WO2010149163A1

WO2010149163A1 - Optical coupler device

Info

Publication number: WO2010149163A1
Application number: PCT/DK2010/050151
Authority: WO
Inventors: Søren AGGER; Thomas Nikolajsen
Original assignee: Nkt Photonics A/S
Priority date: 2009-06-21
Filing date: 2010-06-21
Publication date: 2010-12-29

Abstract

The present invention relates to an optical component comprising an acceptance fibre, e.g. a photonic crystal fibre, for propagation of pump and signal light, at least one pump delivery fibre and an optical assembly, e.g. a reflector element, that directs pump light from the pump delivery fibre into the accep-tance fibre. The pump delivery fibre furthermore has a guiding structure (3012, 3013) embedded within the pump core (3014) in order to collect light leaking into the fibre, thereby avoiding damage to the pump source. It is an object of the invention to provide a fibre coupler for coupling at least one light source into a multi-clad (e.g. double clad) optical fibre, which has practical advantages with respect to handling, loss and back reflection.

Description

OPTICAL COUPLER DEVICES, METHODS OF THEIR PRODUCTION AND

USE

The present invention relates in general to coupling of light from one or more input waveguides to an output waveguide or output section of a waveguide having other physical dimensions and/or optical properties than the input waveguide or waveguides.

The present invention relates to an optical component comprising an acceptance fibre, e.g. a photonic crystal fibre, for propagation of pump and signal light, at least one pump delivery fibre and a reflector element that reflects pump light from the at least one pump delivery fibre into the acceptance fibre. The invention further relates to methods of producing the optical component, and articles comprising the optical component, and to the use of the optical component. The invention further relates to a rod-type optical fibre. The present invention is based on properties of multi-clad, such as double clad, optical fibres with relatively high numerical aperture, such as e.g. photonic crystal fibres (PCF).

The invention may e.g. be useful in applications such as fibre lasers or amplifiers, where light can be coupled efficiently from one or more pump sources to an acceptance fibre, e.g. a double clad fibre, using the optical component. The invention specifically addresses optical fibre amplifiers where pump light and signal light are propagating in different directions (counter-propagating pump) within a double-clad optical fibre.

Optical fibres are today used in numerous applications that span very diverse fields of optics. These fields include telecommunications, medicine, sensors, lasers, amplifiers and many others.

About 10 years ago, a new family of optical fibres appeared, called double clad fibres (also known as double cladding fibres). Such fibres receive a large interest due to their potential for use in high power amplifiers and lasers. They consist of two waveguides embedded into each other; an inner and an outer guiding region. The inner guiding region may be a single mode core for guiding signal light, whereas the outer region may be a multi mode core, also called inner cladding (or pump core), for guiding pump light.

The term 'double clad' or 'double cladding' optical fibre is in the present context taken to refer to an optical fibre comprising at least two cladding regions extending in a longitudinal direction of the optical fibre, at least one of which may be used for propagating light, e.g. pump light, this cladding region therefore is also termed 'a pump core'. The term is NOT intended to exclude the use of optical fibres comprising more than two such cladding regions. Different cladding regions are e.g. differentiated by different optical properties (such as refractive indices) of their background materials, a cladding region comprising micro-structural elements differing from a cladding region NOT comprising any, cladding regions comprising different micro-structural elements differing from each other (the micro-structural elements of the respective cladding regions differing in any property having an influence on the propagation of light at the appropriate wavelength, e.g. by a different size of the micro-structural elements (if not interspersed), by different materials of the micro-structural elements (e.g. voids, solid or liquid), regularly arranged vs. irregularly arranged, etc.), etc.

One use for double cladding fibres is to efficiently convert low quality, low brightness light from e.g. semiconductor lasers (lasers providing pump light) to high quality, high brightness light (signal light). This can be done for both laser and amplifier configurations. For laser configurations the signal light is generated through stimulated emission and within a cavity (which may be formed from fibre Bragg gratings and/or external mirrors). For amplifier configurations, a seed signal is coupled to the single mode core and amplified through stimulated emission.

Brightness is defined as optical power per solid angle per unit area, also termed luminance and measured in the Sl-units of Candela/m² or W/steradian/m². For multi mode fibres, conservation of brightness means that the NA multiplied with the waveguide diameter is a constant before and after the coupling/conversion. The brightness conversion can be implemented by doping the core with an optically active material, e.g. a rare earth dopant and pumping this with pump light, e.g. multi mode light. The rare earth atoms will absorb the pump light and re-emit the energy at lower photon energies. Since the emission will happen through stimulated emission, this light will be guided in the doped core. A single mode operation is may be used, but multi-mode operation is also relevant.

This conversion method can be very efficient (up to around 80 %) and the brightness can be improved by more than a factor of 100. Such light sources are often used as popular alternatives to high brightness solid state lasers, since they are less bulky and far more efficient.

Double clad fibres can be provided in various types (micro-structured as well as non-micro-structured fibres) that are all relevant to the present invention. These types include all-glass fibres (see e.g. Wang et al., Electronics Letters, Vol. 40, No. 10, 2004), polymer clad fibres (see e.g. Martinez-Rios et al., Optics Letters, Vol. 28, No. 18, 2003) and photonic crystal fibres (see e.g. WO 03/019257)

Photonic crystal fibres (PCFs) have recently emerged as an attractive class of fibres, where various properties may be tailored in new or improved manners compared to conventional (solid, non-micro-structured) optical fibres. PCFs are generally described by Bjarklev, Broeng, and Bjarklev in "Photonic crystal fibres", Kluwer Academic Press, 2003. The fabrication of PCFs is e.g. described in chapter IV, pp. 115-130.

In recent years, PCFs have been developed to also show double cladding features. Here, a ring of closely spaced air holes (air-clad) will define the multi mode inner cladding. Fibres with air-cladding and their fabrication are e.g. described in US-5,907,652 and WO 03/019257 that are incorporated herein by reference. The Numerical Aperture (NA) of PCFs can take values from below 0.2 all the way up to more than 0.8. In some embodiments the value lie around 0.6. A common problem in fibre optics is to launch light into a fibre efficiently. Often the source of light and the fibre to couple into have different divergence angles (numerical aperture (NA)) and spot/core sizes. A specific problem is to launch light from a pump-diode-laser with a large spot size and relatively low numerical aperture into a double clad fibre laser with a small area and large numerical aperture.

The traditional method of solving this problem is to use bulk optics. An example can be seen in Fig. 1 , where pump light from a single source, for example a fibre 10 delivering a pump light, is to be coupled into a single end of a PCF 11 (a PCF chosen only as an example of a double clad fibre). The first (slow) lens 12 collimates the light 13 from the pump fibre, whereas the second (fast) lens 14 focuses the light into the inner cladding of the PCF. This approach has the disadvantage such a solution may have only a coupling efficiency of 80-90 % , has h igh reflections, is sensitive to mechanical drift and instability and sensitive to contamination. Finally, such solution makes packaging design for a commercial device complicated and expensive.

The solution of bulk optics has a number of problems. One problem is related to difficulties in achieving coupling with low loss. Another problem is to achieve good coupling for a wide range of wavelengths. A third problem is mechanical stability. Fabrication of devices using bulk optics is also relatively complicated. Furthermore, reflection from the multiple glass surfaces may degrade performance of the system.

In order to couple light from multiple pump lasers to a double clad fibre, a common approach is to use a coupler known as a so-called tapered fibre bundle (also known as fused, tapered fibre bundles). Such couplers have been developed by a number of optical component supplier companies, such as ITF, SIFAM, OFS, JDSU and Nufern - and are described in for example US-5,864,644 or in US-5,935,288.

An example of a tapered fibre bundle is shown in Fig. 2. Several fibres 20 are bundled together and heated to temperatures near melting and tapered 21 . Using a taper, light from each fibre that delivers pump light (pump fibre that may support an NA between 0.15 and 0.22) will merge and as the fused region tapers down in dimensions, the NA slowly (adiabatically) increases (which may up to around 0.45 or even higher). The tapered region may be surrounded directly by air - resulting in an unprotected silica-glass interface. The fused, tapered end of the coupler may be spliced to a double clad fibre.

The problem with fused, tapered fibre bundles is that it is difficult to couple pump light efficiently into a high NA double clad fibre (NA higher than 0.3). It is thus an object of the invention to provide a fibre coupler for coupling one or more light sources into a multi-clad (e.g. double clad) optical fibre, the coupler being improved with respect to the prior art fibre couplers. It is a further object to provide a fibre coupler which is improved with respect to low loss.

A further problem of fused, tapered fibre bundles is that it is difficult to package these, since the tapered region comprises an uncoated waveguide region. This region may be solid glass surrounded by air (the waveguide structure for the pump light in the tapered region) that is fragile and difficult to package. It is thus an object of the invention to provide a component for pump multiplexing that is less fragile and simpler to package.

The bundle of fibres 20 may also comprise a single mode fibre (which may be placed in the centre of the bundle of fibres 20). Such a fibre may serve for feed-through of signal light. This component is known as an all-fibre signal- pump multiplexer and may be used in fibre amplifier configurations. The single mode fibre comprises a single mode core and may be a single clad fibre. For these signal-pump multiplexers also the single mode fibre is tapered. Such signal-pump multiplexers may be used for co- or counter- propagating pump light.

A further problem of fused, tapered fibre bundles is that signal light can be reflected back into the pump delivery fibres - causing damage to the lasers that deliver the pump light. One way of reducing the amount of reflected signal light is to use the signal-pump multiplexer in a configuration, where pump and signal light is counter-propagating. However, even in such a configuration, problems have been found for commercial available signal- pump multiplexers for signal average powers levels of around 10 mW (the exact level depends on the quality of the multiplexer and the specifications of the signal light (e.g. continuous wave, pulse, pulse duration)). It is thus an object of the invention to provide a component for signal-pump multiplexing that has a low reflection of signal light into pump delivery fibres.

The objects of the invention are achieved by the invention described in the accompanying claims and as described in the following.

One object of the invention is to provide an optical component having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis is provided, the optical component comprising: a. a first optical fibre comprising a pump core having a numerical aperture NAi at a first fibre end of said first optical fibre; b. at least one second optical fibre arranged in relation to said first optical fibre, where at least one of said at least one second optical fibres comprises a pump core which at a second fiber end has a numerical aperture NA₂ that is smaller than NAi; c. a reflector element comprising an end-facet with a predetermined profile for reflecting light from said second fibre end of at least one second optical fiber into said pump core of said first fibre.

One object of the invention is to provide a multi-core optical fiber having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis. The multi-core optical fiber comprises a first pump core being adapted to guide a pump signal at a pump wavelength in at least one first pump mode, said first pump core having at said pump wavelength a numerical aperture NAi at a first fiber end of said multi-core optical fiber; a first outer cladding surrounding said first pump core, and at second pump core having at said pump wavelength a numerical aperture NA₂ at said first fiber end of said multi-core optical fiber, where NA₂ is smaller than NAi. The first pump mode of said first pump core has substantially no modal overlap with said second pump core at a pump wavelength λ_p in the range of 300nm to 2500nm. One object of the invention is to provide an optical device comprising a multi- core optical fiber according to the present invention, and an end-cap arranged in relation to the first end of said multi-core optical fiber, wherein said end cap is arranged in contact with said first end of said fiber such that pump signals emerging from said second pump core is coupled into said end cap when emerging at said first end of said multi-core optical fiber.

One object of the invention is to provide an optical component comprising a multi-core optical fiber according to any of claims 76 to 105, or an optical device according to claims 106 to 111 , and a reflector element comprising an end-facet with a predetermined profile for reflecting light from said second pump core at said first fiber end into said first pump core.

In an imaging pump coupling configuration, there may always be a leak of the signal light path to the pump light path due to imperfect isolation. That is, a fraction of a signal guided in the core region of a first optical fiber may be coupled into the pump core of a second optical fiber arranged to deliver pump power to a pump core of the first optical fiber. Semiconductor diodes without protection may suffer catastrophic damage/facet damage by too high signal intensity/energy coming from such a leaking signal light.

The inventors have realized that a method of mitigating this is to use a signal leak light waveguide to route the leaking signals away from the diodes. That is a guiding structure may be arranged to collect that leaking signal thereby lowering the signal power that otherwise leaks to the pump light sources. This leak light waveguide may also be used as a signal light monitor signal.

One object of the present invention is to provide an optical component having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis, the optical component comprising a first optical fiber, a at least one second optical fiber, and an optical assembly. Said first optical fiber comprising a first fiber end, a pump core with a first numerical aperture at said first fiber end, and a core region different from said pump core, said core region being adapted for propagating light at a signal wavelength, λ_s The at least one second optical fiber comprising a first fiber end and a second fiber end, said at least one second optical fiber being arranged in relation to said pump core of said first fiber. The at least one second fiber comprising a pump core with a second numerical aperture at its second fiber end that is smaller than the first numerical aperture. The second optical fiber comprising a guiding structure embedded within the pump core, said guiding structure extending from said second end to said first end of said second optical fiber and being capable of propagating light at a first wavelength, λi. The optical assembly being arranged to direct pump light from said second fiber end of said second optical fiber into said pump core of said first optical fiber

One object of the present invention is to provide an optical system for launching pump light into a pump core of a first optical fiber, said optical system comprising an optical component, a plurality of pump light sources, and an optical combiner. The optical component comprising a second optical fiber comprising a second optical fiber first end, a pump core and a guiding structure embedded in said pump core. The optical combiner comprising an input section, an intermediate section and an output section. The input section comprising a least one monitor optical fiber and at least one input optical fiber for connecting the optical combiner to said pump light sources. In the intermediate section, the input optical fiber and the monitor optical fiber are bundled. The output section comprises an output end, at which a monitor region of the bundle corresponding to said monitor fiber is substantially optically decoupled from an input region corresponding to the input optical fibers, such that an optical signal collected by the monitor region at the output end is guided through said combiner substantially without transfer of optical power to the input fiber. The output end is arranged in optical connection to the second optical fiber first end in such a way that at least a part of a signal propagating in said guiding structure is coupled into the monitor region, and such that pump light guided in said input fibers is delivered to said pump core of said second optical fiber via said input region

One object of the present invention is to provide an optical system for launching pump light into a pump core of a first optical fiber, said optical system comprising an optical component, a plurality of pump light sources, and an optical combiner. The optical component comprising a second optical fiber comprising a second optical fiber first end, a pump core and a guiding structure embedded in said pump core The optical combiner comprising an input section comprising a plurality of input optical fibers for connecting the optical combiner to said pump light sources; an intermediate section over which the input optical fibers are bundled; and an output section comprising an output end and an input region corresponding to the input optical fibers. The output end being arranged in optical connection to said second optical fiber first end in such a way that pump light guided in said input fibers is delivered to said pump core of said second optical fiber via said input region, and such that less than about 50% of the optical power of a signal propagating in said guiding structure of said optical component is coupled into the input region.

In an embodiment the optical component comprises a plurality of second optical fibres and at least some (e.g. all) of the number of second fibres surround the first fibre.

In an embodiment, NA₂ is that is smaller than NAi. This has the advantage of optimizing the amount of light that can be reflected into the pump core of the acceptance fibre.

