WO2004083921A1 - Optical fibre and method of launching pump power into a fibre, and a manufacturing method - Google Patents

Optical fibre and method of launching pump power into a fibre, and a manufacturing method Download PDF

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Publication number
WO2004083921A1
WO2004083921A1 PCT/FI2004/000156 FI2004000156W WO2004083921A1 WO 2004083921 A1 WO2004083921 A1 WO 2004083921A1 FI 2004000156 W FI2004000156 W FI 2004000156W WO 2004083921 A1 WO2004083921 A1 WO 2004083921A1
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WO
WIPO (PCT)
Prior art keywords
fibre
amplifier
pump
fibres
core
Prior art date
Application number
PCT/FI2004/000156
Other languages
French (fr)
Inventor
Oleg Okhotnikov
Original Assignee
Optoelectronics Research Centre
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
Application filed by Optoelectronics Research Centre filed Critical Optoelectronics Research Centre
Publication of WO2004083921A1 publication Critical patent/WO2004083921A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES 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
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2821—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using lateral coupling between contiguous fibres to split or combine optical signals
    • G02B6/2835—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using lateral coupling between contiguous fibres to split or combine optical signals formed or shaped by thermal treatment, e.g. couplers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES 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
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2808—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES 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
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06729—Peculiar transverse fibre profile
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES 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
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • H01S3/094019—Side pumped fibre, whereby pump light is coupled laterally into the fibre via an optical component like a prism, or a grating, or via V-groove coupling

Definitions

  • the present invention relates to an optical fibre combiner comprising at least one amplifier-core fibre, and at least one pump fibre in contact with the amplifier-core fibre for providing a side-coupled pump power to the amplifier-core fibre.
  • the invention also relates to a method of launching pump power into an optical fibre combiner, the fibre combiner comprising at least one amplifier-core fibre and at least one pump fibre in contact with the amplifier-core fibre for launching a side- coupled pump power in the amplifier-core fibre.
  • the invention further relates to a method for manufacturing an optical fibre combiner comprising at least one amplifier-core fibre and at least one pumping fibre in contact with the amplifier-core fibre for launching a side-coupled pump power in the amplifier-core fibre.
  • Optical fibres are widely used in different applications as guidelines of optical power to a destination.
  • Laser devices are normally used for these applications as optical sources of high output power.
  • the lasers can produce a kilowatt-level of an average output power.
  • the high- power lasers are used for treatment of different kind of materials, such as metal sheets, stone, concrete, etc.
  • the high-power laser beam can be used for drilling, cutting, shaping and soldering materials much more accurately and effectively than with devices based on mechanical treatment of material.
  • the powerful light of the laser has to be guided properly and with as high efficiency as possible to a destination. Otherwise the transfer losses lead to excess heating of the fibre and decrease the optical power at the output of the fibre.
  • the high-power fibre lasers are pumped by high- power multimode laser diodes via a cladding element (cladding fibre) surrounding a single-mode core element.
  • the cladding element has different (lower) refractive index than the core element.
  • the cladding element itself is also covered by another cladding layer having still lower refractive index.
  • the signal light beam of the laser is propagating through the core element and the pumping light beam is launched into the cladding element.
  • This type of fibres used for building high-power lasers and amplifiers are known as cladding pumped fibres. Based on the effect of total internal reflection (TIR), the pump light is guided by the pump cladding, i.e. "cladding element".
  • the ray traces of the pump modes cross the fibre core, which contains the absorbing material used as a gain medium. While the pumping light traverses the core element of the gain fibre, it absorbs into it and provides the amplification of the lasing light beam propagating in the core element.
  • the coupling (launching) of the light beam of the pumping laser into the cladding element of the fibre is difficult to arrange. In principle, three different methods are known for the coupling. They are shown in Figs. 1a to 1c.
  • the pumping light beam is coupled into the fibre at one end of the fibre.
  • This arrangement requires that a dichroic mirror is used so that the lasing light beam is launched into the fibre by reflecting it at the dichroic mirror, while the pumping light beam is injected through the mirror into the cladding element of the fibre, or vice versa.
  • the fibre comprises a separate pump guiding fibre for the coupling of the pumping light.
  • the pumping light beam is injected into the pump fibre from the pumping laser source.
  • the pump fibre directs the pumping light beam to the cladding element of the gain-core fibre via special multimode fibre coupler or fibre launcher. This kind of arrangement allows multipoint pumping by using multiple coupler configuration, but the manufacturing of such fibre couplers is quite complicated procedure.
  • the fibre comprises a reflecting notch in the form of a V-groove in the cladding element.
  • the pumping light is injected substantially perpendicularly to the notch, which reflects the pumping light beam to the cladding layer (waveguide or element).
  • This kind of arrangement also allows multipoint pumping, but the manufacturing of the notch requires special tools and the shape of the notch has to be very accurately made such that the reflection at a correct angle with respect to the fibre axis can be achieved.
  • the optical pumping of the fibre amplifier can also be achieved by a so-called side-pumping technique.
  • This technique several bare fibres used for pumping are arranged (wound) around the amplifier-core fibre.
  • Fig. 2a discloses such a fibre coupling technique as a side view
  • Fig. 2b discloses such a fibre coupling technique as a cross sectional view.
  • the pump fibres and the amplifier-core fibre are twisted.
  • the bundle of the pump fibres and the amplifier-core fibre are surrounded by an external low-refractive-index coating.
  • the pump light launched into the pump fibre propagates through this fibre and gradually couples to the (amplifier) gain-core fibre because of optical contact between a pump fibre and a gain-core fibre.
  • the launching of the pumping light beam is easier to arrange with this kind of so-called side-coupling than with the techniques shown in Figs. 1a to 1c.
  • the efficiency of the side- pumping technique may further be improved with some off-set in the fibre core position within cladding element, i.e. when the core is not centrally placed inside the cladding.
  • pump light coupling per unit length of the gain-core fibre is quite low with this technique because pump and gain-core fibres represent touching spheres (in cross-sectional view) and, therefore, the total area of the coupling surface is small.
  • This feature sets the limit for the laser and amplifier length, which is rather long with this coupling method: long length of the fibre is needed to absorb significant fraction of the pump light.
