WO2003038486A2 - An optical light source - Google Patents

An optical light source Download PDF

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Publication number
WO2003038486A2
WO2003038486A2 PCT/GB2002/004912 GB0204912W WO03038486A2 WO 2003038486 A2 WO2003038486 A2 WO 2003038486A2 GB 0204912 W GB0204912 W GB 0204912W WO 03038486 A2 WO03038486 A2 WO 03038486A2
Authority
WO
WIPO (PCT)
Prior art keywords
optical
light source
fibre
optical light
cladding
Prior art date
Application number
PCT/GB2002/004912
Other languages
French (fr)
Other versions
WO2003038486A3 (en
Inventor
Alam Shaiful
Anatoly Grudinin
Kalle Yla-Jarkko
Ian Godfrey
Paul Turner
Jonathan Moore
Christophe Codemard
Ray Horley
Jayaunta Kumar Sahu
David Richardson
Lars Johan Albinsson Nilsson
Cyril Renaud
Romeo Selvas-Aguilar
Original Assignee
Southampton Photonics Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to GB0126007.4 priority Critical
Priority to GB0126007A priority patent/GB0126007D0/en
Priority to GB0203146.6 priority
Priority to GB0203146A priority patent/GB0203146D0/en
Priority to GB0222622.3 priority
Priority to GB0222622A priority patent/GB0222622D0/en
Application filed by Southampton Photonics Limited filed Critical Southampton Photonics Limited
Publication of WO2003038486A2 publication Critical patent/WO2003038486A2/en
Publication of WO2003038486A3 publication Critical patent/WO2003038486A3/en

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Classifications

    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/02Optical fibre with cladding with or without a coating
    • G02B6/036Optical fibre with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03638Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/02Optical fibre with cladding with or without a coating
    • G02B6/02295Microstructured optical fibre
    • G02B6/02314Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/02361Longitudinal structures forming multiple layers around the core, e.g. arranged in multiple rings with each ring having longitudinal elements at substantially the same radial distance from the core, having rotational symmetry about the fibre axis
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/02Optical fibre with cladding with or without a coating
    • G02B6/02295Microstructured optical fibre
    • G02B6/02314Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/0238Longitudinal structures having higher refractive index than background material, e.g. high index solid rods
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/24Coupling light guides
    • G02B6/255Splicing of light guides, e.g. by fusion or bonding
    • G02B6/2552Splicing of light guides, e.g. by fusion or bonding reshaping or reforming of light guides for coupling using thermal heating, e.g. tapering, forming of a lens on light guide ends
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/24Coupling light guides
    • G02B6/26Optical coupling means
    • G02B6/262Optical details of coupling light into, or out of, or between fibre ends, e.g. special fibre end shapes or associated optical elements
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/02Optical fibre with cladding with or without a coating
    • G02B6/02057Optical fibre with cladding with or without a coating comprising gratings
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/02Optical fibre with cladding with or without a coating
    • G02B6/02295Microstructured optical fibre
    • G02B6/02314Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02319Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by core or core-cladding interface features
    • G02B6/02333Core having higher refractive index than cladding, e.g. solid core, effective index guiding
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/02Optical fibre with cladding with or without a coating
    • G02B6/02295Microstructured optical fibre
    • G02B6/02314Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/02376Longitudinal variation along fibre axis direction, e.g. tapered holes
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/24Coupling light guides
    • G02B6/42Coupling light guides with opto-electronic elements
    • G02B6/4201Packages, e.g. shape, construction, internal or external details
    • G02B6/4202Packages, e.g. shape, construction, internal or external details for coupling an active element with fibres without intermediate optical elements, e.g. fibres with plane ends, fibres with shaped ends, bundles
    • G02B6/4203Optical features
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00Semiconductor lasers
    • H01S5/005Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping

Abstract

An optical light source comprising a laser diode (1), a beam shaping optics (2), and an amplifying fibre (3), wherein the amplifying 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).

