WO2009003484A2

WO2009003484A2 - A tapered fibre optical coupler comprising an annular guiding region

Info

Publication number: WO2009003484A2
Application number: PCT/DK2008/000256
Authority: WO
Inventors: Peter M. W. Skovgaard; Martin Dybendal Maack; Thomas Nikolajsen
Original assignee: Crystal Fibre A/S
Priority date: 2007-07-05
Filing date: 2008-07-07
Publication date: 2009-01-08
Also published as: WO2009003484A3

Abstract

The present invention relates to an optical coupler having at least one longitudinal axis, said optical coupler comprising a first segment having first cross sections perpendicular to the longitudinal axis and a second segment having second cross sections perpendicular to the longitudinal axis said first and second segments sharing a common border interface, whereinsaid said first segment comprises an internal volume with refractive index nv selected from the group of a filling material, a vo.d or a combination thereof, where said internal volume is tapered whereby it is possible to combine light from multiple fiber with reduced loss of brightness.

Description

An optical connector, an optical assembly, a light source and a method for manufacturing an optical connector

The present invention relates to an optical connector, an optical assembly, a light source and a method for producing said optical connector.

In many applications of laser light it is advantageous to have high power. While high power lasers may be obtained, such lasers are often cumbersome in use and/or expensive. Furthermore, some applications may require more power than what is available from a single laser.

One approach to overcome such limitations is combining the beam of two or more lasers. Several approaches for combining exist in the art including spectral combining where multiple beams of different wavelength are combined by a wavelength sensitive device, such as a WDM, a prism, a dichroic mirror or a volume Bragg grating. Similarly, beams with non- overlapping polarization may be combined by a polarization sensitive device.

Another approach is coherent combining wherein the interference of the individual laser beams is applied to obtain a high combined power. Finally, spatial combining may be applied, where the individual beams are brought together spatially in a new common spatial distribution or mode. Spatial combining is attractive for some applications as it allows the combination of beams with common wavelength while avoiding the intricate design of a coherent combiner.

For some applications it is attractive to incorporate spatial combination in a fibre optic design, i.e. combining light from two or more feed fibres in a single fibre. This is often advantageous as fibre optics can generally be made more rugged compared to a setup in free space and fibre optics allows simple manipulation of beam paths. Such incorporation involves the challenge to minimize the reduction of brightness of the beams to be combined prior and during said combination, where brightness is defined as the optical power per area and per solid angle. In this context brightness may be reduced by either losses such as due to unwanted reflections, leakage of the light due to undesired location or the like, but likely more importantly by letting modes from one or more feed fibers expand unnecessarily. Such loss of brightness, once introduced, is often unrecoverable.

The object of the present invention is to provide an optical connector which is improved compared with the prior art connectors e.g. with respect to one or more of the properties described above.

Accordingly, the present invention relates to an optical connector having at least one longitudinal axis, said optical connector comprising a first segment having first cross sections perpendicular to the longitudinal axis and a second segment having second cross sections perpendicular to the longitudinal axis, said first and second segments sharing a common border interface, wherein

said second segment comprises a waveguide, having a core with an average effective refractive index n_c and a cladding with an average effective refractive index n_d and

said first segment comprises an internal volume with refractive index n_v selected from the group of a filling material, a void or a combination thereof, where said internal volume is tapered, said taper ending at the intersection of the common border interface and

the first cross sections comprise a volume region, as the cross section of the internal volume, and at least one guiding region surrounding said volume region, said guide region having an average refractive index n^ where n_v< ng__r.

For embodiment where the refractive index n_v varies over the volume region, n_v is taken as the effective refractive index for light guided by the guide region or for which the connector is intended. In one embodiment the guide region is located at the periphery of said volume region whereas other embodiments may have at least part of said first cross sections comprise at least one inner cladding region located at the periphery of said volume region.

In the present context the term taper refers to a reduction of the outer diameter of the object, such as by stretching, collapsing or reduction (e.g. etching).

The longitudinal axis of the optical connector is defined in an unbent connector and is preferably substantially parallel to the core of the waveguide of the second segment. However, for a bent connector the longitudinal axis is taken to follow the bending.

The first and second cross sections are planes perpendicular to the centre axis. Different areas of the cross section performing a function are referred to as regions, such as the guide region being the cross section of a layer or structure designed to guide the injected light toward the core of the waveguide of the second segment. Similarly, a guide region of the second segment is the cross section of the respective core of the waveguide of the second segment. Commonly there will be a single waveguide in the second segment, but in principle the second segment may comprise multiple waveguides and each wave guide may have multiple cores.