  • the patent publication US 6,529,318 discloses a total internal reflection coupler and method for side-coupling pump light into a fibre.
  • One aim of the present invention is to provide a specialty fibre and a method in which the pumping efficiency is increased, and a simple and cost effective manufacturing method using an improved side-coupling approach for the pump/signal light combiner.
  • the invention is based on the idea that the outer shape of the pump fibres and the amplifier-core fibre are better adapted to each other to improve the coupling efficiency of the pumping light beam from the pump fibre to the amplifier-core fibre.
  • the adaptation is performed so that the contacting interface between the amplifier-core fibre and the pump fibre is increased significantly by making the shape of their claddings to be of the reciprocal form.
  • the cross-sections of the cladding of the pump and/or gain-core fibres are shaped to increase the optical contact surface contrary to the point-to- point coupling for fibres with circularly-shaped claddings.
  • the fibre according to the present invention is primarily characterised by that the cross-section of at least one of the fibres is made different from a circle, and that the cross-section of the pump fibres and the amplifier-core fibre are adapted to each other so that the interface between the amplifier-core fibre and the pump fibre is a surface.
  • the method according to the present invention is primarily characterised by that the pumping light is directed by the pump fibre via a contact surface area to the amplifier-core fibre, the surface area being formed between the pump fibre and the amplifier-core fibre by forming the cross-section of at least one of the fibres different from a circle, and by adapting the cross-section of the pump fibres and the amplifier-core fibre to each other in a reciprocal way so that the interface between the amplifier-core fibre and the pump fibre is a surface.
  • the manufacturing method according to the present invention is primarily characterised by that the cross-section of at least one of the fibres is made different from a circle, and that the cross-section of the pump fibres and the amplifier-core fibre are adapted to each other so that the interface between the amplifier-core fibre and the pump fibre is a surface.
  • the present invention provides significant advantages to prior art fibres and pumping methods.
  • the cross sections of the amplifier-core fibre and the pump fibres suit well to each other thus increasing the surface area through which the pumping light can be coupled into the cladding of the amplifier-core fibre. Therefore, the pumping efficiency of the method and the fibre combiner according to the present invention is higher than in prior art.
  • the losses of the fibre are also reduced which reduces inter alia the excess heat generation in the fibre, and also the output power can be increased and/or the power of the pumping lasers can be reduced.
  • the number of pumping fibres and, therefore, the output power are scalable to a desired value.
  • the doping level of the amplifier-core fibre can be arbitrary high.
  • the fibre according to the present invention is relatively easy to manufacture by shaping the fibre preform before fibre drawing process. Owing to a high pumping efficiency of the gain-core fibre and, therefore, an increased rate of pump absorption per unit length of the fibre, highly-doped short-length fibre devices can be designed using proposed method based on cladding-pump geometry. Additionally, thermal fusing of the fibres allows for reliable and environmentally stable pump/signal combiner.
  • Figs. 1a to 1c depict some prior art pump light coupling methods for the fibre
  • Fig. 2a depicts as a side view a fibre combiner in which the side- pumping technique is used
  • Fig. 2b depicts the fibre combiner of Fig. 2a as a cross sectional view
  • Fig. 3a depicts a fibre combiner according to an advantageous embodiment of the present invention as a cross-sectional view
  • Fig. 3b depicts the cross sections of the amplifier-core fibre and the pumping fibres of the fibre combiner of Fig. 3a
  • Fig. 4a depicts a fibre combiner according to an advantageous embodiment of the present invention as a cross-sectional view
  • Fig. 4b depicts the cross sections of the amplifier-core fibre and the pumping fibres of the fibre combiner of Fig. 4a,
  • Fig. 5a depicts a fibre combiner according to an advantageous embodiment of the present invention as a cross-sectional view
  • Fig. 5b depicts the cross sections of the amplifier-core fibre and the pumping fibres of the fibre combiner of Fig. 5a,
  • Fig. 6a depicts a fibre combiner according to an advantageous embodiment of the present invention as a cross-sectional view
  • Fig. 6b depicts the cross sections of the amplifier-core fibre and the pumping fibres of the fibre combiner of Fig. 6a,
  • Fig. 7 depicts a fibre combiner according to an advantageous embodiment of the present invention as a side view
  • Figs. 8a and 8b depict a manufacturing method according to an advantageous embodiment of the present invention as a simplified diagram.
  • Figs. 9a and 9b depict the cross sections of the fibre combiner that is based on circularly-shaped fibres before thermal fusing and after strong thermal fusing that resulted in significant change in the fibre shapes.
  • Figs. 10a and 10b depict the configurations based on series and parallel connections of the combiners to scale the output power of the amplifier or laser to a higher value.
  • Fig. 3a there is a cross section of a fibre combiner 1 according to an advantageous embodiment of the present invention.
  • the fibre combiner 1 comprises an amplifier-core fibre 2 (gain fibre) and at least one pump fibre 3 (pump guide). Typically the fibre combiner 1 comprises more than one pump fibre 3.
  • each pump fibre 3 is placed in contact with the amplifier-core fibre 2.
  • the contacting region between the amplifier-core fibre 2 and any of the pump fibre 3 is called as a coupling interface 4.
  • the cross section of the amplifier-core fibre 2, the cross section of the pump fibres 3, and/or the cross section of both the amplifier-core fibre 2 and the pump fibres 3 are formed different from a circular shape.
  • the cross section of the amplifier- core fibre 2 and the cross sections of the pump fibres 3 are adapted to each other so that the coupling interface 4 forms an enlarged-area surface.
  • a cross-section of the fibre combiner 1 at a location in which the amplifier-core fibre 2 and at least one of the pump fibres 3 are in contact with each other, is considered in more detail, it can be seen that the form of the contacting region (i.e. the cross-section of the coupling interface 4 at that location) is a curve.
  • the curve can be a straight line, a curved line, such as an arc, a line having one or more angles, etc.
  • Figs. 3b, 4b, 5b and 6b depict the cross-sections of the amplifier- core fibre 2 and the pump fibres 3 of the fibre combiners 1 of Figs. 3a, 4a, 5a and 6a, respectively.