In an embodiment, the optical axis of the optical component substantially coincides with the optical axis of the first optical fibre. In an embodiment the optical axis of the first optical fibre substantially coincides with the optical axis of at least one, such as a majority, such as all or the second optical fibres. In an embodiment, the angle between the optical axes of the first and second optical fibres is less than 5°, such as less than 2°, such as less than 1 °, such as less than 0.5°, such as less than 0.2°, such as less than 0.1 °.

In an embodiment, an end-facet of the reflector element is rotation symmetric around an axis which is offset relative of the optical axis of the optical component. The offset axis may be parallel to the optical axis of the optical component and it may be offset by a distance in the range of about 0.1 to about 3.0 times the outer diameter of the first optical fiber, such as in the range of about 0.2 to about 2.5 times the outer diameter of the first optical fiber , in the range of about 0.3 to about 2.0 times the outer diameter of the first optical fiber, in the range of about 0.5 to about 1.5 times the outer diameter of the first optical fiber, in the range of about 0.8 to about 1.2 times the outer diameter of the first optical fiber. The predetermined profile of an end-facet of the reflector element provides reflection of pump light from the pump core of at least one of the second fibres into the pump core of the first fibre, whereby coupling of pump light from the second fibres to the first fibre is obtained. The predetermined profile of an end-facet of the reflector element can minimize unintentional reflection of light (e.g. signal light) from the first fibre into the (pump core) of the second fibre(s). An optical component according to the invention need no tapering and splicing of pump fibres.

In an embodiment, the reflector element faces the first and second fibre ends. In an embodiment, an end-facet of the reflector element faces the first and second fibre ends.

The term 'a pump core' is in the present context taken to mean a region of an optical fibre suitable for propagating light at a pump wavelength λ_p, the pump light being suitable for pumping an optically active material in a fibre to bring an electron of the optically active material in an excited state from which it may decay to a lower state by the excitation of light. In the present application a 'pump core' is present in the 'first' as well as in the 'second' optical fibre. In the 'first' optical fibre (also termed 'acceptance fibre') the pump core may be a region surrounding a central region of the optical fibre, a cladding region may surround a core region (where the core region which may comprise the optically active material). In the 'second' optical fibre (also termed 'pump fibre') the pump core is a region of the fibre adapted for propagating pump light, e.g. a core region of a multimode fibre.

In an embodiment, the reflector element has a first end-facet facing said first end of the first optical fibre and said second ends of said at least one second optical fibre, and said first and/or said second end-facet has/have a predetermined profile.

In an embodiment, the reflector element comprises first and second opposing end facets.

The term 'a reflector element having first and second opposing end facets' is in the present context taken to mean that the first and second facets are located relative to each other so that when the reflector element is positioned in the optical component with its first end facet facing the first and second fibre ends thereby intersecting the optical axis of the first fibre, the optical axis will also intersect the second facet of the reflector element (if the optical axis is continued from the first end facet towards the second end facet).

It is to be understood that the pump light from the second fibres may be reflected from either one of the first and second end facets of the reflector element or from both.

In an embodiment, said end-facet reflects a predetermined fraction of light from said second fibre end(s). In an embodiment said end-facet reflects a predetermined fraction of light from said first fibre end.

In an embodiment, the pump core of the first and at least one of the second optical fibres are adapted for propagating pump light at a pump wavelength

In an embodiment, an end-facet or at least a part of the end-facet of the reflector element is adapted to reflect light at the pump wavelength λ_p.

In an embodiment, the first fibre comprises a core region different from said pump core, the core region being adapted for propagating light at a signal wavelength λ_s different from said pump wavelength λ_p.

In an embodiment, an end-facet or at least a part of the end-facet of the reflector element is adapted to reflect at least a fraction of light at the signal wavelength λ_s. In an embodiment, an end-facet or at least a part of the end- facet of the reflector element is adapted to transmit at least a fraction of light at the signal wavelength λ_s. In an embodiment, an end-facet of the reflector element is adapted to reflect light at said signal wavelength λ_s and said pump wavelength λ_p differently.

In an embodiment, different partial areas of an end-facet of the reflector element are adapted to reflect light at the signal wavelength λ_s and said pump wavelength λ_p differently. In an embodiment, an end-facet of the reflector element has a reflectivity in an area around the central optical axis of the first optical fibre to allow propagation of a predetermined fraction of light (e.g. amplified light) from the first optical fibre.

In a particular embodiment, at least a part of an end-facet of the reflector element has a coating for increased reflection of pump l ight. In an embodiment, an end-facet or at least a part of the end-facet of the reflector element has an anti-reflective (AR) coating for minimizing reflection of light at the pump wavelength λ_p. In an embodiment, a first end-facet or at least a part of the first end-facet has an anti-reflective (AR) coating for minimizing reflection of light at the pump wavelength λ_p and a second end-facet or at least a part of the second end-facet has a coating for increased reflection of light at the pump wavelength λ_p.

In a particular embodiment, a majority of the area of an end-facet of the reflector element has a coating for increased reflection of pump light.

In a particular embodiment, an end-facet of the reflector element is un-coated in a region around the centre of the end-facet.

In an embodiment an end-facet of the reflector element has a coating with higher reflective coefficient for pump light at a wavelength λ_p than for signal light at a wavelength λ_s, where λ_p is different from λ_s.

In a particular embodiment, the coating is a dielectric or a metallic coating.

In a particular embodiment, an end facet of the reflector element is adapted to focus the pump light in the pump core of the first fibre a distance L_f from the first end of the first fibre.

In a particular embodiment, the reflector element comprises a plano-convex element comprising said reflective end facet(s). In other words, the reflector element comprises a first plane end-facet facing the first and second ends of the first and second optical fibres, respectively, and a second opposing end- facet having a (partial) spherical profile. The predetermined profiles of the first and second end-facets of the reflector element are thus plane and (partial) spherical, respectively.

In a particular embodiment, an end-facet of the reflector element facing the first fibre end of the first fibre and the second fibre end(s) of the at least one second optical fibre is a curved surface formed into a bulk material.

In a particular embodiment, an end-facet of the reflector element is rotation symmetric around a longitudinal axis of the optical component.

In a particular embodiment, parts of an end-facet of the reflector element adapted to reflect pump light from the at least one second optical fibre have a spherical shape.

In a particular embodiment, the predetermined profile of an end-facet of the reflector element is adapted to provide a focal length that is substantially equal to 0.5 times the radius of the spherical shape.

In a particular embodiment, an end-facet of the reflector element is asphehcal.

In a particular embodiment, parts of an end-facet of the reflector element adapted to reflect pump light from the at least one second optical fibre have an aspherical shape.

The shape of the pump core of the first and/or second fibres may be substantially circular. Alternatively, the shape may have any other convenient form, e.g. elliptical, D-shaped, star-shaped, polygonal, etc.

In an embodiment, a majority or all of the second fibres have NA₂ < NAi.

In an embodiment comprising a plurality of second optical fibers, the numerical aperture at their second ends of the second optical fibres is equal for all second optical fibres. Alternatively, the numerical aperture at their second ends of the second optical fibres may be different for some of the second optical fibres surrounding the first optical fibre. In a particular embodiment, NAi is higher than 0.22, such as higher than 0.30, such as higher than 0.45, such as higher than 0.55, such as higher than 0.8.

In a particular embodiment, the first fibre is a double clad fibre comprising a signal core.

In a particular embodiment, the signal core comprises rare earth dopants for amplifying signal light in response to pump light in a pump core of the first fibre.

In a particular embodiment, the first fibre is an all-glass double clad fibre, a polymer-clad double clad fibre or a PCF double-clad fibre.

In a particular embodiment, the first fibre is a PCF double-clad fibre comprising a core region for propagating light at a signal wavelength, an inner cladding region - termed a pump core - surrounding the core region for propagating light at a pump wavelength and an air cladding comprising at least one ring of relatively large holes surrounding the inner cladding region.

In the context of the present invention, the phrase "air cladding" refers to a cladding comprising at least one 'layer' or ring of relatively large and relatively closely spaced holes adapted to confine light at the relevant wavelength to the (inner) cladding region surrounded by the air cladding

In a particular embodiment, the holes of the air cladding are collapsed over a length L₀ from the first end of the first fibre.

In a particular embodiment, the focus distance L_f of the pump light in the pump core of the first fibre is substantially equal to the length L₀ over which the holes of the air cladding are collapsed.

In a particular embodiment, first fibre and the second fibres are fused together over at least a part of their length. In a particular embodiment, the first fibre end and the second fibre end(s) are directly connected (e.g. but-coupled or glued of fused) to an end-facet of the reflector element.

In an embodiment, the at least one second optical fibre is located along the periphery of the first optical fibre. In an embodiment, the outer surface of the at least one second optical fibre touch the outer surface of the first optical fibre over a part of their longitudinal extension. In an embodiment, one or more intermediate layers of material is/are located between the outer surface of the first optical fibre and the outer surfaces of the at least one second optical fibre. In an embodiment, an intermediate layer has the form of an intermediate tube surrounding the first optical fibre and thus located between the first optical fibre and said at least one second optical fibre arranged in relation to said first optical fibre.

In an embodiment, the outer diameter (or largest outer cross-sectional dimension) of a second optical fibre is smaller than the corresponding dimension of the first optical fibre.

In an embodiment comprising a plurality of second optical fibres, the outer diameter (or largest outer cross-sectional dimension) of the second optical fibres is equal for all second optical fibres. Alternatively, the outer diameter (or largest outer cross-sectional dimension) of the second optical fibres may be different for some of the second optical fibres.

In an embodiment, the optical component comprises 1 second optical fibre arranged in relation to said first optical fibre. In the context the phrase "arrange in relation to" refers to the situation wherein the optical fibers that are arrangted relative to each other can coupled light between their core regions through said reflector element. In an embodiment, the first and second optical fibers are substantially parallel and separated by a distance that is shorter than the largest outer diameter of the first and second optical fibers.

In an embodiment, the number of second optical fibres is 2 or 3 or larger than or equal to 4, such as larger than or equal to 6, such as larger than or equal to 8, such as in the range from 10 to 24, such as larger than or equal to 12, such as larger than or equal to 20, such as larger than or equal to 40, such as larger than or equal to 80.

In an embodiment, the number of second optical fibres surrounding the first optical fibre is larger than the maximum number of secondary optical fibres being able to all contact the outer periphery of the first optical fibre. In an embodiment, the second optical fibres are located around the first optical fibre in one or more layers (e.g. in 2 or 3 layers). In an embodiment, the outer diameter (or largest outer cross-sectional dimension) of the second optical fibres are different from layer to layer of the second optical fibres. In an embodiment, the numerical aperture of the second optical fibres at their second ends are different from layer to layer of the second optical fibres surrounding the first optical fibre.

In an embodiment, the second optical fibres are located around the first optical fibre in a symmetric manner, i.e. so that the geometrical arrangement of first and second optical fibres when viewed in a corss section perpendicular to a longitudinal axis has some kind of symmetry, e.g. rotational symmetry (such as n-fold, n > 2) around a central axis of the first optical fibre or mirror symmetry around a plane through a central axis of the first optical fibre.

In an embodiment, the second optical fibres are located around the first optical fibre in an asymmetric manner.

In an embodiment, the second optical fibres are supported by a holding element. In an embodiment, the holding element supports the first optical fibre.

In a particular embodiment, the first fibre end and the second fibre ends are mounted in a mounting tube, whereby the first fibre and the second fibres are fixated and protected. In a particular embodiment, the first fibre end and the second fibre ends and the reflector element are mounted in a mounting tube, whereby the first fibre and the second fibres and the reflector element are fixated and protected.

In a particular embodiment, the reflector element additionally comprises one or more elements selected from the group comprising i) an optical element comprising a material, which is substantially optically transparent at a pump wavelength λ_p at least over a part of its area; ii) an optical element comprising a material, which is substantially optically transparent at a signal wavelength λ_s at least over a part of its area; iii) an optical element, which reflects at least a fraction, such as substantially all, of the light at a signal wavelength λ_s at least over a part of its area; iv) an optical element, which transmits at least a fraction, such as at least 60%, of the light at a signal wavelength λ_s at least over a part of its area; v) an optical element, which collimates light at said signal wavelength λ_s, vi) an optical element, which focuses light at said signal wavelength λ_s. wherein said one or more optical elements is/are optically coupled to said first fibre end and/or said second fibre end(s) in its assembled state.

The reflector element can be made configurable so that one or more of the optical elements can be 'easily added or removed, thereby easily adapting the optical component to various applications or requirements.

In an embodiment of the invention, the optical component is used in a fibre amplifier.

In an embodiment of the invention, the optical component is used in a fibre laser. In a particular embodiment, the laser or amplifier comprises an amplifying optical fibre comprising an optically active material, e.g. one or more rare earth elements, e.g. Yb and/or Er.

In a particular embodiment, the amplifying optical fibre is a double clad fibre, e.g. a standard fibre or a photonic crystal fibre.

In a particular embodiment, the amplifying optical fibre is optically coupled to the first optical fibre of the optical component, such as is equal to the first optical fibre of the optical component.

In a particular embodiment, the fibre laser or amplifier comprises first and second optical components according to the invention. In a particular embodiment, the two optical components share a common amplifying optical fibre.

In a particular embodiment, the first optical component comprises a mirror element with a h igh reflector and the second optical fibre assembly comprises a mirror element with an output coupler for laser light.

One object of the invention is to provide a method of producing an optical component is provided, the method comprising: a. providing first optical fibre comprising a pump core having a numerical aperture NAi at a first fibre end of said first optical fibre; b. arranging at least one second optical fibre in relation to said first optical fibre, where at least one of said at least one second optical fibres comprises a pump core which at a second fiber end has a numerical aperture NA₂ that is smaller than NAi; c. providing a reflector element having an end-facet with a predetermined profile and a reflective coating, and orienting said reflector element such that pump light from at least one of said at least one second optical fibre is reflected from the end-facet into the pump core of said first fibre.

In an embodiment the reflector element is arranged so that the end-facet of the reflector element is rotation symmetric around an axis which is offset relative of the optical axis of the optical component. The offset axis may be parallel to the optical axis of the optical component. The offset axis and the optical axis may be offset by a distance in the range of about 0.1 to about 3.0 times the outer diameter of the first optical fiber, such as in the range of about 0.2 to about 2.5 times the outer diameter of the first optical fiber, in the range of about 0.3 to about 2.0 times the outer diameter of the first optical fiber, in the range of about 0.5 to about 1.5 times the outer diameter of the first optical fiber, in the range of about 0.8 to about 1 .2 times the outer diameter of the first optical fiber.

In an embodiment, at least one (such as all) of the said at least one second fibres are positioned in relation to said first optical fibre such as around the first optical fibre.

In an embodiment, the end-facet is provided with a reflective coating at least over a part of its area.

In an embodiment, the reflector element is arranged to comprise a first end- facet and second end-facet with a predetermined profile and a reflective coating and wherein the pump light is reflected from the second end-facet into the pump core of said first fibre.

In a particular embodiment, the first and second fibres are mounted in a mounting tube at least over a part of their length.

In a particular embodiment, the reflector element is mounted in the mounting tube.