The above connector preferably comprises an input end and an output end. The input end is preferably configured to accept light beams injected preferably from one or more fibres referred to as feed fibres or just optical fibres or just fibres. These fibres are preferably arranged to be butt coupled to the connector preferably so that the cores of the feed fibre comprises a cross-sectional overlap with one or more guiding regions of the connector. The feed fibres are preferably fusion spliced to the connector or adhesively connected according to any suitable methods in the field of optics. The waveguide of the second segment preferably comprises an output end which in one embodiment is configured to inject light into another waveguide, preferably adhesively connected to the second segment or any additional segment following the second segment and in another embodiment is configured for emitting light according to the application for which the connector is designed, such as for example combination of output beams from two or more fiber lasers. The output end may further comprise an optical fibre connector, such as and FC/PC connector or any other optical fibre connector known in the field. In one embodiment a delivery fiber is adhesively connected to the second section. In one embodiment, the waveguide of the second section is a delivery fiber.

Alternatively, light may be injected into the optical connector i.e. by lens coupling or other couplings known in the art, such as for example butt- coupling.

In the present context the optical connector is presented as having a first and a second segment. However, it should be noted that the connector may further comprise any number of segment located before the first segment and following the second segment without departing from the scope of the invention.

By this invention the guide region may be designed so as to minimize any undesired expansion of the light to be combined, such as from feed fibres. The guide region preferably has an average refractive index, ng__r, which is higher than the surrounding material, such as the internal volume, so that light is guided by total internal reflection in the guide region. Preferably the guide region is kept at a minimum size while capable of supporting the light from the desired amount of feed fibres or other feeding elements injecting light into the connector. It is also preferable that the cross sectional size of the core of the waveguide of the second segment is kept as close to the sum of the core of the feed fibres as possible. Too small a guide region will result in light being lost in the process of injecting light into the connector and too large a core will reduce the brightness of the combined beam. In one embodiment the optimum size is equal to the sum of the areas of the cores of the feed fibres, but this size may not always be desirable from a manufacturing and/or application perspective. Accordingly, the area of the guide region is preferably larger than 50% of the sum of the cores of the feed fibres for which the connector is designed, such as larger than or equal to 60%, such as larger than or equal to 70%, such as larger than or equal to 80%, such as larger than or equal to 90%, such as larger than or equal to 100%, such as larger than or equal to 110%, such as larger than or equal to 120%, such as larger than or equal to 130%, such as larger than or equal to 140%, such as larger than or equal to 150%, such larger than of equal to 160%, such larger than of equal to 170%, such as larger than or equal to 200%.

In the above discussion of the size of the guide region, it is assumed that the refractive index of the cores of the feed fibres is substantially equal to n_c and that the index contrast between core and cladding is substantially equal as well. In one embodiment substantially matching refractive indices are preferable such as to to avoid undesired reflections. Accordingly, in one preferred embodiment the refractive index of the guide region is substantially equal to the refractive index of at least one feed fibre.

To ensure confinement of injected light to the guide region it is preferred that the first cross sections further comprise a cladding region with an average effective refractive index n_cLr surrounding said volume region and at least one guide region.

The waveguide of the second segment is preferably an optical fibre waveguide or similar in configuration, i.e. a centre core surrounded by a cladding and optionally an outer cladding. In one embodiment the waveguide is, or is similar to, a conventional optical fibre wherein core and/or cladding is a solid material such as glass, plastic or polymer. In another preferred embodiment the waveguide is, or is similar to, a microstructured optical fibre wherein the core and/or cladding comprises optical microstructures. Microstructured optical fibres are well-known in the art, see e.g. A. Tϋnnermann et al., "The renaissance and bright future of fibre lasers", J. Phys. B 38, 2005, pp. 681-693. In one embodiment the waveguide of the second segment comprises multiple cores. Alternatively said waveguide may be a planar waveguide such as a slab waveguide, a rectangular waveguide or a channel waveguide. In principle, the waveguide of the second segment may be any waveguide suitable for carrying the desired wavelength preferably at a size suitable for carrying the spatial combination of the light from the feed fibres. Too small a core will result in light being lost in the process of spatial combination and too large a core will reduce the brightness of the combined beam. In one embodiment the optimum size is equal to the sum of the areas of the cores of the feed fibres, but this size may not always be desirable from a manufacturing and/or application perspective. In principle the guide region of the seconds section may assume any shape and it is commonly an advantage to substantially match the shape of the collapsed or almost collapsed guide region of the first segment with the shape and size of the core region of the second section at the common border interface. Accordingly, the cross sectional area of the core of the waveguide of the second segment is preferably larger than 50% of the sum of the cores of the feed fibres for which the optical connector is designed, such as larger than or equal to 60%, such as larger than or equal to 70%, such as larger than or equal to 80%, such as larger than or equal to 90%, such larger than of equal to 100%, such larger than of equal to 110%, such larger than of equal to 120%, such larger than of equal to 130%, such larger than of equal to 140%, such larger than of equal to 150%, such larger than of equal to 160%, such larger than of equal to 170%, such larger than of equal to 200%. As discussed above, these factors assume substantially identical average refractive indices for core and cladding in feed fibres and the waveguide of the second segment. In yet another embodiment the secondary segment comprises at least one tapered section, said taper affecting at least the core of the waveguide. In one embodiment such tapering is advantageous for matching the waveguide to a secondary waveguide at the output.