Description

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 lOOmW to 10W, 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. 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 areas, 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 large 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 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, which 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 back 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. 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 50 W of optical pump power. The laser diode can
emit 0.5W to 5W of optical pump power.
The cladding can have an outer diameter in the range lOum to lOOum. The
cladding can have an outer diameter in the range 15um to 50um.
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 amplifier, a laser or a distributed feedback laser, and wherem 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 are 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 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 earth 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 lOum.
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 980nm, and wherein the
optical light source comprises a second port, the optical light source being an optical
amplifier for 975nm to 980nm radiation. It is preferable that the waveguide is configured to emit optical radiation in a wavelength range from 975nm to 980nm, 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 1050nm. 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
15μm to 75 μm. The cladding may have an outer diameter in the range 25 μm to 35μm.
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. 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 for pumping an optical fibre amplifier 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 fibre;
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; 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; 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 JAC 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 absorbed 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 channels;
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; 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 15dB;
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
port;
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 amplifying 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. 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 laser diodes,
diode bars and/or diode stacks.
The laser diode 1 can emit 0.1W to 50 W of optical pump power. The laser diode 1
can emit 0.5W to 5W 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 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 lOum to lOOum. The
cladding 6 can have an outer diameter in the range 15um 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 and/or cladding 6 can be doped with germanium, phosphorous, boron, aluminium and/or 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 form 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 core 5. 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 apluralify of amplifying optical devices 33 via an optical fibre 31, apluralify 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 11, the microstructured mesh 111 may be sealed at either
of end 112, 113 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 112 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 flat 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 ferrule.
The reflecting material 123 may be a metal such as chrome, silver or gold, and the metal 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 back through the
amplifying fibre 3.
Figure 4 shows feedback 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 lOum.
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 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 1050nm. 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 15μm to 75 μm. The
cladding may have an outer diameter in the range 25 μm to 35μm.
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. The holes 135 are formed from the inside of capillaries used to fabricate the amplifying
fibre 130. The holes 139 are formed from the interstitial spaces between the capillaries
used to fabricate the amplifying fibre 130. hi certain embodiments, the amplifying fibre
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 136 can
be lOμm to 500μm for the minor axis, and 150μm to lOOOμm for the major axis.
Figure 14 shows an amplifying fibre 140 in which the core 5 and region 131 is
offset from the center of the cladding 141. The amplifying fibre 140 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 5 is
substantially central with respect to the outer circumference of the outer jacket 143 (note
the centre lines 149 shown in Figure 14). 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 131 that surrounds the core 5. This 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 977nm. 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. 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 1 lOOnm) 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 lasing at 977 nm one has to ensure that the gain at 1040 nm is lower than the
threshold for spurious lasing 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 μm [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. K Sahu, et al.
Electron. Lett. 37, 1116 (2001)]. 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 nm 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 μm
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 μm. 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 in l00 μm diameter fibre) are not suitable for the practical
realization of cost-effective fibre based pump sources.
The realization of a 976 mn 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 Pth should be small compared with the available pump power Pp and second the slope
efficiency η with respect to launch power must be high. 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 = kN0AdΨd(λ){[σe(λ) + σa(λ)]n2 - σa(λ)}L , (1)
where k = 4.343, o is the concentration of active ions, L is the fibre length, σe and
σa are the emission and absorption cross sections, respectively and n2 is the fraction of
active ions that are excited. Finally Ψd is the value of the normalized modal intensity
averaged over doped area Ad (in other words if P is the incident pump power then PΨd is
the average intensity in the doped area). It can be shown that the unwanted gain at 1030
nm can be expressed as
G1030 = 0.25G976 + 0.72βαgp , (2)
where β = Ψd s/ΨdP = Aciadding/ Core and cc0pP is the pump absorption.
The 1030 nm gain is proportional to the cladding-to-core area ratio Adding /Acore
and grows rapidly with pump absorption oc0p p. Thus in order to suppress lasing at 1030 nm one has to ensure that G1030 < 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 μm 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 μm.
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 togetiier 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 μm
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 key 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% and preferably greater than 70%); there should be high pump absorption; the pump cladding area should be below 600 μm2 (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 co-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
μm 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 μm. To ensure a high NA the mesh
203 was fabricated with two layers of silica capillary tubes stacked around a preform inside a silica jacket. The strand thickness - ie the diameters of the silica capillary tubes in the
resulting JAC fibre 200 was 1 to 2 μm. 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 ring
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-known 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 run 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 are 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. 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 JAC 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 25 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. 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 271, 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 splitter 2710. The pump splitter 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 -3 IdBm. Figure 28 shows the
gain 282 measured at -1 IdBm and the gain 283 measured at -3 IdBm. Figure 29 shows the
noise figure 291 measured at -1 IdBm. 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. 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 0.5um 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. This
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 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 0.5m and 5m 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 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 311 and from being fed back into the fiber 300. One option is to make the 975 nm highly 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 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 977mn (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 coatings that provide broadband antireflection in the wavelength range from around 91 Onm
to lOOOnm. 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 1 lOOnm 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 1020nm to 1 lOOnm and configured to reflect the 1020nm to 1 lOOnm
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. Both sources 310, 320 have benefits as well as some drawbacks. The structure of
the ASE-source 310 is simple as no external feedback 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, Technical digest 28th European Conference on Optical Communication ECOC-2002, Copenhagen, paper 08.3.5 (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 RIN
of the ASE-source 310 is below— 130 dB/Hz and thus should not generate any extra
contribution to RUST 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 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 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 40m W 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 thin-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 lμs, 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 lOdB 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 and input into the EDFA 360. The total input power of the EDFA 360 was OdBm, i.e. the
power per channel was -15dBm. 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 -15dBm (one channel remaining).
For 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 figure 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, 15mW, 16mW and 17mW 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 laser diode 365. Figure 43 shows the polarisation dependent gain (PDG) 431, 432 versus
wavelength 433 measured using the ASE source 310 and a conventional 980
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 500ns.
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 0.5dB for 15dB 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 15dB. The settling time 443 for adding 15dBm of input optical power and dropping 15dBm of
optical power was less than lOOμs for both cases. The gain-clamping power with a single
remaining channel (-15dBm 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 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. 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 (MCND), 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 jacket 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-circular 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, 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 and/or 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.