One object of the invention is to provide a method of producing an optical component is provided, the method comprising the steps of: a. providing first optical fibre comprising a pump core having a numerical aperture NAi at a first fibre end of said first optical fibre; b. arranging at least one second optical fibre in relation to said first optical fibre, where at least one of said at least one second optical fibres comprises a pump core which at a second fiber end has a numerical aperture NA₂ that is smaller than NAi, c. fusing said first end and second ends together to form an end-cap; d. shaping an end-facet of said end-cap to a predetermined profile; e. coating said end-facet having a predetermined profile with a metallic or dielectric coating such that pump light from at least one of said second fibres is reflected from the second end-facet into the pump core of said first fibre.

One object of the invention relates to the use of an optical component as described above and in the claims or in the detailed description is provided. In an embodiment, use of such an optical component in a laser or amplifier is provided.

One object of the invention is to provide a stiff optical fibre is provided, the stiff optical fibre having a longitudinal direction and comprising a core region, and a cladding region surrounding the core region wherein for a length L with a volume V_L of the solid parts of the stiff optical fibre, the cross-section of the fibre has a profile adapted to provide an improved ratio of axial stiffness to volume compared to a corresponding length of solid fibre with a circular outer form circumscribing said profile.

Various aspects of a stiff or substantially inflexible optical fibre, including a method of its manufacturing by a stack and draw method, is described in WO 02/010817, which is incorporated herein by reference.

In an embod iment, the stiff optical fibre comprises a jacket reg ion surrounding the cladding region wherein the jacket region is adapted to provide axial stiffness to the fibre. In an embodiment, the jacket region does not substantially contribute to the guiding properties of the fibre but is mainly present for mechanical reasons (to minimize bending (loss) of the fibre).

In an embodiment, when viewed in a transversal cross section, the maximum outer dimension D_stlff of the stiff optical fibre is more than 5 times the maximum dimension D_ciad of the cladding region, such as more than 10 times, such as more than 30 times, such as more than 50 times, such as more than 100 times, the maximum dimension D_ciad of the cladding region. In an embodiment, the stiff optical fibre comprises one or more further cladding regions surrounding the first cladding region. In an embodiment, the first cladding region is arranged to propagate light at a pump wavelength λ_p to provide a pump core for the fibre. In an embodiment a second cladding region surrounding the first cladding region comprises an air-cladding in the form of at least one ring of air holes with a narrow bridge width between each air hole in a circumferential direction of the first cladding region, thereby providing confinement of light (e.g. pump light) to the first cladding region.

Various aspects of 'rod-type' optical fibres are e.g. discussed by Limpert et al. in Optics Express, Vol. 13, No. 4, 21 February 2005, pp. 1055-1058 and in Optics Express, Vol. 14, No. 7, 3 April 2006, pp. 2715-2720.

In an embodiment, the stiff optical fibre is a rod-type optical fibre, comprising a first cladding region surrounding the core region and a second cladding region or jacket region surrounding the first cladding region wherein - in a transversal cross section of the fibre - a maximum dimension D_COre of the core region is larger than 20 μm and a maximum outer dimension D_rocι of the rod- type fibre is larger than 700 μm, wherein the ratio of a maximum outer dimension D_ciadi of the first cladding to D_rocι is in the range from 0.01 -0.5, such as in the range from 0.05 to 0.4, such as in the range from 0.1 to 0.3.

In an embodiment, the rod-type fibre comprises an air-cladding in the form of at least one ring of air holes for confining light (e.g. pump light) to the first cladding region. In an embodiment, the second cladding region comprises an air-cladding.

In an embodiment, the second cladding region is equal to the jacket region.

In an embodiment, a jacket region surrounds the second cladding region.

In an embodiment, the term 'a stiff optical fibre' is defined by a bending test in which the force required for bending the stiff optical fibre to a specific radius of curvature (e.g . a 180° bend over a cylinder having such radius) is determined. In an embodiment, a force of more than 0.1 N is required to bend the stiff optical fibre to a radius of curvature of 1 m. In other embodiments, a force of more than 0.5 N, such as more than 1 N, such as more than 5 N, such as more than 10 N is required to bend the stiff optical fibre to a radius of curvature of 1 m.

In an embodiment, the stiff optical fibre cannot be bent to a radius of curvature of less than 1 m without mechanical damage (e.g. fracture).

In an embodiment, the stiff optical fibre is a micro-structured optical fibre.

In an embodiment, the stiff optical fibre comprises an optically active material, e.g. Yb and/or Er.

By profiling the stiff or rod-type optical fibre, e.g. by - starting from a design with a circular outer periphery and a substantially solid outer (second) cladding or jacket region - removing some of the volume of the material (which may be glass) constituting the outer cladding or jacket, e.g. by changing the outer periphery and/or making longitudinally extending holes in the interior of the stiff or rod-type fibre (e.g. in the outer or second cladding or jacket region), the stiffness of the stiff or rod-type fibre may be maintained in the face of a smaller volume of material used for the stiff or rod-type fibre. Further, the surface area of the stiff or rod-type fibre can be optimized (increased), thereby improving the cooling possibilities of the fibre (i.e. the ability to transport heat away from the core and/or (first) cladding regions of the fibre).

In a particular embodiment, the stiff or rod-type fibre comprises one or more longitudinally extending holes. In an embodiment, the longitudinally extending hole or holes has/have a maximum dimension that is larger than the core region of the fibre, such as larger than twice as large, such as larger than 4 times as large as the core region.

In a particular embodiment, the outer periphery of the stiff or rod-type fiber has a non-circular form. In a particular embodiment, the outer periphery of the stiff or rod-type fiber comprises n edges and n vertices, the outer periphery e.g . having a polygonal form.

In a particular embodiment, the edges are non-linear, such as concave with respect to the core region of the fibre.

In a particular embodiment, the profile of the stiff or rod-type fibre is optimized to have a large surface to provide improved dissipation of heat from the fibre.

In a particular embodiment, the profile of the stiff or rod-type fibre is optimized to support one or more optical fibres, e.g. pump fibres.

In a particular embodiment, the core region is adapted for propagating light at a signal wavelength λ_s. In a particular embodiment, the inner cladding region is adapted for propagating light at a pump wavelength λ_p.

In a particular embodiment, D_ciad or D_ciadi is in the range from 100 μm to 400 μm.

In a particular embodiment, D_COre is larger than 50 μm, such as larger than 70 μm, such as larger than 100 μm, such as larger than 150 μm, such as larger than 200 μm, such as larger than 300 μm.

In a particular embodiment, D_stlff or D_rocι is larger than 0.7 mm, such as larger than 1 mm, such as larger than 1.2 mm, such as larger than 1.5 mm, such as larger than 2 mm, such as in the range from 0.7 mm to 3 mm.

In a particular embodiment, the ratio of D_COre to D_ciad or D_COre to D_ciadi is in the range from 0.5 to 0.95, such as in the range from 0.6 to 0.8, such as in the range from 0.7 to 0.75.

In a particular embodiment, an optical component according to an object of the invention comprises a stiff or rod-type optical fibre according to an object of the invention. In the present context, the 'core region' is defined - when viewed in a cross section perpendicular to a longitudinal direction of the fibre - as a light- propagating part of the fibre.

The refractive index n_x is generally the conventional refractive index of a homogeneous material. The effective refractive index n_Θff,_x is the index that light at a given wavelength, λ, experiences when propagating through a given material that may be inhomogeneous (meaning that the material complex e.g. comprises two or more sub-materials, which may be a background material of one refractive index and one or more types of features (often termed micro-structural elements in the present application) of different refractive index/indices). For homogeneous materials, the refractive and the effective refractive index will naturally be similar.

For optical fibres according to the present invention, the most important optical wavelengths are in the ultra-violet to infrared regime (e.g. wavelengths from approximately 150 nm to 11 μm). In this wavelength range the refractive index of most relevant materials for fibre production (e.g. silica) may be considered mainly wavelength independent, or at least not strongly wavelength dependent. However, for non-homogeneous materials, such as fibres comprising micro-structural elements, e.g. voids or air holes, the effective refractive index may be very dependent on the morphology of the material. Furthermore, the effective refractive index of such a fibre may be strongly wavelength dependent. The procedure of determining the effective refractive index at a given wavelength of a given fibre structure having voids or holes is well-known to those skilled in the art (see e.g. Broeng et al, Optical Fibre Technology, Vol. 5, pp. 305-330, 1999).

For a given cross sectional dimension of the reflector element, the allowable distance from the first end of the multi-core optical fiber is to the reflector element increases with reduced NA of the second pump core. A device wherein said second pump cores have relatively low NA values may thus have an improved coupling efficiency from said second pumpt cores to said first pump core. The same argument is applicable to the case where second pump fibers are arranged in relation to a first optical fiber and a reflector elements couples l ight from said second pump cores into the inner cladding/pump core of the first optical fiber.

To prevent damage at the first end of the multi-core optical fiber when high power pump signals exits the second pump core and enters the first pump core, an end-cap is arranged in contact with the first end of the multi-core optical fiber. The optical signal propagating through the second pump core(s) diverges whereby the beam intensity is reduced at the far end of the end cap compared to the intensity at the first end of the multi-core optical fiber. Thereby the optical power threshold for the pump signal in the second pump core causing damages to the device is reduced.

In one embodiment of the the multi-core optical fiber according to the present invention said first pump core and said second pump core are arranged at a minimal cross sectional distance relative to each other, said minimal cross sectional distance being in the range of about 10 μm to 2000 μm, such as in the range of about 20 μm to about 1500 μm, such as in the range of about 30 μm to about 1000 μm, such as in the range of about 40 μm to about 750 μm, such as in the range of about 50 μm to about 500 μm, such as in the range of about 75 μm to about 350 μm.

The multi-core optical fiber may further comprise a second cladding region surrounding said first outer cladding. The second pump core of the multi-core optical fiber may be arranged to surround said second cladding region.

In one embodiment, second pump core of the multi-core optical fiber comprises at least one second pump core feature. The second pump core may comprise a plurality of second pump core features. Said plurality of second pump core features may be arranged in at least one chain surrounding said first outer cladding. The chain may comprise second pump core features that are substantially in contact or spaced apart. When spaced apart, the second pump core features may be substantially equidistantly spaced around the first outer cladding.

In one embodiment, the second pump core of the multi-core optical fiber comprises at least one cross sectional coherent substantially ring-formed second core region. The longitudinal axis of the ring formed second core region may be arranged outside said first pump core.

In one embodiment, the second pump core of the multi-core optical fiber is arranged to surround said first outer cladding. The second pump core may contactly surround the first outer cladding.

In one embodiment, second cladding region of the multicore optical fiber comprises a second cladding low-index region.

In one embodiment the multi-core optical fiber comprises a third cladding region surrounding said second pump core. Said third cladding region may comprise a third cladding low-index region that may comprise a substantially coherent ring of down-doped basis material contactly surrounding said cross sectional coherent substantially ring-formed second core region.

The second cladding low-index region and/or the third cladding low-index region may comprise a down-doped region comprising fiber basis material doped with an index lowering dopant. The first pump core and/or the second pump core may comprise a substantially pure fiber basis material. The second pump core may comprise an up-doped region comprising fiber basis material doped with an index raising dopant.

In one embodiment, the fiber basis material comprises silica. The index raising dopant may comprise germanium and the said index lowering dopant may comprise fluorine.

The second cladding low-index region may comprise a substantially coherent ring of down-doped basis material being contactly surrounded by said cross sectional coherent substantially ring-formed second core region.

In one embodiment, the first outer cladding and/or the second cladding low- index region and/or the third cladding low-index region comprises an air cladding. The presence of an air cladding may ensure that the region it surrounds has a large numerical aperture. The numerical aperture may be adjusted by increases the bridge thickness between the air holes of the air cladding.

In one embodment, NA₂ is below about 0.25, such as below about 0.2, such as below about 0.15, such as below about 0.12, such as below about 0.1 , such as below about 0.07, such as below about 0.04. In one embodiment NAi is in the range of about 0.22 to about 1.8, such as in the range of about 0.3 to about 1.8, such as in the range of about 0.45 to about 1.8, such as in the range of about 0.55 to about 1.8, such as in the range of about 0.8 to about 1.8.

The multi-core optical fiber according to the present invention may further comprise a third core region surrounding said third cladding region. Further core regions may be arranged concentrically with further cladding regions arranged in between said further core regions. The further core regions and cladding regions may be arranged such that pump modes of said further pump core has substantially no modal overlap with other pump cores of the multi-core optical fiber at pump wavelengths in the range of 300nm to 2500nm.

In one embodiment, the multi-core optical fiber according to the present invention comprises a signal core arranged to be surrounded by said first pump core. The signal core may be single mode at a signal wavelength λ_s in the range of 300nm to 2500nm.

In one embodiment, the signal core and/or the first pump core comprises an optically active material, such as Ytterbium and Erbium

In one embodiment, the multi-core optical fiber further comprising stress elements embedded is said first pump core to provide a stress field in the signal core region to making it birefringent or enhancing its birefringence properties.

In one embodiment of the optical device comprising the multicore optical fiber and an end-cap, the extension of said end cap in the direction along said longitudinal axis is in the range of about 1 mm to about 100nm, such as about 2 mm to about 50 mm, such as about 3 mm to about 35 mm, such as about 5 mm to about 25 mm. The end cap may comprise a substantially cylindrical rod comprising substantially pure fiber basis material.

A second end of said multi-core optical fiber may be arranged in relation to a first end of a fiber bundle, wherein at least a part of the fibers of said fiber bundle are arranged to launch a pump signal into said second pump core, and one fiber is arranged to launch light into said signal core. The multi-core optical fiber may be arranged in relation to said fiber bundle by butt coupling or by splicing. In one embodiment, the fiber bundle comprises a section at said fiber bundle first end, wherein the outer diameter changes along the longitudinal axis.

In one embodiment, the optical component comprising the multicore optical fiber comprises a reflector element which comprises an aperture substantially at its cross section centre, said aperture being arranged to allow at least a part of an optical signal propagating in said signal core to pass through said reflector element.

In one embodiment, the guiding structure comprises a first region having a first effective refractive index, and a second region surrounding said first region and having a second effective refractive index which is smaller than said first effective refractive index at least at λi. The second region of said guiding structure may comprise down doped silica material, such as Fluorine doped silica glass.

The difference between the first and second effective refractice indices at λi may providing the guiding property of the guiding structure.

In one embodiment, the second region of said guiding structure comprises an air cladding.

The numerical aperture of said first region of said guiding structure may be in the range of about 0.02 to about 0.22, such as in the range of about 0.04 to about 0.20, such as in the range of about 0.06 to about 0.15, such as in the range of about 0.08 to about 0.12. The numerical aperture of said guiding structure may be below the numerical aperture of the pump core of said second optical fiber.

In one embodiment, the guiding structure is substantially circular in said cross section and the diameter of the first region may be in the range of about 5 μm to about 200 μm, such as in the range of about 10 μm to about 150 μm, such as in the range of about 12 μm to about 125 μm, such as in the range of about 15 μm to about 100 μm, such as in the range of about 20 μm to about 75 μm, such as in the range of about 25 μm to about 60 μm, such as in the range of about 30 μm to about 50 μm.