  • the cross-section of the amplifier-core fibre 2 is formed as a circle.
  • the cross-section of the pump fibres 3 is formed as a circle from which a part is removed. What remains after the removal (at the modified section of the pump fibre 3) is a curve having a radius of curvature substantially equal to the radius of the amplifier-core fibre 2.
  • the split section 3.1 (Fig. 3b) of the pump fibre 3 is a curved plane and it is arranged to be placed towards the amplifier-core fibre 2.
  • the cross-section of all the pump fibres 3 are formed as a circle, and the cross-section of the amplifier-core fibre 2 is formed as a circle from which multiple substantially equal parts are removed.
  • the number of modified sections is equal to the number of pump fibres 3 of the fibre combiner 1. What remains after the removal at each modified section of the amplifier-core fibre 2 is a set of curves having a radius of curvature substantially equal to the radius of the pump fibre 3. Therefore, the modified (split) sections of the gain-core fibre are reciprocal to the shape of the pump fibres.
  • the split sections 2.1 (Fig. 4b) of the amplifier-core fibre 2 are curved surfaces towards which the pump fibres 3 are to be placed.
  • the cross-section of the amplifier-core fibre 2 is formed as a polygon (pentagon in Fig. 5a and hexagon in Fig. 6a).
  • the cross-section of the pump fibres 3 is formed as a split circle. It can be formed, e.g. by cutting a slice out of a circular fibre preform.
  • the split part 3.1 (Figs. 5b, 6b) of the pump fibre 3 is a plane and it is arranged to be placed on one side 2.1 (Figs. 5b, 6b) of the polygon.
  • the number of the sides of the polygon is equal to the number of pump fibres 3 in the fibre combiner 1.
  • One advantage of the present invention is that different number of pump fibres 3 can be used in different embodiments. Therefore, the output power can be scaled to a desired value by inter alia changing the number of pump fibres 3.
  • the radius of the pump fibres 3 and/or the amplifier-core fibre 2 can also be modified to adapt the optical properties of the fibre combiner 1 to a specific device or application.
  • an advantageous embodiment for manufacturing the fibre combiner 1 according to the present invention will be described in more detail with reference to Figs. 8a and 8b.
  • the manufacturing process is a complex, multiphase process, so only the main details necessary for understanding the manufacturing method will be described.
  • a first preform 6 for the amplifier-core fibre 2 and a second preform 7 for the pump fibres 3 are fabricated by one of the techniques for preform fabrication, e.g. flame hydrolysis or chemical vapour deposition.
  • the materials for the preforms 6, 7 are selected according to the desired properties of the fibres 2, 3.
  • the first preform 6 advantageously comprises a core element 6.1 and a cladding element 6.2.
  • the core element 6.1 is e.g. a silica which is doped with one or more suitable dopants, such as rear-earth ions (for example, with erbium, ytterbium, neodymium, etc.) and refractive index-rising element, such as germanium.
  • the cladding element 6.2 may also be doped with refractive index-rising dopant, such as germanium.
  • the refractive index of the cladding element is lower than the refractive index of the core element, thus forming the fibre wavegude for the signal light.
  • the waveguide of the gain-core fibre could be a single- mode waveguide, when high-quality of the beam at the output of the device is required for specific application.
  • the second preform 7 is preferably made of pure silica.
  • the pump waveguides are multimode that allow for efficient launching of the light from high-power broad-area semiconductor pump diodes.
  • the refractive index of a cladding of a gain-core fibre could be higher than the refractive index of the pump fibre, thus further enhancement of the power transfer from pump fibre to the gain-core fibre can be achieved.
  • the preforms 6, 7 are cylindrical pieces of structural glass with circular cross-section. Then, according to the present invention, the shape of at least one of the preforms 6, 7 will be modified to achieve the desired shape of the outer surface. For example, if the cross-section of the amplifier-core fibre 2 differs from the circle, the first preform 6 is shaped accordingly. Respectively, if the cross-section of the pump fibre 3 differs from the circle, the second preform 7 will be shaped so that the cross-section of the preform 7 after the shaping process is substantially similar to the cross-section of the pump fibre 3. As described above, the figures 3b, 4b, 5b and 6b depict some advantageous embodiments of the fibres, wherein the preforms 6, 7 will be shaped to have substantially similar shapes or reciprocal shapes.
  • the shaping of the preforms 6, 7 can be done mechanically, e.g. by grinding or cutting, depending on inter alia the desired shape for the preforms 6, 7.
  • the processing continues with fibre 2, 3 drawing from the preforms 6, 7.
  • the preform 6, 7 is re-drawn into a fibre 2, 3 having a desired diameter, which is much smaller than the diameter of the preform. It is important to mention, that in the drawing process the cross-section of the fibre 2, 3 remains substantially the same as the cross-section of the (shaped) preform 6, 7.
  • all the pump fibres 3 can be formed from the same second preform 7 by cutting the drawn fibre into pieces with required lengths.
  • the amplifier-core fibre 2 is also cut providing the desired length.
  • an assembly template 8 can be used to arrange the fibres into the proper configuration.
  • the assembly template 8 comprises a hole 8.1 , 8.2 for each fibre 2, 3.
  • the amplifier-core fibre 2 is directed through the centre hole 8.1
  • each pump fibre 3 is directed through one outer hole 8.2.
  • the assembly template 8 guides the amplifier-core fibre 2 and the pump fibres 3 towards each other so that they become contacted at a right position.
  • the combination (bundle) of the fibres 2, 3 is then directed through a first aperture 9 to e.g. a heating zone 10, and output from the heating zone through a second aperture 11.
  • a first aperture 9 to e.g. a heating zone 10
  • the heating zone e.g. a heating zone
  • the fibre shape could also be changed during thermal fusing process.
  • the physical mechanism of this change in the fibre shape is owing to the forces of surface tension in the viscous melted glass. As it is well-known from the physics of liquids, these forces tend to minimise the total area of the surface of the viscous liquid substance. In fact, this happens when fabricating so- called well-fused biconical couplers.
  • the fibre combiner 1 of this invention using specially and reciprocally shaped amplifier-core fibre 2 and pump fibre 3 to increase the coupling surface 4, can also be made, in principle, without shaping the preforms prior to fibre drawing.
