflectors such as dichroic mirrors or fibre Bragg
gratings, where
the wavelength selection arises from the shape of the emission cross-section.
Alternatively, a laser can be formed using wavelength selective feedback from
at least one
of these reflectors. Wavelength selective feedback can be achieved using a
filter such as a
fibre Bragg grating. It is also possible to simply pump a Ytterbium doped
fibre in order to
realise a source of amplified spontaneous emission.
Figure 31 shows an ASE source 310 comprising a laser diode 311 emitting at
915nm, optics 312, the JAC fibre 300, an optical fibre 313. The JAC fibre 300
was 3.25m
long. The length is very dependent upon fibre design and the amount of pump
power that
is launched into the fibre. Depending on Yb concentration and disposition, a
length
between O.Sm and Sm is acceptable. The optical fibre 313 is a photosensitive
single mode
fibre comprising a photosensitive waveguide 3111 comprising a core and a
cladding.
Photosensitive fibres for the manufacture of fibre Bragg gratings are
available from many
different suppliers. Optionally, a fibre Bragg grating 3110 (or other
reflector) can be
written into the fibre 313 in order to reflect pump radiation at 915nm back
into the fibre
300 in order to increase the pump absorption and thus increase the output
power. Note that
the fibre 313 should preferably have a photosensitive cladding and a
photosensitive core in
order that the fibre Bragg grating 3110 can be configured to reflect the pump
light, most of
which would be propagating as cladding modes. This option was not implemented
in this
experiment. Also note that there is no need to provide either the fibre 313 or
for
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29
photosensitivity if there is no intention of writing a grating into the fibre
313. If the fibre
313 is not provided, then the JAC fibre 300 should be antireflection coated
and/or cleaved
at an angle to prevent back-reflections.
Referring again to Figure 31, the optical fibre 313 is shown cleaved at an
angle 314
in order to prevent the signal out 315 reflecting back into the JAC fibre 300.
This makes
the output 315 nearly uni-directional even with a simple perpendicular cleave
(4%
reflecting) in the pump launch end 316 of the JAC fibre 300. The optics 312
comprised
both cylindrical and spherical lenses which may be a graded refractive index
(GRIN) lens
and preferably at least one dichroic filter 319 that is highly transmissive
between 900-
950nm to allow the 915nm pump radiation to be transmitted from the laser diode
311 to the
JAC fibre 300, and highly reflective between 970nm-1070nm to attenuate any
unwanted
signals being fed back to the laser diode 311. The dichroic filter 319 can be
configured at
an angle so that the reflected light between 970nm- 1070nm is not reflected
into the fibre
300. Such unwanted signals can damage a laser diode.
It is possible to configure one of the at least one dichroic filters 319 as an
end-
mirror for the JAC fibre 300, that is highly-reflecting at around 975 to
980nm. In such case,
additional measures are preferable to prevent light in the 1020 -1100 nm
wavelength
range from reaching the diode 31 l and from being fed back into the fiber 300.
One option
is to make the 975 nm lughly reflective filter highly transmissive in the
range 1020 -1100
nm. That suppresses feedback into the fiber in the 1020 - 1100 nm range. It
can be
combined with a rejection filter between the dichroic cavity-filter and the
diode 311 that is
highly reflective in the 1020 -1100 nm wavelength range (and optionally at
around 975 to
980 nm), and configured at an angle such that it does not reflect light back
into the fiber
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300. Alternatively, the rejection filter can be positioned between the 975 nm
highly
reflective filter and the fiber 300. In that case, the rejection filter must
not reject 975 nm
radiation; i.e., it should be highly transmissive at 975 nm.
There are many possible variations of arrangements of the at least one
dichroic
filter 319 that perform the essential tasks, namely reflecting 975 nm light
back into the
fiber, transmitting 915 nm pump light from the diode 311 to the fiber 300, and
preferably
rejecting light in the 1020 -1100 nm wavelength range (i.e., does not feed it
back into the
fiber 300 and prevents it from reaching, and damaging, the pump diode 311). If
necessary,
975 nm rejection filters can also be used, outside the design path for 975 nm
light that
prevents 975 nm light firm reaching and damaging the diode 311.