The signal wavelength λ_s may be substantially equal to the first wavelength λi .

In one embodiment, the pump core of said first optical fiber and the pump core of said second optical fiber are capable of propagating pump light at a pump wavelength λ_p, different from said signal wavelength λ_s.

In one embodiment, the pump core of said second optical fiber is surrounded by an air cladding.

The optical assembly may comprise one or more optical elements, such as lenses and reflector elements, arranged to handle the signal and pump light when outside the optical fibers

In one embodiment, the optical assembly comprises a reflector element. The reflector element may comprise an end-facet with a predetermined profile. Ther predeterm ined profile may comprise a cu rved section . The predetermined profile may comprise a substantially planar section

The end-facet of said reflector element may be rotation symmetric around an axis, which is substantially parallel to and offset relative of the longitudinal, optical axis of the optical component. THe end-facet of said reflector element may be rotation symmetric around an axis which is substantially coinciding with the longitudinal, optical axis of the optical component

The end-facet of the reflector element may be capable of reflecting light at said signal wavelength λ_s and said pump wavelength λ_p differently.

In one embodiment, the reflector element is arranged to reflect pump light from said second fiber end of said second optical fiber into said pump core of said first fiber.

The ratio between the reflection coefficient of light at said signal wavelength and the reflection coefficient of light at said pump wavelength from said reflector element, may be less than about 0.5, such as less than about 0.1 , such as less than about 0.01 , such as less than about 0.001 , such as less than about 0.0001

In one embodiment, the guiding structure is arranged to collect signal light reflected from said reflective element onto said second fiber end of said second optical fiber.

In one embodiment, the reflector element is arranged to reflect signal light from said core region of said first optical fiber away from said second fibre end of said second optical fiber. The guiding structure may be arranged to collect signal light leaking through said reflective element.

The ratio between the reflection coefficient of light at said pump wavelength and the reflection coefficient of light at said signal wavelength from said reflector element may be less than about 0.5, such as less than about 0.1 , such as less than about 0.01 , such as less than about 0.001 , such as less than about 0.0001. The monitor optical fiber may be connected to a detector arranged to monitor the optical power of the signal collected by the guiding structure from the first end of said first optical fiber.

In one embodiment, a major part of the optical power of the signal, which is directed onto said second fiber second end from said core region of said first optical fiber is collected by said guiding structure and delivered to said monitor optical fiber via said monitor region, such as more than about 60% of the optical power, such as more than about 70% of the optical power, such as more than about 80% of the optical power, such as more than about 90% of the optical power, such as more than about 95% of the optical power, such as more than about 98% of the optical power.

In one embodiment, the first optical fiber is a PCF dou ble-clad fiber comprising a core region for propagating light at a signal wavelength, an inner cladding region - termed a pump core - surrounding the core region for propagating light at a pump wavelength and an air cladding comprising at least one ring of relatively large holes surrounding the inner cladding region.

The input section maty comprises a plurality of input fibers, such as six, arranged to surround sad monitor fiber.

In one embodiment, less than about 40% of the optical power of a signal propagating in said guiding structure of said optical component is coupled into the input region, such as less than about 30%, such as less than about 20%, such as less than about 10%, such as less than about 5%, such as less than about 2%, such as less than about 1 %, such as less than about 0.5%, such as less than about 0.1 %.

In one embodiment, the longitudinal axis is arranged substantially in the center of said core region of said first optical fiber. In one embodiment, the longitudinal axis is arranged substantially at the interface between said first optical fiber and a second optical fiber.

The optical combiner may be designed in many ways as known to the skilled person

The optical system may comprise a signal power monitor arranged to detect the power of the signal collected and guided by the guiding structure of the second optical fiber.

In the context of the present application, the phrase "wherein said first pump mode ... has substantially no modal overlap with said second pump core" refers to the situation, wherein the scalar electrical field E(x, y) intensity overlap of the first pump mode(s) to the second pump core is less than about 5%, such as less than about 2%, such as less than about 1 %, such as less than about 0.5%, such as less than about 0.1 %

The scalar electrical field E(x, y) intensity overlap of the first pump mode(s) to the second pump core is given by the confinement factor r_pump-core-2:

Further objects of the invention are achieved by the embodiments defined in the dependent claims and in the detailed description of the invention.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other stated features, integers, steps, components or groups thereof.

ITEMS 1. An optical component having a longitudinal, optical axis, and a cross section perpendicular to the long itudinal axis, the optical component comprising: a. a first optical fibre comprising a pump core having a numerical aperture NAi at a first fibre end of said first optical fibre; b. at least one second optical fibre arranged in relation to said first optical fibre, where at least one of said at least one second optical fibres comprises a pump core which at a second fiber end has a numerical aperture NA₂ that is smaller than NAi; c. a reflector element comprising an end-facet with a predetermined profile for reflecting light from said second fibre end of at least one second optical fiber into said pump core of said first fibre.

2. An optical component according to item 1 wherein the reflector element comprises first and second opposing end-facets, said first end-facet facing said first fibre end and said second fibre ends, and said first and/or said second end-facet having a predetermined profile.

3. An optical component according to item 1 or 2 wherein the pump core of the first and at least one of the second optical fibres are adapted for propagating pump light at a pump wavelength λ_p.

4. An optical component according to item 3 wherein an end-facet of the reflector element is adapted to reflect light at the pump wavelength λ_p.

5. An optical component according to any one of items 1 -4 wherein the first fibre comprises a core region different from said pump core, the core region being adapted for propagating light at a signal wavelength λ_s different from said pump wavelength λ_p.

6. An optical component according to item 5 wherein an end-facet of the reflector element is adapted to reflect at least a fraction of light at the signal wavelength λ_s. 7. An optical component according to item 5 or 6 wherein an end-facet of the reflector element is adapted to reflect light at said signal wavelength λ_s and said pump wavelength λ_p differently.

8. An optical component according to any one of items 5-7 wherein different partial areas of an end-facet of the reflector element are adapted to reflect light at the signal wavelength λ_s and said pump wavelength λ_p differently.

9. An optical component according to any one of items 1 -8 wherein an end- facet of the reflector element has a reflectivity in an area around the central optical axis of the first optical fibre to allow propagation of a predetermined fraction of light from the first optical fibre.

10. An optical component according to any of the items 1 to 9, wherein at least a part of an end-facet of the reflector element has a coating for increased reflection of pump light.

11. An optical component according to any of the items 1 to 10, wherein a major part of the area of an end-facet of the reflector element has a coating for increased reflection of pump light.

12. An optical component according to any of the items 1 to 1 1 , wherein an end-facet is un-coated in a region around the centre of the end-facet.

13. An optical component according to any of the items 1 to 12, wherein an end-facet of the reflector element has a coating with higher reflective coefficient for pump light at a wavelength λ_p than for signal light at a wavelength λ_s, where λ_p is different from λ_s.

14. An optical component according to any of the items 1 to 13, wherein the coating is a dielectric or a metallic coating.

15. An optical component according to any of the items 1 to 14 wherein an end facet of the reflector element is adapted to focus the pump light in the pump core of the first fibre a distance L_f from the first end of the first fibre. 16. An optical component according to any of the items 1 to 15, wherein the reflector element comprises a plano-convex element comprising said relective end facet(s).

17. An optical component according to any of the items 1 to 15, wherein an end-facet of the reflector element facing the first fibre end of the first fibre and the second fibre ends of the second fibres is a curved surface formed into a bulk material.

18. An optical component according to any of the items 1 to 17 wherein an end-facet of the reflector element is rotation symmetric around a longitudinal axis of the optical component.

19 An optical component according to any of the items 1 to 17 wherein an end-facet of the reflector element is rotation symmetric around an axis which is offset relative of the optical axis of the optical component.

20. An optical component according to item 19 wherein said offset axis is parallel to the optical axis of the optical component.

21. An optical component according to items 19 or 20 where the offset axis and the optical axis are offset by a distance in the range of about 0.1 to about 3.0 times the outer diameter of the first optical fiber, such as in the range of about 0.2 to about 2.5 times the outer diameter of the first optical fiber , in the range of about 0.3 to about 2.0 times the outer diameter of the first optical fiber, in the range of about 0.5 to about 1.5 times the outer diameter of the first optical fiber, in the range of about 0.8 to about 1.2 times the outer diameter of the first optical fiber.

22. An optical component according to any of the items 1 to 21 wherein parts of an end-facet of the reflector element adapted to reflect pump light from the second fibres have a spherical shape.

23. An optical component according to item 22, wherein the predetermined profile of an end-facet of the reflector element is adapted to provide a focal length that is substantially equal to 0.5 times the radius of the spherical shape. 24. An optical component according to item 23 wherein parts of an end-facet of the reflector element adapted to reflect pump light from the second fibres have an asphehcal shape.

25. An optical component according to any of the items 1 to 24 wherein a majority or all of said at least one second fibre have NA₂ < NAi.

26. An optical component according to any of the items 1 to 25, wherein NAi is higher than 0.22, such as higher than 0.30, such as higher than 0.45, such as higher than 0.55, such as higher than 0.8.

27. An optical component according to any of the items 1 to 26, wherein the first fibre is a double clad fibre comprising a signal core.

28. An optical component according to any of the items 1 to 27, wherein the signal core comprises rare earth dopants for amplifying signal light.

29. An optical component according to 27 or 28, wherein the first fibre is an all-glass double clad fibre, a polymer-clad double clad fibre or a PCF double- clad fibre.

30. An optical component according to item 29 wherein the first fibre is a PCF double-clad fibre comprising a core region for propagating light at a signal wavelength, an inner cladding region - termed a pump core - surrounding the core region for propagating light at a pump wavelength and an air cladding comprising at least a ring of relatively large holes surrounding the inner cladding region.

31. An optical component according to item 30 wherein the holes of the air cladding are collapsed over a length L₀ from the first end of the first fibre.

32. An optical component according to item 31 wherein the focus distance L_f of the pump light in the pump core of the first fibre is substantially equal to the length L₀ over which the holes of the air cladding are collapsed. 33. An optical component according to any of the items 1 to 32, wherein first fibre and the second fibres are fused together over at least a part of their length.

34. An optical component according to any of the items 1 to 33, wherein the first fibre end of the first fibre and the second fibre ends of the at least one second optical fibre are directly connected to the first end-facet of the reflector element.

35. An optical component according to any of the items 1 to 34, wherein the number of second optical fibres is 1 or 2 or 3 or larger than or equal to 4, such as larger than or equal to 6, such as larger than or equal to 8, such as in the range from 10 to 24, such as larger than or equal to 12, such as larger than or equal to 20, such as larger than or equal to 40, such as larger than or equal to 80.

36. An optical component according to any of the items 1 to 35, wherein a plurality of said second optical fibres are located around the first optical fibre in one or more layers (e.g. in 2 or 3 layers).

37. An optical component according to item 36, wherein the numerical aperture of the plurality of second optical fibres at their second ends are different from layer to layer of the second optical fibres surrounding the first optical fibre.

38. An optical component according any one of items 1 to 37, wherein the optical component further comprises a mounting tube surrounding the at least one second optical fibres and the first optical fibre, whereby the optical fibres are fixated and protected.

39. An optical component according to any of the items 1 to 38, wherein the optical component further comprises a mounting tube surrounding the at least one second optical fibre and the first optical fibre and the reflector element, whereby the fibres and the reflector element are fixated and protected. 40. An optical component according to any one of items 1 to 39 wherein the reflector element additionally comprises one or more elements selected from the group comprising i) an optical element comprising a material, which is substantially optically transparent at a pump wavelength λ_p at least over a part of its area; ii) an optical element comprising a material, which is substantially optically transparent at a signal wavelength λ_s at least over a part of its area; iii) an optical element, which reflects at least a fraction, such as substantially all, of the light at a signal wavelength λ_s at least over a part of its area; iv) an optical element, which transmits at least a fraction, such as at least 60%, of the light at a signal wavelength λ_s at least over a part of its area; v) an optical element, which collimates light at said signal wavelength

vi) an optical element, which focuses light at said signal wavelength λ_s. wherein said one or more optical elements is/are optically coupled to said first fibre end and/or said second fibre end(s) in its assembled state.

41. An optical component according to item 40 wherein said reflector element is configurable in that one or more of said elements can be 'easily added or removed.

42. A fibre amplifier comprising an optical component according to any of the items 1 to 41.

43. A fibre laser comprising an optical component according to any of the items 1 to 41.

44. A fibre laser or amplifier comprising an optical component according to any one of items item 1 -41 and an amplifying optical fibre comprising an optically active material, such as Ytterbium and Erbium 45. A fibre laser or amplifier according to item 44 wherein the amplifying optical fibre is a double clad fibre.

46. A fibre laser or amplifier according to item 44 or 45 wherein the amplifying optical fibre is a standard fibre.

47. A fibre laser or amplifier according to any one of items 44 to 46 wherein the amplifying optical fibre is a photonic crystal fibre.

48. A fibre laser or amplifier according to any one of items 44 to 47 wherein the amplifying optical fibre is optically coupled to the first optical fibre of the optical component,

49. A fibre laser or amplifier according to any one of items 44 to 48 wherein the amplifying optical fibre is equal to the first optical fibre of the optical component.

50. A fibre laser or amplifier according to any one of items 44 to 49 comprising first and second optical components according to any one of items 1 -41 sharing a common amplifying optical fibre.

51. A fibre laser or amplifier according to item 50 wherein the first optical component comprises a mirror element with a high reflector and a second optical fibre assembly comprises a mirror element with an output coupler for laser light.

52. A method of producing an optical component, the method comprising: a. providing first optical fibre comprising a pump core having a numerical aperture NAi at a first fibre end of said first optical fibre; b. arranging at least one second optical fibre in relation to said first optical fibre, where at least one of said at least one second optical fibres comprises a pump core which at a second fiber end has a numerical aperture NA₂ that is smaller than NAi; c. providing a reflector element having an end-facet with a predetermined profile and a reflective coating, and orienting said reflector element such that pump light from at least one of said at least one second optical fibre is reflected from the end-facet into the pump core of said first fibre.

53. A method of producing an optical component according to item 52 wherein the reflector element is arranged to comprise a first end-facet and second end-facet with a predetermined profile and a reflective coating and wherein the pump light is reflected from the second end-facet into the pump core of said first fibre.

54 A method according to item 52 wherein the end-facet of the reflector element is rotation symmetric around an axis which is offset relative of the optical axis of the optical component.

55. A method item 54 wherein said offset axis is parallel to the optical axis of the optical component.

56. A method according to items 54 or 55 where the offset axis and the optical axis are offset by a distance in the range of about 0.1 to about 3.0 times the outer diameter of the first optical fiber, such as in the range of about 0.2 to about 2.5 times the outer diameter of the first optical fiber , in the range of about 0.3 to about 2.0 times the outer diameter of the first optical fiber, in the range of about 0.5 to about 1.5 times the outer diameter of the first optical fiber, in the range of about 0.8 to about 1 .2 times the outer diameter of the first optical fiber.