  • the required shaping of the fibres can be obtained from initially circularly-shaped fibres 2 and 3 by sufficient fusing of the arranged assembly of the fibre combiner 1 in high-temperature zone.
  • the temperature treatment in this procedure is basically similar to that used for fabrication of well- fused biconical fibre couplers.
  • the assembled fibres 2 and 3 with circular shape two non-restrictive examples shown in Fig.
  • Fig. 9a should be strongly fused together within heating zone, where the shape of each fibre can be changed creating substantially monolithic glass structure, two non-restrictive examples shown in Fig. 9b.
  • the fusing temperature should be high enough to achieve the "collapse" of the solitary fibres in the assembled bundle and to create monolithic glass structure.
  • the strong thermal fusing of the circularly-shaped fibres is used primarily for changing the shapes of the fibres to obtain well-fused and well-coupled fibres.
  • the bundle of the amplifier-core fibre 2 and the pump fibres 3 can be surrounded by a cover 12 for protecting the fibre combiner 1 against environmental effects.
  • the cover 12 can be formed, for example, of a low diffractive index (e.g. plastic) tube.
  • the material of the cover 12 can be such a material which decreases its dimensions while heated. This allows the cover 12 to tighten the bundle of the fibres and prevent the penetration of undesirable substances. It is also possible that the bundle of the fibres is covered by a low-index plastic gel before placing it into the cover 12.
  • Fig. 7 depicts an advantageous embodiment of the fibre combiner 1 of the present invention as a side view.
  • elements 13 are also shown that needed to accomplish with the amplifier-core fibre 2 a fibre laser or amplifier, e.g. signal source, optical isolators, laser mirrors, etc.
  • a pumping laser 14 delivers the pumping power to the fibre combiner 1.
  • the light beam of the pumping laser(s) 14 can be coupled to the fibre combiner 1 by using one or more pump fibres 3 depending on the number of pump lasers needed to achieve the required level of output power of the fibre laser or amplifier.
  • each pump laser 14 can be coupled to one pump fibre 3.
  • the pump laser 14 in the form of a laser array (not shown) can be used for optical pumping of the fibre combiner 1.
  • the pump fibres 3 can be coupled to the laser array so that one pumping element of the laser array is coupled to one pump fibre 3. Furthermore, it is possible that the pump fibres 3 are pumped from both ends.
  • the element 13 providing the signal light for the fibre combiner 1 based optical amplifier is coupled at one end of the amplifier-core fibre 2.
  • the element 13 providing the optical feedback signal for the fibre combiner 1 based laser is coupled at one end of the amplifier-core fibre 2.
  • the element 13 provides an optical isolation from external reflections for the fibre combiner 1.
  • the further power scaling of the fibre devices based on the proposed combiner can be achieved by the concatenating the above described combiner in series, as shown in Fig. 10a, or in parallel, as shown in Fig. 10b.
  • the latter configuration (Fig. 10b) could, however, result in some penalties to the output beam quality.
  • fibre combiner 1 including one amplifier-core fibre 2
  • the fibre combiner 1 can also include more than one amplifier-core fibres 2.

Abstract

The invention relates to an optical fibre combiner (1) comprising a amplifier-core fibre (2), and at least one pump fibre (3) in contact with the amplifier-core fibre (2) for providing a side-coupled pump power to the amplifier-core fibre. The cross-section of at least one of the fibres (2, 3) is made different from a circle. Further, the cross-section of the pump fibres (3) and the amplifier-core fibre (2) are adapted to each other so that the coupling interface (4) between the amplifier-core fibre (2) and the pump fibre (3) in contact with the amplifier-core fibre (2) is a surface. The shaping of the fibre claddings can be performed by either changing the form of the preform(s) (6, 7) from which the fibres (2, 3) are drawn, or using a thermal fusing process after the drawing, during the assembling of the fibre combiner (1) from the initially circularly-shaped pump fibre(s) (3) and the amplifier-core fibre (2). The invention also relates to a method of launching pump power into an optical fibre combiner (1), and a method for manufacturing the optical fibre combiner (1) comprising a amplifier-core fibre (2) and at least one pumping fibre (3) in contact with the amplifier-core fibre for providing a side-coupled pump power to the amplifying fibre (2).

Description

Optical fibre and method of launching pump power into a fibre, and a manufacturing method
Field of the Invention
The present invention relates to an optical fibre combiner comprising at least one amplifier-core fibre, and at least one pump fibre in contact with the amplifier-core fibre for providing a side-coupled pump power to the amplifier-core fibre. The invention also relates to a method of launching pump power into an optical fibre combiner, the fibre combiner comprising at least one amplifier-core fibre and at least one pump fibre in contact with the amplifier-core fibre for launching a side- coupled pump power in the amplifier-core fibre. The invention further relates to a method for manufacturing an optical fibre combiner comprising at least one amplifier-core fibre and at least one pumping fibre in contact with the amplifier-core fibre for launching a side-coupled pump power in the amplifier-core fibre.
Background of the Invention
Optical fibres are widely used in different applications as guidelines of optical power to a destination. Laser devices are normally used for these applications as optical sources of high output power. The lasers can produce a kilowatt-level of an average output power. The high- power lasers are used for treatment of different kind of materials, such as metal sheets, stone, concrete, etc. The high-power laser beam can be used for drilling, cutting, shaping and soldering materials much more accurately and effectively than with devices based on mechanical treatment of material. However, there are some problems to cope with when using high-power laser devices. The powerful light of the laser has to be guided properly and with as high efficiency as possible to a destination. Otherwise the transfer losses lead to excess heating of the fibre and decrease the optical power at the output of the fibre.
At present most of the high-power fibre lasers are pumped by high- power multimode laser diodes via a cladding element (cladding fibre) surrounding a single-mode core element. The cladding element has different (lower) refractive index than the core element. The cladding element itself is also covered by another cladding layer having still lower refractive index. The signal light beam of the laser is propagating through the core element and the pumping light beam is launched into the cladding element. This type of fibres used for building high-power lasers and amplifiers are known as cladding pumped fibres. Based on the effect of total internal reflection (TIR), the pump light is guided by the pump cladding, i.e. "cladding element". The ray traces of the pump modes cross the fibre core, which contains the absorbing material used as a gain medium. While the pumping light traverses the core element of the gain fibre, it absorbs into it and provides the amplification of the lasing light beam propagating in the core element. There are some disadvantages with these kind of fibres. The coupling (launching) of the light beam of the pumping laser into the cladding element of the fibre is difficult to arrange. In principle, three different methods are known for the coupling. They are shown in Figs. 1a to 1c.