Claims

Claims
1. 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.
2. An optical light source according to claim 1 wherein the beam shaping optics
comprises a first lens.
3. An optical light source according to claim 2 where the first lens is formed on the
end of the amplifying optical fibre.
4. An optical light source according to any one of the preceding claims wherein the
beam shaping optics comprises a second lens.
5. An optical light source according to claim 4 wherein the second lens is a cylindrical
lens.
6. An optical light source according to claim 5 wherein the cylindrical lens is a
cylindrical microlens which has a shape designed to transform some particular
given input light distribution into some desired output light distribution.
7. An optical light source according to claim 5 or claim 6 wherein the cylindrical lens has a uniform refractive index profile or a graded refractive index profile.
8. An optical light source according to claim 1 wherein the laser diode is a multimode
laser diode.
9. An optical light source according to any one of the preceding claims wherein the
laser diode emits 0.1W to 50W of optical pump power.
10. An optical light source according to claim 9 wherein the laser diode emits 0.5W to
5W of optical pump power.
11. An optical light source according to any one of the preceding claims wherein the
cladding has an outer diameter in the range lOμm to lOOμm.
12. An optical light source according to claim 11 wherein the cladding has an outer
diameter in the range 15μm to 50μm.
13. An optical light source according to any one of the preceding claims wherein the
core and/or cladding is doped with at least one of germanium, phosphorous, boron, aluminium and fluoride.
14. An optical light source according to any one of the preceding claims wherein the core is configured to be a single mode waveguide.
15. An optical light source according to any one of the preceding claims wherein the
optical pump power facilitates optical radiation from the rare earth dopant in the waveguide.
16. An optical light source according to any one of the preceding claims wherein the optical radiation from the rare earth dopant in the waveguide is coupled to an
amplifying optical device, wherein the amplifying optical device is one of an
optical amplifier, a laser or a distributed feedback laser, and wherein the amplifying optical device is configured to be pumped by the optical radiation.
17. An optical light source according to any one of the preceding claims wherein the
optical radiation from the rare earth dopant in the waveguide is coupled to a
plurality of amplifying optical devices via an optical coupler, and wherein the
amplifying optical devices are configured to be pumped by the optical radiation.
18. An optical light source according to any one of the preceding claims wherein the
cladding is circular.
19. An optical light source according to any one of claims 1 to 17 wherein the cladding
is substantially rectangular.
20. An optical light source according to any one of claims 1 to 17 wherein the cladding
has a non-circular shape.
21. An optical light source according to any one of the preceding claims wherein the
core is centrally located in the cladding.
22. An optical light source according to any one of claims 1 to 20 wherein the core is
offset from the centre of the cladding.
23. An optical light source according to any one of the preceding claims wherein the
amplifying optical fibre comprises a microstructured mesh surrounding the
cladding.
24. An optical light source according to claim 23 wherein the amplifying optical fibre
has two ends, and wherein the microstructure mesh is sealed in at least one of the
ends of the amplifying optical fibre.
25. An optical light source according to any one of the preceding claims and
comprising feedback means for providing feedback in the waveguide, the
waveguide being a laser.
26. An optical light source according to claim 25 wherein the feedback means is a