It is preferable to seal the end 316 of the JAC fibre 300 as shown in Figure
30 by
heating the fibre 300 in order to prevent moisture from ingressing into the
holes 307. The
end 316 can then be cleaved (as shown) or left with a curved surface, or first
lens 71 as
described with reference to Figure 11. The holes 307 were collapsed at the
other end 317
of the JAC fibre 300 when the fibres 300, 313 were fusion spliced together. It
may also
preferable to deposit the dichroic mirror 319 on the fibre end 316.
Figure 32 shows a fibre laser 320. The fibre laser 320 is similar to the ASE
source
310 but comprises a fibre Bragg grating 323 in the photosensitive single mode
fibre 322
with reflectivity of approximately 10% at 977nm (although the reflectivity
could have been
advantageously reduce to around 1%), and the optics 321 comprises a
cylindrical and
focussing lenses and a broadband dichroic filter 322 that provides feedback
into the laser
320. The JAC fibre 300 was 0.75m long, but 0.25m to 2m may be more preferable
for
different fibre designs. Preferably the cylindrical and spherical lenses are
coated with
CA 02465522 2004-04-29
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31
coatings that provide broadband antireflection in the wavelength range from
around 910nm
to 1000ntn. Preferably, the broadband dichroic filter 322 should provide high
transmission
at 915nm and high reflectivity at 975 to 980nm. It may also be beneficial to
provide high
rejection in a wavelength range of around 1020nm to 1100nm to prevent these
longer
wavelengths either being fed back into the fibre 300 and causing instabilities
or into the
laser diode 311. High rejection can be provided with an additional dichroic
filter having
high reflectivity at 1020rim to 1100nm and configured to reflect the 1020nm to
1100nm
light out of the signal and/or pump path (see discussion with respect to
Figure 31). The
broadband dichroic mirror 322 is preferably deposited on the end of the JAC
fibre 300 after
the air holes are sealed by application of heat (which can be achieved for
example by
placing the fibre 300 into an electric arc). Alternatively, the dichroic
mirror 322 can be
deposited on a thin glass plate and then attached to the end 316 of the JAC
fibre 300, for
example using solder. The laser 320 may optionally comprise a reflector 324
for reflecting
back pump energy at 915nm in order to increase pump absorption. The reflector
324 may
be a fibre Bragg grating, or may be implemented with a narrowband dichroic
mirror place
between the JAC fibre 300 and the fibre 313 - for example, deposited on the
end 317 of the
JAC fibre 300. The latter implementation is preferable because the reflector
324 is
preferably a multimode pump reflector that is configured to reflect the 915nm
light
propagating in the cladding 303
At the output end in Figure 32 the 975 to 980 nm reflectivity should be in the
range
0.2 - 20%, and the 1020 -1100 nm reflectivity should be as low as possible,
and
preferably lower than the reflectivity at 970nm. It is advantageous to reflect
back the pump
light with the reflector 324.
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32
Both sources 310, 320 have benefits as well as some drawbaclcs. The structure
of
the ASE-source 310 is simple as no external feedbaclc is required to produce
emission at
977 nm. Since the output is seeded by spontaneous emission, the relative
intensity noise
RIN is essentially white, and the output is essentially unpolarized even in
the presence of
weak polarizing effects. The drawbacks of the ASE-source 310 are a lower
efficiency and
an inherent sensitivity to back-reflections. This sensitivity to back
reflections can be
resolved using an isolator attached to the output. On the contrary the fibre
laser 320 is less
sensitive to back-reflections and has lower threshold and higher efficiency
than the ASE-
source 310. However, the structure is more complex and there are high RIN
peaks at the
relaxation oscillations frequency and at frequencies corresponding to the
cavity round trip
time.
Figure 33 shows the measured output power 331 of the ASE source 310 and the
output power 332 of the laser 320 plotted against the absorbed power 333.
Figure 34
shows the measured output power 341 of the ASE source 310 and the measured
output
power 342 of the laser 320 plotted against wavelength 343. Figure 35 shows the
relative
intensity noise RIN 351 of the ASE source 310 and the RIN 352 of the laser 320
plotted
against frequency 353. The suppression of emission at around 1040 nm is more
than 20
dB for both the ASE and laser sources 310, 320. The spectral width of the ASE
source 310
is 3 to 4 nm and the centre wavelength is situated at 976 nm, which is near
the peak of the
980 nm absorption band of erbium-ions in silica glass. The spectral width of
the fibre laser
320 was 0.5 nm, mainly determined by the characteristics of the reflective
grating 323.