The shape of the guide region may in principle include any suitable shape preferably providing convenient injection of light into the connector. In one preferred embodiment the guide region is multiple regions of substantially the same cross-sectional shape as mode of the multiple light beams injected into the connector e.g. from two or more feed fibres. Each guide region section may then follow along the taper of the internal volume toward the core of the waveguide of the second segment or the sections may spiral more or less. In another embodiment the guide region fills larger regions along the periphery which may facilitate manufacturing and/or simplify connection to the light to be injected due to the larger cross sectional area. In one preferred embodiment, the guide region fills the entire periphery. Furthermore, the volume region, i.e. the cross section of the internal volume, is not necessarily round but may assume any suitable shape, such as butterfly, square, rhomb and combinations thereof. Such shapes may be beneficial relative to an application with a specific pattern of injected light and/or in order to adapt smoothly to the shape of a particular waveguide in the second segment and/or at the output end.

In order to preserve the light injected into the connector it may be preferable that shape changes, such as cross sectional change, of the guide region along the taper are minimal. Alternatively it may be preferable that the guide region maintains substantially the same cross-sectional area along the taper. Accordingly, in one embodiment the cross-sectional area of the guide region(s) is substantially identical in substantially all of the first cross sections. Similarly for the cladding region in one embodiment the cross- sectional area of the cladding is substantially identical in substantially all of the first cross sections. In one embodiment all first cross sections are identical apart from a scaling of the volume region.

In one embodiment the internal volume is incorporated into the connector in order to limit the size of the guide region so that minimal expansion of the light injected into the connector, e.g. from a feed fibre, occurs. In one embodiment the guide region(s) is located at the periphery of the internal volume, so that when the internal volume is tapered, the guide region collapses or nearly collapses into a single cross sectional area, which preferably overlaps with the core of the waveguide of the second segment. A peripheral position is advantageous as physical constraints, such as the cross section of a feed fibre, may provide an inherent lower limit to spacing of the light beams injected into the connector. Accordingly, by the internal volume and the peripherally spaced guide region(s) such beams are guided to a common core (the waveguide of the second segment) without allowing, or at least limiting, expansion of the light in the space between the injected light beams. Commonly optical fibres are 100μm in cross sectional diameter or more and thus require a volume region of a substantial size. Accordingly, in a preferred embodiment the longest cross sectional dimension of a volume region is more than 100 μm, such as more than 150 μm, such as more than 200 μm, such as more than 250 μm, such as more than 300 μm, such as more than 350 μm, such as more than 400 μm, such as more than 450 μm, such as more than 500 μm, such as more than 550 μm, such as more than 600 μm, such as more than 650 μm, such as more than 700 μm, such as more than 750 μm, such as more than 800 μm.

In one embodiment, the waveguide of the second segment comprises at least one inner cladding so that light is guided in a ring shaped guide region such as a donut (doughnut) mode. In this context the term ring shaped refers to a guide region surrounding one or more regions of inner cladding. In principle the ring shaped guide region may have any shape such as round, oval, triangular, square, rhomb etc. Furthermore, the inner cladding may comprise one or more layers and/or features a hole or holes and/or microstructures comprising solids such as glass, in particular doped glass and/or air and/or vacuum. In principle the inner cladding may be configured in any manner suitable for confining light to the ring shaped guide region. In one embodiment the inner cladding comprises silica doped to reduce the refractive index relative to undoped silica. In one embodiment the inner cladding comprises Flour-doped glass.

In one embodiment the first segment is adapted so that at least one guide region substantially matches a guide region in the second segment at the common border interface. In one embodiment said guide region in the first segment substantially matches a ring shaped guide region of the second segment. In one embodiment the internal volume of the first segment comprises material that along the taper of the internal volume is arranged to substantially match the inner shape of said ring shaped guide region of the second segment at the common border interface. In one embodiment said material is arranged to provide an inner cladding region in at least part of the first cross sections. In one embodiment the radial thickness of said layer is arranged so that the layer substantially matches at least the outer dimension of the inner cladding of the second segment.

In the art of waveguides comprising an inner cladding region the diameter of the inner cladding relative to the outer diameter of the ring is referred to as the ID/OD of the waveguide. In one embodiment of a combiner according to invention having a second segment comprising a ring shaped guide region a particular advantage is that the ID/OD may be designed substantially freely. In one embodiment it is desirable to maintain brightness which corresponds to holding the area of the guide region of the first section times the numerical aperture of this waveguide squared is maintained constant. In one embodiment this flexibility in design allows for a given delivery fiber arranged to received light combined by the connector to provide the required numerical aperture, while the invention provides for the option to design the outer diameter of the ring while substantially maintaining brightness. In the art of laser cutting metal a donut mode is in some embodiments known to provide a finer cut when the laser energy is distributed as a warm ring surrounding a darker center, i.e. a donut mode. In one embodiment, the designer may be able to provide the optimum size of the ring while substantially maintaining brightness.