In the system of Fig. 1a the pumping light beam is coupled into the fibre at one end of the fibre. This arrangement requires that a dichroic mirror is used so that the lasing light beam is launched into the fibre by reflecting it at the dichroic mirror, while the pumping light beam is injected through the mirror into the cladding element of the fibre, or vice versa.
In the system shown in Fig. 1b, the fibre comprises a separate pump guiding fibre for the coupling of the pumping light. The pumping light beam is injected into the pump fibre from the pumping laser source. The pump fibre directs the pumping light beam to the cladding element of the gain-core fibre via special multimode fibre coupler or fibre launcher. This kind of arrangement allows multipoint pumping by using multiple coupler configuration, but the manufacturing of such fibre couplers is quite complicated procedure.
In the system of Fig. 1c the fibre comprises a reflecting notch in the form of a V-groove in the cladding element. The pumping light is injected substantially perpendicularly to the notch, which reflects the pumping light beam to the cladding layer (waveguide or element). This kind of arrangement also allows multipoint pumping, but the manufacturing of the notch requires special tools and the shape of the notch has to be very accurately made such that the reflection at a correct angle with respect to the fibre axis can be achieved.
The optical pumping of the fibre amplifier can also be achieved by a so- called side-pumping technique. In this technique several bare fibres used for pumping are arranged (wound) around the amplifier-core fibre. Fig. 2a discloses such a fibre coupling technique as a side view and Fig. 2b discloses such a fibre coupling technique as a cross sectional view. The pump fibres and the amplifier-core fibre are twisted. The bundle of the pump fibres and the amplifier-core fibre are surrounded by an external low-refractive-index coating. The pump light launched into the pump fibre propagates through this fibre and gradually couples to the (amplifier) gain-core fibre because of optical contact between a pump fibre and a gain-core fibre. The launching of the pumping light beam is easier to arrange with this kind of so-called side-coupling than with the techniques shown in Figs. 1a to 1c. The efficiency of the side- pumping technique may further be improved with some off-set in the fibre core position within cladding element, i.e. when the core is not centrally placed inside the cladding. However, pump light coupling per unit length of the gain-core fibre is quite low with this technique because pump and gain-core fibres represent touching spheres (in cross-sectional view) and, therefore, the total area of the coupling surface is small. This feature sets the limit for the laser and amplifier length, which is rather long with this coupling method: long length of the fibre is needed to absorb significant fraction of the pump light. For example, the patent publication US 6,529,318 discloses a total internal reflection coupler and method for side-coupling pump light into a fibre.
Summary of the Invention
One aim of the present invention is to provide a specialty fibre and a method in which the pumping efficiency is increased, and a simple and cost effective manufacturing method using an improved side-coupling approach for the pump/signal light combiner. The invention is based on the idea that the outer shape of the pump fibres and the amplifier-core fibre are better adapted to each other to improve the coupling efficiency of the pumping light beam from the pump fibre to the amplifier-core fibre. The adaptation is performed so that the contacting interface between the amplifier-core fibre and the pump fibre is increased significantly by making the shape of their claddings to be of the reciprocal form. Shortly, to enhance the pump light outcoupling, the cross-sections of the cladding of the pump and/or gain-core fibres are shaped to increase the optical contact surface contrary to the point-to- point coupling for fibres with circularly-shaped claddings. To put it more precisely, the fibre according to the present invention is primarily characterised by that the cross-section of at least one of the fibres is made different from a circle, and that the cross-section of the pump fibres and the amplifier-core fibre are adapted to each other so that the interface between the amplifier-core fibre and the pump fibre is a surface. The method according to the present invention is primarily characterised by that the pumping light is directed by the pump fibre via a contact surface area to the amplifier-core fibre, the surface area being formed between the pump fibre and the amplifier-core fibre by forming the cross-section of at least one of the fibres different from a circle, and by adapting the cross-section of the pump fibres and the amplifier-core fibre to each other in a reciprocal way so that the interface between the amplifier-core fibre and the pump fibre is a surface. The manufacturing method according to the present invention is primarily characterised by that the cross-section of at least one of the fibres is made different from a circle, and that the cross-section of the pump fibres and the amplifier-core fibre are adapted to each other so that the interface between the amplifier-core fibre and the pump fibre is a surface.
The present invention provides significant advantages to prior art fibres and pumping methods. The cross sections of the amplifier-core fibre and the pump fibres suit well to each other thus increasing the surface area through which the pumping light can be coupled into the cladding of the amplifier-core fibre. Therefore, the pumping efficiency of the method and the fibre combiner according to the present invention is higher than in prior art. The losses of the fibre are also reduced which reduces inter alia the excess heat generation in the fibre, and also the output power can be increased and/or the power of the pumping lasers can be reduced. The number of pumping fibres and, therefore, the output power are scalable to a desired value. The doping level of the amplifier-core fibre can be arbitrary high. The fibre according to the present invention is relatively easy to manufacture by shaping the fibre preform before fibre drawing process. Owing to a high pumping efficiency of the gain-core fibre and, therefore, an increased rate of pump absorption per unit length of the fibre, highly-doped short-length fibre devices can be designed using proposed method based on cladding-pump geometry. Additionally, thermal fusing of the fibres allows for reliable and environmentally stable pump/signal combiner.