57. A method according to any of items 52 to 56, wherein the first and second fibres are mounted in a mounting tube.

58. A method according to item 57, wherein the reflector element is mounted in the mounting tube.

59. A method of producing an optical component, the method comprising: a. providing first optical fibre comprising a pump core having a numerical aperture NAi at a first fibre end of said first optical fibre; b. arranging at least one second optical fibre in relation to said first optical fibre, where at least one of said at least one second optical fibres comprises a pump core which at a second fiber end has a numerical aperture NA₂ that is smaller than NAi, c. fusing said first end and second ends together to form an end-cap; d. shaping an end-facet of said end-cap to a predetermined profile e. coating said end-facet having a predetermined profile with a metallic or dielectric coating such that pump light from at least one of said at least one second fibres is reflected from the second end-facet into the pump core of said first fibre.

60. Use of an optical component according to any one of items 1 -41.

61. Use according to item 60 in a laser or amplifier.

62. A rod-type optical fibre having a longitudinal direction and comprising a core region, a first cladding region surrounding the core region and a second cladding region surrounding the first cladding region wherein - in a transversal cross section of the fibre - a maximum dimension D_COre of the core region is larger than 20 μm and a maximum outer dimension D_rocι of the rod- type fibre is larger than 700 μm, wherein the ratio of a maximum outer dimension D_ciadi of the first cladding to D_rod is in the range from 0.05-0.5, and wherein for a length L with a volume V_L of the solid parts of the rod-type fibre, the cross-section of the fibre has a profile adapted to provide an increased ratio of axial stiffness to volume compared to a corresponding length of solid fibre with a circular outer form circumscribing said profile.

63. A rod-type optical fibre according to item 62 wherein the rod-type fibre comprises one or more longitudinally extending holes having a maximum cross-sectional dimension larger than that of the core region.

64. A rod-type optical fibre according to item 62 or 63 wherein the outer periphery of the rod-type fiber has a non-circular form.

65. A rod-type optical fibre according to any one of items 62 to 64 wherein the outer periphery of the rod-type fiber comprises n edges and n vertices, the outer periphery e.g. having a polygonal form. 66. A rod-type optical fibre according to any one of items 62 to 65 wherein the edges are non-linear, such as concave with respect to the core region of the fibre.

67. A rod-type optical fibre according to any one of items 62 to 66 wherein the profile of the rod-type fibre is optimized to have a large surface to provide improved dissipation of heat from the fibre.

68. A rod-type optical fibre according to any one of items 62 to 67 wherein the profile of the rod-type fibre is optimized to support one or more optical fibres, e.g. pump fibres.

69. A rod-type optical fibre according to any one of items 62 to 68 wherein the core region is adapted for propagating light at a signal wavelength λ_s.

70. A rod-type optical fibre according to any one of items 62 to 69 wherein the inner cladding region is adapted for propagating light at a pump wavelength λ_p.

71. A rod-type optical fibre according to any one of items 62 to 70 wherein Dciadi is in the range from 100 μm to 400 μm.

72. A rod-type optical fibre according to any one of items 62 to 71 wherein Dcore is larger than 50 μm, such as larger than 70 μm, such as larger than 100 μm, such as larger than 150 μm, such as larger than 200 μm, such as larger than 300 μm.

73. A rod-type optical fibre according to any one of items 62 to 72 wherein D₁-Od is larger than 0.7 mm, such as larger than 1 mm, such as larger than 1.2 mm, such as larger than 1.5 mm, such as larger than 2 mm, such as in the range from 0.7 mm to 3 mm.

74. A rod-type optical fibre according to any one of items 62 to 73 wherein the ratio of D_COre to D_ciadi is in the range from 0.5 to 0.95, such as in the range from 0.6 to 0.8, such as in the range from 0.7 to 0.75. 75. An optical component according to any one of items 1 -41 comprising a rod-type optical fibre according to any one of items 62-74.

76. A multi-core optical fiber having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis, the multi-core optical fiber comprising: a. a first pump core being adapted to guide a pump signal at a pump wavelength in at least one first pump mode, said first pump core having at said pump wavelength a numerical aperture NAi at a first fiber end of said multi-core optical fiber; b. a first outer cladding surrounding said first pump core, and c. at second pump core having at said pump wavelength a numerical aperture NA₂ at said first fiber end of said multi-core optical fiber, where NA₂ is smaller than NAi; and

wherein said first pump mode of said first pump core has substantially no modal overlap with said second pump core at a pump wavelength λ_p in the range of 300nm to 2500nm.

77. The multi-core optical fiber according to item 76, wherein said first pump core and said second pump core are arranged at a minimal cross sectional distance relative to each other, said minimal cross sectional distance being in the range of about 10 μm to 2000 μm, such as in the range of about 20 μm to about 1500 μm, such as in the range of about 30 μm to about 1000 μm, such as in the range of about 40 μm to about 750 μm, such as in the range of about 50 μm to about 500 μm, such as in the range of about 75 μm to about 350 μm.

78. The multi-core optical fiber according to item 76 or 77 further comprising a second cladding region surrounding said first outer cladding.

79. The multi-core optical fiber according to any of item 76 to 78, wherein said second pump core comprises at least one second pump core feature.

80. The multi-core optical fiber according to item 79, wherein said second pump core comprises a plurality of second pump core features. 81. The multi-core optical fiber according to item 80, wherein said second pump core features are arranged in at least one chain surrounding said first outer cladding.

82. The multi-core optical fiber according to any of items 76 to 81 , wherein said second pump core comprises at least one cross sectional coherent substantially ring-formed second core region.

83. The multi-core optical fiber according to any of items 76 to 82, wherein said second pump core comprises a longitudinal axis that is arranged outside said first pump core.

84. The multi-core optical fiber according to any of items 76 to 82, wherein said second pump core is arranged to surround said first outer cladding.

85. The multi-core optical fiber according to items 84, wherein said second pump core is arranged to surround said second cladding region.

86. The multi-core optical fiber according to any of items 78 to 85, wherein said second cladding region comprises a second cladding low-index region.

87. The multi-core optical fiber according to any of items 78 to 86, further comprising a third cladding region surrounding said second pump core.

88. The multi-core optical fiber according to item 87, wherein said third cladding region comprises a third cladding low-index region.

89. The multi-core optical fiber according to any of items 86 to 88, wherein said second cladding low-index region and/or said third cladding low-index region comprises a down-doped region comprising fiber basis material doped with an index lowering dopant.

90. The multi-core optical fiber according to any of items 76 to 89, wherein said first pump core and/or said second pump core comprises a substantially pure fiber basis material. 91. The multi-core optical fiber according to any of items 76 to 90, wherein said second pump core comprises an up-doped region comprising fiber basis material doped with an index raising dopant.

92. The multi-core optical fiber according to any of items 89 to 91 , wherein said fiber basis material comprises silica.

93. The multi-core optical fiber according to items 91 or 92, wherein said index raising dopant comprises germanium.

94. The multi-core optical fiber according to any of items 89 to 93, wherein said index lowering dopant comprises fluorine

95. The multi-core optical fiber according to any of items 86 to 94, wherein said second cladding low-index region comprises a substantially coherent ring of down-doped basis material being contactly surrounded by said cross sectional coherent substantially ring-formed second core region.

96. The multi-core optical fiber according to any of items 87 to 95, wherein said third cladding low-index region comprises a substantially coherent ring of down-doped basis material contactly surrounding said cross sectional coherent substantially ring-formed second core region.

97. The multi-core optical fiber according to any of items 76 to 96, wherein said first outer cladding and/or said second cladding low-index region and/or said third cladding low-index region comprises an air cladding.

98. The multi-core optical fiber according to any of items 76 to 97, wherein NA₂ is below about 0.25, such as below about 0.2, such as below about 0.15, such as below about 0.12, such as below about 0.1 , such as below about 0.07, such as below about 0.04.

99. The multi-core optical fiber according to any of items 76 to 98, wherein NAi is in the range of about 0.22 to about 1.8, such as in the range of about 0.3 to about 1 .8, such as in the range of about 0.45 to about 1 .8, such as in the range of about 0.55 to about 1.8, such as in the range of about 0.8 to about 1.8.

100. The multi-core optical fiber according to any of items 76 to 99, further comprising a third core region surrounding said third cladding region.

101. The multi-core optical fiber according to item 100, comprising further core regions arranged concentrically with further cladding regions arranged in between said further core regions, wherein pump modes of said further pump core has substantially no modal overlap with other pump cores of the multi- core optical fiber at pump wavelengths in the range of 300nm to 2500nm.

102. The multi-core optical fiber according to any of items 76 to 101 , further comprising a signal core arranged to be surrounded by said first pump core

103. The multi-core optical fiber according to item 102, wherein said signal core is single mode at a signal wavelength λ_s in the range of 300nm to 2500nm.

104. The multi-core optical fiber according to item 102 or 103, wherein said signal core and/or said first pump core comprises an optically active material, such as Ytterbium and Erbium

105. The multi-core optical fiber according to any of items 76 to 104, further comprising stress elements embedded is said first pump core to provide a stress field in the signal core region to making it birefringent or enhancing its birefringence properties.

106. An optical device comprising a. an multi-core optical fiber according to any of items 76 to 105, and b. an end-cap arranged in relation to said first end; wherein said end cap is arranged in contact with said first end of said fiber such that pump signals emerging from said second pump core is coupled into said end cap when emerging at said first end of said multi-core optical fiber. 107. The optical device according to item 106, wherein the extension of said end cap in the direction along said longitudinal axis is in the range of about 1 mm to about 100nm, such as about 2 mm to about 50 mm, such as about 3 mm to about 35 mm, such as about 5 mm to about 25 mm.

108. The optical device according to item 106 or 107, wherein said end cap comprises a substantially cylindrical rod comprising substantially pure fiber basis material.

109. The optical device according to any of items 106 to 108, wherein a second end of said multi-core optical fiber is arranged in relation to a first end of a fiber bundle, wherein at least a part of the fibers of said fiber bundle are arranged to launch a pump signal into said second pump core, and one fiber is arranged to launch light into said signal core.

110. The optical device according to item 109, wherein the multi-core optical fiber is arranged in relation to said fiber bundle by butt coupling or by splicing.

111. The optical device according to any of items 106 to 1 10, wherein said fiber bundle comprises a section at said fiber bundle first end, wherein the outer diameter changes along the longitudinal axis

112. An optical component comprising a. a multi-core optical fiber according to any of items 76 to 105, or an optical device according to items 106 to 111 , b. a reflector element comprising an end-facet with a predetermined profile for reflecting light from said second pump core at said first fiber end into said first pump core.

113. An optical component according to item 1 12, wherein said reflector element comprises an aperture substantially at its cross section centre, said aperture being arranged to allow at least a part of an optical signal propagating in said signal core to pass through said reflector element. BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection with an embodiment and with reference to the drawings in which:

FIG. 1 schematically shows a method for coupling lower NA light from a pump fibre into the higher NA fibre.

FIG. 2 schematically shows a tapered, fused pump multiplexer as it is realised with traditional technology.

Fig. 3 schematically shows an embodiment of the present invention; Fig. 3a shows a longitudinal view, and Fig. 3b shows a cross-sectional view.

Fig. 4 schematically shows the principle of operation of an embodiment of the present invention.

Fig. 5 shows a schematic embodiment of the present invention.

Fig. 6a shows a schematic drawing of steps of a method for producing an optical component according to the invention. Fig. 6b shows an embodiment of a practical assembly (corresponding to step 2 of Fig. 6a).

Fig. 7 shows a schematic drawing of steps of another method for producing an optical component according to the invention.

Fig. 8 shows a schematic drawing of steps of a method of coating an end- facet of a reflective element according to an embodiment of the present invention.

Fig. 9 shows an illustration of a rod-type fibre coupling scheme.

Fig. 10 shows results of ray tracing light from a 105/125μm 0.22 NA pump fibre displaced 702, 5μm from the center of the rod-type fibre via the optimized asphehcal non-rotation symmetric mirror onto the acceptance facet of the rod fibre. Fig. 11 shows an example of an asphehcal profile for an end-facet of a reflector element according to the invention.

Fig. 12 shows a shape of the reflective element.

Fig. 13 shows embodiments of a fibre holding element for holding the first and second optical fibres of the optical component.

Fig. 14 is an illustration of a cross section of a realized optical component with 11 pump fibres and a single centrally located acceptance fibre.

Figure 15 schematically shows the result of ray tracing light from a pump fibre onto the aperture of an acceptance fibre, Fig. 15a illustrating the origin of the traced rays and Fig. 15b showing where these rays hit the aperture of the acceptance fibre.

Fig. 16 schematically shows an assembly forming a laser based on two individually adapted optical components according to the invention.

Fig. 17 shows three embodiments of an optical component according to the invention, Fig. 17a having a plano-convex reflector with a tilted plane surface, Fig . 17b having a plano-convex reflector and a tilted end-facet of the acceptance fibre, Fig. 17c using as a reflector a spherical surface facing the ends of the pump and acceptance fibres, and Fig. 17d and Fig. 17e show other embodiments of a reflective element comprising a single reflective end- facet.

Fig. 18 shows an embodiment comprising more than one ring of pump fibres surrounding the acceptance fibre.

Fig. 19 shows an example of an optical component according to the invention in the form of a coupler unit for a pulse amplifier based on a rod-type fibre.

Fig. 20 shows an example of a rod-type fibre with holes added. Fig. 21 shows an example of a profiled rod-type fibre.

Fig. 22 shows an example of an optical component according to the invention comprising an asphehcal reflector element.

Fig. 23 shows results of back reflection of light into the pump fibres for an embodiment of an optical device according to the invention.

Fig. 24 shows a schematic of one embodiment with one pump fiber and an offset reflective element.

Fig. 25 shows a layout of optical component design

Fig. 26 shows a Raytracing image showing calculated coupling of 100%

Fig. 27 shows a schematic of one embodiment with one multicore optical fiber, a fiber bundle and a reflective element

Fig. 28 shows schematics of multicore optical fiber designs

The figures are schematic and simplified for clarity, and they just show details, which are essential to the understanding of the invention, while other details are left out.

Example 1 , optical component

Fig. 3 schematically shows an embodiment 30 of the present invention; Fig. 3a being a longitudinal view showing how pump light 35 is coupled to a high NA, double clad fibre (first fibre) 31 from an off axis co-directional pump fibre (second fibre) 32 via a reflective element (reflector element) 33 and signal light 36 is coupled out from the high NA, double clad fibre (first fibre) through the reflective element (the reflector element has a coating that provides high reflection of the pump light, but not of the signal light). The first and second fibre(s) are hold together in a mounting tube 34. The number of second fibres may be 3, 6, 12, 18, but it can be any number, such as 3 or larger, such as 6 or larger. Fig. 3b shows a cross section of the optical component 30 including the ends of the first and second optical fibres 31 , 32 (here 7 pump fibres) and the mounting tube 34 (along plane AA' in Fig. 3a) to which the reflector element 33 is optically coupled.