In some applications, such as pumping of distributed feedback DFB fibre lasers
[L.B. Fu et al., Teclnucal digest 28th European Conference on Optical
Communication
CA 02465522 2004-04-29
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33
ECOC-2002, Copenhagen, paper 0.3.5 (2002)], the temporal stability of a Yb-
doped
fibre-based pump source is as important as the wall-plug efficiency and output
power.
Referring to Figure 35, the ASE-source 310 has no cavity and hence its RIN is
white,
without any peaks arising, e.g., from relaxation oscillations or other cavity
effects. The R1N
of the ASE-source 310 is below-130 dB/Hz and thus should not generate any
extra
contribution to RIN of a DFB fibre laser because the RIN will be integrated
over all
frequencies of the pump source. Hence, the ASE-source 310 is an ideal pump
source for
DFB fibre lasers for application in cable television CATV and wavelength
division
multiplex WDM systems. However, as the shot noise limit of the pump absorption
is -153
dB/Hz the RIN below 1 kHz increases with RIN of the pump for all values above
the shot
noise limits. This may be a concern for some sensing applications for DFB
fibre lasers in
which the low-frequency range is of specific interest.
As can be seen from Figure 35, the fibre laser pump source 320 has several RIN
peaks 354, 355, 356. The relaxation oscillation peak occurs at 450 kHz at a
RIN level of-
100 dB/Hz. The RIN peak is dependent on the cavity length and hence on the
position of
the grating output coupler. In our measurements the cavity length was 3.25m.
The
additional peaks in the RIN spectrum 355, 356 are harmonics of the beat
frequency 354 of
the longitudinal modes within the laser cavity. Outside the peaks the RIN 352
of the fibre
laser 320 is very low and limited only by the sensitivity of the measurement
device (~ -145
dB/Hz). Thus by optimising the device length of the fibre laser 320, it should
be a suitable
pump source for DFB fibre lasers in both analogue CATV and digital WDM
systems.
In addition to the flat RIN characteristics, the unpolarized output of the ASE
source
310 is also advantageous for pumping. The RIN noise of DFB fibre lasers can be
induced
CA 02465522 2004-04-29
WO 03/038486 PCT/GB02/04912
34
not only by the RIN of the pump but also from fluctuations in its polarization
state and
frequency.
With 2.5 W of absorbed pump power the laser source 320 was capable of
delivering
1.4 W of output power. To our knowledge, this is the highest output power
obtained from a
single-mode fibre-coupled source at around 980 nm. Both sources 310, 320 are
suitable for
pumping of DFB fibre lasers and other applications that demand low noise
and/or high-
power. Such applications include pumping distributed bragg reflector (DBR)
fibre lasers
and optical amplifiers for telecom, CATV applications, and laboratory
instrumentation.
Figure 36 shows an erbium doped amplifier (EDFA) 360 comprising a preamplifier
361 and a booster amplifier 362 connected with a mid-stage gain-flattening
filter 363. The
EDFA 360 comprises tap couplers 366, an input photodiode 367, an output
photodiode
3619, isolators 368, WDM couplers 369, erbium doped fibre 3610, and thin-film
pass-band
filters 3615. Fibre 3614 provides coupling of residual pump power from the pre-
amplifier
361 to the booster amplifier 362. The EDFA 360 has an input 3616 an output
3617, and a
pump input 3618. Some of the components in the EDFA 360 can be replaced with
similar
components having similar functionality, such as hybrid components comprising
tap
coupler 366, photodiode 367 and an isolator 368.