Description of the Drawings
In the following, the present invention will be described in more detail with reference to the attached drawings, in which
Figs. 1a to 1c depict some prior art pump light coupling methods for the fibre,
Fig. 2a depicts as a side view a fibre combiner in which the side- pumping technique is used,
Fig. 2b depicts the fibre combiner of Fig. 2a as a cross sectional view,
Fig. 3a depicts a fibre combiner according to an advantageous embodiment of the present invention as a cross-sectional view,
Fig. 3b depicts the cross sections of the amplifier-core fibre and the pumping fibres of the fibre combiner of Fig. 3a, Fig. 4a depicts a fibre combiner according to an advantageous embodiment of the present invention as a cross-sectional view,
Fig. 4b depicts the cross sections of the amplifier-core fibre and the pumping fibres of the fibre combiner of Fig. 4a,
Fig. 5a depicts a fibre combiner according to an advantageous embodiment of the present invention as a cross-sectional view,
Fig. 5b depicts the cross sections of the amplifier-core fibre and the pumping fibres of the fibre combiner of Fig. 5a,
Fig. 6a depicts a fibre combiner according to an advantageous embodiment of the present invention as a cross-sectional view,
Fig. 6b depicts the cross sections of the amplifier-core fibre and the pumping fibres of the fibre combiner of Fig. 6a,
Fig. 7 depicts a fibre combiner according to an advantageous embodiment of the present invention as a side view, and
Figs. 8a and 8b depict a manufacturing method according to an advantageous embodiment of the present invention as a simplified diagram.
Figs. 9a and 9b depict the cross sections of the fibre combiner that is based on circularly-shaped fibres before thermal fusing and after strong thermal fusing that resulted in significant change in the fibre shapes.
Figs. 10a and 10b depict the configurations based on series and parallel connections of the combiners to scale the output power of the amplifier or laser to a higher value. Detailed Description of the Invention
In Fig. 3a there is a cross section of a fibre combiner 1 according to an advantageous embodiment of the present invention. The fibre combiner 1 comprises an amplifier-core fibre 2 (gain fibre) and at least one pump fibre 3 (pump guide). Typically the fibre combiner 1 comprises more than one pump fibre 3. In the fibre combiner 1 each pump fibre 3 is placed in contact with the amplifier-core fibre 2. In this description the contacting region between the amplifier-core fibre 2 and any of the pump fibre 3 is called as a coupling interface 4. According to the invention, the cross section of the amplifier-core fibre 2, the cross section of the pump fibres 3, and/or the cross section of both the amplifier-core fibre 2 and the pump fibres 3 are formed different from a circular shape. In addition to that, the cross section of the amplifier- core fibre 2 and the cross sections of the pump fibres 3 are adapted to each other so that the coupling interface 4 forms an enlarged-area surface. If a cross-section of the fibre combiner 1 at a location, in which the amplifier-core fibre 2 and at least one of the pump fibres 3 are in contact with each other, is considered in more detail, it can be seen that the form of the contacting region (i.e. the cross-section of the coupling interface 4 at that location) is a curve. The curve can be a straight line, a curved line, such as an arc, a line having one or more angles, etc. There are different alternatives to form a fibre combiner 1 having the above mentioned inventive properties. In Figs. 3a, 4a, 5a and 6a there are some advantageous embodiments of the fibre combiner 1 of the present invention depicted as a cross-sectional view. Figs. 3b, 4b, 5b and 6b depict the cross-sections of the amplifier- core fibre 2 and the pump fibres 3 of the fibre combiners 1 of Figs. 3a, 4a, 5a and 6a, respectively.
In the embodiment of Fig. 3a, the cross-section of the amplifier-core fibre 2 is formed as a circle. The cross-section of the pump fibres 3 is formed as a circle from which a part is removed. What remains after the removal (at the modified section of the pump fibre 3) is a curve having a radius of curvature substantially equal to the radius of the amplifier-core fibre 2. Hence, the split section 3.1 (Fig. 3b) of the pump fibre 3 is a curved plane and it is arranged to be placed towards the amplifier-core fibre 2.
In the embodiment of Fig. 4a, the cross-section of all the pump fibres 3 are formed as a circle, and the cross-section of the amplifier-core fibre 2 is formed as a circle from which multiple substantially equal parts are removed. The number of modified sections is equal to the number of pump fibres 3 of the fibre combiner 1. What remains after the removal at each modified section of the amplifier-core fibre 2 is a set of curves having a radius of curvature substantially equal to the radius of the pump fibre 3. Therefore, the modified (split) sections of the gain-core fibre are reciprocal to the shape of the pump fibres. Hence, the split sections 2.1 (Fig. 4b) of the amplifier-core fibre 2 are curved surfaces towards which the pump fibres 3 are to be placed.
In the embodiments of Figs. 5a and 6a, the cross-section of the amplifier-core fibre 2 is formed as a polygon (pentagon in Fig. 5a and hexagon in Fig. 6a). The cross-section of the pump fibres 3 is formed as a split circle. It can be formed, e.g. by cutting a slice out of a circular fibre preform. Hence, the split part 3.1 (Figs. 5b, 6b) of the pump fibre 3 is a plane and it is arranged to be placed on one side 2.1 (Figs. 5b, 6b) of the polygon. The number of the sides of the polygon is equal to the number of pump fibres 3 in the fibre combiner 1.
The above described embodiments of the fibre combiner 1 of the present invention are just non-restrictive examples. It is obvious that other shapes for the cross-sections of the fibres 2, 3 can be different from the above presented examples, and the number of pump fibres 3 can be different than 5 or 6.
One advantage of the present invention is that different number of pump fibres 3 can be used in different embodiments. Therefore, the output power can be scaled to a desired value by inter alia changing the number of pump fibres 3. The radius of the pump fibres 3 and/or the amplifier-core fibre 2 can also be modified to adapt the optical properties of the fibre combiner 1 to a specific device or application. In the following, an advantageous embodiment for manufacturing the fibre combiner 1 according to the present invention will be described in more detail with reference to Figs. 8a and 8b. The manufacturing process is a complex, multiphase process, so only the main details necessary for understanding the manufacturing method will be described. A first preform 6 for the amplifier-core fibre 2 and a second preform 7 for the pump fibres 3 are fabricated by one of the techniques for preform fabrication, e.g. flame hydrolysis or chemical vapour deposition. The materials for the preforms 6, 7 are selected according to the desired properties of the fibres 2, 3. The first preform 6 advantageously comprises a core element 6.1 and a cladding element 6.2. The core element 6.1 is e.g. a silica which is doped with one or more suitable dopants, such as rear-earth ions (for example, with erbium, ytterbium, neodymium, etc.) and refractive index-rising element, such as germanium. The cladding element 6.2 may also be doped with refractive index-rising dopant, such as germanium. The refractive index of the cladding element is lower than the refractive index of the core element, thus forming the fibre wavegude for the signal light. The waveguide of the gain-core fibre could be a single- mode waveguide, when high-quality of the beam at the output of the device is required for specific application. The second preform 7 is preferably made of pure silica. The pump waveguides are multimode that allow for efficient launching of the light from high-power broad-area semiconductor pump diodes. The refractive index of a cladding of a gain-core fibre could be higher than the refractive index of the pump fibre, thus further enhancement of the power transfer from pump fibre to the gain-core fibre can be achieved.