To minimize losses and undesired reflection it may be preferable that the taper of the interval volume is comprising at least one essentially continuous graduating section, preferably said taper being essentially continuously graduating. However, in some cases it may be advantageous, e.g. from a manufacturing perspective, to have a step-wise taper comprising one or more steps.

In one embodiment the internal volume is a void filled with air and/or vacuum. In one embodiment the internal volume is filled with a filling material having a refractive index n_v for which n_v< ng__r. In one embodiment the internal volume comprises a void but prior to heating, a plug of filling material is inserted. In one embodiment said plug is arranged to aid in the guiding of light. In one such embodiment the plug comprises glass with relatively lower refractive index than the region adjacent to the , such as Flour doped glass.

The connector and plug is then preferably heated to collapse any spacing around the plug. The internal volume may also be filled with other materials such as gas, a polymer, a liquid material and a combination thereof this having an advantage in providing at least partial control over the refractive index of the internal volume n_gι.

The common border interface of the two segments is in one embodiment defined at a position wherein the waveguide of the second segment may be said to be well defined, i.e. at the transition wherein the light transits from guiding in one or more guide regions located on the periphery of the internal volume to a centre mode i.e. in the core of the waveguide of the second segment.

To facilitate the best transfer of light from the first to the second segment it is preferred that the guide region overlaps with the core of the wave guide of the second segment at the border interface. A substantially cross-sectional match is often preferred to provide a smooth and/or essentially adiabatic transition; however, depending on the method of manufacturing and or application this may not be obtainable and/or desirable. In one embodiment the shape of the guide region of the first section substantially matches the shape of the guide region of the second section at the common border interface. Accordingly, in preferred embodiment said overlap is more than 50%, such as more than 60% overlap, such as more than 70% overlap, such as more than 80% overlap, such as more than 90% overlap, such as more than 99% overlap.

Similarly it is preferable that the cladding region and the cladding of the waveguide of the second segment cross-sectionally overlap the border interface. A perfect cross-sectional match is often preferred to provide a smooth and/or essentially adiabatic transition; however, depending on the method of manufacturing and or application this may not be obtainable and/or desirable. Accordingly, in preferred embodiment said overlap is more than 50%, such as more than 60% overlap, such as more than 70% overlap, such as more than 80% overlap, such as more than 90% overlap, such as more than 99% overlap.

As stated above, the connector is divided into two segments. In ne embodiment the transition between the two segments is substantially continuous and the two segments are preferably produced while attached to each other. In one embodiment the two segments are produced separately and adhesively attached. Preferably by fusion splicing but any suitable optical connection may be applied.

With secured feed fibres and a suitable output end, the optical connector combined with feed fibres may be said to form a new device. Accordingly, the present invention also relates to an optical assembly, such as an optical combiner, such as a signal combiner, comprising an optical connector according to the invention and comprising one or more optical fibres arranged to inject light into one or more guide regions of the first segment of the optical connector. In one preferred embodiment said fibres are arranged in an adapter configured to position the cores of said fibres for injection of light into the guide region(s) of the optical connector. In one embodiment such adapter comprises a tube with one or more longitudinal holes for positioning and/or stacking optical fibres. In one preferred embodiment the feed fibres are stacked in a single circular hole so that for example 6 or more fibres are stacked along the periphery, such as more than 12 or more than 18 fibres etc. To compress the spacing between feed fibres cores the said cores may be stripped of sheath, outer cladding and/or cladding. This may result in a reduction of the necessary guide region as the periphery of the internal volume may be reduced.

Preferably the adapter including optical fibres forms a single output surface suitable for mating with the optical connector. In one embodiment said surface is produce by placing and fixing the fibres in the adapter followed by a cleave and optionally polishing. Preferably the adapter and the optical connector are adhesively connected such as by glue, fusion splicing and the like.

Furthermore, with one or more light sources (often referred to as pumps or signals) attached to the feed fibres, the optical connector, pumps and/or signals, and feed fibres may be said to form a light source. Pumps and signal are preferably relatively inexpensive light sources, such as laser light sources, where a combination of multiple sources to obtain a light source with high power is advantageous. Alternatively, pumps and signals are preferably high power light sources such as thin-disk lasers or fiber lasers. Hence, such light sources comprise laser diodes and diode-pumped solid state lasers

The invention also relates to a method of manufacturing an optical connector according to the invention. As discussed above several approaches exist with regard to manufacturing the invention. In one preferred embodiment the method comprises • providing a preform comprising providing a glass tube having a cross section comprising a cladding region, at least one guide region and a centre volume,

• optionally at least partly filling said centre volume with a filling material,

heating and drawing and/or collapsing of a part of said preform to taper said centre volume. In one embodiment this method provides flexibility in designing the shape of the guide region of the first segment of the connector as tapering of the central volume may be performed by elongation and thereby a reduction of the area of the guide region or collapse while substantially preserves the area of the guide region, or a combination of the two.. In one embodiment said centre volume is tapered so that the first segment comprises first sections wherein the volume region is absent. In another preferred embodiment said centre volume is tapered to a dimension suitable for guiding light either alone or in conjunction with one or more guide regions, i.e. the dimensions of the remaining structure, as opposed to a homogeneous core formed by collapsed guide structure, are preferably optically small so that light guided experience the centre as a single core.