The pump power for both the pre-amplifier 361 and the amplifier 362 is
provided
by the ASE source 310 whose output was split through a 75/25 coupler 364. The
preamplifier 361 is co-pumped with 200mW while the booster amplifier 362 is
counter-
pumped with 600mW of power. A wavelength division multiplexer coupler 3612 was
connected to the output of the ASE source 310. The wavelength division
multiplexing
(WDM) coupler 3612 was selected to couple 977nm radiation from the ASE source
to the
CA 02465522 2004-04-29
WO 03/038486 PCT/GB02/04912
coupler 364, and undesireable longer wavelength emission at 1035nm to the
termination
3613. The termination 3613 is designed to minimize reflection at 1035nm back
into the
ASE source which can have the effect of inducing instabilities or lasing
action. The
termination was implemented with a tight coil of optical fibre, but could have
been
implemented with index matching gel and/or an angle cleave. The WDM coupler
3612
could also have been replaced with another type of filter, such as a blazed
grating designed
to transmit the desired 977nm radiation, and to attenuate greatly radiation
unwanted
radiation at 1035nm.
Because of the slow dynamics of the ASE source 310 it is not possible to
compensate varying signal loads and transients by modulating the pump power.
Instead, a
DFB laser diode 365 at 1570nm (outside the transmission band) with a maximum
output
power of 40mW is used to clamp the gain and control transients in the booster
amplifier
362. The power from the DFB laser diode 365 is added and dropped from the
amplifier
using tlun-film WDM couplers 3615. When channels are dropped, the gain
compression
decreases, causing the output power of remaining channels to increase.
Therefore the
output of the clamping laser 365 is varied by the control electronics 3611
when channels
are added or dropped, so that the available gain (measured using the input
photodiode 367
and the output photodiode 3619) remains constant. The fast electronic response
time,
below 1 ~,s, allows the suppression of fast transients. The advantage of the
gain-clamping
with the laser 365 within the EDFA gain bandwidth is that only 27mW of optical
power is
required to control a l OdB drop of input power.
The EDFA 360 was tested with 32 channels each having different central
wavelengths that were distributed on the 100GHz ITU grid from 1530.33 to
1555.75nm
CA 02465522 2004-04-29 yJ
r ~~~~'~~1~ fs~~~ i ~ ~ ~f .a
p . .
~~ C ~0~2
36
' - and input into the EDFA 360. The total input power of the EDFA 360 was
OdBm, i.e. the
power per channel was -lSdBm. The EDFA 360 had a saturated output power of
+23dBm
in the region 1528nm to 1563nm. The total pump power was set at 800mW for all
conditions. The gain-flattening filter (GFF) 363 was designed such that the
EDFA 360 had
a flat gain with OdBm input power and zero clamping power. The output WDM
spectrums
show a flat gain from OdBm (32 channels) down to -lSdBm (one channel
remaining).
Fox total signal input power below 0 dBm, the power of the gain-clamping laser
diode 365 was adjusted to keep the gain constant at 23 dB. Figures 37, 38, 39
and 40 show
output WDM spectra obtained from the EDFA 360 with a different number of
channels
371. The gain flatness is better than +!- 0.5 dB for the input power range.
The dual-stage
configuration and the high pump power available allow for a noise figure
better than
5.5dB. A commercial EDFA test system based on time-domain extinction was used
for the
noise rgure measurement. The accuracy of the system for the noise figure
measurement is
better than 0.3dB. As an example the characteristics of the EDFA 360 for a
channel at
1550.92nm are shown in Figure 41 where the gain 411 and noise Figure 412 are
plotted
versus total input power 413.
Advantageously, the combination of GFF 363, EDFA design and gain clamping
using a controllable external source 310, allows the control of the gain tilt
of the EDFA
360. Figure 42 shows the gain 421, 422, 423, 424 with eight channels for
"clamping
powers" of lOmW, 1 SmW, l6mW and l7mW respectively. The gain tilt, that is the
variation in gain 421, 422, 423, 424 with wavelength, decreases with
increasing gain
clamping power from the DFB Iaser diode 365.
A~,~EP~3~E~ Sh~~~.~
~~3~~'i'~~~'~ ~~~~'~' ~~UL~ ~~6)
CA 02465522 2004-04-29
WO 03/038486 PCT/GB02/04912
37
Figure 43 shows the polarisation dependent gain (PDG) 431, 432 versus
wavelength 433 measured using the ASE source 310 and a conventional 9~0
semiconductor laser diode (not shown) respectively as the pump source of the
EDFA 360.