After the preform fabrication, the preforms 6, 7 are cylindrical pieces of structural glass with circular cross-section. Then, according to the present invention, the shape of at least one of the preforms 6, 7 will be modified to achieve the desired shape of the outer surface. For example, if the cross-section of the amplifier-core fibre 2 differs from the circle, the first preform 6 is shaped accordingly. Respectively, if the cross-section of the pump fibre 3 differs from the circle, the second preform 7 will be shaped so that the cross-section of the preform 7 after the shaping process is substantially similar to the cross-section of the pump fibre 3. As described above, the figures 3b, 4b, 5b and 6b depict some advantageous embodiments of the fibres, wherein the preforms 6, 7 will be shaped to have substantially similar shapes or reciprocal shapes.
The shaping of the preforms 6, 7 can be done mechanically, e.g. by grinding or cutting, depending on inter alia the desired shape for the preforms 6, 7. After the shaping of the preforms 6, 7 is performed, the processing continues with fibre 2, 3 drawing from the preforms 6, 7. In the well-established drawing process, the preform 6, 7 is re-drawn into a fibre 2, 3 having a desired diameter, which is much smaller than the diameter of the preform. It is important to mention, that in the drawing process the cross-section of the fibre 2, 3 remains substantially the same as the cross-section of the (shaped) preform 6, 7. It should be noted here that the cross-section of all the pump fibres is preferably the same, wherein all the pump fibres 3 can be formed from the same second preform 7 by cutting the drawn fibre into pieces with required lengths. The amplifier-core fibre 2 is also cut providing the desired length.
After the drawing and cutting the fibres 2, 3, pump and gain-core fibres will be combined together (Fig. 8b). In the combining phase, an assembly template 8 can be used to arrange the fibres into the proper configuration. The assembly template 8 comprises a hole 8.1 , 8.2 for each fibre 2, 3. The amplifier-core fibre 2 is directed through the centre hole 8.1 , and each pump fibre 3 is directed through one outer hole 8.2. The assembly template 8 guides the amplifier-core fibre 2 and the pump fibres 3 towards each other so that they become contacted at a right position. The combination (bundle) of the fibres 2, 3 is then directed through a first aperture 9 to e.g. a heating zone 10, and output from the heating zone through a second aperture 11. During the heating process the bundle of the fibres 2, 3 is heated so that the fibres 2, 3 become fused together and are monolithically stable. It is obvious that also other methods can be used to fuse the fibres 2, 3 together.
Under certain fusion conditions, the fibre shape could also be changed during thermal fusing process. The physical mechanism of this change in the fibre shape is owing to the forces of surface tension in the viscous melted glass. As it is well-known from the physics of liquids, these forces tend to minimise the total area of the surface of the viscous liquid substance. In fact, this happens when fabricating so- called well-fused biconical couplers.
Based on this phenomenon, the fibre combiner 1 of this invention using specially and reciprocally shaped amplifier-core fibre 2 and pump fibre 3 to increase the coupling surface 4, can also be made, in principle, without shaping the preforms prior to fibre drawing. The required shaping of the fibres can be obtained from initially circularly-shaped fibres 2 and 3 by sufficient fusing of the arranged assembly of the fibre combiner 1 in high-temperature zone. The temperature treatment in this procedure is basically similar to that used for fabrication of well- fused biconical fibre couplers. To obtain the required shape of the fibre combiner cross-section, the assembled fibres 2 and 3 with circular shape, two non-restrictive examples shown in Fig. 9a, should be strongly fused together within heating zone, where the shape of each fibre can be changed creating substantially monolithic glass structure, two non-restrictive examples shown in Fig. 9b. With this technique, the fusing temperature should be high enough to achieve the "collapse" of the solitary fibres in the assembled bundle and to create monolithic glass structure.
Compared to the method that uses fibre pre-shaping before thermal fusing of the assembled combiner and that requires relatively little temperature treatment just for fixing the fibre assembly, the strong thermal fusing of the circularly-shaped fibres is used primarily for changing the shapes of the fibres to obtain well-fused and well-coupled fibres.
The advantage of this method is clearly in simplified fibre manufacturing procedure. Main technological concerns, however, are shifted in this method to the stage of arranging of fibre assembly from circularly-shaped fibres and following fusing them in the high- temperature. Therefore, strong fusion method requires precise temperature control to achieve strong fibre fusing over the long length with low excess losses.
It should also be noted that contrary to the coupling technique that uses multimode couplers, shown in Fig. 1 b, in this invention a distributed side-coupling that exploits the fusion of the amplifier-core fibre and pump fibres over the whole (or considerable) length of the fibre combiner is presented. The fusing strength of the fibres and, consequently, the coupling efficiency between amplifier-core fibre and pump fibres can be conveniently adjusted depending on the optimised (desired) length and doping level of the fibre laser or amplifier.
The bundle of the amplifier-core fibre 2 and the pump fibres 3 can be surrounded by a cover 12 for protecting the fibre combiner 1 against environmental effects. The cover 12 can be formed, for example, of a low diffractive index (e.g. plastic) tube. The material of the cover 12 can be such a material which decreases its dimensions while heated. This allows the cover 12 to tighten the bundle of the fibres and prevent the penetration of undesirable substances. It is also possible that the bundle of the fibres is covered by a low-index plastic gel before placing it into the cover 12.