The optical component (or assembly) solves some of the above described problems and provides an optical component for fibre amplifiers that allow pumping of the double clad fibre from one end, while the other end of the double clad fibre is freely accessible for coupling in signal light. In an embodiment, the freely accessible end is spliced to an optical fibre. In another embodiment, the freely accessible end is tapered. In another embodiment, the freely accessible end is tapered and spliced to an optical fibre. This eases the in-coupling of signal light into the double clad fibre. In this manner, counter-propagating pumping is obtained. The assembly provides pump combiner/coupler with signal feed-through in one simple optical component. The assembly is robust and protected by the mounting tube. A whole range of further packaging means can be applied, as the waveguiding strucuture is safely embedded within the mounting tube. Also the assembly provides low back reflection to the pump sources. Unintentional reflection of the signal light from the first fibre 31 into the pump delivery fibres 32 is reduced (or eliminated) because - due to the profile of the end-facet 331 of the reflector element 33 - signal light reflected from the mirror surface 331 may return to the cladding of the first fibre instead of to the pump fibre. The reflection can further be reduced by providing a coating (for example a dielectric coating) that provides high transmission of the signal light through the end-facet of the reflector element. Example 2, optical component

The following is a description of realizations of a unit which acts as a combined pump combiner and coupler. Furthermore it is described how such a combiner can be used in the realization of a fibre laser where the unit acts as a combined pump combiner/coupler, high reflector and output coupler.

The example consists of a description of the following elements of an optical component: Reflective element, first (passive pump) and second (active) optical fibres and a fibre holding element for positioning the first and second optical fibres relative to each other as well as a description of the coupler assembly and applications.

In one realization the reflective element consists of a plano-convex element 120 with a plane 122 and a spherical surface 121 , cf. Fig. 12. The spherical surface 121 is coated with a reflective coating. Such a coating could be either a dielectric coating consisting of a stack of thin layers of dielectric material with different reflective index or a metallic coating. In an embodiment the radius of curvature, R, of the spherical surface 121 is chosen close to a value which is twice the center thickness 123 of the element as sketched on Fig. 12. In this case the focus length, f, of the spherical surface is located close to the plane surface of the element. The optical axis 124 of a central beam incident on the reflective element 120 is indicated.

The at least one pump delivery fibre (or at least one second optical fibre) can in general be of any kind suitable for propagating the appropriate amount of pump light energy at the pump wavelength λ_p and with an appropriate numerical aperture, but are may be chosen such that they are compatible with industry standard pump delivery fibres. The fibres may have a core diameter of 105μm and an 1 25μm outer diameter d_out- The fibres are assumed to deliver light with a numerical aperture (NA) into free space of 0.15. Such values for the NA are some values for commercially available pump diodes emitting light in the 915nm to 976nm spectrum. Other multimode pump delivery fibres could be: (d_0Ut[μm]/NA) 100/0.22, 1 15/0.22, 200/0.22, 400/0.22, 600/0.22, etc.

The acceptance fibre (or first fibre) into which the light is to be coupled from the pump delivery (or second) optical fibre(s) can in general be any multi-clad (e.g. double clad) optical fibre having an appropriate NA adapted to the actual configuration of pump fibres and reflective element, but is may be chosen to be an air clad photonic crystal fibre with an NA large enough to capture substantially all the light coupled from the pump fibres under an angle determined by the reflective element. The inner cladding diameter of the PCF (i.e. the diameter of the inner cladding region spatially confined by the air cladding) may be chosen to be larger than the spot size of the focused pump light in the focal plane. The maximum numerical aperture of the incident pump light as well as the spot size is mainly determined by the dimensions of the reflective element and the outer diameter of the PCF fibre.

The pump and acceptance optical fibres can in principle be positioned and held together by any appropriate means, such as glue, mechanical fixation, fusing, etc. A fibre holding element may be used for this purpose. A fibre holding element for holding and positioning the pump and acceptance optical fibres relative to each other can in principle be of any appropriate form fulfilling the geometrical, optical and thermal requirements of the application.

Two embodiments of the fibre holding element are sketched in Fig. 13. Fig. 13a shows an embodiment 130 consisting of a capillary tube 131 with an inner diameter di which substantially equals the sum of the outer diameter of the acceptance fibre, 02, and two times the outer diameter of the pump delivery fibre, d3 allowing the acceptance fibre surrounded by a number of pump delivery fibres to be positioned in the capillary tube. Alternatively, di may be chosen larger than d₂+2d₃ and the capillary tube subsequently collapsed to fix the fibres in the tube or the fibres can be fixed in the capillary tube by glue or the like. Fig. 13b illustrates another realization the fibre holding element 130 consisting of an element 131 with separate holes 135 (diameter > d2), 136 (diameter > ds) for the acceptance fibre and the pump delivery fibres, respectively. The latter facilitates the process of assembling the unit with fibres. Other appropriate embodiments may comprising two concentric tubes (the inner tube having an outer diameter that is smaller than the inner diameter of the outer tube), the central opening being adapted for holding the acceptance fibre and the ring opening between the two tubes adapted for holding one or more layers of pump delivery fibrers.

The fibre holding element may be made of Glass e.g. Siθ2 as this makes it possible to fuse the whole assembly together by heating.

In one realization of the above described embodiment of an optical component, the following elements can be used: The reflective element is in this embodiment chosen to be a gold coated plano-convex, spherical lens from Edmund optics with a center thickness of 800μm and a radius of curvature of 1700μm. The lens is made of LaSFN₉ and has a refractive index of 1.85.

The pump fibre(s) is here chosen to be identical and to be standard multimode fibres with an outer diameter of 125μm and an inner clad diameter of 105μm. It is assumed that the pump light exits the pump fibres with an NA of 0.15.

The (here, single) acceptance fibre is chosen to be an air-clad PCF fibre with an inner clad diameter of 150μm (.i.e. the diameter of the region surrounded by the air cladding, the air cladding comprising at least one 'layer' or ring of relatively large and relatively closely spaced holes adapted to confine light at the relevant wavelength to the (inner) cladding region surrounded by the air cladding) and an outer diameter of 330μm (i.e. the fibre diameter, including an optional outer protective coating, if present). It is assumed that the pump fibres are stacked in a single capillary tube which means that there is no distance between the pump fibres and the acceptance fibre (in other words, the pump fibres contact the acceptance fibre along its periphery over a certain length).

In one geometry, it is possible to stack 11 pump fibres around the acceptance fibre.

Fig. 14 shows a realized optical component assembly 140 with 11 pump fibres 142 surrounding a single acceptance fibre 143. In the cross section shown, the pump fibres do not actually touch the acceptance fibre along its periphery. The intermediate space 144 between the pump fibres 142, the outer tube 141 of the holding element and the acceptance fibre 143 can e.g. be filled with a glue or any other appropriate filling material In the assembly shown in Fig. 14 the facet of the PCF fiber is sealed by collapsing the holes.

The element assembly was aligned with the plane side of the PCX lens described above. Light was launched into different pump fibers and the light coupled back into the PCF fiber was recorded. For each fiber it was possible to couple of the order of 90% of the light back into the PCF fiber as shown in Fig. 23. Taking into account reflections and absorption by the reflecting gold surface this corresponds to perfect coupling verifying the principle of coupling light off axis from a low NA multimode fiber to a high NA multimode fiber.

The results shown in Fig. 23 were obtained by individual alignment for each of the channels. It was not possible to find a single position of the assembly relative to the mirror where all fibers simultaneous coupled with equal efficiency. The reason for this is most likely that the collapse of the PCF fiber is too long.

In the present embodiment, the radius of curvature of the reflecting surface (i.e. the first end facet) of the reflector element is slightly larger than twice the center thickness of the lens the focus of the lens and is thus located inside the fibre.

The holes which define the air clading in the PCF fibre can be collapsed over a controllable length by heating the fibre tip. By doing this, a sealed facet of the PCF fibre is obtained (see e.g. published patent application no. WO 03/032039). This protects the fibre (e.g. against contamination) and in principle allows for gluing the reflective element together with the fibre. Also, by controlling the collapse length the location of the air clad aperture can be aligned with the focus of the lens (cf. WO 03/032039).

In Fig. 15 the result of ray tracing light from a pump fibre 151 onto the acceptance fibre aperture 153 is schematically shown. In Fig. 15a the origin of the traced rays 152 is shown while Fig. 15b shows where these rays hit the aperture 153 of the air clad fibre. The circle in the figure represents that aperture 153 of the air-clad of the acceptance fibre. The points 154 represent the points which are obtained by tracing rays 152 from points along five diagonals 155 of the pump fibre 151 shown in Fig. 15a. From each point five rays are traced corresponding to five different directions with divergence angle given by the NA of the fibre. As seen, the structure in principle allows for perfect coupling from the pump to the acceptance fibre. The above assembly in principle allows for simultaneous coupling of power from 1 1 pump fibres into the acceptance fibre. The following describes how two of such optical component assemblies can be combined to make a fibre laser (1600 in Fig . 16) where the optical component acts as a pump combiner, feedback element and output coupler in one.

A sketch of the laser assembly 1600 is shown in Fig. 16.

In the above mentioned realization the reflector element (mirror) was coated with gold on the convex side (cf. e.g. 121 in Fig. 12) and no coating on the plane side (cf. e.g. 122 in Fig. 12). A reflector element to be used for a laser should have a different coating. The difference is that the mirror element 163, 163', instead of being coated with gold on the reflective side is coated with dielectric coatings on both the convex and the plane side. The mirror element (cf. 163 in Fig. 16) in one end of the laser has a convex side 1631 which is coated with a coating that reflects substantially all light around 915nm 1634 with a high reflectivity while it transmits substantially all light in the range from 1020nm to 1100nm 1635. In this end the plane side 1632 is coated with a dielectric coating which reflects substantially all light in the range from 1020nm to 1100nm 1635 while it transmits substantially all light around 915nm 1634.

In the second end the coatings of the reflector element 163' are the same except that the coating on the plane side 1633 only reflects a specific amount of light may be in the range from 5-20% in the range from 1020nm to 1100nm 1635 (as indicated by the arrows on the plane face 1633 in Fig. 16c). The rest of the light at this wavelength is transmitted out of the assembly.

In this way the first end (cf. left end of Fig. 16a and Fig. 16b) acts as high reflector for a laser cavity while the other end (cf. right end of Fig. 16a and Fig. 16c) acts as the output coupler of the cavity.

The two optical component assemblies constituting the laser, each have their own pump delivery fibres 161 but share the same acceptance fibre 162. The pump fibres may e.g. be standard multimode fibres with characteristics as indicated above. The acceptance fibre 162, which is chosen for th is embodiment, is a double clad PCF fibre comprising an air cladding with dimensions as described above and with a single mode core which is doped with Ytterbium, Yb.

Today a standard pump laser at 915nm can deliver an output power of the order of 8 to 10W. In the configuration shown above there are 22 channels

(2x11 ) via which pump light from such sources can be delivered to the assembly. Hence in the realization above it is possible to deliver of the order of 200W of pump light. With a laser operating with a power conversion efficieny of 75% this would result in a single mode laser with an output power of 150W.

The realization described above can be varied in many ways. In the following some embodiments of parts of the optical component are mentioned. The various embodiments of different parts of the optical component are intended to be freely combined with each other (possibly appropriately adapted to the application in question).

Concerning the reflector or mirror element, the shape of the element is not limited to a spherical element. Better focusing properties can in principle be obtained using aspherical, reflective surfaces, which are designed to optimize the coupling efficiency. It should also be noted that such surfaces do not necessarily need to be rotation symmetric around the axis of the acceptance fibre. Non rotation symmetric structures can in principle be used to optimize coupling from a limited number of pump fibres with a geometry which cannot be efficiently coupled via a rotational symmetric mirror. A schematic example of an optical component 220 according to the invention comprising a aspherical reflector element 224 is shown in Fig. 22. The optical component comprises a centrally located acceptance fibre in the form of a photonic crystal fibre 223 surrounded by a number of pump fibres 221. The acceptance fibre and the pump fibres are surrounded by a holding element 221. The acceptance fibre 223 comprises a core region 2231 surrounded by a pump core 2232, surrounded by an air cladding 2234, which again is is surrounded by an outer cladding region 2233. The air cladding is collapsed over a predetermined distance from the end of the acceptance fibre facing the first end-facet of the reflector element. The pump fibre 222, comprises a pump core 2221 surrounded by a cladding region 2222. The reflector element 224 comprises a first plane end-facet optically coupled to the ends of the acceptance and pump fibres and an asphehcally profiled second end- facet 2241 , 2242, 2243. Parts 2241 , 2243 of the area of the second end-facet are optimized for reflecting light from the pump cores 2221 of the pump fibres 222 into the pump core 2232 of the acceptance fibre 223. Other parts 2242 are adapted for transmitting a specific fraction of the signal light from the core region 2231 of the acceptance fibre to an application. The surface 2242 including an area around the central optical axis of the core region of the acceptance fibre can e.g. be plane and perpendicular to said optical axis. Alternatively, it may be profiled to have a collimating or focusing function. The design of asphehcal reflective surfaces is illustrated in the example below.

Further, the reflector or mirror element does not have to be a plano-convex structure. In principle the plane surface can be replaced by a tilted or curved surface. A tilted surface could be used to reduce reflection of light from the core of the acceptance fibre back into the fibre as shown in Figs. 17a and 17b. The embodiment of an optical component 170 shown in Fig. 17a comprises a holding element in the form of a tube 171 wherein a central acceptance fibre 173 surrounded by a number of pump fibres 172 are inserted. The end facet 1711 of the holding element including the pump and acceptance fibres is tilted compared to the optical axis (longitudinal direction, cf. arrow 177) of the acceptance fibre. The first plane end facet 1742 of the reflector element 174 facing the ends of the pump and acceptance fibres is tilted correspondingly. A tilt angle (relative to a plane perpendicular to the optical axis of the acceptance fibre 173, cf. arrow 177) in the range from 1 ° to 25°, such as in the range from 1 ° to 5° or from 5° to 15°, such as in the range from 8 ° t o 1 2 ° may be used (cf . eg . PCT a ppl ication no . PCT/DK2004/000439 published as WO 2004/111695). The second end facet 1741 of the reflector element 174 is spherical in shape. The acceptance fibre 173 is indicated to be a photonic crystal fibre comprising a core region 1736, an inner cladding region 1731 surrounded by an air cladding 1732. Near the (first) end of the (first) acceptance fibre facing the reflector element, the holes of the air-clad 1732 have been collapsed (e.g. by heating, e.g. in a fusion splicer) over a length L₀ from the end facet thereby providing a length of solid glass fibre 1735 facing the reflector element. The optimization of the lengths L₀ in relation to mode field diameter is e.g. discussed in WO 03/032039. The pump fibres 172 are indicated to have a core region 1721 and a cladding region 1722.

The embodiment shown in Fig. 17b is similar to the one shown in Fig. 17a. The difference is that instead of tilting the end facets of the holding element 171 , pump 172 and acceptance 173 fibres as well as the (first) end facet 1742 of the reflector element, only the (first) end 1733 of the (first) acceptance fibre 173 is tilted, thereby leaving a volume 1734 (e.g. filled with air) between the end facet of the acceptance fibre and the plane face 1742 of the reflector element 174. The reflector element 174 is optically coupled to the pump 172 and acceptance 173 fibres. The reflector element is e.g. joined to the holding element and/or the pump and/or the acceptance fibres by any appropriate joining technique, e.g. by abutment, by a glue, by heating and locally softening the materials at the joint, etc.