Alternatively the elements of the structure guide the light in cooperation. In one embodiment the method further comprises filling the tapered and/or untapered part with a filling material prior, during and/or post said heating and drawing and/or collapsing. In one embodiment said filling material is selected from the group of a solid, a gas, a liquid and a polymer.

In one embodiment the filling material is arranged to provide at least a part of an inner cladding region of the first segment, so that the material may function to confine the light to one or more guide regions. In one embodiment the filling material comprises material with a refractive index substantially identical to a centre cladding of the second segment of the combiner. This may be particularly practical for producing the first and second segments attached to each other.

Particularly for filling by a solid material it may be advantageous to heat the tapered preform and the filling material to let the material around the centre volume collapse around the filling material.

To achieve optically smooth ends suitable for receiving and/or emitting light the method preferably comprises cleaving either end of the tube. Said cleaving is preferably performed after tapering and/or application of filling material and/or heating. Most preferably the cleaving is performed relatively immediately before coupling with any external elements to avoid mechanical damage to the end surface after cleave and/or dust contamination.

In one one embodiment filling the internal volume comprises depositing a layer on at least part of the surface of said volume. In one embodiment said layer is deposited prior to tapering of the tube. This may e.g. be advantageous for producing a connector with a ring shaped guide region in the second region. Here the deposited layer may be arranged to form the inner cladding of the second segment as the tube is collapsed as part of the taper.

In one embodiment filling the volume comprises inserting a plug into said volume. In one embodiment such a plug made out of a solid may provide additional stability to the connector particularly when heating is required in order to e.g. attach feed fibers via a fusion splice. In one embodiment said plug is shaped to substantially match at least part of said tapered centre volume. One advantage of such shaping may be that a more adiabatic transition to a second segment comprising a ring shaped guide region may be enabled. In one embodiment said shaping comprises tapering the plug by stretching and/or etching. In the event that the tube is collapsed at the end of the taper the small cross-sectional diameter of the tip of the tapered plug may provide a more adiabatic transmission relative as the gap between where the plug ends and the tube collapses may be reduced. On the other hand production and mechanical strength of the plug may be challenged with too small a tip. Accordingly, in one embodiment said plug comprises a tip wherein said tip has a cross-sectional diameter equal to or less than 50μm, such as less than 40μm, such as less than 30μm, such as less than 20μm, such as less than 10μm. In this context the term diameter is to taken broadly, i.e. as a typical dimension on the event that the tip has a non-circular cross section.

In one embodiment the plug comprises one or more waveguides. In one embodiment such waveguides are used to provide a pilot beam i.e. a beam of typically visible light which may make it easier to aim an output of combiner, e.g. a laser welding beam. A waveguides aimed at providing a pilot beam may be referred to as a pilot waveguide. In one embodiment the plug comprises a central pilot waveguide.

In one preferred embodiment the guide region comprises one or more sacrificial zones which may be removed in order to reduce guide region, which will further reduce the volume in which injected light may expand which in turn will increase the overall brightness of the combined beam. These sacrificial zones may be removed by any suitable means known from optical processing, such as etching, mechanical removal, sputtering or the like. In one preferred embodiment said etching is performed with HF or with SF6 using an MCVD lathe which provides the possibility of controlling the amount of etched material in a sacrificial zone with high accuracy.

In the context of the present invention the optical connector may be manufactured in any material with suitable optical properties for the wavelength/wavelengths for which is intended. Examples of such material include glass, silica glass, polymers, plastic, doped glass etc.

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

FIG. 1 shows the end facet of the first segment of a connector according to the invention,

FIG. 2 shows an adapter according to the invention,

FIG. 3 shows the end face of the adapter of figure 2,

FIG. 4 shows an alternative end face of a an adapter according to the invention,

FIG. 5 shows an optical connector and adapter according to the invention,

FIG. 6 shows the end face of the second segment of a connector according to the invention. Fig. 7 shows an exemplary manufacture of a connector having a ring shaped guide region in the second segment.

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 may be left out.