reflector.
27. An optical light source according to claim 26 wherein the reflector is formed from a
cleave in the amplifying optical fibre.
28. An optical light source according to claim 26 wherein the reflector is a fibre Bragg
grating.
29. An optical light source according to claim 26 wherein the reflector is a dichroic
filter.
30. An optical light source according to claim 29 wherein the dichroic filter is
deposited on the end of the amplifying optical fibre.
31. An optical light source according to any one of claims 1 to 24 wherein the
amplifying optical fibre is configured as a source of amplified spontaneous
emission.
32. An optical light source according to any one of the preceding claims wherein the
rare earth dopant is contained in the core.
33. An optical light source according to any one of claims 1 to 31 wherein the rare earth
dopant is contained in the cladding.
34. An optical light source according to any one of claims 1 to 31 wherein the rare earth
dopant is contained in both the core and the cladding.
35. An optical light source according to any one of claims 1 to 31 wherein the rare earth
dopant is configured in a region surrounding the centre of the waveguide.
36. An optical light source according to claim 35 wherein the region surrounding the
centre of the waveguide is a ring surrounding the core.
37. An optical light source according to claim 36 wherein the ring has a thickness in the
range 1 to lOμm.
38. An optical light source according to any one of the preceding claims wherein the
rare earth dopant comprises Ytterbium and the laser diode emits at a wavelength
that is absorbed by the Ytterbium.
39. An optical light source according to claim 38 and comprising a dichroic filter that reflects in the wavelength range 975nm to 980nm, and wherein the optical light
source comprises a second port, the optical light source being an optical amplifier for 975nm to 980nm radiation.
40. An optical light source according to claim 38 wherein the waveguide is configured
to emit optical radiation in a wavelength range from 975nm to 980nm, 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.
41. An optical light source according to any one of claims 1 to 37 wherein the rare earth dopant comprises Erbium and the laser diode emits at a wavelength that is absorbed
by the Erbium.
42. An optical light source according to any one of claims 1 to 37 wherein the rare earth
dopant comprises Neodymium and the laser diode emits at a wavelength that is
absorbed by the Neodymium.
43. An optical light source according to any one of claims 1 to 37 wherein the rare earth
dopant comprises Thulium and the laser diode emits at a wavelength that is absorbed by the Thulium.
44. An optical light source according to any one of claims 1 to 37 wherein the rare earth
dopant comprises Praseodymium and the laser diode emits at a wavelength that is absorbed by the Praseodymium.
45. An optical light source according to any one of claims 1 to 37 wherein the rare earth
dopant is 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.
46. An optical amplifier comprising an optical light source according to any one of the
preceding claims.
47. An optical amplifier according to claim 45 and configured to have low polarisation dependent gain.
48. An optical fibre laser comprising an optical light source according to any one of
claims 1 to 38.
49. 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 any one of claims 1 to 38, and coupling the light source to the pump inputs.
50. 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 any one of claims 1 to 38, and coupling the optical light source to the pump inputs.
PCT/GB2002/004912 2001-10-30 2002-10-30 An optical light source WO2003038486A2 (en)

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WO2003038486A3 (en) 2003-08-21
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EP1440495A2 (en) 2004-07-28

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