The ASE source 310 provides a O.ldB reduction in the PDG of the EDFA 360. This
is
particularly significant for high-bit-rate communication systems where low PDG
is
becoming increasingly important.
The transient behaviour of optical amplifiers is very important in network
applications. In particular, the output of the optical amplifier should not
vary if another
wavelength channel is added or dropped. The transient behaviour of the EDFA
360 shown
in Figure 36 was simulated by switching on and off 31 of the 32 wavelength
channels with
an acousto-optic modulator. The output power of the surviving channel at
1550.92nm was
measured using a fiber Bragg grating filter to filter the output power from
ASE and other
unwanted measurement noise, and a fast photodiode connected to an
oscilloscope. The rise
and fall times of the measured optical add-drop power applied at the input
3616 were
below SOOns.
In order to ensure that the power of the surviving wavelength channel does not
vary,
it is necessary to provide control signals from the outputs of the photodiodes
367, 3619 to
the control electronics 3611 which controls the clamping laser 365 in order to
compensate
for changes of input signal power caused by adding and dropping channels. The
high-speed
electronic control of the clamping laser diode 365 enables the overshoot and
undershoot to
be controlled below O.SdB for lSdB of input power added or dropped. This is
demonstrated by the measurement results of Figure 44, which shows the output
power 441
of the EDFA 360 as a function of time 442 when the input power was increased
by lSdB.
CA 02465522 2004-04-29
WO 03/038486 PCT/GB02/04912
38
The settling time 443 for adding lSdBm of input optical power and dropping
lSdBm of
optical power was less than 100~,s for both cases. The gain-clamping power
with a single
remaining channel (-lSdBm input power) was about 35mW at 1570mn.
The EDFA 360 when pumped with the ASE source 310 and when using the
combination of the gain clamping diode 365 and control electronics 3611 has
excellent
characteristics as measured by its low noise, low gain tilt, low polarisation
dependent gain,
and excellent transient behavior.
Figure 47 shows an amplifying optical device 470 comprising a first port 479,
a
second port 4710, a JAC fibre 472 comprising a first end 475 and a second end
476, a
dichroic mirror 471 a lens 473 and a fibre 474. The JAC fibre 472 may be any
of the JAC
fibres described herein. Preferably the JAC fibre 472 is JAC fibre 300 which
is ring-doped
with Ytterbium. The ends 475, 476 are preferably sealed and cleaved as
described with
reference to figure 11. The fibre 474 is preferably an optical fibre
configured to be
singlemoded at 980nm. Pump radiation 478 is coupled from the laser diode 311
which
emits at 915nm, and is coupled through the dichroic mirror 478 and launched
into the JAC
fibre 472. The pump radiation excites the Ytterbium ions, and radiation is
thereupon
emitted from the first and second ports 479, 4710 of the amplifying optical
device 470.
The amplifying optical device 470 can be configured as an ASE source (see
Figure 31 ) or a
fibre laser (see Figure 32). The amplifying optical device 470 can also be
configured as an
optical amplifier for amplifying signals having a wavelength where the JAC
fibre 472
provides gain.
Figure 48 shows an amplifying optical device 480 comprising a pump module 481,
an input beam 482, a thin-film filter 483 comprising a dichroic filter 484,
isolators 485, a
CA 02465522 2004-04-29
WO 03/038486 PCT/GB02/04912
39
first port 486, and a second port 487. The pump module 481 can comprise the
laser diode
311 and optics 312. Alternatively, the pump module 481 can comprise the pump
module
171. The input beam 482 can be in free space, or more preferably, be guided by
a high-
numerical optical fibre such as a JAC fibre having low attenuation at the pump
wavelength.
Such a JAC fibre can be similar to JAC fibre 472 but without the rare-earth
dopant. The
thin-film filter 483 can comprise graded refractive index (GRIN) lenses. The
JAC fibre
472 is preferably doped with Ytterbium, the pump wavelength is preferably
915nm, and the
dichroic mirror 484 preferably has a low attenuation for the pump radiation,
and has a high
reflectivity for wavelengths longer than the pump radiation. The amplifying
optical device
480 is particularly useful for amplifying signals having wavelengths around
976nm to
980nm, and for amplifying signals having wavelengths around 1035nm to 1140nm.