FIG. 1 shows the first segment of a connector 11 according to the invention with a volume region 1 , a guide region 2, a cladding region 3 and an outer cladding 4. In the present example the guide region 2 and the outer cladding 4 are both made of SiO₂ whereas the cladding region 3 is made of flour doped SiO₂ and the volume region 1 is a void. In this example doping level of the cladding region 3 is adjusted to obtain a NA of 0.15 - 0.22 similar to conventional fibres. To adapt to such fibre cores the thickness of the guide region 2 is 20 μm and the thickness of the cladding region is also preferably 20 μm. The inner diameter, i.e. the diameter the outer diameter of the guide region is preferably approximately 330 μm to allow a simple fit of the core diameters of the feed fibres. To obtain good mechanical strength a suitable outer diameter of the outer cladding 4 is in the order of 800μm.

FIG. 2 shows an adapter 22 suitable for injection of light into a connector according to figure 1. The light is injected from six fibres 6 into a connector according to figure with end facets 7 sharing a common end facet 8 with the adapter 22.

FIG. 3 shows one embodiment of an adapter 22 according to figure 2 where six holes are applied to hold the fiber tips 8 in the facet 7. The dimensions x and y are in this example 350μm and 528μm respectively. In this configuration a dark centre fibre is used to distribute the six live fibre ends 8 along the periphery. The parameters of this example are core diameter of 15 μm of the fibres and their NA is chosen to be slightly multimoded to splice well to for example a signal delivery fibre of a fiber laser or fiber laser sub- assembly, where the signal delivery fiber has a core diameter of around 15 μm (see e.g. commercially available product aeroLASE350 from Crystal Fibre A/S).

FIG. 4 shows a cross-section of a preform for producing an adapter with a configuration as in figure 3. The figure shows the adapter prior to fusing, where dark regions indicate gaps or voids. The outer diameter of the individual fibres is visible.

FIG. 5 shows three-dimensional view of a connector 11 and an adapter 22 according to the invention where a plug (for example of flour doped glass) has been inserted so as to avoid a collapse of the void 1 when heatzone 1 is applied to fuse the two components (11 and 22) together. A border interface 9 is indicated separating the first segment 55 and the second segment 56. The border interface 9 is defined where the guide region 2 collapses to a single region. Heatzone 2 has been applied according to the method of manufacture described herein so as to produce the shown taper of the inner volume 1.

FIG. 6 shows the second cross section 66 of the connector 11 where the void 1 of figure 1 has been collapsed (this corresponds to cross section 9 in Figure 5). Exemplary dimensions according to the previous examples are a multimode core 2 of: diameter around 200 μm, NA around 0.06 (exemplary alternatives could be a diameter of around 50 μm and NA around 0.22). The thickness of the cladding 3 is ~60 μm and in the case of an initial 800 μm outer diameter, the outer diameter of the second segment is approximate 740 μm. In the case of an adaptor such the one described in Figure 3, and in the perfect case of no NA up-conversion, the M^Λ2 of the beam leaving cross section 66 is 20. For comparison, consider the case where no internal volume, no collapse and no taper is used. In this case, a MM fiber is used, having a core diameter large enough to cover all cores of the adaptor in Figure 3. Such core diameter would be about 365 μm, resulting in an M^Λ2 of 365/200 = ~37. Thus, in this example, the implementation of the internal volume nearly doubles the beam quality parameter.

FIG. 7 shows an exemplary manufacture of a connector comprising a second segment 56 with a ring shaped guide region 2. Fig. 7a shows a glass tube 74 comprising a cladding region 3 surrounding a guide region 2 both surrounded by an outer cladding 4. The glass tube 74 is in the process of being slid over the tapered plug 73. In Fig. 7b the plug is placed inside the tube 74. In Fig. 7c the glass tube has been collapsed over the plug 73. The first and second segments (55, 56) are seen as marked here by the line 9. A first cross section 75 is shown along with a second cross section 76 where it is noted that the ID/OD is changed substantially from one to the other. In one embodiment the cross sections 75 and 76 are the input and output sides of the connector. The feature of changing the ID/OD may be useful for such applications as laser welding and laser cutting where high intensity laser light is focused on a subject. Such applications often prefer a donut mode output having a particular ID/OD in combination with high intensity laser light.

Claims

1. An optical connector having at least one longitudinal axis, said optical connector comprising a first segment having first cross sections perpendicular to the longitudinal axis and a second segment having second cross sections perpendicular to the longitudinal axis said first and second segments sharing a common border interface, wherein

the first cross sections comprise a volume region, as the cross section of the internal volume, and at least one guide region surrounding said volume region, said guide region having an average refractive index n^ where n_v< ng__r.

2. The optical connector according to claim 1 wherein at least part of said first cross sections comprise at least one guide region located at the periphery of said volume region.

3. The optical connector according to claims 1 or 2 wherein said first cross sections further comprise a cladding region with an average effective refractive index n_cl__r surrounding said volume region and at least one guide region.

4. The optical connection according to claims 1 or 3 wherein at least part of said first cross sections comprise at least one inner cladding region located at the periphery of said volume region.