Further, a reflective element could consist of a simple curved mirror. As e.g. sketched on Fig. 17c, where the reflector element 174 comprises a piece of bulk material with a curved surface 1742 (the first end facet of the reflector element) made as an indentation into the material. The volume 176 between the end facets of the pump 172 and acceptance 173 fibres and the reflecting surface 1742 can e.g. be filled with air or another appropriate gas or liquid or evacuated. The arrows indicate the direction of pump light in the pump fibres 172 and of the pump light reflected into the acceptance fibre 173. The details of the embodiments of Figs. 17a and 17b may of course be combined with the embodiment of Fig. 17c.

Fig. 17d illustrates another feature of the present invention, namely the easy configurability of the reflector element. In Fig. 17d a reflective element 174 comprising a single reflective end-facet 1742 (as in Fig. 17c) for reflecting light from pump fibres into the pump core of a centrally located acceptance fibre is shown. The reflective element has a central opening 1746 (here a wedge-shaped opening 1747 increasing in width with increasing distance from the reflective end-facet 1742) for propagating light from a central part of the acceptance fibre, e.g. amplified signal light at a signal wavelength λ_s from a signal core. The end-facet 1742 is provided with a reflective coating for enhancing the reflectivity of light at a pump wavelength λ_p. The reflector element can be adapted for removing heat generated by the incident (pump and signal) light, e.g. in the form of cooling ribs, Peltier-element(s) and/or cooling channels (e.g. for liquid cooling). In the embodiment shown the reflector element comprises two cooling channels 1743 adapted for flowing water or other liquid. The reflector element 174 may e.g. be made in a single material, such as a metal (e.g. Al or Cu or Ag or Fe (e.g. steel)) or a ceramic material. Alternatively, the reflector element 174 may be made in several pieces, e.g. two, a front piece 1744 comprising the reflecting end-facet 1742 (e.g. made of Al or Ag) and a rear piece 1745 comprising the cooling channel(s) 1743 and a central opening for the signal light (e.g. made of Cu or steel).

The embodiment shown in Fig. 17e is identical to the one in Fig. 17d except that a first optical element 1748 is positioned in front of the reflecting end- facet 1742 of the reflector element 174 and a second optical element 1749 is positioned in the optical output path of the reflector element. The reflector element may easily configured with first and/or second optical elements according to the application in question. The first optical element 1748 comprise a h igh reflector mirror that reflects signal light at a signal wavelength λ_s but substantially transmits light at a pump wavelength λ_p. Alternatively, the first optical element can be adapted to transmit a fraction of signal light at a signal wavelength λ_s. The second optical element 1749 can e.g. comprise a collimating lens or a focusing lens.

Concerning the fibres used there are some important variations in the choice of acceptance fibre which can be used for this purpose. PCF fibres in principle allows for scaling of the NA of the inner clad to extremely high values such as e.g. between 0.6 and 0.9 or even higher. The main limiting factor in utilizing such high NAs is that the mechanical properties of the fibre becomes poor in the sense that cleaving and splicing becomes difficult. In principle the use of mirror couplers provide a practical way of utilizing such high NAs. In order to implement a PCF fibre with a very high NA in a laser assembly, similar to the one described above, no splicing or cleaving of the acceptance fibre is needed. First the air holes in the fibre can be collapsed and subsequently the collapsed region can be cleaved at a specific distance from the start of the collapse. Using ultra high NAs opens for the possibility of scaling the number of pump channels which can be added to an assembly. The principle of this is shown in Fig. 18. The principle is that the higher angle tolerance of the high NA fibre allows for multiple rings of pump fibres to be added to the device.

In principle any number of pump fibres can be mounted around a given acceptance fibre, the arrangement in layers or 'rings' around the acceptance fibre being dependent on the actual geometries (outer dimensions) of the fibres in question. Fig. 18 shows an example of coupling from a second ring of pump fibres 183 to a central acceptance fibre 181 , the second ring of pump fibres being added to the structure describe above, i.e. surrounding a first ring of pump fibres 182. The distance r, (here n, r₂) is the distance from the center of the acceptance fibre 181 to the center of a fibre in the i^th ring of pump fibres. In the present embodiment, the diameter of the second ring 2r₂ equals 335 μm, NA of the pump fibres NA_pump equals 0.15, NA_max of the acceptance fibre equals 0.84, the reflector is a plano-convex PCX 43397 lens from Edmund Optics Inc. (Barhngton, NJ, USA) and the focus of the reflector is arranged to be inside the acceptance fibre a distance (here 30 μm) from the end facet.

Another consequence of the using fibres with higher NA is that it allows you to move the pump fibre further away from the center. This fact could be of major importance in combination with PCF Rod-type fibres.

Rod-type fibres are characterized by a very large single mode core with mode field diameters in the range from 30-1 OOμm or more. The large core is combined with a relatively small diameter of the inner clad which results in an extremely high pump absorption. Such fibres are of principal interest for use as pulse amplifiers as the large core can withstand the extremely large peak intensities of pulses. In order to be able to sustain low loss propagation of large modes the rod-type fibre has to be very stiff in order to reduce micro bending loss. The rod-type fibre therefore may comprise an outer cladding or jacket region that is optimized to provide stiffness to the fibre. In order to use a mirror coupler together with a rod-type fibre a very high NA of the inner clad is therefore desirable. One major challenge in utilizing such rod-type fibres is to find a way of coupling in the pump light while maintaining access to the core in each end. Another issue is that it is desirable to have the output from the amplifier diverge to a large spot size before it exits the fibre material into air. The purpose of this is to limit the power density on the exit facet to prevent damage. Finally it is of major importance to have substantially no light from the core getting into the pump lasers i.e. the isolation between the amplifier signal and the pumps has to be perfect.

In Fig. 19 a sketch of a coupler combined with a rod-type fibre is shown. The optical component 190 comprises a holding element 191 in the form of a tube wherein a rod-type acceptance fibre 193 is centrally located and surrounded by pump fibres 192. The rod-type optical fibre 193 has a core region 1931 surrounded by inner cladding region 1932 and outer cladding or jacket region 1933. The reflector element 194 has reflecting surface(s) 1941 for reflecting the pump light into the first cladding of the acceptance fibre and a central protrusion 1942 for adapting the mode field diameter 1943 of the light from the core of the acceptance fibre to the optical fibre or component that is to receive the light in question. An air-cladding for confining (pump) light to the inner cladding may be located between the inner cladding region 1932 and the outer cladding or jacket region 1933.

Example 3: Optimization of non rotational symmetric asphehc reflector element

This following describes a procedure for designing a reflective end-cap coupler (reflector element) with a given shape or profile of the reflecting end- facet to couple light from a pump delivery fibre (second fibre) into a double- clad fibre (first fibre).

In order to design a suitable reflector we consider a ray 443 leaving an end 422 of the pump fibre 42 (second end) and consider the criteria which has to be fulfilled for the ray 444 to hit the end 413 of the first fibre 41 (first end) at an angle β within the acceptance cone of the first fibre, cf. Fig. 4. Consider a ray 443 leaving the center of the pump core 421 of the pump fibre 42 at the second end 422 with an angle (90-α) to the y axis determined by the NA of the fibre, (referred to as NA_Pump or as NA2). The line can be described by

y = tan(sin ^{^1} NA_pump )x + d = m_vx + d .

This ray has to be reflected by the surface 441 (an end-facet of the reflector element) into a ray 444 which intersects with the center (x-axis) of the first fibre PCF 41 at an angle β determined by NA_PCF (also referred to as NA1 ). A tangent 442 to the reflecting surface 441 in the point of reflection of the ray 443 from the pump core 421 is indicated. The line followed by the reflected ray 444 can be described by

y = tan(sin ^{^1} NA_PCF )x = m₂x .

In general the slope of the reflective surface, in order to be able to reflect a beam back to (0,0) is given by

dy ₌ l f y _| y -d dx 21 x x

A general solution to this equation can be written

where the constant, c, can be found from the intersection point of the two lines defined above.

The shape described above describes only the shape of the surface in the xy plane. To finish off the design the shape of the mirror in the xz plane is given by a z² dependence. χ(y,z) = .

Note that the shape described above does not have rotational symmetry around the x axis. Also the shape of the mirror in the xy plane is aspherical.

As will be evident from the example below a consequence of these features is that the number of pump fibres which can be used is limited. On the other hand the design opens for the possibility of coupling light from pump fibres into the double clad fibre in configurations where spherical reflective surfaces do not provide efficient coupling.

Example 4, Coupling to rod-type fibre

In order to illustrate the use of an aspherical non-rotational symmetric structure we consider coupling from pump fibres to rod-type fibres. As mentioned rod-type fibres are characterized by a small inner clad with a large guiding core inside. In order to keep the fibre rigid enough to avoid microbending losses that out diameter of the fibre is very thick. The large distance between the pump fibre and the inner clad makes it impossible to achive efficient coupling via a sperical mirror shape.

In the following example a structure as shown on Fig. 9 is considered. The cross-sectional view of the optical component 90 shows a rod-type fibre 91 , which is assumed to have a diameter di of 1 .3mm (including outer cladding or jacket region 912). The inner clad 911 of the fibre (possibly spatially limited by a ring of air-holes constituting an air-clad region) is assumed to have a diameter d3 of 150μm. The NA of the inner clad is assumed to be 0.6. The pump fibres 92 are assumed to have an outer diameter 62 of 125μm and an inner clad with a diameter of 105μm. The NA of the light coming out of the pump fibres is 0.22.

Using the algoritm described above the following aspherical shape of the mirror element is found for a fibre displaced 702.5μm from the center of the core rodfibre:

x(y,z) = V5.82 - 10⁶ + 700 - j - / - z² In the equation above it is assumed that the axis of the fibre is the x-axis and the fibre is displaced from the center of the rod along the y-axis.

On Fig. 10 the result of ray tracing light from a 105/125μm 0.22 NA pump fibre displaced 702.5μm from the center of the rod-type fibre via the optimized aspherical non-rotation symmetric mirror onto the inner cladding (911 in Fig. 9) of the rod-type acceptance fibre (91 in Fig. 9) is shown. In the simulation it is assumed that the inner clad is collapsed to a distance of 60μm behind the exit facet of the pump fibre as explained earlier. As seen a perfect coupling is found. The maximum angle of incidence on the facet corresponds to a NA of 0.65.

In order to multiplex light from several fibres an aspherical structure as shown on Fig. 11 can be used. The structure basically consists of four regions similar to the one described by the optimized structure above for a fibre displaced along the ±y-axis as well as along the ±z-axis (see Fig. 4).

The rod fibre may be modified to bring the pump fibres closer to the inner cladding when integrating the coupler with a rod-type fibre. In Fig. 20 an example of this is shown in the form of a cross section of a rod fibre perpendicular to its longitudinal direction. By adding large air holes 204 in the outer cladding 203 of the rod fibre 200 it is possible to provide access channels for the pump fibres close to the core 201 and inner cladding 202 of the acceptance fibre while maintaining the stiffness of the fibre. The access channels 204 may be made in any appropriate size and number (her 4 relatively large holes are made) and each may contain one or more pump fibres adapted to the specific application and reflector element. In the present embodiment the access channels have a diameter similar to that of a pump delivery fibre. Alternatively, each hole or one or more holes may comprise several pump fibres and/or be adapted to act as cooling channels (e.g. by flowing a cooling liquid). The access channels may e.g. be made as part of the manufacturing process of the rod-type fibre (by inserting appropriately sized tubes in the preform) or after fabrication, e.g. using a laser, e.g. a CO2- laser. One further approach to this is to make a fibre with an edged profile. An embodiment of this is shown in Fig. 21. This fibre 210 is similar to the one shown in Fig. 20 except that the outer shell is removed. The outer profile of the fibre can be of any appropriate form (in Fig. 21 represented by four curved edges 213 and four vertices 214) but should be tailored to maintain the stiffness of the fibre and to be practically handled (e.g. by rounding off some or all of the vertices or giving the outer surface of the rod-type fibre any other appropriate profile (compatible with practical handling and relatively high stiffness, e.g. 'I'). The rod-type fibre can thus act simultaneously as a multi-cladding acceptance fibre and a holding element for the pump fibres of an optical component according to the invention. The rod-type fibre based components may be combined with any of the reflector elements discussed above.

It should be mentioned that an additional benefit of such a design is that it improves the thermal properties of the fibre significantly compared to "conventional" rod-type structures which is of importance as thermal effects are a limiting factor in relation to power scaling in such fibres.

Methods of manufacturing an optical component

Fig. 6a shows a schematic drawing of a method for producing an optical component 60 according to the invention. The method comprises: 1. The fibres 61 , 62 are inserted into a silica capillary tube 64 with an inner diameter matching the outer dimensions of the assembly of first 61 and second 62 fibres. The capillary tube 64 may consists of different tapered regions as shown in Fig. 6a. The trumpet region 642 is for guidance of the ends, 628 of the coating surrounding the fibres. Region 643 is for fixing the fibres with their coating 627. The coating of the fibres is optionally removed over a length in the vicinity of the ends 611 , 621 of the fibres that are to face the reflector element 63 (after the cleaving process in step 3). Region 644 is for fixing the uncoated fiber part of the fibers. The centre element 645 is for centering of the first fiber 61 (e.g. a PCF) in the tube. Region 645 has a diameter that closely fits to that of the first fiber 61.

2. The fibres 61 , 62 are fixed within the tube 64, where possible ways of doing this include gluing or fusing the assembly.

3. The ends 61 1 , 621 of the assembled fibres 61 , 62 (and optionally the end 641 of the holding tube 64) is cleaved/cut and/or polished to provide a plane facet for mounting the reflective element 63. The cleaved/cut position may be located in region 644 of the assembly embodiment shown in Fig. 6.A.

4. The reflective element 63 (here a plano-convex element) comprising a reflecting end-facet 631 is attached to the assembly. This can e.g. be done either by gluing or fusing.

5. In an eventual 5 step (not illustrated in Fig. 6), the surface 631 of the reflective element is coated with a reflective coating

Fig. 7 shows a schematic drawing of another method for producing an optical component 70 according to the invention, wherein the mounting tube and the reflector element are integrated into one piece 74. The method comprises

1. Inserting the stacked ends 71 1 , 721 of a first acceptance fibre 71 and surrounding (second) pump fibres 72 into a holding element 74, here in the form of an opening of a capillary tube, which is integrated with a reflector element having first 742 and second 741 end-facets, with predetermined profiles.

2. Arranging that the ends 71 1 , 721 of the fibres optically connect, here abut, the first, here plane, end-facet 742 of the integrated holding and reflector element 74. 3. Coating the second end-facet 741 of the integrated holding and reflector element 74 with a reflective coating 742 over an area of the second end-facet of the integrated holding and reflector element 74, while optionally arranging that a central area 743 of the first 742 and second 741 end-facets are adapted to transmit (at least a fraction of the) light propagated in a central part of the first acceptance fibre.