In
particular, the amplifying optical device as drawn is a 980nm optical
amplifier. The first
port 486 can be the input port of the optical amplifier, in which case the
optical amplifier is
being counter-pumped, or the second port 487 can be the input port, in which
case the
optical amplifier is being co-pumped.
Figure 49 shows an arrangement 490 comprising a pump source 491, the
amplifying optical device 480 configured as an optical amplifier, a coupler
492, and a
plurality of optical amplifiers 493 each comprising an input port 494, an
output port 495,
and a pump port 496. The pump source 491 can be a 980nm semiconductor laser
diode,
the ASE source 310, or the fibre laser 311. The coupler 492 can comprise at
least one
fused fibre coupler, or be configured in planar optics. The optical amplifiers
493 can
comprise erbium-doped fibre amplifiers.
CA 02465522 2004-04-29
WO 03/038486 PCT/GB02/04912
Figure 50 shows a preform assembly 500 comprising a preform 501, a plurality
of
solid rods 502, a plurality of capillaries 503, and an outer jacket 504. The
preform 501
comprises a core 5 and a cladding 6. The core 5 may be rare-earth doped. The
preform
may also comprise a separate rare earth doped region similar to that described
in previous
embodiments. The capillaries 503 are chosen to maximize the fill ratio, that
is, to ensure
that there are no significant gaps between preform 501, rods 502, capillaries
503 and outer
jacket 504. This is achieved by either selecting the preform 501, rods 502,
capillaries 502
and outer jacket 504 to have the correct size, or adjusting their diameters by
etching, by
heating and stretching on a glass lathe, or by reducing their diameter by
drawing on a fibre
drawing tower prior to assembling the preform assembly 500. If the preform 501
is
fabricated using modified chemical vapour deposition (MCVD), then it is
usually
preferable to reduce its diameter using acid etching. This is because acid
etching reduces
the size of the cladding 6 while leaving the dimensions of the core 5
untouched. Preferably
the capillaries 503 have thin walls in order to increase the volume fraction
of air to glass
within the annular region separating the rods 502 from the outer jaclcet 504.
Increasing the
volume fraction results in increased numerical aperture of the cladding of the
resulting
fibre. Figure 51 shows a cross-section of the JAC fibre 510 that is drawn from
the preform
assembly 500. The fibre 510 comprises longitudinally extending holes. 511. The
cladding
6 is non-circulax which is advantageous because of the increased overlap
between cladding
modes and the core 5.
Figure 52 shows a preform assembly 520 comprising a non-circular preform 521,
rods 522, capillaries 523, and an outer jacket 524. The non circular preform
521 can be
fabricated by etching a preform manufactured using modified chemical vapour
deposition,
CA 02465522 2004-04-29
WO 03/038486 PCT/GB02/04912
41
and then milling to the required shape using an ultrasonic drill. Figure 53
shows the
resulting amplifying fibre 530 that is drawn from the preform assembly 520.
The cladding
6 is non-circular which increases the overlap of cladding modes with the core
5 and thus
increases pump absorption. Advantageously, the rods 522 can be stress applying
rods
comprising silica doped with borosilicate. The stress applying rods may also
be doped
with germania in order to raise the refractive index. The resulting fibre 530
would then be
birefringent which is advantageous for polarization maintenance.
The amplifying fibres 510, 530 can be single mode or multimode depending on
the
size of the core 5.
Figures 51 and 53 show two types of amplifying optical fibres that can be
drawn
from the preform assemblies 500 and 520 respectively. However, many different
designs
can be produced from these assemblies. The variations are produce by applying
different
amounts of pressure andlor vacuum to each individual capillary and also to the
interstitial
gaps between the rods and capillaries. In addition, capillaries can be sealed
prior to the
drawing process. These techniques are well documented in the literature
concerning the
manufacture of holey, microstructured, and photonic bandgap fibres.
It is to be appreciated that the embodiments of the invention described above
with
reference to the accompanying drawings have been given by way of example only
and that
modifications and additional components may be provided to enhance the
performance of
the apparatus.
The present invention extends to the above mentioned features taken singularly
or
in any combination.