5. The optical connector according to claims 3 or 4 wherein the area of the cladding region and/or guide region decreases along with the area of the volume region along at least part of the tapered internal volume.

6. The optical connector according to any of the claims 3 to 5 wherein the area of the cladding region and/or guide region is substantially constant along at least part of the tapered internal volume.

7. The optical connector according to any of the preceding claims wherein the cladding region in the first cross sections overlaps with the cladding of the wave guide of the second segment at the border interface.

8. The optical connector according to any of the preceding claims wherein the guide region in the first cross sections overlaps with the core of the wave guide of the second segment at the border interface.

9. The optical connector according to claims 7 and/or 8 wherein said overlap is more than 50%, such as more than 60% overlap, such as more than 70% overlap, such as more than 80% overlap, such as more than 90% overlap, such as more than 99% overlap, such as 100% overlap.

10. The optical connector according to any of the preceding claims wherein waveguide of the second segment comprises at least one inner cladding and a ring shaped guide region

11. The optical connector according claim 10 wherein said ring shaped guide region may have any shape such as round, oval, triangular, square, rhomb.

12. The optical connector according claim 10 or 11 wherein said inner cladding comprises one or more layers and/or features selected from group of a hole or holes, microstructures comprising a solid, microstructures comprising glass, microstructures comprising doped glass, microstructures comprising air and/or vacuum.

13. The optical connector according to any of the claims 10 to 12 wherein the inner cladding arranged to confine light to the ring shaped guide region

14. The optical connector according to any of the claims 10 to 13 wherein the inner cladding comprises silica doped to reduce the refractive index relative to undoped silica.

15. The optical connector according to any of the claims 10 to 14 wherein inner cladding comprises Flour-doped glass.

16. The optical connector according to any of the preceding claims wherein the waveguide of the second segment comprises at least one central core surrounded by a cladding.

17. The optical connector according to any of the preceding claims wherein the first segment is adapted so that at least one guide region substantially matches a guide region in the second segment at the common border interface.

18. The optical connector according to claim 17 wherein said guide region in the first segment substantially matches a ring shaped guide region of the second segment.

19. The optical connector according to claim 17 or 18 wherein the internal volume of the first segment comprises material that along the taper of the internal volume is arranged to substantially match the inner shape of said ring shaped guide region of the second segment at the common border interface.

20. The optical connector according to claim 19 wherein said material is arranged to provide an inner cladding region in at least part of the first cross sections.

21. The optical connector according to claims 19 or 20 wherein has a radial thickness arranged so that the layer substantially matches at least the outer dimension of the inner cladding of the second segment.

22. The optical connector according to any of the preceding claims wherein the waveguide of the second segment is a microstructured optical fibre.

23. The optical connector according to any of the preceding claims wherein the waveguide of the second segment comprises multiple cores.

24. The optical connector according to any of the preceding claims wherein the waveguide of the second segment is a planar waveguide.

25. The optical connector according to any of the preceding claims wherein said taper comprises at least one essentially continuously graduating section, preferably said taper being essentially continuously graduating.

26. The optical connector according to any of the preceding claims wherein said taper is step wise comprising one or more steps.

27. The optical connector according to any of the claims 7 to 26, wherein the transition at the border interface along the core region and core is essentially adiabatic.

28. The optical connector according to any of the claims 4 to 27, wherein the transition at the border interface along the cladding region and cladding is essentially adiabatic.

29. The optical connector according to any of the claims 3 to 28 wherein the refractive index of n_cU substantially equal to n_ώ and/or n^, is substantially equal to n_c.

30. The optical connector according to any of the preceding claims, wherein said internal volume is at least partially filled by a material with an average refractive index n_g) <n_g__r.

31. The optical connector according to claim 30 where said material is selected from the group of a gas, a polymer, a liquid material and a combination thereof.

32. The optical connector according to any of the preceding claims, wherein the volume region in any cross section comprises at least one of the following shapes: circular, oval, butterfly, square, rhomb and combinations thereof.

33. The optical connector according to any of the preceding claims wherein said first cross sections comprises one guide region surrounding the volume region.

34. The optical connector according to any of the preceding claims wherein said first cross sections comprises two or more guide regions surrounding the volume region.

35. The optical connector according to any of the preceding claims wherein the cross-sectional area of the guide region(s) is substantially identical in substantially all of the first cross sections.

36. The optical connector according to any of the preceding claims wherein the cross-sectional area of the cladding is substantially identical in substantially all of the first cross sections.

37. The optical connector according to any of the preceding claims wherein said second segment comprises at least one tapered section, said taper affecting at least the core.

38. The optical connector according to any of the preceding claims further comprising an end configured for receiving light from two or more optical fibre cores.

39. The optical connector according to any of the preceding claims further comprising an end configured for injecting light into a fibre waveguide.