Fig. 7 depicts an advantageous embodiment of the fibre combiner 1 of the present invention as a side view. In Fig. 7 elements 13 are also shown that needed to accomplish with the amplifier-core fibre 2 a fibre laser or amplifier, e.g. signal source, optical isolators, laser mirrors, etc. A pumping laser 14 delivers the pumping power to the fibre combiner 1. The light beam of the pumping laser(s) 14 can be coupled to the fibre combiner 1 by using one or more pump fibres 3 depending on the number of pump lasers needed to achieve the required level of output power of the fibre laser or amplifier. For example, each pump laser 14 can be coupled to one pump fibre 3. The pump laser 14 in the form of a laser array (not shown) can be used for optical pumping of the fibre combiner 1. In this case many of the pump fibres 3 can be coupled to the laser array so that one pumping element of the laser array is coupled to one pump fibre 3. Furthermore, it is possible that the pump fibres 3 are pumped from both ends. The element 13 providing the signal light for the fibre combiner 1 based optical amplifier is coupled at one end of the amplifier-core fibre 2.
The element 13 providing the optical feedback signal for the fibre combiner 1 based laser is coupled at one end of the amplifier-core fibre 2.
The element 13 provides an optical isolation from external reflections for the fibre combiner 1.
The further power scaling of the fibre devices based on the proposed combiner can be achieved by the concatenating the above described combiner in series, as shown in Fig. 10a, or in parallel, as shown in Fig. 10b. The latter configuration (Fig. 10b) could, however, result in some penalties to the output beam quality.
Although the above described embodiments disclosed a fibre combiner 1 including one amplifier-core fibre 2, it is obvious that the fibre combiner 1 can also include more than one amplifier-core fibres 2.
It is obvious for anyone skilled in the art that the present invention is not limited solely to the embodiments presented above but it may vary within the scope of the claims.

Claims

Claims:
1. An optical fibre combiner (1 ) comprising at least one amplifier-core fibre (2), and at least one pump fibre (3) in contact with the amplifier- core fibre (2) for providing a side-coupled pump power into the amplifier-core fibre, characterised in that the cross-section of at least one of the fibres (2, 3) is made different from a circle, and that the cross-section of the pump fibres (3) and the amplifier-core fibre (2) are adapted to each other so that the interface (4) between the amplifier- core fibre (2) and the pump fibre (3) is a surface.
2. The optical fibre combiner (1 ) according to claim 1 , characterised in that the cross-section of the amplifier-core fibre (2) is substantially a circle, and that the cross-section of the pump fibres (3) is made different from a circle.
3. The optical fibre combiner (1 ) according to claim 2, characterised in that the cross-section of the pump fibres (3) is a circle from which a part is removed, and that the edge of the pump fibre (3) at the modified part is a curve (3.2) having a radius of curvature substantially equal to the radius of the amplifier-core fibre (2).
4. The optical fibre combiner (1 ) according to claim 1 , characterised in that the cross-section of the pump fibres (3) is substantially a circle, and that the cross-section of the amplifier-core fibre (2) is made different from a circle.
5. The optical fibre combiner (1 ) according to claim 4, characterised in that the cross-section of the amplifier-core fibre (2) is a circle from which number of parts equal to the number of pump fibres (3) in the fibre combiner (1 ) is removed, and that the edge of the amplifier-core fibre (2) at the modified parts is a curve (2.2) having a radius of curvature substantially equal to the radius of the pump fibres (3).
6. The optical fibre combiner (1 ) according to claim 1 , characterised in that the cross-section of both the amplifier-core fibre (2) and the pump fibres (3) are made different from a circle.
7. The optical fibre combiner (1 ) according to claim 6, characterised in that the cross-section of the amplifier-core fibre (2) is a polygon, that the pump fibres (3) are made by cutting a slice out of a circular fibre, therein a planar surface is formed on the fibre, the planar surface being arranged to be placed on one side (2.1 ) of the amplifier-core fibre (2).
8. The optical fibre combiner (1 ) according to claim 1 , characterised in that it includes pump fibres (3) and a amplifier-core fibre (2), the fibre assembly (2, 3) being subjected to a thermal fusing process, wherein the pump fibres (3) are fused together and with the amplifier-core fibre (2) creating substantially monolithic glass structure around the amplifier-core fibre (2).
9. A method of launching pump power into an optical fibre combiner (1 ), the fibre comprising at least one amplifier-core fibre (2) and at least one pump fibre (3) in contact with the amplifier-core fibre (2) for launching a side-coupled pump power into the amplifier-core fibre (2), characterised in that the pumping light is directed by the pump fibre (3) via a contact surface area to the amplifier-core fibre, the contact surface area being formed between the pump fibre (3) and the amplifier-core fibre by forming the cross-section of at least one of the fibres (2, 3) different from a circle, and by adapting the cross-section of the pump fibres (3) and the amplifier-core fibre to each other in a reciprocal way so that the interface between the amplifier-core fibre and the pump fibre (3) is a surface.
10. The method according to the claim 9, characterised in that the pumping light is coupled at one end of one pump fibre (3).
11. A method for manufacturing an optical fibre combiner (1 ) comprising at least one amplifier-core fibre (2) and at least one pumping fibre (3) in contact with the amplifier-core fibre (2) for launching a side-coupled pump power into the amplifier-core fibre (2), characterised in that the cross-section of at least one of the fibres (2, 3) is made different from a circle, and that the cross-section of the pump fibres (3) and the amplifier-core fibre (2) are adapted to each other so that the interface between the amplifier-core fibre (2) and the pump fibre (3) is a surface.
12. A method according to claim 11 , characterised in that the fibres (2, 3) are made of preforms (6, 7) by drawing, and that the preforms of those fibres for which the targeted shape differs from a circle are shaped before the drawing.
13. A method according to claim 12, characterised in that the drawn fibres (2, 3) are arranged in a bundle, and that the bundle is treated by heating to fuse the pump fibres (3) with the amplifier-core fibre (2).
14. A method according to claim 11 , characterised in that the fibres (2, 3) are made of preforms (6, 7) with circular shape of the cross-section by drawing, and that a thermal fusing process is used after the drawing, during the assembling of the fibre combiner (1 ) from the fibres (2, 3) to achieve the targeted shape of the fibres (2, 3) and the required cross section of the fibre combiner (1 ).
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