A method of coating a reflector element

Fig. 8 shows a schematic drawing of how to coat a reflective element according to an embodiment of the present invention using a metallic coating for improving the reflectivity of the reflector element. The method comprises the following steps: a) A reflective element 83 comprising an end-facet 831 with a predetermined profile is provided e.g. using grinding or injection molding techniques; b) The end-facet 831 is coated with a photoresist 832; c) A central area 833 of the end-facet is exposed with UV-light; d) The photoresist is developed leaving only photoresist at the central area 833 of the end-facet 831 ; e) The end-facet is coated with a reflective coating 834, e.g. a metallic coating, e.g. comprising Au; f) The remaining photoresist is removed, e.g. by a lift-off technique, leaving the central area 833 without any reflective coating 834.

In the present method, a part of the end-facet of the reflector element is left uncoated. In this example, the uncoated central region is obtained using conventional photo-lithographic techniques as known from e.g. integrated electronics and integrated optics manufacturing techniques. Alternative methods include depositing a coating over the whole end-facet of the reflector element and polishing away the coating over a desired region.

The uncoated part may be used in order to ensure low reflectivity of the signal light (e.g. propagated by a signal core) from the reflector element.

Example: one pump fiber

With reference to figure 24 to 26 is described an optical component design with one pump fiber and an offset reflective element. The pump fiber and the signal fiber are held in place in a fused bundle with proper dimensions. Pump delivery and signal fiber are placed in a distance d from the spherical mirror apex. The spherical mirror has a radius of curvature of R. The mirror could be a drum PCX lens with HR coating at the curved surface. The lens can then be fused onto the fused fiber bundle.

The raytrace matrix for a propagation of distance d=R, mirror reflectance and propagation of distance d=R is found to be

This configuration means that all rays have 1 :1 magnification and in theory enables 100% pump coupling, but the system is not rotational symmetric! Bigger R is preferred, but should be maintained as small as possible while fulfilling other constraints. However, since R can be chosen relatively free, it means that there is 'room' for significant beam expansion before reaching the mirror apex and the drum lens can be macroscopic which benefits handling and fusing.

There is overlap between the signal and the pump at the mirror apex, which means that separation of the two optical rays should be taken care of either by dichroic coating or an uncoated area. In the case of an uncoated area, there will be a pump loss of (NA_sιgnai/NAp_Ump)².

Since the system is not rotational symmetric, there is a displacement of the output signal which has to be corrected by an external lens, could be PCX or asphehc. Alternatively, a free form of the mirror/lens could provide plane surface at the signal output. A layout of this optical component design is seen in figure 25, while the Figure 26 shows a Raytracing image of the calculated coupling of 100%

Example

With reference to Figure 27 is described a configuration wherein the multicore optical fiber 2701 according to the present invention is arranged in relation to a reflective element 2703 and a fiber bundle 2702. The fiber bundle used in could be a 14:1 +1 fiber bundle, wherein 14 fibers are arranged to launch an optical signal into the second pump core(s) of the multi-core optical fiber, and 1 usually centrally located optial fiber is arranged to lauch light into the signal core and/or the first pump core of the multi-core optical fiber. The fiber bundle may comprise other numbers of optical fibers for coupling light into the second pump cores as is apparent to the person skilled in the art. At the end opposite to the fiber bundle, a reflector element is arranged to relect optical signals propagating in the second pump core(s) into the first pump core. The reflector element may have a feed-through arranged to allow at least a part of the optical signal from the signal core to pass through the reflector element. Example- multicore optical fiber designs

With reference to Figure 28 are described multicore optical fiber designs according to the present invention. The 4 illustrared designs all comprise a signal core 2801 surrounded by a first pump core 2803 which again is surrounded by a first outer cladding 2802. Different configurations of the second pump core 2805 and surrounding cladding regions are illustrated in Figs. 28a-d. In Figures 28a-c the second pump core comprises a coherent ring concentrically arranged in relation to the first pump core, while in Fig 28d the second pump core 2805 comprises a second pump core feature which comprises a longitudinal axis that is arranged outside said first core region and hence does not surround the first pump core 2803. With the arrangement of fig. 28d the multicore optical fiber may be used together with a reflector element comprising an end-facet that is rotation symmetric around an axis which is offset relative of the optical axis of the multicore optical fiber.

One realization of the silica based multicore optical fiber design according to Fig. 28a could have a signal core diameter of 40 μm, the signal core being doped with Ytterbium, and an air cladding with an outer diameter of 200 μm defining the perimeter of the first outer cladding. The second cladding low- index region and the third cladding low-index region are made of Fluorine doped silica material with a diameter of: 460 μm and 670 μm, respectively. The outer diameter of the fiber is 850 μm.

One realization of the silica based multicore optical fiber design according to Fig. 28b could have a signal core diameter of 40 μm, the signal core being doped with Ytterbium, and an air cladding with an outer diameter of 200 μm defining the perimeter of the first outer cladding. The second cladding low- index region and the third cladding low-index region are made of Fluorine doped silica material with a diameter of: 460 μm and 670 μm, respectively. The outer diameter of the fiber is 700 μm.

One realization of the silica based multicore optical fiber design according to Fig. 28c could have a signal core diameter of 40 μm, the signal core being doped with Ytterbium, and an air cladding with an outer diameter of 200 μm defining the perimeter of the first outer cladding and the inner perimeter of the second core region. The third cladding low-index region is made of Fluorine doped silica material with a diameter of: 480 μm. The outer diameter of the fiber is 550 μm.

One realization of the silica based multicore optical fiber design according to Fig. 28d could have a signal core diameter of 40 μm, the signal core being doped with Ytterbium, and an air cladding with an outer diameter of 200 μm defining the perimeter of the first outer cladding. The second core region is defined by a third cladding low-index region made of Fluorine doped silica material with a diameter of: 125 μm. The second pump core longitudinal axis is arranged outside the first pump core region The outer diameter of the fiber is 600 μm. Optional there may be several of these second pump cores.

These design parameters can be varied, for example, the dimensions of second and third cladding low-index regions.

Example: reduction of signal light leak

Figure 29 shows two coupler designs where the problem of leaking of signal light can be mitigatged with an optical component design according to thre present invention

In Figure 29a the first fiber 2901 and the second optical fiber 2902 are arranged substantially along the same line. The presence of the reflector element 2910 provides an offset in the beam direction and the two optical fibers may be slightly offset. Pump light 2905 from the second optical fiber 2902 is delived to the pump core of the first optical fiber by an optical assembly comprising two lenses. The amplified signal 2909 having propagated in the core region of the first optical fiber exits the first fiber and is directed away from the second optical fiber by the relector element 2910. The incomplete reflection on the signal by the reflector elements results in a leaking signal 2911 propagating towards the second end of the second optical fiber. When the second optical fiber comprises a guiding structure embedded in its pump core, the damage that the leaking signal may do to the pump sources is reduced.

In Figure 29b the first fiber 2901 and the second optical fiber 2902 are arranged along lines having an angle relative to one another. Pump light 2905 from the second optical fiber 2902 is delived to the pump core of the first optical fiber by an optical assembly comprising two lenses and a reflector element 2910. The amplified signal 2909 having propagated in the core region of the first optical fiber exits the first fiber and is substantially transmitted by the relector element 2910. However, an incomplete transmission results in that a fraction of the signal propagateds towards the second end of the second optical fiber 2902. When the second optical fiber comprises a guiding structure embedded in its pump core, the damage that the leaking signal may do to the pump sources is reduced.

Figure 30 shows an optical system according to the present invention wherein the second optical fiber comprises a guiding structure embedded in the pump core 3014. The guiding structure comprises a first region 3012 and a second region 3012 comprising F-doped silica glass. The pump core 3014 being surrounded by an air cladding 3015. The lntermidieate region comprising a monitor region corresponding to the monitor optical fiber 3017 which guidues the collected signal to the signal power monitor 3018

The invention is defined by the features of the independent claim(s). Preferred embodiments are defined in the dependent claims. Some embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.

Claims

1. An optical component having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis, the optical component comprising: a. a first optical fiber comprising a first fiber end, a pump core with a first numerical aperture at said first fiber end, and a core region different from said pump core, said core region being adapted for propagating light at a signal wavelength, λ_s b. at least one second optical fiber comprising a first fiber end and a second fiber end, said at least one second optical fiber being arranged in relation to said pump core of said first fiber, said at least one second fiber comprising a pump core with a second numerical aperture at its second fiber end that is smaller than the first numerical aperture, the second optical fiber comprising a guiding structure embedded within the pump core, said guiding structure extending from said second end to said first end of said second optical fiber and being capable of propagating light at a first wavelength, λi; and c. an optical assembly arranged to direct pump light from said second fiber end of said second optical fiber into said pump core of said first optical fiber

2. The optical component according to claim 1 , wherein said guiding structure comprises a first region having a first effective refractive index, and a second region surrounding said first region and having a second effective refractive index which is smaller than said first effective refractive index at least at λi.

3. The optical component according to claim 3, wherein said second region of said guiding structure comprises down doped silica material, such as F- doped silica glass.

4. The optical component according to claim 2 or 3, wherein said second region of said guiding structure comprises an air cladding.

5. The optical component according to any of claims 1 to 4, wherein the numerical aperture of said first region of said guiding structure is in the range of about 0.02 to about 0.22, such as in the range of about 0.04 to about 0.20, such as in the range of about 0.06 to about 0.15, such as in the range of about 0.08 to about 0.12.

6 The optical component according to any of claims 1 to 5, wherein said guiding structure is substantially circular in said cross section and wherein the diameter of the first region is in the range of about 5 μm to about 200 μm, such as in the range of about 10 μm to about 150 μm, such as in the range of about 12 μm to about 125 μm, such as in the range of about 15 μm to about 100 μm, such as in the range of about 20 μm to about 75 μm, such as in the range of about 25 μm to about 60 μm, such as in the range of about 30 μm to about 50 μm.

7. The optical component according to any of claims 1 to 6, wherein λ_s is substantially equal to λi.

8. The optical component according to any of claims 1 to 7, wherein the pump core of said first optical fiber and the pump core of said second optical fiber are capable of propagating pump light at a pump wavelength λ_p, different from said signal wavelength λ_s;

9. The optical component according to any of claims 1 to 8, wherein the pump core of said second optical fiber is surrounded by an air cladding.

10. The optical component according to any of claims 1 to 9, wherein said optical assembly comprises a reflector element.

11. The optical component according to claim 10, wherein said reflector element comprises an end-facet with a predetermined profile.

12. The optical component according to claim 11 , wherein said end-facet of said reflector element is rotation symmetric around an axis, which is substantially parallel to and offset relative of the longitudinal, optical axis of the optical component.

13. The optical component according to claim 12, wherein said end-facet of said reflector element is rotation symmetric around an axis which is substantially coinciding with the longitudinal, optical axis of the optical component

14. The optical component according to any of claims 11 to 13, wherein said end-facet of the reflector element is capable of reflecting light at said signal wavelength λ_s and said pump wavelength λ_p differently.

15. The optical component according to any of claims 10 to 14, wherein said reflector element is arranged to reflect pump light from said second fiber end of said second optical fiber into said pump core of said first fiber.

16. The optical component according to claim 15, wherein the ratio between the reflection coefficient of light at said signal wavelength and the reflection coefficient of light at said pump wavelength from said reflector element, is less than about 0.5, such as less than about 0.1 , such as less than about 0.01 , such as less than about 0.001 , such as less than about 0.0001

17. The optical component according to any of claims 1 to 16, wherein said guiding structure is arranged to collect signal light reflected from said reflective element onto said second fiber end of said second optical fiber.

18. The optical component according to any of claims 10 to 14, wherein said reflector element is arranged to reflect signal light from said core region of said first optical fiber away from said second fibre end of said second optical fiber.

19. The optical component according to claim 18, wherein the ratio between the reflection coefficient of light at said pump wavelength and the reflection coefficient of light at said signal wavelength from said reflector element, is less than about 0.5, such as less than about 0.1 , such as less than about 0.01 , such as less than about 0.001 , such as less than about 0.0001

20. The optical component according to any of claims 1 to 19, wherein said guiding structure is arranged to collect signal light leaking through said reflective element.

21. The optical component according to any of claims 10 to 20, wherein said predetermined profile comprises a curved section.

22. The optical component according to any of claims 10 to 21 , wherein said predetermined profile comprises a substantially planar section

23. The optical component according to any of claims 2 to 22, wherein the numerical aperture of said guiding structure is below the numerical aperture of the pump core of said second optical fiber.

24 An optical system for launching pump light into a pump core of a first optical fiber, said optical system comprising a) an optical component according any of claims 1 to 23; b) a plurality of pump light sources, c) an optical combiner comprising a. an input section comprising a least one monitor optical fiber and at least one input optical fiber for connecting the optical combiner to said pump light sources; b. an intermediate section over which the input optical fiber and the monitor optical fiber are bundled; and c. an output section comprising an output end, at which a monitor region of the bundle corresponding to said monitor fiber is substantially optically decoupled from an input region corresponding to the input optical fibers, such that an optical signal collected by the monitor region at the output end is guided through said combiner substantially without transfer of optical power to the input fiber; wherein said output end is arranged in optical connection to said second optical fiber first end in such a way that at least a part of a signal propagating in said guiding structure of said optical component is coupled into the monitor region, and such that pump light guided in said input fibers is delivered to said pump core of said second optical fiber via said input region

25. The optical system according to claim 24, wherein said monitor optical fiber is connected to a detector arranged to monitor the optical power of the signal collected by the guiding structure from the first end of said first optical fiber.

26. The optical system according to claim 24 or 25, wherein a major part of the optical power of the signal, which is directed onto said second fiber second end from said core region of said first optical fiber is collected by said guiding structure and delivered to said monitor optical fiber via said monitor region, such as more than about 60% of the optical power, such as more than about 70% of the optical power, such as more than about 80% of the optical power, such as more than about 90% of the optical power, such as more than about 95% of the optical power, such as more than about 98% of the optical power.

27. The optical system according to claim 24 to 26, wherein said first optical fiber is a PCF double-clad fiber comprising a core region for propagating light at a signal wavelength, an inner cladding region - termed a pump core - surrounding the core region for propagating light at a pump wavelength and an air cladding comprising at least one ring of relatively large holes surrounding the inner cladding region.

28 The optical system according to claim 24 to 27, wherein said input section comprises a plurality of input fibers, such as six, arranged to surround sad monitor fiber.

29 An optical system for launching pump light into a pump core of a first optical fiber, said optical system comprising a) an optical component according any of claims 1 to 23; b) a plurality of pump light sources, c) an optical combiner comprising i. an input section comprising a plurality of input optical fibers for connecting the optical combiner to said pump light sources; ii. an intermediate section over which the input optical fibers are bundled; and iii. an output section comprising an output end and an input region corresponding to the input optical fibers; wherein said output end is arranged in optical connection to said second optical fiber first end in such a way that pump light guided in said input fibers is delivered to said pump core of said second optical fiber via said input region, and such that less than about 50% of the optical power of a signal propagating in said guiding structure of said optical component is coupled into the input region.

30 The optical system according to claim 29, wherein less than about 40% of the optical power of a signal propagating in said guiding structure of said optical component is coupled into the input region, such as less than about 30%, such as less than about 20%, such as less than about 10%, such as less than about 5%, such as less than about 2%, such as less than about 1 %, such as less than about 0.5%, such as less than about 0.1 %.