40. The optical connector according to any of the preceding claims wherein the longest cross sectional dimension of a volume region is more than 100 μm, such as more than 150 μm, such as more than 200 μm, such as more than 250 μm, such as more than 300 μm, such as more than 350 μm, such as more than 400 μm, such as more than 450 μm, such as more than 500 μm, such as more than 550 μm, such as more than 600 μm, such as more than 650 μm, such as more than 700 μm, such as more than 750 μm, such as more than 800 μm.

41. An optical assembly comprising an optical connector according to any of the claims 1 to 41 and further comprising one or more optical fibres arranged to inject light into one or more guide regions of the first segment of the optical connector.

42. The assembly of claim 41 , wherein said fibre(s) is attached to the optical connector by one or more of the following: glued, fused, fusion spliced.

43. The assembly of claims 41 or 42 wherein said fibre(s) is arranged in an adapter.

44. The assembly of claims 43 wherein said adapter comprises a tube with one or more longitudinal holes for positioning and/or stacking optical fibres.

45. The assembly of claims 43 and/or 44 wherein 6 or more fibres are stacked on one hole, such as 12 or more fibers are stacked, such 18 or more.

46. The assembly of any of the claims 43 to 45 wherein said adapter including fibres forms a single output surface suitable for mating with said optical connector.

47. The assembly of any of the claims 43 to 46 wherein the adapter and the connector are fused together and/or adhesively connected.

48. The assembly of any of the claims 44 to 47 wherein the cross sectional area of the core of the waveguide of the second segment is preferably larger than 50% of the sum of the cores of the feed fibres for which the optical connector is designed, such as larger than or equal to 60%, such as larger than or equal to 70%, such as larger than or equal to 80%, such as larger than or equal to 90%, such larger than of equal to 100%, such larger than of equal to 110%, such larger than of equal to 120%, such larger than of equal to 130%, such larger than of equal to 140%, such larger than of equal to 150%, such larger than of equal to 160%, such larger than of equal to 170%, such larger than of equal to 200%.

49. A method of manufacturing an optical connector comprising obtaining an optical connector according to any of claim 1 to 40.

50. The method of claim 49 comprising

• providing a preform comprising providing glass tube having a cross section comprising a cladding region, at least one guide region and a centre volume,

• heating and drawing and/or collapsing of a part of said preform to taper said centre volume.

51. The method of claims 49 or 50 comprising producing the first and second segments of the optical connector with said sections attached to each other.

52. The method of any of the claims 49 to 51 comprising producing the first and second segments of the optical connector separately and subsequently joining said sections adhesively.

53. The method of any of the claims 49 to 52, wherein said centre volume is so that the first segment comprises first sections wherein the volume region is absent.

54. The method claim 52 or 53, wherein said centre volume is tapered to a dimension suitable for guiding light either alone or in conjunction with one or more guide regions.

55. The method of any of claims 49 or 54 further comprising filling at least a part of the tapered part and/or untapered part of the centre volume with a filling material prior, during and/or post said heating and drawing /or collapsing.

56. The method of claim 55 wherein said filling material is arranged to provide at least a part of an inner cladding region of the first segment.

57. The method of claim 55 or 56 wherein said filling material comprises material with a refractive index substantially identical to a centre cladding of the second segment of the combiner.

58 The method any of the claims 55 to 57, wherein said filling material comprises one or more selected from the group of a solid, a gas, a liquid, a gel, and a polymer.

59. The method any of the claims 55 to 58further comprising heating at least part of the tube and/or filling material in order to collapse spacing around said filling material.

60. The method of any of the claims 49 to 59 further comprising cleaving either end of the tube.

61. The method of claim 60 wherein said cleaving is performed after tapering and/or application of filling material and/or heating.

62. The method of any of the claim 49 to 61 wherein filling the volume comprises depositing a layer on at least part of the surface of said volume.

63. The method of any of the claim 49 to 62 wherein filling the volume comprises inserting a plug into said volume.

64. The method of claim 63 wherein comprising shaping said plug to substantially match at least pan" of said tapered centre volume.

65. The method of claim 64 wherein said shaping comprises tapering the plug by stretching and/or etching.

66. The method of any of the claims 63 to 65 where said plug comprises a tip wherein said tip has a cross-sectional diameter equal to or less than 50μm, such as less than 40μm, such as less than 30μm, such as less than 20μm, such as less than 10μm.

67. The method of any of claims 63 to 66 wherein said plug comprises arranging at least one waveguide in said plug.

68. The method of claim 67 wherein said waveguide is a pilot waveguide.

69. The method of any of the claim 49 to 68 wherein said guide region comprises one or more solid sacrificial regions.

70. The method of any of the claims 49 to 69 further comprising the etching of one or more parts of the guide region along at least a part of the tube.

71. The method of claim 70 where said etching is performed by at least one of: acid etching, HF etching, SF6 with MCVD lathe.

72. A light source comprising two or more light sources optically connected to an optical connector according to any of the claims 1 to 40.